Design method of a jointless variable-stiffness flexible robotic fish and its bionic robotic fish

Abstract

The present invention discloses a design method for a jointless, variable-rigidity flexible robotic fish and a bionic robotic fish thereof. The method comprises: measuring the physical parameters of the soft tissues and body shape parameters of a real fish; designing a hydrogel material and a drive module for the robotic fish's body tissue and tail muscle tissue based on the measured parameters; preparing a robotic fish based on the designed hydrogel material and drive module; and testing the stealth performance and swimming performance of the prepared robotic fish. Compared with existing bionic robotic fish design methods, the present invention fully considers the physical properties of the fish's soft tissues, the fish's morphological and structural parameters, and the fish's movement patterns, providing a paradigm for the design of bionic robotic fish. Each part is soft and firm, and the bionic features are significantly improved. The present invention has a simple structure and is easy to control. The hydrogel constituting the bionic robotic fish can be prepared by casting in a mold and can be used for large-scale standardized production. The mold design is derived from the bionic object, so the present design method is universal.

Description

Technical Field

[0001] The present invention relates to the technical field of bionic fish, and in particular to a design method for a jointless variable-rigidity flexible robotic fish and a bionic robotic fish thereof. background

[0002] Underwater robots are essential tools for human exploration, development, and utilization of the ocean. Traditional underwater robots are constructed from rigid materials and structures, propelled by propellers or pumps. These robots suffer from significant drawbacks, including low efficiency, poor maneuverability, invisibility, high noise levels, and poor environmental adaptability. Developing new underwater robots using biomimetic technologies is a key area of research. Fish, common swimming organisms, possess advantages such as high efficiency, high maneuverability, low noise levels, and strong adaptability, attracting considerable interest in developing biomimetic robotic fish. However, despite years of exploration and practice, the biomimetic design methods and performance of biomimetic robotic fish remain immature.

[0003] Traditionally, the design of biomimetic robotic fish has relied on using motors to drive the tail's movement, thereby propelling the robot's motion. Motor-driven biomimetic robotic fish often employ multiple joints in series, meaning each joint uses a motor to control its rotation, thereby controlling the overall tail's movement to mimic the fish's motion. This design presents certain challenges, including the high synchronization requirements imposed by multiple motors, the complexity of controlling multiple motors, and the complex tail structure resulting from the large number of motors and joints, which hinders miniaturization. Later, researchers proposed using tension wires to drive the movement of each joint to mimic the continuous movement of a fish's tail. However, the use of metal wires and the retention of rigid joints prevent flexible actuation, which is inconsistent with the relatively soft nature of biological tissue. The retention of joints also renders the robotic tail fragmented, necessitating a skinned design. This results in cavities, which contradict the continuous and compact nature of fish tail tissue.

[0004] Most biomimetic robotic fish use metal or hard materials for their bodies or components, significantly different from the physical properties of real fish soft tissue. In recent years, some researchers have proposed using flexible materials like silicone to improve the biomimetic performance of robotic fish, using hydraulics and shape memory alloys to drive them. However, these designs still do not fully incorporate the physical properties of real fish soft tissue. Biological research shows that the speed of sound and density of fish soft tissue are very close to those of water, making it a natural material for underwater acoustic stealth. Furthermore, fish can use muscle tension to adjust their body stiffness while swimming, and when they are weak, their bodies can become as flexible as paper. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to propose a design method for a jointless variable-stiffness flexible robotic fish and a bionic robotic fish. The present invention fully considers the physical properties of the fish's soft tissue, the morphological and structural parameters of the fish, and the movement mode of the fish to overcome the shortcomings of existing bionic design methods and bionic robotic fish.

[0006] According to one aspect of the present invention, a method for designing a jointless, variable-stiffness, flexible robotic fish is provided, comprising: measuring the physical parameters of the soft tissues and body shape parameters of a real fish; designing a hydrogel material and a drive module for the robotic fish's body tissue and tail muscle tissue based on the measured parameters; preparing a robotic fish based on the designed hydrogel material and drive module; and testing the stealth and swimming performance of the prepared robotic fish.

[0007] In the above technical solution, compared with existing bionic robotic fish design methods, the present invention fully considers the physical properties of fish soft tissue, fish morphological and structural parameters, and fish movement mode, providing a paradigm for the design of bionic robotic fish and having the following advantages:

[0008] (1) Except for the servo used in the drive module, the bionic robotic fish is made entirely of hydrogel materials and has a jointless design. In addition, it has no hollow structure, and each part is soft and firm, with significant improvements in bionic features.

[0009] (2) The hydrogel material can be regulated by the external physical field to adjust its stiffness. Combined with the driving module, the bionic robotic fish can have different motion performances.

