VEHICLE SUSPENSION SYSTEMS

Abstract: A vehicle suspension system has a gas-sprung hydraulic strut (6,7) for each of its four wheels (2,3). Each liquid actuator chamber (8) of the struts (6) at one end of the vehicle is connected (9) to a further chamber (10) in the diagonally opposite strut (7), which further chamber (10) acts in the opposite direction to the actuator chamber (13) of that strut (7). A linear potentiometer (26) within each strut (6,7) provides a signal representative of the strut length to an on-board computer (28), which, when it detects a difference in the signal from any strut sensor (26) from a predetermined value, causes a proportional servo-valve (35) to admit hydraulic liquid from the actuator chamber of that strut at a flow rate proportional to the detected difference to return the strut length towards its predetermined value.

Description: The present invention relates to vehicle suspensions. British Patent specification GB-A-1384454 has proposed a vehicle suspension system (hereinafter referred to as "a suspension system of the type defined") for a vehicle according to the preamble of claim 1. According to the invention, a suspension system of the type defined is modified according to the characterising part of claim 1. The invention will now be further described by way of example with reference to the accompanying drawing in which: Figure 1 shows diagrammatically a suspension system in accordance with the invention; Figure 2 shows a longitudinal section through a rear strut of the suspension shown in figure 1; Figure 3 shows a damper valve assembly in section; and Figure 4 shows a flow chart of the operations performed by the computer. The suspension system shown in Figure 1 serves to support a vehicle structure 1 on a pair of rear wheels 2 and a pair of steered front wheels 3. Each rear wheel 2 is connected to the structure for example by means of a swing axle 4 while each front wheel 3 is connected to the structure 1 by a suspension linkage 5. The structure 1 is also supported on the swing axles 4 and suspension linkages 5 by means of telescopic struts 6 and 7 at the rear and front of the vehicle respectively. Each of the rear struts 6 has a liquid chamber 8 of volume which varies in accordance with the length of the strut and which is connected by a line 9 to a liquid chamber 10 of the front strut on the opposite side of the vehicle. Whereas the liquid chambers 8 are formed in the upper parts of the struts 6, above the piston-forming portion 11 of the struts, the chambers 10 of the front struts 7 are formed below the pistons 12 of the front struts. Accordingly, liquid displaced from a rear chamber 8 by upward movement of the associated wheel 2 relative to the structure 1 will increase the volume of the chamber 10 of the front strut 7 on the opposite side of the vehicle, thereby shortening the effective length of this front strut and thus opposing vehicle roll. Futher variable-volume liquid chambers 13 are bounded within the front struts 7 by the pistons 12, on the opposite side of the pistons to the chambers 10. The necessary resilience for the suspension is provided by two rear gas springs units 14 and one front gas spring unit 15. Each rear gas spring unit 14 has a gas-containing chamber 16 filled with gas to the required pressure to support the vehicle. The gas-containing chamber 16 is separated by a movable partition 17, here in the form of a diaphragm, from a liquid-containing chamber 18 connected to the associated suspension strut liquid chamber 8 by a line 19 to which the associated line 9 may conveniently be connected. Similarly, the front gas spring unit 15 has a gas-containing chamber 21 precharged with gas under pressure and a liquid-containing chamber 22 separated from the gas-chamber 21 by a movable partition 23 in the form of a diaphragm. The liquid-containing chamber 22 is connected by lines 24 to both of the upper liquid chambers 13 of the two front suspension struts 7. In accordance with the invention, each of the struts 6 and 7 has associated with it an extension sensor 26 and 27 respectively in the form of a potentiometer, the resistive element of which may be formed by an electrically conductive plastics material housed within the strut. Each potentiometer 26 and 27 is connected to an on-board computer 28 which cyclicly assesses the amount and direction by which the reading of each potentiometer differs from a predetermined datum value. In order to correct for such departures of the length of the liquid-filled suspension struts 6 and 7 from their predetermined lengths, further liquid under pressure can be admitted to the chambers 8 and 13 from a pressurised supply line 30 or some liquid can be allowed to escape to a reservoir T. An engine-driven hydraulic pump 32 draws liquid from the reservoir T and delivers the liquid under pressure to a hydraulic pressure accumulator 31 and the line 30. The pump 32 may be driven for example from the engine camshaft and is preferably of the constant-pressure variable-delivery kind, for example a swash-plate pump, the pump delivery being controlled in known manner in accordance with the demand on the line 30. The front levelling control valve 33 is connected by a line 34 to the line 24 and is also connected to the pressure supply line 30 and to