Three-degrees-of-freedom joint with force feedback

Abstract

Joint with three degrees of freedom for a robot, comprising a platform (2), three motors (3a, 3b, 3c) each connected to a ring by means of a pinion, each ring being arranged inside a hollow disc (6a, 6b, 6c) stacked on the base, such that each disc (6, 6a, 6b, 6c) is secured to a ring, each disc (6, 6a, 6b, 6c) being moreover itself secured to a disc head (7, 7a, 7b, 7c) extending in the same direction as the stack of the base and discs (6, 6a, 6b, 6c); for each disc head (7, 7a, 7b, 7c), an arm (8, 8a, 8b, 8c) is connected in rotation on the one hand to the disc head (7, 7a, 7b, 7c) and on the other hand to the platform (2), the joint with three degrees of freedom comprising a sphere (S) connected to the base (B) by a cylindrical bar (T), the sphere (S) being arranged at the centre of a cylindrical opening provided in the centre of the platform (2), the sphere (S), the cylindrical bar (T) and the base (B) cooperating in order to relay the forces generated at the platform (2) and the arms (8, 8a, 8b, 8c).

Description

[0001]DESCRIPTION TITLE: Three-degree-of-freedom joint with force transfer Technical field The technical field of the invention is the joints of robotic limbs, and more particularly such joints with three degrees of freedom. Prior art Robotic limbs generally use several joints in order to provide the best possible mobility, like the limbs of a human being or an animal. A joint involves at least one degree of freedom, generally two or three degrees of freedom. By degrees of freedom, we mean the possibility of performing a rotation along a predefined axis. Thus, with two degrees of freedom, a joint allows rotation along two distinct predefined axes, generally orthogonal. With three degrees of freedom, a joint allows rotation along three distinct predefined axes, generally also orthogonal. Depending on their location in the robotic limb,The joint requires a minimum number of degrees of freedom to allow the limb to function. In particular, the joint located in the wrist of a robotic arm requires at least two degrees of freedom. However, the use of a three-degree-of-freedom joint allows for a greater range of movements that are as close as possible to human movement. The state of the art includes the document "Marine Propulsor based on a Three-Degree-of-Freedom Actuated Spherical Joint", Sudki B. et al., Third International Symposium on Marine Propulsors smp'13, Launceston, Tasmania, Australia, May 2013. This document describes a three-degree-of-freedom joint for a marine thruster to replicate the shoulder of marine animals, including the penguin. The joint described includes coaxial axes connected to the motors and has a fixed center of rotation, a working frequency of 2.5 Hz under load,unlimited rotation along the main axis and arbitrary movement within a cone of + / -60°. However, the joint described in this document occupies a significant space in the accompanying frame, particularly due to the placement of the motors. These are arranged outside the joint itself and are configured to rotate three concentric shafts driving the joint. This structure shows that a large part of the internal volume of the joint is under-exploited. The document Gosselin C. et al. Kinematic analysis, optimization and programming of parallel robotic manipulator. McGill University (1985) presents the optimization of parallel joints. The document Asada H. et al., "Kinematic and static characterization of wrist joints and their optimal design," Proceedings. 1985 IEEE International Conference on Robotics and Automation, 1985, pp.244-250,doi: 10.1109 / ROBOT.1985.1087324. is also known in connection with the modeling of wrist joints. Also known is document FR1912398 in the name of the applicant and relating to a three-degree-of-freedom joint for a robot and the corresponding control method. The joint described in this document constituted a major advance in the field of robot joints. However, it quickly became apparent that it was nonetheless subject to various technical problems. A first problem concerns the forces generated at the joint after prolonged use,and leading to aging of the assemblies resulting in the appearance of assembly clearances. These assembly clearances make the control imprecise and can lead to the breakage of part of the joint. A second problem concerns the difficulty of routing the power supply and control cables of the actuators or joints downstream of the three-degree-of-freedom joint. Indeed, the major advantage of the three-degree-of-freedom joint is the possibility of achieving infinite rotation in a direction normal to the plane of the joint output while benefiting from large rotation angles in the other planes, as well as not suffering from gimbal lock. However, as soon as an actuator is located downstream of the joint, the power supply and control cables of this actuator limit the rotations actually accessible, either by their interaction with the arms,either by their interaction with the joint itself. It has been found that such cables break or become tangled around the joint as soon as large amplitude rotations are performed. The advantage of large rotation angles and infinite rotation is thus neutralized. The same applies to the gimbal