A 3-degree-of-freedom joint that transmits force.

Abstract

A 3-degree-of-freedom joint for a robot, comprising a platform (2) and three motors (3a, 3b, 3c) connected to rings by pinions, wherein each ring is positioned inside hollow discs (6a, 6b, 6c) stacked on a base, and each disc (6, 6a, 6b, 6c) is fixed to the ring, and each disc (6, 6a, 6b, 6c) is also fixed to a disc head (7, 7a, 7b, 7c) which extends in the same direction as the stack of the base and discs (6, 6a, 6b, 6c), and each disc head ( With respect to 7, 7a, 7b, 7c), the arms (8, 8a, 8b, 8c) are rotatably connected first to the disk heads (7, 7a, 7b, 7c) and second to the platform (2). The three-degree-of-freedom joint includes a sphere (S) connected to a base (B) by a cylindrical bar (T), the sphere (S) being positioned at the center of a cylindrical opening provided in the center of the platform (2), and the sphere (S), cylindrical bar (T), and base (B) cooperate to achieve the transmission of forces generated in the platform (2) and the arms (8, 8a, 8b, 8c).

Description

Technical Field

[0001] The technical field of the present invention is the joints of robotic limbs, and more particularly such joints with three degrees of freedom.

Background Art

[0002] Robotic limbs usually use several joints to provide the highest possible mobility, like the limbs of humans or animals.

[0003] Joints involve at least one degree of freedom, usually two or three degrees of freedom. Degree of freedom means the ability to perform rotations about a predefined axis. That is, with two degrees of freedom, a joint enables rotations about two different predefined axes that are usually orthogonal. With three degrees of freedom, a joint enables rotations about three different predefined axes that are also usually orthogonal.

[0004] Depending on their location in the robotic limb, joints are required to have the minimum number of degrees of freedom that can move the limb. In particular, joints located at the wrist of a robotic arm are required to have at least two degrees of freedom. However, by using a three-degree-of-freedom joint, it becomes possible to perform the widest possible range of movements similar to human movements to the maximum extent.

[0005] Non-Patent Document 1 is known from the prior art.

[0006] This specification describes a three-degree-of-freedom joint for a marine propulsion device that enables the reproduction of the shoulders of marine animals, particularly penguins. The described joint includes a coaxial shaft connected to a motor and has a fixed rotation center, an operating frequency of 2.5 Hz under load, unrestricted rotation about the main axis, and arbitrary movement within a cone of + / - 60°.

[0007] However, the joint described herein occupies a significant amount of space within the accompanying frame, particularly due to the placement of the motor. The motor is strictly located outside the joint and is configured to rotate three concentric shafts that control the joint. From this structure, it is clear that a large portion of the internal volume of the joint is unused.

[0008] Non-patent document 2 presents an optimization of parallel joints.

[0009] Non-patent document 3 is also known regarding the modeling of the wrist joint.

[0010] Patent document 1, also filed under the applicant's name, is known to exist, relating to a 3-degree-of-freedom joint for robots and a corresponding control method.

[0011] The joints described herein represent a significant advance in the field of robotic joints. However, it quickly became clear that they are still subject to various technical challenges.

[0012] The first challenge relates to the forces generated in the joints at the end of prolonged use, which lead to assembly degradation and, consequently, play in the assembly. This play in the assembly can result in inaccurate control and potentially lead to failure of parts of the joint.

[0013] The second challenge relates to the difficulty of routing the cables that power and control actuators or joints downstream of a three-degree-of-freedom joint. This is because the main advantage of a three-degree-of-freedom joint is that it allows for infinite rotation in a direction perpendicular to the joint's exit face while benefiting from large rotation angles in other directions, all without being plagued by gimbal lock. However, as soon as an actuator is placed downstream of the joint, the cables that power and control this actuator are limited in the rotation they can actually access, either through interaction with its arm or with the joint itself. Such cables have proven to break or become entangled in the joint as soon as large amplitude rotations are performed. The advantages of large rotation angles and infinite rotation are thus negated. The same applies to gimbal lock, where the lock associated with the cables that power and control the actuators downstream of the joint replaces the gimbal lock itself. [Prior art documents] [Patent Documents]

