WO1989012766A1

WO1989012766A1 - Suspension system

Info

Publication number: WO1989012766A1
Application number: PCT/GB1989/000703
Authority: WO
Inventors: Michael Joseph French
Original assignee: The University Of Lancaster
Priority date: 1988-06-22
Filing date: 1989-06-22
Publication date: 1989-12-28
Also published as: AU3967489A; GB8814761D0

Abstract

A suspension system (e.g. for a vehicle) comprises a gas spring (1) and a mass (e.g. the vehicle body) mounted on the spring for damped oscillating movement. Gas spring has a first gas chamber (2) of variable volume which is compressible and expansible by oscillations of the spring, and a second rigid gas chamber (3) which communicates directly with the first chamber (1) via a rapidly openable and closable valve (5, 6). The system incorporates means (11, 12) for detecting a function of changing compression of the first chamber (1). These detection means are operatively associated with the valve (4, 5) to affect closure or opening thereof when the function passes through predetermined values. Opening of the valve when there is a pressure difference thereacross serves to dissipate energy in the subsequent equalising process, and the changes in stiffness of the spring serve to de-tune the system.

Description

SUSPENSION SYSTEM

The present invention relates to a suspension

^•^ system of the type in which a mass is mounted on a gas spring which (in use of the system) will be subject to a random input which the spring is intended at least partially to absorb.

A particular example of such a system is a motor vehicle suspension system in which the mass is represented by the vehicle body and the random input to the spring arises from the travel of the vehicle wheel over uneven or bumpy ground. A further example of such a suspension system is a seat assembly for a vehicle (eg. a lorry) in which the mass is provided by the seat and its occupant and the random input is the movement of the vehicle over an uneven road.

In conventional vehicle suspension systems, a spring of fixed stiffness is used, together with a damper: such designs are a compromise. There is bound to be a resonant frequency at which the dynamic magnifier is large; the damper, which is fitted to reduce the dynamic magnifier, also transmits high frequencies to the body of the car (in the form of harshness, vibration and noise), so that it cannot be made too effective, and therefore the dynamic magnifier remains larger than is desirable. There is a further compromise, in that a softer spring gives an easier ride, but requires more travel in the suspension than is available.

It is known to improve this kind of suspension in -* two ways. One is by the input of power ( 'active' suspension) usually by hydraulic means. Another is by modulating the damper, that is, making it more or less effective from moment to moment. There is however still the disadvantage of harshness, vibration and noise associated with using a damper. Also known are vehicle suspension systems comprising suspension cylinders connected by hydraulic lines to oleopneumatic accumulators. Valve arrangements are provided in the lines such that opening and closing of the valve provides and interrupts communication between the cylinders and the accumulators thus modulating the stiffness of the suspension system. An example of such an arrangement is shown in GB-A-1 410 624 (Pietzsch) in which the valve is opened and closed by control means which is associated with a sensor arrangement which produces an output signal which is a function of the angular pitching speed of the vehicle body. The valve means is opened and closed in response to the output signal in such a way as to reduce the amplitude of the angular pitching oscillations of the vehicle body excited by its movement over the ground. A further example of suspension systems in which suspension cylinders are associated with oleopneumatic accumulators for the purpose of improving ride is disclosed in US-A-4 664 410 (Citroen) .

However, such oleopneumatic systems are comparatively expensive and may require comparatively large pipes for the hydraulic fluid.

According to the present invention there is provided a suspension system comprising a gas spring and a mass mounted on the spring for damped oscillating movement, wherein the gas spring has a first gas chamber of variable volume which is compressible and expansible by oscillations of the spring, and a second rigid gas chamber which communicates directly with the first chamber via a rapidly openable and closable valve, and the system incorporates means for detecting a function of changing compression of the first chamber, said detection means being operatively associated with the valve to effect closure or opening thereof when

. the function passes through predetermined values whereby the opening of the valve when there is a q pressure difference thereacross serves to dissipate energy in the subsequent equalising process, and the changes in stiffness of the spring serving to de-tune the system.

