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35 Throttle servos The past few years have seen a particular evolution in servos for UAVs, through the widespread adoption of CAN bus over serial or PWM connections for data to be fed from an engine’s throttle to its ECU. The CAN comms architecture enables ECUs to measure the temperature of the throttle servo via a thermistor to track its health and performance. The degree of shock and vibration on the throttle can also be logged for real-time warnings or later analysis, by integrating an accelerometer into the servo. Parameters such as current and voltage can also be measured, as a discrepancy in these can be a good indicator of a servo that will soon fail, be that because of its age, a manufacturing defect, an accident or excessive use beyond the manufacturer’s guidelines. Naturally, the viability of these new levels of servo diagnostics has been helped by the ongoing SWaP optimisation of the sensors, electronics and other components on actuator circuit boards. Different CAN standards have been developed for servos in the UAV market, and have come about often as derivations of existing ones for the automotive sector or UAV autopilots. These can vary in a few ways, with perhaps the most typical being a trade- off between a faster and lighter stream of data updates, or a denser but slower data feed. The primary aim of these standards is not so much SWaP optimisation as maximising the reliability of the servo – particularly in the context of throttle servos, which must operate consistently in high- temperature and high-vibration conditions. This is perhaps a natural evolution in UAV systems, as more and more manufacturers forgo components from RC-type suppliers. Using servos from manufacturers for the hobbyist market can be quite dangerous – not to mention uncertifiable – as UAVs get bigger and their engines become more powerful. As a result, UAV throttle servos are increasingly likely to be designed with parts such as brushless motors and metal gears (rather than plastic), and tested for hundreds or thousands of hours for failures due to temperature, shock and vibration. They are also becoming more likely not to use PWM as a command link, as it can provide highly unreliable signals owing to its vulnerability to interference amid the high-noise environment of UAV engines. In addition, it is slower than CAN and limited to one-way comms from the autopilot to the throttle. Level sensors Probes for tracking fuel and oil levels have improved in terms of weight reduction, reliability and robustness in harsh environments, as befits the needs of the UAV market. In the drive for reliability, more and more UAVs are integrating solid-state sensors, rather than ones with floats or other mechanisms that can jam or underperform amid vibration. There are different ways of continuously detecting liquid levels, and one of the most popular is by using capacitance technology. This measures the dielectric constant of the fluid between two surfaces in contact with the fluid (for example, between a tube and a rod running down its inner length), with changes in the capacitance (and the signal voltage) corresponding to changes in the level of fuel or oil. This provides data on key variables such as temperature, which can be fed into the algorithms and electronics that output the fuel or oil readings (as well as variations based on the type of fuel or oil). General improvements in electronics are also enabling level sensor probes and their processors to operate at higher temperatures, with roughly a 20% higher operating temperature possible now compared with five years ago. As fuel and oil tanks run hotter than before, this reduces the degree to which remote electronics are needed, so systems are not subject to errors stemming from the capacitances of stray wires between the electronics and their probes. Having entirely integrated electronics also makes for more compact level sensors, aiding SWaP optimisation of UAV designs. Improvements in design and manufacturing technology have further contributed to producing smaller sensor packages that are a better fit for UAV airframes. They also make it quicker to customise the lengths of probes to fit the depths of different fuel tanks. In addition, some sensor designs are swapping out metals for carbon composites to save weight. However, Engine ancillary systems | Focus Unmanned Systems Technology | April/May 2020 Sensors for fuel and oil levels can now be made using carbon composite to reduce weight on UAVs (Courtesy of Reventec)