Abstract:
An apparatus, a system, and a method for communication between multiple base stations and radio frequency (RF) transponders (RF Tags) is disclosed. A first radio frequency (RF) base station for communicating RF signals with an RF tag communicates external trigger signals with at least a second RF base station, which causes the second RF base station to begin transmission.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority pursuant to 35 U.S.C. 119(e) to U.S. Provisional Application, Serial No. 60/068,122, filed Dec. 19, 1997. 
    
    
     FIELD OF THE INVENTION. 
     The field of the invention is the field of Radio Frequency (RF) Transponders (RF Tags), wherein one or more base stations communicates information to one or more RF Tags which may contain logic and memory circuits for storing information about objects, people, items, or animals associated with the RF Tags. The RF Tags can be used for identification and location (RFID Tags) of objects and to send information to the base station by modulating the load on an RF Tag antenna. More specifically, the invention relates to a radio frequency tagging system that allows multiple base stations to coordinate their activity. This allows the base stations to efficiently cover a larger zone than one base station while still meeting regulatory requirements. 
     BACKGROUND OF THE INVENTION 
     RF Tags can be used in a multiplicity of ways for locating and identifying accompanying objects, items, animals, and people, whether these objects, items, animals, and people are stationary or mobile, and transmitting information about the state of the of the objects, items, animals, and people. It has been known since the early 60&#39;s in U.S. Pat. No. 3,098,971 by R. M. Richardson, that electronic components on a transponder could be powered by radio frequency (RF) power sent by a “base station” at a carrier frequency and received by an antenna on the tag. The signal picked up by the tag antenna induces an alternating current in the antenna which can be rectified by an RF diode and the rectified current can be used for a power supply for the electronic components. The tag antenna loading is changed by something that was to be measured, for example a microphone resistance in the cited patent. The oscillating current induced in the tag antenna from the incoming RF energy would thus be changed, and the change in the oscillating current would lead to a change in the RF power radiated from the tag antenna. This change in the radiated power from the tag antenna can be picked up by the base station antenna and thus the microphone would in effect broadcast power without itself having a self contained power supply. In the cited patent, the antenna current also oscillates at a harmonic of the carrier frequency because the diode current contains a doubled frequency component, and this frequency can be picked up and sorted out from the carrier frequency much more easily than if it were merely reflected. Since this type of tag carries no power supply of its own, it is called a “passive” tag to distinguish it from an active tag containing a battery. The battery supplies energy to run the active tag electronics. An active tag may also change the loading on the tag antenna for the purpose of transmitting information to the base station, or it may act as a transmitter to broadcast the information from the tag antenna directly to the base station. 
     The “rebroadcast” of the incoming RF energy at the carrier frequency is conventionally called “back scattering”, even though the tag broadcasts the energy in a pattern determined solely by the tag antenna and most of the energy may not be directed “back” to the transmitting antenna. 
     In the 70&#39;s, suggestions to use tags with logic and read/write memories were made. In this way, the tag could not only be used to measure some characteristic, for example the temperature of an animal in U.S. Pat. No. 4,075,632 to Baldwin et. al., but could also identify the animal. The antenna load was changed by use of a transistor. 
     Prior art tags have used electronic logic and memory circuits and receiver circuits and modulator circuits for receiving information from the base station and for sending information from the tag to the base station. 
     U.S. Pat. No. 5,214,410, hereby incorporated by reference, teaches a method for a base station to communicate with a plurality of Tags. 
     Prior art tags typically use a number of discrete components connected together with an antenna. However, to substantially reduce the cost of the tags, a single chip connected to an antenna must be used. 
     In a typical configuration, an application controller (ie work station, computer, microcomputer etc.) issues a command to the base station. The base station executes the command and may report results back to the application controller. 
     In some applications the zone in which RF tags may reside is larger than the zone covered by a single base station. Two or more base stations are required for coverage. In order to provide continuous coverage, the zones of two or more of the base stations must overlap. 
     It is important that only one base station of the group covering a large zone be transmitting at any one time. Two base stations transmitting simultaneously could jam a tag in the overlapped area trying to receive a transmission. Further, a transmitting base station could jam another base station receiver trying to detect a low level signal from a tag. The transmissions of the two or more base stations must be coordinated. There are, however, problems associated with the coordination of base stations. One way to coordinate the base stations would be for the application controller to sequentially issue commands to each base station in the group covering the large zone. Problems with having the application controller coordinate the base stations include: 
     1. The application controller must service each base station for each command, leading to a decrease in system performance. Requiring the application controller to continuously control many base stations could overload the processing capacity of the application controller. 
