Force control of direct-drive manipulators for surface following

The surface following by a robot arm is investigated in this paper. The goal is to present control strategies so that the arm can follow a surface at a given speed with desired contact force. First, a discussion on the use of direct-drive manipulators with decoupled and invariant dynamics is given. These are shown to be suitable for such force control applications. In this case, the controller design in either joint space or end effector space would involve nonlinearities due to kinematics only. This would reduce the computation burden and allows for high performance since the kinematic parameters can be estimated with better accuracies than dynamic parameters. Second, to have a better understanding of some fundamental characteristics of force control, such as the effects of actuator dynamics, environment/sensor stiffness on the performance, experiments were done using a one degree-of-freedom direct-drive arm. The use of a high gain inner velocity loop is shown to give the force control good command following and disturbance rejection characteristics. The direct-drive manipulator shows a superior performance over conventional systems as a result of fast actuator dynamics, low friction and no mechanical backlash. The experiments are conducted to investigate the force speed of response and steady state behavior, impact control and surface following performance.