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Programmable Logic Controllers (PLCs) have considerably changed industrial automation by allowing for a great deal of
flexibility and making it easier to develop and implement complex logic for advanced automation. PLC-based control panels are also
significantly smaller than relay-based ones and can use touch LCD (HMIs) that replace rows and rows of light indicators. The ability to interface
with analog sensors was also expanded with the advent of the PLC.
PLCs are generally very well received in the automation industry. Yet, as with any new technology, some in the industry opposed the use of PLCs, preferring instead to continue to use the older relay panel technology. Many discussions and arguments comparing PLC based automation to relay based automation ensued. However, over time, the benefits of PLCs became too difficult to ignore and relay based electrical panels soon fell to the wayside. Indeed, I sometimes find myself talking to colleagues about how I used to walk into huge old electrical panels with a voltmeter to trace a circuit through multiple relays and to try to figure out why one of the indicator lights was dim instead of OFF. I was freshly out of college and wondering, "what did I get myself into?" Luckily, PLCs came to my rescue.
A PLC is a microprocessor based circuit board with many components: transistors, resistors, FETs, oscillators, capacitors, and many more. There is also firmware running to execute the user’s ladder logic, watchdog timers to recover from lockups and I am sure patch after patch and bug fixes all running in the background to make it possible to automate and easily change and add new ladder lines.
Robust they are, but they do have hundreds of components that can and do fail. We are therefore here bringing back that old argument about PLC vs relay panels, but this time, the discussion is about whether there is a place for those old relay panels in today’s automation field. Of course, some will easily say yes, while others will reject the idea of using the full 500ft spool of wire to create logic.
In our opinion, even though we now have reliable PLCs and relays, the fact is that a system with a large number of components will tend to fail compared to a similar system with less of the same components. A control panel using 10 relays and a PLC along with an HMI and indicator lights will most likely have a higher failure rate than a panel using 10 relays and the same number of indicator lights. The second panel will run without those lines of code making the automation possible and will not have that nice touch LCD making the interface user friendly. But it will also run with virtually no maintenance or attention for years and years until it is full of dust and those nice and white manual pages turn yellow with that old paper smell.
Relay-based panels tend to be difficult to build and troubleshoot, but when they run, they just keep running. In short, we believe that given the right application, there is room in the automation field for relay-based electrical panels. For those applications with a small number of inputs and outputs and in need of that extra robustness and safety factor, perhaps the PLC system should take a step back and let the old relay panel do the job. In these scenarios, a PLC or PC can still be used for monitoring and reporting, but the actual automation should be done with the relays and not those many lines of code running in the background and hundreds if not thousands of electronic components to condition the I/O signals for the microprocessor to perform its function. Putting cost aside, applications needing the best possible reliability and transparency should consider the use of relay panels if the number of inputs and outputs is manageable (10 to 15) and the logic is somewhat simple. There comes a level where complex logic makes the relay based automation very difficult and results in problems that can be extremely difficult to troubleshoot.
In short, hardwire logic should be on the table when applicable and beneficial. Here at Meskotech, Inc, we pride ourselves on always looking for the best and most robust automation solutions for our clients. This includes the use of microcontroller, PLC, and PC-based systems, as well as the occasional relay-based panel when the application calls for it.