Raspberry Pi PLC: A Low-Cost, Flexible Solution for Industrial Automation

Jan 17th, 2025

Brief introduction to PLCs and Raspberry Pi

A Programmable Logic Controller (PLC) is an industrial computer designed to automate processes and control machinery in various industries. PLCs are highly reliable, rugged devices capable of handling tasks like managing conveyor belts, monitoring sensors, or operating robotic systems. They are essential for ensuring precision, efficiency, and safety in automation.

The Raspberry Pi, on the other hand, is a versatile, affordable single-board computer that has gained popularity for its compact design and vast range of applications. While it was initially developed as an educational tool, its low cost, GPIO (General-Purpose Input/Output) pins, and compatibility with open-source software make it a promising alternative to traditional PLCs for hobbyists, educators, and small businesses looking to explore automation on a budget.

Current trends in industrial automation

Industry 4.0 and Smart Factories

Integration of advanced technologies like IoT, AI, and machine learning to create interconnected and intelligent production systems.

Real-time data collection and analysis for predictive maintenance, process optimization, and increased efficiency.

Industrial IoT (IIoT)

Use of connected devices and sensors to monitor equipment and processes.

Focus on seamless communication between machines and systems for enhanced productivity and reduced downtime.

https://www.mdpi.com/2079-9292/10/23/3047

Artificial Intelligence and Machine Learning

AI-driven solutions for predictive maintenance, quality control, and decision-making. Machine learning models to optimize production lines and adapt to changing conditions dynamically.

Cobots and Advanced Robotics

Deployment of collaborative robots (cobots) that work alongside human workers. Increasing adoption of robotics for repetitive tasks, precision assembly, and hazardous operations.

Edge Computing

Processing data closer to the source (e.g., sensors and controllers) to reduce latency and improve decision-making speed. Enhanced security and reduced dependency on centralized cloud systems.

Cybersecurity in Automation

Emphasis on securing industrial networks against cyber threats.

Implementation of robust security measures like encrypted communication and anomaly detection.

Definition of PLCs

Basic Working Principles of PLCs

PLCs receive input signals from various sources such as sensors, switches, or other devices. These signals represent real-world conditions like temperature, pressure, motion, or the status of machinery.

Processing (Execution of Logic): The PLC's central processing unit (CPU) processes the input data based on a preprogrammed logic. Logic is typically defined using ladder diagrams, function block diagrams, or structured text programming.

Based on the processed logic, the PLC sends output signals to actuators, motors, valves, or other devices to perform specific actions. For example, turning on a motor, opening a valve, or stopping a conveyor belt.

PLCs operate in a cyclic manner (scan cycle), repeatedly reading inputs, processing logic, and updating outputs, ensuring real-time control.

Communication

Modern PLCs can connect to networks and communicate with other controllers or supervisory systems, enabling integration into larger automation frameworks.

Typical Application Scenarios of PLCs

Manufacturing Automation:
  • Controlling assembly lines, robotic arms, and machinery in factories.
  • Coordinating processes like welding, packaging, or quality control.

Process Control:
  • Regulating variables like temperature, pressure, or flow in industries such as oil and gas, food processing, and pharmaceuticals.

Material Handling:
  • Managing conveyor belts, sorting systems, and cranes in warehouses, airports, or ports.

Energy Management:
  • Automating systems in power plants, such as turbine control, grid management, and load distribution.

Building Automation:
  • Operating HVAC systems, lighting, and elevators in commercial and residential buildings.

Water and Wastewater Treatment:
  • Monitoring and controlling pumps, valves, and filtration systems in treatment plants.

Transportation Systems:
  • Controlling railway signals, traffic lights, and toll booths.
  • Managing systems in subways or airports for seamless operation.

Agricultural Automation:
  • Controlling irrigation systems, greenhouses, and animal feeding systems for optimized agricultural practices.

Advantages of Using Raspberry Pi as a PLC

Cost-Effectiveness: Raspberry Pi is significantly cheaper than traditional PLCs, making it an attractive option for small businesses, startups, hobbyists, and educational projects.

Customizability: The open-source nature of Raspberry Pi allows users to create and customize automation systems tailored to specific requirements, offering unparalleled flexibility compared to proprietary PLC systems.

Wide Software Ecosystem: Compatible with various programming environments such as Python, Node-RED, and Codesys.

Access to numerous libraries and tools for advanced features like data logging, IoT connectivity, and machine learning.

