Uncrewed Systems Technology 046

84 sensing when they are needed and executing them accordingly. “Although we worked with distributed control systems at Ballard, UAVs can’t really afford the weight of anything other than a centralised control architecture,” Robinson says. “Our software on the other hand is highly modular and object-oriented, which allows us to iterate new versions very quickly or trial new components such as different fuel monitoring sensors, fluid valves and coolant pumps. “Most of our functions are standard, closed-loop PID controls, which is useful for instance in the thermal management target of keeping the internal environment at 60 º C. If it drifts above that, the controller pumps water faster to the heat exchanger, or if it drops below that it sends some water to bypass the heat exchanger. It does the same for maintaining humidity, stack health, purged hydrogen recovery, waste water expulsion and related parts.” As well as running all the valves and pumps, the blower and so on, the fuel cell controller also runs the BMS. The fuel cell system almost always integrates a hybrid battery, virtually always a lithium- polymer or lithium-ion battery, which takes the current peaks, troughs and sudden transitions demanded by the UAV motors and outputs a stable voltage and current to the UAV while gently ramping the fuel cell output up or down. The BMS also takes into account battery state of charge (SoC), battery voltage and current, and load (UAV) voltage and current. That helps with managing the fuel cell because it provides a window into how the PEMFC itself needs to be managed. Core data points such as the battery’s health, SoC and cell temperatures can be monitored closely so that the fuel cell’s operations can be adjusted to keep these within normal parameters, batteries being a rather more hazardous component than PEMFCs. “The battery also lets us output a reasonably high voltage to the UAV so that the cables running to the electric motors don’t have to go through a DC- DC converter,” Robinson notes. “If they do, you or the customer then have to size the converter to the maximum load power, rather than to the maximum fuel cell power – which are two very different numbers.” Although the voltage and current ranges are as specified above, technically anything from a 6S to a 16S battery can be used in either the 600U or 1200U, enabling power output supplies from around 20 V up to about 90 V DC. Robinson says, “Based on what output voltage the customer wants from the battery, we’ll then optimise our fuel cell stack for a similar kind of voltage, because we’ll probably need a DC-DC of our own inside the enclosure between the two, and the less that converter is bucking or boosting, the more power- efficient our system will be overall. “Losses are directly proportional to the scale of DC-DC conversion you’re doing. The reason we made our 1200U using two 600 W stacks rather than a single 1200 W stack is so that we can connect them in parallel for UAVs running on lower voltages, or in series for UAVs running on higher voltages. Series gets you twice the voltage, which makes series-circuit architectures generally a closer match to the newer generations of electric motors, and hence more power-efficient.” October/November 2022 | Uncrewed Systems Technology The bipolar plates are made from graphite for ease of machine-cutting and customisation