A Novel Architecture for Compiled-type Software CNC System

PC-based software computer numerical control (CNC) system is the ideal for open CNC system, but there are some problems about the current open CNC system, such as the limitation of interpolation cycle, demanding on high-speed and high precision machining and the limitation of operation system (OS). So advances a novel PC-based architecture for compiled-type software CNC system, in the architecture, the general Windows is adopted as CNC platform and adopts two huge memories acting as data buffer to solve real time problem. And then analyzes the structure and basic principle of compiling system. The hardware has been developed based on this architecture and applied in the CNC system. Introduction Now, with the development of manufacture technology, many machine tool builders and end-users are expecting that CNC system can integrate with upper intelligent function of manufacture system and open the “ black-box” of CNC, and can put their own ideas of product design and management, special process technology and operation technique into CNC, so it is a trend that CNC system develops from a closed architecture and proprietary system to an open and general system. Some open CNC system has been developed at present, there are mainly three styles. The first is PC embedded in the CNC system and only performs human machine interface (HMI) and programming edit tasks, CNC kernel is proprietary. Next, a card added into PC performs the CNC kernel task such as coordinate axes motion control. This CNC is partly open on HMI and programming edits function based on PC. The third is software CNC system (Soft CNC), which is highest in the level of openness and the key of study on open CNC system Problem of the current Open CNC system. In the following, we mainly analyze the problem of PC-Based software CNC at present, because the first and second are not open at strictly speaking. The Contradiction between the Limitation of Interpolation Cycle and Demand on High-speed and High Precision Machining Figure 1 shows a example of sampled-data line interpolation, the next is the relation between interpolation error and interpolation cycle. Interpolation cycle is: ( ) F r T 1000 2 4 60 δ δ − = (1) because of r δ , then interpolation error is: r T F 18 625 2 2 = δ (2) in Eq.1 and Eq.2, T is interpolation cycle (s),F is cutting speed(m/min),r is curvature radius on the interpolation point where the curve is interpolated (mm),δ is interpolation error (mm). From Eq.1, we can conclude that when cutting speed is constant, if want to improve the cutting precision, must shorten interpolation cycle. Also from Eq.2, when interpolation cycle is constant, the cutting speed is higher, then the interpolation error is greater, and then the cutting precision is lower. Key Engineering Materials Vol. 339 (2007) pp 442-446 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland Online available since 2007/May/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.34-16/04/08,11:02:57) Fig.1 Interpolation error schematic It is supposed that δ is 0.0001mm and F is 30m/min which comes up to the standard of high-speed machining as required, when process small curvature radius contour such as r < 15mm, the T < 219us computed from Eq.1, namely interpolation cycle should not be greater than 219μs. It is difficult for the existing PC-based software CNC system to obtain this interpolation cycle, so the performance of PC hardware and software must be improved. So if the same high precision is achieved, the solution is that have to decrease the cutting speed, which may restrict some performance exertion of high-speed machining tool. And also, with the advancement of high-speed and high precision machining, this contradiction will be more obvious. The Contradiction between Universality and Real-time about OS It is necessary that select an openness, general and flexible OS to implement the PC-based software CNC function, and also OS must fulfill the strong real-time requirement of CNC kernel. There are many OS applied in the PC-based software CNC system, such as DOS, Linux +RT-Linux and Windows NT + RTX etc. DOS can be chosen as the platform of CNC because of it’s real-time performance, but it has such many shortcomings including single task OS and bad HMI . RT-Linux is also real time OS (RTOS), but CNC system based on RT-Linux can’t be integrated with CAD/CAM system that be developed on Windows OS, which only exchange data with CAD/CAM by off-line mode. Windows has many advantages such as good HMI, easy operation, wealth resource and easy exploitation .etc, so it is favorable for many researchers. But, general Windows is non-real time OS. Now it is a solution for the contradiction between universality and real-time that real-time kernel such as RTX is extended into Window NT or Windows, the extended real-time kernel can reach microsecond interpolation cycle. However, the price of the extended real-time kernel is very high and increases the cost of CNC system. Therefore, it is ideal solution for the problem that applying Windows OS by the simplest and proper method as PC-based software CNC platform. The Architecture and Analysis of Compiled-type Software CNC System The architecture of compiled-type software CNC system is shown in Fig.2, in this architecture, manual machining program, the program from automatic programming system, the design data from CAD/CAM, self-defined track function and soft PLC can be easily input into CNC system with convenient HMI in PC. After these program or data are fully compiled by compiling system, they are translated into the target-code that be composed of displacement of coordinate axis direction, displacement offset and switching value, also the target-code files, which size is determined by the size of bottom large memory, can be created, and then target-code files make up of target-code files library. Code transmitted control and check system transmits the target-code based on target-code files to bottom system through bus-mastering system. Access memory controller accepts the code and stores the code into huge memory 1 or 2 alternately. Position and I/O control processor also reads the code data through access memory controller from huge memory 1 or 2 alternately and sequentially, and controls the motors and switching components to execute relevant action. So CNC finishes the whole machining task step by step through accessing the target code sequentially. The Structure and Basic Principle of Compiling System. Traditional CNC executes program based on the unit on program segment, interprets a segment, executes a segment and loops until ending the machining process, which is the same as BASIC language interprets and executes its sentence, which is named interpreting-type running mode. Another is named compiling-type running mode. In this mode, manual machining program, the program from automatic programming system, the design data from CAD/CAM, self-defined track function and soft PLC are fully compiled before machining, then are translated into the original Key Engineering Materials Vol. 339 443