We have recently acquired 3 two-axis inclinometers made by Applied GeoMechanics
(AGI). The meters contain two inclinometers mounted orthogonally and also a
temperature probe. The 3 output voltages from each of the three devices will
be fed to a DGH A-D converter which can talk to the VAX on RS232 cables.
Recent experience with such a system on the carousel strain gauge project will
allow cloning of the data acquisition software.
The inclinometers will be mounted permanently except when being calibrated.
Recent tests of our first AGI inclinometer show reduced noise levels :
Tests of the sensitivity of the AGI device in single-ended
mode and high-gain gave
99.3mV/" + 1
cf manufacturer's figure of 97.1.
Differential mode is twice as sensitive and since it includes
built-in protection against noise spikes it is likely to be our preferred
method of operation. The useable range is +80" in
single-ended mode, half that in differential mode.
This is sufficient to meet expected excursions due to track irregularities,
but leaves little room for significant changes in track profile in the future.
The low gain settings are all 10 times coarser, and some experimentation
will reveal whether data of sufficient resolution may be acquired at this
setting : it may be more advantageous to have the greater range offered
by the low gain.
However, calibration within even the narrowest of these ranges ought to be
possible.
The meters need to be zeroed in order to minimize temperature effects
which are proportional to the output voltage : approximately 5%/degree .
Any zero-point change ought, in any case, to be correctable using the
temperature reading for each device.
Resolution, repeatability and stability are also completely adequate at the
0.1" noise level. However, their use has revealed other features of the
antenna not previously detected.
The AGI data show a periodicity of sometimes 
11
and sometimes
6
, and zero-point jumps every minute or so of amplitude 1/3"
attributable to the antenna SERVO. While these effects are small
and probably combatable via appropriate data-taking
strategies, this has prompted questions regarding the ultimate site of the
inclinometers : should we remove them from the TMU platform and the accompanying
SERVO noise, and relocate them on the elevation bearing brackets ?
Mounting brackets have since
been designed and fabricated to allow positioning of the new meters atop
the elevation bearings as well as in their conventional locations, and to
allow for rapid removal and replacement in order to effect calibration
when mounted on the TMU.
However, measurements taken in August 1994 with an AGI inclinometer atop the
LH A-frame showed large (20") swaying of the A-frame as the antenna
is moved from the zenith to the horizon. This would appear to be a
contraindication for the use of this site for regular inclinometry.