Micromod Micro-DCI: 53SL5100B Single Loop Controller - Manual (Page 77)

53SL5100B Single-Loop Controller
INSTRUCTION MANUAL
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10 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 variation
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
element (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
element (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 integralaction. The
default value for Manual Reset is 50%. The minimum value is 0% and the maximum value is 100%.
10.3 Derivative Action (TD)
Derivative action augments proportional action by responding to the rate of change of the process variable. It
is used to make the controller more responsive to sudden process disturbances. The datapoint to set the
Rate parameter is C108; it has a default value of 0 minutes. The minimum value for derivative action is 0.01
minutes and the maximum time is 8 minutes; 0 is off. The derivative time in minutes is the amount of time by
which the proportional action (or proportional plus integral action) is advanced.