Patent Publication Number: US-2007096882-A1

Title: Sensor based selection of radio frequency identification tags

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to the testing and programming of radio frequency identification (RFID) tag devices.  
      2. Background Art  
      Radio frequency identification (RFID) tags are electronic devices that may be affixed to items whose presence is to be detected and/or monitored.  
      The presence of an RFID tag, and therefore the presence of the item to which the tag is affixed, may be checked and monitored by devices known as “readers.” Readers typically transmit radio frequency signals to which the tags respond. Each tag can store a unique identification number. The tags respond to the reader transmitted signals by providing their identification number, bit-by-bit, so that they can be identified.  
      Tags are typically programmed and tested for proper performance prior to being sold. Future demand for RFID tags is estimated to be for over a billion tags a year. Having an accurate high-speed programming and test system that can support such volume is extremely critical. Currently, programming and test systems that can rapidly and reliably handle large volumes of tags do not exist. Current systems are extremely difficult to control and are reaching their limits in terms of the volume of tags that can be reliably programmed and tested.  
      Such systems can suffer from a variety of problems. For example, systems using radiated test signals sometimes unintentionally read adjacent tags, and thus have difficulty identifying a specific “bad” tag from a group of tags.  
      Furthermore, tags are susceptible to tampering by unauthorized sources. For example, an unauthorized source may attempt to read tags, re-program tags, or even “kill” tags, surreptitiously, by communicating with the tags.  
      Thus, what is needed are RFID tag programming and testing schemes which can handle very large volumes of tags, and can program and test the tags rapidly, in a reliable, secure, and repeatable fashion.  
     BRIEF SUMMARY OF THE INVENTION  
      Methods, systems, and apparatuses for selecting radio frequency identification (RFID) tags are described. In aspects of the present invention, a desired tag may be selected for interaction from a group of tags. The selection of a tag enables the interrogating, programming, testing, and/or other processing or operating on the tag, without interference from others of the nearby tags.  
      In an example aspect, a radio frequency identification (RFID) tag includes a substrate, an antenna on the substrate, an integrated circuit (IC) die mounted to the substrate, and a sensor that when stimulated enables a function of the tag.  
      In a further example aspect, a tag selector stimulates a sensor of the tag to enable a tag. A tag processor interacts with the enabled tag. The tag processor can test, program, and/or otherwise interact with the tag, while enabled. In this manner large numbers of tags can be interacted with in close proximity, such as during their manufacture in a web format, because the tag selector dictates which tag(s) are enabled at any one time.  
      These and other advantages and features will become readily apparent in view of the following detailed description of the invention. Note that the Summary and Abstract sections may set forth one or more, but not all exemplary embodiments of the present invention as contemplated by the inventor(s).  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES  
      The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.  
       FIG. 1  shows a plan view of an example radio frequency identification (RFID) tag.  
       FIG. 2  shows a plan view of an example web of tags that is a continuous roll type.  
       FIG. 3  shows an example block diagram of a tag interaction system, according to an embodiment of the present invention.  
       FIG. 4  shows a flowchart providing a process for interacting with tags, according to an example embodiment of the present invention.  
       FIGS. 5-7  show example types of tags, according to embodiments of the present invention.  
       FIG. 8  shows an example web-based tag interaction system, according to an embodiment of the present invention.  
       FIGS. 9-15  show example types of sensors and selector elements, according to embodiments of the present invention. 
    
    
      The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears.  
     DETAILED DESCRIPTION OF THE INVENTION  
      Introduction  
      The present invention relates to the testing of radio frequency identification (RFID) tags. According to embodiments of the present invention, a function of a tag is enabled by stimulation of a sensor of the tag. The enabled tag can be interacted with. For example, the tag can be tested, programmed, killed, interrogated, or otherwise processed or operated on. Other surrounding tags have not been stimulated, and thus do not respond to the attempts to interact with the tag. In this manner large numbers of tags in close proximity can be processed, such as during their manufacture in a web format, because only a selected tag is enabled at any one time.  
      It is noted that references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.  
