Force / position hybrid control dynamics optimization method of four-module eight-connecting-rod redundant drive novel six-degree-of-freedom parallel mechanism

Abstract

The invention provides a force / position hybrid control dynamics optimization method for a four-module eight-connecting-rod redundant drive novel six-degree-of-freedom parallel mechanism. According to the dynamic optimization method, a redundant drive mode is adopted, dynamic optimization of the four-module eight-connecting-rod redundant drive novel six-degree-of-freedom parallel mechanism is achieved, the purposes of reducing the peak-valley ratio, the maximum drive force and the motor power are achieved, and the life safety of laboratory personnel is guaranteed to a certain extent.

Description

Technical Field

[0001] The present invention relates to the technical field of dynamics optimization, and in particular to a force / position hybrid control dynamics optimization method of a novel six-degree-of-freedom parallel mechanism driven by four-module eight-link redundant drive. Background Art

[0002] The Stewart platform was proposed by American scholar Stewart in 1965. Figure 1 The Stewart platform consists of an upper platform, a lower platform and six actuators. By controlling the six actuators, the upper platform can move in six degrees of freedom in space. It has the advantages of stable structure, high precision and good rigidity. It is widely used in industrial robots, flight simulators, medical rehabilitation, CNC machine tools, radio telescopes and other fields.

[0003] However, the working space of the Stewart platform is limited, and the quality of the hydraulic oil will change when a hydraulic cylinder is used. The driving components of the Stewart platform driven by an electric cylinder or a hydraulic cylinder include a lead screw, a lead screw nut, a motor, a reducer, a cylinder body, a piston, a piston rod, etc. These driving components participate in the movement, making the moving mass larger; at the same time, these moving parts are installed with power cables, signal cables, sensors or high-pressure oil pipes, etc., which reduces the reliability of the mechanism. The Stewart platform has no mechanism redundancy and drive redundancy. When an actuator rod breaks, loses power, abnormally engages the brake, loses the enable, or the oil pipe ruptures, the actuator rod will not work properly or completely lose its supporting function, causing the upper platform to collapse or uncontrollably move dangerously, which can easily cause equipment damage and personal injury.

[0004] Therefore, it is of great significance to develop a force / position hybrid control dynamics optimization method for a new six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive. Summary of the invention

[0005] The purpose of the present invention is to provide a force / position hybrid control dynamics optimization method for a novel six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive, so as to solve the problems existing in the prior art.

[0006] The technical solution adopted to achieve the purpose of the present invention is as follows: a force / position hybrid control dynamics optimization method of a novel six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive comprises the following steps:

[0007] 1) Establish a kinematic model. The four-module eight-link redundant drive new six-DOF parallel mechanism includes eight actuator rod assemblies A, eight slide assemblies B, four module assemblies C, a moving platform D and a static platform F.

[0008] The four module assemblies C are arranged on the static platform F. The lines connecting the midline projection points of the four module assemblies C form a rectangle. Each module assembly C is provided with a connection groove. Two slide assemblies B are movably provided in each connection groove. The slide assemblies B are provided with a slide ball joint mounting seat.

[0009] Four groups of moving platform ball joints are evenly distributed along the circumference of the moving platform D. Each group of moving platform ball joints includes two moving platform ball joints E.

[0010] The two ends of the actuating rod assembly A are provided with ball joints. One end of each actuating rod assembly A is hinged on the corresponding moving platform ball joint seat E, and the other end is hinged on the corresponding skateboard ball joint mounting seat.

[0011] During operation, the slide plate assembly B is driven to move, the synchronous belt drives the movable rod assembly A to move, and then drives the movable platform D to move with six degrees of freedom.

[0012] 2) Establish a dynamic optimization model.

[0013] 3) Force-position hybrid control. The force / position hybrid control dynamic optimization method of the new six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive adopts redundant drive mode to achieve dynamic optimization. Six skateboard assemblies B are selected as non-redundant drives, and the position drive mode is adopted. Two skateboard assemblies B are selected as redundant drives, and the torque drive mode is adopted. According to the specified trajectory planning of the dynamic platform D in the workspace, the kinematics and dynamics of the mechanism are solved, the optimal value of the driving force is solved, the position of the six skateboard assemblies B is adjusted by position servo control, and the position of the two skateboard assemblies B is adjusted by force servo control.

