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
Summary

Links to Plots:
the drift in the  cable tensions for the dataset (.ps) (.pdf)
histograms of the data set (.ps) (.pdf)
the results of the fit (.ps) (.pdf)



 A fit was done to the tiedown tensions using data from 2008 and 2009.


The data set :


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):
Correcting for the drifts:

Throwing out bad data

    To remove bad data points the following requirements were put on the data:

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.


Fitting the data.

The fit used:

For each tiedown a fit was done to:

 
tdKips=c0 +c1*(temp-73) + c2*tdPos12+c3*tdpos4+c4*tdpos8+cos(az-tdaz)*(c5*sin(gr)+c6*cos(gr)+c7*sin(ch))
td
c0
constant
C1
tempCoef
C2
td12Pos
C3
td4pos
C4
td8pos
C5
cos(az-tdAz)*sinGr
C6
cos(az-tdAz)*cosGr
C7
cos(az-tdAz)*sinCh
fit
Sigmas
12 5.5732276 -0.7407116 1.4399629 0.4627205 0.8141619 -182.4695759 21.1679523 30.2854401 1.7
4 5.5481072 -0.7608992 0.4903692 1.5331098 0.8986088 -179.9594440 20.8633769 31.4882659 1.6
8 -2.2657032 -0.7431898 0.4191552 0.5005065 1.8854497 -183.3214785 20.9092364 29.3240446 1.5
Fit daytime (06 to 18 hours)
12 5.6455217 -0.7150638 1.4364386 0.4544647 0.7800069 -180.9760537 20.8268316 30.5142980 1.7
4 4.9735031 -0.7403918 0.4726905 1.5324140 0.9163808 -180.9848987 21.0036030 31.1481109 1.7
8 -2.8533123 -0.7076089 0.4001283 0.4995190 1.9007027 -184.8074029 21.4216825 28.7082903 1.6
Fit night time (00-06 and 18-24  hours)
12 14.1768464 -0.7223369 0.9779562 -1.2681817 2.5060998 -183.1928313 21.2055318 30.7845388 1.4
4 15.8548171 -0.7434472 0.3061097 2.5335186 -0.5698448 -177.9378664 20.6767039 28.7994148 1.2
8 7.6089936 -0.7352189 0.1031195 1.1601542 0.9238263 -183.6302476 20.7704556 32.3331326 1.0


Plotting the fit results:

    The plots show the results of the fit (.ps) (.pdf):

Looking at the Fit coefs

Temperature:

TdPosition

kips change while moving all tiedowns 1 inch









td12
td4
td8
TotalKips
all 3 cables
allData
2.7
2.9
2.8
8.4
Daytime
2.7
2.9
2.8
8.4
niteTime
2.2
2.3
2.2
6.7
  

Ratio Dome/ch weights.

    Using the coef's c5,c6, c7 you can compute the ratio of DomeWeight/chWeight:

Where the dome balances the counterWeights (chza=0)

Measuring the Dome za (zaEGr0) that balances the counterweight when zaEch=0
Going from optic axis zaE to center of mass zaCm for the dome.


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
    The fit results were:
AvgTiedown Position vs Temp when platform in focus

c0
c1
tdInch/degF
sigma
td inches
all data
38.5557
-.323769
.54
daytime [6:20] hours
37.9014
-.317351
.62
night: [0:6] and [20:24]
32.4796
-.235531
.21

    The plot shows the fit residual vs hour of day (.ps) (.pdf):

SUMMARY:



Measurement
Result

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 td"/platform" so
    this value is high by a factor of (1.7 day),or (1.4 nite)
[td12,td4,td8]
[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.

Ratio DomeWeight/ChWeight
The ratio makes no assumptions about the dome or
ch weight. It uses the tension fits for ch,dome za
dependence.
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
1.913 degrees
(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)

zaEGr=6.6
zaCmGr=4.56

Reference positions for most common data
with the platform close to focus (1256.22feet)
tdRef    =[14.8594,15.1085,14.8809]
tdOffset=[  -.0902 ,  0.1589  -0.0687]
temp:      73.5F

Notes

1.The fit will not work if the computed value is < 0 (the tiedown cables can not have negative kips).

2.The fit is available in the idl routine tdkips09 (@prfinit)


processing: x101/100217/tdkipsfit.pro

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