Metamorphic robot joint based on linear symmetry 4U mechanism

Abstract

The invention discloses a metamorphic robot joint based on a linear symmetry 4U mechanism, and belongs to the technical field of robots. Comprising a joint connector connecting rod, a joint output connecting rod, a first connecting rod and a second connecting rod, the two ends of the joint connector connecting rod are connected with the tail ends of the 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 hinged in pairs through a U pair with two vertically intersected rotating motion axes, and the 8R mechanism has two different motion modes, namely a two-degree-of-freedom spherical translation and single-degree-of-freedom rotating motion mode and a three-degree-of-freedom fixed-point rotating motion mode. The mechanism is simple in structure and reliable in movement, and the two movement modes have important reference value for the design of a plane assembly operation robot and the design of a fixed-point movement robot.

Description

Technical Field

[0001] The invention belongs to the technical field of robots and relates to a metamorphic robot joint based on a line-symmetric 4U mechanism. Background Art

[0002] Metamorphic robots can achieve different motion and operation modes through a set of mechanical devices without changing the components through their own structure and drive reorganization, so they have been widely used in application scenarios where the operation mode needs to be changed. Compared with traditional robots with a single, fixed operation mode, the biggest difference between metamorphic robots and them is that their body structure can change according to the needs of different operation tasks to adapt to the configuration requirements under different tasks. Generally speaking, the main way to change the body structure of metamorphic robots is to add metamorphic joints to the body structure of metamorphic robots. This type of metamorphic joint itself is a metamorphic mechanism, which can achieve different motion modes through its own structural reorganization. In the multi-mode operation of metamorphic robots, the various motion modes of metamorphic joints in different configurations are used to cooperate with the movement of other joints in the metamorphic robot to achieve a variety of different operation modes of the metamorphic robot. Therefore, the structural reorganization principle and configuration design of metamorphic joints are the core links of the design of the body structure of metamorphic robots. Summary of the invention

[0003] In view of the above problems, the present invention proposes a metamorphic robot joint based on a line-symmetric 4U mechanism, 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.

[0004] The present invention is a metamorphic robot joint based on a line-symmetric 4U mechanism, including a joint interface connecting rod, a joint output connecting rod, two connecting rods and four U pairs. Among them, 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 end threaded holes are opened 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 respectively connected to threaded pins and pin shafts to form revolute pairs; and the motion axes of the two revolute pairs intersect vertically at point O;

[0005] There are two ways to install the threaded pins and pin shafts in the above four U pairs:

[0006] Method 1: The U pair is as follows: the threaded pin is inserted into the through hole from the outer arc surface 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 surface side of the U pair 3, so that the shaft end part is located on the outer side of the U pair.

[0007] The second method of the U pair is: the threaded pin is inserted into the through hole from one side of the inner arc surface 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 one side of the inner arc surface of the U pair, so that the shaft end part is located outside the U pair.

[0008] 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 to form a revolving pair through the threaded parts of the threaded pins and the threaded holes at the left and right ends of the joint interface connecting rod through threaded matching; the first U pair and the second U pair are respectively plugged and matched with the circular through holes at the bottom ends of the two connecting rods through the pin shafts to form a revolving pair. At this time, the two ends of the joint interface connecting rod are respectively connected to the two connecting rods through two U pairs to form a two-degree-of-freedom revolving Hooke's hinge connection relationship with a rotation axis perpendicular and intersecting.

[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; the second U pair and the first U pair are respectively connected through the threaded parts of the threaded pins and the threaded holes at the left and right ends of the joint output connecting rod to form a revolute pair; the second U pair and the first U pair are respectively plugged and matched with 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 revolving Hooke's joint connection relationship with a vertically intersecting rotation axis.

[0010] The metamorphic robot joints of the above structure have two different motion modes, namely, two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation motion mode and three-degree-of-freedom fixed-point rotation motion mode:

[0011] 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:

[0012] Axis A and axis B are perpendicular and orthogonal at one point, axis C and axis D are perpendicular and orthogonal at one point, axis E and axis F are perpendicular and orthogonal at one point, and axis G and axis H are perpendicular and orthogonal at one point. Axis A, axis C, axis E and axis G are parallel to each other, and the distances from axis A to axis C, axis A to axis E, axis C to axis G, and axis E to axis G are all equal. Therefore, during the movement of the mechanism, axes A, axis C, axis E and axis G always form an equilateral parallelogram. Furthermore, axis B coincides with axis D, and axis F coincides with axis H. This allows the equilateral parallelogram formed by axes A, axis C, axis E and axis G to rotate around the common axis of axes F and H, forming a line-symmetric 4U mechanism.

