Why are there aerial drones, no underwater drones, or underwater remote-controlled bombs, or underwater unmanned submarines?

Control problem of underwater unmanned aerial vehicle

Underwater UAV is a rigid body with six degrees of freedom moving in water, and it is a strongly coupled nonlinear system. Because of moving in water, the hydrodynamic (resistance) coefficient is directly proportional to the square of the moving speed; Propeller propulsion is adopted, and the thrust is proportional to the square of propeller speed. All these make the control problem very difficult, especially when fixed-point operation is needed. The above causes the phenomenon of "zero gain and zero damping" at zero speed, which makes the stiffness of the dynamic positioning control system difficult to meet the requirements of fixed-point operation.

2. UAV recycling problem

At present, except for relatively calm inland rivers and lakes, it is unrealistic to recover AUV water surface in deep water, because there are great swells in deep water. The sea trial experience shows that when the water depth is 4000 meters, the swell is more than 3 meters, so it is difficult to ensure that the robot body and the recovery device are on the same level during recovery, which will bring great danger, so it is still an unsolved problem. The recovery of 50 ~ 100 m is the only feasible scheme, which is very important from the point of view of use. Its solution will greatly reduce the danger to personnel and equipment in the recycling process.

3. Communication problem of underwater UAV.

There are two systems directly related to communication: monitoring system and monitoring system. The former mainly refers to the equipment used by underwater unmanned aerial vehicles for underwater search and underwater observation, generally including underwater cameras, pan-tilt and lighting, imaging sonar, acoustic and magnetic positioning systems, etc. The latter mainly refers to the sensing equipment needed to control and ensure the normal operation of underwater unmanned aerial vehicle, which generally includes depth meter, altimeter, directional compass, temperature, pressure, voltage and current. Can be collected by sensors and transmitted by wired or wireless means. However, at present, the only underwater communication means are underwater sound and optical fiber.

4. The energy problem of underwater UAV.

The energy problem has always been the main factor limiting the working scope of AUV, and it is a long-term effort to develop energy with high specific energy rate. In the foreseeable future, natural batteries will be a substitute. Batteries with variable half-life nuclear fuel may be used in the future.