EP1943512A1

EP1943512A1 - Apparatus and method for detection of relative resistivity

Info

Publication number: EP1943512A1
Application number: EP06793878A
Authority: EP
Inventors: Daniel Karlsson; Frans Linander
Original assignee: Rotfinder AB
Current assignee: Rotfinder AB
Priority date: 2005-09-28
Filing date: 2006-09-28
Publication date: 2008-07-16
Also published as: WO2007039537A1

Abstract

The invention relates to an apparatus and a method for detection of relative resistivity in objects, and more particularly for detection of wood decay in living trees. The apparatus comprises: a controllable constant current source (11); a pair of current delivering electrodes (2A; 2B) connected to the constant current source for delivering current to the object (7); a voltage meter (12); a pair of measurement electrodes (3) connected to the voltage meter and connectable to the object for measuring a voltage caused by the delivered current. The apparatus further comprises processor means (10) being connected to an input device (4) for receiving data relative to the diameter of the object; the processor means (10) being arranged to control the constant current source (11), and to evaluate the measurement result in dependence of the diameter of the object.

Description

APPARATUS AND METHOD FOR DETECTION OF RELATIVE

RESISTIVITY

Field of the invention The present invention relates to an apparatus and a method for detection of relative resistivity in objects, and more particularly for detection of wood decay in living trees. A four-point resistivity method is used in which a low-frequency alternating current is applied to a stem of a tree and the induced voltage is measured between two points along the stem. The effective resistivity of the stem is estimated based on stem cross-sectional area. Trees with butt rot generally have an effective resistivity that is at least a factor of two lower than that of healthy trees.

State of the art

A large percentage of living trees have some kind of decay that reduces their economic value. Trees with decay are usually either left to decompose or are used as fuel wood. In 2001, timber losses in Sweden caused by decay equalled 15% of the annual harvest. An accurate method of detecting decay in living trees would be useful both during the harvesting process and when forested land is assessed for sale or for designation as a conservation area. Infection of trees by wood decay fungi occurs mostly in roots, where damage to the bark allows fungal invasion.

Fungal infections can also spread by way of the grafted roots of neighbouring trees. As the fungus invades the stem, it causes movement of water to the area of mycelial growth, thereby mobilizing metal ions released by damaged tree cells. This causes a decrease in the resistivity of the affected wood compared with healthy wood (Shortle and Smith 1987).

The resistivity of living tree stems can be investigated with the relative impedance in situ examination (RISE) method (Bengtsson 1997) for detecting decay in living trees. This simple four-point method is based on estimation of effective resistivity, the difference in induced voltage between two points along a stem. Decay can be detected by comparing the effective resistivity of a single tree to that of other trees measured under similar conditions, i.e., temperature, humidity, site conditions and time of year.

Four-point measurements are made by passing a current through an object with one pair of electrodes, while measuring the voltage difference with another pair of electrodes (Figures Ia and Ib). A healthy tree gives a higher voltage difference than a decaying tree because decay reduces tissue resistivity. The resistance normalized for stem cross-sectional area provides an absolute value of resistivity that is related to the amount of decay. The effective resistivity of wood depends on water content and temperature, making it difficult to use the resistivity of an individual tree for the detection of decay. Instead, resistivity of an individual tree is compared with that of other trees measured under similar conditions.

The four-point resistivity method is previously known from SE 519 442 and SE 519 444 as well as from the paper Nondestructive detection of decay in living trees, Tree Physiology 24, pp.853-858 © 2004 Heron Publishing - Victoria, Canada.

A problem with the prior art is that the induced the current through the tree has to be adapted to the tree being measured so that the measured voltage difference falls within a range that can be measured with accuracy and is not covered by noise. Also, the prior art does not take into account that different fungi cause different resistivity changes, and that the temperature affects the resistivity as well.

Summary of the invention

The present invention solves the problem by adapting the output current to the diameter of the tree being measured.

