Best Targtet Material for Magnetic Sensing? 2

[JuliaDee]

I'm designing a sensor that needs to convert mechanical vibration in a target object to an electrical signal. The situation is very analogous to a guitar pickup, where a magnet or pole piece is surrounded by several thousand turns of wire, except that in my case I have some control over the materials selection of the object whose vibrations are being transduced.

I want to maximize the transfer gain of the whole system (Vout/TargetDisplacement) in order to maximize signal-to-noise ratio and to enable a coil-to-target distance large enough that manufacturing tolerances are not problematic.

I understand that I can increase coil output by increasing magnet strength and/or number of coil turns, but these have some limits due to cost, size, and undesirable damping of the target object. I want to first choose the best target material then find the lowest cost, smallest combination of magnet power and coil size that will do the job.

I would like some guidance on what magnetic parameter(s) is/are most important in the target object for maximizing the induced coil voltage for a given target displacement diatance. I believe I am looking for the highest possible permeability in order to have maximum impact on the flux lines going through the coil as the target object vibrates. Is this correct and are there other parameters I should be looking at?

Thanks,
Julia

 

[IRstuff]

But, why does it need to be magnetic? There are lots of sensing modalities in the universe.

What are your requirements? Is this for school?

TTFN
faq731-376

 

[JuliaDee]

Argh, my first post here and there's a typo in the topic title. Sigh.

Thanks for the reply, IRstuff. No, this is not for school - I have 35+ years of experience in my field (which doesn't usually involve a whole lot of magnetics, hence my inquiry to this fine forum).

There are indeed many sensing modalities, and I've been through quite a few: experimentally, in production, and in my mind, and am pretty darned keen on a magnetic coil for the next iteration. It's been prototyped and is looking very promising; now I just need to work out some details and we'll be off and running.

I require frequency response out to 20kHz, which rules out a lot of techniques. I require a sensor cost in the vicinity of $2.00 in volume, which rules out some more. I require something that can be produced in volume by a fairly low-tech Asian OEM factory, which rules out some more. The magnetic sensor meets all the requirements, I just need to tell the metallurgists what to put into the mix.

I can't tell you all about the dingus because it's confidential to my client.

So what's the most important magnetic parameter when trying to induce the maximum possible current in a coil in a magnetic field by waving a chunk of ferromagnetic material around in the field?

Thanks,
Julia

 

[MagMike]

Since your application is similar to guitar pickups, the material of choice for those is Alnico, usually Cast Alnico 5. You'll get more output from fully sintered NdFeB, but the pickup manufacturers I've worked with say the sound quality isn't as pure. Maybe your application won't be as picky.

The answer to your last question of your 2nd post: The most important magnetic parameter would be the Remanence of the material.

 

[JuliaDee]

Thanks, MagMike, however you have misunderstood the question. I am not asking what type of magnet to use in the sensing coil, I am asking what is the best material for the *target*. In the guitar pickup analogy, that would be akin to asking what is the best material to make the string out of. In my case, however, I am less contstrained (probably) than stringmakers in my materials choices.

Julia

 

[dgallup]

It depends. How much output do you need? How strong a field are you modulating? For weak fields you want the highest permeability. For strong fields you want high saturation flux density. For low cost you want dirt cheap easily manufactured materials. Do you need corrosion resistance? There is no way we can "optimize" your material choice from the info given. Since cost seems to big your biggest driver I would start with cheap low carbon steel like 1010 or even 1215 in an annealed condition. You can get away with these in a lot of non-critical magnetic applications. Only you can decide if you need something better (read more expensive).

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[JuliaDee]

Thanks, dgallup! I'm currently prototyping with cylindrical neodymium N42 magnets from K&J, 0.2 - 0.31" dia, 0.5 - 0.63" long. Typical Br max is approx 13,000 Gauss. A 0.25" coil-target distance should be manufacturable. K&J's field strength calculator tells me I will see 400 Gauss with those parameters.

Electroplated nickel (sorry, I don't know the exact thickness at this point) actually works, marginally, at very close coil-target distance, say, around .062". A piece of 0.010" mu metal works great at distances up to about 0.375".

Re corrosion resitance, we will probably nickel-plate over everything for both appearance and corrosion resistance, if the sense material turns out to be something other than nickel.

Cost is a consideration, yes, but we'll spend what we need (within reason) to to make it work well.

I guess we'll have to experiment, but it's good to know that I need to look at saturation flux density as well as premeability, that's the kind of info I was looking for.

Julia

 

[RyreInc]

Saturation won't be an issue (for most materials) since the field density is pretty low. Permeability is the key parameter.

400G is the field calculated for that distance in air only. The presence of a high-permeability material will re-distribute the field and increase its magnitude in that region.

MagMike, that is a myth regarding AlNiCo!

 

[dgallup]

Yes, at only 400 G saturation is not important. Looks like you want one of the nickel-iron alloys. Carpenter makes quite a few different ones and has lots of technical info available.

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[dgallup]

http://www.cartech.com/techarticles.aspx?id=1624

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[JuliaDee]

Many thanks to all, especially dgallup and RyreInc!

Julia

 

[EdStainless]

If initial permeability is the key then look at GI Si steel, like used in high quality transformers.
You could use electroless Ni to protect from rust.
If you want to boost field (ans output) then try adding pole pieces to the ends of you magnet. You would raise the operating point and you could taper them. It would raise the field strength.

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