Variable stiffness imitation tuna type robotic fish and fish tail stiffness online regulation method

Abstract

The present application relates to a variable stiffness imitation tuna type robot fish and a fish tail stiffness online regulation method, comprising: a robot fish waterproof shell, a driving structure, a side fin, an online variable stiffness flexible fish tail and a control system; the overall shape of the robot fish waterproof shell is designed to imitate the streamlined spindle structure of the scombridae tuna, and the waterproof shell is mainly divided into a rigid fish head shell at the front end and a tail waterproof flexible silica gel film at the rear end, the rigid fish head shell is internally provided with a driving structure and a control system; the variable stiffness flexible fish tail is located inside the tail waterproof flexible silica gel film, the air pressure inside the variable stiffness actuator is adjusted by an air pump to realize the online continuous regulation of the variable stiffness flexible fish tail; the present application can adjust the tail stiffness according to different working conditions such as straight running, turning, cruising and obstacle avoidance, and can balance the high-speed propulsion efficiency and flexible maneuvering performance, and is suitable for underwater inspection, ecological monitoring and complex narrow environment detection scenes.

Description

Technical Field

[0001] This invention belongs to the field of biomimetic underwater robot and intelligent mechatronics control technology, and relates to a variable stiffness tuna-shaped robotic fish and a method for online control of tail stiffness. In particular, it relates to a method for online control of tail stiffness of a tuna-shaped robotic fish based on an aerodynamic particle blocking mechanism to achieve online continuous variable stiffness control for multi-mode efficient swimming scheduling. Background Technology

[0002] The ocean, covering approximately 71% of the Earth's surface, is the largest habitable and resource-rich ecosystem on Earth. With the increasing national strategic needs for marine resource development, marine ecological environment monitoring, and deep-sea underwater engineering operations, extremely stringent requirements are being placed on underwater equipment capable of autonomous, covert, and stable operation in complex and dynamic hydrodynamic environments. Compared to traditional rigid propeller-driven underwater vehicles (AUVs / ROVs), biomimetic robotic fish overcome their inherent drawbacks such as large size, low system energy efficiency, and susceptibility to strong eddy current interference in the surrounding fluid environment. Due to their significant biomimetic advantages in propulsion efficiency, high maneuverability, noise stealth, and ability to navigate confined spaces, utilizing biomimetic robotic fish to replace traditional equipment in underwater inspection, detection, and data acquisition tasks has become a core development direction in the field of modern marine robotics engineering.

[0003] Based on systematic research into fish propulsion dynamics, the body / tail fin propulsion mode is the primary propulsion method for the vast majority of fish to achieve rapid, long-distance swimming. According to the proportion of body length involved in wave propulsion and the distribution of wave amplitude, the BCF propulsion mode can be further subdivided into the anguilliform, subcarangiform, carangiform, and thunniform. Among these, the thunniform, represented by tuna and swordfish, maintains near-rigidity in most areas of its streamlined body during swimming, with intense wave action concentrated at the rapidly swinging caudal peduncle and the high aspect ratio crescent-shaped caudal fin. This mode boasts the highest propulsion efficiency among all aquatic organisms.

[0004] To mimic the remarkable performance of these organisms, existing research on robotic fish has explored various complex propulsion mechanisms. Currently, the mainstream propulsion methods in academia and industry include: multi-servo motor tandem coordinated propulsion, propulsion combined with smart materials, and hydraulic or pneumatic propulsion. Although linearly driven robotic fish have advantages in system simplification, all current research is constrained by a common and extremely difficult-to-overcome technical bottleneck at the structural mechanics level—the design of a fixed-stiffness tail skeleton. In the real world, fish do not swim with constant body stiffness. Instead, they can dynamically and significantly adjust their body stiffness in real time through the intense contraction of muscles distributed along both sides of the spine, in coordination with collagen fibers and ligaments, to perfectly match the hydrodynamic requirements of their current swimming state. Faced with different propulsion frequencies or complex maneuvering commands, fixed-stiffness robotic fish often have to compromise: if rigid materials are used to increase straight-line speed, the turning radius of the robotic fish will be extremely large; if extremely soft materials are used to ensure maneuverability, the tail will experience severe excessive bending due to water resistance during acceleration, with a large amount of input energy dissipated by the parasitic deformation of the structure, resulting in a precipitous drop in propulsion efficiency. The physical contradiction between "efficient propulsion" and "flexible steering" severely limits the application prospects of biomimetic robotic fish in practical marine engineering. Currently, this field urgently needs a novel variable stiffness actuation mechanism that can simultaneously meet the requirements of wide-range continuous adjustment, high-frequency rapid response, lightweight design, and physical reliability. This is precisely the core technical challenge that this invention aims to overcome. Summary of the Invention

[0005] In view of this, in order to solve the problem that existing biomimetic robotic fish generally adopt a fixed stiffness tail skeleton design, which leads to an irreconcilable physical contradiction between efficient propulsion and flexible steering, and that there is currently a lack of a variable stiffness execution mechanism that can simultaneously meet the requirements of wide-range continuous adjustment and high-frequency fast response, thus affecting their applicable scenarios, this invention provides a variable stiffness tuna-like robotic fish and a method for online control of tail stiffness. It innovatively introduces the phase transition mechanism of granular media into soft robot actuators, constructing a novel structure that can dynamically adjust bending stiffness over a wide range, and endowing the robotic fish with the physical property of online control of tail stiffness.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A variable stiffness tuna-shaped robotic fish includes a front rigid fish head shell, a variable stiffness flexible fish tail wrapped in a streamlined waterproof silicone shell, and a fish head and tail fixing plate for connecting the rigid fish head shell and the flexible fish tail.

[0008] The rigid fish head shell contains a control motherboard that transmits command signals, a battery that provides power to various devices, a side fin servo that drives the side fin to rotate, and an air pump and servo mounting plate that are fixedly installed on the fish head and fish tail mounting plates. The servo mounting plate is fixedly installed with a rudder disk and a servo servo that provides rotation power to the rudder disk. A cable tray is installed on the rudder disk.

