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VIS (PR2)

fabian.arellano - 16:51 Monday 26 April 2021 (16571)

Setting the mirror at nominal height (including buoyancy effect in SF).

With Hirata-san.

See pictures in album PR2 Remedying Work.

Becuase the GAS Filter LVDT card seems to have been broken and replaced without calibration, I measured the buoyancy effect with the vernier calliper measuring the difference between the keystone heights before and after venting. I locked the keystone at the initial position, measured it with respect to a reference and then locked it at the final position and measured it again. I did it twice using different reference points:

	Initial	Final	Buoyancy effect
SF LVDT (uncalibrated)	-1029	4297	5326
Vernier ref. 1 (mm)	12.50	10.23	2.27
Vernier ref 2 (mm)	19.83	22.14	2.31

The reference points can be seen in the diagram on this page of the notebook.
The buoyancy effect is 2.3 mm. It seems relatively large. This result requires more thought.

Then we determined the SF fishing rod range. We used a similar measuring method. We moved the keystone to the FR upper limit, locked it and measured the position with respect reference point 2 (see notebook). Then we did the same for the lower limit.

Place	SF LVDT (uncalibrated)	Vernier ref 2 (mm)
Upper limit	8975	26.67
Lower limit	2270	21.13
Total	6,705	5.54

The fishing rod has a relatively large moving range of 5.54 mm. Very good.
I also moved to a higher position the safety nuts at the upper part of the stepper motor shaft.
I changed the stepper motor limits in the medm screen:
- Upper keysone limit: -1,440,000 steps
- Lower keysone limit: 410,000 steps.

Later we proceeded to set the mirror at the nominal height using the laser level. The configuration was as follows:

BF at the current setpoint. Because the LVDT was not calibrated, this setpoint is not so meaningful but we still used it for O3GK. More thought on this is required.
SF LVDT at 6148 (uncalibrated)
SF fishing rod: roughly at mid-range.
Mirror about 5 mm lower than the desired height. See this picture.
Temperarture 23.9 °C.
O3GK temperature: 23.2 °C.

We cannot compensate this 5 mm difference with the SF fishing rod becuase it is already at mid-range. We require the temperature to decrease.

Images attached to this report

Comments to this report:

fabian.arellano - 18:14 Tuesday 27 April 2021 (16584)

See pictures in album PR2 Remedying Work.

In order to have an accurate idea of how much we will need to move the GAS filter keystones up, I measured the height of the mirror under the following conditions:

SF keystone locked at nominal height of 65.5 mm from the base plate per the 3D-CAD. I measured is indirectly with respect to another reference with a vernier calliper and then checked the height with a ruler. See this photo. The SF LVDT readout at this position was 1604.4 units (the LVDT doesn't seem to be calibrated properly).
The BF keystone at the setpoint per the LVDT. This position is arbitrary becuase the LVDT doesn't seem to be calibrated.
Laser level at the vertical centre of the flange.

The mirror was 6.5 mm below the centre of the flange.

Given this large difference I checked the relative position of other components:

The RM, with respect to the security structure, is lower than the nominal height by 2.5 mm. The nominal value per the 3D-CAD is 31 mm (see this screenshot) and the measured value is 28.5 mm (see this picture).
The security structure, with respect to the center of the flange, is lower than the nominal value by 3 mm. The nominal value per the 3D-CAD is 175 mm (see this screenshot) and the measured value is 178 mm (see this notebook page).

The values above add to 5.5 mm. There is 1 mm missing out of the 6.5 mm. Please note that the laser level line is around 2 mm thick as seen by the bare eye. In pictures the thickness may vary depending on the camera.

For the sake of gathering additional information I measured the height of the SF keystone when the LVDT readout was zero: 0.6 mm below the 65.5 mm level. See this notebook page for details.

tomotada.akutsu - 16:30 Wednesday 28 April 2021 (16612)

Why don't you ask to add the makers more?? Those markers must be also used for installation of mid-size baffles...

fabian.arellano - 17:10 Wednesday 28 April 2021 (16607)

With Hirata-san.

See pictures in album PR2 Remedying Work.

Correction to mirror height estimate due to roll

When Tomaru-san and Hirata-san marked the height for the nominal position of the mirror on the chamber and pillars, I forgot to clarify that we needed them at the -X side of the chamber and they did it at the +X side. Becuase of this, we can only use one out of the three markings they made to measure the height of the +X side of the RM. This means that we need to take into account the roll of the RM in order to estimate the height of the centre of the mirror.

This morning we estimated that the +X side of the RM is lower than the -X side by 3.6 mm. See this picture and this other picture. See the notebook pictures for more details.

This means that the previous estimate of the height of centre of the mirror (RM) is not 6.5 mm below nominal, but 4.7 mm instead.

Setting the mirror height with the SF fishing rod

We moved the SF keystone up with the fishing rod while keeping the BF keystone at the setpoint. We successfully made it reach the appropriate height when the SF LVDT readout was 8786 units. The total range of the FR is 5.54 mm as reported before. Then we measured transfer functions for the SF and BF:

At such a height, as can be seen in the first screenshot, the resonance frequencies remain the same as in the reference but the amplitude decreased. This is likely due to the LVDT not being in the linear range.
We moved the SF keystone close to the setpoint (about 200 LVDT units above) and measured the TF again. See the second screenshot. It is exactly as the reference.
As seen in the third screenshot, the BF TF is very good. (Reference has 30 mHz resolution whereas the new measurement has 10 mHz resolution.)

Observations

In order to get the same SF transfer function amplitude when the mirror is high we likely need to adjust the resistances in the LVDT card in order to extend the linear range and do a proper calibration.
We need to check we can accout for the buoyancy correction. Becuase I don't trust the previously reported value of 2.3 mm, I'll consider the one measured in PR3 SF, which is approximately 600 um.

About the SF fishing rod range for buoyancy effect correction

Becuase we can't use the LVDT to measure the keystone displacement in units of µm, we locked the keystone in the place in which the mirror is at the correct height (with the BF at the setpoint) and measured the keystone position with respect to the reference point 2 shown in the diagram in this notebook page. Then we moved it to the upper FR limit and repeated the procedure. Before moving the FR we changed its upper software limit to -1,570,000 steps. This changed added an additional 0.44 mm to the already existing 5.54 range, to give a total of FR range of 5.98 mm.

The result is that we have an additional 1.42 mm of fishing rod range to cope with the buoyancy. This should be enough (600 um in PR3 SF).

I the stepper motor limits in the medm screen are now:

Upper keysone limit: -1,570,000 steps
Lower keysone limit: 410,000 steps.

Images attached to this comment