In a first aspect, the invention provides an apparatus for measurement of relative resistivity in a cylindrical object comprising: a controllable constant current source; a pair of current delivering electrodes connected to the constant current source for delivering current to the object; a voltage meter; a pair of measurement electrodes connected to the voltage meter and connectable to the object for measuring a voltage caused by the delivered current.

According to the invention the apparatus further comprises processor means being connected to an input device for receiving data relative to the diameter of the object; the processor means being arranged to control the constant current source, and to evaluate the measurement result in dependence of the diameter of the object.

Suitably, the processor means is arranged to control the constant current source in dependence of the resistivity of the object. In one embodiment, the apparatus is arranged to perform two consecutive measurements, wherein a factor k is applied to divide the measured value of a further measurement, before evaluating the measurement result, so that the measurement result may be presented as a pair of values.

In a second aspect, the invention provides a method for measurement of relative resistivity in a cylindrical object comprising: delivering current to the object; measuring a voltage caused by the delivered current; inputting data relative to the diameter of the object; controlling the delivered current; and evaluating the measurement result in dependence of the diameter of the object.

In one embodiment, a factor k being applied to divide the measured voltage, before evaluating the measurement result.

In another embodiment, two consecutive measurements are performed, a factor k being applied to divide the measured voltage of a further measurement, before evaluating the measurement result. The further measurement may be performed to detect different attacks on a tree.

The invention is defined in claims 1 and 16, while preferred embodiments are set forth in the dependent claims.

Brief description of the drawings

The invention will be described below in detail with reference to the accompanying drawings, in which: figs. Ia and Ib show the four-point method applied on a healthy tree and a tree with decay, respectively; fig. 2 is a front view in perspective of the apparatus according to the invention with current electrodes; fig. 3 is a rear view in the perspective of the apparatus; fig. 4 is a front view of the apparatus; fig. 5 is a block diagram of components of the apparatus; and fig. 6 is a diagram of measurement results without a factor k applied to the left, and with the factor k applied to the right.

Detailed description of preferred embodiments

As mentioned in the introduction, the present invention is particularly useful for measuring decay in trees. The invention is also applicable to measurements in other objects when it is desired to measure entities depending on the relative resistivity.

Figs. Ia and Ib show a four-point resistivity measurement in a healthy tree and a tree with decay, respectively. Current I is passed through a portion of a tree by attaching suitable current delivering electrodes. The current I gives rise to equi- potential surfaces, as shown by the lines. The current density is represented by arrows. In a decayed tree, there is usually a rot cone at the root end as indicated. The rot cone lowers the relative resistivity in a marked manner. By measuring the voltage ΔV between two measurement electrodes, a value related to the resistivity is obtained. The resistivity is proportional to the square of the tree diameter D, which is accounted for in the previously known four-point method. However, to create the voltage ΔV in a range suitable for measurement also the delivered current I has to be adapted to the tree diameter D. The present invention provides an apparatus in which this and other practical problems are solved. In figs. 2, 3 and 4 various views of the apparatus according to the invention are shown. The apparatus 1 comprises a box housing the electronic components. The apparatus comprises a pair of current delivering electrodes attached by cables 8. The top electrode 2 A is suitable a spike attached to a handle. The lower electrode 2B is suitable an earth rod to be inserted in the ground near the root of the tree to be measured.

A pair of measurement electrodes 3 is attached to the rear of the apparatus, as shown in fig. 3. When performing a measurement the apparatus 1 is held by means of handles 6 and pressed with the electrodes 3 against the stem of a tree. The cables 8 are sufficiently long to allow for measurements at various heights of the stem.

As may be seen from fig. 4, the apparatus is provided with an input means suitably in the form of buttons 4 or the like and a display 5 for showing the result of the measurement.

In fig. 5 the electronic components of the apparatus are shown. The input device 4, the display 5, the pair of current delivering electrodes 2A and 2B and the pair of measurement electrodes 3 are also shown here with the same reference numerals. The key components of the invention are a processor CPU 10, a constant current source 11 and a voltage meter 12. Conveniently the apparatus also includes a clock, a data flash memory, a USB circuit, a USB port, a charging circuit, a battery, a stepup/stepdown circuit and a voltage inverter. The operation of these circuits will be well known to a person skilled in the art and is not described in detail here.

