AU2002237384A1

AU2002237384A1 - A vehicle suspension

Info

Publication number: AU2002237384A1
Application number: AU2002237384A
Authority: AU
Inventors: Keith Leslie Sharp
Original assignee: Bentley Motors Ltd
Current assignee: Bentley Motors Ltd
Priority date: 2001-02-22
Filing date: 2002-02-21
Publication date: 2003-03-06
Anticipated expiration: 2022-02-21

Description

A VEHICLE SUSPENSION

The present invention relates to a vehicle suspension system.

Suspension systems can be divided into three groups; fully active,

slow active and passive.

For fully active systems the operational frequency is from O to

above wheel-hop frequency (10-15Hz), including both body motions and

vibrations. For slow active systems the operational frequency is from 0

to 3-6 Hz. A slow-active suspension generates forces to the suspension

to control vehicle body motions, also referred as a narrow bandwidth

system due to limited operation range. In a switchable system body

motions and ride characteristics are controlled by changing spring or

damper stiffness. This does not generate forces into suspension. (By the

definition; a passive active suspension). In the case of a fully active

suspension, an actuator replaces the conventional passive suspension

elements such as the spring and damper. To achieve good performance,

the actuator control bandwidth typically extends above the wheel-hop

natural frequency (10-15 Hz). Although the technology and knowledge to

design and manufacture a fully active suspension system is already well

known and proven, its feasibility is not. With current technology,

limitations exist in the cost, packaging and power consumption. Also

beyond the actuator bandwidth the noise and vibration is likely to be a

problem, unless some significant flexibility is added in series with the

suspension strut, for example by way of rubber bushes. One way to reduce the force required to control body attitude

changes and consequently the actuator power consumption is to place an

actuator in series with a passive spring, as in narrow bandwidth systems,

also referred as slow-active systems in the literature. With this

arrangement, the lower frequency active control, typically up to 3 Hz, is

applied to react between the sprung mass and passive suspension.

Higher frequencies are isolated by the passive suspension. However,

slow-active systems still partly share the same noise and vibration

disadvantages as the fully active systems, due to the conventional spring

in series.

Many manufacturers have found that the use of an air spring as a

passive element has offered an improvement to the slow active system.

The active part in these systems has usually been separated from the air

spring (adaptive damping, actuator in series, etc.) or it has been a

switchable volume type. The switchable type uses a supplementary air

reservoir(s) to control wheel travel more effectively. A common

disadvantage for all these systems, which use on/off type switching, is

that the amount of variation is limited. If the switching is applied

instantly when required, an unwanted effect may occur. This effect may

be felt as vibration or even worse as a jerk, which may affect the vehicle

behaviour. Two examples of known systems are the air suspension

system with adaptive damping described in GB 2,287,300A and a

switchable valve operated switchable system which changes spring stiffness between two volumes, using an additional air reservoir as

described in EP 0,864,452A.

An object of the present invention is to overcome or mitigate the

above described disadvantages.

According to the present invention there is provided a suspension

unit for a vehicle comprising an actuator and an airspring and a

connection between the actuator and the air spring to enable air to pass

between them.

In a preferred embodiment of the invention, the actuator comprises

a piston disposed in a cylinder. The piston may be driven by an electric or

hydraulic drive. The airspring also comprises a piston and cylinder. The

connection between the actuator and airspring advantageously comprises

a pipe preferably connected between respective cylinders of the actuator

and airspring. Two units are disposed at opposite ends of the or each axle

of the vehicle. Advantageously, the actuators of the two units are

connected to provide an energy save characteristic in which energy is

transferred from one unit to the other under vehicle body roll conditions.

The connection may comprise an airspring or a mechanical spring

preferably a helical spring. The airspring may comprise a pipe connecting

the cylinders of the two actuators. The mechanical spring may

mechanically connect the pistons of the two actuators. An electrical

control unit is advantageously provided to control the operation of the

actuators. Sensors measure various parameters and produce signals which are fed to the ECU. These are evaluated by the ECU and control

signals transmitted to the actuators to control roll. The parameters

include steering wheel angle, lateral acceleration, throttle position, various

body state and driver inputs. The above described actuator may have

other forms for example, airspring diaphragm type or rubber bellows type

actuators may be used.

