Methods for measurement correction in particle volume flow measurements using shadow photography

Abstract

Method for measurement correction in particle volume flow measurements using shadow photography, the method comprising the following steps: - Calibrating a shadow photography recording device; - Generating high-resolution shadow photographs of a particle volume flow using the calibrated shadow photograph recording device; - Evaluating the high-resolution shadow photographs; - Calculating measured values ​​of the particle volume flow rate; and - Correcting calculated measured values ​​of the particle volume flow rate taking into account the value of the gradient slope GS, wherein the value of the gradient slope GS corresponds to the value of a normalized intensity decrease per pixel at a detected particle edge or point edge, the method characterized in that the calibration step comprises: - Determining a focal plane of the shadow photography recording device, in particular along a lens axis of the shadow photography recording device; - Providing a calibration device with a multitude of calibration points with predefined calibration point diameters d p real ; - Generating high-resolution shadow photographs of the calibration device at predefined intervals. i to the focal plane of the shadow photography recording device; and - For each of the multitude of calibration points with predefined calibration point diameters d p real and for each predefined distance z i From the focal plane: Determining calibration point shadows and determining measured calibration point diameters d p roh the calibration point shadows and determining a gradient slope GS of the respective calibration point shadows; - Determining a maximum gradient slope GS max for each of the calibration point shadows with predefined calibration point diameters d p real , where the maximum gradient slope GS max an upper limit for the gradient slope GS of the calibration point shadows as a function of the predefined calibration point diameter d p real , corresponds; - Determining a ratio between the predefined calibration point diameters d p real , the measured calibration point diameters d p roh and the gradient slope GS based on the multitude of predefined calibration point diameters d p real and the corresponding measured calibration point diameter d p roh and the corresponding gradient slopes GS.

Description

[0001] The present invention relates to a method for correcting measured values ​​when measuring a particle volume flow using shadow photography.

[0002] Determining the particle mass concentration within a flow requires precise knowledge of the particle number, particle size, particle velocity, and particle density of the individual particles, as well as precise determination of the measurement volume. With regard to the investigation of particle-sample interactions in supersonic flows, the shadow photography method offers many advantages.

[0003] In state-of-the-art shadow photography methods, particle size is determined by binarizing the raw image of the particle shadows at a given threshold value and then measuring the remaining spots or shadows as a two-dimensional image of the particles. The measured particle size depends on the selected threshold value for binarization. Alternatively, a normalized contrast value and the so-called point spread function width (PSF) are used instead of binarization.

[0004] A disadvantage of existing methods is that particle shadows captured in shadow photography become blurred and appear larger if they lie outside the focal plane of the shadow photography device. This leads, for example, to blurred particles being assumed to be too large.

[0005] Furthermore, these effects depend on the particle size, which means that the measurement volume at which particles have the required sharpness also depends on the particle size.

[0006] A method with the preamble features of claim 1 has already been disclosed in LEE, Sang Yong; KIM Yu Dong: Sizing of spray particles using image processing technique. In: KSME international journal, Vol. 18, 2004, No. 6, pp. 879-894.

[0007] From JP 2009 - 300 182 A a calibration device has become known for calibrating an optical measuring device for measuring a particle size distribution.

[0008] From NISHINKO, K. [et al.]: Stereo imaging for simultaneous measurement of size and velocity of particles in dispersed two-phase flow. In: Meas. Sci. Techn., Vol. 11, 2000, pp. 633-645, a stereo imaging technique for the simultaneous measurement of size and velocity of solid / liquid particles in dispersed two-phase flows has become known.

[0009] The object of the present invention is to provide a method for correcting measured values ​​which corrects the determined particle size and the measuring volume depending on the particle size.

[0010] A further object of the present invention is to compensate for additional influences of the selected parameters, such as the above-mentioned limit value for binarization.

[0011] Furthermore, it is an object of the present invention to provide a method for correcting measured values ​​which takes into account the counting rate of the individual particle shadows and determines a measuring volume in which the counting rate of the particles is maximum or in which detection of all particles to be measured is ensured.

[0012] The problem is solved by a method according to claim 1.

[0013] The method according to the invention for correcting measured values ​​when measuring a particle volume flow using shadow photography comprises the following steps: - Calibrating a shadow photography recording device; - Generating high-resolution shadow photography images of a particle volume flow using the calibrated shadow photography recording device; - Evaluation of high-resolution shadow photography images; - Calculating measured values ​​of the particle volume flow; and - Correction of calculated measured values ​​of the particle volume flow taking into account the value of the gradient slope GS, where the value of the gradient slope GS corresponds to the value of a normalized intensity decrease per pixel at a detected particle edge or point edge.

[0014] The correction method according to the invention can improve the accuracy of the calculated measured values ​​of the particle volume flow.

[0015] Preferably, the method according to the invention for correcting measured values ​​when measuring a particle volume flow by means of shadow photography is a computer-implemented method.

[0016] Preferably, calculating measured values ​​of the particle volume flow comprises calculating a particle number, a particle concentration, a measured particle diameter and / or a particle velocity.

