Anti-lock braking system
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.
History
Anti-lock braking systems were first developed for aircraft in 1929 by the French automobile and aircraft pioneer Gabriel Voisin. 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.
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. 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.
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.
Effectiveness
A 2003 Australian study[1] 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.
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.
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 when driving cars equipped with ABS, with the result that collision rates between ABS and non ABS cars were not significantly different.
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 quick and 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.
See also
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