Abstract
This paper discusses input-dependent stability on torque servo system of robot hands. With an insufficient space to mount actuators inside the finger joints, a finger joint is normally actuated by a tendon-sheath driving system. Although this driving system enables us to construct a finger system with simple mechanism and large flexibility, friction and compliance existing in tendon-sheath driving system generally bring a nonlinear characteristic involving a hysteresis into the dependence of joint torque output from actuator displacement. While this transmission characteristic is close to the well-known backlash behaviour appearing for the gears system, the author newly found input-dependent characteristics existing in the backlash region of the transmission system through the precise experiments. Namely, the characteristic is that the tendon-sheath transmission system behaves as if there were virtual springs in the backlash region and the strength of virtual spring depending on the input. Furthermore, it was confirmed that there are close relationships between the input-dependent backlash characteristics and the input-dependent stability. Based on the experiments, the transmission characteristics are modelled in simple equations, and the system stability is explored by using the techniques of sinusoidal-input-describing-function (SIDF) . Resultantly, we obtained non-dimensional stability-criterion-maps explaining the experimental results successfully.
