vb.net barcode reader source code FIGURE 20-13 The Allegro 3952 is one of several all-in-one H-bridge motor control ICs. in Software

Painting QR Code JIS X 0510 in Software FIGURE 20-13 The Allegro 3952 is one of several all-in-one H-bridge motor control ICs.

FIGURE 20-13 The Allegro 3952 is one of several all-in-one H-bridge motor control ICs.
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20.4 MOTOR SPEED CONTROL
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The better motor control bridges incorporate overcurrent protection circuitry, which prevents them from being damaged if the motor pulls too much current and overheats the chip. Some even provide for current sense, an output that can be fed back to the control electronics in order to monitor the amount of current being drawn from the motor. This can be useful when you need to determine if the robot has become stuck and the motors have stalled. Recall from earlier in this chapter that DC motors will draw the most current when they are stalled. If the robot gets caught on something and can t budge, the motors will stop, and the current draw will increase. Some of the available motor control bridges include the L293D and L298N from SGSThomson; the 754410, an improved version of the L293 from Texas Instruments; and the LM18293 from National Semiconductor. Be sure to also check the listings in Appendix B, Sources, and Appendix C, Robot Information on the Internet, for additional sources of information on motor control bridges.
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20.3.5 RELAY VERSUS BIPOLAR VERSUS FET MOTOR DRIVERS
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When making the decision on what type of motor driver to use in a robot, you should consider the advantages of the different types that have been described so far. Some of the comments may be surprising and will hopefully give you some things to think about when you are designing your own robot. Relay motor control is probably the least efficient and least reliable. Relays are quite costly and large (especially compared to power bipolar and MOSFET transistors) and will fail after a certain amount of time. To make it easier for you, remember to always socket your relays and provide easy access in your robot so that they can be replaced easily. Where relays have an advantage over the transistor motor controllers/drivers is their low motor power loss and voltage drop. It may seem to be a paradox, but relays are often the most effective method of control in small robots that run from radio battery power supplies. Bipolar transistors are probably the cheapest solution but they can be very inefficient in terms of power usage. Remember that there is a definite voltage drop through the transistor, which translates to a power loss (which is the product of the current flowing through the motor and the voltage drop), which can be very significant in low-voltage situations. MOSFET transistors tend to be the most efficient type of motor driver in terms of power loss, but they can be fairly expensive depending on the transistors that are chosen for the application. They must always be handled carefully to ensure they are not damaged by ESD. Despite the limitations, the high efficiency of MOSFETs makes them the preferred motor driver for DC motors.
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20.4 Motor Speed Control
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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. Some of the more popular methods will be covered in this and later chapters.
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WORKING WITH DC MOTORS
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20.4.1 NOT THE WAY TO DO IT
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Before exploring the right ways to control the speed of motors, it is important to understand 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. 20-14. Here, the potentiometer controls the base current to a bipolar transistor controlling the operation of the motor. Like the previous case, 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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