Unmanned Systems Technology 033 l SubSeaSail Gen6 USSV l Servo actuators focus l UAVs insight l Farnborough 2020 update l Transforma XDBOT l Strange Development REVolution l Radio telemetry focus

42 Focus | Servo actuators continue working even if, for example, a car is driven over them – at depths of 6000 m, the resulting 604 bar of pressure is enough to collapse their housings. As a result, it is useful to be able to replace crystal oscillators with semiconductor clocks in UUV actuators. The latter are not quite as precise as the former, but they can still enable reliable operation while withstanding the oil- compensation needed for work at 6000 m. Such servos do require added maintenance though. For example, the oil will still pick up particles from the PCB assembly and gears, so an oil change will be needed at service intervals. So far though, oil-filling is the only proven way to design vehicle servos that can operate at such depths. Quality control It has been remarked by some in the industry that actuators are the most likely component on a UAV to fail. That is not an unfair assessment, given that they can be intermittently (or in some cases routinely) subjected to shear loads that stress their arms, gears, motors and other parts. Designing and testing them for environmental and physical robustness is therefore vital to ensuring their viability for commercial and military use. Testing is particularly important for throttle servos, as they must be especially robust against extreme vibrations and heat. Given the wide range of altitudes and climates that UAVs must endure in their flights, however, it is not unusual for every servo to undergo precisely the same kinds of testing and equipment to ensure a common and high bar for survivability. These include freezers and heat chambers to prove that their components can withstand extreme heat and cold, from -45 to +80 C, rate tables to vibrate them at 4 g for a given time (30 minutes being one standard), and cycling tests to operate them at different loads for extended periods. When loaded, further tests can be conducted to verify a servo’s performance in terms of backlash, force constant, axial or radial stiffness, and other parameters. Unloaded tests can also identify the motor size constant, running friction and other critical specifications. It follows that load cycling can also be conducted under different vibrations and temperatures to gauge their performance in extreme conditions. Aerospace standards such as DO-160 provide clear and proven guidelines for trialling servos towards long-term reliability. Further certification standards such as ISO 9001 and CE guidelines can be vital for ensuring that production and testing are carried out to the quality, precision and cleanliness needed for UAV servos. As aircraft configurations become more complex, however, more in-depth proving of actuator design and operating integrity becomes desirable. For example, the servos used in some helicopter UAVs are X-rayed. In operation, they are typically used with direct couplings to the UAV’s rotor head to enable pitch, roll, and yaw, and are subjected to extremely high and fluctuating loads, as well as far higher than usual vibration. Losing one servo would result in massive loss of control and a high probability of the helicopter’s destruction. By comparison, non-supersonic fixed- wing aircraft can still glide to safety if a flap or aileron is lost. X-ray images are thus inspected in order to visually examine every solder joint, every phase of every motor and so on. This unearths flaws in components that cannot be found using other standard tests and inspections. While this level of testing is labour- intensive and time-consuming, it certifies beyond reasonable doubt that there are no insufficiently bonded electronics that might be shaken loose by vibrations or by shock loads to the helicopter’s rotor head. August/September 2020 | Unmanned Systems Technology For more demanding VTOL UAVs, actuators can be X-rayed for deep inspections of manufacturing quality (Courtesy of Volz) Standards such as DO-160 provide clear and proven guidelines for trialling servos towards long- term reliability