Dynamic variable stiffness bionic robotic fish

Abstract

The invention relates to the technical field of bionic robots, in particular to a dynamic variable-stiffness bionic robotic fish which comprises a fish head, a fish body and a fish tail, and the fish body comprises at least two fish body sections which are sequentially connected in series in the axial direction; x-shaped joints are arranged at the joints of the fish body sections and the fish tail or at the joints of the adjacent fish body sections; a spatial distribution type double-channel driving system is constructed in the fish head and comprises a swing module and an online variable stiffness module which are independent of each other. The swing module comprises a swing driving assembly and a rigid guiding piece extending along the central axis of the fish body. The rigid guide piece penetrates through the central space of each level of fish body section under the driving of the swing driving assembly and is used for applying lateral thrust to the fish body sections to determine a swing track; decoupling control of tail swing and rigidity adjustment can be achieved, so that the swimming frequency and the body rigidity can be independently adjusted in real time, and better propelling efficiency and maneuvering performance are obtained.

Description

A biomimetic robotic fish with dynamic variable stiffness Technical Field

[0001] This invention relates to the field of biomimetic robot technology, and more specifically to a dynamically variable stiffness biomimetic robotic fish. Background Technology

[0002] Fish can efficiently adapt to different swimming speeds and maneuverability requirements by adjusting their body stiffness in real time. Currently, the technological approaches to achieving variable stiffness adjustment in biomimetic robotic fish mainly fall into three categories:

[0003] The first category is variable stiffness mechanisms based on smart materials, such as shape memory alloys and electrorheological fluids. Although these mechanisms have online adjustment capabilities, they generally suffer from high energy consumption, slow response speed, such as cooling lag of thermally actuated materials, complex control systems, and significant susceptibility to underwater ambient temperature, making it difficult to meet the needs of high-frequency dynamic swimming.

[0004] The second category is fluid-driven variable stiffness mechanisms, such as pneumatic or hydraulic ones. These mechanisms change stiffness by adjusting the fluid pressure within the cavity, but fluid drives have limited response bandwidth and often require complex pump and valve systems, which is not conducive to system miniaturization and integration.

[0005] The third category is based on mechanical adjustment, such as tensioning the overall structure and changing the spring preload. These mechanisms alter the overall stiffness by adjusting cable tension or the state of elastic elements, offering advantages such as high efficiency and structural stability. However, existing designs, especially tensioned overall structures, typically tightly couple stiffness adjustment with the joint oscillation drive, using the same set of drive sources to simultaneously control the oscillation frequency and stiffness. This makes it difficult for the robotic fish to achieve independent, decoupled control of the oscillation frequency and stiffness during swimming. Summary of the Invention

[0006] The purpose of this invention is to provide a biomimetic robotic fish with dynamic variable stiffness, which can achieve decoupled control of tail swing and stiffness adjustment, thereby independently and in real time adjusting swimming frequency and body stiffness to obtain better propulsion efficiency and maneuverability.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A biomimetic robotic fish with dynamic variable stiffness includes a head, body, and tail. The body comprises at least two segments connected sequentially along the axial direction. An X-shaped joint is provided at the connection between the body segments and the tail, or at the connection between adjacent body segments. The head contains a spatially distributed dual-channel drive system, including an independent swing module and an online variable stiffness module. The swing module includes a swing drive assembly and a rigid guide extending along the central axis of the body. Driven by the swing drive assembly, the rigid guide passes through the central space of each body segment, applying lateral thrust to the body segment to establish the swing trajectory. The online variable stiffness module includes a variable stiffness drive assembly and flexible transmission components distributed along both sides of the body. Driven by the variable stiffness drive assembly, the flexible transmission components pass through each body segment, applying axial tension to the body segment to adjust the rotational stiffness of the X-shaped joint. The swing module and the online variable stiffness module are spatially isolated, independently controlling the swing frequency and rotational stiffness of the X-shaped joint of the robotic fish.

[0009] The fish head includes a fish head shell and an active servo mount fixedly connected to the fish head shell.

[0010] The oscillation drive assembly includes an active servo motor fixed inside the fish head shell, a servo arm connected to the output shaft of the active servo motor, and a servo arm connecting rod connecting the servo arm and the rigid guide member; the active servo motor drives the rigid guide member to oscillate through the servo arm and the servo arm connecting rod; the variable stiffness drive assembly includes a first drive servo motor and a second drive servo motor installed inside the fish head shell; the first drive servo motor and the second drive servo motor are arranged in series along the central axis of the fish head; the flexible transmission component includes a first rope connected to the first drive servo motor and a second rope connected to the second drive servo motor, the first rope and the second rope being distributed along both sides of the fish body.

