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Figure 17. ROV motion performance test: (A) rescue ROV hanging; (B) ROV buoyancy and balance adjustment; (C) ROV buoyancy state
test; and (D) underwater hovering state of ROV multifunctional manipulator.

Figure 18. Underwater small target search and grasping test of ROV multifunctional manipulator.

4.2. Pool target search and rescue test of ROV multifunctional manipulator
Figure 18 shows the multifunctional search and rescue ROV carrying a multifunctional manipulator to search
and grab underwater targets. It should be noted that, due to the limited number and placement of thrusters,
pitch motion cannot be fully controlled. In the case where the ROV is used to grip a small object in this
scenario, the jaw (garb system presented in Figure 15) mounted in front of the ROV will open and close,
causing instability in the system, and the pitch motion cannot be fully compensated. Our solution is to retract
the manipulator to run stably around the target, and then grasp it through the open manipulator. Because the
manipulator actuator is electric, the center of gravity changes only slightly, and the improved PID controller
can achieve a more stable control.