JP2019060692A - Method for estimating water content in wood and device therefor - Google Patents

Abstract

To obtain the water content in wood with a simple method.SOLUTION: The present invention is proposed in consideration of the above-described circumstances, and is directed to provide a method and a device capable of estimating the water content in the entire sample with a simple device and in a non-destructive manner.MEANS FOR SOLVING THE PROBLEM: The water content estimating method includes: determining, from a database of the water content of wood and propagation speed of a stress wave (hereinafter may be simply referred to as a propagation speed), a formula representing a probability distribution curved surface having a probability density at which the probability on a minus side of two variables becomes zero as the vertical axis on a coordinate plane of a "function of water content-propagation speed," together with a parameter of the formula; and in an above-described state, determining the propagation speed Cof wood to be measured, and obtaining, from the propagation speed C, the estimated value of the water content from a peak point, obtained by the parameter, of an intersection curve between the probability distribution curved surface and a surface perpendicular to the coordinate plane of a straight line corresponding to the propagation speed Con the coordinate plane. The method can also calculate the confidence interval of the water content.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a moisture content estimation method and apparatus for estimating the moisture content of wood.

  From the aspect of conservation of forest resources, promotion of effective use to wooden houses as a structural member of domestic timber is desired. When new wood is used for a new house, if the wood is not completely or partially dried, it may cause distortion of the building after construction. The moisture content of the wood is determined accordingly. Therefore, it would be desirable to be able to assess the moisture content of the wood at various stages of harvesting, lumbering or finished products.

  There are various methods for measuring the moisture content of wood, and devices and instruments corresponding to the methods are commercially available.

  The electrical resistance method utilizes the fact that when a high voltage is applied between needle-like probes, wood exhibits a resistance value corresponding to the moisture content. The probe is needle-shaped and has a configuration of four, and the resistance of the surface layer portion is measured by driving the sample with a hammer and energizing the same. Since the measurement value by this method is influenced by the tree species, it is premised that the tree species is known in advance (for example, see JP-A-11-304741).

  The high frequency method utilizes the fact that the attenuation factor of the high frequency is different depending on the moisture content of wood, but it is necessary to carry out the measurement together with the influence of the thickness and density of the material (for example, Open 2010-237135)).

  In the infrared method, since infrared rays are absorbed by moisture, the sample is irradiated with infrared rays to measure its reflectance. Naturally, the higher the water content, the smaller the reflection intensity.

  In the drying method, a portion to be a sample is cut out from the raw material (20 g or more in JAS) and dried, and the moisture content is obtained from the weight before and after drying (see, for example, JP-A-11-1488892).

Unexamined-Japanese-Patent No. 11-304741 Unexamined-Japanese-Patent No. 11-148892 JP, 2010-237135, A

The electrical resistance method has the disadvantage of damaging the sample since the sample is pierced with a needle-like probe, and it is possible to measure only the periphery (about 3 cm ² ) of the portion abutted by the probe, and the whole wood can be measured The moisture content of can not be measured. In addition, since the measurement result depends on the tree species, it can not be measured with a sample whose tree species is not known.

  In the high frequency method, since the size of the apparatus is increased and the thickness and the density depend, it is necessary to separately obtain such values. There are cases where measurement is performed by reflection of high frequency and measurement by transmission, but in either case, it is a partial measurement only in the peripheral portion of the probe to which high frequency is applied or in the thickness direction of the sample through which high frequency is transmitted.

  Both of the above two methods use handheld probes and are relatively easy to measure, so they are used to obtain rough moisture content at many sites such as wood processing plants. However, there are also errors depending on the equipment used, and also the measured values between the measuring methods may differ significantly, which has the disadvantage of being unreliable.

  The infrared method is effective for measuring the moisture content of the thin sample or the surface of the sample as described above, but has a drawback that the moisture content inside the thick sample can not be measured.

  Furthermore, although the drying method is highly accurate, it is a destructive test for cutting a sample piece out of the material, and while drying takes time, it is naturally necessary to measure the weight before and after drying. In addition, there is a drawback that it can not be measured continuously at the site of lumber or at the site of use.

