• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
  • 同族专利
  • 引用文献
  • 法律状态
  • 优先权
    • 专利摘要:
    The invention relates to a method for locating an RFID tag (1) with at least one RFID reader (2), characterized in that the RFID reader (2) emits a polling signal (3) comprising a wideband signal (4); the RFID tag modulates the interrogation signal with a code signal (5) and reflects it as a reflection signal (6); and the RFID reader (2) receives the reflection signal and extracts therefrom the code signal and a scattered wideband signal; wherein the code signal used is implemented as a substantially mean-value-free digital signal, and the RFID reader determines a transit time by comparing the determined phase position of the received broadband signal with the transmitted broadband signal and the RFID reader from the transit time determines the distance between RFID reader and RFID tag calculated. The invention further relates to a system for locating an RFID tag, which is adapted to carry out the method according to the invention.
    公开号:AT511750A1
    申请号:T1128/2011
    申请日:2011-08-04
    公开日:2013-02-15
    发明作者:Holger Dipl Ing Dr Arthaber
    申请人:Univ Wien Tech；
    IPC主号:
    专利说明:

    • * * 1 ........: SebW Ag / Fl TU Wifen FCifschöfigs- * uhd Wheezer support
    Method and system for locating objects
    The invention relates to a method for locating objects, in particular RFID tags, and to a system for locating RFID tags with at least one RFID reader. Radio frequency identification (RFID) systems have been known in the art for decades and are used to identify or locate objects or animals. An RFID system consists of an RFID tag, which is located on or in the object to be identified or localized and can contain a distinctive code, and an RFID reader for reading this information. RFID tags can be smaller than a grain of rice and glued to objects, attached in other forms or implanted in living things. In addition, it is possible to produce RFID tags via special printing processes. The advantages of this technology result from the combination of the small size, the inconspicuous read-out option and the low price of the RFID tags (sometimes in the cent range). As a result, this technique is suitable, for example, for replacing the still widespread barcodes.
    The coupling of the RFID reader with the RFID tag is done by generated by the RFID reader electromagnetic alternating fields whose intensity and frequency range depending on the application. • Vienna University of Applied Sciences Vienna Feftschüflgs-ttid TTänsfer's support
    Through these alternating fields, data can be transmitted and, at short distances, the RFID tags can be supplied with electrical energy. If longer ranges are to be achieved and the costs of the RFID tags are not critical, active RFID tags with their own energy supply can be used.
    The RFID reader generates a high-frequency electromagnetic alternating field for interrogation, to which the RFID tag is exposed. The radio-frequency energy which it receives via the antenna can serve as an energy supply during the communication process. Active tags can also be powered by a built-in battery. For half-active tags, the battery only takes care of supplying a microchip located on the RFID tag.
    The RFID tag optionally decodes the commands sent by the reader and encodes and modulates its response to the radiated electromagnetic field by changing its antenna impedance and reflecting the field emitted by the reader. It is generally not an active transmission, but a modulated back-scattering of the received signal. Depending on the type, RFID tags work in the range of 125-134 kHz longwave, 13.56 MHz shortwave, UHF 865-869 MHz (European frequencies) and 902 - 928 MHz (US and Asian frequency bands) or the SHF at 2.45 GHz and 5.8 GHz.
    Radio frequency (RF) tags use load modulation, that is, they use shorting to consume some of the energy of the alternating magnetic field. This can detect the reader, but theoretically also a distant receiver. The antennas of an RF tag form an induction coil with several turns.
    Ultra high frequency (UHF) tags, on the other hand, operate in the electromagnetic far field to receive and transmit the response. The antennas are usually linear, folded, circular or spiral dipoles, the chip is usually located in the middle between the dipolar terms of the RFID tag. 3 · .. * * .. · · .. · * .. * • ..50335 Ag / Fl TU Wien Research and transfer support
    The detection of the position of RFID tags is an essential prerequisite for a variety of applications in the field of logistics, goods tracking, person tracking, tolling, access systems, to determine the entitlement of persons, for theft detection, etc. Basically, it is known by comparison the signal transmitted by the RFID reader with the backscattered or reflected by the RFID tag signal to determine the duration of the signal and from there the distance between the RFID reader and the RFID reader. The comparison can be based on a comparison of the field strength, the power, or the phase shift. However, such methods are very susceptible to interference, in particular due to the fact that reflections always occur from other sources, such as obstacles of any kind.
