How Does a PLC Work?
The working of a programmable logic controller can be easily understood as a cyclic scanning method known as the scan cycle.
A PLC Scan Process includes the following steps
- The operating system starts cycling and monitoring 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.
Programmable Logic Controllers continuously monitor the input values from various input sensing devices (e.g. accelerometer, weight scale, hardwired signals, etc.) and produce 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 the backbone. PLCs are available in different shapes and sizes. When more complex control systems are involved, larger PLC racks are required.
A small PLC is equipped with a fixed I/O pin configuration. So, they have gone for a modular-type rack PLC, which accepts different types of I/O modules with sliding and fit-in concepts. All I/O modules will reside inside this rack/chassis.
Power Supply Module
This module provides 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. PLCs generally work on a 24V DC supply, but few use an isolated power supply.
CPU Module and Memory
CPU module has a central processor, ROM & RAM. ROM memory includes an operating system, drivers, and application programs. RAM 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 a single bit or word processor, can be incorporated with a PLC. A bit processor is used to perform logic functions. Word processors are used for processing text and numerical data, as well as controlling and recording data.
The CPU reads the input data from sensors, processes it, and sends the command to controlling devices. As mentioned in the previous discussion, a DC power source requires voltage signals. The CPU also contains other electrical parts to connect cables used by other units.
Input and Output Module
The input and output modules of a PLC are crucial for sensing physical parameters such as temperature, pressure, and flow, allowing it to interact with various industrial processes.
Input devices can be either start and stop pushbuttons, switches, etc, and output devices can be electric heaters, valves, relays, etc. The I/O module helps to interface input and output devices with a microprocessor. The input module of PLC is explained in the below figure.
The input module of PLC has four main functions.
- The input module interface receives the signal from process devices at 220 V AC
- Converts the input signal to 5 V DC that can be used by PLC
- Isolator block is used to isolate/prevent PLC from undergoing fluctuation
- After which the signal is sent to the output end i.e. the PLC
There are two main sections in the input module, namely the power section and the logical section. Both sections are electrically isolated from each other. Initially, the push button is closed. So, 220 V AC supply is given to the bridge circuit through the resistors R1 and R2.
A bridge rectifier (such as a diode bridge rectifier) is used to convert the AC signal into DC and Zener diode is used to provide a low voltage supply to LED. When the light from the LED falls on the phototransistor, it works in the conduction region. Finally, a 5V DC supply is given to the processor.
The output module of PLC works similarly to the input module but in the reverse process. It interfaces the output load and processor. So here, the first section would be the logic session, and the power section comes next. The working of the output module is shown in the figure below
So, when the program logic high signal is generated from the processor, the LED will turn ON and allow the light to fall on a phototransistor. When the transistor goes to the conduction region, it generates a pulse to the gate of the Triac. The isolator block is used to isolate the logic section and control section.
Communication Interface Module
Intelligent I/O modules are used to transfer information between CPU and communication networks. 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 cannot expand the modules. The manufacturer will decide on every input and output.
Modular PLC
This type of PLC permits multiple expansions 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.
PLCs are divided into three types based on output, namely Relay output, Transistor output, and Triac Output PLC. The relay output type best suits AC and DC output devices. Transistor output type PLC uses switching operations and is 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:
- Allen Bradley
- ABB
- Siemens
- Mitsubishi PLC
- Hitachi PLC
- Delta PLC
- General Electric (GE) PLC
- Honeywell PLC
Each of them has its own specific features, strengths, and compatible software for programming and managing their PLC systems.
PLC Applications
PLCs have a variety of applications and uses, including:
- Process Automation Plants (e.g. mining, oil &gas)
- Glass Industry
- Paper Industry
- Cement Manufacturing
- In boilers – Thermal Power Plants
PLC Programming
When using a PLC, it’s important to design and implement concepts based on your particular use case. To do this, we first need to learn 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 are further divided into many sub-classified types.
- Textual Language
- Instruction list
- Structured text
- Graphical Form
- Ladder Diagrams (LD) (i.e. Ladder Logic)
- Function Block Diagram (FBD)
- Sequential Function Chart (SFC)
Although all of these PLC programming languages can be used to program a PLC, graphical languages (like ladder logic) are typically preferred to textual languages (like structured text programming).
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 a simple example of a ladder diagram.
In the above-mentioned example, two pushbuttons 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 the two vertical lines are called rails. Every rung forms the electrical connectivity between the Positive rail (P) and the 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 that, 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 the left and output lines leaving on the right. It gives a relation between the state of input and output
The advantage of using FBD is that any number of inputs and outputs can be used on the functional block. When using multiple inputs and output, you can connect one function block’s output to another’s input. Whereby building a Function Block Diagram.
The figure below shows various function blocks used in FBD programming.
The figure below shows a ladder diagram and its function block equivalent in Siemens notation.
Structured Text Programming
Structured text, a programming language for PLCs, uses simple statements to dictate operations. It is similar to conventional programming but isn’t case-sensitive, using operators to express logic and relationships.