[0010] (3) Most of the volume of the bionic robotic fish is made of highly hydrated hydrogel material. Its physical parameters such as sound velocity, density, and acoustic impedance are almost the same as those of water, which can achieve the function of underwater acoustic stealth;

[0011] (4) The present invention has a simple structure and is easy to control. The hydrogel constituting the bionic robotic fish can be prepared by casting a mold and can be used for large-scale standardized production. The design of the mold is derived from the bionic object, so the design method is universal.

[0012] In some embodiments, the trait parameters include body length, body height, head length, caudal peduncle length, caudal peduncle height, and body width;

[0013] The body length is the distance from the tip of the snout to the base of the tail fin;

[0014] The body height refers to the vertical distance from the front end of the dorsal fin to the midline of the abdomen;

[0015] The head length is the distance from the tip of the snout to the posterior edge of the gill cover;

[0016] The caudal peduncle length refers to the length from the end of the anal fin to the base of the caudal fin;

[0017] The caudal peduncle height refers to the vertical distance of the lowest point of the caudal peduncle;

[0018] The body width refers to the maximum distance between the left and right sides of the fish body.

[0019] In the above technical solution, the present invention fully considers the physical characteristics of the fish's soft tissue, the fish's morphological and structural parameters, and the fish's movement mode, providing a paradigm for the design of bionic robotic fish. According to the research purpose, fish whose tail muscles control the swinging propulsion of the tail fin are selected as bionic objects, such as the Sciaenidae and Cyprinidae. Tissue sections are made on the fish, and the physical parameters of the soft tissue are measured, including but not limited to the speed of sound, density, stiffness, etc. The body shape traits of the fish are measured, including but not limited to body length, body height, head length, caudal peduncle length, caudal peduncle height, body width, etc. Through a large amount of fish body shape trait measurement data, the proportional relationship of the relevant trait parameters is summarized, and the mold of the bionic robotic fish is made according to the above-mentioned fish body shape trait characteristics.

[0020] In some embodiments, the body height accounts for 25%-27% of the body length; the head length accounts for 20%-22% of the body length; the caudal peduncle length accounts for 16%-18% of the body length; the caudal peduncle height accounts for 12%-14% of the body length; and the body width accounts for 17%-19% of the body length.

[0021] The reason for this design in the above technical solution is that a bionic robotic fish with similar physical features to its biomimetic counterpart can fully utilize fluid dynamics, resulting in higher swimming efficiency. Furthermore, the summary of the proportional relationships between fish body shape characteristics facilitates the construction of bionic robotic fish of different sizes with similar appearances.

[0022] In some embodiments, the characteristic parameters further include the length of the tail fin, the expanded area of the tail fin, and the aspect ratio, and the aspect ratio is calculated by the following formula:

[0023]

[0024] in, is the length of the tail fin, is the expanded area of the tail fin, is the aspect ratio.

[0025] In the above technical solution, the movement ability of fish is related to the aspect ratio of its tail fin. Having an aspect ratio similar to that of real fish not only allows the bionic robot fish to imitate the fish in appearance, but also can obtain a more appropriate aspect ratio to optimize the swimming speed and efficiency.

[0026] In some embodiments, the physical parameter is measured by the following method:

[0027] Tissue sections were taken from several real fish and the quality of the sections was measured;

[0028] The slices were placed in a water tank and the density of the slices was measured using the water displacement method;

[0029] A signal transmitting transducer and a signal receiving transducer are set in the water tank, the slice is placed between the two transducers, and the transducers are started to calculate the slice sound velocity;

[0030] The slice acoustic impedance of the sample is calculated based on the slice acoustic velocity and slice density.

[0031] In the above technical solution, based on the above-mentioned measurements of the physical parameters of fish soft tissue, a hydrogel material suitable for fish body tissue and tail muscle tissue was designed. The main physical parameters simulated include but are not limited to sound velocity, density, and stiffness. The sound velocity, density, and acoustic impedance of real fish soft tissue are very close to those of water, so a highly hydrated hydrogel material can be designed to simulate fish tissue. For real fish, the stiffness can be adjusted during both inactivity and movement. A hydrogel material with a stiffness response that can be controlled by external physical fields to achieve different stiffnesses can be designed.

[0032] In some embodiments, designing the hydrogel material and driving module for the body tissue and tail muscle tissue of the robotic fish based on the measured parameters specifically includes:

[0033] Fish tissue is made of polyvinyl alcohol hydrogel with a polyvinyl alcohol content of 12-13 wt%, using a repeated freeze-thaw method for 2-5 cycles;

[0034] The fish tail muscle tissue adopts polyvinyl alcohol / polyacrylamide hydrogel, wherein 4-6 times the mass of polyvinyl alcohol is dissolved in a polyvinyl alcohol solution with a polyvinyl alcohol content of 9-11wt%, and 0.2-0.6wt% of acrylamide monomer is used as initiator ammonium persulfate and 0.2-0.6wt% of acrylamide monomer is used as cross-linker N,N'-methylenebisacrylamide. The tissue is polymerized by irradiating with ultraviolet light with a wavelength of 365 nm and a power of 20 W for 0.3-0.4 hours. After the polymerization is completed, the tissue is dried at 55°C for 7-8 hours. After drying, the tissue is immersed in distilled water for 10-60 minutes.

