Abstract:
A microwave barcode system is provided wherein the barcode comprises a plurality of conductive wires, mounted on a surface, or embedded beneath the surface of the object. A transmitter, comprising a dipole antenna, radiates microwave signals in the direction of the surface, and a microwave reader, comprising a dipole antenna positioned to receive the microwave signals passing through the surface, senses an attenuation of the microwave signal caused by a resonant interaction between the microwave signals and the conductive wires. The conductive wires of the present invention can be mounted in a stand-up fashion, wherein each wire is mounted adjacent to and parallel to, but not touching, its at least one neighbor, or in a lay-down fashion, wherein each wire is mounted coincident with an imaginary line which is also coincident with the remaining wires, and wherein the wires are mounted end-to-end with a gap surrounding each wire end. Both stand-up and lay-down versions of the barcode may be employed simultaneously without interference between the two versions.

Description:
BACKGROUND OF THE INVENTION 
     Barcodes have become a common and useful tool for identifying products, parts, prices, serial numbers, and many other bits of data. Over the years many different barcodes have been used, including the UPC codes found in grocery stores and the Code 39 barcodes used for many non-grocery items. These different barcodes are substantially only alternative ways of encoding information. For example, a narrow bar or space might represent a 0 while a wide bar or space might represent a 1, or a long bar might represent a 0 while a short bar might represent a 1. 
     Barcodes can differ on how they are sensed. Light, usually in the form of a scanning laser beam, or magnetic energy are commonly used to sense barcodes. A problem with light operated barcodes is that the barcode can become obscured, either with contaminates such as dirt or smudged ink, or by an opaque obstruction, such as a barcode-containing document being inside an envelope or under another document. Another manner of sensing barcodes is through magnetics. Magnetic barcodes generally require careful, close placement of a magnetic barcode reader relative to the barcode itself, which is often a difficult task. 
     Thus, barcodes sensed by light cannot be placed where they cannot be scanned by a scanning light beam. For example, they cannot be placed on objects within envelopes, and they cannot be embedded beneath the surface of an object such as paper currency. While it might be possible to accomplish this objective with magnetic barcodes, technical consideration make it a complex task. 
     Therefore, a barcode system that permits reading a barcode even when embedded beneath a surface, or placed within opaque materials such as an envelope, and not requiring highly accurate placement of the barcode reader relative to the barcode would be beneficial. Even more beneficial would be a barcode system which is of a small size and which permits for multiple barcodes at a single physical location. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a microwave barcode system wherein the barcode comprises a plurality of conductive wires, consisting of a plurality of lengths, mounted on a surface of an object, or embedded beneath the surface of the object. A transmitter, comprising a dipole antenna, radiates microwave signals in the direction of the surface. A microwave reader, comprising a dipole antenna, positioned to receive the microwave signals passing through the surface, and senses an attenuation of the microwave signal caused by a resonant interaction between the microwave signals and the conductive wires. 
     The conductive wires of the present invention can be mounted in a standup fashion, wherein each wire is mounted adjacent to and parallel to, but not touching, its at least one neighbor, and/or in a lay-down fashion, wherein each wire is mounted coincident with an imaginary line which is also coincident with the remaining wires, and wherein the wires are mounted end-to-end with a gap surrounding each wire end. Because a microwave reader positioned to read a stand-up barcode cannot detect signals polarized for a lay-down barcode, and a reader positioned to read a lay-down barcode cannot detect signals polarized for a stand-up barcode, both stand-up and lay-down versions of the barcode may be employed simultaneously at a single physical location. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The present invention exists in the construction, arrangement, in combination of the various parts of the device, and steps of the method, whereby the objects contemplated are attained as hereinafter more fully set forth, specifically pointed out in the claims, and illustrated in the accompanying drawings in which: 
     FIG. 1 depicts a block diagram of a microwave barcode system; 
     FIG. 2 illustrates a microwave barcode system, employing dipole antennas with barcode elements in a stand-up configuration; 
     FIG. 3 illustrates a microwave barcode system, employing dipole antennas in a lay-down configuration; 
     FIG. 4 shows a Yagi antenna; 
     FIG. 5 diagrams a system which may read stand-up and lay-down configurations of a microwave barcode reader; 
     FIG. 6 is a more detailed view of the receiver/detector portion of FIG. 5; and 
     FIG. 7 is another embodiment of a receiver/detector which may be implemented in the circuit of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A microwave barcode system has been disclosed in now abandoned U.S. Ser. No. 09/342,396, by Marchand, filed Jun. 29, 1999, entitled MICROWAVE BARCODE, hereby incorporated by reference. 
