In Chapter 3 it has been discussed how an AUV can keep track of how well it has covered a search area, while taking the uncertainty on the INS location measurements into account.
The AUV achieves this by maintaining a coverage map. This coverage map is used by the path planner, which enables the AUV to autonomously plan its coverage path during an op-eration. The location of this planner inside the autonomy framework can be seen in Figure 2-4.
When the coverage path has the shape of a lawnmower pattern, only the straight tracks of this lawnmower pattern influence the coverage in the search area, since the connections between adjacent straight tracks are located outside the search area. In this final thesis project, the coverage path is planned by optimizing the locations of these straight tracks. In order to determine the location of the coverage path through an optimization, it needs to be decided with which parameters the coverage path is described, since these parameters are the parameters that are going to be optimized. One way to describe a lawnmower-shaped coverage path is by defining a parameter for every straight track, where this parameter de-scribes the location of this straight track in the search area. An advantage of describing a coverage path this way is that the straight tracks can be placed on the search area in any possible way. A disadvantage is that the number of parameters that has to be optimized is equal to the number of straight tracks, which means that the optimization algorithm has to optimize a large number of parameters for an average coverage path. A second disadvantage is that the number of required straight tracks is not known beforehand, so the optimization algorithm does not know how many parameters it has to optimize. Therefore, a coverage path is not described with one parameter for the location of every straight track in this final thesis project, but with the following three parameters instead.
• An: The number of straight tracks in the coverage path.
• Ad: The distance between adjacent straight tracks in the coverage path.
• Al: The location of the first straight track of the coverage path in the search area.
The set of the optimization parameters An, Ad and Al is referred to as A. With the three parameters in A every lawnmower-shaped coverage path can be described, provided that the straight tracks are spaced equidistantly. The optimization parameters are depicted in Figure 4-1.
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dA
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dA
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dFigure 4-1: Optimization parameters.
Before A can be determined through an optimization, the AUV needs to obtain an estimate of the coverage that it can achieve with a straight track. Since the underwater conditions are usually unknown before the start of an operation, the AUV has to travel an initial straight track in order to estimate this resulting coverage. For every straight track in the coverage path, as well as for the initial track, four waypoints are generated. These waypoints are shown in Figure 4-2, in both the xy-plane and in the xz-plane. In Figure 4-2 it can be seen that the first waypoint of every straight scanning track is located at the surface, such that the AUV can reach this waypoint using the more accurate GPS location measurements instead of the less accurate INS location measurements. It can also be seen that the AUV surfaces after every straight track to execute a GPS-fix, as discussed in Chapter 3. Since the waypoints at the surface are located at the same relative location for every straight track, the AUV reaches the search area with approximately the same INS location measurement uncertainty for every straight track. This is important when the coverage that results from one straight track is used to predict the coverage that results from another straight track, which is discussed into more detail later in this chapter. The middle two waypoints of every straight scanning track are located at the scanning height of 6 metres above the ocean floor, each located 10 metres outside of the search area. The reason that the AUV starts scanning the ocean floor just outside the area, is because this increases the sonar image quality of the sonar images obtained at the boundaries of the search area. Between the two middle waypoints the AUV is in the scan mode, as discussed in Chapter 2, otherwise the AUV is in the travel mode.
4-1 Coverage Path Planning Problem 43
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(a) The waypoints in the xy-plane.
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(b) The waypoints in the xz-plane.
Figure 4-2: The waypoints for a straight scanning track.
Once the AUV has completed the initial straight track, the resulting coverage of the search area can be used to determine the optimization parameters describing the coverage path through an optimization. The details of this optimization are discussed later in this chapter, for now it is important to note that after the initial track has been completed, the entire coverage path for the remainder of the operation is determined through an optimization. The reason that the entire coverage path is planned instead of only the next straight track, is that this way the total number of required straight tracks can be estimated, which can be placed on the search area in an optimal way. When instead only the location of the next straight track is optimized, it can happen that the final straight track needs to be placed close to the boundary of the search area, thus not fully utilising the potential coverage that can be achieved by following this final straight track.
Although the planner in the autonomy framework plans the entire coverage path after the initial straight track, only the next straight track of this planned coverage path is executed.
Once the AUV has completed this next straight track, the entire remainder of the coverage path is planned again. The reason that the AUV replans the coverage path after every straight track, instead of planning a coverage path once and then following this coverage path for the entire operation, is that after a new straight track a new estimate of the current coverage performance is obtained. Since this latest estimate is assumed to resemble the current cover-age performance more accurately than the previous estimate, replanning after every straight track enables the AUV to adapt its coverage path to changing underwater conditions affecting the AUV’s coverage performance.
After a straight track has been completed, the coverage map maintained in the knowledge base contains the updated probability density functions describing the PODs in every grid cell in the search area. When these probability density functions have been obtained by combining multiple coverage measurements from different straight tracks, it is impossible to determine what coverage has been achieved exactly by the latest straight track only. For this reason, a second coverage map is maintained in the knowledge base, which is reset every time the AUV starts a new straight track. By making use of this partial coverage map, the coverage performance corresponding to the most recent straight track can easily be computed. When a straight track has been close to the boundary of the search area however, this coverage performance can be incomplete, since the coverage is only maintained on the search area.
When this happens, the incomplete coverage performance estimation is rejected, and instead the previously estimated coverage performance is used to replan the coverage path. Incom-plete estimations of the coverage performance near the boundaries of the search area can be avoided by maintaining the coverage in an area larger than the search area, but this increases the computational cost of both maintaining the coverage map and of the path planning.
In the next section, entropy reduction is introduced as the main objective to maximise by the optimization algorithm. The subsequent section discusses how the current estimate of the coverage performance can be used to predict the coverage of the search area in the future. The final section of this chapter discusses how this prediction is used to determine the parameters describing the coverage path through an optimization.