      Tag Selection and Interaction Embodiments  
      The present invention is applicable to any type of RFID tag.  FIG. 1  shows a plan view of an example radio frequency identification (RFID) tag  100 . Tag  100  includes a substrate  102 , an antenna  104 , and an integrated circuit (IC)  106 . Antenna  104  is formed on a surface of substrate  102 . Antenna  104  may include any number of one or more separate antennas. IC  106  includes one or more integrated circuit chips/dies, and can include other electronic circuitry. IC  106  is attached to substrate  102 , and is coupled to antenna  104 . IC  106  may be attached to substrate  102  in a recessed and/or non-recessed location. IC  106  controls operation of tag  100 , and transmits signals to, and receives signals from RFID readers using antenna  104 . Tag  100  may additionally include further elements, including an impedance matching network and/or other circuitry. The present invention is applicable to tag  100 , and to other types of tags, including surface wave acoustic (SAW) type tags.  
      Volume production of RFID tags, such as tag  100 , is typically accomplished on a printing web based system. For example, in such a system, the tags are assembled in a web of substrates, which may be a sheet of substrates, a continuous roll of substrates, or other grouping of substrates. For instance,  FIG. 2  shows a plan view of an example web  200  that is a continuous roll type. As shown in  FIG. 2 , web  200  may extend further in the directions indicated by arrows  210  and  220 . Web  200  includes a plurality of tags  100   a - p . In the example of  FIG. 2 , the plurality of tags  100   a - p  in web  200  is arranged in a plurality of rows and columns. The present invention is applicable to any number of rows and columns of tags, and to other arrangements of tags.  
      On a web, such as web  200 , RFID tags are typically assembled/positioned as close to each other as possible to maximize throughput, thus making the process of reading, programming, killing, and/or testing individual tags difficult. For example, it may be desired to program a tag, such as writing an identification number and/or other data to the tag. Furthermore, it may be desired to run a test algorithm for the tag to test its operation. Because of the close spacing in web  200 , it is very difficult to localize a radiated (e.g., radio frequency) reader field to excite only one tag.  
      According to embodiments of the present invention, a tag selection configuration is used to select individual tags, even for tags positioned in close quarters, so that the selected tags can be interrogated, tested, programmed, killed, or otherwise interacted with, in a more reliable, secure, and repeatable fashion than in conventional schemes. In embodiments of the present invention, a tag selector interacts with a tag by stimulating a sensor of the tag.  
      Embodiments of the present invention are applicable to interacting with tags  100  in web  200 . Tags may also be interacted with in other environments. In embodiments, tags may be interacted with in tag assembly/manufacture environments, in warehouse environments, in retail environments, etc.  
      For example,  FIG. 3  shows an example block diagram of a tag interaction system  300 , according to an embodiment of the present invention. System  300  includes a tag selector  302  and a tag processor  304 . As shown in  FIG. 3 , tag  100  includes a sensor  306 . In embodiments, tag  100  can include any type of sensor, and any number of one or more sensors, as desired for the particular application. Example sensor types are described in detail further below.  
      Example operation of system  300  is described with respect to  FIG. 4 .  FIG. 4  shows a flowchart  400  providing example steps for interacting with tags, according to an example embodiment of the present invention. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion related to flowchart  400 .  
      Flowchart  400  begins with step  402 . In step  402 , a tag is received having a sensor. For example, the tag is tag  100  shown in  FIG. 1 . As shown in  FIG. 3 , tag  100  includes sensor  306 . Sensor  306  can be present anywhere in tag  100 . For example, as shown in  FIG. 4 , sensor  306  can be mounted to, or otherwise formed in or on substrate  102  of tag  100 . Alternatively, sensor  306  can be formed in or on IC die  106 , as shown in  FIG. 5 .  
      In step  404 , the sensor of the tag is stimulated to enable a function of the tag. For example, as shown in  FIG. 3 , tag selector  302  generates a tag sensor stimulus  308 . Tag sensor stimulus  308  stimulates sensor  306  such that sensor  306  is activated, causing a functionality of tag  100  to be enabled. For instance, the stimulus to sensor  306  may change an electrical and/or mechanical feature (e.g., make or break an electrical connection) of tag  100 , to turn on or off an applicable portion of tag  100  to enable the function. For example, the functionality may be a program module of tag  100  that relates to the programming of an identification number and/or other data into tag  100 . In another example, the functionality that is enabled may be a “kill” function of tag  100 , as further described below. In still another example, the full functionality of tag  100  is enabled, such that tag  100  may be interrogated and/or tested. According to embodiments, any portion of the functionality of tag  100  may be configured to be enabled by sensor  306 , depending on the particular application.  
