Active Suspension System-- A
Mechatronic System
INTRODUCTION:
Air suspension was offered as an option
years ago by some car manufactures. However, it was not widely accepted for use
in passenger cars. In recent years, some heavy duty trucks and buses have used
in air-suspension. Now, with electronic air suspension, air springs are making
a comeback.
In air-suspension systems, the four
steel springs are replaced by four rubber cylinder, or air springs. Each rubber
cylinder is filled with compressed air, with supports the car weight. When a
wheel encounters a bump in the road, the air is further compressed &
absorbs the shock.
The electronic air-suspension system is
shown in fig. It includes an electric air compressor, a microcomputer control
module (MCM), four air springs with built-in solenoid valves, three height
sensors (two front and one rear), and the air-distribution system at lines and
fittings.
The height sensors monitor the riding
height, or vehicle trim height. They signal the contract module of any change.
If the height is too high, the control module opens the solenoid valves in the
spring with too much air. This allows some at the air to escape, towering the
car. It height is too low, the control module turns on the air compressor. Then
the control module opens the solenoid valves in the spring until proper trim
height is restored.
The control and operation of system are
very similar to that of automatic-level control system. However air-suspension
systems provides springing for all four wheels, instead at only two rear wheels
as with automatic level control.
Rolling, Brake
Dip, Bouncing and Pitching :-
Centre of
gravity of a vehicle is at a height but retarding and cornering forces are
applied of necessity at road levels. During cornering, a turning couple about
the longitudinal axis of the vehicle is produced due to the centrifugal force
acting at e.g. and the forces at tyre-road contact patch. This results in a motion
called rolling. The left hand side suspensions move out of phase with right
hand side.
Braking causes
a tendency for the nose of the vehicle to dip. This phenomenon is called brake
dip.
Other types of
sprung mass motion are shown in fig. Pitching is defined as the rotating motion
about a transverse line through the vehicle parallel to ground, the front
suspension moves out at phase with the rear. Bounce is defined as the vertical
motion of the center at gravity. The bounce can be front end bound or rear and
bounce. Diagonal pitch is combination of pitch and roll.
Softness of
springing is limited by relation of wheel base and track to c.g. height and the
permissible values of dip and roll. Smaller vehicles usually have relatively
stiffer springs because c.g. height can be reduced that much.
Suspension pitching
and rolling axes should be arranged to pass through the c.g. of the vehicle so
that the nose dip and the roll are confined to those due to tyre deflections
only. However, such axis positions are difficult to obtain in practice.
Road Irregularities and human susceptibility:-
Some
indication of the magnitudes of the disturbances caused by road irregularities
can be gained from surface irregularity of Roads, DSIR Road Research Board
Report, and 1936-7. It appears that surface undulations on medium – quality
roads have amplitudes of 0.005m are characteristics of very good roads. The
average pitch of these undulations is under 4m while most road vehicle wheels
roll forwards at about 2m / rev. In Additional to the conventional tarmac
roads, there are pave and washboard surface, the letter occurring largely on
unsurfaced roads and tracks. Representative replies of these two types of
surface are described in the MIRA proving ground, by A. Fogg, Proc. A.D. Inst.
Mech. Engrs 1955-56.
Obviously the
diameter of the tyre, size of contact patch between tyre and road, the rate of
the tyre acting as a spring, and weight of wheel and axle assembly affect the
magnitude of the shock transmitted to the axle, while the amplitude of wheel
motion is influenced by all these factors plus the rate of the suspension
springs, damping effect of the shock absorbers, and the weights of the unsprung
and sprung masses. The unsprung mass can be loosely defined as that between the
road and the main suspension springs, while the sprung mass is that supported
on these suspension springs, through both may also include the weights of ports
of the springs and linkages.
Two entirely different types of shock are applied to the
wheel; that due to the wheel’s striking a bump, and that caused by the wheel’s
falling into a pot-hole. The formed will be influenced to a major extent by the
geometry of the bump and the speed of vehicle, while the major influence on the
latter, apart from the geometry of the hole, is the unsprung masses and spring
rates, speed being an incidental influenced factor.
