It's a pretty simple hardware design. Each wheel is powered by its own motor. A platform for the lead-acid batteries. Aplatform for the motor control circuitry. A platform for control and sensor circuitry. And sensor platform that looks like it might pass for a head.
I needed this simple design to test a GPS and infrared tracking idea I came up with. The idea is a robot that can follow a beacon. GPS is only good up to a few meters, which is great for outdoors and large distances, but not real useful for closer, indoor purposes. For close up and indoors, the robot can follow an infrared LED. A person could wear a wrist band or something containing a second GPS device which would broadcast its coordinates for the robot. My theory is that the robot would try to minimize the difference between the two GPS readings by navigation, thereby following the user. For closer functioning, the robot would use a combination of infrared beacon (to follow the user), and ultrasonic range finder (to stay within a minimum distance of the user).