Abstract:
Methods and apparatuses for imaging material properties in natural-fiber-containing materials can utilize time-domain data. In particular, images can be constructed that provide quantified measures of localized moisture content. For example, one or more antennas and at least one transceiver can be configured to collect time-domain data from radiation interacting with the natural-fiber-containing materials. The antennas and the transceivers are configured to transmit and receive electromagnetic radiation at one or more frequencies, which are between 50 MHz and 1 THz, according to a time-domain impulse function. A computing device is configured to transform the time-domain data to frequency-domain data, to apply a synthetic imaging algorithm for constructing a three-dimensional image of the natural-fiber-containing materials, and to provide a quantified measure of localized moisture content based on a pre-determined correlation of moisture content to frequency-domain data.

Description:
PRIORITY 
       [0001]    This invention claims priority from, and is a continuation-in-part of, currently pending patent application Ser. No. 12/170,944, filed on Jul. 10, 2008, which is a continuation-in-part of U.S. Pat. No. 7,548,185, filed Sep. 30, 2005. Both are incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with Government support under Contract DE-AC0576RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    This invention relates to imaging material properties in natural-fiber-containing materials. Rapid and accurate characterization of material properties, which can include, but are not limited to, moisture content, fiber structure, and grain structure (e.g., knots or imperfections), can be critical in determining the quality of a material and/or the ability of the material to be processed into various products. Many of the current systems for measuring material properties must contact the natural-fiber-containing materials and/or do not provide images. For example, many traditional systems for measuring moisture content use a “feeler” device that contacts the material to determine electrical conductivity. The feelers can accumulate dirt and wear out. Other systems, including those involving electromagnetics, typically perform spot measurements of moisture content and do not create images of the material showing localized moisture. Still other systems generate maps, but not true images, of fiber structure, grain structure, and/or knot distribution. Accordingly, a need exists for tools to image material properties in natural-fiber-containing materials without contacting the materials. 
       SUMMARY 
       [0004]    Embodiments of the present invention include methods and apparatuses for imaging material properties in natural-fiber-containing materials. The holographic images provide quantified measures of localized moisture content, which can be important in the context of processing the natural-fiber-containing materials. For example, moisture levels must be closely regulated when processing plywood, oriented strand board (OSB), particle board, wood veneer, and composite materials. Similarly, moisture levels must be closely monitored when processing grain, tea, and other products that are moisture-sensitive, dried, and/or roasted. 
         [0005]    While a variety of embodiments of the present invention are contemplated, in one embodiment, one or more antennas and at least one transceiver are configured to transmit electromagnetic radiation between 50 MHz and 1 THz according to a time-domain impulse function. In a preferred embodiment, the frequencies are between 200 MHz and 1 THz. Response signals are collected at times differing from the transmit times, wherein the response signals comprise time-domain data from radiation interacting with the natural-fiber-containing materials. A computing device configured to apply a synthetic imaging algorithm constructs a three-dimensional image of the natural-fiber-containing materials. The computing device is further configured to transform the time-domain data to frequency-domain data and to provide a quantified measure of localized moisture content based on a pre-determined correlation of moisture content to frequency-domain data. 
         [0006]    A conveyance system can pass the natural-fiber-containing materials through a field of view of the array of antennas. Alternatively, the antennas can scan across the natural-fiber-containing materials. 
         [0007]    As used herein, “time-domain data” can refer to data captured over specific time values representing an electronic signal value resulting, from a physical measurement. A series of successive time domain values will describe a signal envelope showing how a transmitted or received signal will unfold over time. In contrast, “frequency-domain data” can refer to data representing a particular frequency or set of frequencies (a “frequency spectrum”) that consists of a signal magnitude and phase. Data in the two domains (time and frequency) can be related to one another through a well-known mathematical process called a Fourier transform. 
         [0008]    As used herein, “natural-fiber-containing materials” can refer to materials that contain, derive from, or are made of naturally occurring fibers from plant, animal, or mineral sources. Examples of naturally occurring fibers from plants include, but are not limited to, the seed, a leaf, bast, fruit, and stock fibers. Examples of naturally occurring fibers from animals include, but are not limited to animal hair, silk fiber, and avian fiber. Examples of naturally occurring fibers from minerals include, but are not limited to, asbestos and ceramic fibers (e.g., glass fibers, aluminum oxide, silica carbide, boron carbide, etc.). Accordingly, exemplary natural-fiber-containing materials can include, but are not limited to, wood, wood products, composite materials containing natural fibers, plants, grain, and tea leaves. 
