You work for an electrical services firm and are on a client's site when your project manager calls and says to see the plant engineer about a production problem your boss thinks you can solve.
The plant engineer takes you to the PLC cabinet of a line that produces plastic panels. It uses heating elements, mixing valves, motors, and other equipment all under PLC control. The problem is variances in the thickness of the product. Sometimes a panel is too thick, sometimes it's too thin, and sometimes its thickness isn't uniform.
The plant engineer says, “We've put everybody on it at one time or another, but production keeps reporting variations. We're defeated. Can you fix this?”
That so many people have worked on the problem is a big clue. This lack of focus and organization tells you many things, including there's been a lot of guesswork. When there's a problem with a PLC-controlled system, the problem is almost never in the PLC but that's usually where untrained repair techs will tinker with things. Since they are unfamiliar with the PLC, that's where the problem “must” be. The plant engineer took you there, which is another clue.
Ask them to restore the PLC programming to where it was before this problem began. While they are doing that, see what kinds of signal simulators and other equipment you need to check each final control element and each sensor. If those all check out, perform loop simulations and look for inductive coupling with power wiring due to improper routing of loop cabling.
