Anti-lock braking system

The anti- lock braking system ( ABS) is a technical system for enhanced driving safety and less wear on the treads of the wheels. In the German Road Traffic Licensing Regulations it is called " Anti-lock " ( ABV). It is used mainly in motor vehicles, but also in railways and aircraft landing gear.

ABS acts during braking against a possible locking of the wheels by reducing the brake pressure. This allows the brakes of motor vehicles and aircraft better steerability and directional stability. In addition, the system can control the wheel slip shorten the braking distance on wet roads. On a dry road and loose surfaces - for example, gravel or snow - may be extended, however, the braking distance. In railways, the system is designed to prevent flat spots that would otherwise lead to a characteristic slapping noise during the ride.

• 4.1 passenger cars
• 4.2 trucks
• 4.3 Motorcycles
• 4.4 Bicycles
• 4.5 Railways

Principle

The maximum deceleration is reached, depending on road conditions and tire at about 8-25 % slip. Mean 20 % brake slip that in which the vehicle travels over the same period, a distance of one meter, the wheels roll only 0.8 meters. After reaching the maximum braking force - so that the slip value at which the maximum delay is achieved - the slip grows with decreasing braking force to continue until the wheel finally blocked ( = 100% braking slip). In the blocked state is only braked by friction. The ABS controls the brake force to each wheel so that the slip during the braking operation at any time is as close as possible at the optimum value. During heavy braking, the slip oscillates about this point of maximum braking force.

When sliding the tire also no transmission of lateral forces is possible. Since the vehicle slides over the entire contact area of ​​the tires, a steering angle causes no change in direction. In addition, at the wheel while blocking by selective abrasion a so-called brake plates arise. Both can be avoided by ABS.

Design and operation

The first ABS systems were three -channel systems, that is, the brake pressure to both rear wheels could only be controlled together. In the current (2013 ) standard comes standard on cars exclusively the 4-channel ABS is in operation, which controls all the wheels individually.

An ABS system is an extension of the previous standardized dual-circuit brake system. Each two of the four wheel brake cylinders are actuated in unison from a respective one of two main brake cylinder. With ABS (4 -channel ) can be independent of this basic design are reduced separately in each of the four wheel brake cylinder pressure.

There is an inductive sensor or on newer vehicles, a Hall effect sensor and a hole or toothed disc with which the speed is measured at each wheel. The brake pressure on one wheel is reduced if the rotational speed is lowered during braking of excessive in comparison with those of the other wheels. The hydraulic part of the ABS solenoid valves are of conventional construction, two attached to the pressure line leading to each wheel. First, the first valve blocks the line. If speed is on, so is omitted from the side with the second, normally closed valve brake fluid, the brake pressure is lowered. The discharged liquid is transported back to a start-up at the same time the electric pump in the space and to the pressure level between the master cylinder and the first solenoid valve. Otherwise, the fluid would be there is missing, and the brake pedal would be depressed further, when the first solenoid valve in accordance with increase in rotational speed will open again, the brake pedal would soon be impossible fully depressed and brakes. There are many such games in a row and working quickly place ( about 10 per second, with 15 motorcycles per second). The pressure increase due to the reverse pumping is perceived on the pedal with your foot as vibrate. The automatic, fast stuttering brakes jerky and rattles the car significantly. According to the basic concept of the hydraulic separation into two circles are two pumps available ( but often driven with only an electric motor ).

Under moderate braking in normal driving, and upon failure of the ABS braking system acts like a normal dual-circuit system. The brake pressure from the master cylinder acts on the open lines undiminished to the wheel cylinders. The two valves ( eight per four-wheeled vehicle ) are kept at rest with spring open ( first valve ) or closed (second valve). The normal brakes work this way even during power outages.

The extensive signal processing is performed in a central control unit. Below a general minimum speed ( about 6 km / h), the ABS is turned off. When the ignition and when turning on the ABS after exceeding the minimum velocity effected self-tests. Detected errors are enrolled in an electronic memory to facilitate troubleshooting defects.

For trucks with air brakes, the ABS works on the same principle. Since an air compressor works permanently, accounting for the return pumps for the exhausted air. The additional valves are located directly at the wheel brake cylinders, because air pressure changes from a central point over long cables were too late with the wheels on. Hydraulic pressure changes, however, have much shorter maturities.

Additional functions

Brake force distribution

Newer versions of the ABS also assume the braking force distribution ( electronic brake force distribution - EBD) between the front and rear axle (4- channel ) systems, replacing previously used mechanical regulator (Electronic Brake Distribution ), some of which are still installed in trucks. It provides several advantages:

• Optimal utilization of the adhesion coefficient on both axles - including diagonal wheel loads, which could not be represented perfectly with mechanical controls.
• Even small braking the rear axle is mitgebremst and removes a known issue: When mechanical brake force distribution, it could happen that the brake discs were cleaned at the rear axle too rarely by applying the brakes and corrosion on the surface or dirt reduced the braking effect.

