Apparatus and method for inspecting glass

The present invention relates to an inspection apparatus for detecting the surface distortion in a sheet of material such as a sheet of glass, for example, and for indicating whether the level of distortion detected in the inspected sheet is unacceptable. The inspection apparatus includes a light source mounted to direct a first beam of light toward one surface of the sheet at an oblique angle of incidence to cause a second beam of light to be reflected therefrom. A light detector is mounted to receive the reflected beam and is responsive to a light pattern produced by the reflected beam of light for generating an output signal representing the width of the light pattern. The width of the light pattern is a function of the surface distortion of the portion of the surface from which the beam is reflected. A control circuit is responsive to the detector output signals for generating an actual distortion signal representing the amount of surface distortion in the inspected portion of the surface. In the preferred embodiment of the invention, a plurality of individual actual distortion signals are generated and are utilized by the control circuit for generating an overall distortion signal representing the overall distortion in the inspected sheet. The overall distortion signal is compared with a reference distortion signal which defines the acceptable level of surface distortion. Based on the comparison, an indicator is provided for indicating whether the inspected sheet is unacceptable.

BACKGROUND OF THE INVENTION 
The present invention relates generally to an inspection apparatus for 
detecting surface distortion in a sheet of material, such as a glass 
plate, for example. More particularly, the present invention concerns an 
apparatus for determining the amount of surface distortion in a glass 
plate as the glass is carried past the inspection apparatus. 
In the known methods of making and shaping glass, defects may inadvertently 
be produced in the glass which render the glass optically imperfect. 
Defects may also be produced in the glass during subsequent manufacturing 
operations such as during a tempering operation, for example. Among the 
optical imperfections that may be produced is surface distortion. Surface 
distortion, as the term is used herein, generally refers to variations in 
surface flatness, i.e. concave and convex portions. 
Surface distortion in glass causes the glass surface to reflect a distorted 
image. For example, convex portions shrink the image and concave portions 
magnify the image. When excessive distortion is present, the distorted 
images detract from the architectural beauty and are therefore not 
desirable. 
In addition to other methods, one approach which has been proposed for 
detecting surface distortion of a piece of glass is disclosed in U.S. Pat. 
No. 3,857,637 to Obenreder. The Obenreder patent discloses an inspection 
apparatus which utilizes a light source and a position sensing 
photodetector for detecting concave and convex portions on the inspected 
surface and the amplitude of such portions. The light source, such as a 
continuous laser, directs a beam of light on the upper surface of a glass 
plate traveling at a constant speed along a predetermined path relative to 
the light source. The position sensing photodetector is mounted to detect 
the portion of the light beam reflected by the upper inspected surface of 
the glass plate. 
If the inspected surface is flat, the reflected portion of the beam will be 
received by the photodetector along a predetermined reference line. When 
the light beam is reflected from concave or convex portions in the 
surface, the reflected beam will be displaced from this reference line. 
The inspection apparatus includes means responsive to the detector output 
signals to produce a surface flatness profile showing the nature of the 
surface curvature, i.e. concave or convex, and the amplitude of the 
curvature. While such an apparatus is capable of determining the surface 
flatness of a sheet of glass, there is no means provided for analyzing the 
data for determining whether the distortion level of an inspected sheet of 
glass is unacceptable. Also, such an apparatus is subject to errors from 
changes in glass position during the measurement process. 
SUMMARY OF THE INVENTION 
The present invention concerns an inspection apparatus for detecting 
surface distortion such as roll corrugation in a sheet of material such as 
a glass plate, for example, and for indicating whether the level of 
distortion in the inspected sheet is greater than a predetermined amount. 
The apparatus includes a light source mounted to direct a first beam of 
light toward one surface of the sheet at an oblique angle of incidence to 
cause a second beam of light to be reflected therefrom. A light detector 
means is mounted to receive the reflected beam and is responsive to a 
light pattern on the detector means produced from the reflected beam of 
light for generating an output signal representing the width of the light 
pattern. The width of the light pattern is a function of the surface 
distortion of the portion of the surface from which the beam is reflected. 
The light pattern received by the detector means will have a predetermined 
reference width when the second beam is reflected from a substantially 
flat portion of the inspected surface. The difference in width of the 
light pattern sensed by the detector means relative to the width of the 
light pattern produced by flat glass represents a variation in the surface 
flatness of the inspected sheet of glass. For example, in the preferred 
embodiment of the invention, increases in width of the light pattern 
represent a detection of a concave portion on the surface, while decreases 
in width of the image represent an indication of a convex portion. A 
control means is responsive to the detector output signal for generating 
an actual distortion signal representing the amount of surface distortion 
in the inspected portion of the surface. 
