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autonomous control of an unstable model helicopter

1. 35 40 Figure 2 9 Cost function as a function of iterations for 20 global then local iterations The horizontal axis is the number of iterations the vertical axis is the cost function in units of square wavelengths Seventy three percent worked 56 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS Half Global 15 20 25 30 35 40 Figure 2 10 Cost function as a function of iterations for alternating 2 global 2 local iterations The horizontal axis is the number of iterations the vertical axis is the cost function in units of square wavelengths Eighty six percent worked Chapter 3 Hardware This chapter describes the hardware used in this project consisting of the helicopter itself the electronics on the helicopter the ground station electronics and some of the support equipment Details of connections power supplies and mounting are supplied in the appendices 3 1 Helicopter The main design objectives which were satisfied to varying degrees when choosing the helicopter were e Payload capability e Inherent dynamics A helicopter in hover is inherently unstable e Low vibration e Mechanical Strength e Control Authority e Durability e Ease of construction e Ease of replacement 57 58 CHAPTER 3 HARDWARE Figure 3 1 The helicopter loaded with electronics It was decided to use a commercially available mode
2. 2 WY ze To 5 5 TSR EXE OE OE N Oo Ns N oO D0 as gt DO Se To Disk Packet Data Packet Driver Driver Acknowledgements Base Station Helicopter 486 Figure 4 6 Data transfer involved in down loading flight data from the helicopter 486 4 2 68HC11 PROGRAMS 85 4 1 4 Post processing Rather than log the calculated positions and attitudes all the GPS raw data is logged This enables one to study the behaviour of the sensor in more detail and study the effects of changes in algorithms as well as generating plots like figure 2 7 In order to reduce the possibility for errors exactly the same GPS software is used to postprocess the raw GPS data as is used on the helicopter The data can be replayed with the current softwarein two ways One way is to replay the data in real time quantised to 1 18 2 second time intervals the other method is to send packets to the GPS program one packet at a time sending a new packet as soon as the current packet has been fully dealt with This latter approach has the advantage of being significantly faster and generally more convenient The results of the processing are saved to files by checking MS DOS environment variables when an interesting value has been calculated to see whether that value should be saved to a file somewhere This means that the only difference between the post processing setup and the helicopter is a few environment variables and th
3. 0 0 0 000002 0 22 ee 128 B 2 6 C classes 2 a 128 B 2 7 Interrupt Usage saaa a 130 B 2 8 Source Files 2 ee 131 Running the program 2 a 131 B 3 1 Serial port specific ommand line options 132 B 3 2 Global Command line options 133 Parallel Port aoaaa ee 135 Packet Protocols 2 2 2 ee 136 B 5 1 GPS packets 2 2 0 0 0 02 02 02 02 ee ee ee eee 136 B 5 2 Radio Packets 2 2 ee ee 136 Interface with an external program 2 2 00004 138 B 6 1 User command list 2 2 0 0 0 0 0 2 20 0200 139 B 6 2 More details on remote exe 2 0 0 0 0 000200000 143 B 6 3 Verification sum com 2 2 0 ee ee ee 144 B 6 4 GPS packet functions 2 2 2 02 0200 145 B 6 5 GPS Units 2 0 0 02 0 2002 02 ee ee ee 146 Log file XMS 2 2 ee 146 B 7 1 Log file format 2 2 ee 147 B 7 2 Sources in log file 2 2 0 0 2 0 00 002 0000 147 Skeleton client program 1 2 2 ee ee ee 147 xi C IBM Box manual C 1 C 2 C 3 C 4 D 1 D 2 D 3 D 4 D 5 E 1 E 2 E 3 E 4 E 5 IBM PC card and serial card C 1 1 Mechanical Description Modifications to serial board Timer 68HC11 board C 3 1 Power for HC11 board C 3 2 Board construction Programs Running on HC11 GPS Box manual Mechanical Description D 1 1 Vibration DB 25 connector Internal Electronics Ground Power Supply Supplier I
4. 60 600700 600720 600740 time teh 600780 600800 600820 time s Figure 5 13 Pilot pitch commands whilst under computer control 105 106 CHAPTER 5 FLIGHT TEST RESULTS Flight Test 2 31 Dec 1994 135 T T T 130 J Y LI Roll joystick input servo units 95 600700 600720 600740 fetes 600780 600800 600820 time s Figure 5 14 Pilot roll commands whilst under computer control some correlation This can be remedied by looking at the effects of the single degree of freedom slews provided about a minute later at the end of the computer controlled time period For altitude the effect of the yaw pitch and roll slews was small compared to the size of the slews themselves so the time constant for damping can be seen from figure 5 6 to be about five seconds Similarly the transient responses for the horizontal displacements have a time constant of about four to five seconds for roll transverse motion mainly east and five to six seconds for pitch longitudinal motion mainly north It is difficult to be more precise without a very detailed analysis and it is difficult to get details of oscillatory behaviour due to the concurrent presence of the eleven second period limit cycle The time constant for yaw variations is more difficult to measure as there is no real slew in yaw Y when the computer is turned on However there is an excellent slew commanded offset put in manually later
5. E S dt 7 Ar s Aus noise in noise in satellite distance Table 2 1 Order of magnitude estimates of the sizes of various quantities Rough size 10710 55 107655 1075s considered 10753 observed 107 107 1073 5 1 107887 Stationary Receiver l 1077s7 Receiver doing 3g accelerations 1078s want accurate to 1071s want accurate to 1079s 1071 s observed 10 observed 10 s 21 22 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS 2 3 2 Position Position is calculated relative to a receiver on the ground in a differential manner To give an intuitive idea of the concepts before going into a more careful analysis of small corrections one has two antennas typically one on the ground the base station and one on the object being sensed Each antenna looks at several common satellites There will be a phase difference in the signal measured between the antennas as shown in figure 2 1 This phase difference will come partially from the distance between the two antennas in the direction of the satellite s Aa and partially from the time offset in the local clocks In matrix form one has the following equation Az J tt e aa an 7 By observing n satellites one gets several of these equations and one builds up a possibly over determined matrix equation which can be solved in a least squared sense to get Aw and 7 as long as n gt 4 S1 1 A 892 1 Ar A 2 2 T Sn 1 AP In practice t
6. Sn 1 9 P Pr On Note that the values on the right hand side of equation 2 10 are the phases directly from the TANS after adjustment by the integers which are assumed known at this point The matrix on the left hand side contains the satellite line of sight unit vectors which are interpolations or extrapolations of the line of sight vectors produced by the TANS The phase derivative is not directly observable but a perfectly good substitute is given by the phase velocity measurement This is not perfectly accurate see equation 2 11 but the error is of size 107 which when multiplied by 7 of up to 107 s gives an error contribution of 10 s which is sub millimetre One way of looking at this initially non intuitive correction term of fort is as a doppler correction term a time offset in the receivers will affect the measured phase differently depending upon the carrier frequency which is affected by doppler which can be considered an interpretation of are 2 3 3 Velocity Having a velocity signal is very nice for control systems as it allows one to add damping As the GPS system fundamentally is designed around position especially with the TANS units and GP B software the easiest way to obtain a velocity estimate is through calculating the difference between the current position and the last computed position and dividing by the time interval This is of course fairly noisy but more importantly for a control system
7. process this byte through I_spi_receive I_spi_transmit Load a character into the SPI transmit register from the SPI buffer or a NULL character if there are none Isci_receive Receives a character from the SCI Can do three things with it e If the character is an unescaped meta character store that in the sci_escape variable if the meta character is an escape or pwm character otherwise it is a null meta character and is ignored e If the character is some PWM data store it in the appropriate location This differs for the master and slave as they expect different amounts of information in this packet e If the character is normal data feed through store the character in the SPI transmit buffer I_spi_receive Basically the same as I_sci_receive except with the SPI SPI_xmit Sticks the byte in the accumulator into the SPI transmit buffer SCI_xmit Sticks the byte in the accumulator into the SCI transmit buffer ClearCOP Clears the Computer Operated Properly timer Next come two tables that contain the addresses of the data that comes in the PWM packets the first for the SPI and the second for the SCI Changing the protocol to send or receive different data just requires adding or modifying the addresses here The I_sXi_receive routines automatically use these tables and their length to implicitly define the protocol It is of course essential that the slave and master protocols match 170 APPENDIX C IBM BOX MANUAL T
8. Ai y bo R s 64 2 57 tE times One iteration of the pseudo global algorithm then consists of minimising with respect to 3N attitude variables and then the J integers As previously the pseudo global algorithm works better than Newton iteration with a poor initial guess such as all attitudes starting the same and the integers starting off as the average of the phases and Newton iteration converges more rapidly when one is near the solution 52 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS As a slight variation on this algorithm the global minimisation can be performed with respect to only azimuth on the first iteration if it is known that most of the angular movement is going to be in azimuth This can increase the rate of convergence Another slight variation is to modify the local iteration step to do a pseudo global step if the matrices used in its calculation ever become too badly ill conditioned which happens occasionally This will increase reliability An algorithm to actually minimise equation 2 54 consists of several iterations of the pseudo global or local algorithms Simulation showed that about 20 iterations of the pseudo global algorithm were needed to get a reasonable first estimate for the local algorithm which is then applied until the cost function stops decreasing A slightly better algorithm still consists of alternating pairs of pseudo global and local iteration steps The reason for dividing into pairs
9. December 1992 Frederick G Edwards and Peter V W Loomis Civil Helicopter Flight Operations Using Differential GPS volume 3 pages 173 193 Institute of Navigation Wash ington D C 1986 K Ferguson J Kosmalska M Kuhl J M Eichner K Kepski and R Abtahi In IEEE PLANS March 1991 F Van Graas and M Braasch Gps interferometric attitude and heading determi nation initial flight test results Navigation Journal of the Institute of Navigation 38 4 297 316 1991 1992 Paul Hudak Conception evolution and application of functional programming languages ACM Computing Surveys 21 3 359 411 1989 P Y C Hwang Kinematic gps resolving integer ambiguities on the fly In IEEE PLANS pages 579 586 March 1990 M Anthony Lewis Andrew H Fagg and George A Bekey The USC autonomous flying vehicle An experiment on real time behavior control In IEEE 1993 Inter national Converence on Robotics and Automation pages 422 429 May 1993 P V W Loomis A kinematic gps double differencing algorithm 13 17 March 1989 G Lu M E Cannon G Lachapelle and P Kielland Attitude determination us ing dedicated and nondedicated multiantenna gps sensors IEEE Transactions on Aerospace and Electronic Systems 30 4 1053 1058 Oct 1994 BIBLIOGRAPHY 193 18 Paul Y montgomery Hirohiko Uematsu and Bradford W Parkinson Analysis of angular velocity determination using GPS Sep 1994 Salt Lake City 19 H S Satz D B Cox
10. PRN signal is modulated onto this signal This signal has a period of one millisecond and the 1023 bits for each satellite are chosen to be orthogonal to each 14 15 other in a correlation sense This means that a receiver can have a bandpass filter that picks out the 1 57542 GHz frequency called L1 down shifts it to a more manageable frequency and then does a correlation with the 1 023 MHz PRN signal for a given satellite that is expected to be visible A phase locked loop then can be used to keep track of the observed phase of this 1 023 MHz signal This phase modulo some number of milliseconds then gives the pseudo range value Further modulated onto this signal is a 50 Hz data signal which contains information for receivers about where satellites are what their status is etc This coupled with knowledge of the satellite geometries allows a receiver to work out which millisecond is being dealt with and compute the position A more precise method of ranging the P code is also available It broadcasts at a different frequency L2 1 2276 GHz and the pseudo random noise with which each satellite modulates the carrier is at a ten times higher frequency so phase measurements are more accurate and only repeats once a week Locking on to this signal is rather difficult until one has a fairly good idea where one is and what time offset the receiver has This is usually accomplished through the C A code first Thus a P code receiver
11. R L Beard and G P Landis Gps inertial attitude estimation via carrier accumulated phase measurements Navigation Journal of the Institute of Navigation 38 3 273 284 1991 20 S Schneider Experiments in the Dynamic and Strategic Control of Cooperating Manipulators PhD thesis Stanford University Stanford CA 94305 September 1989 Also published as SUDAAR 586 21 V W Spinney Applications of the global positioning system as an attitude reference for near earth uses Navigation Journal of the Institute of Navigation April 1976 22 Boleslaw K Szymanski editor Parallel Functional Languages and Compilers ACM Press New York New York 1991 23 S ShouHan Wang and A K Uht Ideograph ideogram framework hardware for eager evaluation Micro 23 125 134 1990 24 K R Zimmerman and R H Cannon Jr GPS Based Control for Space Vehicle Rendezvous In Proceedings of the ASCE Robotics for Challenging Environments Albuquerque NM February 28 March 3 1994
12. Table D 4 Unofficial pin out of the 28 pin inter board connector in the TANS vector and Quadrex D5 SUPPLIER INFORMATION 179 Power Switch O 7V out 12V DC in 12V DC out 270 VZ Power LED on switch GND Figure D 4 Circuit Diagram for the power supply for the ground based electronics Appendix E GPS Program This appendix describes the function of the program GPS which sits logically on top of TSR processing phase measurements producing position and attitude data and using these data to compute servo controls which are then sent to the servos through TSR A full description of what gets computed where and why is given in section 4 3 This appendix contains information more useful to a person trying to modify or understand the program E 1 Interface to TSR There are three main types of communication between GPS and TSR e TSR GPS Data packets from the GPS receivers the ground station or Con sole commands e GPS TSR Initialisation packets for the GPS receivers e GPS TSR LED control indicating status e GPS TSR Servo Control e Initialising above communication LED and servo control are performed through the normal software interrupt scheme detailed in section B 6 Reception and transmission of GPS packets is as described in 180 E 2 PROCESSING OF PACKETS 181 section B 6 4 Whenever the callback function in GPS is invoked by TSR a packet of data is availabl
13. it introduces some extra data delay into the system the data of an 2 3 ASSUME THE INTEGERS ARE KNOWN 25 extra sample ago a tenth of a second is being used in the current estimate of velocity Filters can reduce the effect of the noise but that tends to increase the data delay still further This is very bad for control data delay can easily mean the difference between instability and stability The GPS receivers measure phase directly that is the fundamental measurement But internally they contain a filter that maintains an estimate of the phase velocity at the local update rate that is typically much higher than the rate at which the phase data is sent over the serial port For the TANS units used on the helicopter the internal update rate is more than an order of magnitude higher update rate than the output rate of 10Hz In principle it could be significantly higher This means that the delay in the measured velocity signal could be less if it were computed using these data Paul Montgomery with some help from me modified the TANS software to emit these phase velocities with precision of 0 03Hz six millimetres per second The noise tended to be a bit under one hertz upon experimentation which is comparable to the noise in first difference estimates The signal produced D is the derivative of the measured phase of satellite with respect to the local clock in receiver r Again as for position the difference between local and g
14. obtained attitude using a carrier phase receiver with multiplexed inputs One receiver would then look at each of the four antennas in turn and would measure the phase difference between each antenna and some master antenna This has the advantage over other similar approaches with multiple receivers of avoiding the time difference between the local clocks in the receiver and probably being cheaper to manufacture There is a disadvantage in that high unexpected accelerations can cause cycle slips if the vehicle moves too far unexpectedly in the time period when the moving antenna is disconnected but this is not a problem for movements likely on a helicopter The observed phases the length of the baseline in the direction of the satellite are given by the equation P B R s 2 16 Here b is the vector distance between the master antenna and a slave antenna i as measured in the body base reference frame called baseline i As usual s is a line of sight unit vector to satellite 7 and R is a rotation matrix that maps vectors between the actual reference frame and the base reference frame in which the baseline vectors b were measured Note that a vector dot product notation is used rather than a matrix multiplication notation as it makes it clear that a scalar is being produced and is hopefully easier to read The measured phases will differ from the observed phases in equation 2 16 by several terms e The integer offset whic
15. sn 1 26 1 DZP DIE n E 2 15 where s and r represent the two receivers in either order In practice with the current receivers used on the helicopter it is perfectly fine to ignore the factor of 1 1 7 as the correction terms from the position calculation are small so the net result will be to just overestimate the velocity by a factor of 1 7 which is tiny compared to the noise for anything subsonic The value of 7 can be obtained fairly easily from differencing the clock bias information in the pseudo range position packets from the TANS In practice it is also fine to ignore the factor of 2 which appeared in the leftmost matrix in equation 2 15 as the term it multiplies also has a small effect with the current level of noise This seemingly pointless approximation is mentioned here since there exist significant intermediate computations the Cholsky factorisation of the matrix in 2 3 ASSUME THE INTEGERS ARE KNOWN 27 question from the position calculation step that can be reused saving execution time if the leftmost matrix in equations 2 10 and 2 15 are the same 2 3 4 Attitude Given the relative position of antennas to the order of a centimetre it is possible to use GPS information to determine attitude suggested in 21 with real time work in eg 12 11 9 19 17 Clearly at least three antennas are needed and having more antennas will help increase robustness through data redundancy Cohen 9
16. three things firstly the symbolic name as defined in service h Secondly in italics the commands in remote exe directly attached with that command and thirdly a description of that command B 6 INTERFACE WITH AN EXTERNAL PROGRAM B 6 1 User command list SERVICE_IS_PRESENT return 1 in AX if tsr is present SERVICE_SEND_FILE send the file name in es bx to the remote computer SERVICE_REQUEST_FILE get the file name in es bx from the remote computer SERVICE_ENABLE_OP enable echo of console character output to remote computer SERVICE_DISABLE_OP disable disable echo of console character output to remote computer SERVICE_REM_ENABLE_OP renable enable echo of remote con sole character output to this computer SERVICE_REM_DISABLE_OP rdisable disable echo of remote con sole character output to this computer SERVICE_REMOVE_THYSELF remove Remove TSR Free mem ory and restore interrupt vectors SERVICE_GET_PWMS_IN returns in es bx the address of the 9 byte location where PWM values incoming are stored 140 APPENDIX B TSR USER S MANUAL SERVICE_ GET_PWMS_OUT returns in es bx the address of the 9 byte location where PWM values outgoing are stored SERVICE_SEND_CHAR line Sends a specific character key in bx to the remote com space puter as if the character had escape come from the remote com nl puter s keyboard SERVICE_SEND_PWMS_ENABLE pwmenable enable sending the current
17. 32 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS accomplished via manipulations of amp and 2 assume that d gt 0 and dy gt 0 Divide through by d and make the substitution z V i B d d gt 0and amp ig Then one wants to find z to minimise C x cos2z B cos x 2 23 Actually finding the minimum of this equation can be done efficiently computationally a description of the algorithm is given in section 2 3 6 Given a value of z one can apply the above simplifications backwards to obtain an optimal value of V to minimise equation 2 19 This can then be used to modify the current attitude estimate One then repeats this process for rotations around the other two axes mutatis mutandis and this makes up a single iteration of my pseudo global iteration algorithm An iteration of this algorithm is generally better than the normal linearise and solve method Newton s method when the estimate is far from the true value as it can jump over ridges as are evident in figure 2 2 whereas Newton s method provides quadratic convergence near the true value and is thus superior for later steps of iteration For this reason in practice two iterations of this global algorithm followed by Newton s algorithm until the value of the cost function stops decreasing works very well typically taking one to two Newton iterations In practice this is not a large improvement here Newton s algorithm does work adequately for t
18. GPS attitude sensor may 114 115 eventually compete with the low quality mechanical gyroscopes currently used on model helicopters At the moment the GPS sensor being used runs at an update rate of 10 Hz but this is solely a function of the limitations of the particular hardware we are using this can be vastly increased in principle with more powerful hardware and so a similar system could be used to control an exceedingly unstable helicopter or other system Similarly the hardware currently used can only track up to six satellites concurrently This is not a major disadvantage given the current number of satellites physically in existence but seeing a larger number of satellites would make the algorithms used here even more useful and adding extra channels to the receivers is not difficult in principle In the shorter term there is still an enormous variety of work to improve the system we are using Technology has improved since the helicopter was originally designed and many of the components can be replaced by smaller faster more reliable devices mounted more carefully to avoid vibration damage and thus improve the performance and reliability of the helicopter The current control laws are sufficient to demonstrate the capability of stabilisa tion using GPS but they do not use the full power of the sensor A careful system identification and better control law design should be able to improve the performance significantly and
19. GPS receivers A summary of the power system on the helicopter is shown in figure 3 8 As a last safety resort in case the 68HC11 board fails which it never has yet there is a servo connected directly to the radio receiver which has the sole purpose of shutting off fuel to the engine 3 3 Ground Station Electronics The ground station electronics is much simpler than the helicopter electronics There is a GPS receiver connected to one GPS antenna and to a portable computer The portable computer initialises the GPS receiver and then relays packets from the GPS receiver to a radio data transmitter to be then received on board the helicopter The portable computer has its own battery pack The GPS receiver and radio transmitter are powered by a 12V battery and appropriate regulators All this equipment fits into 3 3 GROUND STATION ELECTRONICS 67 Antenna 9 6kb Radio Modem A SN SCI A Pitch Servo Slave 68hc11 gt T Roll Servo m Throttle Servo Console SPI Radio a i 2 yaw servo Antenna Master 68hc11 Collective Servo E GPS Antennae Fuel Shutoff servo 9 6kb Q O QO Q Ground Diagnostics lt Spier 38 4kb y l COM2 COM1 gt i Serial RS422 y y board tail LED z com4 Attitude GPS Receiver 5 486 computer 38 4 kb D T a el Q 7 seg 38 4 kb LED COM3 Position GPS Figure 3 7 The electronic signal connections on board
20. POSITIONING SYSTEM GPS algorithm to deal with satellites vanishing and appearing In particular note that the above method works equally well if one adds zero rows and columns into the matrix M for satellites that are not present yet or which are no longer present which effectively extends this algorithm forever It does mean the matrix will grow but this problem will be dealt with later This method also allows one to recover from briefly dropping below four satellites being visible in an elegant manner one first of all assumes that nothing much happens in terms of movement while there are not enough satellites in view and then when this method allows one to get an accurate enough cycle slip measurement one accepts it and one has fully recovered Accuracy This brings up a vital question how accurate are these estimates of C Firstly suppose that the measurements were perfect Then equation 2 32 would be exact However there is some noise in so equation 2 32 is not exact Let Ac be the covariance matrix for C and Ag be the covariance matrix for Note that to a reasonable approximation Ag o7 where is around 10 s A more detailed analysis of the covariance matrix would be useful and not difficult to implement since the signal to noise ratio is actually measured by the TANS and sent out over the serial port Define A 5 ma 2 41 k 1 Note that A is a square matrix of width equal to the number of sa
21. This in principle means that one could get differential position accuracies with sub centimetre accuracy which is a very attractive idea Such approaches are called carrier phase methods However the way these phases are measured is usually not synchronously with re spect to the C A or P code as such would be very difficult so it is not known in advance exactly what the phase is in absolute terms one knows that the phase may have changed 463 34 cycles since the last measurement but one does not know exactly how many cycles there are between two different satellites on the one receiver or two different receivers Once one has determined the offset between two receivers the clock bias which will not be an integer number of wavelengths and the integer offsets be tween the satellites on a given receiver one then has an accurate measurement of A This problem of finding out these values is known as the Integer resolution problem The rest of this chapter deals with how carrier phase measurements are used In section 2 3 it is assumed that the integers are known and the methods of converting them to useful forms such as position velocity attitude and attitude rate are detailed In section 2 4 some methods for obtaining these integers are explained The reason for this apparently backward order is that the mathematics and algorithms of the integer resolution methods are more complex than the methods that assume the integers are known
22. This requires that each baseline be able to see three satellites This is a reasonable assumption in 2 4 OBTAINING INTEGERS 51 many applications but it is not reasonable on the helicopter due to the poor visibility of the side antennas due to occlusion and lack of space for ground planes For these reasons it was necessary to come up with a new algorithm and it was for this reason that the pseudo global solution described in detail in section 2 3 5 was developed For the algorithm in section 2 3 5 generalises easily from minimising equation 2 17 to equation 2 54 One iteration of the pseudo global algorithm now consists of doing a global minimisation on the three attitude parameters for each time period sequentially followed by the integers Minimising with respect to an attitude parameter for a given time period follows almost exactly the same steps described in section 2 3 5 Equation 2 19 has now changed to 2 cosW sinW 0 5 b _ sin Y cosy 0 Rae si pr C Y i baselines 2 55 j satellites 0 0 1 costs independent of Y cos Y csin Y 3cos Y sin Y cq cos Y cs sin Y ce 2 56 Equation 2 56 is now in the same form as equation 2 21 and the optimal value of Y can then be obtained in exactly the same way as in section 2 3 5 After processing the 3N angle variables it is necessary to minimise the integers This is straightforward the best estimate of an integer A is given by 4 oo
23. an extra row and column to the appropriate matrices and setting the values there to zero Because of the effects of the Q vectors growing due to the 1 matrix in equation 2 32 it is necessary to start off with some compensation for previous satellites This process would be identical to the process just described for removing a satellite record from existing parts of the matrix 2 4 2 Attitude Most of the integer resolving methods are based upon a known or computable geometry change Such a change can be done very easily to a baseline pair of antennas mounted rigidly to the helicopter just pick the helicopter up and rotate it This causes the angle between the satellites and the baseline vectors to change significantly providing information which can potentially be used to resolve the integers 50 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS In particular if one no longer assumes that the integers are known then equation 2 16 becomes PI b R s AY 2 53 where the new term A is the unknown integer corresponding to the baseline satellite pair ij Noise is still not mentioned here and the line biases are assumed removed If one takes N measurements over a time period in which the helicopter is rotated then one can try to solve in a least squared sense NJ versions of equation 2 53 where I is the number of satellite baseline pairs providing valid information phase mea surements Typically on the helicopter N 2
24. and they rely on a prior understanding of the simpler algorithms 18 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS 2 1 Terminology The following terminology is used throughout the next few sections It assumes some number of receivers usually labeled with superscripts r and s and various visible satellites usually labeled with subscripts 2 and j e c A symbol representing the conversion between the unit seconds and the unit metres Not used here as it is unnecessary and clutters up the formulae e X is quantity X for receiver r e X is quantity X for satellite i e tis absolute GPS time e t is local time for a GPS receiver r e i t is the measured phase for satellite i at receiver r relative to the local clock e D t is the measured doppler for satellite i at receiver r relative to the local clock D t t e 7 is the time offset for a particular receiver t t T is the vector distance from satellite to receiver r e s is the unit vector line of sight from receiver r to satellite 7 It points towards the satellites up in practice e Az is the 3 displacement of receiver r relative to receiver s e Au is the 3 velocity of receiver r relative to receiver s Note that in the following formulae the units involved will not be explicitly men tioned It is assumed that a consistent set of units are used without going into what they are distances can be equally well measured in centimetr
25. as overhead vision of the robot or special markings on the floor or walls do not work Classical solutions involve star tracking a difficult method especially during the day dead reckoning using sophisticated inertial measurement devices expensive quite heavy and prone to drift and extraordinarily sophisticated and complex heavy and expensive vision systems One source of position information that is starting to become very popular is the Global Positioning System GPS which can be used to measure absolute position anywhere on the world using microwave radio transmissions from satellites 1 4 CONTRIBUTIONS 5 Recent developments have made it possible to determine local position with an accuracy of to centimetres using GPS carrier phase measurements e g 8 This then has all of the requirements we were looking for in a sensor light cheap fast drift free accurate and precise Using similar methods it is possible to use GPS to measure attitude in real time e g 12 11 9 17 by the use of several antennas While roll and pitch can be measured with gyroscopes and pendula in a drift free manner with respect to the direction of gravity in the helicopter reference system azimuth is difficult to measure well compasses are the classic instrument but have problems with local anomalies and magnetic equipment Also using just one sensor with no moving parts for all tasks has advantages in terms of overall system cost and