[0013] The above-mentioned axis A and axis B are respectively the two rotational secondary axes between the joint output connecting rod and the left connecting rod; axis C and axis D are respectively the two rotational secondary axes between the joint output connecting rod and the second connecting rod; axis E and axis F are respectively the two rotational secondary axes between the joint interface connecting rod and the first connecting rod; axis G and axis H are respectively the two rotational secondary axes between the joint output connecting rod and the second connecting rod; 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 performs a two-degree-of-freedom spherical translation relative to the joint interface link, and its motion trajectory is always on the spherical surface, and while performing the 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 is also collinear with axis G. At this time, the joint is in a bifurcation position, and the degree of freedom of the joint increases. During the continued movement, axis A and axis G are kept in a collinear relationship, 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 metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention has a changeable operation mode, and can realize two different motion modes of two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation and three-degree-of-freedom space fixed-point rotation through a set of mechanical joints.

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

[0020] 3. The metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention can realize two motion modes of two-degree-of-freedom translation plus single-degree-of-freedom rotation and three-degree-of-freedom space fixed-point rotation, which are two very typical motion modes in robot operation. Its two-degree-of-freedom translation plus single-degree-of-freedom rotation motion mode has important reference value for the design of planar assembly operation robots and the three-degree-of-freedom space fixed-point motion mode has important reference value for the design of fixed-point motion robots. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the overall structure diagram of the metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention.

[0022] Figure 2 This is a structural diagram of the joint interface connecting rod of the metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention.

[0023] Figure 3 This is a structural diagram of the first connecting rod of the metamorphic robot based on the line-symmetric 4U mechanism of the present invention.

[0024] Figure 4 This is a U-joint structure diagram of the metamorphic robot based on the line-symmetric 4U mechanism of the present invention.

[0025] Figure 5 This is a structural diagram of the arc connecting rod of the U pair of the metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention.

[0026] Figure 6 This is the axis distribution A of the two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation motion mode of the metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention.

[0027] Figure 7 The joints of the metamorphic robot based on the line-symmetric 4U mechanism of the present invention are in the axis distribution B of the two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation motion mode.

[0028] Figure 8 This is an equivalent motion mode diagram of the metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention in a two-dimensional spherical translation plus one-dimensional rotation motion mode.

[0029] Fig. 9 It is a distribution diagram of the axis lines when the joints of the metamorphic robot based on the line-symmetric 4U mechanism of the present invention are in bifurcated configurations for switching between different motion modes.

[0030] Fig.10 The joints of the metamorphic robot based on the line-symmetric 4U mechanism of the present invention are in a three-degree-of-freedom space fixed-point rotation motion mode, and the axis distribution A is shown.

[0031] Fig.11This is the axis distribution B of the three-degree-of-freedom spatial fixed-point rotation motion mode of the metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention.

[0032] Fig.12 This is an equivalent motion pattern diagram of the three-degree-of-freedom spatial fixed-point motion pattern of the metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention.

[0033] In the figure:

[0034] 1- Joint interface link 2- Joint output link 3- First link 4- Second link

[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 present invention will be further described below in conjunction with the accompanying drawings.

[0039] The metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention is mainly composed of four parts, such as Figure 1 As shown, it includes a joint interface link 1, a joint output link 2, a first link 3, a second link 4, and four U pairs, which are respectively the first to fourth U pairs 5-8.

[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 four circumferential sides of the cuboid structure are tangent to the cylindrical surface of the joint interface connecting rod 1.

[0041] There are 4 threaded holes designed on the joint interface connecting rod 1. Among them, the two middle threaded holes 101 are located on the rectangular structure in the middle of the joint interface connecting rod 1. The axes of the two middle threaded holes 101 are parallel, both perpendicular to the two opposite sides of the rectangular structure, and both perpendicularly intersect with the axis of the joint interface connecting rod 1; the above two middle threaded holes 101 are used to connect the joint interface connecting rod 1 to the robot component to be connected through screws, and the side wall of the rectangular structure thereon is 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, with circular through holes 301 designed at both ends; the axes of the two circular through holes 301 are parallel, intersecting and perpendicular to the axis of the first connecting rod 3; and the end faces of the two circular through holes 301 are designed to be flat, for contact and cooperation with the outer wall surface of the U-sub end.

