IMU / GPS Positioning System

1. IMU / GPS Positioning System Center for Intelligent Machines and Robotics Rommel E. Mandapat

2. Project Goals • Develop a low cost positioning system by integrating a Honeywell Inertial Measurement Unit with Novatel Differential GPS • Implement the IMU/GPS system on the Navigation Test Vehicle using MAX architecture • The system performance of the IMU/GPS will be evaluated against that of the existing MAPS/GPS

3. Motivation & Approach • For navigation of autonomous ground vehicle, must have reliable real-time positioning system capable of outputting data at over 10 Hz • Inertial Navigation Systems (INS) provide high data rates but drift over time • DGPS provides high-accuracy position data but at lower data rates (1-5 Hz). Also, GPS is susceptible to data loss. • Integration of INS & GPS through an external Kalman Filter will result in high-accuracy position, velocity and attitude data at high rates

4. System Features • Honeywell HG1700AG11 IMU • Novatel RT-20 differential GPS (DGPS) • PC-104 / Single Board computer platform • Primary and Secondary Kalman Filter (KF) • MAX message set • Data w/ 20 cm positional accuracy at 10 Hz

5. Honeywell HG1700AG11 IMU • Strapdown inertial system • Three Ring Laser Gyros (RLG) measure angular changes about roll, pitch and yaw axes • Three single-axis Accelerometers measure velocity changes in the vehicle x, y, and z directions • Inertial message output at 100 Hz (,, , Vx, Vy, Vz) • 1 MHz output clock rate

6. Novatel RT-20 differential GPS • 20-cm CEP real-time kinematic accuracy in differential mode (RT-20) • “on-the-fly” initialization • 5 Hz RT-20 position maximum output rate • L1 C/A code • 12 channel “all in view” parallel tracking • RS-232 connection

7. System Set-up INS - to replace MAPS Honeywell HG1700AG11 Lat Lon Alt    Vn Ve Vd SBC    Vx Vy Vz PC104 Primary Time Lat Lon Alt    Vn Ve Vd IMU 20 Hz 100 Hz KF Align/Calib Secondary To MCU 10-15 Hz Nav Sol KF Primary CPU Novatel RT-20 Secondary CPU Time Lat Lon Alt GPS KF 1-5 Hz

8. Loosely Integrated System • KF-driving-KF set-up • Advantages • low-level modularity (use different inertial sensors) • ability to directly compare systems by mix-matching components (MAPS/Ashtech vs IMU/Novatel) • faster development time • Disadvantages • more difficult to tune both filters since one affects the other • need more computing power • more expensive

9. Inertial Navigation System (INS) • Inputs • raw inertial data from IMU • initial conditions for alignment (initial position from secondary CPU, initial accelerations from IMU) • Processes • calibration (correctly interpret output of sensors) • alignment (initial position, velocity and attitude [PVA]) • IMU data reception & averaging (from 100 Hz to 20 Hz) • navigation solution (current PVA at 20 Hz) • primary KF (smoothing through data loss) • Outputs • current PVA at 20 Hz in Local Geodetic Vehicle (LGV) coordinates

10. Primary CPU Software Setup To PROCMAN of Secondary CPU 20 Hz Using DOS and DMA HOSTCOM 20 Hz Primary KF 20 Hz Alignment Nav Sol 20 Hz Calibration IMU Com

11. Primary CPU Software Setup Using Lynx OS and data polling To PROCMAN of Secondary CPU 20 Hz HOSTCOM PROCMAN 600 Hz 20 Hz 20 Hz IMU Com Primary KF Calibration Align Nav Sol

12. IMU Data Processing • ACB-104 serial communications card set-up to read in SDLC messages on RS-422 interface line • Use Direct Memory Access (DMA) to read in data at 1 MHz (1 Mbps) • DMA driver running in DOS environment • DMA - During a DMA cycle, the DMA controller chip is driving the system bus, transferring data to/from memory from/to an I/O device, bypassing the CPU, thus freeing it up for other functions • IMU data will be averaged from 100 Hz to 20 Hz, in order to lessen processing power needed to output navigation solution and KF corrected PVA data

13. Navigation Solution • IMU input data (delta velocities and delta angles) are in vehicle body coordinates • Input data also includes general and local gravity components • Earth angular rotation rate is considered • WGS84 reference ellipsoid used to model earth • Inertial navigation is affected by Schuler frequency (time required for one revolution around a circular orbit a constant distance from the center of a spherical earth) • navigation solution computes present PVA data from past values and converts them to LGV coordinates • navigation solution becomes model for primary KF

14. Inertial Navigation

15. IMU/GPS Integration • Inputs • current PVA at 20 Hz from INS • current position (LGV) and time from DGPS at 1 Hz • Processes • secondary KF performs smoothing through GPS loss • secondary KF also performs error bounding by constantly updating navigation solution with accurate GPS position data, thus eliminating IMU drift and need for Zero Velocity Update (Zupt) • Outputs • ships out POS messages to MCU at 10-15 Hz

16. HOSTCOM PROCMAN 20 Hz 10-15 Hz 1-5 Hz INS Com GPS Com Secondary KF Secondary CPU Software Setup To MCU 10-15 Hz

17. Extended Kalman Filter • Estimator / Predictor • Evaluates a estimated trajectory close to the actual trajectory using updated system variables • Define an State Error Vector • velocity errors • position errors • attitude errors • accelerometer & gyro calibration coeff • gravity • Propagate incremental and total state vector across each integration step

18. IMU PC104 Novatel RT-20 Freewave radio DC-DC converter IMU/GPS POS shelf

19. MAPS/GPS POS shelf PC104 Ashtech Z-12 GPS receiver Freewave radio Honeywell MAPS

20. NTV GPS Antenna Set-up

21. GPS Base Station Setup

22. IMU/GPS vs MAPS/GPS • Translate IMU/GPS position to MAPS/GPS reference point on ATV (mule) • Test under ideal conditions • solid GPS satellite lock and radio link at all times • autonomous survey from 1-2 hours • Test under adverse conditions • temporarily withhold GPS data at specific areas on the field • completely withhold GPS data • manually introduce erroneous GPS data • Position data will be compared against post-processed data from Ashtech receiver

23. Area A * arrows indicatdirection of sweep Typical Comparison Survey

24. Typical INS (MAPS) Drift

25. GPS Position Convergence

26. System Comparison

27. Project Schedule Aug Sep Oct Nov Dec Jan Feb Mar IMU Comm Nav Sol / Align Primary KF GPS Integration Testing Final Report To date finished Projected completion

28. Thank You