3.3. Safety range of the towed force
The information on the upper and lower bounds of the towed force and the range of controllable depth in accordance with the towing speed and cable length is important in knowing whether the present towed force is normal and the limitation of the depth control. In order to acquire such information, we propose the step-by-step depth control method shown in Fig. 4. The control process is as follows:
Step 1
With given cable length and towing speed, the mother ship tows the towed vehicle until the depth of the towed vehicle approaches certain equilibrium. In this case, the towed vehicle does not carry out depth control.
Step 2
After reaching equilibrium, downward (toward the sea-bottom) stair-type depth control is performed until the depth control cannot be achieved anymore due to the shortage of lifting force for descending. At the ultimate depth, the magnitude of the towed force becomes largest (the upper bound) and the controllable depth also has the maximum value (the upper bound).
Step 3
After Step 2, to obtain the lower bound, upward (toward the surface) stair-type control is performed until the depth control cannot be accomplished due to the shortage of lifting force for ascending.