1. Anti-lock Braking System (ABS)

2. Index What is an Anti-Lock Braking System (ABS)? History of ABS
Motivation for ABS Development
Overview
Principles for ABS Operation
ABS Components Overview
ABS Components
Subaru Impreza ABS Application.
How does ABS work?
How ABS Work (Video)
System Diagram
Anti-Lock Brake Types
ABS Configurations
Design Challenges
Advantages & Disadvantages
ABS Problems
General information
Summary
Common questions

3. What is (ABS)?
Anti-lock braking system (ABS) is an automobile safety system prevent the wheels of a vehicle locking as brake pedal pressure is applied - often suddenly in an emergency or short stopping distance. This enables the driver to have steering control, preventing skidding and loss of traction.

4. History of ABS
1929 :- ABS was first developed for aircraft by the French automobile and aircraft pioneer Gabriel Voisin, as threshold braking on airplanes is nearly impossible.
1936: German company Bosch is awarded a patent 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.

5. History of ABS 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.
Nowadays:- Almost all new cars have ABS.

6. Motivation for ABS Development
Under hard braking, an ideal braking system should:
Provide the shortest stopping distances
on all surfaces
Maintain vehicle stability and steer ability.

7. Figure 1. Sampling of ABS control
Overview
Many different control methods for ABS systems have been developed. These methods differ in their theoretical basis and performance under the changes of road conditions.
ABS Research
Classical Control
Optimal Control
Nonlinear Control
Robust Control
Adaptive Control
Intelligent Control
Figure 1. Sampling of ABS control

8. Principles for ABS Operation
When the brake pedal is depressed during driving, the wheel speed decreases and the vehicle speed does as well. The decrease in the vehicle speed, however, is not always proportional to the decrease in the wheel speed. The non-correspondence between the wheel speed and vehicle speed is called “slip” and the magnitude of the slip is expressed by the “slip ratio” which is defined as follows: Slip ratio = (Vehicle speed – Wheel speed)/Vehicle speed × 100% When the slip ratio is 0%, the vehicle speed corresponds exactly to the wheel speed. When it is 100%, the wheels are completely locking (rotating at a zero speed) while the vehicle is moving. See Fig 2.

9. Figure 2. Illustration of the relationship between braking coefficient and wheel slip

10. The best braking action occurs at between 10-20%.
If vehicle speed and wheel speed is the same wheel slippage is 0%
A lock-up wheel will have a wheel slippage of 100%
Slip ratio
(B) Coefficient of friction between tire and road surface
Icy road
Asphalt-paved road
(3) Control range by ABS

11. Principles for ABS Operation
Figure 2 shows the relationship between braking co-efficient and wheel slip. It is shown that the slide values for stopping/traction force are proportionately higher than the slide values for cornering/steering force. A locked-up wheel provides low road handling force and minimal steering force.

12. Principles for ABS Operation
The main benefit from ABS operation is to maintain directional control of the vehicle during heavy braking in rare circumstances

13. ABS Components Overview
Hydraulic unit.
Interconnecting wiring
Electronic brake control module (EBCM).
The ABS indicator
The rear drum brake.
Two system fuses.
Four wheel speed sensors.

14. ABS Components Overview
(1) ABS control module and hydraulic control unit (ABSCM & H/U).
(2) Two-way connector.
(3) Diagnosis connector.
(4) ABS warning light.
(5) Data link connector (for SUBARU select monitor).
(6) Transmission control module (AT models only).
(7) Tone wheels.
(8) ABS wheel speed sensor.
(9) Wheel cylinder.
(10) G sensor.
(11) Stop light switch.
(12) Master cylinder.
(13) Brake & EBD warning light.
(14) Lateral G sensor (STi).

15. ABS Components Integrated Nonintegrated ABS brake system are
An integrated system has the master cylinder and control valve assembly made together.
Nonintegrated
A nonintegrated has the master cylinder and control valve assembly made separate.

16. ABS systems consist of 4 primary components:
ABS Components
ABS systems consist of 4 primary components:
1- ABS Controller; the brains of the system.  ABS Controllers are a computer that reads the inputs and then controls the system to keep the wheels from locking up and skidding.
2- ABS Speed Sensors; there are generally one on each wheel (sometimes they are located on the differential). 
It detects a change in acceleration in the longitudinal direction of the vehicle and outputs it to the ABSCM as a voltage signal.

17. ABS Components
3- ABS Modulator/Valves; some system have separate valves for each wheel with a modulator to control them.  Other systems they are combined.  In either case they work with the controller and the pump to add or release pressure from the individual wheels brakes to control the braking.
4- ABS Pumps; since the ABS modulator/valves can release pressure from the individual wheels brakes there needs to be a way to restore the pressure when required.  That is what the ABS pumps job is. When the pump is cycling, the driver may experience a slight pedal vibration.  This cycling is happening many times per second and this slight vibration is natural.

