VEHICLE SUSPENSION ARRANGEMENTS & CONTROL FIELD OF THE INVENTION

The present invention is generally directed to vehicle suspension systems and control thereof, such as ride height and natural frequency response control of hydraulic suspension systems.

BACKGROUND OF THE INVENTION

There are known many forms of hydraulic suspension which provide support for substantially the entire sprung mass of a vehicle. One advantage of such hydraulic suspension systems is that the volume of hydraulic fluid is easy to adjust to provide "load levelling" ie to add or remove fluid from hydraulic actuators to compensate for the displacement caused by a change in load acting on resilient means such as hydro-pneumatic accumulators. This adjustment is more beneficial to vehicles with a large change in sprung mass. However, changing the volume of hydraulic liquid to compensate for ride height changes due to changes in load (load levelling) does not change the rate of resilience of the resilient means. If the resilient means incorporate a linear stiffness spring (such as a mechanical or metallic spring) then the stiffness of the hydraulic suspension does not change with load, leading to a significant reduction in natural frequency with increasing load. If the resilient means incorporate a non-linear spring such as a gas spring then the stiffness of the hydraulic system does change with load. As the natural frequency is related to the static gas volume, once load is added and the static gas volume decreased through compression of the gas, the natural frequency increases.

For vehicles with large payloads, such as where the load on one axle can increase by over 100% from the unladen to the laden state, the operating range of gas accumulators can easily be exceeded by the pressure and displacement requirements of the rams supporting the vehicle. For example, the typical operating range of an accumulator is between 10 and 90% oil volume, so if unladen the bladder or diaphragms or piston of an accumulator is required to displace through 40% of the accumulator volume to permit wheel displacements and the gas volume is required to more than halve from unladen to laden states, then keeping within the safe operating range of the accumulator can be extremely difficult or impossible.

With the above in mind, it would be desirable to provide a method and/or system which alleviates vehicle ride height control issues with change in load on a vehicle.

SUMMARY OF THE INVENTION With the aforementioned in view, it has been found desirable to provide a method of controlling ride height of a vehicle having hydro-pneumatic suspension by adjusting volume and/or pressure of an active volume of gas within at least one accumulator.

The term "controlling ride height" within the context of the present invention includes maintaining a desired or preselected ride height and/or controlling variation in ride height within desired bounds (such as a preferred range, which may be set as a range of minimum and maximum values).

The gas volume may be controlled to provide a constant volume of gas, whereby, advantageously natural frequency response of the system remains substantially constant with varying load.

Gas pressure may in addition or alternatively be controlled to control the distribution of loads within four or more separate hydraulic wheel ram volumes, such as to control warp preload or wedge. Preferably gas pressure may also be controlled in a number of additional accumulators, such as for ride height control of multi wheeled/multi axle vehicles e.g. trucks.

Ride height may be controlled within predetermined bounds (such as a preferred range, which may be set as a range of minimum and maximum values) by controlling the volume of gas in the at least one accumulator to a constant static gas volume, thereby maintaining a substantially constant natural frequency response.

Ride height may be controlled to a preset ride height by adjusting gas volume in the at least one accumulator, thereby maintaining the natural frequency response within predetermined bounds (such as a preferred range, which may be set as a range of minimum and maximum values). Ride height may be controlled by controlling a quantity of gas (e.g. amount of molecules of gas) in dependence upon hydraulic pressure, which is indicative of load. This assists to maintain ride height within preferred bounds and/or maintain a constant frequency. (Measuring volume of gas)

One or more embodiments of the present invention provides a method of maintaining the volume of gas in the accumulators of a hydraulic suspension system within a working range across a predetermined or preferred load range. For example, one or more forms of the present invention provides a method of maintaining volume of gas in at least one accumulator of a hydraulic suspension system within a working range across a predetermined or preferred load range by adjusting said volume in dependence on load and/or ride height.

