Achieving Depth Control on “Bluekoi,” a Bio-Inspired Fish-Like Underwater Robot

Abstract

Studying aquatic organisms requires close-range observation that minimizes disruption to natural behaviors. Bio-inspired robotic fish offer a promising low-impact platform for underwater exploration, but achieving depth control in such systems remains challenging. This thesis presents the design, development, and evaluation of a depth control mechanism for the BlueKoi, a bio-inspired robotic fish developed by the SSR Nagpal Lab at Princeton University. Previously lacking vertical maneuverability, the BlueKoi achieved forward swimming and turning through a tuna-inspired tail actuation system. After evaluating several depth control strategies, including a sliding weight, a buoyancy control unit, and upward-facing propulsion, actuated pectoral dive planes were selected given the BlueKoi’s existing size and mass constraints. The final design features three main components: a magnetic coupling “shaft” that transmits actuation from inside the body to an external dive plane through the intact body wall, achieving complete waterproofing; a modular internal bevel gear-servo assembly achieving a dive plane rotation range of -45 to +35°; and an external hydrofoil mounting interface enabling easy hydrofoil interchangeability. Eight hydrofoil variants based on NACA 0012, 0015, 0018, and 0021 profiles of varying span and chord lengths were evaluated in a water channel test at the BlueKoi's approximate minimum swimming speed (0.19 m/s). The system demonstrated reliable waterproofing across sixteen trials, and force measurements confirmed that negative angles of attack consistently produced negative lift, establishing the dive planes' capacity to initiate dives. The NACA 0018 profile with increased span performed best. A free-swim test further confirmed effective depth control, with the BlueKoi executing repeatable dives and ascents at multiple speeds. Depth changes appeared driven primarily by sustained lift forces rather than body pitch, indicating that continuous actuation is required. These results establish the feasibility of actuated dive planes as a depth control mechanism for the BlueKoi, and motivate future work on pitch characterization, actuation refinement, and complementary approaches such as dynamic buoyancy control.