Re: [foxboro] RAMP block question

Chad,

This is a defect in our cascade handling. Would you please enter a CAR on
this? Include the write-up below as the description. We need a customer
entered CAR to give this some priority.


As it turns out, all control blocks that participate in cascade handling
have the following anomaly. 

When the cascade is opened, or closed, or when an initialization request to
the upstream block is made, the downstream block requests the upstream block
to run immediately on the next BPC regardless of the period/phase of the
upstream block. This logic was created many years ago to provide fast
initialization of the cascade when the upstream blocks run slower than the
downstream block.

The flaw in the cascade logic is that the downstream block should check to
see if PRIBLK=1 before issuing a demand run of the upstream block. If this
were done in this case, the RAMP block would not run additional cycles and
the calculation would be correct.


Regards,
 
Alex Johnson
Invensys Process Systems
Invensys Systems, Inc.
10707 Haddington
Houston, TX 77043
713.722.2859 (voice)
713.722.2700 (switchboard)
713.932.0222 (fax)
ajohnson@xxxxxxxxxxx
-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx] On
Behalf Of Airhart, Chad M.
Sent: Tuesday, January 25, 2005 5:18 PM
To: foxboro@xxxxxxxxxxxxx
Subject: [foxboro] RAMP block question

One of our specialists wanted me to put this question out on the list.
He has built a RAMP block that will be used to ramp the remote setpoint
of a PIDE block in a control scheme.  The RAMP is supposed to move 12000
SCF ( actually from 0 to 12 ) in 15 minutes.  He set the ramp rate at
0.8 or 12/15.  When he runs the block as stand alone, and times it, it
works great.  0.8 the first minute, 1.6 the second etc...  When he makes
the one necessary connection to the remote set point of the PIDE it
ramps twice as fast.  The actual output of the ramp is twice as fast
i.e. 1.6 in the first minute instead of 0.8.  Any ideas why?
P.S. KSCALE is set at 1.0, Range in and out is the same, scale is the
same, engineering units are the same.  Period and phase are even
matched.  Every block in compound are on 1 second period phase 0.  Only
5 blocks in the compound.  Version 6.3 system running display manager
and legacy historian.  When he adjusts the ramp rate to 0.4 or half it
ramps at the rate he wants but we want to know why.

Chad M. Airhart
Senior Engineer (Instrument, Electrical and Control Sys)
Lyondell Chemical Co.                             Victoria Plant
Ph. (361)572-2568
Fx. (361)572-2541
Cell. (361)935-3230


 
 
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