Anti locking brake system

Abstract

                  In this fast moving world of ours, thing seem to get faster & faster day by day. Same is the case with the most common means of conveyance-cars. Every day novel technologies are developed to help the cars become more & more powerful implying that they carry more &more load at greater & greater velocities.

                 

                  However it is not enough to create machines that help you move fast, it is equally important that they should stop just as quickly as the move. This paper take an overview of a cutting edge technology developed in the form of ‘anti locking brakes (ABS)’.This new electronic brake assistance system aims at reducing the hazards of sudden retardation from high speeds.

               

                   It attempts to study one of the promising new age technology which assures safety for the new age fast moving automobile.

Introduction

           The basic function of any braking system in an automobile is to help the vehicle stop whenever & wherever required. Followingfigure shows the various components of a general brake system:

              Following are the different type of brakes that have been used over the years for this purpose:
 1] Disc brakes
The main components of a disc brake are:

• The brake pads
• The caliper, which contains a piston
• The rotor, which is mounted to the hub

                  The disc brake is a lot like the brakes on a bicycle. Bicycle brakes have a caliper, which squeezes the brake pads against the wheel. In a disc brake, the brake pads squeeze the rotor instead of the wheel, and the force is transmitted hydraulically  instead of through a cable. Friction  between the pads and the disc slows the disc down.

2] Drum brakes:
Like the disc brake, the drum brake has two brake shoes and a piston. But the drum brake also has an adjuster mechanism, an emergency brake mechanism and   springs.

             As the brake shoes contact the drum, there is a kind of wedging action, which has the effect of pressing the shoes into the drum with more force.
             The extra braking force provided by the wedging action allows drum brakes to use a smaller piston than disc brakes. But, because of the wedging action, the shoes must be pulled away from the drum when the brakes are released. This is the reason for some of the springs. Other springs help hold the brake shoes in place and return the adjuster arm after it actuates.

The ABS concept: 

                  ABS is an acronym for anti-lock braking system, one of the most significant safety advances in automotive engineering in recent decades. First developed and patented in 1936, ABS is actually derived from the German term "antiblockiersystem."

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 one is stuck on ice, one knows that if the wheels are spinning he has no traction. This is because the contact patch is sliding relative to the ice. By keeping the wheels from skidding while the vehicle slows down, anti-lock breaks benefits in two ways: the vehicle stops faster, and the user will be able to steer while he stops.

                 ABS can improve vehicle stability, steerability and stopping capability. Four-wheel ABS prevents wheel lock-up, allowing drivers to maintain stability as well as steering control during an emergency braking situation.

                When a driver operating a four-wheel ABS equipped vehicle steps firmly on the brake pedal, the system automatically modulates the brake pressure at all four wheels, adjusting pressure to each wheel independently to prevent wheel lock-up. With four-wheel ABS, the driver has improved steering control of the vehicle.

The need for ABS

             When the rear wheels of a vehicle lose traction, oversteer can occur. When the front wheels lose traction, it's called understeer. Either way, the driver can lose the ability to turn the car along the intended path.

            When sensors in intelligent stability and handling systems detect oversteer is imminent, the outside front wheel brake is automatically applied to prevent loss of control.         

             Likewise, when the sensors detect understeer is about to occur, they automatically brake the inside rear wheel, helping the driver make the turn and continue forward in the right direction.

 Components of ABS:

Anti-lock brake pump and valves

There are four main components to an ABS 

system:

                                                           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 diffrential , 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.

Working of ABS

                The anti-lock brake controller is also known as the CAB (Controller Anti-lock Brake).There are many different variations and control algorithms for ABS systems. Here, one of the simpler systems is discussed.

                 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 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 the user 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.

The EBD Assistance

             The force required at the front and rear brake pairs varies with the load carried by the vehicle and the urgency of the stop. Front brakes handle the majority of the action, because the vehicle’s weight shifts forward during braking. Rear brakes require less force overall but need more when the rear of the car is weighed down with passengers or cargo. Historically, engineers have designed front/rear proportioning valves to hit the median for rear brakes — enough force for some rear cargo but no so much that the wheels lock up when the car is unladen.

                On cars without EBD, then the driver presses the brake pedel, the force is evenly distributed to the four brakes. On cars with EBD, the brake force is proportioned to each wheel based on driving conditions and availiable traction. In an emergency straight-line braking situation, EBD would provide more brake pressure to the front to avoid the rear brakes from locking. If the driver brakes while steering a corner, it would control the brake forces between the left and right wheels to help maintain stability.

 Speed sensors located on all four wheels notice that one or more wheels have locked, or may lock, during strong braking. Instantaneously the Skid Control ECU calculates which wheels need to be released and which braked, to avoid a skid and signals this to the Brake Actuator. The Brake Actuator then distributes brake pressure to all four wheels as required, enabling you to steer your car under full control.

Types of Anti-Lock Brakes

           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.

             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.

 Benefits of ABS

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 apply the brakes to reduce the acceleration 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.

Detect a flat tire:
          An ABS (anti-lock braking system) is a system that helps a driver to avoid skids during panic stops. In a car with a normal braking system, all four wheels will lock and cause the car to skid if the driver jams on the brakes in a panic situation. The problems with skidding are:

1. The car will actually take longer to stop.
2. The driver loses all control of the vehicle.

          An anti-lock braking system lets a computer monitor the wheels. If one of them locks, the computer can pulse the brake on that wheel so that the wheel keeps spinning. Because the wheels continue to spin, the driver can continue to control the car with the steering  wheel.

           The computer senses rotation using a rotation sensor on each wheel. If the computer were programmed correctly and if there were a light on the dashboard, then the computer could detect a flat tire. What the computer could do is look at different rotational speeds for one out of the four wheels. A flat tire would spin faster than a properly inflated tire, so the computer would look for one tire spinning faster than the other three, on average, over the course of a period of time. Then it could warn the driver by activating the light on the dash.

Increase in tire life:

            As the ABS prevents skidding of the vehicle, the additional frictional wear occurring due to the braking action in prevented from being concentrated at a single position. Locked wheels on dry asphalt or concrete can quickly create flat spots on tires, which can cause an annoying vibration while driving. The big advantage, however, is the maintenance of the tire -- a significant factor in effective stopping.  

Disadvantages of ABS

Gravel and snow:

In gravel and 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.

A Finnish car magazine, Tekniikan Maailma, tested a VW Golf V fitted with non-studded Continental ContiVikingContact 3 tires.

Braking distance from 80-0 km/h:
	locked wheels	ABS
dry pavement	45 m	32 m
snow	53 m	64 m
ice	255 m	404 m

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.

Conclusion

                   The anti locking brake system provides us with an effective means to ensure that our new generation automobiles become safer as they continue to get faster & stronger.They provide the common man an opportunity to have a go at the new age monstrous engines which can churn out loads & loads of horsepowers just at a small movement of the toe.

                   It is a promising new concept which is catching up fast with the automobile owners and manufacturers the world over. The anti locking brake system certainly promises grater speeds and greater driving pleasures at lower risk levels.