Bionic robotic fish with fast straight swimming and maneuverability and adjustable fish body stiffness

Abstract

The bionic robot fish with fast straight swimming and maneuverability and adjustable fish body rigidity belongs to the technical field of underwater tracking target. The propelling device is connected with the swing fish tail device to drive the tail fin to swing, and the turning device is connected with the first fish body joint, the second fish body joint and the swing fish tail device to drive the first fish body joint, the second fish body joint, the swing fish tail device and the tail fin to turn. The torque of the motor located at the fish head is directly transmitted to the tail of the bionic fish by the steel wire flexible shaft, so that the mass of the tail of the bionic fish is greatly reduced, the swing inertia of the tail is reduced, the fish head part is not affected by the reaction force and swings greatly, the swimming mode of the bionic fish is closer to the crescent tail mode that the front half of the fish body keeps still and the rear half swings, the fish body can obtain a higher propelling speed, the propelling and turning can be simultaneously realized, and the maneuverability of the fish body is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of underwater target tracking technology, specifically relating to a biomimetic robotic fish that combines rapid straight-line swimming and maneuverability with adjustable body stiffness. Background Technology

[0002] While conventional underwater vehicles (AUVs) can track and accompany underwater targets, their inefficient maneuverability often limits their ability to navigate complex underwater environments effectively. They cannot evade or escape fixed obstacles (such as corals and reefs) or rapidly approaching objects that could collide with them (such as drifting seaweed or aquatic animals that mistake them for prey). This can lead to damage to their mechanical structures. Therefore, an underwater vehicle that mimics the swimming motion of fish, achieving similar high-speed propulsion and maneuverability, would not only facilitate rapid target tracking but also provide timely responses to unexpected situations, giving it a significant advantage in this field.

[0003] Fish primarily propel themselves using the BCF (Body Fluid Dynamics) mode, a swimming method that relies on body undulations for propulsion. This method offers advantages in efficiency and speed compared to traditional propeller-driven propulsion (for the same body length). Furthermore, when swimming in the BCF mode, the swimming speed of a fish exhibits a significant positive correlation with the frequency of its body undulations, with extremely high linearity. This means that if a highly maneuverable biomimetic fish were designed, the frequency of its body undulations would largely determine its propulsion speed. Therefore, the highest achievable undulation frequency of a biomimetic fish would be considered an important performance indicator.

[0004] Meanwhile, according to relevant research, fish that swim using the BCF (Body-Flutter) pattern in nature exhibit a characteristic value for their swimming speed based on the proportion of the body's undulating or swaying portion to the total body length. Fish capable of high-speed swimming in nature often possess the following characteristics: the front half, or even the first two-thirds, of their bodies remain almost stationary during swimming, while only the rear half, or even the last third, shows significant undulation or swaying. Some scholars refer to this BCF swimming pattern as the crescent-tail pattern. Fish using this pattern not only possess the aforementioned characteristics but also often have crescent-shaped tail fins; typical examples include sharks, tuna, and whales. Imitating this feature presents a challenge for biomimetic fish because the swaying portion in this pattern is significantly located in the rear half of the body. This means that the actuator providing propulsion, or its output end, should be as close as possible to the rear of the fish's body. However, it's important to understand that fish achieve high-speed turning in water by rapidly and dramatically bending their bodies during propulsion. If the propulsion actuator, such as the motor, is located in the rear half of the fish's body, the center of gravity will be shifted rearward. This results in two problems: firstly, it makes it difficult for the front half of the fish to maintain a stable state during propulsion; secondly, the increased weight at the tail makes it significantly more difficult to bend the body and turn by swaying the tail. Therefore, maintaining minimal swaying in the front half of the fish's body and minimizing swaying in the rear half—making crescent-tail propulsion a challenge in the design of bionic fish—is a significant challenge.

