fit for td tension y08_y09.
21feb10 (updated 11nov20)
Links to sections:
Correcting for load cell drift
Histograms of the dataset
Fitting the data
Plotting the fit results
Looking at the fit coefficients
Links to Plots:
in the cable tensions for the dataset (.ps) (.pdf)
the data set (.ps) (.pdf)
of the fit (.ps) (.pdf)
A fit was done to the tiedown tensions using data from 2008
The data set :
- Each tiedown (index 0,1,2 as td12,td4,td8) has two
cables with a load cell on each cable. The tension on a tiedown
is the sum of these two load cells.
- All of 2008,2009, Td data that was interpolated to 2 minute
steps to align with the platform height/temperature
measurements from the distomats. The data input was:
- az,zagr,zach, tdPos[0:2],temp, platformHght, tkKips[0:2]
- Npoints raw: 1,049,909 before throwing out bad
Correcting for load cell drift:
The amplifiers for the load cells drift
over time. Often the drift is in 1 load cell amplifier of the pair
at a tiedown. When this happened, i tried to replace the
drifting load cell with the one for that same tiedown that was not
drifting. To do this:
The plots show the drift in
the cable tensions for the dataset (.ps) (.pdf):
- grab all the data around the most common points:
should have the same kip values over time.
- For each tiedown plot the individual cable tensions for these
- Over plot the cable difference (with an offset).
- Look to see which of the two cables were causing the
difference in tension in the 2 cables.
Correcting for the drifts:
- Black is the difference in cable tensions (cable1-cable2) with
an offsets for plotting for all the data
- Red is the tension is cable 1, green is the tension in cable2
for the common points chosen above.
- Top td12. Green (cable2) has drifted. red (cable 1) is more
- Middle td4: Green cable2 is drifting while red (cable 1) is
- bottom: td8: both tensions are relatively stable.
- Td 12: use (tensionCable1)*2 for td12 tensions
- Td 4: use (tensionCable1)*2 for td4 tensions
- Td 8: use tension Cable1 + tension cable2 for td8 tensions.
Throwing out bad data
To remove bad data points the following
requirements were put on the data:
- 2kips < Tension1cable < 60Kips
- 2kipsmin:Once a cable loses tension, the linear fits can
have trouble (since the tension can no be negative).
- 60kipsmax: to get rid of bad measurements.
- 55F < tempDegF< 100F. This was to get rid of bad
- platformHght > 1255.8 feet (nominal 1256.22).
This is to get rid of bad distomat readings.
- 1049909 Raw data points
- 907934 points after applying filter.
Histograms of the dataset:
Histograms were made of the dataset after
applying the above filters.
The plots show histograms of the
data set (.ps) (.pdf)
after apply the data quality filters.
- Page 1:temp, az,gr,ch positions
- top: temperature. 73.9 F most common.
- 2nd: azimuth position. The most common azimuths were:
168,180,270,285 and 360 degrees.
- 3rd: dome za: 9.9,11, and 15 (the az swings of aeronomy).
- 4th: ch za: 8.8 this is stow. 0 deg is for aeronomy.
- Page 2: tiedown jack pull downs (in inches).
- Page 3: tiedown tensions measured from the load cells.
- this is the sum of the two cable tensions
- top, to bottom is td12,4,8.
- td4 has a higher average tension than the other 2. Td4 pulls
harder than the other two jacks to compensate for the main
cable length differences.
Fitting the data.
The fit used:
For each tiedown a fit was done to:
- Let i=0,1,2 be tiedown 12,4,8
- TensionTd[i]= C0 + C1*(temp-73) +
+ C3*tdPos + C4*tdPos +
+ C6*cos(az-azTd)*cos(domeZa) +
- The cos(az-azTd) projects the moment arm of the azimuth onto
the tiedown direction.
- All 3 td are included in each td fit since if you leave td12
fixed and move td4 and td8, then the tension in td12 will
- The sin(domeza) cos(domeZa) is used to fit for
ampGr*sin(zaGr-zaGr0). The dome has a non-zero za offset because
of the fixed counterweight on the ch size of the azimuth arm.
