Joint with two degrees of freedom

Abstract

Parallel joint (1) with two degrees of freedom for a robot, comprising a base (2) and a head (3) fixed freely in rotation on the base (2), the head (3) further comprising an active surface (3a) fixed freely in rotation in the head (3), the head (3) comprising three bevel gears (6a, 6b, 6c), a first bevel gear (6a) being borne by a first shaft (7a), a second bevel gear (6b) being borne by a second shaft (7b), a third bevel gear (6c) being secured to the active surface (3a) and positioned in such a way as to mesh simultaneously with the first bevel gear (6a) and the second bevel gear (6b), the second shaft (7b) passing through the first bevel gear (6a) and the hollow first shaft (7a), the first shaft (7a) and the second shaft (7b) being coaxial and mechanically connected to a first motor and to a second motor, respectively.

Description

[0001] DESCRIPTION

[0002] TITLE: Two-degree-of-freedom joint

[0003] Technical field

[0004] The technical field of the invention is the joints of robotic limbs, and more particularly such joints with two degrees of freedom.

[0005] Previous techniques

[0006] Robotic limbs typically use multiple joints to provide the best possible mobility, much like the limbs of a human or animal.

[0007] A joint involves at least one degree of freedom, usually two or three degrees of freedom. Degrees of freedom refers to the ability to rotate along a predefined axis. Thus, with two degrees of freedom, a joint allows rotation along two distinct, generally orthogonal, predefined axes. With three degrees of freedom, a joint allows rotation along three distinct, generally orthogonal, predefined axes.

[0008] Depending on their location in the robotic limb, the joint requires a minimum number of degrees of freedom to function. It might seem simpler to use only three-degree-of-freedom joints for each of the joints in a limb. However, such three-degree-of-freedom joints are heavier, more expensive, and more complex to control than their two-degree-of-freedom counterparts. It is therefore advantageous to have two-degree-of-freedom joints in addition to three-degree-of-freedom joints.

[0009] Such two-degree-of-freedom joints are known in the state of the art. The following documents illustrate different variations of these joints.

[0010] The paper Bsili R. et al. “An evolutionary approach for the optimal design of iCub mk. 3 Parallel Wrist” IEEE-RAS 18th International Conference on Humanoid Robots (Humanoids 2018), 08 / 11 / 18, Beijing, China describes a mechanism for a robotic wrist, with two degrees of freedom as well as associated design parameters to maximize the angles achievable according to each degree of freedom.

[0011] The paper Pencic M. et al. "Social Humanoid Robot SARA: Development of the Wrist Mechanism", IOP Conference Series Materials Science and Engineering. 294(1):012079-l-012079-10 describes another two-degree-of-freedom robotic wrist mechanism allowing 115° flexion / extension and 45° lateral deviation.

[0012] Jager, J et al. (2017) “Joint level modeling, characterization and torque control of the SHERPA robotic arm”, MSs report, Robotics and Mechatronics, University of Twente describes a joint included in a robotic arm mounted on a rover. The arm has seven degrees of freedom distributed between a shoulder, an elbow and a wrist. The shoulder and the elbow are presented as two-degree-of-freedom joints, while the wrist has three degrees of freedom. Olaru I. et al. “Novel Mechanical Design of Biped Robot SHERPA Using 2 DOF Cable Differential Modular Joints” IROS: Intelligent Robots and Systems, Oct 2009, St. Louis, MO, USA. pp.4463-4468, (10.1109 / IROS.2009.5354425) describes a two-degree-of-freedom joint with the particularity of relying on the combination of cables and pulleys.

[0013] It is clear from these various documents that state-of-the-art two-degree-of-freedom joints are bulky and expensive.

[0014] The purpose of this application is to resolve these technical issues.

