fit for td tension y08_y09.
21feb10 (updated 30mar10)
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:
the cable tensions for the dataset (.ps) (.pdf)
set (.ps) (.pdf)
of the fit (.ps) (.pdf)
A fit was done to the tiedown tensions using
data from 2008 and 2009.
The data set :
- Each tiedown (index 0,1,2 as td12,td4,td8) has two
with a load cell on each cable. The tension on a tiedown is the
these two load cells.
- All of 2008,2009, Td data that was interpolated to 2 minute
to align with the platform height/temperature measurements
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
time. Often the drift is in 1 load cell amplifier of the pair at a
When this happened, i tried to replace the drifting load cell
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:
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
offsets for plotting for all the data
- Red is the tension is cable 1, green is the tension in cable2
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
were put on the data:
- 2kips < Tension1cable < 60Kips
- 2kipsmin:Once a cable loses tension, the linear fits can
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).
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
the above filters.
The plots show histograms of the
set (.ps) (.pdf) after
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
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 sin(domeza) cos(domeZa) is used to fit for
ampGr*sin(zaGr-zaGr0). The dome has a non-zero za offset because
fixed counterweight on the ch size of the azimuth arm. There is
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)
the fit results:
The plots show the
of the fit (.ps) (.pdf):
- Page 1: 19jan10 while aeronomy did az spins with
- 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
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
- Page 2: Show fit kips for az=182.7, dome going 2-20 degrees.
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
for page 2 motion.
- Black ch=8.8 degrees
- red ch=0 degrees.
- The moment arm direction opposes the moment arm caused by
dome motion. Positive is pulling down at td 12 (while the dome
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
height can move with the temperature change so this is not the
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)
when platform at focus
kips change while moving all
- Kips change when a tiedown is moved by 1 inch.
- moving a tiedown will cause the tension in the other 2 to
(in the same direction) because the average height of the
- The night time fits have some negative numbers (they should
be the sign). The tiedown positions my be interacting with a
number of azimuth positions.
- The total tension change for 1 tiedown inch on all 3 tiedowns
- The tiedown cables move 1.7 inches for every platform inch.
- It should take about 8 kips to move the platform 1 inch.
- 1.7 tiedown inches would be 14.2 kips/platform inch.. .this
a bit high.
all 3 cables
Ratio Dome/ch weights.
Using the coef's c5,c6, c7 you can compute the
- 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
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
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
the za for the center of mass.
from optic axis zaE to center of mass zaCm for the dome
- The zaE in the fit uses the za encoder (optical axis za).
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
- 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
The center of mass is downhill from the encoder za
- 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
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
file size. This made some of the excursions smaller.
- The rms increases around sunup 7-8 am. It then falls
- 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
up, so the main cables are warmer than the temperature sensor
- 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
cables (but 8am is a bit late for that).
|robust rms of fit = 1.7 kips
each td tension.
change with Temperature
This does not leave the platform in focus
|-.75kips/degF at each tiedown
-2.25Kips/degF combine 3 tiedowns
change with tiedown
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
[2.7 ,2.9,2.8]kips all data and daytime
[2.2 ,2.2,2.3]kips nightime
8.4 kips total 3 tds day,all data
6.7 kips total 3 tds night.
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
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
zaCenterMass to zaEncoder
(the encoder za is the same as the optic axis za and is
uphill from the center of mass za)
dome za balances all counter weights (zach=0)
positions for most
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
can not have negative kips).
fit is available in the idl routine tdkips09 (@prfinit)