JP4450127B2 - Internal pressure inspection method for sealed containers - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an internal pressure inspection method for a sealed container, and more particularly to a percussion frequency determination processing method at the time of percussion for internal pressure inspection of a two-piece can.
[0002]
[Prior art]
Conventionally, the percussion method has been widely adopted as a method for nondestructively inspecting the internal pressure of sealed containers, particularly canned milk-containing beverages that are susceptible to spoilage. The percussion method is an inspection method that converts the percussion sound generated when electromagnetic shock is applied to the can lid (can bottom of a two-piece can) into an electrical signal using a microphone, thereby judging whether the internal pressure of the can is good or not. is there. The percussion sound is generally composed of a specific frequency component, and has a characteristic that it is high when the internal pressure of the can (in the case of a negative pressure can) is high and low when the internal pressure is low. Therefore, the percussion instrument digitizes the captured percussion signal and performs frequency analysis using a fast Fourier transform technique. As a result of frequency analysis, the frequency with the highest intensity is handled as the tapping frequency. In other words, the internal pressure of the can can be determined at the inspection frequency from the relationship between the internal pressure of the can and the frequency. Is possible.
[0003]
The punching waveform of the 3-piece can consists of a single sine wave as shown in FIG. Therefore, the frequency analysis result is also a spectrum having a single peak as shown in FIG. Therefore, the peak may be determined as the tapping frequency. In the case of a three-piece can, the punching frequency and the internal pressure change substantially continuously as shown in FIG. 9, so that the inside of the can can be known from the punching frequency and the quality of the internal pressure can be determined. However, in the case of a two-piece can, the percussion waveform is more complicated than that of a three-piece can. FIG. 10 shows a punching waveform at a specific internal pressure in a two-piece can, and has a shape in which two vibration waveforms are synthesized. Looking at the frequency analysis result at this time, a spectrum having a plurality of peaks is observed as shown in FIG. In this example, since the frequency indicating the maximum peak is 2382 Hz, this is processed as the percussion check frequency, but the second peak may be higher depending on the state when the percussion is performed. That is, despite the same internal pressure of the can, two significantly different taping frequencies are generated. This second peak is not always observed and often occurs at a specific internal pressure.
[0004]
In order to investigate the relationship between the can internal pressure and the punching frequency in the two-piece can, the present inventor performed various can types and created a graph of the can internal pressure and the punching frequency. A similar experiment was performed on a three-piece can. In the case of the three-piece can, as shown in FIG. 9, the relationship between the internal pressure and the punching frequency was smooth, and no discontinuous points were found. However, in the case of a two-piece can, as shown in FIG. 12, the relationship between the inspection frequency and the internal pressure may be discontinuous near a specific internal pressure. In the illustrated example, the phenomenon is observed around 0.02 MPa.
[0005]
When discriminating between non-defective products and defective products with a percussion instrument, the upper and lower limits of the percussion frequency are specified. For example, when 0.02 MPa is the minimum internal pressure, the inspection frequency of 2000 Hz is set as the lower limit value. At this time, since the products in the range surrounded by the lower right of the frame in FIG. 12 are 0.2 MPa or more, the inspection frequency is less than 2000 Hz. That is, it is judged as a defective product while being in the non-defective range, and is rejected. That is, wasteful elimination occurs. On the other hand, products in the range enclosed in the upper left of the frame in the figure are products whose internal pressure is 0.02 MPa or less but the percussion frequency is 2000 Hz or more. This is a defective range but is treated as a good product. That is, it is missed. Such problems occur more frequently as the frequency variation with respect to the internal pressure increases.
[0006]
[Problems to be solved by the invention]
As described above, the percussion method is very effective as a non-destructive method for inspecting the can internal pressure, but in the case of a two-piece can, the relationship with the internal pressure of the can becomes discontinuous near a specific internal pressure. There is a case. For this reason, waste rejection or oversight may occur, and the conventional percussion inspection method is mainly used for the internal pressure inspection of a three-piece can. In the case of a two-piece can, the case of a can body having a relatively high internal pressure. Only used.
[0007]
Therefore, the present invention provides a sealed container that can be accurately applied to a conventional inspection device without causing wasteful rejection of defective products or overlooking of defective products even in a low-pressure two-piece can. The purpose is to provide an internal pressure inspection method.
