# measuring slow oscillations with magnetic methods

1. ## measuring slow oscillations with magnetic methods

Need to obtain natural oscillation frequency for several fr4 boards and other non PCB objects. Striking the objects emanates low frequency tone which dampens out rather quickly. The trouble with obtaining its frequency with condenser and other forms of acoustic microphones is their low frequency response does not dwell near natural frequency of objects and if there are mics which can pick up sub 80Hz accustics then those are likely very costly.

Calculating or or employing FEM to obtain their natural frequency isn't ideal since there are several objects I need to measure containing very dissimilar materials comprising the rigid body hence FEM isn't ideal for this. There might be something with infared reflective optics which might be able to carry this out however not sure if this is most simple and practical method. Perhaps a fine powder spread over the surface of object and an acoustic generator is varied until specific patterns appear. But this might not be accurate and its likely patterns will appear regardless of natural frequency. Would adhering a rather small super magnet so as to not distort the natural frequency of objects and picking up the oscillations with a nearby coil be a means to obtaining natural frequency of objects or are there specialty piezo or other pick ups made specifically for these purposes?

2. ## Re: measuring slow oscillations with magnetic methods

Acceleration sensors are the usual tool to measure mechanical oscillations. For the intended low frequency range cheap MEMS accelerometers may be used, they have up to kHz bandwidth.

3. ## Re: measuring slow oscillations with magnetic methods

Hi,

"Low frequency" does not mean anything.
There are natural "low frequencies" with 0.0000224 Hz ... and less

Is it accoustic (relative air pressure), mechanical force, or mechanical movement, or electrical...

As always: a simple sketch could help to clarify..

Klaus

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4. ## Re: measuring slow oscillations with magnetic methods

Originally Posted by KlausST
Hi,

"Low frequency" does not mean anything.
There are natural "low frequencies" with 0.0000224 Hz ... and less

Is it accoustic (relative air pressure), mechanical force, or mechanical movement, or electrical...

As always: a simple sketch could help to clarify..

Klaus
Acoustic and mechanical movement are very related. This case its mechanical oscillation which requires specialty pick ups since they don't output typical acoustical frequencies.

The nice object about adhering magnet is amplifying the induced coil emf outputs a nice trace on a scope without any digital frameworks for sensor modules. I prefer keeping these things analog and it seems infrared is the most simple next to magnetic sensing and it can be done in analog as well. Are MEMS senors are available in analog form?

5. ## Re: measuring slow oscillations with magnetic methods

Are MEMS sensors are available in analog form?
Sure, that's why I suggested it. The advantage is easy calibration of measurement output.

6. ## Re: measuring slow oscillations with magnetic methods

Are there non invasive methods aside from infared? The MEMS sensor requires mounting onto object, they are ideal for several measurements and infact are ideal for lateral displacements such as found in a piezo material.

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7. ## Re: measuring slow oscillations with magnetic methods

Laser interferometer is the industry standard non-invasive method. Unfortunately expensive.

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8. ## Re: measuring slow oscillations with magnetic methods

Couldn't an interferometer be devised from a high powered low junction capacitance IR led and a high speed LED transistor with the emanating LED and receiver each in an enclosed opaque tubing or is phasing and spacing not trivial to get the interferometer working?

9. ## Re: measuring slow oscillations with magnetic methods

Could you go the theremin route, and use the board's motion to modulate an ultrasonic or low-RF oscillator ?

Think 'metal detector'...

But I DO like the interferometer suggestion, provided you can sufficiently constrain your laser-diode's bandwidth.
FWIW, taken to tech-extreme, isn't this how LIGO & Co detect gravitational waves ?

10. ## Re: measuring slow oscillations with magnetic methods

Shine a narrow beam of light onto the moving surface, at an angle so it reflects and creates a dot where you place a photodetector. Arrange your setup so that you get maximum motion of the dot. Connect the photodetector to an amplifier, counter, imager, etc.

The light beam can be a laser, or perhaps merely a pinhole in front of a narrow beam led.

11. ## Re: measuring slow oscillations with magnetic methods

Originally Posted by Nik_2213
Could you go the theremin route, and use the board's motion to modulate an ultrasonic or low-RF oscillator ?

Think 'metal detector'...

But I DO like the interferometer suggestion, provided you can sufficiently constrain your laser-diode's bandwidth.
FWIW, taken to tech-extreme, isn't this how LIGO & Co detect gravitational waves ?
They seem to be employing non semiconducting lasers. https://www.ligo.caltech.edu/page/ligos-ifo

What are the minimum building blocks for mentioned ultrasonic method?

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12. ## Re: measuring slow oscillations with magnetic methods

Originally Posted by Zak28
The nice object about adhering magnet is amplifying the induced coil emf outputs a nice trace on a scope without any digital frameworks for sensor modules. I prefer keeping these things analog
A Forrest Mims book has an earth-movement sensor project exactly like you describe ('Science Projects' Mini-Notebook, pg 40-41). Suspend the magnet on a string. Position a coil of wire just below the magnet. Amplify the signal from the coil. The description says it can detect trains a mile away.

Perhaps it's all right if the magnet is placed on the moving surface, and the coil is suspended on fine wires, or a shielded cable.

13. ## Re: measuring slow oscillations with magnetic methods

A Forrest Mims book has an earth-movement sensor project exactly like you describe ('Science Projects' Mini-Notebook, pg 40-41). Suspend the magnet on a string. Position a coil of wire just below the magnet. Amplify the signal from the coil. The description says it can detect trains a mile away.

Perhaps it's all right if the magnet is placed on the moving surface, and the coil is suspended on fine wires, or a shielded cable.
How much does center hole diameter of an air coil crossover inductor coil type matter for picking up the oscillations?

14. ## Re: measuring slow oscillations with magnetic methods

Originally Posted by Zak28
How much does center hole diameter of an air coil crossover inductor coil type matter for picking up the oscillations?
For a coil, the article recommends a telephone pickup coil, or a 9V relay. Small is sufficient. The magnet has to jiggle easily, so it should be lightweight. Likewise it only requires a small coil (about the same diameter as the magnet) to interact fully with the flux field coming from the magnet's pole.

Do you plan to use a crossover (air) coil as commonly used in hi-fi speakers? Typical size is a few inches in diameter. It is under-utilized if you were to pair it with a small magnet.