Re: [foxboro] 1 in 2 out

Terry,
There are several solutions depending upon conditions that you are
willing to accept.  We have a similar arrangement that controls our four
well field water pumps (except the PID controls pressure instead of
level).  The motor speed controller makes its output available to all
four VFDs.  Each pump speed controller gets the same value.  The
individual pumps are either on or not, it does not matter.  When a new
pump comes on, the pressure increases, so the requested output
automatically adjusts itself and the other pumps slow down.  To smooth
things out, we have a sequence that ramps the new motor speed up to the
setpoint, then puts it to remote.  (The ramp moves slower than the
pressure controller tuning, so the other pumps adjust accordingly.)  In
this case, the pressure PID FEEDBACK and BCALCI come from its own
output.

Another solution is to use a CALC block (and perhaps a SWCH block) to
determine which of speed control values get sent back to the level
controller inputs.  The logic for the CALC block depends on the behavior
you desire in the cascaded loop.  VFD motors have analog and digital
controls.  Do you allow the analog controls to "float" when the digital
control is off?  Or, do you put the PID to MANUAL and give it an output
of zero?  Do you use the HOLD parameter of the speed PID instead of
going to MANUAL?  These are just some of the behaviors involved in
figuring out what you want your CALC logic to be.

Another scenario assumes your level controller gives each of the speed
controllers a different set point.  For instance, the second motor
starts after the first has reached some critical speed.  Here, a CALC
(or MATH) block works a little algebraic magic on the BCALCO of the two
speed controllers--and takes into account the questions in the previous
paragraph--before sending the result back to the BCALCI and FEEDBACK of
the level controller.

There are still other solutions... =20

I guess the short answer is that there is no short answer.  You get to
figure out what you really want it to do, work the math out, figure out
the logic, and program it all in using one or more other blocks before
feeding it back into the level controller. =20

Good luck,

Chuck Jones
Automation Technologist
Tate & Lyle -- Lafayette South Plant



-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx]
On Behalf Of Terry Currie
Sent: Tuesday, May 30, 2006 3:01 PM
To: foxboro@xxxxxxxxxxxxx
Subject: [foxboro] 1 in 2 out


We have a level control cascaded to setpoint of two VFD speed control
setpoints.
How is this configured for BALCI or FEEDBACK?
The problem is one or both pumps may be running.
Terry


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