What is PLC?

    A programmable logic controller, also called a PLC or programmable
    controller, is a computer-type device used to control equipment
    in an industrial facility. The kinds of equipment that PLCs can control
    are as varied as industrial facilities themselves. Conveyor systems,
    food processing machinery, auto assembly lines…you name it and
    there’s probably a PLC out there controlling it.


















    In a traditional industrial control system, all control devices are wired
    directly to each other according to how the system is supposed to
    operate. In a PLC system, however, the PLC replaces the wiring
    between the devices. Thus, instead of being wired directly to each
    other, all equipment is wired to the PLC. Then, the control program
    inside the PLC provides the “wiring” connection between the devices.
    The control program is the computer program stored in the PLC’s
    memory that tells the PLC what’s supposed to be going on in the
    system. The use of a PLC to provide the wiring connections between
    system devices is called softwiring.

    Let's say that a push button is supposed to control the operation of a
    motor. In a traditional control system, the push button would be wired
    directly to the motor. In a PLC system, however, both the push button
    and the motor would be wired to the PLC instead. Then, the PLCs
    control program would complete the electrical circuit between the two,
    allowing the button to control the motor.

    WHY USE PLCS?
    The softwiring advantage provided by programmable controllers is
    tremendous. In fact, it is one of the most important features of PLCs.
    Softwiring makes changes in the control system easy and cheap. If
    you want a device in a PLC system to behave differently or to control
    a different process element, all you have to do is change the control
    program. In a traditional system, making this type of change would
    involve physically changing the wiring between the devices, a costly
    and time-consuming endeavor.

    In addition to the programming flexibility we just mentioned, PLCs
    offer other advantages over traditional control systems. These advantages
    include:
    • high reliability
    • small space requirements
    • computing capabilities
    • reduced costs
    • ability to withstand harsh environments
    • expandability

    A PLC basically consists of two elements:
    • the central processing unit
    • the input/output system

    The Central Processing Unit
    The central processing unit (CPU) is the part of a programmable
    controller that retrieves, decodes, stores, and processes information.
    It also executes the control program stored in the PLC’s memory. In
    essence, the CPU is the “brains” of a programmable controller. It
    functions much the same way the CPU of a regular computer does,
    except that it uses special instructions and coding to perform its functions.
    The CPU has three parts:
    • the processor
    • the memory system
    • the power supply

    The processor is the section of the CPU that codes, decodes, and
    computes data. The memory system is the section of the CPU that
    stores both the control program and data from the equipment connected
    to the PLC. The power supply is the section that provides the
    PLC with the voltage and current it needs to operate.

    The Input/Output System
    The input/output (I/O) system is the section of a PLC to which all
    of the field devices are connected. If the CPU can be thought of as the
    brains of a PLC, then the I/O system can be thought of as the arms
    and legs. The I/O system is what actually physically carries out the
    control commands from the program stored in the PLC’s memory.
    The I/O system consists of two main parts:
    • the rack
    • I/O modules

    The rack is an enclosure with slots in it that is connected to the CPU.
    I/O modules are devices with connection terminals to which the
    field devices are wired. Together, the rack and the I/O modules form
    the interface between the field devices and the PLC. When set up
    properly, each I/O module is both securely wired to its corresponding
    field devices and securely installed in a slot in the rack. This
    creates the physical connection between the field equipment and the
    PLC. In some small PLCs, the rack and the I/O modules come prepackaged
    as one unit.

    All of the field devices connected to a PLC can be classified in one of
    two categories:
    • inputs
    • outputs
    Inputs are devices that supply a signal/data to a PLC. Typical examples
    of inputs are push buttons, switches, and measurement devices.
    Basically, an input device tells the PLC, “Hey, something’s happening
    out here…you need to check this out to see how it affects the
    control program.”
    Outputs are devices that await a signal/data from the PLC to perform
    their control functions. Lights, horns, motors, and valves are all good
    examples of output devices. These devices stay put, minding their
    own business, until the PLC says, “You need to turn on now” or
    “You’d better open up your valve a little more,” etc.
    An overhead light fixture and its corresponding wall switch are good examples
    of everyday inputs and outputs. The wall switch is an input it
    provides a signal for the light to turn on. The overhead light is an output
    Bit waits until the switch sends a signal before it turns on.
    LetÕs pretend that you have a souped-up overhead light/switch circuit
    that contains a PLC. In this situation, both the switch and the light will
    be wired to the PLC instead of to each other. Thus, when you turn on
    the switch, the switch will send its turn on signal to the PLC instead of
    to the light. The PLC will then relay this signal to the light, which will
    then turn on.
What is PLC?
What is PLC?
Learn PLC
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