102 interfaces, partly in software engineering and hardware components but more so in the ongoing technical support that inevitably needs to be provided with the more technically complex buses. Although these types of servos will probably fulfil the requirements for some lower levels of certification, enabling legal flights but with some restrictions, in the long term it is anticipated that their users will gradually switch to more capable and advanced devices, particularly as higher levels of certification become hard requirements for commercial UAV applications such as deliveries over populated areas or long-distance surveys of road networks. Mechanics High-quality steel geartrains are increasingly common in UAV servos for the longer lifespan and higher reliability they can provide. The geartrain is often seen as the most critical mechanical component in the servo, as it is ‘consumed’ the fastest owing to the high amount of friction it has to endure. Therefore, much like a metal mould in manufacturing outlasts a plastic mould by orders of magnitude, using a steel geartrain is often the biggest factor in scaling back the rate at which servos – the most frequently replaced part in most UAVs – have to be replaced. Different design choices can make better use of these parts inside the servo or tailor the actuator for improved capabilities. Back-driving for instance can be desirable for enabling a control surface to return passively to its starting position in the event of a jam or loss of power. Also, designing a geartrain with a lower (or smaller) gear reduction ratio than is standard will make it easier to back-drive, although obviously at a slight cost to torque output. Also, cutting gears to close tolerances is vital to making them safe and efficient, particularly with regard to backlash, so that the servos are not inherently inefficient when actively reversing. Backlash is the amount of lost or wasted motion due to clearances or slack between gear teeth, when movement is reversed and time passes before contact is re-established. For rotary servos, this is often a matter of sourcing high-quality and hence costly gear manufacturers. In general however, linear servos are easier to design with low backlash, down to a few thousandths of an inch, owing to the innate differences in how their geartrains are shaped. That in turn makes them comparatively easier to back-drive. Other measures, such as helically skewing the electric motor to prevent detent torque (produced by the motor when its windings are not energised) from interfering with back-drive forces can be invaluable in guaranteeing good performance. Taken together, techniques such as these will become more important as UAVs are demanded for high speed and dynamics in flight, for instance in emergency medical or industrial repair deliveries or in special forces operations. A faster UAV needs more control authority over its flaps and ailerons, so keeping backlash to a minimum will be an increasingly mission-critical requirement for such applications. Across most of the UAV world though, the key determinant of geartrain quality will be rapid customisations for end-users. As mentioned, lowering or widening the tolerances in a metal geartrain can have an impact on its performance and lifespan, but it will also reduce the price of the actuator. For manufacturers and operators, this will be a bigger benefit than the extra hundreds or thousands of hours between replacement. This will be especially true if the geartrain is set to last far longer than other parts inside the servo, as many engineers would prefer to replace the whole actuator than open it and replace a single part. Future improvements in mechanical parts will probably also include r&d into housing and ruggedisation enhancements, one notable example being a custom injection-moulded molecular sieve, applied as a desiccant block near the control electronics and BLDC stator. This adsorbs moisture – meaning it causes moisture to stick to its surface – at a slow rate over many years, and is a major performance improvement over more conventional silica-gel desiccants. August/September 2023 | Uncrewed Systems Technology Advances in servo ruggedisation include the use of a custom injection-moulded desiccant block close to the electronics and electrics to adsorb moisture over long periods (Courtesy of Ultra Motion)
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