Modeling, observation, and control of hysteresis torsion in elastic robot joints

Hysteresis torsion in elastic robot joints occurs as a coupled nonlinearity due to internal friction, backlash, and nonlinear stiffness, which are coactive inside of mechanical transmission assemblies. The nonlinear joint torsion leads to hysteresis lost motion and can provoke control errors in relation to the joint output at both trajectories tracking and positioning. In this paper, a novel modeling approach for describing the nonlinear input–output behavior of elastic robot joints is proposed together with the observation and control method, which aim to compensate for the relative joint torsion without load sensing. The proposed modeling approach includes the recently developed 2SEP dynamic friction model and Bouc–Wen-like hysteresis model, which is originated from structural mechanics, both arranged according to the assumed torque transmitting structure. The proposed method is evaluated with experiments using the laboratory setup which emulates a single rotary joint under impact of nonlinear elasticities, friction, and gravity.