A 5 cm-Scale Piezoelectric Jetting Agile Underwater Robot

Existing miniature underwater robots, which use electromagnetic actuators or soft actuators, function in shallow waters. However, in the deep sea, the robots face challenges, such as the miniaturization of the pressure protection unit and the driving method for rapid and agile motions. Herein, a jet‐driven method for a high‐pressure underwater environment is proposed by utilizing the high‐frequency vibration of a piezoelectric vibrator. Thereafter, an antihydropressure miniature robot with a body length of less than 5 cm is designed, which can perform the highly agile actions of floating, sinking, hovering, straight driving, and turning under a water pressure of 20 MPa (equal to the pressure under a water depth of 2000 m). A vertical velocity of 2.95 BH/s and a horizontal velocity of 3.22 BL/s are realized by the prototype, and it achieves faster motions than existing miniature underwater robots. Some potential applications have been realized, including small multicorner pipe exploration by carrying a camera, seaweed epidermal cell sampling in designated areas, and large object transportation by swarming, which proves the high maneuverability and agility of the developed robot. These merits make the robot ideal for multitasking operations in narrow environments with high water pressure, such as multiobstacle seabed.

DOI: 10.1002/aisy.202200262 Existing miniature underwater robots, which use electromagnetic actuators or soft actuators, function in shallow waters. However, in the deep sea, the robots face challenges, such as the miniaturization of the pressure protection unit and the driving method for rapid and agile motions. Herein, a jet-driven method for a highpressure underwater environment is proposed by utilizing the high-frequency vibration of a piezoelectric vibrator. Thereafter, an antihydropressure miniature robot with a body length of less than 5 cm is designed, which can perform the highly agile actions of floating, sinking, hovering, straight driving, and turning under a water pressure of 20 MPa (equal to the pressure under a water depth of 2000 m). A vertical velocity of 2.95 BH/s and a horizontal velocity of 3.22 BL/s are realized by the prototype, and it achieves faster motions than existing miniature underwater robots. Some potential applications have been realized, including small multicorner pipe exploration by carrying a camera, seaweed epidermal cell sampling in designated areas, and large object transportation by swarming, which proves the high maneuverability and agility of the developed robot. These merits make the robot ideal for multitasking operations in narrow environments with high water pressure, such as multiobstacle seabed.
Among these functional materials ( Figure S1, Supporting Information), piezoelectric ceramics (PZT) have the advantages of a fast response, high stiffness, high resolution, excellent electromagnetic compatibility, and good controllability. [43][44][45] Moreover, its inherent antipressure characteristic is compatible with the deep-sea environment, which enables it to obtain extensive applications. [46] However, the small vibration amplitude of PZT, usually tens of micrometers, is a significant shortcoming. The design of effective PZT actuators has become a significant topic for the miniaturization of underwater robots. Resonant piezoelectric actuators with rigid structures can transform microvibrations to macromotions based on electromechanical and friction coupling principles. [47] However, when this type of actuator works underwater, water lubrication exists in the friction interface, which may result in performance degradation or failure; that is, they are incapable of playing a stable driving stability. In addition, the rigid PZT can be modified into a soft MFC or PZT film to obtain a large deformation; [48,49] however, bionic fin-driven robots using these soft PZT materials usually have low speed because of their low working frequency. Therefore, a desirable driving method for piezoelectric materials is required to enhance the motion performances and working depth of miniature underwater robots.
Inspired by the water-jet propulsion mechanism of marine animals, we propose an underwater piezoelectric jetting method, which is described as that the high-frequency microvibration of a piezoelectric vibrator can induce a macro-directional jet ( Figure 1a). A miniature underwater robot is easily constructed by directly stacking three jet units, and it achieves several exceptional performances: 1) a compact overall size of 4.8 Â 2.9 Â 4 cm 3 ; 2) five highly agile locomotion modes under a maximum water pressure of 20 MPa, including floating, sinking, hovering motions in the vertical direction, and straight driving, turning motions in the horizontal direction (Movie S4, Supporting Information); 3) a maximum straight velocity of 155 mm s À1 (3.4 BL/s), which is much faster than the reported miniature underwater robots; and 4) good adaptability of the high-pressure environment and with no velocity attenuation at different water pressures. The experiments also demonstrated the high functionality of the individual robot: exploration in a small multicorner pipe by recording videos and sampling seaweed epidermal cells by carrying a homemade needle (Movie S5, Supporting Information). A cooperative system composed of two robots successfully transports a large object by imitating swarming behavior in nature (Movie S6, Supporting Information).
