Abstract
Stiffness of a swimming appendage is the key mediator between thrust generated and its beating frequency. Due to the advantageous role of flexible propulsors, they are widely adopted in previous swimming robots. As an optimal propulsor, stiffness is highly dependent on its beating frequency, and stiffness modulation is crucial when a robot is swimming with multiple beating frequencies. Herein, a novel swimming paddle that can switch two different stiffness states by sliding a laminate inside and its application to a swimming robot is studied. This paddle has 8 articulated joints and 20 passive flaps to achieve drag asymmetry with minimum control effort. A semiempirical model to estimate the stiffness change in good accuracy is also studied. The thrust modulation caused by stiffness change is comprehensively studied by varying frequency and range of motion. In addition, a nontethered swimming robot propelled by a bilateral pair of paddles is developed to investigate when and how the stiffness adjustment is useful. There is a threshold frequency dividing two regimes where one stiffness excels the other stiffness with respect to cost of transport. Finally, it is shown that the paddle thickness is closely related to the necessity of stiffness change mechanism.
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