lock, which is replaced by a lock linked to the power supply and control cables of the actuators downstream of the joint. The aim of the present invention is to solve these technical problems. Disclosure of the invention The subject of the invention is a three-degree-of-freedom joint for a robot, comprising a platform, three motors each connected to a crown via a pinion, each crown being arranged inside a hollow disc stacked on a base, so that each disc is integral with a crown,each disc is furthermore itself secured to a disc head extending in the same direction as the stack of the base and the discs. For each disc head, an arm is connected in rotation on the one hand to the disc head and on the other hand to the platform, the platform being connected to the base and to the hollow discs only by the three arms, the three-degree-of-freedom joint comprises a sphere connected to the base by a cylindrical bar, the sphere being arranged in the center of a cylindrical opening made in the center of the platform, the sphere, the cylindrical bar and the base cooperating in order to provide a return of forces generated at the platform and the arms. Each disc may comprise a bearing support designed to provide passages for the insertion and installation of the pinions,the bearing supports of each disc are stacked on top of each other and fixed to the base by means of screws and tapped holes made in the base, each bearing support receiving a bearing allowing the rotation of the corresponding disc. The bearing support and the corresponding bearing may be included in a disc covering, secured on the one hand to the crown relating to the disc and on the other hand to the corresponding disc head. The disc covering may be provided with a rotation stop, the bearing support of each disc comprises a protrusion located on its lower periphery, the outside diameter of the lower periphery of the bearing support being less than the inside diameter of the corresponding covering, so that a groove is then formed during the assembly of the covering and the bearing support, interrupted by the protrusion,the rotation stop of a disc and the protrusion of a disc immediately above cooperating to limit the rotation of the disc provided with the rotation stop. A disc covering may comprise two stops, so as to be able to adjust, in cooperation with a protrusion, a positive rotation angle of said disc independently of the negative rotation angle of said disc. Two protrusions may be provided on a bearing support, so as to be able to adjust, in cooperation with a stop, a positive rotation angle of said disc independently of the negative rotation angle of said disc. A stop may be repositionable by means of a removable attachment on the corresponding covering and / or a protrusion is repositionable by means of a removable attachment on the corresponding bearing support. One or more teeth of the crown of a disc may be removed,so as to limit the rotation of the corresponding disk. A motor may be provided with means for determining the angular position of its output axis relative to a reference position. The means for determining the angular position may comprise a plurality of magnetic sensors distributed circularly around the cylindrical bar, associated with a magnetic element arranged in each disk head so as to be able to be detected by the magnetic sensors. The means for determining the angular position may comprise sensors in line with each axis of rotation carrying a pinion. The surface of the sphere may be provided with a coating minimizing friction between the sphere and the platform, in particular of the polytetrafluoroethylene type. The sphere, the cylindrical bar and the base are each provided with a through hole, each hole communicating with the others. The invention also relates to a robot member,comprising at least two limb segments joined by a three-degree-of-freedom joint as described above. Another object of the invention is a robot limb comprising at least two limb segments joined by a three-degree-of-freedom joint as described above as well as equipment arranged downstream of the three-degree-of-freedom joint, the power and / or control cable of said equipment being arranged through the hole in the sphere, the hollow cylindrical bar and through the hole in the base, to open between the motors. Brief description of the drawings Other objects, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example and with reference to the appended drawings in which: - figure [Fig 1] illustrates the main elements of a three-degree-of-freedom joint according to the invention,- figure [Fig 2] illustrates the main elements contributing to the force transfer in a three-degree-of-freedom joint according to the invention, - figure [Fig 3] illustrates the main elements contributing to the actuation of a three-degree-of-freedom joint according to the invention, - figure [Fig 4] illustrates the main elements of a disc and a corresponding arm, - figure [Fig 5] illustrates the circulation of cables in a three-degree-of-freedom joint according to the invention, - figure [Fig 6] illustrates the main elements of a disc and a bearing support, - figure [Fig 7] illustrates a bearing support, - figure [Fig 8] illustrates the position of the three-degree-of-freedom joint