Key Technique And Application In Error Compensation For CNC Machine Tools

The error compensation for CNC machine tools is an economical and effective method to improve the machine tool precision. In this dissertation, key technologies and their applications for error compensation such as error modeling, measurement and real time error compensation implementation are studied, based on the projects supported by"The Foundation for the Author of National Excellent Doctoral Dissertation of P.R. China","The cooperative plan on science and technology of Yunnan province"and"The assimilation and innovation plan for introduced technology of Shanghai".The main contents of this dissertation can be described as follows:(1) The volumetric positioning error of CNC machine tools is analyzed. A reference coordinate system is created on the machine body,some sub-coordinate systems are set on each saddle, so the relationship between tool and workpiece can be expressed as the relationship between"tool-machine body"chain which is from tool to machine body and"workpiece-machine body"chain which is from workpiece to machine body, then the synthesis models of four types of 3-axis machine tool are derived by the homogeneous coordinate transformation method which provides a theory base for the error measurement and compensation.(2) The relationship between the values measured with laser interferometer along the line, face and body diagonal directions and positioning errors are studied and analyzed. The advantages and shortcomings of body diagonal direction in accuracy calibration are discussed. A new sequential step diagonal measurement method has been proposed based on the conventional diagonal measurement. In this method, the error in three axis directions can be divided 9 synthesis errors and they can be identified based on the values measured along four body diagonals. The idea of compensating the volumetric error directly based on these 9 synthesis errors are proposed and verified by the measurement and compensation experiment. With this method, the time for error measurement and identification is saved and the efficiency for error compensation is improved.(3) The relationship between the volumetric positioning errors and the temperature fields are studied and analyzed. Volumetric errors under different temperature conditions are measured by the sequential step diagonal measurement method, and the variation curves of the 9 synthesis errors are obtained. The methods for the prediction of volumetric errors under certain temperature condition such as simple interpolation, curve fitting and neural networks are proposed and compared. The error compensation effect is improved by analyzing and predicting the volumetric positioning errors considering not only the position variation but also the temperature variation. The compensation results show that this method can help to further improve the precision of the machine tool.(4) The modeling methods for thermal error modeling is studied and analyzed. A partial least squares neural network modeling method for CNC machine tool thermal errors is proposed. This method uses the neural network learning rule to obtain the PLS parameters instead of the traditional linear method in partial least squares regression so as to overcome the multicollinearity and nonlinearity problem in thermal error modeling. The basic principle of PLSNN is described and the prediction performance and robustness of the new method is validated by the thermal error modeling application on a CNC turning center.(5) The implementation method for real time error compensation is studied and analyzed. The function of the CNC system for external coordinate system shift is analyzed and a real time compensation system has been developed. With hardware and software design, the machine tool error can be calculated and compensated based on the external thermal and position signals. Furthermore, a simplified thermal error compensation device is also proposed. The development of this device provides an effective implementation method for real time error compensation.(6) The real time error compensation technique is implemented on a CY-K360 CNC lathe as a real case in the cooperative research project with CY Group to improve their manufactured machine tools.The positioning errors considering temperature variation and the spindle thermal drifts are measured, predicted and finally compensated by the real time compensation system based on the external coordinate system shift and the precision of the machine is improved significantly.