Programmable Logic Controllers (PLCs): Basics, Types & Applications

What is PLC

What is a PLC?

PLC stands for “Programmable Logic Controller”. A PLC is a computer specially designed to operate reliably under harsh industrial environments – such as extreme temperatures, wet, dry, and/or dusty conditions. It is used to automate industrial processes such as a manufacturing plant’s assembly line, an ore processing plant, or a wastewater treatment plant.

PLCs share many features of the personal computer you have at home. They both have a power supply, a CPU (Central Processing Unit), inputs and outputs (I/O), memory, and operating software (although it’s a different operating software). The biggest differences are that a PLC can perform discrete and continuous functions which a PC cannot do, and a PLC is much better suited to rough industrial environments. A PLC can be thought of as a ‘ruggedized’ digital computer which manages the electromechanical processes of an industrial environment.

PLCs plays a crucial role in the field of automation, using forming part of a larger SCADA system. A PLC can be programmed according to the operational requirement of the process. In the manufacturing industry, there will be a need for reprogramming due to the change in the nature of production. To overcome this difficulty, PLC based control systems were introduced. We’ll first discuss PLC basics before looking at various applications of PLCs.

PLC Basics

PLCs were invented by Dick Morley in 1964. Since then PLC has revolutionized the industrial and manufacturing sectors. There is a wide range of PLC functions like timing, counting, calculating, comparing and processing of various analog signals.

The main advantage of PLC over a “hard-wired” control system is that you can go back and change a PLC after you’ve programmed it, at little cost (just the cost of the programmer’s time). In a hard-wired control system, you’re essentially having to rip out wires and start from scratch (which is more expensive and takes longer). Let’s look at an example to better understand this advantage.

Imagine you have a light connected to a switch. In general, the light operates under two conditions – ON and OFF. Now you are given a task that when you turn ON the switch, the light should glow only after 30 seconds. With this hard-wired setup – we’re stuck. The only way to achieve this is to completely rewire our circuit to add a timing relay. That’s a lot of hassle for a minor change.

Light Switch
Light Switch

This is where a PLC comes into the picture, which doesn’t require any additional wiring and hardware to make sure a change. Rather it requires a simply change in code, programming the PLC to only turn on the light 30 seconds after the switch is turned ON. So, by using a PLC, it is easy to incorporate multiple inputs and outputs. This is just a simple example – a PLC has the ability to control much larger and more complex processes. A PLC can be customized depending on the application and needs of the user.

Light operated by a PLC
Light operated by a PLC

How Does a PLC work?

The working of a PLC can be easily understood as a cyclic scanning method known as scan cycle.

Block Diagram of How A PLC Works
Block Diagram of How A PLC Works

A PLC Scan Process includes the following steps

  • The operating system starts cycling and monitoring of time.
  • The CPU starts reading the data from the input module and checks the status of all the inputs.
  • The CPU starts executing the user or application program written in relay-ladder logic or any other PLC-programming language.
  • Next, the CPU performs all the internal diagnosis and communication tasks.
  • According to the program results, it writes the data into the output module so that all outputs are updated.
  • This process continues as long as the PLC is in run mode.

Physical Structure of PLC

The structure of a PLC is almost similar to a computer’s architecture.

PLC block diagram
PLC Block Diagram

Programmable Logic Controllers continuously monitors the input values from various input sensing devices (e.g. accelerometer, weight scale, hardwired signals, etc.) and produces corresponding output depending on the nature of production and industry. A typical block diagram of PLC consists of five parts namely:

  • Rack or chassis
  • Power Supply Module
  • Central Processing Unit (CPU)
  • Input & Output Module
  • Communication Interface Module

Rack or Chassis

In all PLC systems, the PLC rack or chassis forms the most important module and acts as a backbone to the system. PLCs are available in different shapes and size. When more complex control systems are involved, it requires larger PLC racks. Small sized PLC is equipped with fixed I/O pin configuration. So, they have gone for modular type rack PLC, which accepts different types of I/O modules with sliding and fit in concept. All I/O modules will be residing inside this rack/chassis.

