A device and method for calibrating moisture capacitive tomography of wood structures using a dielectric substitution method.

Abstract

This invention discloses a device and method for calibrating moisture capacitance tomography of wood structures using a dielectric substitution method. Addressing the problem that the uneven distribution of dielectric constant in wood structures due to unstable factors such as texture, hollow areas, and knots prevents calibration using conventional two-point empty / full tube calibration methods, this invention proposes a dielectric substitution method. A two-electrode sampling capacitance sensor is used to sample wood strips with different moisture contents. Simultaneously, the actual moisture content is measured using a drying method, establishing the relationship between moisture content and the capacitance distribution of the sampling capacitance sensor. A uniform mixture of two particles with different dielectric constants is used to form a dielectric medium that substitutes for any dielectric constant of the wood structure, enabling empty / full tube capacitance calibration of the capacitance tomography system. Finally, the relationship between image grayscale and moisture content is established. This invention solves the problems of difficult and inaccurate calibration of non-uniform dielectric capacitance tomography.

Description

Technical Field

[0001] This invention relates to the field of capacitance tomography technology, specifically to a calibration device and method for moisture capacitance tomography of wood structures using a dielectric substitution method. Background Technology

[0002] Electrical Capacitance Tomography (ECT) measures the capacitance between electrodes arranged along the contour of the imaging region. Based on the sensor's sensitive field characteristics and an inversion algorithm, the dielectric constant distribution of the imaging region is reconstructed. This allows for non-contact imaging of the dielectric constant within structures of arbitrary shapes, making it an ideal non-contact measurement technique. A typical two-dimensional ECT uses an array of n capacitor electrodes arranged along the circumference of a circular, insulated, rigid pipe. Each pair of electrodes forms a capacitor, resulting in a total of m = n*(n-1) / 2 capacitor arrays C. The size of these m capacitors is closely related to the dielectric constant distribution within the pipe. By modeling the sensing characteristics within the pipe, a sensitive field S is obtained, representing the relationship between the dielectric constant and the array capacitance. The dielectric constant distribution within the pipe is then reconstructed based on the sensitive field S, the n array capacitances, and a specific inversion algorithm.

[0003] During the above measurements, changes in the dielectric constant of the two-phase flow mixture due to variations in the type or properties of the solid material can lead to errors in the normalized capacitance, affecting the imaging effect. Therefore, medium calibration is necessary. Typically, during calibration, the capacitance tomography sensor is removed and its capacitance data are collected in two states: an empty tube and a full tube. This is a two-point calibration method, also known as empty-full-tube calibration. Taking gas-solid two-phase flow as an example, the first step is to empty the imaging area to obtain the sensor capacitance under an empty tube, resulting in a grayscale value of 0 in the imaging area; this is called empty-tube calibration. The second step is to fill the imaging area with the measured medium to obtain the sensor capacitance under a full tube, resulting in a grayscale value of 1 in the imaging area; this is called full-tube calibration.

[0004] For two-phase flows such as gas-solid particle flow, gas-liquid flow, and liquid-solid flow, two-point calibration methods for capacitance tomography are quite mature. However, in imaging the moisture content distribution of wood structures, the moisture content is greatly affected by the environment, and the species, texture, and wood defects of the wood also lead to uneven distribution of dielectric constant. It is impossible to find or prepare stable wood with uniform and known moisture content for calibration of the grayscale values ​​corresponding to 0 and 1. Furthermore, during the calibration process, it was found that the relationship between image grayscale and moisture content is not linear. If conventional calibration methods are used, nonlinear errors will be introduced, and the nonlinear error of moisture content will increase with the distance from the two calibration points. Therefore, the existing mature empty / full tube calibration methods cannot be used for calibration. Summary of the Invention

[0005] 1. The technical problem to be solved:

[0006] To address the aforementioned technical problems, this invention provides a device and method for calibrating moisture capacitance tomography (ECT) of wood structures using a dielectric substitution method. The method employs a dielectric substitution approach, using a uniform mixture of two particles with different dielectric constants to form particles that can substitute for any dielectric constant in the wood structure, thus solving the problems of difficult and inaccurate calibration with non-uniform media. In this scheme, a sampling capacitance sensor and square wood strip samples with different moisture contents are used to obtain the moisture and capacitance relationship of a specific wood species, establishing a calibration relationship between moisture content and sampling capacitance for use in ECT calibration using the dielectric substitution method.