[0014] Furthermore, in step 1), the position of the end of the mechanism in space is described as follows:

[0015] q=[xyzαβγ] T

[0016] Where [xyz] T is the position of the origin of the moving coordinate system of the end of the mechanism in the fixed coordinate system, [αβγ] T is the posture of the moving coordinate system at the end of the mechanism in the fixed coordinate system.

[0017] Then the generalized velocity at the end of the mechanism can be described as:

[0018]

[0019] The displacement of each driven joint is defined as:

[0020] Z=[z 1 z 2 z 3 z 4 z5 z 6 z 7 z 8 ] T

[0021] Then the speed of each driven joint is:

[0022]

[0023] By Jacobi J -1 The velocity kinematic model is established as:

[0024]

[0025] The velocity kinematics model is differentiated with respect to time on both sides to obtain the acceleration model:

[0026]

[0027] Furthermore, the slide plate assembly B is driven to move by a motor.

[0028] Further, the eight actuator rod assemblies A are marked as A1, A2, A3, A4, A5, A6, A7 and A8 in the counterclockwise direction. A7 and A8 are selected as redundant drive rods. The four module assemblies C are marked as C1, C2, C3 and C4 in the counterclockwise direction. The connecting grooves are arranged on the side opposite to C1 and C4, and on the side opposite to C2 and C3. The eight slide plate assemblies B are marked as B1, B2, B3, B4, B5, B6, B7 and B8 in sequence. B1 and B2 are slidably arranged in the connecting groove of C1, B3 and B4 are slidably arranged in the connecting groove of C2, B5 and B6 are slidably arranged in the connecting groove of C3, and B7 and B8 are slidably arranged in the connecting groove of C4. The eight moving platform ball joint seats E are marked as E1, E2, E3, E4, E5, E6, E7 and E8 in the counterclockwise direction. E1 and E8 are a group, E2 and E3 are a group, E4 and E5 are a group, and E6 and E7 are a group. One end of A1 is hinged on E1, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B1. One end of A2 is hinged on E2, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B2. One end of A3 is hinged on E3, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B3. One end of A4 is hinged on E4, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B4. One end of A5 is hinged on E5, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B5. One end of A6 is hinged on E6, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B6. One end of A7 is hinged on E7, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B7. One end of A8 is hinged on E8, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B8. B2 is arranged above B1. B3 is arranged above B4. B6 is arranged above B5. B7 is arranged above B7. The skateboard ball joint mounting seats of the two skateboard assemblies B installed on the same module assembly C of the skateboard assembly Bi are staggered.

[0029] The technical effect of the present invention is unquestionable: by adopting a redundant drive mode, the dynamic optimization of the new six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive is realized, the purpose of reducing the peak-to-valley ratio, the maximum driving force and the motor power is achieved, and the life safety of laboratory personnel is guaranteed to a certain extent. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a schematic diagram of the Stewart platform;

[0031] Figure 2 It is a schematic diagram of an eight-link redundant drive parallel mechanism;

[0032] Figure 3 This is a schematic diagram of the moving platform;

[0033] Figure 4 It is a force and position control strategy for redundant drive parallel mechanism;

[0034] Figure 5 This is the simulation result of driving force of non-redundant drive parallel mechanism;

[0035] Figure 6 This is the power simulation result of the non-redundant drive parallel mechanism;

[0036] Figure 7 The simulation results of driving force for redundant drive parallel mechanism;

[0037] Figure 8 Power simulation results for redundantly driven parallel mechanisms. DETAILED DESCRIPTION

[0038] The present invention is further described below in conjunction with the embodiments, but it should not be understood that the above subject matter of the present invention is limited to the following embodiments. Without departing from the above technical ideas of the present invention, various substitutions and changes are made according to the common technical knowledge and customary means in the art, which should all be included in the protection scope of the present invention.

