Piezoelectric-Actuated Flexible Undulating Fins for Biomimetic Surface Robot Propulsion: Design, Modeling, and Experimental Evaluation

Small unmanned surface vehicles operating in confined, vegetation-rich, or ecologically sensitive waters require compact propulsion systems that minimize entanglement and unintended contact. Here, we present a 178 mm biomimetic surface robot propelled by bilateral flexible fins actuated by piezoelectric bimorphs. A differential high-voltage circuit drives each bimorph, while compliant fin structures transmit local bending into distributed deformation. We characterized static and dynamic fin kinematics, blocked thrust, and untethered swimming performance. Static fin-tip deflection increased nearly linearly with voltage, with similar spatial deformation profiles across the tested voltages. In dry-bench tests, fin-tip velocity reached a local maximum near 142 Hz. Under water-loaded conditions, both mean blocked thrust and path-averaged swimming speed were highest at 134 Hz, consistent with a shift in the preferred operating frequency following fluid loading and system integration. At 100 V under this condition, the robot produced a mean blocked thrust of approximately 15.3 mN and achieved a path-averaged speed of 43.68 ± 3.46 mm/s (mean ± SD, n = 5). At a differential peak voltage of 100 V and a drive frequency of 134 Hz, full-system battery-side input power was approximately 2.0 W; the blocked-thrust-to-power ratio was approximately 7.8 mN W−1, and the system-level electrical cost of transport (COT) was approximately 17.2. These results show that local piezoelectric bending can be transmitted through compliant fins to generate measurable thrust and repeatable untethered motion, providing a blade-free propulsion approach for compact surface robots.