External

In the past, external sine commutation was a technology that gave analog drives the ability to commutate motors sinusoidally.  With today’s processors, servo drives no longer need an external controller to produce sinusoidal commutation.  Therefore this discussion is for informational purposes or for cases where specialized control is needed.

External sine commutation was developed to satisfy the need for smoother motor operation at a time when most servo drives were built on analog architecture.  With no on-board processing capabilities, the analog drives weren’t up to the task of mapping out the sinusoidal transitions between phases in real-time; so a method was devised that pushed the processing burden over to the digital controller.  The solution: two sinusoidal command signals that correspond to the motor phase currents A and B, respectively, while phase C is calculated in the drive. The overall amplitude of the signals control the motor torque while the relative values between the two signals controls the commutation phase angle.

In summary, with external sine commutation the controller keeps track of the motor position – typically using motor mounted Hall sensors and an encoder along with other constants such as the motor pole count and the number of encoder counts per electrical cycle.  The controller then calculates the required current in each phase of the motor and passes the command directly to the servo drive via the two analog signals.  Additionally the controller is still responsible for the motion control for the rest of the machine.  All of this may sound complicated, but many controllers had this function built-in.