Anti Lock Breaking System

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  • Topic: Anti-lock braking system, Electronic stability control, Automobile
  • Pages : 6 (1244 words )
  • Download(s) : 1056
  • Published : February 4, 2009
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Introduction

An anti-lock braking system (ABS) is a safety system on motor vehicles which prevents the wheels from locking while braking.

The Anti-lock Braking System is designed to maintain vehicle control, directional stability and optimum deceleration under severe braking conditions on most road surfaces

It does so by monitoring the rotational speed of each wheel and controlling the brake line pressure to each wheel during braking. This prevents the wheels from locking up.

A rotating road wheel allows the driver to maintain steering control under heavy braking

Theory

When the car brakes (normally ) wheels exert a forward force on the street with less than maximum static sliding friction.

If the driver brakes very hard it can occur that the maximum static friction is surpassed and the wheels lose their grip and begin sliding

The amount of traction which can be obtained for an auto tire is determined by the coefficient of static friction between the tire and the road. If the wheel is locked and sliding, the force of friction is determined by the coefficient of kinetic friction. A tire that is just on the verge of slipping produces more friction with respect to the road than one which is locked and skidding . So locked wheels are less effective in stopping on a road. But in gravel, sand and deep snow, locked wheels dig in and stop the vehicle more quickly. A locked tire allows a small wedge of snow to build up ahead of it which allows it to stop in a somewhat shorter distance than a rolling tire.

When tire does not slip, it will roll only in the direction it turns. But once it skids, regardless of the angle of the front wheels, the vehicle continues to skid in whatever direction its momentum sends it until either the driver releases the brakes

Basic Operation
The Anti-lock Braking System is designed to maintain vehicle control, directional stability and optimum deceleration under severe braking conditions on most road surfaces

It does so by monitoring the rotational speed of each wheel and controlling the brake line pressure to each wheel during braking. This prevents the wheels from locking up.

The sensors - one at each wheel send a variable voltage signal to the control unit, which monitors these signals, compares them to its program information, and determines whether a wheel is about to lock up.

When a wheel is about to lock up, the control unit signals the hydraulic unit to reduce hydraulic pressure (or not increase it further) at that wheel’s brake caliper. Pressure modulation is handled by electrically-operated solenoid valves

Components

Wheel Speed Sensors

Abs Control Module

Hydraulic
Modulator

Pump Motor &
Accumulator

Wheel Speed Sensors
The wheel speed sensor pickup has a magnetic core surrounded by coil consist of a magnetic pickup and a toothed sensor ring . As the wheel turns, teeth on the sensor ring move through the pickup magnetic field.

This reverses the polarity of the magnetic field and induces an alternating current (AC) voltage in the pickup windings. The number of voltage pulses per second that are induced in the pickup changes in direct proportion to wheel speed. So as speed increases, the frequency and amplitude of the wheel speed sensor goes up

The WSS signal is sent to the antilock brake control module, where the AC signal is converted into a digital signal and then processed

The control module then counts pulses to monitor changes in wheel speed. The number of voltage pulses per second that are induced in the pickup changes in direct proportion to wheel speed. So as speed increases, the frequency and amplitude of the wheel speed sensor goes up

The WSS signal is sent to the antilock brake control module, where the AC signal is converted into a digital signal and then processed

The control module then counts pulses to monitor changes in wheel speed.

ABS Control Module
The ABS...
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