[0035] In the above technical solution, the polyvinyl alcohol hydrogel prepared by the above process has a measured stiffness of 70-90 kPa, and the polyvinyl alcohol / polyacrylamide hydrogel prepared by the same process has a stiffness of 0.5-2 MPa, which is comparable to the body tissue and tail muscles of real fish. The lower stiffness of the polyvinyl alcohol hydrogel facilitates deformation with less force, making it suitable for use in fish body structures. The higher stiffness of the polyvinyl alcohol / polyacrylamide hydrogel facilitates the transmission of torque generated by the servo to the fish tail, thereby generating controlled oscillation.

[0036] In some embodiments, a robotic fish is prepared based on the designed hydrogel material and driving module, specifically comprising:

[0037] According to the body shape parameters, a mold of the bionic robotic fish is produced, including a fish head mold, a fish body mold and a fish tail muscle tissue mold;

[0038] The fish head mold and the fish body mold are used to prepare the robotic fish body tissue. During the second freezing cycle of polyvinyl alcohol, the driving structure is installed inside the fish body. A 2-3 mm thick polyvinyl alcohol hydrogel is brushed on the interface between the fish head and the fish body to connect the fish body structure. Several freezing cycles are then performed to stabilize the interface connection.

[0039] In the above technical solution, the bionic robotic fish developed according to the biomimetic design method provided by the present invention primarily consists of a fish body structure and a drive module. The fish body structure includes an integrally formed tail, torso, and head, with the drive module enclosed within, achieving a jointless design. A mold fabricated based on the fish's shape and characteristics is then poured with a designed hydrogel prepolymer solution, which is then solidified through polymerization methods such as cyclic freezing, heating, and ultraviolet irradiation to form the bionic robotic fish.

[0040] According to one aspect of the present invention, there is provided a robotic fish prepared by the above-mentioned design method of a jointless variable stiffness flexible robotic fish, the robotic fish comprising: a fish body structure, a driving module;

[0041] The driving module includes fish tail muscle tissue and a driving device for driving the fish tail muscle tissue;

[0042] The fish tail imitation muscle tissue is fixed to the tail of the fish body structure, and is driven by a driving device to drive the fish body tail to swing according to a preset frequency.

[0043] In the above technical solution, the present invention fully considers the physical properties of fish soft tissue, the morphological and structural parameters of the fish body, and the movement mode of the fish body, providing a paradigm for the design of bionic robotic fish and having the following advantages:

[0044] (1) Except for the servo used in the drive module, the bionic robotic fish is made entirely of hydrogel materials and has a jointless design. In addition, it has no hollow structure, and each part is soft and firm, with significant improvements in bionic features.

[0045] (2) The hydrogel material can be regulated by the external physical field to adjust its stiffness. Combined with the driving module, the bionic robotic fish can have different motion performances.

[0046] (3) Most of the volume of the bionic robotic fish is made of highly hydrated hydrogel material. Its physical parameters such as sound velocity, density, and acoustic impedance are almost the same as those of water, which can achieve the function of underwater acoustic stealth;

[0047] (4) The present invention has a simple structure and is easy to control. The hydrogel constituting the bionic robotic fish can be prepared by casting a mold and can be used for large-scale standardized production. The design of the mold is derived from the bionic object, so the design method is universal.

[0048] In some embodiments, the driving device includes a turntable, an L-shaped fixing block, and a steering gear; the fishtail imitation muscle tissue is in the shape of a long strip, with both ends fixed to the inside of the fish body structure and the tail by positioning pins, and the inner side and bottom of the middle symmetrical part are fitted and fixed to the turntable by a pair of L-shaped fixing blocks; the L-shaped fixing block is provided with two openings, the bottom opening is a rectangular opening, which is combined with the protrusion of the turntable and fixed, and the top opening is a circular opening, a screw passes through the circular opening, and the two screws are connected and fixed by a joint nut, so that the two L-shaped fixing blocks are pressed tightly against the fishtail imitation muscle tissue to fit the turntable; the turntable includes a turntable structure and a steering wheel; the turntable structure is provided with a pair of protrusions and a top opening that are symmetrical about the center of the circle; the protrusion extends outward from the bottom of the turntable structure and is connected to the L-shaped fixing block, fitting the fishtail imitation muscle tissue; the top opening is four slot-shaped openings for screwing and fixing the steering wheel; the steering wheel is connected to the steering gear and the turntable structure by screws.