      In step  406 , the enabled function of the tag is interacted with. In other words, the functionality of tag  100  enabled by stimulus of sensor  306 , may be interacted with, including partial or full functionality of the tag. As shown in  FIG. 304 , tag processor  304  may be used to interact with the enabled function. Tag processor  304  generates tag interaction signal  310 . Tag interaction signal  310  is shown as bi-directional in  FIG. 3 , but may also be unidirectional. Tag interaction signal  310  interacts with the enabled function of tag  100 . Steps  406   a - 406   c  shown in  FIG. 4  provide examples of how tag interaction signal  310  may interact with tag  100 .  
      For example, as shown in  FIG. 4 , step  406  may include interrogating the tag, as in step  406   a . Thus, for example, tag processor  304  may include RFID reader functionality to read an identification number stored in tag  100 , and/or to otherwise interrogate tag  100 . As shown in  FIG. 7 , tag  100  may include storage  702 . Storage  702  may reside in IC die  106  or elsewhere in or on tag  100 . Step  406   a  can be performed to verify a portion or all of an identification number and/or other data stored in storage  702  of tag  100 .  
      In another example, step  406  may include programming the tag, as in step  406   b . Thus, for example, tag processor  304  may include functionality to write an identification number and/or other data into storage  702  of tag  100 . Thus, in an embodiment, tag processor  304  may include a programming module, having the hardware, software, and/or firmware necessary to program tags.  
      In another example, step  406  may include testing the tag, as shown in step  406   c . Thus, for example, tag processor  304  may include functionality to perform a partial or full diagnostic test of tag  100 . Thus, in an embodiment, tag processor  304  may include a test module, having the hardware, software, and/or firmware necessary to test tags.  
      In another example, step  406  may include killing the tag. For example, in step  404 , tag selector  302  stimulates sensor  306 , such that sensor  306  enables a “kill” functionality of tag  100 . When the kill functionality is enabled, tag  100  may be killed (e.g., be made unable to be communicated with) by a kill signal. For example, when an item attaching tag  100  is sold in a retail environment, it may be desired to kill tag  100  so that it is no longer operational. This may be done to address privacy concerns, so that tag  100  cannot be later read. Thus, at a checkout area, for example, tag  100  may be brought near tag selector  302 , to enable the kill functionality. Then, before the item leaves the store, a kill signal source (e.g., tag processor  304 ) can be used to kill tag  100  that has the kill functionality enabled. In this manner, tags in the vicinity that are not desired to be killed are not accidentally killed by the kill signal source, and only tags associated with items that have been sold are killed.  
      In another example, step  404  may include enabling a communication functionality of the tag, such as after a kill signal source has previously killed the tag. Thus, tag selector  302  stimulates sensor  306  to enable a killed tag  100  to communicate. In this manner, a previously killed tag  100  could be re-used. For example, in step  406 , the tag could be re-programmed, etc., in a warehouse, by a person at home who purchased an item having the killed tag attached thereto, etc.  
      In an embodiment, once the tag selector stimulus is no longer applied to the tag sensor, the function of the tag is no longer enabled. Subsequently, a next tag can be enabled through application of the stimulus. Furthermore, an entity, such as tag processor  304 , can interact with the enabled next tag. This interaction with the next tag can be performed without interference from the prior tag, which is no longer enabled.  
      Alternatively, in an embodiment, after a tag selector stimulus is applied to a tag, the tag is changed to and remains in the enabled state. Thus, in this alternative embodiment, the transition of the tag to the enabled state is permanent. In alternative to this embodiment, a second application of the stimulus can be used to disable the tag once again. Thus, in this embodiment, the tag selector stimulus can be used to toggle a tag between enabled and disabled states. For example, in a possible application, the tag selector stimulus could be applied to each tag twice, to temporarily enable and disable each tag in a web of tags for interaction therewith (e.g., programming, testing, killing, and/or communicating), and subsequently the tag selector stimulus could be applied a third time to each tag to permanently enable the tags to be used in the field once they leave the interaction station. Other mechanisms may be used to enable tags leaving the interaction station for operation in the field, as would be known to persons skilled in the relevant art(s) from the teachings herein. For example, in an embodiment, after testing, programming, etc., the sensor functionality of the tags could be killed by a kill signal or stimulus, to leave the tags permanently in the enabled state.  
