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          A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Anti-lock braking system (ABS)







A control-lamp which lights up while driving simply indicates that that the ABS- function is inactive. The actual brakes themselves are working as before.

Assignment

The electronic anti-lock braking system should prevent excessive wheelspin and skidding, thus ensuring that the car is manoeuvrable.

Function

The 4-port ABS-system consists of two magnetic valves each (see figure) per wheel brake cylinder and two recirculation pumps with one drive. The two recirculation pumps, both driven by an electric motor, are necessary because the two hydraulic circuits of a dual circuit brake system have to be separately controlled. Although the pumps have a low through-flow, they may temporarily achieve very high pressures of up to 150 bar. Of the 8 magnetic valves 4 are normally closed and 4 are open. The system works the same as a normal two-circuit brake system. The springs of the magnet valves are thus installed, to ensure that the position is maintained, also in the event of a power failure.
If the difference in the rotation speed between the wheels - while braking - is too large, Then the feed to the wheel brake cylinder of the slower or locking wheel is blocked by a (formerly open) magnetic valve. If this is not sufficient to balance the wheel-speeds, then the pressure is reduced by opening the other magnetic valve. Simultaneously, the recirculation pump is activated, pumping the brake fluid into the pipe between the master cylinder and the magnetic valve. The regulating, which feels like the brake pedal is pulsating, can take place up to ten times per second.
Of course, the reaction time of the braking system of the wheel is much too slow to perform a full standstill or a full release of the brake in this short time. What happens is, a slippage, alternating between 20% and 35% is maintained. You can read more about this in the additional information.

The wheels on the front axle are always regulated individually, the two rear wheels jointly. In the case of the compressed-air brake, no recirculation pump is necessary because, thanks to the auxiliary power unit, there is always sufficient air-pressure available. In this case, the air is simply blown off. Since the introduction of ABS and the respective wheel-sensors, there is now a simple method of checking the tyre pressure. The rotation speed of the wheels is compared, if when driving in a straight line, one of them is noticeably faster than the other. 09/11




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Translator: Don Leslie - Email: lesdon@t-online.de

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