[0017] In particular, the calculation of measured values ​​of the particle volume flow is not limited to the calculation of the particle number, the particle concentration, the measured particle diameter, and / or the particle velocity; additional or different measured values ​​can also be determined. This allows the selection of the calculated measured values ​​to be chosen according to the current requirements. This increases the flexibility and efficiency of the process.

[0018] According to the invention, the calibration step comprises the following steps: - determining a focal plane of the shadow photography recording device, in particular along a lens axis of the shadow photography recording device; - Providing a calibration device with a plurality of calibration points with predefined calibration point diameters d p real ; - Generate high-resolution shadow photography images of the calibration device at predefined intervals z i to the focal plane of the shadow photography recording device; and - for each of the plurality of calibration points with predefined calibration point diameters d preg , and for each predefined distance z i from the focal plane: Determining calibration point shadows and determining measured calibration point diameters d p roh the calibration point shadows and determining a gradient inclination GS of the respective calibration point shadows; - Determination of a maximum gradient inclination GS max for each of the calibration point shadows with predefined calibration point diameters d p real , where the maximum gradient slope GS maxan upper limit for the gradient inclination GS of the calibration point shadows depending on the predefined calibration point diameter d p real corresponds; - Determine a ratio between the predefined calibration point diameters d p real , the measured calibration point diameters d p roh and the gradient inclination GS based on the multitude of predefined calibration point diameters d p real and the corresponding measured calibration point diameter d p roh and the corresponding gradient inclinations GS.

[0019] Preferably, the ratio of the predefined calibration point diameter d p real and the corresponding measured calibration point diameter d p roh with gradient slope GS and parameters a, b and c from: dprohdpreal=a+b∗e−c∗GSGSmax, a,b,c=f(dpreal). Preferably, the parameters a, b and c depend on the respective predefined calibration point diameter d p real .

[0020] Preferably, the parameters a, b and c result from a compensation calculation or adjustment, in particular a two-dimensional adjustment of the determined ratio from the predefined calibration point diameters d p real and corresponding measured particle diameters d p roh . In other words, the parameters a, b, c are determined by a fit from the determined ratio of the predefined calibration point diameters d p real and corresponding measured calibration point diameter d p roh determined.

[0021] Preferably, the maximum gradient slope GS max with parameters f, g, h and i: GSmax=f+g−f1+(dprealh)i. Preferably, the parameters f, g, h and i are derived from a calibration calculation or adjustment. In particular, the parameters f, g, h and i are derived from a fit of the determined ratio of the predefined calibration point diameters d p real and corresponding maximum measured gradient inclination GS max determined.

[0022] Preferably, the calibration further comprises: - for each of the plurality of predefined calibration point diameters d p real : Determination of a corresponding parameter range, in particular based on the determined ratio of the predefined calibration point diameter d p real and the corresponding measured calibration point diameter d p roh and the corresponding gradient inclination GS, where each measured calibration point diameter d p rohwith corresponding gradient inclination GS, which is within this corresponding parameter range, a corrected calibration point diameter d p korrigiert where the value of the corrected calibration diameter d p korrigiert the value of the respective predefined calibration point diameter d p real this corresponding parameter range; - Limitation of the determined, corresponding parameter ranges by a minimum gradient inclination GS min .

[0023] Preferably, the corresponding parameter range has a predefined width.

[0024] The width of the calibration point diameter d p real corresponding parameter range can be defined as permissible deviation or variation of the measured calibration point diameter d p roh around the calibration point diameter d p realof the parameter range, with a lower limit of the parameter range being d p real minus half the width and an upper limit as d p real plus half the width of the parameter range.

[0025] Preferably, each corresponding parameter range for a predefined calibration point diameter d p real the same width.

[0026] Preferably, the predefined width of the corresponding parameter range has a value of substantially 2.5 µm.

[0027] Preferably, the minimum gradient slope GS min a lower limit for the gradient inclination GS, at which a measured calibration point diameter d p roh with corresponding measured gradient inclination GS clearly one of the corresponding parameter ranges of a predefined calibration point diameter d p realThis ensures that only those combinations of measured calibration point diameter d p roh and corresponding measured gradient inclination GS are included in the calibration, where a clear assignment to one of the calibration points with predefined calibration point diameters can be made.

[0028] Preferably, the value of the minimum gradient slope GS min depending on the measured calibration point diameter d p roh In particular, the value of the minimum gradient slope GS min for a smaller of the measured calibration point diameters d p roh smaller than for a larger measured calibration point diameter d p roh .

[0029] Correction preferably includes: - for each measured particle diameter d p roh: Determine a corresponding gradient slope GS where the corresponding gradient slope GS is greater than the minimum gradient slope GS min ; - Determine the parameter range within which the measured particle diameter d p roh and the corresponding gradient inclination GS; - depending on the determined parameter range: Assigning a corrected particle diameter d p korrigiert for each of the measured particle diameters d p roh , where the value of the corrected particle diameter d p korrigiert the value of the predefined calibration point diameter d p real of the parameter range.

