GB2181521A - Power assisted steering - Google Patents

Abstract

A power assisted steering system comprises a torque sensing device 1, comprising a resiliently deformable C-spring 24, which is mounted to transmit a manual torque applied to a steering column 2 to a pinion shaft 3 of a steering gear and permit limited relative rotation of the steering column 2 and pinion shaft 3. Two strain gauges 27, 28, are mounted on said C-spring 24, to sense the magnitude and eccentricity of deformation of the C-spring 24 resulting from the application of the torque to the steering column 2. The strain gauges 27, 28, are incorporated in an electronic control system comprising a signal conditioning unit 6, an analogue to digital converter 7 and an electronic control unit 8 which controls power assistance means 4, in proportion to the deformation of the C-spring 24, and hence the steering torque applied to the steering column 2. The electronic control system may also control the power assistance means 4 to be responsive to vehicle speed sensed by a speed sensor 37. <IMAGE>

Description

SPECIFICATION A power assisted steering system Technical fieldandbackgroundart The present invention relates to power assisted steering systems and in particularto such systems in which the amount of power assistance provided to an output member of the system is controlled in responsetothe manual torque applied to a rotary steering input during a steering manoeuvre.

Known power assisted steering systems of the type mentioned have encountered problems in constructing simple and accurate means for sensing the manual torque applied to the rotary steering input.

One means of sensing torque at the steering input is disclosed in U.S. Patent No.4,415,054 - TRW Inc., which relates to a steering gear of rack and pinion type with an output member capable of power assisted displacement and the power assistance to which is provided by an electric motor. The amount of power assistance provided by the electric motor is determined electronically in response to signals received from a torque sensor means. The torque sensor means, in essence, comprises a torsion bar which couples the steering inputtothe pinion ofthe steering gear. Two magnets are mounted (circum ferentiaily displaced from each other) on the torsion bar and opposite to a Hall effect device mounted on part of the steering input.When a manual torque is applied to the steering input there is a relative rotation between the magnets and steering input part caused by twisting ofthetorsion bar. This relative rotation causes a change in signals generated by the Hall effect device and these changes are used to control operation of the electric motor and thereby the provision of power assistance.

It is an object ofthe present invention to provide a power assisted steering system responsive to manual torque applied to a rotary steering inputdur ing a steering manoeuvre and which has a relatively compact, simple and inexpensive torque sensing means bywhich the provision of powerassistance is determined.

Statement of invention andadvantages According to the present i nvention there is prov- ided a power assisted steering system comprising a rotary input member and a rotary output member in which a manual torque isto be applied to the input member to effect a steering manoeuvre through the output member, the torque being transmitted to the output member by lost motion means which permits relative rotation between the input and output mem bers; a resilientcomponentwhich deformseccentr can liy in response to the relative rotation between the input and output members; and strain gauge means mounted on the component to sense and be respond sive to its deformation, said strain gauge means being incorporated in an electronic system which controls power assistance means of the steering system sothatthe power assistance which is prov ided is determined, at least in part, by a variation in signals in the electronic system caused by deforma tion ofthe resilient component as detected by the strain gauge means.

Preferablythe input member is in theform of a shaft or steering column of the steering system and the output member is a co-axial pinion of a rackand pinion steering gear in the system. In a preferred form ofthe invention the resiliently deformable component is a C-spring which deforms eccentrically in response to relative rotation between the input and output members. One such preferred form of the invention has projections formed on adjacent ends of the input and output members which engage in the mouth ofthe C-spring. When a torque is applied to the input memberto effect a steering manoeuvre at least part of the torque is transmitted through the C- spring to the output member.The torque transmitted through the C-spring causes that spring to deform eccentrically. The magnitude and eccentricity ofthis deformation will depend upon the magnitude and direction respectively ofthe torque. Preferably two strain gauges are mounted on the C-spring in positions circumferentially displaced from each other so that each gauge responds to the respective magnitude of the strain and the eccentricity of deformation ofthe C-spring in the part on which it is mounted and from the differential in such responses both the mag nitude and direction ofthe torque can be determined.

