Sliding Mode Controller Design for ABS System

Abstract

The principle of braking in road vehicles

involves the conversion of kinetic energy into

heat. This high energy conversion therefore

demands an appropriate rate of heat

dissipation if a reasonable temperature and

performance stability are to be maintained.

While the design, construction, and location

features severely limit the heat dissipation

function of the friction brake, electromagnetic

brakes work in a relatively cool condition and

avoid problems that friction brakes face by

using a totally different working principle and

installation location. By using the

electromagnetic brake as supplementary

retardation equipment, the friction brakes can

be used less frequently and therefore

practically never reach high temperatures.

The brake linings thus have a longer life span,

and the potential "brake fade" problem can

be avoided. It is apparent that the

electromagnetic brake is an essential

complement to the safe braking of heavy

vehicles. In this thesis, a new mathematical

model for electromagnetic brakes is

proposed to describe their static

characteristics (angular speed versus brake

torque). The performance of the new

mathematical model is better than the other

three models available in the literature in a

least-square sense. Compared with old

models that treat reluctance as a constant,

our model treats reluctance as a function of

speed. In this way, the model represents

more precisely the aggregate effect of all side

effects such as degree of saturation of the

iron in the magnet, demagnetizing effects, and

air gap. The software program written in

Matlab can be used to code different brake

characteristics (both static and dynamic) and

evaluate their performance in different road

scenarios. A controller is designed that

achieves wheel-slip control for vehicle

motion. The objective of this brake control

system is to keep the wheel slip at an ideal

value so that the tire can still generate lateral

and steering forces as well as shorter

stopping distances. In order to control the

wheel slip, vehicle system dynamic equations

are given in terms of wheel slip. The system

shows the nonlinearities and uncertainties.

Hence, a nonlinear control strategy based on

sliding mode, which is a standard approach

to tackle the parametric and modeling

uncertainties of a nonlinear system, is chosen

for slip control. Due to its robustness

properties, the sliding mode controller can

solve two major difficulties involved in the

design of a braking control algorithm: 1) the

vehicle system is highly nonlinear with

time-varying parameters and uncertainties; 2)

the performance of the system depends

strongly on the knowledge of the tire/road

surface condition. A nominal vehicle system

model is simulated in software and a sliding

mode controller is designed to maintain the

wheel slip at a given value. The brake control

system has desired performance in the

simulation. It can be proven from this study

that the electromagnetic brake is effective

supplementary retardation equipment. The

application and control of electromagnetic

brakes might be integrated with the design of

vehicles and their friction braking systems so

that an ideal match of the complementary

benefits of both systems might be obtained to

increase safety to a maximum while reducing

vehicle operating costs to a minimum.