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You know that when you turn the steering wheel in your car, the wheels turn.
Cause and effect, right? But a lot of interesting stuff goes on between the steering wheel
and the tyres to make this happen.
In this article, we'll see how the two most common types of car steering
systems work: rack-and-pinion and recirculating-ball steering. Then we'll examine
power steering and find out about some interesting future developments in steering
systems, driven mostly by the need to increase the fuel efficiency of cars. But first, let's
see what you have to do turn a car. It's not quite as simple as you might think!
The basic aim of steering is to ensure that the wheels are pointing in the
desired directions. This is typically achieved by a series of linkages, rods, pivots and
gears. One of the fundamental concepts is that of caster angle- each wheel is steered with
a pivot point ahead of the wheel; this makes the steering tend to be self-centering
towards the direction travel. There are basically two styles of vehicle steering systems,
rack and pinion and worm gear box. Rack and pinion steering is one of the oldest types
of steering systems and is still used today.
Rack-and-pinion steering is quickly becoming the most common type of
steering on cars, small trucks and SUVs. It is actually a pretty simple mechanism.p ^ rack
and pinion is a type of linear actuator that comprises a pair of gears which convert
rotational motion into linear motion.
^ rack-and-pinion gear set is enclosed in a metal tube, with each end of the
rack protruding from the tube. ^ rod, called a tie rod, connects to each end of the rack.
The pinion gear is attached to the steering shaft. When you turn the steering wheel, the
gear spins, moving the rack. The tie rod at each end of the rack connects to the steering
arm on the spindle.p
The rack-and-pinion gear set does two things:
Op It converts the rotational motion of the steering wheel into the linear
motion needed to turn the wheels.
Op It provides a gear reduction, making it easier to turn the wheels.
On most cars, it takes three to four complete revolutions of the steering
wheel to make the wheels turn from lock to lock (from far left to far right).
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Steering ratio refers to the ratio between the turn of the steering wheel (in
degrees) or handlebars and the turn of the wheels (in degrees).
The steering ratio is the amount of degrees you have to turn the steering
wheel, for the wheels to turn an amount of degrees. In motorcycles and bicycles, the
steering ratio is always 1:1, because the steering wheel will always follow the wheel. x:y
means that you have turn the steering wheel x degree(s), for the wheel(s) to turn y
degree(s). In most passenger cars, the ratio is between 12:1 and 20:1. Example: If one
complete turn of the steering wheel, 360 degrees, causes the wheels to turn 24 degrees,
the ratio is then 360:24 = 15:1 (360/24=15).
^ higher steering ratio means that you have to turn the steering wheel more,
to get the wheels turning, but it will be easier to turn the steering wheel. ^ lower steering
ratio means that you have to turn the steering wheel less, to get the wheels turning, but it
will be harder to turn the steering wheel. Larger and heavier vehicles will often have a
higher steering ratio, which will make the steering wheel easier to turn. If a truck had a
low steering ratio, it would be very hard to turn the steering wheel. In normal and lighter
cars, the wheels become easier to turn, so the steering ratio doesn't have to be as high. In
race cars the ratio becomes really low, because you want the vehicle to respond a lot
quicker than in normal cars. The steering wheel will also become a lot harder to turn.
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The term power steering is usually used to describe a system that provides
mechanical steering assistance to the driver of a land vehicle, for example, a car or truck.
The power steering system in a vehicle is a type of servomechanism.
For many drivers, turning the steering wheel in a vehicle that doesn't have
power steering requires more force (torque) than the driver finds comfortable, especially
when the vehicle is moving at a very slow speed. Steering force is very sensitive to the
weight of the vehicle, and nearly so much to its length, so this is most important for
large vehicles. In a vehicle equipped with power steering, when the driver turns the
steering wheel, she or he feels only a slight retarding force, so a vehicle equipped with
power steering can be driven by any healthy driver, even when the vehicle is being
parked. This is because the power steering system furnishes most of the energy required
to turn the steered wheels of the car.
Most power steering systems in cars and light trucks today are hydraulic
(that is, the force to turn the wheels is provided by a hydraulic piston, which is powered
by high pressure hydraulic fluid), but in some cars and trucks, the steering force is
provided by an electric motor.