     2. It is desirable for one application controller to service many zones. In many cases, the base stations should run autonomously until a significant event occurs, such as a tag entering the zone. In a typical case, the application controller and base stations will be on a local area network. Causing the application controller to continuously service many base stations adds unnecessary network traffic, and could overload the capacity of the network. 
     3. With a network application controller in the processing loop, the switching time from one base station turning off to another turning on is long and indeterminate. Tags overlapping the RF field of two or more base stations will see the field drop, and the tags will then reactivate when the power comes up again. This event can cause tags to reset. Indeed, passive tags powered by the RF field will lose energy in the tag energy store and tag electronics will not have enough energy to continue functioning. Either event causes a tag to lose the information that it carries which describes the state of the tag. In the case of multiple tag identification, the tag state prevents a tag from being identified more than once in the algorithm. If a tag in overlapping the RF fields from two base station loses state during the field switching, it will be identified more than once, degrading performance. 
     RELATED APPLICATIONS AND ISSUED PATENTS 
     Related U.S. Patents assigned to the assignee of the present invention include: U.S. Pat. Nos. 5,521,601; 5,528,222; 5,538,803; 5,550,547; 5,552,778; 5,554,974; 5,563,583; 5,565,847; 5,606,323; 5,635,693; 5,673,037; 5,680,106; 5,682,143; 5,729,201; 5,729,697;5,736,929; 5,739,754; 5,767,789; 5,777,561; 5,786,626; 5,812,065; and 5,821,859. U.S. Patent applications assigned to the assignee of the present invention include: application Ser. No. 08/626,820, filed: Apr. 3, 1996, entitled “Method of Transporting RF Power to Energize Radio Frequency Transponders”, by Heinrich, Zai, et al. (now U.S. Pat. No. 5,850,181); application Ser. No. 08/694,606 filed Aug. 9, 1996 entitled RFID System with Write Broadcast Capability by Cesar et al. (now U.S. Pat. No. 5,942,987); application Ser. No. 08/681,741 filed Jul. 29, 1996 entitled RFID Transponder with Electronic Circuitry Enabling and Disabling Capability, by Heinrich, Goldman et al. (now U.S. Pat. No. 5,874,902); and application Ser. No. 09/153,617 1 filed Sep. 25, 1998, entitled RFID Interrogator Signal Processing System for Reading Moving Transponder, by Zai et al. (now U.S. Pat. No. 6,122,329) The above identified U.S. Patents and U.S. Patent applications are hereby incorporated by reference. 
     OBJECT OF THE INVENTION 
     An object of this invention is an improved system of two or more RF base stations covering a single zone. 
     An object of this invention is to allow two or more base stations, once initiated, to coordinate their activities without intervention from an application controller. This greatly decreases the processing requirements of the application controller and traffic on the network connecting the application controller to the base stations. 
     An object of this invention is to allow two or more base stations to coordinate the switching of RF fields on and off in a fast, deterministic manner, so that RF tags overlapping the RF fields will not detect the field switching. 
     SUMMARY OF THE INVENTION 
     The current invention adds one or more circuits for receiving and/or for generating external trigger signals to the signal generator controller which controls a base station RF signal generator. The external trigger signal is sent directly from one base station to another, without intervention of the application controller which controls both base stations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a base station of the invention. 
     FIG. 2 is a block diagram of an apparatus to detect a multi-state trigger input. 
     FIG. 3 is a block diagram of an apparatus for generating a multi-state trigger output. 
     FIG. 4 is a block diagram of a system for using the base stations of the invention. 
     FIG. 5 is a block diagram of a method for using the base stations of the invention. 
     FIG. 6 is a block diagram of a method for continuous identification of tags. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an RF base station  100  having a signal generator  120  driving an antenna  110 . The signal generator  120  is activated and deactivated by a on/off control line  140  driven by the signal generator controller  130 . An optional circulator  170  is shown between the signal generator  120  and the antenna  110  which may take signals received by antenna  110  and route them to optional receiver  180 . Receiver  180  then sends demodulated signals to the base station electronics which may be part of the signal generator controller  130 . 
     The signal generator controller  130  has conceptually an external trigger input  150  and an external trigger output  160 . Although these can be physically separate signals, the preferred embodiments combine them into a bidirectional signal. 
     The signal generator controller  130  is programmed by an application controller (shown later) through input/output  170 . In a typical application, the signal generator controller  130  is programmed to activate the on/off control line  140  after a count N of trigger deactivations, where the count could be zero or greater. 