However, the disadvantage of an inadequate driving force for resisting water flow still exists in the prototype robot, especially under the unpredictable sea conditions. To address this issue, the robot can be carried on a remote operated vehicle (ROV) with a power supply. After arriving in an environment with quasistatic flow, it can detach from the ROV to accomplish the designed tasks, such as seabed exploration of multiple obstacles in narrow areas (slits, valleys, and pits), detection of polyporous structure of

Structure and Operation Principles
The water-jet driving mechanism of marine animals has guided the design of various underwater robots because of its high propulsion efficiency, high agility, and low disturbance. [50,51] The driving force is derived from the self-excited backward jet, formed by the muscle tissue squeezing the fluid in the cavity.
To emulate this mechanism on the premise of miniaturization, we employed an antipressure piezoelectric vibrator as an actuator to present two simple jet units ( Figure 2a): an axial jet unit and a lateral jet unit. The cavity of a jet unit is composed of only a piezoelectric vibrator and a shell with a single nozzle (the detailed structure is described in Figure S3, Supporting Information).
Owing to the high-frequency vibration of the piezoelectric vibrator, a kPa-level pressure fluctuation is generated in the cavity with volume changes, resulting in fluid ejection and ingestion through the nozzle. Considering their operation in narrow areas with high water pressure, underwater robots should have a miniaturized structure and multilocomotion mode. Therefore, by simply superimposing the jet units mentioned above, a miniature underwater robot with a size of 4.8 Â 2.9 Â 4 cm 3 and weight of 50 g was presented ( Figure 2b). This robot consists of two lateral jet units at the middle to realize 2D plane motions, and an axial jet unit at the bottom to realize vertical motions. A buoyancy adjustment part, using multiple deep-sea solid buoyancy blocks/plates wrapped in a shell, is located at the head of the robot. The shell was optimized as a streamlined dome (Figure 2c), considering the resistances of different head shapes ( Figure S2, Supporting Information). It is helpful to make the buoyancy www.advancedsciencenews.com www.advintellsyst.com of the robot slightly smaller than gravity in the standby condition (the details of the adjusting process are explained in Figure S3a, Supporting Information). The shell material of the three parts is PLA. As there is no transmission system, the overall robot can be easily insulated by coating the piezoelectric vibrators with waterproof glue ( Figure S4, Supporting Information). In addition, the robot can be directly immersed in a high-pressure environment without any antihydropressure devices, owing to the antipressure piezoelectric vibrator. There are no friction and wear problems during the working process, and water can dissipate the heat of the piezoelectric vibrator. These characteristics ensure the working stability of the piezoelectric jetting robot.
As the core driving component of the robot, the jetting unit can generate propulsion through its pulse jet caused by the high-frequency vibration of the piezoelectric vibrator. In contrast to the jet process in the marine animals, which is induced by slow ingestion and rapid ejection with low frequency, the high-frequency jetting propulsion mechanism should be systematically analyzed. First, we built a force measuring system (detailed system is explained in Figure S5a, Supporting Information), and the driving force of the bottom part, which originates from the axial jet unit, was measured under a square wave signal with a voltage of 250 V p-p and a frequency of 200 Hz (Figure 2d). The periodic variation in the obtained curve reflects the dynamic ingestion and ejection processes of the jet unit. We integrated the measured driving force, and found that the integral value is positive and increases cumulatively at a constant slope. The slope of the integral curve represents the average driving force (F av ) ( Figure 2e); the result demonstrates that the jet unit can perform propulsion effect. In contrast, the resistance during motion is negligible. Finally, based on the PIV experiment, we obtained the flow field characteristics near the nozzle when the bottom jet unit was in operation (Movie S1, Supporting Information). The simplified schematic of the streamline (Figure 2f ), taken from the tested flow field, illustrates that the fluid in the cavity is ejected at high momentum along the central area of the nozzle, whereas the ambient fluid is ingested at low momentum along the edge area. It can be concluded that the jet unit can yield sustained a propulsion effect due to the positive kinetic energy difference of the fluid during the periodic ingestion and ejection processes. A detailed description of this process is provided in the ( Figure S6, Supporting Information).
