Seminar Report on
Anti-lock braking system
Prepared by
RANJAN NAYAK
[email protected]
JYOTI Jyotiranja
Introduction An anti-lock braking system (ABS) is a safety system on motor vehicles which prevents the wheels from locking while braking. A rotating road wheel allows the driver to maintain steering control under heavy braking, by preventing a locked wheel or skid, and allowing the wheel to continue to forward roll and create lateral control, as directed by driver steering inputs. Disadvantages of the system include increased braking distances under some limited circumstances (snow, gravel, "soft" surfaces), and the creation of a "false sense of security" among drivers who do not understand the operation, and limitations of ABS.
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.
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An early system was Dunlop's Maxaret system, introduced in the 1950s and still in use on some aircraft models.
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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.
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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.
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Chrysler, together with the Bendix Corporation, introduced a crude, limited production ABS system on the 1971 Imperial. Called "Sure Brake", it was available for several years, and had a satisfactory performance and reliability record.
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Ford also introduced anti lock brakes on the Lincoln Continental Mark III and the Ford LTD station wagon, called "Sure Trak" in 1975.
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The German firms Bosch and Mercedes-Benz had been co-developing anti-lock braking technology since the 1930s, and introduced the first completely electronic 4-wheel multi-channel ABS system in trucks and the Mercedes-Benz S-Class in 1978.
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ABS Systems based on this more modern Mercedes design were later introduced on other cars and motorcycles. General Motors introduced the "Trackmaster" ABS on their Cadillac models in 1971 as an option that was operational on the rear wheels for RWD models.
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In 1988 BMW became the world's first motorcycle manufacturer to introduce an electronic/hydraulic ABS system, this on their BMW K100.
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In 1992 Honda launched its first ABS system, this on the ST1100 Pan European.
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In 1997 Suzuki launched its GSF1200SA (Bandit) with ABS
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Today ABS has become a standard equipment even for small cars
Theory When the car brakes (normally), the momentum of the car must be reduced, so a backwards force needs to be transmitted to the car. This is achieved by the wheels exerting a forward force on the street which lies below the threshold of maximum static sliding friction. The wheels keep sticking to the road because of this friction.
If the driver brakes very hard (or accelerates extremely) it can occur that the maximum static friction is surpassed and the wheels lose their grip and begin sliding (or spinning). In this case the dynamic sliding friction (which is less than the maximum static friction) takes over The amount of traction which can be obtained for an auto tire is determined by the coefficient of static friction between the tire and the road. If the wheel is locked and sliding, the force of friction is determined by the coefficient of kinetic friction and is usually significantly less. A tire that is just on the verge of slipping (10 to 20% slippage) produces more friction with respect to the road than one which is locked and skidding (100% slippage). Once traction is lost, friction is reduced, the tire skids and the vehicle takes longer to stop. So locked wheels are less effective in stopping on a road
But in gravel, sand and deep snow, locked wheels dig in and stop the vehicle more quickly. A locked tire allows a small wedge of snow to build up ahead of it which allows it to stop in a somewhat shorter distance than a rolling tire. That is why some vehicles have an on/off switch for deactivating the antilock system when driving on snow. So, antilock brakes do not necessarily reduce the stopping distance, and in fact may actually increase stopping slightly on dry pavement. But on wet or slick pavement, antilock brakes may reduce the stopping distance up to 25% or more, which could be the difference between a safe stop and an accident But what ABS provides is Directional stability which is very crucial. Directional stability also depends on traction. As long as a tire does not slip, it will roll only in the direction it turns. But once it skids, it has about as much directional stability as a hockey puck on ice because, 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. By minimizing the loss of traction, antilock braking helps maintain directional stability and steering control
Basic Operation The Anti-lock Braking System is designed to maintain vehicle control, directional stability and optimum deceleration under severe braking conditions on most road surfaces. It does so by monitoring the rotational speed of each wheel and controlling the brake line pressure to each wheel during braking. This prevents the wheels from locking up. The ABS system has following main components – • Wheel Speed Sensors • Abs Control Module • Hydraulic Modulator • Pump Motor & Accumulator.
