Inspection Method for Honeycomb Filter

Abstract

To provide a method for inspecting the quality of a columnar honeycomb filter having a porous film formed on a cell surface, the inspection method allowing evaluation of the distribution in an in-plane direction of the thickness of the porous film.SOLUTION: A method for inspecting a columnar honeycomb filter includes obtaining a statistic related to a variation in the brightness of a cell on which a porous film is formed, from an image of a pattern of reflected light and image data of a pattern of transmitted light obtained by irradiating a bottom face of the columnar honeycomb filter with light, and comparing the statistic with a predetermined first reference related to the statistic.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present invention relates to a method for inspecting a columnar honeycomb filter.

Background Art

[0002] Exhaust gas discharged from internal combustion engines such as diesel engines and gasoline engines contains particulate matter such as soot (hereinafter referred to as PM: Particulate Matter). Soot is harmful to the human body and its emission is regulated. Currently, in order to comply with exhaust gas regulations, filters typified by DPF and GPF that allow exhaust gas to pass through a porous partition wall with small pores and filter PM such as soot are widely used.

[0003] As a filter for collecting PM, there are a plurality of first cells extending in the height direction from a first bottom surface to a second bottom surface, the first bottom surface being open and having a plugging portion at the second bottom surface, and the first cells being arranged adjacent to each other with a partition wall interposed therebetween, and a plurality of second cells extending in the height direction from the first bottom surface to the second bottom surface, the first bottom surface having a plugging portion and the second bottom surface being open. A wall flow type columnar honeycomb filter is known.

[0004] When industrially producing such a filter, it is desirable to inspect the PN collection efficiency before the filter is shipped to prevent filters that do not meet the required specifications from entering the market. However, since it is not always practical to conduct quality inspection by passing actual exhaust gas through the filter from the viewpoints of time and cost, an alternative simple inspection method has been developed to determine the presence or absence of defects in the plugging portion by observing the bottom surface of the filter irradiated with light.

[0005] Patent Document 1 proposes an inspection method for a columnar honeycomb structure having a plugging portion. The document discloses a method for detecting a defect in the plugging portion of a columnar honeycomb structure, which includes irradiating light onto the first bottom surface of the columnar honeycomb structure having a plugging portion, receiving the light with a light-transmitting projection medium brought into contact with the second bottom surface of the columnar honeycomb structure, and visualizing the light as a light spot on the projection medium.

[0006] Patent Document 2 describes an inspection method including a step of imaging, with a camera, through a light diffusion film disposed parallel to a second bottom surface in a non-contact state with the second bottom surface, a pattern of transmitted light from the second bottom surface corresponding to the arrangement of each eye seal portion of the first cell and the second cell, which is obtained by irradiating light onto the first bottom surface, and a step of detecting a defect of the eye seal portion of the second cell based on an image of the pattern of transmitted light imaged by the camera.

[0007] Patent Document 3 describes an inspection method including a step of imaging, with a camera, a pattern of reflected light from a second bottom surface and generating image data of the pattern of reflected light, a step of distinguishing and storing, in a storage device, position information of each of a cell adjacent to an outer peripheral side wall and a cell not adjacent to the outer peripheral side wall based on the image data of the pattern of reflected light, a step of imaging, with a camera, a pattern of transmitted light from the second bottom surface and generating image data of the pattern of transmitted light, a step of measuring intensities of transmitted light from each of the cells adjacent to the outer peripheral side wall based on the image data of the pattern of transmitted light and the position information and detecting a cell adjacent to the outer peripheral side wall having an abnormal eye seal portion, and a step of measuring intensities of transmitted light from each of the cells not adjacent to the outer peripheral side wall based on the image data of the pattern of transmitted light and the position information and detecting a cell not adjacent to the outer peripheral side wall having an abnormal eye seal portion.

[0008] On the other hand, in recent years, with the strengthening of exhaust gas regulations, more stringent emission standards for PM (PN regulation: particle number regulation of particulate matter) have been introduced, and high PM collection performance (high PN collection efficiency) is required for filters. Therefore, it has been proposed to separately form a porous film (also referred to as a "collection layer") for enhancing the collection efficiency of PM on the cell surface (Patent Document 4). By forming the porous film, it is possible to collect PM while reducing the pressure loss.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

[0010] In conventional simple inspection methods, as taught in Patent Documents 1 to 3, emphasis was placed on detecting defects in the eye seal portion. Detecting defects in the eye seal portion itself is useful when performing a performance inspection of a columnar honeycomb filter. However, the conventional simple inspection method is not sufficient as an inspection method for a columnar honeycomb filter having a porous membrane as proposed in Patent Document 4, and there is room for improvement.

[0011] Specifically, if there is a significant in-plane distribution (a direction perpendicular to the direction in which the cells extend) in the thickness of the porous membrane formed on the columnar honeycomb filter, it significantly affects the PM collection efficiency by the columnar honeycomb filter. Therefore, in inspecting the quality of a columnar honeycomb filter on which a porous membrane is formed, it is desirable to evaluate the in-plane distribution of the thickness of the porous membrane, but this could not be evaluated by the conventional inspection methods.

[0012] The present invention has been made in view of the above circumstances, and in one embodiment, it is an object to provide an inspection method for the quality of a columnar honeycomb filter having a porous membrane formed on the cell surface, which can evaluate the in-plane distribution of the thickness of the porous membrane. [Means for Solving the Problems]

[0013] The present inventors have intensively studied to solve the above problems, and as a result, in the image data of the pattern of transmitted light obtained by irradiating the bottom surface of the columnar honeycomb filter with light, it has been found that the statistic regarding the variation in the luminance of the cells in which the porous film is formed is correlated with the in-plane distribution of the thickness of the porous film. The present invention has been completed based on this finding and is exemplified below.

[0014] [Aspect 1] An inspection method for a columnar honeycomb filter, The columnar honeycomb filter includes: a plurality of first cells extending from an inlet-side bottom surface to an outlet-side bottom surface, having an opening at the inlet-side bottom surface, and having a plugging portion at the outlet-side bottom surface; and a plurality of second cells extending from the inlet-side bottom surface to the outlet-side bottom surface, having a plugging portion at the inlet-side bottom surface, and having an opening at the outlet-side bottom surface. The plurality of first cells and the plurality of second cells are alternately arranged adjacent to each other with a porous partition therebetween. A porous film is formed on the surface of each first cell. Step 1 of irradiating the inlet-side bottom surface with first light, imaging with a camera the pattern of reflected light from the inlet-side bottom surface according to the arrangement of the first cells and the second cells, and generating image data of the pattern of reflected light; Step 2 of storing, in a storage device, the position information of the openings of the plurality of first cells specified based on the generated image data of the pattern of reflected light; Step 3 of imaging with a camera, through a light diffusion film arranged parallel to the inlet-side bottom surface, the pattern of transmitted light from the inlet-side bottom surface obtained by irradiating the outlet-side bottom surface with second light, according to the arrangement of the first cells and the second cells, and generating image data of the pattern of transmitted light; Step 4 of measuring the luminance of a plurality of pixels located within the openings of the plurality of first cells based on the generated image data of the pattern of transmitted light and the position information stored in the storage device; Step 5 of obtaining a statistic regarding the variation in the luminance of the plurality of pixels for which the luminance has been measured based on the result of Step 4; Step 6 of comparing the statistic with a first criterion regarding the statistic determined in advance; An inspection method including the above steps. [Aspect 2] The inspection method according to Aspect 1, wherein the total mass of the porous membrane from the bottom surface on the inlet side of the first cell to the midpoint of the columnar honeycomb filter in the extending direction of the first cell is less than the total mass of the porous membrane from the bottom surface on the outlet side of the first cell to the midpoint of the columnar honeycomb filter in the extending direction of the first cell. [Aspect 3] The inspection method according to Aspect 1 or 2, wherein the statistic is the standard deviation of luminance. [Aspect 4] The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, The first criterion is determined based at least on the correlation between the statistic and the information on the collection efficiency of particulate matter in exhaust gas under predetermined conditions, which was previously obtained for columnar honeycomb filters with the same product number as the said product number. The inspection method according to any one of Aspects 1 to 3. [Aspect 5] The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, The inspection method according to any one of Aspects 1 to 4, further comprising Step 7 of estimating the collection efficiency from the statistic, based at least on the correlation between the statistic and the information on the collection efficiency of particulate matter in exhaust gas under predetermined conditions, which was previously obtained for columnar honeycomb filters with the same product number as the said product number. [Aspect 6] Step 8 of obtaining the average value of the luminance in a plurality of pixels in which the luminance was measured based on the result of Step 4, and Step 9 of comparing the average value with a second criterion regarding the average value of the luminance determined in advance, and The inspection method according to any one of Aspects 1 to 5, further comprising. [Aspect 7] The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, The second standard is determined based on at least the correlation between the average value and information regarding the mass of the porous membrane, which was previously obtained for a columnar honeycomb filter with the same part number as the part number in question, or the correlation between the average value and information regarding the collection efficiency of particulate matter in exhaust gas under predetermined conditions, which was previously obtained for a columnar honeycomb filter with the same part number as the part number in question. The inspection method according to Aspect 6. [Aspect 8] The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific part number should be assigned. The inspection method according to Aspect 6 or 7, further including Step 10 of estimating the mass of the porous membrane from the average value, based on at least the correlation between the average value and information regarding the mass of the porous membrane, which was previously obtained for a columnar honeycomb filter with the same part number as the part number in question. [Aspect 9] The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific part number should be assigned. The inspection method according to any one of Aspects 6 to 8, further including Step 11 of estimating the collection efficiency from the average value, based on at least the correlation between the average value and information regarding the collection efficiency of particulate matter in exhaust gas under predetermined conditions, which was previously obtained for a columnar honeycomb filter with the same part number as the part number in question. [Aspect 10] The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific part number should be assigned. Step 8 of obtaining the average value of the luminance in a plurality of pixels for which the luminance was measured, based on the result of Step 4. The inspection method according to any one of Aspects 1 to 9, further including Step 12 of estimating the collection efficiency from the statistic and the average value, based on at least the correlation between the statistic and information regarding the collection efficiency of particulate matter in exhaust gas under predetermined conditions, which was previously obtained for a columnar honeycomb filter with the same part number as the part number in question, and the correlation between the average value and information regarding the collection efficiency of particulate matter in exhaust gas under predetermined conditions, which was previously obtained for a columnar honeycomb filter with the same part number as the part number in question.

