A variable stiffness rope-driven joint and its stiffness calculation method

Abstract

The present invention belongs to the technical field of rope-driven robots, specifically a variable stiffness rope-driven joint and a method for calculating its stiffness. It includes a base, a swash plate, a cross hinge, and several sets of geometrically variable pulleys; a base tray is provided on the base for rotationally connecting with the cross hinge; the swash plate is rotationally connected to the cross hinge; the geometrically variable pulley includes a rope, a swash plate rope hole provided below the swash plate, a first base rope hole and a second base rope hole provided at the bottom of the base, a slide rail fixed between the bottom of the base and the base tray, and a sliding seat and a spring sleeved on the slide rail; the spring is located between the sliding seat and the bottom of the base; a sliding rope hole is provided on the sliding seat; one end of the rope is fixed to the sliding seat, and then passes through the first base rope hole, the swash plate rope hole, the sliding rope hole, and the second base rope hole in sequence, and the end thereof is connected to the motor for adjusting the position of the swash plate through the rope. The variable stiffness joint of the present invention is easy to model and control and has a large range of joint stiffness variation.

Description

Technical Field

[0001] The invention belongs to the technical field of rope-driven robots, and in particular relates to a variable-rigidity rope-driven joint and a stiffness calculation method thereof. Background Art

[0002] To enable serpentine manipulators to possess variable stiffness, their joints or links often incorporate materials or structures with specialized controlled stiffness properties, such as springs, shape memory alloys, or particle clusters. Chinese invention patent CN113386166A discloses a decoupled variable stiffness joint suitable for hyper-redundant manipulators. This joint controls the temperature of liquid metal within the hollow, accordion-shaped variable stiffness actuator body within the joint, causing it to solidify and secure the corresponding controlled joints, thereby improving the stiffness of the serpentine manipulator. A presentation at the 2017 12th IEEE Conference on Industrial Electronics and Applications, held from June 18 to 20, 2017, described a 2-DOF cable-actuated joint module with variable stiffness. To increase the stiffness range, this proposal proposed and optimized a compact variable stiffness device, and also presented a design optimization method for the 2-DOF cable-actuated joint module to achieve a wide stiffness adjustment range.

[0003] Of these two variable-stiffness joint solutions for snake-like manipulators, the former introduces complex phase-change metal materials and uses temperature control to change the metal phase, thereby varying the joint's stiffness. This stiffness control method is difficult to model and control, and cannot continuously change the joint's stiffness. The latter, on the other hand, introduces a spring-based variable-stiffness joint module, slightly increasing the range of joint stiffness variation, but still suffers from a limited stiffness range. Furthermore, in both solutions, the transmission ratio between the actuator and the joint remains constant. This means that regardless of the degree of external disturbance to the joint, the disturbance is uniformly transmitted to the rope, which is detrimental to the joint's anti-disturbance performance. Summary of the Invention

[0004] The object of the present invention is to provide a variable stiffness rope driven joint and a stiffness calculation method thereof, so as to improve the joint stiffness variation range and anti-disturbance performance.

[0005] To achieve the above-mentioned object, the present invention provides a variable stiffness joint, comprising a base, a swash plate, a cross hinge and several sets of geometrically variable pulley assemblies;

[0006] The base is provided with a base tray for rotationally connecting with the cross hinge; the swash plate is rotationally connected with the cross hinge;

[0007] The variable geometry pulley assembly includes a rope, a swash plate rope hole provided below the swash plate, a first base rope hole and a second base rope hole provided at the bottom of the base, a slide rail fixed between the bottom of the base and the base tray, and a sliding seat and a spring sleeved on the slide rail; the spring is located between the sliding seat and the bottom of the base; and the sliding seat is provided with a sliding rope hole.

[0008] One end of the rope is fixed on the sliding seat, and then passes through the first base rope hole, the swash plate rope hole, the sliding rope hole and the second base rope hole in sequence, and the end is connected to the motor to adjust the position of the swash plate through the rope.

[0009] Furthermore, the rope from the fixed point on the sliding seat to the rope hole of the first base and the rope from the sliding rope hole to the rope hole of the second base are parallel to each other.

