TECHN ICAL FIELD

The present disclosure relates to a suspension assembly for a vehicle and particularly, but not exclusively, to a suspension assembly that includes an adjustable suspension unit. Aspects of the invention relate to a suspension assembly and a vehicle.

BACKGROUND

Active suspension systems have the potential to offer improvements to vehicle ride comfort and body control by modifying the force transfer between the sprung (e.g. car body) and un-sprung (e.g. wheels) masses of a vehicle.

Active suspension systems are able to improve the ride comfort experienced by vehicle occupants though cancellation of the forces transmitted into the vehicle cabin, which are typically caused by the vehicle's interaction with the road. In particular, active suspension systems can control roll, pitch and heave displacements of the vehicle, wherein roll, pitch and heave displacements generally relate to a movement of the vehicle body in a substantially side to side, front to back and vertical direction, respectively. Active suspension systems can also be used to control the handling of a vehicle by altering the roll stiffness distribution across the vehicle. For example, a front-heavy vehicle with low rear roll stiffness (due to, for example, the presence of an undersized rear anti-roll bar) may exhibit a tendency to under-steer when cornering. By actively decreasing the stiffness of the suspension at the front of the vehicle, so that the roll stiffness distribution is rear biased, the under-steer may be reduced. The arrangement of an active suspension assembly in parallel with a passive spring assembly provides an alternative pathway for high frequency road-input forces to reach the body of the vehicle, thereby reducing the ability of the passive spring assembly to absorb shocks in the road. The sizing, cost and power constraints of the suspension system will often dictate that the actuation bandwidth of the active suspension assembly is significantly lower than much of the road-inputs, such that the active suspension assembly is unable to accommodate the road input forces. This results in an increased transmission of higher frequency road inputs to the vehicle cabin. Placing the active suspension assembly in series with the passive spring assembly overcomes much of the high frequency road input transmission problem by removing the additional load paths. However, due to the limited space within a typical wheel architecture of a vehicle, such systems can only provide a limited actuation stroke. For a typical 'in series' arrangement, the active actuator is positioned between the sprung mass and the road spring and this arrangement results in a suspension strut assembly of considerable length, thereby leading to a limited, active stoke length of the active suspension assembly.

Against this background, it is an object of the present invention to overcome or at least substantially alleviate the disadvantages known in the prior art.

SUMMARY OF INVENTION

According to an aspect of the present invention there is provided a suspension assembly for a vehicle, the suspension assembly comprising;

a first suspension unit mountable between a chassis mount assembly and a wheel mount assembly;

a second suspension unit mountable between the first suspension unit and the wheel mount assembly; and,

a control assembly for modifying the displacement of the second suspension unit relative to the wheel mount assembly,

wherein the chassis mount assembly is configured to mount the first suspension unit to a chassis structure of the vehicle and the wheel mount assembly is configured to mount the second suspension unit to the wheel of the vehicle,

wherein the second suspension unit is telescopically movable relative to the first suspension unit.

By means of the invention the proposed arrangement of the suspension assembly allows for the first and second suspension units to accommodate an increased suspension stroke length, in comparison to a conventional in-series suspension assembly, which thereby enables the assembly to accommodate large articulations of the wheel.

In addition, the suspension assembly is able to withstand large body control forces, such as those associated with a vehicle travelling over rough terrain. Advantageously, the first and second suspension units of the suspension assembly have substantially coincident axes and the second unit may be concentrically arranged within the first suspension unit thereby enabling the suspension assembly to be compactly packaged within the wheel architecture of the vehicle. Whilst still allowing the second suspension unit to freely articulate relative to the movement of the first suspension unit.

Low frequency road inputs may be accommodated by the second suspension unit whilst simultaneously transferring high frequency road inputs to the first suspension unit. Thus, the high frequency load forces exhibited by the wheel are not transmitted through the suspension assembly to the chassis of the vehicle.

The first suspension unit may be a passive suspension assembly. Advantageously, the passive suspension assembly may be naturally responsive to the linear motion of a sprung mass, relative to an un-sprung mass, of a vehicle. In other words, the first suspension unit may be configured to respond naturally to inputs from the road, rather than being actively actuated, or activated, in dependence on receiving an actuation input from an actuation assembly.

The first suspension unit may comprise a passive piston housing and a passive piston, wherein the passive piston is slidably received within the passive piston housing to define a passive pressure chamber of the first suspension unit.

An upper surface of the passive piston housing may be fixably attached to the chassis mount assembly.

