42 Focus | Power management modulation (PWM) signal needed to properly measure and modulate the input and output voltages is 8 bits. The controller also includes a battery charging algorithm denoted as the constant current/constant voltage (CC/ CV) algorithm, which is most widely used for lithium-ion and lithium polymer batteries. Battery charging is performed when the electrical energy production is higher than the power consumption of all the potential loads connected to the battery at the same time. For a full battery voltage setpoint, 16.6 V is selected, which for safety reasons is 0.2 V lower than the value of a completely full battery. All external components, such as inductors and transistors, must be assessed and chosen properly. The requirements depend on the known input/output current and voltage ratios on the basis of the chosen switching frequency, which is 202 kHz. This frequency is a compromise between the physical size of the inductor, the input/output capacitors on one side and switching losses on the other. The buck–boost switcher logic requires four transistors. One pair handles switching in buck operation mode, the other handles switching in boost operation mode, and all four transistors handle switching when the output voltage is similar to the input voltage. Transitions between modes depend on the ratio between the controller’s input voltage, output voltage and switching frequency. For two transistors, the main part of the power loss is caused by switching losses, while for the other two the main part of the power loss is caused by ohmic losses. It is therefore important to strike a compromise between transistor RDSon resistance and CRSS capacitance. The chip monitoring the current and voltage communicates over an I2C bus, but an external device is needed to read these measurements during flight. A data logger captures all measurements at a frequency of 1 Hz and saves them on an onboard microSD card. The frequency source is provided by the external device. A pulse per second (PPS) output signal with a very accurate UAV GNSS receiver has been used for this task. Precise knowledge of the frequency of the captured data allows these measurements to be combined with the UAV’s log data. Multi-mode power To extend the battery charge, aircraft range and operational capabilities of a UAV, power can be generated from multiple sources during flight, including solar power itself and wing structural vibrationinduced power generation. These can be harnessed and used to power an electric motor and propeller propulsion system while the UAV is in flight. Ultra-lightweight 3D-printed graphene supercapacitors can be used with the power management system, and the system will optimise power usage throughout the entire vehicle and storage of the excess electricity. The main power management system draws power from three sources. One is the photovoltaic solar energy from the solar cells, while the second is the compressive strain energy harvested through the piezoelectric generators attached to the root of the wing where most bending strain occurs. Mechanical energy from sub-critical flutter in the airframe is harvested from a springmotor/generator device as the third source mounted inside the UAV, where it will convert wing vibrations into electricity. The power management system itself consists of six components – rectifiers, MPPT boards, balance boards, alarms, diodes and supercapacitors. The rectifiers allow the generated power to be fed into the system at an optimal level. The MPPT board maximises battery life and power output while a separate balance board distributes energy August/September 2023 | Uncrewed Systems Technology Implementing a distributed architecture for UAVs (Courtesy of Vicor) To extend battery charge, power can be generated from multiple sources during flight, which can then be used to power a propeller propulsion system
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