VEHICLE STEERING SYSTEM

Abstract: A road vehicle has steerable front and rear wheels, the rear wheels being steered by an electro-hydraulic system responsive to the manual control input to the front wheels. The hydraulic actuator (20) of the system is biased to the straight-ahead steering position by springs (26), so that the rear wheels are centred in the event of electric or hydraulic failure, and an hydraulically restrained latch element (52) is spring biased to then latch the actuator piston (22) in its centred position.

Description: The invention relates to a vehicle steering system particularly suited for use in a vehicle having front and rear wheel steering. Road vehicles of which the rear wheels can be steered as well as the front wheels in many instances employ quite complicated control arrangements for the rear wheel steering gear. Such arrangements can be responsive to a variety of factors, in addition to the steering angle applied by the driver to the front wheel steering gear, for example, the vehicle body movement as sensed by one or more inertial sensors, and electrical inputs derived from such factors are treated in a computer which provides an electric command signal which is converted, for example, electro-hydraulically, into a wheel steering movement. These arrangements are necessarily subject to the possibility of failure and the invention is concerned with the provision of means for minimising or eliminating the adverse effects of such failure. There is thus known from JP-A-59 128 054, a steering control system for a vehicle, comprising a piston assembly having an actuator piston movable within an actuator cylinder and tie rods extending from the piston in opposed directions outwardly of the cylinder for connection to steerably mounted wheels at spaced locations on opposite sides of the vehicle, whereby movement of the actuator piston effects steering movement of the wheels, a servo-valve responsive to a control signal to supply pressure fluid from a pressure fluid source to the actuator cylinder to effect a desired steering position of the wheels, and springs acting to bias the piston assembly to a piston position corresponding to a straight-ahead position of the wheels. According to the present invention there is provided in such a system latch means biassed by a spring to engage and hold the piston assembly in the straight-ahead position, the latch means being held by fluid pressure from the pressure fluid source out of engagement with the piston assembly. In the event of a system failure, the springs biassing the piston urge the system to its central position, so that a vehicle with front and rear wheel steering which incorporates the system for the rear wheels can still be steered, by means of the front wheels only, by way of the conventional direct or power assisted control normally provided, and the latch means latches the system in the central steering position. The steering orientation of the rear wheels is then unaffected for example by road conditions, so that steering by way of the front wheels can be affected wholly as for a conventional vehicle. The invention is further explained below, by way of illustration, with reference to the accompanying drawing, in which: Figure 1 is a schematic plan view of the steering arrangements of a road vehicle having a steerable front and rear wheels in accordance with the invention; Figure 2 is a schematic view showing in detail the rear wheel steering system of the vehicle of Figure 1, the system being shown in a normal operating condition; Figure 3 is a view similar to that of Figure 2 of an alternative form of rear wheel steering system for the vehicle of Figure 1; and Figure 4 is a like view of part of Figure 3 but showing the system in locked position. The front wheels 2 of the road vehicle of Figure 1, which may be for example a passenger motor car, are steerable conventionally. Thus, a steering wheel 4 acts on steering tie rods 5 through a suitable connection box 6 and the steering column 7. A transducer 10 is connected to the steering column 7 so as to measure the steering angle and the transducer output is supplied to a computerised control unit 11 which in turn provides appropriate control signals to an electro-hydraulic rear wheel steering system 12 of which details appear from Figure 2. As well as or instead of the input from the transducer 10, the control unit 11 may have any suitable input or inputs, for example, from a vehicle speed sensor and/or lateral acceleration or yaw angular velocity sensors. The rear wheel steering system 12 comprises a hydraulic actuator 20 in the form of a double-acting hydraulic cylinder 21 containing a piston 22. From the opposed ends of the piston 22, tie rods 24 extend through apertures in the cylinder end walls for mechanical application to the rear wheels 25 of a steering effect in accordance with the piston position. Externally of the actuator 20, helical compression springs 26 around the tie rods 24 act between the cylinder end walls and discs 27 carried by the tie rods, to exert a centering bias on the piston/tie rod assembly. The interiors of the cylinder 21, on either side of the piston 22, are connected with hydraulic fluid pressure and return lines 30 and 31, which extend from a suitable hydraulic pressure source (not shown), through a servo-valve 32. The servo-valve 32 is responsive to signals from the control unit 11 to control the application of hydraulic fluid pressure to the two sides of the actuator piston 22, to effect the required steering action. The electro-hydraulic steering system 12, incorporates fail-safe means whereby the rear-wheel steering is centralised and locked in the centre position on system failure. Thus, there is connected in the hydraulic pressure line 30 and to the return line 31, a solenoid valve 35 which is biassed to the closed position and is energised under control of the control unit 11 during normal operation of the system so as to assume its open position and thus allow fluid pressure to be applied to the actuator 20. Also, a by-pass valve 36, biassed to assume its open position, is located in a by-pass path 39 connecting the pressure lines extending between the servo-valve 32 and the respective working volumes of the actuator cylinder 21. The pressure line 30 is connected to the by-pass valve 36 by a line 40 and the fluid pressure retains this valve in its closed condition during normal operation of the system. Because the solenoid valve 35 is held open during normal operation of the system under control of the control unit 11, it will close in the event of any failure of the electrical supply, or of the control unit, so the actuator 20 and the by-pass valve 36 are cut off from fluid pressure. Failure of hydraulic pressure can be arranged to cause the valve 35 to close also. Additionally, signals are supplied to the control