Abstract:
An area-reading antenna (in RF linkage with myriad RF tags) is dynamically tuned. It may be dynamically tuned as to its transmission of a power/clock field, or dynamically tuned as to its transmission of a data signal, or dynamically tuned as to its reception of a data signal, or some combination of all three. In this way, a base station has an improved ability to communicate with a greater fraction of the RF tags within its reading area, despite detuning of individual tags due to proximity of detuning influences such as metal or liquids.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. application No. 60/806,520 filed Jul. 3, 2006, which application is hereby incorporated herein by reference for all purposes. 
         [0002]    The invention relates generally to radio communications and relates more particularly to communications between a host (base station) and myriad passive radio tags. 
     
     BACKGROUND 
       [0003]    It is not easy to create a system that permits a base station to communicate individually with each of myriad radio tags in an area, particularly where the tags are required to be limited in cost and size and are required to function for a long time without a problem of a battery running down. Consider the particular case where the reading of the tags is intended to be an “area read”, meaning that the tags are distributed about an area and the base station is at some distance from many of the tags and the operator of the system does not have the luxury of being able to assume that a reader is moved to each of the tags seriatim for individual close-up reading. 
         [0004]    Nearly all RF-ID tag systems in the prior art are systems that designed on the assumption that when a reader reads a tag, the reader and tag are close together or (in some systems) the reader has a high-gain antenna pointed directly at the tag and at no other tag. In this way, the tag is bathed in RF energy and responds specifically because it notices that it has been bathed in RF energy. 
         [0005]    With such tags it is impossible to do an “area read”. Stated differently, if some antenna were positioned so as to be RF-linked with myriad tags, then bathing several or most of the tags in RF energy would simply yield an unintelligible burst of radio clutter. With such a system, the reader would receive confused and overlapping responses if the reader were juxtaposed or positioned so that more than one tag is in its reading path. 
         [0006]    One design decision that can help with making “area reads” possible is the use of lower radio frequencies rather than higher frequencies. This is not easy to do but approaches such as those described in [cite to VLF application] can make it possible to use lower frequencies. 
         [0007]    Another design decision that can help with making “area reads” possible is the use of tags [cite to iDot application] that do not simply respond each time they are bathed in RF energy, but that instead respond only after certain conditions are satisfied, such as receipt of a message with particular content or development of a predefined internal state. With such a system there may be a power/clock RF field that provides power and clock information to the myriad tags in the reading area, and then a different data RF field (typically at a higher frequency than the power/clock field) permits messages to be passed to the tags and permits tags to respond. Each tag typically has two distinct antennas, one for for receiving the power/clock and another for sending and receiving data messages. The base station may likewise have two distinct antennas, or may employ a single antenna directed to both purposes. But even with these design decisions, a system designer cannot do as much as one would like to do in terms of area reads over substantial areas and in terms of large numbers of tags that are in varied orientations relative to the base station and that may have been “detuned” due to proximity to metal or liquids or other materials that distort electrical or magnetic fields. One would like to be able to pick up a particular tag to the exclusion of others. One would like to be able to pick up all or nearly all of the tags in an area, rather than only being able to pick up some of them. 
         [0008]    It would thus be very desirable if additional design elements could be devised that would help to facilitate these aspects of performing “area reads”. 
       SUMMARY OF THE INVENTION 
       [0009]    An area-reading antenna (in RF linkage with myriad RF tags) is dynamically tuned. It may be dynamically tuned as to its transmission of a power/clock field, or dynamically tuned as to its transmission of a data signal, or dynamically tuned as to its reception of a data signal, or some combination of all three. In this way, a base station has an improved ability to communicate with a greater fraction of the RF tags within its reading area, despite detuning of individual tags due to proximity of detuning influences such as metal or liquids. 
     
