Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
  • G01N3/34—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces generated by mechanical means, e.g. hammer blows
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/46—Wood
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
  • G01N2203/02—Details not specific for a particular testing method
  • G01N2203/06—Indicating or recording means; Sensing means
  • G01N2203/0617—Electrical or magnetic indicating, recording or sensing means
  • G01N2203/0623—Electrical or magnetic indicating, recording or sensing means using piezoelectric gauges
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2291/00—Indexing codes associated with group G01N29/00
  • G01N2291/02—Indexing codes associated with the analysed material
  • G01N2291/023—Solids
  • G01N2291/0238—Wood
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2291/00—Indexing codes associated with group G01N29/00
  • G01N2291/04—Wave modes and trajectories
  • G01N2291/042—Wave modes
  • G01N2291/0421—Longitudinal waves

Definitions

• the present invention relates to a method of deriving a (surrogate) measure of stiffness and/or strength of sections of the stem of a felled tree (e.g., so as to be determinative of possible destinies of logs to be cut from the stem), a method of log making or log use determination for logs made or to be made from a felled tree with a view to maximizing stiffness and/or strength related extracted value therefrom, procedures involved in such methods and to related apparatus and means.
• the invention also relates to the apparatus useful in such a procedure and to its use.
• the aforementioned patent contemplates, in addition to visual inspection, a non- destructive method of grading logs by a procedure where properties are determined along a selected length of a specimen by means of longitudinal energy wave propagation through the selected length thereof. This was achieved by locating a first sensor in proximity to a surface of the specimen at one end of the selected length thereof and locating a second sensor in proximity to a surface of the specimen at the remaining end of the selected length thereof, impacting one end of the specimen itself to set up an energy wave within the specimen to travel in a longitudinal direction to encounter and then move along the selected length of the specimen between the sensors measuring the time of passage of the energy wave between the sensors and determining mechanical properties of the specimen as related to the measured time of passage of the energy wave through the selected length.
• MOE dynamic modulus of elasticity
• a device of favour in the log forming industry in New Zealand is of a kind typified by that disclosed in US Patent 5457635 of Interpine Export (NZ) Limited (the full content of which is hereby included by way of reference).
• US Patent 5457635 discloses apparatus for determining cut positions in stems of felled trees.
• the apparatus has distance measuring means which provide an output signal representative of length for determining lengths along the stem.
• the apparatus also has diameter measuring means which provides an output signal representative of diameter for determining the diameter of a stem at selected positions along the stem.
• Data entry means are adapted to receive and output selected information relating to the quality of a stem at various positions along the stem.
• Computing means are also provided on the apparatus which (in response to input signals from the distance measuring means, diameter measuring means and data entry means) determines the preferred cutting positions on the stem to optimize the millable timber obtained from the stem.
• Output means allow information determined by the computer means to be presented to the user so that the stem may then be appropriately marked with cut positions.
• Stiffness and/or strength is a characteristic of the structural quality of the wood in the logs and is fundamental to its performance in certain uses.
• the computer software referred to in US Patent 5457635 consists of an optimizing algorithm which works in conjunction with a log type file or library.
• the log type file or library contains specifications for each log type to be considered along with a relative value representing the desirability of each log type.
• This software is presented with the input of measurements of length, diameter and curvature of the stem. There is also provision for the manual inputting of the number and size of knots (as a quality code).
• the input information then allows the algorithm to "fit" the best value combination of logs to be cut from the stem thus providing an optimum solution in terms of the value returned by the stem when cut into logs.
• the present invention recognises that the MOE is a good indicator of the structural characteristics of timber.
• the MOE is related to the speed V of a longitudinal compression wave by the relation
• p is the density of the wooden material.
• the modulus correlates approximately with the density p .
• the density in large measure is due to the free water content, and the value is in the vicinity of 1000kg/m 3 .
• the modulus is determined by the indicator V 2 by measurement (preferably at low frequency as hereinafter described) of the velocity V. Indeed the present invention recognises the value of a measure of V or a function of V owing to its 2nd order use in the MOE formula above.