    To solve these problems, a combined angle and distance measurement can be performed. For example, a plurality of antennas (e.g., an antenna array) may be mounted on a reader and an angular range of the backscattered signal determined therefrom. For an angle determination in a plane, at least two antennas are necessary for this, while at least three antennas are required for an angle determination in two planes. However, such methods are expensive and expensive. Furthermore, a plurality of spatially offset RFID readers can be provided, whereby a triangulation based on the received signal strength is made possible to more precisely limit the source of the reflections.
    However, this requires several stationary RFID readers.
    The technical object of the invention is, inter alia, to provide a method and / or a system which makes it possible to determine the distance between an RFID reader and a selectively selected RFID tag in a simple, robust manner, with the disadvantages The prior art should be resolved and a simple, cheap, reliable system for distance measurement and / or position determination can be realized. As far as possible, commercial RFID tags should be used, and no exceeding of existing radio standards should occur. * · «4: :: :: ::.: S) 39s * Ag / Fi TU Wten ForednJrtgssiPid Tfdnsfei% upport
    This object is achieved by a method in which the RFID reader is a query signal! which comprises a wideband signal; the RFID tag modulates the interrogation signal with a code signal and reflects it as a reflection signal; the RFID reader receives the reflection signal and obtains therefrom the code signal and a scattered wideband signal; the RFID reader determines a transit time of the wideband signal by comparing the wideband signal with the scattered wideband signal; the RFID reader determines the distance between the RFID reader and the RFID tag from the transit time difference.
    In the method according to the invention, a broadband signal with low power (in order to fulfill the radio regulations) is superimposed on the carrier signal transmitted by the RFID reader during the reading phase. While the RFID tag returns its data, the distance between the RFID reader and the RFID tag can be determined by means of correlation. It exploits the fact that the RFID tag changes its antenna input impedance during transmission, while all other reflectors do not - this way the distance to the active RFID tag can be determined.
    The determination of the propagation time of the wideband signal may be effected by correlation of the wideband signal (possibly matched in accordance with the received code signal) with the scattered wideband signal, a matching method (e.g., least squares fit in the frequency domain), or other methods. 5: *: :: :: 333ss * Ag / Fi TU Wten Far% chdrtgs- fjfid TTransfersupport
    This method can be used to determine the distance to a backscatter tag (whether active or passive). By directly measuring the distance, localization can be made from a single antenna or from a handheld device. Also, triangulation methods can be supported with the distance measurement and thus significantly better results can be obtained. Furthermore, angle-estimating methods with multiple antennas can be supplemented by a distance calculation.
    The special procedure makes it possible to differentiate between the reflected signal and disturbing reflections. Furthermore, multiple propagations can be detected and hidden. This reduces ambiguities in the position determination and leads to a significant improvement compared to triangulating methods.
    The invention is used in the RFID reader and requires no new or modified RFID tags. Conventional RFID tags, such as e.g. EPC Class 1 Gen 2 tags (the most widely used standard) are used.
    The wideband signal can be implemented as a direct sequence spread spectrum signal, as an OFDM signal or as a chirp signal. Such signals have the advantage of a low spectral energy density, whereby other signals are not disturbed, and existing standards can be met.
    According to the invention, the code signal can be embodied as a mean-value-free digital signal, and the duration of a partial bit of the code signal can be an integer multiple of the period of the broadband signal. In particular, the duration of a sub-bit may comprise about 1.3 periods of the wideband signal.
    The absence of a mean value of the code signal makes it possible for the broadband signals reflected by interferers to cancel each other out. Specifically, for example, an FMO or Miller coding can be used, but other codings are also provided according to the invention. 6 ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
    The scattered broadband signal according to the invention can be multiplied by the respective sign of the code signal and accumulated over several periods. The multiplication of the received wideband signal according to the invention with the respective sign of the code signal makes it possible to detect the tag involved in the communication. When using a mean-value code signal, interference cancel each other in this summation. However, it is also possible to use a code signal that is not entirely averaged. As long as you have enough zeros and ones, you can also work with imperfectly averaged signals and intelligent summation.
