Prepared for theU.S. Department of Transportation

Federal Highway Administration

AbstractATC uses the same components as ABS, but works at the other end of the speed spectrum getting a vehicle

started under slippery conditions. The basic operation principle of ABS is “limiting the pressure to any wheel which decelerates too rapidly”.

INTRODUCTION

The problem of skidding reveals the one overwhelming weakness of all motor vehicle br aking systems: they depend strongly on the coefficient of static fr iction between the tire and the road. If for any reason the tire momentarily loses its adhesion to the road while the brakes are applied, the fr iction of the brakes against the drums or rotors locks the wheel solidly and the tire begins skidding across the road. In this condition, the braking force of that wheel is dependent on the sliding friction between the tire and the road, which is much less than the static friction. Under wet or icy conditions, the sliding friction is reduced even further, resulting in significantly longer stopping distances. In addition, when the front wheels are in this condition, they cannot be used to steer the vehicle; regardless of the angle of the front wheels, the vehicle continues to skid in whatever direction its momentum sends it until either the driver releases the brakes or the vehicle collides with something solid enough to bring it to a halt.

Generations of dr ivers were taught to handle this condition by quickly applying and releasing, or pumping, the brakes dur ing a skid. However, this training was often lost during panic situations. Additionally, even the calmest and best-trained dr iver could not pump the brakes faster than two or three times per second, which limited the effectiveness of the technique.

Regardless of manufacturer or the type of vehicle, all antilock brake systems operate in a similar manner. Wheel speed sensors are placed on each wheel that is to be controlled. Each speed sensor usually has a toothed wheel, like a gear, that rotates at the Vehicles with Sports ABS have an additional function which independently controls*1 the rearwheels according to the value sent from the lateral G sensor. (STi models)*1: A system which controls the left and right wheel brakes individually.*2: A system which applies the same fluid pressure to both wheels if either of the left or right wheelsstarts to lock.The pressure is determined based on the lower of the frictional coefficients of both.wheels. Brakes are actually energy conversion devices, which convert the Kinetic energy (momentum) of the vehicle into thermal energy.An improvement in ABS technology now allows some system to prevent wheel spin when accelerating on wet or slippery surface. This capability is known as Traction Control .Some of these systems also provides addition control when cornering or changing.

PRINCIPLE OF ABS CONTROLWhen the brake pedal is depressed during driving, the wheel speed decreases and the vehiclespeed does as well. The decrease in the vehicle speed, however, is not always proportional to thedecrease in the wheel speed. The non-correspondence between the wheel speed and vehiclespeed is called “slip” and the magnitude of the slip is expressed by the “slip ratio” which is definedas 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 is100%, the wheels are completely locking (rotating at a zero speed) while the vehicle is moving.The braking effectiveness is represented by the “coefficient of friction” between the tire and roadsurface.The larger the coefficient, the higher the braking effectiveness. The diagram below showsthe relationship between the coefficient of friction and the slip ratio for two different road surface conditions(asphalt-paved road and icy road), assuming that the same tires are used for both the conditionsand the vehicles are moving forward. Although the braking effectiveness (coefficient offriction) depends on the road surface condition as shown and also on the type of the tire, its peakrange generally corresponds to the 8 — 30% range of the slip ratio.The ABS controls the fluid pressure to each wheel so that a coefficient of friction corresponding tothis slip ratio range is maintained.

How Do ABSs Work?

An ABS consists of several key components: electronic control unit (ECU), wheel speed sensors, modulator valves, and exciter rings. Here’s how these components work together:

Wheel speed sensors constantly monitor and send electr ical pulses to the ECU at a rate proportional to the wheel speed. When the pulse rates indicate impending wheel lockup, the ECU signals the modulator valve(s) to reduce and/or hold the brake application pressure to the wheel(s) in question. 7

