Accuracy improvement of on-machine measurement for the parallel kinematic machine considering constraint motion

The parallel kinematic machine (PKM), which is applied Exechon mechanism, is efficiently used for manufacturing industry due to its agile movement, flexibility and high rigidity. On-Machine measurement (OMM) in high-dof manufacturing machines such as the PKM and five-axis machine tools has importantly used for processed part measurement, coordinate system set and machine performance evaluation. In this study, measurement and compensation of touch probe offset, which occurs measurement error of the OMM, are carried out for the PKM. A dependent rotational motion is occurred due to kinematic constraint, and causes non-constant offset of a touch probe. The dependent rotational motion is calculated via inverse kinematics analysis. The probe offset is accurately measured using a master ring with considering the analyzed dependent rotational motion angle. In addition, measurement procedure to eliminate the offset induced measurement error is presented. To verify the proposed technique, circular tests using a master ring and commercial touch probe on the PKM were performed. Circularity measurement deviation of a master was reduced 65% without the PKM’s kinematic error calibration.