Re: [foxboro] PIDE & PIDA
- From: "Rick Rys" <Rys@xxxxxxxxxxxxxx>
- To: <foxboro@xxxxxxxxxxxxx>
- Date: Wed, 29 Mar 2006 19:44:21 -0500
Nurul,
PIDA was developed after some operating experience with the PIDE and is
generally superior. The PIDE is self contained but the PIDA uses the =
FBTUNE
block. Both have a Pretune and Self Tune mode. The self tune for PIDE =
is
using the BMP parameter with a step upset and waits for WMAX time for a
response. The PIDA also has a BMP parameter (actually in the FBTUNE) =
but
uses a bi-polar pulse (better) and only uses the THRESH (noise =
threshold).
You must have enough BMP to exceed the THRESH before the bi-polar pulse =
will
invert. The Self-Tune mode in the E or the A is a unique feature that to =
my
knowledge no other vendor provides. It tunes with the controller in
automatic by detecting peaks. No peaks =3D no tuning changes. It is =
not
uncommon for the controller to tune itself to sleep, but both PIDE and =
PIDA
can tune themselves back from sluggish when an upset comes along. The =
PIDA
is much better but not perfect at sorting out upsets entering the loop =
at
the natural frequency of the loop. PIDE will futilely attempt to tune =
such
upsets out. PIDE criteria is set with damping and overshoot as the =
tuning
objective. PIDA has no settable objective but typically results in a =
nearly
critically damped response. =20
Both algorithms were designed for load upsets rather than so called =
"servo"
or setpoint response. PIDA also has more mode options and the self =
tuning
function is adapted to each including the PIDTAU. The PIDA tuner also
computes the SPLLAG and the KD derivative filter time with the Pretune. =
The
PIDA can use an algebraic method or a fuzzy logic method to determine =
the
tuning. Don't ask me to explain the difference, but the default =
algebraic
method is generally successful. As I recall the algebraic method does
create an internal transfer function process model that is used in the
computation of the tuning parameters.=20
Generally, I like to use the PIDA Pretune function for most tuning. =
Pretune
usually finds excellent tuning settings in one attempt. It tunes =
shortly
after completing it's bi-polar pulse, although sometimes it will fail to
tune and it is not clear why. You can use a Pretune request of 2 to help
force it to tune, but it can still fail to tune in some cases. The =
Pretune
will also compute the PR_TYP (process type) parameter automatically. In =
my
opinion neither the PIDE nor the PIDA can be trusted in the self tune =
mode
without keeping an eye on it. You can however restrict the range of
adjustment on the tuning parameters. PIDA will detect if it really =
screwed
up and will turn itself off and use the fallback tuning settings. In
Self-tune you can also see it identifying peaks/quiet/settling etc in =
the
block displays.
You can make a simple simulation with PIDA, FBTUNE, DTIME, and LLAG =
blocks
and get a little practice before using it in the plant.
Best regards
Rick Rys P.E.
www.R2Controls.com
508-369-5186 Cell
302-836-6540 Delaware Office Days
R2 E-Mail rys@xxxxxxxxxxxxxx
Valero E-Mail: Rick.Rys@xxxxxxxxxx
=20
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