Abstract:
An RFID verifier includes a transmit signal strength indicator (TSSI) and a receive signal strength indicator (RSSI). Using the TSSI, the RFID verifier may determine the amount of power an interrogated RFID tag is illuminated with. Similarly, using the RSSI, the RFID verifier may determine the amount of power returned to the RFID verifier by the RFID tag. By comparing the returned power to the amount used to illuminate the interrogated tag, the RFID verifier may provide an indicia of quality for the interrogated RFID tag.

Description:
FIELD OF INVENTION 
   The present invention relates generally to RFID applications. More particularly, the present invention relates to an RFID reader or interrogator configured to verify RFID transponder operation. 
   BACKGROUND 
   Radio Frequency Identification (RFID) systems represent the next step in automatic identification techniques started by the familiar bar code schemes. Whereas bar code systems require line-of-sight (LOS) contact between a scanner and the bar code being identified, RFID techniques do not require LOS contact. This is a critical distinction because bar code systems often need manual intervention to ensure LOS contact between a bar code label and the bar code scanner. In sharp contrast, RFID systems eliminate the need for manual alignment between an RFID tag and an RFID reader or interrogator, thereby keeping labor costs at a minimum. In addition, bar code labels can become soiled in transit, rendering them unreadable. Because RFID tags are read using RF transmissions instead of optical transmissions, such soiling need not render RFID tags unreadable. Moreover, RFID tags may be written to in write-once or write-many fashions whereas once a bar code label has been printed further modifications are impossible. These advantages of RFID systems have resulted in the rapid growth of this technology despite the higher costs of RFID tags as compared to a printed bar code label. 
   Although RFID systems have certain advantages over bar coding schemes, they share many concerns as well. For example, bar code scanners can merely read a bar code label; they cannot provide a measure of quality. Because a marginal bar code may be readable by one scanner but not another, users have no way of reliably detecting the marginal bar codes using conventional bar code scanners. Thus, bar code verifiers have been used to measure bar code quality metrics such as contrast, average bar deviation, and related quality indicia. Marginal bar code labels may thus be identified by bar code verifiers, thereby assuring users that their products may be reliably identified. The same concern for quality applies to RFID tags as well. However, the backscatter modulation commonly used to read information from passive RFID tags complicates the RFID verification process. In backscatter modulation, the interrogating RF beam itself provides the power for the RFID tag to respond. One verification metric would thus be how well a given RFID tag absorbed RF energy and retransmitted the energy to the RFID reader. But RF energy is absorbed by many objects in an RFID tag&#39;s environment. A conventional RFID reader has no way of determining whether a tag has absorbed RF energy or whether the absorption occurred due to environmental effects. Instead, a conventional RFID reader can merely determine the signal-to-noise ratio (SNR) of the backscattered signal from a passive RFID tag. A marginal RFID tag may be malfunctioning but illuminated with enough RF energy that the backscattered signal provided a sufficient SNR so that the RFID tag&#39;s signal may be decoded correctly. This same marginal RFID tag may be unreadable in less pristine RF environments. If an RFID tag could be verified to a known standard, such marginal RFID tags could be detected and replaced. 
   The need to verify RFID tags to a known standard is exacerbated by other RFID system properties. For example, RFID tags are not what-you-see-is-what-you-get (WYSIWYG) whereas a bar code label is. In other words, it doesn&#39;t matter what type of article a bar code label is affixed to because readability of the label is not affected, for example, by the article&#39;s color. However, the readability of an RFID tag may be strongly affected by the environment in which it is located. Thus, it is not possible to create a golden standard without knowledge of an RFID tag&#39;s context or environment. Moreover, because RFID tags can be physically or electrically damaged in transit, RFID systems are complicated by the need to find a safe position for the RFID tag. The juggling of RFID tag placement with RF absorption from the tag&#39;s environment can be a formidable task. Finally, the programmability of RFID tags requires that the fidelity of the RF link between an RFID reader and the RFID tag being interrogated must be relatively flawless. Accordingly, there is a need in the art to provide an RFID verifier that can more accurately verify operation of RFID tags using context-sensitive quality standards. 