[0011] The active servo in the swing module is fixed on the active servo mounting bracket; a square hole is provided on the fish body segment, a rigid guide passes through the square hole, and a fitting gap is designed between the rigid guide and the inner wall of the square hole, so that the fish body segment swings by pushing the inner wall of the square hole through the rigid guide.

[0012] In one embodiment, the fish head is further provided with a first rope guide and a second rope guide; the first rope passes around the first rope guide and then passes through the fish body segment and is fixed to the fish tail; the second rope passes around the second rope guide and then passes through the fish body segment and is fixed to the fish tail.

[0013] In a preferred embodiment, the first rope is led out from the first drive servo and passes directly through the active servo mounting bracket and the through holes on one side of each fish body segment to be fixed to the fish tail; the second rope is led out from the second drive servo and passes directly through the active servo mounting bracket and the through holes on the other side of each fish body segment to be fixed to the fish tail.

[0014] The X-shaped joint includes a cross-link assembly connecting adjacent body segments or between a body segment and a tail; the cross-link assembly is configured to: limit lateral shear displacement and axial misalignment between adjacent segments, and force adjacent segments to rotate about a fixed axis around a fulcrum formed by the intersection of the cross-link assembly; the X-shaped joint also includes a spring connected between adjacent segments and arranged in parallel with the cross-link assembly to provide initial preload and elastic restoring force.

[0015] The cross linkage assembly includes four X-shaped joint links, which are hinged between eight X-shaped joint link connectors. The four X-shaped joint links form two X-shaped structures that are symmetrically arranged in a cross pattern.

[0016] The springs are installed in a detachable or hook-on manner; along the fish body from front to back, the springs connected between different segment groups are configured to have the same or different elastic coefficients, thereby forming a preset stiffness gradient in the axial direction of the fish body.

[0017] The online variable stiffness module is configured to execute tension modulation logic: the tension provided by the first drive servo to the first rope is denoted as F1, and the tension provided by the second drive servo to the second rope is denoted as F2; ​​during the process of the active servo driving the fish body to swing, the first and second drive servos adjust F1 and F2 in real time, so that the tension of the rope located on the inside of the bend increases, and the tension of the rope located on the outside of the bend decreases or is released in the opposite direction, so as to generate a torque in the same direction as the fish body swing; at the same time, different stiffness modes are set by adjusting the change amplitude of F1 and F2: when high stiffness is required, F1 and F2 are controlled to change differentially with a larger force; when low stiffness is required, F1 and F2 are controlled to change differentially with a smaller force, thereby realizing online stepless adjustment of stiffness.

[0018] The beneficial effects of this invention are as follows:

[0019] By setting up independent swing modules and online variable stiffness modules, the robotic fish's swimming swing frequency and body stiffness are completely decoupled and controlled. These two can be adjusted independently and in real time, providing great flexibility for exploring optimal swimming patterns. Based on the preload principle, the online variable stiffness module directly changes the spring deformation, enabling rapid and wide-range online stiffness adjustment with fast response and unaffected by the underwater environment. An X-shaped joint is used as the core variable stiffness unit; its spatially interlaced linkage structure has self-stabilizing characteristics in the absence of external force and can provide a large range of stiffness variations within a limited space. Using fewer actuators than joints to drive the coordinated movement of multiple joints results in a lightweight and compact overall structure, significantly reducing the weight and volume of the tail, as well as lowering system power consumption and cost. The entire mechanical structure is clear, the control logic is direct, and feedback control ensures accurate dynamic changes in the traction force on both sides, resulting in high system reliability and facilitating accurate online stiffness adjustment.

[0020] In Example 2, in addition to the aforementioned benefits, other benefits are also provided. The elastic coefficients of the springs between different fish body segments can be customized to simulate different fish species, enabling the robotic fish to construct a non-uniform stiffness gradient that conforms to biomechanics, achieving a perfect combination of passive stiffness preset and active stiffness adjustment. The multi-stage, interconnected X-shaped joints allow the robotic fish to form a continuous, smooth arc when oscillating, mimicking the curvature of a real fish's spine. Attached Figure Description

[0021] The present invention will now be described in further detail with reference to the accompanying drawings and specific implementation methods.

[0022] Figure 1 is a schematic diagram of the biomimetic robotic fish structure with dynamic variable stiffness according to the first embodiment of the present invention.

