Patent Application
20070029748
The present invention relates to a steering system for a vehicle and in particular to a means of actively varying the toe angle of the steerable road wheels.
Recently there has been much interest in steering superposition systems where a supplementary steering input can be made to the steering angle of the front wheels by a device which receives inputs from sensed parameters, typically steering wheel angular input from the driver and/or the vehicle speed, resulting in an output from the steering gear which is effectively the sum of the drivers steering wheel input and the input from the superposition device.
A large number of these systems use a planetary gear box and a controlled electric motor to provide additional input to the steering gear. Such a system is described in U.S. Pat. No. 3,831,701 (Pilon et al). In this case, the steering gear is of the recirculating ball type and a planetary gearbox provides additional rotation of the worm. The same principle is applied to rack and pinion steering gears where a planetary gearbox provides additional rotation of the pinion that meshes with the rack. Other systems, such as the device described in WO 02/36410, achieve the supplementary steering input by moving the pinion substantially in the direction of the rack travel.
These superposition devices enable the steering angle of the steerable road wheels to be actively varied, independent of the driver's steering wheel input. However, the nature of these devices mean that the toe angle of the steerable road wheels necessarily remains fixed.
Toe angle is a term used in the automotive industry to define the angular difference in plan view between the steerable road wheels. It is commonly expressed as the difference in distance between the fronts and rears of the wheel rims when the steerable wheels are in the straight ahead position. However, for simplicity in this specification, it will be expressed as angular difference. Positive toe angle, known as toe-in, is where the front of each of the steerable wheels are closer to each other than the rear. The toe angle of most steering systems is fixed to a predetermined value during vehicle assembly or servicing by manually adjusting the length of the tie rods that connect the steering gear to the steering arms.
Steering offset angle is defined in this specification as the angle of the steering wheel, with respect to the centred position of the steering wheel, when the steerable road wheels are in their on-centre position. The on-centre position of the steerable road wheels occurs when the toe-in angle between these wheels is equally distributed about the longitudinal axis of the vehicle, and unless there are any disturbances or road camber the vehicle will be running substantially straight ahead. For example, if zero toe angle is employed, the on-centre position of the steerable road wheels will align with the longitudinal axis of the vehicle. If the steering wheel angle position in this situation is 4.7 deg clockwise from its centred position, then the steering offset angle is defined as being +4.7 deg. For vehicles without steering superposition devices, the steering offset angle is fixed and is usually simultaneously adjusted during vehicle assembly or servicing with the toe angle. Typically, for a conventional steering system, the steering offset angle is set to zero (within a given setting tolerance) and the toe angle is usually set to either zero or a small positive or negative angle (typically in the range −0.5 deg to +0.5 deg).
The steering superposition devices discussed above, when used without any other active steering control means, effectively just vary the steering offset angle. There is a strong motivation, from the point of view of optimizing the vehicle dynamics, to actively vary the steering offset angle during driving. Varying the steering offset angle can be used to counteract the effect of yaw moments acting on the vehicle due to transient cross winds, road camber, and mismatches between the tyre friction characteristics between the left- and right-hand sides of the vehicle. Actively varying the steering offset angle can render these external disturbances much less noticeable to the driver, and hence dramatically improve the vehicle stability.
Also from the point of view of optimizing the dynamics of the vehicle, there is a strong motivation to actively vary the steering ratio during driving. Steering ratio is a term used in the automotive industry to define the ratio between angular movement of the steering wheel and the average angular movement of the two (usually front) steerable road wheels. For low speed driving a low (direct) steering ratio should ideally be employed to increase the manoeuvrability of the vehicle and decrease the lock-to-lock steering turns required for parking manoeuvres. For high speed driving a much higher (less direct) steering ratio should be employed to decrease the (otherwise) excessive yaw response of the vehicle. This increase in steering ratio should be employed at least in the “on-centre” operating region of the steering system associated with high speed driving. However, it is preferable that the steering ratio can be varied throughout the range of movement of the steering gear. Active control of the steering ratio can be achieved by using a steering superposition device to continually vary the steering offset angle.
It is well known that the relationship between the angles through which a pair of steerable road wheels turn should ideally follow the Ackermann principle. Ackermann proposed a relationship between the inner and outer front steerable road wheels during a turn, based on zero tyre slip, such that lines drawn through axes of the two wheels in plan view intersect at the same point on a line extended longitudinally from the rear axle. However this does not necessarily result in the appropriate relationship between the steerable road wheels when the tyres generate slip or the wheels are displaced vertically or horizontally due to road inputs. This can lead to accelerated tyre wear, irregular force feedback and reduced handling potential which may compromise vehicle safety. Furthermore, it is well known that a conventional steering system theoretically only achieves true Ackermann for one steering angle. Actively varying the toe angle can overcome these problems and several methods have been proposed to achieve this, as discussed below.
