GB2171372A

GB2171372A - A power assisted vehicle steering system

Info

Publication number: GB2171372A
Application number: GB08524279A
Authority: GB
Inventors: Frederick John Adams
Original assignee: TRW Steering Systems Ltd
Current assignee: TRW Steering Systems Ltd
Priority date: 1985-02-20
Filing date: 1985-10-02
Publication date: 1986-08-28
Also published as: JPS61191473A; GB8524279D0; GB8524278D0; GB2171372B; GB8504373D0; GB2171371A; GB2171371B

Abstract

A power assisted vehicle steering system has a rack and pinion steering gear 6 incorporating a servo motor 6a controlled by a valve 22. A fluid pressure supply line 3 to the valve 22 includes an axially adjustable spool 5a of a regulating valve 5 which is adjustable by speed responsive means 19, (which may be a solenoid or stepping motor) to increase the restriction to fluid flow to the valve 22 as vehicle speed increases. A flow control valve 7 is responsive to fluid pressure in the line 3 upstream of the valve 5 and to fluid pressure in the line 3 downstream of the valve 5 so that a pressure differential across the valve 5 is sensed by spool 9 which is displaced to open the supply line 3 upstream of the valve 5 and a reservoir 1. This latter opening reduces the supply of pressure fluid available for actuating the servo motor 6a and the available power assistance as the vehicle speed increases. The spool 5a has opposed faces in a recess 5b (which recess throttles flow in the line 3) and fluid pressure on these faces provide opposed forces which balance the spool axially with respect to its adjustment by the speed responsive means 19. <IMAGE>

Description

1 GB 2 171 372 A 1

SPECIFICATION

A power assisted vehicle steering system Technical field & background art

This invention relates to a power assisted vehicle steering system. It has hitherto been proposed and is now recognised as a desirable feature of power assisted vehicle steering that the power assistance 75 which is provided should decrease as vehicle speed increases and vice versa. By this proposal the steering system may provide very little or no power assistance when the vehicle is at high speed and full power assistance will be provided when the vehicle is at low speed, such as during a park ing manoeuvre. Conventionally, a power assisted steering system has a steering gear, a fluid pres sure operated servo motor device for the gear, a control valve controlling fluid pressure to the servo 85 motor device in response to a steering manoeuvre whereby the servo motor responds to assist that manoeuvre, and a fluid pressure supply to the con troi valve, this supply usually being hydraulic fluid pressurised by a pump. A widely accepted manner 90 of reducing power assistance as vehicle speed in creases is to provide the control valve for the servo motor device with a means that is responsive to vehicle speed and reacts to increase the resistance of the control valve to operate in response to a steering manoeuvre as the vehicle speed increases - an example of this proposal is to be found in our G.B. Specification No. 1,465.901. By further propos als, as disclosed for example in Japanese Specifi cation No. 5871262 and G.B. Specification

2,014,795, the fluid pressure supply to the control valve is determined by a regulating or throttle valve which is responsive to vehicle speed whereby, for example, as disclosed in G.B. Specifi cation No. 1,393,046 the fluid pressure within the 105 servo motor is "drained off" to reduce the power assistance as vehicle speed increases. These prior proposed systems have suffered from problems of complexity, reliability, size and cost. More particu larly, where the aforementioned regulating valve 110 throttles the supply of fluid under pressure to the control valve, the accuracy and consistency in ad justment of the regulating valve by a speed re sponsive means may be impaired by pressure variations on that valve which result from the fluid 115 supply during the adjustment, these pressure vari ations can adversely affect the characteristics re quired of the speed responsive control for the regulating means, particularly if the speed respon sive control is a solenoid and the regulating valve 120 is arranged as disclosed in G.B Specification

