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Mag Sensor for Watch Compass

Continuing with the series of Sensors Expo conversations, I had a chance to discuss a couple of topics with Memsic (one of which we’ll talk about in a future entry). Today we’ll look at their magnetic sensor, which relies on AMR – anisotropic magneto-resistance.

Currents generate magnetic fields, but with AMR, the difference in direction by the field generated by current and some external field will impact the resistance of the material. Current is generally run at an angle in stripes of material, resulting in a so-called “barber pole” look. The good news about this technology is that it can read quickly – about 7 ms.

One of the challenges of any magnetometer for use in a compass is that you’re trying to sense the direction of a 200-mG field in an environment of magnetic fields that can be on the order of 4-16 G. So you have to “center” the measurement so that you can detect this small signal within the environment that threatens to overwhelm it. This centering, on the other hand, gives you more flexibility on where to place the magnet, since proximity to large anomalies is less detrimental.

In a new 2D mag sensor recently released, they’re targeting things like watches, so power has to be conserved. The problem is that the magnetized material in the barber pole can gradually lose its magnetization as dipoles lose their alignment. So you need to realign things occasionally – sort of like running a comb through it to straighten it all out.

Now, you could do that automatically, but it takes power. So instead, they have a set/reset function that sets the field and then reverses the field. This lets them re-center the zero point in addition to refreshing the magnet. But this has to be done manually (although I suppose it could be handled by the system integrator – perhaps hitting the “compass” button could first execute a refresh before measuring, which the user would never know).

The other thing they’re doing differently with this device is giving it a longer lifetime. Phones come and go, and historically, mag sensors have had commensurate lifetimes. But for non-phone system makers, it can be frustrating to evaluate a sensor and, when you’re finally ready to go into production or when you’re extending production, to find out that it’s no longer available. So this device will be kept around longer than typical fleeting phone lifetimes.

You can find out more in their announcement.

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