Friction brake mechanism and associated steering column

Abstract

In order to provide a fail-safe operating mode during a transition between an autonomous steering mode and a driver-controlled steering mode in an emergency situation, a steering system comprises a steering shaft, a brake mechanism movable between an released position and a engaged position for angularly locking the steering shaft, and an override such that a non-destructive external torque applied to the steering shaft above a given threshold when the brake mechanism is in the engaged position results in a rotation of the steering shaft.

Claims

1. A steering system comprising a steering shaft and a brake mechanism movable between a released position and an engaged position for angularly locking the steering shaft, wherein the brake mechanism comprises an override such that a non-destructive external torque applied to the steering shaft above a given torque threshold when the brake mechanism is in the engaged position results in a rotation of the steering shaft, wherein the brake mechanism comprises: a friction element non-rotatable relative to the steering shaft; a first locking element provided with an annular friction face that faces the friction element; at least one biasing spring for biasing the first locking element into frictional engagement with the friction element and for applying a predetermined biasing force to the first locking element in the engaged position, the predetermined biasing force being such that the first locking element starts slipping, relative to the friction element, upon application of the non-destructive external torque greater than the torque threshold; and a second locking element and an actuator for operably engaging second locking element with the first locking element for angularly locking the first locking element in the engaged position.

2. The steering system of claim 1, further comprising an adjustable abutment for loading the biasing spring, the adjustable abutment being fixed at an adjustable position in relation to the steering shaft.

3. The steering system of claim 1, wherein the biasing spring is a helicoidal spring wound around the steering shaft.

4. The steering system of claim 1, wherein the torque threshold is more than 1 Nm and less than 13 Nm.

5. The steering system of claim 1, wherein the actuator is an electromechanical actuator.

6. The steering system of claim 1, wherein at least one of the first and second locking elements includes an annular interface in contact with the other of the first and second locking elements in the engaged position.

7. The steering system of claim 1, wherein the first and second locking elements are positively engaged with one another in the engaged position.

8. The steering system of claim 1, wherein at least one of the first and second locking elements includes an annular interface with a number of protrusions and/or recesses distributed over a circumference of the annular interface.

9. The steering system of claim 1, wherein the first and second locking elements are frictionally engaged with one another in the engaged position.

10. The steering system of claim 1, further comprising a bearing between the first locking element and the steering shaft.

11. A steering system comprising a steering shaft and a brake mechanism movable between a released position and an engaged position for angularly locking the steering shaft, wherein the brake mechanism comprises an override such that a non-destructive external torque applied to the steering shaft above a given torque threshold when the brake mechanism is in the engaged position results in a rotation of the steering shaft, wherein the brake mechanism comprises: a friction element non-rotatable relative to the steering shaft; a first locking element provided with an annular friction face that faces the friction element; at least one biasing spring for biasing the first locking element into frictional engagement with the friction element and for applying a predetermined biasing force to the first locking element in the engaged position, the predetermined biasing force being such that the first locking element starts slipping relative to the friction element upon application of the non-destructive external torque greater than the torque threshold; and a second locking element and an actuator for positively engaging the second locking element with the first locking element for angularly locking the first locking element in the engaged position.

12. A steering system comprising a steering shaft defining a steering axis, and a brake mechanism movable between a released position and an engaged position for angularly locking the steering shaft, wherein the brake mechanism comprises an override such that a non-destructive external torque applied to the steering shaft above a given torque threshold when the brake mechanism is in the engaged position results in a rotation of the steering shaft, wherein the brake mechanism comprises: a friction element non-rotatable relative to the steering shaft; a toothed sleeve rotatable relative to the steering shaft; at least one biasing spring for axially biasing the toothed sleeve into frictional engagement with the friction element and for applying a predetermined biasing force to the toothed sleeve, the predetermined biasing force being such that the friction element and the toothed sleeve start slipping upon application of the non-destructive external torque greater than the torque threshold; a locking element; and an actuator for operably engaging the locking element with the toothed sleeve for angularly locking the toothed sleeve in the engaged position.

