GB2233614A - Vehicle four wheel steering system - Google Patents

Abstract

A rack and pinion steering gear (3) responds to a steering input (9) to displace front wheels. The rear wheels are provided with a second steering gear (11) comprising a rotatably mounted cam member (12) rotated by a drive shaft (13) from the first steering gear. The cam member has a peripheral cam track with which engages a pivoted cam follower 17, 19 such as a roller or lever which is coupled by links 23 to the rear wheels and is displaced during pivotal movement of the cam follower (19) to effect steering displacement for the rear wheels as the cam follower is driven over the cam track during rotation of the cam member (12). In an alternative embodiment the cam member is rotatably driven by an electrical stepping motor (36, Fig. 6). The rotational drive which is imparted to the cam member 12 can be dependent on vehicle speed by control provided by an electronic unit (34) which responds to signals derived from a vehicle speed sensor (35). The four wheel speed responsive steering can provide for the rear wheels to be displaced in the same or opposite sense of direction to the front wheels over different speed ranges by controlling the input to the cam member (12) through a reversing clutch/gear (30). <IMAGE>

Description

TITLE UA vehicle steering systems TECHNICAL FIELD & BACKGROUND ART The present invention relates to a vehicle steering system and is particularly concerned with such a system having two steering gears one of which is responsive to a manual steering input to provide a first steering output and effect steering displacement of steerable road wheels in a first set and the second of which is actuated by an input drive responsive to said steering input to provide a second steering output and effect steering displacement of steerable road wheels in a second set which are remote from those in the first set.Systems of the type mentioned have obtained popularity for four wheeled vehicles such as cars and are commonly referred to as "four wheel steering'1 in which the first steering gear provides a steering output to a front pair of vehicle road wheels and the second steering gear provides a steering output to a rear pair of vehicle wheels. Numerous structures have been proposed for the second steering gear which is usually associated with the rear road wheels and for providing the input drive to actuate that gear, typically such drive being derived by output from the first steering gear, for example as proposed in our British Patent No. 1,387,893 and also in U.S. 4,691,932 and DE-3,600,539.Despite the large number of prior proposals for four wheel steering systems, there is nevertheless a need for a relatively simple, compact and reliable system of the type mentioned and it is an object of the present invention to provide such a system.

STATEMENT OF INVENTION & ADVANTAGES According to the present invention there is provided a vehicle steering system having a first steering gear providing a steering output in response to a steering input to effect steering displacement of steerable road wheels in a first set; a second steering gear providing a second steering output to effect steering displacement of steerable road wheels in a second set remote from those in the first set, the second steering gear comprising a cam member mounted to be rotated about a longitudinally extending axis thereof and having a peripheral cam track, a cam follower engaging said cam track and being mounted to exhibit pivotal displacement during movement thereof along the cam track, and link means coupled to the cam follower to be displaced during pivotal movement thereof to provide the second steering output, and wherein rotational drive means is provided which is responsive to to the steering input and imparts controlled rotation to the cam member to cause the cam follower to be driven along the cam track and exhibit said pivotal displacement.

A particular advantage of the present invention is that the cam member and cam follower which are incorporated in the system can be provided by a cam and lever steering unit or a worm and roller steering unit, both of which are well known in the steering art and have proved to be relatively simple, compact, reliable and inexpensive gear structures.

In a four wheeled car or similar vehicle the first set of road wheels will usually be the pair of front wheels and the second set the pair of rear wheels. With such a four wheel steering system it is usual for the steering displacement provided by the second steering gear in response to a particular steering input to be less than the steering displacement provided by the first steering gear for that steering input. A further advantage provided by the invention is that the cam track can be profiled to provide a varying ratio between the rotation (angular displacement) of the cam member and the pivotal displacement of the cam follower which results therefrom at increments of rotation of the cam member.

