Footwear scanning systems and methods

Description of equivalent WO 2014175985 A2

FOOTWEAR SCAN N ING SYSTEMS AND METHODS

0002STATEMENT AS TO RIGHTS TO INVENTIONS MADE U N DER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

0003This invention was made with Government support under

0004Contract DE-AC0576RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

BACKGROUN D OF THE DISCLOSURE

0006Systems have been utilized to enhance security at numerous public venues, including airports, public arenas, courthouses, etc. These systems have been primarily directed towards detecting objects, such as weapons, explosives, etc. which are concealed under clothing of individuals. However, these systems have had difficulty detecting such objects concealed within a heterogeneous environment, such as footwear which typically include different layers of different types of materials.

0007Accordingly, in some applications, individuals being screened are required to remove footwear for different inspection. Divestment of footwear during the scanning process is a significant impediment to efficient screening and may result in reduced throughput at security checkpoints, increased inconvenience, lost revenue and/or lost productivity.

0008At least some aspects of the present disclosure are directed towards methods and apparatus for screening footwear to identify objects therein.

BRIEF DESCRIPTION OF THE DRAWI NGS

0010Fig . 1 is an illustrative representation of a threat detection system according to one embodiment.

0011Fig. 2 is an illustrative representation of a threat detection system according to one embodiment.

0012Fig . 3 is a plan view of a platform of a threat detection system according to one embodiment. Fig. 3A is a perspective view of a platform of a threat detection system according to one embodiment.

0013Figs. 4A and 4B are graphical representations of an antenna array layout of a threat detection system according to one embodiment.

0014Fig. 5 is a plan view of an antenna of a threat detection system according to one embodiment.

0015Fig. 6 is a graphical representation of beam patterns of the antenna of Fig. 5.

0016Fig . 7 is a computational process flow chart according to one embodiment.

DETAILED DESCRIPTION OF THE DISCLOSURE

0018At least some aspects of the disclosure are directed towards apparatus and methods for detecting objects which may be concealed. For example, at least some of the described aspects may be utilized in applications to detect objects, such as weapons, which may be concealed on individuals. In one example possible application, these apparatus and methods may be implemented in checkpoints at public venues, such as airports, public events, sporting events, courthouses, and concerts, to reduce threats posed by the objects and individuals.

0019As described below, the apparatus a nd methods may be utilized to detect objects which may be concealed within heterogeneous articles, such as footwear. Example footwear which may be scanned include shoes, boots, sandals, flip-flops, moccasins, etc. and which may be made of numerous different types of materials.

0020Scanning of footwear poses additional challenges compared with personnel screening. For example, the phenomenology associated with imaging within a heterogeneous medium, such as shoes, dictates limits for imaging system parameters. Scattering, defocusing, and multipath artifacts are significantly exaggerated due to the high contrast index of refraction associated with a boundary of air and the footwear interface. Active millimeter-wave radar imaging is utilized to perform threat detection in non-divested footwear in one implementation . In one embodiment, a full three dimensional reconstruction of the footwear being scanned is generated . Additional details regarding an active near field imaging technique are discussed in Sheen et. al. , Combined Illumination Cylindrical Millimeter- Wave Imaging Technique For Concealed Weapon Detection, Proceed ings of SPI E, 2000, 4032 , p 52-60, the teachings of which are incorporated herein by reference.

0021Allowing the scanning of footwear to detect objects concealed within the footwear without requiring removal of the footwear from the ind ivid ual may provide increased throughput and convenience compared with other scanning arrangements which require divesting of the footwear for scanning purposes.

0022Referring to Fig . 1 , an example threat detection system 1 0 is shown according to one embodiment. The system 1 0 may be installed in one example application at points of ingress/egress of a public facility. A person to be screened enters the system 1 0 which attempts to detect the presence of concealed objects, such as weapons or explosives, upon the individual . As described in additional detail below, system 1 0 includes a footwear scanning system 1 2 which is configured to identify objects which are concealed within footwear of people being screened .

0023I n one described embodiment, individuals may be screened to attempt to locate objects which are concealed within their footwear while the individuals are wearing the footwear and without prior divesting or removal of the footwear from the individuals.

0024Threat detection system 1 0 may also include an additional personnel scanning system 1 4 which is configured to identify objects which are concealed within clothing of the people being screened .

