GB2300892A

GB2300892A - A damper for a vehicle suspension system

Info

Publication number: GB2300892A
Application number: GB9509893A
Authority: GB
Inventors: Richard Hurdwell
Original assignee: Lotus Cars Ltd
Current assignee: Lotus Cars Ltd
Priority date: 1995-05-16
Filing date: 1995-05-16
Publication date: 1996-11-20
Also published as: GB9509893D0

Abstract

A monotube damper 30 comprises compliant means 31, in the form of a closed cell foam block, that allows the damper 30 to be attached to a vehicle body by joints (32, 34, fig 5) which do not include a rubber isolator. The compliant means 31 deforms on application of a force to the damper 30 to permit motion of piston 16 relative to cylinder 17. In another embodiment a twin tube damper 40 is provided with compliant enclosure 41 connected, via a tube 42, to chamber A and a gas spring 43 attached to chamber B by a tube 44 having a compliant hose portion 44A made of rubber. A second embodiment of monotube damper (70, fig 8) has a chamber (A, fig 8) connected to a gas spring (72, fig 8) by a tube (71, fig 8) having a restriction (78, fig 8) which controls fluid flow, thereby avoiding piston bounce. A chamber (B, fig 8) is connected to a compliant fluid container (74, fig 8), comprising a piston (76, fig 8) biased by a spring (77, fig 8). Fluid may flow from the container (74, fig 8) to the chamber (B, fig 8) and vice-versa. Also disclosed is an actuator for an active suspension system (see fig 13).

Description

A DAMPER OR AN ACTUATOR FOR A VEHICLE SUSPENSION SYSTEM The present invention relates to a damper for a vehicle suspension system.

In the past it has been known to provide a passive damper, commonly called a shock absorber, connected between a vehicle body and a vehicle hub assembly. The damper commonly acts in parallel with a spring between the vehicle body and a wheel and hub assembly. The essential components of the damper are a piston which moves through fluid provided in a cylinder, with the viscosity of the fluid providing resistance to motion of the piston.

Some vehicles employ active suspensions which use hydraulic actuators in place of dampers. The hydraulic actuators also have pistons moveable in cylinders. The supply of fluid to at least one side of the piston is controlled, so that motion of the piston can be occasioned by introduction of fluid into the cylinder on at least one side of the piston. An active suspension contrasts with a passive suspension in that the position of the piston within the cylinder and therefore the distance between the wheel and hub assembly and the vehicle body is actively controlled by a suspension computer which controls flow of fluid to and/or from the actuator, whereas the damper reacts passively to force inputs, with the resistive force being proportional to the velocity of motion of the piston.

There are also adaptive suspension systems which comprise damper elements which react passively to input loads but which have variable damping rates. The damping characteristics of the damper are controlled in such systems by a computer which varies the damping rate of the damper.

In all three types of suspension system the damper or actuator is generally connected to the vehicle body by means of a compliant rubber isolator. It has been found necessary to include isolators in the load path between the vehicle body and the vehicle wheel and hub assembly, because both passive dampers and actively controlled hydraulic actuators are not very efficient at dealing with high frequency noise inputs, such as road noise.

Also, neither system is efficient at dealing with very sudden shock load inputs. The rubber isolators are provided both to cope with the high frequency noise inputs and the sudden shock load inputs to the vehicle.

However, the isolators detract from the ability of the actuator to provide ride and handling control of the vehicle.

The present invention provides in a first aspect a damper for a vehicle suspension system comprising a cylinder, a piston moveable in the cylinder and defining with the cylinder first and second variable volume chambers for receiving fluid, connection means to allow a restricted flow of fluid between the first and second variable volume chambers, wherein the cylinder is connectable to one of a vehicle body and a wheel and hub assembly and the piston is connectable to the other of the vehicle body and the wheel and hub assembly, characterised in that there is provided means to provide compliance in both of the first and second chambers.

The present invention provides in a first aspect a damper for a vehicle suspension system comprising a first cylinder, a piston moveable in the first cylinder which defines with the first cylinder first and second variable volume chambers for receiving fluid, connection means which allows flow of fluid into or out of the first and second variable volume chambers and which has restriction means to restrict the flow of fluid, wherein the first cylinder is connectable to one of a vehicle body and a wheel and hub assembly and the piston is connectable to the other of the vehicle body and the wheel and hub assembly, characterised in that compliant means is provided in or connected to both of the first and second chambers, the compliant means deforming on application of force to the damper to allow motion of the piston relative to the first cylinder.

The compliant means can take any form, for instance; a gas spring, probably requiring separation from the hydraulic fluid in the damper or actuator (by means of a membrane or piston); a compliant oil reservoir, compliant hydraulic hoses, or a compliant cylinder housing; inclusion of compliant material (e.g. closed cell foam) in a chamber; or the use of a working fluid in the actuator which has a bulk modulus reduced from that of the usual hydraulic fluid used in dampers and actuators.

The conventional rubber isolators present in the damper load path between a wheel and hub assembly and a vehicle body can be replaced by non-compliant connections if the damper or actuator of the present invention is used. Alternatively, very stiff compliant mounts can be used, of a stiffness which would not provide sufficient attenuation in a normal passive or adaptive suspension system.

Preferably the connection means connects the first and second variable volume chambers and the compliant means can allow motion of the piston relative to the cylinder without the need for flow of fluid between the first and second chambers.

Preferably the compliant means comprises first compliant means connected to the first chamber and second separate compliant means connected to the second chamber.

In a preferred embodiment the compliant means comprises a compliant body located in one of the chambers.

The compliant body could be, for instance, a closed cell foam block. This would have the advantage of cheapness.

In a further preferred embodiment the compliant means comprises a gas spring permanently connected by connection means to one of the chambers.

The use of a gas spring would be more expensive than use of a compliant body, but would provide better compliance characteristics. A gas spring could be provided within a chamber by the use of a floating piston arrangement, or could be connected to a chamber by a suitable conduit.

In an additional preferred embodiment the compliant means comprises a compliant fluid container connected by connection means to one of the chambers.

