NAV420 Series User`s Manual
Contents
1. 9 Appendix D Warranty and Support I nformation 9 1 Customer Service As a Crossbow Technology customer you have access to product support services which include e Single point return service e Web based support service e Same day troubleshooting assistance e Worldwide Crossbow representation e Onsite and factory training available e Preventative maintenance and repair programs e Installation assistance available 9 2 Contact Directory United States Phone 1 408 965 3300 7 AM to 7 PM PST Fax 1 408 324 4840 24 hours Email techsupport xbow com Non U S refer to website www xbow com 9 3 Return Procedure 9 3 1 Authorization Before returning any equipment please contact Crossbow to obtain a Returned Material Authorization number RMA Be ready to provide the following information when requesting a RMA e Name e Address e Telephone Fax Email e Equipment Model Number e Equipment Serial Number e Installation Date e Failure Date e Fault Description e Willitconnect to NAV VIEW Page 36 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w 9 3 2 Identification and Protection If the equipment is to be shipped to Crossbow for service or repair please attach a tag TO THE EQUIPMENT as well as the shipping container s identifying the owner Also indicate the service or repair required the problems encountered and other information considered valuable to the service facility such as2. This dependence of the local magnetic field on input direction is called the soft iron effect The NAV420CA can actually measure any extra constant magnetic field that is associated with the NAV420CA or your system and correct for it The NAV420CA can also make a correction for some soft iron effects The process of measuring these non ideal effects and correcting for them is called hard iron and soft iron calibration Calibration will help correct for magnetic fields that are fixed with respect to the NAV420CA It cannot help for time varying fields or fields created by parts that move with respect to the NAV420CA The NAV420CA accounts for the extra magnetic field by making a series of measurements The NAV420CA uses these measurements to model the hard iron and soft iron environment in your system The correction algorithm is two dimensional The NAV420CA will calculate the hard iron magnetic fields and soft iron corrections and store these as calibration constants in the EEPROM For best accuracy you should do the calibration process with the NAV420CA installed in your system If you do the calibration process with the NAV420CA by itself you will only correct for the magnetism in the NAV420CA itself If you then install the NAV420CA in a vehicle for instance and the vehicle is magnetic you will still see errors arising from the magnetism of the vehicle The NAV420CA will need to be calibrated for hard and soft iron compensation before use
3. 2 Kalman Filter Attitude and Heading Estimation Model 9 3 3 3 Attitude and Heading Initialization sss 10 3 4 Factory Sensor Calibration eee 11 3 5 Conn etiOns ore Rte bie Mole Bet e ded 11 3 5 1 VO Cable 5 eh et a y e ees 12 3 5 2 Power Input and Power Input Ground 12 3 5 3 Case Ground eben Bald Ale 12 3 5 4 Serial Data Interface esee 12 3 5 5 Serial GPS Interf ce oeeie aree eo eon 12 3 5 6 1 PPS Output Interface eene epe 13 3 5 7 GPS Antenna Connection eese 13 3 5 8 NO CONMECHON cte i tette o terere to euet 14 3 5 9 Quick Digital interface connection eese 14 3 6 Measurement Modes eese 14 3 6 1 Scaled Sensor Mode sese 15 3 6 2 AHRS Mode ribera 15 3 6 3 NAV Mode 51 2 etd e eer SES 16 32 BLEPEOC SSIBEGsnu noe eee RUPES 18 Doc 7430 0121 01 Preliminary Page i Crossb w NAV420CA Series User s Manual 3 8 Commands esee etit e e e Redde eed 19 3 8 1 Input Packets onse ete emite ets 19 3 8 2 Output Pac KeS rechter re 20 3 8 3 Command List iaces esed 20 3 9 Data Packet Form ts nne a ere erne E 23 3 10 Vat bisse RHET e odense 25 3 11 Temperature Sensor one rette tei eks 26 3 12 Magnetic Heading eoe toe net oec eee ts 26 4 NAV420CA Operating Tips eee 27 4 1 Mounting the NAV420CA cesser
4. 3 Command List Command Ping Input UU PK Packet Response UU P Packet Description Pings NAV420CA to verify communications The ping command does not have data or a checksum to facilitate human interaction from a keyboard Sending the ping command will cause the unit to send a ping response All bytes sent and received during the ping command and responses are ASCII printable characters Command Request Data Page 20 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Input Packet Response Packet Description Command Input Packet Response Packet Description Command Input Packet Response Description Command Doc 7430 0121 01 Preliminary Crossb w UU GP packet type Checksum Data Packet See Table 5 This command allows the user to poll for non response type output packets The packet type options available are S Scaled Sensor Packet A AHRS Packet N NAV Packet Refer to Table 5 for data packet format in different modes Query Serial Number and Firmware Version UU GP D Checksum Checksum UU D Serial Number Version String This queries the NAV420CA for its serial number and firmware version The serial number contains 4 bytes and should be interpreted as a 32 bit unsigned integer For example the serial number 4003012 would be sent as the four bytes 00 3D 14 C
5. Series User s Manual Crossb w 100Hz navigation algorithm is synchronized to the GPS 1PPS when available Packet data is therefore valid on 10ms boundaries of UTC Factory calibration data stored in EEPROM is used by the DSP to remove temperature bias misalignment scale factor errors and non linearities from the sensor data The firmware inside the onboard processors produces calibrated angular rate measurements calibrated acceleration measurements calibrated magnetometer measurements and the estimated navigation state which includes body attitude roll pitch heading local level horizontal navigation frame position latitude longitude and altitude and velocity The algorithm used to estimate the navigation state is an Extended Kalman Filter EKF trajectory correction approach in which the inertial accelerometers and gyros propagate the state trajectory made up of velocity and body attitude and the supporting sensors GPS and magnetometers provide velocity and earth magnetic field measurements which the filter uses to calculate corrections to the trajectory state and estimate inertial sensor errors and ferrous material effects The DSP performs time triggered trajectory propagation at 100Hz and will synchronize the sensor sampling with the GPS UTC second boundary when available 3 2 NAV420CA Coordinate System The NAV420CA has a label on one face illustrating the NAV420CA coordinate system as shown in Figure 2 With the connecto
6. be within 2 accurate over the NAV420CA operating temperature range The NAV420CA will output the temperature sensor readings Scaled Sensor and AHRS modes in the digital data packet scaled as follows Temp C data 100 2 where data is the 16 bit unsigned integer sent as the temperature information in the data packet The NAV420CA temperature sensor is internal to the NAV420CA and is not intended to measure the ambient temperature The internal temperature of the NAV420CA may be as much as 159C higher than the ambient temperature 3 12 Magnetic Heading Magnetic north is the direction toward the magnetic north pole true north is the direction towards the true North Pole The NAV420CA yaw angle output is referenced to true north in NAV mode and magnetic north in AHRS mode The direction of true north will vary from magnetic north depending on your position on the earth The difference between true and magnetic north is called declination or magnetic variance The World Magnetic Model WMM 2000 is used to calculate the magnetic declination or variance GPS position is used to calculate the tangent frame earth magnetic field vector from the WMM and the declination angle is computed from the WMM horizontal components The magnetometer measurements can then be rotated through the declination angle to provide a true heading measurement The current declination angle used in the NAV420CA can be determined by taking t
