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There will be plenty of times when you ll want the motors in your robot to go a little slower, or perhaps track at a predefined speed. Speed control with continuous DC motors is a science in its own, and there are literally dozens of ways to do it. We ll cover some of the more popular methods in this and later chapters.
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266 WORKING WITH DC MOTORS
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Before exploring the right ways to control the speed of motors, let s examine how not to do it. Many robot experimenters first attempt to vary the speed of a motor by using a potentiometer. While this scheme certainly works, it wastes a lot of energy. Turning up the resistance of the pot decreases the speed of the motor, but it also causes excess current to flow through the pot. That current creates heat and draws off precious battery power. Another similar approach is shown in Fig. 18.11. Here, a transistor is added to the basic circuit, but again, excess current flows through the transistor, and the energy is dissipated as lost heat. There are, fortunately, far better ways of doing it. Read on.
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BASIC SPEED CONTROL
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Figure 18.12 shows a schematic that is a variation of the MOSFET circuit shown in Fig. 18.8, above. This circuit provides rudimentary speed control. The 4011 NAND gate acts as an astable multivibrator, a pulse generator. By varying the value of R3, you increase or decrease the duration of the pulses emitted by the gates of the 4011. The longer the duration of the pulses, the faster the motor because it is getting full power for a longer period of time. The shorter the duration of the pulses, the slower the motor. Notice that the power or voltage delivered to the motor does not change, as it does with the pot-only or pot-transistor scheme described earlier. The only thing that changes is the amount of time the motor is provided with full power. Incidentally, this technique is called duty cycle or pulse width modulation (PWM), and is the basis of most popular motor speed control circuits. There are a number of ways of providing PWM; this is just one of dozens. Fig. 18.13 shows a timing diagram of the PWM technique, from 100 percent duty cycle (100 percent on) to 0 percent duty cycle (0 percent on). It is important to note that the frequency of the pulses does not change, just the relative on/off times. PWM frequencies of 2 kHz to over 25 kHz are commonly employed, depending on the motor. Unless you have a specification sheet from the manufacturer of the motor, you may have to do some experimentation to arrive at the ideal pulse frequency to use. You want to select the frequency that offers maximum power with minimum current draw.
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FIGURE 18.11 How not to vary the speed of a motor. This approach is very inefficient.
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MOTOR SPEED CONTROL 267
+12V +12V 14 1 2 7 4011 (1/4) g 3 s d Q1 D1 C1 0.1 D3 d Q3 s g
Direction control Forward 1 Reverse 0
C2 5 6 4011 (1/4) 4 d g s Q2 D2 D4 Q4 s d g
Motor control On 1 Off 0
4011 (1/4) 10 A
12 13 0.1
4011 (1/4) g 11
d Q5 s
R1 330 LED1
R2 1M
C1 0.033 R3 100K Speed control
Q1-Q5 N-channel power MOSFET (IRF-630 or equiv.)
FIGURE 18.12 A rudimentary speed and direction control circuit using power MOSFETs. Resistor R1 and the LED serve to indicate that the motor is on.
Excessively high PWM frequencies may negate the speed control aspect, whereas excessively low frequencies may cause significant current draw and motor heating. In the circuit shown in Fig. 18.12, R3 is shown surrounded by a dotted box. You can substitute R3 with a fixed resistor if you want to always use a certain speed, or you can use the circuit shown in Fig. 18.14. This circuit employs a 4066 CMOS analog switch IC. The 4066 allows you to select any of up to four speeds by computer or electronic control. You connect resistors of various values to one side of the switches; the other side of the switches are collectively connected to the 4011. To modify the speed of the motor, activate one of the switches by bringing its control input to HIGH. The resistor connected to that switch is then brought into the circuit. You can omit the 3.3K pull-down resistors on the control inputs if your control circuitry is always activated and connected. The 4066 is just one of several CMOS analog switches. There are other versions of this IC with different features and capabilities. We chose the 4066 here because it adds very
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