Organizations: Department of Mechanical Engineering, The University of Tokyo, Tokyo, 113-8656, Japan · Institute of Industrial Science, The University of Tokyo, Tokyo, 153-8505, Japan
Robotic fish have attracted growing attention in recent years owing to their biomimetic design and potential applications in environmental monitoring and biological surveys. Among robotic fish employing the Body-Caudal Fin (BCF) locomotion pattern, motor-driven actuation is widely adopted. Some approaches utilize multiple servo motors to achieve precise body curvature control, while others employ a brushless motor to drive the tail via wire or rod, enabling higher oscillation and swimming speeds. However, the former approaches typically result in limited swimming speed, whereas the latter suffer from poor maneuverability, with few capable of smooth turning. To address this trade-off, we develop a wire-driven robotic fish equipped with a 2-degree-of-freedom (DoF) crank-slider mechanism that decouples propulsion from steering, enabling both high swimming speed and agile maneuvering. In this paper, we first present the design of the robotic fish, including the elastic skeleton, waterproof structure, and the actuation mechanism that realizes the decoupling. We then establish the actuation modeling and body dynamics to analyze the locomotion behavior. Furthermore, we propose a combined feedforward-feedback control strategy to achieve independent regulation of propulsion and steering. Finally, we validate the feasibility of the design, modeling, and control through a series of prototype experiments, demonstrating swimming, turning, and directional control.
Figures & tables
Fig. 1: The biomimetic fish robot based on a 2-DoF crank-slider mechanism
Fig. 2: Hardware design of the robotic fish: (a) rigid head shell; (b) segmented body sections; (c) passive caudal fin; (d) 2-DoF crank–slider actuation mechanism; (e) elastic spine; (f) driving wire; (g) pectoral fins.
Fig. 3: The 2-DoF crank-slider based actuation mechanism.
Fig. 4: Waterproof design of the fish: (a) O-ring based primary waterproof structure for the rigid head; (b) the skin composed of two sections that enclose the entire body of the robotic fish.
Fig. 5: Operating modes of the 2-DoF crank–slider actuation mechanism for directional control. (a) Principle of the symmetric mode; (b) reel output in symmetric mode; (c) principle of the asymmetric mode; (d) reel output in asymmetric mode.
Fig. 6: Dynamic model of the elastic tail.
Fig. 7: Required motor torque for achieving 1Hz oscillation, estimated using the dynamic model.
Fig. 8: Control framework of the robotic fish. (a) Controlled volume and reference frames; (b) block diagram of the control system; (c) swimming direction during body oscillation.
Fig. 9: Prototype of the robotic fish and its key components. (a) microcontroller unit (MCU) and onboard PC; (b) actuation mechanism; (c) waterproof silicone skin; (d) waterproof structure of the rigid head shell; (e) motors used in the prototype.
Fig. 10: Software architecture of the robotic fish.
Fig. 11: Flapping-in-air experiment for validating the theoretical flapping angle of the robotic fish. (a) Motion capture balls attached to each body section to measure position and orientation; (b) comparison between theoretical prediction and experimental data.
Fig. 12: Forward swimming experiment of the robotic fish in symmetric mode. (a) Swimming trajectory recorded in a water tank; (b) real-time position obtained from motion capture; (c) IMU measurements and motor loads during swimming.
Fig. 13: Turning experiment of the robotic fish in asymmetric mode. (a) Circular swimming trajectory in a water tank; (b) real-time position recorded by motion capture; (c) IMU measurements and motor loads during turning.
Fig. 14: Direction control experiment of the robotic fish in asymmetric mode. (a) Swimming trajectory in a water tank; (b) real-time position recorded by motion capture; (c) comparison of reference, actual, and motion-capture angles.
FAMU/FSU College of Engineering, Tallahassee, FL 323109 · Mechanical Engineering, Vanderbilt University, Nashville Tennessee 37235 · Mechanical Engineering, The University of Tennessee, Knoxville, Tennessee 37996