7. possible from the unit but we could not make the unit completely non magnetic You can permanently magnetize perm up components in the NAV420CA if you expose the unit to a large magnetic field You can use a demagnetizer tape eraser to demagnetize the NAV420CA if it gets permed Follow the instructions for your demagnetizer The NAV420CA must not be located within 24 inches of any large moving ferrous metal objects such as landing gear components motors steel control cables or linkage Avoid any metallic objects that may change position between ground operations and flight operations such as landing gear flap actuators and control linkages The NAV420CA should not be located close to high current DC power cables or 400 cycle AC power cables and their associated magnetic fields Wires carrying high currents alternate currents or intermittent currents can cause magnetic variations that will affect the AHRS Keep wires with these characteristics at least 24 inches away from the NAV420CA These wires can include Battery wires Strobe wires Autopilot control wires Position light wires De ice boot wires Air conditioning power wires HF control wires The NAV420CA case is water resistant but you should always try to protect it from moisture and dust Page 28 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w 5 Limitations 5 1 Installation The NAV420CA must be mounted in a location with limited
8. sophisticated Kalman filter algorithm to allow the unit to track orientation accurately through dynamic maneuvers The Kalman filter will automatically adjust for changing dynamic conditions without any external user input No user intervention or configuration is required at power up The BIT message is included in each packet that provides comprehensive information into system health BIT is used for informing the user of the system status and informing the user of system problems The NAV420CA series units are light weight low power fast turn on reliable and accurate solutions for a wide variety of stabilization navigation guidance and measurement applications However it should not be exposed to large magnetic fields This could permanently magnetize internal components of the NAV420CA and degrade its magnetic heading accuracy 1 2 Package Contents In addition to your NAV420CA sensor product you should have e 1CD with NAV VIEW Software NAV VIEW will allow you to immediately view the outputs of the NAV420CA on a PC running Microsoft Windows You can also download this software from Crossbow s web site at http www xbow com e 1 Digital Signal Cable This links the NAV420CA directly to a serial port Only the transmit receive power and ground channels are used e NAV420 Series User s Manual Page 2 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w This contains helpful hints on programming installati
9. the calibration parameters Each parameter should not deviate more than 0 01 between calibrations Calibration Commands The calibration can be completed without the use of Nav view using the following steps The calibration steps are 1 AR wh Apply power to the NAV420CA Wait 60 seconds for initialization to complete Send 0x555553460100010000 to set the packet output rate to zero Send 0x55555743000C to begin the calibration The NAV420CA will respond with 0x5555570C0063 Slowly rotate the vehicle through a 380 degree turn 10 20 degrees per 6 Page 34 second is ideal The unit will respond with Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w 0x555543 lt 2 byte checksum gt Stop the turn Each of the three parameters above are signed 2 byte fixed point numbers scaled by 2112 Thus divide the data by 2 12 These numbers represent the magnetic environment around the NAV420CA in terms of bias offsets G and scale factor percentage measured during the calibration turn 8 Tocommit the calibration to EEPROM send 0x55555743000E If you do not want to commit the calibration do nothing 9 Disconnect power from the NAV420CA 10 Wait 10 seconds 11 Repeat this calibration procedure to verify consistency in the calibration parameters Each parameter should not deviate more than 0 01 between calibrations Doc 7430 0121 01 Preliminary Page 35 Crossb w NAV420CA Series User s Manual
10. with the system 8 2 NAV420CA Hard and Soft Iron Calibration Procedure 8 2 1 Calibration Process Overview using NAV VIEW There are several steps to the calibration process that are repeated until the NAV420CA has collected enough data to compute a hard and soft iron compensation that meets the performance requirements The calibration steps are Page 32 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w Apply power to the NAV420CA 2 Start NAV VIEW software and make sure communication is established 3 Wait 60 seconds for initialization to complete Click PRESS CAL button on the NAV VIEW control panel 5 The following message will appear click Start to begin the calibration 6 Slowly rotate the aircraft through a 380 degree turn 10 20 degrees per second is ideal until the following message appears Doc 7430 0121 01 Preliminary Page 33 Crossb w NAV420CA Series User s Manual 10 11 8 2 2 CONES i Calibration Data X Hard Iron 0 003079 Y Hard Iron 0 000858 Soft Iron Ratio 0 995145 Do you wish to accept this cailbration Stop the turn and write down the three calibration parameters displayed These numbers represent the magnetic environment around the NAV420CA Click Accept to store the calibration or Cancel to ignore the calibration Disconnect power from the NAV420CA Wait 10 seconds Repeat this calibration procedure to verify consistency in
11. 00 i ry Page 30 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w 7 Appendix B NAV420CA Output Quick Reference 7 1 Digital Output Conversion Data is sent as 16 bit signed integer for all but Longitude Latitude Longitude Latitude data is sent as 32 bit signed integer Acceleration Roll Pitch AHRS NAV Mode accel G data 10 2 angle data 180 2 Rate Magnetic Field Scaled Mode rate s data 630 2 mag Gauss data 1 0 2 Longitude Latitude NAV Mode Altitude NAV Mode lat lon data 180 27 alt m data 16384 2 GPS Velocity NAV Mode vel m s data 256 2 Doc 7430 0121 01 Preliminary Page 31 Crossb w NAV420CA Series User s Manual 8 Appendix C Hard and Soft Iron Calibration 8 1 Hard Soft Iron Calibration Introduction The NAV420CA uses magnetic sensors to compute heading Ideally the magnetic sensors would be measuring only earth s magnetic field to compute the heading angle In the real world however residual magnetism in the NAV420CA itself and in your system will add to the magnetic field measured by the NAV420CA This extra magnetic field will create errors in the heading measurement if they are not accounted for These extra magnetic fields are called hard iron magnetic fields In addition magnetic material can change the direction of the magnetic field as a function of the input magnetic field
12. 4 The firmware version is an ASCII string that describes the NAV420CA firmware version Write Set Fields UU WF lt 1 byte num list of field Checksum or of fields fields data SF None This command allows the user to write default power up configuration fields to the EEPROM WF or set the unit s current configuration SF fields which will be lost on power down Writing the default configuration will not take affect until the unit is power cycled Num of fields is the number of words to be written set The list of fields are the field IDs that will be written with the field data respectively The unit will not write to calibration or algorithm fields The unit will not respond Change Baud Rate Page 21 Crossb w Input Packet Response Description Command Input Packet Response Description Command Input Packet Response Description Page 22 NAV420CA Series User s Manual UU WF 0x01 0x0002 Value Checksum None This change the default power up baud rate of the NAV420CA Upon sending the command power cycle the unit for the change to take effect The available Value options are corresponding baud rates are listed below Value Baud Rate 0x00 9600 0x01 19200 0x02 38400 0x03 57600 Change Packet Type UU WFor 0x01 0x0003 Value Checksum SF None This command al
