Optics: measuring and testing – Material strain analysis – By light interference detector
Reexamination Certificate
1999-06-22
2001-04-17
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
Material strain analysis
By light interference detector
C356S491000
Reexamination Certificate
active
06219131
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and to a device for measuring membranes within a transparent material, such as a window. Such stresses may occur, in particular, in a strip of float glass leaving the end of the production line. Incomplete control over the membranes is the main cause of breakage of glass on float glass lines. Measuring the profile of these stresses makes it possible to control them better and to improve productivity.
It is also known to create a certain prestress condition in the peripheral zone of certain windows, such as car windows, in order to give increased strength to this zone, which is generally fragile.
PRIOR ART
A device for measuring stresses in a window is known in the art, which is referred to by the term Sharples device. This device comprises a light source which emits a light beam that passes successively through a rotary polarizer, the window, a quarter-wave plate, an analyzer and a photodiode.
If the transparent material is subjected to local stresses, its refractive index is modified anisotropically, which has the effect of creating local birefringence in the material. The effect of the birefringence is to phase-shift the various polarization components of the light passing through the material.
It can be shown by the theory of photoelasticity that the stress &sgr; existing at a point in the window is given by the formula:
σ
=
λ
2
π
C
0
E
Ψ
where &lgr; is the wavelength of the light,
E is the thickness of the window,
C
0
is the photoelasticity constant of the window,
and &PSgr; is the phase shift introduced by the birefringence of the window because of the existence of a stress.
&PSgr; is measured by rotating the analyzer until the light intensity observed at its output is zero. At this moment, a black band is observed at the place in the window where the phase shift is being measured.
The Sharples instrument is unreliable because the measurements which can be taken with it involve a not insignificant human factor and procedures which differ from one user to another. Specifically, viewing the black line depends on each individual's vision. It has thus been noted that, with the Sharples instrument, there is a large dispersion in the measurements which may be of the order of 2 MPa for a stress value of the order of 5 MPa.
U.S. Pat. No. 2 563 337, belonging to the same Applicant Company, discloses another device for measuring stress which partially overcomes the drawbacks of the Sharples device. This device, which is illustrated by appended
FIG. 1
, comprises a light-emitting device
10
and a light-receiving device
12
which are arranged respectively above and below a transparent material, for example a strip of glass
14
leaving the end of the production line. The strip of glass is assumed to be moving in the direction perpendicular to the plane of the page. The emitting device comprises a light source
16
, an interference filter
18
for filtering the beam and transmitting a monochromatic beam of predetermined wavelength &lgr;, an optical system
20
used to collimate the monochromatic beam, a rotating polarizer
22
driven by a motor
24
via a gear wheel
26
and a quarter-wave plate
28
oriented at 45° to the longitudinal axis of the strip of glass.
The receiving device
12
comprises an analyzer
30
oriented at 45° to the longitudinal axis of the strip of glass, an interference filter followed by a photodiode
32
and an amplifier
34
.
A pyrometer
36
is installed in the vicinity of the receiving device
12
in order to measure the transverse temperature profile of the glass. The outputs of the photodiode and the pyrometer are connected to a microprocessor
38
.
The phase shift &PSgr; is measured by an optical encoder and is stored in the microprocessor. It is therefore possible, using formula (1), to calculate the stress &sgr; at any point in the strip of glass.
The emitting device
10
and the receiving device
12
are mounted on two support arms lying respectively above and below the strip of glass, and move with a to-and-fro movement on these support arms, along guide rails oriented in the transverse direction of the strip of glass. The movement of the two devices must be synchronous so that their optical axes are always in mutual alignment.
The device according to the said patent makes it possible to measure the stresses correctly, but has several drawbacks:
it has a high cost because of the use of two support arms which need to be driven synchronously,
the rotational drive of the optical system formed by the polarizer and the encoder is provided by relatively complicated mechanical transmissions which require continual maintenance,
the frequency of the signal, which is of the order of a few hertz, is too small for an accurate measurement of the phase shift &PSgr; to be made, because at this frequency the light intensity fluctuates with the slightest of perturbations, such as dust, inclusions or irregularities in the layers deposited on the glass.
The object of the present invention is to overcome these drawbacks by providing a method and a device for measuring stresses which perform well and make it possible to simplify the mechanics for translating the units, or even eliminate this completely, so as to reduce the cost involved.
It also relates to a method and to a device for measuring stresses which are not sensitive to fluctuations in intensity due to dust, inclusions in the glass and irregularities in the layers deposited on the glass.
It also relates to a method and to a device for measuring stresses which make it possible to take proper measurements even if the signal is weak in relation to the noise.
In order to describe the invention, the principle on which it is based will be described first.
A birefringent medium will be considered which is subjected to a stress and is placed between a polarizer and an analyzer. It is known that this introduces a phase shift &PSgr;
0
and that the light intensity I measured at the output of the analyzer is a maximum if the polarizer and the analyzer are crossed and are at 45° with respect to the characteristic axes of the birefringent medium. This intensity is given by the formula:
I
=
I
0
2
(
1
+
C
cos
Ψ
0
)
where C is the contrast and I
0
is the transmitted light intensity.
The phase shift &PSgr;
0
may be the sum of several phase shifts introduced by different optical components. If one of these components is a modulator, for example of the photoelastic type, whose characteristic axes coincide with those of the strip of glass, the following condition will be obtained:
&PSgr;
0
=&PSgr;+&PSgr;
m
where
&PSgr; is the phase shift to be measured in the strip of glass,
&PSgr;
0
is the phase shift introduced by the modulator.
Using f
0
to denote the excitation frequency of the modulator, this gives &PSgr;
m
=A
0
cos (2&pgr;f
0
t).
The measured intensity is then:
I
=
I
0
2
{
1
+
C
cos
[
Ψ
+
A
0
·
cos
(
2
π
f
0
t
)
]
}
Rearranging the expression above gives:
2
I
I
0
-
1
=
C
.
cos
ψ
cos
[
A
0
cos
(
2
π
f
0
t
)
]
-
C
.
sin
ψ
sin
[
A
0
cos
(
2
π
f
0
t
)
]
The amplitudes of the components of I at frequencies f
0
and 2f
0
are respectively given by:
B(f
0
)=−CI
O
J
1
(A
0
)·sin &PSgr;
B(2f
0
)=−CI
0
J
2
(A
0
)·cos &PSgr;
where J
N
are the N
th
order Bessel functions.
However, these two components do not make it possible to calculate the phase shift &PSgr; because, in view of practical measurement conditions, the contrast C is equal to an unknown value less than 100%. Similarly, I
0
is unknown given that the absorption by the glass varies from one component to another and that dust, incl
Grente Pascal
Zhang Jingwei
Pennie & Edmonds LLP
Saint-Gobain Vitrage
Turner Samuel A.
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