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with one or more magnets positioned on its outer rim.
A stationary sensor detects the magnet when in passes
and generates a TTL pulse. Figure 10 shows the basic
components of a Hall effect sensor. FIGURE 10: Another form of velocity feedback for a BDC motor is
BEMF voltage measurement. BEMF voltage and
speed are proportional to one another. Figure 11 shows
the locations where BEMF voltage is measured on a
bidirectional drive circuit. A voltage divider is used to
drop the BEMF voltage into the 0-5V range so that it
can be read by an analog-to-digital converter. The
BEMF voltage is measured between PWM pulses
when one side of the motor is floating and the other is
grounded. At this instance in time the motor is acting
like a generator and produces a BEMF voltage
proportional to speed. HALL EFFECT SENSOR
Wheel Magnet Magnet
Sensor Front View FIGURE 11: Side View BACK EMF VOLTAGE MEASUREMENT
VSUPPLY C1 R1 Q1 Q3 CTRL3
BEMF CTRL2 C3 BDC R2 Q2 C2 BEMF C4 Q4 CTRL4
R4 CTRL1 All BDC motors behave slightly differently because of
differences in efficiency and materials. Experimentation is the best way to determine the BEMF voltage for
a given motor speed. A piece of reflect tape on the shaft
of the motor will allow a digital tachometer to...
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This note was uploaded on 01/29/2014 for the course AA AA taught by Professor Aa during the Winter '10 term at ENS Cachan.
- Winter '10