Advantages of the Invention

[0015] Since the in-plane distribution of the thickness of the porous membrane significantly affects the performance of the columnar honeycomb filter, the ability to evaluate the in-plane distribution of the thickness of the porous membrane is important for stably providing high-quality columnar honeycomb filters to the market. In this regard, according to the inspection method of the columnar honeycomb filter according to an embodiment of the present invention, it is expected to be adopted when high collection performance (high PN collection efficiency) of PM is required, and it becomes possible to easily evaluate the in-plane distribution of the thickness of the porous membrane for a columnar honeycomb filter having a porous membrane formed on the cell surface. Therefore, it can be said that the inspection method has extremely high industrial value.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5-1

Figure 5-2

Figure 6

Figure 7

Figure 8

Best Mode for Carrying Out the Invention

[0017] Next, embodiments of the present invention will be described in detail with reference to the drawings. It should be understood that the present invention is not limited to the following embodiments, and design changes, improvements, etc. can be appropriately made based on the ordinary knowledge of those skilled in the art without departing from the gist of the present invention.

[0018] <1. Columnar Honeycomb Filter> A columnar honeycomb filter according to an embodiment of the present invention will be described. The columnar honeycomb filter can be used as a DPF (Diesel Particulate Filter) and a GPF (Gasoline Particulate Filter) for collecting soot attached to an exhaust gas line from a combustion device, typically an engine mounted on a vehicle. The columnar honeycomb filter can be installed, for example, in an exhaust pipe.

[0019] FIG. 1 and FIG. 2 respectively illustrate a schematic perspective view and a cross-sectional view of the columnar honeycomb filter 100. This columnar honeycomb filter 100 includes an outer peripheral side wall 102, and a plurality of first cells 108 that are arranged on the inner peripheral side of the outer peripheral side wall 102, extend parallel from the inlet-side bottom surface 104 to the outlet-side bottom surface 106, have an opening 107 in the inlet-side bottom surface 104, and have a plugging portion 109 in the outlet-side bottom surface 106, and a plurality of second cells 110 that are arranged on the inner peripheral side of the outer peripheral side wall 102, extend parallel from the inlet-side bottom surface 104 to the outlet-side bottom surface 106, have a plugging portion 109 in the inlet-side bottom surface 104, and have an opening 107 in the outlet-side bottom surface 106. In this columnar honeycomb filter 100, the first cells 108 and the second cells 110 are alternately and adjacently arranged with the porous partition wall 112 interposed therebetween, so that the inlet-side bottom surface 104 and the outlet-side bottom surface 106 each exhibit a honeycomb shape.

[0020] When exhaust gas containing particulate matter (PM) such as soot is supplied to the inlet-side bottom surface 104 on the upstream side of the columnar honeycomb filter 100, the exhaust gas is introduced into the first cell 108 and travels downstream within the first cell 108. Since the first cell 108 has a plugging portion 109 on the downstream-side outlet-side bottom surface 106, the exhaust gas permeates through the porous partition wall 112 that separates the first cell 108 and the second cell 110 and flows into the second cell 110. Since the particulate matter cannot pass through the porous partition wall 112, it is collected and deposited within the first cell 108. After the particulate matter is removed, the clean exhaust gas that has flowed into the second cell 110 travels downstream within the second cell 110 and flows out from the downstream-side outlet-side bottom surface 106.

[0021] Figure 3 shows a schematic partial enlarged view when the columnar honeycomb filter 100 is observed in a cross-section perpendicular to the extending direction of the cells 108 and 110. A porous membrane 114 is formed on the surface of each first cell 108 of the columnar honeycomb filter 100 (the same as the surface of the porous partition wall 112 that partitions and forms the first cell 108). By forming the porous membrane 114 on the surface of the first cell 108, it can be expected that the collection performance for PM will be improved.

[0022] When the columnar honeycomb filter 100 is observed in a cross-section perpendicular to the extending direction of the cells 108 and 110, it is usually required that the in-plane distribution of the thickness of the porous membrane 114 is uniform.

[0023] As the flow velocity of the exhaust gas increases, it is known that the flow velocity of the exhaust gas when passing through the partition walls of the columnar honeycomb filter tends to increase as it approaches the bottom surface on the outlet side. The amount of exhaust gas passing through per unit time increases at locations where the flow velocity of the exhaust gas is high. Therefore, increasing the thickness of the porous membrane to increase the contact opportunity with the porous membrane can enhance the PM collection performance. Thus, by increasing the thickness of the porous membrane as it approaches the bottom surface on the outlet side where the flow velocity of the exhaust gas is high, the PM collection performance can be enhanced without increasing the pressure loss more than necessary. For this reason, in a preferred embodiment of the columnar honeycomb filter 100, the porous membrane 114 formed on the surface of each first cell 108 (the same as the surface of the partition wall that forms the first cell) increases in thickness from the bottom surface 104 on the inlet side to the bottom surface 106 on the outlet side. Therefore, in one embodiment, the columnar honeycomb filter has a greater total mass of the porous membrane 114 from the bottom surface 106 on the outlet side of the first cell 108 to the midpoint of the columnar honeycomb filter 100 in the extending direction of the first cell 108 than the total mass of the porous membrane 114 from the bottom surface 104 on the inlet side of the first cell 108 to the midpoint of the columnar honeycomb filter 100 in the extending direction of the first cell 108. FIG. 4 shows a schematic cross-sectional view of a structural example of the first cell 108 of such a columnar honeycomb filter 100.

[0024] The average thickness of the entire porous membrane in the columnar honeycomb filter can be, for example, 4 to 50 μm, and typically 10 to 40 μm. The average thickness of the entire porous membrane can be determined by the measurement method described in JP-A-2022-158915.

[0025] In one embodiment, the porosity of the porous membrane is higher than the porosity of the partition wall. By having the porosity of the porous membrane higher than the porosity of the partition wall, the advantage of suppressing an increase in pressure loss can be obtained. In this case, the difference in porosity (%) between the porous membrane and the partition wall is preferably 10% or more, and more preferably 20% or more.

[0026] From the perspective of suppressing the increase in pressure loss, the lower limit of the porosity of the porous membrane is preferably 70% or more. Also, from the perspective of suppressing the decrease in collection efficiency, the upper limit of the porosity of the porous membrane is preferably 85% or less. Therefore, the porosity of the porous membrane can be, for example, 70 to 85%. The porosity of the porous membrane can be determined by the measurement method described in Japanese Patent Application Laid-Open No. 2022-158915.

[0027] From the perspective of keeping the pressure loss of the exhaust gas low, the lower limit of the porosity of the partition wall is preferably 40% or more, more preferably 45% or more, and still more preferably 50% or more. Also, from the perspective of ensuring the strength of the columnar honeycomb filter, the upper limit of the porosity of the partition wall is preferably 80% or less, more preferably 75% or less, and still more preferably 70% or less. Therefore, the porosity of the partition wall can be, for example, 40 to 80%. The porosity of the partition wall refers to the value measured with a mercury intrusion porosimeter in accordance with JIS-R1655:2003.

[0028] The porous membrane can be composed of ceramics. The porous membrane can contain, for example, one or more ceramics selected from cordierite, silicon carbide (SiC), talc, mica, mullite, celven, aluminum titanate, alumina, silicon nitride, sialon, zirconium phosphate, zirconia, titania, and silica. The main component of the porous membrane is preferably silicon carbide, alumina, silica, cordierite, or mullite. Among them, since a porous membrane that is firmly bonded to each other due to the presence of the surface oxide film (Si2O) and is difficult to peel off can be obtained, the main component of the porous membrane is preferably silicon carbide. The main component of the porous membrane refers to a component that occupies 50% by mass or more of the porous membrane. The porous membrane preferably contains 50% by mass or more of SiC, more preferably 70% by mass or more, and still more preferably 90% by mass or more. There is no particular limitation on the shape of the ceramics constituting the porous membrane, and for example, granular shape can be mentioned.

[0029] Examples of materials that make up the partition walls and outer peripheral side walls of the columnar honeycomb filter include, but are not limited to, porous ceramics. Examples of ceramics include cordierite, mullite, zirconium phosphate, aluminum titanate, silicon carbide (SiC), silicon-silicon carbide composites (e.g., Si-bonded SiC), cordierite-silicon carbide composites, zirconia, spinel, indialite, sapphirine, corundum, titania, silicon nitride, and the like. These ceramics may contain one type alone or two or more types.

