Control of tendon-driven robotic mechanisms by non-linear springs with hysteresis characteristics

Tasks of robot manipulators are expanding from repetitive tasks in nonhuman environment such as factories to various tasks in human environment such as hospitals and offices. Safety and variety of tasks are necessary for manipulators in human environment, which are acquired by adjustment of joint stiffness. There are tendon mechanisms with nonlinear springs as a joint stiffness adjustment mechanism. SAT (Stiffness Adjustable Tendon) has some advantages such as light and cheap among nonlinear springs. On the other hand, conventional controllers based on a characteristics equation of general nonlinear springs do not realize precise tension control due to hysteresis characteristics of SAT. Therefore, a controller suitable for tendon mechanisms with nonlinear springs whose characteristic equations are not available is realized by adding tension error feedback control with disturbance observer and reaction force observer. Moreover, effectiveness of the proposed method is confirmed by simulations.