[0010] Furthermore, the variable geometry pulley groups are three groups, which are evenly spaced and distributed between the base and the swash plate.

[0011] Furthermore, an upper gear and a lower gear are provided at the top and bottom of the cross hinge respectively, a rotating gear meshing with the upper gear is provided on the swash plate, and a gear meshing with the lower gear is provided at the bottom of the base; angular displacement sensors are provided at the meshing points of the upper gear and the lower gear respectively.

[0012] The present invention also provides a method for determining the stiffness of a variable stiffness joint, comprising: constructing a joint stiffness equation and a joint multi-dimensional state set as shown in (1), then applying a particle swarm algorithm to the set, taking the basic structure of the joint as a constraint and minimizing the joint potential energy as a solution goal, solving the joint state, and then substituting it into the joint stiffness equation to obtain the joint stiffness:

[0013]

[0014] Among them, K j is the joint stiffness, K S is the spring stiffness, K c is the rope stiffness, f is the tension on the driving rope, J l is the Jacobian matrix of the joint pose, J l T is the transpose of the Jacobian matrix, and q is the joint angular displacement.

[0015] Furthermore, the joint potential energy is shown in formula (2):

[0016]

[0017] Where E is the joint potential energy, K S is the spring stiffness, x sis the compression of the spring, K c is the rope stiffness, and l is the elongation of the driving rope.

[0018] Furthermore, the multi-dimensional state set of the joint includes a basic structural parameter set of the variable geometry pulley set, a DH parameter set and a particle swarm parameter set.

[0019] Furthermore, the basic structural parameter set of the geometrically variable pulley assembly includes the spatial vector from the first base rope hole (53) to the swash plate rope hole (51) in the geometrically variable pulley assembly j. The space vector from the first base rope hole (53) to the joint rotation center The space vector from the sliding rope hole (52) to the slant plate rope hole (51) The space vector from the sliding rope hole (52) to the joint rotation center j In the initial state, the space vector of the sliding rope hole (52) to the first base rope hole (53) is

[0020] Furthermore, the process of solving the stiffness using the particle swarm algorithm includes:

[0021] Construct the basic equations of the joint geometry variable pulley system l j =L j (f j )·q; where L j (f j ) is the driving rope length l corresponding to the joint angular displacement q to the geometrically variable pulley set j j The Jacobian matrix of ;

[0022] According to the basic equation of geometric variable pulley system, the basic structural parameter set of joint angle-rope tension is constructed;

[0023] Constructing joint kinematic transformation equations Among them, R j is the basic structural parameter set of the variable geometry pulley assembly;

[0024] Initialize particle swarm variable P;

[0025] When the joint potential energy does not converge or the number of iterations does not exceed the upper limit, the table lookup method is used to find and interpolate the data corresponding to the variable P in the basic structure parameter set and substitute it into the joint kinematic transformation equation. Solve the joint potential energy and finally solve the optimal joint parameters;

[0026] Substitute into the joint stiffness equation K j , solve for the joint stiffness.

[0027] In general, the above technical solutions conceived by the present invention have the following technical advantages compared with the existing technology:

[0028] 1. The variable stiffness joint provided by the present invention is realized by disposing several sets of geometrically variable pulley assemblies between the base and the swash plate. Under the antagonistic action of the joint drive rope, the geometrically variable pulley assemblies undergo controlled deformation, thereby changing the triangular shape formed by the rope holes, thereby changing the direction and lever arm length of the force generated by the ropes on the rope holes of the swash plate. When the downward pulling force on the ropes increases, the lever arm length of the torque generated on the joint center increases. Because the force of the ropes increases, the torque generated by the geometrically variable pulley assembly on the joint center is doubled, so that the joint has a larger joint stiffness variation range. Moreover, when the joint is subjected to external disturbances, its joint stiffness will increase with the increase of the external disturbance, and the joint has stronger stability.

[0029] 2. The variable stiffness joint provided by the present invention is composed of a regularly elastic element (i.e., a spring) and a rigid body element. Its mechanical model is easy to calculate, enabling precise control of physical properties such as joint position and stiffness. This allows the motor driver parameters to be calculated based on the required stiffness, allowing for precise joint drive, while also facilitating the optimization of the joint's mechanical structure and parameters.

