Dynamic Stability Issues in Force Control of Manipulators

Robots in force control mode often become unstable during contact with stiff environments, due mainly to the high gain nature of wrist force-sensor feedback. Three methods are presented for achieving stable force control compliant coverings or soft sensors, sell-tuning of force gains after estimation of environmental impedance, and reliance on fast open-loop joint torque control and using tip force sensor feedback in a slow loop to maintain accuracy. The latter method is proposed as the best one. Dynamic stability is analysed with a simple model of the robot and its environment. The analyses are verified by single-link experiments on the MIT Serial Link Direct Drive Arm.