MBED Tracking Antenna System – A Critical Component to a UAS
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1. MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu Page 1 Table of Contents L WMG INO IN ee cetero piter raodo neta tuni ad ances cers ae ad dubitas ieee dee e a eandis 2 2 Duck Operational Guide segui nice abi ie detne tube aa bos pde eoe erste Voca pue etiem dui meme ai dia 3 3 Theory of 8 1 o M 4 4 EIB CIVIC Als PCI er Lio MN Rm 5 5 LED Status Indicatioh EN ER EE TEE 5 6 COMMMUMICAtION PROLOG et e 5 7 CISDUAPIEEMEREINEOEPMUIU 6 8 Component List Cost OVErVIEW sssi ssisarisnis iie eee ense teste nennen tentes entente Eae sette insta SANNER sese inet ne 7 9 RUNNING COQS RR TR 8 10 Formula and Relation Reference cceceeceesceccsssescecessscceecesseecessenscecessaceescesusecessesusecescenscecessueeesesenecsesenenss 12 MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu Page 2 1 Introduction Recently amateur grade Unmanned Aerial Systems UAS have been increasing in popularity In general users want to fly their radio controlled airplanes as if they are in the cockpit by using a camera and a video transmitter often times users wear video goggles or a monitor on the ground to fly their aircraft This pa2. Page 7 8 Component List Cost Overview Component Name Part Number Supplier Specifications Cost mbed microcontroller NXP LPC 1768 mbed org Cortex M3 Core running free at 96MHz with 512KB FLASH 64KB RAM RS232 Level Shifter PRT 08780 Sparkfun com RS232 to input V out 9 95 Compass Module SEN 07915 Sparkfun com 1 degree repeatability 34 95 1 to 20Hz selectable update rate 0 5 degree heading resolution HS635HB Servo azimuth HS 635HB www hitecrcd com Dual Ball Bearing 29 58 Karbonite Gears Pulse Width Control 1500usec Neutral 4 8 6 0 Volts HS255BB Servo HS 255BB www hitecrcd com Pulse Width Control 22 83 elevation 1500usec Neutral 4 8 6 0 Volts Bottom Mount and SPG425A 360 servocity com Max rotation 400 76 98 Custom Potentiometer degrees 32sec 60 Deg Breadboard n a frys com Standard bread board 7 60 Metal Components n a Ind Meta Supply 5051 Alum 14 76 Machining n a SIO Machine Shop 23 25 Fasteners n a Marshals Industrial 5 44 7 60 Total Cost with Shipping 237 22 MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu Page 8 9 Running C C Code File tracking c include lt HMC6352 h gt include lt tracking h gt Magnetometer HMC6352 compass p28 p27 serial ports Serial usb_232 USBTX USBRX tx rx Debugging Port not used in actual Op Serial groundStation p9 p10 Il
3. compas compass sample 10 0 usb 232 printf f Heading Wn compas led2 led2 blink untill heading is set MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu Page 9 erede lec jer kei jor mur xe deve dep dex dep cube if g lon 100 sanDiego 1 led3 ON 100 long is about center of US else sanDiego 0 led4 ON if sanDiego 1 amp amp compas 346 88 amp amp compas 348 88 usb 232 printf f TRUE NORTH 347 8 Magnetic North SD CA n compas led2 OFF reference found enter main loop Ev torads 1 Jy we Qum i1 223 mym5v SS lei a 23768679 Ev servo pulsewidth us Ev uS break if sanDiego 0 amp amp compas lt 11 98 amp amp compas gt 9 98 rusas 2 pnm mst RU NORTH Oe Meqnetilc Norti berdinetan Park MD Niuean Omas led2 OFF reference found enter main loop Ev torads 1 mw WS alae C 22s SSS ein a 2376 79 g Ev servo pulsewidth us Ev uS break end of set up while loop you have now pointed the antenna tracker at TRUE NORTH Start Main loop while 1 This main loop gets plane coordinates from the serial port converts It to s NED vector pointing from the ground static to the airplane and then sends servos to point at it S get ground and plane coordinates gerila Je
