Variable Stiffness Joint and Working Method and Control Algorithm

Abstract

The present invention discloses a variable stiffness joint, a working method and a control algorithm, belonging to the field of exoskeleton robots. It includes a driving motor, an input shaft, a variable stiffness adjustment mechanism and an output connecting piece; the variable stiffness adjustment mechanism includes a motion coupling module and a pre-tightening force adjustment module, which are connected between the input shaft and the output connecting piece for transmitting power and quantitatively adjusting the stiffness characteristics of the joint. The structure of the present invention is compact, and it can change the stiffness characteristics of the joint by three ways, namely, adjusting the pre-tightening force of the spring, changing the topological form of the wire rope winding, and replacing the flexible element, as well as the combination of multiple ways. It can absorb the vibration at the end of the robotic arm and effectively reduce the damage caused by collisions to the human body and the exoskeleton. On the basis of ensuring safety and comfort, it realizes human-robot collaborative operation.

Description

Technical Field

[0001] The present invention relates to a variable stiffness joint, a working method and a control algorithm, and belongs to the technical field of exoskeleton robots. Background Art

[0002] Supernumerary robotic limbs (SRLs) are a new type of wearable human assistance device. Its robotic arm has independent motion performance and can perform autonomous or collaborative operations with the wearer. It has broad application prospects in industrial production, medical rehabilitation, assisting the elderly and disabled, and life services. Compared with other intelligent robots, SRLs are designed to work with humans with a high degree of human-machine integration. Therefore, their wearing comfort and safety are crucial, which requires their joints to have good compliance and be lightweight. Thus, the design of SRL joints has become an important research direction.

[0003] Common compliant joint designs usually add the types and quantities of sensors on the basis of a rigid drive mechanism. By collecting enough data such as position, torque, and speed, a controller capable of efficiently processing these data is designed to control the impedance of the exoskeleton robot. This makes such joints require the sensors, drive, and control circuits to operate fast enough, and the controller is complex, requiring an accurate dynamic model to be established; such joints need to add many additional modules, which hinders the lightweight design of wearable robots; such joints are not inherently compliant, cannot store energy, and cannot absorb the energy generated instantaneously during collisions. These defects have led to the development of new drive technologies represented by flexible actuators. Therefore, designing a lightweight variable stiffness flexible joint structure for application in exoskeleton robots, enabling it to be inherently compliant, absorb impacts, and effectively reduce the damage caused by collisions to the human body and the robotic arm, and realizing a safe and comfortable human-robot collaboration process has broad application prospects.

[0004] It is found through a search of the prior art literature that:

[0005] Chinese Patent (Application No. 202110496547.5): A variable stiffness robot joint structure. This patent mainly provides a variable stiffness mechanism based on a gear-rack assembly, achieving stiffness adjustment within a certain range and improving the safety of robot use. However, this joint uses two motors, and the overall structure is heavy, which is not conducive to the lightweight design of wearable robots.

[0006] Chinese Patent (Application No. 201910768255.5): A Variable Stiffness Lower Limb Exoskeleton Assist Robot. This patent mainly provides a variable stiffness lower limb exoskeleton assist robot for wearing on the human lower limbs. Among them, a pulley and a steel wire rope are used between the input disk and the output disk of the knee mechanism to form a variable stiffness adjustment function. This rope-driven variable stiffness joint has a large structural size, uses two motors, and the friction between the steel wire ropes, between the steel wire ropes and the pulley is extremely large at the crossing of the steel wire ropes, and the energy transfer efficiency is low. Summary of the Invention

[0007] The object of the present invention is: aiming at the above-mentioned shortcomings, the present invention provides a variable stiffness joint, working method and control algorithm suitable for external limb robots, which are compact in structure, light in weight, allow the mechanism to change the stiffness within an adjustable range, can absorb vibration and effectively reduce the damage caused by collision to the human body and the robotic arm, so as to realize a safe and comfortable human-machine cooperation process.

[0008] A variable stiffness joint, characterized in that:

[0009] It sequentially includes a drive motor, an input shaft, a variable stiffness adjustment mechanism and an output connector along the axial direction;

[0010] The variable stiffness adjustment mechanism includes an output flange, the output flange is installed on the input shaft through a rolling bearing, and the axial displacement of the rolling bearing is restricted by a circlip installed at the outer end of the inner ring of the bearing;

[0011] A first right pulley and a first left pulley are installed on the end face of the input shaft along the diameter direction, and all the pulleys on the end face of the input shaft are collectively called the first group of pulleys; in the same end face direction, a second upper right pulley, a second upper middle pulley, a second lower right pulley, a second upper left pulley, a second lower middle pulley, and a second lower left pulley are installed on the end face of the output flange, and all the pulleys on the end face of the output flange are collectively called the second group of pulleys;

[0012] A tension spring is installed on the right side of the output flange through a tension spring support, and a hollow bolt is installed on the left side of the output flange;