26. available bandwidth and the GPS packets take up about fifty percent of the available bandwidth The bandwidth required by the GPS packets can increase temporarily when a pseudo range position packet is transmitted This software is quite simple and has been tested extensively for reliability as the ability to switch to manual control is vital for the safety of the helicopter This software is described in detail in section C 4 4 3 486 software There are potentially five logically distinct 486 computers mentioned above e The 486 on the helicopter e The base station e The down load computer e The post processing analysis computer e The development environment In practice these jobs are shared among a smaller number of computers but there are five separate tasks to be done requiring different but related software Coping with the immense software engineering task of maintaining five separate versions of the one set of software seemed worth avoiding Thus as alluded to in section 4 1 4 a decision was made to use just one set of software with a general architecture on each of those five logical machines The only distinction between the software is in external command line options or environment variables As can be seen in the previous figures in this chapter showing the breakdown of the various software processes into conceptual units and data movement the architecture of each mode is very similar making this scheme feasible and ef
27. byte when first told to put it in the program buffer B 3 RUNNING THE PROGRAM 133 Port number IRQ Number 1 4 2 3 3 7 4 5 Table B 1 Standard interrupt numbers for the serial ports m Debug mistakes Print Parity Error when a packet does not pass the parity check Print when a program misses a byte due to being too slow When a byte is received which is not expected print out M followed by that byte in hex u Debug interrupts Whenever an interrupt routine is called print out the first char acter of the name of the port i Interrupt number Used to set a non standard interrupt number typically 1 to 7 Use after setting port The standard interrupt numbers used in this program are given in table B 1 and are the interrupts used on the helicopter 486 a Address Use to set a non standard address for the serial chip Use after setting the port number Note that like all other numbers this is a decimal input whereas normally port addresses are specified by their hexadecimal addresses The stan dard addresses come from various locations in RAM set up by the BIOS Standard values are given in table B 2 t Timer Print out a dot at the start of processing for the timer interrupt Mainly for debugging purposes B 3 2 Global Command line options These all consist of one letter followed by a number b Base station If true echo received relevant packets 5A ED 84 from the GPS port to the
28. execution the value of b were known but a was not known for whatever reason in the helicopter scenario it may be that b was some extrapolated value like a line of sight vector and a was some phase measurement that is expected to come down a serial link real soon now Then instead of the program waiting idly for a to arrive the program could reshape it s internal structure realising that 1 a b can be converted into 1 b a which can be partially evaluated in advance This could be done either statically via compiler directives explicitly given by the programmer Simplify this function given this expected data or dynamically based on a sophisticated caching system learning what data is expected This of course could be done in an imperative language like C with some contortions but it would be immensely easier to do in a functional language where functions are first class objects and can be manipulated as entities The improvements in functional simplification and prediction arising from such re search could then feed back in turn to the parallel compiler field Unfortunately eager evaluation technology is still poorly understood other than for very short look ahead problems but I believe that there is an exciting opportunity in seeing how this technology could be applied to real time control type problems Appendix A Helicopter manual This appendix gives some of the details of the hardware used that were no
29. helicopter for position Preferably within half a wavelength though search algorithms can be used to increase this limit 2 4 OBTAINING INTEGERS 39 Secondly one can use a searching algorithm If one knows limits on the integers easy to get directly for the attitude integers and by using differential GPS for a rough estimate of the position integers This has the disadvantages of being slow and often inconclusive Another method is by geometry change If the geometry of the baseline the vector distance between two antennas relative to the satellites line of sight vectors changes then the way the phases change over time can give sufficient information to resolve the integers There are two different approaches commonly used change the baseline quickly with the satellites remaining roughly stationary or hold the baseline constant with the satellites moving The former is used when the baseline is short and can be changed in a controlled manner such as when the antennas are mounted on a rigid body and their scalar separations are fixed This was the chosen method for determining the attitude integers The latter method of waiting for satellite motion has been used extensively by surveyors who put down two receivers and wait This method is not particularly ideal for automatic control as it involves a lengthy wait before starting A variation on these methods is to mount a pseudolite basically an equivalent of the transmitting po
30. is stored on a hard disk and can be post processed in the comfort of a warm laboratory to analyse the performance of the helicopter A down loading session typically starts with a user command on the base station which causes a packet to be sent to the helicopter 486 asking it to send the logged data back to the base station This is done as a series of packets which require individual acknowledgements from the base station improving reliability A Picture of the data transfer involved in down loading is given in figure 4 6 For software development reasons it is also possible to down load a file from disk on one computer to disk on another using the same protocol This is useful for updating the software on the embedded 486 on the helicopter 8 Also GPS EXE 82 CHAPTER 4 SOFTWARE Display Control GPS EXE a Att Calc Ww ze E LOS Se Yn oO DO GPS World Model TSR EXE se oe Packet Driver a To Att GPS From 68HC11s To Position GPS lt Figure 4 4 Data movement during initialisation for the 486 on the helicopter 4 1 DATA FLOW 83 Display Control GPS EXE oh REMOTE EXE Ye Att Calc l S z LOS Se nm oO DO GPS World Model TSR EXE TED es Packet Driver To GPS Receiver 4 To radio Figure 4 5 Data movement during initialisation for the ground station 84 CHAPTER 4 SOFTWARE REMOTE EXE r
31. is that a local step can often cause the solution to jump along a valley vastly increasing the cost function and a second local iteration is usually better at repairing this damage than a pseudo global iteration Once a minimum cost has been reached one must decide whether the solution is rea sonable for this algorithm is by no means guaranteed to converge sometimes because the problem is ill posed the geometry does not contain enough information or the loss of precision due to geometric dilution of precision is too high and sometimes because this algorithm is by no means perfect Fortunately there are three methods of checking the result for reasonableness Firstly the value of the cost function must be sufficiently low as to be accounted for by noise and not some more serious problem Secondly the integers AY which were estimated as continuous values should be fairly near integral values Thirdly as these integers are used in the future in operational attitude solutions if the resulting residuals are too high then there is probably a problem with them The way the helicopter system currently works is that the attitude integers are initialised by picking up the helicopter and rotating it After it has acquired the integers the helicopter can be rotated a little more for this additional level of safety The integers could also be acquired from a rotation in flight whilst under manual control This is T An increase on the first
32. is used to preserve bandwidth These 8 bit values are given by the formula 16 bit value min 0 max 255 7 C 1 A description of how the 68HC11 programs work is given here in case they need to be modified Such modification is to be avoided as this program controls the manual override and is this absolutely critical to the safety of the helicopter The HC11s and programs have been weell tested in their current configuration The following descrip tion is given with significant detail as interrupt driven assembly language is difficult to understand Both programs start off with a whole lot of defines These are symbolic names for the special purpose registers on the 68HC11 family They were taken from a program HEXMON ASM by Fred G Martin No HC11 program should leave home without them Next come a list of variables These are global variables with fixed location in RAM A description of these registers follows with a description of how some of the algorithms work 164 APPENDIX C IBM BOX MANUAL timer_overflow This is a 2 byte value which can but isn t be used to times tamp events with a full 32 bit time using these 2 bytes as the upper word and the internal HC11 16 bit timer as the lower 16 bits This variable is incremented by the timer overflow interrupt may_send_spi On the master a timer value that says when it is next time to send a value over the SPI This value is polled in the main loop rather than i
33. know the exact integers for a position offset but instead just obtained some that were fairly close As mentioned previously this can be done by obtaining the time biases from the GPS pseudo range packets and relative positions can be obtained through regular differential GPS or by just assuming that the two antennas start off close so that the distance can be approximated to zero This section will need a more compact notation than previously as it will now be dealing with many measurements and it will not need to go into as much detail on each individual measurement One can now rewrite equation 2 10 in the form S X where S is now a whole matrix of antenna line of sight vectors and doppler corrections X is a four vector of position and time offset and is a vector of all the phases known The now denotes the measurements at some particular time Suppose that the integers were off by some offset C so that the true value of X is actually given by the equation The immediate effect that this would have on the computed position X is an error of S C where the matrix inverse is taken to mean solution in a least squared sense if S is not square The next question is What does this error mean There are two answers The obvious answer is that it is the error in the computed position from the base station For a surveyor this would be the error in the measurement a bad thing For an airplane coming in to land assumin
34. last computed attitude is usually a pretty good starting estimate and only a few iterations are needed An example of a fairly bad case of the operation of this algorithm is given in figure 2 3 This is the normal method used in Cohen 9 Cohen also used Wahba s method for attitude determination to solve this problem which works very efficiently and effectively for a non coplanar array of at least four antennas each with good visibility of the sky This was not used on the helicopter as the antennas did not have sufficient visibility A slightly better algorithm than Newton iteration for this problem is presented in section 2 3 5 2 3 ASSUME THE INTEGERS ARE KNOWN Cost function of two angles 200 EW i 150 _Aan ht SS ANNA FOO ANI ii TT CSOSA AAA AANA M D ESS SRN Il AN 2S a TASS AT 100 O 50 S WANN NAN SOYA r oo AUS NAC ANA AAR LS SSS Lt idsos 0 Roll 9 Pitch Figure 2 2 Cost function of an attitude error as given by equation 2 17 with respect to two of the three parameters 30 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS Error as a function of time 120 T T T T T T T 100 807 7 Residue wavelengths squared 207 J 0 li ll l ll ll ll li l 0 2 4 6 8 10 12 14 16 18 20 Time Figure 2 3 Example of a fairly poor result of Newton iteration of equation 2 17 The vertical axis is the cost the horizontal axis is the
35. means that one fewer integer variables are solved for and all values produced are relative values This has the advantage of reducing the number of variables for which one must solve but has the disadvantage of being messy when one is forced to change satellites An alternative that could work in this situation is to add an extra constraint to equation 2 27 of the form 1 C 0 where 1 is a vector with all elements unity This states that the average value of the integers C must be zero which removes the ambiguity just mentioned Using this form makes it cleaner to deal with satellites appearing and disappearing With this modification equation 2 27 becomes S 0 0 WA AX P 0 So 0 WA AX P 2 28 0 0 Sn WA AX Pn 0 0 0 1l 1 C 0 To solve Ax b in a least squared sense one solves the equation A Aw ATb which in the context of equation 2 28 gives STS 0 gt 0 ST AX STS 0 SPS 0 ST AX STS c 2 29 0 0 e Sts St AX STB S Sy gt Sn 14nI C DS where 1 is this time a matrix where every element is unity 44 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS The bottom line may be simplified by Gaussian elimination to get SiS 0 0 ST AX STS 0 SES 0 ST AX STe 0 0 STS st AX Sa Pn 0 Oe 0 1 M C Xi MS where Te of Mi T S STS s 2 31 This easily decouples into one equation to determine C viz n n ym C S MG 2 32 i l i l This formulation has several nice properties In
36. on this is enlarged in figure 5 15 Note that this slew is significantly faster than previous slews and the sample period is significant on the time scale being used For this reason figure 5 15 shows individual points each point representing one GPS measurement As the GPS measurements are calculated at 10 Hz the distance between measurements is 0 1s Again it is difficult to obtain an exact measurement but one could accuse this system of having a damping time constant 5 2 COMPUTER CONTROLLED PORTION OF FLIGHT 107 Flight Test 2 31 Dec 1994 0 T T T heading degrees a T 1 25 a 4 o 30 600804 600805 600806 ASA 600808 600809 600810 time s Figure 5 15 Slew in heading under computer control on the order of half a second Having come up with these numbers they must be compared to the control system For heading there are three unknown numbers e Moment of inertia of the helicopter around a vertical axis easy to measure e The feedback due to the internal gyroscope and similar factors moderately diffi cult to measure for the gyroscope very difficult for other factors e Force exerted by the tail rotor as a function of blade pitch in a near hover envi ronment with appropriate turbulence Very difficult to measure probably best done via a significant system identification effort Due to the difficulty of measuring these parameters and the very high speed of the response details of the mo
37. probably came from the delays in the system and or or the quantization of the servos The servos were quantised to a motion of about a third of a degree which corresponds to a displacement of about a foot with the current gains which is comparable to the size of the limit cycle The period of this limit cycle can be determined formally by performing an auto correlation of one variable with respect to a time delay Figure 5 10 contains an auto correlation of the roll signal over the time period 600740 600790 This indicates that the period of the limit cycle is eleven seconds The pilot commands that were put in just at the beginning and again at the end of this period of totally autonomous flight are given in figures 5 11 5 12 5 13 and 5 14 Again from figure 1 1 since the system is still under automatic control the manual inputs are commands in lateral position forward position altitude and heading One can see in these graphs the y position impulse at time 600793 s the x position impulse at time 600802 s and the heading impulse at time 600805 s The autonomous system s response to these impulses can be seen in figures 5 9 5 8 and 5 7 respectively After answering the obvious questions of Is it stable yes and Is there a limit cycle yes comes the question How fast is the response to a slew command and how does this compare to the expected behaviour based on the control system and model used Fortunately t
38. radio and fast ports see section B 5 2 GpsPort A derived class from Port which handles the Trimble packet structure see section B 5 1 Multiple_Data_Port A class with the same interface as DataPort but which refer ences both the Radio and Fast ports connection An inelegant class that handles reliable packet based file communication between two computers via a DataPort transmit_connection A derived class from connection that is specifically for the com puter that is sending the file 130 APPENDIX B TSR USER S MANUAL TSR is designed to be fairly easy to provide support for more serial ports or more data formats by adding new classes that inherit from Port B 2 7 Interrupt Usage Under MS DOS there is a vectored interrupt scheme whereby various sources of inter rupts cause control to jump to a memory location specified by an interrupt vector To add a new interrupt service routine one replaces the existing location with the address of the desired new interrupt routine After processing an interrupt one may desire to call chain on to the old interrupt processing routine There are several interrupts used by TSR Serial Ports The serial ports have interrupts that indicate when a byte is received or ready to be transmitted Note that if there is nothing to transmit the ready to transmit interrupt is masked out TSR is intelligent enough to allow several serial ports to share the same interrupt vector but unfor
39. remove the eleven second period limit cycle An adaptive control sys tem would be a challenge and be of immense practical use as it would bring this system significantly closer to a stage where it could be used by an untrained consumer As a straightforward next step the control needs to be generalised to control the helicopter in forward flight where the helicopter s characteristics are quite dissimilar but inherently much easier to control than its hover characteristics The new system that has now been demonstrated for controlling precisely and totally autonomously both the location and attitude of an inherently unstable helicopter is of course the central crucial first component of the larger powerful task level control systems that it now enables systems for carrying out complete tasks specified in graphical object based terms by a human at the task strategy level Go to the location I am pointing to and pick up that object and take it to this location over here and 116 CHAPTER 6 CONCLUSIONS AND FUTURE WORK attach it to this fastener It will open up an exciting broad new era for autonomous helicopters The largest problem with GPS as a sensor is reliability of satellite coverage especially when the helicopter is banking significantly This can be improved in several ways One is the addition of new satellites As the US DoD is unlikely to significantly increase the number of expensive GPS satellites in the near fut
40. the DB25 connector on the GPS box on the helicopter D3 INTERNAL ELECTRONICS 175 an lon data to be sent data from the GPS data ground request to send IBM ready for data the GPS board is ready for data eo DN DW oO F amp F WO MN FB Table D 2 Pin out of the DB9 connectors going to the GPS box on the helicopter power supply DB25 68HC000 GPS board component side Figure D 3 Orientation of the 34 pin IDC connector on the GPS board 28 pin connector 176 APPENDIX D GPS BOX MANUAL B 13 NC NC Ground Pulse per second CNOB TxDA TxDA CNOA TxDB CNIA CNIB CNIB RxDA RxDB NC CNOB CNOA TxDB CNIA RxDA NC co ODN OH F amp F W N FR N e v NN e mae me me e e a F Co o u oa n A W W gt W W gt W p p p W N N Table D 3 Pinout of the serial connector from an upper GPS board D 4 GROUND POWER SUPPLY 177 The connections to each GPS board on the 28 pin connector consist of power which goes to the voltage regulator and serial port Bt which is sent directly to the DB 25 connector Due to the scarcity of 28 pin IDC connectors 34 pin connectors were used A picture of the connector giving orientation is given in figure D 2 with the placing of the 34 pin connector on it shown in figure D 3 D 4 Ground Power Supply This is a box designed for use on the ground It has two purposes in life e Provide about 7V for the radio transmitter e Pro
41. the SPI is not currently transmitting so that the main loop should feel free to start transmitting at its leisure Various other variables are then set up to their default state the upper 16 bits of the timer the serial port escape codes and the serial port transmit buffer pointers together with reasonable average values of the PWM values for the hopefully few milliseconds until these initial values are overwritten Then interrupts are enabled and it is time for the main loop The main loop is really boring as almost all the work is done through interrupts Basically the main loop first checks logical PWM1 this is the physical value on the master and the transmitted value on the slave and if the measured pulse length is less than the average pulse length the input 16 bit values are transferred to the output 16 bit values Otherwise the main loop transfers the 8 bit values from the IBM to the 16 bit values after expanding the 8 bit values via the inverse of equation C 1 via calls to the subroutine Expand The main loop also converts the 16 bit PWM values to 8 bit values through the subroutine Shrink and stores the 8 bit values in the variables toIBMN The computer operating properly timer COP is then reset to say that the program is still working This should reset the device if something goes wrong such as a static charge or power spike This work could automatically be done easily in the interrupt routines whenever one of
42. the helicopter 68 CHAPTER 3 HARDWARE m an Fuel Shutoff servo tail LED seg LED r 5V reg Y 486 computer parallel port a COM2 COM1 5 7 2V Battery _ Antenna 5V battery i Radio Modem E SCI Pitch Servo s aa Slave 68hc11 D Roll Servo re Throttle Servo Console SPI Radio yaw servo SPI Antenna a 5V battery Master 68hc11 a Collective Servo SCI GPS Antennae 299 y Splitter reg Serial RS422 board COM4 COM3 Attitude GPS ad lL Position GPS Receiver Figure 3 8 The power connections on board the helicopter 3 4 OPERATIONS 69 Figure 3 9 Ground Equipment a carrying case except the portable computer A photograph of the complete ground station is given in figure 3 9 and a block diagram of the interconnections is given in figure 3 10 The portable computer doubles as a down loading station When the helicopter lands a serial cable is passed from the portable computer to the helicopter allowing down loading of flight data 3 4 Operations Going out to fly the helicopter is a significant operation as a single mistake flat battery or loose screw could easily cause the helicopter to crash fatally Since the rotor blades have lead in them to increase the blades moment of inertia they are very danger
43. though often messy to perform this with current tools For instance it would be possible to calculate and invert the left most matrix in equation 2 10 for the expected next time step for all possible satellite visibilities or more rea sonably for the most likely ones However this makes the program more complex and harder to understand debug and maintain which is deadly in a large project It would be very nice to have a language where one would be able to state If there is nothing needing to be done try to execute the following block of code knowing this data but not that data That code would in effect then be recompiled with some previously variable expressions replaced by constant expressions Even more convenient would be if the compiler were capable of doing this automatically using a cache like strategy to determine what was most likely to benefit from precomputation A similar sort of problem has been encountered before in functional languages on parallel machines Following is a very brief description of functional languages An excellent survey is given in 13 Functional languages are fairly new and neither well known nor used commonly although Ericsson has developed and used extensively an 118 CHAPTER 6 CONCLUSIONS AND FUTURE WORK impressive functional language called Erlang 4 A functional language like APL Erlang scheme differs from the more common im perative languages in many ways The most obvious differe
44. translation e Coupling from biases The biases due to trim settings are different in the different axes of the helicopter Thus a rotation will change the effective direction of the trim offsets coupling into control commands The first two and similar effects could be compensated for with a good system identification and cross terms in the control law Accounting for the displacement from the tail to the center of mass can be performed as one has both position and attitude it was not done here as there was some doubt as to the effects of the noise coupled in from the angular measurements and the belief that this effect was small compared to other ignored terms The coupling from biases could be dealt with by an integral term in the control loops This was not done as the knowledge of the system is not particularly high and it was desired to obtain a stable system before adding a potentially destabilising term As said before these coupling terms were neglected and effectively four PD con trollers were set up around the four statically controllable degrees of freedom altitude heading and forwards and transverse location These four control loops are presented with simplistic models of the plant in figures 4 8 altitude 4 9 heading 4 10 pitch forward movement and 4 11 roll transverse movement The altitude control loop is the most straight forward both because it is the least unstable and because the obvious solution of a
45. values of the PWMs to the HC11s 18 2 times a second SERVICE_SEND_PWMS_DISABLE pwmdisable disable sending the current values of the PWMs to the HC11s 18 2 times a second REM_SEND_PWMS_ENABLE rpwmenable remotely enable sending the current values of the PWMs to the HC1l1s 18 2 times a second REM_SEND_PWMS_DISABLE rpwmdisable remotely disable sending the current values of the PWMs to the HC1is 18 2 times a second SEND_PWMS_NOW send the current values of the PWMs to the HC11s now B 6 INTERFACE WITH AN EXTERNAL PROGRAM 141 COMPLETE_FILE_TRANSMISSION complete abort the current file recep tion and save whatever has been received Useful if re mote computer dies SERVICE_FUNCTION_GPS_SEND returns in es bx the address of the function one should call to send a GPS or radio packet to some unit See sec tion B 6 4 SERVICE_FUNCTION_GPS_RECEIVE call the function at ad dress es bx whenever a GPS packet is received See sec tion B 6 4 RECORD_RESET rec_reset start recording from scratch RECORD_DISABLE rec_disable do not record a log file RECORD_ENABLE rec_enable do record a log file RECORD_PWMS_IN rec_pwm_in record incoming PWM values RECORD_PWMS_OUT rec_pwm_out record outgoing values REM_RECORD_RESET rem_rec_reset start recording from scratch REM_RECORD_DISABLE rem_rec_disable stop the remote computer from recording a log file REM_RECORD_ENABLE rem_rec_enable start the remote co
46. zero This is a flag maintained by MS DOS e When the DOS idle interrupt is called This means that MS DOS currently has control of the computer but that MS DOS is just sitting in an idle loop waiting for the user to press a key and is not using any static variables which would make it non reentrant When TSR desires to call some MS DOS function TSR waits until one of the above opportunities occurs This may involve keeping a list of things to do at the next op portunity In particular the clib function printf can not be used during an interrupt which makes debugging rather difficult TSR includes a set of routines that maintain a list of things to print out when the timer interrupt is called with indos zero or the MS DOS idle interrupt is called The handles used to access files are associated with the program currently running If TSR is called in an interrupt the handles associated with the current program will not necessarily be the handles of TSR For this reason one must change the process Though the restoration could equally well have been performed as the last action by the callee function B 2 TECHNICAL DETAILS 127 identification number when one desires to access files from inside an interrupt in TSR This can be done with an unofficial MS DOS function encapsulated in SetPID in TSR The process identification number must be set back to the original value before the interrupt returns B 2 3 HIMEM SYS reent
47. 0 and J varies from about 5 to 12 In an aircraft with better visibility than the helicopter J can be significantly higher We are solving for 3N I variables I integers and N rotation matrices so the system is over determined for J gt 3 and sufficient N To find the optimal in a least squared sense attitudes and integers one wants to minimise a cost function of the form e r E EO P R o AY 2 54 tE times 7 baselines j satellites This is a nasty nonlinear equation being much worse than equation 2 17 as it has the same nonlinearities with many more variables typically on the order of 70 It turns out that in this case Newton iteration from an arbitrary starting point tends not to work adequately This problem has been tackled for a single baseline in 7 and improved by using the known constraints between multiple baselines by Cohen 9 with an elegant algorithm to get a first estimate Cohen s algorithm basically computed the changes in position of each baseline and then used the fact that each baseline is a fixed length to determine the geometric position of each baseline From this the attitude matrices can be easily calculated and then these estimates can be used as a first guess in Newton iteration of equation 2 54 to get an accurate solution Cohen s algorithm works quite well when it is applicable but it unfortunately relies on being able to compute explicitly the change in position of each baseline
48. 179 XIX Chapter 1 Introduction Much of the history of engineering technology can be viewed as an increase in the abilities of humans Mechanical devices can increase a human s precision speed leverage shape strength or appendage arrangement Engines increased humans total power supply Large structures allow humans to bend the world into shapes more useful for humans More recently humans have had access to vast quantities of power the need for sheer physical strength in humans is now rare Much of engineering technology has turned towards information Computers and communications allow a human s mind to be more productive than previously and there is now little need for humans to perform tedious calculations Robots allow humans to get away from having to direct the operations of machinery themselves 1 1 What is a robot The science of robotics is still very young In particular most robots are relatively simple and non ambulatory The exceptions to this rule tend to be teleoperated robots that is robots that are controlled usually in real time by a human Teleoperated robots allow a human to interact with environments with which a human could not directly interact and thus serve a vital role but they are not as potentially powerful as fully or even partially autonomous robots that can perform a task without full time human supervision and control 2 CHAPTER 1 INTRODUCTION It is a little surprising that robots
49. 5 96 CHAPTER 5 FLIGHT TEST RESULTS Over a minute under pure computer control hands off Figure 5 1 A test flight performed on 31 Dec 1994 down loaded the flight data summarised in figure 5 1 The reason for going so high before going into computer mode was to allow Steve a chance to regain control of the helicopter before it hit the ground if something went wrong with the computer control such as losing GPS satellite visibility Happily this did not occur Most of the concentration hereafter section 5 2 will be on the performance of the helicopter whilst under computer control In order to appreciate the scales of movement before showing detailed computer performance graphs some flight information over the whole flight is shown in the rest of this section This flight data is for the same flight as shown in figure 5 1 Time in these and all future graphs in this section is given in terms of absolute GPS time in seconds since the beginning of the week which explains the seemingly large constant offset in the following graphs the flight was neer the end of the week Firstly figures 5 2 and 5 3 contain the East components of position and velocity during the whole flight The sequence of actions can be seen from figure 5 2 Firstly the attitude integers were initialised This was performed by physically picking the helicopter up and rotating it causing a little oscillation in position The helicopter then was put down on th
