ANTILOCK BRAKING SYSTEM

ABSTRACT

Anti Lock Braking system is a system on motor vehicles, which prevents the vehicle from locking while braking. Anti Lock Braking system (ABS) is a very important safety feature, because vehicles become very unstable when the vehicles are locked, braking becomes inefficient, and so braking distances become very long. A typical ABS is composed of central electronic unit, four speed sensors, and two or more hydraulic valves on the brake circuit. The electronic unit constantly monitors the rotational speed of each wheel. When it sensors that one or more wheel is rotating slower than the others moves the valves to decrease the pressure on the braking circuit, reducing the braking effect on that wheel.
The ABS equipment may also be used to implement traction control on acceleration of the vehicle. If when accelerating, the tire looses traction with the ground, the ABS controller can detect the situation and apply brakes to reduce the acceleration so that traction is regained. Manufactures usually offer this as a separately price option even though the infrastructure is largely shared with ABS. More sophisticated versions of this can also control throttle levels and brakes simultaneously, leading to what is termed as the Electronic Stability Program.


INTRODUCTION




Anti Lock Braking system was first developed for aircraft. The first ABS for automobiles was developed by Robert Bosch, becoming available in the year 1978. They first appeared in trucks and limousines from Mercedes-Benz. Systems were later introduced on motorcycles. Like a conventional braking system, ABS is more effective when the tires are in good operating condition with good tread and correct tire pressure is maintained. Like any vehicle whether equipped with ABS or not proper seat adjustment is important. Position yourself comfortably for maximum effectiveness with your ABS. For commercial applications the load should be evenly distributed over the rear axle for maximum effectiveness of the ABS.



PRINCIPLES OF SIMPLE BRAKING SYSTEM


The basic principle of brake system is:

1) Friction: Friction is a measure of how hard it is to slide one object over another. The brakes transmit the force to the tires using friction, and the tires transmit that force to the road using friction also.
2) Hydraulics: The basic idea behind any hydraulic system is very simple. Force applied at one point is transmitted to another point using an incompressible fluid, almost always an oil of some sort.
3) Leverage: The pedal is designed in such a way that it can multiply the force from your leg several times before any force is even transmitted to the brake fluid.


In the figure above, a force F is being applied to the left end of the lever. The left end of the lever is twice as long (2X) as the right end (X). Therefore, on the right end of the lever a force of 2F is available, but it acts through half of the distance (Y) that the left end moves (2Y). Changing the relative lengths of the left and right ends of the lever changes the multipliers.

TYPICAL BRAKING SYSTEM




1) Disc Brakes: Disc brakes use a clamping action to produce friction between the rotor and the pads mounted in the caliper attached to the suspension members. Inside the calipers, pistons press the pads due to pressure generated in the master cylinder. The pads then rub against the rotor, slowing the vehicle.
2) Drum Brakes: The working parts of a drum brake are contained inside the brake drum. The drum is attached to the hub of the wheel and revolves with it. Inside the drum are pair of curved brake shoes that are held close to the drum by retractor springs. The shoes and actuator linkages are mounted to the backing plate behind the drum. When the brake pedal is pressed, fluid is pressurized in the wheel cylinder. Pistons in the wheel cylinder then push outwards against shoes, overcoming the retractor spring tension and pressing the shoes against the drum. The friction of the shoes against the drum slows down the wheel. When pressure is removed from the brake pedal, the retractor springs force the shoes back to their normal position.
3) Brake Lines: Brake lines are tubing with copper and lead coatings to prevent rust and corrosion.
4) Booster: The brake booster is a vacuum powered or hydraulic device that
is attached to the master cylinder in what are referred to as power brake system. Brake boosters are designed to assist braking force from the pedal.
5) Brake Pedal: Brake pedal is situated between the accelerator and clutch
pedal. The brake pedal is connected to the master cylinder via push rods. The
pressure created by pressing the pedal is passed to the master cylinder which
in turn pushes the shoes and pads against the brake drums and rotors, thus
slowing the wheel.
6) Emergency Brakes: Emergency brake is a cable-activated system
used to hold the brakes continuously in the applied position. They are also
known as hand brakes. They are self adjusting in most of the systems.
7) Master Cylinder: To increase safety, most modern car brake system
are broken into two circuits, with two wheels on each circuit. If a fluid leak
occurs in one circuit, only two of the wheels will lose their brakes and your
car will still be able to stop when you press the brake pedal. The master
cylinder supplies pressure to both circuits of the car.
In the figure below, the plastic tank you see is the
brake-fluid reservoir, the master cylinder's brake-fluid source. The
electrical connection is a sensor that triggers a warning light when the brake
fluid gets low.







As you'll see here, there are two pistons and two springs inside the cylinder.