[0030] 3. The variable stiffness joint provided by the present invention can be used as the joint part of a rope-driven serpentine robot arm for maintenance in complex environments. This type of working environment has additional requirements for the flexibility, adaptability and load-bearing capacity of the robot arm. It can also be used for adaptive exploration in difficult-to-model environments such as ruins detection. This type of working environment requires the robot arm to have the ability to adaptively change its posture according to the environment, and also requires the robot arm to have a certain load-bearing capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a schematic structural diagram of a variable stiffness joint of the present invention;

[0032] Figure 2 yes Figure 1 Schematic diagram of the structure of the cross hinge;

[0033] Figure 3 A schematic structural diagram of a variable geometry pulley assembly according to the present invention;

[0034] Figure 4 is a partial cross-sectional view of the variable stiffness joint of the present invention;

[0035] Figure 5 2 is a schematic structural diagram of the variable stiffness joint of the present invention at another angle;

[0036] Figure 6 It is a schematic diagram of the process of changing the joint stiffness of the variable geometry pulley assembly of the present invention under the action of the driving rope;

[0037] Figure 7 Schematic diagram of the deformation of the variable geometry pulley assembly of the variable stiffness joint of the present invention when subjected to external disturbance;

[0038] Figure 8 It is the stiffness algorithm framework of the variable stiffness joint proposed by the present invention;

[0039] Figure 9 This is the basic structure of the geometrically variable pulley assembly in the variable stiffness joint proposed by the present invention.

[0040] Throughout the drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0041] 1- Angular displacement sensor; 2- Cross hinge; 3- Swash plate; 4- Base; 5- Variable geometry pulley set;

[0042] 21-upper gear; 22-cross shaft; 23-lower gear; 31-rotating gear; 41-base tray;

[0043] 51- inclined plate rope hole; 52- sliding rope hole; 53- first base rope hole; 54- second base rope hole; 55- spring; 56- rope; 57- slide rail; 58- sliding seat. DETAILED DESCRIPTION

[0044] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0045] See also Figure 1-5 The present invention provides a variable stiffness joint, including a base 4, a swash plate 3, a cross hinge 2 and several groups of geometrically variable pulleys 5; a base tray 41 is provided on the base 4 for rotationally connecting with the cross hinge 2; the cross hinge 2 is also rotationally connected with the swash plate 3.

[0046] The variable geometry pulley assembly 5 includes a rope 56, a swash plate rope hole 51 disposed below the swash plate 3, a first base rope hole 53, and a second base rope hole 54 disposed on the bottom of the base 4, a slide rail 57 secured between the bottom of the base 4 and the base tray 41, and a sliding seat 58 and a spring 55 disposed on the slide rail 57. The spring 55 is located between the sliding seat 58 and the bottom of the base 4. The sliding seat 58 is provided with a sliding rope hole 52. One end of the rope 56 is secured to the sliding seat 58, then passes through the first base rope hole 53, the swash plate rope hole 51, the sliding rope hole 52, and the second base rope hole 54 in sequence. The end of the rope 56 is connected to a motor, allowing the position of the swash plate 3 to be adjusted via the rope 56.

[0047] With this arrangement, the motor drives the extension and contraction of the rope 56. For example, when the rope 56 of one of the variable geometry pulleys 5 is pulled downward (i.e., the force on the rope is increased), the spring 55 is further compressed as the rope force increases. At this point, the rope between the swash plate rope hole 51 and the sliding rope hole 52 becomes increasingly "vertical" as the sliding rope hole moves downward. The moment arm length of the torque generated by the force of the rope 56 on the joint center increases. Furthermore, due to the increased force of the rope 56, the torque generated by the entire variable geometry pulley 5 on the joint center is doubled. The tension of the rope 56 causes the swash plate 3 to rotate relative to the cross hinge 2, thereby achieving position adjustment.

