A metamorphic robot joint based on line-symmetric 4U mechanism

Abstract

The application discloses a metamorphic robot joint based on a line-symmetrical 4U mechanism and belongs to the technical field of robots. The joint comprises a joint interface connecting rod, a joint output connecting rod, a first connecting rod and a second connecting rod, wherein the two ends of the joint interface connecting rod are connected with the ends of two connecting rods through two U pairs respectively, and the top ends of the two connecting rods are connected with the two ends of the joint output connecting rod through two U pairs respectively; the joint interface connecting rod, the joint output connecting rod and the two connecting rods are hingedly connected with each other through the U pairs with two vertical intersecting rotation axes, forming an 8R mechanism, and the 8R mechanism has two different motion modes, namely a two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation motion mode and a three-degree-of-freedom fixed-point rotation motion mode. The application has the advantages of simple structure, reliable motion, important reference value for the design of a planar assembly operation robot and the design of a fixed-point motion robot in the two motion modes.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of robots, and relates to a metamorphic robot joint based on a line-symmetrical 4U mechanism. BACKGROUND

[0002] A metamorphic robot can realize different movements and operation modes through a set of mechanical devices without changing constituent components, and thus is widely applied in application scenarios in which operation modes need to be changed. Compared with a traditional robot with a single and fixed operation mode, the biggest difference between the metamorphic robot and the traditional robot is that the metamorphic robot can change its structure according to different operation tasks to adapt to configuration requirements under different tasks. Generally, the main way to change the structure of the metamorphic robot is to add a metamorphic joint to the structure of the metamorphic robot. The metamorphic joint itself is a metamorphic mechanism, and can realize different movement modes through structural recombination. In the multi-mode operation of the metamorphic robot, the movement modes of the metamorphic joint in different configurations are combined with the movement of other joints in the metamorphic robot, so that different operation modes of the metamorphic robot can be realized. Therefore, the structural recombination principle and configuration design of the metamorphic joint are the core links of the structural design of the metamorphic robot. SUMMARY

[0003] To solve the above problems, the application provides a metamorphic robot joint based on a line-symmetrical 4U mechanism, which can realize two degrees of freedom spherical translation plus one degree of freedom rotation movement and three degrees of freedom spatial fixed-point rotation two different movement modes through a set of mechanical joints.

[0004] The metamorphic robot joint based on the line-symmetrical 4U mechanism comprises a joint interface connecting rod, a joint output connecting rod, two connecting rods and four U pairs. The joint interface connecting rod and the joint output connecting rod are the same in structure, and have a robot connecting hole in the middle part; the two ends are provided with coaxial end thread holes. The two connecting rods are the same in structure, and are designed with circular through holes at two ends; the four U pairs are all arc connecting rods, and are provided with through holes at two ends, and are connected with threaded pins and pin shafts to form rotating pairs; and the movement axes of the two rotating pairs intersect perpendicularly at point O.

[0005] The threaded pins and the pin shafts in the four U pairs have two installation modes:

[0006] Mode one U pair is that the threaded pin is inserted into the through hole from the outer arc surface of the U pair, so that the threaded part of the threaded pin is located on the inner side of the U pair; and the pin shaft is inserted into the through hole from the inner arc surface of the U pair, so that the shaft end part is located on the outer side of the U pair.

[0007] Mode two U pair is that the threaded pin is inserted into the through hole from the inner arc surface of the U pair, so that the threaded part of the threaded pin is located on the outer side of the U pair; and the pin shaft is inserted into the through hole from the inner arc surface of the U pair, so that the shaft end part is located on the outer side of the U pair.

[0008] The joint interface connecting rod left and right ends are connected to the bottom ends of the two connecting rods through the first U pair and the second U pair respectively; the first U pair and the second U pair are connected to the threaded holes of the left and right ends of the joint interface connecting rod through the threaded portions of the threaded pins, and form rotary pairs; the first U pair and the second U pair are connected to the round through holes of the bottom ends of the two connecting rods through the pins, and form rotary pairs. At this time, the joint interface connecting rod is connected to the two connecting rods through the two U pairs at both ends, forming a two-degree-of-freedom rotary Hooke joint connection relationship with a vertical and intersecting rotary axis.

