Compensation of Nonlinear Torsion in Flexible Joint Robots: Comparison of Two Approaches

Flexible joint robots, in particularly those equipped with harmonic drive gears, can feature torsional elasticities with hysteresis. Under heavy loads and large joint torques the hysteresis lost motion can lead to the control errors when reference tracking and positioning of robotic links. In this paper, two approaches for compensating the nonlinear joint torsion with hysteresis are described and compared with each other. Both methods assume the measured states only available on the motor side of the joint transmission. The first approach assumes a rigid-link manipulator model and transforms the desired link trajectory into that of the motor drives by using the inverse of dynamics and hysteresis map. This is inspired by former works on robotic manipulators with linear joint elasticities and extended to the general nonlinear case. The second approach, introduced previously and revised here with further analysis, relies on the modeling of motor drives and inverse hysteresis map and uses generalized momenta for the torque prediction. Both methods are discussed in detail along with numerical example of the two-link planar manipulator under gravity.