   SUMMARY 
   In accordance with one aspect of the invention, an RFID verifier is provided. The RFID verifier includes: a transceiver operable to interrogate with an interrogating signal an RFID tag and to read a resulting signal from the interrogated RFID tag; a transmit signal strength indicator operable to measure the interrogating signal power; a received signal strength indicator operable to measure the power of the signal from the interrogated RFID tag; and a processor operable to compare the measured interrogating signal power and power for the signal from the interrogated RFID tag to obtain a measure of quality for the interrogated RFID tag. Advantageously, this RFID verifier allows the user to create context-sensitive standards. Should the RFID verifier be integrated with a bar code printer, the RFID verifier may use these context-sensitive standards to allow only standard-passing tags to have a bar code label printed without backup and over-striking of the RFID tag. 
   In accordance with another aspect of the invention, a method interrogating the RFID tag with an interrogating RF signal is provided. The method includes the acts of measuring the power of the interrogating RF signal; receiving a modulated RF signal from the interrogated RFID tag; measuring the power of the received modulated RF signal; and comparing the measured powers to provide a measure of quality for the interrogated RFID tag. Advantageously, a user may measure the quality for the interrogated RFID tag from a plurality of locations to determine the optimal location for an RFID reader. Analogously to bar code verification, this optimal location may be used for subsequent verification of additional RFID tags. For example, a second RFID tag may be interrogated with a second interrogating RF signal from the optimal location. By measuring the power of the second interrogating RF signal; receiving a second modulated RF signal from the interrogated second RFID tag; measuring the power of the received second modulated RF signal; comparing the measured powers of the received second modulated signal and the second interrogating RF signal to provide a measure of quality for the interrogated second RFID tag; the interrogated second RFID tag may be classified into one of a plurality of quality grades based upon the measure of quality for the interrogated second RFID tag. The interrogated second RFID tag may then be either accepted or rejected based upon its classification. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an RFID verifier in accordance with an embodiment of the invention. 
       FIG. 2  is a schematic illustration of an RF transceiver for the RFID verifier of  FIG. 1 . 
       FIG. 3  is illustrates an RFID tag antenna emission pattern with respect to an azimuth scan by an RFID verifier in accordance with an aspect of the invention. 
       FIG. 4  is a graph of the signal intensity as a function of range. 
       FIG. 5  illustrates a verifier display having fiducials oriented such that the verifier may be located at a predetermined angular displacement from the RFID tag antenna boresight. 
       FIG. 6  illustrates the verifier display of  FIG. 5  having the fiducials oriented such that the verifier may be located at another predetermined angular displacement. 
       FIG. 7  is a graph of the signal strength profile as a function of angular displacement resulting from a scan between the angular displacements of  FIGS. 5 and 6 . 
       FIG. 8  is a block diagram of an RFID verifier in accordance with an embodiment of the invention. 
       FIG. 9  illustrates a system having a verifier integrated with a bar code printer in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Turning now to the Figures, a block diagram of an exemplary RFID verifier  100  is shown in  FIG. 1 . RFID verifier  100  includes an RF transceiver and processor  105 . As known in the RFID art, transceiver transmits an RF signal  110  to provide power to a passive RFID tag  120 . Having thus been provided energy, passive RFID tag  120  modulates the RF signal  110  and backscatters an encoded RFID signal  125  to RF transceiver  105 . Transceiver  105  includes separate RF antennas  130 , one for transmitting RF signal  110  and another for receiving encoded RFID signal  125 . However, it will be appreciated that other embodiments of RF transceiver  105  could use a single antenna for both transmission and reception. 
   During verification, it is desirable that RFID verifier  100  be located in an optimum location for interrogating RFID tag. For example, RFID tag  120  may include a dipole antenna having a maximum gain in a boresight direction  140 . To get a measure of the quality for RFID tag  120 , verifier  100  should be located such that the maximum gains of antennas  130  are also in the boresight direction  140 . If verifier  100  is not optimally oriented in this fashion, an otherwise acceptable RFID tag  120  may be deemed of low quality simply because antennas  120  and  130  are not oriented to transmit and receive the maximum achievable RF energy. An analogous orientation must be made during verification of bar code labels in that if the bar code verifier is not normally directed to the bar code label, the resulting skew affects the quality of the bar code decryption. It will be appreciated that RFID tag  120  could be provided with fiducials such that a trained technician would understand how to manually orient RFID verifier  105  in the optimal orientation with respect to the tag&#39;s fiducials. 
