In the previous series, we covered EMI and noise design on the FC side, but a drone's propulsion system is established through the cooperation between the FC and the ESC (Electronic Speed Controller). In this new series, we will cover ESC and motor control. In this first article, we will explain the overall picture of how an ESC uses its principles to rotate a brushless motor.
Why is an ESC necessary?
Almost all motors used in drones are brushless DC motors (BLDC). While brushed DC motors rotate just by connecting a power source, brushless motors do not rotate on their own. It is necessary to switch the current to the three-phase coils in a sequence corresponding to the rotor's position, and the role of the ESC is to realize this switching operation using electronic circuits.
In brushed motors, a commutator and brushes performed this switching mechanically, but brush wear, poor contact, and electrical noise were major weaknesses. Brushless motors solve these problems but require an external electronic switching circuit (ESC) instead.
Basic structure of a brushless motor
To understand an ESC, you must first grasp the structure of a brushless motor.
A typical drone BLDC is an outer-rotor type, having coils (stator) on the inside and magnets (rotor) on the outside. Three-phase coils (U-phase, V-phase, W-phase) are wound on the stator and connected in a Y-connection or Δ-connection.
The rotor's magnets consist of multiple magnetic poles with N and S poles arranged alternately. For drone motors, 14 poles (7 N-poles, 7 S-poles) are common. This number of poles affects the relationship between electrical rotational speed and mechanical rotational speed, which will be described later.
When current flows through the stator coils, a magnetic field is generated, and the rotor's magnets are attracted to that magnetic field, causing rotation. By flowing current through the three-phase coils in sequence, a rotating magnetic field is created within the stator, causing the rotor to rotate continuously.
Generation of three-phase waveforms: Inverter circuit
The core of an ESC is the inverter circuit. It generates a three-phase AC waveform from the DC voltage supplied by the battery and supplies it to the motor.
The circuit configuration is a three-phase bridge, combining three sets of "half-bridges" with MOSFETs placed on both the high and low sides. By appropriately switching a total of six MOSFETs, it switches between three states for each phase: positive voltage, negative voltage, and high impedance.
The switching frequency is generally around several tens of kHz. By modulating the duty ratio with PWM, it effectively creates a waveform close to a sine wave or trapezoidal wave. Increasing the switching frequency makes the waveform smoother, but it increases the MOSFET switching loss.
The selection of MOSFETs greatly affects the performance of the ESC. Choose ones with low on-resistance (Rds(on)) that can withstand large currents and have good switching characteristics. For drone ESCs, power MOSFETs with an Rds(on) of about 1 to 2 mΩ are standard.
Detection of rotor position
To create a rotating magnetic field, it is necessary to know the current position of the rotor. If you do not know where the rotor is facing, you cannot decide which phase to flow current through to generate torque.
There are broadly two methods for position detection.
The Hall sensor method places three Hall elements inside the motor to directly detect the magnetic poles of the magnet. While reliable and stable during startup, it increases wiring and complicates the internal structure of the motor.
The sensorless method estimates the rotor position by detecting the back electromotive force (BEMF) induced in the non-energized coils (one of the three phases). It requires only three wires (motor wires only) and has a simple structure, but since the back EMF is zero when stopped, position detection is impossible, requiring a specialized startup sequence.
The sensorless method is overwhelmingly the mainstream for drone ESCs. Running Hall sensor wiring near the propellers carries a high risk of failure, and for drones where high-RPM performance is prioritized, sensorless provides sufficient accuracy.
Drive Methods: Trapezoidal Wave, Sine Wave, FOC
There are several methods for the waveforms that an ESC supplies to a motor, which affect torque characteristics and efficiency.
Trapezoidal wave drive (120-degree commutation, 6-step) is the simplest method. One rotation is divided into six segments, with two phases energized and one phase idle in each segment. It is the traditional method for BLDC ESCs, easy to implement, and has a low computational load, but it causes torque ripple during step switching, resulting in jerky rotation at low speeds. Many drone ESCs use an improved version of this method.
Sine wave drive is a method that supplies a smooth sine wave current to the three phases, resulting in lower torque ripple and smoother rotation at low speeds. It is more complex to control than trapezoidal wave drive and requires high-precision rotor position detection.
FOC (Field-Oriented Control) is an advanced method that controls current to keep the rotor magnetic field and stator magnetic field orthogonal. It extracts maximum torque and efficiency, and FOC-based ESCs are becoming more common in modern high-performance units. FOC is becoming the standard in AM32 (the successor to BLHeli_32), some implementations of Bluejay, and industrial ESCs.
We will cover the details of each method in the latter half of this series, but choosing a drive method is a critical design decision for balancing drone response performance and efficiency.
Communication between FC and ESC
The ESC operates by receiving commands from the FC. The communication protocol between the FC and ESC is a key factor that determines the ESC's response performance and functionality.
PWM is a classic protocol that represents commands with a pulse width of 1 to 2 ms. It is transmitted at rates from 50 Hz to 490 Hz; while simple, it has limitations in resolution and response speed.
OneShot and MultiShot are high-speed versions of PWM that increase response speed by shortening the pulse width.
DShot is a digital protocol that features noise immunity and telemetry functions (returning motor RPM, temperature, voltage, and current to the FC). It is the current mainstream for racing and industrial drones.
DroneCAN is a CAN bus-based protocol that allows multiple ESCs to be connected on a single bus, with bidirectional communication as standard. It is an option that leverages the advantages of the differential signaling discussed at the beginning of this series, and its adoption is increasing in large-scale and industrial aircraft where noise immunity is required.
A detailed comparison of these protocols will be covered in the middle of this series.
Power System Design
An ESC is a high-current circuit that supplies battery voltage directly to the motor. Momentarily, currents of tens to hundreds of amperes flow per motor, passing through battery wires, capacitors, MOSFETs, and motor wires.
A large-capacity electrolytic capacitor is required on the input side. When the inductance of the battery wires combines with MOSFET switching, large voltage spikes occur, which will destroy the MOSFETs if not absorbed by the capacitor. Choose a capacitor with a capacity of several hundred to several thousand µF and low ESR (Equivalent Series Resistance).
The gate driver is a dedicated IC for switching MOSFETs at high speeds. It must be capable of handling high currents to quickly charge and discharge the MOSFET gate capacitance, and it also handles dead-time control (preventing simultaneous on-states of the upper and lower MOSFETs).
These circuit designs will be covered in detail in the latter half of this series.
Summary
An ESC is an electronic commutation circuit for rotating brushless motors, consisting of elements such as an inverter, position detection, drive waveform generation, and communication protocols. The selection of each element determines the performance of motor control and is directly linked to the response performance, efficiency, and reliability of the entire drone. Next time, we will delve deeper into the operating principles of BLDC motors and cover the mechanism of torque generation and its relationship with rotational speed.