The compliant container could take the form of a compliant (eg. rubber) hose. Such hoses are cheaper and simpler than gas springs. The compliant container could also take the form of a container having a piston movable therein and a spring exerting a biasing force on the piston.

The use of compliant hoses as the connection means would enhance performance gain.

In one embodiment the damper comprises additionally: a second cylinder fixed to and at least partially surrounding the first cylinder, a third chamber for receiving fluid which is defined between the exterior of the first cylinder and the interior of the second cylinder, connecting means connecting the first and third chambers, restriction means in the connecting means to restrict flow of fluid between the first and third chambers and mounting means on the exterior of the second cylinder to enable the first cylinder to be connected to one of the vehicle body and the wheel and hub assembly via the second cylinder.

The compliant means can comprise hydraulic fluid of a bulk modulus with a magnitude 30% to 40% of the bulk modulus of uncontaminated hydraulic fluid used in a comparable conventional damper; preferably the hydraulic fluid has a bulk modulus in the range of 400 to 600 N/mm.

Preferably the or each compliant means is arranged to provide the damper with a stiffness in the range of 900 N/mm to 2500 N/mm; preferably approximately 1600 N/mm.

Preferably the stiffness of the compliant means is adjustable by a user of the damper whereby the user can vary the stiffness of the damper to suit different applications or modes of use.

Preferably compliant means can allow the damper to have the same rate of extension for two different extending forces acting on the damper, a first force when the rate of extension is increasing in magnitude and a second higher force when the rate of extension is decreasing in magnitude. Preferably the compliant means can allow the damper to have the same rate of contraction for two different compressive forces acting on the damper, a first force when the rate of contraction is increasing in magnitude and a second force of greater magnitude when the rate of contraction is decreasing.

Preferably the second higher force can be more than 1.25 times greater than the first lower force and more preferably the second higher force can be more than 1.5 times greater than the first lower force. The difference will depend on the rate of increase of contraction or contraction, a slow rate will result in a small difference. Preferably the compliant means is adjustable by a user of the damper whereby the ratio of the magnitude of the first force to the magnitude of the second force is adjustable by the user.

The damper of the present invention further provides a "hysteresis effect", which will be further described later, which has a beneficial effect on vehicle ride.

The present invention provides in a second aspect a passive or adaptive suspension system for a vehicle comprising a damper as described above, first connection means for connecting one of the piston and the cylinder of the damper to a vehicle body and second connection means for connecting the other of the piston and the cylinder to a wheel and hub assembly, wherein the first connection means includes no compliant isolator member.

Preferably the second connection means includes no compliant isolator member.

The present invention is advantageous because there is no lost motion in the suspension arrangement caused by flexing of isolators. Therefore more energy is dissipated directly by the damper, the damper cannot vibrate on its mountings and better control of wheel and hub motion is achieved. The invention also allows control of the hysteresis in the load/velocity characteristic of the suspension. This provides improved isolation of the vehicle from high frequency loading and shock loading.

The invention also frees the design of suspension systems by removing the need for an isolator.

The present invention provides in a further aspect an actuator for an active vehicle suspension system comprising a cylinder, a piston moveable in the cylinder and defining with the cylinder first and second variable volume chambers for receiving fluid, first fluid connection means for connecting at least the first variable volume chamber to valve means for connecting the first variable volume chamber to a source of pressurised fluid or to an exhaust for fluid, characterised in that compliant means is provided in or permanently connected to the first chamber, whereby on application of force to the actuator the compliant means can deform to allow the piston to move relative to the cylinder without flow of fluid through the valve means to or from the first chamber.

Preferably second fluid connection means is provided to connect the second chamber with the valve means and preferably the compliant means is also provided in or connected to the second chamber.

In a preferred embodiment of actuator the compliant means comprises a body of compliant material located in a chamber.

The body of compliant material could, for instance, be a closed cell foam block. This option would be cheap and simple.

In another preferred embodiment of actuator the compliant means comprises a gas spring permanently connected by third fluid connection means to a chamber.

Use of a gas spring would provide good compliance characteristics, but could be expensive. A gas spring could be incorporated in the actuator by use of a floating piston in one chamber, the floating piston separating hydraulic fluid in the chamber from a pocket of gas.

In a further preferred embodiment of actuator the compliant means comprises a compliant fluid container connected by fourth fluid connection means to a chamber.

The compliant fluid container could be a rubber hose.

Preferably the third fluid connection means comprises a compliant hose.

Preferably the or each compliant means has a stiffness in the range of 2000 N/mm to 5000 N/mm.

Preferably the stiffness is approximately 3500 N/mm.

The conventional rubber isolators present in the load path between a wheel and hub assembly and a vehicle body can be replaced by substantially non-compliant connections if the actuator of the present invention is used. Alternatively, very stiff compliant mounts can be used, of a stiffness which would not provide sufficient attenuation in a normal active suspension system.

As mentioned above, prior art active suspension systems are not able to operate at a bandwidth high enough to attenuate so called "secondary" inputs to the vehicle (e.g. road noise). They can also be limited by instability at high closed loop gains. Introducing compliance at least on one side of the actuator piston has several beneficial effects. First the actuator response to secondary inputs is considerably improved.

Secondly, the high frequency secondary inputs are attenuated without the need for compliant isolators connected between the actuator and the vehicle body and/or wheel and hub assembly. Thirdly, it has been found that the use of the compliance enables the gain of the closed loop control system controlling the actuator to be increased considerably without system instability.

This allows lower values of damping force to be provided by the system and allows optimisation of vehicle ride and handling.

The present invention provides in a fourth aspect an active suspension system comprising an actuator as described above, first mechanical connection means connecting one of the piston and the cylinder of the actuator to a vehicle body, second mechanical connection means connecting the other of the piston and cylinder to the vehicle body, wherein the first mechanical connection means does not include any compliant isolator member.

Preferably the second mechanical connection means does not include any compliant isolator member.

The present invention is advantageous because there is no lost motion in the suspension arrangement caused by flexing of isolators. Therefore better control of wheel and hub motion is achieved. The present invention provides improved isolation of the vehicle from high frequency loading and noise and shock loading.

The invention also frees the design of suspension systems by removing the need for an external isolator.