[0014] [Patent Document 1] French Patent No. 1912398 Specification [Non-patent literature]

[0015] [Non-Patent Document 1] Sudki B. et al., "Marine Propulsor based on a Three-Degree-of-Freedom Actuated Spherical Joint," Third International Symposium on Marine Propulsors smp'13, Launceston, Tasmania, Australia, May 2013. [Non-Patent Document 2] Gosselin C. et al., Kinematic analysis, optimization and programming of parallel robotic manipulator, McGill University (1985) [Non-Patent Document 3] 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 [Overview of the project] [Problems that the invention aims to solve]

[0016] The objective of this invention is to solve these technical problems. [Means for solving the problem]

[0017] The object of the present invention is a three-degree-of-freedom joint for a robot, comprising a platform and three motors, each connected to a ring by a pinion, wherein each ring is positioned inside a stack of hollow disks on a base, each disk is fixed to a ring, and each disk is also fixed to itself a disk head extending in the same direction as the base and the stack of disks.

[0018] For each disk head, first, the disk head, second, the platform is rotatably connected to the arm, the platform is connected to the base and the hollow disk only by three arms, the three-degree-of-freedom joint includes a sphere connected to the base by a cylindrical bar, the sphere is arranged at the center of a cylindrical opening provided at the center of the platform, and the sphere, the cylindrical bar and the base cooperate to achieve the transmission of the force generated in the platform and the arm.

[0019] Each disk can include a bearing support designed to provide a passage for inserting and arranging a pinion, the bearing supports of each disk are stacked on top of each other, and are fixed to the base by screws and screw holes provided in the base, and each bearing support receives a bearing that enables the corresponding disk to rotate.

[0020] The bearing support and the corresponding bearing are included in the disk lining and can be fixed first to a ring related to the disk and second to the corresponding disk head.

[0021] A rotation stopper can be provided in the disk lining, the bearing support of each disk includes a protrusion arranged at the peripheral part of its bottom, the outer diameter of the peripheral part of the bottom of the bearing support is smaller than the inner diameter of the corresponding lining, and when the lining and the bearing support are assembled, a groove interrupted by the protrusion is formed at this time, and the rotation stopper of the disk and the protrusion of the disk directly above cooperate to limit the rotation of the disk provided with the rotation stopper.

[0022] The disk lining can include two stoppers so as to be able to adjust the positive rotation angle of the disk independently of the negative rotation angle of the disk in cooperation with the protrusion.

[0023] Two projections can be provided on the bearing support so as to be able to adjust the positive rotation angle of the disk independently of the negative rotation angle of the disk in cooperation with the stopper.

[0024] The stopper can be made relocatable by a removable fixture on the corresponding lining and / or the projection can be relocatable by a removable fixture on the corresponding bearing support.

[0025] One or more teeth of the ring of the disk can be omitted so as to limit the rotation of the corresponding disk.

[0026] The motor can be provided with means for determining the angular position relative to the reference position of its output shaft.

[0027] The means for determining the angular position can include a plurality of magnetic sensors distributed circularly around a cylindrical bar, which is associated with a magnetic element arranged on each disk head so as to be detectable by the magnetic sensors.

[0028] The means for determining the angular position can include sensors arranged alongside each rotating shaft carrying a pinion.

[0029] The surface of the sphere can be provided with a coating, particularly of the polytetrafluoroethylene type, which minimizes the friction between the sphere and the platform.

[0030] The sphere, the cylindrical bar and the base are each provided with a through hole, and each hole communicates with the other holes.

[0031] Another object of the present invention is a robotic limb including at least two limb segments joined by a three-degree-of-freedom joint as described above.

[0032] Another object of the present invention is a robotic limb comprising at least two limb segments joined by a three-degree-of-freedom joint as described above, and equipment positioned downstream of the three-degree-of-freedom joint, wherein cables supplying power to and / or controlling the equipment are arranged to pass through holes in the spherical and hollow cylindrical bars, through holes in the base, and out between the motors.

