Continuous sliding mode control of compliant robot arms: A singularly perturbed approach

Compliant actuators are essential for ensuring safety in physical human-robot interaction. A compact-sized series elastic actuator (SEA), a type of compliant actuators, was developed to guarantee high force control fidelity and low output impedance in our previous work. This study is focused on the control design of a compliant robot arm driven by the developed SEA. The control problem is formulated into a singularly perturbed form which contains a slow rigid robot dynamics and a fast SEA dynamics. To achieve high-precision tracking without serious control chattering, a second-order sliding mode control (SMC) law is proposed such that the equilibrium point of the closed-loop rigid robot dynamics has semiglobal exponential stability. A derivative-type control law is employed such that the equilibrium point of the closed-loop SEA dynamics has global exponential stability. As the SMC law developed is continuous so that the rigid robot dynamics is continuously differentiable, the singular perturbation theory is applicable to establish semiglobal practical exponential stability of the entire system. This study provides the first application of continuous SMC to robots driven by compliant actuators. Experimental results have verified a high-accuracy and high-resolution tracking performance of the robot control system.