[0049] In the above technical solution, the L-shaped fixing block, secured by screws, allows the fishtail-like muscle tissue to fit tightly against the turntable. The fishtail-like muscle tissue is long and strip-shaped, with its ends symmetrically fixed to the fishtail by fixing pins. When the servo is operating, it drives the turntable to rotate. When the turntable rotates to the left, the fishtail-like muscle tissue compresses on the left and stretches on the right, driving the fishtail to bend to the right. The same applies when the turntable rotates to the left. By changing the operating angle and frequency of the servo, the angle and frequency of the fishtail's swing can be controlled, thereby controlling the movement of the bionic robotic fish. By varying the stiffness of the fishtail-like muscle tissue, combined with the operating angle and frequency of the servo, different motion properties of the bionic robotic fish can be achieved.

[0050] In some embodiments, the L-shaped fixing block and turntable structure are made by polylactic acid 3D printing.

[0051] The above-mentioned technical solution utilizes this configuration because the fish-like tail muscle tissue pulls the robotic fish's tail to swing, mimicking the contraction and relaxation of fish muscles. The single-motor design is easy to control, emulating the driving principle. The bionic fish tail muscle tissue is secured solely with an L-shaped fixing block and a pin, minimizing the presence of rigid material within the robotic fish, resulting in a soft and firm overall structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0053] Figure 1 This is a flow chart of one embodiment of a method for designing a joint-free, variable-stiffness flexible robotic fish according to the present invention;

[0054] Figure 2 This is a schematic diagram of measurable properties of a grass carp, which is one embodiment of the design method of a jointless, variable-stiffness flexible robotic fish according to the present invention;

[0055] Figure 3 This is a schematic diagram of the extension of the tail fin of a fish in accordance with one embodiment of the design method of a jointless variable-stiffness flexible robotic fish according to the present invention;

[0056] Figure 4 This is a schematic diagram of an experiment for measuring the sound velocity of a sample in one embodiment of the design method of a jointless flexible robotic fish with variable stiffness according to the present invention;

[0057] Figure 5 This is a diagram showing the position selection of grass carp tissue in one embodiment of the design method of the joint-free variable-stiffness flexible robotic fish of the present invention;

[0058] Figure 6 This is a schematic diagram of measurable properties of a robotic fish according to one embodiment of the design method of a joint-free, variable-stiffness flexible robotic fish of the present invention;

[0059] Figure 7 This is a schematic diagram of the three-dimensional structure of the second embodiment of the design method of the jointless variable stiffness flexible robotic fish of the present invention;

[0060] Figure 8 2 is a schematic structural diagram in the front view of the second embodiment of the design method of the joint-free variable-stiffness flexible robotic fish of the present invention;

[0061] Figure 9 1 is a schematic diagram of the structure in a top view of the second embodiment of the design method of the jointless variable stiffness flexible robotic fish of the present invention;

[0062] Figure 10 This is a schematic structural diagram of the front view direction of the second embodiment of the design method of the jointless variable stiffness flexible robotic fish of the present invention. DETAILED DESCRIPTION

[0063] The present invention will be described in further detail below with reference to the accompanying drawings and examples. It is particularly noted that the following examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. Similarly, the following examples are only some embodiments of the present invention and are not intended to be exhaustive. All other embodiments obtained by those of ordinary skill in the art without creative effort are intended to fall within the scope of protection of the present invention.

[0064] The present invention proposes a design method for a jointless variable-rigidity flexible robotic fish and a bionic robotic fish thereof. The present invention fully considers the physical properties of the fish's soft tissue, the fish's morphological and structural parameters, and the fish's movement mode to overcome the shortcomings of existing bionic design methods and bionic robotic fish.

[0065] Example 1

[0066] The following uses the common underwater grass carp as a bionic object to illustrate the design method, specifically:

[0067] See also Figure 1 ,in Figure 1 This is a flow chart of one embodiment of a design method for a flexible robotic fish with variable stiffness and no joints according to the present invention. It should be noted that the method of the present invention is not limited to the method of the present invention if the results are substantially the same. Figure 1 The process sequence shown is limited. Figure 1 As shown, the method includes:

[0068] S1: Measurement of soft tissue physical parameters and body shape trait parameters of grass carp;

