Abstract:
A bottle stress analysis system using polarized light for determining stress levels of a transparent plastic bottle includes a rotation assembly for rotating the bottle. A polarized light source assembly is insertible into the bottle cavity to direct plane polarized light through the bottle wall and an analyzer assembly is positioned outside the bottle for measuring intensity of light passing through the bottle wall. In order to measure changes in polarization, the analyzer assembly has a polarization analyzer positioned to transmit the light to a photodetector. A mechanism for rotating either the polarized light source or the analyzer to determine maximum and minimum polarized light intensity at the photodetector facilitates determination of stress at a plurality of points along the bottle wall. A computer controlled analysis of the polarized light intensity can be used to produce a stress profile of the bottle, permitting rejection of bottles having abnormal internal stress that are likely to fail under internal pressure loading.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to the use of polarized light to determine stress in transparent articles. More particularly, the present invention relates to an apparatus for measuring stress levels present in blown plastic bottles suitable for containing drinks or foods. 
     Analysis of stress distributions in models composed of transparent materials is commonly employed by engineers to verify calculations of stress concentration in large scale structures. Plane polarized light is directed to pass through transparent plastic models of a structure. As the plane polarized light passes through the model, the light breaks up at every point (birefringence) into two components corresponding to the directions of the two principal stresses at the point. If the light is then passed through another polarizing element (conventionally known as an analyzer), intensity variations or interference bands form that are indicative of principal stresses in the model. This method of stress determination is quite useful because it is non-destructive and can give both quantitative and qualitative information as to stress defects. 
     Stress determination in plastic bottles used as containers for food or drink is important to reduce failure rates of bottles. Many applications of plastic bottles require subjecting the bottles to high temperatures and pressures, both conditions capable of causing failure in a bottle weakened by a stress concentrating defect. Accordingly, determination of stress at an analysis point on a blown plastic bottle using polarized light and taking advantage of the birefringent properties of aligned polymers and polymeric crystals in blown plastic bottles would be advantageous. Plane polarized light passing through the bottle will travel at different velocities, with light polarized parallel to the optical axis of a polymeric crystal travelling at a different velocity than light polarized perpendicular to the optical axis of the crystal. When the light recombines after passing through the crystal, the light may be circularly, elliptically, or linearly polarized. Since light typically passes through several polymeric crystals as the light travels through the bottle wall, output is commonly elliptically polarized, being a linear function of polarization in the light path through the bottle wall. 
     The degree of elliptically polarized light passing through the bottle is a function of both the effective thickness and the birefringence of the polymeric crystals in the bottle wall. Changes in polarization are therefore dependent on wall thickness variations as well as stress variations. Since many crystals having differing orientations and birefringent properties are present in a light path, the initial linear polarization is almost destroyed, with a nearly random polarization of the emergent light. However, a small residual polarization typically remains that is indicative of stress and wall thickness variations. The intensity of polarization is measurable, and multiple measurements can be taken at a plurality of analysis points on the bottle wall to ensure consistency of the bottle wall and reduce chance of bottle wall failure. 
     Accordingly, the present invention provides a bottle stress analysis system using polarized light for determining stress levels of a transparent plastic bottle having a bottle wall that defines a bottle cavity. The bottle stress analysis system includes a rotation assembly for rotating the bottle and a polarized light source assembly insertible into the bottle cavity to direct plane polarized light through the bottle wall. An analyzer assembly is positioned outside the bottle for measuring intensity of light passing through the bottle wall, the analyzer assembly having a polarization analyzer positioned to transmit the light to a photodetector. In addition, a mechanism for rotating one of the polarized light source and the analyzer to determine maximum and minimum polarized light intensity at the photodetector, and consequently measure stress levels at the analysis point, is provided. It will be appreciated that the analysis point may be located at any area of the bottle including the base or finish and is not limited to merely the sidewall of the bottle. 
     In preferred embodiments the rotation assembly of the bottle stress analysis system includes a rotatable platter configured to support the bottle. A platter stepper motor is connected to the platter to rotate the platter a predetermined angular distance and briefly maintain a stationary position until maximum and minimum polarized light intensity is determined. A computer control assembly for automatically controlling operation of the platter stepper motor is also provided, with the computer control assembly controlling the angular distance through which the platter is rotated, and the duration the stationary position is maintained before further rotation. Motors other than a stepper motor could be used with corresponding control means to achieve substantially the same result. 
