The present study focuses on summarizing the recent advancements in the field of fish swimming mode research and bionic robotic fish prototypes based on smart materials. It has been widely acknowledged that fish exhibit exceptional swimming efficiency and manoeuvrability compared to conventional underwater vehicles. In the pursuit of developing autonomous underwater vehicles (AUVs), conventional experimental methods often prove to be complex and expensive. Hence, the utilization of computer simulations for hydrodynamic modelling provides a cost-effective and efficient approach for analysing the swimming behaviour of bionic robotic fish. Additionally, computer simulations can provide data that are difficult to obtain through experimental methods. Smart materials, which integrate perception, drive, and control functions, are increasingly being applied to bionic robotic fish research. However, the utilization of smart materials in this field is still an area of ongoing research and several challenges remain unresolved. This study provides an overview of the current state of research on fish swimming modes and the development of hydrodynamic modelling. The application of four distinct types of smart materials in bionic robotic fish is then reviewed, with a focus on analysing the advantages and disadvantages of each material in driving swimming behaviour. In conclusion, the paper highlights the key technical challenges that must be addressed for the practical implementation of bionic robotic fish and provides insights into the potential future directions of this field.

An analytical hydrodynamics model for a piezoelectric micro-robotic fish with double caudal fins is presented in this paper. The relation between displacement of the piezoelectric actuator and oscillating angle of the caudal fin is established based on the analysis of the flexible four-bar linkage transmission. The hydrodynamics of caudal fins are described by airfoil and blade element theories. Furthermore, the dynamics and kinetics of the whole micro-robotic fish are analyzed and validated by experiments.

Sensors based on optical interferometric displacement measurement have the outstanding advantage of high sensitivity and are widely applied in various measurement fields. In particular, with the development of micro-opto-electromechanical system technology, grating-interferometer-based sensors have drawn considerable attention and have been developed rapidly, due to their miniature structure and integrated nature. This paper reviews the previous research on the sensing principle, structural integration, and signal demodulation optimization of integrated microsensors based on grating interferometers. To improve the microscale sensing principle, the diffraction characteristics of the micrograting require more accurate analysis, when the critical dimension of the optical path approaches the light wavelength. To achieve a highly integrated system, design and microfabrication for integrating the microsensitive structures, micrograting and photoelectric modules are gradually being developed. To improve the performance of optical interference sensors, extending their measurement ranges while maintaining high sensitivity and high precision with limited integrated microsystems is becoming a research hotspot. This article concludes by indicating the current challenges and potential future directions in this field.

Micro-sensors, such as pressure and flow sensors, are usually adopted to attain actual fluid information around swimming biomimetic robotic fish for hydrodynamic analysis and control. However, most of the reported micro-sensors are mounted discretely on body surfaces of robotic fish and it is impossible to analyzed the hydrodynamics between the caudal fin and the fluid. In this work, a biomimetic caudal fin integrated with a resistive pressure sensor is designed and fabricated by laser machined conductive carbon fibre composites. To analyze the pressure exerted on the caudal fin during underwater oscillation, the pressure on the caudal fin is measured under different oscillating frequencies and angles. Then a model developed from Bernoulli equation indicates that the maximum pressure difference is linear to the quadratic power of the oscillating frequency and the maximum oscillating angle. The fluid disturbance generated by caudal fin oscillating increases with an increase of oscillating frequency, resulting in the decrease of the efficiency of converting the kinetic energy of the caudal fin oscillation into the pressure difference on both sides of the caudal fin. However, perhaps due to the longer stability time of the disturbed fluid, this conversion efficiency increases with the increase of the maximum oscillating angle. Additionally, the pressure variation of the caudal fin oscillating with continuous different oscillating angles is also demonstrated to be detected effectively. It is suggested that the caudal fin integrated with the pressure sensor could be used for sensing the in situ flow field in real time and analyzing the hydrodynamics of biomimetic robotic fish.

In this study, based on the principle of grating interferometer-based acoustic sensors, design guidelines for the grating interferometric module were obtained and analyzed considering various factors in order to obtain high sensitivity, and a glass-based grating interference component and its acoustic sensor device were developed. The key parameters of the grating interference structure were extracted, and their effects on the acoustic response sensitivity were quantified for multiple mechanisms. For the development of the acoustic sensor, the grating-on-convex-platform structure and the micromachining processes of the glass-based components were designed and developed. The developed acoustic sensor samples achieved high sensitivity. In particular, the sample suitable for low-frequency application obtained a sensitivity of 0.776 V/Pa @ 1 kHz, and the spectrum of its sensitivity was flat from 50 Hz to 8 Hz with a deviation less than 1.5 dB and a sensitivity of 0.408 V/Pa @ 20 Hz.