5-16
5-3-5
Impact of PID control by CPU scanning
PID control is one process in the CPU scan. The use of PID control increases the scan time of the CPU. The amount of increase
in the CPU scan is proportional to the number of PID controls used and the sample rate for each control.
The run time for one PID control also depends on the number of options selected, such as alarms, square root errors, etc. The
figure on the right shows the expected range of time.
To calculate the increase in scan time, you must know the scan time only for the ladder (without PID control). When the same
PID control load is applied, the high-speed scan time is a slight percentage increase over the low-speed scan time. The average
scan time calculation formula is as follows.
As the above calculation shows, using PID control of the slow sample rate has a very small effect on scan time. Next, we
develop the above equation to examine the impact of adding various numbers of PID controls.
The new equation above computes the total time for each control from 1 to L (the last control). Finally, add "scan time without
PID control" only once. Assume that the SJ-Ether PLC controls four loops. The table below shows the data and total condition
values for each PID control.
PID control
number
Description
Sampling rate
Addition term
1
Steam flow and inlet valve
0.25 seconds
50 μs
2
Water-bath temperature
30 seconds
0.42 μs
3
Dye level, main tank
10 seconds
1.25 μs
4
Steam pressure and pressure vessel
1.5 seconds
8.3 μs
If you sum the addition terms and add the original scan time, then you can do the following:
The CPU simply performs a PID operation when the scan reaches the sample time for updating the data in the PID control
operation. The built-in PID control scheduler applies the following rules:
PID control with a sample rate of
≦
2 seconds is processed at many PID control rates to maintain the sample rate for each
loop. Specifying PID control at a fast sample rate increases the PLC scan time. Therefore, specify a fast sample rate only
if necessary.
The PID control with a sample rate of
≧
2 seconds is processed at the lowest rate necessary to maintain the sample rate
for each loop, one or a few PIDs per scan.
=
×
+
PID
ループ設定時の平均スキャンタイム
ループのない
スキャンタイム
ループのサンプルレート
PID
演算時間
ループのないスキャンタイム
=
X
+
n=1
n=L
PID
ループ設定時の平均スキャンタイム
×
PID
演算時間
ループのないスキャンタイム
ループのないスキャンタイム
n
番目のループのサンプルレート
=
+
=
PID
ループ設定時の平均スキャンタイム
50
μ
S
+
0.42
μ
S
+
1.25
μ
S
+
8.3
μ
S
50
μ
S
50.06
μ
S
Average scan time when
PID loop is set
Scan time
without loop
Sample rate of loop
PID calculation
time
Scan time without loop
Average scan time when
PID loop is set
Scan time without
loop
PID
calculation
time
Scan time without loop
Sample rate of n th loop
Average scan time when PID
loop is set
Summary of Contents for Kostac SJ-Ether Series
Page 1: ...English version of the KOSTAC SJ Ether series user s manual SJ ETHER USER M...
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Page 13: ...1...
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Page 113: ...3 24 Note Refer to 6 3 3 Module Assembly for connections and locks between modules...
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Page 156: ...flashing OFF Not sent H W RX Green flashing Receiving H W OFF Not received H W...
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Page 310: ...Chapter 7 Maintenance...
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