Breakdown of PLC Rack
Breakdown of PLC Rack

Power Supply Module

This module is used to provide the required power to the whole PLC system. It converts the available AC power to DC power which is required by the CPU and I/O module. PLC generally works on 24V DC supply. Few PLC uses an isolated power supply.

CPU Module and Memory

CPU module has a central processor, ROM & RAM memory. ROM memory includes operating system, driver and application program. RAM memory is used to store programs and data. CPU is the brain of PLC with an octal or hexagonal microprocessor. Being a microprocessor based CPU, it replaces timers, relays, and counters. Two types of processors as a single bit or word processor can be incorporated with a PLC. One bit processor is used to perform logic functions. Whereas word processors are used for processing text, numerical data, controlling and recording data. CPU reads the input data from sensors, process it and finally sends the command to controlling devices. DC power source, as mentioned in the previous discussion is required voltage signals. CPU also contains other electrical parts to connect cables used by other units.

Input and Output Module

Have you ever thought about how to sense physical parameters like temperature, pressure, flow, etc? using PLC? Of course, PLC has an exclusive module for interfacing inputs and output, which is called an input & output module. Input devices can be either start and stop push buttons, switches, etc and output devices can be an electric heater, valves, relays, etc. I/O module helps to interface input and output devices with a microprocessor. The input module of PLC is explained in the below figure.

PLC Input Module
PLC Input Module
PLC input module circuit diagram
Circuit Diagram of PLC Input Module

The input module of PLC does four main functions.

  1. Input module interface receives the signal from process devices at 220 V AC
  2. Converts the input signal to 5 V DC that can be used by PLC
  3. Isolator block is used to isolate/prevent PLC from undergoing fluctuation
  4. After which the signal is sent to the output end i.e the PLC

There are two main sections in the input module namely power section and logical section. Both the sections are electrically isolated from each other. Initially push button is closed. So, 220 V AC supply is given to the bridge circuit through the resistors R1 and R2. The bridge rectifier is used to convert the AC signal into DC and zener diode is used to provide low voltage supply to LED. When the light from LED falls on the phototransistor, it works in the conduction region. Finally, 5V DC supply is given to the processor.

The output module of PLC works similar to the input module but in the reverse process. It interfaces the output load and processor. So here the first section would be logic session and power section comes next. The working of the output module is shown in the below figure

PLC Output Module
PLC Output Module

So, here when the logic high signal is generated from the processor, LED will turn ON and allow the light to fall on a phototransistor. When the transistor goes to conduction region, it generates a pulse to the gate of the triac. Isolator block is used to isolate the logic section and control section.

Communication Interface Module

To transfer information between CPU and communication network, intelligent I/O modules are used. These communication modules help to connect with other PLCs and computers which are placed at a remote location.

Types of PLCs

The two main types of PLC are fixed / compact PLC and modular PLC.

Compact PLC

Within a single case, there would be many modules. It has a fixed number of I/O modules and external I/O cards. So, it does not have the capability to expand the modules. Every input and output would be decided by the manufacturer.

Modular PLC

This type of PLC permits multiple expansion through “modules”, hence referred to as Modular PLC. I/O components can be increased. It is easier to use because each component is independent of each other.

Types of PLCs
A Modular Type PLC

PLC are divided into three types based on output namely Relay output, Transistor output, and Triac Output PLC. The relay output type is best suited for both AC and DC output devices. Transistor output type PLC uses switching operations and used inside microprocessors.

According to the physical size, a PLC is divided into Mini, Micro and Nano PLC.

Some of the manufacturers of PLCs include:

PLC Applications

PLCs have a variety of applications and uses, including:

  1. Process Automation Plants (e.g. mining, oil &gas)
  2. Glass Industry
  3. Paper Industry
  4. Cement Manufacturing
  5. In boilers – Thermal Power Plants

PLC Programming

When using a PLC, it’s important to design and implement concepts depending on your particular use case. To do this we first need to know more about the specifics of PLC programming. A PLC program consists of a set of instructions either in textual or graphical form, which represents the logic that governs the process the PLC is controlling. There are two main classifications of PLC programming languages, which is further divided into many sub-classified types.

