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Sensing the Turn

This is yet another note regarding the innumerable sensors on display at the recent Sensors Expo. But rather than jumping straight in, let’s explore a problem: one akin to “shaft encoding.”

Those of you controlling precision motors and such know far better than I do about keeping track of the rotating shaft of the motor. By tracking marks on the shaft, the electronics can keep track of the position of the shaft (not to mention speed and other related parameters).

But can we apply that to, say, a steering wheel to remove the mechanical linkages? After all, a steering wheel is the same thing, only your hands are the motor. So, in theory, it should work. But there’s a catch: When the power goes off, the system loses its mind. So when you power back on, the system doesn’t know where the steering wheel was left last time it was touched.

You could suggest that a piece of the electronics remain powered on (if low enough power) to keep track even when the motor is off. But if you change the battery, or if Jr. Samples decides to apply the skills he learned on that burned out ol’ 52 Chevy pickup in the back 40 and disconnects the battery, then the system loses its mind again. So when the car starts up, it’s like it’s waking from a bad dream and not knowing where it is.

So simple shaft encoding won’t work; we need something that persists with no power. That would suggest a magnet. For instance, you could place a magnet on the shaft and then detect which direction the magnet is facing. Or put a magnet around the shaft and put the sensor on the shaft. But that only works for applications that use at most one turn. That’s certainly not the case for your grandfather’s Oldsmobile, where a simple lane change required 20 turns of the wheel.

So you can add translation to the rotation: put a thread on the shaft and have either the magnet or the sensor ride on a carrier that moves along the thread. So as you execute multiple turns of the wheel, the carrier slides up and down the shaft (rather than rotating with the shaft). We’ve now translated the multiple rotations into a linear distance, and the strength of the sensed magnetic field can tell us how far we’ve traveled. And it will work when the car starts.

This is the approach that AMS has taken on the AS5410 “absolute position” sensor they had on display. Specifically, they put the magnet on the carrier and use a 3D Hall-effect sensor in a fixed position. The thing that apparently makes this a first is that the sensor can reject stray fields using differential techniques. This can actually mean using several magnetic sensors, so it’s a bit more complicated than my simplistic description… but then again, most things are.

You can find out more info here.

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