Abstract:
In order to provide a fail-soft 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.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention relates to a steering system that provides a fail-soft operating mode during a transition between an autonomous steering mode and a driver-controlled steering mode, in particular in an emergency situation. 
       BACKGROUND ART 
       [0002]    To address the issue of an ergonomic and intuitive voluntary transition from an autonomous steering mode to manual steering, it has been suggested in US 2013 002416 to provide a vehicle steering system with an autonomous interface steering wheel movable from a first position to a second position and an autonomous driving electronic control unit configured to receive a signal indicating that the autonomous interface steering wheel has been moved from the first position to the second position and initiate change of operation of the vehicle between the manual steering mode and the autonomous steering mode in response to receiving the signal. However, this system does not address the issue of a safe transition from the autonomous steering mode to the manual or driver-controlled steering mode at the initiative of the autonomous driving electronic control unit, e.g. in an emergency situation. 
         [0003]    In a somewhat different context, U.S. Pat. No. 6,580,989 discloses a so-called hybrid-type steering system selectively operable in one of three modes: steer-by-wire, electronic power assist steering (EPAS), and manual steering. The steer-by-wire system includes a driver interface system (DIS), a road wheel actuator system (RWAS), and a controller for monitoring and implementing a preferred control strategy. The steering system operates normally in a steer-by-wire mode. Operation in the alternate EPAS mode and manual mode is determined by a controller in response to a malfunction in any part of either the DIS or the RWAS. In each of the EPAS mode and manual mode, the controller causes a clutch mechanism to engage, thus creating a mechanical linkage between the steerable member and the rack and pinion system. In the EPAS mode, one of the road wheel actuator or the reaction torque generator is available to assist in the steering operation. Alternatively, in the manual mode, both the DIS and the RWAS are deactivated and the vehicle is steerable through entirely mechanical means. As the driver is required to hold the steering wheel in all three operating modes, no particular measures have to be taken in the transition between the different steering modes. 
         [0004]    Accordingly, there is a need for means to ensure a safe transition from an autonomous steering to a driver-controlled steering in a vehicle when the driver is unaware of the imminence of this transition. 
       SUMMARY OF THE INVENTION 
       [0005]    According to one aspect of the invention, there is provided a steering system comprising a steering shaft and a brake mechanism movable between an released position and a 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 threshold when the brake mechanism is in the engaged position results in a rotation of the steering shaft. During a transition from an autonomous steering mode to driver-controlled steering mode, the brake mechanism can be moved to the engaged position for angularly locking the steering shaft mechanically linked to the ground wheels while the driver regains control of the steering wheel. Once the hands of the driver are on the steering wheel, the vehicle can be steered by applying on the steering wheel a torque above the threshold of the override. 
         [0006]    According to a preferred embodiment, the override comprises a first friction element and a second friction element frictionally engaged with one another in the engaged position. The torque threshold for overriding the brake mechanism is given by the pressure between the first and second friction elements. According to one embodiment, the first friction element non-rotatable relative to the steering shaft, at least in the engaged position. The second friction element is non-rotatable relative to the brake mechanism in the engaged position. The second friction element may be rotatable relative to the brake mechanism in the released position and locked to the brake mechanism (and hence to a dashboard of the vehicle) to engage the brake. 
         [0007]    According to a preferred embodiment, a biasing mechanism is provided for engaging with one another the first and second friction elements. The biasing mechanism preferably comprises at least one biasing spring for applying a predetermined biasing force to at least one of the first and second friction elements. The predetermined biasing force is preferably such that the first and second friction elements start slipping upon application of a torque at least equal to the torque threshold. The torque threshold is preferably more than 1 Nm and less than 13 Nm. An adjustable abutment fixed at an adjustable position in relation to the steering shaft can be provided for loading the biasing spring. A particularly compact mechanism can be achieved if the biasing spring is a helicoidal spring wound around the steering shaft. 
         [0008]    As will be readily understood, the system can be tuned in various manners before it is mounted on the vehicle to define a desired torque threshold for the override: the material of the friction elements can be changed. If a biasing spring is provided, the stiffness of the spring can be changed. The easiest way of adapting the torque threshold without changing any of the parts of the steering system, however, is to change the location of the adjustable abutment. 
         [0009]    According to one embodiment, the brake mechanism comprises a first braking element, a second braking element, and an actuator, for engaging with one another the first and second braking elements in the locking position. The actuator is preferably an electromechanical actuator. 
         [0010]    According to one embodiment, at least one the first and second braking elements includes an annular interface in contact with the other of the first and second braking elements in the engaged position. 
         [0011]    According to one aspect of the invention, the first and second braking elements are positively engaged with one another in the engaged position. In particular, at least one of the first and second braking elements may include an annular interface with a number of protrusions and/or recesses distributed over a circumference of the annular interface. 
         [0012]    According to another aspect of the invention, the first and second braking elements are frictionally engaged with one another in the engaged position. 
         [0013]    According to one embodiment, the override comprises a friction dutch including a first friction element non-rotatable relative to the steering shaft and a second friction element non-rotatable relative to an annular interface of an annular braking element of the brake mechanism. The steering system may further comprise a bearing between the annular braking element and the steering shaft. 
         [0014]    According to another aspect of the invention, there is provided a steering system comprising a steering shaft, a brake mechanism movable between an released position and a engaged position for angularly locking the steering shaft in several locking positions, and an override to allow rotation of the steering shaft upon application of a non-destructive external torque to the steering shaft above a given threshold when the brake mechanism is in the engaged position. 
         [0015]    According to another aspect of the invention, there is provided a method of controlling a vehicle steering system during a transition from an autonomous operation mode to a manual mode, wherein a steering shaft is locked at an actual position at the end of the autonomous operation mode and until a driver manually applies a torque above a given threshold on the steering wheel. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0016]    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: 
           [0017]      FIG. 1  illustrates a steering system according to one embodiment of the invention; 
           [0018]      FIG. 2  is an isometric view of a steering column of the steering system of  FIG. 1 ; 
           [0019]      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; 
           [0020]      FIG. 4  is a more detailed exploded view of the braking system of  FIG. 3 ; 
           [0021]      FIG. 5  is a section of a steering shaft and of the braking system of  FIG. 3 ; 
           [0022]      FIG. 6  is a section of an alternative embodiment of the braking system; 
           [0023]      FIG. 7  illustrates an alternative embodiment of the steering system. 
       
    
    
       [0024]    Corresponding reference numerals refer to the same or corresponding parts in each of the figures. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]    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. 
         [0026]    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  46  back and forth against a toothed sleeve  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  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. 
         [0027]    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. 
         [0028]    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. 
         [0029]    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 . 
         [0030]    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. 
         [0031]    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 dutch  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-soft mode. During this man oeuvre, 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. 
         [0032]    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 . 
         [0033]    In the embodiment of  FIG. 7 , the brake mechanism  36  is inserted in a steering system  10  without 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 . 
         [0034]    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.