[0015] Statement of the invention

[0016] The invention relates to a parallel joint with two degrees of freedom for a robot comprising a base and a head fixed free to rotate on the base, the head further comprises an active surface fixed free to rotate in the head so that the axis of rotation of the active surface is included in a plane normal to the axis of rotation of the head relative to the base, the head comprising three bevel gears, a first bevel gear being carried by a first axis, a second bevel gear being carried by a second axis, a third bevel gear being integral with the active surface and being arranged so as to mesh simultaneously with the first bevel gear and the second gear, the first axis and the second axis being coaxial with each other and with the axis of rotation of the head relative to the base, the first axis being hollow, the second axis passing through the first bevel gear and the first axis,the first axis and the second axis being mechanically connected to a first motor and a second motor, respectively.,

[0017] The joint may include a sensor for measuring the rotation of the head relative to the base and a sensor for measuring the rotation of the active surface relative to the head.

[0018] The second axis may be hollow, the communication cable of the sensor measuring the rotation of the active surface relative to the head then passing through the first axis and the second axis.

[0019] The head may include a passive surface fixed free to rotate relative to the head, and including a lumen in its center such that a cable can pass through the lumen, the first hollow shaft, and the second hollow shaft to exit into the base.

[0020] The first axle and the second axle may be mechanically connected to a first motor and a second motor respectively, via a gear transmission, a motor being connected to a first gear meshing with a second gear connected to the corresponding axle.

[0021] The first axis and the second axis may be mechanically connected to a first motor and a second motor respectively, via a pulley and belt transmission, one pulley being connected to a motor, the other pulley being connected to the corresponding axis, the two pulleys being connected by the belt.

[0022] A belt drive may be connected to the active surface, a first pulley being connected to the active surface and to a holding element, a second pulley being fixedly attached to the active surface and being freely rotatably attached to the holding element, such that its position relative to the active surface is maintained, the two pulleys being connected by a belt.

[0023] The invention also relates to a method for controlling a two-degree-of-freedom joint as described above, in which the two motors are controlled so that they rotate in different directions and at the same speed to rotate the head relative to the base and the two motors are controlled so that they rotate in the same direction and at the same speed to rotate the active surface relative to the head.

[0024] Another object of the invention is a robotic limb comprising at least two segments connected together by a two-degree-of-freedom joint as described above.

[0025] Brief description of the drawings

[0026] Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example and made with reference to the appended drawings in which:

[0027] - figure [Fig 1] illustrates the main elements of the two-degree-of-freedom joint according to the invention,

[0028] - figure [Fig 2] illustrates a sectional view of the two-degree-of-freedom joint according to the invention,

[0029] - figure [Fig 3] illustrates a sectional view of the two-degree-of-freedom joint according to the invention showing the arrangement of the rotation sensors,

[0030] - figure [Fig 4] illustrates a robotic arm comprising the two-degree-of-freedom joint according to the invention,

[0031] - figure [Fig 5] illustrates a first embodiment of the motorization of the joint 1 with two degrees of freedom,

[0032] - figure [Fig 6] illustrates a first embodiment of the motorization of the joint 1 with two degrees of freedom, and

[0033] - figure [Fig 7] illustrates an embodiment of offsetting the mechanical output of joint 1 with two degrees of freedom.

[0034] Detailed description

[0035] In order to resolve the technical problem and to have a joint with two degrees of freedom, the applicant noted that the use of two nested motor axes made it possible, very surprisingly, to have a joint with two degrees of freedom whose motors are arranged on the same side, thus improving the compactness of the joint as well as the distribution of masses in a robotic limb.

[0036] The two-degree-of-freedom joint 1 according to the invention is illustrated in figure [Fig 1]. The joint 1 comprises a base 2 and a head 3 fixed freely in rotation on the base 2.

[0037] The head 3 comprises a spherical part joined with a cylindrical part. The cylindrical part of the head 3 fits into a corresponding opening of the base 2.

[0038] The head 3 further comprises an active surface 3a and a passive surface 3b directly opposite, fixed free to rotate in the head 3, so that the axis of rotation of the active surface 3a and of the passive surface 3b passes through the center of the spherical part of the head 3 and is included in a plane normal to the axis of the cylindrical part of the head 3.

[0039] The movement of the head 3 and the movement of the active surface 3a are based on three bevel gears 6a, 6b, 6c arranged in the head 3. The first bevel gear 6a and the second bevel gear 6b ​​are arranged opposite each other.

