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- Thread starter serbring
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Side:

Please capitalize the word "I" in sentences.

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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?

Side:

Please capitalize the word "I" in sentences.

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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.

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berkeman

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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.

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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.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.

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Resonance occurs when the bode magnitude plot reaches a peak.

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Is it always true? Moreover does resonance occur when the bode phase plot reaches 90°?Resonance occurs when the bode magnitude plot reaches a peak.

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berkeman

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Why would you say 90 degrees?Is it always true? Moreover does resonance occur when the bode phase plot reaches 90°?

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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?Is it always true? Moreover does resonance occur when the bode phase plot reaches 90°?

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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....

- #12

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You are getting close to what I'm hinting at.

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....

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look at this linkWhy would you say 90 degrees?

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?

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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?

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....

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