[0030] In particular, it is assumed that the predefined calibration point diameters d p realcorrespond to the true size of the calibration points and the corresponding parameter ranges are related to the true size of the calibration points. As part of the correction, the measured particle diameters d p roh , which are located in a corresponding parameter range of a predefined calibration point diameter d p real This further improves the accuracy of the measured values.

[0031] Preferably, the accuracy of the determination of the corrected particle diameters d p korrigiert out of: εdpkorr=|∑i=1ndpcorrected,i−dprealdprealn|, σdpkorr=|∑i=1n(dpcorrected,i−dprealdpreal−εdpkorr)2|.

[0032] Preferably, the shadow photography recording device has a detection efficiency CE and the detection efficiency is given by: CE=nprohnpziel, where n p rohthe number of measured calibration point diameters for a predefined calibration point diameter d p real corresponds and n p ziel the number of calibration points for a predefined calibration point diameter d p real on a selected area of ​​the calibration device.

[0033] Preferably, the detection efficiency CE is determined on the basis of the high-resolution shadow photography images of the calibration device, in particular during calibration.

[0034] Preferably, the detection efficiency can have a value between 0% and 100%. In particular, the detection efficiency can be measured at relative intervals z i of greater than 1.5 mm have a value greater than 100%, since in this case blurred particle shadows are incorrectly split and detected as multiple particles.

[0035] Preferably, the value of the detection efficiency CE depends on the predefined distance z i to the focal plane of the shadow photography device and the size of a predefined calibration point diameter d p real . In particular, the fewer particles are detected, the greater the relative distance z i to the focal plane. Where a relative distance z i to the focal plane a distance z i in front of and / or behind the focal plane. The relative distance z i The distance to the focal plane for which the detection efficiency falls below 100% is greater for larger particles than for smaller particles. By determining the detection efficiency, a minimum particle size that can be resolved and the minimum particle size that can be detected with 100% accuracy can be determined.

[0036] Preferably, the shadow photography recording device has an effective measuring volume depth d, wherein the effective measuring volume depth d is particularly dependent on the predefined calibration point diameter d p real and / or the corrected particle diameter d p korrigiert .

[0037] Preferably, the effective measuring volume depth d GS min limited by the value of the minimum gradient inclination GS min .

[0038] Preferably, the effective measuring volume depth d GS min defined as a maximum distance range of the relative distance z i to the focal plane in which at least one calibration point with corresponding calibration point diameter d p real and corresponding gradient slope GS was found, where the corresponding gradient slope GS is greater than or equal to the minimum gradient slope GS min is.

[0039] Preferably, the effective measuring volume depth d GS min equal to or smaller than a maximum measuring volume depth d CE=100 % , where the maximum measuring volume depth d CE=100 % a maximum distance range of the relative distance z i to the focal plane in which the detection efficiency CE has a value of 100%. In other words, it can be assumed that the detection efficiency CE within the effective measurement volume depth d GS min has a value of 100%.

[0040] Preferably, the minimum gradient slope GS min a value such that the value of the effective measuring volume depth d GS min is less than or equal to the value of the maximum measuring volume depth d CE=100 % and where the detection efficiency CE within the effective measuring volume depth d GS min has a value of 100%.

[0041] Preferably, the high-resolution shadow photography recording device has a plurality of predefined image sections, and the calibration is applied to each of the plurality of predefined image sections. This allows, in particular, a spatially resolved calibration and thus, in particular, a spatially resolved determination and correction of the measured values ​​to be performed.

[0042] Preferably, the calibration device comprises a calibration plate made of a glass substrate with a plurality of calibration points at predefined positions on the calibration plate.

[0043] Preferably, the predefined calibration point diameters d p real The calibration points of the calibration device have sizes from 3 µm to 100 µm.

[0044] Preferably, a predefinition of the plurality of predefined calibration point diameters d p realto a calibration point by the corresponding measured calibration point diameter d p roh , and / or by the predefined position of the calibration point on the calibration plate. This allows, in particular, a dependency between a corresponding measured calibration point diameter d p roh and a predefinition of a calibration point diameter d p real a calibration point can be avoided.

[0045] The evaluation preferably includes: - Creating a raw image; - Normalizing the raw image, in particular using a predefined NorRad value to obtain a normalized image; - Applying a noise filter to the normalized image to obtain a noise-reduced normalized image; - binarizing the noise-reduced normalized image, in particular using a predefined threshold BiThr to obtain a binarized image; - identifying particle shadows, where a particle shadow comprises a particle face and a particle edge; - Determining a shadow size area, where the shadow size area corresponds to a number of pixels that a particle shadow contains; - Specifying a minimum shadow size area, where the minimum shadow size area corresponds to a minimum number of pixels that a particle shadow must contain to be detected as a particle shadow; and filtering the binarized image with respect to the minimum shadow size area.