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

[0044] The four U pairs are all arc-shaped plate-shaped connecting rods with through holes at both ends, which are respectively connected to the threaded pin 9 and the pin shaft 10 to form a revolving pair; and the motion axes of the two revolving pairs intersect vertically at point O, so that the motion of the U pair is a Hooke's hinge motion with two perpendicular rotation axes.

[0045] In the first U pair 5 and the second U pair 6 of the above structure, the threaded pin 9 is inserted into the through hole from the side of the outer arc surface of the U pair, so that the threaded part of the threaded pin is located inside the U pair 3; the pin shaft 10 is inserted into the through hole from the side of the inner arc surface of the U pair 3, so that the shaft end part is located outside the U pair. In the third and fourth U pairs, the threaded pin 9 is inserted into the through hole from the side of the inner arc surface of the U pair 3, so that the threaded part of the threaded pin is located outside the U pair 3; the pin shaft 10 is inserted into the through hole from the side of the inner arc surface of the U pair 3, so that the shaft end part is located outside the U pair.

[0046] Based on the above-mentioned component structures, the metamorphic robot joint based on the line-symmetric 4U mechanism of the present invention is obtained by connecting them in the following manner, specifically:

[0047] First, the left and right ends of the joint interface connecting rod 1 are connected to the bottom ends of the first connecting rod 3 and the second connecting rod 4 through the first U pair 5 and the third U pair 7 respectively. The first U pair 5 and the third U pair 7 are respectively connected to the threaded holes on the left and right ends of the joint interface connecting rod 1 through the threaded parts of the threaded pin 9 to form the first rotation pair and the second rotation pair. Subsequently, the first U pair 5 and the third U pair 7 are respectively plugged and matched with the through holes at the bottom ends of the first connecting rod 3 and the second connecting rod 4 through the pin shaft 10 to form the third rotation pair and the fourth rotation pair. At this time, the joint interface connecting rod 1 forms a two-degree-of-freedom rotational Hooke's hinge connection relationship with a rotation axis perpendicular and intersecting between the first connecting rod 3 and the second connecting rod 4 at both ends through the first U pair 5 and the third U pair 7.

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

[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 by a U pair with two perpendicularly 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 sub-axes between the joint output link 2 and the first link 3 in the joint motion of the metamorphic robot are axis A501 and axis B502; the two rotational sub-axes between the joint output link 2 and the first link are axis C503 and axis D504; the two rotational sub-axes between the joint interface link 1 and the first link 3 are axis E505 and axis F506; the two rotational sub-axes between the joint output link 2 and the second link are axis G507 and axis H508. Among them, axis A501, axis C503, axis E505 and axis G507 are all rotational sub-axes formed by connecting the U-sub middle pin 10; the remaining axes are all rotational sub-axes formed by connecting the U-sub threaded pin 9. Then, when the metamorphic robot joint is in the two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation motion mode, its axes will satisfy the following geometric relationship:

[0051] Among them, axis A501 is perpendicular to axis B502 at one point, axis C503 is perpendicular to axis D504 at one point, axis E505 is perpendicular to axis F506 at one point, and axis 507G is perpendicular to axis H508 at one point. Axis A501, axis C503, axis E505 and axis G507 are parallel to each other, and the distance from axis A501 to axis C503, the distance from axis A501 to axis E505, the distance from axis C503 to axis G507, and the distance from axis E505 to axis G507 are all equal, so during the movement of the mechanism, axis A501, axis C503, axis E505 and axis G507 always form an equilateral parallelogram. Further, axis B502 coincides with axis D504, and axis F506 coincides with axis H508. The equilateral parallelogram formed by axis A501, axis C503, axis E505 and axis G507 can rotate around the common axis formed by axis 506 and axis 508 to form a line-symmetrical 4U mechanism.

