A joint with 2 degrees of freedom
The parallel joint design with nested shafts and bevel gears addresses the size and cost issues of two-degree-of-freedom joints, providing a compact and efficient solution for robotic limbs.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- POLLEN ROBOTICS
- Filing Date
- 2023-10-25
- Publication Date
- 2026-04-22
AI Technical Summary
Existing joints with two degrees of freedom in robots are large in size and expensive, making them less desirable than three-degree-of-freedom joints despite their complexity and cost.
A parallel joint design with two degrees of freedom using nested motor shafts, bevel gears, and sensors, allowing for miniaturization and efficient mass distribution, with motors positioned on the same side to reduce size and weight.
The design achieves a compact and cost-effective joint with two degrees of freedom, enabling efficient control and reduced stress on robotic limbs, while allowing for power and control cable integration.
Smart Images
Figure 2026512993000001_ABST
Abstract
Description
Technical Field
[0001] The technical field of the present invention is the joints of the limbs of a robot, and more specifically, such joints having two degrees of freedom.
Background Art
[0002] The limbs of a robot generally use several joints so as to have the most possible mobility, similar to the limbs of a human or an animal.
[0003] A joint generally includes at least one degree of freedom of 2 to 3 degrees of freedom. The degree of freedom means that rotation can be performed around a pre-defined axis. Thus, when having two degrees of freedom, the joint can rotate around two separate pre-defined axes, which are generally orthogonal. When having three degrees of freedom, the joint can rotate around three separate pre-defined axes, which are likewise generally orthogonal.
[0004] Depending on its position in the limb of the robot, a joint requires a minimum number of degrees of freedom to enable it to operate. It might seem easier to use only joints having three degrees of freedom for each joint of the limb. However, such joints having three degrees of freedom are heavier, more expensive, and more complex to control than the corresponding components having two degrees of freedom. Therefore, it is advantageous to have joints having two degrees of freedom supplementarily to the joints having three degrees of freedom.
[0005] Such joints having two degrees of freedom are known from the prior art. The following documents show various deformations of these joints.
[0006] The paper, "An evolutionary approach for the optimal design of iCub mk. 3 Parallel Wrist," by Bsili R. et al., presented at the IEEE-RAS 18th International Conference on Humanoid Robots (Humanoids 2018) on November 8, 2018, in Beijing, China, describes a mechanism for a robotic wrist with two degrees of freedom, as well as design parameters for maximizing the achievable angles for each degree of freedom.
[0007] The paper Pencic M. et al., "Social Humanoid Robot SARA: Development of the Wrist Mechanism," IOP Conference Series Materials Science and Engineering. 294(1):012079-1-012079-10, describes another robotic wrist mechanism with 2 degrees of freedom, allowing for 115° flexion / extension and 45° lateral displacement.
[0008] Jager, J et al. (2017) "Joint level modelling, characterization and torque control of the SHERPA robotic arm," MSs report, Robotics and Mechatronics, University of Twente, describes the joints included in a robotic arm placed on a rover. The arm has 7 degrees of freedom distributed between the shoulder, elbow, and wrist. The shoulder and elbow are in the form of joints with 2 degrees of freedom, while the wrist has 3 degrees of freedom.
[0009] The paper, "Novel Mechanical Design of Biped Robot SHERPA Using 2 DOF Cable Differential Modular Joints" by Olaru I et al., IROS: Intelligent Robots and Systems, October 2009, St. Louis, Missouri, USA, pp. 4463-4468 (10.1109 / IROS.2009.5354425), describes a joint with 2 degrees of freedom, whose characteristics depend on the combination of cables and pulleys.
