Singularity-free path optimization of the parallel test mechanism for artificial hip joints

This paper focuses on the singularity-free optimization for the working path of a 3SPS+1PS type parallel mechanism which is used for the simulation of hip joint motion. Kinematic and dynamic models of the test mechanism were set up with the vector method and the principle of virtual power, respectively. Based on the Jacobian matrix, the kinematic and dynamic performance evaluation indices were defined. The working path of the test mechanism traverses the singular surface twice. Optimization was conducted with the attitude of the virtual reference coordinate system as the design variable, the mean value of the motion transmission performance index and the mean value and the standard deviation of the driving force in the working path as the optimization objectives, and the reachable workspace as the constraint condition. The equivalent working path of the test mechanism after optimization had no singularity. Moreover, the motion transmission performance improved evidently and the driving force changed smoothly.