Abstract:
A transponder that may be used as an RFID tag includes a passive circuit to eliminate the need for an “always on” active RF receiving element to anticipate a wake-up signal for the balance of the transponder electronics. This solution allows the entire active transponder to have all circuit elements in a sleep (standby) state, thus drastically extending battery life or other charge storage device life. Also, a wake-up solution that reduces total energy consumption of an active transponder system by allowing all non-addressed transponders to remain in a sleep (standby) state, thereby reducing total system or collection energy. Also, the transponder and wake-up solution are employed in an asset tracking system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 12/963,773 entitled Methods and Apparatus for Switching a Transponder to an Active State, and Asset Management Systems Employing Same” filed Dec. 9, 2010, now U.S. Pat. No. 8,022,826, which is a divisional of U.S. patent application Ser. No. 11/678,296 entitled “Methods and Apparatus for Switching a Transponder to an Active State, and Asset Management Systems Employing Same” filed Feb. 23, 2007, now U.S. Pat. No. 7,876,225, which claims the benefit of U.S. Provisional Application No. 60/776,046, entitled “Methods and Apparatus for Switching a Transponder to an Active State, and Asset Management Systems Employing Same,” which was filed on Feb. 23, 2006, the disclosure of which is incorporated herein by reference. 
    
    
     GOVERNMENT CONTRACT 
     This work was supported in part by a grant from NASA under Contract No. NNK040AZ9C. The United States government may have certain rights in the invention described herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to transponders, such as RFID tags, and in particular to methods and apparatus for switching a transponder to an active state and asset managements system that employ such transponders. 
     BACKGROUND OF THE INVENTION 
     The use of radio frequency identification (RFID) systems is expanding rapidly in a wide range of application areas. RFID systems consist of a number of radio frequency tags or transponders (RFID tags) and one or more radio frequency readers or interrogators (RFID readers). The RFID tags include one or more integrated circuit (IC) chips, such as a complementary metal oxide semiconductor (CMOS) chip, and an antenna connected thereto for allowing the RFID tag to communicate with an RFID reader over an air interface by way of RF signals. In a typical RFID system, one or more RFID readers query the RFID tags for information stored on them, which can be, for example, identification numbers, user written data, or sensed data. RFID systems have thus been applied in many application areas to track, monitor, and manage items as they move between physical locations. 
     RFID tags can generally be categorized as either passive tags or active tags. Passive RFID tags do not have an internal power supply. Instead, the relatively small electrical current induced in the antenna of a passive RFID tag by the incoming RF signal from the RID reader provides enough power for the IC chip or chips in the tag to power up and transmit a response. Most passive RFID tags generate signals by backscattering the carrier signal sent from the RFID reader. Thus, the antenna of a passive RFID tag has to be designed to both collect power from the incoming RF signal and transmit (or reflect, e.g., backscatter) the outbound backscatter signal. Due to power limitations, the ability to provide devices such as sensors or microprocessors on passive RFID tags is limited. Passive RFID tags do, however, have the advantage of a near unlimited lifetime as they obtain their power from the RF signal sent from the RFID reader. 
     Active RFID tags, on the other hand, have their own internal power source, such as, without limitation, a battery, a fuel cell or what is commonly known as a super capacitor. The internal power source is used to power the IC chip or chips and discrete circuit elements, which typically include an RF receiver, an RF transmitter, and some type of controller, such as microcontroller or other processor, and any other electronics provided on the active RFID tag. As a result, active RFID tags can include relatively high power devices such as sensors, microprocessors, receivers and transmitters. Also, because of the on-board power, active RFID tags typically have longer ranges and larger memories than passive RFID tags. The internal power source, however, also means that active RFID tags typically have a lifetime that is limited by the lifetime of the power source. Thus, periodic maintenance is required. 
     As noted above, multiple active RFID tags may be used to track, monitor, and manage multiple items/assets as they move between physical locations. In such an application, each active RFID tag is affixed to an item/asset that is located in a particular location or environment, such as in a building. Building shall refer to any structure including, without limitation, a warehouse, a hospital, an office building, or even a vehicle. In current RFID systems, the active RFID tags, when deployed in such a manner, are done so in a state where (i) an RF receiver of the tag is in an active state for receiving RF signals, and (ii) the controller is in a low power inactive (sleep) state to preserve power. When one or more of the active RFID tags are to be queried, the RFID reader sends out a wake-up signal that is received by the RF receiver of each tag. Tags may also be on continuously not requiring a wake-up signal. Upon receipt of the signal, the RF receiver in each tag will then send a signal to the controller of the tag that causes it to move from the inactive state to an active (wake-up) state. For example, in RFID systems implemented according to the ISO 18000 Part 7 standard, when one or more tags are to be queried, the reader will send out a 30 KHz tone lasting for a period of approximately 2.5 seconds. Upon receipt of the tone, the RF receiver in each tag will wake-up the controller in the tag. The RFID reader then sends out signals intended for particular ones of the tags. Those particular tags for which the signals are intended will then perform the requested action, and the remaining tags (i.e., those tags not currently of interest to the reader) will move back to a sleep state. 
