AN1905 ST Microelectronics, Inc., AN1905 Datasheet - Page 74

• Manufacturer Part Number: AN1905
• Description: St7mc Three-phase BLDC Motor Control Software Library
• Manufacturer: ST Microelectronics, Inc.

ST7MC THREE-PHASE BLDC MOTOR CONTROL SOFTWARE LIBRARY
7.3.2 Adjusting falling/rising Bemf settings
Rising and falling Back-EMF coefficients allow the delay to be set up between a zero-crossing
event (Z event) and the consecutive commutation event (C event). Once this ‘delay’ is
elapsed, the PWM is applied on the next switch configuration (one of the 6 steps, T0-T3, T0-
T5, etc.).
With the help of this setting, fine tuning of the commutation ‘mechanism’ is permitted: low co-
efficient values (early commutation) may make the system unstable, while high values may
lower the maximum speed of the system.
In order to match any system, and especially when motor windings are not mounted entirely
symmetrically, 2 delay coefficients can be adjusted: rising (respectively falling) delay associ-
ated with rising (respectively falling) Back-EMF voltage (to further reduce dissymmetrical
winding effects, the sensorless method allows to choose the previous MZPRV or current
MZREG value for delay computation).
It has to be noted that ‘rising’ and ‘falling’ definitions are meaningful only in sensorless mode.
In sensor mode, the hall effect sensor outputs can be connected to the demokit in different
ways, so the rising delay may then be associated to a falling-edge sensor output.
Generally, the delay coefficients have to be set in order to run the motor properly at a desired
frequency, and used to balance the step times (equal time duration of the 6 steps, see
Figure
51).
The final application layer could even allow greater flexibility (not implemented in the library):
6 independent delay coefficients (1 per step) in sensorless mode, or 3 delay coefficients in
sensor mode (1 per sensor). This may be required especially when motor windings are
strongly dissymmetrical or when the sensors are not set equally apart from each other (120
degrees in sensor 120 mode with 1 pair pole for example).
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