WO2000014552A1

WO2000014552A1 - Moisture measurement device using microwaves

Info

Publication number: WO2000014552A1
Application number: PCT/US1999/019314
Authority: WO
Inventors: Danny S. Moshe; Alexander Greenwald
Original assignee: Malcam Ltd.; Friedman, Mark, M.
Priority date: 1998-09-03
Filing date: 1999-08-25
Publication date: 2000-03-16
Also published as: AU5583099A

Abstract

A portable device for evaluating the moisture content of small samples of materials. A sample holder (14) is introduced into a covered cavity. Antennas at both sides of the cavity direct a microwave beam through the material sample. Attenuation and phase shifts are used in extracting information concerning the sample composition. In an alternative approach, the sample cavity is subject to a resonance. The entire instrument is self-contained and ready to use either in a stand-alone mode or as a computer peripheral.

Description

MOISTURE MEASUREMENT DEVICE USING MICROWAVES

FTELD AND BACKGROUND OF THE INVENTION The present invention relates to a device for determining the moisture content of material on an analytic scale and, more particularly, to a device for such analytical measurements for calibrating another moisture measuring device on- location.

Many different types of synthetic and organic materials are the basis for the construction of many different manufactured products. These materials must be gathered, transported and stored before being used in the manufacturing process. The manufacturing process itself may require multiple procedures, first to prepare the raw material, and then to use the processed material in the formation of the actual product. Many of these procedures are dependent upon the moisture content of the material. If the moisture content is too high, for example, the material may decompose during storage and transportation, before it can be used. If the moisture content is too low, processing and use of the material may be difficult.

Synthetic and organic materials whose behavior depends upon their moisture content include seeds, hay, tobacco, grains, wood chips and logs, cotton, paper, wool, seeds, pharmaceuticals, synthetic fibers, and other loose organic materials. As an example, cotton can be considered, although it will be appreciated that similar examples could be given for any of the above materials. Cotton is processed to separate the desired cotton from contaminating materials such as seeds, and is then spun into fibers for use in textile manufacture. For such pro- cessing and spinning to be successful, the cotton fibers should have an even moisture content that is neither too high nor too low. For example, fibers with low moisture content are weaker, breaking more frequently.

The optimum moisture content of the cotton fibers for the production of textiles is from 6.5 to 8% before spinning and between 6-10% on the cones or bobbins, depending upon the requirements of the subsequent processing steps. Thus, effective moisture control in the textile mill depends upon accurate measurement of the moisture content of the fibers.

In order to accurately measure the moisture content of such materials, moisture measurements may be performed using microwave radiation. Typically, a microwave radiation source is located on one side of the bobbin or other type of module which contains the material, and an antenna is located on the opposite side of the bobbin, as described in U.S. Patent No. 5,621,330. The radiation source beam is transmitted through a portion of the bobbin and is received by the receiving antenna, which then produces a signal. This signal is used to determine the moisture content of that portion of the bobbin and the mass uniformity of the bobbin.

Other types of devices for measuring the moisture content of material with microwave radiation have also been disclosed in the prior art. However, all of these devices must be calibrated by laboratory standard procedures in order for the moisture measurements to be both accurate and reproducible. Such calibration is typically performed by measuring the moisture content of a small, fixed- volume sample of the material on an analytical scale. Hereinafter, the term "sample" refers to an amount of material ranging from about 5 cubic centimeters to about 2000 cubic centimeters in volume. The moisture content of the sample is then compared to the measured moisture content of a large bulk of the material, and the device is then calibrated according to this comparison.

Unfortunately, the most widely used method for analytical scale measurements of the moisture content of samples of material involves extensive laboratory work by experienced, highly- trained technicians. This prior art method is performed by weighing samples of material on an analytical balance, and then heating the samples to remove all moisture. The dry weight of the samples is then measured. The difference between the "wet", or regular weight of the samples, and their dry weight, is used to calculate the amount of moisture in the samples. The accuracy of this method is clearly dependent upon the ability of the technician to perform these steps carefully and consistently. Thus, this method is highly susceptible to human error and is also highly time consuming, since all of the steps are performed manually.

In addition, this prior art method has the disadvantage of using entirely different equipment than the device which it is intended to calibrate. Thus, the calibration process must also include compensation for these differences.

The process of calibration of these devices would be far more efficient and easier to perform, as well as more accurate, if the process were to be automated. Such automation would sharply reduce, or even eliminate, the potential for human error, as well as being less time consuming. In addition, less skilled workers could perform the calibration. The process could be automated with a small, portable device, which could measure the moisture content of samples of material on- location, thereby obviating the need for collection and transport of samples to a remote laboratory site for testing. Thus, such an analytical scale device for the on- location measurement of the moisture content of material would clearly solve the problems engendered by the current method of calibration.

There is thus a widely recognized need for, and it would be highly advantageous to have, a device for measuring the moisture content of a sample of material, which is portable for on-location measurements, which is automated so that it is easy to use for an ordinary worker and so that the process can be performed quickly, and which is highly accurate and efficient.

SUMMARY OF THE INVENTION According to the present invention there is provided a portable device for measuring a moisture content of a sample of material, comprising: (a) a sample holder for holding the sample of material; (b) a body with an interior space; (c) a cover for being hingedly attached to the body, such that when the cover is closed, the interior space of the body is covered; (d) a cavity for receiving the sample holder, the cavity being located within the body; (e) a transmitting antenna disposed on one side of the cavity for transmitting a source beam of microwave radiation through the sample of material, such that the source beam becomes an exit beam, the transmitting antenna being located within the body; (f) a receiving antenna disposed on a substantially opposing side of the cavity for receiving the exit beam and for producing an antenna signal according to the exit beam, the receiving antenna being located within the body; (g) an attenuation measurer for determining an attenuation of the antenna signal; and (h) a moisture determiner for calculating the moisture content of the sample of material from the attenuation to form a calculated moisture content. Preferably, the device further features: (i) a phase shift measurer for measuring a phase shift of the antenna signal, such that the moisture determiner calculates the calculated moisture content from the phase shift and from the attenuation. More preferably, the device further features: j) a temperature sensor for measuring a temperature of the sample of material, the temperature sensor featuring a probe for insertion into the sample of material, such that the moisture determiner corrects the calculated moisture content for an effect of the temperature. Most preferably, the device further features: (k) a balance for measuring a weight of the sample of material, the balance being located at a bottom of the cavity, such that the moisture determiner corrects the calculated moisture content for an effect of the weight.

