94 Focus | Motor controllers Additionally, effective FOC depends on wave shaping of the input AC current in order to keep a high power factor. But when the commutation frequency runs close to the switching frequency due to very high motor rpms, the ability to wave shape depends on having many switches per commutation instance. Hence for high-speed, highly dynamic applications, effective FOC motor controllers may actually need to be far larger than one hopes for maintaining good control response at the limits. Furthermore, while FOC can provide very smooth, high-fidelity control, achieving that in field operations can be challenging without painstaking efforts to tune the algorithm and controller exactly to how they will be run in practice (and indeed, many UAV manufacturers lack the facilities to do so appropriately). As an example, most manufacturers will tune the FOC algorithm at room temperature conditions, only to fly their UAVs in either significantly more heat, less heat, higher moisture, lower moisture, and so on. Alternatively, there may be manufacturing variations in the motor, propeller, or wiring harnesses which are hard to account for in the tuning process, or the operator may simply push the ESC so hard that it exudes and accumulates an inhospitable environment around itself. These variables could affect the efficiency with which power is transmitted, or with which rotors climb through air, and many other factors, especially given that uncrewed systems increasingly work in harsh and dynamic environments which deviate wildly from the single point of collective operating conditions at which the motor control is typically tuned. As a result, some very high-end ESCs are still engineered for trapezoidal commutation. It is important to note that trapezoidal control can enable much greater power density, a critical requirement for eVTOL UAVs which need to spool four or more e-motors up or down simultaneously at key moments in flight. It also tends to be significantly less expensive than FOC, and does not suffer from the wave shaping issue at high rpms (nor some other edge case nuances). Another alternative route has been to develop new algorithmic approaches, one of which has aimed to produce a version of FOC which can tune itself in real-time, maximising efficiency gains across a wider range of external conditions. This may mean an algorithm which fails to match FOC’s absolute peak performance outputs to the fractions of a percent in controlled lab conditions, but otherwise gains 3-5% power efficiency over each real-world UAV flight. For organisations flying hundreds of hours per year, that could mean considerable improvements in energy or fuel bills. Creating such a commutation algorithm does not require AI training – many still argue (correctly) that AI has no place in actual direct motor control, and must be limited to observation-based analytics of ESCs. Instead, traditional programming rigour remains the best tool for the job, with clear drive responses to every input defined by human programmers, taking account for how environmental variations can affect motor and ESC performance, such that the commutation algorithm can switch modes and maps to compensate in an apt manner. Manufacturing and testing With motor controllers now being manufactured in large batches for longterm uncrewed vehicle manufacturing contracts, ESC producers are honing their best practices for an efficient and transparent production floor. Often, that begins with a close consultation between engineering and production personnel – if done right, the former will make sure to design something manufacturable, and the latter will understand what to configure for in April/May 2025 | Uncrewed Systems Technology Use of MOSFETs and boards with integrated cooling surfaces can help hugely at moving heat out of the transistors, and into pastes, pads and heat sinks (Image courtesy of Hargrave)
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