Note: Descriptions are shown in the official language in which they were submitted.
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DETECTION OF INCLUSIONS IN GLASS
This invenfion relates to a method and apparatus for detecting
inclusions in glass plate, particularly nickel sulphide (NiS) inclusions.
The invention further relates to an apparatus and method for classifying
the detected inclusions.
BACKGROUND TO THE INVENTION
The problems associated with inclusions in glass have been
l0 known for many years. In particular, it has been known for the last thirty
years that heat-strengthened or fully toughened glass plates that have
NiS inclusions are subject to spontaneous failure. In short, the
presence of NiS inclusions can result in shattering of the glass plate.
Unfortunately, these inclusions have proven to be extremely difficult to
exclude from the manufacturing process. They are also very difficult to
detect. This has led to potentially unsafe glass being used in buildings.
The detection and classification of NiS inclusions has proven to
be a very difficult problem. A heat-soaking process has been
developed in which the manufactured glass is maintained at an
elevated temperature. The theory is that almost all glass which
contains inclusions that could cause spontaneous failure will fail during
the heat soak process. This has proven to be incorrect and there are
today many buildings that have been erected using glass that is subject
to spontaneous failure due to the effect of NiS inclusions. In fact, a
number of instances have been recorded of glass falling from multi-
story buildings as the result of failure due to NiS inclusions that have
not been detected by the heat soak process.
A number of approaches have been proposed for identifying NiS
inclusions in glass. One such proposal is found in United States patent
number 4697082, assigned to Flachglas Aktiengesellschaft. The
Flachglas patent describes a process for testing glass for material
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defects by illuminating the glass with a laser-produced flying light spot.
The forward and backward scattering produced by inclusions in the
glass are detected and analysed. Detecting forward and backscattering
is not generally practical once the glass has been erected into a
building. A single sided approach, normally the outside, is required.
Another approach is described in United States patent number
5459330, assigned to Thomson-CSF. This patent describes an
apparatus that illuminates successive cross section planes in a sheet
of glass and uses a camera to detect reflected radiation. Reflections
from the front and back surface of the glass produce two lines in the
image that define the boundaries within which an inclusion may be
located. Luminous points situated between the two lines are detected
as inclusions within the glass. However, the apparatus does not identify
the nature of the inclusion nor is the apparatus suited to scanning of
glass sheets in situ on a building.
A further apparatus for detecting flaws in glass is described in
United States patent number 4597665, assigned to Tencor
Instruments. As in the Flachglas device, the Tencor apparatus
measures reflected laser light above and below the plane of the glass
sheet. In most applications it is impractical to position detectors on both
sides of the glass being tested.
OBJECT OF THE INVENTION
It is an object of the invention to provide a method and
apparatus for detecting inclusions in glass.
It is a further object of the invention to provide a method and
apparatus for classifying inclusions detected in glass sheets.
Further objects will be evident from the following description.
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SUMMARY OF THE INVENTION
In one form, although it need not be the only or indeed the broadest
form, the invention resides in an apparatus for detecting inclusions in glass
comprising:
means for emitting at least two coherent beams of radiation directed at the
glass at one or more known angles;
means for detecting radiation scattered by inclusions within the glass;
means for scanning the coherent beams across the glass; and
means for recording the location of a scattering inclusion.
The apparatus may further comprise means for classifying the
inclusions. The means for classifying the inclusion suitably comprises a
camera that records a pattern of laser radiation scattered from the detected
inclusion. The means for classifying may also include means for
discriminating a pattern that indicates an inclusion is not NiS
The means for classifying may further comprise categorising means for
positively categorising the detected inclusions that are NiS. The categorising
means is suitably a spectroscopic means that analyses radiation scattered
from the inclusion and categorises the scattered radiation by a spectroscopic
signature.