      Thus, tag processor  304  can interact with enabled functions of tag  100  in the manners described above, elsewhere herein, and in any other way. Tag selector  302  and tag processor  304  each include software, hardware, and/or firmware, or any combination thereof, for selecting and interacting with tags, respectively. Tag selector  302  and tag processor  304  may be incorporated together into a computer system. Tag selector  302  and/or tag processor  304  can further include one or more storage devices for storing information regarding system  300  and tags being interacted with, including memory components, disc-based storage, magnetic storage devices, optical storage, etc. Furthermore, tag selector  302  and/or tag processor  304  can together or separately include a user interface, such as including a keyboard, display, graphical user interface (GUI), pointing device, and/or other visual and/or audio indicators, for a user to interact with tag selector  302  and/or tag processor  304  as needed.  
      In embodiments, tag processor  304  generates one or more interrogation signals or test signals to test tags. For example, test controller  302  may communicate with a tag according to any RFID communication protocol. Tag processor  304  may generate the signal(s) according to one or more interrogation/read protocols, as would be known to persons skilled in the relevant art(s), to read/communicate with tags under test. Example such protocols include binary protocols, tree traversal protocols, slotted aloha protocols, and those required by the following standards: Class 0; Class 1; and EPC Gen 2. Any future developed communication algorithms/protocols are also within the scope and spirit of the present invention.  
      As described above, the tag processors described herein can include elements of conventional RFID readers. For example, depending on the particular application, a tag processor may incorporate one or more antennas, power controls, and read and write capabilities of an RFID reader, to conduct the interrogation and/or testing of tags. For instance, example conventional readers having features that are applicable to the embodiments of the present invention include AR400 and XR400 readers sold by Symbol Technologies of Holtsville, N.Y. The AR400 and XR400 are example 4-port readers that may be used in a “multi-channel” testing configuration, such as shown in  FIG. 8 , described further below. Such readers include also reader/printers, such as manufactured by Zebra Technologies Corporation of Vernon Hills, Illinois, and others, that combine tag programming with label printing. Handheld readers are also included, such as sold by Symbol Technologies and others.  
      An enabled tag  100  processes tag interaction signal  310  received from tag processor  304 . The enabled tag  100  generates a corresponding response if appropriate (e.g., when being tested and/or interrogated). Tag processor  304  evaluates the response of tag  100  to determine whether the enabled function responded properly (if a response is expected).  
      For example, in a test interaction, tag processor  304  may evaluate the response of tag  100  to determine whether tag  100  is operating properly. For instance, the test signal(s) of tag processor  304  may have interrogated tag  100  for its identification number. Test controller  302  evaluates whether tag  100  properly responded with its identification number. In further embodiments, data other than the identification number can be read from tag  100 , to test other data, storage elements, and/or features of tag  100 . In embodiments, any type of test may be performed, to test any feature, parameter, characteristic, etc., of tag  100 .  
      If during an example test, the identification number is properly received from tag  100  (and/or the tag otherwise responds properly), tag processor  304  determines that tag  100  has passed the test, and tag  100  can proceed accordingly. For example, in an embodiment, tag processor  304  may provide an indication that tag  100  passed the test by illuminating an indicator light, by displaying test result information on a graphical display, by storing test result information in storage, and/or by taking other action (or no action).  
      If the identification number is improperly received (and/or the tag otherwise responds improperly), tag processor  304  determines that tag  100  did not pass the test, and may not be functioning properly. For example, an improperly functioning tag may generate a response that is incorrect (i.e., is not the response expected from the tag for the particular test being performed, including a non-response). In such a situation, tag processor  304  may provide an indication that tag  100  failed the test by marking tag  100  as defective, by illuminating an indicator light, by displaying test result information on a graphical display, by storing the test result information in storage, and/or by taking other action. In this manner, the failed tag  100  can subsequently be repaired, disposed, or recycled.  
      In embodiments, any number of interactions can be performed with a particular tag, as long as the tag is enabled. Furthermore, in embodiments, multiple tags received in parallel may be interacted with according to embodiments. For example,  FIG. 8  shows a web-based system  800 , according to an embodiment of the present invention. As shown in  FIG. 8 , system  800  includes tag processor  304 , a computer  802 , a motor controller  804 , a selector mount  806 , and one or more selector elements  808 . Three selector elements  808   a - c  are shown in  FIG. 8  for illustrative purposes. However, any number of one or more selector elements  808  may be present, depending on the particular application.  
      In embodiments, system  800  may be incorporated into a tag assembly line (TAL), which may be a partially or fully automated assembly line. In the example of  FIG. 8 , a tag assembly line receives a continuous roll  812  of substrates, as web  200 . Web  200  includes a plurality of substrates arranged in an array. Web  200  has a width in the X-direction (i.e., into the paper of  FIG. 8 ) that is one or more substrates across. Web  200  has a length in the Y-direction that is substantially continuous (e.g., the length of a roll), and typically many substrates long. At one or more locations (not shown in  FIG. 8 ) of the assembly line prior to a tag interaction station, dies  106  are applied to the substrates of web  200 , and further tag assembly may occur, to produce tags  100  in web  200 .  