according to the invention after a command, - figure [Fig 9] illustrates the rotation sensors arranged in a three-degree-of-freedom joint according to the invention,and - figure [Fig 10] illustrates a robotic arm comprising a three-degree-of-freedom joint according to the invention. Detailed description The joint according to the invention comprises a system of three parallel axes controlling the joint. Joint 1 is illustrated by figure [Fig 1]. Joint 1 makes it possible to move a platform 2 along three axes of freedom relative to a base B by controlling three rotating motors 3a, 3b, 3c. Platform 2 is provided with a cylindrical opening in its center. A sphere S is arranged in the center of platform 2, so that the center of sphere S coincides with the center of rotation of platform 2. The surface of sphere S is provided with a coating minimizing friction between sphere S and platform 2, in particular of the PTFE type (acronym for "polytetrafluoroethylene"). Joint 1 comprises base B near which the three motors 3a, 3b are arranged,3c and on which disks 6a, 6b, 6c are stacked. Each disk 6a, 6b, 6c is itself secured to a disk head 7a, 7b, 7c extending in the same direction as the stacking of the base and the disks 6a, 6b, 6c. For each disk head 7, 7a, 7b, 7c, an arc-shaped arm 8, 8a, 8b, 8c is rotatably connected on the one hand to the disk head 7, 7a, 7b, 7c and on the other hand to the platform 2. Figure [Fig 2] illustrates such an arrangement. The connections between the disk heads 7, 7a, 7b, 7c and the corresponding arms 8, 8a, 8b, 8c are located in the same plane. Similarly, the connections between the arms 8, 8a, 8b, 8c and the platform 2 are included in the same plane. Advantageously, the arc shape represents a quarter of a circle. The discs 6, 6a, 6b, 6c form housings and are provided with bearings to facilitate their movement, reduce friction and wear and maintain the alignment of the discs 6, 6a, 6b,6c relative to the base and to each other. The sphere S is connected to a cylindrical bar T itself connected to the base B. Such an arrangement allows the forces applied to the platform 2 to be transferred to the sphere S and, consequently, to the cylindrical bar T and the base B. The arms 8a, 8b, 8c are thus relieved of part of the forces perceived. More precisely, the advantage of such a configuration involving a sphere secured to the frame via the cylindrical bar lies in the fact that the majority of the forces applied to the platform 2 are taken up by the sphere, cylindrical bar and base assembly, thus forming a sort of internal rigid skeleton. The bending forces (perpendicular to the main axis), for example, are completely taken up by the sphere, cylindrical bar and base assembly, thus relieving the disk heads 7, 7a, 7b, 7c and arms 8, 8a, 8b,8c more fragile. This presents a significant advantage when the three-degree-of-freedom joint 1 is used to form a wrist. The forces resulting from the weight of the hand and the grasped object are thus absorbed by the sphere, cylindrical bar and base assembly. In the same way, the pressure forces (in the same direction as the axis of the cylindrical bar T) are also taken up by the sphere, cylindrical bar and base assembly, and only slightly stress the rigidity of the disc heads 7 and arms 8. This presents a significant advantage when the three-degree-of-freedom joint 1 is used to form a neck. The forces resulting from the weight of the grasped head are thus absorbed by the sphere, cylindrical bar and base assembly. Figure [Fig 2] illustrates the sphere S, the cylindrical bar T and the base B. The discs 6a, 6b, 6c, the disc heads 7a, 7b, 7c, the arms 8a, 8b, 8c and the platform 2 are illustrated in transparency. In figure [Fig 3],three motors 3a, 3b, 3c are illustrated, each connected to a crown 4a, 4b, 4c by means of a pinion 5a, 5b, 5c each carried by an axle. The pinions 5a, 5b, 5c are arranged inside the crowns 4a, 4b, 4c. Each pinion 5a, 5b, 5c is thus arranged at a different height from the base B so as to mechanically drive only the corresponding crown 4a, 4b, 4c. Each crown 4a, 4b, 4c is arranged inside one of the hollow discs 6a, 6b, 6c, so that each disc 6a, 6b, 6c is secured to a crown 4a, 4b, 4c. The motors 3a, 3b, 3c are controlled in rotation, which causes a rotation of each disc 6a, 6b, 6c on a circle called the proximal circle. The rotation of each disc 6a,6b,6c causes the rotation of the arm 8a,8b,8c which is mechanically connected to it on another circle, called the distal circle. Each arm 8a,8b,8c then applies a force on the platform 2 so as to change its position. Each pinion 5a,5b,5c and each corresponding axis are arranged in a different 120° sector. This 120° offset angle is also found at the rest position of each disc head 7,7a,7b,7c, each disc head 7,7a,7b,7c being arranged at 120° from the other two. Figure [Fig 4] illustrates a disc 6 comprising a bearing support 9, a covering 10, connected to a disc head 7 itself connected to an arm 8. The assembly formed by the disc 6, the disc head 7 and the arm 8 is associated with one of the three axes or degrees of freedom of the joint. The joint thus comprises three similar assemblies, each associated with a different degree of freedom. The use of three similar assemblies also makes it possible to reduce