An upper portion of the second suspension unit may form the passive piston of the first suspension unit. The first suspension unit may be a coiled spring.

The control assembly may be configured to dynamically adjust the displacement of the second suspension unit, relative to the wheel mount assembly. By arranging the second suspension unit in series with the first suspension unit, the second suspension unit need only be actively displaced at low frequencies and in response to large undulations in the terrain over which the vehicle is travelling, or in response to movements in the body of the vehicle due to changes in the vehicle's momentum, for example, when cornering or accelerating. The second suspension unit may be an active suspension assembly comprising an active piston housing and an active piston, wherein the active piston is slidably received within the active piston housing to define an active pressure chamber.

The control assembly may comprise a hydraulic actuation assembly, which is operable to output and/or receive pressurised hydraulic fluid to and/or from the active pressure chamber of the second suspension unit. The second suspension unit may comprise at least one supply passage for fluidly connecting the active pressure chamber to the hydraulic actuation assembly. The hydraulic actuation assembly may be configured to increase the pressure of the hydraulic fluid within the active pressure chamber in order to exert an extension force upon an internal surface of active pressure chamber, thereby causing the second suspension unit to extend. The hydraulic actuation may be further configured to reduce the pressure of the hydraulic fluid within the active pressure chamber, thereby allowing the second suspension unit to retract in response to the force exerted upon it due to the sprung mass of the vehicle.

The piston may comprise a head portion and a rod portion, wherein the diameter of the head portion spans the width of the active pressure chamber, wherein the diameter of the rod portion is substantially smaller than the diameter of the head portion and wherein the active piston housing of the second suspension unit comprises a cylinder base for receiving the rod portion of the piston. The active piston may be reciprocally movable along a longitudinal axis of the active piston housing to define an upper and a lower sub-chamber of the active pressure chamber.

The upper sub-chamber may be defined by the active piston housing, which is sealably enclosed, at one end, by an upper surface of the active piston and, at an opposite end, by a cylinder head of the active piston housing.

The lower sub-chamber may be defined by an annular volume formed between an inner sidewall of the active piston housing and a peripheral sidewall of the rod portion of the active piston, wherein the annular volume is sealed, at one end, by the cylinder base of the active piston housing and, at an opposite end thereof, by a lower surface of the head portion of the active piston. The maximum extension and retraction force of the second suspension unit is related to the cross-sectional area of the upper and lower sub-chambers, respectively. Accordingly, the maximum extension force of the active piston is greater than the maximum retraction force due to the relatively reduced area ratio of the annular lower sub-chamber (compared to the substantially circular cross section of the upper sub-chamber). Advantageously, the active piston may be configured so as to apply an active retraction force in conjunction with, and complimentary to, the natural retraction force exerted on the suspension assembly due to the sprung mass of the vehicle. Hence, the active piston may be configured so as to provide greater control of the retraction of the suspension assembly in comparison to a single actuating active piston. In this way, the double actuating active piston is able to provide enhanced control of the movement of the sprung mass of the vehicle despite the relative difference in maximum retraction and extension forces exerted by the active piston. The suspension assembly may comprise a damper unit mountable between the chassis mount assembly and the wheel mount assembly.

The damper unit may be arranged in parallel with the second suspension unit. The damper unit may be telescopically movable relative to the second suspension unit. The damper unit may be concentrically arranged within the active piston housing of the second suspension unit. Advantageously, the damper unit, and the first and second suspension units may have substantially coincident axes and the damper unit may be concentrically arranged within the first suspension unit thereby enabling the suspension assembly to be compactly packaged within the wheel architecture of the vehicle.

The damper unit may comprise a damper piston housing and a damper piston; wherein the damper piston housing defines a damper pressure chamber and wherein the damper piston is slidably received within the damper piston housing to define an upper and lower sub-chamber of the damper pressure chamber.

The damper piston housing of the damper unit may form the active piston of the second suspension unit. The damper piston may comprise a head portion and a rod portion, wherein the rod portion is attached, at one end, to an upper surface of the head portion and, at an opposite end thereof, is attachable to the chassis mount assembly. The rod portion of the damper piston may be concentrically disposed within the body of the first and/or second suspension units; wherein the rod portion may be supported by a guiding means of the first and/or second suspension units. By attaching one end of the rod portion to the chassis mount assembly, independently from the first and/or second suspension units, the damper unit may be advantageously actuated in parallel with the other units of the suspension assembly. In this way, the damping of the vehicle's motion may be unaffected by a failure in the active suspension system, which may result, for example, from a loss of actuator fluid.

