As the new year is well underway I find myself in the midst of my major project for this year. For the next few months I will be replacing the control system for the Keck 2 dome. The project is well underway, but the real work remains ahead of me.
Parts for the PLC/2 are still available, but the programming software is a real issue. The software runs on a DOS (as in pre-windows) operating system, and does not run under the emulation modes of later Microsoft systems. You need a real DOS computer, something that is a bit rare these days. If the controller running the Keck 2 dome were to fail, I am not certain we could repair it.
I have the computer that is used to do the programming in my office, an ancient Compaq Portable III. Portable is an odd word to use with this computer, it weighs over 20lbs and is huge by modern laptop standards. This museum piece still works! Last month I booted it up and wandered through the file system. At power up I was greeted by a monochrome amber screen and a DOS prompt. I still remember a few basic DOS commands, enough to check things out. It appears that all of the software and files are still present on the hard drive. Sometime I need to see if I can indeed program the old PLC/2. If something goes wrong in the update I need to be able to revert to the original control system.
Fortunately the process of replacing an old PLC in an operational facility is one that is by no means unique. We are not the only facility with ancient equipment controlling vital machinery. The PLC manufacturer has provided a few ways this can be done. For our system I have decided to keep the older rack and existing I/O modules in place. This avoids having to completely rewire the hundreds of wires connecting the PLC to the dome equipment. I will replace the original PLC/2 processor card with an I/O scanner module. This converts the entire rack into a remote I/O assembly that can be slaved to the new controller. Later the I/O modules could be re-wired to new I/O modules in the new PLC one module and 32 wires at a time.
Thus the plan is simple… Pull the original PLC/2 controller out of the rack. Install the remote I/O scanner module in place of the old PLC. Connect the new ControlLogix PLC to the scanner… Turn it on. It should (famous last words here) take just a few minutes to switch over. If something goes wrong just put the original controller back into the rack. At least that is the plan, I am going to attempt it a few times on the simulator before I do it in the Keck 2 dome.
Building the test set has taken many days of tedious assembly and wiring. it took hours just to fasten all of the switches in place. Bundles of wires need to be run, at least two to each switch or LED indicator. This is all 120Vac circuitry, so everything needs to be done neatly and with proper insulation for high voltage. It was so nice to flip the power switch and watch the lights come on.
What remains ahead is porting the old software to the new controller. This is a straightforward process, with the hardware working there should be no roadblocks to accomplishing this part of the task. With the test set complete I will be able to fully test the new controller and software.
It is that point the whole project becomes much more daunting. I will have to install the new controller in the real dome, replace the old PLC controller with the I/O scanner, then turn it on and try it. There will be a moment when I press the button and watch 700 tons of steel and aluminum move. Trust me, when it works, and it will work, there will be celebration.
wow Andrew. What a daunting task!! I am happy that Keck has such a talented electrical , as well as computer engineer to do the job .. I watched the dome yesterday while up there doing a full slew to point the shutter at the sun to melt the accumulated ice so they could do science last nite . What a smooth
dome spin it was!!
Good luck.
Aloha