production costs, both in digital machining and in plastic injection. Each disc 6,6a,6b,6c comprises a bearing support 9 designed to provide passages for the insertion and positioning of the pinions 5a,5b,5c connected to the motors by axes. Due to the low amount of forces perceived by this part, it can be produced by plastic injection rather than by digital machining. The bearing supports 9 of each disc 6, 6a, 6b, 6c are stacked on top of each other and fixed to the base B by means of screws. These screws are designed to cooperate with tapped holes made in the base B. It will then be understood that each bearing support 9 is fixed to the base B while the discs 6, 6a, 6b, 6c are driven in rotation by the corresponding crown 4, 4a, 4b, 4c. Each bearing support 9 is associated with a bearing making it possible to ensure the rotation of the corresponding disc 6, 6a, 6b, 6c and to maintain its position in the articulation 1. The bearing support 9 and the corresponding bearing are included in a disc covering 10, integral with the crown 4a, 4b,4c relating to the disc. The disc covering 10 is secured to a disc head 7 by means of a shoulder 10a and fixing means. In other words, a first disc 6a is connected to a first arm 8a by means of a disc head 7a, the first disc 6a being driven by a first motor 3a by means of a first crown 4a, and a first pinion 5a. The similar arrangement is provided for the second disc 6b and the third disc 6c for which one end of each arm is assembled to the disc head at a different height from the covering 10. The other end of each arm 8a, 8b, 8c is then connected to the platform 2 while the platform 2 is included in a plane normal to the axis of the cylindrical bar. In a particular embodiment illustrated by the figure [Fig 5], the sphere S, the cylindrical bar T and the base B are each provided with a through hole,each hole communicating with the others. It will be understood that in the case of the cylindrical bar T, the hole is made in the form of an axial bore. The cylindrical bar T is then essentially a hollow tube. The advantages linked to the sphere, cylindrical bar and base assembly described above are also valid here when the cylindrical bar is provided with an axial bore. In the case of equipment, in particular an actuator, arranged downstream of the articulation, the power supply and / or control cable C of this equipment can be arranged through the hole in the sphere S, in the hollow tube T and through the hole in the base B, to open between the motors 3a, 3b, 3c. The cable C circulates in the hollow tube T, between the pinions 5a, 5b, 5c and inside the crowns 4a, 4b, 4c, through the central hole of the bearing supports 9 of each disc 6, 6a, 6b,6c. It thus does not interact with the operation of the joint and is protected from the environment inside the joint. The cable C thus arranged cannot interact with the arms 8a, 8b, 8c, thus protecting the joint from damage. In a particular implementation of this embodiment, the amplitude of rotation of the discs 6, 6a, 6b, 6c is limited so as to avoid damage to the cable C by twisting during rotation of the equipment to which it is connected, equipment arranged downstream of the joint 1. To achieve this, stops and protrusions are arranged in the different discs in order to limit their rotation. More precisely, a disc covering 10 is provided with a rotation stop 10b intended to limit the rotation of the corresponding disc 6, 6a, 6b, 6c. Figure [Fig 6] illustrates a disc 6a comprising a bearing support 9, a covering 10,a rotation stop 10b and secured to a disc head 7a. The other disc heads 7b, 7c are shown but are not secured to the covering 10 of this disc 6a. The bearing support 9 comprises passages 11 for the insertion of the pinions 5a, 5b, 5c and holes 12a, 12, 12c in which the screws for holding the base B are inserted. The hollow tube T extends in the center of the bearing support. In addition to the stops 10b, the bearing support 9 of each disc 6, 6a, 6b, 6c comprises a protrusion 9a located on its lower periphery. Figure [Fig 7] illustrates such a bearing support 9 provided with a protrusion 9a. We find the passages 11 and the holes 12a, 12b, 12c. In order to allow the rotation of the disc fitted with a stop,a space is provided between the covering 10 and the bearing support 9. This is achieved by providing an outside diameter of the lower part of the bearing support 9 smaller than the inside diameter of the corresponding covering 10. A groove is then formed during the assembly of the covering 10 and the bearing support 9, interrupted by the protrusion 9a. A disc 6, 6a, 6b, 6c is thus free to rotate until the stop 10b present on its covering 10 comes into contact with the protrusion 9a of the bearing support 9 of the disc immediately above. The last disc 6c in the stack has its rotation blocked by a similar protrusion arranged in a covering arranged immediately above. Depending on the respective placement of the stops and the protrusions, the rotation accessible to each disc can be limited. For placement of a stop and protrusion,the disc can access a positive rotation angle and a negative rotation angle defined with respect to a rest position. The articulation in its entirety is then limited to a positive angle and a negative angle equal