44 evenly to the graphene supercapacitor, and diodes prevent negative current feedback into the generation devices. An alarm system is implemented to give an alert whenever the output voltage reaches a low level, because it could potentially harm the entire system. The system can be scaled up to accommodate larger craft, reducing the need for heavy battery banks. Fuel cell Fuel cells can potentially deliver up to five times higher energy density than lithium polymer batteries, leading to a major increase in a UAV’s endurance. Beside fuel cells, a supercapacitor can contribute to the power supply process as well, as it offers a very high power density and fast response to the peak power required for take-off and sudden manoeuvres of UAVs. A hybrid energy management system (EMS) can be used to balance the fuel cell, battery and supercapacitor power delivery. The EMS is designed to monitor the power dispatch between the onboard energy sources with voltage and current sensors to track the power flow, plus converters to monitor power source outputs, and has a processing unit that holds the adopted power management strategy. In practical terms, fuel cells can achieve an efficiency of around 60%, which is not as efficient as lithium batteries, at more than 90%. Embedding a supercapacitor as a supplementary power source in a UAV’s hybrid power architecture boosts the power density and provides a faster power response though, both by delivering peak power and absorbing power fluctuations during dynamic flight with rapid changes in power load. The EMS has to include critical constraints such as flight conditions, power requirements, dynamic power response, fuel consumption, battery overheating, state of health and state of charge monitoring and power distribution efficiency. Tethered UASs Some uncrewed vehicles are powered and controlled via a tether from a ground-based power source. Using voltages of 500 to 800 V allows for greater tether lengths, as the reduced losses allow smaller and lighter cabling, enabling a UAV to fly higher or an underwater vehicle to travel further. The PDN inside the vehicle must be capable of converting the tether’s high-voltage power source with high efficiency, high power density and low weight in order to free up payload space. A cabled, or tethered, UAV receives its primary power via a cable bundle from the ground. Control and telemetry can also be transmitted via the cable, or can be wireless like their battery-powered counterparts. In a battery-powered UAV, the local battery discharge must be closely monitored by the vehicle. In contrast, in a UAS the tether-supplied DC power is typically very predictable and reliable. The base station of the UAS is the power provider for the UAV. A manufacturer will want to build a system that will be able to work with different AC or DC power networks, supplying the tethered vehicle with a stable DC bus voltage. The system designer must consider countering disturbances or changes on the power line, as they could affect the UAV’s position in the air by changing the speed of the propeller and so changing the height of the craft. Events such as load dumps and power transients are common while supporting the base station off-grid, for example from a ground vehicle or sea vessel. These events can also create disturbances or even interruptions in the power source for UAVs, so they often include an emergency battery to guarantee a controlled, powered descent from any altitude. That means the UAV itself needs a power supply to regulate the tethered input power and handle the management of the onboard battery. One of the challenges when designing a power supply for a UAV is choosing the most suitable voltage range. The designer has to decide between low voltage (LV, up to 1500 VDC) or safety extra-low voltage (SELV, less than 60 VDC). Supplying SELV from the base station means there are no hazardous voltages, so the cable insulation can be thin and relatively lightweight. On the other hand, transmitting high power within the limited SELV voltage range leads to high current flow. To offset the otherwise unacceptable losses and heating in that August/September 2023 | Uncrewed Systems Technology The power management system developed for the Bramor PPX UAV (Courtesy of C-Astral)
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