AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 14

• Manufacturer Part Number: AN1857
• Description: A 3-Phase ac Induction Motor Control System Based on the MC68HC908MR32
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
Back-EMF
Zero-Crossing
Detection
14
Back-EMF zero-crossing detection enables position recognition when
controlling brushless dc motors. Sensorless brushless dc motor control
is not discussed in this application note. Rather, refer to Sensorless
Brushless dc Motor Using the MC68HC908MR32 Embedded Motion
Control Development System, Motorola document order number
AN1858/D, for more information.
To perform distortion correction when controlling a 3-phase ac induction
motor, it is necessary to provide phase current polarity information to the
MR32. That information is detected, signal conditioned on the power
board, and passed transparently through the optoisolator board to the
control board. The three zero-crossing signals from phases A, B and C
are routed into the three input current sense IS1–IS3 inputs of the MR32.
They are also routed to the back-EMF selection logic, shown in
Figure
interrupt to the MR32 channel 2 of timer A upon each phase zero-
crossing, provided the other input to the NAND gate is at a logic 1. The
three open collector NAND gates shown in
U7C) are wire ORed such that any one of the these outputs transitioning
to a logic 0 will provide an interrupt to the MR32’s timer A.
The system software uses the MUXA, MUXB, and MUXC inputs to the
NAND gates to enable a particular phase to have an ability to interrupt
the processor. During system operation, the software is aware of the
window when a particular zero-crossing interrupt should occur for any
given phase. MUXA, MUXB, and MUXC inputs to the NAND gates are
enabled for each particular phase during its computed zero cross
window. Using this technique, the system is more noise robust,
eliminating noise glitches from triggering false interrupts outside of its
particular “zero cross window.”
Freescale Semiconductor, Inc.
For More Information On This Product,
7. The back-EMF selection logic is designed to provide an
Go to: www.freescale.com
Figure 7
(U7A, U7B, and
MOTOROLA
AN1857