In order that the invention may be more clearly understood,

embodiments thereof will now be described, by way of example, with

reference to the accompanying drawings, in which:-

Figure 1 diagrammatically shows a suspension unit for one wheel of a

vehicle,

Figure 2 diagrammatically shows the unit of figure 1 when disposed

on one side (the outside) of a vehicle subject to a roll

condition,

Figure 3 diagrammatically shows the unit of figure 1 when disposed

on the other side (the inside) of a vehicle subject to a roll

condition,

Figure 4 diagrammatically shows two suspension units on opposite

sides respectively of the same axle when the vehicle is

travelling in a straight line,

Figure 5 diagrammatically shows two suspension units on opposite

sides respectively of the same axle, Figure 6 corresponds to figure 4 for an alternative arrangement to that

shown in figure 4,

Figure 7 corresponds to figure 5 for an alternative arrangement to that

shown in figure 5, and

Figure 8 diagrammatically shows four suspension units for a four

wheeled vehicle with the vehicle in a pitch condition.

Referring to figure 1 , the suspension unit is shown connected to

wheel 1 of a vehicle. The unit comprises an air spring 2 and an actuator

3. The wheel is connected to the vehicle body at point 4 through

suspension member 5. The air spring 2 is connected between suspension

member 5 and a further point 6 on the vehicle. The air spring 2 comprises

a piston 7 connected to member 5 and cylinder 8 connected to the

vehicle body at 6 and between which a flexible seal 9 is disposed. The

actuator 3 comprises a cylinder 10 in which a piston 1 1 is disposed. The

piston 1 1 may be driven in the cylinder 10 by means of an electric or

hydraulic drive 13. The cylinders 8 and 10 are connected by means of a

pipe 1 2 so that the air spring and actuator share a common air volume.

This shared air volume may be charged infinitely within the limits of

actuator 3 operation. This arrangement enables the conventional spring,

which is largely responsible for transmitting vibrations from the wheel to

the vehicle body, to be removed from the system. Because of added

softness in the suspension and spring stiffness adjustability, the system

will yield an improvement in ride compared to known slow-active suspensions, which suffer refinement problems outside the actuators

operation area. The position of the actuator may be varied in dependance

upon the length of the connection pipe 12 although for practical reasons

it is better for this pipe to have a shorter rather than a longer length. The

actuator may be any one of a number of different types. For example,

instead of the cylinder piston type actuator illustrated in figure 1 , an

airspring type of diaphragm actuator or a rubber bellows type of actuator

may be used. Whatever design is chosen, the aim is to compress and

decompress the total volume to specified size, to achieve a target spring

stiffness.

The suspension shown in figure 1 is shown with the spring and

actuator in the positions they would adopt with the vehicle travelling in a

straight line. This is the case whichever side of the vehicle the suspension

is disposed on. The suspension as shown in figures 2 and 3 is shown for

the vehicle when subjected to roll or lateral acceleration, when cornering.

Figure 2 shows the position for the suspension disposed on the outside of

the vehicle and figure 3 the inside of the vehicle. As can be seen in figure

2, the piston 1 1 of the actuator 3, as compared with the normal position

shown in figure 1 , has been driven to displace air from the cylinder 10 to

the cylinder 8. The piston 9 has moved to compress the air in the

cylinder 8 of the air spring 3. If under this roll condition the actuator

pistons of the two suspension units did not move, the suspension would

act like a normal passive airspring suspension. When the vehicle starts to corner sensors sense the lateral acceleration and steering wheel position.

Further sensors sense yaw, the position of the vehicle relative to the

suspension springs, throttle position and other parameters. Signals

representing all these parameters are fed to an electronic control unit

(ECU). The ECU evaluates all of these inputs and produces output signals

for the actuators for roll control. In order to avoid body lift (jacking), due

to added force into the outside wheel units, the inside spring stiffness

must be reduced by an equal amount, so that the total axle force stays

constant. Hence, in figures 2 and 3 the actuator pistons have moved

simultaneously in opposite directions. However, due to the inherent

character of gas the displacements (swept volumes) are not equal. This is

a significant factor, especially when considering the energy save layout.

In terms of energy demand, the outside wheel actuator requires an

amount of energy, to support the vehicles static load and control the roll,

while the inside unit releases the same amount of potential energy. This

energy requirement may be balanced by the energy released by providing

an energy save connection between the two suspension units on opposite

sides of the vehicle. The energy save connection may be provided by an

airspring as shown in figures 4 and 5. The airspring comprises a pipe 20

connecting the cylinders 10 of the actuators 3 of the suspension units 21

and 22 on the inside and outside respectively of the vehicle. Figure 4

shows the positions of the air springs 2 and actuators 3 when the vehicle

is travelling in a straight line. In that condition the springs and actuators of the two suspension units adopt the same or a closely similar position.