[0008]
[Means for Solving the Problems]
In the course of research for solving the above problems, the present inventor examined the frequency analysis result when the can internal pressure was changed from 0.018 to 0.026 MPa for the two-piece can. As a result, the relationship between the frequency and intensity when the internal pressure of the can was 0.018 MPa, 0.022 MPa, 0.026 MPa was as shown in FIGS. From these figures, even if the internal pressure changes from 0.018 MPa to 0.022 MPa, the frequency of the first peak does not change much. However, at 0.22 MPa, it can be seen that a second peak occurs in the vicinity of 2.1 kHz. In other words, a can with an internal pressure of 0.22 MPa has two taping frequencies near 1.8 kHz and 2.1 kHz. Therefore, if the frequency of the first peak is directly used as the inspection frequency, the change in the internal pressure cannot be read. This is a major cause of variations in the inspection frequency. Further, as shown in FIG. 6, in the canned product having an internal pressure of 0.026 MPa, the first peak occurs in the vicinity of 2.2 kHz, and the second peak occurs in the vicinity of 1.9 kHz. That is, the order of the intensity of the first peak and the second peak is not always constant, and may be reversed under slight conditions.
[0009]
As a result of further investigations focusing on the above facts, the present invention eliminates discontinuities by correcting the frequency of the first peak and the second frequency in the frequency analysis result based on a certain mathematical formula. The present inventors have found that wasteful evacuation and oversight can be suppressed to a minimum, and have reached the present invention.
[0010]
That is, the method for inspecting the internal pressure of the sealed container according to the present invention is a method for inspecting the internal pressure of the sealed container by percussion. The sound generated by applying an electromagnetic shock to the elastic wall of the sealed container is detected with a microphone. The percussion waveform obtained by converting to an electrical signal is frequency converted to obtain a percussion sound spectrum, the first peak and the second peak of the obtained spectrum are extracted, and the intensity of the second peak and the first peak. When the ratio is within a predetermined range, the frequency of the first peak is determined as a tapping frequency, and when the ratio is outside the predetermined range, the correction frequency obtained from the first peak and the second peak is determined as a tapping frequency. It is what.
[0011]
The correction frequency fp is calculated using the following equation.
fp = (f ₁ × M ₁ + f ₂ × M ₂ ) / (M ₁ + M ₂ )
Here, f ₁ : frequency of the first peak f ₂ : frequency of the second peak M ₁ : intensity of the first peak M ₂ : intensity of the second peak.
When the following values the second peak value of the ratio M ₂ / M ₁ of the intensity M ₂ and the intensity M ₁ of the first peak of arbitrarily selected in the range of 0.3 to 0.4 is The frequency of the first peak is set as a check frequency, and the correction frequency fp is set as a check frequency when the frequency is equal to or higher than the value.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail. FIG. 1 shows an embodiment of an internal pressure inspection apparatus for carrying out a sealed container internal pressure inspection of the present invention.
In the internal pressure inspection apparatus shown in FIG. 1, a test head 3 having a coil 1 and a microphone 2 is installed above a path through which a can c passes, and when the can reaches under the test head, the photoelectric switch 10 can , And a pulsed current is supplied from the pulse generator 5 to the coil 1 to give an electromagnetic shock. As a result, the lid (can bottom in the case of a two-piece can) vibrates and emits sound, and the sound is converted into an electrical signal by the microphone 2 to obtain a tapping test signal. The test signal is digitized by an A / D converter 7 through an amplifier / filter 6 and the test waveform stored in the waveform memory 8 is input to the microcomputer 9 to perform frequency analysis by a fast Fourier transform technique. The result of the frequency analysis is processed by a microcomputer under the conditions described in detail below, and treated as a check frequency. The determined inspection frequency is compared with a set value set in advance, and if the inspection frequency is within the allowable range, it is determined to be acceptable. Is rejected as a can with poor internal pressure.
[0013]
As described above, in particular, in the case of a two-piece can, a plurality of intensity peaks in the percussion sound spectrum occur, and in particular, an intensity peak with a small intensity difference occurs at two frequencies in a specific internal pressure range. When a frequency with a high intensity is recognized as the inspection frequency, as described above, wasteful disposal of good cans and oversight of defective cans occur. As a measure for solving the problem, in the present embodiment, when a plurality of intensity peaks occur, as shown in FIG. 2, two of the peaks having the largest peak are selected, and the intensity ratio is 0.3 or 0.3. In the case of 4 or more, the frequency obtained by correcting with the frequency and intensity of the second peak as shown in the following equation is adopted as the frequency of the test without setting the frequency of the test as the frequency of the first peak.
[0014]
fp = (f ₁ × M ₁ + f ₂ × M ₂ ) / (M ₁ + M ₂ ) (1)
Here, f ₁ : frequency of the first peak f ₂ : frequency of the second peak M ₁ : intensity of the first peak M ₂ : intensity of the second peak.
