The authors report on the stability characteristics of force-controlled manipulators with a discrete-time implementation. For a rigid arm, three conclusions are derived. Firstly, the system has linear damping, the amount of damping plays a key role in stability. Typically, a rise in contact stiffness can make damping insufficient, causing instability even in the case of a low feedback gain and sampling frequency well above the Nyquist frequency. Secondly, the role of Coulomb friction and stiction in closed-loop stability is clarified. For the case in which Coulomb friction is the dominant source of damping, a sufficient condition for maintaining stability is given. Thirdly, low-pass filtering can eliminate the instability caused by an insufficient sampling frequency, provided there is linear or nonlinear damping in the system. Experiments on a single-degree-of-freedom arm confirmed the conclusions.>

Most force control algorithms become unstable in case of stiff contact between the end-effector and the environment. To cope with this problem, the authors introduce active damping based on force sensor data. Linear active damping is introduced using the force derivative which is obtained either by means of filtered differentiation or by means of a state-space estimator, and both enable stable stiff contact. Nonlinear active damping through a feedforward method is introduced. The novel algorithm is simple and does not require knowledge of the contact stiffness. It enables satisfactory force response which is fast and without overshoot in the case of a high feedback gain with stiff contact. This algorithm has better potential for rejecting positional disturbances than many current algorithms.>