[0040] The third bevel gear 6c is mechanically secured to the active surface 3a so that a rotation imparted to the third bevel gear 6c is also transmitted to the active surface 3a. The third bevel gear 6c meshes simultaneously with the first bevel gear 6a and the second bevel gear 6b.

[0041] The joint thus formed comprises a first degree of freedom in rotation around the connection between the base 2 and the head 3 along the axis of revolution of the cylindrical part of the head 3, and a second degree of freedom in rotation around the connection between the head 3 and the active surface 3a along the axis of revolution of the active surface 3a.

[0042] A rotation according to the first degree of freedom is obtained when the first bevel gear 6a and the second bevel gear 6b ​​rotate in opposite directions.

[0043] A rotation according to the second degree of freedom is obtained when the first bevel gear 6a and the second bevel gear 6b ​​rotate in the same direction. The third bevel gear 6c is then subjected to a rotational speed equal to the rotational speed of the first bevel gear 6a or the second bevel gear 6b.

[0044] Figure [Fig 2] illustrates a sectional view of the two-degree-of-freedom joint 1 according to the invention.

[0045] In addition to the main elements described above, figure [Fig 2] illustrates the internal structure of the base 2, the head 3 and the active surface 3a.

[0046] The active surface 3a has a disc shape and is mechanically connected to the third bevel gear 6c. However, it remains free to rotate relative to the head 3.

[0047] The passive surface 3b similarly has a disc shape and is left free to rotate relative to the head 3.

[0048] A set of bearings 5 ​​contributes to maintaining the active surface 3a and the passive surface 3b in the head 3 while allowing rotation.

[0049] Another set 4 of bearings allows the cylindrical part of the head 3 to be held in place relative to the base 2, while allowing rotation relative to the axis of the cylindrical part of the head 3. Similarly, a different set 4a of bearings allows the cylindrical part of the head 3 to be held in place relative to the first axis 7a, while allowing rotation relative to the axis of the cylindrical part of the head 3.

[0050] The first bevel gear 6a is connected to a first axle 7a connected to a first motor. The second bevel gear 6b ​​is connected to a second axle 7b connected to a second motor.

[0051] The first axis 7a and the second axis 7b are then coaxial so as to allow this arrangement. This is achieved in particular by making at least the first axis 7a in the form of a hollow axis, the second axis 7b being arranged inside the first axis 7a. The second axis 7b passes through the first bevel gear 6a and the second bevel gear 6b ​​to a support bearing 8. A flange 9 is arranged between the second bevel gear 6b ​​and the support bearing 8 in order to provide a rigid coupling between the second bevel gear 6b ​​and the second axis 7b. The first bevel gear 6a is held in position by a shoulder provided in the first axis 7a and against which the first bevel gear 6a is in contact. Another bearing 10 makes it possible to hold the second axis 7b in the first axis 7a while allowing rotation.

[0052] The concentric axis design and the through arrangement of the second axis 7b relative to the first bevel gear 6a and the second bevel gear 6b ​​allows the two motors to be arranged on the same side of the joint. This configuration is very advantageous for use within a robotic arm since the two motors can then be located on the side of the frame supporting the base 2. It will be understood that, by design, the two axes are kept coaxial with each other by the various bearings or rollers. In addition, their diameters are chosen so that friction between the tubes is absent.

[0053] The two-degree-of-freedom joint 1 thus designed provides two axes of freedom on each of which infinite rotation can be achieved.

[0054] Figure [Fig 3] illustrates the sensors arranged in a two-degree-of-freedom joint 1 according to the invention.

[0055] A first rotation sensor 11 is arranged in the base 2 at the interface between the base 2 and the cylindrical part of the head 3. The first rotation sensor 11 comprises a fixed part connected to the base 2 and a movable part connected to the cylindrical part of the head 3. The fixed part is in particular a magnetic sensor configured to measure variations in the magnetic field. The movable part is in particular a magnetic ring provided with at least one encoder. The magnetic sensor detects a variation in the magnetic field when the magnetic ring is rotated during the rotation of the head 3.

[0056] The magnetic sensor and magnetic element assembly is designed in size, distance, intensity and sensitivity so that the magnetic sensors can detect the magnetic element and the position of the magnetic element can be determined based on the intensity measured by each sensor. The first rotation sensor 11 is provided with a connecting cable 12.

