Robust digital friction compensation

Abstract Even though the nonlinear compensator proposed by Southward, et al. can guarantee the closed-loop stability for stick-slip friction systems in continuous-time control, applying this to digital control systems may often generate a limit cycle response in the vicinity of a reference position because of the inherent time delay of sample-and-hold operation. In this paper, a new robust digital friction compensator (RDFC) is proposed, which consists of a digital P controller+hysteresis friction compensator part (position feedback loop) and an analog D controller part (velocity feedback loop). The hysteresis compensator in the position feedback loop increases the phase lead in comparison with Southward’s technique such that the phase lag coming from the time delay of digital control can be compensated for. A modified form of Lyapunov’s stability theorem is employed to verify the asymptotic stability of the RDFC. Stability and control effectiveness are verified analytically and experimentally on a single-axis robot system.