In a further aspect the present invention provides an active suspension system comprising: an actuator connected between a vehicle body and a wheel and hub assembly, the actuator comprising a cylinder and a piston moveable in the cylinder and defining with the cylinder first and second variable volume chambers, a source of pressurised fluid, an exhaust for fluid, valve means connected to the source of pressurised fluid, the exhaust for fluid and the first chamber, and a control system, wherein the control system generates a position or velocity control signal to control the valve means to vary flow of fluid to or from the first chamber and thereby to vary the position or velocity of the piston, characterised in that compliant means is provided in or connected to the first chamber, whereby the compliant means on application of force to the actuator can deform to allow the piston to move relative to the cylinder without flow of fluid through the valve means to or from the first chamber.

In some prior art active suspension systems the control system has either attempted to control the position or velocity of the piston. In systems which control position or velocity it has always to date been felt necessary to have complete control of velocity and position. The present invention goes against the accepted teaching of the art in that complete control is not possible with the compliant means connected to a chamber, but nevertheless the invention finds an improved system.

In a further aspect the present invention provides an active suspension system which comprises: an actuator connected between a vehicle body and a wheel and hub assembly, the actuator comprising a cylinder and a piston movable in the cylinder and defining with the cylinder first and second variable volume chambers, a source of pressurised fluid, an exhaust for fluid, valve means connected to the source of pressurised fluid, the exhaust for fluid and the first chamber, and a control system, wherein the control system generates a force control signal to control the valve means to vary flow of fluid to or from the first chamber to thereby vary force applied by the actuator, characterised in that compliant means is provided in or connected to the first chamber, whereby the compliant means can deform on application of force to the actuator to allow the piston to move relative to the cylinder without flow of fluid to or from the first chamber.

Preferably the active suspension system has sensor means for measuring a plurality of variables, including force transmitted from the actuator to the vehicle body, and for generating signals indicative thereof and electrical or electronic processsor means for processing the signals indicative of measured variables using a plurality of algorithms which include a plurality of scaling parameters and for producing a velocity demand signal demanding a rate of extension of contraction of the actuator, wherein the processor means varies at least one of the scaling parameters in a manner dependent on the measured force signal or on the velocity demand signal in order to vary the rate at which the magnitude of the velocity demand signal increases with increasing magnitude of measured force.

The rate of increase of magnitude of the unmodified proportional and linear velocity demand signal with increasing magnitude of the measured force can be changed from one value to another value when a threshold value of magnitude of the velocity demand signal is reached.

Alternatively, the rate of increase of magnitude of the velocity demand signal with increasing magnitude of measured force can be changed from one value to another value when a threshold value of magnitude of the force signal is reached. Furthermore, the rate of increase of magnitude of the velocity demand can be varied continuously as a continuous function of the velocity demand signal or the measured force signal.

In a further aspect the present invention provides an active suspension system comprising: an actuator connected between a vehicle body and a wheel and hub assembly, the actuator comprising a cylinder and a piston moveable in the cylinder and defining with the cylinder first and second variable volume chambers, a source of pressurised fluid, an exhaust for fluid, valve means connected to the source of pressurised fluid, the exhaust for fluid and the first chamber, and controlled by a velocity control signal sensor means for measuring a plurality of variables, including force transmitted from the actuator to the vehicle body, and for generating signals indicative thereof, and a control system which includes electrical or electronic processor means for processing the generated signals indicative of measured variables using a plurality of algorithms which include a plurality of scaling parameters and for producing a velocity demand signal demanding a rate of extension or contraction of the actuator, characterised in that the processor means varies at least one of the scaling parameters in a manner dependent on the force signal or the velocity demand signal in order to vary the rate at which the magnitude of the velocity demand signal increases with increasing magnitude of measured force.

In the past active suspensions have generated velocity control signals to the valve means which increased at a rate linearly proportional the measured force and the velocity demand signal. This linear velocity demand response was considered beneficial. The applicant has now appreciated that the handling of a vehicle with active suspension can be improved by having a velocity response with a non-linear characteristic which is in some way dependent on the magnitude of the measured force or the magnitude of the measured signal or the linear velocity demand signal.

The rate of increase of magnitude of the velocity demand signal with increasing magnitude of the measured force can be changed from one value to another value when a threshold value of magnitude of the velocity demand signal is reached. Alternatively the rate of increase of magnitude of the velocity demand signal with increasing magnitude of the measured force can be changed from one value to another value when a threshold value of magnitude of the force signal is reached. Furthermore the rate of increase of magnitude of the velocity control signal can be varied continuously as a function of the measured force or the linear velocity demand signal.

Preferred embodiments of the present invention will now be described with reference to the accompanying figures in which; Figure 1 shows a vehicle suspension system of the prior art, Figure 2 shows a conventional monotube damper according to the prior art, Figure 3 shows a conventional twin tube damper according to the prior art, Figure 4 shows a monotube damper according to the present invention, Figure 5 shows the monotube damper of figure 4 in use, Figure 6 shows a twin tube damper according to the present invention, Figure 7 shows the twin tube damper of figure 6 in use, Figure 8 shows a second embodiment of monotube damper according to the present invention, Figure 9 shows the monotube damper of figure 8 in use, Figure 10 is a schematic showing a damper according to the invention attached to a vehicle about to pass over a road bump.

Figure 11 is a graph illustrating the hysterisis properties of a damper according to the invention.

Figure 12 shows an actuator for an active suspension system according to the prior art, Figure 13 shows an actuator for an active suspension system according to the present invention, Figure 14 shows a portion of an active suspension system according to the present invention, Figure 15 is a graph illustrating a non-linear velocity response of an active suspension system, Figure 16 is a schematic diagram showing part of an active suspension system with the non-linear velocity response shown in Figure 15, Figure 17 is a schematic diagram showing part of an alternative active suspension system with the non-linear velocity response of Figure 15, Figure 18 is a graph illustrating a second nonlinear velocity response of an active suspension system, and Figures 19 and 20 are graphs showing how a gain G of the Figure 17 active suspension system can be continuously varied.

Figure 1 shows an example of a conventional passive suspension system for a vehicle. There can be seen in the figure a wheel and hub assembly 10 which is mounted on a suspension system comprising a swing arm 11 which is pivotally connected by a pivot 12 to a vehicle body 13.

