Despatch P1400+ Temperature Controller - Manual (Page 60)

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9.2 Dual Control Tuning (PID with Primary and Secondary Outputs)
This simple tuning technique balances the need to reach setpoint quickly, with the wish to limit
setpoint overshoot at start-up and during process changes. It determines values for the Primary
Proportional Band ( Pb_P ), Secondary Proportional Band ( Pb_S ) , Integral Time Constant ( ArSt )
and Derivative Time Constant ( RAte ) that allow the PID control algorithm to give acceptable
results in most applications that use dual control (e.g. Heat & Cool).
CAUTION:
This technique is suitable only for processes that are not harmed by large fluctuations
in the process variable.
1. Tune the controller using only the Primary Control output as described in the Single Control
Tuning section above.
2. Set Pb_S to the same value as Pb_P and monitor the operation of the controller in dual
control mode. If there is a tendency to oscillate as the control passes into the Secondary
Proportional Band, increase the value of Pb_S . If the process appears to be over-damped
in the region of the Secondary Proportional Band, decrease the value of Pb_S .
3. When the PID tuning values have been determined, if there is a kick to the process
variable as control passes from one output to the other, set the Overlap/Deadband
parameter to a positive value to introduce some overlap. Adjust this value by trial and
error until satisfactory results are obtained.
9.3 Valve Control Tuning (PI with Linear Outputs)
This tuning technique is used when controlling a modulating valve requiring a linear (mA/VDC)
signal for its positioning circuitry. It determines values for the Primary Proportional Band ( Pb_P ),
and Integral Time Constant ( ArSt ). The Derivative Time Constant ( RAte ) is normally set to zero
(OFF). This PI Control minimises valve wear while giving optimal process control.
CAUTION:
This technique is suitable only for processes that are not harmed by large fluctuations
in the process variable.
1. Set the setpoint to the normal operating process value (or to a lower value if overshoot
beyond this value is likely to cause damage).
2. Set the Primary Proportional Band to the minimum value (i.e. set Pb_P = 0.5%).
3. Set the Integral Time Constant to the maximum value (i.e. set ArSt = 99.59).
4. Set the Derivative Time Constant to OFF (i.e. set RAte = 0.00).
5. Using manual control ensure that the valve is positioned away from its end stops.