[0046] Preferably, the normalization comprises: generating a reference image by applying a sliding maximum filter of a predefined size to the raw image, wherein the predefined size corresponds in particular to a predefined normalization radius NorRad and creating a normalized image by dividing the raw image by the reference image.

[0047] The invention will be explained in more detail below using preferred embodiments with reference to the accompanying drawings.

[0048] They show: Fig. 1 a flowchart of an exemplary embodiment of the method according to the invention; Fig. 2 a diagram of the measured calibration point diameters and gradient inclination to predefined calibration point diameters according to an embodiment of the method according to the invention; Fig. 3 a diagram of the parameter spaces of the predefined calibration point diameters and the measured calibration point diameters assigned to these parameter spaces with corresponding gradient inclinations according to an embodiment of the method according to the invention; Fig. 4 shows a diagram of the detection efficiency as a function of a distance to the focal plane of the shadow photography recording device and the size of a predefined calibration point diameter according to an embodiment of the method according to the invention; Fig. 5 shows a diagram of the gradient slope as a function of a distance to the focal plane of the shadow photography recording device and the size of a predefined calibration point diameter according to an embodiment of the method according to the invention; Fig. 6 a diagram of the maximum measurement volume depth and the effective measurement volume depth limited by the minimum gradient inclination as a function of the size of a predefined calibration point diameter according to an embodiment of the method according to the invention.

[0049] In the method according to the invention, shown in the Fig. 1, the procedure comprises the following steps: In step S01, a shadow photography recording device is calibrated.

[0050] According to the embodiment of the invention, calibration step S01 comprises determining a focal plane of the shadow photography recording device. In particular, the focal plane of the shadow photography recording device is determined along an objective axis of the shadow photography recording device.

[0051] According to the invention, step S01 of calibrating further comprises providing a calibration device, wherein the calibration device has a plurality of calibration points with predefined calibration point diameters d p real In other words, each of the plurality of calibration points has a predefined diameter or a predefined size. In particular, it is assumed that the predefined calibration point diameters d p realcorrespond to the true calibration point diameters. In particular, the value of a predefined calibration point diameter d p real the size or diameter of the calibration point.

[0052] According to the invention, in step S01 of calibration, high-resolution shadow photography images of the calibration device are taken at predefined intervals z i to the focal plane of the shadow photography recording device. Preferably, the generation of high-resolution shadow photography images of the calibration device takes place at predefined intervals z i in front of and / or behind the focal plane of the shadow photography device. In other words, the predefined distances z i to the focal plane, especially relative distances z i to the focal plane of the shadow photography device.

[0053] According to the invention, the high-resolution shadow photography images are then used to determine for each of the plurality of calibration points with predefined calibration point diameters d p real and for each predefined distance z i Calibration point shadows are determined from the focal plane. Furthermore, measured calibration point diameters d p roh the calibration point shadows and a determination of a gradient inclination GS of the respective calibration point shadows. In other words, based on the high-resolution shadow photography images of the calibration device, for each of the calibration points with a predefined calibration point diameter d p real a measured calibration point diameter d p roh with corresponding gradient inclination GS. In particular, the value of the gradient inclination GS is proportional to the relative distance z i from the focal plane (see Fig. 5). In particular, the size of a measured calibration point diameter d p roh from the size of the predefined calibration point diameter d p real of the respective calibration point or be identical to it.

[0054] According to the invention, for each of the calibration point shadows with predefined calibration point diameters d p real a maximum gradient inclination GS max The maximum gradient GS max corresponds to an upper limit for the gradient inclination GS of the calibration point shadows depending on the predefined calibration point diameter d p real .

[0055] According to the invention, based on the plurality of predefined calibration point diameters d p real and the corresponding measured calibration point diameter d p rohand the corresponding gradient inclinations GS a ratio between the predefined calibration point diameters d p real , the measured calibration point diameters d p roh and the respective gradient slope GS. In particular, this establishes a connection or correlation between a predefined calibration point diameter d p real a calibration point and the measured calibration point diameter d determined from the high-resolution shadow photography image p roh and the corresponding gradient inclinations GS.

[0056] According to one embodiment of the present invention, the ratio of the predefined calibration point diameter d preg , and the corresponding measured calibration point diameter d p roh with gradient slope GS and parameters a, b and c from: dprohdpreal=a+b∗e−c∗GSGSmax. a,b,c=f(dpreal).

[0057] Preferably, the parameters a, b and c can be determined from a compensation calculation or an adjustment, in particular a two-dimensional compensation calculation or a two-dimensional adjustment of the determined ratio from the predefined calibration point diameters d p real and corresponding measured calibration point diameter d p roh determine.

[0058] According to one embodiment, the maximum gradient inclination GS max with parameters f, g, h and i: GSmax=f+g−f1+(dprealh)i, where the parameters f, g, h and i also result from a calibration calculation or an adjustment. In particular, the parameters f, g, h and i are derived from a fit of the determined ratio of the predefined calibration point diameters d p real and corresponding measured calibration point diameter d p roh determined.

