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20.4.2 BASIC PWM SPEED CONTROL
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The best way to control the speed of a DC motor is to chop the power being passed to it into different sized chunks. The most common way of doing this is to set up a recurring signal; the time for power to be passed to the motor should be a set fraction of this period. This method is known as pulse width modulation or PWM motor control and its basics can be seen in Fig. 20-15. The time the power signal is active is known as the pulse width, and the duty cycle is the percentage of the pulse width as part of the PWM signal s period. One issue that you should always be aware of is that when a motor s speed is controlled using a PWM, it will vibrate at the speed of the PWM (due to the coils being turned on and off according to the speed control), which could result in an annoying hum. To avoid this, you should run your PWM either at subaudible (60 Hz or less) or superaudible (18 kHz or more) frequencies. Subaudible frequencies are best for small motors that are usually found in toys, while superaudible frequencies should be used for high-performance or efficiency motors (basically anything that wouldn t be considered in a toy).
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FIGURE 20-14 How not to vary the speed of a motor. This approach is very inefficient as the voltage drop through the transistor along with the current through the motor and transistor will cause a lot of power (heat) to be dissipated.
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20.4 MOTOR SPEED CONTROL
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FIGURE 20-15 BASIC features of a PWM signal.
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It is interesting to note that the actual power delivered to the motors follows a square law with respect to the duty cycle instead of the expected linear relationship. The reason for power to drop off as the square of the duty cycle can be seen from going back to the DC electricity power equation: Power = V i = V2/R By halving the time the voltage is applied (a 50 percent duty cycle), the actual power available to the motor is only one-quarter of that available when the duty cycle is 100 percent (or full on). As noted previously, throttling the PWM is not a linear process. When you are setting up your robot, you will have to make sure that the actual power-to-duty cycle relationship is understood as you will discover that if a linear range is used your robot will probably only be active when the PWM duty cycle is 50 percent or more. Creating a PWM signal is not as easy as you would expect. In the previous versions of this book, a PWM generator circuit was built from a relaxation oscillator, not understanding that while the pulse width would change, so would the period, resulting in a constant PWM duty cycle for a varying frequency. What is desired is a varying PWM pulse width for a constant frequency. The first demonstration circuit (Fig. 20-16 with parts list in Table 20-7) uses a 555 to allow a potentiometer to brighten or dim an LED. You will find that the circuit works reasonably well, although it is impossible to completely turn off the LED. This is due to the 555 s inability to provide less than a 50 percent duty cycle signal. In Fig. 20-17, the operating signals are shown for different potentiometer settings. The left side of the oscilloscope trace shows the LED dimmed, a 17 percent duty cycle (the time the signal is low, which is when current can flow through the LED) versus the LED at its brightest, a 49 percent duty cycle.
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WORKING WITH DC MOTORS
8 R4 100k Pot C2 0.01 uF 2
9 Volt Battery
4 LED
IC1 R1 1k
R2 10k
R3 10k 6 C1 0.1 uF 1
FIGURE 20-16 555-based PWM generator circuit, which can be used to dim an LED.
The oscilloscope pictures in Fig. 20-15 probably do not look like what you would expect for two reasons. First, the LED was turned on when the 555 output was low. This allowed the greatest dynamic duty cycle range, which provided the most visible changes in the LED. Second, the PWM pulse width remained constant while the frequency changed: this is a function of the operation of the 555 timer and must be taken into account if this chip is used as a PWM generator.
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