cpsdiff corrections

main/CPSdiff.tex

@@ -24,7 +24,7 @@
 \chapter{Reducing differential motion of aLIGO seismic platforms}
 \label{CPSdiff}
 During 2019, I spent some months working at the LIGO Hanford site (Washington, USA). This experience allowed me to be critically involved in the complicated life of a gravitational-wave interferometer. In particular, I was given the opportunity to study how to improve LIGO performances at low-frequency, focussing on the reduction of seismic motion of the platforms where the optics are located.\\
-In this chapter I will demonstrate how we can modify seismic control configuration of LIGO in order to obtain different and possibly better performance for seismic motion stabilization, faster and longer locking mode and, ultimately, more gravitational waves detections. The detailed computations included in this chapter are original and partially presented to the LIGO community and stored in LIGO DCC \cite{proposal} \cite{technote1} .\\
+In this chapter I will demonstrate how we can modify seismic control configuration of LIGO: in particular, this study should help reducing the differential motion between the chambers, making them move in sync, and help reducing and stabilizing the rms motion of the auxiliary sensors, through an LSC offload. The final goal is to obtain different and possibly better performance for seismic motion stabilization, faster and longer locking mode and, ultimately, more gravitational waves detections.  The detailed computations included in this chapter are original and partially presented to the LIGO community and stored in LIGO DCC \cite{proposal} \cite{technote1} .\\
 This work has been developed in collaboration with LIGO Hanford and LIGO Livingston laboratories, Stanford University, MIT and UoB and completed at UoB during 2020.\\
 This chapter is partially including some technical notes I shared with LIGO collaboration and the contents of this study have been presented at conferences and workshops \cite{chiatalk}.\\
 Essential information about the sections of LIGO involved in this study has been exposed in detail in Chapter \ref{LIGO}.
@@ -45,7 +45,7 @@ Other ways to improve duty cycle is to increase the observable volume: this can
 \begin{figure}[h!]
 \centering
 \includegraphics[scale=0.35]{images/duty_cycle.png} \includegraphics[scale=0.55]{images/dutypele.jpg}
-\caption[LHO duty cycle during O3b]{Example of duty cycle for Hanford Observatory, during O3b (Figure taken from \cite{kisseltalk1}) For almost 20\% of the running time the detector was not locked, which means that it was not observing. It is important to minimise this number, so more gravitational waves can be detected.}
+\caption[LHO duty cycle during O3b]{\textbf{Up:} Example of duty cycle for Hanford Observatory, during O3b \cite{kisseltalk1}. For almost 20\% of the running time the detector was not locked, which means that it was not observing. Of this 20\%, the \textbf{bottom} chart shows the causes of the lockloss: the main ones are seismic and "unknown" The study presented here could possibly reduce both.}
 \label{duty}
 \end{figure}
 
@@ -55,15 +55,15 @@ In particular, during O3 run, it was observed that the chambers in the corner st
 On another side, reducing the differential motion between the chambers means to reduce a source of noise at low frequency (5-30 Hz), as we will show in the next section: this would improve the sensitivity of the interferometer.
 
