Wide Area Loop Coil Design

e

cat6 is 22-24g, 10meter with all 8 wires in series is ~4-6Ohm and with the twist it is a bit longer so it will in the ball park

he mentioned using a car amp, they often do 4Ohm or even 2Ohm

-Lasse

om

at

eld

big

?

uct

has

t

the

me

,

all

e

e a

rm'

he

is

me

y

at

a similar error.

Then they've got three scan coils (X,Y and Z) to set up the field gradients to move the imaged point (or points) around the sample space.

was in Sydney on Monday and Tuesday giving a pair of IEEE distinguished lec tures on MR imaging - I went to the Monday one. It was rather more detailed than the one I attended at EMI Central Research in 1979, but didn't give m e the satisfaction I'd got back in 1979 from asking why they weren't using a super-conducting magnet, or the amusement I got from the answer that expl ained why it was a bad idea.

Apparently the scan coils have to deliver some 3.5kW of reactive power, and need to be very linear, which means that they draw 35kW of real power. The amplifier racks are water-cooled.

--
Bill Sloman, Sydney

tried a 'simple' Helmholtz using single turn of 18 Awg

Not bad guess. the 'drop' in field from axisymmetric r,z AXIS 0,0 to 0,0.5 53 ppm RADIAL 0,0 to 0.5,0 18.7 ppm

looks like you should have made the coil larger to 'swamp' out the physical variation tolerances, eh?

PS: I sent you the model and plot values to your above address. You can RUN the model, then plot around for yourself

double click on file.fem select the 'gear' icon, left click program analyzes when done, select the 'pair of glasses' left click the results show up there

On a sunny day (Mon, 30 Mar 2015 16:59:49 +1100) it happened snipped-for-privacy@accessnet.com wrote in :

when you use many turns very thin wire the temperature will have a big effect on the resistance. If you need to calibrate with some field = current, you need a sense resistor:

control voltage ---- + multi turn loop opamp --matching network ---O-- -- - | | | ----------------------------------| | Rsense | /// Als plz calculate L for that size of loop and turns, you may be surprized.

You could be much better of with a single turn thick wire or copper tube, and series resistor, drive some more power, and even then for only one frequecy. The above diagram eleminates much of the problems, power amp ICs with +/- input exists, or are easy to make with discrete.

ampere turns

You might want to use plastic brackets instead of metal.

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

I sent a file, HH.zip to your above email address which immediately bounced! Then, renamed HH.piz, sent again and didn't seem to bounce, BUT you may have to look for the email in your SPAM folder, let me know if you got it.

note, that 6+ uT field came from 1A through the single turn 18 Awg wire.

I'm still not getting a single original posting from Mark Grainger. At least people copy enough on replies.

I'm working with a design using 6 ft diameter coils right now and the stupid inductance you get from small number of turns at that diameter is impressive. Takes around 3kV to drive it to get the required field. So now we've got a personnel hazard on top of a more complex design!

The text values for plotting were arbitrarily set at steps of 50mils. You can use any plot program like the one in octave [free Matlab clone] or put into an Excel spreadsheet.

Shim the coil?! Brilliant. hmmm, 20 mils out of 12 inches is approx 1700 ppm ratio! Uniformity of the field is going to far outstrip that ratio. For that 1 inch cube inside your 12 inch diameter coil set, you only get around 53 ppm maximum variation! Yep, construction tolerances ate you alive. Assuming physical tolerances are constant, the only way to reduce the ratio is to make the coil set larger, or find which physical tolerance causes the most problem and work on that. But, from your description the Project is done and you wish to satisfy curiosity.

You can take that model and start shifting the physical placements within your ability to make the coils to see how much variation you get in the field strength. Oops have to be careful doing such. Moving the placement changes the mesh, and just changing the mesh can make results vary more than what you'd get from moving a part. There is a 'work-around' I routinely get on the order of better than 0.1 to 1 ppm numerical resolution for slightly shifted blocks of material. The 'workaround' makes the original .fem image look a bit like a "Nude Descending a Staircase" painting, and use a lot of LUA script to walk through the progression of motion, but insures the results are accurate to EACH other with a more coarse mesh, saving time on a RUN.