Human
sensitivity to these disturbances is very complex, and a more detailed
discussion can be found in Car Suspension and Handling by Donald Bastow,
Pentech Press, London, 1980. It is widely held that vertical frequencies
associated with walking speeds between 2.5 and 4 mph that is, 1.5 to 2.3 Hz –
are comfortable, and that fore-and-aft or lateral frequencies of the head
should be less than 1.5 Hz Dizziness and sickness is liable to be experienced
if the inner ear is subjected to frequencies between 0.5 and about 0.75 Hz.
Serious discomfort may be felt in other important organs at frequencies between
5 and 7 Hz.
Springing of
the Car :-
If the front,
and rear wheel axles were allowed to run in bearings fixed rigidly to the
frame, the result would be extremely uncomfortable, the maximum speed of the
car would be very limited, and the engine and transmission, as well as the
bodywork, would be subjected to severe stresses, which in time would no doubt
result in the fracture or breakdown of one or other of the working parts. It
has become recognized, as a result of the long experience, that all types of
vehicles used for locomotion, including railway trains, motor vehicle
horse-drawn vehicles, pedal cycles and even children’s prams, must be provided
with some means of insulating the wheels and axles from the rest of the
vehicle, so that the road or rail shocks received by the wheels when traveling
over uneven ground will not be transmitted appreciably to the other parts. The
axles at railway carriages run in gunmetal axles boxes which can slide
vertically is guides (known as ‘horn plates’) in the carriage frames, stiff
spring bear down on the tops of these boxes and absorb most of the rail shocks;
i.e., spring-insulate the carriage frames from the wheels and axles. The
familiar leaf springs of horse-drawn vehicles serve also for the same purpose.
Similarly, the pneumatic tyres and the spring saddles of pedal cycles afford a
fair degree of insulation from road shocks.
The object of
the springing, or as it is terms, the suspension system, then is :
1)
To protect the occupants from road
shocks.
2)
To reduce the stresses due to road
shocks on the mechanism of the car , and
3)
To maintain the body on an even keel
when traveling over rough ground, or when turning so that any rolling, pitching
or vertical movement tendency is minimized. The ideal suspension system would
be that which allowed the road wheels to travels over rough uneven ground at
any speed, whilst maintaining the body perfectly level; all the wheels would
therefore move up and down relatively to the body.
The Principles of Motor Springing:-
Before
outlining the usual methods of springing cars it should be mentioned that in the
earlier days of motor-cars, members of the engine and transmission, and in some
cases the frame themselves, were opt the fracture through ‘fatigue’ of the
metal under rapidly alternating stresses caused by road shocks, so that the
importance at protecting these parts will appreciated.
The important
principles underlying the satisfactory springing of motor vehicles are firstly,
the reduction, to a minimum, of weight of the wheels and others parts receiving
the road shocks this is usually termed ‘reduction of unsprung weight’.
Secondly, the reduction of rolling or pitching of the body, to a minimum, by
suitable design and attachment of the springs. It is usual to mount the body
frame on the springing system at four points – generally at the corners of the
rectangle formed by the framed members. Thirdly, it has become recognized that
it is not yet possible to absorb satisfactorily the larger and also the smaller
road impacts with one springing device, so that auxiliary attachments, or
subsidiary members of the main springs are provided to look after the minor
shocks; these are termed shock absorbers.
Basic Function
of the Suspension SYSTEM:-
1) To Provide Suitable Riding and
Cushioning Properties –
The
frame should have a high degree of isolation from the axle so that the effect
of road and tyre irregularities and wheel out of balance forces are not
transmitted to the vehicle frame.
2) To Provide Good Road
Since the
basic functions of driving, cornering and braking are obtained by virtue of the
road tyre contact area, the suspension system should always maintain the wheels
in contact with road to obtain these functions which would otherwise be lost.
In addition to
these basic functions the vehicle suspension system must perform a No. of
Complex functions which may conflict with each other. These functions are
summarized below. These are general considerations which are applicable to
passenger cars as well as heavy duty commercial vehicles as the case may be.
3) The suspension system must support the vertical
load imposed by the weight of the vehicle, plus the body and payload weight.