  The present invention has been proposed in view of the above-mentioned conventional circumstances, and an object thereof is to provide a method and an apparatus capable of estimating the moisture content of the whole sample nondestructively with a simple apparatus.

  The present invention adopts the following means in order to achieve the above object.

First, from the database of the moisture content of wood and the propagation velocity of stress waves (hereinafter sometimes referred to simply as propagation velocity), the probability of the negative side of the bivariate on the coordinate surface of "moisture content-function of propagation velocity" is zero An expression representing a probability distribution curved surface with the probability density as y-axis as the vertical axis is determined together with its parameters. In the above state, obtains the propagation velocity C ₀ wood to be measured, from the propagation speed C _0, the probability distribution and the plane perpendicular to the coordinate plane of the straight line corresponding to the propagation velocity C ₀ on the coordinate plane An estimated value of the moisture content is obtained from the peak point of the crossing curve of the curved surface obtained from the above parameters.

  The above method can be realized by using an apparatus provided with the following propagation speed measurement means, storage means, and estimated value calculation means.

  Probability distribution with vertical axis as probability density that probability of minus side of bivariate becomes zero on coordinate plane of "function of moisture content-propagation velocity" from database of moisture content of wood and propagation velocity in the above storage means The equation representing the surface is stored along with its parameters.

The propagation velocity measuring means determines the stress wave propagation velocity C ₀ of the wood to be measured. Also, the estimated value calculation means, from the propagation speed C _0, a plane perpendicular to the coordinate plane of the straight line corresponding to the propagation velocity C ₀ on the coordinate plane with the交曲line of the probability distribution curved surface, from the parameter An estimated value y ₀ of water content is determined from the obtained peak point.

  The probability distribution surface is a surface on which the probability on the minus side is zero. A bivariate lognormal distribution, a gamma distribution, a Poisson distribution or the like can be used as this type of probability distribution. Each parameter of the equation representing the bivariate lognormal distribution is determined by the Hamiltonian Monte Carlo method. Also, as a function of the propagation velocity, the following example adopts the square of the reciprocal of the propagation velocity.

  Since the moisture content of wood can be easily estimated nondestructively simply by determining the stress wave propagation velocity of the wood to be measured by the above method, and since the probability distribution is used, the credit interval of the moisture content is also obtained. It can be calculated. Furthermore, the measurement can be made to be continuous, and there is an effect that it can be used at the production site and use site of wood products.

Water content measuring device used in the present invention. Probability distribution curve of moisture content MC and propagation speed C of wood. Probability distribution curve with bivariate normal distribution function. Probability distribution curve with bivariate lognormal distribution function. The figure which shows the difference | error of the estimation by this invention, and a true value.

<Principle>
The moisture content of wood is divided into a region affected by water (bonded water) bonded to the fiber tissue and a region in which the water remains as free water to the cell tissue. The former is a phenomenon in a region where the water content is low, and the Young's modulus increases (the strength increases) as the water content decreases, but the increase or decrease in free water does not affect the Young's modulus. Thus, the branch point of the form of water contained in wood is called FSP, and the water content is approximately 28%. From this simple model of water content and water content dependence of Young's modulus, it can be seen that the water content is proportional to the square of the reciprocal of the propagation speed above FSP. We will use this fact below.

The applicant has taken advantage of the fact that the relationship E = v ² ((v: propagation velocity of stress wave) holds between Young's modulus E and density 、, and Young's modulus E-density 平面 plane of many wood samples. Taking a density distribution curved surface of the existence probability (density) above, the method of simultaneously determining the Young's modulus and the density of the sample from the intersection curve of the curved surface and the straight line corresponding to the above equation. It proposes in application 2016-193588.

  As described above, that the water content and the Young's modulus have a correlation, it is presumed that the water content of wood has some relationship with the stress wave propagation speed.

Therefore, a bivariate lognormal distribution is used to create a surface of probability density of multiple data on the coordinate surface of the "water content-propagation velocity function" of the wood sample, and the propagation speed of a specific wood sample We will try to estimate the moisture content from C ₀ below.