[0009] The flexible fishtail contains a variable stiffness actuator based on a pneumatic particle blocking mechanism, joint rings spaced along the axial direction of the variable stiffness actuator, and pull lines running through the left and right sides of the joint rings. One end of the pull line is wound around a cable reel, and the other end is fixed to the end of the fishtail or the base of the tail fin. The variable stiffness actuator includes an outer variable stiffness hose and an inner variable stiffness hose that are coaxially nested and supported by a support spring. The inner variable stiffness hose is filled with filler particles. An air chamber cover is provided at the free end of the variable stiffness actuator away from the fishtail. An outer air chamber hose and an inner air chamber hose connected to an air pump are provided on the air chamber cover. The end of the outer air chamber hose away from the air pump is connected to an annular sandwich air chamber formed by the outer and inner variable stiffness hoses. The end of the inner air chamber hose away from the air pump is connected to an inner air chamber inside the inner variable stiffness hose.

[0010] The rigid fish head shell is a closed, rigid, transparent, and waterproof shell. A large-capacity cylindrical battery pack is located at its front bottom, serving as an underwater ballast tank and providing power to the entire robotic fish. The control board is located in the middle of the rigid fish head shell, directly above the battery, and is fixed by a support plate. It receives external control commands or internal attitude detection signals and outputs servo drive signals and air pump control signals according to a preset control strategy, thereby achieving tail swaying, tail stiffness adjustment, side fin attitude adjustment, and multi-mode swimming control. The air pump forms a circulating air path through an annular interlayer air chamber between the outer and inner variable stiffness hoses of the variable stiffness actuator, and an inner air chamber inside the inner variable stiffness hose. The air pump controls the evacuation, pressure holding, and pressure release of the annular interlayer air chamber and the inner air chamber.

[0011] In this invention, the core component that imparts variable stiffness characteristics to the robotic fish is a Particle Jamming Variable Stiffness Actuator (PJVSA) located in the central axis region of the flexible tail. This actuator abandons traditional mechanical stiffness adjustment mechanisms, instead employing a spatial structure combining a flexible substrate, an elastic support, and discrete particle media. Specifically, the PJVSA uses coaxially nested inner and outer double-layered high-polymer silicone tubing, with a supporting spring embedded axially between the inner and outer tubing walls as a framework. The inner silicone tubing cavity is filled with specific polystyrene (PS) microspheres. The physical mechanism of the actuator's variable stiffness is as follows: when the system is at rest or connected to ambient air pressure (0 kPa) via a valve, the particle media within the cavity are in loose contact, allowing the particles to slide or rearrange relative to each other under external force. At this point, the actuator exhibits extremely low bending stiffness on a macroscopic scale, with a highly compliant overall structure, primarily relying on an internal spring skeleton for weak support to maintain its shape. When the micro-pump extracts air from the chamber, creating a high vacuum negative pressure (up to 80 kPa), external water pressure or atmospheric pressure causes the flexible hose to contract rapidly inward, forcefully compressing the originally loose PS microspheres inside. Due to the exponential increase in the normal contact pressure between the particles, a huge dry friction locking force network is formed between the particles and between the particles and the tube wall. The originally discrete particle group is instantly locked into a "solid-like" block structure with extremely high shear modulus through a "blocking phase transition," thereby increasing the actuator's macroscopic equivalent bending stiffness by orders of magnitude.

[0012] Furthermore, the robotic fish has a top fin on its back to improve its posture stability, side fins on both sides to assist in posture adjustment, and a tail fin at the end of its tail to generate propulsion and steering torque during its swing.

[0013] Furthermore, the variable stiffness actuator is provided with multiple joint rings at axial intervals. Each joint ring has a central constraint hole at its center for the variable stiffness actuator to pass through, and lateral guide holes on the left and right sides for the pull wire to pass through.

[0014] The articulated ring is used to constrain the position of the pull line relative to the flexible fishtail, so that the tightening of the pull line can be effectively converted into the bending deformation of the flexible fishtail.

[0015] Furthermore, the outer contour of the joint ring is set as a wavy, toothed, or concave-convex contour to enhance the connection stability between it and the outer flexible shell and prevent the flexible shell from slipping relative to the fish tail during high-frequency swinging.

[0016] Furthermore, the joint ring, pull wire, and variable stiffness actuator are covered with a waterproof silicone fish tail. The silicone fish tail and the tail part at the tail fin position are both made of silicone, which together form a waterproof shape that integrates the back and tail of the robotic fish, and gives the tail a continuous streamlined outer contour.

[0017] Furthermore, the filler particles are polystyrene microspheres with a particle size of 2 mm, and the wire diameter of the support spring is set to 0.9 mm.

[0018] Furthermore, a filter screen is embedded in the air chamber cover. The filter screen is located at the connection between the external air chamber hose, the internal air chamber hose and the air chamber cover to prevent filling particles from entering the air pipe or air pump, thereby ensuring the stable operation of the air circuit system.

[0019] Furthermore, a turntable-side groove is provided on the cable tray, and a vertical plate is provided between the cable tray and the fish head and fish tail fixing plate to facilitate the opening of cable holes and fixation on the servo fixing plate. The pull wires pass through the corresponding cable holes and are then wound in the opposite direction and fixed in the turntable-side groove of the cable tray.

[0020] The beneficial effects of this invention are as follows:

[0021] 1. The variable stiffness tuna-inspired robotic fish structure disclosed in this invention achieves, for the first time, wide-range and large-amplitude online smooth stiffness control within a single continuous structure. Based on the principle of diffuse blockage phase transition, this invention enables the variable stiffness actuator (PJVSA) to achieve stiffness step changes without altering any physical topology or relying on complex mechanical gear structures. Testing has verified that within a continuous air pressure adjustment range of 0-80 kPa, the equivalent absolute stiffness at the tail of the variable stiffness actuator can precisely jump from an initial compliant state to a dense and rigid state, resulting in a significant change in stiffness. The adjustment response time is in the millisecond range, providing a solid hardware foundation for the robotic fish's real-time adaptation to fluid environments.

