If you are wondering about or considering a PLC career, follow along with a day on a project. The names have been changed and some details changed to safeguard security.
"Yes boss, we are still determining the problem with the damper control on the exhaust." "We traced our electrical connections yesterday, that is not the problem." "The electrician has told me that the damper actuator has a small panel with switches and LED indicators." It’s hard to read (he is about 15 feet in the air wearing a safety harness tethered to a gantry crane).
I have to get more info on the actuator, so I leave the building and head to my laptop. It is times like these when I fully appreciate the ease of connecting to the internet and the availability of the documents on manufacturer’s websites. Back to the damper, I run through the setup procedure with the electrician but he is having some difficulty programming the actuator due to the flexing printed circuit board and him worrying about breaking it.
"Okay guys the wiring is good and the actuator setup is done, but it still doesn’t move…maybe it is jammed." So while they get to work on that, I look at some of the I/O while online with the PLC.
The temperature and humidity still looked good so the new sensors appear to be working. I had scaled the analog inputs using the manufacturer data in the PLC modules rather than in the PLC’s program. With the analog output I simply scaled 0-100% to the 4ma to 20ma output. That is the way the program was written and much easier to relate to while you are programming. For instance, say I need a valve half open, 50% or 12ma; which is easier?
I cannot move forward until we free up that damper, so I get up to go take a look, walk out the office and turn back around. I forgot my hardhat. Yes, when in the field or on the manufacturing floor safety gear is required and many employers take it very seriously for good reason. Consider safety as part of a PLC career. Back at the building, they have the coupling apart and can’t move it. "So it is jammed!" It is a new system and suspected the possibility last, I wish we proved its movement first.
A large pipe wrench frees the damper. I return to the PLC, command the actuator to rotate to verify it operates before we bolt everything together …this problem cost us several hours of troubleshooting, dis- assembly, repair and reassembly! So goes commissioning!
I verify that the position feedback for the exhaust damper is working (it is another 4-20ma input scaled to 0-100%) It is very important that this damper is proven open before we command the blower on. The 60 hp blower can over pressurize the system’s design if the damper is closed. I command the blower on…but over the radio the electrician says, "It’s not running!" What now? I am worried that this might be another several hour ordeal. Luckily it was just in OFF mode in the MCC and at the local explosion proof disconnect switch. Whew! I dislike missing the dates for completion and thus having to travel back to the site again. A PLC career will very likely require travel, less so, if your work is in a manufacturing plant where you are based.
We set up the Air damper actuator yesterday, cycled and positioned it (verified both visually and through a 4-20ma position feedback to our PLC) while online with the PLC.
With all of the new I/O checked for operation, it is time to test the program written for this system.
I have to remove some actual conditions and change the values for set-points to get it to start. Very often the programming will be done before coming to the site to commission. There is less pressure and distraction at the office, hopefully, then at site. Progress picks up speed, a couple of tweaks to timing, some additional code, and start up and shutdown of the new system is now checked and complete.
A PLC career is very rewarding, especially when you are directly involved from project beginning to end. If you like computers and programming, plus enjoy knowing how machines work it could be for you. Beware though, that you will not be just sitting in your comfortable office all the time, you will also be "getting your hands dirty" so to speak. That is the part that makes it so rewarding, seeing first-hand the new system come alive.