In a further form, the invention resides in a method of detecting
inclusions in glass including the steps of:
directing at least two coherent beams of laser radiation at the surface of the
glass;
scanning the laser radiation over the glass;
detecting radiation scattered from inclusions in the glass; and
recording the coordinates: of the inclusions. .,~
Suitably, the method further includes the steps of detecting first
detected radiation scattered from a first beam of laser radiation, detecting
second detected radiation scattered from a second beam of laser radiation,
and only recording the coordinates of an inclusion if the first detected
radiation
and the second detected radiation are separated in a known way.
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The second beam of laser radiation may be a reflection of the
first beam.
The method preferably includes the further steps of:
viewing the detected inclusion with a camera; and
classifying the inclusion according to the pattern of laser radiation
scattered from the inclusion.
In preference, the step of classifying classifies the inclusions
that are not NiS.
The method may also include the further steps of: spectroscopically
analysing the radiation scattered from the detected inclusions; and
categorising the identified inclusions as NiS by a spectroscopic
signature.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are described with
reference to the following figures in which:
FIG 1 is a sketch of an apparatus for detecting inclusions
in glass;
FIG 2 is a schematic side view of a first embodiment of a
detection unit;
FIG 3 is a schematic side view of a second embodiment
of a detection unit;
FIG 4 is a schematic side view of a classification unit;
FIG 5 is a sketch showing the pattern of radiation
scattered from a smooth inclusion;
FIG 6 is a sketch showing the pattern of radiation
scattered from a rough inclusion;
FIG 7 is a sketch showing another view of the pattern of
radiation scattered from a smooth inclusion;
FIG 8 is a sketch of a detector head incorporating the
detection units of FIG 2; and
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FIG 9 is a sketch of a detector head incorporating the
detection units of FIG 3;
FIG 10 is a timing diagram showing the method of
determining size and depth of an inclusion.
5
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG 1 there is shown a sketch of an apparatus 1 for
detecting inclusions in glass sheets 2. The apparatus 1 includes a
detector head 3 that can be scanned over the area of the glass sheet 2.
In the preferred embodiment the detector head 3 incorporates multiple
detection units, such as 4. The glass sheet 2 may be scanned in situ on
a building or on a production line.
The mechanical frame 5 of the apparatus is made from extruded
aluminium for strength and light weight. The frame is made to a
suitable size to suit the glass sheets to be tested. It will be appreciated
that glass sheets are manufactured in a vast number of different sizes
and shapes. It is therefore necessary that the mechanical frame has a
degree of modularity for ease of adaptation to different situations. The
frame 5 is clamped to the window 2 using vacuum activated suction
pads 5a.
The detector head 3 is movable in the X-axis on a transverse rail
6. Movement of the detector head 3 is effected by a stepper motor 7
and belt drive 8. The belt 8 is a toothed belt to substantially eliminate
slippage. The X position of the detector head 3 relative to a starting
position is therefore determinable by counting of the stepper motor
pulses.
The transverse rail 6 and detector head 3 are movable in the Y-
axis on lateral rails 9. A constant torque motor 10 drives a belt 11 to
move the transverse rail 6. An encoder on the belt 11 provides
3o positional information that is combined with the stepper motor
information to calculate the coordinates of the detector head 3 at any
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position within the range of movement of the detector head 3 on the
transverse rail 6 and the transverse rail 6 on the lateral rails 9. Support
rails 12 provide rigidity to the frame 5.
Turning now to FIG 2, a first embodiment of the detector head 3
consists of at least one detection unit 4 which includes a laser module
13 that emits a beam of coherent radiation. A suitable laser is a
semiconductor laser operating at 635nm with a power of approximately
20mW. For reasons described below, the laser is modulated at a
known frequency, for example 455kHz. It will be appreciated that the
specific laser is not critical to the apparatus but will be chosen to suit
the particular application.
A lens 14 shapes the output of the laser 13 to produce a line of
radiation that has a length of approximately 10mm at the surface of the
glass and a width of 100~m. A prism 15 directs the laser beam 16 into
the glass plate at an angle of between 30 and 60 degrees, depending
on the thickness of the glass.