[0022] 2. The variable stiffness tuna-inspired robotic fish structure disclosed in this invention fundamentally overcomes the physical contradiction between "propulsion efficiency" and "maneuverability" in existing robotic fish, achieving a significant increase in high-speed straight-line swimming speed. During straight-line swimming, because this invention can increase the tail stiffness (80 kPa) at the optimal operating frequency (3 Hz), the robotic fish's tail exhibits excellent reverse momentum exchange capability when slapping the water flow. Experiments have confirmed that, under high stiffness conditions, the robotic fish achieves a breakthrough straight-line cruising speed of 50.02 cm / s, representing an astonishing 55.05% increase compared to the system without variable stiffness (i.e., under low stiffness 0 kPa conditions). This not only verifies the theoretical predictions of slender body hydrodynamics but also demonstrates the decisive role of high stiffness in overcoming fluid resistance.

[0023] 3. The variable stiffness tuna-inspired robotic fish structure disclosed in this invention endows the underwater vehicle with extremely strong maneuverability in confined environments, enabling agile turning and obstacle avoidance. This is often a weakness of traditional rigid AUVs, but by releasing the negative pressure (0 kPa) of the variable stiffness actuator, the robotic fish of this invention immediately transforms into a flexible, soft-bodied creature. With extremely high compliance, the robotic fish generates a tremendous steering torque at 3 Hz drive, achieving an exceptionally high maximum turning angular velocity of 93.91° / s. This figure represents a 193.73% improvement compared to its turning ability under rigid, high-stiffness (80 kPa) conditions. This characteristic ensures the robotic fish's absolute survivability and maneuverability in complex deep-water environments such as near reefs and pipeline networks.

[0024] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0025] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0026] Figure 1 This is a schematic diagram of the overall structure of the variable stiffness tuna-shaped robotic fish of the present invention;

[0027] Figure 2 This is a schematic diagram of the drive mechanism in the present invention;

[0028] Figure 3 Schematic diagram of servo turntable and cable tray;

[0029] Figure 4 This is a schematic diagram of the biomimetic side fin of the present invention;

[0030] Figure 5 This is a schematic diagram of the connection device between the air pump and the variable stiffness actuator in this invention;

[0031] Figure 6 This is a schematic diagram of the variable stiffness actuator of the present invention;

[0032] Figure 7 This is a simplified model for PJVSA modeling in this invention;

[0033] Figure 8 This is a cross-sectional view of the PJVSA model in this invention.

[0034] Figure labeling: 1-Control mainboard; 2-Battery; 3-Air pump; 4-Side fin; 5-Top fin; 6-Joint ring; 7-Silicone fish tail; 8-Pull line; 9-Filling particles; 10-Support spring; 11-Fish tail fin retainer; 12-Cut fin; 13-Cable bundle; 14-Servo mounting plate; 15-Cable bundle hole; 16-Fish head and tail mounting plate; 17-Turntable side groove; 18-Servo servo; 19-Servo disc; 20-Side fin servo; 21-External air chamber hose; 22-Internal air chamber hose; 23-Air chamber cover; 24-Filter screen; 25-Outer layer variable stiffness hose; 26-Inner layer variable stiffness hose. Detailed Implementation

[0035] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0036] like Figure 1 The variable stiffness tuna-inspired robotic fish shown is designed to mimic the streamlined, spindle-shaped structure of a tuna. The front of the robotic fish is a rigid head shell, while the rear is a flexible tail. The rigid head shell houses the control system and tail drive structure. A silicone tail 7 covers the back and tail of the robotic fish. The silicone tail 7 uses silicone to encapsulate the variable stiffness actuator. A mold inside the flexible tail leaves sufficient space for the movement of the pull wire 8, creating a low-resistance, streamlined structure that conforms to the biomimetic shape of a tuna. The flexible tail contains a joint ring 6, a pull wire 8, and a variable stiffness actuator based on a pneumatic particle blocking mechanism. An air pump regulates the air pressure inside the actuator to achieve continuous online adjustment of the flexible tail's stiffness.

[0037] The rigid head shell at the front and the flexible tail at the rear of the robotic fish are connected by a head-tail fixing plate 16. The head-tail fixing plate 16 serves two purposes: firstly, it provides a mechanical connection between the rigid head shell and the flexible tail; secondly, it provides a transitional mounting position for the tail drive cable 8, air lines, and electrical wires. A top fin 5 is located on the back of the robotic fish to improve its posture stability during swimming. Side fins 4 are symmetrically located on both sides of the robotic fish and connected to the servo motors 20 that drive the side fins, assisting the robotic fish in posture adjustment, steering control, and motion stability control. A tail fin 12 is located at the end of the flexible tail and interacts with the water during tail swaying to generate propulsion and steering torque.

[0038] Combination Figure 1As shown, the internal control system of the rigid fish head shell includes a control motherboard 1, a battery 2, an air pump 3, a servo motor 18, a side fin servo motor 20, and corresponding power management units, communication units, attitude detection units, and pneumatic control units. The control motherboard 1 is electrically connected to the servo motor 18, the side fin servo motor 20, the air pump 3, and the pneumatic control unit. The control motherboard 1 receives external control commands or internal attitude detection signals and outputs servo motor drive signals and air pump control signals according to a preset control strategy, thereby realizing tail swaying, tail stiffness adjustment, side fin attitude adjustment, and multi-mode swimming control. The battery 2 is located at the front of the rigid fish head shell. On the one hand, it powers the control motherboard 1, the air pump 3, the servo motor 18, the side fin servo motor 20, and the communication module. On the other hand, it serves as an internal counterweight component of the robotic fish, which helps to adjust the overall center of gravity and buoyancy, improving the attitude stability of the robotic fish during underwater movement. The air pump 3 is connected to the variable stiffness actuator at the tail through an air pipe. The air pump 3 may include a miniature air pump, a miniature negative pressure pump, and an air distribution solenoid valve. Under the control of the main control board 1, the air pump 3 pumps, maintains, or depressurizes the particle-filled cavity inside the variable stiffness actuator to adjust the internal air pressure state of the variable stiffness actuator, thereby realizing the online continuous adjustment of the bending stiffness of the flexible fishtail.

