Passive Phase-Oriented Impedance Shaping for Rapid Acceleration in Soft Robotic Swimmers

TL;DR

This paper demonstrates that passive impedance shaping using constrained-layer damping significantly improves acceleration and velocity in soft robotic swimmers by passively optimizing force-motion phase relationships during propulsion.

Contribution

It introduces a passive impedance shaping method with constrained-layer damping to enhance transient propulsion in soft robots, avoiding complex control strategies.

Findings

Five-fold increase in peak acceleration

Three-fold increase in terminal velocity

Enhanced thrust and phase alignment across Strouhal numbers

Abstract

Rapid acceleration and burst maneuvers in underwater robots depend less on maintaining precise resonance and more on force--velocity phase alignment during thrust generation. In this work, we investigate constrained-layer damping (CLD) as a passive mechanism for frequency-selective impedance shaping in soft robotic swimmers. Unlike conventional stiffness-tuning approaches, CLD selectively amplifies the dissipative component of bending impedance while preserving storage stiffness, passively shifting the impedance composition toward dissipative dominance as actuation frequency increases. We characterize this behavior through dry impedance measurements, demonstrate that CLD enhances thrust and alters force--motion phase relationships across Strouhal numbers in constrained propulsion tests, and validate that passive impedance shaping yields a nearly five-fold increase in peak acceleration and a three-fold increase in terminal velocity in unconstrained swimming trials. These results establish phase-oriented passive impedance modulation as a simple, control-free pathway for improving transient propulsion in soft robotic systems.