In one embodiment of the present invention the current I delivered to the tree is dependent upon the diameter of the tree. Generally, the more current is needed, the larger cross sectional area the current has to pass. Thus, the current I should be proportional to the square of the diameter. Of course, a tree is not perfectly circular, but it is a sufficiently good approximation. If the tree has an irregular shape, e.g. is more elliptic in cross section, an average diameter can be used.

For a reliable reading only three ranges of tree diameters may be sufficient. In an embodiment of the present invention the following ranges are used:

1. Diameter 10 - 19 centimeter: I = 0,08 mA

2. Diameter 20 - 39 centimeter: I = 0,31 mA

3. Diameter 40 - 80 centimeter: I = 1,22 mA

Suitably the input device 4 comprises a separate button for each range. The user may easily select the correct range by visual inspection of the tree. In an alternative, the input device is graduated e.g. in 5 cm steps, and the user inputs the tree diameter in cm. Then the apparatus selects a suitable current range.

In a further embodiment, a suitable current range is selected automatically by the apparatus. Starting from the lowest current range, the current source 11 is controlled to feed current through the measured object. If the measured voltage reading is too low with respect to the obtained voltage level, the next current range is tried, until a reading falling in a suitable interval is obtained. In this way, the current need not be dependent on the tree diameter, so that the measurement is always performed in the most suitable current range. However, the user must still select the correct tree diameter which is needed in the calculation of the measurement result.

The processor 10 controls the constant current source 11 by means of a pulse width modulated (PWM) square wave. The frequency is suitably 500 Hz, but this is not critical. 300 Hz has also been used, while frequencies approaching direct current may encounter problems.

In alternative embodiments, the input device 4 may comprise a potentiometer, suitably graduated with a scale directly referring to the diameter of the tree. The potentiometer may be a continuously adjustable potentiometer or a resistor network arranged in suitable stages or implemented by means of analog switches controlled by digital signals. It may also be a digital component including a multiplying digital to analog converter.

The voltage ΔV is measured by means of the fixed measurement electrodes 3, which are connected to the voltage meter 12. Suitably the voltage meter comprises an instrument amplifier, a rectifier and a bandpass filter. The maximum output voltage from the current source 11 is approximately 4V, and the measured voltage usually falls within the range of 0 - 3.3 V using the current I values as set forth above. The resistivity value is also proportional to the cross sectional area of the tree. Therefore, the measured voltage value is multiplied by the square of the tree diameter D to obtain a measurement value R. By using a suitable constant scale factor, the measurement value R is in the range from 0 - 255, the larger value corresponding to the larger relative resistivity and a healthier tree.

By performing measurements of a great number of trees, a suitable scale has been defined to evaluate individual trees. The scale is particularly valid for spruce. A lower scale value or limit has been selected as 50. If the measured value R < 50, then the tree is assigned scale mark 10. This corresponds to a decayed tree with a high certainty (disregarding other factors influencing the relative resistivity).

A higher scale value or limit is selected as 220. If the measured value R > 220, then the tree is assigned scale mark 0. This corresponds to a healthy tree.

The measurement values 51 - 219 are assigned to scale marks 9 - 1 with a uniform spacing. Scale marks 9 - 1 represent trees with varying decreasing decay.

However, the resistivity of a tree is affected by several factors besides the diameter, such as the tree characteristics including the kind of metal ions accumulated, different types of pathogenic attacks from different fungi or other organisms, the surrounding temperature, the variety and species of the tree. For instance, an attack from the fungus Heterobasidion spp results in a higher decrease of the resistivity than an attack from the fungi Armillaria or Stereum Sequiholefum. At lower temperatures, below approximately 5⁰C and down to -5⁰C, the resistivity increases with a factor of approximately 2.

