Dome Control PLC Replacement

The time came.

I pressed the button.

It moved.

700 tons of steel and aluminum smoothly rotated until I hit the stop button.

Nothing crashed or seriously broken.

It even moved the right direction.

The risk of breaking something was a real possibility. A mistake here could leave the dome damaged for days or weeks and the telescope useless. I reviewed my plan one more time before starting. A fully written plan with a step by step testing method carefully thought out and reviewed by the other engineers in the department. The plan also had a whole section of “what can go wrong” risk analysis, with risk mitigation steps. What if the brakes release but the motors do not come on? What if the old controller fails when removed from the system?

After a fitful night’s sleep and a long ride to the summit the moment came when I brought the new controller online. Swapping out the connections with the old controller and applying power, the correct indicator lights came on. Even better, as I tested each of the controls in turn the dome and shutters operated perfectly. It worked from the control panel, from the radio controller, under computer mode from the control room. I even tracked the telescope and dome together for an hour without trouble. The tracking was excellent, within 0.1° the whole time.

One minor bug appeared. The dome lights went off when switching to remote mode. Upon turning the switch I was plunged into darkness. Hmmmm… That was not supposed to happen. On the scale of things a very minor problem.

At the end of the day I swapped the connections back to the old controller. There was still more testing to do before the new controller would be used at night. That will come on an engineering night July first.

A second day of testing has gone smoothly as well. Arriving early I was able to run the shutters through their full range of motion before the Sun rose too high in the sky. The lights bug was squished and tested, the lights remaining on or off despite actuation of the local/remote switch. I finished the last items on my checklist without finding any issues.

This is by far the largest piece of machinery I have ever worked on… Seven hundred tons and one hundred feet in diameter, fifteen heavy electric motors, brakes, and three different control methods. My usual trade is in equipment that can fit on a work bench. Too see the dome and shutters move smoothly as the controller sequences the brakes and motors is a pleasure indeed. This was a pretty serious effort, taking many man-weeks of work to accomplish, a lot of learning along the way. I return down the mountain with a sense of relief and accomplishment.

Now to finish the documentation.