50. 5 19 heading 5 20 displacement north and 5 21 displacement east being the analogues of figures 5 6 5 7 5 8 and 5 9 respectively The 31 January 1995 flight was significantly shorter than the 31 December 1994 flight and a smaller proportion of the flight was spent hovering at a single point a very large and long command to move in position was ordered by the pilot which is the main point of interest of these data The y translation command shown in figure 5 22 does come up very cleanly A nine metre slew east was commanded at time 250475 s and released ten seconds later The helicopter slewed as shown in figure 5 21 and then cleanly returned to the starting point The response rate is consistent with the previous estimates of the dynamic response in transverse motion This second flight verified the control stability demonstrated in the first flight and added a demonstration of movement to a commanded position rather than just stable recovery from a commanded transient The altitude data figure 5 18 was unfortunately skewed by a change in the set point of the throttle lever on the console during the initial slew and an unintentional jump around the 250470 second mark The January flight was unfortunately interrupted by an excessively large commanded 5 2 COMPUTER CONTROLLED PORTION OF FLIGHT 111 Flight Test 31 Jan 1995 T heading degrees 3 T 20 fi fi 1 1 1 250450 250460 250470 250480 250490 tim
51. 5 produced ringing 5 2 COMPUTER CONTROLLED PORTION OF FLIGHT 109 Flight Test 2 31 Dec 1994 T 15 T T T T T T T Commanded bank angle Measured bank angle 10 F 4 5 is i A i D HA s t 1 1 y gt o i ao deed 2 H N ha a Pe aA aA M Me whit Akiva I HETT i A A r MAN at i adil AM tek mA A A AASA it MI Wy p il val i wh ARIA Mia a An er ea Afi rote MA Ne it EEY t avi Wong 1 enon 1 EA A an wert Wyn Ar vo Yy H AN id et a r y A W URLi towl W i yf yt I h 1 _ 3 Rh Og i FH i OER fit Li Wy ik y i i 10 F y 4 f 15 L 1 L 1 if 1 L L 600720 600730 600740 600750 600760 600770 600780 600790 600800 600810 time s Figure 5 16 Bank angle commanded by the computer with an offset from position feedback versus actual measurements Refer to figures 1 1 and 4 11 Flight Test 2 31 Dec 1994 0 7 0 65 F 0 6 F 0 55 F Correlation Coefficient 0 45 0 4 L 1 1 1 1 1 1 1 1 1 0 6 0 4 0 2 0 0 2 0 4 0 6 0 8 1 1 2 1 4 1 6 delay s Figure 5 17 A correlation between the time delayed angle command and the observed o during computer control 110 CHAPTER 5 FLIGHT TEST RESULTS Flight Test 31 Jan 1995 132 T T T 131 5 131 130 5 130 129 5 Altitude m 129 128 5 128 L L L L 250450 250460 250470 250480 250490 time s 127 5 Figure 5 18 Altitude under computer control for flight of 31 Jan 1995 altitude
52. 6 from the 68HC11s and is proportional to the servo actuation it would engender with a range of 0 to 255 This is just the manual part of the signal see figure 1 1 to the servo it does not include the autopilot part There are several things to note During over a minute of the time when the com puter is under control the pilot is totally hands off The joystick signal is constant in figure 5 5 Then after a minute of hands off hovering the pilot put in a large impul sive command To this disturbance the autonomously controlled helicopter responded quickly and gracefully as shown by figures 5 2 position and 5 4 attitude There was also some preliminary manipulation near the start figure 5 5 before the computer was turned on for the final time there were a few false starts when Steve the pilot was trying to get a feeling for the initial transient when going into computer mode Also the no signal command in figure 5 5 is actually significantly greater than the average command This affects the computer control hover position for the simple reason that the control software is set up to bias its command with the console s command allowing the pilot to induce transient disturbances such as the one pointed out in figure 5 5 In computer mode the console joysticks effectively command position The signifi cant difference between the no signal command and the average command required for hover ends up as a bias
53. 820 time s Figure 5 7 Heading under computer control in flight performed on 31 Dec 1994 101 102 north displacement m CHAPTER 5 FLIGHT TEST RESULTS Flight Test 2 31 Dec 1994 84 T T T T T 82 80 H 78 744 Automatic response to manual command 70 F 68 66 L L L L 600700 600720 600740 600760 600780 600800 600820 time s Figure 5 8 Displacement north under computer control in flight performed on 31 Dec 19 94 east displacement m Flight Test 2 31 Dec 1994 10 T T T T T 15 20 F 25 4 Automatic response to manual command 30 35 40 1 L i L L 600700 600720 600740 600760 600780 600800 600820 time s Figure 5 9 Displacement east under computer control in flight performed on 31 Dec 19 94 5 2 COMPUTER CONTROLLED PORTION OF FLIGHT 103 5 7 at time 600805 figure 5 12 affected slightly both horizontal displacements as the point on the helicopter being measured was the tail antenna which intrinsically couples rotation into position as mentioned in section 4 4 A distinctive pattern about these graphs is the oscillation that continues after the initial transients have well died away This oscillation occurs with the same roughly eleven second period in all four degrees of freedom linked through the cross couplings as mentioned in section 4 4 From the data the horizontal motion is along a northeast southwest line The limit cycle behaviour
54. AUTONOMOUS CONTROL OF AN UNSTABLE MODEL HELICOPTER USING CARRIER PHASE GPS ONLY A DISSERTATION SUBMITTED TO THE DEPARTMENT OF ELECTRICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY By Andrew Richard Conway March 1995 Copyright 1995 by Andrew Richard Conway All Rights Reserved ii I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and in quality as a dissertation for the degree of Doctor of Philosophy Robert H Cannon Jr Department of Aeronautics and Astronautics Principal Adviser I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and in quality as a dissertation for the degree of Doctor of Philosophy Gene F Franklin Department of Electrical Engineering I certify that I have read this dissertation and that in my opinion it is fully adequate in scope and in quality as a dissertation for the degree of Doctor of Philosophy Ronald N Bracewell Department of Electrical Engineering Approved for the University Committee on Graduate Studies iii iv Abstract This thesis contains the results of my experiments in using carrier phase Global Positioning System GPS techniques to totally control an inherently unstable model helicopter for the first time In the process a new algorithm for determi
55. C11 Since the data coming from the data radio is position phases and it is thus necessary for this information to arrive at the 486 as quickly as possible it is necessary for this data to pass through the 68HC11s as quickly as possible 4 1 2 Initialisation Each GPS receiver needs to be initialised to send out the correct types of data at the correct rate This initialisation is the only data that ever needs to be sent to the position GPS receivers On the helicopter this is automatically set up to occur when the helicopter 486 boots up and starts running the flight software On the ground This is very useful for debugging but is not actually on the helicopter The computation program can run on any 486 PC with suitable serial ports and was developed and tested with the aid of a computer monitor This is actually on the helicopter GPS EXE 4 1 DATA FLOW 79 Display A Control GPS EXE n gt Att Calc i ToT E LOS N oO DO GPS World Model TSR EXE aR Packet Driver From Position GPS To radio Figure 4 2 Breakdown into conceptual units of the software running on the ground station 486 during normal operations Lines indicate data movement 80 CHAPTER 4 SOFTWARE From RC Radio From RC Radio 486 Data Radio modulator modulator To Servos To Servos Figure 4 3 Data Flow in the 68HC11 servo control processors An undashed line indica
56. I did not want and which the TANS hardware sent out which caused increased data lag and decreased sample rate Ben Tigner is responsible for a small part of the code in the program remote exe which reads the control system gains in from a data file and some of the wiring on the helicopter Bruce Woodley is responsible for some of the wiring on the helicopter and the LEDs on the tail Steve Morris is responsible for all the model helicopter building and flying Gad Shelef is responsible for some of the boxes Godwin Zhang is responsible for much of the soldering of electronics The people at the GP B laboratory showed me their code Whilst the implementa tion used for the helicopter was written entirely by me except as explicitly mentioned above some of the processing algorithms part of section 2 3 2 and the linearisation of the attitude cost function around the current attitude s part of sections 2 3 4 and 2 4 2 are very similar in principle although the way data is treated and stored was very different for reasons related to the needs for real time control programme maintenance 12 CHAPTER 1 INTRODUCTION and generality future growth directions and the ability to do position velocity and attitude angular velocity simultaneously Many people were responsible for the effort needed to actually go out set up the equipment and fly the helicopter Having said that the following things were my work e Some key fundamental and
57. If this pointer is non zero the pointer is assumed to be a user function that processes a GPS packet This function can be set by using the SERVICE_FUNCTION_GPS_RECEIVE command of TSR section B 6 1 A good way to crash the machine is to set this pointer to an invalid address or to a temporarily valid address then finish the program and neglect to set it back to NULL The first thing this function should do is restore the local DS variable if any global variables are to be manipulated which is very very likely There is a similar function that enables one to send a packet to a particular GPS unit The address of this function can be obtained through the SERVICE_FUNCTION_GPS_SEND routine The DS register should be restored after calling this function Both functions have the same assumed prototypes typedef void far send_fn int unit int command int length char far data typedef void far rec_fn int unit int command int length char far data The unitis a number which states to which GPS unit this packet pertains see section B 6 5 The command is the packet identification number the length is the length of the packet and the data parameter points to the contents of the packet One can also use this function to send and receive stuff directly from the radio port Any packet with a command not understood by TSR for its internal use that is received will be passed on to an external program through the same function 146 APPE
58. NDIX B TSR USER S MANUAL B 6 5 GPS Units There are three types of GPS units as used in various places such as log files packet reception and packet transmission One can also send packets to the radio port using the same functions by using the special unit types UNIT_RADIO and UNIT_FAST They are given symbolic constants in service h They are e LOCAL_GPS value 0 The local GPS connected to the GPS port e BASE_GPS value 1 The GPS packets being sent through the Radio port e ATT_GPS value 2 The local attitude GPS port e UNIT_RADIO value 3 The serial connection to the radio e UNIT_FAST value 4 The fast serial connection B 7 Log file XMS The XMS memory can store a log of the following events e GPS packets received from any source e PWM data e User information e Timing information This data can then be retrieved from the program in three ways e Replay in real time using timing information accurate to 1 18 2 seconds e Save directly to a local file on disk e Do a file transfer of the file EMM dat exact capitalisation vital For this exact file name instead of trying to find a disk file with that name the program will send the data in XMS memory exactly as if the XMS memory were a file This is useful for extracting debugging information from an embedded system after a test flight B 8 SKELETON CLIENT PROGRAM 147 B 7 1 Log file format The log file consists of a series of entries Ea
59. NG SYSTEM GPS Minimum x in cos 2x B cos x 0 3 5 5 3N X 25 2 2 ISS TI SSS CULERES EES SSS SSS SEERA SSS SSS SS 2 SA SSS SS 15 Figure 2 6 A graph of z giving the minimum of cos 2x B cos x as a function of B and changing and non differentiable though still continuous Indeed except near this point with multiple nearby zeros of C x it can be shown that Newton iteration works very well given a sufficiently close initial estimate For a smooth function like this a small 2 D table lookup with interpolation works adequately and efficiently as an initial guess For the purposes of a lookup table the infinite range of B can be mapped into a finite range through an index function like B 1 B as the estimates do not change much as B approaches infinity In the actual implementation the algorithm firstly checks to see if the values of B and were close to B 4 and 0 If they were very close the algorithm uses the bisection algorithm which is guaranteed to work although it will not converge as quickly as Newton iteration If they were fairly close the algorithm uses a small local 2 3 ASSUME THE INTEGERS ARE KNOWN 37 Figure 2 7 A scatter plot of the values of B and for which the minimum of equation 2 23 was needed during a typical flight table lookup followed by Newton iteration if they were not close to the trouble spot a more coarse table lo
60. PD controller using vertical position Steve the pilot could trim the throttle and leave it at a fixed value for an extended period of time with the other three loops closed All other loops required constant pilot feedback when not closed 4 4 CONTROL 91 Altitude lt prop _ Climb rate 3 m s lt 5 m Figure 4 8 Altitude control loop with GPS feedback on position and velocity Gains are given in terms of percentage of the normal full control actuation Based on figure 1 1 and velocity is reasonable For the heading control loop yaw and yaw rate signals could be used It was decided not to use yaw rate as the existing gyroscope and to a small degree the tail rotor itself provided negative feedback of yaw rate at a higher update rate with less delay Thus the only GPS feedback on heading was the yaw angle The two horizontal controllers were the most complex One method of imagining the system is of an inner control loop controlling attitude inner dashed box in figures 4 10 and 4 11 whose control signal is commanded by an outer PD loop controlling position outer dashed box in figures 4 10 and 4 11 The outer controller is fairly simple use the current attitude signal to resolve the absolute error in horizontal position and velocity into the body axis and use this in the control loop The inner control loop commanding attitude is more like the heading contro
61. PROGRAMS RUNNING ON HC11 165 independent of interrupt latency time timer value is copied into here When the PWM signal goes low this stored time is subtracted from the new latched time to get pwmNin OCN_wenthigh for N between 0 and 4 The timer value when the PWM signal for output capture OC number N went high Add pwmNout to see when OCN should go low and PWM_LONG_DELAY to see when OCN should go high next sXi_tans_buf X is C or P A short 40 byte buffer for stuff to be transmitted over the SXI sXi_trans_from The location in the above circular buffer to start transmitting from Note that this value is a pointer to a memory location not an offset into an array sXi_trans_end The location in the above buffer where one has run our of data to transmit This is also a pointer The buffer is empty when this is equal to the previous variable The buffer is full when equal to one less than this value This convention means that one character of the buffer is wasted The alternative is to assign an extra variable to distinguish between full and empty which also wastes one byte of RAM sXi_escape A flag used in the SXI receiving routine that indicates whether the previous character transmitted was a meta character escape If this is a 0 nothing is abnormal If this is a 1 the next character should be considered as normal data even if the next character happens to be a meta character If this is 2 N then N characters have been rece
62. RALLEL_OSC command The COP timer is decremented every clock tick 18 2 times per second and is reset with the SERVICE_SET_PARALLEL_COP command which resets the COP timer to the value in the bx register In the helicopter there is one bit like this which was once connected to a buzzer which sounded when an oscillating signal fails This can be used to indicate catastrophic failure of the GPS or anything else The third method of affecting the output is is similar to the COP and is called by a similar name CON Computer Operating Nicely This method has a counter in the same way as the COP whose value is reset to the bx register by the SERVICE_SET_PARALLEL_CON command The effect of this mode is slightly different to the COP mode in that this command sets particular bits high or low depending upon whether CON mode is sat isfied This action is set by the command SERVICE_SET_PARALLEL_CON_ACT The bits affected are the zero bits in the BH register and the proper behaviour for these bits 136 APPENDIX B TSR USER S MANUAL is set in the BL register In the helicopter the relevent bit is connected to two 7000 millicandela LEDs and used to send back non critical information to someone on the ground B 5 Packet Protocols B 5 1 GPS packets These are the standard Trimble protocol Each packet starts with a DLE 10 hex character and ends with the two character sequence DLE ETX 03 hex The first byte after the first DLE is the packet
63. Radio and Fast ports The default is on 134 APPENDIX B TSR USER S MANUAL Port Number Location in RAM to find address Normal address 1 400 3F8 2 402 2F8 3 404 3E8 4 406 2E8 Table B 2 Standard base addresses for serial ports All addresses are in hexadecimal Normally the port address in the third column is stored in the memory address given by the second column i Set the software interrupt service vector default 6A hex This is the software inter rupt through which a user program can talk to TSR d Debug ftp If true print out the sequence number of each packet as it is received during file transfers The default is on x Amount of XMS memory in kilobytes to use This is typically used to hold a log of events The default is 2000 kilobytes c Character Output If true anything printed normally to the console will be sent over the Radio and Fast serial ports where they will be printed out on the remote computer s screen This makes using the embedded system a little less horrible In DOS versions above 3 3 for many commands not all characters will be sent due to the way things are printed The DOS version on the remote computer is irrelevant The default is off y Replay enable This replays the contents of XMS as soon as TSR is loaded This is not a particularly useful option directly e Extended memory countdown This prevents writing to XMS memory for a short time parameter 18 2 seconds This delay can
64. T_PWMS_OUT gps_send send_fn tsr_get_ptr SERVICE_FUNCTION_GPS_SEND tsr_send_ptr SERVICE_FUNCTION_GPS_RECEIVE gps_rec void send_gps int unit int command int len char data call this function to send some packet somewhere gps_send unit command len data _DS restore_ds the tsr messes up _DS void main B 8 SKELETON CLIENT PROGRAM restore_ds _DS8 tsr_init for do_something tsr_send_ptr SERVICE_FUNCTION_GPS_RECEIVE OL disable gps_rec 151 Appendix C IBM Box manual This is a description of the functionality of the box containing the 486 IBM PC com patible card 1 the serial card 2 and the 68HC11 card The 486 card is connected to the serial card via a normal PC bus connection The 68HC11 card is functionally and electrically distinct and will be treated separately C 1 IBM PC ecard and serial card The IBM PC card contains a 486DX33 with 4MB of RAM and E PROM that emulates a 360kB diskette The card is model number PCA 6143 and is available from Advantech and J amp K Technology Advantech are much better at answering technical questions J amp K can be cheaper The serial board is from Advantech and contains two RS422 serial ports The card comes with 16450 chips no FIFO which were replaced with 16550 chips which have a FIFO The FIFO makes the serial ports much more reliable at high speeds as the 486 does not have to respond as quickly to the serial po
65. al physical time in this case may be t 10 001s For this reason it is desired to extrapolate the measurements to some reference real time such as t 10s in order to be able to compare the phases from two receivers One can then express the differences in measured phases from two receivers at equal local times t and t in terms of the relative displacement between two antennas at physical time which happens to be equal to and t for convenience P P rt t 7 0 4 0 77 s 77 O 2 5 a t r s t 7 O 2 6 F Aa t s t O rD s 2 7 Az t st 7 1 Lar Sr 0 2 8 Az t s t 7 1 air 0 2 9 where F r t r t and r T 1 77 t Knowing the exact value of OF is meaningless only the difference O O O has any physical meaning One surprising a 7 t multiplying T depends on result is the seeming assymetry in that the factor 1 one arbitrary receiver r This assymetry comes from the fact that the definition of 7 24 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS is also slightly assymetric and these two assymetries cancel out to the level of precision relevent here This now gives a set of equations similar to the naive equation 2 2 which can be again solved in a least squared sense with very little extra effort s 144071 PI Pi O s2 1 405 2 Ax 3 t bh t Og 2 10
66. an explicit type prevents mistakes of omission of swabbing These types are defined in swabtype h and are used in tans h E 4 2 General Utility The file environ h contains definitions for a few useful classes that make the program a little easier to use 184 APPENDIX E GPS PROGRAM env Replaces a default string by an environment variable if the environment variable is set print_to_me Opens a file pointer at the start of the program if given a non null ini tialisation Typically used for postprocessing with a filename coming from env above no_numerror Prevents normal behavious of numeric errors crash the program during the scope of a variable of this type If invoked with arguement s check for errors and print out useful diagnostics If invoked with no arguments ignore errors The former is useful for debugging the latter is useful during a sanity check of an incoming packet a corrupted packet does not cause the program to crash E 4 3 Matrix A matrix class has been defined in matriz c and matriz h This class is designed and tuned for ease of use and efficiency of operation for the types of operations used in attitude computation E 4 4 Attitude computation There are a large number of variables needed in the attitude computation process Rather than send all as parameters or have global variables these variables are kept in local structures Some additional useful classes are also defined Following are the
67. and the contents of config sys in figure C 2 Version 3 3 of DOS is used for small size It is now possible to obtain a new version of the PCA 6143 with a 1 44MB solid state disk Disk space is not currently a major problem C 1 1 Mechanical Description The boxis not particularly complex at least compared to the GPS box chapter D This box is made out of two pieces of aluminium bent and screwed together using locknuts pop riveted onto one section The box was designed and built by Gad Shelef The box masses 0 26 kilograms excluding electronics and mounting with dimensions 15cm by 20 5cm by 4 5cm This box is designed to be mounted from the top having four lock 154 APPENDIX C IBM BOX MANUAL PC card RS422 card Figure C 3 Layout of serial ports on the IBM box Pin Name DCD RxD TxD DTR GND DSR RTS CTS RI co ODN OH F amp F W N FR Table C 1 Pin out for the RS232 port on the IBM card nutted holes This is connected to the front of the helicopter through a piece of wood which absorbs some high frequency vibration There are four DB9 connectors on one side a DB9 and a DB25 connector on the side and a power cable connection to the outside world The four DB9 connections on the side are all serial connections from the PCA 6148 and serial card These connections are shown in figure C 3 Some details of the serial ports are given in table C 3 The pin outs for the two serial ports connected to the R5422 card are give
68. ates by setting the divisor appropriately in this case to four However it is now necessary to get an appropriate frequency clock to the serial chip The clock input needs to be sixteen times the desired baud rate in this case 38400 baud or 0 6144 MHz The standard input on IBM PC compatible cards is three times this frequency i e 1 8432MHz On the card used in thei project this frequency is supplied by a monolithic crystal oscillator It would be sufficient to replace this oscillator with a 0 6144 MHz oscillator should such things happen to be available Unfortunately this frequency is not readily available so the 1 8432 MHz oscillator was replaced by a 1 2288 MHz oscillator which is widely available which was followed by a divide by two counted to reduce the 1 2288 MHz square wave to the desired frequency 0 6144 MHz C 3 TIMER 68HC11 BOARD 157 A spare 7474 was found on the serial card and was used as the divide by two counter Thus this modification did not require any additional electronics mounted on the board only a replacement of the oscillator a few cut PCB traces and some new wires C 3 Timer 68HC11 board This board contains two Motorola MC68HC811E2 microcontrollers Each has four timer channels which are used to receive the pulse width modulated signals from the normal remote control radio Depending upon the state of a certain input input 1 the 68HC11s can either pass the radio signals directly through or put o
69. be presented separately starting with the flight of 31 December 1994 described above Firstly should be the overall plots of the four degrees of freedom being controlled altitude figure 5 6 heading figure 5 7 displacement north figure 5 8 and displace ment east figure 5 9 Note that there were two attempts to go into computer mode this is why there are two initial slews in each of these graphs The first transition was abandoned due to concern about the size of the slew and the exact position of the set point Each figure also shows the effect of the three later slews in roll then pitch then yaw at the end of the test Since the helicopter was facing close to north the zero for heading readings the slew in roll figure 5 9 at time 600794 s figure 5 14 affected mostly the east component of displacement and the slew in pitch figure 5 8 at time 600801 figure 5 13 affected mostly the north component of displacement The slew in heading figure 5 2 COMPUTER CONTROLLED PORTION OF FLIGHT Altitude m heading degrees Flight Test 2 31 Dec 1994 74 f T T 72 70 68 66 64 62 60 58 56 54 L L 1 600700 600720 600740 600760 600780 600800 600820 time s Figure 5 6 Altitude under computer control in flight performed on 31 Dec 1994 Flight Test 2 31 Dec 1994 10 T T T 25 4 J 30 i 1 I 600700 600720 600740 600760 600780 600800 600
70. be used to let the engine GPS etc warm up and not record useless debugging information Default 38 about 2 seconds s Save to RAM for file transfers When this instance of TSR is on the receiving end of a file transfer with another instance of TSR on a remote computer buffer all the B 4 PARALLEL PORT 135 data to XMS before writing to disk After writing TSR then verifies and tries up to 5 times to repair mistakes This allows packets to not be lost due to slow disk drives This is on by default B 4 Parallel Port The parallel port is used as eight logic signals rather than as a strobed device for data transfer There are three ways of using the parallel port The simplest is to set or reset some bits directly This is done with the SERVICE_SET_PARALLEL command which enables one to set particular bits specified by zeros in the mask register BH to particular states specified by register BL In the helicopter this would be suitable for controlling a motor used to pull up the disk The second method of affecting the output is through the COP computer operating properly This is designed to warn of catastrophic failure in either TSR or the program running above it TSR is designed so that some program must keep telling TSR that the computer is still running safely and whilst this is happening TSR will oscillate some of the bits at 9 2 Hz half the timer interrupt frequency The particular bits can be set by the SERVICE_SET_PA
71. ch entry consists of the following fields in order 1 1 byte Source Where this data is coming from such as GPS or automatic data capture at each time event See section B 7 2 2 1 byte Unit Which unit this refers to Such as which GPS unit section B 6 5 3 1 byte Identification What the data is Such as which packet number for GPS 4 1 byte length Length of the following data 5 even number of bytes data The data wanted followed by a filler byte if length is odd B 7 2 Sources in log file The currently defined sources in service h are SOURCE_SAVEITS value 10 Timing information and regularly saved data SOURCE_GPS value 11 GPS packets SOURCE_DUMP value 12 Data specifically instructed to be stored through a direct call to the service interrupt B 8 Skeleton client program An example skeleton client program is included below pragma option ms 3 DRACO OR OO OOO EEEE EE EEE E OR EE OK I E E E E E E kk Ak 2k a AN Interface c Interface to the serial TSR 148 APPENDIX B TSR USER S MANUAL For the Helicopter Project c Andrew Conway 1993 Witttirererrr rrr ere rer errr ere E E E EE E E E er errr ee er er ee ee E E E include lt dos h gt include lt string h gt include lt stdlib h gt include lt stdio h gt include service h include interface h include lt conio h gt int serial_driver_present returns 1 iff the tsr is loaded O ot
72. cknowledgements section but they were not formally working on the project The work described in the following chapters is my own or necessary to include for completeness in order to understand my own On the GPS side the people at the GP B laboratory gave me advice and aid but did generally not work directly with me with the exception of Paul Montgomery Paul is working on a similar aerial vehicle an airplane This poses problems that are different from a helicopter s in terms of vibration payload flight length other sensors stability wing flexure ability to hover and method of landing However very many of our 1 6 PEOPLE WORKING ON THIS PROJECT 11 problems are similar and he decided to use the programs I had written see appendices B and E In return he added to the functionality of these programs in several ways e He found a few bugs e He gave me invaluable help with the attitude calculation code in testing debug ging and understanding the original problems and hardware e He was largely responsible for the velocity calculation work Paul made the mod ifications to the Trimble TANS units and I did the processing to convert doppler to velocity e He added wing flexure and angular velocity code into my attitude code Whilst I did not directly use either of these features they could be useful in the future e He was also largely responsible for removing and compressing some of the excess information that
73. classes defined in attcomp h quaternion A unit magnitude quaternion used for convenient attitude definition and manipulation direction A three component vector used to indicate a direction rotation_matrix A three by three rotation matrix used to represent a rotation for efficient computation baselines A structure encapsulating the surveyed locations and line biases of the four GPS antennas E 4 C CLASSES 185 att_basic Basic information needed for both attitude computation and integer resolu tion intsolve Everything required to perform attitude integer resolution attsolve Everything required to perform attitude computation E 4 5 GPS world model The GPS world model is fairly complex containing a variety of heterogeneous data These data are collected in one overall structure gps with detailed sub objects to make it easy to maintain The following classes are defined in gps h history A general template class for maintianing a time history of some type of data sv Measurements on a single satellite at a single receiver with a single antenna posn_packet Measurements of a single receiver posn_coarse Pseudorange GPS position information los The line of sight vector for a single satellite losbuf Information needed to interpolate or extrapolate line of sight vectors for any time for one satellite los_store All knowledge of satellite line of sight vectors posn_history History of receiverd phase data for on
74. copter taking off as seen through the shield Chapter 4 Software As seen in the hardware chapter there are several computers in this system e The 486 on the helicopter e The two 68HC11s e The ground station These computers have to do different things at different times and there is also post processing to be done This chapter describes the design functionality and philosophy of the programs running on these machines This chapter starts off section 4 1 with a description of data flow and computation what computations or transfer has to be performed to which data and where and when should this be done Section 4 2 deals with a summary of what is performed on the 68HC11 processors and then section 4 3 gives a summary of the software running on the 486 processors the main helicopter processor the ground station and post processing Section 4 4 describes the system model and control system used 4 1 Data Flow The two most important questions when designing a computer program architecture are 73 74 CHAPTER 4 SOFTWARE e What data is needed where does it come from and where does it go e What algorithms are needed The algorithms question has been mainly answered in chapter 2 This section will deal with the first question The issue of where data is going depends upon the current mode of the system The most important mode is the operational mode and that will be presented first as the system should be designe
