Stiffness-variable actuator, knee exoskeleton and control method of stiffness-variable actuator

Abstract

This invention provides a variable stiffness actuator, a knee exoskeleton, and a control method for the variable stiffness actuator, belonging to the field of rehabilitation robot technology. The variable stiffness actuator includes a variable stiffness component, which consists of a zero-length frame hinged four-bar linkage, an elastic link assembly, a stiffness adjustment mechanism, and a T-screw assembly. The input and output disks have coaxial rotation centers. The elastic link assembly includes a linear spring and a Bowden wire. The T-screw assembly is used to change the geometric configuration parameters of the four-bar linkage, achieving stepless adjustment of the stiffness range. The knee exoskeleton includes a rotation drive assembly, a thigh assembly, a lower leg assembly, and the variable stiffness actuator. The control method includes establishing a stiffness model, real-time estimation of human-machine interaction torque, compliant control based on gait phase, and adjustment of the stiffness mode. This invention achieves stepless stiffness reconfiguration over a wide range and structural lightweighting, enabling precise compliant control without the need for torque sensors.

Description

Technical Field

[0001] This invention relates to the field of rehabilitation robot technology, and in particular to a variable stiffness actuator, a knee exoskeleton, and a control method for the variable stiffness actuator. Background Technology

[0002] Currently, mainstream knee exoskeleton drive technologies can be divided into three categories: rigid drive, series elastic variable stiffness actuator (SEA), and variable stiffness actuator (VSA). Rigid drive has a simple structure and is easy to control, but lacks compliance and has poor human-machine interaction safety. Series elastic variable stiffness actuator achieves basic compliance by introducing elastic elements, but its stiffness is fixed and cannot adapt to the variable stiffness requirements at different pace states. Variable stiffness actuator can dynamically adjust stiffness, which theoretically best meets the needs of human movement, but existing technologies still have many shortcomings.

[0003] In the design of variable stiffness joints, there have been many attempts in the existing technology. Chinese patent application CN111376306B discloses a variable stiffness joint for robots, which achieves variable stiffness by changing the deformation of the leaf springs through changes in the internal mechanical structure during joint rotation. However, it suffers from drawbacks such as a one-to-one correspondence between rotation angle and stiffness value, limited adjustment range, and poor versatility. Chinese patent application CN107773384B discloses a variable stiffness lower limb exoskeleton robot, which controls the overall stiffness of the exoskeleton through a variable stiffness actuator, exhibiting a certain degree of compliance. However, it lacks specific design for the knee joint, making it difficult to meet the requirements for fine-grained stiffness control at the joint level. Chinese patent application CN110652425B discloses a variable stiffness lower limb exoskeleton assistive robot, which constructs a variable stiffness mechanism through drive pulleys and steel cables. However, stiffness adjustment relies on discrete switching of the number of pulley sets, making stepless continuous adjustment impossible. Chinese patent application CN113524143A discloses a variable stiffness knee joint and lower limb exoskeleton robot, which uses piezoelectric materials to adjust the elastic coefficient, but the stiffness adjustment range is narrow and the requirements for the drive circuit are high. US11035743B2 discloses a series elastic variable stiffness actuator system, which achieves a compact axial structure design, but it is essentially still in the category of fixed stiffness and cannot achieve dynamic stiffness adjustment.

[0004] In summary, existing variable stiffness exoskeleton knee joint technologies generally suffer from the following problems: 1. Limited stiffness adjustment range: most solutions struggle to achieve a wide range of stiffness coverage from high compliance to high rigidity on a single device; 2. Limited stiffness adjustment methods: some solutions rely on discrete switching or angle-stiffness coupling, failing to achieve truly stepless continuous adjustment; 3. The contradiction between structural complexity and wearability: while adversarial or multi-motor solutions offer rich functionality, their weight and size are insufficient for wearability requirements; 4. Torque sensing relies on expensive sensors, increasing system cost and failure risk. Therefore, there is an urgent need for a knee exoskeleton solution that is compact, has a steplessly reconfigurable stiffness range, is cost-effective, and highly safe, to meet the needs of all scenarios, from rehabilitation training to daily walking assistance. Summary of the Invention

[0005] The purpose of this invention is to provide a variable stiffness actuator, a knee exoskeleton, and a control method for the variable stiffness actuator, in order to solve the problems existing in the prior art, such as limited stiffness adjustment range, discrete adjustment methods, complex structure, poor wearing comfort, and high cost due to reliance on torque sensors.