When you press the brake pedal, it pushes on the primary piston through a linkage. Pressure builds in the cylinder and lines as the brake pedal is depressed further. The pressure between the primary and secondary piston forces the secondary piston to compress the fluid in its circuit. If the brakes are operating properly, the pressure will be the same in both circuits.

When the first circuit leaks, the pressure between the primary and secondary cylinders is lost. This causes the primary cylinder to contact the secondary cylinder. Now the master cylinder behaves as if it has only one piston. The second circuit will function normally, but the driver will have to press the pedal further to activate it. Since only two wheels have pressure, the braking power will be severely reduced.

8) Combination Valve:
The valve does the job of three separate devices:
• The metering valve
• The pressure differential switch
• The proportioning valve

Metering Valve:
The metering valve section of the combination valve is required on cars that have disc brakes on the front wheels and drum brakes on the rear wheels. The disc brake pad is normally in contact with the disc, while the drum brake shoes are normally pulled away from the drum. Because of this, the disc brakes are in a position to engage before the drum brakes when you push the brake pedal down.
The metering valve compensates for this, making the drum brakes engage just before the disc brakes. The metering valve does not allow any pressure to the disc brakes until a threshold pressure has been reached. The threshold pressure is low compared to the maximum pressure in the braking system, so the drum brakes just barely engage before the disc brakes kick in.
Having the rear brakes engage before the front brakes provides a lot more stability during braking. Applying the rear brakes first helps keep the car in a straight line.
Pressure Differential Switch:
The pressure differential valve is the device that alerts you if you have a leak in one of your brake circuits. The valve contains a specially shaped piston in the middle of a cylinder. Each side of the piston is exposed to the pressure in one of the two brake circuits. As long as the pressure in both circuits is the same, the piston will stay centered in its cylinder. But if one side develops a leak, the pressure will drop in that circuit, forcing the piston off-center. This closes a switch, which turns on a light in the instrument panel of the car. The wires for this switch are visible in the picture above.

Proportioning Valve:
The proportioning valve reduces the pressure to the rear brakes. Regardless of what type of brakes a car has, the rear brakes require less force than the front brakes.
The amount of brake force that can be applied to a wheel without locking it depends on the amount of weight on the wheel. More weight means more brake force can be applied. If you have ever slammed on your brakes, you know that an abrupt stop makes your car lean forward. The front gets lower and the back gets higher. This is because a lot of weight is transferred to the front of the car when you stop. Also, most cars have more weight over the front wheels to start with because that is where the engine is located.
If equal braking force were applied at all four wheels during a stop, the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force. If the proportioning valve were set to 70 percent and the brake pressure were 1,000 pounds per square inch (psi) for the front brakes, the rear brakes would get 700 psi.


















Terminology in ABS:

1) ABS: Stands for Anti-Lock Braking system.

2) ABS Actuator: The work hose of the ABS system, it’s the piece that actually releases pressure in the ABS channel to modulate the brakes.

3) ABS Channel: The channel is the hydraulic lines from the ABS actuator to the wheels. ABS channel can consist of 1 or 2 wheel that will pulse/modulate when the ABS actuator performs its work.

4) ABS Sensor: It measures the speed of the wheel.

5) ABS Pulse or Modulation: When the ABS actuator engages and pulses/modulates the brake pressure being applied to the ABS channel. This is the method by which ABS works. When brakes pulses, they are prevented from locking because they rotate for a split second.










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. 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:
• Speed sensors
• Pump
• Valves
• Controller

Speed Sensor:
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:
• In position one, the valve is open; pressure from the master cylinder is passed right through to the brake.
• 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.
• 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.



Types of ABS:
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.
• 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.


Advancements:
1) Traction Control system: In order to maintain directional stability and prevent the vehicle breaking away, the wheels should neither lock up during braking, nor should they spin when driving off. Wheel-speed sensors continuously monitor each wheel. TCS evaluates the wheel-speed sensor signals and immediately recognizes when a wheel is about to start slipping. In this case TCS is activated and regulates the drive torques of the driven wheels. This electronic wheel-slip control selectively brakes individual wheels and intervenes in the engine management.
2) Intelligent Stability and Handling Systems: Intelligent stability and handling systems provide you with greater control of your vehicle when loss of control is imminent. These systems have sensors that detect the direction your vehicle is going and compare it to the direction you are steering the vehicle. When the system detects a discrepancy between your intended path and the direction the vehicle is actually traveling, the system will intervene to help bring the movement of the vehicle back in line with your intentions.


Warning Alarms in ABS:
When you start your vehicle an ABS indicator light will illuminate on the instrument panel for a few seconds. This is normal. When the ABS lights stays on, means the ABS is not working. However, the conventional braking system is working. We recommend that you take your vehicle to your nearest dealer for servicing as soon as possible.


