Antilock braking system report


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Antilock braking system report

  3. 3. P3 SRAM Cell23 CONCLUSION 1824 REFRENCES 19 Introduction A brake is one of the most important parts of any type of vehicle. Brake is used to retard or stop a vehicle. Here Kinetic energy transferred into Heat energy. The kinetic energy increases with the square of the velocity. So, K.E.=1/2mv² .An anti-lock braking system is a safety system that allows the wheels on a motor vehicle to continue interacting tractively 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. Stopping a car in a hurry on a slippery road can be very challenging. Anti-lock braking systems (ABS) take a lot of the challenge out of this sometimes nerve-wracking event. In fact, on slippery surfaces, even professional drivers cant stop as quickly without ABS as an average driver can with ABS. 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. 3
  4. 4. P3 SRAM CellHISTORY 1936-: Bosch and Mercedes-Benz partner - R&D into ABS. 1972: WABCO partners with Mercedes-Benz developing first ABS for trucks. 1978: First production-line installation of ABS into Mercedes and BMW vehicles. 1981: 100,000 Bosch ABS installed. 1936: German company Bosch is awarded a patent for “Apparatus for preventing lock-braking of wheels 1985: First ABS installed on US vehicles. 1986: 1M Bosch ABS installed. 1987: Traction control - in conjunction with ABS - used on passenger vehicles. 1989: ABS hydraulic unit combined with standard hydraulic brake unit. 1992: 10M Bosch ABS installed. 1995: Electronic Stability - in conjunction with ABS and TCS - for passenger cars. 1999: 50M Bosch ABS installed. 2000: 6 of 10 new cars on the road are ABS equipped. 2003: 100M Bosch ABS installed. 4
  5. 5. P3 SRAM Cell HOW IT WORKSA 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 constantlymonitors the rotational speed of each wheel; if it detects a wheel rotating significantlyslower than the others, a condition indicative of impending wheel lock, it actuates thevalves to reduce hydraulic pressure to the brake at the affected wheel, thus reducing thebraking force on that wheel. The wheel then turns faster. Conversely, if the ECU detects a wheel turningsignificantly faster than the others, brake hydraulic pressure to the wheel is increased sothe braking force is reapplied, slowing down the wheel. This process is repeatedcontinuously and can be detected by the driver via brake pedal pulsation. Some anti-locksystem can apply or release braking pressure 16 times per second. 5
  6. 6. P3 SRAM Cell The ECU is programmed to disregard differences in wheel rotative speed below acritical threshold, because when the car is turning, the two wheels towards the centre ofthe curve turn slower than the outer two. For this same reason, a differential is used invirtually all roadgoing vehicles. If a fault develops in any part of the ABS, a warning light will usually beilluminated on the vehicle instrument panel, and the ABS will be disabled until the faultis rectified. The modern ABS applies individual brake pressure to all four wheels through acontrol system of hub-mounted sensors and a dedicated micro-controller. ABS is offeredor comes standard on most road vehicles produced today and is the foundation for ESCsystems, which are rapidly increasing in popularity due to the vast reduction in price ofvehicle electronics over the years. Modern electronic stability control (ESC or ESP) systems are an evolution of theABS concept. Here, a minimum of two additional sensors are added to help the systemwork: these are a steering wheel angle sensor, and a gyroscopic sensor. The theory ofoperation is simple: when the gyroscopic sensor detects that the direction taken by the cardoes not coincide with what the steering wheel sensor reports, the ESC software willbrake 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 helpsin the operation of Cornering Brake Control (CBC), since this will tell the ABS thatwheels on the inside of the curve should brake more than wheels on the outside, and byhow much. The ABS equipment may also be used to implement a traction controlsystem(TCS) on acceleration of the vehicle. If, when accelerating, the tire loses traction,the ABS controller can detect the situation and take suitable action so that traction isregained. More sophisticated versions of this can also control throttle levels and brakessimultaneously. 6
  7. 7. P3 SRAM CellCOMPONENTSThere are four main components to an ABS: speed sensors, valves, a pump, and acontroller. 7
  8. 8. P3 SRAM Cell Speed Sensor :These sensors use a magnet and a coil of wire to generate a signal. The rotation of thewheel or differential induces a magnetic field around the sensor. The fluctuations of thismagnetic field generate a voltage into the sensor. A schematic of this system is shown infigure below. The ABS controller interprets this signal.Since the voltage inducted on the sensor is a result of the rotating wheel, thissensor can become inaccurate at slow speeds. The slower rotation of thewheel can cause inaccurate fluctuations in the magnetic field and thus causeinaccurate readings to the controller. 8
