ADAPTIVE SHOCK ABSORBERS AND DAMPING VALVES THEREFOR

Abstract: An adjustable damping valve for incorporation in the bypass passage (9) of an adaptive shock absorber, comprises at least one inlet (12) through which hydraulic fluid medium enters the adjustable damping valve (10) and an outlet (20) through which it exits the adjustable damping valve (10). A controllable valve (15, 16, 17) controls the flow of hydraulic fluid medium through the adjustable damping valve (10) between the said at least one inlet (11) and the said outlet (20). The controllable valve (15, 16, 17) comprises a valve member (15) which is moveable against resilient biasing means (19) urging it into co-operation with a port or orifice (13) and the controllable valve (15, 16, 17) is adapted, such that, in use, its response to a control signal is delayed and the valve member (15) remains closed until the force developed across it drops below a predetermined thereshold level.

Description: DESCRIPTION ADAPTIVE SHOCK ABSORBERS AND DAMPING VALVES THEREFOR The present invention relates to adaptive shock absorbers of the kind which comprise a working cylinder divided by a working piston secured on a piston rod into two working chambers filled with damping fluid and which have damping or throttle valves in the working chamber and/or the working piston. More specifically, the present invention relates to adaptive shock absorbers of the kind set forth and which have a bypass passage in parallel with the damping valves in which is incorporated one or more adjustable damping valves for controlling the flow of hydraulic fluid medium through the bypass passage. The present invention also relates to adjustable damping valves for incorporation in the bypass passage of adaptive shock absorbers of the kind set forth. It is known from GB-A-2164723 to provide an adaptive shock absorber in which the damping characteristic can be adjusted by means of an adjustable damping valve incorporated in a bypass passage between the two working chambers of the adaptive shock absorber. The adjustable damping valve comprises a housing having a port in one of its end walls and at least one other opening, a valve member which is moveable longitudinally within the damper valve housing under the control of an electromagnet and which co-operates with the port, and a pressure dependent spring loaded disc valve arrange co-axially and in series with the valve member. The valve member and the pressure dependent spring loaded disc valve control the flow of hydraulic fluid medium through the damper valve housing 5 between the port and the other opening or openings, and hence regulate the flow of hydraulic fluid medium through the bypass passage of the adaptive shock absorber. It will be understood that as the valve member is opened or closed it alters the piston rod force which the 10 shock absorber exerts on the vehicle structure. As a consequence, the vehicle structure responds by radiating undesirable noise into the passenger compartment. The level of noise is dependent on the rate of change of piston rod force and hence, by decreasing the rate of I 5 change, the level of noise can be minimised. One way of achieving this is to delay the operation of the valve member until the shock absorber piston velocity is very slow. It is generally appreciated that a solution to the 2° noise could be realised by attention to the control software which drives the longitudinally moveable valve member, but to achieve the same effect by this means requires the presence in the suspension system of an additional transducer, to detect near-zero suspension 2 * -* * velocity and further complicates the electronic controller. It is an object of the present invention to provide an adjustable damper valve for incorporation in the bypass passage of an adaptive shock absorber, which, in use, will not switch from a high to a low setting, when the adaptive shock absorber is developing significant force. It is another object of the present invention to provide an adaptive shock absorber which incorporates an adjustable damping valve having the benefit of the above stated object. According to a first aspect of the present invention there is provided an adjustable damping valve for incorporation in the bypass passage of an adaptive shock absorber, the adjustable damping valve comprising at least one inlet through which hydraulic fluid medium enters the adjustable damping valve and an outlet through which it exits the adjustable damping valve, and a controllable valve which controls the flow of hydraulic fluid medium through the adjustable damping valve between the said at least one inlet and the said outlet, characterised in that the controllable valve is adapted, such that, in use, its response to a control signal is delayed and the valve member remains closed until the force developed across it drops below a predetermined threshold level. Thus, the adjustable damping valve according to the present invention will switch from a high to a low setting only when the pressure developed by the damping piston falls to an insignificant level. The force threshold at which the controllable valve - A - opens is given by the following equation: F threshold = (Fsol - Fsp) x working surface area orifice area of the damping piston where Fsol is the force required to open the controllable valve and, Fsp is the restoration bias which biases the controllable valve towards a closed position. Preferably the controllable valve comprises a valve member which is moveable against resilient biasing means urging it into co-operation with a port or orifice. Preferably, the controllable valve is controlled by an electromagnet which is both simple and economical to implement. Conveniently, the armature of the electromagnet comprises the valve member. However, it may also be controlled by pneumatic or hydraulic actuators, flexible cables, or any other convenient control means. In the case of an electromagnetically controlled valve the force (Fsol) required to open the controllable valve is determined in particular by the magnetic circuit efficiency the current in the electromagnet, the cross- sectional area of the valve member and the valve member lift. By appropriate selection