[0006] To achieve the above objectives, the present invention provides a variable stiffness actuator, including a variable stiffness component, the variable stiffness component comprising: The zero-length frame hinged four-bar linkage consists of an input plate, an output plate, a flexible linkage assembly, and a frame. The rotation centers of the input plate and the output plate are coaxial and coincident, forming a frame length of zero for the zero-length frame hinged four-bar linkage. The input plate and the output plate are made of titanium alloy. The flexible linkage assembly includes a linear spring and a Bowden wire. The stiffness adjustment mechanism includes a pulley block, which is mounted on the input and output discs, and the Bowden cable passes through the pulley block. The stiffness range adjustment mechanism, including a T-type lead screw assembly, is fixedly installed on the rod or frame of the zero-length frame hinged four-bar linkage and is used to change the geometric configuration parameters of the zero-length frame hinged four-bar linkage.

[0007] Preferably, the T-type lead screw assembly includes a lead screw drive motor, a T-type lead screw, a slide rail, a slider, and an output wheel. The output end of the lead screw drive motor is driven and connected to the T-type lead screw. The slider is threadedly engaged with the T-type lead screw and slidably mounted on the slide rail. The output wheel is rotatably mounted on the slider. One end of the linear spring is connected to a spring horizontal bearing bracket, and the other end is connected to a Bowden wire, which passes around the output wheel.

[0008] Preferably, the pulley block includes an input fixed pulley block fixedly mounted on the input disk and an output fixed pulley block fixedly mounted on the output disk. The Bowden cable passes through the input fixed pulley block and the output fixed pulley block in sequence. One end of the Bowden cable is connected to the drive wheel, and the other end is connected to the linear spring.

[0009] Preferably, the variable stiffness component further includes an input motor, the output end of which is driven and connected to the input disk; the lower side of the output disk is fixedly connected to the inner ring of the spherical bearing, and the outer ring of the spherical bearing is fixedly connected to the thigh bearing connecting plate.

[0010] The present invention also provides a knee exoskeleton, including the variable stiffness actuator as described above, and further comprising: The rotation drive assembly is fixedly installed inside the thigh assembly; The thigh component is fixedly connected to the variable stiffness component; The lower leg assembly is fixedly connected to the output disk of the variable stiffness actuator and rotates synchronously with the output disk.

[0011] Preferably, the rotation drive assembly includes a rotation drive motor, a gearbox, and a drive wheel. The rotation drive motor is installed inside the thigh assembly. The input end of the gearbox is connected to the output end of the rotation drive motor. The drive wheel is fixedly connected to the output end of the gearbox. One end of the Bowden wire is wound and fixed to the drive wheel.

[0012] Preferably, the thigh assembly includes a thigh plate, a thigh retainer, a thigh retainer plate, a thigh output disc connecting plate, and a thigh bearing connecting plate. One end of the thigh output disc connecting plate is fixedly connected to the variable stiffness assembly, and the other end is fixedly connected to the thigh plate. The thigh retainer and the thigh retainer plate are fixedly installed on the thigh plate, and the thigh bearing connecting plate is fixedly installed on the thigh plate for mounting the bearing. The thigh plate and the thigh retainer plate are made of carbon fiber reinforced composite material or aerospace-grade aluminum-magnesium alloy.

[0013] Preferably, the lower leg assembly includes a lower leg plate, a lower leg output disc connecting plate, a spring horizontal bearing bracket assembly, an L-shaped perforated bracket, and an L-shaped bracket fixing seat. One end of the lower leg output disc connecting plate is fixedly connected to the output disc, and the other end is fixedly connected to the lower leg plate. The spring horizontal bearing bracket assembly is fixedly installed on the lower leg plate to support the end of the linear spring. The L-shaped bracket fixing seat is fixedly installed on the outside of the lower leg plate, and the L-shaped perforated bracket is fixedly installed on the L-shaped bracket fixing seat.

[0014] The present invention also provides a control method for a variable stiffness actuator, applied to the variable stiffness actuator as described above, comprising the following steps: Step S1: Establish the stiffness model of the variable stiffness actuator. The stiffness model describes the functional relationship between the output stiffness and the joint motion state, the spring preload, and the position of the stiffness range adjustment mechanism. Step S2: Based on the stiffness model, the joint deflection angle is obtained by measuring the angle difference between the input disk and the output disk. Combined with the current spring preload and the position of the T-shaped lead screw assembly, the human-machine interaction torque is estimated in real time. Step S3: Based on the sensor to identify the human gait phase, and according to the human-computer interaction torque estimated in step S2, the estimated torque is brought close to zero through closed-loop control, thereby achieving compliant following control with extremely low impedance without the need for additional torque sensors. Simultaneously, the stiffness mode of the variable stiffness actuator is adjusted based on the gait phase recognition results: In the swing phase, the control T-type lead screw assembly is adjusted to make the variable stiffness actuator work in the low stiffness range, so as to realize swing following; In the support phase, the control T-type screw assembly is adjusted to place the variable stiffness actuator in a high stiffness range working position to achieve support assistance; The low stiffness ranges from 0 to 10 N·m / rad, while the high stiffness ranges from 150 N·m / rad to 300 N·m / rad.

