Tuesday, March 22, 2011

AVC Bot: Dead Reckoning

Eeek!  Only 30 days left before race day!!

You may be wondering what happened. Why Data Bus crashed repeatedly.

I've a high level of confidence that GPS error and, to a lesser degree, heading error were the causes. As I continue to collect more and more test run data, GPS plots show pretty substantial position errors, even when the GPS has a very good fix (HDOP of 1.2 or less).

I've been planning all along to supplement GPS fix information with dead reckoning computations and I've finally gotten the software bugs worked out. Here are the plots I collected last night showing errors in GPS and dead reckoning position estimation.

The Plots

In each test run I'm starting at the exact same geographic location: the intersection of a concrete seam and the asphalt at the end of my driveway. I've plotted the point in Google Earth, and I've initialized the dead reckoning code with those coordinates as starting points.

In general, all the dead reckoning plots have the right shape, but seem to get the distances a bit wrong and they appear rotated counter-clockwise by several degrees. The GPS plots at best have the right shape but are off by around 50' in various directions. At worst, the fix data is nearly unusuable.

I'm taking distance and heading measurements every 200ms and the GPS sends fix information just as often. But the GPS data is obviously much noisier, hence the jagged lines as in the plot above and below.

So what does it all mean?  

  • At slower speeds, the wheel encoders effectively have less resolution, increasing error
  • Distance calculation error is propagating throughout the run
  • The compass appears to have an offset of several degrees
  • The GPS may be updating too quickly to get accurate results for the slow speeds traveled
  • Google Earth undoubtedly has some positional error
And what do I do about it?

I could optimize the tradeoff between calculation frequency and encoder resolution. The main distance problem is likely wheel circumference error propagating. More on long paths than short ones.

Last night I drove the robot in a straight line for a known distance and captured the total number of encoder ticks. The original circumference measurement was 3% too large.

I'll calibrate the compass with the onboard calibration and using a compass. I can calibrate it to a known straight feature in Google Earth, too.

To improve GPS readings, I'll try setting the update rate to 1Hz and see if that improves accuracy. I'll reinstall the iGPS-500 and gather data for both units and compare.

Google Earth introduces some error that I'll have to address next.

Beyond Individual Sensors

Ultimately there's a limit to each sensor's accuracy and precision and to calibration methods. Using statistical methods to fuse sensor input should improve accuracy.

I really have no idea if it is possible to get down to sub-meter accuracy navigating a 240m path while driving around barrels and colliding with other robots. I hope so. I better find out pretty darn quickly.

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