mag field sensors

chopper, hmmm wiggle it at 200,000 rpm?

a homemade proton precession magnetometer is easily made from a 2 liter bottle of distilled water, or better to use alcohol. However, these are non-vector sensors, output totally relates to the mag field integrated over the WHOLE volume. Since you're making a cancellation system probably not the type of sensor to use.

make that 'since John is making a ...."

That would be telling.

I said "one to four." I figure that I could make my controller box interface to up to four field sensors, in case that turns out to be useful. And I can sell more sensors.

Unless one is on top, or better

It is.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation

Have a look at those:

There're other interesting devices under the "linear hall effect" entry.

If you need real bandwidth, you can go the dual path route, with such a sensor for DC-LF and a coil for the higher frequencies.

--
Thanks, 
Fred.

Do you need the absolute field, or is the field gradient enough?

George H.

We need the vertical component of the local field, from DC to some hundreds of Hz. Any initial DC offset (ie, the initial average value of the static field) can be ignored or offset, as that would be the baseline for the beginning of an experimental run. Experiments can last hours, so we want to do good field correction over that sort of time span.

I think. After we build some systems, we'll play with algorithms to optimize whatever we're doing. That's a reason to make the thing versatile. I may use a microZed to do DSP, because we could implement huge FIR filters or anything else that makes the corrections better.

It's looking, so far, like a fluxgate would be the best field sensor... nearly hundred-year old technology. It might be fun to design one.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation

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have

Single phase (and 3 phase) synchronous two pole motors are synchronous at

3600 rpm. Cheaper and likely easier for 200,000 rpm, use sine and cosine drive with coils in physical quadrature, same rotating field and no moving parts.

?-)

From what has been gleaned so far, 'in a lab' and 'around an analytical instrument'

The absolute value, vector strength in one axis, DC to several hundred Hertz, near where the instrument's sensitivity is.

Better to build such compensation in, where the sensitivity is. After all, measuring over there and compensating over here, often doesn't work out well. Gradient measurements would be even worse.

Static field ignored?! Then you DO need an AC measurement!

The natural noise in the earth's fields is well published,

E.L. Maxwell and D.L. Stone, "Natural Noise Fields 1cps to 100kc", IEEE Transactions on Antennas an Propagation, Volume AP-11, Number 3, pp

339-43, May 1963. D.A. Chrissan, A.C. Fraser-Smith, Seasonal Variations of Globally Measured ELF/VLF Radio Noise, Technical Report D177-1, Stanford University Dept of Electrical Engineering, STAR Lab, December 1996. R. Barr, D. Llanwyn Jones, C.J. Rodger, "ELF and VLF radio waves", Journal of Atmospheric and Solar-Terrestrial Physics, Volume 62, Issue 18, pp 1689-1718, November 2000. Kenneth Davies, Chapter 9, Propagation of Low and Very Low Frequency Waves, Ionospheric Radio Propagation, New York, N.Y., Dover Publications, Inc., 1966. M. Balser and C.A. Wagner, "Observations of Earth-ionosphere cavity resonances", Nature, Volume 188, pp 638-41, 1960

However, watch out, the noise today is much worse than the noise during those measurements, because the Sun has literally gone nuts in the past year, some of the worst solar flares [which really perturb the field and require astronauts to stay 'inside'] in the history of measuring the storms. However, these are probably still much smaller than man-made stuff. AC mains is a piece of cake to monitor. The worst source will be somebody moving a piece of magnetic equipment around, like rolling a metal chair; or moving a large filing cabinet; or on a nearby road a car, truck, or bus driving by [assuming you have a road nearby]. I once measured a transient of over 2uT up into the 5Hz spectrum for an 18 wheeler going by.

As I said, your measurement is AC. I once designed AC field measuring circuitry to replace the 60+ Hall Effect Sensors used in a pig. At my circuit's 2 mA per sensor compared to 10+ mA per HA Sensor they were ecstatic at the possibility of extending the battery life. The spec was

0.01Hz [typical 0.001Hz] to 2kHz. At those low frequencies, I'd turn on the circuit at 8am and then be able to work with it around noon. Very reluctant to shut off and change a part, too. Talk about 'slow' development cycle.

Couldn't you use fast preset? When working on my 200pV/rtHz preamplifier I had to have a 10000s time constant input filter (keep LF noise low). Of course no one want to connect the instrument and wait several days for it to operate, so on connection it uses a short time constant that settles the bias points in 100ms, then switches to the low noise mode. Works great.

--
Thanks, 
Fred.

200pV/rtHz?! Now THAT's impressive! What was that for?

During development at circuit turn on, I used to 'zap' the circuit up near opeation point. Most irritating of all is that charge migration effect.

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Well this won't work for you but I'll mention it anyway. (cause it's fun) If you take a permenant magnet and one of the little digital scales that go es dwon to near the 0.01g level. Then you can measure the magnetic field g radient in the direction of the earths gravitational field. By weighting th e magnet in both directions. (you need a little syrofoam or plastic cup to set the magnetic above the tray of the scale.)

George H.

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Oh one other random idea. The Earth'f field NMR guys use tihs trick where they use two pickup coils one is small and tightly wound around the sample, and the other is concentric, but much bigger... but with the same turns ar ea. The coils are wired in opposition so external signals are mostly cance lled... I don't know if sometihng similar would work in your case.

george H.

On a sunny day (Mon, 17 Feb 2014 07:59:12 -0800 (PST)) it happened George Herold wrote in :

Does not work for me either: EW

NS

And with the magnet pole in the other direction like this no change either, is the same too:

These are good scales, I bought it to weight diamons before shipping those cross border with my drone,

Any ideas? Seems like a cool experiment. The magnet is attracted a lot by the scales, it does see a metal object held above it.

Hi Jan... OK I'd have to figure out the sensitivity. But put the axis of the magnet along the gravitational field direction. Then you need a field gradient. I just brought another magnetic near it.

The instrument is already super-shielded, but can still stand improvement. In some minority of installations, there is something nearby (a streetcar line, train station, parking garage) that either generates mag fields or varies the earth's field at milligauss levels. We can maybe get a 10:1 improvement if we sense local fields and apply corrections, probably with a bit of DSP to match the correction to the machine's native time-domain response to the fields.

Single-sensor correction has been tested, and helps a lot, but we can't put the sensor inside the (large) instrument. So I figure we could straddle the gadget with 2, 3, or 4 sensors and do a weighted average to approximate the field at the center. That assumes that the magnetic disturbance sources are a reasonable distance away. So, I need a pretty good, fairly low-cost, stable mag field sensor, and so far a fluxgate looks like the best bet. All the semiconductor sensors that I can find are too drifty or too weird.

A fluxgate with a field feedback winding (ie, the fluxgate is a null detector) will be very repeatable and stable.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation

On a sunny day (Mon, 17 Feb 2014 09:44:20 -0800 (PST)) it happened George Herold wrote in :

Cool, I think I get the idea a bit better now, Will try again tomorrow and let you know. Thanks for the help.

I was going to say six sensors (top, bottom, N,S,E,W) but you could do it with four if they weren't all in the same plane.

George H.

H&H call that the "baby scale loop".

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net