There is no counterweight for the ch.
- Three fits were done:
- All data
- 8am to 6pm
- 0 to 6am and 6pm to midnight
daytime (06 to 18 hours)
night time (00-06 and 18-24 hours)
Plotting the fit results:
The plots show the
of the fit (.ps) (.pdf):
- Page 1: 19jan10 while aeronomy did az spins with
dome=15,ch=0,all day long.
- top: Measured temperature.
- middle: measured kips in each tiedown
- Bottom: Measured kips - fitkips for each tiedown.
- td12 (black) has is about 4 kips below the other. This is
because cable2 of td12 is about 4 kips less than cable 1 (it
has a dc offset). Putting this back into the difference, the
fit of td12 is close to the others.
- During the day, the fit does not perform as well as the
evenings. This is probably because the air temperature that
is used is not the same as the cable temperature. At night
these are much closer.
- Page 2: Show fit kips for az=182.7, dome going 2-20 degrees.
The platform is at the focus position.
- top: ch=8.8 degrees stow
- bottom: ch=0 degrees
- Page 3: The North,south moment arm caused by the tiedown
tension for page 2 motion.
- Black ch=8.8 degrees
- red ch=0 degrees.
- The moment arm direction opposes the moment arm caused by
the dome motion. Positive is pulling down at td 12 (while the
dome goes up 180 degrees from td12).
Looking at the Fit coefs
- This shows how the tension in the tiedowns change with
temperature while all else remains constant.. But.. the
platform height can move with the temperature change so this is
not the value that is needed to keep the platform in focus.
- -.75 Kips/degF at each tiedown.
- 2.25Kips (3cables) /degF is the increase in the tension with
- A fit was done for the average tdPos vs temperature when the
platform was at the focus (1256.22feet)
c1*temp when platform at focus
(6am to 8pm)
kips change while moving all
tiedowns 1 inch
- Kips change when a tiedown is moved by 1 inch.
- moving a single tiedown will cause the tension in the
other 2 to change (in the same direction) because the average
height of the platform will be changed.
- The night time fits have some negative numbers (they should
all be the sign). The tiedown positions my be interacting with
a limited number of azimuth positions.
- Total change in tension when all 3 td moved 1 inch.
- prior to 6nov20 i was doing the following
- evaluate the fits at pos N inches and N+1 inches. applied
to all tdpos for each fit. This ended up summing coef C2-C4
for each td.
- This was incorrect since it was counting the increase more
than once for each td. What we really was to sum C2 for
td12, C3 for td4, and C4 for td8.
- so DeltaKIps/Tdinch: are just C2,C3, or C3
all 3 cables
|KipsNeeded to move platform
1 inch (using 1.7tdInc/platformIn)
- The nighttime fit values are a bit low.
- Joe vellozzi quoted 8.6 kips needed to move the platform 1
Ratio Dome/ch weights.
Using the coef's c5,c6, c7 you can compute the
ratio of DomeWeight/chWeight:
- Ratio=sqrt(c5^2 + c6^2)/c6
- Alldata : Wdome/Wch= 6.04
- How to the standard dome, ch weights agree with this ratio:
- The table below shows the dome/ch ratio for the accepted
weights values as well as how much we would have to change the
dome or ch to get a ratio of 6.02.
Where the dome balances the counterWeights (chza=0)
Dome za (zaEGr0) that balances the counterweight when zaEch=0
- let E be encoder za
- let Cm be za for dome center of Mass
- let zaEGr0 be the the za where the dome balances the
counterweight with the ch at za=0
- When the ch is at 0 deg za, the dome balances the counter
weight at zaEGr0 degrees.
- The counter weight includes: 45 kips at 145 feet, the vertex
shelter, stairway to bottom chord, ..etc.
