Multi-Pole Magnetic Encoders for Active Speed-Measurement Systems 1999-01-3402
Automotive wheel speed sensors have generally been based on principles of magnetic field sensing. “Active” sensors, as Hall and Magneto-Resistive, overcome some drawbacks of the previous systems. However, recent conceptual advancements, focusing on the tone-wheel, have proved significant for system performance. The tone-wheel is replaced in fully active systems by a precise ring of steel, bonded to magnetized rubber, where multiple poles are produced in the circumference. The ring serves as an encoder for magnetic sensors, eliminating the need for a large, strong permanent magnet on the sensor - thus significantly reducing size. Active sensor and encoder allow detectable speeds down to zero, improved accuracy, and significantly larger air-gaps and allowable tolerances. These properties, in turn, provide major advantages in manufacture and assembly costs. Usage of elastomeric compounds provides excellent mechanical, dynamic and environmental behavior. Furthermore, many speed-sensing systems are integrated at shaft-ends or in the vicinity of bearings, where seals are required. Elastomeric encoder technology allows combination of multiple functions, as sealing, into a single component - thereby reducing the number of parts.