A six-degree-of-freedom underactuated parallel variable stiffness platform

Abstract

The application is a six-degree-of-freedom underactuated parallel variable stiffness platform, and the main structure comprises a six-degree-of-freedom industrial robot and a parallel variable stiffness robot, wherein the parallel variable stiffness robot mainly comprises a screw block linear actuator, a variable stiffness branch chain, a fixed bottom plate, a moving platform and a baffle, and the three screw block linear actuators and the three variable stiffness branch chains are core mechanisms.The motion of the six-degree-of-freedom underactuated parallel variable stiffness platform is realized through large structural deflection of a flexible connecting rod, passive Cartesian compliance caused by inherent elasticity of the flexible connecting rod can change contact stiffness, thereby reducing contact force, timely processing assembly deviation, efficiently completing reliable multi-task assembly tasks of the robot, and a stiffness / motion hybrid control method is adopted for the robot controller, so that expected force can be obtained and expected directional deformation can be realized.

Description

Technical Field

[0001] This invention belongs to the field of industrial robot technology and relates to a six-degree-of-freedom underactuated parallel variable stiffness platform. Background Technology

[0002] In the production of engineering machinery, automobiles, and electronic products, assembly is the most crucial task. Assembly accuracy plays a decisive role in the overall quality and efficiency of the product; however, assembly is primarily a manual operation. The rapid and repeatable positioning accuracy of robotic systems reduces production costs and improves processing efficiency and product yield. Therefore, researching high-precision automated robotic assembly technology is of great significance for improving the automation level of product manufacturing and saving human resources.

[0003] When robots perform assembly tasks, they cannot accurately determine the contact status of parts due to the inability to accurately acquire position. In addition, products are being updated and replaced at an increasingly rapid pace, and there are many different types of parts to be assembled. Since the stiffness and position information of the manipulated object are usually insufficient and inaccurate, when the robot performs stiffness interaction operations, the assembly task often fails due to deviations between parts, and may even cause damage to the parts.

[0004] Therefore, the primary goal of researchers is to control the forces interacting with the robot. Robotic compliant assembly primarily applies active and passive compliant techniques in robot compliance research. Many studies have used active control methods to improve assembly performance. Active compliant control techniques include stiffness control methods, impedance control, hybrid force / position control methods, adaptive fuzzy control, adaptive neural network control, variable stiffness control, and adaptive impedance control. However, the complexity and high cost of active control systems, and their reliance on precise dynamic models of the robotic arm, make widespread adoption challenging. Therefore, research into low-cost, highly adaptable passive compliant devices has been underway for some time. Since robotic arms cannot respond to shocks faster than the control cycle, these control systems must use slower operating speeds and longer control cycles to protect the robotic arm and assembly components from shocks. Active compliance control offers a high success rate and accuracy, even when the shape or size of the assembly changes, because it is implemented in software. However, this requires sophisticated control technology and a deep understanding of the control rules or algorithms by the operator. Furthermore, this control method cannot respond to shocks faster than the control cycle. Therefore, for the safety of the robot and the assembly parts, it requires low operating speed and a long control cycle.

[0005] A further idea is to utilize the inherent mechanical compliance of the mechanism. A remote center compliant mechanism, composed of a shear linkage with low lateral stiffness, provides compliance to the mechanical end effector. This mechanism absorbs reaction forces generated during assembly and compensates for misalignment between assembled components through its passive compliance and deformation. Furthermore, its simple structure allows for rapid assembly without damaging parts. However, due to the limited contact length of semi-assembled components, there are limitations. It is generally recommended that the pin tip be located within 1 / 3 of the compliance center of the device, i.e., the point where passive lateral and angular motions are decoupled and fixed. Therefore, when assembled components change, a remote center compliant mechanism with appropriate compliance must be replaced, resulting in poor versatility and an inability to sense contact information.