         [0009]    In some embodiments, the array of antennas comprises a linear array of interlaced antennas. Interlacing the antennas allows the overall number of antennas included in the array to be reduced because some of the antennas can be switched to transmit or receive according to a pattern or sequence. Additionally, the array of antennas can be configured for cross polarization, which can minimize the effect of surface reflections off of the natural-fiber-containing materials. In still other embodiments, the electromagnetic radiation comprises circularly polarized radiation. In those instances when the natural-fiber-containing materials exhibit fiber structure, grain structure, and/or knots (e.g., wood veneer, plywood, OSB, particle board, etc.) circularly polarized radiation can be used to optimally image the fiber structure, grain structure, and/or knots. Furthermore, higher frequency radiation (i.e., 10 GHz and above) can improve resolution for identifying the fiber structure, grain structure, and/or knots, while lower frequencies (i.e., 1-3 GHz) can be well suited for determination of moisture content. 
         [0010]    Embodiments of the present invention can be arranged to operate in one of at least two modes. In a reflection mode of operation, electromagnetic radiation interacts with the natural-fiber-containing materials and is reflected back in an amount related to the dielectric constant. The array of antennas receives this reflected radiation as data to construct the image. In a through-transmission mode of operation, electromagnetic radiation is transmitted through the natural-fiber-containing materials and can be reflected by a reflector placed behind the natural-fiber-containing materials, relative to the array of antennas. The originally transmitted radiation is attenuated by its interactions with the natural-fiber-containing materials and the attenuated radiation is then used as data to construct the image. Particular embodiments can further comprise a substantially fully reflective reference sample that is passed through the field of view with each one of the natural-fiber-containing materials, or with a batch of natural-fiber-containing materials. The reference sample can provide a reference measurement for moisture content quantification and/or calibration purposes. 
         [0011]    The purpose of the foregoing summary is to enable the United States Patent and Trademark Office and the public generally, especially the scientists, engineers, and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The summary is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. 
         [0012]    Various advantages and novel features of the present invention are described herein and will become further readily apparent to those skilled in this art from the following detailed description. In the preceding and following descriptions, a number of embodiments are described, including the preferred embodiment of the invention, by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of modification in various respects without departing from the invention. Accordingly, the drawings and description of the preferred embodiment set forth hereafter are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0013]    Embodiments of the invention are described below with reference to the following accompanying drawings. 
           [0014]      FIG. 1  is an illustration depicting one embodiment of an apparatus for imaging material properties in natural-fiber-containing materials. 
           [0015]      FIG. 2  includes an image of eight wood coupons having various moisture levels, which image was generated using high-frequency, cross polarized radiation. 
           [0016]      FIG. 3  includes an image of eight wood coupons having various moisture levels, which image was generated using high-frequency, co-polarized radiation. 
           [0017]      FIG. 4  is a scatter plot of the relative amplitude as a function of moisture content for data generated using high frequency, cross polarized radiation. 
           [0018]      FIG. 5  includes an image of eight wood coupons having various moisture levels, which image was generated using mid-frequency, cross polarized radiation. 
           [0019]      FIG. 6  includes an image of eight wood coupons having various moisture levels, which image was generated using mid-frequency, co-polarized radiation. 
           [0020]      FIG. 7  is a scatter plot of the relative amplitude as a function of moisture content for data generated using mid-frequency, cross polarized radiation. 
           [0021]      FIG. 8  is a scatter plot of the relative amplitude as a function of moisture content for data generated using mid-frequency, co-polarized radiation 
           [0022]      FIG. 9  includes an image of eight wood coupons having various moisture levels, which image was generated using low-frequency, cross polarized radiation. 
           [0023]      FIG. 10  is a scatter plot of the relative amplitude as a function of moisture content for data generated using low-frequency, cross polarized radiation. 
           [0024]      FIGS. 11   a  and  11   b  are illustrations depicting two configurations for a time-domain measurement system according to embodiments of the present invention. 
           [0025]      FIG. 12  are illustrations depicting examples of transmitted and response signals in a time-domain measurement system according to embodiments of the present invention. 
           [0026]      FIG. 13  is an illustration of a reconstructed response waveform in a time-domain measurement system according to embodiments of the present invention. 
           [0027]      FIG. 14  is an illustration depicting the quantification of localized moisture content from time-domain measurements according to embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    The following description includes the preferred best mode of one embodiment of the present invention. It will be clear from this description of the invention that the invention is not limited to these illustrated embodiments but that the invention also includes a variety of modifications thereto and embodiments thereof. Therefore the present description should be seen as illustrative and not limiting. While the invention is susceptible of various modifications and alternative constructions, it should be understood that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims. 
