A programmable logic controller (PLC) is an industrial computer that can perform the following tasks:
Monitoring and control of industrial automation applications
Perform tasks related to testing and measurement work.
Perform process-related functions (including functions related to HVAC systems) that are outside the scope of this document.
A PLC receives and processes data from sensors and input devices, then makes logic-based decisions and issues control commands to mechanical or electrical systems. A PLC is an embedded system that combines a computer's processor, memory, and input/output (I/O) devices—much like its competitors, hardwired relay logic and PC-based logic.
In terms of form factor, today's PLCs come in a wide variety of styles, ranging from very simple computers with integrated chip (IC) form factors to large rack-mounted collections of controller sub-components housed in multiple chassis. Simpler, microcontroller-based PLCs, or those employing system-on-a-chip (SoC), can achieve very high reliability and operate with very low input power. In contrast, the most complex PLCs have blurred the lines between PLCs and general-purpose computers used for real-time industrial control... although the former still emphasizes reliability and real-time performance.
PLCs were originally designed to directly replace hardwired control logic based on relays and drum sequencers. These early PLCs simply performed basic operations by converting inputs into outputs. Any machine task requiring proportional-integral-derivative (PID) control was performed by external analog electronics. Now, PID control, and even more complex operations, are standard features of PLC instruction sets.
In fact, the expected functionality of PLCs has been increasing over time, resulting in many PLCs today being quite complex and capable of executing sophisticated adaptive programs. Thanks to Moore's Law, the power of semiconductor chips is constantly increasing while their size is shrinking, enabling smaller controllers to achieve unprecedented intelligence. This trend will continue with full support for motion control, vision systems, and communication protocols. Regarding PLC size, some Programmable Automation Controllers (PACs) integrate the PLC with a PC, replacing the PLC and proprietary control system (running in a proprietary programming language) to suit certain applications. Today, even more PLCs are being integrated into Human Machine Interfaces (HMIs).
PLC-enabled industrial digital environment
Today's industrial automation relies on machine feedback, operational data, and complex interconnections between digital devices.
Control digital devices.
Run advanced functions—such as those related to IIoT connectivity and machine reconfiguration.
It enables people to make decisions about various machines and operating conditions.
Improve overall productivity and workpiece quality.
These automated devices include various information systems to store, process, and serve this data.
Material Requirements Planning (MRP) or Manufacturing Resource Planning (MRP) systems provide information on production planning, scheduling, financial matters, and inventory control. In contrast, historical data systems store time-series data from sensors and instruments, which can be used to create graphs to help operators and management systems understand and manage automation trends. Statistical Process Control (SPC) is a historical database application.
A Human-Machine Interface (HMI) is a machine control panel (or a module that connects wirelessly to a mobile device) that allows operators to view data and issue commands. Closely related to HMI functionality is a Supervisory Control and Data Acquisition (SCADA) system, which enables real-time control and monitoring of the interaction between automated machines and their HMIs and historical databases. SCADA is a type of HMI that can control multiple machines and display data related to multiple devices.
Manufacturing Execution System (MES) includes functions such as schedule management and data collection. In some respects, this system can be viewed as existing between and overlapping with MRP and SCADA.
Enterprise Resource Planning (ERP) systems integrate manufacturing-related MRP, MES, Product Lifecycle Management (PLM), and CRM information systems. An ERP system can be a monolithic software suite handling all these functions, or a core ERP system connecting specialized applications from multiple vendors. Typically, only senior management interacts with the ERP—in large enterprises, most people interact with one of its component systems.
PLCs typically operate at a lower level than these information systems. PLCs send and receive information to and from machines, motors, and sensors. They can also interact with the information layers above, sending data to historical databases or SCADA systems, or receiving control input information from SCADA or HMI systems. More complex PLCs can also perform SCADA and historical database functions... and, increasingly, even HMI functions.
Image: PLCs typically operate at a lower level than automated information systems. (Image credit: Jody Muelaner)
Please note that PLCs are not only used for automation: they are also used to control test platforms (product development) and laboratory measurement tasks.
As mentioned above, automation typically emphasizes diagnostics and requires PLCs to perform decisive real-time operations to achieve real results.
In contrast, PLCs used in measurement tasks place greater emphasis on performing measurement acquisition and other forms of data acquisition quickly and accurately.
For machine automation tasks, PLCs rely on real-time processing, where the latency between input and response output is measured in milliseconds. Beyond the simplest PLC functions, most require a real-time operating system (RTOS). While many PLCs still use proprietary operating systems, there is growing interest in open-standard operating systems. For example, VxWorks is a proprietary real-time operating system widely licensed for industrial control. Several leading robotics manufacturers, including Kuka and ABB, use this operating system. Alternatively, there's FreeRTOS, a free open-source variant released under the MIT open-source license. FreeRTOS includes various Internet of Things (IoT) libraries suitable for a wide range of automation applications. For more details, see the Digi-Key article, "Connecting Your Designs to the Cloud Quickly and Securely Using Amazon FreeRTOS."