The ECU then adjusts pressure, seeking one which gives maximum braking without risking wheel lockup. When the ECU acts to modulate the brake pressure, it will also (on most vehicles) turn off the retarder (if so equipped) until the risk of lockup is over. The ECU continually checks itself for proper operation. If it detects malfunction/failure in the electr ical/electronic system, it will shut down that partof the ABS affected by the problem or the entire ABS depending upon the system and the problem. When this happens, the ABS malfunction lamp lights. An ABS adjusts brake pressure much faster and more accurately than can drivers. It’s faster because: • electr onic controls are very fast and

Some vehicles also use a traction control system in conjunction with the ABS. Traction control helps the ABS improve vehicle traction by minimizing wheel slip on the drive axle during acceleration. If a wheel on the drive axle starts to slip, the traction control system automatically brakes the wheel slightly, transferring engine torque to the wheels with better traction. I f all the drive wheels start to slip, the traction control system may also reduce engine power

a. Automatic Traction Control (ATC) b. Traction Control (TC) c. Automatic Slip Regulation/Anti-Spin Regulation (ASR)

same speed as the vehicle wheel or axle. Mounted close to, but not touching this toothed wheel, is a permanent magnet wrapped with a coil of wire, called the pick-up coil. As each tooth rotates past the permanent magnet, it causes the magnetic field to concentrate and increase slightly. This, in turn, induces a small pulse of current in the coil of wire. The number of pulses per second are directly proportional to the speed of the wheel. The faster the wheel turns, the faster the teeth pass the magnet and the higher the pulse rate. The pulsed output from the wheel speed sensors goes to an electronic controller, which monitors each wheel's speed relative to the speed of the other wheels. As long as the brakes are not being applied and all of the monitored wheels are rotating at roughly the same speed, the system takes no action. If, however, the brakes are being applied and one or more of the monitored wheels suddenly begins to reduce speed at a higher rate than the others indicating a loss of traction with the road and an imminent wheel lockup and skid the controller then activates the antilock system. The antilock brake system on any vehicle is simply an additional monitoring and controlling function superimposed on the existing vehicle brake system. ABS is not a second brake system, nor does it replace the vehicle brake system. When all four wheels on an automobile are monitored and controlled, the system is called a four-channel ABS. If the front two wheels plus the rear axle (but not each rear wheel individually) are to be controlled, the system is called a three-channel ABS. On heavy trucks with two rear drive axles, the ABS is commonly a four-channel system which controls the front wheels and two of the four rear wheels. Trailers pulled by heavy truck tractors may also have their own separate ABS which must interconnect with the ABS on the tractor.

In an automobile, the brakes are actuated by hydraulic pressure. The ABS controller operates solenoid valves built into the high pressure side of the master brake cylinder. These valves are normally open and do not interfere with braking. When the controller senses that a wheel is locking up while braking, it first activates a solenoid to close a valve in the affected wheel's brake line which prevents the pressure from increasing any further. If the locked wheel continues to lose speed, the controller activates a second solenoid which bleeds pressure off the affected brake line, in effect releasing the brake for that wheel regardless of whether the driver is still pushing on the brake pedal. As soon as the wheel regains traction and its speed increases, the solenoids are de-activated, and normal braking resumes. Of course, if the conditions are such that the wheel starts to skid again, the brake will promptly begin to lock up and the ABS will take over. This cycle is repeated 12 to 15 times per second until either the road condition changes or the driver releases the brakes. The driver will be able to detect this rapid cycling as a vibration felt through the brake pedal, but will not have to take any action.

3.1.5). ABS Valves:

There is a valve in the brake line of each brake controlled by the ABS. On some system, the valve has three positions.

a).In position one, the valve is open; pressure from the master cylinder is passed right through to the brake.

b).In position two, the valve blocks the line, isolating that brake from the master cylinder .This prevents the pressure from rising further shouted the driver push the brake pedal harder.

c).In position three, the valve releases some of the pressure from the brake.

What is an ABS?

Antilock braking systems (ABSs) are electronic systems thatmonitor and control wheel slip dur ing vehicle braking. ABS scan improve vehicle control during braking, and reduce stopping distances on slippery (split or low coefficient of fr iction) road surfaces by limiting wheel slip and minimizing lockup. Rolling wheels have much more traction

than locked wheels. Reducing wheel slip improves vehicle stability and control during braking, since stability increases as wheel slip decr eases.

