Unmanned Systems Technology 016 | Hydromea Vertex AUV | Power management systems | Unmanned Space Vehicles | Continental CD-155 turbodiesel | Swift 020 UAV | ECUs | DSEI 2017 Show report

24 surge axis (backwards and forwards). Only the sway (side-to-side) axis is not directly controlled. Sideways motion can be achieved by rolling the Vertex onto one side and then using the horizontally mounted thrusters together, but that sacrifices depth-keeping ability. “Typically we don’t need sideways – sway – movement. On an ROV for camera inspection work perhaps, it would be useful, but for AUVs there is no need,” Schill says. Even without it, he emphasises, Vertex is very manoeuvrable. “For example, if it needs to do a very sharp 180° turn it will use a combination of roll and pitch, almost like a jet fighter roll, because that is faster than a yaw motion. It can also pitch vertically up or down for rapid changes in depth. “As we don’t have a camera or anything that cares about up or down, we are free to spin the vehicle in any direction,” he says. Slimline hubless thrusters Hydromea invented, patented and developed the thrusters and the motors that drive them, and their configuration is unusual. Called Disk Drive Thrusters (DDTs), each one is hubless, with its blades mounted on the inside of a ring through which they push the water, and this ring is driven from its outer rim by an annular motor. A key advantage of this design is that weed and other debris is much less likely to entangle the thruster, according to Schill. “We have tested this, and it just gets sucked through the centre,” he says. The driver for the thruster and motor design, he says, was the desire for manoeuvrability with minimal drag. “If you have a regular thruster, you have a big motor sticking out perpendicular to the flow, perpendicular to the direction we mostly want to travel in, which causes a lot of drag,” he says. “So we needed a very slim, low-profile thruster that we could put into the winglets.” While the same basic design is used for all five thrusters, they are not identical. The horizontally mounted thrusters are designed to work with equal efficiency in both directions, and the left and right rotors are mirror images of one another. The main thrusters meanwhile are asymmetrical to optimise them for forward thrust because that is what they will be providing most of the time. Different numbers of blades have also been tried, with the long-serving original prototype vehicle currently fitted with three-bladed versions in its horizontally mounted thrusters. A second vehicle shown to this writer had five-bladed rotors. All the rotors are 3D-printed plastic, which allowed Hydromea to experiment with different shapes and profiles. The rotors are hydro-lubricated by the surrounding water, making them oil-free and pressure-proof, and eliminating the need for dynamic seals, while direct drive eliminates the need for gears. The motor housing also contains an integrated motor controller with its CAN bus interface. This controller runs Hydromea’s proprietary sinusoidal commutation algorithm, which continuously monitors and optimises the motor timing parameters to ensure smooth and efficient operation across the motor’s full speed range. The controller also performs closed-loop speed control in both directions in response to speed commands from the vehicle’s control system. October/November 2017 | Unmanned Systems Technology The Vertex’s monolithic polymer pressure vessel is enclosed in a syntactic foam ‘wet’ outer hull, which provides much of the vehicle’s buoyancy (Courtesy of Hydromea)