Parts and materials:
  • double-sided foam-core tape, known to RC car racers as "servo tape";
  • four hobby servos (Cirrus CS20, for example);
  • four wheels, Lego for example;
  • sheet aluminum, somewhere around 1/32" thick
  • hot melt glue
  • your chassis, which can be:
    • a RC airplane receiver and battery,
    • your favorite microcontroller board(?),
    • whatever

Construction:

  • Modify the servos for continuous rotation. See Jones and Flynn, or almost any hobby robotics web page for instructions on this.
    (If you use the Cirrus hobby servos, I found a piece of aluminum sheet with a notch in it to be very useful in pulling the gear off the potentiometer shaft. Removing the potentiometer doesn't work very well, so you should drill out the hole slightly in the outdrive gear so it rotates freely on the potentiometer shaft. I hot-melted the potentiometer blade to keep it centered. You can use an RC receiver and transmitter to center it. Mine does drift a bit. Maybe I should have disconnected the pot and substituted a pair of resistors. If you try that let me know how it works and whether you had room for the resistors.)
  • Carefully center each Lego wheel on a servo wheel and hot melt them together. (If you find a source of wheels that fit the standard servo splined shaft, please let me know. You could use the wheels supplied with the servos, but they're small and slippery.)
  • Servo tape the four servos to the chassis.
  • Now you will notice two problems with the suspension of the robot: (1) with the weight on the wheels, they tend to splay out; and (2) the weight will not be evenly distributed among the wheels. This is where the aluminum sheet comes in. Cut a piece for the front two servos and a second piece for the rear two servos. Each piece is taped to the bottom of the servos, so each piece should be almost as wide as the chassis, and as long as a hobby servo. Tape it to the bottom of the servos. Now they will not splay out.
  • To fix the second problem... My robot has a very narrow wheel base, so that the servos are tight together. When I taped the front two servos to the chassis, I only put servo tape at the center 1/4 inch or so. With the aluminum holding them together at the bottom, the two servos rock back and forth as a unit. Now the weight will be evenly distributed. If your robot has a wider wheelbase this will not work, so I suggest you use a third piece of aluminum between the servos and the chassis. If it shorts your PC board and catches fire, send me a picture. (I wonder if servo tape is conductive?)
  • Now you have a mobipulator! Oh yeah, if you want it to move, you will have to connect the cables to the chassis, arrange for power, write code if you used a microcontroller, and I hope your chassis is capable of driving 4 PWM outputs.