Micromod Micro-DCI: 53SL5100A Single Loop Controller - Manual (Page 107)

Single Loop Process Controller Instruction Manual
Instrument Tuning 99
10.0 Instrument Tuning
Tuning the instrument is an iterative process to refine the Proportional Band (PB), Integral (also called
Reset Time [TRI]), and Derivative (TD) parameters of the Control Module 0 (CON-0).
The three parameter datapoints are C106, Proportional Band (PB); C107, Reset Time (TR); and C108,
Derivative Time (TD). Values for these three datapoints determine the proportional, integral, and derivative
action of the instrument so that its output properly alters the final control element setting to correct the
effects of a disturbance to the process or a setpoint change.
10.1 Proportional Action (PB)
In Proportional action there is a comparative relationship between the controlling instrument output signal
magnitude and the calculated error, which is the difference between the process variable and the setpoint.
The proportional action of a controller is set as a percentage of the proportional band (% P.B.). Percent
proportional band is the full scale through which the error signal must vary to cause full scale output varia-
tion due only to proportional control mode response. The proportionality factor between the output signal
and the error is called the gain.
Gain = 100 / % P.B.
If a small percent change in error is required to cause a full scale output variation from the final control ele-
ment (narrow proportional band), then the process system has a high gain. For example, with the setpoint
at 50% of scale and a percent proportional band at 20%, the process variable feedback signal must vary
from 40% to 60% of scale to make the final control element vary through its full stroke. The gain would be
100 / 20 = 5.
If a large percent change in error is required to cause a full scale output variation from the final control ele-
ment (wide proportional band), then the controller is set for low gain. The datapoint to set the % P.B.
parameter is C106; it has a default value of 100%. The minimum value for proportional band is 2% and the
maximum value is 1000%; 0 is off.
10.2 Integral Action (TR)
Integral action augments proportional action to cause the controller to drive the final control element until
the error is completely nulled. It produces a corrective signal proportional to the deviation and the length of
time the controlled variable has not been at setpoint. Integral action is expressed as the length of time in
minutes required for it to produce an output change equal in magnitude to that produced by proportional
action.
The datapoint to set the Reset parameter is C107; it has a default value of 0 minutes (no integral action).
The minimum value for integral action is 0.02 minutes and the maximum value is 200 minutes.
Integral action datapoint C107 must be 0 minutes if Manual Reset, datapoint C111 is used in lieu of integra-
laction. The default value for Manual Reset is 50%. The minimum value is 0% and the maximum value is
100%.