[0039] Embodiment 1:

[0040] This embodiment provides a force / position hybrid control dynamics optimization method for a novel six-degree-of-freedom parallel mechanism with redundant drive of four modules and eight links, see Figure 2 The four-module eight-link redundant drive novel six-DOF parallel mechanism includes eight actuator rod assemblies A, eight slide assemblies B, four module assemblies C, a moving platform D and a static platform F. 2a and 2b represent different views. O-xyz is an absolute coordinate system, and O1-x1y1z1 is a moving coordinate system. In this embodiment, the z-axis can be arranged at any angle.

[0041] The four module assemblies C are arranged on the static platform F. The lines connecting the midline projection points of the four module assemblies C form a rectangle. Each module assembly C is provided with a connection groove. Two slide assemblies B are movably provided in each connection groove. The slide assemblies B are provided with a slide ball joint mounting seat.

[0042] Four groups of moving platform ball joints are evenly distributed along the circumference of the moving platform D. Each group of moving platform ball joints includes two moving platform ball joints E.

[0043] The two ends of the actuating rod assembly A are provided with ball joints. One end of each actuating rod assembly A is hinged on the corresponding moving platform ball joint seat E, and the other end is hinged on the corresponding skateboard ball joint mounting seat.

[0044] During operation, the slide plate assembly B is driven to move, the synchronous belt drives the movable rod assembly A to move, and then drives the movable platform D to move with six degrees of freedom.

[0045] The force / position hybrid control dynamic optimization method of the new six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive adopts redundant drive mode to achieve dynamic optimization. Six skateboard assemblies B are selected as non-redundant drives, and the position drive mode is adopted. Two skateboard assemblies B are selected as redundant drives, and the torque drive mode is adopted. According to the specified trajectory planning of the dynamic platform D in the workspace, the kinematics and dynamics of the mechanism are solved, the optimal value of the driving force is solved, the position of the six skateboard assemblies B is adjusted by position servo control, and the position of the two skateboard assemblies B is adjusted by force servo control.

[0046] Embodiment 2:

[0047] The main contents of this embodiment are the same as those of embodiment 1, wherein see Figure 3 , the kinetic optimization method includes the following steps:

[0048] 1) Establish a kinematic model.

[0049] 2) Establish a dynamic optimization model.

[0050] 3) Force-position hybrid control. According to the specified trajectory planning of the dynamic platform D in the workspace, the kinematics and dynamics of the mechanism are solved, the optimal value of the driving force is solved, and the positions of the six slide assemblies B are adjusted through position servo control, and the positions of the two slide assemblies B are adjusted through force servo control.

[0051] Embodiment 3:

[0052] The main contents of this embodiment are the same as those of Embodiment 1, wherein the eight actuator rod assemblies A are marked in sequence as A1, A2, A3, A4, A5, A6, A7 and A8 in the counterclockwise direction. A7 and A8 are selected as redundant drive rods. The four module assemblies C are marked in sequence as C1, C2, C3 and C4 in the counterclockwise direction. The connecting grooves are arranged on the side opposite to C1 and C4, and on the side opposite to C2 and C3. The eight slide plate assemblies B are marked in sequence as B1, B2, B3, B4, B5, B6, B7 and B8. B1 and B2 are slidably arranged in the connecting groove of C1, B3 and B4 are slidably arranged in the connecting groove of C2, B5 and B6 are slidably arranged in the connecting groove of C3, and B7 and B8 are slidably arranged in the connecting groove of C4. The eight moving platform ball joint seats E are marked in sequence as E1, E2, E3, E4, E5, E6, E7 and E8 in the counterclockwise direction. E1 and E8 are a group, E2 and E3 are a group, E4 and E5 are a group, and E6 and E7 are a group. One end of A1 is hinged on E1, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B1. One end of A2 is hinged on E2, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B2. One end of A3 is hinged on E3, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B3. One end of A4 is hinged on E4, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B4. One end of A5 is hinged on E5, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B5. One end of A6 is hinged on E6, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B6. One end of A7 is hinged on E7, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B7. One end of A8 is hinged on E8, and the other end is hinged on the ball joint mounting seat of the skateboard corresponding to B8. B2 is arranged above B1. B3 is arranged above B4. B6 is arranged above B5. B7 is arranged above B7. The skateboard ball joint mounting seats of the two skateboard assemblies B installed on the same module assembly C of the skateboard assembly Bi are staggered.