Acting between the swing arm 11 and the vehicle body 13 is a road spring 14. A passive damper 15 is connected between the swing arm 11 and the vehicle body 13 and acts in parallel with the road spring 14. The passive damper 15 comprises a piston 16 moveable in the cylinder 17.

The piston 16 is connected by a connecting rod 18 and by a compliant pivot joint 19 to the swing arm 11. The cylinder 17 is connected by a rubber isolator 20 to the vehicle body 13.

An example of a conventional monotube damper is shown in figure 2 and in this damper it can be seen that the piston 16 defines in the cylinder 17 an upper chamber A and a lower chamber B. An orifice 21 is provided in the piston 16 to allow flow of fluid from chamber A to chamber B and vice versa. The orifice 21 is designed to deliberately restrict flow of fluid therethrough to provide damping forces.

A floating piston 22 is provided in the top chamber A and gas is sealed in a chamber C defined between the floating piston 22 and the top of cylinder 17. The floating piston 22 and the chamber C are necessary since the connecting rod 18 reduces the cross-sectional area of the chamber B with respect to the chamber A. Thus, for a given chamber height, chamber A can contain more fluid than chamber B and if fluid is to flow effectively from one chamber to the other some degree of compliance must be provided and this is achieved by means of the floating piston 22 between the chambers A and C.

In the twin tube damper 23 of figure 3 a second cylinder 24 is used instead of a second piston 22. The first cylinder 17 is located within the outer second cylinder 24 and is fixedly attached thereto. Gas is provided in a chamber D defined between the outer cylinder 24 and the inner cylinder 17. The gas is located in the top annular portion of the chamber D. The piston 16 moves within cylinder 17 and has two orifices which are shown as 25 and 26. The orifice 26 is controlled by a one way valve 27.

An orifice 28 is also provided between the bottom chamber B and the chamber D. A one way valve 29 is provided in a second orifice 29 connecting the chamber B and chamber D. In the arrangement it can be seen that the connecting rod 18 will in fact be connected to a vehicle body rather than a swing arm. The provision of rod 18 in chamber A decreases the cross-sectional area of the piston 16 which acts in chamber A in comparison with the cross-section of the piston 16 acting in chamber B.

The gas in chamber D is provided to account for the different cross-sectional areas.

Isolators 19 and 20 (shown in figures 1, 2 and 3) are necessary in a vehicle suspension system when normal prior art passive dampers are used e.g. the dampers 15 and 23 of figures 2 and 3. The dampers 15 and 23 shown in figures 2 and 3 transmit high frequency noise inputs, such as road noise, and also sharp sudden shock loads inputs. The rubber isolators 19 and 20 are needed to provide compliance in the load path between the wheel and hub assembly 10 and the vehicle body 13 to attenuate these inputs. However, the use of the isolators 19 and 20 detracts from the handling and some aspects of the ride of the vehicle, as mentioned above.

A monotube damper 30 according to the present invention is shown in figure 4. The monotube damper 30 of the present invention comprises all of the components of the monotube damper 15 of the prior art and identical components have identical reference numerals.

The difference between the monotube 30 of figure 4 and the monotube 15 of the prior art is the inclusion of compliant means in the lower chamber B, the compliant means comprising a closed cell foam block 31. When the monotube damper 30 of figure 4 is used in place of the monotube damper 15 of figure 2, the need for use of a rubber isolators such as isolators 19 and 20 is eliminated and the cylinder 17 can be connected directly to a vehicle body via a suitable non-compliant articulating joint. This is shown in figure 5 where the damper 30 can be seen connected to a vehicle body by an articulating joint 32 which includes no compliant rubber isolator. The damper 30 is also connected to the swing arm by a joint 34 which does not include an isolator.

In figure 6 a twin tube damper 40 according to the present invention is illustrated. In most respects the damper 40 is identical to the damper 23 shown in figure 3 and identical components have been given the same reference numerals.

The difference between the figure 6 twin tube damper 40 and the figure 3 twin tube damper 23 lies in the provision of compliant means connected to chambers A and B. The chamber A is connected to a compliant enclosure 41 via a tube 42. The chamber B is connected to a gas spring 43 via a tube 44. The tube 44 includes a compliant hose portion 44A, which is made of rubber.

If the twin tube damper 23 shown in figure 3 is used in a vehicle suspension system, then the connecting rod 18 will be connected to the vehicle body via a rubber isolator, shown as 20 in figures 1 and 3. If the twin tube damper 23 is replaced by the twin tube damper 40 of the present invention illustrated in figure 6 then the rubber isolator 20 should preferably not be used and the connecting rod 18 can be connected to the vehicle body via a suitable non-compliant joint. This is shown in figure 7, where the damper 40 can be seen connected to a vehicle body by an articulating joint 45 which does not include a compliant rubber isolator. Also the damper 40 is shown connected to a swing arm of a wheel and hub assembly by a joint 46 which does not include a compliant isolator member.

A second embodiment of monotube damper according to the invention is shown in figure 8. In figure 8 the monotube damper 70 has a chamber A which is connected by a tube 71 to a gas spring 72. The monotube damper 70 also has a chamber B connected by a tube 73 to a compliant fluid container 74. The compliant fluid container 74 comprises a cylinder 75 in which a piston 76 is movable. The piston 76 is biased by a spring 77 which exerts a biasing force on piston 76 when compressed. The piston 76 and cylinder 75 define together a variable volume chamber E into which and out of which fluid can flow to and from the chamber B of damper 70.

In the tube 71 there is provided a restriction 78 which restricts flow of fluid through the tube 71. The restriction introduces a controlled element of damping in the fluid flow; this avoids any problems caused by the piston 16 bouncing within the cylinder 17.

Figure 9 shows the damper 70 in use. The damper 70 is connected to a vehicle body by a joint 79 which does not include a rubber isolator. Also the damper 70 is connected to the swing arm 80 by a joint 81 which does not include a compliant member.

The gas spring 72 could be provided within the cylinder of damper 70, with a dividing wall provided in the chamber A and an orifice in the dividing wall.