13. 630 deg sec 15 16 X Axis Acceleration 10 10 G 17 18 Y Axis Acceleration 10 10 G 19 20 Z Axis Acceleration 10 10 G 21 22 X Axis Magnetic Field 1 1 Gauss 23 24 Y Axis Magnetic Field 1 1 Gauss 25 26 Z Axis Magnetic Field 1 1 Gauss 27 28 Temperature 100 100 oC 29 30 GPS ITOW last 2 bytes 0 65536 msec 3132 BIT 33 34 Checksum 5 3 NAV Mode Packet 0 1 Header 0x5555 2 N Page 24 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w 3 4 Roll Angle 180 180 deg 5 6 Pitch Angle 180 180 deg 7 8 Heading Angle true north 180 180 deg 9 10 Roll Angular Rate 630 630 deg sec 11 12 Pitch Angular Rate 630 630 deg sec 13 14 Yaw Angular Rate 630 630 deg sec 15 16 X Axis Velocity North 256 256 m sec 17 18 Y Axis Velocity East 256 256 m sec 19 20 Z Axis Velocity Down 256 256 m sec 21 22 23 24 Longitude 180 180 deg 25 26 27 28 Latitude 180 180 deg 29 30 Altitude 100 16284 m 3132 GPS ITOW last 2 bytes 0 65536 msec 33 34 BIT 35 36 Checksum 3 10 Timing The NAV420CA default data output is NAV packet at 50 packets per second Depending on the configuration field set for update rate the unit can be made to output at a different constant output rate or not at all for polled operation In some applications using the NAV420CA s digital output requires a p
14. DC unregulated at 350 mA If you are using the cable supplied with the NAV420CA the power supply wires are broken out of the cable at the DB 9 connector The red wire is connected to the positive power input the black wire is connected to the power supply ground DO NOT REVERSE THE POWER LEADS 3 5 8 Case Ground The case is electrically connected to Pin 5 of the DB 15 connector The Pin 5 should be electrically connected to the user s cable shield especially if the chassis does not make good ground contact The case is isolated from the Power Input Ground and should be bolted to a good conducting surface that is grounded 3 5 4 Serial Data Interface The serial interface is standard RS 232 9600 19200 38400 or 57600 baud 8 data bits 1 start bit 1 stop bit no parity and no flow control and will output at a user configurable output rate These settings allow interaction via a standard PC serial port 3 5 5 Serial GPS Interface The GPS receiver outputs data in NEMA 0183 format as defined by the National Marine Electronics Association NMEA Standard For Interfacing Marine Electronic Devices Version 2 20 January 1 1997 Page 12 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w The packets are sent at 9600 Baud 8 bits no parity bit 1 stop bit The GPS receiver outputs the following messages as shown in Table 2 Table 2 GPS Output Packet Format NEMA Record Descri
15. NAV420 Series User s Manual Document 7430 0121 01 Preliminary September 2004 Crossb w Crossbow Technology Inc 41 Daggett Drive San Jose CA 95134 Tel 408 965 3300 Fax 408 324 4840 email info xbow com website www xbow com 2004 Crossbow Technology Inc All rights reserved Information in this document is subject to change without notice Crossbow and SoftSensor are registered trademarks and NAV420CA is a trademark of Crossbow Technology Inc Other product and trade names are trademarks or registered trademarks of their respective holders NAV420CA Series User s Manual Crossb w Table of Contents T Introducti n i onines reete Enn Fr deed ne denen eda 1 1 1 The NAV420CA Navigation and Attitude Sensing Units 1 1 2 Package Contents ete eee Dre ateist 2 27 Ouck Start 5 rcov eT Nee ceu ET 4 2 1 NAV VIEW SoftWare nn nee a 4 2 1 1 NAV VIEW Computer Requirements esee 4 2 1 2 Install NA V VIEW niche pr inre ieee 4 2 2 Conn ctions ian tee e eder ae eo eine 4 2 3 Setting up NAV VIEW arsen dansede henses eene enne 5 2 4 Take Measurements iatera ae nnne nne enne 5 3 NAVA20CA Details iceberg aet 6 3 1 NAV420CA Architecture sees nee 6 3 2 NAVA420CA Coordinate System eese 7 3 3 Attitude and Heading Determination Algorithm 8 3 3 1 Attitude and Heading Processor eee 9 3 3
16. addition of a 3 axis magnetometer inside the NAv420CA allows it to make a true measurement of magnetic heading without an external flux valve With the built in GPS receiver the combined system becomes a low cost INS that can output location velocity and acceleration The Crossbow NAV420CA is a solid state equivalent of a vertical gyro artificial horizon display combined with a directional gyro flux valve and Global Positioning System GPS Crossbow s newest inertial system the NAV420CA combines the latest in low cost MEMS sensors and digital signal processing techniques to provide an inexpensive and compact sized alternative to existing IMU systems Closely coupled integration of the sensors data acquisition elements and a Kalman filter based algorithm allow the NAV420CA to provide an accurate representation of the attitude and heading of an object with improved performance over older technology systems The integration of a GPS receiver provides more information for the extended Kalman filter allowing it to provide better corrections for attitude determination as well as the ability to estimate accelerometer and magnetometer sensor errors Furthermore the digital architecture s flexible interface allows easy integration into most applications The calibrated sensor output angular rate acceleration and magnetic vector allows easy integration for control systems The Crossbow NAV420CA is the latest generation of navigation and atti
17. b w Doc 7430 0121 01 Preliminary Page iii Crossb w NAV420CA Series User s Manual About this Manual The following annotations have been used to provide additional information 4 NOTE Note provides additional information about the topic M EXAMPLE Examples are given throughout the manual to help the reader understand the terminology IMPORTANT This symbol defines items that have significant meaning to the user 6 WARNING The user should pay particular attention to this symbol It means there is a chance that physical harm could happen to either the person or the equipment The following paragraph heading formatting is used in this manual 1 Heading 1 1 1 Heading 2 1 1 1 Heading 3 Normal Page iv Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w 1 Introduction 1 1 The NAV420CA Navigation and Attitude Sensing Units This manual explains the use of the NAV420 Series of products a combined Navigation and attitude and heading reference system to measure stabilized pitch roll and yaw angles in a dynamic environment along with position and velocity Crossbow has been developing low cost solid state systems that measure roll pitch and heading using MEMS technology in commercial industrial and aerospace markets since 1998 The Crossbow Navigation System or NAV420CA uses a 3 axis accelerometer and a 3 axis rate sensor to make a complete measurement of the dynamics of the system The
18. ctions between the NAV420CA and the computer check the power check the serial COM port assignment on your computer 2 4 Take Measurements Once you have configured NAV VIEW to work with your NAV420CA pick what kind of measurement you wish to see Graph will show you the output you choose as a strip chart type graph of value vs time FFT will show you a real time Fast Fourier transform of the output you choose Navigation will show an artificial horizon and the stabilized roll pitch and heading output of the NAV420CA When GPS is available and NAV packet is chosen GPS will show a digital display of latitude longitude and altitude and strip chart type graph of velocity vs time Let the NAV420CA warm up for 60 seconds when first turned on This allows the Kalman filter to estimate the rate sensor biases The NAV420CA needs to be held still motionless during this period Now you re ready to use the NAV420CA Doc 7430 0121 01 Preliminary Page 5 Crossb w NAV420CA Series User s Manual 3 NAV420CA Details 3 1 NAV420CA Architecture The NAV420CA is an intelligent attitude gyro for roll pitch and directional gyro for heading angle measurements in dynamic environments The unit is also a nine axis measurement system that outputs accurate acceleration angular rates and magnetic orientation The NAV420CA uses the latest in solid state sensor technology resulting in superior performance reliability and stability o
