**Introduction to the Principle of Underwater Drone**
An underwater drone, also known as an unmanned underwater vehicle (UUV), is a remotely operated or autonomous device designed to perform tasks underwater. These drones are equipped with visual and sensory systems, allowing them to replace or assist humans in various underwater operations. Since the 1970s, significant advancements have been made, leading to the development of robots capable of working at different depths and performing diverse functions. They are widely used in oil exploration, seabed mineral surveys, salvage operations, pipeline inspections, cable laying, marine farming, and dam monitoring in rivers and reservoirs.
**Classification of Underwater Drones**
Underwater drones can be categorized into three main types: Towed Underwater Vehicles (TUV), Remotely Operated Vehicles (ROV), and Autonomous Underwater Vehicles (AUV). TUVs are typically towed by a surface vessel, ROVs are controlled from the surface via a tether, and AUVs operate independently without direct human control. The evolution of these vehicles has progressed from early submersibles in the 1960s to advanced ROVs in the 1970s, AUVs in the 1980s, and hybrid models that combine the best of both worlds.
**Control Challenges of Underwater Drones**
Underwater drones are complex systems with six degrees of freedom, making their control highly nonlinear and coupled. Due to water resistance, which increases with the square of speed, and thrust that depends on propeller rotation, maintaining stability—especially at low speeds—is extremely challenging. This leads to issues like "zero gain, zero damping," where control becomes unstable when the drone is stationary. Figure 3 illustrates the control mechanism of two underwater drones.
**Key Components of Underwater Drones**
Submersibles are specialized vehicles that carry tools for observation and operation. They are equipped with thrusters for multi-directional movement and cameras with lights for visibility. Sensors such as sonar, compasses, and depth gauges provide critical data. The onboard electronics include a sealed Computer, drive system, and control module.
Repeater systems help deploy and recover submersibles efficiently, reducing drag and interference from the umbilical cable. Hoisting systems manage the deployment and retrieval of equipment, while tethers provide power and communication links between the submersible and the repeater. Armored cables ensure durability and reliability.
Observation equipment, including cameras and imaging sonars, forms the core of the drone’s operational capability. Additional robotic arms and tools can be added for more complex tasks.
**Communication Challenges in Underwater Drones**
Underwater communication remains one of the biggest technical hurdles. Sound waves are the primary medium, but they suffer from high latency and limited range. At 6,000 meters deep, sound travel time can be up to 8 seconds, significantly affecting real-time control. Fiber optics offer faster data rates but are limited in range and mobility. Laser communication shows promise but is still in experimental stages.
Hydrophones, which detect acoustic signals, are used for navigation and positioning. Optical fiber and laser technologies are being explored to improve bandwidth and reduce delays, though each comes with its own set of limitations.
**Energy Supply Issues**
Power is a major constraint for long-term underwater operations. While batteries are currently the most common energy source, future solutions may include advanced fuel cells or nuclear-powered systems. For ROVs, high-voltage power transmission is essential for deep dives, while AUVs rely on onboard batteries. Recycling stations are being developed to extend mission durations, enabling continuous underwater operations.
**Development Prospects and Challenges**
The future of underwater drones lies in the integration of ROV and AUV technologies, enhancing autonomy, precision, and efficiency. Advances in energy storage, positioning systems, and materials will allow deeper and longer missions. Innovations in acoustic technology and artificial intelligence will further expand their capabilities. However, challenges such as communication delay, energy limitations, and environmental adaptability remain key areas for ongoing research and development.
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