U.S. Pat. No. 4,371,191 (Goldberg et al), U.S. Pat. No. 5,143,400 (Miller et al) and JP 9-193827 (Yukimitsu et al) all disclose steering systems having hydraulic cylinders integrated with, or in line with, both tie rods. The hydraulic cylinders are controlled to effectively vary the length of the tie rods and hence actively vary the toe angle. These systems can also be controlled to actively vary the steering offset angle. However, the range of variation in steering offset angle is limited by the constraints of packaging the cylinders in line with the tie rods and hence it is difficult for such systems to provide large variations in steering ratio over the range of movement of the steering gear, particularly in comparison with the variation in steering ratio achievable by steering superposition devices having a planetary gearbox. Also, such systems must have actuators in both tie rods in order that the toe angle can be varied without the necessity of simultaneously varying the steering offset angle.
Heavy vehicles typically have a steering system with a single tie rod connecting both steering arms and a steering gear that is arranged to rotate one of the steering arms directly. U.S. Pat. No. 6,283,483 (Johnson et al) discloses a modification to such a steering system where a hydraulic cylinder is integrated with the single tie rod to actively vary its length. This system can actively vary the toe angle but it has the disadvantage that any variation in toe angle also results in a variation of the steering offset angle. For example, if the toe angle is actively varied whilst the vehicle is running in a straight line then the driver must correct the steering wheel angle to maintain straight ahead running.
It is an object of the present invention to ameliorate at least some of the problems of the prior art.
The present invention consists of a steering system for a vehicle comprising a steering gear having a rack and a pinion in mesh therewith, said rack being laterally displaceable with respect to said vehicle as a function of inputs from a steering wheel through rotation of said pinion, and a steering superposition means, and first and second road wheels steerable by rotation of first and second steering arms respectively by means of first and second tie rods arranged to transmit lateral displacement of said rack to said first and second steering arms respectively, characterised in that lateral displacement of said rack is transmitted to at least one of said steering arms through a linear actuator, and said linear actuator and said steering superposition means are controlled by a control means in response to at least one sensed parameter.
Preferably, lateral displacement of said rack is transmitted to only one of said steering arms through a linear actuator.
Preferably, said linear actuator is integrated with one of said tie rods. Preferably, there is a pivot joint at each end of said tie rod and said linear actuator is disposed between said pivot joints.
Preferably, said linear actuator comprises a ball screw assembly. Preferably, said linear actuator further comprises a hollow armature electric motor.
Preferably, in one embodiment, said steering superposition means provides rotation of said pinion in addition to that provided by said steering wheel. Preferably, said steering wheel is connected to a steering column and said steering superposition means is integrated with said steering column. Alternatively, said steering superposition means is integrated with said steering gear.
Preferably, in another embodiment, said steering superposition means provides lateral displacement of said pinion. Preferably, said steering superposition means is integrated with said steering gear.
Preferably, said linear actuator and said steering superposition means may be controlled such that the toe angle of said road wheels is variable without varying the steering offset angle.
Preferably, said sensed parameter comprises steering wheel rotation. Preferably, said sensed parameter comprises vehicle speed.
The present invention will now be described by way of example with reference to the accompanying drawings, in which:
a shows the tie-rod and integrated linear actuator of the steering system of
b shows an alternative arrangement for integrating a tie rod and linear actuator for a steering system in accordance with the present invention.
a shows how the toe angle is adjusted during straight ahead running without changing the steering offset angle, using the steering system of
b shows how the steering offset angle and toe angle are adjusted to correct for disturbances using the steering system of
c shows how toe angle can be optimised during cornering for each steerable road wheel, using the steering system of
a shows how the turn radius is determined for a vehicle using a typical prior art Ackermann steering arrangement.
b shows how the turn radius of a vehicle can be minimized by using a steering system according to the present invention.
For the sake of brevity and descriptive convenience in the following description, functionally similar components appearing in more than one figure bear common reference numerals in all of the figures, and their initial description made in respect to an earlier figure is generally not repeated in relation to a later figure.
Steering input is applied by the driver through rotation of steering wheel 24, which is connected to steering column 23. Steering superposition device 32 is integrated with steering column 23 and its output is rotationally transmitted to pinion 14 through hookes joints 28, 29 and intermediate shaft 27. Steering superposition device 32 adds incremental rotation of pinion 14 to that provided by rotation of steering wheel 24, and therefore steering superposition device 32 provides incremental lateral displacement of rack 12. Steering superposition device 32 may utilise a planetary gearbox controlled by an electric motor. Such steering superposition devices are well known in the prior art, as discussed in the background. A hydraulic power steering valve or a torque sensor, depending on whether steering gear 10 is respectively a hydraulically power assisted or an electrically power assisted steering gear, would normally be connected upstream of pinion 14 and would typically form part of steering gear 10.
