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1460035692-2005ThesisYihSteerbyWireImplicationsforVehicleHandlingandSafety.pdf (Size: 5.27 MB / Downloads: 23)
Abstract
Recent advances toward steer-by-wire technology have promised significant improvements
in vehicle handling performance and safety. While the complete separation of
the steering wheel from the road wheels provides exciting opportunities for vehicle
dynamics control, it also presents practical problems for steering control. This thesis
begins by addressing some of the issues associated with control of a steer-by-wire
system. Of critical importance is understanding how the tire self-aligning moment
acts as a disturbance on the steering system. A general steering control strategy has
been developed to emphasize the advantages of feedforward when dealing with these
known disturbances. The controller is implemented on a test vehicle that has been
converted to steer-by-wire.
One of the most attractive benefits of steer-by-wire is active steering capability.
When supplied with continuous knowledge of a vehicle’s dynamic behavior, active
steering can be used to modify the vehicle’s handling dynamics. One example presented
and demonstrated in the thesis is the application of full vehicle state feedback
to augment the driver’s steering input. The overall effect is equivalent to changing a
vehicle’s front tire cornering stiffness. In doing so, it allows the driver to adjust a vehicle’s
fundamental handling characteristics and therefore precisely shift the balance
between responsiveness and safety.
Another benefit of steer-by-wire is the availability of steering torque information
from the actuator current. Because part of the steering effort goes toward overcoming
the tire self-aligning moment, which is related to the tire forces and, in turn, the
vehicle motion, knowledge of steering torque indirectly leads to a determination of
the vehicle states, the essential element of any vehicle dynamics control system. This relationship forms the basis of two distinct observer structures for estimating vehicle
states; both observers are implemented and evaluated on the test vehicle. The results
compare favorably to a baseline sideslip estimation method using a combination of
Global Positioning System (GPS) and inertial navigation system (INS) sensors.
Introduction
1.1 Evolution of automotive steering systems
The proliferation of electronic control systems is nowhere more apparent than in the
modern automobile. During the last two decades, advances in electronics have revolutionized
many aspects of automotive engineering, especially in the areas of engine
combustion management and vehicle safety systems such as anti-lock brakes (ABS)
and electronic stability control (ESC). The benefits of applying electronic technology
are clear: improved performance, safety, and reliability with reduced manufacturing
and operating costs. However, only recently has the electronic revolution begun to
find its way into automotive steering systems in the form of electronically controlled
variable assist and, within the past two years, fully electric power assist [5, 40].
The basic design of automotive steering systems has changed little since the invention
of the steering wheel: the driver’s steering input is transmitted by a shaft
through some type of gear reduction mechanism (most commonly rack and pinion or
recirculating ball bearings) to generate steering motion at the front wheels. One of
the more prominent developments in the history of the automobile occurred in the
1950s when hydraulic power steering assist was first introduced. Since then, power
assist has become a standard component in modern automotive steering systems. Using
hydraulic pressure supplied by an engine-driven pump, power steering amplifies
and supplements the driver-applied torque at the steering wheel so that steering effort
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CHAPTER 1. INTRODUCTION 2
is reduced. In addition to improved comfort, reducing steering effort has important
safety implications as well, such as allowing a driver to more easily swerve to avoid
an accident.
The recent introduction of electric power steering in production vehicles eliminates
the need for the hydraulic pump. Electric power steering is more efficient than
conventional power steering, since the electric power steering motor only needs to provide
assist when the steering wheel is turned, whereas the hydraulic pump must run
constantly. The assist level is also easily tunable to the vehicle type, road speed, and
even driver preference [32, 6]. An added benefit is the elimination of environmental
hazard posed by leakage and disposal of hydraulic power steering fluid.
