What can a PLC do?  Why do we use them?

  • The CPU regulates the program, data storage and data exchange with I / O modules.
  • Input and output modules are the means of exchanging data between field devices and CPUs. Indicates to the CPU the exact status of the field devices and also acts as a tool to control them.
  • A programming device is a computer loaded with programming software that allows a user to create, transfer, and make changes to HMI software.
  • Memory provides storage media for the HMI program as well as for different data.

The concept of PLC 

” PLC ” which means ” Programmable Logic Controller “, is clear. The word “programmable” differentiates it from the conventional logic of the relay. It can be easily programmed or changed according to the application requirement. The HMI also outweighed the risk of wiring change.

What can a PLC do? Why do we use them? (in the photo: SIEMENS Simatic S7-1500, credit: fully integratedautomation.com)

The PLC as a unit consists of a processor to perform the control action on the field data provided by the input and output units. In a programming device, the PLC control logic is first developed and then transferred to the PLC.

So what can a PLC do?

  • It can perform retransmission switching tasks.
  • It can perform counting, calculation and comparison of analogue process values.
  • Provides flexibility to modify control logic, whenever needed, in the shortest amount of time.
  • Responds to changes in process parameters within fractions of a second.
  • Improves the reliability of the overall control system.
  • It is cost-effective to control complex systems.
  • It aims to pull simpler and faster
  • Can work with the help of HMI (Human-Machine Interface) compute.

The following is an example of ABB programmed AC500 logic controllers.

Basic component diagram

Figure 1 shows the basic diagram of a common PLC system.

Complete PLC diagram
PLC

As shown in the figure above, the heart of the “PLC” is in the centre, ie the heart of the Processor or CPU (central processing unit).

  • The CPU regulates the SCADA program, data storage and data exchange with I / O modules.
  • Input and output modules are the means of exchanging data between field devices and CPUs. Indicates to the CPU the exact status of the field devices and also acts as a tool to control them.
  • A programming device is a computer loaded with programming software that allows a user to create, transfer, and make changes to PLC software.
  • Memory provides storage media for the HMI program as well as for different data.

PLC system size

They are usually sorted by size:

  • A small system is one with less than 500 analogue and digital I / Os.
  • An intermediate system has I / Os ranging from 500 to 5,000.
  • A system with over 5,000 I / O is considered large.

Components of the PLC system

CPU or processor: The main processor (central processing unit or CPU) is a microprocessor-based system that runs the control program after reading the status of the field inputs and then sends commands to the field outputs.

I / O Section: The I / O modules act as the “Real Data Interface” between the field and the CPU. It knows the real status of the field devices and controls the field devices through the relevant input/output cards.

Programming device: A CPU card can be connected to a programming device via a communication link via a programming port on the CPU.

Operating station: A operating station is commonly used to provide an “operating window” to the process. It is usually a separate device (generally a PC), loaded with HMI (Human Machine Software).

PLC settings

There are two basic configurations that commercial manufacturers offer:

  1. Stable configuration
Stable PLC configuration

2. Modular configuration

Modular type PLC

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