Unmanned Systems Technology 038 l Skyeton Raybird-3 l Data storage l Sea-Kit X-Class USV l USVs insight l Spectronik PEM fuel cells l Blue White Robotics UVIO l Antennas l AUVSI Xponential Virtual 2021 report

66 System operation Such is the difficulty of achieving stable humidity and hence a stable linear power output with open-cathode cells that it is common for them to incorporate current pulsing for their continuous operation. Current pulsing is a form of intermittent short-circuiting of the stack using MOSFETs in the FCC, to drop the stack’s voltage sharply, spike its current and so forcibly rehydrate the cells and boost power output back to its rated levels. For safe current pulsing, the power source and load must be intermittently disconnected. To avoid sudden drops in propulsion during these brief moments, a hybrid battery or supercapacitor large enough to maintain constant power delivery to the vehicle’s motors must be integrated. It could be argued that coupling large batteries – the heaviest energy storage solution – with hydrogen gas tanks, the lightest form of energy storage, is inherently counter-intuitive for aerospace applications. As a consequence of having far more stable humidity over their operating time, the Protium cells can use far smaller batteries or supercapacitors relative to open-cathode cells, reducing overall powertrain mass and bringing the weight benefits of hydrogen gas energy storage to the fore. “Current pulsing can also have detrimental effects on the lifetime of a stack; by not relying on it, we help UAV manufacturers keep their replacement costs low,” Jap adds. “Theoretically, end-users could run our fuel cells on their UAVs with no hybrid battery as a secondary power supply at all, although our FCC is programmed with some key functions relating to how batteries are used that make it potentially useful to install at least a small one on board.” As a final point here, Spectronik has tested its closed-cathode architecture at -15 C, as per customer requests, and developed a cold-start capability for operating the system and powering UAVs at such low temperatures without the need for any external heaters or power sources. “It’s a proprietary technique at the moment, based on the use of hydrogen gas and some internal control systems for creating heat inside the fuel cell to achieve the necessary thermal conditions for the electrochemical reaction,” Jap notes. Control and safety electronics Along with the other electronics, the FCC is housed in an enclosure next to the airbox, where it monitors key health and performance parameters including the stack’s internal temperature, pressure, voltage, current and gas tank pressure (to calculate the fuel level). It also controls the balance of plant, including the blower, cooling fans, manifold internal valves, the hydrogen valves on top of the stack’s upper end plate and the purge solenoid valve. The system used is a PIC32MX microcontroller from Microchip Technologies. It is a 32-bit controller with a 120 MHz DSP-enhanced core, 512 kbytes of flash memory, 128 kbytes of SRAM, a fail-safe clock monitor and advanced memory protection, which supports multiple comms and control interfaces including six UARTs, two I2Cs, and USB 2.0 and CAN 2.0B ports. “We’ve also programmed a few smart features into this controller,” Jap says. “For example, the purge valve is controlled intelligently via our energy accumulator function. June/July 2021 | Unmanned Systems Technology The 0.05 mm gold-plated cooling layer is designed with offset fins for improving the turbulence and hence cooling effect of incoming air Spectronik’s fuel cell electronic controller is programmed in- house using a 32-bit PIC32MX microcontroller with 512 kbytes of flash memory as its core