Six-degree-of-freedom under-actuated parallel variable stiffness platform

Abstract

The invention relates to a six-degree-of-freedom underactuated parallel variable-stiffness platform which mainly structurally comprises a six-degree-of-freedom industrial robot and a parallel variable-stiffness robot, and the parallel variable-stiffness robot is mainly composed of a lead screw sliding block linear actuator, a variable-stiffness branch chain, a fixed bottom plate, a movable platform and a baffle. Wherein the three lead screw sliding block linear actuators and the three variable stiffness branched chains are core mechanisms. Movement of the six-degree-of-freedom underactuated parallel variable-stiffness platform is achieved through large structural deflection of the flexible connecting rods, and due to passive Cartesian flexibility generated by inherent elasticity of the flexible connecting rods, contact stiffness can be changed, so that contact force is reduced, assembly deviation is treated in time, and the service life of the six-degree-of-freedom underactuated parallel variable-stiffness platform is prolonged. Reliable multi-task assembly tasks of the robot are efficiently completed, a robot controller adopts a rigidity / motion hybrid control method, expected force can be obtained, and expected directional deformation is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of industrial robots and relates to a six-degree-of-freedom under-actuated parallel variable stiffness platform. Background Art

[0002] In the production of engineering machinery, automobiles, and electronic products, assembly work is the most important work. Assembly accuracy plays a decisive role in the overall quality and efficiency of the product. However, assembly tasks are mainly manual operations. The fast and repeated positioning accuracy of the robot system reduces production costs, improves processing efficiency and product qualification rate. Therefore, the study of robot high-precision automated assembly technology is of great significance to improving the degree of automation of product production and saving human resources.

[0003] When robots perform assembly tasks, they cannot accurately obtain the position and thus cannot accurately determine the contact status of parts. In addition, product updates are becoming faster and faster, and there are many types of assembly parts. Since the stiffness and position information of the manipulated objects is usually insufficient and inaccurate, when robots perform stiffness interaction operations, the deviation between parts often leads to assembly task failure or even damage to parts.

[0004] Therefore, the main goal of researchers is to control the forces that interact with the robot. Robotic compliant assembly mainly applies active and passive compliance techniques in robot compliance research. Many studies have used active control methods to improve assembly performance. Active compliance 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, whose control algorithms rely on accurate dynamic models of the manipulator, make it challenging to achieve widespread use, so research on low-cost, adaptable passive compliance devices has already begun. Since the manipulator cannot respond to impact faster than the control cycle, these control systems must use slower operating speeds and longer control cycles to protect the manipulator and assembly parts from impact. Active compliant control offers a high success rate and accuracy and can be used even when the shape or size of the assembly changes because it is implemented in software. However, this requires complex control technology and requires the operator to have a deep understanding of the control rules or algorithms. In addition, this control method cannot respond to shocks faster than the control cycle, so for the safety of the robot and the assembly parts, it requires low operating speeds and large control cycles.

[0005] A further idea is to take advantage of the inherent mechanical compliance of the mechanism. The remote center compliant mechanism consists of a shear-link mechanism with low lateral stiffness, which provides compliance to the mechanical end effector. The remote center compliant mechanism can absorb the reaction forces generated during the assembly process and compensate for the misalignment between the assembled parts through its passive compliance and deformation. At the same time, due to its simple structure, the assembly process can be completed quickly without damaging the parts. However, due to the limited contact length of the semi-assembled parts, it has its limitations. It is generally recommended that the tip of the pin should be within 1 / 3 of the center of the device's flexibility, that is, the point where the passive flexibility lateral and angular motion are decoupled and fixed. Therefore, when the assembly parts change, the remote center compliant mechanism with appropriate compliance must be replaced, resulting in poor versatility and inability to perceive contact information.

