The spread of drones is constantly growing, with a variety of applications ranging from the hobby sector, to the commercial and industrial fields, up to the most advanced military applications. Drones have the advantage of being able to be remotely operated, thus flying over areas that would be too difficult, dangerous or inconvenient to reach in person. The applications in the commercial field are manifold: agriculture, monitoring of plants and buildings, shooting areas and even the delivery of packages, medicines or necessity goods. New and further applications of drones will most likely be identified in the coming years, when they will no longer be considered only as toys or gadgets, but as valuable tools to improve the quality of our lives.
Electronic speed control
This article focuses on medium- and high-range drones, normally equipped with brushless DC motors. Motors of this type require careful and continuous regulation of speed and of the relative direction of rotation; in some cases, the possibility of implementing a real dynamic brake is needed as well. The circuit responsible for these functions is the ESC (Electronic Speed Controller), which is typically composed of a power supply stage, a circuit for detecting the current, a microcontroller and a communication interface with the flight controller (Figure 1). The design of an ESC involves a series of important features concerning:
- the topology used for motor control;
- compromise between cost and efficiency (hence the time of flight);
- type of battery installed on the drone;
- required performance (for example, maximum controllable motor speed);
- electromagnetic compatibility (EMC) and immunity to interference.
There are two types of brushless motors suitable for use on drones: DC brushless motors (BLDC) and AC brushless motors (BLAC), also known as permanent magnet synchronous motors (PMSM). The choice of which type of motor to use is typically based on the selected control algorithm: trapezoidal or field-oriented control (FOC). More precisely, a trapezoidal motor control algorithm has the following characteristics:
- motor control based on a six-step commutation sequence;
- detection of the magnetic angle of the rotor, to perform the commutation at the correct angle (each step corresponds to an angle of 60 degrees);
- in the absence of sensors, the commutation angle is estimated by measuring the phase-voltage back EMF.
Instead, a FOC control algorithm has the following features:
- control of the motor through sinusoidal phase voltages or currents (FOC);
- detection of the magnetic field angle of the rotor with a minimum accuracy between 1 and 5 degrees, to always ensure the maximum torque;
- in the absence of sensors, the rotor magnetic angle is estimated based on the motor’s phase voltages and currents.
A sensorless control system, compared to the alternative based on mechanical speed sensors, is often preferred as it keeps the project costs low and helps to improve the robustness of the system.
The PWM signal, used to commutate between the different power stages, varies according to the type of control selected. Figure 2 shows the filtered phase voltages obtained after removing the PWM carrier, referred to the case of trapezoidal and sinusoidal motor control techniques.
The trapezoidal control is affected by an issue (a problem), due to the abrupt variation between each phase, which generates torque ripples and current spikes, reducing the efficiency and producing vibrations. At the electrical level, the DC bus voltage of an ESC is between 7.4V and 22.4V, while the current from the lithium polymer battery (LiPo) is between 10A and 20A. To minimize interference, the PWM frequency used for commutation between phases is between 30kHz and 60kHz. Once the control algorithm has been chosen, it must be decided whether the control shall be performed in open-loop or closed-loop.
The closed-loop control is preferable, since, by measuring the current necessary to make the motor perform the desired displacement, a higher efficiency and accuracy are obtained with respect to the open-loop solution. In the case of trapezoidal control, only one shunt current must be measured, while in the case of sinusoidal control it is necessary to measure up to three shunt currents (Figure 3). When using a closed-loop control, it is necessary to perform a tuning of the algorithm so that the motor remains stable at high rotation speeds (that is, over 12kRPM).
Solutions for commercial drones
The design of an ESC for drones requires high-grade components, specifically designed for driving high-speed motors at maximum speed. Texas Instruments has developed an MCU family, named InstaSPIN, which makes designing three-phase motor control applications easier. InstaSPIN-FOC, suited for sensorless systems, features a fast software encoder with torque and velocity control for any three-phase motor. InstaSPIN-MOTION is instead addressed to sensor systems and provides position, velocity and torque control for any three-phase motor. Besides MCU, TI offers other products suitable for ESC design, such as the NexFET series of power MOSFETs and the DRV8305 three-phase gate driver with three integrated current shunt amplifiers.
InstaSPIN comes with a high voltage motor control kit, a complete reference design for learning and experimenting with digital control of high voltage motors. Based on the revolutionary InstaSPIN-FOC a nd InstaSPIN-MOTION motor control technologies, the platform includes a TI C2000 InstaSPIN 32-bit microcontroller (figure 4). It allows developers to quickly identify, automatically tune, and control a three-phase motor, thus quickly providing a stable and functional motor control system. The kit is an excellent, all-around motor inverter design tool, showcasing sensorless and encoder-based control of the most common types of high voltage, three phase motors, including AC induction (ACI), brushless DC (BLDC), and permanent magnet synchronous motor (PMSM & IPM).
STMicroelectronics also offers a complete reference design for electronic speed controller (ESC), implementing a sensorless FOC algorithm. The STEVAL-ESC001V1 reference design for electronic speed controllers is suitable for entry-level commercial drone designs and drives any three-phase brushless (or PMSM) motor powered by 6S LiPo battery packs, or any equivalent DC supply, up to 30 A peak current. The STEVAL-ESC001V1 allows designers to quickly develop their application thanks to a complete pre-configured firmware package (STSW-ESC001V1), implementing a sensorless Field Oriented Controlled algorithm with 3-shunt current reading, speed control, and full active braking. The reference design board can accept commands from a flight controller through PWM signals, but other communication interfaces like UART, CAN, and I²C are also available.
The reference includes a battery eliminator circuit working at 5V, an NTC sensor for temperature measurement and circuitry for overcurrent and overvoltage protection (OCP/OVP). The small form factor and current capability make this reference design suitable for electronic speed controllers on small and light unmanned aerial vehicles like professional drones. The STSW-ESC001V1 firmware/software package plus STM32 PMSM FOC software development kit – MC library allows optimizing the electronic speed controller design by acting on the field-oriented control parameters embedded in a STM32 MCU and exploit the ST motor profiler to retrieve the relevant motor parameters rapidly. The ST sensorless FOC algorithm fits any three-phase BLDC or PMSM motor application, ensuring longer flight times and optimal dynamic performance.
Electronic speed controllers for drones are essential to provide motor speed control in low and medium size commercial drones. One of the most widely used solution is based on a sensorless FOC technique for controlling 3-phase brushless motors. For a sensored solution, which achieves greater accuracy and better speed controller, a sensorless solution has the advantages of lower costs and reduced weight, two key factors in any low or medium size drone.
>> This article was originally published on our sister site, Power Electronic News: “Electronic Speed Control for Drones.”