34. Motion Control of Wheeled Mobile Robots

This chapter may be seen as a follow up to Chap. 17, devoted to the classification and modeling of basic wheeled mobile robot (WMR) structures, and a natural complement to Chap. 35, which surveys motion planning methods for WMRs. A typical output of these methods is a feasible (or admissible) reference state trajectory for a given mobile robot, and a question which then arises is how to make the physical mobile robot track this reference trajectory via the control of the actuators with which the vehicle is equipped. The object of the present chapter is to bring elements of the answer to this question based on simple and effective control strategies. A first approach would consist in applying open-loop steering control laws like those developed in Chap. 35. However, it is well known that this type of control is not robust to modeling errors (the sources of which are numerous) and that it cannot guarantee that the mobile robot will move along the desired trajectory as planned. This is why the methods here presented are based on feedback control. Their implementation supposes that one is able to measure the variables involved in the control loop (typically the position and orientation of the mobile robot with respect to either a fixed frame or a path that the vehicle should follow). Throughout this chapter we will assume that these measurements are available continuously in time and that they are not corrupted by noise. In a general manner, robustness considerations will not be discussed in detail, one reason being that, beyond imposed space limitations, a large part of the presented approaches are based on linear control theory. The feedback control laws then inherit the strong robustness properties associated with stable linear systems. Results can also be subsequently refined by using complementary, eventually more elaborate, automatic control techniques.