Our inexpensive Truckbot robots have been using tiny $3 micro servos that get modified for continuous rotation. We do this by removing some plastic that prevents the gears from going all the way around and then bypassing the potentiometer inside.
The process took us about 30 minutes the first time but it only takes 5-10 minutes once you’ve gone through it once. You will need several tools and be comfortable soldering small connections. A couple of 250k resistors is also needed to replace what the 5k potentiometer was doing. It used the current position of the servo gear to create a different level of resistance in the circuit. We just want to fake it as always being centered so any command to be in a position other than center will result in forward or backward rotation.
We haven’t found any micro servos for sale that have already been modified, but Fry’s often had larger ones available for under $10. Pololu also carries them in case you’re already ordering from them and want to save on shipping. These don’t fit in the current Truckbot body and use too much power to be run directly from the Arduino’s 5v source. We”re exploring some larger robots and would love to hear about your projects using servos in our discussion group.
For a detailed descriptions of how this works and what steps to take, we recommend checking out Tod’s great article on tobbot.com, which has tons of great details on what to do. We’ve also taken pictures of the servo modification process and put them in a gallery with captions to provide you with additional details.