[0023] Figure 2 is a front view of the biomimetic robotic fish with dynamic variable stiffness according to the first embodiment of the present invention;

[0024] Figure 3 is a cross-sectional view along section AA of the first embodiment of the present invention;

[0025] Figure 4 is a cross-sectional view along section BB of the first embodiment of the present invention;

[0026] Figure 5 is a top view of the biomimetic robotic fish with dynamic variable stiffness according to the first embodiment of the present invention.

[0027] Figure 6 is a cross-sectional view along the CC section line of the first embodiment of the present invention;

[0028] Figure 7 is a schematic diagram of the internal structure of the fish head and body according to the first embodiment of the present invention;

[0029] Figure 8 is a schematic diagram of the feedback control mechanism of the present invention;

[0030] Figure 9 is a schematic diagram of the biomimetic robotic fish structure with dynamic variable stiffness according to the second embodiment of the present invention.

[0031] Figure 10 is a front view of the biomimetic robotic fish structure with dynamic variable stiffness according to the second embodiment of the present invention.

[0032] Figure 11 is a cross-sectional view along section AA of the second embodiment of the present invention;

[0033] Figure 12 is a cross-sectional view along section BB of the second embodiment of the present invention;

[0034] Figure 13 is a top view of the biomimetic robotic fish structure with dynamic variable stiffness according to the second embodiment of the present invention;

[0035] Figure 14 is a cross-sectional view along the CC section line of the second embodiment of the present invention.

[0036] In the diagram: First drive servo 1; Second drive servo 2; Active servo 3; Servo arm 4; Servo arm connecting rod 5; First rope 6; Second rope 7; Rigid guide 8; Fish head shell 9; Active servo mounting bracket 10; First fish body segment 11; Second fish body segment 12; Fish tail 13; First servo wheel 14; First rope guide 15; Second servo wheel 16; Second rope guide 17; Spring 18; X-joint connecting rod connector 19; X-joint connecting rod 20; Bearing 21; Third fish body segment 22; Fourth fish body segment 23. Detailed Implementation

[0037] The present invention will now be described in further detail with reference to the accompanying drawings.

[0038] Example 1:

[0039] As shown in Figures 1 to 8, the structure and function of a biomimetic robotic fish with dynamic variable stiffness will be described in detail below.

[0040] A biomimetic robotic fish with dynamic variable stiffness includes a fish head, a fish body, and a fish tail. The fish head and the fish body are rotatably connected, and the fish body and the fish tail are connected by an X-shaped joint. A swing module for driving the fish body to swing is installed inside the fish head, and an online variable stiffness module for adjusting the rotational stiffness of the X-shaped joint between the fish body and the fish tail is installed inside the fish head to dynamically change the rotational stiffness online.

[0041] By setting up independent swing modules and online variable stiffness modules, the complete decoupled control of the swimming swing frequency and body stiffness of the robotic fish is achieved. The two can be adjusted independently and in real time, providing great flexibility for exploring the optimal swimming mode.

[0042] Based on the principle of preload, the rotational stiffness of the X-joint can be dynamically changed online through the online variable stiffness module. This enables rapid and wide-range online stiffness adjustment with fast response and is unaffected by the underwater environment.

[0043] Using an X-type joint as the core variable stiffness unit, its spatially interlaced linkage structure has self-stabilizing characteristics when there is no external force, and can provide a large range of stiffness variation within a limited space.

[0044] As shown in Figure 1, the fish head includes a fish head shell 9 and an active servo mount 10 fixedly connected to the fish head shell 9. The active servo mount 10 and the fish head shell 9 can be connected by glue. The fish body includes a first fish body segment 11 and a second fish body segment 12 that are rotatably connected to each other. The first fish body segment 11 is rotatably connected to the active servo mount 10. The first fish body segment 11 and the active servo mount 10 can be rotatably connected by bearings. The first fish body segment 11 and the second fish body segment 12 can be rotatably connected by bearings.

[0045] On the left and right sides of the active servo mount 10 where the active servo 3 is mounted, ribs can be added appropriately to increase the internal bending strength of the active servo mount 10. The fish head shell 9 and the active servo mount 10 can also be connected by screws or made into one piece. The active servo mount 10 and the first fish body segment 11, and the first fish body segment 11 and the second fish body segment 12 can also be connected by double-ended studs, which makes installation more convenient.