[0007] 2. Technical Solution:

[0008] A moisture capacitive tomography calibration device for wood structures using a dielectric substitution method is used to calibrate the moisture content distribution of wood structures. It includes mixed calibration particles, multiple sample wood strips with different moisture contents, and a two-electrode sampling capacitive sensor device. The mixed calibration particles are composed of a mixture of high-dielectric-constant and low-dielectric-constant particles, and the overall equivalent dielectric constant of the mixed calibration particles can be changed by adjusting the ratio of the two types of particles.

[0009] The two-electrode sampling capacitance sensor device includes a measuring electrode plate and an excitation electrode plate. The measuring electrode plate includes a sheet-like measuring electrode and a grounded ring protection electrode that surrounds the measuring electrode with gaps. The measuring electrode plate and the excitation electrode plate are placed opposite each other, and the cavity between them serves as the measuring area. The measuring area is slightly larger than the sample wood strip and is used to hold the sample wood strip or fill it with mixed calibration particles. The excitation electrode is connected to an excitation signal generator to generate an excitation AC voltage signal, which enables a uniform alternating electric field to be generated in the measuring area.

[0010] Furthermore, both the excitation electrode and the measurement electrode are rectangular copper foils; the overall size of the measurement electrode and the surrounding grounding ring protection electrode is the same as the shape and size of the excitation electrode; the cross-section of the sample wooden strip is square; and the measurement area between the measurement electrode plate and the excitation electrode plate is cuboid.

[0011] Furthermore, both the excitation electrode and the measuring electrode are rectangular stainless steel sheets; the overall size of the measuring electrode and the surrounding grounding ring protection electrode is the same as the shape and size of the excitation electrode; the measuring area between the measuring electrode plate and the excitation electrode plate is cuboid.

[0012] Furthermore, the length of the excitation electrode is twice its width.

[0013] Furthermore, the sample wooden strip has a side length of 15-30 mm; the excitation electrode has a length of 50-100 mm and a width of 30-50 mm.

[0014] Furthermore, the particles of the mixed calibration particles are a mixture of high dielectric constant particles with a relative dielectric constant greater than 15 and low dielectric constant particles with a relative dielectric constant less than 4.

[0015] A method for calibrating the moisture capacitance tomography of wood structures using a dielectric substitution approach includes the following steps:

[0016] Step 1: Pre-set at least 5 sample wooden strips with different moisture contents to evenly cover the commonly used moisture content measurement range.

[0017] Step 2: Place the sample wood strips with different moisture contents into the measurement area of ​​the calibration device and measure their corresponding capacitance values ​​Cs. At the same time, weigh the sample wood strips to obtain their weights. m w .

[0018] Step 3: Obtain the weight m0 of each sample wood strip under absolute dryness using the drying method; place the wood strips in an oven at 103 degrees Celsius to dry them and remove moisture, continuously monitoring the weight change; when the weight change is less than a preset weight difference within a preset time period, the sample wood strip is considered to be in an absolute dry state, and the weight of the wood strip at this point is the absolute dry weight of the sample wood strip. m 0; Substitute the weight and oven-dry weight of each sample wood strip into the formula: MC=(m w - m0) / m0, calculate the true moisture content MC of the corresponding sample wood strip.

[0019] Step 4: The moisture content MC of the sample wood strips obtained in Step 3 is corresponding to the capacitance value Cs measured in Step 2. The data is then input into a computer to fit the relationship curve, thus obtaining the MC-Cs scale table for the wood.

