A local box manufacturer called ACES in a panic because the machine which produced the starch that glued their corrugated cardboard had suddenly quit working, resulting in a sticky mess. The factory used glue to turn giant rolls of paper into corrugated packaging for large companies like Home Depot and Rubbermaid, and the factory was at a standstill without it. A senior Control Systems Investigator (CSI) from ACES arrived on the scene at midnight.
The glue machine operators informed the CSI that the PLC had gone bad: It was jumping from Step 1 to Step 2 all by itself. After a quick tutorial on how the machine worked, the ACES CSI reasoned that the scale was faulty, and causing the PLC to jump to Stage 2 because it was receiving input that it had 800 lbs of water in the tank — when the tank was actually empty. Not only was the scale obsolete, but closer investigation revealed random reading fluctuations. Because of that clue the CSI knew that the problem wasn’t in the scale head: the problem had to be a result of changing voltage in the sensors.
The operators revealed that the machine had been power-washed earlier that day. The CSI traced the wiring to a junction box held closed with globs of silicone, but the box was dry inside. Then, he followed a pipe up to another box — and when he opened the latch was hit by a gush of water.
The vigorous power washing had sprayed water in the control cabinet.
The CSI applied water-displacing solvent to the sensors and blew them dry with an air hose. The reading on the scales plunged to -1 lbs and remained stable, and the PLC behaved correctly. The glue machine was unstuck and was soon back online producing glue for corrugated boxes. ACES is developing a replacement system for the obsolete scales to avoid downtime in the future.