Design considerations for an underwater soft-robot inspired from marine invertebrates

Recent progress on underwater soft robots: adhesion, grabbing, actuating, and sensing

The research on biomimetic robots, especially soft robots with flexible materials as the main structure, is constantly being explored. It integrates multi-disciplinary content, such as bionics, material science, mechatronics engineering, and control theory, and belongs to the cross-disciplinary field related to mechanical bionics and biological manufacturing. With the continuous development of various related disciplines, this area has become a hot research field. Particularly with the development of practical technologies such as 3D printing technology, shape memory alloy, piezoelectric materials, and hydrogels at the present stage, the functions and forms of soft robots are constantly being further developed, and a variety of new soft robots keep emerging. Soft robots, combined with their own materials or structural characteristics of large deformation, have almost unlimited degrees of freedom (DoF) compared with rigid robots, which also provide a more reliable structural basis for soft robots to adapt to the natural environment. Therefore, soft robots will have extremely strong adaptability in some special conditions. As a type of robot made of flexible materials, the changeable pose structure of soft robots is especially suitable for the large application environment of the ocean. Soft robots working underwater can better mimic the movement characteristics of marine life in the hope of achieving more complex underwater tasks. The main focus of this paper is to classify different types of underwater organisms according to their common motion modes, focusing on the achievements of some bionic mechanisms in different functional fields that have imitated various motion modes underwater in recent years (e.g., the underwater sucking glove, the underwater Gripper, and the self-powered soft robot). The development of various task types (e.g., grasping, adhesive, driving or swimming, and sensing functions) and mechanism realization forms of the underwater soft robot are described based on this article.

Lessons for Robotics From the Control Architecture of the Octopus

Biological and artificial agents are faced with many of the same computational and mechanical problems, thus strategies evolved in the biological realm can serve as inspiration for robotic development. The octopus in particular represents an attractive model for biologically-inspired robotic design, as has been recognized for the emerging field of soft robotics. Conventional global planning-based approaches to controlling the large number of degrees of freedom in an octopus arm would be computationally intractable. Instead, the octopus appears to exploit a distributed control architecture that enables effective and computationally efficient arm control. Here we will describe the neuroanatomical organization of the octopus peripheral nervous system and discuss how this distributed neural network is specialized for effectively mediating decisions made by the central brain and the continuous actuation of limbs possessing an extremely large number of degrees of freedom. We propose top-down and bottom-up control strategies that we hypothesize the octopus employs in the control of its soft body. We suggest that these strategies can serve as useful elements in the design and development of soft-bodied robotics.

An Underwater Glider with Muscle—Actuated Buoyancy Control and Caudal Fin Turning

Underwater robotic gliders exploit gravity and buoyancy for long-distance cruising with ultra-low energy consumptions, making them ideal for open ocean surveying operations. However, the gliding-based motion generation principle also prevents their maneuverability, limiting their use in the short distances that are usually encountered in harbors or coastal scenarios. In this work, an innovative underwater glider robot is developed, enabling maneuverability through the introduction of an efficiently actuated caudal fin with bidirectional turning capabilities. In addition, modular actuator units, based on soft actuated materials, are integrated to control pitch angle by dynamically shifting the center of mass from the center of buoyancy. As a result, the high energy efficiency feature of the gliders is maintained, while high maneuverability is also achieved. The design concept, modeling of key components, and framework for control are presented, with the prototyped glider tested in a series of bench and field trials for validation of its motion performance.

Locomotor transition: how squid jet from water to air

The amazing multi-modal locomotion of flying squid helps to achieve fast-speed migration and predator-escape behavior. Observation of flying squid has been rarely reported in recent years, since it is challenging to clearly record the flying squid's aquatic-aerial locomotion in a marine environment. The existing reports of squid-flying events are rare and merely record the in-air motion. Therefore, the water-air locomotor transition of flying squid is still unknown. This paper proposes the idea of using CFD to simulate the process of the flying squid (Sthenoteuthis oualaniensis (S. oualaniensis)) launching from water into air. The results for the first time reveal the flow field information of squid in launching phase and show the kinematic parameters of flying squid in quantification. Both a trailing jet and pinch-off vortex rings are formed to generate launching thrust, and the formation number L _ω /D _ω is 5.22, demonstrating that the jet strategy is to produce greater time-averaged thrust rather than higher propulsion efficiency. The results also indicate that the maximum flying speed negatively correlates with the launch angle, indicating that a lower launch angle could result in a larger flying speed for the flying squid to escape. These findings explore the multi-modal locomotion of flying squid from a new perspective, helping to explain the trade-off strategy of water-to-air transition, and further enhance the performance of aquatic-aerial vehicles.

