Uncrewed Systems Technology 043 l Auve Tech Iseauto taxi l Charging focus l Advanced Navigation Hydrus l UGVs insight l MVVS 116 l Windracers ULTRA l CES 2022 show report l ECUs focus I Distant Imagery

43 Battery charging | Focus charging stations. The transmitters in the wireless kits are universal though, so a fleet of highly diverse robots can share the same charging stations. Software becomes a key part of such a system. Fleet management software manages a network of charging stations and larger fleets of robots. The software aggregates historical information on every charge cycle for every robot and provides analytics for optimising fleet-wide charging processes and battery performance. Passenger vehicles The design of the coils is also important in the engineering trade-off between the separation and the efficiency of the power transfer. The closer together they are, the tighter the inductive coupling and the higher the efficiency. However, this is a challenge for vehicle designs where the distance can vary, for example from a sportscar to an SUV to light commercial truck to a bus, all with different heights above the ground. Some suppliers have developed techniques to raise the lower pad to reduce this distance, while others lower the coils from the vehicle down, similar to the pin contact system mentioned above. The SAE standard Published in 2020, the ISO 15118 standard defines a unidirectional wireless charging system of up to 11 kW at a frequency range of 81.39-90 kHz. Three power levels are defined – 3.7 kW for WPT1, 7 kW for WPT2 and 11 kW for WPT3. The standard also defines three levels of efficiency coil separations, from 100 to 250 mm, defining a grid-to- battery efficiency of 94% with a maximum coil separation of 250 mm. The end-to-end power conversion efficiency is a combination of the coil design, the coupling and the control electronics, where the conversion frequency is also important. A higher frequency allows for lower losses in the electronics by using wide- bandgap transistors made of materials such as silicon carbide or gallium nitride. The standard defines an 85 kHz coupling frequency, as the 79-90 kHz band is available for use globally, but some wireless charging systems have been developed at 150 kHz, which means the coils have lower losses but the electronics have a little more. The standard also defines the acceptable tolerance for aligning the ground and vehicle coils when the vehicle is driven over the ground pad. This ranges from ±75 mm in the direction of travel and ±100 mm in the lateral direction. The standard also includes a message protocol specification, with messages based on the JSON message definition using IEEE 802.11n wi-fi hardware. This sets out definitions to indicate that Unmanned Systems Technology | April/May 2022 Many UAVs now come with a single charger for the battery, which limits their mission time. Flight times of typically 12 to 15 minutes for a commercial quadrotor then require 60 minutes or more to continue. The development of a quad charger to charge four batteries at the same time however allows a UAV to operate for much longer. Many UAV missions take place well away from mains electricity, so the charger has to be powered by a diesel generator or larger battery system and inverter. The output of these systems are typically modified sine waves, which needs a charger that can cope with an input range of 94 to 240 V. Adding voltage, current and temperature monitoring and a dedicated controller allow the charger to provide the maximum allowable current for the battery. Each UAV supplier has different battery packs, and these can also have different chemistries, so the charger controller needs to be optimised with algorithms for a specific UAV supplier and the inverter input. For reliability, the quad charger is built from four individual chargers, each with its own monitoring circuits so that the failure of one charger or battery does not have an impact on the other three. Using multiple chargers also allows the batteries to be treated more gently and extend their operating lifetime. On average, a charger would charge four batteries from 20% capacity to 90% for better longevity, rather than from empty to 100%. Most operators recharge the batteries at 30-35%, and these would typically charge to 90% in 48 minutes. Charging to 100% would take 58 to 65 minutes, as the last 10% of charge typically takes 20% of charge time. Four chargers turns out to be a practical limit for the generators that power them. Moving to an eight-charger system would require more current than is available. Eight-port systems have been developed, but these use a round-robin system where only four are charged at a time. UAV quad charger Charging four batteries at a time helps to keep UAVs in the air (Courtesy of Colorado Drone Chargers)