respectively to the sum of the positive and negative angles accessible to each disc. In such a configuration the sum of the positive angle and the negative angle of a disc is always equal to 360°. Alternatively, a stop can be arranged on either side of the rest position, so as to be able to adjust the positive rotation angle independently of the negative rotation angle. The positive rotation angle can be equal to or different from the negative rotation angle. It will be understood that in such a variant,a disc covering comprises two stops which each cooperate with a protrusion of the bearing support of the disc immediately above. In such a configuration the sum of the positive angle and the negative angle of a disc is less than or equal to 360°. The same effect will be obtained if there is one stop on a covering and two protrusions on the bearing support immediately above. The stops can also be repositionable by means of a removable fixing on the corresponding covering. This can be, for example, a stop to be inserted into a hole, several holes being provided on the covering, regularly distributed or not. It can also be a stop to be screwed onto tapped holes. Alternatively,the rotation of a disc can be limited by the removal or omission of one or more teeth of the corresponding crown. Figure [Fig 8] illustrates the articulation 1 with three degrees of freedom after a command has been given. It will be noted that the platform is inclined relative to its rest position illustrated by figure [Fig 1]. It will also be noted that the outlet orifice of the sphere S remains clear despite the inclination of the platform 2. Given the angles achievable by the platform 2 in planes normal to the plane of the platform 2, this outlet orifice is always clear so that shearing of the cable C by the platform is not possible. Each motor 3a, 3b, 3c is also provided with means for determining the angular position of its output axis relative to a reference position. Such determination means are illustrated by figure [Fig 9]. In a first embodiment,the angular position determining means comprise a plurality of magnetic sensors 13 arranged on a plate 14, advantageously the printed interface circuit of the magnetic sensors 13. The plurality of magnetic sensors 13 is distributed uniformly over the periphery of the plate 14 so that the gap between two magnetic sensors immediately in succession is constant over the entire periphery. In each disk head 7a, 7b, 7c is installed a magnetic element 15a, 15b, 15c arranged so as to be able to be detected by the magnetic sensors 13. It will be understood that such an arrangement is based on the adequacy between the intensity of the magnetic field emitted by each magnetic element 15a, 15b, 15c, the sensitivity of the magnetic sensors 13, considered alone or in combination,and the distance between each magnetic element and the plurality of magnetic sensors 13. In order to be able to associate the magnetic element of each disk head with a determined position, the articulation 1 is initialized by commanding a small displacement of each disk, one after the other. The position of the magnetic element detected by the magnetic sensors 13 is then associated with the disk set in motion. Once the three disks are set in motion, the three magnetic elements (in the case illustrated here) are identified. Their displacement can then be followed as the commands are transmitted to the articulation. In a second embodiment, the angular position determination means comprise sensors 16a, 16b, 16c in line with each rotation axis carrying a pinion 5a, 5b, 5c. The sensors 16a, 16b, 16c make it possible to carry out a closed-loop control of the rotation of the motors 3a, 3b, 3c. In a third embodiment,the position determining means comprise a combination of magnetic sensors 13 and magnetic elements 15a, 15b, 15c according to the first embodiment and rotation speed sensors 16a, 16b, 16c according to the second embodiment. Figure 5 illustrates such an embodiment, making it possible to detect the position of the disk heads 7 and the arms 8 at the start of the movement of the joint 1 thanks to the combination of magnetic sensors 13 and magnetic elements 15a, 15b, 15c and then to follow their movement precisely thanks to the rotation speed sensors 16a, 16b, 16c. Referring again to figure [Fig 5], it can be noted that the cable(s) C1, C2, C3 for connecting the magnetic sensors 13 and / or the rotation speed sensors 16a, 16b, 16c are arranged inside the joint as for the cable C for power supply and control of equipment downstream of the joint. Referring again to figure [Fig 6],it can be noted that slots are provided in each bearing support for these cables C1, C2, C3, said slots being provided in alignment with the passages 11, the hollow tube T and the stops 12a, 12b, 12c. Just like the cable C, the cables C1, C2, C3 are thus protected by the articulation. Figure [Fig 10] illustrates a robotic arm 20 in which the wrist articulation is achieved by a three-degree-of-freedom articulation 1 according to the invention. It will be noted that the motors 3 are arranged inside the robotic arm 20 so that they are both protected and hidden. The cables C, C1, C2, C3 are also included inside the robotic arm 20 and are therefore also protected.