[0010] These various studies clearly show that joints with two degrees of freedom, developed using cutting-edge technology, are generally large in size and expensive. [Prior art documents] [Non-patent literature]
[0011] [Non-Patent Document 1] 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), November 8, 2018, Beijing, China. [Non-Patent Document 2] Pencic M. et al. “Social Humanoid Robot SARA: Development of the Wrist Mechanism”, IOP Conference Series Materials Science and Engineering. 294(1) :012079-1-012079-10 [Non-Patent Document 3] Jager, J et al. (2017) "Joint level modelling, characterization and torque control of the SHERPA robotic arm," MSs report, Robotics and Mechatronics, University of Twente [Non-Patent Document 4] Olaru I. et al. "Novel Mechanical Design of Biped Robot SHERPA Using 2 DOF Cable Differential Modular Joints," IROS: Intelligent Robots and Systems, October 2009, St. Louis, Missouri, USA, pp. 4463-4468, (10.1109 / IROS.2009.5354425) [Overview of the Initiative] [Problems that the invention aims to solve]
[0012] The purpose of this application is to overcome these technical problems. [Means for solving the problem]
[0013] One object of the present invention is a parallel joint for a robot having 2 degrees of freedom, the joint comprising a base and a head rotatably fixed to the base, the head also comprising a working surface rotatably fixed to the head, so that the axis of rotation of the working surface is perpendicular to the axis of rotation of the head with respect to the base, the head comprising three bevel gears, the first bevel gear supported by a first shaft, the second bevel gear supported by a second shaft, and the third bevel gear fixed to the working surface and arranged to mesh simultaneously with the first and second bevel gears, the first and second shafts being coaxial with each other and with respect to the axis of rotation of the head with respect to the base, the first shaft being hollow, the second shaft passing through the first bevel gear and the first shaft, and the first and second shafts being mechanically connected to a first motor and a second motor, respectively.
[0014] 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 surface of action relative to the head.
[0015] The second axis can be hollow, and a communication cable for a sensor to measure the rotation of the surface of action relative to the head passes through the first and second axes.
[0016] The head may have a passive surface that is rotatably fixed relative to the head, and a port at its center, allowing a cable to pass through the port, a first hollow shaft, and a second hollow shaft and emerge into the base.
[0017] The first shaft and the second shaft can be mechanically connected to the first motor and the second motor, respectively, by a gear transmission mechanism, with one motor connected to a first gear that meshes with a second gear connected to the corresponding shaft.
[0018] The first shaft and the second shaft can be mechanically connected to the first motor and the second motor, respectively, by pulleys and a belt transmission mechanism, with one pulley connected to the motor and the other pulley connected to the corresponding shaft, and the two pulleys connected by a belt.
[0019] The belt transmission mechanism can be connected to the working surface, with a first pulley connected to the working surface and a retaining element, and a second pulley fixed integrally with the working surface and rotatably fixed to the retaining element, so that its position relative to the working surface is maintained, and the two pulleys are connected via a belt.
[0020] Another object of the present invention is a method for controlling a joint having 2 degrees of freedom, as described above, wherein two motors are controlled to rotate in different directions and at the same speed to rotate a head relative to a base, and two motors are controlled to rotate in the same direction and at the same speed to rotate a working surface relative to the head.
[0021] Another object of the present invention is a limb of a robot comprising at least two segments connected together by a joint having two degrees of freedom as described above.
[0022] Other objects, features, and advantages of the present invention will become apparent upon reading the following description, which is made by way of non-limiting example and refers to the accompanying drawings.
Brief Description of the Drawings
[0023] [Figure 1] It is a figure which shows the main elements of the joint which has two degrees of freedom by this invention. [Figure 2] It is a sectional view of the joint which has two degrees of freedom by this invention. [Figure 3] It is a sectional view of the joint which has two degrees of freedom by this invention which shows the structure of a rotation sensor. [Figure 4] It is a figure which shows the robot arm provided with the joint which has two degrees of freedom by this invention. [Figure 5] It is a figure which shows a 1st Embodiment regarding the motor drive of the joint 1 which has two degrees of freedom. [Figure 6] It is a figure which shows a 1st Embodiment regarding the motor drive of the joint 1 which has two degrees of freedom. [Figure 7] It is a figure which shows an embodiment of the offset of the mechanical output of the joint 1 which has two degrees of freedom.
Mode for Carrying Out the Invention
[0024] In order to solve technical problems and have a joint with two degrees of freedom, using two nested motor shafts surprisingly enables having a joint with two degrees of freedom where the motors are arranged on the same side, and thus the applicant has noticed that it enables miniaturization of the joint and mass distribution in the limb of the robot.
[0025] The joint 1 having 2 degrees of freedom according to the present invention is shown in Figure (Figure 1). The joint 1 comprises a base 2 and a head 3 that is rotatably fixed to the base 2.
[0026] Head 3 comprises a spherical portion connected to a cylindrical portion. The cylindrical portion of head 3 is inserted into the corresponding opening in base 2.
[0027] The head 3 further includes an action surface 3a and a passive surface 3b that are fixed to rotate freely and are directly opposite to each other, such that the axes of rotation of the action surface 3a and the passive surface 3b pass through the center of the spherical portion of the head 3 and are contained in a plane perpendicular to the axis of the cylindrical portion of the head 3.
