Speed sensors
The hydraulic unit is located under the master brake cylinder.
1 - The casing of the electric pump, its relay and the block of electromagnetic valves;
2 - Valve block with solenoid valves and brake lines to the wheels.
The figure shows the speed sensor (2) rear wheel positioned horizontally at a precisely calculated distance from the impulse wheel (1), which rotates behind the reflector (3) disc brake.
Each of the four speed sensors in the front and rear wheels reads the speed of the corresponding wheel and sends this information to the electronic control unit. Based on the signals processed in it, the hydraulic unit is controlled.
Speed sensors are mounted on wheels at a short distance from a special gear - impulse wheel. Rigidly connected to the hub, the impulse wheel rotates in front of the speed sensor, depending on the wheel speed. The sensor consists of a magnetic core and a coil. Each tooth of the impulse wheel that passes in front of the sensor causes a momentary increase in voltage in the speed sensor. Thus, a voltage change occurs in the sensor, which, depending on the wheel speed, changes its frequency.
ABS control unit
The ABS control unit is located in the hydraulic unit and processes the speed data from the speed sensors. This information - about 8000 per second at a speed of about 130 km / h - is compared with the programmed values. If the control unit, on the basis of information about various speeds, recognizes the risk of wheel lock, then it «orders» hydraulic unit to reduce the brake pressure to the point that prevents the wheels from blocking.
Reducing the brake pressure eliminates the risk of wheel lockup, the control unit registers changes in the wheel speed information and thus continuously adjusts the brake pressure via a hydraulic unit. Such a change of control signals entering the hydraulic unit occurs depending on the state of the roadway during the entire braking process with a period of one millisecond. If the speed is less than 7 km/h, the ABS does not work.
Hydraulic unit
The main element of the ABS is located in the engine compartment, on the left in front of the main brake cylinder, a hydraulic unit. It includes:
- Electric pump;
- Valve block with solenoid valves for adjusting the pressure in the brake actuator in the event of a risk of wheel blockage.
- When you press the brake pedal, pressure through the valve block, through its own pipeline for each wheel, is transmitted to one front wheel and the rear wheel lying diagonally from it.
The brake fluid, which is supplied by the master brake cylinder through the valve block to the wheels, enters when the braking pressure is reduced, directly from the valve block immediately back to the reservoir. If the pressure in the brake actuator rises again, then the required amount of brake fluid again flows from the reservoir through the hydraulic pump directly into the corresponding brake circuit. You notice this due to the pulsation of the pedal, which begins when the pump and, accordingly, the ABS itself.
Relay
To ensure the operation of the solenoid valves and the electric pump, one relay is installed under the cover of the hydraulic assembly, respectively, each of which is protected by a 15 A fuse in a box under the instrument panel.