Tesla Units and Magnetic Strength

Just to give you a reference. Let's say it's a decent size. You put a dozen steel 1/2-13 nuts in your pants pocket and walk by within 5 feet. They will leave your pants. A normal MRI magnet might be about 3-4T these days.

Anyone able to relate that to something real world?

A Junkyard car-lifting electromagnet is around 1T and requires around 20kW of power.

With a 16T field (and less) nonmagnetic objects can be levitated.

What exactly do you want the magnetic sensor to do? Also, how high is the magnetic being lifted (ie- what range are you trying to measure)?

Can you provide more information on your application (2Hz movement seems awfully quick for a large electromagnet).

You have a very tricky problem, as this magnet is quite likely to polarize any metal shielding you put around your magnetic sensor, making the shielding useless.

I would highly suggest the use of pneumatic sensors and actuators, as these are not affected by magnetic fields. Also if you were to use even an optical sensor, you would need to be careful of the direction you lay your wiring, as the magnetic field will affect it as well, potentially producing false readings.

To give an example, I have some friends who work in an aluminium induction furnace (strong magnetic fiend), and no-one there needs a tool belt, all the metallic flooring, support structures, I-beams are magnetised so strongly that any steel tool will hold it's weight onto the roof.

I will not give advice on how to shield the magnetic sensor if used, as this is not my field of expertise, but I would highly suggest to use pneumatic sensors.

If you have a pacemaker stay away - a long way away from it. Not my area but I would suggest that at that sort of level your sensor would not stand a chance.

I put a position sensor in a 6T magnet a few years back. The only thing that would work well was optical. A previous engineer had rigged this very elaborate mechanical linkage setup that went out and behind shielding a distance of perhaps 20 feet where it was connected to an electrical sensor. It worked if the linkage stayed free. The linkage did not stay free.

It depends on the precision you need the location. Any mechanical linkage will have some hysteresis and running at 2 Hz will complicate things. Any metal will feel large forces, steel very much so.

So you are left with optical. Optical is a mature technology. You will have to make the machine from non magnetic metals or you can have problems. This thing running at 2 Hz will cause heating in metal fillings in your teeth, it will pull the eyeletss out of your shoes (if steel) A number of people have been killed by magnets like this when fools brought metal into the MRI room and it flew into the gap.

http://www.google.ca/search?hl=en&q=mri+%2Baccident+%2Bmagnet&btnG=Google+Search&meta=&aq=f&oq=

Hi RVZ ... I deal with magnetics every day, but these levels are WAY beyond what I encounter. I did find this site with some "interesting facts". http://hypertextbook.com/facts/2000/AnnaWoo.shtml (sorry if this link doesn't work well ... my browser is doing strange things lately. BTW, if you ever find out what they're doing, and you're not at risk of being visited by the MIB, drop a note to us all ... it really has by curiosity ... especially the "2 Hz" portion. Kind regards ...

Doug,

I do regularly work with high magnetic fields. You grasped exactly the concerns here, so you get a GA from me.

The OP cannot use a magnetic sensor for any motions around this device. A myriad of problems will occur during both ramping up of the magnet and during the actual motions. Besides, optical sensing can be considerably much more precise than any magnetic pickup.

So.... You work with hydraulics huh?

We need to talk sometime if you're up for it.

The need for magnetic sensors vs optical, sonic, infra red, etc.. has to do with some secret they have and want to keep under tabs. Do not drink the RED fruit punch if you go to their meeting at the hanger.

JL Mealer
Mealer Companies LLC
http://mealercompanies.com

JFC! There is no secret about optical sensors. You can buy the %$$$#@ things from McMaster if you can live with that level of resolution and you have the room. There are plenty of other sources. There is no secret technology, nothing that you need to get involved in in terms of conspiracies.

You could always use some sort of plastic chain drive if that suits. But, hangars? Come on.

Ahhhh.. I was joking.

JL Mealer
Mealer Companies LLC
http://mealercompanies.com

You know what I like about CR4?

It's that you can start reading a post cos it sounds sort of half interesting or whatever, and then someone like Mr Mealer comes along and whacks you round the head with some really interesting stuff that can lead who knows where.

Mr Mealer, your website says lots but also nothing - but I'm gonna check back in every now and then because, you never know, there just might be something there one day and I would hate to miss out on that.

I may be cynical and sceptical, but hey, I have good reason to be, from being caught before by HHO and all that stuff.

Go for it, sir, I hope it works for you, for the planet, and for the USA.

Just trying to keep the humor flowing!

You're welcome on our site anyday... And up until the 'branding pros' get a hold of it and push my sense of humor and politcal views into secrecy, you may be able to stay entertained.

Then again, with the branding comes our big funding and the details of the MEALER will be allowed to the public. I just hope I am not forced to change the vehicle name to a wingless bird or a long dead inventor who was forced out of business by a crooked General Electric!

JL Mealer
Mealer Companies LLC
http://mealercompanies.com

Magnetics is not my area of expertise, electronics, optics, and measurments is.

So you have a very large magnet of considerable weight. I assume that it will have a motor and cable & pulley system or mechanical connection of some sort or a hydraullic cylinder/s to move it at the 2 Hz rate. I would consider the following options:

1) a rotary optical encoder attached to the drive motor or pulleys

2) or connect the optical encoder to the mechanical driving arm

3) I have built very accurate Laser based thickness gages and that technique could be employed if the more mechanical approch is not feasable.

It sounds like the 7T field is not only very dangerous but would give most sensors some major problems if accuracy is a critera. So I would keep the sensor as far away from the field as possible. Good Luck with you project. ....... Pasesetter........