19. de the analog sensors are sampled converted to digital data temperature compensated corrected for misalignment and scaled to engineering units The digital data represents the actual value of the quantities measured A calibration table for each sensor is stored in the NAV420CA non volatile memory A single data packet can be requested using a serial poll command or the NAV420CA can be set to continuously output data packets to the host The data is sent as signed 16 bit 2 s complement integers In this mode the NAV420CA operates as a nine axis measurement system The scaled sensor outputs are enabled in this mode Note that stabilized pitch roll and yaw angles are not available in scaled sensor mode To convert the acceleration data into G s use the following conversion accel data 10 2 where accel is the actual measured acceleration in G s data is the digital data sent by the NAV420CA and 10 is the G Range for your NAV420CA The data is scaled so that I G 29 81 ms This maximum G range is a default value To convert the angular rate data into degrees per second use the following conversion rate data 630 2P where rate is the actual measured angular rate in sec data is the digital data sent by the NAV420CA and 630 is the Angular rate Range of the NAV420CA This maximum angular rate range is a default value To convert the magnetometer data into Gauss use the following conversion mag data 1 0 2 where mag is th
20. e connected properly to SMA jack located next to the DB 15 connector f IMPORTANT Place the antenna with optimal sky visibility 3 5 8 No Connection During normal operation of the NAV420CA no connection is made to the factory test pins These pins have internal pull up mechanisms and must have no connections for the NAV420CA to operate properly 3 5 9 Quick Digital interface connection On a standard DB 9 COM port connector make the connections as described in Table 3 Table 3 DB 9 COM Port Connections COM Port Connector NAV420CA Connector Pin Signal Pin Signal 2 RxD 1 TxD 3 TxD 2 RxD 5 GND 9 GND Power is applied to the NAV420CA on pins 3 and 4 Pin 4 is power ground Pin 3 should have 9 30 VDC unregulated at 350 mA DO NOT REVERSE THE POWER LEADS 3 6 Measurement Modes The NAV420CA is designed to operate as a complete attitude and heading reference system You can also use this as a nine axis sensor module The NAV420CA can be set to operate in one of three modes Scaled Sensor mode AHRS mode or NAV mode The measurement mode selects the information that is sent in the data packet over the RS 232 interface See the Data Packet Format section for the actual structure of the data packet in each mode The default system operation is NAV mode Page 14 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w 3 6 1 Scaled Sensor Mode In Scaled Sensor mo
21. e modern equivalent of an analog vertical gyro that used a plumb bob in a feedback loop to keep the gyro axis stabilized to vertical The NAV420CA takes advantage of the rate gyros sensitivity to quick motions to maintain an accurate orientation when accelerations would otherwise throw off the accelerometers measurement of the NAV420CA orientation relative to gravity the NAV420CA then uses the accelerometers to provide long term stability to keep the rate gyro drift in check when GPS is down The NAV420CA uses a sophisticated Kalman filter algorithm to track the bias in the rate sensors and accelerometers This allows the NAV420CA to use a very low effective weighting on the accelerometers when the NAV420CA is moved This makes the NAV420CA very accurate in short term dynamic maneuvers regardless of GPS status The NAV420CA outputs the stabilized pitch roll and yaw angles in the digital data packet in AHRS mode To convert the digital data to angle use the following relation angle data SCALE 2 where angle is the actual angle in degrees pitch roll or yaw data is the signed integer data output in the data packet and SCALE is a constant SCALE 180 for roll pitch and yaw 3 6 3 NAV Mode When GPS is up the NAV420CA acts as a complete navigation and attitude and heading reference system The NAV packet outputs the stabilized pitch roll and yaw angles longitude latitude altitude GPS velocity along with the angular rate
22. e EKF In this way the NAV420CA utilizes the GPS receiver s internal ECEF to tangent frame velocity transformation resulting in a computationally efficient measurement update The NAV420CA uses its internal magnetometers to provide a heading measurement update for the EKF Magnetometer measurements are calibrated corrected for hardiron and softiron effects from the local environment and rotated to the tangent frame The World Magnetic Model WMM 2000 is used to calculate the relationship between magnetometer heading and true heading known as magnetic declination or variance GPS position is used to calculate the tangent frame earth magnetic field vector from the WMM and the declination angle is computed from the WMM horizontal components The magnetometer measurements can then be rotated through the declination angle to provide a true heading measurement The true heading measurement s nonlinear relationship to the quaternion attitude is captured by the EKF measurement model to create a linearized filter update If the GPS link is lost or poor the Kalman Filter solution stops tracking accelerometer bias The algorithm continues to apply gyro bias correction and provides stabilized angle outputs The EKF tracking states are reduced to angles and gyro bias only The accelerometers will continue to integrate velocity however accelerometer noise bias and attitude error will cause the velocity estimates to walk off on the order of seconds The a
23. e actual measured magnetic field in Gauss data is the digital data sent by the NAV420CA and 1 0 is the Magnetic field Range of the NAV420CA This maximum magnetometer range is a default value The Kalman filter is enabled in scaled sensor mode however the sensors outputs are not corrected by Kalman filter bias estimates in the scaled sensor output packet Therefore the rate sensor bias will change slightly due to changes in temperature and time 3 6 2 AHRS Mode In AHRS mode the NAV420CA acts as a complete attitude and heading reference system and outputs the stabilized pitch roll and yaw angles along with the angular rate acceleration and magnetic field information The angular rate acceleration and magnetic field values are calculated as described in the Scaled Sensor mode Doc 7430 0121 01 Preliminary Page 15 Crossb w NAV420CA Series User s Manual The Kalman filter operates using an AHRS type filter if GPS is down or a reduced state navigation filter if GPS is up When GPS is down the filter tracks the rate sensor bias and calculates stabilized roll pitch and yaw angles The NAV420CA uses the angular rate sensors to integrate over your rotational motion and find the actual pitch roll and yaw angles When GPS is down the NAV420CA uses the accelerometers to correct for rate sensor drift in the vertical angles pitch and roll the NAV420CA uses the magnetometers to correct for rate sensor drift in the yaw angle This is th
24. e cable supply power the NAV420CA Match red to power and black to ground The input voltage can range from 9 30 VDC at 350 mA 4 Bolt the base of the unit to a grounded surface A good ground is required for EMI and lightning over voltage protection Page 4 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w 6 WARNING Do not reverse the power leads Applying the wrong power to the NAV420CA can damage the unit although there is reverse power protection Crossbow Technology is not responsible for resulting damage to the unit should the reverse voltage protection electronics fail 2 3 Setting up NAV VIEW With the NAV420CA connected to your PC serial port and powered open the NAV VIEW software 1 NAV VIEW should automatically detect the NAV420CA and display the serial number and firmware version if it is connected 2 If NAV VIEW does not connect check that you have the correct COM port selected You find this under the DMU menu 3 Select the type of display you want under the menu item Windows Graph displays a real time graph of all the NAV420CA data FFT displays a Fast Fourier transform of the data Navigation shows an artificial horizon display 4 Youcan log data to a file by entering a data file name You can select the rate at which data is saved to disk 5 If the status indicator says Connected you re ready to go If the status indicator doesn t say connected check the conne
