Vehicle speed sensor

Monday, 22 January 2007

A vehicle speed sensor is a device, which is generally utilized in an automobile to sense vehicle speed and send this information to the vehicle's onboard computer. Many modern automotive vehicles are equipped with electronic control systems that regulate various components of the vehicle. These electronic control systems are utilized to control the components based on information represented by output signals from various sensors for detecting operating conditions. There are a variety of electronic control systems, for example, an electronically-controlled suspension system, an electronically-controlled steering system, an ABS (anti-lock brake system), a TCS (traction control system), an ESP (electronic stability program), etc. In order to control engine performance electronically, it is often necessary to provide a variety of signals to the engine control module. In an electronic control system for a vehicle receives information from a variety of sensors, processes the received information, and drives a corresponding circuit and/or an actuator in such a way that appropriate control operations can be performed. These signals indicate the status of the parameter being detected and to which the control must respond. Among status signals necessary is a signal that is indicative of the vehicle speed. Wheel speed sensors are used for detecting rotation of wheels on a vehicle. With the increase in electronic controls for vehicle systems, a need has been found for numerous speed sensing devices in vehicles with increasingly stringent angular motion discrimination. Contributing to this growing interest in speed sensing devices in motor vehicles is the popularity of anti-lock braking systems (ABS), the advancement of controls for automatic transmissions, and traction control. Various types of wheel speed sensors are presently known for motor vehicle applications. Transmission-mounted vehicle speed sensors can be utilized to sense the speed of a vehicle. Typical transmission speed sensors have a one-piece external housing that is molded around an internal sensing structure. Vehicle speed sensors which use a Hall-Effect device as the signal generating element also have been used. These have the advantage of generating a signal of uniform amplitude over the entire vehicle speed range. Wheel speed sensors attached to wheels of a vehicle, respectively, to detect wheel speeds of a running vehicle are designed to detect the rotation of rotors which rotate together with axles coupled to the respective wheels.

Vehicle anti-lock brake systems are designed to maximize the ability of a vehicle operator to bring a vehicle to a controlled stop on any type of road surface. The system accomplishes this goal by preventing the vehicle brakes from prematurely halting vehicle wheel rotation, regardless of the road surface and the pressure applied to the brake pedal by the vehicle operator. Speed sensing assemblies, which measure the rate of rotation of a vehicle wheel, are critical components of vehicle anti-lock braking systems. In order to control slippage, antilock braking systems establish servo-feedback control of the brake system to minimize slippage during braking. A typical system uses an antilock braking controller to determine incipient wheel slip and modulates brake pressure based on wheel speed information determined from wheel speed sensors. An anti-lock brake system for an automotive vehicle includes a wheel speed detecting device for detecting rotational speeds of a road wheel. The wheel speed detecting device includes a ring-shaped rotor which is press-fitted around a hub at the side of an axle shaft on which the road wheel is mounted. Antilock braking systems require accurate wheel speed signals which vary in accordance with vehicle wheel speed. The wheel speed sensors may use various sensing principles such as magnetic reluctance, Hall effect or optics. In operation, one or more of the vehicle wheel speed sensors provides input to the controller for determining the vehicle speed. The controller monitors the vehicle and vehicle wheel speeds for an indication of an anti-lock braking event. Electrical signals from wheel speed sensors are used by the controller to modulate pressure applied to the vehicle wheels to improve braking performance. In addition, braking application is used to enhance traction through the use of differential braking. Based upon the vehicle speed, the control unit typically determines a slip threshold. Using the vehicle velocity as a reference, slip threshold may be expressed as the difference between a selected velocity and the vehicle velocity. The ABS wheel speed sensor has a target ring attached to the rotatable section of the vehicle wheel bearing and has a sensor mechanism assembly. The sensor assembly includes a sensor which senses the rotation of the target ring, a sensor housing which supports the sensor mechanism, an electrical plug, and a pigtail wire having one end attached and electrically connected to the sensor mechanism and having another end attached and electrically connected to the electrical plug. The electrical plug is adapted for electrical connection to a vehicle ABS computer cable. When a vehicle brakes, the wheel speed sensor detects revolutions of the wheel and outputs a signal indicating the detected revolution of the wheel. This signal is applied to a controller and then the controller provides uniform braking power on the basis of the signal. Wheel speed sensors preferably make a non-contact observation of the rotation of the wheel, both to avoid interference with the wheel's movement and to avoid the difficulty of transmitting a signal from the rotating wheel to the vehicle.

The speed sensor reacts to a stimulus or a signal with a corresponding electrical signal. A typical wheel speed sensor unit includes an encoder mounted on a rotary member of the vehicle, and a sensor mounted on a stationary member so as to oppose the encoder. In a speed sensor device, the component which produces a stimulus corresponding to the rotating shaft speed is called a target wheel or target and the other component which reacts to this stimulus with an electrical signal is called a sensor. The target may be either active or passive. Active targets, also known as encoders, are those that produce alternating magnetic fields which are sensed and related to shaft speed, and incorporate permanent magnetization. Passive targets, known as tone wheels, are not magnetized components, but instead are usually metal rings with teeth or notches, and may be integrated within seals as well. Active encoders normally require the magnetization of an elastomeric ring. This requires that the elastomer be vulcanized with ferrite powder/filaments, thus making it unsuitable for sealing. Active encoders have numerous advantages, including reduction in the size of the target wheel and sensor, weight reduction, reduction in the number of parts, and integration of the components into a small seal package. The active property of the encoder allows use of Hall-effect or magneto-resistive sensors, which are smaller than other sensors used with a passive tone wheel. Wheel speed sensors can also be classified as active or passive, depending on whether they produce a field internally or are supplied with external power to do so. Active sensors include electronic components that are typically powered by a power source associated with the vehicle. A Hall-effect sensor represents one type of active sensor. Passive sensors, on the other hand, need no outside power and usually consist of a coil surrounding a magnetic material. A variable reluctance sensor represents one type of passive sensor. An active sensor requires power from the control system and will operate effectively with larger air gaps and at lower speeds. Active sensors are better suited for traction control systems and in assemblies where the air gap is difficult to accurately control. Both types of sensors are positioned proximate to a circular shaped element having a plurality of teeth, which rotates with the wheel hub. Either type of encoder can be integrated with either type of sensor. However, the combination of an active encoder with an active sensor provides advantages in size and performance. The performance of speed measurement systems is constantly increasing. The use of active encoders with active sensors allows wider band widths of the sensing systems, measuring speeds from zero rpm.

Related Information about Vehicle speed sensor