To take into account various situations, the apparatus is suitably provided with at least two measurement ranges. In the first measurement range, the apparatus is adapted for detecting e.g. Heterobasidion spp during normal temperatures. In a second measurement range, the apparatus is adapted for detecting e.g. Armillaria and/or Stereum Sequiholefum and/or for operation at low temperatures. In the second measurement range a factor k is applied to divide the measured voltage. The factor k is suitably 2.

Figure 6 is a diagram of measurement results without the factor k applied to the left, and with the factor k applied to the right. The bars at A represent measurements for Heterobasidion spp, while the bars at B represent measurements for Armillaria. Bar Al is the measurement result for Heterobasidion spp during normal temperatures. R ranges from 0 to 10, if there is a Heterobasidion spp attack. (10 is the maximum output value even if the resistivity is lower, and 0 is the minimum output value even if the resistivity is higher.) However, at lower temperatures, the resistivity generally is higher as shown by bar A2, resulting in that some trees would be indicated as healthy (R = 0), when in fact they are not. Since the user is aware of the temperature, he should then apply the factor k, raising the bar as shown by the bar A4. Now all trees attacked by Heterobasidion spp are indicated as decayed to a more or less degree. In one embodiment, the input device 4 comprises a separate button for applying the factor k.

If the factor k is used during normal temperatures bar Al would be raised as shown by the bar A3. For Heterobasidion spp this value does not give any particularly useful information. However, an attack by Armillaria, Stereum Sequiholefum or other causes may result in only a small decrease in resistivity as shown by the bar Bl at normal temperatures and B2 at lower temperatures. Such a measurement would most probably result in an indication of a healthy tree. By performing a second measurement with the factor k applied, the user may make a further check. If there is a small decrease in resistivity, the bars Bl and B2 will be raised as shown by the bars B3 and B4, respectively, yielding indications with R > 0, i.e. indicating decayed trees. If the tree is healthy, the resistivity is so high that R will equal 0 even with the factor k applied.

In one embodiment, the apparatus is arranged to make consecutive measurements twice, without and with applying the factor k, so that the output measurement result is a pair of values for each tree. See the table below.

Note that cases 2, 3, and 4, will be distinguished form a healthy tree using the further measurement range with the factor k applied. The values are only illustrative examples and not the results of actual measurements. As another example, if a user reads an output of (0,7) during a normal temperature, he may deduce that the tree is most probably free from Heterobasidion spp but attacked by something else, possibly Armillaria. He will of course also use his professional experience and knowledge about the locality when forming his opinion.

Note that the exact value of 2 for the factor k is selected also for practical reasons as integer values are more easily implemented in the apparatus. An ideal factor, in other words the ratio between two measurement ranges for two expected attacks by different fungi, may actually be e.g. 1.7 or 2.2 etc, but in the general the value 2 yields sufficiently informative results.

Using the inventive principle, further measurement ranges may be provided by applying further factors, e.g. k = 2, 3, ... (integers) or 0.5, 1.7, 2.2, 2.9, ... (decimal numbers) etc.

A person skilled in the art will appreciate that the apparatus may be implemented by means of various combinations of hardware and software as outlined above. The scope of the invention is only limited by the accompanying claims.

Claims

1. An apparatus (1) for measurement of relative resistivity in a cylindrical object comprising: a controllable constant current source (11); a pair of current delivering electrodes (2A, 2B) connected to the constant current source (11) for delivering current to the object; a voltage meter (12); a pair of measurement electrodes (3) connected to the voltage meter (12) and connectable to the object for measuring a voltage caused by the delivered current; characterised by: processor means (10) being connected to an input device (4) for receiving data relative to the diameter of the object; the processor means (10) being arranged to control the constant current source (11), and to evaluate the measurement result in dependence of the diameter of the object.

2. An apparatus according to claim 1, characterised in that the constant current source (11) is controlled so that the delivered constant current is approximately proportional to the square of the diameter of the object.

3. An apparatus according to claim 1, characterised in that the constant current source (11) is arranged to deliver the constant current in three steps in dependence of the diameter of the object.