[0013] The end of the tension spring is connected to the first end of the steel wire rope, and there are two winding methods for the second end of the steel wire rope;

[0014] The first high stiffness mode: the second end of the steel wire rope sequentially bypasses the second lower right pulley, the second upper middle pulley, the first right pulley, the second upper right pulley, the second upper left pulley, the first left pulley, the second lower middle pulley, the second lower left pulley and then passes through the middle hole of the hollow bolt, and the end of the rope is fixed to the end of the hollow bolt with an aluminum sleeve by the press sleeve method. A compression spring is placed between the head of the hollow bolt and the matching nut to prevent the hollow bolt from loosening due to too small pre-tightening force of the steel wire rope; there is no crossing in the above-mentioned steel wire rope winding;

[0015] The second low-stiffness mode: The second end of the wire rope successively bypasses the second right lower pulley, the second right middle pulley, the second right upper pulley, the second left upper pulley, the first left pulley, the second left middle pulley, and the second left lower pulley, then passes through the middle hole of the hollow bolt. The wire rope end is fixed to the tail end of the hollow bolt with an aluminum sleeve by the swaging method. The compression spring is placed between the head of the hollow bolt and the mating nut. There is no crossing in the above wire rope winding;

[0016] Or the second end of the wire rope successively bypasses the second right lower pulley, the second right middle pulley, the first right pulley, the second right upper pulley, the second left upper pulley, the second left middle pulley, and the second left lower pulley, then passes through the middle hole of the hollow bolt. The wire rope end is fixed to the tail end of the hollow bolt with an aluminum sleeve by the swaging method. The compression spring is placed between the head of the hollow bolt and the mating nut. There is no crossing in the above wire rope winding.

[0017] Furthermore, a working method of a variable stiffness joint, characterized by including the following processes:

[0018] The drive motor transmits rotation to the first pulley through the input shaft and drives the wire rope to move. The tension spring expands and contracts accordingly. When the wire rope is tensioned, the rotation of the input shaft is transmitted to the second pulley and the output flange through the wire rope, and drives the output connector to rotate;

[0019] The stiffness characteristics of the joint are changed through three adjustment methods, including: adjusting the pre-tightening force of the compression spring through the mating nut, selecting to change the wire rope winding method, and configuring tension springs of different models.

[0020] Furthermore, a control algorithm for a working method of a variable stiffness joint, characterized by including the following processes:

[0021] The relative rotation angle between the input shaft and the output connector of the variable stiffness joint is the internal deflection angle q of the variable stiffness mechanism, and the internal deflection torque is T q ; The distances from the geometric center A1 of the first right pulley and the geometric center A2 of the first left pulley to the geometric center O of the variable stiffness joint are equal, which is l1; The midpoint C1 of the connection line between the geometric center B1 of the second right upper pulley and the geometric center B2 of the second right middle pulley, and the midpoint C2 of the connection line between the geometric center B4 of the second left upper pulley and the geometric center B5 of the second left middle pulley are equal to the distance o from the geometric center of the variable stiffness joint, which is l3; When the input shaft and the output connector rotate relative to each other, the distances between C1 and A1, and between C2 and A2 are equal, which is l2, and its calculation formula is

[0022]

[0023] The internal tension of the wire rope is the tension received by the tension spring, which is F. During the process of transmitting motion, its equivalent arm of force is L, and its calculation formula is

[0024]

[0025] Where N is the number of working branches, the high stiffness mode corresponds to 2 working branches, and the low stiffness mode corresponds to 1 working branch;

[0026] The equivalent stiffness of the tension spring and the wire rope in series is k, and its calculation formula is

[0027]

[0028] Where k t is the stiffness of the tension spring, and k s is the stiffness of the wire rope;

[0029] The stiffness mathematical model of the variable stiffness joint is

[0030]

[0031] Where F0 is the initial pre-tightening force of the tension spring;

[0032] Based on the given or calculated target output stiffness K tag and the stiffness characteristic curve K(q) of the variable stiffness mechanism, the target deflection angle q of the variable stiffness mechanism can be calculated tag :

[0033] K tag = K(q tag )

[0034] Other robot structural components connected to the drive motor serve as the base of the variable stiffness joint; the angles and torques directly output by the drive motor are θ and T respectively θ ; the robot link connected to the output connecting piece serves as the output end of the variable stiffness joint, and its output angles and torques are α and T respectively α ;

[0035] When the end of the robot link is in fixed contact with the object and the output end of the variable stiffness joint is fixed, the output angle α of the variable stiffness joint is uniquely determined by the position of the variable stiffness joint base; at this time, based on the reaction force F measured by the force sensor at the end of the robot link N，0 , combined with the gravity M and the structural dimension model of the robot link between the variable stiffness joint and the end, the torques generated by the reaction force at the end of the robot and the gravity of the robot link on the variable stiffness joint can be calculated, which are T(F N，0 ) and T(M) respectively, and the output torque T of the variable stiffness joint at the current base position can be further calculated by the principle of virtual work α :