Living Materials Herald a New Era in Soft Robotics

Living beings have an unsurpassed range of ways to manipulate objects and interact with them. They can make autonomous decisions and can heal themselves. So far, a conventional robot cannot mimic this complexity even remotely. Classical robots are often used to help with lifting and gripping and thus to alleviate the effects of menial tasks. Sensors can render robots responsive, and artificial intelligence aims at enabling autonomous responses. Inanimate soft robots are a step in this direction, but it will only be in combination with living systems that full complexity will be achievable. The field of biohybrid soft robotics provides entirely new concepts to address current challenges, for example the ability to self-heal, enable a soft touch, or to show situational versatility. Therefore, "living materials" are at the heart of this review. Similarly to biological taxonomy, there is a recent effort for taxonomy of biohybrid soft robotics. Here, an expansion is proposed to take into account not only function and origin of biohybrid soft robotic components, but also the materials. This materials taxonomy key demonstrates visually that materials science will drive the development of the field of soft biohybrid robotics.

Asymmetry in the jet opening: underwater jet vectoring mechanism by dragonfly larvae

Aquatic Anisopteran dragonfly larvae achieve respiration and propulsion by repetitive water jets flowing through their anal openings. Previous studies have shown that the tri-leaflet anal valves modulate the emerging jet by varying the opening size. We discovered that the valves are also capable of controlling the opening asymmetry by independent retraction of a leaflet. This study shows the effects of their valve asymmetry control on the respiratory and propulsive flows. Furthermore, the effects of size variation are re-evaluated using fluid momentum and power equations. Synchronized dual cameras recorded the valve movement and the flow generated by Aeshnidae sp. During the respiratory jetting, retraction of a single leaflet positions the opening in an off-centred locale, from which diagonally deflected jets emerge. The resulting flow field, together with the opening size modulation, implicates a reduction in the reinhalation of the exhaled jet and partial powering of the refilling process. Instead, during the propulsive jetting, concurrent partial retraction of the three leaflets results in the centred opening. The resulting jet flows straight, which has an implication for lowering form drag. Additionally, the propulsive aperture size control suggests improved thrust production. Our study highlights the significant influence that an asymmetrically positioned jet opening can have on biological jet flow. The findings inspire a new mechanism for jet vectoring that may prove useful for application in the broader engineering field.

A unified multi-soft-body dynamic model for underwater soft robots

A unified formulation that accounts for the dynamics of a general class of aquatic multi-body, soft-structured robots is presented. The formulation is based on a Cosserat formalism where the description of the ensemble of geometrical entities, such as shells and beams, gives rise to a multi-soft-body system capable of simulating both manipulation and locomotion. Conceived as an advanced tool for a priori hardware development, n-degree-of-freedom dynamics analysis and control design of underwater, soft, multi-body, vehicles, the model is validated against aquatic locomotion experiments of an octopus-inspired soft unmanned underwater robot. Upon validation, the general applicability of the model is demonstrated by predicting the self-propulsion dynamics of a diverse range of new viable combinations of multi-soft-body aquatic system.

Fundamentals of soft robot locomotion

Soft robotics and its related technologies enable robot abilities in several robotics domains including, but not exclusively related to, manipulation, manufacturing, human-robot interaction and locomotion. Although field applications have emerged for soft manipulation and human-robot interaction, mobile soft robots appear to remain in the research stage, involving the somehow conflictual goals of having a deformable body and exerting forces on the environment to achieve locomotion. This paper aims to provide a reference guide for researchers approaching mobile soft robotics, to describe the underlying principles of soft robot locomotion with its pros and cons, and to envisage applications and further developments for mobile soft robotics.