21-01-2013, 11:16 AM
ANTI-LOCK BREAKING SYSTEM
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ABSTRACT:
Anti-lock braking system (ABS) is an automobile safety system that allows the wheels on a motor vehicle to maintain attractive contact with the road surface according to driver inputs while braking preventing the wheels from locking up (ceasing rotation) and avoiding uncon-trolled skidding. It is an automated system that uses the principles of threshold braking and cadence brakingwhich were practiced by skillful drivers with previous generation braking systems. It does this at a much faster rate and with better control than a driver could manage.
ABS generally offers improved vehicle control and decreases stopping distances on dry and slippery surfaces for many drivers; however, on loose surfaces like gravel or snow-covered pavement, ABS can significantly increase braking distance, although still improving vehicle control All the work is based on seven General Motors passenger vehicles having ABS as standard equipment for 1992 models but not available for 199 1 models. The ratio of crashes under an adverse condition (say, when the pavement is wet) to under a normal
condition (say, when the pavement is dry) is compared for ABS and non-ABS vehicles. After correcting for such factors as model year effects not linked to ABS, the following associations between ABS and crash risk were found by averaging data from the five states Texas, Missouri, North Carolina, Pennsylvania and Indiana (the errors are one standard error); a (10 k 3)% relative lower crash risk on wet roads compared to the corresponding comparison on dry roads; a (22 Y& 1 l)% lower risk of a pedestrian crash compared to the risk of a non-pedestrian crash; a (39 & 16)% increase in rollover crash risk compared to the risk of a non-rollover crash. Data from the same five states were used to examine two-vehicle rear-end collisions. Using the assumption that side-impact crashes estimate exposure, it was found that for wet roads ABS reduces the
risk of crashing into a lead vehicle by (32 5 8)%, but increases the risk of being struck in the rear by (30 f 14)%. The results from this study and from all available reported
studies are summarized in tabular form.
History:
ABS was first developed for aircraft use in 1929 by the French automobile and aircraft pioneer, Gabriel Voisin, as threshold braking on airplanes is nearly impossible. These systems use flywheel and valve attached to a hydraulic line that feeds the brake cylinders. The flywheel is attached to a drum that runs at the same speed as the wheel. In normal braking, the drum and flywheel should spin at the same speed. However, if a wheel were to slow down, then the drum would do the same, leaving the flywheel spinning at a faster rate. This causes the valve to open, allowing a small amount of brake fluid to bypass the master cylinder into a local reservoir, lowering the pressure on the cylinder and releasing the brakes. The use of the drum and flywheel meant the valve only opened when the wheel was turning. In testing, a 30% improvement in braking performance was noted, because the pilots immediately applied full brakes instead of slowly increasing pressure in order to find the skid point. An additional benefit was the elimination of burned or burst tires. (This citation has no mention of Gabriel Voisin, who was not involved in aviation at the time; neither are there patents to substantiate this claim)
In 1958, a Royal Enfield Super Meteor motorcycle was used by the Road Research Laboratory to test the Maxaret anti-lock brake. The experiments demonstrated that anti-lock brakes can be of great value to motorcycles, for which skidding is involved in a high proportion of accidents. Stopping distances were reduced in most of the tests compared with locked wheel braking, particularly on slippery surfaces, in which the improvement could be as much as 30 percent. Enfield's technical director at the time, Tony Wilson-Jones, saw little future in the system, however, and it was not put into production by the company.
A fully mechanical system saw limited automobile use in the 1960s in the Ferguson P99 racing car, the Jensen FF, and the experimental all wheel drive Ford Zodiac, but saw no further use; the system proved expensive and unreliable
INTRODUCTION:
Stopping safely is one of the most important functions a motor vehicle can perform. Failure of the brake system will almost invariably result in property damage, personal injury, or even death. Consequently, a great deal of consideration has been given to improving the brake system in trucks and passenger cars over the last nine decades. One of the latest improvements is an antilock brake system which, as the name suggests, prevents a vehicle's brakes from locking up and skidding during hard stops on wet or icy roads.
The problem of skidding reveals the one overwhelming weakness of all motor vehicle braking systems: they depend strongly on the coefficient of static friction between the tire and the road. If for any reason the tire momentarily loses its adhesion to the road while the brakes are applied, the friction of the brakes against the drums or rotors locks the wheel solidly and the tire begins skidding across the road. In this condition, the braking force of that wheel is dependent on the sliding friction between the tire and the road, which is much less than the static friction. Under wet or icy conditions, the sliding friction is reduced even further, resulting in significantly longer stopping distances. In addition, when the front wheels are in this condition, they cannot be used to steer the vehicle; 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 or the vehicle collides with something solid enough to bring it to a halt.
the antilock air pressure control valves are located on the vehicle frame rail, near each wheel.