The damper unit may be a semi-active hydraulically actuated damper assembly. The lengths of the first and second suspension units may be substantially proportional to each other.

According to a further aspect of the invention there is provided a vehicle comprising the suspension assembly as described above; a wheel; a chassis structure; a chassis mount assembly configured to mount the first suspension unit of the suspension assembly to the chassis structure of the vehicle, and a wheel mount assembly configured to mount the second suspension unit of the suspension assembly to the wheel of the vehicle.

Within the scope of this application it is expressly envisaged that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic drawing of a side view of a vehicle provided with a suspension assembly according to an embodiment of the invention;

Figure 2 is a schematic drawing of an axial cross-sectional view of a suspension assembly according to another embodiment of the invention;

Figure 3 is a schematic drawing of an axial cross-sectional view of a suspension assembly according to a further embodiment of the invention; and, Figure 4 is a schematic drawing of an axial cross-sectional view of a suspension assembly according to a still further embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS References in the following description to "upper", "lower" and "side", and other terms having an implied orientation, are not intended to be limiting and refer only to the orientation of the parts shown in the accompanying drawings.

Referring to Figure 1 , a suspension assembly 12 for connecting a sprung mass of a vehicle 10 to an unsprung mass of the vehicle is shown. The suspension assembly 12 includes a first suspension unit 14, a second suspension unit 16 and a damper unit 18 arranged in series with each other so as to form a single suspension strut. The first suspension unit 14 is disposed between a chassis mount assembly 20 of the vehicle and the second suspension unit 16, which itself is disposed between the first suspension unit 14 and a wheel mount assembly 22 of the vehicle. The first suspension 14, second suspension 16 and damper 18 units include concentric tubular sections, each arranged to slide into one another.

Referring to Figure 2, the first suspension unit 14 of the suspension assembly 12 is fixedly attached, at an upper end, to an upper part 26a of a chassis structure of the vehicle 10 via a first connection point 20a of the chassis mount assembly 20. The second suspension unit 16 is fixedly attached, at a lower end thereof, to a vehicle wheel 24 via the wheel mount assembly 22. An upper portion of the second suspension unit 16 is concentrically disposed within a lower portion of the first suspension unit 14. The damper unit 18 is attached, at a lower end thereof, to the wheel 24 via the wheel mount assembly 22. The damper unit 18 is further attached, at an upper end, to the upper part 26a of the vehicle chassis structure via a second connection point 20b of the chassis mount assembly 20. A lower portion of the damper unit 18 is concentrically disposed within a lower portion of the second suspension unit 16.

The wheel mount assembly 22 includes a swing arm 28 that is rotatably attached, at one end, to a wheel hub of the wheel 24. The swing arm 28 is further rotatably attached, at an opposite end, to a lower part 26b of the chassis of the vehicle 10, such that the wheel 24 is pivotable about the vehicle chassis 26a, 26b. The swing arm 28 is further rotatably attached to the second suspension unit 16 via an eye 30, which is integrally mounted to the lower portion of the second suspension unit 16. Alternatively, the second suspension unit 16 may be mounted, via the wheel mount assembly 22, directly to the hub of the wheel 24. The vehicle chassis 26a, 26b forms part of the sprung mass of the vehicle 10, whereas the vehicle wheel 24, the wheel mount assembly 22 and the suspension assembly 12, together, form part of the un-sprung mass of the vehicle 10.

The first suspension unit 14 defines a pneumatic "passive" spring assembly, or spring-like assembly, which is naturally responsive to the linear motion of the sprung mass, relative to the un-sprung mass, of the vehicle. In other words, the first suspension unit 14 is configured to respond naturally to the road inputs, rather than being actively actuated, or activated, in dependence on receiving an actuation input from an actuation assembly. The first suspension unit includes a cylindrical piston housing 32 for receiving a piston 34 of the first suspension unit 14. An upper surface 36 of the passive piston housing 32 is attached to the upper part 26a of the chassis structure of the vehicle 10, via the first connection point 20a of the chassis mount assembly 20. A passive pressure chamber 38, suitable for holding pneumatic fluid, is defined by the passive piston housing 32. The passive piston 34 is reciprocally moveable within the passive piston housing 32 so as to vary the volume of the passive pressure chamber 38.