It is a very bad idea to move such a powerful magnet unless at least the following problems have been addressed:

1-The mass and location of metal exposed to the field must be constant at all time to avoid induction of large voltages in the superconducting coil. If not, the coil and amplifier will be destroyed.

2-By their construction, these magnets are not suitable for repeated motions.

3-Any conductors in the vicinity will have large eddy currents induced. This could heat by induction any metal surface nearby. Your pump will have to supply this heating power as it will add a resistance to movement.

4-You hydraulic cylinders will probably seized from the magnetic fields if not properly shielded.

5-Any metal filing will stick to the cylinder surface and increase wear if not totally block it.

I am sure that there are many other challenges that they will face, to the point that my bet is that it will not work the first time, and probably not the second time either. Expect some very expensive modifications. I hope for your company that this is not a fix price to success contract.

Hi,

anything magnetic above 1 to 2 T is either excited by a superconducting coil or a pulsed electrical current.

The strongest permanent magnets have near 1.4T at the poles if operated with nearly no air-gap.

You have to be sure that your entire hydraulic circuit is electrically nonconducting (not possible I think) or will be severely distorted/fragmented by the rate of change of this magnetic field.

Eddy currents are induced in any material by induced voltage U is -dΦ/dt, Φ is the magnetic flux to be measured in Volt x seconds, or Φ = B(Tesla) x A(square-meter).

Tesla is flux density, if multiplied by area resulting in flux Φ. (Tesla is V x s/m²)

If you take a cylindrical coil with a length of 10 cm you have to energize this with 80000 ampere-turns to get an internal magnetic field of 800,000 A/m often symbolised as H.

Flux density in air and air-like materials is B = µ_o x H

µ_o is the permeability of vacuum or 1.2 x 10-6 (V x s)/(A x m)

(x is multiplication symbol).

So 800,000 A/m is in vacuum, air, plastics, wood = 1 T !!!

In any conducting material you would have to wait until the eddy-currents have decayed.

If you penetrate with your flux density of 7 T a metallic plate of Aluminum and if the field is constant nothing happens but you need the above stated current-turns per length or in total 5.6 Mega-amps per meter of coil. (Very soon melting!)

If you change the 7T with a frequency of 2 Hz then the rate of change, being 2π x f or roughly 12.6 rad/s is giving a rate of change of magnetic flux density of 88T/s.

If the coil has an inner dimension of 10 x 10 cm (4 x 4 inch) or 10-2 m2 then the rate of change of flux is dΦ/dt = 0.88 V. This is a circular voltage that is driving a circular current that is directed against the energising current of the coil. (Lenz law).

Let the mean length of this current path be 25 cm, the current flowing in a ringlike shape 3 cm wide, 5mm thick and 25 cm. This ring will have a resistance of 50µOhm.

(30mOhm/meter and square millimeter assumed)

So a voltage of 0.88V is driving a current of 18.000A.

And this a heating power of R x I² = 16200W or 16 KW, quickly melting the aluminum plate.

This is a rough estimate, likely a factor of 2 too low as the aluminum plate will extend to the outside.

These two currents interact also mechanically generating forces that can be expressed as magnetic pressure of 4bar at 1 T, 16 bar at 2 T and nearly 200bar at 7T.

So coil and plate will be subject to 20KN axially and coil also radially.

This is a quick and dirty rough calculation to have an estimate.

So ask about a drawing with the respective flux-densities and the rate of change.

From this you can deduce the allowed positions and possible (if possible) shielding.

(You cannot insulate against a magnetic field as you can insulate against an electric field with a conductive shield and against a voltage with electrical insulators.)

If pulsed as in magnetisers the situation is much worse as the pulses are much shorter.

More on request. Tough problem.

RHABE

www.precision-engineering.de

This might be a small cavity MRI machine, which gives high resolution when coupled with the 2 HZ rate. For bones, amber, etc and people with no metal in the part imaged.

the use of fiberglass-epoxy rod and solid structures will work for the hydraulic cylinders. Carbon fiber as well, if non conductive type. A laser interferometer with a dielectric mirror on the work piece can measure displacement remotely, as can a long fiberglass stick with engraved rulings. It depends on what will serve. Possibly keeping the magnet fixed in place and moving the object being scanned?, if it is a scanner.

You may be near reality with your hypothesis.

But I do not understand the low frequency.

There are existing MRI instriuments for chemical analysis. (Brooker). But these use small sample tubes (a few mm diameter zirconia) spinning with ultrahigh rates (5 to 15 KHz !!!) in a high magnetic DC field.

RHABE

Hi , what is the amplitude or stroke?

Wow, Thank you all for the great responses!

I have not yet signed any non-disclosure agreement, as we are in the initial stages of this job. I actually know very little, other than they are in the power generation field, and the particular unit in question, is in their test lab. The budget they tossed out is unreasonably low for what they need. All I have been asked to do is provide the hydraulic system capable of oscillating the magnet at the 2Hz rate. At this point, it sounds like a high liability job, and I'm not sure its something i wish to pursue. However, we have provided an initial bid for the job (which was about 10X the price they were shooting for), so I will keep you all updated with any information I am at liberty to share. This is definitely one of the more interesting jobs on my plate at this time.

Maybe you think about a (glass-fiber-epoxi) extension

of a hydraulically moved part.

Extension to be located near and in the high magnetic field.

Hydraulic mover at 1 ... 0.1 T or below.

Best shielding: layers of high permeability and high electrical conductivity sheet metal.

RHABE

Power generation and magnet motion will produce many changing mechanical motions and vibrations. When in doubt use Loctite or a similar product on any non-moving thread.