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PLCs can be found in a large number of sites performing the functions of controlling industrial processes, these processes become so different even within the same industrial complex, which are located ranging from simple procedures such as having under optimal levels the temperature value of a closed enclosure, to reach the most complex as an example of the entire sequence of steps to refine crude oil in an oil plant.

The selection of a PLC as a control system depends on the needs of the production process that has to be automated, considering as more important the aspects listed below:


Reduced space - When the place where the control system has to be installed inside the plant is very small, the PLC is the best alternative, since even with all its necessary attachments they can occupy a minimum of space without this being detrimental to the productivity and safety of personnel and facilities.

Periodically changing production processes - There are industries such as the automotive industry that year after year sees the need to change the model of the vehicle that leaves its plants, which is why both the assembly sequence and the readjustment of the tolerance values of the parts with which the vehicle is armed have to be modified. Being the main weapon of these changes, the modifications that suffer the instructions of the program that controls the logic of operation of the PLC.

Sequential processes - It is well known that when an activity is repeated a great number of times during a certain interval of time, it becomes a monotonous activity for man, producing emotional fatigue at a certain moment, causing deconcentration and involuntary induction of errors that can be fatal, both for the integrity of man and for the facilities. With a PLC, this can be avoided by simply implementing control sequences, which, although repeated many times during the day, will not lose the precision with which they have to be done.

Different actuators in the same industrial process - With only one PLC you have the possibility of manipulating actuators of different nature among them, and even more, with the same PLC you can direct different production lines in which each one has assigned to its own actuators, this last one depends on the quantity of outputs and in general of the size as far as its capacity to lodge the program of user.

Verification of the different parts of the process in a centralized way - There are a large number of industries in which the production plant is far from the control room, or also, for example, as in oil plants, there is a need to verify the remote operation of all refineries. With a PLC, the design of communication networks is had in a natural way, so that the information is channeled to a central office from which it is possible to observe remotely how the automatic control system is operating, and the graphical representation of both the sensors and the actuators is visualized by means of monitors.


For those who are beginning to enter the world of PLCs, it is opportune to give them the information of the good and bad of PLCs, so that in this way you have all the elements when selecting the most convenient control system. It is necessary to clarify that although it is possible to automate any process with a PLC, one must not fall into the temptation of turning it into a panacea to solve all the problems that may arise, for example, if we want to control the filling of the water tank that we have installed in our house, the use of a PLC to carry out this activity would be a technological waste in addition to representing a very high cost for a very simple task.

The use of a PLC must be justified for effects of optimizing especially the economic resources that in our days are very important and scarce. The advantages and disadvantages of using a PLC are listed below.
  • More precise control.
  • Faster response time.
  • Flexibility Control of complex processes.
  • Easy programming.
  • Security in the process.
  • Use of little space.
  • Easy installation.
  • Less energy consumption.
  • Better monitoring of the operation.
  • Less maintenance.
  • Fast detection of breakdowns and downtime.
  • Less time in the elaboration of projects.
  • Possibility of adding modifications without increasing costs.
  • Lower cost of installation, operation and maintenance.
  • Possibility of governing several actuators with the same automaton.


  • Specialized labor force.
  • Centralizes the process.
  • Appropriate environmental conditions.
  • Higher cost to control very small or simple tasks.


Although it is not a question of giving a recipe, here are some of the most important aspects that should be taken into account to choose one of the many PLCs that exist in the market.
  • Price according to its function (cheap - expensive, insecure - safe, unprotected - protected, austere - complete).
  • Number of inputs / outputs, and whether these are analog or digital and their ranges of operation.
  • Number of programs you can handle.
  • Number of programs you can run at the same time (multitasking).
  • Number of counters, timers, flags and registers.
  • Programming languages.
  • Specialized software for each PLC model and its ease of use.
  • Software for programming from the PC and need of interface card.
  • Ability to network several PLCs.
  • Support of the PLC manufacturing company in our locality.
  • Service and spare parts
  • Professional training on the control system.
  • Literature in our language.
  • All the criteria previously observed are becoming obvious as we advance in terms of our experience in the handling of PLCs, so here I make a careful invitation not to reverse the learning of this control system, because apart from being a very interesting universe, it is easy to understand to program a PLC as will be observed and verified in subsequent publications.

This concludes another installment that I leave for you with much affection hoping that I can be of use to you and sincerely thanking you for the time you have taken to read my article.

All images used in this article are my own, photographs taken during my professional work. I allow the use of these images as long as they are mentioned to me in the article and the source is cited.

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