Research On Linux-Based CNC System For Micro EDM And Its Related Key Techniques

Owing to the high precision and good surface quality that it can offer, micro Electrical Discharge Machining (EDM) is potentially an important process for the fabrication of micro-tools, micro-components and parts with micro-features. Computer Numerical Control (CNC) system is a key component of the machine tool, and micro-EDM CNC system has a close relationship with the machine arts. Therefore, a micro-EDM CNC system is intensively studied in this dissertation.By analyzing the characteristics of a micro-EDM system, an overall design of the CNC system is taken. By using linear motor and linear encoder, a full loop motion control policy is proposed. It cancels all intermediate transport segments and greatly improves the motion accuracy and sensitivity of the machine tool. On the other hand, it also adds some difficulties when being used in a micro-EDM machine.Apart from the proprietary operating system, Linux offers abundant functionalities, better stability and robustness. So it is an ideal software development platform for a CNC system. By analyzing the characteristics of RTAI and RTLinux, a real-time abstraction layer is designed to provide a uniform API for RTAI and RTLinux. The developed micro-EDM CNC system allows real-time programs to use either RTAI or RTLinux without any code changes.PMAC motion controller is a multi-task real-time system, and it can do sophisticated motion planning calculations. Therefore, a micro-EDM CNC system is developed, in which PMAC is employed as the subordinate computer, and a PC-based platform is applied as the principal computer running on the Linux operating system. A PID + speed feed-forward + acceleration feed-forward motion control policy is proposed, which improves the response speed and accuracy, and without losing stability. The techniques of motion trajectory buffer and motion trajectory segmentation are used to implement backward control of motion trajectory. Under the Linux operating system, the left CNC programs are developed, such as a CNC decoder, a GUI and a database.On-the-machine measurement of electrode dimensions is necessary to achieve good accuracy in micro-EDM. This is because after a part that has been taken out of the machine for measurement is reinstated for more machining, the resetting error will drastically affect the final accuracy of the machined features. Therefore, a micro-EDM machine vision system is developed based on the analysis of the optical system, which is composed of a halogen type light source, a zoom lens, a CCD camera and a bracket which has 6-degree of freedom. It has a resolution of 1.61μm. Based on the Linux operating system, an image capturing program is developed with the V4L2 API, in which the image data is acquired with the method of mmap() memory map. The conversion between the data structure of IplImage and the class of QImage is implemented. Therefore the image can be processed by using OpenCV, as well as showed with Qt. The contour of micro electrodes can be extracted by means of the Canny edge detector. The experiment shows that the relative error between the CNC system and the scanning electron microscope (SEM) is less than±5%.Based on the self-developed machine, experiments of block electrical discharge grinding and micro-EDM milling have been carried out. By using the machine vision function of the CNC system, micro electrodes can be measured and compensated on-line. The experimental results prove its feasibility, stability and reliability.