Since the inspection apparatus detects distortion by measuring the width of 
a light pattern formed by the diode array, rather than by detecting the 
exact position at which the reflected light beam falls on the diode array, 
as disclosed in the above-discussed U.S. Pat. No. 3,857,637 to Obenreder, 
the inspection apparatus is less sensitive to variations in the position 
of the inspected glass plate as compared with the prior art systems. 
In the preferred embodiment of the invention, a plurality of actual 
distortion signals are generated, each representing the distortion in a 
separate one of a plurality of inspected portions of the glass. The 
control means is responsive to the plurality of actual distortion signals 
for generating an overall distortion signal representative of the 
distortion in the entire sheet. The control means stores a predetermined 
reference distortion signal which defines an acceptable level of surface 
distortion and is responsive to the overall distortion signal and the 
reference distortion signal for indicating whether the amount of overall 
distortion exceeds the predetermined level. The present invention can 
include a display means for displaying the value of the overall distortion 
signal to an operator. Also, a strip chart recorder can be provided for 
recording a flatness profile of the inspected sheet. 
In addition to providing an indication of the distortion of the inspected 
sheets, the inspection apparatus can also be utilized to provide other 
control functions. For example, in glass tempering operations, the amount 
of distortion introduced by the tempering operation is a function of the 
temperature maintained in the tempering furnace. Thus, if the inspection 
apparatus is utilized in a tempering system, the apparatus can be 
connected to control the furnace temperature in order to reduce any 
detected distortion.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a combined perspective view and block 
diagram illustrating an inspection apparatus 10 of the present invention 
for use in inspecting individual sheets of glass 12 which have been 
treated by a glass tempering system 14. It should be noted at the outset 
of this description that, while the present invention is described as an 
apparatus for inspecting glass, the apparatus can be utilized in other 
applications wherein it is desired to inspect a reflective surface of an 
article for surface distortion. 
In FIG. 1, the glass tempering system 14 includes a plurality of conveyor 
rollers 18 for conveying the individual glass sheets 12 through the 
system. The rollers 18 are typically coupled to a conveyor drive means 
(not shown) capable of suitably driving the rollers 18. The conveyor 
rollers 18 support the individual glass sheets 12 as they are conveyed 
through a tempering furnace 22 and a cooling or quenching station 24. 
Generally, there are three types of surface distortion which can be caused 
by a glass tempering operation. These include (1) edge deformation wherein 
the edge portions of a tempered sheet do not remain flat with the central 
portion of the sheet, (2) roll corrugation wherein a series of alternating 
convex and concave portions are formed along the glass sheet, and (3) bow 
deformation wherein the glass sheet may be bowed either upwardly or 
downwardly. Since each of the above three types of distortion can result 
in an optically imperfect sheet of glass, the tempering operation must be 
carefully monitored to minimize these types of distortion. The inspecting 
apparatus of the present invention is specifically designed to detect the 
above described types of surface distortion. 
As an individual sheet 12 of glass exits the quenching station 24, the 
glass is inspected by the apparatus 10. The apparatus 10 includes a light 
source 26 having a lamp 28, a "milk glass" diffuser 30 to provide a beam 
of substantially uniform light intensity, and a slit 31 to shape the 
diffused light into the desired shape for effectively directing a light 
beam 32 at an oblique angle onto the glass sheet 12. Typically, the slit 
31 is generally rectangular in shape and the light source 26 is oriented 
such that the longer dimension of the slit 31 is disposed perpendicularly 
to the linear path of the glass sheet 12. 
A portion of the beam 32 is reflected upwardly by the glass sheet 12 as a 
reflected beam 34. In instances wherein the glass sheet 12 is relatively 
transparent, the beam 32 is reflected by both the upper and lower surfaces 
of the glass sheet 12. A flat mirror 36 is positioned in the path of the 
reflected beam 34 and is adapted to direct the reflected beam 34 toward a 
light detector means such as a camera assembly 38. The camera assembly 38 
includes an adjustable aperture 40 and a lens 42 for producing an image in 
a camera unit 44. The camera unit 44 comprises an array of photosensitive 
devices such as photodiodes which, as will be discussed, are utilized to 
determine the width of the light pattern produced on the diode array by 
the reflected beam 34. 
