vb.net barcode reader source code Copyright 2006, 2001, 1987 by The McGraw-Hill Companies, Inc. Click here for terms of use. in Software

Maker QR-Code in Software Copyright 2006, 2001, 1987 by The McGraw-Hill Companies, Inc. Click here for terms of use.

Copyright 2006, 2001, 1987 by The McGraw-Hill Companies, Inc. Click here for terms of use.
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WORKING WITH SERVO MOTORS
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control a servo with your PC, a microcontroller such as the BASIC Stamp, or even a simple circuit designed around the familiar 555 timer integrated circuit. In this chapter R/C servos will be presented along with how they can be put to use in a robot. While there are other types of servo motors, it is the R/C type that is commonly available and reasonably affordable. For simplicity s sake, when you see the term servo in the text that follows understand that it specifically means an R/C servo motor, even though there are other types.
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22.1 How Servos Work
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Fig. 22-1 shows a typical standard-sized R/C servo motor, which is used with flyable model airplanes and model racing cars. The size and mounting of a standard servo is the same regardless of the manufacturer, which means that you have your pick of a variety of makers. Along with the standard-sized servor, there are other common sizes of servo motors also available, which will be discussed later in the chapter. Inside the servo is a motor, a series of gears to reduce the speed of the motor, a control board, and a potentiometer (see Fig. 22-2). The motor and potentiometer are connected to the control board, all three of which form a closed feedback loop. Both control board and motor are powered by a constant DC voltage (usually between 4.8 and 6.0 V, although many will work with power inputs up to 7.2 V). To turn the motor, a digital signal is sent to the control board. This activates the motor,
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FIGURE 22-1 The typical radio-controlled (R/C) servo motor.
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22.2 SERVOS AND PULSE WIDTH MODULATION
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Output Control Arm Gears Position Sensor Potentiometer Control Signal
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Pulse Width to Voltage Converter
Motor Driver
FIGURE 22-2 The internal parts of an R/C servo. The servo consists of a motor, a gear train, a potentiometer, and some control circuit.
which, through a series of gears, is connected to the potentiometer. The position of the potentiometer s shaft indicates the position of the output shaft of the servo. When the potentiometer has reached the desired position, the control board shuts down the motor. As you can surmise, servo motors are designed for limited rotation rather than for continuous rotation like a DC or stepper motor. While it is possible to modify an R/C servo to rotate continuously (as discussed later in this chapter), the primary use of the R/C servo is to achieve accurate rotational positioning over a range of 90 or 180 . While this may not sound like much, in actuality such control can be used to steer a robot, move legs up and down, rotate a sensor to scan the room, and more. The precise angular rotation of a servo in response to a specific digital signal has enormous uses in all fields of robotics.
22.2 Servos and Pulse Width Modulation
The motor shaft of an R/C servo is positioned by using pulse width modulation (PWM). In this system, the servo responds to the duration of a steady stream of digital pulses (Fig. 22-3). Specifically, the control board responds to a digital signal whose pulses vary from about 1 ms (one-thousandth of a second) to about 2 ms. These pulses are sent some 50 times per second. The exact length of the pulse, in fractions of a millisecond, determines the position of the servo. Some servos are very tolerant of varying PWM periods, while others will not work properly or jitter if the pulses come at anything other than 50 times per second. To be on the safe side, always try to maintain 20 ms between the start servo control pulses. At a duration of 1 ms, the servo is commanded to turn all the way in one direction (for
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20 ms 1 ms 2 ms
FIGURE 22-3 A pulse, every 20 ms, varying from 1 ms in duration to 2 ms in duration controls the position of the servo.
example counterclockwise). At 2 ms, the servo is commanded to turn all the way in the other direction. Therefore, at 1.5 ms, the servo is commanded to turn to its center (or neutral) position. As mentioned earlier, the angular position of the servo is determined by the width (more precisely, the duration) of the pulse. This technique has gone by many names over the years. One you may have heard of is digital proportional the movement of the servo is proportional to the digital signal being fed into it. The power delivered to the motor inside the servo is also proportional to the difference between where the output shaft is and where it s supposed to be. If the servo has only a little way to move to its new location, then the motor is driven at a fairly low speed. This ensures that the motor doesn t overshoot its intended position. But if the servo has a long way to move to its new location, then it s driven at full speed in order to get there as fast as possible. As the output of the servo approaches its desired new position, the motor slows down. What seems like a complicated process actually happens in a very short period of time the average servo can rotate a full 60 in a quarter to half second.
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