Abstract
The traditional machine tools with indirect feedback of motor encoders face the accuracy problems caused by geometric errors, like screw pitch error, backlash, structure deformation, assembly errors and thermal errors caused by the change of room temperature and the heat generated by motors. This research uses the Six-Degree Motion Measurement Device (MMD) to measure the pose of the tool relative to the workpiece and to perform the direct pose feedback control. By this way, all geometric and thermal errors within the mechanism chain of the machine tool can be eliminated. This research improves the original design of the Hexapod-type MMD. The top and bottom plate of the new MMD are made of Invar and Quartz, in order to reduce thermal errors caused by the change of room temperature. Furthermore, experiments are conducted to investigate thermal errors of the telescopic measuring ball bars of the MMD caused by room temperature changes and the heat sources in the read heads of the linear scales. Methods for the compensation of thermal errors are proposed, so that the MMD can maintain its measurement accuracy within a certain range of room temperature changes. The developed machine tool with direct pose feedback uses the EtherCAT to read the encoder values of the machine tool and the MMD, and to drive the servo motors. In contrast to the traditional motion card system, the EtherCAT CNC system has the advantages of simple wiring and flexible configuration. Verified by the positioning experiments, the position accuracies on the x, y and z directions of the machine tool with direct pose feedback control are dramatically improved from ±2.6μm, ±17.5μm and ±8.3μm to ±1.7μm, ±1.8μm and ±0.7μm. Verified by the machining experiments, the machining errors on the x and z directions are reduced from 9.5μm and 7.7μm to 2.2μm and 1.7μm.