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Programmable Logic Controller


What is a PLC?

PLC Full form is “Programmable Logic Controller.” A PLC may be a laptop specially designed to work faithfully underneath harsh industrial environments – like extreme temperatures, wet, dry, and/or dust-covered conditions. It's accustomed to automatize industrial processes like a producing plant’s mechanical system, associate degree ore dressing plant, or a waste material treatment plant.

PLCs share several options of the private pc you have got reception. They each have an influence provide, a C.P.U. (Central Process Unit), inputs and outputs (I/O), memory, and operative software package (although it’s a unique operative software). The most significant variations are that a PLC will perform separate and continuous functions that a laptop cannot do, and a PLC is far higher suited to rough industrial environments. A PLC may be thought of as a ‘ruggedized’ computer that manages the mechanical device processes of associate degree industrial setting.

PLCs play a vital role within the field of automation, mistreatment forming a part of a bigger SCADA system. A PLC may be programmed consistent with the operational demand of the method. Within the producing trade, there'll be a necessity for reprogramming thanks to the modification in production. To beat this issue, PLC-based management systems were introduced. We’ll 1st discuss PLC basics before viewing numerous applications of PLCs.


The first Programmable Logic Controllers were designed and developed by Modicon as a relay re-placer for gram and Landis.

  • These controllers eliminated the necessity for rewiring and adding extra hardware for every new configuration of logic.
  • The new system drastically raised the practicality of the controls, whereas reducing the cupboard space that housed the logic.
  • The first PLC, model 084, was fancied by Dick E. W. Morley in 1969
  • The first industrial in PLC, 184, was introduced in 1973 and was designed by Michael Joseph Greenberg.

PLC Basics

PLCs were fictitious by Dick E. W. Morley in 1964. Since then, PLC has revolutionized the economic and producing sectors. There's a broad vary of PLC functions like temporal order, counting, calculative, scrutiny, and process numerous analog signals.

The main advantage of PLC over a “hard-wired” system is that you just will return and alter a PLC once you’ve programmed it at the minimal price (only the value of the programmer’s time). In a very hard-wired system, you’re primarily having to tear out wires and begin from scratch (which is dearer and takes longer). Let’s look into the associate degree example to higher perceive this advantage.

Imagine you have got a lightweight connected to a switch. In general, the sunshine operates underneath 2 conditions – ON and OFF. Currently, you're given a task that after you activate the switch, the sun ought to glow solely once thirty seconds. With this hard-wired setup – we’re stuck. The sole thanks to delivering the goods this can be to utterly wire our circuit to feature a temporal order relay. That’s a lot of problems for a minor modification.

WHAT programing language is used TO PROGRAM A PLC?

While Ladder Logic is that the most typically used PLC programing language, it's not the sole one. The following table lists several words that are wont to program a PLC.

Ladder Diagram (LD)

 Ancient ladder logic may be a graphical programing language. At first, programmed with easy contacts that simulated the gap and shutting of relays, Ladder Logic programming has been dilated to incorporate such functions as counters, timers, shift registers, and mathematics operations.

Function diagram (FBD) - 

A graphical language for depiction signal and information flows through re-usable operate blocks. FBD is incredibly helpful for expressing the interconnection of system algorithms and logic.

Structured Text (ST) – 

A high-level text language that encourages structured programming. It's a language structure (syntax) that powerfully resembles PASCAL and supports a large variety of standard functions and operators. For example;

If Speed1 > one hundred.0 then.
    Flow_Rate: = fifty.0 + Offset_A1;
    Flow_Rate: = one hundred.0; Steam: = ON

Instruction List (IL):

 an occasional level “assembler like” language that's supported similar directions list languages found in an exceedingly big selection of today’s PLCs.

LD    0

Sequential operate Chart (SFC) 

A technique of programming advanced management systems at an additional extremely structured level. An SFC program is an outline of the system, within which the fundamental building blocks ar entire program files. Every program file is made mistreatment one in every one of the different forms of programming languages. The SFC approach coordinates massive, difficult programming tasks into smaller, additional manageable tasks.

Light Switch

This is wherever a PLC comes into the image, that doesn’t need any further wiring and hardware to create positive of a modification. Instead, it requires a straightforward change in code, programming the PLC to solely activate the sunshine thirty seconds once the switch is turned ON. So, by employing a PLC, it's straightforward to include multiple inputs and outputs. Only a direct example – a PLC can management a lot more extensive and a lot of complicated processes. A PLC may be custom-made, looking at the applying and desires of the user.

How will a PLC work?

The operating of a PLC may be simply understood as a cyclic scanning methodology referred to as the scan cycle.