[0033] Other purposes, features, and advantages of the present invention will become apparent upon reading the following description, which is provided only as a non-limiting example and is prepared with reference to the attached drawings. [Brief explanation of the drawing]

[0034] [Figure 1] This figure shows the main elements of the 3-degree-of-freedom joint according to the present invention. [Figure 2] This figure shows the main elements that contribute to force transmission in the three-degree-of-freedom joint according to the present invention. [Figure 3] This figure shows the main elements that contribute to the operation of the 3-degree-of-freedom joint according to the present invention. [Figure 4] This diagram shows the main elements of the disk and its corresponding arm. [Figure 5] This diagram shows the cable routing in a 3-degree-of-freedom joint according to the present invention. [Figure 6] This diagram shows the main elements of the disc and bearing support. [Figure 7] This is a diagram showing a bearing support. [Figure 8] This figure shows the position of the 3-degree-of-freedom joint after a command according to the present invention. [Figure 9] This figure shows a rotation sensor positioned at a 3-degree-of-freedom joint according to the present invention. [Figure 10] This figure shows a robot arm including a 3-degree-of-freedom joint according to the present invention. [Modes for carrying out the invention]

[0035] The joint according to the present invention includes a system comprising three parallel shafts that control the joint. Joint 1 is shown in Figure 1.

[0036] Joint 1 allows platform 2 to be moved relative to base B on three free axes by controlling the rotation of three motors 3a, 3b, and 3c. Platform 2 has a cylindrical opening at its center. A sphere S is positioned at the center of platform 2, with the center of sphere S coinciding with the center of rotation of platform 2. The surface of sphere S is coated with a PTFE type (abbreviation for "polytetrafluoroethylene") coating to minimize friction between sphere S and platform 2.

[0037] Joint 1 includes a base B, with three motors 3a, 3b, and 3c positioned adjacent to it, and disks 6a, 6b, and 6c stacked on the base B. Each disk 6a, 6b, and 6c is also fixed to disk heads 7a, 7b, and 7c, which extend in the same direction as the base and the stack of disks 6a, 6b, and 6c.

[0038] For each disk head 7, 7a, 7b, 7c, arc-shaped arms 8, 8a, 8b, 8c are rotatably connected, firstly to the disk heads 7, 7a, 7b, 7c, and secondly to the platform 2. Figure 2 illustrates this arrangement. The connections between the disk heads 7, 7a, 7b, 7c and the corresponding arms 8, 8a, 8b, 8c are coplanar. Similarly, the connections between the arms 8, 8a, 8b, 8c and the platform 2 are also coplanar. Advantageously, the arc shape forms a quarter of a circle.

[0039] The discs 6, 6a, 6b, and 6c form a casing, which facilitates their movement, reduces friction and wear, and is provided with bearings that maintain the alignment of the discs 6, 6a, 6b, and 6c on the base and on each other.

[0040] The sphere S is connected to the cylindrical bar T itself, which is connected to the base B. This arrangement allows the forces applied to the platform 2 to be transmitted to the sphere S, and by extension to the cylindrical bar T and base B. As a result, the arms 8a, 8b, and 8c have some of the forces they experience reduced. More precisely, the advantage of such a configuration, which includes a sphere fixed to the frame via a cylindrical bar, lies in the fact that most of the forces applied to the platform 2 are absorbed by the assembly consisting of the sphere, cylindrical bar, and base, thus forming a kind of internal rigid skeleton.

[0041] For example, bending forces (perpendicular to the main axis) are completely absorbed by the assembly consisting of a sphere, cylindrical bar, and base, thereby reducing the load on the more fragile disk heads 7, 7a, 7b, 7c and arms 8, 8a, 8b, 8c. This is a significant advantage when forming the wrist using the three-degree-of-freedom joint 1. In this way, the forces arising from the weight of the hand and the grasped object are absorbed by the assembly consisting of a sphere, cylindrical bar, and base.

[0042] Similarly, the pressure (in the same direction as the axis of the cylindrical bar T) is absorbed by the assembly consisting of the sphere, cylindrical bar, and base, resulting in only minimal stress on the rigidity of the disk head 7 and arm 8. This is a significant advantage when forming the neck using the 3-degree-of-freedom joint 1. Thus, the force arising from the weight of the gripped head is absorbed by the assembly consisting of the sphere, cylindrical bar, and base.

