Unmanned Systems Technology 017 | AAC HAMR UAV | Autopilots | Airborne surveillance | Primoco 500 two-stroke | Faro ScanBot UGV | Transponders | Intergeo, CUAV Expo and CUAV Show reports

42 There is an argument that hardware subsystems at the same level of certification as the software are required for the overall system certification – for example, a level D design needs sensors that are also certified to level D. However, it is possible to use a wider range of sensors, and the system, including the autopilot, can be certified to a particular level with the appropriate design process, testing and evidence. That is important, because to control a craft the autopilot has to access various sensors, which are all slightly different and are all affected by temperature. By calibrating the sensors that are integrated in the autopilot as well as others throughout the craft, this variation between controllers is minimised and so all the systems will perform the same. Also, calibration improves the accuracy of the sensors for better UAV performance. If the sensors are calibrated over a wide range of temperatures, these benefits are available across that range. Some manufacturers try to keep their sensors at a known temperature by using a heater, but that adds another potential failure mode to the autopilot as the heater could fail, and increases the complexity of the entire system. A heater also doesn’t help in over-temperature situations, and if the autopilot gets too cold – at high altitudes say – there could be situations where the heater does not produce enough heat. However, a well-designed calibration system will test the autopilot at each temperature as it is being calibrated to demonstrate that it operates properly over a wide range of temperatures. Semiconductor devices such as the controller chip, gyroscope or comms chip developed for consumer devices such as smartphones may not be suitable for use in an autopilot, partly because of the temperature range encountered in UAV applications and partly because of their overall reliability. Industrial devices tend to be the preferred choice here, as they are specified to -40 to +100 C rather than the 0 to 85 C in consumer designs. Automotive specifications are even wider, at -40 to +125 C, and for military devices it is -55 to +125 C. The other major advantage with automotive- and industrial-grade chips is that they have a much longer lifetime. Typically, consumer or cellphone chips go out of production after a few years, but automotive and industrial chips typically stay in production for a decade or so. It takes time to design a sophisticated UAV, and once there are a lot of UAVs out in the market, having to change a key component such as an autopilot involves a lot of engineering work. If an autopilot is using consumer-grade chips then it will have to be redesigned and re-certified when they go out of production. Board manufacture There are also potential hardware reliability issues at the manufacturing stage. An electronics component production line has many stages where small defects can creep in. If the solder paste clumps just a little, for example, and if the heat in the reflow oven isn’t accurate (as there can be cold spots in the oven) then imperfect solder joints can occur on the finished circuit board. These conduct electricity but are mechanically weak, so they pass electrical tests but are likely to fail in real- world use. The industry uses the term ‘infant mortality’ here, where the failure probability of an electronics device is very high when it is new but then declines dramatically after the first 100 hours or so of use.  It is therefore important that an autopilot manufacturer has some process in place to detect these weak solder joints. Typically, manufacturers use ESS (environmental stress screening) to weed them out. An ESS process stresses the circuit board enough to cause these solder joints to break so that they fail the electrical test. Not all circuit boards are the same of course, and this can be an issue for the reliability of the autopilot hardware. The main electronics manufacturers’ trade association (known as the IPC) therefore December/January 2018 | Unmanned Systems Technology The AP10.3 from UAVOS is a small automatic control system designed to be installed into a wing with a depth of 0.51 in, a length of 2.6 in and a width of 1.65 in (Courtesy of UAVOS)