[0048] The rope 56 from the fixed point on the sliding seat 58 to the first base rope hole 53 and the rope 56 from the sliding rope hole 52 to the second base rope hole 54 are parallel to each other. Figure 5 As shown, an inclined slide rail 57 is positioned between the bottom of the base 4 and the top of the base tray 41. A sliding seat 58 is slidably mounted on the slide rail 57. The downwardly sliding end of the sliding seat 58 secures the initial end of the rope 56. A rope hole 52 is then formed along the slide rail 57 through the sliding seat 58. Both the first base rope hole 53 and the second base rope hole 54 are located at the lower end of the slide rail 57, ensuring that the rope 56 remains substantially parallel to the slide rail 57. The second base rope hole 54 extends downward through the bottom of the base 4, while the first base rope hole 53 opens upward at the bottom of the base 4, allowing the rope 56 to ascend to the swash plate rope hole 51. Specifically, when the swash plate 3 and the base 4 are parallel, the line formed by the first base rope hole 53 and the swash plate rope hole 51 is substantially perpendicular to the base. Since the position of the rope holes directly affects the magnitude and direction of the force exerted by the rope on the swash plate 3, in practical applications, the positions of these rope holes can be optimized based on the target stiffness requirements.

[0049] In particular, there are three sets of variable geometry pulleys 5, which are equally spaced between the base 4 and the swash plate 3. Through the coordinated control of the three sets of variable geometry pulleys 5, the control stability and flexibility of the joint can be improved, and the stiffness range can be increased.

[0050] The base tray 41 on the base 4 extends upward and has a bearing hole at its top for engagement with the cross hinge 2. The base tray 41 is typically positioned in the center of the base 4. To facilitate installation of the slide rail 57, the side of the base tray 41 can be tilted. The lower end of the slide rail 57 is positioned at the edge of the base 4, while the upper end is positioned above the base tray 41. This allows the slide rail 57 to tilt upward. Adjusting the tilt angle also allows for control of joint stiffness.

[0051] like Figure 2 As shown, the middle portion of the cross hinge 2 is provided with a cross shaft 22, which is used to cooperate with the base tray 41 and the swash plate 3, respectively, to achieve relative rotation. The top and bottom of the cross hinge 2 are also provided with an upper gear 21 and a lower gear 23, respectively. The swash plate 3 is provided with a rotating gear 31 that meshes with the upper gear 21, and the bottom of the base 4 is provided with a gear that meshes with the lower gear 23. An angular displacement sensor 1 is provided at the meshing point between the upper gear 21 and the lower gear 23. Two sets of joint angular displacement sensors 1 are arranged on the cross hinge 2 and are connected to the swash plate 3 and base 4, respectively, via a gear transmission. This transmits the relative angular displacement between the cross hinge 2, the swash plate 3, and the base 4 to the sensor side.

[0052] The motion of the swash plate 3 is transmitted to the rotating gear 31, which in turn drives the rotation of the upper gear 21. As the swash plate 3 rotates about the cross shaft 22 of the cross hinge 2, it also generates relative motion with the sensor bracket extending from the cross hinge. This relative motion is amplified by the gears and detected by the angular displacement sensor 1. The spatial force exerted on the swash plate 3 by the drive cable 56 causes the cross hinge 2 to rotate relative to the base 4 in one direction and relative to the swash plate 3 in the other direction. This allows for two-directional motion between the base 4 and the swash plate 3, providing greater control flexibility.

[0053] The variable stiffness joint provided by the present invention is a fully active serpentine arm, that is, each joint is independently connected to a group of motors during use, which is easy to model, thereby facilitating accurate calculation and regulation of stiffness.

[0054] like Figure 3 As shown, the working principle of the present invention is as follows: when the variable geometry pulley 5 is pulled by the rope 56 and deformed, the sliding rope hole 52 will move toward the base rope hole 54 under the action of the rope 56, thereby changing the triangular shape formed by the rope holes, thereby changing the direction of the force generated by the rope 56 on the swash plate rope hole 51, and changing the length of the rope's force arm on the swash plate 3, as shown in FIG. Figure 3When the rope 56 of the variable geometry pulley assembly 5 is pulled and moves, and the pulley assembly deforms, the tension generated by the rope segment between the sliding rope hole 52 and the swash plate rope hole 51 increases the moment arm of the cross hinge 2, thereby increasing the torque exerted by the rope on the swash plate to resist external disturbances.