0025Referring to Fig . 2 , additional details regarding the footwear scanning system 1 2 are shown according to one embodiment. The illustrated system 1 2 includes a platform 20, antenna array 22, transceiver 24, motor 26 , computer 28 , and user interface 30. Other embodiments are possible including more, less and/or alternative components.

0026As shown in the example embodiment, an individ ual stands upon the platform 20 during scanning operations with respect to the individ ual's footwear. The platform 20 includes an upper surface 36 which is configured to red uce reflections of electromag netic waves being transmitted throug h an interface between the platform 20 and the individ ual's footwear. Furthermore, the opposite lower surface 38 of the platform 20 is config ured to reduce reflections of the electromagnetic waves being transmitted throug h an interface between the platform 20 and antenna array 22.

0027Although an example embodiment is described herein which utilizes electromag netic radiation to perform the scanning , acoustics or ultrasonic scanning may be utilized in other embodiments.

0028The antenna array 22 is embodied within a movable scanning device and adjacent to lower surface 38 of platform 20 in the depicted embodiment. Antenna array 22 is implemented as a quasi-monostatic array configured to emit electromagnetic waves upwardly through platform 20 and into the footwear of the individual in one embodiment. The electromagnetic waves transmitted by antenna array 22 are reflected downwardly from the footwear and again pass through platform 20 and are subsequently received by the antenna array 22. As described below with respect to Fig . 3, the antenna array 22 may be moved beneath and in contact with platform 20 during scanning operations. Additional details regarding antenna array 22 are d iscussed below with respect to Figs. 4A and 4B and in US Patent Nos. 5,455, 590 and 5 ,557,283, the teachings of which are incorporated herein by reference.

0029Transceiver 24 is a frequency mod ulated continuous wave (FMCW) heterodyne transceiver in one embodiment. Transceiver 24 operates with antenna array 22 to transmit and receive electromagnetic rad iation at frequencies capable of penetrating non- metallic shoe materials to image the contents of the footwear in one embodiment. For example, electromag netic radiation having a bandwidth within a range of about 500 MHz to about 100 GHz may be utilized in one embodiment. The bandwidth may be about 1 GHz to about 40 GHz in one more specific embodiment.

0030A motor 26 is provided to move antenna array 22 beneath the platform 20 during scanning operations in the example embodiment. The antenna array 22 is moved in a linear path below platform 20 in one embodiment described below.

0031A computer 28 is coupled with transceiver 24 and motor 26. Computer 28 controls various operations of scanning system 12 including emission of electromagnetic radiation and movement of antenna array 22 via motor 26. Furthermore, computer 28 may sample and process electrical signals from transceiver 24 which correspond to electromagnetic waves received by antenna array 22 to provide information regarding contents of footwear including objects which are concealed , for example, within the footwear of the individual.

0032Computer 28 includes processing circuitry 40 and storage circuitry 42 in the illustrated example. In one embodiment, processing circuitry 40 is arranged to process data, control data access and storage, issue commands, and control other desired operations. Processing circuitry 40 may comprise circuitry configured to implement desired programming provided by appropriate computer- readable storage media in at least one embodiment. For example, the processing circuitry 40 may be implemented as one or more processor(s) and/or other structure configured to execute executable instructions including, for example, software and/or firmware instructions. Other example embodiments of processing circuitry 40 include hardware logic, PGA, FPGA, ASIC, state machines, and/or other structures alone or in combination with one or more processor(s). These examples of processing circuitry 40 are for illustration and other configurations are possible.

0033Processing circuitry 40 is configured to control operations of systems 10, 12, process signals from transceiver 24 which correspond to received electromagnetic radiation as described above, and provide information regarding the results of scanning operations, including for example, information regarding contents within the footwear being scanned such as anomalies which may indicate the presence of objects of interest within the footwear. In one embodiment, processing circuitry 40 implements data processing such as image reconstruction using a 2D rectilinear Fourier based near field focusing algorithm using a non-free space dielectric constant in one embodiment.

0034Storage circuitry 42 is configured to store programming such as executable code or instructions (e.g. , software and/or firmware), electronic data, databases, sampled data, or other digital information and may include computer-readable storage media. At least some embodiments or aspects described herein may be implemented using programming stored within one or more computer-readable storage medium of storage circuitry 42 and configured to control appropriate processing circuitry 40.