[0036] T α = T(F N，0 ) + T(M)

[0037] When the output end of the variable stiffness joint is in a fixed state, the output torque T of the variable stiffness joint α is equal to the deflection torque T inside the variable stiffness mechanism q . Combining with the stiffness characteristic curve K(q) of the variable stiffness mechanism, the internal deflection angle q0 of the variable stiffness mechanism at the current moment can be further calculated as follows:

[0038] T α = T q = ∫K(q0)dq

[0039] Since the angle θ output by the drive motor is equal to the sum of the internal deflection angle q of the variable stiffness mechanism and the output angle α of the variable stiffness joint, the output angle α0 of the variable stiffness joint at the current base position can be calculated as follows:

[0040] α0 = θ0 - q0

[0041] where θ0 represents the output angle of the drive motor at the current moment. Combining with the target deflection angle q tag of the variable stiffness joint, the target output angle α tag of the drive motor at the current base position can be obtained as follows:

[0042] θ tag = q tag + α0

[0043] The drive motor performs position control to achieve accurate stiffness output. When the base position changes, the reaction force detected by the force sensor at the end of the robot link changes. Repeat the above steps to calculate the output angle of the variable stiffness joint after the base position changes, and the drive motor performs position compensation control.

[0044] Compared with the prior art, the invention has the following beneficial effects:

[0045] 1. The variable stiffness joint in the present invention is reconfigurable and can complete the configuration of different working branch numbers through different wire winding methods of the steel wire ropes, so as to realize the switching between two stiffness adjustment modes.

[0046] 2. A series of different models of tension springs are used for configuration in the present invention. By selecting appropriate springs, the variable stiffness joint can flexibly adapt to different application requirements.

[0047] 3. The variable stiffness joint in the present invention can adjust the pre-tightening force of the compression spring through the matching nuts to change the stiffness within the adjustable range without reinstalling the robot joint.

[0048] 4. The variable stiffness joint in the present invention has a compact structure, is suitable for exoskeleton robots, and can also be quickly integrated with other systems as an independent module.

[0049] 5. The maximum torsional angle of the joint in the present invention is 360 degrees. Compared with the existing variable stiffness joints, it can achieve large-angle rotation. At the same time, the control method of the present invention is simple. The active variable stiffness of the joint can be realized by manually rotating the hollow bolt to adjust the spring pre-tightening force, replacing the traditional stiffness adjustment motor, reducing the weight of the variable stiffness joint, and contributing to the lightweight of the wearable robot.

[0050] 6. In the present invention, the variable stiffness joint is used as the joint of the external limb robot and applied to the operation scenario of the external limb support plate. It can output a specific stiffness without adding additional joint sensors through the control algorithm, so as to achieve stable support for plates with different stiffnesses. When the position of the base changes due to human disturbance in the external limb support state, due to the flexibility of the variable stiffness mechanism, the joint angle changes, but the support point does not shift accordingly. The adaptive stiffness compensation of the external limb end support can be achieved through the control algorithm. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a schematic connection diagram of the variable stiffness joint of the present invention in the support operation example of the external limb robot;

[0052] Figure 2 It is a schematic structural diagram of a variable stiffness joint applicable to an external limb robot of the present invention from one perspective;

[0053] Figure 3 It is a schematic structural diagram of a sectional view of a variable stiffness joint applicable to an external limb robot of the present invention;

[0054] Figure 4 It is a front view of the variable stiffness adjustment mechanism in the high stiffness mode;

[0055] Figure 5 It is a front view of the variable stiffness adjustment mechanism in the low stiffness mode and the first winding method;

[0056] Figure 6 It is a front view of the variable stiffness adjustment mechanism in the low stiffness mode and the second winding method;

[0057] Figure 7 It is a simulation curve of the stiffness model of the variable stiffness joint. (a) The stiffness change curve of the variable stiffness joint when adjusting the pre-tension force with N = 2 and k = 6 N / mm configured; (b) The stiffness change curve of the variable stiffness joint when using different topological forms of wire rope winding with F0 = 20 N and k = 6 N / mm configured; (c) The stiffness change curve of the variable stiffness joint when adjusting the spring stiffness with N = 2 and F0 = 20 N configured;

[0058] Figure 8Schematic diagram of the structure in the external limb support state;

[0059] Figure 9 Schematic diagram of the structure of the first joint (variable stiffness joint);

[0060] Figure 10 Output stiffness control method for the end of an external limb robot based on a variable stiffness joint;

