METHOD AND DEVICE FOR MEASUREMENT OF PROPERTIES OF WOOD

The invention relates to a method for the measurement of properties of wood which is specified in the preamble of claim 1. Furthermore, the invention relates to a device for the measurement of properties of wood which is specified in the preamble of the first device claim.

Mechanic harvesting of fuel wood has become more and more common in recent years. Harvested standing timber is principally wood of small diameter, the base diameter being about 5-25 cm. The harvested wood usually goes for processing through chipping or chopping, whereby a cut surface created in the wood in connection with felling does not have similar quality requirements as a cut surface of raw wood going to the production of sawn timber. Because of this, at the felling heads of a harvester developed for harvesting fuel wood there is most usually a cutting knife or equivalent operating in the so-called guillotine style, cutting with a pressing motion. Then, the volume of harvested wood is not measured with similar measuring devices determining wood volume as when harvesting delimbed log or pulpwood, but usually the volume of harvested wood is estimated either from a stack on the roadside by measuring the perimeter or by defining the weight of wood on a weigher fastened to the crane of a forwarder or a timber carrier, sometimes also a harvester. The specific weight of wood is strongly dependent on its water content, due to which considerable inaccuracies are related to determining the volume of wood from its weight. In harvesting fuel wood, moisture content or net energy content has also been used in addition to volume as the basis of measuring and pricing wood, but this is awkward.

The object of the invention is to introduce a method and a device for measuring the properties of wood by means of which above disadvantages are eliminated. Particularly, the object of the invention is to introduce a method and a device by means of which the diameter, cut surface area and volume of a tree can be reliably determined when cutting the tree.

The object of the invention is provided with a method and a measuring device which are characterised by what is presented in the claims.

In the volume measurement principle described here, the harvester measures the diameter or area of the cut surface of a tree being cut directly from the operation of the cutting knife, of which data it is also possible to deduce the total volume of cut wood. The measurement takes place during cutting and the data is immediately

available. Particularly, the method and the device according to the invention are applicable to measuring and determining the wood diameter, cut surface area and wood volume. Furthermore, the method and the device are applicable to the simultaneous measurement and determination of the combined diameter, cut surface area and volume of two or more tree trunks.

The invention will now be described in more detail with reference to the accompanying drawings in which

Fig. 1 schematically shows the structure and method of a measuring device, Fig. 2 shows thrust required by a cutting means as a function of time, Fig. 3 shows the connection of cutting pressure and diameter with unseasoned spruce, and

Fig. 4 shows the connection of cutting pressure and diameter with unseasoned birch.

A method according to the invention and the operating principle and structure of a measuring device according to the invention are schematically shown in Fig. 1. The cutting device includes a cutting means 1 and a power unit 4 for moving the cutting means. Furthermore, the measuring device includes a monitoring and calculation apparatus connected to the parts of the device, which is not shown in the figures. As the cutting means, there is a means appropriate for the purpose provided with a cutting blade, such as e.g. a cutting knife, plate or some other known means. As the power unit, there is a device appropriate for the purpose which is arranged to move the cutting means against a tree 2 with required force for cutting the tree. As such a device, a hydraulic cylinder is usually used, but also other power units are possible. The power unit, such as the hydraulic cylinder, is arranged to press the cutting means against the tree, whereby the tree is cut in the so-called guillotine style. The travel of the cutting means can be linear (arrow 3) or rotatory in relation to the tree. The pressing force of the hydraulic cylinder is directly proportional to the pressure of hydrostatic fluid which in Fig. 1 is measured by a pressure gauge 6 set in a pipeline 5 bringing fluid in the hydraulic cylinder. In another embodiment, the pressing force can be measured by a power sensor directly connected to the cutting blade.