Figure 5 shows the position of the air springs 2 and actuators 3 when the

vehicle is in a roll condition. In that condition, the piston of the actuator

3 of the outside suspension unit 22 moves to displace air to the

corresponding airspring 2 while the piston of the actuator 3 of the inside

suspension unit 21 moves to increase the effective shared volume

between the actuator 3 and the airspring 2.

A similar situation obtains in the alternative embodiment shown in

figures 6 and 7. In this embodiment the normal straight line position is

shown in figure 6 and the roll position in figure 7. The airspring

constituted by pipe 20 of figures 4 and 5 is replaced by a coil spring 24.

This coil spring acts in a manner similar to the airspring of figure 4 and 5

to transfer energy from the actuator 3 of the inside suspension unit 21 to

the actuator 3 of the outside suspension unit 22 under roll conditions as

shown in figure 7.

The system may also be employed to control pitch which occurs,

for example during braking. Referring to figure 8, a four wheeled vehicle

is shown comprising four suspension units respectively associated with

the four wheels of the vehicle. The front wheels are referenced 31 and

32 and associated front suspension units 33 and 34 and the rear wheels

are referenced 35 and 36 and associated rear suspension units 37 and

38. Pitch is controlled by actuating the pistons 1 1 of the actuators 3 of

the front suspension units 32 and 33 to displace air to the corresponding air springs 2 while the pistons 1 1 of the actuators 3 of the rear

suspension units 37 and 38 are displaced to increase the shared volumes

of the actuators 3 and corresponding air springs under the control of the

ECU 39. During the operation energy save as between front and rear axle

units cannot be used to reduce energy demand. Indeed the energy save

can have a negative effect on the energy requirement depending upon the

execution of the middle spring. In a similar fashion to the control of pitch

by altering axle stiffness, the front and rear roll couples can be adjusted

to alter vehicle cornering characteristics between under and over steer.

It will be appreciated that the above embodiments have been

described by way of example only and that many variations are possible

without departing from the scope of the invention.

Claims

1 . A suspension unit for a vehicle comprising an actuator and an

airspring and a connection between the actuator and the air spring

to enable air to pass between them.

2. A suspension unit as claimed in claim 1 , in which the actuator

comprises a piston and cylinder.

3. A suspension unit as claimed in claim 2, in which the piston is

adapted to be driven by an electric drive.

4. A suspension unit as claimed in claim 2, in which the piston is

adapted to be driven by a hydraulic drive.

5. A suspension unit as claimed in any preceding claim, in which the

airspring comprises a piston and cylinder.

6. A suspension unit as claimed in any preceding claim, in which the

connection between the actuator and the airspring comprises a

pipe.

7. A suspension unit as claimed in claim 6, in which the actuator and

airspring each comprises a piston and cylinder and the pipe is

preferably connected between the cylinders.

8. A suspension system comprising two suspension units each as

claimed in any one of the preceding claims.

9. A suspension system as claimed in claim 8, in which the two units

are disposed at opposite ends respectively of the axle of a vehicle.

10. A suspension system as claimed in claim 8 or 9, in which a

connection connects the actuators of the two units together to

provide an energy save characteristic in which energy is transferred

from one unit to the other under vehicle roll conditions.

1 1 . A suspension system as claimed in claim 10, in which the

connection comprises an airspring.

12. A suspension system as claimed in claim 1 1 , in which the airspring

comprises a pipe connecting the two actuators.

13. A suspension system as claimed in claim 12, in which, where the

actuators each comprise a piston and cylinder, the pipe connects

the two cylinders.

14. A suspension system as claimed in claim 10, in which the

connection comprises a mechanical spring.

15. A suspension system as claimed in claim 12, in which the

mechanical spring is a helical spring.

16. A suspension system as claimed in claim 14 or 15, in which, where

the actuators each comprise a piston and cylinder device, the

mechanical spring mechanically connects the two pistons.

17. A suspension system as claimed in any of claims 8 to 16, in which

an electrical control unit is provided to control operation of the

actuators.

18. A suspension system as claimed in claim 17, in which sensors are

provided to measure parameters and feed representative signals to

the electrical control unit.

19. A suspension system as claimed in claim 18, in which the electrical

control unit is connected to the actuators and is operative to

receive signals from the sensors and transmit control signals to the

actuators to control roll.

20. A suspension system comprising two pairs of suspension units as

claimed in any of claims 8 to 1 9, in which the pairs are respectively

adapted to be associated with the front and rear wheels of a four

wheel vehicle and the actuators of the units are operative under the

control of an electrical control unit to control pitch of the vehicle.