[0069] Measure the measurable traits of several grass carp of different masses and obtain the proportional relationship between the measurable traits. Figure 2 Measurable traits include: body length 1, body height 2, head length 3, caudal peduncle length 4, caudal peduncle height 5, and body width 6. Body length 1 refers to the distance from the tip of the grass carp's snout to the base of the caudal fin; body height 2 refers to the vertical distance from the front end of the grass carp's dorsal fin to the midline of the abdomen; head length 3 refers to the distance from the tip of the grass carp's snout to the posterior edge of the gill cover; caudal peduncle length 4 refers to the length from the end of the grass carp's anal fin to the base of the caudal fin; caudal peduncle height 5 refers to the vertical distance from the lowest point of the caudal peduncle; and body width 6 refers to the maximum distance between the left and right sides of the fish. Statistical results show that the proportional relationship between the measurable traits of grass carp is as follows: body height 2 accounts for 25%-27% of body length 1; head length 3 accounts for 20%-22% of body length 1; caudal peduncle length 4 accounts for 16%-18% of body length 1; caudal peduncle height 5 accounts for 12%-14% of body length 1; and body width 6 accounts for 17%-19% of body length 1. In this embodiment, this arrangement is designed so that a bionic robotic fish with similar physical features to the biomimetic object can fully utilize fluid dynamics, thereby achieving higher swimming efficiency. Furthermore, summarizing the proportional relationships between fish body shape characteristics facilitates the construction of bionic robotic fish of different sizes but with similar appearances.

[0070] In addition, the locomotion ability of fish is related to the aspect ratio of their tail fin. See 3. The formula for aspect ratio is:

[0071] Where b is the length of the tail fin, and A is the expanded area of the tail fin. The measured aspect ratio of all grass carp ranges from 3.1 to 3.4. A mold for a biomimetic robotic fish was fabricated based on the aforementioned fish body shape characteristics. In this implementation, the locomotion ability of fish is correlated with the aspect ratio of their tail fin. Having an aspect ratio similar to that of fish not only allows the hydrogel robotic fish to mimic fish in appearance, but also achieves a more appropriate aspect ratio to optimize swimming speed and efficiency.

[0072] Tissue sections were taken from several grass carps and physical parameters such as sound velocity, density, acoustic impedance, and stiffness were measured. Figure 4 The method for measuring the sound velocity of grass carp tissue samples is demonstrated. A signal transmitting transducer 8 and a signal receiving transducer 9 are placed in a water tank filled with water, and a sample 10 to be measured is placed between the two. The width of the sample is recorded as The signal generator 11 generates five 500 kHz pulse signals, and the transducer 9 is connected to the oscilloscope 12. The time delay ∆𝑡 of the received signal before and after the sample 10 is placed is measured. The sound velocity of the sample can be calculated according to the following formula:

[0073] in, - the speed of sound in water, — thickness of the grass carp tissue sample, ∆𝑡 — time delay between receiving the signal before and after sample 10 is placed. The density of the tissue sample is calculated by dividing its mass by its volume, where the mass is obtained by weighing and the volume is measured using the displacement method. The acoustic impedance of the tissue sample is calculated by multiplying the sound velocity and density. Based on the measured stress-strain curve, the Young's modulus is calculated within the linear small deformation range, which provides the stiffness of the tissue sample. Based on the above-mentioned measurements of the physical parameters of fish soft tissue, a hydrogel material suitable for fish body tissue and tail muscle tissue is designed. The key physical parameters to be simulated include, but are not limited to, sound velocity, density, and stiffness. The sound velocity, density, and acoustic impedance of real fish soft tissue are very close to those of water, making it possible to design a highly hydrated hydrogel material to simulate fish tissue. To achieve adjustable stiffness for real fish both in motion and during inactivity, a hydrogel material with a stiffness response that can be manipulated by an external physical field can be designed to achieve varying stiffness.

[0074] In this embodiment, the physical properties of fish soft tissue, the morphological and structural parameters of the fish, and the movement patterns of the fish are fully considered, providing a paradigm for the design of bionic robotic fish. Based on the research objectives, fish whose tail muscles control the swinging and propulsive movement of the tail fin are selected as bionic objects, such as the Sciaenidae and Cyprinidae. Tissue sections are taken from the fish, and the physical parameters of the soft tissue are measured, including but not limited to the speed of sound, density, and stiffness. The body shape traits of the fish are measured, including but not limited to body length 1, body height 2, head length 3, caudal peduncle length 4, caudal peduncle height 5, body width 6, etc. Based on a large amount of fish body shape trait measurement data, the proportional relationship between the relevant trait parameters is summarized, and a mold for the bionic robotic fish is produced based on the above-mentioned fish body shape trait characteristics.

[0075] S2: Design hydrogel materials that mimic grass carp body tissue and tail muscle tissue;

[0076] Based on the measured physical parameters of grass carp soft tissue, a hydrogel material with similar physical parameters was designed. The fish tissue was simulated using a polyvinyl alcohol hydrogel with a polyvinyl alcohol content of 12.5 wt%. It was prepared using a repeated freeze-thaw method with two to three freeze cycles. The fish tail muscle tissue was simulated using a polyvinyl alcohol / polyacrylamide hydrogel. To prepare this hydrogel, five times the mass of acrylamide monomer was dissolved in a 10 wt% polyvinyl alcohol solution. Then, using ammonium persulfate as an initiator (0.4 wt% acrylamide monomer) and N,N'-methylenebisacrylamide as a crosslinker (0.4 wt% acrylamide monomer), the hydrogel was polymerized under 365 nm ultraviolet light at a power of 20 W for 0.3-0.4 hours. After polymerization, the hydrogel was dried at 55°C and then soaked in distilled water for several minutes. The length of the drying and soaking time can be used to controllably adjust the stiffness of the hydrogel. The longer the drying time, the greater the stiffness of the hydrogel, and the longer the soaking time, the smaller the stiffness of the hydrogel.