 \subsection{ISI stabilization}
-Differential motion affects the ISI of the HAM and BSC chambers in the CS: these are the platforms that we want to stabilize. Several sensors are responsible for sensing the seismic motion, in all degrees of freedom of each stage. They are T240, L4C, GS13 and CPS \cite{kisselthesis}.
-\begin{figure}[h!]
-\centering
-\includegraphics[scale=0.9]{images/isi.png}
-\caption[Example of ISI scheme]{Example of ISI inertial sensor scheme (figure taken from \cite{adl2}). All the main inertial sensors involved in the seismic isolation are shown in their locations. The CPSs are position sensors located between stages to measure the relative position.}
-\label{isi}
-\end{figure}
+Differential motion affects the ISI of the HAM and BSC chambers in the CS: these are the platforms that we want to stabilize. Several sensors are responsible for sensing the seismic motion, in all degrees of freedom of each stage. We have already introduced the seismic sensors in Section \ref{ligosei} of Chapter \ref{LIGO}: they are T240, L4C, GS13 and CPS \cite{kisselthesis}.\\
+%\begin{figure}[h!]
+%\centering
+%\includegraphics[scale=0.9]{images/isi.png}
+%\caption[Example of ISI scheme]{Example of ISI inertial sensor scheme (figure taken from \cite{adl2}). All the main inertial sensors involved in the seismic isolation are shown in their locations. The CPSs are position sensors located between stages to measure the relative position.}
+%\label{isi}
+%\end{figure}
 \noindent
-In particular, CPS sensors are placed on every stage of every chamber: it is easy to compare motion between HAM and BSC chambers through the signal of a device sensing the same motion on every chamber \cite{kisseltalk2}.\\
+In particular, CPS sensors are placed on every stage of every chamber: it is easy to compare the motion between HAM and BSC chambers through the signal of a device sensing the same motion on every chamber \cite{kisseltalk2}.\\
 The idea which should stabilize ISIs to follow the ground motion is to lock the chambers to each other, in order to make them move on a synchronized way, following a common motion given by a driver chamber (or block of chambers).
 
 \paragraph*{Role of the mode cleaner}
@@ -80,7 +80,9 @@ We started our design on the chambers on the X arm. Along this direction, the In
 In the next section we will demonstrate that CPS are good witnesses to sense differential motion and that they also can be used to lock the chambers with each other.
 
 \section{Sensing differential motion via CPS}
-The Capacitive Position Sensors (CPS) measure the relative motion between two stages of the isolation system. On HAM chambers they are set between HEPI and ground, and between Stage 1 and HEPI. On BSC chambers they also measure the relative motion between Stage 1 and Stage 2.  The plots in Fig. \ref{diff} show the differential motion seen by the CPS between BSC and HAM chambers: the sensors put in evidence that the HAM chambers have a more synchronized motion with respect to the motion between HAM and BSC and BSCs only. This means that the block of HAM chambers on X arm is more stable relatively to the other blocks and can be used as driver for the other chambers, with the mode cleaner acting as witness. We then projected the CPS of the X axis chambers to the suspension point in order to obtain PRCL and Input Model Cleaner Length (IMCL) traces like as they would be sensed by the CPS. For BSCs, we decided to sum the contributions of the CPSs on stage 1 and stage 2 and to project this sum to the suspension point.
+The Capacitive Position Sensors (CPS) measure the relative motion between two stages of the isolation system. On HAM chambers they are set between HEPI and ground, and between Stage 1 and HEPI. On BSC chambers they also measure the relative motion between Stage 1 and Stage 2.  The plots in Fig. \ref{diff} show the differential motion seen by the CPS between BSC and HAM chambers: the sensors put in evidence that the HAM chambers have a more synchronized motion with respect to the motion between HAM and BSC and BSCs only. This means that the block of HAM chambers on X arm is more stable relatively to the other blocks and can be used as driver for the other chambers, with the mode cleaner acting as witness. We then projected the CPS of the X axis chambers to the suspension point in order to obtain PRCL and Input Model Cleaner Length (IMCL) traces like as they would be sensed by the CPS. For BSCs, we decided to sum the contributions of the CPSs on stage 1 and stage 2 and to project this sum to the suspension point.\\
+\noindent
+One of the main differences between the behaviour of CPS IMCL and CPS PRCL is that the former is involving only the HAM chambers. Since HAM2 and HAM3 have a very good common motion, IMCL can be considered more stable with respect to PRCL, which instead involves also BSCs. Indeed, CPS PRCL is following only the BSCs, at frequencies below 0.03 Hz \ref{sus}.\\
 
 \begin{figure}[H]
 \centering
@@ -92,15 +94,13 @@ The Capacitive Position Sensors (CPS) measure the relative motion between two st
 \end{figure}
 