75. d and one of the attitude board s inputs This introduces a 3dB loss which in practice is acceptable The standard connections used for these five antennas are reading from the connector nearest the side of the box Front left looking towards the front right rear and position Unfortunately the antenna splitter does not fit inside the box There are four holes for mounting When replacing the screws used here do not use too long screws as they can cut into the thin coaxial cables between the antenna connections and the circuit boards There is one large hole for a DB25 connector providing electrical connections to the 171 172 APPENDIX D GPS BOX MANUAL outside world as will be described in section D 2 There is one hole for mounting a TO 220 type package an LT1085CT 5 low dropout five volt regulator to provide a regulated 5V supply from the 7 2V nominal battery pack see figure D 1 This regulator will work down to an input voltage of about 6V The metal flange of this regulator needs to be heat sinked but may not be connected to the metal chassis as the chassis is grounded and a PNP transistor is used in the LT1085CT 5 in order to obtain the desired low drop out voltage with the disadvantage of forcing this metal flange to be at a positive voltage Thus there is a mica insulator between the box and the LT1085CT 5 The regulator is held in place with a nylon screw and a metal nut D 1 1 Vibration The helicopter is subject t
76. d and radio controlled helicopters are very difficult to fly as mentioned previously The word robot itself came from a Czechoslovakian play meaning working man with overtones of slavery and monotony and was popularised by science fiction authors then became commonplace 4 CHAPTER 1 INTRODUCTION Helicopters have the significant advantage over winged aircraft in that they can hover which is of vital importance for observation based tasks Model helicopters do however come with some severe disadvantages e They tend to be unstable with fairly rapid dynamics e They are less efficient than winged aircraft and thus cannot carry as much payload which restricts the amount of electronics and other hardware that can be added e Hobby helicopters are limited in size restricting the payload size to several kilo grams e They have severe vibration problems so the control electronics must be very rugged e They do not have much spare volume available for bulky objects e They crash easily spectacularly and thoroughly In this project we used as a basis the largest available hobby store type RC helicopter as the starting point It is described in section 3 1 1 3 Why GPS As mentioned previously position is a physical quantity that is particularly hard to sense This is especially true on board an aerial vehicle as one needs to sense in three dimensions and many of the solutions that can be done indoors such
77. d load the stack with the address of the base 128 APPENDIX B TSR USER S MANUAL of this preallocated block whenever an interrupt is called taking care when one has recursive interrupts This is implemented in TSR via some assembly code B 2 5 Compiler problems The compiler used is Borland C V3 1 It is generally a nice compiler allowing one to access registers directly though the compiler seems to misbehave occasionally when using this feature by passing a 16 bit register such as _SI as an argument to a function taking an 8 bit argument Do not do this There is also a subtle bug associated with using the pascal calling convention with inline functions containing local variables with destructor methods Do not do this B 2 6 C classes Many C classes are used to encapsulate various parts of the program The program was originally written in vanilla C and has been partly converted to C to enhance readability no_interrupt No interrupt will occur during the scope of a variable of this class The constructor checks to see if interrupts are enabled and if so disables interrupts and stores the old interrupt enable state The destructor restores the old state allowing safe nesting of instances of this class This class allows one to isolate a sensitive piece of code as follows no_interrupt here masks interrupts do_something_sensitive if blah return do_something_else The interrupt enable flag i
78. d to give maximum performance in this mode Before operations it is necessary to initialise various parts of the helicopter and after operations it is necessary to down load information from the helicopter so that the flight can be analysed These who modes both require distinct data movements Lastly after a complete flight it is necessary to process the data that has been retrieved to make it into a usable form As an extra consideration it should be easy to maintain and inter operate the soft ware to perform all these tasks for overall simplicity 4 1 1 Operational During operations the 486 on the helicopter does all the calculations and produces control signals which are sent to the 68HC11s Figure 4 1 shows the movement of data on the helicopter 486 which is about to be discussed The lowest level of software operation of the 486 is the packet driver This conceptual unit manages the serial ports collating packets and sending these packets up to the higher layers as appropriate Two GPS receivers position and attitude are connected directly to the 486 and are managed directly with this layer The packet driver also has to deal with information from the 68HC11s On the receiving side this is split up into GPS packets from the ground station and the values of the RC console controls from the pilot All received valid packets are stored in memory for later down loading The helicopter 486 needs to have the GPS information from all t
79. data that they need over the SPI Note that when I say sends I really mean stick in the buffer to be sent later automagically The master send the slave a 5 byte packet containing the four IBM commanded PWM values and the one PWM value the slave needs to know to determine its mode The slave send the master its four PWM values to be sent on to the IBM LICN N is 1 to 4 Interrupt routine called every time one of the input capture pins PWM inputs changes state See the description of variables ICN_wenthigh for what these functions do ILOCN N is 1 to 4 Interrupt routine called every time one of the output capture pins changes state due to its timer coming due This routine sets up the timer for the next time output capture pin N ought to change state L_serial Interrupt routine called every time as serial event occurs finished sending or just received a character If finished sending get a new character out of the buffer and send it Otherwise send a NULL character If receive call Isci_receive C 4 PROGRAMS RUNNING ON HC11 169 Lspi Interrupt called when the SPI has finished transmitting a character On the master this interrupt means that it is now allowable to send a new character flag_send_spi is set to 1 On the slave this interrupt means that the SPI is ready to accept a new byte to transmit so call _spi_transmit to send it On both the master and slave this interrupt means that a byte has just been received so
80. del for heading will have to wait upon a very detailed system identification For altitude the situation is similar The horizontal degrees of freedom are much more amenable to analysis and in many ways are significantly more interesting being the most unstable There are two rather strong and not particularly valid assumptions that one can make in order to work with these parameters Firstly there is the assumption made previously that the roll and pitch loops are decoupled from each other and from the other loops Secondly there is 108 CHAPTER 5 FLIGHT TEST RESULTS the assumption that these loops can be decoupled into an attitude control loop which is poorly understood but operates on a fast dynamic scale with respect to the outer loop of position control This is not a good assumption for high performance control but it is a reasonable approach to do a first order check of the control system Looking at the outermost closed loop for the transverse movement loop in figure 4 11 one basically has a transfer function from input disturbance to position of 1 s gAs gB where A is the velocity feedback term 0 035sm7 and B is the position feedback term 0 017m This transfer function has a conjugate pair of poles in the left hand side of the s plane with real component 0 17s here s stands for seconds which is eminently consistent with the observed disturbance decay time constant of five to six seconds The fore and aft pitch loo
81. ds or so which give a unit vector s pointing to a given satellite for a specific time and also pseudo range position information The pseudo range position information is the absolute position in space according to C A measurements i e conventional GPS It also gives a fairly accurate estimate of the local time offset 7 accurate to the order of a hundred metres roughly 3 1077s The phase measurements come in one of two forms depending on the type of EPROMs used For position and velocity calculations the GPS board looks at just one antenna and measures the phase of the signal from each of up to six visible satellites relative to the local clock and modulo an integer offset It also produces an estimate of the rate of change of phase Note that the 10 Hz output rate is precise to the accuracy Or in the same way that a physicist may say that an electron may have a mass of 0 5 MeV implicitly assuming a division by c 20 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS of the local clock which is about one millisecond This means that the phase will be sampled ten times a second at times up to one millisecond off the exact GPS time It turns out that corrections need to be made for the fact that two different receivers will produce information at slightly different times For attitude the board looks at four antennas one of which is designated the master antenna The phase and phase rate estimates are then produced for each of up to s
82. e The contents of this packet are copied to a temporary storage place for later processing see section E 2 This is done so that the interrupt function can return quickly and so that the rest of the program does not have to worry about the stack segment differing from the data segment which may be the case during an interrupt Most of this processing is done in the files interfac c and serbuffe c The file serbuffe c does the buffering of the packets and is the only file in the application that has to deal with the stack segment and the data segment not being the same E 2 Processing of packets At the heart of the GPS program is a big loop which polls for new packets Polling may sound a little inefficient why poll when one could interrupt The main reason is that due to the non multitasking structure of MS DOS triggering a time consuming calculation inside an interrupt is difficult without a polling loop Fortunately this is not a problem as polling for packets is not as time critical as polling for individual bytes and that usually one doesn t actually want to interrupt existing processing when a new packet comes in The main loop also checks the keyboard for a q character to see if GPS should quit or for various other characters to do various debugging commands These are not documented here as they change often and are not interesting When a packet is received in the polling loop the packet is checked to see if it should be di
83. e s Figure 5 19 Heading under computer control for flight of 31 Jan 1995 displacement north m L 250450 250460 250470 250480 250490 time s 148 1 L I Figure 5 20 North displacement under computer control for flight of 31 Jan 1995 112 CHAPTER 5 FLIGHT TEST RESULTS Flight Test 31 Jan 1995 6 T T T T T actual commanded displacement east m L 250450 250460 250470 250480 250490 time s Figure 5 21 East displacement under computer control for flight of 31 Jan 1995 Flight Test 31 Jan 1995 140 T T T T T 135 J 130 J 120 7 115 F 7 TM J 105 1 1 1 1 1 250450 250460 250470 250480 250490 time s Manual roll servo command servo units Figure 5 22 Pilot commanded y translation whilst under computer control for flight of 31 Jan 1995 Each unit of command represents a 31 5 cm change in the set point 5 2 COMPUTER CONTROLLED PORTION OF FLIGHT 113 slew in heading at time 250498 s which caused simultaneously large pitch roll and yaw actuations This caused loss of GPS satellite visibility and thus the end of the computer control Manual control was reenabled and Steve landed the helicopter safely Very high speed slews will have to be pre limited in future control systems The pitch and roll actuations came from the effect of the zero point biases changing with heading Chapter 6 Conclusions and Future Work Thi
84. e computers P code is a GPS signal that is more precise than C A code Proportional Differential A type of feedback consisting of a signal proportional to x and a signal proportional to Pseudo Random Noise Pulse Width Modulated A signal whose value is determined by the length of a pulse Random Access Memory Fast solid state storage Remote Radio Controlled teleoperated A common serial communications standard A serial communications standard less common than RS232 but more reliable Selective Availability Serial Communications Interface A standard serial port on the 68HC11 Serial Peripheral Interface A less standard serial port on the 68HC11 190 SV TANS TSR US DoD APPENDIX F LIST OF ACRONYMS Space Vehicle Synonym for satelite Trimble Advanced Navigation System The gps receiver hardware operating with modified EPROMS provided by the GP B lab at Stanford Terminate and Stay Resident A type of program under MS DOS that returns control to the system but leaves the code resident in memory activated by interrupts United States Department of Defense organisation that put up the GPS satelites and maintain selective availability Bibliography 1 PCA 6143 Half size All in One 486SX DX DX2 CPU Card With Flash ROM Disk User s Manual 750 East Arques Avenue Sunnyvale CA 94086 U 5 A 2 PCL 743 Dual Port RS 422 RS 485 Interface Card User s Manual 750 East Ar ques Ave
85. e console figure 3 3 are two joysticks Each joystick has two degrees of freedom giving four controlled axes To a simplistic approximation these four controls are thrust and three angular movements Thrust is typically controlled by a vertical movement on the left joystick and is actuated by two different servos One servo controls the throttle to the engine affecting the total power The other servo controls the blade pitch on the rotors attempting to match the engine and rotor impedances for optimal efficiency There is a nonlinear mapping inside the radio console figure 3 4 that maps engine throttle to blade pitch and broadcasts both signals to the receiver on the helicopter In practice our helicopter has so much extra weight on it that in or near hover the throttle is always operating near the upper half of its range and the blade pitch varies very little Heading is typically controlled by a horizontal movement on the left joystick and is actuated by one servo on the helicopter which changes the blade pitch and thus thrust of the tail rotor In practice it is exceedingly difficult for even a skilled human to control heading directly this way and practically all model helicopters are designed to 3 1 HELICOPTER 61 Transmitter throttle collective curve 180 T T T T T 160 4 120 4 collective 100 F J 60 i L L L L 0 50 100 150 200 250 300 throttle Figure 3 4 RC Transmitter mapping from throttle to blad
86. e engine requires connecting a glow plug and twisting a shaft connected directly to the motor with an electronic starter The equipment to do this plus equip ment to pump fuel in and out of the fuel tank is carried in a tray as shown in figure 3 2 The fuel tank runs the helicopter for twelve minutes in a near hover out of ground effect Vibration is a large problem Helicopters are generally less efficient than airplanes so a large engine is needed which causes significant vibration which is a problem 3 1 1 Actuators The helicopter contains two rotors powered off the same engine The large main rotor above the helicopter is rigidly mounted to the frame of the helicopter and provides the thrust necessary to keep the helicopter in the air Translational movement is ac complished by tilting the plane of this rotor disk causing thrust to be rotated off the vertical The small tail mounted rotor exists largely to compensate for the torque on the helicopter caused by drag on the main rotor Normally a model helicopter is flown by a human holding a console attached to a radio transmitter running at a frequency near 72MHz and with a range of hundreds of 60 CHAPTER 3 HARDWARE Figure 3 3 The radio console with antenna retracted for controlling an RC helicopter metres A receiver on the helicopter then sends commands to servos on the helicopter which actuate control servos as commanded by the pilot on the ground The main controls on th
87. e existence of the video display A drawing of data transfer pathways during post processing is given in figure 4 7 Note how close this is to figure 4 1 The graphs in chapter 5 were generated using this method 4 2 68HC11 programs The 68HC11 programs are written in assembly language for efficiency and are almost entirely interrupt driven For low propagation delay reasons the 68HC11s do not process the packet structure of the incoming serial stream but rather pass the data straight through unless the data contains information intended for the 68HC11 as indicated by the meta packet structure described in section B 5 2 The same meta packet structure is used to insert the pilot s servo command values into the serial stream at a nominal rate of 30Hz This data rate could drop if the serial link utilisation is so high that the transmit buffer has not cleared since the last 86 CHAPTER 4 SOFTWARE Display Disk File A A Control GPS EXE TG ad Ww To oO E D E N oO DO GPS World Model TSR EXE OF Seog Packet Driver Figure 4 7 Data movement for the postprocessing software Undashed lines indicate data movement Dashed lines indicate computation 4 3 486 SOFTWARE 87 servo command packet This feature prevents the GPS packets from being slowed down more than about twelve milliseconds due to passing through the 68HC11s Normally the servo commands take about thirty percent of the
88. e ground and left there for almost two minutes while the 5 1 FLIGHT PROFILE 97 Flight Test 2 31 Dec 1994 T 5 T T T T T T 0 y fe eae f N Take Off Initialisation Ground oO E 40 8 Land E Start 5 aai Computer p Control 20 F Response to command 25 H input 30 Computer Bu control End i Computer 35 L Bias Error A Contra L L 40 1 1 1 i i 600500 600550 600600 600650 Time 8 600750 600800 600850 600900 Ime Ss Figure 5 2 East component of displacement in flight performed on 31 Dec 1994 engine was started and we scrambled behind the safety of the shield At take off Steve the pilot flew the helicopter west as well as up When the computer control was engaged the helicopter slewed about seven metres west and five metres north and five metres up After this slew which compensated for bias errors that will be explained later the helicopter hovered autonomously for about one minute within a volume of diameter about a metre Steve then introduced some purposeful transient disturbances by biasing the position set point to test stability in terms of transients The helicopter s autonomous recovery was quick and smooth as the data show Lastly Steve returned the helicopter to manual control and landed near the takeoff point Pictures of north and vertical displacement and velocity have the same form as figure 5 2 It is especially easy to see the increased stability of the helicopter under c
89. e h define PACKET_SYNC 0 define PWM_from_6811 1 define FILE_COMING 2 define FILE_DATA 3 define FILE_COMING_ACK 4 define FILE_DATA_ACK 5 define PACKET_SCREEN 6 define PACKET_KEY 7 define REQUEST_FILE 8 define PACKET_ENABLE_OP 9 define PACKET_DISABLE_OP 10 define PACKET_PWM_OUTPUTS 11 define FILE_ALL_RECEIVED 12 define REM_PWM_SEND_EN 13 define REM_PWM_SEND_DIS 14 define RECCOM_RESET 50 define RECCOM_DISABLE 51 define RECCOM_ENABLE 52 define RECCOM_PWMS_IN 53 define RECCOM_PWMS_OUT 54 define ENABLE_HC11_SEND_PWMS 60 define DISABLE_HC11_SEND_PWMS 61 define GPS_PHASES Oxa0 define GPS_ECEF Oxat define GPS_LATT Oxa2 define GPS_MANY_PHASES Oxa3 define GPS_RAW_CODE_MEASUREMENTS Ox5A 138 APPENDIX B TSR USER S MANUAL B 6 Interface with an external program An external program can call commands and get data from TSR via the interrupt service vector default 6A hex One calls this routine with some value in the accumulator AH which specifies the required action TSR performs that action and returns There are various definitions in the file service h which contain symbolic definitions for the commands used below together with some useful type declarations There is a program called remote exe which allows a user to perform most of these commands manually The program takes a series of arguments which are all either commands or arguments to the previous command In the list below there are listed
90. e pitch Units are propor tional to servo actuation have a small cheap gyroscope effectively connected to the tail servo in such a manner that it provides negative feedback of heading angular velocity making the helicopter significantly easier to control This gyroscope is cheap and light and has terrible long term performance but it provides rapid and low phase delay feedback There exist helicopters that use a mechanical feedback system similar to Hiller rotors which will be mentioned soon and which have the same effect as and thus replace the gyroscope This gyroscope has hereafter been considered part of the plant rather than the control system as it is difficult to remove and then a human pilot could not fly this helicopter The transmitter also couples a portion of the thrust control into this servo command partially compensating for the coupling between thrust and heading increasing thrust tends to increase main rotor drag requiring additional tail thrust The right joystick typically controls the other two attitude values pitch and roll They are actuated by one servo each which tilts the swash plate a cam around the main drive shaft in two degrees of freedom The swash plate has runners on it that cause the blades to change pitch according to the height of the swash plate portion immediately 62 CHAPTER 3 HARDWARE below the current position of the blades This means that the blades will generate more lift in some orientat
91. e receiver Used to maintain inte gers coarse_histroy History of pseudorange position data Used for clock bias measure ments and stability check time_manager Manage time for sanity checks for received phase data and deduction of the uppermost bits of a time given the lowermost bits att_sv Measurements of a single satellite at four antennas with one receiver 186 APPENDIX E GPS PROGRAM att_meas One measurement for all satellites at four antennas with one receiver att_history A time history of pahse measurements at four antennas with one receiver Used for integer maintenance and resolution match_svs A class with phases from the ground and helicopter receivers matched ready for position computation position A computed position gps The GPS world model containing all previous structures E 4 6 Control For ease of manipulation a few structures are used in the control system gains A list of the gains used in the control system packet_gains The packet sent from the basestation containing the gains Includes a checksum and the destination rotated_position The current position and velocity error in body coordinates together with the current attitude Used together with gains to produce the control signals E 5 Debugging files and postprocessing Postprocessing is performed by running GPS with various environment variables set to the names of files requested A list of the environment variables and the conte
92. ected at run time For instance a piece of C code like double select int which double a double b if which return a else return b select some_var sin x cos x 6 1 AN ASIDE ON LANGUAGES 119 will cause both sin and cosg to be evaluated when only one actually needs to be evaluated Of course it is simple in this example to avoid this problem but as pro grams become more complex it becomes messy to go avoid this sort of situation Lazy evaluation also enables some nice programming expressions where the full arguments to a function may not even be computable in a finite time The process of evaluation in imperative languages like C is called strict evaluation and for many functions it is more efficient than lazy evaluation as one does not have to check whether a variable has been evaluated before using it For this reason many functional languages allow a mixture of lazy and strict functions A function would be strict in certain arguments if all those arguments are guaranteed to be needed for the evaluation of the function This can usually be checked by the compiler A compiler then generates code that basically has the job of evaluating expressions Some main expression starts the process off and then its arguments are evaluated immediately if strict otherwise whenever needed This process continues recursively and thus eventually everything that needs to be is computed This type of evaluation is very easy to
93. elp in the EE department This was quite a large project and many people contributed directly to it Above all I would like to thank Robert Cannon for showing me how to manage such a project which was something I had no real ability in Immense thanks go to Steve Morris heli copter virtuoso who built maintained and flew the helicopters under trying conditions and often with little notice as well as having a large impact upon the control design and for teaching me about helicopters Steve s innate conservatism counterbalanced my overactive optimism well and let us fly a dangerous fragile object safely and without major crash Thanks also to Ben Tigner and Bruce Woodley who saved me a large quantity of time and frustration by helping with many of the immense number of tasks that had to be done perfectly to give the helicopter any chance at reliability Thank you also for putting up with my poor abilities as a delegator Similar thanks to Gad Shelef and Godwin Zhang for their help and skills in mechanical and electronic con struction Thanks also to Gerardo Pardo Castellote Eric Olsen Gordon Hunt Kortney Leabourne HD Stevens Jay Littlefield and Jeff Russakow for helping in the field or other significant actions Many thanks also to everyone else in the Aerospace Robotics Laboratory for constant helpful suggestions especially Larry Pfeffer who seems to know a little bit about everything On the GPS side I was fortunate to have the GP B labo
94. es metres inches sec onds or wavelengths Each is a well defined unit of distance with a conversion factor in between Using seconds as a distance measurement or metres as a time measurement 2 2 GPS RECEIVER DATA 19 may seem strange think light second instead of second and think of a second as just another unit of lenght with conversion factors left out of formulae in the same way that conversion factors between inches and miles are left out of formulae Cluttering up formulae with conversion constants adds to the length of already often cumbersome formulae and makes the interpretation of relative magnitudes of times and distances more difficult This unit system also makes many quantities dimensionless which makes them significantly more easy to work with 2 2 GPS Receiver data The GPS receivers used are TANS units the lower boards from a TANS Quadrex or Vector They are manufactured by Trimble and sold to Stanford at a large discount They use EPROMs originally programmed at the GP B laboratory and last modified by Paul Montgomery The receivers communicate to the external world through a serial line and a packet protocol section B 5 1 One sends the receivers various initialisation packets and they then automatically send back packets containing phase measurements at 10Hz together with some other information This extra information contains some diagnostic information plus line of sight vectors every thirty secon
95. es would be useful For instance to do a directory on the remote computer remote line dir To send the file config sys to the remote computer remote send config sys To get the virtual file EMM dat see section B 7 remote get EMM dat B 6 3 Verification sum com There is a program sum com which computes and prints two checksums for its argument This can be used to check that a data transfer went correctly The first checksum is just the sum of every byte in the program modulo 2 the second checksum has a pseudo random weighting for each byte in the program so that this checksum can detect most swapped bytes Typical usage A sum sum com Checksums for sum com 829 C763 sum com has two other uses if it is invoked with no arguments the computer will B 6 INTERFACE WITH AN EXTERNAL PROGRAM 145 beep four times This can be a handy diagnostic aid occasionally If sum com is invoked with three arguments it will consider the first two arguments to be files to read and the third a file to write sum com will produce the third file from the logical bitwise and operation applied to each byte This can be used to bypass an unfortunate bug in writing the EEPROM circuitry on the 486 board the board in the helicopter will occasionally writes FF bytes unexpectedly B 6 4 GPS packet functions When a GPS packet is received from either the GPS ports or the data ports from a remote GPS unit a certain pointer is checked
96. est to people trying to reproduce or extend some of the engineering in this work rather than just understand it I have tried to keep this in the appendices which are intended to be used as a reference Chapter 2 Global Positioning System GPS The Global Positioning System is a set of satellites in non geostationary orbits which broadcast signals that can be used to determine one s position anywhere on earth with an accuracy on the order of metres It was put up by the United States of America s Department of Defense in a program run by Bradford Parkinson The principle of operation is fairly simple A receiver on the ground picks up signals from several satellites Due to timing information embedded in the signals the receiver can determine the time offset between the local clock in the receiver and the clock on the satellite This offset is the sum of the distance between the satellite and the receiver and the time error in the local receiver the time error in the satellite is very small due to very good and constantly resynchronised clocks With four or more satellites the receiver can then solve for the the four variables of position x y z and local clock offset t The details of operation are more complex The most common method of operation is to use the C A coarse acquisition signal which is broadcast from all satellites on a 1 57542 GHz carrier wave To distinguish between satellites a 1 023 MHz pseudo random noise
97. ficient 88 CHAPTER 4 SOFTWARE Having made that decision it was then decided to break the software that runs on the 486 computers into three main programs The lowest level program TSR EXE contains the packet driver and the data storage previously discussed TSR EXE also manages timings file transfers console emulation and some other useful utilities for using an embedded processor A detailed description of TSR EXE is given in appendix B This program contains most of the IBM PC hardware specific operations making the rest of the software more convenient to develop On all computers TSR EXE runs in the background and other programs communi cate to in by e Giving it a command like Down load flight data from the remote machine e Explicitly sending a packet such as GPS initialisation to some logical unit such as the attitude GPS board e Being given a packet from some logical unit such as the remote GPS position receiver TSR EXE takes care of physical to logical port mappings baud rates and other commu nication parameters for different setups through command line parameters TSR EXE can also be set up to automatically retransmit certain GPS packets functioning as the base station in the background On top of this sits one of two application programs The smaller REMOTE EXE allows a user to send commands directly to TSR EXE like Down load flight data from the remote machine or Send the following ga
98. g the base station is at a fixed position relative to the runway this is the error in the distance to the ground also a bad thing For an autonomous helicopter this is the error in the position of the base station which is not necessarily a bad thing If the helicopter is trying to hover for instance it does not 2 4 OBTAINING INTEGERS 41 matter whether it knows exactly where it is relative to the base station it only cares about its distance relative to where it is trying to hover and if that was calculated via the same process with the same error then these errors cancel out and it is irrelevant For this reason there are a lot of tasks that an autonomous vehicle can do without knowing the integers exactly The trouble comes as the time span gets greater Although C is time independent S is not as the satellites are moving This messes up the whole situation as the relative error from previous positions will be sj S 1 C which will grow as tz and t get further apart However by this stage there has been some significant satellite movement by definition and one can use the surveyors method of resolving integers through satellite motion The method about to be described is based on the ideas outlined above For early time the satellites have not moved much and it is a reasonable assumption that the relative error due to incorrect integers is small As the satellites move more one builds up an estimate of C and