[0006] Active compliance requires precise touch environment recognition, accurate force sensors and complex control algorithms. Any error of any size may cause problems such as surface deterioration, wedging and clogging. In contrast, passive compliance is not adaptable to complex environments and relies solely on its consistent stiffness, which limits its practical application. Combining the advantages of two different compliance strategies, the robot 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 compliant control, which combines the advantages of passive and active compliant control strategies. While rigid robots can provide less dependent precise positions, flexible manipulators are better able to monitor and control environmental pressure. Summary of the invention

[0007] In order to achieve the above-mentioned purpose, the present invention provides a six-degree-of-freedom under-actuated parallel variable stiffness platform, which solves the problems existing in the prior art. Unlike rigid robots, the end effector movement of the variable stiffness flexible assembly robot is achieved through the large structural deflection of the flexible link rather than through the relative movement of the rigid link. The main advantage of the variable stiffness robot is the passive Cartesian compliance generated by the inherent elasticity of the flexible link, which can change the contact stiffness and thus reduce the contact force, and the structure is simple and easy to manufacture. In addition, assembly deviations can be handled in a timely manner, thereby efficiently completing the robot's reliable multi-task assembly tasks. It provides higher flexibility and compliance 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-mentioned purpose, the technical solution adopted by the present invention is: a six-degree-of-freedom under-actuated parallel variable stiffness platform, including a six-degree-of-freedom industrial robot and a parallel variable stiffness robot; the parallel variable stiffness robot includes a screw slider linear actuator, a variable stiffness branch chain, a fixed bottom plate, a mobile platform and a baffle, wherein the variable stiffness branch chain 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 chain slider, a motor fixing plate, a linear stepping 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 sleeved on 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 The linear stepper motor is connected to one end of the sleeve, and the other end of the sleeve is connected to the fixed top plate. The other end of the fixed top plate is installed with the actuator end fixing block, the actuator end connecting joint passes through the actuator end fixing block, and the actuator end connecting block is sleeved on the actuator end connecting joint; the screw slider linear actuator includes a base, a motor plate, a coupling, a support seat, an actuator slider, a slide plate, a screw, a linear rail, and a bearing plate; the motor plate and the bearing plate are respectively installed on the left and right ends of the base, 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, the slide plate is fixed above the actuator slider, the screw passes through the coupling and the actuator slider, and the left and right ends are respectively connected to the motor plate and the bearing plate; the parallel variable stiffness robot controller adopts a stiffness / motion hybrid control method, the core of the method is the proportional-differential control method, when performing assembly tasks, even in the case of large position deviations, the desired force can be obtained and the desired directional deformation can be achieved.

[0009] Furthermore, the fixed bottom plate adopts a bilaterally symmetrical non-equilateral hexagonal structure, including three identical long sides and three identical short sides.

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

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

[0012] Furthermore, there are a total of three variable stiffness branches, one end of each variable stiffness branch is connected to the mobile platform through a platform end connecting block and a platform end connecting joint, and the other end is connected to the screw slider linear actuator through an actuator end connecting block and an 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 chain. Each lead screw slider linear actuator and each variable stiffness branch chain are connected in this way.

[0014] Furthermore, the variable stiffness branch chain 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 chain slider, the motor fixing plate and the fixed top plate, and then connect the entire variable stiffness branch chain mechanism.

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

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

[0017] Furthermore, the controller of the parallel variable stiffness robot adopts a stiffness / motion hybrid control method. When there is a position deviation between the axis and the hole assembly, according to Calculate the stiffness, K is the sum of the stiffness of the three variable stiffness branches, K j is the variable stiffness branch stiffness, C j is the jth flexible branch, is the compliance of the strain stiffness branch, J j,p is the spatial Jacobian matrix. The shaft-hole contact force can be regarded as a virtual spring with stiffness. If the robot is deformed 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, the stiffness can be reduced by mapping the desired force to the slider position, and the slider position at each controller can be solved to generate the desired force at the mobile platform of the parallel variable stiffness robot.

[0018] Furthermore, the controller adopts a proportional-differential control method, uses the measured position of the variable stiffness branch chain and the reference contact position of the parallel variable stiffness robot to calculate the stiffness force, and then uses the spring force to determine the necessary deviation from the initial unchanged position of the parallel variable stiffness robot, which is used as the input of the controller to achieve slider position tracking and contact stiffness control. In order to address the problem of excessive contact force caused by assembly position errors of shaft and hole parts, the stiffness controller can achieve the desired bandwidth by selecting appropriate controller gains.