4. flown between 255 and 285 degrees from true north the Azimuth rotation is flipped to by subtracting 180 degrees from that particular MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu Page 5 position The elevation flips and now operates in the 90 to 180 degree region to accommodate the Azimuth rotation With this set up the aircraft can freely make circles around the ground station and the tracking system will follow Generally it s not recommended to take off south of your ground station due to this rotational flip but it s possible More testing is necessary to see the effect on signal strength during takeoff 4 Electrical Specification Min Nom Max Units Input Voltage 4 5 5 6 V Input Current 200 800 1200 mA NiCd or NiMH Battery 1 2V cell 4 4 5 cells 5 LED Status Indications LED1 Should blink which indicates receipt of a correct communication string per change in state LED2 Will blink while the system is not aligned to true north is off during operation LED3 Based on received message magnetic declination is 12 San Diego CA otherwise off LED4 Based on received message magnetic declination is 10 Lexington Park MD otherwise off 6 Communication Protocol The communication string is an array of bytes of size 26 There is a start by
5. is obtained The latitude longitude and altitude LLA is converted to Earth Centered Earth Fixed Coordinates ECEF using Formula1 The two ECEF coordinate vectrors are subtracted to obtain a single ECEF vector that points from the ground to the aircraft This new ECEF vector is then converted to North East Down NED Formula2 Reference Frame so that the azimuth and elevation angles can be obtained from the NED vector Azimuth angle is obtained by taking the inverse tangent of North and East component of the NED vector Formula3 and elevation angle is obtained using all three components using Formula4 Servos had to be calibrated to obtain a relationship between the pulse width and angle that pulse width produces This was done by using a tilt meter and sending servo pulses by a separate function which is included in the Code Section A relationship was found between the pulse width and angle using the polyfit function in Matlab this relationship is available for reference Formula5 This relationship depends on mechanical set up of the system and will be different for any new system designed from this document Once the mapping from angle to pulse width is known it is implemented in software and the pulse width is sent to the elevation and azimuth servos Unfortunately a true continuous rotation in azimuth was not possible due financial constrains Instead a 360 degree servo rotation is implemented If at any time the aircraft will be
6. ER TES leds DigitalOut ledl LED1 DigitalOut led2 LED2 DigitalOut led3 LED3 DigitalOut led4 LED4 int main wgs84 g for ground p for plane lat deg lon deg alt m ilex cows jp leu jp lem jp eli ep leno qp lom Gp eus everything in meters ecef NED is north east down ecef xyz is the vector pointing from the ground to the plane in ecef NED is the North East Down vector from ground to plane iGO Joi Sk Jel We 18 ap Wj Sp Cl Wp Cl Gels Xe Geile Wp ecer gt IN I IP if f groundStation pos in Radians loat g latR g lonR Azimuth and Elevation Angles Radians florens Bey IND Servo commands Tat An Sp aw US bool sanDiego Set up serial settings Bits S 8N1 is default usb 232 baud 57600 groundStation baud 57600 HMC6352 Magnetometor Settings Continuous mode periodic set reset 20Hz rate compass setOpMode HMC6352 CONTINUOUS 1 20 Set up Leds OFF ledl led2 led3 led4 OFF Set up Servos for Azimuth and Elevation PwmOut Az servo p21 An BSrVvo perice 0 020 7 PwmOut Ev _servo p22 Ev_servo period 0 020 Set up Initial Servo Position Az torads 90 Ev torads 30 is WS chine Gustave se il isle p i SiO ahs E y us lime 12223 my umy 599 19 sp 2370651789 6 while 1 set up while set up servo for turning on ies SSueVO jowlllsSigalcheln Wis Vaz GS P Ev servo pulsewidth us Ev uS wait 0 05