[0039] Combination Figure 2 and Figure 3 As shown, the tail drive structure inside the rigid fish head shell includes a servo motor 18, a servo motor mounting plate 14, a servo disc 19, a cable tray 13, cable holes 15, a turntable side groove 17, and high-strength cables 8. The servo motor 18 is fixed inside the rigid fish head shell of the robotic fish via the servo motor mounting plate 14. The output shaft of the servo motor 18 is connected to the servo disc 19, which has a cable tray 13 for winding the cables 8. The cable tray 13 has a turntable side groove 17. The cable tray 13 is located between the servo disc 19 and the flexible fish tail, and is used to guide and bundle the cables 8 on both sides. A vertical plate is provided between the cable tray 13 and the fish head / tail mounting plate 16 to facilitate the opening of the cable holes 15 and to be fixed on the servo motor mounting plate 14. The high-strength cables 8 on both sides pass through the corresponding cable holes 15, are wound in the opposite direction, and are fixed in the turntable side groove 17 of the cable tray 13.

[0040] The pull lines 8 are symmetrically arranged on both sides of the flexible fish tail, passing sequentially through the constraint holes on the outer sides of multiple joint rings 6 distributed in the variable stiffness flexible fish tail. The rear end of the pull lines 8 is fixed to a fixed position at the end of the fish tail or the base of the tail fin 12. The pull lines 8 on both sides are arranged in a reverse winding manner on the cable tray 13, so that when the rudder 19 drives the cable tray 13 to rotate, one side of the pull line 8 is tightened and the other side of the pull line 8 is released by an equal amount. When the servo motor 18 rotates back and forth according to the periodic signal output by the control motherboard 1, the rudder 19 drives the pull lines 8 on both sides to alternately tighten and release through the cable tray 13. The pull line 8 on the tightened side generates a lateral tension on the joint ring 6, causing the flexible fish tail to bend to that side; at the same time, the pull line 8 on the other side is released, without significantly hindering the bending of the flexible fish tail. Through the periodic forward and reverse rotation of the servo motor 18, the flexible fish tail can generate continuous left and right swings, thereby driving the tail fin 12 to slap the water and generate propulsion.

[0041] This embodiment adopts a single servo wire drive structure, which has advantages such as compact structure, simple transmission chain, lighter weight, and lower waterproofing difficulty compared to the multi-servo series drive method. By using the joint ring 6 to spatially constrain the pull wire 8, the fish tail can maintain a relatively smooth and continuous bending shape when driven by the pull wire.

[0042] Combination Figure 1 , Figure 5 , Figure 6 , Figure 7 and Figure 8As shown, the flexible fishtail internally houses a joint ring 6, a pull wire 8, and a variable stiffness actuator based on a pneumatic particle blocking mechanism. The variable stiffness actuator, arranged axially along the fishtail, is the core structure for online adjustment of the fishtail stiffness. This variable stiffness actuator employs a pneumatic particle blocking structure, comprising an outer air chamber hose 21, an inner air chamber hose 22, an air chamber cover 23, a filter screen 24, filling particles 9, a support spring 10, an outer variable stiffness hose 25, and an inner variable stiffness hose 26. Specifically, the outer variable stiffness hose 25 and the inner variable stiffness hose 26 are coaxially nested, supported by a spiral support spring 10. The inner variable stiffness hose 26 is filled with filling particles 9, which are polystyrene microspheres. The air chamber cover 23 is located at the end of the variable stiffness actuator and seals the annular particle cavity formed by the outer variable stiffness hose 25, the support spring 10, the inner variable stiffness hose 26, and the filling particles 9. The air chamber cover 23 is equipped with an external air chamber hose 21 and an internal air chamber hose 22, both connected to the air pump 3. One end of the external air chamber hose 21 is connected to the air pump 3, and the other end is connected to the annular sandwich air chamber formed by the outer variable stiffness hose 25 and the inner variable stiffness hose 26. One end of the internal air chamber hose 22 is connected to the air pump 3, and the other end is connected to the inner air chamber inside the inner variable stiffness hose 26. A filter screen 24 is located in the air path connection area, i.e., at the connection between the internal air chamber hose 22, the external air chamber hose 21, and the air chamber cover 23, to prevent the filling particles 9 from entering the air pipe or the air pump 3, thereby ensuring the stable operation of the air path system.

[0043] The particle size of the filler granules 9 is set to 2 mm, the wire diameter of the support spring 10 is set to 0.9 mm, and the wall thickness of the external air chamber hose 21 is set to 1.0 mm. Under the condition that the air pump 3 provides a maximum negative pressure of approximately 80 kPa, the variable stiffness actuator can achieve a large stiffness adjustment range. The above parameters are only one preferred embodiment; those skilled in the art can adaptively adjust the particle size, spring wire diameter, hose wall thickness, cavity size, and negative pressure range according to the size of the robotic fish, target swimming speed, tail swing amplitude, drive frequency, and hydrodynamic load requirements.

[0044] The joint rings 6 are spaced apart along the axial direction of the variable stiffness actuator. Each joint ring 6 has a central constraint hole for the variable stiffness actuator to pass through, and lateral guide holes for the pull wire 8 to pass through on both sides of the joint ring 6. The joint rings 6 are used to constrain the position of the pull wire 8 relative to the flexible fish tail, so that the tightening of the pull wire 8 can be effectively converted into the bending deformation of the flexible fish tail. At the same time, the outer side of the joint ring 6 can be provided with a wavy, toothed, or concave-convex contour to enhance the connection stability between it and the outer silicone fish tail 7, and prevent the silicone fish tail 7 from slipping relative to the fish tail during high-frequency swinging. The silicone fish tail 7 covering the joint rings 6, the pull wire 8, and the fish tail portion at the position of the variable stiffness actuator and the tail fin 12 is all made of silicone material, forming an integrated waterproof shape of the back and tail of the robotic fish, and giving the tail a continuous streamlined outer contour. The silicone fish tail 7 can be made of silicone, rubber, or other flexible waterproof materials.