[0015] The preferred stiffness model is as follows: ; in, Indicates the output stiffness; Indicates the joint deflection angle; This indicates the position of the stiffness range adjustment mechanism, i.e., the displacement of the slider in the T-screw assembly; Indicates the equivalent stiffness of the spring; Indicates the lever arm coefficient; This indicates the spring preload.

[0016] Therefore, the present invention employs the aforementioned variable stiffness actuator, knee exoskeleton, and control method for the variable stiffness actuator, and the beneficial technical effects are as follows: (1) This invention achieves a wide-range stepless reconfiguration of the output stiffness range through the collaborative design of a stiffness range adjustment mechanism and a zero-length frame hinge four-bar linkage, solving the problems of limited stiffness adjustment range and discrete adjustment methods in the prior art. Specifically, this invention uses a stiffness range adjustment mechanism to continuously change the geometric configuration parameters of the four-bar linkage, thereby achieving a wide range of stiffness coverage from high compliance to high rigidity without changing the elastic element or the pulley block configuration. This design breaks through the bottleneck of limited stiffness adjustment range of traditional variable stiffness actuators and overcomes the defects of existing technologies that rely on discrete switching and cannot achieve stepless continuous adjustment, so that the same variable stiffness actuator can meet both the compliance requirements in the early stage of rehabilitation and the high rigidity support requirements in the walking support period.

[0017] (2) This invention is based on the coaxial layout and modular structure design of a zero-length frame hinged four-bar linkage, which significantly improves wearing comfort and structural lightweighting while ensuring functional integrity. The design of the input and output disks having their rotation centers coaxially coincident perfectly adapts to the natural movement of the human knee joint, avoiding motion interference caused by traditional offset structures; key structural components are made of lightweight and high-strength materials, which significantly reduces the rotational inertia of moving parts. At the same time, the stiffness range adjustment mechanism uses self-lubricating materials for pairing, achieving maintenance-free operation.

[0018] (3) This invention proposes a sensorless torque estimation and gait adaptive control method based on a stiffness model, which achieves high-precision compliant interactive control without the need for a torque sensor, reducing system cost and failure risk. By establishing a theoretical model describing the relationship between output stiffness and joint motion state, spring preload, and the position of the stiffness range adjustment mechanism, the human-machine interaction torque can be estimated in real time using motion parameters collected by the encoder; then, based on the gait phase recognition results, the variable stiffness actuator is controlled to enter a low stiffness mode in the swing phase to achieve transparent following, and switches to a high stiffness mode in the support phase to provide stable support, and the estimated torque is brought close to zero through closed-loop control. This method overcomes the high cost and easy damage problems caused by the reliance on torque sensors in traditional schemes, and achieves low-cost and high-reliability compliant interaction while ensuring control accuracy. Attached Figure Description

[0019] Figure 1 A schematic diagram of the overall structure of the knee exoskeleton including the variable stiffness actuator; Figure 2 This is a schematic diagram of the mechanical structure of the variable stiffness actuator proposed in this invention. Figure 3 A diagram showing the connection relationships between a zero-length four-bar linkage, a linear spring, and a T-screw. Figure 4 This is a front view of the rotation drive assembly structure; Figure 5 This is a perspective view of the rotation drive assembly structure; Figure 6 This is a schematic diagram of a variable stiffness component structure; Figure 7 This is a schematic diagram of the lower leg assembly and the T-shaped lead screw assembly.

[0020] Figure Labels 1. Rotary drive assembly; 11. Rotary drive motor; 12. Gearbox; 13. Drive wheel; 2. Thigh assembly; 21. Thigh plate; 22. Thigh retainer; 23. Thigh retainer plate; 24. Thigh output disc connecting plate; 25. Thigh bearing connecting plate; 3. Variable stiffness assembly; 31. Input motor; 331. Input fixed pulley block; 332. Output fixed pulley block; 32. Input disc; 33. Pulley block; 34. Output disc; 35. Bowden cable; 36. T-screw assembly; 361. T-screw; 362. Screw drive motor; 363. Slide rail; 364. Slider; 365. Output wheel; 37. Linear spring; 38. Spherical bearing; 4. Lower leg assembly; 41. Lower leg plate; 42. Lower leg output plate connecting plate; 43. Spring horizontal bearing bracket assembly; 44. L-shaped bracket with holes; 45. L-shaped bracket fixing seat. Detailed Implementation

[0021] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0023] Example 1 I. Variable stiffness actuator structure.

