99 the standard in high-end autonomous aircraft (it already has) but which variety of CAN will win out, as a few are in use and more lie on the horizon. All these trends and others feed into the rule that quality is king in servo designs. Adopting high-end mechanical and electronic components is widespread across servos for the UAV market. Large servo architectures The airborne autonomy market offers plenty of designs for air taxis, HAPS and heavy cargo UAVs, whose manufacturers and prospective operators want to deploy them in operations over populated regions. That means OEMs must think beyond the reliability of their platforms and instead about ‘safety’, as defined by regulators and other aviation authorities. In the context of servo actuators, maximising their reliability has often been skewed towards adding more and more control surfaces to a UAV’s wings and tail. For instance, instead of one pair of ailerons and one pair of flaps, a design engineer might plan for three pairs of each or more across a given wingspan so that if one breaks in mid-flight, others can pick up the slack, in a flight control sense. However, given the maintenance overheads this might bring (due to having far more servos to replace per aircraft), it is becoming more common to produce inherently redundant versions without causing an equivalent increase in operating inventory and labour costs. Inside these servos, there are often dual control and power electronics, BLDC motors and geartrains acting on a common output shaft. They would therefore be dual redundant, mechanically and electronically, enabling robust UAV platforms and good protection against the overwhelming majority of failure modes. As far as the regulators are concerned though, the reliability of this type of ‘duplex’ architecture is not necessarily synonymous with safety. To aviation authorities, safety for UAV servos (as with commercial airline servos) can be achieved with a single lane of electronics, motors and gearboxes, paired with a dual-redundant command link. In this type of ‘simplex’ architecture, one lane is responsible principally for taking commands from the autopilot and using them to drive the servo’s electric motor. A second lane is responsible for monitoring the first, and shutting it down if it is found to be generating or propagating errors. Most failure modes as defined by the regulations are therefore covered by simply having the servo monitor itself when working, and shutting itself down when it is not. Simplex servo architectures are expected to become increasingly common among large autonomous aircraft, given that they can meet certification requirements while avoiding the much higher manufacturing costs and complexities of dual-redundant servos, and their far more stringent testing requirements. With two of almost every electronic and mechanical part, there are many more failure modes and permutations than in single-lane, dualcommand servos. In some cases though, large autonomous aircraft manufacturers will still opt for duplex servos in the belief that they are innately safer. In fact they go so far as to over-spec and bulk-order actuators Servos | Focus It is becoming more common to produce inherently redundant servos without causing an increase in operating inventory and labour costs Uncrewed Systems Technology | August/September 2023
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