[0009] The top ends of the two connecting rods are connected to the left and right ends of the joint output connecting rod through the second U pair and the first U pair respectively; the second U pair and the first U pair are connected to the threaded holes of the left and right ends of the joint output connecting rod through the threaded portions of the threaded pins, and form rotary pairs; the second U pair and the first U pair are connected to the through holes of the top ends of the two connecting rods through the pins, and form rotary pairs. At this time, the joint output connecting rod is connected to the two connecting rods through the second U pair and the first U pair at both ends, forming a two-degree-of-freedom rotary Hooke joint connection relationship with a vertical and intersecting rotary axis.

[0010] The metamorphic robot joint has two different motion modes, which are a two-degree-of-freedom spherical translation plus a single-degree-of-freedom rotation motion mode and a three-degree-of-freedom fixed-point rotation motion mode:

[0011] In the two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation motion mode of the metamorphic robot joint, the relationships of the rotary axes are as follows:

[0012] The axes A and B are perpendicular and orthogonal to a point, the axes C and D are perpendicular and orthogonal to a point, the axes E and F are perpendicular and orthogonal to a point, and the axes G and H are perpendicular and orthogonal to a point. The axes A, C, E and G are parallel to each other, and the distances between the axes A and C, the axes A and E, the axes C and G, and the axes E and G are equal. Therefore, the axes A, C, E and G always form an equilateral parallelogram during the motion of the mechanism. Further, the axes B and D coincide, and the axes F and H coincide. The equilateral parallelogram formed by the axes A, C, E and G can rotate around the common axes F and H, forming a line-symmetric 4U mechanism.

[0013] The above-mentioned axis A and axis B are the two rotational secondary axes between the joint output link and the left link respectively; axis C and axis D are the two rotational secondary axes between the joint output link and the second link respectively; axis E and axis F are the two rotational secondary axes between the joint interface link and the first link respectively; axis G and axis H are the two rotational secondary axes between the joint output link and the second link respectively; axis A, axis C, axis E and axis G are all rotational secondary axes formed by U-secondary pin connection; the remaining axes are all rotational secondary axes formed by U-secondary threaded pin connection.

[0014] During the movement, axis B will be parallel to axis F, and axis D will be parallel to axis H; the joint output link will perform two-degree-of-freedom spherical translation relative to the joint interface link, and its motion trajectory is always on the spherical surface. While performing two-degree-of-freedom spherical translation, the joint output link always has a rotational motion around its own axis.

[0015] The three-degree-of-freedom fixed-point rotation motion mode is:

[0016] When the movement reaches the point where axis A is perpendicular to axis B, axis C is perpendicular to axis D, axis G is perpendicular to axis H, and axis E is perpendicular to axis F; and axis B, axis D, axis F and axis H are collinear, and axis A and axis G are also collinear, then the joint is in a bifurcation position, and the joint freedom increases. During continued movement, axis A and axis G are kept collinear, and the common straight line is straight line A, axis B and axis D are collinear, and the common straight line is straight line B, axis F and axis H are collinear, and the common straight line is straight line C; when the three common straight lines intersect at one point, the movement of the joint output link relative to the joint interface link will be a three-degree-of-freedom spatial fixed-point rotation. At this time, the joint output link has a rotation in any direction in three-dimensional space, that is, a typical rigid body fixed-point motion in three-dimensional space, and its rotation center is the intersection of the common lines A, B, and C.

[0017] The advantages of the present invention are:

[0018] 1. The present invention provides a metamorphic robot joint based on a line-symmetric 4U mechanism with a variable operation mode, which can realize two different motion modes of two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation and three-degree-of-freedom spatial fixed-point rotation through a set of mechanical joints.

[0019] 2. The joint structure of the metamorphic robot based on the line-symmetrical 4U mechanism of the present invention only includes 8 connecting rods and 4 Hooke's hinge mechanical components, with a simple structure and reliable movement.

[0020] 3. The two degrees of freedom translation plus single degree of freedom rotation movement and three degrees of freedom space fixed-point rotation movement modes of the variable cell robot joint based on the line-symmetrical 4U mechanism can be realized, which are very typical in robot operation, and the two degrees of freedom translation plus single degree of freedom rotation movement mode has important reference value for the design of planar assembly operation robot and the three degrees of freedom space fixed-point movement mode has important reference value for the design of fixed-point movement robot. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 The overall structure diagram of the variable cell robot joint based on the line-symmetrical 4U mechanism.

[0022] Figure 2 The joint interface connecting rod structure diagram of the variable cell robot joint based on the line-symmetrical 4U mechanism.

[0023] Figure 3 The first connecting rod structure diagram of the variable cell robot based on the line-symmetrical 4U mechanism.