[0024] like Figures 1-3 As shown, the variable stiffness actuator includes a variable stiffness component 3. The variable stiffness component 3 is located on the outside of the knee joint and consists of a zero-length frame hinge four-bar linkage, an elastic link assembly, a stiffness adjustment mechanism, and a stiffness range adjustment mechanism.

[0025] The zero-length frame hinged four-bar mechanism adopts a zero-length frame design, meaning that the rotation centers of the input disk 32 and the output disk 34 coincide on the same axis, forming a structure with zero frame length to perfectly adapt to the natural rotational movement of the human knee joint. The lower side of the output disk 34 is fixedly connected to the inner ring of the joint bearing 38, and the outer ring of the joint bearing 38 is fixedly connected to the thigh bearing connecting plate 25.

[0026] The flexible linkage assembly includes a linear spring 37 and a Bowden wire 35. One end of the linear spring 37 is connected to the input disc 32 via the Bowden wire 35, and the other end is connected to a spring-mounted bearing bracket. When the knee joint moves and produces a deflection angle, the input disc 32 and the output disc 34 rotate relative to each other, stretching the linear spring 37 via the Bowden wire 35, causing it to deform and thus generating a flexible torque. The linear spring 37 is shot-peened, introducing residual compressive stress into the spring surface, significantly improving its fatigue strength and preventing breakage during high-frequency reciprocating motion.

[0027] The stiffness adjustment mechanism includes a pulley block 33. The pulley block 33 includes an input fixed pulley block 331 fixedly mounted on the input disk 32 and an output fixed pulley block 332 fixedly mounted on the output disk 34. A Bowden line 35 passes sequentially through the input fixed pulley block 331 and the output fixed pulley block 332. One end of the Bowden line 35 is connected to the drive wheel 13 of the rotation drive assembly 1, and the other end is connected to the linear spring 37. By changing the number of pulley blocks 33, the equivalent stiffness of the linear spring 37 can be amplified. When performing low-stiffness tasks, a smaller number of pulley blocks (…) are used. Configuration; When performing high-stiffness tasks, use a large number of pulley blocks (maximum). When reconfiguration is required, it can be done simply by replacing and rewinding the Bowden wire; there is no need to disassemble the actuator.

[0028] The stiffness range adjustment mechanism includes a T-screw assembly 36, integrated within the mechanism and fixedly mounted on the rods or frame of a zero-length frame hinged four-bar linkage. The T-screw assembly 36 includes a screw drive motor 362, a T-screw 361, a slide rail 363, a slider 364, and an output wheel 365. The output end of the screw drive motor 362 is driven by the T-screw 361. The slider 364 is threaded onto the T-screw 361 and slidably mounted on the slide rail 363. The output wheel 365 is rotatably mounted on the slider 364, and a Bowden wire 35 passes around the output wheel 365. The surface of the T-screw 361 is treated with a Teflon coating to reduce friction, and a slider nut with self-lubricating properties (such as PEEK material) is used to ensure smooth and maintenance-free adjustment. The lead screw drive motor 362 drives the T-type lead screw 361 to rotate, which drives the slider 364 to move along the slide rail 363. The slider 364 serves as the lever arm adjustment fulcrum of the linear spring 37. By changing its position, the effective lever arm length of the input disk 32 is continuously adjusted, thereby changing the geometric configuration parameters of the zero-length frame hinge four-bar mechanism and realizing stepless adjustment of the output stiffness range.

[0029] The variable stiffness component 3 also includes an input motor 31, the output end of which is connected to the input disk 32 for driving the input disk 32 to rotate.

[0030] II. Knee exoskeleton.

[0031] This embodiment also provides a knee joint exoskeleton, such as Figures 1 to 7 As shown, the device includes the aforementioned variable stiffness actuator, as well as a rotation drive assembly 1, a thigh assembly 2, and a lower leg assembly 4.