  9. 9. P3 SRAM Cell Valves : The valves within an ABS serve three distinct functions. The first function of thevalves is to open and allow the hydraulic fluid from the brake pedal or the pump to reachthe braking system. The second function of the valves is to maintain the current pressureprovided to the braking system. This is accomplished by closing the valve to resist furtherpressure from the brake pedal. The third function of these valves is to reduce the amountof hydraulic pressure at the braking system. This is accomplished by opening the valvesto allow the hydraulic fluid to be released from the braking system. A picture of astandard ABS valve and pumping system is show in figure below. The majority of problems with the valve system occur due to clogged valves.When a valve is clogged it is unable to open, close, or change position An inoperable valve will prevent the system from modulating the valves andcontrolling pressure supplied to the brakes. 9
  10. 10. P3 SRAM Cell Pumps : The pump in the ABS is used to restore the pressure to the hydraulic brakes aftervalves have released it. A signal from the controller will release the valve at the detectionof wheel slip. After a valve releasethe pressure supplied from the user, the pump is used to restore a desired amount ofpressure to the braking system. The controller will modulate the pumps status in order toprovide the desire amount of pressure and reduce slipping. A picture of the pumpingsystemis shown in figure above.Similar to the valves, the major limitation or mode of failure is due toblockage within the pump. A blockage within the pump will prevent thepump from supplying the correct pressure to the pumping system. Controller : 10
  11. 11. P3 SRAM Cell The controller is an ECU type unit in the car which receives information from each individual wheel speed sensor, in turn if a wheel loses traction the signal is sent to the controller, the controller will then limit the brakeforce (EBD) and activate the ABS modulator which actuates the braking valves on and ff. Use of ABS There are many different variations and control algorithms for use in an ABS. One of the simpler systems works as follows: 1. 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 a 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 km/h) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.2. The ABS controller knows that such a rapid deceleration is impossible, so it reduces thepressure to that brake until it sees an acceleration, then it increases the pressure until it sees thedeceleration again. It can do this very quickly, before the tire can actually significantly changespeed. The result is that the tire slows down at the same rate as the car, with the brakes keeping thetires very near the point at which they will start to lock up. This gives the system maximumbraking power. 3. When the ABS system is in operation the driver will feel a pulsing in the brakepedal; this comes from the rapid opening and closing of the valves. This pulsing also tells the 11
  12. 12. P3 SRAM Celldriver that the ABS has been triggered. Some ABS systems can cycle up to 16 times per second . 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. There are mainly three types of ABS. 12
  13. 13. P3 SRAM Cell Four-channel, Four-sensor ABS This is the best scheme. There is a speed sensor on all four wheels and a separatevalve for all four wheels. With this setup, the controller monitors each wheel individuallyto 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 speedsensor and a valve for each of the front wheels, with one valve and one sensor for bothrear 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 bothachieve maximum braking force. The rear wheels, however, are monitored together; theyboth have to start to lock up before the ABS will activate on the rear. With this system, itis possible that one of the rear wheels will lock during a stop, reducing brakeeffectiveness.One-channel, One-sensor ABSThis 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 rearwheels are monitored together and they both have to start to lock up before the ABSkicks in. In this system it is also possible that one of the rear wheels will lock, reducingbrake effectiveness. This system is easy to identify. Usually there will be one brake line going through aT-fitting to both rear wheels. You can locate the speed sensor by looking for an electricalconnection near the differential on the rear-axle housing. 13
  14. 14. P3 SRAM CellEffectivenessA 2003 Australian study by Monash University Accident Research Centre found thatABS: • Reduced the risk of multiple vehicle crashes by 18 %. • Reduced the risk of run-off-road crashes by 35%.. 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 thatwould be easily possible without the benefit of ABS. In real world conditions even analert, skilled driver without AB even through the use of techniques like threshold braking,to match or improve on the performance of a typical driver with a modern ABS-equippedvehicle. ABS reduces chances of crashing, and/or the severity of impact. Therecommended 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, whereappropriate, to steer around obstructions. In such situations, ABS will significantlyreduce the chances of a skid and subsequent loss of control. In gravel, sand and deep snow, ABS tends to increase braking distances. On thesesurfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this fromoccurring. Some ABS calibrations reduce this problem by slowing the cycling time, thusletting the wheels repeatedly briefly lock and unlock. Some vehicle manufacturersprovide an "off-road" button to turn ABS function off. The primary benefit of ABS onsuch surfaces is to increase the ability of the driver to maintain control of the car rather 14