[0043] Figure 2 shows a sphere S, a cylindrical bar T, and a base B. Disks 6a, 6b, 6c, disk heads 7a, 7b, 7c, arms 8a, 8b, 8c, and platform 2 are shown transparently within this figure.

[0044] In Figure 3, the three motors 3a, 3b, and 3c are connected to rings 4a, 4b, and 4c respectively by pinions 5a, 5b, and 5c, which are supported by shafts. The pinions 5a, 5b, and 5c are positioned inside the rings 4a, 4b, and 4c. Therefore, each pinion 5a, 5b, and 5c is positioned at a different height from the base B so as to mechanically drive only the corresponding rings 4a, 4b, and 4c.

[0045] Each ring 4a, 4b, and 4c is positioned inside one of the hollow disks 6a, 6b, and 6c, and each disk 6a, 6b, and 6c is fixed to the rings 4a, 4b, and 4c.

[0046] Motors 3a, 3b, and 3c are controlled to rotate, causing each disk 6a, 6b, and 6c to rotate on a circumference called the proximal circle. The rotation of each disk 6a, 6b, and 6c causes the mechanically connected arms 8a, 8b, and 8c to rotate on another circumference called the distal circle. At this time, each arm 8a, 8b, and 8c applies force to the platform 2 to change its position.

[0047] Each pinion 5a, 5b, 5c and its corresponding spindle are positioned in a different 120° sector. This 120° offset angle is also seen in the idle position of each disk head 7, 7a, 7b, 7c, with each disk head 7, 7a, 7b, 7c positioned 120° from the other two.

[0048] Figure 4 shows a disk 6 including a bearing support 9 and a lining 10 connected to a disk head 7 which is itself connected to an arm 8. The assembly formed by the disk 6, disk head 7, and arm 8 is associated with one of the three axes or degrees of freedom of the joint. Thus, the joint includes three similar assemblies, each associated with a different degree of freedom. By using three similar assemblies, it is also possible to reduce manufacturing costs in both digital fabrication and plastic injection molding.

[0049] Each disc 6, 6a, 6b, and 6c includes a bearing support 9 designed to provide a passage for inserting and positioning pinions 5a, 5b, and 5c, which are connected to the motor by a shaft. Because the amount of force acting on this component is small, it can be manufactured by both plastic injection molding and digital machining. The bearing supports 9 of each disc 6, 6a, 6b, and 6c are stacked on top of each other and secured to the base B by screws. These screws are designed to work in cooperation with screw holes provided in the base B. It will be understood that while each bearing support 9 is fixed to the base B, the discs 6, 6a, 6b, and 6c rotate by their corresponding rings 4, 4a, 4b, and 4c.

[0050] Each bearing support 9 is associated with a bearing that ensures the rotation of the corresponding discs 6, 6a, 6b, and 6c and maintains their position within the joint 1. The bearing supports 9 and their corresponding bearings are contained within the disc lining 10 and are fixed to rings 4a, 4b, and 4c relating to its discs. The disc lining 10 is fixed to the disc head 7 by shoulders 10a and fixing means.

[0051] In other words, the first disk 6a is connected to the first arm 8a by the disk head 7a, and the first disk 6a is driven by the first motor 3a by the first ring 4a and the first pinion 5a. A similar arrangement is provided for the second disk 6b and the third disk 6c, with one end of each arm connected to the disk head at different heights in the lining 10. The other end of each arm 8a, 8b, and 8c is then connected to the platform 2, which is contained in a plane perpendicular to the axis of the cylindrical bar.

[0052] In a particular embodiment shown in Figure 5, the sphere S, cylindrical bar T, and base B each have through holes, each hole communicating with the others. In the case of the cylindrical bar T, it will be understood that the holes are provided in the form of axial perforations. The cylindrical bar T is then substantially a hollow tube. The advantages of the assembly consisting of the sphere, cylindrical bar, and base described above are also valid when the cylindrical bar has axial perforations.