[0061] Explanation of reference numerals: First joint drive motor 1, input shaft 2, variable stiffness adjustment mechanism 3, output connecting piece 4, second joint drive motor 5, motor support base 6, motor protection shell 7, motion coupling module 30, pre-tightening force adjustment module 31, output flange 301, tension spring support 302, tension spring 303, steel wire rope 304, first set of pulleys 305, first right pulley 305-1, first left pulley 305-2, snap ring 306, shoulder-mounted rolling bearing 307, hollow bolt 311, matching nut 312, compression spring 313, aluminum sleeve 314, second set of pulleys 315, second upper right pulley 315-1, second upper middle pulley 315-2, second lower right pulley 315-3, second upper left pulley 315-4, second upper middle pulley 315-5, second lower left pulley 315-6, geometric center A1 of the first right pulley, geometric center A2 of the first left pulley, geometric center O of the variable stiffness joint, geometric center B1 of the second upper right pulley, geometric center B2 of the second upper middle pulley, midpoint C1 of the line connecting the geometric centers of the second upper right pulley and the second upper middle pulley, geometric center B4 of the second upper left pulley, geometric center B5 of the second upper middle pulley, midpoint C2 of the line connecting the geometric centers of the second upper left pulley and the second upper middle pulley, output angle α of the first joint, output angle α0 of the first joint at the current moment, weight M of the first link, length L of the first link, distance l between the centroid of the first link and the center of the first joint, end support force F N and end support force F at the current moment N，0 and end support force curve F N (t), estimated stiffness K of the curved plate tag deflection angle q of the variable stiffness joint, deflection angle q0 of the variable stiffness joint at the current moment, target deflection angle q of the variable stiffness joint tag output angle θ of the first joint drive motor, output angle θ0 of the first joint drive motor at the current moment, target output angle θ of the first joint drive motor tag output torque T of the first joint drive motor θ deflection torque T of the variable stiffness mechanism q output stiffness K(q) of the variable stiffness mechanism, output torque T of the first joint α . Detailed implementation method

[0062] Please refer to Figure 1 and2 As shown, a variable stiffness joint applicable to an external limb robot includes a first joint drive motor 1, an input shaft 2, a variable stiffness adjustment mechanism 3, and an output connector 4; in the external limb robot, the first joint drive motor 1 is connected to a motor protection housing 7 and a motor support seat 6, and the output connector 4 is fixed to the motor protection housing 7 and a second joint drive motor 5.

[0063] As Figure 2 shown, 3 the variable stiffness adjustment mechanism 3 is connected between the input shaft 2 and the output connector 4, and includes a motion coupling module 30 and a preload adjustment module 31, which are respectively used for transmitting power and adjusting the stiffness characteristics of the joint.

[0064] As Figure 2 shown, 3 as shown in Fig. 4, the motion coupling module 30 includes an output flange 301, a tension spring support 302, a tension spring 303, a wire rope 304, a first pulley 305, a snap ring 306, a shoulder rolling bearing 307, and a second pulley 315. The first pulley 305 includes a first right pulley 305-1 and a first left pulley 305-2; the second pulley 315 includes a second upper right pulley 315-1, a second upper middle pulley 315-2, a second lower right pulley 315-3, a second upper left pulley 315-4, a second lower middle pulley 315-5, and a second lower left pulley 315-6.

[0065] The preload adjustment module 31 includes a hollow bolt 311 and a matching nut 312, a compression spring 313, and an aluminum sleeve 314.

[0066] The input shaft 2 is fixedly connected to the first joint drive motor 1, and the output connector 4 is fixedly connected to the output flange 301.

[0067] The output flange 301 is mounted on the input shaft 2 through a rolling bearing 307, and the axial displacement of the rolling bearing 307 is restricted by a snap ring 306 installed at the outer end of the inner ring of the bearing;

[0068] The first right pulley 305-1 and the first left pulley 305-2 are mounted on the end face of the input shaft 2 along the diameter direction, and all the pulleys on the end face of the input shaft 2 are collectively referred to as the first group of pulleys 305; in the same direction, the second upper right pulley 315-1, the second upper middle pulley 315-2, the second lower right pulley 315-3, the second upper left pulley 315-4, the second lower middle pulley 315-5, and the second lower left pulley 315-6 are mounted on the end face of the output flange 301, and all the pulleys on the end face of the output flange 301 are collectively referred to as the second group of pulleys 315;

[0069] The tension spring 303 is mounted on the right side of the output flange 301 through a tension spring support 302, and the hollow bolt 311 is mounted on the left side of the output flange 301;

[0070] The end of the tension spring 303 is connected to the first end of the wire rope 304, and there are two winding methods for the second end of the wire rope 304;

[0071] High stiffness adjustment mode ( Figure 4 ): The second end of the wire rope 304 successively bypasses the second right lower pulley 315-3, the second right middle pulley 315-2, the first right pulley 305-1, the second right upper pulley 315-1, the second left upper pulley 315-4, the first left pulley 305-2, the second left middle pulley 315-5, and the second left lower pulley 315-6, and then passes through the middle hole of the hollow bolt. The end of the rope is fixed to the tail end of the hollow bolt with an aluminum sleeve by the swaging method. The compression spring 313 is placed between the head of the hollow bolt and the matching nut 312 to prevent the hollow bolt from loosening due to too small a pre-tightening force of the wire rope 304; there is no crossover in the above wire rope 304 winding;