[0046] As shown in Figure 3, an active servo motor 3 is fixedly connected to the active servo motor mounting bracket 10, a servo motor arm 4 is fixedly connected to the output shaft end of the active servo motor 3, a servo motor arm connecting rod 5 is fixedly connected to the servo motor arm 4, and a rigid guide 8 is installed on the servo motor arm connecting rod 5.

[0047] The second fish body segment 12 is provided with a square hole, and the rigid guide 8 passes through the square hole provided on the second fish body segment 12; the rigid guide 8 is made of rigid material;

[0048] In use, the swing module includes an active servo motor 3, a servo arm 4, a servo arm connecting rod 5, a rigid guide 8, an active servo motor mounting bracket 10, and a second fish body segment 12. Under the combined action of the active servo motor 3, the servo arm 4, the servo arm connecting rod 5, the rigid guide 8, the active servo motor mounting bracket 10, and the second fish body segment 12, the robotic fish swings left and right along its body. In the initial stage, the servo arm 4, the servo arm connecting rod 5, the rigid guide 8, and the second fish body segment 12 are all symmetrical about the central axis of the robotic fish. When the swing module moves, the output end of the active servo motor 3 rotates, driving the servo arm 4, the servo arm connecting rod 5, and the rigid guide 8 to rotate around the central axis of the output end of the active servo motor 3 at the same angular velocity. The rigid guide 8 swings within the square hole inside the second fish body segment 12. When the end of the rigid guide 8 contacts the longitudinal wall of the square hole, the second fish body segment 12 swings along with the rigid guide 8. The first fish-body segment 11 rotates naturally along the central axis of the two bearings 21 connecting the first fish-body segment 11 to the active servo mount 10. The second fish-body segment 12 rotates naturally along the central axis of the two bearings 21 connecting the first fish-body segment 11 and the second fish-body segment 12. The fish tail 13, along with the spring 18, X-joint connecting rod 19, X-joint connecting rod 20, and the fish tail 13, oscillate with the rotation of the second fish-body segment 12. The output shaft of the active servo 3 rotates to drive the fish body to dynamically oscillate around its longitudinal central axis, thereby generating propulsion.

[0049] The X-shaped joint connects the second body segment 12 and the swinging end of the tail 13 to provide swing support. As shown in Figure 1, the X-shaped joint includes eight X-shaped joint connecting rods 19. Four X-shaped joint connecting rods 19 are fixedly connected to the second body segment 12, and four X-shaped joint connecting rods 19 are fixedly connected to the tail 13. Four X-shaped joint connecting rods 20 are hinged between the eight X-shaped joint connecting rods 19. The four X-shaped joint connecting rods 20 form two X-shaped structures that are symmetrically arranged in a cross pattern. Four springs 18 are distributed on both sides of the four X-shaped joint connecting rods 20, and the joint retains only the longitudinal swing freedom.

[0050] One end of the first X-shaped joint link 20 is hinged to the upper end of the X-shaped joint link connector 19 on the right side of the second fish body segment 12, and the other end of the first X-shaped joint link 20 is hinged to the upper end of the X-shaped joint link connector 19 on the left side of the fish tail 13; one end of the second X-shaped joint link 20 is hinged to the upper end of the X-shaped joint link connector 19 on the left side of the second fish body segment 12, and the other end of the second X-shaped joint link 20 is hinged to the upper end of the X-shaped joint link connector 19 on the right side of the fish tail 13; One end of the third X-joint link 20 is hinged to the lower end of the X-joint link connector 19 on the right side of the second fish body segment 12, and the other end of the third X-joint link 20 is hinged to the lower end of the X-joint link connector 19 on the left side of the fish tail 13; one end of the fourth X-joint link 20 is hinged to the lower end of the X-joint link connector 19 on the left side of the second fish body segment 12, and the other end of the second X-joint link 20 is hinged to the lower end of the X-joint link connector 19 on the right side of the fish tail 13;

[0051] A spring 18 is connected between the two X-shaped joint connecting rods 19 on the right side, and a spring 18 is connected between the two X-shaped joint connecting rods 19 on the left side.

[0052] The online variable stiffness module includes a first drive servo motor 1, a second drive servo motor 2, a first rope 6, a second rope 7, a fish tail 13, a first servo motor wheel 14, a first rope guide 15, a second servo motor wheel 16, a second rope guide 17, a spring 18, an X-shaped joint connecting rod connector 19, and an X-shaped joint connecting rod 20. Both the first drive servo motor 1 and the second drive servo motor 2 are fixedly connected inside the fish head shell 9. The first servo motor wheel 14 is fixedly connected to the output shaft of the first drive servo motor 1, and the second servo motor wheel 16 is fixedly connected to the output shaft of the second drive servo motor 2. The first rope 6 is knotted and wound around the first servo motor wheel 14, and the second rope 7 is knotted and wound around the second servo motor wheel 16.

