Despatch MIC1422 Controller - Manual (Page 63)

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SECTION 12: AUTOTUNE MODE
12.1 AutoTune Mode and Description
The Auto-Tune Mode is used to optimize tuning while the instrument is operating.
To access the Auto-Tune Mode, with the instrument in Control Mode, enter the Setup
Mode. Press the SCROLL key until Auto is displayed. To engage Auto-Tune, press the
UP key. The AT LED will light red.
Note: If the Enable Pre-Tune parameter in the Tune Mode is enabled, then on
power-up, the unit will automatically engage the Pre-Tune Mode prior to engaging
the Auto-Tune Mode when power is restored.
To disengage the Auto-Tune Mode, access the Auto-Tune Mode. Press the DOWN
key. The AT LED will turn off.
To return to Control Mode, press the SCROLL key until CtrL is displayed, then press
the DOWN key.
12.2 How Auto-Tune Works
This instrument uses a pattern recognition algorithm, which monitors the process error
(deviation signal). Figure 12-1 shows a typical temperature application involving
process start up, a setpoint change and a load disturbance. The deviance signal is
shown shaded and overshoots have been exaggerated for clarity.
The auto-tuning algorithm observes one complete deviation oscillation before
calculating a set of PID values. Successive deviation oscillations cause values to be
recalculated so that the controller rapidly converges on optimal control.
When auto-tuning controllers are switched off, the final PID terms remain stored in the
controller's nonvolatile memory, and are used as starting values at the next switch on.
The stored values are not always valid, if for instance, the controller is brand new or the
application has changed. In these cases the user can utilize an extra facility on the
auto-tuning controllers called "Pre-Tune". Figure 12-2 shows how 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.