One or more embodiments of the present invention provides a hydraulic suspension system for a vehicle, the vehicle including a body and at least one forward pair of laterally spaced wheels and at least one rearward pair of laterally spaced wheels, the hydraulic suspension system including: at least one wheel ram for at least one laterally spaced pair of wheels, the at least one wheel ram providing substantially all the support of the load on said laterally spaced wheels, the at least one wheel ram including at least a compression chamber, at least one accumulator including a liquid chamber and a gas chamber separated by a moveable wall, the liquid chamber forming a liquid volume connected to the compression chamber of the at least one wheel ram forming a wheel ram compression volume, the gas chamber forming an active gas volume, wherein gas is supplied to and released from the active gas volume to adjust the quantity of gas in the active gas volume to compensate for load changes on the vehicle or on at least one said pair of laterally spaced wheels.

The hydraulic suspension system may provide substantially all of the support for the vehicle body. The adjustment of the active gas volume may be made in dependence on the hydraulic pressure in one or more wheel rams to thereby compensate for load changes. In such arrangements, the pressure in the hydraulic suspension (which can be measured to determine payload and total mass of vehicle) may be used to control adjustment of the quantity of gas in the active gas volume. Alternatively or in addition, the active gas volume may be adjusted to maintain a constant volume of gas in the accumulator for any static load condition of the vehicle. This would maintain a constant natural frequency of the suspension which is a benefit, but other methods or system arrangements of the

present invention may, though potentially less desirable, be employed to achieve a similar result without needing to measure the actual volume of the active gas volume by measuring displacement of a moveable wall, such as a piston, diaphragm or bladder. In a further alternative form of the present invention, the quantity of gas in the active gas volume may be adjusted to correct the ride height of the vehicle. In this case gas can be displaced into or out of the active volume of gas in the accumulator until the desired ride height is achieved - this only requires simple height sensors, so is a cost effective and useful method of control. The adjustment of the quantity of gas in the active volume of gas may be continuously varied, i.e., with great resolution, as opposed to continually adjusted during motion to compensate for dynamic motions. Whilst this is achievable, adjustment is preferable when static or when in a steady state condition, such as travelling in a straight line at constant speed, with no significant accelerations being continually present on the vehicle body (other than gravity). Alternatively the quantity of gas in the active volume of gas may be adjusted between a set number of discrete states, which may be as few as two (unladen and fully laden).

The adjustment may alternatively be made through driver selection, which may be beneficial to anticipate a load change and enable pre-emptive action or to permit a change of stiffness (and response frequencies), but is not preferred in some applications.

In order to facilitate the adjustment of the active gas volume, it may be advantageous to provide a precharge adjustment device comprising a primary volume separated by a moveable wall into a liquid volume and a gas volume. This can permit a conventional liquid pressure/displacement control system (oil tank and pump for example) to be used to adjust the quantity of gas molecules in the active gas volume of the accumulator. The gas volume total between the accumulator and the precharge adjustment device can be checked and serviced like a conventional hydro-pneumatic accumulator, preventing the need to provide such a facility on the vehicle.

The precharge adjustment device may be used to continually vary the quantity of gas in the active gas volume or to simply adjust the quantity of gas in the active gas volume between two states. For example, the precharge

adjustment device can be used such that its liquid volume is at a minimum and its gas volume is at a maximum when the vehicle is in an unladen condition, and such that its liquid volume is at a maximum and its gas volume is at a minimum when the vehicle is in a fully laden condition. Damping may be provided to damp or restrict the flow of liquid between the wheel ram and the accumulator. The damping may be single stage or multistage. The damping can also be switched or varied in dependence on the load (or a pressure indicative of the load).

As a minimum level of damping is required for axle and body control while the vehicle is unladen, this can be provided in, on or near to the wheel ram chamber. This minimum level of damping can include a fixed single or multistage damper valve. The additional damping which is required for increased loads can be located remotely mounted to the vehicle body or to the accumulator. This additional damping can be switchable or variable, any power and/or control wires for this being less vulnerable on the body than on the wheel ram.

Despite the quantity of gas in the active gas volume of the accumulator being controlled, it can be desirable to control the volume of liquid in the wheel ram compression volume. Therefore, this liquid volume can be adjusted (especially to correct for volume changes due to temperature changes and/or leakage).

