UWE Bristol Engineering showcase 2015

Overview of Four Wheel Steering:
Adam Green
Beng Mechanical Engineering
The concept of Four Wheel Steering (FWS) is a
chassis control method that was first considered for
mass production in the mid-1980’s by some
Japanese manufacturers. The systems did not last
long in production and were discontinued in
production vehicles until Porsche reintroduced the
ability to steer the rear wheels In their high-end
performance cars. Considering the purpose of these
cars is to be fast it makes sense that there is an
advantage to be gained in performance by using
FWS. Therefore the purpose of this project is to
investigate to what extent the advantages are to be
gained from using FWS for a performance vehicle.
Vehicle Equations of Motion:
Using the bicycle vehicle model the equations of
motion are deduced from Newton’s second law of
motion and the manipulation of simple laws of
geometry. The equations are rewritten using state
space form to allow them to be modelled more
efficiently in Simulink. The inputs to the equations
are front and rear wheel steering angle, and the
outputs are lateral velocity and yaw rate.
Rear Wheel Steering Control Strategies:
Proportional FWS - Steering the rear wheels as a
constant function of the front wheels
FWS with Feedback Control - Steering the rear
wheels in order to achieve a desired yaw rate value.
FWS with Rear Advance - Steering the rear
wheels out of phase at the beginning of turn in,
followed by in phase instantaneously afterwards.
Four Wheel Steering for Performance Vehicles
Vehicle Modelling & Simulation:
An improved response is defined as less yaw
oscillation, decreased time to steady state, less yaw
overshoot and greater lateral acceleration. If the
use of FWS depicts any of these desired
characteristics, then an advantage can be seen from
using FWS.
Vehicle model
Simulink vehicle model
Simulation Results:
Proportional FWS – TWS Yaw Rate Comparison
FWS with Feedback Control Simulink Vehicle
Model
FWS with Feedback Control – TWS Yaw Rate
Comparison
It is seen on the graph that FWS with Feedback
Control shows decreased settling time, less yaw
rate oscillation and less yaw rate overshoot. This
shows greater vehicle stability through a corner
when compared to TWS. Therefore an improved
vehicle response has been achieved through use of
FWS.
Global Displacement Vehicle Path Modelling:
What can be seen from the S-bend turn modelled
is that FWS adheres to the desired vehicle path
more effectively than TWS. This provides further
evidence for the lower stability of a vehicle using
TWS compared to FWS and thus provides further
evidence for the advantages that are gained from
using FWS in a performance vehicle.
Project Supervisor
Dr Benjamin Drew
Project summary
The project investigates ways of how to improve a
performance/motorsport vehicle’s handling
performance through the use of FWS. Vehicle
response to different rear axle steering control
strategies shall be modelled using Simulink
simulation software and the results shall be compared
to that of a TWS vehicle. The potential benefits that
can be seen from using an integrated chassis control
system are discussed and analysed. Conclusions based
on the simulation results were used to conclude to
what extent FWS has a positive impact on vehicle
response.
Project Objectives
• Develop an understanding of what FWS is and
how FWS has been used in the past
• Produce a Dynamic vehicle model with FWS using
Simulink simulation software.
• Investigate different rear axle control strategies and
their effects on vehicle system response
• Compare lateral velocity, lateral acceleration and
yaw rate system responses to conventional TWS
• Plot TWS, FWS and desired vehicle paths using
global positioning coordinates
• Analyze the results to deduce if an improved
response has been achieved through the use of
FWS
• Conclude to what extent FWS has effected the
vehicle response and if an improved response has
been achieved.
Project Conclusion
The results achieved in the investigation were that in
theory there are significant advantages to be gained
for a performance/motorsports vehicle using FWS.
This was shown through the improved response to
yaw rate oscillations, time taken and stability through
transient conditions shown both on the yaw rate
graphs and vehicle path plots. However the results are
bound by the simplicity of the vehicle model used in
the investigation. Introducing further degrees of
freedom to the vehicle model is predicted to further
improve vehicle response.