Telescope operational limits apr10

Last mod: 29apr10

Intro
Memos, tables, presentations from Amman & whitney
Min td tensions required vs za
Allowable za range vs temp
Minimum tensions required for various dome, ch configurations.
Summary

Plots

The minimum tiedown tension required vs za (.ps) (.pdf)
Allowable dome za range vs temperature (.ps) (.pdf)
Astronomy observing with the dome: ch sits at stow (.ps) (.pdf):
Astronomy observing with the ch: dome sits at 8.469 degrees (.ps) (.pdf)
Aeronomy observing with the ch: ch sits at za=0 (.ps) (.pdf)
Aeronomy observing with the ch: ch sits at za=15 (.ps) (.pdf)
Aeronomy observing with the ch: dome sits at za=12 (.ps) (.pdf)
Testing: astronomy using the dome with ch tracking dome above 8.83 degrees za (.ps) (.pdf)
Testing: astronomy using the dome with the ch sitting at 19.9 degrees za (.ps) (.pdf)

Intro

A broken beam was found on the telescope 03feb09. A splint was installed and verified on  23mar10 to bring the beam back to its original strength. An analysis using the current weights and operational motion of the telesecope performed by the engineering firm ammann & whitney resulted in new operational limits for the telescope (see email sent 29mar10). The summary of the new limits are:
•  The maximum allowable unbalanced load on the azimuth arm is now a function of the minimum tension in a tiedown corner. As the tiedown tension increases, the maximum allowable unbalanced load can increases to a maximum value of 15000 kip-feet.
• Fitting a linear curved to their plot gives:
• MaxUnbalancedMom= -645.933 + 325.837*tdCornerKips
• Inverting the function to give minimum required tiedown tension for a given unbalanced momment:
• MinTdCornerTension= 1.982 + .003069*MaxUnbalancedMomKips
An analysis was then done for most of the beams on the platform and azimuth arm using full telescope motion. The beams similar to the one that broke (6 sets of 2) showed stresses above the allowable limit. A few other beams were overstressed (but to a lesser amount). The plan is to reinforce the 6 sets of 2 beams similar to the one that broke so they are no longer overstressed by full telescope motion.
A further analysis is in progress checking that the cable tension safety factor is within the allowable range.

The sections   contain:
• These are the communications, analysis, and presentations from the engineering firm
• This shows the min tiedown tension required vs za.
• The allowed za range vs temperature.
• How the tensions in the tiedowns are changing vs az,za and temperature for various observing configurations (dome tracking, ch tracking, aeronomy observing).

Memos, Tables, presentations from Amman & whitney:

Platform reinforcement sep10:

Platform reinforcement jun10:REINFORCEMENT U1-L2-SK-3.pdf

17may10: procedure for re-enforcing broken member (u2-u2l3) (.pdf)
29apr10: A&W presentation from 29apr10 meeting (.pdf)

21may10: A&W non-linear analysis of cables:
27apr10: tensions from A&W modelling for 29apr10 meeting in NY.
• Notes:
• STAAD is the  modeling program they used in 2010.
• For STAAD (forces 20000) means they used an unbalanced moment of 20000 kip-fit on the azimuth arm.
• member labeling: U is uppaer, L is lower Un-Lm is diagonal upper to lower
• U0 is at the corner of the triangle, U1 is the next joint in from the corner on the upper chord.
• U0-U1 is the chord that connects U0 with U1.
• Bracing trusses are the  3 sections that make the triangle into a hexagon (part way in from the vertices of the triangle).
• platform truss diagonol U2L3-L2 is the beam that broke.
• column terminology
• gross area does not include holes (bolts).
• Fy yield failure,  usually 1.6 max allowable.
• ksi Kips per square inch. if < 16 ksi then no fatigue (infinite life).
• I moment of intertia, Fcr critical failure??.
• Platform Truss table (.pdf)
• Platform Stress table (.pdf)
• Feed arm truss table (.pdf)
29mar10

Min tiedown tension vs zenith angle

To figure out the minimum tiedown tension required vs za:
• Compute unbalance(za) for dome moving  (ch at stow) and ch moving (dome at stow)
• clip to 15000 kip feet
• fit curve from Amman and whitney: maxUnbalance vs tdTension
• invert to get tdTensionMin = c0 + c1*unbalance
• Compute minTdtension= c0 + c1*unbalance(za)
The plots show the minimum tiedown tension required vs za (.ps) (.pdf):
• Top: dome tracking, ch at stow. They are balanced around dome=8 deg za
• Bottom: ch tracking, dome at stow. They are balanced around ch=12 deg za.

Allowable dome  za range vs temperature.

Using the A&W limits of ap10, compute the za range vs temperature for the gregorian dome for the configurations:
• Ch at stow, dome moves in za
• Ch at stow until dome moves above 8.8 degrees za, then ch moves at equal za as dome
• Ch at 0 degrees, dome moves in za 8.2 upward.
To compute the allowable range for each configuration:
• For a given config, compute the unbalanced moment as the dome moves in za.
• use the dome pointing at and away from corner 8 since this corner has the lowest tensions (see there is the extra weight in the corner from the ubolt cut/repair).
• For temperature ranges 65,70,75,80,85,90 compute the kips that are needed to keep the platform in focus.
• Limit the allowable range to for each config so that the tiedown kips are >= to the minimum kips required for a given unbalance (see above).
The plots show the allowable dome za range  vs temperature for various ch configurations (.ps) (.pdf)
• Top ch at stow.
• middle: ch tracks dome position when dome above 8.4 degs za
• bottom: ch sits at za=0 (aeronomy). The dome must be at least 8.2 degrees away from the ch to prevent collision.