[0077] The acoustic velocity, density, and acoustic impedance measurements of the polyvinyl alcohol hydrogel and polyvinyl alcohol / polyacrylamide hydrogel are as follows:

[0078] Table 1 Comparison of the acoustic velocity, density and acoustic impedance of the hydrogel and grass carp tissue

[0079]

[0080] The stiffness of the polyvinyl alcohol hydrogel was measured to be 70-90 kPa, and the stiffness of the polyvinyl alcohol / polyacrylamide hydrogel was 0.5-2 MPa. Figure 5, which is on the same order of magnitude as grass carp body tissue and tail muscles. The lower stiffness of the polyvinyl alcohol hydrogel facilitates deformation with less force, making it suitable for use in fish body structures. The higher stiffness of the polyvinyl alcohol / polyacrylamide hydrogel facilitates the transfer of torque generated by the servo to the fish tail, resulting in controlled oscillation.

[0081] S3: Design of the driving module of the bionic grass carp robot;

[0082] In this embodiment, the robotic fish comprises a body structure and a drive module. The drive module includes fish-like tail muscle tissue and a drive device that drives the tail muscle tissue. The fish-like tail muscle tissue is fixedly attached to the tail of the body structure and driven by the drive device to cause the tail of the fish to oscillate at a preset frequency. The detailed design details are described in the second embodiment and will not be elaborated here.

[0083] S4: Overall formation of the bionic robotic grass carp;

[0084] The fish body structure and tail musculature mimicking the above-mentioned molds are finalized using 3D printing, making them easy to manufacture and promising the potential for large-scale standardized production. The molds are divided into a fish head mold, a fish body mold, and a tail musculature mimicking mold. The head and tail musculature mimicking molds are of corresponding shapes. The body mold includes a body-shaped mold and an internal structure mold. The internal structure mold is constructed based on the shape of the driver module, providing space for the driver module to be added to the fish body. The body-shaped mold also includes space for the tail musculature mimicking the grass carp's shape. During the second polyvinyl alcohol freezing cycle, the driver structure is installed within the body structure. A 2-3 mm thick layer of polyvinyl alcohol hydrogel is applied to the interface between the head and body to connect the body structure. The molds are then subjected to another freezing cycle to stabilize the interface connection. The bionic robotic fish developed using the biomimetic design method provided in this embodiment primarily consists of a fish body structure and a drive module. The body structure includes an integrally formed tail, torso, and head, encasing the drive module, achieving a jointless design. A mold constructed based on the fish's shape and characteristics is then poured into a designed hydrogel prepolymer solution, which is then solidified through polymerization methods such as cyclic freezing, heating, and UV irradiation to form the bionic robotic fish.

[0085] In this example, refer to 6. The proportions of the bionic robotic grass carp's traits are as follows: body height (17 cm) accounts for 26.1% of body length (16 cm); head length (18 cm) accounts for 21.7% of body length (16 cm); caudal peduncle length (19 cm) accounts for 17.2% of body length (16 cm); caudal peduncle height (20 cm) accounts for 13.6% of body length (16 cm); and body width (21 cm) accounts for 18.2% of body length (16 cm). The aspect ratio of the bionic robotic grass carp is 3.17. The bionic robotic grass carp has similar structural proportions and an aspect ratio to those of the grass carp.

[0086] S5: Testing the stealth and swimming performance of a jointless, variable-stiffness, flexible stealth biomimetic grass carp robot

[0087] The developed jointless, variable-stiffness, flexible stealth bionic grass carp was placed in water and its stealth performance was tested at various angles using sonar detection equipment (BioSonics, Seattle, WA, USA). The results showed that the backscatter intensity of the bionic grass carp was very weak at any orientation, making it difficult to distinguish in the echo image. Furthermore, by adjusting the stiffness of the fish-like tail muscle tissue and, in conjunction with the control end, adjusting the oscillation frequency and rotation amplitude of the servo, the bionic grass carp could achieve different swimming performances under different tail muscle stiffness conditions.

