The ability to quickly change the speed and rotational direction of its motors is the key to a drone's performance, which makes the electronic speed control (ESC) module a very important subsystem. Faster motor-speed changes also make for greater stability, which saves energy thereby enabling longer flight times.
Typically, non-military drones use three-phase, brushless motors (wound stators with permanent magnet rotors). While these motors have several advantages - including better reliability - over older DC motors that used mechanical commutation and brushes, they do require complex electronic control circuits. But, thanks to state-of-the-art microcontrollers, computationally intensive control schemes can be implemented to achieve better dynamic performance, such as field-oriented control (FOC), which decouples torque from magnetization control and maintains efficiency over a wide range of speeds.
Knowing the position of the rotor is fundamental to implementing FOC; to generate maximum torque, the angle between the rotor field and the stator field must be equal to 90°. Sensors can measure the rotor's position, or -- when space is at a premium - the position can be derived, using an algorithm based on various current and voltage measurements, in which case it is called 'sensorless.'
All this may sound complicated. But a new reference design eases the pain. The "High-Speed Sensorless-FOC Reference Design for Drone ESCs" implements an ESC that can be used for unmanned aerial vehicles (UAV) or drones, or battery-operated power tools, for that matter. It supports two to six LiPo cells and can even estimate temperature from winding resistance changes to protect the motor during temporary overloads.