West Control Solutions Pro-EC44 - Manual (Page 108)

Pro-EC44 2-Loop Graphical Profile Controller & Recorder
Pro-EC44 Product Manual - 59540-1 October 2013 Page 101
Pre-tune is selected from the automatic tuning menu. It will not engage if either primary or
secondary outputs on a controller are set for On-Off control, during setpoint/profile ramping
or if the process variable is less than 5% of the input span from the setpoint.
Note: To pre- tune a cascade, first select “Cascade-Open” to tune the PID set(s)
on the slave. After the slave has successfully tuned, remember to pre-tune the
master/slave combination (this time select “Cascade-Closed”). The cascade
remains open until you do this.
Auto Pre-Tune
As a single-shot operation, pre-tune will automatically disengage once complete, but can be
configured to run at every power up using the auto pre-tune function. If auto pre-tune is
selected, a Standard Pre-tune will attempt to run at every power up, applying new tuning
terms to the current Active PID set. Auto pre-tune will not be able to test the process if at the
time the controller is powered up, either primary or secondary outputs are set for On-Off
control, during setpoint/profile ramping or if the process variable is less than 5% of the input
span from the setpoint. Auto pre-tune is not possible with cascade control mode.
Self-Tune
If engaged, self-tune uses a pattern recognition algorithm to continuously monitor and adjust
for control deviation. It optimises the tuning by applying new PID terms to the current Active
PID set while the controller is operating. In VMD control mode, derivative is not applied by
self-tune, and the controller is optimised for PI control.
Figure 59. Self-Tune Operation
The diagram shows a typical application involving a process start up, setpoint change and
load disturbance. In each case, self-tune observes one complete oscillation before
calculating new terms. Successive deviations cause the values to be recalculated converging
towards optimal control. When the controller is switched off, these terms are stored and used
as starting values at switch on. The stored values may not always be ideal, if for instance the
controller is new or the application has changed. In this case the user can use pre-tune to
establish new initial values for self-tune to fine-tune.
Use of continuous self-tuning is not always appropriate. For example frequent artificial load
disturbances, such as where an oven door is often left open for extended periods, might lead
to calculation errors. In standard control mode, PI & D are all calculated, which may not suit
all processes. Self-Tune cannot be engaged if the instrument is set for on-off control or with
cascade control mode.
Te
mperatu
re
Setpoint 1
Setpoint Change
Load Disturbance
Time
Setpoint 2
Start-up