[0006] Active compliance requires precise tactile environment recognition, accurate force sensors, and sophisticated control algorithms. Errors of any magnitude can lead to problems such as surface degradation, wedging, and clogging. Conversely, passive compliance is ill-suited to complex environments, relying solely on its consistent stiffness, thus limiting its practical applications. By combining the advantages of both compliance strategies, robots can exhibit optimal compliance characteristics, reduce control problems, and achieve high-precision assembly. Therefore, a continuously variable stiffness parallel mechanism is designed to provide composite compliance control, combining the advantages of both passive and active compliance control strategies. While rigid robots can provide more precise positioning with less dependence, flexible robotic arms are better able to monitor and control environmental pressures. Summary of the Invention

[0007] To achieve the above objectives, this invention provides a six-degree-of-freedom underactuated parallel variable stiffness platform, solving the problems existing in the prior art. Unlike rigid robots, the end effector motion of the variable stiffness flexible assembly robot is achieved through the large structural deflection of the flexible links rather than the relative motion of the rigid links. The main advantage of the variable stiffness robot is the passive Cartesian compliance generated by the inherent elasticity of the flexible links, which can change the contact stiffness, thereby reducing the contact force, and it has a simple structure and is easy to manufacture. In addition, it can handle assembly deviations in a timely manner, thus efficiently completing reliable multi-task assembly tasks. It provides higher compliance and flexibility than traditional rigid robot assembly, and the proposed variable stiffness provides consistent contact force within the range of motion without the need for a complex robot controller.

[0008] To achieve the above objectives, the technical solution adopted by this invention is: a six-degree-of-freedom underactuated parallel variable stiffness platform, comprising a six-degree-of-freedom industrial robot and a parallel variable stiffness robot; the parallel variable stiffness robot includes a lead screw slider linear actuator, a variable stiffness branch, a fixed base plate, a moving platform, and a baffle, wherein the variable stiffness branch includes a platform end connecting block, a platform end fixing block, a platform end connecting joint, a platform end fixing plate, a Ni-Ti alloy rod, a guide rod, a variable stiffness branch slider, a motor fixing plate, a linear stepper motor, a bushing, a fixed top plate, an actuator end fixing block, an actuator end connecting block, and an actuator end connecting joint; the platform end connecting block is fitted onto the platform end connecting joint, the platform end connecting joint passes through the platform end fixing block, the platform end fixing block is installed at one end of the platform end fixing plate, the other end of the platform end fixing plate is connected to one end of the Ni-Ti alloy rod, the other end of the Ni-Ti alloy rod is connected to one end of the motor fixing plate, and the other end of the motor fixing plate is connected to... A linear stepper motor is connected to one end of a bushing, and the other end of the bushing is connected to a fixed top plate. An actuator end fixing block is installed on the other end of the fixed top plate. The actuator end connecting joint passes through the actuator end fixing block, and the actuator end connecting block is fitted onto the actuator end connecting joint. The lead screw slider linear actuator includes a base, motor plate, coupling, support seat, actuator slider, slide plate, lead screw, linear guide, and bearing plate. The left and right ends of the base are respectively equipped with motor plate and bearing plate. The linear guide is fixed on the base. The support seat and actuator slider are installed on the linear guide. The coupling is fixed inside the support seat. The slide plate is fixed above the actuator slider. The lead screw passes through the coupling and actuator slider, and its left and right ends are connected to the motor plate and bearing plate, respectively. The parallel variable stiffness robot controller adopts a stiffness / motion hybrid control method. The core of the method is the proportional-derivative control method. When performing assembly tasks, even with large positional deviations, the desired force can be obtained to achieve the desired directional deformation.

[0009] Furthermore, the fixed base plate adopts a left-right symmetrical non-equilateral hexagonal structure, including 3 identical long sides and 3 identical short sides.

[0010] Furthermore, there are a total of 3 lead screw and slider linear actuators, which are evenly distributed on the fixed base plate, and each lead screw and slider linear actuator is installed on the short side of the fixed base plate.

[0011] Furthermore, the baffle is an equilateral hexagonal structure, installed in the center of the fixed base plate, wherein three non-adjacent sides coincide with the surface of the bearing plate of the lead screw slider linear actuator, thereby determining and maintaining the position of the lead screw slider linear actuator on the fixed base plate.

[0012] Furthermore, there are a total of 3 variable stiffness branches. One end of each variable stiffness branch is connected to the moving platform through the platform end connecting block and the platform end connecting joint, and the other end is connected to the lead screw slider linear actuator through the actuator end connecting block and the actuator end connecting joint.

[0013] Furthermore, the platform end connecting block and the platform end connecting joint are connected to the slide plate, establishing a connection between the lead screw slider linear actuator and the variable stiffness branch. Each lead screw slider linear actuator and each variable stiffness branch are connected in this way.