[0032] The rotation drive assembly 1 is installed inside the thigh assembly 2, serving as the power source for the pretensioning force of the exoskeleton. The rotation drive assembly 1 includes a rotation drive motor 11, a reduction gearbox 12, and a drive wheel 13. The rated output torque of the rotation drive motor 11 is 0.4 N·m, and the reduction ratio of the reduction gearbox 12 is 60:1. The rotation drive motor 11 is fixedly installed inside the thigh assembly 2. The input end of the reduction gearbox 12 is connected to the output end of the rotation drive motor 11, and the drive wheel 13 is fixedly connected to the output end of the reduction gearbox 12. One end of the Bowden wire 35 is wound and fixed to the drive wheel 13. Because the transmission method of the Bowden wire is prone to loosening, a pretensioning mechanism is required to pretension the Bowden wire. After the rotation drive motor 11 increases torque through the reduction gearbox 12, it drives the drive wheel 13 to rotate, winding the Bowden wire 35 wound on the drive wheel 13, shortening the length of the Bowden wire 35, thus achieving pretensioning of the Bowden wire and adjusting the initial pretensioning force of the linear spring 37.

[0033] The thigh assembly 2, serving as a proximal support structure, is fixedly connected to the variable stiffness assembly 3. The thigh assembly 2 includes a thigh plate 21, a thigh retainer 22, a thigh retainer plate 23, a thigh output disc connecting plate 24, and a thigh bearing connecting plate 25. To reduce the burden on the wearer, the thigh plate 21 and thigh retainer plate 23 are preferably made of carbon fiber reinforced polymer (CFRP) or aerospace-grade aluminum-magnesium alloy, reducing the weight of the structural components by more than 60% while ensuring strength. Specifically, the carbon fiber reinforced polymer is a prepreg-molded composite material composed of T300 or T700 grade carbon fiber and an epoxy resin matrix, with a fiber volume content of 55%-65% and a density of 1.5-1.6 g / cm³. 3 Tensile strength ≥1500MPa, elastic modulus ≥120GPa; or made of aerospace-grade aluminum-magnesium alloy, specifically 7075-T6 aluminum alloy, with tensile strength ≥500MPa, yield strength ≥400MPa, and density approximately 2.8g / cm³. 3 However, this invention is not limited to the materials specifically listed above. Any lightweight, high-strength structural material with comparable specific strength (e.g., ≥150 kN·m / kg) or specific stiffness (e.g., ≥20 MN·m / kg) can be applied to the thigh plate and / or calf plate of this invention, achieving the same technical effects of reducing structural weight and decreasing rotational inertia. One end of the thigh output disc connecting plate 24 is fixedly connected to the variable stiffness assembly, and the other end is fixedly connected to the thigh plate 21. The thigh retainer 22 and thigh retainer plate 23 are fixedly mounted on the thigh plate 21, and the thigh bearing connecting plate 25 is fixedly mounted on the thigh plate 21 for mounting the bearing.

[0034] The lower leg assembly 4 is fixedly connected to the output disk 34 of the variable stiffness actuator and rotates synchronously with the output disk 34. The lower leg assembly 4 includes a lower leg plate 41, a lower leg output disk connecting plate 42, a spring horizontal bearing bracket assembly 43, an L-shaped perforated bracket 44, and an L-shaped bracket fixing seat 45. One end of the lower leg output disk connecting plate 42 is fixedly connected to the output disk 34, and the other end is fixedly connected to the lower leg plate 41. The spring horizontal bearing bracket assembly 43 is fixedly installed on the lower leg plate 41 to support the end of the linear spring 37. The L-shaped bracket fixing seat 45 is fixedly installed on the outside of the lower leg plate 41 by countersunk screws, and the L-shaped perforated bracket 44 is fixedly installed on the L-shaped bracket fixing seat 45 to limit and fix the sensor on the slide rail.

[0035] The input disk 32 and output disk 34 are made of titanium alloy, which reduces the rotational inertia of the moving parts. For example, TC4 titanium alloy (Ti-6Al-4V) has a density of approximately 4.43 g / cm³. 3 The tensile strength is ≥900MPa, the yield strength is ≥800MPa, and the elastic modulus is approximately 110GPa. However, the present invention is not limited to the specific materials mentioned above. Any lightweight high-strength metal material or composite material with a comparable specific strength (e.g., ≥200kN·m / kg) can be applied to the input and output disks of the present invention, and can also achieve the technical effects of reducing rotational inertia and improving response speed.

[0036] III. Control methods and principle modeling.

[0037] This embodiment also provides a control method for the above-mentioned variable stiffness actuator, which is modeled based on the following principle: (a) Geometric kinematic model.

