Despatch P1400+ Temperature Controller - Manual (Page 89)

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Pre-Tune Type: Controller Definition
The Pre-Tune facility artificially disturbs the start-up pattern so that a first approximation of the PID
values can be made prior to the setpoint being reached. During Pre-Tune, the controller outputs full
Primary Power until the process value has moved approximately halfway to the setpoint. At that
point, power is removed (or outputs full Secondary Power for Dual Control), thereby introducing an
oscillation. Once the oscillation peak has passed, the Pre-Tune algorithm calculates an
approximation of the optimum PID tuning terms proportional band(s), automatic reset and rate. The
process is shown in the diagram below.
When Pre-Tune is completed, the PID control output power is applied using the calculated values.
Pre-Tune limits the possibility of setpoint overshoot when the controller is new or the application has
been changed. As a single-shot operation, it will automatically disengage once complete, but can be
configured to run at every power up using the Auto Pre-Tune function.
The Pre-Tune feature on Valve Motor Drive controllers always sets the Rate parameter to zero
(OFF) because derivative action is not usually desirable in these applications.
Pre-Tune will not engage if either primary or secondary outputs on a controller are set for On-Off
control, during setpoint ramping or if the process variable is less than 5% of the input span from the
setpoint. Pre-Tune Operation
Figure 38. Pre-Tune Operation
Also refer to Auto Pre-Tune , Automatic Reset , Control Type , On-Off Control , Input Span , PID ,
Primary Proportional Band , Process Variable , Rate , Secondary Proportional Band , Self-Tune ,
Setpoint , Setpoint Ramping , Tuning and Valve Motor Drive Control .
Primary Output Power Limit Type: Controller Parameter
Used to limit the power level of the Primary Output and may be used to protect the process being
controlled. It may be adjusted between 0% and 100%. This parameter is not applicable if the
primary output is set for On-Off control.
Display code is
OPhi , default value = 100%
Also refer to On-Off Control .
Control Power
Pre-Tune
engaged
here
+100% Power (HEAT output)
Initial PV
Process Variable
SP – Initial PV
2
Setpoint
-100% Power (Cool output)