Abstract:
The invention relates to a joystick ( 1 ) for controlling an aircraft, including a frame ( 2 ), a lever ( 3 ), a mechanical linking assembly ( 4 ) for connecting the lever to the frame enabling a rotation of the lever ( 3 ) relative to the frame ( 2 ) about a first rotation axis (X), in which the mechanical linking assembly ( 4 ) includes a first pivot joint ( 9 ), the first pivot joint ( 9 ) including a first portion, a second portion movably mounted relative to the first portion, and at least two flexible blades, each flexible blade having an end that is attached to the first portion and another end that is attached to the second portion, and being resiliently deformable in order to enable a rotation of the second portion relative to the first portion along the first rotation axis (X) and to generate a return torque which hinders the rotation of the second portion relative to the first portion.

Description:
FIELD OF THE INVENTION 
     The invention relates to a joystick for controlling an aircraft. 
     PRIOR ART 
     Conventional aircraft are known, the flight controls of which are controlled by shifting a control lever connected mechanically to the orientable flight controls of the wings. 
     Aircraft for which orientation of the flight controls is performed by electric and/or hydraulic actuators controlled by sensors for sensing movement of the lever are also known. On such devices, the lever is therefore not linked mechanically to the flight controls and the pilot does not sense any resistance from the lever, which lets him estimate movements of the flight controls and the forces they undergo. 
     But some control levers are equipped with active and/or passive devices for simulating a feedback force on the lever. 
     Also, in some planes equipped with electronic flight controls, the control stick has been replaced by a control device called a “joystick”. More compact than a conventional control stick, the joystick is generally integrated into a pilot&#39;s seat armrest and comprises a lever which the pilot operates solely by the movement of his wrist. Installing joysticks has freed up the space between the pilot and the dashboard so that other equipment can be installed. 
     The joystick generally includes a set of springs for exerting a return force on each of the axes of rotation of the lever (roll axis and pitch axis) and to return the lever to a neutral position when the pilot exerts no force on the lever. 
     As the joystick is controlled by way of the wrist, return forces to be generated are much weaker than return forces generated on traditional control levers. 
     At the same time, the sensitivity of the pilot to the performance of the joystick is increased. It is therefore important to be able to generate return forces according to a law of force defined precisely and stably (that is, reproducible). However, the existence of friction in existing mechanisms tends to deteriorate behaviour of the joystick as is sensed by the pilot. 
     In particular, when the joystick is equipped with force sensors, friction occurs between the pieces of the mechanism causing bias and hysteresis in the measurements made by these sensors, such that the force feedback cannot be generated precisely. 
     Apart from friction problems, the usual passive devices are often a source of non-linearity of the elastic force, or even of coupling force, caused by displacement of an axis on the force sensed on the normally independent second axis. There are so many phenomena which contribute to imprecision of the force feedback. 
     SUMMARY OF THE INVENTION 
     An aim of the invention is to propose a joystick for controlling an aircraft, which is simple, robust and has no risk of jamming, to create return forces with good precision and stably. 
     This aim is attained, within the scope of the present invention, by a joystick for controlling an aircraft, comprising:
         a frame,   a lever,   a mechanical linking assembly of the lever au frame enabling rotation of the lever relative to the frame according to a first axis of rotation,   wherein the mechanical linking assembly comprises a first pivot joint, the first pivot joint comprising a first part, a second part mounted mobile relative to the first part, and at least two flexible blades, each flexible blade having a end fixed to the first part and another end fixed to the second part and being resiliently deformable to enable rotation of the second part relative to the first part according to the first axis of rotation and generating a return torque tending to oppose rotation of the second part relative to the first part.       

     The structure of the pivot joint used in the mechanical linking assembly guides the lever in rotation without creating friction and at the same time generates a force feedback on the lever. 
     Also, the structure of the joint allows integrating sensors right inside the articulation. 
     Also, several pivot joints can be assembled in series to produce complex laws of force feedback. 
