A mystery surfaced at a snack-chip maker, but American Controls & Engineering Service Inc., Rose Hill, Kan., played the successful private eye.

The snack-food manufacturer's tortilla press would run fine for a while. Yet sometimes, during the middle of a cycle, it would come down but not return to the up position. By cycling power to the machine, it would resume operation, sometimes for hours, sometimes just minutes.

There were clues, though. First, an American Controls engineer inspected the PLC code, which confirmed that the “up” solenoid was energized. Then, he ruled out hydraulic failure.

On further investigation, the engineer found a third solenoid labeled “bypass.” The customer could not provide hydraulic system prints, so the engineer performed a detailed inspection of the PLC code, discovering that bypass must be energized during up or down movement. During the stall, the PLC output light was on, but there was no power to the bypass solenoid. This pointed directly to the perpetrator: failure of the PLC relay output. And the relay contacts inside the PLC had failed.

The perp met its match when the engineer revised the machine design, installed a PLC TRIAC output card and remote-mounted relays to handle the load of the hydraulic solenoids. (TRIAC stands for triode for alternating current, which is a type of silicon device or chip.) Now, if a failure should occur, the client simply replaces a $10 relay instead of a $500 output card.

Case closed.