    Likewise, the invention provides that the summation takes place only over a period. For example, a day can take a long time " 1 " and then return "0" for a long time - just one period. Likewise, according to the invention, it is provided that from the day a " 0101010101 ... " Episode or the like can be requested for distance measurement. Then the distance measurement would not be performed with a data signal, but with a dedicated auxiliary signal.
    The distance between the RFID readers and the RFID tag can be determined by a method according to the invention and the position of the RFID tag can be calculated from the known positions of at least one of the RFID readers.
    The data transmission between the RFID reader and the RFID tag can be done by modulated backscatter. According to the invention, it is also provided that the RFID tag is designed as a semi-active tag which uses, for example, active reflectors (RX antenna + modulator + amplifier + TX antenna), as prescribed in the Austrian truck toll system.
    The carrier signal can be designed as a high-frequency carrier signal. It can have a frequency of 850 MHz to 950 MHz. The RFID tag may be a passive RFID tag according to EPC CJassl Gen2. 7 J Η:; ; I * »5payi Ag / Fl TU Wt6n Fdi% heart and teaser support
    The RFID reader can additionally make an angle estimation for locating the RFID tag. For this purpose, it can be provided that several antennas or an antenna array are provided on the RFID reader.
    According to the invention, the RFID tag can carry out the modulation of the interrogation signal with a code signal either by varying its antenna impedance or in the form of an active modulation.
    Furthermore, the invention extends to a system for locating an RFID tag with at least one RFID reader, wherein the RFID reader comprises a data processing unit and at least one antenna, and the RFID tag comprises at least one antenna and a logic, wherein the RFID tag Tag is arranged to reflect an interrogation signal with a code signal modulated as a reflection signal, the RFID reader is adapted to emit an interrogation signal comprising a carrier signal modulated broadband signal, and from the reflection signal, the code signal and a scattered wideband signal to win, the data processing unit is arranged to determine a transit time difference between the wideband signal and the scattered broadband signal, and to calculate therefrom the distance between the RFID reader and the RFID tag.
    The system may further comprise a data processing unit connected to or connectable to the RFID readers and a database, wherein the positions of the RFID readers are stored in the database and the data processing unit for calculating the position of the RFID tags from the positions of the RFID readers and the distances between the RFID readers and the RFID tags is set up. * · «· * · Φ t a λ t β: :: 50398 Ag / Fi * .. * * .. * *. JO * WiÖn " l% chijngs- and transfer support
    On the RFID reader, several antennas can be provided for angle estimation in one or two planes.
    When using a spread spectrum signal as a broadband signal, the period of the spread spectrum sequence used according to the invention can be chosen so that regardless of the data rate of the backscattered signal (can vary by about +/- 20%) always at least a full period of the spread -Spectrum sequence fits into a sub-bit of the code signal used. In this case, exactly one period can be taken from the data signal sub-bit center for summing the backscattered signals. Since it is not known in advance how fast the day will just crawl back, these spread-spectrum periods always start in a different position. During the summation, the signal excerpt is therefore initially cyclically rotated (one knows from the position as it should be), so that in the summation always same .Spread Spectrum -Perioden be added.
    The RFID tag may be implemented as a passive, active or semi-active RFID tag and configured to reflect the interrogation signal as a reflection signal by modulating its antenna impedance or by actively modulating it with a code signal.
    Further features of the invention will become apparent from the claims, the description and the drawings.
    In the drawing, the subject invention is illustrated schematically in embodiments. Show it
    Fig. 1: a schematic representation of an embodiment of the invention;
    2 shows a schematic representation of the signal curves in a method according to the invention;
    3 shows a schematic representation of the detail III from FIG. 2; ♦ * t * * «t φ · · · 9 · * # · # · # * · ***!« · * 50398 Ag / Fl * · · * * *. * * * «Tl / Wiferf FofschCings and transfer support
    4 shows a schematic representation of the detail IV from FIG. 3;
    FIGS. 5a-5b show a schematic illustration of an embodiment of the summation of a selected RFID tag and a jammer according to the invention;
    6a-6b: further schematic representation of embodiments according to the invention.
    Fig. 1 shows an embodiment of a system according to the invention for locating an RFID tag 1 with an RFID reader 2. The RFID tag 1 comprises an antenna 13, a logic 14 and devices for power supply (rectifier), for decoding the received signal and for modulating the antenna impedance. Optionally, the logic may be connected to a memory, as shown in this embodiment.
