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Bode plot

  1. Apr 5, 2014 #1
    What frequency are they referring to when they plot magnitude versus frequency in a bode plot of a control system? I don't understand where frequency comes from in control systems. I've researched this and I'm still very confused by what is meant by frequency.
  2. jcsd
  3. Apr 5, 2014 #2


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    Staff: Mentor

    Consider a DC-DC power supply. The feedback voltage monitors the output voltage, and the pulse-width of the "On Time" is adjusted to supply the right amount of current to keep the output voltage in regulation.

    Now make the load impedance vary with time -- that happens all the time with real loads. You can do a Bode plot of the output voltage variation versus the variation of the load impedance. You will find that at some frequencies, the DC-DC will no longer be able to maintain output voltage regulation, because the feedback control loop is not able to keep up with regulation at those frequencies.
  4. Apr 5, 2014 #3
    So it's an instantaneous measurement of input to response events that are being caused by system noise or a ramp sequence?
  5. Apr 6, 2014 #4

    jim hardy

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    wow - i can't parse that sentence.

    Bode Plot is a graph.
    It's a graph of the system's frequency response.
    It's a graph of the ratio of output signal amplitude to input signal amplitude at various frequencies. That ratio is the system Gain.

    Designers would generate the graph analytically using the transfer function of the system.
    Experimenters might generate it empirically using an oscilloscope and signal generator.
    A student of control systems should do both.

    The Bode plot also has a second line, phase shift.
    For a hypothetical sinewave input(or a real one for that matter) the output will usually lag behind.
    The amount of that lag, expressed in degrees , is the phase shift.

    As you know a sine wave has a frequency associated with it.
    It's the assumed frequency of a hypothetical sine wave input to the system, or the actual frequency of a real one .
    If you generate your Bode plot empirically as mentioned above, you will apply real sinewave inputs at various frequencies, plot the results and connect the dots.

    One reason Bode plots are so popular is they tell you at a glance how close your system is to instability.
    If a system has gain of 1 or more and phase shift of 180 degrees at any frequency it WILL oscillate at that frequency. Mother Nature is heartless.
    You can see clearly on the Bode plot whether your system approaches that condition.

    So a Bode plot , which can be made analytically, can save you from building a machine that is predetermined to self destruct.

    hope this helps.

    old jim
    Last edited: Apr 6, 2014
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