accuracy of manufacture than would have been needed with a geared system. 
The stepper motors used are Astrosyn 35 PMC 006. The size of motor was 
determined by measuring the static torque on the end of the shafts, and 
from this data the supplier was able to recommend a suitable motor. The 
sizing of the motors allowed a fairly large tolerance, as facilities were 
not available to accurately measure the static torque.  Àn important 
feature of the drive is a de-clutching arrangement to disconnect the 
motors. It is almost impossible to turn the motors when connected 
through the lead screws, due to the magnetism in the stepper motor. 
The pitch of both X and Y lead screws on the coordinatograph is ! mm. 
The stepper motors give 200 steps per revolution, and drive the lead 
screws through 2:1 reduction gearing, giving 100 steps per revolution of 
the lead screw. Each step is therefore equivalent to a 40 micron movement 
on the table. 
Stepper Motor Driver Unit 
  
The stepper motors are driven by a Digiplan Ltd 'Digidrive' 1055 Driver 
Unit. This unit takes pulses from the microcomputer and converts them 
into an appropriate level of power output to drive the motors. An 
important consideration with stepper motors is to decide the running 
speed in relation to the requirement that the motor/load must be able to 
start/stop instantaneously. A speed within the instantaneous start/stop 
range may be too slow for normal operation while a higher speed may over- 
load the motor at start, or lead to overrun when stopping. A pulse 
generator was used to experiment with different speed settings, and it 
was found that a speed of 2 cms a second could be obtained, while still 
providing instant start/stop. This was considered fast enough for the 
drawing work required of the system, and considerably simplified the 
writing of the software, as it was not necessary to write in a program 
for accelerating and decelerating the system. 
Interface 
A small interface also mounted on an S100 card was required to make the 
pulse signal of the microcomputer compatible with the signal required by 
the stepper motor driver. This card also carries an output to control the 
pen up/pen down commands which must synchronise with the movement of the 
coordinatograph. The timing for the pulses is derived from the micro- 
computer's own clock. This interface was built by the University of York, 
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