When you think about drones, you might think of the quad-rotor with its four individual electric motors, each driving a propeller. When discussing these motors, design engineers emphasize they are just one part of an aerospace system. The motors must be integrated with a power source (battery), control microelectronics and software, aircraft sensors such as accelerometers, and a payload.
But why a quad-rotor configuration for a vertical takeoff and landing (VTOL) drone? Why not a helicopter with the main rotor and a tail rotor to cancel torque? A helicopter rotor is more complex; each blade needs to have its angle of attack in the air changed in sync with the team of blades that increase lift-to-rise as they rotate. Changing the blade angles differentially via a mechanical swash plate produces motion in the direction you desire. Mechanically changing blade angles also adds component weight.
The rotors in a quad-rotor are simple props, as the blade angle is fixed, and are controlled by throttling each motor. Two rotors rotate clockwise and the other two counterclockwise, cancelling torque effects. Each rotor tilts at a slight angle. Throttling the rotors together increases or decreases lift and the aircraft rises or descends. Differential throttle applications control pitch, yaw, and roll. When a drone tilts, added throttle input sends the aircraft in the desired direction.
Some of the more expensive drones have a hex-rotor design. This allows continuous flight even if one rotor fails.
Degrees of Capability
A typical consumer drone can range in price from $25 to well over $1,000. While even the cheapest drone has some imaging capacity and can serve as a flight trainer, the ones with the highest prices usually offer better imaging, video quality, and features such as collision avoidance sensors. Most of the consumer models can only fly for 20–25 minutes and are geofenced to somewhere over 2km. The US Federal Aviation Administration (FAA) regulations limit a consumer drone’s lift to 122m above ground-level altitude—as piloted aircraft must be above 152m—and mandates that the craft must remain within its user’s sight.
One of the latest consumer drones is the Parrot ANAFI. According to Parrot Drones’ R&D propulsion team leader, Edouard Rosset, the consumer must have an enjoyable flight experience for a drone to be a successful product. To this end, ANAFI development design engineers decided to focus on a package with high portability, ease of use and flying, and quality images (with 4K video and high dynamic-range (HDR) images).
Brushless Motors Play a Crucial Role
Drone motors, as part of what Rosset terms the “vehicle power transmission chain,” play a crucial role in converting electrical power into mechanical power via the props. The design team set a critical weight range for the motor to keep the drone as light as possible, thereby increasing endurance and overall portability. They created a brushless, permanent-magnet outrunner motor with high-output power for a maximum speed of 54 kilometers per hour, a wind resistance up to 49 kilometers per hour, and for optimal efficiency while hovering.
By nature, a brushless motor commutates or switches the current (and the resulting magnetic field) in its armature coils electronically, thus producing torque from a reaction against the magnetic field of its permanent magnets. On the contrary, a classic brushed motor mechanically switches current as its coils rotate within the fixed permanent-magnetic field. The brushes touch each subsequent coil winding as the coils rotate. Without brushes (and the associated wear from friction), there are fewer parts in a brushless type.
However, an outrunner brushless configuration fixes the armature coils, and a ring of permanent magnets rotates around the coils. An inrunner type has the rotating magnets within the coils. Because an outer ring of magnets results in a larger air gap surface area for the electromagnetic field to pass through, at a greater radius from the axis of rotation, the outrunner arrangement inherently produces more torque.
Why Brushless: Think Efficiency and Performance
Rosset comments, “A brushless motor is much more resistant to dust and moisture. Winding wires are isolated and there is no contact in the sensitive areas.” In general, he says, “Efficiency is one of the key performance indicators for a motor. To reach this objective, [the motor] has to be adapted precisely to the propeller to offer good heat dissipation. In the ANAFI, this is exactly what we did. That’s why the windings are more visible, to keep the coils cool, maximizing motor efficiency.”
As part of the power transmission chain, “The motor control algorithm needs to be custom made for a motor,” Rosset says. “It is the only way to maximize the efficiency while hovering [and] pushing the drone at top speeds. It needs to be designed to control the motor in every flight phase with a fast response time to enhance flight control.”