[0053] Input the end position of the mechanism and solve the kinematics and dynamics of the redundantly driven parallel mechanism. The desired position input of the position driving slide of the redundantly driven parallel mechanism is X = [x 1 x 2 x 3 x 4 x 5 x 6 ] T , the expected driving force input F of the force-driven slide of the redundant drive parallel mechanism = [F 7 F 8 ] T , the redundant drive parallel mechanism position drive slide actual position input Actual driving force input of the force-driven slide of the redundant drive parallel mechanism Select 1 to 6 as non-redundant drives and adopt the position drive mode, select 7 to 8 as redundant drives and adopt the torque drive mode, and feed back the position error of each position drive slide through the position drive 1 to 6 servo system, and feed back the driving force error of each position drive slide through the force drive 7 to 8 servo system, so as to effectively adjust the driving force of the redundant drive parallel mechanism and improve the stability of the structure.

[0054] like Figure 5 , Figure 6 , Figure 7 and Figure 8 , the maximum driving force of the non-redundant drive parallel mechanism is F=6129.9N, and the maximum power is O=5404.6W; the maximum driving force of the redundant drive parallel mechanism is F=1966.4N, and the maximum power is P=3074.6W, then the maximum driving force is reduced by ε=67.9%, and the maximum power is reduced by ρ=43.1%.

[0055] The mechanism of this embodiment has n=6 degrees of freedom, m=8 drives, and its redundancy r=2. Based on force-position hybrid control, n=6 non-redundant drive joints are used as position drive mode, and r=2 redundant drive joints are used as torque drive mode.

[0056] The specific steps of the kinetic optimization method are as follows:

[0057] 1) Establish kinematic model:

[0058] Define the position of the dynamic coordinates of the end of the mechanism in space, which can be described by generalized coordinates:

[0059] q=[xyzαβγ] T

[0060] Where [xyz] T is the position of the origin of the moving coordinate system of the end of the mechanism in the fixed coordinate system, [αβγ] T is the posture of the moving coordinate system at the end of the mechanism in the fixed coordinate system.

[0061] Then the generalized velocity at the end of the mechanism can be described as:

[0062]

[0063] The displacement of each driving slide assembly is defined as:

[0064] Z=[z 1 z 2 z 3 z 4 z 5 z 6 z 7 z 8 ] T

[0065] Then the speed of each driven joint is:

[0066]

[0067] By Jacobi J -1 The velocity kinematic model is established as:

[0068]

[0069] The velocity kinematics model is differentiated with respect to time on both sides to obtain the acceleration model:

[0070]

[0071] 2) Establish a dynamic optimization model:

[0072] The generalized forces of the parallel mechanism in the working space are solved by Lagrangian dynamic equations:

[0073]

[0074] Among them, Q i is the generalized force of the parallel mechanism in the working space, L=EU is the Lagrangian function, E is the kinetic energy of the system, and U is the potential energy of the system.

[0075] According to the principle of virtual work, the relationship between the generalized force and the driving force F is expressed as follows:

[0076] Q=J T F

[0077] Power is the product of driving force and speed:

[0078] P=Fv

[0079] The mechanism of this embodiment has 6 degrees of freedom and 8 drives, so J is a non-square matrix. Since the number of unknowns to be solved is greater than the number of equations, there are infinite solutions in theory. Using the least squares norm to solve, we get:

[0080] F=GQ

[0081] Where G = J(J T J) -1 .