The variable volume chamber may for example be a bellows or a piston and cylinder arrangement.

In a preferred embodiment of the invention, the detection means comprises sensors (eg. accelerometers and/or extension transducers) the outputs of which are sent to a microprocessor which controls opening and closing of the valve (eg. in accordance with the criteria of the type described more fully below) . In another embodiment, the detection means may be provided by an actuator arrangement mechanically connected to the variable volume chamber such that compression and subsequent expansion of the chamber in accordance with predetermined criteria serves to open and close the valve.

By opening and closing the valve, the spring is switched between a relatively 'soft' condition (valve open) and a relatively stiff condition (valve closed). The stiffness of the spring is thus modulated in accordance with predetermined conditions. The advantage of modulating the spring (as compared to a damper or a hydraulic actuator in prior art suspension systems) is that the system switches its ^*natural frequency between two values, a higher one when the

-* valve is closed, and a lower one when it is open. In effect, it no longer has a natural frequency when ^s suitably operated and this results in a large reduction in the dynamic magnifier even when no damper is fitted. The suspension system of the invention dissipates the energy input from the road by opening the valve when there is a pressure difference over it without the disadvantages of harshness, vibration and noise associated with the dampers used in prior art suspension systems. It may be necessary to use a damper (in addition to the gas spring) for alleviating problems associated with dynamic tyre loads (so called 'wheel hop'). However since the damper is only required to deal with the dynamic tyre loads (and is not required to reduce motion of the vehicle body) the design of the damper is considerably simplified and its action can be milder.

The invention will be further described by way of example and with reference to the accompanying drawings, in which

Fig. 1 is a diagrammatic representation of a first embodiment of car suspension system in accordance with the invention;

Fig. 2 illustrates operation of the suspension system when the car wheel encounters a bump;

Fig. 3 is a frequency response curve for the suspensions system of the invention as compared to a conventional form of suspension; and

Fig. 4 illustrates a second embodiment of suspension system of the invention.

Fig. 1 shows the suspension system for one wheel of a motor vehicle and comprises an air spring 1 (shown much magnified) subdivided into a variable volume chamber 2 and a fixed volume chamber 3. The two chambers 2 and 3 may communicate with each other via a valve which as shown in the drawing comprises a port 4 which may be opened and closed (thereby sealing chambers 2 and 3 from each other) by a valve plug 5 operated by an actuator 6. This actuator 6 is such that the port 4 may be opened or closed very rapidly, eg. within 30 milliseconds. Actuator 6 may for example be a solenoid or servo mechanism. Other forms of valve may be used, for example butterfly valves, slide valves etc.

The chamber 2 is a bellows arrangement with flexible walls 7 and a rigid closure 8. The flexible walls 7 provide the variable volume capability for chamber 2. The rigid closure 8 is associated with a wheel 9 of the vehicle in any suitable way, eg. an attachment to a suitable point on a trailing arm or bottom wishbone.

The rigid chamber 3 is attached in any suitable way to the body of the vehicle.

Although Fig. 1 shows chamber 3 directly above the chamber 2, this is not essential and chamber 3 may be at any suitable disposition relative to chamber 2 (provided that the two chamber may be in communication via a valve) so as to facilitate accommodation within the vehicle or access to the valve plug 5 or actuator 6. For example, chamber 3 may be to one side of chamber 2.

An accelerometer 11 is provided on the car body for measuring the upward or downward acceleration of the body (as it moves over uneven road surfaces) and an extension transducer 12 is provided for measuring the vertical position of the wheel relative to the car body. Signals from the accelerometer 11 and transducer 12 are processed by a microprocessor 13 which serves to control actuator 6 for opening and closing aperture 5 in accordance with the criteria discussed below.

Mechanical bump and rebound stops for arresting excessive upper or lower travel of the wheel 9 may be provided in conventional manner.

It should be appreciated that a suspension system as described is provided for each wheel of the motor vehicle, save that a single microprocessor 13 may receive and process signals from all accelerometers 11 and extension transducers 12 as well as operating all actuators 6.

The operation of the illustrated air spring will now be described in more detail.

