ControllingDevices0

ControllingDevices0 - Generating Digital Signals...

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Generating Digital Signals Introduction Are a number of different kinds of digital signals That may be required in an embedded application Applications may include 9 Control of various kinds of motors 9 Interface to some form of display 9 Control for a piece of equipment such as Printer, keyboard, CDROM, or video imager Let’s examine representative examples of several more common of these applications Motors and Motor Control Ability to control different kinds of motors Important in a host of contemporary applications Ranging from ¾ Assembly robots to ¾ Remotely controlled vehicles to ¾ Precision positioning of medical instruments Motors typically found in such applications fall into three categories DC motors Servo motors Stepper motors Let’s look at each DC Motors Accompanying figure gives a high-level diagram Basic components of a DC motor These comprise Stationary permanent magnet called a stator Movable (rotating) electromagnetic called a rotor System to connect power to the electromagnetic called brushes and a commutator Operation of the motor proceeds as follows When a voltage is applied across the electromagnetic Magnetic poles of the rotor Are attracted to the opposite poles of the stator - 1 of 21 -
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Thereby causing the rotor to turn As the rotor turns The electromagnet becomes polarized In the opposite direction Poles of the rotor are now Repelled by nearer poles Attracted to the opposite poles of the permanent magnet Causing the rotor to turn once again Observe The commutator is a split ring Against which the brushes make physical contact One portion of the commutator Connected to one end of the electromagnet Other portion Connected to the opposite end Through the action of the commutator Direction of the field in the electromagnet continually switched Causing the rotor to continue to move +V +V Commutator Electromagnet Rotor Brushes The actions of 9 Commutator 9 Brushes 9 Electromagnet Illustrated through the simple model in adjacent figure The brushes are fixed however as the rotor rotates Commutator - which is attached to the rotor Acts like a switch connecting the voltage source First one way then the opposite way across the electromagnetic Thereby changing its polarization When power is applied DC motor has the ability to continuously turn through 360 degrees In one direction If the applied voltage is held constant Speed of the motor is also held constant - 2 of 21 -
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Increasing or decreasing the applied voltage Will have a corresponding affect on the motor’s speed Using scheme called pulse width modulation PWM The average magnitude of the applied voltage Can effectively be controlled Thereby so can the motor’s speed Will learn how to do this shortly As the speed of the motor decreases so does its torque If the polarity of the applied voltage is reversed Motor will run in the opposite direction Will use a circuit called an H bridge to manage the reversal Generally a DC motor is not used for positioning tasks
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This document was uploaded on 03/29/2012.

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ControllingDevices0 - Generating Digital Signals...

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