Tunabot Flex: a tuna-inspired robot with body flexibility improves high-performance swimming

Abstract

Tunas are flexible, high-performance open ocean swimmers that operate at high frequencies to achieve high swimming speeds. Most fish-like robotic systems operate at low frequencies (≤3 Hz) resulting in low swim speeds (≤1.5 body lengths per second), and the cost of transport (COT) is often one to four orders of magnitude higher than that of tunas. Furthermore, the impact of body flexibility on high-performance fish swimming remains unknown. Here we design and test a research platform based on yellowfin tuna (Thunnus albacares) to investigate the role of body flexibility and to close the performance gap between robotic and biological systems. This single-motor platform, termed Tunabot Flex, measures 25.5 cm in length. Flexibility is varied through joints in the tail to produce three tested configurations. We find that increasing body flexibility improves self-propelled swimming speeds on average by 0.5 body lengths per second while reducing the minimum COT by 53%. The most flexible configuration swims 4.60 body lengths per second with a tail beat frequency of 8.0 Hz and a COT measuring 18.4 J kg−1 m−1. We then compare these results in addition to the midline kinematics, stride length, and Strouhal number with yellowfin tuna data. The COT of Tunabot Flex’s most flexible configuration is less than a half-order of magnitude greater than that of yellowfin tuna across all tested speeds. Tunabot Flex provides a new baseline for the development of future bio-inspired underwater vehicles that aim to explore a fish-like, high-performance space and close the gap between engineered robotic systems and fish swimming ability.

Tuna robotics: hydrodynamics of rapid linear accelerations

“…Peak power measurements are indicated by mechanical power peak P M , initial electrical power peak P (1) E (maximum electrical power value at first spike during acceleration), secondary electrical power peak P (2) E (maximum electrical power value of the oscillating power signal after initial spike in power), and t peak denotes the time to reach the initial electrical power peak. of the four body joints [16]. Furthermore, we found that the minimum body amplitude of (4:39 + 0:48) 10 À3 BL (mean ± s.d.)…”

“…Tunabot Flex improved the mechanical and bioinspired designs of its predecessor, including variable body flexibility, while using the same external dimensions and motor. Added body flexibility was accomplished using a design that also enabled the generation of constant tail beat amplitudes at all tested frequencies, and the addition of body segments greatly increased swimming performance [16]. This body segment design involves small gaps between segments that open and close during swimming (figure 1), but visualization of flow in these regions indicated that water rapidly moved both in and out and did not reduce swimming performance.…”

“…This body segment design involves small gaps between segments that open and close during swimming (figure 1), but visualization of flow in these regions indicated that water rapidly moved both in and out and did not reduce swimming performance. In addition, White et al [16] demonstrated that steady swimming performance improved even when the number of gaps increased, and that passive body drag variations among models with different numbers of gaps were negligible.…”

“…Complete details of power measurement methodology (e.g. circuitry and calculations) and tail beat frequency control are presented in [16].…”

“…In this study, we use a newly developed robotic platform inspired by yellowfin tuna (the Tunabot Flex [16]) to extend current analyses of acceleration patterns in fish swimming by inducing linear accelerations from an initial rest position at zero velocity. Thunniform swimmers heavily rely on liftbased propulsion generated by their lunate-shaped caudal fins.…”