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The idea of magnetoresistive sensors first came into being in 1856. More precisely, it was the discovery of magnetoresistance (now more commonly called ordinary magnetoresistance) that started the inevitable progress towards the creation of magnetoresistive sensors. Amazing isn't it how something so seemingly insignificant changed the way we live now.
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Magnetoresistance is the property of a material to change the value of its electrical resistance when a magnetic field is applied. Sounds fairly simple, and the process is relatively easy to understand. Whether or not we'd actually be able to build our own magnetoresistive sensors is another story.
The man who discovered this effect was William Thomson, perhaps better known as Lord Kelvin. You will know him when you refer to the Kelvin scale of absolute temperature measurement. The symbol: K in the Kelvin scale is an absolute temperature scale where absolute zero (the absence of all thermal energy) is referred to as 0 K. While Kelvin managed to discover the magnetoresistance effect, he was never able to successfully lower electrical resistance in anything by more than 5 percent.
Interestingly enough it was until recently that scientists have discovered materials showing giant magnetoresistance, collossal magnetoresistance and magnetic tunnel resistance. There is also Anisotropic Magnetoresistive technology on the horizon as well.
And What They Do
Giant magnetoresistive and anisotropic magnetoresistive technologies are used to make Hall-effect sensors, with a higher level of sensitivity. This means even minute changes in magnetic fields are picked up on right away.
Like 0 K in terms of the Kelvin scale of temperatures, magnetoresistive sensors are considered to be true zero-speed sensors. They are available with your choice of outputs as well. Note: these sensors operate best at longer distances.
As with any technology, there may be minor glitches, and you will find one with the magnetoresistive sensors. They apparently are noted for giving false signals. This would indicate they might not be suitable for using too close to big motors/generators. Having said that, if you have a sensor custom built by a company that knows what it's doing, your specific requirements are taken into consideration for the final product.
New Technology
We referred to AMR a bit earlier in this article. AMR is one of the newer kids on the block. It stands for Anisotropic Magnetoresistive technology. This simply means the reliance of electrical resistance on the angle bewteen the direction of the electrical current and the position of the magnetic field. In 25 words or less, electrical resistance has a maximum value when current direction is parallel to any magnetic field that's applied.
The AMR effect is actually being used in a wide variety of sensors that measure the Earth's magnetic field, and electrical current. The current is determined by measuring the magnetic field around the conductor. AMR is also used for linear position, traffic detection and angle sensing.
GMR, also referred to earlier, which stands for giant magnetoresistive effect is seen in thin film structures. They are made up of alternating ferromagnetic and nonmagnetic metal layers. The simplified explanation of this effect is that it produces a significant decrease in electrical reistance while in the presence of a magnetic field. Magnetoresistive sensors are now used by hard disk drive manufacturers.
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