[0057] A second rotation sensor 13 is arranged in the active surface 3a of the head, so as to measure its position relative to a rest position or relative to a magnetic element secured to the rest of the head 3.

[0058] When the two-degree rotation joint 1 is equipped with these sensors, it is thus possible to determine the absolute or relative position of each part of the joint 1 so that closed-loop control of each degree of freedom is possible.

[0059] In a particular embodiment, the second axis 7b is hollow like the first axis 7a, thus providing a preferred path for running different cables. This path is notably used for the passage of a connection cable for the second rotation sensor 13. This path can also be used to run a cable 15 connecting equipment or actuators arranged downstream of the joint 1. The cable 15 then emerges through a slot provided in the center of the passive face 3b. Such a cable 15 notably makes it possible to power and control the equipment or actuators arranged downstream. This is particularly important when the joint 1 with two degrees of freedom is used as a shoulder or elbow in a robotic arm as illustrated in the figure [Fig 4], in which at least one actuator 20, in this case a wrist joint, is arranged downstream of the joints 1 with two degrees of freedom.The two-degree-of-freedom joint 1 arranged in the elbow is also located downstream of the two-degree-of-freedom joint arranged in the shoulder and benefits from the circulation of its power and control cable inside the hollow axes of this shoulder joint. In general, a robotic limb can thus comprise a two-degree-of-freedom joint 1 arranged between two segments of the limb.

[0060] The importance of the bearings 5 ​​between the head 3 and the passive face 3b will also be understood. Indeed, when it is integrated into a robotic limb such as that illustrated in figure [Fig 4], the limb segment following an articulation 1 with two degrees of freedom is fixed to the active surface 3a. This limb segment is also fixed to the passive surface 3b in order to share the support forces and to prevent the active surface 3a alone from supporting them. Once the limb segment is fixed to the passive surface 3b, the latter must be provided with freedom of rotation so as to follow the rotational movement imparted by the active surface 3a to the limb segment. The presence of bearings 5 ​​makes it possible to achieve this.

[0061] The upstream and downstream interface of the joint will now be discussed.

[0062] Figure [Fig 5] illustrates a first embodiment of the motorization of joint 1 with two degrees of freedom.

[0063] A first motor 21a is mechanically connected to the first axis 7a via a first set of gears 22a allowing a reduction to be achieved.

[0064] Similarly, the second motor 21b is mechanically connected to the second axle 7b via a second set of gears 22b to provide a reduction.

[0065] The two motors 21a, 21b are thus arranged in the extension of the joint 1, which is advantageous in the case of an articulation between two limbs, such as the elbow or the knee. The size of the system is reduced because the motors are integrated in the forelimb.

[0066] In a particular embodiment, the first motor 21a and the second motor 21b have the same characteristics, the two sets of gears 22a, 22b then having the same reduction ratio.

[0067] In a second embodiment as regards the motorization of the joint 1 with two degrees of freedom, illustrated by the figure [Fig 6], the first axis 7a is connected to the first motor 21a by a first set of pulleys and belt 23a. Similarly, the second axis 7b is connected to the second motor 21b by a second set of pulleys and belt 23b. Such an arrangement makes it possible to offset the motors 21a, 21b and to modify the center of mass or the size of the robot in the vicinity of the joint 1. This is particularly advantageous in the case of a joint 1 used for a shoulder or a hip insofar as the motors 21a, 21b can then be arranged in the chassis (i.e. the torso) of the robot.

[0068] Figure [Fig 7] illustrates a rotation offset at the output of the joint. In such an embodiment, the output of the joint, corresponding to the active surface 3a included in the joint head 3, is connected to a first pulley 25a. A second pulley 25b is arranged at the location of the rotation offset. A belt 25c is arranged so as to transmit the rotation of the first pulley 25a to the second pulley 25b.

