ELECTRIC STEERING SYSTEM WITH MECHANICAL BACK-UP DEVICE

Abstract: A steering system (1) for an automotive vehicle, comprises a steering device (4) to be operated by a driver, a rotatable connection (5) having a drive end (6) connected to the steering device and a driven end (8) connectable to a linkage (3) of the steering wheels (2) of the vehicle. The drive end of the rotatable connection is provided with a first sensor (10), the driven end of the rotatable connection is provided with a second sensor (11) and an electric motor (16), the first and second sensors being connected to a control device (12), said control device controlling the electric motor on the basis of signals emanating from the first and second sensors.

Description: Electric steering system with mechanical back-up device The invention is related to a steering system for an automotive vehicle, comprising a steering device to be operated by a driver, a rotatable connection having a drive end connected to the steering device and a driven end connectable to a linkage of the steering wheels of the vehicle. Such a steering system is known, and usually comprises a purely mechanical rotatable connection, e.g. a tubular shaft. Said shaft is also provided with an absorber mechanism for protecting the driver in case of a frontal collision. Although such prior art steering system performs satisfactorily, it nevertheless has disadvantages as well. The short tubular shaft still may present a potential hazard to the driver. Furthermore, its application in the surrounding vehicle construction imposes limitations, on the vehicle design as well. Its routing from the steering wheel to the linkage is rather straightforward, and can hardly be varied or routed along a different path. Usually, also a power assist device is connected to the steering system so as to alleviate the steering torque to be exerted by the driver on the steering wheel. Although generally hydraulic power assist devices are applied in this connection, electric power assist devices have been proposed as well. Such electric power assist devices have the advantage that no hydraulics are needed. The hydraulic oil, which presents environmental disadvantages, and the somewhat cumbersome lay out of hydraulic connections can then be dispensed with. The object of the invention is to provide a steering system which can be applied in a more versatile way, and which does not have the disadvantages of the prior art hydraulically assisted steering devices. This object is achieved in that the drive end of the rotatable connection is provided with a first sensor, the driven end of the rotatable connection is provided with a second sensor and an electric motor, the first and second sensors being connected to a control device, said control device controlling the electric motor on the basis of signals emanating from the first and second sensors. The electric motor can drive the driven end of the rotatable connection in several ways. First of all, the electric motor can act as a power assist device, analogous to the hydraulic assist devices. The rotatable connection in that case still delivers some torque for steering purposes. However, it is also possible to provide the full steering torque through the electric motor. In that case, the rotatable connection merely acts as a mechanical backup of the fully electric steering system in the case of e.g. an electric power failure. Preferably, the first sensor is incorporated in a first bearing and the second sensor is incorporated in a second bearing, said bearings being connected to the rotatable connection for supporting said rotatable connection with respect to the vehicle. Such sensorised bearings constitute standard available, proven components, and can easily and reliably be incorporated in the steering system. In case the rotatable connection can be carried out as a mechanical backup drive, it can be a flexible torque cable, e.g. a Bowden cable. Such cable can be adapted more easily to the space constraints within a vehicle. Nevertheless, such cable may also be combined with a shaft portion. Furthermore, the drive end of the rotatable connection can be provided with an accelerating gear drive and the driven end of the rotatable connection can be provided with a decelerating gear drive. Alternatively, a standard steering column can be used as a mechanical back-up drive. As a result of the gear drives in question, the rotatable connection has a relatively high rotational speed. Thereby, it becomes possible to apply a rotatable connection, such as a Bowden cable or shaft, with relatively small cross-sectional dimensions without impairing the torsional stiffness of the steering system. Moreover, a relatively high number of revolutions of the rotatable connection makes it possible to apply relatively simple sensors with only small resolution. The relatively high number of resolutions which exhibits the rotatable connection makes that such simple, economic sensors still can provide the required angular accuracy. The sensors are positioned between the accelerating and the decelerating gear drives. The combined ratio can be selected to fit the steering design, but is often suitably selected as 1. The electric motor can be positioned between the accelerating and the decelerating gear drives as well, other positions however are also possible, e.g. at the decelerating gear. Thus, an electric motor without gear reduction can be applied directly to the rotatable connection and to the linkage. The invention will now be described further with reference to an embodiment - 1 m in the figure. The figure shows a schematic lay out of an electric steering system 1 according to the invention, which is connected to the wheel 2 of a vehicle through a mechanical linkage 3 which is known per se and which will therefore not be described further. The steering system 1 comprises a steering wheel 4 as well as a flexible cable 5 which extends between the steering wheel 4 and the linkage 3. This cable can for instance be carried out as a so-called Bowden cable, that is a flexible cable with a stationary hollow outer tubular cable, in combination with a rotationally stiff inner cable accommodated within the outer cable. Such Bowden cables are known per se as well, and will therefore not be detailed further. At its drive end 6 the Bowden cable is connected to an accelerating gearbox 7, which in turn is connected to the steering wheel 4. At its driven end 8 the Bowden cable is connected to a decelerating gearbox 9, which in turn in connected to the linkage 3. The combined gear ratio of the accelerating gearbox 7 and the decelerating gearbox 9 can be 1; other ratios are possible as well. At the output end of the accelerating gear box 7, a sensorised bearing 10 is provided, and a similar sensorised bearing 11 is provided at the input end of the decelerating gear box 9. These sensorised bearings 10, 11 are connected to a control unit 12. Upon rotating the steering wheel 4, the sensorised bearing 10 senses the accelerated rotation at the output end of the accelerating gearbox 7. The control unit 12 receives signals from this sensorised bearing 10 and also from the sensorised bearing 11 at the input end of the decelerating gearbox 9. In case differences are measured, the control unit through line 15 delivers a control signal to the electric steering motor 16 so as to provide the desired rotational speed at the input end of the decelerating gear box 9, which of course depends on the rotations of the steering wheel 4. Preferably, the electric steering motor 16 provides the full rotational torque required driving the linkage system. In that case, the Bowden cable 5 functions merely as a mechanical back up, which would take over the steering control in case of an electric power failure. The steering device is connected to a force feedback motor 18 so as to simulate the forces which are exerted on the vehicle wheels during driving. This force feedback motor is controlled through line 17 by the control unit 12. The force feedback motor 18 ~ + ains a signal related to the power which is needed to drive the steering motor 16. The more power is delivered by the steering motor 16 for a steering action of the wheels 2, the more torque is then transferred to the steering wheel 4 by the force feedback motor 18.