[0024] Figure 4 The U pair structure diagram of the variable cell robot joint based on the line-symmetrical 4U mechanism.

[0025] Figure 5 The circular arc connecting rod structure diagram of the U pair of the variable cell robot joint based on the line-symmetrical 4U mechanism.

[0026] Figure 6 The axis distribution A of the two degrees of freedom spherical translation plus single degree of freedom rotation movement mode of the variable cell robot joint based on the line-symmetrical 4U mechanism.

[0027] Figure 7 The axis distribution B of the two degrees of freedom spherical translation plus single degree of freedom rotation movement mode of the variable cell robot joint based on the line-symmetrical 4U mechanism.

[0028] Figure 8 The equivalent movement mode diagram of the two degrees of freedom spherical translation plus single degree of freedom rotation movement mode of the variable cell robot joint based on the line-symmetrical 4U mechanism.

[0029] Figure 9 The axis distribution diagram of the bifurcation configuration when the variable cell robot joint based on the line-symmetrical 4U mechanism is in different movement mode switching.

[0030] Figure 10 The axis distribution A of the three degrees of freedom space fixed-point rotation movement mode of the variable cell robot joint based on the line-symmetrical 4U mechanism.

[0031] Figure 11Axis distribution B of three degrees of freedom space fixed-point rotation motion mode of metamorphic robot joint based on line-symmetrical 4U mechanism of the application.

[0032] Figure 12 Equivalent motion mode chart of three degrees of freedom space fixed-point motion mode of metamorphic robot joint based on line-symmetrical 4U mechanism of the application.

[0033] In the figure:

[0034] 1-joint interface connecting rod 2-joint output connecting rod 3-first connecting rod 4-second connecting rod

[0035] 5-first U pair 6-second U pair 7-third U pair 8-fourth U pair

[0036] 101-middle threaded hole 102-end threaded hole 501-axis A 502-axis B

[0037] 503-axis C 504-axis D 505-axis E 506-axis F 507-axis G 508-axis H DETAILED DESCRIPTION

[0038] The application will be further described below in combination with the drawings.

[0039] The metamorphic robot joint based on line-symmetrical 4U mechanism of the application is mainly composed of four parts, as shown in the figure, including joint interface connecting rod 1, joint output connecting rod 2, first connecting rod 3, second connecting rod 4, and four U pairs, i.e. first to fourth U pairs 5 to 8. Figure 1

[0040] The joint interface connecting rod 1 is a cylindrical connecting rod, and the middle part is designed as a cuboid structure coaxial with the joint interface connecting rod 1. The radial cross section of the cuboid structure is a square, and the circumferential four sides of the cuboid structure are tangent to the cylindrical surface of the joint interface connecting rod 1.

[0041] Four threaded holes are designed on the joint interface connecting rod 1. Two middle threaded holes 101 are located on the middle cuboid structure of the joint interface connecting rod 1, and the axes of the two middle threaded holes 101 are parallel, both perpendicular to the opposite sides of the square structure and both perpendicular to the axis of the joint interface connecting rod 1. The above-mentioned two middle threaded holes 101 are used to connect the joint interface connecting rod 1 and the robot components to be connected through screws, and the side walls of the square structure thereon are used to cooperate with the wall surface of the robot connection position for positioning. The other two end threaded holes 102 are designed on the two end faces of the joint interface connecting rod 1. The two end threaded holes 102 are coaxial with the joint interface connecting rod 1 and are used for the connection between the joint interface connecting rod 1 and the U pair.

[0042] ​The first connecting rod 3 is a cylindrical connecting rod as a whole, and two circular through holes 301 are designed at two ends; the axes of the two circular through holes 301 are parallel and intersect and are perpendicular to the axis of the first connecting rod 3; and the end faces of the two circular through holes 301 are designed as planes for contact and cooperation with the outer wall surface of the U-shaped sub.

[0043] The joint output connecting rod 2 is the same in structure and size as the joint interface connecting rod 1. The second connecting rod 4 is the same in structure and size as the first connecting rod 3.

[0044] The four U-shaped sub are arc-shaped plate structure connecting rods, two ends of which are provided with through holes for being connected with threaded pins 9 and pin shafts 10 to form rotating sub; and the motion axes of the two rotating sub are perpendicular and intersect at point O, so that the motion of the U-shaped sub is the hooke joint motion of two rotating axes being perpendicular.