the reservoir 31. Similarly, two rear levelling control valves 35 are connected by lines 36 to the lines 19 and are also connected to the pressure supply line 30 and the reservoir 31. Each of the three level control valves 33, 35 is electrically controlled by the computer 28 to which it is connected and is constructed to pass a flow of liquid at a flow rate determined by the control signal supplied to it by the computer 28. In the case of the two rear control valves 35, the flow rate into the associated strut chamber 8 from the line 30 or out of the chamber 8 to reservoir 31 is controlled by the valve 35 so as to be proportional to the amount by which the strut length differs from the predetermined value corresponding to a predetermined value of ride height. Liquid is admitted or withdrawn in accordance with whether the volume of the chamber 8 is reduced below the predetermined value or has increased above it respectively. The computer 28 controls the single front levelling valve 33 effectively in accordance with the arithmetic mean of the readings of the two front potentiometers 27, the flow rate delivered by the valve 33 being proportional to the amount by which this arithmetic mean differs, positively or negatively from the predetermined value. The suspension system may be damped in known manner by means of the damping characteristics of the gas spring units 15 and 16. By programming the computer 28 to assess the length of the struts sufficiently frequently (e.g. sixty four times per second), the computer may assess the degree of damping which is achieved by the suspension. By providing the gas spring units 15 and 16 with variable damping control valves, the computer can be arranged to damping of the suspension in accordance with the requirements. In order to compensate for the effect of radial compression of the vehicle tyres on the wide height, pressure sensors 41 and 42 may be connected to the hydraulic lines 24 and 19. Thus, for example, during heavy braking of the vehicle, with consequently greater load on, and compression of, the front tyres, the pressure in the line 24 will be substantially increased. The computer 28 can be accordingly programmed to adjust the datum position for the front struts 7 to cause the valve 33 to admit further liquid from the line 30 and thus maintain the ride height of the front of the vehicle despite the tendancy of the vehicle to "dive" under heavy braking. Similarly, the front pressure sensor 41 and outer pressure sensor 42 when the vehicle is cornering hard, will cause the computer to cause the outer rear strut 6 to be lengthened to compensate for radial compression of the outer tyres. Figure 2 shows one of the rear suspension struts 6 in axial section. The strut comprises a lower piston portion 61 slidable in an upper, barrel assembly 62 (with appropriate sealing means S), both assemblies having eyes 63 and 64 for articulation to a swing axle and the vehicle structure respectively. The body 26a of the potentiometer 26 has its upper end secured in a recess in the barrel assembly 62 which is formed with a passage 65 for leadwires (not shown) for connecting the potentiometer 26 to the computer. The slider (not shown) within the casing 26a is connected by a rod 26b to the lower part of the plunger 61. At its uppermost end, the housing 26a carries a peg 66 which is clamped by a set screw 67. The variable-volume chamber 8 within the strut 6 is connected by a transverse bore 68 both to a port 69 for connection to the lines 9 and 19, a pressure sensor (not shown) connected to the computer, and to the passage 36 which is here formed by an extension of the bore 68 and leads into the leveling control valve 35. The valve 35 is a series E050 minature proportional servo-valve supplied by Moog Controls Limited of Ashchurch, Tewksbury, GL20 8NA, England. The valve has a polarized torque motor which moves a pilot valve element between two opposed inlet pilot orifices to throttle the flow through one or other of these orifices. The resultant differentials in the back pressures in these orifices are applied to opposite ends of a hydraulic servo spool-valve controlling the flow to or from the passage 36. Figure 3 shows a damper construction inserted between the line 19 and the gas spring 14 which is here in the form of a hydraulic pressure accumulator. The liquid port 71 is mounted in one end of a bore 72 extending through a damper housing 73 to a threaded connection 74 for the line 9. A disk-type damper valve 75 is slidable in an enlarged portion 76 of the bore 75 between two opposed shoulders. An adjustable throttle valve assembly 77 having an adjustable screw 78 with a conical end provides a preset flow resistance between passages 79 and 80 on opposite sides of the damper valve 75. A non return valve formed by a ball 81 allows flow through the valve 77 only in one direction. The resistance to flow in the opposite direction is independently adjustable by means of a similar assembly 82 connected in the opposite direction between the passages 79 and 80. Figure 4 shows a flow chart of the computing operations carried out by the computer. It will of course be appreciated that the arrangement shown in the drawings may be used where the wheels 2 are the front, steered wheels and the wheels 3 are the rear wheels.