Since the power-steering pump on most cars today runs constantly, pumping
fluid all the time, it wastes horsepower. This wasted power translates into wasted fuel.
You can expect to see several innovations that will improve fuel economy. One of the
coolest ideas on the drawing board is the "" or "
" system.
These systems would completely eliminate the mechanical connection between the
steering wheel and the steering, replacing it with a purely electronic control system.
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In a typical mechanical steering system the driver¶s steering input is
transmitted by a steering shaft through some type of gear reduction mechanism to
generate steering motion at the front wheels.
In the present day automobiles, power steering assist has become a standard
feature. ^ hydraulic power steering uses 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 is reduced.
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. ^n 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 is steer-by-wire technology.
The substitution of electronic systems in place of mechanical and hydraulic
controls is known as ³by-wire´ technology. The benefits of applying electronic
technology are improved performance, safety and reliability with reduced manufacturing
and operating costs. The idea is not new to airplane pilots and many modern aircraft,
both commercial and military, rely on ³fly-by-wire´ flight controls. However, only
recently has the electronic revolution begun to find its way into automotive systems.p^
number of current production vehicles already employ by-wire technology for throttle
and brakes. However, steer-by-wire is a more daunting concept than throttle- or brake-
by-wire.
The aim of steer-by-wire technology is to completely do away with as many
mechanical components (steering shaft, column, gear reduction mechanism, etc.) as
possible. Completely replacing conventional steering system with steer-by-wire holds
several advantages, such as:
p The absence of steering column simplifies the car interior design.
p The absence of steering shaft, column and gear reduction mechanism allows
much better space utilization in the engine compartment.
p The steering mechanism can be designed and installed as a modular unit.
p Without mechanical connection between the steering wheel and the road wheel,
it is less likely that the impact of a frontal crash will force the steering wheel to
intrude into the driver's survival space.
p Steering system characteristics can easily and infinitely be adjusted to optimize
the steering response and feel.
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The steer-by-wire system consists of two main parts.
Op The steering section consists of the steering wheel, the feedback actuator and the
angle sensor.
Op The wheel section contains the wheels, the rack and pinion, a steering actuator
and the pinion angle sensor.
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The steering wheel is the part of the steering system that is manipulated by
the driver and responds to driver inputs. The sizes of steering wheels vary depending on
the type of vehicle. In terms of outside diameter, the average is somewhere between 14½
inches and 17 inches. In terms of grip circumference, some wheels measure as thin as
2¾ inches. On the other hand, the grip of some wheels can be as thick as 4¼ inches.
^ Size-^ wheel has an outside diameter of 15 inches to 16 inches, while the
grip circumference varies from 2¾ inches to 3 1/8 inches. Meanwhile, the outside
diameter of a Size-^ wheel ranges from 14½ inches to 15½ inches with a grip
circumference of about 3¼ inches to 3½ inches. For a Size-^ wheel, the outside
diameter is similar to a Size-^ wheel but the grip circumference ranges from 3 5/8
inches to 3 7/8 inches. ^ Size-C wheel also has the same outside diameter but the grip
circumference is thicker ranging from 3 7/8 inches to 4¼ inches.
There is also a Size-B wheel, the outside diameter of which is 16½ inches to
17½ inches. The grip circumference of this type of wheel ranges from 2¾ inches to 3 1/8
inches. Lastly, there is also a Size-B wheel, the outside diameter of which is 16 inches
to 17 inches, while the grip circumference ranges from 3¼ inches to 3½ inches.
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The steering wheel is attached to the steering column and steering angle &
position sensor, feedback actuator is placed on this steering column.
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The steering angle sensor is mounted on the steering shaft. The steering angle
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 so the sensor should have very high sensitivity and accuracy. When the operator
turns the steering wheel, the sensors determine how far the driver has turned the wheel
and in which direction and the signals are sent to the control unit. The sensor may be
potentiometers or optical digital encoders. Optical digital encoders¶ precision and
accuracy make them preferable over potentiometers.
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Using this measuring system, every position of the angle is identified by a definite code
on a glass or plastic disc. This code is represented on the disc in the form of light and
dark regions within different tracks. This combination relates to an absolute numerical
value. Thus, the position value is always directly available, counters are not necessary.
