17.1 Programmable Logic Controllers (PLCs)
4 min read•august 7, 2024
Programmable Logic Controllers (PLCs) are the backbone of industrial automation. They're like the brains of a factory, controlling machines and processes. PLCs use special programming languages and hardware to make sure everything runs smoothly and safely.
PLCs have a unique way of operating. They constantly scan inputs, run their program, and update outputs. This cycle happens super fast, allowing PLCs to respond quickly to changes. They also use timers and counters to keep track of time and events.
PLC Programming Languages
Ladder Logic and Function Block Diagrams
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- (LD) uses graphical representation resembling electrical ladder diagrams
- Consists of contacts (inputs) and coils (outputs) connected by lines
- Follows left-to-right, top-to-bottom execution order
- Widely used for simple control tasks and easy to understand for technicians familiar with electrical diagrams
- (FBD) represents control logic using interconnected graphical blocks
- Each block performs a specific function (arithmetic, logic, timers, etc.)
- Data flows from inputs to outputs through connected blocks
- Suitable for complex control algorithms and data processing
Structured Text and PLC Programming Software
- (ST) is a high-level, text-based programming language similar to Pascal or C
- Supports complex data types, loops, and conditional statements
- Offers flexibility and power for advanced control tasks and calculations
- Requires more programming expertise compared to graphical languages
- provides an integrated development environment (IDE) for creating, editing, and debugging PLC programs
- Includes tools for configuring PLC hardware, managing projects, and monitoring real-time data
- Examples of PLC programming software: Siemens TIA Portal, Rockwell RSLogix, Schneider Electric SoMachine
PLC Hardware Components
Input/Output Modules and PLC Hardware Architecture
- Input/Output (I/O) modules interface the PLC with field devices
- Input modules convert signals from sensors (switches, proximity sensors, etc.) into digital or analog values the PLC can process
- Output modules convert PLC control signals into appropriate forms (voltage, current, etc.) to drive actuators (valves, motors, lights, etc.)
- I/O modules are available in various types (digital, analog, relay, etc.) and configurations (number of channels, signal range, etc.)
- PLC hardware architecture typically consists of a power supply, , memory, and I/O modules
- Power supply provides regulated power to PLC components
- CPU executes the control program, processes data, and manages communication
- Memory stores the control program, data, and configuration settings
- I/O modules are connected to the CPU through a backplane or network
Industrial Sensors and Actuators
- Industrial sensors convert physical quantities into electrical signals for PLC input
- Examples: limit switches for position detection, photoelectric sensors for presence detection, temperature sensors for process monitoring
- Sensors are selected based on the type of quantity to measure, accuracy, response time, and environmental conditions
- Industrial actuators convert electrical signals from PLC outputs into physical actions
- Examples: solenoid valves for fluid control, electric motors for motion control, indicator lights for status display
- Actuators are chosen based on the type of action required, power consumption, response time, and compatibility with the PLC output
PLC Operation and Control
Scan Cycle and Real-Time Control
- PLCs operate in a continuous , repeatedly executing the control program
- Input scan: PLC reads the status of input devices and updates input memory
- Program execution: PLC executes the control logic based on the input status and program instructions
- Output scan: PLC updates the output devices based on the results of program execution
- Housekeeping: PLC performs internal diagnostics, communication, and other background tasks
- Real-time control ensures the PLC responds to input changes and updates outputs within a deterministic time frame
- Scan time is the duration of one complete scan cycle and depends on the program complexity and PLC performance
- PLCs are designed for fast scan times (typically in milliseconds) to meet the real-time requirements of industrial control applications
Timers and Counters
- Timers are used to introduce time delays or measure elapsed time in PLC programs
- On-delay timers (TON) activate the output after a specified delay when the input is true
- Off-delay timers (TOF) maintain the output for a specified delay after the input becomes false
- Retentive timers (RTO) accumulate time whenever the input is true, retaining the elapsed time even if the input becomes false
- Counters are used to count events or quantities in PLC programs
- Up-counters (CTU) increment the count value each time the input transitions from false to true
- Down-counters (CTD) decrement the count value each time the input transitions from false to true
- Retentive counters (CTUD) can both increment and decrement the count value based on separate inputs, retaining the count value even if the PLC is powered off
Key Terms to Review (19)
Automated manufacturing: Automated manufacturing refers to the use of control systems, such as computers or robots, to operate equipment in manufacturing plants, including machinery, processes in factories, boilers, and heat treating ovens. This process enhances efficiency by minimizing human intervention and increasing production speed and consistency while reducing errors and costs.
CPU: The CPU, or Central Processing Unit, is often referred to as the brain of a computer or microcontroller, executing instructions from programs by performing basic arithmetic, logic, control, and input/output operations specified by the instructions. In microcontroller systems, the CPU orchestrates all tasks and processes, managing interactions with various peripherals and components to ensure smooth functionality. In the context of programmable logic controllers, the CPU plays a crucial role in processing inputs from sensors and executing control commands to drive outputs for industrial automation processes.
Function Block Diagram: A function block diagram (FBD) is a graphical representation used to design and implement control systems in programmable logic controllers (PLCs). This diagram allows users to visualize the relationships between different functions and components, making it easier to understand and troubleshoot complex systems. The FBD notation helps in structuring the program logic using function blocks, which can include inputs, outputs, and internal logic that processes information.
