Publication 1762-RM001C-EN-P
Process Control Instruction 19-21
Output Limiting with Anti-Reset Windup
You may set an output limit (percent of output) on the control variable.
When the instruction detects that the control variable has exceeded a
limit, it sets an alarm bit (bit LL for lower limit, bit UL for upper limit), and
prevents the control variable from exceeding either limit value. The
instruction limits the control variable to 0 and 100% if you choose not to
limit.
Select upper and lower output limits by setting the limit enable bit (bit
OL), and entering an upper limit (CVH) and lower limit (CVL). Limit
values are a percentage (0 to 100%) of the control variable.
The difference between selecting output alarms and output limits is that
you must select output limiting to enable limiting. Limit and alarm values
are stored in the same words. Entering these values enables the alarms,
but not limiting. Entering these values and setting the limit enable bit
enables limiting and alarms.
Anti-reset windup is a feature that prevents the integral term from
becoming excessive when the control variable reaches a limit. When the
sum of the PID and bias terms in the control variable reaches the limit, the
instruction stops calculating the integral sum until the control variable
comes back in range. The integral sum is contained in element, IS.
The Manual Mode
In the MANUAL mode, the PID algorithm does not compute the value of
the control variable. Rather, it uses the value as an input to adjust the
integral sum (IS) so that a smooth transfer takes place upon re-entering
the AUTO mode.
In the MANUAL mode, the programmer allows you to enter a new CV
value from 0 to 100%. This value is converted into a number from 0 to
16383 and written to the Control Variable address. If your ladder program
sets the manual output level, design your ladder program to write to the
CV address when in the MANUAL mode. Remember that the new CV
value is in the range of 0 to 16383, not 0 to 100. Writing to the CV percent
(CVP) with your ladder program has no effect in the MANUAL mode.
PID Rung State
If the PID rung is false, the integral sum (IS) is cleared and CV remains in
its last state.
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