Speed Sensor Integration in Hub Units Steven F. Brown SKF-USA COPYRlGHT SoaiV OF AUTOMOTNE ENGINEERS, INC. ABSTRACT Advancements in vehicle wheel speed sensing technology present automotive engineers with unique design opportunities. Both the integration of speed sensors into wheel bearing hub units and the introduction of new speed sensor technologies will be discussed. INTRODUCTION Since 1938, when the first wheel bearing hub unit was introduced, hub units have incorporated an increasing number of functions. Today's hub unit is a sealed, pre- greased, preset unit, which may also transmit driveline torque, provide mounting for the brake rotor and wheel, and serve as a suspension structural member. Due to the location and mechanical precision of hub units, they are also a natural location for the monitoring of vehicle wheel speed. Christian Rigaux SKF-France Currently used vehicle wheel speed sensors have several problems, including cost, assembly, and signal quality. By integrating the wheel speed sensors with hub bearing units (HBU), these problems can be solved. In addition, alternative sensor technologies which expand the capabilities of wheel speed sensors will be presented. CURRENT CONFIGURATION OF WHEEL SPEED SENSORS Most wheel speed sensors today are used solely to provide wheel lock-up information to the anti-lock braking (ABS) system. The ABS system computer processes this information and determines if the brake fluid pressure to any one wheel should be modulated to prevent wheel lock-up. Downloaded from SAE International by University Of Newcastle, Wednesday, August 08, 2018In a conventional arrangement, an inductive sensor is mounted on the knuckle or the axle housing. The sensor uses a target wheel, or impulse ring, which is mounted on a rotating member. For driven wheels, the rotating member is usually the outboard constant velocity joint or the axle shaft. For non- driven wheels, the impulse ring is usually mounted on the inboard side of the hub. These types of arrangements have several inherent problems: Tolerances on sensor location and impulse ring diameter must be closely held to maintain the required air gap. Assembly and signal verification testing required at the vehicle assembly plant. Sensor and impulse ring are exposed to possible damage from the outside environment. High system cost and relatively low reliability. The impulse ring has traditionally been a machined or sintered metal ring of either magnetic stainless or carbon steel. Some pressed sheet metal rings, which offer cost and weight advantages, are being introduced now. The sensor normally employed is an passive, or variable reluctance, type. It is composed of a magnet, pole piece, and a coil of wire which are contained in a housing. This passive sensor produces a quasi-sinusoidal signal of amplitude that varies with impulse ring rotational speed. While this sensor has been used by many automobile manufacturers, it has the following limitations: Relatively large size (i.e. 18 mm diameter). Quite sensitive to air gap variation. Signal amplitude varies directly with impulse ring rotational speed . Very low signal amplitude at low vehicle speeds (i.e. 5 krnthr). Relatively large size (i.e. 18 mm diameter). Quite sensitive to air gap variation. - Signal amplitude varies directly with impulse ring rotational speed . Very low signal amplitude at low vehicle speeds (i.e. 5 kmthr). At these low speeds, the signal becomes virtually unreadable and is therefore not useable at wheel speeds approaching lock-up. This trait also makes the transmission of the signal more susceptible to connector terminal corrosion. However, alternative technologies which can enhance and expand the performance of wheel speed sensors are available. ALTERNATIVE WHEEL SPEED SENSOR TECHNOLOGIES The development of electronic components has expanded the options available for sensor