[0059] According to a preferred embodiment, the calibration step S01 further comprises determining a corresponding parameter range for each of the plurality of predefined calibration point diameters d p real . Preferably, the corresponding parameter range is determined based on the determined ratio of the predefined calibration point diameter d p real and the corresponding measured calibration point diameter d p roh and the corresponding gradient inclination GS, where each measured calibration point diameter d p roh with corresponding gradient inclination GS, which is within this corresponding parameter range, a corrected calibration point diameter d p korrigiert The value of the corrected calibration point diameter d p korrigiert the value of the respective predefined calibration point diameter d p realthis corresponding parameter range.

[0060] Preferably, the determined corresponding parameter ranges are limited by a minimum gradient inclination GS min In other words, the measured calibration point diameters d p roh to a predefined calibration point diameter d p real whose corresponding gradient slope GS is smaller than the minimum gradient slope GS min , are excluded from further consideration. This ensures that the assignment of a corrected calibration point diameter d p korrigiert based on a clear relationship between a predefined calibration point diameter d p real and a corresponding measured calibration point diameter d p roh This further improves the accuracy of the calibration.

[0061] Preferably, the minimum gradient slope GSmin a lower limit for the gradient inclination GS, at which a measured calibration point diameter d p roh with corresponding measured gradient inclination GS, clearly one of the corresponding parameter ranges of a predefined calibration point diameter d p real This ensures that only those combinations of measured calibration point diameter d p roh and corresponding measured gradient inclination GS are included in the calibration, where a clear assignment to one of the calibration points with predefined calibration point diameters can be made.

[0062] Preferably, the value of the minimum gradient slope GS min depending on the measured calibration point diameter d p roh In particular, the value of the minimum gradient slope GS minfor a smaller of the measured calibration point diameters d p roh smaller than for a larger measured calibration point diameter d p roh .

[0063] Preferably, each of the corresponding parameter ranges has a predefined width. Preferably, the predefined width of the respective corresponding parameter ranges has a value of substantially 2.5 µm. The predefined width is not limited to 2.5 µm and can also have a value less than or greater than 2.5 µm.

[0064] In step S02, high-resolution shadow photography images of a particle volume flow are generated using the shadow photography recording device calibrated in step S01.

[0065] In step S03, the high-resolution shadow photography images are evaluated. According to one embodiment of the present invention, step S03 of evaluation further comprises creating a raw image. This is followed by normalization of this raw image, in particular based on a predefined value NorRad to obtain a normalized image. Furthermore, to obtain a noise-reduced normalized image, a noise filter is applied to the normalized image. The noise-reduced normalized image is then binarized, in particular based on a predefined threshold value BiThr to obtain a binarized image. Particle shadows are then identified, wherein a particle shadow comprises a particle surface and a particle edge. Furthermore, the shadow size area is determined, wherein the shadow size area corresponds to a number of pixels that contain a particle shadow.Furthermore, a minimum shadow size area is defined, where the minimum shadow size area corresponds to the minimum number of pixels that must contain a particle shadow to be detected as a particle shadow. Furthermore, the binarized image is filtered with respect to the minimum shadow size area.

[0066] In step S04, the measured values ​​of the particle volume flow are calculated. The calculation of measured values ​​of the particle volume flow preferably includes the calculation of a particle number, a particle concentration, a measured particle diameter, and / or a particle velocity. In particular, the calculation of measured values ​​of the particle volume flow is not limited to the calculation of the particle number, the particle concentration, the measured particle diameter, and / or the particle velocity, and additional or different measured values ​​can also be determined. This allows the selection of the calculated measured values ​​to be selected depending on the current requirement. This increases the flexibility and efficiency of the method.

[0067] In step S05, the calculated measured values ​​of the particle volume flow are corrected taking into account the gradient slope GS. The gradient slope GS corresponds to the value of a normalized intensity decrease per pixel at a detected particle edge or point edge.

[0068] According to a preferred embodiment, the correcting step S05 further comprises the following steps: Determine a corresponding gradient slope GS for each measured particle diameter d p roh . The corresponding gradient slope GS is greater than the minimum gradient slope GS min . This means that only those measured particle diameters d p roh and corresponding gradient inclination GS are taken into account, for which a clear assignment to a predefined calibration point diameter d is made in step S01 of the calibrationp real could be made. Preferably, the parameter range is also determined within which the measured particle diameter d p roh and the corresponding gradient slope GS. In particular, each parameter range corresponds to a corresponding predefined calibration point diameter d p real .

[0069] Preferably, depending on the determined parameter range, a corrected particle diameter d is then assigned p korrigiert for each of the measured particle diameters d p roh . Here, the value of the corrected particle diameter d p korrigiert the value of the predefined calibration point diameter d p real of the parameter range. In particular, it is assumed that the predefined calibration point diameters d p realcorrespond to the true size of the calibration points and the corresponding parameter ranges are related to the true size of the calibration points. As part of the correction, the measured particle diameters d p roh , which are located in a corresponding parameter range of a predefined calibration point diameter d p real This can further improve the quality of the measurement.

