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Sometimes, it can be difficult to clearly understand the differences between PLCs, PACs and industrial PCs because, at the end, all of them are used for the same purpose, controlling a process, but they all do it in different ways. Some people even think that these three systems compete with each other, but they are actually used for different specific purposes since they have their strengths and weaknesses.
Industrials PCs are not difficult to differentiate from the rest, they are usually used as the brain of a whole system, or maybe for a simple data base used to store data from many other devices, using the power and versatility of general-purpose operative systems, like Windows and Linux, and the power of more powerful CPUs like Intel Core or Intel Xeon.
On the other hand, PLCs and PACs stand in a more blurry line, PACs are supposed to be the best of both worlds, the power and trust that PLCs offer along more powerful CPUs, larger memories, and other advantages, but with the continuous advance in technology, PLCs have advanced to such degree that they do offer a great amount of power and functionality as well, that is why PLCs have changed their route of development and marketing in the last few years, and now offer more compact and cost saving designs.
To be able to have a better understanding of the differences between these three systems, it is worth coming back to the era of their conception to have a better look at the roll that they were made to fill, since, as we said before, with the increasing development of the technology, the differences between them are becoming less noticeable and obvious at glance.
At the end of the 60s, the industry was still working with the old relays and other components important for combinational logic, but these were incredible hard to work with in every point of view, the necessity for a change to a more reliable system brough the invention of the first PLC, which it basically was a smaller and reprogrammable version of the old relay racks.
A few years later, even if PLCs were the standard for the industry, they had a few disadvantages that were born from their lack of CPU power. For instance, the lack of motion control and inability to do complex processing tasks. This resulted in a need for the addition of PCs to the system, which were used as a kind of command center for the different PLCs used in a system.
Industrial PCs were introduced in the 90s, but they had an apparent slow start since the first versions were not suited for harsh environments which were very common in the industry, not only that, there was a lack of trust in their operative systems which had serious stability problems, and would be a huge problem at the moment of updating or changing something in the system.
Then we have PACs, they came into the market as a more powerful alternative to the original PLCs, with two or more integrated microcontrollers, which gave the possibility to use a more complex logic system, to have a better integration with other platforms like SCADA, and to add many other functionalities like motion control, all the things that the industry were looking for but without the lack of reliability of the industrial PCs.
Today, industrial PCs are still in the same spot, the have huge computing capabilities but has the downside of having a general-purpose OS, but some improvements in their systems have given more trust to the industry. On the other hand, PLCs have improved to such degree, that many times is hard to see the difference between them and PACs, that is why PLC lines are changing to provide other advantages that PACs can’t offer, like small footprint or integrated and full featured systems in just a piece of hardware.
A deeper look into the characteristics of modern PLCs, will show to everybody the wide range of features that they are able to provide, some might say that they look a lot as the first PACs, from capable analog I/O to advanced communication protocols, the more advanced are capable of remote I/O and motion control, two features that were previously only available in PAC systems.
This brings up the question: how do they differentiate from PACs? Well, manufactures have been changing the target of most PLC product lines, offering not only a wide range of functionality but other advantages like small footprint, and an incredible decrease in price over the last couple of decades, PLCs are now commonly seen in small systems like elevators, vending machines, home automation, and some small projects in farms.
Since PLCs have been targeting smaller projects where scalability is not the most important thing, PACs have taken the place in the bigger automation projects, they provide great scalability, huge number of control I/O, all the necessary CPU heavy tasks, the possibility to add other PACs for redundancy, and much more. They might not be the smallest, but they surely provide an incredible number of features, reliability and upgradability.
Right now, PACs are much more expensive than PLCs, and this shows how the PLCs have been giving more space to PACs in the big budget industry, while PLCs are the go-to option in small scale projects, this might also have to do with the huge development that the industry of microcontrollers and embedded system have had in the last few years, where even a cheap Raspberry Pi can do what a PLC can with the right hardware.
Industrial PCs have been gaining trust thanks to the development put into them to make them more suitable for industrial environment, not only in the hardware side (which was actually fixed at the end of the 90s), but in the software side, for instance, there are some IPCs capable of setting tasks to work in parallel to the OS, this means that even if there are some changes needed in the OS, it is possible to make them without interfering with the control tasks.
The use cases for IPCs are basically anything that needs the CPU power and the versatility of operative systems like Windows or Linux, for instance, some are used as a control center for other different control devices like PLCs and PACs, another example is to use them as data base center for the devices of a control system.
Well, that depends, you have to carefully study the characteristics and requirements of the project, and depending on that information the best system shall be chosen. The most important requirements would be how complex it is, the scalability needed, the support given by the manufacturer, and, lastly, the cost.
In some cases, the cost might be the most important thing, but when choosing the right system for your project, going for an initial big investment might prove cheaper in the long run, having in mind that if the system needs to grow in some manner, a device with high scalability is a better choice than a device that will be completely replaced and would end up being a bigger expense.
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