Joint Force Analysis And Moment Efficiency Index Of Cable-Driven Rehabilitation Devices

Cable-driven rehabilitation devices (CDRDs) have been studied by many researchers in the past decade. While a CDRD rotates a human joint by generating an assistant moment about the axis of the joint, it also creates a resultant force acting on the joint as long as the assistant moment is nonzero. Such a joint force may cause excessive joint wear or even break the joint if it exceeds a threshold. Thus, it is critical to analyze not only the assistant moment generated by a CDRD to rotate the joint, but also the joint force to have a safe and effective rehabilitation training. This paper studies how a CDRD with three degrees of freedom (DOFs) and four cables exerts a joint force on a general three-DOF human joint. The kinematics and dynamics models of the CDRD are established and the joint force needed to provide the assistant moment is derived mathematically at first. Then, an index to evaluate the efficiency of a CDRD in providing assistant moment (i.e., moment efficiency) is proposed. Lastly, a case study of the flexion and extension of the knee assisted by a CDRD is presented to demonstrate the derivation of the joint force and the usage of the moment efficiency index. The moment efficiency index not only promotes the safety of rehabilitation training, but also provides a guideline for the design of CDRDs.