[0070] Preferably, the accuracy of the determination of the corrected particle diameters d p korrigiert out of: εdpkorr=|∑i=1ndpcorrected,i−dprealdprealn|, σdpkorr=|∑i=1n(dpcorrected,i−dprealdpreal−εdpkorr)2|.

[0071] In step S05, the calculated measured values, in particular the measured particle diameter, are corrected taking into account the value of the gradient slope GS, wherein the value of the gradient slope GS corresponds to the value of the normalized intensity decrease per pixel at a detected point edge or particle edge. According to one embodiment of the present invention, the correction step S05 further comprises the following steps: determining a corresponding gradient slope GS for each measured particle diameter d p roh , where the corresponding gradient slope GS is greater than the minimum gradient slope GS min . Furthermore, the parameter range is determined within which the measured particle diameter d p roh and the corresponding gradient inclination GS are located.

[0072] Then, each of the measured particle diameters d p roh a corrected particle diameter dp korrigiert The assignment is made depending on the determined parameter range. The value of the corrected particle diameter d p korrigiert the value of the predefined calibration point diameter d p real of the corresponding parameter range.

[0073] The Fig. Figure 2 shows the relationships between sixteen different predefined calibration point diameters d determined according to an embodiment of the present invention p real with a size between 5 µm and 100 µm and the corresponding measured calibration point diameters d p roh and the corresponding gradient inclinations GS.

[0074] Furthermore, the Fig. 2, the maximum gradient inclinations GS determined according to an embodiment of the present invention max and the determined minimum gradient inclination GS min shown. In the Fig. 2 the 16 different predefined calibration point diameters d p real each represented by 16 different symbols. The value of the gradient slope GS depends on the distance of a calibration point from the focal plane of the shadow photography device. In particular, the value of the gradient slope GS decreases with increasing distance from the focal plane. As shown in the Fig. 2, it depends on the size of a predefined calibration point diameter d p real and in particular on the optical properties of the shadow photography recording device, whether the value of a measured calibration point diameter d p roh becomes larger or smaller with decreasing gradient diameter GS.

[0075] In the Fig. 2 is the minimum gradient slope GS minrepresented as a dashed line corresponding to a lower limit for the gradient inclination GS at which a measured calibration point diameter d p roh with corresponding measured gradient inclination GS clearly one of the corresponding predefined calibration point diameters d p real can be assigned.

[0076] As in the Fig. 2, the value of the maximum gradient slope GS max of the size of a predefined calibration point diameter d p real away.

[0077] The Fig. 3 shows the calibration point diameters d for each of the plurality of predefined calibration point diameters p real determined corresponding parameter ranges according to an embodiment of the present invention. The Fig. The corresponding parameter ranges shown in Figure 3 have a width of 2.5 µm. The determination of a respective corresponding parameter range is carried out in particular based on the determined ratio between the predefined calibration point diameter d p real and the corresponding measured calibration point diameter d p roh and the corresponding gradient inclination GS. Then, each measured calibration point diameter d p roh , which is within a corresponding parameter range to one of the predefined calibration point diameters d p real is located, the value of this corresponding predefined calibration point diameter d p real As described in the Fig. 3, the determined corresponding parameter ranges are determined by the minimum gradient slope GS min limited.

[0078] The shadow photography recording device further has a detection efficiency CE. The detection efficiency CE results from the number n p roh the measured calibration point diameter d p roh for a predefined calibration point diameter d p real and the number n p ZieL of calibration points for a predefined calibration point diameter d p real on a selected area of ​​the calibration device: CE=nprohnpziel.

[0079] Preferably, the detection efficiency CE is determined based on the high-resolution shadow photography images of the calibration device, particularly during calibration. In particular, the detection efficiency can assume a value between 0% and 100%. In particular, the detection efficiency can be measured at relative intervals z iof greater than 1.5 mm, a value greater than 100% is recommended, as in this case, blurred particle shadows are incorrectly split and detected as multiple particles. In particular, the detection efficiency rate depends on the value of the predefined calibration point diameter d p real and the relative distance z i to the focal plane of the shadow photography device. In particular, the greater the relative distance z, the fewer particles are detected i to the focal plane. The relative distance z i The distance to the focal plane for which the detection efficiency falls below 100% is greater for larger particles than for smaller particles. By determining the detection efficiency, a minimum particle size that can be resolved and the minimum particle size that can be detected with 100% accuracy can be determined.

[0080] The Fig. Figure 4 shows a detection rate CE determined according to an embodiment of the present invention as a function of the distance z i to the focal plane of the shadow photography device and the size of a calibration spot diameter d p real . As in the Fig. 4, the detection rate CE for a predefined calibration point diameter d p real depending on the relative distance to the focal plane z i of the shadow photography device. The detection efficiency rate decreases for smaller predefined calibration point diameters d p real already at a smaller distance z i from the focal plane of the shadow photography device than for a larger predefined calibration point diameter d p real . The distance range for each of the individual predefined calibration point diameters d p realfor which the detection efficiency CE is 100% defines the maximum measuring volume depth d CE=100% of the shadow photography device. When determining the maximum measurement volume depth d CE=100 % Those values ​​with a detection efficiency CE greater than 100% are not further taken into account.

