Re: [foxboro] PIDA parameters
- From: "Ashley Davey" <Ashley.Davey@xxxxxxxxxxxx>
- To: <foxboro@xxxxxxxxxxxxx>
- Date: Tue, 27 Sep 2005 07:30:59 +0200
Jim and Adam
Thank you.
I am sure now that I need to change the MODOPT from 6 to 5 on some of my loops.
Regards
Ashley Davey
Systems Engineer
African Products (Pty) Ltd
Gauteng
Republic of South Africa
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>>> Adam.Pemberton@xxxxxxxxxxxx 04:01:45 27/09/05 >>>
Wow. Great answer Jim.
One comment I would make which is already alluded to by Jim is that the
ideal (interacting) form is easier to tune. It has the advantage that
(for non-integrating processes) if the INT term is set right (generally
set to the time constant (read 63% point) of the open loop process) the
PBAND term can then be used to speed up or slow down the speed of
response. So if you find that you control is too aggressive, make PBAND
bigger. If it's too slow, make PBAND smaller (within limits). Because
the total integral action is a function of PBAND and INT, the integral
action will be adjusted appropriately.
One other comment. IMC methodology states that for a first order process
(example lag or integrator on it's own) a PI controller (MODOPT=4) is
required. If it's second order (similar order lag and integrator, 2 lags
in cascade, or resonant system) use a PID (MODOPT=5). The only other
situation would be a dead-time dominant process (dead-time > 2x lag time
const) where the dead-time is well known. In this circumstance you could
benefit from a PITAU or PIDTAU option. But get your dead-time right!!
Regards
Adam
PS: In academia a Lag is different from a dead-time. A lag starts moving
as soon as it's input changes, and reaches 63% of its new steady state
value after one time constant. A dead-time does not respond to an input
change at all for the period of the dead-time.
-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [ mailto:foxboro-bounce@xxxxxxxxxxxxx
< mailto:foxboro-bounce@xxxxxxxxxxxxx > ] On Behalf Of James D Murphy
Sent: Monday, 26 September 2005 11:57 PM
To: foxboro@xxxxxxxxxxxxx < mailto:foxboro@xxxxxxxxxxxxx >
Subject: Re: [foxboro] PIDA parameters
...
There are three different forms of the PID controller. They are the
standard or "non-interacting" form, the series or "interacting" form and
the parallel form. The standard form is the ideal form where the terms
are noninteracting in the time domain. The series or "interacting"
algorithm applies the gain term to both integral and derivative terms
(think of a PD and PI controller in series); this is effectively how
older pneumatic and some analog controllers worked. It is the most
restricted form of the three. The parallel form is the most general,
"mathematician's" form and is the most flexible of the three. However,
it is also the form where the parameters have little physical
interpretation. It is mostly used when tuning the PID algorithm
mathematically.
Jim Murphy
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