WO2017072404A1

WO2017072404A1 - Method for determining properties and defects of wood while cutting it

Info

Publication number: WO2017072404A1
Application number: PCT/FI2016/050736
Authority: WO
Inventors: Markku Tiitta
Original assignee: Puumit Oy
Priority date: 2015-10-26
Filing date: 2016-10-19
Publication date: 2017-05-04
Also published as: FI127963B; FI20150296A

Abstract

The present invention relates to a method for the determination of the properties and defects in wood while cutting it. With the method in accordance with the invention, electrodes (1) are used to create a dynamic electric field in wood (2) in order to measure the electrical complex impedance using at least two frequencies for calcu- lating the parameters required for determining properties and defects of wood in the effective electric field.

Description

METHOD FOR DETERMINING PROPERTIES AND DEFECTS OF WOOD

WHILE CUTTING IT

The present invention relates to a method in accordance with claim 1 for the determination of properties of wood while cutting it. Detection of wood decay in green wood using electrical method has been known for decades (US3864627). Using a well-known method, a hole is drilled into tree, an electrode is put into the hole and by measuring the change in electrical conductance between the electrodes, it is possible to detect the position of decay inside a tree. Nowadays when tree is cross-cut using a harvester, the estimation of decay is made by visual analysis from cross-cutting of a tree.

Electrical methods have been widely used for the determination of wood moisture content because the effect of moisture on the electrical properties of wood is significant (WO2007028856A1, US2003146767). At low moisture levels, for example, the electrical properties of wood change exponentially rather than linearly as a func- tion of the moisture content. Two commonly used methods for measuring the moisture content of wood is the determination of the electrical resistance of wood and the measurement of the dielectricity of wood. When ohmmeters are used, the electrodes are inserted into the wood, meaning that the material must be partially broken. With measuring methods based on dielectricity, surface contact is usually enough, which makes this method non-destructive. In practice, the reliability of meters relying on surface contact is poor, which is largely due to the fact that the method is extremely sensitive to surface moisture.

The maximum limits for the measuring range of ohmmeters are determined by the saturation point of the wood fibres. At moisture contents exceeding the fibre satura- tion point, the cell cavities of wood contain free water in addition to the water bound to the fibres, and thus the electrical resistance of wood does not, beyond this point, really change as a function of the moisture content. With methods based on dielectricity, it is possible to measure wood moisture levels at moisture contents exceeding the fibre saturation point up to over 100%. In theory, the dielectric constant of wood increases until the cell cavities are completely filled with water. In reality, the accuracy of dielectric moisture meters decreases when the moisture content exceeds the grain saturation point, which is, among other things, due to the fact that spatial variation in moisture is usually high and a method relying on a single measurement frequency is highly sensitive to variations in the wood surface moisture content. The object of the present invention is to provide a method for the determination of properties and defects of wood while cross-cutting it in order to classify wood and to use the information in further processing. More specifically, the object of the invention is to provide a method that allows early classification of wood quality prop- erties to increase the efficiency of wood use, processing, and yield and in addition allows the determination of, e.g. superior quality wood material to be utilized in the most demanding applications of wood.

The object of the invention is achieved with a method characterized by which is presented in the claims. With the method in accordance with the invention, electrodes are used to create dynamic electric field in wood, measure the complex electrical impedance using at least two frequencies and determine the properties and defects of wood material in the effective electric field. When a tree is cross-cut, a chain saw/cutting blade cuts the tree. When electrodes are attached to the cross-cutting system dynamic electric field is generated into wood during the cross-cutting process and then a continuous electrical spectral response is measured. The complex electrical spectral response is affected by the wood properties and the response can be used to determine wood properties and defects and the results can be utilized in the further processing. Thus, it is possible to classify the wood according to its properties at the very early stage of wood processing and logistics in forest industries to increase the efficiency of wood processing and use.

With the method in accordance with the invention, electrodes are used to create a dynamic electric field at alpha and beta dispersion frequency range in wood when cross-cutting a wood. When electrical parameters from both dispersion ranges are measured simultaneously, wood properties are determined more accurately and thus the properties can be taken account in further processing of wood and/or the forest to be cut.