99. generic GPS algorithms Invention and implementation of the pseudo global minimisation algorithm sections 2 3 4 and 2 4 2 Invention of the double differencing algorithm section 2 4 1 used for deter mining position in automatic control Some of the new velocity work Some of the work to make the GPS position packets faster e Running the project e Working out the system design e Implementing almost all the software 68HC11 servo controllers Ground and helicopter based serial drivers GPS algorithms debugging and post processing aids control software IBM pseudo multitasking work e Making the servo controller and switch HC11 board appendix C 3 e Lots of little details too small to write up e Making sure everything worked simultaneously and together 1 7 OUTLINE OF THESIS 13 1 7 Outline of thesis Chapter 2 describes how GPS works and the algorithms that are used in processing the GPS information Chapter 3 describes the actual hardware used in this project Chapter 4 describes the actual software used in this program including communica tions diagnostics implementations of the algorithms in chapter 2 control and overall structure Chapter 5 describes the experimental flight test results of this project Chapter 6 describes the new conclusions that can be drawn and gives some ideas for future work This thesis may occasionally contain a few technicalities that are only of inter
100. h will be assumed known and already accounted for in this 28 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS section e The line bias phase difference due to the differing lengths of antenna cables This can be calibrated in advance and accounted for by subtraction from the measured phase All future measured phases will be assumed to have had this calibrated value subtracted from them already e Noise To calculate the attitude represented by the rotation matrix R given several phase measurements one wishes to solve a set of equations like equation 2 16 As R can be considered to be specified by three independent parameters one needs at least three phases In practice there tend to be significantly more than three available and one solves this equation in a least squared sense Thus one wants to find a R that minimises the cost function CR 5 y bi Rs y 2 17 i baselines j satellites This is a nasty nonlinear equation in effectively three variables A typical plot of the cost function in terms of two of three Euler angles parameterising R is given in figure 2 2 One easy efficient solution that often works for solving nonlinear functions of several variables is to iterate over the process of linearise around the current best guess and solve Newton s method It turns out that this works fairly well for this problem though it can be a little slow if the initial guess is bad Fortunately the initial guess of the
101. hat gt 0 Showing that C x has only one zero between 5 and ax is equivalent to showing that 2sin2a and B sin x have only one point of intersection in this region Figure 2 4 shows this situation for an example with B 4 and 0 1 Suppose one starts considering the point a and moves backwards At the first point x where the two graphs cross i e u B sin x 2sin 2z the gradients must differ Specifically it is necessary that B sin z Bcos a VB u is greater than 2 sin 2x 4cos2x 2 2 u At a second hypothetical crossing point this gradient relation would reverse That is VB u lt 22 u A glancing touch point of inflection in C 2 has effectively the same requirement 2 3 ASSUME THE INTEGERS ARE KNOWN 35 high 7 low while high low gt mid high low 2 if C mid gt 0 then high mid else low mid return mid Figure 2 5 Bisection algorithm to solve equation 2 23 Claim this pair of gradient relations never occurs Proof Firstly note that 2 sin 2x is negative if z lt 3r So 42 sin 2 can never be greater than L B sin x so both intersections must occur with x gt r So one needs to find a pair of values of u such that for a small u VB wu gt 2 2 u and for a large u JB u lt 2 22 u The first condition is clearly never satisfied for B l
102. have not had as large an impact upon the world as many futurists have predicted given the rapid exponential growth possible with complete automation and therefore the enormous economic incentives to build such devices There are several reasons for such lack of impact Perhaps the most important stumbling block has been the lack of artificial intelli gence The muscles and joints of a human body are easily within today s technology to reproduce and improve upon but the ability to do something intelligent then is rather difficult One step down from full intelligence and much more feasible today is to enable robots to perform a small number of short tasks This has ended up being fairly suc cessful and many tedious tasks have been automated However there are still immense problems with the overwhelming complexity of describing in computer code tasks that seem quite simple to a human Complexity also comes into the ability to control systems that are not very simple Control theory is quite good for simple perfectly rigid linear systems but is still being developed for more complex systems One further step down a robot must be aware of its current state and the state of its environment in order to make reasonable future movements and this turns out to also be surprisingly difficult One of the hardest state variables to measure is position of the robot in the environment as there are few physical properties that are intrinsic to pos
103. he 28 pin connector on the GPS board for table D 4 two DB9 RS422 connectors pin out in table D 2 to the DB 25 connector with only a crimp connector D 3 Internal Electronics If an upper board 3 were used the connections would be given in table D 3 Otherwise connections to the GPS boards are via the 28 pin connectors on the lower boards Note that the upper board if used needs at least 12V as the upper board contains an on board stitching power regulator The 7 2V and corresponding ground go to an LT1085CT 5 with a bypass capacitor circuit diagram in figure D 1 The 5V output is used to power both GPS boards 174 eo ODN DW oO F amp F W NH FR NO we SO WS WY NO HB BB mnm meae rnae Re Er am oe e Wo N FPF OD o DN Dt FF WwW NY KF D to to NC position from position from position to position to position attitude from attitude from attitude to attitude to attitude to attitude to position from position from position to position to position from attitude from attitude to attitude to attitude APPENDIX D 6V minimum Nominal 7 2V 6V connected to pin 1 IO Ground TX data from GPS TX RX data to GPS RX IO Ground TX TX4 RX4 RX GND 12V GND 7 2V N CNOB OK to send stuff to GPS CNOB CNIB OK for GPS to send stuff CNIB NC CNOB OK to send stuff to GPS CNOB CNIB OK for GPS to send stuff CNIB N N GPS BOX MANUAL Table D 1 Pin out of
104. he master and slave protocols are different as the input for one is output for the other In particular any PWM values coming up from the ground station are almost certainly nonsense and are stored somewhere harmless The ability to understand this protocol is implemented on the slave anyway since it is symmetrical to do so from the point of view of the slave and master programs and from the point of view of the TSR program running on the IBM computers in the helicopter and on the ground The ground program can send PWM values to the HC1l1s during debugging without crashing the slave HC11 Besides someday someone might want to change it Lastly comes the list of interrupt vectors and the interrupt routines used This list was also based upon Fred G Martin s program Appendix D GPS Box manual This is a technical description of the connections for the TANS boards in the box on the helicopter D 1 Mechanical Description The box masses 1 46 kg total including electronics with dimensions 19 5 cm by 13 cm by 9 cm It contains two TANS GPS lower boards and optionally one vector upper board These two boards are used for attitude and position The box was designed and built by Gad Shelef There are five holes for antennas to come out These are designed for the four antennas to be connected to the attitude board and one antenna for the position board It is sensible to have a splitter allowing one antenna to service both the position boar
105. he offset biases which cause the actual set point to be several metres away from the expected hover point where the helicopter was when the computer control switch was thrown give excellent slews as they represent a true step function input to the control system It is unfortunate that they are all simultaneous which could cause 104 CHAPTER 5 FLIGHT TEST RESULTS Flight Test 2 31 Dec 1994 T 0 8 F J 0 4 H J 0 2 4 Correlation Coefficient 0 2 H 4 0 4 i 1 1 L L 5 Delay s Figure 5 10 Autocorrelation of roll signal whilst under computer control and stable showing the 11 second period limit cycle Flight Test 2 31 Dec 1994 260 T T T 240 4 220 7 200 4 180 4 Throttle joystick input Servo units 160 F H E ee 140 600700 600720 600740 fee 600780 600800 600820 time s Figure 5 11 Pilot throttle commands whilst under computer control 5 2 Yaw joystick input servo units Pitch joystick input Servo units COMPUTER CONTROLLED PORTION OF FLIGHT Flight Test 2 31 Dec 1994 125 T T T T T 120 4 115 F 5 110 F 4 105 F 100 95 90 85 80 600700 600720 600740 tee 600780 600800 600820 time s Figure 5 12 Pilot heading commands whilst under computer control Flight Test 2 31 Dec 1994 140 T T T T T 130 7 120 j U 110 H 100 l 90 H 4 80 4 70
106. hen indicate that the integer valid flag is set This will remain set until a cycle slip is detected rare or until visibility to the satellite has been lost The position receivers take the simpler route of sending out a cycle slip flag and assuming that packets will not be lost or that the 486 will be able to detect errors some other way The position receivers need no feedback from the 486 during normal operation The receivers use pseudorange to determine their position sufficiently accurately for the calculation of very accurate LOS vectors from the known satellite orbits The base station does not send up LOS vectors as it would be a waste of bandwidth over the radio link 4 1 DATA FLOW 17 to the worst fit to equation 2 53 is declared invalid If new satellites have come into view or an integer has been previously invalidated the attitude calculation unit attempts to determine the integer based upon the just computed attitude This allows satellites to enter and leave visibility cleanly The position calculation conceptual unit is invoked whenever a pair of packets from the base station and the GPS position receiver arrive with matching time stamps If at least four satellites are visible on both the helicopter and base station antennas and the corresponding integers are known then the algorithm in section 2 3 2 is applied The solution x y z is passed up to the control level and is also used to determine the integers f
107. herwise AH SERVICE_IS_PRESENT geninterrupt 0x6a return _AX 1 void tsr_do_int int command do a tsr command that has no arguments _AH command geninterrupt 0x6a void tsr_send_key char c B 8 SKELETON CLIENT PROGRAM 149 send a keystroke to the remote computer _AH SERVICE_SEND_CHAR _BH 0 _BL c geninterrupt 0x6a typedef void far voidptr voidptr tsr_get_ptr int command call a tsr command that returns a far pointer _AH command geninterrupt 0x6a return MK_FP _ES _BX void tsr_send_ptr int command void far v call a tsr command that takes a pointer _ES FP_SEG v _BX FP_OFF v _AH command geninterrupt 0x6a period_get far pget the PWM input vales from the HC11s period_send far psend the PWM input vales sent to the HCiis send_fn gps_send pointer to function that sends packets int far restore_ds the normal value of the _DS register 150 APPENDIX B TSR USER S MANUAL void far gps_rec int unit int command int length char far data int i _DS restore_ds called from elsewhere _DS may be wrong do_something void tsr_init void if serial_driver_present printf Andrew you are getting even more absent minded n printf LOAD THE SERIAL DRIVER BEFORE RUNNING THIS n exit 10 pget period_get far tsr_get_ptr SERVICE_GET_PWMS_IN psend period_send far tsr_get_ptr SERVICE_GE
108. his problem The reason for introducing this algorithm is it s value for solving a more complex cost function as appears in section 2 4 2 2 3 6 Finding the minimum of equation 2 23 One knows that B gt 0 from the way B is determined in practicet and one can clearly assume that is between 0 and 27 If gt 7 one can make the transformations z x and 2r and one ends up solving the same equation form except with between 0 and 7 so one can assume without loss of generality that 0 lt lt a Furthermore if Tf d is near zero and d is not then the solution is trivial If both are near zero then the result is not important anyway as V has no real effect on the cost In any case one could change the sign of B via adding to 2 3 ASSUME THE INTEGERS ARE KNOWN 33 gt 5 we can make the further substitutions x m x and x leaving the same form of equation again except with 0 lt lt 4 If B is zero then there are two identically correct solutions 5 Assume hereafter that B gt 0 So now the problem is reduced to solving equation 2 23 for B gt Oand0 lt lt F The value of that minimises this function can be narrowed down by a few obser vations Claim cos x lt 0 for the minimum value of x Proof by contradiction Suppose x gave the minimum and cos x gt 0 Then consider y 7 Then cos 2y cos 2a and cos y lt 0 due to the per
109. hree receivers the attitude receiver on the helicopter the position receiver on the helicopter and the position receiver at the base station These GPS packets are brought into an internal 4 1 DATA FLOW 79 Display A A Control GPS EXE wales w To a 2 g LOS wy Se 2 nm Oo DO GPS World Model TSR EXE To 68HC11s PaaS ak Packet gt Driver z To Att GPS From 68HC11s From Att GPS From Position GPS Figure 4 1 Breakdown into conceptual units of the software running on the 486 on the helicopter during normal operations Undashed lines indicate data movement Dashed lines indicate computation This performs the computer control sections in figure 1 1 76 CHAPTER 4 SOFTWARE data structure given the grand title of GPS World Model which performs various sanity checks to detect badly corrupted packets maintains a history and synchronises the phase measurements from the helicopter and ground position receivers The GPS World Model conceptual section also sends responses back to the Attitude GPS receiver to manage the integer valid flags The GPS position and attitude determination algorithms require knowledge of the line of sight LOS vectors s to the satellites There is conceptually a portion of the GPS World model that accepts LOS vector packets from the two helicopter receivers checks to see that they are sane and then uses a history to interp
110. identification number Intermediate DLE characters that occur naturally in the data must be escaped by proceeding them with an extra DLE character B 5 2 Radio Packets Note This is not important to know unless it is intended to intercept these packets en route The packets sent over the radio link involve a different packet structure They start off with a synchronisation character of 55 hex Next comes the packet identification number one byte followed by the packet length one byte unsigned followed by the data At the end is a parity byte which is the logical bytewise exclusive or of each of the preceding bytes in the packet This protocol is overlaid with a higher level meta packet protocol that enables a device between two PCs running this program to send real time data to one of them preempting existing packets without corrupting them To do this any of the hex values ESCCHAR 27 decimal NULLCHAR 26 decimal or PWMCHAR 25 decimal must be escaped if they occur by preceding them with ESCCHAR NULLCHARs should be ignored if encountered in a stream unescaped this is to support the 68HC11 SPI communications interface which is not totally asynchronous The PWMCHAR precedes a series of 9 bytes containing information on current PWM values sent to or from a 68HC11 B 5 PACKET PROTOCOLS 137 The following packet identification numbers for the main packet structure are currently supported as shown in this extract from servic
111. iming errors make this process a little more complex A more detailed analysis follows If the local clock is fast then the satellites will seem to be broadcasting slowly and so will seem to be reducing The effect of this will be that 7 which will be increasing if the local clock is fast will be subtracted from If the satellite moves away from the receiver will increase in the direction s so s will decrease and will decrease the same amount Thus the phase measured at the antenna ignoring noise etc is given by Ort T t t r s t 7 OF 2 3 2 3 ASSUME THE INTEGERS ARE KNOWN 23 x T t i s t 7 Hes 0 2 4 where O is some constant offset Note that the notation of something as a function of time is somewhat confusing as when or r is mentioned it is the phase measured at the local time t whereas and s are functions of physical real time This is because T and are only accessible at given times whereas x and s are dynamic external quantities In typical use one has a receiver which produces a phase measurement at a local believed time such as t 10s In actual fact the clock bias may be such that the measurement was actually taken at real physical time t 9 998s To form a differential pair one has a reading from a second receiver at the same believed time t 10s according to the second receiver s local clock However the actu
112. ins to the helicopter Details are given in section B 6 2 GPS EXE is the most complex It contains e The GPS World Model which is responsible for initialising the GPS receivers maintaining the attitude integers and sychronising data This includes the Line of Sight unit e Position Computation responsible for initialising the GPS position integers and computing three dimensional position 4 4 CONTROL 89 e Attitude Computation responsible for initialising the GPS attitude integers and computing the three attitude degrees of freedom e Control which takes the pilot s console commands and the six position and atti tude values and computes servo commands e Display which shows what is going on in the system on a video monitor either as text or a graphical representation of the helicopter s current position and orienta tion File output from post processing can be considered part of this unit GPS EXE is designed to work in real time always processing the most recent data as soon as it can to get as low data delay from computation as possible Of the algorithms in chapter 2 position determination is the fastest running in a couple of milliseconds maximum Most of the execution time is getting the right data in the right place The attitude determination can be fairly slow depending upon the data Taking longer than 50 milliseconds is unusual The really slow algorithm is the attitude integer resolution which ma
113. iodic nature of the cosine function so C y lt C x violating the assumption that x gave the minimum value This means that lt a lt on Claim 0 lt x lt r Proof by contradiction Let r lt x lt 2a and let y 2r x Then C x C y cos z cos x 2 sin sin x lt 0 so y is at least as good as z Now look at the derivatives of C C x 2sin2z Bsin 2 24 C x 4cos2x B cos x 2 25 One requires x such that C 2 0 and C x gt 0 Claim z a 7 Proof by contradiction Suppose t gt x gt a Then sin x lt 0 and sin2z lt 0 so C x gt 0 so this region is not possible Claim x 5 b 3 Proof by contradiction Suppose that Z lt x lt 5 Then sin2z gt 0 and sin x gt 0 so C x lt 0 so this region is also not possible 34 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS 4sin x 0 1 2sin2x 2 3 4 Figure 2 4 An example of the intersection of two sine waves This leaves the only possible interval as lt x lt m with the information that C x is a continuous function which is less than zero just before 5 and greater than zero just after 7 This means that the bisection algorithm will always be able to find a zero of C x in this range The last thing to show is that there is only one zero of C x in this range Firstly assume t
114. ions with respect to the helicopter than others This will cause a torque on the rotor disk which will tilt the rotor disk and thus the helicopter which is rigidly attached The tilted disk will then provide thrust in a horizontal direction causing the helicopter to accelerate in that direction Since these two controls affect the blade pitch in varying amounts over each cycle they are commonly referred to as the cyclic controls in contradistinction to the collective effect on the blade pitch from the thrust control which is usually implemented by moving the whole swash plate vertically As with the tail rotor it is difficult for a human pilot to control this directly and so a Hiller paddle a small blade rotating perpendicularly to the main rotor is used to provide negative feedback mechanically This has a similar effect to the gyroscope on the tail servo and can be considered part of the plant rather than part of the control system In summary as far as a control system is concerned there are effectively four actua tors which near to hover can be considered to actuate angular rate due to the feedback from the Hiller paddles and gyroscope and total thrust The system is not long term stable in attitude velocity or position Altitude velocity is actually quite stable hav ing some negative feedback on velocity due to the interactions between the rotor blade pitch air passing through the rotor due to a vertical velocity and the gravitatio
115. is about twice as complex as a C A code receiver There are of course various sources of error in the system The most obvious of these is how accurately two receivers can measure the phase of the C A or P code signal This turns out in practice to be within a few metres for C A receivers with a higher accuracy for P code Note that from here on this thesis refers to times and T so one second distances interchangeably assume that the speed of light is equal to one is equivalent to roughly 299 792 458 metres This is a perfectly valid system of units often used in physics Another major source of errors is the atmosphere which bends and slows down radio signals This leads to errors in the tens of metres To prevent undesirables anyone except the US DoD from obtaining too high an accuracy for positioning whilst still providing a useful service to the commercial world the US DoD manipulates the transmitted signals with extra noise known as Selective Availability SA which artificially produces further errors in the pseudo range measurements This One light second per second 16 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS Satelite direction s Data Antenna A Base Antenna Figure 2 1 Picture of the phase difference A for one satellite observed between two antennas reduces accuracy to the order of a hundred metres or so which is sufficient for many commercial uses To obtain higher accuracy in a loca
116. ition in an easily computable manner in an arbitrary environment This lack of position information makes mobile robots particularly difficult Most animals seem to be able to cope in a world without a direct position sensor through a variety of sensors such as vision hearing including sonar and smell and with an incredibly complex and poorly understood processing stage inside the brain that can collate enormous quantities of information cross reference with a complex world view and produce a result suitable for tasks such as eating grass remembering how to get to the water hole and killing other animals This system while having the undeniable advantage of working well is not used directly in robots for several reasons Firstly we do not know how to do it Secondly even if we did modern computers and storage capabilities may not be up to the task Thirdly there are some tasks we may wish done 1 2 WHY A HELICOPTER 3 by robots that require significantly more accurate measurements Most importantly there are alternatives While the typical science fiction characterisation of a robot is a humanoid object there is no more requirement for a robot to have legs rather than wheels or wings rather than turboprops than there is for it to metabolise carbohydrates or proteins rather than burn high grade hydrocarbons or use electricity stored in myriad ways Similarly there is no reason why robots have to use the same senses as animals when
117. iv List of Figures 1 1 1 2 1 3 1 4 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 Lateral location portion of the GPS based autopilot 6 The stadium at Georgia Institute of Technology where the aerial robotics contest is held aoaaa ee 7 The arena in which the aerial vehicle must fly in the AUVS contest 8 The six foot diameter rings containing the disks to be picked up 9 Picture of the phase difference A for one satellite observed between two antennas sooo e e e 16 Cost function of an attitude error as given by equation 2 17 with respect to two of the three parameters oaoa aa a 29 Example of a fairly poor result of Newton iteration of equation 2 17 The vertical axis is the cost the horizontal axis is the number of iterations 30 An example of the intersection of two sine waves 34 Bisection algorithm to solve equation 2 23 2 2 2 020 0004 35 A graph of x giving the minimum of cos 2x B cos x as a function of Band 2 ee 36 A scatter plot of the values of B and for which the minimum of equation 2 23 was needed during a typical flight 2 000 37 Cost function as a function of iterations for local Newton iteration The horizontal axis is the number of iterations the vertical axis is the cost function in units of square wavelengths Forty five percent worked 54 XV 2 9 Cost function as a function of iterations fo
118. ived of a PWM packet The next eight definitions are the HC11 bits used to represent which bits are active in the TFLG1 register PWM_LONG_DELAY is the time in units of 0 54 5 between sending a packet containing the PWM values to the IBM SPI_Delay is the time between characters being sent over the SPI This is arbitrarily set to 0 6m which is designed to be fairly slow to give the slave plenty of time to respond to the last SPI event the SPI is not buffered in 166 APPENDIX C IBM BOX MANUAL the 68HC11 whilst still being significantly faster than the 9600 baud SCI about 1m between characters so that the SPI does not cause serial characters to be lost Next come three equates for the meta characters used in the serial communications See section B 5 2 for details of their usage note this section like a few others referred to in this document is currently in a different document the TSR manual under the heading Radio Packets Now comes the initialisation portion of the main code The stack register is set up to the top of RAM and the X register is set up to point to the register set This enables the use of faster and smaller instructions later on to refer to registers The SPI is enabled as master for the master and slave for the slave Check interrupts and then the SPI and SCI interrupts are enabled The PWM outputs are then set up to trigger at different times This means that they will stay out of synchro
119. iven with a transistor amplifier courtesy of Bruce Woodley A 2 Eight LEDs are attached to the parallel port on the 486 Through a 1502 resistor A 2 ELECTRONIC CONNECTION 7 2V 680 2N2222 Figure A 2 LED driver 123 Appendix B TSR User s manual B 1 Introduction TSR EXE is a short terminate and stay resident program for use on an MS DOS type machine TSR is designed to convert a cheap IBM PC type single board computer which has poor support for embedded control and communications into something that is not so unpleasant to use TSR basically controls serial ports handling all the low level details so that a higher level program can just deal with packets being received and transmitted asynchronously TSR also has a software interrupt handle which can be used to command file transfers keyboard emulation and console emulation TSR can also store data in XMS memory This data can then later be saved to a file locally restored from a local disk sent down a serial port and replayed in real time for debugging purposes TSR can make use of the FIFO buffer in a 16550 UART TSR is particularly designed for talking to Trimble TANS GPS receivers and can understand the packet structure of these devices MS DOS s form of pseudo multitasking Typically only useful when used in conjunction with the replaying facility 124 B 2 TECHNICAL DETAILS 125 B 2 Technical Details This section is designed for
120. ix satellites visible for each of the three other antennas relative to the master antenna The antennas are linked in a clever multiplexed manner described in detail in 9 so they all share a common clock and thus there is only an integer ambiguity and no time ambiguity as well It is possible for one Trimble board to do both but this was not done in this project due to the lack of available software to run on the Trimble boards 2 3 Assume the integers are known For the moment assume that the integer or time offsets are known so that one does not have to worry about them and can deal with just the operational solutions that have to be done in real time with as small phase delay as possible due to either communication or computation To be useful on the helicopter these computations must execute in a few milliseconds and not consume excessive memory 2 3 1 On the Size of quantities At several times second order terms will be neglected in expansions in what follows To justify these approximations it is worthwhile having a close look at the size of various quantities These are given in table 2 3 1 Note that for consistency all quantities are given in units of seconds A conversion into seconds for a centimetre level of precision and a hundred metres distance over which the helicopter is expected to operate are given 2 3 ASSUME THE INTEGERS ARE KNOWN Term 1 cm 100m Az at dt dD dt d amp z S dt dS dt
121. l loop and since the fly bar gives negative angular velocity feedback only angular GPS feedback was used The innermost block in figures 4 10 and 4 11 is shown as K s which 92 CHAPTER 4 SOFTWARE Heading gyro prop Heading rate A 0 2 deg deg Figure 4 9 Heading control loop with GPS feedback on attitude yaw The gain is given in terms of tail rotor pitch degrees per degree of yaw error Based upon figure 1 1 is not meant to be taken very literally but rather represents a complex plant with an integral type behaviour and stability with appropriate values of negative feedback of angle The feedback gains are in terms of degrees blade pitch or the rotors or Hiller paddles per degree of helicopter inclination 4 4 CONTROL etc eS Py yet nf a fe Pe yw Ne ee gp oh ee neg a Bg tal E beta Sete Seed aE el NS pal i Bye ng the Penta Se AOE NS Bete het A In yg gy re ee nh 93 gt 15s Velocity Position N Q 9 gt Q 3 x 2 A Figure 4 10 Longitudinal position pitch control loop with GPS feedback on position velocity and attitude pitch Specific longitudinal version of figure 1 1 94 A CHAPTER 4 SOFTWARE Velocity Position Figure 4 11 Lateral position bank angle control loop with GPS feedback on position velocity a
122. l environment it is possible to use a differential form of GPS One uses two antennas and two receivers situated nearby both receiv ing the same signal The difference in the pseudo ranges A see figure 2 1 is then significantly more accurate as the effects of selective availability and atmospheric noise cancel out exactly Now the main limitation on differential accuracy is how accurately the two receivers can measure the phase of the C A code Note that this does not help one obtain a more accurate global position it is entirely a differential measurement Later receivers were produced that were capable of measuring the phase of the 1 5 GHz carrier at least relative to some local clock As this has a much shorter wavelength than either the C A or P codes it is economically feasible to measure this phase with 17 much higher accuracy in terms of position and in fact it can be done relatively easily to give accuracies on the order of millimetres This has been done for a long time now a reasonable description is given in 9 Basically the carrier is downconverted to an intermediate frequency of about 4 MHz An in phase and quadrature correlation coefficient are then measured between this 4 MHz new carrier and the pseudo random noise for the particular satellite being tracked modulated by a reference 4 MHz local oscillator A phase locked loop is then placed around this structure and the phase is tracked with updates every millisecond
123. l helicopter as they are readily available reliable and fairly inexpensive This severely constrains some of the other objectives as there is a limited range of such helicopters and they are designed for hobby fliers who prize aerobatic ability over payload capability and stability Given the decision to use a commercially available model helicopter payload became the most significant criterion The helicopter chosen was an Excel 60 PRO which is one of the largest of the hobby model helicopters A still larger helicopter would be prohibitively expensive as it would be out of the mass consumer market range The helicopter loaded up with electronics is shown in figure 3 1 This helicopter masses about five kilograms fueled and can carry a payload of five to six kilograms more and still fly reasonably out of ground effect in California weather There are three main power sources for propulsion available for model helicopters electric glow fuel and gasoline The electric helicopters are clean and relatively easy to maintain and use but are unsuitable due to their very low payload and flight time Gasoline engines have advantages due to efficiency allowing very long flight times but they are harder to use as the engines are heavier hotter and tend to vibrate more For these reasons glow fuel was used 3 1 HELICOPTER 59 Figure 3 2 Hardware needed to start the engine and perform field maintenance and adjustments Starting th
