Tunable stiffness in fish robotics: mechanisms and advantages

Abstract

One of the emerging themes of fish-inspired robotics is flexibility. Adding flexibility to the body, joints, or fins of fish-inspired robots can significantly improve thrust and/or efficiency during locomotion. However, the optimal stiffness depends on variables such as swimming speed, so there is no one “best” stiffness that maximizes efficiency in all conditions. Fish are thought to solve this problem by using muscular activity to tune their body and fin stiffness in real-time. Inspired by fish, some recent robots sport polymer actuators, adjustable leaf springs, or artificial tendons that tune stiffness mechanically. Models and water channel tests are providing a theoretical framework for stiffness-tuning strategies that devices can implement. The strategies can be thought of as analogous to car transmissions, which allow users to improve efficiency by tuning gear ratio with driving speed. We provide an overview of the latest discoveries about 1) the propulsive benefits of flexibility, particularly tunable flexibility, and 2) the mechanisms and strategies that fish and fish-inspired robots use to tune stiffness while swimming.

Research Development on Fish Swimming

“…Fish can modulates body stiffness actively to adjust its swimming performance through muscles, tendons and other biological tissues [179][180][181]. Through testing biological structures, Long Jr supported the biological hypothesis that fish swimming behaviors are controlled by the body stiffness and the stiffness can be altered by the vertebral column [182].…”

Energy-efficient tunable-stiffness soft robots using second moment of area actuation

“…Zhong et al [20] have shown experimentally that tuning the stiffness of their tuna-like robot allowed them to double their swimming efficiency at speeds from 0 to 2 body lengths per second. Quinn and Lauder [15] explain how fish tune their stiffness through muscle contraction, fin and tail shape alteration, and skin surface changes. They compare this to current swimming robot stiffening methods including structural control [21], mechanical control [22], and intrinsic rigidity control [23].…”

“…Flexibility is important for efficient animal swimming, and by extension, swimming robots [15]. By being flexible, swimmers are able to make use of resonance.…”

Energy-Aware Control of Euler–Bernoulli Beams by Means of an Axial Load

“…I N THE last years, evidences that controlled mechanical systems would benefit from the possibility of actively changing their stiffness properties have been accumulated in the literature. These benefits include energy-efficiency, prevention of safety hazards, and more generally the possibility of providing new functionalities to the controlled system [1]- [3]. For an extensive discussion regarding this theme see [4], where the authors indicated their vision on distributed variable stiffness actuation combined with embedded sensing, which is embodied in this work using as plant the continuum mechanics of an Euler-Bernoulli beam.…”