4) It must Provide Adequate Stability
and Resistance to Sideways and Roll Over –
This is
especially important for commercial trucks where substantial variations in the
vertical center of gravity location can occur and also, in certain operations.
Swaying, shifting and surging loads may be encountered cornering causes a
tendency for the vehicle to roll.
5) It must Transfer Driving and Braking
Forces between Frame and Axles –
The suspension system must provide means to transfer the
longitudinal forces generated during acceleration or deceleration.
6) It must resist Drive and Brake
Torque Wind-up –
When the driving and braking torques are applied to the
ground through the tyre-road contact areas. The front suspension springs have a
tendency to ‘wind-up’. Due to the spring wind-up, any point on the unsprung
components other than the center of rotation is displaced. This will cause
steering wheel rotation or change the angular position of the road wheel.
Spring wind-up also displaces the tie-rod towards the engine and may affect the
clearance between tie-road and engine exhaust system, and other components.
7) It must Resist the Cornering Effects
–
When
negotiating a Curve or a Turn. Normally a vehicle has a tendency to continue in
straight line and when the front wheels are turned forces are generated that
cause the vehicle to turn. The cornering forces cause a weight shift which
results in compression on one spring and release of another which may result in
a rotation of axle in the plan view. This is called axle role steer. Another
effect such load transfer can cause is the displacement of the steering arm
ball from its normal load position and may result in what is called compliance
steer.
8) It must maintain proper positioning of
the castor on steering axle so that proper steering geometry is maintained. It
should also maintain axles in alignment parallel to each other and
perpendicular to the front.
9) In case of drive axles, the suspension
system must provide for limited movement of drive shaft slip splines and in
case of tandem axles the load transfer between the axles should be minimum.
Classification
of Suspension SYSTEM:-
According to
elements used for suspensions.
- Laminated or leaf springs.
- Coil Springs.
- Torsion bars.
- Air Springs.
- Rubber Springs.
- Hydro-elastic Springs.
According to
point of application.
- Front axel Suspension.
- Rear Axel Suspension.
Desirable
Characteristics of a Suspension SYSTEM:-
The following
are the desirable characteristics of a vehicle suspension below:
1) Maximum Deflection Consistent with
Required Stability –
In order to provide good cushioning
ability together with better riding qualities, the suspension system must
provide maximum deflection. However, it should be consistent with the vehicle
stability requirements.
2) Compatibility with other Vehicle
Components –
Suspension
system alone cannot completely determine the actual ride provided in a vehicle
tyres, frame stiffness, wheelbase. Steering linkage all affect vehicle ride and
hence the suspension system must be compatible with these components.
3) Minimize Wheel Hop –
For the purpose of suspension
analysis the vehicle weight is divided into sprung weight and unsprung weight.
Spring weight is the weight of the vehicle that is supported on springs; rest
is called unsprung weight. Frame and components attached to it come into the
definition of sprung weight while wheels and wheel axles comes into unsprung
weight. The resonance frequency. This wheel top frequency should be minimum
wheel hop frequency and its amplitude greatly affect the road holding and
hence, the cornering and braking obtainable because cornering and braking
forces are at necessity applied at the road level.
4) It must provide sprung mass frequency
that is relatively constant between laden and unloaded conditions. Furthermore,
this natural frequency must not be in resonance with tyre rpm or with pavement
expansion strips. Typical values of sprung mass frequency for passenger cars
varies from 0.75 C/S to 2.5 C/S.
Alternative to this is provision of
a variable rate spring which will be effective on a wide range of loading and
under varying conditions
5) It must have low maintenance and
operating costs. It’s initial cost should also be low.
6) The total weight of the suspension
system should be minimum.
7) It should minimise tyre wear.
Structure of
Active Suspension SYSTEM:-
Fig. shows the
active suspension system for Toyota Soarer and Fig. shows the hydraulic circuit for the system of
Fig.
The active
suspension system for Soarer has four functions – ride comfort control, vehicle
attitude control, height control and stability (Maneuverability) control. These
functions are carried out by controlling hydraulic cylinder which have gas
springs support each wheel.
In the
relatively low frequency band of less than 2 Hz, the pressure control valve
receives pressure supply and discharge signals from the electric sensors, such
as a G-sensor and controls the system.
