Patent Application
20090167322
The field of the invention relates generally to systems for classifying a substance and, more particularly, to an electromagnetic classification system for classifying a substance.
At least some known classification systems use electromagnetic signals to facilitate classifying a substance. One known classification system is a dielectrometry monitoring apparatus that three-dimensionally profiles and color images material contents of an article that is carried through an interrogation region. The apparatus includes a stationary, collimated, microwave transmitter/receiver antennae array and associated electronics for measuring the dielectric constant of the material contents to produce data representative of the dielectric material configuration and contents of the article. High-speed GaAs gates, switching devices, and microstrip delay lines are used to perform timed, depth-wise sampling of data regarding the dielectric constant material characteristics. As such, timed microwave pulses are directed towards the article to determine the dielectric constant of material within the article. The system compares the dielectric constant data to predefined criteria to determine whether the data profile of the material is indicative of any of a variety of contraband or hazardous conditions.
Another known classification system includes an interrogation region through which a target that is to be screened passes and a transmitter/receiver array that operates adjacent the interrogation region for illuminating the target with a pulsed microwave beam. As such, the microwave beam is time-dependent. The system monitors a dielectric response received from the target, and includes electronics that produce output data that is interpretable to identify the presence of contraband associated with the illuminated target based on the dielectric response of the target.
Yet another known classification system facilitates classify liquids based on measured dielectric properties of a liquid by transmitting an electromagnetic signal, typically at microwave frequencies, at the liquid. However, using microwave dielectrometry alone may cause a relatively large number of false alarms and/or false negatives. Such dielectrometers discriminate between liquids by generally classifying them on the basis of high, or generally benign, and low, or generally hazardous, dielectric constants, which may generate false alarms and/or false negatives. For example, cooking oil, generally not considered a hazardous material, has a low dielectric constant may be classified as hazardous while a mixture of hydrogen peroxide and acetone, considered an explosive material, may be classified as safe based on its high dielectric constant. Some known improvements to such classification systems have been made by using an imaginary part of the dielectric constant, or the “loss tangent”. However, dielectrometry, when used by itself, has other limitations in addition to that discussed above, such as calibration of such a system for the shape, size, and/or type of material of the container having the liquid therein.
A known dipole antenna 10 is shown in
When a flat, perfect conductor 20 is positioned near antenna 10 and spaced a distance d from antenna 10, the interaction between antenna 10 and conductor 20 can be modeled by assuming an image antenna 22 is spaced distance d from an opposite side of conductor 20 as antenna 10. Image antenna 22 is a replacement for a ground plane conducting surface generated by the ground proximate to antenna 10. The current Iimage flowing in image antenna 22 is equal and opposite the current Iactual of antenna 10.
Other known classification systems use magnetic resonance and/or quadrupole resonance to classify objects and/or substances. Such systems may be referred to herein as “resonance classification systems,” and/or variations thereof. One known type of resonance classification system is a nuclear magnetic resonance (NMR) system. The NMR system uses the quantum mechanical magnetic properties of an atom's nucleus and the nuclear magnetic resonance to study molecules in a material. For example, at least some known NMR systems examine magnetic nuclei by aligning the nuclei of a substance using an applied constant magnetic field and disturbing the resulting alignment of the nuclei using an alternating magnetic field. The response of the nuclei to the alternating electric field is used to determine a resonant absorption of a material that is detected by the NMR system. Different atoms within a molecule resonate at different frequencies at a given field strength and, as such, the observation of the resonance frequencies of a molecule allows a user to discover structural information about the molecule. Furthermore, the time relaxation characteristics of the NMR response of a single nuclear species, for example, the hydrogen nucleus, as measured by the spin-spin relaxation time, commonly denoted as T2, and the spin-lattice relaxation time, commonly denoted as T1, yields information about the chemical environment of that nuclear species. Such information may be used in classifying a substance. Moreover, the NMR response may also yield other parameters of the substance, including, but not limited to, the diffusion coefficient of a liquid and, in some cases, chemical shift information, that may be used to classify the substance.
Another known resonance classification system is a nuclear quadrupole resonance (NQR) system. Known NQR systems are related to the NMR systems described above. The NQR system is used to detect atoms having nuclei with a nuclear quadrupole moment. Unlike NMR systems, at least some known NQR systems perform inspections in an environment without a static or DC magnetic field. At least one known NQR system includes a radio frequency (RF) power source, a coil to produce a magnetic RF excitation field, and a detector circuit to detect a RF NQR response being emitted from a component of an object and/or substance. The NQR response is generated by the interaction of a quadrupolar charge distribution within the object and/or substance with an electric field gradient supplied by a non-uniform distribution of electron density (from bonding electrons). Compared to the chemical shift measured in NMR, the interaction is much larger; however, the response averages to zero in a liquid phase.
