This thesis presents research into an improved active steering system technology for a
passenger car road vehicle, based on the concept of steer-by-wire (SBW) but possessing
additional safety features and advanced control algorithms to enable active steering
intervention. An innovative active steering system has been developed as 'Semi-Active
Steering' (SAS) in which the rigid steering shaft is replaced with a low stiffness resilient
shaft (LSRS). This allows active steer to be performed by producing more or less steer angle
to the front steered road wheels relative to the steering wheel input angle. The system could
switch to either being 'active' or 'conventional' depending on the running conditions of the
vehicle; e.g. during normal driving conditions, the steering system behaves similarly to a
power-assisted steering system, but under extreme conditions the control system may
intervene in the vehicle driving control. The driver control input at the steering wheel is
transmitted to the steered wheels via a controlled steering motor and in the event of motor
failure, the LSRS provides a basic steering function. During operation of the SAS, a reaction
motor applies counter torque to the steering wheel which simulates the steering 'feel'
experienced in a conventional steering system and also applies equal and opposite counter
torque to eliminate disturbance force from being felt at the steering wheel during active
control operation.
The thesis starts with the development of a mathematical model for a cornering road
vehicle fitted with hydraulic power-assisted steering, in order to understand the relationships
between steering characteristics such as steering feel, steering wheel torque and power boost
characteristic. The mathematical model is then used to predict the behaviour of a vehicle
fitted with the LSRS to represent the SAS system in the event of system failure. The
theoretical minimum range of stiffness values of the flexible shaft to maintain safe driving
was predicted.
Experiments on a real vehicle fitted with an LSRS steering shaft simulator have been
conducted in order to validate the mathematical model. It was found that a vehicle fitted with
a suitable range of steering shaft stiffness was stable and safe to be driven. The mathematical
model was also used to predict vehicle characteristics under different driving conditions
which were impossible to conduct safely as experiments.
Novel control algorithms for the SAS system were developed to include two main criteria,
viz. power-assistance and active steer. An ideal power boost characteristic curve for a
hydraulic power-assisted steering was selected and modified and a control strategy similar to
Steer-by-Wire (SBW) was implemented on the SAS system.
A full-vehicle computer model of a selected passenger car was generated using
ADAMS/car software in order to demonstrate the implementation of the proposed SAS
system. The power-assistance characteristics were optimized and parameters were determined
by using an iteration technique inside the ADAMS/car software. An example of an open-loop
control system was selected to demonstrate how the vehicle could display either under-steer
or over-steer depending on the vehicle motion.
The simulation results showed that a vehicle fitted with the SAS system could have a
much better performance in terms of safety and vehicle control as compared to a conventional
vehicle. The characteristics of the SAS system met all the requirements of a robust steering
system. It is concluded that the SAS has advantages which could lead to its being safely fitted
to passenger cars in the future.
Keywords: steer-by-wire, active steering, innovative, power-assisted steering, steering
control, flexible shaft, steering intervention, system failure, safety features
