SDSS Trip Report
Windbaffle Servo System Commissioning at APO
May 13-16, 1998

Paul C. Czarapata
Fermi National Accelerator Laboratory
Beams Division / Headquarters
Batavia, IL   USA    60510

On this trip I concentrated on the continued commissioning of the Windscreen Servo System. The new Altitude motor (with brake) was installed and wired. The brake control was implemented and tested. The Windscreen altitude brake is applied whenever both directional inhibits are asserted. This is the only mode that makes sense since if only one inhibit is asserted (at zenith for example) the windscreen must still be able to move off the limit. The E-Stop generates a bi-directional inhibit and thus asserts the brake.

I took an additional look at the noise being picked up on the LVDT’s for both Azimuth and Altitude and found that there was a ground loop through the iso-buffer that was inducing the noise. I removed the buffer and went directly to the ADC of the PLC. This change brought the operational noise down to a few hundred counts or less with all servos on. This appears to be an acceptable level.

The altitude loop gain was set for the proportional part of the PID loop. Additional tuning will be needed for the I and D terms when the actual windscreen is installed and wind loading and other structural resonance’s can be explored. Based on what I observed I believe the servo system will be "stiff" enough to meet the tolerances at the telescope truss.

One operational difficulty that will have to be addressed is the functioning of the telescope servo amplifier interlocks. The interlock for the bi-directional inhibit require someone press the reset on the front of the amplifier. I have spoken to Peter Prieto and he knows of the problem and has a new interlock PAL to install.

On departing the site, the windscreen system was operational in both Azimuth and Altitude and had tracked the telescope motion for over twelve hours while work on the telescope servo system progressed. A small amount of my time was spent on other miscellaneous system issues.

Questions regarding this trip report should be addressed to Paul Czarapata by phone at (630) 840-3106 or by e-mail at

Last modified 05/27/98