     The application controller initiates an application command. If the count N programmed into the signal generator controller  130  is zero, the signal generator controller activates the signal generator  120  immediately. Otherwise, the signal generator controller  130  continuously polls the trigger input  150 , counts deactivations, and activates the signal generator  120  after the count N is reached. 
     The signal generator controller  130  could be implemented as a state machine using logic gates, a programmable logic device (PLD), or an application specific integrated circuit (ASIC). Creating a state machine using these techniques is well known. 
     Alternatively, the signal generator controller  130  could be implemented using a micro controller running a software polling program. Writing a program to poll a general purpose micro controller input and activate a general purpose output uses well known techniques. 
     FIG. 2 shows a detail of a preferred implementation of the trigger input circuit. In a preferred embodiment, the trigger input  150  is a multi-state signal wire. The states can be detected using a comparator with a fixed threshold. 
     Comparator  220  compares the trigger input  150  to a threshold  240  which is set to a point between the states “none active” and “one active.” The comparator  220  distinguishes these two states. 
     Comparator  210  compares the trigger input  150  to a threshold  230  which is set to a point between the states “one active” and “more than one active.” The comparator  220  distinguishes these two states. 
     Thus the output of the circuit of FIG. 2 distinguishes three states. The transition of signal  260  from “one active” to “none active” is counted as a trigger deactivation. The assertion of the signal  250  in the state “more than one active” indicates that more than one base station has its signal generator activated. This can be used as an error condition. 
     FIG. 3 shows a detail of a preferred implementation of the trigger output circuit. In a preferred application, the “none active/one active” input  310  is actually the RF field on/off signal  140 . The input  310  turns on or off a fixed current source  330 . The current source  330  drives the trigger out signal  160 , the current sinking into a fixed resistive terminator  320 . 
     When the input  310  indicates “none active”, the current source is off, no current flows through the terminator  320 , and the voltage at trigger out  160  is zero. When the input  310  indicates “one active”, the current source is on, current flows through the terminator  320 ,and the voltage at trigger out  160  is a fixed non-zero level. 
     In a preferred implementation, multiple base stations have their trigger out signal  160  bused together, but there is only one terminator  320 . If more than one base station drives current into the terminator, a higher voltage indicating “more than one active” results on line  160 . 
     FIG. 4 shows a preferred implementation of a system of multiple base stations  100  connected to a single application controller  410 . For example, three base stations  102 ,  104 , and  106  are shown, along with an optional article detector  480 . In the preferred embodiment, the base stations  102 ,  104 , and  106  and application controller  410  are connected by a local area network  420 , but other connections are also contemplated. 
     In the preferred embodiment, the external trigger in  150  and external trigger out  160  are a single bidirectional signal, and the trigger from the first, second, and third base stations are connected to a trigger bus  430  and a resistive terminator  320 . Two RF tags  440  with associated tag electronics  450 , and tag antennas  460  are shown receiving RF radiation  470  from base station  104 . The tags  440  may optionally be relatively moving with respect to the base stations with a tag velocity  480 . 
     One typical application, shown by the block diagram of FIG. 5, is to identify all tags currently in the field. In this case, the first base station A would be programmed by the application controller in step  510  to turn on immediately, the second base station B after one trigger negation, and the third base station C after two negations. The application controller sends out the identification command in step  520 . Once the base stations receive the identification command, the first base station A activates its RF field, performs its algorithm, sends out an external trigger negation signal on line  150 / 160 , and turns off in step  530 . The second base station B, sensing the first trigger negation, activates its RF field in step  540  and follows the same path. Likewise, when the second base station B finishes sending and sends out the second trigger negation signal, the third base station C activates in step  550 . After all the base stations have completed their turns, the process is ended in step  560 . 
     The timing between one base station turning off, and another base station turning on is critical. The time elapsed must be less than the time t Max  where a passive tag  440  (a tag without a separate battery to provide the tag electronics  450  with power) in the RF field loses so much energy that the tag electronics  450  no longer function. The time t Max  must also be less than a time t Min  where the tag can distinguish that the base station transmission has terminated, since the tags are programmed to reset themselves a time t Min  after transmission ceases from the base station. The time t Max  is preferably 1 millisecond, more preferably 100 microseconds, and most preferably 30 microseconds. 
     This setup need only be performed once in making a single pass through the identification algorithm. 
     Several important points are to be noted: 
     1. Only one command need be sent by the application controller  410 . This minimizes network  420  traffic and application controller  410  processing. 
