ABS (Anti-lock Braking System)
Because ABS has been popular since the mid-80s, I suppose most of you have already known its theory. Anyway, for the sake of those new joining car enthusiasts, I think it would be better to describe it briefly here.
Basic theory
You might think that optimal braking is implemented by completely locking all the wheels. No, law of physics tells us that the coefficient of friction between the ground surface and a static object is always greater than a moving object. If the tyres are sliding on the road surface, the friction between road and wheel will not be maximum. Therefore, the maximum braking occurs when the wheels are braked up to the level that the wheels just do not slide.
To ensure the shortest stopping distance, ABS applies intermittent braking in very high frequency. This avoid complete lock up of wheels, thus gives the name "Anti-Lock Braking System".
Another advantage of ABS is letting the driver to keep controlling the car during braking. Before ABS appeared, cars lock up during braking, thus unable to be steered to avoid collision. With ABS, while slowing down the car, the driver can simultaneously try to steer away from the obstacle in front.
To implement anti-lock braking, ABS system employs speed sensors for individual wheels. If the wheel speed detected differs from the vehicle speed, that means the wheel is sliding, thus the computer will signal the corresponding brake to loose until sliding disappear. The computer will also compare the speed of all wheels, if one or more of them run considerably faster than others, that means the car is losing control, it will apply more brake to that wheel to correct the driving path.
A Brief History
Let me share with you the little bit information I gathered. ABS was originated in aeroplanes. It was developed in order to shorten the distance necessary for landing. It did not appeared in road cars until 1966, when Jensen FF (the first 4WD road car) installed a system developed by Dunlop. That system, called Maxaret, did not employ computer as well as wheel speed sensors. It just employed electronic sensors to avoid locking the disc brakes. Anyway, road testers immediately found its superiority over conventional brakes.
What's next. Sorry, my information becomes incomplete since then. The following is the information bits I got :
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BMW applied ABS to its road cars in 1979. Then motorcycle in 1987.
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Bosch launched the modern computerised ABS in the early 80s. Mercedes and BMW included it as option of their top of the range.
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In 1985, Ford Granada Scorpio took it as standard equipment, while Chevrolet Corvette made it a very common option. As production scale increased, ABS became cheaper and popular.
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In the mid-80s, Lucas Girling and AP also developed their low price ABS for cars like Ford Escort and Fiat Uno. Both served only the front wheels.
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Today, even mini cars offer ABS as standard.
Significance of ABS
Not only enhance braking, ABS sensors, computer and hydraulic pump also serve as the hardwares for Traction Control, Electronic Stability Control and Artificial LSD (read these topics in the following paragraphs). If not ABS is so popular, these new technology might not have appeared.
How Anti-Lock Brakes Work
by Karim Nice
Inside This Article
1.
Introduction to How Anti-Lock Brakes Work
2.
The ABS System
3.
Anti-Lock Brake Types
4.
ABS Questions
5.
Lots More Information
6.
See all Under the Hood articles
Stopping a car in a hurry on a slippery road can be very challenging. Anti-lock braking systems (ABS) take a lot of the challenge out of this sometimes nerve-wracking event. In fact, on slippery surfaces, even professional drivers can't stop as quickly without ABS as an average driver can with ABS.
Anti-Lock Brake Image Gallery
Location of anti-lock brake components. See more anti-lock brake images.
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In this article, the last in a six-part series on brakes, we'll learn all about anti-lock braking systems -- why you need them, what's in them, how they work, some of the common types and some associated problems.
by Karim Nice
Inside This Article
1.
Introduction to How Anti-Lock Brakes Work
2.
The ABS System
3.
Anti-Lock Brake Types
4.
ABS Questions
5.
Lots More Information
6.
See all Under the Hood articles
The ABS System
The theory behind anti-lock brakes is simple. A skidding wheel (where the tire contact patch is sliding relative to the road) has less traction than 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 (see Brakes: How Friction Works for 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 to steer while you stop.
There are four main components to an ABS system:
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Speed sensors
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Pump
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Valves
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Controller
Anti-lock brake pump and valves
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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:
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In position one, the valve is open; pressure from the master cylinder is passed right through to the brake.
