Biomimetic Mantaray robot toward the underwater autonomous -- Experimental verification of swimming and diving by flapping motion -

This study presents the development and experimental verification of a biomimetic manta ray robot for underwater autonomous exploration. Inspired by manta rays, the robot uses flapping motion for propulsion to minimize seabed disturbance and enhance efficiency compared to traditional screw propulsion. The robot features pectoral fins driven by servo motors and a streamlined control box to reduce fluid resistance. The control system, powered by a Raspberry Pi 3B, includes an IMU and pressure sensor for real-time monitoring and control. Experiments in a pool assessed the robot’s swimming and diving capabilities. Results show stable swimming and diving motions with PD control. The robot is suitable for applications in environments like aquariums and fish nurseries, requiring minimal disturbance and efficient maneuverability. Our findings demonstrate the potential of bio-inspired robotic designs to improve ecological monitoring and underwater exploration.

💡 Research Summary

The paper presents the design, fabrication, and experimental validation of a biomimetic manta‑ray robot that propels itself by flapping its pectoral fins. The authors argue that conventional screw‑propelled underwater vehicles disturb the seabed and generate turbidity, which is undesirable for ecological monitoring and visual inspection tasks. Inspired by the median‑and‑pectoral‑fin (MPF) locomotion of manta rays, the robot uses two servo motors per fin to generate a combined flapping (X‑axis) and feathering (Y‑axis) motion, reproducing the 90° phase offset that maximizes thrust in natural manta rays.

The mechanical design adopts the NACA 0020 airfoil shape for both the control box and the fins, reducing hydrodynamic drag. The fins consist of a rigid acrylic‑resin core and a flexible rubber‑like resin periphery, fabricated with a Stratasys Objet Eden260VS 3‑D printer. The overall dimensions are 360 mm × 750 mm × 70 mm, with a mass of 2.15 kg. The electronics comprise a Raspberry Pi 3 Model B, an Arduino Nano, an MPU‑9050 IMU, and an LPS33HW pressure sensor. Power is supplied by separate batteries, giving roughly 50 minutes of operation. Communication between the Pi and the Arduino occurs via USB; Wi‑Fi (augmented by a USB dongle) enables remote monitoring.

Control is based on a simple proportional‑derivative (PD) law that corrects the yaw angle measured by the IMU. The error signal (θ_err = θ_yaw − θ_d) drives adjustments to the left and right flapping amplitudes: θ_flR = θ_flR0 − k_p θ_err − k_d Δθ_err, θ_flL = θ_flL0 + k_p θ_err + k_d Δθ_err. Flapping and feathering angles follow sinusoidal trajectories: θ_fl = θ_flmax sin(2πft), θ_fe = θ_femax sin(2πft − π/2), with θ_flmax = 30°, θ_femax = 45°, and f = 0.75 Hz, ensuring the 90° phase difference.

Two sets of experiments were conducted in a 15 m × 2 m × 0.485 m pool. The first test examined surface swimming in a straight line. Without PD control, the robot deviated significantly, showing mean trajectory errors up to 10 cm and a standard deviation of 5 cm. With PD control, the mean error dropped to 2.8 cm (max 5.2 cm) and the standard deviation fell to 1.2 cm. Yaw variations remained within roughly –6° to +4°, and the forward speed stabilized at about 20 cm s⁻¹ after the initial 5 s. The second test added a diving phase while maintaining straight motion. Up to a distance of 200 cm the PD controller kept the robot on course, but the robot then collided with the pool bottom, causing a sudden change in yaw and a loss of control. This highlighted the limitation of the simple PD scheme when faced with large external disturbances such as impact or rapid pressure changes.

The authors compare their platform with prior manta‑ray robots that used soft actuators, dielectric elastomers, or multi‑stable structures. Their contribution lies in (1) adopting a realistic airfoil geometry for drag reduction, (2) demonstrating that low‑cost, off‑the‑shelf electronics combined with a straightforward PD controller can achieve stable flapping propulsion, and (3) providing quantitative performance data (≈20 cm s⁻¹ speed, <5 cm trajectory error) for both surface and submerged operation.

Limitations identified include the relatively short battery life, torque constraints of the hobby servos, and the inability of the PD controller to handle abrupt disturbances such as collisions or sudden depth changes. The authors suggest future work on more sophisticated control strategies—model‑predictive control, adaptive or reinforcement‑learning based controllers—and on integrating gliding phases to recover energy, thereby extending mission endurance. They also envision adding more degrees of freedom (e.g., tail fin, additional pectoral segments) to enable complex maneuvers like somersaults or rapid turns.

In conclusion, the study validates that a biomimetic manta‑ray robot driven by flapping fins can provide low‑disturbance, efficient locomotion suitable for aquarium, fish‑nursery, or shallow‑water ecological monitoring. With further advances in control algorithms and energy management, such platforms could become viable autonomous underwater vehicles for a broad range of scientific and environmental applications.