The equipment requirements of FMVSS 121 specify that ABSs on truck-tractors and full trailers must control the brake pressures to at least one front axle and one rear axle. The ABSs on semi-trailers and dollies must control at least one axle of the vehicle. Additionally, the ABSs on tractors must control one of the rear axles with two modulator valves so that the brake pressure on one end of the axle is independent of the brake pressure on the other end. The performance requirements of FMVSS 121 can require an ABS on additional axles.

E: EBD (ELECTRONIC BRAKE FORCE DISTRIBUTION)The EBD system utilizes the function of the conventional ABS. This system prevents premature locking of the rear wheels by using electronic control instead of a proportioning valve. The system provides a feature to optimize the brake force distribution to the front and rear wheels according to change in loading conditions or displacement of the center of gravity during deceleration.The ABSCM calculates the optimum brake force distribution from the difference in speed between the front and rear wheels. Based on the results, the ABSCM controls the rear wheel brake pressure through the ABS hydraulic unit (H/U) to achieve the optimum brake force distribution suitable for the driving condition.The proportioning valve is disused on EBD equipped vehicles.

(4) (5)

(3)

(1)

... ------ ---------- (6)

(2)

ABS00019

(1) Brake force to rear wheels

(2) Brake force to front wheels

(3) Optimum brake force distribution for light loaded

conditions (4) Optimum brake force distribution for

heavy loaded conditions (5) EBD controlled brake

force distribution

(6) PCV controlled brake force distribution (reference)

TABLE 1: ABS Features and BenefitsFEATURE BENEFIT

Control of steering, drive and trailer wheelIncreases steer ing ability and vehicle stabilityduring brakingRs educes possibility of jackknifing and trailer swingReduces tire flatspotting

Fail-safe electrical/electronic systemIf the electr ical/electronic system fails, the

ABS is shut off, returning the vehicle to normal braking. On some systems, the ABS is only shut off at the affected wheels.

Traction control An optional feature that controls excessive

9

wheel spin during acceleration, reducing thepossibility of power skids, spins or jackknifes.

Self-diagnosing systemBuilt-in system makes maintenance checks

quick and easy.

Diagnostic tool compatibility ABSs are compatible with industry standard hand- held and computer-based diagnostic tools. Blink codes and other diagnostic schemes can also be used fortroubleshooting, if other tools are not available.

ABS Malfunction Indicator Lamp Informs the driver or technician that an ABSfault has occured. The warning lamp may also transmit blink code infor mation. It doesnot signal all possible faults.

With OR Without

ABS keeps the wheels from fully locking up while braking to allow the driver to maintain steering control. There are two basic types of ABS. Rear Wheel ABS prevents only rear wheels from locking. Four Wheel ABS prevents all four wheels

from locking. Without ABS, too much force applied to the brake pedal can cause a wheel to stop turning ( lockup) and begin skidding, greatly reducing the capability of the driver to steer or maintain stability. With ABS, the wheels are kept rolling and the driver maintains steering control within the limits of traction. Many light trucks use rear-wheel ABS to prevent rear wheel lockup.