[0005] Although the swimming speed of fish is linearly correlated with the frequency of body undulations within a certain range—that is, the swimming speed increases proportionally with the increase in undulation frequency—this linear relationship has an upper limit. Studies have shown that if the fish's body stiffness (the resistive torque generated between joints during relative angular displacement, with the direction opposite to the angular displacement and the magnitude showing a certain linear or non-linear positive correlation with the magnitude of the angular displacement, and the ratio of the magnitude of the resistive torque to the angle of rotation of the fish joints is the fish's body stiffness) and the fish's body undulation frequency achieve a certain matching relationship, the fish's propulsion efficiency will be increased several times over. Therefore, the adjustability of the fish's body stiffness and the matching degree between the fish's body stiffness and the swimming frequency jointly determine the upper limit of the swimming speed of the bionic fish, which plays a crucial role in the swimming performance of the bionic fish.

[0006] Current biomimetic robotic fish are mainly driven in the following three ways:

[0007] 1. The fish's body is bent by pulling a rope with a servo motor. The fish's body oscillates periodically as the servo motor rotates in both directions, thus propelling the fish. By controlling the program to make the servo motor's oscillation asymmetrical, the fish can be turned.

[0008] 2. Set a servo motor (such as a servo motor) at each joint that needs to bend. By controlling the rotation angle of the motors on each joint, the fish's posture can be controlled. This can also achieve the purpose of periodic undulation of the fish and propel the fish. The fish can also be turned by adjusting the control logic.

[0009] 3. By using a DC motor in conjunction with a special transmission mechanism such as a slot mechanism, the rotational motion of the motor can be converted into the periodic oscillating motion of the fish body, thereby achieving the propulsion of the fish body.

[0010] Application number CN202310440448.4, entitled "A Robotic Fish with a Multi-directional Follow-up Tail Fin," describes a typical biomimetic robotic fish that uses a servo motor to pull a rope to achieve body undulation. Since servo motors are known for their low commutation frequency, this design is limited by the low commutation frequency of the servo motor, preventing the biomimetic robotic fish from achieving a higher body undulation frequency, resulting in a lower propulsion speed. Related research indicates that at the same propulsion speed, a greater degree of body curvature leads to a faster turning speed. This design, with its larger body turning angle, is more suitable for turning biomimetic fish than propulsion.

[0011] The application, with application number CN202310222338.0 and titled "A Bionic Robotic Fish," is a typical bionic fish with a stepper motor or servo motor set in each joint. Although steering can be achieved by controlling the rotation angle of each motor, this approach is also limited by the low commutation frequency of the servo motor, making it difficult to achieve a high swing frequency. Furthermore, since each joint is equipped with a servo motor, the inertia of the fish's swing is relatively large, making it impossible to achieve a high propulsion speed.

[0012] The application, CN202320174299.7, entitled "A Structure for Brushless Motor-Driven Oscillation of the Tail Fin of a Robotic Fish," is a typical solution that uses a specific transmission mechanism to convert the rotational motion of a motor into the oscillation motion of a fish tail. This solution can achieve high-frequency oscillation of the fish tail, and the turning of the fish body can also be achieved by changing the angle of the servo motors installed at several joints before the propulsion motor. However, this solution has a major drawback: it does not take into account the joint stiffness between the fish body joints and cannot adjust it. Therefore, it is impossible to properly match the stiffness of the fish body with the oscillation frequency, and it is impossible to adjust the swimming mode under different stiffnesses. Moreover, as mentioned earlier, the rear-mounted motor will be unfavorable for the bionic fish to swim in the crescent tail propulsion mode, and the rearward shift of the center of gravity will also increase the difficulty of turning the fish body. Summary of the Invention

[0013] In order to solve the above problems, the present invention provides a biomimetic robotic fish that combines fast straight swimming and maneuverability with adjustable body rigidity.

