Points of consideration

When choosing a seafloor mapping technology, we can present the following points of consid-eration:

• The data needs

The data needs depend on what kind of data is needed to survey and are the most impor-tant factor. For example, a side scan sonar can produce a high-resolution image over a large area but does not collect topographic data or information about the seafloor com-position.

• Price

Price is an important and limiting factor. The price of a system might entirely disqualify an otherwise perfect solution. It should always be minimized as long as the requirements are reached.

• Range

The area that a survey unit can cover can be important or redundant depending on the application. The range can even be detrimental because it often comes at the cost of ac-curacy, price or both.

• Adaptability

If the surveying equipment is to be implemented into a specific or existing system, it needs to be adaptable and have software integration possibilities.

• Bandwidth needs

When survey data is collected, it needs to be saved in the ROV or sent to a computer for further processing or plotting. The system bandwidth needs to be able to handle data coming from the survey equipment while not throttling the communication system.

Evaluating the price categories

Different companies making hydrographic surveying equipment were contacted. After talking to different system providers, it was decided that we would use a side-scan sonar. Side-scan sonar systems come in a wide price range and can be classified into three categories.

• Hobby levelsystems are mostly used for fishing and hobby experiments. The systems are cheap and usually cost around 700€for the transducer systems, and has a decent resolu-tion. However, these kinds of systems are difficult to adapt to existing software solutions, and they often only have a depth rating of around 10 meters. The systems are almost always integrated incorporate systems with ready to deploy displays, software and hard-ware. These supplementary systems often increase the cost by thousands of euros.

• Medium levelsystems offer increased adaptability and often have custom made specifi-cations like housing or depth rating to fit your system. In addition, these systems often provide an API. With a provided API, it is possible to implement a sonar image into our GUI. The price of these systems is from around 1 500€to around 20 000€.

• High-end systemsare the most accurate, with great depth rating, resolution and range and adaptability. However, the price matches, from 20 000€to over 100 000€.

Primary goals

When choosing a sonar, we considered these points to be the main needs of the ROV project:

• Runde Miljøsenter wanted to find lost fishing equipment. Therefore resolution and range were relatively important.

• The system’s price should be as low as possible while achieving the other requirements.

• The adaptability needs to be good to implement the sonar into our system easily.

• The required bandwidth of the sonar needs to be acceptable to the bandwidth of the tether system on the ROV. The Slepe-ROV has a bandwidth of around 10/100 over ethernet, which the sonar system would need to share with other systems in the ROV.

For the Towed ROV, the high-end systems are too expensive to consider. Therefore the choice was between the hobby level and the medium level. As we reached out to a few engineers from Garmin, we were told that these systems would not support being implemented into our system.

In addition, the depth rating of these systems where low and therefore would not support the depths we wanted to reach. Therefore using these kinds of systems would require modification for them to fit the ROV system.

This would include making sure that the transducer where depth rated for at least 50 meters and decoding the image sent from the electronics to implement it with our API. Therefore, it was decided that the hobby level systems were not a good fit for the ROV system.

Main alternatives

In the end, we seriously considered three different companies, each with multiple systems and available customization’s.

• Imagenex:

Has the smallest size and weight and the widest range. The Imagenex sonar uses serial communication, but they are also the most expensive [50].

• DeepVision:

The electronics of the DeepVision sonar can be potted, which would increase the adapt-ability for future systems. This would also present the possibility for connectors instead of penetrators for cable entry into the ROV body. Making it easier to add and remove the transducers in the future and decrease the amount of watertight space required for future ROV bodies. This would reduce the chance that they could be damaged during trans-portation. DeepVision also Provides a cross-platform software API for integrating into our

system. The Deep vision OEM module sends data using ethernet and is powered by POE [26].

• StarFish:

Includes software API for ease of integration. The electronics box is large and would take a lot of space in the ROV body. But it can provide pre-made fastening brackets, making it easier to mount the system. StarFish also have a "single unit" system where the entire system could be attached to the ROV with a few screws. But the electronics Uses USB communication and would need to be coded into the RPi system in the ROV [87].

Both DeepVision and Starfish provided unique advantages. Deepvision with their potted and smaller electronics and possibilities for connectors, and StarFish with their simple bracket sys-tem and lower price. In the end, the group went with the DeepVision syssys-tem, mainly because of the advantages of connectors with the potted electronics, the small size of the electronics, and the ease of integration.

4.6.2 Implementation

The plan for the Side-Scan Sonar was only to implement it to our software system and get it up and running. Therefore it was built a small electronic box to keep water out while testing, also brackets were made; these can be glued onto the ROVs flange. Connection of the sonar to the electronics module and Ethernet can be found in Appendix [F]. After a recommendation from the manufacturer, these were mounted, so they were crossing each other as shown in Figure 4.20. For both sonars, penetrators were added and sealed with epoxy. More about that in section 4.7.1.

Figure 4.20: Side-Scan Sonar setup