[0020] Step 5: Adjust the ratio of the two calibration particles to form a preset low dielectric constant mixed calibration particle l, fill the measurement area of ​​the sampling capacitance sensor with it, and measure its capacitance Cs. l Then, refer to the scale table in step four to obtain the corresponding moisture content (MC). l Simultaneously, the mixed particles are used for empty tube calibration of ECT to obtain the corresponding empty tube capacitance with a grayscale of 0 in the ECT image; the ratio of the two particles is adjusted to form a preset high dielectric constant mixed calibration particle h, which is then used to fill the sampling capacitance sensor to measure its capacitance Cs. h Then, refer to the MC-Cs scale table for wood from step four to obtain the corresponding moisture content MC. h The mixed particles are then used for full-tube calibration of the ECT image to obtain the full-tube capacitance with a grayscale value of 1 in the corresponding ECT image.

[0021] Step 6: Reconstruct the grayscale image based on the ECT calibration in Step 5, and convert the image grayscale into a water content scale based on the grayscale and water content relationship in Step 5.

[0022] Further, step six specifically involves: replacing the coordinate axis representing capacitance values ​​in the MC-Cs calibration table for wood with a grayscale coordinate axis, while keeping the coordinate axis representing moisture unchanged; based on the empty and full tube calibration results from step five, finding the two points with corresponding grayscale values ​​of 0 and 1 in the relationship curve; and dividing the two grayscale values ​​of 0 and 1 equally between them, thus converting the MC-Cs calibration table for wood into the MC-g calibration table.

[0023] Furthermore, in step one, the moisture content of the five sample wood strips of the same type with different moisture contents ranged from 15% to 50%, and the difference in moisture content between any two samples was greater than 5%.

[0024] 3. Beneficial effects:

[0025] (1) This solution addresses the issue that the unstable factors such as wood texture structure, hollowness, and wood knots cause uneven internal moisture content, which in turn leads to uneven distribution of dielectric constant, making it impossible to use the conventional two-point hollow-full tube calibration method. The solution proposes a medium substitution method, which uses a uniform mixture of two particles with different dielectric constants to form a material that can replace any dielectric constant of the wood structure, thus solving the problems of difficult and inaccurate calibration of non-uniform media.

[0026] (2) In this scheme, a sampling capacitance sensor device for calibration is designed. The moisture and capacitance relationship of a certain wood species is obtained by using square wood strip samples with different moisture contents that are adapted to the sampling capacitance sensor, forming a calibrated relationship between moisture content and sampling capacitance, which is used for ECT calibration by the medium substitution method. This device has a simple structure, small size, is easy to implement, and calibrates absolute moisture content in a non-contact manner.

[0027] (3) In this scheme, the ratio of two types of particles is adjusted to replace the non-uniform wood structure with particles with uniform dielectric constant distribution for ECT calibration, and the water content range of the imaging can be arbitrarily adjusted, thus solving the calibration problem of wood structure water imaging ECT. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the measuring electrodes of the calibration device of the present invention;

[0029] Figure 2 This is a schematic diagram of the excitation electrodes of the calibration device of the present invention;

[0030] Figure 3 This is a cross-sectional view of the calibration device in this invention;

[0031] Figure 4 The table below shows the indexing table of MC-Cs in a specific embodiment;

[0032] Figure 5 This is a schematic diagram of the substitution method in this scheme.

[0033] Reference numerals: 1. Measuring electrode plate; 11. Sheet-shaped measuring electrode; 12. Grounding ring protection electrode; 13. Gap between sheet-shaped measuring electrode and protection electrode; 2. Excitation electrode plate. Detailed Implementation

[0034] The present invention will now be described in detail with reference to the accompanying drawings.

[0035] As attached Figure 1-3 As shown, a moisture capacitive tomography calibration device for wood structures using a dielectric substitution method is used to calibrate the moisture content distribution of wood structures. It includes mixed calibration particles, multiple sample wood strips with different moisture contents, and a two-electrode sampling capacitive sensor device. The mixed calibration particles are composed of two types of particles with high dielectric constant and low dielectric constant, and the overall equivalent dielectric constant of the mixed calibration particles can be changed by adjusting the ratio of the two types of particles.

