TYPES OF PLC

 

The two major types of PLC

When it comes to the types of PLC, these two are the most common answer that you will find in any source from the internet simply because they are the least subtle of all the classifications available.

1. Fixed ( Integrated or Compact) PLC
2. Modular PLC

1. Fixed/Integrated/Compact PLC

This type of PLC is most commonly called the Fixed I/O PLC.

“Fixed I/O” actually stands for Fixed “Input/Output”. When you buy Compact PLCs, you will notice that the input section and the output sections of the PLC are integrated into the microcontroller itself.

This means that every type of output or input is fixed and is determined by the manufacturer.

Furthermore, the number of inputs and outputs may not be expanded in this type of PLC.

2. Modular PLC

The modular PLC is a type that allows multiple expansions of the PLC system through the use of modules, hence the term “modular”.

Modules give the programmable logic controller additional features like increased number of I/O units, and they are usually easier to use because each component is independent of each other.

The power supply, communications module, Input/Output module are all separate to the actual microcontroller so you have to manually connect them to each other to create your PLC control system.

A type of modular PLC is the rack-mounted or rack mount PLC. In a rack mount PLC, the communications module of the PLC resides in the rack itself, so all connections are centralized.

 Advantages of using Modular PLC

The modular PLC is the most desirable type of PLC especially for large industrial systems with a lot of devices to take input from or control. Here are some of the advantages that the Modular PLC has over the Fixed PLC:

Scalability

The modular PLC, because you can always add modules over and over again, provides greater scalability not only for your PLC control system but also to the company that uses the controller.

By using the Modular PLC, the processes become more centralized as the modules are only added to 1 programmable logic controller.

Because of the Modular PLC’s design, the company would be able to expand their assembly lines, for example, by adding more similar output devices that perform similar tasks to be controlled by the same controller.

This would not be possible with the Fixed I/O PLC. With the Fixed I/O PLC, you would be limited only to the capacity of that single device that performs the control and input scans in your control system.

With the Fixed I/O PLC, however, you gain economic advantage if your control system would only consist of a handful of devices such that only one device would be sufficient to handle it. That assumes that the system would not require additional devices in the next years to come.

Number of I/O

Of course, because the Modular PLC is scalable, the number of input devices to be scanned by the PLC, as well as the number of output control capability that it has, is way higher than the Fixed I/O PLC.

Even a single added module for the Modular PLC is sufficient enough to exceed the I/O capacity of a single same sized Fixed PLC.

Easier Fault Detection

Because each function is separated by each module of the Modular PLC, the fault detection becomes far easier than that of the Fixed I/O.

The latter requires that you have the knowledge of the built-in, proprietary circuitry of the PLc that you are using.

In the Modular PLC, however, you have one less problem because if for example, ALL of the outputs do not work and the PLC does not receive input from the input devices, it may be that the I/O module is the problem.

It is far more convenient to troubleshoot a Modular PLC because each component can be treated independently from the whole PLC system.

Less Downtime

As stated in the last point, you may have realized that the Modular PLC is easier to repair because the modules have separate systems.

This means less downtime for the assembly lines being controlled by the PLC, because troubleshooting will take less time than that of the Fixed I/O.

Because modules can be bought separately, a MAJOR advantage of this is you can absolutely use a backup set of modules to immediately replace a broken module once that happens.

Usually, these modules or even the PLC are designed to handle industrial conditions so that would not happen very often. But it is absolutely worth it to have a backup just in case!

With the Fixed PLC, because it would be more difficult to troubleshoot the PLC system, more time would be required for the system to work again.

Also, if the PLC happens to require some servicing by the manufacturer (which takes a lot of time again), you would have to replace the PLC altogether to get your system working again.

Basically, a backup PLC instead of a backup module for Fixed PLCs.

Module Combinations

This greatly expands the capabilities of the Modular PLC because you can mix and match the different modules however you want to achieve a system goal.

Basically, the Modular PLC offers more customization options, and you can even create a more complex system of devices which performs operations that you may have never even imagined a PLC could do.

Memory

Just like our very own Desktop Computers, Modular PLCs are capable of having memory expansions for higher storage capacity.

Blog Posted By : J.JAGAN RUPCHAND.M.E.,  Lecturer / EEE