0035The computer-readable storage medium may be embodied in one or more articles of manufacture which can contain, store, or maintain programming , data and/or digital information for use by or in connection with an instruction execution system including processing circuitry 40 in one embodiment. For example, computer-readable storage media may be non-transitory and include any one of physical media such as electronic, magnetic, optical, electromagnetic, infrared or semiconductor media. Some more specific examples of computer- readable storage media include, but are not limited to, a portable magnetic computer diskette, such as a floppy diskette, a zip disk, a hard drive, random access memory, read only memory, flash memory, cache memory, and/or other configurations capable of storing programming , data, or other digital information.

0036User interface 30 is configured to interact with a user including conveying data to a user, for example via a display (e.g. , displaying visual images and text for observation by the user) as well as receiving inputs from the user, for example via a mouse and/or keyboard. User interface 30 is configured as a graphical user interface (GU I ) in one embodiment. User interface 30 may be configured differently in other embodiments.

0037In one embodiment, the user interface 30 depicts results of the scanning operations, such as anomalies or objects which are concealed within the subject or article being scanned. The user interface 30 may generate an indication that the individual being scanned is not a risk or may indicate that the individual should be checked further in one embodiment.

0038Referring to Fig. 3, a plan view of the platform 20 is shown. In addition, the antenna array 22 is also shown in the plan view for discussion purposes of example scanning operations while in the described embodiment the antenna array 22 lies below the platform 20 and would not be visible in the plan view. The platform 20 is physically coupled to antenna array 22 and the bottom of the footwear during scanning operations in one embodiment. The geometry and dielectric constant of dielectric material of platform 20 are designed to optimize energy transmission from the antenna array 22 into the platform 20 and into the footwear and back through the platform 20 into the antenna array 22 in one embodiment.

0039Threat detection system 10 uses an imaging system with sufficient resolution to isolate scattering contributions from separate scatterers in one embodiment. As mentioned above, electromagnetic radiation within an example bandwidth of 1 -40 GHz is utilized in one embodiment. The use of this radiation enables scanning operations which provide a suitable combination of range and cross-range resolution while still penetrating and imaging inner contents of footwear. Platform 20 is utilized in one described arrangement to improve penetration of the electromagnetic waves into heterogeneous articles (such as the soles of shoes) and to reduce reflections of the electromagnetic energy. Electromagnetic radiation of other frequencies may be used in other embodiments.

0040An individual being scanned would stand on the surface 36 of platform 20 in one example such that their footwear is positioned at locations 50 during scanning of the footwear. In one embodiment, the antenna array 22 includes a plurality of antennas which are shown in Figs. 4A and 4B in a linear configuration in one example. In the described example, the antennas may be arranged in the array 22 along the y axis. Following the initiation of a scan, the antenna array 22 is moved along the x axis (orthogonal to the y axis) while electromagnetic radiation is transmitted into and received from the footwear positioned at locations 50. Following the completion of a scan, the antenna array 22 returns to the position shown in Fig. 3 for a subsequent scan, perhaps of another individual.

0041The example scanning geometry corresponds to the arrangement of the antennas upon the array 22 and movement of the antenna array 22 during scanning operations. In the described example, a 2D scanning aperture is utilized corresponding to the linear arrangement of antennas of the array 22 along the y axis and the rectilinear movement of the array 22 along the x axis during the transmission and reception of the electromagnetic radiation. The described example rectilinear scanning geometry was selected for scanning of footwear due to the generally rectangular shape of the soles of the footwear and the path-length variation as a function of this geometry resulted in tolerable multipath imaging artifacts (e.g., the artifacts appear as a physically thicker sole of the footwear).

0042In one embodiment, platform 20 is configured to enhance coupling of electromagnetic radiation emitted from the antenna array 22 though the platform 20 into the footwear of the individual being scanned (as well as the return path through the platform 20 into the antenna array 22). In one embodiment, the physical construction of the platform 20 as well as the materials of the platform 20 itself are configured to enhance propagation of electromagnetic radiation between the antenna array 22 and the footwear with reduced reflections at the interfaces of the platform 20 with respect to the antenna array 22 and footwear 50.