[0038] The core transmission mechanism of the variable stiffness actuator can be simplified to a zero-length frame four-bar model, where the frame length is... .set up: The effective length of the input rod is adjusted via the T-screw assembly 36. Let the slider displacement be... ,but , This is the initial length; The length of the output rod is a constant. ; The length of an elastic link (a link consisting of linear springs or springs in series); : Joint deflection angle, which is the angle between input disk 32 and output disk 34.

[0039] According to the law of cosines, the real-time length of the elastic link... With joint deflection angle The relationship is: .

[0040] (ii) Torque and stiffness model.

[0041] Let the physical stiffness of linear spring 37 be... The pulley system has a ratio of 33. The equivalent stiffness of the system at the end of the Bowden line is... For physical stiffness Multiplied by 10, i.e., equivalent stiffness .

[0042] Let the initial preload of the spring be... (correspond (Spring force at the time). When the joint deflects. At that time, the deformation of the elastic element for: ; Note: This assumes... At this point, the spring is at its shortest compression state or its natural extension state, which is the equilibrium point. .

[0043] Restoring torque (output torque) acting on the output joint Due to spring force The output torque is determined by the lever arm generated by the mechanical structure. Based on the principle of virtual work, the output torque... for: ; in, This represents the elastic potential energy stored in a spring due to its deformation; spring force. Geometric Jacobian term (lever arm coefficient) Derived from geometric relations: ; Output torque formula: ; Output stiffness of variable stiffness actuator Defined as the derivative of torque with respect to angle To simplify calculations, under small-angle deflection, the mechanism stiffness is mainly determined by geometric parameters and preload, and its theoretical stiffness model can be approximated as: ; The core innovation of this invention lies in: changing the T-shaped lead screw assembly 36 This directly changed the length of the input rod. This, in turn, changed the Jacobian term. .because and It exhibits a nonlinear positive correlation and modulates Stiffness can be significantly increased or decreased. The reference value and adjustment range are used to achieve "adjustment of variable stiffness range".

[0044] (III) Example of calculating the assist effect.

[0045] To verify the technical effect of the present invention, a specific calculation example is given below. Assume the physical stiffness of the linear spring... Pulley system ratio Then the equivalent stiffness The output rod is 40mm long.

[0046] When the knee joint is flexed to Calculate the maximum output torque at different T-screw positions when (typically squatting or raising the leg to its maximum angle).

[0047] Condition A: Low stiffness / compliant mode (early recovery / oscillating phase).

[0048] Adjusting the T-screw to make (Longest lever arm), preload .

[0049] Length of elastic link ; Spring deformation ; Spring force .

[0050] Condition B: High stiffness / strong support mode (late recovery / support phase).

[0051] Setting the T-screw to make (Shortest lever arm), preload .

[0052] Length of the flexible link: .

[0053] Spring deformation .

[0054] Spring force .

[0055] Lever arm coefficient .

[0056] Output torque .

[0057] In the fields of rehabilitation robots and exoskeletons, biomechanics and the definition of required ranges indicate that "low stiffness" is typically defined as the range of 0–10 N·m / rad. This range corresponds to the swaying phase of human walking or passive traction training in the early stages of rehabilitation. "High stiffness" is typically defined as the range of 150 N·m / rad to 300 N·m / rad. This range corresponds to the support phase of human walking or the process of moving from sitting to standing.

[0058] (iv) Calculation of stiffness adjustment range.

[0059] Near the equilibrium position (small angle) ) Calculate stiffness adjustment capability. Based on the stiffness characteristics of the zero-length frame model, as... (Singularity), extremely sensitive to changes in stiffness. Under the small-angle approximation, the theoretical stiffness can be expressed as: ; By adjusting the T-screw assembly 36, the change (Increasing from 42mm to 85mm), and combined with the adjustment of preload, the overall stiffness adjustment range of the system can cover from 3.8 N·m / rad (high compliance) to 210 N·m / rad (high rigidity). This range effectively covers the human movement needs from "highly compliant oscillation" to "highly rigid support".

[0060] (v) Control methods and procedures.

[0061] Based on the above principles, the control method in this embodiment includes the following steps: Step S1: Establish a stiffness model: Establish a theoretical stiffness model for the variable stiffness actuator to describe the nonlinear functional relationship between the output stiffness and the joint deflection angle, spring preload, and the position of the T-type lead screw assembly at position 36.

[0062] Step S2, Sensorless Torque Estimation: Based on the theoretical stiffness model established in step S1, the current joint deflection angle is obtained by measuring the angle difference between the input disk 32 and the output disk 34 through the encoder; combined with the current spring preload (determined by the driving state of the rotation drive component 1) and the position of the T-shaped lead screw component 36 (obtained by the encoder of the lead screw drive motor 362), the human-machine interaction torque is estimated in real time by substituting it into the stiffness model.

