Re: [foxboro] cascade/constraint control

Hi Greg,
we've done something simular with flow and pressure control.


Var. setpoint--+  BCALCI<------BCALC1
               |  |            |
Flow     AIN-->PIDE----->|     |
                         |OUTSEL----->AOUT(valve control)
Pressure AIN-->PIDE----->|     |
               |  |            |
Fixed setpoint-+  BCALCI<------BCALC2

In our case we need to maintain a minimum pressure. OUTSEL is set to
pass the highest output to the AOUT.
The pressure PID block has a fixed setpoint based on a minimum pressure.
The flow PID block setpoint is based on flow demand.

As the demand for flow is dropping, the valve closes, which will cause a
reduction in pressure. This will result in a higher output on the
pressure PID block, forcing the valve to maintain a position that
retains the minimum pressure.

I'm sure something like the above can be done with flow and temperature.

Cheers,

Frits Schouten.
NZSteel


-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx]
On Behalf Of Gregory A Hurwitt
Sent: Wednesday, 24 October 2007 9:07 AM
To: foxboro@xxxxxxxxxxxxx
Subject: [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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