Anti-lock Braking System

Presentation Outline What is an Anti-Lock Braking System (ABS)? Motivation for ABS Development History of ABS Overview Principles for ABS Operation ABS Components Overview ABS Components How does ABS work? System Diagram ABS Configurations Design Challenges Limitations Summary

1. What is ABS? “An anti-lock brake system is a feedback control system that modulates brake pressure in response to measured wheel deceleration, preventing the controlled wheels from becoming fully locked.”

2. Motivation for ABS Under hard braking, an ideal braking system should: provide the shortest stopping distances on all surfaces maintain vehicle stability and steerability Anti-lock braking systems were developed to best meet these needs.

3. History of ABS 1936: German company Bosch is awarded a patent for an “Apparatus for preventing lock-braking of wheels in a motor vehicle”. 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. 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. Overview Anti-Lock Braking Systems (ABS) are designed to maintain driver control and stability of the car during emergency braking. Locked wheels will slow a car down but will not provide steering ability. ABS allows maximum braking to be applied while retaining the ability to 'steer out of trouble‘ . The operation of ABS can slightly reduce stopping distance in some cases like on wet road surfaces, but it can increase the stopping distance in others, as may be the case in deep snow or gravel.

An ABS system monitors four wheel speed sensors to evaluate wheel slippage. Slip can be determined by calculating the ratio of wheel speed to vehicle speed, which is continuously calculated from the four individual wheel speeds. During a braking event, the function of the control system is to maintain maximum possible wheel grip on the road - without the wheel locking - by adjusting the hydraulic fluid pressure to each brake by way of electronically controlled solenoid valves.

For passenger car applications, the majority of ABS components are often housed together in a single, under-hood mounted module.

5. Principles for ABS Operation Design Goals: attain minimum stopping distance maintain stability and steerability Design Solution: Develop a system that rapidly modulates the braking force under hard braking conditions to: maintain the ideal tire slip percentage to maximize braking force (≈ 15%), recalling that Fbrake = µBN permit the vehicle to be steered with stability maintained by preventing skidding

6. ABS Components Overview Brake Master Cylinder, Hydraulic Modulator Unit with Pump and Valves Vehicle’s Physical Brakes Typical ABS Components: Wheel Speed Sensors (up to 4) Electronic Control Unit (ECU)

ABS Components Wheel Speed Sensor (WSS) Hydraulic Modulator Unit

a) Wheel Speed Sensor (WSS) Teeth on the sensor ring rotate past the magnetic sensor, causing a reversal of the magnetic field polarity, resulting in a signal with frequency related to the angular velocity of the axle.

Electronic Control Unit The signal from the WSS is proportional to angular velocity. By differentiating this signal, acceleration of each wheel is known. If a wheel is decelerating too quickly the brake pressure is modulated. A fifth input to the ECU is from a brake pedal switch. This signal can shift program execution from a standby to an active state

b) Hydraulic Modulator Unit The hydraulic modulator unit contains the ABS pump as well as solenoid valves for each brake line. The fifth line - far right - is from the brake master cylinder, which is connected to the brake pedal.

Valves and Brakes The valves modulate the brake pressure up to 20 times per second, effectively realizing the ideal tire slip percentage. ABS ‘pumps’ the brakes much faster than any driver could.

7. How does ABS work? Basically, there are sensors at each of the four wheels (or in the case of the less sophisticated three-channel system, one on each of the fronts and only one for the pair of rears). These sensors watch the rotation of the wheels. When any one of the wheels stops rotating due to too much brake application, the sensors tell the car's computer, which then releases some of the brake line pressure that you've applied - allowing the wheel to turn again. Then, just as fast as it released the pressure, the computer allows the pressure to be applied again - which stops the rotation of the wheel again. Then it releases it again. And so on. With most ABS, this releasing and re-application - or pulsing - of the brake pressure happens 20 or more times per second. Practically speaking, this keeps the wheel just at the limit - the threshold - before locking up and skidding. ABS prevents you from ever locking up the brakes and skidding - no matter how hard you apply the brakes. Obviously, this is going to mean much more steering control. With ABS, all you have to do in an emergency is quickly squeeze the brake pedal as hard as you can and hold it there. And when I say hard, I mean HARD. Let the system do the finesse work for you. This may not be as easy as it sounds. After years of being told (and practising) not to press too hard on the brake pedal, this may not feel very natural.

8. System Diagram

Depending on the ABS application, there are several typical layouts. 9. ABS Configurations Depending on the ABS application, there are several typical layouts.

10. Design Challenges Because of the safety-related nature of the ABS application, special emphasis is placed on functions designed to detect system faults and ensure that a fail-safe state occurs during faults. These functions may be implemented with techniques such as dual processors and specialized self test and watchdog modules. ABS systems typically include eight electric solenoid valves and other electrical loads, such as relays and pump motors, that require considerable drive currents. Electronic drive circuitry is often required to sense any failure of these components during operation.

11. Differences between ABS and Rear Anti-lock Brakes. Finally, there is a great difference between ABS and Rear Anti-lock Brakes. ABS is on all four wheels. Rear Anti-lock Brakes, as the name suggests, are only on the rear wheels. This system is often used on pick-up trucks and vans, and is a less-expensive compromise. It is used because the weight load on the rear of a truck or van can vary so much. The brakes needed to handle a heavily loaded vehicle are too effective for when it is lightly loaded. Therefore, the rear brakes are much more susceptible to locking-up.

12. Limitations Three points should be obvious, but don't appear to when looking at the type of crashes some drivers have with ABS-equipped vehicles. Contrary to popular belief, ABS :- does not allow you to drive faster; does not allow you to brake later; and does not allow you to corner faster.

13. Summary An ABS typically consists of: Up to 4 wheel speed sensors An ABS ECU A Brake Master Cylinder, with accompanying Hydraulic Modulator Unit and Solenoid Valves The vehicle’s physical brakes An ABS is designed to modulate braking pressure to attain the peak coefficient of kinetic friction between the tire and the road, and to avoid total wheel lockup. An ABS, under hard braking conditions, is designed to reduce stopping distances under most conditions, while maintaining vehicle stability and steerability.

You have to understand that ABS does not raise the traction limits of your vehicle. It only helps you stay within the limits. Period. So, don't get caught up in believing that ABS will allow you to drive any faster, brake later or go around a corner any faster - or allow you to safely follow any closer.

Presented by :- Sambit Sen E.C.E.,6th Sem. 5340187