124. l units interfac c Communicates with TSR and the user lookup h The lookup table to get a first approximation to the minimum of equation 2 23 mat_math c Some matrix routines used in gps c mat_math h Prototypes for functions in mat math c matrix c Some matrix functions used in attcomp c matrix h Prototypes for functions in matrix c E 4 C CLASSES 183 nogr c Creates a text display of helicopter status c f gr c radiocom h Contains the gains structure uplinked from the base station serbuffe c Buffers the GPS packets serbuffe h The interface between serbuffe c and interfac c service h The interface between tsr c and interfac c setup h Calculates checksum on gains packet Also used by remote exe swabtype h Data types to safely manage the difference between big endian and little endian architectures tans h Packet definitions from the TANS E 4 C Classes This section contains a brief description of the purpose of most of the classes used in this program They will de divided up by their source E 4 1 Data Swabbing The packets coming from the TANS receivers contain multibyte values in the opposite order in memory to the order expected by the IBM Through the explicit type conversion operators of C it is possible to make data types INTEGER LONG SINGLE and DOUBLE which are stored in memory in the format used by the TANS and which can be implicitly converted to an IBM usable type in calculations Having them as
125. lobal time messes up the algebra a little In particular ser 1 ir 80 Tar d r r r A ETD 2 11 d r r d r gei J U ar Differentiating equation 2 10 with respect to time gives s 1 i0 SIP Si 3 i s2 1 05 t Au 03 12 L t 8 LE Az T p T Sn 1 F t SL DL 8 C 2 12 The matrix on the left hand side of the equation is easily obtainable it has not changed since the position calculation Its derivative on the right hand side can also be 26 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS calculated relatively straightforwardly as they contribute only a very small correction term to the velocity calculation The rate of change of line of sights is easy if one can interpolate one can get a reasonable time rate of change The second derivative of phase comes from first differences of D Again this term is so tiny that the differences between D and are insignificant The position and time offset come from the position calculation which should be done just before the velocity calculation This just leaves the phase derivatives to be calculated Unfortunately the local clock issue means that one cannot quite just say D Firstly note that to first order D D 1 5 1 70 2 13 1 1 62 6 76 2 14 so that one may update this equation to s 1 207 t DI Dit 51 St s2 1 26 t Au i D3 t D amp t 8 LE Az 7 7 T
126. local iteration is common and is not counted This is indicated by beeping and on a seven segment LED display 2 4 OBTAINING INTEGERS 53 not done due to desire to check that the integer resolution worked and to the difficulty in performing such a maneuver manually To increase reliability ever further here are two possible suggestions Firstly one can store the phase measurements for the seconds preceding and during integer resolution and use them for verification Secondly it is straightforward to compute the partial derivative of changes in A to input phases and this can be used to get an estimate of the expected noise in the integers This can be used to improve the accuracy of the decision as to whether they are too far off integer values to be reasonable Neither were implemented as they did not seem necessary and would increase the time for integer resolution the first involves storage and replay the second involves inversion of a fairly large matrix Some results of simulation are shown in figures 2 8 2 9 and 2 10 The simulated data is for a roughly 180 degree turn with twenty time steps and J averaging 6 5 and never dropping below 5 Note that Cohen s algorithm requires J to be at least 9 and only some specific patterns of 9 really work The base case for comparison is Newton s method figure 2 8 In practice this did work a reasonable proportion of the time 45 percent Figure 2 8 shows how the cost functions beha
127. ment e Dealing with doppler corrections in the time offset between receivers e Most importantly the overall philosophy of producing instantly usable position data that can be efficiently updated later in real time If one has four satellites and a moving vehicle there is not much one can do But with more than four satellites one has redundant information which can be used to get the estimate for C Collecting equation 2 26 over time gives the following matrix equation AX S 0 0 z S AX 0 Sy 0 z P 2 27 AX 0 0 Sn I B C If one actually tries to solve this equation one finds that there is a vector that comes very close to being in the null space of the matrix on the left hand side of equation 2 27 This vector corresponds to all the integers being increased by a constant amount 6 and all the time biases being decreased by the same amount As the integers trying to be determined in equation 2 27 are typically small being just the residues after a fairly good approximation with conventional GPS the effects due to the doppler corrections will be tiny causing it to be unreasonable and pointless to try to distinguish between offsets in all the integers or all the receiver time errors 2 4 OBTAINING INTEGERS 43 One method commonly used to solve this problem is to explicitly prevent this from happening by only calculating the integers relative to one base integer called the double difference approach e g 14 This
128. mer to bx SERVICE_SET_PARALLEL_CON 153 pset_con Reset the CON timer to bx SERVICE_SET_PARALLEL_CON_ACT 154 pset_con_act Set the state when CON is used Bits not set in BH are affected set to BL iff CON is valid SERVICE_SEND_BYTE_DIRECT Send the byte in BL to unit AL directly without buffer ing Used mainly for com patibility with other Trim ble software and drive exe Otherwise use of this func tion is not recommended B 6 2 More details on remote exe remote exe is an executable file that calls TSR interrupt service routines and does basically nothing else The above table gives most of the commands each command is a single argument that specifies which service function is to be performed Some of 144 APPENDIX B TSR USER S MANUAL the commands key save load send and get take a following parameter For all other than key the next parameter is the string passed to the service routine For key each character in the following parameter is sent one at atime The special commands space nl and escape send a space new line and escape key press respectively The command line has a slightly different flavour This command takes the rest of the arguments as key presses and sends them across interspersed by spaces and followed by a new line For instance the following two lines are equivalent remote key copy space key tsr exe space key newtsr exe nl remote line copy tsr exe newtsr exe Some exampl
129. mputer recording a log file 142 APPENDIX B TSR USER S MANUAL REM_RECORD_PWMS_IN rem_rec_pwm_in tell the remote computer to save the received PWM val ues 18 2 times a second REM_RECORD_PWMS_OUT rem_rec_pwm_out tell the remote computer to save the being sent PWM values 18 2 times a second RECORD_ADD_SAVEIT Save the bx characters at memory address es dx 18 2 times a second with identi fication al RECORD_DUMP_BLOCK Save the bx characters at memory address es dx now with identification al and unit dx HC11SEND_PWMS_ENABLE tell the HC11s to send PWM data HC11SEND_PWMS_DISABLE dis hc11 tell the HC11s not to send PWM data Useful for more efficient data transmission for debugging HC11 REM_SEND_PWMS_ENABLE rem_en_hel1 remotely tell the HC11s to send PWM data HC11_ REM_SEND_PWMS_DISABLE rem_dis_hel 1 remotely tell the HC11s not to enable send PWM data RECORD_REPLAY replay Replay the log file RECORD_REPLAY_DISABLE Stop replaying the log file B 6 INTERFACE WITH AN EXTERNAL PROGRAM 143 RECORD_REPLAY_LOADFILE load the log file from the lo cal file es dx RECORD_REPLAY_SAVEFILE save the log file into the local file es dx SERVICE_SET_PARALLEL Set the parallel port to BL logically ORed with BH and current contents SERVICE_SET_PARALLEL_OSC pset_osc Set the parallel port to oscil late bits set in BL at 9 1 Hz as long as COP timer is OK SERVICE_SET_PARALLEL_COP pset_cop Reset the COP ti
130. n in table D 2 The PCA 6143 RS232 connection pin out is given in table C 1 The PCA 6143 RS422 pin out is the same as the other RS422 pin outs The to power cables go to an LT1085CT 5 5V 3A low dropout regulator as used in the GPS box These input wires have to supply a bit over 6V They are connected C l IBM PC CARD AND SERIAL CARD 155 Pin Name o ON Don fF w NY Q Z J Table C 2 Pin out for the RS232 port on the 68HC11 card Port Description IO address interrupt use 1 RS232 on PCA 6143 4 HC11 and radio 2 RS422 on PCA 6143 3 unused 3 RS422 on serial card 7 GPS position 4 RS422 on serial card 5 GPS attitude Table C 3 Information on the serial connections to 7 2 volt batteries which provides a small safety margin I tried experimenting with switching voltage regulators but the resulting electro magnetic noise reduced the radio range to an unsatisfactory level The circuit diagram for the regulator is identical to that given in the GPS box description in figure D 1 Power demand varies from a normal 2A up to about 3A on occasions depending on what the CPU is doing This power supplies only the IBM and serial cards not the HC11 card The HC11 s supply is dealt with in section C 3 1 The passive backplane connection between the IBM and serial cards is provided by a one inch long ribbon cable between the two cards and IDC connectors This connector has a tendency to slip off under the helicopter s vibration so the b
131. n which the aerial vehicle must fly in the AUVS contest a field shown in figure 1 3 The aim of the contest is to make a totally autonomous vehicle that will take off from inside the 15 foot by 15 foot start square in the upper right of figure 1 3 fly over to a black six foot diameter nonmetallic ring in which there are six orange three inch diameter ferromagnetic four ounce disks see figure 1 4 The aerial vehicle then must pick up one of these disks optionally landing in the six foot diameter ring in the process fly over to the far ring drop the disk and repeat to pick up all the disks then return to the starting point and land A suitable vehicle then needs to be accurately controlled and probably capable of hovering We decided that an aerial vehicle capable of performing this task would be a fine design target This contest has been running for several years now The best entrant so far has been Georgia Institute of Technology s helicopter which used an external vision system to locate the helicopter in a small area They managed to hover fairly accurately and stably Last year s winner was the USC team who obtained long term position using an on board vision system 15 which was not working during the contest and used sonar 15 1 5 CONTEST Figure 1 4 The six foot diameter rings containing the disks to be picked up 10 CHAPTER 1 INTRODUCTION range sensors to obtain altitude roll and pitch Another fairly
132. nal field From a pilot s point of view a model helicopter is unstable enough to require full time concentration and control just to maintain stability 3 2 Helicopter Electronics To measure attitude of the helicopter at least three GPS antennas are needed As a larger number of antennas helps mitigate poor visibility and generally increase relia bility four antennas were used The antennas white rectangles can be seen in figure 3 1 with the master antenna on the tail where visibility is best enlargement in figure 3 5 one of the three slave antennas mounted at the front of the helicopter as high as The maximum number supported by the receiver used 3 2 HELICOPTER ELECTRONICS 63 Figure 3 5 The master antenna on the tail of the helicopter possible to avoid occlusion problems and the other two mounted off to the sides The accuracy and reliability of the GPS attitude determination system is largely dependent upon the geometry of the antennas generally speaking the farther apart and the less collinear the antennas are the better the accuracy of measurements This is difficult on a helicopter body which is as small as possible excluding the long tail In order to have the side mounted antennas significantly out of the line of the other two antennas a piece of honeycomb aluminium was used to hold them rigidly a significant distance away from the helicopter body See figure 3 6 for details There are two GPS receiver
133. nce is that functional lan guages have no assignment operator A variable can only ever contain one value This is not as restrictive as it sounds loops in imperative languages become recursive functions in functional languages which can in principle be optimised by a decent compiler into code as efficient as imperative languages Indeed such compilers are just starting to appear This removes the existence of global state and means that the compiler can recognise that f a is always the same whenever a or f are computed This property is called referential transparency This is not the case in languages such as C Consider for example the random function or I O functions These have to be handled in functional languages by passing state into the the function explicitly This is often inconvenient but it has some huge advantages One advantage is that it is convenient for functions themselves to be considered as variables and passed around as parameters and operated on Being able to operate on functions is actually an immensely powerful ability Time and asynchronous events can also be handled smoothly e g 4 5 Another major benefit of referential transparency is the ability to use lazy evaluation Lazy evaluation means that a value does not have to be evaluated until it is needed The reason this is useful is that often expressions are written down and never used What is more this inefficiency is often done in a manner that cannot be det
134. nd bank angle Specific form of figure 1 1 Chapter 5 Flight Test Results The helicopter has flown under autonomous closed loop GPS control has hovered stably and has shown an excellent ability to deal with command induced transients on the order of ten metres Only GPS signals were used to control both helicopter location and attitude Due to the significant effort of setting up equipment for a flight and troubles with vibration causing unreliability there have been to date only two flights with all loops closed by GPS using the control gains given in figures 4 8 4 9 4 10 and 4 11 This chapter describes the results of these flights The graphs in this chapter come from GPS data down loaded from the helicopter after the flights 5 1 Flight Profile A typical test flight is shown in figure 5 1 The helicopter took off under manual control was flown by the pilot Steve Morris up to a relatively safe high location where the computer control switch was thrown The helicopter repositioned a few metres due to the biasing errors in the zero point trim adjustments and then hovered autonomously Hands off After about a minute Steve commanded some transients manually via perturbations on the control console which acts as a position control in computer mode to demonstrate the closed loop system s transient recovery from disturbances Lastly Steve put the helicopter back into manual mode and landed the helicopter whence we 9
135. ndirectly through effects on c The effect on covariances is similar although the algebra is a little more complex Suppose that the partial covariance matrix Ay is split up as t T Ay 2 48 T A The implicitly defined by equation 2 44 vector Q may get longer as more satellites come into view as the unity matrix implicitly grows as more satellites are added The extra places in Q will always be unity however and so just storing the value of Q for the current configuration will be sufficient to account for its effects on future satellite entries 2 4 OBTAINING INTEGERS 49 Then one can say N 52 Aa a 1 Q7ACQ 2Q7 Ane 2 49 Aae An e a OTA 2 50 Ane R agg T a Q 2 51 2 52 _ 2 T Impl _ T Araig Anta 71QQt a TQ QT from which one can in a similar way isolate the covariance of the lost satellite from new satellites One must keep some extra housekeeping information for old satellites but this need not be dealt with every iteration and so does not slow down the system too badly Also this makes the memory consumption grow roughly linearly with number of satellite changes rather than quadratically as would be the case if one did not perform removals like the ones shown here Gaining a Satellite As mentioned previously gaining a satellite is straightforward If it were not for the slight contortions described above when losing a satellite it would just be a matter of adding
136. nformation GPS Program Interface to TSR 2 2 Processing of packets Source filles C Classes 0 E 4 1 Data Swabbing E 4 2 General Utility E 4 3 Matrix aeaaeae E 4 4 Attitude computation E 4 5 GPS world model E 4 6 Control Debugging files and postprocessing F List of Acronyms xii 152 152 153 156 157 158 158 163 171 171 172 172 173 177 177 180 180 181 182 183 183 183 184 184 185 186 186 188 Bibliography 191 xiii List of Tables 2 1 3 1 3 2 B 1 B 2 C 1 C 2 C 3 C 4 D 1 D 2 D 3 D 4 Order of magnitude estimates of the sizes of various quantities Frequencies Used 2 ee Lifetimes of consumables 0 0 ee a Standard interrupt numbers for the serial ports Standard base addresses for serial ports 0 0 Pin out for the RS232 port on the IBM card 0 Pin out for the RS232 port on the 68HC11 card anaana ee Information on the serial connections 0 0 0000 HC11 board DB25 pin out check that IN OUT numbers are correct Pin out of the DB25 connector on the GPS box on the helicopter Pin out of the DB9 connectors going to the GPS box on the helicopter Pinout of the serial connector from an upper GPS board Unofficial pin out of the 28 pin inter board connector in the TANS vector and Quadrex 2 x
137. ng with the parallel port serial h Contains constants for the serial port service h Contains definitions to enable a caller program to communicate with TSR stack h Contains functions to switch over to a local stack tsr h Contains the Port class definitions B 3 Running the program TSR is a small about 30k standard EXE file which can be run directly from the command line TSR then prints out the status of all the ports and extended memory which TSR is using and then goes into the background letting other programs run TSR takes many command line options These all are single strings separated by spaces Each string is one command and consists of either one or two letters followed by a number The letter s represent the parameter being varied and the number represents the value of the parameter For on off parameters 0 represents off and 1 represents on The one letter commands are global commands the two letter commands give values to particular serial ports An environment string GPS_TSR_ARG can contain a list of strings separated by spaces which will also be searched for command line options There are internal defaults for settings which can be superceded by settings in GPS_TSR_ARG which will be superceded by command line options 132 APPENDIX B TSR USER S MANUAL B 3 1 Serial port specific ommand line options These commands all contain two letters followed by a number The first of these letters specifies the pa
138. ning the unknown integer wavelength offsets for attitude calculation was devised The helicopter is capable of hovering autonomously It uses four GPS antennas on the helicopter and a ground reference station to determine position and attitude to precisions of roughly a centimetre and a degree both at a ten Hertz update rate The new algorithm for integer resolution allows integers to be resolved in a computa tionally efficient manner with fewer satellites in view than previous algorithms allowing use in a greater number of applications This thesis describes the overall problems approaches and philosophy of design then contains detailed descriptions of the various logical parts of the project A descrip tion is given of GPS carrier phase approaches and how position velocity attitude and attitude rate can be calculated and a description of the new algorithms that make this possible The hardware used in this project is then described followed by the software for flight and analysis The results of flight tests are given and then some conclusions and suggestions for further work in this valuable arena are presented The appendices contain comprehensive technical details of the hardware and software vi Acknowledgements I would like to thank Professors Robert Cannon and Stephen Rock for their vital advice as well as efforts in finding funding and experts in a variety of fields I would like to thank Professor Gene Franklin for his h
139. ntegers are known 0 00200004 20 2 3 1 On the Size of quantities 2 ee ee ee 20 2 3 2 Position 2 ee 22 2 3 3 Velocity aoaaa aa ee 24 2 34 Attitude 2 ee ee 27 2 3 5 Pseudo global attitude solution 0 31 2 3 6 Finding the minimum of equation 2 23 32 2 4 Obtaining Integers 2 4 1 Position 2 4 2 Attitude Hardware 3 1 Helicopter 3 1 1 Actuators 3 2 Helicopter Electronics 3 3 Ground Station Electronics 3 4 Operations Software 4 1 DataFlow 4 1 1 Operational 4 1 2 Initialisation 4 1 3 Data Retrieval 4 1 4 Post processing 4 2 68HC11 programs 4 3 486 software 4 4 Control 2 3 7 Attitude Rate Flight Test Results 5 1 Flight Profile 5 2 Computer Controlled Portion of Flight 0 00 Conclusions and Future Work 6 1 An aside on Languages Helicopter manual A l Suppliers A 2 Electronic connection 57 57 59 62 66 69 73 73 74 78 81 85 85 87 89 B TSR User s manual 124 B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 Introduction 2 ee 124 Technical Details 2 ee 125 B 2 1 Segment Registers 2 0 0 0 0 0 000 0000 125 B 2 2 DOS reentrancy 2 2 2 0 0 0 0 ee ee 126 B 2 3 HIMEM SYS reentrancy 02 a 127 B 2 4 Stack size 2 ee 127 B 2 5 Compiler problems
140. nterrupted flag_send_spi On the master a flag that is zero iff the master is currently sending a message on the SPI pwmNin For N between 1 and 4 gives the 16 bit value of the PWM signal on input N for this chip The pulse width is measured in units of 0 545 This value is maintained by interrupts pwmNout For N between 1 and 4 gives the 16 bit value of the PWM signal to be produced on output N for this chip The pulse width is measured in units of 0 545 This value is maintained by interrupts SendPWMs A flag byte sent from the IBM regularly or whenever the IBM decides to do so If this byte is non zero the IBM wishes to have regular updates of the PWM signals IBMpwmN For N between 1 and 8 These are the 8 logical PWM commands from the IBM Connections 1 4 are not needed by the slave so they are not stored toIBMN for N between 1 and 8 These are the 8 logical PWM input values to be sent to the IBMs The master knows all of these since the master HC11 is sent the slave s values the slave only knows its own plus the value of channel 1 which is needed for the mode pass through IBM controlled selection ICN_wenthigh for N between 0 and 4 The timer value when the PWM signal for input capture IC number N went high These are maintained by interrupts The punNin values are maintained by interrupt routines that are called whenever one of the PWM inputs changes state If the PWM went high the latched and thus C 4
141. nts of the files produced if the environment variable is set follows GPS_ATTSOLVE_FILE GPS attitude solution and debugging information GPS_BPHI_FILE B and values for which equation 2 23 needs to be solved Used to create figure 2 7 GPS_ID_FILE Some information useful for system identification GPS_INTSOLVE_FILE Debugging information for integer resolution E 5 DEBUGGING FILES AND POSTPROCESSING 187 GPS_LOS_FILE Debugging information of line of sight vectors GPS_POSN_FILE GPS Position calculations GPS_SNR_FILE Information on SNRs for satellites GPS_VEL_FILE GPS velocity calculation GPS_XMS_FILE Information saved explicitly by GPS to XMS during the flight Appendix F List of Acronyms This appendix contains a list of the acronyms used in this thesis which may be unfamiliar to some of the readers 486 An Intel microprocessor 68HC11 A Motorola microcontoller AUVS Association for Unmanned Vehicle Systems Conversion factor between time and distance the speed of c light 1 C A Coarse Acquisition The most commonly used GPS code FIFO First In First Out A buffer Gravity Proble B A large project at Stanford that also GP B works with GPS GPS Global Positioning System 188 HC11 Ll MS DOS PD PRN PWM RAM RC R5232 RS422 SA SCI SPI 189 68HC11 1 575420 0 GHz GPS C A carrier frequency MicroSoft Disk Operating System A common operating system for IBM PC compatibl
142. nue Sunnyvale CA 94086 U S A rev a2 edition Dec 1989 3 TANS VECTOR Four Antenna GPS Attitude Determination System SPECIFICA TION AND USER S MANUAL 645 North Mary Avenue Sunnyvale CA 94086 revision a edition June 1993 4 J L Armstrong B O Dacker S R Virding and M C Williams Implement ing a functional language for highly parallel real time applications In Software Engineering for Telecommunication Systems and Services pages 157 163 1992 Albert Benveniste and Paul Le Guernic Hybrid dynamical systems theory and the E signal language IEEE Transactions on Automatic Control 35 5 535 546 May 1990 6 K M Black R G Martinez W Page and J Fitzer Evolution and design of the 1993 university of texas at arlington autonomous aerial vehicle In Proceedings June 1993 7 R Brown and P Ward A gps receiver with built in precision pointing capability In IEEE PLANS pages 83 93 March 1990 191 192 8 n a BIBLIOGRAPHY Clark E Cohen Boris Pervan H Stewart Cobb Dave Lawrence J David Powell and Bradford W Parkinson Real time cycle ambiguity resolution using a pseudo lite for precision landing of aircraft with gps In Second International Symposium on Differential Satellite Navigation Systems March 30 April 2 1993 Amsterdam Clark Emerson Cohen Attitude Determination Using GPS PhD thesis Stan ford University Department of Aeronautics and Astronautics Stanford CA 94305
143. number of iterations 2 3 ASSUME THE INTEGERS ARE KNOWN 31 Note that the baselines and the line biases can be obtained through a self survey over a period of several hours 9 2 3 5 Pseudo global attitude solution For reasons which will become clear in section 2 4 2 the linearise and solve method used in section 2 3 4 was not entirely satisfactory to solve equation 2 17 It would be useful to have an approach that converged more rapidly in cases when the initial guess was very bad The following section describes an algorithm that works well for bad initial guesses Consider an estimate of attitude R Consider the problem of finding the best value of V such that the new rotation matrix R defined by cosUW sinW 0 Ri sin cos 0 R 2 18 has the smallest cost function 2 cos sinW 0 C R V 5 b siny cosy 0 R 7 OY 2 19 i baselines j satellites jet 0 0 i 5 a cos V azsin V a3 2 20 i baselines j satellites amp cos Y csin Y c3cos Y sin Y c4cos Y cs sin Y ce 2 21 d cos 2V amp dzcos Y 2 ds 2 22 where a ci d and are numbers that can be readily and efficiently calculated for given R phases baselines b and line of sight unit vectors s One wants to find a value of Y that minimises this So one wants to find the minimum of d cos 2W E1 dz cos Y 2 as da is irrelevant to the minimisation Without loss of generality
144. ny forever hopefully reducing simultaneous transients at the servos Note that the two HC11s may end up syncronised but avoiding this is more work than it is worth The SCI is set up to transmit a null character so that when the SCI has finished sending it will cause an interrupt All the serial lines are constantly transmitting some thing This is necessary for the SPI and not particularly harmful for the SCI and there are significant benefits in the simplicity of having both protocols the same This arrangement also makes the interrupt structure on the SCI rather trivial as one does not always have to turn the transmit ready interrupt off whenever the buffer gets empty and turn the interrupt back on when a character ends up in the buffer It would arguably be a good idea to not send null characters as it has the two disadvantages of allowing the receiver to get out of synchronisation as well as causing the receiver to respond unnecessarily to interrupts but the first is not a problem during normal operation though the first packet ever sent may be corrupted and the effect upon execution time of the extra interrupts is negligible All interrupts are now cleared and enabled and the timer is enabled This sets in track automatic maintaining of almost all of the variables mentioned above as most of the program is really in the interrupts C 4 PROGRAMS RUNNING ON HC11 167 The master now loads spi_escape with a non zero value to say that
145. o extreme vibration problems These have caused surface mount integrated circuits on the circuit boards to break off Shock mounting the box is difficult for various reasons ranging from the physical difficulty and expense of changing the design to picking an appropriate frequency that will not cause unwanted and de structive oscillations There has been significant damage caused through attempting to shock mount heavy objects too loosely causing unwanted and unstable dynamic modes This has to be done carefully The currently being tried solution is to apply an epoxy adhesive to the most likely to be stressed of the components vis the surface mount integrated circuits with a large mass to perimeter ratio This basically means the 68HC000 chips the 86C681 UARTs and the ADC0808 analog to digital converter plus the sockets for the EPROMS This has improved the situation but the vibration protection still requires significant im provement for long term reliability D 2 DB 25 connector The female DB25 connector provides both power and signals for the boards inside The pin out is given in table D 1 This pin out is chosen so that one can easily connect to D3 INTERNAL ELECTRONICS 173 7 2V from batteries 5V to GPS boards LT1085CT 5 22uF Tantalum Ground from DB25 Ground to GPS boards Figure D 1 Power Supply for the GPS box circuit diagram 68HC000 GPS board component side ho Figure D 2 Orientation of t
146. of memory for later down loading Four minus the number of satellites visible 78 CHAPTER 4 SOFTWARE Above everything is a display which consists of a video display if connected and some status LEDs on the parallel port if connected The ground GPS operation can be considered as largely a subset of the above op eration as shown in figure 4 2 The packet driver only has to deal with input from the one position GPS receiver and has the main job of sending the data from the GPS receiver straight back out another serial line to the radio transmitter to be sent up to the helicopter The packet driver can also save the GPS packets if required and can send the packets to a display module which can be very useful as a ground diagnostic to check that the ground station is working properly and that there are sufficient satellites in view The 68HC11s have the basic functions of passing a data stream between the 486 on the helicopter and the data radio horizontal lines in figure 4 3 and of passing servo commands between the radio console and the servos vertical lines in figure 4 3 The 68HC11s also allow data to cross couple the pilot s commands are inserted into the data stream sent to the 486 and computer commanded values from the 486 are extracted out of the data stream from the 486 and may be sent to the slave 486 depending upon the value of a switch set by the pilot and transmitted as one of the servo values to the master 68H
147. okup and then Newton iteration is used With this arrangement only one or two Newton iterations were required for about five digits accuracy in z Note that the Newton iteration mentioned here is only in one variable and consists of a dozen odd operations it is not nearly as computationally expensive as a multidi mensional Newton iteration The purpose of the two separate tables is to conserve memory by not having a fine lookup table were it not needed An equivalent alternative to the two tables would be a complex index function of B and expanding the phase space near B 4 6 0 A scatter graph of the values of B and sent to this algorithm during a typical flight is shown in figure 2 7 This demonstrates that in practice the values of B and are not particularly congregated around B 4 and 0 so the average efficiency of this approach is very high since the bisection algorithm is rarely resorted to 38 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS In summary it is possible to find the minimum of equation 2 23 in a computation ally efficient manner 2 3 7 Attitude Rate In a similar manner to velocity Paul Montgomery et al 18 obtained attitude rate signals This involves a computationally simple step after the attitude calculation It is mentioned here for completeness as it can be and is measured and calculated on the helicopter Details are not presented here as this implementation was done by Paul and is n
148. olate or extrapolate to obtain an LOS vector for each satellite in view for the current time Sitting on top of the GPS World model and LOS calculation stages are the imple mentations of the GPS algorithms given in chapter 2 The attitude calculation algorithm is invoked each time that a new raw phase packet comes from the attitude GPS receiver The attitude calculation unit checks to see if the integers have already been calculated If not this unit checks to see if there has been sufficient phase change in the recent history to make an assumption of the existence of significant angular motion reasonable If there is sufficient phase change then the integer resolution algorithm section 2 4 2 is attempted If the cost function equation 2 54 is small enough at the calculated minimum and if the estimated integers are close enough to mathematical integers then the integers are assumed correct and remembered If the attitude integers are known then the attitude computation conceptual unit calculates the actual attitude and feeds roll pitch and yaw up to the control system If the error in equation 2 17 is higher than expected from noise the integer corresponding Instead of sending out a packet which may be lost to indicate a cycle slip the attitude receiver sends out a phase valid signal on each packet The 486 then responds with a packet saying that it has noticed that the integer is not valid The attitude receiver may t
