Differentials

By using a differentials you provide power to two wheels with one motor, and allow a certain independance of movement between the two wheels. For instance, if one wheel is stopped then the other one can continue to rotate.

This behaviour is different from two wheels connected via a simple shaft, as in this case the movement of the twoo wheels are fixed to each other.

In terms of roboteering, a differential (also known as a diff) allows us to power and steer our robot in a new, and possibly more nimble, way. As far as I know, Firestorm is the only robot to have done this so far. For links to this robot see this photograph and this movie

Now for an explanation of differentials, courtesy of an old Lego differential kit

Both wheels moving at the same speed

This movie shows the differential ring gear driving both axles with no impediment. The pinion gear is stationary

Left wheel locked

Because the left wheel is locked the pinion gear moves and transfers the movement from the left axle to the right axle, which moves at twice the speed.

Wheels moving in opposite directions

Now both wheels are spinning in opposite directions, the ring gear is stationary, and the pinion gear is transferring all its movement to both axles equally.

If a motor were attached to the pinion gear, or to one of the axles (which is how Firestorm did it) then the robot would rotate. This would be an alternative way of powering a robot.