18. Subaru Impreza ABS Application.
ABS control module and hydraulic control unit
ABS control module section
Valve relay
Motor relay
Motor
Front left inlet solenoid valve
Front left outlet solenoid valve
Front right inlet solenoid valve
Front right outlet solenoid valve
Rear left inlet solenoid valve
Rear left outlet solenoid valve
Rear right inlet solenoid valve
Rear right outlet solenoid valve
Automatic transmission control Module
Diagnosis connector

19. Subaru Impreza ABS Application.
(16) Data link connector
(17) ABS warning light
(18) Stop light switch
(19) Stop light
(20) G sensor
(21) Front left ABS wheel speed sensor
(22) Front right ABS wheel speed sensor
(23) Rear left ABS wheel speed sensor
(24) Rear right ABS wheel speed sensor
(25) IGN
(26) Battery
(27) Brake warning light
(28) Parking brake warning light
(29) Brake fluid level switch
(30) Lateral G sensor (STi)
(31) Driver-controllable center differential control unit

20. Figure 3. ABS Operating Diagram
How does ABS work?
Figure 3. ABS Operating Diagram

21. How does ABS work? Simplicity
We will discuss how one of the simpler system works.
Sensors at each of the four wheels  sense the rotation of the wheel.
Too much brake application  wheel stop rotating
Sensors  ECU  releases brake line pressure  wheel turns again.
then ECU applies pressure again  stops the rotation of the wheel releases it again and so on
NB:
This releasing and re-application or pulsing of brake pressure happens times per second or more.
This keeps the wheel just at the limit before locking up and skidding no matter
ABS system can maintain extremely high static pressure and must be disabled before attempting repairs.

22. How does ABS work? Solenoid Valve
Solenoid Valve Assembly:
Is a pair of valves that can:
A. Increase pressure
B. Hold pressure steady
C. Decrease pressure

23. How does ABS work? Solenoid Valve
A. Increase pressure
During pressure increase mode of operation fluid is allowed to flow through both solenoids to the brake caliper
Solenoid 1
Pressure increase
Solenoid 2
Pressure decrease/Vent solenoid
Brake line under pressure
Brake fluid line not under pressure

24. How does ABS work? Solenoid Valve
B. Hold pressure steady
During Pressure Hold mode of operation both solenoids are closed and no additional fluid is allowed to flow to brake calipers.
Solenoid 1
Pressure increase
Solenoid 2
Pressure decrease/Vent

25. How does ABS work? Solenoid Valve
C. Decrease pressure
During Pressure Vent mode the pressure increase solenoid is closed. The Vent solenoid opens allowing fluid to vent into an accumulator chamber
Solenoid 1
Pressure increase
Solenoid 2
Pressure decrease/Vent

26. Figure 4. ABS Block Diagram
System Diagram
Figure 4. ABS Block Diagram

27. ABS Types
ABS brakes are either
1 Channel
3 Channel
4 Channel

28. ABS Types 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.

29. ABS Types 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.

30. ABS Types 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.

31. Configurations of ABS Types

32. Depending on the ABS application, there are several typical layouts.
ABS Configurations
Figure 5.
Depending on the ABS application, there are several typical layouts.

33. Design Challenges
ABS control is highly nonlinear control problem due to the complicated relationship between its components and parameters. The research that has been carried out in ABS control systems covers a broad range of issues and challenges. Many different control methods for ABS have been developed and research on improved control methods is continuing. Most of these approaches require system models, and some of them cannot achieve satisfactory performance under the changes of various road conditions. While soft computing methods like Fuzzy control doesn’t need a precise model. A brief idea of how soft computing is employed in ABS control is given.

34. Design Challenges Fuzzy control :
Is Intelligent control systems can be used in ABS control to emulate the qualitative aspects of human knowledge with several advantages such as robustness, universal approximation theorem and rule-based algorithms.

35. Advantages & Disadvantages
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

36. Advantages & Disadvantages
Stop Times - Anti-lock brakes are made to provide for surer braking in slippery conditions. However, some drivers report that they find the stopping distances for regular conditions are lengthened by their ABS system, either because there may be errors in the system, or because noise of the ABS may contribute to the driver not braking at the same rate.
Delicate Systems - It's easy to cause a problem in an ABS system by messing around with the brakes. Problems include disorientation of the ABS system, where a compensating brake sensor causes the vehicle to shudder, make loud noise or generally brake worse.

37. Advantages & Disadvantages
Cost - An ABS can be expensive to maintain. Expensive sensors on each wheel can cost hundreds of dollars to fix if they get out of calibration or develop other problems. For some, this is a big reason to decline an ABS in a vehicle.
System damage - A variety of factors can cause the system to be less effective, and can present with everything from shuddering of the vehicle to loud noises while trying to stop

38. ABS Problems Problems with ABS
The sensors on the wheels might get contaminated by metallic dust. When this condition occurs the sensors become less efficient in picking up problems. In modern ABS systems, two more sensors are added to help:
wheel angle sensor,
gyroscopic sensor

39. ABS Problems
The idea behind this is that when the gyroscopic sensor detects that the car’s direction is not the same as what the wheel sensor reports, the ABS software will cut in to brake the necessary wheel in order to help the car go the direction the driver intends.

40. General information.
Statistics show that approximately 40 % of automobile accidents are due to skidding.
Skidding , vehicle instability, steer inability and long distance stopping, 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.

41. Summary
The antilock braking system controls braking force by controlling the hydraulic pressure of the braking system, so that the wheels do not lock during braking.
The antilock braking system prevents wheels locking or skidding, no matter how hard brakes are applied, or how slippery the road surface. Steering stays under control and stopping distances are generally reduced.

42. Common Questions What is ABS? Why is that important?
How do I know whether my vehicle has ABS?
Will I notice anything when the ABS is working or not?
Does ABS change the way I should use the brakes?
How does ABS work?
Do cars with ABS stop more quickly than cars without?
Are all antilock systems the same?
How can I familiarize myself with ABS?