In the previous article, we explained the operating principles of BLDC motors. This time, we will focus on the drive waveforms that the ESC supplies to the motor. The three methods—trapezoidal wave, sine wave, and FOC (Field Oriented Control)—all affect torque smoothness, efficiency, responsiveness, and implementation difficulty. As the mainstream for drone ESCs gradually shifts from BLHeli-based trapezoidal wave control to FOC-based systems (such as AM32), understanding the essence of each method becomes the foundation for design decisions.
What Does the Drive Method Determine?
The choice of drive method determines the current waveform that the ESC sends to the three-phase coils. This affects the following factors:
Torque ripple (fluctuations in instantaneous torque), efficiency (lower losses for the same output), rotational quality at low speeds, behavior during startup, the computational power required by the ESC, the accuracy requirements for rotor position information, and acoustic characteristics (pitch noise).
For drones, performance in the high-RPM range where maximum efficiency and torque are achieved, response speed, and ripple (vibration/noise) during hovering are particularly important.
Trapezoidal Wave Drive (120-degree commutation, 6-step)
This is the most classic and simplest method. One electrical cycle is divided into six segments, and in each segment, two of the three phases are energized, while the remaining phase is set to high impedance.
The commutation patterns are switched in order: (U+ V-), (U+ W-), (V+ W-), (V+ U-), (W+ U-), and (W+ V-). The current waveform flowing through each phase is close to a rectangle, and it is called a trapezoidal wave because the line-to-line voltage waveform is trapezoidal.
Advantages:
- Implementation is very easy, and the computational load is light (can be implemented even on 8-bit MCUs)
- Rotor position only needs to be determined with 60-degree accuracy (sensorless BEMF detection is sufficient for practical use)
- Transition from startup to the speed range where BEMF can be acquired is relatively easy
Disadvantages:
- Torque fluctuates momentarily when switching steps, causing about 12% ripple
- Rotation is jerky at low speeds (below several hundred RPM) and lacks smoothness
- The coil is abruptly disconnected when switching commutation segments, resulting in significant noise (pitch noise)
In the drone industry, BLHeli_S (8-bit MCU) and BLHeli_32 (32-bit MCU) have long been the representatives of this method. In racing drones, performance in the high-RPM range is prioritized, and the ripple issue is relatively less noticeable, so it remains the mainstream today.
Sine Wave Drive
This method supplies a smooth sine wave current to the three phases. It can be viewed as a continuous interpolation of the six steps of a trapezoidal wave.
It acquires the rotor position with high resolution (via encoder or high-precision BEMF estimation) and generates current commands using a sine curve based on that position. All three phases are energized simultaneously, with each phase having a 120-degree phase difference.
Advantages:
- Torque ripple is significantly smaller than that of trapezoidal waves (a few percent or less)
- Rotates smoothly even in the low-speed range
- Low noise, with less noticeable pitch noise
Disadvantages:
- Requires rotor position to be known with an accuracy of 1 to several degrees in electrical angle
- Computational load is heavier than trapezoidal waves, requiring at least a 32-bit MCU
- Sensorless implementation requires high BEMF estimation accuracy, and the startup sequence is complex
While sine wave drive is commonly used in industrial motors, it is positioned between trapezoidal wave and FOC in drone ESCs, and there are actually few products that use pure sine wave drive. In many cases, it is developed further into FOC.
FOC (Field-Oriented Control)
A further evolution of the sine wave drive method, representing the pinnacle of modern motor control.
The essence of FOC lies in decomposing the three-phase current into a 'component parallel to the rotor magnetic field (d-axis current)' and a 'component orthogonal to the rotor magnetic field (q-axis current)' for control. Since torque is generated only by the q-axis current, directly controlling the q-axis current makes it possible to 'generate the target torque with the minimum current'.
The core of the coordinate transformation consists of two steps. The Clarke transformation converts the three phases (U, V, W) into two-phase (α, β) fixed coordinates, and the Park transformation converts these into rotating coordinates (d, q) that follow the rotor's rotation. After control, the inverse transformation is used to return to the PWM duty cycle.
Advantages:
- Current for the same torque is minimized, resulting in maximum efficiency
- Torque ripple is minimal (can reach 1% or less)
- Smooth across the entire range from low to high speeds
- Field weakening control allows for rotation speeds exceeding the rated speed (though rarely used in drones)
Disadvantages:
- High computational load, requiring a high-performance MCU (STM32G4, F4 or higher)
- Rotor position accuracy is extremely critical; for sensorless operation, advanced observers (flux estimators, sliding mode observers, etc.) are required
- Complex gain tuning (PI controllers for both d-axis and q-axis)
- Startup sequence is even more complex (smooth transition from open-loop startup to FOC)
FOC is increasingly being adopted in AM32, Bluejay (partial implementation), industrial ESCs, and motor controllers for robotics. In drone ESCs, the shift to FOC is being led by aircraft intended for payload transport, industrial inspection, aerial photography, and long-endurance flight rather than racing. This is because improvements in efficiency directly impact battery consumption and flight time.
Differences in Current Sensing
The choice of drive method also affects the current sensing method on the ESC board.
With trapezoidal wave drive, control is possible if the current of the two energized phases is known, so current measurement using a single shunt resistor (DC-side low-side shunt) is sufficient.
With sinusoidal drive and FOC, it is necessary to grasp the instantaneous current of each phase in real-time, so either a shunt resistor is placed on each of the three phases (three-phase shunt method), or a method is used where the DC-side shunt is synchronized with PWM timing to extract each phase separately (single-shunt timing sampling).
If the drive method is not decided at the hardware design stage, the shunt placement and ADC configuration may become inappropriate, leading to a situation where the method cannot be changed later.
Which method should be chosen?
The choice is determined by the aircraft's application and performance requirements.
For racing and FPV drones, maximum torque and response speed are the top priorities, while hovering efficiency and quietness are secondary. BLHeli_32 (trapezoidal wave) has long been the standard, but even in the recent racing scene, the transition to AM32 (which can also implement FOC) is progressing.
For industrial and payload aircraft, efficiency and quietness provide direct value. For inspection, surveying, and delivery aircraft that require long flight times, the efficiency advantage of FOC directly translates to flight time. AM32 or proprietary FOC ESC implementations are chosen.
For aerial photography aircraft, FOC or high-precision sinusoidal drive is advantageous because low-frequency torque ripple and motor noise affect camera image jello (distortion due to rolling shutter) and vibration noise.
For educational and research purposes, trapezoidal wave is easy to learn and suitable as a first step for DIY ESCs. There is also a path to develop into FOC with open-source implementations like SimpleFOC.
Summary
The differences between trapezoidal wave, sinusoidal wave, and FOC can be organized into four categories: torque ripple, efficiency, computational load, and hardware requirements. Trapezoidal wave is simple and strong at high speeds, while FOC is smooth and highly efficient across the entire range, but requires stricter hardware and control design. The world of drone ESCs is currently in a transitional period, moving from the trapezoidal wave dominance for racing to the spread of FOC, and choosing according to the aircraft's application is important. Next time, we will compare ESC-FC communication protocols—PWM, OneShot, DShot, and DroneCAN—from the perspectives of response performance and functionality.