The industrial automation project I just finished is a showcase for a certain manufacturer. I used its PLC, HMIs, VFDs, Ethernet-networked IO, sensors, and safety products. It was a one-stop shopping trip. In only two instances did I have to go to a different manufacturer for a particular component; the showcase manufacturer simply did not make those components. I’m very proud of the way the system came together almost exactly as we initially designed it. We did run into some surprising Sherlock Ohms-type cases though.

The one I wish to discuss involved its safety system. E-Stop buttons, light curtains, and guard switches feed into a bank of modular safety relays to create several safety zones. E-stop buttons and guard doors will interrupt the entire system, but breaking the light curtain on the right side will not interrupt production on the left (and vice versa). The modular safety relays use input modules, a base module, and output modules (sort-of a mini-PLC in approach).

The odd behavior involved resetting a time-delay output module. Sometimes, that module would not reset, even though the base module had passed the reset signal to it and reported all output modules energized. The symptoms were difficult to trace; the system had no safety events active, the safety circuit reported all was well, but no motion. Several times, I found myself digging through the program looking for a chunk of logic that wasn’t working the way I expected.

On the one hand, it was a relief to discover the logic I’d created was fine, that it was a hardware component that would sometimes not reset when told. On the other, there was no reason for this to happen, and the sporadic nature was not helping. Eventually, our supplier was able to produce a tech notice published by the manufacturer describing exactly the symptoms we encountered.

Unfortunately, there was no easy fix for the component. The manufacturer said it was a defect in the firmware, but that the time and cost involved with fixing the firmware of a safety product weren’t justified when they were in fact planning to roll out the successor to that safety product line.