FOUR WHEEL STEERING SYSTEM
1Kudumula Harish, 2D Sunny
1Chaitanya Bharathi
Institute of Technology,Mechanical Engg ¾
2 Chaitanya
Bharathi Institute of Technology,Mechanical Engg 3/4
ABSTRACT
This paper deals with the details of four wheel
steering (4WS) system.With the help of this system all the four wheels can be
turned to any direction using the steering. Thus the vehicle can be controlled
more effectively especially during cornering and parking. Also the speed of the
vehicle can be increased or decreased.
Four-wheel steering requires less driver input for any steering
maneuver if all four wheels are steering the vehicle. As with two-wheel steer
vehicles, tire grip holds the four wheels on the road. However, when the driver
turns the wheel slightly, all four wheels react to the steering input, causing
slip angles to form at all four wheels. The entire vehicle moves in one
direction rather than the rear half attempting to catch up to the front.
This system finds application in off-highway vehicles
such as fork lifts, agricultural and construction equipment and mining
machinery. It is also useful in passenger cars, mainly SUVs.
1. INTRODUCTION
Four-wheel steering, 4WS,
also called rear-wheel steering or all-wheel steering, provides a means to
actively steer the rear wheels during turning maneuvers. It
should not be confused with four-wheel drive in which all four wheels of a
vehicle are powered. It improves handling and help the vehicle make tighter
turns.
Production-built
cars tend to understeer or, in few instances, oversteer. If a car could automatically
compensate for an understeer/oversteer problem, the driver would enjoy nearly
neutral steering under varying conditions. 4WS is a serious effort on the part
of automotive design engineers to provide near-neutral steering.
The front wheels do most of the steering.
Rear wheel turning is generally limited to 50-60 during
an opposite direction turn. During a same direction turn, rear wheel steering
is limited to about 10-1.50.
When both the front and
rear wheels steer toward the same direction, they are said to be in-phase and
this produces a kind of sideways movement of the car at low speeds. When the
front and rear wheels are steered in opposite direction, this is called
anti-phase, counter-phase or opposite-phase and it produces a sharper, tighter
turn.
2. WHY FOUR-WHEEL STEERING SYSTEM?
To
understand the advantages of four-wheel steering, it is wise to review the
dynamics of typical steering maneuvers with a conventional front -steered
vehicle. The tires are subject to the forces of grip, momentum, and steering
input when making a movement other than straight-ahead driving. These forces
compete with each other during steering maneuvers. With a front-steered
vehicle, the rear end is always trying to catch up to the directional changes
of the front wheels. This causes the vehicle to sway. As a normal part of
operating a vehicle, the driver learns to adjust to these forces without thinking
about them.
When turning, the driver is putting into
motion a complex series of forces. Each of these must be balanced against the
others. The tires are subjected to road grip and slip angle. Grip holds the
car's wheels to the road, and momentum moves the car straight ahead. Steering
input causes the front wheels to turn. The car momentarily resists
the turning motion, causing a tire slip angle to form. Once the vehicle begins
to respond to the steering input, cornering forces are generated. The vehicle
sways as the rear wheels attempt to keep up with the cornering forces already
generated by the front tires. This is referred to as rear-end lag, because
there is a time delay between steering input and vehicle reaction. When the
front wheels are turned back to a straight -ahead position, the vehicle must
again try to adjust by reversing the same forces developed by the turn. As the
steering is turned, the vehicle body sways as the rear wheels again try to keep
up with the cornering forces generated by the front wheels.
The idea behind four-wheel steering is
that a vehicle requires less driver input for any steering maneuver if all four
wheels are steering the vehicle. As with two-wheel steer vehicles, tire grip
holds the four wheels on the road. However, when the driver turns the wheel
slightly, all four wheels react to the steering input, causing slip angles to
form at all four wheels. The entire vehicle moves in one direction rather than
the rear half attempting to catch up to the front. There is also less sway when
the wheels are turned back to a straight-ahead position. The vehicle responds
more quickly to steering input because rear wheel lag is eliminated.
3. TYPES OF 4WS
There
are three types of production of four-wheel steering systems:
3.1
Mechanical 4WS
3.2
Hydraulic 4WS
3.3
Electro-hydraulic 4WS
3.1
Mechanical 4WS
Figure 1. Mechanical 4WS
In a straight-mechanical type of 4WS, two
steering gears are used-one for the front and the other for the rear wheels. A
steel shaft connects the
two steering gearboxes and
terminates at an eccentric shaft that is fitted with an offset pin. This
pin engages a second offset pin that fits into a planetary gear.
