No shame to shim.

Won't a resistive shim only work properly when the coils are run at the same voltage that you tested? An adjustable current- fraction shim would work at any current. Would be built into the power supply.

GH

The coils are in series, I'm stealing current from one of them.

George H.

The NMR people have several superconductive shims inside the magnet, persistant currents, and a dozen or two room temp shims with programmable power supplies. They want line widths around 1 part in

Teach them how to shim too!

Good, but the amount you're stealing is dependent on the voltage = on the current, and so correct only for your setup at the time. If it was instead a fixed fraction, it'd be universal.

Hi Win, I'm failing to see your objection. I am stealing a fixed fraction of the current. About 5(ohms)/2k(ohms) a few parts in 1000. Since there are 150 turns/ side, it's like taking away ~1/3 of a turn. The assumption is that there is some small manufacturing difference between the coils. I tested it over a range of currents from ~0.2 amps to 2 amps. At all currents the 'shim' improved the LW. Though at low currents the LW is also bounded by the local 60 Hz AC magnetic fields, so less of an improvement.

George H.

I'm only at the part in 10^4 level... at best. Here's some data.

George H.

Well, my guess is that the coils are SLIGHTLY different, maybe the packing of the turns was just a little different between them. So, what you are doing is to tweak them so that the field is the same. Is that right?

If so, then the only reason it might cause a problem is if the coils change again, maybe as epoxy cures or something, that causes them to drift a little. Anyway, if the device is that sensitive, then it may require re- shimming when deliverd to the customer.

Jon

Jon

It may not be worth it for your purposes, but you could null out local static (like earth's) and AC fields.

Right, We asked the coil winder to do the coils in pairs, but the simplest idea is that one side has a bit smaller average radius. (We asked the bobbins to be made in pairs too.. )

Hmm, the average customer will never know. ~1/2-1/3 of the coils are sent off with a mechanical shim. Dang!! you know that is something I didn't document... well there may be something in my note book, but nothing systematic. (head slap)

I have no idea the long term LW of a coil pair. It's copper wire on phenolic bobbins with aluminum support parts. no glue or epoxy. There's certainly thermal changes in the copper resistance, you can put ~100W through the coils... and they get hot after a while. Anyway I'm correcting a manufacturing error by playing off copper resistance against a metal film resistor. Otherwise, I put these coils on the shelf with ~10 other pairs. (Hmm some of those are just bad for missing a turn, but there may be a few shimable pairs, some Friday.)

George H.

We have a set of vertical coils to tune out that component of the Earth's field. And then align the optical (z) axis with the horizontal component of the E's field. And use the big coils to add or subtract to the horizontal part.

In cities I find the DC field is pointing in different directions all the time. (and probably changing in magnitude) So our above scheme is far from ideal.

Re: AC magnetic field cancellation, boy that would be great for my customers. You can do a lot at DC with a single coil, or magnet. At AC I expect the magnitude and direction of the field is changing. I could tie some single coil to an AC transformer with series resistor/ pot. It needs an undergrad to hold the coil in place while someone takes the data.

But maybe you have some other idea?

George H.

The magnetic field is proportional to current, not voltage. In the classical power factor sense, the current/voltage phase difference depends of motor loading. With power electronics, the current waveform differs significantly from sine, so some kind of comb filter might help.

Even in countries with predominantly single phase domestic low voltage feeds, the medium voltage feed is three phase and I would expect that for instance neighboring apartments could have different mains voltage phase and hence different current phase and hence magnetic field resultant.

Oh right, I'd need a phase adjustment too. I'd forgotten about that. Thanks George H.

I was thinking that a small sense coil (or two or three) could detect local AC fields and some current could be squirted into your big coils. But your coils only cover two axes. Is the sample sensitive to fields in all three? If so you'd need a third field coil to null the field.

Combine the AC nulling with active shimming. That could be fun.

Whether this hassle is worth it depends on how valuable line width is, or whether shimming and nulling would be sellable options.

In NMR the samples live deep inside a superconductive magnet which is a pretty good shield. Seeing 60 or 120 Hz spurs on a spectrum is like having flies in the soup.

Hmm that is a good question! I think at moderate applied fields, (away from the zero field region*) that the position (frequency) of the resonance depends only on the total B field. So that the orthogonal components of the AC field only add as the sum of squares to the total field and can be ignored. And then it's only the component along the optical axis that I care about. I've got a modulation input for the main coils so I can try this. I'll need a 60 Hz phase shifter. (A 90 degree All pass and mixer/ summer?) OK another project for some Friday.

George H.

A sense coil (like a little drum core inductor) will make a voltage that is proportional to the rate of change of field. The current into your big coil makes field linearly. So you want an integrator to do the phase shift. The integrator will work at all frequencies (except DC!) so handles line harmonics too.

That sounds right. Years ago a made a gizmo. (multi-turn air coil (~100 turns, ~5" diameter.) put in a aluminum box with foam support and a bnc to outside, isolated I think. IIRC I tried to calibrate with the OP signal which seemed to be about right.. The signal was noisy. I was thinking of sending in some pick-off from a transformer. (that misses thing's like induction motors or what ever it is that sends ripples through an AC line synced signal.) with an amplitude and phase knob into the coils. A factor or 2-3 in LW is 2-3 in signal. Always a good thing.

Sensing the local field would be good, but no magnetic materials.

Hey that would be a fun thread bend. How to measure LF local E-M fields, LF is something below ~ 10kHz.

E-fields are easy, stick paper clip into a 'scope input, and hang on. (where's the human body RC 3 dB point? ~100 pF and 10 k ohm?

George H.