Crucial to drone design is the ability to control the speed and rotation of the motors.Most drones are made byBrushless DC Motorpowered by, requiring constant adjustment of speed and direction of rotation.The electronic speed control (ESC) module performs these functions, including the power stage, current sensing circuit, microcontroller, and communication interface with the flight control system, thus forming the basis of the drone.This article describes important factors to consider when designing an ESC and market development solution.
The design of an ESC requires careful evaluation and analysis of characteristics, which can be summarized as follows:
- Batteries installed on drones
- available budget
- Electromagnetic Compatibility (EMC) and Immunity
There are two types of brushless motors that can be installed on drones: brushless DC motors (BLDC) and brushless AC motors (BLAC), also known as permanent magnet synchronous motors (PMSM).The choice of which type of motor to use is influenced by the control algorithm chosen, which can be trapezoidal control or field-oriented control (FOC).The trapezoidal motor control algorithm has the following key features:
- Motor Control Based on Six-Phase Switching Sequence
- Rotor magnetic angle detection for setting the correct angle;Each step corresponds to a 60° angle
- In sensorless control systems, the switching angle is estimated by measuring the back EMF phase voltage
The FOC control algorithm, on the other hand, has the following characteristics:
- Motor control via sinusoidal phase voltage or current (FOC)
- Rotor angle detection accuracy of 1° to 5° ensures the algorithm is always capable of delivering maximum torque
In sensorless control systems, the magnetic angle of the motor is estimated from the motor phase voltages and currents.Its position is determined by monitoring certain electrical parameters of the motor without using additional sensors.The most common type used in drones is the brushless DC motor because of its small size, relatively low cost, durability, and robustness.
Most drones have at least four motors, with four-motor versions being the most commonly used.The ESC is responsible for controlling the speed of each motor, so the most common drone architectures involve dedicating one ESC to each motor.All ESCs must be able to communicate with each other directly or indirectly through the flight controller in order to control the drone easily.The direction each motor spins is also important: in a quadcopter, one pair of motors turns in one direction, while the other pair turns in the opposite direction.
The motor control technique most commonly used by ESC manufacturers is field oriented control, a technique for controlling motor torque and speed.When implemented properly, FOC can handle even rapid acceleration changes without creating instability, allowing drones to perform complex maneuvers while maximizing efficiency.
The block diagram in Figure 1 below shows a FOC architecture that includes the following components:
- Current controller consisting of two integral-proportional controllers
- Optional external loop speed controller and reference current generator
- Clarke, Park, and inverse Park transformations for converting from a stationary frame to a rotating synchronous frame
- Space Vector Modulator Algorithm to Convert vα and vβ Commands into Pulse Width Modulated Signals Applied to Stator Windings
- Protection and auxiliary functions, including startup and shutdown logic
- Optional observer to estimate rotor angular position if sensorless control is required
A motor control engineer designing a FOC performs several tasks, including developing a controller architecture with two PI controllers for the current loop, optimizing the gains of all PI controllers to meet performance requirements, and designing a space vector modulator to control the PWM.
Once the control algorithm (trapezoidal or FOC) has been chosen, the next step is to choose between an open-loop or closed-loop control system.In open-loop control, a synchronous motor (BLDC or BLAC) is driven by a control signal and is assumed to follow the commanded control action.In a closed-loop control system, a circuit checks that the motor is moving as expected.If not, the control system automatically compensates for over- or under-motion by reducing or increasing the current.
When using a closed-loop or open-loop (sensorless) control system, current and voltage must be measured for feedback signals.Figure 2 shows a typical measurement setup, suitable for both trapezoidal and sinusoidal control systems.With trapezoidal control using a sensorless algorithm that uses three-phase voltages to calculate the rotor angle.
