Re: PID controller: dummy questions

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.arch.embedded
Subject	Re: PID controller: dummy questions
Date	2020-06-26 11:07 +0200
Organization	A noiseless patient Spider
Message-ID	<rd4dsp$c79$1@dont-email.me> (permalink)
References	<rd2odq$oiv$1@dont-email.me> <rd4dct$9dc$1@dont-email.me>

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On 26/06/2020 10:59, pozz wrote:
> Il 25/06/2020 19:55, pozz ha scritto:
>> I'm trying to implement a PID controller in C for an embbeded
>> system. This is the first time. I think I haven't fully understand
>> the theory.
> 
> I read all your answers, but I didn't get the point.
> 
> I'm thinking of a system where a process variable (PV, such as a
> liquid flow) must match a desired set point (SP, such as 1 lt/min).
> The controller can change a control variable (CV, such as a voltage
> applied to an electronic valve).
> 
> If we need 1 lt/min we start opening the valve increasing the
> voltage control. When we see 1 lt/min on a flowmeter, we stop
> increasing the voltage that stay indefinitely at, for example, 1.0V. 
> At the steady state, when the error is zero, the control voltage *is 
> not* zero, but 1.0V.

You are describing an "I" controller, not a "P" controller.

For long-term steady state regulation, the "I" bit of "PID" is typically
the most important.  But lacking a "P" part means you will react slowly
to changes.  And if you lack the "D" part, you need to keep the "I" part
slow to avoid oscillations.

> 
> The proportional term (P) of a PID controller can't mimic this
> "manual control", because when the error is zero (we found the right
> voltage on the valve), the P term drops to zero and the valve would
> close.

That is correct.

That is why a pure "P" controller is often not a suitable choice.  It
can never achieve a zero-error steady state.

> 
> When I use my brain, I will do:
> 
> - the error is high, rotate the knob fast - the error is low (we are
> approaching the desidered value), rotate the knob slowly - the error
> is zero, ok stop rotating the knob
> 
> The P term of a PID controller works in a different way:
> 
> - the error is high, high voltage control, open the valve high - the
> error decreases, low voltage control, the valve starts closing - the
> error is zero, close the valve completely (P=0)
> 
> 
> Maybe my brain works as an integrative-only controller?

Brains are not consistent enough to be classified as PID controllers of
any kind.

Thread

PID controller: dummy questions pozz <pozzugno@gmail.com> - 2020-06-25 19:55 +0200
  Re: PID controller: dummy questions Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-06-25 21:14 +0200
  Re: PID controller: dummy questions DJ Delorie <dj@delorie.com> - 2020-06-25 15:29 -0400
    Re: PID controller: dummy questions David Brown <david.brown@hesbynett.no> - 2020-06-26 08:43 +0200
  Re: PID controller: dummy questions Richard Damon <Richard@Damon-Family.org> - 2020-06-25 22:21 -0400
  Re: PID controller: dummy questions pozz <pozzugno@gmail.com> - 2020-06-26 10:59 +0200
    Re: PID controller: dummy questions David Brown <david.brown@hesbynett.no> - 2020-06-26 11:07 +0200
    Re: PID controller: dummy questions Rick C <gnuarm.deletethisbit@gmail.com> - 2020-06-26 09:10 -0700
    Re: PID controller: dummy questions Richard Damon <Richard@Damon-Family.org> - 2020-06-26 12:29 -0400
      Re: PID controller: dummy questions Rick C <gnuarm.deletethisbit@gmail.com> - 2020-06-26 12:00 -0700
        Re: PID controller: dummy questions Richard Damon <Richard@Damon-Family.org> - 2020-06-26 22:24 -0400
          Re: PID controller: dummy questions Paul Rubin <no.email@nospam.invalid> - 2020-06-26 21:41 -0700
            Re: PID controller: dummy questions DJ Delorie <dj@delorie.com> - 2020-06-27 01:15 -0400
              Re: PID controller: dummy questions Paul Rubin <no.email@nospam.invalid> - 2020-06-26 23:12 -0700
                Re: PID controller: dummy questions Rick C <gnuarm.deletethisbit@gmail.com> - 2020-06-27 00:07 -0700
                Re: PID controller: dummy questions pozz <pozzugno@gmail.com> - 2020-06-27 16:53 +0200
                Re: PID controller: dummy questions David Brown <david.brown@hesbynett.no> - 2020-06-27 17:31 +0200
                Re: PID controller: dummy questions Rick C <gnuarm.deletethisbit@gmail.com> - 2020-06-27 10:09 -0700
                Re: PID controller: dummy questions Rick C <gnuarm.deletethisbit@gmail.com> - 2020-06-27 15:44 -0700
                Re: PID controller: dummy questions Richard Damon <Richard@Damon-Family.org> - 2020-06-27 12:34 -0400
                Re: PID controller: dummy questions DJ Delorie <dj@delorie.com> - 2020-06-27 11:51 -0400
            Re: PID controller: dummy questions Rick C <gnuarm.deletethisbit@gmail.com> - 2020-06-26 23:06 -0700
              Re: PID controller: dummy questions Paul Rubin <no.email@nospam.invalid> - 2020-06-26 23:18 -0700
                Re: PID controller: dummy questions Rick C <gnuarm.deletethisbit@gmail.com> - 2020-06-27 00:23 -0700
            Re: PID controller: dummy questions Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2020-06-27 10:28 +0300
            Re: PID controller: dummy questions Richard Damon <Richard@Damon-Family.org> - 2020-06-27 18:13 -0400
    Re: PID controller: dummy questions Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-06-26 22:30 +0200
      Re: PID controller: dummy questions Phil Martel <pomartel@comcast.net> - 2020-06-28 13:10 -0400

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