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Natural frequency determination

  1. Mar 24, 2010 #1
    I have estimated some transfer frequency functions of complex mechanical suspension. It is shown the gains reach a very high value at a 0.8Hz. How could i know if the 0.8Hz isthe suspension natural frequency?
     
  2. jcsd
  3. Mar 24, 2010 #2
    This question does not even make sense, do you know what a natural frequency is? Also, what is a transfer frequency functions? Do you mean you have a transfer function, obtained analytically via frequency sweeps?

    Side:
    Please capitalize the word "I" in sentences.
     
  4. Mar 25, 2010 #3
    I have applied a frequency sweep and then i have measured the accelleration at fixed frame (called a) and at the suspended frame (called b). After that i have calculated the ration (b/a) from fourier transform of a and b. is f1 a natural frequency of suspension if the phase of B/A=90° in f1? Now the question is more clear?
     
  5. Mar 25, 2010 #4
    Capitalize "I" when you use it in sentences. Goodness, gracious :rolleyes:.

    The word for (b/a) is ratio, not ration. Fourier is named after someone, it is capitalized as well. When you type in a proper English sentence, I will try and help you.
     
  6. Mar 25, 2010 #5

    berkeman

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    Staff: Mentor

    It does sound like it is a resonant frequency, if you are getting amplitude gain there. Is it the only resonant frequency you have found in your sweep? You should be able to start to calculate the resonant frequency based on the spring constant and the unsprung mass, I would think.
     
  7. Mar 27, 2010 #6
    thank you for your answer. The question is my suspension isn't so simple and so it is difficult to say if it is a real resonant frequency. So I would want to know if there is a general way to estimate a natural frequency suspension.
     
  8. Mar 27, 2010 #7
    Resonance occurs when the bode magnitude plot reaches a peak.
     
  9. Mar 29, 2010 #8
    Is it always true? Moreover does resonance occur when the bode phase plot reaches 90°?
     
  10. Mar 29, 2010 #9

    berkeman

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    Staff: Mentor

    Why would you say 90 degrees?
     
  11. Mar 29, 2010 #10
    Rather than give you the answer, I want you to arrive to it on your own. Reflect on the derivation of terms when we define the natural frequency, damping ratio, etc, for any classic single input, single output (SISO) system. Hint: look at the equations of motion in those cases. What can you say about them? How does that apply to your complex system?
     
  12. Apr 5, 2010 #11
    When I don't know the values in my system (ie mass compliance, damping), I often modify a known value (ie mass) and observe the effect. For a second order system, it's easy to ascertain the baseline mass based upon the added mass and frequency shift.

    Then, knowing the baseline mass and the baseline resonance, you can compute the compliance.

    Of course it gets much more tricky if you're coupled into another system....
     
  13. Apr 5, 2010 #12
    You are getting close to what I'm hinting at.
     
  14. Apr 7, 2010 #13
    look at this link

    http://mechatronics.technion.ac.il/rotordynamics/pdf/lesson_1b.pdf [Broken] page 6. For w=wn the phase is 90°. Is it true also for a MIMO system?
     
    Last edited by a moderator: May 4, 2017
  15. Apr 7, 2010 #14
    i think i have understood. I take a siso system and i apply it a frequency sweep. After that i try to increase the mass and apply another time a frequency sweep. Then with a system identification method i can estimate the natural frequency, isn't it?
     
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