Figure 2: ESC with sensorless motor control.On the right is Texas Instruments’ ESC high-speed sensorless FOC reference design for drones, and on the left is its block diagram.(Source: Texas Instruments)
The mechanical simplicity and aerodynamic stability of the UAV are related to the coordinated use of electric motors and their maneuvering.In a quadcopter, one pair of motors located diagonally across the structure turns in the same direction as the other two, but in opposite directions.If all four motors are turning at the same speed, the drone can climb, descend or maintain level flight.If the diagonal pair turns faster than the other pair, the drone will rotate around its center of gravity and stay within the same horizontal plane (Figure 3).
Figure 3: The drone uses different combinations of rotor speeds to perform maneuvers.(Source: STMicroelectronics)
If you change the speed of the head (or tail) rotors, the drone will go up or down like a fixed wing aircraft dive.Adjusting the torque left or right causes the drone to roll, causing it to spin around its axis.The proper rotor speed is varied by the UAV’s flight control system to achieve the flight altitude required to accomplish the desired maneuver.
For control engineers, speed correction is a common control loop feedback problem that can be solved with proportional, integral, derivative (PID) controllers.
Designing an ESC for a drone requires high quality components specifically designed to control high RPM motors (12,000+ RPM).Texas Instruments (TI) has developed a family of MCUs called InstaSPIN that simplify the design of three-phase motor control applications.InstaSPIN-FOC is suitable for sensorless systems with fast software encoders for torque and speed control of any three-phase motor.InstaSPIN-MOTION targets sensorless systems and provides position, speed and torque control for any three-phase motor.
Complete reference designs for these oscilloscopes are available from TI, including InstaSPIN-FOC and InstaSPIN-MOTION motor control technologies.The platform includes a 32-bit TI C2000 InstaSPIN microcontroller.It allows developers to identify, automatically tune and control three-phase motors, quickly delivering a stable and powerful motor control system.
STMicroelectronics offers a complete ESC reference design implementing a sensorless FOC algorithm.The STEVAL-ESC001V1 ESC reference design is suitable for entry-level commercial drone designs, and can drive any three-phase brushless motor (or PMSM) powered by a 6S Li-polymer battery pack or any equivalent DC power source with a peak current of up to 30 A .With a complete pre-configured firmware package (STSW-ESC001V1), the STEVAL-ESC001V1 allows designers to quickly develop their application implementing a sensorless field-oriented control algorithm with three-shunt current reading, speed control and fully active braking.The STSW-ESC001V1 firmware/software package plus the STM32 PMSM FOC software development kit MC library allows optimizing the ESC design by acting on the FOC parameters embedded in the STM32 MCU and quickly retrieving the relevant motor parameters using the ST Motor Analyzer.ST’s sensorless FOC algorithm can be adapted to any three-phase BLDC or PMSM motor application, providing longer flight time and best dynamic performance (Figures 4 and 5).
Figure 4: Block diagram of ST’s STEVAL-ESC001V1 solution (Source: STMicroelectronics)
Figure 5: ST’s STEVAL-ESC001V1 board (Source: STMicroelectronics)
ShouldHoverGamesA drone development platform that can be used to build any autonomous vehicle, from drones and rover drones to modular and flexible NXP hardware/software solutions.The development kit is essentially based on a microprocessor with Linux and Open CV and various accompanying sensors for guided flight.
Flight controllers ensure that the drone remains stable.The board is open source and other external sensors can be plugged in to optimize operation according to function.
One of the IoT connections must be used to implement the LiPo battery and country-specific telemetry radio.To get the full functionality of the kit, you need to choose which of the two available telemetry radios to purchase.With telemetry, you can connect to the vehicle in real-time during flight and can view the status of the drone, load and control autonomous waypoints and make any necessary changes while in flight.Telemetry data is sent to the control station, but is also stored in the flight unit.
Kit components also include DC-DC power module, GPS NEO-M8N module with stand, safety switch, buzzer, bright RGB status LED, SEGGER J-Link EDU Mini/FTDI USB-TTL-3V3 cable/with cable Debug breakout board, BLDC brushless motor 2212 920 kV, and ESC motor controller 40 A OPTO (Figure 6).
Reviewing editor: Liu Qing