11.2 Soft underwater robots

Soft underwater robots are revolutionizing marine exploration and research. These flexible, adaptable machines mimic aquatic organisms, using soft materials to navigate complex underwater environments efficiently. They offer unique advantages over traditional rigid robots in terms of maneuverability and interaction with delicate marine life.

From fish-like swimming robots to octopus-inspired manipulators, these bio-inspired designs showcase innovative actuation methods and sensing technologies. Challenges like durability and energy efficiency drive ongoing research, promising exciting advancements in underwater robotics and marine science applications.

Soft materials for underwater environments

• Soft materials are well-suited for underwater environments due to their compliance and adaptability to complex and dynamic conditions
• Underwater soft robots can be designed to mimic the morphology and behavior of aquatic organisms, enabling efficient locomotion and manipulation
• Key properties of soft materials for underwater use include high stretchability, low modulus, and resistance to water and biofouling

Bioinspired underwater soft robots

Fish-like swimming robots

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• Fish-like swimming robots mimic the undulatory motion of fish, using soft actuators to generate propulsive forces
• These robots can achieve high swimming efficiency and maneuverability by exploiting the fluid-structure interactions between their soft bodies and the surrounding water
• Examples include the MIT fish robot and the FILOSE robot (Fish-Like Omnidirectional Soft Electronic Robot)

Octopus-inspired soft manipulators

• Octopus-inspired soft manipulators are designed to mimic the dexterity and adaptability of octopus arms
• These manipulators typically consist of soft, continuum structures with multiple degrees of freedom, allowing them to conform to objects and perform complex grasping tasks
• Examples include the Octobot developed by Harvard University and the soft robotic octopus arm created by the Italian Institute of Technology

Jellyfish-like pulsed-jet propulsion

• Jellyfish-like robots use pulsed-jet propulsion, a mechanism inspired by the swimming motion of jellyfish
• These robots typically have a soft, bell-shaped body that contracts and expands to expel water, generating thrust
• Examples include the Robojelly developed by Virginia Tech and the Cyro robot created by Festo

Actuation of underwater soft robots

Hydraulic actuation

• Hydraulic actuation uses pressurized fluid to drive the motion of soft robots
• This actuation method provides high force output and precise control but requires a fluid supply system and can be affected by the surrounding water pressure
• Examples of hydraulically actuated underwater soft robots include the Octobot and the fish-like robot developed by the University of Auckland

Pneumatic actuation

• Pneumatic actuation uses compressed air to inflate and deflate soft chambers, generating motion
• This actuation method is simple and lightweight but can be affected by the compressibility of air and the surrounding water pressure
• Examples of pneumatically actuated underwater soft robots include the soft robotic fish developed by MIT and the soft robotic jellyfish created by Florida Atlantic University

Dielectric elastomer actuators (DEAs)

• DEAs are a type of soft actuator that consists of a thin elastomeric film sandwiched between two compliant electrodes
• When a voltage is applied, the electrostatic attraction between the electrodes causes the elastomer to compress and expand, generating motion
• DEAs have high energy density and fast response times but require high voltages and can be affected by the presence of water
• Examples of DEA-based underwater soft robots include the swimming robot developed by the University of Bristol and the jellyfish-inspired robot created by the University of California, Los Angeles

Sensing in underwater soft robots

Soft strain sensors

• Soft strain sensors are used to measure the deformation and motion of soft robots
• These sensors are typically made of stretchable, conductive materials such as carbon-filled silicone or liquid metal-filled channels
• Examples of soft strain sensors used in underwater robots include the resistive strain sensors used in the Octobot and the capacitive strain sensors used in the fish-like robot developed by the University of Auckland

Soft pressure sensors

• Soft pressure sensors are used to measure the pressure distribution on the surface of soft robots, which can be useful for detecting contact with objects and estimating hydrodynamic forces
• These sensors are typically made of soft, conductive materials that change their electrical properties in response to pressure
• Examples of soft pressure sensors used in underwater robots include the resistive pressure sensors used in the soft robotic fish developed by MIT and the capacitive pressure sensors used in the octopus-inspired manipulator created by the Italian Institute of Technology