The next step in steering system evolution—to completely do away with the steering
column and shaft—represents a dramatic departure from traditional automotive
design practice. The substitution of electronic systems in place of mechanical or hydraulic
controls is known as by-wire technology. This idea is certainly not new to
airplane pilots [46]; many modern aircraft, both commercial and military, rely completely
on fly-by-wire flight control systems (Figure 1.1). By-wire technology paved
the way for high performance aircraft designed to have a degree of maneuverability
never before possible. If not for the intervention of flight control computers, some of
these planes—because they are inherently unstable—could not be flown by human
pilots without crashing.
1.2 Technical advantages of steer-by-wire
A number of current production vehicles already employ by-wire technology for the
throttle and brakes (Figure 1.2) [21]. A few supplement conventional front steering
with rear steer-by-wire to improve low speed maneuverability and high speed stability
[7, 53]. Completely replacing conventional steering systems with steer-by-wire, while a
more daunting concept than throttle- or brake-by-wire for most drivers, holds several
advantages. The absence of a steering column greatly simplifies the design of car
interiors. The steering wheel can be assembled modularly into the dashboard and
located easily for either left- or right-hand drive. The absence of a steering shaft
allows much better space utilization in the engine compartment. Furthermore, the
entire steering mechanism can be designed and installed as a modular unit. Without
a direct mechanical connection between the steering wheel and the road wheels, noise,
vibration, and harshness (NVH) from the road no longer have a path to the driver’s
hands and arms through the steering wheel. In addition, during a frontal crash,
there is less likelihood that the impact will force the steering wheel to intrude into
the driver’s survival space. Finally, with steer-by-wire, previously fixed characteristics
like steering ratio and steering effort are now infinitely adjustable to optimize steering
response and feel.
Undoubtedly the most significant benefit of steer-by-wire technology to driving
safety and performance is active steering capability: the ability to electronically augment
the driver’s steering input. As a part of fully integrated vehicle dynamics control,
the first active steering system for a production vehicle was recently introduced in
the 2004 BMW 5-Series. While not yet a by-wire system, this feature demonstrates
the sort of handling improvements that can be made to a vehicle equipped with true
steer-by-wire. Similar to electronic stability control (ESC) systems that have been
available for several years, active steering is able to maintain vehicle stability and maneuverability
by interceding on behalf of the driver when the vehicle approaches its
handling limits, such as during an emergency maneuver, or when driving conditions
call for a change in steering response.
Statistical and empirical studies have shown a substantial reduction in the accident
rate for vehicles equipped with ESC [4, 10, 12, 25, 28, 38]. However, active
steering and steer-by-wire technology take vehicle control one step further. In current
ESC systems, a computer analyzes information from multiple vehicle sensors
and intervenes on behalf of the driver to prevent potentially catastrophic maneuvers
by either selectively braking individual wheels or reducing engine power. Because
these types of systems are motivated by safety, their engagement sometimes interrupts
the continuity of driving feel and therefore limits the vehicle’s performance
envelope. Steer-by-wire introduces the possibility that one can indeed have the best
of both worlds: improved driving safety and handling performance. Instead of intruding
suddenly, a steer-by-wire system smoothly integrates steering adjustments during
an emergency maneuver to maintain stability [7]. The benefits go beyond stability
control: for example, a large, heavy vehicle can be made to feel as responsive as a
smaller, lighter vehicle during normal driving. The ability to actively steer the front
wheels allows artificial tuning of a vehicle’s handling characteristics to suit the driver’s
preference.
Furthermore, in some cases it is actually advantageous to utilize steering instead
of differential braking to generate yaw moment, because steering requires less friction
force between the tires and ground. Consider the case when the rear tires have reached
their limits of adhesion during cornering, e.g. a rear wheel slide; the only means of
control are the front wheels. This situation typically leads to a spinout or, with
poorly timed steering inputs, a violent fishtailing that is nearly impossible to control.
To generate a correcting yaw moment, one can either apply braking to the outside
front wheel or counter steer the front wheels (Figure 1.3). The moment generated by
differential braking is:
M = Fx
t
2