Re: [foxboro] cascade/constraint control
- From: "FoxPat" <fox@xxxxxxxxxxxxxxx>
- To: <foxboro@xxxxxxxxxxxxx>
- Date: Wed, 24 Oct 2007 08:21:41 +0200
Greg,
As already suggested it is better to use the OUTSEL block.
We use a lot of the 'override' or 'constraint' controls.
An important parameter that affects the behaviour of this scheme drastically
is the LIMOPT parameter of the PIDA (we only use PIDA, but I believe the PID
block has the same functionality regarding LIMOPT).
If you leave the LIMOPT at 1 you might see some strange behaviour and tuning
might not be made as robust as you would like.
When leaving LIMOPT at 1 the integral action of the 'not-selected'
controller will freeze as soon as it becomes 'not selected'. This means that
the output of the TC could be very close to the output of the LC while the
temperature is still significantly above setpoint, and as such, the P action
of the temperature controller (which does not freeze) could already effect
the setpoint of the FC when temperature drops but still is not below
setpoint.
We typically set the LIMOPT op both controllers (the LC and the FC in your
case) to 3. This means that the temperature controller will move it's output
away from the output of the Level controller so it will not already start to
interfere due to the P action of the temperature controller, even though the
temperature is still above setpoint. The smaller the offset on the TC
becomes, the closer the output of the LC will 'track' the output of the LC
and (if LC would have no offset), it will take over exactly at the point
where the offset of the TC changes sign (temperature drops below setpoint).
If you can spare the time I would suggest you build a small simulation test
configuration, set-up some trend display (use foxanalyst if you have it) and
do some 'step-testing'.
Be aware that control is a 'dynamic' thing and therefore what you see does
not always have a straightforward explanation.
I guess in your case tuning of both loops will be quite different (TC versus
LC) ?
If tuning of both controllers is significantly different (e.g. you could
have TC as PID and LC as PI with very long integral time) it will not make
things easier but it should still work.
You might find that, even though the temperature controller is above
setpoint it will still be the 'selected' controller if the LC also has a
significant offset.
I have seen configurations where output limits of the LC and/or TC where
'back-calculated' (with a +offset for the High Limit and a -offset for the
Low Limit) to make sure the LC and TC do not 'wind-up'.
Al of this is really not necessary as the PID(A) algorithms take care of al
of this.
It should work like a charm!
Success,
Patrick Martens
Total Raff. Ned. N.V.
-----Oorspronkelijk bericht-----
Van: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx]
Namens Gregory A Hurwitt
Verzonden: dinsdag 23 oktober 2007 22:07
Aan: foxboro@xxxxxxxxxxxxx
Onderwerp: [foxboro] cascade/constraint control
I have a control scheme that will normally run as level-flow cascade. If
the temperature of the downstream equipment falls too low, then the flow
must be restricted. I plan to use a constraint control scheme:
+-----------+ +-----------+ +-----------+
| | | | | |
| LC | --------> | SIGSEL | --------> | FC |
| | | (low) | | |
+-----------+ +-----------+ +-----------+
^
+-----------+ |
| | |
| TC | -----------------+
| |
+-----------+
What are the recommended FBK and BCALCI connections for the LC and TC? How
do I ensure that the TC does not wind up while it is not selected?
____________________________________________________________________________
__________
Greg Hurwitt
Engineering Associate
E-Mail: gregory.hurwitt@xxxxxxxx
Postal Address:
BASF Corporation
602 Copper Road
Freeport, TX 77541
USA
BASF - The Chemical Company
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