Functional Safety: Functional safety is a part of overall safety that ensures systems operate correctly and safely in response to inputs, preventing dangerous failures. It focuses on the electronic and software components of systems to ensure they behave as intended, even in the presence of faults. It also emphasizes the importance of risk assessment, validation, and verification throughout the lifecycle of the system to guarantee safety standards are met.
HMI Development Tools: HMI development tools are software applications and frameworks that facilitate the design, creation, and deployment of human-machine interfaces (HMIs) used in industrial automation and control systems. These tools enable engineers and developers to build user-friendly interfaces that allow operators to interact with machines, programmable logic controllers (PLCs), and other automated systems, thereby improving efficiency and usability.
IEC 61131: IEC 61131 is an international standard that defines the programming languages and software architecture for programmable logic controllers (PLCs). It provides guidelines for various programming languages, including Ladder Diagram, Function Block Diagram, Structured Text, Instruction List, and Sequential Function Charts, facilitating interoperability and standardization in industrial automation systems.
Input Module: An input module is a crucial component in programmable logic controllers (PLCs) that facilitates the reception of signals from various input devices, such as sensors and switches. This module converts the signals into a format that the PLC can process, allowing it to perform control tasks based on real-world conditions. Input modules play a vital role in the overall functioning of PLC systems by providing essential data for decision-making and automation processes.
Ladder logic: Ladder logic is a programming language used to develop software for programmable logic controllers (PLCs). It visually resembles a ladder, where each rung represents a control circuit, making it easy for technicians to read and understand. Ladder logic uses symbols to represent inputs and outputs, enabling the creation of complex control systems for industrial automation.
Micro PLC: A micro PLC is a compact and efficient programmable logic controller designed for smaller-scale automation tasks. It offers features similar to larger PLCs but in a smaller form factor, making it ideal for applications with limited space and budget constraints. Micro PLCs are often used in simple control tasks, providing essential functionalities for controlling machinery and processes in various industries.
Modbus: Modbus is a communication protocol developed in 1979 that allows devices such as PLCs, sensors, and computers to communicate with each other in industrial environments. It is widely used in automation and control systems due to its simplicity and reliability, making it essential for data exchange between devices in various industrial applications. Modbus supports multiple communication modes including serial (RTU and ASCII) and TCP/IP, enabling flexible integration into existing network architectures.
NFPA 79: NFPA 79 is the National Fire Protection Association's standard for the electrical and electronic equipment of industrial machinery. It aims to ensure safety in industrial settings by providing guidelines for the design, installation, and maintenance of electrical systems to prevent fire hazards. This standard is particularly relevant when considering how programmable logic controllers (PLCs) are integrated into machinery, as it sets safety protocols that influence their operation and reliability.
Output module: An output module is a crucial component of a programmable logic controller (PLC) that is responsible for sending signals to external devices or actuators based on the processed data from the PLC. It converts the control logic processed within the PLC into physical actions, such as turning on a motor or activating a relay, facilitating interaction between the PLC and the machinery it controls.
Plc programming software: PLC programming software is a specialized tool used to create, modify, and maintain the control logic and configuration of Programmable Logic Controllers (PLCs). This software enables users to program PLCs using various programming languages, including ladder logic, function block diagrams, and structured text, allowing for the automation of industrial processes and machinery.
Process control: Process control refers to the methods and technologies used to manage and regulate various industrial processes, ensuring they operate efficiently, safely, and within desired parameters. This involves monitoring variables such as temperature, pressure, and flow rate, and making adjustments as needed through control systems. Key features include feedback mechanisms, automation, and the use of controllers to maintain stability and optimize performance.
Profibus: Profibus is a standard for fieldbus communication in automation technology, particularly within industrial environments. It facilitates the connection and communication between devices like sensors, actuators, and controllers, enabling efficient data exchange in systems such as programmable logic controllers and SCADA systems. Its ability to support real-time data transfer makes it essential for modern industrial networks.
Real-time processing: Real-time processing refers to the capability of a system to process data and respond to inputs or events within a strict time constraint, ensuring that the output is delivered in a timely manner. This is crucial in applications where delays can lead to system failures or unsafe conditions. It involves both hardware and software components that work together to monitor, analyze, and react to data as it comes in, making it essential for applications ranging from industrial automation to vehicle control systems.
Relay PLC: A Relay PLC is a type of programmable logic controller that uses electromechanical relays for its switching operations. These devices serve as the backbone for industrial automation, controlling machinery and processes through a series of electrical signals and logical functions. With their reliable switching capabilities, relay PLCs have been widely used in various applications before the advent of more advanced solid-state devices.
Scan cycle: A scan cycle refers to the process through which a programmable logic controller (PLC) continuously reads input signals, processes data based on its programmed logic, and updates output signals in a repetitive manner. This cycle is crucial for the real-time operation of PLCs, allowing them to monitor various conditions and control processes efficiently. The scan cycle typically consists of distinct phases: input scanning, program execution, and output updating, which together ensure the timely response of the system to changing conditions.
Structured text: Structured text is a high-level programming language used primarily in the context of programmable logic controllers (PLCs) for industrial automation. It allows users to write code that is organized in a clear and logical manner, making it easier to understand, maintain, and modify. This language is part of the IEC 61131-3 standard, which defines programming languages for PLCs, and offers features such as variables, control structures, and functions to create complex control logic efficiently.