With a four wheel steering system it may be considered advantageous for the rear wheels to be steered in the same sense of direction to the steering which is applied to the front wheels when the vehicle is at high speed and for the rear wheels to be steered in the opposite sense of direction to the front wheels when the vehicle is at low speed. At high vehicle speeds it is usual that relatively small steering inputs only are applied to the system to displace the front wheels within a small range of steering angles whereas at low vehicle speeds relatively large steering inputs are frequently applied to the system to displace the front wheels within a large range of steering angles.With this in mind the cam track can be profiled to provide a steering system in which, as the cam follower moves along the cam track in response to a progressively increasing steering input, the second steering output to the rear wheels initially displaces those wheels to steer in the same sense of direction as the front wheels and subsequently displaces the rear wheels to steer in the opposite sense of direction to the front wheels (so that, in effect the direction in which the rear wheels are steered will reverse as the steering input and thereby the steered angle of the front wheels is increased). Although such a system may not be responsive to vehicle speed (as discussed hereinafter) it does lend itself towards providing the advantageous characteristics mentioned above.

The rotational drive means that is responsive to the steering input can be provided by a rotational output derived from the first steering gear. In a typical system the first steering gear may comprise a pinion which is rotatable in response to the steering input and engages a rack member that is displaceable during rotation of the pinion to provide the first steering output and the rotational output for the drive means can be derived from rotation of the pinion. The rotational drive means may be derived from a shaft that extends from and is rotatable with the pinion; alternatively the rack member can engage an auxiliary pinion which is rotated in response to displacement of the rack member and the rotational output to provide the drive means can be derived from rotation of the auxiliary pinion.

Alternatively the rotational drive means can be provided by an electric motor, typically a stepping motor, which is actuated to provide the controlled rotation to the cam member by control from switch means that is responsive to the steering input.

The relatively simple steering system of the present invention can readily be adapted to what may be regarded as a more sophisticated speed responsive system which includes vehicle speed sensing means to which the rotational drive means is responsive so that during a steering manoeuvre the controlled rotation that is imparted to the cam member is dependent upon the prevailing speed of the vehicle. This speed dependency may be such that the controlled rotation which is imparted to the cam member is in one sense of direction for one predetermined range of vehicle speed and is in the opposite sense of direction for a further range of vehicle speed. A vehicle speed responsive system as aforementioned and which provides both directional control over predetermined ranges of vehicle speed for the second set of road wheels as aforementioned and also variations in the steering output which is provided for the second set of road wheels in accordance with variations in vehicle speed can conveniently be provided in the system where the rotational drive means to the cam member comprises an electric stepping motor having an electronic control unit which controls actuation of the stepping motor to effect a required steering displacement for the second set of road wheels in accordance with signals which are derived from the switch means that is responsive to the steering input (and may thereby be indicative of the extent and direction of the steering input torque which is applied during a steering manoeuvre) and also from a vehicle speed sensor.

Where the rotational drive means is provided by the rotational output derived from the first steering gear as aforementioned, the rotational output will typically comprise a rotatable shaft; a reversing clutch or gear unit can be provided in the rotatable shaft which is controlled by an electronic unit in response to signals derived from the vehicle speed sensor and switch means which is responsive to the steering input so that the clutch means is actuated, for example, to displace the second set of wheels in an opposite steering direction to the first set of wheels during a steering manoeuvre at a low range of vehicle speed and to displace the second set of road wheels in the same sense of steering direction as the road wheels in the first set for a higher range of vehicle speeds.

Conventionally both the first and second steering gears will each have a neutral condition in which zero steering displacement is provided, usually when the front and rear vehicle wheels steer straight (parallel to the fore-and-aft centre line of the vehicle). Where the system of the present invention is provided with characteristics which vary in accordance with vehicle speed as aforementioned whereby for a given steering input the second set of road wheels effects steering displacement in one sense of direction for one predetermined range of vehicle speed and in the opposite sense of direction for a further predetermined range of vehicle speed, there may be provided a still further predetermined range of vehicle speed between those previously mentioned during which the second steering gear is maintained in its neutral condition irrespective of the steering input which is applied.It is preferred that the second steering gear is provided with biasing means which biases that gear to its neutral condition, for example so that in the event of the rotational drive means failing when the second steering gear is displaced from its neutral condition, that gear will automatically revert to its neutral condition on a fail-safe principle.

