WO2021122038A1

WO2021122038A1 - Vehicle steering

Info

Publication number: WO2021122038A1
Application number: PCT/EP2020/084495
Authority: WO
Inventors: Ian Gordon Murray
Original assignee: Gordon Murray Design Limited
Priority date: 2019-12-17
Filing date: 2020-12-03
Publication date: 2021-06-24
Also published as: GB201918613D0; GB2590423A

Abstract

Power assistance for the steering system of a vehicle is often desirable but can adversely affect the steering feedback provided to the driver. We disclose a steering system for a vehicle, comprising a steering column (10) connectable at one end (12) to a steering wheel and at another end (14) to a pinion of a rack and pinion assembly, and an electrical motor (26) connected to the steering column via a clutch (16, 18) and driven to as to assist the steering when the clutch (16, 18) is engaged, and a control unit (14) for the clutch (16, 18) arranged to selectively disengage the clutch (16, 18) above a pre-set speed no greater than 30km/h. Thus, the control unit (40) can disengage the clutch (16, 18) above the pre-set speed, allowing for the provision of steering assistance at low vehicle speeds typically used for parking manoeuvres and the like and allowing the suspension systems of the vehicle to be designed and optimised for un-assisted driving. At higher speeds, the driver is unlikely to be carrying out a tight manoeuvre and it is therefore reasonable to assume that steering inputs will be more measured and being made at least partly in response to feedback via the steering wheel. Other triggers for disengagement are discussed. Suitable forms for the clutch (16, 18) include a friction clutch (16, 18) and a dog clutch.

Description

Vehicle Steering

FIELD OF THE INVENTION

The present invention relates to steering systems for vehicles.

BACKGROUND ART

Vehicles require some form of steering system so that they can be controlled to drive in a direction that is useful. Aside from specialist solutions such as skid steering, the usual way to do this is to adjust the direction of one or more of the wheels on which the vehicle is running. This will usually be two wheels at the front of the vehicle, although other solutions exist such as vehicles which adjust the direction of the rear wheels instead of or in addition to that of the front wheels. This adjustment is commonly by way of a rack and pinion arrangement; other solutions existed but have largely been supplanted by the rack and pinion.

It is common to provide a power steering system to help the driver steer the vehicle by augmenting the steering effort needed to turn the steering wheel, making it easier for the driver to turn. The vehicles of most manufacturers have become heavier over the years, and as tyre sizes have generally increased, this has resulted in greater steering effort being needed. As a result, some form of power assistance is highly desirable or essential in many modern vehicles. Typically this is by way of either hydraulic actuators or electric motors.

Hydraulic power steering systems for cars augment steering effort via an actuator, a hydraulic cylinder that is part of a servo system. These systems have a direct mechanical connection between the steering wheel and the linkage that steers the wheels; should power- steering system failure occur, this means that the vehicle can still be steered using manual effort alone. Hydraulic systems require a pump driven by the engine, which draws power from the engine and needs to be accommodated in the engine bay where it can be driven, usually via the serpentine belt. The pump, actuator and associated high-pressure lines also all add weight.

Electric power steering systems use electric motors to provide the assistance instead of a hydraulic system. As with hydraulic types, power to the actuator (motor, in this case) is controlled by the rest of the power-steering system. Electrical assistance has advantages in that no hydraulic pump is needed to provide a supply of high-pressure hydraulic fluid, but some argue that electrical systems do not offer adequate feedback to the driver through the steering column. Steering feedback or "feel" is important in the context of sports and high- performance vehicles as it provides the driver with information as to the conditions being experienced by the steered wheels in an immediate and tactile form. Experienced drivers can use this feedback to detect the onset of understeer and the like, and to sense the overall stability of the vehicle and how close it is to the limits of its performance envelope. Efforts in the field of electrical steering assistance have to date been directed at improving this feedback so that it is more closely aligned with the feedback received from a hydraulically-assisted steering system.

