  Nonholonomic wheeled mobile robots are often required to implement algorithms
designed for holonomic kinematic systems. This creates a velocity tracking
problem for the actual wheeled mobile robot. In this paper, we investigate the
issue of tracking the desired velocity in the least amount of time, for a
differential drive nonholonomic wheeled mobile robot. If the desired velocity
is a constant, the Pontryagin Maximum Principle can be used to design a
control. A control is designed for the cases when the wheel can be commanded
speeds and torques. When the desired velocity is smoothly time-varying, we
propose a hybrid structure and study its properties.
