Despite the emergence of plastics in the last half-century, bottles are still widely used as vessels for containing liquids. Recently, bottles of minimum thickness have been manufactured in order to lighten the weight for ease of conveyance, minimize the cost of their production, and save resources. However, bottles must be manufactured in such a way as to prevent their breakage during conveyance and use. Breakage is often due to a lack of uniformity in the bottle's thickness. Thus, a real-time and non-contact method for measuring bottle thickness is required for their systematic mass production. For
this purpose, some non-destructive methods have been developed using light An
optical system is usually easy to construct and has sufficient resolution for
practical use. However, the methods described in the Ref. The
previous methods
This
method uses both reflected and scattered lights on the bottle surface and is
based on the diffraction of these lights at each boundary. Figure 5.57 shows
the optical principle for this method. The scattered light on both the outer
and inner surfaces of the bottle is focused onto a CCD array by means of both
lenses, as shown in Fig. 5.57(a): we call this "the image method". On
the other hand, the reflected light on both the outer and inner surfaces of the
bottle reaches the CCD array directly as shown in Fig. 5.57(b): we call this
"the reflection method". Both image and reflection methods have
successfully been used for measuring surface displacement and surface roughness
in our previous paper, where the optical basis has been described in detail
The reflection method usually has higher sensitivity than the imaging method. It cannot, however, be applied to rough surfaces and surfaces with local curvature, because the reflected light on such a surface cannot be captured on the CCD array as shown by the dotted line in Fig. 5.57(b). By contrast, the image method shown in Fig. 5.57(a) can be successfully used in such situations. Thus, the technique in this paper is based fundamentally on the image method, with the reflection method used as a supplementary method to detect a dislocated bottle center as described in 5. 11. 2. C. When an irradiated surface is dislocated mainly due to the change of bottle thickness and the dislocation of the bottle center, the surface and thus the image position on the CCD array changes, which makes it possible to measure the bottle thickness.
The image method is first analyzed using a fixed bottle center and only regarding the two-lens system. Figure 5.58 shows the optical principle for measuring bottle thickness. As
described above, the
thickness can be calculated by a light-ray tracing of the diffraction at each
boundary. The optics are so arranged that the irradiation positions S
where R Bottles
are usually manufactured using a mould, thus ensuring that the outer radius R
The analysis described in section 5. 11. 2 B under the condition of a fixed center is not, however, practical, because the bottle's center is dislocated in practically every measurement, which leads to measurement error. In this section, it is shown how both reflection and image methods are used for measuring bottle thickness under the condition where the bottle center is dislocated. Thus, this method essentially eliminates measurement error. Figure
5.59 shows the optical principle for measuring bottle thickness under the
condition of a dislocated center. Scattered and reflected light are, in a
practical sense, detected at different directions from each other by means of a
half-mirror between the surface and a focusing lens, as shown in Fig. 5.59(c);
the scattered light is detected on the CCD The
position of the bottle center is analyzed by means of the position of both the
reflected light on the CCD
where, As
described in 5. 11. 2. B, the outer radius is assumed to be practically
constant, and any change of thickness can then be determined by means of the
inner radius R
where t
Figure 5.60 shows the schema of our practical system for measuring bottle thickness under the condition of a dislocated center by a combination of the image and reflection methods. The system is composed of two main subsystems, i.e., the data processing
subsystem shown in Fig. 5.60(a) and the sensor head shown in Fig. 5.60(b). The data obtained by both CCD arrays are A/D converted and amplified by means of an A/D converter. The data acquisition and data transmission are controlled by means of the CPU. Such data lead to a FPGA (field programmable gate array), and then to a DSP (digital signal processor) through a data transmitter and receiver. This DSP is the interface for fast-signal transmission with low-voltage differential signaling. The data are processed at the DSP, and the results including judgment of the bottle's acceptability are displayed on a monitor. Both
the scattered and reflected light from the outer and inner surfaces is received
on the CCD The
measurement frequency of this system depends on the data acquisition speed of
the CCD array and the data-processing speed. A data-processing speed which is
between a few tens and a few hundreds kHz is sufficiently high. Then, the measurement
frequency depends on the data-acquisition speed of the CCD array: CCD arrays
with the speed of a few kHz are now on the market. Some measurements, usually a
few tens measurements, are smoothed to obtain an accurate result because each
signal will have a noise due to vibration caused by on-line measurement
A
simulation experiment was performed using the computer software "Zemax
Table
5.6 shows the position detected on the CCD array for an optical arrangement
involving combinations of different incident angles and dislocations. The
resolution
This resolution varies
depending on the optical arrangement, such as incident angle Table
5.7 shows the resolution obtained from the data in Table 5.6. The mean
resolution for this arrangement was between 10 and 50
In conclusions, the following results were obtained.