[0036] The two-electrode sampling capacitance sensor device includes a measuring electrode plate 1 and an excitation electrode plate 2. The measuring electrode plate includes a sheet-like measuring electrode 11 and a grounded ring protection electrode 12 that surrounds the measuring electrode with gaps. The measuring electrode plate and the excitation electrode plate are placed opposite each other, and the cavity between them serves as the measuring area. The measuring area is slightly larger than the sample wood strip and is used to place the sample wood strip or fill it with mixed calibration particles. The excitation electrode is connected to an excitation signal generator to generate an excitation AC voltage signal, which can generate a uniform alternating electric field in the measuring area.

[0037] Furthermore, both the excitation electrode and the measurement electrode are rectangular copper foils; the overall size of the measurement electrode and the surrounding grounding ring protection electrode is the same as the shape and size of the excitation electrode; the cross-section of the sample wooden strip is square; and the measurement area between the measurement electrode plate and the excitation electrode plate is cuboid.

[0038] As attached Figure 1 As shown in the figure, 13 represents the gap between the sheet-like measuring electrode and the grounding ring protection electrode.

[0039] Furthermore, both the excitation electrode and the measuring electrode are rectangular stainless steel sheets; the overall size of the measuring electrode and the surrounding grounding ring protection electrode is the same as the shape and size of the excitation electrode; the measuring area between the measuring electrode plate and the excitation electrode plate is cuboid.

[0040] Furthermore, the length of the excitation electrode is twice its width.

[0041] Furthermore, the sample wooden strip has a side length of 15-30 mm; the excitation electrode has a length of 50-100 mm and a width of 30-50 mm.

[0042] Furthermore, the particles of the mixed calibration particles are a mixture of high dielectric constant particles with a relative dielectric constant greater than 15 and low dielectric constant particles with a relative dielectric constant less than 4.

[0043] A method for calibrating the moisture capacitance tomography of wood structures using a dielectric substitution approach includes the following steps:

[0044] Step 1: Prepare at least 5 sample wooden strips with different moisture contents to evenly cover the commonly used moisture content measurement range;

[0045] Step 2: Place the sample wood strips with different moisture contents into the measurement area of ​​the calibration device and measure their corresponding capacitance values ​​Cs. At the same time, weigh the sample wood strips to obtain their weights. m w .

[0046] Step 3: Obtain the weight m0 of each sample wood strip under absolute dryness using the drying method; place the wood strips in an oven at 103 degrees Celsius to dry them and remove moisture, continuously monitoring the weight change; when the weight change is less than a preset weight difference within a preset time period, the sample wood strip is considered to be in an absolute dry state, and the weight of the wood strip at this point is the absolute dry weight of the sample wood strip. m 0; Substitute the weight and oven-dry weight of each sample wood strip into the formula: MC=(m w - m0) / m0, calculate the true moisture content MC of the corresponding sample wood strip.

[0047] Step 4: The moisture content MC of the sample wood strips obtained in Step 3 is corresponding to the capacitance value Cs measured in Step 2. The data is then input into a computer to fit the relationship curve, thus obtaining the MC-Cs scale table for the wood.

[0048] Step 5: Adjust the ratio of the two calibration particles to form a preset low dielectric constant mixed calibration particle l, fill the measurement area of ​​the sampling capacitance sensor with it, and measure its capacitance Cs. l Then, refer to the scale table in step four to obtain the corresponding moisture content (MC). l Simultaneously, the mixed particles are used for empty tube calibration of ECT to obtain the corresponding empty tube capacitance with a grayscale of 0 in the ECT image; the ratio of the two particles is adjusted to form a preset high dielectric constant mixed calibration particle h, which is then used to fill the sampling capacitance sensor to measure its capacitance Cs. h Then, refer to the MC-Cs scale table for wood from step four to obtain the corresponding moisture content MC. hThe mixed particles are then used for full-tube calibration of the ECT image to obtain the full-tube capacitance with a grayscale value of 1 in the corresponding ECT image.

