Multi-turn sensors that remember their absolute positions even after power has been turned off, and then restored, are essential for the next generation of electronic car stability systems. The sensors promise to greatly reduce loss of control, resulting in accidents, as well as a host of other applications.

The technologies involved are quite advanced. The RSM 2800 EuroSensor from Variohm uses the Giant Magneto Resistance effect, used in hard disk drives, where thin films of alternate magnetic and non-magnetic layers are laid down in what is described as spiral arms.
Sixteen resistive strips are laid out in a square spiral arrangement. As the magnet on the shaft is rotated, different strips are switched on and off. This means one set is used for the first rotation, a second set for the second rotation and so on. The switching is non-volatile so the device remembers which set of strips it is using.

As a result, the 28mm diameter device provides 12-bit absolute position resolution over a range of up to 16 turns. Voltage or current output is standard, and a digital version with 0.1º resolution is also available.

Rated temperature range is from –40 to +85°C, or up to 125°C on request. The housing is plastic and the input shaft stainless steel, with a choice of indexing flat or push-on coupling. Supply voltages are 24VDC and 5VDC.

As well as motor vehicles, the device is suitable for valve and flap position detection and also a low cost absolute feedback system for servo controls. The sensor can also be fitted to the drum of a cable extension transducer.