     The joystick according to the invention can also have the following characteristics:
         the second part of the first pivot joint is mobile in rotation relative to the first part of the first pivot joint from a rest position according to a first direction of rotation only, and the first pivot joint comprises a first stop arranged to prohibit travel of the second part in a second direction opposite the first direction,   the mechanical linking assembly comprises a first set of pivot joints, including the first pivot joint and a second pivot joint, the second pivot joint comprising a first part and a second part mounted mobile in rotation relative to the first part from a rest position according to the second direction of rotation only, and the second pivot joint comprises a second stop arranged to prohibit travel of the second part in the first direction opposite the second direction,   the first stop is arranged such that when the first joint is in the rest position, the blades of the first pivot joint are flexed and exert non-zero return torque on the second part of the first pivot joint,   the first stop is adjustable in position so as to enable adjusting the non-zero return torque,   at least one pivot joint comprises a first couple of blades and a second couple of blades, each couple of blades generating a return torque according to the first axis of rotation,   the first couple of blades and the second couple of blades are arranged symmetrically relative to each other,   at least one pivot joint comprises also an elastic element connecting the first part and the second part together and when the blades of the pivot joint are not flexed, the elastic element is in traction between the first part and the second part, such that the elastic element generates a return force tending to cause rotation of the second part relative to the first part,   the mechanical linking assembly comprises a first chain of pivot joints, including the first pivot joint and a third pivot joint mounted in series with the first pivot joint, the first part of the first joint or respectively the second part of the first joint, being mounted integral with the second part or respectively of the first part of the third pivot joint,   the first pivot joint comprises a first stop arranged to limit travel of the second part of the first pivot joint in a first direction of rotation, and the third pivot joint comprises a third stop arranged to limit travel of the first part of the third pivot joint in the first direction of rotation, such that when the lever is driven in rotation relative to the frame in the first direction of rotation, the first pivot joint and the third pivot joint are successively stopped,   the first stop and/or the third stop is adjustable in position so as to enable adjusting the travel from which the first pivot joint and/or the third pivot joint is stopped,   the mechanical linking assembly comprises a strain or deformation sensor mounted on one of the blades, the blade serving as a test body for the sensor,   the mechanical linking assembly enables rotation of the lever relative to the frame according to a second axis of rotation, perpendicular to the first axis of rotation, and the mechanical linking assembly comprises a fourth pivot joint, the fourth pivot joint comprising a first part, a second part mounted mobile relative to the first part, and at least two flexible blades, each flexible blade connecting the first part and the second part together and being resiliently deformable to enable rotation of the second part relative to the first part according to the second axis of rotation and generate a return torque tending to oppose the rotation of the second part relative to the first part,   the mechanical linking assembly comprises a second chain of pivot joints, including the fourth pivot joint and a fifth pivot joint mounted in series with the fourth pivot joint, the first part of the fourth joint or the second part of the fourth joint, being mounted integral with the second part or respectively with the first part of the fifth pivot joint.       