     
       DESCRIPTION OF THE DRAWING 
         [0010]    The invention will be described with respect to a drawing in several figures. 
           [0011]      FIG. 1  shows a dynamically tuned base station antenna and a plurality of RF tags. 
           [0012]      FIG. 2  shows one type of capacitor ladder which can be used as part of a tuning system for an antenna. 
           [0013]      FIG. 3  shows a base station with separate antennas for power/clock and data, each dynamically tuned, and in RF linkage with a plurality of RF tags. 
           [0014]      FIG. 4  shows a microcontroller for a base station according to the invention. 
           [0015]      FIG. 5  shows an antenna tuner for a base station according to the invention. 
           [0016]      FIG. 6  shows a receiver for a base station according to the invention. 
           [0017]      FIG. 7  shows a transmitter for a base station according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  shows a base station  10  having a an antenna  11  and a plurality of RF tags  12   a,    12   b,    12   c,    12   d.    
         [0019]    It should be appreciated that the RF design environment of a system such as that shown in  FIG. 1  is extremely different from most RF design environments. In many design environments the designer has the luxury of being able to assume that the antenna is manufactured specifically for a particular purpose and is impedance-matched to the transceiver. No such luxury is available to the designer of an area-read system, where the antenna is often best thought of as basically a random wire that happens to cover the area to be read. 
         [0020]    In many design environments the designer is able to assume that the devices being communicated with are stationary. No such luxury is available to the designer of such an area-read system. 
         [0021]    In many design environments the designer is able to assume that the devices being communicated with have plenty of power (e.g. cell phones with rechargeable batteries). No such luxury is available to the designer of such an area-read system. 
         [0022]    In some design environments the designer is able to assume that the base station (and/or its antenna) is moved from place to place, being in close proximity with only one device at a time. No such luxury is available to the designer of such an area-read system. 
         [0023]    In some design environments the designer is able to assume that the base station antenna is a high-gain antenna, pointing to only one device at a time (for example a highway toll collection system with a directional antenna pointed directly at a single lane of traffic). No such luxury is available to the designer of such an area-read system. 
         [0024]    In accordance with the invention, an antenna tuning system is provided. In this exemplary embodiment variable capacitors  13 ,  14  are provided in series with and in parallel with the antenna  11 , and are controlled by a control means  15 . The control means  15  acts to match the antenna  11  to the radio transmitter and receiver of the base station  10 . 
         [0025]    At first glance one might think of the system of  FIG. 1  as simply an antenna tuner of a traditional sort, such as the type of antenna tuner used by an amateur radio operator to couple a transceiver to a “random-wire” antenna. Such antenna tuners are well known. With such an antenna tuner, the tuner is adjusted once to provide an impedance match (thus optimizing the SWR or standing-wave ratio), and then the setting remains the same for as long a the transceiver is being used at a particular frequency. Only when the operator shifts to a different frequency or a different frequency band (e.g. from 40 meters to 160 meters) would the tuner be adjusted away from its previous settings. But as will be appreciated in the discussion which follows, the invention calls for quite different activities. 
         [0026]      FIG. 2  shows one type of capacitor ladder which can be used as part of a tuning system for an antenna. The capacitor ladder has a series of capacitors  16 , each twice as large (in capacitance) as the next. Switches  17  determine which capacitors are in-circuit and which are not, thus permitting control means  15  to select a particular capacitance value at a particular time, and to change the value from time to time. Another approach is to use a variable capacitor (e.g. with moving blades in greater or lesser engagement) controlled by a servo motor. Those skilled in the art can readily review the substantial prior art in the area of antenna tuners and can pick and choose from among that prior art to achieve antenna tuning as needed here, all without departing from the invention. It is not, for example, required that two capacitors  13 ,  14  be used, as other reactive elements (and other combinations of reactive elements) can be employed to achieve desired matching and tuning. 
         [0027]      FIG. 3  shows a base station  18 ,  19  with separate antennas  20 ,  21  for power/clock and data, each dynamically tuned, and in RF linkage with a plurality of RF tags  24   a,    24   b,    24   c,  and  24   d.  In such a system there are matchers/tuners  22 ,  23 , each matching a respective antenna  20 ,  21  with its respective transmitter or receiver or transceiver  18 ,  19 . 
         [0028]    An exemplary base station according to the invention is shown in several parts—a microcontroller ( FIG. 4 ), an antenna tuner ( FIG. 5 ), a receiver ( FIG. 6 ), and a transmitter ( FIG. 7 ). The function of the base station will now be discussed in some detail. 
         [0029]    External connectors. In this exemplary embodiment there are several external connectors. These are:
       RS232 connector  31  ( FIG. 4 )   programming port J 6  ( FIG. 4 )   power connector J 5  ( FIG. 7 )   antenna connector  20  ( FIG. 5 )       
 