   To eliminate the need for a trained technician who would appreciate, for example, that if a tag&#39;s antenna is a dipole antenna, how to properly orient RFID verifier  100  with respect to this dipole&#39;s boresight, embodiments of RFID verifier  100  will include intelligence to assist an operator in the proper orientation. For example, RFID verifier  100  may include an image processor  150  coupled to a lens assembly  155  and a display  160 . Depending upon the desired orientation of verifier  100 , image processor  150  would orient fiducials  165  on display  160  such that an image of RFID tag  120  is centered within these fiducials  165 . Alternatively, as will be explained further herein, verifier  100  may include a GPS unit (not illustrated) so as to assist in the proper orientation of verifier  100 . 
   To provide an indicia of tag quality, verifier  100  includes a received signal strength indicator (RSSI)  170  and a transmitted signal strength indicator (TSSI)  175 . Rather than use relative indicia such as SNR or bit error rate, RSSI  170  should be configured such that it provides a calibrated indication of the received signal strength. In this fashion, processor  105  may compare transmitted power for RF signal  110  as provided by TSSI  175  to received power for encoded RFID signal  125 . For example, based upon this comparison, RFID tags could be sorted into “A” level, “B” level, and “C” level categories. Advantageously, this comparison can be made for RFID tags that are on packages in a production setting. Marginal RFID tags may then be immediately detected and replaced as necessary. 
   Turning now to  FIG. 2 , a schematic illustration for an exemplary embodiment of a superheterodyne transceiver  105  is shown. It will be appreciated, however, that baseband or homodyne architectures may also be implemented. A low noise amplifier (LNA)  200  amplifies a received RF signal denoted as RF in  (signal  125  in  FIG. 1 ) to provide an input to an RF multiplexer (MUX)  205 . After coupling through RF MUX  205 , the amplified received RF signal is downconverted to IF in a mixer  210  responsive to a local oscillator (LO) signal. The downconverted analog IF signal from mixer  210  may then be digitized in an analog-to-digital converter (ADC)  215  to provide a digital IF signal. A digital transceiver  250  decodes the digital IF to identify the RF tag being interrogated. In addition this decoding may be used to provide other indicia of quality such as bit error rate (BER). 
   As discussed in the background section, verification based upon a relative variable for the received RF signal such as SNR would be error prone because the resulting RFID verifier would have no way of distinguishing, for example, an otherwise-acceptable RFID) tag located within an RF absorbing environment from an unacceptable RFID tag located in a pristine RF environment. To provide an accurate received signal strength indication, an LNA  220  matched to LNA  200  amplifies a reference signal from a reference oscillator  225  to provide an amplified reference signal to RF MUX  205 . Thus, through operation of RF MUX  205 , either the amplified reference signal or the amplified received RF signal is downconverted in mixer  210  and then digitized in ADC  215 . Reference oscillator  225  is calibrated such that if MUX  205  selects the amplified reference signal, the resulting digitized IF reference signal is also of known power because the gain of LNA  220  is known. In this fashion, the digitized IF received signal may be compared to a digitized reference IF signal of known power such that an absolute power value for the digitized IF received signal may be obtained through the comparison. 
   An analogous measurement is made for an RF signal (denoted as RF out ) that will couple to the transmitting antenna  130  ( FIG. 1 ) to provide transmitted RF signal  110 . To provide RFout, transceiver  100  generates a digital IF signal  229  that is converted into analog form in a digital-to-analog converter (DAC)  230 . It will be appreciated that reference oscillator  225  may be formed in an analogous fashion: the generation of a reference digital IF signal that is then upconverted according to a reference RF signal. Digital IF signal  229  is up converted to RF in a mixer  235  responsive to the LO signal. A power amplifier  240  amplifies the resulting RF signal to provide RF out . It will be appreciated that transmitting antenna  130  has internal losses such that the power of transmitted RF signal  110  is less than the power in RF out . It is desirable, however, to accurately know the power of the transmitted RF signal  110  to properly verify an RFID tag. Thus, TSSI  175  receives both the input to power amplifier  240  and its output. Because the gain of power amplifier  240  is known, TSSI  175  can then calculate the power for transmitted RF signal  110 , thereby accounting for any losses introduced by transmitting antenna  130 . 
   It will be appreciated that numerous processing architectures may be used to process the received digital IF signal and to generate the transmitted digital IF signal. For example, transceiver  250  including digital filters, I/Q demodulators, and a digital signal processor may be used to process and generate these signals. Higher-level functions would be implemented within a microprocessor  260 . An input/output and user interface module  270  allows a user to interact with microprocessor  260 . Regardless of the particular architecture implemented, the use of RSSI  170  and TSSI  175  enables an accurate verification of RFID tags. 
