AN1915D Freescale Semiconductor / Motorola, AN1915D Datasheet - Page 12

• Manufacturer Part Number: AN1915D
• Description: 3-Phase BLDC Motor Control with Quadrature Encoder using DSP56F80x
• Manufacturer: Freescale Semiconductor / Motorola

12
Target Motor Theory
3.1.4 Position Alignment
3.1.5 Speed Control
We can see that the commutation interrupt is called six times per electrical revolution. Note that the
commutation interrupt is called in the same moment as when we use Hall Sensors. Thus we can use the
same control routines, which are included in the SDK.
Since the Quadrature Encoder doesn’t give the absolute position, we need to know exactly the rotor
position before the motor is started. One possible, and very easily implemented, method is the rotor
alignment to a predefined position. The motor is powered by a defined static voltage pattern and the
rotor aligns to predefined position. This alignment is only done once, during the initial motor start up.
The
position counter are set to +/- 30 electric degrees from the alignment position, in order to preset the
commutation sector. (according to 3.1.3.3) The next voltage vector is set to be orthogonal to the
alignment position. The resultant direction of the voltage vector is given by polarity of applied voltage.
Commutation ensures the proper rotor rotation of the BLDC motor, while the motor speed only
depends on the amplitude of the applied voltage. This amplitude is changed by the PWM technique.
The required speed is controlled by a speed controller, implemented as a conventional PI controller.
The PI controller compares the actual speed to the required speed and, using the difference, calculates
duty cycle with the voltage amplitude required to correct the discrepancy.
Figure 3-13
shows the position of the aligned rotor. After alignment, the compare registers of the
3-Phase BLDC Motor Control with Quadrature Encoder
Freescale Semiconductor, Inc.
Figure 3-13. Alignment Rotor Position
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