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**Introduction to the Principle of Underwater Drones**
An underwater drone, also known as an Unmanned Underwater Vehicle (UUV), is a remotely operated or autonomous device designed to perform tasks underwater. It is equipped with visual and sensory systems, allowing it to replace or assist human divers in various underwater operations. These drones can be controlled from the surface or operate independently, making them ideal for deep-sea exploration, maintenance, and scientific research. Since the 1970s, underwater drones have seen significant development, leading to the creation of advanced robots capable of working at different depths and performing a wide range of functions, such as oil and gas exploration, seabed mapping, salvage operations, pipeline inspections, and marine habitat monitoring.
**Classification of Underwater Drones**
Underwater drones are typically categorized into three main types: Towed Vehicles (TUV), Remotely Operated Vehicles (ROVs), and Autonomous Underwater Vehicles (AUVs). TUVs are towed by a ship or platform, while ROVs are controlled via a tethered cable from the surface. AUVs, on the other hand, operate independently without a direct connection to the surface. Over time, these technologies have evolved, with early submersibles in the 1960s giving way to more advanced ROVs in the 1970s and AUVs in the 1980s. Today, hybrid models that combine the advantages of both ROVs and AUVs are becoming increasingly common.
**Control Challenges of Underwater Drones**
Operating an underwater drone is a complex task due to its six degrees of freedom and the nonlinear nature of water dynamics. The resistance encountered in water increases with the square of the speed, and the thrust generated by the propellers also depends on the square of their rotational speed. This creates a highly coupled and unpredictable system, especially when the drone needs to maintain a fixed position. At low speeds, the phenomenon of "zero gain, zero damping" occurs, making it difficult to control the drone’s position accurately. This challenge is a key area of ongoing research. Figure 3 illustrates the control mechanism of two underwater drones.
**Key Components of Underwater Drones**
Submersibles are designed to carry observation tools and work equipment. They feature a frame structure with buoyancy blocks that ensure neutral buoyancy when fully loaded. Propellers are placed in horizontal, vertical, and lateral directions to allow three-dimensional movement. A gimbal with a camera and lights is often installed at the front of the frame. Common sensors include imaging sonar, compasses, depth pressure sensors, and altimeters. The onboard electronics consist of an underwater computer, drive system, and control module housed in a sealed chamber. Additional monitoring sensors track power, pressure, temperature, and water leakage.
Repeater systems help deploy and retrieve submersibles efficiently, reducing resistance from the umbilical cable. The hoisting system is typically gate-shaped and hydraulically driven, with storage mechanisms for cables and equipment. The tether connects the submersible to the repeater, using buoyant materials to reduce drag. Armored main cables provide mechanical, energy, and data transmission between the system components. Observation equipment includes cameras and sonar, forming the basic system for underwater operations.
**Communication Issues in Underwater Drones**
Underwater communication remains one of the biggest challenges due to the limited effectiveness of traditional methods. Currently, only acoustic and fiber-optic communication are viable. Acoustic signals are widely used but suffer from high latency, especially at great depths. For example, at 6000 meters, it can take up to 8 seconds for a signal to travel. Fiber optics offer higher data rates but are limited in range and flexibility. Optical communication using blue-green lasers is still experimental and has limitations in power consumption and practicality.
Hydrophone systems, which use passive acoustic detection, can identify underwater sound sources and are useful for navigation. However, they are limited in range and require careful calibration. As underwater drones become more advanced, improving communication reliability and distance will be crucial for expanding their operational capabilities.
**Energy Challenges for Underwater Drones**
Power supply is a critical factor limiting the range and endurance of underwater drones. While batteries are currently the most common power source, future advancements may involve nuclear fuel or other high-energy options. In the meantime, charging stations can be used to extend mission duration. ROVs typically rely on power supplied through a tether, with voltage levels increasing as depth increases. AUVs, however, are self-contained and use high-energy batteries such as silver-zinc or lithium-based systems.
**Future Development and Challenges**
The next generation of underwater drones is expected to combine the strengths of ROVs and AUVs, offering greater autonomy, precision, and adaptability. Advances in energy technology, such as proton exchange membrane fuel cells, could significantly improve performance. Precise positioning systems, including underwater GPS and acoustic navigation, are also being refined. Zero-visibility navigation, using laser and acoustic technologies, aims to overcome the challenges of poor visibility in deep waters.
Material science continues to play a vital role, with the need for strong, lightweight, and corrosion-resistant structures. Operational technologies, such as robotic manipulators and virtual reality interfaces, will enhance the ability of drones to perform complex tasks. Acoustic communication improvements will enable longer-range operations, while intelligent systems will bring us closer to fully autonomous underwater vehicles. Recycling technology is also essential, as retrieval operations are often affected by harsh sea conditions. Overall, the future of underwater drones looks promising, with continuous innovation driving their expansion into new applications.