Field of the invention

The present invention relates to a hydraulic damper for a motor vehicle and in particular to a damper for an adaptive damping system having a variable damping rate.

Background of the invention

Hydraulic dampers comprise a piston and cylinder unit in which hydraulic fluid can flow at a regulated rate through passages connecting the working chambers on each side of the piston. By selectively blocking and unblocking certain passages, the extent of damping can be varied.

In any suspension system with a fixed damping rate, the selection of the damping rate calls for a compromise between comfort and handling. In general terms, soft damping improves comfort while hard damping improves handling.

Adaptive damping systems use switchable dampers to vary the damping rate dynamically as a function of the driving conditions and in some systems the switching between settings may be effected very rapidly.

When fast acting variable dampers are switched to a harder setting, it has been found that the sudden pressure rise within the damper creates a force pulse at the ends of the unit and hence a noise in the vehicle.

To mitigate this problem, it has already been proposed to reduce the difference between the settings, but this also reduces the benefits of an adaptive damping system. A better proposal has been to sense damper movement and to time the switching of the setting of the damper to coincide with an instant when there is no movement within the damper, at which time the pressure rise will be minimised. This

solution calls for the switching instant to be delayed and requires an additional sensor to detect the movement or internal pressure within the damper.

Summary of the invention

With a view to mitigating the foregoing disadvantages, the present invention provides a controllable hydraulic damper comprising a pressure tube filled with a liquid, a piston movable in the tube and connected to a rod passing through one end of the tube, the piston defining two chambers in the tube with the rod passing through the first of the two chambers, first valve means in the piston to permit liquid to flow from one chamber to the other at a controlled rate during piston movement, a reservoir connected by second valve means to the second chamber and connected by a return, circuit containing third valve means to the first chamber, the level of fluid in the reservoir being raised and lowered during movement of the piston as a result of the changes in the intrusion volume of the piston rod, at least one of the valve means being controllable to vary the flow resistance therethrough and thereby vary the stiffness of the damper, wherein a resilient element is provided within the fluid circuit in communication with the chamber on the higher pressure side of the piston to suppress noises occurring when the damper is abruptly changed to a higher stiffness, the resilient element serving to increase the volume of the fluid circuit by an amount significantly smaller than the intrusion volume of the piston rod when the pressure acting on the element exceeds a predetermined value.

To avoid the resilient element being compressed by the pressures normally occurring in- the pressure circuit of the hydraulic damper, a preloaded or pre-compressed element may be used. In this way, the element will only become active when a certain minimum pressure, corresponding to the

preload, is exceeded, which pressure may vary between vehicles.

There are several suitable locations at which the resilient element may be positioned, it being only necessary that it should be on the higher pressure side of the damper when the softer setting is selected.

The resilient element may take various forms, such as a gas bladder or a ball. It is preferred that it should comprise a closed cell foam plug, preloaded by clamp washers. The element may be embedded in the damper push rod or be housed within bellows or a preformed pipe designed to change volume under rapid increases in pressure.

The invention is not restricted to any given configuration of the damper and is equally applicable to twin tube non-pressurised and pressurised dampers and the switching between settings may be carried out by internal or external control elements.

Brief description of the drawings

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

Figure 1 is a schematic representation of the pressure circuit of a variable rate hydraulic damper, Figure 2 is a graph showing the variation of pressure with time during switching to a harder setting in a conventional damper, and

Figure 3 is a graph similar to that of Figure 2 showing the effect of incorporating a compressible element in the pressure circuit of the damper.

Description of the preferred embodiment

The twin-tube damper shown in Figure 1 comprises an outer tube 10 that contains a hydraulic fluid reservoir 12 and an inner pressure tube or cylinder 14 within which a piston 16 connected to a push rod 21 reciprocates. The pressure tube 14 is totally filled with hydraulic fluid. Valves 18 and 20 are mounted in the piston to permit hydraulic fluid to flow between the working chambers on opposite sides of the piston and further valves 22 and 24 in the base of the inner pressure tube 14 permit fluid flow between the reservoir 12 and the lower working chamber of the pressure tube 14. The required damping of the movements of the push rod 21 results from the viscous losses in pumping hydraulic fluid between the working chambers of the inner pressure tube 14.

Because of the area of the push rod 21, when the piston moves downwards, as viewed, the volume of fluid displaced from the lower working chamber is more than that required to fill the upper working chamber and hydraulic fluid is pumped into the reservoir 12 through the non-return valve 22 in the base plate of the inner tube 14. Conversely, when the piston 16 moves upwards, as viewed, the fluid flowing through the piston 16 is not sufficient to occupy the increased volume of the lower working chamber and fluid is drawn in from the reservoir 12 through the non-return valve 24 in the base plate of the inner pressure tube 14.

As so far described, the damper only has one setting determined by the non-return valves and orifices in the piston 16 and the base plate of the inner pressure tube 14. To permit the damping rate to be modified for an adaptive damping system, in this embodiment, a by-pass tube 32 connects the reservoir 12 to the upper working chamber of the inner pressure tube 14 and contains a non-return valve 34 and a solenoid operated valve 36.

For as long as the solenoid valve 36 is closed, the damper operates in the manner previously described, this corresponding to the hard setting of the damper. When however the solenoid valve 36 is open, fluid can now also flow out of the upper working chamber directly into the reservoir 12 through the by-pass tube 32 without being forced through the narrower opening. in the piston 16 resulting in a softer setting with less damping.

Such adaptive dampers are in themselves well known and when they are switched to a harder setting, it has been found that the sudden pressure rise within the damper creates a force pulse at the ends of the unit and hence a noise in the vehicle.

To mitigate this problem, the described embodiment of the invention provides for the inclusion of a pre-loaded, compressible closed cell member in communication with the pressure circuit of the damper, the drawing showing four alternative locations for such a member. For example, a plug 40 may be mounted in the push rod 21 or a plug 41 in the top seal assembly 42 of the inner pressure tube 14. In this case, the plug 40 may be preloaded by a grub screw formed with a bore to expose the member to the pressure in the inner tube, which grub screw may have a sized orifice to regulate the flow of fluid.

As an alternative, the compressible closed cell member may be in the form of a ring 44 surrounding the push rod 21 or a ring 45 positioned at the top of the upper working chamber immediately below the top seal assembly 42. A collar 46, 47 may be used to pre-load the ring 44, 45.

It should be mentioned that the total change in volume brought about by the compressible element should be small and significantly smaller than the intrusion volume of the piston rod 21.

Because of its small size, the compressible element does not affect the damping force. The effect of including such a compressible member in the high pressure circuit is shown by comparing the graphs in Figures 2 and 3. These show the variation of pressure with time during a switching to a hard setting. In Figure 2, without the inclusion of a compressible member, the pressure rises very steeply and overshoots the hard setting value. However, in Figure 3, with the compressible member present, the pressure surge is absorbed by the compressible member and, without affecting the final pressure, increases the time taken for the transition from typically 5 ms to 8 ms thereby reducing the shock and the noise that it causes.

The volume change of the compressible member is carefully selected so as to provide an imperceptible effect on the convention hard setting values.

Though the invention has been specifically described by reference to a separate compressible member, it is possible to provide a resilient element in other ways. For example, the by-pass tube 32 may include a non-circular section that can be deflected when the pressure rises above a predetermined level to increase the volume of the circuit on the higher pressure side of the piston 16.