In the
intermediate frequency band of 2 – 6 Hz, a spool valve in the pressure control
valve senses the pressure changes and mechanically (Mechanical servo function )
operates to keep the line pressure constant, thereby preventing the
transmission of vibrations to the vehicle body.
As shown in
Fig, oil pressure generated in the oil pump is temporarily accumulated in the
accumulator via an attenuator which reduces pressure pulsation. A pressure
control valve in the integrated valve unit control high pressure to necessary
levels and supplies pressure to each hydro-pneumatic cylinder or returns the
oil in the hydraulic cylinders.
Generally,
line pressure from the oil pump is changes according to the oil consumed by the
pressure control valve. In this system, there is a PC valve in the pump. The PC
valve balance the discharged flow rate and oil consumption properly, so that
the line pressure is kept constant.
Air Springs :-
A volume of
air, enclosed either in a cylinder fitted with a piston or in a flexible
bellows, can be used as a spring, as shown in fig. Under the static load, the
air is compressed to a predetermined pressure, and subsequent motion of the
piston either increases or decreases the pressure and consequently increases or
decreases the force acting on the piston. If this force is plotted against the
piston travel, a curve similar to the compression curve of an engine indicator
card will be obtained, so obviously the rate at which the force varies with the
piston travel becomes greater as the air pressure increases. It follows that,
whereas with a metal spring, equal increments of force result in equal
increments of deflection the rate of an air spring is not constant. This
varying rate is an advantage in that a low rate can be obtained for small
deflections from the mean riding position while keeping the total rise and fall
of the axles within reasonable limits.
Air springs
are fairly widely employed on vehicles whose laden and unladen weights differ
greatly. This includes principally tractors for semi-trailers, the semi-trailers
themselves and large drawbar trailers. They are also used to some extent on
coaches, more especially in continental Europe
and the USA
In
double-wishbone type suspensions a rubber bellows, circular in section and
having two convolutions, is generally used and simply replaces the coiled
spring of the conventional design. Rubber bellows type springs are used also in
the Dunlop Stabilair suspension, as shown in fig. Alternatively a metal
air-container in the form of an inverted drum is fixed to the frame and a
piston, or plunger, is attached to the lower wishbone. Since the piston is
considerably smaller than the drum, sealing is affected by a flexible diaphragm
secured to its periphery and the lip of the drum. This construction enables the
load deflection characteristics of an air spring to be varied considerably by
using profiled guides, such as E and F in fig, to control the form assumed by
the diaphragm, and thus it’s effective area, as the inner member moves relative
to the other one.
Elongated
convoluted bellows such as are indicated in fig, have been used in trucks and
coaches, with radius rods to deal with the driving and breaking torques and
thrusts, and a panhard rod for lateral location.
Development
of variable displacement oil pump for active air suspension :-
Introduction –
The subject of
this report is to outline our development of a variable displacement pump for
automobiles. The pump has reduced the pressure pulsation to realize a quiet
system. This was accomplished by simulation analyses. Experiments
were conducted on a pressure
sensing control valve ( hereinafter, PC valve ) having stable
controllability in a board area if revolutions from 600 r.p.m. to 6000 r.p.m.
and off supply from 5 lit / min. to 23 lit / min.
It has been
our desire to realize compatibility between stability and ride comfort in an
automobile. Recently, due to the increased advances of electronics technology
and integration with hydraulic technology, active control has been adapted to
the classis system.
Active
suspension gives the driver soft ride comfort running on rough roads and
stability which the driver expects. In order to actively and instantly restrain
or control undesired movements generated by the vehicle vibrations and inclinations,
it is necessary for the vehicle itself to provide a power source which can
always supply energies necessary for controlling.
On the other
hand, running on a smooth road such as a highway, the vibrations or attitude
changes of vehicle do not occur so often and not much energy is needed.
Therefore, it is necessary developed hydraulic system with an efficient energy
supply.
Active
suspension was first introduced on a commercial basis in the 1989 model
vehicles ( TOYOTA CELICA AND NISSAN INFINITY ).
