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32 and the associated motor controllers. While each battery has its own integral battery management system, ASV has developed some simple power distribution electronics of its own that draw power for the payloads and other systems from whichever of the two batteries has more charge, to help balance the loads on them. Other than that, the batteries are not interconnected and remain independent for the sake of simplicity. The only load that cannot be powered independently by either battery is the propulsion, as each pod takes power from the battery in its own hull only. That keeps the wiring simpler and lighter, minimising the number of connectors. Torqeedo offers two propeller models for the system – one optimised for high thrust, the other for high speed – and ASV plans to test and compare them soon. While different propellers could be used for different missions, ASV is more focused at the moment on finding the best compromise between the two standard options. “The one we have appears to cover the whole speed range for us,” Daltry says. “But for applications like surveying across a fast-running river, or current monitoring, you might want to push up to 7 or 8 knots to crab across a current and measure it. That is why we want a propeller that can deliver at both ends of that speed spectrum.” The decision to choose all-electric propulsion was driven by the need for simplicity, and was enabled by the energy density of the latest lithium-ion batteries. Control and autonomy While the thrusters provide steering as well as propulsion, they are not themselves steerable, nor does the boat have a rudder. With all directional control provided by differential thrust, there is no need for any actuators. Daltry acknowledges that this arrangement does not provide redundancy, but argues that because the Torqeedo drives are very reliable components, an operator would be unlucky to experience a failure. “It is only really external factors – and extreme ones at that – that are likely to cause a problem,” he says. “We didn’t want to burden customers with the cost of something that is highly redundant. We wanted something that was simple and just does the job. We are talking about an entry-level price point.” User interface The C-Cat 3 uses ASV’s standard ASView control system packaged for outdoor operations. The user interface computer is normally a rugged laptop in this kind of application, and the company tends to use Panasonic Toughbooks. The PC is connected to a couple of Peli cases, one containing a radio to which the antennas plug in, and the other housing a network interface box and a power supply for the radios. There is also a small PC that acts as a server that can run multiple user interfaces in a client-server architecture, enabling vehicle control to be switched between them, either to share the workload on a long mission or to allow remote supervision. “For example, if you were operating from a large mothership that was doing February/March 2018 | Unmanned Systems Technology Each hull has a self-contained propulsion pod from Torqeedo, with yaw control through differential thrust, each powered by its own battery (Author’s image)