9 reasons why PLC is used over Microcontrollers

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Ever since the semiconductors were invented, the electronics industry grew exponentially at an exceedingly high rate in terms of the number of electronic devices produced each year.

Microprocessors, which are made of millions of semiconductors, are now more powerful than ever before. From 8-bit processing, microprocessors now support up to 64-bit processing and are actually THE standard ones to use these days.

From the production of cheaper, but more powerful processors, microcontrollers have also become the rage in the market, especially the Arduino and the PIC Microcontrollers.

What is a Microcontroller?

A microcontroller is a compact integrated circuit designed to manage devices that consist a control system. They are embedded devices that have processor, memory, and I/O control in one unit.

Microcontrollers are VERY inexpensive, and can also perform similar processing tasks that resemble the ones in the industry. In fact, I just checked Amazon and saw microcontrollers priced at less than $10.

They can perform operations from simple ones to advanced, such as:

  • Mathematical operations
  • Logical processing
  • Serial communications
  • Wireless communications
  • Pre-processing of data for Machine Learning
  • Controlling devices for a DIY Smart Home
  • Reading data from both Analog and Digital devices

There are basically endless possibilities for the use of these devices.

Microcontrollers have indeed high capabilities as well as the flexibility to become used in every control system that comes to mind.

You may think “Microcontrollers are the IDEAL devices for companies! Decreased costs, processing and control capability, more flexibility and customization–these companies are stupid for not replacing the PLCs!”. That’s also what I thought.

You see, the basic PLC was actually designed to be future proof and is still being developed by the different makers. They even have different types of PLC that suit the control system to their size. Having said that, here are some of the reasons why the PLC is used over the microcontrollers.

User-friendly

PLC over Microcontrollers easy

This is perhaps one of the top reasons out there to use PLC or any device for that matter.

Anyone can easily learn the basics of PLC and immediately start using it because it has a very short learning curve. This means that the time that it will take you in getting from step 0 to step 1, where you actually start programming a PLC is WAY SHORTER than when you are going to use the microcontroller.

You just have to know the basics of PLC and how it works, the different devices, how to program them, how to connect them, and you’re set.

In my experience, you need to have at least a background on electronics before you can make a microcontroller (the stand alone versions, e.g PIC) work.

There are a lot of pins that you should be aware of–whether they should be grounded or not, where they should be connected or shorted to, etc. This would not be a problem if you had the PIC programmer, though.

Later, you would see that these pins can become a problem once applied to a system.

Okay, it’s not a big deal, but, when you just have to look at the datasheet over and over again just to see the pin configuration, it sucks!

You may not have to experience the same event if you use pre-built controllers such as the Arduino, an open-source microcontroller. Because it is open-source, information is EVERYWHERE about the things that you can do for this device.

However, this advantage is overcome by the PLC because it is proprietary.

You see, even though the disadvantage of PLC is the limited resources about it available on the Internet, the companies that make PLC make sure that every customer gets EVERYTHING that they need.

When you buy programmable logic controllers, user manuals come with it. They have detailed instructions on how to use the product that they make so that the consumer can use it to its full potential immediately.

Even when you buy input devices for logic controllers, user manuals ALSO come with them. This is perhaps the advantage of a proprietary device.

Also, you do not anymore need to have a ton of interfaces for the different devices that you will control. For example, controlling a 24Vdc device requires an interface for the microcontroller just because it can only produce a small amount of voltage.

Using a PLC just ELIMINATES this problem, because the PLC is designed to work with industrial devices immediately.

Also, because companies routinely change the personnel assigned to the maintenance of the control system, it is assured that the PLC would be easily understandable in terms of the electrical connections as compared to the microcontroller equipped with a ton of interfaces.

Ease of programming

This should be under the User-Friendly heading, but I put it here anyway because it was getting too long. Anyways, PLC programming is graphical, hence it is easier to understand even for newbies and non-technical staff.

It’s actually a great bonus because it would not take a lot of time for them to understand the program uploaded to the PLC.

In the microcontroller’s case, some already use Flowcharts in programming, but usually they are programmed using lower level language like C or Mnemonics–yes, the ones that computer engineers use.

I looked at some PLC forums and what’s actually common for them is that the ladder logic diagram was the easiest program to comprehend because it’s basically just “Connect the two vertical lines, and the line, called “rung” is ON”.

In an event of failure, I read that these same people noticed that even electricians who had zero experience in programming a PLC themselves, have been helped by the graphical characteristic of the program in their troubleshooting tasks.

Perhaps the greatest programming advantage is that even if someone else did the program, it was easy for the PLC program to be modified by another person in a short period of time.

In microcontrollers, let’s say the Arduino (easier to program), you would have to know the functions of each line that the previous programmer had entered before you can start modifying the program.

In my experience, when I program a microcontroller, it’s just very confusing to have a different “style” of code in front of you, especially if the variables were not properly named. I had to start again from scratch if I wanted to “debug” the code.