    
    
     
       PRESENTATION OF DRAWINGS 
       Other characteristics and advantages will emerge from the following description which is purely illustrative and non-limiting and must be considered with respect to the appended figures, in which: 
         FIG. 1  schematically illustrates a joystick according to a first embodiment of the invention, 
         FIGS. 2A and 2B  schematically illustrate a pivot joint which can be used in the joystick according to claim  1 , 
         FIGS. 3A to 3C  schematically illustrate respectively the pivot joint in rest position, the pivot joint in loaded position and a law of variation of the couple generated by the pivot joint as a function of an angle of rotation about its principal axis, 
         FIG. 4  schematically illustrates a first variant of the pivot joint, 
         FIGS. 5A and 5B  schematically illustrate respectively a second variant of the pivot joint and a law of variation of the couple generated by the joint as a function of an angle of rotation, 
         FIGS. 6A and 6B  schematically illustrate respectively a third variant of the pivot joint and a law of variation of the couple generated by the joint as a function of an angle of rotation, 
         FIGS. 7A and 7B  schematically illustrate a fourth variant of the pivot joint, 
         FIGS. 8A and 8B  schematically illustrate respectively an assembly in series of two pivot joints and a law of variation of the couple generated by the assembly as a function of an angle of rotation, 
         FIG. 9A  schematically illustrates the joystick according to the first embodiment in a configuration in which the lever is inclined according to a first axis of rotation, 
         FIG. 9B  schematically illustrates the joystick according to the first embodiment in a configuration in which the lever is inclined according to a second axis of rotation, perpendicular to the first axe, 
         FIGS. 9C and 9D  schematically illustrate a law of variation of the couple generated by the joint as a function of an angle of rotation respectively according to the first axis and according to the second axe, 
         FIG. 10  schematically illustrates a joystick according to a second embodiment of the invention, 
         FIG. 11A  schematically illustrates the joystick according to the second embodiment in a configuration in which the lever is inclined according to a first axis of rotation, 
         FIG. 11B  schematically illustrates the joystick according to the second embodiment in a configuration in which the lever is inclined according to a second axis of rotation, perpendicular to the first axis, 
         FIG. 11C  schematically illustrates a law of variation of the couple generated by the joint as a function of an angle of rotation according to the first axis or according to the second axis. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a joystick  1  according to a first embodiment of the invention. 
     The joystick  1  comprises a frame  2  intended for example to be integrated into a pilot&#39;s seat armrest, a lever  3  mounted mobile in rotation relative to the frame  2 , and a mechanical linking assembly  4  of the lever on the frame for generating a return of force on the lever according to two axes of rotation X and Y. 
     The lever  3  has a general elongated form according to a longitudinal direction (axis Z′). 
     The mechanical linking assembly  4  comprises two support pieces  5  and  6  mounted fixed relative to the frame  2 , an intermediate piece  7  mounted mobile in rotation relative to the support pieces  5  and  6  about the first axis X, and a connecting piece  8  on which is fixed the lever  3 , the connecting piece  8  being mounted mobile in rotation relative to the intermediate piece  7  about the second axis Y. The second axis Y is perpendicular to the first axis X. Also, when the lever  3  is in neutral position, the axes X, Y and Z′ are orthogonal to each other. 
     The mechanical linking assembly  4  also comprises a first couple of pivot joints  9 ,  10  and a second couple of pivot joints  11 ,  12 . 
     The intermediate piece  7  has the form of a cross. More precisely, the intermediate piece  7  comprises four arms  13  to  16  extending from a common point of attachment  17 , each arm being connected at the level of its free end to a respective pivot joint  9  to  12 . 
     The first couple of pivot joints includes a first pivot joint  9  mounted between the support piece  5  and the intermediate piece  7  and a second pivot joint  10  mounted in parallel to the first pivot joint  9 , between the support piece  6  and the intermediate piece  7 . The pivot joints  9  and  10  enable rotation of the intermediate piece  7  relative to the frame  2  about the axis X. 
     The second couple of pivot joints includes a third pivot joint  11  mounted between the intermediate piece  7  and the connecting piece  8  and a fourth pivot joint  12  mounted in parallel to the third pivot joint  11 , between the intermediate piece  7  and the connecting piece  8 . The pivot joints  11  and  12  enable rotation of the intermediate piece  7  relative to the frame  2  about the axis Y. 
     The mechanical linking assembly  4  enables rotation of the lever  3  relative to the frame  2  simultaneously about the axis X and about the axis Y, for example letting the pilot control the aircraft for roll and pitch. 
       FIGS. 2A and 2B  schematically illustrate a pivot joint  9  which can be used in a joystick according to  FIG. 1 . 
     The pivot joint  9  comprises a first part  91  and a second part  92  mounted mobile relative to the first part  91 . The first part  91  is fixed to the support piece  5  (that is, the frame  2 ) and the second part  92  is fixed to the intermediate piece  7 . 
