Abstract:
The invention concerns a vehicle steering column ( 2 ) comprising a manual control member ( 3 ) for applying a torque on the column ( 2 ) drive shaft ( 8 ), the column being equipped with a device for measuring the torque applied to the drive shaft ( 8 ), characterised in that the torque measuring device includes a sensing element without requiring a torsion bar to be located along the drive shaft ( 8 ) axis and thus producing a break therein.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the technical field of vehicle steering columns. Steering column conventionally means a tubular element, fixed to the body of the vehicle, under the dashboard, which guides and supports the drive shaft connected to the steering wheel. 
     This steering wheel is then a manual control member, connected to the steered wheels and used by the driver to steer the vehicle. 
     2. Description of the Prior Art 
     The invention also concerns decoupled steering columns. Unlike conventional steering columns, decoupled steering columns are not associated with a steering box converting the circular movement of the steering wheel into angular displacement of the steering rod which brings about turning of the wheels. 
     On the contrary, for these decoupled steering systems, there is no direct mechanical link between the steering wheel and the wheels resting on the ground, whether the vehicle is real or belongs to a simulation system. 
     This simulation can be for play purposes, connected with training in driving schools or perhaps connected with an interactive driving simulation for the requirements of motor vehicle manufacturers. 
     In such simulators, the restitution of forces at the steering wheel by virtue of a mechanism generating a load torque on the steering wheel according to the type of vehicle to be simulated, whether or not fitted with power-assisted steering, must take into account the driving conditions to be recreated. 
     Measurement of the torque applied to the steering wheel is consequently essential for providing a good simulation, in real time. 
     Measurement of the torque on the steering wheel shaft is also very important in servo-or power-assisted steering. 
     This is because initiation of the assistance depends notably on the torque applied by the driver on the steering wheel. 
     The torquemeter, or torque sensor, used in servo-steering outputs a signal indicative of the turning torque exerted by the driver on the steering wheel and therefore on the drive shaft of the vehicle steering column. 
     This signal is conventionally sent to a steering assistance computer which initiates the assistance, by controlling for example an electric motor, in the case of electric servo-steering. 
     The majority of torquemeters comprise a torsion bar. 
     Under isotropic linear elasticity, the torque exerted in terms of pure torsion on a solid cylindrical bar of circular section varies, for a given material, as a function of the fourth power of the diameter of the bar, at a fixed angle of torsion. 
     Consequently, the fact of making an area of reduced cross section in the steering column allows a concentration and amplification of torsional deformation in this area, used for measuring the torque, to the detriment of the rigidity and mechanical strength of said torsion bar. 
     Examples of torsion bar torquemeters for servo-steering can be found in the following documents: 
     WO-97 08 527, EP-453 344, EP-325 517, FR-2 738 339 originating from the applicant, 
     as well as in the following other documents: 
     patent applications in Japan published under numbers JP-43 17 862, JP-50 77 743, JP-57 19 81 71, JP-59 07 58 64, JP-57 08 77 62, JP-59 11 85 77, JP-61 14 674, JP-62 13 43 71, JP-62 09 44 70, JP-63 29 037, JP-63 09 36 73, JP-30 79 473; 
     European patents published under numbers EP-369 311, EP-396 895, EP-418 5 763, EP-515 052, EP-555 987, EP-562 426, EP-566 168, EP-566 619, EP-652 424, EP-638 791, EP-673 828, EP-681 955, EP-728 653, EP-738 647, EP-765 795, EP-738 648, EP-770 539, EP-802 107; 
     PCT international patent applications published under numbers 87/02 319, WO-92/20 560, WO-95/19 557, WO-96/06 330; 
     patent applications in France, the United Kingdom and the United States published under the following numbers: GB-2 306 641, FR-2 705 455, U.S. Pat. Nos. 4,874,053, 4,907,668, 4,984,474, 5,123,279, 5,394,760, 5,515,736, 5,578,767, 5,585,573, 5,616,849, 5,641,916. 
     Torsion bar torquemeters are large in size. Their installation requires two additional bearings to lessen notably spurious flexion effects. 
     The document FR-2 724 018 relates to a torque sensor comprising an elongation measurement gauge device. This gauge device is disposed on a measuring body, subject to mechanical tensions under the action of a torque. 
     The document EP-442 091 describes an installation for measuring the angle of rotation or the torque of a rotary or fixed element of a machine, having a torsion element in the form of a spoked wheel connected to a number of measuring elements, at least one spoke of the spoked wheel being cut so that the parts of the spoke or spokes are applied one against the other upon the displacement by a predetermined flexion of the other spokes. The measuring device uses eddy currents. 