     The multiple active RFID tag arrangement just described presents at least two power management problems. First, each active RFID tag that is deployed is required to have at least one component, i.e., an RF receiver, in an active, relatively high power consuming state at all times so that it can listen for the wake-up signal. Second, when the RFID reader needs to query one or more particular tags, all of the tags that are deployed are woken up (for example, according to the ISO 18000, Part 7 standard), i.e., their controllers are caused to move to an active, relatively high power consuming state. Only when a particular tag determines that the query in question is not intended for it will it then move back to the sleep state. As will be appreciated, these problems result in unnecessary use of power from the power source (e.g., battery) of each tag, and therefore decreases the lifetime of each tag. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes at least two problems associated with (1) current active RFID tags, and (2) current active RFID tag wake-up protocols. The first problem is that in current RFID tags, an active RF receiving element must always be awake to anticipate a wake-up signal for the balance of the tag electronics. The present invention uses a passive circuit to eliminate the need for an “always on” active RF receiving element to anticipate a wake-up signal for the balance of the tag electronics. This solution allows the entire active RFID tag to have all circuit elements in a sleep (standby) state, thus drastically extending battery life or other charge storage device life and thus essentially eliminating shelf maintenance on the active RFID tag. The second problem is that in current active RFID tag systems, the electronics of all of the RFID tags in a system are awakened in response to wake-up signals even if the signal is not intended for a particular tag or tags. The present solution provides a major energy saving circuit that eliminates the need to wake up all of the RFID tags in response to each wake-up signal. This circuit thus reduces total energy consumption of an active RFID tag (or sensor) system or collection of devices by allowing all non-addressed tags (sensors) to remain in a sleep (standby) state, thereby reducing total system or collection energy. This second circuit can be used in conjunction with the first passive circuit mentioned above or in conjunction with any existing active RFID tag (sensor) systems. Thus, the elements of the present invention capitalize on the benefits of an active RFID tag while eliminating the problems discussed above, thus moving active RFID tags closer to a passive tag operation. 
     In one embodiment, the present invention relates to a transponder apparatus, such as, without limitation, an RFID tag, that includes an electronic device, such as a processing unit (e.g., microprocessor or microcontroller), that is capable of being in an inactive, sleep state and an active state, a power source, such as a battery, in electronic communication with the electronic device for providing power to the electronic device, and a switch having an antenna for receiving at least one RF signal. The switch converts the at least one RF signal into at least one DC signal. The at least one DC signal causes the electronic device to move from the inactive, sleep state to the active state. Preferably, the switch does not require power from the power source or another power source within or connected to the transponder apparatus. The switch may, in one embodiment, include a rectifying circuit, such as a charge pump, for converting the at least one RF signal into the at least one DC signal. The switch in that embodiment may also further include a matching network electrically connected to the antenna, wherein the charge pump is electrically connected to an output of the matching network. In yet another embodiment, the transponder apparatus may further include an RF transmitter and/or an RF receiver that is/are in electronic communication with the processing unit. 
     In one particular embodiment, the antenna is tuned to a particular frequency or range of frequencies. In this embodiment, the at least one RF signal has a frequency that is substantially equal to the particular frequency or is within the range of frequencies. 
     The at least one DC signal may be provided to the electronic device to directly cause the electronic device to move from the inactive, sleep state to the active state. Alternatively, the transponder apparatus may further include a filtering circuit in electronic communication with the switch, wherein the at least one DC signal is provided to the filtering circuit. The filtering circuit provides a wake-up signal to the electronic device to cause the electronic device to move from the inactive, sleep state to the active state only if the at least one DC signal and/or the at least one RF signal has a predetermined format, such as a predetermined number of bursts each having a predetermined duration. 
     In another embodiment, the transponder apparatus further includes one or more additional switches that each has an additional antenna for receiving at least one additional RF signal. Each additional switch converts the at least one additional RF signal into at least one additional DC signal. The at least one RF signal and each of the at least one additional RF signals have different frequencies. In this embodiment, a logical combination of the at least one DC signal and one or more of the at least one additional DC signals causes the electronic device to move from the inactive, sleep state to the active state. Alternatively, the electronic device may be caused to move from the inactive, sleep state to the active state only if the at least one DC signal and the at least one additional DC signals are created in a particular sequence. 
     In another embodiment, the present invention relates to a method of moving an electronic device, such as a processing unit, included in a transponder apparatus from an inactive, sleep state to an active state, wherein the electronic device consumes power from a power source of the transponder apparatus in the active state. The method includes receiving at least one RF signal and converting the at least one RF signal into at least one DC signal without consuming power from the power source or another power source within or connected to the transponder apparatus. The method further includes providing a wake-up signal to the electronic device in response to receipt of the at least one RF signal that causes the electronic device to move from the inactive, sleep state to the active state. The wake-up signal of the method may, in one particular embodiment, be the at least one DC signal. Alternatively, the method may include determining whether the at least one DC signal or the at least one RF signal has a predetermined format, wherein the providing step comprises providing the wake-up signal (which is separate from the at least one DC signal) to the electronic device to cause the electronic device to move from the inactive, sleep state to the active state only if it is determined that the at least one DC signal or the at least one RF signal has the predetermined format. The predetermined format may include a predetermined number of bursts, each of the bursts having a predetermined duration. 
     In another embodiment, the method further includes receiving at least one additional RF signal and converting the at least one additional RF signal into at least one additional DC signal without requiring the consumption of power from the power source or another power source within or connected to the transponder apparatus. In this embodiment, the at least one RF signal and each of the at least one additional RF signals have different frequencies, and the providing step comprises providing the wake-up signal to the electronic device to cause the electronic device to move from the inactive, sleep state to the active state only in response to a logical combination of the at least one DC signal and one or more of the at least one additional DC signals. Alternatively, the wake-up signal may be provided to the electronic device only if the at least one DC signal and the at least one additional DC signals are created in a particular sequence and/or if the at least one RF signal and each of the at least one additional RF signals are received in a particular sequence. 
     Another aspect of the present invention relates to a system for tracking a plurality of assets that includes a central computer system that maintains a plurality of records relating to the assets, and a plurality of transponders, wherein each of the transponders is associated with a respective one of the assets and stores an identifier identifying the particular asset with which it is associated. Each of the transponders includes an electronic device capable of being in an inactive, sleep state and an active state, a power source in electronic communication with the electronic device for providing power to the electronic device, and a switch having an antenna for receiving at least one RF signal that is generated at the direction of the central computer system. The switch converts the at least one RF signal into at least one DC signal that causes the electronic device to move from the inactive, sleep state to the active state. When the electronic device is in the active state, the transponder generates and transmits a response signal including the identifier identifying the particular asset with which the transponder is associated. The response signal is then used to update a particular one of the records maintained by the central computer system relating to the asset identified by the identifier in the response signal. Preferably, the switch in each of the transponders does not require power from the power source of the transponder or another power source within or connected to the transponder. The transponders may be any of the various embodiments described above. As such, individual transponders or groups of transponders may be selectively awakened. 
