Kinematic calibration on a parallel kinematic machine tool of the Stewart platform by circular tests

This paper presents a methodology to calibrate kinematic parameters on a Hexapod-type parallel kinematic machine tool of the Stewart platform. Unlike in the case of conventional serial kinematic feed drives, on parallel kinematic feed drives the tool position and orientation can be only indirectly estimated from angular position of servo motors. Therefore, for high-accuracy motion control of parallel kinematic feed drives, it is the most critical issue to calibrate kinematic parameters such as the reference length of struts and the location of base joints. This paper demonstrates a calibration method based on circular tests to measure the machine's contouring accuracy in a circular operation. To optimize the positioning accuracy of a parallel kinematic feed drive over the entire workspace, it is important to evaluate the machine's global positioning error in circular tests by using the specialized jig plate. The effectiveness of the calibration is experimentally validated on a commercial parallel kinematic machining center.