How ABS Works?

[McPatrick]

The Anti-lock Braking System (ABS) operates as follows: During braking, ABS rapidly determines the wheel lock-up state based on speed signals from each wheel speed sensor. It closes the normally open input solenoid valve on the wheel that begins to lock, maintaining constant braking force. If the wheel continues to lock, it opens the normally closed output solenoid valve, causing the braking pressure on that wheel to rapidly decrease as it connects directly to the brake fluid reservoir, preventing complete wheel lock-up due to excessive braking force. This keeps the braking state at the optimal point (with a slip ratio S of 20%), achieving the best braking effect and maximum driving safety. The brake fluid in the front chamber of the master cylinder is under dynamic pressure, pushing the reaction sleeve to the right, which in turn pushes the booster piston, moving the brake pedal push rod to the right. Therefore, when ABS is active, the driver can feel vibrations in the pedal and hear some noise. After the vehicle decelerates, once the ABS computer detects that the wheel lock-up state has disappeared, it closes the main control valve, returning the system to normal braking operation. If the accumulator pressure drops below the safety limit, the red brake fault indicator and amber ABS fault indicator light up. In this case, the driver must apply greater force with a deep pedal press to achieve effective braking on both front and rear wheels. ABS Classification: ABS can be categorized by manufacturer or by control channel. The following mainly introduces the channel classification method. In ABS, a brake pipeline capable of independent brake pressure regulation is called a control channel. ABS devices are classified into four-channel, three-channel, two-channel, and one-channel systems. Performance Characteristics: The anti-lock braking system enhances safety during emergency braking. In other words, vehicles without ABS may experience wheel lock-up during emergency braking, rendering the steering wheel immobile, thereby increasing the risk of severe consequences. Advantages: When a wheel is about to reach the next lock-up point, the brake fluid pressure causes the system to repeat its action, operating 60–120 times per second, akin to continuous braking and releasing, similar to mechanical "cadence braking." Thus, ABS prevents loss of steering control and wheel skidding during emergency braking, ensuring wheels do not lock up and preventing tires from rubbing against the ground at a single point, thereby increasing friction and achieving over 90% braking efficiency. It also reduces brake wear, doubling the lifespan of brake drums, discs, and tires. Vehicles equipped with ABS achieve slip resistance performance of 80%–90% on dry asphalt, 10%–30% in rain, and 15%–20% in snow. Limitations: ABS itself has limitations and cannot defy certain physical laws. In two scenarios, ABS does not provide the shortest braking distance: first, on smooth dry roads when braking is performed by an experienced driver; second, on loose gravel, soil, or deep snow surfaces. Additionally, on dry surfaces, the latest ABS systems can control the slip ratio within 5%–20%, but not all ABS systems operate at the same rate or degree. Although four-wheel ABS can brake the vehicle in the shortest possible distance, if braking is initiated too late to stop completely before colliding with an obstacle, it cannot prevent the accident.