[0045] In the first U-shaped sub 5 and the second U-shaped sub 6, the threaded pin 9 is inserted into the through hole from the outer arc surface side of the U-shaped sub, so that the threaded part of the threaded pin is located at the inner side of the U-shaped sub 3; and the pin shaft 10 is inserted into the through hole from the inner arc surface side of the U-shaped sub 3, so that the shaft end part is located at the outer side of the U-shaped sub. In the third U-shaped sub and the fourth U-shaped sub, the threaded pin 9 is inserted into the through hole from the inner arc surface side of the U-shaped sub 3, so that the threaded part of the threaded pin is located at the outer side of the U-shaped sub 3; and the pin shaft 10 is inserted into the through hole from the inner arc surface side of the U-shaped sub 3, so that the shaft end part is located at the outer side of the U-shaped sub.

[0046] Based on the structure of each component, the joint based on the line-symmetrical 4U mechanism of the metamorphic robot of the present application is obtained by being connected in the following manner:

[0047] First, the joint interface connecting rod 1 is connected with the bottom ends of the first connecting rod 3 and the second connecting rod 4 through the first U-shaped sub 5 and the third U-shaped sub 7 at the left and right two ends respectively. The first U-shaped sub 5 and the third U-shaped sub 7 are connected through the threaded part of the threaded pin 9 and the threaded hole at the left and right two ends of the joint interface connecting rod 1 in threaded cooperation, to form the first rotating sub and the second rotating sub. Then, the first U-shaped sub 5 and the third U-shaped sub 7 are connected through the pin shaft 10 and the through hole at the bottom end of the first connecting rod 3 and the second connecting rod 4 in plug-in cooperation, to form the third rotating sub and the fourth rotating sub. At this time, the joint interface connecting rod 1 is connected through the first U-shaped sub 5 and the third U-shaped sub 7 at the two ends, and a rotating axis perpendicular and intersecting is formed between the first connecting rod 3 and the second connecting rod 4, to form a two-degree-of-freedom rotating hooke joint connection relationship.

[0048] Furthermore, the top ends of the first connecting rod 3 and the second connecting rod 4 are connected to the left and right ends of the joint output connecting rod 2 through the fourth U pair 8 and the second U pair 6 respectively. The fourth U pair 8 and the second U pair 6 are respectively connected to the threaded holes on the left and right ends of the joint output connecting rod 2 through the threaded portion of the threaded pin 9, forming the fifth rotation pair and the sixth rotation pair. The fourth U pair 8 and the second U pair 6 are respectively plugged into the through holes at the top ends of the first connecting rod 3 and the second connecting rod 4 through the pin shaft 10, forming the seventh rotation pair and the eighth rotation pair. At this time, the two ends of the joint output connecting rod 2 form a two-degree-of-freedom rotational Hooke's hinge connection relationship with a rotation axis perpendicularly intersecting with the first connecting rod 3 and the second connecting rod 4 respectively through the fourth U pair 8 and the second U pair 6.

[0049] Therefore, the metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention can be regarded as an 8R mechanism in which the joint interface link 1, the joint output link 2 and the first link 3 and the second link 4 are articulated through a U pair with two perpendicular intersecting rotational motion axes. For the 8R mechanism, its spatial axis distribution is as follows: Figure 6 shown.

[0050] The metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention has two different motion modes, namely, a two-degree-of-freedom spherical translation plus a single-degree-of-freedom rotation motion mode and a three-degree-of-freedom fixed-point rotation motion mode. The distribution relationship of the motion axis of the metamorphic robot joint in the two-degree-of-freedom spherical translation plus a single-degree-of-freedom rotation motion mode is as follows: Figure 6 and Figure 7 As shown, the two rotational secondary axes between the joint output link 2 and the first link 3 during the metamorphic robot joint motion are axis A501 and axis B502; the two rotational secondary axes between the joint output link 2 and the first link are axis C503 and axis D504; the two rotational secondary axes between the joint interface link 1 and the first link 3 are axis E505 and axis F506; and the two rotational secondary axes between the joint output link 2 and the second link are axis G507 and axis H508. Axis A501, axis C503, axis E505, and axis G507 are all rotational secondary axes formed by connecting the U-joint center pin 10; the remaining axes are all rotational secondary axes formed by connecting the U-joint threaded pin 9. When the metamorphic robot joint is in a two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation motion mode, its axes will satisfy the following geometric relationships:

[0051] Wherein, the axis A501 and the axis B502 are perpendicular to a point, the axis C503 and the axis D504 are perpendicular to a point, the axis E505 and the axis F506 are perpendicular to a point, the axis G507 and the axis H508 are perpendicular to a point. The axis A501, the axis C503, the axis E505 and the axis G507 are parallel to each other, and the distance between the axis A501 and the axis C503, the distance between the axis A501 and the axis E505, the distance between the axis C503 and the axis G507, the distance between the axis E505 and the axis G507 are equal, so that the axis A501, the axis C503, the axis E505 and the axis G507 always form an isosceles parallelogram during the movement of the mechanism. Further, the axis B502 and the axis D504 coincide, and the axis F506 and the axis H508 coincide. So that the isosceles parallelogram formed by the axis A501, the axis C503, the axis E505 and the axis G507 can rotate around the common axis formed by the axis 506 and the axis 508, forming a line-symmetric 4U mechanism.

[0052] At the same time, during the movement, the axis B502 will be parallel to the axis F506, and the axis D504 is parallel to the axis H508. When the robot joint axis satisfies the above geometric relationship, the metamorphic robot joint movement mode is as shown in Figure 8 , that is, the joint output link 2 moves relative to the joint interface link 1 with two degrees of freedom spherical translation, which is a translational motion, and the motion trajectory is always located on the spherical surface shown in Figure 8 , and at the same time of two degrees of freedom spherical translation, the joint output link 2 always has a rotation motion around its own axis.

[0053] Figure 9 The axis distribution diagram when the metamorphic robot joint moves to the bifurcation configuration of the two different movement modes. At this time, the robot joint has two movement mode tendencies. As shown in Figure 9 . At this time, the axis A501 and the axis B502 are perpendicular to each other, the axis C503 and the axis D504 are perpendicular to each other, the axis G507 and the axis H508 are perpendicular to each other, and the axis E505 and the axis F506 are perpendicular to each other. And the axis B502, the axis D504, the axis F506 and the axis H508 are collinear, and the axis A501 and the axis G507 are also collinear. Compared with the movement mode in Figure 6 and Figure 7 , at this time, the axis A501 and the axis G507 are collinear, and the axis B502, the axis D504, the axis F506 and the axis H508 are collinear. The above special axis arrangement causes the degree of freedom of the metamorphic robot joint to change suddenly, and the degree of freedom of the joint increases.

[0054] When the metamorphic robot joint moves to Figure 9When the motion bifurcation configuration is shown, if the axis A501 and the axis G507 continue to maintain a collinear relationship during the continued movement of the joint, the common straight line is straight line A, the axis B502 and the axis D504 are collinear, the common straight line is straight line B, the axis F506 and the axis H508 are collinear, and the common straight line is straight line C. As shown in Figure 10 and Figure 11 When the three common straight lines intersect at a point, at this time, the motion of the joint output connecting rod 2 relative to the joint interface connecting rod 1 will be a three-degree-of-freedom spatial fixed-point rotation, and the equivalent motion is as shown in Figure 12 At this time, the joint output connecting rod 2 can realize rotation in any direction in the three-dimensional space, that is, the typical rigid body fixed-point motion in the three-dimensional space, and the rotation center is the intersection point of the common straight lines A, B and C.

Claims

1. A metamorphic robot joint based on a line-symmetric 4U mechanism, characterized by: It includes a joint interface link, a joint output link, two connecting rods and four U pairs; The joint interface connecting rod has the same structure as the joint output connecting rod, with a robot connection hole in the middle and coaxial threaded holes at both ends. The two connecting rods have the same structure, and circular through holes are designed at both ends; The four U-pairs are all arc-shaped connecting rods with through holes at both ends, which are connected to the threaded pins and the pin shafts respectively to form a revolving pair; and the motion axes of the two revolving pairs intersect vertically at point O; the threaded pins and the pin shafts in the four U-pairs have two installation methods: Method 1: The U pair is as follows: the threaded pin is inserted into the through hole from the outer arc side of the U pair, so that the threaded part of the threaded pin is located on the inner side of the U pair; the pin shaft is inserted into the through hole from the inner arc side of the U pair 3, so that the shaft end is located on the outer side of the U pair; Method 2: The U pair is as follows: the threaded pin is inserted into the through hole from the inner arc side of the U pair, so that the threaded part of the threaded pin is located outside the U pair; the pin shaft is inserted into the through hole from the inner arc side of the U pair, so that the shaft end is located outside the U pair; The left and right ends of the joint interface connecting rod of the above structure are connected to the bottom ends of the two connecting rods through the first U pair and the second U pair respectively; the first U pair and the second U pair are respectively connected by threaded parts of the threaded pins and the threaded holes at the left and right ends of the joint interface connecting rod to form a revolute pair; the first U pair and the second U pair are respectively plugged into the circular through holes at the bottom ends of the two connecting rods through the pin shafts to form a revolute pair; at this time, the two ends of the joint interface connecting rod are connected to the two connecting rods through the two U pairs to form a two-degree-of-freedom revolving Hooke's hinge connection relationship with a rotation axis perpendicular to and intersecting with each other; The top ends of the two connecting rods are connected to the left and right ends of the joint output connecting rod through the second U pair and the first U pair respectively; the second U pair and the first U pair are respectively connected to the threaded holes on the left and right ends of the joint output connecting rod through the threaded parts of the threaded pins to form a revolute pair; the second U pair and the first U pair are respectively plugged into the through holes at the top ends of the two connecting rods through the pin shafts to form a revolute pair; at this time, the two ends of the joint output connecting rod are respectively connected to the two connecting rods through the second U pair and the first U pair to form a two-degree-of-freedom rotational Hooke's hinge connection relationship with a rotation axis perpendicularly intersecting.

