Optics: measuring and testing – Angle measuring or angular axial alignment – Photodetection of inclination from level or vertical
Reexamination Certificate
2002-10-10
2003-12-16
Tarcza, Thomas H. (Department: 3662)
Optics: measuring and testing
Angle measuring or angular axial alignment
Photodetection of inclination from level or vertical
C702S154000
Reexamination Certificate
active
06665062
ABSTRACT:
TECHNICAL FIELD
The present invention concerns an inclinometer, in other words a device intended to measure inclination variations.
In particular, the invention can be used in mines, drillings, in the civil engineering field, for the surveillance of civil engineering structures (for example bridges and tunnels) and buildings (in particular, historical monuments), and anywhere where a precise control of the stability of a structure is required, particularly over the long term.
STATE OF THE PRIOR ART
Diverse methods of auscultation are customarily used to monitor the amplitude and the rate of distortion, whether horizontal or vertical, of structures, of the surface of the ground and of accessible sub-soil parts in various categories of construction.
Customarily, in civil engineering, instruments called “extensometers” are used to monitor distortions. Various extensometers are commercially available.
As regards rotation measurements, electric inclinometers, also called “tiltmeters” are already known, which are used to monitor the change in inclination of points located on the ground or in the ground or placed on a structure. These inclinometers comprise a sensor that is sensitive to gravity (a pendulum) and arranged in an appropriate housing. Several types of inclinometers are known.
For example, a mechanical inclinometer is known, which comprises a beam and a bubble level, with an adjustment for levelling at one of its ends. This beam is fastened onto two reference spheres anchored on the system to be measured. The levelling adjustment is carried out by adjusting the bubble level and a tachometer dial is used to carry out the measurement. The measurement range is typically several degrees. The precision is approximately ±0.013 millimeters for a beam 200 millimeters long (i.e. around 60 &mgr;rad) and is reduced to ±0.13 millimeters for a beam 900 millimeters long (i.e. around 150 &mgr;rad).
An inclinometer comprising an accelerometric sensor is also known. The measurement is carried out by placing the inclinometer in a reproducible position on a flat reference piece. One takes a first reading then one turns the sensor by 180° and one then takes a second reading. The flat reference piece is metallic or ceramic and must be securely fastened onto a surface that one wishes to control. The measurement range goes from −30° to +30° and the precision is typically ±250 rad.
Moreover, an inclinometer comprising a pendulum fastened to the upper part of said inclinometer is also known. The inclination of the body of said inclinometer induces bending stresses on one piece. These bending stresses are monitored by two vibrating cord sensors fastened to each side of the piece. In another configuration, the two vibrating cord sensors are fastened between the pendulum and the cover of the inclinometer. This configuration makes it possible, thanks to a differential mounting, to disregard temperature effects.
The measurement range is typically from −0.110 to +0.110 with a precision of around 0.5% of the full scale (around ±10 &mgr;rad to ±100 &mgr;rad).
Furthermore, an inclinometer comprising an electrolytic level sensor is known. In a first embodiment, a glass measuring cell containing a liquid that conducts electricity (mercury for example) is sealed at its two ends. The measuring precision of this type of sensor is average. Moreover, its thermal sensitivity is high. As a result, it is not very suitable for geotechnical or civil engineering applications.
In a second embodiment, a vacuum is created in the measuring cell. The performance levels are better. Nevertheless, a specific calibration is necessary for each cell and re-calibrations are required. Such inclinometers are expensive, especially when they are used in a network.
We will now consider optic fibre sensors and particularly the advantages of such sensors, such as the insensitivity to electromagnetic perturbations.
First of all, it is worth recalling several facts concerning photo-induced fibre Bragg gratings.
A Bragg grating photo-induced in an optic fibre consists of a periodic structure formed by a modulation of the refraction index of the core of the optic fibre. This type of structure behaves practically like a mirror for a very fine spectral band around a characteristic wavelength &lgr;
B
(wavelength for which there is a phase tuning between the multiple reflections within the grating) and remains transparent to all other wavelengths. In fact, the multiple waves reflected at these other wavelengths are not in phase, interfere destructively in reflection and are thus transmitted.
The characteristic wavelength, called “Bragg wavelength”, is defined by Bragg's law:
&lgr;
B
=2
.N
eff
.&Lgr;
where &Lgr; is the pitch of the effective index network n
eff
.
The final characteristics of a photo-induced Bragg grating depend on the induction parameters such as the type of laser (wavelength, operating conditions) and the luminous intensity used, the wavelength &lgr; at which this network is induced, the effective index n
eff
of the optic fibre in which it is induced, the amplitude of the modulation or variation of index An and the period &Lgr; of said index variation.
These parameters determine the characteristic magnitudes of the Bragg grating, namely: the Bragg wavelength &lgr;
B
, the reflectivity R
max
at &lgr;
B
, and the width at mid-height of the reflectivity peak, as well as the propensity of the grating to withstand large temperature variations or considerable extensions, which is an important aspect for the use of said Bragg grating as a transducer.
We will now consider such a Bragg grating transducer. Given Bragg's law that characterises this grating, the characteristic wavelength &lgr;
B
depends on the temperature and the stresses (&sgr;x, &sgr;y, &sgr;z) applied to the fibre in which the grating is formed.
It is normal to separate the three contributions, namely the temperature variations &Dgr;T, the extensions &egr;=&Dgr;L/L along the axis of the core of the fibre and the hydrostatic pressure variations &Dgr;P, according to the equation:
&Dgr;&lgr;
B
/&lgr;
B
=a′.&Dgr;T+b′.&egr;+c′.&Dgr;P
where a′, b′ and c′ are coefficients that depend on the characteristics of the fibre and, to a lesser extent, on its temperature. In practice, they can be assimilated to constants, independent of the temperature, over a large range of operation.
Thus, a precise measurement of &Dgr;&lgr;
B
(variation of &lgr;
B
compared to an initial reference) makes it possible to determine the amplitude of the variation of the phenomenon that has induced this variation of &lgr;
B
. Beyond its simple role as a spectral filter, the Bragg grating thus constitutes a “transducer”, since it transforms the changes in an influence quantity into a spectral shift proportional to these changes.
We will now consider the response of the Bragg grating to a temperature variation. When the grating is subjected to such a variation, it dilates or contracts, which modifies its pitch. Moreover, since the refractive index of a material depends also on the temperature, these two phenomena bring about a variation &Dgr;&lgr;
B
of the characteristic wavelength such that:
Δ
λ
B
λ
B
=
Δ
(
n
Λ
)
n
Λ
=
[
1
Λ
ⅆ
Λ
ⅆ
T
+
1
n
ⅆ
n
ⅆ
T
]
Δ
T
=
a
′
·
Δ
T
In the case of silica, the coefficient a′ is substantially equal to 7.8 10
−6
/° C.
By making a=a′.&lgr;
B
, one can then state:
&Dgr;&lgr;
B
=a.&Dgr;T
We will now consider the response of the Bragg grating to these distortions. As we have seen, stresses are also likely to modify the characteristic wavelength of the grating.
We can define the variation of the Bragg wavelength as a function of an extension as follows:
Δ
λ
B
λ
B
=
Ferdinand Pierre
Rougeault Stephane
Andrea Brian
Commissariat a l'Energie Atomique
Tarcza Thomas H.
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