Design, control, and implementation of an agile swimming robotic fish with marine applications

Abstract

Fishes have mastered the art of swimming over the many centuries through evolution. Experimental biologists and fluid dynamicists have studied their swimming biomechanics, propulsion, and maneuverability since the early 1900s. While there have been significant developments in the theoretical and investigative segments on how fish swim, the development of its robotic counterparts was at its nascent stage. Most existing designs of robotic fish often faced a trade-off between fast swimming and directional control, with most results confined to laboratory environments. The design of the robot fish dubbed Snapp, an agile bio-inspired swimming robotic fish, attempts to integrate both speed and maneuverability, exhibiting high-speed swimming and comprehensive 3-dimensional maneuverability in open waters, marking a significant advancement in aquatic robotics. The innovative aspect of Snapp lies in its novel cyclic-differential method integrated into its propulsion mechanism. This method synergizes propulsion with yaw-steering, enabling swift and precise course corrections. Additionally, Snapp features two independently controlled pectoral fins, crucial for advanced pitch and roll control. The core challenge was replicating real fish’s seamless, coordinated movement in a mechanical structure. This required a deep exploration of the integration between different fields of fluid dynamics, biomechanics, and robotics, leading to the development of a system that could mimic the undulatory swimming patterns of fish. The scotch-yoke mechanism employed in Snapp converts rotary motor motion into a sinusoidal lateral movement, effectively capturing the essence of fish-like undulatory motion. The specially designed compliant tail-fin encompasses the attempt to embed mechanical intelligence to simplify the complex gait. Control over oscillation parameters such as frequency and amplitude enables Snapp to emulate the swimming dynamics of real fish closely. Integrating a cyclicdifferential method for yaw turning and the independent control of pectoral fins for pitch and roll movements allows Snapp to navigate with unprecedented control and agility in aquatic environments. This control system, combined with the advanced design of the propulsion mechanism, positions Snapp at the forefront of robotic fish technology. In our evaluations conducted in open water environments, Snapp demonstrated substantial improvements in speed and maneuverability. It achieved swimming speeds of up to 1.5 m/s (1.7 body lengths per second) and performed complex maneuvers, including figure-8 and S-shaped trajectories. Snapp is capable of instantaneous yaw changes of 15 degrees in just 0.4 seconds, a minimal turn radius of 0.85 meters, and maximum pitch and roll rates of 3.5 rad/s and 1 rad/s, respectively. These results highlight Snapp’s potential for wide-ranging applications in open seas, including ecological monitoring, underwater infrastructure inspection, and marine research. A specific application would be for coastal ocean monitoring, where Snapp can swim against rough waves and complex coast terrain.