109 the body can be lengthened by adding a separate housing that adds about 23 cm of length to the bay and around 3.2 kg of extra payload capacity. Under pressure With time and money at a premium for its XPRIZE effort, the team based the pressure hull of the proof-of-concept vehicle on a scuba tank. “We needed a pressure hull that could withstand at least 3000 psi for round one of the competition,” Daum says. “We decided to use aluminium scuba tanks and modify them by cutting out the bottom and having a pressure flange machined for them. That worked well, and our pressure vessel successfully passed a tank test to 3200 psi.” That 3000 psi requirement would mean a depth of more than 2000 m; however, the production Barracuda is designed to operate down to 600 m, where the pressure is typically about 880 psi, depending on temperature and salinity. An aluminium pressure hull is therefore unnecessary, so the production vehicle’s structure is formed from a high-impact plastic sealed by 4 in end caps from Blue Robotics. The team has completed preliminary pressure testing to 800 psi, and Daum is confident that the vehicle will be good for at least 1000 psi. One of the major engineering challenges facing the team centred on packaging, particularly in fitting all the components required into the volume available in the slim hull. Another challenge was sourcing affordable through-hull connectors that could take the water pressure. Home-grown propulsion Affordable thrusters that were suitable for mounting on the dive planes and that could provide the performance the company needed were not available at the time, so the team developed its own, and still makes them. The motor is a commercially available waterproof high-torque brushless unit, but Tampa replaces the standard bearings with hybrid ceramic ones, dips the electrical windings in insulating resin, and 3D-prints its own propellers. The motor mounts are also the company’s own design. As well as propelling the vehicle forwards and backwards, the thrusters can also run at different speeds and in opposite directions to provide steering and to enable the vehicle to turn through 360o within its own length, eliminating the need for a rudder. “If we have an obstacle in front when we are in a tight corner, for example, we can back out of the situation, turn and then avoid it,” Larson says. “That is one of the things that makes the vehicle unique among the mini and micro AUVs.” The thrusters are the only external moving parts, as even the dive planes are fixed. Pitch control is provided by using the battery as a moveable weight to shift the vehicle’s centre of gravity fore and aft. The thrusters and all the externally mounted sensors and antennas, for GPS and wi-fi for example, are served by power and data cables that meet waterproof connectors in the rear pressure plate, minimising the number of penetrations that would otherwise require extra strengthening. The team designed its own lithium polymer battery, a 6S, 26 V unit rated at 23 Ah. This delivers current to a 26 V bus and a 5 V bus to power loads including the thrusters, navigation and comms systems and the sensors. The propulsion system enables the Barracuda to cruise at between 1 and 6 knots, with a maximum run time of between 4 and 5 hours depending on the mission, the payload and the chosen speed. Computing power and obstacle avoidance The onboard computing power to manage the vessel comes from an Nvidia Jetson Nano CPU/GPU AI platform running the Robot Tampa Deep Sea Barracuda | Digest Uncrewed Systems Technology | August/September 2023 The proof-of-concept vehicle built for the XPRIZE competition featured a pressure hull based on a modified aluminium scuba tank to withstand pressure at 2000 m, while the production vehicle has a polymer pressure hull rated for 600 m
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