[0030] The columnar honeycomb filter may carry a PM combustion catalyst that aids in the combustion of PM such as soot, an oxidation catalyst (DOC), an SCR catalyst and an NSR catalyst for removing nitrogen oxides (NOx), and a three-way catalyst capable of simultaneously removing hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx).

[0031] The bottom surface shape of the columnar honeycomb filter is not particularly limited. For example, in addition to round shapes such as circular, elliptical, racetrack-shaped, and oval-shaped, it can be polygonal shapes such as triangular and quadrilateral. The columnar honeycomb filter 100 in FIG. 1 has a circular bottom surface shape and is cylindrical as a whole.

[0032] The height of the columnar honeycomb filter (the length from the bottom surface on the inlet side to the bottom surface on the outlet side) is not particularly limited and may be appropriately set according to the application and required performance. There is also no particular limitation on the relationship between the height of the columnar honeycomb filter and the maximum diameter of each bottom surface (the maximum length among the diameters passing through the center of gravity of each bottom surface of the columnar honeycomb filter). Therefore, the height of the columnar honeycomb filter may be longer than the maximum diameter of each bottom surface, or the height of the columnar honeycomb filter may be shorter than the maximum diameter of each bottom surface.

[0033] There is no limitation on the shape of the cells in a cross-section perpendicular to the extending direction of the cells, but it is preferably a quadrilateral, hexagon, octagon, or a combination thereof. Among these, a square and a hexagon are preferred. By making the cell shape like this, the pressure loss when flowing a fluid through the columnar honeycomb filter can be reduced.

[0034] From the viewpoint of suppressing the pressure loss, the upper limit of the average thickness of the partition walls in the columnar honeycomb filter is preferably 0.305 mm or less, more preferably 0.254 mm or less, and even more preferably 0.241 mm or less. However, from the viewpoint of ensuring the strength of the columnar honeycomb filter, the lower limit of the average thickness of the partition walls is preferably 0.152 mm or more, more preferably 0.178 mm or more, and even more preferably 0.203 mm or more. Therefore, the average thickness of the partition walls can be, for example, 0.152 mm to 0.305 mm. In this specification, the thickness of the partition wall refers to the length of the line segment connecting the centers of gravity of adjacent cells when the line segment crosses the partition wall in a cross-section perpendicular to the extending direction of the cells. The average thickness of the partition wall refers to the average value of the thicknesses of all the partition walls.

[0035] The cell density (the number of cells per unit cross-sectional area perpendicular to the extending direction of the cells) is not particularly limited, but for example, it can be 6 to 2000 cells per square inch (0.9 to 311 cells / cm 2 ), more preferably 50 to 1000 cells per square inch (7.8 to 155 cells / cm 2 ), and particularly preferably 100 to 400 cells per square inch (15.5 to 62.0 cells / cm 2 ). The cell density is calculated by dividing the number of cells on one bottom surface (including the sealed cells) by the area of the bottom surface excluding the outer peripheral side walls.

[0036] The columnar honeycomb filter can also be provided as an integrally molded product. Further, the columnar honeycomb filter can also be provided as a segment joined body by joining segments of a plurality of columnar honeycomb filters each having an outer peripheral side wall to each other on the side surfaces to integrate them. By providing the columnar honeycomb filter as a segment joined body, the thermal shock resistance can be enhanced.

[0037] <2. Manufacturing method of columnar honeycomb filter> The columnar honeycomb filter having the plugging portion and the porous film can be produced by a known manufacturing method and will be exemplarily described below. First, after kneading a raw material composition containing a ceramic raw material, a dispersion medium, a pore former, and a binder to form a green body, the green body is extruded and molded into a desired columnar honeycomb structure having a plurality of first cells and second cells. After drying the columnar honeycomb structure, plugging portions are formed on both bottom surfaces of the columnar honeycomb structure, and then the plugging portions are dried to obtain a columnar honeycomb structure having the plugging portions. The columnar honeycomb structure is usually provided as a ceramic fired body by performing debinding and firing thereafter.

[0038] Next, a porous film is formed on the surfaces of the plurality of first cells of the columnar honeycomb structure that has undergone the firing process. First, while injecting an aerosol containing ceramic particles in a direction perpendicular to the inlet-side bottom surface toward the center of the inlet-side bottom surface of the columnar honeycomb structure, a suction force is applied to the outlet-side bottom surface to suck the injected aerosol from the inlet-side bottom surface and attach the ceramic particles to the surfaces of the first cells. As the ceramic particles, the above-described ceramic particles constituting the porous film are used.

[0039] The median diameter (D50) of the ceramic particles in the aerosol in the volume-based cumulative particle size distribution measured by the laser diffraction / scattering method is preferably 0.5 to 5.0 μm, and more preferably 1.0 to 3.0 μm. By injecting extremely fine ceramic particles, it becomes possible to increase the porosity of the obtained porous film.

[0040] When carrying out the step of attaching ceramic particles to the surface of the first cell, in order to increase the thickness of the porous membrane from the inlet-side bottom surface to the outlet-side bottom surface of the columnar honeycomb structure filter, it is preferable to increase the suction force of the outlet-side bottom surface and increase the flow rate of the aerosol flowing through the columnar honeycomb structure. Specifically, it is preferable that the lower limit of the average flow rate of the aerosol flowing into the columnar honeycomb structure (= aerosol flow rate / area of the inlet-side bottom surface) is 2 m / s or more, and more preferably 4 m / s or more. Further, in order to maintain the high porosity of the porous membrane, it is preferable that the upper limit of the average flow rate of the aerosol flowing into the columnar honeycomb structure is 80 m / s or less, and preferably 60 m / s or less.

[0041] Thereafter, the columnar honeycomb structure having ceramic particles attached to the surface of the first cell is heat-treated under the condition of keeping it at a maximum temperature of 1000 °C or more for 1 hour or more, typically, under the condition of keeping it at a maximum temperature of 1100 °C to 1400 °C for 1 hour to 6 hours, whereby the columnar honeycomb filter is completed. The heat treatment can be carried out, for example, by placing the columnar honeycomb structure in an electric furnace or a gas furnace. By the heat treatment, the ceramic particles are bonded to each other, and the ceramic particles are baked onto the partition walls in the first cell, and a porous membrane is formed on the surface of the first cell. When the heat treatment is carried out under oxygen-containing conditions such as air, a surface oxide film is generated on the surface of the ceramic particles, and the bonding of the ceramic particles to each other is promoted. Thereby, a porous membrane that is difficult to peel off can be obtained.

[0042] <3. Inspection method of columnar honeycomb filter> The inspection method of the columnar honeycomb filter according to the present invention can use the above-described columnar honeycomb filter as an inspection object. The inspection method of the columnar honeycomb filter 100 according to the present invention, in one embodiment, performs the following steps 1 to 6. Additionally, one or more of steps 7 to 12 may be performed.

[0043] (Step 1) In step 1, the entrance-side bottom surface 104 is irradiated with first light, and a camera images the pattern of the reflected light from the entrance-side bottom surface 104 according to the arrangement of the first cell 108 and the second cell 110, generating image data of the pattern of the reflected light. Based on the image data of the pattern of the reflected light obtained by performing step 1, it is possible to identify the positions of the openings 107 of the plurality of first cells 108 on the entrance-side bottom surface 104.

[0044] (Step 2) In step 2, the position information of the openings 107 of the plurality of first cells 108 identified based on the generated image data of the pattern of the reflected light is stored in the storage device 302. Since the porous film 114 is formed on the surfaces of the plurality of first cells 108, it is important to store the position information of the openings 107 of the plurality of first cells 108. The position information is not particularly limited as long as it can identify the positions of the respective openings 107 of the plurality of first cells 108. For example, it can include information on the two-dimensional coordinates of the pixels constituting the contours of the respective openings 107, whereby it is possible to identify the positions, shapes, and sizes of the respective openings 107. The contour of the opening 107 is defined by the porous partition wall 112 excluding the porous film 114 (i.e., the porous partition wall 112 in a state where the porous film 114 does not exist) and can be identified, for example, by binarization processing. The position information of the openings 107 of the plurality of first cells 108 is used in step 4 described later.

[0045] (Step 3) By performing step 2, it is possible to obtain the position information of the openings 107 of the plurality of first cells 108 on which the porous film 114 is formed. However, even if step 2 is performed, the thickness information of the porous film 114 formed on the surfaces of the plurality of first cells 108 cannot be obtained. Therefore, in step 3, the pattern of the transmitted light from the entrance-side bottom surface 104 according to the arrangement of the first cell 108 and the second cell 110, which is obtained by irradiating the exit-side bottom surface 106 with second light, is imaged by a camera through a light diffusion film arranged parallel to the entrance-side bottom surface 104, generating image data of the pattern of the transmitted light.