[0053] For equipment located downstream of the joint, particularly actuators, the cable C supplying power to and / or controlling this equipment can be routed through a hole in the sphere S, through a hollow tube T, through a hole in the base B, and out between the motors 3a, 3b, and 3c. The cable C passes through the central hole of the bearing support 9 of each disc 6, 6a, 6b, and 6c, and through the hollow tube T inside the pinions 5a, 5b, and 5c and the rings 4a, 4b, and 4c. This does not interfere with the operation of the joint and is protected from the environment inside the joint. The cable C, routed in this manner, also does not interfere with the arms 8a, 8b, and 8c, thus protecting the joint from damage.

[0054] In one particular implementation of this embodiment, the amplitude of rotation of disks 6, 6a, 6b, and 6c is limited to prevent damage to the cable C by twisting when the equipment to which the cable C is connected, located downstream of the joint 1, rotates.

[0055] To implement this, various stoppers and protrusions are placed on the discs to limit their rotation.

[0056] More precisely, the disk lining 10 is provided with a rotation stopper 10b intended to restrict the rotation of the corresponding disks 6, 6a, 6b, and 6c. Figure 6 shows disk 6a fixed to disk head 7a, including bearing support 9, lining 10, and rotation stopper 10b. Other disk heads 7b and 7c are also shown, but are not fixed to the lining 10 of disk 6a.

[0057] The bearing support 9 includes a passage 11 for inserting pinions 5a, 5b, and 5c, and holes 12a, 12b, and 12c into which screws for retention in the base B are inserted. The hollow tube T extends to the center of the bearing support.

[0058] In addition to the stopper 10b, the bearing support 9 for each disc 6, 6a, 6b, 6c includes a projection 9a located on its lower periphery. Figure 7 shows such a bearing support 9 with the projection 9a. The passage 11 and holes 12a, 12b, 12c are visible.

[0059] A space is provided between the lining 10 and the bearing support 9 so that the disc with the stopper can be rotated. This is achieved by making the outer diameter of the lower part of the bearing support 9 smaller than the inner diameter of the corresponding lining 10. When the lining 10 and the bearing support 9 are assembled, a groove interrupted by the projection 9a is formed at this time.

[0060] Discs 6, 6a, 6b, and 6c rotate freely until a stopper 10b on their lining 10 contacts a projection 9a on the bearing support 9 of the disc directly above it. The last disc in the stack, 6c, is prevented from rotating by a similar projection located on the lining directly above it. Depending on the arrangement of the stoppers and projections, it is possible to restrict the rotation accessible to each disc. Due to the arrangement of the stoppers and projections, the discs can access positive and negative rotation angles defined relative to the idle position. The joint as a whole is then restricted to positive and negative angles equal to the sum of the positive and negative angles accessible to each disc, respectively. In such a configuration, the sum of the positive and negative angles of the discs is always equal to 360°.

[0061] In one modification, stoppers can be positioned on both sides of the idle position so that the positive rotation angle can be adjusted independently of the negative rotation angle. The positive rotation angle may be equal to or different from the negative rotation angle. In such a modification, the disc lining includes two stoppers, each cooperating with a projection on the bearing support of the disc directly above it. In this configuration, the sum of the positive and negative angles of the disc is 360° or less.

[0062] The same effect could be achieved by placing one stopper on the lining and two protrusions on the bearing support directly above it.

[0063] The stopper can also be repositioned by a removable fastener on the corresponding lining. This could be, for example, a stopper to be fitted into a hole, where multiple holes are provided on the lining, some regularly spaced and some not. It could also be a stopper to be screwed into a screw hole.

[0064] Alternatively, the rotation of the disk can be restricted by removing or omitting one or more teeth on the corresponding ring.

[0065] Figure 8 shows the 3-degree-of-freedom joint 1 after the command has been executed. It should be noted that the platform is tilted relative to its idle position shown in Figure 1. It should also be noted that the exit opening of the sphere S remains open despite the tilt of platform 2. This exit opening is always open so that cable C is not sheared by the platform, due to the angle that platform 2 can achieve in a plane perpendicular to the plane of platform 2.

[0066] Each motor 3a, 3b, and 3c is also provided with means for determining the angular position of its output shaft relative to a reference position. Such determination means are shown in Figure 9.

[0067] In the first embodiment, the means for determining the angular position includes a plurality of magnetic sensors 13, which are arranged on a plate 14, preferably on the interface printed circuit of the magnetic sensors 13. The plurality of magnetic sensors 13 are uniformly distributed around the plate 14, such that the spacing between two adjacent magnetic sensors is constant throughout the perimeter.