The planetary gear meshes with the
matching teeth of an internal gear that is secured in a fixed position to the
gearbox housing. This means that the planetary gear can rotate but the internal
gear cannot. The eccentric pin of the planetary gear fits into a hole in a
slider for the steering gear.
A 120-degree turn of the steering
wheel rotates the planetary gear to move the slider in the same direction that
the front wheels are headed. Proportionately, the rear wheels turn the steering
wheel about 1.5 to 10 degrees. Further rotation of the steering wheel, past the
120degree point, causes the rear wheels to start straightening out due to the
double-crank action (two eccentric pins) and rotation of the planetary gear.
Turning the steering wheel to a greater angle, about 230 degrees, finds the
rear wheels in a neutral position regarding the front wheels. Further rotation
of the steering wheel results in the rear wheels going counter phase with
regard to the front wheels. About 5.3 degrees maximum counter phase rear steering
is possible.
Mechanical 4WS is steering angle
sensitive. It is not sensitive to vehicle road speed.
3.2
Hydraulic 4WS
Figure 2. Hydraulic 4WS
The
hydraulically operated four-wheel-steering system is a simple design, both in
components and operation. The rear wheels turn only in the same direction as
the front wheels. They also turn no more than 11/2 degrees. The system
only activates at speeds above 30 mph (50 km/h) and does not operate when the
vehicle moves in reverse.
A two-way hydraulic cylinder mounted on
the rear stub frame turn the wheels. Fluid for this cylinder is supplied by a
rear steering pump that is driven by the differential. The pump only operates
when the front wheels are turning. A tank in the engine compartment supplies
the rear steering pump with fluid.
When
the steering wheel is turned, the front steering pump sends fluid under
pressure to the rotary valve in the front rack and pinion unit. This forces
fluid into the front power cylinder, and the front wheels turn in the direction
steered. The fluid pressure varies with the turning of the steering wheel. The
faster and farther the steering wheel is turned, the greater the fluid
pressure.
The fluid is also fed under the same
pressure to the control valve where it opens a spool valve in the control valve
housing. As the spool valve moves, it allows fluid from the rear steering pump
to move through and operate the rear power cylinder. The higher the pressure on
the spool, the farther it moves. The farther it moves, the more fluid it allows
through to move the rear wheels. As mentioned earlier, this system limits rear
wheel movement to 11/2 degrees in either the left or right direction.
3.3
Electro-hydraulic 4WS
Figure 3. Electro-hydraulic 4WS
Several 4WS systems combine computer
electronic controls with hydraulics to make the system sensitive to both
steering angle and road speeds. In this design, a speed sensor and steering
wheel angle sensor feed information to the electronic control unit (ECU). By
processing the information received, the ECU commands the hydraulic system
steer the rear wheels. At low road speed, the rear wheels of this system are
not considered a dynamic factor in the steering process.
At moderate road speeds, the rear
wheels are steered momentarily counter phase, through neutral, then in phase with
the front wheels. At high road speeds, the rear wheels turns only in phase with
the front wheels. The ECU must know not only road speed, but also how much and
quickly the steering wheel is turned. These three factors - road speed, amount
of steering wheel turn, and the quickness of the steering wheel turn - are
interpreted by the ECU to maintain continuous and desired steer angle of the
rear wheels.
The basic working elements of the
design of an electro-hydraulic 4WS are control unit, a stepper motor, a swing
arm, a set of beveled gears, a control rod, and a control valve with an output
rod. Two electronic sensors tell the ECU how fast the car is going.
The yoke is a major mechanical
component of this electro-hydraulic design. The position of the control yoke
varies with vehicle road speed. For example, at speeds below 33 mph (53 km/h),
the yoke is in its downward position, which results in the rear wheels steering
in the counter phase (opposite front wheels) direction. As road speeds approach
and exceed 33 mph (53 km/h), the control yoke swings up through a neutral
(horizontal) position to an up position. In the neutral position, the rear
wheels steer in phase with the front wheels.
The stepper motor moves the control
yoke. A swing arm is attached to the control yoke. The position of the yoke
determines the arc of the swing rod. The arc of the swing arm is transmitted
through a control arm that passes through a large bevel gear. Stepper motor
action eventually causes a push-or-pull movement of its output shaft to steer
the rear wheels up to a maximum of 5 degrees in either direction.
