Zero Crossing Detection Method; Startup Procedure; Figure 13: Bemf Measuring During The Off-Time And Adc Sampling - ST STM32 User Manual

UM2124
3.3.5

Zero crossing detection method

In order to synchronize the BLDC motor with the driver, when the BEMF signal becomes
equal to the motor neutral voltage a zero crossing point occurs.
There are two different scenarios that depends on the high frequency PWM signal
generated to modulate the high-side devices that change the way of measuring the BEMF
value, in particular:
1. during PWM T-on time applied to the high-side switch, the motor neutral voltage
becomes equal to the bus voltage divided by 2.
2. During PWM T-off time applied to the high-side switch, the motor neutral voltage
becomes 0 V.
The zero crossing can be detected during both the on and the off time of the PWM signal.
The six-step fw library v.1.0 and v.1.1 are only based on T-off mode and the detection of
the zero crossing point is performed by comparing the BEMF converted value to a fixed
threshold.
the BEMF measuring during the T-OFF of T1 switch when the phase C is floating and the
ADC sampling method measures the BEMF signals, Vbus, temperature, motor current etc.
3.3.6

Startup procedure

To perform a sensorless control on the BLDC motor, the BEMF must reach an appreciable
value to be readable. Since the BEMF amplitude depends on the rotor speed, it requires
the motor to rotate at the speed above a specific threshold. Thus, it is important to
implement an asynchronous startup strategy.
The following phases are necessary to start the motor up and begin the closed loop control
with BEMF detection:
• alignment phase
• ramp generation phase
Figure 13: "BEMF measuring during the OFF-time and ADC sampling"

Figure 13: BEMF measuring during the OFF-time and ADC sampling

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Implementation of six-step motor control
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