25. e eene 27 5 Linit tions xam tee ooi nARD Te eel ie ex 29 5 1 Inst llatiOn 2 e ERREUR 29 29 2 eAdlignmentaz i uscito eleme Ib 29 5 3 Operation in Magnetic Environment seen 29 5 4 Range Limitations eese enne 29 6 Appendix A Mechanical Specifications esee 30 6 1 NAV420CA Outline Drawing eene 30 7 Appendix B NAV420CA Output Quick Reference 31 7 1 Digital Output Conversion eee 31 8 Appendix C Hard and Soft Iron Calibration sssss 32 8 1 Hard Soft Iron Calibration Introduction sss 32 8 2 NAV420CA Hard and Soft Iron Calibration Procedure 32 82 1 Calibration Process Overview using NAV VIEW 32 8 2 2 Calibration Commands esee 34 9 Appendix D Warranty and Support Information 36 9 1 Customer Servicen tere nere 36 9 2 Contact DIrectoEy oinaren eri te tire HO iie aioe 36 9 3 Return Procedure ru seiner bee e ente 36 9 3 1 AUChOFZ tlOn oet dere o e ere qp edes 36 9 3 2 Identification and Protection esee 37 9 3 3 Sealing the Container esee 37 9 3 4 Marking gear E RSS 37 9 3 5 Return Shipping Address eee 37 9 4 Waranty cuisse ho eee RT Hs 37 Page ii Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Cross
26. er The tangent frame is used as the navigation frame and earth rate is not considered By reducing the navigation filter to this minimalist form we are able to sample the sensor set at I kHz and run the navigation filter at 50 Hz 3 3 1 Attitude and Heading Processor The data processor attitude estimation algorithm provides stable Euler roll pitch and yaw angles For improved accuracy and to avoid singularities when dealing with the cosine rotation matrix a quaternion formulation is used in the algorithm to provide attitude propagation The body angular rates are then sensed by the gyros and a differential equation describing the propagation of the quaternion is integrated using Runge Kutta algorithm to obtain the propagated quaternion The cosine rotation matrix is obtained from the quaternion which then defines the attitude roll pitch and yaw angles With GPS up the NAV420 tracks rate sensor bias accelerometer bias quaternion attitude and tangent frame velocities Propagating tangent frame velocities directly avoids transformations to and from non Euclidean coordinate systems Eliminating position states from the navigation filter nearly cuts processing requirements in half because the state covariance update is n The state model can seamlessly switch between the reduced navigation filter and an AHRS type filter based on the health of the GPS receiver 3 3 2 Kalman Filter Attitude and Heading Estimation Model The Kalman f
27. he difference between yaw angle output in the NAV packet and AHRS packet Page 26 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w 4 NAV420CA Operating Tips 4 1 Mounting the NAV420CA The NAV420CA should be mounted as close to the center of gravity CG of your system as possible This will minimize any lever effect If it is not mounted at the center of gravity then rotations around the center of gravity will cause the NAV420CA accelerometers to measure acceleration proportional to the product of the angular rate squared and the distance between the NAV420CA and the CG The NAV420CA will measure rotations around the axes of its sensors The NAV420CA sensors are aligned with the NAV420CA case The mounting holes of the NAV420CA case are used as a reference for aligning the NAV420CA sensor axes with your system You should align the NAV420CA case as closely as possible with the axes you define in your system Errors in alignment will contribute directly to errors in measured acceleration and rotation relative to your system axes The NAV420CA should be isolated from large vibration if possible NAV420CA performance is tested to 2G random vibration from 20 Hz to 2 kHz Larger vibration will make the accelerometer readings noisy and can therefore affect the angle calculations In addition if the magnitude of the vibration exceeds the range of the accelerometer the accelerometer output can saturate This can cause er
28. he sum by hex 10000 3 The remainder should equal the checksum The packet also contains the packet type field and the BIT word output Refer to section 3 7 for details about the BIT word processing Table 5 NAV420CA Series Data Packet Format 5 1 Scaled Sensor Mbde Packet Bytes Description Range Units 0 1 Header 0x5555 2 SG 3 4 X Axis Acceleration 10 10 G 5 6 Y Axis Acceleration 10 10 G 7 8 Z Axis Acceleration 10 10 G 9 10 Roll Angular Rate 630 630 deg sec 11 12 Pitch Angular Rate 630 630 deg sec 13 14 Yaw Angular Rate 630 630 deg sec Doc 7430 0121 01 Preliminary Page 23 Crossb w NAV420CA Series User s Manual 15 16 X Axis Magnetic Field 1 1 Gauss 17 18 Y Axis Magnetic Field 1 1 Gauss 19 20 Z Axis Magnetic Field 1 1 Gauss 21 22 X Axis Temperature 100 100 Le 23 24 Y Axis Temperature 100 100 oc 25 26 Z Axis Temperature 100 100 1G 27 28 CPU Board Temperature 100 100 oC 29 30 GPS ITOW last 2 bytes 0 65536 msec 31 32 BIT 33 34 Checksum 5 2AHRS Mode Packet 0 1 Header 0x5555 2 A 3 4 Roll Angle 180 180 deg 5 6 Pitch Angle 180 180 deg 78 Heading Angle mag north 180 180 deg 9 10 Roll Angular Rate 630 630 deg sec 11 12 Pitch Angular Rate 630 630 deg sec 13 14 Yaw Angular Rate 630
29. hm 00 GPS up full accuracy NAV Accuracy 01 low accuracy NAV high accuracy AHRS 02 low accuracy AHRS 03 AHRS initialization 3 8 Commands The NAV420CA has a simple command packet structure You send a command to the NAV420CA over the RS 232 interface and the NAV420CA will execute the command All communications to and from the unit are packets that start with a single word alternating bit header 0x5555 This is the ASCII string UU All communications packets except for the ping command and response end with a single word checksum The checksum is calculated in the following manner 1 Sum all packet contents except header and checksum 2 Divide the sum by 0x10000 3 The remainder should equal the checksum NAV VIEW is a very good tool to use when troubleshooting your device NAV VIEW formulates the proper command structures and sends them over the RS 232 interface You can use NavView to verify that the NAV420CA is functioning correctly NavView does not use any commands that are not listed here 3 8 1 Input Packets All communications sent to the unit except for the ping command are input packets with the following format 2 byte command variable length data 2 byte checksum gt This generalized input structure allows input commands to carry data for advanced user interaction All input packets can be no longer than 128 bytes All 2 byte input commands consist of a pair of ASCII characters A
30. ilter attitude correction approach achieves improved performance due to its ability to estimate the attitude errors and gyro bias states The advantage with this approach is that an absolute attitude error estimate is provided to the trajectory to correct any errors due to physical noise disturbances and gyro errors as well as a characterization and tracking of the gyro biases which in effect provides an online rate sensor calibration The filter model is an Extended Kalman Filter formulation made up of two components a linearized attitude error and gyro bias state model and a nonlinear attitude quaternion error measurement model The state model predicts where the attitude errors and gyro bias states will Doc 7430 0121 01 Preliminary Page 9 Crossb w NAV420CA Series User s Manual propagate based on input data from the gyros and the measurement model corrects this prediction with the real world attitude error measurements obtained from the accelerometer gravity and magnetometer earth magnetic field reference This balance of state modeling with real world observables gives the Kalman filter the adaptive intelligence to assign appropriate confidence levels on its two components The NAV420CA blends GPS magnetometer and accelerometer tilt measurements into the EKF update depending on the health and status of the associated sensors The NAV420CA uses the GPS receiver s tangent frame velocity estimate as a direct measurement update for th
