Issue 37 Unmanned Systems Technology April/May 2021 Einride next-gen Pod l Battery technology l Dive Technologies AUV-Kit l UGVs insight l Vanguard EFI/ETC vee twins l Icarus Swarms l Transponders l Sonobot 5 l IDEX 2021 report

88 Digest | EvoLogics Sonobot 5 if that meant users would then have to worry about all the problems such as blades breaking, seaweed clogging or draft requirements that made shallow- water surveys difficult.” The company has therefore combined the bulk of the advantages of jet propulsion with the efficiency gains of propeller drive thrust by integrating two BLDC motors into the floats – one in each, mounted in grooves above the keel line – that connect to propellers running in the water. “With this configuration, if you hit an object or debris the motors are kept safe, and we can add a grid or grille to provide even further protection against flotsam or seaweed,” Dr Lange adds. “These are major issues in the rivers and small lakes where our customers operate.” Each motor’s housing is filled with oil, and is designed with slip ring sealings and a spring-loaded cap for pressure tolerance. This is notably similar to the motors and servos used in UUV thrusters, and indeed the company intends for these motors to be used on their AUVs and ROVs as well, simplifying parts inventories for users who own and operate fleets of EvoLogics’ different vehicles. “We use a spring-loaded cap so that the pressure inside the motor is always higher than the water pressure outside, ensuring that if there’s a breach in the sealing, oil always leaks out before water leaks in,” Dr Lange notes. “That said, the motor’s parts will tolerate being immersed in water for up to 100 hours, thanks to specially treated permanent magnets and special coverings on the coils. “That makes them very effective in failsafe terms, and it also reduces friction a lot, so cooling the motors isn’t a concern.” The motors have been engineered by a third-party manufacturer to EvoLogics’ specifications to be as efficient as is feasible at their size. Each produces a static thrust of 85 N at 500 W of power, or 120 N at 1000 W. Compared with its predecessors, the Sonobot 5’s thrusters have also been designed with greater strength and durability in mind, to reduce long-term maintenance requirements. Overall, they give the USV a top speed of 5 m/s (18 kph), while typical surveys will probably be carried out at speeds of between 0.5 and 1.5 m/s. The control architecture uses differential thrust for steering, rather than adding in servos for rudders, which might present another potential vulnerability to collisions or moving-part malfunctions. “We also designed a new motor controller from scratch with better cooling, as the Sonobot 3 had needed a complex water-flow system for cooling the powertrain and we’d hoped to remove that to improve design simplicity,” Dr Lange says. “And we’ve spent a long time looking for a sensorless, sinusoidal drive, in order to have smooth power and torque output at the slow speeds needed for detailed hydrographic surveys, and without the noise or current peaks that come with trapezoidal control. “Going with that latter commutation April/May 2021 | Unmanned Systems Technology The motors are mounted in grooves above the keel line to protect them from impacts and seaweed The Sonobot 5’s shape keeps wind resistance low, minimising disturbances to its survey paths