  Here, as a function of the propagation velocity, in the following description, the square of the reciprocal of the propagation velocity C is taken, and the reason is as described above. The reason for adopting the bivariate lognormal distribution will be described later.

<Probability surface>
FIG. 2 (a) shows a scatter plot of moisture content and propagation velocity based on a clear database of moisture content MC (%) of wood and propagation velocity C (m / s) of stress wave.

  In the present invention, it is assumed that the parameters of the probability distribution are determined using the Hamiltonian Monte Carlo method. In order to adopt a bivariate normal distribution as a function form of a well-known probability distribution in this method, it is desirable that the scatter states be able to be linearly approximated.

From scatter diagram of FIG. 2 (a), the attempt to convert the distribution state capable linear approximation obtained, the water content as described above - the square (C ^-2) (units of inverse propagation velocity s ² / m ² By taking the above, a substantially linear scatter plot is obtained as shown in FIG. 2 (b). However, since it becomes a very small value when expressing the square of the reciprocal of propagation velocity (C ^-2 ) as it is, it is multiplied by 10 ^-8 .

  Furthermore, FIG. 3 draws a probability distribution map by bivariate normal distribution from the scatter diagram shown in FIG. 2 (b), with the axis perpendicular to the coordinate plane of “function of moisture content-propagation velocity” as the probability axis. . In the state of FIG. 3, the water content MC is present even in a negative region (see the arrow in FIG. 3), so it is inconvenient to use a bivariate normal distribution function as the distribution function. Therefore, a bivariate lognormal distribution function shown in the following equation (1) is adopted as the distribution function. This makes it possible to obtain the probability distribution chart shown in FIG. The parameters for determining the specific forms of the above two probability distribution functions can be obtained by applying the Hamiltonian Monte Carlo method of Bayesian statistics.

  Here, μ is an average, σ is a standard deviation, and R is a correlation coefficient. Also, each subscript x and y indicates its attribute (ie, x is propagation velocity and y is moisture content).

The five parameters μ _x , μ _y , σ _x , σ _y and R in the above equation (1) can be determined by the Hamiltonian Monte Carlo method (HMC method) of Bayesian statistics, and the specific probability distribution function (1) equation The results obtained by applying the equation (2) are shown in Table 1.

When a probability distribution curved surface of the reciprocal of the moisture content-the propagation velocity C is drawn based on the equation (1) whose parameters are determined as described above, it becomes as shown in FIG. Here, the propagation velocity C ₀ is measured for a specific sample, and the reciprocal square power C ₀ ⁻² = a straight line perpendicular to the moisture content-inverse velocity squared coordinate plane of a constant straight line The position of the peak (d) of the intersection curve (c) between b) and the probability curved surface is the estimated moisture content y ₀ to be determined for the specific sample.

  Here, the position of the peak is given by the following equation (3).

y: Water content to be determined, x: inverse power of propagation speed squared × 10 ^-8
Table 2 shows the estimated value of the moisture content calculated by said method based on the propagation speed measured about several samples. As a comparison, the water content obtained by the drying method is also described.

Further, since the probability distribution is used, the credit interval for the water content obtained as described above can also be calculated, and Table 2 shows the 70% credit interval. Here, taking sample No. 153 as an example, it means that "the moisture content is 34.9% or less with a probability of 85%" can be said.
That is, the moisture content can be expressed in another expression by using the upper limit of the credit interval.

  FIG. 1 shows an example of a water content estimation device according to the present invention. Basically, it comprises the propagation velocity measuring means 10, the storage means 20 and the estimated value calculating means 30.

The propagation velocity measuring means 10 is provided with a pair of needle-like probes 11 and 11, and the two probes 11 and 11 are struck on a specific sample while keeping a predetermined distance, and one of the probes 11 is a hammer etc. When it is struck with a ball, the time from the detection of the impact sound by the one probe 11 to the detection of the impact sound by the other probe 11 is measured. This results in the stress wave propagation velocity C ₀ propagating wood propagation velocity measuring means 10 is obtained.

The propagation velocity C ₀ of the specific sample obtained in this manner is input to the estimated value calculation means 30. On the other hand, in the storage means 20, the above-mentioned probability curved surface is stored in advance as the above-mentioned equation (1) and five parameters, and further, equation (3) for obtaining the position of peak (water content) is also stored.