[0072] Low stiffness adjustment mode ( Figure 5 ): The second end of the wire rope 304 successively bypasses the second right lower pulley 315-3, the second right middle pulley 315-2, the second right upper pulley 315-1, the second left upper pulley 315-4, the first left pulley 305-2, the second left middle pulley 315-5, and the second left lower pulley 315-6, and then passes through the middle hole of the hollow bolt. The end of the rope is fixed to the tail end of the hollow bolt with an aluminum sleeve by the swaging method. The compression spring 313 is placed between the head of the hollow bolt and the matching nut 312 to prevent the matching nut 312 from loosening due to too small a pulling force of the wire rope 304. There is no crossover in the above wire rope 304 winding;

[0073] Or another winding method ( Figure 6 ): The second end of the wire rope 304 successively bypasses the second right lower pulley 315-3, the second right middle pulley 315-2, the first right pulley 305-1, the second right upper pulley 315-1, the second left upper pulley 315-4, the second left middle pulley 315-5, and the second left lower pulley 315-6, and then passes through the middle hole of the hollow bolt. The end of the rope is fixed to the tail end of the hollow bolt with an aluminum sleeve by the swaging method. The compression spring 313 is placed between the head of the hollow bolt and the matching nut 312. There is no crossover in the above wire rope 304 winding.

[0074] The working principle of the present invention is as follows: When the variable stiffness joint of the robot of the present invention is in use, the first joint drive motor 1 drives the input shaft 2 to rotate. The rotation of the input shaft 2 is transmitted through the first pulley 305. When the steel wire rope 304 is tightened, the rotation of the input shaft 2 is transmitted to the second pulley 315 and the output flange 301 through the steel wire rope 304, and drives the output flange 301 to rotate. When the input shaft 2 rotates, since the steel wire rope 304 first rotates and tightens with the input shaft 2 and then transmits the power to the second pulley 315 and the output flange 301, the output flange 301 does not move when the input shaft 2 just starts to rotate, causing the input shaft 2 to generate a deflection angle relative to the output flange 4, and this deflection angle gradually increases; when the tension on the steel wire rope 304 is equal to the elastic force of the tension spring 303, the deflection angle reaches the maximum value, and the output flange 301 rotates synchronously with the input shaft 2.

[0075] As Figure 4 , the process of establishing the variable stiffness joint stiffness model is as follows:

[0076] The angle of relative rotation between the input shaft 2 and the output connecting member 4 of the variable stiffness joint is the deflection angle q inside the variable stiffness mechanism, and the internal deflection torque is T q ; The distances from the geometric center A1 of the first right pulley 305-1 and the geometric center A2 of the first left pulley 305-2 to the geometric center O of the variable stiffness joint are equal, which is l1; The midpoint C1 of the connection line between the geometric center B1 of the second upper right pulley 315-1 and the geometric center B2 of the second upper middle pulley 315-2, and the midpoint C2 of the connection line between the geometric center B4 of the second upper left pulley 315-4 and the geometric center B5 of the second upper middle pulley 315-5 are equal to the distance from the geometric center of the variable stiffness joint, which is l3; When the input shaft 2 and the output connecting member 4 rotate relative to each other, the distances between C1 and A1, and C2 and A2 are equal, which is l2, and its calculation formula is

[0077]

[0078]

[0079] where k t is the stiffness of the tension spring, and k s is the stiffness of the steel wire rope;

[0080] The internal tension of the steel wire rope 304 is the tension received by the tension spring 303, which is F:

[0081] F = 2Nk·(l2 - l3 + l1) + F0

[0082] where F0 is the initial pre-tightening force of the tension spring;

[0083] During the process of transmitting motion, its equivalent arm is L, and its calculation formula is

[0084]

[0085] where \(n\) is the number of working branches. The high - stiffness mode corresponds to \(n = 2\) working branches, and the low - stiffness mode corresponds to \(n = 1\) working branch.

[0086] Then, the joint torque \(\tau\) required to balance the wire rope tension is:

[0087] \(\tau = L\cdot F=2NkL\cdot(l_2 - l_3 + l_1)+F_0L\)

[0088] The stiffness mathematical model of the variable - stiffness joint is

[0089]

[0090] Based on the given target output stiffness \(K\) tag and the stiffness characteristic curve \(K(q)\) of the variable - stiffness mechanism, the target deflection angle \(q\) of the variable - stiffness mechanism can be calculated tag :

[0091] \(K\) tag \(=K(q\) tag )

[0092] In the present invention, the variable - stiffness joint integrates all components within a cylindrical space with a radius of 48 mm and a height of 60 mm. It has a compact structure, is suitable for external - limb robots, and can also be quickly integrated with other systems as an independent module.

