**Introduction to the Principle of Underwater Drones**
An underwater drone is a remotely operated or autonomous device designed for tasks beneath the water surface. It is equipped with visual and sensory systems, allowing it to perform various operations such as inspection, exploration, and salvage. These drones are typically controlled from the surface or operate independently using advanced algorithms. Since the 1970s, significant advancements have been made in underwater robotics, leading to the development of machines capable of functioning at different depths and performing complex tasks. Applications include oil and gas exploration, seabed mapping, pipeline inspections, marine farming, and dam monitoring.
**Classification of Underwater Drones**
Underwater drones can be broadly categorized into three types: Towed Underwater Vehicles (TUV), Remotely Operated Vehicles (ROV), and Autonomous Underwater Vehicles (AUV). TUVs are towed by a surface vessel, ROVs are controlled via a tether, and AUVs operate independently without direct human intervention. The evolution of these systems has progressed from early submersibles in the 1960s to modern hybrid vehicles that combine the advantages of both ROVs and AUVs. This advancement has significantly improved the efficiency and range of underwater operations.
**Control Challenges of Underwater Drones**
Controlling an underwater drone is complex due to its six degrees of freedom and the nonlinear dynamics caused by water resistance. The system's behavior is influenced by factors such as hydrodynamic drag, which increases with the square of the speed, and thrust, which depends on the propeller's rotation speed. These characteristics make maintaining stability and precision difficult, especially when operating at zero speed, where "zero gain, zero damping" phenomena occur. This challenge is critical for applications requiring precise positioning. Figure 3 illustrates the control mechanism of two underwater drones.
**Key Components of Underwater Drones**
Submersibles are designed to carry tools and sensors for observation and manipulation. They feature a frame structure with buoyancy blocks to maintain neutral buoyancy when fully loaded. Propellers are placed in multiple directions to enable three-dimensional movement. A gimbal with cameras and lights is often installed for better visibility. Sensors like sonar, compass, depth gauges, and altimeters are essential for navigation and data collection.
Repeater units help manage the tether and ensure efficient deployment and retrieval of the submersible. Hoisting systems and tethers provide mechanical connections and power transmission. Armored cables are used to enhance durability and reduce resistance. Additional tools and manipulators can be attached for specialized tasks.
**Communication Issues in Underwater Drones**
Underwater communication remains a major challenge. Acoustic signals are the primary method, but they suffer from delays and limited bandwidth. At a depth of 6000 meters, sound travel time can be up to 8 seconds, making real-time control difficult. Optical fiber offers higher data rates but is limited by distance and flexibility. Laser communication is still experimental, with challenges in power consumption and practicality. Hydrophones are used for passive acoustic tracking, helping guide the drone toward specific targets.
**Energy Challenges for Underwater Drones**
Power supply is a critical limitation for long-duration missions. While batteries are commonly used, future technologies like proton exchange membrane fuel cells may offer higher energy density and efficiency. ROVs typically rely on high-voltage power supplied through tethers, while AUVs use onboard batteries. Improving energy efficiency and extending operational ranges remain key research areas.
**Future Prospects and Technological Challenges**
The next generation of underwater drones will integrate the strengths of both ROVs and AUVs, enhancing autonomy and functionality. Advances in energy storage, positioning systems, and materials will improve performance and reliability. Technologies like acoustic navigation, laser imaging, and intelligent control systems are expected to revolutionize underwater exploration. Despite progress, challenges such as communication delay, energy limitations, and deep-sea pressure continue to shape the future of this field.
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