[0028] The movement of the head 3 and the movement of the working surface 3a depend on three bevel gears 6a, 6b, and 6c located on the head 3. The first bevel gear 6a and the second bevel gear 6b are located on the front of the mating surface.
[0029] The third bevel gear 6c is mechanically fixed to the working surface 3a, and therefore the rotation transmitted to the third bevel gear 6c is also transmitted to the working surface 3a. The third bevel gear 6c simultaneously meshes with the first bevel gear 6a and the second bevel gear 6b.
[0030] The joint formed in this manner has a first degree of freedom that rotates along the axis of rotation of the cylindrical portion of the head 3, with respect to the connection between the base 2 and the head 3, and a second degree of freedom that rotates along the axis of rotation of the working surface 3a, with respect to the connection between the head 3 and the working surface 3a.
[0031] A first degree of freedom of rotation is achieved when the first bevel gear 6a and the second bevel gear 6b rotate in opposite directions.
[0032] A second degree of freedom of rotation is achieved when the first bevel gear 6a and the second bevel gear 6b rotate in the same direction. The third bevel gear 6c then receives a rotational speed equal to the rotational speed of either the first bevel gear 6a or the second bevel gear 6b.
[0033] Figure (Figure 2) shows a cross-section of a joint 1 having 2 degrees of freedom according to the present invention.
[0034] In addition to the main elements mentioned above, Figure 2 shows the internal structure of the base 2, head 3, and working surface 3a.
[0035] The working 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.
[0036] Similarly, the passive surface 3b has a disk shape and is free to rotate relative to the head 3.
[0037] A pair of bearings 5 works to hold the working surface 3a and the passive surface 3b in the head 3 while allowing rotation.
[0038] Another bearing set 4 allows the cylindrical portion of the head 3 to be held in place relative to the base 2 while allowing rotation of the cylindrical portion of the head 3 relative to its axis. Similarly, another bearing set 4a allows the cylindrical portion of the head 3 to be held in place relative to the first shaft 7a while allowing rotation of the cylindrical portion of the head 3 relative to its axis.
[0039] The first bevel gear 6a is connected to the first shaft 7a, which is connected to the first motor. The second bevel gear 6b is connected to the second shaft 7b, which is connected to the second motor.
[0040] The first shaft 7a and the second shaft 7b are therefore coaxial to enable this configuration. This is achieved in particular by making at least the first shaft 7a a hollow shaft, and the second shaft 7b is positioned inside the first shaft 7a. The second shaft 7b passes from the first bevel gear 6a and the second bevel gear 6b to the support bearing 8. A flange 9 is positioned between the second bevel gear 6b and the support bearing 8 to provide a firm connection between the second bevel gear 6b and the second shaft 7b. The first bevel gear 6a is then mounted on the first shaft 7a and held in place by the shoulders with which the first bevel gear 6a makes contact. Another bearing 10 allows the second shaft 7b to be held on the first shaft 7a while allowing it to rotate.
[0041] The concentric shaft design, and the through-shape of the second shaft 7b to the first bevel gear 6a and the second bevel gear 6b, allows the two motors to be positioned on the same side of the joint. This configuration is highly advantageous for use in a robotic arm, as it allows the two motors to be located on the side of the frame supporting the base 2. It will be understood that the design ensures the two shafts are held coaxially with respect to each other by different bearings. In addition, their diameters are selected to avoid friction between the shafts.
[0042] A joint 1 with two degrees of freedom designed in this way can have two free axes, each of which can rotate infinitely.
[0043] Figure (Figure 3) shows a sensor placed in a joint 1 having 2 degrees of freedom according to the present invention.
[0044] The first rotation sensor 11 is provided on the base 2 at the interface between the base 2 and the cylindrical portion of the head 3. The first rotation sensor 11 comprises a fixed portion connected to the base 2 and a movable portion connected to the cylindrical portion of the head 3. The fixed portion is, in particular, a magnetic sensor configured to measure fluctuations in the magnetic field. The movable portion is, in particular, a magnetic ring equipped with at least one encoder. The magnetic sensor detects fluctuations in the magnetic field when the magnetic ring is rotated as the head 3 rotates.
[0045] The magnetic sensor and magnetic element set is designed with respect to size, distance, intensity, and sensitivity, and so the magnetic sensor can detect the magnetic elements and determine the position of the magnetic elements according to the intensity measured by each sensor. The first rotation sensor 11 is equipped with a connecting cable 12.