of the design parameters for the controllable valve it is possible to adjust the force threshold at which it switches from a high setting to a low setting so that it only occurs when the damping piston ceases to develop significant force. In practice, this means that no downward switching takes place (that is to say from a high setting to a low setting) until the damping piston velocity has fallen to a value approaching zero. It follows from this that there will be no significant force at the point of attachment of the adaptive shock absorber to the vehicle when the damping piston velocity is insignfican . It will be understood that the adjustable damping valve according to the present invention operates as described hereinbefore when the adaptive shock absorber piston is moving in either the upward (rebound) or the downward (compressive) direction, whenever switching from a higher to a lower setting is selected. The fact that switching is being delayed does not matter from the viewpoint of suspension control when switching from a high to a lower setting. Preferably, a control valve is provided immediately downstream of the controllable valve and is adapted to have a pressure versus flow characteristic to suit the control requirements of the vehicle. The control valve may take the form of a poppet valve. This is spring biased towards a closed position and opens only when the pressure across it exceeds a predetermined level, when it imposes a pressure drop related to the piston velocity by the control requirements of the vehicle. However, it is envisaged that the control valve could also take the form of an orifice. It will be understood that the control valve must be located downstream of the controllable valve because it is, of course, essential that the pressure upstream of the controllable valve holds the valve member thereof against its seat. Should the control valve be placed upstream of the controllable valve it would trap pressure between itself and the 5 controllable valve and prevent the controllable valve lifting from its seat. The adjustable damping valve according to the present invention differs from conventional damping valves in that it is deliberately designed so as to be 10 held down by the pressure developed in the hydraulic fluid by the damping piston. Only when this has dropped below the threshold set by the design perameters outlined above will the controllable valve open to allow switching from a high to a low setting. Conventional damping • *- 5 valves are usually balanced inasmuch as the valve member is designed so that the upstream end thereof is exposed to the same pressure as the downstream end (usually a through bore is provided from one end of the valve member to the other) , and as a consequence the valve has little 20 tendancy to move in either direction. The adjustable damping valve according to the present invention may be arranged in parallel with the conventional damping or throttle valves in the damping piston and/or in the base of the adaptive shock absorber. 25 Another possibility is that the adjustable damping valve may be arranged in a lockable adaptive shock absorber without conventional damping valves being present in the damping piston or in the base of the cylinder. The adjustable damping device according to the present invention can be used in twin tube shock absorbers, including McPherson struts and semi-struts, and in monotube shock absorbers. 5 An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a longitudinal section through a twin tube adaptive shock absorber comprising an adjustable damping 10 valve in accordance with a first aspect of the present invention; and Fig. 2 is a section through an adjustable damping valve in accordance with a first aspect of the present invention. •* - *■ -' The twin tube adaptive shock absorber shown in Fig. 1 is a suspension unit for use in a vehicle suspension system. The adaptive shock absorber comprises a working cylinder 1 and a damping piston 2 carried by a piston rod 3. The free end 4 is adapted, in use, to be secured to 20 one part of a vehicle, for example, the vehicle body. Similarly, the working cylinder 1 is adapted, in use, to be secured to a second part of a vehicle, for example, a vehicle suspenion member. The damping piston 2 is arranged for sliding movement within the working cylinder 5 1 and divides the working cylinder 1 into an upper working chamber 5 and a lower working chamber 6. The damping piston 2 is further provided with one or more damping valves (not shown) which allow hydraulic fluid medium to flow from the lower working chamber 6, in front of the damping piston 2, to the upper working chamber 5, on each downward stroke of the damping piston 2. Arranged in the base of the working cylinder 1 is one or more 5 further damping valves (not shown) , through which hydraulic fluid medium displaced by the damping piston 2 on its downward stroke, and not passing back into the upper working chamber 5 through the valves in the damping piston 2, is forced into a reservoir space 7 which is 0 formed between the working cylinder 1 and an outer casing 8. The upper working chamber 5 is also connected to the reservoir space 7 through a bypass passage 9 and an adjustable damping valve 10 which will now be described 5 in greater detail with reference to Fig. 2 of the accompanying drawings. Fig. 2 is a section through an adjustable damping valve 10 which is incorporated in the bypass passage 9 of a twin tube adaptive shock absorber. The adjustable 0 damping valve 10 comprises an outer housing 11 which is adapted, in use, to be secured in the outer casing 8 of the adaptive shock absorber. In use, hydraulic fluid medium enters the adjustable damping valve 10 through a feed bore 12 which is aligned with the bypass passage 9 ^ and flows through a port or orifice 13, past a spring loaded poppet valve 14, and into the reservoir space 7 of the adaptive shock absorber through a port 20 in the forward end of the adjustable damping valve 10. The port 13 can be closed by an axially moveable valve member 15 which forms the armature of, and is therefore controlled by, an electromagnet 16 which also comprises a coil 17. A restoring spring 19 exerts a permanent