    The RFID reader 2 has a transmitting part TX and a receiving part RX, which are each equipped with an antenna 11. Alternatively it can also be provided that the RFID reader 2 has no separate antennas, but a common antenna and a circulator, directional coupler or the like for separating the transmitting and receiving direction.
    The RFID reader 2 first sends out an interrogation signal 3, which is received by the RFID tag 1, modulated with a data signal and scattered back as a reflection signal 6.
    At the same time the interrogation signal 3 is reflected by other interferers in the transmission range, whereby the reception at the RFID reader 2 is impaired. In addition, the reception at the receiving part RX of the RFID reader 2 is also strongly superimposed by the coupling K with the transmitting part TX. 10: • · «« «* • ···································································································································································
    Fig. 2 shows a schematic representation of the waveforms in a method according to the invention. The transmitted power PTX of the interrogation signal 3 transmitted by the RFID reader 2 initially shows a query sequence in the form of a sequence of signal packets which serve for initialization and for powering the RFID tag 1.
    The interrogation signal 3 is composed of a narrow-band radio-frequency carrier signal and a superimposed first broadband signal 4, the radio-frequency carrier signal having a frequency of approximately 865 MHz. The broadband signal 4 is designed as a direct-sequence spread-spectrum signal, wherein the specific choice of the spreading code is initially irrelevant.
    The broadband signal 4 modulated onto the carrier signal is schematically indicated in FIG. 2 as high-frequency oscillation. The power received by the receiving part of the RFID reader 2 is schematically shown as Prx. Here, the interrogation sequence is repeated, followed by the actual reflection signal 6. The signal received by the receiving part RX consists primarily of the signal coupled through the direct path K, as well as of numerous interference signals and the wanted signal of the RFID tag. The detail area III shown in FIG. 2 is shown in more detail in FIG.
    FIG. 3 shows a schematic representation of the detail III of the PRX from FIG. 2. It should be noted that this is a simplified representation of a process that is actually sketchable only in the complex plane, and only for the basic understanding of the method serves.
    The reflection signal 6 is generated by the RFID tag 1 by varying its antenna impedance with a data signal 8. With the data signal 8, the RFID tag 1 transmits, for example, its own invariable serial number, data of the designated object or other information.
    11. ·····. * · · * ·. 50398 Aq / FI * «* * ** * * *». TLTWiSn Feeding and Transfer Support
    The data signal 8 itself is not used directly for modulation, but transmitted in the form of a mean-value code signal 5. For example, in FIG. 3, the transmission of the bit sequence " 0 1 " This code signal 5 is a scattered broadband signal 7 superimposed, which is not identical in time with the transmitted broadband signal 4 due to the finite term between the RFID reader 2 and the RFID tag 1. The size ratios shown are purely exemplary and do not limit the invention in any way. For example, it is possible that the broadband signal 4 or the scattered broadband signal 7 has a higher amplitude than the code signal 5. The detail IV is explained in more detail in FIG.
    FIG. 4 shows a schematic representation of the detail IV of FIG. 3. Shown is a full period of the code signal 5. In the present case, a Miller sequence with M = 4 was used for the code signal 5, but other mean-free codings of the Data signal provided. The exemplary code signal 5 comprises the 8 time sections or partial bits T0 to T7. The scattered broadband signal 7 is superimposed on the code signal 5 and has the period T on. In the present case, the period T and the duration of a Miller period (partial bits) Tj are chosen such that approximately two periods of the broadband signal 4 and 7, respectively, fit into a period (a partial bit) T i. This has the advantage that in the following demodulation the scattered Breitbandsigna! 7 more robust can be reconstructed. It should be noted, however, that more periods do not necessarily lead to a more robust reconstruction, since then the bandwidth increases and several band-limiting elements are involved in the communication (antennas).
    5a shows a schematic representation of the processes in the RFID reader 2 for calculating the transit time t of the broadband signal 4. The received reflection signal 6 is assumed. This passes through a decoder, which detects the sign of the code signal 5. At the output of the decoder the values +1 or -1 can be present. This is not the data signal 8, but only the code signal 5. The value at the output of the decoder is multiplied by the reflection signal 6 and then there is a summation. 12 :: :: ::.:; ·; · 50398 Ag / Fi "TU WienVorscrtüngs and Transfersupporl
    This method is shown by way of example in FIG. 5b. For the selected RFID tag, the decoder detects at the time periods Τι, T3, T4 and T & (see Fig. 4) has a negative sign, and multiplies the reflection signal 6 by -1.
