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40 Focus | Motor controllers appropriate aerospace or automotive standards for design quality, ruggedness, environmental survivability and so on in order to fit the increasingly regulation- aware unmanned market. Microcontrollers from STMicroelectronics and Texas Instruments in particular have proven popular among ESC manufacturers, for their capabilities, form factors and standards-compliant design parameters. Lastly, a growing trend in ESC software design is allowing configurability by the end-user, whose unmanned vehicles must sometimes be tailored and re- customised at short notice depending on customer requirements. This requires exposing certain settings to unmanned systems engineers that they can adjust from their own interfaces – assuming they are competent software and electrical engineers themselves, which indeed a fast-growing number of motor controller buyers are. These configurations particularly include settings for motors’ rpm rates, dynamics and switching frequencies, which if done incorrectly can put undue load on the ESCs and batteries, shortening the lifespan of both. However, motor controller developers are also increasingly enhancing their documentation to provide guidance and troubleshooting towards easier configurability. Thermal management As electric powertrain engineers know, power electronics produce a lot of heat, and the heat builds up over time and with rising switching frequencies. That can mean further copper losses, potentially damaging the ESC’s components as their maximum thermal ratings are breached. Much of the cooling of motor controllers will come from the propeller wash in the aircraft on which they are installed. End-users are typically diligent in positioning their ESCs downwind of the propeller, or incorporating a liquid- cooling system, including pumps, radiators and filters, for all the extra weight that entails. However, there are also key steps that ESC manufacturers can take at their end to help reduce heat build-ups. Designing the layout of the MOSFETs, gate drivers, capacitors and other aspects of the PCBs for thermal efficiency is a critical first step here. For example, the way in which connections and components are soldered onto circuit boards affects how much resistance the ESC exhibits and how much heat it dissipates. Passive cooling systems such as aluminium heat sinks are also a well- established technology, in addition to aluminium housings for their thermal conductivity and acceptable strength-to- weight ratio. Layers of thermally conductive silicone pads, already widely used in EV batteries and inverters, are also now being installed in ESCs for smaller unmanned vehicles. These extract heat from the inner circuit board components to the outer housing, where air or water cooling circuits can dissipate it. Graphite pads are also available for this purpose, although they are more electrically conductive than silicone and therefore may need to be isolated. Testing The thermal stability of a motor controller can be further verified by tests such as thermal imaging, especially while testing them with spinning or jammed motors, and derating them at high ambient temperatures. Motor controllers are increasingly put through their paces by mounting them on a dynamometer with an electric motor and propeller, and powered by a stationary and continuous DC supply, to achieve a desirable measure of endurance in line with automotive and aerospace industry standards. This might include several thousand hours of operation, or thousands of cycles or consecutive starts without failures. Also, running the ESCs to their rated current peaks, as well as back down at three or four times the transient response end-users might need, are ideal tests for gauging product safety in operations taking place BVLOS or near populated areas. In addition to these propulsion-specific tests, industry-standard vibration and temperature screening – sometimes going as far as what are known as highly accelerated stress screening tests – is now widely run on prototypes of selections of manufactured ESCs. Rate tables and thermal chambers are key to verifying their continued performance in the extremes of cold, heat and shock they might experience. Conclusion Motor controllers have made huge progress over the past five years December/January 2021 | Unmanned Systems Technology Various inspections and tests during and after manufacturing are important for verifying the quality and MTBF of aerospace-grade ESCs (Courtesy of APD)