How do i find resonance? and how would the resonant frequency be measured?

Question

Mentioned above is my exact question. This question has been asked before, but no answer was given. I love the idea of being able to ask all questions in a central location like this, but, the answer given to the question in question was not an answer at all. It was a definition/history lesson. I'd really appreciate if someone could just simply answer the question please. I already know what resonance is, if I didn't, I never would have asked the question listed above. I don't much care about the history of the word or who discovered it. I only care about how to find it. "HOW" being the operative term here. How should I test for it both mechanically and electrically? 2 steel pipes of different thickness, one resonates at this and the other resonates at that, how do I find it? 2 LCR circuits of different values, both with different resonant frequencies, how do I find them?

Thanks,

Michael

Answer 1

"2 LCR circuits of different values, both with different resonant frequencies, how do I find them"

Use a variable frequency generator, and an AC meter or oscilloscope.  Put a resistor in series with the circuit and apply the variable frequency, with the meter or scope across the circuit. Series resonance, you will see a peak at the resonant frequency. Parallel resonance, you will see a dip at the resonant frequency. Watch out for the capacitance of the meter or scope shifting the resonance.

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Answer 2

The previous answer is correct for electrical circuits. Actually, any system, electrical or mechanical has various resonant modes.

To find the resonant modes in a mechanical system, say, a fan or turbine, install a vibration pickup transducers axially and radially and run it at different speeds, i.e., frequencies. Connect the transducers to an x-y (frequency vs. amplitude) type chart recorder and observe the spikes that occur. They are all resonant modes.

Critical speed is the one at which the largest spike occurs. It is important because the system may self destruct if left running at this speed. It also may be more than one.

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If it is a pipe, you could excite it with a vibration/sound transducer at different frequencies, transversally and longitudinally, and even torsionally, and again, measure the response with a vibration pickup transducer, e.g., a microphone, and record the response on an x-y chart recorder.

There are test benches designed specifically for these type of analysis. The responses are also called "signature" and each system has a different one.

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It is not easy to predict resonant modes in complex structures. But, altering the symetry of, say, a pipe, most likely will kill/dampen the main resonant modes.

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Answer 3

The usual way is to excite the circuit in some fashion. If the circuit is electronic ( made of inductors and capacitors ) it may be parallel or series. A parallel circuit exhibits high impedance at resonance and low impedance off resonance. A series circuit exhibits low impedance on resonance and higher impedance off resonance.

One possible way to excite the circuit is with a signal generator and use a detector in a voltmeter mode across the circuit. The voltage will rise to a peak at resonance in the case of the parallel resonant circuit. Because of loading, the series circuit will cause the voltage to drop at resonance. However, this method will load the circuit with parasitics and so the result will only be indicative.

Another possible way is to use a Dip Meter. This uses a oscillatory circuit which indicates the loading of the oscillator by a meter when the oscillator coil is brought near the resonant circuit. The oscillator is swept through the range of frequencies the may be resonant on and a dip in the reading indicates the resoance point. Used to be called a Grid Dip Meter but they usually use FET's now.

You can also use the phase of the circuit. The Phase will measure zero on resonance and some other value off resonance.