🫠Underwater Robotics Unit 1 – Underwater Robotics: Marine Environments

Key Concepts and Terminology

• Underwater robotics involves the design, development, and deployment of autonomous or remotely operated vehicles (ROVs) for marine exploration and tasks
• Bathymetry measures the depths and shapes of underwater terrain using sonar or other techniques
• Salinity refers to the concentration of dissolved salts in seawater, typically around 35 parts per thousand (ppt) in the open ocean
• Thermocline is a distinct layer in the water column where temperature changes rapidly with depth
• Acoustic communication uses sound waves to transmit data underwater, as radio waves are quickly attenuated in water
• Degrees of freedom (DOF) describe the independent ways a robot can move in three-dimensional space, typically including translation (surge, sway, heave) and rotation (roll, pitch, yaw)
• Remotely operated vehicles (ROVs) are tethered underwater robots controlled by a human operator on the surface
• Autonomous underwater vehicles (AUVs) are untethered robots that can navigate and perform tasks independently based on pre-programmed instructions or adaptive algorithms

Marine Environment Characteristics

• Pressure increases linearly with depth in the ocean at a rate of about 1 atmosphere (atm) per 10 meters, requiring robust pressure housings for underwater robots
• Temperature in the ocean varies with depth and location, ranging from near-freezing in polar regions and deep waters to over 30°C (86°F) in some tropical surface waters
  • Thermoclines can create distinct layers with different temperatures and densities, affecting underwater vehicle buoyancy and acoustic propagation
• Salinity variations can occur due to freshwater input from rivers, precipitation, or ice melt, impacting seawater density and conductivity
• Currents and tides can vary in strength and direction, affecting robot navigation and station-keeping
• Visibility underwater is often limited due to absorption and scattering of light, with blue wavelengths penetrating the deepest
• Biological factors such as biofouling (accumulation of organisms on surfaces) and animal interactions (e.g., curious dolphins) can impact robot performance and maintenance
• Acoustic properties of seawater, including sound speed and attenuation, vary with temperature, salinity, and pressure, affecting sonar performance and communication

Types of Underwater Robots

• Remotely Operated Vehicles (ROVs) are tethered to a surface vessel and controlled by a human operator
  • ROVs typically have a umbilical cable that provides power, communication, and control signals
  • Work-class ROVs are larger, more powerful systems used for industrial tasks like offshore construction and maintenance
• Autonomous Underwater Vehicles (AUVs) operate independently without a tether, following pre-programmed missions or adaptive algorithms
  • Gliders are a type of AUV that uses changes in buoyancy and wings to achieve efficient long-range travel with minimal power consumption
  • Hybrid ROV/AUV systems can switch between tethered and autonomous modes for flexibility in different mission scenarios
• Unmanned Surface Vehicles (USVs) operate on the water surface and can be used for tasks like communication relay, sensor deployment, or coordinating with underwater robots
• Biomimetic robots take inspiration from marine animal locomotion and sensing, such as fish-like swimming or octopus-inspired manipulation
• Soft robotic systems use compliant materials and actuators for enhanced safety, adaptability, and bio-compatibility in delicate marine environments

Sensors and Data Collection

• Acoustic sensors, including sonar and acoustic Doppler current profilers (ADCPs), use sound waves to measure distances, detect objects, and quantify water velocity
  • Multibeam echosounders provide high-resolution bathymetric mapping by emitting multiple sonar beams in different directions
  • Side-scan sonar produces imagery of the seafloor or underwater structures by scanning perpendicular to the robot's path
• Optical sensors, such as cameras and laser scanners, provide visual data for inspection, mapping, and object recognition
  • Stereo camera systems enable 3D reconstruction and depth perception
  • Structured light projectors can enhance underwater imaging by providing active illumination and texture
• Environmental sensors measure water properties like temperature, salinity, pressure, pH, and dissolved oxygen concentration
  • Conductivity, Temperature, Depth (CTD) sensors are commonly used to profile water column characteristics
• Chemical sensors, such as mass spectrometers and fluorometers, can detect specific compounds or indicators of biological activity
• Inertial Measurement Units (IMUs) combine accelerometers, gyroscopes, and sometimes magnetometers to estimate robot orientation and motion
• Acoustic modems enable wireless communication between underwater robots and surface vessels or shore stations