The sensors - one at each wheel since1985, but at both front wheels and one at the rear differential on earlier
models – send a variable voltage signal to the control unit, which monitors these signals, compares them to its program information, and determines whether a wheel is about to lock up. When a wheel is about to lock up, the control unit signals the hydraulic unit to reduce hydraulic pressure (or not increase it further) at that wheel’s brake caliper. Pressure modulation is handled by electrically-operated solenoid valves. 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, like a gear, that rotates at the same speed as the vehicle wheel or axle. Mounted close to, but not touching this toothed wheel, is a permanent magnet wrapped with a coil of wire, called the pick-up coil (see illustration). As each tooth rotates past the permanent magnet, it causes the magnetic field to concentrate and increase slightly. This, in turn, induces a small pulse of current in the coil of wire. The number of pulses per second is directly proportional to the speed of the wheel. The faster the wheel turns, the faster the teeth pass the magnet and the higher the pulse rate. The pulsed output from the wheel speed sensors goes to an electronic controller, which monitors each wheel's speed relative to the speed of the other wheels. As long as the brakes are not being applied and all of the monitored wheels are rotating at roughly the same speed, the system takes no action. If, however, the brakes are being applied and one or more of the monitored wheels suddenly begins to reduce speed at a higher rate than the others—indicating a loss of traction with the road and an imminent wheel lockup and skid—the controller then activates the antilock system.
The antilock brake system on any vehicle is simply an additional monitoring and controlling function superimposed on the existing vehicle brake system. ABS is not a second brake system, nor does it replace the vehicle brake system. When all four wheels on an automobile are monitored and controlled, the system is called a four-channel ABS. If the front two wheels plus the rear axle (but not each rear wheel individually) are to be controlled, the system is called a three-channel ABS. On heavy trucks with two rear drive axles, the ABS is commonly a four-channel system which controls the front wheels and two of the four rear wheels. Trailers pulled by heavy truck tractors may also have their own separate ABS which must interconnect with the ABS on the tractor. In an automobile, the brakes are actuated by hydraulic pressure. The ABS controller operates solenoid valves built into the high pressure side of the master brake cylinder. These valves are normally open and do not interfere with braking. When the controller senses that a wheel is locking up while braking, it first activates a solenoid to close a valve in the affected
wheel's brake line which prevents the pressure from increasing any further. If the locked wheel continues to lose speed, the controller activates a second solenoid which bleeds pressure off the affected brake line, in effect releasing the brake for that wheel regardless of whether the driver is still pushing on the brake pedal. As soon as the wheel regains traction and its speed increases, the solenoids are de-activated and normal braking resumes. Of course, if the conditions are such that the wheel starts to skid again, the brake will promptly begin to lock up and the ABS will take over. This cycle is repeated 12 to 15 times per second until either the road condition changes or the driver releases the brakes. The driver will be able to detect this rapid cycling as a vibration felt through the brake pedal, but will not have to take any action. The ABS will minimize the skid and will allow the driver to maintain directional control of the vehicle. The brakes on a heavy truck are actuated by air pressure, rather than hydraulic pressure. The antilock brake system on a truck works in a manner similar to the ABS on an automobile, except the antilock air pressure control valves are located on the vehicle frame rail, near each wheel.
Components WHEEL SPEED SENSORS
The wheel speed sensors (WSS) consist of a magnetic pickup and a toothed sensor ring (sometimes called a "tone" ring). The sensor(s) may be mounted in the steering knuckles, wheel hubs, brake backing plates, transmission tail shaft or differential housing. On some applications, the sensor is an integral part of the wheel bearing and hub assembly. The sensor ring(s) may be mounted on the axle hub behind the brake rotor, on the brake rotor itself, inside the brake drum, on the transmission tail shaft or inside the differential on the pinion shaft. The wheel speed sensor pickup has a magnetic core surrounded by coil windings. As the wheel turns, teeth on the sensor ring move through the pickup magnetic field. This reverses the polarity of the magnetic field and induces an alternating current (AC) voltage in the pickup windings. The number of voltage pulses per second that are induced in the pickup changes in direct proportion to wheel speed. So as speed increases, the frequency and amplitude of the wheel speed sensor goes up. The WSS signal is sent to the antilock brake control module, where the AC signal is converted into a digital signal and then processed. The control module then counts pulses to monitor changes in wheel speed.
