| With the constant improvement of manufacturing industry over the past few years, five-axis CNC machine tool as high-end manufacturing equipment that can process complex profile is continuously expanding its application area. In this process, increasingly high machining precision is required. Supported by National Science and Technology Major Project “High-end CNC Machine and Basic Manufacturing Equipment(2009ZX04014-22 & 2011ZX04015-31)”, “National Natural Science Foundation of China” and “National Doctoral Program Foundation of Institutions of Higher Education of China”, this object of study is concerned with two turntable five-axis NC machine tool produced by Shenyang Machine Tool Co., Ltd. Thereby, high-precision measuring, robust modeling and real-time compensation of multiple errors within a full temperature range(including volumetric position errors of translational axis, rotatory axis angular errors and different kinds of thermal errors) are discussed. A real-time compensation system based on multiple MCU is developed for the HEIDENHAIN iTNC530 CNC system. A series of online prediction and real-time compensation tests for the errors are carried out.The test result verifies that error modeling and compensation methods proposed in this paper can efficiently improve the machining precision of five-axis CNC machine tool.This study consists of four main parts, covering the key technologies of synthesis real-time compensation of multiple errors of the five-axis CNC machine tool and the calidation tests.Part I: Error components analysis and synthesis error model of the five-axis CNC machine tool:Based on the structure of the machine tool, the error components are analyzed and sorted by the affect factors. With the Homogenous Coordinate Transformation method, by comparing the transformation matrix under ideal and actual conditions, the synthesis error model of the five-axis CNC machine tool is established. This model can express the relationship between the final machining errors and the error components of the motion parts. By further analysis of the error components, the synthesis error model can be simplified, and it would be more suitbale to apply the simplified model during the compensation process later.Part II: High precision measuring method of multiple errors of five-axis CNC machine tool within a full temperature range:(1) On basis of improved method of step-diagonal measurement, a high precision measuring method of the volumetric errors of the three translational axes of five-axis CNC machine tool within a full temperature range are proposed. By analysis of measurement principle and measurement precision of convetional step-diagonal measurement, the methods of improving the two major factors that have the greatest impact upon measurement precision–set-up errors and thermal conditions mismatch are proposed respectively: a new technology of face-diagonal measurement method for identifying set-up errors is first proposed, thereby the influence of set-up errors can be eliminated; optimized procedures of step-diagonal measurement based on the concept of repeatable warm process of the machine is then proposed to eliminate the influence of thermal conditions mismatch, and the installation frequency of laser interferometer can be reduced as well. By comparison with direct error measurement method, it proves that the measuring efficiency and accuracy can both be greatly improved by step-diagonal measurement method.(2) Rotary positioning errors of two rotatory axis on the five-axis CNC machine tool are measured by laser, precise polygonal mirror kit and high-precision sinusoidal rotary encoder. Rotary positioning errors are measured for the C-axis of the machine tools that with the larger rotary motion range by laser, precise polygonal mirror kit; Rotary positioning errors are measured for the A-axis of the machine tools that with the smaller rotary motion range by high-precision sinusoidal rotary encoder. The obtained rotary positioning errors can provide the data foundation for modeling for rotary axis errors.(3) Standard mandril is applied instead of machining cutting tools during the measurement of thermal errors of spindle. Special fixture with non-contact displacement sensor integrated with multi-channel signal acquisition system can efficiently and synchronously collect the errors and corresponding temperature data of the spindle of in all directions. The obtained data can provide the data foundation for modeling for the thermal errors of the spindle.Part III: Optimal modeling for multiple error components of the five-axis CNC machine tool within a full temperature range(1) After analysis of the causes and change mechanism of all kinds of errors, an idea of modeling for the errors that both are related to the position of the machine tool and the temperature status of the machine tool based on error separation are raised.The errors are decomposed into the static manufacturing errors related to coordinate position and thermal effected error related to the temperature status. And then the models for the two decomposed parts are respectively established. Finally, those models are superposed to generate the integrated prediction model that can predict errors under any coordinate position and any temperature condition. By comparison with the predicted errors and measured errors, it can be proved the prediction precision of the established models.(2) A model for the errors only related to the thermal conditions of the machine tool is proposed based on the least squares support vector machine by Bayesian inference. To obtain the optimal parameters based on MAP, Bayesian inference is applied to optimal selection of LS-SVM regularization parameters and kernel function parameters in the model training process. By compared with the prediction result of other models, it has been proven that BLS-SVM model has a high prediction precision and has high robustness and generalization ability especially the working condition of machine tool changes.(3) For the static geometrical errors only related to the positions of the machine tool, simple fitting methods are used during the modeling process.Part IV: Real-time compensation method and validation tests of the five-axis CNC machine tool within a full temperature range(1) Compensation implementation method based on a self-developed external compensation system. A real-time compensation system is developed for the HEIDENHAIN CNC system. This system is independent from the machine tool CNC system, and can automatically and circularly execute on-line prediction and real-time compensation. All the compensation process will not impede CNC machining coordinate and programs of workpiece machining process.(2) Compensation test of multiple error components of five-axis CNC machine tool within a full temperature range: Based on the real-time compensation system on-line prediction and real-time compensation tests of the main error components are carried out. By compared with the changes with and without compensation, it has been proven the compensation effect is good with proposed modeling, on-line prediction and compensation implementation method.(3) Synthesis compensation tests during machineing process of the five-axis CNC machine tool. Based on each measured value of error, iteration and simplification of classified and established error models, the simplified synthesis compensation model for multiple errors is used to implement on-line prediction and real-time tests for the synthesis machining errors during the real machining process of two typical parts, one is a standard ball part, another is an impeller. By comparing the changes before and after compensation, it has been proven that the proosed compensation method can improve the machining precision of the five-axis CNC machine tool significantly. All the experimental results are tested and approved by East China measurement institute and Quality Testing of Machine Tool Products for Mechanical Industry Center... |
PDF Full Text Request