[0046] The second rotation sensor 13 is positioned on the working surface 3a of the head and measures its position relative to the stationary position of the head 3 or to a magnetic element fixed to the rest of it.
[0047] When joint 1, which has 2 degrees of freedom, is equipped with these sensors, it becomes possible to determine the absolute or relative position of each part of joint 1 so that closed-loop control of each degree of freedom is possible.
[0048] In one particular embodiment, the second shaft 7b is hollow, similar to the first shaft 7a, and therefore provides a favorable path for the circulation of various cables. This path is particularly used for passing the connection cable of the second rotation sensor 13.
[0049] This path can also be used to circulate cables 15 connecting equipment or actuators located downstream of joint 1. The cables 15 then emerge through a port provided at the center of the passive surface 3b. Such cables 15 are particularly useful for supplying power to and controlling equipment or actuators located downstream. This is especially important when joint 1 with degrees of freedom 2 is used in a robotic arm as a shoulder or elbow, as shown in Figure 4, where in this specification it is a wrist joint, and at least one actuator 20 is located downstream of joint 1 with degrees of freedom 2. Joint 1 with degrees of freedom 2 located at the elbow is also located downstream of joint 1 with degrees of freedom 2 located at the shoulder and benefits from the circulation of its power supply and control cables inside the hollow axis of this shoulder joint. In general, a robotic limb can thus comprise joint 1 with degrees of freedom 2 located between two segments of the limb.
[0050] The importance of the bearing 5 between the head 3 and the passive surface 3b will also be understood. In fact, when integrated into a robot's limb, such as shown in Figure 4, the limb segment following the joint 1, which has 2 degrees of freedom, is fixed to the working surface 3a. This limb segment is also fixed to the passive surface 3b so as not to be supported by the working surface 3a alone, and to share the supporting force. As soon as the limb segment is fixed to the passive surface 3b, the latter needs to have freedom of rotation in order to follow the rotational motion imparted to the limb segment by the working surface 3a. The presence of the bearing 5 makes this possible.
[0051] The upstream and downstream interactions of the joint will be discussed next.
[0052] Figure (Figure 5) shows a first embodiment relating to motor drive of joint 1 having 2 degrees of freedom.
[0053] The first motor 21a is mechanically connected to the first shaft 7a by a first set of gears 22a in order to reduce speed.
[0054] Similarly, the second motor 21b is mechanically connected to the second shaft 7b by a second set of gears 22b in order to reduce its speed.
[0055] The two motors 21a and 21b are therefore positioned as extensions of joint 1, which is advantageous for joints between two limbs, such as the elbow or knee. Since the motors are integrated into the forelimb, the overall size of the system is reduced.
[0056] In one particular embodiment, the first motor 21a and the second motor 21b have the same characteristics, and therefore the two sets of gears 22a and 22b have the same reduction ratio.
[0057] In a second embodiment of motor-driven joint 1 having 2 degrees of freedom, as shown in Figure 6, the first shaft 7a is connected to the first motor 21a by a first set of pulleys and belt 23a. Similarly, the second shaft 7b is connected to the second motor 21b via a second set of pulleys and belt 23b.
[0058] Such a configuration allows for the offsetting of motors 21a and 21b and modification of the robot's center of mass or overall size in the vicinity of joint 1. This is particularly advantageous when joint 1 is used for the shoulder or waist, insofar as motors 21a and 21b can be located in the robot's chassis (i.e., torso).
[0059] Figure (Figure 7) shows the rotational offset output from the joint. In this embodiment, the joint output corresponding to the working surface 3a included in the joint head 3 is connected to the first pulley 25a. The second pulley 25b is positioned at the location of the offset rotation. The belt 25c is positioned to transmit the rotation from the first pulley 25a to the second pulley 25b.
[0060] The belt 25c can only perform its role of transmitting between the two pulleys 25a and 25b when minimal tension is applied to it. In addition, the belt 25c has limitations on the torsion it can tolerate, and therefore the two pulleys 25a and 25b must remain substantially in the same plane. To satisfy these limitations, a retaining element 25d is mechanically fixed to the bearing to allow axial rotation of the pulleys 25a and 25b. The retaining element 25d ensures that the pulleys 25a and 25b can be held in the appropriate relative position for driving by the belt 25c. Such a retaining element 25d is also fixed to the joint head 3 to maintain the relative position of the second pulley 25b and the joint head 3, while allowing rotation of the working surface 3a connected to the pulley 25a.
