Traction control system for an automobile via engine torque control.

Abstract

Control of vehicle traction is of utmost importance in providing safety and obtaining

desired vehicle motion in longitudinal and lateral vehicle control. Vehicle traction control systems

can be designed to satisfy various objectives of a single vehicle system or a platoon of vehicles in an

automated highway system, which include assuring ride quality and passenger comfort.

Vehicle traction force directly depends on the friction coefficient between road and

tire, which in turn depends on the wheel slip as well as road conditions. From control

point of view, we may influence traction force by varying the wheel slip. Wheel slip is a

nonlinear function of the wheel velocity and the vehicle velocity. Different objectives of traction

control, give different target slips to be followed.

Simulation study shows that longitudinal controllers, which do not take traction into account

explicitly (termed as traction less or passive controllers), cannot handle external disturbances well;

on the other hand, longitudinal traction controllers (termed as active controllers) give satisfactory

results with the same disturbances. Simulations show how some of the vehicle performance

objectives are met by using traction controllers.

Introduction

Traction refers to the maximum frictional force that can be produced between surfaces without

slipping. In auto mobiles traction is responsible for the movement of vehicle. In the design of

wheeled or tracked vehicles, high traction between wheel and ground is more desirable than low

traction, as it allows for more energetic acceleration without wheel slippage.

Traction control is a technology designed to help your vehicle maintain traction, no matter how slippery

the road surface. Technically, it is a mechanical, hydraulic, or electric system that maintains or

controls traction to any wheels driven by the engine. Unlike mechanical traction control systems of

the past such as limited slip differentials, today’s systems are nearly all computer-controlled as they

actively watch wheel slip. An option formerly reserved for performance, you can find traction control on

all types of vehicles today.

The purpose of the Traction Control System is to prevent wheel spin from occurring due to acceleration. The

maximum torque that can be transmitted to the wheels is determined by the coefficient of friction generated between

the road and the tires. If torque exceeds that level, the wheels are likely to spin. Conditions for Traction operation

may include slippery road surfaces, acceleration while cornering and hard acceleration.

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 which leads to slipping of the vehicle and loss of  power which results in uncontrolled

cruising. Difference in slip may occur due to turning of a vehicle or differently varying road

conditions for different wheels. And thus need to be controlled for a safer way to cruise.

Modeling of the Open Loop System (plant model) ?????????????????????????????????????

Simulink is used to simulate the complete control system, including the control algorithm in addition

to the physical plant of traction control.

A common approach is to generate a linear approximation of the plant and then use the linearized

model to design a controller using analytical techniques. Simulink can be employed for generating

the linearized model and MATLAB can be employed for designing the controller as described in the

Introduction page.

Design of the Control System (present your methodology)

Vehicle parameter values are given as shown in appendix

Assuming that desired acceleration slip = 0.2

The road surface is covered with packed snow. The peak friction coefficient is 0.3 at slip

0.2. 60% static weight is distributed on front axle and 40% on rear axle.

Neglect the dynamic weight transfer due to deceleration & steering and no engine

intervention.

Assuming, Rising time, Tr = 70 ms and Max Overshoot < 10 %.

So we get,

2(rad / s)

Desired Characteristics equation, D(s) s2 3.2s 4 0 ---------------- (i)

Now,

si

ik

kth ) 0.2

1.5

Now PID controller was preferred for the Brake based Traction Control System. [you can

use any other controller]

Close loop characteristics equation is given by –

1

0.8

n

0.3(

th

bK

1PID(s) G(s) 0

1 (K P sI )(s a

) 0

s^2 (a bKP )s bK I 0 --------------------- (ii)

a ( R

siFzi si g)

where, i

b i

Now compute normal force at contact patch,

Normal force at contact patch of frontal wheel, Fzfi 0.6

1740 g kg-m/s2

= 5120.82 kg-m/s2

Normal force at contact patch of rear wheel, Fzri 0.4

1740 g kg-m/s2

= 3413.88 kg-m/s2

R

I

I

2

2

PID Controller for Front Wheel

Transfer function, G(s) s

b

a s 955.67

Where, a = 955.67 & b = 0.35

Now comparing the coefficients of equations (i) & (ii)

K P 2721.4

K I 11.43

Since the Kp is negative, the desired performance is not reachable and tuning the gains

which yields reasonable performance are:

K P 1220

K I 19650

P ID co nt r oller f or R ear W h e e l

Transfer function, G(s) s

b

a s 642.02

Where, a = 642.02 & b = 0.35

Now comparing the coefficients of equations (i) & (ii) –K I

1825

Again since Kp is negative , the desired performance of the system is not reachable so we

need to tuned the gains to just find a reasonable performance close to what we firstly

wished to.

The tuned gains which give us similar step response for this section as well.

K P 960

K I 16450

Significant Steps in Model Building in SIMULINK

0.35

0.35

11.43PK

Simulations Results

PI controller for Front Wheel Simulations

Open Loop step response:

Closed Loop step response:

PID controller for Rear Wheel Simulations

Open Loop step response:

Closed Loop step response step

Discussion

Conclusion

Traction control helps limit tire slip in acceleration on slippery surfaces. Powerful rear-drive cars

from the sixties often had a primitive form of traction control called a limited slip differential

helping to reduce, but not eliminate wheel spin. While limited-slip rear axles are still in use in many

Front- and rear-drive vehicles today, the device can't completely eliminate wheel slip. Hence, a more

sophisticated system was needed.

In modern vehicles, traction-control systems utilize the same wheel-speed sensors employed by the

antilock braking system. These sensors measure differences in rotational speed to determine if the

wheels that are receiving power have lost traction. When the traction-control system determines that

one wheel is spinning more quickly than the others, it automatically applies brake to that wheel to reduce

its speed and lessen wheel slip. In most cases, individual wheel braking is enough to control wheel

slip. Therefore for drivers who routinely drive in snowy and icy conditions, traction control is a must-

have safety feature.

References (if any)

Appendix (if needed)