The passive pressure housing 32 further includes an inner cylindrical sidewall 40, which is enclosed, at one end, by an upper surface of the passive piston 34 and, at an opposite end, by a cylinder head 42 of the piston housing 32. A flexible member, or diaphragm 44, is crimped at one end to the underside of the cylinder head 42, whilst at a distant end, the diaphragm 44 is sealingly attached around a peripheral surface of the passive piston 34. The diaphragm 44 forms a rolling lobe around the piston, thereby sealing the piston within the piston housing of the first suspension unit so as to prevent pneumatic fluid from leaking out into the surrounding environment. An upper surface of the passive piston 34 forms an annular abutment surface, which provides a flat surface for contacting with an inner surface of the cylinder head 42 of the passive piston housing 32. In embodiments, the inner surface of the cylinder head 42 may comprise a bump stop, disposed between the annular abutment surface of the passive piston 34 and the inner surface of the cylinder head 42, wherein the bump stop is configured to cushion the impact between the two contact surfaces.

During operation of the suspension assembly 12, and in particular upon compression of the first suspension unit 14, the passive piston 34 is urged into the passive piston housing 32, thereby reducing the volume of the passive pressure chamber 38 and increasing the pressure of the pneumatic fluid therein. The compression of the pneumatic fluid causes the diaphragm 44 to extend along the side of the passive piston 34, thereby relieving the increase in pressure of the pneumatic fluid. Reciprocally, upon expansion of the first suspension unit 14, the passive piston 34 is withdrawn from the housing, and the resulting increase in the volume of the passive pressure chamber 38 causes the diaphragm 44 to retract up, inside the passive piston housing 32.

The diaphragm 44 may be formed of any elastomeric material including, for example, rubber. The pneumatic fluid may be any compressible gas including, for example, air.

The second suspension unit 16 is a dynamically adjustable hydraulically actuated suspension unit, or "active" suspension assembly, which is actively responsive to the linear motions of the sprung mass, relative to the un-sprung mass, of the vehicle. The second suspension unit 16 is configured to actively respond to road inputs, in dependence on receiving an actuation input from a control assembly 60 of the second suspension unit. Additionally, the control assembly 60 is configured to modify the displacement of the second suspension unit 16.

The second suspension unit 16 is telescopically movable relative to the first suspension unit 14 so as to slide within it. The second suspension unit 16 includes a cylindrical active piston housing 50 for receiving a piston 52 of the second suspension unit 16. An active pressure chamber 54, suitable for holding hydraulic fluid, is defined by the active piston housing 50.

The active piston 52 is slidably received within the active piston housing 50 and is reciprocally moveable along a longitudinal axis of the housing so as to vary the volume of the active pressure chamber 54.

The control assembly 60 is a hydraulic actuation assembly, which is operable to output pressurised hydraulic fluid to the active pressure chamber 54, via a supply passage 58 and an inlet port of the active piston housing 50. The hydraulic actuation assembly 60 is also operable to receive pressurised hydraulic fluid from the active pressure chamber 54.

The circumferential surface of the active piston 52 is further provided with a guiding means (not shown) for guiding the motion of the piston within the housing and for maintaining the piston's position when at rest. The guiding means is a guide ring, which is located around the circumferential surface of the piston and is configured to provide a minimal level of friction at the interface between the piston and the housing of the second suspension unit. The guiding ring also provides a defined gap between the circumferential surface of the piston and the inner surface of the piston housing to enable the sealing means to function.

In the embodiment shown, the active pressure chamber 54 includes only a single chamber. Alternatively, the chamber may comprise a number of fluidly interconnected sub-chambers, each suitable for receiving pressurised hydraulic fluid. The active pressure chamber 54 is sealably enclosed, at one end, by the upper surface of the active piston 52 and, at an opposite end, by a cylinder head 56 of the active piston housing 50. The upper surface of the active piston 52 further provides an annular abutment surface for contacting with an inner surface of the cylinder head 56.

The damper unit 18 is a passive damper, or shock absorber, comprising a cylindrical damper piston housing 70, which is closed at one end by a cylinder base 72 and at an opposite end by a cylinder head 74. The damper piston housing 70 defines a pressure chamber for receiving hydraulic fluid. The damper unit 18 is disposed between the chassis 20 and wheel 22 mount assemblies of the vehicle 10 and is arranged in series with both the first 14 and second 16 suspension units. The damper piston housing 70 forms a part of the active piston 52 of the second suspension unit 26a, 26b. Consequently, the damper piston housing 70 is telescopically movable relative to the active piston housing 50.

A piston 78 of the damper unit 18 includes a head portion 78a and a rod portion 78b, wherein the rod portion 78b is attached, at its lower end, to an upper surface of the head portion 78a and, at an opposite end, to the upper part 26a of the vehicle chassis, via a second connection point 20b of the chassis mount assembly 20. In this configuration, the damping unit 18 is isolated from the first 14 and second 16 suspension assemblies. Thus, any damping performed by the damping unit 18 would be unaffected by a loss of hydraulic fluid of the second suspension unit 16.