[0045] The stiffness adjustment of the variable stiffness actuator is based on the pneumatic particle blocking mechanism. When the inner air chamber inside the inner layer of the variable stiffness hose 26 of the actuator is connected to the external air pressure or is in a low pressure differential state, the contact force between the filling particles 9 is small, and the particles can undergo relative slippage and rearrangement under external load. At this time, the variable stiffness actuator exhibits a low bending stiffness state, and the overall compliance of the flexible fishtail is relatively high, making it suitable for performing highly maneuverable actions such as small-radius turns, obstacle avoidance, and rapid attitude adjustment.

[0046] When the air pump 3 evacuates air from the inner air chamber containing the filling particles 9 through the internal air chamber hose 22, creating a negative pressure inside the inner air chamber, the normal contact force between the filling particles 9 and between the filling particles 9 and the cavity wall increases significantly. As the contact force increases, the friction between the particles strengthens, and the originally loose particle group gradually forms a solid-like structure with overall load-bearing capacity. At this time, the variable stiffness actuator enters a high stiffness state, which can effectively suppress excessive bending of the fish tail under hydrodynamic loads and improve the propulsion efficiency of the robotic fish when moving straight at high speed.

[0047] When the control board 1 controls the air pump 3 or the solenoid valve to release the negative pressure, the internal pressure of the particle chamber is restored, the contact constraint between the filling particles 9 is weakened, the particles regain their relative sliding ability, and the variable stiffness actuator returns to its low stiffness state. Thus, the robotic fish can achieve online, continuous, and reversible adjustment of the stiffness of its flexible tail without changing the mechanical topology of the tail.

[0048] In this embodiment, the robotic fish can switch between different swimming modes according to different task requirements, including high-speed straight-line mode, low-speed cruising mode, fast turning mode, and obstacle avoidance mode in complex environments.

[0049] In high-speed straight-line mode, the control board 1 controls the air pump 3 to evacuate air from the inner air chamber containing the filling particles 9 inside the variable stiffness actuator, creating a high negative pressure. For example, under a negative pressure of 80 kPa, the filling particles 9 become significantly blocked, increasing the overall bending stiffness of the flexible fish tail. Simultaneously, the control board 1 controls the servo motor 18 to rotate reciprocally at a preset frequency and amplitude, causing the pull line 8 to drive the fish tail to swing left and right. At this time, the high-stiffness fish tail can reduce parasitic deformation under hydrodynamic loads, allowing for more efficient reverse momentum exchange generated by the tail fin 12 slapping water, thereby improving the straight-line propulsion efficiency and cruising speed of the robotic fish.

[0050] In rapid turning mode, the control board 1 controls the air pump 3 or the air distribution solenoid valve to depressurize the variable stiffness actuator, bringing the inner air chamber containing the filling particles 9 close to atmospheric pressure. The filling particles 9 then return to a loose, rearrangeable state, reducing the stiffness of the flexible fishtail. At this time, the control board 1 controls the servo motor 18 to output biased or asymmetrical oscillation, causing the fishtail to bend significantly to one side. The low-stiffness fishtail provides a larger instantaneous bending amplitude, enabling the robotic fish to obtain a greater steering torque, thereby reducing the turning radius and improving maneuverability in complex environments.

[0051] In low-speed cruise mode, the control board 1 can control the variable stiffness actuator to be in a medium stiffness state and reduce the oscillation frequency of the servo motor 18 to reduce energy consumption, which is suitable for long-term underwater inspection or environmental monitoring tasks.

[0052] In complex environment obstacle avoidance mode, the control motherboard 1 can adjust the side fin angle, tail stiffness, and tail swaying frequency in real time according to external control commands, inertial measurement unit feedback, or preset path planning signals. When the robotic fish needs to avoid obstacles, cross narrow areas, or perform on-the-spot turning maneuvers, the system reduces tail stiffness and increases tail offset swaying; when the robotic fish completes obstacle avoidance and resumes straight swimming, the system increases tail stiffness and resumes symmetrical periodic swaying.

[0053] Combination Figure 4 As shown, the side fins 4 are symmetrically arranged on both sides of the robotic fish, and their roots are connected to the output terminals of the side fin servo motors 20. The control motherboard 1 controls the rotation angle of the side fin servo motors 20 to adjust the sway angle of the side fins 4 relative to the body of the robotic fish.

[0054] When the robotic fish swims in a straight line, the two side fins 4 maintain a symmetrical angle to improve its roll and pitch stability. When the robotic fish needs to turn, the two side fins 4 can use differential control, that is, one side fin 4 increases the angle of attack while the other side fin 4 decreases the angle of attack, thereby generating additional turning torque in conjunction with the tail swing. When the robotic fish needs to surface, dive, or correct its attitude, the side fins 4 can synchronously change their angle to provide pitch adjustment torque.

[0055] The top fin 5 is positioned at the midline of the back of the robotic fish. During the robotic fish's swimming process, it plays a directional stabilizing role similar to the dorsal fin of a biological fish, which can reduce yaw disturbances and roll swaying when the robotic fish swims at high speed.

[0056] The assembly process of this variable stiffness tuna-shaped robotic fish may include the following steps:

[0057] First, the control motherboard 1, battery 2, air pump 3, servo motor 18, and side fin servo motor 20 are installed inside the rigid fish head shell and positioned using corresponding mounting brackets. Battery 2 is located at the front or bottom of the fish head shell to adjust the overall center of gravity. The control motherboard 1 is fixed inside the fish head shell by a support plate and is electrically connected to the servo motor 18, side fin servo motor 20, air pump 3, and pneumatic control unit.