[0082] The driving force and power of the non-redundant driven six-degree-of-freedom parallel mechanism are obtained by driving with 6 non-redundant driven actuators, where the maximum driving force is maxF 1 , the maximum power is maxP 1 The driving force and power of the redundantly driven six-DOF parallel mechanism are obtained by driving with 6 non-redundantly driven actuator rods and 2 redundant rods, where the maximum driving force is maxF 2, the maximum power is maxP 2 . F 1 , P 1 , F 2 , P 2 Take absolute values.

[0083] Maximum driving force reduction:

[0084]

[0085] Maximum power reduction:

[0086]

[0087] 3) Force and position hybrid control:

[0088] In this embodiment, the number of drives of the mechanism is greater than the number of degrees of freedom of the mechanism. 1 to 6 are selected as non-redundant drives, and the position drive mode is adopted. 7 to 8 are selected as redundant drives, and the torque drive mode is adopted. The control system completes the kinematics and dynamics of the mechanism according to the specified trajectory planning of the motion platform in the workspace, and then uses the optimization algorithm to solve the optimal value of the driving force. Finally, the positions of the 6 driving joints and the driving forces of the 2 driving joints are adjusted by the position servo control system and the force servo control system respectively.

[0089] This embodiment has the following advantages: 1) The lower platform of the mechanism occupies a small plane space. The X / Y dimensions of the mechanism are small. 2) The Z-direction stroke is large. Increasing the length of the guide rail can achieve a large Z-direction stroke. 3) The number of moving parts of a single branch chain is minimal. The moving parts only include the essential upper platform, connecting rod, and slide plate. The motor, reducer, lead screw, bearing and other components are fixedly installed on the module base and do not belong to the moving parts of the mechanism. 4) The moving parts are not electrified. There are no electrified or oil-supplied devices on the moving parts. The motor power cable, brake cable and encoder, limit switch, grating ruler and other signal cables are all fixedly installed on the module base. 5) Improve safety. Two of the eight actuator rods are redundantly driven actuator rods. When any one or two actuator rods fail, the platform safety can still be guaranteed, and the upper platform will not collapse or produce uncontrollable dangerous movements. 6) Improve the position accuracy of the upper platform. Eliminate the kinematic pair clearance through redundant drive and force / position hybrid control. 7) Improve the rigidity of the mechanism. Redundant actuator rods can greatly improve the overall rigidity of the mechanism. 8) Balance the driving force and reduce the motor power. Based on redundant drive and force / position hybrid control, the driving force of each actuator rod and the driving motor power are balanced to achieve the effect of "peak shaving and valley filling". Compared with the Stewart platform with the same technical parameters, the maximum driving force and motor power can be greatly reduced. If a balancing cylinder is added to each slider, the effect of "peak shaving and valley filling" will be better.

[0090] Embodiment 4:

[0091] The main contents of this embodiment are the same as those of Embodiment 1, wherein this embodiment establishes kinematic and dynamic models, analyzes the dynamic mechanism of force / position hybrid control and eight-bar redundant drive, and realizes the dynamic optimization of the new six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive based on the least-squares optimization algorithm, thereby achieving the purpose of reducing the maximum driving force and motor power.

Claims

1. A force / position hybrid control dynamics optimization method for a new type of six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive, characterized in that: The following steps are involved: 1) Establish a kinematic model; wherein, the four-module eight-link redundant drive novel six-DOF parallel mechanism includes eight actuator rod assemblies (A), eight slide assemblies (B), four module assemblies (C), a moving platform (D) and a static platform (F); The four module assemblies (C) are arranged on a static platform (F); the midline projection points of the four module assemblies (C) are connected to form a rectangle; each module assembly (C) is provided with a connecting groove; two slide assemblies (B) are movably provided in each connecting groove; the slide assemblies (B) are provided with a slide ball joint mounting seat; The moving platform (D) is evenly distributed with 4 groups of moving platform ball joint seats along the circumferential direction; each group of moving platform ball joint seats includes two moving platform ball joint seats (E); Both ends of the actuating rod assembly (A) are provided with ball joints; one end of each actuating rod assembly (A) is hinged to the corresponding moving platform ball joint seat (E), and the other end is hinged to the corresponding skateboard ball joint mounting seat; When working, the driving slide assembly (B) moves, the synchronous belt drives the moving rod assembly (A) to move, and then drives the moving platform (D) to move with six degrees of freedom; 2) Establish a dynamic optimization model; 3) Force-position hybrid control; The force / position hybrid control dynamics optimization method of the new six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive adopts redundant drive mode to achieve dynamic optimization; 6 slide assemblies (B) are selected as non-redundant drives, and the position drive mode is adopted; 2 slide assemblies (B) are selected as redundant drives, and the torque drive mode is adopted; according to the specified trajectory planning of the moving platform (D) in the workspace, the kinematics and dynamics of the mechanism are solved, the optimal value of the driving force is solved, the position of the 6 slide assemblies (B) is adjusted by position servo control, and the position of the 2 slide assemblies (B) is adjusted by force servo control.