[0014] Furthermore, the variable stiffness branch has a total of 4 Ni-Ti alloy rods, 2 guide rods and 4 bushings, which connect the platform end fixing plate, the variable stiffness branch slider, the motor fixing plate and the fixed top plate, thereby connecting the entire variable stiffness branch mechanism.

[0015] Furthermore, the actuator end connecting block and actuator end connecting joint are connected to the mobile platform, establishing a connection between the variable stiffness branch and the mobile platform. Each variable stiffness branch and the mobile platform are connected in this way.

[0016] Furthermore, the base and lead screw connect the motor plate, coupling, support base, actuator slider, linear guide and bearing plate, thereby connecting the entire lead screw slider linear actuator.

[0017] Furthermore, the controller of the parallel variable stiffness robot employs a stiffness / motion hybrid control method. When there is a positional deviation between the shaft and hole assembly, according to... Calculate the stiffness, where K is the sum of the stiffnesses of the three variable stiffness branches. j For the stiffness of the variable stiffness branch, C j For the j-th flexible branch, To ensure the compliance of the strain stiffness branch, J j,p J is the spatial Jacobian matrix. T j,p Given the transpose of the spatial Jacobian matrix, the shaft-hole contact force can be considered as a stiff virtual spring. If the robot deforms during assembly and handling, the controller will generate the desired virtual spring force to achieve the desired directional deformation and reduce the contact force. At the same time, by mapping the desired force to the slider position, the stiffness is reduced, and the slider position at each controller is solved, thereby generating the desired force at the moving platform of the parallel variable stiffness robot.

[0018] Furthermore, the controller employs a proportional-derivative control method, calculating the stiffness force using the measured position of the variable stiffness branch and the reference contact position of the parallel variable stiffness robot. Then, the necessary deviation from the initial unchanged position of the parallel variable stiffness robot is determined by the spring force, which serves as the input to the controller. This achieves slider position tracking and contact stiffness control. To address the problem of excessive contact force caused by assembly position errors of shaft and hole parts, the stiffness controller achieves the desired bandwidth by selecting an appropriate controller gain.

[0019] Furthermore, the assembly end effector measures the contact force through a force sensor and transmits the measured force to the stiffness controller. After calculation by the proportional-derivative control method, the signal is sent to the robot controller and the linear stepper motor. The linear stepper motor drives the variable stiffness branch slider to move, changing the effective length of the Ni-Ti alloy rod, thereby changing the stiffness of the overall parallel variable stiffness robot and realizing the control of the contact force.

[0020] The beneficial effects of this invention are:

[0021] 1. The variable stiffness branch can generate large deformation and stiffness changes in the horizontal and vertical directions, and the six-degree-of-freedom underactuated parallel variable stiffness platform can achieve a wide range of precise motion and variable stiffness contact.

[0022] 2. Unlike traditional rigid robots, the compliance of the six-degree-of-freedom underactuated parallel variable stiffness platform is achieved through large deflection deformation of coupled flexible branches, which has higher flexibility and compliance. It has passive compliance when performing pick-and-place tasks and can complete the tasks efficiently.

[0023] 3. Even under large external loads and positional deviations, the six-degree-of-freedom underactuated parallel variable stiffness platform has sufficient positioning accuracy and reliable assembly performance, and can avoid excessive contact forces.

[0024] 4. By effectively controlling the limb linear actuator, the position and orientation of the robotic arm end effector can be precisely determined, thereby completing the desired task. At the same time, by using bolts to adjust the stiffness of the variable stiffness chain, the stiffness of the moving platform can be changed, thus giving it more flexible functions.

[0025] 5. The six-degree-of-freedom underactuated parallel variable stiffness platform is constructed entirely from readily available technologies and materials. The robot's base is mainly composed of acrylic sheets, the connecting components are composed of aluminum and 3D-printed nylon materials, the four Ni-Ti alloy rods with the same cross-section are composed of nickel-titanium metal compounds, and the variable stiffness branches are connected to the mobile platform through 3D-printed nylon parts. The mobile platform is custom-produced from acrylic sheets.

[0026] 6. The six-degree-of-freedom underactuated parallel variable stiffness platform can adjust the contact stiffness to reduce the contact force and torque, and transition from rigid contact to compliant contact. Even with a large deviation in the horizontal position, it can ensure the correct and compliant assembly of the shaft and hole components. In addition, the variable stiffness branch slider can quickly return to the initial position, increasing the stiffness and contact stiffness of the compliant robot.

