POSITION AND CONSTRAINT FORCE CONTROL OF A VEHICLE WITH 2 OR MORE STEERABLE DRIVE WHEELS

Since a vehicle with two or more steerable drive wheels is always trav eling in a circle about an instantaneous center of rotation, the motio n of the wheels is constrained. The wheel translational velocity divid ed by the radius to the center of rotation must be the same for all wh eels. When the drive wheels are controlled independently using positio n control, the motion of the wheels may violate the constraints and th e wheels may slip. Consequently, substantial errors can occur in the p osition and orientation of the vehicle. A vehicle with N steerable dri ve wheels has N holonomic constraints on the steering angles, (N - 1) nonholonomic constraints on the wheel velocities, and one degree of fr eedom. We have developed a new approach to the control of a vehicle wi th N steerable drive wheels. The novel aspect of our approach is the i ntroduction of variables to control the constraint forces. To control the vehicle, we have one variable to control motion and (N - 1) variab les that can control the constraint forces to reduce errors. Recently, Kankaanranta and Koivo developed a control architecture that allows t he control variables for force and position to be decoupled. In the wo rk of Kankaanranta and Koivo the control variables for force are an ex ogenous input. We have made the control variables for force endogenous by defining them in terms of the errors in satisfying the nonholonomi c constraints. We have applied the control architecture to the HERMIES -III robot and have measured a dramatic reduction in error (more than a factor of 20) compared to motions without constraint force control.