Sinusoidal commutation and field oriented control

Sinusoidal commutation and field oriented control are more advanced approaches that vary the stator angle continuously, rather than in discrete 60 degree steps. To do this they generally use a position encoder rather than a Hall sensor for feedback. An alternate approach is to measure the back-EMF (electro motive force) from each coil to determine the angle of the rotor. Although not as precise, and only usable after rotation has been established, this technique has the advantage that it requires no sensors.

Figures 3A and 3B provide a control flow overview of both sinusoidal commutation and field oriented control. They differ significantly in that sinusoidal commutation “vectorizes” the motor torque command into phase commands before the current loop, while field oriented control performs a current loop directly on the Q (quadrature) force, and uses two special transforms, known as Park and Clarke transforms, to reference the D/Q frame to the phased A/B frame used in sinusoidal commutation.

Park & Clarke Transform

This difference in approach has negligible effect at low rotation speeds. But becomes significant at high rotation speeds. The reason is that in sinusoidal commutation, as the motor rotates, the current commands for each phase will contain an increasing amount of high frequency sinusoidal variation. All control loops have some lag, and so the current loop operating on these signals will lag the desired current, which means that the location of the useable force vector (Q) will lag the desired force vector. The higher the rotation speed, the greater the phase lag.

Field oriented control (3A) and sinusoidal commutation (3B)

Field oriented control avoids this problem because the current loop operates on the Q force and the D force, which are independent of motor rotation speed. After the output command of the Q and D loops are determined, they are then referenced to the A and B phase command using the Park and Clarke transforms, and converted into specific voltage commands for each coil of the motor.

For this reason, if both types of control are available, field oriented control is preferred over sinusoidal commutation. One advantage that sinusoidal commutation does have over field oriented control however is that the process of vectorization (splitting a single torque command into specific commands for each phase of the motor) is separate from the current control. This means that if you are using an off-the-shelf motion card with separate amplifiers, you will probably use sinusoidal commutation rather than field oriented control. This is because many “dumb” amplifiers support multi-phase A and B inputs, but few provide support for field oriented control connected to an external motion card.