Terminal sliding-mode based force tracking control of piezoelectric actuators for variable physical damping system

The need for safe human-robot interaction in emerging robotic applications has recently driven the development of new range of actuation systems spanning from variable stiffness drives to variable damping or full variable impedance joints. Concerning the provision of variable physical damping in compliant actuators, the main objective is to improve the control of compliant joint. In a particular class of these variable physical damping actuators (VPDAs), the level of generated damping is realized by creating a friction force which is actively controlled by piezoelectric actuators (PEAs). Therefore, to effectively control the damping output, the accurate force control of the PEAs is required. However, difficulties to precisely control the generated piezoelectric force stem from its highly nonlinear behavior such as hysteresis and creep effect. This paper presents a novel practical force tracking controller for PEAs with unknown hysteresis behavior. The proposed control consists of two elements: terminal sliding-mode based desired dynamics injection and model-free compensation for nonlinear dynamics of PEAs, which allow fast convergence and extraordinary robustness to the closed-loop system. The stability of the overall system is proved in the sense of Lyapunov. On the real prototype of VPDA, the proposed control scheme is experimentally verified and analyzed by comparison to other controllers, demonstrating improved force tracking performance of PEAs in VPDA system.