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INTRODUCTION A traction control system (TCS), also known as Anti-Slip Regulation (ASR), is typically (but not necessarily) a secondary function of the anti-lock braking system on production vehicles , and is designed to prevent loss of traction of the driven road wheels, and therefore maintain the control of the vehicle when excessive throttle is applied by the driver and the condition of the road surface (due to varying factors) is unable to cope with the torque applied. The basic idea behind the need of a traction control system is the difference between the slips of different wheels or an apparent loss of road grip that may result in loss of steering control over the vehicle. Difference in slip may occur due to turning of a vehicle or differently varying road conditions for different wheels. At high speeds, when a car tends to turn, its outer and inner wheels are subjected to different speed of rotation, that is conventionally controlled by using a differential . A further enhancement of the differential is to employ an active differential that can vary the amount of power being delivered to outer and inner
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INTRODUCTION

A traction control system (TCS), also known as Anti-Slip Regulation (ASR), is typically (but not necessarily) a secondary function of the anti-lock braking system on production vehicles, and is designed to prevent loss of traction of the driven road wheels, and therefore maintain the control of the vehicle when excessive throttle is applied by the driver and the condition of the road surface (due to varying factors) is unable to cope with the torque applied.

The basic idea behind the need of a traction control system is the difference between the slips of different wheels or an apparent loss of road grip that may result in loss of steering control over the vehicle. Difference in slip may occur due to turning of a vehicle or differently varying road conditions for different wheels. At high speeds, when a car tends to turn, its outer and inner wheels are subjected to different speed of rotation, that is conventionally controlled by using a differential. A further enhancement of the differential is to employ an active differential that can vary the amount of power being delivered to outer and inner wheels according to the need (for example, if, while turning right, outward slip (equivalently saying, 'yaw') is sensed, active differential may deliver more power to the outer wheel, so as to minimize the yaw (that is basically the degree to which the front and rear wheels of a car are out of line.) Active-differential, in turn, is controlled by an assembly of electromechanical sensors collaborating with a traction control unit.

Background

In July 1998, the National Transportation Safety Board (NTSB) published a report

entitled "Multiple Vehicle Crossover Accident, Slinger Wisconsin February 12, 1997 "

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(report number PB98-916203 NTSB HAR-98/01) which detailed the results of an

accident investigation it had conducted.

This accident occurred on a roadway with patches of ice and involved a tractor-double

trailer combination (“doubles”) losing control, crossing the median and striking a flatbed

tractor trailer traveling in the opposite direction. The flatbed tractor-trailer then crossed

the median and struck a van traveling in the opposite direction. In the report the NTSB

concluded, among other things, that:

• The initial loss of stability was the result of wheel spin-up on the doubles

combination’s single drive axle.

• At the speed and under the conditions in which the accident took place,

antilock brake and traction control technology would have given the

doubles truck driver more time to respond to the loss of stability.

In effect, NTSB felt that if the doubles vehicle had been equipped with a traction control

system, the accident might not have occurred.

At the end of their report, NTSB made a number of recommendations to various

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organizations in government and industry. One of the recommendations to the National

Highway Traffic Safety Administration (NHTSA) was to “Work together with the

Federal Highway Administration, the American Trucking Associations, the International

Brotherhood of Teamsters and the Motor Freight Carrier Association to conduct

laboratory and truck fleet testing to assess the safety benefits of adding traction control

devices to antilock brake systems (ABS) and report your findings to the National

Transportation Safety Board (H-98-15)” [recommendation number].

The report that follows describes the project that was initiated by the NHTSA to address

the NTSB recommendation relative to traction control systems and testing.

A Vehicle Traction Control system helps the driver maintain control of the vehicle during acceleration, particularly on a slippery road. The Traction Control system reduces wheel slip and maintains traction of all drive wheels by individually applying brake to the slipping drive wheel and reducing the engine torque.

Different vehicles use different design, but the common purpose of the Traction Control System remains the same - prevent drive wheels from slipping during acceleration.