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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.
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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 a computer in the car. It watches the speed sensors and controls the valves.
ABS 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 for decelerations in 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 the pressure to 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 the tire can 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 a pulsing in 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.
Anti-lock braking system From Wikipedia, the free encyclopedia
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An anti-lock braking system (ABS) is a system on motor vehicles which prevents the wheels from locking while braking. The purpose of this is to allow the driver to maintain steering control under heavy braking and, in some situations, to shorten braking distances (by allowing the driver to hit the brake fully without the fear of skidding or loss of control). Disadvantages of the system include increased braking distances under certain conditions and the creation of a "false sense of security" among drivers who do not understand the operation and limitations of ABS.
Since it came into widespread use in production cars (with "version 2" in 1978), ABS has made considerable progress. Recent versions not only handle the ABS function itself (i.e. preventing wheel locking) but also traction control, brake assist, and electronic stability control, amongst others. Not only that, but its version 8.0 system now weighs less than 1.5 kilograms, compared with 6.3 kg of version 2.0 in 1978.
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Contents
[hide]
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1 History
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2 Operation
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3 Effectiveness
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4 Traction control
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5 Risk compensation
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6 Design and selection of components
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7 See also
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8 External links
|  [edit] History
Anti-lock braking systems were first developed for aircraft in 1929 by the French automobile and aircraft pioneer Gabriel Voisin, as threshold braking an airplane is nearly impossible. An early system was Dunlop's Maxaret system, introduced in the 1950s and still in use on some aircraft models, in 1936 the German Companies Bosch and Mercedes-Benz pioneered the first electronic version for use on Mercedes Benz cars.[citation needed] This version which was made of more than 1000 analogue electronic parts was still fairly slow.
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, in automobile use, somewhat unreliable. However, a limited form of anti-lock braking, utilizing a valve which could adjust front to rear brake force distribution when a wheel locked, was fitted to the 1964 Austin 1800.
ABS brakes on a BMW motorcycle
The first true electronic 4-wheel multi-channel ABS was co-developed by Chrysler and Bendix for the 1971 Imperial. Called "Sure Brake", it was available for several years and had a satisfactory performance and reliability record. Ford also introduced anti lock brakes on the Lincoln Continental Mark III and the Ford LTD station wagon, called "Sure Trak". The German firms Bosch and Mercedes-Benz had been co-developing anti-lock braking technology since the 1930s; They first appeared in trucks and the Mercedes-Benz S-Class. ABS Systems were later introduced on other cars and motorcycles.
[edit] Operation
The anti-lock brake controller is also known as the CAB (Controller Anti-lock Brake).
A typical ABS is composed of a central electronic unit, four speed sensors (one for each wheel), and two or more hydraulic valves on the brake circuit. The electronic unit constantly monitors the rotation speed of each wheel. When it senses that any number of wheels are rotating considerably slower than the others (a condition that will bring it to lock[1]) it moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel. The wheel(s) then turn faster and when they turn too fast, the force is reapplied. This process is repeated continuously, and this causes the characteristic pulsing feel through the brake pedal.
The sensors can become contaminated with metallic dust and fail to detect wheel slip; this is not always picked up by the internal ABS controller diagnostic.
One step beyond ABS are modern ESC systems. Here, two more sensors are added to help the system work: these are a wheel angle sensor, and a gyroscopic sensor. The theory of operation is simple: when the gyroscopic sensor detects that the direction taken by the car doesn't agree with what the wheel sensor says, the ESP software will brake the necessary wheel(s) (up to three with the most sophisticated systems) so that the car goes the way the driver intends. The wheel sensor also helps in the operation of CBC, since this will tell the ABS that wheels on the outside of the curve should brake more than wheels on the inside, and by how much.
^ The electronic unit needs to determine when some of the wheels turn considerably slower than any of the others because when the car is turning the two wheels towards the center of the curve inherently move slightly slower than the other two – which is the reason why a differential is used in virtually all commercial cars.
[edit] Effectiveness
A 2003 Australian study[2] by Monash University Accident Research Centre found that ABS:
•Reduced the risk of multiple vehicle crashes by 18 percent, but that it
•Increased the risk of run-off-road crashes by 35 percent.
