To guarantee flexibility and safety of robots is important for contact interaction with external environment and force transmission among internal actuation systems. Compliance control has become the key issue for robotic systems (e.g., exoskeletons) to achieve such characteristics. Usually, stiffness of the force/torque controller determines the compliance quality during the force transmission from the actuator to the joint. In order to obtain configurable stiffness in compliance control in exoskeletons, designing tracking control strategies under variable (time-varying) stiffness are urgently demanded. In this paper, we present a time-varying stiffness control method for the knee exoskeleton. This approach can control the joint of the knee exoskeleton with time-varying stiffness in the actuation level, which may enhance compliance quality of the knee exoskeleton more safe. Simulation results validate the presented control strategy.
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