UST035

36 Focus | Motor controllers connected to the motor for this purpose. The smooth control output of this approach is ideal for dynamic applications at low motor speeds, but as revs go up, the probability of errors increases greatly owing to the limited gain and frequency response of its controllers, as well as rapid build-ups of back EMF. FOC commutates the motor by calculating voltage and current vectors based on motor current feedback, rather than relying on Hall effect sensors, encoders or resolvers for motor position calculation. It has yet to achieve the popularity of the other approaches, partly because of the work and complexity involved in ensuring its firmware matches customer requirements. For example, switching frequencies of 150 kHz are possible with FOC, compared with the 30 kHz with trapezoidal, but that puts considerable strain on the transistors in the ESC circuitry. Large, high-power transistors are particularly susceptible to such issues, and as a result an ESC designed for FOC might incorporate 20-30 smaller, low-power transistors and gate drivers, along with additional sensors required for FOC, potentially resulting in a motor controller of prohibitive size and weight. However, FOC undeniably yields many advantages over trapezoidal and other forms of control. As well as the higher switching frequencies that enable more precise dynamic motor control over speed and torque, it is the most energy efficient over the widest operating range. It can also produce speeds of up to 150,000 rpm in some cases, which is critical for high-end small UAVs. Power electronics Silicon MOSFETs remain the most widely used transistor for switching in ESCs. They are appropriate for the voltage architectures and current draws of smaller air vehicles weighing less than 25 kg (for compliance with FAA Part 107 and similar UAS regulations), and electric motor voltages up to 200 V are almost always controlled by ESCs using MOSFETs. The MOSFETs chosen for use in an ESC should be specified with sufficient voltage ‘headroom’ to ensure they can handle the spikes of current drawn by the motor, which could go from 5 to 500 A in an instant. For a UAV with a 50 V powertrain, for example, MOSFETs rated to 70 V or more ought to be sufficient. For similar reasons, the lowest possible electrical resistance is desirable to minimise the internal heat from copper losses. The MOSFET’s gate charge, which is the total charge needed to drive the transistor, and input capacitance – its ability to hold charge at the point of power input – should also be as low as possible, to minimise switching losses. The reverse recovery time of the MOSFET’s internal body diode should be considered as well. When switching (particularly at high slew rates – changes in voltage or current per unit of time), any overshoots of voltage or current will December/January 2021 | Unmanned Systems Technology Some newer motor and commutation architectures are enabling speeds of over 150,000 rpm – ideal for electric compressors in turbochargers and PEMFC intakes (Courtesy of Aeristech) ESCs for lift motors and servos are generally the same, although the latter must control position as well as velocity (Courtesy of A-M-C)