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Ring Laser Gyroscope for accurate angle metrology
Abstract
An apparatus for high precision angle metrology,
based on a mid-scale ring laser gyroscope, is in development at
the Italian Metrologic Research Institute (INRIM). The aim is to
build a portable instrument with an accuracy in the range of
some tens nanoradiants.
To measure the angular motion parameters of various kinds of objects, digital angle converters are widely used, which differ in the output electrical interface using a parallel or serial method of code transmission. Currently, there is no single approach to the metrological assessment of the accuracy characteristics of these digital angle converters. Such an integral assessment can be the code reliability of the converter, which characterizes the probability that the value of the measured angle corresponds to the calculated quantum, the code of which is read. Now there is an analytical method for calculating the code reliability used for digital angle converters with parallel code transmission. The article proposes an algebraic method for calculating the code reliability. Unlike the analytical method, it lacks the disadvantages of approximating the error histogram of the digital angle converters by random variable distribution functions. The conducted studies have shown that the algebraic method of calculating the code reliability can be used for the digital angle converters, with parallel and serial code transmission. It is shown that in order to fi nd the unevenness of the angular scale of the digital angle converters using a dynamic goniometer with an incremental angle sensor, it is necessary to center the array of measured error values. The results obtained can be used in measuring technology and metrology for digital angle converters calibration. They are of interest to specialists working in the fi eld of metrology of angular measurements, measuring equipment, digital angle converters manufacturers.
We consider issues of metrological assessment of the accuracy characteristics of digital angle transducers designed to measure the angular movement parameters of various objects. It is shown that currently there is no unified approach to such an assessment, and this is due to the differences in electrical data output interfaces of transducers using parallel or serial code transmission. It is noted that the integral of the transducer encoding reliability can serve as such an assessment. The reliability confidence value characterizes the probability that the value of the measured angle corresponds to the calculated quantum, the code of which has been read. We describe an analytic method for computing encoding reliability for digital angle converters with parallel code transmission. A deficiency in this method is the need to approximate the histogram of the error of the digital angle transducer using distribution functions of random variables. An algebraic method is proposed for calculating encoding reliability that avoids the disadvantage of the analytic method described above. We establish that the algebraic method for calculating encoding reliability can be used for transducers with both parallel and serial code transmission. A methodology is proposed for computing encoding reliability, used to find the integral characteristic of a digital angle transducer with serial and parallel interfaces. It is shown that to find the non-uniformity of the angular scale in a digital angle transducer using a dynamic goniometer with incremental angle encoder requires centering an array of measured error values. The results obtained will be useful in calibrating digital angle transducers.
For the purpose of realizing the dynamic angle measurement and calibration of the inertial equipment (such as rotary table), a dynamic angle measurement system based on the autocollimator and the fiber optic gyroscope (FOG), which can be used as an angle standard device, is designed in this paper. In this system, the scale factor of the FOG can be calibrated in the field application based on the autocollimator with the precision of
$1^{\prime \prime }$
in the range of ±0.05°. Furthermore, the angle measurement deviation between the FOG and the rotary table can be limited in
$\pm 3^{\prime \prime }$
when the scale factor of the FOG is calibrated by the autocollimator when necessary. This research can guarantee the long-term accuracy of the scale factor of the FOG in the field application and it will greatly promote the applications of the FOG as an angle standard device in the field of dynamic angle measurement.
The dynamic laser goniometer (LG) implementation for noncontact measurements of an object's angular position is presented. One of the possible implementations involves determining the time dependence of the scanning mirror's angular position. Another application is aimed at determining the oscillatory movement parameters on the test table. The results obtained in the course of the research show that the dynamic LG makes it possible to calibrate various kinds of test beds making angular oscillations or angular movement of arbitrary law. © 2016 Society of Photo-Optical Instrumentation Engineers (SPIE).
The report presents results of implementation of the dynamic laser goniometer in the mode of non-contact measurements of an object’s angular position. One of obtained results is connected with determination of the time dependence of the scanning mirror angular position. Another kind of implementation result is determination of parameters of a test table oscillatory movement. The obtained results shows that the use of the LG makes it possible to calibrate various kinds of test-beds making angular oscillations or angular movement of some other law.