According to preferred features of the present invention, the device further includes: (1) a screen for displaying the calculated moisture content. Preferably, the device further features: (m) an information input unit for inputting information about a type of the material and a form of the material, such that the moisture determiner corrects the calculated moisture content for an empirical effect of the type of the material and the form of the material. Most preferably, the information input unit is selected from the group consisting of a touch sensitive screen, a keyboard and a keypad.

According to other preferred embodiments of the present invention, the sample holder is constructed from a substance for reducing an effect of reflectance of the source beam. Preferably, the material is selected from the group consisting of cotton fiber, silk fiber, wool fiber, pharmaceutical material, seeds, paper, tobacco and synthetic fiber.

According to still other preferred embodiments of the present invention, the source beam is repeatedly transmitted through the sample of material by the transmitting antenna such that a plurality of antenna signals is produced, such that the attenuation measurer determines the attenuation for each of the plurality of antenna signals to form a plurality of attenuation values, the plurality of attenuation values being filtered to form the attenuation for the calculation of the calculated moisture content by the moisture determiner.

According to another preferred embodiment of the present invention, the cavity is substantially triangular in shape.

According to still another preferred embodiment of the present invention, the cavity is a microwave resonator, the transmitting antenna is a microwave radiation broadband generator-synthesizer for producing the source beam, and the receiving antenna is a signal receiver for receiving the exit beam.

Preferably, the microwave resonator is substantially cylindrical in shape. More preferably, the microwave resonator is constructed from metal. Most preferably, the microwave resonator features an entry for receiving the sample holder and a plurality of metal pieces being located substantially opposite from the entry.

Also preferably, the sample holder rotates within the microwave resonator for measuring the moisture content of the sample of material.

Hereinafter, the term "material" includes any material which can be sampled and which has a moisture content measurable with microwave radiation. For example, cotton, wool, silk and synthetic fibers, as well as paper, seeds, grains, wood in any form including logs and chips, and other loose organic material, all fulfill these requirements, as do pharmaceutical materials, such as powdered medications. Hereinafter, the term "module" refers to any structure of material, including bales and bobbins. Hereinafter, the term "on-location" refers to the ability of a calibration method to be performed at the site from which samples are taken, substantially without the need to transport the samples to a remote location. Hereinafter, the term "portable" refers to the ability to be transported from one location to another, without the need for special lifting and moving equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1A is a partially cut-away view of an exemplary portable device for performing a moisture measurement on a sample of material according to the present invention; FIG. IB shows an exemplary sample holder for the device of Figure 1A; FIG. 1C shows a series of exemplary calibration curves which could be obtained by using the device of Figure 1 A with the exemplary sample holder of Figure IB; FIG. ID shows a schematic block diagram of an exemplary embodiment with a reference signal; FIG. IE shows a schematic block diagram of a compensator according to the present invention; and FIG. IF shows another preferred embodiment of a portion of the present invention;

FIGS. 2A and 2B show two side views of the device of Figure 1 according to the present invention; FIG. 3 is a flow chart of the method of calculating the moisture content of the material; and

FIGS. 4A-4C show a third embodiment of the device of the present invention for moisture measurements with a resonator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of a device which can be used to measure the moisture content of a sample of material, and which can then perform the necessary calculations in order to calibrate an on-location device for performing such moisture measurements on a large scale. The principles and operation of a device according to the present invention may be better understood with reference to the drawings and the accompanying description.

Referring now to the drawings, Figure 1A shows an exemplary device for performing the moisture measurement of a sample of material according to the present invention. A device 10 for measuring the moisture content of a sample of material is shown. Device 10 features a cavity 12 for receiving a sample holder 14. Sample holder 14 holds a sample of material 16. Although both sample holder 14 and cavity 12 are shown as being substantially cubic, substantially any suitable, complementary pair of geometrical configurations could be used, such that cavity 12 is able to receive sample holder 14, as described with reference to Figures IB and ID below.

Device 10 features a microwave radiation source 18, shown on one side of cavity 12, and at least one receiving antenna 20, shown on a substantially opposing side of cavity 12. Microwave radiation source 18 preferably includes at least one source antenna 22 for transmitting a source beam. For the purposes of illustration, one source antenna 22 and one receiving antenna 20 are shown, although preferably a plurality of different types of source antenna 22 and receiving antenna 20 are included, which are respectively capable of transmitting and receiving a plurality of different frequencies, as shown in Figure ID for example. The source beam is directed through sample 16, sample holder 14 and cavity 12, and then exits as an exit beam. The exit beam is then received by receiving antenna 20. Preferably, both source antenna 22 and receiving antenna 20 are high frequency, at least 1 GHz and optionally up to about 50 Ghz, according to the application. Also preferably, both source antenna 22 and receiving antenna 20 have high gain for greater sensitivity.

After receiving antenna 20 has received the exit beam, receiving antenna 20 produces an antenna signal. The antenna signal then goes to an attenuation unit 24. Attenuation unit 24 includes an attenuation measurer 26, which measures the attenuation of the antenna signal. As the source beam passes through sample 16, sample holder 14 and cavity 12, the source beam is attenuated. However, the attenuation caused by sample holder 14 and the material lining cavity 12 can be determined in the absence of any sample. The extent of the remaining attenuation is determined by the elementary mass of sample 16, which is the mass of the material of sample 16 encountered by the source beam, and by the moisture content of the material of sample 16 encountered the source beam. Thus, attenuation measurer 26 is actually correcting the measurement for the effect of sample holder 14 and the material lining cavity 12.