[0015]
In the above formula, when M ₂ / M ₁ is equal to or less than a value arbitrarily selected within the range of about 0.3 to 0.4, the difference between the first peak and the second peak is large, and thus the frequency of the first peak It was confirmed that a discontinuity hardly occurred between the internal pressure and the inspection frequency even when the above was adopted as the inspection frequency as it was. Therefore, in that case, the first peak is directly adopted as the inspection frequency without performing the correction by the above formula. If the intensity ratio M ₂ / M ₁ is greater than or equal to the above selected value, the frequency fp corrected by the equation (1) is set as the tapping test frequency. Whether the corrected frequency fp or the first peak frequency f ₁ is used as the percussion frequency is determined according to the experiment of the present inventor, and is not always a constant value, depending on the type of the sealed container. Although there is a slight deviation, if it is judged as described above based on a specific value arbitrarily selected within the range of M ₂ / M ₁ in the range of 0.3 to 0.4, all can types It was confirmed that the quality of the internal pressure can be accurately determined.
[0016]
The two-piece can whose internal pressure is in the range of 0.01 MPa to 0.09 MPa is tested by the apparatus shown in FIG. 1 while incorporating the correction formula as an arithmetic expression in the microcomputer 9 and correcting the test frequency according to the above conditions. did. As a result, the results shown in FIG. 3 were obtained for the internal pressure and the inspection frequency. The figure corresponds to FIG. 12 described above. As shown in the figure, the discontinuity disappears and the frequency with respect to the internal pressure continuously changes. As a result, defective cans and good cans do not exist at a specific frequency, and wasteful elimination and oversight can be prevented.
[0017]
The preferred embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment. For example, the internal pressure inspection method of the present invention is particularly effective for a method of punching a two-piece can body. However, the present invention is not limited to a two-piece can. By adopting, it is possible to make a good test. It can also be applied to other methods for inspecting the internal pressure of sealed containers.
[0018]
【The invention's effect】
As is clear from the above description, according to the present invention, even in a two-piece can in which a plurality of intensity peaks due to percussion inspection occurs, a relationship in which the frequency with respect to the internal pressure continuously changes can be created. The internal pressure of the can can be accurately inspected by the percussion method without wasteful disposal of non-defective products and oversight of defective products. Moreover, since it can be easily applied to a conventional percussion inspection apparatus, it is economical.
[Brief description of the drawings]
FIG. 1 is a schematic view of an embodiment of a percussion inspection apparatus for carrying out an internal pressure inspection method for a sealed container according to the present invention.
FIG. 2 is a diagram illustrating the principle of the present invention.
FIG. 3 is a graph showing the relationship between the internal pressure of a two-piece can and an inspection frequency in the embodiment of the present invention.
FIG. 4 is a two-piece can sound detection spectrum with an internal pressure of 0.018 MPa.
FIG. 5 is a two-piece can sound detection spectrum with an internal pressure of 0.022 MPa.
FIG. 6 is a two-piece can tap sound spectrum with an internal pressure of 0.026 MPa.
FIG. 7 is a waveform diagram of a three-piece can.
8 is a percussion sound spectrum obtained by frequency analysis of the percussion line diagram of FIG.
FIG. 9 is a graph showing the relationship between the internal pressure of three pieces and the inspection frequency.
FIG. 10 is a waveform diagram of a 2-piece can.
11 is a tap sound spectrum obtained by frequency analysis of the tap waveform diagram of FIG.
FIG. 12 is a graph showing the relationship between the internal pressure of two pieces and the inspection frequency.
[Explanation of symbols]
3 Taping Head 5 Pulse Generator 6 Amplifier / Filter 7 A / D Converter 8 Waveform Memory 9 Microcomputer 10 Photoelectric Switch 11 Setter / Display

Claims (3)

In a method for inspecting the internal pressure of a sealed container by inspecting the internal pressure of the sealed container, the frequency of the percussion waveform obtained by converting the sound generated by applying an electromagnetic shock to the elastic wall of the sealed container into an electrical signal using a microphone The percussion sound spectrum is obtained by conversion, the first peak and the second peak of the obtained spectrum are extracted, and when the intensity ratio of the second peak and the first peak is within a predetermined range, the first peak A method for inspecting an internal pressure of a sealed container, wherein the frequency is set as a check frequency, and when the frequency is outside the predetermined range, the correction frequency fp obtained by the following equation is used as the check frequency.
fp = (f ₁ × M ₁ + f ₂ × M ₂ ) / (M ₁ + M ₂ )
Where f _{1 is} the frequency of the first peak
f ₂ : second peak frequency
M ₁ : intensity of the first peak
M ₂ is the intensity of the second peak. Wherein the following cases value selected freely between the value of the ratio M ₂ / M ₁ is 0.3 to 0.4 of the intensity of the second peak M ₂ and the intensity M ₁ of the first peak, the the frequency of the first peak and the hit test frequency, in the case of more than the value internal pressure inspection method of a sealed container according to claim 1, characterized in that the hit test frequency the correction frequency fp. The internal pressure inspection method for a sealed container according to claim 1 or 2, wherein the sealed container is a two-piece can.

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