- We measure zaEGr0 from the fit coefficients C5 and C6:
- grAmp*sin(zaEGr-zaEGr0)=grAmp*[cos(zaEGr0)sin(zaEGr) -
sin(zaEGr0)cos(zaEGr)]=[c5*sin(zaEGr) + c6*cos(zaEGr)]
- grAmp=sqrt(c5^2 + c6^2)
- Using the 3 separate td fits we get:
- td12: zaEGr0=6.6, td4:zaEGr0=6.6,td8: zaEGr0=6.5 degrees
- zaEGr0= 6.5 deg
(encoder/optic axis za) where the optic axis za is
uphill from the za for the center of mass.
Going from optic axis zaE to
center of mass zaCm for the dome
- The zaE in the fit uses the za encoder (optical axis za).
This is 1.1113 degrees uphill from the dome centerline.
dome weighing of 27feb03 showed a discrepancy in the
uphill, downhill weights of the dome:
- uphill 48,040, downhill 167,620 lbs.
- Assuming the lift points were +/= 10.75 feed from the dome
centerline, and the radius for the 10.75 foot lift
separation is 425 feet:
- Center of mass dome=asin[(48-167)/(48+167)*10.75
/425ft]=-.802 degrees downhill from the dome centerline.
- Combining the optic axis and weight unbalance gives:
- zaCmGr= zaEGr-1.913
degrees. The center of mass is downhill from the
- zaCmGr0=6.5 - 1.913 = 4.59
Standard tiedown position for focus.
When the platform is in focus (1256.22 feet), the
average tiedown position is only a function of the temperature. This
equation can then be used to predict the tiedown positions for
any temp when the platform is in focus.
The dataset to do this used the following specs:
The fit was done using data all day. It was also
done using 6:20 hrs for daytime and [0:6],[20:24] for nightime
- tdRefPos=[14.9969,15.2455,15.0182] (td12,4,8) at 73 degF
- Require the platform to be within .1 inches of 1256.22
- require the tiedown offsets to be within .1 inches of the
The fit results were:
AvgTiedown Position vs
Temp when platform in focus
|daytime [6:20] hours
|night: [0:6] and [20:24]
The plot shows the
residual vs hour of day (.ps) (.pdf):
- the data was median filtered by a factor of 20 to cut down on
the file size. This made some of the excursions smaller.
- The rms increases around sunup 7-8 am. It then falls
during the day.
- when the residual increases, the tiedowns need to be pulled
down more, so the main cables are colder than the temperature
- When the residual drops below 0, the tiedowns need to be
pushed up, so the main cables are warmer than the temperature
- The drop during the day is explained by the cables getting
heated by the sun.
- The jump up at sunrise (7,8 am) having the cable colder is a
bit harder to explain. It could be some kind of condensation
on the cables (but 8am is a bit late for that).
|quality of fit
|robust rms of fit = 1.7 kips
for each td tension.
|Kips change with Temperature
This does not leave the platform in focus
|-.75kips/degF at each tiedown
-2.25Kips/degF combine 3 tiedowns
|Kips change with tiedown position.
move all 3 tiedowns together 1 td inch
- This does not leave the platform in focus
- 8.4 kips should move the platform 1".. 1.7
this value is high by a factor of (1.7 day),or (1.4
[1.4,1.5,1.9] ]kips all data and daytime
4.86 kips total 3 tds day,all data
4.43 kips total 3 tds night.
using the all data values and 1.7 tdinch/platform inch, it
8.25 kipstotal to move the platform 1 inch.
The ratio makes no assumptions about the dome or
ch weight. It uses the tension fits for ch,dome za
|6.02 (average of all
data, and nightime meaurements.
- Using dome=215,ch=35 ==> ratio=6.14
- Using dome=210.7, ch=35 ==> ratio=6.02
- using dome=215, ch=35.7 ==> ratio=6.02
|Offset zaCenterMass to zaEncoder
(the encoder za is the same as the optic axis za and is
uphill from the center of mass za)
|Where dome za balances all counter weights (zach=0)
|Reference positions for most common data
with the platform close to focus (1256.22feet)
tdOffset=[ -.0902 , 0.1589 -0.0687]
fit will not work if the computed value is < 0 (the
tiedown cables can not have negative kips).
fit is available in the idl routine tdkips09 (@prfinit)
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