0043In one embodiment, the platform 20 comprises a material which corresponds to an article to be scanned. More specifically, it is desired to provide the platform 20 comprising material which propagates the electromagnetic radiation of interest (i .e. , propagates electromagnetic energy within the frequency range of interest to implement the scanning) as well as having an impedance or dielectric constant which is similar to or matched with the footwear being scanned to minimize reflections at the interface of the footwear and the platform 20 and which may result in artifacts.

0044The thickness (i .e. , the dimension between surfaces 36, 38) and material composition of platform 20 may be selected corresponding to the wavelengths of electromagnetic radiation being utilized . In general , the thickness may be red uced for materials having increased electric permittivity. In example embodiments, the platform 20 may have a thickness between 0-25 cm and an electric permittivity between 2-1 0, and perhaps 2-5 for use in scanning of footwear having rubber soles.

0045I n one embodiment, the platform 20 comprises a homogeneous solid substrate having a dieiectric constant or electrical permittivity which is matched to/corresponds with the dielectric constant or electrical permittivity of the footwear being scanned . If the type of sole of the footwear is known, then the material of the platform 20 may be selected to have a dielectric constant which is similar to or the same as the dielectric constant of the soles of the footwear. For example, the platform 20 may comprise a solid rubber substrate if the footwear has a rubber sole. In another example, various different types of footwear may be scanned (e.g . , in a public setting) and the material of platform 20 may be selected to correspond to an average dielectric constant of numerous different types of footwear.

0046In addition , the thickness of the platform 20 may also be selected to enhance propagation of the electromagnetic radiation while reducing the presence of artifacts. In one embodiment, it is desired to have a thickness between opposing surfaces 36, 38 of at least a plurality of wavelengths of the electromagnetic rad iation being utilized (e.g . , a thickness greater than at least 5 wavelengths in one embodiment). Referring to Fig . 3A, the platform 20 may have a conformal upper surface 36 in one embodiment to reduce reflections at the interface of the footwear and the platform 20. For example, different soles of different footwear have various different types of treads or patterns and it is desired to reduce the presence of air gaps between the platform 20 and the footwear being scanned to red uce reflections of the electromagnetic energy at the interface of the footwear and platform 20. In one embodiment, upper surface 36 of platform 20 is config ured to conform to d ifferent surface features (e.g . , different tread patterns) of different soles of footwear to reduce the presence of the air gaps at the interface and which may reduce reflections of the electromagnetic energy compared with , for example, a rigid flat surface.

0047As discussed above, the platform 20 may comprise a substrate 52 of homogeneous material having an impedance which is matched to or corresponds to the footwear to be scanned in an example footwear scanning application . Furthermore, a plurality of members 54 may be provided at surface 36 to assist with reduction of air gaps at an interface of the footwear and the surface 36 of the platform 20. In one embodiment, the members 54 are free to move and may fill open spaces between the tread features of the soles of the footwear providing a conformal upper surface 36 and which reduces the presence of air gaps between the footwear and the platform 20 compared with a relatively hard , non-conformal surface.

0048In one embod iment, the members 54 comprise the same material as material of the substrate 52 to reduce or minimize reflections of the electromagnetic radiation passing between the substrate 52 and the members 54. I n a more specific example embodiment for utilization in a footwear scanning application, the substrate 52 and members 54 may comprise the same type of rubber which may be selected to have a similar dielectric constant to soles of footwear. Furthermore, a thin protective retention layer, film or membrane 56, such as a d urable plastic, latex, rubber, etc. , may be provided over the members 54 to contain them upon the upper surface 36. I n one embod iment, the film 56 has a thickness of approximately 1 0 mils.

0049Referring to Fig . 4A, a layout of antennas of the antenna array 22 is shown according to one embodiment. Add itional details regarding example antenna arrays 22 which may be utilized are discussed in a US Patent Application having Serial No. 1 3/629,849, entitled Apparatus for Synthetic Imaging of an Object, naming David M . Sheen as inventor, filed September 28, 201 2, and the teachings of which are incorporated herein by reference.

