What is a Programmable Logic Controller (PLC)?

In the realm of industrial automation, Programmable Logic Controllers (PLCs) serve as the backbone, orchestrating and managing the intricate processes that drive modern manufacturing and production facilities. From automotive assembly lines to food processing plants, PLCs play a pivotal role in streamlining operations, enhancing efficiency, and ensuring precision in diverse industrial settings. This article delves into the fundamentals of PLCs, exploring their functionalities, applications, and significance in today’s automated landscape.

Why we use PLC instead of usual of using microcontrollers?

One reason is to avoid rewiring.
Second reason is easiness to START and STOP.
Third reason is easiness to add modules.
Forth reason is easiness to debug.
Fifth reason is easiness to visually program.
There are more.

With normal microcontrollers, such as Atmel (including mainstream Arduino) or ESP32 or STM32, we of hard-wire the sensors for a particular circuit. It is good when we control a pump with a relay at home. Problem starts when we want to add a new feature to that pump control system or add another pump.

Other than the problem of rewiring, we have a limited number of I/O with one microcontroller. We solve this issue with multiplexing for things such as LED output or button input but for time critical PWM, that way become unreliable.

OpenPLC is an open-source industrial controls platform that allows the transformation of these well known microcontrollers into a programmable logic controller (PLC). Actually we can convert Arduino, ESP32 etc to PLC. Below is a basic example of a ladder logic program for a PLC to control a motor. This example assumes that the motor is connected to an output module of the PLC.

This is not how we work with the general purpose microcontrollers. And now Arduino offers Arduino PLC IDE:

What is a Programmable Logic Controller (PLC)?

Before PLC, there were relay boxes. In that way, controlling complex logic based on wiring and hardware used to become too much difficult. When solid state things and computers started to grow, PLC took birth. The logic in case of PLC is software based. We need to write the same snippet of C/C++ program to start/stop a motor controlled by a relay.

In the simplest case, a programmable logic controller has inputs, outputs, an operating system (firmware) and an interface via which the user program can be loaded. The user program determines how the outputs are to be switched depending on the inputs. There are PLC rack, PLC modules supporting DIN rail.

The operating system ensures that the user program always has the current state of the encoders available. Based on this information, the user program can switch the outputs so that the machine or system works in the desired way.

The PLC is connected to the machine or system with sensors and actuators. In addition, there are status displays. The sensors are connected to the inputs of the PLC and communicate to the PLC what is happening in the machine or system. Examples of sensors are e.g. buttons, light barriers, incremental encoders, limit switches, or even temperature sensors, level sensors, etc. The actuators are connected to the outputs of the PLC and offer the possibility of controlling the machine or system. Examples of actuators are contactors for switching on electric motors, electric valves for hydraulics or compressed air, but also modules for drive controls (motion control, speed control with controlled acceleration or deceleration, stepper motor controls).

A PLC can be implemented in very different ways, e.g. as a single device (“assembly”), as a PC plug-in card, as a software emulation, etc. Modular solutions are widely used, in which the PLC is composed of individual plug-in modules (also referred to as assemblies). The topic of drive control (motion control, speed control with controlled acceleration or deceleration) is increasingly associated with the PLC.

Stand-alone devices offer advantages in miniaturization and are less expensive for smaller automation tasks. A modular design offers the typical advantages of a modular system: high flexibility, expandability, cost savings in larger plants through the use of many identical modules that can be produced in large quantities.

In many areas, the PLC has replaced the previously hardwired arrangement of relays (connection-programmed control). Cost-effective assemblies, standardization of the control task and high flexibility are characteristic of the PLC concept.

In addition to the core task (control and regulation), current PLC assemblies are increasingly taking on other tasks: visualization (design of the human-machine interface), alarming and recording of all operating messages (data logging).

Increasingly, the sensors and actuators are also connected to the PLC via a fieldbus and no longer discreetly. This reduces the wiring effort. For some time now, not only sensors and actuators, but also parts of the PLC such as input and output modules have been connected to a central station via a bus and (bus) interface modules (decentralized peripherals). In modern systems, the bus systems are replaced by networks or supplemented by them. Compared to bus systems, networks (Ethernet) are more flexible and faster.

Finally, there is also an increasing connection to a company’s administrative computers (vertical integration). For example, up-to-date data on production statuses, inventories, etc. are always available (“networked factory”).

The difference between a modern PLC and a process control system is becoming smaller and smaller. Further technical development is as stormy as in computer technology in general.

