Unmanned Systems Technology 042 | Mayflower Autonomous Ship | Embedded Computing | ElevonX Sierra VTOL | UUVs insight | Flygas Engineering GAS418S | Ocean Business 2021 report | Electric motors | Priva Kompano

91 developments enabling all these benefits and more, it is worth looking into how they are achieved, and what directions electric motor designs and parts for the unmanned vehicles market are being taken. Configuration choices Before going into detail about the technologies though, a quick overview of the current state of the art in electric motor architectures – particularly those aspects that have stayed largely the same over the past 3-6 years – might be useful. By and large, the motors used in unmanned vehicles are still either brushless DC (BLDC) or permanent magnet AC (PMAC). In both, a stator of wound copper electromagnets uses a switching magnetic field (the current for which is controlled electronically) to turn a rotor of permanent magnets. The main difference between BLDC and PMAC is that the former is driven by a trapezoidal waveform from its motor controller, while the latter is driven sinusoidally. When it comes to selecting the right motors for a new unmanned vehicle, the best approach for determining the optimum size, weight, power, torque and so on is still stringent design planning, simulation and consultation with a motor manufacturer. Beyond these, however, a few key configuration choices continue to be debated. For one, different manufacturers and customers will have different preferences regarding whether the e-motor is an inrunner – in which the rotor is internal to the stator – or an outrunner, in which the rotor is external to the stator. At the time of our previous focus on electric motors ( UST 21, August/ September 2018), outrunners were generally the more popular. They do indeed have certain inherent advantages over inrunners: with an internal stator, the stator teeth face outwards and are thus much easier to wind with copper than if the teeth are facing inwards. They also usually use fewer parts, waste less material in terms of sheet metal production for the same motor size, and make it easier to bond magnets in precise positions. These properties make outrunners easier and hence less costly to manufacture. Also, they generally produce higher torque and weigh less than inrunners of equivalent size and power. However, outrunners have been prone to poor heat dissipation, requiring them to be designed with open enclosures, making them less robust against harsh environments containing dust or moisture, and against shocks and vibrations. Their often 40-60% higher moments of inertia can also reduce their response and agility in larger motor designs and more dynamic applications. Propulsion systems designers have therefore tilted more towards inrunners, as the demand to be able to carry out heavy weight-carrying and highly dynamic missions has grown. Such missions include surveillance over Electric motors | Focus The best approach for determining the optimum motor size, weight, torque and so on for a new unmanned vehicle is still stringent planning Unmanned Systems Technology | February/March 2022 Hand-winding of motors is still highly valued for greater customisability and copper packing in e-motors compared with most automated systems (Courtesy of Plettenberg)