The active
suspension wad further installed in the 1991 model ( TOYOTA SOARER ) which
abolish metal springs for supporting the vehicle body in order to improve
attitude control during critical turning of the vehicle. The suspension system
without metal springs, known as the Full Hydro-Pneumatic System, requires twice
as much as energy as the previous active suspension system installed in the
CELICA.
Thus, focus
was placed on the swash plate type variable displacement pump to efficiently
supply energy.
Design of variable Displacement Pump :-
It is
necessary for the pump to determine the proper values on response for oil
consumption changes and maximum discharged flow rate in order to instantly and
accurately control the automobile for ride comfort and vehicle attitude.
Discarded Flow Rate –
The oil
consumption conditions were preliminarily tested under various running modes,
from relatively constant running conditions at high speed cruising to running
on rough roads. It was found from the test that the all consumption wad
changeable from 5 ¶ /
min during high speed cruising to 18 lit / min on rough road running. Based on
these results and considering more severe conditions, the maximum oil is set at
23 lit / min ( 2000 r.p.m. ). It was further set to 7 lit / min ( 600 r.p.m. )
for the height control under the engine idling conditions. These two levers
were targeted. ( as shown in fig. )
Response of Variable Displacement –
A change at
high speed will create a delay of about 0.2 sec. from the actual operation of
steering wheel to the initiation of vehicle inclination. If such lane changes
are repeatedly made, rolling control is made by using oil in the accumulator.
During this time, compensation for this oil consumption is balanced by
increasing the discharge flow rate. ( as shown in fig. )
In addition to
these specifications, the following two points were considered for automobile
use :
Synthetic oil
of low viscosity was used to keep the viscosity stable under the environmental
temperature between –30 0C and 120 0C.
Secondly,
the pump pulsation was reduced ( which is the original cause of the vibration )
to reduce the noises in the passenger compartment for comfortable driving.
Structure
of the Variable Displacement Oil Pump and Operation Thereof :
In order to
make the pump compact, the active suspension and power steering pumps are
placed in tandem and separated by oil seals. The pumps are driven by the
engine.
As shown in
fig, the active suspension pump is of axial piston type having 9 cylinders and
also comprised of a PC valve for swash plate angle control and swash plate. The
pump is always kept the pressure at 11.7 MPa pressure.
In order to
reduce the pressure pulsation, the shape of the pump outlet portion is modified
and an attenuator is mounted on the pump to absorb the pulsation immediately
after the discharging. This attenuator includes metal bellows to prevent gas
from pressure nitrogen gas introduced to make a seal.
Operation
:
1) When the oil supply in the suspension
system is reduced or the number of pump revolutions is increased, the pump
pressure increases.
2) Then, the PC valve is moved upward ( as
shown in fig. ) to increase the pressure in the swash plate control cylinders
which moves the actuator piston to the left to reduce the swash plate angle.
3) Thus, the
oil flow from the pump is reduced to keep the line pressure constant. As above,
if the pump pressure decreases, the line pressure is also kept constant. As
explained, when high discharge flow rate is unnecessary, the flow rate is
reduced, thereby reducing the horse power and heat loss.
Control System
for Active Air Suspension Systems :-
Suspension
control entails more than just regulation of the vertical movement of the
wheels. The many factors that have to be taken into account include comfort of
the occupants, roll, both longitudinal and lateral weight transfer, and the
maintenance of contact pressure between the wheels and the ground consistent
with good stability and handling.
In a
fully-active system there is a pump and hydraulic fluid reservoir and
generally, one hydraulic actuator and one control valve for each wheel or pair
of wheels. There may also be one or more hydraulic accumulators, to supplement
the rate of flow from the pump to cater for sudden deflections of the
suspension. The control valves may be all in one unit. They execute commands
from an on-board computer which is served with information by sensors. The
computer may be programmed to make instantaneous responses to changes inroad
surface or equilibrium ( speed, roll, bake dive, or acceleration squat ) as
indicated by the sensors or, in a simpler control arrangement, it may issue its
commands periodically, to adjust the system to suit average conditions over
periods of seconds or even minutes, depending on the precision of control
required. Correction can be made also to under or oversteer, by adjustment of
the front: rear roll stiffness ratio, even automatically while the car is being
driven.