 \noindent
-One of the main differences between the behaviour of CPS IMCL and CPS PRCL is that the former is involving only the HAM chambers. Since HAM2 and HAM3 have a very good common motion, IMCL can be considered more stable with respect to PRCL, which instead involves also BSCs. Indeed, CPS PRCL is following only the BSCs, at frequencies below 0.02 Hz (\ref{sus}).\\
-\noindent
-Fig. \ref{sus} shows the plots of PRCL and ICML as sensed by CPS projection to the suspension point. These projections indicate that reducing the differential motion as seen by the CPSs will help to reduce the residual motion seen by the optical cavities.\\
+Fig. \ref{sus} shows the plots of PRCL and ICML as sensed by CPS projection to the suspension point. These projections indicate that reducing the differential motion as seen by the CPSs will help to reduce the residual motion seen by the optical cavities. We expect this to be effective in a range of frequencies between 0.1 and 0.5 Hz.\\
 
 \begin{figure}[h!]
 \centering
 \includegraphics[scale=0.3]{images/CPS_SUSPOINT_IMCL.PNG}\\
 \includegraphics[scale=0.3]{images/CPS_SUSPOINT_PRCL.PNG}
-\caption[CPS suspoint projections]{CPS suspension point (suspoint) projections: IMCL and PRCL. The calibrated PRCL trace used as comparison has been de-whitened.}
+\caption[CPS suspoint projections]{CPS suspension point (suspoint) projections: IMCL and PRCL. These plots show that the CPS sensors are good to monitor the motion of the optics at suspension points. In particular, the PRCL sensed by the CPS matches the calibrated PRCL trace between 0.6 and 0.2 Hz. Below this value we were expecting the traces to match: the un-match is due to PRCL calibration issues under solving. The IMCL is currently monitored by the GS13, and the plot shows that even CPS can be a good sensor for it. Hence we can use these sensors to monitor the motion of the optics, additionally to the relative motions between the platforms.}
 \label{sus}
 \end{figure}
 
@@ -149,16 +149,12 @@ $x_{p}$ & plant motion\\
 We can compute the signal $\textit{$d_{2}$}$ which will be the CPS offset to send to HAM3 chamber. In this case, HAM2 will drive HAM3 to follow its motion. Defining K = PC:\\
 \begin{equation}
 \centering
-d_{2} = x_{p_{2}} -  x_{g} 
-\end{equation}
-
-\begin{equation*}
-\centering
 \begin{split}
+d_{2} = x_{p_{2}} -  x_{g}\\
 &= K[L_{2}(SN_{g} + Sx_{g} + d_2) + H_2N_{1_2} + H_2(x_g + d_2)] + Px_g - x_g\\
 &= KL_{2}S(N_{g} + x_{g}) + KL_{2}d_2 + KH_2N_{i_2} + KH_2x_g + KH_2d_2 + Px_g - x_g.
 \end{split}
-\end{equation*}\\
+\end{equation}\\
 Since L$_2$ + H$_2$ = 1, we get:\\
 \begin{equation*}
 \centering
@@ -277,15 +273,15 @@ All this analysis has been performed through Matlab software.\\
 \end{figure}
 