We propose an implementation strategy for the agile motion to control the proposed robot in executing complex tasks ( Figure 3a, Movie S2, Supporting Information). The driving force, generated by applying an excitation voltage to the piezoelectric vibrator, is along the inner normal direction of the corresponding nozzle, and can be adjusted by changing the voltage amplitude. By controlling the driving force of the jet unit in the bottom part (F 1 ), gravity (F g ), and buoyancy (F b ) of the robot, the motions of floating (F 1 þ F b > F g ), sinking (F 1 þ F b < F g ), and hovering (F 1 þ F b = F g ) can be realized; by coordinately tuning the driving forces of the jet units in the middle part (F 2 and F 3 ), the motions of straight driving (F 2 = F 3 ), turning counterclockwise (F 2 < F 3 ), and turning clockwise (F 2 > F 3 ) can be produced. To verify the proposed motion strategies, the robot realizes successfully motion along "S" in a water tank by the cooperation of multiple jet units ( Figure S7 and Movie S3, Supporting Information). The agility of the robot matched the jellyfish and squid by controlling only the excitation voltages of the jetting units.

Motion Characteristics
The driving performance of the jet units depends on the vibration characteristics of the piezoelectric vibrator, which are controlled by the excitation signals. A comparison of the driving characteristics under four types of waveforms indicated that the largest average driving force occurred under the square wave voltage, which was selected as the excitation signal in the following experiments (Figure 3b). Quantitative evaluations of the motion capability can provide a reference for on-demand operations. By applying varying frequencies on the jet unit of the bottom part ( Figure S8a, Supporting Information), it is observed that the robot can float as fast as possible at a frequency of 300 Hz, which is close to the simulated and tested resonant frequencies of the lateral unit ( Figure 3d; the specific simulated model and tested process are illustrated in Figure S9, Supporting Information). Similar to nature animals, [52,53] the resonance of the jetting unit can improve motion ability. Thereafter, the floating, hovering, and sinking motions are measured by applying signals with different voltage amplitudes. According to the tested results, a maximum floating velocity of 118.2 mm s À1 (equal to 2.95 BH/s) and a maximum sinking velocity of 69 mm s À1 (equal to 1.73 BH/s) are obtained in shallow water (Figure 3c). In addition to moving in the vertical direction, the robot can achieve horizontal motion by the middle part. Under a voltage of 210 V p-p , the velocity-frequency curve for straight driving reached a peak at a frequency of 130 Hz ( Figure S8b, Supporting Information), which was lower than the preferred motion frequency in the bottom part. A maximum straight velocity of 150 mm s À1 (equal to 3.22 BL/s) was measured. The reason for the frequency difference is that the bending direction of the vibrator of the lateral unit is perpendicular to the jet nozzle axis, indicating that the influence of water is more evident in attenuating its resonant frequency when compared with the axial unit (Figure 3e). The experimental results of the two units were in good agreement with the simulated results, as the deviations were within 10%. As expected, the optimal turning capability also appears at a frequency of 130 Hz ( Figure S8c, Supporting Information). The turning angular velocity reaches 6.5 rad s À1 and the turning radius decreases to 5.41 mm with an increase in the voltage, which proves that the robot can turn easily and quickly.