[0081] In the Fig. 5, the maximum gradient inclination GS determined according to an embodiment of the present invention max depending on the relative distance z i to the focal plane of the shadow photography device and the respective size of a predefined calibration point diameter d p real shown.

[0082] In the Fig. 5 those measured calibration point diameters d p roh whose gradient slope GS is smaller than the minimum gradient slope GS min, shown in gray. The effective measurement volume depth depends on the predefined calibration point diameter d p real and limited by the value of the minimum gradient inclination GS min . Depending on the size of a predefined calibration point diameter d p real corresponds to the effective measuring volume depth d GS min the distance range from the focal plane of the shadow photography device within which a corresponding measured calibration point diameter d p roh has a corresponding gradient slope GS that is greater than the minimum gradient slope GS min . It is always the case that the effective measuring volume depth d GS min is equal to or smaller than the maximum measuring volume depth d CE=100 % , where the detection efficiency CE has a value of 100%.

[0083] The effective measuring volume depth d for different boundaries is given in the Fig. 6. The Fig. 6 shows a comparison of the maximum measurement volume depth d CE=100 % and the effective measuring volume depth d GS min , which is determined by the minimum gradient inclination GS min is limited depending on the predefined calibration point diameter d p real . Since for all predefined calibration point diameters d p real applies that the maximum measuring volume depth d CE=100 % is greater than or equal to the effective measuring volume depth d GS min , it can be assumed that all particles within the effective measuring volume depth d GS min Preferably, the minimum gradient slope GS min a value such that the value of the effective measuring volume depth d GS min is less than or equal to the value of the maximum measuring volume depth d CE=100 % and where the detection efficiency CE within the effective measuring volume depth d GS min has a value of 100%.

[0084] The downward pointing triangles represent the maximum effective measuring volume depth d CE=100 % , which can be approximated by linear fitting or determined by a linear fit. The gradient slope GS min limited effective measuring volume depth d GS min is indicated by circles and reaches a maximum at about 1.8 mm for predefined calibration point diameters with a size in the range of about 20 µm. The gradient slope GS min limited effective measuring volume depth d GS min then decreases for particles with a size of 30 µm and approaches the value of 1 mm for larger predefined calibration point diameters d p real . As in the Fig. 6, the value of the gradient slope GS min limited effective measuring volume depth d GS min for predefined calibration point diameters d GS minwith a size of 100 µm at about 1 mm.

[0085] This creates a method for correcting measured values ​​when measuring particle volume flow using shadow photography, improving the accuracy of the calculated measured values, such as the determined particle size and the measured volume. Furthermore, a method is created that compensates for the additional influences of the selected parameters, such as the limit value for binarization, and takes into account the counting rate of the individual particle shadows.