 \subsection{Contributions from CPS and inertial sensors}
-To calculate the CPS signal contribution, we need the ground motion and we used the ITMY STS (Streckheisen Tri-axial Seismometer) signal on X direction. This is going to be the same motion for every chamber, since there is only one sensor in the Corner Station to measure it, because it has been found that the ground motion is the same everywhere in the Corner Station. From this signal, we separate the contribution given by the tilt ($\theta_g$) from the microseismic frequency (0.08 Hz). Then we subtract the tilt to obtain the ground motion $x_g$ from the STS:\\
+To calculate the CPS signal contribution, we need the ground motion and we used the ITMY STS (Streckheisen Tri-axial Seismometer) signal on X direction. This is going to be the same motion for every chamber, since there is only one sensor in the Corner Station to measure it, because it has been found that the ground motion is the same everywhere in the Corner Station. This signal includes a contribution from the tilt motion, dominating in the STS spectrum in the microseismic frequency (< 0.15 Hz); we call it $\theta_g = \theta \cdot g/\omega^2 $ and we separate it from the ground motion $x_g$:\\
 \begin{equation}
 x_g = STS - \theta_g.
 \end{equation}\\
-The CPS signal has been then computed summing in quadrature the contributions given by tilt, ground motion and CPS noise (N$_{cps}$), and applying the sensor correction filter:\\
+The CPS signal has been then computed summing in quadrature the contributions given by tilt, ground motion and CPS noise ($N_{cps}$), and applying the sensor correction filter:\\
 \begin{equation}
 CPS_{inj}=\sqrt{(\theta_g\cdot SC)^2+(x_g\cdot (1-SC))^2+(N_{cps})^2}.
 \end{equation}\\
-Figure \ref{cps_inj} shows the CPS signal and all its contributions.
+Figure \ref{cps_inj} shows the CPS signal and all its contributions. Since $\theta_g$ effects one STS, this injected signal is the same at all CPS sensors.
 
 \begin{figure}[h!]
 \centering
@@ -295,7 +291,7 @@ Figure \ref{cps_inj} shows the CPS signal and all its contributions.
 \end{figure}
 
 \noindent
-To calculate the platform motion of the BSC, we used data from the ITMX ISI along X direction. This is the signal from the T240 sensor. As before, we separate the tilt contribution ($BSC\theta_p$) from the signal and to obtain the inertial sensor contribution for the BSC chambers we sum in quadrature the contributions from tilt and T240 noise:
+To calculate the platform motion of the BSC, we used data from the ITMX ISI along X direction. This is the signal from the T240 sensor. We applied the same technique and we separate the tilt contribution ($BSC\theta_p$) from the signal, and to obtain the inertial sensor contribution for the BSC chambers we sum in quadrature the contributions from tilt and T240 noise:
 \begin{equation}
 T240_{inj} = \sqrt{{BSC\theta_p}^2+{N_{T240}}^2}.
 \end{equation}\\
@@ -304,7 +300,7 @@ Figure \ref{t240_inj} shows the T240 signal and its contributors.
 \begin{figure}[h!]
 \centering
 \includegraphics[scale=0.3]{images/t240inj.png}
-\caption[BSC contributions]{Plot of all the single BSC contributions computed from the inertial sensor involved in this chamber.}
+\caption[BSC contributions]{Plot of all the single BSC contributions computed from the inertial sensor involved in this chamber. We are certain that the T240 is dominated by tilt effects below 80 mHz, and by sensor-noise at higher frequencies. We interpolated these two bands together to determine an effective input disturbance from the T240.}
 \label{t240_inj}
 \end{figure}
 
@@ -327,9 +323,9 @@ In order to compute the platform motion for the single chambers in isolation and
 
 \begin{figure}[h!]
 \centering
-\includegraphics[scale=0.3]{images/bscblend.png}
+\includegraphics[scale=0.45]{images/bscblend.png}
 \includegraphics[scale=0.3]{images/hamblend.png}
-\caption[Blending filters]{Plots of all possible costs built with different combinations of blending filters. The orders of magnitude are indicated by the low and high pass indices l and h of the binomial filter in \ref{binomial} and the are going between l =[1,4] and h = [1,4].}
+\caption[Blending filters]{Plots of all possible costs built with different combinations of blending filters. The orders of magnitude are indicated by the low and high pass indices l and h of the binomial filter in \ref{binomial} and the are going between l =[1,4] and h = [1,4]. The plateau on both plots is given by the fact that the SC filter is dominating over those frequencies, which induces issues in the choice if the blending frequency. This makes a new way to evaluate blending filter in presence of the SC filter necessary.}
 \label{blend}
 \end{figure}
 

main/LIGO.tex

@@ -121,6 +121,7 @@ Technical noises arise from electronics, control loops, charging noise and other
 This thesis focuses on the improvement of the seismic isolation system, which noises affect the inertial sensors placed on the suspension benches and the stabilization of the resonant cavities, which in turn limit the sensitivity of the detector in the low frequency bandwidth. The goal is to provide solutions to reduce seismic motion and improve the detector sensitivity.
 