    This undoes the modulation in the RFID tag and has the same wideband signal at each time slot. In contrast, signals from interferers that were not modulated with the code signal 5 cancel each other out. This demonstrates the need to use a mean or near-zero signal. The closer the mean value of the code signal 5 is to zero, the more accurate the subsequent detection of the broadband signal 7. Moreover, the number of summation effects also influences the accuracy of the method: the more periods are added, the more accurately the wideband signal 7 is reconstructed. For this purpose, it may be provided that the RFID reader requests a multi-digit random number before the actual query, in order to be able to reconstruct the broadband signal clearly therefrom. Of course, any response from the tag, especially the actual payload, can be used for this purpose.
    From the knowledge of the scattered broadband signal 7 and the transmitted broadband signal 4, the RFID reader 2 can determine by correlation in a known manner the phase shift or the transit time of the broadband signal 4, and calculate therefrom the desired time t. This is also shown schematically in Fig. 5b.
    Finally, Figs. 6a-6b show further embodiments of the system according to the invention. In the embodiment according to FIG. 6a, the RFID tag 1 is fastened to an object 17 to be located. There are three RFID readers 2 are provided, whose positions are known. By carrying out the method according to the invention, each RFID reader 2 can determine the distance to the RFID tag 2. In addition, it can be provided that the RFID readers 2 have multiple antennas to perform an angle estimation. From the knowledge of angle and distance, an improved location can be made possible. f 3 · · · jj · ♦ · ··· * 50398 Ag / Fl ·· * * ** ** * * * * tlj Weft and transfer support
    Likewise, in an exemplary embodiment according to FIG. 6b, it may be provided that the RFID readers 2 are connected to a central data processing unit 15, which in turn is connected to a database 16 which contains the position data of the RFID readers 2. From the position data and the distances calculated by the RFID readers 2, the central data processing unit 15 calculates the position of the RFID tag 1 and the associated object 17.
    The invention is not limited to the illustrated embodiments, but of course includes other embodiments of the invention. In particular, the invention is not limited to the use of passive UHF RFID tags, but also includes active RFID tags, semi-active RFID tags, and / or RFID tags operating in other frequency ranges, with modulated backscatter communication or other methods can be done.
    14 I 50398 Ag / Fl * * * * «« tü Vienna & R & D clipping and transfer support
    LIST OF REFERENCES 1 RFID tag 2 RFID reader 3 Interrogation signal 4 Broadband signal 5 Code signal 6 Reflection signal 7 Broadband signal 8 Data signal 9 Data processing unit 10 Memory 11 Antenna 12 Transceiver 13 Antenna 14 Logic 15 Data processing unit 16 Database 17 Object
    权利要求:
    Claims (17)
    [1]
    1). * * * * · # 9 ** # 50398 Acj / Fl ** * * * * ♦ * * ♦ · tl3wTen ^ orsclxings and transfer support Claims 1. A method for locating an RFID tag (1) with at least one RFID reader (2), characterized in that a. the RFID reader (2) emits an interrogation signal (3) comprising a wideband signal (4); b. the RFID tag (1) modulates the interrogation signal (3) with a code signal (5) and reflects it as a reflection signal (6); c. the RFID reader (2) receives the reflection signal (6) and extracts therefrom the code signal (5) and a scattered wideband signal (7); d. the RFID reader (2) determines a transit time of the wideband signal (4) by comparing the wideband signal (4) with the scattered wideband signal (7); e. the RFID reader (2) determines the distance between the RFID reader (2) and the RFID tag (1) from the transit time difference.
    [2]
    2. The method according to claim 1, characterized in that the wideband signal (4) is designed as a direct sequence spread spectrum signal, as an OFDM signal or as a chirp signal
    [3]
    3. The method according to claim 1 or 2, characterized in that the comparison in step d) is performed as a correlation or in the form of a matching method.
