05-12-2012, 11:46 AM
Adaptive vehicle skid control
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Abstract
In this paper, adaptive vehicle skid control, for stability and tracking of a vehicle during slippage of its wheels without braking, is
addressed. Two adaptive control algorithms are developed: one for the case when no road condition information is available, and
one for the case when certain information is known only about the instant type of road surface on which the vehicle is moving. The
vehicle control system with an adaptive control law keeps the speed of the vehicle as desired by applying more power to the drive wheels
where the additional driving force at the non-skidding wheel will compensate for the loss of the driving force at the skidding wheel, and
also arranges the direction of the vehicle motion by changing the steering angle of the two front steering wheels. Stability analysis proves
that the vehicle position and velocity errors are both bounded. With additional road surface information available, the adaptive control
system guarantees that the vehicle position error and velocity error converge to zero asymptotically even if the road surface parameters
are unknown.
Introduction
Initially, active control of vehicle dynamic systems is
used to reduce fuel consumption and emissions [1]. In current
automotive technology, active control is also used to
increase the safety and the reliability of the vehicle ride
and handling. Special control systems are designed for different
parts of the vehicle dynamics [2–4]. Anti-lock brake
systems (ABS) are designed to prevent skidding of the
wheels during braking [5,6], while traction control systems
(TCS) are accomplishing the same objective, preventing the
slipping of the wheels, during acceleration [6]. When the
vehicle is on a slippery surface, because of the drop at
the coefficient of road adhesion, the drive wheels may slip.
System structure and dynamics
In order to demonstrate the adaptive controller design
for the wheel slip/skid problem, an electrical vehicle is
designed, which is shown in Fig. 1. The back wheels of
the vehicle have in-wheel electrical motors and the speed
of the back wheels can be controlled separately. The front
wheels are used for the purpose of steering only and are
accompanied by steer by wire system.
For low speed applications, kinematic steering (a.k.a.
Ackermann steering) is used to model the motion of the
vehicle where the sideslip angles are small so that, the
wheels are assumed to be pure rolling. A single-track model
(a.k.a. bicycle model) is used to simplify the analysis. The
wheels on an axle are represented by a single wheel with
the double of cornering stiffness.
In this new study, dynamic steering is used, considering
the sideslip angles are big enough to create cornering
forces. A single-tack model is not applicable since the back
wheels are controlled separately and the cornering forces
effecting each front and back wheel can be different because
of the possible different road conditions acting on each
wheel.
Adaptive wheel skid control
System operation
Driver assistance systems can be categorized into three
different groups by their application area [4]. Longitudinal
control involves keeping the vehicle at a designated distance
from the prior vehicle maintaining a relatively constant
speed. This is accomplished by using cruise control
and adaptive cruise control systems, where radar is added
to a standard cruise control to measure the distance and
the speed of the prior vehicle to adjust the following distance
and the speed of the vehicle. Anti-lock brake systems
(ABS) and traction control systems (TCS) can be categorized
in the longitudinal vehicle control systems, where
these systems are designed to prevent skidding in the longitudinal
dynamics.
Lateral control of the vehicle is achieved by look-down
and look-ahead reference systems and accomplishes the
low speed steering of the vehicle. Lane keeping, lane changing,
turning, avoiding obstacles, steering over correction
because of a flat tire and lane departure problems are
solved by lateral control algorithms.
Concluding remarks
In this research, adaptive vehicle skid control concept is
developed. Dynamic equations of an electric vehicle are formulated
and the effect of the road friction coefficient is studied.
When the road condition information is not available,
the boundedness of the position and velocity error is
proved. In the second part of the research, the performance
of the controller is examined when the road condition is
measured (but the road friction coefficients are assumed
not to be known). Simulation results show the effectiveness
of the controller schemes. The effect of a flat tire and rollover
resistance of the vehicle, by selecting the steering angles
dfl and dfr differently, will also be studied for future research.