West Control Solutions 8700+ - Manual (Page 153)

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Glossary 59305, Issue 7 – March 2014
PLC Type: General Definition
This stands for Programmable Logic Controller. A microprocessor based device used in
machine control. It is particularly suited to sequential control applications, and uses “Ladder
Logic” programming techniques. Some PLC’s are capable of basic PID control, but tend to
be expensive and often give inferior levels of control.
Also refer to PID.
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
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.
Control Power
Pre-Tune
engaged
here
+100% Power (HEAT output)
Initial PV
Process Variable
(PV)
SP – Initial PV
2
Setpoint
(SP)
-100% Power (Cool output)