The Physics of ABS Brakes
Anti-lock braking systems were originally developed for use on aircraft braking systems. Early aircraft systems were mechanically operated. In the 1960s, automotive engineers began development of mechanical ABS for racing applications and experimental vehicles. The first operational electronic system was offered by Mercedes-Benz in 1978. ABS has also been developed for motorcycles.
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Brake Basics
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When the brake pedal is depressed, a series of hydraulic and mechanical actions are initiated. Brake fluid in the master cylinder is forced through brake lines to all four wheels. A proportioning valve determines how much fluid pressure should be sent to the front and rear brakes. On ABS-equipped vehicles, the proportioning valve may be overruled by braking needs determined by brake sensors at each wheel. Fluid pressure at each brake forces brake shoes against a brake drum, or brake pads against brake rotors to slow and stop the vehicle.
ABS Components
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In addition to the standard brake components, ABS components include an electronic controller, sensors at the master cylinder, wheel speed sensors at each wheel, sensors at the brake pedal, hydraulic control unit and an interface to the vehicle's electronic control module. Traction control systems share all the brake system components and add a few more such as steering angle sensor, yaw rate sensor, lateral acceleration sensor and pressure generator. Traction control, or a vehicle stability system, monitors pressure to individual brakes to aid in vehicle control when drivers respond to road emergencies.
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Sensing Wheel Motion
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The electronic controller uses inputs from the wheel sensors to measure wheel speed. When the controller receives a signal that at least one wheel is turning slower or has stopped during braking, system pressure will be reduced to that wheel, reducing wheel skid or sliding on the road surface. With all four wheels rolling equally, the driver should be able to steer away from or around obstacles in the road. Drivers may feel slight brake pedal pulsation during hard braking as pressure is released and reapplied by the system.
Road Surfaces
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On dry, hard road surfaces, ABS provides the quickest means of stopping a vehicle. Wheel sensors are able to quickly sense when the wheels have locked up and are skidding. On gravel or snow-covered roads, ABS-equipped vehicles may have a greater stopping distance because the system will not allow wheels to dig into the road surface and stop the vehicle. Advanced systems sense slippery road conditions and reduce brake cycling speeds so the wheels can gain traction.
Applied Physics
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Hydraulic pressure is transformed into mechanical action as brake shoes and brake pads are moved to stop the vehicle. Brake pads and brake shoes create friction that slows and stops the vehicle. ABS wheel sensors measure wheel speed and create an electric signal informing the electronic controller of the need to reduce brake pressure. Traction control sensors measure lateral movement, vehicle side-to-side motion and driver steering actions to apply or release brake pressure to individual wheels.
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References
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