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Author Topic: PID Output  (Read 8945 times)

RBPLC

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PID Output
« on: July 26, 2020, 09:32:13 PM »
In the shown photo I've got a TC reading ambient temperature. When I change the .SP to slightly below the current ambient temp, the forward acting loop is still giving slight output with 0 Rate and 0 Reset. Why is this? I would expect that by setting the setpoint to anything lower than the current .PV would yield zero output from the loop. What am I missing?
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franji1

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Re: PID Output
« Reply #1 on: July 27, 2020, 10:05:28 AM »
The Bias term is non-zero, hence it is affecting the .Output calculation.  The Bias can become non-zero based on your "bumpless" transition behavior option or previous tuning constants.
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RBPLC

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Re: PID Output
« Reply #2 on: July 28, 2020, 07:19:27 PM »
What's the likelihood of adding a tickbox that would keep (force) the bias term at zero on a manual/auto transition? For many of the ways I would use the PID, it would be nice to keep it at zero during a transition. Not sure if this would be useful or not to the general user base. For now I'll just one shot the bias term to zero during the transition.
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Controls Guy

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Re: PID Output
« Reply #3 on: July 28, 2020, 09:09:37 PM »
I was thinking sort of the same thing.  If Ki is set to zero (at least from Do-More), maybe clear bias by default?
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Controls Guy

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Re: PID Output
« Reply #4 on: July 28, 2020, 09:10:58 PM »
Not continuously keep it at zero, just one-shot if the user clears Ki.
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ADC Product Engineer

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Re: PID Output
« Reply #5 on: July 29, 2020, 08:26:10 AM »
Quote from: RBPLC on July 28, 2020, 07:19:27 PM
What's the likelihood of adding a tickbox that would keep (force) the bias term at zero on a manual/auto transition? For many of the ways I would use the PID, it would be nice to keep it at zero during a transition. Not sure if this would be useful or not to the general user base. For now I'll just one shot the bias term to zero during the transition.

This is a fairly common issue.  If Mark/Bob can do add a tickbox or a structure field to force bias to zero, I would lend it my vote.
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