Auto Questions: How Anti-Lock Brakes Work

Confused about your anti-lock braking system (ABS)? This article may remove some of the mystery.

The advent of power brakes on automobiles was a huge step forward in vehicle safety. Even the weakest driver was now able to apply maximum braking force and control even the largest vehicle. But as with every advance, power brakes created a new problem. Cars stop fastest when their wheels do not skid. The shortest stopping distance is achieved when the brakes are applied such that the wheels are almost but not quite skidding. When power assist gave everyone the ability to apply maximum force to the brakes, skids became even more likely. During panic stops or on slick roads, drivers were more likely to apply excessive force and go into a dangerous skid. Drivers were taught to pump the brake pedal, applying and releasing the brakes as quickly as possible to maintain control and stop as quickly as possible. But this was a stop-gap solution at best and engineers were actively working on a better fix.

When computers became viable components of automobiles, the idea of automatically controlling the application of the brakes became a real possibility. The concept was to have the computer pump the brake pedal instead of the driver. Obviously the computer could do it faster and more accurately and so minimize skidding. The problem was how to provide the proper information to the computer and how to interpret its output. The first step was to develop sensors that could be mounted on each wheel. The use of computers in cars spurred the development of sensors that could provide relevant information to the computers. In this case, the solution was to adapt magnetic sensing technology that had already been successfully used in others areas of the vehicle. These sensors tell the computer how fast the wheels are spinning and the computer also has information about how fast the vehicle is moving. When the brakes are applied, the computer monitors wheel speed and vehicle speed. If it detects that a wheel has stopped spinning but the vehicle is still moving, this means that the wheel is skidding. The computer then releases the brake on that wheel until the skid stops and then reapplies the brake. Each wheel is monitored and controlled separately. The computer can release and apply the brakes many times in just a few seconds. Even the best drivers can't match this performance. The driver maintains steering control and still achieves the shortest possible stopping distance.

The first systems were installed in the 1970'2 on semi-tractor trailers where skidding can lead to jack-knifing and serious accidents. The most noticeable result of the rapid application and release of the brakes was a significant vibration that was especially pronounced in the trailer. If you've ever applied the brakes hard in a vehicle equipped with anti-lock brakes you probably felt the pulsing in the brake pedal. This vibration is greatly amplified in a large truck. In fact, during one of the early tests of the system, a truck fully loaded with bags of ball bearings went into a panic stop from 60 miles per hour. The system performed perfectly but the vibrations broke open several of the sacks and coated the test track with ball bearings. Early on, truckers complained of the noise and vibration more as an embarrassment when they inadvertently braked too hard and activated the system. But the systems have proved themselves and are a standard feature on trucks and cars today and it would be difficult to find a driver that doesn't have a positive opinion of them.



Although the concept of anti-lock braking systems is simple, implementation is not straightforward at all. We tend to forget how harsh the automotive environment is for electrical systems. Automobiles are expected to perform without problems in temperatures ranging from -40 degrees F to 140 degrees F. Battery voltages can range from a few volts under cold starting or battery failure condition to 50 volts or more when other failures occur. This doesn't count electrical surges and noise cause by other equipment. Sensors mounted on wheels are subjected to mud, snow, ice and water not to mention gas, oil and road debris. Safety systems such as brakes must not fail under any or all of these conditions. If a failure does occur, the brakes must continue to function so that normal stopping can occur. But these problems were overcome and as the power and speed of computers has increased, the capabilities and reliability of the braking systems have also improved. It is a tribute to the design talents of automotive engineers that such systems are commonplace on today's vehicles.

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