Active wheel speed sensors

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

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. 

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 difference between the systems is lower than the one ofthe system reliability shown inFigure 4. Att=15 years the failure probability only differs about 31.5% (ABS to ESP 1) respectively 1.8% (ABS to ESP 2). ESP 1 is a lot worse because of the hydraulic unit with ECU and the active wheel speed sensor, which were used for the first time. 

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

The ETCS is composed by several subsystems cormected to the central safety processing unit called European Vital Computer (EVC). The EVC contains an on-board odometry system that performs an estimation of the speed and traveled distance based on the measurements provided by a set of sensors, such as wheel speed encoders, accelerometers or Doppler radars, and the Balise Transmission Module (BTM) that gives the exact position as the train passes the eurobalises placed in the railway track. If the estimations exceed the authorized values, the EVC automatically activates an emergency brake. 

Since the system activates an emergency brake when the values estimated by the odometry system exceed the authorized limits, the odometry algorithm has to provide accurate and reliable measurements. Thus, the algorithm is usually based on a fault-tolerant sensor-fusion approach. In this case, we design the algorithm following one of the approaches described by Malvezzi et al. in [16]. The algorithm estimates the speed of the train and the traveled distance with the information provided by an accelerometer that measures the acceleration of the train and two encoders that measure the speed of a different wheel each. 

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