Optionally and preferably, at least a part of the antenna signal also goes to a phase shift determiner 28, which determines the phase shift of the antenna signal. The phase shift is the difference between the phase of the source beam and the phase of the exit beam, which is actually the phase shift caused by the passing of the source beam through sample 16, sample holder 14 and the material lining cavity 12. However, as for the determination of the attenuation, the effect of sample holder 14 and the material lining cavity 12 on the phase shift can be determined in the absence of sample 16, so that the portion of the phase shift caused by sample 16 can then be calculated.

Preferably, device 10 features a cover 30 which is shown as being hingedly connected to a body 32 of device 10, although other configurations are also possible. Cover 30 can protect both the delicate electronics of device 10 and sample 16 while moisture measurements are being made. More preferably, cover 30 is made from a material which is substantially impenetrable to microwave radiation, such as Eccosorban-73™ (Millimeter Wave Technology Inc.) Most preferably, body 32 is also lined with the same material, in order to reduce or substantially eliminate interference from ambient radiation, such as signals from cellular phones and other electronic devices in the environment.

In addition to interference from ambient radiation, the measurement of moisture with microwave radiation can also be affected by reflectance of the microwaves within body 32 of device 10. Preferably, both the material and the structure of sample holder 14 is able to reduce reflectance of the microwave radiation. For example, sample holder 14 could be constructed from a suitable plastic or from a mixture of plastic and an absorbent material for absorbing microwaves. Although the presence of the absorbent material would tend to increase the signal to noise ratio, the geometrical placement of the sections of material would preferably tend to decrease reflectance of the microwave radiation, thereby increasing the proportion of radiation which passes through sample 16 within sample holder 14, thus increasing the signal to noise ratio by decreasing interference caused by the reflected waves. More preferably, the material lining cavity 12 is also able to reduce reflectance.

According to preferred embodiments of the present invention, device 10 preferably also features a temperature sensor 34 and a balance 36, which can also be a touch sensor. Temperature sensor 34 is attached to cover 30, and features a probe 38 which is inserted into sample 16 when cover 30 is closed. Temperature sensor 34 then determines the temperature of the material of sample 16, which can be used to compensate for the effect of temperature on the measurement of the moisture content of sample 16, as described in more detail in Figure 3 below.

Balance 36 is located at the bottom of cavity 12 as shown, such that when sample holder 14 is placed within cavity 12, balance 36 is able to determine the weight of sample holder 14 and sample 16 together. Since the weight of sample holder 14 is known, the weight of sample 16 can be calculated. The weight of sample 16 can then be used to compensate for the effect of the weight of the material on the measurement of the moisture content, as described in more detail in Figure 3 below. Preferably, balance 36 is a touch sensor. Optionally and more preferably, cavity 12 is lined with teflon or another surface which has reduced friction, which would permit an optical balance to be used to determine the weight of sample 16 (not shown). However, substantially any device for measuinrg the weight of sample holder 14 and sample 16 could benefit from the inclusion of teflon in the lining of cavity 12. Once the attenuation and the phase shift have been determined, along with any other compensatory measurements such as temperature and weight of the material, these measured factors are then used by a moisture determiner 40 to determine the moisture content of sample 16, as shown in more detail in Figure 3 below. The results are then displayed on a screen 42.

Preferably, the user is able to enter information with an information input unit 44. Information input unit 44 could be a small keypad or even a keyboard, as shown, or a touch sensitive screen, for example. Information input unit 44 preferably enables the user to enter identifying information about sample 16, such as the name of the user, the date and time, the type of the material, and the location of the bulk of material, for example. Other useful and preferred information includes, but is not limited to, the type of material of the sample, and the form of the material of the sample. The sample could be in the form of a powder, fibers, leaves, seeds, or threads, for example. If the material includes some type of fibers, the information also preferably includes the direction of the fibers, whether parallel or perpendicular. The type and form of the material of the sample are particularly important as compensating factors for the calculation of the moisture content of the material, as shown in Figure 3 below.

A number of variations are possible for the device of Figure 1 A. For example, sample holder 14 is preferably substantially triangular in shape in order to enable measurements to be made dynamically down the length of sample of material 16 with varying widths, as shown in Figure IB. The variation of the width of sample of material 16 enables the effect of the amount of sample of material 16 to be removed from the calculations of the moisture content as sample holder 14 is moved past a microwave radiation source 18 (described in greater detail below).

The moisture content of sample of material 16 can then be determined from a series of calibration curves, as shown in Figure 1C. Briefly, a plurality of exemplary calibration curves are shown, each of which is a graph of the amplitude of the received microwave radiation (y-axis) against time (x-axis). As sample holder 14 of Figure 1A is moved between source 18 and receiving antenna (receiver) 20, multiple moisture measurements are made. As shown, each moisture measurement ^corresponds to a different portion of sample of material 16 being measured. Each such portion has a different thickness and therefore a different attenuation coefficient α, which can be determined according to the calibration curves. Thus, the true moisture content of the material can be determined according to a plurality of such calibration curves.

Alternatively and preferably, the moisture content of the material could be determined from an equation, in which the moisture content is a function of the attenuation coefficient, as described in further detail below. A similar effect can be obtained by rotating sample holder 14 within cavity

12, as shown in the preferred embodiment of sample holder 14 illustrated in Figure IE. In this embodiment, sample holder 14 is preferably a cylindrical resonator cell rotating between microwave radiation source 18 and at least one receiving antenna 20, as described in further detail below. Figure ID shows a schematic block diagram of a preferred configuration of the device of the present invention, which features microwave antennas capable of transmitting or receiving a plurality of frequencies, including at least a first frequency of microwaves denoted F_h and a second frequency of microwaves denoted F₂. In addition, this embodiment features a reference signal for correcting the measured moisture values. It should be noted that the physical configuration of the device is substantially similar to that of Figure 1A, although certain components have been omitted for the sake of clarity.

