- 191126 - loadcell 1 td8 problem was a loose cable in the
hoffman box up by the a/d converterr(or signal conditioning
The load cells were re calibrated by
luis,willie, and osvaldo around 13:00 on 15Nov19. Prior to the
- td12 loadcells had a difference of about 5 kips (when the
avg tension was 20 kips)
- td4 : ldcell1 had a +10kips offset.
- td8 : loadcell 1 looked to be jumping.
td tensions 11-17Nov19
The first plots show the td tensions for
191111 thru 191117 (.pdf)
- the calibration was done 191115 around 13:00
- You can see how the tensions varied before and after the
Calibration results table
table shows the results of the calibration (.pdf)
- The calibration removes and dc offset, and then sets the
gain from a known resistance from the load cell calibration
- The pre column was measured before any adjustments. the
pos column was after the adjustments.
- the pre column on the cal off row shows the dc offsets
prior to the adjustments.
- Historically we've had large dc offset drifts.
- The red boxes show problems.
- td4 ldc1 calon tension:
- The calon,off was set to the correct voltage, but the
little star did not read the correct kips (.02 kips/cnt)
- This must be a problem between the amplifier output
and the a/d results.
Load cell differences before,after calibration
Td tensions and load cell
differences for 15Nov19 (.pdf)
- Page 1: ldcell tensions and ldc1-ldc2 differences
- top: td tensions before the calibration
- 2nd: ldc1 - ldc2 before the calibration
- 3rd: td tensions after the calibration.
- td8-1 stayed close to 0 after the calibration.
- bottom: ldc1 - ldc2 after the calibration.
- The red lines are linear fits to the ldc1 -ldc2 plots
- Page 2: linear fits to ldc1 - ldc2
- The 0 crossing is the relative dc offset for the 2. the
slope is the relative gain error between the 2.
- top: td12
- the calibration fixed the 5 kips dc offset
- at 34 kips the difference remained the same -7 kips
- middle: td4
- The 10 kip offset was corrected (ldc1)
- The slope error remains the same (since the on-off
voltage remained the same pre and post.
- bottom: td8
- no dc offset pre post,
- the large slope is from ldc1 remaining at 0.
Gain, offset difference moving td's 3 -> 22 inches
On 19Nov19 (after the load cell calibration)
the tiedowns were driven 3 to 22 inches and back at constant
- The ch was at stow and the dome was at 8 deg za
- this should be close to the azimuth arm balance
- the pointing routine tdcalib was used (helpdt tdcalib).
- it drove the tiedowns for 3 to 22 inches at .06
- This should have put a uniform load change on the 3
- It can be used to test the loadcell calibration.
The plots shows the tension change as
the td's moved 3->22inches (.ps)
- Top: the td tension for the motion.
- the colors are td 12(black), td4 (red), td8 (green)
- The solid lines are load cell1. the dashed lines are
- You can see that td4 load cell 1 is way off in tension.
- middle: plot loadcell 1 - loadcell 2 for each td during
the moves vs the average tension in a cable.
- the blue lines are linear fits to the differences. The 0
crossing would be the dc offset.
- Bottom: linear fit to td tension vs time
- a linear fit was done to each ramp in the top figure.
- The slope is tdTension/second. This can be used to
check the gain calibration of the load cell amplifiers.
- Since the velocity was constant it was also:
tdKips/.06 tdinches (we moved at .06 inches/second)
- Since the rate was the same for all td's, the slopes
should have been equal
- The plot shows the slope/.06 to give tension per tdInch.
The lope of the downward ramp was multiplied by -1.
- black lines: load cell 1
- red lines: load cell 2
- td4 load cell 1 is way off.
- the load cell 2 slopes are very close together , the
loadcell 1 slopes are jumping around
- On 01dec17 we
also measured that the ldcell 2 gain were all the same
while the ldcell 1 gains differed
- My guess is that there is something going on with the
little star reading the a/d converters.
- Using the average slope for load cell 2:
- Moving the tiedowns 1 inch changes the tension by
- There are 1.7 td inches/platform inch.
- We get 9.3 Tdkips/platform inch of motion
- The loadcells were recalibrated on 191115.
- Problems that persist
- td4 loadcell 1 is way off.
- When the caldeflection voltage was set correctly , the
kips reading was still off.
- voltage dif expected: 2.459 V, voltage dif measured:
- kips expected: 80.56, kips measured 60.58
- This is a connection problem ldcell amp to a/d, or an
a/d reading problem
- td8 loadcell 1. is intermittently dropping out
(for hours at a time)
- it then jumps back to the correct value
- a constant velocity ramp 3->22inches and back was used
to verify the gain settings of the tiedowns.
- all of load cell 2's look to be calibrated correctly
- We've seen this back in 01dec17 that loadcell 2's are
all the same, but loadcell 1's are off. this may be a
little star problem.
- Using loadcell 2 values, we get 9.3
tdKips/PlatformInch. This is close to what we've
- I think the original design spec was 8.4 kips/platform
inch (although we've always been a bit off from this).
- a vxWorks macro: tcalib can be used to drive
the tiedowns from 3 to 22 inches and back
- probably best to put dome at za=8 deg before starting
- see helpdt tdcalib for a description.