Low Temperature Coefficient Magnetics
Low Temperature Coefficient Magnetics
(OP)
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?
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?





RE: Low Temperature Coefficient Magnetics
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RE: Low Temperature Coefficient Magnetics
http://www.gizmag.com/nist-laser-power-radiation-p...
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.
RE: Low Temperature Coefficient Magnetics
RE: Low Temperature Coefficient Magnetics
RE: Low Temperature Coefficient Magnetics
RE: Low Temperature Coefficient Magnetics
But I see the advantage of what you’re suggesting.
RE: Low Temperature Coefficient Magnetics
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
RE: Low Temperature Coefficient Magnetics
RE: Low Temperature Coefficient Magnetics
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).
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P.E. Metallurgy, Plymouth Tube
RE: Low Temperature Coefficient Magnetics
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.
RE: Low Temperature Coefficient Magnetics
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
RE: Low Temperature Coefficient Magnetics
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?
RE: Low Temperature Coefficient Magnetics
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.