[0014] The technical solution adopted in this invention is:

[0015] A biomimetic robotic fish, combining rapid swimming and maneuverability with adjustable body rigidity, includes an upper outer shell, a lower outer shell, a first body joint, a second body joint, a tail-swinging device, and a tail fin. The upper outer shell is a waterproof shell, which, together with the lower outer shell, forms the main power unit. The upper and lower outer shells are mounted vertically to form the waterproof shell. The first and second body joints, the tail-swinging device, and the tail fin are sequentially and rotatably connected at the rear end of the waterproof shell. A propulsion device is connected to the tail-swinging device, driving the tail fin to perform tail-swinging operations. A steering device is connected to the first and second body joints and the tail-swinging device, driving the first and second body joints, the tail-swinging device, and the tail fin to perform fish-body turning operations.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] 1. Firstly, for underwater tracking, this biomimetic robotic fish can adjust its tracking distance and angle with the target through its rapid straight-line swimming ability and extremely high flexibility, ensuring the accuracy and effectiveness of tracking, such as protecting aquatic animals and underwater targets like divers and submersibles. Furthermore, its strong maneuverability allows it to promptly perform obstacle avoidance and escape maneuvers in response to unexpected situations. In addition, the biomimetic robotic fish's body rigidity is adjustable, allowing it to switch swimming modes (such as high-speed or energy-saving modes) according to the application, thereby better completing the tasks at hand.

[0018] Bluefin tuna is a highly maneuverable deep-sea fish. This biomimetic fish achieves similar high maneuverability by mimicking the body shape and movement patterns of bluefin tuna, greatly facilitating its functions such as quickly tracking schools of fish without disturbing them.

[0019] 2. This invention innovatively uses a flexible steel wire shaft to directly transfer the torque of the motor located at the fish's head to the tail of the bionic fish, greatly reducing the mass of the tail end and thus reducing the tail's swing inertia. This allows the fish's head to swing more significantly without being affected by reaction forces, making the bionic fish's swimming pattern closer to the crescent tail pattern where the front half of the fish body remains stationary while the back half undulates. This allows the fish to achieve a higher propulsion speed. Moreover, thanks to the flexibility of the flexible steel wire shaft, the bending of the fish body is not affected. Even when the fish body is bent, the torque of the DC motor can still be transferred to the tail, achieving simultaneous propulsion and turning, greatly improving the fish's maneuverability.

[0020] Furthermore, considering the impact of the stiffness between the fish's joints on its swimming performance, the stiffness of the joints is adjusted by using elastic ropes installed between them, making the swimming pattern of the bionic fish adjustable and easy to achieve. Simultaneously, the use of servo-driven ropes allows the bionic fish to perform large-scale turns even at high speeds.

[0021] Because there is still a considerable amount of space at the front of the fish's head, cameras and other equipment can be installed in the reserved space, making underwater observation feasible. All metal parts used, except for those made primarily of aluminum, have undergone electroplating for rust prevention. This means that, apart from the flexible steel shaft which needs periodic replacement, other metal parts do not require frequent replacement, greatly simplifying the later maintenance of the equipment. Attached Figure Description

[0022] Figure 1 This is an isometric view of the present invention;

[0023] Figure 2 This is a top view of the present invention;

[0024] Figure 3 This is a main sectional view of the present invention;

[0025] Figure 4 This is a schematic diagram illustrating three possible orientations of the fish body in this invention;

[0026] Figure 5 This is a schematic diagram illustrating the principle of the fish tail swaying in this invention;

[0027] The components include: 1. Upper shell of the fish head; 2. Lower shell of the fish head; 3. Waterproof servo; 4. Servo rocker arm; 5. Lithium battery pack; 6. DC motor; 7. Microcontroller and motor driver; 8. Coupling 1; 9. Flexible steel wire shaft; 10. Swinging fish tail device; 11. Tail fin; 12. Tail fin elastic rope; 13. Three-stage elastic rope; 14. Second fish body joint; 15. Second-stage elastic rope; 16. First fish body joint; 17. First-stage elastic rope; 18. Servo drive rope; 101. Fish tail base; 102. Optical shaft bolt; 103. Connecting shaft; 104. Rotation lever; 105. Optical shaft bolt and nut; 111. Fixing bolt and nut; 112. Tail fin base. Detailed Implementation

[0028] To better understand the purpose, structure, and function of this invention, the invention will be described in further detail below with reference to the accompanying drawings.

