Best way to measure magnetic waves for electromagnetic induction

[josh_TMS]

Hi all,

Looking to measure some magnetic waves being generated at an electric coil. Freq is between 0-20kHz and magnitude is pretty small <1T. Any have suggestions for the best tool to measure and log data of this magnetic waveform?

Googling around, I found meters like this: [Possible spam link removed by Mentors]
I thought maybe even a simple hall sensor but not sure.

What do you suggest?

Thanks!

 

[berkeman]

Welcome to PF.

Hi all,

Looking to measure some magnetic waves being generated at an electric coil. Freq is between 0-20kHz and magnitude is pretty small <1T. Any have suggestions for the best tool to measure and log data of this magnetic waveform?

Googling around, I found meters like this: [Possible spam link removed by Mentors]
I thought maybe even a simple hall sensor but not sure.

What do you suggest?

Thanks!

It looks like the PF AI software didn't like you posting a link to a small company like that in your first few posts here. I did see the link though, and the small hand-held instrumtent looks okay. Kind of expensive, though, depending on the accuracy you need.

Can you say more about your application? Do you really need to measure all the way down to DC fields, or would a calibrated pickup coil for AC only work instead? What resolution do you need? What logging software would you prefer to use, or you can use anything that is supplied by the manufacturer (like the software supplied with the unit at your link)?

 

[josh_TMS]

Thanks Berkeman!

I actually don't have a great background here. Many many years ago, I did study one year of electrical engineering before switching to mechanical. I'm trying to hack together my own TMS machine mostly for fun and as a way to get back into things (inspired by success of treatment for depression).

The slowest this TMS machine gets is one magnetic pulse every 2 seconds, so 0.5Hz. I'm not sure if I need a DC sensor for that? I think maybe because the actual pulse is quite quick (<100ms) and only the duration between the pulses is long (2 seconds), that I maybe only need to measure AC.

I would need resolution of 0.01mT or better as the lowest magnitude is 0.1mT so I think my resolution should be 10% of that at least.

Any software is OK, I would love if I could get raw data so I can process it myself also on MatLab.

 

[berkeman]

The slowest this TMS machine gets is one magnetic pulse every 2 seconds

Sorry, what is a TMS machine? And why is it generating magnetic pulses?

 

[josh_TMS]

Sorry, what is a TMS machine? And why is it generating magnetic pulses?

https://en.wikipedia.org/wiki/Transcranial_magnetic_stimulation

I'm not planning on building one that can achieve therapeutic levels, just using this as inspiration to build a small wave generator.

 

[berkeman]

https://en.wikipedia.org/wiki/Transcranial_magnetic_stimulation

Interesting. It looks like they mainly use a damped sinusoid waveform (capacitor discharge into the transducer coil(s)).

I'm not planning on building one that can achieve therapeutic levels, just using this as inspiration to build a small wave generator.

What are you going to use to drive your coil and monitor that drive waveform?

BTW, here is the link to the hand-held instrument that you posted in your OP that was deleted as possible spam by our AI. I don't think you posted this as an attempt to spam.

https://www.alphalabinc.com/product/mr3/

 

[Tom.G]

If you don't mind doing the data reduction, a low-cost Digital Oscilloscope with logging capabilities may be more cost effective. (or 3 of them if you need 3-axis data)
(https://www.google.com/search?q=low+cost+digital+oscilloscope)

Since both your TMS pulse and the meter you are considering are intermittent (pulsed, sampled), you would ideally synchronize them some way. If un-synchronized, you will be getting partial, or no, readings for most of the samples.

Have Fun!

Cheers,
Tom

 

[josh_TMS]

If you don't mind doing the data reduction, a low-cost Digital Oscilloscope with logging capabilities may be more cost effective. (or 3 of them if you need 3-axis data)
(https://www.google.com/search?q=low+cost+digital+oscilloscope)

Since both your TMS pulse and the meter you are considering are intermittent (pulsed, sampled), you would ideally synchronize them some way. If un-synchronized, you will be getting partial, or no, readings for most of the samples.

Have Fun!

Cheers,
Tom

Interesting idea. How do I use an oscilloscope to measure magnetic fields?

 

[josh_TMS]

Interesting. It looks like they mainly use a damped sinusoid waveform (capacitor discharge into the transducer coil(s)).What are you going to use to drive your coil and monitor that drive waveform?

BTW, here is the link to the hand-held instrument that you posted in your OP that was deleted as possible spam by our AI. I don't think you posted this as an attempt to spam.

https://www.alphalabinc.com/product/mr3/

Right now, I don't have the drive circuitry. Working through everything in parallel. I asked about measurement because I'm trying to see what tools I need to buy for building and validation.

 

[Tom.G]

Interesting idea. How do I use an oscilloscope to measure magnetic fields?

Well... obviously you need a sensor with an electrical output that depends on the magnetic field. I think @berkeman mentioned the two easy ones, a Hall element or a coil of wire.

