AN1931D Freescale Semiconductor / Motorola, AN1931D Datasheet - Page 21

• Manufacturer Part Number: AN1931D
• Description: 3-Phase PM Synchronous Motor Vector Control
• Manufacturer: Freescale Semiconductor / Motorola

MOTOROLA
The proposed algorithm combines both methods. The algorithm simultaneously measures the number
of quadrature encoder pulses per constant period, and an accurate time interval between the first and
last pulse is counted during that constant period. The speed can then be expressed as:
where:
The algorithm requires two timers for counting pulses and measuring their period, and a third timer as
a time base; see
counts a system clock divided by 2. The values in both timers can be captured by each edge of the
Phase A signal. The time base is provided by timer D0, which is set to call the speed processing
algorithm every 900 s. An explanation of how the speed processing algorithm works follows.
First, the new captured values of both timers are read. The difference in the number of pulses and their
accurate time interval are calculated from actual and previous values. The new values are then saved
for the next period, and the capture register is enabled. From that moment, the first edge of Phase A
signal captures the values of both timers (A2, A3) and the capture register is disabled. This process is
repeated on each call of the speed processing algorithm; see
speed
k
N
T
Figure
Freescale Semiconductor, Inc.
3-Phase PM Synchronous Motor Vector Control
4-3. Timer A2 counts the pulses of the quadrature encoder, and timer A3
For More Information On This Product,
Figure 4-4. Speed Processing
calculated speed
scaling constant
number of pulses per constant period
accurate period of N pulses
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speed
=
k N
---------- -
T
Figure
4-4.
System Concept
(EQ 4-1.)
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