     The first part  91  and the second part  92  each have a general cylindrical form and are positioned relative to each other with their axes of revolution combined, the two parts  91  and  92  having a cylindrical section of same internal radius relative to this common axis of revolution. 
     The pivot joint  9  also comprises two flexible blades  93  and  94 , each flexible blade connecting the first part  91  and the second part  92  together. The two flexible blades include a first blade  94  extending parallel to a first plane and a second blade  93  extending parallel to a second plane, orthogonal to the first plane. The first plane and the second plane pass through the common axis of revolution of the parts  91  and  92 . 
     Each flexible blade  93 , respectively  94 , has a first end fixed to the first part  91  and a second end fixed to the second part  92 . More precisely, each end of the blade  93 , respectively  94 , is linked mechanically by a complete link (or housing) to one of the parts  91 ,  92 . 
     The blades  93 ,  94  are resiliently deformable in flexion to enable rotation of the second part  92  relative to the first part  91  according to an axis of rotation corresponding to the common axis of revolution of the parts  91  and  92 , and generate return torque tending to oppose rotation of the second part  92  relative to the first part  91 . 
     So, the blades  93  and  94  ensure guiding in rotation of the second part  92  relative to the first part  91 , according to a single degree of liberty (rotation according to a single axis of rotation). 
     The blades  93  and  94  work in pure flexion, and each blade can be used as a test body to support a sensor  950 , such as a strain gauge for example. This produces measuring directly representative of the torque or travel nearest the origin and consequently, not perturbed by friction or play. 
       FIGS. 3A to 3C  schematically illustrate the functioning of the pivot joint  9 . 
     In  FIG. 3A , the pivot joint  9  is in a rest position. In this position, the flexible blades  93 ,  94  are not flexed and generate no return torque. 
     In  FIG. 3B , the second part  92  is driven in rotation relative to the first part  91 . The flexible blades  93  and  94  (shown in dots) are flexed and, because of their elasticity, on the second part  92  generate return torque substantially proportional to the angle of rotation θ of the second part  92  relative to the first part  91 . 
       FIG. 3C  schematically illustrates the law of variation of the return torque generated by the pivot joint as a function of the angle of rotation θ. The return torque generated by the pivot joint  9  is proportional to the angle of rotation θ in both directions of rotation, in a range of angular travel between −θmax and +θmax. In this range, the law is linear and symmetrical relative to the rest position 0 of the pivot joint. The maximum angles of rotation −θmax and +θmax correspond to the end positions of the pivot joint in which the second part  92  is stopped against the first part  91 . 
     The structure of the pivot joints  10  to  12  shown in  FIG. 1  is identical to what has just been described for the pivot joint  9  in relation to  FIGS. 2A, 2B, 3A, 3B and 3C . 
       FIG. 4  schematically illustrates a first variant of the pivot joint  9  of  FIGS. 2A and 2B . 
     In this variant, the pivot joint  9  comprises four blades  93  to  96 . The blades include a first couple of blades  93 ,  94  and a second couple of blades  95 ,  96 , the couples of blades being arranged symmetrically relative to each other. 
     Each couple of blades comprises a first blade  93  (respectively  96 ) extending parallel to a first plane and a second blade  94  (respectively  95 ) extending parallel to a second plane orthogonal to the first plane. 
     The first blades  93  and  96  (or lateral blades) extend on either side of the second blades  94  and  95  (or central blades), the second blades  94  and  95  being arranged side by side. 
     This first variant provides security in case of cracking of one of the blades. In case of accidental cracking of one of the blades, it is possible to retain functioning in downgraded mode by retaining the guide function of the pivot link, without loss of travel, and with functional stiffness diminished by a quarter only (one blade in four in nominal rotation mode). The blades must be separated from each other beyond their housing so that any breakage of one blade does not spread to the other blade. 
       FIG. 5A  schematically illustrates a second variant of the pivot joint  9 . 
