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VIS (BS)
mark.barton - 15:19 Thursday 21 March 2019 (8480) Print this report
BS IP Fix

With Terrence and Fabian.

Today we implemented the fix to the BS IP mentioned yesterday in klog 8476. Recent IP TFs had shown a frequency of 39 mHz, which is on the edge of instability, and yesterday the IP started behaving unpredictably as we tried to center it.

We removed the tank lid and locked the IP.

We did an audit of the arc weights: there were 11 x 38 mm x 120°, 2 x 10 mm x 120° and 4 x 3 mm x 120°.

First we tried reverting to just the 11 x 38 mm x 120° (189.2 kg). This gave f = 120 mHz, which is too high (it would not have properly filtered the microseismic peak).

We tried adding back the 2 x 10 mm x 120° (9.06 kg). This gave f = 67 mHz, which would have been OK, but we decided to be a bit more conservative because of the time pressure.

So we tried removing the  2 x 10 mm x 120° and adding just the 4 x 3 mm x 120° (5.43 kg). This gave f = 80 mHz. The Q was reasonable and the IP had a good springy feel when pressed gently in L or T.

The IP position was then H1=558, H2=2600, H3=1642, L=-1220, T=184 and Y=2690.

We turned on the non-IP damping and the IP position did not change, indicating that nothing had started/stopped touching, even as the payload came to its correct height.

We used FR adjustments to get L≈T≈0. The IP responded predictably, unlike yesterday.

We used FR adjustments to get IP Y to -3900 µrad, which is what it had been after Nakano-san adjusted it on 3/18 up until yesterday afternoon. After we also undid the TILT QPD adjustment of yesterday, the TILT OL was roughly centered, indicating that we had successfully recovered the alignment.

We had plenty of FR range, so we decided to offload the IP yaw to F0 yaw. Manually and by eye, we applied about (-)3900/17778 = 0.22 turns clockwise to the F0 stepper shaft, and then used FR adjustments to get the IP to Y=0. After a few more rounds of tiny adjustments, we got a good state, with small values of IP-Y, IM-Y and OL-Y. 

We decided to arbitrarily zero the OL pitch by adjusting the TILT QPD in vertical. We only needed a small adjustment, which might be a good sign, but it could be a coincidence: up until yesterday it was at quite a different position in L (and there is some natural L->P coupling) and probably touching somewhere.

After we replaced the tank lid, the state was still very good:

  • IP H1=99, H2=155, H3=166, L=-36, T=-20, Y=234
  • IM L=-85, T=-128, V=123, R=-10, P=-738, Y=377
  • OL TILT P=-28, Y= 22
  • OL LEN "P"=21, "Y"=41
  • OL DIAG L=-130, P=-49, Y=41

The IP FR positions were -10000, -15000, -15000.

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