According to another aspect of the present invention, a hydraulic suspension system includes: at least one front left wheel ram, at least one front right wheel ram, at least one back right wheel ram and at least one back left wheel ram, each wheel ram being double-acting including a compression chamber and a rebound chamber, the compression chamber of the least one front left wheel ram being connected to the rebound chamber of the at least one front right wheel ram forming a front left compression volume, the compression chamber of the least one front right wheel ram being connected to the rebound chamber of the at least one front left wheel ram forming a front right compression volume, the compression chamber of the least one back right wheel ram being connected to the rebound chamber of the at least one back left wheel ram forming a back right compression volume, the compression chamber of the least one back left wheel ram being connected to

the rebound chamber of the at least one back right wheel ram forming a back left compression volume, and at least one accumulator in fluid communication with a respective fluid volume of each compression volume, said at least one accumulator including a liquid chamber and a gas chamber separated by a moveable wall, the liquid chamber forming a liquid volume connected to the respective compression volume, the gas chamber forming a respective active gas volume. This arrangement can be applied to vehicles with multiple front axles and/or multiple back axles, with each axle set or group therefore having two compression volumes. The wheel ram arrangement of each axle set or group thereby providing higher roll stiffness than heave stiffness and no pitch stiffness between adjacent axles in the same set or group, pitch stiffness coming from the use of two (a front and a rear) or more axle sets or groups.

In such a hydraulic suspension system, gas may be supplied to and released from the active gas volume to adjust the quantity of gas in the active gas volume to compensate for load changes on the vehicle or on at least one said pair of laterally spaced wheels.

Damping may be provided for at least one chamber of each wheel ram and/or for each accumulator. The damping may be single or multi-stage, fixed, switchable or variable and it may be distributed between locations on wheel rams and remote. Remote damping can provide the switchable or variable additional damping required at higher loads.

For example, at least a portion of the damping may be provided in, on or near to the wheel ram. In addition or alternatively, at least a portion of the damping may be located remotely of the wheel ram.

Embodiments of the present invention are particularly applicable with respect to vehicles having a significant change of load. One or more such embodiments will hereinafter be described. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

Figure 1 is a schematic showing a first possible embodiment of the invention including a system for adjusting the precharge of an accumulator.

Figure 2 is a schematic diagram of one possible arrangement of wheel rams for a truck according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

Referring initially to Figure 1 , a set of control components for adjusting the spring rate of a hydraulic suspension is shown. Only one wheel ram is shown. This may be located at any point between the wheels (not shown) and body of the vehicle (not shown). For example it can be provided between the centre of a beam axle and the body to provide support for the body over one axle, or it can be provided nearer the wheel, acting between the wheel geometry and the body (with another similar ram being provided in the corresponding position on the opposite side of the vehicle). The wheel ram 10 includes a cylinder body forming a cylinder bore 11 in which a piston 12 is located forming a compression chamber 20 and a rebound chamber 21. The piston has a seal and/or bearing 13 at the sliding interface between piston and bore. A rod 15 is also attached to the piston 12 and passes out of the cylinder body at the end cap where seals, bearings and wipers can be provided individually or as one piece 17. In the orientation shown, the rod is connected to the body of the vehicle (such as through some form of resilient mount or other joint) and the cylinder body is connected to the wheel geometry, although obviously the orientation can be reversed such that the cylinder body is connected to the body of the vehicle and the rod is connected to the wheel geometry.

The rebound chamber 21 can be connected to other wheel rams or to the compression chamber 20, through the piston in which case the piston can include a damper valve to provide a damping force. The compression chamber of the wheel ram is connected to an accumulator 26 by a hydraulic line 22 (hose, conduit, passage or combination of any of these) which can include a damper valve 24. The accumulator includes an accumulator body including a bore 27 and a piston 28. The piston has a seal and/or bearing 29 at the sliding interface between piston and bore, the seal forming two chambers, a fluid chamber 31 and a gas chamber 32. The accumulator 26 is of the piston type, although other types can be used such as those incorporating a bladder or diaphragm. As the rod 15 of the wheel ram is displaced with respect to the wheel ram cylinder body 11 , fluid is displaced into

the fluid chamber 31 of the accumulator displacing the accumulator piston 28 and compressing the gas volume in the gas chamber 32.

