Issue 39 Unmanned Systems Technology August/September 2021 Maritime Robotics Mariner l Simulation tools focus l MRS MR-10 and MR-20 l UAVs insight l HFE International GenPod l Exotec Skypod l Autopilots focus l Aquaai Mazu

86 Focus | Autopilots low-drift, broad-range IMUs into their autopilot system without tacking on excessive weight, power consumption or volume. Beyond enabling modular carrier boards and triple-redundant IMUs, autopilot manufacturers continue to take increasingly broad approaches to navigational redundancy. Quadruple- redundant autopilots are now available, as well as some triple-redundant versions that go so far as to combine IMUs from different models and manufacturers. Building a redundant architecture from different IMU models (with different designs and performance specifications from each other) provides not only a fallback if an accelerometer or gyroscope fails, but could also cover each other’s weaknesses, and enable wider dynamic sensing ranges. This latter quality can enable flight control to be maintained under very extreme flight conditions and g -forces, such as those experienced by tube-launched UAVs. Lone IMUs in such craft can quickly become saturated when exposed to such severe (and difficult-to-measure) dynamics. Hardware design often begins with drawing up a circuit schematic that can then be laid out in a suitable CAD program. The quality of both the software and design practices are critical here to ensure the engineer can pay close attention to details such as signal integrity, which includes ensuring the correct routing of information between I/Os and APIs, planning of the layer stack and minimising cross-talk between data links. Another crucial issue that is sometimes undervalued during hardware layout is input protection, as the power and signal inputs must be protected against the various types of faults that can occur in the power bus. Undervoltages, overvoltages, reverse voltages, electrostatic discharges and surges are all very real possibilities owing to generator behaviour, battery misplacement or other concerns potentially arising from human error or subsystem interplay. Components on the circuit board can therefore be severely damaged during operation if the pinouts are not designed or selected to weather multiple electrical malfunctions. With these issues in mind, it has become common for autopilot manufacturers to choose carefully and invest heavily when it comes to connectors. High levels of Mil- Std testing and certification are desirable here for ensuring the integrity of power and comms in extremes of vibration, g -forces, moisture and temperature. Aviation authorities are also adopting strong views on the quality assurance standards for the companies supplying these parts, so compliance with ISO 9001 is becoming increasingly essential. As well as designing for standard power and comms buses between modules and interfaces, it is now deemed highly valuable that an additional system for onboard diagnostics be included in an autopilot’s architecture. For example, measurements for the voltage, current and power between parts should be checked constantly and autonomously to ensure appropriate action can be taken if the protections are being overtaxed by some unforeseen events affecting the power system. Once the autopilot hardware design is fully laid out, it can be sent to contract manufacturers for fabricating the board and assembling the PCB with the necessary electric and electronic components. These processes are typically carried out to IPC standards such as IPC-6012 (for building PCBs) and IPC-A-610 (for assembly). In the latter case, IPC-A-610 Class 2 is often adhered to for consumer electronics PCBs, but it allows for certain types of defects to lower production times and costs. It is therefore critical that autopilot boards are assembled to IPC-A-610 Class 3 or an equivalent to prevent the types of issues that often affect consumer UAVs, tablets or smartphones from arising in professional-grade UAVs flying cargo or cameras over populated areas. Class 3 PCBs are the type most often used in commercial aerospace, as well as life support systems, space systems and military avionics. As well as tolerating far fewer defects, standards for Class 3 also specify helpful practices such as using copper wrap plating to prevent delaminations from the harsh thermal gradients that unmanned systems are increasingly subject to. August/September 2021 | Unmanned Systems Technology One reason for the widespread use of ADS-B Out was its early adoption among the open-source community – ADS-B transponders and certified GNSS systems can be tightly integrated with autopilots such as these (Courtesy of uAvionix)