The stiffness of the compliant means of a damper of the invention will be chosen to give an effective compliance to the damper arrangement similar to or slightly stiffer than a comparable conventional damper and isolator arrangement. Typically for a light passenger vehicle the stiffness will be in the range 900 N/mm to 2500 N/mm, with a preferred median of around 1600 N/mm, when calculated as an effective stiffness at the wheel.

The selection of a suitable stiffness for the compliant means could be made after calculating the natural frequency of the spring/damper arrangement, but more typically the stiffness would be chosen emperically by testing.

The use of compliant means provides a beneficial "hysterisis" effect, as illustrated by figures 10 and 11.

Figure 10 shows schematically a wheel 50 connected to a vehicle body 51 by a damper 52, which is a damper according to the invention (eg. damper 15, damper 23 or damper 70). The vehicle body 51 is moving in the direction of the shown arrow and thus the wheel 50 must pass over a bump. There can be seen on the bump various points A to I. Illustrated on the graph of figure 11 is a curve showing the relationship between the vertical component of velocity of the wheel 50 (ie. the velocity towards and away from the vehicle body) and the force transmitted by the damper 52 to the vehicle body 51. A negative velocity indicates that the wheel 50 is moving upwardly, towards the vehicle body 51, and a positive velocity indicates that the wheel 50 is moving downwardly, away from the vehicle body.A negative force indicates a compressive force required to close the damper 51 and a positive force indicates a force required to extend the damper 51. The points A to I are marked on the graph.

As the wheel 50 first engages the bump the wheel 50 is given an upward (negative velocity) which increases in magnitude from point A to point C and decreases in magnitude as the top of the bump is reached from point C to point E. It can be seen from a comparison of the points B and D that the compressive force on the damper 51 at point B is less in magnitude than the compressive force on the damper at point D, even though the damper has the same velocity (ie. is contracting at the same rate) at both points.This is advantageous since it leads to the result that the vehicle body is subjected to a more gradually increasing force than would be provided by a normal damper; the force/velocity characteristics of a known damper are given by the dotted line 60 in figure 7 and it will be seen that the force on a normal damper for a given velocity is the same whether the velocity is increasing or decreasing. With use of a damper according to the invention, the driver experiences less of a jerk on encountering a bump.

After the wheel 50 has reached the top of the bump, the damper 52 is extended under the action of a road spring 53 acting in parallel with the damper 52. This happens from E to I. Again the hysterisis effect results in two different expansive forces for the same rate of extension of the damper, depending on whether the rate of extension is increasing or decreasing in magnitude. This again is advantageous in lowering the downward jerk force on the vehicle body as the wheel passes the top of the bump.

Some prior art dampers do exhibit some degree of hysterisis, but this is not deliberate. The present invention deliberately introduces hysterisis and the hysterises is then tunable to meet specific vehicle needs.

The stiffnesses of the compliant means of the invention will preferably be adjustable so that the properties of the damper can be adjusted to suit different vehicle needs.

Whilst in the embodiments described fluid flows from one chamber to another via a restriction through the piston of the damper this is not a necessary feature of the invention: instead a restriction could be provided to allow fluid flow out of one chamber to another suitable container.

Whilst in the embodiments described above the compliant means comprises gas springs, compliant enclosures and compliant inserts, it is also envisaged by the applicant that compliant means could comprise compressible hydraulic fluid which is more compressible than the usual hydraulic fluid used for dampers or deliberately compliant sides for the cylinders of the dampers.

The bulk modulus of hydraulic fluid usually used for dampers is 1380 N/mm2, which falls on contamination by air to 780 N/mm2. The present invention would use hydraulic fluid with a bulk modulus of around 30% to 40% of the standard bulk modulus, typically around 500 N/mm2.

Alternatively gas springs or compliant containers would be connected to the fluid to lend the fluid an effective stiffness of around 500 N/mm2.

Whilst in the above specification we have only mentioned monotube and twin tube dampers, it should be appreciated that the present invention could be used with any form of damper.

Figure 12 shows an actuator of a known active suspension system. There can be seen in the figure an actuator 110 which comprises a piston 111 moveable in a cylinder 112. The piston 111 is connected by a connecting rod 113 to a joint 114 for connection to a swing arm of a wheel and hub assembly of a vehicle (not shown in the figure). The cylinder 112 is connected by a rubber isolator 115 to a vehicle body (shown at 116).

The piston 111 defines in the cylinder 112 an upper chamber A and a lower chamber B. A rod 118 extends upwardly from the piston 111 through a dividing wall 119 into an empty chamber C defined between the dividing wall 119 and the top of the cylinder 112. The rod 118 is necessary to equalise the face areas of the two sides of piston 111.

The chamber A is provided with a fluid pipeline 121 which allows hydraulic fluid to flow into and out of the chamber A. Similarly the chamber B is provided with a fluid pipeline 122 which allows hydraulic fluid to flow into and out of chamber B. Both pipelines 121 and 122 will be connected to a servo-valve.

The rubber isolator 115 is necessary in an active suspension system if the actuator of figure 12 is used.

An active suspension system is not efficient at attenuating high frequency noise inputs to the vehicle, such as road noise and is not efficient at attenuating sharp sudden shock loads to the vehicle. The rubber isolator 115 is needed to provide compliance in the load path between a wheel and hub assembly and the vehicle body 116 to deal with road noise and sharp inputs.

However, the use of the isolator 115 detracts from the ride and handling of the vehicle, as mentioned above.

An actuator 130 according to the present invention for an active suspension system is shown in figure 13.

The actuator 130 is similar to the conventional actuator 110 of figure 1 described above and like components will be given identical reference numerals. Thus it can be seen that the piston 111 and the cylinder 112 define between them two chambers; an upper chamber A and a lower chamber B. The upper chamber A is connected by a tube 121 to a servo-valve (123 in figure 14) which controls the flow of fluid to and from the chamber A. The lower chamber B is connected by a tube 122 also to the servo-valve 123 which controls flow of fluid to and from the bottom chamber B. The servo-valve 123 will be controlled by an active suspension control system as is well known in the prior art to control motion of the piston 11 in the cylinder 112.