     FIG. 1 sets forth a block diagram of a microwave barcode system  10 . Such a system includes a microwave transmitter  12  which emits a polarized microwave signal  14  toward a substrate  16  having a set of conductive lines (wires)  18  which act as the microwave readable element or barcodes. When the microwave signal  14  strikes the microwave readable elements  18 , the microwave signal is partially scattered and partially attenuated. The scattered portion  20  of the microwave signal can be sensed by a sensor  22 . If the sensor  22  receives a scattered signal it determines that a microwave-readable element exists. In that case, sensor  22  produces a predetermined output signal, such as a one. Therefore, useable data is obtained through the use of the above-noted system. Wires  18  may be located on the surface, or below the surface of substrate  16 . 
     To obtain transmission and reception of the microwave signal, appropriate receptors and receivers of the signal must be used. However, it had been considered that severely large receptors and transmitters would be necessary to transmit and receive the signals. A device which may be used as transmitters or receptors is a horn antenna and another is wave guides. While a level of success is obtained using these elements as transmitters and receivers, certain deficiencies existed such as bulkiness of the elements and the lack of spatial resolution in an obtained signal. 
     Turning to FIG. 2, illustrated is a microwave barcode system  30  including a barcode  32  made up of a set of conductive lines (wires)  34 , lined up on or below a surface, parallel to each other. In the stand-up barcode the wires are oriented parallel to each other and perpendicular to the direction of the barcode  32 . The barcode  32  is read by sliding it between two dipole antennas, a transmitting dipole antenna  36  and a receiving dipole antenna  38 , one on each side of the barcode  32 . The wires  34  and antennas  36 ,  38  illustrated in FIG.  2  and other Figures are not drawn to scale. A presence and length of each wire  34  in barcode  32  can be determined by detecting absorption of microwaves  40  in the gap between the antennas  36 ,  38  as a function of the frequency of the microwaves  40  used. 
     With reference now to FIG. 3, a microwave barcode system  50  illustrates a lay-down microwave barcode  52  made up of a set of conductive lines (wires)  54 , lined up end-to-end, coincident with a line  56  in the direction of the barcode  52 . The wires  54  are arranged with a gap  58  between adjacent ends  60  so that there is no contact between any two wires  34 . As with the stand-up barcode, the lay-down barcode  52  is read by sliding it between two antennas, a transmitting dipole antenna  62  and a receiving dipole antenna  64 , one on each side of the barcode  52 . A presence and length of each wire  54  in the barcode can be determined by detecting attenuation of microwaves  66 , caused by a resonant interaction of the microwave signal with each wire  54 , in the gap between the antennas  62 ,  64  as a function of the frequency of the microwaves  66  used. The lay-down microwave barcode has the advantage of using less “real estate” than the stand-up barcode. 
     Each of the wires  34 ,  54  in each barcode  32 ,  52  encode one or more bits of data, depending on how many lengths of wire are used. If, for example, 4 different lengths are used, each wire can encode 2 bits of data comprising a binary number. Four bits of data can also be encoded using only 3 different lengths of wire and the absence of a wire at each location. For example, using 3 different lengths of wire and the absence of a wire at each location, the absence of a wire could represent the binary value 00, the shortest length of wire could represent binary value 01, the median length of wire could represent binary value 10, and the longest length of wire could represent binary value 11. Alternately, each length of wire and the absence of a wire at each location could be used to represent letters of an alphabet. 
     Because of the small size and relative simplicity of the dipole antennas  36 ,  38 ,  62 ,  64 , and because transmitting antennas  36 ,  62  produce polarized microwave signals, while the receiving antenna  38 ,  64  are sensitive to the polarization of the microwave signals, a combination of stand-up and lay-down barcodes are also possible. The lay-down detector cannot detect the stand-up barcode, and vice versa, the two kinds of barcodes  32 ,  52  can be placed at the same location. The two sets of antennas  36 ,  38 ,  62 ,  64  will then be used to read the two barcodes  32 ,  52 . This concept is understandable by superposing FIGS. 2 and 3. 
     The use of dipoles for both the transmitter  36 ,  62  and receiver  38 ,  64  antennas has the advantage of compactness, simplicity and good performance. Previously used horn antennas and waveguides were more bulky and did not have good spatial resolution. 
     The length of the antennas  36 ,  38 ,  62 ,  64  are approximately one half of the wavelength of the microwave signal used. With reference again to FIGS. 2 and 3, the distance between antennas  38 ,  64  and the barcodes  32 ,  52  are typically less than one wavelength. Because multiple frequencies will typically be used, the length of the antenna will be a compromise. The length of the transmitting antennas  36 ,  62  and receiving antennas  38 ,  64  need not be the same, and good performance over the range of frequencies is achieved by tuning one antenna of a pair at one end of the frequency range and the other antenna at the other end of the frequency range used. 
