The Design Aspects of a Microcomputer controlled NC Machine System

Abstract

Computer numerically controlled (CNC) machine tools are proving their value by helping their users to remain competitive. The advent of microprocessors and thus microcomputers should have encouraged the smaller firms using numerically controlled (NC) machine tools to join the CNC trend but they are often prevented from doing so mainly owing to the cost of the special engineering involved in a dedicated microprocessor machine system. One of the ways by which CNC can be introduced would be to acquire a microcomputer the time of which could be shared between the control of an existing NC machine via large scale integrated (LSI) units and possibly the execution of some management functions. This thesis describes the design of an economic control system for a milling machine in 2½ (or 3) axes with stepping motors controlled from a microcomputer in open loop. On the existing machine, the X-axis has a hydraulic motor and the Y- (and Z-) axes have hydraulic cylinders and rams. Each axis is actuated by a stepping motor in a servo loop. The axis servos thus formed are shown to be linear, the velocity following error varying proportionally with the stepping rates. The whole CNC system can be built in-house because the high cost of special engineering can be considerably reduced by the use of programmable LSI's in the microcomputer interface which output interpolation pulses to the stepping motors for accelerations, decelerations and programmed feedrates. They count the pulses and provide interrupts to the 16-bit microcomputer. Machines with direct stepping motor drives can also use these interfaces. The microcomputer operating system is generated on floppy diskettes (DOS) which also contains the control file. Assembly language subroutine calls are included in the main program which is in FORTRAN. The use of a high-level language for the microcomputer enables the programming of the milling machine to be by manual data inputs (MDI) in computer question-and-answer mode. Chapters 6 and 7 are devoted to recent developments which are evidence of the interest in NC LSI's and are shown to be complementary to this study.