2. The metamorphic robot joint based on a line-symmetric 4U mechanism according to claim 1, characterized in that: The joint interface connecting rod and the joint output connecting rod are cylindrical rods with a rectangular parallelepiped section in the middle, and the robot connection hole is opened at the rectangular parallelepiped section.

3. The metamorphic robot joint based on a line-symmetric 4U mechanism according to claim 1, characterized in that: The two connecting rods are cylindrical rods with circular through holes designed at both ends; the axes of the two circular through holes are parallel and perpendicular to the axis of the first connecting rod; and the end surfaces of the two circular through holes are designed to be flat.

4. The metamorphic robot joint based on a line-symmetric 4U mechanism according to claim 1, characterized in that: It has two different motion modes: two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation and three-degree-of-freedom fixed-point rotation. Among them, the relationship between the rotation sub-axes when the metamorphic robot joint is in the two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation motion mode is: Axis A is perpendicular to axis B and intersects at a point, axis C is perpendicular to axis D and intersects at a point, axis E is perpendicular to axis F and intersects at a point, and axis G is perpendicular to axis H and intersects at a point; axis A, axis C, axis E and axis G are all parallel to each other, and the distance from axis A to axis C, the distance from axis A to axis E, the distance from axis C to axis G, and the distance from axis E to axis G are all equal, so during the movement of the mechanism, axis A, axis C, axis E and axis G always form an equilateral parallelogram; further, axis B502 coincides with axis D504, and axis F506 coincides with axis H508; so that the equilateral parallelogram formed by axis A, axis C, axis E and axis G can rotate around the common axis formed by axis F and axis H, forming a line-symmetrical 4U mechanism; The above-mentioned axis A and axis B are respectively the two rotational secondary axes between the joint output link and the left link; axis C and axis D are respectively the two rotational secondary axes between the joint output link and the first link; axis E and axis F are respectively the two rotational secondary axes between the joint interface link and the first link member; axis G and axis H are respectively the two rotational secondary axes between the joint output link and the second link; axis A, axis C, axis E and axis G are all rotational secondary axes formed by U-joint pin connection; the remaining axes are all rotational secondary axes formed by U-joint threaded pin connection; During the motion, axis B will be parallel to axis F, and axis D will be parallel to axis H. The joint output link will perform a two-degree-of-freedom spherical translation relative to the joint interface link, and its motion trajectory will always be on the spherical surface. In addition, while performing the two-degree-of-freedom spherical translation, the joint output link will always have a rotational motion around its own axis. The three-degree-of-freedom fixed-point rotation motion mode is: When the movement reaches the point where axis A is perpendicular to axis B, axis C is perpendicular to axis D, axis G is perpendicular to axis H, and axis E is perpendicular to axis F; and axis B, axis D, axis F and axis H are collinear, and axis A and axis G are also collinear, then the joint is in a bifurcation position, and the joint freedom increases. During continued movement, axis A and axis G are kept collinear, and the common straight line is straight line A, axis B and axis D are collinear, and the common straight line is straight line B, axis F and axis H are collinear, and the common straight line is straight line C; when the three common straight lines intersect at one point, the movement of the joint output link relative to the joint interface link will be a three-degree-of-freedom spatial fixed-point rotation. At this time, the joint output link has a rotation in any direction in three-dimensional space, that is, a typical rigid body fixed-point motion in three-dimensional space, and its rotation center is the intersection of the common straight lines A, B, and C.

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