[0055] like Figure 7 As shown, when the joint is subjected to external disturbances, if the rope is not actively adjusted, the variable geometry pulley set 5 will undergo a certain deformation according to the degree of external disturbance. At this time, one or two of the three variable geometry pulley sets at the joint will be under tension, while the others will be under compression. The pulley sets that are deformed by traction will generate an additional torque on the center of the joint that increases with the increase of the disturbance, while the remaining pulley sets will generate a torque that decreases with the increase of the disturbance. The direction of these torques after superposition is opposite to the direction of the external torque applied to the joint, which can offset the disturbance caused by the external force applied to the joint, thereby increasing the stiffness of the joint.

[0056] Figure 8 This paper demonstrates the stiffness determination method for the variable geometry pulley system proposed in this invention. Specifically, when a small torque in a specified direction is applied to a joint with no external load, balanced internal tension, and minimum local energy, the joint will deflect. This deflection is in the same direction as the applied torque, and the ratio of the deflection to the torque is the joint stiffness.

[0057] When a joint is deflected by an external force, the elastic elements within the joint will deform to adapt to the structural changes, while storing or releasing a certain amount of energy. At the moment of deflection, the energy stored in the joint as a whole will instantly rise to a large value, and then gradually slide to a low value during the force balance process of the various geometrically variable pulley groups within the joint. If an equation is constructed based on the relationship between the elastic potential energy stored in the ropes and elastic elements of each group of geometrically variable pulleys in the joint and the joint deflection angle, and then through an optimization solution to minimize the total joint energy, the joint deflection angle can be calculated based on the size of the external disturbance torque, and then the relationship between the joint stiffness, rope tension, and external disturbance torque can be obtained.

[0058] The stiffness adjustment method proposed in the present invention is implemented as follows:

[0059] according to Figure 9 The basic structure of the geometrically variable pulley group is shown, and the rope-pulley group node linkage equation is established as follows:

[0060]

[0061]

[0062] where Δxj is the compression of the spring in the variable geometry pulley j, is the tension on the rope in j, ∠SMO is the angle formed by the swash plate rope hole 51, the sliding rope hole 52 and the joint rotation center, is the initial preload on the rope in j, K S The joint rotation center is the center of the swash plate 3 rotation axis.

[0063] S j is the torque exerted by the rope on the joint in the variable geometry pulley j, is the space vector from the first base rope hole 53 to the swash plate rope hole 51 in j, is the space vector from the first base rope hole 53 to the joint rotation center, is the space vector from the sliding rope hole 52 to the swash plate rope hole 51, is the space vector from the sliding rope hole 52 to the joint rotation center.

[0064] l j is the length of the driving rope corresponding to the geometrically variable pulley set j, j is the space vector from the sliding rope hole 52 to the first base rope hole 53 in the initial state, l dj j is the fixed length of the rope in the drive transmission part, q is the joint angular displacement, L j (f j ) is from q to l j The Jacobian matrix of .

[0065] The above formula can be used to construct the relationship between the force acting on the swash plate 3 in the joint and the cable tension.

[0066] The joints are modeled using the robotic arm DH parameter method as follows:

[0067]

[0068] c(θ)=cosθ i ; s(θ)=sinθ i ;

[0069] c(α)=cosα i-1 ; s(α)=sinα i-1

[0070] in

[0071]

[0072] On this basis, the kinematic equation of the joint is constructed as follows:

[0073]

[0074] Where R is the joint origin rotation transformation matrix, and r is the joint origin offset matrix.

[0075] Combined with the rope-pulley node linkage equation established above, the Jacobian matrix J from the joint drive rope to the joint posture can be constructed l as follows:

[0076]

[0077] Wherein, L refers to the formula l j =L j (f j )·q by L j (f j ), that is, is the transformation matrix from the joint driving rope to the joint angle.