Claims

CLAIMS 1. Three-degree-of-freedom joint for a robot, comprising a platform (2), three motors (3a, 3b, 3c) each connected to a crown (4, 4a, 4b, 4c) via a pinion (5a, 5b, 5c), each crown (4, 4a, 4b, 4c) being arranged inside a hollow disc (6a, 6b, 6c) stacked on a base (B), so that each disc (6, 6a, 6b, 6c) is secured to a crown (4, 4a, 4b, 4c), each disc (6, 6a, 6b, 6c) is furthermore itself secured to a disc head (7, 7a, 7b, 7c) extending in the same direction as the stacking of the base (B) and the discs (6, 6a, 6b, 6c), for each disc head (7,7a,7b,7c), an arm (8,8a,8b,8c) is rotatably connected on the one hand to the disc head (7,7a,7b,7c) and on the other hand to the platform (2), the platform being connected to the base (B) and to the hollow discs only by the three arms (8,8a,8b,8c),characterized by the fact that the three-degree-of-freedom joint comprises a sphere (S) connected to the base (B) by a cylindrical bar (T), the sphere (S) being arranged in the center of a cylindrical opening made in the center of the platform (2), the sphere (S), the cylindrical bar (T) and the base (B) cooperating in order to provide a force transfer of the forces generated at the level of the platform (2) and the arms (8, 8a, 8b, 8c).