During travel of the vehicle over bumps on the road, the port 4 is rapidly opened and closed in accordance with criteria such as those described below. With port 4 open, the rigid member 8 works against the total volume of air in the communicating chambers 2 and 3 and as a result the spring may be considered to be soft. Conversely, with port 4 closed, the rigid member 8 works only against the volume of air in chamber 2 and the spring is of higher stiffness. The natural frequency of the suspension is inversely proportional to the square root of the volume of air against the rigid member 5 works. Thus with port 4 open, the natural frequency of the suspension is lower then when the aperture 5 is closed.

The accelerometer 11 and extension transducer 12 provide signals from which various quantities such as the extension x of the airspring (as well as the derivatives x and x) and the vertical movement y of the vehicle body (as well as the derivatives y and y) are obtained either directly or by digital or analogue integration or differentiation. In turn, various criteria are computed from these values of x, y and/or their derivatives. When these criteria pass through certain values, the port 4 is rapidly opened or closed, depending on which criterion is involved.

A typical criterion for operating the actuator 6 is when

A_r x + B_rx > C_r (i)

Alternatively second derivatives may also be used -o give an inequality of the form.

-1

It is also possible to use an inequality including values based on both x and y, eg.

In the above inequalities (l)-(3), A, B, and C are positive or negative constants which may be periodically varied to suit prevailing road conditions .

A simple view of the manner in which the suspension system operates will now be described with reference to the vehicle wheel travelling along a smooth road and encountering a single short bump (Fig. 2) . The lower portion of the figure depicts a bump on a road, whereas curve A in the upper portion .depicts movement of the car as the wheel goes over the bump. Also shown in Fig. 2 is curve B which depicts the movement of a car body provided with a conventional suspension of intermediate stiffness .

During travel of the vehicle the accelerometer 11 and transducer 12 continuously monitor upward acceleration of the wheel 9 and extension of the air spring 1 respectively and send appropriate signals to the microprocessor 13.

On a perfectly smooth road, there is no upward or downward movement of wheel 9 and the valve remains open so that chambers 2 and 3 are in communication and the spring is soft. When the wheel encounters a short bump, the wheel 9 begins to rise producing an increasing load in spring 1 which causes the car body to move upwards. Since^' the valve is still open (and the spring soft) the upward acceleration of the car

SUBSTITUTE SHEET body is less that would be the case for a conventional suspension of intermediate stiffness.

It is however necessary to arrest the rising wheel and this is effected by rapidly shutting the valve as soon as the signals from accelerometer 11 and transducer 12 bring a first criterion into effect. This first criterion may take any one of the forms discussed above and will be such that either rapid upward acceleration (albeit with only a relatively short displacement of the air spring) or a relatively large displacement of the spring (albeit with a relatively low upward acceleration of the wheel 9) will cause the valve to be closed. The position at which the valve is considered to shut is shown on curve A of Fig. 2. Closure of the valve causes the upward acceleration of the car body to increase slowly as the air pressure rises more rapidly in the smaller volume of chamber 2 (as compared to the combined volume of chambers 2 and 3) .

When the wheel comes to a stop or very shortly thereafter a second criterion comes into play and the valve is rapidly opened. As a result, pressures equalise between chambers 2 and 3 reducing the rise of the car body and dissipating part of the energy by the bump. Consequently, the rise of the car body is less than would be the case for the conventional suspension of intermediate stiffness (compare curves A and B in Fig. 2).

As the wheel comes off the bump back onto the level road, the car body will start moving downwardly after the wheel. A third criterion is then invoked to close the valve rapidly so that the increasing pressure in the smaller chamber 2 slows the downward movement of the body. A fourth criterion then comes into play so as once again to open the valve. The third and fourth criteria thus together serve to reduce the fall of the body and eliminate overshoot.

• The four criteria thus described serve to improve the ride under 'normal' conditions when the wheel does not approach the limits of its travel. A further four criteria may be provided to cater for larger than average bumps when contact with the mechanical bump or rebound stop might otherwise occur. This further set of four criteria might thus be regarded as 'electronic bump stops' and they would be characterised by high values of the C constant as compared with those in the 'normal' criteria. By using non-linear forms of criteria it may be possible to combine the aforementioned eight criteria into one set of four criteria, but this may be less economical in processing.