The dimensions of the slit 31 of the light source 26, and the relative 
position between the camera assembly 38 and the glass sheet 12 are 
selected such that, when the portion of the surface of the glass sheet 12 
which reflects the beam 32 is flat and contains no distortion, the image 
produced in camera unit 44 will have predetermined reference dimensions. 
It has been found that a slit 31 of 1 inch across and 53/4 inches in 
length used with a 500 watt quartz iodine lamp provides a suitable light 
source, while a conventional Reticon camera provides a suitable camera 
assembly. 
The manner in which the camera assembly 38 is utilized to detect variations 
in the surface flatness of the glass sheet 12 will now be discussed. 
Basically, the system functions to measure the amount of surface 
distortion in a sheet of material by directing a light beam onto the 
surface to be inspected, and subsequently determining the width of a light 
pattern formed on the camera diode array by the reflected portion of the 
light beam. The width of the light pattern is a function of the amount of 
surface distortion in the inspected area of glass. When the glass is 
relatively flat and contains no distortion, the light pattern on the diode 
array of the camera unit 44 will have a predetermined width. However, as 
surface distortion is detected, the width of the light pattern as sensed 
by the camera diode array will vary. These variations in width of the 
light pattern can be utilized to determine the amount of surface 
distortion in the inspected glass. 
Referring to FIG. 2, the light beam 32 is directed downwardly onto the 
surface of the glass sheet 12 and is reflected upwardly toward the flat 
mirror 36. The flat mirror 36 directs the reflected beam 34 through the 
camera aperture 40, and the camera lens 42 causes an image to be focused 
in a plane P offset from a camera diode array 44a by a distance S. As 
surface distortion is detected, the distance the image plane is spaced 
from the diode array 44a varies to cause the width of the light pattern as 
seen by the diode array 44a to vary. As will be discussed, such variations 
in width of the light pattern are a function of the surface distortion in 
the inspected glass. 
Referring to FIGS. 3a through 3c, there is shown the manner in which the 
spacing between the diode array 44a and the plane in which the image is 
focused varies as surface distortion is detected in the glass sheet, thus 
causing the width of the light pattern as seen by the camera diode array 
44a to vary. In FIG. 3a, which illustrates the position of an image formed 
by a beam reflected from a relatively flat portion of the glass sheet, the 
image is focused in a plane P1 which is spaced a distance S1 from the 
diode array 44a. As shown in FIG. 3b, when the beam is reflected from a 
concave portion of the glass sheet, the image is focused in a plane P2 
spaced a distance S2 from the diode array 44a which is greater than the 
distance S1 of FIG. 3a. As shown in FIG. 3c, when the beam is reflected 
from a convex portion of the glass sheet, the image is focused in a plane 
P3 spaced a distance S3 from the diode array 44a which is less than the 
distance S1 of FIG. 3a. 
The camera unit 44 functions to generate electrical waveform signals, 
similar to waveforms W1 through W3 located along the left hand portions of 
FIGS. 3a through 3c, which represent the intensity and width of the light 
pattern formed on the diode array 44a by the reflected beam. The magnitude 
of each portion of the waveforms is a function of the intensity of the 
light pattern incident on the respective portion of the diode array. In 
FIGS. 2 and 3a through 3c, the shaded portion represents the portion of 
the beam wherein a plane parallel to the image plane will have an area of 
relatively uniform light intensity. In FIGS. 3a through 3c, since the 
camera diode array 44a is not located along the plane in which the image 
is focused, the light pattern formed on the diode array 44a is out of 
focus and is blurred around the periphery of the pattern. The inclined 
edge portions of the waveforms W1 through W3 represent the blurred 
portions of respective light pattern. 
In order to determine a diode count representative of the width of the 
light pattern seen by the diode array 44a, those diodes which generate a 
signal above a predetermined threshold voltage level Vt can be counted. In 
FIG. 3a, the waveform W1 consists of a portion at a uniform voltage level 
V1 having a width M1 and inclined portions which decrease to zero volts at 
a width D1. The number of diodes which generate a signal above the 
threshold voltage Vt is represented by width N1. Thus, N1 represents the 
diode count obtained when the beam is reflected from a relatively flat 
portion of the glass surface. In FIG. 3b, the waveform W2 consists of a 
portion at a uniform voltage level V2 having a width M2 and inclined 
portions which decrease to zero volts at a width D2. In this case, the 
diode count obtained is represented by width N2. In FIG. 3c, the waveform 
W3 includes a portion of width M3 having a uniform voltage level V3 and 
inclined portions which decrease to zero volts at a width D3. The diode 
count in FIG. 3c is represented by width N3. 
