ANTI LOCKING BRAKES
An anti-lock braking system (ABS, from German: Antiblockiersystem) is a safety system that allows the wheels on a motor vehicle to continue interacting with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up (that is, ceasing rotation) and therefore avoiding skidding.
An 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, an ABS can significantly increase braking distance, although still improving vehicle control.
Since initial widespread use in production cars, anti-lock braking systems have evolved considerably. Recent versions not only prevent wheel lock under braking, but also electronically control the front-to-rear brake bias. This function, depending on its specific capabilities and implementation, is known as electronic brake force distribution (EBD), traction control system, emergency brake assist, or electronic stability control (ESC).
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.
The system known as anti-lock brakes helps drivers to have better control of a vehicle in some road conditions where braking may be necessary. In vehicles without anti-lock brake systems, drivers who encounter slippery conditions may have to pump brakes to make sure they do not spin out of control because of locked up wheels. ABS systems work to coordinate wheel activity with a sensor on each wheel that regulates brake pressure as necessary, so that all wheels are operating in a similar speed range. Here are some of the main benefits of an ABS system.
Despite the fact that anti-lock brakes are proven to be a safety featurein most situations, and insurers consider them to significantly lower risk for a vehicle, not all drivers are sold on this option for a car or truck. Here are some of the down sides that drivers claim about this kind of brake system
The 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. An early system was DUNLOP'S MAXARET system, which was introduced in the 1950s and is still in use on some aircraft models. These systems use a 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.
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.
One manufacturer‘s name is synonymous with the recent history of ABS. Bosch was the first manufacturer in the world to begin manufacturing ABS in series production in 1978 following a ten-year period of development. The first millionth ABS was produced eight years following production startup. By 1996 Bosch had supplied 25 million systems and the figure rose to more than 40 million in 1998.
The 50 millionth ABS unit rolled off the production line of Bosch`s IMMENSTADT (ALLGÄU) plant in Germany at the end of July. Every workday more than 40,000 of these electronic safety systems leave Bosch`s 17 production facilities located in Germany and abroad. This amounts to more than a third of all such systems produced around the globe today.
In 1999, Bosch-produced ABS systems will be fitted to 92% of the passenger cars produced in Germany, 67% of passenger cars in Western Europe, 74% in North America, 62% in Japan and 55% worldwide. In the beginning, ABS was mostly found in top-of-the range cars but by1999, up to 86% of all compact cars produced in Western Europe and up to 30 percent of even smaller cars were equipped with ABS
Chrysler, together with the BENDIX Corporation, introduced a computerized, three-channel, four-sensor all-wheel ABS called "Sure Brake" for its 1971 Imperial. It was available for several years thereafter, functioned as intended, and proved reliable. In 1971, General Motors introduced the "TRACKMASTER" rear-wheel only ABS as an option on their Rear-wheel drive Cadillac models. In the same year, Nissan offered an EAL (Electro Anti-lock System) as an option on the Nissan President, which became Japan's first electronic ABS.
In 1988, BMW introduced the first motorcycle with an electronic-hydraulic ABS: the BMW K100. Honda followed suit in 1992 with the launch of its first motorcycle ABS on the ST1100 Pan European. In 2007, Suzuki launched its GSF1200SA (Bandit) with an ABS. In 2005, Harley-Davidson began offering ABS as an option for police bikes. In 2008, ABS became a factory-installed option on all Harley-Davidson Touring motorcycles and standard equipment on select models
In the early stage of ABS it had very few signals to control its behavior. It calculates the wheel speed and captures the vehicle speed by releasing breaks while wheel speed is 0 through the same speedometer. And it treats all 4 wheels the same way. This system worked fine in general road conditions yet there were identified problems. In the case of vehicle is driving down a hill, the vehicle speed will never be 0 while it release the break to capture it due the energy transformation from potential energy to kinetic energy. Hence there was no way to totally stop a vehicle which is driving down the hill by using break paddles. The problem were quickly solved using a simple logic that if wheel speed were less that certain level it will be locked by break till the driver releases the break paddle.
Additionally, if all four wheels are not in same road conations, this kind of break system yet result a skid. If two wheels are on snow and others on clean road, once the driver applies break it will turn the vehicle in an uncontrollable manner no matter it has ABS. Having the objective of solving this problem modern ABS systems collect data separately from all four wheels and apply ABS accordingly.
In traditional ABS systems collect the vehicle speed data from the same speedometer while uses to collect wheel speed data which is not always an accurate a mechanism. It always requires releasing the breaks and letting the wheels to spin to capture the relevant data. Modern ABS system are been designed to capture the vehicle speed using GPS data which could be identified as a proper mechanism, yet it could be less accurate. However, it doesn’t require releasing the break to capture vehicle speed.
Figure-1 ABS brakes on a BMW motorcycle
OPERATION OF ABS
The anti-lock brake controller is also known as the CAB (Controller Anti-lock Brake).
A typical ABS includes a central electronic control unit (ECU), four wheel speed sensors, and at least two hydraulic valves within the brake hydraulics. The ECU constantly monitors the rotational speed of each wheel; if it detects a wheel rotating significantly slower than the others, a condition indicative of impending wheel lock, it actuates the valves to reduce hydraulic pressure to the brake at the affected wheel, thus reducing the braking force on that wheel; the wheel then turns faster. Conversely, if the ECU detects a wheel turning significantly faster than the others, brake hydraulic pressure to the wheel is increased so the braking force is reapplied, slowing down the wheel. This process is repeated continuously and can be detected by the driver via brake pedal pulsation. Some anti-lock system can apply or release braking pressure 16 times per second.
The ECU is programmed to disregard differences in wheel relative speed below a critical threshold, because when the car is turning, the two wheels towards the center of the curve turn slower than the outer two. For this same reason, a differential is used in virtually all road going vehicles.
If a fault develops in any part of the ABS, a warning light will usually be illuminated on the vehicle instrument panel, and the ABS will be disabled until the fault is rectified.
The modern ABS applies individual brake pressure to all four wheels through a control system of hub-mounted sensors and a dedicated micro-controller. ABS is offered or comes standard on most road vehicles produced today and is the foundation for ESC systems, which are rapidly increasing in popularity due to the vast reduction in price of vehicle electronics. Over the year