On high-traction surfaces such as bitumen, or concrete many (though not all) ABS-equipped cars are able to attain braking distances better (i.e. shorter) than those that would be easily possible without the benefit of ABS. Even an alert, skilled driver without ABS would find it difficult, even through the use of techniques like threshold braking, to match or improve on the performance of a typical driver with an ABS-equipped vehicle, in realworld conditions. ABS reduces chances of crashing, and/or the severity of impact. The recommended technique for non-expert drivers in an ABS-equipped car, in a typical full-braking emergency, is to press the brake pedal as firmly as possible and, where appropriate, to steer around obstructions. In such situations, ABS will significantly reduce the chances of a skid and subsequent loss of control.
In gravel and deep snow, ABS tends to increase braking distances. On these surfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this from occurring. Some ABS calibrations reduce this problem by slowing the cycling time, thus letting the wheels repeatedly briefly lock and unlock. The primary benefit of ABS on such surfaces is to increase the ability of the driver to maintain control of the car rather than go into a skid — though loss of control remains more likely on soft surfaces like gravel or slippery surfaces like snow or ice. On a very slippery surface such as sheet ice or gravel it is possible to lock multiple wheels at once, and this can defeat ABS (which relies on detecting individual wheels skidding). Availability of ABS relieves most drivers from learning threshold braking.
But part of the answer is that on HEAVY snow, locked wheels can be useful because they gather up a "wedge" of snow which helps to slow the vehicle. ABS allows this wedge to clear every time the wheels are unlocked. The same can apply on sand in some conditions.
Note, however, that this somewhat simplistic test compares ABS with locked wheels. A good driver with a car with a decently designed braking system, designed to minimize the chances of accidentally locking the brakes during a "panic stop", would fare better under these conditions.
A June 1999 NHTSA study found that ABS increased stopping distances on loose gravel by an average of 22 percent [1].
Other tests shows results that differ from those above when braking on ice. An independent test, with a 1989 Dodge Omni, a small economy car, and a 1995 Pontiac Grand Am equipped with ABS (Mid Sized family Vehicle) The Pontiac matched or had shorter stopping distances on the glare ice, despite being heavier. However, since the vehicles, brakes and tires were different, this is not a completely valid comparison.
When activated, some earlier ABS systems caused the brake pedal to pulse noticeably. As most drivers rarely or never brake hard enough to cause brake lockup, and a significant number rarely bother to read the car's manual, this may not be discovered until an emergency. When drivers do encounter an emergency that causes them to brake hard and thus encounter this pulsing for the first time, many are believed to reduce pedal pressure and thus lengthen braking distances, contributing to a higher level of accidents than the superior emergency stopping capabilities of ABS would otherwise promise. Some manufacturers have therefore implemented Mercedes-Benz's "brake assist" system that determines that the driver is attempting a "panic stop" and the system automatically increases braking force where not enough pressure is applied. Nevertheless, ABS significantly improves safety and control for drivers in most on-road situations.
[edit] Traction control
Main article: Traction control
The ABS equipment may also be used to implement traction control on acceleration of the vehicle. If, when accelerating, the tire loses traction with the ground, the ABS controller can detect the situation and take suitable action so that traction is regained. Manufacturers often offer this as a separately priced option even though the infrastructure is largely shared with ABS. More sophisticated versions of this can also control throttle levels and brakes simultaneously.
[edit] Risk compensation
ABS brakes are the subject of some widely cited experiments in support of risk compensation theory, which support the view that drivers adapt to the safety benefit of ABS by driving more aggressively.
The two major examples are from Munich and Oslo. In both cases taxi drivers in mixed fleets were found to exhibit greater risk-taking behaviour when driving cars equipped with ABS, with the result that collision rates between ABS and non ABS cars were not significantly different.
