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.
Citation
Master of Science at the University of Nairobi, 1982Publisher
University of Nairobi College of Biological and Physical Sciences (CBPS)