Measuring frequency of a vibrating component using magnets 1

[Shockdesigner331]

Hello all. I'm a mechanical engineer with no experience using magnets and need some help. Here's what I'm trying to do:
Small scale TMD that needs to be tuned to a certain frequency (Tuned Mass Damper, basically just a mass on the end of a beam suspended in damping fluid, inert silicone). This is a very small TMD (fits in palm of hand) and therefore we can't place an accelerometer on the mass, plus, each production TMD will need to be tuned to differing frequencies. The fact that it is in a hermetically sealed chamber filled with fluid eliminates the use of capacitance type proximity sensors. So my idea is to mount a permanent magnet into the mass itself. The case the TMD is located in is to be made from aluminum, with a distance from the magnet to the outside of the case of only 1/4" (could probaly make that smaller). The use some sort of pickup on the outside of the case to measure the change in magnetic field when the mass is moving. Is this far fetched? Any recomendations on magnet type? I'd like to have a sensor that could non-permanently be mounted on the case while testing, any pointers on sensors? Any knowledge you could share would be greatly appreciated.

 

[BobM3]

Hall effect devices will sense magnetic fields. Their output can be on/off or proportional to the magnetic field. Check out

http://www.Digikey.com

and do a search on "hall effect". They even sell magnets that can be used with the sensors.

 

[MagMike]

Shockdesigner: It sounds quite do-able. I'd recommend using SmCo or NdFeB magnets, they'll give you a strong signal for a Hall-effect device and/or a search coil.

 

[IRstuff]

I think that you'll have an additional disturbance. In order for there to be a disturbance in the magnetic field, the TMD would have to interact with and therefore perform work on the magnetic field. The net effect has to be a change in the motion and energy of the TMD.

I'm not sure why you ruled out capacitive means.

Another possibility is to see if an accel or strain gage could sense the pressure waves that are generated by the TMD inside the package.

TTFN

FAQ731-376

 

[prex]

Shockdesigner331,
you should also know that the moving magnet inside an aluminum case will have an additional damping due to eddy currents. This might however be a beneficial effect for you...
Another thought: if the distance of the magnet to the hall sensor is 1/4", the amplitude of the oscillations should be bigger than that for the sensor to go on-off. Otherwise you can use a proportional sensor, but this requires more complex signal handling.
Another option would be to have an iron piece on back of sensor to have a stronger change in magnetic field, but this in turn adds extra forces on the magnet.

prex

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[MagMike]

Prex: Correct me if I'm wrong, but I have the impression that the magnet was going to be inside the aluminum case, with the TMD. If this is correct, then eddy currents shouldn't be an issue.

 

[Shockdesigner331]

Thanks for all the info so far.
Yes, the magnet is going to be inside the aluminum case on the TMD.
I ruled out capacitance prox. sensors because of cost, and these units are hermitically sealed and adding a sensor with wires would compromise that. (Let me know if I'm off base there)
For those reasons I thought the magnet idea would work. I am a little worried about the small deflections of the TMD. (±.05" max, tuning freq ~ 1hz) I think this means that I'd have to go with a proportional sensor. I took a look at digikey, talk about a lot of choices... I think the path forward is to purchase a magnet and sensor and experiment (changing the gap, etc). Are there any recomendations as to a good sensor for this app?

 

[sreid]

See Allegro for Hall Sensor Specs.

http://www.allegromicro.com/en/Products/Categories/Sensors/

Probably can buy from Digikey after selection.

The aluminum case is going to "short circuit" a lot of the magnetic field due to eddy currents. But some Hall sensors can detect very small field changes.

 

[MagMike]

sreid: If the magnet is in the aluminum case, vibrating at the same rate, why would there be eddy currents?

 

[handleman]

The magnet is inside the case, vibrating. The case does not vibrate. Any time you have a magnet moving in relation to a conductor in that type of arrangement it will cause eddy currents and have a damping effect on the motion.

 

[Shockdesigner331]

if the magnet does cause some damping it will be acceptable (there's damping fluid anyways). What should I be looking for in a sensor that can detect small changes?

 

[BobM3]

Small changes in frequency or small changes in magnetic field strength? As long as the frequency response of the hall device is above the frequency you want to measure then you should be able to detect frequencies (and changes in frequency) fairly well. If the amplitude of the vibration is important then things get more difficult. The hall device would have to be mounted consistently relative to the magnet from unit to unit. The magnets would also have to be consistent.

Are you going to use a scope or spectrum analyizer to look at the output and determine the frequency? If so, that might be the limit to detecting small frequency changes.

 

[MagMike]

handelman: Thanks for the clarification (a star for you). I'm well aware of the conditions that'll produce eddy currents, I just didn't realize in this application that there would be relative motion between the magnet and the aluminum case.