Research On Preprocessing Technology In CNC System

Preprocessing technology, composed of error checking of NC code, decoding, tool radius compensation and acceleration & deceleration control, is the base of the motion control in CNC and determine the quality of the CNC performance to some extent. The theoretical study and practice of the preprocessing technology in CNC has important theoretical significance and practical value.The structural characteristic of NC programming language is analyzed, and the description model (regular expression) of NC programming language is constructed from the perspective of grammar, which are the theoretical basis for the recognition model of NC programming language.Based on the theory of finite automaton, Lexical recognition model, grammar recognition model and decoding model of NC code are constructed. The state transition diagram and the corresponding state transition function and its algorithm for error checking and decoding technology are constructed also, and the correctness of the models are proved according to the graph theory's theorem. On the system platform of RTOS, the implementation of error checking and decoding technology are discussed in detail with examples.The cutter radius compensation technology is put forward based on direction vector. By introducing direction vector, the four connecting modes of liner-liner, liner-arc, arc-arc and arc-liner are simplified a kind of liner-liner, and lots of arithmetic formulae in the different connecting modes are transformed into few uniform ones, which realized the function of the C-function cutter radius compensation accurately and fast. At the same time, the concept of direction vector is applied in 3D manufacture, the theory and implementation of there-dimensional cutting tool radius compensation limited to the case of liner-liner in CNC system are studied. According to the spatial relationship of the programming outline, the algorithm for the path of cutter center of the program profile is developed, and the formula for calculating the switching point of the different transfer cases is analyzed in detail. The function of the 3D cutter radius compensation is achieved based on direction vector.The impact on parts contour error because of speed gain mismatch is discussed, and the contour error models of line and arc are built. The relationship between contour error and processing speed is drawn, which is the strong theoretical basis to choose the maximum speed in the acceleration and deceleration control. The acceleration & deceleration control technology of index rule based on least deviation is put forward, in which the motor feed frequency is controlled real-time through direct calculation method, but not consulting table method. The strategy combines the advantages of the least deviation method, index rule control method and real-time calculation method, and the acceleration and deceleration control of line interpolation, arc interpolation and screw thread interpolation is achieved. The dynamic characteristics incongruousness of each axis and position loop gains are considered as the main reasons of contour error in the thesis,The foregoing research achievements are applied in the CNC system for mill and lathe named TDNC-L4, which presents a preliminary practical verification of relevant theoretical achievements.