     To effectively provide a source of directed, linearly polarized light, the bottle stress analysis system includes polarized light source assembly. The polarized light source assembly includes a periscope insertible through a neck of the bottle into the bottle cavity, the periscope holding a light source and a polarizer that transmits plane polarized light, with the polarizer being positioned to transmit plane polarized light at a 45° angle with respect to an axis of symmetry of the bottle. The light source can optionally be a light emitting diode (LED) connected to a light source intensity control to vary intensity of light emitted by the LED. A lens (or multiple lens system) for focusing light emitted by the light emitting diode, and a mirror for reflecting converging, plane polarized light toward a stress analysis point on the bottle coincident with the focal point are also positioned in the periscope. To further increase control of the system, the light source intensity control is connected to a computer control assembly for automatically controlling operation of the light source intensity control. 
     After light has passed from the periscope and through the bottle wall, the bottle stress analysis system utilizes an analyzer assembly for linearly polarizing the light. In addition to a polarizing element (the analyzer) the analyzer assembly includes a lens for focusing polarized light passing through the polarization analyzer onto the photodetector. The photodetector is configured to have a varying electric current in response to variations in light intensity, and further includes an electrical converter for converting the electrical current to an electrical voltage, and an amplifier for amplifying that voltage. A computer control assembly having an analog to digital converter converts this elecrical voltage output of the amplifier into computer readable digital format. 
     The rotating means for rotating the polarization analyzer includes an analyzer rotation assembly for rotating the polarization analyzer relative to a fixed plane of polarization of light emitted by the polarized light source assembly. An analyzer stepper motor is connected to the polarization analyzer to rotate the polarization analyzer a predetermined angular distance and briefly maintain a stationary position until maximum and minimum polarized light intensity is determined. Again, to increase ease of operation and speed of bottle stress analysis, a computer control assembly for automatically controlling operation of the analyzer stepper motor is provided. The computer control assembly controls the angular distance through which the polarization analyzer is rotated, as well as the duration the stationary position is maintained before further rotation. 
     The various features and advantages of the invention will become more apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a bottle stress analysis system in accordance with the present invention, the bottle stress analysis system including a rotatable platter supporting a bottle through which polarized light is directed and its intensity analyzed; 
     FIG. 2 is a representative graphic output presented by a computer display monitor to indicate stress levels at a predetermined height around a bottle, with accept/reject limits being indicated as horizontal lines; and 
     FIG. 3 is a representative graphic output presented by a computer display monitor to indicate low, high, and marginal stress regions for samples of bottles taken from several experimental lots. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A bottle stress analysis system 10 as illustrated in FIG. 1 is useful for measuring relative stress level as a function of light polarization changes in light passing through transparent containers such as bottle 12. Bottle 12 is a conventional plastic bottle commonly used to hold, for example, carbonated beverages, purified or mineral waters, or fruit juices. As is readily apparent from the illustration, the bottle 12 has a bottle wall 14 surrounding a bottle cavity 16. Access to the bottle cavity is through a bottle neck 18 that can be easily closed off by a removable plastic screw cap or other sealing mechanism. The bottle 12 is blow mold formed by rapid pneumatic expansion of heated plastic parisons that expand to contact walls of a mold cavity. This method of formation imparts a stress induced crystal alignment to the microscopic polymeric crystals (and long chain polymeric molecules) in the bottle wall. 
     Aligned polymers and polymeric crystals exhibit a high degree of birefringence. This birefringence causes polarized light to become circularly or elliptically polarized upon passing through the polymeric crystal since light polarized parallel to and perpendicular to the optical axis of a biaxial crystal travels at different velocities. Depending on the thickness of the material, the transmitted light can be circularly, elliptically or linearly polarized. If there is more than one polymeric crystal present in the beam path, the output is the sum of the various degrees of elliptical polarization. The degree of polarization is a function of the effective thickness and birefringence of the crystal. polarization changes measured by the bottle stress analysis system are caused by a combination of the wall thickness variations as well as the stress variations. However, since variations of thickness among the polymeric crystals give many possible degrees of elliptical polarization, and since the number of polymeric crystals in any section through the bottle wall 14 is quite large, total light output not attributable to stress variations in the bottle wall will be approximately randomly polarized. 