Description

DESCRIPTION OF THE FIGURES

(1) Other advantages and features of the invention will become more clearly apparent from the following description of specific embodiments of the invention given as non-restrictive examples only and represented in the accompanying drawings, in which:

(2) FIG. 1 illustrates a steering system according to one embodiment of the invention;

(3) FIG. 2 is an isometric view of a steering column of the steering system of FIG. 1;

(4) FIG. 3 is an exploded view of a part of the steering system of FIGS. 1 and 2, showing the assembly of a braking system on the steering column;

(5) FIG. 4 is a more detailed exploded view of the braking system of FIG. 3;

(6) FIG. 5 is a section of a steering shaft and of the braking system of FIG. 3;

(7) FIG. 6 is a section of an alternative embodiment of the braking system;

(8) FIG. 7 illustrates an alternative embodiment of the steering system.

(9) Corresponding reference numerals refer to the same or corresponding parts in each of the figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(10) Referring to FIG. 1, a vehicle is provided with a steering system 10 including a steering wheel 12, a steering shaft 14, a pinion gear 16 that rotates with the steering shaft 14 and meshes with a toothed rack 18 for moving tie rods 20 linked to steerable ground wheels 22. The steering wheel 12 is connected to the steering shaft through a mechanical or electrical clutch mechanism 26. A steering motor 28 is connected with the steering shaft and controlled by a steering control unit 30. Rotation and/or torque sensors 32, 34 are provided on the steering wheel 12 and on the steering shaft 14 and connected to the control unit 30. As such, this steering system 10 is known from US 2013002416, which is incorporated herewith by reference.

(11) The steering system 10 is further provided with a friction brake mechanism 36, which is illustrated in detail in FIGS. 2 to 5. The brake mechanism 36 comprises an actuator 38 received in an actuator housing 40 clamped with a C-shaped clamp 42 to a jacket tube 44 in which the steering shaft 14 is mounted for rotation about a steering axis 100. The actuator 38, which can be of any suitable construction, moves a toothed lock bolt (e.g., a second locking element) 46 back and forth against a toothed sleeve (e.g., a first locking element) 48 in a radial direction respective to the steering axis 100. A needle bearing 50 is provided to guide the toothed sleeve 48 in rotation about the steering axis 100. The toothed sleeve 48 is pressed fitted onto an outer race ring of the needle bearing. The toothed sleeve has a first annular planar face 52 in frictional contact with a flat abutment ring (e.g., a friction element) 54 press-fitted onto the steering shaft 14, and a second annular face 56 opposed to the first annular face 52. A compression spring 58 is loaded between the second annular face 56 and an adjustable thrust washer 60, which is connected to the steering shaft 14 by press-fitting at a proper distance from the abutment ring 54 to achieve a desired axial force through the compression spring 58 at the interface between the toothed sleeve 48 and the abutment ring 54. As will be apparent to those skilled in the art, the brake mechanism does not impart any axial thrust on the steering shaft, since both the adjustable thrust washer and the abutment ring sit on the steering shaft.

(12) A switch 62 may be provided to allow the driver to switch between a driver-controlled steering mode and an autonomous steering mode. This switch 62 may be located on the dashboard or integrated to the steering wheel 12, e.g. as disclosed in US2013002416.

(13) In the driver-controlled steering mode and in the autonomous steering mode, the brake mechanism is disengaged, i.e. the toothed lock bolt 46 is out of engagement with the toothed sleeve 48, which can rotate together with the abutment ring 54, the compression spring 58, the adjustable thrust washer 60 and the steering shaft 14 as one unit.

(14) In the driver-controlled steering mode, the clutch 26 is engaged and the steering wheel 12 is mechanically connected to the steering shaft 14 and the ground wheels 22. The motor 28 may or may not be used as power assistance for supplementing the torque delivered by the driver on the steering wheel 12.

