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in the "Secret Sauce"
Winning Strategies at the DARPA Grand Challenge
by Mark Helmlinger
All of the sensor systems can produce "false positives", seeing objects that are not really there. There are even "negative obstacles," like the canyon in Thelma and Louise! Figuring out which sensor to believe over another, how to integrate all of the inputs to produce a map of the road ahead in memory, and then how to choose the right path to the next waypoint, those are the real problems. The control software at this point is so complicated that only practice can tell if the right choices are being made by the bot. There are variables in the code that can be adjusted to influence those choices, but the bots had an amazing capacity to surprise their creators with unplanned behavior -- especially the losers.
This is where the rubber meets the road. And where the servo meets the steering wheel! Some teams, like Team DAD, had it down so smooth, you'd swear someone was actually behind the wheel. Other teams, like the Golem Group, had lightning reactions, charging up to obstacles and swerving at the absolute last moment. Still others kind of meandered around. All teams had to be sure not to get into an overcorrection loop, where each action requires an opposite reaction. That's when a bot can shake itself to death or bounce like a pinball from guardrail to guardrail.
Many teams could not reverse. TerraMax sure could! One of the reasons it took 12+ hours to complete the course was that it in spite of its steerable rear axles, it often made K-turns around sharp corners, backing up many times for one turn. Of course what was impressive was that it knew when to turn like that and successfully did so for TWO DAYS.
Up until the National Qualifying Event held a week before the final race, Carnegie-Mellon was the team to beat. Stanford handily demonstrated its winning strategy at the NQE, and it was then that it was evident there would be a real horse race in the Mojave. Also at the NQE, after making it past many narrow obstacles with (relative) grace and surprising agility (that 16-ton monster can stop on a dime!) no one doubted that TerraMax would (eventually) finish. The real dark horse was Team Gray.
The vehicle for Team Gray was a Ford Escape Hybrid. It's most interesting and visible feature was a huge table of solar panels mounted on the top. At about 300W of capacity, they were used as the only way to charge the 24V battery system that powered the LADAR. Computing power came from a main microprocessor that was shock-mounted and designed to run on 12V. The team soon learned that the hybrid drivetrain has almost no starting torque. During trials, the Escape could not pull its chase vehicle out of the mud, but the team hauling on a rope did. The Escape also once came to a stop in sand, piled up a couple of inches in front of each wheel, and could not get out. Couldn't even spin the wheels!
Nonetheless, the Escape had enough of the right stuff to finish the race. (This may say more about the course than the vehicle.) In many ways, the Gray's strategy represented a minimum approach (and certainly the smallest budget). Only the right technology and software was brought to bear on the problem. Team Gray just might have a thing or two to show the others.
Will there be a race next year? It's unlikely. DARPA got what it wanted, the technology to build autonomous vehicles for use in combat situations. Considering the difficulty of the course, these bots are ready for duty in Iraq, but not a place like Afghanistan. Maybe next year, someone will organize an Extreme Challenge
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October 16, 2005