TAILIEUCHUNG - Building Robots Part 12

196 POWER + APPENDIX B. HARDWARE POWER + Figure : The H-Bridge with Left-to-Right Current Flow To facilitate control of the H-bridge circuit, enable circuitry as depicted in Figure is typically used. In this circuit, the inverters ensure that the vertical pairs of transistors are never enabled simultaneously. The Enable input determines whether or not the whole circuit is operational. If this input is false, then none of the transistors are enabled, and the motor is free to coast to a stop. By turning on the Enable input and controlling the two Direction inputs, the motor can be made to turn in. | 196 APPENDIX B. HARDWARE Figure The H-Bridge with Left-to-Right Current Flow To facilitate control of the H-bridge circuit enable circuitry as depicted in Figure is typically used. In this circuit the inverters ensure that the vertical pairs of transistors are never enabled simultaneously. The Enable input determines whether or not the whole circuit is operational. If this input is false then none of the transistors are enabled and the motor is free to coast to a stop. By turning on the Enable input and controlling the two Direction inputs the motor can be made to turn in either direction. Note that if both direction inputs are the same state either true or false and the circuit is enabled both terminals will be brought to the same voltage Power or Power respectively . This operation will actively brake the motor due to a property of motors known as back emf in which a motor that is turning generates a voltage counter to its rotation. When both terminals of the motor are brought to the same electrical potential the back emf causes resistance to the motor s rotation. The SGS-Thomson Motor Driver Chip A company named SGS-Thomson makes a series of chip called the L293 that incorporates two H-bridge motor-driving circuits into a single 16-pin DIP package. . THE MOTOR DRIVERS 197 Figure The H-Bridge with Enable Circuitry 198 APPENDIX B. HARDWARE Block Diagram of the L293 Motor Driver Chip Enable Motor 1 Direction A Direction B Enable Motor 2 Direction A Direction B Logic Reference . _ Voltage Motor Power Motor Ground Figure The SGS-Thomson L293 Motor Driver .

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