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

32 Dossier | SubSeaSail Gen6 USSV feel what the boat is doing, Todter admits. “In our case the response is quite slow, as you have to wait 30 s to a minute to find out whether the new heading is better or worse,” he says. “So it’s a bit ponderous, but it always gets where it’s going.” Control systems theory still applies though, and the software has to take into account the tendency to oversteer, for example. So it is important to measure the rate of turn with the IMU and/or the magnetic compass and use that information in programming the autopilot so that it can command rudder angles and rates that will allow it to turn smoothly and hold the vessel on course. Self-regulating wingsail The autopilot’s job is made easier by the fact that the rudder is the only effector it has to control, thanks to the aforementioned patented mechanism that enables the sail to rotate and set itself at an angle of attack that powers the boat regardless of its heading – so long as that heading is not too close to the wind. “We don’t have to control the wing, we just have to point the boat,” Todter says. The mechanism consists of a cam with a spring-loaded tensioner/follower that generates torque that exactly opposes the torque that wind pressure acting on the sail applies to the mast. Ensuring that these forces are balanced when the sail reaches the correct angle is a function of the cam profile, the spring rate and the length of the tensioner arm. The wingsail is rigid and is mounted on a sleeve in the centre of the cam, and the whole assembly is free to rotate around the fixed mast. The cam has a dwell portion of constant radius that can be thought of as the cam’s base circle, and a nose portion that can be considered as the cam’s lobe, which is divided into symmetrical left and right profiles that meet at a point opposite the base. When the tensioner is on the dwell portion of the cam, it generates no torque. That coincides with the wing being in a ‘no-go’ point of sail, which is a range of directions or angles with respect to the wind in which a sailing boat cannot sail. It is when the tensioner is on the nose portion of the cam that it generates a force that opposes the torque created by the wind on the wingsail. These positions coincide with the angles between the wind and the wingsail that allow the latter to generate thrust. Control station and operating modes While the 28 kg Gen6 vessel can be operated by one person, it is typically run by a team of two, and two are needed for launch & recovery. There is also a smaller, 14 kg Gen6 Mini that can be launched/recovered by one person. The operator interface runs on a laptop or tablet, enabling complete mission August/September 2020 | Unmanned Systems Technology Crew deploying the Gen6 from a support boat gives an idea of scale. The spring-loaded cam follower arm is just visible between the float module deck and the base of the wingsail The mechanism that automatically sets the sail to the best angle to the wind for thrust generation is based on a cam (302), which is subject to opposing torque forces from the wind and a spring-loaded arm (208), while 110 carries the mast