4. An apparatus according to any one of the previous claims, characterised in that the input device (4) is graduated with a scale directly referring to the diameter of the object.

5. An apparatus according to claim 4, characterised in that the input device (4) is connected to a potentiometer.

6. An apparatus according to claim 4, characterised in that the input device (4) is connected to the processor means (10).

7. An apparatus according to claim 1, characterised in that the processor means (10) is arranged to control the constant current source (11) in dependence of the resistivity of the object.

8. An apparatus according to claim 7, characterised in that the processor means (10) is arranged, to control the current source (11) to feed current through the measured object, starting from a low current range, and if the measured voltage reading is too low, a higher current range is tried, until a reading falling in a suitable interval is obtained.

9. An apparatus according to any one of the previous claims, characterised in that the processor means (10) is arranged to calculate the measurement result as a measurement value R proportional to the measured voltage and to the square of the diameter of the object.

10. An apparatus according to claim 7, characterised in that the measurement result is assigned to a scale that may be presented on a display on the apparatus.

11. An apparatus according to claim 8, characterised in that the object is a tree, and that the maximum measurement value is 255 and the minimum measurement value is 0, the scale being defined as:

R < lower limit is assigned scale mark 10 corresponding to a decayed tree, R > higher limit is assigned scale mark 0 corresponding to a healthy tree, lower limit < R < higher limit is assigned scale marks 9 to 1 in equal steps corresponding to trees with decreasing decay.

12. An apparatus according to any one of the previous claims, characterised in that the apparatus is arranged to perform two consecutive measurements, wherein a factor k is applied to divide the measured voltage of a further measurement, before evaluating the measurement result, so that the measurement result may be presented as a pair of values.

13. An apparatus according to claim 12, characterised in that the factor k equals

2.

14. An apparatus according to any one of the previous claims, characterised in that the pair of measurement electrodes (3) is attached to the apparatus with a fixed distance between the measurement electrodes (3).

15. An apparatus according to any one of the previous claims, characterised in that the pair of current delivering electrodes consists of one top electrode (2A) attachable to the stem of a tree, and one lower electrode (2B) comprising an earth rod.

16. A method for measurement of relative resistivity in a cylindrical object comprising: delivering current to the object; measuring a voltage caused by the delivered current; characterised by: inputting data relative to the diameter of the object; controlling the delivered current, and evaluating the measurement result in dependence of the diameter of the object.

17. A method according to claim 16, wherein the delivered current is controlled so that it is approximately proportional to the square of the diameter of the object.

18. A method according to claim 16, wherein the delivered current is controlled in dependence of the resistivity of the object.

19. A method according to claim 18, wherein the delivered current is controlled to be fed through the measured object, starting from a low current range, and if the measured voltage reading is too low, a higher current range is tried, until a reading falling in a suitable interval is obtained.

20. A method according to any one of claims 16 to 19, wherein the measurement result are calculated as a measurement value R proportional to the measured voltage and to the square of the diameter of the object.

21. A method according to claim 20, wherein the object is a tree, and the maximum measurement value is 255 and the minimum measurement value is 0, the measurement result being assigned to a scale defined as: R < lower limit is assigned scale mark 10 corresponding to a decayed tree, R > higher limit is assigned scale mark 0 corresponding to a healthy tree, lower limit < R < higher limit is assigned scale marks 9 to 1 in equal steps corresponding to trees with decreasing decay.

22. A method according to any one of claims 16 to 21, wherein a measurement is performed, a factor k being applied to divide the measured voltage of the measurement, before evaluating the measurement result.

23. A method according to claim 22, wherein the object is a tree and the measurement is made at temperatures below approximately 5⁰C.

24. A method according to any one of claims 16 to 21, wherein two consecutive measurements are performed, a factor k being applied to divide the measured voltage of a further measurement, before evaluating the measurement result.

25. A method according to claim 24, wherein the measurement result is presented as a pair of values.

26. A method according to claim 22 or 23, wherein the further measurement is performed to detect different attacks on a tree.

27. A method according to any one of claims 22 to 26, wherein the factor k equals 2.