23-05-2012, 01:47 PM
BRUSH-LESS DC MOTOR DRIVE FOR STEER-BY-WIRE AND ELECTRIC POWER STEERING APPLICATIONS
BRUSH-LESS DC MOTOR DRIVE FOR STEER-BY-WIRE AND ELECTRIC POWER STEERING APPLICATIONS.PDF (Size: 469.16 KB / Downloads: 48)
Abstract:
Due to the growing demand of cars,
trucks, and sport utility vehicles (SUVs) in
today’s society, there becomes the need to
create automobiles that are optimized in
different aspects. In current cars, many
mechanical systems are robbing the
conventional internal combustion engine from
valuable horsepower. For instance, the steering
systems of the most later model vehicles could
be observed to be one of these resourcerobbing
systems because of their inefficient use
of either the rack and pinion system or the recirculating
ball system. In either case, they are
both mechanical systems that incorporate the
use of hydraulics (power steering), which prove
to be very inefficient systems. The implementation
of the electric power steering and steerby-
wire systems brings hope in creating
steering systems for most vehicles, which will
still be equipped with power steering. Ths is a
much safer and more efficient choice for the
future production of cars. This paper gives a
state-of-the-art review of automotive steering
systems, which present numerous application
opportunities for advanced electric motor
drives, specially brush-less DC (BLDC) motor
drives.
I. CONVENTIONAL STEERING SYSTEMS
The rack and pinion steering systems could be
observed to be the most common type of steering
found on most cars, small trucks, and SWs. This
system in actuality is a very simple system that
could be broken down into its individual parts, as
shown in Figure 1. The rack and pinion gear-set is
enclosed in a metal tube with both of the ends
sticking out. A tie rod is then attached to these
overhanging ends on the rack. The tie rod is then
connected to the steering arm that is attached to
the automobile’s wheel, which allows it to turn in
the desired direction. The pinions gear is directly
attached to the steering shaft, which is connected
to the steering wheel. As a result, when the
steering wheel is turned, the pinion is turned,
which cause the rack to move either left or right
and as a desired outcome, it causes the wheels to
turn in the specified direction.
The rack and pinion gearing system actually only
accomplishes two goals. It converts the rotational
motion that the driver applies to the steering
wheel into a linear motion, which is needed to
turn the wheels. In addition, due to the large size
of the steering wheel in comparison to the small
pinion in the rack and pinion system, a gear size
reduction is created, which makes it much easier
to turn the wheels. Although rack and pinion
steering system could be considered to be the
most commonly found steering system, it is not
the only system used.
Figure 1. Rack and pinion steering system
(Courtesy of How Stuff Works).
The re-circulating ball system, as shown in Figure
2, is also a very popular system, which could be
found on many trucks and SWs. It is
significantly different than the rack and pinion
system; however, it could still be broken down
into its base components, which are steering
wheel, re-circulating ball gearbox, and pitman
arm. The steering wheel, directly connected to the
re-circulating ball gearbox, turns the pitman arm.
The pitman arm is connected to the track rod,
which connects to the tie rods directly controlling
the wheels they are connected to. They are located
at both of the ends on the track. The box could be
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observed as two parts. The fust piece is a block of
metal with a threaded hole drilled on the inside
and gear teeth cut on the outside. The teeth
located on the outside of the block are what
engage the gear that moves the pitman arm. The
steering wheel connects to a threaded rod that
sticks inside the hole of the block of metal. When
the steering wheel is turned, it turns the bolt,
which is held in a fixed position, therefore,
causing the block to move up and down the rod
that results in the gear turning the wheels to move.
The re-circulating ball steering gearbox contains
a worm gear. Instead of having the shaft make
direct contact with the threads in the block, all the
threads are filled with ball bearings. These ball
bearings circulate through the gear as it turns in
the box. The result of using these ball bearings is
that they reduce friction and wear in the gear
system. They also help to reduce slop, which is a
free movement feeling that is experienced when
changing the direction of the steering wheel.
Figure 2. Re-circulating ball steering system
(Courtesy of How Stuff Works).
11. CONVENTIONAL POWER STEERING SYSTEMS
Power steering is very similar for both the
rack and pinion and re-circulating ball steering
systems. The power steering system, as shown in
Figures 3 and 4, primarily consists of a rotary
valve, a steering gear, and a pump that is powered
by a belt driven pulley. In order to obtain the
hydraulic power necessary for steering assistance,
a rotary-valve pump is used. This pump is driven
by a belt and pulley system driven by the car's
engine. Inside the pump is a set of retractable
vanes similar to propeller blades on a fan that spin
inside an oval casing. As these vanes spin, they
pull low-pressure hydraulic fluid from the return
line and force it out at an outlet as a much higher
pressure. This amount of flow that is created by
the vanes is determined by the car's engine speed.
