| With fast development of economy in recent years, China has become one of the compelling manufacture countries in the world. But the level of China's manufacturing still lags behind the world's first-class level. Due to the limit of equipment manufacturing, it is hard to meet the needs of the entire manufacturing industries. What we should do is to improve the accuracy of the machine tools in order to raise the level of the equipment manufacturing; therefore, it is crucial to change ordinary machine tools to CNC machine tools. Modern numerical control machine tools all employ a CNC (Computerized Numerical Control) device– a computer with an operation numerical control system, which can execute part or all of the functions of the numerical control devices according to the numerical program of the parts. Economic CNC machine tools, simple, cheap and very suitable for the machine tools'numerical alteration, are under discussion and research in this thesis.To improve the machining accuracy, it is necessary to eliminate the machine tools'errors. There are two basic ways to eliminate the errors: errors avoidance and errors compensation. In this thesis, the second one is chosen. Errors compensation is a method to measure the errors and use appropriate methods to compensate it, instead of unrealistically avoiding it as the first way indicates. After compensating the errors, the machine tools could produce more accurate workpiece with the improved position precision. And we can also heighten using frequency of the current machine tools.There are two types of compensations in terms of ways of realizing: hardware compensation and software compensation. On account of the inherent deficiencies of hardware compensation, most studies in the thesis will focus on the software compensation, which measures the errors between the actual direction and the ideal direction, and superposes the reverse errors to the order of the system to realize the error compensation.NC machine tools are very complicated, with many projects to be checked up, let alone all manner of errors. What comes into this thesis is the position error, which is the synthesis formed by various error sources in the non-loaded situation. In response to the position error is position precision, the definition of which is as follows: the error between the actual direction and the ideal direction when the executants of the machine tools perform unidirectional or bidirectional orientation for many times at different locations while moving along one of the axes of coordinates. Because the error is random and discrete, it should be estimated in a statistical way. There are three standards for evaluating the position precision: the repetitive position precision of the axis, the position precision of the axis and the reverse discrepancy of the axis. Error-separation is implemented as for the last two. First, measure the position precision, and then measure the reverse discrepancy. This can simplify the process of measurement and compensation.The method raised in the thesis is mainly for the compensation for the error of the servo drive system of NC machine tools. Economical CNC machine tools'servo drive part belongs to the open-loop control. As there is no feedback in the open-loop system, the systematic error cannot be compensated. If we add a compensation (revised) part to an open-loop system to measure the errors between the actual direction and the ideal direction, and superpose the reverse errors to the directive of the system to achieve the error compensation, then the accuracy of the open-loop servo system will be improved.The experimental table designed includes the mechanical system as well as the measure and control system. The hardware and software of the control system have been designed and analyzed. The 55BF004 three-phase VR step motor will be selected, and it will drive the mobile table through a pair of gears and ball bearing lead screw. The equivalent of the pulse is 0.005mm, the step angle of the step motor is 0. 75, and the speed ratio of the gears is 0.667. The measure and control system is composed of control system, measure system, drive system and alarm system. The control system is a master-slave system. The master PC is a yanhua IPC-610 IPC, which mainly completes input, output, display, data receiving and other functions. The slave one is an AT89c52 SCM, which mainly controls the speed of the step motor and the direction of displacement. Configuration software is implemented to design IPC software system, and C51 is implemented to design SCM software system. Serial communication is employed between IPC and SCM by using EIA RS-232-C (electronics industry association recommended standard 232-c) standard. A displacement grating sensor (produced by Changchun Institute of Optics,Fine Mechanics and Physics, Chinese Academy of Sciences) is used to measure distance, and it is connected with a GS8220 Series Digital Readout (produced by Beijing CSCA Automation Technology Company), which is used to deal with the data. Good effect in motor driving is generated by a SH-3F075 step driver (produced by Beijing Sidate Electrical and Mechanical Technology Company), which can also save time and effort.The experiment has been completed on the experimental tables. The experiment shows that the positive position error is reduced by 90%, from 106μm before the compensation to 10μm after that. Before compensation, the maximum of reverse discrepancy is 117μm, the minimum is 107μm. After compensation, it decreases to 8μm. The effect of compensation is obvious. Then, an analysis is applied to the position precision of the experimental table, which has been compensated according to national standard GB10931-89 Method of Assessing the Position Precision of Digital Control Machine Tools, with following results– the reverse discrepancy is 8.0μm; the repetitive position error is 7.82μm; the position error (unidirectional) is 18.55μm; the position error (bidirectional) is 26.35μm. We can see from the statistics that the position error is quite random and discrete, and the position precision is relatively low at the end of the stroke. Through compensation, the position precision increases by 82% and the reverse discrepancy decreases by 92%. Compensation experiment has met the presupposition, but there is still some work to be further studied and some deficiencies to be improved to make the method realizable in application.
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