     When a polarized light source assembly 30 is positioned inside the bottle cavity 16 to direct polarized light 48 outward through the bottle wall 14, the light 48 can be directed at 45° to the axis of symmetry of the bottle. Due to the symmetrical flow during molding the light is polarized at 45° to the major optical axis. The average polarization of the light after passing through the bottle is near zero, with the plastic in the bottle has destroyed virtually all of the initial polarization of the light beam. If the bottle were perfectly homogeneous the final polarization angle should be small and constant. The polarization angle change for linearly polarized light is measured by passing the light that is transmitted through the bottle through an analyzer assembly 50 that includes a polarizing analyzer 52. This polarizing analyzer 52 is rotated to determine the maximum and minimum positions of light intensity, and those intensities are quantified. The difference between the maximum and minimum intensities divided by their sum is the calculated polarization. The polarization measured through the bottle divided by the polarization with no bottle present (the initial polarization of the source and source polarizer) is the relative polarization change R. The relative polarization change R is plotted and analyzed for the measured bottle. When measured around the circumference of a bottle, R is not constant, as would be expected for a homogeneous bottle with no substantial stress induced polarization. There are various levels of apparently random fluctuations. This occurs because the bottles are not uniform. Each bottle contains thickness and stress variations caused by the molding process which are the underlying cause of the variations in R. 
     Annealing of the bottle 12 reduces its residual stress variations, and therefore reduces the fluctuations in R. Stress differences between small areas of the bottle wall 14 (the high spatial frequency differences) are the first to be relieved during annealing. Because a priori calculations of acceptable stress levels based on polarization curves is not possible, the bottle stress analysis system determines a correlation between stress and to observed polarization values. These correlations are established by measuring several bottles at various states during the annealing process. These correlation values are not invariable and should be calibrated for each mold and new batch of resin. However, as a general rule the lower the fluctuations in R the better the annealing of the bottle. 
     As illustrated in FIG. 1, directing polarized light through the bottle wall 14 is simple and convenient with use of the polarized light source assembly 30. The polarized light source assembly includes a light source 32 conveniently constructed from a red light emitting diode (LED) producing substantially monochromatic light at 660 nanometers. Alternatively, light emitting devices producing monochromatic light between about 450 and 1200 nanometers are acceptable, as are full or continuous spectrum light sources with monochromatic filters. The light source 32 is controlled by a light source intensity control 34 positioned outside the bottle cavity 16. Non-polarized light 36 is emitted by the light source 32, focused by relay lenses 38, and passed through a linear polarizer 40 having an optical axis at 45° with respect to the axis of the bottle 12. The angle 45° is chosen because it tends to reduce the effects of other system errors such as wobble in the mechanical system and misalignment. Other angles from 0° to 360° have been tried and given usefull results. Low cost, general purpose linear photographic polarizers that are useful through the visible light and near infrared spectral regions can be used. Alternatively, nicol prisms or other conventional optical polarizers may be used. The linearly polarized light 48 is reflected off a mirror 42 and directed to a focal point at about the position of the bottle wall 14. After passing through this predetermined analysis point 80 on the bottle wall 14, the light is received by the analyzer assembly 50. 
     As illustrated in FIG. 1, the light source 32, relay lenses 38, polarizer 40, and mirror 42 are permanently mounted in a periscope 46. The periscope 46 is typically formed from metal or plastic, and has a generally cylindrical construction with a radius sufficiently small to allow its easy insertion through the bottle neck 18 and into the bottle cavity 16. The periscope 46 is formed to define a periscope aperture 44 through which polarized light 48 reflected from mirror 42 can pass toward the bottle wall 14. The periscope 46 is mounted for vertically directed reciprocal motion with respect to a base (not shown), allowing the periscope 46 to be raised and lowered. When the periscope 46 is raised, a bottle 12 to be analyzed for stress can be prepositioned below the periscope 46. Lowering the periscope 46 allows initialization measurements (no bottle present) or polarization measurements (with bottle wall 14 present between the polarized light source assembly 30 and analyzer assembly 50). Raising or lowering of the periscope 46 can be accomplished manually or automatically. 