     In one particular embodiment, the system further includes a network with which the central computer system may communicate, a plurality of wireless access points in electronic communication with the network, and a plurality of interface devices. Each of the interface devices is adapted to (i) wirelessly communicate with at least one of the wireless access points, (ii) receive the response signal transmitted by a particular one or more of the transponders, and (iii) generate and transmit to the at least one of the wireless access points at least one second response signal that includes each identifier that was included in each response signal received by the interface device. Each at least one second response signal is transmitted to the central computer system through the network. The central computer system uses the at least one second response signal received from one or more of the interface devices to update one or more of the records. 
     In another particular embodiment, the system further includes a network with which the central computer system may communicate, and a plurality of interface devices. Each of the interface devices is adapted to (i) communicate with the network, (ii) receive the response signal transmitted by a particular one or more of the transponders, and (iii) generate and transmit to the network, through a wired or wireless connection, at least one second response signal that includes at least each identifier that was included in each response signal received by the interface device. Each at least one second response signal is transmitted to the central computer system through the network, and the central computer system uses the at least one second response signal received from one or more of the interface devices to update one or more of the records. 
     In either of these two just described embodiments, the assets are located within an environment such as one or more building (e.g., a hospital), and each of the interface devices may be associated with a particular location within the environment. In addition, each of the second response signals may include an identification of the interface device from which it was transmitted, and the central computer system may use the identification included in each second response signal to update in the records a location of one or more of the assets. 
     In one particular embodiment, each of the transponders has a code associated therewith, and wherein for each of the transponders in order for the at least one RF signal received by the transponder to be converted in at least one DC signal that will cause the electronic device of the transponder to move from the inactive, sleep state to the active state, the at least one RF signal received by the transponder must be formatted according to the code associated with the transponder. In another particular embodiment, each of one or more groups of selected ones of the transponders have a code associated therewith, and wherein for each of the groups of transponders in order for the at least one RF signal received by each transponder in the group to be converted into at least one DC signal that will cause the electronic device of the transponder in the group to move from the inactive, sleep state to the active state, the at least one RF signal received by the transponder in the group must be formatted according to the code associated with the group of transponders. Thus, individual transponders or groups of transponder may be selectively awakened. 
     In an alternative embodiment, the present invention relates to a system for tracking a plurality of assets that includes a central computer system maintaining a plurality of records relating to the assets, a network, wherein the central computer system is in electronic communication with the network, and a plurality of interface devices each being adapted to communicate with the network. In addition, the system includes a plurality of transponders, wherein each of the transponders is associated with a respective one of the assets and stores an identifier identifying the particular asset with which it is associated. Each of the transponders is adapted to receive from one of the interface devices at least one RF signal that is generated at the direction of the central computer system and in response thereto to generate and transmit a response signal including the identifier identifying the particular asset with which the transponder is associated. Each of the interface devices is adapted to (i) receive the response signal that is transmitted by each of a particular one or more of the transponders, and (ii) generate and transmit to the network at least one second response signal that includes each identifier that was included in each response signal received by the interface device. In each case, the at least one second response signal is transmitted to the central computer system through the network, and the central computer system uses the at least one second response signal received from one or more of the interface devices to update one or more of the records. For a group the transponders, the at least one RF signal that is generated at the direction of the central computer system and that causes each of the transponders in the group to generate and transmit the response signal including the identifier identifying the particular asset with which the transponder is associated may be common to the transponders in the group. 
     The at least one second response signal in each case may be a plurality of second response signals, wherein each of the second response signals corresponds to a respective one of the response signals that was received by the interface device and includes the identifier that was included in the corresponding one of the response signals. The central computer system may further be adapted to cause the generation of the at least one RF signal particular to one or more of the transponders in a sequential fashion, wherein corresponding response signals and second response signals are generated and transmitted in a corresponding sequential fashion. Location information associated with each interface device may be used to update the location of each of the assets as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the principles of the invention. As shown throughout the drawings, like reference numerals designate like or corresponding parts. 
         FIG. 1  is a block diagram of an active RF transponder according to one embodiment of the present invention; 
         FIG. 2  is a schematic diagram of a burst switch according to an aspect of the present invention; 
         FIG. 3  is a block diagram of an RF transponder according to an alternative embodiment of the present invention; 
         FIG. 4  is a schematic illustration of a code that may be required to awaken the transponder  50  shown in  FIG. 3 ; 
         FIG. 5  is a block diagram of an RF transponder according to a further alternative embodiment of the present invention; 
         FIG. 6  is a block diagram of an alternative RF transponder that is similar to the RF transponder shown in  FIG. 1  but that further includes an RF receiver; 
         FIG. 7  is a block diagram of an alternative RF transponder that is similar to the RF transponder shown in  FIG. 3  but that further includes an RF receiver; 
         FIG. 8  is a block diagram of an RFID system according to an aspect of the present invention; 
         FIG. 9  is a block diagram of an asset management system according to a further aspect of the present invention; and 
         FIG. 10  is a block diagram of an interface device forming a part of the asset management system shown in  FIG. 9 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a block diagram of an RF transponder  5  according to one embodiment of the present invention. The RF transponder  5  includes a burst switch  10 , which is described in more detail in connection with  FIG. 2 . The burst switch  10  is in electronic communication with a processing unit  15 , which may be, without limitation, a microprocessor, a microcontroller, or some other type of processor device. The processing unit  15  may further be another type of electronic device, such as a CMOS device or any other electronic circuit element provided on, for example, a semiconductor substrate or printed circuit board (PCB), which performs a particular function or functions. The processing unit  15  is capable of being placed into an inactive, sleep state where the current drawn by it is at a minimum. In addition, the processing unit  15  may be woken up, i.e., moved from the inactive, sleep state to an active state, upon receipt of an external input signal. An RF transmitter  20  is in electronic communication with the processing unit  15 . The RF transmitter  20  may be a separate transmitter component, or may be part of a transceiver component that is capable of both transmitting and receiving RF signals. The RF transmitter  20  is, in response to commands received from the processing unit  15 , able to transmit RF signals through an antenna  25  connected thereto. Like the processing unit  15 , the RF transmitter  20  is capable of being placed into an inactive, sleep state where the current drawn by it is at a minimum, and can be woken up by receipt of an external input signal provided by the processing unit  15 . The RF transponder  5  also includes a battery  30  which provides the power required for the operation of the processing unit  15  and the transmitter  20 . The battery  30  may alternatively be replaced by another power source, such as, without limitation, a fuel cell or a super capacitor. 
