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PLCs Explained in Fewer than 100 Words

03-Apr-2023
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A programmable logic controller, or PLC, is a computerized device used in industrial and manufacturing processes to automate and control equipment. It helps to improve efficiency, accuracy and safety by controlling various aspects of the machinery, such as speed, temperature, pressure and more. PLCs use programming languages to execute specific tasks and are highly adaptable to various applications. With input/output capabilities, it monitors sensors and adds control algorithms to take actions based on the data it receives. They have revolutionized factories and other industrial settings by automating complex processes, improving efficiency and reducing errors.

Download the Mitsubishi Electric PLC catalogue, MELSEC iQ-R here and visit their website for their full range of PLCs. Read along for special bonus content and learn more about PLC FAQs!

PLC FAQ

What are PLC used for?

PLC, or Programmable Logic Controllers, are specialized computer systems that are widely used in industrial automation and control applications. They are designed to monitor inputs, make decisions based on predefined logic, and control outputs in order to automate various processes in industries such as manufacturing, energy, transportation, and more. Here are some common uses of PLCs:

  1. Process Control: PLCs are extensively used to control and regulate industrial processes, such as temperature control, pressure control, flow control, and level control. They monitor sensors and feedback devices, and based on predefined logic, they adjust actuators or output devices to maintain desired process parameters.
  2. Sequential Control: PLCs excel at executing predefined sequences of operations. They are used to control the sequence of events in machines or processes, ensuring that each step is executed in the correct order. This is crucial in applications like conveyor systems, assembly lines, and packaging machines.
  3. Motion Control: PLCs can control various types of motion systems, such as motors, drives, and actuators. They can execute precise positioning, speed control, and synchronization tasks, making them suitable for robotics, CNC machines, and automated material handling systems.
  4. Safety Systems: PLCs play a vital role in ensuring the safety of workers and equipment. They are used in safety-critical applications to monitor emergency stop buttons, safety interlocks, and other safety devices. In the event of a hazardous condition, the PLC can take immediate actions, such as stopping machinery or activating alarms.
  5. Data Acquisition and Logging: PLCs can collect data from sensors and other devices connected to them, allowing for real-time monitoring and analysis of various parameters. They can log data for performance analysis, troubleshooting, and optimization of industrial processes.
  6. Human-Machine Interface (HMI): PLCs often integrate with HMIs, which are graphical interfaces used by operators to interact with the control system. HMIs provide a visual representation of the process, allow operators to monitor variables, make adjustments, and receive alarms or alerts.

Overall, PLCs are flexible and robust control systems that are widely used to automate and optimize industrial processes, improve efficiency, enhance safety, and enable precise control and monitoring.

What is PLC used in Process Control?


PLCs are extensively used in process control applications to monitor and regulate various parameters within industrial processes. Here are some specific examples of how PLCs are used in process control:

  1. Temperature Control: PLCs are commonly used to monitor temperature sensors and control heating or cooling devices. They can adjust the power supplied to heaters or control the opening and closing of valves to maintain the desired temperature within a process.
  2. Pressure Control: PLCs can monitor pressure sensors and control valves, pumps, or compressors to maintain optimal pressure levels in a process. They can regulate the flow of fluids or gases to ensure that pressure remains within a specified range.
  3. Flow Control: PLCs are used to monitor flow meters and control valves or pumps to regulate the flow rate of liquids or gases in a process. They can adjust the speed or opening of actuators to maintain the desired flow rate.
  4. Level Control: PLCs are employed to monitor level sensors, such as ultrasonic or pressure sensors, and control pumps, valves, or other devices to maintain the desired level of liquids or bulk materials in tanks or containers.
  5. pH Control: In processes where pH levels need to be controlled, PLCs can monitor pH sensors and adjust the dosing of chemicals to maintain the desired pH level.
  6. Mixing and Blending Control: PLCs can control the mixing and blending of different components in a process. They can regulate the opening and closing of valves or adjust the speed of mixers to achieve the desired ratios and homogeneity.
  7. Batch Control: PLCs are often used in batch processes to control the sequence of operations, monitor various parameters, and ensure that each step is executed correctly. They can coordinate the timing and sequencing of valves, pumps, heaters, and other equipment involved in the batch process.

These are just a few examples of how PLCs are used in process control applications. PLCs offer the flexibility to adapt to specific process requirements, provide precise control, and enable real-time monitoring and adjustment of parameters to optimize the process and ensure efficient and reliable operations.

What are 3 types of PLC?