A multiple- frequency system 46 has a multiple-frequency transmitter 48 for sequentially transmitting microwave radiation at a plurality of frequencies. The frequency to be transmitted is selected by a frequency controller 50. Transmitter 48 then causes a transmitting antenna 52 to transmit microwave radiation at the desired frequency. The transmitted microwave radiation then passes through a sample of material 54, which is contained with a sample holder substantially as described above (not shown). This radiation is received by a receiving antenna 56. Receiving antenna 56 sends a signal to a signal receiver 58. Signal receiver 58 is preferably a heterodyne receiver. Substantially simultaneously, a reference signal is sent from transmitter 48 to a reference receiver 60, which is also preferably a heterodyne receiver. Signal receiver 58 sends a measurement signal ( "I.F. 1") to a detector 62, while reference receiver 60 sends a reference signal ( "I.F. 2") to detector 62. Detector 62 uses the reference signal to determine the correct attenuation of the measurement signal, and then passes both signals to a phase detector 64, which determines the correct phase shift for the measurement signal.

The phase shift is obtained by sequentially transmitting microwave radiation of at least two different frequencies Fj, F₂ through the material, and "hopping" or alternating at least between these two frequencies at each point in the material.

The phase shift is calculated as described with the equations below: 1. λ=C/F; /= (wave length of radiation); ε'= (dielectric constant of material)

2π 2. P,=

^~λ *^lε' *£;

4. P,= K*F₁; P₂= K*F₂; K= — * J * l ; P_rP₂= Δ p = κ*(F_rF₂);

5. n, is the portion of the result of the expression (Pj/360) to the left of the decimal point; ifP_m, P_m2 then n₂ = n,; fP_m, < P_m2 then n₂ = n₁ - 1;

7. Pg= (phase including nπterm) = v * 360, in which v is the portion of the result of

the expression the left of the decimal point;

8. Corrected Phase-shift = P_g + P_ni[ = P_cor]

Note that Fj, F₂ are the frequencies of the microwave radiation, as described above; Pj is the phase shift of microwave radiation at frequency F , P₂ is the phase shift of microwave radiation at frequency F₂; K

is the difference between the phase shift of the radiation at frequencies F, and F₂; P_m P_m are the measured phase shifts with frequencies F_h F₂, respectively; Pg is the gross phase shift difference; and nj and n₂ are greater than 0.

Although these equations both describe the corrected phase shift, P_C0I. , and can be used for its calculation, the refinements of the calculations must be done according to empirically observed properties of the material itself and effects of the surrounding environment, substantially as described in Figure 3.

The two frequencies of microwave radiation should be chosen in order to enable sufficient dynamic range such that measurements of different ranges of moisture content can be made substantially without saturation. Thus, if saturation occurs with the higher of the two frequencies, the measurements can be made at the lower of the two frequencies, thereby avoiding saturation. For fibrous material, preferably the values of Fj and F₂ are substantially greater than 24 Ghz. By contrast, for seeds, preferably the values of Fj and F₂ are substantially less than 16 Ghz.

Specifically, a particularly preferred embodiment of the present invention for increasing the sensitivity of the measurement is shown in Figure IF. This preferred embodiment shares certain features which multiple-frequency system 46 shown in Figure ID, as indicated by the identical reference numerals. However, certain features have also been added. The device shown in Figure IF also features a splitter 49 for splitting the microwave radiation from transmitter 48 into two portions. A first portion, indicated as A_0meas, is transmitted by transmitting antenna 52. A second portion, indicated as A_rφ is passed to a bridge summation apparatus 63. Bridge summation apparatus 63 also receives the signal from receiving antenna 56, shown as A_meas. The combined signal, A_res, is then passed to a detector 65. The performance of the device according to this preferred embodiment of the present invention could be described according to the following set of equations.

(Equation 1)

Aref = Aref * _e ^~j*^pr (Equation 2)

— j. — *(W)*£+ i*β*(W)*£

Ameas = Aomeas * β ' ^{J \} > (Equation 3)

In these equations, A_res is the amplitude of the resultant signal, after corrections for the effect of the material lining the cavity have been made; A_ref is the amplitude of the reference signal; A_meas is the amplitude of the measured signal; A_omeas is the amplitude of the signal from the receiving antenna; a*(W) is the attenuation coefficient, which depends upon the moisture content of the material; β*(W) is the phase coefficient, which also depends upon the moisture content of the material; / is the length of the material; and _re/-is the phase shift of the reference signal. From these three equations, the following equation for the amplitude of the resultant signal can be obtained:

The sensitivity of the amplitude vector which results from the moisture content of the material (W) is as follows.

From the above equation, clearly the maximal value of the deπvative is obtained when the following conditions are true:

Figure 1 E is a schematic block diagram of an exemplary compensator 66, which receives the known attenuation caused by sample holder 14 and the mateπal lining cavity 12 (shown as "Attenuatιon₂"), as well as the measured attenuation (shown as "Attenuation_!") The measured attenuation is affected by the known attenuation caused by sample holder 14 and the mateπal lining cavity 12, as well as by the attenuation caused by sample 16 alone and by the attenuation caused by the moisture withm sample 16. Compensator 66 removes the effect of the known attenuation caused by sample holder 14 and the mateπal lining cavity 12 from the measured attenuation in order to obtain the true attenuation (shown as

"Attenuatιon₃"), which only includes the attenuation caused by the matenal itself and by the moisture content of the matenal.

