Signal 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. 

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-... 

The wheel speed is calculated based on the input from a Hall Effect sensor. Such a sensor may produce a sine wave signal with a frequency that is proportional to the rotational speed of the wheel, as depicted in Fig. 8. 

The result of applying the oscillation fault model may vary depending on the implementation of the Hall Effect sensor signal conditioner. In our implementation, we assume that the number of zero crossings will increase as an effect of oscillations superimposed on the Hall Effect sensor signal, as shown in Fig. 9. This will cause the estimated wheel speed to be higher than the actual wheel speed (154 RPM in Fig. 9 instead of 98 RPM in Fig. 8). Since we inject faults directly into the behavior model, which takes the estimated wheel speed as input, we use the offset fault model to represent an oscillation superimposed on the Hall Effect sensor signal. 

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