Design of ABS

An antilock brake system is designed for a specific vehicle application. A truck which does not pull a trailer, like a cement mixer , would have a slightly different ABS than a truck tractor which pulls one or more trailers. Likewise, an antilock brake system for a trailer would have a different designith wheel speed sensors. On some applications, each wheel is equipped with its own speed sensor. This type of arrangement would be called a "four wheel, four channel" system since each wheel speed sensor would give its input into a separate control circuit (the word "channel" here actually refers to each individual electr onic circuit rather than the individual hydraulic brake circuits). On other applications, fewer sensors are used. Many four-wheel ABS systems have a separate wheel speed sensor for each front wheel but use a common speed sensor for both rear wheels. These are called "three channel" systems. The rear wheel speed sensor is mounted in either the differential or the transmission. The sensor reads the combined or average speed of both rear wheels. This type of setup saves the cost of an additional sensor and reduces the complexity of the system by allowing both rear wheels to be controlled simultaneously. Another variation is the "single channel" rear-wheel only ABS system that is used on many rear-wheel drive pickups and vans. Fords version is called "Rear Antilock Brakes" (RABS) while GM and Chrysler call theirs "Rear Wheel Anti-Lock" (RWAL). The front wheels have no speed sensors and only a single speed sensor mounted in the differential or transmission is used for both rear wheels. Rear-wheel antilock systems are typically used on applications where vehicle loading can affect rear wheel traction, which is why it is used on. Because the rear-wheel antilock systems have only a single channel, they are much less complex and costly than their three- and four-channel, four-wheel counterparts.

ANTILOCK BRAKE CONFIGURATIONS

On some applications, each wheel is equipped with its own speed sensor. This type of arrangement would be called a "four wheel, four channel" system since each wheel speed sensor would give its input into a separate control circuit (the word "channel" here actually refers to each individual electr onic circuit rather than

the individual hydraulic brake circuits).

On other applications, fewer sensors are used. Many four-wheel ABS systems have a separate wheel speed sensor for each front wheel but use a common speed sensor for both rear wheels. These are called "three channel" systems. The rear wheel speed sensor is mounted in either the differential or the transmission. The sensor reads the combined or average speed of both rear wheels. This type of setup saves the cost of an additional sensor and reduces the complexity of the system by allowing both rear wheels to be controlled simultaneously. Another variation is the "single channel" rear-wheel only ABS system that is used on many rear-wheel drive pickups and vans. Fords version is called "Rear Antilock Brakes" (RABS) while GM and Chrysler call theirs "Rear Wheel Anti-Lock" (RWAL). The front wheels have no speed sensors and only a single speed sensor mounted in the differential or transmission is used for both rear wheels. Rear-wheel antilock systems are typically used on applications where vehicle loading can affect rear wheel traction, which is why it is used on. Because the rear-wheel antilock systems have only a single channel, they are much less complex and costly than their three- and four-channel, four-wheel counterparts.

WHEEL SPEED SENSOR

The wheel speed sensor has two main components: the exciter and the pickup. The exciter is a ring with notched teeth. The pickup is commonly called “the sensor.” It contains a wire coil/magnet assembly, The wheel speed sensors (WSS) consist of a magnetic pickup and a toothed sensor ring (sometimes called a "tone" ring). The sensor(s) may be mounted in the steering knuckles, wheel hubs, brake backing plates, transmission tailshaft differential housing or on wheel bearing and hub assembly. The sensor ring(s) may be mounted on the axle hub behind the brake rotor, on the brake rotor itself, inside the brake drum, on the transmission tailshaft or inside the differential on the pinion shaft. The wheel speed sensor pickup has a magnetic core surrounded by coil windings. As the wheel turns, teeth on the sensor ring move through the pickup magnetic field. This reverses the polar ity of the magnetic field and induces an alternating current (AC) voltage in the pickup windings. The number of voltage pulses per second that are induced in the pickup changes in direct proportion to wheel speed. So as speed increases, the frequency and amplitude of the wheel speed sensor goes up. The WSS signal is sent to the antilock brake control module, where the AC signal is converted into a

digital signal and then processed. The control module then counts pulses to monitor changes in wheel speed. On applications where the wheel speed sensor is not part of thehub or wheel bearing assembly, it can be replaced if defective. Sensor problems can be caused by an accumulation of debris on the end (they are magnetic), incorrect air gap or faults in the wiring or connectors.