     Light passing through the analysis point 80 on the bottle wall 80 is received by the analyzer assembly 50. The analyzer assembly 50 includes a polarizing analyzer 52, a convergently focusing detector lens 54, a photodetector 56 and an amplifier 58. The polarizing analyzer 52 can be of identical construction to the polarizer 40, and like polarizer 40 can be formed from standard, general purpose photographic polarizers. The light passing through this second polarizing analyzer is focused by detector lens 54 onto the photodetector 56. The photodetector is conveniently a large area (typically 0.250 inches in diameter) silicon photodetector having a response curve that covers the emission spectrum of the polarized light and has peak response at approximately 850 nanometers. Typically, the response of such conventional photodetectors is linear with respect to intensity within a few percent and is uniform across its surface. This uniformity reduces the current output changes that are due to position changes of the light image. Easily visible surface irregularities, as well as other variations in the bottle wall, should be avoided when determining an analysis point 80 because such irregularities can cause the image to move on the photodetector, resulting in apparent light intensity changes. The photodetector converts the light to an electrical current, which is then converted to a voltage signal and amplified by the amplifier 58. The Amplifier 58 can also include electronic filters for reducing electronic noise and signal drift. 
     To determine the maximum and minimum polarized light intensities (which give the amount of rotation of the plane of polarization), the analyzer 52 of the analyzer assembly 50 is rotated with respect to the polarizer 40. An analyzer rotation assembly 50 includes an analyzer stepper motor 62, an analyzer drive link 64, and motor control 66. The stepper motor 62 allows the analyzer to be slightly rotated, held still momentarily for photodetector measurements to be taken, and rotated to a new position for further measurements. By controlling the amount of rotation, the precision of maxima or minima determination can be easily enhanced or decreased. 
     To change the position of the analysis point 80 the periscope 46 can be raised or lowered to different vertical positions with respect to the bottle wall 14. Alternatively, a series of analysis points can be obtained by rotating the bottle 12 while holding the periscope level constant. As shown in FIG. 1, this is accomplished by positioning the bottle 12 on a rotation assembly 20. The rotation assembly 20 includes a platter 22, a platter stepper motor 24, a motor control 26, and a platter drive link 28. Like the analyzer 52, the platter 22 and the bottle 12 resting upon it can be slightly rotated, held still momentarily for photodetector measurements to be taken, and rotated to a new position for further measurements. By controlling the amount of rotation, the precision of stress determination along a radius of the bottle 12 can be easily enhanced or decreased. 
     Operation of the bottle stress analysis system 10, reduction of photodetector intensity data, and presentation of stress analysis data for a bottle 12 can be augmented by use of a computer control assembly 70. The computer control assembly 70 includes a conventional electronic computer 72 linked to external signals and machinery by parallel port 74, analog to digital convertor 76, and digital to analog convertor 78. The analog to digital convertor 76 is connected to the amplifier 58 to receive output from photodetector 56. The amplified voltage signal produced by amplifier 58 is converted to a digital signal and passed to the control computer by an analog-to-digital converter board 76. The light intensity data is then processed by the control computer 72. In addition the control computer 72 can control the intensity of the LED output using a digital-to-analog converter to drive the voltage controlled light source intensity control 34. The intensity of the light source 32 is controlled by varying the current. The control computer 72 can also control the rotation of the bottle and the analyzer polarizer by using the stepping motors 26 and 66 coupled to it through the parallel data output port 74. 
     Analysis of light intensity maxima and minima and display of calculated stress levels can be done with the following exemplary computer program suitable for controlling operation of many conventional personal, mini, and mainframe computer systems. Generally the computer program is subdivided into control and analysis subprograms with the computer controlling initialization and operation of system 10, and analyzing data to present a human readable output showing bottle stress levels. The following program, written in the C language, can be used to control and analyze data produced by the bottle stress analysis system. ##SPC1## 
     To better understand operation of the system in a typical application, a step by step discussion of system operation and data reduction is presented as follows 
     A. The analyzer 52 is rotated so that maximum light intensity is achieved on the photodetector 55, with the analyzer 52 being aligned with the plane of light polarization produced by the polarized light source assembly 30. Typically, the rotational step resolution of the analyzer rotation assembly 60 for rotating analyzer 52 is about 0.75°. The exact rotational step size is not critical but it should be fine enough so that the polarizer can be rotated fairly close to the true maximum. 
