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50 Digest | Advanced Navigation Hydrus System architecture At the very front of the Hydrus sits an underwater 4K camera and an optical modem, behind which is a ring composed of LEDs and forward-facing sonar transducers. To the rear of these sensing systems, at the approximate midsection of the hull, are the UUV’s eight thrusters – four for forward and reverse thrust as well as turning, with four more disposed in the roll axis for dynamic steering assistance. The rear half of the vehicle contains a DVL (Doppler velocity log), a USBL transceiver, a water sampling sensor suite and an acoustic modem. “It was quite a challenge to balance the UUV for things like buoyancy, EMI, acoustics and so on, and the paths taken to resolve those are collectively the real reason why it now looks so different to what we’d originally planned all those years ago,” Orr comments. “If you’re trying to optimise for a camera, LED array, forward sonar, downward sonar, rearward USBL and so on, without the ease of say a work-class ROV chassis to keep them all neatly mounted and spaced apart, you need to dedicate serious work to connecting, synchronising and deconflicting them all. “Which we did – which is why we’re able to run most systems from a single FPGA, and rely on a single small battery pack for 2-3 hours of continuous operation. It’s also how we realised that a torpedo shape would actually be really problematic if we wanted this level of subsystem and functionality on the UUV.” The length of the vehicle was extended several times over its development cycles, primarily for buoyancy, and most subsystems are produced in-house for ease of mechanical and electrical integration, with only the transducer ceramics outsourced to Steiner & Martins in the US. This architecture makes the Hydrus highly manoeuvrable and consistently intelligent in its navigation autonomy, suiting it to close-up inspections of complex reefs as well as underwater structures such as shipwrecks and infrastructure foundations. This capability was proven during a trial run at a wharf on the Ningaloo Reef in Western Australia, which is notorious for being very challenging to survey autonomously. “The wharf is a protected marine site: there’s a lot of aquatic life around it that surveyors have to be careful around, but the wharf’s owners are also legally required to survey it regularly,” Orr says. “They’d largely given up on using any kind of ROV or AUV to inspect its underwater foundations, because it’s a really complex series of beams with lots of rocks and seagrass that can cause damage or jam propellers. Previous attempts had resulted in crashes, tethers getting tangled, and similar problems. “So they really didn’t think the Hydrus would work. They had divers on site expecting to have to recover the UUVs and fill in for them at some point, but our system worked perfectly. Two of our units surveyed the foundations over a day; our obstacle avoidance and dynamic mission planning meant they could pull it off out of the box, and by the end they’d captured everything the owners needed.” Hull configuration The Hydrus is built as a fully sealed and encapsulated design around its components, which are integrated tightly enough that effectively no connectors and very few cables are used for the electronics. This reduction in part count has also cut maintenance requirements and helped bring down the overall cost of the system. “We’re starting off at a price of US$45,000, which is what most people pay nowadays just for their DVL and INS package, whereas the Hydrus has its own DVL, INS, sonar, underwater camera, computer, powertrain and two modems,” Orr says. “We’ve worked hard at integrating everything really tightly to eliminate every unnecessary overhead in the architecture.” As mentioned, the hull is shaped largely around the components, with optimisation of the sonar, vision, comms and so on taking precedence over drag reduction, particularly since fast speeds would be detrimental to survey data quality. The outer hull is injection-moulded plastic, while the inner hull features a small quantity of syntactic foam for buoyancy and to prevent any unwanted air pockets, as well as a urethane layer for watertight sealing. April/May 2022 | Unmanned Systems Technology Initial designs of the Hydrus resembled a conventional torpedo- shaped AUV; the design has since evolved to tightly package the vehicle around its subsystems