2. Three-degree-of-freedom joint according to claim 1, wherein each disc (6,6a,6b,6c) comprises a bearing support (9) designed to provide passages for the insertion and placement of the pinions (5a,5b,5c), the bearing supports (9) of each disc (6,6a,6b,6c) are stacked on top of each other and fixed to the base (B) by means of screws and tapped holes provided in the base (B), each bearing support (9) receiving a bearing allowing the rotation of the disc (6,6a,6b,6c) corresponding.

3. Three-degree-of-freedom joint according to claim 2, in which the bearing support (9) and the corresponding bearing are included in a disc covering (10), integral on the one hand with the crown (4a, 4b, 4c) relating to the disc and on the other hand with the corresponding disc head (7, 7a, 7b, 7c).

4. Three-degree-of-freedom joint according to claim 3, in which the disc covering (10) is provided with a rotation stop (10b), the bearing support (9) of each disc (6, 6a, 6b, 6c) comprises a protrusion (9a) located on its lower periphery, the outside diameter of the lower periphery of the bearing support (9) being less than the inside diameter of the corresponding covering (10), so that a groove is then formed during the assembly of the covering (10) and the bearing support (9), interrupted by the protrusion (9a),the rotation stop (10b) of a disc and the protrusion (9a) of a disc immediately above cooperating in order to limit the rotation of the disc provided with the rotation stop (10b).

5. Three-degree-of-freedom joint according to claim 4, in which a disc covering (10) comprises two stops (10b), so as to be able to adjust, in cooperation with a protrusion (9a), a positive rotation angle of said disc independently of the negative rotation angle of said disc., 6. A three-degree-of-freedom joint according to claim 4, wherein two protrusions are provided on a bearing support, so that, in cooperation with a stop (10a), a positive rotation angle of said disc can be adjusted independently of the negative rotation angle of said disc.

7. A three-degree-of-freedom joint according to any one of claims 4 to 6, wherein a stop is repositionable by means of a removable attachment on the corresponding covering and / or a protrusion is repositionable by means of a removable attachment on the corresponding bearing support.

8. A three-degree-of-freedom joint according to claim 1 to 3, wherein one or more teeth of the crown of a disc are removed, so as to limit the rotation of the corresponding disc. 9.A three-degree-of-freedom joint according to claim 1 to 8, wherein a motor (3a, 3b, 3c) is provided with means for determining the angular position of its output axis relative to a reference position.

10. A three-degree-of-freedom joint according to claim 9, wherein the angular position determining means comprise a plurality of magnetic sensors (13) distributed circularly around the cylindrical bar (T), associated with a magnetic element (15a, 15b, 15c) arranged in each disc head (7a, 7b, 7c) so as to be able to be detected by the magnetic sensors (13).

11. A three-degree-of-freedom joint according to any one of claims 9 or 10, wherein the angular position determining means comprise sensors (16a, 16b, 16c) in line with each rotation axis carrying a pinion (5a, 5b, 5c). 12.Three-degree-of-freedom joint according to any one of claims 1 to 11, wherein the surface of the sphere (S) is provided with a coating minimizing friction between the sphere (S) and the platform (2), in particular of the polytetrafluoroethylene type.

13. Three-degree-of-freedom joint according to any one of claims 1 to 12, wherein the sphere (S), the cylindrical bar (T) and the base (B) are each provided with a through hole, each hole communicating with the others.

14. Robot limb, comprising at least two limb segments joined by a three-degree-of-freedom joint (1) according to any one of claims 1 to 13. 15.Robot limb comprising at least two limb segments joined by a three-degree-of-freedom joint (1) according to claim 13 as well as equipment arranged downstream of the three-degree-of-freedom joint (1), the power and / or control cable of said equipment being arranged through the hole in the sphere (S), the hollow cylindrical bar (T) and through the hole in the base (B), to open between the motors (3a, 3b, 3c).

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