At least some known NMR and/or QNR systems detect whether a material is a metal or non-metal material; however, benign substances, such as water, and hazardous substances, such as gasoline, may emit similar responses in such systems such that the hazardous substance is not distinguished from the benign substance by using only resonance classification systems.
In one aspect, a method for classifying a substance is provided. The method includes transmitting an electromagnetic signal at the substance, measuring a portion of the electromagnetic signal reflected by the substance, determining a reflection coefficient of the substance using the measured portion of the electromagnetic signal, and outputting a classification of the substance based on the determined reflection coefficient.
In another aspect, a classification system is provided. The classification system includes a resonance classification system and an electromagnetic classification system including an antenna and a measurement device communicatively coupled to the antenna. The measurement device generates a signal representative of a measurement of a reflected signal. The classification system also includes a control system operatively coupled to the resonance classification system and the electromagnetic classification system. The control system is configured to output a classification of a substance based at least partially on a reflection coefficient determined using the signal generated by the measurement device.
In still another aspect, an electromagnetic classification system is provided. The electromagnetic classification system includes an antenna and a measurement device communicatively coupled to the antenna. The measurement device generates a signal representative of a measurement of a reflected signal. The electromagnetic classification system also includes a control system operatively coupled to the measurement device. The control system is configured to output a classification of a substance based at least partially on a reflection coefficient determined using the signal generated by the measurement device.
The embodiments described herein provide systems and a method for classifying a substance. In one embodiment, an electromagnetic classification system transmits an electromagnetic signal at or towards an object including a substance therein. Using a signal reflected from the substance, the electromagnetic classification system identifies whether the substance includes an aqueous or non-aqueous mixture, such as a solution. As used herein, the term “aqueous solution” refers to a solution in which the solvent is water. A “solvent,” as used herein, refers to a liquid that dissolves a solid, liquid, and/or gaseous substance to result in a solution. As used herein, the term “non-aqueous solution” refers to a solution having a solvent other than water. Examples of non-aqueous solutions include organic solutions, such as acetone, ethanol, methane, isopropanol, propane, alcohols, glycols, aromatic hydrocarbons, and aliphatic hydrocarbons, and/or inorganic solutions, such as liquid ammonia and sulfur dioxide. The substance within the object may be further identified using a resonance classification system to determine if the solution includes explosives, narcotics, weapons, and/or other contraband present within the object.
A technical effect of the systems and method described herein is to distinguish flammable liquids from non-flammable liquids. An embodiment of a method uses a reflection coefficient to facilitate classifying a substance as aqueous or non-aqueous. As used herein, the term “reflection coefficient” refers to a ratio of a reflected wave to an incident wave at a point of reflection. The ratio may be a ratio of the voltages, currents, intensities, and/or amplitudes corresponding to the reflected and incident waves at an antenna's input terminal. Embodiments of the systems and method described herein may be used to facilitate avoiding misclassification of benign liquids as volatile liquids, and vice versa, by discriminating between aqueous and non-aqueous mixtures. For example, as described above, a resonance classification system detects a first property of a material, such as whether the material is metallic or non-metallic. Further, by determining the reflection coefficient of the substance using electromagnetic signals, a benign substance, such as water, which may be indistinguishable from some hazardous substances, such as gasoline, using only resonance classification systems, is distinguished from a hazardous substance because the benign substance is aqueous and the hazardous substance is non-aqueous.
At least one embodiment of the present invention is described below in reference to its application in connection with and operation of a system for inspecting a substance. However, it should be apparent to those skilled in the art and guided by the teachings herein provided that the invention is likewise applicable to any suitable system for scanning objects including a substance including, without limitation, containers, people, cargo, crates, boxes, drums, baggage, containers, luggage, and suitcases, transported by water, land, and/or air, as well as other containers and/or objects.
In the exemplary embodiment, resonance classification system 52 is a magnetic resonance system or a quadrupole resonance system, as described above. Resonance classification system 52 may be positioned upstream r downstream of EM classification system 100. In the exemplary embodiment, EM classification system 100 includes a signal source 102, a transmission line 104, a directional coupler 106, an antenna 108, a detector 110, and a measurement device 112. In the exemplary embodiment, signal source 102 generates electromagnetic signals at a predetermined wavelength λ. More specifically, signal source 102 generates S-band and/or X-band signals at a fixed frequency. As used herein, the term “S-band” refers to an electromagnetic signal between about 2 GHz and about 4 GHz, and the term “X-band” refers to an electromagnetic signal between about 8 GHz and about 12 GHz. Further, in the exemplary embodiment, signal source 102 generates a steady state signal. As used herein, the term “steady state” refers to a signal that is in a substantially stable condition and that does not change with respect to time.
