# Development of Modular Bio-Inspired Autonomous Underwater Vehicle for Close Subsea Asset Inspection
## 7 Initial Testing and Lessons Learned
### Testing Acoustic Communication and Rangefinding  
The RoboFish prototype uses an M64 Acoustic Modem [43]. Because this modem was still a Beta version during the initial testing, a number of in-water trials were conducted to establish whether the two pairs of RoboFish uses were working. Both modems were functional, and a point-to-point acoustic link was established and packets transmitted over it successfully. Apart from minor issues in the beginning, mainly with wiring and serial port configurations, the modem's Channel 3, which is between 93.75 khz and 125.00 khz, offered a very reliable acoustic link over 50-80 m range in open water, as well as inside a compact water tank of 302 L. Channel 1 had a lower signal strength causing a shorter range. Channel 4 was more unpredictable, as it worked but with a shorter range and was slightly unstable. Channels 6 and 7 were not tested, as they would give a shorter range and were not required at this stage. These parallel channels can be used by RoboFish for networking in the future, as it is possible to switch between channels to enable communication between more than two modems without packet collisions (but not at the same time).  
The minor wiring and interface issues were related to the 3.3V UART to USB serial converter. A pair of Blue Robotics' BLUART USB-to-serial converters [59] were used. To avoid such issues, the converter and the modem need to be common-grounded. The UART TX from the modem needs to be connected to the UART RX on the converter board and similarly for the RX pins. The modems need to work in water to avoid unwanted overheat. A blinking light about every 2 s on the modem will indicate it is powered, but no link is established. The head of the RoboFish is designed so that it has the modem fitted outside.  
The range finder was also tested and is currently fully operational in RoboFish. Its readings will be integrated in the final mission-oriented control system. Distances read by this Rangefinder can be read from a displaying interface running on the topside computer. This window consists of four important components as shown in Figure 19:  
* _Distance Readout:_ The Distance Readout presents the distance to the target in the latest measurement. The reading that is shown in Figure 19 was the distance to the floor in a testing tank during RoboFish's initial trials. The confidence measurement for the newest range reading is presented below the distance reading and is colour-coded based on strength as follows: green = 100%, yellow = 50% and red = 0%.
* _Distance Axis:_ This vertical axis represents the distance from the transducer built in the Echo-sounder. It starts from the top of the window, which represents zero distance from the face of the transducer and runs down vertically with the distance to the farthest object being at the bottom. Its scale automatically adjusts to indicate a live scanning range of the rangefinder.
* _Return Plot:_ The Return Plot presents the echo strength against the distance of the newest profile sample. The stronger an echo is, the wider its trace appears.
* _Waterfall:_ The Waterfall is a 3D trace presenting consecutive profile samples. The X axis is time; and Y axis is new distance reading shifting from right to left as a new echo arrives.