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37 that interlocks with the output shaft. When the motor’s rotor turns, it spins the leadscrew but the ball nut remains still in its rotational axis, instead producing a linear motion – forwards or backwards, depending on the direction of rotation. As the geartrain or screw train provides the critical mechanical output, it makes sense that they should be made of a strong material. Hobby-grade actuators are often made from plastic, which is lightweight, inexpensive and has low backlash – the error between the commanded input position and the resulting output position – but they often break down. Vibration imparts a particular stress to gears, and as a result, plastic gear servos rarely last for much more than 20 hours of operation. They are even dangerously prone to failure within the first hour of operation. Gears made from alloys of brass, aluminium and (sometimes, in small quantities) titanium are occasionally used as a compromise between strength and weight. These produce very low backlash but often degrade in performance after 50-100 hours. Steel gears are prone to higher backlash than the alternatives, and are more expensive as they need higher quality tooling and process controls. However, they can provide the longest lifespan – from 2000 to 10,000 hours as a matter of course – and as a result have a much lower ratio of cost to service life than alternative material gears. The servo housing is the largest component and thus the heaviest, with aluminium 6061 being widely used for industrial and military-grade servos. For UAVs needing extreme weight optimisation (such as HALE pseudo- satellites) some future servo designs will have carbon fibre structural parts to save weight where possible. For such systems, overall weight can be reduced by 30-40%. Demands for low weight have also resulted in highly miniaturised actuators weighing less than 35 g that can be manufactured in bulk and designed with a minimum of internal wiring. Wire harnesses being shaken loose is a perennial concern in UAVs of all sizes, but smaller UAVs incorporating smaller servos naturally have less inertia, so both the UAVs and servos can be more heavily subjected to shock and vibration from turbulence hitting aerofoils or from revving motors. While a good PCB assembly will reduce the amount of wires needed, and is typically fastened securely to the housing, many components – such as temperature and position sensors – must be wired to the motor, adding potential points of failure. Wire harnesses can be secured further by potting the connections with an epoxy, reducing the ability of wires to swing about within their internal free space as vehicles are shaken around. Motion control and position sensing In the unmanned systems market, the brushes on a brushed high-end DC motor often wear out after 500 hours of use, although they can be cycled on a test bench to determine with good predictability how long they will last. However, they also produce EMI through sparking, which can be detrimental to internal electronics and signal integrity, as new unmanned vehicle designs carry increasing Servo actuators | Focus Steel gears have the longest lifespan – from 2000 to 10,000 hours – so they have a much lower ratio of cost to service life Unmanned Systems Technology | August/September 2020 Servo actuators consist broadly of a power and signal input, a PCB assembly with control and power electronics, an electric motor, a geartrain and an output shaft (Courtesy of Futaba)