tailieunhanh - McGraw-Hill - The Robot Builder's Bonanza Part 9

Tham khảo tài liệu 'mcgraw-hill - the robot builder's bonanza part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 266 WORKING WITH DC MOTORS NOT THE WAY TO DO IT 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. . 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. BASIC SPEED CONTROL Figure shows a schematic that is a variation of the MOSFET circuit shown in Fig. 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. 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 .