149. om a different program Typically TSR will require that the DS register point to the start of the TSR data space whereas the calling program will have DS set to point to the program s own data space See section B 6 4 for an example of the use of such a call To compensate for this the callee function must set the DS register correctly before doing anything else loading DS from a far variable Looking at the assembly language output of the compiler for a sample function call can help make this problem more comprehensible When the callee function has returned 126 APPENDIX B TSR USER S MANUAL the DS register must be restored Restoring the DS register to an appropriate value for the caller program is the responsibility of the caller program Dealing with the DS and other registers during a software interrupt is handled automatically by the processor and compiler B 2 2 DOS reentrancy Unfortunately MS DOS is not reentrant This means that if MS DOS is executing some function and then an interrupt occurs the code in the interrupt handler may not perform any MS DOS calls Since TSR must perform various MS DOS functions such as writing to the console and accessing disk files non reentrancy is a significant problem There are some occasions when an interrupt handler may safely access MS DOS functions There are two guaranteed methods of finding such occasions e When a certain memory location pointed to by indos in this program is
150. ome reason to bank up old data the main computer is always ready to accept data and the radio is never ready to accept data A corresponding flow of data exists in the opposite direction due to the main com puter sending the computed servo command values to the master HC11 which extracts the four PWM values handled by the master HC11 and passes the other four PWM values on to the slave HC11 The slave HC11 will then pass any data packets sent on to the radio which will promptly ignore these data as the radio is half duplex and set up to only receive on the helicopter as it is much more important for the helicopter to get information from the ground than vice versa The IBM cannot command the HC11s to change between computer and manual control this can only be commanded by a switch on the remote control console C 3 1 Power for HC11 board Power for this circuit comes independently of the power for the 486 computer The power source comes directly from a 4 8V battery pack with a lifetime longer than the 486 batteries lifetime This is because the HC11 board is more important for helicopter survival There are two 4 8V battery packs used one to power the servos and one to power the radio and HC11 board This is done to prevent the very large power glitches in the servos from affecting the relatively delicate HC11s C 3 2 Board construction This is a custom board designed by me and built by Godwin Zhang The circuit diagram i
151. omputer control by looking at the attitude of the helicopter Figure 5 4 shows the pitch of the helicopter throughout the flight Note the relatively very small oscillations when under computer control Graphs of roll and yaw have the same form as figure 5 4 Figure 5 5 shows one of the pilot s manual commands pitch to the helicopter during the flight When under computer control this joystick input commands longitudinal 98 Velocity m s pitch degrees CHAPTER 5 FLIGHT TEST RESULTS Flight Test 2 31 Dec 1994 4 T T T T T T T 3 S A under pure computer 2 f control J 1 H sA A 4 2b 4 3 L L 1 i 1 1 L 600500 600550 600600 600650 a 600750 600800 600850 600900 Ime S Figure 5 3 East component of velocity in flight performed on 31 Dec 1994 5 Flight Test 2 31 Dec 1994 Manual 10 F Initialisation Te Land ye Command 5b nesponsg iy 4 ob Pure computer 4 control Ground A fal any 15 H 4 20 4 25 H 4 30 600500 600550 600600 600650 ree 600750 600800 600850 time s Figure 5 4 Theta angle tilted forwards pitch in flight performed on 31 Dec 1994 5 1 FLIGHT PROFILE 99 position as shown in figure 1 1 The quantity plotted vertically in figure 5 5 and other figures showing servo joystick commands is the signal received by the 48
152. onverted to RS 232 signal levels by a MAX 232 The timer outputs are capable of driving the servos directly All timer inputs and outputs are sent out the the DB25 connector The eight timer inputs come from the normal hobby remote control radio receiver They are buffered by a 74HC244 immediately after coming out of the radio This chip is on a tiny piece of PC board covered in heat shrink tubing and aluminium foil The purpose of this buffering is to prevent noise from the HC11 board or noise picked up in the wires from passing into the sensitive radio receiver This buffer has a large effect upon the usable range The connectors to the radio and servos are standard three pin remote control hobby style The inner pin is 5V connected to the red wire the leftmost pin on a male connector when the key is upwards is the signal orange wire and the right pin is ground brown wire A picture is given in figure C 6 The connectors to the radio are female the connectors to the servos are male The DB25 connector also contains connections to go to the DB9 RS232 connection on the IBM PC board This is the only electrical connection between the boards and this connection is done externally due to the shape of the IBM PC card The female DB25 connector s pin out is given in table C 4 All unused inputs are connected to 5V with pull up resistors to prevent current drain and possible damage due to floating inputs The SPI connections have a
153. opter has landed at 38400 baud There is no down linking of data from the helicopter during flight in order to reduce weight and potential radio frequency 66 CHAPTER 3 HARDWARE interference problems This means that all computation is done on the helicopter Should the ground station vanish the helicopter would then be unable to compute GPS position but would still be capable of attitude stabilisation To provide feedback to the helicopter operators the parallel port on the 486 card drives a seven segment LED display providing detailed status and a pair of high effi ciency LEDs on the tail which flash every time a position fix is generated A summary of the electronic interconnections on the helicopter excluding power supplies is given in figure 3 7 Electrical power for the above circuitry comes from three batteries The normal method of powering the radio receiver and servos on an RC helicopter is via a small 4 8V to 5V battery pack containing four NiCd batteries Two of these are used on this helicopter one powering the RC radio receiver and HC11s and the other powering the servos This is all the circuitry needed for manual control to function so if something goes wrong with the rest of the circuitry the helicopter should still be controllable The third battery is significantly larger than the other two being 7 2V with a ca pacity of 2 3Ah This supplies the data radio and via 5V low dropout regulators the 486 serial card and
154. or new satellites that have come into view If at least one but fewer than four satellites are in view and the corresponding integers are known then there is a significant problem as this corresponds to the case of the helicopter having previously been flying along happily receiving at least four satellites and then losing sufficient satellites to compute position In the hope that this situation is temporary an attempt to keep track of position is made by assuming that one or more of the position variables x y z remain constant This is not a marvelous solution but it can cope with small outages while the helicopter is under manual control This could in time be recovered from totally through the algorithm given in section 2 4 1 If position integers are not believed known and there are fewer than four satellites visible then nothing is done If there are at least four satellites in view then the integers are estimated as if the base station and helicopter antennas are colocated as discussed in section 2 4 1 There are three inputs to the control conceptual unit during normal operations the position and attitude from the just mentioned calculations and the console values from the pilot sent up from the packet driver Whenever one of these changes the control unit calculates new control values and sends these commanded values back to the packet driver which packages them up and sends them off to the 68HC11s and to the storage area
155. ot significantly different from 18 This signal was not used for control on the helicopter for several reasons which will be discussed in section 4 4 2 4 Obtaining Integers So far this discussion has assumed that the integer offsets were known As the phase integrity is maintained once the phase locked loop has locked onto the signal the integers have only to be determined once There are several ways to obtain the integers Firstly one can just declare the integers by fiat If one knows the relative position of the antennas accurately one can get sufficiently accurate time bias information from the pseudo range packets and then say This is your position calculate the integers from that This has the disadvantage of requiring the initial offsets to be known as accurately as possible which is actually easy to do on the ground with the antennas in a known position For the attitude integers this would involve putting the helicopter in a known attitude For the position integers this would mean putting the helicopter and base station in a known relationship Even if the initial position is out by a few metres this is unlikely to be a disaster as the drift rate it would introduce from satellite motion would be fairly low Another problem is that the imputed integers when new satellites arrive would not lie close to integer boundaries so a drift could come from there Still we have had considerable success with this approach with the
156. ous in the event of a crash Since our helicopter has significant modifications it is more likely than most model helicopters to crash whilst close to the ground and people For this reason there is an eight foot high Lexan shield behind which we stand when flying the helicopter figure 3 11 There are spare batteries for the helicopter and ground station to allow multiple flights and a large battery to replace the 7 2V battery on board the helicopter during down loading on the ground A table giving the rough lifetimes of various consumables is given in table 3 2 A photograph of the helicopter just taking off is given in figure Transparent exceedingly strong plastic 70 9600 baud COM1 486 computer Laptop with batteries COM2 x gi CHAPTER 3 HARDWARE Radio Modem 7 2V reg 12V battery Antenna GPS Antenna O To helicopter for flight data retreival y Yy Position GPS Receiver e 38400 baud Figure 3 10 Electronics in the ground station 3 4 OPERATIONS 71 Figure 3 11 The safety shield Consumable Rough Lifetime Computer memory for flight data 8 minutes Fuel 12 minutes 7 2V helicopter battery 30 minutes Portable computer battery 30 minutes 4 8V helicopter batteries over 1 hour Ground station 12V battery 2 hours Table 3 2 Lifetimes of consumables 72 CHAPTER 3 HARDWARE Figure 3 12 The heli
157. ox contains wooden blocks designed to keep the connector in place The DB 9 connector on the bottom comes from the HC11 board and is an RS232 156 APPENDIX C IBM BOX MANUAL port pin out in table C 2 designed to connect to the data radio The DB25 connector contains all the other connections to the HC11 board to and from the outside world this connector s pin out is given in table C 4 C 2 Modifications to serial board The serial boards used in this project have been modified to allow reception at 38400 baud and transmission at 9600 baud The reason for this strange arrangement is that the TANS boards are capable of transmitting at 38400 baud but are incapable of receiving reliably at that speed Normally a serial post on an IBM PC compatible is fixed to one baud rate In order to allow this multiple baud rate fix hardware changes were necessary The 16550 serial chips used in this board have separate inputs for both the receive and transmit clocks The input for the transmit clock goes via a programmable timer and then is sent back to an output pin Typically in an IBM PC system this transmit clock output pin is connected to the receiver clock input pin so that both run at the same speed If the connection between the transmitter clock output and the receiver clock input is broken and the receiver clock input is connected directly to the transmitter clock input then the transmit and receive ports can run at different baud r
158. p is very similar except the gains are effectively slightly larger due to the factor of 0 75 in the attitude feedback loop This increases the magnitude of the real value of the pole to 0 22s which is again eminently consistent with the observed disturbance decay time constant of four to five seconds This decoupling looks reasonable and was in fact motivated by looking at figure 5 16 which shows the commanded angle from the outer loop at the top and the actual measured angle below Note that the commanded value does not take the pilot s stick commands into account which is the reason for the discrepancy in the large value at time 600795 s Looking at enlargements of figure 5 16 indicates a delay of about 0 5s from a subjec tive view This can be easily made more rigorous by performing a correlation between the commanded actuation delayed by a certain time step and the measured angle A graph of this correlation over the time period 600740 600790 is provided in figure 5 17 which strongly supports the value of 0 55 as the response time of the system Another test flight similar to the one described in detail above was performed a month later at NASA Ames Research Center 31 January 1995 the results were very similar to the results discussed above Graphs of the results are given in figures 5 18 This value was chosen during experiments with stability enhancement with feedback on roll and pitch angles only A higher value than 0 7
159. parallelise to some extent 22 One has a list of expressions that need evaluation based upon strictness definitions and sends them off to nearby processors Due to referential transparency the order of evaluation does not matter However lazy arguments are a problem here as they may or may not need to be computed On a parallel machine it is eminently possible that there may be some processors with nothing to do It is possible to then pass them at a low priority the job of evaluating lazy arguments which may not be needed Then if these arguments are needed they will be used If not then no time has been wasted as those nodes have nothing else to do This type of evaluation is called eager or speculative evaluation and has been used in microprocessors at the hardware level for a long time for tasks such as optimising jumps 23 The reason for mentioning all of this is that this situation is in many ways similar to the real time control situation one has idle time that is being wasted while new sensor data are being received It should be possible to bring some of the compiler technology over from eager evaluation on parallel machines to eager evaluation of a 120 CHAPTER 6 CONCLUSIONS AND FUTURE WORK blocked single processor machine Furthermore one can go significantly further simplifying expressions when some sub expressions are known but not others For instance if one had a function evaluating 1 a b and at some point in
160. particular M is easy to calculate at each time step in real time this computation only involves inverting a 4 by 4 array which has to be Cholsky factorised in the position calculation algorithm anyway plus a few small matrix multiplications and additions Also one only has to keep the current sum of each side rather than the detailed past history which enables this calculation to be done easily in real time without excessive memory or time constraints Indeed a new estimate of C can easily be done many times a second We have no need to store data for each time step as one has no need to calculate the values of X and one does not need them to calculate C It might be worthwhile at this point to step back and look at the big picture in particular matrix M This derivation so far proceeded ignoring the number of satellites in view whereas this is in reality an obviously critical phenomenon yet it seems to not take part in the above equation We would expect that with three or fewer satellites one does not have enough information to compute Ag properly let alone corrections With exactly four one has just enough for Ag but none spare for corrections With five or more satellites in view one ought to be able to do something 2 4 OBTAINING INTEGERS 45 Suppose that there are m satellites in view Then S will be a m x 4 matrix In particular if m lt 4 then S t will have rank less than 4 so S7 t S t will also have rank less than fo
161. position bank angle control loop with GPS feedback on posi tion velocity and bank angle Specific form of figure 1 1 2 A test flight performed on 31 Dec 1994 0 0 East component of displacement in flight performed on 31 Dec 1994 East component of velocity in flight performed on 31 Dec 1994 Theta angle tilted forwards pitch in flight performed on 31 Dec 1994 80 82 83 84 86 91 92 93 94 96 97 98 98 Pilot command directly affecting pitch in flight performed on 31 Dec 1994 100 Altitude under computer control in flight performed on 31 Dec 1994 Heading under computer control in flight performed on 31 Dec 1994 Displacement north under computer control in flight performed on 31 Dec 101 101 5 9 5 10 5 11 5 12 5 13 5 14 5 15 5 16 5 17 5 18 5 19 5 20 5 21 5 22 A l A 2 C 1 C 2 C 3 C 4 C 5 C 6 Displacement east under computer control in flight performed on 31 Dec Autocorrelation of roll signal whilst under computer control and sta ble showing the 11 second period limit cycle 2 0 0 Pilot throttle commands whilst under computer control Pilot heading commands whilst under computer control Pilot pitch commands whilst under computer control Pilot roll commands whilst under computer control Slew in heading under computer control Bank angle commanded b
162. pter during normal operations Undashed lines indicate data movement Dashed lines indicate computation This performs the com puter control sections in figure 1 1 aoaaa 75 4 2 Breakdown into conceptual units of the software running on the ground station 486 during normal operations Lines indicate data movement 79 xvi 4 3 4 4 4 5 4 6 4 7 4 8 4 9 4 10 4 11 5 1 5 2 5 3 5 4 5 5 5 6 5 7 5 8 Data Flow in the 68HC11 servo control processors An undashed line indicates unconditional data flow a dashed line indicates possible data flow oaa ee Data movement during initialisation for the 486 on the helicopter Data movement during initialisation for the ground station Data transfer involved in down loading flight data from the helicopter 486 Data movement for the postprocessing software Undashed lines indicate data movement Dashed lines indicate computation Altitude control loop with GPS feedback on position and velocity Gains are given in terms of percentage of the normal full control actuation Based on figure ld 2 ee Heading control loop with GPS feedback on attitude yaw The gain is given in terms of tail rotor pitch degrees per degree of yaw error Based upon figure 1 1 2 ee ee ee Longitudinal position pitch control loop with GPS feedback on posi tion velocity and attitude pitch Specific longitudinal version of figure Ld ee ee Lateral
163. r 20 global then local iterations The horizontal axis is the number of iterations the vertical axis is the cost function in units of square wavelengths Seventy three percent worked 55 2 10 Cost function as a function of iterations for alternating 2 global 2 local iterations The horizontal axis is the number of iterations the vertical axis is the cost function in units of square wavelengths Eighty six percent worked 2 0 56 3 1 The helicopter loaded with electronics 204 58 3 2 Hardware needed to start the engine and perform field maintenance and adjustments ooa ee 59 3 3 The radio console with antenna retracted for controlling an RC helicopter 60 3 4 RC Transmitter mapping from throttle to blade pitch Units are propor tional to servo actuation osoo ee 61 3 5 The master antenna on the tail of the helicopter 0 63 3 6 The positions of the four GPS antennas on the helicopter and their rela tive distances ooo ee 64 3 7 The electronic signal connections on board the helicopter 67 3 8 The power connections on board the helicopter 0 68 3 9 Ground Equipment 0 a 69 3 10 Electronics in the ground station 2 2 000 70 3 11 The safety shield 2 20 2000 020000002 2 ee ee ee 71 3 12 The helicopter taking off as seen through the shield 2 002 72 4 1 Breakdown into conceptual units of the software running on the 486 on the helico
164. radio PNE E I ye Gain oe en Swashplate y Helicopter Vp Helicopter V l servo dynamics dynamics i Sensed bank angled GPS a attitude ne sensing ye Sensed location System a i and rate We y Se ay GPS a position a00 a sensing system l l I Figure 1 1 Lateral location portion of the GPS based autopilot 1 5 CONTEST 7 Figure 1 2 The stadium at Georgia Institute of Technology where the aerial robotics contest is held and control all six degrees of freedom position and attitude e The system was demonstrated in two successful flights in which the helicopter hovered under pure computer control hands off for over a minute and then also recovered quickly and gracefully from major intentional disturbances super imposed via abrupt manual command signals 1 5 Contest This work was partially motivated by an annual contest run by the Association for Unmanned Vehicle Systems AUVS the International Aerial Robotics competition held in Atlanta Georgia on the Georgia Institute of Technology campus A photograph of the stadium the contest is held in is given in figure 1 2 This contest is designed to encourage and test totally autonomous aerial vehicles in 8 CHAPTER 1 INTRODUCTION 15 lt gt A 3 high barrier Start ee j Drop off disks Pick up disks y 4 gt 120 Figure 1 3 The arena i
165. rancy There may be a similar problem with HIMEM SYS or compatible memory manager XMS is used to store data for later extraction and post processing It is possible though rare for something to be using XMS which gets interrupted by TSR which then calls HIMEM SYS again This may cause the system to crash There are three main contenders for thing using XMS at the same time as TSR e TSR itself the XMS routines enable interrupts which means one can not avoid reentrancy problems by disabling interrupts This is fixed via a static variable to check reentrancy e The MS DOS system itself especially if COMMAND COM is loaded in the high memory area HMA This is dealt with by the rather restrictive assumption that one can not use XMS whilst DOS is in use A normal memory buffer is used while XMS is not available When XMS becomes available the information is coppied from the temporary buffer to XMS Some other application program Caveat emptor There is no protection imple mented against this Do not use other programs that use XMS with TSR loaded and receiving and storing data or there is a chance of a crash B 2 4 Stack size There are occasions when the stack usage by TSR during an interrupt is too great to use the local stack belonging to the application currently being processed This is especially true when COMMAND COM is being reloaded A solution to this is to use a piece of preallocated memory for this purpose an
166. ratory s GPS experimenters around Thanks to Professor Bradford Parkinson Clark Cohen and Trimble Navigation for their support of this project Many thanks to Stu Cobb Hiro Uematsu and Kurt Zimmerman for explaining what the black boxes from Trimble were supposed to do Especial thanks to Paul Montgomery who worked very closely with me on several parts of the GPS software making suggestions finding bugs explaining the contents of the black boxes and being almost entirely responsible for the velocity measurements This work was funded and therefore made possible by the Boeing Company and vii NASA Ames Research center Thanks to Shoreline Amphitheatre for letting RC en thusiasts fly in in their parking lot and to NASA Ames for letting us fly on an Ames runway Many thanks to the readers especially R Cannon who worked so quickly and dili gently Lastly and perhaps most importantly thank you to my friends in the Bay Area for making life here much more pleasant vill Contents Abstract v Acknowledgements viii 1 Introduction 1 1 1 What is a robot aaua aa a 1 1 2 Why a helicopter anaoa 00000000 2 ae 3 1 3 Why GPS a 4 1 4 Contributions aoaaa ee 5 1 5 Contest 2 2 ee 7 1 6 People working on this project 2 2 0 000 10 1 7 Outline of thesis 2 0 2 2 ee ee 13 2 Global Positioning System GPS 14 2 1 Terminology 2 0 eee 18 2 2 GPS Receiver data oaao ee 19 2 3 Assume the i
167. reliability Thus it was decided to use GPS for computing attitude as well Due to the difficulty of obtaining a drift free sensor just using normal several metre accuracy differential GPS has been studied for normal helicopters e g 10 and is being used in the emerging field of unmanned aerial vehicles in conjunction with other faster more accurate local inertial devices GPS is discussed in detail in chapter 2 The control system used in the present research is shown for one degree of freedom in schematic form in figure 1 1 This figure shows an innermost attitude control loop surrounded by a position control loop with position commanded by the radio console 1 4 Contributions The research described in this dissertation has resulted in fundamental experimental and algorithmic contributions to automatic control that have been demonstrated in new successful experiments e New efficient algorithms and approaches to use real time data from GPS sensors for control of an unstable plant have been developed e These new algorithms allowed for the first time the full autonomous control of an intrinsically unstable model helicopter in hover using only GPS signals to sense Lateral location CHAPTER 1 INTRODUCTION command jc Bank Servo Swashplate location Manual sine signal position angle y Joystick angie o Input command by
168. rt interrupts The PCA 6143 contains one R5422 port and an RS232 port unfortunately neither of which have FIFOs buffering their data The PCA 6143 is designed to plug into a passive backplane The easiest way to pro gram the EEPROM on the PCA 6143 is to plug the 486 card into a passive backplane 750 East Arques Avenue Sunnyvale CA 94086 U S A 408 245 6678 243224 Christy St Fremont CA 94538 510 226 9484 152 C l IBM PC CARD AND SERIAL CARD 153 tsr c1 bO Rni x2048 remote rec_pwm_in rec_enable set GPS_POSN_FILE set GPS_LOS_FILE set GPS_PH_FILE set GPS_MAT_FILE gps Figure C 1 Contents of autoexec bat on the embedded PC DEVICE HIMEM SYS Figure C 2 Contents of config sys on the embedded PC containing a video card we have these things and connect an ordinary PC keyboard and monitor The PCA 6143 contains both a floppy and a hard disk drive controller Connecting this to an ordinary PC disk drive completes a working system indistinguish able from a normal desktop IBM PC compatible The DIP swithched should be set up so that the external floppy is drive A and the emulated E PROM disk is drive B One can then load software onto drive B When finished convert the solid state disk back to drive A and the board will be ready for use The EEPROM virtual floppy disk currently contains the TSR described in chapter B and the GPS program described in chapter E The contents of the autoexec bat is given in figure C 1
169. rticular logical port being controlled G GPS Port Typically used to interface to a TANS Quadrex A Attitude GPS Port Typically used to interface to a TANS Vector R Radio Port Used to interface between a ground station and the computer on the remote vehicle Typically connected to a radio F Fast Port Very like the radio port but typically used when the vehicle has landed and one can connect a high speed serial link directly to the vehicle for faster down loads Typically not used during operations The second letter represents the parameter being controlled for that serial port s Set speed to the specified baud value If the parameter is zero set the serial chip baud rate divisor to 4 If you do not know what this means ignore this The purpose of this action is to allow a specially modified serial board see section C 2 to run at 9600 baud transmit and 38400 baud receive One needs special hardware for this p Set parity 0 means no parity 24 means even parity and 8 means odd parity b Set number of bits Must be 5 to 8 e Set number of stop bits Must be 1 or 2 n Set port number 0 means deactivate this port 1 4 means COM1 COM4 respectively d Debug received packets Print out the hex packet ID for a packet received on this channel r Debug received data As soon as a byte is received print it in hex t Debug sent data As soon as something is loaded into the serial chip print the byte out in hex Also print out the
170. rtion of a satellite on the ground and have an airplane fly over observing the phases from both the satellites above and the pseudolite below 8 This is truly perfect for airplanes coming in to land at an airport the integers relative to a receiver on the runway can be computed as the airplane lands but it is not ideal for an autonomous helicopter for several reasons There are all sorts of pragmatic reasons about mounting of antennas and pseudolites in the field but also it means that the helicopter has to fly a significant distance before it can resolve the integers This is not really possible for a supposedly fully autonomous vehicle although there are possible methods of finessing this problem Anyway avoiding having to use a pseudolite to resolve the position integers would make the helicopter significantly more useful More useful still of course is to have pseudolites around that both add information replacing poorly seen satellites and provide useful geometrical information for resolving integers Having the option of both is of course the best In practice at present the motion method is used for attitude and an initial guess 40 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS method is used for the position A time based guess refinement method described in section 2 4 1 can be used to offset problems due to drifting position These solutions will now be described in more detail 2 4 1 Position Suppose that one did not
171. s one for calculating position and one for calculating attitude These two receivers are mounted inside an aluminium box discussed in detail in appendix D The GPS antenna on the tail is connected to both GPS receivers through an antenna splitter The antennas contain a preamplifier built into them and powered through a DC bias on the coaxial cable connecting them to the receivers so the splitter contains a DC block stopping the attitude board from supplying power to the tail antenna The other three GPS antennas are all connected directly to the attitude GPS receiving board The box containing the GPS boards is mounted to the honeycomb aluminium previously mentioned 64 CHAPTER 3 HARDWARE Master antenna Antenna 2 Antenna 3 Antenna 1 Figure 3 6 The positions of the four GPS antennas on the helicopter and their relative distances 3 2 HELICOPTER ELECTRONICS 65 Purpose Frequency From To RC console 72 110 MHz Pilot Helicopter Data communications 461 2625 MHz Ground Station Helicopter GPS 1575 42 MHz Satellites Helicopter amp Ground Station Table 3 1 Frequencies Used The GPS receivers produce raw phases to convert these phases into useful position and attitude information a significant amount of computation must be performed This is done on a 486 IBM PC compatible mounted in a box in the front of the helicopter A modified serial port card attached to the 486 card is used to communicate with the GPS recei
172. s given in figure C 4 and the circuit diagram for the connector is given in figure C 5 The circuit contains two 68HC811E2 microcontrollers These contain on chip E7PROM to store the program a standard serial port SCI a nonstandard serial port SPI and four timer channels each The reset signal is generated by a low voltage detector 34064 the clock signal by For the amounts of current typically drawn from each battery C 3 TIMER 68HC11 BOARD 159 PWM OUTPUT u ul uL pao PAL PA PAO xT Paa RESET PAS an IRQ PAT GND GND pao 3 PA Pea ua Be a DEAS U1 PAT PEO zee u2 Pat eta pA U2 PAS pee oar U2 PAG PEA Pc2 PES Pca PEG sive AR 2sur GeHCalIE2 eeHceliE2 10K 11 Aerospace Robotics Labortory Dept of Aeronautics amp Astronautics Stanfe ersity Stanford CA 94305 Tele design name Size Document Number ate April 27 1994 Figure C 4 Circuit diagram for the HC11 board 160 APPENDIX C IBM BOX MANUAL Figure C 5 Circuit diagram for the connector to the HC11 board radios Servos and 486 C 3 TIMER 68HC11 BOARD 161 Figure C 6 Servo pin out for male connector an 8MHz oscillator for each of the microcontrollers Each SCI port is c