       FIG. 2  is a schematic diagram of the burst switch  10 . The burst switch  10  includes an antenna  35 , which, in the embodiment shown in  FIG. 2 , is a square spiral antenna. The antenna  35  is electrically connected to a matching network  40 , which in turn is electrically connected to a voltage boosting and rectifying circuit preferably in the form of a charge pump  45 . Charge pumps are well known in the art. Basically, one stage of a charge pump essentially doubles the effective amplitude of an AC input voltage and stores the resulting increased DC voltage on an output capacitor. The voltage could also be stored using a rechargeable battery. Successive stages of a charge pump, if present, will essentially increase the voltage from the previous stage resulting in an increased output voltage. The matching network  40  matches the input impedance of the charge pump  45  to the impedance of the antenna  35  for optimal performance of the antenna  35  and optimal charge pump  45  output voltage. In one particular embodiment, the matching network  40  is an LC tank circuit formed by, for example, the inherent distributed inductance and inherent distributed capacitance of the conducing elements of the antenna  35 . The antenna  35  is tuned to receive RF signals having a particular frequency or range of frequencies. The RF signals that are received by the antenna  35  are provided, in the form of an AC signal, to the charge pump  45  through the matching network  40 . The charge pump  45  essentially amplifies and rectifies the received AC voltage signal and outputs the resulting DC signal. These operations are performed without requiring the consumption of power from the battery  30  or any other power source within or connected (physically) to the RF transponder  5 . 
     Referring again to  FIG. 1 , in operation, the RF transponder  5  is deployed in a state wherein the processing unit  15  and the transmitter  20  are in the inactive, sleep state. As such, the draw on the battery  30  will be at a minimum. When it is desired to “wake-up” the RF transponder  5 , an RF signal of an appropriate frequency is transmitted to the RF transponder  5  by, for example, an RFID reader or other suitable device. The RF signal is received by the burst switch  10 , and as described above, the RF signal is used to produce a DC signal. The DC signal that is produced is provided to the sleep input (pin) of the processing unit  15 , which causes the processing unit  15  to move from the inactive, sleep state to its active state. In the active state, the processing unit  15  is able to perform any action that is required, such as waking up the RF transmitter  20  and causing it to transmit a signal that contains information such as an identifier for the RF transponder  5 . When finished (or after some predetermined period of time), the processing unit  15  can return to an inactive, sleep state until subsequently woken up as described herein. As will be appreciated, the burst switch  10  as shown in  FIG. 2  is designed to produce a DC signal of an appropriate level for input into the sleep input of the processing unit  15  through appropriate selection of the parameters of the antenna  25 , the matching network  40  and/or the charge pump  45 . 
     A shortcoming of the RF transponder  5  shown in  FIG. 1  is that spurious RF energy (noise) received by the burst switch  10  could inadvertently cause the processing unit  15  to move to the active state, thereby consuming power when not needed. Also, if a number of similar RF transponders  5  (i.e., similar meaning the antenna  35  of each is tuned to the same frequency or frequency range) are deployed together, an RF signal that is transmitted by a reader will activate all of the RF transponders  5 , even if they are not all currently of interest to the reader. In other words, there is no way to selectively activate one or more of them without also activating the remaining ones of them. 
       FIG. 3  is a block diagram of an RF transponder  50  according to an alternative embodiment of the present invention that addresses the shortcomings of the simple RF transponder  5  shown in  FIG. 1 . As seen in  FIG. 3 , the RF transponder  50  is similar to the RF transponder  5  in that it includes a burst switch  10 , a processing unit  15 , an RF transmitter  20  connected to an antenna  25 , and a battery  30 . However, the RF transponder  50  further includes a low power filtering circuit  55 . Specifically, as shown in  FIG. 3 , the DC output of the burst switch  10  is provided to the filtering circuit  55 , and the output of the filtering circuit  55  is provided to the sleep input of the processing unit  15 . The function of the filtering circuit  55  is twofold. First, the filtering circuit  55  prevents spurious RF energy (noise) from inadvertently causing the processing unit  15  to move from an inactive, sleep state to an active state. Second, the filtering circuit  55  provides a mechanism by which the particular RF transponder  50  in which the filtering circuit  55  is included can be selectively woken up, i.e., have its processing unit  15  selectively moved to an active state. The filtering circuit  55  performs these functions by causing a wake-up signal to be sent to the sleep input of the processing unit  15  only if a particular sequence or pattern (i.e., format) of RF signals is received by the burst switch  10 . 