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Let's see how the Electronic Traction Control System works in a typical front-wheel drive car (In a front-wheel drive vehicle the engine power is sent to the front wheels, so the front wheels are the drive wheels).

Imagine you are accelerating from a stop on an icy road in a front-wheel drive vehicle without Traction Control. If you accelerate very gently, you might be OK, but if you press the gas pedal just a bit harder, one or both front wheels could lose traction and begin to spin on ice, so the vehicle would be very hard to control.

The Traction Control is designed to help in a situation like this. Once the Traction Control System senses that any of the drive wheel(s) starts slipping, it reduces the engine torque and shortly applies the brake to the slipping wheel(s) to slow it down just enough to regain traction, thus helping you to control your vehicle during acceleration.

Typical Traction Control system is based on vehicle's Anti-lock braking system (ABS) and uses many of ABS components. It utilizes the ABS wheel speed sensors to monitor the speed of all four wheels. When Traction Control system senses that the wheel looses traction (begins to rotate faster) during acceleration, it applies the brakes to that wheel using the ABS hydraulic module and commands the Engine Control Module to reduce the engine power. When Traction Control system operates, you could probably feel that the engine power is reduced and hear some buzzing noise similar to that of ABS. The Traction Control light may also flash.

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EARLY TRACTION CONTROL

Powerful rear-drive cars from the sixties often had a primitive form of traction control called a limited slip rear differential.

Volvo 760 pioneered and introduced electronic traction control to the market.

In 1971, the Buick division of GM introduced Max Trac, which used an early computer system to detect rear wheel spin .

Cadillac also introduced the ill fated Traction Monitoring System (TMS) in 1979 on the redesigned Eldorado. It was criticized for it's slow reaction time and extremely high failure rate.

Traction control is part of a series of three braking technology developments that began appearing in vehicles in the mid-eighties.

In chronological order, these developments are: anti-lock brakes, aka ABS (1978), traction control (1985), and stability control (1995).

The foundation of ABS and Traction Control were already in place when Bosch pioneered Stability Control with their Electronic Stability Program (ESP) in the Mercedes Benz E Class in 1995.

Today TCS has become a standard equipment even for small cars.

THEORY

Traction refers to the maximum static friction that could be produced between given surfaces without slipping. If the driver full acceleration it can occur that the maximum static friction is

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surpassed and the wheels lose their grip and begin sliding. In the design of wheel-propelled vehicles, higher traction between wheel and ground is generally more desirable than lower traction, The coefficient of traction is identical to the coefficient of static friction except maximum static friction is named traction.

The basic idea behind the need of a traction control system is the difference between the slips of different wheels or an apparent loss of road grip that may result in loss of steering control over the vehicle. Difference in slip may occur due to turning of a vehicle or differently varying road conditions for different wheels. At high speeds, when a car tends to turn, its outer and inner wheels are subjected to different speed of rotation, that is conventionally controlled by using a differential. A further enhancement of the differential is to employ an active differential that can vary the amount of power being delivered to outer and inner wheels according to the need (for example, if, while turning right, outward slip (equivalently saying, 'yaw') is sensed, active differential may deliver more power to the outer wheel, so as to minimize the yaw (that is basically the degree to which the front and rear wheels of a car are out of line.) Active-differential, in turn, is controlled by an assembly of electromechanical sensors collaborating with a traction control unit.

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Traction between two surfaces depends on several factors including:

Material composition of each surface.

Macroscopic and microscopic shape or "roughness".

Normal force pressing contact surfaces together.

Size of surface area at contact.

Contaminants at the material boundary including lubricants and adhesives.

The traction force is given by:

Traction Force = Driving Torque /Radius of Wheel.

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Basic operation

The Traction control System is designed to maintain vehicle control, directional stability and optimum deceleration under all conditions on most road surfaces

The sensors monitor the speed of the wheels. The sensors generate a signal that is proportional to wheel speed, so by comparing wheel speeds the TCS can detect changes that indicate a wheel is losing traction, skidding or spinning.

