METHOD AND SYSTEM FOR AUTOMATIC DETERMINATION OF TIMBER QUALITY IN FROZEN OR UNFROZEN CONDITION

DESCRIPTION

Technical Field

The present invention relates to a method and Installation for automatic determination of the quality of wooden logs and sawn timber. Quality in this context refers to wood density, strength and/or modulus of elasticity. This determination is used, for example, in modern mills for automatic proper and accurate sorting of logs for various purposes and various sawlog products.

Background Art

Resonance based technology is known and used for non-destructive quality classifying logs and boards. A hammer impulse against the end of a log for example generates acoustic waves in the log which are measured for various parameters such as velocity gradients, propagation, patterns, resonance frequency etc. Such acoustic impulse/resonance methods are described in US 6 782 732 (Huang et al.) and WO 2007/01 1296 A1 (Lindstrom), and are marketed for sawmill use by for example Fibre-Gen (Hitman LG 640) and A-sort AB.

Certain researchers have described the use of microwave scanning to estimate certain properties of a log such as density, moisture content or temperature. See for example "Microwave penetration in wood using imaging sensor", L. Hansson et al. Measurement 38 (2005) 15-20 and L Hansson et al "Finite element modeling (FEM) simulation of interactions between wood and microwaves", J Wood Sci (2006) DOI 10.1007/s 10086-005-0794-8.

Winter use of microwave scanning of timber is dealt with in "How do microwaves respond to winter conditions?" a Technical Report submitted by Olle Hagman et al to the 58 ^th Annual Meeting of the Forest Products Society, Grand Rapids Michigan 27-30 June 2004. It relates that the results of microwave scanning of sawn timber are skewed and unreliable when there is the possibility that a log or board or portion thereof is frozen requiring calibrations for each of those situations. Such calibrations are even impossible at around 0° C.

The acoustic/resonance based methods for determining log quality are simple and reliable as long as there is no chance that the logs or boards are frozen, completely or in the interior. Infrared measurement of temperature is only accurate as to the surface temperature. Frozen log portions give completely deceptive results, indicating a much higher log quality than is the case. A log which should have been sent to the chipper or to a low quality cutting pattern might be sent through the sawmill only to result in boards of bad quality.

It would be a great advantage to be able to make the automatic acoustic resonance quality determination reliable even for use when the logs or portions thereof may still be frozen when entering the sorting yard.

This is made possible by the method and installation according to the invention as disclosed in independent Claims 1 and 6.

Brief Description of Drawings

The present invention will be described in the following with reference to a non- limiting example, illustrated in the accompanying drawings, of which:

Fig.1 shows schematically an exemplary process in accordance with the invention whereby winter logs are quality sorted before breakdown.

Fig. 2 shows microwave scanning of pine timber at 3 different temperatures as well as transmission through air.

Fig. 3 shows acoustic resonance measurement of similar pine timber.

Fig. 4 is a plotting of the maximum acoustic frequencies for a large number of pieces of timber when frozen and unfrozen.

Accoustic resonance methods are used for example to sort logs before breakdown into different machine stress grades (MSG), for improved yield upon sawing, or into G1 and G2 veneer grades. Inaccurate sorting results in poorer sawmill yields. Fig. 1 shows how the system according to the invention could be set up for sorting prior to the individual logs entering the saw mill. An individual log 1a is taken from arriving logs 1 and is subjected to microwave scanning 2 followed by acoustic resonance measurement. It is also, of course, possible to perform these two steps in the reverse order. The results of the microwave scanning and the acoustic resonance measurement of the individual log are processed by computer means 4 and an instruction is sent to the log sorting station to direct the log to the proper line based on its quality, very low quality logs being sent to the chipper line to make biofuel, fiberboard or pulp.