Furthermore, one of the newer systems, the emergency wheel. Emergency wheels have a smaller circumference and rotate faster, so that the ABS in case of emergency braking can process the feedback from the speed sensor correctly.

Meanwhile, ABS is increasingly supplemented by ESC ( vehicle dynamics control ).

Yawing moment

When the vehicle is on a road with different friction coefficients between the right and left side ( for example, snow or wet leaves at the edge of the road ), would adjust a moment about the yaw axis to the side with the higher static friction at a sudden emergency braking without ABS ( yaw moment ) because the side of the vehicle is braked more on the catchy underground as the page on the smooth surface. In the vehicle -tuning the ABS manufacturers should ensure that the brake pressure of the wheel is not as fast rises sharply on rough ground, as it would be physically possible. Gives the driver additional reaction time to react by braking. The time it takes for the ABS to come on rough ground to full braking power is dependent on the manufacturer 's philosophy. It is usually shorter than with limousines for sports cars. Due to the slowdown in construction of the braking force is also the braking distance is extended. However, this is taken in view of the advantages in terms of control of the vehicle into account.

Meanwhile, there are also systems that replace the yawing moment by an automatic intervention in the steering. In an active counter-steering by means of a superposition steering system ( active steering ) the extension of the braking distance by the yawing moment is omitted.

Off-road ABS

Offroad - ABS is offered mainly in SUV auxiliary function, which on loose surfaces largely cancels out the previous system-dependent stopping distance. By interval -like blocking one or more wheels, the braking effect of the wedge is used which forms the front of the wheel brakes. On other road surfaces, the system works like a conventional ABS. The additional function was introduced in 2006 in the VW Touareg under the name ABSplus and is now also available from other manufacturers. The modified brake control may be some manufacturers also be activated manually.

History

The first use of a lock device was tested in aviation, so that the aircraft remained on track after landing when braking from high speed. As early as 1920 put the French automobile and aircraft pioneer Gabriel Voisin a hydraulically functioning system. In the 1950s, several commercial aircraft were equipped with the Maxaret Anti -Skid the British company Dunlop

1928 received the German Karl Wessel a patent on a braking force regulator for automobiles, but he never got beyond the paper stage. In 1936, Bosch was a patent on a device for preventing the hard braking the wheels of a motor vehicle. The devices consisted of about 1000 analog components and were very unwieldy and slow. With the digital technology, the amount of the components could be reduced to about 140 pieces. This ABS was ready for.

In 1969, the first generation of an ABS ( electronically controlled anti-lock braking system ) on the Frankfurt Motor Show by the U.S. company ITT Automotive ( until 1967 Alfred Teves ) was presented.

In 1970 Citroen shortly before the launch of the Teldix ABS ( Teldix for Telefunken - Bendix ), which eventually failed due to financial problems of the automobile manufacturer and the first oil crisis. In 1975, the Bosch patents and licenses of Teldix.

Passenger cars

The first car with a mechanical ABS was the Jensen FF with Dunlop Maxaret ABS from the year 1966 1969 Ford equipped the Lincoln Continental Mark III with an acting only on the rear wheels ABS system called Sure- Track Brake system.; as the Ford Thunderbird. From 1971 to 1973, the Chrysler Group offered its luxury model Imperial surcharge with a Sure -called electronic anti-lock brake system Bendix on, which included the front and rear wheels. 1978, Bosch introduced its electronic ABS on the market; same time, the term ABS from Bosch was legally protected. Other manufacturers call their systems partially as ABV (automatic blocking ). First of ABS for the S-Class W 116 Mercedes- Benz was available, then for the BMW 7 Series E23. 1985 Ford Scorpio was the first passenger car, the disposal standard with ABS. As of 1987, Volkswagen and the VW Golf II, III and Passat Transporter T3 to high surcharges to the system available (golf: 1800 DM Passat: 2200 DM, Transportation: 3720 DM Price, 1989). End of 2003, about 90 percent of licensed vehicles in Germany ABS. Due to a commitment by the European automobile industry (ACEA ) all vehicles less than 2.5 tonnes gross vehicle weight are fitted as standard with ABS since 1 July 2004. The Japanese automobile associations have submitted identical obligations.

Truck

Since 1981, Mercedes -Benz offers the ABS for compressed air brakes, developed together with the company WABCO. Since 1987, all coaches and, since 1991, all trucks are equipped with ABS. Since January 1991 trucks over 3.5 tonnes GVW and buses with more than eight seats may be authorized only with ABS.

