Getting Started is a series dedicated to helping you enter the field of automation. These are aimed at the true beginner.
Not knowing the difference between relay and relay ladder logic (RLL) is understandable on your first week or so. Many parts are identical, as RLL is modelled after relay logic. Normal relays have coils and contacts. The contacts are common, normally open and/or normally closed. Some relays will instead have separate inputs that will cause them to latch (on or off). Timing relays also exist.
Figure 1: Common symbols for relay logic and RLL
Consider the absolute basics using a simple example: a garage door opener. You have three buttons: open, close and stop. If you press open, it opens until it hits the limit switch. If you press close, it closes until you hit the closed limit switch. If you press stop, the system stops running.
Figure 2: Simple garage door logic.
In this case, the logic is really simple. If you are making a simple system that you want anyone to be able to work on, you want to use relays to the extent possible. This would be an utter waste (in the author's opinion) of a PLC. PLCs are good for complex systems with some degree of calculations involved, otherwise they end up adding delays to resolving issues for everyone who cannot go online with the program.
PLC's also work just a little differently. Most end up doing (roughly) three things, repeatedly:
Scan inputs
Calculate things
Write outputs
They also do not behave quite the same as a real circuit. Consider Figure 3.
Figure 3: Every PLC trainer's dream. To my knowledge, no one has found a use case for this circuit, outside of PLC training classes.
If you read this as a PLC would, you get the following:
Coil A is energized on when (CR-1 AND CR-2) OR (CR-1 AND CR-3 AND CR-5) OR (CR-4 AND CR-5)
Logic only flows one way in RLL, but electricity flows both ways in real life. Hence, if this were a relay circuit, you could also energize coil A if (CR-1 AND CR-3 AND CR-5) OR (CR-4 AND CR-3 AND CR-2).
One popular demonstration with RLL is the old "light box" trick, where you have two different rungs operate the same line of ladder. It works with synchronous scanning, where the last rung to define a coil causes the output. (How would you have two rungs of ladder power the same coil in relay logic?)
That is probably enough for one evening. Enjoy!
Comments