(A)

(4)

(5) (6)

(B) (2)

(3)

-,I

II

--, - -ABS00018

(A) Front (1) Sensor body (4) High-speed

(B) Rear (2) Pole piece (5) Low-speed

(3) Tone wheels (6) Permanent magnet

ABS Electronic Controll Module

TThe ECU processes all ABS infor mation and signal functions. It receives and interprets voltage pulses generated by the sensor pickup as the exciter teeth pass by, and uses this information to determine: • I mpending wheel lock-up and • When/how to activate the ABS modulator valves. The ECU connects to the following ABS components: wheel speed sensors, ABS modulator valves, power source, ground, warning lamps, blink code switch, J1587* diagnostic connector, and retarder control device (usually by relay or the J1922**/ J1939*** datalink.) The ECU also makes self- diagnostic checks dur ing normal operation.Dur ing braking, the ECU uses voltage pulses from each wheel speed sensor to determine wheel speed changes. If the ECU determines that the pulse rate of the sensed wheels indicates imminent lock-up, it cycles the ABS modulator valves to modify brake air pressure as needed to provide the best braking possible. he ABS electronic control module (which may be referred to as an EBCM "Electr onic Brake Control Module" or EBM "Electronic Brake Module") is a microprocessor that functions like the engine control computer. It uses input from its sensors to regulate hydraulic pressure during braking to prevent wheel lockup. The ABS module may be located in the trunk, passenger compartment or under the hood. The key inputs for the ABS control module come from the wheel speed sensors

and a brake pedal switch. The switch signals the control module when the brakes are being applied, which causes it to go from a "standby" mode to an active mode.

HYDRAULIC MODULATOR The hydraulic modulator or actuator unit contains the ABS solenoid valves for each brake circuit. The exact number of valves per circuit depends on the ABS system and application. Some have a pair of on-off solenoid valves for each brake circuit while others use a single valve that can operate in more than one position. PUMP MOTOR & ACCUMULATOR A high pressure electric pump is used in some ABS systems to generate power assist for normal braking as well as the reapplication of brake pressure during ABS braking. In some systems, it is used only for the reapplication of pressure during ABS braking. The pump motor is energized via a relay that is switched on and off by the ABS control module. The fluid pressure that is generated by the pump is stored in the "accumulator." The accumulator on ABS systems where the hydraulic modulator is part of the master cylinder assembly consists of a pressure storage chamber filled with nitrogen gas. Should the pump fail (a warning light comes on when reserve pressure drops too low), there is usually enough reserve pressure in the accumulator for 10 to 20 power-assisted stops. After that, there is no power assist. The brakes still work, but with incrspecifics. B. Modulator Valves ABS modulator valves regulate the air pressure to the brakes during ABS action. When not receiving commands from the ECU, the modulator valve allows air to flow freely and has no effect on the brake pressure. The ECU commands the modulator valve to either: • change the air pressure to the brake chamber, or • hold the existing pressure. However, it cannot automatically apply the brakes, or increase the brake application pressure above the level applied by the driver. The modulator valve typically contains two solenoids. The modulator valve and relay valve may be incorporated into a single unit. The modulator valve may also be separate, inserted into the service line to the brake chamber(s) after any relay valve, located as close as practicable to the chamber(s) itself. When the modulator valve is separate, it has to control more air flow and, therefore, includes two larger diaphragm valves which are controlled by the solenoids. It usually has three ports: the supply port, the delivery port and the exhaust port. • Typically, when an ECU controlling a separate modulator valve detects impending wheel lockup, it activates the solenoids to close the supply port and open the exhaust port. When enough air is vented to prevent wheel lockup, the exhaust valve will close and the ECU will depending on the

situation either:

. ABS comes in about FOUR different TYPES:-

5.1). 1 or 2 channel 2-wheel (Rear ABS)

This ABS is usually prevalent on trucks .It consists of 2 ABS sensors on the rear wheels and one or two ABS channels to pulse the rear wheel together (1 channel) or separately (2 channel).

5.2). 2 channel 4- wheel Criss Cross

This is the ABS system present on the 91-94 Sentra.It consists of 4 ABS sensors (one on each wheel) and 2 ABS channels arranged in a Criss-Cross(left front & right rear, right front & left rear).When the right rear wheels lock up, the left front wheel & right rear wheel are pulsed together.