[0058] Next, the outer variable stiffness hose 25 and the inner variable stiffness hose 26 are coaxially fitted together. A spiral support spring 10 is added to the annular cavity formed by the two to support the entire structure. Particles 9 are filled into the inner air chamber inside the inner variable stiffness hose 26 of the variable stiffness actuator. An air chamber cover 23 and a filter screen 24 are installed at the end of the variable stiffness actuator to form a sealed, air-extractable structure. Subsequently, one end of the outer air chamber hose 21 and the inner air chamber hose 22 are connected to the annular sandwich air chamber and the inner air chamber, respectively, and the other end is connected to the air pump 3 inside the rigid fish head shell.

[0059] Next, multiple joint rings 6 are sequentially fitted along the axis of the variable stiffness actuator, so that the variable stiffness actuator passes through the central constraint hole of the joint ring 6. The pull wires 8 on the left and right sides are respectively passed through the lateral guide holes of each joint ring 6, and the rear end of the pull wires 8 is fixed at the tail fin retainer 11 at the end of the fish tail or at the fixed position at the root of the tail fin 12.

[0060] Then, pass the front end of the pull cable 8 through the cable bundle hole 15 and fix it to the corresponding groove of the servo disc 19 or the cable bundle disc 13 in a symmetrical, reverse-winding manner. Adjust the initial tension of the pull cable 8 so that the flexible fishtail remains naturally straight when the servo motor 18 is at the mid-position angle.

[0061] Finally, the silicone fish tail 7 is wrapped around the joint ring 6, the pull wire 8, and the variable stiffness actuator, and sealed to the fish head and tail fixing plate 16 and the rear end of the rigid fish head shell. The tail fin 12 is installed on the end of the flexible fish tail, forming a stable connection with it. After assembly, the fish head shell, air passage interface, silicone fish tail 7 connection, and tail end are waterproofed and sealed.

[0062] The variable stiffness tuna-shaped robotic fish provided in this embodiment introduces a particle blocking phase change mechanism into the tail structure by setting a variable stiffness actuator inside the flexible tail, thereby realizing online continuous adjustment of the tail's bending stiffness.

[0063] In this embodiment, the robotic fish adopts a single servo motor cable drive method. Through the cooperation of the servo motor 18, servo disc 19, cable tray 13, cable 8 and joint ring 6, the flexible fish tail can produce smooth and continuous left and right swinging. At the same time, the air pump 3 is used to adjust the internal air pressure of the variable stiffness actuator, so that the filling particles 9 switch between a loose state and a blocked state, thereby allowing the flexible fish tail to switch between a low stiffness compliant state and a high stiffness support state.

[0064] Compared to traditional fixed-stiffness robotic fish, this invention can adjust the tail stiffness according to different hydrodynamic environments and motion tasks, exhibiting excellent adaptability in various working conditions such as high-speed straight-line movement, low-speed cruising, rapid turning, and obstacle avoidance in complex environments. This invention features a compact structure, simple drive mechanism, wide stiffness adjustment range, and fast response speed, making it suitable for underwater inspection, ecological monitoring, narrow space exploration, and research on biomimetic underwater robot platforms.

[0065] The variable stiffness actuator achieves online control of the torque through the pressurization and depressurization of the air pump 3, thereby enabling online control of the variable stiffness flexible fishtail. The fifth principle of the variable stiffness fishtail:

[0066] The pneumatic particle-blocking variable stiffness tail actuator is subjected to both external bending moment and internal resisting bending moment during bending. Under air pressure, the contact state of the particle medium inside the actuator changes, making the overall equivalent stiffness of the structure adjustable. Simultaneously, constrained by the flexible matrix material, the actuator exhibits typical continuous curvature characteristics during bending. Based on the deformation characteristics of the flexible material, the bending shape of the actuator can be approximated as a circular arc with a bending radius of R, an arc length of L, and a bending angle of θ; therefore, θ = L / R holds true. When the air pump inflates, the resulting macroscopic resisting bending moment... Frictional torque M caused by particle blockage p Air pressure causes torque M a Silicone superelastic restoring torque M e and the spring torsional restoring torque M s It is composed of four non-linear superpositions:

[0067]

[0068] Among them, the frictional torque caused by particle blockage:

[0069] The bending moment generated by particle blockage essentially originates from the friction within the particle system and between it and the cavity walls. When no pressure difference is applied to the particle cavity, the particles are in a loosely packed state with weak contact forces between them, resulting in low overall shear resistance. The particles can freely rearrange within the cavity, causing the actuator to exhibit low stiffness and making it prone to bending deformation. As the pressure difference increases, the particle system gradually transforms from a loose state into a "solid-like" structure with overall load-bearing capacity, significantly hindering bending deformation.

[0070] The total frictional torque M between the particles and the inner layer of the variable stiffness hose p :

[0071]

[0072] M p d1, d2, and d3 are the equivalent bending moment generated by particle blockage; d1, d2, and d3 are the radial thicknesses of the outer variable stiffness hose, the annular sandwich air chamber, and the inner variable stiffness hose, respectively; r is the radius of the inner air chamber; R is the equivalent radius of curvature after the actuator is bent; and μ is the equivalent coefficient of friction between the particle and the cavity wall. and These are the parameter variables along the axial and circumferential directions, respectively; the air chamber and the particle chamber are supplied with air pressure P, respectively. a and P g Two of them satisfy P a =-P g , P represents the pressure difference between the two. d =P a -P g =2P a =2P g .

[0073] The torque caused by air pressure:

[0074] The force on the PJVSA actuator under air pressure mainly comes from two parts: one is the torque M generated by the air pressure on the closed surface at the end of the structure. a1， M a2 Secondly, the torque M caused by the rigidification of air pressure. ph .