     In the preferred embodiment, the filtering circuit  55  is a state machine that will generate a wake-up signal only if a particular pre-set “code” is received from the burst switch  10 , wherein the code is a particular sequence of a certain number of voltage “bursts” (i.e., voltage signals of a certain (although possibly varying) length or duration; in this case, the bursts are DC signals, but bursts as used herein may also refer to RF signals of a certain (although possibly varying) length or duration) from the burst switch  10  each having a particular length expressed as a multiple of some pre-set unit of time, such as 1 millisecond.  FIG. 4  shows an example of a 4 element code that may be required to be output by the burst switch  10  and received by the filtering circuit  55  in order for the filtering circuit  55  to generate a wake-up signal for waking up the processing unit  15 . In the example of  FIG. 4 , the code that must be received is a 4 burst code consisting of a burst of length 5 (e.g., 5 milliseconds), followed by a burst of length 2 (e.g., 2 milliseconds), followed by a burst of length 4 (e.g., 4 milliseconds), followed by a burst of length 6 (e.g., 6 milliseconds). In effect, the code is 5 2 4 6. As will be appreciated, the code scheme of  FIG. 4  is meant to be exemplary only, and any number of bursts of any possible length and any base length unit of time may be used for a particular code without departing from the scope of the invention. 
     In operation, the filtering circuit  55  will count (possibly on a dedicated counter) the number of separate bursts received and the length of each burst (the length of each burst may be stored in a register or any suitable memory). When the count reaches the pre-set number, e.g., 4, the registers (or memory) are checked for the proper code (i.e., has the proper sequence of burst lengths been received). If the code is determined to be correct, the filtering circuit  55  will generate a wake-up signal for the processing unit  15 . As will be appreciated, the required code may be generated by an RFID reader by generating a sequence of an appropriate number of RF bursts wherein each RF burst is of a particular time. As described in connection with  FIG. 2 , each such RF burst will result in a corresponding DC voltage (DC burst) being output by the burst switch  10  having a length equal to the length of the RF burst. Thus, in order to generate the 5 2 4 6 code described above, an RF reader must output an RF burst having a length of 5 (e.g., 5 milliseconds), followed by an RF burst having a length of 2 (e.g., 2 milliseconds), followed by an RF burst having a length of 4 (e.g., 4 milliseconds), followed by an RF burst having a length of 6 (e.g., 6 milliseconds). 
     The filtering circuit  55  thus solves the noise problem by requiring a particular sequence of RF bursts before the processing unit  15  is awakened. The filtering circuit  55  also allows a number of RF transponders  50  to be deployed and selectively and independently awakened. In particular, each transponder  50  (or set of transponders  50  to be grouped and awakened together) that is deployed at a location can be provided with a unique code. In order for an RFID reader to wake up a particular transponder  50  (or set of grouped transponders  50 ), the RFID reader will need to generate the appropriate RF bursts. As an alternative, any particular RF transponder  50  may be provided with more than one code that would enable it to be awakened, wherein one code may be used to awaken the RF transponder  50  individually, and another code may be used to awaken it as part of a group of particular transponders  50 . 
     As seen in  FIG. 3 , the filtering circuit  55  is connected to the battery  30  for power purposes. Preferably, the filtering circuit  55  is a device or component that may enter a low power sleep state. The filtering device  55  will remain in a sleep state until a burst is received, at which time it will move to an active state (the burst is the wake-up signal), count the burst, measure its duration, and return to sleep until the next burst is received. As a result, minimal power is consumed by the filtering circuit  55 . As will be appreciated, the filtering circuit  55  thus may be any low power electronic device that can be turned on for a short period of time, increment a counter, measure a burst length, and then go back to sleep. 
       FIG. 5  is a block diagram of an RF transponder  60  according to a further alternative embodiment of the present invention that includes an alternate arrangement for addressing the shortcomings of the simple RF transponder  5  shown in  FIG. 1 , i.e., the noise problem and the inability to discriminate among multiple transponders. As seen in  FIG. 5 , the RF transponder  60  is similar to the RF transponder  5  in that it includes a processing unit  15 , an RF transmitter  20  connected to an antenna  25 , and a battery  30 . However, the RF transponder  60  includes multiple burst switches  10 A,  10 B,  10 C, and  10 D wherein the antenna  35  of each burst switch  10 A,  10 B,  10 C,  10 D is tuned to a different frequency or range of frequencies (although only four burst switches  10  are shown, more or less than four may be employed to suit the particular needs of the application in question without departing from the scope of the present invention). In addition, as represented by passive logic combination  65 , the burst switches  10 A,  10 B,  10 C,  10 D are topologically interconnected in manner that implements a selected logical combination, such as an AND, an OR, or any other logic operation or combination of operations. It will be appreciated that each burst switch  10 A,  10 B,  10 C,  10 D will only output a DC signal if it receives an RF signal of the appropriate frequency (each referred to as a “burst switch frequency” for convenience). Thus, the passive logic combination  65  can be chosen to only provide a wake-up signal to the processing unit  15  if a particular combination of the burst switch frequencies is received. For example, the passive logic combination  65  could be implemented as an AND such that all of the burst switch frequencies must be received for a wake-up signal to be sent to the processing unit  15 . Alternatively, the passive logic combination  65  could be implemented with a series of ANDs and ORs such that any two, or any three of the burst switch frequencies or a particular two or a particular three of the burst switch frequencies must be received for a wake-up signal to be sent to the processing unit  15 . 
     Thus, because particular burst switch frequencies must be received to wake-up the processing unit  15 , the arrangement shown in  FIG. 5  prevents spurious RF energy (noise) from inadvertently causing the processing unit  15  to move from an inactive state to an active state. In addition, the arrangement shown in  FIG. 5  may also be used to provide a mechanism by which the particular RF transponder  60  in which it is included can be selectively woken up, i.e., have its processing unit  15  selectively moved to an active state. Specifically, a number of transponders  60  may be deployed with different burst switch frequencies and/or different passive logic combinations  65  such that an RFID reader can generate appropriate RF signals to selectively wake-up certain ones of the RF transponders  60 . For example, one RF transponder  60  could be deployed wherein all of the burst switch frequencies are required to wake it up, another RF transponder  60  could be deployed wherein a particular two of the burst switch frequencies are required to wake it up, another RF transponder  60  could be deployed wherein a different particular two of the burst switch frequencies are required to wake it up, another RF transponder  60  could be deployed wherein a particular three of the burst switch frequencies are required to wake it up, and so on. 