• Apparatus such as the PILODYNTM (density penetrometer which estimates only outer wood density) procedure as a measure of wood characteristics is therefore far less attractive as an indicator of wood performance for structural or other purposes than is the longitudinal wave propagation V or function of V (e.g., elapsed time values) procedure hereinafter described to interpolate to logs along the stem length a surrogate stiffness and/or strength (and thus stiffness and/or strength related value enhancing destiny).
• V or function of V e.g., elapsed time values
• the present invention also recognises the value of the simple relationship where V can be determined by determining (particularly at low frequency) an equivalent fundamental resonance frequency (f 0 ) using the relationship
• V resonant frequency x twice the length (L) of the stem.
• the present invention therefore in a first aspect consists in a method of log making (or at least cut positioning for log making) from the stem of a felled tree with a view to maximizing stiffness and/or strength related extracted value therefrom, which method includes inducing from an end of the stem of the felled tree a sonic wave (as hereinafter described) to travel along the length of the stem of the felled tree, reflecting repeatedly at the free ends, deriving by reference to the tree stem length (or multiples thereof) and the appropriate resonant frequency for the stem V or V 2 or a function of V or V 2 , and then using that output of V or V 2 or a function of V or V 2 (i) as a factor in determining cut positions for a number of logs from the tested felled tree stem, and
• steps (i) and/or (ii) there is reference to a library of known stiffness and/or strength characteristics [whether MOE, V 2 , V, elapsed time, resonant frequency or some other surrogate value] in relation to known sonic wave travel values typical or generic of trees of the felled tree condition.
• the deriving of the function of V or V 2 value comprises the steps of at one end of the stem of a felled tree (preferably after the removal of all branches), inducing an energy input (e.g., by striking or otherwise) and detecting at that same end the resonant frequencies using spectral analysis of the sequence of multiply reflected returns of the input energy and deriving from the spectrum by appropriate calculations a value of V or a function of V by reference to the known length of the stem.
• US Patent 5396799 discloses the use of impacting means to induce the sonic or acoustic wave and a detection of the passage of the wave past a laterally positioned piezo electric transducer based detector. Those or alternative forms of detection and/or wave propagation can be utilised by the present invention.
• the present invention envisages detection at the same end from which the impact is induced so as to minimise the complexity of the process.
• V then becomes a manual or automatic input to computing means as will hereinafter be described.
• Such computing means may, for example, be a derivative of the form of apparatus described in the aforementioned US Patent 5457635.
• the present invention in a further aspect consists in a method of determining a value indicative of stiffness and/or strength or useful for that purpose (e.g., V or a function of V) [which may optionally be squared to provide a better surrogate value of stiffness and/or strength] which comprises inducing into one end of the stem of a felled tree an acoustical input (e.g., by striking or other means) and by detecting subsequent reflected sound therefrom or otherwise using that input to provide a measure of resonant frequencies and by reference to the length of the stem, determining a value indicative of V or V 2 or a function or V or V 2 (e.g.; MOE) by reliance on the resonant frequencies.
• a value indicative of stiffness and/or strength or useful for that purpose e.g., V or a function of V
• a value indicative of stiffness and/or strength or useful for that purpose e.g., V or a function of V
• the present invention consists in a method of log making or log use determination for logs made or to be made from a felled tree with a view to maximizing stiffness and/or strength related extracted value therefrom, said method comprising
• the log making optimisations may be those disclosed in Interpine Export (NZ) Limited US Patent 5,457,635, the full content of which is here introduced by way of reference.
• sonic wave is to be considered synonymous with any energy wave of a vibrational kind that may be induced along the length of the stem of a tree (which might variously be referred to as an induced stress wave, an induced energy wave, an induced acoustic wave, etc).
• the present invention consists in a method of providing an indicator of or from which stiffness and/or strength can be estimated, which method involves an operative use of apparatus as hereinafter described or as disclosed in our New Zealand Patent Specification No. 337015 filed 30 July 1999.
• the present invention consists in a method of providing an indicator of or from which stiffness and/or strength can be estimated for a felled log of known or measurable length L, said method comprising or including the steps of striking an end of the felled log whilst having sensing means in contact with the log end to detect one or both of (i) the impulse and at least one echo of the impulse resulting from the striking of that same log end and (ii) multiple echoes of the impulse resulting from the striking of that same log end, processing the output of at least said sensing means in processing means to derive, using an echo or echoes sensed by said sensing means, a said indicator, and displaying said indicator or any derivative thereof received from said processing means, optionally thereafter appropriately marking or otherwise indicating the fate of the log on the basis of the displayed indicator, said process being further characterised in that said processing means tests frequency transformed data derived from time based echo data with a view to deriving a measure or good estimate of fundamental frequency f 0 , L has been, is or can
• Figure 1 shows an MOE measuring instrument including an accelerometer sense head as it is preferably used against a log end in conjunction with a hammer and data interpretation devices to yield a result such a result to be used
• Figure 2 illustrates schematically the types of spectra derived from long and short stems plotted as fh/Nfo against harmonic number
• FIG 3 illustrates how whole stem velocity information, combined with a knowledge of typical velocity profiles along a steam, can predict velocities within logs subsequently cut from the stem
• Figure 4 shows echo decay
• Figure 5 shows a preferred sensing head
• Figure 6 is a block diagram of the preferred electronic hardware.
• Stiffness and/or strength measurement is a parameter which has had recent prominence, both in regard to log and timber stiffness and/or strength and the implications it has for the basic constituent fibres of the wood. Measurement of stiffness and/or strength using so- called stress wave timers, that is to say electronic instruments which detect the time of flight of a sonic impulse along or across a piece of wood have been in use for many years.
• the curve drawn is the resulting prediction of speed along that stem. Also shown in Figure 3 as the stepped line are speeds subsequently measured in the sequence of logs made from that stem. Clearly in this example, a combination of reference information and stem-average measurement has enabled a considerable improvement to be made in velocity or stiffness and/or strength estimation along the stem prior to making cuts.
• V is the velocity of longitudinal waves along the log or beam and p is the mass density of the wood, including its water content.
• p is the mass density of the wood, including its water content.
• the present invention in its preferred form recognises that accurate measurement of the sonic velocity of logs or stems can be made in a time of a second from the identification of impact-induced resonances found by Fourier analysis and a good estimate of the stiffness and/or strength modulus found.
• the three elements of apparatus required are the measuring head, the signal acquisition and processing hardware, and the algorithms needed to interpret the resonance data. In this respect see Figure 1.
• Low battery drain e.g. operation for at least one shift on a battery
• Figure 5 shows the sensing head, comprising a piezo-style accelerometer 8 mounted on a body 9 which contains a cable entry 10 for the wires to the accelerometer 8, and a switch 11.
• the wires are further protected mechanically by flexible tubing 12 which also prevents water ingress to the head and which extends to the electronic unit to be described.
• the frequency response of the accelerometer may be chosen for the nature of the log expected. For normal forest work, a frequency response of 10 to 3000 Hz is adequate, but wider ranges may be advantageously used, particularly if the instrument is to be used in research applications. It is preferable that the accelerometer incorporates a charge amplifier, since connection to the electronic unit may then be made through a cable of any length.
• the purpose of the switch 11 is to activate the signal acquisition circuits immediately prior to striking the log under test. It is desirable that the accelerometer is compliantly mounted on the body, for example on a pad of silicone rubber 13, as this enables the operator to press the head against the timber face and maintain good contact independently of any hand movement. If the accelerometer mount is rigid, spurious acceleration signals may be generated if the flat face of the accelerometer is inadvertently rocked against the timber.
• a thin cap 14 of material such as neoprene rubber is fixed over the end of the head so as to be in contact with the accelerometer end face. The purpose of this is to provide some protection for the accelerometer against inevitable build up of debris such as resin from the logs under test. The cap may be cleaned or replaced. Tests have shown that 1mm of a hard rubber does not significantly impair collection of acoustic signals from logs.
• Signals may be collected reliably with this head regardless of the nature of the cross-cut face; for example, the deep ridges produced by the hydraulic saws in automatic harvesters such as the WARRATAHTM generate signals no different from more even surfaces. It is not necessary to embed the detector in the wood to achieve coupling, a fact that considerably speeds up the sounding operation. Experience has shown that neither placement of the head or the blow is critical. This is understandable since the system analyses many tens of reflections of the acoustic pulse in modes which incorporate the entire log, so the precise nature of the initial shock becomes unimportant. This is in clear distinction from so-called stress wave testers, where a single transit time of an acoustic pulse is measured.
• the gain of the analogue amplifier is made to increase at a constant logarithmic rate , for example 20 to 60dB, over the course of the event to partially offset the natural attenuation.
• Amplifier offset voltages must be carefully controlled with such a strategy to prevent dc contamination of the final spectrum.
• high resolution A/D converters typically 14 bit, are used so that useful resolution can still be obtained where the signal has fallen into the microvolt range (but is still above the noise background) .
• the level 100ms after this might be 3mV, which would give some resolution on a 14 bit converter set to 3 V scale, since the least significant bit is 0.19mvolt. However, signals beyond the 100ms time frame would quickly fail to be digitized.
• time-dependent gain extends the period over which signals can be usefully digitised. 20 dB of gain over the 100 ms described above would raise the signal at that time to 30mv, enabling useful digitization to be considerably extended.
• FIG. 6 A block diagram of the electronic hardware is drawn in Figure 6.
• the accelerometer 15 is coupled to an analogue amplifier 16 which incorporates a gain control function.
• the state of the entire instrument is controlled by two programmable logic devices numbered 18 (the event controller) and 19 (the intelligent power controller). When powered up, only parts of these PLDs are operative, and since they are not switching, standing current is very low.
• the enable switch 20 When the enable switch 20 is closed the PLD 18 turns on the Analogue section 16 and the A/D converter 17 , and digitised samples from the accelerometer are fed to the signal register 4(b) in the PLD. If the signal exceeds a threshold, the event detector 4(c) assumes that the log or sample has been struck. The event starts the logarithmic increase in the analogue amplifier gain, and initiates the Intelligent power controller PLD 5. which powers up the microprocessor 21.
• the microprocessor 21 records a number of digitised values over an ensuing time. Typically, 2048 readings will be taken over 400 ms, following which the analogue amplifier and A/D converter are turned off. The data are then Fourier transformed following appropriate windowing and filtering. The particular data record described combination will yield a maximum frequency of 2.5kHz with a resolution of 2.5Hz, which suits forest applications, but could be changed to suit other needs.
• the power spectrum is then analysed by the processor 21 using algorithms discussed in the next section to extract a fundamental resonance f 0 , and an answer displayed in the liquid crystal unit 22. This can consist of a single value for velocity, (assuming a prior log length has been entered into the unit), using the formula
• V - 2 f 0 L where L is the length, or the value can be converted to a speed class, and the code for that class displayed, for example "green" to indicate a colour marker to be used.
• the microprocessor Having initiated the display, the microprocessor returns to hibernation mode to save current, and reactivates after a time of for example 30s to turn the display off under the control of the intelligent power controller 19.
• the unit is configured to deliver the minimum necessary information to operating crews, but clearly the full detail of spectral information, which may be required for R and D operations, is potentially available.
• the logic of the controller 19 is configured so that by keyboard entries, it is possible to send the spectral information via serial port 24 to an external computer for graphical display or data recording. Conversely, data received at the serial port activates the power controller and thence the processor, so that the serial port can be used to control the operation of the device from an external computer.
• V 2Lf n
• V is the speed of longitudinal compressional motions along the member, and since the lateral boundaries are stress free, is given by the well known relation
• samples may not be slender - a four metre saw log with a diameter of 50cm is a considerably "fatter" than a sawn beam 100 by 50mm, and because ofthe excitation spectrum and the log shape, few harmonics will be detected in the log compared with the sawn wood.
• a decision on which frequency should be identified as the fundamental may be less clear for the log. We have found that this can be exacerbated by the presence of unwanted noise spikes in the spectrum, or unwanted resonances arising from less than optimum hammer blows. Situations of poor spectra have been found to be inevitable in some physical locations, for example when obtaining spectra from the logs of cross-cut stems, when the log faces are relatively inaccessible.
• stems In the case of stems, the departure can be enormous. Since stems are "slender" many harmonics can be excited in the region below 1 OOOHz, and the lowest member of the series, if the fundamental, has been observed to be as much as 40% above the value implied by the higher harmonics. This would lead to a difference of two in the predicted value of stiffness and/or strength.
• the modeling shows that it is the low harmonics which are raised above the value expected from the wood modulus, while the high harmonics still indicate stem stiffness and/or strength.
• the harmonic frequency tends to fall lower than expected. Because for stems, the frequency at which this is predicted to occur is high, the effect is unlikely to be seen and indeed we have not observed it.
• Tapered-log modeling shows that it is the taper per wavelength which is important.
• the imbalance or asymmetry occurring in the oscillating mass and spring forces about each node in the log is the underlying cause of frequency shift.
• the taper per wavelength in the N th harmonic is only 1/N of that in the fundamental.
• the higher harmonics are much less affected by the taper and yield the correct stiffness and/or strength.
• Modeling shows, and our experience confirms, that to a reasonable approximation, if the fundamental resonance frequency is raised by a factor ke "1 over its value expected on the basis ofthe stem length and stiffness and/or strength, the N th harmonic will be raised by a factor ke "N over its harmonic value.
• the lowest member f coincides with f 0 if the log is slender and non-tapered.
• This background of observation and modeling results provides the basis of the algorithms used to analyse stem spectra. While a velocity can be judged by an operator from a screen display of spectra, an automatic system needs to allow for noise peaks, non harmonic effects, and perhaps most confusing to an automatic process, missing spectral peaks which confuse the identification of a series. The algorithm must reject occasional noise peaks in the spectrum, which means that as many as possible ofthe resonant peaks should be identified, since random noise spikes will not occur in harmonic ratios.
• the identification system first considers only spectral signals above a threshold, for example those within 20% of the power of the largest spectral peak. It may be advantageous to smooth data in the frequency domain before doing this if signals are noisy to limit the number of peaks to be considered.
• the possible range of frequencies for a fundamental is calculated and spectral peaks sought within that range.
• the search is done within velocity windows whose ranges are less than 2:1. Within such a window, the range of possible fundamental frequencies cannot overlap the consequent second harmonic range, and so allows fundamental and second harmonic to be distinguished. If no successful identification is ultimately made within this window, subsequent searches are made within modified velocity windows. This is generally not required.
• Most green p. radiata logs have velocities between 2.5 and 4km/s which fulfills the velocity criterion.
• a filter comb For each potential candidate for a fundamental resonance, a filter comb is constructed. For example, if the peak to be tested had a frequency of 300 Hz, a comb consisting of 300, 600, 900, Hz is constructed, and the energy measured within that comb by adding the power at the comb frequencies. For short logs, a deviation of a few percent is allowed, i.e. energy is considered to be part of the comb if it falls within a predetermined band about the expected centre, to take account ofthe effects described earlier which are encountered in practice.
• a useful variation of this procedure which takes into account the stacking effect, is to base the comb search on the second harmonic, since this is relatively little affected by stacking, and to allow deviations from harmonic to fall mainly at the fundamental frequency.
• the velocity, and modulus, are then calculated from the second harmonic by assuming that this is the frequency 2f 0 .
• This procedure is repeated for all peaks which are candidates for the fundamental within its allowed frequency range.
• the preferred identification is that spectral peak whose comb accounts for the greatest quantity of spectrum power.
• a numerical confidence measure which follows from this procedure is the ratio of the power accounted for in the peaks within the comb to the sum of power in other peaks plus the background noise level.
• f N is the frequency ofthe N th harmonic
• k is a constant between 0 and 1, which must be determined.
• This procedure will sometimes yield two values of k which generate equal summed powers.
• a second measure is therefore taken at each value of k to express how closely the comb is fitted. This is the sum ofthe deviations of each peak from its comb centre frequency. The choice is made on the basis ofthe most power and the best comb fit.
• next candidate resonance for the fundamental is then tested, and classed as a better identification or not on the basis of both the resonance power accounted for, and the closeness of fit to the comb.
• computation time is acceptably short.

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• Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)
• Investigating Strength Of Materials By Application Of Mechanical Stress (AREA)

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• 1998
  • 1998-12-17 NZ NZ33343498A patent/NZ333434A/xx not_active IP Right Cessation
• 1999
  • 1999-08-17 WO PCT/NZ1999/000134 patent/WO2000036413A1/en not_active Application Discontinuation
  • 1999-08-17 AU AU54539/99A patent/AU751539B2/en not_active Ceased
  • 1999-08-17 EP EP99940745A patent/EP1159610A4/de not_active Withdrawn
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