The results of analysis and experimental investigation of a laser goniometer (LG), working in the mode of the noncontact measurement of an object's angular position, are presented. The important feature of this approach is the very wide range of high-accuracy measurements. In this case, the LG, characterized by the accuracy of ~0.1 arc sec, has big advantages in comparison to photoelectrical autocollimators which have a rather narrow range of measured angular positions. Our results indicate that the use of a laser dynamic goniometer makes it possible to measure constant angles with an accuracy of 0.05 to 0.1 arc sec in the range of possible angles of 15 to 20 deg. If the measured angle is varying, the residual measurement error contains an additional component, which is equal to ~0.2 arc sec, induced by the nonflatness of the optical polygon's faces and by the problems with statistical averaging of the measurement results.
We propose an under-ground experiment to detect the general relativistic
effects due to the curvature of space-time around the Earth (de Sitter effect)
and to rotation of the planet (dragging of the inertial frames or
Lense-Thirring effect). It is based on the comparison between the IERS value of
the Earth rotation vector and corresponding measurements obtained by a
tri-axial laser detector of rotation. The proposed detector consists of six
large ring-lasers arranged along three orthogonal axes.
In about two years of data taking, the 1% sensitivity required for the
measurement of the Lense-Thirring drag can be reached with square rings of 6
$m$ side, assuming a shot noise limited sensitivity ($ 20 prad/s/\sqrt{Hz}$).
The multi-gyros system, composed of rings whose planes are perpendicular to one
or the other of three orthogonal axes, can be built in several ways. Here, we
consider cubic and octahedron structures. The symmetries of the proposed
configurations provide mathematical relations that can be used to study the
stability of the scale factors, the relative orientations or the ring-laser
planes, very important to get rid of systematics in long-term measurements,
which are required in order to determine the relativistic effects.
We present a fully active-controlled He-Ne ring laser gyroscope, operating in
square cavity 1.35 m in side. The apparatus is designed to provide a very low
mechanical and thermal drift of the ring cavity geometry and is conceived to be
operative in two different orientations of the laser plane, in order to detect
rotations around the vertical or the horizontal direction. Since June 2010 the
system is active inside the Virgo interferometer central area with the aim of
performing high sensitivity measurements of environmental rotational noise. So
far, continuous not attempted operation of the gyroscope has been longer than
30 days. The main characteristics of the laser, the active remote-controlled
stabilization systems and the data acquisition techniques are presented. An
off-line data processing, supported by a simple model of the sensor, is shown
to improve the effective long term stability. A rotational sensitivity at the
level of ten nanoradiants per squareroot of Hz below 1 Hz, very close to the
required specification for the improvement of the Virgo suspension control
system, is demonstrated for the configuration where the laser plane is
horizontal.
The concept of constructing precision laser goniometer systems, based on integrating a ring laser and an optical angle sensor with the holographic principle of angular scale recording is considered. The concept implies the application of the cross-calibration procedure, aimed at determination of systematic components of the errors of angle sensors, used in the system. The results of the presented system studies demonstrate the error of angular measurements amounting to ~0.01''. The results of implementing the proposed concept in the creation of a standard system of the plane
angle unit of rigid rotation and the measuring and computing complex for automated control of digital angle transducers with high digit capacity are briefly presented.
Over the last two decades a series of large ring laser gyroscopes have been built having an unparalleled scale factor. These upscaled devices have improved the sensitivity and stability for rotation rate measurements by six orders of magnitude when compared to previous commercial developments. This progress has made possible entirely new applications of ring laser gyroscopes in the fields of geophysics, geodesy, and seismology. Ring lasers are currently the only viable measurement technology, which is directly referenced to the instantaneous rotation axis of the Earth. The sensor technology is rapidly developing. This is evidenced by the first experimentally viable proposals to make terrestrial tests of general relativistic effects such as the frame dragging of the rotating Earth.
Large ring lasers for rotation sensing
- K U Schreiber
- J.-P R Wells
K. U. Schreiber and J.-P. R. Wells, Large ring lasers for rotation
sensing, Rev. Sci. Instrum. 84, 041101 (2013)