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When traction control is needed, it applies the brakes on the drive wheel(s) that is losing traction. Slowing the wheel allows it to regain traction. At the same time, torque is shifted through the open differential to the opposite wheel that still has traction

When a wheel is about to slip, the control unit signals the hydraulic unit to reduce hydraulic pressure (or not increase it further) at that wheel’s brake caliper. Pressure modulation is handled by electrically-operated solenoid valves

When the traction control computer (often incorporated into another control unit, like the anti-lock braking system module) detects one or more drive wheels spinning significantly faster than another, it will use the ABS to apply brake friction to the wheels that are spinning too fast. This braking action on the slipping wheel(s) will cause power to be transferred to the wheels that are not due to the mechanical action within a differential. all-wheel drive vehicles also often have an

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electronically controlled coupling system in the transfer case or transaxle that is engaged (in an active part time AWD), or locked up tighter (in a true full-time set up that drives all the wheels with some power all the time) to supply the non-slipping wheels with (more) torque.

This often occurs in conjunction with the powertrain computer reducing available engine torque by electronically limiting throttle application and/or fuel delivery, retarding ignition spark, completely shutting down engine cylinders, and a number of other methods, depending on the vehicle and how much technology is used to control the engine and transmission.

Automatic traction control (ATC) systems (also called ASR for automatic slip regulation)

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are currently available as regular production options on most heavy commercial vehicles

(power units). These systems are integrated with ABS which is now mandatory for all air-braked vehicles and vehicles with hydraulic brakes having a GVWR in excess of 10,000 lbs. These systems utilize components of the ABS as well as additional components specific to the ATC.

The ABS wheel speed sensors are used to determine drive axle(s) slip by comparing the speed of the drive axle(s) wheels to the speed of the wheels on the steering axle. When the speed of the drive axle(s) exceeds that of the steering wheels by some predetermined amount, the traction control software in the ABS electronic control unit (ECU) can command either of two different events: 1) a reduction of engine speed (RPM) and 2) application of the drive axle brakes on one side of the drive axle(s).

ATC serves two primary operational functions:

1. Improves mobility on low friction surfaces allowing the vehicle to start from a stop when the vehicle is on a grade.

2. Prevents skidding while driving at highway speeds caused by over-speeding and spinning the drive wheels.

To improve mobility (Item 1), both the engine speed reduction and brake apply functions are utilized. Skidding and loss of stability at higher speeds (Item 2) is controlled via engine speed reduction only since brake application at higher speeds could result in brake overheating. The report which follows deals only with the engine speed reduction function of ATC.

Components

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Wheel Speed Sensors

TCS Control Module

Hydraulic Modulator

Pump Motor & Accumulator

Wheel Speed Sensors

The wheel speed sensor pickup has a magnetic core surrounded by coil consist of a magnetic pickup and a toothed sensor ring.

As the wheel turns, teeth on the sensor ring move through the pickup magnetic field.

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This reverses the polarity 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 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.

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TCS Control Module

The control module is a microprocessor and uses input from its sensors to regulate hydraulic pressure during braking to prevent wheel slip. The key inputs are 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

When braking is needed, the control module kicks into action and orders the hydraulic unit to modulate brake pressure as needed.

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Hydraulic Modulator

The hydraulic modulator or actuator unit contains the ABS solenoid valves for each brake circuit .

Traction control typically adds an extra solenoid valve in the ABS modulator for each drive wheel's brake circuit. 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

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Pump Motor & Accumulator

A high pressure electric pump is used in some TCS systems to generate power assist for normal braking as well as the reapplication of brake pressure during traction control.

The fluid pressure that is generated by the pump is stored in the "accumulator."

The accumulator on TCS systems where the hydraulic modulator is part of the master cylinder assembly consists of a pressure storage chamber filled with nitrogen gas.