(15) In the autonomous steering mode, the clutch 26 is disengaged and the motor 28 applies a driving torque to the steering shaft 14 irrespective of the position of the steering wheel 12. In the autonomous steering mode, the driver may regain at least partial control of the steering by turning the steering wheel 12 by a predetermined angle, which is detected by the sensor 32. A haptic feedback (e.g. click) may be provided to inform the driver that his or her request to regain partial or total control of the steering system 10 has been registered and executed. Various transition strategies can be configured in response to the input of the driver and of various other parameters of the vehicle: the clutch 26 may or may not remain disengaged, and if not, the motor 28 may or may not be used as power steering assistance.

(16) If on the other hand the autonomous steering mode has to be interrupted as a matter of urgency at the initiative of the steering control unit 30, e.g. because of a malfunction of the motor 28 or of a sensor, before the driver has had time to regain control of the steering, the autonomous steering control unit generates an alarm to inform the driver and at the same time reengages the clutch 26 and the brake mechanism 36. The toothed lock bolt 46 engages the toothed sleeve 48 so as to prevent its rotation. The toothed sleeve 48 is biased by the loaded compression spring 58 in frictional engagement with the flat abutment ring 54 and prevents rotation of the steering shaft 14. As a result, the rack and pinion transmission 18, 16, tie rods 20 and ground wheels 22 remain in the position assumed at the beginning of the transition, to give the driver time to regain control of the steering wheel 12. As soon as the driver imparts torque to the steering wheel 12, the sensor 32 detects the applied torque and the control unit disengages the toothed lock bolt 46 to release the steering shaft 14. Should however the toothed lock bolt 46 remain engaged, e.g. because of a malfunction of the sensor 32, of the actuator 38 or of the control unit 30, the driver can override the brake mechanism 36 by applying a torque that is higher than the rated frictional torque resulting from the axial force of the compression spring 58 onto the toothed sleeve 48, to maneuver the vehicle to a full stop in a fail-safe mode. During this maneuver, the brake mechanism 36 remains engaged, which means that the driver has to continuously overcome the rated frictional torque of the brake mechanism. For this reason, the rated frictional torque of the brake mechanism should preferably be both of a sufficient magnitude to prevent rotation of the steering shaft before the driver has gripped the steering wheel, and sufficiently low to allow the driver to turn the steering wheel.

(17) In the embodiment illustrated in FIG. 6, the brake mechanism 36 is mounted on a sleeve 70 to create a stand-alone module. The sleeve 70 can be press fitted or welded on the steering shaft 14.

(18) In the embodiment of FIG. 7, the brake mechanism 36 is inserted in a steering system 10 without a clutch, including a steering wheel 12, a steering shaft 14, a pinion gear 16 that rotates with the steering shaft 14 and meshes with a toothed rack 18 for moving tie rods 20 linked to steerable ground wheels 22. A steering motor 28 is connected with the steering shaft and controlled by a steering control unit 30. The steering wheel 12 is connected to a feedback motor 80. Rotation and/or torque sensors 32, 34 are provided on the steering wheel 12 and on the steering shaft 14 and connected to the control unit 30.

(19) Various other modifications are contemplated. Depending on the packaging area available, the compression spring 58 can be replaced with a Belleville washer. The bearing 50 for guiding the toothed sleeve can be of any type, e.g. a roller bearing with or without cage, or a sliding bearing to achieve weight reduction or meet space requirements. Tuning can be achieved by proper selection of the materials and surface roughness or friction coefficient at the friction interface between the flat abutment ring 54 and the toothed sleeve 48. In particular, plastic, rubber, ceramic and/or organic resin can be used as well as steel. The adjustable thrust washer and/or abutment ring can be replaced with nuts if the shaft has provision for external threads. This allows a very precise tuning of the slipping torque.