[0049] Step 6: Reconstruct the grayscale image based on the ECT calibration in Step 5, and convert the image grayscale into a water content scale based on the grayscale and water content relationship in Step 5.

[0050] Further, step six specifically involves: replacing the coordinate axis representing capacitance values ​​in the MC-Cs calibration table for wood with a grayscale coordinate axis, while keeping the coordinate axis representing moisture unchanged; based on the empty and full tube calibration results from step five, finding the two points with corresponding grayscale values ​​of 0 and 1 in the relationship curve; and dividing the two grayscale values ​​of 0 and 1 equally between them, thus converting the MC-Cs calibration table for wood into the MC-g calibration table.

[0051] Furthermore, in step one, the moisture content of the five sample wood strips of the same type with different moisture contents ranged from 15% to 50%, and the difference in moisture content between any two samples was greater than 5%. Specific implementation examples:

[0053] In this embodiment, wood strips with different moisture contents (approximately 15%, 20%, 30%, 40%, and 50%) were prepared for calibration. The wood strips were of similar size, with a cross-section of 20mm x 20mm. Using this method, the following results were obtained: Figure 4 The corresponding data of the moisture content MC of the wood strip and the capacitance Cs of the sampling sensor are shown. The relationship curve between the two is fitted to form a scale table of their relationship.

[0054] Replacing low-moisture-content wood strips with mixed particles of low equivalent dielectric constant, the ECT system was calibrated for low dielectric constant, yielding the empty tube capacitance Cl. Then, mixed particles of high equivalent dielectric constant were formed, and their capacitance values ​​were sampled and measured. The mixing ratio was adjusted to form particles with the capacitance equal to that of the desired high-moisture-content wood. This was then used for ECT high dielectric constant dielectric calibration, yielding the full tube capacitance Ch. After normalizing the capacitance, the image grayscale was calculated using an image reconstruction algorithm. Cl corresponds to an image grayscale of 0; Ch corresponds to an image grayscale of 1. The curve between grayscale 0 and 1 represents the relationship between wood moisture content and grayscale, thus completing the ECT system calibration. (Appendix) Figure 5 This is a chart that reflects the conversion relationships between the various parameters in this scheme.

[0055] Although the present invention has been disclosed above with reference to preferred embodiments, these are not intended to limit the invention. Any person skilled in the art can make various changes or modifications without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention should be defined by the scope of the claims of this application.