According to studies, e.g. resin content, decay resistance, decay rate, moisture content, density and strength can be determined from fresh and dried wood using im- pedance spectroscopy. When conducting continuous scanning of wood, the position of the beginning and the end of decay and heartwood can be simply determined by monitoring the change in electrical parameters. The electrical properties of decayed wood are very different from sound sapwood or heartwood. Also, the electrical properties of dead heartwood are significantly different from sapwood. It is essen- tial to use at least two different frequencies from alpha and beta dispersion region to measure and compensate the effect of moisture content and density.

In another preferred embodiment of the invention, the electrical complex spectrum is measured with one or several electrodes placed on the chainsaw guide bar, cross- cutting blade and/or electrode(s) in its immediate vicinity. In the method, the properties and their proportions and distributions are continuously determined when cross-cutting the wood. The determined impedance parameters are differently sensitive to various wood properties and defects.

In another preferred embodiment of the invention, the electric field is measured from a multitude of positions of chainsaw guide bar and/or blade.

With the method in accordance with the invention, the method is used to determine, e.g. heartwood and sapwood proportion in the cross-cut, decay distribution in the cross-cut, decay resistance, chemical properties, moisture distribution, resin content in the cross-cut and/or the cross dimension with and without the bark at the position of the cross-cut.

With the method in accordance with the invention, the cross-cutting of tree and the changes in the electrical spectrum are determined continuously. Thus the borders of e.g. decay and heartwood can be determined. After the tree has been cross-cut and the complex electrical spectral response is measured from the whole cross-section, wood properties can be quantitatively determined from the calculated electrical parameters. In the impedance spectroscopy method (hereinafter the IS method), a variable electric field is generated inside the specimen to be measured by means of electrodes. When electrodes are at the position of e.g. sapwood, the local properties of sapwood can be determined, and correspondingly, e.g. local properties of heart- wood and/or decay can be determined.

In another preferred embodiment of the invention the electrical impedance spectrum is measured using the cross-cutting blade as an active electrode and the passive electrode can be connected, e.g. to sapwood or ground. Then the whole blade affects the measurement and then it is possible to achieve a very good contact with the tree or wood to be measured and cross-cut. The determination of, e.g. decay proportion, decay rate, heartwood proportion and resin content is possible by comparing the spectral responses at the different stages of cutting. For example, the decay proportion of the cross-section results in decreased impedance and the heartwood proportion results in increased impedance. When using this type of measurement system, it is possible to achieve reliable and durable measurement solution without contact problems. On the other hand, the sensitivity to small objects is reduced because of the large electric field. The electrical properties of the object affect the result when they are in the effective electric field which is determined by the used electrode configuration. The functionality of the method is especially affected by the fact that the sensitivity is the highest for the electrical properties of the material in the near vicinity of the active electrode.

With the method in accordance with the invention, it is possible to determine resin content, decay resistance, decay, moisture content, density and the strength of a liv- ing tree when cross-cutting it. The impedance measurement can be conducted using small impedance chip and thus the measurement module is inexpensive and small. The measurement rate has been developed and it is possible to conduct more than thousand measurements per second and thus the measurement rate is enough for the current industrial application. The method can be integrated e.g. to the harvester cross-cutting head.

Resin acid content and decay resistance of pine heartwood has a significant correlation and it can be determined using electrical impedance spectroscopy. The correlation is clear both in fresh and dried wood. According to studies, resin acid content, decay resistance, decay rate, moisture content, density and strength of wood can be determined using electrical impedance spectroscopy. When practical scanning measurements were conducted on the cross-section of cut wood from surface to the pith, it was possible to determine the border of sapwood and heartwood and also the difference of resin content between the heartwood samples which indicates the potential of the method to estimate the resin acid content and decay resistance. For ex- ample the resin acid content of pine heartwood has a known relation to the decay resistance. The properties of fresh wood have known relations to the properties of dried wood and processed wood. Thus, by using the method, it is possible to estimate the properties of the end product in the beginning of the industrial wood processing chain. In the following, the invention is presented in greater detail with reference to the attached drawings where

Figure 1 is a schematic of one embodiment of the method in accordance with the invention where the measurement is conducted using two electrodes in the chainsaw guide bar and Figure 2 is a schematic of another embodiment of the method in accordance with the invention where the active electrode is connected in the chainsaw guide bar and the blade is used as a passive electrode and

Figure 3 is a schematic of another embodiment of the method in accordance with the invention where the active electrode is the blade.

