How some sensors use giant magnetoresistance

So, what if an application needs a sensor to track an axis 360° and beyond? Absolute encoders need a power source and (for some designs) can be prohibitively costly while potentiometers can lack sufficient resolution. So, for applications not satisfied by these other options, multiturn shaftless rotary sensors are a leading option. These multiturn sensors:

• Track linear or rotary actuators’ drive-spindle positions over many turns.
• Supply control systems with data to actuate low-cost fluid-power and electromechanical drives sans gearing.
• Monitor powered conveyor rollers and drums in spool-related applications.
• Monitor angles prompted by steering-wheel rotations in automotive applications.
• Detect how open roll-up doors are in warehouse settings.

Many sensors billed as multiturn variations leverage the giant magnetoresistance or GMR effect. GMR is a quantum mechanical phenomenon in which a layered structure’s electrical resistance changes with the layers’ relative magnetizations. With parallel magnetizations, electron scattering and resistance are low. With antiparallel magnetizations, they are high.

Some multiturn sensors also feature a domain wall generator or DWG with boundaries between magnetic regions to work like tiny switchable zones. These boundaries change how easily electricity flows so the sensor can record movement counts via magnetic changes — and in some cases, store that data for years.

Within the sensor electronics, a reference layer with fixed magnetization sits atop other sensor and memory layers (complete with an inward-spiraling track) that limit magnetization to 0° or 180° with shape anisotropy. The large size of the DWG pool of magnetic domains prevents shape anisotropy and allows its magnetization direction to align with field of the sensor’s target magnet.

When the magnet rotates with the machine axis to which it’s attached, it causes the DWG to release magnetic-domain groups (aligned to either 0° or 180°) into the track … and then the cycle repeats at 360° to affect subsequent sensor track segments.

The segments’ resistances are measured and processed by an integrated circuit; the number of segments possessed by the inward-spiraling track determines how many axis rotations the sensor can count.

In fact, some variations of these multiturn shaftless sensors can track 16 turns and beyond with 16-bit resolution and ±0.25% linearity.