Impact of hysteresis lost motion on the sensorless torsion control of elastic robotic joints

The sensorless torsion control of elastic robotic joints, which has been developed in the previous works, allows one to compensate for the joint load positioning errors without applying auxiliary joint-output sensing. In this paper we investigate and evaluate experimentally the impact of hysteresis lost motion on the performance of sensorless torsion control. For that purpose, the hysteretic torsion-torque relationship, detected during the quasi-static measurements, is simplifying approximated by the static map of nonlinear stiffness without losses. Taken into consideration two boundary operation states, i.e. with maximal and minimal steady-state loads, and that due to the gravity, we show for which cases a non-accounting for hysteresis lost motion becomes significant. The experimental evaluation is accomplished on a stand-alone joint testbed with gear elasticities.