He concludes by highlighting, “It’s best to have a deep understanding of the electromagnetic, mechanical, and electrical behavior of the motor chosen for a drone—while minimizing its size and weight.”
Rosset sees motors (like electronics) continuing the trend of miniaturization, suggesting that “even if brushless technology reaches a maturity the ultimate goal is to keep performance high while reducing size and weight.”
Eyes in the Sky
Drones designed to provide situational awareness for first responders and military units may look like consumer drones in size and configuration. But these specialized drones incorporate higher performance batteries, motors, and manufacturing methods for greater capabilities and reliability. These aircraft must operate in all kinds of weather, while consumer types usually cannot function in precipitation, high winds, and temperature extremes.
An example of these compact military platforms is the InstantEye series from InstantEye Robotics. Versions of these aircraft can transmit encrypted images from 4km in distance, with endurance up to one hour using optional mission batteries. They can speedily reach a point of interest at 90kph and quickly climb at 610 meters per minute. Such performance numbers (which are wind and payload-dependent) would make a consumer drone pilot envious because his or her DJI Phantom 3 typically travels at less than 60kph with a maximum rate of climb of approximately 300 meters per minute.
Regarding the factors that influence drone design, InstantEye’s Chief Systems Engineer, Philly Croteau, states, “You cannot just talk about one component and optimize it because all components must be considered together and design compromises and trade-offs made to get an overall effective platform.”
He emphasizes that the result must be cost-effective, meet performance goals, and be easy to use and train people to use. Cost-effectiveness and high performance add value, and low training time means that potential operators can learn system operations even in a combat zone.
Considering drone performance, Croteau focuses on factors that maximize lift capacity. Thrust from the motors and props as well as efficient production not only affect flight but vehicle stability and control. Likewise, the propulsion system must have robustness for reliability in a combat or first-response situation.
Croteau specifically highlights the ruggedness advantage of brushless motors over brushed types. “With brushes, there is more friction at high [revolutions per minute (rpm)], leading to contact wear,” he writes. Some arcing could also be present that could lead to electromagnetic interference, influencing the aircraft electronics. He notes that the InstantEye has coatings for corrosion prevention, such as on contacts.
With this in mind, a designer must be adroit in his or her selection and application of a motor so that there is no adverse impact to the vehicle mass. Creating an easy change out solution for drone motors and seals further improves drone uptime in the field.
Like Rosset, Croteau stresses the importance of motor and electronics heat dissipation. “The motor is positioned to be cooled in the prop flow…” he says, “placing the controller close to the motor, or using a single heat sink in the airflow [under] the prop tip.” Because everything in aircraft design is a tradeoff, Croteau notes that the mass of a heat sink should be minimized and positioned relative to the center of gravity for minimal effect on inertia and controllability. Furthermore, he says that efficient drone electronics that sip power with the least amount of heat generation will drain the battery at a slower rate for longer flight times.
Croteau comments on motor control as well, noting that the motor controller is critical to the efficiency for the quick-switching [metal-oxide semiconductor field-effect transistor (MOSFET)] that each controller signals in driving a motor. He says that InstantEye designers use a control algorithm that generates a trapezoidal signal that approximates a sine wave to produce less heat, and his team specifically chose MOSFETs because they experience the least amount of heat loss when switching.
InstantEye developers studied hawk moths in developing their flight control algorithm, noting the insects’ reaction rates and how the insects recovered after running into trees. This study has contributed to drone stability in high winds and gusts for steady camera feeds.
Croteau predicts that several technology trends will influence drone design. “One [trend will be a] reduced cost of small brushless motors because of greater automation possible in drawing wire and winding coils, which previously were hand wired. Another [will] be the ability to pitch the prop blades like a helicopter to add another degree of freedom for more controllability and stability,” he says. The latter might be done via shape-memory materials to avoid mechanical complexity.