[0046] The image data of the pattern of transmitted light obtained in Step 3 reflects the in-plane distribution of the thickness of the porous membrane 114 of the first cells 108. That is, when light is irradiated from the bottom surface 106 on the outlet side, the light transmittance decreases for the first cells 108 with a large thickness of the porous membrane 114, while the light transmittance increases for the first cells 108 with a small thickness of the porous membrane 114. Therefore, for example, if the thickness of the porous membrane 114 formed on the surface of the first cells 108 is uniform in the in-plane direction, the intensity of the transmitted light from the bottom surface 104 on the inlet side of each first cell 108 becomes uniform. Conversely, if the thickness of the porous membrane 114 formed on the surface of the first cells 108 is non-uniform in the in-plane direction, the intensity of the transmitted light from the bottom surface 104 on the inlet side of each first cell 108 becomes non-uniform.

[0047] (Step 4) Therefore, in Step 4, based on the image data of the pattern of transmitted light generated in Step 3 and the position information stored in the storage device 302, the luminance of a plurality of pixels located within the openings 107 of the plurality of first cells 108 is measured. In order to improve the evaluation accuracy, in Step 4, it is preferable to measure the luminance of 90% or more of the number of pixels located within the openings 107 of each of the first cells 108 for 90% or more of the number of the first cells 108 included in the columnar honeycomb filter. It is more preferable to measure the luminance of 99% or more of the number of pixels located within the openings 107 of each of the first cells 108 for 99% or more of the number of the first cells 108 included in the columnar honeycomb filter. It is even more preferable to measure the luminance of all the number of pixels located within the openings 107 of each of the first cells 108 for all the number of the first cells 108 included in the columnar honeycomb filter.

[0048] With only the image data of the transmitted light pattern obtained in Step 3, the boundaries of the respective openings 107 of the first cells 108 tend to be unclear. However, by performing Step 2, accurate position information of the plurality of first cells 108 in which the porous film 114 is formed can be obtained. Therefore, by superimposing the position information of the openings 107 of the plurality of first cells 108 obtained in Step 2 on the image data of the transmitted light pattern generated in Step 3, it becomes possible to accurately measure the brightness of the plurality of pixels located within the openings 107 of the plurality of first cells 108. FIG. 6 shows an example of an image in which the contours (indicated by dotted lines) of the openings 107 of the plurality of first cells 108 are superimposed on the image of the transmitted light pattern.

[0049] (Step 5) In Step 5, based on the results of Step 4, a statistic regarding the variation in brightness among the plurality of pixels for which brightness has been measured is obtained. Examples of the statistic regarding the variation in brightness include the standard deviation, the coefficient of variation, and the variance. Among these, the standard deviation is preferable because it can be evaluated in the same unit as the average value of the brightness described later. In Step 5, instead of obtaining a statistic regarding the variation in brightness for each first cell 108, a statistic regarding the variation in brightness is obtained using all the pixels for which brightness has been measured in Step 4 as the population. At this time, a histogram showing the brightness distribution may be generated with brightness on one coordinate axis and the number of pixels on the other coordinate axis. If the thickness of the porous film 114 formed on the surface of the first cell 108 is uniform in the in-plane direction, the variation in brightness will be small. Conversely, if the thickness of the porous film 114 formed on the surface of the first cell 108 is non-uniform in the in-plane direction, the variation in brightness will be large. Therefore, by obtaining the variation in brightness, it is possible to evaluate the distribution of the thickness of the porous film 114 of the columnar honeycomb filter in the in-plane direction.

[0050] In addition, it has been found that the in-plane distribution of the thickness of the porous membrane 114 of the columnar honeycomb filter has a correlation with the collection efficiency of the columnar honeycomb filter for particulate matter. For example, when the degree of non-uniformity in the in-plane direction of the thickness of the porous membrane 114 increases, the collection efficiency of the columnar honeycomb filter for particulate matter tends to decrease. This means that there is a significant correlation between the statistic regarding the variation in luminance and the information regarding the collection efficiency of particulate matter in the exhaust gas under predetermined conditions. Therefore, it is also possible to estimate the collection efficiency of the columnar honeycomb filter for particulate matter by obtaining the variation in luminance.

[0051] (Step 6) In Step 6, the statistic regarding the variation in luminance obtained in Step 5 is compared with a first criterion regarding the statistic determined in advance. The first criterion may be appropriately set according to the specifications required for the columnar honeycomb filter. For example, it is possible to determine whether the inspection regarding the in-plane distribution of the thickness of the porous membrane 114 passes or fails, and thus whether the inspection regarding the collection efficiency of particulate matter passes or fails, based on whether the first criterion is satisfied. Therefore, in one embodiment, the columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, and the first criterion is determined based at least on the correlation between the statistic and the information regarding the collection efficiency of particulate matter in the exhaust gas under predetermined conditions, which has been obtained in advance for the columnar honeycomb filter having the same product number as the said product number.

[0052] As a product, columnar honeycomb filters are usually assigned a part number according to specifications. Therefore, when evaluating the statistic related to the variation in luminance, the inspection accuracy can be improved by conducting the evaluation based on the data of columnar honeycomb filters with the same part number as the columnar honeycomb filter to be inspected. In this specification, the columnar honeycomb filters with the same part number refer to columnar honeycomb filters with the same design specifications other than the design specifications related to the porous membrane. For example, columnar honeycomb filters with the same design in terms of the overall shape, the material of the partition wall, the cell shape in the cross-section perpendicular to the flow path direction of the cell, the cell density (the number of cells per unit cross-sectional area), the partition wall thickness (nominal value based on the specifications of the base), the porosity of the partition wall, and the material and structure of the end-sealing part are of the same part number. The design specifications related to the porous membrane may or may not be the same.

[0053] It is desirable that the correlation between the statistic obtained in advance for a specific part number of columnar honeycomb filters and the information on the collection efficiency of particulate matter in exhaust gas under predetermined conditions is high. Specifically, it preferably has a correlation with a determination coefficient of 0.7 or more, and more preferably has a correlation with a determination coefficient of 0.8 or more. To obtain such a high correlation, it is desirable to appropriately set the conditions when measuring the collection efficiency and acquire a sufficient number of data on the combination of the statistic related to the variation in luminance and the information related to the collection efficiency. Also, as will be described later, the average value of luminance also affects the collection efficiency. Therefore, when acquiring data, a high correlation can be obtained by collecting data between columnar honeycomb filters with similar average luminance values.

[0054] An example of a specific method for setting the first reference will be described. A plurality of columnar honeycomb filters having the same part number as the part number to be assigned to the columnar honeycomb filter to be inspected, and having different in-plane distributions of the thickness of the porous film, are prepared, and for these, the above-mentioned statistic and the collection efficiency of particulate matter in the exhaust gas under predetermined conditions are measured. Next, the results are plotted on a two-dimensional coordinate system in which one coordinate axis represents the statistic and the other coordinate axis represents the collection efficiency to obtain a regression equation. As the regression equation, for example, a linear regression equation, an exponential regression equation, a logarithmic regression equation, or a polynomial regression equation can be used. Thus, when a regression equation between the statistic regarding the variation in luminance and the collection efficiency is obtained, the statistic that can satisfy a predetermined collection efficiency can be determined based on this regression equation, and this can be used as the first reference. The step of comparing the statistic with the first reference may be performed manually by a human or automatically using a computer.

[0055] (Step 7) As described above, by obtaining the variation in luminance, it is also possible to estimate the collection efficiency of the columnar honeycomb filter for particulate matter. Therefore, for example, in one embodiment of the inspection method according to the present invention, the columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific part number should be assigned, and at least based on the correlation between the statistic obtained in advance for the columnar honeycomb filter having the same part number and the information regarding the collection efficiency of particulate matter in the exhaust gas under predetermined conditions, the method further includes Step 7 of estimating the collection efficiency from the statistic.

[0056] (Step 8) By performing Step 4, it is possible to obtain the luminance information of a plurality of pixels located within the openings 107 of the plurality of first cells 108. Therefore, based on the result of Step 4, Step 8 of obtaining the average value of the luminance in the plurality of pixels for which the luminance has been measured may be performed.

[0057] The mass (adhesion amount) of the porous film 114 formed on the surface of the first cell 108 is correlated with the average value of the luminance. For example, if the mass of the porous film 114 formed on the surface of the first cell 108 increases, the transmitted light decreases, so the average value of the luminance decreases. Conversely, if the mass of the porous film 114 formed on the surface of the first cell 108 decreases, the average value of the luminance increases. Therefore, by obtaining the average value of the luminance, it is possible to evaluate whether the mass of the porous film 114 formed on the surface of the first cell 108 is appropriate. Furthermore, by obtaining the average value of the luminance, it is also possible to estimate the mass of the porous film 114.

[0058] Also, it has been found that the mass of the porous film 114 of the columnar honeycomb filter is correlated with the collection efficiency of the columnar honeycomb filter for particulate matter. For example, when the mass of the porous film 114 decreases, the collection efficiency of the columnar honeycomb filter for particulate matter tends to decrease. This means that there is a significant correlation between the average value of the luminance and the information on the collection efficiency of particulate matter in the exhaust gas under predetermined conditions. Therefore, by obtaining the average value of the luminance, it is also possible to estimate the collection efficiency of the columnar honeycomb filter for particulate matter.