[0063] Step S3, Gait Recognition and Compliant Control: The human gait phase is identified based on an IMU sensor or plantar pressure sensor. Based on the human-machine interaction torque estimated in Step S2, closed-loop control is used to bring the estimated torque close to zero, thus achieving extremely low impedance compliant following control without the need for an additional torque sensor. Simultaneously, the stiffness mode of the variable stiffness actuator is adjusted according to the gait phase recognition results: during the swing phase or early rehabilitation phase, the lead screw drive motor 362 drives the T-shaped lead screw 361 to rotate, moving the slider 364 to the maximum arm position (increasing...). This allows the variable stiffness actuator to operate in a low stiffness range suitable for swing following, reducing human-machine interaction; in support phase or walking mode, the control slider 364 moves to the small lever arm position (reducing...) This allows the variable stiffness actuator to operate within a high stiffness range suitable for support assistance, preventing knee collapse.

[0064] IV. Working principle.

[0065] When wearing, the flexible cushioning layer (such as silicone or TPU) on the inside of the thigh plate 21 and calf plate 41 is tightly fitted to the limb and secured with straps to ensure comfortable wear without pressure.

[0066] During system operation, variable stiffness adjustment is achieved through "geometric configuration reconstruction": the lead screw drive motor 362 drives the T-shaped lead screw 361 to rotate according to the control signal, which drives the slider 364 to move along the slide rail 363, thereby changing the effective lever arm length of the linear spring 37. In the support phase or walking mode, the slider moves in the opposite direction to reduce the lever arm (e.g., to 42mm), significantly increasing the output stiffness (approximately 210 N·m / rad) to prevent knee collapse. In the swing phase or early rehabilitation phase, the slider moves to increase the lever arm (e.g., to 85mm), reducing stiffness (approximately 5 N·m / rad) to reduce human-machine aggression. Thanks to the self-locking characteristic of the T-screw, the stiffness setting can be maintained even after the motor is powered off, eliminating the need for continuous power consumption.

[0067] Furthermore, during operation, the controller combines the joint deflection angle, current slider position, and preset stiffness model collected by the encoder to estimate the human-machine interaction torque in real time. Based on the gait phase recognition results, the variable stiffness actuator is controlled to enter a low stiffness mode in the swing phase to achieve transparent following, and switches to a high stiffness mode in the support phase to provide stable support. Through closed-loop control, the estimated torque is brought close to zero, achieving highly transparent compliant control and gait adaptive adjustment without the need for expensive torque sensors.

[0068] It is worth noting that all contents not described in detail in this invention are existing technologies and are well known to those skilled in the art.

[0069] Therefore, this invention employs the aforementioned variable stiffness actuator, knee exoskeleton, and variable stiffness actuator control method. By continuously adjusting the geometric configuration parameters of the zero-length frame hinge four-bar linkage through the T-shaped lead screw assembly, it achieves a wide-range stepless reconfiguration of the output stiffness range. While ensuring a compact and lightweight structure, it realizes sensorless torque estimation and gait adaptive compliant control based on a theoretical stiffness model. This effectively solves the problems of limited stiffness adjustment range, discrete adjustment methods, complex structure, poor wearing comfort, and high cost due to reliance on torque sensors in the prior art.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A variable stiffness actuator, characterized by, Including variable stiffness components, which include: The zero-length frame hinged four-bar linkage consists of an input plate, an output plate, a flexible linkage assembly, and a frame. The rotation centers of the input plate and the output plate are coaxial and coincident, forming a frame length of zero for the zero-length frame hinged four-bar linkage. The input plate and the output plate are made of titanium alloy. The flexible linkage assembly includes a linear spring and a Bowden wire. The stiffness adjustment mechanism includes a pulley block, which is mounted on the input and output discs, and the Bowden cable passes through the pulley block. The stiffness range adjustment mechanism, including a T-type lead screw assembly, is fixedly installed on the rod or frame of the zero-length frame hinged four-bar linkage and is used to change the geometric configuration parameters of the zero-length frame hinged four-bar linkage.

2. The variable stiffness actuator of claim 1, wherein, The T-type lead screw assembly includes a lead screw drive motor, a T-type lead screw, a slide rail, a slider, and an output wheel. The output end of the lead screw drive motor is connected to the T-type lead screw drive. The slider is threaded with the T-type lead screw and is slidably mounted on the slide rail. The output wheel is rotatably mounted on the slider. One end of the linear spring is connected to the spring horizontal bearing bracket, and the other end is connected to the Bowden wire, which passes around the output wheel.