[0068] In each disk head 7a, 7b, and 7c, magnetic elements 15a, 15b, and 15c are installed and positioned so that they can be detected by magnetic sensors 13. It will be understood that such arrangement is based on a fit between the strength of the magnetic field emitted by each magnetic element 15a, 15b, and 15c, the sensitivity of the magnetic sensors 13, considered individually or in combination, and the distance between each magnetic element and the multiple magnetic sensors 13. To enable associating the magnetic elements of each disk head with a given position, joint 1 is initialized by sequentially requesting small movements of each disk. The position of the magnetic element detected by the magnetic sensors 13 is then associated with the disk that is subsequently operated. When three disks are operated, three magnetic elements (as shown here) are identified. Their movement can then be tracked according to commands sent to the joint.

[0069] In the second embodiment, the means for determining the angular position includes sensors 16a, 16b, and 16c along the respective rotation axes supporting the pinions 5a, 5b, and 5c. The sensors 16a, 16b, and 16c enable the implementation of closed-loop control of the rotation of the motors 3a, 3b, and 3c.

[0070] In the third embodiment, the means for determining the position includes a combination of the magnetic sensor 13 and magnetic elements 15a, 15b, and 15c according to the first embodiment, and the rotational speed sensors 16a, 16b, and 16c according to the second embodiment. Figure 5 shows one such embodiment, in which the combination of the magnetic sensor 13 and magnetic elements 15a, 15b, and 15c detects the position of the disk head 7 and arm 8 when the joint 1 starts to operate, and then the rotational speed sensors 16a, 16b, and 16c can accurately track their movement.

[0071] Referring again to Figure 5, it can be noted that cables C1, C2, and C3, which connect the magnetic sensor 13 and / or rotational speed sensors 16a, 16b, and 16c, are located inside the joint, as is cable C, which supplies power to and controls the equipment downstream of the joint.

[0072] Referring again to Figure 6, it can be noted that each bearing support has openings for these cables C1, C2, and C3, and these openings are formed alongside the passage 11, the hollow tube T, and the stoppers 12a, 12b, and 12c. Just like cable C, cables C1, C2, and C3 are thus protected by the joints.

[0073] Figure 10 shows a robotic arm 20 in which the wrist joint is implemented by a 3-degree-of-freedom joint 1 according to the present invention. It should be noted that the motor 3 is located inside the robotic arm 20 and is protected and concealed. Cables C, C1, C2, and C3 are also included inside the robotic arm 20 and are therefore protected as well. [Explanation of Symbols]

[0074] 1 joint 2 Platforms 3a, 3b, 3c motors 4a, 4b, 4c rings 5a, 5b, 5c pinion 6, 6a, 6b, 6c disks 7, 7a, 7b, 7c disk heads 8, 8a, 8b, 8c arms 9 Bearing support 9a protrusion 10 Lining 10a Shoulder 10b Stopper 11 aisles 12a, 12a, 12c holes 13 Magnetic Sensor 14 plates 15a, 15b, 15c Magnetic elements 16a, 16b, 16c sensors 20 Robot Arms

Claims

1. A three-degree-of-freedom joint for a robot, comprising a platform (2) and three motors (3a, 3b, 3c) connected to rings (4, 4a, 4b, 4c) respectively by pinions (5a, 5b, 5c), wherein each ring (4, 4a, 4b, 4c) is positioned inside hollow discs (6a, 6b, 6c) stacked on a base (B), and each disc (6, 6a, 6b, 6c) is fixed to the rings (4, 4a, 4b, 4c). Each disk (6, 6a, 6b, 6c) is also fixed in itself to disk heads (7, 7a, 7b, 7c) that extend in the same direction as the stacking of the disks (6, 6a, 6b, 6c) on the base (B), and for each disk head (7, 7a, 7b, 7c), arms (8, 8a, 8b, 8c) are rotatably connected, firstly to the disk head (7, 7a, 7b, 7c) and secondly to the platform (2), and the platform extends to the base (B) only by the three arms (8, 8a, 8b, 8c). A three-degree-of-freedom joint connected to the hollow disk, wherein the three-degree-of-freedom joint includes a sphere (S) connected to the base (B) by a cylindrical bar (T), the sphere (S) being positioned at the center of a cylindrical opening provided at the center of the platform (2), and the sphere (S), the cylindrical bar (T), and the base (B) cooperating to achieve the transmission of forces generated in the platform (2) and the arms (8, 8a, 8b, 8c).