Transmission line 104 couples signal source 102 to antenna 108 for transmission of the signal generated by signal source 102 to antenna 108. Transmission line 104 is configured to transmit or guide radio-frequency energy between a first point and a second point. Transmission line 104 may be, for example, a wire, a two-wire line, a coaxial wire, and/or a hollow pipe or waveguide. In the exemplary embodiment, transmission line 104 is coupled to antenna 108 at a feedpoint 114, as described above. At feedpoint 114, transmission line 104 has a predetermined impedance ΩF in free space, as described above. The feedpoint impedance ΩF varies as antenna 108 is positioned with respect to a surface, such as a surface 116 of object 60 having substance 62 therein. More specifically, in the exemplary embodiment, the feedpoint impedance ΩF varies as antenna 108 is positioned with respect to a surface having a dielectric constant that is substantially different from the medium immediately surrounding antenna 108 and/or from a material that has a relatively high conductivity.
In the exemplary embodiment, antenna 108 is a dipole antenna, as described above. More specifically, antenna 108 is slightly shorter than the predetermined wavelength λ. For example, antenna 108 has a length λ/2, as described above. Further, in the exemplary embodiment, antenna 108 has an impedance ΩA such that, in free space, impedance ΩF is at resonance. Antenna 108 and transmission line 104 are matched such that resonance occurs when antenna 108 exhibits zero feedpoint reactance X. As used herein, the term “reactance” refers to the imaginary part of electrical impedance Z. In the exemplary embodiment, impedance ΩA is approximately equal to impedance ΩF (i.e., ΩF≅ΩA≅Ω) to achieve resonance in free space. Antenna 108 is configured to transmit the electromagnetic signal from signal source 102 at or towards object 60 and substance 62 therein, and to receive the electromagnetic signal reflected from substance 62. Further, EM classification system 100, in the exemplary embodiment, includes a conductive surface 118 spaced a predetermined distance from antenna 108. Conductive surface 118 facilitates achieving resonance and/or increasing the directionality of antenna 108. Moreover, in the exemplary embodiment, antenna 108 may be movable with respect to object 60 and/or stationary with respect to object 60. For example, antenna 108 and/or EM classification system 100 may be hand-held and movable with respect to a person, and/or may be stationary such that a container is moved pass antenna 108 and/or through EM classification system 100.
Directional coupler 106 is in communication with antenna 102 and samples a signal reflected from feedpoint 114 and/or a signal transmitted to feedpoint 114. In the exemplary embodiment, directional coupler 106 is coupled within transmission line 104. Directional coupler 106 is also in communication with detector 110. As such, directional coupler 106 is communicatively coupled between antenna 108 and detector 110, and between signal source 102 and antenna 108. Directional coupler 106 transmits the sampled signal(s) to detector 110, which converts the sampled signal to a direct current (DC) signal, for example.
In the exemplary embodiment, detector 110 is in communication with measurement device 112 to transmit the DC signal to measurement device 112. Further, in the exemplary embodiment, detector 110 functions as a diode to prevent current from flowing from measurement device 112 to directional coupler 106, transmission line 104, and/or antenna 108. Measurement device 112 receives the DC signal from detector 110 and converts the DC signal to human and/or machine readable form. In the exemplary embodiment, measurement device 112 measures a power reflected from substance 62 through antenna 108 and/or a phase of a signal reflected from substance 62 through antenna 108. Measurement device 112 is in communication with control system 54 such that control system 54 may further process the signal transmitted from measurement device 112.
In the exemplary embodiment, control system 54 includes, without limitation, one or more integrated circuit, a processor, a computer, a microcontroller, a microcomputer, a programmable logic controller, an application specific integrated circuit, and/or any other suitable components. As shown in
In the exemplary embodiment, control system 54 is configured to output a classification of substance 62 within object 60 based on a reflection coefficient determined using an output of measurement device 112, as described in more detail below. Classification of substance 62 may include, but is not limited to including, an indication whether substance 62 within object 60 is aqueous or non-aqueous, an indication of whether object 60 includes a metallic or a non-metallic material, an indication of a threat level of a material within object 60, an image of object 60, and/or any other suitable classification of substance 62 and/or object 60. Control system 54 is also configured to output a classification of substance 62 and/or object 60 using an output of measurement device 112 and an output of resonance classification system 52. Further, control system 54 is configured to determine an impedance caused by a signal reflected from substance 62, as described in more detail below.
Additionally, in the exemplary embodiment, the results of classification system 50 are output to a memory, such as storage device 64, a drive, a display device, such as display device 66, and/or any other suitable component. In alternative embodiments of classification system 50, one or more control systems 14 may be used to classify substance 62 within object 60.