As a safety precaution, a relief-valve is part of the
pump system to make sure that the pressure does
not get too high, specifically when the engine is
revving hard causing a large amount of fluid to he
pumped out.
Figure 3. Conventional power steering system
(Courtesy of How Stuff Works).
Steering
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Figure 4. Typical hydraulic assisted steering
layout (Courtesy of How Stuff Works).
In these power steering systems, a cylinder with a
piston in the middle is attached to the rack (rack
and pinion) or track rod (re-circulating ball). On
the piston are two fluid ports, located on the
opposite sides of the piston. These ports are
connected to the lines that provide the flow of
hydraulic fluid needed to the specified side of the
piston forcing the piston to move, as a result,
creating assistance to the steering. As a safety
precaution, it should be noted that in the case of a
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power steering malfunction or failure, manual
(non-assisted) steering would always be available.
111. ADVANCEMENTS IN POWER STEERING SYSTEMS
the torsion bar signals the rotary valve to allow
fluid flow from Line 1 to Line 3 and from Line 4
to Line 2. That forces the hydraulic piston towards
the driver side providing most of the force needed
to make a rie-h t-hand turn. Similarlv, to make a
left-hand turn, the rotary valve allows fluid flow
from Line 1 to Line 4 and from Line 3 to Line 2
causing the hydraulic piston to move towards the
With the increase in size and weight of
automobiles, power steering has been developed
to assist motorists with slow speed maneuvering "nQCPnnPI &,io p..,"'..i"L I L V 1 . such as parking. A power steering system
provides most of the torque necessary to
overcome the friction between the wheels and the
road. As was explained, the most common power
steering system in use today is hydraulic power
steering. A simplified diagram for such a system
is illustrated in Figure 5.
Figure 5. Hydraulic power steering system.
The major components in Figure 5 include the
hydraulic pump and reservoir, rotary valve,
steering wheel, torsion bar, rack and pinion,
hydraulic piston, and the hydraulic lines that
connect the system together. The hydraulic pump
and reservoir, powered by the engines drive-belt,
provide pressurized hydraulic fluid to the system
through Line 1. The rotary valve directs fluid flow
from Line 1 to Line 2, Line 3, or Line 4
depending on the desired steering operation.
Steering commands are inputted through the
steering wheel to the torsion bar. The torsion bar
has two functions; one is to convert torque to
lateral force via the rack and pinion, the other is to
signal the rotary valve to assist the steering
maneuver.
For a right-hand turn, the driver turns the steering
wheel clockwise (directions are referenced to the
driver in the driver seat). That spins the torsion
bar in the same direction causing the rack to move
laterally towards the driver side. At the same time,
Whenever the wheels are in the centered position,
no power steering assistance is needed. The
hydraulic fluid simply circulates through Line 1
and Line 2 resulting in a major inefficiency for the
hydraulic power steering system. In fact, the
hydraulic pump is always working, even when it
is not needed for steering assistance.
In an aim to improve the efficiency of a hydraulic
power steering system, the engine drive-belt can
be replaced with an electric motor. The electric
motor provides power to the hydraulic pump only
when turning of the wheels is desired. This
greatly improves the efficiency of the power
steering system; hut, it adds cost and time to the
manufacturing process along with weight to the
system. Therefore, a step further has been taken to
develop a new type of power steering system,
namely electric power steering.
IV. ELECTRIC POWER STEERING SYSTEMS
It has been made aware that, in all the current
conventional steering systems, the steering wheel
and the column count as a major source for injury
to the driver in automobile front-end collisions.
As a result, numerous amount of energyabsorbing
and non-intrusion designs have been
developed. Energy-absorbing columns were
developed in order to serve two purposes. First,
they must stop the wheel and column from being
pushed to the rear or the car in the event that car is
crushed from a frontal impact. Secondly, the
energy-absorbing column must be able to provide
the driver with a tolerable amount of resistance as
hdshe thrusts forward and strikes the wheel with
hisher chest. Although the idea does not seem to
pose any alarm or concern, one major problem
that has risen is that the collapse of the column
due to a kontal impact should not obstruct its
ability to provide a proper "ride down" for the
driver's chest.
The problem with the power steering system is
that the rotary-valve pump must provide a
sufficient flow of hydraulic fluid when the engine
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IS at an idling state. As a direct result of this, the
pump moves a much larger amount of fluid than is
needed when the engine runs at faster speeds. It is
noted that, when a vehicle is being driven and the
steering wheel is not being turned, it is necessary
for both the hydraulic lines to provide the same
amount of pressure to the steering gear. When the
steering wheel is turned, a spool valve is turned in
one direction or the other and ports open to allow
the flow of the high-pressure fluid needed in the
appropriate line.