2,014,795. It is an object of the present invention to alleviate the above mentioned disadvantages in the provision of a power assisted vehicle steering sys tem of the kind generally discussed above. 125 Statement of invention & advantages According to the present invention there is provided a power assisted vehicle steering system which comprises a steering gear; fluid pressure operated servo motor 130 means for said gear; a control valve controlling fluid supply for said servo motor means in response to a steering manoeuvre whereby said servo motor means responds to assist that man- oeuvre; a fluid pressure supply to said control valve, the control valve being of the open centre type having a neutral condition in which the fluid pressure supply communicates through the valve with low pressure or reservoir; a regulating valve in the fluid pressure supply, the regulating valve being controlled by vehicle speed responsive means which reacts to adjust the regulating valve to decrease the flow of pressurised fluid to the control valve as vehicle speed increases and vice versa; a flow control valve which is responsive to a fluid pressure differential upstream and downstream of the regulating valve in the fluid pressure supply and reacts in response to a predetermined pressure differential to provide communication between the fluid pressure supply upstream of the regulating valve and low pressure or reservoir, and wherein the regulating valve is arranged so that, during adjustment thereof, fluid pressure forces to which that valve is subjected from the fluid pressure supply are balanced with respect to the control of the valve by the speed responsive means whereby the adjustment of the regulating valve by the speed responsive means is substan- tially independent of fluid pressures in the fluid pressure supply.

Both the regulating valve which controls the fluid pressure supply to the control valve of the servo motor and the regulating valve can be of very simple, reliable and compact construction such as spool valves; of particular note however is that the forces to which the regulating valve is subjected (during its adjustment) from fluid pressure in the supply are balanced with respect to the speed responsive control so that, unlike the proposal in G.B. 2,014,795, the adjustment of the regulating valve by the speed responsive means is achieved independently of the fluid pressure variations in the supply. Consequently the accuracy in the control afforded by the speed responsive means can be determined solely in accordance with the characteristics of that means. Preferably the regulating valve comprises a throttle member which is adjustable axially by the speed responsive means progressively to open or close the fluid pressure supply for the servo motor means as the vehicle speed progressively decreases or increases respectively, the throttle member having axially opposed faces which are subjected to fluid pressure in the fluid pressure supply during adjustment of the regulating valve and the fluid pressure on the opposed faces provide axially opposed faces to maintain the throttle member balanced axially with respect to the speed responsive means.

The regulating valve (or the aforementioned throttle member thereof) may be solenoid controlled or controlled by a stepping motor (in each case an appropriate electrical system including a vehicle speed sensor being provided). By use of a stepping motor to provide control of the regulating 2 GB 2 171 372 A 2 valve that valve can be adjusted incrementally and accurately consistent with successive steps of the motor as the vehicle speed increases or decreases. Preferably the or a throttle member of the regulat- ing valve is adjustable axially to open or close the fluid pressure supply as appropriate and the throt tle member is controlled in its axial adjustment by incremental rotation of an output from the step ping motor.

The flow control valve is preferably connected in 75 parallel with the fluid pressure supply to bridge the regulating valve so that a displaceable member of the flow control valve is responsive to the pressure differential in the fluid pressure supply upstream and downstream of the regulating valve and that 80 valve member is displaced in response to a prede termined pressure differential to open, or increase, communication between the fluid pressure supply upstream of the regulating valve and low pressure return or exhaust. Preferably the flow control valve 85 responds to progressively open communication between the fluid pressure supply and the low pressure return or exhaust as aforementioned. The predetermined opening characteristics of the flow control valve may be determined, at least in part, 90 by the preloading of that valve, for example by ap propriate spring or other biasing means. It will be apparent from the aforegoing that as the regulat ing valve closes to reduce the fluid supply which is available to actuate the servo motor there will be 95 an effective decrease in the power assistance avail able at that time. Simultaneously with the regulat ing valve closing, a pressure differential will develop across the flow control valve to open, or increase, the communication between the fluid supply upstream of the regulating valve and the low pressure return or reservoir so that a propor tion of the fluid supply will be directed to the low pressure return or reservoir while the remaining proportion is directed through the regulating valve 105 to be available to operate the servo motor means.

This latter proportion is determined as necessary to provide the required characteristics of the servo motor means for the particular vehicle speed and characteristics of the regulating valve.

Drawing One embodiment of a power assisted vehicle steering system, constructed in accordance with the present invention, will now be described, by way of example only, with reference to the ac companying illustrative drawings in which:

Figure 1 diagrammatically shows the system ap plied with a conventional rack and pinion steering gear and utilising a solenoid control, and Figure 2 shows a modification of the system in 120 Figure 1 utilising a stepping motor control.