     SUMMARY OF THE INVENTION 
     The invention relates to a torque measurement system not having the drawbacks of the systems known in the prior art for steering columns and capable of being installed, possibly reversibly, at any point on the steering column, this device being insensitive to electromagnetic interference and being rigid, consisting of a single piece, with high inertia and of small size and low cost. 
     To that end, the invention relates to a vehicle steering column comprising a manual control member for applying a torque on the drive shaft of the column, said column being provided with means for measuring the torque applied to said drive shaft, said measuring means comprising a sensing element with no torsion bar placed along the axis of the drive shaft and producing a break therein. 
     In one embodiment, the column is of the decoupled type. 
     In another embodiment, the column has a steering box converting the rotating movement of the drive shaft into angular displacement of the steering rod which brings about turning of the wheels. 
     The column has assistance means whose initiation depends notably on the torque applied to the drive shaft by operation of the control member. 
     Assistance calculation means are provided, sending signals to the means for controlling the assistance means, depending on the turning torque applied to the drive shaft. 
     The means for measuring the torque applied to the drive shaft are formed from an assembly having a certain rigidity comprising: 
     a displaceable first outer ring; 
     a second outer ring, substantially unstressed, placed at a distance from the displaceable outer ring; 
     an inner ring immovably attached rotation-wise to a column on which the torque to be measured is applied, the displaceable outer ring being capable of being immovably attached rigidly to the means applying the torque to be measured on the column, the displaceable outer ring being assembled with the inner ring by at least one elastically deformable means; 
     the substantially unstressed outer ring being assembled with the inner ring by at least one substantially unstressed means; 
     the sensor comprising means of measuring the small relative displacement of the displaceable outer ring with respect to the substantially unstressed outer ring when a torque is applied on the column by the means. 
     Other objects and advantages of the invention will emerge during the following description of embodiments, a description which will be given with reference to the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a  and  1   b  are block diagrams of vehicle steering systems comprising a torque measurement system according to the invention; 
     FIG. 2 is an exploded perspective view of a steering column having a torque sensor with flexion sensing element, integrated with the steering wheel, according to one embodiment of the invention; 
     FIG. 3 is a perspective view of the sensing element depicted in FIG. 2; 
     FIG. 4 is a front view of a flexion sensing element capable of taking the place of that depicted in FIGS. 1 and 2; 
     FIG. 5 is a view of a torsion sensing element capable of taking the place of the sensing elements depicted in FIGS. 1 to  4 ; 
     FIG. 6 is an exploded perspective view of a vehicle steering column comprising a sensing element as depicted in FIG. 5; 
     FIG. 7 is a sectional view of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is made first of all to FIG. 1 
     The steering device  1 , a block diagram of which is depicted in FIG. 1, is intended to be incorporated into a vehicle, for example a motor car. 
     This device  1  comprises a steering column  2 . This column  2  can be telescopic, as applicable, and/or of variable inclination with respect to the vehicle body and the driver. 
     A steering wheel  3  is associated, possibly so as to be removable, with this steering column. 
     Steering wheel means a manual control member used by the driver to steer the vehicle. 
     In one embodiment, the steering column  2  is associated with a steering box  4 , for example a rack, converting the circular movement of the steering wheel  3  into angular displacement of the steering rod which brings about turning of the wheels  5 . 
     This embodiment corresponds to diagram a) of FIG.  1 . 
     In another embodiment, the wheels  5  are not connected by a series of mechanical elements with the steering wheel  3 . The steering is then referred to as decoupled. 
     This embodiment corresponds to block diagram b) of FIG.  1 . 
     A torque measurement system  6  is associated with the steering device  1 . As this torque measurement system is capable of being installed, possibly so as to be removable, at any point of the steering device, no one-to-one link has been depicted between this measurement system  6  and the different elements of the steering device  1  in the block diagrams of FIG.  1 . 
     Reference is now made to FIG. 2 which is an exploded perspective view of a steering column having a torque sensor with flexion sensing element, added on to the steering wheel, according to one embodiment. 
     The steering device  1  depicted in FIG. 2 has a control assembly  7  under the steering wheel. 
     The control assembly  7  under the steering wheel comprises a central block having a through hole  7   b  of diameter such that the assembly  7  can be fitted on the drive shaft  8  of the steering column  2 . 
     In the present embodiment, the end part of the drive shaft is of substantially circular cross section, the through hole  7   b  also being circular in cross section, i.e. perpendicular to the main axis D of the steering device  1 . 
     The steering device  1  comprises a rotary connector  9  provided with two pins and a central through hole. 
     The steering device  1  also comprises a torquemeter whose sensing element is added into the hub  10  of the steering wheel  3 . 
     As applicable, as depicted in FIG. 2, the steering wheel  3  has in its central part an air-bag casing  11 . 
