Unmanned Systems Technology 016 | Hydromea Vertex AUV | Power management systems | Unmanned Space Vehicles | Continental CD-155 turbodiesel | Swift 020 UAV | ECUs | DSEI 2017 Show report

74 More data on the operating environment in terms of temperature, altitude, aircraft attitude and other parameters is also wanted, and all this information provides greater freedom to develop complex control strategies. The use of heavy fuels such as JP8/ kerosene in spark-ignited engines also places additional demands on the ECU, and one approach to meeting them is to add some extra timers and controls for heat cycles during start-up, and for temperature management via spark timing and fuel enrichment. Control strategies Engine control strategies have changed very little in the past 20 years or so, because the principles engines work on have not changed, so neither have the requirements for opening and closing an injector, and timing a spark based on programmed knowledge of the engine and measurements from sensors. There are three basic control strategies that ECUs can use – N-alpha, speed- density and mass airflow. In the first, N stands for engine rpm and alpha for throttle position, and the ECU looks up the fuelling – how long the injector should be open – from a table based on those two measured parameters. Speed-density refers to engine speed and the density of the air. One industry insider interviewed for this article says it should be referred to as the ideal gas law control system because it uses the relationship between air pressure, volume and temperature, and a gas constant to calculate the mass of air going into the engine so that the ECU can look up the fuelling value from a table to get the desired air-fuel ratio. The mass airflow strategy is similar, but uses input from a sensor that measures mass airflow directly, which simplifies the ECU’s job because it does not have to calculate it. Limits to precision While modern UAV engines have to be controlled very precisely in terms of air-fuel ratio and spark timing to meet fuel consumption and power requirements, the limits to precision are in the ancillary hardware rather than the ECUs themselves. As one developer puts it, if the injector, sensors or pump are fluctuating in pressure or sensor value, then it becomes more difficult for the ECU to control the engine precisely. However, modern ECU electronics are more than precise enough for the engines they control, as the complexity of the algorithms and the peripherals typically used do not require higher-end processors. However, future implementation of more complex algorithms such as advanced diagnostics and prognosis is likely to demand more precision. Hybrid control Although hybrid power plants make greater demands on their control systems, they do not necessarily lead to the use of extra drivers in the ECU, as individual controllers for the engine and the electric motor are typically retained. However, if an autopilot makes torque or rpm demands, there must be an additional supervisory controller to apportion the response to the demand between the engine and the electric motor. One developer suggests that a smart architecture would combine the internal combustion ECU and the hybrid supervisor into a single unit. While that is unlikely to alter the ECU hardware specification very much, it will impose new software requirements. However, if the engine control ECU does not have at least 30% spare capacity, a microprocessor upgrade would probably be needed to contain the additional hybrid functionality, the developer says. Another says that most of its hybrid customers have their own control system, into which they plug the ECU. They prefer the linear throttle control and smooth predictability of an engine with electronic fuel injection over the large variations from a carburetted engine. Some hybrid systems feature a generator control unit (GCU) and an ECU, with the former exercising control over the latter, while some of the latest systems combine the ECU and GCU into a single unit, particularly in the smaller UAVs. One ECU developer working on a new hybrid UAV power plant uses its latest ECU and a separate power control unit (PCU). The ECU is unaffected by whether it is turning a propeller or a generator, as it is the PCU that manages the hybrid aspect. This PCU is connected to the engine, the batteries and the load, which October/November 2017 | Unmanned Systems Technology With hybrid UAVs, developers face a choice between separate controllers or a single integrated controller (Courtesy of Quaternium)