173. s project has accomplished two main aims The first aim and the original one for the project was to demonstrate that GPS all by itself is a suitable sensor for total real time control of a helicopter The second aim was to contribute some significant improvements to GPS algorithms for the situations that arise in automatic control of a vehicle like a helicopter Aim one was demonstrated by the totally autonomous hover control for the first time of an inherently unstable unmanned helicopter both position and attitude were controlled using GPS receivers and antennas as the only sensors Aim two was accomplished through the improved integer resolution algorithms for attitude which allows the use of antennas with poor visibility and the dynamical method for dealing with position integers which allows a vehicle to not have to wait to resolve position integers One of the nicest features of GPS as a sensor is that it is completely electronic In particular as semiconductor technology advances the sensor electronics will become smaller and smaller without the limits that mechanical devices would impose The main limit of size will be the GPS antennas themselves Applying Pournelle s law Silicon is cheap but iron is expensive the overall cost bulk mass and raw cost will soon become very low This will make putting this sort of system on small vehicles exceedingly cost competitive Indeed it is plausible that a
174. s returned to its previous state on either the termination of the block of code that is the scope of the variable here or upon the early exit at the return statement This implementation has the advantage of removing the possibility of forgetting to re enable interrupts at one of the possibly many exit B 2 TECHNICAL DETAILS 129 points of shortening the program source and of general cleanliness change_pid Like no_interrupt except this class sets the process identification number throughout the scope of a variable of this class This class requires that a global variable MyPID be initialised before this class is used Interrupt Does a partial job of encapsulating an interrupt Does not contain any code but does take care of keeping track of old vectors to make interrupts easier to deactivate Transmit_Buffer Maintains a cyclic character buffer with interrupt prevention at ap propriate locations so that an instance of this class can be used in a reentrant manner Used to buffer data to be transmitted to serial ports as well as to the console Port Basic class to represent a serial port Handles everything except packet structure During an interrupt the appropriate Port class member function needs to be ac tivated explicitly and told the address of its service routine as interrupts do not provide a parameter giving a pointer to the class DataPort A derived class from Port which implements the data packet structure for the
175. series 1kQ resistors in case of a software failure forcing both chips to think that they are the master or both the slave and thus both driving a connected line 162 Pin co ODN OH F amp F W N FR ND N DD i e e meme mae ra e wo N e oo o u oa an oB WO Ne D 25 Function Out 1 Out 2 Out 3 Out 4 5V GND RxD from IBM TxD to IBM 5V In 1 In 2 In 3 In 4 Out 5 Out 6 Out 7 Out 8 GND NC NC GND In 5 In 6 In 7 In 8 APPENDIX C IBM BOX MANUAL Table C 4 HC11 board DB25 pin out check that IN OUT numbers are correct C 4 PROGRAMS RUNNING ON HC11 163 C 4 Programs Running on HC11 There are two HC11s each runs a slightly separate program There are several reasons for the differences e The Motorola SPI interface is inherently non symmetric requiring one device to be the master and one device to be the slave e Only one device the master talks to the 486 e Only one device reads the PWM signal that means pass through or block There are eight logical PWM inputs and outputs as seen by the external IBM These are numbered 1 to 8 The first four are the master channels 1 to 4 the second four are the slave channels 1 4 Channel 1 controls whether the PWM values will be passed straight through or the IBM supplied values will be used Internally the timers are quantized to 16 bit values 0 545 and in pass though mode the 16 bit values are used When talking with the IBMs only an 8 bit range
176. signal in the control As there is no integral control implemented this bias ends up causing a bias in the control loop which has the effect of causing the set point to be several metres away from the nominal place This is the reason for the initial slews in figure 5 2 when the computer control was turned on This will be easy to take care of in the next control version Another observation from figure 5 5 is that when in manual control the pilot does actually work continuously commanding large actuations Other pilot commands look similar to those for pitch 1 Almost north in this flight 100 CHAPTER 5 FLIGHT TEST RESULTS Flight Test 2 31 Dec 1994 T 140 T T T T T T 130 4 120 E i j 4 5 110 L Ground Computer S control a 1007 J S Ss 90 4 Manual Prac X position command 70 4 Manual 60 control x L L L 1 i 50 i i 600500 600550 600600 600650 Tine e 600750 600800 600850 600900 Ime S Figure 5 5 Pilot command directly affecting pitch in flight performed on 31 Dec 1994 5 2 Computer Controlled Portion of Flight Having shown in the previous section the behaviour of the helicopter at large this section will focus on the portion of the flights under computer control The two flights with computer control will
177. someone planning to modify TSR or to write a similar pro gram It is not necessary for the casual user to read this section other than for interest in some of the design considerations It is assumed that the reader of this section is familiar with C the Borland C compiler and has some knowledge of the 80x86 and MS DOS architecture TSR is written in C using the Borland C V3 1 compiler and is compiled in tiny mode with the following optimizations 3 Ogilt It uses C for data encap sulation rather than for multiple inheritance so the program should be comprehensible to someone with a moderate understanding of C The comments are not extensive though the variable names are generally long and meaningful The source is optimised for readability then speed then program size B 2 1 Segment Registers Using the tiny memory model rather than a large model reduces code size by almost a factor of two and increases execution speed Unfortunately the tiny model is harder to use as during interrupts the stack segment will differ from the data segment This means that local variables on the stack will not be representable by near pointers so one must compile with the option mt which specifies that CS 5S This means that all the library routines which take an address such as strlen and open cannot use local variables Another problem with segment registers occurs when a function in one program such as TSR is called directly fr
178. splayed After that the packet is passed to a great big gps data structure into which the information in the packet is inserted in a structured manner This structure keeps track of the state of the world as far as is knows by the GPS receivers If the busy loop runs out of packets to process GPS checks to see if anything signifi cant has happened to the GPS world structure since GPS last did a computation If so a new position attitude and or control computation is performed as appropriate This may sound overly complex but has the features of e Returning from the interrupt routine quickly 182 APPENDIX E GPS PROGRAM e Dealing with queues of packets quickly if GPS is getting new packets faster than GPS can process every packet which doesn t happen in practice on the CPU we are using currently GPS ignores old packets and only deals with the latest data e GPS responds to packets with computations in practice very rapidly E 3 Source files GPS is made from many source files They are listed as follows attcomp c Computes attitude from phases and attitude integers attcomp h Structures used in attcomp c control c Computes control values environ c Some generally useful utilities environ h Prototypes for useful utilities gps c Manage the GPS world model and compute GPS position gps h The GPS world structures gr c Creates a graphic display of the helicopter status helicont h Interconverts raw servo commands and physica
179. successful team was the University of Texas at Arlington who had a tail sitter 6 which used a vision system which also did not work at the contest We believe that the GPS solution herein is a better long term solution to the sensor problem in many respects as it works over a large volume of space is much easier to set up and works in better orientations GPS has the disadvantage of not working indoors or in an obstructed environment but even that is not an insurmountable problem with pseudolites 24 as mentioned in chapter 6 1 6 People working on this project This project was funded by The Boeing Company and The NASA Ames Research Cen ter Professors Robert Cannon and Stephen Rock are the Stanford professors running the project I worked full time on this project being the main organiser worker and system integrator Dr Steve Morris worked part time as a pilot helicopter builder and control system advisor Dr Ben Tigner and Bruce Woodley helped me part time with various time consuming parts of the project it takes a long time to wire up the connectors needed for this project reliably etc Both Ben and Bruce worked on var ious related projects which I have not mentioned in this thesis as I had little direct involvement with them and they are not necessary to the understanding of this project Bruce Woodley will be taking over control of the project when I leave Many other people have helped in this project as listed in the a
180. sume that the entry to be removed has index number 1 Then one has a matrix n a QT 14 30M 2 44 k 1 Q R implicitly defining a Q and R whence a QT C1 hy 2 45 Q R c h which produces the equations ac Q c h whence c a h a Q e 2 46 48 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS and Qc Re h Eliminating c gives R a QQ c h a hQ 2 47 which is in the form of a matrix equation as equation 2 32 This has some very important consequences given that e a Q and h would never change with further readings e Equations 2 46 and 2 47 are true for any R and h and in particular they remain true if R and h grow due to the addition of new readings e The matrix a QQ which is effectively added to R is independant of the two things that may change R and h except for a possible extension which is easy to deal with e The vector a h Q which is effectively added to h similarly never changes The result of these facts is that it does not matter if the additions to R and h are done at the point of time when the satellites are lost to view or at some later time when the integers are calculated Given this property the corrections a QQ and a h Q may as well be added in directly now avoiding the need to save and process these values separately Note that taking equation 2 46 out of the main matrix sum does not prevent later data from improving the accuracy of c i
181. t 2 whilst the second is never satisfied for B gt 4 For 2 lt B lt 4 the first condition is satisfied for u gt 16 B 3 and the second for u lt 16 B 3 These last two inequalities are in the opposite direction to that needed for two intersections so there cannot be a second intersection If 6 0 then C a 2sin 2x B sin z sinz 4cose B which has a single solution in the range of interest of x 7 if B gt 4 and the second solution cos B 4 if B lt 4 Checking second derivatives shows that the second solution is the correct solution if it exists otherwise the first solution is the correct solution The preceding few paragraphs have proved that the bisection algorithm works for finding a solution of C x 0 as long as gt 0 and it is actually fairly easy to see that it will actually work for 0 as well if it is written in the form shown in figure 2 5 This is a simple robust algorithm that converges in n iterations to get n bits of accuracy which is slower than would be desired given the number of times this problem needs to be solved A graph of the results of this algorithm is shown in figure 2 6 Looking at this graph one sees that the best value of is a continuous function of B and with no unpleasant behaviour except near the point 0 and B 4 where the change from two nearby zeros of C 2 to one zero makes the behaviour rapidly 36 CHAPTER 2 GLOBAL POSITIONI
182. t included in chapter 3 as they were not considered vital to an understanding of the project but are useful to people continuing on this project A 1 Suppliers Unusual parts for the electronic boxes GPS and 486 are given in appendices D and C respectively Helicopter equipment can be obtained most readily from Helicopter World locally The portable computer was purchased from PromoX Systems The data radios 3 and antennas came from Metric Systems Most miscellaneous GPS equipment came from Trimble Navigation A 2 Electronic connection Most of the electrical connections are given in figures 3 7 3 8 and 3 10 and are discussed in chapter 3 From the exterior the helicopter appears to be several boxes attached together In 1521 Sinclair Frontage Rd Milpitas CA 408 942 9525 Sunnyvale CA San Diego CA 619 736 0020 645 North Mary Avenue Sunnyvale CA 94086 121 122 APPENDIX A HELICOPTER MANUAL Figure A 1 Location of parts on board the helicopter particular with reference to figure 3 7 the 486 card the serial card and the 68HC11s are in one box appendix C and the GPS receivers are in a separate box appendix D A photo of the helicopter with electronics labeled is given in figure A 1 The 8 bit parallel port on the 486 is used to drive a seven segment display directly with the seven least significant bits The most significant bit is used to control a pair of high power LEDs on the tail dr
183. tellites in view so it is reasonable to calculate A in real time Equation 2 32 now becomes C A M S 2 42 i l which makes calculating covariances straightforward Ac A gt Misa AT 2 43 i l So there are now three things that need to be maintained as time goes on 2 4 OBTAINING INTEGERS 47 e The matrix y Mi e The vector H S _ M e The matrix Ay 7 M Ao M7 For a six channel receiver like the TANS this involves only 78 floating point numbers that need to be stored for an unlimited length flight This storage has not dealt with satellites rising and setting As mentioned previously the above mathematics is identical if some measurements do not contain some particular satellite with the missing rows and columns in the sum M just becoming zero Thus satellites can rise and set with the same mathematics just steadily larger growing matrices as the number of integers being calculated increases so there are computational reasons why one would not want to work this way We would like some way of getting rid of old entries when some integer lock has been lost as there will never be any more entries corresponding to that integer In this way the matrices will never get larger than m by m for an m channel receiver the TANS is 6 channel Losing a Satellite Suppose a satellite has been lost whether it be due to setting occlusion or something else To make the notation simpler without loss of generality as
184. tes unconditional data flow a dashed line indicates possible data flow 4 1 DATA FLOW 81 station this is done whenever the display program is run Another initialisation step needs to be done the control gains need to be sent to the 486 For flexibility in testing in the field the control gains are not hard coded rather they are sent up from the base station to the helicopter through the same protocols as the GPS base station data When a gains packet is received on the helicopter it does a few sanity checks and a second weighted checksum to make sure that the packet is valid accidentally letting through a bad gains packet would be very very bad The packet is then stored and used later for control There are actually two different types of gains packets that can be sent up corre sponding to two different positions of the gain select switch on the pilot s console These are sent separately and saved in separate locations Pictures of the data movements for initialisation are given in figures 4 4 and 4 5 for the helicopter 486 and ground station respectively 4 1 3 Data Retrieval During the flight all the GPS base station and servo packets passing through the helicopter 486 packet handler are logged together with some timing information The logging is into extended memory on the helicopter 486 After the flight it is possible to attach a cable to the helicopter and down load this in formation to the base station where it
185. the input values is changed but that would have the disadvantage of significantly increasing the length of the interrupt routines and thus increasing the chance of missing an SPI event on the slave due to not being able to respond in time That is all the slave mainloop does The master mainloop has one more thing to do to check to see whether it is time to perform another SPI transfer There are two criteria that must be met e The time must be greater than the variable may_send_spi e The flag flag_send_spi must be nonzero 168 APPENDIX C IBM BOX MANUAL If these are satisfied the subroutine Ispi_transmit is called This is checked twice to reduce latency The variable may_send_spi is set to a time SPI_Delay after the current time to indicate when the next SPI character can be set The flag flag_send_spi is not set until the character has finished being transmitted I will now describe the various subroutines most of which are interrupt routines Expand Expands an 8 bit value from the IBM into a 16 bit value giving the pulse length in units of 0 545 by multiplying by 4 and adding PWM_MINIMUM Shrink Opposite of Expand I_timer_over Interrupt routine called whenever the timer overflows once every 32m8 Increments timer_overflow On the master the interrupt routine checks to see of the IBM wants PWM values If so this routine sends the IBM a packet of the 8 PWM values Both the timer and the slave then send to each other the
186. the particular needs in compiler code generation relevent In the Stanford Aerospace Robotics Laboratory this Task Level Control capability has already been demonstrated by many systems such as two dimensional free flying robots using GPS receivers and pseudolites in 24 6 1 AN ASIDE ON LANGUAGES 117 to real time control Consider a program whose sole task is to compute GPS positions based on received phase measurements and line of sight vectors The conventional method of solving this would be to wait until a matching pair of phase packets have arrived find which satellites are visible and set up equation 2 10 and solve it Then wait for the next packet pair The thing to notice here is that there are some calculations that have to be performed as quickly as possible on order to not introduce any more delay than necessary into the control loop and then there is a pause when no useful computation is done In multi user computer systems this time based irregularity is not a problem the load is shared amongst many users who are unlikely to want all the computation at once and overall throughput is biased fairly highly with respect to responsiveness Unlike multi user computers in embedded computer control systems and single user personal computers it is usually preferable to use otherwise unallocated time to precompute as much as possible of the calculation about to be done before all the data have arrived It is perfectly possible
187. there are other viable alternatives such as radio based systems There are advantages to the animal approaches legs can go over rocks wings are exceedingly efficient and can be turned on and off easily carbohydrates are readily available and vision accomplishes many other useful things but they do not preclude useful robots being built using other technologies 1 2 Why a helicopter This thesis contains the results so far of Stanford s experiments in building an au tonomous helicopter as a robot a long way from C3PO in Star Wars but useful none the less The immediate question is Don t remote control RC airplanes and helicopters already exist and the answer is of course Yes One can buy excellent RC models from a hobby shop for a reasonable amount of money However they are just teleoperated devices they could potentially be made significantly easier to use In particular RC helicopters being inherently unstable are particularly difficult to control and need a very skilled pilot concentrating full time on flying Aerial robots are unlikely to be particularly useful in manufacturing but they are useful in transportation observation and working in impassable terrain for ground based vehicles or hazardous environments like a volcano Perhaps the most immediate application to the commercial world is the investigation of power lines for electricity suppliers it is difficult to observe from the groun
188. tunately the hardware on IBM PC type machines rarely supports this as the interrupt lines are not usually driven by open collector type outputs This interrupt chains if the function to which one would chain is part of TSR This allows sharing of interrupts Service Interrupt This is the interrupt that the higher level user program calls to interact with TSR See section B 6 for a list of the possible commands that can be sent to TSR If the value in register AH is not recognised by the service routine this interrupt chains Clock Interrupt This is called 18 2 times per second The handler inserts timing information in the log and tries to do pending MS DOS operations such as console output and file loading or saving if indos is zero This interrupt handler always chains Character Output This is asoftware entry point for routines that write to the console These are intercepted so that these characters can be sent to an external computer if desired This interrupt handler always chains B 3 RUNNING THE PROGRAM 131 Dos Safe This interrupt is called whenever MS DOS is in an idle loop and thus one may safely use MS DOS functions The interrupt handler does any MS DOS commands that are needed This interrupt handler always chains B 2 8 Source Files There is one main C file for TSR called tsr c which includes several files emm h Contains function for dealing with XMS memory parallel h Contains functions for deali
189. ur As this is a four by four matrix it will be uninvertible and thus M will be undefined as expected If m 4 then S t will almost certainly be invertible unless the satellites are very close which is unusual Then M will turn out to be zero which means absolutely no new information will be supplied as suspected If m gt 4 then M will actually make sense Some properties of M should now be worth pointing out assuming that m gt 4 It will clearly be symmetric It will be traceless as trace ABC trace BCA and trace A B trace A trace B so trace M trace zs srs st 2 33 trace 7 STS s s 2 34 trace Z T 2 35 0 2 36 It will have determinant zero as each column of S is an eigenvector of M with eigenvalue 0 M S z s s S s S 2 37 Si Si srs STS 2 38 Si S 2 39 2 40 Having zero determinant is not a surprise otherwise one could solve for C with only one time measurement which would be solving n 4 unknowns with n equations However the sum of several M matrices will not necessarily have a zero determinant Note that a significant change in S is needed to have a non small determinant as the determinant of the sum is second order in the change in S Also the above derivation has implicitly assumed that the number of satellites re mains constant throughout the calculation In practice it would be nicer to allow the 46 CHAPTER 2 GLOBAL
190. ure and other organisations are even less likely other approaches need to be looked at The most promising of these is pseudolites These are transmitters which appear to a GPS receiver as identical to a satellite albeit lacking in orbital parameters and a high accuracy clock These have been used 24 indoors to replace satellites in principle they could be used to augment poor satellite visibility Another promising approach to increase reliability is to augment the pure GP S sys tem with cheap light low quality inertial units which can be used to maintain control and an estimate of current position to make up for brief satellite outages All in all using GPS for high precision rapid update automatic control is likely to become very important in the next decade 6 1 An aside on Languages As sensors become more and more abstract and the role of computers in processing sensors in automatic control systems becomes more and more significant the number of lines of programs involved in the low level innermost loops of many projects is increasing This means that it is worthwhile considering the effects of software engineering tools on such projects There is of course a huge software engineering effort required in the higher more intelligent levels of control but the tools for that work are quite similar to tools for other problems and there has been extensive work on this serious problem e g 20 This section is concerned with
191. uses it to make corrections preventing the error from ever getting too large Using this algorithm very little effort is required to initialise the helicopter one just takes it out to where one wants to fly puts the base antenna somewhere nearby it does not matter much where maybe turns the helicopter around to initialise the attitude integers and then it is ready to go It doesn t know the integers exactly yet but it does not need them immediately This would save considerable effort by the ground staff over other approaches and in particular makes the helicopter much more usable for applications such as power line inspection in remote areas without a pseudolite mounted in a handy position Since this algorithm relies on satellites beyond the four minimum to determine po sition it is very helpful to have more than six satellites visible For this reason a GPS receiver with more channels than the Trimble TANS may be useful Algorithm This algorithm is similar in many regards to proposals for on the fly kinematic position ing 16 14 which differ from the normal surveying technique of putting down antennas 42 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS and waiting for the satellites to move by allowing one antenna to move The differences with this approach are e The emphasis on real time implementability e The ability to deal with changing satellites or temporary outages leading to cycle slips in a poor visibility environ
192. ut their own control signals This provides the ability for the operator to switch the helicopter over to manual control at any stage assuming that this circuit is reliable The circuit has a secondary feature of passing through R 232 signals from an external radio to the main computer Data flow can be visualised as starting from the data modem radio at 9600 baud This passes to the slave 68HC11 through its SCI serial port The slave 68CH11 then sends this data plus the values of its PWM inputs to the master 68HC11 over the two devices SPI serial port The SPI port is rather nonstandard and synchronisation is very time critical The SPI runs at about 16000 baud The master 68HC11 adds in its own PWM data and sends all the PWM values on to the main computer the 486 card via its SCI connection at RS232 voltage levels No RS232 handshaking is used it is assumed that the devices the HC11s are connected to can always accept data This is for several reasons e It would require extra complexity e The HC11s don t have enough RAM to buffer the data for long e Even if the 68HC11s did have sufficient RAM delaying data for that length of time would almost certainly be wrong for other reasons due to the need for up to date information for control This 7474 was normally used to enable or disable the output drivers when the board was acting as an RS485 port This facility was not needed 158 APPENDIX C IBM BOX MANUAL e Even if there were s
193. ved during this algorithm a hundred examples are shown simulta neously Using twenty iterations of the pseudo global algorithm instead of Newton s algorithm on exactly the same test cases worked 73 percent figure 2 9 and the alter nation worked 86 percent of the time figure 2 10 Reducing the number of satellites visible to 5 or 6 reduces these figures to 29 58 and 70 percent respectively Of more importance this algorithm was tested in practice on real data on the he licopter where it performed admirably One rotation was usually sufficient to initialise the attitude The largest practical problem was putting the helicopter down without obscuring the antennas The integer resolution process takes under a second on the 33MHz 486 in the helicopter 54 CHAPTER 2 GLOBAL POSITIONING SYSTEM GPS 200 T T Local 150 F 4 100 F 4 50 0 30 35 40 Figure 2 8 Cost function as a function of iterations for local Newton iteration The horizontal axis is the number of iterations the vertical axis is the cost function in units of square wavelengths Forty five percent worked 2 4 OBTAINING INTEGERS 55
194. vers over an RS 422 link at 38400 baud The box containing the 486 and the serial card is described in appendix C The 486 then has to be able to command the servos On a normal RC helicopter there is a receiver on the helicopter which has several outputs Each output goes to a servo sending it a value via pulse width modulation This has been modified so that each output goes to a board containing a pair of 68HC11 microcontrollers which decode this pulse width modulated stream The 68HC11s then look at one of the radio outputs which is driven by a switch on the RC console If that switch is in one direction the helicopter is in manual mode and the radio signals are all passed directly through to the servos and the helicopter can be flown by a pilot as normal Otherwise the 68HC11s use the values commanded by the 486 to generate the pulse lengths and command the servos The 68HC11s communicate with the 486 over a 9600 baud RS 232 serial link There is also a data radio on the helicopter This is needed to receive the measured GPS phases from the ground station This communicates via RS 232 at 9600 baud and this signal goes through the 68HC11s This is done to save serial ports though in principle it could be connected to the other serial port on the 486 A list of the radio communications and frequencies used on the helicopter is given in table 3 1 There is a last serial port RS 422 on the 486 which is used to down load flight data after the helic
195. vide a convenient supply with on off switch for the ground GPS receiver A car or motorcycle 12V battery is then typically connected to the input of this box and then the switched 12V output is connected to the GPS receiver and the regulated output to the data radio transmitter Both can then be switched on or off cleanly This box speeds up setting up time and convenience The simple circuit diagram is given in figure D 4 D 5 Supplier Information The GPS boards come from Trimble Navigation contact Mark Crotty 408 481 2221 Trimble gave Stanford a large discount The LT1085CT 5 regulators are rare practically no retailers have them They can be obtained most easily through Arrow Electronics 408 441 9700 credit card line 800 833 3557 fax 612 946 4835 They have a 50 minimum order There are two serial ports on the TANS Port B can do everything needed 178 APPENDIX D GPS BOX MANUAL pin pin on DB25 function poition attitude board to 5V reg 5V NC CNIA CNIA CNOA CNOA TxDA TxDA RxDA4 RxDA CNIB CNIB CNOB CNOB TxDB TxDB RxDB4 RxDB I O Ground I O Ground co ODN OH F amp F W N FR e e e e e l DO a Be W N e D NO NN e Be eae o N 1 pulse per second output 10MHz IOP clock output No N Ww N Proportional power input to ADC Battery output 3 5V NR N w R High Quality Pulse per Second output N D reset signal NC Ground y N Oo N
196. y take almost a second Fortunately this typically only has to be performed once although GPS EXE continues responding to other GPS tasks such as position computation whilst attitude integer resolution is taking place This is important if the helicopter is regaining attitude integers in the air through large angular motions which are likely to cause satellite loss and recovery at the position receiver For postprocessing the same program GPS EXE is used taking as argument the name of the file containing flight data downloaded at the end of a flight This data is then processed as if the packets were being received again By saving various computations such as position to a disk file the downloaded information can be analysed 4 4 Control The control system used was a very primitive one relying on a very simplistic model of the helicopter with no cross coupling between degrees of freedom and feedback loops based on figure 1 1 In actuality the cross coupling can take several forms such as 90 CHAPTER 4 SOFTWARE e Coupling from controls Increasing thrust increases torque on the main rotor and causes a yaw motion e Coupling from motion Forwards velocity causes increased lift on the forward moving blade and decreased lift on the backward moving blade causing a roll motion and overall increased lift e Coupling from sensor The sensor is on the tail of the helicopter so an angular motion of the helicopter shows up as a
197. y the computer with an offset from position feedback versus actual measurements Refer to figures 1 1 and 4 11 A correlation between the time delayed angle command and the observed during computer control 2 2 ee ee Altitude under computer control for flight of 31 Jan 1995 Heading under computer control for flight of 31 Jan 1995 North displacement under computer control for flight of 31 Jan 1995 East displacement under computer control for flight of 31 Jan 1995 Pilot commanded y translation whilst under computer control for flight of 31 Jan 1995 Each unit of command represents a 31 5 cm change in the set point 2 6 a Location of parts on board the helicopter 0 048 LED driver 2 ee Contents of autoexec bat on the embedded PC 2 Contents of config sys on the embedded PC 0 0 Layout of serial ports on the IBM box 08 Circuit diagram for the HC11 board 0 Circuit diagram for the connector to the HC11 board radios Servos and 486 ee Servo pin out for male connector 0 22 200200004 Xvi 104 109 122 D 1 Power Supply for the GPS box circuit diagram 2 173 D 2 Orientation of the 28 pin connector on the GPS board for table D 4 173 D 3 Orientation of the 34 pin IDC connector on the GPS board 28 pin connector175 D 4 Circuit Diagram for the power supply for the ground based electronics

autonomous control of an unstable model helicopter

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