West Control Solutions 8700+ - Manual (Page 123)

1 /
4
-DIN, 1 /
8
-DIN & 1 /
16
- DIN Controllers & Indicators - Product Manual
Page 118
Manual Tuning 59305, Issue 7 – March 2014
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 (

), Secondary Proportional Band (

), Integral Time
Constant (

) and Derivative Time Constant (

) 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

to the same value as

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

. If the process appears to be over-damped
in the region of the Secondary Proportional Band, decrease the value of

.
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.
Valve Control Tuning (PI with VMD or Linear Outputs)
This tuning technique is used when controlling a modulating valves, either with a Valve Motor
Drive Controller, or if a standard controller if the valve requires a linear (mA/VDC) signal for
its positioning circuitry. It determines values for the Primary Proportional Band (

), and
Integral Time Constant (

). The Derivative Time Constant (

) is normally set to zero
(OFF). This PI Control minimises valve wear whilst giving optimal process control.
CAUTION:
This technique is suitable only for processes that are not harmed by large
fluctuations in the process variable.
1. Check that the Motor Travel Time (

) matches the time taken for the valve to travel
between its physical end stops. Adjust if required.
2. Check that the Minimum Motor On Time (

) matches the minimum drive effort needed
to initiate valve movement when stationary. Adjust if required.
3. Set the setpoint to the normal operating process value (or to a lower value if overshoot
beyond this value is likely to cause damage).
4. Set the Primary Proportional Band to the minimum value (i.e. set

= 0.5%).
5. Set the Integral Time Constant to the maximum value (i.e. set

= 99.59).
6. Set the Derivative Time Constant to OFF (i.e. set

= 0.00).