Ideally,
perhaps, sensors would defect the rise and fall of the ground in front of each
wheel, so that the suspension could be made to deflect a precisely equal amount
and thus keep the vehicle riding at a constant height above the ground. A speed
sensor would be necessary, too, so that the computer could synchronise the
movements of the suspension with the passage of the measured surface profile
under the wheels. A further refinement might be a transducer to measure the
actual response of the wheel so that, by taking into account hardness or
softness of the surface, the dynamic loading on the tyres could be limited.
Such a system,
however, is impracticable at the current state of the art, and it is simpler to
use a pressure transducer in each hydraulic actuator to signal to the computer
any increase or decrease in the load applied by the ground to the wheel.
Responding virtually instantaneously to this signal, the control system can
direct fluid to flow either into or out of the actuator as necessary.
The sensors
actually used may include one transducer on each axle to measure the variations
in height of the sprung mass above it under varying loads, a speed sensor, a
yaw detection gyroscope or steering motion transducer, or a lateral
accelerometer for measuring tendency to roll, a longitudinal accelerometer to
detect braking and acceleration forces, and an accelerometer or strain gauge on
each hub for assessing the quality of the road surface. In some instances a
degree of simplification has been obtained by using for the rear axle only one
height sensor and one actuator, the motions of two rear wheels being made
interdependent through a hydraulic interconnection. Alternatives to some of the
above-mentioned sensors might be transducers sensing the displacements of the
throttle, brake pedal, steering gear and axle.
A typical
fully active suspension system and a semi-active system as shown in fig,
neither, however, is a standard option in a quantity-produced vehicle. Since if
studied in relation to the basic principles already outlined, are self
explanatory, it is not proposed to describe then in detail here, however, some
comments are necessary regarding Figs.
Fig. shows a
single suspension unit in the AP system. An increase in the static load on the
right in the illustration, to be deflected upwards about its pivot. This of
course is provided the relative movement between suspension arm and body is
slow, so that the coil spring and damper in the linkage between the suspension
arm and the lever are not compressed. The consequent upward deflection of the
lever pulls the spool valve to the left, causing it to direct hydraulic fluid
the hydraulically damped gas spring, thus extending it until the ride-height
returns to what it was before.
Rapid upward
movement of the lever, on the other hand, are opposed by the inertia of the
offset mass, so the coil spring and damper are compressed and there is little
or no movement of the spool valve. In these circumstances the gas spring
performs as in a conventional suspension system and, since little or no fluid
motion is involved, the energy consumption by the engine driven pump is
correspondingly small. To obtain this effect, the coil spring and damper have
to be turned so that the force exerted on the offset pendulous mass gives it
the same vertical acceleration as that imparted to the body of the vehicle by its
gas spring and integral damper. This system has two major advantages. First,
the body does not sink down on its suspension when it is switched off.
Secondly, it does not need an electronic control.
The AP system
can be turned to take into account not only the lateral weight transfer but
also the vertical deflection of the tyres during cornering. In some instance,
however, especially for heavy vehicles, it may react too slowly. This cam be
overcome by arranging for a steering input, as shown in Fig. 35.32. A
double-acting ram, actuated by the steering mechanism, transfers hydraulic
fluid from one side to the other of the vehicle through the dampers in the
links between the suspension arms and the levers actuating the spool valve.. it
does this in a direction such as to lift the lever on the outer and lower that
on the inner side of the turn.
Further
information on active suspension in general, including design calculation, can
be obtained from two papers, by Sharp and Hassan, Proc. I. Mech. E., Vols. 200
D3, 1986, and 201 D2, 1987.
** General Description of the Control
Systems :-
Figure shows
the system configuration of the Active Suspension and the Active 4WS.
Basic Control
Strategy :-
Fig shows the
control block diagram of the Slow Active Suspension system various sensors are
installed to get information on the vehicle, the longitudinal, lateral and
vertical acceleration of the body, the relative strokes between sprung and
unsprung mass, steering angle, and vehicle speeds etc.
The basic
control strategy is based on three functions shown below.
