SCALE FACTORS
30
pulse is received on the count input. It is when the
10th pulse is received that the display changes
from 4 to 5. However, if the counter is used in the
Reset to Preset mode, the display shows 5 when
the Reset key is pressed. The first pulse received
changes the display to show 4, and the ninth pulse
changes the display to 0. But, it is the TENTH
pulse that causes the output to change state. This
is because after the ninth pulse, there is a remain-
der of 0.5000 counts in the counter and, therefore,
the value in the counter is not actually zero until
after the next pulse.
HOW SCALE FACTORS AFFECT PROCESSES
When the use of Scale Factors results in partial
count remainders, those remainders can affect the
manner in which the process being controlled will
function. For example, if a Scale Factor of 1.3000
is entered into a control and a Preset of 15 is used
in the independent mode, a table as shown in Fig-
ure 25 results.
The control is used in the Reset mode. When
reset, the counter starts at zero and counts to the
Preset value. If the Auto Recycle mode is imple-
mented, the counter recycles when the Preset
value is reached. But, with a Preset of 15, the
counter has actually accumulated 15.6000 counts.
Thus, when it recycles, a value of 0.6000 counts
remains. When the next pulse is received, 1.3000
counts is added and the count value is 1.9000.
The “Second Cycle Display” column shows the
displayed value for the second cycle.
It is obvious from the last column that slightly more
counts are accumulated for the second part than
were accumulated for the first. If this table were
carried out for the third part, we would find that the
third part is cut off one pulse too early. Clearly, the
carryover of the remaining partial count causes
problems in these types of applications.
As a solution, a function code has been provided
which allows the choice of whether the remaining
partial count is carried over into the next cycle or
not. Function 83, Scaler Reset on Recycle, allows
selection of this option. If function 83 has a value
of “0” entered, the scaler is not reset when an Auto
Recycle occurs. If a value of “1” is entered, the
scaler is reset each time an Auto Recycle occurs.
This forces any remaining partial count to be reset
to zero, eliminating the problem described above.
The unit is shipped from the factory with the Scaler
Reset on Recycle Mode enabled (Function 83 has
a value of “1”).
It should be noted that the remaining partial count
is typically an extremely small part of the total
length of the part being produced (typically less
than 1%). In those applications where the mea-
surement system may be chosen, the rule of
thumb is that the measurement device should
have a minimum of twice the resolution (generate
at least twice as many pulses per unit of measure)
as the desired part accuracy.
For example, if a 10.00 inch part is to be made and
the tolerance of the part may be plus or minus 0.02
inches, the measurement system should generate
at least one pulse for each 0.01 inches of material
being measured. Thus, after the display shows
10.00 inches (1000 counts), there may be a
remaining partial count of 0.400 due to the use of
a Scale Factor. The percentage of error is calcu-
lated by 0.400/1000. This yields 0.04% error.
Even though the error is so small, compensation
should still be made for the extra partial count at
the end of a part by entering a “1” in Function 83.
This is because the error is cumulative; that is,
PULSES
RECEIVED
RESULT
CALCULATED
DISPLAY
VALUE
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
0.0000
0.5000
1.0000
1.5000
2.0000
2.5000
3.0000
3.5000
4.0000
4.5000
5.0000
5.5000
6.0000
6.5000
7.0000
7.5000
8.0000
0
0
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
Figure 24.
Pulses Received versus Display
Value Using Scale Factor of 0.5000
