Identification of strut and assembly errors of a 3-PRS serial-parallel machine tool

In a serial-parallel type machine tool, the parallel spindle platform plays the key role in manipulating three directions of movement. Spatial symmetry of the 3-PRS loops is essential to the machine's systematic accuracy. Currently, however, there is no effective instrument capable of measuring the symmetrical errors of the corresponding joints and strut lengths during structure assembly. In this study, an experimental method is proposed to identify the mechanism symmetric errors of a 3-PRS serial-parallel machine tool during the test run. It is based on the differentiation of the inverse kinematics equations. The mechanism errors could be derived by an identification model. With the aid of a developed 3D laser ball bar to detect the spatial position and orientation of the spindle platform, and three laser Doppler scales to measure three sliders' positions simultaneously, the length errors of three struts and the symmetrical errors of the R-joints and S-joints can be identified by the optimization technique. This technique can help shop floor engineers to tune the symmetrical errors of the 3-PRS mechanism during machine assembly. (C) 2004 Elsevier Ltd. All rights reserved.