The goal of the experiments was to verify the final approach algorithm and system validity. Figure 23 describes the initial start point for the final docking approach. It shows a top view (left) and a side view (right) of the initial start conditions. The dock was placed within viewing range of the camera. The center of the dock was placed at a depth of 1.5m. The dock was introduced by (Lee et al, 2003), (Park et al, 2007). The dock was funnel-shaped. This shape makes it possible for the AUV to dock successfully through sliding even if she approaches obliquely. The dock used an external power source.
Because robustness against disturbance has not yet been developed and this attempt was during the early stages of development, some restrictions were applied. There was no current and there were no waves. The dock was fixed on the basin floor. The water was clean. ISiMI was operated using a wired LAN communication. RF wireless communication was not suitable to receive the large amount of image data necessary. The wireless LAN was disconnected when the AUV submerged. The R.P.M. of the thrust propeller was invariant and the forward speed was about 1.0m/ s. The relation between R.P.M and speed was determined by (Jun et al. 2008). There was no speed control. Experiments without the attitude keeping control and experiments with the attitude keeping control were conducted separately.
A. Underwater docking experiment without the attitude keeping control Only the vision-guidance control was applied. No distance estimation was applied. ISiMI depended on the camera until contact with the dock. In Fig. 24, pixel errors are plotted against time. A pixel error is defined as deviation between the origin and the estimated center of the dock center in the image coordinate. The pixel errors decreased and were regulated during the first 9 seconds of the test. However, between seconds 9-15, there were discontinuous oscillations. These oscillations were caused by the defect of the image processing system to process, not by actual motions of the AUV, i.e. one more light moved out of the camera viewing range. The AUV became confused and it could not find the center of the dock. This occurred when the AUV was in the second stage area. To estimate the center precisely, all five lights were required, but in this area, the AUV could not see all of them. It was found that the AUV had some head-on collisions with Light #5 or the inner plane of the dock. She performed imprecise final approaches and suffered collisions with the dock. Fig. 25 is a sequence of continuously grabbed images taken by an underwater camera. (a) ISiMI starts, (b) she cruises to (c) the dock, (d) an imprecise approach near the dock, (e) after a collision, she rebounded and (f) she could not enter the dock. Thus, it was proven that the vision-guidance control was not unnecessary during this part of the docking procedure.
Position Error in the image coordinate
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