[0053] The first rope 6 and the second rope 7 have high modulus and low creep properties;

[0054] The fish head shell 9 is fixedly connected to a first rope guide 15 and a second rope guide 17. The first rope 6 passes around the first rope guide 15 and through the left through hole on the active servo bracket 10 and the left through hole on the second fish body segment 12, and is fixedly connected to the fish tail 13. The second rope 7 passes around the second rope guide 17 and through the right through hole on the active servo bracket 10 and the right through hole on the second fish body segment 12, and is fixedly connected to the fish tail 13.

[0055] The first drive servo motor 1, the second drive servo motor 2, the first rope 6, the second rope 7, the fish tail 13, the first servo motor wheel 14, the first rope guide 15, the second servo motor wheel 16, the second rope guide 17, the spring 18, the X-joint connecting rod 19, and the X-joint connecting rod 20 work together to achieve online dynamic changes in the rotational stiffness of the robotic fish. This module is named the online variable stiffness module. In the initial stage, the four identical springs 18 are stretched to provide a certain initial stiffness, and the fish tail 13 is located at the central axis of the robotic fish. When the output shaft of the first drive servo motor 1 rotates, the first servo motor wheel 14 rotates synchronously, thereby driving the first rope 6 wound around the first servo motor wheel 14 to move laterally along the tangential direction below the first rope guide 15. The first rope 6 pulls the fishtail 13 to swing to the left, causing the spring 18 and the X-shaped joint connecting rod 19 to retract. When the output shaft of the second drive servo 2 rotates, the second servo wheel 16 rotates synchronously, thereby causing the second rope 7, which is wrapped around the second servo wheel 16, to move laterally along the tangential direction below the second rope guide 17. The second rope 7 pulls the fishtail 13 to swing to the right, causing the spring 18 and the X-shaped joint connecting rod 19 to retract.

[0056] The force provided by the first drive servo motor 1 to the first rope 6 is denoted as F1, and the force provided by the second drive servo motor 2 to the first rope 7 is denoted as F2. The rotation of the first drive servo motor 1 and the second drive servo motor 2 causes F1 and F2 to change dynamically. When F1 is greater than F2, the fish tail 13 swings to the left; when F1 is less than F2, the fish tail 13 swings to the right. The cycle of F1 and F2 is synchronized with the oscillation module of the robotic fish. When the active servo motor 3 drives the second fish body segment 12 to swing to the left, the first drive servo motor 1 is controlled to retract the first rope 6, increasing F1, while the second drive servo motor 2 is controlled to release the second rope 7, decreasing F2. Conversely, when the active servo motor 3 drives the second fish body segment 12 to swing to the right, F2 is increased while F1 decreases. Through this dynamic differential tension control, the direction of the tension torque generated by the rope is always consistent with the oscillation direction of the fish body, thereby boosting the oscillation of the fish tail, improving the instantaneous explosive force and propulsion efficiency of the robotic fish's forward swimming, and enabling the robotic fish to achieve efficient and stable straight-line swimming. To change the rotational stiffness, the degree of change of F1 and F2 needs to be controlled to achieve the change in stiffness.

[0057] In order to enable F1 and F2 to reach the preset stiffness online, a feedback control mechanism as shown in Figure 8 is established.

[0058] Since the left and right drive units have similar structures, the following description will not be distinguished: Set the target tension value F of the cable. target The main controller calculates the torque requirement into a corresponding target current command I based on the radius of the servo rotor and the torque constant of the motor. targetThe data is then sent to the motor driver. The driver runs a current closed-loop control algorithm: firstly, it samples the actual operating current I of the motor in real time using a current sensor. actual The regulator calculates the deviation ΔI between the current output and the target current command. Then, based on this deviation, it calculates a compensation amount and dynamically adjusts the duty cycle of the pulse-width modulation signal output to the motor, thereby correcting the drive voltage. Through this high-frequency closed-loop regulation, the system ensures continuous operation at I... actual Real-time follow I target This state allows the servo motor to output a stable electromagnetic torque. Ultimately, this torque is converted into a preset physical traction force on the rope via the servo motor wheel, achieving independent, dynamic, and high-precision loading of F1 and F2.

