Unmanned Systems Technology 007 | UMEX 2016 report | Navya ARMA | Launch & recovery systems | AIE 225CS | AUVs | Electric motors | Lethal autonomous weapons

72 coils. The coils are usually connected in three main sections or phases, which is why most brushless motors have three power cables connected to them, one per phase. The way in which a motor is wound controls the distribution of the magnetic flux density throughout the motor. This in turn controls the waveform shape of the back-electromotive force (EMF), which in turn dictates how best to drive the motor, whichever commutation method is chosen. Every electric motor has to have some sort of controller, which will have differing features and complexity depending on the task the motor will be performing. A typical controller contains three frequency-controlled three-phase outputs that are controlled by a logic circuit. Simple controllers use comparators to determine when the output phase should be advanced or delayed, while more advanced controllers use a microcontroller to manage acceleration, control speed and fine-tune efficiency. Controllers that sense rotor position based on back-EMF have extra challenges in initiating motion because no back-EMF is produced when the rotor is stationary. This rotor position is usually determined by beginning rotation from an arbitrary phase, and then skipping to the correct phase if it is found to be wrong. This can cause the motor to run backwards briefly, adding even more complexity to the start-up sequence. Other sensorless controllers are capable of measuring winding saturation caused by the position of the magnets to infer the rotor position. Magnetic attraction Magnets on electric motors come in various configurations, denoted by N and P numbers where the number before the N denotes the number of electromagnets in the stator, while the number before the P shows how many permanent magnets are in the rotor. Most outrunner brushless motors follow the 12N14P configuration. There are some specialist low-Kv multirotor motors with a higher number of electromagnets and permanent magnets that allow the motor to create more torque more efficiently, but because of this greater number they are more expensive. One area of particular interest in improving the performance of electric motors is the use of magnets using rare earth elements, which are produced from naturally occurring minerals with unique magnetic properties. The advantage of the rare earth compounds over other magnets is that their crystalline structures have very high magnetic anisotropy, which means that a crystal of the material is easy to magnetise in one particular direction but resists being magnetised in any other direction. These minerals are expensive and in limited supply though, so there is a good deal of academic and industry research into developing alternative technologies. One such programme is looking at the use of a switched reluctance motor, where the magnetic force is created by running current through a series of wire coils that form a circle around a steel rotor. As each coil is turned on and off, the rotor realigns itself with the new magnetic field, causing it to spin. The rare earth element neodymium is the most widely used type of magnet material. Neodymium magnets are of the permanent type and are produced from a blend of neodymium, iron and boron. The material has the strongest magnetic properties of the permanent rare earth magnets currently readily available. Neodymium magnets are produced in two forms – sintered and bonded. Sintered magnets are the more popular as they have a greater performance, whereas bonded magnets are generally found in applications that require special shapes. There are two configurations of flux motors – radial and axial. In radial flux motors the rotor is on the inside, or occasionally on the outside, of the stator windings. In an axial flux motor, the rotor is in front of the stator windings. The benefits of an axial flux motor are that it can be made thinner and lighter, allowing it to fit better into certain geometries and change direction quicker. The form factor of an axial tends to be used where a pancake design is required – that is, a hub motor. Axial also encourages high torque due to its relatively larger diameter than an equivalent radial and can be run without a gearbox, so is ideal for wheel hubs. Radials are however by far the most common form of motor off the shelf, and where you have a long thin space, like a fuselage, then the radial form factor suits and is the preferred motor for most UAV applications. Best of both There is one novel application of electric motors for unmanned systems on the market. Most brushed motors have the magnets on the case and the windings on the rotating shaft, while traditional April/May 2016 | Unmanned Systems Technology Compact 7.4 V brushless motor (Courtesy of Graupner)