Normally as load is added to the vehicle the wheel ram would compress and displace fluid into the accumulator, compressing the gas volume until its pressure increased sufficiently to react the load applied to the wheel ram. In hydro-pneumatic suspension systems such as this the ride height is typically then levelled by supplying more hydraulic fluid to compensate for the change in gas volume required to increase pressure to react the increased load. In this case while the average ride height may remain constant for any load change, if the load change is significant in proportion to the unladen state of the vehicle, then the compression of the accumulator gas volume will be significant. Accumulators have safe operating ranges and pressures which can easily be exceeded if the load changes and wheel ram displacements are significant.

However if the gas volume of the accumulator is adjusted to return the accumulator piston to the unladen steady state position after a load had been added, then there are several advantages: if the fluid in the ram, hydraulic line and accumulator fluid chamber is relatively constant in volume then the ride height will be maintained through adjusting the accumulator gas volume; the accumulator piston can be maintained at mid position so the stiffness characteristic does not end up in the extremely non-linear region caused by the gas volume being compressed to a fraction of its initial volume; the natural frequency of the wheel ram system remains the same; the accumulator can be maintained within a safe operating range.

To control the amount of gas in the accumulator gas chamber 32, the chamber is connected by a gas line 34 through a gas valve 35 to a precharge adjustment device 37. The gas valve is typically a normally closed solenoid actuated valve. The precharge adjustment device is a convenient method of changing the gas volume in the accumulator 26 without requiring a gas pump to control gas at high pressure. It can also permit the accumulator gas precharge to be adjusted without needing to know the volume of gas in the accumulator. The precharge adjustment device includes a body including a bore 38 and a piston 39. The piston has a seal and/or bearing 40 at the sliding interface between piston and bore, the seal forming two chambers, a gas chamber 42 and a fluid chamber

43. The precharge adjustment device can be similar in construction to an accumulator as shown here so can be of the piston type as shown here or of any other known type such as those incorporating a bladder or diaphragm.

For vehicles such as bulk carriers (i.e. earth moving trucks, grain trucks, mining trucks and truck for moving other specific vehicles such as an armoured tank carrier or road grader carrier) where there are essentially only two load states, unladen and fully laden and where the payload is greater than the unladen mass of the vehicle, maintaining the gas volume relatively constant in the accumulator(s) connected to the wheel ram(s) can be straightforward. In such cases the precharge adjustment device can be designed such that between the two states the piston 39 moves from one end to the other of the bore 38. So when the vehicle is unladen, the gas chamber 42 is at its maximum size, the fluid chamber 43 is at its minimum size and the piston 39 is in the position shown in Figure 1. If the gas valve is used to equalise the pressure in the gas volume 42 in the precharge adjusting device 37 to the gas volume 32 in the accumulator 26, then closed for operation of the vehicle, the accumulator piston 28 can be in the required static position and the volume and pressure in the accumulator will provide the required natural frequency of the vehicle body. When the vehicle is fully laden, the gas chamber 42 is at its minimum size, the fluid chamber 43 is at its maximum size and the piston 39 is at the opposite end of the bore 38. Again the gas valve needs to be operated to ensure that the static pressure is equalised between the gas volume 42 in the precharge adjusting device 37 and the gas volume 32 in the accumulator 26 while load adjustment is occuring.

There are many detailed methods that can be used to control the accumulator gas chamber volume. For example, for the bulk carrier application where the precharge adjustment device is only controlled between the two states, the adjustment can be made slowly through restricting the flow through the gas valve once the load change has occurred, or alternatively, the adjustment can be made during the loading time, so during loading the gas valve will be open and the piston 39 controlled.

The piston 39 is controlled by hydraulic control of the hydraulic volume 43 using a typical hydraulic control arrangement 50. In figure 1 this arrangement 50 includes a hydraulic pump 51 , a tank 52 and a dump or return valve 53 and

controls the volume of fluid in the hydraulic volume via control line 44. Either the pump is actuated to increase the volume of hydraulic fluid in the fluid chamber 43 of the precharge adjustment device or the dump valve is actuated to reduce the volume of hydraulic fluid in the fluid chamber 43 of the precharge adjustment device. The pump may have a non-return valve in series to prevent reverse flow (leakage through the pump) if required and a pressure relief valve may be provided. The dump valve is typically a normally closed solenoid actuated valve. The gas valve can be actuated whenever the pump or dump valve are actuated, or it can be controlled to be open continuously during a control routine, or closed and only opened for short periods to achieve the pressure equalisation.