A piston 78 of the damper unit 18 includes a head portion 78a and a rod portion 78b connecting to the head portion 78a at its lower end. The head portion 78a spans the width of the pressure chamber 80 and is guided therein through cooperation between the head portion 78a and an inner wall of the damper piston housing 70. The head portion 78a of the piston divides the pressure chamber 80 into two sub-chambers; an upper chamber 80a, through which the rod portion 78b extends, and a lower chamber 80b. The rod portion 78b is attached, at its upper end, to the upper part 26a of the vehicle chassis, via a second connection point 20b of the chassis mount assembly 20. In other words, the upper end of the rod portion 78b is connected to the chassis mount assembly 20 at a connection point that is independent of the first damper suspension unit 14. Consequently, any road-input forces, relating to the linear motion of the vehicle wheel 24, are independently transmitted through the damper unit 18 and/or the suspension units 14, 16 of the suspension assembly 12.

The damper piston 78 is slidably received within the damper piston housing 70 and is also reciprocally moveable along a longitudinal axis of the damper piston housing 70. An upper surface of the damper piston forms an annular abutment surface, which thereby provides a flat surface for contacting with an inner surface of the cylinder head 74 when the damper unit 18 is fully extended. A lower surface of the piston forms a circular abutment surface, which forms a flat surface for contacting with an inner surface of the cylinder base 72, when the damper unit 18 is fully retracted. The damper piston 78 is fitted with a two-way valve (not shown) and is received within the damper piston housing 70 so as to define an upper 80a and a lower 80b pressure chamber.

A first annular guide 82 is disposed within the cylinder head 74 of the damper piston housing 70, through which the piston rod 78b is received. A rubber seal is arranged adjacently to the piston guide to prevent any hydraulic fluid from escaping the damper pressure chamber 80. During operation of the suspension assembly 12, a substantially upward movement of the wheel 24 results in the damper piston being displaced in a downward motion, relative to the damper piston housing 70, which in turn causes hydraulic fluid to flow from the lower 80b to the upper 80a damper chamber, through the two way valve of the damper piston.

A sealing unit is further provided on the circumferential surface of the damper piston head 78a which, during movement of the piston, slides along the curved inner surface of the damper piston housing 70, in order to maintain fluid separation of the upper 80a and lower 80b chambers. The damper piston rod 78b is disposed, concentrically, within the body of the first 14 and second 16 suspension units. The piston rod 78b is further received through a second annular guiding means 84, which is disposed within a circular hole in the head of the passive piston 34 of the first suspension unit 14. The second annular guiding means 84 is configured to provide a minimal level of friction at the interface between an inner surface of the passive piston head 34 and the damper piston rod 78b. In addition, the second annular guiding means 84 comprises a rubber seal, which is configured to allow movement of the damper piston rod 78b, relative to the piston head 34, whilst simultaneously preventing hydraulic fluid from escaping the active pressure chamber 54 of the first suspension unit 14. A third damper rod guiding means 86 is disposed within an opening in the cylinder head of the first suspension unit 16, and is configured for receiving the damper piston rod 78b whilst preventing pneumatic fluid from escaping the passive pressure chamber 38 of the first suspension unit 14.

The damper piston 78 may be a solid, integrally formed piston. Alternatively the damper piston 78 may be a composite structure including an outer portion and an inner core in which the piston rod 78b is rigidly affixed to the head portion 78a by any suitable means.

Turning now to the operation of the suspension assembly 12, in use, the first suspension unit 14 supports the weight of the vehicle 10 and allows the wheels 24 to follow the undulations of the road surface, thereby maintaining the stability of the vehicle 10 and ensuring that the driver has control of vehicle 10 at all times. The first suspension unit 14 attenuates any high frequency inputs from the road by complying with the movements of the wheel 24. By absorbing the force of the high frequency road inputs, the first suspension unit 14 isolates any road shocks from the vehicle chassis 26a, 26b and thereby improves the comfort provided for the vehicle occupants.

The second suspension unit 16 is configured to control the ride height of the vehicle 10 and is further operable to control the position of the vehicle body 10, relative to the wheels 24. In doing so, the displacement of the second suspension unit 16 provides control of the vehicle's primary body motions (i.e. roll, pitch and heave) by maintaining the geometry during acceleration, braking and cornering manoeuvres of the vehicle 10.