IoT Integration: Raspberry Pi can seamlessly connect to cloud platforms, enabling real-time monitoring, remote control, and predictive analytics.

Ieal for implementing Industry 4.0 and IoT-based automation solutions.

Multiple Input/Output (IO) Options: Equipped with GPIO pins for basic IO functions and expandable with HATs (Hardware Attached on Top) or shields for industrial-grade connectivity.

Open-Source Licensing: Eliminates dependency on proprietary software or hardware, reducing long-term costs and offering full control over the system.

Components Needed

Hardware

Raspberry Pi and Expansion Boards for Digital/Analog I/O

 ✓ Digital I/O Boards
Provide additional digital input and output pins. Useful for controlling relays, sensors, and actuators.
Examples: PiFace Digital, Automation HAT.

 ✓ Analog I/O Boards
Add analog input/output functionality to the Raspberry Pi, which lacks native support for analog signals. Essential for reading data from analog sensors (e.g., temperature, pressure) and controlling devices like analog actuators.
Examples: ADC/DAC HATs like the ADS1115 or MCP3008-based boards.

✓ Relay Boards
Enable control of high-voltage devices such as motors, lights, or industrial machinery.
Examples: PiRelay, Sainsmart 4-Channel Relay Module.

✓ Industrial I/O Boards
Designed for harsh environments with features like opto-isolation, overvoltage protection, and compatibility with industrial signal levels.
Examples: Sequent Microsystems Industrial HAT, UniPi Neuron.

✓ Communication Protocol Boards
Support industrial communication protocols like Modbus, CAN, or RS-485.
Examples: PiCAN2, RS485 HAT.

Digital and Analog I/O Modules

 ✓ Digital I/O Modules

https://www.makerfabs.com/4-channel-a-d-module-for-raspberry-pi-3.html

Digital I/O modules are used to interface with devices that operate using binary signals (on/off, high/low). These modules expand the number of digital inputs/outputs and provide compatibility with industrial voltage levels.

Key Features:
  • Support for TTL (3.3V or 5V), 12V, and 24V signals.
  • Control devices like relays, switches, LEDs, and solenoids.
  • Opto-isolation for protection against voltage spikes.

 ✓ Analog I/O Modules
Analog I/O modules are essential for processing signals with varying values, such as those from sensors measuring temperature, humidity, or pressure.

Key Features:
  • ADC (Analog-to-Digital Converter) for converting analog inputs into digital signals.
  • DAC (Digital-to-Analog Converter) for generating analog outputs.
  • Common resolutions: 10-bit, 12-bit, or higher for precise readings.

Raspberry Pi OS, Codesys, and OpenPLC: Platforms for Automation

Raspberry Pi OS

Overview
  • Raspberry Pi OS is the recommended operating system for Raspberry Pi devices..

Codesys
Overview
  • Codesys is a professional-grade development environment for PLC programming, supporting standard IEC 61131-3 languages such as Ladder Diagram (LD), Function Block Diagram (FBD), and Structured Text (ST).
  • It can be installed on Raspberry Pi to transform it into an industrial-grade PLC.

OpenPLC
Overview
  • OpenPLC is an open-source platform designed specifically for PLC programming and automation.
  • It supports IEC 61131-3 programming languages, making it a cost-effective alternative to traditional PLC software.

Development environment:

The development environment for Raspberry Pi automation using Python, Node-RED, and MQTT offers a versatile and powerful approach to building IoT and real-time control systems. One of most effective languages for Raspberry Pi implementation is Python due to its usage of high level commands and availability of numerous libraries. It enables direct control of GPIO pins for interfacing with sensors, actuators, and other devices, as well as robust data processing capabilities. Libraries like RPi.GPIO and gpiozero allow efficient management of hardware, while others like paho-mqtt integrate seamlessly with MQTT for communication.

Node-RED, a flow-based development tool, complements Python by providing a visual programming interface. It simplifies the creation of automation workflows through drag-and-drop nodes, enabling users to design complex systems without extensive coding expertise. Node-RED supports GPIO control, HTTP requests, and integration with cloud services, making it a practical tool for orchestrating data flows and building real-time dashboards for monitoring and control.

MQTT serves as the backbone of communication in this environment. It is a lightweight protocol ideal for resource-constrained devices like Raspberry Pi. MQTT uses a publisher-subscriber model, enabling efficient and real-time messaging between devices. This makes it a natural choice for IoT applications where reliable communication is crucial.