7. TE g lom totloar m Sce Od em S c TO erem ec TS d rer S ep RN g alrt totloar msc 24 megl megla megla e float tofloat uchar b0 uchar bl uchar b2 uchar b3 teke loll os lee Jo fe float fo float J bs return fc File tracking h define PI 3 14159274 define ON 1 define OFF 0 typedef unsigned char uchar float torads flost deg float todegs float rad Gets string from groundStation and converts to Lat Lon Alt youl gece closes i9 lat closes jp lom Cloete p elt Cloete gy let loere cj lom Cloer ey alot e converts lat lon alt to ECEF earth centered earth fixed coordinates woe Jie exer clei lac i oati lhon E loata eE oaee OAE Ww OaE e TADP converts 4 bytes to float float tofloat uchar bO uchar bl uchar b2 Uchar b3 this is a testing function it allows me to input a puse width in uS and output it to the servo s Allows me to get y Mx B it s not used in the main program but is usefull void Servo Calio p MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu Page 12 10 Formula and Relation Reference 1 Latitude Longitude Altitude to Earth Centered Earth Fixed Coordinate System a Define the flatness f and eccentricity e of the ellipsoid as follows MAE142 notes _ Semi major axis 6378137 m a semimajor axis f a g lo l b
8. g 1900 1120 uS and the elevation Servo is defines from 0 180 deg 0500 2400 uS blackout region where the azimuth will swing back to Az PI And Ev pi Ev df Az gt UIAPIAT2 ee Az lt q9 PT 12 Az Az PI my Bl mos 270 90 aie UA SS Se D fale wee x 2S INCL INF DPI now get the uS the values below are from polyfit matlab from your servo calibrations to the second degree is WS ausus O 9195 ava us p eel vas ae SIO 413 B Hy u Qa 12223 mysmy SENG sp 2376318 6 Send Servo Commands Az servo pulsewidth us Az uS Ev servo pulsewidth us Ev uS 2 MEUS ONES sjouealionese 1 Fae aie Haya i Az uS 22525 Ev Sa Az EDS Nn coes Aa AA S Tees v VIRUS usb 232 printf Hi suelo 232 sxseneie QU Uc wae Jeabdwenrla 2 258 Ruin HP ee Ehe was ios a r Ni m Dp Ute P AES ON joram Sie Gallet OP ES plans We jor latc jer lemp jo ale Jp Pikes eee Wn Vp 1 4 1 We jo E Giaowacl We p iene y lem p allie Ewounnel eer Vn Vp Cp BW BB E oe Fh oe oe oe i vaa 232 prit u I wie 232 jorin 7 ttn Ue Leta Le oe end main wee Merser Elus lac clocit bomp slozi eile wiheelcs xp Wilko WW edens A WGS84 ellipsoid constants float a 6378137 earth semi major axis float esqrd 0 006694379990141 Eccentricity float Ns calculation lst toreds lat lon torads lon The length of normal N to the ellip
9. ith any autopilot system or GPS only system as long as the communication protocol is met This system was developed with the sUAS competition and UCSD team in mind but the system is not specific to any ground station Because magnetic declination changes with geo position this particular system can accurately be used only in San Diego or Lexington Park Maryland which is where the competition is held Magnetic declination is the angle between magnetic north and true north and it not only varies with location but also with time This system can be easily adapted for any location if the user so chooses by changing the code when the magnetic declination is known for a particular location and time MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu Page 3 2 Quick Operational Guide The use of this system needs to be included at the end of the pre flight system checklist before taking off For the purpose of this guide let s assume the user is at this stage and is ready to take off The user needs to complete the following steps for easy operation Connect a Serial Cable to the system DB9 connector Communications should already be established at the ground station based on the communication protocol Connect the power cable from the battery to the system LED1 should be blinking LED2 should be blinking LED1 is a meas