The actuator of the present invention differs from the actuator of figure 12 in that compliant means is connected to both chamber A and chamber B. A compliant gas spring 131 is connected by a tube 132 to chamber A.

The tube 132 itself comprises a compliant portion 132A, in the form of a rubber hose. The compliance in chamber B is provided by a compliant enclosure 133 connected by a tube 134 to chamber B and the compliance is also by a closed cell foam insert 135 located in the bottom chamber B.

The stiffnesses of the compliant means 131, 133 and 135 are chosen to lend to the actuator arrangement a stiffness similar to or slightly stiffer than the stiffness of an isolator used in a comparable conventional system. For a car the stiffness would be typically in the range of 2000 N/mm to 5000 N/mm and preferably around 3500 N/mm. The degree of compliance for a particular suspension could be chosen by calculating the natural frequency of the system, but would more typically be chosen emperically through testing.

The cylinder 112 is connected directly to the vehicle body 116 with no compliant isolator interposed between the load cell 117 and the vehicle body. In the prior art active suspension system of figure 12 it was necessary to connect the rubber isolator 115 between the cylinder 12 and the vehicle body 116 to provide attenuation of high frequency loads which could not be dealt with by the active control system which is necessarily of limited bandwidth. In the present invention a rubber isolator such as 115 is not necessary and the cylinder 112 is connected directly to the vehicle body. This has numerous advantages as mentioned before.

In figure 14 there can be seen part of an active suspension system for a vehicle according to the invention. The figure shows the actuator 130 for one wheel and hub assembly only, however it should be appreciated that the suspension system will have one actuator for each wheel and hub assembly of the vehicle.

The above described actuator 130 is shown connected to the servo-valve 123 which regulates flow of fluid to the chambers A and B from a source of pressurised fluid 124 (commonly a pump) or to an exhaust for fluid 125 (commonly a sump). A controller 126 is provided to control the servo-valve 123 in a manner well known in the prior art.The controller 126 will process signals generated by sensors such as: a longitudinal accelerometer 150 for measuring the longitudinal acceleration of the vehicle; a lateral accelerometer 151 for measuring lateral acceleration of the vehicle; a speed sensor 152 for measuring vehicle speed; a yaw gyrometer 153 for measuring the yaw rate of the vehicle; a load cell 117 (and other load cells for each wheel and hub assembly of the vehicle); L.V.I.T.'s (such as 154) for measuring the extension of each actuator; a wheel and hub accelerometer 155 for measuring the acceleration of the wheel and hub assembly 127 towards and away from the vehicle body 116 (a wheel hub accelerometer will be provided for each wheel and hub assembly of the vehicle).

The controller 126, typically a digital electronic processor, will control a servo-valve associated with each actuator of the vehicle.

The use of compliant means in an active suspension actuator also has the advantage that the servo-valve 123 can be located further from the actuator than in the prior art systems. In the prior art active suspension systems it was necessary to locate a servo-valve as close to an actuator as possible to minimise delay in response caused by inertia of fluid in the pipes connected to the actuator. Since the actuator of the present invention has compliant means the problem of delay becomes less critical and a servo-valve can be located away from its associated actuator with the advantages of convenience of packaging and provision of a less hostile environment for the servo-valve.

The controller 126 of the preferred embodiment of the active system will be arranged to output a velocity demand signal to the servo-valve 123 which then controls flow of fluid to control the velocity of the piston 11.

In the present invention the piston velocity will not be controlled exactly by the control system, as in the prior art, due to the compliant means provided, but this has been found to have beneficial effects, as described above.

Controllers such as controller 126 have been described in many of the applicant's earlier patent specifications and in many patent specification of other individuals and corporation. Thus its working will not be described in detail here. However, the standard function of the controller has been improved in one respect by the present invention. The improvement can be implemental in many different ways, two of which are given now as examples.

In the first implements the calculation of a velocity demand signal by the controller is performed by the following equation: I X Dx .- -. F-k dc C where Dxd = demanded velocity F = force x = displacement c = constant k = constant This equation is carried out in four different modes of Heave, Pitch, Warp and Roll, with the Force and Displacement inputs resolved into each mode and a corresponding modal Dxd produced.

In most systems k and c are fixed constants, although in some systems they are varied in start-up conditions or at the limit of actuation motion (in order to give "bump stops"). However, in the prior art systems there has been in normal operating conditions a linear relationship between force and demanded velocity. This is illustrated by the dotted line in figure 15.

In the present invention either or both of the constants k and c are varied either with force F or velocity Dxd to give the force/demanded velocity relationship shown as a solid line in figure 15. The gradient of the solid line in figure 15 can be seen to change at distinct thresholds. The velocity demand increases at a first rate with increasing magnitude of force from zero then the velocity demand increases at faster rate after the point A illustrated in figure 15 (i.e. after a force or velocity threshold is reached).

The faster rate of increase of velocity with increases in force is maintained until point B when the rate of increase of velocity with force slows down (i.e. the gradient of the line decreases). Thus the velocity response is non-linear over the complete range of forces and velocities. It has been found that a non-linear response gives better suspension performance than the linear response of the prior art systems. Different parts of the ride of the vehicle can be tuned to specific needs.

Alternatively the rate of increase of the mangitude velocity demand signal with increasing magnitude of measured force signal could be continuously varied and this is shown in Figure 18 where the dotted line indicates the linear response of the prior art and the solid line shows a non-linear response of the present invention.

A schematic illustration of how a non-linear velocity response can be provided in an active system is shown in figure 15, which shows diagrammatically part of the controller 126. In the figure it can be seen that the box 200 receives a force signal F and a displacement signal X. The box 200 then produces a velocity demand signal Dxd in accordance with equation above. The box 200 will receive modular force and displacement signals and then generate a modular velocity demand signal. The constants c and k and supplied to the box 200 from a memory map 201. The memory map 201 supplies values of k and c is dependence on the force signal F. The memory map of the box 201 can be programmed as desired in order to achieve a suitable non-linear force demanded velocity response.