         [0029]    Referring to  FIGS. 1   a  and  1   b , illustrations depict one embodiment of an apparatus for imaging material properties in natural-fiber-containing materials.  FIG. 1   a  depicts generally an embodiment of the present invention including an imaging workstation  100  that receives and process data from a holographic transceiver  104  having an antenna array  103  configured for transmission and reception. An exemplary imaging workstation can comprise a scanner controller, data acquisition hardware and software, holographic imaging software, and reconstruction software. The antenna  103  can be mechanically and/or electronically scanned across a sample  102  using, at least in part, a rectilinear scanner  105 . As illustrated, the sample  102  comprises 8 swatches of plywood and a metal reference plate. 
         [0030]    Referring to  FIG. 1   b , holographic radar data collection is performed by electronically or mechanically scanning a transceiver or antenna array  106 , which can be arranged linearly or two-dimensionally across a planar (i.e., two-dimensional) aperture  109 . The microwave/millimeter-wave transceiver  106  emits a diverging beam that interacts with the imaging target  108  and then measures the amplitude and phase of the scattered wavefront at each sampled position  107  and frequency over the scanned aperture  109 . This dataset is three-dimensional with dimensions consisting of the two aperture dimensions and the frequency dimension. Wavefront reconstruction techniques can be used to mathematically focus these data using computer-based image reconstruction algorithms, and using the frequency dimension to recover the third spatial dimension. The resolution achievable with these imaging techniques is limited by diffraction of the waves, which is determined by the wavelength range and bandwidth of operation. The lateral resolution of the system can be described approximately by the following equation, where λ c  is the mid-band wavelength and F #  (F-number) is the range divided by the aperture size. 
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         [0031]    Depth resolution can be described by Equation 2, where c=3×10 8  m/s is the speed of light and B is the bandwidth of the system. For example, a millimeter-wave imaging system that operates over the 25-35 GHz frequency range with a 1 meter square aperture and a 1 meter range has mid-band wavelength of 1 cm, F # =1, and bandwidth B=10 GHz. This results in a lateral resolution of 0.5 cm and a depth resolution of 1.5 cm. 
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         [0032]      FIGS. 2-10  include data obtained using the apparatus depicted in  FIG. 1 . Multi-frequency microwave scans of 16 coupons of wood, approximately ⅛ inch (3 mm) thick, 9 inches long (with the grain), and 9.5 inches wide (across the grain), were made. The weight percent of moisture (i.e., the difference between wet and dry wood divided by the dry weight) in the coupons was determined by weighing before and after scanning, then drying and weighing after the scanning was completed. 
         [0033]    The scans covered the ranges from 2 to 4 GHz and 8 to 18 GHz. The scan radiofrequency (RF) modes included linear polarized, co-polarized, and cross-polarized configurations. The linear scans were either polarization aligned with the grain direction (“Aligned”) or polarization perpendicular to the grain direction (“Cross”). Co-polarized scans were designated Right-Right (“RR”) and cross-polarized as “LR”. 
         [0034]    The lowest-frequency data set in each scan was used to create a mask defining the location of each coupon in the scan. The masks were used to create, for each scan, a text file containing the average amplitude of each coupon for each frequency. For each rf mode, the text files were imported into a spreadsheet. The data was normalized to the average reflected amplitude from the metal plate for the respective set. The normalized data were used to create plots of average amplitude (Y axis) as a function of moisture content (X axis). 
         [0035]    Overall, the plots show an increase in reflectivity with moisture content, up to a certain level. This level, and behavior above that level, depend on both frequency and polarization. At the low end (e.g., 5%-9%), the plots generally show an irregular cluster. On a larger scale, the linear and cross-polarized data show an increase in reflectivity up to about 80%, and level or slightly declining after that. The co-polarized data show an increase to about 60% and then a decrease of comparable slope out to 120% (where the data end). 
         [0036]    Only one scan was done at low frequency (e.g., 2-4 GHz). It was done in LR mode. The correspondence of reflected amplitude to moisture content was nearly linear to about 100%. However, the spatial resolution appeared to be rather low. Since the scan was done on larger increments than the other scans (i.e., 50 points versus 200 to 240 points), it is hard to discriminate the loss in radar resolution from the loss in scan resolution. Specifics regarding the data are discussed in the following paragraphs. 
         [0037]    Referring first to  FIG. 2 , eight coupons of wood having varying degrees of moisture have been imaged using cross polarized, high-frequency (e.g., 17.6 GHz) electromagnetic radiation in the reflection mode of operation. A reflector comprising a metal plate was placed in the lower left corner as a reference. Adjacent to the image is a photo of the grid of coupons that indicates the moisture content in each coupon. The moisture content was determined as a weight percent by weighing before and after scanning, then drying and weighing after scanning. Comparison of the image with the photo shows that image brightness is strongly correlated with moisture content in each of the coupons. 
