Wheel speed sensors

Navigation systems record the position of a vehicle using satellite-based location systems (GPS) and match the determined position with the navigation system s digital street map. Yaw-rate sensors, in combination with wheel-speed sensors, permit interpolation in situations where no satellite reception is possible, such as when driving through tunnels or if the GPS signals cannot be clearly interpreted due to multiple reception as a result of reflections from houses in urban situations, for example. 

Wheel-speed sensors do not monitor speed directly they sense the movement of the circumference of the tire. This is done with 48 or 32 pulses per turn, and some correction factor for tire sizes. This information comes from driven and non-driven wheels, and means four independent sources of information describing road surface, friction coefficient, cornering etc. In all cases the information is passed through the electronic control unit (ECU). ABS and all advanced systems are safety systems that do not allow the electrical signal to be split before it goes to the ECU. This is necessary to make a fail-safe system. 

Each system has its own specific sensor requirements [2], There are two main categories of wheel-speed sensor passive and active. 

The input circuit of an ECU for active wheel-speed sensors has to provide... 

There are two variants of power supply for wheel-speed sensors on the market. 

Fig. 7.9.6 Active wheel-speed sensor input stage...

The specific sensor construction takes into account the space requirements, the possible assembly hole dimension, and the sensor reading types bottom read and side read. Therefore a big selection of sensor versions regarding the housing geometry are available. Fig. 7.9.8 gives an overview of possible versions for passive and active wheel-speed sensors. 

Fig. 7.9.9 a) Cap sensor consists of direct connected wheel-speed sensor, cap, and multipole tone wheel b) clip sensor with depending fixture (without screw) c) cap-ring sensor... 

The cap sensor includes a direct connected sensor and a protection cap mounted to the hub. The clip sensor can be mounted by its special geometry without a flange and screw. Present wheel-speed sensors are manufactured by overmolding the sensor elements (ASICs) directly with polyamide thermoplastic material, which fulfills the current needs. The requirements regarding temperature will increase for future car projects. This means a challenge for the development of materials and joining methods. 

Wheel-Speed Sensor Installation, Cable Layout, and Connector... 

Wheel-speed sensors are available with or without (direct connected sensors) cable. Cable routing in the car is important and many types of sensor configuration (cable layout) are possible. Grommets and clips can fix the cable to the car body. Expansion bellows of plastic protect the cable from damage. Fig. 7.9.12 shows a complete WSS consisting of sensor head, connector, grommets, clips, and expansion bellows. The fixture to the car body with clips and grommets is shown in Fig. 7.9.13. 

Fig. 7.9.10 a) Wheel bearing with integrated multipole tone wheel b) wheel-speed sensor with in-bearing application (tone wheel not visible)... 

Joining the wheel-speed sensor to the cable tree is done with a connector. The demands on the connector are low contact-resistance, safe connection by connector pins under the influence of dynamic impact, and pin material resistant to corrosion. Depending on the installation area, the further requirement of tightness may be necessary. Fig. 7.9.14 shows three examples of connectors. 

Fig. 7.9.12 Wheel-speed sensor with fixings on the cable...

Fig. 7.9.14 Examples of wheel-speed sensor connectors a) and b) molded connector c) direct connection at the sensor head...

Active wheel-speed sensors have been available for years. New features in modem cars to enhance comfort require more information about the behavior of the car. One important piece of information is the rotational direction of the wheels, and hence the direction of the car. The Bosch DFlli was the first sensor to deliver this information, and it was used in the Bosch EHB system in 2001. The sensor transmits this additional information by modulation of the pulse width of the speed signal. The sensor contains three Hall elements two for speed measurement by the difference principle, and a third arranged in such a way that an addi-... 

Fig. 7.9.17 Arrangement of tone wheel and wheel-speed sensor, airgap range, possible deflection D, and deflection direction...

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