  15. 15. P3 SRAM Cellthan go into a skid, though loss of control remains more likely on soft surfaces like gravelor slippery surfaces like snow or ice. On a very slippery surface such as sheet ice orgravel, it is possible to lock multiple wheels at once, and this can defeat ABS (whichrelies on comparing all four wheels, and detecting individual wheels skidding).Availability of ABS relieves most drivers from learning threshold braking. A June 1999 National Highway Traffic Safety Administration (NHTSA) study foundthat ABS increased stopping distances on loose gravel by an average of 22 % accordingto the NHTSA, "ABS works with your regular braking system by automatically pumping them. Invehicles not equipped with ABS, the driver has to manually pump the brakes to preventwheel lockup. In vehicles equipped with ABS, your foot should remain firmly planted onthe brake pedal, while ABS pumps the brakes for you so you can concentrate on steeringto safety." When activated, some earlier ABS systems caused the brake pedal to pulsenoticeably. As most drivers rarely or never brake hard enough to cause brake lock-up,and a significant number rarely bother to read the cars manual, this may not bediscovered until an emergency. When drivers do encounter an emergency that causesthem to brake hard, and thus encounter this pulsing for the first time, many are believedto reduce pedal pressure, and thus lengthen braking distances, contributing to a higherlevel of accidents than the superior emergency stopping capabilities of ABS wouldotherwise promise. Some manufacturers have therefore implemented a brakeassist system that determines that the driver is attempting a "panic stop" (by detecting thatthe brake pedal was depressed very fast, unlike a normal stop where the pedal pressurewould usually be gradually increased, Some systems additionally monitor the rate at theaccelerator was released) and the system automatically increases braking force where notenough pressure is applied. Hard or panic braking on bumpy surfaces, because of thebumps causing the speed of the wheel(s) to become erratic may also trigger the ABS.Nevertheless, ABS significantly improves safety and control for drivers in most on-roadsituations. 15
  16. 16. P3 SRAM Cell Anti-lock brakes are the subject of some experiments centred arround riskcompensation theory, which asserts that drivers adapt to the safety benefit of ABS bydriving more aggressively. In a Munich study, half a fleet of taxicabs was equipped withanti-lock brakes, while the other half had conventional brake systems. The crash rate wassubstantially the same for both types of cab, and Wilde concludes this was due to driversof ABS-equipped cabs taking more risks, assuming that ABS would take care of them,while the non-ABS drivers drove more carefully since ABS would not be there to help incase of a dangerous situation. A similar study was carried out in Oslo, with similarresults. • Do anti-lock brakes really work? Anti-lock brakes really do help you stop better. They prevent wheels from locking upand provide the shortest stopping distance on slippery surfaces. But do they really preventaccidents? This is the true measure of the effectiveness of ABS systems. The Insurance Institute for Highway Safety (IIHS) has conducted several studiestrying to determine if cars equipped with ABS are involved in more or fewer fatalaccidents. It turns out that in a 1996 study, vehicles equipped with ABS were overall noless likely to be involved in fatal accidents than vehicles without. The study actuallystated that although cars with ABS were less likely to be involved in accidents fatal to theoccupants of other cars, they are more likely to be involved in accidents fatal to theoccupants of the ABS car, especially single-vehicle accidents. There is much speculation about the reason for this. Some people think that drivers ofABS-equipped cars use the ABS incorrectly, either by pumping the brakes or by releasingthe brakes when they feel the system pulsing. Some people think that since ABS allowsyou to steer during a panic stop, more people run off the road and crash. 16
  17. 17. P3 SRAM CellElectronic Stability Control 17
  18. 18. P3 SRAM CellAdvantages It allows the driver to maintain directional stability and control over steering during braking Safe and effective Automatically changes the brake fluid pressure at each wheel to maintain optimum brake performance. ABS absorbs the unwanted turbulence shock waves and modulates the pulses thus permitting the wheel to continue turning under maximum braking pressure Disadvantages It is very costly. 18
  19. 19. P3 SRAM Cell Maintenance cost of a car equipped with ABS is more. Disadvantages It is very costly. Maintenance cost of a car equipped with ABS is more. Conclusion Statistics show that approximately 40 % of automobile accidents are due to skidding. These problems commonly occur on vehicle with conventional brake system which can be avoided by adding devices called ABS. If there is an ABS failure, the system will revert to normal brake operation. Normally the ABS warning light will turn on and let the driver know there is a fault.19
  20. 20. P3 SRAM CellRefrences• Prof. Raul. G. Longoria, ME, University of Texas, Austin• .Maruti Suzuki Swift Dzire VXi has hit the roads with powerful Anti-Lock Braking System, an essential safety feature.• Maruti Suzuki Swift Dzire VXi by devika rajpali / Electronics community . 20
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