Pololu 3pi robot
From Hamsterworks Wiki!
For Christmas one day my wife got me this robot.
The 3pi is a two wheeled + one caster robot. It's about the size of a drink coaster, and powered by 4 AAA batteries. The only sensors are 5 reflective ones on the front edge - perfect for line following and maze solving.
There are fitted with either AMTEL's ATmega168 or ATmega368P micro-controller - the later serial numbers have the mega368 part. Sadly mine is a ATmega368, so once the all the libraries are loaded their isn't much room left.
The standard tool set is WinAVR or AVR Studio using the libraries available at http://www.pololu.com/docs/0J20, but using the instructions at http://www.pololu.com/docs/0J17/3 you can make it into an Arduino compatible.
Top speed is around 2m per second. And it has enough torque to have a bit of wheelspin when moving from a dead stop.
One really great feature is a DC-DC converter to take the 5V from the batteries and step it up to 9V to drive the motors. Because of this you get constant speed no matter what state of charge the battery is.
I've done some testing with a small bit of code that runs the motors at a given speed for one or two seconds, and then measuring the distance. pretty much:
set_motors(x,x) delay_ms(1000); set_motors(0,0);
Speed was very linear, with a pretty good rule of thumb being:
speed = (x - 40) * 2
Because of this is very easy to repeatable accuracy in distance traveled to within a few percent without rotary encoders - just don't accelerate too fast that your wheels slip.
I've also tried to navigate by dead reckoning after self-calibrating, but it is very hard to get reliable performance due to difference in slack and play in gears when calibrating and actually going somewhere. I could most probably model this better with a lot of thinking about pertinent factors.
The robot is great. but my Orangutan programmmer isn't that good. You have to jump through hoops to get it to work with Windows 7 as there is no signed driver available. A much better option would be the Pololu USB programmer (http://www.pololu.com/catalog/product/1300)