[0052] At the same time, during the movement, axis B502 will be parallel to axis F506, and axis D504 will be parallel to axis H508. When the robot joint axes satisfy the above geometric relationship, the movement mode of the metamorphic robot joint is as follows: Figure 8 As shown, the joint output link 2 performs a two-degree-of-freedom spherical translation motion relative to the joint interface link 1, which is a translation motion, and the motion trajectory is always located at Figure 8 On the spherical surface shown in , and while performing two-degree-of-freedom spherical translation, the joint output link 2 always has a rotation motion around its own axis.

[0053] Fig. 9 The axis distribution diagram of the metamorphic robot joint when it moves to the bifurcation configuration of two different motion modes. At this time, the robot joint has two motion mode tendencies. Fig. 9 As shown in . At this time, axis A501 is perpendicular to axis B502, axis C503 is perpendicular to axis D504, axis G507 is perpendicular to axis H508, and axis E505 is perpendicular to axis F506. In addition, axis B502, axis D504, axis F506 and axis H508 are collinear, and axis A501 is also collinear with axis G507. Figure 6 and Figure 7 In the motion mode, the axis A501 is added to be collinear with the axis G507, and the axis B502, axis D504, axis F506 and axis H508 are collinear. The above special axis arrangement causes the degrees of freedom of the metamorphic robot joints to change suddenly, and the degrees of freedom of the joints increase.

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

Claims

1. A metamorphic robot joint based on a line-symmetric 4U mechanism, characterized in that: It includes a joint interface link, a joint output link, two links 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 end 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 arc-shaped connecting rods with through holes at both ends, which are respectively connected with threaded pins and pin shafts to form a revolving pair; and the motion axes of the two revolving pairs intersect vertically at point O; the threaded pins and 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 surface 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 surface side of the U pair 3, so that the shaft end part is located on the outer side of the U pair; The second method of the U pair is: the threaded pin is inserted into the through hole from one side of the inner arc surface 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 one side of the inner arc surface of the U pair, so that the shaft end part 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 to form a revolving pair through the threaded parts of the threaded pins and the threaded holes at the left and right ends of the joint interface connecting rod through threaded matching; the first U pair and the second U pair are respectively plugged and matched with the circular through holes at the bottom ends of the two connecting rods through the pin shafts to form a revolving pair; at this time, the two ends of the joint interface connecting rod are respectively 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 and intersecting; 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 revolving Hooke's joint connection relationship with a rotation axis perpendicularly intersecting.

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

3. A metamorphic robot joint based on a line-symmetric 4U mechanism as claimed in claim 1, characterized in that: The two connecting rods are cylindrical rods with circular through holes 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 faces of the two circular through holes are designed to be planes.

4. A metamorphic robot joint based on a line-symmetric 4U mechanism as claimed in claim 1, characterized in that: It has two different motion modes, namely two-degree-of-freedom spherical translation plus single-degree-of-freedom rotation motion mode and three-degree-of-freedom fixed-point rotation motion mode; 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 one point, axis C is perpendicular to axis D and intersects at one point, axis E is perpendicular to axis F and intersects at one point, and axis G is perpendicular to axis H and intersects at one point. Axis A, axis C, axis E and axis G are parallel to each other, and the distances from axis A to axis C, axis A to axis E, axis C to axis G, and axis E to axis G are all equal. Therefore, during the movement of the mechanism, axis A, axis C, axis E and axis G always form an equilateral parallelogram. Furthermore, axis B502 coincides with axis D504, and axis F506 coincides with axis H508. This allows the equilateral parallelogram formed by axis A, axis C, axis E and axis G to 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 two rotational secondary axes between the joint output connecting rod and the left connecting rod; axis C and axis D are respectively two rotational secondary axes between the joint output connecting rod and the first connecting rod; axis E and axis F are respectively two rotational secondary axes between the joint interface connecting rod and the first connecting rod member; axis G and axis H are respectively two rotational secondary axes between the joint output connecting rod and the second connecting rod; 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 movement, axis B will be parallel to axis F, and axis D will be parallel to axis H; the joint output link performs a two-degree-of-freedom spherical translation relative to the joint interface link, and its motion trajectory is always on the spherical surface, and while performing the two-degree-of-freedom spherical translation, the joint output link always has 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 is also collinear with axis G. At this time, the joint is in a bifurcation position, and the degree of freedom of the joint increases. During the continued movement, axis A and axis G are kept in a collinear relationship, 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.

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