Development of a Stiffness‐Adjustable Articulated Paddle and its Application to a Swimming Robot

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.

New Insights into Sea Turtle Propulsion and Their Cost of Transport Point to a Potential New Generation of High-Efficient Underwater Drones for Ocean Exploration

“…We compare the observed flow features against the turtle's thrust and lift forces flipper angle of attack (AoA), flipper velocity and power consumption, to offer insight into the ways in which each feature influences the animal's propulsion effort and cost of transport (COT). We compare the COT obtained by Cornelia against the current state-of-the-art swimming robotics [21][22][23][24][25][26][27][28][29], and we find Cornelia to produce extremely low values for the COT, pointing to the effectiveness of the sea turtle propulsive strategy.…”

Design and Analysis of a Novel Bionic Tensegrity Robotic Fish with a Continuum Body

“…Kwak et al proposed a stiffness-adjustable paddle with a sliding laminate-based method and integrated it into an untethered swimming robot. By displacing a flexible sheet with rigid elements to offset the alignment state with the opposite layer, two different stiffness states can be achieved [ 30 ]. However, these proposed variable stiffness mechanisms usually require an extra structure increasing system complexity.…”

“…As mentioned above, the robotic fish with discrete mechanisms usually integrate flexible components with only a single stiffness [ 18 , 19 , 20 ], the continuum robotic fish with stiffness variation mechanism commonly employ an extra structure [ 27 , 28 , 29 , 30 ], and the latest emerged robotic fish with tensegrity structure also adopt the pre-programmed stiffness and rarely pay attention to the maneuverability [ 34 , 35 , 36 ]. Toward these problems, there are two primary contributions of this study: First, based on a novel modular tensegrity structure, we propose a bionic flexible and continuum fish body that can realize the fish-like oscillation and online stiffness variation simultaneously.…”

Design and Reality-Based Modeling Optimization of a Flexible Passive Joint Paddle for Swimming Robots

“…Aquatic animals are adept at using passive changes in their body structures to adapt to or enhance their interactions with fluids, generating complex behaviors from simple movement patterns [ 1 , 2 ]. Flexible propellers can guide the fluid towards the preferred axis without much loss [ 3 ], with simple control modes and high mechanical efficiency [ 4 , 5 ]. This has motivated researchers to take a great interest in flexible propellers.…”