[0069] The belt 25c can only perform its transmission role between the two pulleys 25a, 25b if a minimum tension is applied to it. In addition, the belt 25c is limited in the torsion that it can accept, so that the two pulleys 25a, 25b must remain substantially in the same plane. In order to satisfy these constraints, a holding element 25d is mechanically secured to bearings allowing the rotation of the axes of the pulleys 25a, 25b. A holding element 25d makes it possible to maintain the pulleys 25a, 25b in relative positions appropriate for driving by the belt 25c. Such a holding element 25d is also secured to the head 3 of the joint so as to maintain the relative positions of the second pulley 25b and the head 3 of the joint while allowing the rotation of the active surface 3a connected to the pulley 25a.

[0070] As with the embodiment illustrated in Figure [Fig 6], this embodiment has the advantage of shifting the center of mass of the system. This is advantageous for joints between a forelimb and a hindlimb such as the knee or the elbow. Indeed, the center of mass of the joint is then located closer to the joint between the forelimb and the torso, reducing the stresses on the latter. The motors can then be used more efficiently for the same restored force or resized downwards for a gain in mass and cost.

Claims

CLAIMS 1. Parallel joint (1) with two degrees of freedom for a robot comprising two motors (5a, 5b), a base (2) and a head (3) fixed free to rotate on the base (2), the head (3) further comprises an active surface (3a) fixed free to rotate in the head (3) so that the axis of rotation of the active surface (3a) is included in a plane normal to the axis of rotation of the head (3) relative to the base (2), the head (3) comprising three bevel gears (6a, 6b, 6c), a first bevel gear (6a) being carried by a first axis (7a), a second bevel gear (6b) being carried by a second axis (7b), a third bevel gear (6c) being integral with the active surface (3a) and being arranged so as to mesh simultaneously with the first bevel gear (6a) and the second gear (6b), the first axis (7a) and the second axis (7b) being coaxial with each other and with the axis of rotation of the head (3) relative to the base (2), the first axis (7a) being hollow,the second shaft (7b) passing through the first bevel gear (6a) and the first shaft (7a), the first shaft (7a) and the second shaft (7b) being mechanically connected to the first motor (5a) and the second motor (5b), respectively., 2. Joint according to claim 1, comprising a sensor for measuring the rotation of the head (3) relative to the base (2) and a sensor for measuring the rotation of the active surface (3a) relative to the head (3).

3. Joint according to claim 2, in which the second axis (7b) is hollow, the communication cable of the sensor for measuring the rotation of the active surface relative to the head then passing through the first axis (7a) and the second axis (7b).

4. Joint according to claim 3, in which the head (3) comprises a passive surface (3b) fixed free to rotate relative to the head (3), and comprising a lumen in its center so that a cable can pass through the lumen, the first hollow axis (7a) and the second hollow axis (7b) to exit in the base (2).

5. Articulation according to any one of claims 1 to 4, wherein the first axis (7a) and the second axis (7b) are mechanically connected to the first motor (5a) and the second motor (5b) respectively, by means of a gear transmission, one of said motors being connected to a first gear meshing with a second gear connected to the corresponding axis.

6. Articulation according to any one of claims 1 to 4, wherein the first axis (7a) and the second axis (7b) are mechanically connected to the first motor (5a) and to the second motor (5b) respectively, by means of a pulley and belt transmission, one pulley being connected to one of said motors, the other pulley being connected to the corresponding axis, the two pulleys being connected by the belt.

7. Joint according to any one of claims 1 to 6, in which a belt transmission is connected to the active surface (3a), a first pulley (25a) being connected to the active surface (3a) and fixed freely in rotation on a holding element (25d), a second pulley (25b) being fixed integrally to a remote active surface (3ab) and being fixed freely in rotation on the element holding (25d), so that its position relative to the active surface (3a) is maintained, the two pulleys being connected by a belt (25c).

8. Method for controlling a joint (1) with two degrees of freedom according to any one of claims 1 to 7, in which the two motors are controlled so that they drive the axes (7a, 7b) so that they rotate in different directions and at the same speed to rotate the head (3) relative to the base (2) and the two motors are controlled so that they drive the axes (7a, 7b) so that they rotate in the same direction and at the same speed to rotate the active surface (3a) relative to the head (3).

9. Robotic member comprising at least two segments connected together by a joint (1) with two degrees of freedom according to any one of claims 1 to 7.

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