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71 The USVs To adapt that ability to a USV, the Ultras use four motor-propellers in a vectored- thrust configuration, with each one pointing outwards in a such a way as to provide a clockwise rotation. “The layout resembles that on an underwater ROV,” Walton explains. “If you want the craft to move at a 45° angle, you’d need to use just two thrusters; to move forwards or backwards, they’d all be working. Also, the yaw can be changed very quickly by using all four motors to spin the USV clockwise on the spot.” So far, the company’s development prototype, the Ultra-1, has been trialled in harbour surveys at Newlyn and Mevagissey, in Cornwall, England, as well as various bridge surveys. It was built using an inflatable dinghy to save costs and quickly test the propulsion concept, but a professional-grade version, the Ultra-2, was due to be ready by the end of 2018, to replace the Ultra-1 and act as the company’s main survey craft. The smallest USV in the series, the 20 kg Ultra-S, will carry the autopilot, control software and comms systems for the larger USV (saving the team having to buy, integrate and calibrate two or three lots of such equipment). Before deploying the Ultra-2, the team will remove the Ultra-S thrusters and plug them into the larger vessel’s power and payload systems via an RS-485 interface. However, the Ultra-S might sometimes be deployed independently if surveys are needed in gaps that only its 80 x 50 cm frame can navigate. The catamaran-shaped Ultra-2 will deploy survey equipment from the empty centre of gravity (CoG) between its pontoons, as this will further reduce sensor movements as the craft pitches and rolls about the centre of its body. The 3 m long hulls are constructed from roto-moulded foam-filled polyethylene plastic for durability and buoyancy. Without its sensors, the Ultra-2 should weigh 200 kg, with 100 kg for the payload and the other 100 kg for batteries. The craft uses 10 kWh worth of Torqeedo 26-104 battery packs, with lithium nickel manganese cobalt oxide cells in waterproof enclosures rated to IP67. These power the four Torqeedo Cruise 2.0 pod thrusters, with each 2 kW drive capable of delivering 5 hp of thrust in their respective vectors, for a top speed of 8 knots, a cruising speed of 4 knots and 12 hours’ endurance. More important than speed and endurance, however, is the control afforded to the Ultras by the company’s propulsion architecture. “The vectored layout lets us rapidly correct for external forces such as currents or waves rolling the USVs and their sensors about,” Walton notes. “It might not be especially efficient for travel, but when taking extremely point-dense measurements of assets 50-60 m away, we need the Ultra-2 to be stable at 0.5 knots, to prevent movements of even just a few degrees as they could severely degrade the reliability and accuracy of the survey data.” Typically the Leica P40 lidars and R2Sonics 2024 multi-beam (or Teledyne Reason P50) sonars used by the company measure 100 and 50 times per second respectively. The slower the Ultra-2 moves, the more points it captures per unit of distance travelled. With a conventional thruster set-up, however, slower movement means loss of steerage, survey line-keeping and heading. With the Ultras though, the vectored thrusters can maintain heading to ±1° and steerage within 10 cm of the survey line, even at 1 knot, parallel with quay walls or other critical survey objectives. Sonar integration differs between the USVs. The Ultra-S sonar is fixed in Ultrabeam Hydrographic Ultra-2 | In operation Unmanned Systems Technology | December/January 2019 The company’s propulsion and control layout relies on four thrusters pointing outwards in a clockwise rotation, similar to an ROV – a system originally inspired by the way quadcopter UAVs maintain their position in strong winds and gusts In the Ultra-1 prototype, the sonar would be pivoted into the water, with the smaller Ultra-S sitting in the middle of its inflatable hull, which houses the autopilot, comms and control systems to be used by the larger vessel