[0027] 7. The six-degree-of-freedom underactuated parallel variable stiffness platform has a simple structure, does not require a complex robot controller, is easy to manufacture and use, has a high safety factor, and will not cause harm to the human body or the working environment.

[0028] 8. The six-degree-of-freedom underactuated parallel variable stiffness platform controller can define a user-defined stiffness at the output end of the parallel variable stiffness robot. In automated assembly tasks, it can control the stiffness and contact position in a specific direction, which can realize flexible interaction between parts and avoid excessive contact force that may cause assembly failure and damage to parts. Attached Figure Description

[0029] Appendix Figure 1 This is an overall structural diagram of the six-degree-of-freedom underactuated parallel variable stiffness platform of the present invention;

[0030] Appendix Figure 2 This is a structural diagram of a parallel variable stiffness robot based on the six-degree-of-freedom underactuated parallel variable stiffness platform of the present invention.

[0031] Appendix Figure 3 This is a diagram of the variable stiffness branch structure of the six-degree-of-freedom underactuated parallel variable stiffness platform of the present invention.

[0032] Appendix Figure 4 This is a structural diagram of the lead screw-slider linear actuator of the six-degree-of-freedom underactuated parallel variable stiffness platform of the present invention;

[0033] Appendix Figure 5 This is a schematic diagram of the contact stiffness control of the six-degree-of-freedom underactuated parallel variable stiffness platform of the present invention.

[0034] In the attached diagrams: 1. Lead screw and slider linear actuator; 2. Variable stiffness chain; 3. Fixed base plate; 4. Moving platform; 5. Baffle; 6. Platform end connecting block; 7. Platform end fixing block; 8.

[0035] 9. Platform end connecting joint; 10. Platform end fixing plate; 11. Ni-Ti alloy rod; 12. Guide rod; 13. Variable stiffness support chain slider; 14. Motor fixing plate; 15. Linear stepper motor; 16. Bushing; 17. Fixed top plate; 18. Actuator end fixing block; 19. Actuator end connecting block; 20. Actuator end connecting joint; 21. Base; 22. Motor plate; 23. Coupling; 24. Support base; 25. Actuator slider; 26. Slide plate; 27. Lead screw; 28. Linear rail; 29. ​​Bearing plate; 30. 6-DOF industrial robot; 31. Assembly end actuator; 32. Parallel variable stiffness robot. Detailed Implementation

[0036] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0037] like Figure 1 As shown, the six-degree-of-freedom underactuated parallel variable stiffness platform includes a six-degree-of-freedom industrial robot and a parallel variable stiffness robot, which together form the working environment for assembling parts.

[0038] like Figures 2 to 4 As shown, the parallel variable stiffness robot includes three lead screw-slider linear actuators and three variable stiffness branches. The positions of the three lead screw-slider linear actuators are determined and fixed by bolts and baffles. The included angle between adjacent actuators is 2π / 3. One end of each variable stiffness branch is connected to the actuator slider of one of the lead screw-slider linear actuators via bolts, actuator end connecting blocks, and actuator end connecting joints. One end of each variable stiffness branch can slide on the lead screw-slider linear actuator via the actuator slider, used to adjust the position of one end of the variable stiffness branch on the lead screw-slider linear actuator. The relative positions on the linear actuators; the other ends of the three variable stiffness branches are connected to the moving platform through bolts, platform end connecting blocks, and platform end connecting joints, forming a moving platform with three branches evenly distributed on the same plane. Because the lead screw slider linear actuator is actively propelled, the proximal end of the variable stiffness branch can be controlled by its position and direction. Through effective control of the lead screw slider linear actuator, the position and attitude of the end effector of the robotic arm can be accurately determined. In conjunction with a 6-DOF industrial robot, six different input variables can control the output motion and flexibility of the 3-DOF parallel platform.

[0039] Furthermore, the platform end connecting block of the variable stiffness branch is fitted onto the platform end connecting joint, the platform end connecting joint passes through the platform end fixing block, the platform end fixing block is installed at one end of the platform end fixing plate, and four Ni-Ti alloy rods are installed at the other end of the platform end fixing plate. The other end of the Ni-Ti alloy rods is connected to one end of the motor fixing plate, the other end of the motor fixing plate is connected to the linear stepper motor and four bushings, the other end of the bushings is connected to the fixed top plate, and the actuator end fixing block is installed at the other end of the fixed top plate. The actuator end connecting joint passes through the actuator end fixing block, and the actuator end connecting block is fitted onto the actuator end connecting joint. The middle part of the variable stiffness branch is provided with a guide rail, which passes through the variable stiffness branch slider to fix and guide the stroke of the variable stiffness branch slider. The entire variable stiffness branch is connected. The position of the slider of each variable stiffness branch is adjusted by adjusting the linear stepper motor, thereby adjusting the end stiffness of a single variable stiffness branch.