[0088] Example 2

[0089] See Figures 7 to 10 The robotic fish was prepared based on the method of Example 1, as follows:

[0090] The bionic grass carp robot designed in this embodiment includes a body structure 22 and a drive module. The drive module includes fish tail muscle tissue 23, a turntable 24, an L-shaped fixing block 25, and a servo 26. The fish tail muscle tissue 23 is long and strip-shaped, with its ends secured to the tail of the body structure via locating pins 27. The inner and bottom sides of the symmetrical middle portion are secured to the turntable 24 via a pair of L-shaped fixing blocks 25. The L-shaped fixing blocks 25 have two openings: a rectangular bottom opening that engages and secures with a protrusion on the turntable 24, and a circular top opening through which screws pass. The two screws are connected and secured via a joint nut, pressing the two L-shaped fixing blocks 25 against the fish tail muscle tissue 23 against the turntable 24. The turntable 24 comprises a turntable structure and a steering wheel. The turntable structure is provided with a pair of protrusions symmetrical about the center of the circle and a top opening. The protrusions extend outward from the bottom of the turntable structure and connect to the L-shaped fixing block 25, which fits the fishtail musculature 23. The top opening comprises four slot-shaped openings for screwing the steering wheel. The steering wheel is connected to the servo 26 and the turntable structure via screws. The L-shaped fixing block 25 and turntable structure are manufactured using 3D printing, using commonly used 3D printing materials such as polylactic acid (PLA). This design is based on the fact that the process of the fishtail musculature 23 pulling the robotic fish's tail to swing simulates the process of fish muscle contraction driving the tail's swing. The single-motor design is easy to control and is biomimetic in its driving principle. Using only the L-shaped fixing block 25 and fixing pin 27 to secure the biomimetic fishtail musculature 23 minimizes the presence of rigid material in the robotic fish, making the robotic fish overall soft and compact.

[0091] Specifically, the bionic robotic fish works as follows:

[0092] The L-shaped fixing block 25 is fixed with screws so that the fishtail-mimicking muscle tissue 23 fits tightly against the turntable 24. The fishtail-mimicking muscle tissue 23 is in the shape of an elongated strip, with both ends symmetrically fixed to the fishtail by fixing pins 27. When the servo 26 is working, it drives the turntable 24 to rotate. When the turntable 24 rotates to the left, the left side of the fishtail-mimicking muscle tissue 23 is compressed and the right side is stretched, driving the fishtail to bend to the right. The same applies when the turntable 24 rotates to the left. By changing the working angle and frequency of the servo 26, the angle and frequency of the fishtail swing can be controlled, thereby controlling the movement of the bionic robotic fish. By changing the stiffness of the fishtail-mimicking muscle tissue 23, combined with the working angle and frequency of the servo 26, different movement performances of the bionic robotic fish can be achieved.

[0093] This embodiment fully considers the physical properties of fish soft tissue, fish morphological and structural parameters, and fish movement patterns, providing a paradigm for the design of bionic robotic fish and having the following advantages:

[0094] (1) Except for the servo used in the drive module, the bionic robotic fish is made entirely of hydrogel materials and has a jointless design. In addition, it has no hollow structure, and each part is soft and firm, with significant improvements in bionic features.

[0095] (2) The hydrogel material can be regulated by the external physical field to adjust its stiffness. Combined with the driving module, the bionic robotic fish can have different motion performances.

[0096] (3) Most of the volume of the bionic robotic fish is made of highly hydrated hydrogel material. Its physical parameters such as sound velocity, density, and acoustic impedance are almost the same as those of water, which can achieve the function of underwater acoustic stealth;

[0097] (4) The present invention has a simple structure and is easy to control. The hydrogel constituting the bionic robotic fish can be prepared by casting a mold and can be used for large-scale standardized production. The design of the mold is derived from the bionic object, so the present design method is universal.

[0098] The above descriptions are only some embodiments of the present invention and do not limit the scope of protection of the present invention. Any equivalent device or equivalent process transformation made by using the contents of the description and drawings of the present invention, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A design method for a jointless flexible robotic fish with variable stiffness, characterized in that: include: Measure the physical parameters of the fish's soft tissue and body shape parameters; the physical parameters are measured by the following method: Tissue sections were taken from several real fish and the quality of the sections was measured; The slices were placed in a water tank and the density of the slices was measured using the water displacement method; A signal transmitting transducer and a signal receiving transducer are set in the water tank, the slice is placed between the two transducers, and the transducers are started to calculate the slice sound velocity; Calculate the slice acoustic impedance of the sample based on the slice acoustic velocity and slice density; Design the hydrogel material and driving module for the robotic fish's body tissue and tail muscle tissue based on the measured parameters; Prepare a robotic fish based on the designed hydrogel material and driving module; The stealth performance and swimming performance of the prepared robotic fish were tested.