     2. The switch over among base stations is performed independent of application controller  410  processing and network  420  traffic. The switching time can be minimized and a worst case maximum switchover time t Max  can be specified. 
     A second typical application is continuous identification shown in FIG.  6 . In this variation, base station A is told by the application controller in step  610  that it is first in a ring of three base stations. Base station B is told that it is second of three, and base station C the third of three. Then the application controller sends out the identification command in step  620 . 
     Once base stations A, B, and C receive the identification command, base station A becomes active in step  630 . Base station B is programmed to start after one trigger, and base station C after two triggers. Once base station A completes, rather than terminating as before, it is reprogrammed to start again after two triggers. Base stations B and C are programmed identically after they complete the first round. Control passes to base station B in step  640 , and base station C in step  650 . The system returns to step  630  to cycle continuously. The result is a continuous identification, with control passing to base stations A, B, C, A, B, C and so on indefinitely. Once again, there is no application controller  410  processing or network  420  traffic required as the identification process passes from base station to base station. 
     The actual termination of the process of FIG. 6 can be programmed to be after a tag is identified, after a preset time limit, after a command from the application controller, or other application determined criteria. 
     A third typical application is processing a moving tag as shown schematically in FIG.  4 . Again the setup would be to transmit from base station A followed by base station B followed by base station C if the tag were known to always move in the direction where it would enter the zone of base station A followed by zone B followed by zone C, As an example, tags on a conveyer belt would enter the relevant zones sequentially. Base station A, would first try to process the tag for a time corresponding to the time that a tag would be in the zone of base station A. Once Base station A completed its attempt, control would pass to base station B and then to base station C. In this application, the first trigger signal might come from a detector  480  which detected a moving object moving into the zone covered by base station A. 
     In one preferred embodiment, the signal generator controller is a digital state machine whose input is the trigger and a programming mechanism. The output of the state machine is the RF field on/off control. 
     In another preferred embodiment, the signal generator controller is a micro controller programmed to insure guaranteed response time. The trigger is a software readable input and the RF field on/off control is a software controlled output. When suitably programmed, the micro controller continuously polls the trigger input. 
     In either preferred embodiment, the signal generator controller can be programmed to 
     ignore the trigger input and turn on the RF field, 
     turn on the RF field after the trigger activates and deactivates one or more times, or 
     turn off the RF field and record an error if the trigger indicates that more than one base station&#39;s RF field is on. 
     In one preferred embodiment, the trigger output is a single signal line with multiple states. 
     In another preferred embodiment, the trigger output is a set of signal lines, each line having only two states. 
     In another preferred embodiment, the trigger signal is a resistively terminated wire, and the driver is a current source. The resulting voltage across the termination is proportional to the number of base stations driving the trigger. 
     In a preferred embodiment, the trigger signal is bidirectional, a single wire connected in a bus architecture to each base station. This single wire is both detected by and potentially driven by each base station. 
     In a preferred system embodiment, multiple base stations covering a zone have their trigger circuits connected together in a bus structure. Each base station is programmed to turn on its RF field based on a programmable count of trigger signals. The result is that the application controller need only initiate the first base station in the sequence, after which time the other base stations will activate in turn without further intervention from the application controller. The sequence can be a series, with the application command terminating after the final base station in the series completes, or it can be a loop, with the command continuing indefinitely until an event is detected. The event can be the detection of tags meeting a certain criteria, reaching a time limit, an object moving out of the zone covered by the tags, or the application controller indicating that the loop should terminate. 
     In a preferred sequence, base stations which have overlapping fields are adjacent in the programmed sequence, so that tags within the overlap region do not detect the field switching. 
     In an alternate preferred sequence, base stations turn on in a sequence related to the typical movement of the RF tag in the zone, so that the speed at which moving tags are located is optimized. 
     Given this disclosure, equivalent embodiments of this invention will be apparent to those skilled in the art. These embodiments are also within the contemplation of the inventors. 
     Example of equivalent embodiments include, but are not limited to: 
     A multiple two state digital wire encoding of the trigger state  430  rather than the multi-state analog encoding disclosed. 
     A base station  100  with separate trigger in  150  and trigger out  160  rather than the bidirectional unified trigger signal  420  disclosed. 
     A chaining of trigger signals rather than the bus architecture  420  disclosed. 
     A star arrangement where a trigger pulse is sent from a base station, from the application controller, or from an event detector to a central location, which then sends a trigger pulse to all the base stations.