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32 Based on those test results, the team decided to make a new version that was slightly bigger, as well as making a few other changes. “For example, the older design had a one-atmosphere closed space on top that would hold the vacuum pump and the connected systems inside,” Doroudgar says. “For the Mk 2 though we just have a little canister that holds only the associated electronics, and that is now installed in the UUV, but everything else is outside and exposed to seawater, including the pump, the altimeter, the junction box and so on. “Also the silt-handling tests were conducted in water tanks using fine, washed sand and a sticky cohesive mud, to test across those extremes. It’s actually quite difficult to accurately mimic the seafloor for testing– you rarely know what type of silt you’re going to be operating over, and even if you do, the conditions can vary greatly.” The silt tests were aimed at investigating two principle characteristics that would determine the anchor’s effectiveness at securing to the seafloor – the silt’s internal cohesion (how much it would hold together against the pulling force of the caisson), and its adhesion (how much it would stick to the caisson’s rim). “We got good results with both in terms of holding capacity, although we did find the mud was more susceptible to lateral forces than the sand,” Doroudgar says. Performance tests have shown that the suction anchor will securely hold up to 140 kg of force – far more than the 40 kg-f needed to keep the Solus-LR in place – and theoretically the UUV can stay in place indefinitely once the saltwater beneath the caisson has been pumped out. To disengage the anchor, the pumping systems operate in reverse, injecting water back into the caisson at roughly the same speed and duration as the securing process. Once the inner and outer water pressures begin to equalise, the caisson slips from the seafloor – the tension from the tether also helps here – and it can then be retracted back into the Solus-LR. While the load sensor in the tether’s pulley helps the UUV to determine when to reel in the anchor, an acoustic homing sensor atop the caisson detects its proximity to the UUV. The combined measurements enable the anchor to return safely into its bay. The future The second round of the Solus-LR’s sea trials are set to end by this March or so. By the end of June, Cellula aims to have started its month-long pilot programme, the aim of which is to run the UUV fully submerged for an entire month and to cover 2000 km without refuelling. “It’s ambitious as far as UUV trial runs go, but it’s how we want to introduce Solus-LR to the marketplace,” Woodroffe says. “It’ll mean considerable work and potential risks, but we’re determined to do something that no- one else is doing, and to show the Canadian government that their funding has paid huge dividends.” February/March 2020 | Unmanned Systems Technology Dossier | Cellula Robotics Solus-LR Cellula’s control software is used mainly when guiding the Solus-LR in and out of a harbour (Author’s image) The Solus-LR’s suction anchor is built around a carbon-steel caisson that sinks into the seafloor as water is pumped out of it (Author’s image)