Unmanned Systems Technology 028 | ecoSUB Robotics AUVs I ECUs focus I Space vehicles insight I AMZ Driverless gotthard I InterDrone 2019 report I ATI WAM 167-BB I Video systems focus I Aerdron HL4 Herculift

32 in a manner that can be easily varied. It did that by adding hollow nylon ribs to the internal equipment chassis that serve to stiffen it, and to carry variable amounts of lead shot as ballast. “What that means is we can install a really heavy sensor payload if we want, because we can take the ballast out,” Sloane says. In terms of shape, the most complex and critical nylon part is the tail section. This forms the hydrodynamic duct that contains the rudder and the two-bladed propeller, which are also 3D-printed nylon components. CFD helped determine the basic form of the vehicle at the beginning of development and towards the end for hydrodynamic refinement of the duct and the propeller, with dramatic results. Using OpenFoam v6 to optimise the design of this area gave a 20% improvement in efficiency, translating into greater speed, range or endurance within the energy budget. “That was always part of the plan,” Sloane says, “but we left it right until the last knockings to make sure everything else was design-stable.” In this process, the combination of CFD and 3D printing proved to be a powerful one. The designers went through multiple iterations with different duct lengths and profiles, and propellers with different numbers of blades and blade profiles from which to choose. They then pitted the best of the new designs against the original in a race along a test tank at Southampton University. “We took two otherwise identical vehicles with identical motors and settings, one with the old duct and one with the new, and the new one just steamed away. The difference was amazing,” Sloane says. Propulsion and steering To avoid the pressure hull being penetrated, which invites leaks, both the propeller and rudder are driven through magnetic couplings, so the only things that pass through the aluminium case are magnetic lines of force. (Aluminium is non-ferrous; magnetic coupling won’t work through ferrous metals.) A single PWM-controlled DC motor from Maxon drives a shaft, around which a series of magnets is arranged. This shaft spins inside a rearward tubular extension of the pressure case. Outside that extension is a hollow shaft containing a matching set of magnets. It is this second shaft to which the propeller is attached. In the larger vehicles, the motor drives the first shaft via a gearbox, but the other details are the same as in the μ5, only scaled up. For yaw control, a linear actuator moves an arm with magnets attached up and down inside a vertical tubular extension of the pressure case, around which is another tubular arm with magnets inside and a linkage that converts the vertical motion to lateral motion for the rudder. Of the two magnetically coupled systems, the motor-propeller relationship is the more complex to manage. If the magnetic coupling is to engage, the motor has to be brought up to speed gradually on start-up, as simply giving it full power from a standing start will cause the couple to drop out and the motor will spin rapidly without turning the propeller. Likewise, speed changes when the vehicle is underway also have to be smooth and gradual. “It takes a whole lot of software and testing to do that,” Sloane says. This software has to be adaptive because the coupling’s characteristics vary with water conditions, he explains, some making it harder to spin the propeller, some making it easier. “It would change if you have a slightly different density in the water or if you’re working near the surface and have lots of cavitation bubbles,” he says. “All the October/November 2019 | Unmanned Systems Technology The shape of the rear section with its ducted propeller and rudder was refined late in the design process using advanced CFD, yielding a 20% improvement in efficiency (Author’s image)