unit 11 by two position sensing transducers 49, in the form of linear variable displacement transformers, which are connected between a central external flange on the cylinder 21 and respective ones of the discs 27. The control unit 11 is arranged to effect closure of the solenoid valve 35 if the transducers 49 supply inconsistent outputs or if the actuator piston position is not in accordance with the position commanded by the unit. Closure of the solenoid valve 35 or loss of pressure in the line 40 for any other reason results in a pressure drop to the by-pass valve 36, enabling this to open to connect together the two working chambers of the actuator cylinder 21. The cylinder 22 is thus able to move freely, and the compression springs 26 will centralise the cylinder position and thus that of the wheels 25. The illustrated rear wheel steering system of the invention also includes means whereby the steering actuator 20 is then positively locked in its central position. The actuator piston 22 is thus provided with axially central peripheral groove 50, and the cylinder 21 has a cylindrical extension portion 51 extending from the axial centre of the cylinder with its axis perpendicular to the cylinder axis. A lock pin 52 is slidably received within the extension portion 41 and is urged by a compression spring 54 acting between the remote end of the pin and the end wall of the extension portion to move into the cylinder. The extension portion 51 is however in communication with the pressure line 30 through a branch line 55, and the fluid pressure normally holds the lock pin 52 out of the main cylinder, with the spring 54 compressed. Loss of pressure in the branch line 55 due for example to closure of the solenoid valve 35 causes the pressure in the cylinder extension portion 51 to drop to a level at which it is no longer sufficient to overcome the force of the spring 54 which will consequently urge the pin 52 into the groove 50 in the piston, so that the steering tie rods are locked against movement. The inner end of the pin 52 is tapered, to facilitate entry of the pin into the groove 50, the side walls of which are correspondingly frusto-conical, so that entry of the pin has a centering effect. Thus, closure of the solenoid valve 35 results in prompt centralisation and locking of the rear wheel steering system. When the solenoid valve 35 is subsequently energised, hydraulic pressure reaches both the by-pass valve 36 and the cylinder extension portion 51, to close the by-pass valve and to return the steering lock pin 52 to its withdrawn position, thereby enabling resumed control of the rear wheel steering by way of the servo valve 32 and actuator 20. In the modified rear wheel steering system 60 shown in Figures 3 and 4, parts identical to those of the system 12 shown in Figure 2 are indicated by the same reference numerals. The system 60 comprises a double-acting hydraulic actuator 61 with a cylinder 62 within which slides a piston member 64 having tie rod portions 65 at its outer ends which protrude externally of the cylinder ends. A sealing portion 66 of the piston member 64 separates the cylinder interior into two parts communicating with the pressure and return lines 30, 31 under control of the servo-valve 32. At each end, the actuator 61 mounts a housing 70 containing a compression spring 71 acting between the adjacent actuator end and a washer 72 trapped within the housing by an outer end flange thereof and acted upon by the associated tie rod portion 65 by way of a flange 74. The springs 71 thus exert a centering force on the piston member 64, tending to return the system to the central or straight-ahead condition shown in Figure 3. Linear variable displacement transformers 75 extend between the right hand housing 70 of the actuator 61 and a side arm 76 on the associated tie rod 65. These transformers 75 constitute an alternative to the transformers 49 of Figure 2 for providing to the control unit 11 signals representing tie rod position. Like the system 12, the system 112 incorporates means for releasably locking the system in the straight-ahead condition on system failure. Adjacent the right-hand spring housing 70, the actuator 61 has a sleeve portion 80 defining an annular chamber 81 extending around the cylinder 62. The chamber 81 has a part adjacent the sleeve 70 by which it normally receives fluid pressure by way of the branch line 55 extending from the pressure line 30. A sleeve piston 82 is slidably received around the cylinder 62, with one end extending into the annular chamber 81 to be acted upon by the fluid pressure therein. The opposite end of the piston 82 protrudes from the chamber 81 and is acted upon by a compression spring 84 retained by a flange 85 at the exterior of the cylinder 62. The chamber 81 communicates with interior of the cylinder 62 by means of a series of holes in the cylinder wall through which balls 87 can move radially between the chamber 81 and an annular groove 89 provided in the piston member 64 in sliding engagement with the cylinder wall. The sleeve piston 82 carries a radially projecting side arm 90 externally of the chamber 81 by which it controls the position of the by-pass valve 36, in the by-pass 39 which is provided with a retraction 92. In the position shown in Figure 3, pressure in the branch line 55 acts on the sleeve piston 82 to urge it leftwards, compressing the compression spring 84. The piston member 64 can then move to left or right by action of fluid pressure on the two sides of the sealing portion 66 under control of the servo-valve 32, the balls 87 being urged outwardly from the groove 89, into the chamber 81 by the sides of the groove to permit the piston member movement. In this position of the piston 82, the side arm 90 keeps the by-pass valve 36 in its closed condition. If the fluid pressure supply should fail for example because of closure of the solenoid valve 35, the compression spring 84 is able to move the sleeve piston 82 to the right, its inner end being shaped to then urge the balls 87 radially inwardly. The side arm 90, in this position of the piston 82, causes the by-pass valve 36 to open, so that the piston member 64 is free to move under the centering effect of the springs 71, the rate of return to center being regulated by the restriction 92. When the center position, shown in Figure 4, is reached the balls 87 enter the groove but still engage the edges of the holes in the cylinder wall, so that the piston member 64 is latched to the cylinder 62. The slot and groove 89 are so disposed that the system is then in its central or straight-ahead condition. When fluid pressure is restored, pressure in the branch line 55 moves the sleeve piston 82 back to the position shown in Figure 3 so that normal operation can recommence. The invention can of course be embodied otherwise than as specifically described.

