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PID Control Period (V5020)
The PID Control Period specifies the time period that the output will be applied to. This value is also
used as the sample rate for the PID calculation. It is read by the temperature controller when bit 0 of the
Mode Word is set and only applies to PID control. The default PID Control Period is 2.0 seconds
(V5020=20).
If 1 second is specified as the PID Control Period then there will be 50 (1/.020) 20 millisecond periods. If
the PID output is 100% then the output will be on for all 50 periods. If the PID output is 50% then the
output will be on for 25 periods. If the PID output is 25% then the output will be on for 12 periods. The
output time is accurate to within one PLC scan time.
a)
Maximum Control Period
When the Output is a mechanical relay, the Control Period may be increased to lengthen relay
life. As a rule of thumb, the PID update rate should be 4x to 10x faster than the loop deadtime.
The deadtime of the loop is approximately the temperature speed of response to a change in
output. For example, assume the output is increased 20% (V5002=V5002+200) at time zero.
Thirty seconds later the temperature has increased 1 degree. Thus the maximum Control Period
is safely 30/10x = 3 seconds (V5020=30). Therefore a shorter Control Period of 2 seconds
should be used.
b)
Minimum Control Period
Use 2 seconds (V5020=20) for the minimum Control Period unless the temperature speed of
response to a change in output dictates a smaller value (see Maximum Control Period above).
The minimum value which may be specified is .5 seconds (V5020=5).
How tightly a fast loop can be controlled is limited by the PLC scan time. The actual time
proportioned output will be on from 0 to +1 scan time more than the calculated output on time.
Normally the scan time is not a problem because it is small compared to the Control Period. For
example, the output error for a .5 second Control Period will range from 0 msec (0%) to +20
msec (+4%) for a 20 msec PLC scan time.
However, the scan time can introduce a significant source of error if the scan time is large and
the Control Period is small. For example, the output error for a .5 second Control Period can be
up to 25% for a 125 msec PLC scan time.
As a rule of thumb, the Control Period should be at least 10 times the PLC scan time. In the last
example, it should be 1.2 seconds minimum.
ON/OFF Hysteresis (V5021)
The ON/OFF Hysteresis is a deadband around the SetPoint where the output will not change. The
minimum hysteresis is .1. The default hysteresis values are .5 EC and 1.0 EF. This value only applies
to ON/OFF control. This value is read by the temperature controller when bit 0 of the Mode Word is set.
2.20
INSTALLATION AND WIRING
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