Hydraulic External Pre-isolation at LIGO
Livingston
Contributed by Joe Giaime, Louisiana State University and LIGO Livingston
Excess Ground Noise at
LLO
We have known
for a few years that the low-frequency ground vibration at the LIGO Livingston
Observatory is significantly greater than that experienced at our Hanford site.
As shown by the figure at left, in the 1 to 3 Hz band, the ground typically shakes
about seven times more at Livingston than it does at Hanford. The graph is a
cumulative histogram of the fraction of one-minute duration measurements of r.m.s.
velocity in the 1 to 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
frequencies, all the way down to 0.1 Hz.
This extra motion makes
the Livingston detector's mirror-positioning controls work much harder to
acquire and maintain optical resonance in our four-kilometer Fabry-Perot arm cavities.
Consequently, the bulk of the Livingston data taken during the first three LIGO science
runs came from night and weekend operation, when forestry and other local human
activities were not in progress. 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 coarse 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 use in the proposed Advanced LIGO
detector upgrade, but about two years ago we realized 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 one 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 outcomes.
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 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 two years' worth of the minds and
labor of dozens of LIGO Lab and LSC collaboration members to bring HEPI
from the prototype stage to where we are now, a fully
installed, almost completely commissioned system at Livingston. Here is a
brief summary:
- Decades of R&D on quiet hydraulics with Dan DeBra at Stanford,
focusing 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.
