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	FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
		Digest Janitor: pbabcock at hbd.org


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Contents:
  Re: PID's ("Jeff Dieterle")


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Date: Fri, 16 Mar 2012 12:05:56 -0400
From: "Jeff Dieterle" <jddieterle at hughes.net>
Subject: Re: PID's

"It just occurred to me that perhaps Jeff's confusion about stability stems
from thinking that the on/off cycling of the output gets fed back to the
input of the controller i.e. that the PV signal has modulation on it. It
doesn't. The thermal mass of the water or mash is a big low pass filter.
Though I'm cycling on a 30 second cycle the thermal time constant of the
load is minutes. Does that help?"

Yes to a degree, my experience with closed loop control was in the high
pressure hydraulic industry with speed control loops on extrusion presses,
more responsive to error. In your situation using the foot pedal versus
closed loop control, an expensive option to stay closed loop is replacing
the steam pedal with an electrically actuated variable steam control valve
with its own cmd/fdbk positioning loop independent of the pid controller.
The valve positioning loop would be commanded by the 4-20ma output from the
pid, the mash temp would be the PV and target temp would be the SP.   



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