     In this case, the rotary connector  9  can serve as both the connection for the airbag casing  11  and the connection between the electronic circuit  12  of the servosteering torquemeter and the assistance means. 
     In other embodiments, the steering device  1  has no air-bag casing. 
     The steering wheel  3  has a rim  13  and a frame  14 . 
     In the embodiment depicted, the frame  14  appears in the form of a transverse arch  14   a  connected to the rim  13  by four inclined arms  14   c.    
     The transverse arch  14   a  has through holes  14   b  made in it, allowing the passage of screws  15  for fixing-the hub  10  on the steering wheel  3 . 
     The hub  10  will now be described with reference to FIGS. 3 onwards. 
     In the embodiment depicted in FIG. 3, the hub  10  comprises a flexion sensing element. 
     A cylindrical inner ring  16  and two outer rings  17   a  and  17   b  are connected by elastic braces deformable flexion-wise  18   a  and non-deformed braces  19 . More precisely, the rear outer ring  17   a,  fixed to the hub  10  of the steering wheel  3  by screws  15  passing through holes  20 , is connected to the inner ring  16  by means of elastic braces deformable flexion-wise  18   a.    
     In the embodiment depicted, there are  4  of these deformable braces  18   a,  evenly distributed, perpendicular to the main axis D. 
     The front outer ring  17   b  is connected to the inner ring  16  by means of nondeformed radial braces  19 . 
     In the embodiment depicted, there are the same number of these non-deformed braces  19  as there are braces elastically deformable flexion-wise  18   a,  the braces  18   a,    19  being situated substantially along two radial planes perpendicular to the main axis D. 
     In other embodiments, not depicted, there are two, three or four braces  19 . 
     In other embodiments, not depicted, the outer ring  17   b  is connected to the inner ring by a plate or an annular wall. 
     The deformable outer ring  17   a  of the hub  10  is rigidly linked to the arch  14   a  of the steering wheel  3 , the screws  15  passing through the holes  14   b  of the arch  14   a  in order to reach the holes  20  provided in the fixing lugs  21  of the outer ring  17   a.    
     At the same time, a screw  22  fixes the steering wheel  3  on to the column  2 , the control assembly  7  under the steering wheel, the rotary connector  9 , and the hub  10  being sandwiched between the arch  14   a  of the steering wheel  3  and the steering column, over a length corresponding to the end p art of the drive shaft  8  of the column  2 . 
     The electronic circuit  12  is added on, or affixed, to the hub  10 . 
     When the driver exerts a force on the rim  13  of the steering wheel  3 , the rear outer ring  17   a  immovably attached to the steering wheel  3  brings about a flexion-wise deformation of the braces  18   a,  said deformation being all the greater, the larger the load torque on the column  2 . 
     As for the front outer ring  17   b,  this remains substantially unstressed. 
     Its position can consequently serve as a reference basis for measuring the displacement of the rear outer ring  17   a.    
     The front outer ring  17   b  carries sensors  23  of small displacements. These sensors perform a local detection and a variable number thereof can be disposed. 
     In the embodiment depicted, there are two of these sensors  23  and they are disposed in housings  24  made in the front ring  17   b,  in line with the rear ring  17   a.    
     These sensors  23  can be of any suitable type: optical, capacitive, electromagnetic, magnetoresistive, etc. sensors. 
     In one embodiment, these are Hall effect sensors. The magnetic field generators then have, in on e emb odime nt, a magnetized structure with antiparallel magnetization directions and are immovably attached to the drive shaft of the steering column. The Hall probes thus deliver a signal proportional to the torsional torque as a result of the relative angular shift of the field generators with respect to the probes. 
     Each field generator can be constituted by two parallelepipedal magnets contiguous along one of their lateral faces, the respective induction vectors being of opposite senses, perpendicular to said attachment face. 
     In another embodiment, these sensors are magnetoresistive (MR) or giant magnetoresistive (GMR) sensors. 
     Although a single Hall effect probe  23  is sufficient for measuring small displacements, a number of probes can, for reasons of reliability, be disposed in the measurement air gap  25  in order to create redundancy. 
     Each of the Hall effect probes  23  can have its own associated electronic circuit. 
     By comparison or combination of the signals delivered by 2, 3 or 4 different probes, it is possible to detect any failure of one of the probes and provide the torquemeter with excellent reliability. 
     The thickness, length and width of the braces, as well as their number, their angular distribution, and the material used to make them, influences, as will be clearly apparent to persons skilled in the art, the following characteristics: 
     moment of inertia; 
     maximum stress in the braces, for a given maximum torque, for example breaking stress; 
     value of the measurable minimum torque, for a technique of measuring given small displacements. 
