Robot Modeling and Control, 2nd Edition

CHAPTER 10FORCE CONTROL

In previous chapters we considered the problem of tracking motion trajectories using a variety of elementary and advanced control methods. These position control schemes are adequate for tasks such as materials transfer, spray painting, or spot welding where the manipulator is not interacting significantly with objects in the workplace (hereafter referred to as the environment). However, tasks such as assembly, grinding, and deburring, which involve extensive contact with the environment, are often better handled by controlling the forces¹ of interaction between the manipulator and the environment rather than simply controlling the position of the end effector. For example, consider an application where the manipulator is required to wash a window, or to write with a felt tip marker. In both cases a pure position control scheme is unlikely to work. Slight deviations of the end effector from a planned trajectory would cause the manipulator either to lose contact with the surface or to press too strongly on the surface. For a highly rigid structure such as a robot, a slight position error could lead to extremely large forces of interaction with disastrous consequences (broken window, smashed pen, damaged end effector, etc.). The above applications are typical in that they involve both force control and trajectory control. In the window washing application, for example, one clearly needs to control the forces normal to the plane of the window and position ...