Claims

1. A method for moisture capacitance tomography calibration of wood structures using a dielectric substitution method, implemented using a moisture capacitance tomography calibration device for wood structures using a dielectric substitution method; the moisture capacitance tomography calibration device includes mixed calibration particles, multiple sample wood strips with different moisture contents, and a two-electrode sampling capacitance sensor device; the mixed calibration particles are composed of two types of particles with high dielectric constant and low dielectric constant, and the overall equivalent dielectric constant of the calibration mixed particles can be changed by adjusting the ratio of the two types of particles; the two-electrode sampling capacitance sensor device includes a measuring electrode plate and an excitation electrode plate; the measuring electrode plate includes a sheet-like measuring electrode and a grounded ring protection electrode that surrounds the measuring electrode with gaps; the measuring electrode plate and the excitation electrode plate are placed opposite each other, and the cavity between them serves as the measuring area, which is slightly larger than the sample wood strips, for placing the sample wood strips or filling them with mixed calibration particles; the excitation electrode is connected to an excitation signal generator to generate an excitation AC voltage signal, which can generate a uniform alternating electric field in the measuring area; characterized in that: Includes the following steps: Step 1: Prepare at least 5 sample wooden strips with different moisture contents to evenly cover the commonly used moisture content measurement range; Step 2: Place sample wood strips with different moisture contents into the measurement area of ​​the sampling capacitance sensor and measure their corresponding capacitance value Cs. At the same time, weigh the sample wood strips to obtain their weight m. w ; Step 3: Obtain the weight m0 of each sample wood strip under absolute dryness using the drying method; place the wood strips in an oven at 103 degrees Celsius to dry them, removing moisture, and continuously monitor their weight changes; when the weight change is less than a preset weight difference within a preset time period, the sample wood strip is considered to be in an absolute dry state, and the weight of the wood strip at this point is the absolute dry weight m0 of the sample wood strip; substitute the weight and absolute dry weight of each sample wood strip into the formula: MC=(m w - m0) / m0, calculate the true moisture content MC of the corresponding sample wood strip; Step 4: Correspond the moisture content MC of the sample wood strips obtained in Step 3 to the capacitance value Cs measured in Step 2, input the data into a computer to fit the relationship curve, thus obtaining the MC-Cs scale table for wood; Step 5: Adjust the ratio of the two calibration particles to form a preset low dielectric constant mixed calibration particle l, fill the measurement area of ​​the sampling capacitance sensor with it, and measure its capacitance Cs. l Then, refer to the scale table in step four to obtain the corresponding moisture content (MC). l Simultaneously, the mixed particles are used for empty tube calibration of ECT to obtain the corresponding empty tube capacitance with a grayscale of 0 in the ECT image; the ratio of the two particles is adjusted to form a preset high dielectric constant mixed calibration particle h, which is then used to fill the sampling capacitance sensor to measure its capacitance Cs. h Then, refer to the MC-Cs scale table for wood from step four to obtain the corresponding moisture content MC. h The mixed particles are then used for full tube calibration of the ECT to obtain the full tube capacitance with a gray level of 1 in the corresponding ECT image. Step 6: Reconstruct the grayscale image based on the ECT calibration in Step 5, and convert the image grayscale into a water content scale based on the grayscale and water content relationship in Step 5.

2. The method for calibrating moisture capacitance tomography of wood structures using the medium substitution method according to claim 1, characterized in that: Step six specifically involves replacing the coordinate axis representing capacitance values ​​in the MC-Cs scale table for wood with a grayscale coordinate axis, while leaving the coordinate axis representing moisture unchanged. Based on the calibration results of the empty and full tubes in step five, find the two points with corresponding gray values ​​of 0 and 1 in the relationship curve; divide the two points with gray values ​​of 0 and 1 equally, and the MC-Cs scale table of wood can be converted into the MC-g scale table.

3. The method for calibrating moisture capacitance tomography of wood structures using the medium substitution method according to claim 1, characterized in that: In step one, the moisture content of the five sample wood strips of the same type with different moisture contents ranged from 15% to 50%, and the difference in moisture content between any two samples was greater than 5%.

4. The method for calibrating moisture capacitance tomography of wood structures using the dielectric substitution method according to claim 1, characterized in that: Both the excitation electrode and the measuring electrode are rectangular copper foils; the overall size of the measuring electrode and the grounding ring protection electrode surrounding it is the same as the shape and size of the excitation electrode; the cross-section of the sample wooden strip is square; the measuring area between the measuring electrode plate and the excitation electrode plate is cuboid.

5. The method for calibrating moisture capacitance tomography of wood structures using the medium substitution method according to claim 1, characterized in that: Both the excitation electrode and the measuring electrode are rectangular stainless steel sheets; the overall size of the measuring electrode and the surrounding grounding ring protection electrode is the same as the shape and size of the excitation electrode; the measuring area between the measuring electrode plate and the excitation electrode plate is cuboid.

6. The method for calibrating moisture capacitance tomography of wood structures using the dielectric substitution method according to claim 4 or 5, characterized in that: The length of the excitation electrode is twice its width.

7. The method for calibrating moisture capacitance tomography of wood structures using the dielectric substitution method according to claim 4 or 5, characterized in that: The sample wooden strip has a side length of 15-30mm; the excitation electrode has a length of 50-100mm and a width of 30-50mm.

8. The method for calibrating moisture capacitance tomography of wood structures using the dielectric substitution method according to claim 1, characterized in that: The mixed calibration particles are a mixture of high dielectric constant particles with a relative dielectric constant greater than 15 and low dielectric constant particles with a relative dielectric constant less than 4.

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