A flexible passive joint for robotic fish pectoral fins: Design, dynamic modeling, and experimental results

Abstract

Pectoral fins are important actuation mechanisms in achieving maneuvering and propulsion for robotic fish. Existing designs predominantly adopt a rigid joint connecting the actuator to the pectoral fin, which requires differential actuation speeds in the power and recovery strokes in order to produce thrust and thus limits the overall actuation frequency. To address this problem, in this paper we propose a novel design of a flexible joint, which enables the pectoral fin to sweep back passively along the fish body in the recovery stroke, to minimize the drag, while maintaining the prescribed motion in the power stroke. A dynamic model is presented for a robotic fish propelled by pectoral fins incorporating such flexible joints, where the pectoral fin mechanism is modeled by two rigid segments connected with a pair of torsional spring and damper. This design results in a net thrust even with the same recovery and power stroke speed within each fin beat cycle, which simplifies the fin control. Experimental results on a robotic fish prototype are presented to validate the effectiveness of the proposed model, and to demonstrate the significant advantage of the proposed fin joint over the rigid joint. Furthermore, modeling analysis and experimental investigation are used to examine the influence of the joint length and stiffness on the locomotion performance.

Design and dynamic modeling of electrorheological fluid-based variable-stiffness fin for robotic fish

“…Fish propel in water by moving different fins or deforming the body [1,2], which has inspired the development of robotic fish that accomplish locomotion in ways that emulate those of biological fish [3][4][5][6][7][8]. Compared with rigid fins, flexible fins and fin joints introduce additional dynamic behavior that can be exploited to enhance robotic fish performance [9][10][11][12][13][14][15][16][17][18][19]. The optimal flexibility, however, typically changes with factors such as fin-beat frequency or amplitude [7,8,20,21].…”

Bio-inspired flexible joints with passive feathering for robotic fish pectoral fins

“…However, this method tends to significantly slow down the fish in the extended recovery stroke period [33]. This issue was addressed in [34], where the authors proposed a design of a passive joint for the rowing motion, which enables the pectoral fin to sweep back passively (along the same rowing axis) in order to minimize the drag force during the recovery stroke.…”

“…Finally, the writing has been polished throughout the paper. In another line of work by the authors [34,38], an alternative design of flexible joints for pectoral fins was proposed, where the flexible rowing joint allows the fin to sweep back passively (along the same rowing axis) during the recovery stroke. That work, which does not involve feathering motion, is complementary to the current paper with minimal overlap.…”