SCALE FACTORS
32
In the above formula, the Theoretical Distance is
the distance that would be measured if the mea-
suring wheel were new or within design tolerance
of new. For stretch or shrinkage applications, it is
the amount of material fed into the process before
the stretching or shrinkage occurs. The Measured
or Calculated Distance is the length which results
upon completion of the part or process. For exam-
ple, if the counter is intended to produce 12.00
inch parts but the parts come out of the machine
only 11.93 inches long, the Measure distance is
11.93 inches. (The Theoretical Distance in this
example is 12.00 inches.) Figure 26 shows graphi-
cally what takes place in this application.
The shaft encoder in Figure 26 produces 600
pulses per revolution. Doubling is used in the
counter to result in 1200 pulses per revolution. The
measurement wheel is intended to have a 12.00
inch circumference. This should result in 1 pulse
per 0.01 inches. Since a 12.00 inch is desired, a
Preset of 1200 is entered into the counter with a
scale factor of 1.0000.
However, when the process is run, the parts con-
sistently come out of the machine only 11.93
inches long. The counter is counting 1200 pulses
and the output of the counter is energized at that
time. Obviously, the wheel is not the 12.00 inch cir-
cumference which it should be. Rather than
replacing the measurement wheel, a scale factor
can be entered to compensate for the discrepancy.
Using the formula on the previous page, the scale
factor is calculated by:
11.93
″
(Measured)
Scale Factor =
12.00
″
(Theoretical)
= 0.9942
With this scale factor entered, the display still
shows 12.00 counts for each part, but each pulse
received is worth only 0.9942 counts. Thus, more
than 1200 pulses are received by the counter for
each part being produced and the part is made to
the correct length.
For applications where the material is stretched or
shrunk, the measurement device may be located
on the front end of the process where the unaf-
fected material is fed in. Yet the counter can have
a scale factor entered which allows it to measure
the finished parts. Figure 27 shows a typical pro-
cess which results in material stretch.
Figure 26.
Wheel Wear Correction Application