[0075] The torque M caused by the air pressure at the tip of the outer variable stiffness hose a1 :

[0076]

[0077] The torque M caused by the air pressure at the tip of the inner variable stiffness hose a2 :

[0078]

[0079] Consider the annular air chamber formed between two coaxial flexible tubes, which is filled with a uniform air pressure P. a When no bending occurs, the structure is perfectly axisymmetric about the neutral axis, and the effects of gas pressure in the circumferential direction cancel each other out, producing no net bending moment. When the structure undergoes bending deformation under external forces, the geometry of the gas chamber changes with the curvature; the outer side of the chamber tends to stretch while the inner side tends to compress. The gas needs to continuously do work on the chamber boundary to resist this bending-induced shape change, thus macroscopically manifesting as an equivalent bending moment that resists bending, i.e., the pressure stiffening effect.

[0080] Torque caused by air pressure rigidity :

[0081]

[0082] Total torque M caused by air pressure a :

[0083]

[0084] The torque caused by the silicone:

[0085] The silicone hose serves as the main flexible load-bearing structure of the actuator, and its mechanical response directly affects the overall bending behavior. Ignoring radial stress and assuming that circumferential stress is significantly less than axial stress, the axial stress is taken as the only non-zero principal stress and denoted as s. Under external load, the structure undergoes planar bending, and a constant curvature assumption is adopted along its length: κ = θ / L, where θ is the overall bending angle and L is the effective length. The radius of curvature of the neutral layer is assumed to be R = L / θ.

[0086] Since the cross section of the variable stiffness actuator is a coaxially symmetric annulus, under the pure bending approximation, the neutral axis is taken to pass through the geometric center of the cross section, and the polar coordinates of the cross section (ρ, β) are established, where ρ is the radial distance to the center of the cross section, and β is the circumferential angle related to the normal of the bending plane (sinβ>0 corresponds to the outer bending side, and sinβ<0 corresponds to the inner bending side).

[0087] The torque M caused by silicone e :

[0088]

[0089] Where d1, d2, and d3 are the radial thicknesses of the outer variable stiffness hose, the annular sandwich air chamber, and the inner variable stiffness hose, respectively; r is the radius of the inner air chamber; and c is the initial shear modulus of the material.

[0090] The torque caused by the spring:

[0091] In the PJVSA structure described above, the spring acts as an internal support and restoring element, not only maintaining the structural shape and limiting excessive deformation, but also providing additional elastic restoring torque during actuator bending. When the actuator undergoes bending deformation, the built-in spring will generate corresponding torsional and bending coupled deformation. There is a clear mechanical relationship between its end rotation angle and the bending moment it is subjected to, and the mechanical behavior of the spring in the bending state can be equivalent to the deformation problem of a torsion member.

[0092] The torque M caused by the spring s :

[0093]

[0094] θ is the bending angle at the end of the spring; M is the bending moment of the external load acting on the spring; a is the stress correction factor; b is the rod length correction factor. The spring prestress is denoted by n; the effective number of coils is denoted by D2; the mean diameter of the spring is denoted by φ; the geometric correction factor for the spring is π / 3; G is the shear modulus of the spring material; I p =πd 4 / 32 is the polar moment of inertia of the cross section; d is the spring wire diameter.

[0095] The total equivalent stiffness of the variable stiffness brake PJVSA is represented by the torque caused by particle interference, air pressure, elastic stress, and spring. Total equivalent stiffness:

[0096]

[0097] δ is the rotation angle caused by the external torque.

[0098] The stiffness of the fishtail can be adjusted online by changing the gas pressure of the variable stiffness brake PJVSA.

[0099] The power output system of the robotic fish of this invention adopts a single-motor cable-driven architecture. A high-torque servo motor, installed in front of the waterproof bulkhead at the rear of the fish's head, has its output shaft connected to a double-rail front-end rudder with upper and lower cable grooves. In the flexible tail region, multiple independent rigid joint rings are evenly distributed along the central axis formed by the support springs. These joint rings have a special wavy or gear-shaped cross-sectional outer contour to securely support the external flexible waterproof silicone skin. Simultaneously, each joint ring has a large-diameter central constraint hole at its center for a variable stiffness actuator (including a central main spring and a particle cavity) to pass through, and symmetrical lateral guide holes on both sides of this central hole for high-strength cables to pass through. The high-strength cables on both sides are wound in the opposite direction by the front-end rudder and sequentially pass through the lateral guide holes of each of the aforementioned discrete joint rings, finally being fixed to the rigid end plate at the base of the tail fin. The servo motor rotates and periodically creates a path difference between the two pull wires, thereby forcing each joint ring to deflect relative to the spring axis, which in turn drives the internal variable stiffness actuator and the flexible silicone shell that encloses it to produce a smooth and continuous biomimetic fish tail swing.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. 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 be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A variable stiffness tuna-shaped robotic fish, characterized in that, It includes a front rigid fish head shell, a variable stiffness flexible fish tail wrapped in streamlined waterproof silicone, and a fish head and fish tail fixing plate (16) for connecting the rigid fish head shell and the flexible fish tail. The rigid fish head shell is equipped with a control motherboard (1) that transmits command signals, a battery (2) that provides power to each device, a side fin servo motor (20) that drives the side fin (4) to rotate, and an air pump (3) and a servo motor mounting plate (14) that are fixedly installed on the fish head and tail mounting plate (16). The servo motor mounting plate (14) is fixedly installed with a servo disk (19) and a servo servo motor (18) that provides rotation power to the servo disk (19). The servo disk (19) is equipped with a cable tray (13). The flexible fishtail is equipped with a variable stiffness actuator based on a pneumatic particle blocking mechanism, joint rings (6) spaced apart along the axial direction of the variable stiffness actuator, and pull wires (8) passing through the left and right sides of the joint rings (6). One end of the pull wire (8) is wound on the cable tray (13), and the other end is fixed to the end of the fishtail or the root of the tail fin (12). The variable stiffness actuator includes an outer variable stiffness hose (25) and an inner variable stiffness hose (26) that are coaxially nested and supported by a support spring (10). The inner variable stiffness hose (26) is internally The variable stiffness actuator is filled with filler particles (9) and has an air chamber cover (23) at the free end away from the fish tail. The air chamber cover (23) is provided with an external air chamber hose (21) and an internal air chamber hose (22) that are connected to the air pump (3). The end of the external air chamber hose (21) away from the air pump (3) is connected to the annular sandwich air chamber formed by the outer variable stiffness hose (25) and the inner variable stiffness hose (26). The end of the internal air chamber hose (22) away from the air pump (3) is connected to the inner air chamber inside the inner variable stiffness hose (26).

