GB2189444A

GB2189444A - Improvements relating to power assisted steering

Info

Publication number: GB2189444A
Application number: GB08709936A
Authority: GB
Inventors: Alan Peter Ogburn
Original assignee: TRW Transportation Electronics Ltd
Current assignee: TRW Transportation Electronics Ltd
Priority date: 1986-04-26
Filing date: 1987-04-27
Publication date: 1987-10-28
Also published as: GB8709936D0

Abstract

A main pump (2) circulates hydraulic fluid through a variable power assist mechanism (4) which is responsive to vehicle speed so that assistance is smaller the higher the speed. This assistance is generated by hydraulic fluid branched off from the main circuit to act on a control valve (7) which moves in conjunction with a steering rack (10). The branch has its own continuously operating pump (14) and the amount of fluid passed to the control valve is determined by a modulating valve (17) which in turn is governed by an electric controller (18) with a vehicle speed input. <IMAGE>

Description

SPECIFICATION Improvements relating to power assisted steering This invention relates to power assisted steering .

In particular, it concerns assistance which varies with veh icle speed. It is generally accepted that when travelling at high speed down a motorway, for example, it is not really necessaryto have power steering, even in a large heavy car, but when parking or driving slowlythrough heavytraffic, it is advantageousto have such help.

This graduated form of assistance is known as a SpeedProportionalFeeI(SPF)system.

In one currentform, an SPF system has a steering pump which delivers hydraulicfluid to a variable power assist mechanism associated with the steering rack, there being a returnforthefluidtothe reservoir from which the pump draws. Thevariable power assist mechanism has valve means which are subject to fluid sunder pressure derived from this hydraulic circuit in a graduated manner dependent on vehicle speed, the fluid from the pump therefore being directed th roug h the va Ive means to assist the steering also in a speed related manner.Such a system will be referred to as an SPF system of the kind described.

Present practice is to use fluid bled off from the return to the reservoir. This is directed through a control unitwhich receives as an electrical inputa signal related to vehicle speed. The bled-off return fluid is pressurised in relation to this speed and delivered to the valve meansto stiffen the steering in a speed related manner.

Such a control unit is an expensive item, and it is the aim of this invention to provide a less costly alternative which could work equally well.

According to one aspect of the present invention there is provided an SPF system of the kind described wherein an auxiliary pump draws hydraulicfluidfrom the samesource asthe main steering pump and its output is delivered via pressure modulating valve means to thevariable power assist mechanism.

For a better understanding of the invention, one embodiment will now be descri bed, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a blockcircuitdiagram of a knownSPF system, and Figure2 is a biockcircuitdiagram of an SPF system according to this invention.

Figure 1 shows a known Speed Proportional Feel (SPF) power steering system. Many of the components will also befound in the embodiment of Figure 2 and the same references will be used there. The system has a reservoir 1 from which a pump 2 draws oil to circulateitvia a pipe 3 to a variable power assist mechanism 4 and thence backto the reservoir 1 via a pipe 5. When the system is idling, the mechanism 4 presents a small restriction to the flow of oil in the pipe 3, but when assistance is being supplied bathe mechanism 4, the pressure in the pipe 3 rises corresponding to the steering load. A pressure relief valve 6 in a bypass to the pump 2 prevents the pressure in the pipe 3 rising to a level which may cause damage to the system.

In the mechanism 4, the oil is delivered to a valve block 7 which can direct the pressurised oil through one orthe other of lines 8 and 9 to a steering rack 10, the opposite line then being a return communicating with pipe 5. When the steering rack is moved in one direction, this causes fluid to be admitted in the sense to help that movement. In an SPF system, the valve block7 is arrangedto give maximum assistance when the vehicle speed is zero, while as that speed increases the valve block is adjusted progressively to reduce the hydraulic assistance, thereby stiffening the steering.This response is achieved by a controller 11 which uses fluid bled off fromthereturn pipe5through branch 12. ltisan electrically operated device, powered from the vehicle's battery, whose main input is a signal representing vehicle speed. As indicated, there is a further input denoted "firm/soft" which is driver selectable so that the feel of the steering for speeds throughoutthe vehicle's range can be stiffor relaxed according to choice.The controller 11 delivers fluid to the valve block 7 via line 13 at a pressure reiatedto speed. Whenthevehicle is stationary the pressure will be zero, while at maximum speed it may riseto 5 5 x 105Pa. This increasing pressure has an effect on the valve system increasingly to reduce the hydraulic assistance and progressively change overto manual steering.

It would be possible to invert this arrangement so that maximum pressureis applied at zero speedand vice-versa,the effecton the steering still being to increase the stiffness as speed rises.

In Figure 2 a different approach is taken, and there is no branch offthereturn 5. Instead, the controlling fluid is supplied from a secondary pump 14, drawing direct from the reservoir 1 and having a bypass valve 15 corresponding to thevalve 6for the main pump 2. The pumps 2 and 14will conveniently be in a common housing and have a common drive, such as by belt from engine crankshaft.The output of the pump 14 is directed by line 16 to a modulating valve 17 governed byan electronic controller 18.Thishas inputssimilartothose ofthecontroller 11, and its action is to vary the setting of the valve 17 according to speed. However, as compared with the conventional form of controller 11, this arrangement has been found to be considerably more economical in the consumption of electrical power. Asthevalve 17 opens progressively more oil can be forced through line 18, thus stiffening the steering. When the valve 17 is closed the bypass valve 15 will allow fluid to circulate through the pump 14.

