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DCP200 Profile Controller & Recorder - Product Manual
51-52-25-150, Issue 1 – April 2009 Glossary Page 159
Self-Tune
Self-Tune continuously optimises tuning while a controller is operating. It uses a pattern
recognition algorithm, which monitors the control deviation. The diagram shows a typical
application involving a process start up, setpoint change and load disturbance.
Figure 53. Self-Tune Operation
The deviation signal is shown shaded, and overshoots have been exaggerated for clarity.
The Self-Tune algorithm observes one complete deviation oscillation before calculating a
new set of PID values. Successive deviation oscillations cause the values to be recalculated
so that the controller converges on optimal control. When the controller is switched off, these
PID terms are stored, and are used as starting values at the next switch on. The stored
values may not always be ideal, if for instance the controller is brand new or the application
has changed. In these cases, the user can utilise Pre-Tune to establish new initial values.
Self-Tune will then fine-tune these values as it monitors any control deviation.
Use of continuous self-tuning is not always appropriate. For example applications which are
frequently subjected to artificial load disturbances, for example where an oven door is likely
to be frequently left open for extended periods, can lead to errors in the calculations. In
addition, because Self-Tune tunes for full PID control, it is not recommended for valve control
applications, which normally require PI control.
Self-Tune cannot be engaged if the instrument is set for On-Off Control.
Also refer to: Control Deviation, Modulating Valves. On-Off Control, Pre-Tune, PI, PID,
Setpoint and Tuning.
Sensor Break Pre-Set Power
If a thermocouple or RTD breaks, or it is disconnected, the instrument detects this condition
within 2 seconds, and sets the control output(s) to the value defined by the Sensor Break
Pre-Set Power Output parameter in the Control Configuration sub-menu. Process alarms
behave as though the PV has gone high.
Non-zero based linear inputs (e.g. 2 to10V or 4 to 20mA, but not 0 to 20mA) can also detect
a sensor break condition, setting the power to the Pre-Set Power value. Process alarms
behave as though the PV has gone low.
Also refer to: Input Range, Linear Input, RTD and Thermocouple.
Te
mperature
Setpoint 1
Setpoint Change
Load Disturbance
Time
Setpoint 2