   Prior to verification of an RFID tag, the optimum range between the tag and verifier  100  should be determined. This optimal range may be experimentally determined or be provided by the manufacturer of the RFID tag being verified. A user of verifier  100  may perform an experimental determination by making received signal strength measurements at varying ranges in the boresight direction  140  for RFID tag  120 . 
   These range-varying measurements may be better explained with respect to a typical antenna emission pattern for an RFID tag dipole antenna as shown in  FIG. 3 . As can be seen from the emission pattern, transmitted RF energy from the antenna drops off as angular displacements are made from boresight direction  140 , which is denoted as the ideal read path in  FIG. 3 . For example, measurements made at the angular displacements denoted as azimuth  1  and azimuth  2  will mischaracterize the transmitted signal strength. However, measurements made in the boresight direction  140  will measure the strongest emissions from the RFID antenna. An exemplary graph of measurements made along boresight direction  140  is shown in  FIG. 4 . It can be seen that transmitted signal strength from the RFID tag peaks at an ideal read position A. Should measurements occur any closer to RFID tag  120  than position A, near-field effects decrease the transmitted signal strength. Similarly, should measurements occur at ranges further than position A from RFID tag  120 , far-field effects decrease the transmitted signal strength. A typical range for ideal read position A is approximately three meters. However, it will be appreciated that an ideal read position for a given RFID tag will depend upon the type of antenna being implemented within the given RFID tag. 
   Having determined the ideal read position, the corresponding range from RFID tag  120  to verifier  100  may be used to size fiducials  165  such that a user may readily manually orient verifier  100  at the proper range by aligning fiducials  165  with RFID tag  120 . It will be appreciated that verifier  100  may be configured with varying sets of fiducials  165  corresponding to varying types of RFID tags being verified. Depending upon the particular RFID tag being verified, a user could, for example, select from a pull down menu the appropriate fiducials  165 . 
   Having been configured with the appropriate fiducials  165 , a user may manually locate verifier  100  such that RFID tag  120  is centered within fiducials  165 , thereby assuring that verifier  100  is located at the range of the ideal read position A from RFID tag  120 . By introducing the appropriate skew to fiducials  165 , a desired angular displacement from boresight direction  140  may be achieved. It will be appreciated that the alignment of fiducials  165  is with respect to RFID tag physical landmarks such as the tag outline. If the RFID tag antenna is assumed to be aligned in a precise fashion with the tag physical landmarks, then the alignment of fiducials  165  with the physical landmarks of the RFID tag produces a corresponding alignment with the RFID tag antenna. In such a case, fiducials  165  may be oriented such that by aligning them with the physical landmarks of the RFID tag being verified, a user will locate verifier  100  at the ideal read position A. However, the alignment of an RFID tag antenna may be skewed or unknown with respect to the physical landmarks. In such a case, verifier  100  may be configured to locate fiducials  165  within display  160  such that a user will scan across the transmitted RF beam from RFID tag  120  to find the maximum antenna gain direction  140 . Verifier  100  may then locate fiducials  165  appropriately so that a user will align verifier  100  in the maximum antenna gain direction  140 . Because the ideal range has already been predetermined and accounted for in the dimensions of fiducials  165 , verifier  100  will then be at the ideal read position A discussed with respect to  FIG. 3 . 
   This scanning procedure may be better understood with reference to  FIGS. 5 and 6 .  FIG. 5  shows an exemplary arrangement of fiducials  165  within display  160  such that when RFID tag physical landmarks  500  are aligned within fiducials  165 , verifier  100  is offset from the maximum antenna gain direction  140 . For example, fiducials  165  may be arranged such that verifier  100  is displaced to read at position azimuth  1  as shown in  FIG. 3 . After the received signal strength is measured at read position azimuth  1 , fiducials  165  may be aligned within display  160  as seen in  FIG. 6  such that a user will be forced to scan across the antenna beam to another read position such as the read position for azimuth  2  in  FIG. 3 . As the user scans across the antenna beam, the verifier  100  continues to sample the antenna beam to measure received signal strength. In this fashion, a profile of the received signal strength may be expected as seen in  FIG. 7 . To form this profile, verifier  100  may monitor the location of the physical landmarks  500  within display  160  as at the time of each measurement. For example, if a user scans slowly in a first portion of the scan and then scans more rapidly in a second portion of the scan, the profile should reflect that the measurement points within the first portion are more closely spaced than the measurement points in the second portion of the scan. By correlating the time of each measurement with the position of physical landmarks  500  within display  160  at each measurement time, each measurement may be located at the correct angular displacement as seen in  FIG. 7 . Verifier  100  may then analyze the profile to determine the maximum antenna gain direction  140 . Having located maximum antenna gain direction  140 , verifier may locate fiducials  165  within display  160  such that a user will be position verifier  100  at the ideal read position A. Verification of RFID tag  120  may then proceed as discussed herein. 