(1) The system consists of two main subsystems: one is a sensor head consisting of a laser as a light source, two CCD arrays for detecting the position of both reflected and scattered light and a focusing lens to image the light scattered on the CCD array, and the other is a data-processing subsystem consisting of a personal computer with an A/D converter and amplifier, and a monitor for displaying a thickness. (2) The resolution of
this method, i,e., the smallest measurable change in thickness, was between 10
and 50
A
small change in bottle thickness, First,
the small change in the outer radius,
The small change of the
outer radius,
where
which can be obtained by
means of the law of sines for the triangle
which leads to the following relation by means of (A. 1):
Next,
we obtain the following relation for
where
which leads to a following relation:
where
as shown in Fig. 5.58. Finally,
we obtain the relation for
which leads to:
where
and r
The
dislocated center position of the bottle,
The angle
The angle r
The distance
The distance
Likewise, the distance
The angle
Then by means of these
values, the distance
The angle
Angle
The center dislocation
In
this appendix, the inner radius R
where the angle
Angle
The distance
Similarly, the distance
The angle
The distance
The distance
Using these values, the
angle
Then the distance
from which, the angle
The small angle
from which the angle
The distance
The distance
The distance
Further, the distance
The angle
where,
We then calculate some
values required for the calculation of R The
distance
The distance
Further, the distance
Using the above obtained
values, the inner radius R
Thin
films, for an example a lap film, and thin coatings, for examples an insulating
vanish and a ceramic coating, have recently seen increasing use in many
industries such as semiconductor and IC industries, the packaging industry, and
the automobile industry. In these cases, accurate and real-time measurement of
the thickness of coatings and films is very important for determining
durability and cost performance. Many methods using light have been developed
and realized In
addition to these methods, we have proposed an optical method using laser
interference at many incident angles for direct measuring of the thickness of a
thin film or a coating. In
this section, a practical system for measuring film thickness essentially in
real time has been devised which involves a laminar-like laser light and a CCD
camera. The system's performance is also discussed.
The
basic principle of this method depends on a multi-wave laser interference of
both light reflected on the upper and lower surfaces of the coating, as was
shown in the previous paper
This eliminates the need
of a mechanism for varying the incident angles as required for the previous
method.
The maximum variable
range of the incident angle, (
where The
reflected lights on both the upper and lower surfaces of the film interfere
with each other and result in a sign-like intensity distribution R on the CCD
array sensor as is shown in a cut in Fig. 5.64, which is a basis of this
method. The thickness of the film, h, can be expressed as follows:
where
The
above description was a basic principle of this method, which uses a sheet
laser light. The largest problem involved in this method was a large fluctuation
in the light intensity distribution on the photo-receiver array, which makes an
accurate determination of The
method was further improved so as to measure the film thickness essentially in
real time by means of a laminar-like laser light. Figure 5.65 expresses the
optics for this method, which uses spatial smoothing. The laminar-like laser
light with a width W is focused onto a line on the film, and is reflected onto
the cylindrical lens SL
The
method in this study requires only two or three successive incident angles Figure
5.66 shows an example of the interference pattern on the CCD camera, i,e., the
light intensity distribution, to obtain spatial smoothing.
each horizontal line is
arisen from each corresponding position on a line O'OO'' on the film, and all
the data on a line O'OO'' are recorded on the CCD camera. Each light intensity
distribution can, basically, be used for the determination of However,
the angle
A practical system was constructed. Figure 5.68 shows the appearance of this system, and Table 5.8 its main performance.
The measurement frequency of a film thickness was determined mainly by the sample number for smoothing, N, and the speed of a 16 bit A/D converter in the processing circuit. The frequency was about 50 Hz, which is almost enough for practical use. The
measurable minimum thickness of the film is 2.4 A
variable range of incident angle,
Film
thickness can be calculated by means of Eq. (5.54) or (5.55). Thus, two or
three consecutive
This determines the
measurable minimum thickness h
where On
the other hand, the measurable maximum thickness is determined by the spatial
resolution of an incident angle
where d is the spatial
resolution of the CCD camera in the horizontal direction, and f
In conclusions, the following results were obtained.
(1) The system consists of two main sub-systems; one is an optical system composed of a laser having a wavelength of 405 nm, two cylindrical lenses, and a CCD camera, and another is a data processing system. (2) The main performance
of this system is as follows, for a variable angle of incident light of 33
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