A Soft Robotic Model to Study the Effects of Stiffness on Fish-Like Undulatory Swimming

Abstract

Fish swim using a combination of active and passive movements. Active swimming is generated by muscles that produce an undulatory wave passing down the body of fish. The undulating body imparts momentum to the water and the fish moves forward. Passive movement, in contrast, occurs when fluid pushes on the body, resulting in undulatory motion without muscular involvement. Animals may simultaneously generate active movement while also experiencing the consequences of morphological characteristics such as fins that generate passive movement. Passive models of fish are easily manufactured, but not sufficiently complex to realistically model fish swimming. Hard robotic models can be active and complex, but they are difficult to design and manufacture. We have designed two soft robotic fish models (pneufish) using pneumatic actuators, or pneunets, that produce active swimming by deforming a central plastic backbone. This apparatus can have either a single pair of actuators (the duo model) on each side, or two pairs of pneumatic actuators per side forming a quad configuration. Using these active pneumatic models, we demonstrate an important interaction between activation frequency and stiffness, and that variable longitudinal stiffness between anterior and posterior pneunets can lead to an increase in thrust without simultaneously increasing lateral forces. Lateral forces are an unavoidable consequence of undulatory body motion, but can be reduced by appropriate phasing of body and fin motion.