National Instruments NI-Motion User Manual page 281

Appendix A Sinusoidal Commutation for Brushless Servo Motion Control
Direct Set
Determining the Counts per Electrical Cycle of the Motor
Caution Counts per electrical cycle must be set correctly to avoid overheating and
damaging your motor.
Commutation Frequency
NI-Motion User Manual
Direct set is an initialization method where the controller sets the current
position as the specified phase angle. This initialization method is
recommended only for a custom system with known initial phase angle.
Whenever the axis is enabled, the controller must perform the phase
initialization procedure to determine the phase.
The controller needs to know the counts per electrical cycle of the motor to
determine the commutation phase. The motor manufacturer usually gives
this specification. In many cases, the information also may be specified as
the number of poles.
To convert from the number of poles to the number of counts per electrical
cycle, use the following formula:
counts per electrical cycle
The controller updates the command voltage and the commutation phase
every update period. To commutate brushless motors smoothly, the
controller must update the phase at least six times per electrical cycle.
Therefore, the commutation frequency is limited by the update rate of the
control loop. To calculate the maximum commutation frequency supported
at a particular PID update rate, use the following formula:
commutation frequency
counts per revolution
= ----------------------------------------------------------- -
counts per electrical cycle
= ------------------------------------------------------------- -
A-2
×
2
number of poles
×
PID rate 6
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