The telescopic nature of the suspension assembly 12 facilitates the compact packaging of the first 14 and second 16 suspension units whilst enabling a large active stroke length of the second suspension unit 16. Furthermore, by arranging the second suspension unit 16 in series with the first suspension unit 14, the second suspension unit 16 need only displace at low frequencies and in response to large undulations in the terrain over which the vehicle 10 is travelling, or in response to any movements in the body of the vehicle 10 due to changes in the vehicle's momentum, for example, when cornering or accelerating. Perturbations in the road surface cause the wheel 24 to oscillate in a substantially linear motion. The road inputs are transmitted, via the wheel mount assembly 22, to the damper piston housing 70 causing it to oscillate relative to the damper piston 78. The energy needed to pump the fluid through the piston valve provides the damping action. The damper unit 18 absorbs a proportion of the energy stored in the suspension, thereby inhibiting the sprung and un-sprung masses from oscillating as the vehicle 10 passes over perturbations in the road, which may destabilise the vehicle 10 if left un-damped. In embodiments, the damper unit 18 comprises a pressurised gas reservoir (not shown), which is separated from the lower 80b chamber by a floating piston (not shown). The volume of the gas reservoir is configured to compensate for the associated volume of the damper piston rod 78b as it is received inside the damper piston housing 70. The compression and expansion of the gas held within the pressurised gas reservoir results in a progressive change in the characteristics of the damping unit 18. For example, during a compression stroke of the damper unit 18 the force required to depress the piston 78 into the piston housing 70, at a constant speed, will increase at an increasing rate as the piston 78 moves further inside the housing 70. Now turning to the actuation of the suspension assembly 12, the second suspension unit 16 is operated by means of the hydraulic actuation assembly 60. The actuation assembly 60 includes an integrated valve block 66, mounted to the second suspension assembly unit 16, which is fluidly connected to a low-pressure reservoir 64 via a motor driven hydraulic pump 62, wherein the reservoir 64 and the pump 62 are each housed within the vehicle body 10. The pump 62 drives hydraulic fluid from the low-pressure reservoir 64, through a flexible hose 63, to a high- pressure accumulator 66 located in the valve block 68 of the hydraulic actuation assembly 60.

A check valve 68a allows hydraulic fluid to flow from the pump 62 to the accumulator 66 but prevents the hydraulic fluid from returning to the pump 62, thereby enabling the pressure of the hydraulic fluid in the accumulator 66 to increase over time. In the event that the hose 63 ruptures the actuator can be locked in position by returning the proportional valve 68b to a central position.

A control module (not shown) monitors the pressure in the accumulator via a pressure sensor. When the pressure in the accumulator 66 drops below a pre-determined threshold value the control module sends a control signal to the pump 62, instructing it to increase the flow of fluid from the low pressure reservoir 64 to the accumulator 66. If the pressure in the accumulator 66 exceeds the pre-determined threshold value, the control module instructs the pump 62 to reduce the fluid flow. The control module may further comprise means to prevent the control module from switching on and off at high frequency in response to, for example, very small fluctuations in pressure. The supply passage 58 is provided within the body of the piston housing 50 of the second suspension unit 16. Alternatively the supply passage 58 may comprise a separate tube or pipe. An opening of the first supply passage 58 is located at the cylinder head of the active pressure chamber 54. In embodiments, the opening of the fluid connector is located along the cylindrical sidewall of the housing at a proximal position to the cylinder head. The supply passage 58 is configured to fluidly connect the accumulator 66 of the hydraulic actuation assembly 60 to the pressure chamber 54 of the second suspension unit 16.

A proportional valve 68b regulates the flow of fluid into and out of the active pressure chamber 54. In order to extend the second suspension unit 16, the proportional valve 68b allows hydraulic fluid to flow along the fluid channel 58 to the pressure chamber 54, thus increasing the pressure in the chamber 54. The increased pressure in the chamber 54 exerts a downward force on the upper surface of the active piston 52, thereby causing it to protrude out of the piston housing 54 and thus causing the second suspension unit 16 to extend. In the event of the flexible hose 63 rupturing, the actuator can be locked in position by positioning the proportional valve to a central position. In embodiments, the actuation assembly 60 may comprise a leakage flow sensor, configured to provide a signal to a control module in dependence on detecting a leakage of hydraulic fluid. Upon receiving the signal from the leakage flow sensor, the control module may control the proportional valve 68b to inhibit the flow of hydraulic fluid to and from the pressure chamber 54.