Electric type of power steering system completely
does away with all hydraulic components. It
provides steering assistance with an electric motor
that directly assists steering maneuvers only when
turning is desired. The electric motor may be
mounted to assist lateral motion, as shown in
Figure 6 (a), or to assist circular motion, as shown
in Figure 6 (b). The control system for the electric
motor consists of the typical components for an
electric motor drive. The controller uses torque
commands and currentlvoltage feedback to
control the power electronic converter. The
converter then outputs the voltage necessary to
carry out the desired steering operation.
V. STEER-BY-WIRE
Steering systems in today's modern vehicles have
adapted many new ideas and technologies, which
has ultimately directed them to experiment with
the idea of steering by wire. The following is the
chronological order of the essential steps and
breakthroughs that were taken that created the
path for the discovery of the steer-by-wire steering
system.
Due to the constant pumping of ffuid in the
power-steering pump on most cars today, much
valuable horsepower is wasted. This wasted
horsepower translates into wasted fuel that
everyone could do without. The variabledisplacement
power-steering pump is a device that
is already utilized in a few of the cars in
production today. This pump reduces the amount
of fluid that is being pumped at higher speeds,
when it is not necessary to have the assistance of
power steering, thereby, causing a reduction in the
amount of power that is consumed from the
engine.
Taking the next step forward, the
development of electro-hydraulic systems came
about. In this system, an electric motor with
variable speeds would be used to power the
hydraulic steering pump. This would allow the
motor to tum off when no steering maneuvers are
undertaken, therefore, causing a reduction in the
amount of power that is being consumed.
Passengi Si& Driver side
FmnlTi FmoN'Ti
(a)
Passenger Sldc
Fmnl T i
I -
(h)
Figure 6. Electrically-assisted power steering
Once again, if we were to take the technology of
steering one step further, the development of
electric power steering would be explored. In this
system, all the hydraulic equipment would be
eliminated completely. Instead of having
hydraulics assist in power steering, an electric
motor would be directly mounted on the rack to
assist in steering. Electronic sensors would be
strategically places to the steering wheel sending
signals down to a control system that controls the
electric motor on the rack. This would allow the
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electric motor to provide the proper amount of
assistance.
The gradual evolution and breakthrough
discoveries in automotive steering finally lead to
the discussion of the steer-by-wire system. In this
system, the mechanical connection that is placed
between the steering wheel and the steering would
be eliminated completely. As a replacement, there
would be a purely electronic control system. This
form of steering would contain sensors that would
send signals to tell the car what the driver wants
the wheels to do. It is even possible for motors in
the steering system to provide the driver feedback
on what the car is experiencing.
VI. COMPARISON OF CONVENTIONAL STEERING VERSUS STEER-BY-WIRE
The major benefit with the current manual
steering systems (rack and pinion, re-circulating
ball) is that we are using technology that has been
well developed and known; however, the major
disadvantage to this is the high steering effort that
is due to the increasing from-axle loads and tire
widths. To resolve this problem, hydraulic power
steering was developed. Although hydraulic
power steering reduces steering effort and allows
for manual steering in case of a hydraulic pressure
decrease, there is the issue that it requires the use
of hydraulics and the large amount of energy that
is consumed because of the directly driven boost
pump. Generally, traditional steering systems are
no longer acceptable due to numerous reasons. To
begin with, the steering column is a major
inconvenience when pertaining to crash smcture
development. The steering assembly also heavily
influences the engine compartment package. It is
important to note that the disposal of hydraulics
becomes an important issue. The driver support
system also only allows for longitudinal
interventions (brakes-ABS, engine moment-ASC,
brakes and engine movement-DSC). Lateral
interventions through steering would greatly
improve active safety.
With the development of electric power steering,
there becomes the ability for variable steering
assistance, which results in less energy being
consumed. As a major added bonus, there is no
longer the need for hydraulics. The problem with
this system, however, is that there is no driver
independent steering assistance and there is still
the presence of the inconvenient steering column.
This system would not be considered as a steerby-
wire system due to the mechanical connection.
A steer-by-wire system with mechanical or
hydraulic backup is also a steering system that
could be observed. The benefit to this system is
that there is driver independent steering
assistance, however, there is the disadvantage of
needing a safety case for switchover and there is
no guarantee in the function of the backup. When
having a mechanical backup system, the
hydraulics are eliminated; however,. the steering
column as well as the clutch assembly will still be
present; this would then result in package
difficulties.
If we were to choose to have a hydraulic backup,
this would allow for the elimination of the
steering column, however, would now require
hydraulics and a boost pump. This would
ultimately lead to the electric redundant steer-bywire
system. In this system, there would be no
steering column and hydraulics, which therefore
would eliminate the need for a hydraulic or
mechanical switchover mechanism. This system
would take advantage of all the steer-by-wire
technology. As a result, there would only be the
need for one system technology instead of three
(standard, backup, and switchover mechanism).
The only problem with this option would be the
requirement of a safety case for the redundant
electric system.