Low Temperature Coefficient Magnetics

[Overrun]

Our group has developed and is hoping to improve the thermal properties of a DC voice coil instrument that measures the pressure (power) of a high energy laser upon the device armature. A position detector monitors incipient displacement and modulates the coil current to balance the laser pressure. The Δ current signal is rather down in the noise mud.

The concern is heat finding its way into the instrument and affecting the “fixed” magnetic field. The magnet is Alnico 5 and the soft magnetics are 12L14, neither being optimized –heat treating? - for thermal stability. Tc and the “contribution” of the higher Tc component, rather than efficiency would seem to be the prime concern.

Any thoughts?

 

[IRstuff]

I don't know much about how you intend to do this measurement, but I would suspect that the measurement would still be quite crude, given the need to suspend the voice coil somehow, which has to take away from whatever force a laser might generate. Something along the likes of the original Cavendish gravitational constant experiment might be more applicable:

http://en.wikipedia.org/wiki/Cavendish_experiment,

or, for that matter, even a Crookes radiometer

http://en.wikipedia.org/wiki/Crookes_radiometer

with a capacitive encoder

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[Overrun]

The concept is sound;

http://www.gizmag.com/nist-laser-power-radiation-pressure-measuring/29595/

The instrument is capable of accurately resolving to a part in several million. However, over relatively long periods of time a CW laser imposes a rather large heat load. I’m looking for conventional expertise on enhancing the balance of permeability, Tc, coil current etc.

Obviously my practical knowledge of magnetics is rather scant and I’d rather not reinvent the wheel. Or maybe this is new and it’s on me to deal with it. Thanks for your consideration in any event.

 

[Compositepro]

Use a solenoid coil in place of the alnico magnet. You are using a voice coil already because of the very accurate relationship between magnetic field strength and current, which can be very accurately measured.

 

[Overrun]

Compositepro; small world, we’ve been experimenting with that. Two coils would be only current dependent. However, the coils would get a bit toasty and would seemingly still require soft iron to complete the flux path, wouldn’t they?

 

[Compositepro]

Not for generating very small forces. I would think that standard analytical balances, which commonly use this principle, use two coils rather a permanent magnet. No need to reinvent the wheel; see what they do.

 

[Overrun]

Analytical balances are what we do and why NIST came to us. Over the many years the internally generated heat has been successfully dealt with. The external laser heat source is a new challenge to the magnetics Tc. Lower Tc soft iron and/or a better balance between the magnet, permeability etc. is what I’m looking at. The two coil approach provides fields solely as a function of current but dumps a good deal of heat into the structure. At such levels the heat causes other problems in nonmagnetic component growth as well as with the magnetics.

But I see the advantage of what you’re suggesting.

 

[EdStainless]

One approach is to use Alnico 5 and a small trim coil to compensate the field.
There are also shunt materials that have temperature coef. such that they shunt less field at higher temps. This is what they tune traveling wave tubes with for radar use.

You do need to better optimize your steel unheattreated 12L14 is not your best option.

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P.E. Metallurgy, Plymouth Tube

 

[Overrun]

Thanks for the input. It appears that 12L14 isn’t optimum for permeability or consistency, but may OK for its permeability temperature coefficient. Greater permeability from another alloy with the same Tc would also have a greater adverse effect from temperature change with the same Tc.

 

[EdStainless]

NO, you need as much field as you can get because either using a trim coil or shunts will reduce the field.
For shuts look for Thermoperm or Calmalloy.
Their Currie point is just above room temp (various compositions have diff Tc). So as they are fairly magnetic at RT, and put across the gap they will shunt field, as they warm near Tc they will rapidly become less magnetic, shunting less field (as the magnet weakens).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MagBen]

What is the temperature range for applications, and what was the temperature you magnets "saw" the heating?

AlNico has a very small temp coefficient on magnetics, the problem sounds to be more related to the soft magnet. Tc should be a big concern as long as Tc is much higher than the application temperature. The stability is the key, unanneal is obviously a relatively unstable status, "heating" can change strain level to change permeability. Also, the material type seems not ideal due to so-called "magnetic aging" even at low temperature, which degrades permeability. The carbon precipiation is the culprit for that effect, so carbon steel may not be ideal.

Ed's idea for compensating shunt is good. The materials are called temperature compensating alloys in Carpenter Technology Corp. which offers couple of grades to cover a wide temperature range -40 to 140C. These alloys are normally used in watt-hour meters, speedometers for permanent magnet, and voltage regulation for ectromagnet compensation. The flux density, or permeablity decreases (can be linearly by design) with temperature.

 

[Overrun]

The device has several effective compensation schemes. My guess –the laser thing is a new use- is that the temperature flux rather than heat-soaked change per se is the new concern. Uneven growth in the mechanical suspension is a likely culprit that’s also being addressed. There’s also perception. A steady offset isn’t noticed but drift well within accuracy specs is disconcerting. The temperature change probably is (well) less than 0.10⁰ F at room temperature.

My concern with the 12L14 is that it’s a rather plain vanilla soft iron. Not being a certified magnetic material is a bit of a red flag. Slow metallurgical change isn’t that much of concern in that our instruments are zeroed between measurements and calibrated regularly –moving the device affects calibration at the higher resolutions. Put simply, is 12L14 about as good as we’re going to get for short term change in permeability with temperature change? With temperature flux and uneven heating of various instrument components, it’s more effective to minimize drift than to compensate for it.

Your comments are appreciated

 

[MagBen]

You are looking at permeability temperature coefficient at a temperature range of less than 0.1F? I doubt even the most accurate measurement system can detect any difference in magentics!

How much high could your external laser heat source increase the temperature of magnets? Material unstability could only be from here. Since you zeroed each measurements, long term metallurgical changes should be an issue. given the Tc of carbon steel is much higher than RT, changes in magnetics at a very narrow temeprature range is very trivial. Are you sure this was a magnetics related issue?

 

[Overrun]

MagBen, the laser power meter is itself used to determine and detect thermal anomalies. By first measuring the laser and then applying the same force from a nonthermal source –the drift is not seen in the second instance- the source can be established. However, as mentioned above, thermal expansion is also suspect. May well be both.

I thought the soft iron temperature/permeability specs would be known for the common materials used. But it’s fair to say that our use pushes the boundary and we may have to do the work. Not finding anything this side of a paywall may have been a clue.