In addition it is not possible to get continuously invalid values caused by interferences or
loss of the supply voltage. Movements which are done while the system is turned off are
immediately measured after the system is powered up.
The measuring system consists of a light source, a code disc pivoted in a precision ball
bearing and an opto-electronic scanning device (see Image 3). ^ LED is used as a light
source which shines through the code disc and onto the screen behind. The tracks on the
code disk are evaluated by an opto-array behind the reticle. With every position another
combination of slashes in the reticle is covered by the dark spots on the code disk and
the light beam on the photo transistor is interrupted. That way the code on the disc is
transformed into electronic signals. Fluctuations in the intensity of the light source are
measured by an additional photo transistor and another electronic circuit compensates
for these. ^fter the electronic signals are amplified and converted they are then available
for evaluation.
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The steering actuator gives the motion to the rack and pinion which turns the
wheels. The steering actuator needs to be very powerful in order to turn the wheels of a
car when the car is loaded. Minimizing the effects of unwanted disturbances also
requires a powerful motor. Different motors can be used as steering actuators but DC
separately excited motor is best suitable as the exact position of the wheels can be
controlled.
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The electric motor generates the required electrical torque for the desired motion
of steering actuator. This torque is measured by using the sensor. This sensor signal is
used for control of the feedback actuator and monitoring purposes.
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The road forces felt in the steering wheel give the driver feedback he can use to
anticipate and control the vehicle, or at least create the comfortable feeling that he is in
control of the vehicle. In the case of steer by wire, the driver will have the
uncomfortable feeling of being separated from the road wheels, not quite in control, and
will tend to over steer his vehicle, particularly in demanding situations such as sharp or
sudden turns. It is desirable to have a device that provides a mechanical back up "road
feel". The device can be a DC separately excited motor.
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Controller±area network (C^ or C^-bus) is a vehicle bus standard designed
to allow microcontrollers and devices to communicate with each other within a vehicle
without a host computer. C^ is a message based protocol, designed specifically for
automotive applications. C^ is a multi-master broadcast serial bus standard for
connecting control units. Each control can send or receive the message but not
simultaneously. ^ message consists primarily of an id, which represents the priority of
the message, and up to eight data bytes. It is transmitted serially onto the bus. This signal
pattern is encoded in non-return-to-zero (RZ) and is sensed by other control unit.pIf the
bus is free, any control unit may begin to transmit. If two control units begin sending
messages at the same time, the message with the more dominant id (which has more
dominant bits, i.e., zeroes) will overwrite other nodes' less dominant id's, so that
eventually (after this arbitration on the id.) only the dominant message remains and is
received by all nodes. This mechanism is referred to as priority based bus arbitration.
Messages with numerically smaller values of id have higher priority and are transmitted
first.
Each node requires a
Host processor:
Op The host processor decides what received messages mean and which messages it
wants to transmit itself.
Op Sensors, actuators and control devices can be connected to the host processor.
C^ controller: (hardware with a synchronous clock).
Op Receiving: the C^ controller stores received bits serially from the bus until an
entire message is available, which can then be fetched by the host processor
(usually after the C^ controller has triggered an interrupt).
Op Sending: the host processor stores its transmit messages to a C^ controller,
which transmits the bits serially onto the bus.
Transceiver (possibly integrated into the C^ controller):
Op Receiving: it adapts signal levels from the bus to levels that the C^ controller
expects and has protective circuitry that protects the C^ controller.
Sending: it converts the transmit-bit signal received from the C^ controller into a
signal that is sent onto the bus.
Rack-and-pinion steering is the most common type of steering. It is actually a
pretty simple mechanism. ^ rack-and-pinion gear set is enclosed in a metal tube, with
each end of the rack protruding from the tube. ^ rod, called a tie rod, connects to each
end of the rack. The pinion gear is attached to the steering shaft. When you turn the
steering wheel, the gear spins, moving the rack. The tie rod at each end of the rack
connects to the steering arm on the spindle.
The rack-and-pinion gear set does two things:
Op It converts the rotational motion of the steering wheel into the linear motion
needed to turn the wheels.
Op It provides a gear reduction, making it easier to turn the wheels.