Navigation and Control Systems

• Underwater localization estimates a robot's position and orientation using sensor data and prior knowledge of the environment
  • Dead reckoning integrates velocity or acceleration measurements over time to track position, but is subject to drift
  • Acoustic positioning systems, such as Long Baseline (LBL) or Ultra-Short Baseline (USBL), use acoustic beacons to triangulate robot position
• Simultaneous Localization and Mapping (SLAM) techniques build a map of the environment while simultaneously estimating the robot's pose within it
• Path planning generates feasible and efficient trajectories for the robot to follow based on mission objectives and environmental constraints
  • Sampling-based methods, such as Rapidly-exploring Random Trees (RRT), can effectively explore high-dimensional configuration spaces
• Obstacle avoidance uses sensor data to detect and safely navigate around unexpected objects or hazards
• Feedback control systems continuously monitor robot state and adjust actuators to maintain desired position, orientation, or velocity
  • Proportional-Integral-Derivative (PID) controllers are commonly used for low-level control of thrusters, control surfaces, or other actuators
• Adaptive control techniques can automatically tune controller parameters to account for changes in robot dynamics or environmental conditions

Design Challenges in Marine Robotics

• Pressure resistance requires careful design and selection of materials, seals, and connectors to withstand extreme hydrostatic pressures at depth
  • Syntactic foam and pressure-compensated oil-filled housings are commonly used for buoyancy and pressure tolerance
• Corrosion resistance is critical for long-term operation in harsh saltwater environments, often requiring sacrificial anodes, cathodic protection, or inert coatings
• Hydrodynamic efficiency impacts robot range, speed, and maneuverability, requiring optimization of hull form, propulsion, and control surfaces
• Communication bandwidth is limited underwater, as acoustic signals have lower data rates and higher latency compared to radio or optical methods in air
• Energy storage and management are critical for untethered robots, as batteries have lower energy density than fossil fuels and recharging at sea is challenging
  • Solar panels, wave energy converters, or fuel cells can provide alternative power sources
• Biofouling can degrade sensor performance, increase drag, and accelerate corrosion, requiring antifouling coatings, materials, or active cleaning systems
• Fault tolerance and redundancy are important for robots operating in remote or hazardous environments where immediate human intervention is infeasible

Applications and Case Studies

• Ocean exploration and mapping
  • AUVs and ROVs have been used to discover and characterize hydrothermal vents, cold seeps, and other deep-sea features
  • Seafloor mapping with multibeam sonar and photogrammetry enables high-resolution bathymetry and habitat characterization
• Marine archaeology
  • ROVs have located and documented historic shipwrecks, such as the Titanic and the ancient Greek Antikythera wreck
• Offshore industry
  • Work-class ROVs perform inspection, maintenance, and repair tasks on oil and gas platforms, pipelines, and subsea infrastructure
  • AUVs conduct geophysical surveys for oil and gas exploration, as well as pre-installation and post-lay pipeline inspection
• Environmental monitoring
  • Gliders and other long-endurance AUVs collect data on ocean circulation, climate change, and marine ecosystem health
  • Chemical and biological sensors on underwater robots can detect pollutants, harmful algal blooms, or invasive species
• Search and recovery
  • ROVs and AUVs assist in locating and recovering drowning victims, black boxes from aircraft crashes, or lost maritime cargo
• Military and security
  • Mine countermeasures (MCM) robots detect and neutralize underwater mines
  • Harbor security robots monitor for intrusions or anomalous activities

Future Trends and Innovations

• Swarm robotics involves coordinating large numbers of simple robots to perform complex tasks through emergent behaviors and distributed sensing
  • Bio-inspired swarm algorithms, such as schooling fish or ant colonies, can enable robust and scalable multi-robot systems
• Soft robotics and smart materials can enhance robot adaptability, safety, and efficiency through compliant structures and novel actuation methods
  • Shape memory alloys, electroactive polymers, and fluidic elastomer actuators are promising technologies for soft underwater manipulation and locomotion
• Autonomous intervention capabilities, such as dexterous manipulation and decision-making, can enable robots to perform more complex tasks without human oversight
  • Machine learning techniques, such as deep reinforcement learning, can allow robots to adapt to uncertain environments and improve their skills over time
• Persistent autonomy aims to extend robot mission durations and distances through advances in energy management, fault tolerance, and self-maintenance
  • In-situ power generation, such as underwater recharging stations or energy harvesting from currents or temperature gradients, can reduce reliance on onboard batteries
• Human-robot interaction methods, such as natural language processing, gestures, and augmented reality displays, can make underwater robots more intuitive and effective tools for human operators
  • Haptic feedback systems can provide a sense of touch and force for remote manipulation tasks
• Miniaturization of sensors, actuators, and computing hardware can enable smaller, cheaper, and more agile underwater robots for distributed sensing and exploration
  • Microrobots, such as robotic plankton or bacteria, could revolutionize environmental monitoring, drug delivery, and other microscale applications in the marine realm