DRAWINGS Embodiments of four wheel vehicle steering systems 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 diagrammaticaliy illustrates a first embodiment of the system in which rotational drive to the cam member of the second steering gear is provided by a rotational output derived from the first steering gear; Figure 2 is an axial section of one form of the second steering gear conventionally known as a cam and lever type steering gear; Figure 3 is a section taken on the line III-III of the gear shown in Figure 2; Figure 4 is an axial section of a further form of the second steering gear conventionally known as a worm and roller type steering gear; Figure 5 shows the system of Figure 1 modified to provide vehicle speed responsive characteristics;; Figure 6 diagrammatically illustrates a further embodiment of the steering system in which the rotational drive to the second steering gear is provided by an electric stepping motor and which system may include vehicle speed responsive characteristics, and Figures 7, 7A and 8, 8A respectively illustrate modifications to the cam tracks incorporated in the second steering gears of the systems in Figures 1 and 6.

DETAILED DESCRIPTION OF DRAWINGS The steering systems illustrated are applied to a four wheel vehicle having a front pair of steerable road wheels 1 and a rear pair of steerable road wheels 2. Steering displacement of the front ,wheels 1 is provided by a conventional rack and pinion gear 3 having a pinion 4 rotatably mounted in a rack housing 5 with its teeth engaging those of a rack bar 6 so that rotation of the pinion drives the rack bar 6 longitudinally to provide the required steering displacement. The ends of the rack bar are coupled through conventional steering linkages (including universal joint 7 and tie rods 8) to the front wheels 1. A steering column 9 is coupled to the pinion 4 through a universal joint 10 to provide a steering input which effects in rotation of the pinion in conventional manner. The rack and pinion unit 3 may be power assisted.

Steering displacement of the rear wheels 2 is effected by a second steering gear 11 comprising either a cam and lever gear as shown in Figures 2 and 3 or a worm and roller gear as shown in Figure 4 and both of which gears may be of conventional form. The gear in Figures 2 and 3 includes a cam member 12 which has a drive shaft 13 and is mounted in a housing 14 to be rotatable about its longitudinally extending axis 13A. The member 12 has a longitudinally extending peripheral cam track in the form of a coarse pitch worm groove 15 with which engages a cam follower in the form of a pin 16 on a pivotally mounted lever 17. The lever 17 has a shaft 18 by which it is pivotally mounted in the housing 14 and which shaft 18 carries a rocker arm 19 that is displaced in unison with the lever 17 during pivotal movement thereof.In the structure of the gear shown in Figure 4, the cam follower is in the form of a roller or wheel 20, the outer circumferential edge of which engages in the worm groove 15. The roller 20 is rotatably mounted on a shaft 21 in a cage 22 which is pivotally mounted on a shaft (not shown) in the housing 14 and which shaft connects with a rocker arm in a similar manner to that shown at 18 and 19 in Figure 3. It will be appreciated that upon rotation of the shaft 13 in the gears of Figures 2 to 4, the cam follower (16, 20) will be driven axially of the shaft as it is displaced along the worm groove 15 to pivotally displace the component (17, 22) which carries the cam follower and effect in pivotal or rocking movement of the rocker arm 19. The rocker arm 19 has connected thereto rods 23 which form part of a steering linkage to the rear wheels 2.Accordingly it will be appreciated that during rotation of the shaft 13 for the rear steering gear 11, the resultant pivotal movement of the rocker arm 19 will effect in longitudinal displacement of the rods 23 to impart steering displacement to the rear wheels. In practice the steering gears 3 and 11 will each have a neutral condition corresponding to the wheels of the vehicle being steered straight ahead (parallel to the foreand-aft direction of the vehicle) and in the rear steering gear 11 this will typically correspond to the cam follower 16 or 20 engaging the cam track 15 at the mid-length position thereof as shown in Figures 2 and 4. The pitch of the helical groove 15 will usually be symmetrical about the aforementioned neutral condition. Furthermore, the pitch of the helical groove can vary to provide a varying ratio between the rotation or angular displacement of the cam member 12 and the pivotal displacement of the rocker arm 19 which results therefrom at increments of rotation of the cam member 12 as may be considered appropriate to provide predetermined steering characteristics for the rear wheels 2.