Method 300 includes transmitting 302 an electromagnetic signal at or towards substance 62. More specifically, in the exemplary embodiment, an S-band and/or an X-band signal is transmitted 302 at or towards the substance using an antenna, such as antenna 108 (shown in
Method 300 also includes determining 318 a reflection coefficient of substance 62 using the measured portion of the electromagnetic signal. More specifically, in the exemplary embodiment, the reflection coefficient is determined 318 by modeling 320 the interaction between the antenna and flat surface that is a perfect conductor by assuming an image antenna is spaced distance d from an opposite side of the perfect conductor as the antenna, as described above and shown in
Z
feedpoint
=Z
self
+Z
mutual*(Iimage/Iactual), Eq. (1)
where Zfeedpoint is the impedance at the feedpoint of the actual antenna, Zself is the impedance between the antenna and itself, and Zmutual is the impedance induced between the image antenna and the actual antenna. Since Iimage=−Iactual, Zfeedpoint=Zself−Zmutual. As such, when the spacing between the image antenna and the actual antenna is approximately equal to zero, Zmutual approaches Zself and Zfeedpoint is approximately equal to zero.
When the flat surface is a non-perfect conductor, the image current Iimage is calculated 324 using:
I
image
=I
actual*Γ, Eq. (2)
where Γ is the reflection coefficient. The reflection coefficient approaches unity when either the dielectric constant ε of the non-perfect conductor becomes large (i.e., ε>>1), or the conductivity of the non-perfect conductor is sufficiently high that the skin depth is small compared to a wavelength. As used herein, the term “dielectric constant” refers to a measure of the ability of a material to store electrical energy, and the term “skin depth” refers to a measure of the distance needed for a current to decrease to 1/e of its original value, where e is the known mathematical constant. Conversely, if the conductivity of the non-perfect conductor is low enough (i.e., meets the criteria for a good dielectric) and the non-perfect conductor has a low dielectric constant, then the reflection coefficient Γ will approach zero, giving the equation:
in which case the antenna acts as if it were in free space.
Further, if the resistance of the antenna ΩA is matched to the resistance of the transmission line ΩF such that resistance ΩA is approximately equal to resistance ΩF (i.e. ΩF≅ΩA≅Ω) to achieve resonance in free space, as described above, and the change in Zfeedpoint is known, the reflection coefficient of the antenna Γfeedpoint can be derived 326 using:
Using the above-described calculations, the reflection coefficient of a material, such as the non-perfect conductor, can be determined 318. The above-described calculations are one example of determining 318 the reflection coefficient of the substance by determining 322 an impedance caused by the measured portion of the electromagnetic signal reflected from the substance. The reflection coefficient is used to determine 328 a characteristic of the substance, for example, whether the substance is aqueous or non-aqueous. For example, water has a reflection coefficient of about −1 normal to its surface, and accordingly, has a dielectric constant of about 80 at frequencies lower than approximately 20 GHz. In contrast, a petroleum product generally has a reflection coefficient of −0.16 normal to its surface of approximately and a dielectric constant of approximately 2 at frequencies lower than approximately 20 GHz. As such, water can be distinguished from other substances, such as petroleum products, using the reflection coefficient of the substance being classified, scanned, and/or tested.
More specifically, by determining 318 the reflection coefficient of the material, a dielectric constant of the material may optionally be determined 330 using a suitable relationship between the reflection coefficient and the dielectric constant. A classification of substance 62 within object 60 based on the determined reflection coefficient is then output 332 to a memory, such as memory 64 (shown in
By using the above-described method and systems a relatively benign substance may be distinguished from a relatively volatile substance. For example, by using both a resonance classification system and an EM classification system, a relatively benign liquid, such as vegetable oil, may be classified as such. More specifically vegetable oil has a relatively low dielectric constant. Vegetable oil may be flammable but, because it has a low vapor pressure, vegetable oil is at a low hazard level. However, vegetable oil is also characterized by a relatively high viscosity, and will be determined to be benign by using both the resonance classification system and the EM classification system. Further, by using the EM classification system in conjunction with the resonance classification system, the classification is facilitated to be more robust, as compared to using either classification alone. In addition, the above described classification system enable the dielectric measurement to be simplified, as compared to known systems that use electromagnetic signals for classification. For example, the classification using the above-described classification system is less likely to be influenced by bottle characteristics, and the design of the antenna can be simplified to facilitate lowering the cost of implementation, as compared to known systems that use electromagnetic signals for classification.
Exemplary embodiments of systems and a method for classifying a substance are described above in detail. The systems and method are not limited to the specific embodiments described herein. For example, the method may also be used in combination with other classification systems and/or classification methods, and is not limited to practice with only the classification systems as described herein.
Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.
While various embodiments of the invention have been described, those skilled in the art will recognize that modifications of these various embodiments of the invention can be practiced within the spirit and scope of the claims.
