By Steve Bush 25th October 2021
Allegro Microsystems is advocating GMR – giant magneto-resistive – effect sensors instead of Hall effect for automotive rotary wheel position sensing.
The company, which makes both Hall and GMR sensors, argues that GMR has lower noise and therefore reduces angular jitter when measuring the position of rotating shafts.
“GMR is fundamentally a lower-noise magnetic sensing technology than Hall effect for magnetic sensor ICs,” Allegro told Electronics Weekly. “Both GMR and Hall are effectively transducers that convert magnetic flux density into voltage. Allegro GMR provides more than 50 times the voltage per gauss and as ICs always have a noise floor – such as from thermal noise – GMR provides higher SNR.”
It sees Hall sensors producing a speed accuracy within 0.5 to 2% if speed is deduced from two adjacent output pulses, and them operating with an allowable air gap of ~2 to 3mm – depending on the target type. “These sensors have met the needs of standard braking systems,” said Allegro
Again depending on the sensed target, the company sees its GMR wheel speed sensors being used with 4 to 8mm, and being capable of providing signals where jitter can equate to as little as 0.02% of speed.
“The percentages for Hall effect and GMR are with the same general conditions, using a typical ring magnet used for automotive wheel speed sensing, near the max air gap capability of a Hall effect speed sensor, and at 150°C – the worst-case for jitter,” Allegro told Electronics Weekly, adding: “There’s no significant disadvantage with Allegro GMR versus Hall effect.”
Allegro’s GMR uses a spin valve architecture (left), which involves both a reference layer with a fixed magnetic orientation and a free layer that aligns with the external magnetic field applied.
The magnetic orientation in the element affects the spin state of the electrons, which affects the rate of electron scattering and consequently changes the resistance of the element.
When a magnetic field is applied in the same direction as the reference layer, the GMR element has a relatively low resistance. Resistance becomes higher when the field applied is perpendicular to the reference layer.
Such magnetic sensors are either ‘back-biased’ with an in-package magnet for use with non-magnetised ferromagnetic cogs or toothed rotating targets, or do not have a magnet in the package and are used with alternating-pole ring magnet targets.
Example Allegro GMR automotive rotational sensors (right) are A19350 for sensing ring magnets, or ATS19480 and ATS19580 for plain ferromagnetic targets – the latter includes a direction output.
Inside, magnetic fields are measured differentially to cut common-mode noise and environmental effects. All are developed in line with ISO 26262 functional safety.
Wheel position sensor are used in anti-lock braking, traction control systems – including torque vectoring – and driver assistance systems such as autonomous parking and collision avoidance.
Tagged with: automotive GMR Hall sensing
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