[0029] This invention application is for a biomimetic robotic fish that combines rapid straight swimming and maneuverability with adjustable body stiffness, targeting the field of tracking and accompanying underwater organisms or underwater submersibles. It uses rapid straight swimming and maneuverability in combination with the adjustment of the fish's joint stiffness to switch swimming modes to track and accompany underwater moving targets such as divers and underwater biomimetic fish formations in real time.

[0030] This biomimetic fish incorporates and summarizes the advantages of common biomimetic fish designs, considering the impact of adjustable body stiffness on swimming performance. A parallel drive scheme using DC motors and servo motors is implemented. Symmetrically distributed elastic ropes between each passive joint provide stiffness. The servo motor used is a standard, high-response, waterproof hollow cup servo motor, small in size (only 33.4mm high) and capable of a 60° rotation within 80ms, thus enabling the fish to turn. Alternatively, a servo motor with an independently designed transmission box could be used for even higher response and turning speeds. The DC motor transmits torque via a flexible steel shaft to the tail-swinging device, enabling high-frequency tail swaying. The passive stiffness of the body joints can be adjusted by changing the stiffness of the elastic ropes. The overall structure is compact, with a length of only 34cm, exhibiting agile, accurate, and highly maneuverable movements.

[0031] Specifically, such as Figures 1-5As shown, the biomimetic robotic fish of the present invention, which combines rapid straight-line swimming and maneuverability with adjustable body rigidity, includes an upper outer shell 1, a lower outer shell 2, a first body joint 16, a second body joint 14, a tail-swinging device 10, a tail fin 11, a propulsion device, and a steering device. The upper outer shell 1 is a waterproof outer shell for the fish head, and together with the lower outer shell 2, it constitutes the power unit. The upper outer shell 1 and the lower outer shell 2 are installed vertically to form the waterproof outer shell for the fish head. The first body joint 16, the second body joint 14, the tail-swinging device 10, and the tail fin 11 are rotatably connected and installed in series from front to back at the rear end of the waterproof outer shell for the fish head. The propulsion device is connected to the tail-swinging device 10 to drive the tail fin 11 to perform tail-swinging operations. The steering device is connected to the first body joint 16, the second body joint 14, and the tail-swinging device 10 to drive the first body joint 16, the second body joint 14, the tail-swinging device 10, and the tail fin 11 to perform fish body turning operations.

[0032] like Figure 1 As shown, the fish body joint of the present invention is not limited to two sections, but can also be configured with three, four or more sections.

[0033] The fish's propulsion system consists of two parts: propulsion and steering.

[0034] Propulsion is achieved through propulsion devices, such as... Figure 2 , Figure 3 As shown, the propulsion device includes a DC motor 6, a coupling 8, and a flexible steel wire shaft 9. The general method is to convert the rotational motion of the output shaft of the DC motor 6 into the swinging motion of the fish tail. First, the output shaft of the DC motor 6 is connected to one end of the flexible steel wire shaft 9 through the coupling 8. At the same time, the other end of the flexible steel wire shaft 9 is connected to the connecting shaft 103 of the swinging fish tail device 10 through the coupling 2. The torque of the output shaft of the DC motor 6 is transmitted to the tail of the fish body, which drives the rotation lever 104 in the swinging fish tail device 10 to rotate.

[0035] like Figure 5 As shown, the swinging fish tail device 10 includes a connecting shaft 103, a rotary lever 104, optical axis bolts and nuts 105, a fish tail base 101, two optical axis bolts 102, a tail fin elastic rope 12, and a fixing bolt and nut 111. The front end of the fish tail base 101 is rotatably connected to the second fish body joint 14 through a rotating shaft. The tail fin 11 includes a fixing bolt and nut 111 and a tail fin base 112. The tail fin base 112 is rotatably connected to the rear end of the fish tail base 101 through optical axis bolts and nuts 105, and the rotation of the tail fin 11 is provided with passive stiffness by two symmetrically distributed tail fin elastic ropes 12. Two vertically arranged optical axis bolts 102 are installed between the fish tail bases 101. The rear end of the rotary lever 104 is inserted between the two optical axis bolts 102, and the front end of the rotary lever 104 is connected to the other end of the steel wire flexible shaft 9 through the connecting shaft 103.

