Wheel Sensors

Courtesy of VOLVO CARS CORPORATION

The wheel sensors are installed on the spindle joints of the front and rear wheels. The wheel sensors are active sensors. The task of the wheel sensors is to supply the control module information on the rotation speed of each wheel. The control module uses the signals from the wheel sensors to calculate wheel speed and the acceleration and retardation of the car.

The wheel sensors are supplied with 12 V. When the magnetic pulse wheel rotates, the sensor generates a pulsed current (quadratic wave). The strength of the current depends on the position of the pulse wheel. The signals from the coil in the sensor are then affected by the resistor, which is sensitive to magnetic fields. This means that the output signal is a current which oscillates between 7 mA and 14 mA. The frequency increases with speed.

The magnetic wheels are integrated in the inner wheel bearing seal and the front wheel sensor is radially located in the spindle housing in relation to the magnetic wheel. The rear wheel sensors are axially located in relation to the magnetic wheels.

Both the front and rear magnetic wheels have 88 poles, 44 north poles and 44 south poles. In the event of a fault with any of the wheel sensors, the following functions will be cancelled or will not engage:

• Anti-lock brake system (ABS)
• Stability control (SC)
• Traction control (TC)
• Active yaw control (AYC)
• Adaptive cruise control (ACC)
• Collision warning and collision mitigation by braking (CMbB)
• City Safety  (only applies to XC60)

Electronic brake force distribution (EBD) is available if one wheel sensor is faulty. Electronic brake force distribution (EBD) is disengaged if there is a fault in more than one wheel sensor simultaneously.

The control module calculates the vehicle speed using the signals from the wheel sensors.

There are diagnostics for the wheel sensors.

Direction-sensitive wheel sensor (structure week 201046-) (does not apply to S80L) 

With the introduction of City Safety  a direction-sensitive wheel sensor is needed so that Brake Control Module (BCM) knows in which direction the vehicle is moving. This is so that the brake mitigation will not engage, e.g., during reversing, if a vehicle approaches from the front and this enters the active area for the lidar for Closing Velocity Module (CVM).

The direction-sensitive wheel sensor is located on the right front wheel spindle and, using the wheel sensor's signal, the Brake control module (BCM) can calculate the wheel speed as well as the vehicle's direction.

The sensor is supplied with 12V. When the magnetic rotor rotates, the sensor generates a pulsed current (square wave), where the current amplitude depends on the position of the magnetic rotor. Compared to an ordinary wheel sensor, each square pulse is replaced by a series of pulses, pulse trains, that is included within the ordinary wheel sensor's longer pulse length.

The length of the pulse train is max. 10 pulses. The beginning and end of each pulse train are indicated by a high pulse of approx. 28mA. Between these start and end pulses there are 8 lower pulses (7-14 mA). This included pulse train is really a digital word with 8 bits. The direction is given in one of these bits.

This detection is possible since the sensor has 2 adjacent elements that register pulse change when north and south poles pass. Then the direction can be determined by knowing which element reacts first to a pulse change. Other bits inform about status and other control information. Using start and end pulse it is also possible to know what the wheel speed is.