10. je dex jo ellie lar dex e su convert airplane to ecef lla2ecefip lat p lon p alt p x pov pz convert ground to ecef Ila2ecefiqg lat g lon og alt dg x dg vv g 2 subtract plane from ground to get the difference vector pco M jobs G ep ecet y jg y p wp Gr BS Pw Ec Ner Nonas b g lonR torads g lon translation ecef xyz to north east down NZI NE cnm clt Ee esa cnilicm M eme eplleic salin eillom cos enit e sin glon cos glon 0 5 COs leler eos kelon E eos elat s eLan NE snm reni NED TE2L ecef S NEL cea an ecos lo TOn eeen LE Sea e ea on ecer y cos i Cati ecer ap I ceia en Eee s a cos G_lomR acer yy D cosl lavn ecos loni ecet COS Calais soni mp on eC Cty eat me mmc Ecchi p 1 D we are on the ground looking up D N I atan2 N E abs atan2 D sqrt N N E E this will always be positive ti q I if plane is on the ground next to ground station point north ieg lar e let Ee lon p ton Ev 0 Az PI 2 Aling Az is from 0 to 2 pi no negative numbers MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu Page 10 if Az lt 0 Az AZ wp sene from here on out it depends on the how the servo pulse width relates to angle her my azimuth servo is defined from 270 gt 90 de
11. ng position as soon as the reference north is set at all times keep external objects away from the rotational parts of the system this may cause harm to the user and the gear mechanism MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu Page 4 3 Theory of Operation The very first process the system completes after establishing the commutation is it sets the magnetic declination by retrieving the location of the ground station from the communication message The detailed communication protocol is discussed on page 5 For example in San Diego the magnetic declination is approximately 12 this is a known number that is stored in software At this time a North reference point must be set and the system must be aligned to the True North axis by using a magnetometer An indication to the user is made by blinking LED2 that the system is not set a secondary visual indication is given by the elevation servo which is pointed 30 degrees up As soon as the user points the gimbal to True north which is calculated from the magnetometer and the known declination the user is prompted to stop moving when LED2 is turned off and the elevation servo will point to zero elevation Now the system enters it main operational loop Anew communication string is read in and the latitude longitude and altitude of the aircraft and the ground station
12. rticular Tracking Antenna was developed for such a purpose but in particular it was developed to help the UCSD AUVSI team for their annual sUAS Student Unmanned Aerial System Competition which takes place in Maryland The UCSD team also uses an camera and video transmitter but their objective is to find and identify targets More about the competition can be found on this website http 65 210 16 57 studentcomp2010 Both of the above applications require an analog or digital video signal broadcasting from the aircraft With a stationary application this is a task easily accomplished with the use of a flat panel antenna since they are light cheap and directional and with line of sight work very well For a highly dynamic mobile application a directional antenna such as a flat panel antenna is problematic The UCSD team uses a 14 dBi flat panel antenna with a 30 degree vertical and horizontal beam width that means to get their video signal they would have to fly within 30 degrees of the antenna which not practical In the past the team has dedicated a freshman to point the antenna at the aircraft at all times the developed Tracking Antenna System removes the need for a human gimbal and replaces it with a smart gimbal that receives position information of the airplane as a binary signal through the ground station of the autopilot and then commands an azimuth and elevation servo to point the flat panel antenna at the aircraft This system can be used w
13. semiminor axis Semi minor axis 6356752 3142 m a b f f f flatness e eccentricity Flattening 298 257223563 Eccentricity 0 0818191908426 b The length of normal N to the ellipsoid extending from the surface of the ellipsoid to its intersection with the z axis of the ECEF is E a N q jj L q latitude vVl e sim gp c Covert X N h cos cos A Z N 1 e h sin qp o latitute longitude Y N h cos sin A h altitude above ellipsoid 2 Conversion to North East Down Coordinate System from ECEF MAE142 notes Ny y sinwcosA sin sin coso A IY Ipg 7 sin cosA 0 D Iz cosqQcosA cosqosinA sin p f latituteof NED frame origin 3 Azimuth Angle Calculation N Az tan rads A longitude NED frame origin 4 Elevation Angle Calculation Ev tan rads 5 Function that maps Angle to Pulse Width constants obtained with Matlab s polyfit function AZys 9195Az 121 18Az 1308 uS Evys 1 223Ev 589 16Ev 2377 uS