     As applicable, means can be provided to limit the displacement of the ring  17   a.  For example, two limit stop braces can extend radially in a transverse direction from the inner ring  16  to the deformable ring  17   a.    
     These limit stops make it possible, as applicable, to avoid a plastic deformation of the braces  18   a.    
     The sensing element can be made out of a material chosen from amongst the group comprising: steel, cast iron, aluminium alloys, magnesium alloys. 
     A 35NCD16 steel, a spheroidal graphite iron, or a 7000 series aluminium can for example be considered. 
     The sensing element can be cast or machined, depending on the material used, the geometry of the braces, and the acceptable cost notably, as will be apparent to persons skilled in the art. 
     When the sensing element is cast in aluminium or in magnesium alloy, the casting can be performed with a metal insert having grooves for mounting the sensing element on the drive shaft  8 . 
     Reference is now made to FIG. 5 which illustrates one embodiment of a torsion sensing element hub. 
     The hub  10  has an unstressed outer ring  17   b,  with a substantially cylindrical outer peripheral surface. 
     This ring  17   b  is provided with two housings  24 , made in two diametrically opposite areas of extra thickness  26 . 
     Between these areas of extra thickness  26 , the inner surface of the ring  17   b  is substantially cylindrical. 
     The ring  17   b  is assembled with the inner ring  16  by at least one brace  19 , a plate or equivalent. 
     In the embodiment depicted, two radial braces  19 , made in one piece with the inner ring  16  and the unstressed outer ring  17   b,  link these two rings  16 ,  17   b.    
     These braces  19  are, in the embodiment depicted, of square section substantially constant from their foot  27  to their head  28  and are substantially aligned. 
     The inner ring  16  has a through hole defining a grooved fixing sleeve  29  and, on the opposite side, a surface  30  for resting on the end part of the drive shaft  8 , when the sensing element is intended to be incorporated at the end part of the drive shaft  8 . 
     A tube, deformable torsion-wise,  18   b  connects the inner ring  16  to the displaceable outer ring  17   a.    
     As applicable, this tube can be punched axially, the axial openings thus made separating braces deformed under axial torsion. 
     This displaceable outer ring  17   a  is assembled rigidly with the hub applying the torque to the drive shaft  8 , namely the steering wheel  3 . 
     Screws  15  provide, via the holes  20 , the fixing of the hub  10  on the transverse central plate  31  of the frame  14  of the steering wheel  3 . 
     This plate  31  is provided with holes  14   b  corresponding to the holes  20  in the hub  10 . 
     The frame  14  of the steering wheel has, just as in the embodiments depicted in FIG. 2, a number of inclined arms  14   c  connecting the transverse central plate  31  to the rim  13  of the steering wheel  3 . 
     When the hub  10  is mounted on a steering wheel  3 , in the manner depicted, the deformable outer ring immovably attached to the plate  31  and therefore to the rim  13  of the steering wheel  3  is displaced rotationally with respect to the unstressed outer ring  17   b.    
     Measurement of this small displacement, for example by means of Hall probes  23  placed in the housings  24  and magnets  23 ′ fixed opposite on the plate  31 , allows measurement of the torque applied by the driver and allows control of the assistance after processing of the signal by the electronic circuit  12 . 
     This electronic circuit  12  comprises, in one embodiment: 
     a current input for supplying the Hall probes with power; 
     a circuit for filtering the signal coming from the probes, in order to eliminate the background noise; 
     a module providing analogue to digital conversion of the signal; 
     a module for checking and compensating for drift of the signal emitted by the Hall probes, as a function of the temperature, for example in a range −40° C. to +80° C.; 
     a security module regularly testing the correct operation of each probe. 
     As applicable, the electronic circuit  12  has a module making it possible to fix the initiation threshold for the steering assistance, said threshold corresponding to a given torque value, or perhaps a wireless or contactless signal transmission module. 
     The torque sensors as just depicted with reference to FIG. 2 onwards can, in other embodiments, not be added on to or integrated with the steering wheel, but be placed at any point whatsoever between the steering wheel and the steering column universal joints. 
     Persons skilled in the art, as will be clearly apparent to them, must take into account the following factors, notably: 
     any electric assistance motor must be placed before the steering column lock; 
     the volumes laid down by crash tests (simulated accident tests or impact tests) and the ergonomics of the driving position influence the general size of the steering column  2 . 
     The torque measurement system according to the invention can thus be installed, possibly so as to be removable, at any point on the steering column. This torque measurement system has a sensing element which is robust, rigid, of small dimensions and separable from the geometry of the column. 
     The potential for standardization is thus increased. The structure of the torque sensor makes it possible to avoid repeated integration studies for the torquemeter depending on the types of column envisaged.