The strain gauge means are incorporated in an el ectronic control circuit which may also include a signal conditioning unit, an analogue to digital converter, and an electronic control unit. The primary purpose of the signal conditioning unit is to set a datum level (which preferably corresponds to the signals generated by the strain gauge means when no torque is applied to the input member) and all the signals from the strain gauge means are preferably related to this datum level. Preferably the electronic control unit is of a digital type, and therefore the signals from the strain gauge means which are of analoguetype are digitally encoded bythe analogueto digital converter.The electronic control unit is preprogrammed to control the powerassistance means in response to the magnitude and direction of the torque applied to the input member.

In a preferred form of the invention the power assistance means is an electric motor similarlyto that disclosed in U.S. 4,415,054. However, itwill be realised that the power assistance means may be of other type, for example an hydraulic motor con- trolled by way of a fluid flow valve which is in turn controlled by the electronic control unit.

In the preferred form of the steering system mentioned above where a C-spring is provided between the input and output members, it is also preferred thatthe C-spring will rotationally bias the input and output members relative to each other and to a central position or neutral condition corresponding to zero manual input torque (that is when a steering manoeuvre is not being effected).It will be appreciated that large angles of relative rotation between the input and output members are undesirable and it has been found that the use of a C-spring as the resilient component requires onlysmall angles of relative rot ation between the input and output members to det ermine, from the reaction of the strain gauge means, therelativetorquewhich is applied to the input member and thereby determine the actuation required ofthe power assistance means. Preferably the Cspring is pre-stressed so that a particular magnitude oftorque has to be applied to the input member before any deformation ofthe spring will occur.In this way the system may be primarily sensitive to relatively largetorques applied to the input member corresponding to situations where power assistance is considered desirable; also by pre-stressing the Cspring the self centering characteristics between the input and output members may be enhanced.

Although the preferred form of the present invention employs a C-spring as the resilient component, itwill be realised that other resiliently deformable components might be used, for example a torsion bar of helical coil spring or a combination of either or both ofthesewith a C-spring.

The electronic control unit may also receive signals from sources other than the strain gauge means, such as a vehicle speed sensor, which additional signals can be integrated with the signals from the strain gauge means and the resultant signals employed to control the power assistance means, for example to reducethe power assistance available as the vehicle speed increases.

During a steering manoeuvre the input and output members will be rotated togetherwith the strain gauge means and a problem may be encountered in effecting the electrical connection of the strain gauge means in the electronic control system since, if ordinary connecting cables are used, they will tend to wrap around the input or output member during a steering manoeuvre; one proposed solution to this problem is to effect the connections by way of brushes. However, another and preferred solution is to utilise a coiled, flat connecting cable, the coils of which are looselywound around one of the input or output members and linked tothestrain gauge means.The cords of the cable are arranged to tighten when the input member is rotated in one direction (but not tightened to the extent that the cable or connections thereof will be damaged) and when the input member is rotated in the opposite direction, the cable coils will slacken but be contained in a housing ofthe input/output members.

Drawings One embodiment of a power assisted steering system constructed in accordance with the present invention will now be described, by way of example only, with reference to the accompanying illustrative drawings in which: Figure 1 diagrammatically shows (in part section) the steering system having a torque sensing coupling between a steering column and a pinion shaft of a rack and pinion steering gear, and Figures is a viewofthe torque sensing coupling shown in Figure 1 taken along the line ll-ll.

Detailed description of drawings The power assisted steering system shown in Figure 1 has a torque sensing device generally indi cated at 1 coupling together a steering input column 2 and a co-axial pinion shaft 3. The pinion shaft 3 extends from a substantially conventional rack and pinion steering gear 4 (illustrated diagrammatically).

When a torque is applied to the steering column 2the torque sensing device 1 generates analogue electronic signals (analogeous to the torque applied) which are transmitted bya flat cable 5to a signal conditioning circuit 6. The torque sensing device 1 is constructed (as will be described hereinafter) such that an electronic signal is constantly generated, even when no torque is applied to the steering column 2. Thefunction ofthesignal conditioningcircuit6, isto compare these signals to a null ordatum level. The datum level may be established on assembly of the steering system and with no torque applied to the steering column 2. Thus a fluctuation in the signals caused by a change in torque on the steering column 2 wili subsequently be compared to the datum level.

