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100 in real-world conditions without going to actual flight tests. Environmental tests can also be critical to checking that barometric pressure sensors, manifold air sensors and others are still being read correctly. Integration ECU manufacturers these days are working hard to keep up with the requests of UAV OEMs, and much of that work is centred on software adjustments. Perhaps the best known of these is the introduction and widespread adoption of CAN bus as the preferred comms protocol between the ECU, its input sensors, its output actuators and the autopilot. As discussed at length previously in this magazine, CAN has been adopted for autonomous systems from the automotive market so that the former can benefit from the bidirectionality, transparency, smart prioritisation and security of signal processing that the protocol brings to the data running between subsystems. In the case of ECUs, this enables in-depth, real-time monitoring of the performance and health of sensors, servos, generators and so on, at both the autopilot and GCS levels. Operators and AI algorithms written for automating engine management might be able to achieve far safer and more precise engine management as a result of this. To achieve an effective CAN comms and management system, an ECU engineer must determine the role of every bit in every packet of telemetry in every output function. Each could refer to a different parameter or different error thresholds for those parameters, or caps on related parameters, among others. Once assigned, these can be compiled in an interface control document (ICD) and shared with the end-user so that they can fully understand how data is transmitted between the ECU and its inputs and outputs, and hence how they might adjust the software accordingly. And while high-end ECUs are more often being designed around CAN bus as their interfacing protocol, it is worth noting that the UAV market is not settling on one particular variant of CAN. Customer requests run beyond just the commonly used SAE J1939 and J2284 standards originally set for buses/trucks and passenger cars, respectively. Others include UAVCAN, which features a highly deterministic and safety-oriented protocol capable of running on 32 kbytes of ROM and 8 kbytes of RAM – meaning ECUs can use it without needing any added memory – as well as CANopen and other, less well-known variations. In any case, choosing CAN in the software comms has a critical benefit for the hardware integration of the ECU. With more data able to be transmitted on the bus, fewer wires are needed around the airframe – it is not unusual to find that switching from PWM to CAN reduces the number of wires running from the ECU by a third. As a result of this CAN-capable ECU, end-users reduce their part counts, expenses and vehicle weight. Heavy fuel and hybrid engines Another key evolution in ECU technology has been the reprogramming and remapping of engine management software for spark-ignited heavy-fuel engines. Theoretically, one could take a running gasoline engine, switch it to a heavy-fuel feed, and it would continue running – but the actual process of using heavy fuel, whether in a dedicated or multi-fuel engine, is far more complicated as far as the ECU is concerned, even when not accounting for the necessary changes to combustion chambers, heaters and so on. Transients for instance will not be handled well unless an ECU’s software has been rewritten from the ground up, to ensure the way in which heavy fuel atomises, disperses and pools is accounted for in the parameters of injection control and other outputs. Some further issues must be tackled for effective heavy-fuel management. One is that heavy fuel cannot April/May 2022 | Unmanned Systems Technology Focus | ECUs Bench-testing and engine cycling enable inspections of ECU performance before going to flight tests (Courtesy of Performance Electronics)