1. An SPF system of the kind described wherein an auxiliary pump draws hydraulicfluidfromthe same source as the main steering pump and its output is delivered via pressure modulating valve means to the variable power assist mechanism.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements relating to power assisted steering This invention relates to power assisted steering . In particular, it concerns assistance which varies with veh icle speed. It is generally accepted that when travelling at high speed down a motorway, for example, it is not really necessaryto have power steering, even in a large heavy car, but when parking or driving slowlythrough heavytraffic, it is advantageousto have such help. This graduated form of assistance is known as a SpeedProportionalFeeI(SPF)system. In one currentform, an SPF system has a steering pump which delivers hydraulicfluid to a variable power assist mechanism associated with the steering rack, there being a returnforthefluidtothe reservoir from which the pump draws. Thevariable power assist mechanism has valve means which are subject to fluid sunder pressure derived from this hydraulic circuit in a graduated manner dependent on vehicle speed, the fluid from the pump therefore being directed th roug h the va Ive means to assist the steering also in a speed related manner.Such a system will be referred to as an SPF system of the kind described. Present practice is to use fluid bled off from the return to the reservoir. This is directed through a control unitwhich receives as an electrical inputa signal related to vehicle speed. The bled-off return fluid is pressurised in relation to this speed and delivered to the valve meansto stiffen the steering in a speed related manner. Such a control unit is an expensive item, and it is the aim of this invention to provide a less costly alternative which could work equally well. According to one aspect of the present invention there is provided an SPF system of the kind described wherein an auxiliary pump draws hydraulicfluidfrom the samesource asthe main steering pump and its output is delivered via pressure modulating valve means to thevariable power assist mechanism. For a better understanding of the invention, one embodiment will now be descri bed, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a blockcircuitdiagram of a knownSPF system, and Figure2 is a biockcircuitdiagram of an SPF system according to this invention. Figure 1 shows a known Speed Proportional Feel (SPF) power steering system. Many of the components will also befound in the embodiment of Figure 2 and the same references will be used there. The system has a reservoir 1 from which a pump 2 draws oil to circulateitvia a pipe 3 to a variable power assist mechanism 4 and thence backto the reservoir 1 via a pipe 5. When the system is idling, the mechanism 4 presents a small restriction to the flow of oil in the pipe 3, but when assistance is being supplied bathe mechanism 4, the pressure in the pipe 3 rises corresponding to the steering load. A pressure relief valve 6 in a bypass to the pump 2 prevents the pressure in the pipe 3 rising to a level which may cause damage to the system. In the mechanism 4, the oil is delivered to a valve block 7 which can direct the pressurised oil through one orthe other of lines 8 and 9 to a steering rack 10, the opposite line then being a return communicating with pipe 5. When the steering rack is moved in one direction, this causes fluid to be admitted in the sense to help that movement. In an SPF system, the valve block7 is arrangedto give maximum assistance when the vehicle speed is zero, while as that speed increases the valve block is adjusted progressively to reduce the hydraulic assistance, thereby stiffening the steering.This response is achieved by a controller 11 which uses fluid bled off fromthereturn pipe5through branch 12. ltisan electrically operated device, powered from the vehicle's battery, whose main input is a signal representing vehicle speed. As indicated, there is a further input denoted "firm/soft" which is driver selectable so that the feel of the steering for speeds throughoutthe vehicle's range can be stiffor relaxed according to choice.The controller 11 delivers fluid to the valve block 7 via line 13 at a pressure reiatedto speed. Whenthevehicle is stationary the pressure will be zero, while at maximum speed it may riseto 5 5 x 105Pa. This increasing pressure has an effect on the valve system increasingly to reduce the hydraulic assistance and progressively change overto manual steering. It would be possible to invert this arrangement so that maximum pressureis applied at zero speedand vice-versa,the effecton the steering still being to increase the stiffness as speed rises. In Figure 2 a different approach is taken, and there is no branch offthereturn 5. Instead, the controlling fluid is supplied from a secondary pump 14, drawing direct from the reservoir 1 and having a bypass valve 15 corresponding to thevalve 6for the main pump 2. The pumps 2 and 14will conveniently be in a common housing and have a common drive, such as by belt from engine crankshaft.The output of the pump 14 is directed by line 16 to a modulating valve 17 governed byan electronic controller 18.Thishas inputssimilartothose ofthecontroller 11, and its action is to vary the setting of the valve 17 according to speed. However, as compared with the conventional form of controller 11, this arrangement has been found to be considerably more economical in the consumption of electrical power. Asthevalve 17 opens progressively more oil can be forced through line 18, thus stiffening the steering. When the valve 17 is closed the bypass valve 15 will allow fluid to circulate through the pump 14. CLAIMS

2. An SPF system substantially as hereinbefore described with reference to Figure 2 of the accompanying drawing.