To achieve this goal, ABS solves the problem of limiting the pressure in the brake actuator (therefore, the braking torque in the wheel brakes) the value at which, based on the conditions of interaction between the wheel and the road surface, it is possible to create maximum braking forces on the wheels of the car.
The principle of ABS control of the braking torque in wheel brake mechanisms is based on the fact that during the blocking of the wheel, its angular velocity decreases sharply.
The wheel braking control process is cyclic. This is due to the inertia of both the wheel itself and the brake drive, as well as the ABS elements. The quality of regulation is assessed by the range in which the ABS maintains the slip of the braking wheel (this range depends on the frequency with which the system can cycle through). A large amplitude of slip coefficient values impairs braking comfort (appears «jerking» car), and the structural elements of the car in this case experience additional loads.
ABS must:
- provide a minimum braking distance in accordance with the regulated standards;
- do not interfere with smooth (without jerks) braking;
- have adaptability, i.e. adapt to changing external conditions (for example, changing the state of the road surface);
- not interfere with braking in case of failure;
- signal its failure and carry out diagnostics of the malfunctions that have arisen;
- meet the general requirements for vehicle structural elements (reliability, low cost, etc.).
Regardless of the design, ABS includes the following elements:
- sensors that provide primary information about the angular velocity of the braking wheel (or about the deceleration of the braking wheel, or about the pressure of the working fluid in the brake actuator - depending on the functioning algorithm);
- Control block (usually electronic), which processes the information coming from the sensors and gives a command to the actuators;
- actuators, which, depending on the command coming from the control unit, reduce or hold the pressure in the wheel brake drive at a constant level.
Angular velocity sensors are usually inductive-frequency in all ABS (are, in fact, pulse generators), consisting of a rotor in the form of a toothed disk or toothed (or perforated) a ring made of a magnetic alloy rotating together with the wheel or elements of its drive, and an inductor mounted motionless at some distance from the teeth of the rotor (this distance is often referred to as the air gap). When the teeth of the rotor pass by the coil, as a result of changes in its electromagnetic field, an alternating current voltage is created with a frequency and amplitude directly proportional to the angular velocity of the rotor (and hence the wheels).
car ABS «built-in»: ABS elements are an addition to the service brake system (the service brake system described above includes wheel brakes, a master cylinder, a vacuum booster, connecting hoses and tubes, an emergency brake fluid level sensor, a warning lamp) in the form of special devices - wheel speed sensors, hydraulic unit, electronic brake control unit, ABS fuse relay, connecting wiring (with connectors), as well as the ABS malfunction indicator lamp with the module for controlling this lamp.