Opinion on instrumentation

Oh please share pic's! (and names.)

Sounds like a lot of work... are you going to try and make this design fly? Maybe you can just harvest the physics parts of it... is it a flux gate, hall effect, something else?

Are you making one or lots? We've got a fluxgate thing from England that puts out a frequency that's related to the B field.. you could turn it into analog with a bit of work.

What kind of resolution are you looking for? (and how much scratch)

(I wish you lived a bit closer, I come up and rip into it with you. :^)

Sounds like fun. At least post some horror pix.

Aren't there 3-axis chips?

There are 1-axis fluxgates for prices like $20 or something. Three of them?

Accuracy? Are there Hall effect chips that'll do this for you?

Take a look at TI DRV425.

There are plenty of compass chips with SPI output, it would be cheap and easy to add an arduino board with analog outputs.

Pretty nice- offset and drift is quite respectable.

Just have to come up with an orthogonal mounting of three- that doesn't create too much field itself- for a decent sensor head.

--sp

Legos?

I searched on Ebay for " 3-axis magnetometer ". The cheapest was $1.29 including shipping. Probably not what you need, but you might look at both Ebay and AliExpress. It is incredible what is available.

Dan

Machined aluminum? SLA 3D printing even maybe.

--sp

The bandwidth is related to the AC excitation frequency - may not be easy to change that. If the excitation frequency is 8-10 times the bandwidth you need, it becomes a simpler matter to open up the LP filters.

The scheme is to drive the (usually toroid) core into saturation with AC and phase detect at the 2nd harmonic to resolve asymmetry in the B-H loop. The phase detector output is fed back to a dc coil aligned with the measurement axis to null the 2nd harmonic. The current fed back is then proportional to the external field.

It's not hard to make one that resolves a few milligauss on one or two axes with stuff most engineers have laying around. Getting to a few microgauss takes a bit of engineering.

Flux gate magnetometer is what come to mind when flying leads, glue and tape are mentioned. You may have some other method in use in your instrument but most of those are much cleaner in implementation.

No, Legos is better. You're talking about bandwidth and magnetism, conductive material near the probe has to be minimized. Ditto magnetic material (so a mass of hotmelt glue, or just about anything transparent, is ideal: no conduction, no magnetization).

On Fri, 15 Jan 2016 22:07:24 -0800 (PST), whit3rd Gave us:

Delrin, G10, FR4, etc.

On a sunny day (Fri, 15 Jan 2016 15:32:35 -0500) it happened Spehro Pefhany wrote in :

You do not say anything about sensitivity and accuracy. Solid state 3 axis magneto sensors are cheap modules on ebay. Here I use that, combined with 3 axis position sensor, for compass:

BTW wires and glue is good, somebody did rel work. And that they do not reply is because they may well think .. :-) Would you?

Do it! Or, if you have a spare one, hock it at Dave Jones?

Tim

Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:

Hi, all:-

So we have this 3-axis magnetometer kicking around - probe plus a benchtop instrument with multiple displays. It's good for measuring the earth's field. This one has the option to have the bandwidth really low, presumably to reject 60Hz hum, and they also made a high bandwidth version.

I asked the company what parts I would have to change to make it high bandwidth but they're ignoring me. So I took the cover off and it's a real horrorshow inside- fly wires, fixes, hot glue all over the place.

So should I do a tear-down page or video exposing their shoddy workmanship for all to see while reverse-engineering their design (their website looks like they have pretensions of being a legitimate company) or should I just forget about it?

Looks like it might take an hour or three to figure out what they are doing- it's got some OPAxxx op-amps and a bunch of other stuff (looks needlessly complex, actually).

I need to end up with a 3-axis magnetometer with analog outputs for a couple gauss. Preferably without spending too much scratch.

--sp

We use some black delrin that machines very nicely, that's what I was going to suggest. After that perhaps some phenolic.. but that's messier.

George H.

Interesting thanks. It does sound like a lot of work, for all three axes you need a few toroids and three (helmholtz) coils to cancel the field... Hmm and if there is any magnetic materiel around that would interfere with the canceling field.

I don't suppose you know of a good article describing the technique and trade-offs.

George H.

On Sat, 16 Jan 2016 07:59:37 -0800 (PST), George Herold Gave us:

Astronomically speaking, of course... ;-)

I don't have anything I could point at right now but:

Once over lightly here:

The old standby - Review of Scientific Instruments:

If you search for 'flux gate magnetometer' this pops up:

A Simple Field Detector for a Dc Permeameter Charles Q. Adams Rev. Sci. Instrum. 31, 1119 (1960);

A two?core flux?gate magnetometer is employed in a simple balanced bridge circuit as the dc field detector of a single strip permeameter. The sensitive magnetometer and simple bridge circuit eliminated the necessity of the complex filter and selective amplifier used in earlier systems and allows operation independent of oscillator frequency stability. The H?measuring system is designed to measure fields from 1 moe to about 10 oe with a sensitivity of ?25 ?a/moe. Economy and simplicity of construction circuitry and operation are its chief advantages.

(note that a permeameter measures a differential field - probably why two cores were used)

I've seen a number of hobbyist-level projects for FG compasses and such in various places - one using cmos logic to generate the excitation and reference for the phase detector (which was a 74xx86 XOR) and a cheap opamp for gain and filtering but I can't turn up a reference.

The last half of this youtube shows a typical sensor: