A PLC is not the only choice for controlling a process. Sticking with only basic relays may be of a benefit depending upon your application. Yet, on the other hand, a computer might be the way to go. The PLC vs. PC debate has been going on for a long time. More often though it doesn't come down to an "either or" situation but involves a mix of technologies.
PLC vs. Relay
When I first started programming PLCs it was still questionable if a PLC was necessary over just relay control. With PLC prices going down, size shrinking, and performance of PLCs improving over the years this has become less of a battle. Yet the designer has to ask themselves if a PLC is really overkill for their application. Some questions should be asked.
- Is there a need for flexibility in control logic changes? Will there be frequent control logic changes? Will there be a need for rapid modification?
A lot companies believe they will never change a design but more often then not ideas and goals do change and modifications will need to be made. Do you want to do that in hardware (relays) or software (PLC)?
- Must similar control logic be used on different machines?
It's so much easier to download a program then build another panel.
- Is there a need for future growth?
A PLC can easily accept a new module in a slot or get an expansion base.
- Is there a need for high reliability?
PLCs are seen as more robust over individual components.
- Is downtime a concern?
Any change or troubleshooting on a relay system means the system might have to go offline. Changes in a PLC can often be made online with no downtime.
- Are space requirements important?
Based on the number of relays a PLC can be a real space saver.
- Are increased capability and output required?
PLCs can be faster then their mechanical counterparts.
- Are there data collection and communications required?
Only possible with a PLC or computer.
- What are the overall costs?
There's a certain price point comparison but in these days it's very low in favor of a PLC.
PLC vs. Dedicated Controller
A dedicated controller is a single instrument that is dedicated to controlling one parameter such as a PID controller measuring a temperature for heating control. They have the advantages of an all in one package, typically with display and buttons. This can be a very good thing to use in simple applications. A PLC these days can compete price wise and functionally with these controllers especially if you more then one controller is needed. PLCs offer a greater degree of flexibility too because the can be programmed to handle all sorts of different scenarios.
PLC vs. PC (Personal Computers)
The PLC vs. PC debate has been going on for years and I'm not going to attempt to give the definitive answer. They both have their pros and cons. What often happens is that the two are used for their strengths in different parts of the factory.
| Â |
PLC |
PC |
| Environment |
The PLC was specifically designed for harsh conditions with electrical noise, magnetic fields, vibration, extreme temperatures or humidity. |
Common PCs are not designed for harsh environments. Industrial PCs are available but cost more. |
| Ease of Use |
By design PLCs are friendlier to technicians since they are in ladder logic and have easy connections. |
Operating systems like Windows are common. Connecting I/O to the PC is not always as easy. |
| Flexibility |
PLCs in rack form are easy to exchange and add parts. They are designed for modularity and expansion. |
Typical PCs are limited by the number of cards they can accommodate and are not easily expandable. |
| Speed |
PLCs execute a single program in sequential order. The have better ability to handle events in real time. |
PCs, by design, are meant to handle simultaneous tasks. They have difficulty handling real time events. |
| Reliability |
A PLC never crashes over long periods of time. ("Never" may not be the right word but its close enough to be true.) |
A PC locking up and crashing is frequent. |
| Programming languages |
Languages are typically fixed to ladder logic, function block or structured text. |
A PC is very flexible and powerful in what to use for programming. |
| Data management |
Memory is limited in its ability to store a lot of data. |
This is where the PC excels because of it's hard drive. Any long term data storage, history and trending is best done on a PC. |
| Cost |
Just too hard to compare pricing with so many variables like I/O counts, hardware needed, programming software, etc. |
Hybrids of PLC/PCs are common now (e.g. WinPLC). This type of hardware tries to mix the two platforms using the strengths of both. So the CPU might be able to run Windows CE or Linux in a rack that can accept common I/O modules.
Off the Shelf vs. Build Your Own
For some manufacturers the choice these days is coming down to buy a PLC or make your own. The benefits of PLCs have become so widely known that manufacturers looking to cut cost can engineer their own solutions and build them more cost effectively. An example of this is the Divelbiss "PLC on a Chip" with the accompanying EZ LADDER programming software.
Tools for PLC programming
PC versus PLC, Relay control, PID or other controller
This article is an excellent comparison of the pro and con when deciding what kind of control to use for a particular task.
During my career, I have used PLCs to replace relays in control cabinets, opting to use relays only in applications where different voltages were being switched (Input or Output) or when a PLC output could not handle the current requirements of the application. I've had my turn at rewiring and/or troubleshooting some pretty complicated (and sometimes extremely messy) relay control panels, often eventually replacing these with a PLC - a much cleaner, more compact option.
I have only seen a PLC "crash" in a manner similar to a PC one time in more than 20 years. This unit was pretty new, and in active service. We didn't lose control functions, but the communications interfaces failed and there was no way to access the PLC or go online with it to troubleshoot. We simply replaced the processor module and downloaded the application and were back in business. Had that happened to a PC, the turnaround might have been quite a bit longer.
Dedicated controllers, like single-loop PID controls are fine if you only have a few of them to install or service. For me, these are useful because programming a PID block can sometimes be difficult. The dedicated controller can handle the PID, and even have contacts to be able to send or receive discrete signals to/from a PLC. Many have communications interfaces too, which allow them to pass or receive more complex data from the PLC controls.
In closing, I've worked with all of the control devices mentioned in the article, and I think the PLC is tops for most applications. Where data storage is required, yes, the PC is best, but with the nice HMI programs available, and the ethernet interfaces available for PLCs, storing data on a PC host is easy, giving controls folks lots of options to choose from to do our jobs. Thanks for a great article, it really rings true.
PLC advantages.
One of the most useful advantages a PLC has is the ability to simulate I/O. Assuming the program structure buffers raw I/O to a local buffer/structure, it is fairly easy to simulate devices such as motors or valves that have feedback. We do this all the time while panel development/manufacture is taking place. This allows us to debug the routines to the point where all we need to do is test the panel for correctness and then to an I/O check once the field devices are wired.
i just came across this site and even with my years of PLC experience find it quite useful. I am also recommending some junior/newbie engineers to visit this site.
Frank
PLC Crash
How did you determine that the PLC "crashed" "in a manner similar to a PC" when " there was no way to access the PLC or go online with it to troubleshoot." I have seen over 100 failed PLCs and/or components in the past 10 years. I can only assume that your hardware environment is not nearly as harsh as what I've been exposed to.
A PLC does not really
A PLC does not really "crash" like a PC. It is designed not too. It runs one program endlessly and therefore doesn't have the memory issues like PCs. That said, hardware failures can occur and typically a PLC has a red CPU light on the front to indicate a major hardware fault.
You can troubleshoot a PLC through it's programming software or an touch screen.
Your failure rate is very high. PLCs are electronics so manufacturers specify environments that are dust and moisture free. Also, high temperatures can cause premature failures.