31. information The roll pitch yaw and angular rate values are calculated as described in the Scaled Sensor and AHRS modes The Kalman filter operates in NAV mode when GPS is up to track the rate sensor bias accelerometer bias NED velocities and stabilized roll pitch and yaw angles In NAV mode the NAV420CA uses the angular rate sensors to integrate over your rotational motion and find the actual pitch roll and yaw angles The NAV420CA uses the accelerometers indirectly to correct for rate Page 16 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w sensor drift in the vertical angles pitch and roll the NAV420CA uses the magnetometers to correct for rate sensor drift in the yaw angle the NAV420CA uses the GPS velocity to correct the NED velocity trajectory accelerometer bias rate sensor drift and attitude The NAV420CA uses a sophisticated Kalman filter algorithm to track the bias in the rate sensors and accelerometers This allows the NAV420CA to use a very low effective weighting on the Kalman filter measurements from GPS and magnetometers when the NAV420CA is moved This makes the NAV420CA very accurate in extended dynamic and static maneuvers when GPS is up The NAV420CA outputs GPS Longitude Latitude Altitude and NED velocity in the NAV digital data packet when GPS is up The GPS Longitude Latitude is a 4 byte data and to convert it into degrees lon lat data SCALE 2 where lon lat is the actual Longi
32. lerometer rate gyro and magnetometer sensor bias shifts over temperature 40 C to 71 C are compensated and verified using a thermal chamber and rate table 3 5 Connections The NAV420CA has a male DB 15 connector The signals are as shown in Table 1 15 Pin D Connector Male Pinout 0 0 60 6 Table 1 Connector Pin Assignments Pin Signal RS 232 Transmit Data RS 232 Receive Data Positive Power Input Vcc Power Ground Chassis Ground NC Factory use only RS 232 GPS Tx RS 232 GPS Rx Signal Ground c jo jo o m o nmj Docit 7430 0121 01 Preliminary Page 11 Crossb w NAV420CA Series User s Manual 10 1 PPS OUT 11 NC factory use only 12 NC factory use only 13 NC factory use only 14 NC factory use only 15 NC factory use only 3 5 1 VO Cable The user must provide a shielded cable with the shield connected to the I O connector shell in order to provide the required EMI protection The cable sent with the unit is intended to provide the user with the ability to test the unit right out of the box and will not provide adequate shielding for all environments Case ground see below must be used to provide full EMI protection 3 5 2 Power Input and Power Input Ground Power is applied to the NAV420CA on pins 3 and 4 Pin 4 is ground Pin 3 should have 9 to 30 V
33. lows the user to change the measurement mode If you want change the packet type only temporarily use SF instead of WF in the command packet above The available Value options are corresponding power up modes are listed below Value Measurement Mode Ss Scaled A Angle N NAV Change Packet Output Rate UU WForSF 0x01 0x0001 Value Checksum None This command allows the user to change the packet output rate If you want change the packet type only temporarily use SF instead of WF in the command packet above The available Value options are corresponding power up modes are listed below Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w Value Output Rate 0x00 Quiet 0x01 100 Hz 0x02 50 Hz 0x04 25 Hz 0x05 20 Hz 0x10 10 Hz 0x50 2 Hz When the output rate is set to Quiet the unit goes into Polled mode 3 9 Data Packet Format In general the digital data representing each measurement is sent as a 16 bit number two bytes except for GPS latitude longitude The GPS latitude longitude is sent as 32 bit number four bytes The data generally represents a quantity that can be positive or negative Each data packet will begin with a two byte header hex 55 55 and end with a two byte checksum The checksum is calculated in the following manner 1 Sum all packet contents except header and checksum 2 Divide t
34. magnetic material near the unit The NAV420CA should be mounted as close to the center of gravity CG of your system as possible 5 2 Alignment The NAV420CA must successfully complete a hard iron and soft iron calibration to reach full accuracy Refer to Appendix C of this manual for detailed instructions 5 3 Operation in Magnetic Environment Introduction of large ferrous or magnetic material objects close to the NAV420CA after alignment will affect the heading performance Maintain at least 24 inches of distance between moving ferrous metal or magnetic material and the NAV420CA 5 4 Range Limitations The internal sensors in the NAV420CA are limited to maneuvers of less than 200 deg sec and less than 10 Gs acceleration in bank pitch and heading Over range of a sensor is indicated in the data packet BIT message Over range will start a new initialization cycle of the NAV420CA and will require 60 seconds of straight and level condition to reinitialize the NAV420CA The NAV420CA like all magnetometer and magnetic compass based systems will not perform properly at the magnetic North and South Poles The NAV420CA will not operate properly in low gravitational fields and magnetic fields encountered during space flight Doc 7430 0121 01 Preliminary Page 29 Crossb w NAV420CA Series User s Manual 6 Appendix A Mechanical Specifications 6 1 NAV420CA Outline Drawing roll pitch Connector side View z 3
35. on valuable digital interface information including data packet formats and conversion factors Doc 7430 0121 01 Preliminary Page 3 Crossb w NAV420CA Series User s Manual 2 Quick Start 2 1 NAV VIEW Software Crossbow includes NAV VIEW software to allow you to use the NAV420CA right out of the box and the evaluation is straightforward Install the NAV VIEW software connect the NAV420CA to your serial port apply power to your unit and start taking measurements 2 1 1 NAV VIEW Computer Requirements The following are minimum capabilities that your computer should have to run NAV VIEW successfully e CPU Pentium class e RAM Memory 64MB minimum 128MB recommended e Hard Drive Free Memory 17MB e Operating System Windows 98 NT4 2000 XP 2 1 2 Install NAV VIEW To install NAV VIEW in your computer 1 Insert the CD Support Tools in the CD ROM drive 2 Findthe NAV VIEW folder Double click on the setup file 3 Follow the setup wizard instructions You will install NAV VIEW and a LabVIEW Runtime Engine You will need both these applications If you have any problems or questions you may contact Crossbow directly 2 2 Connections The NAV420CA is shipped with a cable to connect the NAV420CA to a PC Serial port 1 Connect the 15 pin end of the digital signal cable to the port on the NAV420CA 2 Connect the 9 pin end of the cable to the serial port of your computer 3 The additional black and red wires on th
36. ption GGA Global positioning system fixed data GLL Geographic position latitude longitude GSA GNSS DOP and active satellites GSV GNSS satellites in view RMC Recommended minimum specific GNSS data VTG Course over ground and ground speed 3 5 6 1 PPS Output Interface The NAV420 synchronizes to GPS 1PPS internally The 100Hz navigation algorithm is synchronized to the GPS 1PPS when available Packet data is therefore valid on 10ms boundaries of UTC The 1PPS is also output on the 1PPS OUT Pin 10 This is open collector output The Figure 3 shows the sequential order of the signal present at 1 PPS OUT pin The 1 PPS signal is aligned to the sampling clock of 23 104 MHz This results in a timing resolution of 43 ns GPS 8 30 00 GPS 8 30 01 TIMEPULSE 1000 ms M Figure 3 1PPS Output Signal 3 5 7 GPS Antenna Connection A GPS needs to receive signals from as many satellites as possible A GPS receiver doesn t work properly in narrow streets and underground parking lots or if objects or human beings cover the antenna Poor visibility may Docit 7430 0121 01 Preliminary Page 13 Crossb w NAV420CA Series User s Manual result in position drift or a prolonged Time To First Fix TTFF A good sky visibility is therefore a prerequisite Even the best receiver can t make up for signal loss due to a poor antenna in band jamming or a bad RF board design The NAV420CA unit ships with an external active antenna that must b
