New load cells were installed
in td12-1, td4-2 and td8-1 on 28nov17. We calibrated all 6 load
cells on 01dec17.
To check the calibration:
- the ch was put at stow and the dome at 8 degrees za. This
is close to balance for the azimuth arm.
- Each tiedown should see similar tensions (but they could
- the tiedowns were driven (in tracking mode) 21" -> 2"
-> 21" at .05 inches per second.
- all 3 tiedowns move together, so the change in tension
at each tiedown should be the same.
The plots show the td tensions
after the calibration (.ps) (.pdf):
- We finished the calibration around 14:00
- Page 1:
- Top tiedown tensions for 01dec17 (after the calibration)
- Each color is a different loadcell tension.
- the ramps around 14-15 hours are when the tiedowns
were cycled between 2 and 21 inches.
- Bottom: Tension difference between cables on a tiedown
- Each color is a different td.
- Black: td12-1 - td12-2
- red: td4-1 - td4-2
- green: td8-1 - td8-2
- The tension in each cable should be the same (since
the yokes can pivot)
- The tensions are not the same, and they change with
the tension in a tiedown.
- A linear fit was done to the difference vs the average
tension in the two cables.
- So the difference in td4 cable tension changes .3
kips for each kip in a cable
- At 30 kips cable tension, the difference is 10
- Page 2: look at ramping tiedown position 2 to 21 inches.
- We moved the tiedowns 21" to 2" and back .. twice
- the tiedowns moved at .05" per second in tracking
- So the change in tension should have been the same for
all of the tiedowns
- Top:over plot the tensions in the 6 ldcells during the
- Each color is a different load cell
- At 14.5 we were at 2 inches jack pull down. This was
the minimum tension.
- the tensions were not the same because
- The dome/ch positioning did not guarantee that all
td tensions started the same. There could have
been an offset.
- 2nd-bottom frame. plot tensions from each tiedown
- black is load cell 1, red is load cell 2.
- For each load cell I removed the average tension at
14.5 hours (when it was at a minimum)
- The plot variation should then show problems with
the gain calibration.
- The slope of the tension is definitely different for
- Page 3: over plot ldcell1, ldcell2 for all tds
- The black curves are the loadcell 1 tensions for td
- the red curves are the loadcell 2 tensions for td 12,4,8
- all of the curves should lie on top of each other since
the change in tension should be the same.
- The black curves (ldcell1) show lots variation.
- The red curves lay on top of one another.. The gain
calibration must be very good.
- 3 load cells were replaced on 28nov17
- All 6 load cells were recalibrated on 01dec17
- The difference in the tensions for the 2 cables at one
tiedown got up to 10 Kips ( when 30 kips was inn each cable)
- When scanning the tiedowns 2 to 21 inches:
- The calibration for ldcell2 on on 3 tiedowns gave
exactly the same results..
- The tensions in ldcell1 for each tiedown varied a lot.
- Something is going on causing the gain in loadcell 1 of
each td to be incorrect.
- The problem is not noise, since the value does not jump
- load cell 1 is always plugged into the leftmost slot of
each amplifier baseboard.
- loadcell 1 is read before loadcell 2 on the a/d
- I thinks there is a single a/d and a mux that is used
to read the different inputs.