Soft PLCs are software packages that can be run on a PC. In practice, industrial-grade PC versions are often chosen, although this is also possible with commercially available personal computers. The peripheral connection (sensors and actuators) is carried out either serially via bus systems or with input and output cards that are now commercially available. These systems work without a special bit processor and sometimes do not yet reach the necessary speed. They are therefore only suitable for non-time-critical applications. A slot PLC is an installation card for the PC. This usually contains a bit processor and the PLC operating system.

 
Functionality
 

At its core, a PLC executes a program to automate control functions based on input from sensors and other devices, and then produces corresponding output signals to control machines or processes. These programs are typically written in specialized languages such as ladder logic, function block diagrams, or structured text, tailored to the specific requirements of the application.

 
Key Components
 

The brain of the PLC is CPU (Central Processing Unit), responsible for executing control programs and managing system operations. Input Modules interface with external sensors and devices to receive signals representing the state of the system. Output Modules interface with actuators and other devices to produce control signals that affect the operation of the system. Power Supply provides the necessary electrical power to the PLC and its components. Communication Interface allows the PLC to communicate with external devices, such as Human-Machine Interfaces (HMIs) or supervisory control systems.

 
Applications
 

PLCs find widespread application across various industries, including manufacturing, automotive, energy, pharmaceuticals, and more. Regulating variables such as temperature, pressure, flow rate, and level in industrial processes.
Precisely controlling the movement of motors, actuators, and robotic arms in manufacturing and assembly operations. Sequencing operations in machinery and equipment to ensure proper coordination and synchronization. Collecting and logging data from sensors and devices for monitoring, analysis, and optimization of processes. Implementing safety interlocks and emergency shutdowns to protect personnel and equipment from hazardous conditions.

Advantages of PLCs

PLCs offer flexibility in programming, allowing for easy modification and adaptation to changing process requirements. Built to withstand harsh industrial environments, PLCs are highly reliable and durable.
PLC systems can be easily expanded or upgraded to accommodate changes in production capacity or technological advancements. PLCs provide advanced diagnostic capabilities, enabling quick troubleshooting and fault detection. PLCs can seamlessly integrate with other automation components and systems, such as SCADA (Supervisory Control and Data Acquisition) systems, MES (Manufacturing Execution Systems), and ERP (Enterprise Resource Planning) systems, for comprehensive control and management of industrial operations.

Challenges and Considerations

While PLCs offer numerous benefits, they also present certain challenges and considerations. Developing and debugging PLC programs requires specialized knowledge and skills.

Regular maintenance and firmware updates are necessary to ensure optimal performance and reliability. With increasing connectivity, PLCs are vulnerable to cyber threats, requiring robust security measures to safeguard against unauthorized access and attacks. The initial investment in PLC hardware and software can be significant, although the long-term benefits often justify the expenditure.

Since the PLC can also process analogue signals (via DA/AD converters), it has also conquered the field of control engineering. There are standardised solutions for almost every control task (e.g. as function modules). In this respect, the term “control” no longer corresponds to the strict definitions of open and closed loop control.

One technical limit is the potentially explosive areas. Here, either pneumatic signals or intrinsically safe circuits are used. A commercially available PLC can be encapsulated in accordance with the regulations. Of course, it is then not accessible during operation. Furthermore, a solution must be found for the connection of the sensors and actuators (e.g. pressure-resistant bulkhead fittings). In general, it can be said that a PLC is rarely used in potentially explosive areas. There are now input and output cards to which intrinsically safe circuits can be connected. There are also intrinsically safe data buses. The PLC itself is then located in a control room outside the hazardous area.

The use of a PLC is also not very useful for control tasks with very few inputs and outputs. Example: Star-delta start of an engine. Furthermore, there are machines that have to cope with very time-critical tasks. Such machines usually have very rare and highly specialized sensors. Example: the detection of a thread break in a high-performance weaving machine. The use of a PLC would require expensive field tests here.

The PLC has enormous advantages in terms of flexibility. This is precisely why it is not very suitable for “tamper-proof” controls. This has nothing to do with safety in the sense of avoiding danger to people and the environment, but with keeping unauthorized persons away.

Conclusion

Programmable Logic Controllers (PLCs) are indispensable tools in industrial automation, empowering businesses to enhance productivity, efficiency, and safety across a wide range of applications. As technology continues to evolve, PLCs will remain at the forefront of innovation, driving the next generation of smart manufacturing and industrial processes. With their versatility, reliability, and adaptability, PLCs are poised to play a pivotal role in shaping the future of industrial automation.