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Control system - open loop transfer function and open loop gain

  1. Nov 20, 2012 #1
    control system -- open loop transfer function and open loop gain

    Is there any difference between open loop transfer function and open loop gain ?
     
  2. jcsd
  3. Nov 21, 2012 #2

    CWatters

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    Re: control system

    What if the transfer function was

    y = Acos(xt + θ)

    where θ was a phase shift.

    What would the gain be?

    What if the open loop gain varies with frequency?
     
  4. Nov 21, 2012 #3

    rude man

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    Re: control system

    A transfer function gives gain and phase shift at any given frequency. Gain is the first part of that.
     
  5. Nov 22, 2012 #4
    Re: control system -- open loop transfer function and open loop gain

    Does this mean that transfer function can provide with negative value due to phase shift !
     
  6. Nov 22, 2012 #5

    NascentOxygen

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    Re: control system -- open loop transfer function and open loop gain

    I think you will find some authors make no distinction, both terms equating to G(s)H(s).
     
  7. Nov 22, 2012 #6

    NascentOxygen

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    Re: control system -- open loop transfer function and open loop gain

    There has to be a negative sign somewhere in the loop over the frequency band of interest*, either in G(s) or else H(s), otherwise your loop gain will be +ve and when the loop is closed the system will be unstable.
     
  8. Nov 22, 2012 #7

    rude man

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    Re: control system -- open loop transfer function and open loop gain

    Most assuredly yes.

    Consider H(s) = 1/s2. Letting s = jω you get a gain of -1/ω2, in other words a phase shift of 180 degrees.

    (This is two integrators in cascade FYI).
     
  9. Nov 23, 2012 #8
    Re: control system -- open loop transfer function and open loop gain

    That would be the loop gain since H(s) is normally in the feedback path :)

    Open loop transfer function and open loop gain are almost used interchangeably. It's assumed to be the 'A' part (or 'G' part) of your standard feedback loop diagram.

    Loop gain is the gain after making one lap around the feedback loop from the output of the summer to the input of the summer and is Aβ (or GH) of your standard feedback loop diagram.
     
  10. Nov 23, 2012 #9

    NascentOxygen

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    Re: control system -- open loop transfer function and open loop gain

    Correct. ... equating both to G(s).
     
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