 \section{LIGO seismic isolation system}
+\label{ligosei}
 Every optic needs to be stable with respect to seismic motion, because movements in the mirrors will cause unwanted displacement of the laser beam on the optical surface, resulting in noise during the laser travel into the cavities and then at the output. The main mirrors (test masses and beam splitter) are suspended from a stabilized bench and every suspension chain is placed in vacuum chambers called \textit{Basic Symmetric Chamber} (BSC). The auxiliary optics are placed on optical benches enclosed in the \textit{Horizontal Access Module} (HAM) chambers.
 
 \begin{figure}[h!]
@@ -149,6 +150,16 @@ The BSCs have a similar design as the HAMs, but they have two stages of ISI to s
 \label{bsc}
 \end{figure}
 
+\paragraph{The sensors on the chambers}
+The devices dedicated to monitoring the seismic motion are inertial and displacement sensors, which are horizontal and vertical, according to the different motion they need to sense. Currently, no sensors for tilt motion are installed on the platforms. Actuators are paired to each sensors, for active isolation of the sensed noise. The vertical displacement sensors are called Capacitive Position Sensors and are placed between every stages of every chamber: they measure the relative motion between the platforms. These are the sensors we will use in Chapter \ref{CPSdiff}. The vertical and horizontal inertial sensors with the dedicated actuators are placed on the platforms, underneath the optical tables, measuring the seismic motion in the horizontal and vertical directions. The position and the use of these sensors are different for HAM and BSC chambers, depending on the number of stages and the presence of the suspensions. The calibration and the specific role of each sensor into the seismic isolation system can be found in \cite{kisselthesis}, with references to the covered range of frequencies in \cite{kisseltalk3}.
+
+\begin{figure}[h!]
+\centering
+\includegraphics[scale=1]{images/seifig.png}
+\caption[Example of HAM-ISI scheme]{Example of ISI inertial sensor scheme for a HAM chamber (figure taken from \cite{kisselthesis}). All the main inertial and displacement sensors involved in the seismic isolation are shown in their locations. The CPSs are the displacement sensors located between stages to measure the relative position. For the BSC chambers, the setup is similar.}
+\label{isi}
+\end{figure}
+
 \paragraph{Stabilizing the ISI}
 Part of the work presented in this thesis focussed on the enhancement of the performances of the active isolation system of the ISIs for both BSC and HAM chambers.\\
 Active isolation implies a sensing system of the noise to reduce and a control system to compensate the disturbance. Each platform includes relative position sensors, inertial sensors and actuators, working in all degrees of freedom.\\

main/main.tex

@@ -219,8 +219,11 @@ Beginning of Gravitational Wave Astronomy}
 
 \bibitem{mat} F. Matichard et al, \textit{Seismic isolation of Advanced LIGO: Review of strategy, instrumentation and performance}, Class. Quantum Grav. 32 185003, 2015
 
+\bibitem{kisseltalk3} J. Kissel, \textit{On	Seismic	Isolation in 2nd Generation	Detectors}, GWADW talk, 2012, dcc.ligo.org/LIGO-G1200556
+
 \bibitem{lsc} K. Izumi, D. Sigg, \textit{Advanced LIGO: length sensing and control in a dual recycled interferometric gravitational wave antenna}, 2017 Class. Quantum Grav. 34 015001
 
+
 %oplev
 
 \bibitem{mca} C. Di Fronzo \textit{Optical sensors for improving low-frequency performance in GW detectors}, Mid-Course Assessment, University of Birmingham, 2018