After a half way successful entry in the AVC 2014 competition I finally have the time to write a few words.
The rover is build upon a rather cheap RC car, that however is a very reasonable platform for the project, the Turnigy 1/16 Monster Beetle.
The core computing platform is a Stellaris Launchpad with RobG's excellent LCD booster pack, which doubles for me as LCD and microSD unit.
There is a GY-87 IMU, which consists of a MPU6050 Accel/Gyro and a HMC5883L Magnetometer, plus a BMP180 Bario (which I did not use, regrettably). The SW on the LM4F120 is able to read the MPU6050 at 1Khz and the HMC5883L at 75Hz (which is more than 50% of the I2C bus bandwidth).
There is also a uBlox6 based GPS uint, the VK16U6 (in case you are wondering). It' operates at 4Hz update rate. Why 4Hz ? The ublox6 has an advertised max update rate of 5Hz. However at 5Hz SBAS/WAAS is not working, at least for me, and not with 12 satellites in perfect view, plus either PRN133 or PRN137 (if I recall correctly). Going back to 4Hz solved that ;-)
There is a 13 state EKF on board that is run at 250Hz (in the pre-race configuation). The path follower is a simple "Pure Pursuit", with position input, and using the magnetometer for heading estimation. The steering is done via rather simple poportional control derived from the steering error, rather than a PID ... mainly because I ran out of time programming/tuning the PID, and because it turned out to be good enough.
The vehicle was tested at 20Mph speed and finetuned for that upper boundary. The contollering mechanism did not try to adjust the speed other than an initial gradual ramp up to top speed, and the breaking at the end. Experiments showed that flipping the vehicle in gradual turns was not an issue, and that the slippage compensated for the high speed good enough.
In the AVC 2014 race config we dialed the speed back to 12Mph and 15Mph respectively. We did one run using the EKF, and the subsequent runs using GPS only (with forward predicition).
The downside was a mechanical failure which causes us to fail on one of the 3 rounds. He an image as to what the underlying issue was:
The vehicle jumps, but still tries to navigate in mid-air. In one of those jumps the front left C-Bracket got nuked, as the front wheels were pointing all the way left ...
Perhaps next year, at AVC 2015 there are more entrants using a TI Tiva ;-)