2. The force / position hybrid control dynamics optimization method of the new type of six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive according to claim 1 is characterized by: In step 1), describe the position of the end of the mechanism in space: q=[xyzαβγ] T Where [xyz] T is the position of the origin of the moving coordinate system of the end of the mechanism in the fixed coordinate system, [αβγ] T is the posture of the moving coordinate system of the end of the mechanism in the fixed coordinate system; Then the generalized velocity at the end of the mechanism can be described as: The displacement of each driven joint is defined as: From=[from 1 With 2 With 3 [z4z5z6z7z8] T Then the speed of each driven joint is: By Jacobi J -1 The velocity kinematic model is established as: The velocity kinematics model is differentiated with respect to time on both sides to obtain the acceleration model:

3. The force / position hybrid control dynamics optimization method of the new type of six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive according to claim 1 is characterized by: The slide assembly (B) is driven to move by a motor.

4. The force / position hybrid control dynamics optimization method of the new type of six-degree-of-freedom parallel mechanism with four-module eight-link redundant drive according to claim 1 is characterized by: The 8 actuator rod assemblies (A) are marked as A1, A2, A3, A4, A5, A6, A7 and A8 in the counterclockwise direction; A7 and A8 are selected as redundant drive rods; the 4 module assemblies (C) are marked as C1, C2, C3 and C4 in the counterclockwise direction; the connecting grooves are arranged on the side opposite to C1 and C4, and on the side opposite to C2 and C3; the 8 slide plate assemblies (B) are marked as B1, B2, B3, B4, B5, B6, B7 and B8 in sequence ; B1 and B2 are slidably arranged in the connecting groove of C1, B3 and B4 are slidably arranged in the connecting groove of C2, B5 and B6 are slidably arranged in the connecting groove of C3, and B7 and B8 are slidably arranged in the connecting groove of C4; the 8 moving platform ball joint seats (E) are marked as E1, E2, E3, E4, E5, E6, E7 and E8 in the counterclockwise direction; E1 and E8 are a group, E2 and E3 are a group, E4 and E5 are a group, and E6 and E7 are a group; one end of A1 is hinged On E1, the other end is hinged to the skateboard ball joint mounting seat corresponding to B1; one end of A2 is hinged to E2, and the other end is hinged to the skateboard ball joint mounting seat corresponding to B2; one end of A3 is hinged to E3, and the other end is hinged to the skateboard ball joint mounting seat corresponding to B3; one end of A4 is hinged to E4, and the other end is hinged to the skateboard ball joint mounting seat corresponding to B4; one end of A5 is hinged to E5, and the other end is hinged to the skateboard ball joint mounting seat corresponding to B5; one end of A6 is hinged to E6, and the other end is hinged to the skateboard ball joint mounting seat corresponding to B5; One end of A7 is hinged on E7, and the other end is hinged on the skateboard ball joint mounting seat corresponding to B7; one end of A8 is hinged on E8, and the other end is hinged on the skateboard ball joint mounting seat corresponding to B8; B2 is arranged above B1; B3 is arranged above B4; B6 is arranged above B5; B7 is arranged above B7; the skateboard ball joint mounting seats of the two skateboard assemblies (B) installed on the same module assembly (C) of the skateboard assembly Bi are staggered.

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