1,720,971 research outputs found
Modeling, estimation and control of ring laser gyroscopes for the accurate estimation of the earth rotation
Full text linkHe − Ne ring lasers gyroscopes are, at present, the most precise devices for absolute angular velocity measurements. Limitations to their performances come from the non-linear dynamics of the laser. Accordingly to the Lamb semi-classical theory of gas lasers, a model can be applied to a He–Ne ring laser gyroscope to estimate and remove the laser dynamics contribution from the rotation measurements.
We find a set of critical parameters affecting the long term stability of the system. We propose a method for estimating the long term drift of the laser parameters, and for filtering out the laser dynamics effects, e.g. the light backscattering. The intensities of the counterpropagating laser beams exiting one cavity mirror are continuously measured, together with the monitor of the laser population inversion. These quantities, once properly calibrated with a dedicated procedure, allow us to estimate cold cavity and active medium parameters of the Lamb theory. Our identification procedure, based on the perturbative solutions of the laser dynamics, allow us for the application of the Kalman Filter theory for the estimation of the angular velocity.
The parameter identification and backscattering subtraction procedure has been verified by means of a Monte Carlo studies of the system, and then applied to the experimental data of the ring lasers G-PISA and G-WETTZELL. After the subtraction of laser dynamics effects by Kalman filter, the relative systematic error of G-PISA reduces from 50 to 5 parts in 103, and it can be attributed to the residual uncertainties on geometrical scale factor and orientation of the ring. We also report that after the backscattering subtraction, the relative systematic errors of G-WETTZELL are reduced too.
Conversely, in the last decade an increasing attention was drawn to high precision optical experiments, e.g. ring laser experiments, which combine high sensitivity, accuracy and long term stability. Due to the experimental requirements, position and orientation of optical elements and laser beams formation must be controlled in the field of nano-positioning and ultra-precision instruments. Existing methods for beam direction computing in resonators, e.g. iterative ray tracing or generalized ray transfer matrices, are either computationally expensive or rely on overparametrized models of optical elements.
By exploiting the Fermat’s principle, we develop a novel method to compute the beam directions in resonant optical cavities formed by spherical mirrors, as a function of mirror positions and curvature radii. The proposed procedure is based on the geometric Newton method on matrix manifold, a tool with second order convergence rate that relies on a second order model of the cavity optical length. As we avoid coordinates to parametrize the beam position on mirror surfaces, the computation of the second order model does not involve the second derivatives of the parametrization.
With the help of numerical tests, we show that the convergence properties of our procedure hold for non-planar polygonal cavities, and we assess the effectiveness of the geometric Newton method in determining their configurations with an high degree of accuracy and negligible computational effort.
We also presents a method to account for the (ring laser) cavity deformations due to mirrors displacement, seen as the residual motions of the mirrors centers after the removal of rigid body motions. Having the cavity configuration and the model to account for mirrors movements, the calibration and active control of the optical cavity can be addressed as a control problem. In fact, our results are of some importance not only for the design and simulation of ring laser gyroscopes, but also for the active control of the optical cavities.
In the final part of this work we detail a complete model including the simulation of the physical processes of interest in the operation of a ring laser gyroscope. Simulation results for the application of the model to the ring laser GP2 are presented and discusse
A fast nonlinear model predictive control strategy for real-time motion control of mechanical systems
No full textComputing Laser Beam Paths in Optical Cavities: An Approach Based on Geometric Newton Method
No full textIn the last decade, increasing attention has been drawn to high-precision optical experiments, which push resolution and accuracy of the measured quantities beyond their current limits. This challenge requires to place optical elements (e.g., mirrors, lenses) and to steer light beams with subnanometer precision. Existing methods for beam direction computing in resonators, e.g., iterative ray tracing or generalized ray transfer matrices, are either computationally expensive or rely on overparameterized models of optical elements. By exploiting Fermat’s principle, we develop a novel method to compute the steady-state beam configurations in resonant optical cavities formed by spherical mirrors, as a function of mirror positions and curvature radii. The proposed procedure is based on the geometric Newton method on matrix manifold, a tool with second-order convergence rate, that relies on a second-order model of the cavity optical length. As we avoid coordinates to parametrize the beam position on mirror surfaces, the computation of the second-order model does not involve the second derivatives of the parametrization. With the help of numerical tests, we show that the convergence properties of our procedure hold for non-planar polygonal cavities, and we assess the effectiveness of the geometric Newton method in determining their configurations with high degree of accuracy and negligible computational effort
Optimization of the geometrical stability in square ring laser gyroscopes
No full textUltra-sensitive ring laser gyroscopes are regarded as potential detectors of the general relativistic frame-dragging effect due to the rotation of the Earth. Our project for this goal is called GINGER (gyroscopes in general relativity), and consists of a ground-based triaxial array of ring lasers aimed at measuring the rotation rate of the Earth with an accuracy of 10?14 rad s?1. Such an ambitious goal is now within reach, as large-area ring lasers are very close to the required sensitivity and stability. However, demanding constraints on the geometrical stability of the optical path of the laser inside the ring cavity are required. Thus, we have begun a detailed study of the geometry of an optical cavity in order to find a control strategy for its geometry that could meet the specifications of the GINGER project. As the cavity perimeter has a stationary point for the square configuration, we identify a set of transformations on the mirror positions that allows us to adjust the laser beam steering to the shape of a square. We show that the geometrical stability of a square cavity strongly increases by implementing a suitable system to measure the mirror distances, and that the geometry stabilization can be achieved by measuring the absolute lengths of the two diagonals and the perimeter of the ring
Characterization of a Nanometer Displacement Gauge for the Dimensional Control of Large Optomechanical Structures
No full textWe present the design, implementation, and
characterization of a heterodyne laser interferometer for nanometer
displacement metrology. The purpose is to monitor the 3-D
shape of a large optomechanical structure planned for future
general relativity experiments. Reaching the target 10−11-m
displacement uncertainty over 7-m distances and many days’
integration periods is a challenging task. The solution here
investigated consists of a nonpolarizing Mach–Zehnder layout,
featuring an optical cancelable circuit and a holey folding mirror.
The instrument working principle and the method for online
phase reconstruction are presented, as well as the complete
hardware configuration used. The several sources of noise are
investigated mathematically and, whenever possible, verified
experimentally. The displacement gauge was tested up to one
day of continuous data acquisition, showing nanometer-level
performance down to 100 mHz, while air index variations and
mechanical instabilities are currently the main limiting factors at
lower frequencies. This experiment has brought into light many
technical issues that will constitute precious “lessons learned” for
the future improvements of the system
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Compensation of the laser parameters fluctuations in large ring laser gyros: a Kalman filter approach
No full textHe-Ne ring laser gyroscopes are, at present, the most precise devices for
absolute angular velocity measurements. Limitations to their performance come
from the non--linear dynamics of the laser. Following the Lamb semi-classical
theory, we find a set of critical parameters affecting the time stability of
the system. We propose a method for estimating the long term drift of the laser
parameters and for filtering out the laser dynamics effects from the rotation\ud
measurement. The parameter estimation procedure, based on the perturbative
solutions of the laser dynamics, allow us to apply Kalman Filter theory for the
estimation of the angular velocity. Results of a comprehensive Monte Carlo
simulation and results of a preliminary analysis on experimental data from the
ring laser prototype G-Pisa are shown and discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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