vibrating coil gradiometer

That's very simple. Strong field, no problem at all.

Not a good idea.

[...]

Use GMR sensor ICs from Honeywell. With two sensors, few analog parts and proper callibration, you will be able to resolve the gradients at the order of 0.001 of the Earth field. GMR is as simple as it gets; there are of course better sensors and technologies.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

When working with magnetic fields, remember they are LOW impedance. Take advantage of that. It's not like you're making a voltmeter that must have high impedance.

You could try 'chopping' Earth's DC field by using a spinning loop coupled to a 'slip ring' transformer for non-contact transfer of signal. [make the loop AND the coupling transformer, use air core] Spin the loop at some 'proper' harmonic that is synchronous to AC mains.

Speed of the spin determines the number of turns and knowing the spinning speed you can synchronously detect the signal and really lower the noise floor. maybe a cheap optical coupler/interrupter [take apart an old mouse] as the loop is spun to provide the sampling pulse.

To analyze what's going on in the fields, get a copy of femm 4.2 If you need help to start on the learning curve, let me know.

PS: Circa 1989, I made a portable 4 inch diameter AC magnetometer, that had a 'flat' sensitivity of 1V per uT over spectrum from 5 Hz to

1MHz.[max 2uT] Gently rocking the meter would peg the output as it moved through that 50uT field. The noise floor was in the range of 5nT [on a scope the output signal was a crisp line and not thickened by noise] and you could 'see' vehicles go by on the street adjacent to our lab as the metal deflected the Earth's field.

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Oh, those might work, thanks (HMC1001, HMC1002) they're not GMR though, but some Anisotropic MagnetoResistive (AMR) technology. Not cheap (~$15 each) Or were you suggesting a different sensor?

We've got some hall sensors from Allegro (A1324), but they are way too noisy.

George H.

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Hi Robert, thanks for that. I bet your 1989 magnetometer cost more than a few hundred dollars.

A colleague has done the field gradient measurment, by weighting a permenant magnet. (First with the magnetic moment pointing up and then down.) He claims that a gradient of 1uT/m gives a weight difference of ~ 1 milli gram (out of a 100 gram magnet) Which can be done with some of these cheap balances. This doesn't give all the components of the gradient, but it might be enough.

George H.

-Lasse

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Grin, Thanks Lasse, I'd like the gradiometer so our users would know where NOT to put this,

George H.

There is really no problem to measure weak magnetic fields. The problem is to measure weak gradients on the background of uniform but strong magnetic field of the Earth. What your colleague measured is probably caused by misalignment of the magnet axis wrt Earth magnetic field. So the magnet essentially acted like a compass. Compassing is common problem for all mechanical magnetic gizmos, so sensitivity to gradient is limited by mechanical misalignment.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

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Hi Vlad, I hope it's OK if I disagree with you. There is certainly misalignment between the B field and magnetic moment. But a uniform B field can only cause a torque on the magnet. (So there is certainly some 'compassing' when one is weighing the magnet, but that pushes down more on one side and less on the other... no net force.) The force on the magnet is caused by the gradient in the field. You can see this pretty easily with a spring, magnet and rather strong field gradients.

Figure 2 down the bottom of the page is for a single coil, along the axis. It's fun to write down the field as a function of z, differentiate to get dB/dz and then see how that fits the data.

(I've ordered some of the honeywell sensors, it be great to be able to measure both the field and the gradient... but it's a bit of a pain getting two sensors zero'd I've got a little mu metal tube that is maybe close to zero field inside.)

George H.

One way to do rotating coil would be with two coils: One fixed and external one to sense the varying field and the other one being a rotating short circuited ring. The rotating ring will see an induced emf=w.B.s.sin(theta) which shorted will induce a current in the loop, which in turn will produce a rotating B component, which will be sensed by the enclosing sense coil.

I did not run the figures, you know how to do that, so I don't know if the figures are practical or not.

A gradiometer could be build from this with two rotating rings and one big enclosing coil, with turns in one sense for half its length, then the same turns count in the other sense for the second half length. You sense the field gradient emf across the whole coil...

Lock-ins should give you more than enough sensitivity, but again I did not run the figures.

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Thanks, 
Fred.

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Wow! I think I see it. Does this really work? (I get baffled by changing B fields sometimes.)

I'll have to put some real numbers in for the rotating coil (wire size/ resistance). If I'm not mistaken, it will be the current in the rotating coil that I need to calculate.