[0093] Non - standard parts such as the input shaft 2 and the output flange 301 in the present invention are made of 3D - printed nylon material. The joint weighs only 225 g, which is beneficial to the lightweight of the structure and is more suitable for external - limb robots. Due to the characteristic that 3D printing technology can print complex parts, the main structure of the present invention adopts an integrated part design, greatly reducing the installation error.

[0094] The design of the lightweight variable - stiffness joint of the present invention is essentially based on a four - link mechanism, which is realized through the first pulley 305, the second pulley 315 and the wire rope 304, forming a basic working branch. Each set of working branches includes the wire rope 304, a first pulley 305 installed at the end of the input shaft 2 and two second pulleys 315 of the output flange 301. When the input shaft 2 rotates relative to the output flange 301, the length of the wire rope 304 wound around the first and second pulleys changes, driving the stretching spring to expand and contract, thereby realizing the change of the joint stiffness.

[0095] For an embodiment of the present invention, the system stiffness during operation changes non-linearly with the mechanism deflection angle θ; when the same torque is input to the joint, the joint stiffness changes with the spring pre-tension F0, the number of working branches N used, and the spring stiffness k. Therefore, the variable stiffness mechanism of the present invention can change the stiffness characteristics of the joint in three ways, including adjusting the pre-tension of the tension spring 303, changing the winding mode of the wire rope 304, and configuring different types of tension springs 303.

[0096] The pre-tension of the tension spring 303 of the light variable stiffness joint of the present invention is adjusted by a pre-tension adjustment module, which is a manual, non-disassembly joint, and stepless adjustment method within a range of 20 mm. The specific operation method is: use a tool to fix the matching nut 312 in the pre-tension adjustment module 31, and rotate the hollow bolt 311 to pull the wire rope 304 to stretch and contract the tension spring 303, thereby adjusting the pre-tension of the spring. Under the conditions of using a tension spring with a stiffness of 6 N / mm and two sets of working branches, the mechanism stiffness simulation model with different spring pre-tensions is as Figure 7 (a) shown. The system stiffness at zero deflection angle of the mechanism increases with the increase of the spring pre-tension; as the deflection angle of the mechanism increases, the influence factor of the spring pre-tension on the system stiffness gradually decreases.

[0097] The light variable stiffness joint of the present invention has two wire rope winding topologies, namely using one set and two sets of working branches. As Figure 5 、 6 shown, it is the topology form of using one set of working branches, corresponding to the low stiffness adjustment mode; as Figure 4 shown, it is the topology form of using two sets of working branches, corresponding to the high stiffness adjustment mode. Under the conditions of setting the pre-tension of the tension spring to 20 N and using a tension spring with a stiffness of 6 N / mm, the mechanism stiffness simulation model with different wire rope winding topologies is as Figure 7 (b) shown. At the same mechanism deflection angle, the joint system stiffness in the high stiffness adjustment mode is nearly twice that in the low stiffness adjustment mode.

[0098] The output flange of the light variable stiffness joint of the present invention is designed with a special groove at the installation position of the spring support column, allowing the placement of a tension spring with an outer diameter of less than 12 mm and a stretching range of 50 mm. At the same time, there is a series of different types of tension springs 303 for configuration, and the joint does not need to be disassembled during configuration, which is convenient and fast. Under the conditions of using the high stiffness adjustment mode and setting its pre-tension to 20 N, the mechanism stiffness simulation model with different stiffness springs is as Figure 7 (c) shown. The system stiffness at zero deflection angle of the mechanism does not change with the spring stiffness; as the deflection angle of the mechanism increases, the influence factor of the spring stiffness on the system stiffness gradually increases.

[0099] By selecting the above three adjustment methods and coupling multiple methods, the requirements for the stiffness characteristics of the shoulder vertical joint of the exoskeleton robot under different working conditions can be met.

[0100] The present invention integrates a variable stiffness mechanism into the joint of an exoskeleton robot and applies it to the operation scenario of the exoskeleton supporting a plate. It can output a specific stiffness without adding additional joint sensors through a control algorithm, thereby achieving stable support for plates with different stiffnesses. When the position of the base changes due to human disturbance in the exoskeleton support state, due to the flexibility of the variable stiffness mechanism, the joint angle changes, but the support point does not shift accordingly. Through the control algorithm, adaptive stiffness compensation for the end support of the exoskeleton can be achieved.

