This page has links to hurricane maria's affect
- 170920: Eye passed over ao 11:30
az swings with tilt sensors to see if things have changed.
pointing model installed right after maria.
telescope gain variation pre/post maria
compare sband narrow gain curves pre/post maria.
steps to realign optics after maria
Tilt sensor data during hurricane maria
The tilt sensors are mounted on the rotary
- The pitch tilt sensor is aligned along the azimuth arm
direction when the floor is at 75 degrees
- It has a range of +/- 15 degrees.
- The roll tilt sensor is perpendicular to the pitch sensor. It
has a range of +/- 2 degrees.
- The rotatory floor is level at a zenith angle of 10 degrees.
- During hurricane maria, the floor was at 75 degrees. The
gregorian was at 8.4792 degrees
- The azimuth arm was bolted to the platform during the
- The gregorian dome was pinned to the azimuth arm.
- Tilt sensor data is sampled at 5 Hz.
- Data was taken from 19:38 19sep17 till 18:33 20sep17
The first plots shows the tilt sensor data for the entire time
- Black is the pitch tilt sensor
- red is the roll tilt sensor. it clipped.
- the eye passed over ao around 11:30 on 20sep17 (20.48
The 2nd plot has the tilt values and wind speed,direction for
20sep17 (.ps) (.pdf)
- Top: pitch tilt sensor for hour of day
- middle: Wind speed on platform (3 sec avg) vs hour of day
- bottom: wind direction
- The eye passed over ao around 11:30 (11.5 on x axis).
The last plot has the spectrum of the pitch tilt sensor by hour
for 20sep17 (.ps) (.pdf)
- The spectra were computed for each hour of the day (till
- The fftlen = 5*3600 samples
- the spectrum was computed as:
- keep fftlen/2 points (since real data)
- When plotting smooth by 11 channels to give 11/3600=3
- The maximum bandwidth is 2.5 Hz.
- The spectra are plotted by hour with offsets for display.
- 0-1 hours at the bottom (black)
- 17-18 hours at the top grayish blue.
- Some of the strongest frequency components have been
flagged with vertical dotted lines:
- .553 Hz
- .641 Hz (strongest)
- .805 Hz (crosshead?)
- 1.037 Hz
- 1.357 Hz
- this drifted to lower freq as the wind got stronger. It
- Any frequencies above 2.5 Hz would alias back down.
- You can see the oscillation amplitudes increased until 11:00
when the eye passed. They pick up again around 12:00
- oscillation frequencies measured:
|drifted to lower freq
Tiedown tensions during hurricane maria.
Tiedown cables connect a cement block on the
ground to each corner of the platform. There are 2 cables from
each corner down to the corresponding cement block. The cables are
attached to the block with a 150 ton jack that is computer
controllable. The two cables at each corner connect to the jack
with a pin/loadcell that measures the tension in each cable.
- Prior to the hurricane the td jacks are moved all the
way up (24 ") to release as much tension as possible.
- The load cell output level is in the millivolt range. An
amplifier is used to kick the signal up to a/d levels (volts).
- We've always had trouble with the stability of these
- There is a calibration resistor the lets you calibrate the
amplifier. It has a gain and dc offset adjustment.
- Historically we've found that the gain tends to be stable,
but the dc level drifts.
- The a/d used to measure the signal is set to go 0 to
+volts. When the dc level drifts below 0 volts, part of
the signal is cutoff.
- During the hurricane 1 amplifier at each corner had a large dc
offset and was unusable.
- td12_2, td4_1,td8_2 had non zero values.
- for each td corner i used the the above tensions ,
doubling them to get the total kips/corner.
- the tiedown tensions are read once a second.
The first plots shows the td tensions for 20sep17 (.ps) (.pdf)
- The data is sampled at 1 Hz.
- the plots show the total tension on each corner.
- I've removed the median value (since the dc offset differed)
- The bottom portion of the plots are clipped.
- the a/d clipped at 0 volts. this was caused by the dc offset
- Top: td 12
- middle: td4
- bottom: td8
- after 14.5 the value went offscale. I think the cable from
loadcell to cabinet had problems
- Td 12 and td4 had similar amplitudes. Td8 were a bit higher.
This may have been a amp gain problem.
- You can see the hurricane eye passing around 11.5
- The peak amplitude for 12,4 was around 20kips.
- Since it was an oscillation the tension was probably
changing by +/-20 kips
The second set of plots has the spectra of the td tension by
hour of day (.ps) (.pdf)
- The spectra was computed as spc=abs(fft(kipshr))
- I computed the spectra for each td corner separately
- The spectra were smoothed by 11 channels giving 11/3600=
- td12 and td 4 spectra were added together
- td8 spectra started to get pretty noisy after 6am so i
didn't include it
- 1 Hz sampling gives a maximum bandwidth of .5 Hz.
- Any oscillations higher than this will alias back down.
- the spectrum for each hour is plotted with an offset for
- 0-1 hours at the bottom (black)
- The vertical dotted lines flag some of the stronger
- .320 Hz
- (strongest during the hurricane)
- not present prior to wind picking up
- strongest prior to wind
- This is close to the computed oscillation freq of
- The wind died 11-12 when the eye passed overhead.
- The td tensions
- picked up around 6am
- peaked around 10:00
- died around 11-12 when the eye passed overhead
- picked up again until dying away aroud 19 hours.
- td12 and td 4 and similar amplitudes with a max of 15-20 kips.
- td8 values were larger. Not sure whether this was real, or a
gain calibration of the loadcell amp.
- Measured oscillation frequencies:
- Anything about .5 Hz will alias down
|strongest oscillation. Only present
with high winds. Not present prior to hurricane
|present prior to winds
|present prior to winds. This is not an
alias harmonic of .358*2. That would show up at
td,tilt sensor summary
- The td kips measurements had problems with calibration.
- td12,td4 showed a maximum oscillation of 15-20 kips per td
- the measured oscillation frequencies:
|Strongest during winds. Not present
with low winds
|present without high winds
|preset without high winds
|present without high winds, broad
during high winds
|present without winds. Strongest with
|only present with high winds (although
this is the crosshead frequency for the receivers on
|broad during high winds.
- .641Hz tilt sensor strongest value is the 2nd harmonic of
the td .32Hz
- I don't think the td value is aliased.
- I wonder if the rocking along the azimuth arm is doubling
the vertical oscillation freq.
- Could be if the oscillations in individual td cables were
out of phase.