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Symptom:
Have
you observed that having a positioner on a valve controlling a fast process
makes the loop unstable or hard to control?
Probable cause:
A loop
becomes unstable or hard to control if the loop gain is too high. In this
example, the positioner in a fast process may make the loop gain high enough
at the loop natural frequency to cause instability.
Corrective action:
A positioner is the
inner loop of a cascade control setup. To obtain good control, the inner
loop of cascade control must be faster than the outer loop. The block
diagram below describes this positioner cascade control.
There are two general
areas where corrective action can stabilize a positioner in a fast fluid
process, such as flow.
Electronic
Control: Reducing the controller gain to a lower value will
usually stabilize this control. If Lambda tuning is used, use a
conservative Lambda in order to arrive at the correct gain value. (See
Loop Tip #7). This is usually possible because most electronic
controllers have a large adjustment range on the controller gain.
Pneumatic Control:
If controller tuning with reduced gain is not successful, the best approach
is to remove the positioner. This is because most pneumatic controllers do
not have a large gain adjustment range, and therefore cannot obtain a low
enough gain value. If this is done, the control valve will now have a larger
frictional deadband, which would require a higher controller gain to help go
through the deadband.
In either case, the
controller reset should be kept fast to match the process. |
