AN1915D Freescale Semiconductor / Motorola, AN1915D Datasheet - Page 10

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

10
Target Motor Theory
Note:
3.1.3.3 Direct conversion of Quadrature Encoder signals to a commutation sector
Figure 3-10. Separation of One Electrical Revolution into Six Commutation Sectors
position very quickly. The frequency of scanning depends on the maximal rotor speed, the number of
pole pairs and on the required precision of commutation moment detection. The same scan frequency
as used in the PWM (16kHz) is satisfactory for the common applications.
In this case, the rotor position can be scanned in the moment of a PWM reload interrupt. The algorithm
translates the actual position into the one of the six sectors. If a change of sector is detected, the
commutation is performed.
A different method is the direct conversion of Encoder output to commutation sectors. The conversion
is provided completely automatically by hardware. The advantage of this method is that an interrupt
rate depends on the actual motor speed, while in the previous method, there is a constantly high
interrupt rate in order to scan the actual motor position. The direct conversion operates in following
way:
The electrical revolution is divided into six sectors, as in
the quadrature counter, which has its inputs connected to the Quadrature Decoder. After rotor
alignment to known position, both compare registers are set to values that correspond to sector borders
and the counter is set to zero, see
count the pulses of the Quadrature Encoder. Nothing happens until the counter reaches one of the
compare values.
The
counting, two pole pairs (500 x 4 / 2 = 1000 pulses per el. revolution)
Figure 3-10
3-Phase BLDC Motor Control with Quadrature Encoder
Freescale Semiconductor, Inc.
considers 500 pulses per mech. revolution, both rising and falling edges
For More Information On This Product,
Figure
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3-11. When the motor starts to move, the counter starts to
Figure
3-10. The rotor position is scanned by
MOTOROLA