Detailed description of drawings Hydrauliefluid from a reservoir 1 is pressurised by a pump 2 and directed by way of a supply conduit 3 to a control valve 22 of a rack and pinion steering gear 6. The gear 6 and control valve 22 may be of conventional type, the former having a double acting piston and cylinder type servo motor 6a to which hydraulic fluid is directed from the conduit 3 by the control valve 22 to provide assistance to the steering manoeuvre. The control valve 22 is of the open centre type having a neutral condition when a steering effort is not applied thereto in which the fluid pres- sure supply from conduit 3 is directed to a low pressure return conduit 23 which communicates with the reservoir 1. The control valve 22 will usually be of the rotary type having relatively rotatable valve components which are spring loaded relative to each other to be self centralising to the neutral condition as discussed, for example, in our G. B. Specifications Nos. 1,465,901 and 1,603,198.

Located in the supply conduit 3 is a regulating valve 5 comprising an axially displaceable spool 5a having an annular recess 5b which is adjustable relative to the conduit 3 to vary the restriction to the flow of hydraulic fluid through the supply conduit. It will be noted that the annular recess 5b of the spool has axially opposed faces of equal area and that the variations in restriction to fluid flow by the valve 5 is effected solely through the recess 5b; accordingly the variations in fluid pressure from the supply conduit and on the spool 5a within the recess 5b are applied equally and oppositely to the aforementioned opposed faces so that the spool is balanced axially as regards the effect of fluid pressure thereon in the conduit 3. The force necessary to displace the spool 5a is therefore independent of any differential in pressure which develops in the conduit 3 upstream and downstream of the valve 5. The spool 5a is displaceable under control of a solenoid 19 connected to a controller 20 which responds to signals from a sensor device 21 that is responsive to the speed of a vehicle within which the system is incorporated. In addition to vehicle speed, the sensor device 21 may be responsive to parameters other than vehicle speed such as lateral acceleration (which may be taken from the speed of rotation applied to effect a steering input through a steering wheel) and other characteristics which may affect the desirability for a variation in available power assistance at a particular instant. The controller 20 is arranged to control the spool 5a so that when the vehicle is stationary or slow moving the restriction by the regulating valve to fluid flow through the conduit 3 is at a minimum (thereby providing maximum fluid pressure supply to the control valve 22 for actuation of the servo motor 6a) and when the vehicle is at high speed the spool 5a is adjusted to provide a maximum restriction to fluid flow through the con duit 3 (thereby providing a minimum or no fluid pressure supply to the control valve 22 and mini mum or no power assistance facility from the servo motor). Preferably the adjustment of the spool 5a by the controller 20 between the afore mentioned minimum and maximum restrictions is progressive as the vehicle speed progressively in creases.

Communicating with the supply conduit 3 to be responsive to a fluid pressure differential devel oped in the supply conduit upstream and down stream of the regulating valve 5 is a flow control valve 7. The valve 7 has a housing 8 within which is located a displaceable spool cup 9. Located in 3 GB 2 171 372 A 3 the housing 8 on one side of the cup 9 is a fluid chamber 10 and on the other side of the cup a spring chamber 11. A spring 1 la is located in the chamber 11 to bias and preload the cup 9 in a sense which contracts the chamber 10. Displace ment of the spool cup 9 in the housing 8 controls the opening and closing of an annular port 12 to a conduit 13 which communicates with the reservior 1 by way of conduit 23. The chamber 10 communi cates by way of a conduit 14 with the supply con duit 3 upstream of the regulating valve 5 and the spring chamber 11 communicates by way of a con duit 15 with the supply conduit 3 downstream of the regulating valve 5.