[0078] According to the joint kinematic transformation process described above, based on the virtual displacement method, the joint energy conservation equation can be established as follows:

[0079]

[0080] where f T is the driving rope tension, δl is the driving rope displacement, S e T is the external torque on the joint, δq is the joint angular displacement, δx s is the spring compression. The spring stiffness K is introduced into this equation s , rope stiffness K c , and transform the equation to derive the joint stiffness K j The equation is as follows:

[0081]

[0082] The formulas described above indicate that joint stiffness can be adjusted by adjusting the drive cable tension, drive cable displacement, spring stiffness, and joint geometry, and the relationships between these variables can be analyzed using the formulas described above. However, the joint stiffness equations show that joint stiffness is directly related to the joint cable tension, cable stiffness, and spring stiffness within the joint, and these factors are coupled to each other. This makes it difficult to directly solve the joint stiffness using the above equations analytically. In engineering applications, simulation and numerical fitting are often used to resolve these complex coupled variable relationships.

[0083] Based on the above equations, the present invention innovatively introduces a particle swarm optimization solution algorithm. By solving the states of each set of geometrically variable pulleys in the joint under different joint angles and rope tensions, a multi-dimensional joint state set is established. The particle swarm method is then used to solve the joint state and further solve the joint stiffness, with the basic structure of the joint as the constraint and the joint potential energy minimization as the solution goal. The core formula of this method is the joint elastic potential energy formula, such as:

[0084]

[0085] Where E is the elastic potential energy of the joint, K S is the spring stiffness, x s is the compression of the spring, K c is the rope stiffness, and l is the compression of the drive rope. This is primarily composed of the elastic potential energy stored in the spring and the rope. When a joint is subjected to an external impact, the energy absorbed by it causes elastic deformation of the joint's drive rope and spring, resulting in a step-like increase in the joint's internal elastic potential energy. Due to friction between the joint's internal components, the joint's elastic potential energy gradually decreases until it reaches a stable state, which is the target state for the solution. The specific implementation process is as follows:

[0086]

[0087] Among them, the basic parameter set R of the joint angle-rope tension space is j The space vector from the first base rope hole (53) to the swash plate rope hole (51) in the geometrically variable pulley assembly j is included. The space vector from the first base rope hole (53) to the joint rotation center The space vector from the sliding rope hole (52) to the slant plate rope hole (51) The space vector from the sliding rope hole (52) to the joint rotation center j In the initial state, the space vector of the sliding rope hole (52) to the first base rope hole (53) is

[0088] The joint stiffness is determined according to the above method, and the driving parameters of the joint can be regulated according to the stiffness, and the structural optimization design can also be performed.

[0089] In summary, the present invention provides a rope-driven joint with a large controllable joint stiffness variation range and a control method thereof. Under the antagonistic action of the joint driving rope, the geometrically variable pulley group will undergo controlled deformation, and the degree of deformation can be calculated based on the tension on the rope, the displacement of the rope, and the angular displacement fed back by the joint angular displacement sensor. The control method mainly includes a method for determining the stiffness of this type of variable stiffness joint, which is specifically manifested in an inverse kinematics calculation process based on the joint angular displacement-rope displacement, introducing the transformation equation of the geometrically variable pulley group, and establishing the joint energy transformation equation through the virtual displacement principle. Based on the local minimization principle of the joint energy, the particle swarm optimization method is used to solve the joint posture. The present invention overcomes the shortcomings of the variable stiffness rope-driven joint in the prior art, such as the difficulty in modeling and controlling, and the small stiffness variation range, and provides a variable stiffness joint that is easy to model, controllable, and has a large joint stiffness variation range, thereby constructing a fully driven rope-driven serpentine manipulator with both flexibility, adaptability, and load-bearing capacity.