3. The variable stiffness actuator of claim 1, wherein, The pulley system includes an input fixed pulley system fixedly mounted on the input plate and an output fixed pulley system fixedly mounted on the output plate. The Bowden cable passes through the input fixed pulley system and the output fixed pulley system in sequence. One end of the Bowden cable is connected to the drive wheel, and the other end is connected to the linear spring.

4. The variable stiffness actuator of claim 1, wherein, The variable stiffness assembly also includes an input motor, the output end of which is connected to the input disk drive; the lower side of the output disk is fixedly connected to the inner ring of the spherical bearing, and the outer ring of the spherical bearing is fixedly connected to the thigh bearing connecting plate.

5. Knee exoskeleton, characterized in that, Including the variable stiffness actuator as described in any one of claims 1 to 4, further comprising: The rotation drive assembly is fixedly installed inside the thigh assembly; The thigh component is fixedly connected to the variable stiffness component; The lower leg assembly is fixedly connected to the output disk of the variable stiffness actuator and rotates synchronously with the output disk.

6. The knee exoskeleton of claim 5, wherein, The rotation drive assembly includes a rotation drive motor, a gearbox, and a drive wheel. The rotation drive motor is installed inside the thigh assembly. The input end of the gearbox is connected to the output end of the rotation drive motor. The drive wheel is fixedly connected to the output end of the gearbox. One end of the Bowden wire is wound and fixed to the drive wheel.

7. The knee exoskeleton of claim 5, wherein, The thigh assembly includes a thigh plate, a thigh retainer, a thigh retainer plate, a thigh output disc connecting plate, and a thigh bearing connecting plate. One end of the thigh output disc connecting plate is fixedly connected to the variable stiffness assembly, and the other end is fixedly connected to the thigh plate. The thigh retainer and thigh retainer plate are fixedly installed on the thigh plate, and the thigh bearing connecting plate is fixedly installed on the thigh plate for mounting the bearing. The thigh plate and thigh retainer plate are made of carbon fiber reinforced composite material or aerospace-grade aluminum-magnesium alloy.

8. The knee exoskeleton of claim 5, wherein, The lower leg assembly includes a lower leg plate, a lower leg output disc connecting plate, a spring horizontal bearing bracket assembly, an L-shaped perforated bracket, and an L-shaped bracket fixing seat. One end of the lower leg output disc connecting plate is fixedly connected to the output disc, and the other end is fixedly connected to the lower leg plate. The spring horizontal bearing bracket assembly is fixedly installed on the lower leg plate to support the end of the linear spring. The L-shaped bracket fixing seat is fixedly installed on the outside of the lower leg plate, and the L-shaped perforated bracket is fixedly installed on the L-shaped bracket fixing seat.

9. A control method for a variable stiffness actuator, characterized by, Applied to the variable stiffness actuator as described in any one of claims 1-4, the method includes the following steps: Step S1: Establish the stiffness model of the variable stiffness actuator. The stiffness model describes the functional relationship between the output stiffness and the joint motion state, the spring preload, and the position of the stiffness range adjustment mechanism. Step S2: Based on the stiffness model, the joint deflection angle is obtained by measuring the angle difference between the input disk and the output disk. Combined with the current spring preload and the position of the T-shaped lead screw assembly, the human-machine interaction torque is estimated in real time. Step S3: Based on the sensor to identify the human gait phase, and according to the human-computer interaction torque estimated in step S2, the estimated torque is brought close to zero through closed-loop control, thereby achieving compliant following control with extremely low impedance without the need for additional torque sensors. Simultaneously, the stiffness mode of the variable stiffness actuator is adjusted based on the gait phase recognition results: In the swing phase, the control T-type lead screw assembly is adjusted to make the variable stiffness actuator work in the low stiffness range, so as to realize swing following; In the support phase, the control T-type screw assembly is adjusted to place the variable stiffness actuator in a high stiffness range working position to achieve support assistance; The low stiffness ranges from 0 to 10 N·m / rad, while the high stiffness ranges from 150 N·m / rad to 300 N·m / rad.

10. The control method of a variable stiffness actuator according to claim 9, wherein, The stiffness model is as follows: ； in, Indicates the output stiffness; Indicates the joint deflection angle; This indicates the position of the stiffness range adjustment mechanism, i.e., the displacement of the slider in the T-screw assembly; Indicates the equivalent stiffness of the spring; Indicates the lever arm coefficient; This indicates the spring preload.

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