5.3). 3 channel 4-wheel

This is the more common ABS system in cars. It consists of 4 wheel sensors and 2 channels in the front (LF, RF) and one channel for the rear wheels .When one of the front wheels lock up, it pulses independently of the other wheels. When one of the rear wheels lock up, it is pulsed together with the other rear wheel, similar to a very fast pulling and releasing of the emergency brake.

5.4). 4 channel 4-wheel

This is the ABS system present on the 95+200SXI Sentra.It consists of 4 ABS sensors and 4 ABS channels. All wheels pulse independently of each other, like it should be.

So, the long and short of it is that a 4 channel system will work better because only the wheel that is locked pulses (loses braking power), while the other 3 wheels continue to do braking for the car. This results in more stability while braking and possibly shorter distances.

7. DEVELOPMENT & FUTURE IMPLEMENTATION OF ABS:-

7.1).TRACTION CONTROL SYSTEM :- (TCS)

An electro-mechanical control system designed to monitor and influence wheel dynamics, and ultimately vehicle dynamics during acceleration maneuvers. In order of priority, these systems are intended to enhance vehicle 1) stability (RWD applications) 2) steerability (FWD applications) and 3) launch performance (all applications). Typical systems consist of 3-4 wheel speed sensors, an ECU

containing the algorithm processing the wheel speed information, a series of solenoid-driven valves, and a pump-motor subsystem which can be actuated to build, hold, and release brake fluid pressure from the wheel-end brake components.

7.2).STABILITY CONTROL SYSTEM :- (SCS)

Stability control systems are especially beneficial on wet, snowy, or icy conditions, although they do offer handling benefits during emergency maneuvers on dry pavements. The design intent of stability control is to keep the vehicle going in the direction the driver is steering the car. To do this, the brakes are applied on one wheel to help steer the car in the correct direction. For example, if poor traction causes the front end of the car to slip sideways when you are going around a corner, the computer will apply the wheel brake on the inside of the corner causing the car to turn and slow down. If the back end of the car slips sideways; the brake on the outside of the corner is applied to bring the car back into line. The system works when the car starts to slide on a straight road the same as it does when turning corners.

7.3).ELECTRONIC STABILITY PROGRAM :- (ESP)

An electro-mechanical control system designed to monitor and influence wheel dynamics, and ultimately vehicle dynamics during any vehicle state (braking, accelerating, or coasting). In addition to the sensors and hardware used during ABS and TCS, ESP typically utilizes the additional input from a steering angle sensor, a yaw rate sensor, and a lateral/longitudinal accelerometer when determining both 1) the driver's intended heading and 2) the vehicle's actual heading. Once the system has determined a significant difference (error) between (1) and (2) above, the solenoid-driven valves and pump-motor subsystem can be actuated to build, hold, and release brake fluid pressure at individual wheel-end brake components, creating asymmetric (cross-vehicle) brake forces in an attempt to create yaw moments, turning the vehicle toward the driver's intended path.

7.4).ELECTRONIC BRAKE FORCE DISTRIBUTION :- (EBD)

An electro-mechanical control system designed to monitor and influence rear wheel dynamics, and ultimately foundation brake (front-rear) balance. In so many words, the EBD utilizes the ABS hardware to function as an "intelligent brake proportioning valve." Unlike a traditional mechanical proportioning valve which is limited by design to one kneepoint and slope, the EBD algorithm relies on closed-loop feedback to continuously monitor rear wheel slip, adjusting brake line pressure to the rear wheels as appropriate.

10. REFERENCES:-

1).William H. Crouse and Donald L. Anglin, Automotive Mechanics, 10th edition,Tata Mc-Graw Hill Publishing Limited,pp: 729-748.

2). K. Newton, W. Steeds and T.K. Garrett, The Motor Vehicle, 11th edition ,Butterworth International Editions, pp: 730-749.

3).Erich J. Schulz, Diesel Equipment-I, 3rd print 1988, pp: 206-234.

4).The journals of Society of Automotive Engineers India (SAE India).

5). W1860BE.book Page 4 Tuesday, January 28, 200