Online Magnetorheological Damping Modulation of a Robotic Tuna for Enhanced Speed and Efficiency

Biological tunas are exceptional open-ocean swimmers, maintaining superior swimming capabilities. A key to this achievement is their powerful contraction of the peduncle tendon muscle, which effectively transmits energy to the caudal fin. Drawing inspiration from this biomechanical principle, this article develops a robotic tuna prototype that integrates a lightweight magnetorheological (MR) damper that emulates caudal keel muscle functionality. An increase in the excitation current in the inner coil of the damper results in an increase in the shear stress of the MR fluid, thereby enhancing the output damping torque. Experimental results demonstrate a tunable damping torque range from 86.6 to 213.5 mN$\cdot$m. A Lagrangian-based dynamic model is employed to analyze the effects of current intensity on propulsion performance. The load-bearing capacity of the peduncle structure is evaluated across three weight loads (300, 400, and 500 g), demonstrating improvements of 92.4%, 50.1%, and 31.64% with online current adjustment, respectively. Furthermore, swimming experiments reveal a maximum speed of 0.64 BL/s and a minimum cost of transport of 45.18 J/m/kg. Compared to a single elastic mechanism, the proposed MR damping modulation mechanism enhances propulsion performance, especially increasing speed by a factor of 2.38 and efficiency by a factor of 1.56 at a high frequency of 4.5 Hz.