In the embodiment of Figure 1 the shaft 13 of the cam member is coupled to be rotatably driven from the pinion 4 and such coupling comprises a transmission shaft 24 connected at one end through a universal joint 25 to an extension shaft of the pinion 4 and at its other end through a universal joint 26 to a gear unit 27 that provides a step down ratio from the shaft 24 to the shaft 13 of say 4:1. The gear unit 27 conveniently comprises a toothed belt 27a driving between a sprocket 28 rotatable with the transmission shaft 24 and a larger diameter sprocket 29 rotatable with the shaft 13. It will be appreciated that an intermeshing gear wheel drive can be substituted for the belt drive.With the structure shown in Figure 1 and in response to a steering input torque which is applied to the steering column 1 to effect a steering manoeuvre, the rack dnd pinion gear 3 will effect in steering displacement of the front wheels 1 while the steering input torque is transmitted through rotation of the transmission shaft 24 and the gear unit 27 to effect in pivotal displacement of the rocker arm 19 and thereby steering displacement of the rear wheels 2. The latter steering displacement will generally be considerably less than that provided for the front wheels 1; furthermore, the system in Figure 1 can be set up so that the rear wheels 2 will be steered in a similar sense or in the opposite sense of direction to the steering which is applied to the front wheels 1.

The steering system of Figure 1 can be modified to include a facility whereby the steering displacement of the rear wheels is responsive to the speed of the vehicle in the sense that, depending upon vehicle speed, the displacement of the rear wheels may be either in the same sense or in the opposite sense of steering displacement as the front wheels. Such a modification is shown in Figure 5 where the transmission shaft 24 is coupled to drive the input shaft 13 of the cam member 12 through a reversible gear unit 30. The unit 30 has a clutch 31 rotatable with the shaft 24 and displaceable into engagement to transmit drive to either of two belt drive gears 32 or 33 one of which is reversed relative to the other (as indicated by the cross belt 27b for unit 33).Displacement of the clutch member 31 into engagement with either the belt gear 32 or the belt gear 33 imparts rotational drive to the shaft 13 in one or the opposite senses of direction.

Displacement of the clutch member 31 may be effected electrically, for example by energisation of a solenoid, by control from an electronic unit 34. The control unit 34 derives signals from a vehicle speed sensing device 35.

Typically the control unit 34 in response to signals derived from the speed sensor 35 will displace the clutch member 31 so that for a low range of vehicle speeds the clutch member will engage and transmit drive to the gear unit 32, for a high range of vehicle speeds the clutch member will engage the gear unit 33 to provide reverse drive to the shaft 13, and for an intermediate range of vehicle speeds the clutch member will be disengaged from both gear units 32 and 33 so that no steering displacement is effected for the rear wheels (irrespective of the steering input torque which may be applied to effect steering displacement of the front wheels).

In the embodiment of the steering system shown in Figure 6 rotational drive for the shaft 13 of the cam member 13 is provided by an electrical stepping motor 36, the actuation of which is controlled by an electronic control unit 34 which receives signals from the vehicle speed sensing device 35. In addition, the control unit 34 receives signals from a switch device 37 on the steering column 9 and which is responsive to the steering input torque applied to the system and provides an indication of the direction of the steering input and the duration thereof. The control unit 34 responds to the signals derived from the switch device 37 and the speed sensor 35 to actuate the stepping motor 36 to impart drive to the steering gear 11 and thereby control the steering displacement of the rear wheels.

