Re: [foxboro] PH controller in Waste Water Treatment
- From: "Rick Rys" <Rys@xxxxxxxxxxxxxx>
- To: <foxboro@xxxxxxxxxxxxx>
- Date: Wed, 22 Mar 2006 07:45:25 -0500
Robert,
Took a quick look at the MFA Controller. Looks like the early Foxboro =
analog
design (H-line/Spec200) with 3-break points.
http://www.cybosoft.com/technologies/mfaph.html. It is not an =
improvement
over what you have built into IA PIDA which incidentally was originally
developed for the 761/762/743 controller family. This function was =
built
because it was one of the processes where EXACT (self tuner) was not a =
good
solution and would improve capability. The PIDA links to a CHAR block =
for
this function just like the 761 did. An important point which MFA gets =
wrong
is that pH control is not actually logarithmic except for pure distilled
water with ideal strong acid/base. The non-linear gain problem can only
start with a detailed titration analysis of the waste water, as the
titration can change when something new is flushed down the drain. =
Computing
titration curves is mathematical nightmare of non-linear equations fit =
only
for the simplest of solutions. If you have a consistent titration curve =
then
characterization can help. Generally characterizing both the Setpoint =
and
Measurement is the best way to go as error characterization is only =
valid
for a particular setpoint. Model predictive controllers have some
difficulty with linearizing the controller error. Linearizing the valve =
(or
pump) can be done separately. Few people have actually done this =
procedure
correctly, but the tools are there. See
http://www.r2controls.com/761_linearization.htm. Some pH control =
problems
require reagent delivery of 10 million units of reagent plus or minus =
1.0
unit to keep within setpoint range . With a varying load and process
deadtime you have can have a very difficult control problem that is not
solved by controller alone regardless of it's sophistication. The =
design of
the reaction vessels, electrode placement, and reagent metering are
essential here. Some pH control loops are quite easily handled with
ordinary PID or even on-off control. Which kind do you have??
Rick Rys P.E.
www.R2Controls.com
508-369-5186 Cell
302-836-6540 Delaware Office Days
Valero E-Mail: Rick.Rys@xxxxxxxxxx
=20
_______________________________________________________________________
This mailing list is neither sponsored nor endorsed by Invensys Process
Systems (formerly The Foxboro Company). Use the info you obtain here at
your own risks. Read http://www.thecassandraproject.org/disclaimer.html
foxboro mailing list: //www.freelists.org/list/foxboro
to subscribe: mailto:foxboro-request@xxxxxxxxxxxxx?subject=join
to unsubscribe: mailto:foxboro-request@xxxxxxxxxxxxx?subject=leave
Other related posts:
