Patent Application
20130340892
The present invention relates to an improved method and apparatus for maximising value when breaking down a tree stem, log, cant, flitch or slab to sawn timber.
U.S. Pat. No. 6,889,551 discloses a method of lumber break down to maximise the value of the lumber recovered from a log or similar by a system which includes determining an acoustic velocity in the log to predict an average modulus of elasticity, determining density profile information, and utilising the stiffness profile in cutting the log, typically by generating a sawing pattern for the log.
It is an object of the present invention to provide an improved method and apparatus for breaking down a tree stem, log, cant, flitch or slab to sawn timber, or to at least provide the public with a useful choice.
In one aspect in broad terms the invention comprises a method of breaking down a stem, log, cant or slab which includes:
Preferably determining density profile information using x-ray radiation includes moving the stem, log, cant or slab through at least one beam of x-ray radiation or moving at least one source of x-ray radiation relative to the stem, log, cant or slab.
Preferably determining density profile information includes measuring x-ray radiation energy after propagating through a stem, log, cant or slab.
Preferably the method further comprises generating a sawing pattern from the determination of the position of minimum density.
In a second aspect the invention may broadly be said to consist of a method of breaking down a stem, log, cant or slab comprising the steps of:
In one embodiment the method further comprises cutting the stem, log, cant or slab by locating a centre of the sawing pattern in a predetermined position relative to the determined position of minimum density of the stem, log, cant or slab.
In one embodiment the step of predicting the stiffness profile across the stem, log, cant or slab comprises calculating an initial profile of stiffness across the stem, log, cant or slab from an elasticity model of the stem, log, cant or slab, and determining a stiffness profile using the acoustic velocity value for the stem, log, cant or slab, the density profile information and the initial stiffness profile.
Preferably the method further comprises validating the stiffness profile by calculating a velocity of a compression wave in the stem, log, cant or slab from a velocity profile derived from the density profile information and the stiffness profile, and comparing the calculated velocity with the acoustic velocity value.
Preferably the method further comprising adjusting the stiffness profile according to the comparison and validating the adjusted stiffness profile in accordance with the steps of claim 3.
In one embodiment the step of determining density profile information includes subjecting the stem, log, cant or slab to x-ray radiation.
Preferably determining density profile information further includes measuring the x-ray radiation energy level after propagating through the stem, log, cant or slab.
The x-ray radiation may comprise a collimated beam, a diverging beam, a converging beam, or any combination thereof.
The x-ray radiation may be provided by one or multiple radiation source(s).
Preferably the method further comprising calibrating the x-ray radiation to an energy level prior to subjecting the stem, log, cant or slab to the x-ray radiation. Preferably calibrating the x-ray radiation comprises passing a control stem, log, cant or slab through the x-ray radiation and adjusting the x-ray radiation energy level to an appropriate energy level for the control.
Preferably subjecting the stem, log, cant or slab to x-ray radiation comprises moving the stem, log, cant or slab through at least one beam of x-ray radiation. Alternatively subjecting the stem, log, cant or slab the step to x-ray radiation includes moving at least one source of x-ray radiation relative to the stem, log, cant or slab.
In one embodiment the step of determining an acoustic velocity value of the stem, log, cant or slab comprises applying a force to the stem, log, cant or slab and measuring a frequency of vibration resulting from the applied force.
Preferably the method further comprises forming one or more laser marker lines on the stem, log, cant, or slab in accordance with the sawing pattern and cutting the stem, log, cant or slab with a cutting machine in accordance with the marker lines.
In a third aspect the invention may broadly be said to consist of a method of determining a stiffness profile of a stem, log, cant or slab comprising the steps of:
In a fourth aspect the invention may broadly be said to consist of a system for breaking down a stem, log, cant or slab comprising:
Preferably the system further comprises an output monitor for displaying the predicted stiffness profile across the stem, log, cant or slab.
Preferably the measurement system comprises at least one x-ray radiation source and at least one x-ray radiation detector configured to locate on either side of the stem, log, cant or slab in a density measurement position of the stem, log, cant or slab in use, the at least one source configured to apply x-ray radiation energy through the stem, log, cant or slab and the at least one detector configured to receive and measure an energy level of x-ray radiation propagating through the stem, log, cant or slab, and the measurement system further comprising at least one processor configured to determine the density profile information from one or more energy levels measured by the at least one detector across the stem, log, cant or slab.
Preferably the at least one source is located above the stem, log, cant, or slab in the density measurement position, and the at least one detector is located underneath the stem, log, cant or slab in the density measurement position.
Preferably the measurement system comprises a compressed air driven hammer located adjacent the stem, log, cant or slab in an acoustic velocity measurement position of the stem, log, cant or slab in use and configured to strike the stem, log, cant or slab to stimulate vibration in said stem, log, cant or slab, and an accelerometer configured to locate against an end of the stem, log, cant or slab in the acoustic velocity measurement position in use, and output data relating to a frequency of the vibration, the measurement system further comprising at least one processor configured to determine the acoustic velocity value from the frequency data of the accelerometer.
Preferably the system further comprises a cutting system including:
Preferably the system further comprises a transport system configured to convey the stem, log, cant or slab in use first to a measurement stage associated with the measurement system and then to a cutting stage associated with the cutting system.
The term “comprising” as used in this specification and claims means “consisting at least in part of”. When interpreting each statement in this specification and claims that includes the term “comprising”, features other than that or those prefaced by the term may also be present. Related terms such as “comprise” and “comprises” are to be interpreted in the same manner.
As used herein the term “and/or” means “and” or “or”, or both.
The invention is further described with reference to the accompanying figures in which:
In the method of the invention an acoustic or sonic velocity measure obtained for a log or cant is combined with a radial density profile for the log or cant which will typically be green, i.e. undried and typically freshly cut and thus high moisture content, to derive a radial profile of its MoE. This MoE stiffness profile can be used to estimate the dry MoE of timber sawn from the sample and to determine how to saw the log or cant to maximise recovery of high value timber for structural applications. The position of minimum density in the log or cant is determined and used to locate the centre of a sawing pattern at the minimum density position of the log or cant and/or in the calculation of a sawing pattern.
As disclosed in U.S. Pat. No. 6,889,551 when a wood stem or log or cant receives an acoustic impulse, by striking the sample, with a hammer for example, the speed of longitudinal waves can be calculated from the formula
V=2f0L 1
where L is the sample length, f0 the fundamental or lowest longitudinal mode, and V the desired speed of longitudinal compression (i.e. sound) waves. V is in turn related to the modulus of elasticity E, or MoE, by the expression
V
2
=E/ρ 2
where ρ is the material density of the wood. Thus for velocity and in particular from f0, it is possible to determine an MoE value or value indicative of MoE for the sample. Any suitable system for measuring acoustic velocity may be used.
In equation 2 the relevant density is simply the mass to volume ratio, including the mass of water. It is known that the acoustic-measured MoE remains constant as timber dries from green until the Fibre Saturation Point is reached; in further drying to equilibrium moisture content (about 12% in New Zealand) the sonic modulus increases by perhaps 20%.
The preferred procedure is intended for use with green or undried wood, first because sawing decisions clearly relate to green timber, and second because the water content at this stage largely determines the density. It does this overwhelmingly in the sapwood, and partially in the drier heartwood. A dry MoE can be estimated from a wet value by simply increasing the values by about 20%.
In some forms of the invention the radial velocity profile implied by the MoE and density is integrated across the sample, and the MoE profile is first shifted up or down, by a maximum of 10% for example, to seek agreement with the measured log or cant acoustic velocity. If agreement is not reached within this range, the outer MoE is then clamped, and the core MoE value raised or lowered to generate agreement. (The outer MoE has been found to be more tightly defined by log or cant acoustic velocity than the core MoE.)
Regardless of the basic dry density, the density of the outer sapwood in green p. radiata is typically around to 1050 kg/m3, while that of the drier inner wood is more variable but typically around 550 kg/m3. The result is that the acoustic velocity in this species is not a strong function of radius. The velocity in the weak inner wood is raised by its lightness while the velocity in the stronger outer wood is lowered because of its higher density. The velocity at any location is found to be not far from the average velocity for the whole log or cant. The location of particular interest is the zone near the bark where it is known that all p. radiata trees have a density of about 1050 kg/m3. Combining this density with the acoustic velocity for the whole log or cant gives an estimate of the MoE of the wood near the bark. The MoE information can be refined if more information on the wet density is available. The approach is to begin with a first radial profile based on equations formulated from data which indicate the likely core and bark values of MoE and a radial profile of wet density measured for each log or cant). The density and MoE profiles define a radial profile of acoustic velocity whose appropriately weighted average should equal that measured sonically for the whole log or cant. If the computed velocity does not agree with the measured velocity corrections to the MoE must be made as will be described in detail below.
Variations are possible. For example, constraints can be put on the radial MoE profile to prevent non-physical results occurring, and other modifications to a parabolic profile can be incorporated. These will depend on knowledge of the particular species likely to be encountered. It is known from the literature that corewood MoE correlates with dry (and wet) density, so when core MoE is changed, a corresponding change in the density may be made.
An accurate density profile is desirable. One reason is because the cant may have a position of minimum density, or heartwood, or pith, located in the geometrical centre relative to its lateral width.
It should be appreciated that when the specific sawing pattern is evaluated in response to a determination of the true position of minimum density, the sawing pattern will include a method for evaluating one or more of the measured width, height, acoustic velocity and determined density profile information, and outputting a pattern indicative of cut placement or desired structural timber sizes.
Preferably the length of timber is a stem, log, or cant and characteristics that the density model is based upon include the density of an outer portion of the stem, log, or cant, the density of an inner portion of the stem, log, or cant, and a transition between the outer and inner densities at a radial position determined by the equation:
R
core
=aD−b
where Rcore is the radius of the transition, D is the diameter of the timber, and a, b are characteristic parameters previously determined for the wood species.
Preferably the method further comprises calculating a velocity of the compression wave in the timber calculating a velocity profile of a compression wave in the timber using
Preferably the length of timber is a cant and V(R) is averaged using:
Preferably the length of timber is a log and V(R) is integrated using:
Preferably the method further comprises utilising the stiffness or elasticity profile in determining the placement of sawing points or a sawing pattern for a stem, log, or cant.
Preferably the method further comprises utilising the elasticity or stiffness profile in sawing side slabs from a stem or log to form a cant or slab.
Data relating to MoE characteristics of the stem, log or cant being analysed is also obtained and used to formulate characteristic MoE equations for the species as indicated at 55. Such data will typically have been measured from analysis of the stem, log or cant to be sawn. The density, acoustic velocity, dimensional and measured MoE information is then used to calculate an MoE profile across the stem, log or cant as indicated at 56. The calculated MoE profile for the stem, log or cant may be output for use as indicated at 57 or alternatively used to provide information to a log breakdown sawing system or a manual saw operator, or a cut placement or a sawing pattern for the stem, log or cant to maximise the value or value as structural timber obtained. This general method can be carried out individually for each stem, log or cant that is processed.
The velocity (Vmeas) of the plane compression wave in the stem, log or cant is determined as indicated at 60 using a suitable acoustic technique. A density profile across the radius of the stem, log or cant is then measured as indicated at 61. Examples of both a measured density profile 70a and estimated density profile 70b are shown in
An initial MoE/elasticity profile is then determined as indicated at 62 using the measured velocity, Vmeas and an appropriate model which is formulated through experimentation. For p. radiata this involves determining MoEmax corresponding to the sapwood elasticity and MoEmin corresponding to the heartwood elasticity. An approximately parabolic curve which fits the data is then formulated which enables an initial estimate of the elasticity at all points across the diameter of the timber to be calculated. The resulting initial elasticity profile 71 (see
At this point an iterative process is undertaken to refine the initial estimate of the MoE/elasticity profile to determine an MoE/elasticity profile which reflects more accurately the actual elasticity across the timber. In general terms this process involves adjusting the initial elasticity profile until, using the estimated or measured density profile, the average of the calculated velocity profile Vav more closely approximates the measured velocity of the plane compression wave in the stem, log or cant to within the desired accuracy. More particularly, Vav and Vmeas are compared as indicated at 64 to see if these are equal or if they differ. If Vav≠Vmeas then some adjustment of the initial MoE is performed. During adjustment of the MoE profile, MoEmax is preferably not moved by more than 10% from its measured value.