DESIGN OF ABS:
An antilock brake system is designed for a specific vehicle application. A truck which does not pull a trailer, like a cement mixer, would have a slightly different ABS than a truck tractor which pulls one or more trailers. Likewise, an antilock brake system for a trailer would have a different design.
ABS for automobiles may be even more specific and may be designed for a particular
Regardless of manufacturer or the type of vehicle, all antilock brake systems operate in a similar manner. Wheel speed sensors are placed on each wheel that is to be controlled. Each speed sensor usually has a toothed wheel that rotates at the same speed as the vehicle wheel or axle. If the brakes are applied and one or more of the monitored wheels suddenly begins to reduce speed at a higher rate than the others, the controller activates the antilock system.
brand name and model of car. Since ABS components must fit and function along with existing vehicle components on each model, the design and manufacturing process of a new antilock brake system is carried out in partnership between the automobile manufacturer and the ABS supplier.
RAW MATERIALS:
The toothed wheel or gear in the speed sensor is made of soft iron, usually cast. Iron is chosen because of its high magnetic permeability and low magnetic reluctance. Magnetic reluctance is roughly equivalent to electrical resistance, and sometimes the toothed wheel is called the reluctor. The function of the toothed wheel is to allow the permanent magnet's field to easily pass through each tooth to cause a momentary concentration of field strength which induces a current in the pick-up coil. The pick-up coil has a permanent magnet in the core, wrapped with a coil of copper wire.
The controller usually employs transistors known as hot-side drivers which control the power side of the circuit rather than the ground side. These transistors produce more heat than is usual in an electronic circuit. Rather than being placed in a plastic or stamped steel housing, they are attached to a cast aluminum housing with a finned heat sink to dissipate the heat.
The hydraulic brake pressure solenoids used in automobiles have a standard construction of copper coil elements with steel valves and bodies. They are housed in the same casing as the brake system master cylinder which is usually cast from aluminum.
The electrical wiring is copper, often with cross-linked polyethylene insulation. To prevent radio frequency interference (RFI), in which high-power radio signals might be received through the wiring and cause the system to activate, all wiring is either shielded or the wires are run as twisted pairs to cancel out the effects of radio waves. Connectors are plastic with internal copper contacts.
SPEED SENSORS:
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 the differential, provide this information.
VALVES:
There is a valve in the brake line of each brake controlled by the ABS. On some systems, the valve has three positions:
In position one, the valve is open; pressure from the master cylinder is passed right through to the brake.
In position two, the valve blocks the 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 valve releases some 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.
CONTROLLER
The controller is an ECU type unit in the car which receives information from each individual wheel speed sensor, in turn if a wheel loses traction the signal is sent to the controller, the controller will then limit the brake force (EBD) and activate the ABS modulator which actuates the braking valves on and off.
CONCLUSION :
under test track conditions. The conclusions we can draw from the statistics are that earlier ABS suffered from a lack of driver training, and since prior driver training taught techniques counterproductive to ABS function, an increase in accidents was observed. But as driver training comes inline with ABS function, a decrease in accidents is being observed and will likely continue to improve. These analyses suggest that the introduction of ABS does not seem to have reduced the number of automobile accidents where they were expected to be effective. Kahane stated that involvements in multi-vehicle crashes on wet roads were significantly reduced by 24 percent, and nonfatal crashes by 14 percent (with ABS). However, these reductions were offset by a statistically significant increase in the frequency of single-vehicle, run-off-road crashes (rollovers or impacts with fixed objects), as compared to cars without ABS. Fatal run-off-road crashes were up by 28 percent and nonfatal crashes by 19 percent. It is unknown to what extent, if any, this increase is due to ABS or other causes. It is also unknown to what extent, if any, this increase is due to drivers incorrect usage of ABS or incorrect responses by drivers to their ABS. In comparison, some benefits were observed for light vehicles other than automobiles (pickup trucks, sport utility vehicles, and vans), equipped with two-wheel ABS instead of the four-wheel ABS used on most automobiles. Two-wheel ABS has been effective in reducing the risk of nonfatal run-off-road crashes for almost every type of light truck. Nonfatal rollovers were reduced by 30 to 40 percent. Side impacts with fixed objects were reduced by 15 to 30 percent. Frontal impacts with fixed objects were reduced by 5 to 20 percent.