Saturday, March 16, 2013

WCRS Robotgames - 2013

I don't know how I missed it, since I've been checking all the time, but the 2013 Robotgames have been announced on the WCRS site!

I've been doing a fair amount of work to understand how best to get a whole 'schwack of Atmel processors working together, mostly using an SPI bus for heavy lifting, and I2C for talking to individual sensors and really tiny custom sensor PCBs. Most of that has been targeted for the Dagu Rover 5 platform, mostly because it's small enough to run around the house, and because it's a low cost / low barrier to entry for trying out ideas.

But the Rover 5 has one big limit - it's small! Too small for the WCRS Off-Road challenge, I think. And it's tracked, with 4 motors (instead of two), so I actually dedicated an entire AT328 to keeping the motor/encoders running at the same speed to stop it from throwing it's tracks. But that's only half a solution, because any small gravel or twigs that get jammed between the tracks and wheels can cause trouble. It's does drive dead-straight, however :)

So it's time for a bigger build! But what form should it take?

I started down the path of designing a tracked robot, but problems with keeping the tracks clear aside, even finding tracks big enough are a problem. And the ground clearance for tracked vehicles is low - it's great for tanks - the low profile makes them harder to shoot - but it's not the greatest if you want to cross rough terrain.

There are other designs that have more ground clearance; big wheels, for example, but really big wheels take an enormous amount of torque to turn, and there are limits to motor size + battery weight that can fit within the size rules for the off-road race and stay stable.

How about the JPL/NASA designs for 6 wheeled rovers using the rocker-bogie suspension? Those have amazing ground clearance and stability. But they are kinda ... slow.

Really, their limit is more based on a very tight budget for electricity and a huge desire to not 'overdrive' their ability to safely command the rovers. They use small, highly geared motors which move the rovers at just the right speed that the autonomous systems can keep them out of trouble. After all, they do cost a lot of taxpayer money, and it would be bad to put a dent in the fender of the family car. Or drive off a cliff.

But here on earth, for a short race, and a tough little rover, could faster speeds be used?

Kind of. The rocker-bogie design isn't well suited to 'dynamic' vehicle behaviors. By 'dynamic' I mean moving fast enough that you can bounce around, get some air over a jump, etc. So by moving their rovers slowly, NASA keeps the dynamic portion of the vehicles suspension system to a minimum. However, a  faster rover could be slowed when it's time to climb over a rock, instead of running into it full speed.

Here is a good video of testing the design in the JPL Mars Yard:

Could they move faster? On some of the long drives of the MER rovers Spirit and Opportunity I bet they wished they could. The ground was certainly flat enough and free enough of rocks - if they had a bigger power budget I think they could have used a slightly different motor gearing and really put the pedal to the metal.

I'm betting with a little cleverness to slow the rover before a large obstacle is mounted, I could really crank out the speed through the flats, so the 6 wheel design might work in a race scenario.

Now for the details - in the next post ... ;)

Here are some additional links and videos:

Pathfinder / Sojourner This little guy really captured my attention when it landed. It outperformed all expectations, but was finally let down by the lander, which seems to have failed first and cut off communications with Sojourner, who may have driven around until it also failed, trying to call home.

Spirit and Opportunity  Bigger than Sojourner, the MER rovers are going into the history books along with the Voyager probes as well exceeding their design lifetimes; too bad Spirit got stuck. I hope they can re-establish contact with Opportunity after the 2013 solar conjunction, when the Sun gets between Earth and Mars.  Here is document describing the suspension and considerations in a bit more detail.

MSL / Curiosity Even bigger still, this rover didn't even need a lander, although it must have set a record for a crazy-complicated EDL (Entry, Decent, and Landing) system. Check this out:

Here is a longer video with more of the rover and surface operations.

And if you aren't already, you just have to follow @Sarcasticrover on twitter! And the author is a fellow Calgarian, so that's bonus points!

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