Stabilityofhybridposition/forcecontrol Appliedtomanipulatorswithflexiblejoints

Hybrid position/force control enables a robot to apply specified forces to a constraint while moving along a prescribed path. Flexibility of the motor transmissions (joint flexibility) is a major concern in the design of hybrid controls, particularly with regard to system stability. While much work on hybrid control has been toward the development of advanced schemes specifically for flexible-joint robots, little is known about how joint flexibility affects the stability of simple well-established hybrid controls. This paper examines the stability of the most basic hybrid control, which requires no robot dynamic model. For both rigid and compliant constraints, we prove that the closed-loop system is always asymptotically stable. Yet we also show that joint flexibility can destabilize a well-known model-based control. We then propose a control that achieves asymptotic tracking when applied to rigid-joint manipulators and is asymptotically stable when applied to flexible-joint manipulators. Experiments on a constrained flexible-joint robot support the key theoretical results.