vb.net barcode reader source code The Fundamentals of DC Motors in Software

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20.1 The Fundamentals of DC Motors
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There are many ways to build a DC motor. By their nature, all DC motors are powered by direct current hence the name DC rather than the alternating current (AC) used by most motorized household appliances. By and large, AC motors are less expensive to manufacture than DC motors, and because their construction is simpler they tend to last longer than DC motors. The most common form of DC motor is the permanent magnet type, so-called because it uses two or more permanent magnet pole pieces (called the stator). The turning shaft of
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WORKING WITH DC MOTORS
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Electromagnet on Armature Rear Bearing
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Permanent Magnet South Pole Motor Brush and Power Connection
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Front Bearing
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Driveshaft
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Commutator
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Motor Brush and Power Connection
Permanent Magnet North Pole
FIGURE 20-1 The basic parts of a DC motor.
the motor, or the rotor, is composed of windings that are connected to a mechanical commutator. Internally, metal brushes (which can wear out!) supply the contact point for the current that turns the motor. Fig. 20-1 provides a side view of the motor while Fig. 20-2 shows the different parts of a motor that has been taken apart. There are normally three sets of windings to a DC motor as illustrated in Fig. 20-3. The brushes can only come into contact with two of the windings commutators, resulting in two rotors being magnetized at any given time. These magnetized rotors create an unbalanced force in the DC motor, causing them to be pulled toward their respective permanent magnet poles, and torque on the motor shaft. The placement of the commutators and the brushes causes the motor windings to change polarity as the rotor turns (again shown in Fig. 20-3). Other types of DC motors exist as well, including the series wound (or universal) and the shunt wound DC motors. These differ from the permanent magnet motor in that no mag-
Motor Housing with Front Bearing
Permanent Magnets
Motor Armature
End Cap with End Bearing
Driveshaft
Motor Coils
Commutator
Brushes
FIGURE 20-2 A disassembled DC motor, showing its basic parts.
20.2 REVIEWING DC MOTOR RATINGS
60 Degrees Later Position Initial Position with Forces with Changes in Electromagnet Indicated Forces Indicated
FIGURE 20-3 Two views of the three winding DC motor shaft at different positions. As the shaft turns, the polarity of the different windings change due to the changing position of the brushes relative to the commutators.
nets are used. Instead, the stator is composed of windings that, when supplied with current, become electromagnets. One of the prime benefits of most, but not all, DC motors is that they are inherently reversible. Apply current in one direction (the and on the battery terminals, for example), and the motor may spin clockwise. Apply current in the other direction, and the motor spins counterclockwise. This capability makes DC motors well suited for robotics, where it is often desirable to have the motors reverse direction, such as to back a robot away from an obstacle or to raise or lower a mechanical arm. Not all DC motors are reversible, and those that are typically exhibit better performance (though often just slightly better) in one direction over the other. For example, the motor may turn a few revolutions per minute faster in one direction. Normally, this is not observable in the typical motor application, but robotics isn t typical. In a robot with the common two-motor drive (see 18, Principles of Robot Locomotion ), the motors will be facing opposite directions, so one will turn clockwise while the other turns counterclockwise. If one motor is slightly faster than the other, it can cause the robot to steer off course. Fortunately, this effect isn t usually seen when the robot just travels short distances, and in any case, it can often be corrected by the control circuitry used in the robot.
20.2 Reviewing DC Motor Ratings
Motor ratings, such as voltage and current, were introduced in 19, Choosing the Right Motor for the Job. Here are some things to keep in mind when considering a DC motor for your robot:
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