Adaptive Impedance Control for Compliantly Actuated Robots with a Unified Safety Measure

Compared with stiff actuators, compliant actuators are known to offer a range of advantages such as high force fidelity, low output impedance, and tolerance to shocks, and its application can be found in both industry and healthcare. The overall compliantly actuated robot consisting both the rigid-link dynamics and the actuator dynamics is a high-order system with couplings between each other, which opens up challenges in the development of robot control schemes. While several progress is achieved in the control of compliantly actuated robots, the open issue of safety is not systematically addressed. This paper presents a new continuous adaptive impedance controller for compliantly actuated robots, which integrates two interaction modes, i.e. Desired-Impedance Mode and Safety-Stop Mode, in a smooth and stable manner. The transition between two modes is embedded in the controller and automatically realized according to the variation of a unified safety measure. The proposed controller allows the robot to safely interact with environment under a desired impedance model and also enables it to be stopped immediately whenever unforeseen and unexpected safety issues arise. The stability of closed-loop system is rigorously proved with Lyapunov methods, with the consideration of both the rigid-link dynamics and the actuator dynamics. Experimental results on an upper-limb rehabilitation robot are presented to validate the proposed controller.