Figures 2 A and 2B show two side views of device 10 of Figure 1 A Figure 2 A shows a view of the exteπor of one side of device 10, with cover 30 closed Screen 42 and information input unit 44 are shown attached to the exteπor of one side of body 32 of device 10, for easy access by the user (not shown) Preferably, two handles 46 are attached as shown for facilitating transport of device 10. which is intended to be portable Figure 2B shows a cut-away side view of device 10, showing sample holder 14 inside cavity 12. Balance 36 is also shown. Figure 3 shows a flow chart of the calculations for determining the moisture content of the sample. The attenuation is used to determine the raw moisture content of the material. Optionally and preferably, the phase shift is used to correct the raw moisture content to calculate the moisture content of the material more accurately. The attenuation, and the phase shift if measured, are used in combination with empirically determined correction factors to calculate the final moisture content of the material.

The first step in the flow chart is the transmission of microwave radiation through the material, which can be performed using the device essentially as described above. Microwaves are transmitted through the sample so that they pass through the sample and are received on the other side. Preferably, this step is repeated at least once so that a plurality of transmissions through the sample is done. More preferably, microwaves of a plurality of different frequencies are transmitted in order to eliminate at least part of the interference, for example as shown and described with reference to Figure IB above. The attenuation and optionally the phase shift are calculated, as shown in step 2, by an attenuation measurer and a phase shift measurer, respectively.

The flow chart now branches into two parts. The right branch shows the steps used in calculating the raw moisture content of the material, while the left branch shows the steps for the determination of the density of the material. For clarity, steps in the right (moisture content) branch will have the letter "a" appended; e.g., "3a", "4a", etc. Steps in the left (density) branch will have the letter "b" appended; e.g., "3b", "4b", etc.

Following the right branch, in step 3a an algorithm is used to filter the data points obtained for the attenuation. Each transmission of microwaves through the sample enables a separate measurement of the attenuation to be made as described above in Figure 1. Each such measurement is a separate data point. These data points must be filtered, since otherwise artefacrual data could be obtained. Filtering can be done by averaging the data points, or by selecting the median of the points, for example, since all measurements are replicates.

Once the data has been filtered, the attenuation is preferably corrected for the effect of the weight of the material and the sample holder, more preferably according to the vector of multiple measurements made for each sample, as shown in step 4a. This correction is preferably performed by compensating the attenuation with the ratio of a standard weight to the actual weight, for example by multiplication when the material is tobacco, and produces a weight-corrected attenuation value.

Next, in step 5 a, the weight-corrected attenuation value is preferably corrected for temperature, to produce a temperature-corrected attenuation value. The correction is performed by adding the weight-corrected attenuation value to the factor a(l - Ts/Te), where Ts is the standard temperature, and Te is the measured temperature of the material, in order to produce the temperature-corrected attenuation value. The value of a is empirically determined according to the type of material. More preferably, the temperature is substantially continuously monitored by the temperature sensor, so that each measurement of the attenuation can be corrected with the temperature value taken as the transmission of microwaves was made. The temperature-corrected attenuation value thus is compensated for the effect of measurements at different temperatures.

In step 6a, the temperature-corrected attenuation value is used to calculate a raw moisture value for the material. Preferably, various empirical factors are included in the calculation, such as the type of material being examined and the form of the material. This raw moisture value will be used in the determination of the final moisture value for the material. However, the final moisture value is preferably determined by also incorporating the density of material into the calculations. The density of the material is preferably calculated as shown in the left branch of the flow chart. Turning back now to the left and optional branch of the flow chart, in step

3b, the data points are preferably filtered as for step 3a. Next, in step 3b, the density is calculated from the phase shift, in accordance with empirical information from a database. The empirical information includes the type of material and the form of the material. Additionally, the database preferably contains "fuzzy descriptors" which are used to find the correct phase region and to determine the proper relationship between measured phase shift values and calculated density values, such that fuzzy logic is used to determine the density of the material. These "fuzzy descriptors" are obtained by collecting phase shift data from an analysis of material with known density, and then comparing the calculated density values with the true, known density values of the test material. From this analysis, the proper correlation between the measured phase shift values and the calculated density values can be determined. Since this correlation depends upon the material or materials from which the test shape is constructed, such an analysis must be performed for substantially every desired material in order to obtain these essentially empirical correlations.

Finally, in step 7, the optional density value, and the raw moisture value, which is calculated in step 6a, are combined to determine the final moisture value by a moisture determiner. The equation for calculating the final moisture value includes the density, as well as an empirically determined correlation factor. The correlation factor depends upon the type and form of material, and was empirically determined through experimentation. The final moisture value is then output, for example by displaying on the display screen of the device of Figures 1 and 2. An example of specific coefficients and moisture values for the material cotton is as follows. The measured signal from the receiver antenna can be determined according to the following equation:

Erec = ETR * _e-^a{W)£ * s l ωt + β* (W) * t *- P pstd

In this equation, the value ofE_rec is the recorded signal from the receiver antenna; E_TR is the true received signal; Wis the final moisture value; / is the length of the sample; T is the temperature at which the measurement was made; ω is the frequency of microwave radiation; p_std is the standard calibrated density; /? is the density of the material; a is the attenuation coefficient; and ?is the phase coefficient. The values of a and β are given by the following equations: β = Kι * W

The values for Kj and K₂ depend upon the type of material and can be empirically determined. For example, for cotton fibers, when the measured temperature was 20 °C, the density was 100 kg/m , and the frequency was 10 GHz, the resultant value of K] was 0.18 dB per one percent of moisture, and the resultant value of ^ was 10 degrees per one percent of moisture.

These equations are used collectively to determine the final moisture value according to the type of material. For cotton fibers, under the conditions described above, when a was 2.1 dB/m and β was 1050 degree/m, the final moisture value was about 7%.