In this state, when the stress wave propagation velocity C ₀ is obtained from the propagation velocity measuring means 10, the estimated value computing means 30 reads out the equation (3) and the five parameters from the storage means 20 and sets the peak position. The corresponding moisture content y ₀ will be obtained. The moisture content y ₀ obtained in this manner is displayed on the display unit 40.

  In addition to the above, there is a hammering method as the propagation speed measuring means.

  That is, when one end of wood as a sample is hit with a hammer, the sample longitudinally vibrates at the natural frequency f of the sample. The relationship between the natural frequency f and the stress wave propagation speed C is C = 2Lf, where L is the length of the sample. Therefore, the stress wave speed can be obtained by obtaining the natural frequency.

  In the above, although the square of the reciprocal of the stress wave propagation velocity is used as one axis of the probability distribution curve, it is particular about this if it is a function that can obtain a linear approximation with the water content of the other axis It is not a thing. In addition, although bivariate lognormal distribution is adopted as the probability distribution, the present invention is not limited to this, and the probability on the negative side becomes zero for any element of two variables, for example, gamma distribution , Poisson distribution, etc. can be used.

  As described above, the present invention can obtain the water content of practical level wood only by determining the propagation speed, and also uses the probability distribution curve, so the credit interval for the obtained water content is also obtained. It can be calculated. Therefore, the value of use at production sites and distribution sites utilizing wood resources is considered to be large.

11 · · · probe 10 · · propagation speed measuring means 20 · · storage means 30 · · estimated value calculating means 40 · · display unit

Claims (10)

From the moisture content of wood and the stress wave propagation velocity database, the probability density with which the probability of the minus side of the bivariate becomes zero on the coordinate surface of "moisture content-function of propagation velocity" is taken as the axis perpendicular to that plane Determining an expression representing a probability distribution surface together with its parameters;
Determining the propagation velocity C ₀ of the wood to be measured;
Than the propagation speed C _0, the straight line of the coordinate plane to a plane perpendicular to the probability distribution curved surface of交曲line corresponding to the propagation velocity C ₀ on the coordinate plane, the water content from the peak point obtained from the parameter Obtaining an estimated value;
A method for estimating the moisture content of wood, comprising:   The method for estimating moisture content of wood according to claim 1, wherein a credit interval of the value is determined in addition to the estimated value of moisture content.   The method for estimating moisture content of wood according to claim 1, wherein a square of an inverse number of stress wave propagation speed is used as a function of the stress wave propagation speed.   The method for estimating the moisture content of wood according to claim 1, wherein a formula representing a bivariate lognormal distribution is used as the probability distribution in which the probability on the negative side of the bivariate is zero.   The method according to claim 4, wherein each parameter of the bivariate lognormal distribution is determined by a Hamiltonian Monte Carlo method. From the moisture content and propagation velocity database of wood, probability distribution with the probability density at which the probability on the minus side of the bivariate becomes zero on the axis perpendicular to that surface on the coordinate surface of "function of moisture content-propagation velocity" Storage means storing an expression representing a curved surface together with its parameters;
A propagation velocity measuring means for determining stress wave propagation velocity C ₀ of wood to be measured;
Estimated value calculation means for obtaining an estimated value of water content from the propagation speed C ₀ and a peak point on the probability distribution curved surface obtained from the parameter;
Moisture content estimation device of wood characterized by having.   The wood moisture content estimation device according to claim 6, wherein a credit interval of the value is determined in addition to the estimated value of the moisture content.   The wood moisture content estimation device according to claim 6, wherein a square of an inverse number of the stress wave propagation speed is used as a function of the stress wave propagation speed.   The moisture content estimation device for wood according to claim 6, wherein a formula representing a bivariate lognormal distribution is used as the probability distribution in which the probability on the negative side of the bivariate is zero. 10. The wood moisture content estimation apparatus according to claim 9, wherein each parameter of the bivariate lognormal distribution is determined by a Hamiltonian Monte Carlo method.

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