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Sonar mosaicing of chain scenario

After the first successful tests with the ARIS sonar, the UdG team worked towards reproduction of the chain scenario of PANDORA project. A chain of 13 links and a total length of about 7 meters has been built simulating a real mooring chain.

chain scenario

Reproduction of the chain scenario at UdG’s water tank.

Before the first year review of the project, we conducted some experiments inside the UdG water tank to simulate inspection of the chain by means of sonar.

Girona-500, equipped with ARIS, was manually teleoperated along the chain gathering images at a short range to generate afterwards an acoustic mosaic of high resolution.

The following video summarizes the mosaicing process of the sonar images:

The figure below shows the obtained full chain mosaic:

chain mosaic

ARIS Forward-Looking Sonar: first tests

Last month the UdG team received a new piece of equipment for the PANDORA project: the ARIS Forward-Looking Sonar (FLS). This sonar generates high-resolution acoustic images at a near-video rate, and can play a key role on those underwater inspections where the water visibility does not allow the use of optical cameras. In the chain scenario of the PANDORA project, the process of cleaning the chain is prone to generate turbidity in the water which can difficult the algorithms taking control of the cleaning process itself as well as the subsequent inspection. By using a forward-looking sonar we plan to work with acoustic images and overcome this lack of visibility.
We have worked towards the development of an algorithm for the generation of acoustic mosaics and we have done some preliminar tests with the ARIS sonar in the water tank of the UdG. Although the sonar is still not integrated to Girona-500, it has been attached to the vehicle and we have used the ARIS commercial software to gather images of several small objects placed on the bottom of the water tank. The robot was driven in a zig-zag trajectory along three different tracklines gathering around 1500 different sonar frames. The developed mosaicing algorithm was able to register successfully a high number of frames, including many loop closures, achieving the consistent mosaic shown in the figure below.

Girona-500 with ARIS

Girona-500 with ARIS installed for the experiments.

Mosaic generated from ARIS frames

Mosaic generated from the ARIS frames.

Panel and Valve Detection

One of the problems to be addressed in the project is the detection and localisation of an underwater panel. The aim is to have the vehicle perform a valve-turning task autonomously once the location of the panel is known. Detection of the panel is performed by comparing images from the camera with a pre-defined template. A series of images of the panel are taken and the “best” image is selected for use as the template.

Panel template is taken from a close-up image of the panel. A mask is used to highlight the static marks on the panel and ignore the valve handles.

Valve detection is performed by first detecting the panel and making known of the rigid geometry to localise the valves with respect to the centre of the panel. To detect the orientation of the valves, a Hough transform is applied first to detect lines within a bounding box around each valve. The orientation of each valve is then obtained by searching for lines of specified minimum length within the bounding box.

The figures show the detected panel and valves highlighted with white lines at approximate distances 2m and 1m. The detection of valve orientation at large distances can be inaccurate and is only considered at short distances.