[0036] The slot mechanism used in this design involves inserting a rotary lever 104 between two optical shaft bolts 102 that replace the slot. As the rotary lever 104 rotates, it actuates the optical shaft bolts 102, causing them to bear force. Since the two optical shaft bolts 102 are fixed to the fish tail base 101, and the caudal fin base 112 is rotatably connected to the fish tail base 101 with a certain joint rigidity, the fish tail base 101 and the caudal fin 11 will swing around the pivot located at the front end of the swinging fish tail device 10 due to the rotation of the rotary lever 104. As the rotary lever 104 rotates periodically, the caudal fin 11 and the fish tail base 101 will also complete periodic swinging. The specific swinging principle is detailed in the appendix. Figure 5 .

[0037] like Figure 3 As shown, the DC motor 6 is powered by the lithium battery pack 5. The DC motor 6 uses a microcontroller and a motor driver 7 to control the current. The DC motor 6, the lithium battery pack 5, the microcontroller and the motor driver 7 are all sealed inside the outer shell 1 of the fish head.

[0038] Since the DC motor 6, lithium battery pack 5, microcontroller and motor driver 7, receiver and other components are not waterproof, they are all integrated into the outer shell 1 of the fish head and sealed. At the same time, a skeleton sealing ring is set at the output shaft of the DC motor 6 for dynamic sealing to prevent water from entering the outer shell 1 of the fish head.

[0039] The DC motor 6 uses a microcontroller and a motor driver 7 to control the current. The signal controlling the microcontroller is sent to the signal receiver by a wireless remote control, thereby realizing the high and low frequency changes of the fish tail swing.

[0040] Because there is still a considerable amount of space at the front of the fish's head, cameras and other equipment can be installed within this reserved space, making underwater observation feasible. All metal parts used, except for those made primarily of aluminum, have undergone electroplating for rust prevention.

[0041] For steering, a waterproof servo motor 3 is used.

[0042] like Figures 2-4 As shown, the steering device includes a waterproof servo motor 3, a servo arm 4, and two servo drive ropes 18. The servo arm 4 is mounted on the shaft of the waterproof servo motor 3 via a toothed groove. The front ends of the two servo drive ropes 18 are respectively fixed to both ends of the servo arm 4. The other ends of the two servo drive ropes 18 pass sequentially through the first fish-body joint 16 and the second fish-body joint 14 and are fixed to the swaying fish-tail device 10.

[0043] Therefore, when the waterproof servo motor 3 rotates, it pulls the servo motor drive rope 18, which in turn drives the two passive joints, the first fish body joint 16 and the second fish body joint 14, as well as the swinging fish tail device 10, to bend the fish body, thereby achieving fish body turning during propulsion by bending the fish body.

[0044] The inner walls of the first fish-body joint 16 and the second fish-body joint 14 are both provided with through grooves for the servo drive rope 18 to pass through.

[0045] like Figures 1-3 As shown, since the stiffness of the fish body joints is crucial to the swimming performance of fish, M2 bolts and nuts are installed on both sides of the lower shell 2 of the fish head, the first fish body joint 16, the second fish body joint 14, and the swinging fish tail device 10 to fix the first-level elastic rope 17, the second-level elastic rope 15, and the third-level elastic rope 13 to provide stiffness for the fish body joints. At the same time, M2 bolts and nuts are installed on both sides of the adjacent ends of the tail fin 11 and the swinging fish tail device 10 to fix the tail fin elastic rope 12. The stiffness of each level of elastic rope can be changed as needed, such as in energy-saving mode or high-speed mode.

[0046] This biomimetic fish can serve as an underwater robot, performing tasks such as underwater surveying, exploration, and target following and companionship. This invention provides a unique solution for rapid, straight-line swimming and maneuvering of a biomimetic fish. Furthermore, this biomimetic fish retains ample space for mounting cameras, sensors, and other tools to expand its functionality. In addition, this biomimetic fish can also serve as an experimental platform for research into the swimming mechanisms of fish.