[0019] Furthermore, the assembly end effector measures the contact force through a force sensor, transmits the measured force to the stiffness controller, and after calculation by the proportional-differential control method, sends the signal to the robot controller and the linear stepper motor. The linear stepper motor drives the variable stiffness branch chain slider to move, changes the effective length of the Ni-Ti alloy rod, and then changes the stiffness of the overall parallel variable stiffness robot to achieve contact force control.

[0020] The beneficial effects of the present invention are:

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

[0022] 2. Different from traditional rigid robots, the compliance of the six-degree-of-freedom under-actuated parallel variable stiffness platform is achieved by coupling the large deflection deformation of the flexible branch chain. It has higher flexibility and compliance, has passive compliance when performing pick-and-place tasks, and can complete the task efficiently.

[0023] 3. Even in the presence of large external loads and position 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. Through the effective control of the limb linear actuator, the position and posture of the end effector of the robot arm can be accurately determined to complete the desired task. At the same time, the stiffness of the variable stiffness branch chain can be changed by using bolts to change the stiffness of the moving platform, making it more flexible.

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

[0026] 6. The six-degree-of-freedom under-actuated 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 in the case of large horizontal position deviations, the correct and compliant assembly of the shaft and hole components can be ensured. In addition, the variable stiffness branch chain slider can be quickly reset to the initial position, increasing the stiffness and contact stiffness of the compliant robot.

[0027] 7. The six-degree-of-freedom under-actuated 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 and the working environment.

[0028] 8. The six-degree-of-freedom under-actuated 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 in a specific direction and the contact position, which can achieve flexible interaction between parts and avoid excessive contact force that may cause assembly failure and damage to parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Attached Figure 1 It is an overall structural diagram of the six-degree-of-freedom under-actuated parallel variable stiffness platform of the present invention;

[0030] Attached Figure 2 It is a structural diagram of a parallel variable stiffness robot of a six-degree-of-freedom underactuated parallel variable stiffness platform of the present invention;

[0031] Attached Figure 3 It is a structural diagram of a variable stiffness branch chain of the six-degree-of-freedom underactuated parallel variable stiffness platform of the present invention;

[0032] Attached Figure 4 It is a structural diagram of the lead screw slider linear actuator of the six-degree-of-freedom under-actuated parallel variable stiffness platform of the present invention;

[0033] Attached Figure 5 It is a schematic diagram of contact stiffness control of the six-degree-of-freedom under-actuated parallel variable stiffness platform of the present invention.

[0034] In the above figures: 1. Screw slider linear actuator; 2. Variable stiffness branch chain; 3. Fixed bottom plate; 4. Mobile platform; 5. Baffle; 6. Platform end connecting block; 7. Platform end fixing block; 8. Platform end connecting joint; 9. Platform end fixing plate; 10. Ni-Ti alloy rod; 11. Guide rod; 12. Variable stiffness branch chain slider; 13. Motor fixing plate; 14. Linear stepper motor; 15. Bushing; 16. Fixed top plate; 17. Actuator end fixing block; 18. Actuator end connecting block; 19. Actuator end connecting joint; 20. Base; 21. Motor plate; 22. Coupling; 23. Support seat; 24. Actuator slider; 25. Slide plate; 26. Screw; 27. Linear rail; 28. Bearing plate; 29. ​​6-DOF industrial robot; 30. Assembly end actuator; 31. Parallel variable stiffness robot. DETAILED DESCRIPTION

[0035] The specific implementation modes of the present invention are further described in detail below with reference to the accompanying drawings.

[0036] like Figure 1 As shown, the six-degree-of-freedom under-actuated parallel variable stiffness platform includes a six-degree-of-freedom industrial robot and a parallel variable stiffness robot, which together constitute a working environment for assembling parts.