Existing sorting systems usually utilize only acoustic resonance measurement to determine log quality, a hammer impacts the end of the log and microphones record the frequencies and amplitudes of the resulting resonances. The frequency of greatest amplitude can be used as a good indication of the strength and density of the log, a higher resonant frequency corresponding to higher density. When looking at a sample reading shown in Fig. 3 for example, three harmonic peaks a, b, and c are registered with the maximum amplitude at peak a at a frequency of about 700 Hz, corresponding to a certain stiffness in the wood, indicating strength and quality. This measurement is used to automatically steer the log to the line where it will provide the best possible product yield either as veneer of different thicknesses, sawn boards from various cutting patterns or as chips.

A problem with acoustic resonance measurement sorting arises when some of the logs may be frozen when measured. A frozen log often gives a frequency reading when frozen which is twice that of the same log when thawed. It is the frequency of greatest amplitude which is taken by the known system as indicative of log quality. Thus, when referring to Fig. 3, the frequency of 700 Hz would be taken as indicative of the log quality, even if the log were in a frozen state. This anomaly is shown in Fig. 4 which plots the highest resonant frequencies of some 50 logs both when frozen (the Y-axis) and when thawed (the X-axis). Most of the logs register double the frequency when frozen as when thawed (the relevant state for quality). All of the frozen logs registering the double frequency will be sent into the higher quality lines and ultimately produce sawn products which may have to be discarded because the strength of the wood is too low.

It has now been recognized according to the present invention that combining acoustic resonance measurement with microwave scanning may provide a way to more accurately sort logs even in a winter environment. According to the present invention, it is recognized that the microwave scanning of a log which is frozen either completely or only in the middle produces a typical pattern of attenuation. This is illustrated clearly in Fig. 2, showing a microwave scanning pattern across a piece of timber 12, the lateral position of the microwave reading in the timber being shown on the x-axis 13 and the amplitude of the signal being shown on the y-axis 14. The microwave pattern through air is the curve 8, which shows very little distortion. The curve 11 where the wood temperature is +8° C and the curve 10 where the wood temperature is 0° C display a typical attenuation dip for unfrozen wood. The curve for deep frozen wood at -30° C is labeled 9 and displays very little attenuation.

This difference in attenuation is exploited in the present invention which utilizes this phenomenon together with the usual acoustic resonance measurement to determine automatically when a log is frozen and discount the acoustic resonance reading thereof, indicating a high quality log.

Fig. 4 shows the effect of freezing on the acoustic resonance measurements for various qualities of logs. Experience makes it possible to classify logs receiving an acoustic measurement rating of below 600 Hz as being correctly measured and not having an incorrectly doubled value because it is frozen, since very few logs ever resonance measure in the unfrozen state below 300Hz. Logs having a resonance measurement of greater than 700 Hz can generally be assumed to have a doubled value, i.e. by being frozen. The problematical resonance values are in the 600- 700 Hz range, i.e. determining whether a particular log is a low quality log (for example 350 Hz when in the unfrozen state) or is a high quality log (700 Hz) which is not frozen. The present invention uses a supplementary microwave scanning and evaluation (significant attentuation=unfrozen, minimal attenuation=frozen) to determine whether the log is frozen and halve the acoustic resonance value if so.

The person skilled in the art will recognize that although the above described embodiment is directed to certain specified frequency ranges, these ranges may vary, but still within the claimed scope of the invention, depending on the type of wood measured impact method etc. These ranges may be determined by testing and calibration.

Although the example is described as being applied at a sawmill, the person skilled in the art will immediately understand that this method and system according to the invention can be applied with certain or all of the steps performed already in the woods and not at a sawmill.

The person skilled in the art will of course recognize that the present invention may be implemented in a variety of different manners within the scope of the claims, for example by utilizing the microwave attenuation result only when a certain range of frequencies are obtained from the acoustic resonance measurements and disregarding it for others. It is of course expected that feed-back information at the end of the sawmill line, i.e. actual quality evaluations will be utilized to further trim the sorting process to provide even better economy for winter operation.