In a similar manner to the embodiment of Figure 5, the control unit 34 in Figure 6 can provide for steering displacement of the rear wheels in the same sense or the opposite sense of direction to those of the front wheels over high and low speed ranges of the vehicle and also for zero steering displacement over an intermediate speed range of the vehicle when a steering input is applied to the system as detected by the switch device 37. This speed responsive facility is optional in Figure 6 and if required the sensor 35 can be omitted so that the motor 36 rotates the cam member 12 solely in response to signals derived from the switch 37.

The steering systems of Figures 1 and 6 can also be modified in such a way that, without being responsive to the speed of the vehicle, they may provide advantageous steering characteristics which are normally associated with speed responsive steering as previously discussed. This can be achieved by appropriate profiling of the cam track 15 so that in response to a steering input which progressively increases the steering displacement of the front wheels 1 from their neutral condition, the movement of the cam followers 16 or 20 along the cam track sequentially causes the rear wheels 2 to be steered initially in the same sense of direction as the front wheels, the steering displacement of the rear wheels 2 then to reverse, and subsequently to be steered in the opposite sense of direction to the front wheels 1.It is recognised that at high vehicle speeds the front wheels 1 will be displaced through relatively small steering angles from their neutral condition and this may correspond with the rear wheels 2 being steered through the cam track control in the same sense of direction as the front wheels whereas at low vehicle speeds the front wheels 1 are likely to be displaced through large steering angles from their neutral condition and to an extent that the rear wheels are steered through the cam track control beyond their reversal stage and in the opposite sense of direction to the front wheels. Examples of suitably profiled cam tracks 15A and 15B on the cam member 1; which will provide the aforementioned reversal of steering direction for the rear wheels 2 are shown in Figures 7 and 8 respectively.

Figures 7A and 8A respectively show the cam tracks in Figures-7 and 8 developed in a flat plane in which when the cam follower 16 engages with the respective tracks at the intersection of the ordinate Y and abscissa X the second steering gear 11 will be in its neutral condition. It will be seen from Figure 7 and 7A that the cam track 15A is a discontinuous groove, generally of helical form, on the cam member 12. However the geometry of the track 15A is such that when the cam member is rotated partially in either sense of direction to displace the follower 16 from its neutral condition, the follower 16 will initially be displaced axially of the cam member in one sense of direction, will reverse its direction of axial displacement in the region 15C of the track and subsequently be displaced axially in the opposite sense of direction as the cam member approaches 1800 of angular rotation. The steering output to the rear wheels 2 will correspond with the aforementioned axial displacement of the cam follower where, because the track 15A extends from the ordinate Y first to one side and then to the opposite side of the abscissa X, the rear wheels can be displaced sufficiently to steer in the same sense and then in the opposite sense of direction to the front wheels.

The cam track 15B in Figures 8 and 8A is a continuous groove around the cam member 12 and its geometry is such that as the cam follower 16 moves along the track from the neutral condition of the gear it will again be displaced axially first in one direction, reverse at 15C and then be displaced axially in the opposite direction (as the cam member approaches 1800 of angular rotation from the neutral condition). However, the cam track 15B extends in each direction from the neutral condition at the ordinate Y to one or the other side only of ,the abscissa X, consequently following reversal at 15C the rear wheels 2 will not be displaced sufficiently to steer in the opposite sense of direction to the front wheels 1.It will be apparent from Figure 8A that when the cam follower 16 engages the track 15C following rotation of the cam member 12 through 1800 the rear steering gear 11 will provide steering characteristics which correspond to those of the gear being in its neutral condition where the rear wheels present zero steering angle (although the front wheels will then be directed to a relatively large steering angle).

In the embodiments as above described and illustrated, it is preferred that the steering gear unit 11 is biased to its aforementioned neutral condition (for example by biasing springs applied to the rocker arm 19) so that in the event of a failure in the input drive to the rear wheel steering gear, that gear will revert automatically to its neutral condition. Such biasing may also serve to revert the steering gear 11 to its neutral condition in the speed responsive system where there is an intermediate range of vehicle speed during which zero steering displacement is provided for the rear wheels irrespective of the steering input which is applied to the system.

