Each wheel has a wheel speed sensor that measures wheel speed. The sensor consists of a magnetic core and a winding and is installed at a certain distance from the toothed rotor. The rotors for the front wheels are mounted on the outer CV joints of the drive shafts, the rotors for the rear wheels are mounted on the hubs. As the hub rotates, the sensor's magnetic field changes, causing an alternating voltage whose frequency changes with wheel speed.
The signals from the sensors are sent to an electronic control device that can accurately determine whether the wheel is accelerating or decelerating relative to the recommended speed. The information from the electronic control device is sent to a hydraulic modulator which contains four traction relays, each acting on one valve for one brake, and they all operate independently of each other in three different modes:
Pressure increase stage:The solenoidal intake valves are open, brake pressure from the master cylinder is applied directly to the brake calipers.
Constant pressure stage: The solenoidal inlet and outlet valves are closed, the pressure in the brake system in the calipers is maintained at a constant level, even if the pressure of the master cylinder increases.
Pressure reduction stage:The solenoid inlet valve is closed to prevent further brake pressure going to the caliper and in addition the outlet valve is open to relieve existing pressure and release the brake. Fluid returns to the master cylinder through the return, pumping up the hydraulic modulator.
The braking cycle is described for one wheel, all four wheel systems work in a similar way, although independently.
Wheel speed is measured by wheel speed sensors and processed by an electronic control device. By comparing the signals received from each wheel, the control unit can determine the recommended speed, and detect any deviation from this speed that could lock the brake. If a blockage is detected, the control unit enters the constant pressure stage, and no further increase in pressure in the brake system affects this brake. If the blockage is still there, the pressure reduction stage is activated to unlock the wheel. The control unit again switches to the constant pressure stage until the wheel speed increases to a certain value, then the cycle repeats, the control unit again switches on the pressure increase stage. This control cycle repeats continuously and rapidly until the brake pedal is released or the vehicle comes to a halt.
An additional circuit in the electronic control device monitors the functioning of the system and informs the driver of damage by means of an alarm lamp. If damage occurs, the system must be switched off, allowing normal braking without ABS.
Pic. 9.38 Schematic arrangement of the anti-lock braking system (Chapter 27)
1 - Wheel speed sensors
2 - Brake calipers
3 - Hydraulic modulator
Pros - Solenoid valves
Зb - Return pump
4 — the Main brake cylinder
5 - Electron. control device
6 - Control lamp
7 - ABS switch
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