Force measurement time-delays and contact instability phenomenon

In this paper we consider stability issues of manipulators in contact with a rigid environment, i.e. submitted to a unilateral constraint Of the form F(q) greater than or equal to 0, where q is the vector of generalised coord inates. The interaction force feedback loop contains time-delays which may induce instability phenomena. Sufficient delay-dependent conditions are der ived tc, guarantee that the robot's tip remains in contact with the surface . These conditions are found hy taking into account the fact that, due to t he unilateral nature of the constraint, the intervention force must have co nstant sign during the whole task. We analyse the cases of proportional and proportional-integral force feedback.