     B. The light source 32 intensity is varied until the measured plane polarized light output is at a set value approximately equal to 75% of the full scale output. This is done so that consecutive runs will be taken at the same light intensity in order increasing consistency of stress measurements. 
     C. The analyzer 52 is rotated 360° four times and the detected light output measured as a function of angle in order to determine the two maxima and two minima. 
     D. The analyzer 52 is rotated 360° and the intensity is measured to determine the background transmission and polarization with no bottle present. 
     E. The periscope 46 is raised and the bottle 12 is placed onto the platter 22 and held firmly in position. The periscope 46 is then lowered into the bottle cavity 16 of the bottle 12 so that the plane polarized light will pass through the bottle wall and fall onto the photodetector. 
     F. The analyzer 52 is rotated to a first light intensity maxima and the light intensity is measured as a function of bottle angle as the bottle is rotated 360° by rotating the bottle platter. The bottle is rotated in 0.2° steps. After rotating one step the bottle is stopped and the light intensity is measured. The bottle is then rotated the next step. The speed of rotation is approximately 10 milliseconds per step. The step size and rotation speed are set such that the detector electronics will have time to settle and so that the angular resolution will be fine enough to measure the variations in the bottle. The exact speed and resolution can be chosen to to give a good signal on the particular bottle of interest. 
     G. The analyzer 52 is rotated to the first light intensity minima and the intensity as a function of bottle angle is again measured. 
     H. The analyzer 52 is then rotated to the second light intensity maxima and second minima and the intensity angular function si measured as before for each. 
     I. The bottle is removed and the background is measured again. 
     J. The polarization intensity is calculated for each point measured around the bottle. For each point, i, the polarization intensity, P(i)=[Imax(i)-Imin(i)]/[Imax(i)+Imin(i)], where: Imax(i) is the average of the first and second maxima and Imin(i) is the average of the first and second minima at point i. 
     K. Next the change in polarization is calculated for each point around the bottle, Rp(i)=P(i)/P(n), where P(n) is the polarization with no bottle present. With the current system changes in polarization can be measured to better than one part in 10,000 or better than 0.01%. This data is saved in a data file along with other information such as time, data, comments, background level, average level etc., 
     L. The slope of the relation Rp(i) as a function of angle is examined and areas of relatively large changes of slope are categorized as point defects. These regions are eliminated from the data by replacing them with a straight line going from the point proceeding the region in question to the first point after that region to give Rpm(i). 
     M. The low frequency portion of the relation Rp(i) is calculated be applying a digital lowpass filter to the data with the point defects removed, Rrm(i). The cutoff of this filter is currently 200 points. This is equivalent to a spatial wave length of 40°. Rlf(i) will be used to reference this low frequency portion of the relation. 
     N. The spatial spectral energy of Rrm(i) around the bottle is calculated by determining the root-mean-square of the data with the point defects removed. The high frequency portion of the relation is found by subtraction the low frequency portion, Rlf(i), from the whole, Rrm(i). The energy for this relation, Rhf(i), containing spatial frequencies less than 40° in extent (high frequency stress) is determined by using root-mean-square relation. 
     M. The various forms of Rp(i) are plotted on the computer monitor for realtime examination by an operator. These include the total Rp(i), Rp(i) with point defects moved Rrm(i), the low frequency component (Rlf) of Rrm(i) and the high frequency component (Rhf) of Rrm(i). FIG. 2 illustrates this display on a computer. Although not shown, the display can be color coded to enhance viewability. 
     O. The high frequency energy and total stress energy are plotted on a graph such as illustrated in FIG. 3 of total stress versus high frequency stress and compared to standard ranges to indicate the proper annealing. In general the smaller the high frequency stresses the better the annealing. However this is dependent on the resin and the molding process used. There is also a relationship between the total and high frequency stresses. FIG. 3 shows this relationship, with selected experimental bottles stress levels plotted and the regions used to determine the proper degree of annealing. 
     Although the invention has been described in detail with reference to certain preferred embodiments and specific examples, variations and modifications exist within the scope and spirit of the invention as defined in the following claims.