As an alternative to the above-noted method of producing a non-linear force/velocity demand response, a variable gain could be applied to velocity demand signals generated for the actuators. This is shown schematically in figure 17. A velocity demand signal Dxd for an actuator is produced by an active suspension control system (not shown) and then the velocity demand signal Dxd is multiplied by a variable gain G to produce a modified velocity demand signal Dxdl which is then relayed to the actuator. The variable gain G is varied by the box 301 which maps the velocity demand signal Dxd on to a mapping table to produce a suitable value of G. The mapping table 301 can be programmed to provide a desired nonlinear velocity response.Alternatively the gain G can be varied as a continuous function of measured force or measured velocity. Figures 19 and 20 show how G can be continuously varied.

Whilst in the above description the compliant means provided in or connected to the chambers of the actuator are gas springs compliant enclosures and compliant inserts, it is also envisaged by the applicant that the compliant means could comprise compressible hydraulic fluid used instead of substantially incompressible hydraulic fluid used in the known art or the compliant means could comprise compliant sides for the cylinder 112.

The pipes 121 and 122 could be compliant hoses which provide the necessary compliance for the actuator.

A gas spring could both be provided in an actuator of the invention by replacing the fixed dividing wall 19 with a floating piston.

Whilst we have only described use of the invention in the form of an equal area actuator for an active suspension system (both sides of piston 11 being of equal area), it should be appreciated that the invention can also be used in any form of actuator in an active suspension system.

Claims

CLAIMS:

1. A damper for a vehicle suspension system comprising a first cylinder, a piston moveable in the first cylinder and defining with the first cylinder first and second variable volume chambers for receiving fluid, connection means which allows flow of fluid into or out of the first variable volume chamber, wherein the first cylinder is connectable to one of a vehicle body and a wheel and hub assembly and the piston is connectable to the other of the vehicle body and the wheel and hub assembly, characterised in that compliant means is provided in or connected to both of the first and second chambers, the compliant means deforming on application of force to the damper to allow motion of the piston relative to the first cylinder.

2. A damper as claimed in Claim 1 wherein the connection means connects the first and second chambers and the compliant means can allow motion of the piston relative to the first cylinder without the need for flow of fluid between the first and second chambers.

3. A damper as claimed in Claim 1 or Claim 2 wherein the compliant means comprises first compliant means connected to the first chamber and separate compliant means connected to the second chamber.

4. A damper as claimed in any one of the preceding claims wherein the compliant means comprises a compliant body located in one of the chambers.

5. A damper as claimed in any one of the preceding claims wherein the compliant means comprises a gas spring permanently connected by connection means to one of the chambers.

6. A damper as claimed in any one of the preceding claims wherein the compliant means comprises a compliant fluid container connected by connection means to one of the chambers.

7. A damper as claimed in Claim 5 or Claim 6 wherein the connection means comprises a compliant hose.

8. A damper as claimed in any one of the preceding claims comprising additionally: a second cylinder fixed to and at least partially surrounding the first cylinder, a third chamber for receiving fluid which is defined between the exterior of the first cylinder and the interior of the second cylinder, connecting means connecting the first and third chambers, restriction means in the connecting means to restrict flow of fluid between the first and third chambers and mounting means on the exterior of the second cylinder to enable the first cylinder to be connected to one of the vehicle body and the wheel and hub assembly via the second cylinder.

9. A damper as claimed in any one of the preceding claims wherein the compliant means comprises hydraulic fluid of a bulk modulus with a magnitude 30% to 40% of the bulk modulus of uncontaminated hydraulic fluid used in a comparable conventional damper.

10. A damper as claimed in Claim 9 wherein the hydraulic fluid has a bulk modulus in the range of 400 to 600 N/mm.

11. A damper as claimed in any one of the preceding claims wherein the or each compliant means is arranged to provide the damper with a stiffness in the range of 900 N/mm to 2500 N/mm.

12. A damper as claimed in Claim 10 wherein the or each compliant means is arranged to provide the damper with a stiffness of approximately 1600 N/mm.

13. A damper as claimed in any one of the preceding claims wherein the stiffness of the compliant means is adjustable by a user of the damper whereby the user can vary the stiffness of the damper to suit different applications or modes of use.

14. A damper as claimed in any one of the preceding claims wherein there is provided compliant means which can allow the damper to have the same rate of extension for two different extending forces acting on the damper, a first force when the rate of extension is increasing in magnitude and a second higher force when the rate of extension is decreasing in magnitude.

15. A damper as claimed in any one of the preceding claims wherein there is provided compliant means which can allow the damper to have the same rate of contraction for two different compressive forces acting on the damper, a first force when the rate of contraction is increasing in magnitude and a second force of greater magnitude when the rate of contraction is decreasing.

16. A damper as claimed in Claim 14 or Claim 15 wherein the second higher force can be more than 1.25 times greater than the first lower force.

17. A damper as claimed in Claim 16 wherein the second higher force can be more than 1.5 times greater than the first lower force.

18. A damper as claimed in any one of Claims 15, 16 or 17 wherein the compliant means is adjustable by a user of the damper whereby the ratio of the magnitude of the first force to the magnitude of the second force is adjustable by the user.

19. A passive or adaptive suspension system for a vehicle comprising a damper as claimed in any one of the preceding claims, first connection means for connecting one of the piston and the cylinder of the damper to a vehicle body and second connection means for connecting the other of the piston and the cylinder to a wheel and hub assembly, wherein the first connection means includes no compliant isolator member.

20. A passive or adaptive suspension system as claimed in Claim 19 wherein the second connection means includes no compliant isolator member.

21. A damper substantially as hereinbefore described with reference to and as shown in the accompanying figures 2 to 5.

22. An actuator for an active vehicle suspension system comprising a cylinder, a piston moveable in the cylinder and defining with the cylinder first and second variable volume chambers for receiving fluid, first fluid connection means for connecting at least the first variable volume chamber to valve means for connecting the first variable volume chamber to a source of pressurised fluid or to an exhaust for fluid, characterised in that compliant means is provided in or connected to the first chamber, whereby on application of force to the actuator the compliant means can deform to allow the piston to move relative to the cylinder without a need for flow of fluid through the valve means to or from the first chamber.

23. An actuator as claimed in Claim 22 wherein second fluid connection means is provided to connect the second chamber with the valve means and wherein the compliant means is also provided in or connected to the second chamber.

