The necessary building blocks for an AUV devoted to pipeline and cable tracking are depicted in figure 1, and explained in the following paragraphs.
The navigation module is usually referred to the on board sensory systems. It comprises the data fusion necessary to locate precisely in 3D the AUV rigid body and the target position. The usual components within the navigation system are a global position system (GPS), an inertial navigation system (INS), a compass, a depth sensor, and others. Thus, the navigation system provides the dynamic mission planner system, the guidance system and the control system with accurate data to achieve their objectives.
The guidance module is frequently associated to a low-level trajectory generation. When the waypoints for the robot are defined, a trajectory to reach them is necessary in order to feed the controllers set points.
The control module is regarded to the feedback loops allowing the vehicle to describe the trajectory as close as possible to the proposed path given by the guidance module. In effect, assuming that the navigation system yields a clear perception of the AUV's positions, speeds and headings, and the guidance system gives a suitable trajectory to reach a waypoint, there is still remaining a module capable of maintaining the vehicle as close as possible to the prefixed trajectory. Established in this way, the problem to solve at this stage is a control problem to command the vehicle actuators (propellers, rudders and pumps). A mission planner, according to the robot's application, is also necessary to accomplish the task in an autonomous way. A key component of the mission plan is the path planning. Special sensor acquisitions (snapshots, videos, water samples, and others) or special actions (debris grasping) may also be considered within a mission planner. The mission plan consists of two kinds of objectives: long term static objectives and short term changing ones. The first ones, are given beforehand, in a rigid way through a human-machine interface, and then conform a Static Mission Planning, (SMP). The short term objectives can be changed on-line, and constitute the Dynamic Mission Planning (DMP), and varies as the vehicle movement progresses in the real world. This is also known as mission replanning, in response, for instance, to different obstacles to be avoided, based on data from a forward looking sonar and the pipeline or cable position. The artificial intelligence based DMP is a core development in the AUV presented in this article, so it will be explained in more detail in a following section.
When the underwater installation is constructed, legacy data (LD) are archived containing pipeline's or cable's positions, depths, a corridor width and forbidden zones. They should be on-line accessed by the DMP and the obstacle avoidance system (OAS). Pipeline trajectory may be estimated from special sensors like a multi-beam echo sounder (MBE), a side-scan sonar (SSS), a magnetic tracker (MAG), a DIDSON sonar (DID), cameras (CAM) and others. This information is combined in a sensor fusion module (SFM) yielding a position and direction estimate of the target to be inspected. From these data, the DMP is able to decide a trajectory according to different situations like searching a pipeline, following it, navigating closer to it, or recognizing other objects surrounding it. This desired trajectory is defined as a collection of four (4) waypoints to be reached by the vehicle. The Path Planner in this architecture only decides if the desired trajectory given by the DMP is possible or not, according to the outcome of the OAS. Then the waypoints belonging to the desired and possible trajectory are inputs to the guidance module.
The Obstacle Avoidance System (OAS) receives data from a forward-looking sonar (FLS). When an obstacle is detected, a near possible waypoint is proposed to correct the desired trajectory from the DMP.
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