What do amusement park rides, factory assembly lines, sawmills, food processing plants, and robotic work cells all have in common? They all rely on Programmable Logic Controllers (PLCs) for automation and control.
A programmable logic controller (PLC) is an industrial computer used to monitor equipment, process input signals, execute programmed instructions, and control machinery or manufacturing processes. Designed for industrial environments, PLCs provide reliable, repeatable control across applications ranging from individual machines to complex production systems.
As a Rockwell Automation Gold Level System Integrator, Concept Systems designs, implements, and modernizes PLC control systems for manufacturers across a wide range of industries.
Let’s explore how PLCs work, the components that make up a PLC system, and why they’re essential to industrial automation.
How Does a PLC Work?
PLCs come in various models and configurations, each designed to meet specific automation needs. While different manufacturers offer unique functionalities, the core operation of a PLC remains the same: continuously monitoring inputs, executing logic, and controlling outputs.
In the United States, Allen-Bradley PLCs from Rockwell Automation dominate the market. For large-scale control applications, the ControlLogix PLC stands out, offering world-class process, safety, and motion control capabilities.
A PLC is a task-driven control system with four basic operational steps for every device. They operate in a continuous loop to execute their control programs. The cycle consists of these 4 basic steps:
- Input Processing: Identifies the status of all input devices that are connected to the PLC.
- Program Execution: Implements the user-created PLC Programming.
- Output Processing: Energizes or de-energizes all connected output devices.
- Housekeeping: Communicates with programming terminals and internal diagnostics.
The order, complexity, and timing of these steps depend on the model and manufacturer of PLC being used and how the PLC programming is setup. For detailed information from Rockwell Automation on Tasks and scans see: Logix 5000 Controllers Tasks, Programs, and Routines
PLC Operation Example: A Conveyor System
Consider a conveyor that moves products through a manufacturing line. A sensor detects when a product reaches a designated position and sends an input signal to the PLC. The PLC evaluates that signal against its programmed logic and sends an output command to start, stop, or adjust the conveyor.
This continuous interaction between inputs, logic, and outputs allows the system to respond automatically as production conditions change.
What are input/output (I/O) devices?
Input and output (I/O) devices are essential components that allow the PLC to interact with the external environment. These devices can be either active or passive elements, depending on the function of the process and device. Active devices require an external power source to function and often include amplification or signal conditioning components. The field devices are usually physically wired into the PLC I/O modules.
Input Devices: Digital and Analog Inputs
Input devices transmit signals to the PLC, originating from various sensors and switches that monitor environmental changes. A PLC can receive various types of input signals. The two most common types are Digital Inputs (DI) and Analog Inputs (AI). A DI has two states: ON or OFF (1 or 0). Some common AI signals include 4-20mA current loops and 0-10V voltage signals.
Input devices are often used for human interaction with the process. Push buttons, like a start button, are commonly mounted on a Human Machine Interface (HMI) panel. These Switches are generally DI passive devices. Some other examples of DI’s are limit switches, toggle switches, and auxiliary contacts. DI Passive devices are usually less expensive and less complicated than active devices. Some examples of active digital input devices are proximity switches, temperature switches, level switches and pressure switches. Inputs with 24VDC are widely used in industrial automation due their safety and compatibility. However, many older systems still use 120VAC input signals.
Analog inputs can be supplied by passive or active devices as well. Analog input signals to a PLC can vary continuously over a range of voltage or current, providing detailed information about connected devices. These signals are essential for applications that require precise measurements rather than simple on/off detection. Some of the commonly used passive devices are thermocouples and resistive temperature detectors (RTDs). However, active devices are more commonly used to supply the PLC with analog inputs. Some examples of these devices include temperature sensors, pressure transducers, level sensors, and flow meters.
Output Devices: Digital and Analog Outputs
Output devices are elements that take signals from a PLC and carry out actions accordingly. They play a crucial role in implementing control commands within various industrial processes. Two Common signals from the PLC to an output device are Digital Output (DO) and Analog Output (AO).
Digital Output signals are binary (on/off) and are used to control output devices like stack lights, solenoids, relays, valves, motor starters, horns and alarms. The digital outputs are generally 24VDC or 120VAC depending on the actual device being controlled. Low voltage devices can reduce the risk of electrical shock.
Analog output signals from a PLC can continuously vary within a range, allowing for precise control of connected devices. These signals are crucial for applications that need fine adjustments rather than just on/off control. Some common devices are control valves, Variable Frequency Drives (VFDs), heaters, and hydraulic actuators.
Key Hardware Components of a PLC System
PLC Chassis and CPU
A PLC chassis, often referred to as a rack, is the physical structure that houses the components. It is a modular system that allows for various configurations. The chassis facilitates communication between all the modules through the backplane.
The Central Processing Unit (CPU), or main processor, is a microprocessor-based component within a PLC. It monitors the status of field inputs, runs the control program, and issues commands to the field outputs. This sequence enables the PLC to efficiently oversee and regulate various industrial processes. For Large PLC Control Systems, Allen-Bradley and Rockwell Automation offer the ControlLogix 5580 Controllers.
I/O Modules
There are several types of I/O modules, with the most common being Analog Output (AO), Analog Input (AI), Digital Output (DO), and Digital Input (DI). I/O modules act as the real data interface between field devices and the PLC. A PLC knows the real status of field devices and controls them with relevant I/O cards.
Human Machine Interface (HMI)
An HMI operating station is used to provide an “operating window” to the PLC process. It is generally a separate device, like a PC and monitor, that is loaded with Human Machine Interface Software. A common Rockwell solution for an HMI is FactoryTalk View.
PLC Programming Software
Rockwell Automation’s PLCs are mainly programmed using the Studio 5000® environment, a robust suite of tools crafted to simplify the development and upkeep of automation systems.
Why Use a PLC in Industrial Automation?
PLCs offer several advantages for industrial automation:
✔ Eliminate the need for rewiring when modifying control logic.
✔ Compact and scalable, allowing for customized I/O configurations.
✔ Capable of advanced control functions, such as counting, timing, and analog processing.
✔ Easy troubleshooting with built-in diagnostics.
✔ Seamless integration with HMIs and industrial networks.
✔ Cost-effective for complex automation solutions.
PLC Upgrades and Modernization with Concept Systems
From legacy PLC replacements to new control system integration, Concept Systems helps manufacturers improve reliability, maintainability, and performance through industrial controls engineering.
As a Rockwell Automation Gold Level System Integrator, our team works with technologies including Allen-Bradley ControlLogix, CompactLogix, and FactoryTalk to develop control solutions around each facility’s operational needs.
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