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35 outputting 10 kW or higher; such power throughput would quickly destroy hobby- grade ESCs, and they could certainly not achieve the response times needed for agile, controlled flight. For example, a large UAV with an 81 cm propeller must have a transient response time of less than 300 ms to enable propellers to spool up as fast as possible with minimal latency – ideally within 100 ms – between the ESC receiving a new command and outputting it to the motor. Meanwhile smaller UAVs, such as one weighing 2 kg and with 22 cm propellers, need an input-to-output transient response in the tens of milliseconds, to ensure that the propeller can spool up from 10% to 100% rpm speed within 0.05 seconds. Longer than that can result in oscillations in the feedback loop from the flight controller, or saturating the motors with current. Modern ESCs are optimised to prevent such issues, and to handle at least double or triple the sustained current draw rating of their motors to ensure consistent safe output amid high switching speeds, current spikes and temperatures. With UAV engineering turning more and more towards bespoke powertrain components and designs, ESC suppliers are also becoming more proficient at tailoring motor controllers to fit unmanned aircraft operations. Information on a UAV’s MTOW and endurance, as well as the kinds of propeller, motor, energy storage and comms network, are key here. With closer partnership, further key parameters can be optimised, for current ratings, duty cycles and more, to ensure precise and energy-efficient lift and cruise across the full range of operations being performed by UASs these days. Commutation For an electric motor to produce constant torque in a given direction, the magnetic field in the stator windings must be switched repeatedly and precisely, in a process known as commutation. The quality of commutation firmware is increasingly the greatest factor in how ESCs are able to minimise their latencies and transient response times. There are various commutation techniques, with the choice often depending on the supplier and its ESCs. The most commonly used in unmanned systems include six-step (also known as 120 º commutation or just trapezoidal commutation, for the shape of the AC waveform generated via switching) and 12-step (also known as 150 º commutation). To a lesser extent there are sinusoidal and field-oriented control (FOC) methods as well. Six-step trapezoidal commutation is the most used and widely understood technique, as it guarantees compatibility and a modicum of user-friendliness with at least 98% of electric motors on the market without needing any particular tuning. It is a simple technique, so it requires less processing power than other methods, making it potentially well-suited to how UAV designs are evolving. More and more commercial- and military- grade UAVs are designed as ‘hybrid quadrotors’: fixed-wing craft with four vertical lifting motors, which spend only a few minutes per mission in quadrotor- lift mode, flying in fixed-wing mode for between four and 12 hours. That means energy efficiency over long periods – a hallmark of newer commutation approaches – may not be as important as high power density. Although 12-step commutation also produces a trapezoidal waveform, the key difference between it and six-step is that the latter energises two motor phases at a time, with the third phase being empty, whereas the former energises all three phases at once. Ostensibly that gives higher levels of speed (and its control) with a given electric motor. However, torque ripple and build-ups of counter-electromotive force – known as back EMF, which is induced by and opposes the high- frequency changes in the current – are also higher with this technique. Sinusoidal commutation enables a smooth, stepless current output, without the torque ripple or spikes typical of the two other methods. It results in a sinusoidal switching waveform, hence its name. Achieving this requires more accurate (and less coarse) feedback on rotor position than that provided by Hall effect sensors, which are sufficient for trapezoidal commutation. An encoder or similar device has to be Motor controllers | Focus Unmanned Systems Technology | December/January 2021 Although field-oriented control can be challenging to engineer, it offers the most energy efficient commutation over the widest operating range (Courtesy of Aveox)