High-Pressure Environment Test
The harshness of the deep-sea environment is characterized by strong corrosiveness and extreme hydrostatic pressure. www.advancedsciencenews.com www.advintellsyst.com The piezoelectric vibrators were sealed with silica gel to effectively prevent corrosion. The driving mechanism of the jet unit and fabrication method of the robot indicates that it is theoretically compatible with a high-pressure environment. A high-pressure experimental system for simulating the deep-sea environment was built to test the performance (Figure 4a; the experimental setup is shown in Figure S10, Supporting Information). First, the multilocomotion mode of the robot under a water pressure of 20 MPa (equal to the pressure at a water depth of 2000 m) was effectively realized (Figure 4b, Movie 4). Thereafter, we obtained the floating performance of the robot under different water pressures (Figure 4c,d), and the results demonstrated that the velocity deviations were within 9.91%. Specifically, the motion performance of the piezoelectric jetting robot was slightly affected by water pressure. It can be concluded that the designed jet units, based on piezoelectric high-frequency microvibration, have great potential for operating in deep-sea environments, and they can fully utilize the advantages of the fast response and anti-pressure property of piezoelectric vibrators.

Application Tests of Our Robot
The overall size of our robot is on the centimeter scale, and it can realize highly agile motions. We tested the robot in typical scenarios to illustrate its potential for practical applications. One of them is the exploration of a multicorner pipe with a diameter of 11 cm (Figure 5a and MovieS5, Supporting Information, part I). First, the robot, carrying the camera, moves slowly to the target point. Subsequently, based on the feedback of the carried camera and combined with the control strategies described above, the robot agilely explores the entire multicorner pipe. Finally, it realizes a homeward voyage after exploration. Another typical application of our piezoelectric jetting robot is sampling of marine life (Figure 5b and Movie S5, Supporting Information, Part II). First, the robot, carrying a camera and sampling needle, approached the sampling target based on feedback from the camera. Then, the sampling process is conducted when the sampling needle contacted the target. Finally, the robot returned to its initial position using the samples. These results demonstrate that the proposed robot can complete multifunctional tasks in narrow areas (pipelines, corals, canyons, trenches). Thus, it is reasonable and efficient to use a single robot for these tasks. The swarming operation is an effective solution when the task is impossible for an individual robot, such as large object transport, similar to the behavior of a biological swarm. [54,55] We conducted collective performance tests. A square box that is approximately 2 times larger than a robot is successfully transported to the appointed location with the www.advancedsciencenews.com www.advintellsyst.com cooperation of the two robots ( Figure 5c and Movie S6, Supporting Information). Initially, two robots equipped with push rods were placed randomly away from the box. [51] Thereafter, they attempted to search and contact different surfaces of the object (t = 32 s), and push the target box collaboratively. During the pushing process, the driving forces of the two robots are adjusted in real time to compel the box to move straightly into storage in less time. Therefore, the proposed robot exhibits high agility and maneuverability, which are beneficial for the swarm operation of multiple robots.

Conclusion
Existing miniature underwater robots have insufficient agility to operate in narrow areas with high pressure, limiting further exploitation and scientific research on marine resources. In this study, we introduced a miniature piezoelectric jetting robot with antihydropressure capacity and multilocomotion mode. The robot, up to 5 cm in body length, effectively achieved vertical (floating, hovering, and sinking) and horizontal (straight driving and turning) motions under a water pressure of up to 20 MPa. Good adaptability was demonstrated by the consistent floating velocities under different water pressures. The proposed driving mechanism in a high-pressure environment was explained and verified. The maximum tested floating speed of 2.95 BH/s (118.2 mm s À1 ), straight speed of 3.22 BL/s (150 mm s À1 ), and turning angular velocity of 6.5 rad s À1 are noticeable. The proposed robot exhibits the merits of miniature size, high agility, and high mobility in comparison to previously reported bionic soft underwater robots and electromagnetic underwater robots ( Figure 6). [56][57][58][59][60][61][62][63][64][65] Moreover, its motion performance is comparable to or better than that of some typical marine animals. The robot presented in this work exhibited an exceptional operating ability. Specifically, by carrying a camera and sampling needle, the detection of small multicorner pipes and the sampling of seaweed epidermal cells were successfully accomplished. Furthermore, a simple swarm composed of two robots was programmed to realize the collaborative operation of pushing a large box. All experiments verified the practicability of the robot, allowing it to serve in challenging environments, such as narrow areas in the deep sea, to implement complex tasks. The cm-scale robot operation without cable is the research target in the future.

