As I walked into the control room the first thing I heard was, “Why can’t I commission this transmitter?” The technician was pointing to the screen where he was trying to commission some Foundation fieldbus transmitters that had been taken out for service. At least this time he’d remembered to decommission them first.
“I don’t know, let me take a look,” I responded as I started sizing up the situation. The first thing I noticed was that the three transmitters were appearing and disappearing from the utility. That’s never a good sign. “Have you checked the voltage on the segment?” I asked. Of course the answer was no, so I directed him to go to the rack room and check it.
“The voltage is fine. Well above the 18 V minimum,” he reported back in a few minutes.
I suggested that he go check it at the transmitters. Since the transmitters were located a few hundred yards from the control room, it was a little while before I heard anything, but then the radio crackled with, “I’ve only got 5 V at the transmitters.” A short while later, he walked back into the control room and said, “I found a seagull nest on the cable tray and they’d pecked through the wiring.” OK, we had found the problem, but he’d spent several hours trying to commission devices that were never going to work. He was a trained instrument technician, so why hadn’t he immediately started by checking the wiring?
This pattern has been repeated thousands of times in my career, even before the advent of computerized controls, but it seems that it has become much worse with the advent of smart instruments. People who have worked in the field for years seem to have suddenly forgotten the basics of their jobs. A good connection is required for digital communication just like it was for sending an analog signal. Adequate voltage is required to generate the signal just like it was for analog devices. The tools for finding these issues are still the same as they always have been, a VOM and your eyes. Sure, those tools have been augmented by things like handheld communicators and diagnostic programs, but there’s still no substitute for basic blocking and tackling.
The cure for this is of course training, training, and more training. The more people work with the tools, the better they become at quickly separating what the computer can and cannot do. If your diagnostic program can only communicate intermittently with the device you’re trying to diagnose, then stop trying to use it and go see what’s causing it to come and go.
At the same plant, I had another technician say he couldn’t see the transmitter he’d just connected to the bus, but he could see the other one on the same fieldbus segment. These two transmitters were being used to control a pair of parallel pumps. So I walked out to the area with him, and the indicator on the transmitter was showing the pressure in the line so it had adequate voltage. Looking closer at the device, I realized that it was an analog transmitter! Then I realized that the transmitter that I was seeing from the computer was in the wrong line! The technician had not been trained to decommission transmitters before removing them if there was a possibility that they wouldn’t be put back in their original location. As a result, the transmitter had come back on line with its original ID, so the control system thought it was still controlling the B pump when it was now connected to the A piping.
It’s no wonder that the customer didn’t want anything to do with fieldbus devices again. But that wasn’t the answer; the answer was, to paraphrase an old political adage, “Train early and train often.”