The logical control system has been in the mix even before solid logic circuit started. Its sole purpose to help electromechanical relays do its work. Electromechanical relays are used even today, but most companies prefer a far more modern logiccontrol device. Relays are now only used in applications that require switching with high voltage power. Systems that require on and off switching controls are very important in most machines, but it is rare to find built-in electromechanical relays and discrete logic gates. So instead, companies use a digital computer program to do all thefunctions.
In the 60’s, a company called Bedford Associates invented device that later became the widely used PLC or Programmable Logic Controller. It was first named MODICOM or Modular Digital Controller. Bedford Associates became the only manufacturer, designer, and supplier of these devices. When PLC became popular, other engineering companies started to design and manufacture their own version of PLC to meet the high demand in the market. PLC is used to replace electromechanical relays in performing certain logical tasks. The PLC can copy the connection of relays in doing certain tasks.
PLC interprets the signals whether it is a high or low frequency using its input terminals. With its output terminals, its power lights, motors, and devices that use it’s on or off control. To make PLC user-friendly, the program language was designed like a ladder logic diagram. Industrial electricians and engineers can easily read it since they are used to read logic language. They can easily adapt to PLC since it performs like the control function they are accustomed to. PLC is just normal computers made for industrial purposes. The PLC is designed to replace electromechanical control relays. Most Programmable Logic Controller uses 120 volts just like its predecessor, it can also input or output, low
To know more about the history of PLC, visit: https://en.wikipedia.org/wiki/Ladder_logic
AC voltage signals being used in some logiccircuits.
PLC models differ depending on the signal connection used or what are the programming standards. But all of them are similar in a way that they allow generic instructions used by PLC programmers. An opto-isolator device or commonly known as Light-Emitting Diode is used to connect both input and common station inside the PLC housing. This LED emits a high signal to the computer’s mainframe using a 120-volt power. When the power flows through the input and common station, an LED light will indicate that the output is on.
Computer programs are the ones who made the logic PLC uses in its control system. It which output to turn on using input conditions. There is no switch relay or relay coil working inside PLC, although it the programs used are ladder logic diagram. All programs that the programmers used are entered using a personal computer that is connected to every PLC port in the system. There is no power relayed when the button is not pressed. So unless there’s a power going through the coil, the output remains unenergized. All power and connecting wires that appears in the computer’s screen arevirtual, it means no actual hardware. There’s no actual circuitry inside the PLC. They are all computer language made by programmers.
Everyone should know that personal computer is not a necessary component for PLC to run. Once the program is inputted, PLC will still run the program. PLC is very versatile and powerful comes from the fact that it alters the behavior of the control system. Although it is a programmable mechanism, PLC can always alter the behavior by making the commands different depending on how they want to use it. The program can be altered without changing the electrical components.
There are advantages in using a PLC in software instead of in hardware. The signals that are inputted can be used in the PLC program multiple times. Since it is a programmable control, you can use as many contacts as possible because the output is just a small speck in the PLC’s memory data and it can be reused. The button connected to the input will serve as the “ON” button while the other button will serve as the “OFF” button. There’s another contact that uses output status coil like a seal so the motor contact can be energized continuously after the program is turned on. When the ON button is pushed, the contact will be energized at the same time, the other contact that is connected to the OFF button will close inside the program. The closing of the OFF contact will send a 120-volt power thru motor coil. IF we release the ON button, the motor coil will run continually since the coil status is in place, thus the power will continue to flow to the motor coil. To make the motor stop, the OFF button should be pressed, making the current flow in the OFF contact. When the OFF button is released the ON contact will return to its standard state. Until the ON button is pressed again, the motor coil will not run.
To have a better understanding about PLC diagram, visit: http://www.ecmweb.com/content/what-know-about-plc-ladder-diagram-programming
The most important part of understanding PLC is that the fail-safe model of the PLC is as important as its controlled electromechanical system. Programmers with PLC training should always monitor the effect of the open wiring of the PLC device being used. One of the advantages when using PLC is that electromechanical relays can’t copy the PLC. It is being monitored remotely and controlled using a digital personal computer network. Since a personal computer is connected to other computers, the PLC can communicate with them very easily. PLC is used in almost all modern facilities around the world because itis powerful, versatile and can easily control since it is fully automated. Although the program itself is not cheap, it will save you money in the long run since you only have to maintain via a personal computer. We can say that the PLC can make electromechanical relays obsolete in the near future, but it is a good thing, right? Facilities using PLC can save money for maintenance and labor.