2. The variable stiffness tuna-shaped robotic fish as described in claim 1, characterized in that, The variable stiffness actuator is provided with multiple joint rings (6) at axial intervals. Each joint ring (6) has a central constraint hole for the variable stiffness actuator to pass through, and lateral guide holes for the pull wire (8) to pass through on the left and right sides of the joint ring (6).

3. The variable stiffness tuna-shaped robotic fish as described in claim 2, characterized in that, The joint ring (6), the pull wire (8), and the outer side of the variable stiffness actuator are covered with a waterproof silicone fish tail (7). The silicone fish tail (7) and the fish tail part at the position of the tail fin (12) are both made of silicone, which together form a waterproof shape of the back and tail of the robot fish and make the tail part have a continuous streamlined outer contour.

4. The variable stiffness tuna-shaped robotic fish as described in claim 3, characterized in that, The air chamber cover (23) is fitted with a filter screen (24). The filter screen (24) is located at the connection between the external air chamber hose (21), the internal air chamber hose (22) and the air chamber cover (23) to prevent filling particles (9) from entering the air pipe or air pump (3) and to ensure the stable operation of the air circuit system.

5. The variable stiffness tuna-shaped robotic fish as described in claim 4, characterized in that, The cable tray (13) has a turntable-side groove (17). A vertical plate is provided between the cable tray (13) and the fish head and tail fixing plate (16) to facilitate the opening of cable holes (15) and to be fixed on the servo fixing plate (14). The pull wire (8) passes through the corresponding cable holes (15) and then winds in the opposite direction and is fixed in the turntable-side groove (17) of the cable tray (13).

6. The variable stiffness tuna-like robotic fish as described in claim 8, characterized in that, The outer contour of the joint ring (6) is set as a wavy, toothed or concave-convex contour.

7. The variable stiffness tuna-shaped robotic fish as described in claim 1, characterized in that, The robotic fish has a top fin (5) on its back to improve its posture stability, and side fins (4) on both sides to assist in posture adjustment. The tail of the robotic fish has a tail fin (12) at the end to generate propulsion and turning torque during the swinging process.

8. The method for online adjustment of the tail stiffness of a variable stiffness tuna-shaped robotic fish according to any one of claims 1 to 7, characterized in that, Macroscopic resistance to bending moment of variable stiffness actuator under external load Frictional torque M caused by particle blockage p Air pressure causes torque M a Silicone superelastic restoring torque M e and the spring torsional restoring torque M s It consists of four nonlinear superpositions, satisfying the mechanical equilibrium equations: The variable stiffness actuator achieves the effect of applying pressure and depressurization to the particles by pressurizing and depressurizing the air pump (3). p Air pressure causes torque M a Silicone superelastic restoring torque M e and the spring torsional restoring torque M s This allows for online control of the variable stiffness flexible fishtail.

9. The online control method for fishtail stiffness according to claim 8, characterized in that, Particle blockage friction torque M p The total frictional torque between the particles and the inner variable stiffness flexible hose is calculated using the following formula: Where d1, d2, and d3 are the outer layer variable stiffness hose, the annular sandwich air chamber, and the inner layer, respectively, which are the radial thicknesses of the variable stiffness hose; r is the radius of the inner air chamber; R is the equivalent radius of curvature after the actuator is bent; L is the arc length; θ is the bending angle; θ = L / R; and μ is the equivalent friction coefficient between the particle and the cavity wall. and These are the parameter variables along the axial and circumferential directions, respectively; the air chamber and the particle chamber are supplied with air pressure P, respectively. a and P g Two of them satisfy P a =-P g , P represents the pressure difference between the two. d =P a -P g =2P a =2P g ; The torque exerted by air pressure on the variable stiffness actuator consists of three parts: first, the torque M exerted by air pressure on the outer variable stiffness hose around the base origin O on the outer hose end face. a1 Secondly, the torque M exerted by the air pressure on the end face of the inner flexible hose around the origin O at the base is... a2 Thirdly, the torque caused by the rigidity of air pressure. ; ， Among them: the torque M caused by the air pressure at the top of the outer layer variable stiffness hose a1 : The torque M caused by the air pressure at the tip of the inner variable stiffness hose a2 : Torque caused by air pressure rigidity : Establish the polar coordinates (ρ, β) of the cross-section, where ρ is the radial distance to the center of the cross-section, and β is the circumferential angle related to the normal of the bending plane (sinβ>0 corresponds to the outer bending side, and sinβ<0 corresponds to the inner bending side); the torque M caused by the silicone. e : Where d1, d2, and d3 are the radial thicknesses of the outer variable stiffness hose, the annular sandwich air chamber, and the inner variable stiffness hose, respectively; r is the radius of the inner air chamber; and c is the initial shear modulus of the material. Spring torsional restoring torque M s : Where θ is the bending angle at the end of the spring; M is the bending moment of the external load acting on the spring; a is the stress correction factor; and b is the rod length correction factor. The spring prestress is denoted by n; the effective number of coils is denoted by D2; the mean diameter of the spring is denoted by φ; the geometric correction factor for the spring is π / 3; G is the shear modulus of the spring material; I p =πd 4 / 32 is the polar moment of inertia of the cross section; d is the spring wire diameter.

10. The method for online control of fishtail stiffness as described in claim 9, characterized in that, The total equivalent stiffness of a variable stiffness brake is represented by the torque caused by particle interference, air pressure, elastic stress, and spring. The formula for calculating the total equivalent stiffness is: Here, δ is the rotation angle caused by the external torque. The stiffness of the fish tail can be adjusted online by changing the gas pressure of the variable stiffness brake.

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