37. r facing you and the mounting plate down the axes are defined as x X axis from face with connector through the NAV420CA C Y axis along the face with connector from left to pitch right Z axis along the face with the connector from top to bottom Figure 2 NAV420CA Coordinate System The axes form an orthogonal right handed coordinate system An acceleration is positive when it is oriented towards the positive side of the coordinate axis For example with the NAV420CA sitting on a level table it will measure zero g along the x and y axes and 1 g along the z axis Gravitational acceleration is directed downward and thus will be defined as negative for the NAV420CA z axis The position output from GPS is represented in Latitude Longitude and Altitude LLA convention This is a most commonly used spherical co Doc 7430 0121 01 Preliminary Page 7 Crossb w NAV420CA Series User s Manual ordinate system The GPS velocity is defined in North East and Down reference frame The users can convert this into Cartesian coordinate system called Earth Centered Earth Fixed ECEF ECEF uses three dimensional XYZ coordinates in meters to describe the location of a GPS user or satellite Several online resources are available to help users with this transformation For example refer to the application note on Crossbow website http www xbow com Support appnotes htm The angular rate sensors are aligned with the
38. recise understanding of the internal timing of the device The processor internal to the NAV420CA runs in a time triggered loop collecting data from the sensors processing the data and then producing the output packet The data is streamed to the user through a parallel process on the serial port If GPS 1PPS is available the internal time triggered loop is synchronized to the IPPS second boundary Therefore data packets contain information valid at 10ms boundaries of the GPS UTC time pulse The unit goes through three processes in one data cycle First the sensors are down sampled from 1kHz to 100Hz Second the unit processes the data for output using the calibration and navigation algorithms After processing the data the NAV420CA will buffer the serial data Third the unit actually transfers the data out over the RS 232 port at the selected baud rate while processing continues GPS ITOW International Time of Week is available in selected output packets The internal GPS is operating at 4Hz and thus ITOW is updated at that rate Only the lower two bytes of GPS ITOW is available due to baud rate limitations Doc 7430 0121 01 Preliminary Page 25 Crossb w NAV420CA Series User s Manual 3 11 Temperature Sensor The NAV420CA has onboard temperature sensors The temperature sensors are used to monitor the internal temperature of the NAV420CA to allow for temperature calibration of the sensors The temperature sensor is specified to
39. rors in the accelerometer output The NAV420CA should be isolated from magnetic material as much as possible Magnetic material will distort the magnetic field near the NAV420CA which will greatly affect its accuracy as a heading sensor Because the NAV420CA is using Earth s weak magnetic field to measure heading even small amounts of magnetic material near the sensor can have large effects on the heading measurement Bad materials include anything that will stick to a magnet iron carbon steel some stainless steels nickel and cobalt Use a magnet to test materials that will be near the NAV420CA If you discover something near the NAV420CA that is magnetic attempt to replace it with something made from a non magnetic material If you cannot change the material move it as far as possible from the NAV420CA Even small things such as screws and washers can have a negative effect on the NAV420CA performance if they are close NAV420CA can correct for the effect of these magnetic fields by using hard and soft iron calibration routine as explained in Appendix C Good materials include brass plastic titanium wood aluminum and some stainless steels Again if in doubt try to stick a magnet on the material If the magnet doesn t stick you are using a good material Doc 7430 0121 01 Preliminary Page 27 Crossb w NAV420CA Series User s Manual DO NOT try to stick a magnet to the NAV420CA We have removed as much magnetic material as
40. s a semantic aid consider the following single character acronyms P packet F fields these are settings or data contained in the unit R read pertains to default non volatile fields G get pertains to current fields or settings W write pertains to default non volatile fields Doc 7430 0121 01 Preliminary Page 19 Crossb w NAV420CA Series User s Manual S set pertains to current fields or settings G and S refer to current fields Modifying current fields with S take effect immediately and are lost on a power cycle R and W refer to default power up fields These fields are stored in non volatile memory and determine the unit s behavior on power up Modifying default fields take effect on the next power up and thereafter There are 4 user input commands PK GP WF and SF NOTE The NAV420CA commands are case sensitive 3 8 2 Output Packets All communications received from the unit except for the ping response are output packets with the following format UU lt 1 byte packet type variable length data 2 byte checksum gt All packet types will be single printable ASCII characters All output packets can be no longer than 128 bytes There are 5 output packet types P D S A and N The P is response type packet which is sent in response to Ping request The remaining packets are available using the get packet command polling or can be configured for continuous fixed rate output 3 8
41. s used for informing the user of the system status and informing the user of system problems The following information is supplied in the BIT byte fields of the data packet The table 4 contains the actual bit definition present in the two byte output BIT field in the angle mode data packet see section 3 9 below Table 4 Bit Message Definition Bit BIT Data Description 0 Reserved 1 Reserved 2 Reserved 3 Turn 0 Yaw rate magnitude lt 0 4 deg sec Detect 1 Unit is executing a turn 4 Comm 0 No serial port transmit communication failure Transmit has been detected Error 1 A serial port transmit communications failure has been detected such as overrun parity 5 Reserved 6 GPS Status 0 GPS 3D solution is valid 1 GPS unlocked or data packet not present Reserved Algorithm 0 30 sec initialization complete Initialization 1 Not ready waiting for power up and initialization completion 9 PPS Signal 0 GPS 1 PPS signal locked Lock 1 GPS 1 PPS signal not locked 10 Reserved 11 MagAlign 0 Magnetometer hardiron softiron alignment Valid valid 1 Magnetometer hardiron softiron alignment invalid 12 User Port 0 No user port receive communication failure Comm has been detected Page 18 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w Receive Error A user port receive communications failure has been detected 13 Reserved 14 15 Algorit
42. se same axes The rate sensors measure angular rotation rate around a given axis The rate measurements are labeled by the appropriate axis The direction of a positive rotation is defined by the right hand rule With the thumb of your right hand pointing along the axis in a positive direction your fingers curl around in the positive rotation direction For example if the NAV420CA is sitting on a level surface and you rotate it clockwise on that surface this will be a positive rotation around the z axis The x and y axis rate sensors would measure zero angular rates and the z axis sensor would measure a positive angular rate The magnetic sensors are aligned with the same axes definitions and sign as the linear accelerometers For example when oriented towards magnetic North you will read approximately 0 25 Gauss along X 0 0 Gauss along Y and 40 35 Gauss along Z direction North America Pitch is defined positive for a positive rotation around the y axis pitch up Roll is defined as positive for a positive rotation around the x axis roll right Yaw is defined as positive for a positive rotation around the z axis turn right The angles are defined as standard Euler angles using a 3 2 1 system To rotate from the body frame to an earth level frame roll first then pitch and then yaw 3 3 Attitude and Heading Determination Algorithm The attitude and heading determination algorithm is divided into two separate entities Gyro meas
