Unmanned Systems Technology 033 l SubSeaSail Gen6 USSV l Servo actuators focus l UAVs insight l Farnborough 2020 update l Transforma XDBOT l Strange Development REVolution l Radio telemetry focus

order, loitering or submerging at any point as commanded. The software has been written in Embedded C, runs on a proprietary computer and sails the vessel between waypoints without any direct information on the direction or speed of the wind. “If you are a sailor you understand very quickly that knowing the wind direction is important,” Todter says. “You can’t sail directly into the wind, but you can go almost anywhere else at different headings to the wind and at different performance levels.” Todter wanted to simplify the autonomous sailing system by eliminating the need for an anemometer and its associated cost and complexity. “Our aim from the outset has been simplicity and low cost, while trying to make the vessel almost disposable from the military’s point of view,” he says. The only navigation sensors SubSeaSail uses are a GNSS receiver with an integral IMU and a digital magnetic compass. This combination enables the software to infer the direction and strength of the wind through a ‘perturb and test’ methodology, which Todter describes as new territory in control system operation. “The simplest thing to do is to head straight for your next waypoint and see what happens,” he says. “If you are making progress, the assumption is you are not sailing directly into the wind. “Then you try to improve your performance by sailing either a bit left or a bit right of your current heading. The measure of performance is called the ‘velocity made good’ [VMG], which is the speed directly towards your goal.” Because velocity is a combination of speed and direction, if the vessel is heading directly towards the waypoint, the VMG and the speed are the same. If the boat is moving at the same speed but at some angle to the direct line between the boat and the waypoint, the VMG will be lower than the speed. However, if the boat’s heading allows the sail to generate more thrust than it could on a direct approach to the waypoint, and the boat’s speed is higher, then the VMG is also likely to be higher. Essentially, the perturb-and-test technique programmed into the autopilot’s logic uses a kind of directed trial and error to maximise the VMG. Todter says, “In some cases you can’t get anywhere near where you want to go, so you have to head off at quite wide angles and then figure out when it is appropriate to tack and head 90 º from where you were going to put you on course for the waypoint.” A human sailor without an anemometer but with a cloth sail on the boat will be able to make efficient progress more quickly by steering progressively closer to the wind. They will watch the sail and steer away when it starts to flutter and lose its ideal curve, and generally SubSeaSail Gen6 USSV | Dossier STRUCTURES • Structural design • Certification • Load envelope calculations • Composites & metallics • FEA static & dynamic analysis • Fatigue verification • Failure root cause analysis • Composites fractography AERO & HYDRODYNAMICS • Vehicle configuration • Performance prediction • Optimisation • Vehicle dynamics • Control surface design • Flutter analysis • Gust response • Cavitation & ventilation • CFD & panel methods PROTOTYPING & PRODUCTION • Materials & process down-selection • Manufacturing drawings & process specification • Vendor interaction • Manufacturing oversight • Test design: from coupons to complete vehicles INNOVATION & IP • Inventive design solutions • Patent generation & review • Board level advisory • Technical due diligence Contact our independent engineering consultancy for: Design & Analysis from Concept to Delivery Helping you accelerate your UXV projects & achieve your goals +44 (0)1273 571932 www.aerotrope.com [email protected] Aerotrope Ltd., Unit 2 Level 5 S, New England House, New England Street, Brighton, East Sussex BN1 4GH, UK