[0040] Furthermore, motor plates and bearing plates are respectively installed on the left and right ends of the base of the lead screw linear actuator. The linear rail is fixed on the base, the support seat and the actuator slider are installed on the linear rail, the coupling is fixed inside the support seat, and the slide plate is fixed above the actuator slider. The lead screw passes through the coupling and the actuator slider, and its left and right ends are connected to the motor plates and bearing plates respectively. The slide plate is connected to the actuator end connecting block and the actuator end connecting joint to support and fix the variable stiffness chain.

[0041] Furthermore, during assembly tasks, a force sensor is installed on the assembly end effector to measure the contact force generated during assembly. The measured force is transmitted to the stiffness controller, which calculates the force using the proportional-derivative control method and sends the signal to the robot controller and the linear stepper motor on the variable stiffness branch. The linear stepper motor drives the variable stiffness branch slider to move, changing the position of the variable stiffness branch slider on the Ni-Ti alloy rod. By changing the effective length of the Ni-Ti alloy rod, the stiffness of the variable stiffness branch is changed, thereby changing the stiffness of the overall parallel variable stiffness robot and achieving contact force control.

[0042] like Figure 5 As shown, the robot controller calculates the stiffness force using the measured position of the cantilever beam and the robot's reference contact position. Then, it uses the spring force to determine the necessary deviation from the robot's initial, unchanged position, using this as the input to the robot controller, which then transmits the signal to the undeformed parallel variable stiffness robot. The proportional-derivative control algorithm implements slider position tracking and contact stiffness control. To address the problem of excessive contact force caused by assembly position errors in shaft and hole-type parts, it is necessary to select an appropriate controller gain to ensure the stiffness controller achieves the desired bandwidth.

[0043] The examples described above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of protection of the present invention.

Claims

1. A six-degree-of-freedom underactuated parallel variable stiffness platform, comprising a six-degree-of-freedom industrial robot (29), an assembly end effector (30), and a parallel variable stiffness robot (31); The parallel variable stiffness robot (31) includes a lead screw slider linear actuator (1), a variable stiffness chain (2), a fixed base plate (3), a moving platform (4), and a baffle (5); The variable stiffness branch (2) includes a platform end connecting block (6), a platform end fixing block (7), a platform end connecting joint (8), a platform end fixing plate (9), a Ni-Ti alloy rod (10), a guide rod (11), a variable stiffness branch slider (12), a motor fixing plate (13), a linear stepper motor (14), a bushing (15), a fixed top plate (16), an actuator end fixing block (17), an actuator end connecting block (18), and an actuator end connecting joint (19); the platform end connecting block (6) is fitted onto the platform end connecting joint (8), the platform end connecting joint (8) passes through the platform end fixing block (7), and the platform end fixing block (7) is installed on... One end of the platform end fixing plate (9) is connected to one end of the Ni-Ti alloy rod (10), the other end of the Ni-Ti alloy rod (10) is connected to one end of the motor fixing plate (13), the other end of the motor fixing plate (13) is connected to one end of the linear stepper motor (14) and the bushing (15), the other end of the bushing (15) is connected to the fixed top plate (16), the other end of the fixed top plate (16) is installed with the actuator end fixing block (17), the actuator end connecting joint (19) passes through the actuator end fixing block (17), and the actuator end connecting block (18) is fitted on the actuator end connecting joint (19); The linear actuator (1) includes a base (20), a motor plate (21), a coupling (22), a support base (23), an actuator slider (24), a sliding plate (25), a lead screw (26), a linear guide (27), and a bearing plate (28). The motor plate (21) and the bearing plate (28) are respectively installed on the left and right ends of the base (20). The linear guide (27) is fixed on the base (20). The support base (23) and the actuator slider (24) are installed on the linear guide (27). The coupling (22) is fixed inside the support base (23). The sliding plate (25) is fixed above the actuator slider (24). The lead screw (26) passes through the coupling (22) and the actuator slider (24), and its left and right ends are respectively connected to the motor plate (21) and the bearing plate (28). The parallel variable stiffness robot (31) controller adopts a stiffness / motion hybrid control method. The core of the method is the proportional-derivative control method. When performing assembly tasks, even if there is a large positional deviation, the desired force can be obtained to achieve the desired directional deformation.

2. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: The fixed base plate (3) adopts a non-equilateral hexagonal structure with left and right symmetry, including 3 identical long sides and 3 identical short sides.

3. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: There are a total of 3 lead screw and slider linear actuators (1), which are evenly distributed on the fixed base plate (3), and each lead screw and slider linear actuator (1) is installed on the short side of the fixed base plate (3).

4. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: The baffle (5) is an equilateral hexagonal structure and is installed in the center of the fixed base plate (3). Three of its non-adjacent sides coincide with the surface of the bearing plate (28) of the lead screw and slider linear actuator (1), thereby determining and maintaining the position of the lead screw and slider linear actuator (1) on the fixed base plate (3).

5. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: There are a total of 3 variable stiffness branches (2). One end of each variable stiffness branch (2) is connected to the moving platform (4) through the platform end connecting block (6) and the platform end connecting joint (8), and the other end is connected to the lead screw slider linear actuator (1) through the actuator end connecting block (18) and the actuator end connecting joint (19).

6. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: The platform end connecting block (6) and the platform end connecting joint (8) are connected to the slide plate (25), establishing a connection between the lead screw slider linear actuator (1) and the variable stiffness branch (2). Each lead screw slider linear actuator (1) and each variable stiffness branch (2) are connected in this way.

7. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: The variable stiffness branch (2) consists of a total of 4 Ni-Ti alloy rods (10), 2 guide rods (11) and 4 bushings (15), which connect the platform end fixing plate (9), the variable stiffness branch slider (12), the motor fixing plate (13) and the fixed top plate (16), thereby connecting the entire variable stiffness branch (2) mechanism.

8. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: The actuator end connecting block (18) and actuator end connecting joint (19) are connected to the moving platform (4), establishing a connection between the variable stiffness branch (2) and the moving platform (4). Each variable stiffness branch (2) and the moving platform (4) are connected in this way.

9. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: The base (20) and lead screw (26) connect the motor plate (21), coupling (22), support base (23), actuator slider (24), linear guide (27) and bearing plate (28), thereby connecting the entire lead screw slider linear actuator (1).

10. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: The controller of the parallel variable stiffness robot (31) adopts a stiffness / motion hybrid control method. When there is a positional deviation between the shaft and the hole assembly, according to... Calculate the stiffness, where K is the sum of the stiffnesses of the three variable stiffness branches. j For the stiffness of the variable stiffness branch, C j For the j-th flexible branch, To ensure the compliance of the strain stiffness branch, J j,p J is the spatial Jacobian matrix. T j,p For the transpose of the spatial Jacobian matrix, the shaft hole contact force can be regarded as a stiff virtual spring. If the robot deforms during assembly and handling, the controller will generate the desired virtual spring force to achieve the desired directional deformation to reduce the contact force. At the same time, by mapping the desired force to the slider position, the stiffness is reduced, and the slider position at each controller is solved, thereby generating the desired force at the moving platform (4) of the parallel variable stiffness robot (31).

11. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: The controller adopts a proportional-derivative control method, calculates the stiffness force using the measured position of the variable stiffness branch (2) and the reference contact position of the parallel variable stiffness robot (31), and then determines the necessary deviation from the initial unchanged position of the parallel variable stiffness robot (31) by the spring force, which is used as the input of the controller, realizing slider position tracking and contact stiffness control. In response to the problem of excessive contact force caused by assembly position error of shaft and hole parts, the stiffness controller reaches the desired bandwidth by selecting an appropriate controller gain.

12. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 11, characterized in that: The assembly end effector (30) measures the contact force through a force sensor and transmits the measured force to the stiffness controller. After calculation by the proportional-derivative control method, the signal is sent to the robot controller and the linear stepper motor (14). The linear stepper motor (14) drives the variable stiffness branch slider (12) to move, changing the effective length of the Ni-Ti alloy rod (10), thereby changing the stiffness of the overall parallel variable stiffness robot (31) and realizing the control of the contact force.

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