[0037] like Figures 2 to 4 As shown, the parallel variable stiffness robot comprises three lead screw slider linear actuators and three variable stiffness branch chains. The positions of the three lead screw slider linear actuators are determined and fixed by bolts and baffles. The angle between adjacent actuators is 2π / 3. One end of each variable stiffness branch chain is connected to an actuator slider of a lead screw slider linear actuator by bolts, an actuator end connecting block and an actuator end connecting joint. One end of each variable stiffness branch chain can slide on the lead screw slider linear actuator through the actuator slider, so as to adjust one end of the variable stiffness branch chain on the lead screw slider linear actuator. The other ends of the three variable stiffness branches are connected to the mobile platform through bolts, platform end connecting blocks and platform end connecting joints to form a mobile platform with three branches evenly distributed on the same plane. Because the 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 screw slider linear actuator, the position and posture of the end effector of the robotic arm can be accurately determined. In conjunction with a 6-DOF industrial robot, 6 different input variables can be realized to control the output motion and flexibility of the 3-DOF parallel platform.

[0038] Furthermore, the platform end connection block of the variable stiffness branch chain is sleeved on the platform end connection joint, the platform end connection joint passes through the platform end fixing block, the platform end fixing block is installed on one end of the platform end fixing plate, the other end of the platform end fixing plate is installed with four Ni-Ti alloy rods, 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 shaft sleeves, the other end of the shaft sleeve is connected to the fixed top plate, the other end of the fixed top plate is installed with the actuator end fixing block, the actuator end connection joint passes through the actuator end fixing block, the actuator end connection block is sleeved on the actuator end connection joint, the middle part of the variable stiffness branch chain is provided with a guide rail, the guide rail passes through the variable stiffness branch chain slider, so as to fix and guide the stroke of the variable stiffness branch chain slider, and the entire variable stiffness branch chain is connected. Each variable stiffness branch chain adjusts the position of the slider through the linear stepper motor, thereby adjusting the end stiffness of a single variable stiffness branch chain.

[0039] Furthermore, the motor sheet and the bearing sheet are installed at the left and right ends of the base of the screw slider linear actuator respectively, 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, the slide plate is fixed above the actuator slider, the screw passes through the coupling and the actuator slider, and the left and right ends are connected to the motor sheet and the bearing sheet respectively. The slide plate is connected to the actuator end connection block and the actuator end connection joint to support and fix the variable stiffness branch chain.

[0040] Furthermore, when performing assembly tasks, a force sensor is installed on the assembly end effector, and the contact force generated during the assembly process is measured by the sensor. The measured force is transmitted to the stiffness controller. After calculation by the proportional-differential control method, the signal is sent to the robot controller and the linear stepper motor on the variable stiffness branch chain. The linear stepper motor drives the variable stiffness branch chain slider to move, thereby changing the position of the variable stiffness branch chain slider on the Ni-Ti alloy rod. The stiffness of the variable stiffness branch chain is changed by changing the effective length of the Ni-Ti alloy rod, thereby changing the stiffness of the overall parallel variable stiffness robot and realizing contact force control.

[0041] like Figure 5 As shown, the robot controller calculates the stiffness force using the measured position of the cantilever beam and the reference contact position of the robot, and then uses the spring force to determine the necessary deviation from the initial unchanged position of the robot, which is used as the input of the robot controller to transmit the signal to the undeformed parallel variable stiffness robot. The proportional-differential control algorithm realizes the slider position tracking and contact stiffness control. In order to solve the problem of excessive contact force caused by the assembly position error of the shaft and hole parts, it is necessary to select a suitable controller gain so that the stiffness controller can achieve the desired bandwidth.

