AN1913D Freescale Semiconductor / Motorola, AN1913D Datasheet - Page 18

• Manufacturer Part Number: AN1913D
• Description: 3-phase BLDC Motor Control with Sensorless Back-EMF ADC Zero Crossing Detection using DSP 56F80x
• Manufacturer: Freescale Semiconductor / Motorola

Control Technique
5.4 Sensorless Commutation Control
18
The non-fed phase “branch” voltage U
presented BLDC Motor Control application synchronizes the Back-EMF Zero Crossing detection with
PWM. The AD conversion of phase branch voltages is triggered in the middle of PWM pulse. Then the
voltage for Back-EMF is sensed at the time moments because the non-fed phase branch voltage is
already stabilized.
This section presents sensorless BLDC motor commutation with the Back-EMF Zero Crossing
technique.
In order to start and run the BLDC motor, the control algorithm has to go through the following states:
Figure 5-5
without the position feedback. When the rotor moves, the Back-EMF is induced on the non-fed phase
and acquired by the ADC. As a result, the position is known and can be used to calculate the speed and
process the commutation in the Running state.
•
•
•
Alignment
Starting (Back-EMF Acquisition)
Running
Figure 5-4. Back-EMF Zero Crossing Synchronization with PWM
shows the transitions between the states. First, the rotor is aligned to a known position
BLDC control with BEMF Zero Crossing using ADC
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va
is disturbed at the PWM switching edges. Therefore the
u
u
Zero Crossing Sample
s/w flag
va
vb