Tradeoff between safety and performance for humanoid rehabilitation robot based on stiffness

With the application of the robots more extensive, human-robot coexistence in the shared workspace is required on many occasions (e.g. home service, nursing care, industry, etc.). Therefore, human safety within human-robot coexistence represents the key requirement. Most safety studies have focused on collision analysis and safety evaluation, but not on the consideration of inherent safety and design of a safer manipulator with the safety evaluation results. In this paper, a humanoid rehabilitation robot is developed with compliant joints, for which we propose a method to describe the nonlinear model of human-robot collision with effective mass and effective stiffness of robot's end-effecter(EE), which relate the important design parameter joint stiffness to collision safety. Under the condition of satisfying rehabilitation task, the safety can be guaranteed by adding passive mechanical compliance and compromising performance. Meanwhile, the influence of joint compliant on the modal frequencies is analyzed, and the lowest order modal frequency and safety evaluation results based task are used as performance tradeoff conditions. Finally, design criteria and constraint conditions of spring in compliant joint are given.