制动防抱死系统背后的理论是简单。一个skidding wheel(where the tire contact patch is sliding relative to the road) has lesstractionthan a non-skidding wheel. If you have been stuck on ice, you know that if your wheels are spinning you have no traction. This is because the contact patch is sliding relative to the ice (seeBrakes: How Friction Worksfor more). By keeping the wheels from skidding while you slow down, anti-lock brakes benefit you in two ways: You'll stop faster, and you'll be able tosteerwhile you stop.
There are four main components to an ABS system:
- Speed sensors
- Pump
- Valves
- 控制器
速度传感器
The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The speed sensors, which are located at each wheel, or in some cases in thedifferential, provide this information.
Valves
There is a valve in the brake line of eachbrakecontrolled by the ABS. On some systems, the valve has three positions:
- In position one, the valve isopen; pressure from themaster cylinderis passed right through to the brake.
- In position two, the valveblocksthe line, isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal harder.
- In position three, the valvereleasessome of the pressure from the brake.
Pump
Since the valve is able to release pressure from the brakes, there has to be some way to put that pressure back. That is what the pump does; when a valve reduces the pressure in a line, the pump is there to get the pressure back up.
控制器
The controller is a computer in the car. It watches the speed sensors and controls the valves.
一个BS at Work
There are many different variations and control algorithms for ABS systems. We will discuss how one of the simpler systems works.
The controller monitors the speed sensors at all times. It is looking fordecelerationsin the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.
The ABS controller knows that such a rapid deceleration is impossible, so it reduces thepressureto that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before thetirecan actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires very near the point at which they will start to lock up. This gives the system maximum braking power.
When the ABS system is in operation you will feel apulsingin the brake pedal; this comes from the rapid opening and closing of the valves. Some ABS systems can cycle up to 15 times per second.