Fig. 2 shows a thrust pressure curve of a travel cylinder of the cutting means. This clearly illustrates that when the cutting means, such as a cutting knife 3, is moved towards the tree, it is first able to move freely, whereby the thrust required for

moving is small. When the cutting means meets the surface of the tree, the thrust required for moving the cutting means increases substantially, varies according to the properties of wood and then decreases again to the initial level after the tree has been cut. As the travel speed of the cutting knife is known for the whole time of cutting, it is possible to estimate a diameter A (in Fig. 1) of the tree in the travel direction of the cutting knife from the duration of the increase of pressing force. This can also be calculated from a dead travel section C (in Fig. 1) of the cutting knife between the blade of the cutting knife and the surface contact of the tree by deducting this from the total travel length of the knife. As the force required for moving the cutting knife is at its greatest when the most part of the blade length of the cutting knife takes part in cutting the tree, it is possible to estimate a diameter B (in Fig. 1) perpendicular against the travel of the cutting knife of the tree from the greatest pressing force of the cutting knife during cutting. In the case of hydraulic transmission, this can be calculated from the highest pressure peak of hydraulic fluid during cutting. From these dimensions, of one separately or using two dimensions together, it is possible to calculate the area of the cut surface of wood when presuming the cross section of wood circular or elliptic. The area of the peak (in Fig. 2) of the force curve, the pressure curve in the case of hydraulic transmission, during cutting reflects the cut surface area of wood as such.

The ratio of the maximal hydraulic pressure to the cut diameter of spruce and birch in the cutting tests of a prototype of the measuring device is shown in Figs. 3 and 4, from which it is seen that in the typical diameter range 4-8 cm the statistical 95-% confidence interval of the diameter of wood is about 5 mm, i.e., about 10% of the diameter.

The wood volume can be calculated from the diameter of the stub using the Laasasenaho equations, among others. The standard error of volume deduced from the stub diameter with the Finnish wood species pine, spruce and birch is respectively 17.2%, 18.7% and 18.8%. If length data of standing timber is added to the measurement, the standard error is respectively 7.2%, 7.6% and 8.5%. If the cut diameter obtained from the height of 6 m is added to the calculation, the standard error is respectively 3.5%, 3.4% and 3.5%. If the tree trunk is cut as a cut-to-size log at two cutting points, the volume of the log produced can be calculated by applying the model of a tapered cylinder. When measuring a larger wood volume, e.g. when calculating daily output, it is possible to add all cross-section measurements based on cutting and, when the combined total length of the wood grade is observed, the

average total wood volume can be worked out. Then, a measuring error related to the measurement of one log is avoided and the measuring accuracy of a large wood volume is improved.

Comparing Figs. 3 and 4, it is found that with spruce the cutting pressure equates about 30% thicker wood than with birch. This is because the compression strength perpendicular against the birch grains is about double compared to Finnish soft wood species. From this follows that the measuring device is able to distinguish between birch and the softer wood species by comparing the maximal cutting force of the cutting knife with the travel-directional diameter data defined from the free travel when the tree is assumed circular of its trunk. This can be utilised when wishing to register the proportion of birch from the other wood. The use value of birch is somewhat different as fuel wood compared to other common wood species.

When utilising the method and apparatus according to the invention, it is possible to take two or more (thin) trunks into the cutting device, whereby when cutting them the pressing force of the power unit is measured in a way described above during the travel of the cutting means and the cutting of the trees and from the pressing force the property values of trees are calculated. This way, it is possible to quicken the harvesting of wood and simultaneously also to be able to obtain data reliable enough and to monitor the felling work. Equivalently, if a single tree branches before topping, in which there is thus a multi-trunk cutting event, the method gives also in this case data on the combined wood area of the topping point and the determination can be used in the calculation of the total wood volume.

The measurement principle of the device includes confusing factors which are mainly caused by the biological-physical properties of wood. The elasticity of wood varies somewhat according to moisture and temperature, but this effect can be minimised in the measuring device by compiling reference values which observe the seasons and the outside temperature. A branch, rot or other flaw of wood occurring at the cutting point can give an incorrect measuring result which can interfere with the measuring result of a single merchantable log, but the effect of these local flaws on the measuring result of a total wood batch decreases due to averaging as the number of measuring events increase.

With this method, the practical measuring accuracy in defining the volume of merchantable wood is about 10-30%. Even though the variation is this large, the measuring result can be used e.g. in estimating a daily cut output.

The invention is not limited to the described advantageous embodiments, but it can vary within the scope of the inventive idea presented in the claims.