For test and measurement tasks, PLCs rely on real-time processing, where the delay between field device measurements and data collection is measured in milliseconds. The era when engineers had no choice but to use interface converters and transmission channel systems is long gone. Now, peripheral intelligent devices and I/O components have enhanced and simplified signal acquisition through digital and analog inputs.
Today's engineers also have more choices. These choices are based on standardized interfaces and interoperable components with cross-manufacturer compatibility.
Consider only I/O components with integrated PLC functionality. These products are compatible with configurable HMIs running Windows or Linux operating systems and featuring Ethernet connectivity—but lack easy recalibration capabilities for field devices generating low-voltage analog signals, or do not provide analog I/O for such devices. Such I/O components can also be used with a specially configured PLC, specifically designed to collect data from remote I/O devices… or collect data directly from sensors via their onboard I/O.
The T7 Multi-Functional Data Acquisition Device (DAQ) includes Ethernet, USB, Wi-Fi, and Modbus connectivity, enabling it to work with a wide range of field devices, industrial HMIs, and PLCs. In particular, the Modbus/TCP connectivity provides controllability through various third-party software and hardware options, achieving openness and flexibility—which in turn allows industrial system architects and R&D engineers to choose data collection and automation applications without being constrained by vendor preferences.
Of course, PLCs are not the only option for machine automation or test and measurement. As all industrial controls become increasingly complex, some vendors have begun classifying certain hardware as Programmable Automation Controllers (PACs) to indicate greater functionality. And in many cases, a single piece of hardware may contain multiple processors. In fact, PLCs themselves are becoming increasingly complex—so there's no hard and fast rule about when hardware performing PLC functions constitutes a PAC. Most PACs integrate PLC and PC functionality and are used in complex automation systems, characterized by multiple PC-based applications, HMIs, and historical databases. A key difference is that PACs are more readily adopted by developers due to their more open architecture compared to traditional control technologies.
However, another option today is the modular PLC. This type of PLC consists of modules that perform different functions. All PLCs must include a CPU module, which includes a processor to run the operating system and programs, and memory. In addition, it may include a separate power supply module and additional input/output (I/O) modules. A PLC may include both digital and analog I/O modules. Another module may be needed for network communication.
PLCs can be integrated—with all modules housed in a single chassis—or modular. Integrated PLCs are more compact, while modular PLCs offer more functionality and typically allow multiple modules to be easily connected by plugging them directly into each other or using a common rack as a bus. Each module is addressed based on its position on the bus. While the physical support of the rack may conform to standards such as DIN, the data bus is usually proprietary to the PLC manufacturer.
The role of PLC in the Internet of Things
As interest in Industry 4.0 (also known as IIoT) grows, industrial users are increasingly looking to connect their industrial controllers to their company's network using Internet protocols. This means communicating using Transmission Control Protocol (TCP) and Internet Protocol (IP) or simply TCP/IP. However, the IIoT trend is not just about the use of Internet protocols... it's also about machine learning and big data. As PLCs become more powerful (more advanced control makes PLC functionality a feature), they offer more host capabilities such as vision systems. Internet connectivity also allows engineers (through the system PLC) to leverage cloud-based algorithms to process extremely large datasets (also known as big data) for machine learning.
In practical applications, EtherCAT, a control automation technology, excels in IIoT PLC functionality. It's an Ethernet-based communication protocol suitable for real-time control applications with cycle times of less than 0.1 milliseconds—the fastest industrial Ethernet technology capable of synchronization with nanosecond-level accuracy. Another significant advantage is the flexibility of EtherCAT network topologies, eliminating the need for network hubs and switches. Devices can be easily connected in ring, linear, star, or tree configurations. PROFINET is a competing standard with similar capabilities.
Conclusion
The current trend towards increasingly complex data collection and industrial control will continue. This means that PLCs used for industrial automation, as well as testing and measurement, will increasingly resemble PACs and integrate with SCADA and historical databases. Internet protocols and open standards such as EtherCAT are also being steadily adopted for PLC communication. This connectivity will, in turn, stimulate greater market adoption of Industry 4.0 technologies, such as big data analytics and machine learning, partly due to the ability to allocate the required processing power and memory to:
Cloud-based computing
Edge devices capable of data processing
In addition to these trends, there is still a need for more reliable and energy-efficient traditional PLCs to perform relatively simple test, measurement, and control functions.
