GMR
Sensing wheel speed with GMR CHRISTINE GRAHAM | ALLEGRO MICROSYSTEMS
Giant magnetoresistance sensors are strong candidates for the high-accuracy wheel-speed sensing necessary in autonomous vehicle systems.
GMR and current sensing
The purpose of a braking system has evolved from simplistic stopping to critical functions in vehicle safety through highly accurate feedback to traction control (TCS) and advanced driver assistance systems (ADAS). These advances have been made possible in part due to the technological developments in magnetic wheelspeed sensing. Integrated circuits for magnetic sensing have evolved from simple 100-transistor designs to those containing hundreds of thousands of transistors. The Hall effect is a common transducer technology used in sensor designs. The Hall transducer interfaces with processing circuitry to produce a switching square wave representing either speed or speed and direction for the braking control unit’s processing algorithms. Hall effect-based wheel-speed sensors have helped move braking system technologies from standard anti-lock braking (ABS) capabilities to traction control (i.e. anti-slippage) and full stability control (vehicle maneuverability). The Hall effect, discovered in 1879 by Sir Edwin Hall, refers to the voltage produced when an electrical current flows through a conductive plate while it sits in a magnetic field. The resulting voltage is perpendicular to and proportional to the current and magnetic field applied. This Hall voltage is small in amplitude and susceptible to temperature variations. So it takes special signal processing to effectively integrate it into a sensor IC. Wheel speed sensors monolithically integrate Hall transducers on a single silicon substrate with signal conditioning circuitry. These plus compensating algorithms provide a reliable output square wave to the control unit. The Hall IC is either used in combination with a back-biasing magnet to sense a ferromagnetic target (such as an iron-based gear) or the IC directly senses a magnetic encoder ring (a.k.a. ring magnet), also known respectively as back biased (gear) or front biased (ring magnet). eeworldonline.com | designworldonline.com
Allegro — A&CV HB 08-19_v2.indd 39
A typical GMR sensor chip. The GMR resistors are connected in a Wheatstone bridge with half the elements positioned under one magnetic condition, the other half under another.
GMR and ring magnet applications Direction of rotation
Air gap
Branded face of package
A&B
Pin 8
C&D
Ring magnet
Pin 1
Element pitch Automotive speed sensing applications such as ABS may use a ring of magnetic material with alternating north and south magnetization. The GMR sensor may be placed under this material such that the plane of the die is horizontal. The spacing between the four Wheatstone bridge-connected GMR resistor elements creates a differential magnetic field sensed by these sets of elements based on where the ring magnet is in its rotational cycle.
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DESIGN WORLD — EE NETWORK
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8/8/19 7:46 AM