Examples of such efforts include GB2262491A, which discloses an electrically assisted steering system in which assistance is applied to the steering via a clutch that is actuated in response to the level of steering effort. The road speed of the vehicle is taken into account in deciding the level of assistance that is provided, but the system is active whenever the vehicle is rolling and the engine is running. EP0901217A discloses an electrically assisted steering system which responds to the road speed of the vehicle by reducing the amount of assistance provided as the speed increases. Some assistance is always provided, however, and the assistance motor remains connected to the steering drive. EP0276005A discloses an electrically assisted steering system controlled by an electronic control unit which takes road speed into account. The level of assistance is gradually reduced in steps, akin to EP0901217A, from a high level that applies at low speeds, through gradually reducing levels at open road speeds, and at a speed towards the upper end of the vehicle's speed range the assistance is withdrawn entirely and a clutch disengages the electrical assistance. JPH08133106A discloses a clutch-controlled electrically-assisted steering system in which the clutch is disengaged in response to a high-speed rotation of the steering wheel. This allows the driver to overwhelm the steering assistance in an emergency situation (for example). The presence of the clutch in this arrangement allows the assistance motor to operate at a lower speed, as it does not need to cater for emergency steering; this is said to reduce noise and wear.

Meanwhile, a small number of vehicles retain a direct, unassisted steering system. These tend to be either very simple kit cars, or minimalist sports cars with relatively low- powered engines and hence narrower tyres. Examples include the Caterham® 7 and Ariel Atom.

SUMMARY OF THE INVENTION

A higher-powered sports car is likely to use larger-section tyres in order to provide the necessary traction to deal with the power and torque output of the engine. These in turn mean that some power assistance for the steering system is essential even when care is taken to ensure the car is lightweight. However, it is in such vehicles that steering feedback is important.

In our view, the direction adopted by the existing art is wrong in principle. Such vehicles are designed on the assumption that they will have electrically power-assisted steering, and the problem that is then addressed is how to tailor the operation of the assistance system so as to provide both an appropriate level of assistance and an acceptable level and nature of driver feedback. We wish to take a different approach; a vehicle should be lightweight enough and have its suspension optimised so that it does not generally need steering assistance. That ensures that feedback to the driver through the steering system is ideal, as none is removed via the steering assistance.

In practice, the specific combination of very low speeds and tight cornering (i.e. parking manoeuvres) is likely to be difficult to make comfortable, so we propose a system that introduces power assistance in this specific situation, rather than one that defaults to power assistance and then tries to minimise its effect when feasible. The present invention therefore provides a steering system for a vehicle, comprising a steering column connectable at one end to a steering wheel and at another end to a pinion of a rack and pinion assembly, and an electrical motor connected to the steering column via a clutch and driven to as to assist the steering when the clutch is engaged, and a control unit for the clutch arranged to selectively disengage the clutch above a pre-set vehicle speed no greater than 30km/h.

This allows for the provision of steering assistance at the low vehicle speeds typically used for parking manoeuvres and the like. Indeed, we prefer that the pre-set speed is lower still, such as 25, 20 or 15km/h. At these speeds, significant steering input is often needed but there is minimal need for steering feedback. Above the preset speed(s), the driver is unlikely to be carrying out a tight manoeuvre and it is therefore reasonable to assume that steering inputs will be more measured and being made at least partly in response to feedback via the steering wheel. The control unit can sense the vehicle speed via an input receiving information from the vehicle's other systems, which usually includes the vehicle speed. It could additionally react to a manual input such as a switch, or to an algorithm taking into account a range of other factors such as steering angle, yaw angle and other stability control sensors, etc. This choice of a very low cut-off, significantly less than that of EP0276005A, enables the vehicle to be provided with a suspension design and setup which is tailored to unassisted driving at open road speeds, without regard for the consequences at slow speeds. In turn, the level of steering assistance provided at parking speeds can be set at a level which is enhanced significantly (relative to known systems) to compensate for what may be an adverse suspension setup, without regard for the consequential loss of driver feedback at road speeds.

