To achieve this goal, ABS solves the problem of limiting the pressure in the brake drive (therefore, the braking torque in the wheel brake mechanisms) 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 vehicle wheels.
The principle of regulating the braking torque in wheel brake mechanisms using ABS is based on the fact that when a wheel locks, its angular velocity decreases sharply.
The process of regulating wheel braking is cyclical. 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 ABS maintains the slip of the braking wheel (this range depends on the frequency at which the system can perform a duty cycle). A large amplitude of the slip coefficient values worsens comfort during braking (the car starts to "jerk"), and the vehicle's structural elements in this case experience additional loads.
ABS must:
- ensure a minimum braking distance in accordance with the regulated standards;
- do not interfere with smooth (without jerks) braking;
- be adaptable, i.e. adapt to changing external conditions (for example, changes in the condition of the road surface);
- do not interfere with braking in the event of failure;
- signal its failure and diagnose any malfunctions that arise;
- meet the general requirements for vehicle design 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 drive - depending on the operating algorithm);
- control unit (usually electronic), which processes information coming from sensors and gives commands to actuators;
- actuators that, depending on the command received from the control unit, reduce or maintain the pressure in the wheel brake drive at a constant level.
The angular velocity sensors in all ABS systems are usually inductive frequency sensors (are, in essence, pulse generators), consisting of a rotor in the form of a toothed disk or toothed (or perforated) rings made of magnetic alloy rotating together with the wheel or its drive elements, and an induction coil mounted stationary at some distance from the rotor teeth (this distance is often referred to as the air gap). When the rotor teeth pass 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 therefore the wheels too).
ABS of the car is "built-in": ABS elements are an addition to the working brake system (the above described service brake system includes wheel brake mechanisms, a master brake cylinder, a vacuum booster, connecting hoses and tubes, a brake fluid emergency level sensor, and a control lamp) in the form of special devices - wheel speed sensors, hydraulic unit, electronic brake control unit, ABS fuse relay, connecting electrical wiring (with connectors), as well as the ABS malfunction indicator lamp with the control module for this lamp.