Programmable logic controller: operating principle

 A Programmable Logic Controller (PLC) is defined as an electronic machine. It is programmable by someone who has a non-computer specialty. But precisely, it is intended to control automatic systems in an industrial environment in real time. Programmable logic controllers (PLC).

They appeared in the United States to meet the requirements of the automotive industry to improve automatic type production lines which could follow the trend of technologies.

The first American manufacturers of the PLC are the companies Allen-Bradley and Modicon. Among the advantages of an API compared to other devices:

  •     He is reliable.
  •     Simple in implementation, the programming language is very simple.
  •     There is flexibility for adaptation.
  •     Maintenance and troubleshooting is very simple and done by technicians with electromechanical training.
  •     It is possible to integrate it into a production system.

So, in this article we will see the answers to these questions:

  •     What are the areas of use of APIs?
  •     What is the nature of the information processed by the API?
  •     The shape of the API's internal architecture?

Areas of use of APIs

PLCs are used in almost all industrial fields to control different systems and machines such as conveyors, product packaging, etc. Or production lines, for example in the automotive and food industry. It can also perform process control operations such as metallurgy and chemistry. It is also found in the field of the construction of industrial and tertiary buildings and also to control the temperature (hot and cold), lighting, alarms and security.

Generally, the programmable logic controller processes three types of information:

  • All or nothing (T.O.R.) or logic type: we have information that must be equal to 0 or 1. Detectors and push buttons deliver this type of information.
  • Analog type: the information operates in a range of values ​​or a well-defined interval. It is characterized by its continuity, this information is delivered by a temperature pressure sensor.
  • Numeric type: the information is in the form of binary codes (1001110). The computer or an intelligent system that can deliver this type of information.

The structure of a programmable logic controller

A programmable logic controller (PLC) is a tool:

  •     Adapt to the surgical part. Industrial inputs/outputs, functional processing.
  •     Current logic (sequence and/or combination).
  •     Specialized coupler, relocation.
  •     Adapt to environment, temperature, humidity, vibration, electrical and electromagnetic noise.
  •     Suitable for design and production tasks. Describe through dialogue tools

And implement the application in a simplified language suitable for debugging tasks,

• The dialog tool allows occasional editing and modification, program transfer, Simulation, dynamic visualization, etc…

• Suitable for normal and degraded mode operation tasks.

The Functional Structure of a Programmable Logic Controller

The 5 main functions of a programmable logic controller are:

  •     Detection from sensors distributed on the machine,
  •     Motion controls for pre-actuators and actuators,
  •     Operational dialogue, programming and production supervision.

  •     These functions use different communication modes depending on their particularity.

  • Or "wire to wire" link of logic input/output modules (discrete), for example, Or “serial” or “parallel” link for dedicated coupler and programming module.
  • The Functional Structure of a Programmable Logic Controller

The Hardware Structure of a Programmable Logic Controller

The API comes in the form of a set of functional blocks, articulated around a channel. Communication: the Bus. Thanks to this modular organization, the configuration becomes more and more flexible. We can see that for one.

In a different case of application, to meet the imposed specifications, the PLC must contain a certain number of components.

This modular organization produces a common hardware structure, including:

  • Form the base frame of the metal frame of the PLC with a "backplane" card (Bus + connector)
  • The power supply is responsible for providing the energy necessary for the normal operation of PLCs.
  • A central unit or CPU, comprising the processor, the basic elements of the PLC, and the memory card is independent or integrated into the central unit and the input/output manager
  • A central unit or CPU, comprising the processor, the basic elements of the automaton, and the memory card is independent or integrated into the central unit and the input/output manager.
  •     Discrete or dedicated I/O coupler, depending on the application.

the rack

However, they are metal structures that can hold many electronic boards of the same size, depending on the given spacing. Equipment can take the following forms:

  • Compact closed system with multiple I/Os defined by the manufacturer, generally reserved for low-end and small PLCs.
  • A main rack and one (or more) expansion racks, connected by PLC bus or expansion coupler if the number of inputs/outputs is high.
  • The Hardware Structure of a Programmable Logic Controller

PLC power supply

It provides the energy necessary for the normal operation of automata. The main voltages used in the PLC are +12v and ±5v, which are suitable for the operation of internal electronic boards.

In order to be able to act directly on the PO in the event of a fault, some power modules are fitted with contacts and indicator lights.

The power supply module in the reserved slot of the main chassis.

We distinguish :

  • Emergency power supply module, which provides the necessary energy from a 220V power source.
  • The power supply continues to supply the necessary energy from the External power supply (24 or 48V).

To choose a PLC power supply module. It is necessary to see its configuration and the energy balance at the level of the coupler installed.

Central unity

The central unit is the heart of the PLC, it includes all the necessary equipment

 The internal logic function of the API:

  1.     Processor,
  2.     Memory,
  3.     Input/output manager.

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