Control of differential motion between adjacent advanced LIGO seismic isolation platforms [electronic resource]
Keywords: Mechanical Engineering.

Gravitational wave detection will provide further insight into areas that are inaccessible by traditional electromagnetic astronomy methods such as black holes. The Laser Interferometer Gravitational-wave Observatory (LIGO) is a large project designed to detect directly gravitational waves from astrophysical sources through the use of three, long-baseline interferometers and is funded by the National Science Foundation (NSF). We have demonstrated a prototype system, the Seismic Platform Interferometer (SPI), which could improve the operational reliability of the observatory. Initial LIGO, operating for 1 year of science data collection, had no known event detections. To improve the estimated detection event rate by a factor of about 1,000, the LIGO project is currently installing the Advanced LIGO (aLIGO) upgrade increasing the sensitivity of the detectors. Commissioning work for aLIGO has already started with the upgrade scheduled to come on-line in 2015. Several significant changes are being made to improve the detectors' performance. One of these changes is the installation of an upgraded seismic isolation system. This is necessary to increase the attenuation of ground motion to the suspended optics to 10 orders of magnitude at 10 Hz -- an order of magnitude improvement over current LIGO. Seismic isolation starts with quiet Hydraulic External Pre-Isolators (HEPI) outside the vacuum system, Internal Seismic Isolation (ISI) platforms inside the vacuum envelope, and then as many as four stages of pendulums culminating at the final proof mass optic. As part of aLIGO, thirty Internal Seismic Isolation (ISI) platforms, actively controlled in six degrees of freedom, are being installed to support and align each of the optics that are part of the interferometer. These platforms are controlled relative to inertial space utilizing seismometers. At low frequencies, however, the horizontal feedback seismometers cannot distinguish horizontal accelerations from the component of gravity, g, due to tilt. This tilt-horizontal coupling is one of the factors limiting the low frequency performance of the ISI system. Several solutions could address the problems caused by the tilt-horizontal coupling in the feedback inertial sensors. One set of solutions involves measuring the tilt independently and then subtracting its effect from the horizontal inertial sensors' signal. Possible solutions in this set include measuring tilt rate with a ring laser gyroscope and integrating to obtain the tilt. Alternatively, two or more linear inertial sensors (accelerometer or seismometer) could be spatially arranged in such a way as to obtain tilt from the difference of their signals. One method would involve two horizontally separated vertical inertial sensors. In this arrangement, tilt is the differential signal between the two instruments. A vertical seismometer with immunity to atmospheric pressure changes was designed and prototyped for this purpose. We have developed and demonstrated a different approach to address the excess motion at low frequency imposed by the tilt-horizontal coupling in the inertial sensors. An auxiliary sensor, the Seismic Platform Interferometer (SPI) was designed, prototyped, and demonstrated in the Stanford Engineering Test Facility (ETF) measuring and controlling the differential displacement between adjacent platforms at low frequencies. The dynamic range requirement of subsequent sensors, such as the main LIGO interferometer, is reduced by controlling this motion. While LIGO was operational, motion reduction would help simplify lock acquisition of the main interferometer as it effectively offloads some of the necessary control to isolation stages closer to the ground. This offloading is also helpful in that these stages utilize actuators that are better impedance matched to control low frequency motion and zero frequency offsets and alignments than actuators acting directly on the test masses, optics, and suspension systems. In order to sense the excess motion resulting from tilt coupling into the horizontal control loops, the SPI needs to measure the differential length between platforms. The measurement of differential pitch and yaw also becomes necessary because the attachment point for the suspension to the optics is not co-located at the center of rotation of the platform but is approximately 1 m above it and up to 1 m to the side. The SPI prototype signal was then used to control the differential motion of two actively controlled isolation platforms in the ETF. These platforms were separated by a distance of 8.9 m with one being a two-stage system and the other a single stage platform. The two-stage platform served as the host platform. Both of the platforms were locked together at low frequencies through the prototype SPI in differential length, pitch, and yaw signal by controlling the remote platform. The differential displacement motion reduction was recorded on two independent GS-13 seismometers and displayed an order of magnitude reduction between the frequencies of 100 mHz to 5 Hz. The SPI prototype demonstrated success in measuring and controlling differential pitch, yaw, and length between two active seismic isolation platforms. The in-loop RMS differential motions were reduced by 11 times in pitch, 24 times in yaw, and 4,418 times in length by the SPI enabled control. The SPI therefore provides a solution to induced, unwanted motion at low frequency caused by the tilt-horizontal coupling in the feedback seismometers of the active ISI platforms in Advanced LIGO. It is recommended that an SPI system be implemented in aLIGO. The work contained in this thesis does not necessarily reflect the scientific opinion of the LIGO Scientific Collaboration (LSC) as it was not required to be technically reviewed by the LSC.