Mobile Base Description

The mobile base on Brak is designed as a synchronous drive system. This design was chosen to give the flexibility of non-holonomic drive systems, but is generally regarded as being more complex to build than turtle, tricycle, or Ackerman steering ("car-like") designs. In the synchronous drive system all wheels are used in steering and driving the robot. Translational motion is obtained by driving all wheels at the same rate and rotational motion is by steering all wheels at the same rate (hence everything is synchronized). Most synchronous drive systems use three wheels to eliminate the need for a suspension system (all wheels will always be in contact with the ground, or else you've already fallen over). The complexity of this system is making sure that all wheels turn and steer at the same rate. More on this later.

If you have a browser with VRML capability, try the VRML model of Brak we're developing.

As mentioned, the synchronous drive system allows the robot to perform holonomic motions. In simplest terms this means it's easy for BRAK to parallel park. The mobile base is capable of turning in place so it can follow paths consisting of straight line segments meeting at any angle. Of course it has to come to a complete stop before turning to do this, but nonetheless it can do it. In addition, the choice of a circular platform means that Brak's "width" is the same regardless of which direction it's moving.

The downside of the synchronous drive system (besides its complexity) is the platform doesn't have a "front" or "back" or "side": these labels are relative to the direction the base is steering at a given time. This means that any control of sensors on the platform which are "robo-centric" need to compensate for the steering orientation. For example, a camera which is supposed to observe the path in front of the robot must be able to pan (rotate around an axis parallel to the steering axes) to match the steering orientation.