On applications where the wheel speed sensor is not part of the hub or wheel bearing assembly, it can be replaced if defective. Sensor problems can be caused by an accumulation of debris on the end (they are magnetic), incorrect air gap or faults in the wiring or connectors. ABS CONTROL MODULE
The ABS electronic control module (which may be referred to as an EBCM "Electronic Brake Control Module" or EBM "Electronic Brake Module") is a microprocessor that functions like the engine control computer. It uses input from its sensors to regulate hydraulic pressure during braking to prevent wheel lockup. The ABS module may be located in the trunk, passenger
compartment or under the hood. It may be a separate module or integrated with other electronics such as the body control or suspension computer. On the newer ABS systems (Delphi DBC-7, Teves Mark 20, etc.), it is mounted on the hydraulic modulator. The key inputs for the ABS control module come from the wheel speed sensors and a brake pedal switch. The switch signals the control module when the brakes are being applied, which causes it to go from a "standby" mode to an active mode. When ABS braking is needed, the control module kicks into action and orders the hydraulic unit to modulate brake pressure as needed. On systems that have a pump, it also energizes the pump and relay.
Like any other electronic control module, the ABS module is vulnerable to damage caused by electrical overloads, impacts and extreme temperatures. The module can usually be replaced if defective, except on some of the newest systems where the module is part of the hydraulic modulator assembly. HYDRAULIC MODULATOR
The hydraulic modulator or actuator unit contains the ABS solenoid valves for each brake circuit. The exact number of valves per circuit depends on the ABS system and application. Some have a pair of on-off solenoid valves for each brake circuit while others use a single valve that can operate in more than one position. On Delco VI ABS systems, small electric motors are used in place of solenoids to drive pistons up and down to modulate brake pressure. On some systems, the individual ABS solenoids can be replaced if defective, but on most applications the modulator is considered a sealed assembly and must be replaced as a unit if defective. Hydraulic modulator has a hydraulic modulator block including a reservoir and a damper; and an electronic control block detachably attached to the surface of the hydraulic modulator block. The reservoir and the damper are overlapped with each other such that a surface of the hydraulic modulator block becomes entirely substantially flat. With this, the hydraulic modulator becomes simple in construction. The hydraulic modulator has a solenoid valve; an electronic control circuit board; and an electric wiring pattern prepared by pressing a metal plate having a first major surface formed with a first tin layer and a
nickel layer and a second major surface formed with a second tin layer, into a three-dimensional shape such that the electric wiring pattern is formed with (1) a first terminal having a surface that is formed with the first or second tin layer electrically connected with the solenoid valve, (2) a connector terminal having first and second surfaces respectively formed with the first and second tin layers, and (3) a second terminal having a surface that is formed with the nickel layer electrically connected with the electronic control circuit board. Thus, each terminal has a secure electrical connection with another member.
PUMP & ACCUMULATOR
A high pressure electric pump is used in some ABS systems to generate power assist for normal braking as well as the reapplication of brake pressure during ABS braking. In some systems, it is used only for the reapplication of pressure during ABS braking. The pump motor is energized via a relay that is switched on and off by the ABS control module. The fluid pressure that is generated by the pump is stored in the "accumulator." The accumulator on ABS systems where the hydraulic modulator is part of the master cylinder assembly consists of a pressure storage chamber filled with nitrogen gas. Should the pump fail (a warning light comes on when reserve pressure drops too low), there is usually enough reserve pressure in the accumulator for 10 to 20 power-assisted stops. After that, there is no power assist. The brakes still work, but with increased effort. On ABS systems that have a conventional master cylinder and vacuum booster for power assist, a small accumulator or pair of accumulators may be used as temporary holding reservoirs for brake fluid during the hold-release-reapply cycle. This type of accumulator typically uses a spring loaded diaphragm rather than a nitrogen charged chamber to store pressure.
Anti-Lock Brake Types There are several ABS systems used by the auto manufacturers, every make and model chooses what system is best suitable for their vehicle depending on size and weight, we will show you the most common systems used today.
