PID control of gas chromats

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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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?
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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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