Hydraulic External Pre-isolation at
LIGO
Livingston
Joe Giaime, Louisiana State
University and LIGO Livingston, 20
September 2004.
Excess
Ground
Noise at LLO
We have
known for a few years that the low-frequency ground vibration at the
LIGO site in Livingston, Louisiana, is significantly greater than that
at our Hanford site. As shown in the figure at left, in the 1-3 Hz
band, the ground typically shakes about 7 times more at Livingston than
it does at Hanford. The graph is a cumulative histogram of
the
fraction of 1 minute duration measurements of r.m.s. velocity in the 1
- 3 Hz band yielding a result greater than the value on the horizontal
axis. Livingston's noise is also worse than Hanford's at even
lower freqeuncies, all the way down to 0.1 Hz.
This extra motion makes the LIGO Livingston
detector's mirror-positioning controls work much harder to acquire and
maintain optical resonance in our 4 km Fabry-Perot arm
cavities.
Consequently, the bulk of the Livingston data taken during the first
three LIGO
science runs came from nights and weekends, when forestry and
other
local human activity was not going on. Our detector only
worked
reliably when ground noise in the troublesome 0.1 - 3 Hz band was
below average, comparable to the levels at Hanford.
The new Hydraulic External
Pre-Isolation (HEPI) system is a
replacement
for fine and course actuation stages that support the LIGO payload in
each tank, and was designed to augment the existing seismic isolation
system. HEPI had been under development at Stanford for the
proposed Advanced LIGO detector upgrade, but we realized about two
years ago that we had to accelerate our work and install an external
stage in Livingston to allow round-the-clock detector operation.
HEPI
Technique
HEPI uses several of the techniques known as
"active seismic isolation"
to lower the vibration level on its payload. The payload is supported through
the HEPI system by
four piers bolted to the floor.
- At each tank corner pier,
there is a sensor/actuator set mounted
to the payload, vertical and horizontal.
- Each degree of freedom
(positions and angles) is servo-controlled
with respect to HEPI displacement sensors and geophones. This
feedback reduces the vibration by a factor of a few in the 1-3 Hz band.
- The displacement sensor
signal is corrected for floor motion,
as measured by a low-noise broadband seismometer, the Streckeisen
STS-2., in each direction. Sensor correction further reduces
the
payload vibration, at frequencies all the way down to 0.1 Hz.
Quiet Hydraulics
As implied by its name, HEPI uses forces generated
by hydraulic
pressure to partially cancel
the forces from ground vibration. The actuator is essentially
a
hydraulic Wheatstone bridge (2); viscous fluid is forced through it by
a pump (1). Small deviations among the resistive elements of
the
bridge create a pressure difference between (C1) and (C2), which
appears across an actuation plate (5) within a set of flexing bellows
(4). Up to 1 mm of flex is available, without any sliding
friction or non-laminar fluid flow.
Interferometric
test of HEPI using
LLO's 4-km X arm
During the first two weeks of
August 2004, we tested HEPI
performance by using the single-arm interferometer configuration of the
LIGO detector. The graph below shows a number of interesting
things.
The green solid trace is the
velocity spectral density of the
arm length changes due to vibration that are being corrected by feeding
back directly to the test masses, and so represents the effect of
troublesome ground vibration, with HEPI turned off. The
dashed
green line is the accumulated root-mean-squared velocity, (in units
of meter per second) calculated right-to-left. So, nearly
4 µm/s
of
disturbance is present, and other statistics indicate that the day
these data were taken was at the 95th percentile in ground
noise.
We
know from experience that our
detector won't work in its two-arm gravitational-wave-hunt mode on days
that are this noisy. The
1 µm/s dashed magenta
line
shows the RMS level above which locking is difficult.
The blue lines show the same
things, with HEPI turned on in the the
end and inner test mass payloads in our X arm. Noise is
reduced
along the entire troublesome band, and the RMS velocity is brought down
to a level that would make operations possible!
Design,
installation
and
commissioning
It has taken the minds and labor of
dozens
of
LIGO Lab and LSC collaboration members for two
years to
bring
HEPI from the prototype stage to where we are now, a fully installed,
nearly completely commissioned system at Livingston. Here is
a
brief summary:
- Decades of R&D on
quiet hydraulics with Dan DeBra at
Stanford, focussing on use of laminar flow oil to actuate machine tool
assemblies.
- Recent development &
prototyping of zero-stiction balanced
bellows quiet hydraulic actuators, by DeBra, Hardham, Lantz et al,
intended for use in Advanced LIGO pre-isolation stage. 2-DOF
test
stand experiment.
- Study by Hua et al of
effective control filter techniques for
‘sensor correction’ active seismic isolation at
sub-hertz frequencies.
- Design of third-generation
actuator, payload suspension springs,
and external housing for HEPI by Hardham, Hammond, Mason, Kern, Lacour,
etc.
- Tests at LASTI (ongoing) by
Mason, Hardham, Coyne, Lantz,
Mittleman, Ottaway, Sarin, Macinnis, etc. New
‘safe’ fluid in
use, tested at CIT.
- Re-implementation of control
system and electronics for LIGO/VME
environment and GDS by Bork, Sarin, Abbott(s), etc.
- Mass production and
installation at LLO, by Kern, Abbott, Spjeld,
Lacour, Traylor, Overmier, Mailand, Hanson, Carter, and many more.
- Hardware/software
commissioning at LLO by Abbott, Traylor,
Overmeir, Hanson, Fyffe, Wooley, Sellers, Parameswariah, etc.
- Controls commissioning/
testing at LLO by Mittleman, O’Reilly,
Coyne,
Lantz, Giaime, Frolov, etc.