Motorcycles

The first ABS for motorcycles was introduced in 1985 by Lucas Girling. The first series was the company manufacturer FTE automotive is based in boars / Lower Franconia (then a division of the company FAG ball Fischer). Meanwhile, ABS was first introduced in 1988 as an option on the BMW K models and then cost 1980 DM charge. Today also Bosch, Nissin, Continental Teves and ABS Brembo deliver. The Federal Ministry of Transport, Building and Urban Development recommends to prescribe ABS law. The same call on the representative of the German delegation in a working group of the Economic Commission for Europe. The scientific basis of the claim is a study of the Federal Highway Research Institute. The way the project encounters sometimes criticism.

Meanwhile, increases the range of bikes and scooters that are optional or standard with ABS. Recent studies of the ADAC go in extrapolations from 160 not killed motorcyclists per year and up to 6900 motorcycle accidents with injuries could have been prevented in 2009 if the driver would have had ABS on board. The ADAC criticized the inadequate dissemination of ABS for motorcycles. On motorcycles under 250 cc ABS is even circulated only less than one percent of the models. For the heavier machines, there is the security system at just over 25 percent of two-wheelers in Europe.

The motorcycle ABS is until now (as of 2010 ) designed to maintain stability at straight-ahead hard braking. Newer systems are considered restricted curve fit, fully suitable for curves systems are still in development. The systems are different depending on the manufacturer. The pressure modulation is performed either via electronically controlled solenoid valves (eg BMW, Ducati, Kawasaki, KTM, Suzuki, Yamaha), plunger system (BMW old, Honda) or magnetic system (Honda). The rule frequencies are far apart, depending on system development. The first systems (ABS I) amounted to a maximum of 7 control operations per seconds, the latest systems can have 15 control processes. Below 4 km / h ( BMW, Honda, etc. ) or 10 km / h ( Honda SH 300), the system is shut down regularly due.

Studies confirm the interest in purchasing ABS, availability and economic viability provided. The supplement is 600 to 1050 Euro. In addition, each model differently increased maintenance costs compared to conventional brake systems. In addition, some systems can be properly maintained in an approved workshop using special equipment.

Bicycles

In 2004, the company Biria introduced an antilock braking system for bicycles, the 1-4-2 Safety Brake. This affects the torque support the coaster brake is a mechanism with cable, in addition to the handbrake lever on the front brake. About brake the front wheel, the rear wheel loses contact with the ground, whereupon the brake torque is lost at the rear wheel and the front brake is loosened. Advantage of this method is that no additional driving resistances occur without applying the brakes and the pedal brake acts on both brakes. In related patent describes "a certain ABS - effect ".

The Italian company Brovedani 1995 had an ABS brake in cantilever style on offer. The principle is, by revolving on the rim rollers are coupled by a balance lever with the brake shoe to constantly change its distance to the rim, and thus to achieve an acceleration- independent ABS effect.

Railways

In rail vehicles are for several decades as " anti-slip " designated systems in use, which are intended to prevent, inter alia, the wheels from locking while braking. This is done primarily but not from driving dynamic considerations, but so that the wheel and rail are protected against wear by sliding.

Pros and Cons

With a solid track and the common material combinations (rubber with asphalt / concrete), the smallest possible stopping distance still rolling wheel, and not with locked wheel in certain different conditions (snow, loose sand ) results in the shortest possible stopping distance is achieved with locked wheels, but the steering ability is then lost. When braking aid here offers ABS on a compromise: While it may come with ABS to an extension of the braking distance, but the blocking with his negative consequences can be avoided.

In principle, the steerability of the vehicle can be obtained when braking without ABS. Sport riders dominate the corresponding control by meter the brake pressure or brake - as imitated in ABS - stuttering operate. Both measures meets the ABS automatically, while average human reflex and skill usually not sufficient to react properly in dangerous situations. By the ABS in modern motor vehicles, the brakes is controlled individually for each wheel, and close to optimal.

• The vehicle remains steerable even during hard braking, so that the obstacle can be avoided.
• Especially on wet roads, the vehicle has better braking and shorter stopping distances.
• Protection of the tire, as the tire wear evenly distributed over the circumference. In contrast, can be greatly eroded at a lock braking the tire at one point. This results in braking plates, which cause an irregular operation and strong noise.
• Better controllability on split-friction road by yawing moment.
• The braking force for each wheel can be optimally controlled

• On road surfaces with loose surfaces such as sand, gravel or snow, it comes without ABS with locked wheels into a wedge formation in front of the wheel This wedge can lead to a reduction in braking distance. If this is not chock up, increases in general, the braking distance.