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Electric/Magnetic

360 Degree Rotary Position Sensing with Novel Hall Effect Sensors

March 1, 2006 By: Vincent M. Hiligsmann, Melexis Sensors

A new breed of Hall sensors can sense all three magnetic flux density components at a single point.


Adding an integrated magnetic concentrator to Hall effect sensors enables high-accuracy 360° rotary position sensing. The Triaxis Hall technology, based on integrated magnetic concentrators (IMCs), enables the development of small, cost-effective, high-accuracy, noncontact rotary position sensors. Melexis' MLX90316 is the first member of the Triaxis family and is intended to solve long-standing challenges in 360° position sensing.

How Does it Work?

Conventional horizontal (or planar) Hall sensors are sensitive only to the magnetic flux density applied orthogonally to the IC surface. In contrast, the Triaxis Hall sensor can actually sense all three components of the flux density at a single spot.

This is made possible by adding a 200 µm dia. and 25 µm thick IMC centered above a cross-shaped arrangement of two pairs of planar Hall plates. The Hall plates are the sensitive elements (Figure 1A).

Figure 1. A top view of the Triaxis Hall sensor (A) showing the IMC (yellow) and planar Hall plates (blue). A cross section (B) along one axis of a Triaxis sensor shows the IMC and planar Hall plates and magnetic flux lines
Figure 1. A top view of the Triaxis Hall sensor (A) showing the IMC (yellow) and planar Hall plates (blue). A cross section (B) along one axis of a Triaxis sensor shows the IMC and planar Hall plates and magnetic flux lines

The IMC material is amorphous and it is deposited and structured onto the silicon wafer level using photolithography and etching techniques in a postprocessing step.

The IMC converts the flux density applied parallel to the surface of the chip (B//) into orthogonal components (B_1) that can be sensed through the Hall plates underneath (Figure 1B). The conversion is linear as long as the IMC material does not become saturated, which occurs with a parallel magnetic flux density >70 mT. If saturation occurs, the linearity of the sensor is affected but not irreversibly; linearity returns once the flux density is back in normal range. The IMC does not show hysteresis.

Each pair of Hall plates measures the orthogonal flux density applied to them directly or through the presence of the IMC structure. If we take each pair of plates and subtract the signals within them, any orthogonal component (i.e., BZ) of the flux density cancels out, leaving the only parallel components (i.e., BX and BY). Adding the signals eliminates the horizontal components; therefore only the orthogonal component is sensed. Consequently, through a simple operation, we can measure all three components of the flux density. Hence the name Triaxis Hall.

Triaxis Hall in Action

As a noncontact rotary position sensor IC, the MLX90316 uses only the parallel components of the flux density (i.e., BX and BY) as applied to the IC by a rotating magnet (sidebar).

Figure 2. A block diagram of the MLX90316
Figure 2. A block diagram of the MLX90316

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