Helmholtz coils, one suspended directly above the other.

What would the field geometry look like if the coils are driven so currents oppose, e.g. one CW and one CCW.

Do the fields resolve to a "point" of concentrated field intensity at the center?

Mark Granger

Grin, Well bigger coils are more expensive, and less field per amp. I read some theory papers years ago, IIRC the inhomgeniety goes as the volume "size" d over the coil diameter D to the fourth power. (d/D)^4 which is about what I see.

Thanks.

George H.

OK that's wonderful. (it will be Monday before I look.) If I can't find it you can send it to my gmail account. snipped-for-privacy@gmail.com..

Why such big coils? A big volume? Is this for NMR imaging?

Yeah just curiosity. The shim was in the angle between the coils. (not in x or y direction... though I tried tweaking those too.) So the 12" diameter coils have three ~6" posts, to set the spacing between the coils. I shimmed under one of the posts to correct the field. The exact reason why this worked is a bit of a mystery.. I've got some hand wavy ideas..

Hmm sounds like you really need to know how to "fly" the simulation. (Ain't that always the case.)

Anyway thanks again.

George H.

Maybe brass or aluminum brackets. A little piece of iron can really ruin your whole day.

To OP, I'm not sure what sort of repeatably you need, but maybe think about a processed wood product which will be more dimensionally stable than than hard wood.. grains and things.

George H.

The weight of the wood will make the joints creak and the hexagon will go all eggshape. For rigidity, it'd be best to use inner-ring/outer-ring and make a circular truss, connecting them with zigzag members.

Three meters is a VERY BIG circle, one might consider getting enough

2-inch or larger PVC pipe, and forming it to shape while circulating boiling water; you'd end up with a large Hula-hoop.

Based upon all the helpful suggestions ...

I think will purchase some 2 x 2 hardwood and cut a saw kerf in the top edge, about 1 inch deep. Then make a hexagon using metal brackets.

Into that I will slide vertically some 6 wire ribbon cable. No fasteners needed. Cut an exit slot mid way in one peice of wood, bring out both ends, and wire the turns in series.

Paint it black and it will look like something from DARPA that cost ten grand.

Mark Granger

I sent it to the teachspin.com address. Do you need it resent?

no. 'mine/tunnel' communication. You'd be surprised how many miles [with ENOUGH diameter] you can send stuff, especially through salt layers and salt water. If you get a chance do a search for ELF to submarines and stuff. The Navy [it's my understanding] still uses E Field at ELF tocommunicate by dragging a LONG wire behind an airplane, same at the sub. Although not as effective does have a better form-factor when flying, or swimming ;)

you're welcome. let me know whether you got the file.

PS: After being burnt so badly by errors, or poorly stated assumptions, in those equations in articles, I found using femm was far more likely to yield 'trusted' results, PLUS, the tiny little equation usually has so many restrictions, it's useless, ...or I immediately think of another question [which the article doesn't answer] Just easier to ANALYZE, and then poke around at wht's going on on your own. Need an equationj? Export to octave and then curve fit. And! you can even get 'off-axis' plotting [lines to the side], which, as far as I know, is usually left out of simple articles.

Fun... this type of stuff?

George H.

The coil has a circular form, but there's no reason it shouldn't be held in position on a simple flat sheet of wood or plexiglass.

Fix the coil rigidly on the flat form. Suggest silver or tin-plated copper stranded hookup wire for low frequency. Most teflon-coated stranded hookup wire is silver plated. For high frequency, use litz.

RL

It's supposed to be 10 feet across, though.

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

yeh, get something like this:

throw away the base and stitch the wire onto the fabric

-Lasse

wow! few details there. no frequencies, no distance, nothing. yes, and no.

This is for fairly short range, less than 1500 ft. Actually serves a slightly different function. but the idea is to have the fields 'penetrate' everywhere. At low enough frequencies they go right through a Faraday cage, well a thin one.

Hinges?

RL