A Hall sensor puts out a millivolt-range of voltage directly proportional to the magnetic field it is in. If you want a 3-axis measurement you need 3 Hall sensors, one for each axis.

A coil of wire, or solenoid, will generate a voltage depending on the rate of change of a magnetic field. This means a bit more processing to get the intensity of the field.

The following approaches are used in digital clamp-on Ammeters that will read DC current flow. The common existing ones on the market are probably too large for your usage, but here is how they operate. Maybe you can modify one to fit your needs.

A alternate way of using a coil of wire is to insert a core of magnetic material and drive an AC current thru the coil. As the core reaches magnetic saturation from the external field, the waveform of the AC current will become non-symmetrical. This non-symmetry can also be used to detect the polarity of the external field.

There are two common ways to sense the distortion of the coil current:
1) If you put a resistor in series with the coil, the voltage drop across the resistor will reflect the coil current.
2) If you put a second winding on the coil (a "sense winding"), the voltage induced in it will reflect the changes in the coil drive current.

In either case, you then subtract this sense voltage from the drive voltage to determine how far into saturation the core is driven. Advantages are that the sense circuitry can be isolated from the coil drive circuitry, and that additional gain can be achieved by increasing the number of turns on the sense winding.

That should give you enough options to keep you researching for a while!

Cheers,
Tom

 

[josh_TMS]

Well... obviously you need a sensor with an electrical output that depends on the magnetic field. I think @berkeman mentioned the two easy ones, a Hall element or a coil of wire.

A Hall sensor puts out a millivolt-range of voltage directly proportional to the magnetic field it is in. If you want a 3-axis measurement you need 3 Hall sensors, one for each axis.

A coil of wire, or solenoid, will generate a voltage depending on the rate of change of a magnetic field. This means a bit more processing to get the intensity of the field.

The following approaches are used in digital clamp-on Ammeters that will read DC current flow. The common existing ones on the market are probably too large for your usage, but here is how they operate. Maybe you can modify one to fit your needs.

A alternate way of using a coil of wire is to insert a core of magnetic material and drive an AC current thru the coil. As the core reaches magnetic saturation from the external field, the waveform of the AC current will become non-symmetrical. This non-symmetry can also be used to detect the polarity of the external field.

There are two common ways to sense the distortion of the coil current:
1) If you put a resistor in series with the coil, the voltage drop across the resistor will reflect the coil current.
2) If you put a second winding on the coil (a "sense winding"), the voltage induced in it will reflect the changes in the coil drive current.

In either case, you then subtract this sense voltage from the drive voltage to determine how far into saturation the core is driven. Advantages are that the sense circuitry can be isolated from the coil drive circuitry, and that additional gain can be achieved by increasing the number of turns on the sense winding.

That should give you enough options to keep you researching for a while!

Cheers,
Tom

Thanks Tom, very helpful. Is there any difference between a Hall effect sensor (https://www.ti.com/lit/ds/symlink/d...=https%3A%2F%2Fwww.ti.com%2Fproduct%2FDRV5056) and these handheld meters ( https://www.digikey.com/en/products/detail/hirst-magnetic-instruments-ltd/GM08/15195149 )? Or am I just paying extra for a screen and the power circuitry?

 

[Tom.G]

You would be getting your moneys worth with that Hirst Gaussmeter. You will save a BUNCH of time doing circuit design and debugging, calibration, and writing software for data logging.

The question would be that if you are using pulsed stimulation, is that device fast enough.

In case you haven't found it, here is a link to the manufacturers website:
https://www.gaussmeter.co.uk/GM08-Gaussmeter

Cheers,
Tom

 

[Klystron]

Strongly agree with @Tom.G that an oscilloscope provides excellent interpetration of electromagnetic signals from a wide variety of transducers, adaptable for a range of projects.

Instead of multiple O-scopes, if required, consider acquiring one 'scope with multichannel and/or multi-axis inputs. Multichannel allows the operator to switch among different signals while multi-axis allows one to mix or otherwise combine different axes on one display.

If budget constrained, consider finding a used or surplus working O-scope at reduced cost. Years ago, I helped a USAF group acquire surplus high end O-scopes and sundry test equipment released from NASA space program for little more than shipping and re-calibration costs. My cousin outfitted his backyard metallurgy test lab with surplus digital equipment acquired from expired or downsizing companies in Silicon Valley, including sets of transducers to connect his electron microscope to an O-scope at reasonable cost.

I used to frequent surplus electronic equipment stores in Silicon Valley that sold and even calibrated used lab equipment for budget conscious engineers, teachers and hobbyists.

 

[sophiecentaur]

Interesting idea. How do I use an oscilloscope to measure magnetic fields?

A magnetic probe (search coil with a lot of turns on it) will produce a reasonable input voltage for a scope if you're dealing with field changes in the tens of ms region. Faster pulses, higher output. An easy DIY solution, if you are only making comparative measurements.