Soft tactile sensors

• Soft tactile sensors are used to detect contact and measure the force distribution on the surface of soft robots
• These sensors are typically made of soft, conductive materials that change their electrical properties in response to touch
• Examples of soft tactile sensors used in underwater robots include the resistive tactile sensors used in the octopus-inspired manipulator created by the Italian Institute of Technology and the capacitive tactile sensors used in the soft robotic gripper developed by Harvard University

Modeling and control of underwater soft robots

Fluid-structure interaction (FSI) modeling

• FSI modeling is used to simulate the complex interactions between soft robots and the surrounding fluid environment
• This approach combines computational fluid dynamics (CFD) and finite element analysis (FEA) to predict the deformation of soft structures and the resulting fluid flow
• Examples of FSI modeling applied to underwater soft robots include the simulation of fish-like swimming robots and the modeling of octopus-inspired manipulators

Nonlinear control strategies

• Nonlinear control strategies are used to handle the complex, nonlinear dynamics of underwater soft robots
• These strategies include model-based control, such as feedback linearization and sliding mode control, and model-free control, such as reinforcement learning and adaptive control
• Examples of nonlinear control strategies applied to underwater soft robots include the use of feedback linearization for controlling fish-like swimming robots and the application of reinforcement learning for controlling octopus-inspired manipulators

Adaptive control for unknown environments

• Adaptive control strategies are used to enable underwater soft robots to adapt to unknown and changing environments
• These strategies involve online parameter estimation and control law adaptation to compensate for uncertainties and disturbances
• Examples of adaptive control strategies applied to underwater soft robots include the use of model reference adaptive control for fish-like swimming robots and the application of adaptive neural network control for octopus-inspired manipulators

Fabrication techniques for underwater soft robots

Molding and casting

• Molding and casting are widely used techniques for fabricating soft robots, involving the use of 3D printed molds and the casting of soft, elastomeric materials such as silicone
• This approach allows for the creation of complex, three-dimensional structures with embedded sensors and actuators
• Examples of underwater soft robots fabricated using molding and casting include the Octobot and the soft robotic fish developed by MIT

3D printing of soft materials

• 3D printing of soft materials, such as silicone and hydrogels, enables the direct fabrication of soft robotic structures with high precision and repeatability
• This approach allows for the integration of multiple materials with different mechanical properties and the incorporation of functional components such as sensors and actuators
• Examples of underwater soft robots fabricated using 3D printing include the fish-like robot developed by the University of Auckland and the octopus-inspired manipulator created by the Italian Institute of Technology

Multi-material fabrication methods

• Multi-material fabrication methods involve the combination of different materials with distinct mechanical and functional properties to create complex, heterogeneous structures
• These methods include multi-material 3D printing, embedded 3D printing, and hybrid fabrication techniques that combine 3D printing with traditional manufacturing processes
• Examples of underwater soft robots fabricated using multi-material methods include the fish-like robot with embedded soft sensors developed by the University of Auckland and the octopus-inspired manipulator with integrated soft actuators and sensors created by the Italian Institute of Technology

Challenges and opportunities in underwater soft robotics

Durability and longevity of soft materials

• Soft materials used in underwater robots are subjected to harsh conditions, including high pressures, abrasion, and biofouling, which can limit their durability and longevity
• Research efforts are focused on developing new materials and coatings that can withstand these conditions and extend the operational lifetime of underwater soft robots
• Opportunities exist for the development of self-healing and self-cleaning materials that can autonomously repair damage and maintain functionality

Energy efficiency vs traditional underwater robots

• Underwater soft robots have the potential to be more energy-efficient than traditional, rigid robots due to their ability to exploit fluid-structure interactions and their lower weight and inertia
• However, the actuation and control of soft robots can be more complex and energy-intensive, requiring the development of efficient power systems and control strategies
• Opportunities exist for the integration of energy harvesting mechanisms, such as piezoelectric and triboelectric generators, to enable long-term, autonomous operation of underwater soft robots

Potential applications in marine research and exploration

• Underwater soft robots have the potential to revolutionize marine research and exploration by enabling the study of delicate marine ecosystems and the exploration of previously inaccessible environments
• Soft robots can be designed to minimally disturb their surroundings, allowing for non-invasive sampling and monitoring of marine life
• Opportunities exist for the development of soft robotic systems for deep-sea exploration, environmental monitoring, and marine archaeology, as well as for the inspection and maintenance of underwater infrastructure such as pipelines and cables