  Flexible joint robots, in particularly those which are equipped with
harmonic-drive gears, can feature elasticities with hysteresis. Under heavy
loads and large joint torques the hysteresis lost motion can lead to
significant errors of tracking and positioning of the 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 signals available only on the motor side of joint transmissions. 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
dynamics and inverse hysteresis map. The second approach relies on the modeling
of motor drives and inverse hysteresis and uses the generalized momenta when
predicting the joint torsion. Both methods are discussed in details along with
a numerical example of two-link planar manipulator under gravity.
