Research On Dynamic And Real-Time Error Compensation For Two Turntable Five-Axis NC Machine Tool

Five-axis NC machine tool, which is one of the typical multi-axis machine tools, is able to carry on the processing of free surface with complex geometric shape with higher accuracy because of its ability for providing shiftable and rotational motions simultaneously for the cutting tools. It has been used widely in the field of precision manufacturing due to its high processing flexibility.In this paper, we proved that error compensation technology based on software is a both economical and effective method to improve processing accuracy of five-axis NC machine tool through the investigation on error measurement, mathematical modeling and compensation implementation. This study was supported by the National Science and Technology Major Project"High-level NC Machine Tools and Basic Manufacturing Equipment"and the five-axis NC machine tool was developed jointly by Shanghai Jiaotong University and Shanghai Electric Central Research Institute.The main content of this paper are as follows:(1) Established a comprehensive mathematical modeling for five-axis NC machine tool. On the analysis of basic structure of five-axis NC machine tool and main error sources of processing system, the error sources of geometric error, thermal error and cutting force error were pointed out respectively as well as their manifestation in each moving axis and main-axis errors. In total 97 error elements were obtained, and these had been simplified to 42 error elements by linear superposition of geometric error, thermal error and cutting force error of same movement part in the same movement freedom.A comprehensive mathematical modeling for five-axis NC machine tool was established based on hypothesis of small angle and the homogeneous coordinate transformation theory of rigid motion. As with two-rotary-table five-axis machine tool as an example, the calculation formulas of position error and orientation error were deduced. The modeling and formulas developed lay a foundation for error compensation implementation and can be extended to other five-axis NC machine tools with different structures.(2) Presented an efficient error measurement method of five-axis NC machine tool. This method was presented based on the body diagonal CNC machine precision testing method recommended by ISO230-6 and in the cooperation with OPTODYNE company. By using laser doppler displacement measuring instrument, four body diagonals were measured step by step, nine groups of relationships between the errors and the movements of error compensation were obtained. Compared with the traditional error measurement methods, same test results can be realized with less testing time and procedures, thus highly improved the error compensation efficiency.(3) Investigation on temperature measuring point optimization of five-axis NC machine tool. 24 sensors were placed in machine tool for the temperature measurement of thermal dynamic process and an optimal strategy was established based on the thermal characteristic analysis and correlation theory, four thermal sensitive positions were obtained by screening. This method can provide same measurement result as using gray system theory, but with a more simple process. The thermal error model based on these four temperature points simplified the structure, reduced the calculation data quantity and improved prediction accuracy and robustness.(4) Established a thermal error model of five-axis NC machine tool based on artificial neural network. Online modeling method and model which adapted to changing conditions of machine tool were proposed based on the learning ability and extrapolating characteristics of neural network theory. This modeling was based on the thermal error model of neural network, combining with time sequence theory, fuzzy theory and self-learning ability of artificial neural network. The good prediction performance of this model was verified by experiments and comparison with conventional polynomial mathematical model.(5) Presented a step-by-step decoupling compensation strategy for five-axis machine tool. The mathematical expression of position error of each moving axis and rotation axis movements in workpiece coordinate system at any time was deduced by setting workpiece coordinate system as basic coordinate system and applying homogeneous coordinate transformation theory. Based on the coupling characteristics of simultaneously movements of shiftable axis and rotary axis, a step-by-step compensation strategy was established: first to compensate posture error by adjusting actual posture of workpiece to ideal posture through the rotation movements of rotary axis; Second to compensate position error through shifting movements of shiftable axis, and set up error compensation mathematical model for five-axis NC machine tool.The processing accuracy was obviously improved through simulation analysis and the experiment of real-time compensation processing on curved surface part. At the same time, the movement interference on compensation processing was effectively avoided. This error compensation mathematical model and the feasibility and validity of real-time compensation make compensation process simple, feasible, reliable and effective.(6) Verified the proposed error compensation for five-axis NC machine tool by experiment. The accuracy of the error model and the adaptability of the compensation strategy were verified with different external coordinate offset function and the developed error compensation device. This compensation device can realize the real-time error compensation by adding error compensation data into the signal of servo loop signal through the offset of external coordinate without any modifications on NC command and CNC system hardware and software. Applying the laser doppler step-by-step body diagonal measuring method, the error compensation based on G code was verified as well, this can provide good compensation effect on low cost NC machine tool.