Programmable logic controller-plc


What is Programmable logic controller (PLC)
A programmable logic controller (PLC) is a special data processor used as controller for machines in industrial processes. As a part of process control, a programmable logic controller is used to monitor input signals from a variety of input points which report events and conditions occurring in a controlled process. During the execution of a stored control program, they read inputs from the controlled process and, per the logic of the control program, provide outputs to the controlled process. A PLC typically consists of an input section, a logic (or processing) section, an output section, and a power supply. A PLC is typically programmed with a sequential program for controlling a machine, such as a pressing or marking machine that continuously repeats the same motions during, for example, an automated assembly process or other manufacturing process. Programmable logic controllers can be programmed using different development platforms. One common development platform is called ladder logic. Ladder logic is a programming language based on executing commands on a line-by-line permission system. Each ladder logic program comprises one or more ladder logic statements. These ladder logic statements are often termed “rungs.” Each ladder logic statement defines the relationship between an output variable and one or more input variables. Programmable logic controllers are used throughout industry to control and monitor a wide range of machines and other movable components and systems. A PLC can monitor such input conditions as motor speed, temperature, pressure, volumetric flow and the like.

Re: Programmable logic controller (PLC)

My comments
A programmable logic controller, PLC or programmable controller is a small computer used for automation of real-world processes, such as control of machinery on factory assembly lines. Where older automated systems would use hundreds or thousands of relays and cam timers, a single PLC can be programmed as a replacement.

The PLC is a microprocessor based device with either modular or integral input/output circuitry that monitors the status of field connected “sensor” inputs and controls the attached output “actuators” (motor starters, solenoids, pilot lights/displays, speed drives, valves, etc.) according to a user-created, logic program stored in the microprocessor’s battery-backed RAM memory. The functionality of the PLC has evolved over the years to include capabilities beyond typical relay control; sophisticated motion control, process control, Distributed Control System and complex networking have now been added to the PLC’s list of functions.

EXAMPLES
A simple program could maintain the level of water in a tank between two float switches by opening and closing an electric valve. A slightly more complex arrangement could involve a scale under the tank (as an input) and a flow controller (as an output) allowing water to flow at a controlled rate. A typical industrial application might control several tanks in a process such as sewage treatment. Each tank might be watched for a variety of conditions such as being too full or too empty or having the wrong pH.



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