Claims

[1] Method for correcting measured values when measuring a particle volume flow using shadow photography, the method comprising the steps: - Calibrating a shadow photography recording device; - Generating high-resolution shadow photography images of a particle volume flow using the calibrated shadow photography recording device; - Evaluation of high-resolution shadow photography images; - Calculating measured values of the particle volume flow; and - Correcting calculated measured values of the particle volume flow taking into account the value of the gradient slope GS, wherein the value of the gradient slope GS corresponds to the value of a normalized intensity decrease per pixel at a detected particle edge or point edge, the method being characterized in that the calibration step comprises: - determining a focal plane of the shadow photography recording device, in particular along a lens axis of the shadow photography recording device; - Providing a calibration device with a plurality of calibration points with predefined calibration point diameters d p real ; - Generate high-resolution shadow photography images of the calibration device at predefined intervals z i to the focal plane of the shadow photography recording device; and - For each of the plurality of calibration points with predefined calibration point diameters d p real and for each predefined distance z i from the focal plane: Determination of calibration point shadows and determination of measured calibration point diameters d p roh the calibration point shadows and determining a gradient inclination GS of the respective calibration point shadows; - Determination of a maximum gradient inclination GS max for each of the calibration point shadows with predefined calibration point diameters d p real , where the maximum gradient GS max an upper limit for the gradient inclination GS of the calibration point shadows depending on the predefined calibration point diameter d p real , corresponds; - Determine a ratio between the predefined calibration point diameters d p real , the measured calibration point diameters d p roh and the gradient inclination GS based on the multitude of predefined calibration point diameters d p real and the corresponding measured calibration point diameter d p roh and the corresponding gradient inclinations GS. [2] The method of claim 1, wherein calculating measured values of the particle volume flow comprises: calculating a particle number, a particle concentration, a measured particle diameter and / or a particle velocity. [3] Method according to claim 1 or 2, wherein the ratio of the predefined calibration point diameter d p real and the corresponding measured calibration point diameter d p roh with gradient slope GS and parameters a, b and c results from: dprohdpreal=α+b∗e−c∗GSGSmax, a,b,c=f(dpreal). [4] Method according to claim 3, wherein the parameters a, b and c are derived from a compensation calculation or adjustment, in particular a two-dimensional adjustment of the determined ratio from the predefined calibration point diameters d p real and corresponding measured calibration point diameter d p roh result. [5] Method according to one of claims 1 to 3, wherein the maximum gradient inclination GS max with parameters f, g, h and i results in: GSmax=f+g−f1+(dprealh)i. [6] Method according to claim 5, wherein the parameters f, g, h and i result from a compensation calculation or adjustment. [7] Method according to one of the preceding claims, wherein the calibration further comprises: - For each of the plurality of predefined calibration point diameters d p real : Determination of a corresponding parameter range, in particular based on the determined ratio of the predefined calibration point diameter d preg , and the corresponding measured calibration point diameter d p roh and the corresponding gradient inclination GS, where each measured calibration point diameter d p rohwith corresponding gradient inclination GS, which is within this corresponding parameter range a corrected calibration point diameter d p korrigiert where the value of the corrected calibration point diameter d p korrigiert the value of the respective predefined calibration point diameter d p real this corresponding parameter range; - Limitation of the determined corresponding parameter ranges by a minimum gradient inclination GS min . [8] The method of claim 7, wherein the corresponding parameter range has a predefined width. [9] The method of claim 8, wherein the predefined width of the corresponding parameter range has a value of substantially 2.5 µm. [10] Method according to one of claims 7 to 9, wherein the minimum gradient inclination GS mincorresponds to a lower limit for the gradient inclination GS at which a measured calibration point diameter d p roh with corresponding measured gradient inclination GS clearly one of the corresponding parameter ranges of a predefined calibration point diameter d preg , can be assigned. [11] Method according to one of claims 7 to 10, wherein the value of the minimum gradient inclination GS min depending on the measured calibration point diameter d p roh is. [12] A method according to any one of the preceding claims, wherein the correcting comprises: - For each measured particle diameter d p roh : Determine a corresponding gradient slope GS, where the corresponding gradient slope GS is greater than the minimum gradient slope GS min ; - Determine the parameter range within which the measured particle diameter d p rohand the corresponding gradient inclination GS; - Depending on the determined parameter range: Assigning a corrected particle diameter d p korrigiert for each of the measured particle diameters d p,roh , where the value of the corrected particle diameter d p korrigiert the value of the predefined calibration point diameter d p real of the parameter range. [13] Method according to one of claims 7 to 11, wherein the accuracy of the determination of the corrected particle diameters d p korrigiert results from: εdpkorr=|∑i=1ndpcorrected,i−dprealdprealn|, σdpkorr=|∑i=1ndpcorrected,i−dprealdprealn|, [14] A method according to any one of the preceding claims, wherein the shadow photography recording device has a detection efficiency CE and the detection efficiency is given by: CE=nprohnpziel, where n p rohthe number of measured calibration point diameters for a predefined calibration point diameter d p real corresponds and n p ziel the number of calibration points for a predefined calibration point diameter d p real on a selected area of the calibration device. [15] Method according to one of the preceding claims, wherein the shadow photography recording device has an effective measurement volume depth d, wherein the effective measurement volume depth d is in particular dependent on the predefined calibration point diameter d p real and / or the corrected particle diameter d p korrigiert . [16] Method according to claim 14 or 15, wherein the effective measurement volume depth d GS min is equal to or smaller than a maximum measuring volume depth d CE=100 % , where the maximum measuring volume depth d CE=100 % a maximum distance range of the relative distance z ito the focal plane in which the detection efficiency CE has a value of 100%. [17] Method according to one of claims 7 to 16, wherein the minimum gradient inclination GS min has a value such that the value of the effective measuring volume depth d GS min is less than or equal to the value of the maximum measuring volume depth d CE=100 % and where the detection efficiency CE within the effective measuring volume depth d GS_min has a value of 100%. [18] A method according to any one of the preceding claims, wherein the high-resolution shadow photography recording device has a plurality of predefined image sections and the calibration is applied to each of the plurality of predefined image sections. [19] Method according to one of the preceding claims, wherein the evaluation comprises: - Creating a raw image; - Normalizing the raw image, in particular using a predefined NorRad value to obtain a normalized image; - Applying a noise filter to the normalized image to obtain a noise-reduced normalized image; - binarizing the noise-reduced normalized image, in particular using a predefined threshold BiThr to obtain a binarized image; - identifying particle shadows, where a particle shadow comprises a particle face and a particle edge; - Determining a shadow size area, where the shadow size area corresponds to a number of pixels that a particle shadow contains; - Specifying a minimum shadow size area, where the minimum shadow size area corresponds to a minimum number of pixels that a particle shadow must contain to be detected as a particle shadow; and filtering the binarized image with respect to the minimum shadow size area.

Images