Cute, but maybe instead of changing the 'sign' of the stationary sense coil, I could have the rotating coils wired in the opposite direction. (Still the rotating coils are going to have to be pretty well matched.)

OK here's another idea. I was thinking about using Vlad's field sensing chips to measure the gradient. If I use two of them, the problem is always getting them 'matched'. So how about if I just shake one chip around and look at the AC signal at the shake frequency? I'll want to keep it nice and 'flat' as I shake it. It's not clear to me if unintended rotations (as I shake it) will give signals at f or 2f....

(Hmm looks like f unless I get pointed along the field direction)

George H.

Use a PIC. Match the sensors digitally. That works very well; to the accuracy limited by self noise.

Unintended rotations will result in compassing.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

.....

That one is easy... The interesting one is the total flux seen by the enclosing coil which is, well... better measured I think :-)

Wired in the opposite direction? I'd sure be interested to have a picture of this: just look at what it is with the one turn coils :-)

I also think that rotating them in opposite direction won't make a difference.

One turn rings lathed from copper tubing?

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Thanks, 
Fred.

Don't tell the customers, more like $24 Copper is EXPENSIVE! I even got away with cannibalizing a cheap meter to mount it on, so I didn't have to spring $$ for a display..

Weight difference? I wonder if that could be done using MEMS? Combine with stress sensor and voila!

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OK I'm still trying to get my head around how this works. But I think I'm starting to see it. Even if this isn't used to measure field gradients it might be fun to build one and see it work. "I'm not as smart as other's and it's nice to have some data to help guide my thinking"*

Yeah, sorry about that. Sticking my foot in my mouth again. (But I have no shame and will certainly do it again.)

Do I gain anything by having multiple turns? If not then how about a copper disk?

I was first thinking I wanted the stationary pickup coil with it's axis along the field direction, but now I'm thinking it needs to be at right angles?

And thanks for the 'crazy' idea Fred. Do you know if this has ever been made?

George H.

OK then I've got to calibrate each sensor separately. And then worry about DC offset drift with temperature. I'm not saying it's not possible.....but I need to measure differences of ~1-2 parts per thousand in the field.

It's worse than that, it gives me a signal. Roughly proportional to the rotation angle. One good thing about vibrating the sensor is that it moves the signal up in frequency, away from all the 1/f crud.

George H.

It doesn't take a zero-field region to make the adjustment; just a way to reverse the sensor orientation, and make sure if it reads +0.457 gauss at zero degrees, it reads -0.457 gauss at 180 degrees.

Nonmagnetic materials are important, too; switches, resistor leads, nuts 'n bolts, all will have to be degaussed if they are near the sensor.

You just have to match one sensor against the other in the uniform field.

Done that. It isn't very difficult.

That's why I prefer digital correction. Linear and nonlinear effects could be taken care off.

IIRC those NMR sensors generate fairly uniform noise; at least the ones I tried.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

Hi whit3rd, I've done that for a rough zero. Plenty good most of the time when I only care about the field to a few percent or so. But I'm contemplating a gradient measurement. I'd like to measure the gradient over a length about equal to the sample size ~10 cm. And I need to measure the gradients at the ~ 1uT/m (10mG/m... 1mG/10cm) level. So something better than 1 mG. Now flipping over is fine... (as long as you think carefully about how it's flipped) But a further issue is that the field in the old building that I work in jumps around at the few mG level... kinda randomly as elevators and fork lifts move around. (I guess I could take the sensors to my house in the country for zero adjustment.... then one worries about temperature changes...) Anyway the mu-metal tube reduces the effects of a changing local field.

Yeah been there, all the brass bolts are screened for magnetic effects beofre being used in the 'non-magnetic' instuments. (Old carbon comp resistors are non-magnetic BTW)

George H.

I didn't think of copper disks but it should work. Anyway field calculations aren't trivial anymore...

That and multi-turns might be a win... or not:

Emf goes with the number of turns. Then :

1) as well as the resistance (for a constant wire gauge). Thus current is constant with turns and the generated field goes with the number of turns.

2) Now if you only have a defined section to wind your coil, it is another matter as the total resistance goes with turns squared, then the generated field stays constant.

Say you have an X,Y,Z orthonormal axis system Your pick up coil will have a Z axis. Then draw your "chopping" coil in the X-Y plane and have it rotating on, say X axis. That way you'll have generate a rotating field in the Y-Z plane (well for the loop on axis component). Now you just keep the Z axis component of that for the pick up coil...

No, I just dreamed that up. Hey, if you patent it, put my name on it :-) Ooops, to late...

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Thanks, 
Fred.