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Min tensions for various dome , ch configurations.

To figure out the az, za limits i:
• Computed the tiedown tension vs azimuth angle (0..360 degrees) for a fixed dome za,ch position, and temperature.
• I used the tension in tiedown 8 since it is a little lower than the other two tiedowns.
• plotted these tiedown tensions for dome za's  1.06 to 19.69 degrees
• over plotted (in red) the minimum tiedown tension allowable for the given azimuth arm unbalanced moment.
• The tiedown tensions should stay above the red line to be within the operational limits.
• Each page contains the results for 6 different temperatures : 65, 70, 75, 80, 85, and 90 degF.
The computations were done for 4 configurations of the ch with the dome moving, and 1 config of the dome with the ch moving. All of the computations assume that the platform is in focus:

• Astronomy observing with the dome: ch sits at stow (.ps) (.pdf):
• dome moves in za, ch sits at stow (za=8.83)
• Page 1:
• Top: azimuth arm unbalanced moment vs dome za when ch at stow
• The dome shouldn't go above za=17 since this is 15000 kip-feet.
• Bottom: maximum allowable unbalanced load vs tiedown corner tension.
• Page 2:
• Tiedown tensions vs dome za for complete azimuth spins.
• The kip variation at each za is from the azimuth spin.
• The red line shows the minimum allowable tension.
• frames top to bottom have temps: 65, 70,75, 80,85, 90 deg F
• Notes:
• the 15000 kip-feet limit is hit at za=17 (that's why the red line goes up above there).
• For 75 F we can run from 3.5 to 12 degrees za.
• Astronomy observing with the ch: dome sits at 8.469 degrees (.ps) (.pdf):
• ch moves in za while dome sits at 8.469 degrees.
• Page 1:
• We can move ch 0 to 20 degrees without hitting the 15000 kip-feet limit.
• Page 2:
• The ch can move 0 to 20 degrees za with restriction up to 80 deg F.
• At 85 DegF the ch must stay above za=4.
• at 90 DegF the ch must stay above 9 deg za (if tracking).
• Note: the daytime cable temperatures may be higher than the air temps reported.
• Aeronomy observing with the ch: ch sits at za=0 (.ps) (.pdf)
• ch sits at za=0, dome can be positioned above 8.4 degrees za.
• Page 1:
• With the ch at 0 za, the dome should be at or above 8.4 degree za to avoid the collision limit.
• With the gr at 15 za, the unbalanced limit is 15000 kip-feet so the dome may never go higher than za=15 with ch at 0.
• Page 2:
• At 75 deg F the dome can go up to 10 deg za. This assumes nighttime temperatures.
• During the day there are azimuth where the ch can not be at 0 deg for any dome za.
• The temps are for the air temp sensor. The cable temperatures during the day can get higher than this.
• For daytime observing, you probably should leave the azimuth arm fixed with the dome pointing opposite a td corner:
• az=60, 180, or 300 deg az
• Aeronomy observing with the ch: ch sits at za=15 (.ps) (.pdf)
• Ch sits at za=15. dome moves za=3 to 15 depending on temp.
• Page 1:
• dome can now go to za=18 (depending on temp).
• Page 2:
• For 75 F the dome can move za's 5 to 13 degrees.
• Aeronomy observing with the ch: dome sits at za=12 (.ps) (.pdf)
• dome sits at za=12. Shows how high you need the ch vs temp.
• Page 2:
• For 75F the dome must stay above za=10.
• This config is illegal for temps above 83F
• Testing: astronomy using the dome with ch tracking dome above 8.83 degrees za (.ps) (.pdf)
• This set shows that using the ch as a moving counter balance when the dome it above 8.3 degrees does not help much.
• Page 1:
• We can now get to za=19.69 with 15000 kip-feet (but see the td tensions on the next page)
• Page 2:
• For 75 F we can now get to 12.5 degrees za.
• The ch has moved from 8.83 -> 12.5 = 3.6 degrees.
• The dome weights 6 times more than the ch so the limit only moves up 3.6/6=.6 degrees.
• The lower limit doesn't change since the ch sits at 8.83 when the dome is below 8.83 degrees in both cases.

Summary :

• Balance position ch at stow: dome za=8, ch za=8.834
• Dome weighs 6 times the ch weight.
• For the same moment: moving  dome 1 degree, need to move the ch 6 degrees (assuming sin(za)=za .. max err 2% 0 to 20 deg).
• Equations for loads and tensions:
•  MaxUnbalancedMom= -645.933 + 325.837*tdCornerKips
• MinTdCornerTension= 1.982 + .003069*MaxUnbalancedMomKips
• Slaving the ch to the dome when dome is above 8.83 degrees.
• this doesn't buy you much since the ch is 1/6 the weight of the dome.

 mode dome za ch za temp Time Astronomy using Dome 3.5 - 12 8.834 < 76F after 7pm,before 7am daytime above 85 you should probably move dome to za=8.4 Astronomy using ch 8.469 0-20 < 80F at 85F ch > 4 degrees Aeronomy using Ch 8.4-10 0 <76F daytime probably best to move dome to 8.4 za and az to 60,180, or 300 degrees.
• Note: The temperatures used  above are air temperatures. They come from extrapolating the fits to the nighttime tension fits vs temperature. The cable temperatures during the day are higher than the air temperatures. Using the temperatures in the above table during the day may result in unbalanced loads outside the allowable range.

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