The detection unit 4 also includes at least one sensor 17 and
sensor electronics 18 that measures the radiation scattered from any
inclusions in the glass. Silicon photodiodes have been found to be
suitable sensors although other arrangements, such as fibreoptic
collectors coupled to a photomultiplier tube, CCD cameras, video, and
sensor arrays, would also be suitable.
The scattered radiation 19 is collected by objective lens 20 and
focussing lens 21. The lenses 20, 21 image the scattered radiation
onto the sensor 17. As the incident laser beam is a line, a cylindrical
lens (not shown) may optionally be used to focus the scattered
radiation to a spot at the sensor. The inclusions within the glass act
essentially as point source scatterers so the imaging optics can image
the scatterer onto the sensor with good efficiency.
As can be seen in FIG 2, the reflection 22 from the front surface
24 and the reflection 23 from the back surface 25 of the glass 2 are not
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collected by the lenses 20, 21. Furthermore, scattering from the front
surface 24 and back surface 25 is not imaged by the collection optics
onto the sensor 17. Although some surface scatter will be collected, the
optical and physical arrangement spatially filters the radiation reflected
or scattered from the front and back surface. Thus the signal obtained
from the sensor electronics 18 is primarily due to radiation scattered
from within the body of the glass.
As described in more detail below, each inclusion will produce
two signals. One signal is produced when the inclusion is illuminated
directly by the laser beam 16. Another signal is produced when the
laser beam is reflected from the back surface 25 and the reflected
beam illuminates the inclusion. The presence of both signals is
confirmation that the inclusion occurs in the bulk of the glass rather
than on the front or back surface. If only a single signal is detected the
position is not recorded. Furthermore, the two signals must be
separated by an expected amount (time or distance) for the position of
the scatterer to be recorded. This criteria helps to avoid false results
caused by anomalous signals generated by strong scatterers on the
surface of the glass sheet.
A method of determining the size and depth of the inclusion is
described below.
In addition to the geometric arrangement, various filters are
employed to improve the signal to noise ratio of the detector head
including notch filters to reduce the background.
The prism 15 is positioned closely adjacent the surface of the
glass. The prism 15 is spring loaded within the detector unit 4 and may
move between outer stop 26 and inner stop 27 on bearing 28. This
arrangement ensures that the prism 15 is always in a known position
relative to the body of the detector unit 4 within the detector head 3. As
described below, the detector head 3 rolls on the front surface 24 of the
glass 2. The position of the laser beam 16 relative to the glass 2 is
PCT/AU00/01032
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8 Received 18 June 2001
therefore always known so the coordinates of an inclusion that causes
scattering from within the glass can be accurately recorded.
As mentioned above, the laser is modulated, preferably at a frequency
of 455kHz. Modulating the laser allows for lock-in detection of the scattered
radiation and reduces the effect of background light. Furthermore, the rate of
scanning may not be constant over the entire scan area and therefore an
independent timing mark is required for determining size and depth. This
aspect is described in detail below with reference to FIG 10.
An alternative embodiment of a detector unit 30 is shown in FIG 3. The
primary difference from the first embodiment is the incorporation of two laser
housings 31, 31a. Each housing mounts a laser 32, 32a and corresponding
focussing lenses 33, 33a. The laser beams 34, 34a are directed to the glass
sheet 2 by adjustable mirrors 35, 35a.
It will be appreciated that a person skilled in the art would understand
that the arrangement of FIG 3 may be achieved using a single laser and a
beam splitter to produce two laser beams.
The mirrors 35, 35a are adjustable to account for different glass
thickness. The optimal angle for interrogating the glass sheet with the laser
beams is dependent upon the thickness of the glass and the mechanical
arrangement of the apparatus, particularly the amount of space available
between the laser and the surface of the glass. The inventor has found that an
angle of between 40 and 45 degrees is best for 1 Omm thick glass.
