SparkFun AVC build log 2015-06-14

It’s less than a week until the SparkFun AVC, so I figured it’s probably time for a quick update. There have been a lot of things I still need to get working (such as a physical start button), which is why I haven’t been writing much, but I figured this would be a good break.

Progress

Since my post a few months ago, I’ve gotten a few things checked off my to do list. Here’s what I had written, with things that are completed marked off:

  • Critical
    • Control the car from the onboard computer
    • Complete sensor data processing
    • Complete basic navigation to waypoints
    • Add a physical start button
  • Nice to have
    • Include basic collision detection and recovery
    • Use a Kalman filter for position estimation
    • Add a remote kill switch
    • Implement obstacle avoidance
    • Consider the shortcut, non-GPS option, and ramp and hoop
    • Estimate speed from accelerating from a standstill
    • Look at faster motors and other upgrades
    • Get remote monitoring working

Reading data from the iSUP800F module was a bit tricky. It’s modal in that it either emits NMEA (i.e. GPS messages), or binary messages of a proprietary binary format. The binary format contains raw magnetometer, accelerometer, temperature, and pressure readings. I wanted to use the raw magnetometer readings rather than relying on it converting these measurements into compass readings (more on that later), so my code needed to constantly switch the mode on the device.

Collision detection is very basic. I had wanted to include accelerometer readings to detect a collision, but for now, if the reported GPS speed is 0 for more than 2 seconds, then a collision is reported and the car backs up, turns, and tries again.

I had wanted to do some basic image processing to avoid the barrels, go under the hoop, and go over the jump, but one of my camera mounts ended up snapping after I ran into a curb during a test run. I still have the camera on board for recording a first person view of runs, but I don’t think it’s secured well enough to do any kind of avoidance.

Compass

In years past, a lot of people seemed to have compass problems, myself included. A lot of people (again, including myself) tended to turn left immediately and drive into the hay bails. From looking at my logs, it looked like my car was picking up some interference, because it thought it was facing 90 degrees clockwise of where it actually was.

I saw similar problems when I was testing this year. Debugging this problem was aided immensely by remote monitoring. I was able to reproduce this problem and found it occurred consistently when the car would drive over certain parts of my test runs. The compass would visibly deflect as the car drove past a point and then return back after the car had put some distance between itself and the point. It appeared that there was something in the ground that was interfering with the magnetometer.

I started logging the raw magnetometer readings and found that when there was significant deflection, the magnitude of the readings increased dramatically. I figured that I could monitor these magnitudes and drop readings when they are far away from the expected magnitude. I added some code to monitor the mean and standard deviation of the magnitude readings while I was calibrating the hard iron offsets of the compass; then I added code to drop readings if they were more than 2 standard deviations from the mean. This helped, but a lot of readings were being dropped, even though they appeared legitimate, i.e. the heading looked correct. Pushing that up to 4 standard deviations seemed to provide a better balance.

Demo

Part of the requirements for entering the AVC is providing a demo video or pictures a few weeks before the competition to prove that you have some progress. Here was mine:

The car mostly works and is following waypoints! I still need to do more testing, investigate some better waypoint following techniques, and get a physical button wired up.

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