[0059] Example 2

[0060] A biomimetic robotic fish with dynamic variable stiffness includes a fish head, a fish body, and a fish tail. The fish head and the fish body are connected by an X-joint, and the fish body and the fish tail are connected by an X-joint. A swing module for driving the fish body to swing is installed inside the fish head. An online variable stiffness module for adjusting the rotational stiffness of the X-joints between the fish head and the fish body and the fish body and the fish tail is installed inside the fish head and the fish body.

[0061] As shown in Figure 9, the fish head includes a fish head shell 9 and an active servo motor mounting bracket 10 fixedly connected to the fish head shell 9. The active servo motor mounting bracket 10 and the fish head shell 9 can be connected by bolts.

[0062] The fish body includes a first body segment 11, a second body segment 12, a third body segment 22, and a fourth body segment 23. The first body segment 11, the second body segment 12, the third body segment 22, and the fourth body segment 23 are connected by X-joints. The first body segment 11 is connected to the active servo mount 10 by an X-joint. The first body segment 11 and the second body segment 12 are connected by an X-joint. The second body segment 12 and the third body segment 22 are connected by an X-joint. The third body segment 22 and the fourth body segment 23 are connected by an X-joint. The fourth body segment 23 is connected to the tail 13 by an X-joint. The X-joints provide swing support.

[0063] As shown in Figures 12 and 13, the X-shaped joint includes eight X-shaped joint link connectors 19, and four X-shaped joint links 20 are hinged between the eight X-shaped joint link connectors 19. The four X-shaped joint links 20 form two X-shaped structures that are arranged symmetrically and intersecting each other. A spring 18 is connected between the two X-shaped joint link connectors 19 on the right side and between the two X-shaped joint link connectors 19 on the left side. The four springs 18 are distributed on both sides of the four X-shaped joint links 20, and the joint retains only the longitudinal swing degree of freedom.

[0064] As shown in Figure 12, an active servo motor 3 is fixedly connected to the active servo motor mounting bracket 10, a servo motor arm 4 is fixedly connected to the output shaft end of the active servo motor 3, a servo motor arm connecting rod 5 is fixedly connected to the servo motor arm 4, and a rigid guide 8 is installed on the servo motor arm connecting rod 5.

[0065] The third fish body segment 22 is provided with a square hole, and the rigid guide 8 passes through the square hole provided on the third fish body joint 22; the rigid guide 8 is made of rigid material.

[0066] In the initial state, the rigid guide 8 is located on the central axis from the first fish-body segment 11 to the third fish-body segment 22, and there is a designed fit clearance between the rigid guide 8 and the inner wall of the square hole of the third fish-body segment 22. The spring 18 is in a state of force equilibrium, forcing each fish-body joint to follow the rigid guide 8 to laterally deflect and maintain in a neutral position.

[0067] During the movement of the mechanism, the active servo motor 3 drives the rigid guide member 8 to sweep in the horizontal plane. The rigid guide member 8 first passes through the aforementioned mating gap, and then contacts and presses against the inner wall of the third fish-body joint 22 on one side, applying an active lateral torque. The rigid guide member 8 and the spring 18 form a push-pull cooperative relationship.

[0068] Pushing phase: The rigid guide 8 overcomes the resistance of the spring 18 and pushes the inner wall to bend the fish body.

[0069] Return phase: When the rigid guide 8 swings back to center and disengages from one side of the inner wall, the stretched spring 18 on the bent outer side releases its elastic potential energy. This elastic tension drives the fish body segment to actively return to its original position, ensuring its trajectory closely follows the movement trend of the rigid guide 8. This alternating combination of rigid pushing drive and elastic passive return eliminates motion lag caused by gaps in the fit. Adjacent fish body segments are strictly geometrically constrained by the X-shaped joint link 20. The X-shaped link restricts axial misalignment and torsion between segments, forcing them to rotate only around the fulcrum formed by the intersection of the X-shaped links, thus converting the lateral thrust of the rigid guide 8 into a smooth, wavy bending motion.

[0070] The online variable stiffness module includes a first drive servo motor 1, a second drive servo motor 2, a first rope 6, a second rope 7, a fish tail 13, a first servo motor wheel 14, a second servo motor wheel 16, a spring 18, an X-shaped joint connecting rod connector 19, and an X-shaped joint connecting rod 20. The first drive servo motor 1 and the second drive servo motor 2 are connected in series and fixedly connected inside the fish head shell 9 along the central axis of the fish head. The first servo motor wheel 14 is fixedly connected to the output shaft of the first drive servo motor 1, and the second servo motor wheel 16 is fixedly connected to the output shaft of the second drive servo motor 2. The first rope 6 is knotted and wound on the first servo motor wheel 14, and the second rope 7 is knotted and wound on the second servo motor wheel 16.