This valve or sensor may either be upstream or downstream of the steering superposition device. For simplicity in this description, no valves, torque sensors or power assistance means are shown.
It should be noted that steering systems in accordance with the present invention may use other types of steering superposition devices to that shown in
It should also be noted that means other than hookes joints 28, 29 and intermediate shaft 27 may be used to transmit rotation from steering column 23 to pinion 14. For example, one or both the hookes joints may substituted for a rag joint or a simple non-compliant metal sleeve arrangement. Alternatively the entire connection member may comprise a simple non-compliant metal sleeve member which enables direct rotational connection of steering column 23 to pinion 14.
Road wheels 20a and 20b are mounted on hub assemblies (not shown) that each pivot about a substantially vertical axis. Steering arms 21a and 21b are attached to the hub assemblies. Road wheels 20a and 20b are steered by rotation of steering arms 21a and 21b respectively. Tie-rods 15a and 15b are connected to steering arms 21a and 21b respectively by outer pivot joints 17, and are connected to the ends of rack 12 through inner pivot joints 16. Tie-rods 15a and 15b transmit the lateral displacement of rack 12 to steering arms 21a and 21b thereby steering road wheels 20a and 20b.
Tie-rod 15b is a conventional solid tie rod that transmits displacement of rack 12 directly to steering arm 21b. Tie-rod 15a, however, has linear actuator 25 integrated with it such that the length of tie-rod 15a is variable.
An advantage of having only one tie rod integrated with a linear actuator, compared with the prior art that has linear actuators in both tie rods, is that it is easier to package in the limited space available in a vehicle for the steering system.
Referring again to
a, 4b and 4c explain the operation of the steering system shown in
b shows how the steering offset angle and toe angle are actively varied, without driver input, to keep the vehicle travelling straight ahead when encountering an external disturbance such as road camber irregularities or cross winds. Before encountering the disturbance, the angular dispositions of road wheels 20a and 20b are indicated by lines 1. In this example, to maintain straight ahead running during the disturbance it is desired to rotate road wheels 20a and 20b by angles A and B counter-clockwise respectively, as indicated by lines 3a and 3b. To achieve this correction, steering superposition device 32 outputs a counter-clockwise incremental rotation of pinion 14, which laterally moves rack 12 incrementally to the right thus turning road wheel 20b inwards by angle B. At the same time, linear actuator 25 extends by an incremental amount, as required, to create the desired difference in angular disposition between road wheels 20a and 20b. The extension of linear actuator 25 is proportional to the difference between angles A and B. Therefore, the toe angle has varied by the difference between angles A and B. Steering wheel 24 has not been rotated by the driver, so the steering offset angle has effectively varied by the average of angles A and B.
c shows how toe angle can be optimised for each road wheel during cornering. In this example the driver has rotated steering wheel 24 such that road wheels 20a and 20b have angular dispositions 4a and 4b respectively. In this example it is desired to incrementally rotate road wheels 20a and 20b by angles A and B counter-clockwise respectively, as indicated by lines 5a and 5b. To achieve this correction, steering superposition device 32 outputs a counter-clockwise incremental rotation of pinion 14, which laterally moves rack 12 incrementally to the right thus turning road wheel 20b inwards by angle B. At the same time, linear actuator 25 extends by an incremental amount required to create the difference in angular disposition of road wheels 20a and 20b equal to the difference between A and B. Therefore, the toe angle has varied by the difference between angles A and B. During this correction, steering wheel 24 has not been rotated by the driver, so the steering offset angle has effectively varied by the average of angles A and B.
a and 5b show how individually varying the toe angle of the steerable wheels, such as shown in
An example of this condition is when half of the vehicle is on a high-μ surface 44 such as dry bitumen, and the other half of the vehicle is on a low-μ surface 45 such as gravel or ice. It is well known in the art of vehicle dynamics that, during braking on this split-μ surface, there is an inherent compromise between maintaining stability of the vehicle and achieving a minium stopping distance. This is a result of the fact that the road wheel which is on the high-μ surface generates a higher braking force which creates a yaw moment which turns the vehicle towards the high-μ surface. The present invention enables the steer angle of each steerable wheel to be actively modified individually such that a high brake force can be applied to both steerable wheels without creating an unstable yaw moment on the vehicle. This is achieved as described with reference to
It should be understood that the present invention can also be applied to “centre take-off” steering gears where the tie-rods are attached to the rack near its middle, rather than its ends.
Although the present invention has been described in various embodiments in this specification, it should be recognised that departures may be made from the embodiments without departing from the scope of the invention.
The term “comprising” as used in this specification is used in the inclusive sense of “including” or “having”, and not in the exclusive sense of “consisting only of”.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003905623 | Oct 2003 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU04/01394 | 10/14/2004 | WO | 10/17/2006 |