[0059] (Step 9) In Step 9, the average value of the luminance obtained in Step 8 is compared with a second criterion regarding the average value of the luminance determined in advance. The second criterion may be appropriately set according to the specifications required for the columnar honeycomb filter. For example, it is possible to determine the pass / fail of the inspection regarding the mass of the porous film 114, and thus the pass / fail of the inspection regarding the collection efficiency of particulate matter, based on whether the second criterion is satisfied. Therefore, in one embodiment, the columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, and the second criterion is determined based at least on the correlation between the average value and the information on the mass of the porous film obtained in advance for the columnar honeycomb filter of the same product number, or the correlation between the average value and the information on the collection efficiency of particulate matter in the exhaust gas under predetermined conditions obtained in advance for the columnar honeycomb filter of the same product number.

[0060] It is desirable that the correlation between the previously obtained average value and the information regarding the mass of the porous membrane for the columnar honeycomb filter of a specific product number be high. Specifically, it is preferable to have a correlation with a determination coefficient of 0.7 or more, and more preferably a correlation with a determination coefficient of 0.8 or more. In order to obtain such a high correlation, it is desirable to acquire a sufficient number of data regarding the combination of the average value of the luminance and the information regarding the mass of the porous membrane.

[0061] Also, it is desirable that the correlation between the previously obtained average value and the information regarding the collection efficiency of particulate matter in the exhaust gas under predetermined conditions for the columnar honeycomb filter of a specific product number be high. Specifically, it is preferable to have a correlation with a determination coefficient of 0.7 or more, and more preferably a correlation with a determination coefficient of 0.8 or more. In order to obtain such a high correlation, it is desirable to appropriately set the conditions when measuring the collection efficiency and acquire a sufficient number of data regarding the combination of the average value of the luminance and the information regarding the collection efficiency. Also, as described above, the statistical quantity regarding the variation in luminance also affects the collection efficiency. Therefore, when acquiring data, a high correlation can be obtained by collecting data for columnar honeycomb filters having the same statistical quantity regarding the variation in luminance.

[0062] (Step 10) As described above, it is also possible to estimate the mass of the porous membrane by obtaining the average value of the luminance. Therefore, for example, in one embodiment of the inspection method according to the present invention, the columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, and based at least on the correlation between the average value and the information regarding the mass of the porous membrane previously obtained for the columnar honeycomb filter having the same product number as the said product number, Step 10 of estimating the mass of the porous membrane from the said average value is further included.

[0063] (Step 11) As described above, by obtaining the average value of the luminance, it is also possible to estimate the collection efficiency of the particulate matter by the honeycomb filter. Therefore, for example, in one embodiment of the inspection method according to the present invention, the honeycomb filter to be inspected is a honeycomb filter to which a specific product number should be assigned, and at least based on the correlation between the average value and the information regarding the collection efficiency of the particulate matter in the exhaust gas under predetermined conditions, which has been previously obtained for the honeycomb filter with the same product number as the said product number, the method further includes step 11 of estimating the collection efficiency from the average value.

[0064] (Step 12) As described above, both the statistic regarding the variation in luminance and the average value of the luminance have a significant correlation with the information regarding the collection efficiency of the particulate matter by the honeycomb filter. For this reason, the estimation accuracy of the collection efficiency of the particulate matter by the honeycomb filter can be increased by considering both of them. Therefore, in one embodiment of the inspection method according to the present invention, the honeycomb filter to be inspected is a honeycomb filter to which a specific product number should be assigned, step 5 of obtaining the statistic regarding the variation in luminance in a plurality of pixels in which the luminance has been measured based on the result of step 4, step 8 of obtaining the average value of the luminance in a plurality of pixels in which the luminance has been measured based on the result of step 4, the correlation between the statistic and the information regarding the collection efficiency of the particulate matter in the exhaust gas under predetermined conditions, which has been previously obtained for the honeycomb filter with the same product number as the said product number, and the correlation between the average value and the information regarding the collection efficiency of the particulate matter in the exhaust gas under predetermined conditions, which has been previously obtained for the honeycomb filter with the same product number as the said product number, and the method further includes step 12 of estimating the collection efficiency from the statistic and the average value, at least based on these correlations.

[0065] A model for estimating the capture efficiency can be constructed by statistical analysis based on data associating one or both of a statistic related to the luminance variation and the average value of the luminance with information related to the capture efficiency of particulate matter in the exhaust gas under predetermined conditions. For example, a regression equation as described above can be obtained for each group with a similar average luminance value based on the data, or the data can be used for machine learning as teacher data to construct the model. Known learning models such as neural networks and support vector machines can be used for machine learning. When using a neural network for learning, deep learning may be used.

[0066] <4. Inspection Device for Columnar Honeycomb Filter> FIGS. 5-1 and 5-2 show schematic side views for explaining the configurations of inspection devices 10 and 20 suitable for carrying out the inspection method according to an embodiment of the present invention described above. The inspection device 10 shown in FIG. 5-1 and the inspection device 20 shown in FIG. 5-2 differ in the presence or absence of the light diffusion film 14, but other configurations are the same. The configuration of the inspection device shown in FIG. 5-1 can be suitably used when imaging the pattern of reflected light in step 1 with the camera 16. The configuration of the inspection device shown in FIG. 5-2 can be suitably used when imaging the pattern of transmitted light in step 3 with the camera 16.

[0067] When carrying out the inspection method according to the present invention, for example, step 1 is carried out using the inspection device 10 shown in FIG. 5-1, and then, with the columnar honeycomb filter set in the inspection device, the light diffusion film 14 is arranged to change to the configuration of the inspection device 20 shown in FIG. 5-2, and step 3 can be carried out. Alternatively, step 3 can be carried out using the inspection device 20 shown in FIG. 5-2, and then, with the columnar honeycomb filter set in the inspection device, the light diffusion film 14 is removed to change to the configuration of the inspection device 20 shown in FIG. 5-1, and step 1 can be carried out.

[0068] In one embodiment, the inspection devices 10 and 20 are a light irradiator 12a for irradiating the first light to the inlet side bottom surface 104, A light irradiator 12b for irradiating the second light onto the exit-side bottom surface 106, a camera 16 for imaging the pattern of the reflected light from the entrance-side bottom surface 104 and the pattern of the transmitted light, are provided.

[0069] Also, in one embodiment, the inspection apparatuses 10 and 20 include a computer 300 having an image processing unit 301, a storage device 302, an arithmetic unit 303, and a display unit 304.

[0070] The image processing unit 301 can generate image data of the pattern of the reflected light and image data of the pattern of the transmitted light imaged by the camera 16. Further, the image processing unit 301 can specify the positions of the respective openings 107 of the plurality of first cells 108 based on the generated image data of the pattern of the reflected light.

[0071] The storage device 302 can store the position information of the openings of the plurality of first cells 108 specified based on the generated image data of the pattern of the reflected light. The storage device 302 can store the generated image data of the pattern of the transmitted light. The storage device 302 can store a first criterion regarding a statistical quantity related to the variation in the predetermined luminance. The storage device 302 can store a second criterion regarding the average value of the predetermined luminance. Also, the storage device 302 can store a model (program) for estimating the collection efficiency based on at least one or both of the statistical quantity and the average value.

[0072] The calculation unit 303 can measure the luminance of a plurality of pixels located within the apertures 107 of the plurality of first cells 108 based on the image data of the generated transmitted light pattern and the position information stored in the storage device 302. The calculation unit 303 can calculate a statistic regarding the variation in luminance among the plurality of pixels for which the luminance has been measured, the average value of the luminance, and other statistics. The calculation unit 303 can determine whether or not the statistic regarding the variation in luminance among the plurality of pixels for which the luminance has been measured satisfies the first criterion. Further, the calculation unit 303 can determine whether or not the average value of the luminance among the plurality of pixels for which the luminance has been measured satisfies the second criterion. Further, the calculation unit 303 can calculate the predicted collection efficiency based on at least one or both of the statistic and the average value by using a model for estimating the collection efficiency.

[0073] The display unit 304 can display an image of the reflected light pattern and an image of the transmitted light pattern based on the image data of the reflected light pattern and the image data of the transmitted light pattern generated by the image processing unit 301. The display unit 304 can display a statistic regarding the variation in luminance among the plurality of pixels for which the luminance has been measured, and the average value of the luminance. The display unit 304 can display various graphs such as a luminance histogram. The display unit 304 can display the result of whether or not the first criterion determined by the calculation unit 303 is satisfied. The display unit 304 can display the result of whether or not the second criterion determined by the calculation unit 303 is satisfied. The display unit 304 can display the predicted collection efficiency calculated by the calculation unit 303. The display unit can be configured by a display such as an LCD or an organic EL.

[0074] In one embodiment, the inspection apparatuses 10, 20 can include a housing 13 for accommodating the light irradiators 12a, 12b, the cameras 16, and the columnar honeycomb filter 100. The housing 13 can be configured to block external light. Thereby, inspection can be performed in a dark environment, and the inspection accuracy can be improved.

[0075] The light irradiator 12a is installed at a position where it can irradiate the first light onto the inlet-side bottom surface 104 of the columnar honeycomb filter 100. In the inspection apparatus 10 shown in FIGS. 5-1 and 5-2, the light irradiator 12a is installed above the inlet-side bottom surface 104 of the columnar honeycomb filter 100 and is configured to irradiate the first light downward. The light from the light irradiator 12a is preferably irradiated isotropically onto the inlet-side bottom surface 104. For this reason, for example, a method of irradiating light directly above the inlet-side bottom surface 104 using ring illumination or coaxial illumination as the light irradiator 12a, and a method of symmetrically irradiating light onto the inlet-side bottom surface 104 with the central axis of the columnar honeycomb filter 100 as the center of symmetry using one or more pairs of light irradiators 12a as shown in the drawing are preferable.

[0076] The light irradiator 12b is installed at a position where it can irradiate the second light onto the outlet-side bottom surface 106 of the columnar honeycomb filter 100. In the inspection apparatus 20 shown in FIGS. 5-1 and 5-2, the light irradiator 12b is configured to irradiate the second light upward. And the columnar honeycomb filter 100 is configured to be arranged directly above the light source of the light irradiator 12b with the outlet-side bottom surface 106 positioned downward. Regarding the light source, although it also depends on the diffusion angle of the light from the light source and the distance from the outlet-side bottom surface 106, from the viewpoint of irradiating the outlet-side bottom surface 106 with uniform light, it is preferable that the light source facing the outlet-side bottom surface 106 spreads over an area range larger than the outlet-side bottom surface 106. For example, it is possible to irradiate light with an output such that the illuminance of the outlet-side bottom surface 106 is 10,000 lx or more.

[0077] The light sources of the light irradiators 12a and 12b are not particularly limited, and examples thereof include LEDs, incandescent bulbs, halogen lamps, etc. These light sources can generally irradiate diffused light. There is also no particular limitation on the wavelength of the light to be irradiated, as long as it is a wavelength at which the camera has light reception sensitivity. Therefore, it is also possible to irradiate white light. There is no particular limitation on the output of the light to be irradiated either, but when the height of the columnar honeycomb filter 100 is large and when the eye-sealing portion is deep, it is preferable to have a strong output in order to ensure the transmitted light intensity with a short exposure time or to mitigate the influence of disturbances.

[0078] In one embodiment, the columnar honeycomb filter 100 may be placed in contact with the light irradiator 12b. Contacting the columnar honeycomb filter 100 with the light source is advantageous from the viewpoints of the intensity of the transmitted light and the uniformity of the light. Further, in another embodiment, the columnar honeycomb filter 100 may be arranged, for example, with a light-transmissive substrate sandwiched therebetween so as not to be in contact with the light irradiator 12b. Examples of the light-transmissive substrate include a transparent substrate and a translucent substrate, but a translucent substrate such as ground glass is preferable in order to eliminate the non-uniformity of the light from the light source.

[0079] The camera 16 is disposed at a position where it can image the pattern of the reflected light. When the first light is irradiated from the light irradiator 12a onto the entrance-side bottom surface 104 of the columnar honeycomb filter 100, the camera 16 can image the pattern of the reflected light from the entrance-side bottom surface 104 according to the arrangement of the first cell 108 and the second cell 110. In the inspection apparatus 10 shown in FIGS. 5-1 and 5-2, the camera 16 is disposed above, preferably directly above, the entrance-side bottom surface 104 of the columnar honeycomb filter 100, and the lens 17 is directed toward the entrance-side bottom surface 104, that is, downward. The camera 16 for imaging the pattern of the reflected light may be either an area camera or a line camera, but an area camera is preferred for reasons such as high imaging tact, wide illumination width, and small equipment size. From the viewpoint of improving the inspection accuracy, the camera for imaging the pattern of the reflected light preferably has a pixel resolution of 40 μm / pix or less (the horizontal and vertical lengths of one pixel are 40 μm or finer), and can typically have a pixel resolution of 5 to 40 μm / pix, more typically 20 to 40 μm / pix.

[0080] Based on the imaging result by the camera 16, the computer 300 can generate image data of the pattern of the reflected light. Next, the computer 300 identifies the position information of the openings 107 of the plurality of first cells 108 based on the generated image data of the pattern of the reflected light, and stores the identified position information in the storage device 302.

[0081] Further, the camera 16 is disposed at a position where it can image the pattern of transmitted light. The camera 16 can image the pattern of transmitted light from the entrance-side bottom surface 104 according to the arrangement of the first cell 108 and the second cell 110 when the second light is irradiated from the light irradiator 12b to the exit-side bottom surface 106 of the columnar honeycomb filter 100. In the inspection apparatus 20 shown in FIGS. 5-1 and 5-2, the camera 16 is disposed above, preferably directly above, the entrance-side bottom surface 104 of the columnar honeycomb filter 100, and the lens 17 is directed toward the entrance-side bottom surface 104, that is, downward. The camera for imaging the pattern of transmitted light may be either an area camera or a line camera, but an area camera is preferred for reasons such as high imaging tact, wide illumination width, and small equipment size. From the viewpoint of improving inspection accuracy, the camera for imaging the pattern of transmitted light preferably has a pixel resolution of 40 μm / pix or less (the horizontal and vertical lengths of one pixel are 40 μm or finer), and typically can have a pixel resolution of 5 to 40 μm / pix, more typically 20 to 40 μm / pix.

[0082] The camera that images the pattern of transmitted light may be the same as or different from the camera that images the pattern of reflected light. However, when switching between the camera that images the pattern of transmitted light and the camera that images the pattern of reflected light, if the angles of both cameras are different, it is necessary to perform position correction by image processing so that the cell positions recognized by both cameras match. In the inspection apparatus shown in FIGS. 5-1 and 5-2, the camera that images the pattern of transmitted light is the same as the camera that images the pattern of reflected light.

[0083] When imaging the pattern of transmitted light with the camera 16, the light diffusion film 14 is used. By imaging the pattern of transmitted light from the entrance-side bottom surface 104 through the light diffusion film 14, a wide-range transmitted light image can be obtained due to the light diffusion effect, and it is also possible to obtain a transmitted light image of the entire end face at once.

[0084] It is difficult to image the pattern of the reflected light from the entrance-side bottom surface 104 through the light diffusion film 14 with the camera 16. Therefore, when imaging the pattern of the reflected light, it is desirable to avoid the light diffusion film 14 from the entrance-side bottom surface 104 so as not to interfere with the imaging.

[0085] In the configuration of the inspection apparatus shown in FIG. 5-2, the light diffusion film 14 is arranged parallel to the entrance-side bottom surface 104 in a non-contact state with the entrance-side bottom surface 104 of the columnar honeycomb filter 100. The light diffusion film 14 may be in contact with the entrance-side bottom surface 104, but it is preferable to arrange it in a non-contact state because the light diffusion film 14 is not damaged by contact with the entrance-side bottom surface 104. The fact that the light diffusion film 14 is arranged "parallel" to the entrance-side bottom surface 104 is a concept that includes not only strict mathematical parallelism but also approximate parallelism within a range that has no substantial influence on the inspection accuracy. Exemplarily, when the average angle formed by the light diffusion film 14 and the entrance-side bottom surface 104 is 0° to 5°, it is included in the concept of parallelism here.

[0086] From the viewpoint of improving the inspection efficiency, it is preferable to arrange the light diffusion film 14 so as to cover the entire entrance-side bottom surface 104. For this reason, in one embodiment, the area of the main surface of the light diffusion film 14 is larger than the area of the entrance-side bottom surface 104 of the columnar honeycomb filter 100.

[0087] There is no particular limitation on the distance between the light diffusion film 14 and the entrance-side bottom surface 104. However, if it is too close, there is a risk of contact due to vibration or the like. Therefore, the lower limit of the distance is preferably 1 mm or more, more preferably 2 mm or more, and even more preferably 3 mm or more. Also, if the distance between the light diffusion film 14 and the entrance-side bottom surface 104 is too far, the transmitted light of adjacent cells overlaps and the inspection accuracy decreases. Therefore, the upper limit of the distance is preferably 100 mm or less, more preferably 30 mm or less, and even more preferably 15 mm or less. In this specification, the distance between the light diffusion film 14 and the entrance-side bottom surface 104 refers to the length until the straight line extended in the normal direction from the center of gravity of the entrance-side bottom surface 104 contacts the light diffusion film 14.

[0088] From the perspective of facilitating inspection, the diffusion angle of the light diffusion film 14 is preferably 10° to 90°, more preferably 20° to 60°, and even more preferably 20° to 30°. The diffusion angle of the light diffusion film 14 can be adjusted, for example, by a method of adding a light scattering medium inside the film, a method of adjusting the surface roughness, etc. In this specification, the diffusion angle of the light diffusion film 14 is defined as the angle at which the brightness (illuminance) is reduced by half with respect to the direction (the normal direction of the film surface) showing the maximum brightness (illuminance) when white light is irradiated perpendicularly to the film surface. The angle at which the brightness (illuminance) is reduced by half can be specified by using an illuminometer equipped with a pinhole that is sufficiently smaller than the irradiation range of white light in front, and searching for the angle at which the brightness is reduced by half with the same irradiation position and measurement distance of white light to the light diffusion film (14).

[0089] There is no particular limitation on the thickness of the light diffusion film 14, but if it is too thick, the transmitted light of adjacent cells overlaps and the inspection accuracy decreases. Therefore, it is preferably 50 mm or less, more preferably 3 mm or less, and even more preferably 1 mm or less. If the thickness of the light diffusion film 14 is too thin, the strength is insufficient and wrinkles, bending, or breakage are likely to occur. Therefore, it is preferably 0.2 mm or more, and more preferably 0.5 mm or more.

[0090] There is no particular limitation on the material of the light diffusion film 14, but from the perspective of performing stable inspection, it is preferably a hard material so that wrinkles and bending do not occur. For example, glass, plastic (such as polycarbonate), etc. can be mentioned.