[0101] As Figure 8 、 9 、shown in Figure 10, when the end of the exoskeleton robot contacts the plate, the contact force information F N (t) is obtained through the feedback of the force sensor installed at the end of the exoskeleton robot, and the stiffness K tag of the plate is obtained after Kalman filtering processing; based on the stiffness characteristic curve K(q) of the variable stiffness mechanism, the deflection angle q tag required for the corresponding stiffness is obtained:

[0102] K(q tag ) = K tag

[0103] In this scenario, the position of the end of the exoskeleton robot can be fixed, and the output angle θ0 of the variable stiffness joint is determined by the position of the base; since the position of the base is only affected by the human posture, the output angle θ0 of the variable stiffness joint under a certain human posture is uniquely determined; based on the support reaction force F N，0 at the current moment, the torque T(F N，0 ) generated on the variable stiffness joint, and the torque T(M) generated by the gravity of the output link of the variable stiffness joint on the variable stiffness joint, the output torque T α of the variable stiffness joint under the current human posture is calculated by the principle of virtual work:

[0104] T α = T(F N，o ) + T(M)

[0105] When the robot is in the support state, the output torque T θ of the first joint drive motor, the output torque of the variable stiffness joint, and the deflection torque inside the variable stiffness mechanism are equal:

[0106] T θ = T q = T α

[0107] Combined with the stiffness characteristic curve K(q) of the variable stiffness mechanism, the deflection angle q0 of the variable stiffness mechanism at the current moment can be further calculated as follows:

[0108] ∫K(q0)dq = T q

[0109] Since the angle θ output by the driving motor 1 is equal to the sum of the internal deflection angle q of the variable stiffness mechanism and the output angle α of the variable stiffness joint, the output angle α0 of the variable stiffness joint in this human posture can be calculated as follows:

[0110] α0 = θ0 - q0

[0111] where θ0 represents the output angle of the driving motor 1 at the current moment. Combining with the target deflection angle q of the variable stiffness joint tag , the target output angle θ of the driving motor 1 required for the corresponding human posture and specific stiffness can be obtained tag :

[0112] θ tag = α0 + q tag

[0113] The driving motor 1 performs PID position control to achieve accurate stiffness output. When the human body is disturbed, the force sensor detects the end force F N changes, and the above steps are repeated to calculate the joint output angle α after the human posture changes, and the driving motor 1 performs position compensation control.

[0114] There are many specific implementation methods and ways for the present invention. The above description is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and modifications can be made, and these all belong to the protection scope of the present invention.

Claims

1. A variable stiffness joint, characterized in that: It sequentially includes a driving motor (1), an input shaft (2), a variable stiffness adjusting mechanism (3), and an output connecting member (4) along the axial direction; The variable stiffness adjusting mechanism (3) includes an output flange (301), and the output flange (301) is mounted on the input shaft (2) through a rolling bearing (307), and the axial displacement of the rolling bearing (307) is restricted by a snap ring (306) mounted at the outer end of the inner ring of the bearing; A first right pulley (305-1) and a first left pulley (305-2) are mounted on the end face of the input shaft (2) along the diameter direction. All the pulleys on the end face of the input shaft (2) are collectively referred to as the first group of pulleys (305); in the same end face direction, a second upper right pulley (315-1), a second upper middle pulley (315-2), a second lower right pulley (315-3), a second upper left pulley (315-4), a second upper middle pulley (315-5), and a second lower left pulley (315-6) are mounted on the end face of the output flange (301). All the pulleys on the end face of the output flange (301) are collectively referred to as the second group of pulleys (315); A tension spring (303) is mounted on the right side of the output flange (301) through a tension spring support (302), and a hollow bolt (311) is mounted on the left side of the output flange (301); The end of the tension spring (303) is connected to the first end of a wire rope (304), and there are two winding methods for the second end of the wire rope (304); The first high stiffness mode: The second end of the wire rope (304) sequentially bypasses the second lower right pulley (315-3), the second upper middle pulley (315-2), the first right pulley (305-1), the second upper right pulley (315-1), the second upper left pulley (315-4), the first left pulley (305-2), the second upper middle pulley (315-5), and the second lower left pulley (315-6), then passes through the middle hole of the hollow bolt, and the end of the rope is fixed to the tail end of the hollow bolt with an aluminum sleeve by the press sleeve method. A compression spring (313) is placed between the head of the hollow bolt and the mating nut (312) to prevent the hollow bolt from loosening due to too small a pre-tightening force of the wire rope (304); there is no crossing in the above wire rope (304) winding; The second low stiffness mode: The second end of the wire rope (304) sequentially bypasses the second lower right pulley (315-3), the second upper middle pulley (315-2), the second upper right pulley (315-1), the second upper left pulley (315-4), the first left pulley (305-2), the second upper middle pulley (315-5), and the second lower left pulley (315-6), then passes through the middle hole of the hollow bolt, and the end of the rope is fixed to the tail end of the hollow bolt with an aluminum sleeve by the press sleeve method. A compression spring (313) is placed between the head of the hollow bolt and the mating nut (312). There is no crossing in the above wire rope (304) winding; Or another winding method: The second end of the wire rope (304) successively winds around the second right lower pulley (315-3), the second right middle pulley (315-2), the first right pulley (305-1), the second right upper pulley (315-1), the second left upper pulley (315-4), the second left middle pulley (315-5), and the second left lower pulley (315-6), then passes through the middle hole of the hollow bolt. The wire rope end is fixed to the tail end of the hollow bolt with an aluminum sleeve by the swaging method. The compression spring (313) is placed between the head of the hollow bolt and the mating nut (312). There is no crossing in the above wire rope (304) winding.

