PID control of gas chromats

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Bruce Varley's Avatar (by Gravatar) posted on
August 14, 2012, 9:13 am
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After many years, I've come across a discontinuous analyser again, not a
chromat but behaves the same. Takes a sample, and about 15 minutes later the
analysis output updates. I vaguely recall an old trick when applying PIDs to
these types of signals, it was to apply two time constants in series to the
analog sampled output, then use that signal as the PID input.

What I can't recall is whether the value of each time constant should be
half the sample time of the analyser or the full sample time. Has anyone
come across this, can recall what the rule of thumb is?

We're obviously looking at more sophisticated algos, but there are
attractions in this case to using a standard error-based controller.



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Tim Wescott's Avatar (by Gravatar) posted on
August 20, 2012, 1:56 pm
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Re: PID control of gas chromats

On Tue, 14 Aug 2012 21:13:21 +0800, Bruce Varley wrote:


No recollection here, but it sounds like the solution you recall would
make sense if you wanted to get a pretty good shot at PID control with a
continuous-time controller without humongous discontinuities in the
drive.  Ditto if the PID controller was sampled but had a fixed sampling
rate much higher than your instrument output.

Keeping that in mind, a 1st-order lowpass would give you exponentially
decaying spikes from your derivative, while a 2nd-order lowpass would
round out the derivative action into (hopefully) nice bumps.  I know
you're smart enough to go from there to a solution.

Being as I come from the embedded control world, where one is neither
limited nor supported by a PLD or other pre-made rectilinear box with
predefined functionality, I would just make a PID controller that sampled
synchronously with the measurement output.  This would make any
discontinuities from the proportional and derivative action take the form
of 15-minute long, presumably-small rectangular pulses.  If a square edge
every 15 minutes was an issue, I'd follow the controller with a filter
that smooths out the output to whatever degree desired, with a settling
time within that 15 minute interval.

Hey, by the way -- how common is the name Varley, and if it's rare, do
you have any relatives in BC who fly model airplanes?

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

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Bruce Varley's Avatar (by Gravatar) posted on
August 21, 2012, 4:06 am
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Re: PID control of gas chromats



Yep, we're looking at synchronous solutions, but given the application
requirements, I suspect that a simple 2-lag solution may work fine. It's
simple and it keeps the operator interface consistent.

A dynamic model suggests that 2 lags, each with TC = 0.5 * sample time,
works OK, the resulting output is smooth enough to allow a controller with
some derivative to be used.


No idea, rellies pop up from time to time in other places. If there is one
in BC I hope they'll get in touch, it's a great part of the world.



 

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