[0091] In order to stably fix the light diffusion film 14 in the apparatus, in one embodiment, the inspection apparatus can include at least one of the following instruments (a) to (c). (a) A tension device having at least a pair of gripping parts for applying tensile stress to the light diffusion film. (b) Two transparent plates for sandwiching the light diffusion film. (c) A transparent plate for attaching a light diffusion film.

[0092] In the configuration of the inspection apparatus 20 shown in FIG. 5-2, a tension device 18 having a pair of gripping portions 18a is provided. For example, the tension device 18 can be configured such that a pair of gripping portions 18a sandwich opposite end portions of the light diffusion film 14 and can pull in opposite directions. By applying a tensile stress to the light diffusion film 14, it is possible to prevent wrinkles and bending from occurring even when the light diffusion film 14 is thin or soft, so that a stable inspection can be carried out. Further, the tension device 18 can further include a pair of gripping portions 18a so that it can also pull simultaneously in the in-plane direction orthogonal to the pulling direction by the pair of gripping portions 18a. As a result, the light diffusion film 14 is pulled in four directions, so that the light diffusion film 14 can be held more stably.

[0093] In the configuration of the inspection apparatus shown in FIG. 5-2, each gripping portion 18a has a clamp mechanism including a pair of gripping plates facing each other, one above the other. In one embodiment, the tension device 18 can include driving means for transmitting a pressing force for gripping the light diffusion film 14 and a tensile stress for pulling the light diffusion film 14 outward to the gripping portion 18a. As the driving means, any known means can be adopted. For example, as the driving means for transmitting the pressing force, a spring or an electric cylinder or the like can be used, and as the driving means for transmitting the tensile stress, a turnbuckle or an electric cylinder or the like can be used.

[0094] When the camera 16 images the pattern of the transmitted light from the entrance - side bottom surface 104, if the outer surface of the outer peripheral side wall 102 of the columnar honeycomb filter 100 is illuminated by the light from the light irradiator 12b or the light from outside the inspection apparatus, the luminance of the opening 107 of the first cell 108 near the outer peripheral side wall 102 tends to become bright, and there is a possibility that the inspection accuracy may decrease. Therefore, when imaging the pattern of the transmitted light from the entrance - side bottom surface 104 with the camera 16, it is preferable to circumferentially cover the outer peripheral side wall 102 of the columnar honeycomb filter 100 with a light - shielding annular member 15. The light - shielding annular member 15 is preferably made of an elastic material such as rubber and elastomer so as to be able to flexibly correspond to the size of the columnar honeycomb filter 100.

[0095] As the light - shielding annular member 15, a balloon chuck can be preferably used. When using a balloon chuck, the pressing force from the balloon is likely to be dispersed over the entire contact surface with the columnar honeycomb filter 100, and it is difficult for a large local pressure to be applied. Therefore, the columnar honeycomb filter 100 is less likely to be damaged during fixation. In addition, the pattern of the reflected light from the entrance - side bottom surface 104 may be imaged with the camera 16 in a state where the outer peripheral side wall 102 of the columnar honeycomb filter 100 is circumferentially covered with the light - shielding annular member 15, but it is not particularly necessary. A preferred embodiment of the balloon chuck is as described in Japanese Patent Application Laid - Open No. 2021 - 156775.

Example

[0096] Hereinafter, examples for better understanding of the present invention and its advantages are illustrated, but the present invention is not limited to the examples.

[0097] <Test 1: Verification of the correlation between the standard deviation of luminance and the PM collection efficiency> (1 - 1. Manufacture of columnar honeycomb filter) According to the conventional method, extrusion molding, drying, degreasing, and firing of the green body were carried out to produce a large number of cordierite columnar honeycomb structures having eye-sealing portions. In these columnar honeycomb structures, the eye-sealing portions were formed such that the first cells and the second cells were alternately and adjacently arranged with a porous partition wall interposed therebetween. The specifications of these columnar honeycomb structures are common as follows and are intended to be given the same part number. Overall shape: cylindrical with a diameter of 132 mm and a height of 120 mm Cell shape in a cross-section perpendicular to the flow path direction of the cell: square Cell density (number of cells per unit cross-sectional area): 200 cells / inch 2 Average thickness of the partition wall: 8 mil (0.203 mm) (nominal value based on the specifications of the base metal) Porosity of the porous partition wall: 55% Porosity of the porous membrane: 80%

[0098] Next, a porous membrane was formed on the surfaces of a plurality of first cells of each columnar honeycomb structure to obtain a columnar honeycomb filter. The method for forming the porous membrane is as follows. First, while injecting an aerosol containing ceramic particles in a direction perpendicular to the inlet-side bottom surface toward the center of the inlet-side bottom surface of the columnar honeycomb structure, a suction force was applied to the outlet-side bottom surface to suck the injected aerosol from the inlet-side bottom surface and deposit ceramic particles made of SiC on the surfaces of the first cells. Thereafter, a porous membrane was formed by heat-treating the columnar honeycomb structure having ceramic particles deposited on the surfaces of the first cells.

[0099] In the above manufacturing process, when carrying out the step of attaching ceramic particles, the suction flow rate and suction time of the aerosol were varied. As a result, a number of columnar honeycomb filters with different in-plane distributions of the thickness of the attached porous film were obtained while keeping the mass of the porous film of the columnar honeycomb filter constant. For each of the columnar honeycomb filters, the total mass of the porous film from the outlet-side bottom surface of the first cell to the midpoint of the columnar honeycomb filter in the extending direction of the first cell was greater than the total mass of the porous film from the inlet-side bottom surface of the first cell to the midpoint of the columnar honeycomb filter in the extending direction of the first cell.

[0100] (1-2. Luminance measurement test) For each of the many columnar honeycomb filters manufactured above, the luminance was measured according to the following procedure. First, the outer peripheral side wall 102 of the columnar honeycomb filter 100 was circumferentially covered with a light-shielding annular member 15 (balloon chuck), and then it was set in the inspection apparatus 10 having the configuration shown in FIG. 5-1. Next, using a pair of light irradiators 12a, first light was symmetrically irradiated onto the inlet-side bottom surface 104 with the central axis of the columnar honeycomb filter 100 as the center of symmetry. As the light source of the light irradiator 12a, ring-shaped LED illumination was used. While irradiating the first light, the pattern of the reflected light from the inlet-side bottom surface 104 according to the arrangement of the first cell 108 and the second cell 110 was imaged with a camera 16 having a pixel resolution of 30 μm / pix in both the horizontal and vertical directions, and image data of the pattern of the reflected light was generated. Based on the generated image data of the pattern of the reflected light, the position information of the openings 107 of all the first cells 108 of the columnar honeycomb filter 100, specifically, the information of the two-dimensional coordinates of the pixels constituting the contour of the opening 107 (defined by the porous partition wall 112 excluding the porous film 114) was specified by binary processing, and the position information of the openings 107 of the plurality of specified first cells 108 was stored in the storage device 302 of the computer 300.

[0101] Next, while the honeycomb filter 100 was set in the inspection apparatus 10, the light diffusion film 14 was placed on the inlet-side bottom surface 104 and the configuration was changed to that of the inspection apparatus 20 shown in Fig. 5-2. At this time, the distance between the light diffusion film 14 and the inlet-side bottom surface 104 was set to 5 mm. As the light diffusion film 14, one with a thickness of 0.75 mm, a diffusion angle of 20°, and a material of polycarbonate was used.

[0102] Next, using the light irradiator 12b, the second light was irradiated onto the outlet-side bottom surface 106 of the honeycomb filter 100. The light irradiator 12b has a light source facing the outlet-side bottom surface 106 that spreads over an area range larger than the outlet-side bottom surface 106, and irradiated light with an output such that the illuminance of the outlet-side bottom surface 106 becomes 70,000 [lx]. As the light source of the light irradiator 12b, LED surface illumination was used. While irradiating the second light, the pattern of the transmitted light from the inlet-side bottom surface 104 according to the arrangement of the first cell and the second cell was imaged by the same camera 16 as the camera 16 that captured the pattern of the reflected light through the light diffusion film 14, and image data of the pattern of the transmitted light was generated.

[0103] Based on the generated image data of the pattern of the transmitted light and the position information stored in the storage device 302, the luminance of a plurality of pixels located within the openings 107 of the plurality of first cells 108 was measured. Here, for all the first cells 108 of the honeycomb filter 100, the luminance of all the pixels located within the openings 107 of each first cell 108 was set as the measurement target of the measurement luminance. After that, the standard deviation of the luminance of all the pixels for which the luminance was measured was obtained.

[0104] (1-3. PM Collection Test) Regarding the numerous columnar honeycomb filters manufactured above, the following PM collection tests were respectively carried out. An aerosol containing oil particles such as DEHS (bis(2-ethylhexyl) sebacate) particles with a particle size of about 100 to 1000 nm injected from an aerosol generator was supplied to the columnar honeycomb filter at a flow rate of 500 to 10,000 L / min for 30 seconds to collect PM on the columnar honeycomb filter, and the PN (number of emitted particles) in the exhaust gas was measured by a PN counter at the inlet side (upstream side in the gas flow direction) and the outlet side (downstream side in the gas flow direction) of the columnar honeycomb filter. The collection efficiency was calculated by the formula (number of particles on the inlet side - number of particles on the outlet side) / number of particles on the inlet side × 100 (%).

