WHT primary mirror axial load cell
test
During the aluminising of the WHT mirror, when the
mirror is removed from the cell, there is an opportunity to test and adjust the
radial load cells.
We need to connect the
mains power to the mirror support controller. This can be done applying power
to cable: 139TCC12 with the blue mains test cable from AF2, kept in the AF2
cupboard. Connect a DVM to the "Force" terminals of the Digital Mirror
Support Controller (DMSC) card. This is the output of the strain gauge
amplifier of the mirror support system. (See for details the Technical Manual
of the mirror support kept in the ops room) Now take a reading of the DVM with
no force applied to the radial load cells. Then repeat this with a know weight holding steady on top of each radial load cell.
The weight I used was 6.35 kg. Results of test carried out on 8th of Feb. 2012:
|
Load-cell |
Zero
force (V) |
6.35kg
force (V) |
mV/kg |
|
North
(0 deg.) |
0.251 |
2.923 |
421 |
|
S/E
(120 deg.) |
0.374 |
3.061 |
423 |
|
S/W
(240 deg.) |
-0.096 |
2.569 |
420 |
This file: http://www.ing.iac.es/~ops/private/operations/WHT
primary mirror axial load cell test.htm
Jürg Rey, 10 February, 2012
Measurements done 17th and 18th of May 2016
Same set-up and procedure as explained above, but now
the weight is 7.2 kg approximately. The measurements have been done by Amado
and Diego.
Ambient temperature: 7.0 ºC
Electronics rack external temperature: 15.1 ºC
A better estimation of the block mass is derived from
its measures; it’s a square prism made of lead with a central hole: 8.89 x
10.34 x (6.97 + 6.84)/2 cm. The last measure takes into account the trapezoidal
section of the height and width of the prism. The diameter of the hole is 1.24
cm. Being the lead density of 11.34 g/cm3,
the mass is:
mass= [8.89
* 10.34 * (6.97 + 6.84)/2 – π*0.622*(6.97 + 6.84)/2]*11.34= 7.10
kg
The first measurement of the zero force was done with
the rack just connected, i.e. cold; no significant difference was detected
except for board/load cell WHT-3.
The measurements have been repeated several times in
sequence, first without any mass and then with the 7.2 kg mass on the top of
the load cell stick. All the voltages measured in volts. As a general rule the repeatability
of the measurements is about 10 mV; though board WHT-2 showed more noise and
less stability in the measurements. The last measurement with the mass was done
10 minutes after the previous one.
|
Load-cell |
Board ID |
Zero force (COLD) |
Zero force (WARM) |
7.1kg force |
Zero force |
7.1kg
force |
Zero force |
7.1kg
force |
Slope mV/kg |
|
North (0 deg.) |
WHT-3 |
0.254 |
0.332 |
3.221 |
0.317 |
3.236 |
0.318 |
3.264 |
411 |
|
S/E (120 deg.) |
WHT-1 |
0.353 |
0.360 |
3.303 |
0.359 |
3.301 |
0.359 |
3.303 |
415 |
|
S/W (240 deg.) |
WHT-2 |
-0.166 |
-0.130 |
2.794 |
-0.130 |
2.791 |
-0.132 |
2.789 |
400 |
This file is located at:
http://www.ing.iac.es/~eng/ops/wht/WHT%20primary%20mirror%20axial%20load%20cell%20test.htm
Diego Cano, 18-May-2016