Overview: For the Fluids & Mechatronics Laboratory, this project aimed to improve the propulsive efficiency of bio-inspired underwater vehicles by mimicking fish fin elasticity. I designed a tunable stiffness mechanism for the lab vehicle's hydrofoil fin that emulates the variable stiffness of actual fish fins, improving thrust during flapping propulsion.
Background:
Fish display efficient swimming capabilities not replicable using traditional propellers. Studying and replicating these natural unsteady propulsors can improve underwater unmanned vehicle design. Biological fins varying in stiffness both spatially and temporally, through musculoskeletal control, optimizing thrust. The lab's current fin mechanism involves a rigid 3D-printed hydrofoil that actuates ("flaps") about a shaft at its leading edge (Fig. 1). They required a way to temporarily manipulate the fin's stiffness through the effective torsional spring constant of this shaft.
Objective:
Develop a tunable stiffness mechanism for a robotic hydrofoil fin that can vary its torsional stiffness in real-time during flapping motions for future experimental studies.
Methodology:
The fin's leading edge is fixed to a shaft. I designed a new mechanism employing a flexible PLA beam fixed to this shaft (Fig. 2, 3). A pin carriage can move along the length of this beam via motorized belt drive, changing the effective beam length and, consequently, the equivalent torsional spring constant of the fin's shaft. The carriage was belt-driven by an Arduino-controlled stepper motor, housed within my design. This whole mechanism and its electronics had to be mountable on the existing experimental rig consisting of a shaft-heaving mechanism fixed to a larger stationary frame (Fig. 2).
Detailed CAD models were produced to optimize the dimensions for material efficiency and functionality. The actuator (a stepper motor) was programmed via Arduino to be able to adjust the effective fin stiffness at any moment (Fig. 3). To be able to achieve desired fin stiffness values, however, the underlying elastic behavior of the 3D-printed stiffness mechanism (beam and pin) needed to be modeled mathematically based on experimental data.
I derived the mechanism’s torsional stiffness as a function of pin position by hand using Euler–Bernoulli beam theory, then experimentally via static calibration tests, where known torques were applied to the fin (Fig. 4). Angular deflections were measured to establish the torque-deflection relationship of the fin shaft. Data were recorded to determine the effective torsional stiffness (k_eq) at different pin positions along the beam (Fig. 5,6).
Key Results:
Calibration tests confirmed the linear behavior of the stiffness mechanism with an R² value of 0.98 in the torque vs. deflection regression models. Hence, the bending beam mechanism is equivalent to a torsional spring attached to the central shaft, by Hooke's law, and the derived elastic function (pin position vs. k_eq) can be used to program this mechanism to modulate the effective fin stiffness in any time-varying pattern (Fig. 6).
Conclusion:
These results demonstrate that the hydrofoil stiffness modulator can reliably vary fin stiffness, enabling experimental analysis of bio-inspired propulsion. This mechanism can be used to significantly enhance propulsive efficiency by optimizing thrust in sync with the flapping cycle, with implications for the design of next-generation underwater vehicles. My work, from detailed CAD design to rigorous experimental validation, contributes a replicable approach for integrating tunable mechanical properties in dynamic fluid environments. The lab can now use my mechanism in their fin propulsion rig to understand the hydrodynamic impact of variable fin stiffness in water channel tests.
Hydrofoil_Stiffness_Modulator_Poster_Prasanna_Krishnamoorthy.pdf