During use of the system as above described and with the vehicle stationary or at low speeds when maximum power assistance is required for a steering manoeuvre, the spool 5a will present the minimum restriction to the flow of fluid in the sup ply conduit 3. There will therefore be a negligible 85 pressure differential between the upstream and downstream sides of the regulating valve 5 and as sensed through the conduits 14 and 15 so there will be little or no pressure differential on the spool cup 9 and the spool will be maintained by the spring 11 a to close the port 12. Consequently, the output from pump 2 is directed solely through the conduit 3 to be available for providing power as sistance by the servo motor 6a. As the speed of the vehicle increases the solenoid controlled spool 95 5a is adjusted axially to progressively close com munication between the annular recess 5b and the fluid in conduit 3 and thereby to increase the re striction to flow of fluid through the supply conduit 3. This restriction to flow results in a pressure dif- 100 ferential developing between the upstream and downstream sides of the regulating valve 5 in the supply conduit 3. As a consequence the spool cup 9 is subjected to a fluid pressure differential be tween the chambers 10 and 11; this will result in a 105 pressure force on the cup 9 that progressively in creases until it overcomes the biasing effect of the spring 1 la and displaces the cup 9 against the spring biasing and opens the chamber 10 to com munication with the low pressure return conduit 110 13. In this latter condition a proportion of the hy draulic fluid from the pump 2 by-passes the control valve 22 and is directed by way of conduits 14 and 13 back to the reservoir 1. The output of the pump 2 is preferably arranged to be constant irrespective 115 of vehicle speed or vehicle engine speed, so that the reduction in the fluid which is available through the supply conduit 3 for actuating the servo motor will decrease as the restriction pre sented by the regulating valve 5 increases with the 120 increase in vehicle speed and thereby there will be an effective reduction in the power assistance which is provided by the servo motor.

During the aforementioned axial displacement of the spool 5a to vary the restriction to fluid flow in the conduit 3, the effect of the pressure variations in the conduit 3 are balanced within the spool re cess 5b as previously discussed; consequently the pressure variations in the conduit 3 are without ef fect on the control exhibited by the solenoid 19 and on the spool 6a so that the accuracy and characteristics of that control are independent (to a practical extent) of pressure in the supply conduit 3.

In the modification shown in Figure 2 the axial displacement of the spool 5a for varying the re striction to fluid flow through the supply conduit 3 is achieved by an electrical stepping motor 30 (rather than by the solenoid 19 of Figure 1). The motor 30 may be of conventional type (as will be well known to persons skilled in the art) which rotationally drives a screw spindle 31 in response to signals from the controller 20. In response to signals from the controller, the stepping motor will rotate the spindle in one or the opposite directions and through successive angular displacements or steps with considerable accuracy. The spindle 31 screw threadedly engages within an axially extending screw threaded blind bore 32 in an end of the spool 5a and the spool is restrained from rotating within its cylinder by a pin 33 in the valve housing which sliclably engages within an axial track 34 in the cylindrical surface of the spool. Consequently when the motor 30 is stepped to rotate the spindle 31, the spool 5a will be driven axially relative to its cylinder and in the appropriate direction to increase or decrease the restriction to fluid flow through the conduit 3 which is presented by the annular recess 5b in communicating to that conduit. In use the vehicle speed sensor device 21 and controller 20 will impart signals to the stepping motor 30 in accordance with vehicle speed to cause the motor to step and rotate the spindle and thereby drive the spool incrementally to a position of the valve 5 which is predetermined for the prevailing vehicle speed. For example, if the vehicle is capable of 100 m.p.h. the controller 20 may cause the motor 30 to step successively and incrementally for each increase in vehicle speed of 10 m.p.h. so that the motor will have ten steps each step, say, rotating the spindle 31 through half a revolution; the aforementioned five revolutions of the spindle 31 will adjust the valve 5 from a minimum restriction in the conduit 3 (possibly where the recess 5b is fully open to the conduit 3) to a maximum restriction in the conduit 3 (possibly when the conduit 3 is closed by the full diameter of the spool 5a) to provide a variation in power assistance progressively and incrementally from full assistance to no assistance (and vice versa as the vehicle speed decreases). The axially balanced fluid forces on the spool 5a within the recess 5b (as previously discussed) together with the precision which is available in displacing the spool 5a by the stepping motor through the engaging screw threads permits extremely accurate control of the fluid flow throttling effect within the supply conduit 3 in accordance with vehicle speed.

