calibrate magnetic sensors

[alansimpson]

I am looking for an a product to produce predictable field intensity to be used to calibrate hall sensors ideally up to 3T (30,000 Gauss)

I would like it adjustable.

I am thinking of something like a "C" shaped core with a gap into which the sensor is placed and a coil driven by DC current.

Thanks

 

[EdStainless]

The devices that I have seen are not adjustable. They are permanent magnet based.
I suppose that you could build an electromagnet and use a high precision DC power supply to drive it.

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Plymouth Tube

 

[IRstuff]

I think that's normally done with a Helmholtz coil setup.

TTFN

FAQ731-376

 

[alansimpson]

Hi EdStainless.

Could you tell me where to get PM based system? Might do in some application.

Thanks

 

[MagMike]

I hope Ed doesn't mind me jumping in.

A number of gaussmeter manufacturers (particularly ones that make laboratory quality instruments) offer reference magnets. Here are some links:

http://www.lakeshore.com/mag/ga/gm410oi.html

http://www.maginst.com/specifications/reference_magnets.htm

I have a few old ones from Walker (not sure if they make them any more), but usually reference magnets only go up to a few thousand gauss. Beyond that, you'll need a full electromagnet (Helmholz coils can only produce a 1 to 2 kGauss). However, 30,000 Gauss in an electromagnet will be tough to produce.

Just a few months ago my company made a 30,000 Gauss (3 Tesla) permanent magnet dipole. It fits in the palm of your hand.

 

[EdStainless]

I would have pointed him the same places Mike, thanks.
I built 2T PM jobs in the past, so it isn't too great of a stretch. The problem is that is the active sensor area is 0.125" x 0.125", then you need good field uniformity over an area about 0.250" x 0.250". this implies that your pole faces need to be about 0.500" diameter. It ends up being larger than you would think at first.
At least with a cal magnet you only need an air gap of 0.125".

There are a couple of ways of building these. A "C" yolk is easiest. Putting the magnets and poles inside of a tubular return structure is better. No chance of picking up debris or of damage to the magnets. Just a little window cut in the side for insertion of the probe.

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Plymouth Tube

 

[ArtWagner]

MagMike, your 3 T dipole sounds interesting. How is it done?

AlanSimpson: to make your field source adjustable, you would need current in a coil. 3 T is larger than saturation level of steels, for example, 1018 steel is around 1.8 T. Thus the coil structure could not benefit much from steel and the electromagnet would have to basically have an "air" core. Thus, the ampere-turns required to produce your adjustable field would likely be large, making your electromagnet large and heavy. This is how it seems to me.

 

[MagMike]

ArtWagner: We used fully sintered NdFeB magnets in quadrature (superposition) with a high permeability pole piece to achieve 3 Tesla. It was a bit of a challenge to put together because of the forces invovled, but proper fixturing helped. When I get back to the office later this week I'll see if I can put up a picture of it.

 

[MagMike]

Here is a picture of the 3 Tesla dipole.

 

[EdStainless]

I have built some with SmCo 2:17 where we used many magnet pieces to shape or buck the field. You can reach real high fields with these but they are a bear to build and the field uniformity may not be very good.
I would suggest against using NdFe since the temp coef is so high. We could measure the changes based on the temperature of the room.

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Plymouth Tube

 

[MagMike]

EdStainless: Good point. The discussion had strayed from the OP's question and I should have started a new topic.

The 3 Tesla dipole we constructed was not built as a reference magnet. That customer needed 3 Tesla minimum for their application and we achieved 3.1 Tesla in that design.