After processing by the signal conditioning circuit 6, signals therefrom are transmitted to an analogue to digital encoding unit7 bywhich the analogue signals are encoded to digital form.

After digital encoding the signals from the unit7 are transmitted to an electronic control unit 8 which is arranged to control an electric motor 9 in response to the electronic signals transmitted from the torque sensing device 1. The electric motor 9 is capable of providing power assistance to the displacement of the rack (not shown) of the steering gear 4, forex- ample, as disclosed in U.S. 4,415,054, to assist in the steering manoeuvre intended by rotation ofthe column 2.

Thus, the amount of power assistance provided by the electric motor 9 is controlled in accordance with the magnitude and direction ofthe torque which is applied to the steering column 2 during a steering manoeuvre. Thetorquesensing device 1 comprises an elongate annular housing 10 having a cylindrical co-axial bore 11. An end part 12 ofthe pinion shaft 3 is received co-axiallywithin the major length of the bore 11 through end 13 ofthe housing. The end part 12 ofthe pinion shaft 3 forms the male part of a malel female type lost motion rotary coupling. The end part 12 engages in an end part 15 of steering column 2 which part 15 is formed as the female part ofthe lost motion coupling and extends co-axially partway into the bore 11.The lost motion coupling is constructed to inhibit relative radial displacement between the pinion shaft3 and the steering column 2, while permitting restricted relative rotational motion therebetween.

The pinion shaft 3 is rotatably mounted in the bore 11 by means of bearings 16seated in an annularrecess 17 formed in the end 13 of the housing 10. The end part 15 of the steering column 2 is rotatably mounted in the bore 11 by means of a bearing 18 seated in an annular recess 19 in the end 14 ofthe hous- ing and in an annular recess 20 on the end part 15.

Ashaft part 22 ofthe pinion shaft3 adjacent the end 12 has a diameter greaterthan that of the end 12 and is located within a cylindrical chamber26formed by an annular recess within the end 15 of steering column 2. A C-spring 24 is mounted on the shaft part 22 to extend partway around the circumference of that shaft part. Chamber 26 has a diameter sufficiently greaterthan the external diameter ofthe C spring to allow for deformation (radial enlargement) of the C-spring. A pin 21 extends radially from shaft part 22 within the mouth 23 of the C-spring 24 and a pin 25 extends axially from the end 15 within the mouth of the C-spring 24 (as can best be seen in Figure 2).The location ofthe pins 21 and 25 in the mouth of the C-spring restrains relative rotational movement between the pinion shaft3 and steering column 2.However, some relative rotational movement between the steering column 2 and pinion shaft 3 will occurwhen sufficient torque is applied to the steering column 2to cause the C-spring to deform resiliently (by pressure applied thereto through the pins 21,25) and to enlarge radially into chamber26.

The C-spring 24 is pre-stressed when it is initially mounted on the shaft part 22 and this has advantageous effects; firstly it will be necessary for a predetermined minimumtorqueto be applied to the steering column before deformation ofthe C-spring 24 (and thus relative rotation between the shafts 2 and 3) occurs. This minimum torque will, predominantly, be governed by the magnitude of prestressing ofthe C-spring and may be pre-calculated to provide the required characteristics ofthe steering system. Secondly, efficient self-centering between the pinion shaft3 and steering column 2 can be provided when manual torque is removed from the steering column 2.

Sensing ofthe magnitude and direction ofatorque applied to the steering column 2 is achieved by sensing the consequential change in strain in the external circumferential part of the C-spring 24 caused by deformation ofthe C-spring as a resultoftheapp- lication of a torque which exceeds the aforementioned minimum torque. Sensing of this strain is achieved by means of two strain gauges 27 and 28 which are mounted on the radially outer surface of the C-spring 24, one on each side of a position diametricallyoppositetheC-spring mouth 23. The change in strain and hence an indication ofthe torque applied to the steering column could be deter mined by use of a single strain gauge, using the two strain 27 and 28 however enables both the magni tudeanddirectionofthetorquetobedetermined.

This is because the deformation of the C-spring is eccentric and the changes in its strain are not constant along its external circumferential part length.