0050I n the illustrated example embod iment, the antennas 60 are arranged in a plurality of columns 62 , 64 and the antennas 60 in column 62 are utilized to transmit electromagnetic waves or radiation into the platform 20 and footwear thereon and the antennas 60 in column 64 are configured to receive the emitted electromagnetic waves or radiation reflected from the footwear being scanned and again passing through platform 20. The illustrated example antenna array is a 3:4 array including a ratio of three transmit antennas to four receive antennas. Other layouts of antennas 60 may be utilized in other embodiments.

0051A virtual sample point of the array corresponds to a transmit and receive pair of the antennas 60 and is located at the midpoint between the pair. These possible virtual sample points provided by the depicted layout are illustrated as Xs in Fig . 4A for a plurality of different possible pairs of antennas 60. I n one embodiment, it is desired to select pairs of the antennas 60 which provide sample points for use in scanning which are spaced from one another along the y axis by a distance of approximately one quarter of the wavelength of the electromagnetic radiation being utilized . Once identified , these pairs of antennas 60 are used to provide transmission and reception of the electromagnetic radiation and the data resulting therefrom is sampled and processed to implement the scanning of the contents of the footwear or other object being scanned . The antennas 60 may be fed by transceiver 24 using an absorptive based electronic switch in one embodiment. Referring to Fig . 4B , another layout of the antenna array 22 is shown at increased zoom compared with the layout of Fig . 4A. The antennas are arranged in respective transmit and receive columns 62 , 64 and the possible virtual sample points which may be utilized by the illustrated layout are shown as dots 66. As mentioned above, the selected pairs of transmit and receive antennas 60 provide the sample points (dots 66) spaced apart from one another approximately one q uarter of the wavelength of the electromagnetic radiation being utilized.

0052Referring to Fig . 5 , one example embodiment of an antenna 60 is shown. Antenna 60 utilized as a transmit antenna emits electromagnetic radiation having freq uencies corresponding to the frequencies of electrical signals received from transceiver 24 and an antenna 60 utilized as a receive antenna outputs electrical signals to transceiver 24 as a result of received electromag netic radiation. The antennas 60 of array 22 are individually implemented as a circular cross-polarized antenna having a 3 dB antenna gain pattern in the illustrated embodiment. More specifically, the antenna 60 is a cavity- backed dual-arm Archimedean spiral antenna for use with electromagnetic radiation within a 1 -40 G Hz bandwidth in one embodiment. The depicted antenna 60 provides a relatively wide beamwidth over the applicable bandwidth. I n one embodiment, antenna 60 has a 51 .85 degree (or greater) half-power beamwidth .

0053The illustrated antenna 60 incorporates a coaxial cable to microstrip transition which feeds a wideband exponentially tapered balun which is utilized to create balanced differential spatial lines which are soldered to the feed points of the two antenna arms in one embodiment. The antenna is mounted inside a cylindrical aluminum housing 70 embedded within radar absorbing material 72 within the cavity to red uce backward -directed energy produced by the antenna. I n one embodiment, the material 72 may be the same as the material of platform 20. Furthermore, a thin coating of the same material may also be provided over the antennas 60 of the array 22 to protect the antennas 60. Other antenna configurations such as rectilinear may be utilized in other embodiments.

0054The field distribution for the illustrated antenna 60 includes a relatively high amplitude current distribution at the center region of the antenna 60 and lower amplitude current distribution at the outer regions of the antenna 60 at 20 GHz.

0055As mentioned above, the antennas of the array 22 may be circular cross-polarized antennas 60 where the transmit and receive antennas transmit and receive on opposite circular polarizations, respectively. For example, the transmitting antennas may be left- hand circularly polarized and the receiving antennas may be right- hand circularly polarized in one embodiment. A circular cross- polarized system reduces imaging artifacts due to multipath in reflectance based imaging modalities (e.g. , millimeter-wave) by receiving electromagnetic waves of opposite circular polarized handedness relative to the transmitted waves. Since the polarity of a circularly polarized wave is reversed upon reflection from a surface, this example described system only receives waves that have experienced an odd number of reflections. This is useful for reducing imaging artifacts associated with relatively high amplitude signals that have experienced an even number of reflections.

0056Additional details regarding antennas 60 which may be utilized to implement scanning operations are discussed in US Patent Nos. 7,253,766, 7,034,746, and 7,986,260, the teachings of which are incorporated herein by reference.

