| This study is sponsored by several National Key Science and Technology Special Projects,National Natural Science Foundation of China and Specialized Research Funder for Doctoral Program of High Education.This research is carried out on a large gantry type machine tool.Firstly,multi-error elements which affect the machining precision are analyzed and measured.Secondly,multi-error elements models are established.Then,according to the homogeneous coordinate transformation theory,a comprehensive error model is proposed.Finally,a real-time error compensation system is developed to improve the machining precision and verify the accuracy of the comprehensive error model.The main contents of this study are listed as follows:(1)Geometric errors and thermal errors of large machine tools are analyzed.The meanings,causes and main effects of these errors,as well as the solutions to decrease them are described.Finally,straightness errors and the thermally induced positioning errors are regarded as the main error sources for large machine tools.All the geometric errors are measured by a laser interferometer measurement system.Besides,the principles and methods of measurements are demonstrated,and also comparisons between the proposed measurement methods and traditional methods are conducted.Based on the analyzations and the measurement results,it is concluded that the accuracy of the machine tool is mainly affected by straightness errors and thermally induced positioning errors,which are regarded as the main error sources needed to be compresated in this study.(2)To model and compensate the thermally induced positioning errors of ball-screw systems which are positioned by rotary encoders,a mathematical exponential expression is established to model the transient temperature of the screw shaft and the time by carrying thermal characteristic analysis on the screw shaft.Then based on the recorded warming and cooling time,the transient temperature of the screw shaft is calculated.According to the thermal expansion theory,a model for thermally induced positioning errors of the ball-screw system is established.However,since the machining conditions are extremely complicated and varying,the thermally induced positioning errors of the ball-screw system can’t be perfectly expressed by one single error model.So the screw shaft is divided into several evenly distributed heat regions and in each region the thermally induced positioning errors are modeled by the error modeling method mentioned above.The thermally induced positioning error model of the whole ball-screw system is generated by the superposition of error models of all the regions.(3)To compensate the positioning errors of long stroke axes which are positioned by grating scales,a thermally induced positioning error model is proposed.Several temperature sensors are installed on the grating scale and the temperature data are real-time recorded.The temperatures of every position on the grating scale are calculated via interpolation methods.According to the thermal expansion theory,the thermal elongations of every position on the grating scale can be calculated by integrating the variations of position and temperature.Combining the geometric positioning error model,the thermally induced positioning error model of the grating scale is established.(4)To compensate the straightness errors which vary irregularly,a method which applies the B-Spline curves to fitting and modeling the errors is proposed.The straightness errors of the machine tool are fitted by B-Spline curves which are capable of superior ability in data fitting and local adjustable.In addition,the accuracies of error models are evaluated and it can be improved by adding control nodes.A spacial straightness error model is established by the superposition of single straightness error models.Finally,the spacial straightness error value in each moving axes direction is calculated.(5)For CNC systems which equipped with Ethernet ports,an error compensation system which based on the industrial computer is developed,which including data communication module,reading error data module,temperature acquisition module,error modeling module and human-machine interface module.For CNC systems which are not equipped with Ethernet ports,another error compensation system which based on PLC and I/O modules is developed,and the compensated data and position data are interchanged between the compensation system and the CNC system via I/O modules.(6)To verify the veracity of the error measurement,the accuracy of the error modeling and the effectiveness of the error compensation system,experimental tests are conducted.Firstly,the straightness errors and the positioning errors are effectively compensated based on the error compensation system,which means the spatial straightness error model and the thermally induced positioning error model are correct.Secondly,spatial body diagonals positioning error measurements are conducted before and after compensation.The results show that the spatial body diagonal positioning accuracy is significant improved.Therefore,the spatial performance of the machine tool is improved after error compensation. |
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