  1. Textual Language
    • Instruction list
    • Structured text
  2. Graphical Form
    • Ladder Diagrams (LD) (i.e. Ladder Logic)
    • Function Block Diagram (FBD)
    • Sequential Function Chart (SFC)

Due to the simple and convenient features, graphical representation are much preferred to textual languages.

Ladder Logic

Ladder logic is the simplest form of PLC programming. It is also known as “relay logic”. The relay contacts used in relay controlled systems are represented using ladder logic.
The below figure shows the simple example of a ladder diagram.

Ladder Logic
PLC Ladder Logic

In the above-mentioned example, two push buttons are used to control the same lamp load. When any one of the switches is closed, the lamp will glow. The two horizontal lines are called rungs and two vertical lines are called rails. Every rung forms the electrical connectivity between Positive rail (P) and Negative rail (N). This allows the current to flow between input and output devices.

Functional Block Diagrams

Functional Block Diagram (FBD) is a simple and graphical method to program multiple functions in PLC. PLCOpen has described using FBD in the standard IEC 61131-3. A function block is a program instruction unit which, when executed, yields one or more output values. It is represented by a block as shown below. It is represented as a rectangular block with inputs entering on left and output lines leaving at the right. It gives a relation between the state of input and output

Functional Block
Function Block

The advantage of using FBD is that any number of inputs and outputs can be used on the functional block. When using multiple input and output, you can connect the output of one function block to the input of another. Whereby building a Function Block Diagram.

Example Functional Block Diagram
Example Functional Block Diagram

The figure below shows various function blocks used in FBD programming.

Functional block programming
Functional Block Programming

The figure below shows a ladder diagram and its function block equivalent in Siemens notation.

Ladder to functional block
Ladder to functional block [Source]
Ladder to Functional Block
Ladder to functional block diagram [Source]

Structured Text Programming

Structured text is a textual programming language that utilizes statements to determine what to execute. It follows more conventional programming protocols but it is not case sensitive. A series of statements (logic) is constituted of expressing assignments and relationships using several operators. The structures text operators are listed below in the image.

Structured Text Programming
Structured Text Programming

PLC Programming Examples

A signal lamp is required to be switched on if a pump is running and the pressure is satisfactory, or if the lamp test switch is closed. In this application, if there should be an output from the lamp inputs from both pump and pressure sensors are required. Hence,
AND logic is used. OR logic is used for the test input condition, it is required to give an output of lamp on regardless of whether there is a signal from the AND system. By using END or RET instruction in the ladder diagram, we can tell PLC has reached the end of the program. The function block diagram and the ladder diagram are shown below in the figure.

PLC Program to Test Glowing of Lamp
PLC Program to Test Lamp Glowing

As another example, consider a valve which is to be operated to lift a load when a pump is running and either the lift switch is operated or a switch operated indicating that the load has not already been lifted and is at the bottom of its lift channel. OR logic is used for two switches and an AND logic is used with two switches and the pump. Valve will be operated only if the pump is ON and two switches are operated.

PLC Program to Operate Valve
PLC Program to Operate Valve

Consider a drinks machine that allows the selection of tea or coffee, milk or no milk, sugar or no sugar, and will supply the required hot drink on the insertion of a coin. From the below-shown figure, it is seen that either tea or coffee is selected using the first OR logic gate. The first AND gate give an output when either Tea or coffee is selected and a coin is inserted into the machine. The output from this AND gate is given to second AND gate. The second AND gate operate only when hot water combines with tea. Milk and sugar are optional additions which can occur after a coin has been inserted.

Functional Block Diagram of Drinking Machine
FBD for Drinking Machine
Ladder Logic for Drinking Machine
Ladder Logic for Drinking Machine Application

About Vidya Muthukrishnan

Ms M.Vidya has been teaching professionally for the past 5 years, and is currently employed as an Assistant Professor in the Department of Instrumentation and Control Engineering at the Sri Krishna College of Technology. She has completed her B.Tech Electronics and Instrumentation from SASTRA University and M.Tech in Biomedical Engineering from VIT University Vellore.