The role of damping and low pass filtering in the stability of discrete time implemented robot force control

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