Study On Motion Controller Of Hybrid Polishing Machine Tool

Regarded as the fusion discipline between modern precision processing technology and automations, the automation of free-form surface polishing is very important to promote the developments of automobile, machine, electronics and some high and new technology industries. By assistant means of computer technique, the technology of polishing automatically is developed dramatically among countries all over the world, in order to overcome the low efficiency problem widely existing in the process.At the end of 2004,"The Hybrid Polishing Machine Tool"was appraised at provincial level. Through catering the curvature of each machined point, the polishing normal pressure could be controlled. The coupling and the nonlinear nature of long distance movement could be made up, by applying corresponding algorithm of partitioning tool-path planning too. Relative to the cost of whole CNC equipment, the former PMAC (Programmable Multiple Axises Controller) was expensive. Also considering the closed function library provided by Delta Tau, a new open CNC system was designed independtently for the sake of the polishing characteristics of the machine tool.Studying on the hybrid polishing machine tool, the motion control platform (fig.1) was formed. The mainly hardware included the motion controller, servo actuator, AC servo motor, the hybrid polishing machine tool, PC, PCI9054 card and XDS510 emulator etc. This framework issured that the system matched the open structure of"NC +PC".Under the operate system of Windows, the host computer designed the user interface using VC, mainly completing the communication between the PC and DSP, the program input and compilation, the pre-handling of velocity, the analog simulation of processing parts and receiving and handling reports or alarm signals form DSP. The submit computer was DSP system, mainly being responsible for the executive commands from PC and real-time missions related to the machine tool.Concerning the software structure of the motion controller, several domestic methods existed: integrated control method, design method based on interrupt, the foreground and background method and transplanting the RTOSμc/os-â…¡etc. According to actual circumstance of the item and the comparison of various methods, the foreground and background design method was chosen in the early times. Analyzing the function request of the motion controller, the result mainly comprised of initializing the parameters of the DSP system, the communication between PC and motion controller, deciding the basic protocol model, the explanation and compilation models for the delivered codes, the tools compensation module, the interpolation module and position/speed servo control modules etc. According to the functional request of the software, the flow charts of the main function and service subprograms based on real-time interrupt were drew up. The main function mainly completed initialization for controller, hardware panels and global variables, and setting up the basic parameters of timers and other outside facilities. Service subprograms based on cputimer1 and cputimer2 was activated in term of the type of interrupt. The cputimer1 subprograms implemented manual adjustments, and the subprogram of cputimer1 was for processing procedures automatically.Besides the completion of the whole structural design, the indispensable modules of tools compensation and linear interpolation for CNC were developed. Now the theoretical research findings and simulation analysis of two modules are as follows.1. The theoretical research findings of two modulesResearch on the questions of radius compensation and length compensation inspace for polishing machine tool, two specialized modules were developed, that were compounded with the 5-axis NC post-processing procedures before interpolation. So the correct NC programs were made to increase the processing efficiency of operators and accuracy of parts. Formula derivation (1²) were used to calculate radius compensation vector and length compensation vector of the hybrid polishing machine tool in space.Where R is the radius of the selected tool; R1 the radius of corners of filleted cutter; i , j ,k the components of the normal vector of processing point; l3 the length of vertical spindle with horizontal rotational degree of freedom; l 4 the length of the supporting bar of tools; ? 4 the horizontal rotational degree of freedom; ? 5 the plumb rotational degree of freedom; ( x , y ,z ), ( )x0 , y 0 ,z 0, ( x M , y M ,z M) the coordinate values of processing point, the core-point of tools after special radius compensation, the core-point of moving platform mechanism after special radius compensation and special length compensation.To verify the correctness of the software structure, special linear interpolation module was developed. After promoting and optimizing the traditional DDA to special interpolation algorithm, many problems such as inefficience, redundant empty steps and low accuracy were alleviated. It was proved that the distance from apoint to a straight line in space was less than 2 2 pulse equivalent. Formula (3) was used to calculate the displacement of each coordinate axis.2. The experimental simulation resultsFig.2 shows the workpiece model built by CATIA. The workpiece surface simulation after special compensation is shown in Fig.3. The fine interpolation results is shown in Fig.4(See below).3. Conclusions and discussionsAs shown in Fig.3, compared to surface 1, the basic shapes of surface 2 after special radius compensation and surface 3 after special length compensation have not changed yet. For surface 2, it is not a 2mm migration caused by selected tool radius in Z-axis direction, but compensation both in X-axis and Y-axis happened according to the changes of the normal vectors on the processing surface. The data of special length compensation is changing by the attitude angles (φ4 andφ5).After fine interpolator, no obvious changes happen in the shape of surfaces. A total of 819 coordinates appear in Fig.4(a). The information of each interpolation point can be seen clearly. As a result of the selection of a higher accumulator register, 8370 points were figured out after fine inperpolator. So each point is not clearly shown in Fig.4(b). It was proved that the interpolation accuracy was still less than 2^1/2/2 pulse equivalent, after inputing the data of each point to the formula (the distance from a point to a straight line in space). So the conclusion can be made errors can be seen still within the permissible range.Fig.4. the rough and precise interpolation results Fig.3. the workpiece surface simulation after special compensation...