Programmable logic controller , PLC

                               PLC     
                                  Programmable logic controller

Introduction : 

Programmable logic controllers, also called  programmable controllers or PLCs, are solid-state members of the computer family,using integrated circuits instead of electromechanical  devices to implement control functions. They are able to storing instructions,such as sequencing, timing, counting, arithmetic, data manipulation,and communication, to control industrial machines and processes.
 
In this, an automated system a the PLC is commonly regarded  as such the heart of the control system. With a control application program (stored within the PLC memory) in the execution, the PLC constantly monitors the state of the system through the field input devices' as such feedback signal. It will then depend on the program logic to the determine a course of action to be carried out at the field output a devices.

Before the advent of solid-state logic circuits, logical control electromechanical relays.

A relay control panel is comprised of a systems were designed and built exclusively around design, but have been replaced in many of their former roles as logic level control devices, relegated most often to those applications single to thousands of relays. Relays are far from obsolete in modern demanding high current and/or high voltage switching.

Systems and processes requiring " on/off " control abound in the modern commerce and  a industry, but that control systems are rarely built from either electromechanical relays or discrete  logic gates.

Instead, digital computers fill the need, which may be programmed to do a variety of logical functions.

The PLC may be used to control a simple and repetitive task, or a few of them may be interconnected together with other host controllers or host computers through a sort of  the communication network, in order

to integrate the control of a analyzable process.

 the Programmable controllers have a many definitions.However, PLCs can be thought of in simple terms as industrial computers with the specially designed a architecture in both their central units (the PLC itself) and their a interfacing circuitry to field devices (input/output connections to the real world)

 Input Devices in PLC:

Push-buttons, keypad and toggle switches, which form the basic man-machine interface, are types of manual input device. 

On the other hand, for detection of work-piece, monitoring of moving mechanism,checking on pressure and or liquid level and many others, the PLC will have to tap the signal from the specific automatic sensing devices like , limit switch, photoelectric sensor, level sensor ,proximity switch and so on. 

as we can say that theTypes of input signal to the PLC would be of ON/OFF logic or analogue. These input signals of it ,are interfaced to PLC through the  various types of PLC input module.

Intelligence of an automated system is greatly depending on the PLC ability of  to read in the signal from the various types of a automatic sensing and the manual input field devices.

 Output Devices in PLC:

 The most commonly controlled devices are as buzzers ,motors, solenoids, indicators,relays   etc.,

an automatic system is without complete and the PLC system is almost paralyzed without means of interface to the field output devices. 

Through activation of motors and solenoids the PLC can control from a simple select and place system to a much complex servo positioning system. These type of output devices  of PLC are the mechanism of an automated system . its direct effect on the system performance.

However, the other output devices  as the pilot lamp, a buzzers and a alarms are just meant for notifying purpose. the input signal interfacing, signal from output devices are interfaced to the PLC through the wide range of PLC output module.

Characteristic Functions of a PLC:

A programmable controller is currently defined by the National Electrical Manufacturers Association (NEMA) as a digital  electronic device that
 uses a programmable memory ,to store the instructions and to

implement the specific functions  as   timing,  counting,   logic, sequence, 
 and arithmetic to control the machines and the processes.

 However, this definition is so broad as to encompass nearly every solid-state 
device used in manufacturing, from a simple timer to a  mainframe computer.
 Instead of this definition it is more useful  to examine  the most important and indispensable characteristics of a PLC that impersonate its unique aspects
 which are:

It is the field programmable by the user. This characteristic allows the user to
 write and change programs in the field without again writing(rewriting) or 
sending the unit back to the manufacturer for this purpose. 

 It moderate(contains )the programmed functions. PLCs contain at least logic, timing, counting, and memory functions that the user can access through some type of
 control-oriented programming language. 

 It scans the memory and inputs and outputs ( I/O ) in adeterministic manner.
 This critical feature allows to the control engineer to determine precisely 
how the machine or process will reply (respond )to the program. 

 It can provides a error checking and diagnostics. A PLC will periodically run internal tests of its memory,processor, and I/O systems to ensure that what it is doing to the machine or process is what it was programmed to do.  

it can be observed (monitored) . A PLC will provide some form of monitoring capability, either through indicating lights that can show the status of inputs and outputs, or
 by external device that can display program execution status.

PLC Basics:

A PLC consists of a Central Processing Unit (CPU) it containing processor,  application memory and executive memory , Input and Output Interfacing modules, which are directly connected to the field I/O devices. 

The program controls the PLC so that when the signal of  input from an input device is turned on, the allow response is made. The response commonly,involves turning on or off an output signal to some sort of output devices.

A simplified model of a PLC The input modules convert the high-level signals that come from  the field devices to logic-level signals that the PLC's processor can read directly.
 The logic solver are reads inputs and decides  what states of the output  should be, 
based on the user's program logic. 

The output modules convert the logic-level output signals from the logic solver into the high-level signals that are needed by the various field devices.
 we know,The program loader is used to enter the user's program  into
 the memory or convert (change)it and to monitor the execution of the program.  

Construction of a PLC:

Some PLCs are integrated into a single unit, whereas others are modular.  Integrated PLCs  are sometimes called  shoebox or  brick

PLCs because of their small size. If an integrated PLC is available with the capabilities that a user needs, it is usually the most economical option.  Modular PLCs  consist of optional components required for a more complex control application, as selected and assembled by the user.

The  Modular PLCs include the following components, as

1- The CPU module, containing the CPU and its memory.

2-  Input and output modules (I/O modules), to allow  the PLC to read sensors and control actuators.

3-  A power supply module, to provide power to the CPU and often to provide power to
     drive sensors   and low-power actuators connected to I/O modules.

4-  A rack or bus, so that the CPU module can exchange data with I/O modules. In some PLCs, 
     this  component isn't required because each module plugs directly  into     its neighboring module.

(A PLC system with these components is all that is needed to control an automated system. Since a PLC must be programmed before it can be used, another component is required:  

• A  programming unit is necessary to create the user-program and send it 
  to a PLC CPU module's   memory)