[foxboro] PIDA documentation (Was: PH controller in Waste Water Treatment)

"This option is seldom used and probably not
well documented,..."

So, how many out there really make full use of the capabilities of the 
PIDA block, particularly with regard to self/adaptive tuning 
(FBTUNE/FFTUNE, etc.)?  Documentation and training on how to do this is 
sorely lacking, and I haven't had/taken the time to experiment with it 
enough to teach myself.  IMHO, this is a 1-day training class Foxboro 
ought to offer; The EXACT/PIxTAU/etc. capabilities are distinguishing 
features of the I/A system, and Foxboro out to play them up by helping 
their customers get the most out of them (especially now that competitors 
are adding interesting bits along these lines to their own DCS 
controllers).


Corey Clingo
BASF Corp.





"Johnson, Alex P (IPS)" <alex.johnson@xxxxxxxxxxxxxxxx> 
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Re: [foxboro] PH controller in Waste Water Treatment






Robert,

I'm not control expert but I've lived with, worked with, and sat next to
lots of them. With that in mind, I'd like to make a couple of comments on
the threads of response that you have received that I think deserve a
comment:

1) There was a comment about the suitability of the PIDA algorithm and our
control block set for various types of process control problems - large 
dead
times, long lags, and combinations of both.

The PIDA algorithm when used with the PIDTAU option is quite capable of
dealing with long dead-times. This option is seldom used and probably not
well documented, but if for some reason your process has a long dead time 
we
do have a solution in our block.

Long lag processes aren't too bad in general if the DT is short. [Yes, 
this
is a general statement and there may be exceptions.]

The real problem is long lags and large dead times. Those problems need
extra help beyond our PID algorithm.


2) General comment on pH control

Here's what I've learned from those who really do understand pH control:

It is really, really easy to design a physical system for pH control that
simply cannot control pH regardless of the algorithm used.


The real issue with pH control is almost always related to the process
equipment. At the end of the day, if the pH in the inflow can vary 2:1, 
you
need a range of neutralizing flow that can support a 100:1 change in flow.
That's a bit wide for most flow valves and generally requires separate 
tanks
with separate neutralization flows and lots of mixers and so on.

No control algorithm is going to control the pH if the physical equipment
can't meet the process demands.

Are you certain that your physical plant can meet the need?


Regards,

Alex Johnson
Invensys Systems, Inc.
10900 Equity Drive
Houston, TX 77041
713.329.8472 (voice)
713.329.1700 (fax)
713.329.1600 (switchboard)
alex.johnson@xxxxxxxxxxxxxxxx





 
 
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