The use of the precharge adjusting device permits a fixed, sealed quantity (in gas molecule terms) of gas to be used between the active gas volume in the accumulator and the gas volume in the gas precharge adjusting device. The actual quantity of gas in the active gas volume of the accumulator can be varied by shunting gas from or into the precharge adjusting device which can itself be powered either by control of a hydraulic volume as shown in figure 1 , or using a lead screw type electronic plunger or any other known means. Although the precharge adjusting device is advantageous in many ways (no gas pump required, adjustment can use means already fitted to vehicle or hydraulics as used for the wheel rams, can use min and max positions for simple unladen and fully laden conditions) it is of course possible to use any other means to adjust the quantity of gas in the active gas volume.

The single-acting ram shown in Figure 1 can be used to control two laterally spaced wheel assemblies through various known geometrical arrangements. Alternatively one ram can be used per wheel assembly (or pairs of longitudinally adjacent wheels which are close together such as those on one side of the dual axle arrangement). However, if a pair of laterally spaced single acting rams is used they have a common roll and heave stiffness.

In Figure 2 the wheel rams 101-110 are part of a roll control system for a truck, utilising similar wheel rams to those shown at 10 in Figure 1. As the rod diameters are large, the push-out force from the rams will be significant so the rams will provide vertical support of the body and heave stiffness as well as roll stiffness. To provide higher roll stiffness than heave stiffness (as is usually

required of a truck when attempting to minimise roll angle and centre of mass lateral shift for stability whilst minimising vertical accelerations for comfort and protection of the load) the known principle of laterally cross connected double- acting rams is applied. That is, if two double acting rams are hydraulically connected from the compression chamber of a first to the rebound chamber of a second forming a first circuit and from compression chamber of the second to rebound chamber of the first forming a second circuit, like motions (i.e. both compression or both rebound) of the two rams displaces a bore volume minus an annular volume {i.e., result is the rod volume) into or out of any attached resilience, whereas dissimilar motions (i.e. one compression and one rebound) of the two rams displaces a bore volume plus an annular volume into or out of any attached resilience, providing a higher stiffness for dissimilar motions than for like motions. When this principle is applied to a pair of laterally spaced rams, higher roll stiffness than heave stiffness is provided. In this case the principle is extended to all the wheel rams at one end of the truck with all similar chambers of the rams on one side of the vehicle being interconnected to provide zero pitch stiffness of those rams without affecting the heave stiffness, similar to the effect of the walking beam geometry sometimes used between longitudinally adjacent wheel assemblies. So this hydraulic interconnection between longitudinally adjacent wheel rams can provide the same stiffness benefits as a walking beam without the geometry limitations. Alternatively one or more of the rams on each side could activate more than one set of wheels through a walking beam arrangement for example.

So the compression chambers 201 and 202 of the rearward and forward front left wheel rams (101 and 102 respectively) are connected to the rebound chambers 213 and 214 of the forward and rearward front right rams (103 and 104 respectively) and to an accumulator 261 forming a first fluid volume, in this case a front left fluid volume (as it includes the front left compression chambers). Similarly, compression chambers 203 and 204 of the forward and rearward front right rams (103 and 104 respectively) are connected the rebound chambers 211 and 212 of the rearward and forward front left wheel rams (101 and 102 respectively) and to an accumulator 263 forming a second fluid volume, in this case a front right fluid volume (as it includes the front right compression

chambers). This four ram front arrangement provides support for the front of the truck with a higher roll stiffness than heave stiffness and with zero pitch or warp stiffness within that four wheel ram set (although with no link to the suspension at the other end of the vehicle, pitch and warp stiffness are still present in the overall vehicle suspension).

