Contactless torque sensors.

[firewalker]

Does anyone know the main "secret" for the contactless torque sensors? Mainly the one used in vehicles steering wheels? Does it have to do with the fact that a rod under torque moves slightly from it's center of rotation?

Alexander.

[Rerouter]

It has to do with the material not being a perfectly rigid shaft, so there is a torsion between 2 measuring points, this torsion is what is measured.

To give a bigger example, think of a vehicles driveshaft, in trucks where it gets much longer its not uncommon for one end to be 90 degrees rotated from the other end while accelerating. Its still within the safe material limits, but it does twist up when it is under load.

[firewalker]

Yes but how? It sue isn't strain resistors. If I understand correctly some papers it has to do with magnetic field and the way a rod move of center when under torque.

[Rerouter]

The type I am aware use a ring magnet on the shaft and measures it siilar to a normal hall effect positioning sensor,

The ones your looking at may well measure how off center it gets, but that seems like it would need a lot more force to accomplish unless they knew there tolerances where almost perfect. (A steering wheel connects via a shaft to a universal joint generally just past the firewall, its possible they manufacture a slight eccentricity to make it walk off center.)

[Andy Watson]

I don't know what is used in car steering mechanisms but years ago we used a system that was based on magnostriction to measure the torque of a gearbox drive shaft. Essentially the system induced a magnetic field into part of the shaft and then measured the response as if it were a transformer. I believe a similar system is commonly used on ships' propeller shafts. The actual measurement relies on the reverse of magnostriction - I don't know if it has a name.
This patent shows something similar:
http://www.freepatentsonline.com/4939937.html