Final approach algorithm

It was first suggested by Deltheil et al (2000) that a vision system is suitable for docking because it offers simplicity, stealthiness and robustness. In this chapter, a final approach algorithm based on vision-guidance is suggested. It was supposed that the AUV could be guided to the dock by controlling only yaw and pitch. This final approach algorithm generates reference yaw and reference pitch and makes the AUV track them. The docking stage begins when the AUV arrives within 10-15 m in front of the dock. The docking stage of the return process is subdivided here into two stages because there exists an area where the dock lights are out of the camera viewing range when the AUV is close to the dock. Figure 20 shows the first and second stages. During the second stage the AUV is about 1.4m from the dock, and the lights of the dock are out of the range of the camera. The essential difference of the second stage is the manner of generating reference yaw for steering motion and reference pitch for diving motion. During both parts of the docking stage, a conventional Proportional-Derivative (PD) control is applied to track the references. Values of these gains were tuned by trial-and-error using the results of the simulations and underwater experiments. A. The first stage

In this stage, reference yaw and pitch were generated based on vision-guidance. All dock lights were located in the viewing range of the CCD camera. This vision-guidance controller generated reference yaw and reference pitch using the estimated center of the dock. A discrepancy between the estimated dock center and the origin of the image coordinates became an error input of the vision-guidance controller. Fig. 21 is a block diagram of the vision-guidance control. A Proportional-Integral (PI) controller was used to generate reference yaw and pitch from the position error. To eliminate steady-state error, I-control was used. By conducting repeated underwater experiments, values of the PI gains were tuned.

Fig. 20. The 1st stage and the 2nd stage of docking approach B. The second stage

When the distance estimated by the image processing became smaller than a pre-specified threshold value, the second stage began. In this area, the last reference yaw and pitch

Fig. 20. The 1st stage and the 2nd stage of docking approach B. The second stage

When the distance estimated by the image processing became smaller than a pre-specified threshold value, the second stage began. In this area, the last reference yaw and pitch become fixed. Because the AUV is very close to the dock, it was supposed that changing yaw or pitch could be dangerous and keeping the final references would be plausible. This method is referred to as 'attitude keeping control.' (Park et al., 2007) During this phase, ISiMI becomes blind and simply tracks these final fixed references until contacting the dock. Fig. 22 shows a flow chart of the final approach algorithm.

Fig. 21. The vision-guidance control algorithm. Po is the origin of the image coordinate frame. Pc is the estimated center of the dock. 0 is pitch, y is yaw. QKf and yKf are generated reference pitch and yaw, respectively.
Fig. 22. Flow chart of the final approach algorithm.
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