1) PID feed-back control of the height’s
deflection –
PID
(Proportional, Integral, Differential) feed-back control is executed to reduce
the deflection between the height target given by Selector Switch etc. and present
height by relative stroke sensors. The objective of this control is to adjust
static height corresponding to height target and to compensate feed-forward
errors.
2) PI feed back control of vertical
acceleration –
Proportional value of vertical acceleration
to compensate the lag of servo response and integrate that equivalent to
vertical velocity are used for food-back control in order to realize Sky-hook
damping. This damping is effective for improving ride comfort as shown in Fig,
and it can also compensate feed-forward errors.
3) PD feed-forward control of
longitudinal and lateral acceleration –
The movements of sprung turning i.e.
the pitch and roll, can be estimated from the longitudinal and lateral
acceleration. Inertial moments are cancelled by this feed-forward control, and
then equivalent pitch and roll stiffness of the suspension can be raised up to
keep the vehicle’s attitude flat as shown in fig.
Effect of Integrated
Control :-
The effects of
integrated control are shown in fig, Plotted are the magnitudes of
acceleration, deceleration by braking and lateral acceleration when the
vehicles turns different radius of curves on a test course. The outermost solid
line shows the limits of tire traction. The inner area enclosed by solid lines
shown the distribution of acceleration and deceleration of a vehicle that has
no integrated control system. The outer solid line shows the distribution of
acceleration and deceleration of a vehicle that has an integrated control
system.
As shown in
this diagram the vehicle with an integrated control system provides higher
acceleration and deceleration than the vehicle without integrated control. It
is clearly seen that the control vehicle has improved performance with higher
stability and control.
The overall ECU
configuration involving the Active Suspension, Active 4WS, ABS and TRC is shown
in fig, integrated control is accomplished through exchanging several bits of
information within this electronic control unit.
The effects of
the integrated control systems are shown bellow.
The Importance of Feed-Forward Control
–
Fig. shows the
conceptional structure of the vehicle’s attitude control. The fluctuation of
the vehicle attitude f
is determined by the equivalent disturbance force to the vehicle D Fd and the robustness of the vehicle system H(s) which
depends on the feed-back characteristics. That is
f = D Fd / H(s)
Generally an
order to reduce the change in the vehicle’s attitude, it is necessary to make a
more robust system or to decrease the equivalent disturbance force using more
correct feed-forward canceling. However, comparing to a Full Active Suspension
system with robust feed-back control, a Slow Active Suspension system cannot
have enough robust feed-back control due to the lack of response, so
feed-forward control becomes more important in order to attain good performance
of vehicle’s attitude control.
Therefore for
a suitable control of vehicle’s attitude in the Slow Active Suspension system,
a more precise estimation of the disturbance force and a more accurate
compensation of the rag are necessary.
Hence we
developed new feed-forward control algorithms described hereinafter for the
Slow Active Suspension to improve the vehicle’s attitude control.
Effects of the
Active Suspension :-
Balance of
Frequency Response between Yaw and Roll – From wheel steering angle
proportional control combined with yaw rate feedback controlled by the Active
4WS, significantly improves the steering response and convergence of yawing
after a lane change. However, it tends to generate a lateral acceleration at
high frequency causing an ordinary vehicle to develop a fast transient roll
(initial roll from sharp steering and rollback) resulting in an uncomfortable
feeling to the driver. With the attitude control of the Active Suspension,
there is a positive feeling of stability with a high level of dynamic balance
in the yaw and roll direction.
Stability
& controllability in Lower and Higher
Ranges
The use of the
tires relative to slip angle and load is very important for the integrated
control by the two systems. The important characteristic of the Active
Suspension is that it does not only control the attitude of the vehicle when
turning, but also improves critical controllability by roll stiffness
distribution control.
Figure shows
the effects of the rear steering control and roll stiffness distribution
control on controllability and stability. As shown, the Active 4WS produces a
large control effect in the range of less than 0.5G, while the Active
Suspension produces a large control on roll rigidly distribution in higher G
range. The new SOAERE provides sharp and stable steering performance in the
range of les than 0.5G by implementing the Active 4WS. Improved performance by
additional steering in a turn by the Active suspension’s roll rigidly
distribution control is obtained in higher G range.