This final moisture value can be then be used to calibrate the on-location device. The advantage of the device of the present invention is that the entire process of obtaining the final moisture value, after the sample has been placed into the sample holder and then into the cavity, is entirely automatic. The device of the present invention can be operated by a relatively unskilled user, yet gives highly accurate and sensitive results. Furthermore, the device of the present invention is far less sensitive to the presence of chemicals in the sample of material than the prior art method, which requires heating of the material to remove the moisture. In addition, the small size of the sample within a known sample holder volume reduces sensitivity to temperature and moisture deviations within the material. Finally, the closed nature of the device of the present invention reduces sensitivity to other ambient factors.

In another preferred embodiment, a resonator cell is featured as shown in Figures 4A-4C, described briefly below. A more extensive description is provided in U.S. Patent Application No. 08/xxx,xxx, entitled "Method And Device For Highly Accurate, High Speed, Real Time, Continuous Or Stationary, In-Line, Non- Invasive, Three Dimensional, Multi-Slice Density Deviation And Density Measurements And Calculations Of Homogeneous Or Non-Homogeneous Fibrous Yarn, Slivers, Or Pad Material", filed on xx, xx, 1998, the teachings of which are incorporated by reference as if fully set forth herein.

Figure 4A is a schematic diagram of a preferred embodiment which features a resonator cell for receiving the sample holder containing the sample of material. Sample of material 16 is placed within sample holder 14, in a substantially similar manner as previously described above for Figure 1A. However, instead of placing sample holder 14 into a cavity which features transmitter and receiver antennas as in Figure 1A, sample holder 14 is now placed into a cylindrical microwave resonator 68. Microwaves of controllable, specified frequencies are generated by a microwave radiation broadband generator- synthesizer 70. Broadband generator- synthesizer 70 continuously generates, in a sweeping mode, microwave radiation of frequencies within pre-set ranges, and feeds the microwaves through a microwave input port 72, into resonator 68.

The range of frequencies of microwave radiation passes through material 16 such that the radiation becomes transmitted microwaves. The transmitted microwaves are continuously received by and further transmitted through a microwave output port 74, into signal receiver 76. From signal receiver 76, the microwave signals go to an attenuation measurer 78, simultaneous to continuously received values of material temperature received from a strategically located external temperature sensor 80. Attenuation measurer 78 is similar in function to the attenuation measurer of Figures 1A and 3. The attenuation values are corrected substantially as described for Figure 3, for example according to the value of the measured density.

Figure 4B shows a close-up of resonator 68 with sample of material 16 in holder 14. This embodiment illustrates and describes, but is not limited to, a microwave resonator of substantially cylindrical shape, it being understood that other such shapes are possible within the scope of the present invention. Resonator 68 is comprised of a hollow piece of cylindrically shaped metal, having typical dimensions of 44 mm height and 38 mm diameter, preferably constructed of high purity copper or invar alloy, or an equivalent material thereof. The entire inner surface of resonator 68 is preferably anodized with high purity gold or silver. Gold and silver are the preferred anodizing metals for their superior conductivity and anti-corrosive properties compared to base metals, such as copper or invar. This anodized metal design enhances resonator performance and quality characteristics. Resonator 68 features a resonator cover 82 having a typical diameter of about 20 mm for a base plate diameter of 38 mm. Preferably resonator cover 82 is composed of the same anodized metal as the principle microwave resonator. Alternatively and preferably, resonator cover 82 is composed of teflon. Resonator cover 82 acts as a cutoff waveguide for preventing leakage of microwave radiation outside resonator 68. Resonator cover 82 is critically important for containing the microwave radiation inside resonator 68, thereby maintaining high quality, Q, of resonator 68 (i.e., high amplitudes of the electromagnetic field associated with the microwave signals inside resonator 68).

Inside, and preferably at one end, of resonator 68, are located pieces of metal 84, preferably two as shown in this example, with typical dimensions of length, a = 7.3 mm; height, b = 6.2 mm; and width, c = 1.6 mm, and composed of the same anodized metal as the microwave resonator and waveguides. Explanation of their function is as follows. Resonator quality, Q, is defined as the ratio of resonance frequency, f_r, to twice the resonance frequency full width half amplitude bandwidth, 2(delta)f, i.e., Q = f_r / [2(delta)fj, where the resonance frequency and resonance frequency full width half amplitude bandwidth are measured at half-height of the microwave with maximum amplitude, where A_m and A_m/2 correspond to maximum amplitude, and amplitude at half-height of maximum amplitude, respectively. Inside resonator 68, during operation under ideal conditions, the type of resonance modes are preferably double-degenerate, whereby each double-degenerate mode of the microwave radiation has two different electromagnetic distributions at the same resonance frequency, f_r, i.e., A_m and A'_m. For two double-degenerate components, a difference in the amplitude at resonances exists such that two components of the same mode have different couplings with signal input and output ports 72 and 74. In practice, these components exhibit different resonance frequencies, f_r and f _r, in addition to different amplitudes, A_m and A'_m, respectively, due to structural and/or material imperfections, asymmetries, and/or non-homogeneities of the resonator and/or cutoff waveguides. This phenomenon leads to error in resonance frequency bandwidth received by signal receiver 76, in that 2(delta)f ≠ 2(de)f, which leads to error in the determination of the quality, Q, of resonator 68, ultimately causing error in measurements of moisture content and in calculations (compensated) of the density of sample of material 16. Elimination of this error is accomplished with metal pieces 84. Metal pieces 84 are positioned inside the resonator at the point of highest electric field strength of the component having the lower amplitude. The presence of metal pieces 84 causes a decrease in the resonance frequency of the component with the smaller amplitude, A'_m, resulting in separation of the two resonance curves, enabling a correct reading of the resonance frequency full width half amplitude bandwidth, 2(delta)f, and thus enabling the true quality of the resonator signal to be processed. Moreover, this correction enables the calculation of highly accurate values of moisture content. The preceding corrective design in resonator 68 is an example only; each microwave resonator configuration requires its own corrective design, based upon the specific material and structural imperfections, asymmetries, and/or non-homogeneities.