2. The design method of a jointless variable stiffness flexible robotic fish according to claim 1, characterized in that: The trait parameters include body length, body height, head length, caudal peduncle length, caudal peduncle height, and body width; The body length is the distance from the tip of the snout to the base of the tail fin; The body height refers to the vertical distance from the front end of the dorsal fin to the midline of the abdomen; The head length is the distance from the tip of the snout to the posterior edge of the gill cover; The caudal peduncle length refers to the length from the end of the anal fin to the base of the caudal fin; The caudal peduncle height refers to the vertical distance of the lowest point of the caudal peduncle; The body width refers to the maximum distance between the left and right sides of the fish body.

3. The design method of a jointless variable stiffness flexible robotic fish according to claim 2, characterized in that: The body height accounts for 25%-27% of the body length; the head length accounts for 20%-22% of the body length; the caudal peduncle length accounts for 16%-18% of the body length; the caudal peduncle height accounts for 12%-14% of the body length; and the body width accounts for 17%-19% of the body length.

4. The design method of a jointless variable stiffness flexible robotic fish according to claim 2, characterized in that: The characteristic parameters also include the length of the tail fin, the expanded area of the tail fin, and the aspect ratio, which is calculated by the following formula: in, is the length of the tail fin, is the expanded area of the tail fin, is the aspect ratio.

5. The design method of a jointless variable stiffness flexible robotic fish according to claim 1, characterized in that: Based on the measured parameters, the hydrogel materials and drive modules for the robotic fish's body and tail muscles are designed, including: Fish tissue is made of polyvinyl alcohol hydrogel with a polyvinyl alcohol content of 12-13 wt%, using a repeated freeze-thaw method for 2-5 cycles; The fish tail muscle tissue adopts polyvinyl alcohol / polyacrylamide hydrogel, wherein 4-6 times the mass of polyvinyl alcohol is dissolved in a polyvinyl alcohol solution with a polyvinyl alcohol content of 9-11 wt%, and 0.2-0.6 wt% of acrylamide monomer is used as initiator ammonium persulfate and 0.2-0.6 wt% of acrylamide monomer is used as cross-linker N,N'-methylenebisacrylamide. The tissue is polymerized by irradiation with ultraviolet light of wavelength 365 nm and power 20 W for 0.3-0.4 hours. After the polymerization is completed, the tissue is dried at 55°C for 7-8 hours. After the drying is completed, the tissue is immersed in distilled water for 10-60 minutes.

6. The design method of a jointless flexible robotic fish with variable stiffness as claimed in claim 4, characterized in that: The robotic fish is prepared based on the designed hydrogel material and driving module, specifically including: According to the body shape parameters, a mold of the bionic robotic fish is produced, including a fish head mold, a fish body mold and a fish tail muscle tissue mold; The fish head mold and the fish body mold are used to prepare the robotic fish body tissue. During the second freezing cycle of polyvinyl alcohol, the driving structure is installed inside the fish body. A 2-3 mm thick polyvinyl alcohol hydrogel is brushed on the interface between the fish head and the fish body to connect the fish body structure. Several freezing cycles are then performed to stabilize the interface connection.

7. A jointless, variable-rigidity flexible robotic fish, characterized in that: The robot fish is prepared by the design method of a jointless variable stiffness flexible robotic fish according to any one of claims 1 to 6, and comprises: a fish body structure, a driving module; The driving module includes fish tail muscle tissue and a driving device for driving the fish tail muscle tissue; The fish tail imitation muscle tissue is fixed to the tail of the fish body structure, and is driven by a driving device to drive the fish body tail to swing according to a preset frequency.

8. The jointless, variable-stiffness flexible robotic fish according to claim 7, characterized in that: The driving device includes a turntable, an L-shaped fixing block, and a steering gear; the fish tail imitation muscle tissue is in the shape of a long strip, and its two ends are fixed to the inside of the fish body structure and the tail by positioning pins, and the inner side and bottom of the middle symmetrical part are fitted and fixed to the turntable by a pair of L-shaped fixing blocks; the L-shaped fixing block is provided with two openings, the bottom opening is a rectangular opening, which is combined and fixed with the protrusion of the turntable, and the top opening is a circular opening, and a screw passes through the circular opening. The two screws are connected and fixed by a joint nut, so that the two L-shaped fixing blocks are pressed tightly against the fish tail imitation muscle tissue to fit the turntable; the turntable includes a turntable structure and a steering wheel; the turntable structure is provided with a pair of protrusions and a top opening that are symmetrical about the center of the circle; the protrusion extends outward from the bottom of the turntable structure and is connected to the L-shaped fixing block, fitting the fish tail imitation muscle tissue; the top opening is four slot-shaped openings for screwing and fixing the steering wheel; the steering wheel is connected to the steering gear and the turntable structure by screws.

9. The jointless, variable-rigidity flexible robotic fish according to claim 8, characterized in that: The L-shaped fixing block and turntable structure are made by polylactic acid 3D printing.

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