Force controlled contour following on unknown objects with an industrial robot

Abstract

This paper deals with controller features which improve force controlled contour following. This kind of robot force control may by used in surface finishing tasks like polishing, deburring or grinding. The already introduced proportional controller with positive position feedback brought very good results in force control of a position controlled robot in both impact and contact phase. If the characteristics of the environment which should be finished with the robot tool are not constant, unfavorable contact forces may occur which can damage the tool or the workpiece. For the purpose of adapting the current inclination angle between robot end-effector and environment we investigate the insertion of an additional integrator into the force controller. Thereby, it will be possible to reduce static control errors. However, more attention should be paid during its parameterization with respect to the stability boundary of the closed loop control. Another novel feature in this paper is the variation of the end-effector velocity as a function of the force control error. With this idea force peaks can be decreased or the loss of contact between robot end-effector and environment can be avoided, e.g. when the inclination angle of the environment changes. All algorithms proposed in this paper are successfully verified by practical experiments.