Similarly at the rear there is a multi-axle arrangement, in this case having three wheel rams on each side of the vehicle. The principle of laterally cross connected double-acting rams is again applied and further extended to encompass the six rear wheel rams. So the compression chambers 205, 206 and 207 of the forward, middle and rearward back right wheel rams 105, 106 and 107 respectively are interconnected to each other and to the rebound chambers 218, 219, 220 of the rearward, middle and forward back left wheel rams 108, 109 and 110 and to an accumulator 265 forming a third fluid volume, in this case a back right fluid volume (as it includes the back right compression chambers). Similarly the compression chambers 208, 209 and 210 of the rearward, middle and forward back left wheel rams 108, 109 and 110 respectively are interconnected to each other and to the rebound chambers 215, 216 and 217 of the forward, middle and rearward back right wheel rams 105, 106 and 107 and to an accumulator 268 forming a fourth fluid volume, in this case a back left fluid volume (as it includes the back left compression chambers). This six ram back arrangement provides support for the back of the truck with higher roll stiffness than heave stiffness and with zero pitch or warp stiffness, within that six wheel ram set. Again, as there is no link to the four rams at the front of the vehicle, pitch and warp stiffness are still present in the overall vehicle suspension. Although the pitch stiffness is desirable to stabilise the vehicle about the pitch axis, removing the warp stiffness can be advantageous. The warp stiffness can be removed by providing further interconnections between the front and the rear fluid volumes. These interconnections can take many forms. For example a load distribution device can be provided such as that shown in FR 2 663 267 from Renault s.a.s, details of which are incorporated herein by reference. Alternatively a load distribution device with additional modal resilience can be provided, such as that shown in the applicant's US patent number 6,270,098 (details of which are

incorporated herein by reference) connected between the four fluid volumes of the present invention.

The simplest form of such interconnections however is to provide a diagonal connection between the four fluid volumes such that the first (or front left) and third (or back right) fluid volumes are interconnected to form a first diagonal fluid volume, and the second (or front right) and fourth (or back left) fluid volumes are interconnected to form a second diagonal fluid volume. This interconnection between the wheel rams at four corners of a vehicle is as described in the applicant's passive ride control US patent 6,761 ,371 and active ride control US patent 6,519, 517 details of both of which are incorporated herein by reference. While these two prior ride control patents disclose separate support means, much of the application can be applied to the present invention also. However, with any of these interconnections between the four fluid volumes, fluid communication is required along the length of the vehicle. This generally restricts their application to smaller vehicles or to applications where low speed removal of warp stiffness is highly beneficial (such as in an earth moving or mining truck which drives off-road) as sufficient dynamic response is unlikely for faster events such as riding over a rock or other single-wheel input at high speed. Alternatively the simplest front to rear diagonal connections can be provided with valves to control their operation similar to the applicant's International Patent Application Publication Number WO2006/092012, details of which are incorporated herein by reference.

As in a truck the dynamic response of any front to rear interconnection will be poor and potentially the front axles can be on a separate body to the rear axles (such as the two front axles in the present invention being the two rear axles of a prime mover or a dolly and the three rear axles being connected to a trailer) only the separate front and rear arrangements of Figure 2 will now be discussed, being the most typical application. However some hydraulic interconnection (even if just to use a common fluid pressure source) and certainly some electrical interconnection is still likely in this situation.

Each compression chamber (201-210) is provided with a compression damper valve 241-250. This compression damper valve can be a single-direction damper valve (i.e., providing damping in only one direction with a non-return

valve in parallel effectively providing little to no damping force in the opposite direction. The compression damper valve can in that case provide damping in only the compression direction or only the rebound direction. Alternatively the compression damper valve can provide damping in both directions (either similar damping rates or different damping rates from compression to rebound). The compression damper valve can be a simple restriction, a multi-stage valve with a flexing shim stack and/or a sprung blow off, or an electronically, pneumatically or hydraulically controlled valve. As the hydraulic system provides the support for the vehicle, the hydraulic pressure can be sufficient for the compression damper to be the only damper required for the wheel ram (i.e., it can generate sufficient rebound damping force without cavitation). Similarly the rebound damper valve 251-260, if provided, can be single or bi-directional, simple, staged or controlled and can be the only damper provided for the wheel ram. Alternatively no dampers are provided at the wheel ram, or both compression and rebound dampers are used in combination for the wheel ram. Compression and/or rebound dampers can be provided remotely from each wheel ram, either for each wheel ram, or for two or more wheel rams together.

