Unmanned Systems Technology Dec/Jan 2020 | Phoenix UAS | Sonar focus | Construction insight | InterGeo 2019 | Supacat ATMP | Adelan fuel cell | Oregon tour | DSEI 2019 | Copperstone Helix | Power management focus

92 operating in line of sight for example it can be quickly landed without much risk of damage to people, property or payload. Many more applications for UAVs are now beyond visual line of sight (BVLOS) though, and the autopilot needs more information about the power system if it is to avoid a catastrophe. A lot of the DC-DC converters used in unmanned designs have no security, and are designed for lightweight industrial applications rather than aerospace certification. Also, many designs do not implement redundancy, or do so with duplicate converters that are not part of the power management system. Many of the BVLOS applications are for hybrid VTOL UAVs that typically use battery-powered motors for vertical lift, shifting to a traditional propeller driven by a combustion engine for horizontal flight. That presents a major challenge for the power management in combining the different power systems. With that in mind, a 900 W fully redundant power distribution unit (PDU) has been developed so that a failure of the power supply would not cause a catastrophic error, especially for UAVs. This is combined with a power management controller and onboard charger (OBC). The PDU has been designed with full redundancy in other areas as well. The servo power supply is duplicated, as are the avionics and the battery pack. The design also doubles up on the silicon field effect (FET) transistors and inductors used for the power conversion. However, there are potentially common elements in a design that can lead to a reliability risk. For example, power management systems all use a common 12 V accessory rail, which can fail, and that needs to be considered. The power management system also shouldn’t be vulnerable to a software failure, a bug or a power supply microprocessor crashing. The microprocessor in the power system is therefore only used for telemetry configuration and monitoring. Instead, a programmable memory chip (an EEPROM) is connected to the power supply via a digital-to-analogue converter to set the voltage on power-up. The microprocessor can reprogram the EEPROM over a CAN bus to change the voltage, but it isn’t necessary to control the power system. Rather than using two separate power supplies, the redundant switched-mode power supply design uses banks of interleaved buck converters, which draw current from the main high-voltage bus at different times to limit the EMI. If a FET, inductor or capacitor in one of the buck converters fails then the others spread the load, ensuring that system power is still available. To avoid reliability problems, the capacitors are polymer electrolytic or ceramic such as the X7R type with automotive or aerospace grades, rather than electrolytic capacitors that can fail if the temperature varies too much. There are three temperature sensors on the PDU board, one at each end and one in the middle. If one of the buck converters fails then the temperature will increase. There are also two redundant fans with separate supplies to keep the temperature down. The way the air is routed through the PDU casing is a matter of discussion for the designers, who want to organise the airflow so that the failure of a fan will not create hotspots. There are two typical failure modes for the PDU. The main one is where one of the FETs fails and shorts out. This is followed quickly by an open circuit, so the fault tends to clear, but clearing that fault takes a lot of current in a short time and with some heating. The PDU’s designers say this has been seen in testing but not in the field. The PDU reports voltages, currents and temperature to the autopilot or a remote operator to make the decisions about whether to shut down parts of the system. It deliberately does not include local decision-making, to limit the number of components that can fail, such as a microprocessor. The 1.6 kW OBC unit provides the AC-DC supply to the PDU, and charges the onboard batteries while flying in horizontal mode. That ensures there is power to bring the UAV safely to the ground if the horizontal-flight engine fails. The OBC handles the battery charging, engine start and electric power generation. It converts a three-phase output from an onboard alternator to DC of a variable voltage depending on the engine speed, provides a stable voltage rail and charges the battery. It can also connect to the PDU. All of this gives the system plenty of time – two minutes – to land safely and ensures that the redundant autopilots still have power to do so. Connectors are also critical to the design and reliability of the power management system. For example, one PDU design for a December/January 2020 | Unmanned Systems Technology A wireless mesh can be used to connect battery management sensors, but reliability and security are key issues currently being addressed (Courtesy of Analog Devices)