0057Referring to Fig. 6, a simulated spiral antenna beam gain pattern is shown in the two principal planes at 20 GHz as a function of angle. The desired RHCP upper curves 82, 84 and undesired LHCP lower curves 86, 88 are shown.

0058Referring to Fig . 7, one method for implementing a scan of a subject such as footwear is shown according to one embodiment. The illustrated method may be implemented by processing circuitry 40 in one implementation. Other methods are possible including more, less and/or alternative acts. At an act A1 0 , the processing circuitry initiates scanning operations by initiating movement of the antenna array and controlling the emission of electromagnetic waves from transmit antennas of the antenna array. Additional details of this act according to one embodiment are described in US Patent Nos. 5 ,455,590, 5 ,557,283 and 7, 505 ,692, the teachings of which are incorporated herein by reference.

0059At an act A1 2, the processing circuitry controls sampling operations. In one embod iment, processing circuitry controls sampling of analog-to-digital conversion circuitry of the computer which is utilized to convert intermediate frequency sig nals from the transceiver into sig nal vectors which may be utilized for image reconstruction . Additional details of this act according to one embodiment are described in US Patent Nos. 5 ,455 ,590 , 5,557,283 and 7,505,692 , the teachings of which are incorporated herein by reference.

0060At an act A1 4, the processing circuitry implements a generalized synthetic aperture focusing technique (GSAFT) to generate a three dimensional image volume where each voxel value is proportional to a scatterer reflectance value. Add itional details of this act according to one embodiment are described in US Patent Application Serial No. 1 3/629,849 incorporated by reference above and Mensa , D . , High Resolution Radar Cross-Section Imaging, Norwood , MA, Artech House, 1 991 , the teachings of which are incorporated herein by reference.

0061At an act A1 6 , preprocessing may be performed where voxels are extracted for mesh generation using an amplitude threshold value or a derivative based threshold value in example implementations. In one amplitude thresholding example, the threshold may be varied to select different voxels for use in the surface reconstruction and seg mentation and contrast enhancement may be performed . In one specific embodiment, only voxels having intensity values above a threshold are utilized . Thresholding may be used to isolate high intensity scatterers in one embod iment. At an act A1 8, a surface mesh is created using image processing software, such as VolumeRover, available from the Computational Visualization Center. Depth information can be preserved and used to convey the location of high intensity reflectors present in an imaged scene. This example software uses multiple surface reconstruction algorithms to create a smooth surface given an input volumetric point cloud. In one embodiment, preprocessing algorithms may be used to convert point cloud values to be proportional to the feature of interest and the new point clouds can be imported into surface reconstruction modules for segmentation.

0062At an act A20 , information is provided regarding contents within the footwear, perhaps concealed . In one more specific example, an image of the footwear and the contents within the footwear resulting from the previous processing is displayed includ ing objects which may be concealed within the footwear itself or between the footwear and the feet of the individual. An individual of interest may be selected for further inspection if a possible threat is observed in an image.

0063The image may also be processed using feature extraction and classification algorithms to identify objects of interest. Additional details are discussed in McMakin , D. , Keller, P . , Sheen, D. , Hall, T. , Dual-Surface Dielectric Depth Detector for Holographic Millimeter- Wave Security Scanners, Proc. SPIE . 7309, Passive Millimeter-Wave Imaging Technology XI I 73090G , May 1 , 2009; McMakin, D. , Hall , T. , Sheen, D . , Holographic Radar Imaging Privacy Techniques Utilizing Dual-Frequency Implementation, Proc. SPI E . 6943, Sensors, and Command , Control, Communications, and Intelligence (C3I ) Technologies for Homeland Security and Homeland Defense VI I 69430P, April 3, 2008; Keller, P. , McMakin , D . , Sheen , D. , McKinnon , A. D. , Summet, J . , Privacy Algorithm for Airport Passenger Screening Portal, Proc. SPIE. 4055, Applications and Science of Computational Intelligence II I 476 , March 30, 2000; and McMakin, D. , Sheen, D. , Hall, T. , Kennedy, M . , Foote, H . , Biometric Identification using Holographic Radar Imaging Techniques, Proc. SPI E. 6538 , Sensors, and Command , Control , Communications, and Intelligence (C3I) Technologies for Homeland Security and Homeland Defense VI 85380C, April 27, 2007, the teachings of which are incorporated herein by reference.