     In this variant, the second part  92  of the pivot joint  9  is mobile in rotation relative to the first part  91  of the pivot joint from a rest position (position of the joint when no force is applied to the joint), according to a first direction of rotation only (arrow A). For this purpose, the pivot joint  9  comprises a stop  97  arranged to prohibit rotation of the second part  92  relative to the first part  91  in a second direction of rotation (arrow B), opposite to the first direction. 
       FIG. 5B  schematically illustrates the law of variation of the return torque generated by the pivot joint  9  as a function of an angle of rotation θ of the second part relative to the first part. The return torque generated by the pivot joint  9  is proportional to the angle of rotation θ in the first direction of rotation, in a range of travel between 0 and +θmax. In this range, the law is linear. 
       FIG. 6A  schematically illustrates a third variant of the pivot joint. 
     In this variant, the stop  97  is arranged such that when the joint is in a rest position (that is, no force is being applied to the joint), the blades  93 ,  94  of the pivot joint  9  are flexed and exert on the second part  92  of the joint non-zero return torque tending to keep the second part  92  supported against the stop  97 .  FIG. 6B  schematically illustrates the law of variation of the return torque generated by the pivot joint  9  as a function of the angle of rotation θ. The return torque generated by the pivot joint  9  is proportional to the angle of rotation θ in the first direction of rotation (arrow A), in a range of travel between +θ1 and +θ2, with +θ2&gt;+θ1&gt;0. In this range, the law is linear. The extreme angles of rotation +θ 1  and +θ 2  correspond respectively to the position in which the second part  92  is supported against the stop  97  and to the position in which the second part  92  is supported against the first part  91  (the first part  91  constituting a stop for the second part  92 ). 
     The position of the stop  97  can be adjusted (for example by means of a threaded element) so as to adjust the angle+θ 1  to then adjust the minimal actuation torque C. 
       FIG. 7A  schematically illustrates a fourth variant of the pivot joint  9 . 
     This fourth variant is here identical to the second variant, but could apply to any other variant. In the fourth variant, the pivot joint  9  also comprises an elastic element  98 , such as a spring for example, connecting the first part  91  and the second part  92  together. The elastic element  98  is held elongated over the entire range of angular travel of the second part  92  relative to the first part  91 . 
     The elastic element  98  is arranged between the central blades  94 ,  95  and extends in a direction forming an angle of 45 degrees relative to the blades  93 ,  94 ,  95  and  96 . 
     As is illustrated in  FIG. 7B , when the blades  93  to  96  of the pivot joint  9  are not flexed, the elastic element  98  passes through the axis of rotation of the second part  92  relative to the first part  91 , such that even though the elastic element  98  is in traction between the first part  91  and the second part  92 , it exerts no torque between the latter. In other words, the elastic element  98  is in a position of equilibrium when the blades  93  to  96  are not flexed, and it therefore exerts no force between the first part  91  and the second part  92 . 
     Since the second part  92  is shifted in rotation relative to the first part  91 , the resilient force generated by the elastic element  98  no longer passes through the axis of rotation of the pivot joint  9 , and it tends to favour rotation in the same direction as that of the second part  92  relative to the first part  91 . 
     Because of this arrangement, the elastic element  98  generates negative return torque which compensates, at least in part, the positive return torque generated by the blades  93  to  96 . Selecting appropriately the characteristics of the elastic element  98  makes it possible to design a pivot joint  9  without friction and having zero stiffness near the neutral position. 
     Also, it is possible to provide adjusting means  980  of the tension of the elastic element  98  (for example a threaded element cooperating with the first part  91 ), for adjusting the resulting stiffness of the pivot joint  9 . 
       FIG. 8A  schematically illustrates an assembly in series of two pivot joints  9  and  109 , for forming a chain of pivot joints. In this assembly, the first part  1091  of the pivot joint  109  is fixed to the second part  92  of the pivot joint  9 . 