43. the list of information provided to request the RMA number Place the equipment in the original shipping container s making sure there is adequate packing around all sides of the equipment If the original shipping containers were discarded use heavy boxes with adequate padding and protection 9 3 8 Sealing the Container Seal the shipping container s with heavy tape or metal bands strong enough to handle the weight of the equipment and the container 9 3 4 Marking Please write the words FRAGILE DELICATE INSTRUMENT in several places on the outside of the shipping container s In all correspondence please refer to the equipment by the model number the serial number and the RMA number 9 3 5 Return Shipping Address Use the following address for all returned products Crossbow Technology Inc 41 Daggett Drive San Jose CA 95134 Attn RMA Number XXXXXX 9 4 Warranty The Crossbow product warranty is one year from date of shipment Doc 7430 0121 01 Preliminary Page 37 Crossb w Crossbow Technology Inc 41 Daggett Drive San Jose CA 95134 Phone 408 965 3300 Fax 408 324 4840 Email info xbow com Website www xbow com
44. ttitude tracking performance will degrade and the filter will become susceptible to false gravity acceleration errors typical of the AHRS only EKF formulation The UTC packet synchronization will drift due to internal clock drift 3 8 3 Attitude and Heading Initialization Immediately after power up the NAV420CA uses the accelerometers and magnetometers to compute the initial angles roll pitch and yaw The roll and pitch attitude will be initialized using the accelerometer s reference of gravity and yaw will be initialized using the leveled magnetometers X and Page 10 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w Y axis reference of the earth s magnetic field For this same reason the NAV420CA needs to be held motionless for 60 sec upon power up for the initialization to complete 3 4 Factory Sensor Calibration A calibration procedure performed at the factory will provide correction parameters for the following static and dynamic errors for each sensor The software will then apply these parameters to each sensor to provide a correction for the errors e Rate sensors are calibrated for temperature bias scale factor and misalignment e Accelerometers are calibrated for temperature bias scale factor and misalignment e Magnetometers are calibrated for bias and scale factor Sensor errors are compensated for these effects using a proprietary algorithm from data collected during manufacturing Acce
45. tude Latitude data is the signed integer data output in the data packet and SCALE is a constant SCALE 180 to convert into degrees The GPS Altitude is a 2 byte data and to convert it into meters If data lt 400 alt unsigned data SCALE 2 Else alt signed data SCALEy2 where alt is the actual Altitude data is the signed or unsigned integer data output in the data packet and SCALE is a constant SCALE 16384 to convert into meters Note in Table 5 3 that the range on altitude is not symmetric about zero Thus a range check must be performed to interpret the altitude data correctly The velocity is a 2 byte data and to convert it into m sec vel data SCALE 2 where vel is the actual velocity measured in m sec data is the signed integer data output in the data packet and SCALE is a constant SCALE 256 m sec If the GPS signal is lost or poor the velocity is computed using free integration of accelerometer outputs the accuracy of which will degrade if the GPS signal is lost for extended periods of time The last known good GPS position is sent in the data packet while the system believes there is a GPS outage The yaw angle output will continue with reference to true north using the last known magnetic declination angle Doc 7430 0121 01 Preliminary Page 17 Crossb w 3 7 BIT Processing NAV420CA Series User s Manual The BIT message in each packet provides comprehensive information into system health BIT i
46. tude and heading reference systems in the DMU family It has a sophisticated mechanical and electrical design and provides stable roll pitch and heading measurements under high dynamic conditions Furthermore the device provides a self tuning system that automatically compensates for bias in all three gyros and accelerometers The system can generate accurate attitude and heading data based on measurements obtained from commercially available MEMS performance sensors The Doc 7430 0121 01 Preliminary Page 1 Crossb w NAV420CA Series User s Manual sensors are factory calibrated on Crossbow test equipment including calibrated ovens and rate tables The sensor and system calibration parameters and temperature compensation curves are written to non volatile memory in the system The NAV420CA has an RS 232 serial link Data may be requested via the serial link as a single polled measurement or may be streamed continuously The NAV420CA uses angular rate sensors and linear acceleration sensors that are bulk machined MEMS devices Solid state MEMS sensors make the NAV420CA both responsive and reliable The magnetic sensors are state of the art miniature fluxgate sensors Fluxgate sensors make the NAV420CA sensitive and responsive with better temperature performance than other technologies such as magneto resistive sensors When combined together with the built in GPS receiver it provides a complete navigation solution The NAV420CA utilizes a
47. ured angular rate information is integrated in an attitude trajectory propagation model and attitude errors and gyro biases are estimated in a Kalman Filter Attitude and Heading Estimation Model Both gyros and accelerometers suffer from bias drift misalignment errors acceleration errors g sensitivity nonlinearity square terms and scale factor errors The magnetometers are also susceptible to magnetic disturbances which corrupt their measurement of the earth magnetic field These errors typically known as hardiron and softiron effects are calibrated out once the system is installed in its final mounting position The largest error in attitude and heading propagation is associated with the gyro bias terms The Kalman filter attitude estimation component provides an on the Page 8 Doc 7430 0121 01 Preliminary NAV420CA Series User s Manual Crossb w fly calibration for the gyros by providing corrections to the attitude processor trajectory and a characterization of the gyro bias state The accelerometers provide an attitude reference using gravity and the magnetometers provide a heading reference using the earth s magnetic field vector The NAV420 runs a 7 state AHRS type EKF when GPS is down and a 13 state reduced navigation filter loosely coupled when GPS is up The quality of the NAV420 s low cost MEMS inertial sensors makes double integration for position tracking of marginal value and thus only velocities are tracked in the filt
48. ver time and operating environments The NAV420CA consists of the following subsystems 1 Inertial Sensor Array This is an assembly of three accelerometers three gyros rate sensors and four temperature sensors 2 A three axis fluxgate magnetometer board used to compute heading 3 AWAAS capable GPS receiver for position and velocity measurement 4 A digital processing PCB which receives the signals from the inertial sensors and magnetometers This unit converts these signals to digital data filters the data computes the attitude solution monitors and processes all BIT data and transmits the results to the user These blocks are shown in the system block diagram below in Figure 1 Digital Outputs X Y Z Acceleration Roll Pitch Yaw Rate X Y Z Magnetic Field X Y Z High Speed Gyros Sampling amp DSP L L L L MEMS Roll Pitch Yaw Angle T Position Velocity n X YIZ Sensor n Built In Test Accelerometers Compensation E L MEMS Full State L XIVIZ Kalman Filter a m Magnetometers a n L Flux Gate GPS Antenna L LJ n L L L L L L a Temperature GPS Receiver Power Power Input Conditioning 9 TO 30 VDC a a Figure 1 NAV420CA System Architecture The NAV420 analog sensor set is converted to digital data at 1 kHz The sensor data is filtered and down sampled by a DSP using FIR filters The Page 6 Doc 7430 0121 01 Preliminary NAV420CA
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