      Once tags  100  have been assembled in web  200  to the extent that they are functional, they can be interacted with using system  800 , for programming, test, etc. Computer  802  is coupled through a communications link  810  to motor controller  804 . Computer  802  provides control signals to control operation of motor controller  804  over communications link  810 , and may receive feedback from motor controller  804  over communications link  810 , if appropriate for a particular application. Motor controller  804  causes roll  812  and/or further wheels and/or spools coupled to web  200  to advance web  200 .  
      In the embodiment of  FIG. 8 , computer  802  and sensor elements  808  include functions of tag selector  302  of  FIG. 3  further described above. Computer  802  is coupled to selector mount  806  through a communications link  820 . Selector mount  806  is a mount for a plurality of selector elements  808   a - 808   c . Selector elements  808   a - 808   c  are each configured to provide a stimulus (similar to tag sensor stimulus  308  described above) to a corresponding tag of web  200 , when instructed by computer  802 . Typically, a single one of selector elements  808  provides a stimulus at any one time, so that one tag is interacted with at a time, but multiple simultaneous stimuli are possible in some embodiments (e.g., when shielding is used to shield individual tags on the web, etc.). Note that although selector elements  808  are shown being applied to a top side of web  200  in  FIG. 8 , alternatively, selector elements  808  could be applied to a bottom side of tags  100  of web  200 .  
      A single width row of selector elements  808  can be present to operate on a row of tags  100  of web  200 , or a two-dimensional array of selector elements  808  can be present in system  800 , to operate on a multiple rows of tags  100  web  200 . Web  200  can be periodically or continuously advanced, such that subsequent rows of tags can be operated on in a similar fashion by selector elements  808 . This process can continue until interaction with all the tags of web  200  is complete.  
      Alternatively, a single selector element  808  may be present in system  800 . In such an embodiment, the single selector element  808  may be directed (e.g., aimed) or moved (e.g., by selector mount  806 ) as needed to operate on tags  100  at different positions on web  200 . For example, in a laser selector embodiment, a scanning laser could be used (e.g., to provide a heat pulse), enabling tags one at a time on web  200  by being sequentially aimed at the tags.  
      Computer  802  is coupled to tag processor  304  through a communications link  830 . Tag processor  304  is configured to provide tag interaction signal  310 , under control of computer  802 , to interact with a particular tag  100  of web  200  that is enabled by a sensor element  308 . If appropriate, tag processor  304  is configured to receive responses from the particular tag  100  being interacted with. Tag processor  304  may radiate tag interaction signal  310  to a tag through the air, as shown in  FIG. 8 , or may make indirect or direct contact with the tag, depending on the particular application.  
      Computer  802  uses selector elements  808  to sequentially stimulate each tag  100  of web  200 , one at a time, to sequentially enable a function of each tag  100 . Tag processor  304  sequentially interacts with each stimulated tag  100  to interact with the tag function while enabled. In this manner, system  800  allows separate interaction with each of tags  100  of web  200 .  
      Once tags  100  are interacted with (e.g., programmed and tested), further processing may be performed on tags  100 , including processing tags  100  into label format, singulation of web  200  into separate tags, removal of failed tags, etc.  
      Note that selector mount  806  of  FIG. 8  is shown for illustrative purposes, and that any type of mount may used, as would be understood by persons skilled in the relevant art(s), including individual mounts for each selector element, etc.  
      System  800  is shown for illustrative purposes, and not for purposes of limitation. Embodiments of the present invention may be implemented in a variety of systems. For example, label printers exist that print a bar code label, while programming a RFID tag embedded in the label. In such an application, the label printer (hand-held or otherwise) may include a selector element  808 , such as a heating head, that is pulsed to enable programming of the tag of a label currently being spooled and printed. Thus, a label currently being spooled over a test head of the label printer can be tested without impacting other tags on the label spool. Further systems and applications for selection and interaction with tags will become known to persons skilled in the relevant art(s) from the teachings herein.  
      As described above, a variety of types of sensors  306  may be present in tags  100 . Thus, various corresponding types of selector elements  808  may be used to produce a corresponding tag sensor stimulus  308  to stimulate the sensors.  FIGS. 9-15  show example types of sensors  306  and corresponding selector elements  808 , according to embodiments of the present invention.  