     Also, in the example shown in  FIG. 8A , just one of the pivot joints  9  comprises a stop  99  limiting the travel of the pivot joint. 
     The position of the stop  99  can be adjusted (for example by means of a threaded element cooperating with the first part  91  of the pivot joint  9 ) so as to adjust the angle +θ 1  for change of stiffness of the assembly. 
       FIG. 8B  schematically illustrates the law of variation of the return torque generated by the assembly of pivot joints as a function of the angle of rotation θ of the second part  1092  of the pivot joint  109  relative to the first part  91  of the pivot joint  9 . 
     The resulting law of variation has a double slope. More precisely, the stiffness follows a law of variation defined by sections. 
     In a first range of travel between 0 and θ 1 , the two pivot joints  9  and  109  are driven in rotation simultaneously. The resulting return torque generated by the assembly of joints  9  and  109  is a combination of individual return torques generated by the two pivot joints  9  and  109 . This resulting return torque is proportional to the angle of rotation θ of the second part  1092  of the pivot joint  109  relative to the first part  91  of the pivot joint  9  with a first stiffness resulting from the combination of individual stiffness of the two pivot joints  9  and  109  in series. 
     When the angle of rotation θ reaches θ 1 , the second part  92  of the pivot joint  9  comes into contact against the stop  99  such that the second part  92  can no longer be driven in rotation relative to the first part  91 . 
     In a second range of travel between θ 1  and θ 2 , the return torque generated by the assembly of the joints  9  and  109  varies in a linear manner with a second stiffness equal to individual the stiffness of the pivot joint  109  alone. 
     When the angle of rotation θ reaches 02, the second part  1092  of the pivot joint  109  arrives stopped against the first part  1091  of the pivot joint  109  and the second part  1092  can no longer be driven in rotation relative to the first part  1091 . No rotation of the assembly is possible beyond θ 2 . 
     The same functioning can be obtained by assembling in series pivot joints  9  and  109  having the same angular travel, but different degrees of stiffness. In fact, with equal travel, the supplest pivot joint arrives stopped before the stiffest pivot joint. When the supplest pivot joint reaches its stop, the stiffness of the assembly, initially less than each of the two degrees of stiffness, will be returned to the stiffness of the stiffest pivot joint which will not yet have reached its stop. 
       FIG. 9A  schematically illustrates the joystick  1  according to the first embodiment in a configuration in which the lever  3  is inclined by an angle θ according to the second axis of rotation Y. 
       FIG. 9C  schematically illustrates the law of variation of the couple generated by the two joints  11  and  12  on the lever  3  as a function of the angle of rotation θ of the lever  3  relative to the frame  2  about the axis Y. 
       FIG. 9B  schematically illustrates the joystick  1  according to the first embodiment in a configuration in which the lever  3  is inclined by an angle θ according to the first axis of rotation X. 
       FIG. 9D  schematically illustrates a law of variation of the couple generated by the two joints  9  and  10  on the lever  3  as a function of the angle of rotation θ of the lever  3  relative to the frame  2  about the axis X. 
       FIG. 10  schematically illustrates a joystick  1  in keeping with a second embodiment of the invention. 
     This second embodiment is identical to the first embodiment, except that the first pivot joint  9  and the second pivot joint  10  have been replaced by a first chain of pivot joints  9 ,  109 ,  209 ,  309  and a second chain of pivot joints  10 ,  110 ,  210 ,  310  mounted between the support pieces  2  and the intermediate piece  7 . 
     Similarly, the third pivot joint  11  and the fourth pivot joint  12  have been replaced by a third chain of pivot joints  11 ,  111 ,  211 ,  311  and a fourth chain of pivot joints  12 ,  112 ,  212 ,  312  mounted between the intermediate piece  7  and the connecting piece  8 . 