[0090] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A variable stiffness joint, characterized in that: It comprises a base (4), a swash plate (3), a cross hinge (2), a plurality of sets of geometrically variable pulleys (5) and a motor; The base (4) is provided with a base tray (41) for rotationally connecting with the cross hinge (2); the swash plate (3) is rotationally connected with the cross hinge (2); The variable geometry pulley assembly (5) comprises a rope (56), a swash plate rope hole (51) provided below the swash plate (3), a first base rope hole (53) and a second base rope hole (54) provided at the bottom of the base (4), a slide rail (57) fixed between the bottom of the base (4) and the base tray (41), and a sliding seat (58) and a spring (55) sleeved on the slide rail (57); the spring (55) is located between the sliding seat (58) and the bottom of the base (4); and the sliding seat (58) is provided with a sliding rope hole (52). One end of the rope (56) is fixed to the sliding seat (58), and then passes through the first base rope hole (53), the swash plate rope hole (51), the sliding rope hole (52) and the second base rope hole (54) in sequence, and the rear end is connected to the motor, so as to adjust the position of the swash plate (3) through the rope (56).

2. The variable stiffness joint according to claim 1, characterized in that: The rope (56) from the fixed point on the sliding seat (58) to the first base rope hole (53) and the rope (56) from the sliding rope hole (52) to the second base rope hole (54) are parallel to each other.

3. The variable stiffness joint according to claim 1, characterized in that: The variable geometry pulley groups (5) are three groups, which are distributed at equal intervals between the base (4) and the swash plate (3).

4. The variable stiffness joint according to claim 1, characterized in that: An upper gear (21) and a lower gear (23) are respectively provided at the top and bottom of the cross hinge (2); a rotating gear (31) meshing with the upper gear (21) is provided on the swash plate (3); and a gear meshing with the lower gear (23) is provided at the bottom of the base (4).

5. The variable stiffness joint according to claim 4, characterized in that: An angular displacement sensor (1) is provided at the meshing position of the upper gear (21) and the lower gear (23).

6. A method for determining the stiffness of a variable stiffness joint according to any one of claims 1 to 5, characterized in that: include: Construct the joint stiffness equation and joint multi-dimensional state set shown in (1), and then use the particle swarm algorithm in this set, with the basic structure of the joint as the constraint and the joint potential energy minimization as the solution goal, to solve the joint state, and then substitute it into the joint stiffness equation to obtain the joint stiffness: Among them, K j is the joint stiffness, K S is the spring stiffness, K c is the cable stiffness, f is the tension on the driving cable, Jl is the Jacobian matrix of the joint pose, J l T is the transpose of the Jacobian matrix, and q is the joint angular displacement.

7. The stiffness determination method according to claim 6, characterized in that: The joint potential energy is shown in formula (2): Where E is the joint potential energy, K S is the spring stiffness, x s is the compression of the spring, K c is the rope stiffness, and l is the elongation of the driving rope.

8. The stiffness determination method according to claim 6, characterized in that: The multi-dimensional state set of the joint includes a basic structural parameter set of a geometrically variable pulley set, a DH parameter set and a particle swarm parameter set.

9. The stiffness determination method according to claim 8, characterized in that: The basic structural parameter set of the geometrically variable pulley assembly includes the space vector from the first base rope hole (53) to the swash plate rope hole (51) in the geometrically variable pulley assembly j. The space vector from the first base rope hole (53) to the joint rotation center The space vector from the sliding rope hole (52) to the slant plate rope hole (51) The space vector from the sliding rope hole (52) to the joint rotation center j In the initial state, the space vector of the sliding rope hole (52) to the first base rope hole (53) is 10. The stiffness determination method according to any one of claims 6 to 9, characterized in that: The process of solving stiffness using the particle swarm algorithm includes: Construct the basic equations of the joint geometry variable pulley system l j =L j (f j )·q; where L j (f j ) is the driving rope length l corresponding to the joint angular displacement q to the geometrically variable pulley set j j The Jacobian matrix of ; According to the basic equation of geometric variable pulley system, the basic structural parameter set of joint angle-rope tension is constructed; Constructing joint kinematic transformation equations Among them, R j is the basic structural parameter set of the variable geometry pulley assembly; Initialize particle swarm variable P; When the joint potential energy does not converge or the number of iterations does not exceed the preset value, the table lookup method is used to find and interpolate the data corresponding to the variable P in the basic structure parameter set and substitute it into the joint kinematic transformation equation. Solve the joint potential energy and finally solve the optimal joint parameters; Substitute into the joint stiffness equation K i , solve for the joint stiffness.

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