Claims (20)

1. A vehicle steering system,having a first steering gear providing a steering output in response to a steering input to effect steering displacement of steerable road wheels in a first sett a second steering gear providing a second steering output to effect steering displacement of steerable road wheels in a second set remote from those in the first set, the second steering gear comprising a cam member mounted to be rotated about a longitudinally extending axis thereof and having a peripheral cam track, a cam follower engaging said cam track and being mounted to exhibit pivotal displacement during movement thereof along the cam track, and link means coupled to the cam follower to be displaced during pivotal movement thereof to provide the second steering output, and wherein rotational drive means is provided which is responsive to the steering input and imparts controlled rotation to the cam member to cause the cam follower to be driven along the cam track and exhibit said pivotal displacement.

2. A system as claimed in claim 1 in which the cam track is profiled to provide a varying ratio between the rotation of the cam member and the pivotal displacement of the cam follower which results therefrom at increments of rotation of the cam member.

3. A system as claimed in either claim 1 or claim 2 in which the cam track comprises a groove and the cam follower comprises a lever or roller member engaging with said groove.

4. A system as claimed in any one of the preceding claims in which the cam track is profiled so that, in response to a steering input which progressively increases steering displacement of the wheels in the first set, the second steering output initially effects in steering displacement of the wheels in the second set in one sense of direction then reverses and subsequently effects in steering displacement of the wheels in the second set in the opposite sense of direction.

5. A system as claimed in claim 4 in which the cam track is profiled so that the initial steering displacement of the wheels in the second set is in the same sense of steering direction as the wheels in the first set.

6. A system as claimed in claim 5 in which the steering displacement of the wheels in the second set subsequent to said reversal is sufficient for those wheels to be steered in the opposite sense of steering direction to the wheels in the front set.

7. A system as claimed in any one of the preceding claims in which the rotational drive means comprises a rotational output derived from the first steering gear.

8. A system as claimed in claim 7 in which the first steering gear comprises a pinion rotatable in response to the steering input and engaging a rack member which is displaced during rotation of the pinion to provide said first steering output and said controlled rotation is derived from rotation of the pinion.

9. A system as claimed in claim 8 in which the rotational output is derived from a shaft extending from the pinion.

10. A system as claimed in claim 8 in which the rack member engages an auxiliary pinion which is rotated in response to displacement of the rack member and the rotational output is derived from rotation of the auxiliary pinion.

11. A system as claimed in any one of claims 1 to 6 in which the rotational drive means comprises electric motor means which is actuated by control from switch means that is responsive to said steering input.

12. A system as claimed in any one of the preceding claims and comprising vehicle speed sensing means, said rotational drive means being responsive to the sensing means whereby the controlled rotation which is imparted to the cam member is dependent upon the prevailing speed of the vehicle. ,

13. A system as claimed in claim 12 in which said controlled rotation is in one sense of direction for one predetermined range of vehicle speed and is in the opposite sense of direction for a further predetermined range of vehicle speed for the same steering input.

14. A system as claimed in either claim 12 or claim 13 and comprising an electronic control unit which is responsive to signals derived from the vehicle speed sensing means and provides an output signal which controls the rotational drive means.

15. A system as claimed in claim 14 when dependent upon claim 7 in which the electronic control unit controls actuation of a clutch/reversing gear which determines the direction of rotational drive that is applied to the cam member in response to a steering input.

16. A system as claimed in any one of the preceding claims in which the second steering gear has a neutral condition in which zero steering displacement is provided to the road wheels in the second set and said second steering gear is provided with biasing means which biases that gear to its neutral condition.

17. A vehicle steering gear substantially as herein described with reference to Figures 1 to 4 of the accompanying illustrative drawings.

18. A vehicle steering gear as claimed in claim 17 and including the modifications substantially as herein described with reference to Figure 5 of the accompanying illustrative drawings.

19. A vehicle steering gear as claimed in claim 17 and including the modifications substantially as herein described with reference to Figures 7, 7A, 8 and 8A of the accompanying illustrative drawings.

20. A vehicle steering gear substantially as herein described with reference to Figure 6 of the accompanying illustrative drawings. ,