Thus, for example, if the steering column is rotated clockwise as indicated by arrow 29 in Figure 2, the strain sensed bystrain gauge 28will fall morethan the strain sensed by strain gauge 27 and the electronic control unit 8 can therefore assess from the inputthereto that the torque is applied in a clockwise direction and respond accordinglyto provide the appropriate power assistance. A complementary re sponse will occurwhen an anti-clockwise torque is applied.

Adja cent to the shaft part 22 and remote fromthe end 12 is a part30 ofthe pinion shaft 3 which hasa diameter slightly greaterthan that of the shaft part 22. Fixedly mounted on the shaft part 30 is a co-axial pulley part 31 having radial flanges 32. Fixedly moun ted on the circumference ofthe pulley part 31 is a cable connector 33 which secures one end oftheflat cable 5to the pulley part 31. The cable is loosely coiledaboutthepulleypart31 from the cableconnec- tor33 (for example, to extend twice around the pulley part 33) and then passes through a port34inthe housing 1 Oto which it is fixedly secured to prevent the cable from being drawn out of the housing.From the housing 10 the cable 5 connects to the signal conditioning circuit 6. The strain gauges 27 and 28are connected to the cable 5 by means of wires 36 which extend from those gauges through the adjacent flange 32 to the cable connector33. The housing 10 is conveniently mounted on a housing of the steering gear 4. The coiled cable 5 as described above permits the steering column 2and pinion shaft3to be rotated together and relative to the housing 10 to effect steering from lock-to-lock while the coils of the cable are tightened or loosened (within the housing 10) without detriment to the electrical connections at 33.

The elctronic control unit may also respond to inputs 37 otherthan from the torque sensing device 1, such as signals from a speed sensor capable of sensing the speed of the vehicle, for example so that the available powerassistance decreases astheveh- icle speed increases.

Claims (13)

1. A power assisted steering system comprising a rotary input member and a rotary output member in which a manual torque isto be applied to the input memberto effect a steering manoeuvre through the output member, the torque being transmitted to the output member by lost motion means which permits relative rotation between the input and output members; a resil ient component which deforms eccentr- ically in response to the relative rotation between the input and output members; and strain gauge means mounted on the component to sense and be responsiveto its deformation, said strain gauge means being incorporated in an electronic system which controls power assistance means ofthe steering system so that the power assistance which is provided is determined, at least in part, by a variation in signals in the electronic system caused by deforma tion of the resilient component as detected by the strain gauge means.

2. A system according to claim 1 wherein at least two strain gauge means are mounted in spaced relationship on the resilient componentto be capable of sensing both the magnitude and eccentricity of deformation ofthe component.

3. A system according to either of claim 1 or claim 2 wherein the resilient component comprises a Cspring.

4. A system according to claim 3 wherein said input and output members are co-axial said C-spring being mounted coaxially with said input and output members and, abutment means is provided which projects one each of said input and output members to engage with the ends of said C-spring to deform the C-spring during relative rotation between the input and output members.

5. A system according to any one of the preced ing claims wherein the resilient component is prestressed on assembly so that a predetermined value oftorque has to be applied to effect deformation of the resilient component.

6. A system according to any one of the preceding claims wherein the resilient component comprises a torsion bar.

7. A system according to any one ofthe preceding claims wherein said strain gauge means are electrically connected to the electronic system by flexible electrical conduits which are coiled about one of said inputor output members to compensate for rotation of said input and output members.

8. A system according to any one ofthe preceding claims wherein the electronic system includes a signal conditioning unit, an analogue to digital converter and an electronic control unit capable of controlling the power assistance means.

9. A system according to any one ofthe preceding claims and including a speed sensor capable of sensing the speed of a vehicle in which the system is installed, said sensor providing signals to the or a control unitwhich controls the power assistance means to be responsive to vehicle speed.

10. A system according to any one ofthe preceding claims wherein the input member comprises a steering column.

11. A system according to any one of the preceding claims wherein the output member comprises a pinion, of a steering gear.

12. A system according to any one of the preceding claims wherein the power assistance means comprises an electric motor controlled by said electronic system.

13. A system according to claim 1 and substantially as herein described with reference to the accompanying illustrative drawings.