2. Each disc (6, 6a, 6b, 6c) includes a bearing support (9) designed to provide a passage for inserting and positioning the pinions (5a, 5b, 5c), the bearing supports (9) of each disc (6, 6a, 6b, 6c) are stacked on top of each other and secured to the base (B) by screws and screw holes provided in the base (B), and each bearing support (9) receives a bearing that enables rotation of the corresponding disc (6, 6a, 6b, 6c), the three-degree-of-freedom joint according to claim 1.

3. The three-degree-of-freedom joint according to claim 2, wherein the bearing support (9) and the corresponding bearing are included in the disc lining (10) and are fixed first to the ring (4a, 4b, 4c) associated with the disc and second to the corresponding disc head (7, 7a, 7b, 7c).

4. A rotation stopper (10b) is provided on the disc lining (10), the bearing support (9) of each disc (6, 6a, 6b, 6c) includes a projection (9a) located on the bottom periphery thereof, the outer diameter of the bottom periphery of the bearing support (9) is smaller than the inner diameter of the corresponding lining (10), and when the lining (10) and the bearing support (9) are assembled, a groove interrupted by the projection (9a) is formed at this time, the rotation stopper (10b) of the disc and the projection (9a) of the disc directly above it cooperate to restrict the rotation of the disc on which the rotation stopper (10b) is provided, the three-degree-of-freedom joint according to claim 3.

5. The three-degree-of-freedom joint according to claim 4, comprising two stoppers (10b) such that the disc lining (10) can cooperate with the projection (9a) to adjust the positive rotation angle of the disc independently of the negative rotation angle of the disc.

6. The three-degree-of-freedom joint according to claim 4, wherein two protrusions are provided on the bearing support so that, in cooperation with the stopper (10a), the positive rotation angle of the disk can be adjusted independently of the negative rotation angle of the disk.

7. A three-degree-of-freedom joint according to any one of claims 4 to 6, wherein the stopper is repositionable by a removable fastener on the corresponding lining and / or the projection is repositionable by a removable fastener on the corresponding bearing support.

8. A three-degree-of-freedom joint according to any one of claims 1 to 3, wherein one or more teeth of the ring of the disk are omitted to restrict the rotation of the corresponding disk.

9. A three-degree-of-freedom joint according to any one of claims 1 to 8, wherein the motors (3a, 3b, 3c) are provided with means for determining the angular position of their output shafts with respect to a reference position.

10. The three-degree-of-freedom joint according to claim 9, wherein the means for determining the angular position includes a plurality of magnetic sensors (13) distributed in a circular manner around the cylindrical bar (T), which are associated with magnetic elements (15a, 15b, 15c) positioned on each disk head (7a, 7b, 7c) so as to be detectable by the magnetic sensors (13).

11. The means for determining the angular position includes sensors (16a, 16b, 16c) along each rotation axis supporting the pinions (5a, 5b, 5c), as described in claim 9 or 10, for a three-degree-of-freedom joint.

12. The 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, particularly of the polytetrafluoroethylene type, that minimizes friction between the sphere (S) and the platform (2).

13. The three-degree-of-freedom joint according to any one of claims 1 to 12, wherein through holes are provided in the sphere (S), the cylindrical bar (T), and the base (B), and each hole communicates with the other holes.

14. A robotic limb comprising at least two limb segments joined by a three-degree-of-freedom joint (1) as described in any one of claims 1 to 13.

15. A robotic limb comprising at least two limb segments joined by a three-degree-of-freedom joint (1) as described in claim 13, and equipment positioned downstream of the three-degree-of-freedom joint (1), wherein cables supplying power to and / or controlling the equipment are arranged to pass through holes in a spherical body (S) and a hollow cylindrical bar (T), through holes in a base (B), and exit between motors (3a, 3b, 3c).

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