3200 Series
User Manual
Part No HA028651 Issue 11.0 July-12 59
11.2.3
Derivative Term ‘TD’
Derivative action, or rate, provides a sudden shift in output as a result of a rapid change in error. If the measured value
falls quickly derivative provides a large change in output in an attempt to correct the perturbation before it goes too far.
It is most beneficial in recovering from small perturbations.
The derivative modifies the output to reduce the rate of change of error. It reacts to changes in the PV by changing the
output to remove the transient. Increasing the derivative time will reduce the settling time of the loop after a transient
change.
Derivative is often mistakenly associated with overshoot inhibition rather than transient response. In fact, derivative
should not be used to curb overshoot on start up since this will inevitably degrade the steady state performance of the
system. Overshoot inhibition is best left to the approach control parameters, High and Low Cutback, section 11.2.5.
Derivative is generally used to increase the stability of the loop, however, there are situations where derivative may be the
cause of instability. For example, if the PV is noisy, then derivative can amplify that noise and cause excessive output
changes, in these situations it is often better to disable the derivative and re-tune the loop.
If set to Off(0), no derivative action will be applied.
In 3200 controllers derivative is calculated on change of PV. For applications such as furnace temperature control, it is
common practice to use Derivative on PV to prevent thermal shock caused by a sudden change of output as a result of a
change in setpoint.
11.2.4
Relative Cool Gain ‘R2G’
The proportional band parameter ‘PB’ adjusts the proportional band for the heating output. Relative cool gain adjusts the
cooling proportional band relative to the heating proportional band. If the rate of heating and rate of cooling are widely
different it may be necessary to manually adjust Relative Cool Gain to achieve the optimum settings for the cooling
proportional band. A nominal setting of around 4 is often used.
Note, This parameter is set automatically when Auto-tune is used unless the parameter ‘AT.R2G’ is set to ‘No’ - see
section 11.4.
Temperature
Time
SP
Propor Integral
response
Temperature
Time
SP
Response with derivative action
included
Summary of Contents for XST-2-0-12-1V1-E28
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