Measurement and monitoring of temperature of sample of material 16 are accomplished by employing a strategically located in-line temperature sensor 80. A thermocouple, thermoresistor, or alternatively, an infrared device can be used as temperature sensor 80. Temperature sensor 80 is strategically positioned to monitor, in a real time, continuous or stationary, in-line, and non-invasive mode, the temperature of sample of material 16. The temperature sensor output lead of 80 is connected to attenuation measurer 78.

In the present invention, the relative geometry of the microwave resonator device illustrated in Figure 4B is such to enable the rotation of either sample of material, microwave resonator, or the direction of the microwave radiation in the resonator, during real-time, in an in-line, non-invasive mode of density measurement and control. The ability to rotate the sample holder is also an optional but preferred feature of the other embodiments of the device of the present invention, as previously described, since such rotation allows multiple moisture measurements to be made at different locations or "slices" of the sample of material.

Microwave radiation generator-synthesizer 70 can optionally include a number of features which are designed to maximize the sensitivity of resonance frequency shift (density) and resonator quality (moisture content) measurements of fibrous materials. One such feature is an electric field director which determines the mode of the electric field strength, established by the microwave source, through input port 72 relative to sample of material 16 inside microwave resonator 68, such that the mode of the electric field partially determines the magnitudes of frequency shifts and changes in resonator quality (i.e., changes in signal amplitude). Maximum frequency shift and change in resonator quality, due to the presence of sample of material 16, occur when the electric field strength of the source microwave radiation is parallel to the test material (not shown). When the electric field strength is substantially perpendicular to sample of material 16, minimum frequency shift and change in resonator quality are obtained. The electric field director determines the direction of the electric field strength. The measurement and calculation of the moisture content of the material is performed as described in Figure 4C. Figure 4C is a flow chart of the method of calculating density and moisture content of samples of material for the resonator of the present invention. The overall calculation method used in achieving the objectives of the present invention is based on the evaluation of two arrays (of time samples), D and W, representing an array of resonator resonance frequency shift / density, and an array of resonator quality / moisture content, respectively. Numerical evaluation of the arrays D and W is determined from evaluation of iterative correlation functions, F_; (i.e., F, through F₈), which in turn are functions of resonator frequency shift, resonator quality, material temperature, and four empirically determined structure sensitive (i.e., material shape and type) correlation functions, S„ denoted by Si, S₂, S₃, and S₄. The correlation functions, F, and S„ represent sets of linear and non-linear functions, each functional form uniquely determined from correlations of calculated density and moisture content values to true, known density and moisture content values, for a variety of material structural shapes and types. Correlation data are obtained from calibration plots and numerical analysis of density vs resonance frequency shift or moisture content vs resonator quality data, made from data generated during calibration measurements recorded at standard conditions, using reference materials of known densities, moisture content, temperatures, and of well characterized structure shapes and types. The correlation functions, F, and S„ provide proper correlations between calculated and known density and moisture content values, according to different sample material structural peculiarities or uniqueness, and are critical in achieving high accuracy and reproducibility in calculations of final density and moisture content shown on the display unit. Numerical results of these calculations represent averages and standard deviations of density (and optionally, moisture content) of n slices of material, whereby n is an adjustable parameter proportional to the scan rate of the resonator device.

Both arrays, D and W are used in calculating density and moisture content averages and standard deviations. The D and W arrays are directly proportional to material density, and moisture content, which in turn are functions of resonator resonance frequency, f_r, resonator quality, Q, respectively, material temperature, T_m, and all four of the empirically determined structure sensitive correlation functions, S, , (Si, S₂ , S₃, and S₄). Moreover, evaluation of array W, providing structural and temperature compensated average and standard deviation values of moisture content, i.e., W_A and W_SD, are themselves used in compensating and correcting initial average and standard deviation values of density, i.e., D_A and D_S , leading to highly accurate moisture content compensated values of density average and standard deviation, i.e.,

D _A and D _SD, per n frequency shift datapoints (i.e., per n slices of analyzed sample).

The first step in the flow chart represents the real time, in-line, continuous or stationary mode, multi-slice scanning of n slices of the test material using one of the preferred embodiments (a) or (b) (Fig. 1 and 3, or 4) of the device of the present invention. Multi-slice scanning of the test material, positioned inside the microwave resonator/waveguide device, is performed by continuously transmitting a sweeping set of microwaves of known frequencies into the resonator, and identifying and detecting the affected microwaves by the respective microwave receiver components. The scanning step provides the necessary raw data for the initial determination of the resonator resonance frequency shift/density array, D₀, and the resonator quality array, W₀, step (1). Following step (1), an algorithm is used to evaluate, filter and eliminate unreasonable data points from the calculations of initial frequency shift and resonator quality, step (2). The raw frequency data must be filtered in order to eliminate system noise and/or artificial readings of resonance frequency and/or frequency bandwidths. Values of density array, D and quality array, W_b are calculated from the appropriate functions, F_b and F₅, respectively, following data filtering, step (3). The flow chart now branches into two parts. The right branch shows the steps used in calculating the density of the material, while the left branch shows the steps used in calculating moisture content of the material. For clarity, steps in the right (density) branch, and steps in the left (moisture content) branch, are assigned the numbers 3a - 3c, and 4a - 4c, respectively. Proceeding with density calculations, the next value of density array, D , is evaluated from the function, F₂, of D, and S[ (structure correlation function), step (3a). Calculated values of density can now compensated and adjusted for temperature. In step (3b), the next value of density array, D₃, is evaluated from the function, F₃, of D₂ and T, where the dependency of F₃ on T is empirically determined, and as an example, is proportional to (1 - T_m/T_s ), where T_m and T_s are the 'actual' and 'standard' material temperatures, respectively. In step (3c), initial values of the magnitude of the material density array, D, including average density, D_A and density standard deviation, D_SD, are evaluated from the function, F₄, of D₃ and S₂, another structure sensitive empirical correlation function.