[0042] The above examples are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are included in the protection scope 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) comprises a lead screw slider linear actuator (1), a variable stiffness branch chain (2), a fixed base plate (3), a mobile platform (4) and a baffle (5); The variable stiffness branch chain (2) comprises a platform end connection block (6), a platform end fixing block (7), a platform end connection joint (8), a platform end fixing plate (9), a Ni-Ti alloy rod (10), a guide rod (11), a variable stiffness branch chain slider (12), a motor fixing plate (13), a linear stepping motor (14), a shaft sleeve (15), a fixed top plate (16), an actuator end fixing block (17), an actuator end connection block (18), and an actuator end connection joint (19); the platform end connection block (6) is sleeved on the platform end connection joint (8), the platform end connection joint (8) passes through the platform end fixing block (7), and the platform end fixing block (7) is installed on the platform end connection joint (8). One end of the platform end fixing plate (9), the other 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 the linear stepping motor (14) and one end of the shaft sleeve (15), the other end of the shaft sleeve (15) is connected to the fixed top plate (16), the other end of the fixed top plate (16) is installed with an 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 sleeved on the actuator end connecting joint (19); The screw slider linear actuator (1) comprises a base (20), a motor sheet (21), a coupling (22), a support seat (23), an actuator slider (24), a slide plate (25), a screw (26), a linear rail (27), and a bearing sheet (28); the motor sheet (21) and the bearing sheet (28) are respectively installed at the left and right ends of the base (20); the linear rail (27) is fixed on the base (20); the support seat (23) and the actuator slider (24) are installed on the linear rail (27); the coupling (22) is fixed inside the support seat (23); the slide plate (25) is fixed above the actuator slider (24); the screw (26) passes through the coupling (22) and the actuator slider (24); and the left and right ends are respectively connected to the motor sheet (21) and the bearing sheet (28); The controller of the parallel variable stiffness robot (31) adopts a stiffness / motion hybrid control method, the core of which is a proportional-differential control method. When performing assembly tasks, even in the presence of large position deviations, the desired force can be obtained and the desired directional deformation can be achieved.

2. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: The fixed bottom plate (3) adopts a bilaterally symmetrical non-equilateral hexagonal structure, comprising three identical long sides and three identical short sides.

3. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: There are three screw slider linear actuators (1) in total, which are evenly distributed on the fixed base plate (3), and each screw slider linear actuator (1) is installed at the short side end 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), wherein three non-adjacent sides coincide with the surface of the bearing plate (28) of the screw slider linear actuator (1), so as to determine and maintain the position of the screw 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 three variable stiffness branch chains (2) in total. One end of each variable stiffness branch chain (2) is connected to the mobile platform (4) via a platform end connecting block (6) and a platform end connecting joint (8), and the other end is connected to the lead screw slider linear actuator (1) via an actuator end connecting block (18) and an 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 chain (2). Each lead screw slider linear actuator (1) and each variable stiffness branch chain (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 chain (2) has a total of 4 Ni-Ti alloy rods (10), 2 guide rods (11) and 4 shaft sleeves (15), which connect the platform end fixing plate (9), the variable stiffness branch chain slider (12), the motor fixing plate (13) and the fixed top plate (16), thereby connecting the entire variable stiffness branch chain (2) mechanism.

8. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: The actuator end connection block (18) and the actuator end connection joint (19) are connected to the mobile platform (4), establishing a connection between the variable stiffness branch chain (2) and the mobile platform (4). Each variable stiffness branch chain (2) and the mobile 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 the lead screw (26) connect the motor plate (21), the coupling (22), the support seat (23), the actuator slider (24), the linear rail (27) and the 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 position deviation between the shaft and the hole assembly, according to Calculate the stiffness, K is the sum of the stiffness of the three variable stiffness branches, K j is the variable stiffness branch stiffness, C j is the jth flexible branch, is the compliance of the strain stiffness branch, J j,p is a spatial Jacobian matrix. The shaft-hole contact force can be regarded as a virtual spring with stiffness. If the robot is deformed 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 can be reduced, and the slider position at each controller can be solved, thereby generating the desired force at the mobile 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-differential control method, uses the measured position of the variable stiffness branch chain (2) and the reference contact position of the parallel variable stiffness robot (31) to calculate the stiffness force, and then uses the spring force to determine the necessary deviation from the initial unchanged position of the parallel variable stiffness robot (31), which is used as the input of the controller to achieve slider position tracking and contact stiffness control. In order to solve the problem of excessive contact force caused by the assembly position error of the shaft and hole parts, the stiffness controller can achieve the desired bandwidth by selecting a suitable controller gain.

12. The six-degree-of-freedom underactuated parallel variable stiffness platform according to claim 1, characterized in that: The assembly end effector (30) measures the contact force through a force sensor, transmits the measured force to a stiffness controller, and after calculation by the proportional-differential control method, sends a signal to a robot controller and a linear stepper motor (14). The linear stepper motor (14) drives the variable stiffness branch chain slider (12) to move, thereby 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 contact force control.

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