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Motion Control Resources

What’s a PLC (Programmable Logic Controller)?

by Fabrice Boisset, General Manager, Europe, Middle East, Africa Region

Robotic Welding

A PLC (programmable logic controller) is a digital computer used for industrial automation to automate different electro-mechanical processes. It was introduced to eliminate issues such as high power consumption that arose from the use of relays to control manufacturing processes. It consists of a programmed microprocessor whose program is written on a computer and later downloaded via a cable to the PLC. The program is stored in a non-volatile PLC memory.

How does a PLC work?

The programmable logic controller receives information from connected input devices and sensors, processes the received data, and triggers required outputs as per its pre-programmed parameters. Based on its inputs and outputs, a PLC can easily monitor and record runtime data like operating temperature, machine productivity, generation of alarms when a machine fails, automatic start and stop processes and more. This means that PLCs are robust and flexible manufacturing process control solutions that are adaptable to most applications.

PLC hardware

PLC hardware components include:

  • CPU: checks the PLC regularly to prevent errors and performs functions like arithmetic operations and logic operations.
  • Memory: system ROM permanently stores fixed data used by the CPU while RAM stores the input and output device information, timer values, counters, and other internal devices.
  • O/P section: this section gives output control over devices like pumps, solenoids, lights, and motors.
  • I/O section: an input section that tracks on field devices like switches and sensors.
  • Power supply: though most PLCs work at 24 VDC or 220VAC, some have isolated power supplies.
  • Programming device: is used to feed the program into the processor’s memory.

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PLC key features

Key features of a programmable logic controller include:

  • I/O: The CPU retains and processes data while the input and output modules connect the PLC to the machinery. I/O modules provide the CPU with information and trigger specified results. I/O modules can be analog or digital. Note that I/O can be mix-matched to achieve the right configuration for an application.
  • Communications: Apart from input and output devices, PLCs must connect with other system types. For instance, a user may need to export application data recorded by the PLC to a SCADA (supervisory control and data acquisition) system designed to monitor several connected devices. A PLC provides different communication protocols and ports to facilitate communication between the PLC and the other systems.
  • HMI: Users require a HMI (human machine interface) to interact with a PLC. The operator interfaces can be large touchscreen panels or simple displays that allow users to input and review PLC information in real-time.

PLCs will continue to grow in prominence due to the current Industry 4.0 and the industrial internet of things hype. These movements require programmable logic controllers to communicate via web browsers, connect to the cloud via MQTT and to databases via SQL. As a result, PLCs will become an increasingly important part of modern machine automation.

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