    [4]
    4. The method according to any one of claims 1 to 3, characterized in that the code signal (5) is designed as a mean value free digital signal and the duration of a partial bit of the code signal (5) is an integer multiple of the period of the broadband signal (4). ι £ 50398 Ag / Fi * ·· * * * * * " * * * n! l * Wlen * f * ocation and transfer support
    [5]
    5. The method according to any one of claims 1 to 4, characterized in that the code signal (5) encodes any data signal (8), wherein FMO sequences, Miller sequences or another encoding is used.
    [6]
    6. The method according to any one of claims 1 to 5, characterized in that in step c) the scattered broadband signal (7) multiplied by the sign of the code signal (5) in the respective time segment and summed over one or more periods.
    [7]
    7. A method for determining the position of an RFID tag (1), wherein the position of one or more RFID readers (2) is known, characterized in that the distance between the RFID readers (2) and the RFID tag (1) determined by a method according to one of claims 1 to 6 and from the known positions of at least one of the RFID readers (2), the position of the RFID tag (1) is calculated.
    [8]
    8. The method according to any one of claims 1 to 7, characterized in that the data transmission between the RFID reader (2) and the RFID tag (1) is effected by modulated backscattering or dedicated modulators.
    [9]
    9. The method according to any one of claims 1 to 8, characterized in that the carrier signal is designed as a high-frequency carrier signal and preferably has a frequency of 850 MHz to 950 MHz.
    [10]
    10. The method according to any one of claims 1 to 9, characterized in that it is the RFID tag (1) is a passive RFID tag according to EPC Classl Gen2.
    [11]
    11. The method according to any one of claims 1 to 10, characterized in that the RFID reader (2) for locating the RFID tag (1) additionally makes an angle estimate. 1 >; J * *: **: * '* * 50398 Ag / Fl * · * · · TUAA / ierrForscfiungs- and transfer support
    [12]
    12. The method according to any one of claims 1 to 11, characterized in that the RFID tag in step b) performs the modulation either by varying its antenna impedance or in the form of an active modulation.
    [13]
    13. The method according to any one of claims 1 to 12, characterized in that the RFID tag is sent out a dedicated auxiliary signal for distance measurement.
    [14]
    14. System for locating an RFID tag (1) with at least one RFID reader (2), wherein the RFID reader (1) a data processing unit (9) and at least one antenna (11), and the RFID tag (1 ) comprises at least one antenna (13) and logic (14), characterized in that the RFID tag (1) is adapted to reflect a query signal (3) modulated with a code signal (5) as a reflection signal (6) RFID reader (2) is arranged to emit an interrogation signal (3) comprising a broadband signal (4), and from the reflection signal (6) the code signal (5) and a scattered broadband signal (7) to win, the data processing unit (9 ) is arranged to determine a transit time difference by comparing the broadband signal (4) with the scattered broadband signal (7), and to calculate therefrom the distance between the RFID reader (2) and the RFID tag (1). * · * · 50398 Ag / Fl · * »» # a TlTWierf research and transfer support
    [15]
    15. System according to claim 14, characterized in that the system further comprises a data processing unit (15) connected or connectable to the RFID readers (1) and a database (16), wherein the positions of the RFID readers (1) in the Database are stored and the data processing unit (15) for calculating the position of the RFID tags (1) from the positions of the RFID readers (2) and the distances between the RFID readers (2) and the RFID tags (1) set is.
    [16]
    16. System according to claim 14 or 15, characterized in that a plurality of antennas are provided for angle estimation in one or two planes on the RFID reader.
    [17]
    17. System according to any one of claims 14 to 16, characterized in that the RFID tag is designed as a passive, active or semi-active RFID tag.



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    法律状态:
    优先权:
    申请号 | 申请日 | 专利标题
    ATA1128/2011A|AT511750B1|2011-08-04|2011-08-04|METHOD AND SYSTEM FOR LOCATING OBJECTS|ATA1128/2011A| AT511750B1|2011-08-04|2011-08-04|METHOD AND SYSTEM FOR LOCATING OBJECTS|
    EP12751014.7A| EP2739989B1|2011-08-04|2012-07-31|Method and system for locating rfid tags|
    PCT/EP2012/064957| WO2013017596A1|2011-08-04|2012-07-31|Method and system for locating objects|
    US14/237,131| US9471820B2|2011-08-04|2012-07-31|Method and system for locating objects|
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