Like optical encoders hall effect sensors require a

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Unformatted text preview: el 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 Hall Effect Sensor Front View FIGURE 11: Side View BACK EMF VOLTAGE MEASUREMENT VSUPPLY C1 R1 Q1 Q3 CTRL3 R3 Motor 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.

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