[0071] The first rope 6 and the second rope 7 have high modulus and low creep properties;

[0072] Inside the fish head shell 9, the first rope 6 is led out from the first servo wheel 14, passes directly through the left through hole on the active servo mounting bracket 10 and the left through holes on each fish body segment, and is finally fixedly connected to the fish tail 13; the second rope 7 is led out from the second servo wheel 16, passes directly through the right through hole on the active servo mounting bracket 10 and the right through hole on each fish body segment, and is finally fixedly connected to the fish tail 13.

[0073] In the initial state, the first rope 6 and the second rope 7 are under basic tension, the spring 18 remains balanced, and the X-joint link 20 is in a neutral position. During mechanism movement, the online variable stiffness module and the swing drive module work closely together.

[0074] When the active servo 3 controls the rigid guide 8 to swing to the left, the first drive servo 1 controls the first rope 6 and the second drive servo 2 controls the second rope 7 in an active differential operation: the first drive servo 1, as the active tensioning side, performs a winding action, pulling the first rope 6; the second drive servo 2, as the cooperating side, performs a releasing action, loosening the second rope 7 to allow the fish body to extend. If the guide rod swings to the right, the winding and releasing actions of the front and rear servos are interchanged. This logic ensures that the cable tension is always actively applied to the inside of the fish body's bend, thereby most effectively controlling the joint shape.

[0075] During the aforementioned active oscillation process, the degree of retraction of the active tension-side servo is adjustable, which directly determines the rotational stiffness of the joint. In high-stiffness mode, the first drive servo 1 performs a large-amplitude retraction, applying a very large pulling force. This forces the left spring 18 to compress violently, generating a huge preload inside the joint, making the fish body "rigid" and difficult to change shape by external disturbances. In low-stiffness mode, the first drive servo 1 performs a small-amplitude retraction, applying a smaller pulling force. At this time, the left spring 18 is only slightly compressed, the preload inside the joint is small, and the fish body is in a "flexible" state. By adjusting the magnitude of the retraction pulling force of the active side servo, stepless adjustment of the joint stiffness can be achieved.

[0076] When the cable is subjected to high tensile force to obtain high stiffness, the fish body segments are subjected to enormous axial pressure. At this time, the X-shaped joint link 20 plays a crucial role in shear resistance and guidance due to its cross-support structure. It can effectively lock the lateral shear freedom between adjacent segments, preventing the fish body from being misaligned or twisted due to excessive cable tension on one side. This structure ensures that the cable tension can be purely and efficiently converted into the axial compression of the spring 18, thereby ensuring the stability and safety of rotational stiffness adjustment.

[0077] The oscillation frequency of the robotic fish is determined by the speed of movement of the active servo motor 3 and the rigid guide 8; while the stiffness of the left and right sides of the robotic fish is determined by the tension of the cables of the first drive servo motor 1 and the second drive servo motor 2. At the same oscillation frequency, the rotational stiffness of the fish body can be freely changed by increasing or decreasing the cable tension. Therefore, the oscillation frequency and stiffness change are independent of each other, achieving complete decoupled control.

[0078] It should be understood that the above division between functional units is only for illustrating the functional logic. In the specific implementation process, each functional unit can be implemented by an independent hardware module or integrated into the same processing module. This disclosure does not limit this.

Claims

1. A dynamically variable stiffness biomimetic robotic fish, comprising a head, body, and tail, characterized in that: The fish body includes at least two fish body segments connected in series along the axial direction; an X-shaped joint is provided at the connection between the fish body segment and the tail (13), or at the connection between adjacent fish body segments; a spatially distributed dual-channel drive system is constructed inside the fish head, including an independent swing module and an online variable stiffness module: the swing module includes a swing drive component and a rigid guide (8) extending along the central axis of the fish body, the rigid guide (8) passes through the central space of each level of fish body segment under the drive of the swing drive component, and is used to apply lateral thrust to the fish body segment to establish the swing trajectory; the online variable stiffness module includes a variable stiffness drive component and flexible transmission components distributed along both sides of the fish body, the flexible transmission components pass through each level of fish body segment under the drive of the variable stiffness drive component, and are used to apply axial tension to the fish body segment to adjust the rotational stiffness of the X-shaped joint; the swing module and the online variable stiffness module are spatially isolated from each other, and independently control the swing frequency and rotational stiffness of the robot fish.

