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A metal or magnetic post on each wheel passes by a Hall effect sensor, triggering an electronic signal at each rotation. The sensor is mounted on a stationary part of the suspension near the wheel. A Hall effect sensor is a reliable solid-state device that sends an electrical impulse when a magnet comes near. Since it doesn't require physical contact, it works at high speeds and doesn't wear out. The faster the wheel spins, the faster electronic pulses go to a computer for processing.
A specialised computer gets the speed data from all four wheels and other sources, determining the driving conditions. Normally, they will all be spinning at the same speed. During a turn, the outer wheels will spin slightly faster. If some wheels are spinning and others aren't, that spells trouble. The computer will perform a variety of actions to try to remedy the problem. It works fast, monitoring the car and reacting in thousandths of a second.
Anti-lock braking (ABS) was one of the first uses of wheel speed sensors. If you step hard on the brakes and one wheel locks up, the car can skid out of control. The computer senses that the brakes are being applied forcefully, some wheels are spinning and one is not. It intervenes by releasing and reapplying the brake pressure on the locked-up wheel, allowing it to spin again and preventing or reducing the skid. Again, this is a fast process, occurring dozens of times a second.
In 2009, more automobiles and trucks are benefiting from automatic stability control systems. As with ABS, a computer monitors the wheels, engine and brakes to maintain vehicle control. On snowy ground, for example, some wheels may lose traction, leading to a skid or "fishtail." Here, also, the computer sees that some wheels are spinning normally, but one is spinning too fast. It may apply the brake on the wheel, slowing it down, or reduce the engine throttle. The computer keeps it up until wheelspin is brought under control.
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