Claims: A steering control system for a vehicle, comprising: a piston assembly having an actuator piston (22;64) movable within an actuator cylinder (21;80) and tie rods (24;65) extending from the piston in opposed directions outwardly of the cylinder for connection to steerably mounted wheels (25) at spaced locations on opposite sides of the vehicle, whereby movement of the actuator piston effects steering movement of the wheels, a servo-valve (32) controlling the supply of a pressure fluid from a pressure fluid source (P) to the actuator cylinder to effect a desired steering position of the wheels, and springs (26;71) acting to bias the piston assembly to a piston position corresponding to a straight-ahead position of the wheels, characterised by latch means (52;87) biassed by a spring (54;84) to engage and hold the piston assembly in the straight-ahead position, the latch means being held by fluid pressure from the pressure fluid source (P) out of engagement with the piston assembly. A system as claimed in claim 1 wherein the latch means comprise a spring (54) biassing a latch pin (52) to engage a formation (50) of the piston (22) located between the surfaces thereof on which the pressure fluid controlled by the servo-valve (32) acts, the fluid pressure of the pressure fluid source (P) being applied to the latch pin to hold the latch pin spaced from the piston. A system as claimed in claim 2 wherein at least one of the latch pin (52) and the piston formation (50) has a tapered configuration whereby movement of the latch pin to engage the piston urges the piston into the straight-ahead position thereof. A system as claimed in claim 1 wherein the latch means comprises a latch piston (82) movable in a latch cylinder (80) and held by fluid pressure from the pressure fluid source (P) in a first position, a spring (84) biassing the latch piston to a second position, and balls (87) held by the latch piston in the second position thereof in latching engagement with the actuator piston (64). A system as claimed in claim 4 wherein the balls (87) are movable by the actuator piston (64) out of the latching engagement when the latch piston (82) is in its first position. A system as claimed in claim 4 or 5 wherein the latch piston (82) and cylinder (80) are annular and surround the actuator piston (64), and wherein the balls are receivable in an annular groove (89) in the actuator piston. A system as claimed in any preceding claim having a by-pass (39) by-passing the supply of the pressure fluid from the servo-valve to the actuator cylinder, a valve (36) in the by-pass biassed to its open position, and means applying fluid pressure from the pressure fluid circuit to hold the by-pass valve in its closed position. A system as claimed in claim 4, 5 or 6 having a by-pass (39) by-passing the supply of the pressure fluid from the servo-valve to the actuator cylinder, and a by-pass valve (36) in the by-pass, the by-pass valve being held closed by the latch piston (82) in the first position thereof and being held open by the latch piston in the second position thereof. A system as claimed in any preceding claim having a control unit (11) for supplying the control signal in response to inputs dependent on at least one vehicle condition. A system as claimed in claim 9 wherein a solenoid valve (35) is biassed to a position to interrupt pressure fluid supply to the servo valve (32) and is maintained in a position to supply pressure fluid by a signal from the control unit (11). A system as claimed in claim 9 or 10 having sensors (49;75) for supplying to the control unit (11) inputs dependent on the steering position for comparison with predetermined or command dependent information. A system as claimed in claim 11 wherein the sensors comprise position sensing transducers (49;75) connected between the actuator cylinder (21;80) and the tie rods (24;65). A vehicle comprising manually controllable steerable front wheels (2), and steerable rear wheels (25) controlled by a steering system as claimed in claim 9, 10, 11 or 12 wherein the at least one vehicle condition comprises vehicle steering control to the steerable front wheels, vehicle speed, vehicle acceleration, or vehicle attitude.

Inventors: WILLIAMS, DAVID ALLEN, WRIGHT, PETER GODFREY
Applicants: GROUP LOTUS PLC
Application year: 1986
Main classification: B62D7
IPC codes: B62D5, B62D7