2. The working method of the variable stiffness joint according to claim 1, characterized in that It includes the following processes: The driving motor (1) transmits the rotation to the first group of pulleys (305) participating in the rope winding through the input shaft (2), drives the wire rope (304) to move, and the tension spring (303) expands and contracts accordingly. When the wire rope (304) is tightened, the rotation of the input shaft (2) is transmitted to the second group of pulleys (315) and the output flange (301) participating in the rope winding through the wire rope (304), and drives the output connecting piece (4) to rotate; The stiffness characteristics of the joint are changed through three adjustment methods, including: adjusting the pre-tightening force of the compression spring (313) through the mating nut (312), selecting to change the winding method of the wire rope (304), and configuring tension springs (303) of different models.

3. The control algorithm of the variable stiffness joint working method according to claim 2, characterized in that It includes the following processes: The relative rotation angle between the input shaft (2) and the output connecting member (4) of the variable stiffness joint is the internal deflection angle q within the variable stiffness mechanism, and the internal deflection torque is T q ; The distances from the geometric center A1 of the first right pulley (305-1) and the geometric center A2 of the first left pulley (305-2) to the geometric center O of the variable stiffness joint are equal, denoted as l1; The midpoint C1 of the line connecting the geometric center B1 of the second right upper pulley (315-1) and the geometric center B2 of the second right middle pulley (315-2), and the midpoint C2 of the line connecting the geometric center B4 of the second left upper pulley (315-4) and the geometric center B5 of the second left middle pulley (315-5) are equidistant from the geometric center O of the variable stiffness joint, denoted as l3; When the input shaft (2) and the output connecting member (4) rotate relative to each other, the distances between C1 and A1, and between C2 and A2 are equal, denoted as l2, and its calculation formula is The internal tension of the wire rope (304) is the tension received by the tension spring (303), which is F. During the process of transmitting motion, its equivalent arm of force is L, and its calculation formula is where N is the number of working branches. The high stiffness mode corresponds to 2 working branches, and the low stiffness mode corresponds to 1 working branch; The equivalent stiffness of the series connection of the tension spring and the wire rope is k, and its calculation formula is where k t is the stiffness of the tension spring, and k s is the stiffness of the wire rope; The stiffness mathematical model of the variable stiffness joint is where F0 is the initial pre-tightening force of the tension spring; Based on the given or calculated target output stiffness K tag With the stiffness characteristic curve K(q) of the variable stiffness mechanism, the target deflection angle q of the variable stiffness mechanism can be calculated tag : K tag = K(q tag ) Other robotic structural components connected to the drive motor (1) serve as the variable stiffness joint base; the angle and torque directly output by the drive motor (1) are θ and T respectively θ ; the robotic link connected to the output connector (4) serves as the variable stiffness joint output end, and its output angle and torque are α and T respectively α ; When the end of the robot link is in fixed contact with the object and the output end of the variable stiffness joint is fixed, the output angle α of the variable stiffness joint is uniquely determined by the position of the variable stiffness joint base; at this time, based on the reaction force F measured by the force sensor at the end of the robot link N,0 , combined with the gravity M of the robot link between the variable stiffness joint and the end and the structural dimension model, the torques generated by the reaction force at the end of the robot and the gravity of the robot link on the variable stiffness joint can be calculated, which are T(F N,0 ) and T(M), respectively. Further, the output torque T of the variable stiffness joint at the current base position can be calculated by the principle of virtual work α : T α = T(F N,0 ) + T(M) When the output end of the variable stiffness joint is in a fixed state, the output torque T of the variable stiffness joint α is equal to the deflection torque T inside the variable stiffness mechanism q Combined with the stiffness characteristic curve K(q) of the variable stiffness mechanism, the internal deflection angle q0 of the variable stiffness mechanism at the current moment can be further calculated: Since the angle θ output by the driving motor (1) is equal to the sum of the internal deflection angle q of the variable stiffness mechanism and the output angle α of the variable stiffness joint, the output angle α0 of the variable stiffness joint under the current base position can be calculated: α0 = θ0 - q0 where θ0 represents the output angle of the drive motor (1) at the current moment, combined with the target deflection angle q of the variable stiffness joint tag , the target output angle θ of the drive motor (1) at the current base position can be obtained tag : θ tag = q tag + α0 The position is controlled by the driving motor (1) to achieve accurate stiffness output; When the base position changes, the robot link end force sensor detects that the reaction force changes. Repeat the above steps to calculate the output angle of the variable stiffness joint after the base position changes, and the driving motor (1) performs position compensation control.

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