Experimental Section
Objectives of the Study: This study aims to realize the miniaturization of underwater robots to the centimeter level, and to achieve an agile and high-pressure environment adaptable operation. We proposed an underwater piezoelectric jetting method, and designed two kinds of jet units using this method. A miniature underwater robot with antihydropressure and multilocomotion modes was developed. A series of experiments were conducted to verify the application potential in real confined environments with high water pressures.
Materials and Fabrication: The miniature underwater robot consists of three piezoelectric jet units. Each jet unit is composed of PLA shells, piezoelectric vibrators, and buoyancy plates/blocks. The shell was made of PLA, and printed using a 3D printer (Z600, HORI, China). The piezoelectric vibrator was composed of a piezoelectric ceramic (PZT-5H, Zibo Yuhai Electronic Ceramics, China) and copper sheet. The material properties of the piezoelectric ceramic and copper sheet are listed in Table S1, Supporting Information. The fabrication process ( Figure S4a, Supporting Information) of the piezoelectric vibrator was as follows. 1) The negative electrode surface of the piezoelectric ceramic was evenly coated with the epoxy adhesive (E-HP120, Henkel Loctite, USA), and then pasted on a copper sheet. 2) After the epoxy adhesive was solidified for approximately 24 h (compressive strength ≥100 MPa and shearing strength ≥87.3 MPa), wires were welded on the positive surface of the piezoelectric ceramic and copper sheet. 3) To prevent short-circuit and seawater corrosion, the piezoelectric vibrator was coated with insulating soft silica gel (K-5707B, Kafuter, China) for approximately 12 h for curing. The connections between the piezoelectric vibrators, shells, and buoyancy blocks/plates were realized using epoxy adhesive (E-HP120, Henkel Loctite, USA), and approximately 24 h were required for solidification.
Control Platform: The piezoelectric jetting underwater robot adopted cable control, and the cables were used to apply voltage to the piezoelectric Figure 6. Performances comparison of the proposed robot with some marine animals, bionic soft underwater robots, and traditional electromagnetic underwater robots. The performances include three aspects: body length, velocity/body length ratio, and operating depth.
www.advancedsciencenews.com www.advintellsyst.com vibrators. Voltages were generated by a function generator (DG 4162, RIGOL, China), and then amplified by a power amplifier (E00.A3, CPREMORROW, China). The performance of the jet unit was controlled by varying the output voltage amplitude of the function generator. Performance Test Method: The driving performance of the robot primarily includes the driving force and motion velocity. As shown in Figure S5a, Supporting Information, the driving force of the jet unit was measured using two types of force sensors (MODEL 8111, ARIZON, China; MODEL 6023, ARIZON, China) and the corresponding data acquisition devices. The procedure for testing the driving force was as follows: first, two sensors were connected to the robot head by epoxy and fixed on rigid profiles in the water tank; the function generator and power amplifier (ATA-4051, Aigtek, China) were adopted to excite the bottom jet unit, and then the sensors converted the applied external force into corresponding signals, which were amplified by a transmitter (BSFY-1, BUFSON, China) and a charge amplifier (MODEL 6091, ARIZON, China), respectively; Finally, an analog-to-digital converter (9215, National Instruments, USA) transferred the amplified analog signals to digital signals, which were received and processed by a personal computer. Figure S5b,c, Supporting Information, illustrates the data-solving process.
The motion velocity test adopted a video-recording method. 1) Several white-scale boards were arranged around the motion range. 2) The entire motion process of the robots was captured into video by the surrounding cameras. 3) We selected the time periods (Δt) during which the robots moved smoothly, and measured the moving distance (Δl). 4) The motion velocities were obtained (Δl/Δt). For experiments under high pressure, the robot was placed in a high-pressure experimental system, including a camera and lamp, and the velocities were obtained using the video method. More details of the high-pressure experimental system are presented in Figure S10, Supporting Information.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.