It should be appreciated that the criteria thus far described are illustrative only and that other criteria may be employed for controlling operation of the valve.

A further way of regarding the action of the suspension resides in the fact that, with the valve shut, the air spring has a higher natural frequency that when the valve is open. By opening and shutting the valve in accordance with the aforementioned criteria, the spring is switched from one frequency to the other in ways which effectively detune it, thus almost removing the high dynamic magnifier (ie. the ratio of car body vertical travel compared to the

* height of the bump) associated with the natural frequency in conventional suspensions. This is

^? illustrated in the computer simulated plot shown in

Fig. 3 in which curve C is the frequency response of the suspension system shown in Fig. 1 and curve D is that of a conventional suspension. It will be seen that the peak of primary resonance (in the conventional suspension) is almost completely removed by the detuning effect.

The suspension system described in Fig. 1 is based on the use of sensors (accelerometer 11 and transducer 12) as well as microprocessor 13. However, it is possible to make use of a less refined system for effecting opening and closure of the valve, and such a system is shown in Fig. 4. The system of Fig. 4 incorporates a similar air spring to Fig. 1, mounted on the rigid closure 8 and locating within chamber 2 is a cylinder 40 on which is mounted a loosely fitting piston 41. The piston is biased resiliently out of cylinder 40 by a spring 42 and carries a rod 43 which operates the valve plug 44. This arrangement gives an analogue approximation to a criteria as illustrated by equation (1) above in terms of extension only.

A number of modifications may be made to the suspension system described. For example, it may be desirable in some cases to provide more than one rigid chamber with each such rigid chamber communicating independently with the chamber of variable volume by a respective controllable valve.

Such an arrangement may be advantageous in a vehicle whose total weight varies widely. Both valves may be controlled in accordance with criteria as described above. Alternatively, one of the valves may be used solely to change the average stiffness of the suspension system.

It should also be appreciated that the pneumatic tyre of the wheel also act as a spring. To eliminate problems with wheel 'hop¹, it is possible for the suspension system of the invention to be modified to include specially designed dampers.

Claims

1. A suspension system comprising a gas spring and a mass mounted on the spring for damped oscillating movement, wherein the gas spring has a first gas chamber of variable volume which is compressible and expansible by oscillations of the spring, and a second rigid gas chamber which communicates directly with the first chamber via a rapidly openable and closable valve, and means are provided for detecting a function of changing compression of the first chamber, said detection means being operatively associated with the valve to effect closure or opening thereof when the function passes through predetermined values whereby the opening of the valve when there is a pressure difference thereacross serves to dissipate energy in the subsequent equalising process, and the changes in stiffness of the spring serving to de-tune the system.

2. A suspension system as claimed in Claim wherein the first gas chamber comprises a bellows arrangement.

3. A suspension system as claimed in Claim 1 wherein the first gas chamber comprises a piston and cylinder arrangement.

4. A suspension system as claimed in any one of Claims 1 to 4 wherein the detection means comprises sensors which are associated with a microprocessor which controls opening and closing of the valve.

5. A suspension system as claimed in Claim 4 wherein the sensors comprise accelerometers and/or extension transducers.

6. A vehicle having a plurality of wheels whereof at least some of the wheels of the vehicle are associated with a suspension which comprises a gas spring mounted between the wheel and the body of the vehicle, wherein the gas spring has a first gas chamber of variable volume which is compressible and expansible by oscillations of the spring, and a second rigid gas chamber which communicates directly with the first chamber via a rapidly openable and closable valve, and means are provided for detecting a function of changing compression of the first chamber, said detection means being operatively associated with the valve to effect closure or opening thereof when the function passes through predetermined values whereby the opening of the valve when there is a pressure difference thereacross serves to dissipate energy in the subsequent equalising process, and the changes in stiffness of the spring serving to de-tune the system.