Introduction to PLC
PLC, short for Programmable Logic Controller, is a rugged industrial computer designed to perform automation tasks in harsh environments. These conditions may include extreme temperatures, dust, moisture, or vibrations commonly found in manufacturing and processing industries.
PLCs have transformed industrial automation since their invention in 1964 by Dick Morley, offering robust control over complex systems with precise reliability.
PLC Fundamentals
Why PLCs Are Important in Industry
PLCs play a crucial role in industrial automation by performing operations such as:
- Timing
- Counting
- Calculating
- Comparing
- Processing analog signals
They are used to control everything from assembly lines to power plants.
Structure of a PLC: Major Components
A standard PLC system is built from five main components:
1. Rack or Chassis
This acts as a frame or base that houses all the other modules like the CPU, I/O modules, and power supply. It enables communication between all internal modules.
2. Power Supply Module
The power supply provides the necessary electrical power to all PLC components.
3. Central Processing Unit (CPU)
The CPU is the brain of the PLC, responsible for executing control instructions. It typically uses an octal or hex-based microprocessor to process logic.
4. Input and Output (I/O) Modules
These modules connect the PLC to field devices:
Common Input Devices:
- Switches and push buttons
- Limit switches
- Proximity and photoelectric sensors
- Temperature, pressure, and level switches
Common Output Devices:
- Valves and solenoids
- Motors and fans
- Horns and alarms
- Pumps and relays
5. Communication Interface Module
This module allows the PLC to exchange data with external systems, using intelligent I/O networks and protocols.
Types of PLC: Compact vs Modular
🔹 Compact PLC
- Fixed I/O capacity (non-expandable)
- Ideal for small-scale operations
- Cheaper but difficult to repair
- Best suited for standalone tasks
🔹 Modular PLC
- Expandable I/O capability
- Used in large-scale industries
- Easier to repair (independent modules)
- Higher cost but more flexible and scalable
Popular PLC Brands and Their Programming Software
| Brand | PLC Software |
|---|---|
| Allen Bradley | RSLogix |
| Siemens | Simatic Manager |
| Mitsubishi | MELSOFT Series |
| Omron | CX Programmer |
| Honeywell | HRA |
| GE (General Electric) | Durus |
| Fatek | WinProLadder |
| Hitachi | EH-150 |
| ABB | AC010, AC500 |
| Bosch Rexroth | Bosch Software Suite |
PLC Programming Languages
PLCs support various programming languages, categorized into:
1. Textual Languages
- Instruction List (IL)
- Structured Text (ST)
2. Graphical Languages
- Ladder Diagram (LD)
- Function Block Diagram (FBD)
- Sequential Function Chart (SFC)
These languages are defined under IEC 61131-3 standard, ensuring universal compatibility.
Timer Instructions in PLC
Timers are crucial for time-based automation tasks. Common types include:
🔸 TON (Timer ON Delay)
Activates after a preset time delay once the condition becomes true.
🔸 TOFF (Timer OFF Delay)
Activates after a preset time once the condition becomes false.
🔸 RTO (Retentive Timer ON)
Stores the elapsed time even if the condition becomes false or a power cycle occurs.
Timer Terms:
- Timer Base: Time interval base unit (e.g., ms, sec)
- Preset Value: Target delay duration
- Accumulated Value: Current time counted
- Timer Number: Identifier or tag for each timer
Common Flowchart Symbols in PLC Programming
| Symbol | Name | Function |
|---|---|---|
| 🔷 Rectangle | Process | Represents internal operations or logic processing |
| 🔶 Parallelogram | Input/Output | Used for data input or output |
| 🔷 Diamond | Decision | Represents decision-making (Yes/No, True/False) |
| ⭕ Circle | Connector | Links flowchart sections, avoiding line intersection |
| 🔁 Double Rectangle | Predefined Process | Indicates subroutine or interrupt logic |
| 🟢 Oval | Terminal | Start or end of the logic flow |
| ➡️ Arrow | Flow Line | Indicates direction of process flow |
Ladder Diagram of PLC
Ladder Diagram (LD) is one of the most commonly used graphical programming languages in PLC systems. It visually represents logic in a format similar to electrical relay logic diagrams, making it easy for electricians and automation engineers to understand and implement control processes.
