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7.3 Operation of the PID Function
7.3.1
Automatic Operation
When the PID function block is called, it compares the current CPU time with the last PID solution
time stored in the reference array. If the interval between the two times is equal to or greater than
the Sample Period (word 3 of the reference array) and also equal to or greater than 10 ms, the PID
algorithm is solved using this time interval. Both the last solution time and CV output are updated. In
Automatic mode, the output CV is copied to the Manual Command parameter (word 14 of the
reference array).
Note: If you call the PID block in Auto mode after a long delay, you may want to use SVC_REQ 16 or
SVC_REQ 51 to load the current CPU time into the stored
the reference array). This will update the last PID solution time and avoid a large step change
of the integral term. Another method to prevent the step change is to copy the PV value to
the SP before placing the loop into Auto.
7.3.2
Manual Operation
The PID function block is placed in Manual mode by providing power flow to both the Enable and
Manual input contacts. The output CV is set from the Manual Command parameter. If either the UP
or DN inputs have power flow, the Manual Command word is incremented (UP) or decremented (DN)
by one CV count every PID Sample Period. For faster manual changes of the output CV, it is also
possible to add or subtract any CV count value directly to/from the Manual Command word (word 14
of the reference array).
The PID function block uses the CV Upper Clamp and CV Lower Clamp parameters to limit the CV
output. If a positive Minimum Slew Time (word 12 of the reference array) is defined, it is used to limit
the rate of change of the CV output. If either CV Clamp or the rate of change limit is exceeded, the
value of the integral (reset) term is adjusted so that CV is at the limit. The anti-reset-windup feature
assures that when the error term tries to drive CV above (or below) the clamps for a long period of
time, the CV output will move off the clamp immediately when the error term changes sufficiently.
This operation, with the Manual Command tracking CV in Automatic mode and setting CV in Manual
mode, provides a bump-less transfer from Automatic to Manual mode. The CV Upper and Lower
Clamps and the Minimum Slew Time always apply to the CV output in Manual mode and the integral
term is always updated. This assures that when a user rapidly changes the Manual Command value
in Manual mode, the CV output cannot change any faster than the slew rate limit set by the
Minimum Slew Time, and the CV cannot go above the CV Upper Clamp limit or below the CV Lower
Clamp limit.
In order to assure a bump-less transfer from Manual back to Automatic mode, the user program
should copy the PV to the SP before switching back to Automatic mode. This allows the algorithm to
update the last sample period time and prepare to re-calculate CV based upon the new Auto Mode
SP commanded.
GFK-2950C
Chapter 7. PID Built-In Function Block
February 2018
(word 24 of
Previous Solution Time
353
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