What are the different types of PLC?

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As with every piece of technology in the world, every device must come to a point where it must be able to be scaled, or designed for more specific applications: large-scale, small-scale, medium-scale, analog, digital, direct current, alternating current–you name it. The basic Programmable Logic Controller has adapted to these technological advances by branching out into different types that suit each specific application and hence maximizing economical resources of each consumer.

The types of PLC may be classified according to some parameters. However, you must be reminded that some overlaps may apply thus creating a combination of PLC types per manufacturer.

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.

Fixed/Integrated/Compact PLC

Types of PLC: Fixed I/O
Types of PLC: Fixed I/O (Photo from blog.cetrain.isu.edu)

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.

Modular PLC

Types of PLC: Modular PLC
Types of PLC: Modular PLC (Photo from blog.cetrain.isu.edu)

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.

What are the 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.

An analogy for the Fixed I/O and Modular I/O

The best analogy that I could think of is a Laptop computer as the Fixed I/O PLC and the Desktop computer as the Modular I/O PLC.

Laptop computers usually have a fixed RAM and Hard Disk Memory and are usually more expensive to even upgrade.

The compatibility of the Hard Disk and the RAM is also an issue in laptop computers if expansions happen to be available.

Desktop computers have slots that allow the user to just plug in or replace existing parts to upgrade their computer.

The RAM or the Hard Disk may easily be replaced by just removing the existing one from its respective connections and replacing it with the new one you bought somewhere.

Although laptops usually have greater advantages that the PC for the same price (because they are portable), the limits of their processing power are quite low compared to desktop computers.

Like the Fixed PLC, it may initially be more superior to the Modular PLC at the start, but in the long term, the Modular PLC will still offer a greater economic advantage in highly demanding applications.

Type of PLC according to Output

Switching operations in the primitive stages of the PLC was Relay-type. This means that there is an electromechanical switch being controlled using the Programmable Logic Controller to power the output devices.

Through years of development, developers have found greater advantages in using other methods of controlling the outputs using different switching components: Triac and Transistors.

However, the type of load must be considered first before you choose the type of output that the PLC should have.

Relay Output

Types of PLC: Relay output
Types of PLC: Relay output (Photo from plcs.net)

Relay outputs are suitable for both alternating current, and direct current output devices.

It’s basically a Relay as an output, and the PLC controls the switching of the relay by running current through its coils.

If you did not know yet, having current through a coil creates a magnetic field in the middle of the coil which attracts the metallic contacts of the relay.

However, one of the problems with relays is the mechanical wear and tear that it experiences during repetitive switching operations.

Hence, you should realize by now that the Relay type output is more suitable in infrequent switching operations e.g. activating a continuous-running device such as a motor.

Transistor Output

Types of PLC: Transistor Output
Types of PLC: Transistor Output (Photo from plcs.net)

Transistors are semiconductor devices that are used for switching operations and are used inside microprocessors in a micro- or nano- scale.

In transistors, no mechanical or moving component is involved, so faster switching is achieved using this type of output.

A switching device that does not have moving components is called a solid-state device. You may have already heard this in storage devices: Solid-state drives.

Of course, you should already expect higher speeds from solid state drives as you would expect in solid state outputs of the PLC.

Anyway, only DC outputs may be handled by transistors because of their construction: they only allow one direction of current to flow after they have been switched.

One advantage of this output type is that the PLC uses optoisolation to switch the transistor: hence isolating the PLC from the output’s electrical source.

Triac Output

The triac is also a solid-state device that is an equivalent of two “mirrored” transistors (specifically, the bipolar junction transistor).

Because the current can now flow in two directions, this type of PLC output may be used in controlling outputs that use Alternating Current.

Analog Output

The previously mentioned types of PLC outputs are designed for switching operations. This means that they are digital in nature because they only operate in the ON or OFF condition.

Analog outputs are in a different realm. Usually, the Analog Output PLCs control speed of continuously running devices such as motors or turbines. Of course, the voltage/current ranges vary depending on the manufacturer and/or modules.

Types of PLC according to Size

Larger PLCs are usually designed to handle a higher number of devices, even in both Modular and Fixed types.

However, using these larger PLCs may not always be the most efficient device to become the controller of your automated system.

Manufacturers of the PLC have created smaller and smaller PLCs to suit smaller applications. In buying the Fixed type of PLC, this is usually the most considered factor because as you have learned earlier, Fixed PLCs are more efficiently used in small-scale applications that do not require a large expansion in the future.

Mini PLC

Mini PLCs have usually have 128 to 512 I/O points which are already a LOT for a control system.

For small control system that is only predicted to scale up a little bit, Mini PLCs are ideal to use instead of larger PLCs (above 512 I/O points).

Micro PLC

Micro PLCs are the ones that have 15 to 128 I/O points. They are most commonly used in very small automation or control systems like amusement rides.

Why so? Because amusement rides require that the controls are near the ride itself so this eliminates the requirement that there should be a “Central PLC” for all the rides in the amusement park.

In addition, because each amusement ride can only add a couple of devices here and there for safety purposes, they would not require a high number of I/O points so the Micro PLC will already suffice.

Pico/Nano PLC

Pico PLCs have less than 15 I/O points.

Usually, they are seen in PLC trainer systems because they are very easy to use (and usually comes with a display panel) and they do not look intimidating to beginners who want to know the basics of PLC first.

The thing that is really desirable in Pico/Nano PLC devices is the simplicity and compactness as they are usually just the size of your hand.

Safety PLCs

The safety PLC is a special type of PLC that conforms to the IEC 61508 – Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems.

Basically, this type of PLC functions just like the normal logic controller except that it almost does not fail.

That sounds too good to be true because it is. No real-world scenario is ideal. Hence, Safety PLCs are equipped with “redundancies” such that if it DOES fail, it does so in a safe manner.

The only thing to keep in mind, though, is when you use safety PLCs, you must have experienced and educated maintenance personnel to examine these PLCs.

In addition, it is generally more expensive than the normal PLC because of the added safety circuitry.

Other than that, if those were not a problem, then it is more convenient in the long run to use safety PLCs instead of normal PLCs.

Conclusion

Different types on PLCs are available at the market and it is actually a vital information to learn before you actually connect an input or output device to a PLC or even put the PLC in your control system if you are the one responsible for the design. Some may connect an AC output to a transistor based PLC and that would already become a problem. If you have read up to this point, you now have a brief overview of the different types of PLCs that are cost-efficient in their specific application.

I suggest that you check the PLC programming guide next to learn more about PLCs!

Thank you for reading!

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