Safety design and performance analysis of humanoid rehabilitation robot with compliant joint

Human safety represents the key requirement in physical human-robot interaction (pHRI). However, the majority of current research works focus on the safety evaluation. There is no definite way to design a robot that not only meets the performance but also has inherent safety. A humanoid robot for human joint movement rehabilitation is developed, to fulfill the requirement of both safety and its performance. A nonlinear model of human-robot collision with effective mass and stiffness of robot’s end-effecter (EE) is proposed. An important parameter involved in the model is the joint stiffness of the robot, which has an inherent direct effect on the safety. The influence of joint compliance on the modal frequencies is analyzed, and the kinematic performance of the robot is estimated roughly by the lowest order modal frequency. The design method of compliant joint stiffness is put forward, which can balance the safety and kinematics performance requirements. When the compliance is utilized intentionally in joints to improve safety, the question whether or not rigid performance of compliant joint can be achieved or approached by control is arose naturally. Such a challenge is addressed by the cascade control, where outer position loop with link-side position feedback is constructed. Here, restoring torque in the elastic transmission is taken as virtual control that is, in turn, set as the reference command of the inner torque loop. Intuitively, once the virtual control is reproduced as rapidly as possible in the inner torque loop, quasi-rigid performance is exhibited in the outer position loop as if there were no series elasticity in compliant joint. The stability criterion is derived, and the virtual stiffness and dynamic performance are examined. Finally, experiments are performed to validate effectiveness of the suggested control scheme.