Emergency Lane Change Performance :-
Emergency lane
change performance is one of the characteristics that best demonstrate the
effect of integrated control of the Active Suspension and Active 4WS. Speedy
response and convergence of yaw and lateral acceleration are essential.
Controllability in the non-linear range is also momentarily required.
Figure shows a comparison of performance in terms of
approach speed, steering angle and yaw rate change in an emergency avoidance
situation.
The quick response to steering by the integrated control of
the Active 4WS and Active Suspension enable the vehicle to change lanes
smoothly with good stability and without excess yaw
Adjustable and
Self-Adjusting Suspension :-
When steel
torsion-bar springs are used, some method of adjusting the standing height at
the suspension is needed. This is because, owing to the multiplying effect of
the lever arm connected to the active end of the torsion bar, even a small
tolerance on the angular relationship between the fittings at its ends can make
a significant difference to the attitude of the vehicle. Moreover, it is
generally difficult to maintain tight tolerance on the angular relationship
between the ends, especially when the bar has been overstressed, or scragged,
to increase its fatigue resistance. The adjustment device is generally a screw
stop against which a short lever on the static end of the bar bears.
There are also
variants of this principle, in which a warm-and-wheel drive is used, the wheel
being on the Static end at the torsion bar and the worm on a spindle that can
be rotated by the driver whilst sealed in the vehicle. Whereas the screw type
adjustment is for the initial setting on the production line and only rarely
used when the vehicle is being serviced, the worm and wheel or other mechanism
– sometimes actuated by a small electric motor – is employed also for adjusting
the fore-and-aft trim of the vehicle to cater for variations in the load
distribution – for example, when heavy luggage is carried in the boot. While
provision for such manual adjustment system is uncommon, automatic adjustment
is the norm for air-suspension.
There are two
distinctly different types of automatic adjustment system for air suspension.
One is the Citroen arrangement as shown in fig, in which on engine-driven
hydraulic pump supplies fluid under pressure to an accumulator and thence
through leveling valves to combined are spring and strut – damaged units. This
is the constant mass system, in which the mass of the air, or an inert gas,
enclosed in the spring is constant. The principle is illustrated
diagrammatically, but greatly simplified, in fig, where the hydraulic
accumulator is omitted and a floating piston P is depicted instead of the
flexible diaphragm of the Citroen system, and the hydraulic damping system is
omitted from the chamber O. The constant mass at gas A is compressed above the
floating piston.
Space O,
between the floating piston P and the piston b attached to the axle, is filled
with oil O, which moves up and down with the piston b and P, the air being
correspondingly compressed or expanded. If the load in increased so that the
assembly C, which is fixed to the body B, moves downwards, the valve V opens
port D, so oil from the pump E passes into the space O. The piston P and
assembly C, together with the body, therefore move upwards, and this continues
until the port D closes again. Similarly, if the load decreases, the port F is
opened and oil escapes from the space O until the port F closers again.
Similarly, if the load decreases, the port F is opened and oil escapes from the
space O until the port F closes again. This the basic ride height of the
suspension can be kept constant. This self-adjusting action is damped so that
the motions between the body and axle due to irregularities of the road do not
influence the basic setting of the ride height.
Advantages of the Active SUSPENSION: -
Figure shows a
circuit diagram of the Hydraulic system. In Figure, a block diagram of the
control system is shown. The features of the system are as follows.
1) The normal coil spring is not used in
this suspension, thus eliminating its vibration. This improves riding comfort
at how frequencies. The spring function has been replaced by the
Hydro-pneumatic Suspension system. The pressure control range has been widened
to maintain a flat vehicle position even while turning.
2)
The variable displacement piston
pump is used to reduce energy consumption during non turning maneuvers, but
still have sufficient flow in the turn ( as shown in fig. ). It features nine
pistons to reduce pressure pulsation.
3) Vertical G sensors are used for skyhook
damping.
4) Active Suspension has four control
functions as follows.
- Riding comfort control to absorb road surface
irregularities.
- Vehicle attitude control to maintain a constant
vehicle attitude at all times.
- Stability and controllability control to ensure
stability in turning and straight
driving.
- Vehicle height control to maintain a constant height
regardless of the load. These functions operate mutually for overall
control.
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