Basics of PLC - The Workhorse of Industrial Automation

Introduction

• PLC - Programmable Logic Controller
  • Special purpose computer used in industrial automation.
  • No display, keyboard, printer, or hard drive.
  • Traditionally replaces relay panels.

Advantages of PLCs over Relays

• Relays:
  • Turn on and off devices.
  • Time-consuming setup.
  • Complicated to change logic.
  • High failure rate.
  • Consume more electricity, produce heat and soot.
• PLCs:
  • More reliable, less downtime.
  • Smaller, less energy consumption.

Components of a PLC

• CPU Module
• Input/Output Devices (I/O)
  • May be part of the CPU or separate modules.
  • Connected by backplane or data cables (can be miles away).

Capabilities of a PLC

• Digital and Analog I/O
• Numerical Inputs and Outputs
  • Includes calculation capabilities for SPC values and PID controllers.

Programming a PLC

• Programming Application
  • Runs on a standard desktop/laptop.
  • Communicates using Ethernet or proprietary bus.
  • Recent preference for Ethernet and USB.
• Ladder Logic
  • Different implementations by different manufacturers.
  • Varies in capabilities, methods, and logic processing order.

Standardization Attempt

• IEC 61131 Standard
  • Defined by American National Standards Institute (ANSI).
  • Describes several programming languages with various strengths and weaknesses.
  • Local standards or customer specifications usually dictate the language used.

Conclusion

• Overview of PLC structure and its importance in industrial control systems.
• Additional resources and training available at realpars.com