[0105] (1-4. Evaluation of correlation) With the collection efficiency (%) on the horizontal axis and the standard deviation of luminance on the vertical axis, the data obtained from the luminance measurement test and the PM collection test were plotted on a two-dimensional coordinate system. The results are shown in Fig. 7. Fig. 7 shows an approximate straight line created based on the least squares method. From Fig. 7, it can be seen that there is a high correlation (determination coefficient R 2 = 0.8769) between the standard deviation of luminance and the collection efficiency. Therefore, for example, if an inspection is carried out with the standard deviation of luminance being 15 or less as the first criterion, columnar honeycomb filters with a high possibility of insufficient collection efficiency can be excluded.

[0106] <Test 2: Verification of the correlation between the average value of luminance and the PM collection efficiency> (2-1. Manufacture of columnar honeycomb filters) Numerous columnar honeycomb structures having the same specifications as in Test 1 were manufactured.

[0107] Next, a porous membrane was formed on the surfaces of the plurality of first cells of each columnar honeycomb structure to obtain a columnar honeycomb filter. The method for forming the porous membrane is as follows. First, while injecting an aerosol containing ceramic particles in a direction perpendicular to the inlet-side bottom surface toward the center of the inlet-side bottom surface of the columnar honeycomb structure, a suction force was applied to the outlet-side bottom surface to suck the injected aerosol from the inlet-side bottom surface and attach ceramic particles made of SiC to the surfaces of the first cells. Thereafter, a porous membrane was formed by heat-treating the columnar honeycomb structure having ceramic particles attached to the surfaces of the first cells.

[0108] In the above manufacturing process, when carrying out the step of attaching ceramic particles, the suction time of the aerosol was variously changed. As a result, a number of columnar honeycomb filters with different masses of the attached porous membrane were obtained. For any of the columnar honeycomb filters, the total mass of the porous membrane from the outlet-side bottom surface of the first cell to the midpoint of the columnar honeycomb filter in the extending direction of the first cell was greater than the total mass of the porous membrane from the inlet-side bottom surface of the first cell to the midpoint of the columnar honeycomb filter in the extending direction of the first cell.

[0109] (2-2. Luminance measurement test) Regarding the number of columnar honeycomb filters manufactured above, for all the first cells of each columnar honeycomb filter, in the same procedure as in Test 1, the luminance of all the pixels located within the openings of each first cell was measured, and the average value of the luminance of all the pixels for which the luminance was measured was obtained.

[0110] (2-3. Mass measurement of porous membrane) Regarding the number of columnar honeycomb filters manufactured above, the mass of the attached porous membrane was measured from the mass difference between the columnar honeycomb structure before the formation of the porous membrane and the columnar honeycomb filter after the formation of the porous membrane.

[0111] (2-4. PM collection test) For some of the numerous columnar honeycomb filters manufactured above, a PM collection test was carried out in the same procedure as in Test 1, and the collection efficiency was calculated.

[0112] (2-5. Evaluation of correlation) With the mass of the porous membrane on the horizontal axis and the average luminance on the vertical axis, the data obtained from the luminance measurement test, the mass measurement of the porous membrane, and the PM collection test were plotted on a two-dimensional coordinate system. The results are shown in Fig. 8. From Fig. 8, it can be seen that there is a high correlation (coefficient of determination R 2 = 0.9479) between the mass of the porous membrane and the average luminance. Also, for the columnar honeycomb filter with an average luminance of about 70 - 80 (the mass of the porous membrane is about 3.3 g), the collection efficiency was about 67%, while for the columnar honeycomb filter with an average luminance of about 35 (the mass of the porous membrane is about 6.6 g), the collection efficiency was about 97%. From this, it can be seen that there is also a significant correlation between the average luminance and the collection efficiency. Therefore, for example, if an inspection is carried out by setting the condition that the average luminance is 40 or less as the second criterion, columnar honeycomb filters with a high possibility of insufficient collection efficiency can be excluded. In combination with the first criterion set in Test 1, it may be determined as a qualified product only when both the first criterion and the second criterion are satisfied.

Explanation of symbols

[0113] 10: Inspection device 12a: Light irradiator 12b: Light irradiator 13: Housing 14: Light diffusion film 15: Light-shielding annular member 16: Camera 17: Lens 18: Tension device 18a: Gripping part 20: Inspection device 100: Columnar honeycomb filter 102: Outer peripheral side wall 104: Inlet side bottom surface 106: Outlet side bottom surface 107: Opening 108: First cell 109: Eye stopper part 110: Second cell 112: Porous partition wall 114: Porous membrane 300: Computer 301: Image processing unit 302: Memory device 303: Arithmetic unit 304: Display unit

Claims

1. A method for inspecting a columnar honeycomb filter, wherein the columnar honeycomb filter includes: a plurality of first cells extending from an inlet-side bottom surface to an outlet-side bottom surface, having an opening at the inlet-side bottom surface, and having a plugging portion at the outlet-side bottom surface; and a plurality of second cells extending from the inlet-side bottom surface to the outlet-side bottom surface, having a plugging portion at the inlet-side bottom surface, and having an opening at the outlet-side bottom surface, and the plurality of first cells and the plurality of second cells are alternately and adjacently arranged with a porous partition wall therebetween, a porous film is formed on the surface of each of the first cells, Step 1 of irradiating the inlet-side bottom surface with first light, imaging, with a camera, a pattern of reflected light from the inlet-side bottom surface corresponding to the arrangement of the first cells and the second cells, and generating image data of the pattern of the reflected light; Step 2 of storing, in a storage device, position information of openings of the plurality of first cells identified based on the generated image data of the pattern of the reflected light; Step 3 of imaging, with a camera through a light diffusion film arranged parallel to the inlet-side bottom surface, a pattern of transmitted light from the inlet-side bottom surface obtained by irradiating the outlet-side bottom surface with second light, and corresponding to the arrangement of the first cells and the second cells, and generating image data of the pattern of the transmitted light; Step 4 of measuring the luminance of a plurality of pixels located within the openings of the plurality of first cells based on the generated image data of the pattern of the transmitted light and the position information stored in the storage device; Step 5 of obtaining a statistic regarding the variation in luminance among the plurality of pixels for which the luminance has been measured based on the result of Step 4; Step 6 of comparing the statistic with a first criterion regarding the statistic determined in advance; and the inspection method includes the above steps.

2. The inspection method according to Claim 1, wherein the total mass of the porous film from the outlet-side bottom surface of the first cell to the midpoint of the columnar honeycomb filter in the extending direction of the first cell is greater than the total mass of the porous film from the inlet-side bottom surface of the first cell to the midpoint of the columnar honeycomb filter in the extending direction of the first cell.

3. The inspection method according to Claim 1 or 2, wherein the statistic is the standard deviation of luminance.

4. The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, and the first criterion is determined based at least on a correlation between the statistic and information regarding the collection efficiency of particulate matter in exhaust gas under predetermined conditions, which has been obtained in advance for columnar honeycomb filters of the same product number as the product number of the columnar honeycomb filter to be inspected, and the inspection method according to Claim 1 or 2.

5. The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, The inspection method according to claim 1 or 2, further comprising step 7 of estimating the collection efficiency from the statistic amount, based at least on the correlation between the statistic amount obtained in advance for columnar honeycomb filters with the same product number as the said product number and the information on the collection efficiency of particulate matter in exhaust gas under predetermined conditions.

6. Step 8 of obtaining the average value of the luminance in a plurality of pixels in which the luminance is measured, based on the result of step 4; Step 9 of comparing the average value with a second standard regarding the average value of the predetermined luminance; The inspection method according to claim 1 or 2, further comprising these steps.

7. The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, The second standard is determined based at least on the correlation between the average value and the information on the mass of the porous membrane obtained in advance for columnar honeycomb filters with the same product number as the said product number, or the correlation between the average value and the information on the collection efficiency of particulate matter in exhaust gas under predetermined conditions obtained in advance for columnar honeycomb filters with the same product number as the said product number. The inspection method according to claim 6.

8. The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, The inspection method according to claim 6, further comprising step 10 of estimating the mass of the porous membrane from the average value, based at least on the correlation between the average value and the information on the mass of the porous membrane obtained in advance for columnar honeycomb filters with the same product number as the said product number.

9. The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, The inspection method according to claim 6, further comprising step 11 of estimating the collection efficiency from the average value, based at least on the correlation between the average value and the information on the collection efficiency of particulate matter in exhaust gas under predetermined conditions obtained in advance for columnar honeycomb filters with the same product number as the said product number.

10. The columnar honeycomb filter to be inspected is a columnar honeycomb filter to which a specific product number should be assigned, Step 8 of obtaining the average value of the luminance in a plurality of pixels in which the luminance is measured, based on the result of step 4; Based on at least the correlation between the statistical quantity obtained in advance for the columnar honeycomb filter with the same part number and the information on the collection efficiency of particulate matter in the exhaust gas under predetermined conditions, and the correlation between the average value obtained in advance for the columnar honeycomb filter with the same part number and the information on the collection efficiency of particulate matter in the exhaust gas under predetermined conditions, the inspection method according to claim 1 or 2, further comprising step 12 of estimating the collection efficiency from the statistical quantity and the average value.

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