A sensor 17 and sensor electronics 18 detect radiation scattered from
the bulk of the glass sheet in the manner described previously. Lenses 36, 37
collect scattered radiation 38 from the bulk of the glass and direct it to the
sensor 17. As with the first embodiment, the,f~nt and back surface reflections
are not collected by the lenses
Unlike the first embodiment, the optical elements are fixed within the
detection unit 30 (except for the adjustability of the mirrors 35, 35a) and
the
unit 30 is spring loaded to remain close to the surface of the glass. This
simplifies the detection unit arrangement compared to the first embodiment.
AMENDED Si ;~.
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In the second embodiment, the laser beams 34, 34a are counter-
propagating. As the detector head 3 is scanned across the surface of
the glass sheet 2, any inclusion will produce scattering from first one
laser beam 34 and then the other 34a. The scattering signals are
analogous to the forward scattering and backward scattering signals
described with reference to FIG 2, but both signals are of
approximately the same magnitude. In the embodiment of FIG 2 the
backward scattering signal is considerably weaker than the forward
scattering signal. For this reason the second embodiment is preferred.
As with the first embodiment, the position of a scatterer is only
recorded if two signals are detected with an expected separation. The
expected separation may be within a certain number of steps of the
stepper motor or within a certain period of time, depending on the rate
of scan of the detector head.
In the second embodiment it may be convenient in some
applications to modulate each laser beam at a different frequency, say
455kHz for laser beam 34 and 370kHz for laser beam 34a. The
electronics 18 can then be configured to discriminate between signals
from each laser thereby providing an extra dimension of discrimination
for accurately recording the position and nature of a detected inclusion.
For example, the first embodiment will not be able to correctly identify
two inclusions that exist in the glass at a separation equal to the
spacing between the laser beams at the inclusion. This is because the
second scattering signal could be backwards scattering from the first
inclusion or forwards scattering from the second inclusion. However,
frequency discrimination allows the system to differentiate between the
received signals.
The schematic diagram of FIG 3 shows the laser beams 34, 34a
as counter-propagating and meeting at the rear surface of the glass. It
will be appreciated that this is not an essential arrangement. Each laser
beam may be directed to the glass at a different angle but recording of
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the calculation of the position of the detected inclusion will require
adjustment to account for the geometric arrangement.
Similarly, the laser beams 34, 34a may be directed at the glass
sheet at the same angle but may be arranged to be slightly separated
5 at the rear surface. This will require a slightly wider field of view of the
collection optics. Although FIG 2 and FIG3 show the collection optics
as collecting scattered radiation from a distinct point, it will be
appreciated that this is merely indicative, in fact the collection optics
collect any scattered radiation from within the field of view of the optics.
10 The inventor envisages that both lasers could be adjacent and
directing coherent radiation at the glass. This is not a preferred
arangement.
The detector head 3 may include at least one classification unit
40, shown in FIG 4. The classification unit 40 consists of a CW laser 41
with beam shaping optic 42 that directs a beam 43 of coherent laser
radiation towards the known position of an inclusion. The scattered
radiation 44 is collected by lens 45 and viewed by a video camera 46.
The inventor has found that the scattering from certain inclusions will
cause a visible pattern. Inclusions that are smooth scatterers, such as
air bubbles, produce specular reflections in all directions. Constructive
interference is visible as bright lines in the video image and is
indicative of an air bubble or similar specular reflector. A sketch of a
typical image of a smooth scatterer is shown in FIG 5.
In contrast, a rough scatterer does not produce a regular
constructive interterence pattern but instead produces the pattern
shown in FIG 6. It can be seen in FIG 6 that there are bright spots that
result from constructive interference and dark spots resulting from
destructive interference but does not show the regular bands indicative
of a specular reflector. The video image may therefore be used to
classify a detected inclusion into either a smooth scatterer or a rough
scatterer. Since it is known that NiS in glass has a rough texture, any
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detected inclusion that produces a regular interference pattern can be
rejected as not being NiS.
In some instances it may be appropriate to use the camera to
capture a wider field of view of the inclusion. This will result in imaging
of the inclusion in the manner shown in FIG 7. The information content
is essentially the same, but is presented differently.
