HOMEBREW Digest #5917 Fri 16 March 2012

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  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. Return to table of contents
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