Simulate tiedown motion after shimming the rails.
feb,2002
The tiedown positions needed to correct the current
pitch, roll , focus errors are shown in: tiedown
positions to correct for pitch, roll, and focus. These plots show that
the tiedowns do not have enough range to do the correction.
We will have to remove some of the pitch and roll error by shimming and
possibly rotating the dome. Rotation of the dome is done by redrilling
the mount points for the dome. Shimming can be done on the elevation rails
and the azimuth rails. A simulation was done using the pitch, roll, focus
model computed from the aug02 surveys. It looked at two things:
-
Correct the pitch and roll by just rotating the dome (redrilling the mount
points). Assume there are no errors in the redrilling.
-
Redrill the mount points to just correct the average roll error, and then
reshim the elevation rails from 10 to 20 degrees za. Assume a residual
error on redrilling of .02 degrees and a shimming error of .04 degrees.
Do not include the focus change caused by the shimming (note.. after the
fact, jose maldonado said that they could probably redrill the dome holes
to 1/16 of an inch. Over the 12 foot span this is about .025 deg).
-
Redrill the mount points to correct the roll by .12 degrees. Shim the elevation
rail to remove a linear pitch from za=10 to za=20 of .14 degrees. Assume
it is done exactly. Include the focus change caused by shimming the rails.
Figure 1 Top: This is the pitch and roll before (solid) and after (dashed)
the redrilling of the mount points. The pitch correction of .04 degrees
and roll correction of .12 was chosen after trying various combinations.
-
Figure 1 Center: This is the tiedown position needed to correct the pitch,roll,
focus error before and after the redrilling. The tiedown limits are overplotted
in blue.
-
Figure 1 Bottom: The tension in 1 tiedown block before and after redrilling.
The limits are +120 kips and 0 kips (a blue line is drawn at 5 kips for
reference).
-
Figure 2 plots the tiedown position (top), tension in one block (center),
and total tension in the 3 tiedowns (bottom) for temperatures: 60, 70,
80, and 90 degrees F. At 60 deg the tdPosition limits are hitting
(tiedowns all the way down). They are also hit (on the other side)
at temperatures of 90 def F at 19 deg za.They start to lose tension around
17 deg za for 90deg F. At 60F the total tension limit of 240 kips is exceeded.
This method does not modify the focus. We can probably run between 70F
and 85F. The exact range will depend on the errors in drilling (and the
accuracy of the model).
These plots set the average roll error to +/- .02 degrees
and set the average pitch error (above 10 degrees za) to +/-.04 degrees.
The azimuth dependence of the error is unchanged. All 4 permutations
of the errors are plotted to show how the td range changes with the shimming/positioning
error.
-
Figure 1 shows what you get by just reshimming the elavation rails above
za=10 deg. Top: The current pitch versus za is plotted as the solid black
line. The red dashed line is the pitch corrected so that the average pitch
error above 10 degrees za is +.04 degrees. The dashed green line is the
same with the average pitch set to -.04 degrees. At each za is a complete
az spin worth of values. The range of the vertical lines is what is meaningful
(the horizontal lines are just the values at az=0). .04 degrees was chosen
as a "reasonable" shimming error (although there hasn't been any feedback
from the people that actually have to do the work).
-
Figure 1 Center: This is the tiedown positions to correct for the 3 options:
current (black), +.04 average pitch (red), -.04 average pitch (green).
It was computed for 72 deg F (the tiedowns need to move 1.7 inches for
every 5 degrees F). The horizontal lines are the tiedown limits. It is
illegal to cross a limit (and they do!). There is also a large vertical
range used at this 1 temperature which means that the temperature
range where it could work is small.
-
Figure 1 Bottom: The tiedown tension in kips (for 1 block) for these three
settings. The maximum allowed value is 120 kips. The minimum should probably
stay above 5 kips to insure that we don't lose tension in a cable.
-
Figure 2 has the roll error and then the results from reshiming for pitch
and rotating for roll. Top: This shows the current roll (black) and the
roll after setting the average roll to +.02 deg (red) and -.02 deg (green).
This would be done be redrilling the holes for the dome mount. The tighter
spec of .02 deg was used because it should be easier to redrill the
holes than shim the rails. Of course this ignores the fact that to reshim
the pitch you mess with the current roll.
-
Figure 2 Center. These are the tiedown positions for the current error
(black) and then the four permutations of (pitch,roll) corrections: (+.04p,+.02r
Red), (-.04p,-.02r Green), (+.04p,-.02r Blue), (-.04p,+.02r pink). The
vertical range needed has decreased appreciably. The only permutation that
exceeds a limit is the green (-.04p,-.02r) above za of 16 degrees.
-
Figure 2 Bottom. The tensions on a single block for the 5 choices.
None of them will cause a problem at 72F.
-
Figure 3 Top. The (+.04p,-.02r Blue) option of figure 2 was replotted for
temperatures 60,70,80,and 90 degF. This option works better than the others
in colder weather. At 60deg F the tiedown limit is exceed (tiedowns are
pulled all the way down).
-
Figure 3 Center: The tension on a tiedown block is plotted for the same
option for temps 60-90 F. At 90F the cable will start to lose tension at
za=17 (before it hits the motion limit). At 60F the tension gets up to
about 105 kips (120 kips lifts the block).
-
Figure 3 Bottom: This is the total tension on the platform from the tiedowns
versus za for the 4 temperatures (it should be horizontal lines, but the
model isn't perfect..). The maximum permitted tension is 240 kips. We hit
that at 60 degF.
The model shows that adjusting the pitch is not enough,
we must also remove the average roll. This is probably done the easiest
by redrilling the holes (if not , you'll probably have to reshim all the
way down to low za). We can emphasize the low temperature or high
temperature by leaving a residual pitch at high za. This needs to be decided.
Using the radius of 869.781 feet the platform is too high. Adding weight
to the platform could give us more range at lower temperatures. Before
doing this we should verify that the shorter radius of 869.781 is correct
(the focus curves we've done do not support this).
The shimming will modify the focus by up to 1 inch at za=19.5 degrees
(this has not been included here).
These plots show the results from rotating the dome
by .12 degrees in roll and then doing a linear ramp in pitch of 0
to .14 degrees from za=10 to 20 degrees. This is probably the corrections
that we will do. No errors are included in the positioning.
-
Figure 1 has the data for 72 degrees F. There is a linear ramp in pitch
(10->20deg za of .14 deg). The average roll has also been removed by rotating
the dome (.12 degrees). The change in focus from the shimming has been
included.
-
Figure 2 plots the data for temperatures 60,70,80, and 90 F. The
position limits are hit at 70F at za=5 and the kips go to < 5 kips for
temp=90F. The total kips at 60F is greater than the allowed 240 kips.
These computations include the offset
of the reflector relative to AO9. It also assumed that the focus curve
measured aug01 using the theodolite was the correct focus. This would apply
the measured focus errors at 72deg and the platform at 1256.35 feet. The
radius of curvature used for the reflector was 869.781 feet. The theoretical
focus correction using this radius does not agree with the focus curves
done by moving the platform.
processing: idl/prf/sim1.pro,sim.pro,sim1.cmp,sim.cmp
home_~phil