30-05-2013, 11:45 AM
Steer-by-Wire Control System
Steer-by-Wire.pdf (Size: 1.93 MB / Downloads: 139)
Abstract
The automotive industry has already implemented many advanced computer systems in
an attempt to increase safety and comfort of drivers. In parallel with these advancements
we see a big shift from mechanical systems to electrical systems and steer-by-wire is another
implementation that is very promising in terms of safety and functionality. Already, there
are some commercial prototypes of such ‘by-wire’ systems[1] and there is a lot of research,
both academic[2] and comercial[3], in the field. For my Engineering Senior Design Projet at
Swarthmore College, I chose to work on a steer-by-wire system to gain more insight into control
theory and I thought the double-control system that provided the crucial feedback to the driver
was an interesting engineering problem.
Introduction
Definition and Benefits of Steer-By-Wire
A steer-by-wire system aims to eliminate the physical connection between the steering wheel and
the wheels of a car by using electrically controlled motors to change the direction of the wheels and
to provide feedback to the driver.
Today’s automobiles benefit more and more from the many uses of electronic systems. The
integration of a steer-by-wire system can enhance these systems in many ways. In particular, the
handling and the safety of the cars can be improved significantly. Since a steer-by-wire system is
easily modifiable, different drivers will be able to adjust the system to accommodate their styles
and this will enhance handling. I addition, disabled people and the elderly will benefit immensely
from steer-by-wire because they will be able to situate the steering wheel to meet special needs.
Traction control systems are very closely tied with driving safety and they can be enhanced with
steer-by-wire vastly. For instance, in a situation where the car starts oversteering (when the rear
of the vehicle heads towards the outside of the corner), the natural instinct of many inexperienced
drivers is to turn the steering wheel towards the inside, which in turn causes more oversteer. A
steer-by-wire system could be modified to take control in a situation like this to steer to the outside.
Since there are virtually no physical connections between the steering wheel and the wheels,
a steer-by-wire system can be implemented on different cars easily. The steering wheel could be
placed on either side of a car (or anywhere else). Both of these improvements would reduce costs
of production and allow a wider range of designs.
Design
System Overview
The steer-by-wire system consists of two main parts. The steering section consists of the steering
wheel, the feedback actuator and the feedback actuator angle sensor. The wheel section contains
the wheels, the rack and pinion, a steering actuator and the pinion angle sensor. Figure 2 shows
the system components. In my system I only demonstrated the double control mechanism and did
not implement it in a rack and pinion configuration.
The feedback angle sensor provides the steering actuator with its primary input signal and the
pinion angle sensor provides the feedback motor primary signal. The small size of the feedback
motor lets the driver rotate the steering wheel with little difficulty. As soon as the driver starts
steering, the control mechanism tries to push the steering wheel back into place (and the wheels
into the position dictated by the current position of the steering wheel) and this mimics the resistive
force of a real steering wheel. However, changing the proportional constant of the feedback motor
can make it harder/easier for the driver to steer and allows for adjustable steering (with some
drawbacks such as more vibration).
Angular Sensors
The angular sensors of the system are very crucial and they need to be very accurate because little
perturbations or errors ultimately make the control of the system much harder for a driver. In a real
implementation of a steer-by-wire system then would have be very high sensitivity and accuracy
in order to minimize risks. In my project I used two optical digital encoders that were used in a
previous project. These are BEI Duncan’s EX-11 and MX-15 encoders and both of the sensors are
seen in Figure 5.
Computer Modeling
Alhough the control system can be done in hardware, it is much easier to implement the control
system in software. I chose to use Matlab because I was familiar with it from previous classes.
There were some simulations of steer-by-wire systems done using Matlab[4] so I knew it would be
a great choice.
Simulink Modeling
Most of the modeling and control is done using SimuLink and Matlab . The Real-Time Workshop
in Simulink allows the user to design the controller in blocks and then compiles the scheme and
runs the program in real-time as long as the computer is on. As opposed to past years, when the
DAQ Board could only be used for a limited number of data samples (and therefore time), the
Real-Time Workshop is a huge improvement.
In the steer-by-wire system, I implement the model seen in Figure 11. Although the controls of
both motors seem similar, their inherent differences require different control parameters. The input
to both controllers is governed by the difference in the rotary sensor readings. In other words, the
error signal is the difference in the angular sensor voltages. Both systems use PID controllers but
the parameters vary. Also, since integration causes more noise, the integration constant is very low.