14. soid m sepul eack 9 eioi Je siol Je g alti cosilath cos lon Walii cos4 at ern lon Cil esepecl IN ap eub sulin beue NK KZ float torads float deg return deg PI 180 in rads float todegs float rads return rads 180 PI in degs void servo calib void PwmOut servo p22 servo period 0 020 char pwm uS 5 int pwm uS i while 1 MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu Page 11 usb 232 printf n Enter PWM in uS 0600 2400 DE Ueo 2327 scam ssi Wn aS e pwm uS i atof pwm uS servo pulsewidth us pwm uS i WISI 2S cjeweslsuewe wer vo matere unb Wa owm wis 25 5 void getlla float amp p lat i Tloare p lon loatt p elt leatt G lat sleoate o lon rloatk G elt uchar msg 26 do while msg 0 groundStation getc Oxff wait for the start of communication string for int i l 1 lt 26 Itt msg i groundStation getc fill message string while msg 25 Oxfe make sure the last character is OxFE otherwise re read ledl led1 blink airplane pacc ie relloeic use l4 ise Sl Scr rm TUTO s p lom tonlist Sel MSC mse Moll wee Sil e p alt toriet msg 12 mse ase LO rr S TET ground g lar comlenic mscr sem S cT rms cp TS erm TES
15. te OxFF and an end byte OxFE The data part of the communication string is the Latitude Longitude and Altitude in Decimal Degrees and Meters respectfully of the aircraft and the ground station OxFF Plane Lat Plane Lon Plane Alt Ground Lat Ground Lon Ground Alt OxFE Each individual data field is a 4 byte in a single precision floating format As shown in the example below Plane Latitude in Decimal Degrees Plane Latitude as a single precision floating point format 39 995972 OxEO OxFB Ox1F 0x42 By reversing the order and combining the 4 bytes to a single 32 byte float you can obtain the Plane Latitude in Decimal Degrees MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu Page 6 7 Wiring Diagram Magnetometer NiMH 4 Cell av gt VIN GND Battery MBED Micro Controller SCL PIN2 VIN SDA PIN1 GND PIN40 3 3V PIN10 RX PIN28 SCL PIN27 SDA RS232 to 3 3V Level Shifter PIN21 PWM VIN PIN22 PWM GND TX Azimuth Servo RS232 IN VIN RS232 GND GND SIG Elevation Servo VIN GND SIG MBED Tracking Antenna System A Critical Component to a UAS User Manual and Technical Documentation UCSD ECE Dept Daniel Bedenko dbedenko ucsd edu
16. ure of communication and protocol and is set to blink for each correct communication string LED3 or LED4 should be ON depending on where the user is located LED3 for San Diego and LED4 for Lexington Park Maryland The elevation servo should be pointing the system to 30 degrees LED2 should be blinking and this means that the system is not correctly aligned to True North Loosen the yaw screw on the tripod and slowly rotate the system around the yaw axis around to where you think North should be as soon as the user is within a degree of True North LED2 will stop blinking and the elevation servo will return to zero elevation Tighten the yaw screw at the tripod At this point if the system is moved the above steps need to be repeated Connect the Coax cable to the Antenna from your video receiver and zip tie a meter or so of cable length to the leg of the tripod so that the rotational components are provided with adequate strain relief on the Antenna Check reception of signal complete the rest of the pre flight check list and you are ready to take off as soon as there is some distance between the aircraft and the ground station the Tracking system will track the aircraft CAUTION It is not recommended resetting the system while the aircraft is in the air If the system is moved you can power cycle it and reset it to True North reference using the above instructions but if this happens during flight the antenna will go to the tracki
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