Structure of a Ladder Diagram
The diagram is made of "rungs" that resemble the steps of a ladder. Each rung represents a logical operation or control instruction.
- Left rail: Represents the power supply (L+ or live)
- Right rail: Represents the return path (neutral)
- Rungs: Horizontal lines with input and output symbols that represent the control logic
Basic Ladder Diagram Symbols
| Symbol | Name | Description |
|---|---|---|
| --| |-- | Normally Open Contact | Allows current when input is TRUE |
| --|/|-- | Normally Closed Contact | Allows current when input is FALSE |
| --( )-- | Coil | Activates an output when rung logic is TRUE |
| --[\]-- | Timer | Delays activation based on preset time |
| --[CTU]-- | Counter | Counts upward when input is TRUE |
Simple Example: Start/Stop Motor Circuit
--| |----|/|----------------( )--- Start Stop Motor
Explanation: When the Start button is pressed and the Stop button is not pressed, the Motor coil is energized.
Advantages of Ladder Diagram:
- Easy to learn and implement
- Best suited for simple to moderately complex logic
- Visually intuitive like electrical wiring diagrams
Conclusion
PLCs are the backbone of industrial automation, offering unmatched flexibility, reliability, and scalability. Whether it's a compact controller for a single machine or a modular PLC managing an entire factory line, these systems ensure precise and efficient operation.
As industries embrace Industry 4.0 and smart manufacturing, knowledge of PLC systems and programming is becoming more essential than ever.
Frequently Asked Questions (FAQs) about PLC
Q1. What is a PLC and where is it used?
Answer: A PLC (Programmable Logic Controller) is an industrial computer used to control machines and processes in manufacturing, automation, energy, and other sectors. It is known for its durability and real-time processing capability.
Q2. Who invented the PLC and when?
Answer: The PLC was invented by Dick Morley in 1964. He is considered the father of PLC technology.
Q3. What are the main components of a PLC system?
Answer: The main components are: Power Supply, CPU, Input/Output Modules, Communication Interface, and the Rack or Chassis that holds them together.
Q4. What are common input and output devices used in PLC?
Answer: Common input devices include push buttons, limit switches, and sensors. Output devices include motors, valves, relays, fans, and alarms.
Q5. What is the difference between Compact and Modular PLC?
Answer: Compact PLCs have a fixed number of I/Os and are used for small applications. Modular PLCs have expandable I/Os and are used in large-scale industrial systems.
Q6. Which programming languages are used in PLC?
Answer: PLCs use graphical languages like Ladder Diagram (LD), Function Block Diagram (FBD), and Sequential Function Chart (SFC), as well as textual languages like Structured Text (ST) and Instruction List (IL).
Q7. What is a Ladder Diagram in PLC programming?
Answer: A Ladder Diagram is a graphical representation of logic control that resembles a ladder. It’s easy to understand and widely used in industries for PLC programming.
Q8. What are TON, TOFF, and RTO timers in PLC?
Answer: TON is Timer ON Delay, TOFF is Timer OFF Delay, and RTO is Retentive Timer ON. These timers are used for time-based automation in PLCs.
Q9. Which are the most popular PLC brands?
Answer: Top PLC brands include Siemens, Allen Bradley, Mitsubishi, Omron, Honeywell, GE, Hitachi, and ABB.
Q10. Is learning PLC programming difficult?
Answer: Not at all. PLC programming is quite beginner-friendly, especially using Ladder Diagrams. With basic electrical logic knowledge, one can start learning PLC easily.