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simplified mathematics behind wheel slip

Suppose a FWD vehicle weighs 1000 kg. Its engine produces 100 Nm torque at some RPM. It wheels have radii of 0.2 m.

m = 1000kg; T = 100Nm; r = 0.2m

Assuming 60%; of car's weight is on front wheels,

each wheel carry a load of 1000*0.6/2 = 300 kg.

Approximating g = 10 m/s2 and coeff. of friction of dry road = 0.9,

the tractive force on each wheel comes at = u*m*g = 0.9*300*10 = 2700 N

Assume car's 2nd gear ratio 0.5 and final drive is 0.25.

Then 100 Nm engine torque will appear on drive shaft as = 100/0.5/0.25 = 800 Nm

Torque at each wheel = propulsive force * radius of wheel

For FWD cars, torque is applied at two front wheels.

So propulsive force at each wheel = (800/2)/0.2 = 2000 N

The wheel slip occurs if propulsive force is greater than tractive force.

. This simple calculation shows:

Why it is said that you can easily skid on wet and on ice (where friction coeff. is very low)!

Why it is advised that you should use higher gears (thus less available torque on wheels) while driving on snow.

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Why 4WD has greater traction (its torque is shared by 4 wheels instead of 2 so propulsive force at each wheel is 50 percnt; less compared to 2WD, leads to less prone to skidding)

Oversteer

If the vehicle oversteers, the rear wheels lose traction and the vehicle tends to follow the red dotted line.

Oversteer can throw the car into a spin.

The car is said to oversteer when the rear wheels do not track behind the front wheels but instead slide out toward the outside of the turn.

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Causes of oversteer

The tendency of a car to oversteer is affected by several factors such as mechanical traction, aerodynamics and suspension, and driver control.

oversteer is the condition when the slip angle of the rear tires exceeds that of the front tires. This occurs because the rear tires must handle both the lateral cornering force and engine torque.

UNDERSTEER

If the vehicle understeers, the front wheels lose traction and the vehicle tends to follow the red dotted line.

In simpler words understeer is the condition in which the front tires do not follow the trajectory the driver is trying to impose while taking the corner, instead following a more straight line trajectory

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understeer happens when the front tires have a reduction in traction during a cornering situation, causing the front-end of the vehicle to have less mechanical grip and become unable to follow the trajectory in the corner.

The difference between the circle the wheels are currently tracing and the direction in which they are pointed is the slip angle.

If the slip angles of the front and rear wheels are equal, the car is in a neutral steering state.

If the slip angle of the front wheels exceeds that of the rear, the vehicle is said to be under steering.

If the slip angle of the rear wheels exceeds that of the front, the vehicle is said to be oversteering.

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Uses

In road cars: Traction control has traditionally been a safety feature in high-performance cars,

In race cars: Traction control is used as a performance enhancement, allowing maximum traction under acceleration without wheel spin.

In motorsports: In Formula One, an effort to ban TC has led to the change of rules for 2008: every car must have a standard ECU, issued by FIA, which is relatively basic and does not have TC capabilities

A.B.S vs T.C.SABS and traction control operate similarly. In fact, the ABS control unit is the basic "building block" for traction control

A.B.S T.C.S

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ABS works by sensing slippage at

the wheels during braking,

and continually adjusting

braking pressure to ensure maximum contact between the tires and the road.

ABS can control individual wheels’ braking forces

The hydraulic modulator or actuator unit contains the ABS solenoid valves for each brake circuit

The TCS traction control prevents the driven wheels spinning during drive-off.

Traction Control controls individual wheels’ acceleration forces.

Traction control typically adds an extra solenoid valve in the ABS modulator for each drive wheel's brake circuit.

ADVANTAGES

Traction control improves safety during bad weather driving and reduces wear on tires by preventing excessive wheel slip.

Major roles of the traction control system (TCS) are to guarantee the acceleration performance

Directional stability even in extreme road conditions, under which average drivers may not control the car properly.

The TCS traction control prevents the driven wheels spinning during drive-off.

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