Subject: FW: PRIBLK - Our special friend
Date: Mon, 27 Mar 2000 09:06:03 +1000
From: "O'Brien, Con (EUPRO)" <O'[EMAIL PROTECTED]>
To: "'Foxboro DCS mail list'" <[EMAIL PROTECTED]>
Hi Con,
I talked with one of the instructors here at Foxboro about this problem
with bumpless transfer on split range or sequential valve control. The
problems with the BCALCO not working with CHARC come because on
initialization CHARC keeps last good BCALCO until it receives an
initialization acknowledgement from the upstream block. If PRIBLK is not
set there will never be an initialization acknowledgement, so BCALCO does
not update (this is explained more explicitly in the last couple paragraphs
of the CHARC block detail in the ICC Blocks document).
I tried a number of different control schemes to work around this. It is
posible to deliver the initialization acknowledgements to the CHARC blocks
by using a CALC block downstream of the PID block and using the PRO command
to propigate the status information out to both CHARC blocks. You can get
the BCALCI information from one CHARC block back up through the CALC block
to the PID with PRI, but you can't combine that with the info from the
other CHARC block.
The best solution I have come up with uses BIAS blocks with PRIBLK = 0 to
terminate the downstream controls (BIAS-->CHARC-->AOUT) for initialization
(so BCALCO works on the charc blocks) and use a CALC block to monitor the
ERROR/BAD/MA status of the downstream blocks and send that indication into
the INITI on the PID block. The same CALC block can monitor for an
upstream condition upstream and force the downstream controls to track or
hold. The CALC block reads BCALCO values from the CHARC blocks to compute
PID BCALCI and FBK any way it is desired.
I'm not very satisfied with this solution, but have not been able to come
up with a better one for both split range valves and for sequential valves.
The field engineers I have talked with so far say they just use a CALC
block to split out the signals to the valves and don't use PRIBLK, but they
are typically linearizing the flow response with valve sizing and
positioners or living with non-linear flow response rather than using CHARC
blocks. When I get a chance I'm
asking TAC to see if they have a model answer for this question.
Regards,
Kevin FitzGerrell
Fairbanks Gold Mining, Inc.
---------------original message------------------
Hi Kevin,
This is indeed the problem I've been having. I'd like to be able to
manipulate the initialization bits directly from a CALC block, so that in
the case of two parallel control streams, I could logically combine the
initialisation requests.
I'd also be interested to hear what the official word is on the CHARC
BCALCO.
Thanks,
Con
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