PLCs are commonly found in manufacturing, automotive, oil and gas, transportation, and many other industries. We even find them in everyday devices like washing machines and traffic lights. Today’s industrialized world certainly relies on PLCs, but what are they?
TechTarget provides a definition: “A programmable logic controller (PLC) is a small, modular solid state computer with customized instructions for performing a particular task. PLCs, which are used in industrial control systems (ICS) for a wide variety of industries, have largely replaced mechanical relays, drum sequencers and cam timers.” Essentially, a PLC is a type of computer. Rather than focusing on ease of use and a GUI (graphical user interface) like a PC would, a PLC prioritizes resilience to extreme conditions, operational uptime, and maximum efficiency.
The first PLC was created by Dick Morley in 1968 for General Motors. Since then, industries around the world have adopted this technology to automate processes and improve outcomes.
Here’s how it works. First, the PLC receives inputs from switches, push-buttons, sensors, encoders, and the like. This then triggers outputs when the software performs its logic on the inputs, and the PLC engages valves, motor starters, solenoids, actuators, pumps, and more. These I/O modules can be either analog or digital, and they are at the heart of a PLC’s functionality.
The PLC can then communicate with other devices and systems. For instance, it might export data to a SCADA (supervisory control and data acquisition) system that monitors an array of devices.
In effect, a PLC continuously cycles through the following four-step process:
1. Input Scan
2. Program Scan
3. Output Scan
This means that it receives the input, puts that input through its logic, and then determines an output based upon the result of that function. During housekeeping is where we find communications, as well as internal diagnostics and other similar tasks.
In order to operate a PLC in real-time, users depend on HMIs (Human Machine Interfaces). These allow an operator to use anything from a text-based terminal to a GUI touchscreen to review and input information in the PLC.
Uses and Advantages of PLCs
According to All About Circuits, “The purpose of a PLC was to directly replace electromechanical relays as logic elements, substituting instead a solid-state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks.”
The main advantages are, therefore, reliability, flexibility, and scalability. PLCs are built to withstand extreme temperatures, strong vibrations, high humidity, and more. Furthermore, since they are not reliant on a PC or network, a PLC will continue to independently function without any connectivity.
Flexibility and scalability are the main draws to the PLC. “Since the PLC is a programmable device, we can alter its behavior by changing the commands we give it, without having to reconfigure the electrical components connected to it,” explains All About Circuits. Instead of going through the expensive and laborious task of changing hardware, engineers only need to alter the software to change the PLC’s functionality.
PLCs may be used for countless repeatable processes because they don’t rely on any mechanical parts and they can gather and use information. PLCs are versatile. We find them controlling the flow of water in cooling and hydroelectric systems, engaging machinery in manufacturing settings, and even directing traffic flows in stop lights. These are but a few of the myriad possibilities in the world of PLCs.
PLCs also differ from other computers in the languages that coders use to write software for them. There are five languages that the IEC (International Electrotechnical Commission) supports in their IEC 61131–3 international standard.
PLCopen summarizes, “IEC 61131–3 defines, as a minimum set, the basic programming elements, syntactic and semantic rules for the most commonly used programmable languages. This includes the graphical languages Ladder Diagram (LD) and Functional Block Diagram (FBD), and the textual languages Instruction List (IL) and Structured Text (ST), as well as the Sequential Function Chart (SFC) language, used to structure the internal organization of a program. Via decomposition into logical elements, modularization and modern software techniques, each program is structured, increasing its re-usability, reducing errors and increasing programming and user efficiency.”
The most common language is ladder logic. Because it was originally modeled on the relay-logic of legacy physical devices, OT (Operational Technology) engineers easily and quickly adopted it. Rather than using lines of written code, both Ladder and Function Block use graphical logic to control a PLC.
On the other side of the spectrum, we have IL, a low-level assembly language, and ST, a high-level programming language more akin to C than other PLC languages. These languages bring their own unique advantages, such as executing complex tasks that utilize algorithms and mathematical functions, but their use is not as widespread.
For most of its lifespan, PLC software has been proprietary and restrictive. However, with the emergence of the Industrial Internet of Things (IIoT) in Industry 4.0, we’re beginning to ask more from our PLCs. These devices need to communicate on the Internet, use SQL to query databases, and even connect to enterprise clouds.
This means that tomorrow’s industries are going to need a new, more flexible brand of PLCs. Phoenix Contact is working on a solution called PLCnext, the first of a new generation of open control PLCs. On PLCnext, engineers can seamlessly combine IEC 61131–3 OT languages with common IT languages like Python, C++, and Java to meet the demands of an increasingly connected world.
PLCs vs Microcontrollers
One last point that we need to consider is the distinction between PLCs and microcontrollers. “A microcontroller,” writes TechTarget, “is a compact integrated circuit design to govern a specific operation in an embedded system. A typical microcontroller includes a processor, memory and input/output (I/O) peripherals on a single chip.” Arduino and Raspberry Pi are the two biggest names, and they are both open-source devices with large followings.
Dave Reneker from ControlDesign decided to put them to the test. “Some industrial users might envision these platforms as a substitute for an entry-level PLC,” he posits. “After all, if an Arduino can control a robot for a STEM competition entry, why can’t it control an industrial robot, or a simple machine? If it’s possible to buy an Arduino for as little as $20, why spend hundreds on a PLC?”
In his experiment, he built and programmed a water pumping system on both an Arduino and a PLC in order to compare their relative advantages and disadvantages. Here’s what he found.
As far as cost is concerned, the Arduino has a much lower sticker price. “But when all the ancillary components necessary to make the Arduino useful in this relatively simple application are added,” he concludes, “the hardware cost will narrow or disappear.” Moreover, since programming the Arduino was more laborious, a PLC turns out to be cheaper to install and maintain.
Moreover, PLCs are much more durable. This is crucial for industrial settings. As EEEnthusiast explains, “Microcontrollers often fail due to short circuit faults, static shock from the environment, physical damage, and moisture and fluid damage.”
While microcontrollers may be great learning environments and perfectly suited for individuals who want to automate domestic processes, PLCs are the clear winner for industrial settings.
PLC technology has drastically changed the manufacturing and industrial world, and we expect this trend to continue. Advances in both hardware and software components are driving growth across all industrial sectors.
As more complex tasks become subject to automation and devices become further connected through both edge and cloud computing, PLCs must adapt and expand to meet this horizon.