Do’s of ABS:
1) Keep your foot on the brake. Maintain firm and continuous pressure on the brake while steering to enable four-wheel ABS to work properly. Avoid pumping the brake, even if the brake pedal is pulsating. In light trucks that are equipped with rear-wheel anti-lock brakes, however, the front wheels can still lock up the same as conventional brakes. If that happens, the driver should ease up on the brake pedal with just enough pressure to allow the front wheels to roll again so you can steer.
2) Allow enough distance to stop. Follow three seconds or more behind vehicles when driving in good conditions. Allow more time if conditions are hazardous.

3) Practice driving with ABS. Become accustomed to pulsations that occur in the brake pedal when ABS is activated. Empty parking lots or other open areas are excellent places to practice emergency stops.


Don’ts of ABS:

1) Drive an ABS-equipped vehicle more aggressively than vehicles without ABS. Driving around curves faster; changing lanes abruptly or performing other aggressive steering maneuvers is neither appropriate nor safe with any vehicle.

2) Pump the brakes. In four-wheel ABS-equipped vehicles, pumping the brake turns the system on and off. ABS pumps the brakes for you automatically, at a much faster rate, and allows better steering control.
3) Be alarmed by mechanical noises and/or slight pedal pulsations while applying the brake in an ABS-equipped vehicle.
These conditions are normal and let the driver know ABS is working.
Tips for driving with ABS:
1) Drive safely, because your anti-lock brakes are only as good as the driver using them. Anti-lock brakes cannot compensate for driving faster, more aggressively, or maintaining unsafe following distances. They cannot guarantee recovery from a spin or skid prior to an emergency braking situation. Avoid extreme steering maneuvers while your anti-lock brake system is engaged.
2) Anti-lock brake systems can stop more quickly than conventional brakes on wet paved surfaces and on icy or packed snow-covered roads. Stopping distances can be longer on loose gravel or freshly fallen snow, although drivers won't experience the lock-up of the wheels usually associated with conventional hard braking. Therefore, drivers will still have the ability to steer around objects in front of them—such as another car.


3) Remember that you can steer while you are braking with four-wheel anti-lock brake systems. Steer clear of hazards while keeping your foot firmly on the brake pedal. Be aware that your vehicle will not turn as quickly on a slippery road as it would on dry pavement.








Comparison Between 4wheel ABS, TCS, Intelligent Stability & Handling Systems:

4 Wheel ABS Traction Control Intelligent Stability and Handling Systems
Prevents wheel lock-up under many road conditions X X
Allows driver to maintain control when brakes are fully applied X X
Sensors detect impending wheel lock-up X X
Pumps the brakes like a driver would, only much faster and more effectively X X
Engages when the driver stomps on the brake pedal X X
Prevents unwanted wheel spin in low traction situations X X
Adjusts vehicle acceleration when driving in low-traction situations, such as rain or snow X X
Helps drivers accelerate safely X X
Detects a vehicle's position in relation to steering input with use of sensors X
Monitors and compares a vehicle's movement with the direction a driver is steering X
Automatically brakes specific wheels, allowing a driver to maintain steering control during a skid

Benefits of ABS:
1) Anti-lock braking systems (ABS) use electronic controls to prevent the wheels from locking.
2) Helps the driver to maintain better steering control, especially on uneven surfaces and wet pavement.
3) You can steer around the object that you are heading towards even while applying maximum brake force.

Limitations of ABS:
1) Cost of around $500n to $1000 is needed to add ABS to the car.
2) The maintenance cost of car equipped with ABS is more.
3) ABS cannot compensate for rash and fast driving.

Conclusion:
The car equipped with ABS has more advantage than the car with conventional braking system. But always keep in mind that ABS is as good as the driver using them. ABS cannot compensate for rash and fast driving. Like the conventional brake equipped car, the car having ABS should also be driven with care.
With the advent of new technologies like Traction Control Systems (TCS) and Intelligent Stability & Handling Systems (IS&HS), the possibilities of accidents can be drastically reduced. Though these newer technologies are costlier at present they are efficient tools to ensure safe driving. Soon these technologies will become mandatory for all vehicles.
Surveys done in various countries have shown that the car equipped with ABS is less susceptible to accidents. In developed countries ABS has become mandatory for all passenger cars as well as heavy vehicles. Soon ABS systems and its advanced versions are set to become an essential tool for ensuring safe driving


Bibliography:
1) Automobile Engineering by Kirpal Singh.
2) About ABS by Ken Prate.
3) How Anti Lock Braking System works by Karim Nice.
4) www.how stuffworks.com