The clutch is intended to provide a releasable connection between the assistance motor and the steering column. This allows the assistance motor to be disconnected from the steering column when not in use, leaving a steering feel that is transmitted through the steering column unaffected by the assistance motor. Suitable forms for the clutch include a friction clutch and a dog clutch.

The electrical motor will usually include reduction gears, which we prefer are located between the electrical motor and the clutch so that with the clutch disengaged, the gears do not affect the steering feel. Indeed, we prefer that there are no gears between the clutch and the steering column. We prefer that the one end of the steering column is connected directly to a steering wheel boss, and also that the other end of the steering column is connected to a constant- velocity joint for connection to pinion of a rack and pinion assembly. This is a straightforward way of ensuring that there is a direct mechanical connection from the steering wheel to the steered wheels, thus giving a "pure" steering feel when the assistance motor is disengaged.

The present invention also provides a vehicle including a rack and pinion assembly controlling the direction of at least two steered wheels, the pinion being driven by a steering system as defined above.

In this way, the invention yields a approach where we can take advantage of the high level of assistance that an electric system can provide at a light weight, but not worry about the impact on driver feel.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which;

Figure 1 shows a schematic illustration of a steering system according to the present invention, in a high-speed state; and

Figure 2 shows a schematic illustration of a steering system according to the present invention, in a low-speed state.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to figure 1, this shows a schematic illustration of the principle behind the present invention. A steering column 10 extends at one end 12 towards a steering wheel (not shown). Usually, the steering wheel is connected to the column 10 (or to an intermediate shaft linked to the column 10) via a steering wheel boss. The wheel itself may be fixed to the boss, or it may be removable to allow for easier ingress and egress by the driver. The other end 14 of the steering column 10 extends towards a constant-velocity (CV) joint (not shown) which drives the input to a rack and pinion arrangement, either directly or indirectly.

The column 10 carries a circular first friction plate 16, which is securely attached to the column 10 and therefore rotates with it. This friction plate 16 is designed to be rigid and to accept the forces imposed by the steering assistance, so comprises a solid disc on the surface of which a friction material may be provided. This faces a driven friction plate 18 which is mounted on the column 10 via a bearing 20 and thus rotates freely on the column 10. This is also a solid disc provided with friction material on at least the side facing the first friction plate 16; at its outer circumferential edge it is toothed 22 and engages with the output drive 24 of an electric power-assisted steering motor 26. The motor 26 may be conventional compared to existing electric power-steering motors and may include gearing 26a in order to control the rotational speed and torque of the output drive 24 as required.

As shown in figure 1, the driven friction plate 18 is not urged against the first friction plate 16 and thus no torque will be transmitted from the motor 26 to the column 10. More importantly, if the driver operates the steering wheel and thus rotates the steering column 10, this will not be transmitted to the motor 26 and any gearing within it, and thus the motor 26 will not contribute to the feedback received by the driver from the steering wheel.

A pressure plate 28 is also carried on the column 10, in this case freely rotating around the column on a bearing 30. In an alternative arrangement, the pressure plate 28 could be rotationally locked to the column 10 via (for example) a spline arrangement allowing a small degree of movement of the pressure plate along the axis of the column 10. The pressure plate is located on the opposite side of the driven friction plate 18 to the first friction plate 16, so that they can sandwich the driven friction plate 16. A collet 32 is also mounted on the column 10 via a bearing 34 and can be urged against the pressure plate 28 by an operating arm 36 so as to sandwich the driven friction plate between the pressure plate 28 and the first friction plate 16. The operating arm is driven by an actuator 38 which is controlled by a control unit 40 in response to a signal 42. Thus, when the control unit activates, the actuator 38 moves the operating arm 36 and thus urges the pressure plate 28 via the collet 32 to sandwich the driven friction plate 18 between the pressure plate 28 and the first friction plate 16. This links the motor 26 to the column 10 and means that steering assistance can be provided by the motor in the usual manner, as shown in figure 2. When the control unit releases the actuator 38, releasing pressure on the pressure plate 28 and disengaging the clutch, as shown in figure 1, the motor 26 is isolated mechanically from the column 10 and the driver's steering 'feel' is unaffected by the presence of the steering assistance motor 26. Figures 1 and 2 show a single-plate clutch for simplicity and clarity, but in practice a multi-plate clutch may be adopted so as to transfer the same torque with a smaller-diameter structure. This may assist in packaging the arrangement within the vehicle.

