AN1914 Freescale Semiconductor / Motorola, AN1914 Datasheet - Page 21

• Manufacturer Part Number: AN1914
• Description: 3-Phase BLDC Motor Control with Sensorless Back EMF Zero Crossing Detection Using DSP56F80x
• Manufacturer: Freescale Semiconductor / Motorola

Freescale Semiconductor, Inc.
Control Technique
5.4.2.1 Algorithms BLDC Motor Commutation with Zero Crossing Sensing
All these processes are provided by new algorithms which were designed for these type of applications
within SDK. They are described in Motor Control.pdf, chapter BLDC Motor Commutation with
Zero Crossing Sensing (see [12.1]).
From pictures an overview of how the commutation works can be understood. After commuting the
motor phases there is a time interval (Per_Toff[n]) when the shape of Back-EMF must stabilized (after
the commutation the fly-back diodes are conducting the decaying phase current, therefore sensing of
the Back-EMF is not possible). Then the new commutation time (T2[n]) is preset. The new
commutation will be performed at this time if the Back-EMF zero crossing is not captured. If the
Back-EMF zero crossing is captured before the preset commutation time expires, then the exact
calculation of the commutation time (T2*[n]) is made based on the captured zero crossing time
(T_ZCros[n]). The new commutation is performed at this new time.
If (for any reason) the Back-EMF feedback is lost within one commutation period corrective actions
are taken in order to return to the regular states.
The flow chart explaining the principle of BLDC CommutationControl with BEMF Zero Crossing
Sensing is shown in
Figure
5-7.
DSP56F80x MC PWM Module in Motor Control Applications
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