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Automotive safety technology: Anti-lock brakes

hydraulic schematic
hydraulic schematic

Perhaps the oldest and most important bit of automotive safety technology is the brake system. After all everything that goes must also eventually stop. Conventional brake systems work by applying a friction force to a rotating part of the drive system to slow that rotation and bring the vehicle to a stop. In the earliest days, vehicles used cable operated brakes like you find on a bicycle. Since the early decades of the twentieth century, pretty much all vehicles have migrated to hydraulic brake systems.

Hydraulic fluids have the wonderful property of being incompressible. If you fill a tube with hydraulic fluid and push with a force on one end, you get the same force out the other end. This works well in a car where you can send the brake fluid through tubes to all four corners from a single source. When the driver presses the brake pedal, it pushes a piston in the master cylinder which pushes brake fluid through the tubes to the four corners. There the fluid is used to squeeze brake pads against the rotors or shoes against the inside of a drum. In either case friction is generated which slows the car.

The only problem with this picture is that the same pressure is applied to all four corners which produces more or less the same braking force. This works great if the car is going in a straight line on a uniform surface at a constant speed. Ultimately however, the amount of deceleration a vehicle can produce is limited by the friction force between the tire and the road. That force is determined by multiplying the friction coefficient by the vertical force on the tire both of which are variable. The friction coefficient depends on the tire and road surface. A summer tire on asphalt might have a friction coefficient of 1.0 or more while the same tire might have a coefficient of 0.3 on gravel or 0.1 on ice. Road surfaces can also be inconsistent such as patchy ice, slush or puddles in the rain.

The vertical load also changes. When the brakes are applied the weight transfers to the front of the car putting more load on the front tires and reducing the load on the rear. Similarly as the body rolls when the car is going around a corner, weight shifts from the inside to the outside.

If the braking force exceeds the friction force that can be produced between the tire and road, the tire will lock up and stop rotating, instead just sliding along the road. When this happens, the tire can no longer provide directional control for steering. Up until the late 1970s, drivers would have to modulate or pump the brake pedal to reduce the braking force so that steering control could be maintained. Unfortunately a driver only has one control, the brake pedal that reduces all of the wheel's braking force and it's difficult for a driver to judge where the limit of adhesion is to maintain optimum braking.

Enter the microprocessor and solenoid valves. Beginning with the 1978 Mercedes-Benz S-Class, Bosch introduced the modern anti-lock brake system (ABS). These systems have evolved over the latest three decades as they have become almost universally applied, but the basic principle remains the same. ABS consists of three main components, the hydraulic actuator, the electronic control unit (ECU) and the wheel speed sensors.

Sensors measure the rotational speed of each of the four wheels and send the signals to the ECU. The software in the ECU compares the signals and calculates an estimated vehicle speed. If any of the four corners are slowing down significantly faster than the vehicle and the driver is applying the brakes, ABS can start to intervene.

The hydraulic actuator contains pairs of solenoid valves in the hydraulic fluid flow path between the master cylinder where the pressure is generated when the driver applies the brake pedal and the brakes at the wheels. The so-called apply or isolation valve is normally open and as the driver presses the pedal, the fluid flows through to the wheels and back to the master cylinder when the pedal is released.

When excessive wheel slip is detected the isolation valve is closed on the wheel or wheels that are showing signs of impending lock up. This prevents any more brake force from being generated on that wheel even if the driver continues to press the pedal. If the wheel speed does not recover the second solenoid valve in the circuit, the dump valve opens up to reduce the pressure in that individual wheel circuit. Brake fluid flows through the dump valve into a low pressure accumulator where a pump picks it up and returns it upstream of the isolation valve. Both the isolation and dump valves typically only open for about 3-30 milliseconds depending on the conditions.

Once the wheel speed recovers, the isolation valve will open again to start applying pressure back to the wheel. The goal is to maintain just the right amount of pressure at each wheel to maintain it at its limit of adhesion for maximum braking force and the shortest stopping distance while preserving the driver's ability to steer. The isolation and dump valves will cycle the brake pressure up and down on each wheel from 2-4 times per second. While a driver may be able to replicate that cycle rate, only ABS can control the individual wheel brakes independently.

Thanks to the wheel speed sensors the ABS can also detect sudden changes in surface condition such as patchy surfaces, or transitions from low to high or high to low traction. ABS can also handle what is known as a split-coefficient stop and keep the vehicle stable so that the driver doesn't lose control. A split-coefficient is a condition where one side of the car is on a lower traction surface than the other such as if two wheels drop off onto a gravel shoulder or get into slush/ice on the edge of the road.

In this condition, if the brakes are applied for maximum braking on the high traction surface, it will put a force on one side of the vehicle that will tend to make it spin which can be difficult for the driver to control. ABS will generally, limit the initial braking force on the high traction side and then gradually build it up in a way that allows the driver time to react.

In the next part of this series we'll look at the next stage in the evolution of modern brake systems, traction control.


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