Claims: A vehicle suspension system for a vehicle having a structure supported on at least two pairs of wheels (2,3) by means of a liquid-filled telescopic suspension strut comprising a piston slidable in a barrel (6,7) for each wheel of first and second pairs of wheels, first and second valves (35) controlled according to signals from sensors (26) responsive to the vertical position of respective wheels (2) of a first wheel pair to admit liquid under pressure or release liquid to or from a chamber between said piston and barrel of each of the respective struts of the first wheel pair to resist vehicle roll, a third valve (33) controlled according to signals from sensor means responsive to the vertical position of the structure relative to the second pair of wheels (3) to admit or release liquid under pressure to or from a first chamber (13) between said piston and barrel of each of the struts (7) of the second pair, the struts (7) of the second pair each having a second liquid chamber (10) between said piston and barrel the volume of which varies with up-and-down movement, each such second chamber (10) being in communication with the chamber (8) of a strut (6) of the first pair so that the pressure in said second chamber (10) of each strut of the second pair varies with the pressure in the chamber (8) of one of the struts of the first pair, the arrangement being such that the changes of pressure in said second chambers (10) produce roll resistance at the second pair of wheels (3) in the same sense as the roll resistance produced at the first pair of wheels (2), wherein the system includes means for controlling the flow rate through each valve (33,35) to or from the associated struts in such a manner that the direction of flow is such as to restore the associated strut or struts to their datum positions, wherein the flow rate to each of the struts (6) of the first pair of wheels is proportional to the sensed departure of the respective piston of each of the struts (6) from its datum position, characterised in that each strut has a sensor (26) associated therewith for indicating the amount and sense of departure of the piston from a datum position and in that the flow rate to the struts (7) of the second pair of wheels (3) is controlled in such a manner that it is proportional to the algebraic sum of the sensed departures of the respective pistons of each of the struts (7) from its datum position. A suspension system according to claim 1 and including damping means for the suspension, means (28) for monitoring and assessing the instantaneous lengths of the struts sufficiently frequently to thereby assess the damping characteristics of the suspension, means for comparing the assessed damping characteristics with predetermined values and means to reduced differences between the assessed and predetermined damping. A suspension according to claim 1 or 2, wherein pressure sensors (41,42) are arranged to sense the pressures in the struts, and the flow rate control means are arranged to modify the flow rates through the valves (33,35) to compensate for changes in radial compression of the vehicle tyres corresponding to changes in the sensed strut pressures. A suspension system according to any of claims 1 to 3, wherein the sensors (10) are electrical sensors for producing and delivering to a computer (28) an output signal representative of the length of the respective strut (6,7), and the said valves (33,35) are electrically controlled hydraulic proportional servo-valves controlled by the computer (28). A suspension system according to claim 4, wherein the sensors (10) comprise potentiometers within the struts.

Inventors: DERNIE, FRANK WILLIAM
Applicants: WILLIAMS GRAND PRIX ENGINEERING LIMITED
Application year: 1988
Main classification: B60G17
IPC codes: B60G21, B60G17