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Fig. 24. Position error (unit: pixel) in the image coordinate. The vision-guidance control was applied through all intervals. (1) t = 0-9seconds : Errors are decreasing. In this interval, all 5 lights were in the viewing range of the camera. The AUV was able to estimate the center precisely. (2)t = 9-15seconds : one or more lights were out of the viewing range of the camera. Precise estimation of the center became impossible. The oscillation was caused by the defect of information from image processing rather than actual motion of the AUV.

Fig. 24. Position error (unit: pixel) in the image coordinate. The vision-guidance control was applied through all intervals. (1) t = 0-9seconds : Errors are decreasing. In this interval, all 5 lights were in the viewing range of the camera. The AUV was able to estimate the center precisely. (2)t = 9-15seconds : one or more lights were out of the viewing range of the camera. Precise estimation of the center became impossible. The oscillation was caused by the defect of information from image processing rather than actual motion of the AUV.

Fig. 25. Docking : Grabbed images by an underwater camera (Arrows indicate moving directions of the AUV): (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.

B. Underwater docking experiment with the attitude keeping control

The attitude keeping controller was applied when ISiMI was near the dock. Image processing was used to estimate both the location of the center and the distance to the dock. The patterns were similar to that of Fig. 24 during the first 9 seconds of the test. Oscillations of the sort encountered during the first test were anticipated after 9 seconds. However, after the vision-guidance control was stopped, the reference yaw and pitch were fixed by the attitude keeping controller. In Fig. 26, the solid lines are the yaw(the upper graph) and pitch(the lower graph) measured by AHRS. The short-dash lines are the generated reference yaw and pitch. After 9 seconds, the references were fixed. The long-dash lines show the fixed references and the AUV tracked them. Fig. 27 shows the moment of docking. The

Fig. 26. Final approach: (upper) Yaw, Ref. yaw and fixed ref. yaw (lower) Pitch, Ref. pitch and fixed ref. pitch are shown respectively. After 9 seconds, ref. yaw and ref. pitch were fixed. (1)t = 0-9seconds : The vision-guidance control was applied. In this interval, all 5 lights were in the viewing range of the camera. (2)t = 9-15seconds : At t = 9.4seconds, the attitude keeping control began and the references were fixed.

Fig. 26. Final approach: (upper) Yaw, Ref. yaw and fixed ref. yaw (lower) Pitch, Ref. pitch and fixed ref. pitch are shown respectively. After 9 seconds, ref. yaw and ref. pitch were fixed. (1)t = 0-9seconds : The vision-guidance control was applied. In this interval, all 5 lights were in the viewing range of the camera. (2)t = 9-15seconds : At t = 9.4seconds, the attitude keeping control began and the references were fixed.

Fig. 27. Docking: (left) The original photograph. The original photograph was sharpened anad the edge of ISiMI was emphasized to make her more easy to recognize. The white arrow indicates ISiMI (right).

original photo was sharpened and the edge of IsiMI was emphasized in order to make her more easy to recognize. The photo shows that ISiMI was going into the dock with a more precise approach.

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