         [0038]    Referring to  FIG. 3 , the same grid of coupons was imaged using co-polarized radiation. The results indicate that high-frequency, co-polarized radiation is effective in revealing grain structure, but is not as suitable as cross polarized radiation for quantifying moisture.  FIG. 4  includes a scatter plot of the relative amplitude as a function of moisture content. The relative amplitude was measured as the brightness in the image obtained using high-frequency, cross polarized radiation. As described below, low-frequency radiation is more effective for quantifying moisture levels. 
         [0039]    Referring to  FIG. 5 , an image of the same grid of coupons was generated using mid-frequency (e.g., 11.8 GHz), cross polarized radiation. Compared to the image in  FIG. 2 , mid-frequency radiation results in lower image detail. However, grain pattern is still evident. However, the brightness of the coupon images correlates well with moisture levels. Referring to  FIG. 6 , a mid-frequency, co-polarized image provides grain structure, although it lacks the detail contained in the high-frequency images. Referring to the scatter plots and  FIGS. 7 and 8 , which show relative amplitude as a function of moisture content from cross polarized and co-polarized images, respectively, it is evident that while lower frequencies can generate stronger correlations between brightness and moisture level, co-polarized radiation is not suited for tracking moisture levels over the entire range shown, though it could be used over low and high ranges separately. 
         [0040]    Referring to  FIG. 9 , an image of the same grid of coupons was generated using low-frequency (e.g., 2.1 GHz), cross polarized radiation. The low-frequency image has low resolution, but correlates extremely well with moisture levels, as shown in  FIG. 10 .  FIG. 10  includes a scatter plot showing relative amplitude as a function of moisture content from the data generated by low-frequency, cross polarized radiation, and shows that the low-frequency radiation is highly effective at tracking moisture content. 
         [0041]    The data in  FIGS. 2-10  demonstrate that localized moisture content, grain structure, image resolution, and other properties of natural fiber materials do not predictably correlate to amplitude across frequencies and/or polarizations. According to embodiments of the present invention, collection of data in the time-domain is based on transmission and reception of impulses of millimeter-wave radiation. The impulses comprise a plurality of frequencies. Therefore, the time-domain data acquired at multiple frequencies is first transformed into frequency-domain data. The frequency-domain data can then be used to quantify localized moisture content based on pre-determined correlations between frequency-domain data and moisture content. 
         [0042]    In one example, a time-domain signal is generated at one or more antennas and a response signal is captured at one or more antennas after interacting with a natural fiber material sample. The response signal can be a reflection from, or a transmission through, a natural-fiber-containing material. The response signal is sampled in time and is transformed to a frequency response. The measured moisture levels versus frequency, which can be determined as described above, is then used to construct a map of the localized moisture level for the sample. 
         [0043]      FIGS. 11   a  and  11   b  show two configurations for a time domain measurement system. An antenna  1100  is driven with a series of pulses (ST) in a controlled fashion. A moisture-containing sample  1102  is illuminated with this pulse, and a receiving antenna  1101  picks up the signal  1104  after it interacts with sample (SR). As shown in  FIG. 11   a , the response signal  1104  can be received after reflecting off a metal back reflector  1105 , called reflection mode. Alternatively, as shown in  FIG. 11   b , the signal can be picked up after passing through the sample once (transmission mode); reflection mode has the advantage of providing a clear propagation distance from transmitted antenna to received distance. Both the transmitted and response waveforms vary over time; the response waveform is shifted in time with respect to the transmitted waveform according to the effective time the signal takes between the antennae. 
         [0044]    Referring to  FIG. 12 , the transmitted signal (ST)  1201  serves as a reference signal for sampling the response signal (SR)  1202  at different time locations within the response waveform. The ST is periodic, and if we consider the rising edge of the waveform as the time at which a pulse is transmitted, then this rising edge can serve as a “reset” time in which to begin capturing the response waveform. At specific delay times from this reset time, the response signal (SR) can be sampled. Using successively greater delayed sampling times allows for the construction of a single period of the response waveform as shown in  FIG. 13 . Using sampling of SR in this fashion can be less technically difficult than capturing the response waveform in one period, although this direct capture is also embodied by the present invention. 
         [0045]    To obtain a moisture measurement, the received and reconstructed response signal (SR)  1401  is transformed to the frequency domain using Fourier analysis as shown in  FIG. 14 . The Fourier transform gives a spectral amplitude, i.e., a set of amplitude values over frequency  1402 . The moisture content values over frequency can be measured using single frequency measurements and the corresponding response amplitude values as described elsewhere herein. These values can be used to construct a table of pre-determined correlations  1403  that can be applied to one or more subsequent time-domain measurements. The moisture content value for the sample can be derived from the table of these responses using one of several algorithms, the simplest of which is to take a single frequency/amplitude pair and its corresponding moisture content value. 
         [0046]    While a number of embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The appended claims, therefore, are intended to cover all such changes and modifications as they fall within the true spirit and scope of the invention.