14-02-2013, 11:30 AM
PROGRAMMABLE LOGIC CONTROLLER
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Introduction to PLC
A digitally operating electronic apparatus which uses a programming memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic to control through digital or analog modules, various types of machines or process.
History and Origin
Developed to replace relays in the late 1960s
PLC began in the 1970s, and has become the most common choice for manufacturing controls.
The PLC was invented in response to the needs of the American automotive manufacturing industry (primarily General motors).
Costs dropped and became popular by 1980s
Now used in many industrial designs
I/O Module
The I/O interface section of a PLC connects it to
external field devices.
The main purpose of the I/O interface is to condition the various signals received from or sent to the external input and output devices.
Input modules converts signals from discrete or analog input devices to logic levels acceptable to PLC’s processor.
Output modules converts signal from the processor to levels capable of driving the connected discrete or analog output devices.
PROCESSOR
The processor module contains the PLC’s microprocessor, its supporting circuitry, and its memory system.
The main function of the microprocessor is to analyze data coming from field sensors through input modules, make decisions based on the user’s defined control program and return signal back through output modules to the field devices. Field sensors: switches, flow, level, pressure, temp. transmitters, etc. Field output devices: motors, valves, solenoids, lamps, or audible devices.
PLC Operations
Basic Function of a Typical PLC
Read all field input devices via the input interfaces, execute the user program stored in application memory, then, based on whatever control scheme has been programmed by the user, turn the field output devices on or off, or perform whatever control is necessary for the process application.
This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning.
Advantages of PLC’s
Less wiring.
Wiring between devices and relay contacts are done in the PLC program.
Easier and faster to make changes.
Trouble shooting aids make programming easier and reduce downtime.
Reliable components make these likely to operate for years before failure.