J. Kissel, J. Warner

Based on the results of B&K hammering the H1 ISI HAM6 blades before and after installing the new blade tip and flexure dampers, I've retuned and re-installed the pre-existing tuned-mass dampers. Lots of details, results, discussion and techniques below. As Jim mentions in LHO aLOG 26482, this concludes the installation of the HAM-ISI damping mechanisms for H1 ISI HAM6. 

Thanks to all who've have helped!

We will close up the chamber tomorrow as planned.

Details
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Blade Characterization

Over the past few days, as we've been installing the new Blade Tip Dampers (D1500469)
 and Flexure Dampers (D1500200) -- see LHO aLOGs 26482, 26473, 26456, and 26452 -- we've been gathering before-and-after B&K transfer function of the blade-spring & flexure system without the eLIGO tuned-massed dampers.

The results of the this characterization are shown in the attachment 2016-04-07_H1ISIHAM6_DampingInstall_BandKResults_BladeSpring_UndampedvsDamped.pdf. (Recall, for your convenience, which blade is which from the attachment HAM6ISI_Sensor_and_Blade_Names.pdf).

As expected, all three blades' first bending mode has moved down from ~153 Hz to ~139 Hz due to the extra mass of the blade-tip damper. Note, sadly, only Corner 3's "after" measurement has any coherence above ~300 Hz because we didn't starting using the B&K hammer's hard tip until that last corner. That means we don't have a quantitative metric for how much the dampers reduced each blade's higher frequency modes we expected to damp (at ~450 Hz and ~900 Hz) -- but at least we can see the end result from corner three. However, we have no reason to believe that these dampers aren't working at these frequencies as expected. Once pumped down, we'll make a comparison between the GS13 ASDs before and after the vent, and that'll be our final assurance that all has gone well.

We definitely need to use the hard-tip on the B&K hammer for all future installs and characterization of ISI damping mechanisms.

Tuned Mass Damper Tuning

Once we'd identified the new first bending mode frequency, the smaller, narrow-band tuned mass dampers needed retuning. I took them into the optics lab, set up a crude shadow sensor system with a HeNe laser and On-Trac QPD, hooking up the output to an SR785. See attachment 2016-04-07_TMDTuningPics.pdf for pics of the setup. 

The tuning itself was pretty easy. I went in armed with a set of calipers to try to a priori predict the increase in length needed to decrease the resonance to the desired frequency, but the tightening of the restraining bracket turned the frequency tuning to a guess-and-check game regardless. Pre- and Post measuring after tuning was finished showed an increase in length of 0.6+/-0.015 [in] to 0.66+/-0.015 [in]. 

A comparison of the resonant frequency and Q is shown explicitly in 2016-04-07_H1ISIHAM6_BladeTMD_Tuning.pdf. I was able to preserve the Q of about 10, and moved the frequency down to 139 [Hz] as desired. Given that I knew the frequency of each blade's first bending mode, and the resonance of each tuned mass damper, I matched each blade to the appropriate TMD and they're thus annotated in the legend by on which blade they were installed.

Tuned Mass Damper Installation

Nothing much to report here, the installation went smoothly. With the TMD in hand, I reached through the outer wall and "picture" access panel to secure the magnetically attached device to the blade. It took a few pictures and adjustments to get the final location "right" as shown in 2016-04-07_TMDInstalledPics.pdf (though they probably don't *have* to be so precisely oriented and located). Installing the TMD on Corner 1's blade was the "most difficult," in that it's a little uncomfortable and awkward climbing in chamber, sitting down, and reaching into the ISI, but it's totally doable even for someone of my stature.