In the form shown it is determined as indicated at 65 if the MoE profile has already been adjusted such that MoEmax has been moved more than say 10%. If it has not, and this will be the case in the first iteration, then the entire MoE profile is moved upwards or downwards by a small amount. To do so it is then determined as indicated at 66b whether Vav>Vmeas. If it is not then it is assumed that the calculated initial MoE profile is too “low” and the MoE profile is shifted upwards as indicated at 68b.
To do so it is determined as indicated at 66a whether Vav>Vmeas. If it is not then it is assumed that the initial MoE profile is too “low”. In this case the MoEmin value is increased 67a by a small amount to produce a revised MoE profile 83 with a “flatter” shape as shown in
Steps 60-63 shown in
The length of timber is struck at one end as indicated at 90 with an impact device such as a hammer which induces a range of standing compression waves along the length of the timber. The impact device may be activated automatically using a machine or alternatively may involve manually striking the end of the timber with the hammer. A transducer is then used to detect the compression waves within the stem, log or cant as indicated at 91. The transducer can be any suitable device, such as a piezo-electric accelerometer or the like which is mounted on or near one end of the timber being examined. The output of the transducer is analysed by a processor to determine the frequency of the fundamental component f0 as indicated at 92 using a suitable signal processing technique. The length of the stem, log or cant is measured as indicated at 93 and this value along with the fundamental frequency is utilised to determine the velocity of the plane wave by way of equation 1 as indicated at 94. This velocity Vmeas, gives a good indication of the velocity of the plane compression wave in the sapwood as discussed previously. This is only one way of finding the compression wave velocity and other suitable techniques known to those skilled in this area of technology could be utilised.
R
core=0.5405D−116 3
where Rcore is the transition radius and D is the stem or log diameter. The radius of the transition point is calculated as indicated 97 using equation 3 to produce the density profile 70b as indicated at 98. The transition point 73 is indicated on the density profile 70b in
Alternatively, and preferably, the wet density profile is used directly from the measurement by microwave or x-ray assessment and the MoE profile directly processed from received ray intensity information.
The initial MoE can be defined by:
MoE(R)=MoEmin+(MoEmax−MoEmin)(R/Rmax)2 8
where R is the radius and Rmax is the radius of the stem or log. Once equations have been determined for the model, MoEmax and MoEmin are calculated as indicated at 100 using equations 5 and 6 and these values are utilised to calculate the initial MoE(R) indicated at 101 using equation 8. It will be appreciated that a maximum value of MoE(R) may be specified, for example 13 GPa as noted before, to avoid unrealistic values being calculated.
Once the initial MoE has been determined an average velocity is calculated 63 as shown in
The wet density at each radial point is determined through either a model or measurement as described earlier and the MoE at each radial point is determined from the initial MoE calculated using equation 8. The average, Vav of the velocity profile V(R) over the entire radius of the timber is then determined 103. For a cant this is preferably done by integrating the V(R) from the centre of a cant to the maximum radius Rmax as follows:
For a stem or log the increasing area of wood at a given speed as the radius increases means that the average velocity is found by:
The integration equations assumes a symmetrical stem or log, however this could easily be adapted for non-symmetrical geometries. The average velocity Vav, is then used in combination with the measured velocity Vmeas, to refine the MoE profile as described previously with reference to
A source of x-ray radiation 205 is located above the transport system 112 that supports the cants 104 for delivery to the gangsaw 115. Preferably the x-ray source 205 provides a collimated beam of x-ray radiation. However, other x-ray sources having divergent or convergent beams may be used in circumstances that will be discussed later. Preferably the x-ray source 205 is connected to the computer 106 by a wired or wireless link 208 so that it can be energised remotely. Alternatively the x-ray source 205 includes a local energy source. The x-ray source 205 is located above the transport system 112 so that cants 104 pass beneath before being sawn. An x-ray radiation detector 206 is located beneath the x-ray source 205 and preferably beneath the transport system 112. The detector 206 is arranged to detect x-ray radiation that has been emitted by the x-ray source 205 and radiated through the cant 104. The changing density of the cant wood causes a varying magnitude of x-ray radiation absorption. The resultant strength of the radiation received by the detector 206 is therefore relative to density of the cant material. Information pertaining to the density of wood across the cant can be directly inferred from the energy received by the x-ray detector.