     In the example shown in  FIG. 10 , each chain of pivot joints comprises four pivot joints mounted in series, including:
         two pivot joints mobile in rotation from a pre-stressed position according to a first direction of rotation only (arrow A), specifically the pivot joints  9 ,  109 ,  10 ,  110 ,  11 ,  111 ,  12 ,  112 ,   two pivot joints mobile in rotation from a pre-stressed position according to a second direction of rotation only (arrow B), the second direction of rotation being opposite the first direction of rotation, specifically the pivot joints  209 ,  309 ,  210 ,  310 ,  211 ,  311 ,  212 ,  312 .       

     For this purpose, the pivot joints  9 ,  109 ,  10 ,  110 ,  11 ,  111 ,  12 ,  112  comprise a stop  97 , such as that which is shown in  FIG. 5A or 6A , hampering travel of the joint in the second direction of rotation. 
     Similarly, the pivot joints  209 ,  309 ,  210 ,  310 ,  211 ,  311 ,  212 ,  312  comprise a first stop  97 , such as that which is shown in  FIG. 5A or 6A , prohibiting travel of the joint in the first direction of rotation. 
     Also, each of the pivot joints  9 ,  10 ,  11  and  12  comprises a second stop  99  limiting travel of the joint in the first direction of rotation, such as what is shown in  FIG. 8A . 
     Similarly, each of the pivot joints  209 ,  210 ,  211  and  212  comprises a second stop  99  limiting travel of the joint in the second direction of rotation, such as that is shown in  FIG. 8A . 
       FIG. 11A  schematically illustrates the joystick  1  according to the second embodiment in a configuration in which the lever  3  is inclined relative to the frame  2  about a second axis of rotation Y. 
     As is illustrated in this figure, when the lever  3  is inclined in a second direction (arrow B) about the axis Y, only the pivot joints  211 ,  311  and  212 ,  312  work. The pivot joints  11 ,  111  and  12 ,  112  are stopped. 
     Inversely, when the lever  3  is inclined in a first direction (arrow A), opposite the second direction, only the pivot joints  11 ,  111  and  12 ,  112  work. The pivot joints  211 ,  311  and  212 ,  312  are stopped. 
       FIG. 11B  schematically illustrates the joystick  1  according to the second embodiment in a configuration in which the lever  3  is inclined relative to the frame  2  according to the first axis of rotation X. 
     As is illustrated in this figure, when the lever  3  is inclined in a second direction (arrow B), only the pivot joints  209 ,  309  and  210 ,  310  work. The pivot joints  9 ,  109  and  10 ,  110  are stopped. 
     Inversely, when the lever  3  is inclined in a first direction (arrow A), opposite the first direction, only the pivot joints  9 ,  109  and  10 ,  110  work. The pivot joints  209 ,  309  and  210 ,  310  are stopped. 
     As is illustrated in  FIG. 11C , this second embodiment produces a law of variation of the return torque generated according to the two axes X and Y having an initiation threshold, that is, the pilot must exert on the lever  3  a couple greater than a threshold value C to drive the lever in rotation according to each of the directions of the axes X and Y (combination of stiffness of pivot joints  9 + 109 ,  209 + 309  and  10 + 110 ,  210 + 310  according to each of the directions of the axis X and combination of stiffness of pivot joints  11 + 111 ,  211 + 311  and  12 + 112 ,  212 + 312  according to each of the directions of the axis Y). 
     Also, the law of variation has a double slope. More precisely, the stiffness of the assembly has a law of variation by sections. In a first range of travel between 0 and θ 1 , the return torque generated by the chain of pivot joints has a linear variation as a function of the angle of rotation θ with a first stiffness. In a second range of travel between θ 1  and θ 2 , the return torque generated by the chain of pivot joints has a linear variation with a second stiffness (stiffness of pivot joints  109 ,  309  and  110 ,  310  according to each of the directions of the axis X and stiffness of pivot joints  111 ,  311  and  112 ,  312  according to each of the directions of the axis Y). 
     In this way, combination of several pivot joints assembled in series produces laws of complex force return, the characteristics and the assembly of the pivot joints able to vary as a function of the preferred law of force feedback.