Figures 1-3 show a cross-section of wood (2) and a wood cross-cutting head including a chainsaw guide bar (3). Figures 1-2 show the electrode(s) (1). In Figure 3 there is no separate electrode but the cutting blade is used as an active electrode. Additionally, the schematic 1-3 show the impedance measuring device 4 to which the electrodes are connected. Furthermore, the system includes the control unit 5, to which the measuring device has been connected. The control unit incorporates the necessary technology for modifying the received signals, processing the data, determining the properties of the wood according to predetermined calibrations and sends the output based on the determinations to be used for further processing. The number and the size of the electrodes may vary from one embodiment to another as preferred and they can be placed in the desired and appropriate manner relative to the cutting blade and the cross-cutting process of wood. It is essential that there is no low ohmic direct path between the active and passive electrode. The active and/or passive electrode may be capacitive to overcome the short circuit problem. The following section provides a description of the invention by means of examples.

Example 1 : Determining wood quality while cross-cutting a living tree

The following section provides a description of the first embodiment of the invention with reference to Figure 1 in which properties of wood 2 are measured by using electrodes 3 attached into the chainsaw guide bar or cross-cutting head of a harvester. In this example, the electrodes 1 are 10 mm diameter round film electrodes which are attached in to the chainsaw guide bar 3 and electrically isolated from it. One electrode is active and another one is connected to the signal earth. Before the measurements are started, the impedance measurement is calibrated using open and closed circuit corrections. The electrodes are connected to the impedance spectrometer 4 with which the impedance measurement can be carried out. The electrical complex impedance is determined based on the reference value and the measurement reading in the control unit S at each measuring frequency. In this case, the control unit 5 consists of a computer including an analogue/digital converter that converts the measured signals suitable for use in the control software. The computer includes a software application that monitors the measurement channels for the impedance. The same computer is used for analysing the data and it sends the wood quality data to be used for further processing of the wood. The determined quality data may be e.g. monitored, saved and used for the classification of the wood.

For each species of wood, there are predetermined calibration values which are used as a reference to analyse the measured data. There are specific limit values for ex- ample for detecting decay and heartwood and these limit values are based on theoretical calculations that are also verified experimentally.

Measurement is activated before cutting the tree in order to obtain initial parameters when the chainsaw guide bar and electrodes are in the air. The IS method can be used for assessing the wood moisture content while at the same time by using other frequency or frequencies of complex impedance spectrum to determine decay, decay rate , heartwood content and decay resistance when cross-cutting the wood. Once cutting starts, the changes taking place in the impedance spectrum are monitored at least at two frequencies in terms of the electrical impedance and phase. In this case the determination takes place at the frequencies 4 kHz, 20 kHz, and 80 kHz. Additionally, if present the position of the borders of decayed region and/or heartwood region are determined from the continuous scanning data.

Quality properties of wood can be determined, e.g. by measuring impedance modulus and phase angle at the frequencies 4 kHz, 20 kHz, and 80 kHz. In this example, the quality classification is based on the measured values and their variability in the scanning measurement data of the cross-cut wood. To enhance the speed of the classification the simplest way is to use pre-determined limit values for each frequency and thus when any measured value exceeds the limit, the wood can be classified according to that. When using small film electrodes the spatial accuracy and resolution is high and the effective measuring depth is determined according to the electrode size and the distance between the electrodes. Thus it is possible to accurately determine the borders of heartwood, sapwood and decay. Each measurement parameter has limit values where the parameter value may vary to belong to the certain quality class.

Example 2. Application of multiple variable methods In this example of an embodiment of the invention, the method is similar to example 1 with reference made to Figure 1 through 3, except that the method used for analysing the measurements is a multiple variable method. The multiple variable method may, for example, involve the use of multiple variable regression, principal component regression (PCR) or partial least squares (PLS). By making measurements on the calibration samples, it is possible to formulate a multiple variable matrix that shows, in separate columns, the actual quality class measured from each calibration sample as well as the values for the specified impedance spectroscopy parameters. The quality class of wood can be determined by means of a standard destructive test method after the cutting and measurement. By using one of said multiple variable methods, the error between the determined quality function application and the actual quality can be minimized. This yields a multiple variable matrix that contains the factors for the parameters as well as the constant factors. The quality of wood is determined by measuring the impedance spectroscopy parame- ters while cross-cutting the wood and using the quality functions based on the specified matrix factors.