The above initial calculated values of density are to be compensated/corrected for moisture content of the material, determined from resonator quality data according to the left branch of the flow chart. Following elimination of unreasonable values of resonator quality, step (2), and calculation of quality array W_b by using the function, F₅ of W₀, step (3), the initial value of the moisture content array, W₂, is calculated from the function, F₆, of j and S₃, step (4a). The next value of moisture content array, W₃, includes compensation and adjustment for temperature. W₃ is evaluated from the function, F₇, of W₂ and T, where the dependency of F₇ on T is proportional to (1 - T_m / T_s), similar to that of F₃, in the above density calculations. In step (4c), the corrected values of the magnitude of material moisture content array, W, including average moisture content, W_A and moisture content standard deviation, W_SD, are evaluated from the function, F₈, of W₃ and S₄. These values of moisture content are used to compensate the above initial calculated values of density, step (5), in order to obtain final moisture content compensated average and standard deviation values of mateπal density, D _A and D _SD, step (6), which are ready for display.

This embodiment of the present invention is particularly preferred because the resonator cell has high sensitivity for very small volumes of material, such as from about 5 cubic centimeters to about 50 cubic centimeters. In particular, the resonator cell can aid in the measurement of the moisture content of pharmaceutical powders or synthetic materials. The moisture content of pharmaceutical powders is particularly difficult to measure, since these powders tend to have low moisture content, such that the resonator is preferred over the antennas for measuring the moisture content of such material.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims

WHAT IS CLAIMED IS:

1. A portable device for measuring a moisture content of a sample of material, comprising:

(a) a sample holder for holding the sample of material;

(b) a body with an interior space;

(c) a cover for being hingedly attached to said body, such that when said cover is closed, said interior space of said body is covered;

(d) a cavity for receiving said sample holder, said cavity being located within said interior space of said body;

(e) a transmitting antenna disposed on one side of said cavity for transmitting a source beam of microwave radiation through the sample of material, such that said source beam becomes an exit beam, said transmitting antenna being located within said body;

(f) a receiving antenna disposed on a substantially opposing side of said cavity for receiving said exit beam and for producing an antenna signal according to said exit beam, said receiving antenna being located within said body;

(g) an attenuation measurer for determining an attenuation of said antenna signal; and

(h) a moisture determiner for calculating the moisture content of the sample of material from said attenuation to form a calculated moisture content.

2. The device of claim 1, further comprising:

(i) a phase shift measurer for measuring a phase shift of said antenna signal, such that said moisture determiner calculates said calculated moisture content from said phase shift and from said attenuation.

3. The device of claim 2, further comprising:

(j) a temperature sensor for measuring a temperature of the sample of material, said temperature sensor featuring a probe for insertion into the sample of material, such that said moisture determiner corrects said calculated moisture content for an effect of said temperature.

4. The device of claim 3, further comprising:

(k) a balance for measuring a weight of the sample of material, said balance being located at a bottom of said cavity, such that said moisture determiner corrects said calculated moisture content for an effect of said weight.

5. The device of claim 4, further comprising:

(1) a screen for displaying said calculated moisture content.

6. The device of claim 5, further comprising:

(m) an information input unit for inputting information about a type of the material and a form of the material, such that said moisture determiner corrects said calculated moisture content for an empirical effect of said type of the material and said form of the material.

7. The device of claim 6, wherein said information input unit is selected from the group consisting of a touch sensitive screen, a keyboard and a keypad.

8. The device of claim 2, wherein said sample holder rotates within said cavity for measuring the moisture content of the sample of material at a plurality of slices of the sample of material.

9. The device of claim 8, wherein a density of the sample of material is determined from said phase shift for each of said plurality of slices to form a plurality of densities.

10. The device of claim 9, wherein said plurality of densities are compared to determine a deviation of said densities.

11. The device of claim 10, wherein the moisture content of the material is corrected according to said deviation of said densities to form a corrected moisture content of the sample of material.

12. The device of claim 1, wherein said sample holder is constructed from a substance for reducing an effect of reflectance of said source beam.

13. The device of claim 1, wherein the material is selected from the group consisting of cotton fiber, silk fiber, wool fiber, pharmaceutical material, seeds, paper, tobacco and synthetic fiber.

14. The device of claim 1 , wherein said source beam is repeatedly transmitted through the sample of material by said transmitting antenna such that a plurality of antenna signals is produced, such that said attenuation measurer determines said attenuation for each of said plurality of antenna signals to form a plurality of attenuation values, said plurality of attenuation values being filtered to form said attenuation for said calculation of said calculated moisture content by said moisture determiner.

15. The device of claim 1 , wherein said sample holder is substantially triangular in shape.

16. The device of claim 1, wherein said cavity is a microwave resonator, said transmitting antenna is a microwave radiation broadband generator-synthesizer for producing said source beam, and said receiving antenna is a signal receiver for receiving said exit beam to form a resonator signal.

17. The device of claim 16, wherein said microwave resonator is constructed from metal.

18. The device of claim 17, wherein said microwave resonator features an entry for receiving said sample holder and a plurality of metal pieces being located substantially opposite from said entry.

19. The device of claim 16, further comprising:

(i) a phase shift measurer for measuring a phase shift of said resonator signal, such that said moisture determiner calculates said calculated moisture content from said phase shift and from said attenuation.

20. The device of claim 19, wherein said sample holder rotates within said microwave resonator for measuring the moisture content of the sample of material at a plurality of slices of the sample of material.

21. The device of claim 20, wherein a density of the sample of material is determined from said phase shift for each of said plurality of slices to form a plurality of densities.

22. The device of claim 21, wherein said plurality of densities are compared to determine a deviation of said densities.

23. The device of claim 22, wherein the moisture content of the material is corrected according to said deviation of said densities to form a corrected moisture content of the sample of material.