2. The biomimetic robotic fish with dynamic variable stiffness according to claim 1, characterized in that: The fish head includes a fish head shell (9) and an active servo mount (10) fixedly connected to the fish head shell (9).

3. The biomimetic robotic fish with dynamic variable stiffness according to claim 2, characterized in that: The swing drive assembly includes an active servo (3) fixed inside the fish head shell (9), a servo arm (4) connected to the output shaft of the active servo (3), and a servo arm connecting rod (5) connecting the servo arm (4) and the rigid guide (8); the active servo (3) drives the rigid guide (8) to swing through the servo arm (4) and the servo arm connecting rod (5); the variable stiffness drive assembly includes a first drive servo (1) and a second drive servo (2) installed inside the fish head shell (9); the flexible transmission component includes a first rope (6) connected to the first drive servo (1) and a second rope (7) connected to the second drive servo (2), the first rope (6) and the second rope (7) being distributed along both sides of the fish body.

4. The biomimetic robotic fish with dynamic variable stiffness according to claim 3, characterized in that: The active servo (3) in the swing module is fixed on the active servo mounting bracket (10); a square hole is provided on the fish body segment, and a rigid guide (8) passes through the square hole. A fitting gap is designed between the rigid guide (8) and the inner wall of the square hole. The fish body segment is driven to swing by pushing the inner wall of the square hole through the rigid guide (8).

5. The biomimetic robotic fish with dynamic variable stiffness according to claim 4, characterized in that: The fish head is also provided with a first rope guide (15) and a second rope guide (17); the first rope (6) passes around the first rope guide (15) and then passes through the fish body segment and is fixed to the fish tail (13); the second rope (7) passes around the second rope guide (17) and then passes through the fish body segment and is fixed to the fish tail (13).

6. The biomimetic robotic fish with dynamic variable stiffness according to claim 4, characterized in that: The first drive servo (1) and the second drive servo (2) are arranged in series along the central axis of the fish head; the first rope (6) is led out from the first drive servo (1), passes directly through the active servo mounting bracket (10) and the through hole on one side of each fish body segment, and is fixed to the fish tail; the second rope (7) is led out from the second drive servo (2), passes directly through the active servo mounting bracket (10) and the through hole on the other side of each fish body segment, and is fixed to the fish tail.

7. The biomimetic robotic fish with dynamic variable stiffness according to claim 1, characterized in that: The X-shaped joint includes a cross-link assembly connecting adjacent fish body segments or between a fish body segment and a fish tail; the cross-link assembly is configured to: limit lateral shear displacement and axial misalignment between adjacent segments, and force adjacent segments to rotate about a fixed axis around a fulcrum formed by the intersection of the cross-link assembly; the X-shaped joint also includes a spring (18), which is connected between adjacent segments and is arranged in parallel with the cross-link assembly to provide initial preload and elastic restoring force.

8. The biomimetic robotic fish with dynamic variable stiffness according to claim 7, characterized in that: The cross linkage assembly includes four X-shaped joint links (20), which are hinged between eight X-shaped joint link connectors (19). The four X-shaped joint links (20) form two X-shaped structures that are symmetrically arranged in a cross configuration.

9. A biomimetic robotic fish with dynamic variable stiffness according to claim 7, characterized in that: The spring (18) is installed in a detachable or hook-on manner; along the fish body from front to back, the springs (18) connected between different segment groups are configured to have the same or different elastic coefficients, thereby forming a preset stiffness gradient in the axial direction of the fish body.

10. A dynamically variable stiffness biomimetic robotic fish according to claim 3, characterized in that: The online variable stiffness module is configured to execute tension modulation logic: the tension provided by the first drive servo (1) to the first rope (6) is denoted as F1, and the tension provided by the second drive servo (2) to the second rope (7) is denoted as F2; ​​during the process of the active servo (3) driving the fish body to swing, the first drive servo (1) and the second drive servo (2) adjust F1 and F2 in real time, so that the tension of the rope located on the inside of the bend increases, and the tension of the rope located on the outside of the bend decreases or is released in the opposite direction, so as to generate a torque consistent with the direction of the fish body swing; at the same time, different stiffness modes are set by adjusting the change amplitude of F1 and F2: when high stiffness is required, F1 and F2 are controlled to perform differential changes with a larger force; when low stiffness is required, F1 and F2 are controlled to perform differential changes with a smaller force, thereby realizing online stepless adjustment of stiffness.

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