Hierarchical Tracking Control of Car-Like Mobile Robots

A hierarchical tracking controller for a car-like mobile robot subject to nonholonomic constraints is designed in this paper. The special structure of the derived equations of motion makes it possible to separate the design into three levels: motion planning, kinematic, and dynamic. In the proposed scheme, a fuzzy inference engine in the kinematic level is used to change the desired trajectory computed from the motion planning level. A sliding mode controller is adopted to track the new reference values of privileged coordinates in the dynamic level, which subsequently drives the non-privileged coordinates to the desired values. From the simulation results, it is shown that such hierarchical tracking control which simultaneously takes kinematics and dynamics into consideration indeed effectively solve the tracking problem. All the variables can be steered to their desired values asymptotically, which is assured by the skew-symmetric property of the reduced Appell equation.