Claims

1. A power assisted vehicle steering system which comprises a steering gear; fluid pressure operated servo motor means for said gear; a con130 trol valve controlling fluid supply for said servo 4 GB 2 171 372 A 4 motor means in response to a steering manoeuvre whereby said servo motor means responds to as sist that manoeuvre; a fluid pressure supply to said control valve, the control valve being of the open centre type having a neutral condition in which the fluid pressure supply communicates through the valve with low pressure or reservoir; a regulating valve in the fluid pressure supply, the regulating valve being controlled by vehicle speed responsive means which reacts to adjust the regu- 75 lating valve to decrease the flow of pressurised fluid to the control valve as vehicle speed in creases and vice versa; a flow control valve which is responsive to a fluid pressure differential up stream and downstream of the regulating valve in 80 the fluid pressure supply and reacts in response to a predetermined pressure differential to provide communication between the fluid pressure supply upstream of the regulating valve and low pressure or reservoir, and wherein the regulating valve is 85 arranged so that, during adjustment thereof, fluid pressure forces to which that valve is subjected from the fluid pressure supply are balanced with respect to the control of the valve by the speed re sponsive means whereby the adjustment of the regulating valve by the speed responsive means is substantially independent of fluid pressure in the fluid pressure supply.

2. A system as claimed in claim 1 in which the regulating valve comprises a throttle member which is adjustable axially by said speed respon sive means progressively to open or close the fluid pressure supply for the servo motor means as the vehicle speed progressively decreases or increases respectively, said throttle member having axially opposed faces which are subjected to fluid pres sure in the fluid pressure supply during adjustment of the regulating valve and the fluid pressure on said faces provide axially opposed forces to main tain the throttle member balanced axially with re spect to the speed responsive means.

3. A system as claimed in either claim 1 or claim 2 in which the adjustment of regulating valve is controlled by an electrical system which corn prises a vehicle speed sensor.

4. A system as claimed in claim 3 in which the regulating valve is a solenoid controlled spool valve.

5. A system as claimed in claim 3 in which ad justment of the regulating valve is incremental and controlled by an electrical stepping motor.

6. A system as claimed in claim 5 in which the regulating valve comprises the or a throttle mem ber which is adjustable axially to open or close the fluid pressure supply for the servo motor means as the vehicle speed progessively decreases or in creases respectively, axial adjustment of said throt tle member being controlled by incremental rotation of an output from said stepping motor.

7. A system as claimed in claim 6 in which the 125 throttle member is restrained from axial rotation and the output from the stepping motor screw threadedly engages said throttle member so that rotation of the output during stepping of the motor drives the throttle member axially for its adjust- 130 merit.

8. A system as claimed in any one of the preceding claims in which the flow control valve is connected in parallel with the fluid pressure supply to bridge the regulating valve and has a displaceable member which is responsive to the pressure differential in the fluid pressure supply upstream and downstream of the regulating valve, said valve member being displaced in response to said pressure differential to open, or increase, communication between the fluid pressure supply upstream of the regulating valve and low pressure return or exhaust.

9. A system as claimed in claim 8 in which biasing means is provided to preload the displaceable member against its displacement in a sense to open, or increase, said communication between the fluid pressure supply upstream of the regulating valve and low pressure return or exhaust.

10. A system as claimed in claim 6 in which said biasing means comprises spring means which preloads the displaceable member relative to a housing for that member.

11. Asystem as claimed in anyone of claims 8 to 10 in which the displaceable member partly defines a first fluid chambe.. which communicates with the fluid supply upstream of the regulating valve and partly defines a second fluid chamber which communicates with the fluid supply down- stream of the regulating valve, said fluid chamber being alternately expanded and contracted respectively during displacement of the displaceable member in response to variations in said pressure differential and said displaceable member control- ling the opening and closing of a port to said first fluid chamber through which port that first fluid chamber can communicate with the low pressure return or exhaust.

12. A system as claimed in any one of the pre- ceding claims in which the flow control valve responds to progressively open communication between the fluid pressure supply and the low pressure return or exhaust.

13. A power assisted vehicle steering system as claimed in claim 1 and substantially as herein described with reference to Figure 1 of the accompanying illustrative drawings.

14. A system as claimed in claim 13 and modified substantially as herein described with refer- ence to Figure 2 of the accompanying illustrative drawings.

Printed in the UK for HMSO, D8818935, 7186, 7102. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.