Example 3. Cross-cutting blade as an active electrode

In this example of an embodiment of the invention, the method is similar to example 1 and 2 with reference made to Figure 2, except that the cross-cutting blade is used as an active electrode and the passive electrode (signal ground) is connected e.g. to sapwood or ground. The ground connection can be made by using electrode connected to the cross-cutting machine to achieve the connection to sapwood or ground. In this example, the effective dynamic electric field is spatially larger but still the sensitivity is the highest in the near vicinity of the active electrode. The electrical properties of materials near the active electrode have the highest effect on the measurement. The spatial size of the electric field depends on how the electrodes are connected. In practice, the electrical properties of the region between the electrodes has the highest impact to the response. Because the cutting blade is the active electrode it is possible to achieve a very good contact with the wood. When using this type of measurement system, it is possible to measure larger spatial partition of wood compared with the example 1. On the other hand, the spatial accuracy of detecting borders is lower than in example 1.

Example 4. Cross-cutting blade as a signal ground electrode

In this example of an embodiment of the invention, the method is similar to exam- pie 2 with reference made to Figure 3, except that the cross-cutting blade is used as an signal ground electrode and the active electrode(s) are connected e.g. to the chainsaw guide bar. The electrical properties of materials near the active electrode have the highest effect on the measurement. When using this type of measurement system, it is possible to measure larger spatial partition of wood compared to the example 1 but smaller than in the example 3. On the other hand, the spatial accuracy of detecting borders is lower than in example 1 but better than in example 3. The electrical properties of the material in the vicinity of the active electrode has the major effect on the response.

Example S. Impedance spectroscopy model application In this example of an embodiment of the invention, the method is similar to examples 1 through 4 or examples 6 and 7, and reference is made to Figure 1-3. In this embodiment, a broad impedance spectrum is measured extending at least from the frequency band of 10 kHz to 100 kHz. An electrical model, the parameters of which are used for determining the quality, is fitted in the complex spectrum. For example, the Gauss-Newton or Nelder-Mead optimization method may be used in the model application.

Example 6. Multiple electrodes application

In this example of an embodiment of the invention, the method is similar to examples 1 through 5 or example 7, and reference is made to Figure 1-3. In this embodi- ment, several IS electrodes are used for the purpose. Thus it is possible to achieve more accurate spatial resolution and detection of borders compared with the methods using one electrode pair.

Example 7. Various wood product applications

In this example of an embodiment of the invention, the method is similar to exam- pies 1 through 6, and reference is made to Figure 1-3. In this embodiment, the wood to be cut is not a living tree but the method can be applied to other types of wood- based materials. When cutting the material, the properties and defects of material can be determined. The materials include different types of wood products e.g. sawn timber, planed timber, massive timber, glued wood products and composites. The practical applications of the invention are not limited to the above examples; instead, the invention and its embodiments may be varied within the scope of protection provided by the claims.

Claims

CLAIMS:

1. A method for the determination of the properties and defects of wood while cross-cutting it, characterized in that electrodes (1) are used to create at least one dynamic electric field in wood (2), measure the complex electrical imped- ance using at least two frequencies for determining properties and defects of wood in the effective electric field.

2. A method in accordance with claim 1, characterized in that the method is used to monitor cross-cutting of wood by electrical impedance spectroscopy method at alpha and beta dispersion regions (100 Hz - 1 MHz).

3. A method in accordance with claim 1 or 2, characterized in that the electrical complex spectrum is measured using one or several electrodes placed on the cross-cutting blade and/or chainsaw guide bar and/or with the electrodes in the immediate vicinity of them.

4. A method in accordance with one of the claims 1 to 3, characterized in that the electric field is measured from several measuring points on the cross- cutting blade and/or chainsaw guide bar.

5. A method in accordance with one of the claims 1 to 4, characterized in that measured impedance spectral responses are different for different properties and defects.

6. A method in accordance with one of the claims ltoS, characterized in that the proportions and distributions of wood properties are determined based on the measurement taking place continuously while cross-cutting of a wood.

7. A method in accordance with one of the claims 1 to 6, characterized in that the proportion of sapwood and heartwood is determined from the cross-cut.

8. A method in accordance with one of the claims 1 to 6, characterized in that the moisture, density and/or decay distribution of wood is determined from the cross-cut.

9. A method in accordance with one of the claims 1 to 6, characterized in that decay resistance, chemical constituents, moisture content, density, strength and/or resin acid content of wood are determined from the cross-cut.

10. A method in accordance with one of the claims lto6, characterized i n that the diameter of the wood with and without the bark is determined from the cross-cut.