Advanced driver assistance systems require higher accuracy at similar, or in some cases larger, air gap distances between the sensor and the sensed target. Giant magneto-resistance (GMR) is a strong candidate to meet these enhanced requirements. Front-biased magnetic encoder rings are the most common target type in today’s light vehicle applications and the most common implementation for GMR. The GMR replaces the Hall effect as the sensing transducer. In principle, GMR and Hall are both magnetic sensors, but the two differ in fundamental operation and capability.
The GMR resistors are connected in a Wheatstone bridge with half the elements positioned under one magnetic condition, the other half under another.
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.
The fundamental principle of the GMR effect is based on electron spins. In a magneto-resistor, electron scattering rates rise or fall as a function of the interaction of the spin state of the electrons and the magnetic orientation of the medium where the electrons are traveling. Electron scattering increases the mean free path of the electron flow, effectively altering the resistance of the medium. Simply put, the GMR transducer’s resistance changes in the presence of a magnetic field.
Earlier generations of MR technology limited sensing angle, linear range, and sensitivity. These limitations have been overcome in Allegro’s state-of-the-art patented GMR technology, providing greater range, and higher sensitivity. The sensing orientation allows the GMR sensor to be a drop-in replacement for Hall effect sensors.
The fundamental principle of the GMR effect is based on electron spins. In a magneto-resistor, electron scattering rates rise or fall as a function of the interaction of the spin state of the electrons and the magnetic orientation of the medium where the electrons are traveling. Electron scattering increases the mean free path of the electron flow, so the GMR transducer’s resistance changes in the presence of a magnetic field.
Historically, the GMR was oriented 90° out of phase from the Hall effect sensor making it difficult to swap the technologies. Another issue seen in earlier MR designs is that of discontinuities in the signal at close air gaps. The signal perturbation directly correlates to higher edge jitter, reducing the wheel speed sensor’s output accuracy. Nearly a decade has been spent overcoming the signal discontinuities in wheel speed sensors now available, such as Allegro’s A19250 and A19350 GMR wheel speed sensors. In addition to low jitter, these sensors offer large operating air gaps which extend far beyond the capability of Hall transducers.
Like Hall effect technology, GMR is monolithically integrated onto a single silicon substrate with signal conditioning circuitry. However, the resulting GMR signal is magnitudes higher than a Hall signal with much greater signal-to-noise and one-tenth the jitter. These qualities let GMR sensors sense objects at much bigger air gaps.