       FIG. 9  shows tag  100  including a temperature sensor  906 . In  FIG. 9 , selector element  808  is a heat source  902  that applies heat  904  to temperature sensor  906  to stimulate temperature sensor  906 . Heat source  902  can be any heat source, including a source of radiated heat and conducted heat, including a heated head, or a hot gas flow nozzle. In another example, heat source  902  may be a laser  1002 , such as shown in  FIG. 10 . As shown in  FIG. 10 , laser  1002  (such as a low power laser) emits a laser beam  1004  used to heat temperature sensor  906 , to stimulate temperature sensor  906 . Temperature sensor  906  can be any type of component or material that suitably changes a measurable characteristic with temperature, including a thermistor, a metal (e.g., expands (has a suitable coefficient of thermal expansion, CTE), changes in electrical conductivity, etc.) or other material. In an example embodiment, temperature sensor  906  can be a temperature gradient sensing device in IC die  106  that detects a small but sudden rise in temperature from a heating head of heat source  902 .  
       FIG. 11  shows tag  100  including an optical sensor  1106 . In  FIG. 11 , selector element  808  is a light source  1102  that emits light  1104  to optical sensor  1106  to stimulate optical sensor  1106 . Light source  1102  can be any type of applicable light source, including a light bulb, light emitting diode, laser, etc. In example embodiments, optical sensor  1106  can be one or more photodetectors, such as semiconductor photodiodes or phototransistors that are fabricated into IC die  106 .  
       FIG. 12  shows tag  100  including a magnetic sensor  1206 . In  FIG. 12 , selector element  808  is a magnetic field source  1202 , such as a magnet (including an electromagnet) that generates a magnetic field  1204  to stimulate magnetic sensor  1206 . Magnetic sensor  1206  can include any type of material that suitably changes a measurable characteristic in a magnetic field, including a Hall effect device, a metal (e.g., that bends), permalloy, or other material.  
       FIG. 13  shows tag  100  including a vibration sensor  1306 . In  FIG. 13 , selector element  808  is a vibration source  1302 , that may include a contact member  1308  for making contact with tag  100 , that generates a vibration  1304  to stimulate vibration sensor  1306 . For example, vibration source  1302  can be any source that can provide any suitable vibration frequencies, including ultrasound frequencies. Vibration sensor  1306  can be any type of vibration sensor, including a piezo-electric membrane, a micro-electrical-mechanical system (MEMS) element fabricated into IC die  106  or otherwise formed on or mounted to tag  100 , or any other type of vibration sensor, including an ultrasonic sensor.  
       FIG. 14  shows tag  100  including a pressure sensor  1406 . In  FIG. 14 , selector element  808  is a pressure source  1402  that provides a pressure  1404  to stimulate pressure sensor  1406 . Pressure source  1402  may be any type of pressure source, and may include a contact member  1408  for making contact with tag  100  (as shown in  FIG. 14 ), a gas source to apply a directed gas pressure, etc. Pressure sensor  1406  can be any type of pressure sensor, including a strain gauge, a piezo-electric sensor, a switch, etc.  
      Note that contact members  1308  and  1408 , when present, may include a spring and/or other shock-absorption mechanism, to prevent damage to tag  100  when they make contact therewith.  
       FIG. 15  shows a cross-sectional view of a MEMS cantilever  1502  formed in or on a substrate  1504 , which may be die  106 , substrate  102 , or other portion of tag  100 . Cantilever  1502  may be used as vibration sensor  1306  or pressure sensor  1406 . For example, when cantilever  1502  is vibrated, or when sufficient pressure is applied to cantilever  1502 , an end  1506  of cantilever may make contact with substrate  1504  to activate the sensor. For instance, a contact area  1508  on end  1506  of cantilever  1502  may be an electrically conductive material that makes electrical contact with a contact area  1510  on substrate  1504  when cantilever  1502  bends, to create an electrical current path, thereby allowing cantilever  1502  to operate as a switch. Cantilever  1502  can be formed in a variety of ways, including standard photolithography and other MEMS fabrication techniques.  
      In embodiments, the tag selection techniques described herein allow interaction with tags in an independent and sequential manner. The tag selection techniques also reduce the possibility of tags being read, re-programmed, or killed by unauthorized sources, because interaction with the tags requires application of the tag selector stimulus.  
      Conclusion  
      While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.