Configuration bifurcation and self-motion analysis of 3SPS + 1PS bionic parallel test platform for hip joint simulator

A hip joint simulator with a 3SPS + 1PS spatial parallel manipulator as the core module is proposed. Compared to traditional serial test platform, it brings better performance, as also as its inherent defect—singularity. In order to analyze the motion behavior of the manipulator near singular points, the entire configuration paths corresponding to single input parameter, are traced with the help of the closed-form solution of the forward position. The investigation reveals the reason to cause forward singularity and uncontrolled degree-of-freedom (DOF) of the manipulator, and it is verified by 3-D motion simulation. Then, a Configuration Stability Index is proposed to measure the controllability of DOF and a qualitative and quantitative analysis of self-motion is presented using its equivalent motion. The results indicate that: at all six bifurcation points, the uncontrolled DOFs show a similar direction to Z-axis, with slightly offsets to the other two axes. It means that the platform has an unstable rotation around Z-axis and a weak load-ability in that direction. Moreover, the direction of uncontrolled DOF almost stays constant under different control precisions. The research provides an analytical basis to achieve the certain motions at singular points and improve the motion stability for parallel manipulator.