In the presently described configuration, the second suspension unit 16 is operable as a single acting actuator, in which case the suspension assembly 12 relies upon the weight of the sprung mass (i.e. the vehicle body 10) to provide the compressive force needed to retract the second suspension unit 16.

During the operation of the suspension assembly 12, the actuation of the second suspension unit 16 results in the active cancellation of low frequency road inputs. The time it takes for the hydraulic actuation assembly 60 to detect an input and then react (i.e. by pumping fluid into the active pressure chamber 54) determines the maximum road input frequency that the second suspension unit 16 is able to actively accommodate. The maximum road input therefore represents an upper limit of the operational bandwidth of the second suspension unit 16.

Referring to Figure 3, in a further embodiment of the invention, the first suspension unit 1 14 is provided with a double acting suspension actuator. The active piston 152 of the second suspension unit 1 16 separates the volume contained within an active piston housing 150 into an upper 154a and lower 154b sub-chamber of the active pressure chamber 154. Consequently, the piston includes a head portion 152a and a rod portion 152b, wherein the head portion 152a spans the width of the active pressure chamber 154 and the diameter of the rod portion 152b is substantially smaller than the diameter of the head portion 152a. In addition, the active piston housing 150 of the second suspension unit 1 16 further includes a cylinder base 157 for receiving the rod portion 152b of the active piston 152. The upper sub-chamber 154a is formed within a cylindrical sidewall of the active piston housing 150, which is sealably enclosed at one end by the upper surface of the active piston 152 and at an opposite end by a cylinder head 156 of the second suspension unit 1 16. The lower sub- chamber 154b defines a substantially annular chamber, formed between the cylindrical sidewall of the active piston housing 150 and a circumferential sidewall of piston rod 152b. The lower sub-chamber 154b is sealed at one end by the cylinder base 157 and at an opposite end by a lower surface of the piston head 152a.

The upper surface of the active piston 152 forms an annular abutment surface, which provides a flat surface for contacting with an inner surface of the cylinder head 156, when the second suspension unit 1 16 is fully retracted. In embodiments, the inner surface of the cylinder head 156 may comprise a bump stop, disposed between the annular abutment surface of the active piston 152 and the inner surface of the cylinder head 156. The bump stop is configured to cushion the impact between the two contact surfaces. A lower surface of the piston head 152 forms an annular abutment surface, which provides a flat surface for contacting with the inner surface of the cylinder base, when the second suspension unit is fully extended.

Further sealing and guiding elements (not shown) are provided at the interface between the circumferential surface of the piston rod and the inner surface of the cylinder base. The sealing element of the cylinder base provides a means of fluid separation between the lower sub- chamber 154b and the exterior environment, thereby preventing the leakage of hydraulic fluid from the suspension assembly 1 12.

As with the previous embodiment, a first supply passage 158a is provided for fluidly connecting the accumulator 166 of the hydraulic actuation assembly 160 to the upper sub-chamber 154a. An opening of the first supply passage 158a is located at the cylinder head 156 of the active pressure chamber 154. In addition to the first supply passage 158a, a second passage 158b is provided for fluidly connecting the accumulator 166 to the lower sub-chamber 154b for which an opening is provided along the sidewall of the active piston housing 150. The opening being proximal to the cylinder base 157 of the housing so as to maximise the retraction stroke length of the second suspension unit 1 16.

The difference in pressure across the two sides of the head portion 152a of the active piston 152 causes the second suspension unit 1 16 to extend and retract. In addition to the extension actuation, as described in the previous embodiment, hydraulic fluid may be pumped into the lower sub-chamber 154b via the second supply passage 158b, thereby increasing the pressure in the lower sub-chamber 154b of the suspension unit. The subsequent upward force, exerted on the lower surface of the piston head 152a causes the piston 152 to move toward the cylinder head 156, which results in the retraction of the second suspension unit 1 16.

The second suspension unit 1 16 is operated under closed loop control such that the proportional valve is operable to regulate the flow of hydraulic fluid, into and out of the upper 154a and/or lower 154b sub-chambers of the active pressure chamber 154. For example, upon increasing the pressure in the upper sub-chamber 154a, by pumping hydraulic fluid from the accumulator 166 and into the sub-chamber, a downward force is exerted on an upper surface of the piston head 152a such that it is forcibly urged in a downward direction. Consequently the volume of the upper sub-chamber 154a is increased, whilst simultaneously reducing the volume of the lower sub-chamber 154b. This, in turn, forces hydraulic fluid to flow from the lower sub- chamber 154b, along the second supply passage 158b, to the hydraulic actuation assembly 160. Upon receiving the hydraulic fluid from the lower sub-chamber 154b, the proportional valve is operable to direct the fluid to either return to the low-pressure reservoir 164 or to the high pressure accumulator 166. Referring now to Figure 4, the suspension assembly 212 according to yet another embodiment of the invention is shown in which a damper unit 218 is arranged non-concentrically with the first 214 and second 216 units of the suspension assembly 212.