     In an alternative embodiment of the RF transponder  60 , instead of providing the passive logic combination  65 , the burst switches  10 A,  10 B,  10 C, and  10 D could be combined and biased with respect to one another such that the burst frequencies must be received in a particular pre-set order for a wake-up signal to be sent to the processing unit  15 . In such an arrangement, each burst switch  10  following a first one of the burst switches  10  would require the preceding burst switch  10  to be energized before it would be capable of outputting a DC signal. In this sense, the arrangement of burst switches  10 A,  10 B,  10 C,  10 D functions like an electronic combinational lock, and as such is able to prevent noise from inadvertently waking up the processing unit  15  and is able to allow the RF transponder  60  in which it is implemented to be selectively woken-up. 
       FIG. 6  is a block diagram of an alternative RF transponder  5 ′ that is similar to RF transponder  5  shown in  FIG. 1  except that it further includes an RF receiver  70  connected to an antenna  75 . The RF receiver  70  may be caused to move from an inactive, sleep state to an active state by the burst switch  10  in order allow for further communication with the processing unit  15  via the RF receiver  70 . The communications may be according to an established standard, such as the ISO 18000 Part 7 standard. Similarly,  FIG. 7  is a block diagram of an alternative RF transponder  50 ′ that is similar to RF transponder  50  shown in  FIG. 3  except that it also further includes an RF receiver  70  connected to an antenna  75 . The RF receiver  70  in this embodiment may be caused to move from an inactive, sleep state to an active state by the burst switch  10  and filtering circuit  55  in the manner described elsewhere herein in order to allow for further communication with the processing unit  15  of the RF transponder  50 ′ via the RF receiver  70 . Again, the communications may be according to an established standard, such as the ISO 18000 Part 7 standard. 
       FIG. 8  is a block diagram of an RFID system  80  according to an aspect of the present invention. The RFID system  80  includes a plurality of RF transponders  85  deployed in a particular location, such as within a building. The RF transponders  85  may be, without limitation, any of the RF transponder embodiments described herein, such as RF transponder  5 , RF transponder  5 ′, RF transponder  50 , RF transponder  50 ′ or RF transponder  60 . The RF transponders  85  may also be an RF transponder as described in co-pending U.S. provisional application Ser. No. 60/673,715 entitled “Method and Device for Reducing Power Consumption of Active RFID Tags,” owned by the assignee of the present invention, the disclosure of which is incorporated herein by reference, or any other type of known or later developed suitable RF transponder. The RFID system  80  further includes an interrogator unit  90  which is in electronic communication with a host (central) computer system  95 . Under the control of the host computer system  95 , the interrogator unit  90  generates the RF signals (e.g., bursts) that are required to selectively awaken one or more of the RF transponders  85  in the manners described elsewhere herein. Once awakened, each RF transponder  85  may simply transmit some identifying information to the interrogator unit  90  to confirm its presence at the location, or, in those embodiments that permit (e.g., RF transponders  5 ′ and  50 ′), each RF transponder  85  may receive further communications from the interrogator unit  90  (for example, according to the ISO 18000 Part 7 standard) and respond accordingly. Thus, due to the power conserving capabilities of the RF transponder  5 , the RF transponder  5 ′, the RF transponder  50 , the RF transponder  50 ′ and the RF transponder  60  described elsewhere wherein, the RFID system  80  is able to operate with minimal power consumption and therefore an extended lifetime. In order to avoid collisions in one embodiment, the RF interrogation response signals are transmitted one at a time in a sequential manner, such as according to an order determined by the unique identifier of each RFID tag  110 . Other collision avoidance mechanisms are also possible. 
       FIG. 9  is a block diagram of an embodiment of an asset management system  100  according to a further aspect of the present invention. The asset management system  100  enables centralized, remote location tracking of a number of assets  105  within a particular location  115 , such as, for example and without limitation, a hospital or another environment. The assets  105  may be any type of physical item, including both movable items and items that are permanently or temporarily fixed in place. For example, in a hospital application, the assets  105  may be various types of medial equipment, such as, without limitation, a crash cart, an EKG machine, a wheel chair, a gurney, an oxygen dispenser, a staff member, or a patient. Each of the assets  105  has an RFID tag  110  physically associated therewith, preferably by physically attaching the RFID tag  110  to the asset  105 . Each RFID tag  110  is preferably any of the RF transponder embodiments described herein, such as RF transponder  5 , RF transponder  5 ′, RF transponder  50 , RF transponder  50 ′ or RF transponder  60 . 
     The asset management system  100  further includes a central asset management computer system  120  that is connected to a main network  125 . The asset management computer system  120  may include, without limitation, a PC or another suitable computing device that is provided with one or more software applications for implementing the system described herein. As seen in  FIG. 9 , a number of wireless access points  130  are in electronic communication, preferably wired communication, with the main network  125  and are dispersed throughout the location  115 . Each wireless access point  130  is capable of receiving a signal from the main network  125 , and thus from the asset management computer system  120 , and wirelessly transmitting that signal within a particular defined area. In addition, each wireless access point  130  is capable of receiving wireless signals from within its particular defined area and transmitting those signals to the main network  125 , and thus to the asset management computer system  120 . The main network  125  and wireless access points  130  thus form a wireless network for the location  115 . In the preferred embodiment, the wireless network for the location  115  is a WiFi network that is implemented according to the IEEE 802.11 family of standards, or another suitable standard. 
