The Case of the Gummed-Up Glue Machine
The box manufacturer was panicked. The factory used the glue to turn giant rolls of paper into packaging for large companies such as Home Depot and Rubbermaid. And now it was at a standstill. And it was midnight.
Just like in television’s CSI show, bad stuff happens outside business hours. American Control & Engineering Service Inc., in Rose Hill, Kan., sent its own crime scene investigator to the site.
The Clues. The glue machine operators noted that the PLC had gone bad. It was jumping from Step 1 to Step 2 “all by itself.” After getting a quick tutorial on how the machine worked, the ACES engineer reasoned that the scale was faulty. The PLC skipped Stage 1 because it was receiving input that it had 800 lb of water in the tank - when the tank was actually empty. Not only was the scale obsolete, but closer investigation revealed random reading fluctuations.
The Problem. That was the clue that broke the case. The problem was not in the scale head. It had to be a result of changing voltage in the sensors. After asking a few questions, the ACES engineer learned that the starch-making machine had been power-washed earlier that day. Wiring led to a junction box held closed with globs of silicone, and the box inside was dry. Then the engineer followed a pipe to another box, and when he opened the latch, water poured out. Vigorous power washing that invaded the control cabinet was the culprit.
The Solution. The ACES engineer applied water-displacing solvent to the sensors and blew them dry with an air hose. The reading on the scales plunged to -1 lb and remained stable. The PLC behaved correctly. The fix “unstuck” the starch-making machine, which soon was back online producing glue for corrugated boxes.
American Control & Engineering now is developing a replacement system for the plant’s obsolete scales, and factory operators have learned a power-washing lesson. Case closed.