• Open and closed systems: Open anti-lock system : Open anti-lock system is one in which the brake fluid released from the brakes during ABS stop is not returned to the brake during the ABS stop; instead, the fluid is stored in an accumulator during the ABS stop, then returned to the master cylinder reservoir afterwards. A disadvantage of the open systems is that the brake pedal will drop during a long ABS stop as fluid flows from the brake lines. Some open systems have a pump that restores fluid to the master cylinder to keep the pedal from sinking, but the pump is not involved in the actual anti-lock function. This type is used in simple-real wheel-only ABS designs. Closed system: Closed system has some means, generally an electrically powered pump, to restore hydraulic pressure that's bled off during an ABS stop. The pump supplies fluid to an accumulator, where it's stored under pressure until is needed to increase brake line pressure. In some cases, pump pressure is applied to the brakes during the ABS stop, with the
amount and timing of pressure application controlled by a solenoid valve.
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By The Number Of Channels : Anti-lock braking systems by the number of channels -- that is, how many valves that are individually controlled -- and the number of speed sensors
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.
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.
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
• Integrated systems and Non-integrated systems Integrated systems: An integrated system gets its name from the fact that the major hydraulic components like the brake booster and the hydraulic modulator are integrated into a unit with the master cylinder. Other components, such as the accumulator and hydraulic modulator, may also be part of the assembly. Many of these systems have no vacuum booster. In such systems, the ABS pump provides brake boost as well as the pressure necessary for anti-lock brake operation. The pump forces fluid into one or more accumulators, where is stored at very high pressures, typically 2000 to 3000 psi until it is needed. On systems without a vacuum booster, the booster is a valve, controlled by the driver's foot on the brake pedal, which regulates the amount of boost applied. Non-integrated systems:
Non-integrated systems, also known as "add-on" ABS, are installed in conventional brake systems between the master cylinder and the wheel brakes. A vacuum booster is used. The master cylinder is very much alike, or in some cases identical, to the master cylinder used with nonanti-lock brakes. The hydraulic modulator is installed near the master cylinder. The brake fluid lines from the master cylinder connect to the hydraulic modulator. Brake lines run from the hydraulic modulator to each of the wheel brakes. During normal braking, it's as if the hydraulic modulator weren't there, hydraulic pressure from the master cylinder flows uninterrupted through the modulator to the brakes. During an ABS stop, the hydraulic modulator rapidly changes the hydraulic pressure at the wheel brakes, holding it steady, reducing it, or letting it increase. Fluid pressure is reduced by allowing some of the high pressure to return to its source. This low pressure fluid in an ABS system is commonly referred as "decayed" fluid.
• Hydraulic circuits: Primary: The primary circuit is normally operated by the master cylinder piston closest to the rear of the master cylinder, and thus in direct contact with the booster pushrod. Secondary: The secondary circuit is operated by the master cylinder piston closest to the front of the master cylinder.
In the case of front-rear split circuits, the primary circuit operates both front brakes and the secondary circuit operates both rear brakes, in diagonally split circuits, the primary operates one front brake and diagonally opposite rear, while the secondary operates the remaining two wheels.
Additional developments Modern Electronic Stability Control (ESC or ESP) systems are an evolution of the ABS concept. Here, a minimum of two additional sensors are added to help the system work: these are a steering 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 does not coincide with what the steering wheel sensor reports, the ESC software will brake the necessary individual wheel(s) (up to three with the most sophisticated
systems), so that the vehicle goes the way the driver intends. The steering wheel sensor also helps in the operation of Cornering Brake Control (CBC), since this will tell the ABS that wheels on the inside of the curve should brake more than wheels on the outside, and by how much
Disadvantages • Increased braking distances under some limited circumstances (ice, snow, gravel, "soft" surfaces), • Creation of a "false sense of security" among drivers who do not understand the operation, and limitations of ABS. •
The anti-lock brakes are more sensitive on the damper condition. the influence of the worn components on the performance of the vehicle with anti-lock brakes is more significant than without anti-lock brakes, the stopping distance with defective shocks is by meters longer for the presented simulation scenario.