Additionally dampers 331-340 can be provided for the accumulators. These can permit higher roll damping forces to be generated and can be used as the sole source of damping in some applications. However as the wheels generally need to have motions damped out using damper valves provided at the wheel rams to permit good yet damped response of the wheel rams, they are usually preferred to accumulator dampers, although both can be used in combination. As the damping requirements vary significantly with significant changes in vehicle load, it is preferable that the compression, rebound and/or accumulator damper valves are varied or switched between different levels of damping. As the wheel ram dampers can be remote from the wheel rams, these permit any electrical connections to be more protected than those on the wheel rams (and permits a simpler cheaper cylinder design). However, providing all the wheel damping remote from the wheel rams (in a manifold block) gives high pressures in the flexible hoses that connect the wheel ram to the remote damper manifold as all of the support roll and damping pressures can be additive and a further

disadvantage is that any expansion of the hose with pressure is effectively a spring in series with the damper and reduces the control that the damper can exert on the wheel. It can therefore be beneficial to provide some of the wheel ram damping (compression and/or rebound direction acting on compression and/or rebound chambers) at the wheel ram and an additional variable amount remotely. In the case where the load condition is switched between minimum and maximum only, the remote damper manifold block can include a damper valve with a bypass valve so that unladen the remote damper is bypassed and only the wheel damper operates. Then under the high load condition, the bypass is closed and the wheel damper valve works in series with the additional damper valve in the damper manifold block to provide increased body mass damping and increased wheel damping as determined through modelling and tuning.

The quantity of gas in the active gas volume 321 , 323, 325, 328 can be adjusted in dependence on several factors. For example, it can be adjusted in dependence on the actual volume of the active gas volume. This volume can be measured and adjusted such that under static conditions the active gas volume in the accumulator is constant regardless of the load. This maintains a constant vehicle body heave natural frequency of the hydraulic suspension system. However measurement of the actual volume of the active gas volume can require complex sensors and although it will then maintain a constant natural frequency of the suspension system, the ride height can then vary due to changes in volume of the hydraulic liquid due to temperature changes or leakage.

The quantity of gas in the active gas volume can be adjusted in dependence on the hydraulic pressure (indicative of the load) in one or more wheel rams. In this case pressure transducers are required to monitor the pressures in the wheel rams and displacement transducers are required to measure the volume of gas in either the active gas volume or the gas volume 421 , 423, 425, 428 of the precharge adjustment device. While this may seem more complex with no additional benefits, there is the advantage that for vehicles which are usually operated in one of two conditions, unladen or fully laden, the control and sensing is extremely simple (sensing of the gas volume in the precharge adjustment device can be simpler than sensing the active gas volume in an accumulator which may be of different construction and experiences more

pressure fluctuations due to its dynamic operational requirements. When the vehicle is unladen, the gas volume 421 , 423, 425, 428 in the precharge adjustment device is at a maximum, so if one extreme of travel of the precharge adjustment device is designed to give that required maximum volume, the active gas volume needs to be equalised in that condition. Similarly if fully laden the precharge adjustment device is designed such that its gas volume 421 , 423, 425, 428 is at the required minimum volume, the active gas volume again needs to be equalised in that condition. For safety and practicality, the precharge adjustment device gas volume and the active gas volume of the accumulator cannot be instantly equalised, so either the gas valve (or valves 351 , 353, 355, 358) need to be open during the change of load and the corresponding adjustment, or the gas valve must be highly restricted in flow or time or operation (short pulses of time open only).

The quantity of gas in the active gas volume 321 , 323, 325, 328 can be adjusted in dependence on other measures also, or instead. For example, the ride height of the vehicle can be adjusted using solely adjustment of the active gas volume. In this case, the active gas volume can vary due to temperature changes or hydraulic liquid leakage, so while it is possible to make quick changes for continual changes of load using this simple height sensor input, it is not an ideal sole solution but is excellent if the hydraulic liquid volume in the compression volumes has been checked. Another issue is that system pressures need to be considered to ensure that in any system having more than three hydraulic ram volumes, there are minimal unnecessary warp of wedge forces where a roll moment on a front axle assembly is conflicting with an opposite roll moment on a rear axle assembly. If the front and rear axle assemblies are on different bodies (i.e., the front axle assemblies can be on the rear of the tractor unit of a truck or on a dolly between trailers and the rear axle assemblies can be on the rear of a trailer unit or body) then a connection is required between the different bodies to ensure that any adjustment does not provide a large warp or wedge load between the front and rear axle assemblies. This connection can be hydraulic if the wheel ram assemblies of the front and rear axle assemblies are hydraulically linked. However if the vehicle is regularly split [i.e., the trailer is regularly unhitched from the tractor unit or dolly) then the provision of the

hydraulic connections can be more complex than the benefits of such a connection justify. As noted earlier, the benefits of a front to rear hydraulic link may be beneficial to reduce warp loads in low speed warp motions, but not of sufficient benefit for higher speed wheel motions to justify the complexity. In that case all that is required is a simple electronic link between the bodies to permit communication between the electronic control units which control the adjustment of hydraulic and active gas volumes.