To further classify the detected inclusions a spectroscopic
technique may be employed. The spectroscopic technique involves
spectroscopic analysis of the scattered radiation to categorise the
inclusion according to spectroscopic signature. For example, the
scattered radiation may have a distinctive Raman scattered signal that
categorises the inclusion as NiS. Alternatively, a different
categorisation head may be employed that includes a laser emitting
radiation in the blue region of the spectrum and a spectrometer that
measures fluorescence from NiS.
To increase the efficiency of scanning a glass sheet, a number
of detection units and classification units can be mounted in a single
detector head. A front view of a typical detector head is shown in FIG 8.
The detector head 3 includes three detection units 4 and two
classification units 40. Also visible in FIG 8 are rollers 41 that roll on
the surface of the glass so that the position of the detector head
relative to the glass stays constant, irrespective of imperfections in the
glass surface.
The detector head 39 for the second embodiment is shown in
FIG 9 and is slightly different than the detector head 3 to account for
the second laser in the detection unit 30. The detector head includes
three detection units and two classification units. Four rollers 42
support the detection head 39 against the glass.
In operation, the detector head 3 is advanced in the X direction
to sweep out a 10 mm strip of the glass for each detection unit, than
moved along the Y axis before scanning the next strip. It has been
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found that scanning rates of 2.5 minutes per square meter is
achievable with the preferred embodiment.
As mentioned above, the coordinates of detected inclusions in
the glass are recorded. Not all inclusions will be NiS. In fact, only a
small percentage of detected inclusions will be NiS in a typical glass
sheet. After the glass has been scanned the detector head is returned
to the coordinates of all detected inclusions and the classification unit
is used to classify the inclusion.
In a production line situation the framework can be fixed with the
glass sheet being passed under the framework on a conveyor. The
data obtained can be integrated with the glass cutting operation to
optimise the cut to eliminate known and suspected NiS inclusions. The
operation of the apparatus is similar in either application.
When testing glass already installed on a building the need for
accurate registration requires an additional dimension. Typically, the
apparatus is fitted to existing equipment for facade maintenance from
the outside. The position of the facade maintenance equipment can
then be recorded by window number or some other suitable parameter.
Because the laser radiation is modulated, it is possible to exploit
the time base of the laser pulses to estimate the size and depth of an
inclusion from the signal obtained from the scattered radiation. It will be
appreciated that two signals, as shown in FIG 10, will be detected for
each inclusion for either the first, single laser embodiment, or the
second, double laser embodiment. A primary scattering signal 50 will
be detected as the first laser beam crosses the inclusion. As the scan
continues the second laser beam or the reflection of the laser beam
from the back surface will be scattered by the inclusion thereby giving a
secondary scattering signal 51. The time difference Ot between the
primary scattering signal 50 and the secondary scattering signal 51
indicates the depth of the inclusion. The time that secondary scattering
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signal 51 occurs after the primary scattering signal 50 depends upon
the rate of scan of the detector head 3, 39.
As shown in FIG 10, each scattered signal has a width ~w. The
width of the signal, or more specifically the number of scattered laser
pulses, gives an indication of the size of an inclusion. A large inclusion
will scatter more laser pulses for a given scan rate than a small
inclusion. Given that the pulse rate is known, and the rate of advance
of the carriage is known, it is a simple matter to calculate the size of the
inclusion that equates to a given number of scattered pulses.
It has been found that inclusions of greater than or equal to
50~m size can be detected with the apparatus. Furthermore, these
inclusions can be classified as not NiS or otherwise with a high degree
of accuracy.
It is convenient to operate the apparatus from a conventional
personal computer. The personal computer provides the signals to stop
and start the motors, turn the lasers on and off, and record the
coordinates of detected inclusions. These functions could also be
performed by a purpose built controller.
Throughout the specification the aim has been to describe the
invention without limiting the invention to any specific combination of
features.
