Input-dependent stability on joint torque control of robot hand

Input-dependent stability was observed during torque control experiments using the first joint of the Darmstadt-HAND. The friction and compliance existing in tendon-sheath driving systems bring a hysteresis characteristic into the dependence of joint torque output on actuator displacement. While this transmission characteristic is close to the well-known backlash behavior appearing for the gears situated between a motor and load shaft, input-dependent characteristics were found to exist in the backlash region of the transmission system in the case of the use of a practical tendon length. The observed characteristic shows springlike behavior even in the backlash region and it also depends on input. Through experiments, it was confirmed that there are close relationships between the input-dependent backlash characteristics and the input-dependent stability. Based on the experiments, a description is given of the transmission characteristic in simple model equations, and the system stability is explored by using the sinusoidal-input-describing-function technique. A nondimensional stability-criterion map explaining the experimental results is shown