Learning to Turn: Deformation Control of a Novel Flexible Fishtail Actuated by Artificial Muscles

Achieving agile, fish-like turning remains a critical challenge in the development of robotic fish. The flexible deformation of a fish’s body significantly contributes to its turning ability. Consequently, researchers have devoted substantial efforts to enhancing the compliance and agility of robotic fish. This paper presents a novel flexible fishtail, inspired by the musculoskeletal structure of real fish, which can control the deformation of its flexible components during turns, improving the turning performance of robotic fish. The fishtail primarily consists of a carbon fiber plate with attached artificial muscles. These muscles can contract and stretch, causing the carbon fiber tail to bend and deform. To enable precise deformation control, a dynamic model of the fishtail is established based on Hamilton’s principle. Subsequently, deep reinforcement learning methods such as deep deterministic policy gradient and soft actor-critic are employed to develop a deformation controller under joint deflection. Cross-validation ensures the controller’s accuracy and robustness. The experimental results demonstrate that this controller facilitates the deformation control of the flexible tail across various turning amplitudes and speeds, allowing the fishtail to exhibit adjustable compliance or resistance to fluids during turning motion. Consequently, this enhances the agility of the robotic fish.

I. Introduction

Natural selection has endowed fish with exceptional swimming abilities, enabling high-speed, high-maneuverability, and high-efficiency underwater propulsion. Among these abilities, the capability for rapid, small-radius turning is particularly remarkable, with many fish able to execute sharp turns even in confined spaces. For fish employing the body and/or caudal fin (BCF) propulsion mode [1], the ability to actively bend and deform their tails and the compliance of their caudal fins are crucial for agile turning. These fish control the shape of their bodies and fins through complex muscular systems and flexible tendons, allowing the turning torque of the tail to be efficiently transmitted along the body to the caudal fin, interacting with the surrounding fluid to achieve rapid turns.