Together, Python, Node-RED, and MQTT create a seamless ecosystem. Python can manage hardware interactions and perform advanced computations, while MQTT ensures efficient communication of data between devices or cloud platforms. Node-RED ties everything together by enabling visual workflow management and real-time data visualization. For example, in a home automation system, Python could read temperature data from a sensor, publish it to an MQTT broker, and Node-RED could subscribe to the data, display it on a dashboard, and trigger actions like turning on a fan.

This environment is scalable and flexible, suitable for both simple and complex automation tasks. Python provides the depth needed for advanced functionality, Node-RED enhances ease of use and visualization, and MQTT facilitates seamless communication. Whether for prototyping, IoT applications, or full-scale automation systems, this combination delivers a robust, integrated solution.

Software Configuration

Installing and Configuring OpenPLC on a Raspberry Pi

OpenPLC is a free and open-source platform that transforms Raspberry Pi into a powerful programmable logic controller (PLC) compliant with IEC 61131-3 standards. Below are the steps to install and configure OpenPLC on a Raspberry Pi.

Prepare the Raspberry Pi
1. Hardware Requirements:
   o Raspberry Pi (Model 3B, 4, or newer recommended).
   o SD card with Raspberry Pi OS at least 16 GB..
2. Update the System
3. Install Required Dependencies: Install essential packages for building and running OpenPLC:
sudo apt install git build-essential python3-pip apache2 php libmodbus-dev libsnmp-dev -y

Install OpenPLC
1. Clone the OpenPLC Repository: Navigate to your home directory and clone the OpenPLC repository:
git clone https://github.com/thiagoralves/OpenPLC_v3.git

cd OpenPLC_v3

2. Run the Installer Script: OpenPLC provides an installation script that automates the setup process. Run the script with:
sudo ./install.sh rpi

3. Set Permissions: After installation, ensure the OpenPLC runtime has the necessary permissions to interact with GPIO:

sudo chmod a+rw /dev/mem

Configure OpenPLC
1. Start the OpenPLC Runtime: Launch the OpenPLC runtime by running:
sudo ./start_openplc.sh
This will start the OpenPLC web interface and the runtime system.

2. Access the Web Interface: Open a web browser on your local network and navigate to:
http://<raspberry_pi_ip>:8080

Log In: Default username: openplc Default password: openplc

You should change the default credentials for security under the "Settings" tab in the web interface.

3. Load a PLC Program:
    o Go to the "Programs" tab in the OpenPLC web interface.
    o Upload a PLC program in one of the supported formats (e.g., Ladder Logic, Function Block Diagram, Structured Text).
    o Compile and deploy the program.

Optional - Enable Modbus Communication

1. Activate Modbus Server: OpenPLC includes a Modbus TCP/RTU server for industrial communication. Enable it under the "Settings" tab.

2. Test Modbus: Use tools like modpoll or a Modbus-compatible SCADA system to communicate with OpenPLC.

Troubleshooting
   • Web Interface Not Accessible: Ensure the Apache server is running:
   • sudo systemctl restart apache2
   • GPIO Issues: Check permissions for GPIO access and ensure the correct pin mapping in the OpenPLC configuration.
   • Program Errors: Validate your PLC program for syntax errors before uploading.

Limitations

    × Industrial Suitability: Raspberry Pi lacks industrial-grade durability and certifications, making it less reliable in harsh environments.
    × Limited GPIO: The number of GPIO pins may restrict complex systems, requiring expansion boards.
    × Real-Time Performance: Raspberry Pi's real-time capabilities are not as robust as dedicated PLCs.
    × Software Stability: OpenPLC, being open-source, may have fewer features and support compared to commercial PLC software.

Conclusion

Raspberry Pi, with its low cost, flexibility, and compatibility with open-source platforms, offers an exciting alternative to traditional PLCs for automation projects. While it may not yet fully replace industrial-grade PLCs in harsh environments, it provides an excellent platform for prototyping, educational purposes, and small to medium-scale applications. By leveraging tools like Python, Node-RED, and OpenPLC, users can build custom automation solutions that integrate seamlessly with IoT ecosystems. As open-source industrial automation continues to evolve, Raspberry Pi’s potential as a programmable controller will only grow, making it an invaluable tool for hobbyists, educators, and innovative businesses.