The electronically controlled, 4WS
system regulates the angle and direction of the rear wheels in response to
speed and driver's steering. This speed-sensing system optimizes the vehicle's
dynamic characteristics at any speed, thereby producing enhanced stability and,
within certain parameters, agility.
4. ACTUAL
4WS
The actual 4WS system
consists of a rack and pinion front steering that is hydraulically powered by a
main twin-tandem pump. The system also has a rear-steering mechanism,
hydraulically powered by the main pump. The rear-steering shaft extends from
the rack bar of the front-steering assembly to the rear-steering-phase control
unit.
The rear steering is comprised of the
input end of the rear-steering shaft, vehicle speed sensors, and steering-phase
control unit (deciding direction and degree), a power cylinder, and an output
rod. A centering lock spring is incorporated that locks the rear system in a
neutral (straight-ahead) position in the event of hydraulic failure.
Additionally, a solenoid valve that disengages the hydraulic boost (thereby
activating the centering lock spring in case of an electrical failure) is
included.
5. FAIL-SAFE MEASURES
All 4WS systems have fail-safe measures.
For example, with the electro-hydraulic setup, the system automatically counteracts
possible causes of failure: both electronic and hydraulic, and converts the
entire steering system to a conventional two-wheel steering type. Specifically,
if a hydraulic defect should reduce pressure level (by a movement malfunction
or a broken driving belt), the rear-wheel-steering mechanism is automatically
locked in a neutral position, activating a low-level warning light.
In the event of an electrical failure,
it would be detected by a self-diagnostic circuit integrated in the four wheel-steering
control unit. The control unit stimulates a solenoid valve, which neutralizes
hydraulic pressure, thereby alternating the system to two-wheel steering. The
failure would be indicated by the system's warning light in the main instrument
display.
On any 4WS system, there must be
near-perfect compliance between the position of the steering wheel, the
position of the front wheels, and the position of the rear wheels. It is
usually recommended that the car be driven about 20 feet (6 meters) in a
dead-straight line. Then, the position of the front/rear wheels is checked with
respect to steering wheel position. The base reference point is a strip of
masking tape on the steering wheel hub and the steering column. When the wheel
is positioned dead center, draw a line down the tape. Run the car a short distance
straight ahead to see if the reference line holds. If not, corrections are
needed, such as repositioning the steering wheel.
Even severe imbalance of a rear wheel
on a speed sensitive 4WS system can cause problems and make basic
troubleshooting a bit frustrating.
6. ADVANTAGES
OF 4WS
The vehicle's cornering behavior
becomes more stable and controllable at high speeds as well as on wet or
slippery road surfaces.
The vehicle's response to steering
input becomes quicker and more precise throughout the vehicle's entire speed
range.
The vehicle's straight-line stability
at high speeds is improved. Negative effects of road irregularities and
crosswinds on the vehicle's stability are minimized.
Stability in lane changing at high
speeds is improved. The vehicle is less likely to go into a spin even in situations
in which the driver must make a sudden and relatively large change of
direction.
By steering the rear wheels in the
direction opposite the front wheels at low speeds, the vehicle's turning circle
is greatly reduced. Therefore, vehicle maneuvering on narrow roads and during
parking becomes easier.
7. APPLICATIONS OF 4WS
Some of the vehicles in which the 4WS is
applied are:
7.1
Chevrolet Suburban 2500:
Chevrolet
Suburban 2500
7.2 Jeep Hurricane:
Jeep Hurricane
8. CONCLUSION
Thus the four-wheel steering system
has got cornering capability, steering response, straight-line stability, lane
changing and low-speed maneuverability. Even though it is advantageous over the
conventional two-wheel steering system, 4WS is complex and expensive. Currently
the cost of a vehicle with four wheel steering is more than that for a vehicle
with the conventional two wheel steering. Four wheel steering is growing in popularity
and it is likely to come in more and more new vehicles. As the systems become
more commonplace the cost of four wheel steering will drop.
REFERENCES
1.
“Automotive Technology-A Systems Approach”, Jack
Erjavec.
2.
“Automotive Suspension and Steering Systems”,
Thomas W Birch.
3.
“Automotive Service-Inspection, Maintenance,
Repair”, Tim Gilles.
4.
http:\\www.howstuffworks.com
5.
http:\\www.howhurricaneworks.com
6.
http:\\www.thecarconnection.com
7.
http:\\www.theautochannel.com
8.
http:\\www.delphiauto.com
No comments:
Post a Comment