Research On The Geometric Error Measurement And Error Compensation Of The Numerical Control Machine Tools

The volumetric error identification and error compensation have been researched in this dissertation. The measurement method and error compensation model in the error field in the plane (two-dimensional) have been discussed and developed. The main contributions of this dissertation are shown as follows: The recent statuses in this subject are analyzed carefully. The characters of the geometric error have been discussed, including the relativity to the original point, dependence of the commanded position and continuity with the error field. These characters are verified by using the high accuracy linear and grid encode made in HEIDENHAI cooperation. A relay measurement method which can be used to calibrate the error field of the plane on the CNC machine tools has been developed; its accuracy and error accumulation rule has been studied. The results show that the relay method can be used for the geometric error measurement on the lower and medium stage machine tools. This presented an efficient calibration method using small high accuracy equipment to measure volumetric error of the large working space of CNC machine tools and will be found wide application in the industry. The disadvantages of the Double Ball Bar (DBB) are discussed. The DBB can only be used to measure the radial error. The device called two Dimensional Ball Bar (2D-BB) is developed, which can identify not only the radial error but also the rotation angle error when the machine is commanded moving in a circle(these values can be changed in the direction of the X axis and Y axis of the machine). Error analysis and accuracy calibration of the device have been made. The accuracy correlation with the grid encode has shown that the repeatability of the 2D-BB are 1.1 micro-meters in the direction of X axis and 1.6 micro-meters in the direction of Y axis. The root mean square deviation is 0.3 micro-meters in the direction of X axis and 0.4 micro-meters in the direction Y axis separately. The device can be used for the geometric error measurement because its high accuracy and repeatability. A method for error measurement, accuracy determination and pitch error compensation on medium stage CNC machine tools based on step gauge has been proposed and has been embedded in the HuaZhong numerical control system. The backlash error is detected by using the linear encode VM101 system which was made in HEIDENHAIN cooperation. The error variations are less than 2 micro-meters after the compensation when the measurements are applied in the same point. The results have shown that the backlash error is vanished. The error measurements and modeling of the machine is easier after the backlash error vanished. Polynomial model and artificial neural network (ANN) model are developed, which has presented the basic steps for soft-error compensation. As the application of relay measurement method, the error field of the plane on a milling center is mapped by using the 2D-BB and grid encode separately. The parameter of the model is recognized through Least Square Method (LSM). The accuracy between the different measurements method and different models are compared, the root mean square deviation is 2.3 micro-meters in the direction of X axis and are 1.4 micro-meters in the direction of Y axis between the two polynomial models. The results show that all of the measurements method and math's models are well uniformity and can be used in the error measurement in the lower or medium stage CNC machine tools. By applying the ANN model, the position error of circular trajectory is decreased by 72 percent after compensation. The error field on the plane is compensated using the polynomial model; the error value is at least decreased by 51.8 percent after the compensation. The roundness of the workpiece is detected by the DQR-1 roundness measuring instrument. The results show that the accuracy is 22.0 micro-meters, but the accuracy is decreased to 14.2 micro-meters after the polynomial model compensation and to 12.6 micro-meters after the ANN model compensation. The results declared that the accuracy is enhanced after the compensation in the error field on the plane of the CNC machine tools.