A Self Programming. Numerical-controlled Drill For Printed Circuitboards

Abstract

The numerical control of an X-Y co-ordinate table by the Tuscan microcomputer is presented as a problem involving interface skills, numerical control and signal processing. The main objective of the project 1S to develop the electronic circuits and software required for a numerically controlled tool for drilling circuitboards. The novelty of the system lies in the use of a self-programming scheme by which the tool automatically scans a predrilled master board and processes the data thus acquired to generate a numerical control program which will supervise the drilling of subsequent boards. Conventional practice is followed in that the drilling head is fixed and the board is mounted on an X-Y co-ordinate table, driven by stepper motors. Interfacing consists of the design of a general purpose Input/ Output interface card for the S100 bus. Further hardware is developed to process the signals from this Input/Output card and provide drive signals for the stepper motors which position the co-ordinate table. Hardware required to interface the sensing unit. the drill and error detection signals is also developed. A·power supply is designed to -, provide an unregulated high cu~rent output for the stepper motor drives and tightly regulated decoupled supplies for the signal processing hardware. The software needed comprises 2 programs one scanning the master circuitboard to generate a numerical control sequence for the second which supervises drilling. The principal software routines have been tested in a rather slow Basic program and flowcharts and subroutine listings are provided for a proposed fast program with extensive realtime processing. The most important parameters for the stepper motor are the maximum speed at which synchronisation can be maintained and the resolution. The stepper motors were chosen on the basis of their resolution being appropriate to the accuracy required for the printed circuitboard layout of holes. The maximum speed is limited by the characteristics of the stepper co-ordinate table and the power drive circuit which is a transistor bridge. These factors are examined theoretically, using an exponential rise model for coil current. Tests have been carried out to determine the optimal drive speed. A· bilevel drive technique is considered as a measure to increase maximum speed. The circuitboard is scanned using an infra-red emitter-sensor unit. exeriments have been carried out to determine the accuracy and reliability of the system for the range of hole sizes likely to be used on circuitboards and for different values of emitter-sensor separation. The results have been used to determine the optimal emitter-sensor separation and the tolerances on the data that is obtained from the scanning.