1 Overview

When an inductor moves through a magnetic field, a voltage is induced in it. A brushless motor is a collection of three inductors spinning in a circular arranged magnetic field, and an electrical voltage is induced in the motor’s coils. BEMF is the induced voltage that always occurs when the motor spins. Ideally, the BEMF occurs at any non-zero velocity, but in practice, due to noise and limited measurement resolution, a minimum rotation per minute (RPM) is required to have a measurable BEMF which can provide the rotor’s position through ZCD detection.

The BEMF voltage has three main parameters: Amplitude, frequency, and phase.

The amplitude is proportional to the rotor's speed and is motor-specific.

The frequency is proportional to the rotor's speed and the number of pole pairs. The motor's RPM can be calculated using the BEMF frequency and equations that capture the motor characteristics.

The phase is important for synchronization between the stator's rotating field and the rotor. The phase and frequency are used as feedback for drive synchronization (phase and speed alignment with the motor) and for implementing control loops.

For proper BEMF measurement and ZCD detection, the drive signals for all three phases must be disabled simultaneously to not influence the measurement. Even in Trapezoidal Drive, this is not achieved, as only one phase is not driven. The other two are still being driven and will generate noise. The ZCD is determined using analog and digital filters, along with a Majority detection algorithm. For more information, see Majority Detection algorithm as a reference. These filters are required to reduce the noise and have a close to accurate ZCD identification, but they introduce a delay between the actual ZCD and the measured ZCD.

For Sensorless Sinusoidal drive, all three phases are conducting continuously and there is no undisturbed measurement window that can be used for direct motor BEMF measurement. Thus, the go-to FOC method measures each coil current and recreates the BEMF through a mathematical model, which requires performant hardware and is not applicable for 8-bit MCUs. The dominant method for Sinusoidal Drive on 8-bit MCUs is given by a sensored solution with a Phase-Locked Loop (PLL) synchronization approach.

This technical brief introduces a new Sinusoidal Drive method, which creates a direct BEMF acquisition window that complies with the requirement for the no phase measurement point disturbance from the drive signals, allowing for a simple and unfiltered ZCD detection that is closer to the actual motor coil current ZCD.

The new Sinusoidal Drive method requires Pulse-Width Modulation (PWM) rising event alignment and dead time control, achieved with pure software or a combination of software and hardware-capable peripherals. Thus, the method presented can be used in any 8-bit MCU that can generate complementary PWM signals. However, with dedicated hardware peripherals, as found in Microchip's AVR® EB Family, the code size is drastically reduced, and interrupt time is shortened.