Block Diagram of however A PLC Works
A PLC Scan method includes the subsequent steps
The software starts athletics and observation of your time.
The C.P.U. Starts reading the info from the input module and checks the standing of all the inputs.
The C.P.U. Starts capital punishment, the user or computer program written in relay-ladder logic or the other PLC-programming language.
Next, the C.P.U. Performs all the inner designation and communication tasks.
According to the program results, it writes the info into the output module so that all outputs are updated.
This method continues as long because the PLC is in run mode.
Physical Structure of PLC
The structure of a PLC is sort of kind of like a computer’s design.

PLC diagram

Programmable Logic Controllers unendingly monitors the input values from numerous input sensing devices (e.g., measuring equipment, weight scale, hardwired signals, etc.) and produces corresponding output looking on the character of production and trade. A typical diagram of PLC consists of 5 elements, namely:

  1. Rack or chassis
  2. Power provides Module
  3. Central process Unit (CPU)
  4. Input & Output Module
  5. Communication Interface Module

Rack or Chassis
In all PLC systems, the PLC rack or chassis forms the first necessary module and acts as a backbone to the system. PLCs are on the market in numerous shapes and sizes. Once a lot of complicated management systems are concerned, it needs more massive PLC racks. Small-sized PLC is provided with a set I/O pin configuration. So, they need to be gone for standard kind rack PLC, that accepts differing kinds of I/O modules with slippery and slot in construct. All I/O modules are residing within this rack/chassis.

Breakdown of PLC Rack

Power provides Module

This module is employed to produce the specified power to the complete PLC system. It converts on the market AC power to DC power that is needed by the C.P.U. and the theme/O module. PLC typically works on a 24V DC provide. Few PLC uses associate degree isolated power provide.

CPU Module and Memory

CPU module incorporates a processor, ROM & RAM. Storage memory includes software, driver, and computer program. RAM is employed to store programs and knowledge. C.P.U. is that the brain of PLC with associate degree positional notation or polygon microchip. Being a microprocessor-based C.P.U., it replaces timers, relays, and counters. 2 forms of processors as one bit or word processing system may be incorporated with a PLC. One bit processor is employed to perform logic functions. Whereas word processors ar used for process text, mathematical knowledge, dominant and recording experience. C.P.U. Reads the computer file from sensors, processes it, and eventually sends the command to dominant devices. DC power supply, as mentioned within the previous discussion, is needed voltage signals. C.P.U. Additionally contains alternative electrical elements to attach cables employed by alternative units.

Input and Output Module


Have you ever thought of a way to sense physical parameters like temperature, pressure, flow, etc.? mistreatment PLC? After all, PLC has an associate degree exclusive module for interfacing inputs and output, which is termed an associate degree input & output module. Input devices may be either begin and stop pushbuttons, switches, etc. and output devices may be an electrical heater, valves, relays, etc. I/O module helps to interface input and output devices with a microchip. The input module of PLC is explained within the below figure.

  1. PLC Input Module
  2. PLC input module circuit diagram
  3. Circuit Diagram of PLC Input Module
  4. The input module of PLC will four main functions.

Input module interface receives the signal from method devices at 220 V AC
Converts the signal to five V DC which will be employed by PLC
Isolator block is used to isolate/prevent PLC from undergoing fluctuation
After that, the message is shipped to the output finish, i.e., the PLC
There are 2 main sections within the input module, specifically the ability section and also the logical section. Each chapter is electrically isolated from one another. At first, the electrical switch is closed. So, 220 V AC provides is given to the bridge through the resistors R1 and R2. The bridge rectifier is employed to convert the AC signal into DC, and also a lot of zen diodes are used low voltage supply to a diode. Once the sunshine from diode falls on the phototransistor, it works within the conductivity region. Finally, a 5V DC provides is given to the processor.

The output module of PLC works kind of like the input module, however, within the reverse method. It interfaces the output load and processor. Therefore here, the primary section would be a logic session, and also, the power section comes next. The operating of the output module is shown within the below figure.

PLC Output Module

So, here once the logic high signal is generated from the processor, the diode can activate and permit the sunshine to fall on a phototransistor. Once the semiconductor goes to the conductivity region, it makes a pulse to the gate of the Triac. Isolator block is employed to isolate the logic section and management section.

Communication Interface Module

To transfer data between C.P.U. and communication networks, intelligent I/O modules are used. These communication modules facilitate to attach with alternative PLCs and computers that are placed at a foreign location.

Types of PLCs

The two primary forms of PLC ar fastened / compact PLC and standard PLC.

Compact PLC:-

Within one case, there would be several modules. It's a set variety of I/O modules and external I/O cards. So, it cannot expand the modules. Each input and output would be determined by the manufacturer.

Modular PLC:-

This type of PLC permits multiple enlargements through “modules,” thence brought up as standard PLC. I/O ports may be exaggerated. It's easier to use as a result of every element is freelance of every alternative.