     The asset management system  100  also further includes a number of interface devices  135  that are dispersed throughout the location  115 . Each interface device  135  is located within the range of at least one of the wireless access points  130 . As described in greater detail below, each interface device  135  is capable of receiving wireless (RF) signals from and transmitting wireless (RF) signals to the associated wireless access point  130  according to the appropriate protocol. In addition, each interface device  135  is capable of transmitting RF signals to the RF tags  110  that are in proximity thereto and receiving RF signals from those RF tags  110 . In particular, based upon control signals received from the asset management computer system  120  through the main network  125  and the appropriate wireless access point  130 , each interface device  135  is capable of transmitting one or more RF signals to the burst switch  10  of the associated RF tags  110  (in the manner or manners described elsewhere herein in connection with the embodiments of the RF transponder  5 , the RF transponder  5 ′, the RF transponder  50 , the RF transponder  50 ′ and the RF transponder  60 ) for purposes of causing the processing unit  15  of the associated RF tags  110  to move to an active state. In addition, each interface device  135  is capable of receiving response signals from the associated RF tags  110  after they have been awakened. In this respect, the interface devices  135  function like RFID readers or interrogators. For reasons that will be explained hereinafter, each interface device  135  is provided with an identifier that uniquely identifies it. Such identifiers enable the asset management computer system  120  to associate each interface device  135  with a particular location within the location  115 , such as a particular room or wing in a building. This may be done in the form of a table stored by the asset management computer system  120 . Thus, each interface  135  can be located or found to be non-functional through the asset management system  100  itself. 
       FIG. 10  is a block diagram of an embodiment of the interface device  135  shown in  FIG. 9 . The interface device  135  includes a processing unit  140 , which may be, without limitation, a microprocessor, a microcontroller, or some other type of processor device. The processing unit  140  is electrically connected to a power interface  145  which provides power thereto. The power interface  145  is adapted to be coupled to an AC source, such as a wall outlet, in order to receive an AC voltage. The power interface  145  converts the AC voltage into a DC signal that is suitable for use by the processing unit  140 . A wireless network transceiver  150  is provided in electronic communication with the processing unit  140 . The wireless network transceiver  150  is adapted to receive wireless (RF) signals from and transmit wireless (RF) signals to one or more wireless access point  130  according to the appropriate protocol, such an 802.11 protocol, using an appropriate frequency, such as 2.45 GHz. In addition, a tag transceiver  155  is provided in electronic communication with the processing unit  140  for enabling the processing unit  140  to transmit appropriate RF signals to the associated RFID tags  110  and to receive appropriate response signals from associated RFID tags  110 . Thus, as will be appreciated, each interface device  135  functions as an interface between the two communications systems, i.e., the wireless network implemented by the wireless access points  130  and the wireless communications links to the RFID tags  110 . In an alternative embodiment, a separate (dedicated) transmitter may be provided in each interface device  135  for sending the required signals to the burst switch  10 , and the RF transceiver  155  may be used for other communication with the RFID tags  110 . 
     In one particular embodiment of the asset management system  100 , each of the RFID tags  110  is an RF transponder  5  (or, alternatively, an RF transponder  5 ′). The burst switch  10  of each of the RF transponders  5  has an antenna  35  that is tuned to a particular frequency or frequency range, such as 433 MHz. In this embodiment, the asset management computer system  120  stores one or more files, such as, without limitation, one or more files in a database, that include for each asset  105  an identification of the asset type (e.g., crash cart, EKG machine, etc.) and a unique identifier for the asset  105 . The unique identifier may be, without limitation, a serial number. The RFID tag  110  (i.e., transponder  5 ) associated with each asset  105  stores the unique identifier for the asset  105 . The unique identifier may be stored in a memory of the RFID tag  110  that is part of the processing unit  15  thereof or that is separate from but in electronic communication with the processing unit  15  thereof. When deployed, the processing unit  15  of each RFID tag  110  is in a sleep state, and will remain in that state until awakened as described below. 
     In operation, this particular embodiment of the asset management system  100  is adapted to track and maintain an inventory of each asset  105  including the particular location of each asset  105  within the location  115 . To do so, the asset management computer system  120  periodically or on demand generates an asset interrogation signal. The asset interrogation signal is sent to the main network  125  and then to each wireless access point  130 . Each wireless access point  130  then wirelessly transmits the asset interrogation signal according to the appropriate protocol, such as an 802.11 protocol. The wirelessly transmitted asset interrogation signal is received by each interface device  135  that is within the range of each wireless access point  130 . In response to receipt of the asset interrogation signal, each interface device  135  generates a second RF interrogation signal having a frequency that will be picked up by the antenna  35  of the burst switch  10  of each RFID tag  110 . As described elsewhere herein, when the burst switch  10  of each RFID tag  110  receives the second RF interrogation signal, a DC signal is generated that causes the processing unit  15  of each RFID tag  110  to move to an active state. Each such processing unit  15  is adapted to then cause an RF interrogation response signal of an appropriate frequency (e.g., 433 MHz) to be generated by the associated transmitter  20  in the RFID tag  110 . Each RF interrogation response signal includes the unique identifier stored by the RFID tag  110  that generated the RF interrogation response signal. In order to avoid collisions, the RF interrogation response signals are, in one embodiment, transmitted one at a time in a sequential manner, such as according to an order determined by the unique identifier of each RFID tag  110 . Other collision avoidance mechanisms are also possible. 
     The RF interrogation response signals are then received by the respective interface devices  135  (i.e., the interface device  135  that is in proximity to the RFID tag  110  that generated the RF interrogation response signal). Each interface device  135  compiles a list of RF interrogation response signals that is has received, and transmits a second interrogation response signal for the corresponding particular location  137  ( FIG. 9 ) according to the chosen protocol of the wireless network that is implemented. The second interrogation response signal generated and transmitted by each interface device  135  will include the unique identifier of the interface device  135  and the list of RF interrogation response signals complied by the interface device  135 . The second interrogation response signals are then received by the associated wireless access points  130  and transmitted to the asset management computer system  120  through the main network  125 . 