[0061] Similar to the embodiment shown in Figure 6, this embodiment has the advantage of offsetting the center of mass of the system. This is advantageous for joints between forelimbs and hindlimbs, such as the knee or elbow. In fact, the center of mass of the joint is located in that case near the joint between the forelimb and the torso, reducing the stress on them. In that case, motor drives can be used more efficiently or reduced for the same force output, resulting in gains in mass and cost. [Explanation of Symbols]
[0062] 1 joint 2 bases 3 heads 3a Working surface 3b Passive surface 4 bearings 4a Bearing 5 bearings 6a First bevel gear 6b Second bevel gear 6c Third cap tooth 7a First axis 7b The second axis 8 Support bearings 9 flanges 10 bearings 11. First rotation sensor 12 connection cables 13. Second rotation sensor 15 Cables 20 Actuators 21a First motor 21b Second motor 22a First set of gears 22b Second set of gears 23a First set of pulleys and belts 23b Second set of pulleys and belts 25a First pulley 25b Second pulley 25cm belt 25d holding element
Claims
1. A parallel joint (1) for a robot having 2 degrees of freedom, It comprises two motors (5a, 5b), a base (2), and a head (3) that is rotatably fixed to the base (2). The head (3) also includes the working surface (3a) which is rotatably fixed to the head (3) such that the axis of rotation of the working surface (3a) is perpendicular to the axis of rotation of the head (3) with respect to the base (2), and the head (3) includes three bevel gears (6a, 6b, 6c), the first bevel gear (6a) being supported by a first shaft (7a), the second bevel gear (6b) being supported by a second shaft (7b), and the third bevel gear (6c) being fixed to the working surface (3a) and the first bevel gear (6a) and A parallel joint (1) is arranged to mesh simultaneously with the second bevel gear (6b), the first shaft (7a) and the second shaft (7b) are coaxial with each other and with respect to the rotation axis of the head (3) relative to the base (2), the first shaft (7a) is hollow, the second shaft (7b) passes through the first bevel gear (6a) and the first shaft (7a), and the first shaft (7a) and the second shaft (7b) are mechanically connected to the first motor (5a) and the second motor (5b), respectively.
2. The joint according to claim 1, further 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 working surface (3a) relative to the head (3).
3. The joint according to claim 2, wherein the second axis (7b) is hollow, and a communication cable for the sensor for measuring the rotation of the working surface relative to the head passes through the first axis (7a) and the second axis (7b).
4. The joint according to claim 3, wherein the head (3) comprises a passive surface (3b) that is rotatably fixed to the head (3) and has a port at its center, and a cable is allowed to pass through the port, the first hollow shaft (7a), and the second hollow shaft (7b) and emerge into the base (2).
5. The joint according to any one of claims 1 to 4, wherein the first shaft (7a) and the second shaft (7b) are mechanically connected to the first motor (5a) and the second motor (5b), respectively, by a gear transmission mechanism, and one of the motors is connected to a first gear that meshes with a second gear connected to the corresponding shaft.
6. The joint according to any one of claims 1 to 4, wherein the first shaft (7a) and the second shaft (7b) are mechanically connected to the first motor (5a) and the second motor (5b), respectively, by a belt and pulley transmission mechanism, one pulley is connected to one of the motors, the other pulley is connected to the corresponding shaft, and the two pulleys are connected by the belt.
7. The joint according to any one of claims 1 to 6, wherein a belt transmission mechanism is connected to the working surface (3a), a first pulley (25a) is connected to the working surface (3a) and rotatably fixed to a retaining element (25d), and a second pulley (25b) is fixed integrally with the offset working surface (3a) and rotatably fixed to the retaining element (25d), so as to maintain its position relative to the working surface (3a), and the two pulleys are connected via a belt (25c).
8. A method for controlling a joint (1) having 2 degrees of freedom as described in any one of claims 1 to 7, A method wherein the two motors are controlled to drive the shafts (7a, 7b) in different directions and at the same speed to rotate the head (3) relative to the base (2), and the two motors are controlled to drive the shafts (7a, 7b) in the same direction and at the same speed to rotate the working surface (3a) relative to the head (3).
9. A robotic limb comprising at least two segments connected together by a joint (1) having 2 degrees of freedom as described in any one of claims 1 to 7.