[0047] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A biomimetic robotic fish that combines rapid straight-line swimming and maneuverability with adjustable body rigidity, characterized in that: The device includes an upper shell (1) for the head, a lower shell (2) for the head, a first body joint (16), a second body joint (14), a tail-swinging device (10), a tail fin (11), a propulsion device, and a steering device. The upper shell (1) is a waterproof shell for the head, and together with the lower shell (2), it forms a power assembly. The first body joint (16), the second body joint (14), the tail-swinging device (10), and the tail fin (11) are sequentially connected and rotated from front to back at the rear end of the waterproof shell. The propulsion device is connected to the tail-swinging device (10) to drive the tail fin (11) to perform tail-swinging operations. The steering device is connected to the first body joint (16), the second body joint (14), and the tail-swinging device (10) to drive the first body joint (16), the second body joint (14), the tail-swinging device (10), and the tail fin (11) to perform fish body turning operations. The propulsion device includes a DC motor (6), a coupling (8), and a flexible steel wire shaft (9). The output shaft of the DC motor (6) is connected to one end of the flexible steel wire shaft (9) via the coupling (8), and the other end of the flexible steel wire shaft (9) is connected to the connecting shaft (103) of the swinging fish tail device (10) via a coupling (2). This transmits the torque of the DC motor (6) output shaft to the tail of the fish, causing the rotary lever (104) inside the swinging fish tail device (10) to rotate. The swinging fish tail device (10) includes a connecting shaft (103), a rotary lever (104), a fish tail base (101), two optical axis bolts (102), an optical axis bolt and nut (105), a tail fin elastic rope (12), and a fixing bolt and nut (111). The front end of the fish tail base (101) is rotatably connected to the second fish body joint (14) through two rotating shafts. The tail fin includes a fixing bolt and nut (111) and a tail fin base (112). The tail fin base (112) is connected to the second fish body joint (14) through the optical axis bolt and nut. The mother (105) is rotatably connected to the rear end of the fish tail base (101). Two tail fin elastic ropes (12) are fixed to the fixing bolts and nuts (111) to provide joint rigidity for the rotation joint between the tail fin base (112) and the fish tail base (101). There are two vertically arranged optical axis bolts (102) at the front of the fish tail base (101). The rear end of the rotary lever (104) is inserted between the two optical axis bolts (102). The front end of the rotary lever (104) is connected to the other end of the steel wire flexible shaft (9) through the connecting shaft (103).

2. The biomimetic robotic fish with both rapid straight-line swimming and maneuverability and adjustable body rigidity as described in claim 1, characterized in that: The DC motor (6) is powered by the lithium battery pack (5). The DC motor (6) uses a microcontroller and a motor driver (7) to control the current. The DC motor (6), the lithium battery pack (5), the microcontroller and the motor driver (7) are all sealed inside the outer shell (1) of the fish head.

3. The biomimetic robotic fish with both rapid straight-line swimming and maneuverability and adjustable body rigidity as described in claim 1, characterized in that: The steering device includes a waterproof servo (3), a servo rocker arm (4), and two servo drive ropes (18). The servo rocker arm (4) is mounted on the shaft of the waterproof servo (3) through a toothed groove. At the same time, the front ends of the two servo drive ropes (18) are respectively fixed to both ends of the servo rocker arm (4), and the other ends of the two servo drive ropes (18) pass through the first fish body joint (16) and the second fish body joint (14) in sequence and are fixed to the swinging fish tail device (10).

4. The biomimetic robotic fish with both rapid straight-line swimming and maneuverability and adjustable body rigidity as described in claim 3, characterized in that: The inner walls of the first fish-body joint (16) and the second fish-body joint (14) are provided with through grooves for the servo drive rope (18) to pass through.

5. The biomimetic robotic fish with both rapid straight-line swimming and maneuverability and adjustable body rigidity as described in claim 3, characterized in that: Bolts and nuts are installed on both sides of the lower shell (2) of the fish head, the first fish body joint (16), the second fish body joint (14), and the swinging fish tail device (10) to fix the first-level elastic rope (17), the second-level elastic rope (15), and the third-level elastic rope (13) to provide rigidity for the fish body joint.

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