Types of PLCs

A standard kind PLC
PLC ar divided into 3 sorts supported to output, correctly Relay output, semiconductor output, and Triac Output PLC. The relay output kind is best suited to each AC and DC output devices. Semiconductor output kind PLC uses switch operations and used within microprocessors.

According to the physical size, a PLC is split into mini, small, and Nano PLC.

Some of the makers of PLCs include:

  1. Allen Bradley
  2. ABB
  3. Siemens
  4. Mitsubishi PLC
  5. Hitachi PLC
  6. Delta PLC
  7. General electrical (GE) PLC
  8. Honeywell PLC

PLC Applications

PLCs have a spread of applications and use, including:
  1. Process Automation Plants (e.g., mining, oil &gas)
  2. Glass trade
  3. Paper trade
  4. Cement producing
  5. In boilers – Thermal Power Plants

PLC Programming

When employing a PLC, it’s necessary to style and implement ideas looking at your specific use case. To try and do this, we tend to 1st got to grasp a lot of concerning the specifics of PLC programming. A PLC program consists of a collection of directions, either in a matter or graphical type, that represents the logic that governs the method the PLC is dominant. There are 2 main classifications of PLC programming languages that ar any divided into several sub-classified sorts.

  1. Textual Language
  2. Instruction list
  3. Structured text
  4. Graphical type
  5. Ladder Diagrams (LD) (i.e. Ladder Logic)
  6. Function diagram (FBD)
  7. Sequential perform Chart (SFC)

Due to the natural and convenient options, the graphical illustration is far most popular to matter languages.

Ladder Logic

Ladder logic is the simplest type of PLC programming. It's additionally referred to as “relay logic.” The relay contacts employed in relay controlled systems are painted mistreatment ladder logic.
The below figure shows the natural example of a ladder diagram.

PLC Ladder Logic

In the example mentioned earlier, 2 pushbuttons are accustomed to management an equivalent lamp load. Once anybody of the switches is closed, the lamp can glow. Horizontal lines are known as rungs, and two vertical lines are known as rails. Each level forms the electrical property between Positive rail (P) and Negative rail (N). This enables this to flow between input and output devices.

Functional Block Diagrams

A functional diagram (FBD) may be an easy and graphical methodology to program multiple functions in PLC. PLCOpen has represented mistreatment FBD within the standard IEC 61131-3. A perform block may be a program instruction unit that, once dead, yields one or a lot of output values. It's painted by a neighborhood, as shown below. It's portrayed as a rectangular block with inputs coming into on left and output lines feat at the proper. It offers a relation between the state of input and output.

Functional Block


The advantage of mistreatment FBD is that any variety of inputs and outputs may be used on the purposeful block. Once mistreatment multiple input and output, you'll be able to connect the production of 1 perform block to the information of another. Whereby building a perform diagram.

Example purposeful diagram
The figure below shows numerous perform blocks employed in FBD programming.

Functional block programming
The figure below shows a ladder diagram and its perform block equivalent in Siemens notation.

Ladder to purposeful block
Ladder to persistent block [Source]
Structured Text Programming
Structured text may be a matter of programing language that utilizes statements to see what to execute. It follows a lot of standard programming protocols; however, it's not case sensitive. A series of reports (logic) is ingrained in expressing assignments and relationships mistreatment many operators. The structures text operators are listed below within the image.

Structured Text ProgrammingPLC Programming Examples

A signal lamp is needed to be switched on if a pump is running and also the pressure is satisfactory. If the light takes a look at the switch is closed. During this application, if there ought to be associate degree output from the lamp inputs from each pump and pressure sensors are needed. Hence,
AND logic is employed. OR logic is used for the take a look at input condition, it's required to convey associate degree output of the lamp. There's a symptom from the AND system. By mistreatment finish or douse instruction within the ladder diagram, we will tell PLC has reached the tip of the program. The perform diagram and also the ladder diagram ar shown below within the figure.

PLC Program to check Glowing of LampPLC Program to check Lamp Glowing

As another example, contemplate a valve that's to be operated to elevate load in the pump. This is running, and either the elevate switch is performed. A switch operated, indicating that the amount has not already been raised and is at rock bottom of its elevate channel. OR logic is employed switches associate degreed an AND logic is used with two switches and also the pump. A valve is operated, providing the pump is ON, and 2 switches are operated.

PLC Program to work Valve

Consider a drinks machine that enables the choice of tea or occasional, milk or no milk, sugar or no sugar, and can provide the specified hot drink on the insertion of a coin. From the below-shown figure, it's seen that either tea or occasional is chosen mistreatment the primary OR computer circuit. The first gate provides associate degree output once either Tea or occasional is selected, and a coin is inserted into the machine. The output from this gate is given to the second gate. The second gate operates only if quandary combines with tea. Milk and sugar are ex gratia additions which may occur once a coin has been inserted

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