The distortion value for an inspected portion of glass can be calculated by 
determining the difference between the diode count obtained for the 
inspected portion and the diode count representing a flat portion of the 
sheet of glass. As the image moves away from the diode array, as is the 
case in FIG. 3b for concave surface distortion, the width of the light 
pattern on the diode array 44a becomes wider, causing more diodes to be 
illuminated and increasing the diode count as compared to the flat glass 
diode count. The distortion value associated with the light pattern 
produced in FIG. 3b is calculated by subtracting the flat glass diode 
count N1 of FIG. 3a from the diode count N2 of FIG. 3b. As the image moves 
toward the diode array, as shown in FIG. 3c for convex surface distortion, 
the width of the light pattern on the diode array 44a is decreased, thus 
decreasing the diode count as compared to the flat glass diode count. The 
distortion value associated with the light pattern produced in FIG. 3c is 
determined by subtracting the flat glass diode count N1 from the diode 
count N3 of FIG. 3c. Thus, in the embodiment shown in the drawings, a 
positive distortion value indicates a detection of a concave surface, 
while a negative distortion value indicates a detection of a convex 
surface. 
Referring again to FIG. 1, the reflected beam 34 is sensed by the camera 
unit 44 which, in turn, generates an output signal which is a function of 
the width of the light pattern produced in the camera unit 44. The output 
signal generated by the camera unit 44 is supplied to a microcomputer 46. 
The microcomputer 46 periodically samples the output signal of the camera 
unit 44 and determines an individual distortion value for the inspected 
portion of the glass sheet by subtracting the flat glass diode count from 
the diode count obtained from the inspected portion. After the individual 
distortion values for an entire glass sheet have been calculated, the 
microcomputer 46 processes the data to determine an actual overall 
distortion value representative of the overall distortion in the glass 
sheet being inspected. 
While the actual overall distortion value can be determined in a number of 
ways, one method which has been used is to compute the average of a 
predetermined number of the maximum individual distortion values obtained 
for the inspected sheet. The actual overall distortion value can then be 
compared to a previously programmed reference distortion value which 
defines an acceptable level of distortion. Based on the result of the 
comparison, the microcomputer can provide the operator with an indication 
as to whether the inspected sheet is acceptable. 
The microcomputer 46 is connected to a display device 60 which can be 
located at an inspection station (not shown) to display the results of 
current and previous microcomputer distortion value computations. The 
microcomputer 46 can also be connected to a conventional strip chart 
recorder 62 which records a profile representing the flatness of the 
inspected sheet. An alarm 64 can be connected to receive an actuation 
signal from the microcomputer 46 to alert an operator in the event a 
particular sheet of glass has been determined to be unacceptable. 
Also, in order to obtain an accurate distortion value for an inspected 
portion of a sheet of glass, it is necessary that the entire light pattern 
fall on the camera diode array. The microcomputer 46 can monitor the 
output signal from the camera 44 to determine whether the entire light 
pattern falls on the diode array 44a. If the entire light pattern is not 
on the diode array, the microcomputer can generate a signal to inform the 
operator of this condition. 
In addition to providing an indication of the distortion of the inspected 
sheets, the inspection apparatus can also be utilized to provide further 
control functions. For example, in glass tempering operations, the amount 
of distortion introduced by the tempering operation is a function of the 
temperature at which the tempering furnaces are maintained. Thus, if the 
inspection apparatus is utilized in a tempering system, the apparatus can 
be connected to control the furnace temperature in order to reduce any 
detected distortion. For example, in FIG. 1, the microcomputer 46 is 
connected to a temperature control circuit 66 which generates a control 
signal to a heating means 68. In the event the inspection apparatus 
detects a predetermined number of unacceptable glass sheets, the 
temperature of the tempering furnace can be decreased to reduce the 
surface distortion of the sheets. 
It should be noted that, while the inspection apparatus has been described 
for use in detecting distortion in a relatively flat surface of a 
reflective material, it will be appreciated that the inspection apparatus 
can also be adapted to detect distortion in a curved sheet of material. In 
these instances, the apparatus is programmed to compare the width of the 
light pattern produced by the reflected beam with a predetermined width 
representing the desired curvature of the inspected portion of the sheet. 
In accordance with the provisions of the patent statutes, the principles 
and mode of operation of the present invention have been discussed in what 
is considered to represent its best embodiment. However, it should be 
understood that the invention may be practiced otherwise than as 
specifically illustrated and described without departing from its spirit 
or scope.