[edit] Design and selection of components
Given the required reliability it is illustrative to see the choices made in the design of the ABS system. Proper functioning of the ABS system is considered of the utmost importance, for safeguarding both the passengers and people outside of the car. The system is therefore built with some redundancy, and is designed to monitor its own working and report failures. The entire ABS system is considered to be a hard real-time system, while the subsystem that controls the selfdiagnosis is considered soft real-time. As stated above, the general working of the ABS system consists of an electronic unit, also known as ECU (electronic control unit), which collects data from the sensors and drives the hydraulic control unit, or HCU, mainly consisting of the valves that regulate the braking pressure for the wheels.
The communication between the ECU and the sensors must happen quickly and at real time. A possible solution is the use of the CAN bus system, which has been and is still in use in many ABS systems today (in fact, this CAN standard was developed by Robert Bosch GmbH, for connecting electronic control units!). This allows for an easy combination of multiple signals into one signal, which can be sent to the ECU. The communication with the valves of the HCU is usually not done this way. The ECU and the HCU are generally very close together. The valves, usually solenoid valves, are controlled directly by the ECU. To drive the valves based on signals from the ECU, some circuitry and amplifiers are needed (which would also have been the case if the CAN-bus was used).
The sensors measure the position of the tires, and are generally placed on the wheel-axis. The sensor should be robust and maintenance free, not to endager it's proper working, for example an inductive sensor. These position measurements are then processed by the ECU to calculate the wheel-spin.
The hydraulic control unit is generally located right next to the ECU (or the other way around), and consists of a number of valves that control the pressure in the braking circuits. All these valves are placed closely together and packed in a solid block. This makes for a very simple layout, and is thus very robust.
The central control unit generally consists of two microcontrollers, both active simultaneously, to add some redundancy to the system. These two microcontrollers interact, and check each other's proper working. These microcontrollers are also chosen to be power-efficient, to avoid heating of the controller which would reduce durability. The software that runs in the ECU has a number of functions. Most notably, the algorithms that drive the HCU as a function of the inputs, or control the brakes depending on the recorded wheel spin. This is the obvious main task of the entire ABS-system. Apart from this, the software also needs to process the incoming information, e.g. the signals from the sensors. There is also some software that constantly tests each component of the ABS system for its proper working. Some software for interfacing with an external source to run a complete diagnosis is also added. As mentionned before the ABS system is considered hard real-time. The control algorithms, and the signal processing software, certainly fall in this category, and get a higher priority than the diagnosis and the testing software. The requirement for the system to be hard real-time can therefore be reduced to stating that the software should be hard real-time. The required calculations to drive the HCU have to be done in time. Choosing a microcontroller that can operate fast enough is therefore the key, preferably with a large margin. The system is then limited by the dynamic ability of the valves and the communication, the latter being noticeably faster. The control system is thus comfortably fast enough, and is limited by the valves.
Anti-Lock Brake Types
Anti-lock braking systems use different schemes depending on the type of brakes in use. We will refer to them by the number of channels -- that is, how many valves that are individually controlled -- and the number of speed sensors.
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Four-channel, four-sensor ABS - This is the best scheme. There is a speed sensor on all four wheels and a separate valve for all four wheels. With this setup, the controller monitors each wheel individually to make sure it is achieving maximum braking force.
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Three-channel, three-sensor ABS - This scheme, commonly found on pickup trucks with four-wheel ABS, has a speed sensor and a valve for each of the front wheels, with one valve and one sensor for both rear wheels. The speed sensor for the rear wheels is located in the rear axle.
This system provides individual control of the front wheels, so they can both achieve maximum braking force. The rear wheels, however, are monitored together; they both have to start to lock up before the ABS will activate on the rear. With this system, it is possible that one of the rear wheels will lock during a stop, reducing brake effectiveness.
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One-channel, one-sensor ABS - This system is commonly found on pickup trucks with rear-wheel ABS. It has one valve, which controls both rear wheels, and one speed sensor, located in the rear axle.
This system operates the same as the rear end of a three-channel system. The rear wheels are monitored together and they both have to start to lock up before the ABS kicks in. In this system it is also possible that one of the rear wheels will lock, reducing brake effectiveness.
This system is easy to identify. Usually there will be one brake line going through a T-fitting to both rear wheels. You can locate the speed sensor by looking for an electrical connection near the differential on the rear-axle housing.
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