     As shown in FIG. 4, it is possible to improve on the directionality of the antennas by using a Yagi dipole antenna structure  70 , which includes a dipole antenna  72 , a one-half or greater wavelength reflector  74  and director  76  elements. Adding these elements reduces the bandwidth of the antennas, and this should be considered in the design. 
     Readers for both stand-up and lay-down barcodes have been constructed and tested. With reference to FIG. 5, a barcode system  80  was tested using a 4-wire barcode  82 . While microwave frequencies of interest range from 1 gigahertz to 200 gigahertz, the microwave frequency range used in the test was 10 gigahertz to 18 gigahertz. The wires  84  used had a length in the range of 0.3″ to 0.5″ and were placed at a distance of 0.5″ from each other. The antennas  86 ,  88  were placed about 0.5″ from each other, and the barcode  82  was placed in the center of the two antennas  86 ,  88 . The transmitter  90  used a voltage controlled oscillator (VCO), and the receiver/detector  92  was a broadband Schottky detector. The detector  92  and VCO  90  were interfaced to a PC  94  using an ADC/DAC interface card  96 . Use of the lay-down barcode  82  gave the best results in terms of ability to read the barcode and error rates. An advantage provided by using a dipole antenna as the receiving antenna  88  is that a Schottkyo diode  98  can be placed directly on the antenna  88 , bridging receiving elements  100 ,  102  of antenna  88 , thus providing signal detection before any losses are encountered in connecting cables. 
     With further attention to FIG. 6, detector  92  of FIG. 5 is shown in more detail. Detector  92  includes dipole antenna  88  including receiving elements  100  and  102 . The total length of antenna  88  is one half the microwave radiation wavelength. Schottky diode  98  is connected directly between and in series with receiving elements  100  and  102 . The leads of Schottky diode  98  comprise part of the length of the receiving elements  100 ,  102 . A one-quarter wavelength resonator  104  is connected to one lead of Schottky diode  98  in a position perpendicular to receiving element  100 . Another one-quarter wavelength resonator  106  is connected to the remaining lead of the Schottky diode in a position perpendicular to receiving element  102 . A capacitor  108  is connected between the remaining ends of quarter wavelength resonators  104  and  106 . The two leads of capacitor  108  are connected through a pair of wires  110 ,  112  to the analog digital converter  96 . 
     The arrangement illustrated in FIG. 6 comprises a very small and inexpensive detector. In fact, for the wavelengths used between 12 and 18 gigahertz, the total length of the receiving elements  100 ,  102  is less than one centimeter. An advantage of detector  92  is that the detector is located at the receiving elements and there are no signal losses before signal detection. The spatial resolution is limited primarily by the wavelength of the microwave radiation. For example, at 15 gigahertz, one half of the wavelength is approximately one centimeter, which is the length of the receiving elements  100 ,  102 . 
     Turning to FIG. 7 another antenna configuration is shown. The detector  120  includes a dipole antenna  122  with receiving elements  124  and  126 . The total length of antenna  122  is one half of the microwave radiation wavelength. Antenna  122  is connected to a coaxial cable  128  which is in turn connected to a Schottky diode detector  130 . The Schottky diode detector  130  in turn is connected by a pair of wires  132  to analog digital converter  96 . Connecting dipole antenna  122  to the detector diode  130  through a length of coaxial cable contributes to signal loss and also makes for a more bulky apparatus than the embodiment shown in FIG.  6 . 
     An advantage of a microwave barcode reader using a dipole antenna is the insensitivity to dirt or ink present on the document. It is also possible to read microwave barcodes on documents placed inside envelopes, thus providing a simple means for determining that the document contained in the envelope belongs to the same person or persons addressed on the face of the envelope before mailing the document-containing envelope. 
     Another advantage offered by the present invention is embedding microwave barcodes within a material of which a document is comprised, paper currency for example. The barcode is invisible to persons handling the currency but could be easily detected by a simple and compact microwave barcode reader using dipole antennas. 
     It should be appreciated that, while the microwave barcode has been described using wires to make the barcode, other methods of producing the barcode would produce essentially the same results. For example, a conductive ink can replace the wires provided that the conductivity of the ink is sufficient to produce adequate attenuation due to resonance. Metallic foils could also be used in place of wires. 
     It should also be appreciated that, while stand-up and lay-down barcode schemes have been described, other schemes can also be used with the present invention. For example, another way of encoding information is to use wires of equal lengths, mounted at one angle to represent one datum value and mounted at another angle to represent another datum value. A plurality of angles could be used. The varying absorption of the microwave signal by the equal-length wires at different angles, because of the polarization of the microwave signal, would make it possible to determine the angle of each wire. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.