West Control Solutions Pro-EC44 - Manual (Page 107)

Pro-EC44 2-Loop Graphical Profile Controller & Recorder
Pro-EC44 Product Manual - 59540-1 October 2013 Page 100
Automatic Tuning
To automatically optimise the controllers tuning terms for the process, you can use Pre-Tune,
Self-Tune or Auto Pre-Tune independently for each control loop.
Note: Automatic tuning will not engage if either proportional band is set to On/Off
control. Also, pre-tune (including an auto pre-tune attempt) will not engage if the
setpoint is ramping, if a profile is running, or if the Process Variable is <5% of
span from setpoint.
Pre-Tune
Pre-tune performs a single disturbance of the normal start-up pattern so that a good
approximation of the ideal PID values can be made prior reaching setpoint. It automatically
stops running when the test is complete. The user chooses which PID set the new tuning
terms will be applied to, but this selection does not change the selected
“active PID set”. This
allows tuning of any PID set for future use before return to control with the current PID set.
In VMD mode, derivative is not applied by pre-tune, and the controller is optimised for PI
control. In standard control mode, PI & D are all calculated, which may not suit all processes.
There are two pre-tune modes with different process test points . The first is “Standard Pre-
Tune ” which tests the process response half-way from the activation point (the process value
when pre-tune began running) to the current setpoint . The second type is “Pre-Tune at
Value
” which allows the user to specify the exact point at which the process test will occur.
CAUTION: Consider possible process over-shoot when selecting the value
to tune at. If there is a risk of damage to the product or equipment select a
safe value.
During pre-tune, the controller outputs full primary power until the process reaches the
specified test point. Power is then removed (full secondary power applied for dual control),
causing an oscillation which the pre-tune algorithm uses to calculate the proportional
band(s), integral and derivative time. The pre-tune process is shown below.
Figure 58. Pre-Tune Operation
Oscillation Peak
Test Point specified, or
for “standard” pre-
tune
Control Power
Pre-Tune
engaged
here
+100% Power (HEAT output)
Initial PV
Process Variable (PV)
Pre-Tune Value set or
for Std Pre-Tune =
SP – Initial PV
2
Setpoint (SP)
-100% Power (Cool output)