Also, for different input and output devices, different libraries would not be required. This is because, as you have learned in this post, the PLC was designed to be compatible with the existing industrial devices.

Modular Design

There are two types of PLC, the Compact PLC and the Modular PLC. The Modular PLC, as the name implies, have different modules that you can just “plug n play” in order to start using them.

Expanding your system will only take a very short amount of installation time because the modules are just basically plugged in to the rack mount of the PLC. Modules can also include: Memory, I/O points, Communications.

Just as you thought, this design allows the PLC to be scalable and hence allows your control system to expand along with the business.

With the microcontroller, however, they are usually limited to a number of I/O units (and they are not many).

Also, not only you could add components to easily upgrade your system’s capacity, but because of the modular design you are also relieved of some troubleshooting problems that come with fixed designs like the microcontrollers.

When fault happens in a PLC control system using a Modular PLC, you can easily identify which module caused the problem, replace it by a spare one, and start the operations again.

The faulty module will be sent to the service center for repair. It’s that easy.

Safety systems

PLC over Microcontrollers safe

As I have discussed in my post: “What are the different types of PLC?”, there are these special types of PLCs called the Safety PLC.

Safety PLCs have integrated safety functions called Redundancies that accomplish two objectives:

  • Maximum uptime: zero failure
  • In an event of failure, safely do so

Not only that, even normal PLCs already have safety systems (which make the Safety PLCs even safer).

Diagnostic features are installed in all PLC devices such that before the system even starts, it detects errors that the operator or programmer may have overlooked in his PLC Commissioning.

Robustness
PLC over Microcontrollers robust

Let’s face it: Microcontrollers would not be able to handle the extreme conditions in the work areas in the industry.

Because they come unshielded, they are subject to electromagnetic interference because radio waves are present everywhere and hence cause a disturbance in the unit via electromagnetic induction, electrostatic coupling, or conduction.

Also, they would not be able to handle high temperatures for a long period of time.

Power fluctuations may also easily destroy your microcontroller.

In PLCs, however, these are not even a problem. They are immune to noise, and can easily handle the high temperatures in the industrial setting, along with the power fluctuations through regulation.

The PLC is an industrial grade device! If you look at ALL descriptions of the PLC on the internet or in any reference material, you would see that the PLC is a “ruggedized computer designed to perform industrial control”.

Maintenance

The PLC requires very low maintenance because they are robust.

Again, any maintenance electrician that may be assigned to the PLC control system can easily check the whole system because the connections are just easier, the program is highly readable, and it is standardized.

Historical Inertia

PLC over Microcontrollers historical inertia

Programmable Logic Controllers are systems that have already proven themselves through the years of application and development.

It solved the major problem of control using the Relays, and it just continued to develop to suit companies that deal with a wide number of controlled devices.

Nowadays, PLCs can even be networked using peripherals such as the Ethernet, or other communication devices. This makes a more complex system out of the basic PLC setup.

Also, because the PLC was used since a very long time ago, companies are less likely to migrate to a microcontroller based setup when the spares that they have been designed for PLC systems.

Even so, new equipment for PLC is still compatible with the old ones that were manufactured years ago. This makes the PLC more cost-efficient in the long run than the microcontrollers.

Upfront costs are less important

PLC over Microcontrollers upfront costs

For companies that are making millions and millions of dollars every year, buying a PLC control system that is a thousand bucks more expensive than the microcontroller based ones would still be the likely option.

Why? Because even though the upfront costs are high, the maintenance costs are EXTREMELY low.

It goes back again to the previously mentioned reasons: the PLC is easy to learn so it does not need a programming expert, it is robust, the compatibility issues are gone, safety mechanisms are present–just everything that a company wants for a device that LASTS is already in the PLC.

Microcontrollers are cheaper, yes, but their qualities are just not the ones that companies are looking for in the industrial side. It’s really quantity over quality with high productivity for them (and people should try to apply this in their lives, too!).

In the long run, the PLC would produce less downtime than the microcontroller-based ones thus generating more assets for the company with a lower maintenance cost.

Standardization

In companies, they want more standardization rather than higher customization approach to control system designs.

With the help of IEC61131 standard, all companies worldwide already have a guideline to follow.

In PLC systems, this eliminates confusion in troubleshooting or in programming, especially when there are different people involved in the same task.

As what I’ve mentioned earlier, companies let people check the PLCs in a concurrent fashion. This means that a standard must be followed and complied to strictly to avoid confusion which what generally causes larger problems in the long run.

Conclusion

Overall, PLCs are still king in the industrial side of automation and control systems. Although microcontrollers may be used in highly specialized systems, they are just not suited for the battles that the PLCs face in the industry. PLCs have less complexity, cost-efficient, and robust–that’s generally the reason why they are preferred for industrial control systems.

Thank you for reading!

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