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8 Platform one August/September 2017 | Unmanned Systems Technology Alta Devices and PowerOasis are developing a reference design for integrated solar and lithium-ion battery power systems for small UAVs (writes Rory Jackson). It will allow UAV makers to quickly develop and manage the generation, storage and management of solar energy for unmanned aircraft systems. The reference architecture will target 2-4 m (6.5-13 ft) span UAVs, using lithium-ion batteries with five to seven cells. The battery power management system will be developed by PowerOasis in the UK, linked to the specialist lightweight solar cells developed by Alta in the US. PowerOasis’ energy management software is designed to be equipment- agnostic, supporting most industry standard protocols including RS-232, RS-485, Ethernet, CAN bus and BACnet connectors for energy control and general equipment control standards, according to John O’Donaghue of PowerOasis. “We also have a number of analogue control interfaces for measuring currents and voltages directly, and then in addition we always end up writing custom device drivers for proprietary pieces of equipment,” he said. “Something like an autopilot might typically have an RS-485 serial comms interface, but the actual registers it uses on that are proprietary to each individual autopilot vendor. It’s critical therefore that we keep our power management platform flexible and agnostic, so that it’s capable of controlling a lot of different things.” A third-party subcontractor will be used to construct the power management hardware, with design for manufacture to ensure the product can be run down an electronics assembly line with minimal defects. PowerOasis will also test the operation and fault cases of its power controller system before passing it on to Alta Devices. In addition to providing its gallium arsenide solar cells, Alta Devices will be working with end-users to integrate the reference architecture into their UASs. “We had gathered considerable data from UAV designers who sought us out, hoping to convert their power system into a solar-electric hybrid, on such things as how big these platforms typically are, how much solar each one uses, and the types of batteries each company is using,” said Rob Parenti at Alta Devices. “That gave us something of a ‘design matrix’ of where the most value was being sought, and that was how we came up with the initial design parameters of a UAV with a 2-3 m wingspan, around the size of the AeroVironment Puma, that can carry around 2 kg of payload, and with about six to 12 hours of endurance. “When you looked at the internal electrical components, most of them ran on 5s to 7s lithium-ion batteries, so that really narrowed the design down to an input voltage for the maximum power point tracker and the solar that could sit on that,” Parenti said. The reference design is expected to be launched in September 2017, with internal testing over the winter months and flight testing in the summer of 2018. “The company we plan to use for the initial testing has a lot of internal computational fluid dynamics and wind tunnel capabilities. We’re planning to use both of those to get plenty of modelling simulation, and see how it all matches up in the end with the actual outdoor flight testing,” said Parenti. Solar charge for lithium-ion Power supplies The Puma UAV uses solar cells from Alta Devices (Courtesy of Alta Devices)