During braking, as the slip ratio rises, the ABS system maintains an ideal slip ratio of 0.10 to 0.30 based on the road-tire friction characteristics. For example, when the vehicle is running normally on an ideal road surface, the slip ratio is 0 when the wheels are locked, the slip ratio is 1. It is the ratio of the difference between the vehicle speed and the wheel speed to the vehicle speed. Slip ratio is a means of calculating and expressing the locking status of a wheel. This type of ABS is commonly seen in pickup trucks. One Channel, One Sensor ABS - One valve and speed sensor located on the rear axle monitor both the rear wheels. There is one valve and one sensor for both the rear wheels. Three Channel, Three Sensor ABS - The front wheels each have a sensor and a valve. Maximum braking force is achieved with this type. After the wheel slip returns to normal, the valve solenoids are de-energized and the hydraulic modulator resumes the regular braking process.Īntilock braking systems can be classified based on the number of channels and number of sensors employed.įour Channel, Four Sensor ABS - This type of ABS uses a speed sensor and separate valves for each of the four wheels. The excess brake fluid is returned to the master cylinder through the return pump (E). Once the control unit detects any excessive wheel slip, the corresponding outlet valve is opened to release the pressure to the accumulator (D) and prevent possible wheel lockup. Then both the inlet and outlet valves remain closed to hold this pressure and provide sufficient brake torque for wheel brake cylinders. When operating in normal conditions, the outlet valve (C) of the hydraulic modulator is closed and the inlet valve (A) stays open until the pressure reaches the desired value. The architecture of the ABS system (including the hydraulic modulator) is illustrated in the figure below. The main components of these systems include regular brake parts (such as the brake pedal, hydraulic cylinders and lines), wheel speed sensors, and a hydraulic modulator operated by an electronic control unit. They are now often paired with other systems such as Electronic Brake-force Distribution (EBD), Electronic Stability Control (ESC) and Traction Control to further increase vehicle control and driver safety. ABS systems monitor the wheel speed in real time and regulate the brake pressure automatically in order to prevent wheel lockup and improve the driver's control of the vehicle. This can result in longer stopping distances or even a loss of steerability. Braking hard, particularly on ice or wet roads, can cause one or more wheels to lock and skid over the surface. These systems play a significant role in improving the safety of modern vehicles. Antilock Braking Systems Basic DescriptionĪntilock Braking Systems (ABS) prevent wheel lockup by modulating the braking pressure.
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