The first suspension unit 214 is fixedly attached, at its upper end, to the upper part 226a of the vehicle chassis structure, via the first connection point 220a of the chassis mount assembly 220. The second suspension unit 216 is fixedly attached, at its lower end, to the vehicle wheel 224 via a first connection point 222a of the wheel mount assembly 222.

The damper unit 218 is attached, at its lower end, to the wheel 224 via a second connection point 222b of the wheel mount assembly 222. The damper unit 218 is further attached, at its upper end to the upper part 226a of the vehicle chassis structure via a second connection point 220b of the chassis mount assembly 220. The swing arm 228 of the wheel mount assembly 222 is rotatably attached, at one end, to the wheel hub of the wheel 224 and, at its opposite end, to the lower part 226b of the vehicle chassis. Such an arrangement is particularly well suited for use in a multi-link suspension, such as would commonly be used to suspend a rear wheel 224 of a premium vehicle.

The second suspension unit 216 is provided with a double acting suspension actuator. The head portion 252a of the active piston 252 separates the volume contained within the active pressure chamber 254, into a first 254a and a lower 254b sub-chamber. However, due to the fact that the damper is arranged in parallel with the actuator, the rod portion 252b of the active piston 252 is advantageously provided with a reduced diameter, relative to the diameter of the head portion 252a. Consequently, the volume of the lower sub-chamber 254b is increased relative to the upper sub-chamber 254a such that the relative extension and compression forces exhibited by the second suspension unit 216 are equivalent. Such an arrangement also means that the hydraulic fluid flow requirements for the lower sub-chamber 254b are similar to the first 254a and hence, the relative control strategies for operating the second suspension unit 216 in either compression or extension are equivalent.

In embodiments, the suspension assembly may be mounted between the sprung and un-sprung mass of a vehicle. The suspension assembly may be actively operated in order to counteract any undesirable motion of the sprung mass relative to the un-sprung mass. In particular the assembly may be operable to control both rotary (roll and pitch) and linear (heave) displacements of the vehicle body. The suspension assembly may be further operated to control vehicle handling by altering the roll stiffness distribution across the vehicle, particularly in vehicles that exhibit poor handling characteristics due to, for example, the presence of an undersized front/rear anti-roll bar. The assembly may alternatively be used in vehicles which do not have front and/or rear anti-roll bar(s).

In any embodiment of the invention, the first suspension unit 14 may be any kind of passive elastomeric spring such as a coiled spring, or helical spring, made of any elastomeric material, which may include, for example, metal alloys, plastics and/or composite materials. In such embodiments, the actuator assembly of the second suspension unit may be used to provide self-levelling functionality for the vehicle. In embodiments, the damper unit 18 may be a 'twin-tube' damper or any other type of damper that is known in the art. The damper unit 18 may be a semi-active or adaptive damper, which may vary the firmness of the damped motion of the suspension assembly 12 in order to match the changing road conditions or to resist the movement of the vehicle 10. Control of the semi- active damper and the second suspension unit 16, 1 16, 216 may be coordinated in order to produce an optimal net force from the suspension assembly 12. By operating the semi-active damper in sympathy with the second suspension unit 16, 1 16, 216 the stability of the vehicle 10 and/or comfort of the passengers may be optimised. In embodiments of the invention, the high-pressure accumulator 66 and/or valve block 68 of the actuation assembly 60 may be housed in the vehicle body 10, thereby reducing the un-sprung mass of the vehicle.

In embodiments, the control assembly comprises a separate pump/motor unit for each wheel of the vehicle. Alternatively the control assembly may comprise an axle pump motor unit, whereby a single motor drives a separate pump for each active suspension assembly. In a yet further embodiment, a single motor may drive a single pump, the output from which may be selectively divided between a first and second suspension assembly located at an opposite end of the vehicle axis.

The length of the first 14 and second 16 suspension units, of the suspension assembly 12, may be substantially proportional to each other.

Aspects and embodiments of the invention are set out in the accompanying claims. It will be appreciated by someone skilled in the art that the invention could be modified to take many other alternative forms without departing from the scope of the claims.