There are several types of PLCs available in the market, each with its own specific characteristics and capabilities. Here are three common types of PLCs:

  1. Modular PLC: Modular PLCs are designed with a modular architecture, allowing users to customize and expand the system based on their specific application requirements. They consist of a rack or chassis that can accommodate various modules such as input modules, output modules, communication modules, and specialized function modules. This flexibility makes modular PLCs suitable for large-scale and complex automation systems where customization and scalability are essential.
  2. Compact PLC: Compact PLCs, also known as small-scale PLCs or brick PLCs, are compact units that integrate the CPU, power supply, input/output (I/O) modules, and other essential components into a single compact package. They have a smaller form factor and are often used in applications where space is limited or a smaller number of I/O points are required. Compact PLCs are cost-effective, easy to install, and suitable for simpler control tasks or standalone machines.
  3. Rack-Mount PLC: Rack-mount PLCs are designed to be mounted in standard equipment racks or cabinets. They are typically used in applications where multiple PLCs need to be installed in a centralized control room or cabinet. Rack-mount PLCs offer high scalability and are suitable for applications that require a large number of I/O points or complex control systems. They can be easily expanded by adding modules to the rack, allowing for easy customization and future expansion.

It's important to note that this is not an exhaustive list, and there are other types of PLCs available, such as nano PLCs, modular compact PLCs, and high-performance PLCs. The choice of PLC type depends on factors such as the complexity of the control system, the number of I/O points, the space available, the scalability requirements, and the specific needs of the application.

Why PLC is used in automation?

PLCs (Programmable Logic Controllers) are used in automation for several reasons due to their unique capabilities and characteristics. Here are some key reasons why PLCs are widely used in automation:

  1. Flexibility: PLCs offer a high degree of flexibility in automating various industrial processes. They can be programmed and reprogrammed to accommodate changes in production requirements or process parameters. This flexibility allows for easy adaptation to different applications and the ability to modify control logic without making significant changes to the hardware.
  2. Reliability: PLCs are designed to operate in harsh industrial environments with resistance to factors such as temperature variations, electrical noise, and vibrations. They are built for robustness and durability, ensuring reliable operation even in demanding conditions. This reliability is crucial in maintaining continuous and uninterrupted automation processes.
  3. Scalability: PLC systems can be easily scaled up or down based on the requirements of the automation application. They offer modular designs that allow for the addition or removal of I/O modules and other components, enabling easy expansion or reduction of the control system as needed. This scalability is valuable for accommodating changes in production volumes or the addition of new equipment.
  4. Integration: PLCs can integrate with various sensors, actuators, and other devices commonly used in industrial automation. They can communicate with a wide range of field devices, such as motors, drives, sensors, and HMIs (Human-Machine Interfaces). This integration capability allows for seamless communication and control across the entire automation system, ensuring efficient coordination and operation.
  5. Real-time Monitoring and Control: PLCs offer real-time monitoring and control capabilities, enabling operators to monitor process variables, receive alarms and alerts, and make adjustments as necessary. PLCs can quickly process input signals, execute control algorithms, and generate output signals with minimal delay, ensuring timely and precise control of industrial processes.
  6. Safety: Safety is a critical aspect of automation systems, especially in industrial settings. PLCs can integrate safety features, such as emergency stop circuits, safety interlocks, and safety monitoring, to ensure the protection of personnel, equipment, and the environment. PLCs can quickly respond to safety events and take appropriate actions to prevent accidents or hazardous situations.

Overall, PLCs are widely used in automation due to their flexibility, reliability, scalability, integration capabilities, real-time control, and safety features. They provide a powerful and adaptable platform for automating industrial processes, improving efficiency, reducing human error, and enhancing overall productivity.

What is a Redundant PLC?

A redundant PLC, also known as a redundant programmable logic controller, is a configuration of PLC(s) that provides increased system reliability and availability. Redundancy is implemented to ensure that the control system remains operational even in the event of a component failure, thus minimizing downtime and preventing disruptions in critical industrial processes.

In a redundant PLC setup, two or more PLCs are interconnected to work in parallel, continuously monitoring each other's status and exchanging data. The redundant PLCs operate in sync, executing the same control logic and processing the same inputs and outputs. Redundancy can be achieved at various levels, including CPU, power supply, and I/O module redundancy.

If a failure occurs in one of the redundant PLCs or any the associated components, the remaining operational PLC(s) take over control seamlessly, ensuring the uninterrupted operation of the automation system. The failed component can be replaced or repaired without affecting the overall system performance.

Redundant PLC configurations typically employ fault detection mechanisms, such as heartbeat signals or watchdog timers, to monitor the health and status of the redundant components. These mechanisms allow the redundant PLCs to detect failures or abnormalities and initiate the switchover to the backup PLC(s) if necessary.

The use of redundant PLCs enhances system reliability, improves fault tolerance, and increases the overall availability of critical automation systems. It helps prevent single points of failure and ensures continuous operation even in the face of hardware failures or other disruptions. This is particularly important in industries where downtime can result in significant financial losses, safety hazards, or damage to equipment.

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