     Upon receipt of the second interrogation response signals, the asset management computer system  120  is able to update the location of each asset  105  in its records. In particular, each second interrogation response signal that is received will include a list of unique identifiers that, as described above, uniquely identify each asset  105 . Each second interrogation response signal will also include the identifier of the interface device  135  that sent it, thus identifying the location of that interface device  135 . As a result, the asset management computer system  120  can use this information to associate a particular location within the location  115  with each asset  105 . 
     As described elsewhere herein, one of the shortcomings of the RF transponders  5  and  5 ′ is that they could be inadvertently awakened by spurious RF noise. This could present a problem for the embodiment of the asset management system  100  just described as the RFID tags  110 , being RF transponders  5  or  5 ′ in that embodiment, could be caused to inadvertently send RF interrogation response signals in response to noise. This problem is addressed in an alternative embodiment of the asset management system  100  in which each of the RFID tags  110  is an RF transponder  50  (or, alternatively, an RF transponder  50 ′) that will be awakened by the same burst code, e.g., 5 2 4 6. In this particular embodiment, operation of the asset management system  100  is similar to that described above. However, in this embodiment, the second RF interrogation signal that is generated by each interface device  135  upon receipt of the asset interrogation signal from a wireless access point  130  will be an RF signal consisting of the appropriate RF bursts sufficient to cause the burst switch  10  of each RFID tag  110  to create the required burst code for the filtering circuit  55  of the RFID tag  110 . As described elsewhere herein, that code, when received by the filtering circuit  55 , will cause a wake-up signal to be generated for the associated processing unit  15 , which, in response, will wake-up and generate the appropriate RF interrogation response signal. Thus, in this embodiment, the adverse affects of noise are minimized. 
     A further shortcoming of the RF transponders  5  and  5 ′ is that there is no mechanism for discriminating among a number of them when deployed, i.e., there is no way to selectively cause only certain ones of them to respond. As result, the embodiment of the asset management system  100  that utilizes the RF transponder  5  or  5 ′ will be required to interrogate all of the RFID tags  110  each time an inventory update is desired, as opposed to only interrogating selected RFID tags  110  and thus selected assets  105 . As will be appreciated, while this will still gather the necessary asset location information, it will cause battery power for certain of the RFID tags  110  to be unnecessarily consumed. 
     Thus, according to a further aspect of the present invention, a further alternative embodiment of the asset management system  100  is provided in which selected ones and/or selected groups of the RFID tags  110  may be interrogated. In this particular embodiment, each of the RFID tags  110  is an RF transponder  50  (or, alternatively, an RF transponder  50 ′) that may be awakened by a burst code that is unique to that RFID tag  110 . For example, each individual RFID tag  110  may be assigned a unique 4 element burst code as described elsewhere herein (such as 5 2 4 6) (a  4  element burst code is merely an example, and it should be understood that the burst code may have more or less than 4 elements). As a result, each of those RFID tags  110  may be selectively, individually interrogated by the asset management computer system  120  in the manner described elsewhere herein using the appropriate burst code in order to determine the current location thereof. In addition, one or more of the RFID tags  110  may also be adapted to be awakened by a particular burst code that is common to a selected group of RFID tags  110 . In other words, certain groups of RFID tags  110  (and thus certain groups of assets  105 ) may also be assigned a second burst code that may be used to awaken each of the RFID tags  110  in the group. For example, all assets  105  of type one (e.g., crash carts, or assets on floor one of a building) may be assigned the burst code 4 2 4 3, all assets  105  of type two (e.g., EKG machines, or assets on floor two of a building) may be assigned the burst code 3 1 4 2, etc. As a result, the location of all assets  105  in a particular group, such as crash carts, can be readily determined, if desired, by the asset management system  100  using a single burst code. 
     As will be appreciated, in the embodiment of the asset management system  100  just described, each asset interrogation signal that is sent by the asset management computer system  120  will need to include information that identifies the particular burst code that is to be used for that interrogation. The interface devices  135  will then use that information to generate the appropriate second RF interrogation signals that are transmitted. When multiple assets  105  or specified groups thereof are to be interrogated in this manner, the asset management computer system  120  will preferably generate and transmit the appropriate asset interrogation signals in a sequential fashion in order to avoid signal collision problems (the responses will also be sent in a similar, corresponding sequential fashion). Thus, according to an aspect of the present invention, the asset management computer system  120  maintains a table or similar record that links each asset  105  with the code or codes that may be used to awaken the RFID tag  110  associated with the asset  105 . That same table or other record will also preferably separately list specified asset groups (e.g., crash cards, EKG machines, assets in a particular wing, etc.) and the common code that is assigned to each group so that such common codes may be readily accessed. 
     In yet a further alternative embodiment of the asset management system  100 , each of the RFID tags  110  is an RF transponder  60  that, as described elsewhere herein, is able to be awakened by a particular combination or sequence of burst switch frequencies. This embodiment is similar to the embodiment of the asset management system  100  described above that employs the RF transponders  50  or  50 ′, except that the burst codes are replaced by specified combinations or sequences of burst switch frequencies. The basic operation of the asset management system  100  otherwise remains essentially the same. 
     The present invention therefor provides a number of embodiments of RF transponders and assets management systems employing the same that minimize the power that is consumed by each transponder. As a result, the lifetime of each RF transponder may be maximized. 
     While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. For example, the majority of the description contained herein describes the burst switch  10  as awakening a processing unit  15 . It should be appreciated that the burst switch  10  may be utilized to awaken any type of electronic device that is capable of entering an inactive, sleep state. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.