Pressure transducers (not shown) are generally provided for the hydraulic pressure of the front and rear compression volumes so that the loading (load magnitude and position) can be ascertained. This can permit the loading of the vehicle to be carried out in such a way that the maximum design load can be carried in the vehicle, maximising the payload carried safely and maximising the efficiency of the truck. It should be noted that the controllers can permit the vehicle to be levelled when the centre of mass does not lie on the vehicle centre line, although the controller can issue a warning if the load is too eccentric. Similarly the controller can adjust the roll angle to ensure that the centre of mass of an eccentric load then lies over the vehicle centre line, although this can only be done within preset limits of load eccentricity.

The levelling control can have several different modes. For example, on initial startup of the truck, the pressure in the hydraulic systems of the wheel ram assemblies and the precharge adjustment devices can be minimal or near zero to permit the total gas volume to be checked. If the total gas volume (in the active gas volume of the accumulator and the gas volume of the precharge adjustment device) is found to be too low, a service warning can be generated. To this end pressure, transducers 361 , 363, 365, 368 are preferably provided on either the active gas volume or the gas volume of the precharge adjustment device 371 , 373, 375, 378. For the front accumulators the pressure transducers 361 , 363 are shown on the active gas volume - this effectively permits the pressure in the compression volumes to be measured. However in this position the transducers are subject to all dynamic pressure fluctuations of the hydraulic suspension, so at the rear the pressure transducers 365, 368 are shown in their alternative position on the precharge adjustment device as an alternative.

Once this initial check is completed, the vehicle can be set to the correct ride height to introducing hydraulic liquid into the compression volumes (front left and right and rear left and right) using control valves 581 , 583, 585, 588 (and releasing liquid as necessary using corresponding dump valves 571 , 573, 575, 578). For the front compression volumes these control valves are connected to the accumulator side of the compression volumes and at the rear they are shown on the wheel ram side of the accumulator damper valves. If during this procedure the vehicle is detected to be laden, then the hydraulic volumes of the precharge adjustment devices will need to be controlled in reaching the correct set up of the hydraulic system. Ideally however the vehicle will normally initially be unladen so the compression volumes can be correctly set, then the precharge adjustment device can be the primary means to correct for ride height as the vehicle is loaded. As the unladen vehicle is loaded, the gas valves 351 , 353, 355, 358 can be opened and the hydraulic volumes 431 , 433, 435, 438 filled to compensate for the load change using control valves 551 , 553, 555, 558 to add hydraulic liquid and valves 531 , 533, 535, 538 to release hydraulic liquid. Indeed before each loading cycle the compression volumes can be adjusted prior to loading (as a maintenance routine), then the hydraulic volumes 431 , 433, 435, 438 of the precharge adjustment device used to compensate solely for the load as it is added. Each let and right pair of compression volumes can be controlled by independent controllers which communicate to the controllers of the other pairs of compression volumes. Alternatively a single ECU can control two or more pairs of compression volumes.

It is preferable that the hydraulic suspension system is the primary support means for the vehicle body. If some initial unladen body mass is supported by separate springs for example, the ratio of change in force required from the wheel rams is then greater for the same additional load, which will result in either deflection of the static position of the accumulator moveable wall away from an idea range, or change in the natural frequency of the suspension system and vehicle, both of which defeat aims of the present invention.

It should be appreciated that while the present invention is describing in relation primarily to trucks, it can be applied to other vehicles, such as trains.

It should be understood that the rods may be much smaller so that they provide little to negligible push out force and therefore provide little support. In this case the pressures in the compression volumes and the position of the precharge adjustment device will need to be controlled in response to inputs from the vehicle support means indicative of the load on the vehicle.