Input-dependent stability of joint torque control of tendon-driven robot hands

The input-dependent stability observed during torque control experiments using the first joint of the Darmstadt-HAND is discussed. Friction and compliance existing in tendon-sheath drive systems introduce a hysteresis nonlinearity between the joint torque output and the actuator displacement. Although this transmission characteristic is close to the well-known backlash behavior of the gears situated between a motor and a load shift, this hysteresis loop exhibits input-dependent characteristics in the backlash region of the transmission system, with springlike behavior within a portion of the backlash region. Experiments confirmed that there is a close relationship between the input-dependent backlash characteristics and the input-dependent stability. Based on these experiments, the authors describe the transmission characteristic using a simple model and explore the system stability using sinusoidal-input-describing-functions (SIDF). A nondimensional stability-criterion-map that successfully predicts the experimental results is presented