force on the valve member 15 so that when the electromagnet 16 is de- energised the valve member 15 is biased towards the port 13 to close same. The spring loaded poppet valve 14 is biased towards a normally closed position by means of a spring 21. A seal 22 prevents hydraulic fluid medium from leaking out between the damper valve 10 and the outer casing 8. It will be understood that for the valve member 15 to open to allow hydraulic fluid medium to flow through the port 13; Fsol > Fresid, where, Fsol is the force pulling the valve member away from the port and is determined in particular by the current in the electromagnet, the cross-sectional area of the valve member and the valve member lift and Fresid is the residual force pushing the valve member against the port and is determined by the piston force (in rebound or compression) , the cross-sectional area of the annular space 5 (annular area) in rebound or of the piston rod 3 in compression, the cross-sectional area of the port (orifice area), and the force exerted by the restoring spring (Fsp) such that Fresid = Piston force orifice area Fsp Annulus area for the rebound phase, or Fresid Piston force x orifice area + Fsp Annulus area for the compression phase If Fresid exceeds Fsol then the valve member 15 will not open. Since Fresid is proportional to the piston force it follows that at very high piston forces the valve member 15 will remain shut, thereby keeping the damping valve on the highest setting until the piston force drops to the point where Fresid < Fsol and the valve member 15 can open. The delay in switching from the higher setting (damping member 15 closed) to the lower setting (damping member 15 open) is advantageous, for it is the transition from higher setting to lower setting at high piston forces which gives rise to the greatest damper switching noise. By ensuring that the damping member 15 only switches when the piston force drops below a predetermined threshold, damper switching noise is much reduced. The use of a force threshold for hard (high) to soft (low) damper changes has little, if any, effect on the operation of the adaptive shock absorber for the change from high to low setting is only a return to normal operation after control has been applied. Rapid damper valve operation is only required in the change from low to hard setting, that is to say, soft to hard suspension, and since this is actually aided by oil flow it occurs much more effectively at high piston forces. By rearrangement of the equations given hereinabove it is possible to arrive at the equation for the threshold at which Fsol equals Fresid. This is F threshold (compression) = (Fsol - Fsp) x (work area of the damper orifice area piston - the cross- sectional area of the or, piston rod) F threshold (rebound) = (Fsol - Fsp) x rebound annulus orifice area of damper It follows from the formula given above that by varying the force of the restoring spring and/or the valve member pull force, the required force threshold can be set for high to low switching and also to give the fastest low to high switch time. In the context of the valve calculation, it should be understood that the orifice area is already fixed by the needs of the shock absorber setting and so it is not a variable in this calculation. It has to appear in the calculation because it will vary between one vehicle application and another, necessitating the electromagnetic force and possibly the spring force to change in sympathy. The adjustable damping valve provides an improved response speed when closing as a consequence of responding to the pressure drop across it. This is due to the pressure accelerating the valve plunger towards the seat. This pressure, and therefore the improvement in response speed, is proportional to the piston velocity, because the higher the velocity the greater is the pressure across the valve; and the increasing response speed with respect to damper velocity is an important advantage in many control strategies.

Claims: CLAIMS 1. An ad ustable damping valve for incorporation in the bypass passage (9) of an adaptive shock absorber, the adjustable damping valve (10) comprising at least one inlet (12) through which hydraulic fluid medium enters the adjustable damping valve (10) and an outlet (20) through which it exits the adjustable damping valve (10) and a controllable valve (15, 16, 17) which controls the flow of hydraulic fluid medium through the adjustable damping valve (10) between the said at least one inlet (11) and the said outlet (20), characterised in that the controllable valve (15, 16, 17) is adapted, such that, in use, its response to a control signal is delayed and the valve member (15) remains closed until the pressure developed across it drops below a predetermined threshold level. 2. An adjustable damping valve according to claim 1 , characterised in that the controllable valve (15, 16, 17) comprises a valve member (15) which is moveable against resilient biasing means (19) urging it into co-operation with a port or orifice (13). 3. An adjustable damping valve according to claim 1 or 2, characterised in that the controllable valve (15, 16, 17) is controlled by an electromagnet (16). 4. An adjustable damping valve according to claim 3 characterised in that the armature (15) of the electromagnet (16) comprises the valve member (15). 5. An adjustable damping valve according to any preceeding claim characterised in that a control valve (14) is provided immediately downstream of the controllable valve (15, 16, 17) and is adapted to have a pressure versus flow characteristic to suit the control requirements of the vehicle. 6. An adjustable damping valve according to claim 4 characterised in that the control valve (14) comprises a poppet valve. 7. An adjustable damping valve according to any preceding claim characterised in that the pressure upstream of the valve accelerates it towards the closed condition. 8. An adaptive shock absorber in combination with an adjustable damping valve according to any preceeding claim.

Inventors: PEARCE PAUL TOWERSEY, PEARCE, PAUL, TOWERSEY
Applicants: MONROE EUROP UK LTD, MONROE EUROPE (UK) LIMITED
Application year: 1990
Main classification: F16F9
IPC codes: F16F9, F16K31