24. An actuator as claimed in Claim 23 wherein the compliant means comprises first compliant means connected to the first chamber and second separate compliant means connected to the second chamber.

25. An actuator as claimed in any one of Claims 22 to 24 wherein the compliant means comprises a body of compliant material located in a chamber.

26. An actuator as claimed in any one of the claims 22 to 25 wherein the compliant means comprises a gas spring permanently connected by third fluid connection means to a chamber.

27. An actuator as claimed in any one of Claims 22 to 25 wherein the compliant means comprises a compliant fluid container connected by fourth fluid connection means to a chamber.

28. An actuator as claimed in Claim 26 or Claim 27 wherein the fourth fluid connection means comprises a compliant hose.

29. An actuator as claimed in any one of claims 22 to 28 wherein the compliant means has a stiffness in the range of 2000 N/mm to 5000 N/mm.

30. An actuator as claimed in Claim 29 wherein the compliant means has a stiffness of approximately 3500 N/mm.

31. An active suspension system comprising an actuator as claimed in any one of claims 22 to 30, first mechanical connection means connecting one of the piston and the cylinder of the actuator to a vehicle body, second mechanical connection means connecting the other of the piston and cylinder to the vehicle body, wherein the first mechanical connection means does not include any compliant isolator member.

32. An active suspension system as claimed in Claim 31 wherein the second mechanical connection means does not include any compliant isolator member.

33. An active suspension system comprising: an actuator connected between a vehicle body and a wheel and hub assembly, the actuator comprising a cylinder and a piston moveable in the cylinder and defining with the cylinder first and second variable volume chambers, a source of pressurised fluid, an exhaust for fluid, valve means connected to the source of pressurised fluid, the exhaust for fluid and the first chamber, and a control system, wherein the control system generates a position or velocity control signal to control the valve means to vary flow of fluid to or from the first chamber to thereby vary the position or velocity of the piston, characterised in that compliant means is provided in or connected to the first chamber, whereby the compliant means on application of force to the actuator can deform to allow the piston to move relative to the cylinder without flow of fluid through the valve means to or from the first chamber.

34. An active suspension system comprising: an actuator connected between a vehicle body and a wheel and hub assembly, the actuator comprising a cylinder and a piston movable in the cylinder and defining with the cylinder first and second variable volume chambers, a source of pressurised fluid, an exhaust for pressurised fluid, valve means connected to the source of pressurised fluid, the exhaust for fluid and the first chamber, and a control system, wherein, the control system generates a force control signal to control the valve means to vary flow of fluid to or from the first chamber to thereby vary force applied by the actuator, characterised in that compliant means is provided in or connected to the first chamber, whereby the compliant means on application of force to the actuator can deform to allow the piston to move relative to the cylinder without flow of fluid through the valve means to or from the first chamber.

35. An actuator for an active suspension system substantially as hereinbefore described with reference to and as shown in the accompanying figures 13 and 14.

36. An active suspension system as claimed in any one of claims 31 to 34 which has sensor means for measuring a plurality of variables, including force transmitted from the actuator to the vehicle body, and for generating signals indicative thereof and electrical or electronic processor means for processing the signals indicative of measured variables using a plurality of algorithms which include a plurality of scaling parameters and for producing a velocity demand signal demanding a rate of extension or contraction of the actuator, wherein the processor means varies at least one of the scaling parameters in a manner dependent on the measured force signal or on the velocity demand signal in order to vary the rate at which the magnitude of the velocity demand signal increases with increasing magnitude measured force.

37. An active suspension system as claimed in claim 36 wherein the scaling parameter is varied when a threshold value of magnitude of the velocity demand signal is reached such that the rate of increase of magnitude of the velocity demand signal with increasing magnitude of the measured force is changed from one value to another value.

38. An active suspension system as claimed in claim 36 wherein the scaling parameter is varied when a threshold value of magnitude of the force signal is reached such that the rate of increase of magnitude of the velocity demanded signal with increasing magnitude of the measured force signal is changed from one value to another value.

39. An active suspension system as claimed in claim 36 wherein the processing means produces a first velocity demand signal by using a first plurality of algorithms which have scaling parameters which are independent of the measured force signal and the first velocity demand signal and the processing means then multiplies the first velocity demand signal by a gain which is varied with variations in the magnitude of the first velocity demand signal in order to produce a second velocity demand signal which is then relayed to the valve means.

40. An active suspension system comprising: an actuator connected between a vehicle body and a wheel and hub assembly, the actuator comprising a cylinder and a piston moveable in the cylinder and defining with the cylinder first and second variable volume chambers, a source of pressurised fluid, an exhaust for fluid, valve means connected to the source of pressurised fluid, the exhaust for fluid and the first chamber, sensor means for measuring a plurality of variables including measuring force transmitted from the actuator to the vehicle body, and generating signals indicative thereof, and a control system which includes electrical or electronic processor means for processing the signals indicative of measured variables using a plurality of algorithms which include a plurality of scaling parameters and for producing a velocity demand signal demanding a rate of extension or contraction of the actuator, characterised in that the processor means varies at least one of the sealing parameters in a manner dependent on the force signal or on the velocity demand signal in order to vary the rate at which the magnitude, the velocity demand signal increase with increasing magnitude of measured force.

41. An active suspension system as claimed in claim 40 wherein the scaling parameters is varied when a threshold value of magnitude of the velocity demand signal is reached such that the rate of increase of magnitude of the velocity demand signal with increasing magnitude of the measured force is changed from one value to another value.

42. An active suspension system as claimed in claim 40 wherein the scaling parameter is varied when a threshold value of magnitude of the force signal is reached such that the rate of increase of magnitude of the velocity demand signal with increasing magnitude of the measured force is changed from one value to another value.

43. An active suspension system as claimed in claim 40 where the processing means provides a first velocity demand signal by using a first plurality of algorithms which have scaling parameters which are independent of the measured force signal and the first velocity demand signal and the processing means then multiples the first velocity deamdn signal by a gain which is varied with the variations in the first velocity demand signal in order to produce a second velocity demand signal which is then relayed to the valve means.