The cause of kinematic instability in hybrid position/force control: contact compliance

The instability of Raibert and Craig´s (1981) hybrid position/force control, sometimes called `kinematic instability´ in the literature, has been the subject of numerous research articles. Although this control was proposed for robots in contact with rigid kinematic constraints, stability analyses have either neglected the constraint or modelled it as a spring. In this paper, we prove that Raibert and Craig´s hybrid control is never unstable when applied to rigid constraints. We therefore conclude that the cause of the previously reported `kinematic instabilities´ was the assumed compliance (or, in the case of some studies, absence) of the constraint