Claims: Claims 1. Steering system (1) for an automotive vehicle, comprising a steering device (4) to be operated by a driver, a rotatable connection (5) having a drive end (6) connected to the steering device (4) and a driven end (8) connectable to a linkage (3) of the steering wheels (2) of the vehicle, characterised in that the drive end (6) of the rotatable connection (5) is provided with a first sensor (10), the driven end (8) of the rotatable connection (5) is provided with a second sensor (11) and an electric steering motor (16), the first and second sensors (10, 11) being connected to a control device (12), said control device (12) controlling the electric steering motor (16) on the basis of signals emanating from the first and second sensors (10, 11). 2. Steering system according to claim 1, wherein the first sensor (10) is incorporated in a first bearing and the second sensor (11) is incorporated in a second bearing, said bearings being connected to the rotatable connection (5) for supporting said rotatable connection (5) with respect to the vehicle. 3. Steering system according to claim 2, wherein the rotatable connection (5) comprises a flexible torque cable such as a Bowden cable. 4. Steering system according to claim 2 or 3, wherein the rotatable connection (5) comprises a shaft. 5. Steering system according to any of the preceding claims, wherein the drive end (6) of the rotatable connection (5) is provided with an accelerating gear drive (7) and the driven end (8) of the rotatable connection (5) is provided with a decelerating gear drive (9). 6. Steering system according to claim 5, wherein the sensors (10, 11) are positioned between the accelerating and the decelerating gear drives (7, 9). 7. Steering system according to claim 5 or 6, wherein the combined gear ratio of the ""--.derating and of the decelerating gear drives (7, 9) is about 1. 8. Steering system according to claim 6 or 7, wherein the electric steering motor (16) is positioned between the accelerating and decelerating gear drives (7, 9). 9. Steering system according to any of the preceding claims, wherein the steering device (4) is connected to a force feedback motor (18). 10. Steering system according to claim 9, wherein the force feedback motor (18) is controllable (17) by means of the control device (12). 11. Steering system according to claim 9 or 10, wherein the force feedback motor (16) is positioned between the accelerating and decelerating gear drives (7, 9).

Inventors: WITTMEIJER HANS HENNING, WITTMEIJER, HANS, HENNING
Applicants: SKF ENG & RES CENTRE BV, WITTMEIJER HANS HENNING, SKF ENGINEERING & RESEARCH CENTRE B.V., WITTMEIJER, HANS, HENNING
Application year: 2001
Main classification: B62D5
IPC codes: B62D1, B62D5