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

30 the original attracted military interest as a means of transporting useful quantities of fuel and other materiel as cargo. It was for this role that the second generation represented the most substantial variation in the configuration of SubSeaSail USSVs – a catamaran. In this role, the ability to bring the whole of the vessel to the surface for loading and unloading is essential. “You can’t really do that with a monohull because it tips over when you bring it to the surface, so we built a catamaran version,” Todter says. “It sailed mostly under water, but featured a ballast water pumping system so it would come up and sail on the surface as well.” After the Gen2 catamaran demonstration, SubSeaSail turned its attention back to monohull configurations, making the prototypes larger, faster and more powerful, and capable of longer missions. The next step was to develop an autopilot to enable it to sail by itself with minimal sensor input, which presented challenges of its own. Submerging and surfacing By this time, the company had what it considered to be a viable monohull vessel, but it also wanted to demonstrate the ability to submerge completely in order to shelter from bad weather or escape hostile attention. It therefore developed a version with a variable ballast system based on the one developed for the catamaran. This pumps water into a set of three ballast tanks so that the vessel can submerge and hold the chosen depth before surfacing on command. In each tank is a bladder containing air that becomes compressed by the water pumped into the tank to make the boat submerge. To force the water out, the control system opens a set of valves and the air pressure in the sealed bladders forces the water out again and brings the boat to the surface. The ballast pumping system has to be accurate and responsive because of the boat’s sensitivity to small changes in buoyancy. “It will sink as soon as it is neutral, but if it’s 10 g over that weight it will just keep going, it will sink forever,” Todter says. “So there is another control algorithm in the computer that uses input from a water pressure sensor to work out the depth and decide whether to pump more water in or let some out.” If ballast control is not sufficiently quick and accurate, the boat is likely to oscillate around its depth target dramatically, so the pumping system, water pressure sensing and control algorithm are critical to stable depth-keeping. Unlike in a manned submarine, this is a closed system that does not require a snorkel to replenish the air supply or an air compressor and a system of high- pressure storage tanks. One challenge with developing this buoyancy management system was sourcing the miniature submersible pumps, valves and tanks. While there are, for example, suitable tanks available off the shelf, they were never exactly the right combination of size, volume and pressure capability. “There are some compromises in that, and I think the next place we will go in that regard is to develop our own tanks so that we can optimise them,” Todter says. Deducing the wind The Gen6 vehicle has a basic level of autonomy in that it will accept a set of waypoints that can be uploaded to it via wi-fi or an Iridium satcom system, and follow them in the programmed August/September 2020 | Unmanned Systems Technology The upper float module is the part that pierces the surface. It is kept as small as practical to minimise wave- making drag The ‘perturb and test’ autonomous sailing method is based on finding headings that maximise the ‘velocity made good’ towards the next waypoint without the need for direct wind measurement