🫠Underwater Robotics Unit 4 – Underwater Robot Propulsion Systems

Key Concepts and Terminology

• Propulsion generates thrust to move an underwater robot through the water
• Thrust is the force that propels the robot forward and overcomes drag
• Drag is the resistance force acting on the robot as it moves through the water
• Efficiency is the ratio of useful work performed to the total energy expended
• Cavitation occurs when bubbles form and collapse around a propeller, causing damage and reducing efficiency
• Bollard pull is the maximum static thrust a propulsion system can generate
• Propulsive power is the power delivered by the propulsion system to the water
  • Calculated as the product of thrust and velocity $P = T \times v$
• Advance ratio is the ratio of the robot's speed to the propeller's rotational speed
  • Affects propeller efficiency and cavitation

Underwater Propulsion Basics

• Propulsion systems convert energy into thrust to move the robot through the water
• Thrust is generated by accelerating a mass of water in the opposite direction of desired motion
• Conservation of momentum explains how the acceleration of water produces a reaction force (thrust) on the robot
• Propulsive efficiency depends on factors such as propeller design, motor efficiency, and hydrodynamic losses
• Drag forces, including skin friction and form drag, oppose the robot's motion and must be overcome by thrust
• Streamlining the robot's body can reduce form drag and improve overall efficiency
• Matching the propulsion system to the robot's size, weight, and intended speed is crucial for optimal performance
• Propeller selection involves considering diameter, pitch, number of blades, and material

Types of Propulsion Systems

• Propellers are the most common type of propulsion system for underwater robots
  • Consist of rotating blades that push water to generate thrust
  • Can be fixed-pitch or variable-pitch for improved control and efficiency
• Thrusters are self-contained propulsion units that include a motor, propeller, and nozzle
  • Provide precise control and maneuverability
  • Can be positioned and oriented in various configurations (e.g., vectored thrust)
• Jet propulsion systems use a pump to expel water through a nozzle, creating thrust
  • Offer high efficiency and low noise compared to propellers
  • Suitable for high-speed applications and environments with debris
• Biomimetic propulsion systems mimic the swimming mechanisms of aquatic animals
  • Examples include undulating fins, oscillating foils, and jellyfish-inspired pulsed jets
  • Offer high maneuverability and efficiency at low speeds
• Hybrid propulsion systems combine multiple propulsion technologies for improved performance
  • Example: a combination of propellers and thrusters for hovering and long-range travel

Design Considerations

• Propulsion system selection depends on the robot's size, weight, speed, and mission requirements
• Matching the propulsion system to the robot's hydrodynamic characteristics is essential for optimal performance
• Propeller design factors include diameter, pitch, number of blades, and material
  • Larger diameters and higher pitch generate more thrust but require more power
  • Increasing the number of blades improves efficiency but may increase complexity and cost
• Thruster placement and orientation affect the robot's maneuverability and stability
  • Vectored thrust configurations allow for precise control in multiple degrees of freedom
• Streamlining the robot's body reduces form drag and improves overall efficiency
• Proper sealing and waterproofing of the propulsion system components are critical for reliable operation
• Vibration and noise reduction techniques, such as using balanced propellers and isolation mounts, improve stealth and minimize interference with sensors
• Modularity and ease of maintenance should be considered for field operations and repairs

Power Sources and Energy Efficiency

• Underwater robots typically rely on batteries or tethered power for propulsion
• Lithium-ion batteries are commonly used for their high energy density and long cycle life
  • Proper battery management systems ensure safe and efficient operation
• Tethered power systems provide a continuous power supply but limit the robot's range and maneuverability
• Fuel cells and hybrid power systems are emerging technologies for extended endurance missions
• Energy efficiency is crucial for maximizing mission duration and range
• Propulsive efficiency can be improved through optimized propeller design, motor selection, and hydrodynamic streamlining
• Power management techniques, such as duty cycling and sleep modes, conserve energy during periods of low activity
• Regenerative power systems, such as solar panels or turbines, can harvest energy from the environment to extend mission duration
• Minimizing the robot's weight and drag reduces the power required for propulsion

Control and Navigation

• Propulsion control systems regulate the speed, direction, and thrust of the robot
• Motor controllers, such as electronic speed controllers (ESCs), provide precise control of propeller or thruster motors
• Feedback systems, including tachometers and current sensors, monitor the propulsion system's performance and enable closed-loop control
• Navigation systems, such as inertial measurement units (IMUs) and Doppler velocity logs (DVLs), provide position and velocity information for autonomous operation
• Acoustic positioning systems, like ultra-short baseline (USBL) or long baseline (LBL), enable precise underwater localization
• Sensor fusion techniques combine data from multiple sensors to improve navigation accuracy and robustness
• Autonomous control algorithms, such as proportional-integral-derivative (PID) controllers or model predictive control (MPC), enable the robot to maintain desired trajectories and adapt to environmental disturbances
• Remote control interfaces allow operators to manually control the robot's propulsion and navigation during tethered operations

Real-World Applications

• Underwater robots with advanced propulsion systems are used in a variety of industries and research fields
• Offshore oil and gas exploration and maintenance
  • Inspection, repair, and maintenance of subsea infrastructure
  • Surveys and mapping of seafloor for pipeline routing and site selection
• Marine research and conservation
  • Monitoring and sampling of marine ecosystems and habitats
  • Tracking and studying marine life behavior and migration patterns
• Underwater archaeology and cultural heritage preservation
  • Documenting and exploring submerged historical sites and shipwrecks
  • Non-invasive imaging and mapping of delicate artifacts
• Military and defense applications
  • Mine countermeasures and explosive ordnance disposal
  • Surveillance and reconnaissance missions in contested waters
• Aquaculture and fisheries management
  • Inspection and maintenance of fish farms and aquaculture facilities
  • Stock assessment and habitat mapping for sustainable fisheries
• Environmental monitoring and disaster response
  • Monitoring of water quality, pollution, and climate change impacts
  • Rapid deployment for search and recovery operations in underwater environments

Future Trends and Innovations

• Advances in materials science and manufacturing techniques enable lighter, stronger, and more efficient propulsion components
• Biomimetic propulsion systems inspired by aquatic animals offer new possibilities for efficient and maneuverable underwater locomotion
• Swarm robotics and cooperative control algorithms enable coordinated operation of multiple robots for large-scale surveys and monitoring
• Integration of artificial intelligence and machine learning techniques for adaptive and autonomous propulsion control
• Development of hybrid propulsion systems that combine the advantages of different technologies for improved performance and versatility
• Wireless underwater power transfer and charging technologies extend the operational range and endurance of untethered robots
• Advancements in energy storage and harvesting technologies, such as high-capacity batteries and fuel cells, enable longer mission durations
• Miniaturization of propulsion components allows for the development of smaller, more agile, and cost-effective underwater robots for a wider range of applications