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The Basics of Programmable Logic Controllers

What do amusement park rides, factory assembly lines, and light fixtures have in common? They are all controlled by a Programmable Logic Controller (PLC), a digital computer used to control machinery by constantly monitoring input and output devices.
PLCs are designed for multiple input and output arrangements, and can withstand various temperature ranges, electrical noises, vibrations, and impacts. Integrating a PLC into any production line or mechanical process is highly beneficial. PLCs enable operation processes to be changed or replicated, while simultaneously collecting and communicating important information.

How do PLCs work?

There are four basic operational steps for every PLC:

  1. Input Scan: Identifies the status of all input devices that are connected to the PLC.
  2. Program Scan: Implements the user-created program logic.
  3. Output Scan: Either energizes or de-energizes all connected output devices.
  4. Housekeeping: This includes communications with programming terminals and internal diagnostics.

What are input/output devices?

Input Device: An input device is a piece of computer hardware equipment used to provide data, and control signals to an information processing system (IPS).
Some examples of input devices include:

  • Switches and push buttons
  • Sensing devices
  • Limit switches
  • Proximity sensors
  • Photoelectric Sensors
  • Condition sensors
  • Vacuum switches
  • Temperature switches
  • Level switches
  • Pressure switches

Output Devices: An output device is any piece of hardware used to communicate the results of data processing carried out by an IPS, and translate the information into an understandable form.

  • Valves
  • Motor starters
  • Horns and alarms
  • Stack lights
  • Control relays
  • Pumps
  • Printers
  • Fans

What are the fundamentals of a PLC system?

CPU or processor: The Central Processing Unit, or main processor, is a microprocessor-based system. It executes the control program after reading field input status, then sends out commands to field outputs.
I/O section: I/O modules act as the Real Data Interface between field and CPU. A PLC knows the real status of field devices and controls them with relevant I/O cards.
Programming device: CPU cards can be connected with programming devices through a communication link via a programming port on the CPU.
Operating station: An operating station is used to provide an “operating window” to the PLC process. It is generally a separate device, like a PC, that is loaded with Human Machine Interface Software.

Why should I use a PLC?

  • PLCs eliminate the need for rewiring and adding additional hardware for each new logical configuration.
  • These devices increase the functionality of controls and do not take up much physical space.
  • Since PLCs are sectional, they can be mixed and matched, so you can choose the best combination of input and output devices for your specific operation.
  • PLCs can perform relay-switching tasks, as well as count, calculate, and compare analog process values.
  • A PLC’s flexibility makes it easy to modify control logic at any time.
  • PLCs are cost-effective for controlling complex systems.
  • PLCs provide easy trouble-shooting capabilities.
  • PLCs can work seamlessly with Human-Machine Interface computers.