   In an alternative embodiment, rather than employ a visual orientation approach as just discussed, verifier  100  may be configured with a global positioning system (GPS)  800  as seen in  FIG. 8 . To perform an antenna beam scan, a user may first measure the coordinates of RFID tag  120 . Knowing these coordinates, verifier  100  may then calculate the coordinates of the read position at azimuth  1  and  2  as discussed with respect to  FIG. 3 . The user would be instructed to move verifier  100  accordingly such that it scans across the antenna beam to form a profile as discussed with respect to  FIG. 7 . 
   Regardless of how the ideal read position discussed with respect to  FIG. 3  is determined, a verifier may then be located at this ideal position. This is akin to locating a bar code label verifier normally with respect to the bar code label surface. It will be appreciated that having found the ideal read position, the verifier being located at this ideal read position need not be configured to include any imaging capability as discussed with respect to  FIGS. 5 ,  6 , and  7 . Instead, a verifier that merely possesses the TSSI end RSSI capabilities discussed with respect to  FIG. 2  is sufficient. This verifier may classify tags according to levels of quality as discussed previously. Having been mounted at the ideal read position, the verifier may then be integrated or associated with a bar code printer. Bar code labels printed by the bar code printer supplement or duplicate RFID tag information as known in the art. An article having an RFID tag would also have a bar code label as printed by the bar code printer. However, because the verifier is associated with the bar code printer, articles having RFID labels that are not of a suitable quality level may be rejected immediately. An exemplary printer/verifier system  900  is shown in  FIG. 9 , Articles having RFID tags  905  are transported sequentially past a verifier  910 . When each RFID tag  905  is sequentially located such that verifier  910  is at the ideal read location, the transportation is momentarily stopped so that the ideally-located RFID tag  905  may be verified. For example, article  920  has been stopped so that its RFID tag  905  is the one being verified. An article  925  has already bad its tag verified. Thus, a bar code label  930  from a bar code printer has been applied to article  925 . After article  920  has had its tag verified, an article  940  may be transported to the ideal read location, stopped, and have its tag verified, and so on. Those articles whose RFID tags  905  are not of suitable quality will be identified so that their RFID tags  905  may be replaced. 
   Consider the advantages of system  900 —because the verification of RFID tags is context dependent, another verifier may be used to determine the worst-case scenario for subsequent verification of articles such as article  920 . Having been shipped to a customer or intermediate location such as a warehouse, a user at these subsequent locations will want to be assured that the previously-verified RFID tags  905  are still readable. The use of a verifier as discussed previously may determine the levels of quality and thus the transfer function from the production facility to these subsequent locations. For example, the transfer function may be such that an “A” level tag at the production facility becomes a “B” level tag in the context of a customer&#39;s warehouse. Similarly, a “B” level tag may become a “C” level tag under this transfer function. If the user determines that only “B” level tags are acceptable at its warehouse, then system  900  at the production facility will only pass “A” level tags given this transfer function. 
   As just described, verifier  910  bases its quality gradations for the verified tags solely upon the RF energy interrogation of the tag being verified. However, it will be appreciated that these gradations may also be affected upon other contextual information. For example, a user of verifier  910  may recognize that a certain class of articles are having their RFID tags verified. Alternatively, this recognition may be automated through a machine reading of SKU information. Given this contextual information, verifier  910  may alter its gradations accordingly. For example, whereas the same verified quality for one type of article may be classified as an “A” grade, this same verified quality for another type of article may be classified as a “B” grade. 
   It will be appreciated that numerous modifications may be made to the preceding description. For example, the scanning process may be automated. In an automated embodiment, a verifier may be movably located on a mechanized positioner. The verifier would control the mechanized positioner so that a scan may be performed. Accordingly, although the invention has been described with respect to particular embodiments, this description is only an example of the invention&#39;s application and should not be taken as a limitation. Consequently, the scope of the invention is set forth in the following claims.