SparkFun AVC 2015 recap

Another year, another AVC. This was my second year competing, and although I had a lot more time to prepare, things still didn’t go very well. I figured I’d give a recap of the things that went well, the things that didn’t, and plans for next year.

Last-minute modifications

SparkFun allowed teams to do some practice runs the day before the competition. I attended, and the main thing that I discovered was that the fence surrounding the course was about 5 cm off the ground, and my vehicle had a propensity to get itself wedged underneath whenever it ran into it. The suspension on the Grasshopper lets the wheels kind of bow inward while moving the chassis down; this caused the tires to move just enough to get wedged underneath or pushed all the way to the other side of the fence, causing the vehicle to get stuck. I didn’t have any padding or other bumpers on the front, and this ended most of my runs. I ended up going to Solid State Depot afterwards and attaching a clear plastic bump plate to prevent the vehicle from getting wedged again. I zip tied it to the front bumper of the Grasshopper so that it was sloped up and forward.

The morning of the competition, they allowed some more practice runs. The bump plate worked well in preventing the vehicle from getting wedged, but my zip ties snapped almost immediately. I ended up duct taping the plate to the bumper instead. Based on some more testing, I also fiddled with my collision recovery algorithm. Unfortunately, I didn’t have time to test this…


So! On to my first heat. My vehicle took off the line and then turned slightly to the left and into the fence. It ended up stopping there, and due to a bug in my last-minute code changes, it never tried to back up and try again. Another competitor made a good suggestion: drive the vehicle straight for a few seconds in order to get a better GPS heading, and then switch to the normal navigation algorithm. This should hopefully avoid other competitors and other navigation problems.

The second heat revealed a problem with my bump plate: because it was mounted at an angle, it had the tendency to drive over and on top of smaller obstacles. Another vehicle drove in front of me right at the start and hit the curb, and my vehicle ended up running into them and getting high-centered on top of them. I think I also damaged their vehicle… Sorry!

My third run ended similarly. The car appeared to be navigating well but was driving very closely along the fence. The fence itself was weighted down by some sandbags and my car made it over one, but got high-centered on the second.

Practice runs

There was some good news in the midst of all of this though. In between heats, we were allowed to do some test runs, and I did collect some interesting data. And, there were even some successful runs!


Here’s what the course looked like. I’ve highlighted the starting line in blue; the course starts travelling northwest and then makes 4 right turns to return to the starting line. Each red dot represents a single GPS reading. The first problem that I saw was that the GPS readings started a little to the south of the starting line, but eventually corrected. My vehicle initially drove into the curb at the start (because it thought that it needed to drive further north) but detected a collision and successfully recovered.

The next weird thing I saw was a jump in the GPS data that I highlighted in green. At first, I thought this was due to incorrect readings from the GPS module, but after looking at the logs, the module just failed to switch into NMEA mode. The module I used is modal, and you need to switch between having it report magnetometer readings and NMEA messages. For whatever reason, this switching failed, but my car used dead reckoning to interpolate the gaps, so there was no problem.


The next jump occurred as I was nearing the second turn. The GPS started reporting coordinates about 200 meters south, so my car tried to drive north and ended up running into the fence. The GPS jumped a few times after that, first to the west of the course, then to the inside of the course, and finally near to the fence perimeter, which is close to where my vehicle was. I’m not too sure how to deal with these discrepancies. One idea that was suggested my a member of Solid State Depot is to ignore data after it jumps; this would work for a while and I could fill in the gaps using dead reckoning, but how do you recover from this error? If the GPS readings jump back but are 5 meters off from your estimate, is the estimate wrong, or is the GPS wrong? I haven’t come up with a good plan, so I decided to scrap this idea. Another idea is to define the fence perimeter around the course and ignore readings outside of it. This would help with obviously bad readings, but the readings near the start and end of my runs were close to the vehicle’s position but outside of the fence, so this strategy would discard some somewhat useful data. Maybe I could fudge readings that are just outside of the fence to appear as just inside the fence and provide the Kalman filter with a confidence inversely proportional to the distance outside of the fence.


My second test run looked a lot better. The car didn’t run into anything and made it around the course in about 1:15. Unfortunately it stopped right before the finish line and claimed that it had reached all of its waypoints. I’m not sure why it stopped there, so I set up for a third test run.


This test run also went well: no collisions, the GPS data looked sound, and vehicle made it around in a similar time. However, like last time, it stopped before the finish line. For later runs, I just added more waypoints past the first turn, which worked well in further testing.


Here are both test runs overlaid each other in white, along with the path that I was trying to follow in yellow. I’m surprised with how closely they follow each other, especially at the start and between the 2nd and 3rd turns. I think the vehicle was turning too wide past some waypoints and ended up overcompensating as it approached the next waypoint.

Crowd pleaser!

This year, I dressed my vehicle up as a blue shell from Mario Kart, and dressed myself as Luigi. I was inspired after watching this video. This was a lot of fun and seemed to get a good reaction from the crowd. My family also dressed up! At the end of the day, I won the crowd pleaser award. Yippee!


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