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Servos and Pulse Width Modulation
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The motor shaft of an R/C servo is positioned by using a technique called pulse width modulation (PWM). In this system, the servo responds to the duration of a steady stream of digital pulses. Specifically, the control board responds to a digital signal whose pulses vary from about 1 millisecond (one thousandth of a second, or ms) 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. Note that it is not the number of pulses per second that controls the servo, but the duration of the pulses that matters. The servo requires about 30 to 60 of these pulses per second. This is referred to as the refresh rate; if the refresh rate is too low, the accuracy and holding power of the servo is reduced.
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SERVOS AND PULSE WIDTH MODULATION
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FIGURE 20.1 The typical radio-controlled (R/C) servo motor.
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FIGURE 20.2 The internals of an R/C servo. The servo consists of a motor, a gear train, a potentiometer, and a control circuit.
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At a duration of 1 ms, the servo is commanded to turn all the way in one direction (let s say counterclockwise, as shown in Fig. 20.3). 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 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 it 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. The actual length of the pulses varies between servo brands, and sometimes even between different models by the same manufacturer. The 1 2 ms range is typical but is by no means set in stone. When you are purchasing a servo brand for a model airplane or car, you should typically mate it with a radio receiver made by the same company to ensure compatibility. Since you re not likely to use a radio receiver with the R/C servos in your robot, you ll need to do some experimenting to find the optimum pulse width ranges for the servos you use. This is just part of what makes robot experimenting so fun!
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Length of pulse changes to control position of servo
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1.0 ms Same period for all signals
Servo position
FIGURE 20.3 Pulse width modulation is used to control the position of the output shaft of the servo motor.
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The Role of the Potentiometer
The potentiometer of the servo plays a key role in allowing the motor to set the position of its output shaft. The potentiometer is physically attached to the output shaft (and in some servo models, the potentiometer is the output shaft). In this way, the position of the potentiometer very accurately reflects the position of the output shaft of the servo. Recall that a potentiometer works by providing a varying voltage to a control circuit, as shown in Fig. 20.4. As the wiper inside the potentiometer moves, the voltage changes. The control circuit in the servo correlates this voltage with the timing of the incoming digital pulses and generates an error signal if the voltage is wrong. This error signal is proportional to the difference between the position of the potentiometer and the timing of the incoming signal. To compensate, the control board applies the error signal to turn the motor. When the voltage from the potentiometer and the timing of the digital pulses match, the error signal is removed, and the motor stops.
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