In a further alternative, the friction-plate clutch arrangement could be replaced with a dog clutch. These involve toothed or other profiled surfaces that are moved into engagement in order to lock the driven element onto the column 10. For example, the friction surfaces of the first friction plate 16 and the driven friction plate 18 could be replaced with engagement formations to lock the two plates together when engaged. Alternatively, the driven plate could be slid longitudinally onto and off a spline on the column 10. Dog clutches are commonly used (for example) to engage the correct gear of a manual transmission and will therefore be familiar to the skilled person.

Thus, via the above-described mechanisms the drive motor 26 can be selectively engaged with the steering column 10. This allows the steering assistance to be provided when desired and withdrawn when not. The vehicle designer thus has an additional freedom to provide a 'pure' steering feel when needed or desired, unaffected by interference from or mechanical/induction drag within the motor 26 and its associated gearing.

The signal input 42 to the control unit 40 includes a speed sensor output, along with (potentially) other relevant information. When the control unit senses a speed below a threshold of no more than 30kph, such as (for example) 25, 20 or 15kph, the clutch can be engaged and steering assistance provided. Above that speed, the clutch is disengaged. Of course, the driver could be given the ability to vary the preset speed if desired, or offered a manual override.

In another configuration, the input 42 to the control unit 40 could comprise multiple factors including speed, steering angle, yaw angle and the output of other stability control sensors, the status of the stability control system and when it is actively intervening, and the like. Thus, steering assistance could be provided at low speeds, removed by disengaging the clutch as speed increases above the threshold, and re-applied temporarily during emergency situations. In a further configuration, the driver could be provided with a simple on/off choice such as a switch, allowing them to disengage steering assistance when they would prefer to go without.

Of course, the various configurations could be combined in various ways, such as driver-controlled 'off' switch which is over-ridden in an emergency or at very low speeds.

It will of course be understood that many variations may be made to the above- described embodiment without departing from the scope of the present invention.

Claims

1. A steering system for a vehicle, comprising a steering column connectable at one end to a steering wheel and at another end to a pinion of a rack and pinion assembly, and an electrical motor connected to the steering column via a clutch and driven to as to assist the steering when the clutch is engaged; and a control unit for the clutch, arrange to selectively disengage the clutch above a pre-set speed no greater than 30km/h.

2. A steering system according to claim 1 in which the clutch is a friction clutch.

3. A steering system according to claim 1 in which the clutch is a dog clutch.

4. A steering system according to any one of the preceding claims further comprising reduction gears between the electrical motor and the steering column, located between the electrical motor and the clutch.

5. A steering system according to claim 4 in which there are no gears between the clutch and the steering column.

6. A steering system according to any one of the preceding claims in which the one end of the steering column is connected directly to a steering wheel boss.

7. A steering system according to any one of the preceding claims in which the other end of the steering column is connected to a constant-velocity joint for connection to pinion of a rack and pinion assembly.

8. A steering system according to any one of the preceding claims in which the control unit disengages the clutch above a pre-set vehicle speed no greater than 15km/h.

9. A vehicle including a rack and pinion assembly controlling the direction of at least 2 steered wheels, the pinion being driven by a steering system according to any one of the preceding claims.