The fish pectoral fins, which play a vital role for fish swimming mobility, can perform three de-gree-of-freedom movements (rowing, feathering, and flapping motions), promoting that a lot of bionic robotic pectoral fins have been proposed and developed. However, these developed robotic pectoral fins driven by electromagnetic motors or smart materials still cannot fully realize the aforementioned three movement modes. To solve this problem, a novel piezoelectric robotic pectoral fin based on the converse piezoelectric effect and friction drive principle is proposed in this study, to achieve the three motion modes. Using a piezoelectric actuator, the robotic pectoral fin can be driven to move with three degree-of-freedom motion modes. Firstly, the overall structures of the proposed piezoelectric robotic pectoral fin and the designed piezoelectric actuator are explained in detailed. Additionally, a finite element simulation and a combination of vibration measurement and impedance analysis experiments are carried out to verify the effectiveness of the proposed piezoelectric actuator. Finally, an experi-mental investigation is conducted to evaluate output performances of the robotic pectoral fin proto-type. Experimental results indicated that 1) the maximum average velocities of the rowing and flap-ping motions of the pectoral fin prototype under an excitation voltage of 550 Vpp are 290 deg/s and 241.7 deg/s, respectively, and the maximum rotation speed of the feathering motion is 6.03 deg/s; 2) The maximum output forces of the rowing and flapping motions of the pectoral fin are 2.156 N and 2.107 N, respectively; 3) Rowing motion start stop response times are13 ms and 10.6ms, flapping motion start and stop response times are 14.2 ms and 9.2 ms, and feathering motion start/stop response times are 34 ms and 56.8 ms, respectively.

The flapping-motion of the caudal fin allows the fish to swim with high efficiency and mobility, particularly in terms of persistence, propulsion, and acceleration. This has led to theoretical and practical research on the development of robotic caudal fin thrusters that offer similar properties and performance. However, the current caudal fin thrusters are driven by electromagnetic motors, which require a transmission system that makes them difficult to miniaturize, and need protection against water intrusion. To address these issues, this paper proposes a novel piezoelectric caudal fin thruster with a fully open structure that has no chambers in any of its parts. The converse, piezoelectric effect and direct friction drive principle are used to make a rotation unit for the piezoelectric actuator drive and achieve a reciprocating motion that makes the caudal fin flap. The proposed piezoelectric caudal fin thruster has an open and simple structure. It has a weight of 30 g, a length of 89 mm, and a thrust of 0.07 N. It is easy to miniaturize and is lighter, smaller, and more efficient than previously reported caudal fin thrusters that were based on ionic polymer–metal composites and shape memory alloys. Experimental results verified the effectiveness of the proposed design, which can be easily scaled up or down in size depending on the operating situation.

Unmanned Underwater Vehicles (UUVs) play a vital role in marine exploration, and can achieve an extension of human hands and eyes to help researchers reach inaccessible and complex underwater spaces. However, the developed UUVs are driven by electromagnetic motors that also make it difficult to miniaturize due to the existence of the transmission system. Additionally, electromagnetic motors need to be protected to against water intrusion, especially in the deep sea. Therefore, in order to solve the above-mentioned problems, a novel structure-functional integration piezoelectric thruster for miniature UUVs is proposed in this study. Based on the converse piezoelectric effect and the direct friction drive principle, rudders and propellers can be alternately driven by a piezoelectric actuator, constructing the thrust unit and the steering unit of the piezoelectric thruster, respectively. Therefore, this makes the structure and function of the piezoelectric thruster integrated. Finite element simulations are first conducted to determine the geometrical sizes of the proposed actuator. The efficiency of the designed piezoelectric actuator is then confirmed using an underwater vibration measurement. Finally, experimental evaluations of the output performance of the piezoelectric thruster are performed. In the propulsion mode, the maximum rotation speed and thrust of the positive and negative propeller of the prototype with an excitation voltage of 600 Vpp were 404 rpm/0.10 N and 413 rpm/0.11 N, respectively. In the cooperative working mode, 20.00 kHz was used as the driving frequency, and the yaw and pitch rudders had the maximum average angle velocities of 92 deg/s and 90 deg/s for an excitation voltage of 600 Vpp, respectively. The maximum rotation speed and thrust of the positive and negative propellers of the prototype with an excitation voltage of 600 Vpp were 133 rpm/0.01 N and 132 rpm/0.01 N, respectively. According to experimental findings, the prototype thruster exhibits excellent mechanical properties.

The efficient swimming performance of fish is a miracle of nature, and the bionic robot fish powered by smart materials can replicate the movement pattern of fish to a greater extent. To achieve a simple, flexible, and controllable underwater robot, a novel robot fish driven by piezoelectric bimorphs is proposed in this study, which has similar swimming patterns to the body and/or caudal fin propulsion (BCF) swimming mode and can achieve straight travel and steering underwater. The validity and feasibility of the principles were verified by wet mode simulation. A prototype is manufactured and tested for underwater vibration characteristics to confirm the motion pattern of the caudal fin of the robot fish. It has a weight of 13.3 g, a length of 150 mm. The maximum uniform speed of the robot fish prototype is 53 mm/s, and the thrust is 2.213 mN, and its maximum efficiency is 0.864%.