Note that in some embodiments a control cant may need to pass through the x-ray beam such that the energy received by the detector can be calibrated or referenced to a known energy level. In other embodiments an x-ray energy measurement may be taken at the detector, or at least before the beam passes through a cant, such that the energy sensed at the detector can be calibrated or referenced to a known energy level. In other embodiments the energy sensed at the detector is not calibrated or referenced to the x-ray radiation source and only relative measurements are taken. In other embodiments the x-ray attenuation information is compared with density profile information of known types of wood.
In a variation of the above embodiment, the beam of x-ray radiation moves above the cant instead of having the cant move beneath the beam of x-ray radiation. A moveable x-ray radiation source may be facilitated by mounting the x-ray source on a translation stage that moves transverse to the direction cants are transported through the system. Alternatively the x-ray radiation source may be pivoted to scan the beam across the cant. Alternatively an x-ray beam reflector may be located proximate the x-ray source such that the beam direction can be reflected to scan as desired. Movement of the x-ray source, or at least the beam, relative to the cant negates the requirement for cants to move transversely in the transport system. This may be advantageous in sawmills where cants are generally only transported in a longitudinal direction or it is otherwise not practical to install equipment to facilitate transverse movement of a cant.
In some embodiments, the x-ray radiation may be provided by a pencil beam radiation source, or by a radiation source having a diverging beam such that the beam adequately spans the width of a cant when incident. In such instances where a diverging beam is desired, the detector may be a number of discrete devices arranged to receive x-ray radiation at discrete locations, or a continuous detection device arranged to receive a broad beam width.
In preferred embodiments, the x-ray radiation source 205 is a single source of x-ray radiation. However, in other embodiments it may be desirable to have multiple sources of x-ray radiation arranged to provide density information at various lateral and/or longitudinal positions of the cant being scanned. In such circumstances where multiple beams are used, the density profile information retrieved may be averaged such that an average position of minimum density along the cant is determined. Other statistical measures may be employed on multiple readings to establish the most desirable position of minimum density and subsequently generate a sawing pattern from.
In such circumstances where a large deviation in the density profile is discovered along the length of a cant, it may be desirable to cut the cant in two at some longitudinal position. A sawing pattern best suited for each section can then be utilised.
Further arrangements of the x-ray radiation source and detectors suitable for providing a cross-sectional scan of a cant for retrieving density information will be apparent to those skilled in the art.
The inventors have determined that the density information determined from the absorption profile of x-ray radiation at a single longitudinal position on a cant generally corresponds to the density along the entire longitudinal length of the cant. A single scan therefore provides adequate information for determining whether the position of minimum density is offset from the geometrical centre.
Preferably a final determination of the sawing pattern is based on two measures, a measure of acoustic speed/velocity and the density profile information provided by the x-ray scan. The determination of the sawing pattern may be made automatically by a processor of the system or manually by an operator. The sawing pattern depends on the true MoE profile which includes information on the position of minimum density.
To illustrate the effectiveness of the invention a trial was conducted to compare a batch of equal quality cants sawn using both the process for determining the true position of minimum density and the traditional process where the position of minimum density is assumed to be at the geometrical centre of the cant. Approximately 1000 logs were divided into two groups according to their acoustic velocity measure being high or low and thus being designated either high or low quality. The cants having high velocity were those with a mean acoustic velocity of 3.39 km/sec. These cants were the subject of the test since they are more suitable for producing structural grade timber.
The logs were sawn and wing boards were diverted from the board flow so they would not dilute any effect of the sawing treatment applied to the cants. Using a determination of x-ray density and acoustic velocity from the in line cant acoustic velocity measure, the average stiffness of the cants was determined.
In its various aspects, the method of determining the stiffness/MoE profile and/or of determining a sawing pattern of the invention can be embodied in a computer-implemented process, a machine (such as an electronic device, or a general purpose computer or other device that provides a platform on which computer programs can be executed), processes performed by these machines, or an article of manufacture. Such articles can include a computer program product or digital information product in which a computer readable storage medium containing computer program instructions or computer readable data stored thereon, and processes and machines that create and use these articles of manufacture.
Where in the foregoing description reference has been made to elements or integers having known equivalents, then such equivalents are included as if they were individually set forth.
Although the invention has been described by way of example and with reference to particular embodiments, it is to be understood that modifications and/or improvements may be made without departing from the scope or spirit of the invention as defined by the accompanying claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 599764 | May 2012 | NZ | national |