         [0034]    In an exemplary embodiment one circuit board carries the circuitry for  FIGS. 4 ,  6 , and  7 , and a separate circuit board carries the circuitry for  FIG. 5 . Transmitter circuitry  FIG. 7  includes a Complex Programmable Logic Device (CPLD) U 6 . Microcontroller circuitry  FIG. 4  includes a microcontroller  27 . 
         [0035]    Receive data path. A received RF signal is received on a loop antenna, omitted for clarity in  FIG. 5 , but attached to a pair of terminals at connector  20 . Relays  21 ,  22 , and  23  are controlled by transistors  47 ,  46 , and  45  respectively, and comprise a four-to-one multiplexer which connects one of four possible antennas to the rest of the circuitry. Selector lines  24  control the multiplexer, and pass through connector  25  to  FIG. 4 , where they are selected by microcontroller  27 . 
         [0036]    The received signal is matched (tuned) by capacitors selected by relays  34 ,  35 ,  36 ,  37 ,  38 . These capacitors are connected, or are not connected, in parallel, so as to provide an optimal standing-wave ratio between the antenna and the transmitter ( FIG. 7 ) and receiver ( FIG. 6 ). The relays are controlled by control lines  44  which pass through connector  25  to  FIG. 4 , where they are selected by microcontroller  27 . In this way, any of thirty-two different capacitance values may be selected for antenna-matching purposes. The received RF signal passes through connector  67  to  FIG. 7 . 
         [0037]    In  FIG. 7 , the received signal at connector  67  is coupled to signal line  48 , called “recsig”. This signal line passes to signal line  48  in  FIG. 6  which is the radio receiver. The received signal is filtered and reaches transformer  49  which develops two phases of the received signal,  180  degrees out of phase with each other. Multiplexers  50  and  55  are each a triple two-channel multiplexer, or the equivalent of three single-pole-double throw switches. Control line  60  determines which of the two phases is provided to analog path  51 . Control line  61  determines which of the two phases is provided to analog path  52 . 
         [0038]    A first integrator receiving the analog path  51  and feeds output line  53 . A second integrator receives the analog patch  52  and feeds output line  54 . Discharge line  57  determines whether the capacitors of the integrators are discharged (reset). Output lines  53 ,  54  pass to  FIG. 4  and thence to microcontroller  27 . Microcontroller  27  optionally provides the received signal as a TTL signal to jumpers  29  and thence to RS232 level shifter  28 , and thence to RS232 connector  31  to external equipment omitted for clarity in  FIGS. 4 ,  5 ,  6 , and  7 . 
         [0039]    Transmit data path. External equipment, omitted for clarity in  FIGS. 4 ,  5 ,  6 , and  7 , may pass a serial data signal through connector  31  ( FIG. 4  to RS232 level shifter  28  and thence to microcontroller  27 . Microcontroller  27  sends a transmit signal on line  64  to  FIG. 7 , where IC U 6  develops quadrature A and quadrature B signals QA and QB. These signals drive a transformer which is in turn coupled with connector  67 . RF signal at connector  67  passes to  FIG. 5 , where it is coupled with the above-mentioned antenna tuner capacitors and then to one or another of four possible antennas at connector  20 . 
         [0040]    In use, the base station described here can interact with myriad RF tags, carrying out a method such as the following:
       transmitting a power/clock field from the antenna at a first frequency below 1 MHz, thereby providing power and clock to a plurality of the RF tags;   while the power/clock field is being transmitted, first transmitting a data signal from the antenna at a second frequency, the second frequency higher than the first frequency and below 1 MHz, and receiving a response from at least a first one of the RF tags at the antenna while not receiving a response from at least a second one of the RF tags;   tuning the antenna differently than during the first transmitting;   while the power/clock field is being transmitted, for a second time transmitting a data signal from the antenna at the second frequency, and receiving a response from the at least a second one of the RF tags at the antenna.       
 
         [0045]    The first and second frequencies may both be below 450 kHz. 
         [0046]    The tuning of the antenna yielding the response from the at least a second one of the RF tags may be based upon stored information regarding tuning that previously yielded a response from the at least a second one of the RF tags. 
         [0047]    The method may be such that the first transmitting also does not receive a response from at least a third one of the RF tags, and may be such that the second transmitting also does not receive a response from the at least a third one of the RF tags, in which case the following may be carried out:
       tuning the antenna differently than during the first transmitting and differently than during the second transmitting;   while the power/clock field is being transmitted, for a third time transmitting a data signal from the antenna at the second frequency, and receiving a response from the at least a third one of the RF tags at the antenna.       
 
         [0050]    Yet another method that may be carried out is:
       transmitting a power/clock field from the first antenna at a first frequency below 1 MHz, thereby providing power and clock to a plurality of the RF tags;   while the power/clock field is being transmitted, first transmitting a data signal from the second antenna at a second frequency, the second frequency higher than the first frequency and below 1 MHz, and receiving a response from at least a first one of the RF tags at the second antenna while not receiving a response from at least a second one of the RF tags;   tuning the second antenna differently than during the first transmitting;   while the power/clock field is being transmitted, for a second time transmitting a data signal from the second antenna at the second frequency, and receiving a response from the at least a second one of the RF tags at the second antenna.       
 
         [0055]    It will be appreciated that this base station is able to carry out its activities in a way that is optimized for a particular tag, despite its possible detuning due to nearby metal objects or other interfering conditions, based for example upon a stored record of the circumstances that permitted successful communication with that particular tag in the past. 
         [0056]    The base station is able to select among several antennas, each of which may be oriented differently in three-dimensional space, and one of which may couple more effectively with a particular RF tag than the others of the antennas. 
         [0057]    The base station is able to carry out impedance matching (by means of the antenna tuner of  FIG. 5 ) to couple effectively between a particular antenna and the radio transmitter and receiver ( FIGS. 6 and 7 ). In particular, antennas can be “random wire” and can nonetheless be tuned effectively to the radio transmitter and receiver. 
         [0058]    The base station is able to shift its transmit and receive frequencies dynamically, so as to pick up RF tags that have been detuned as mentioned above. 
         [0059]    Those skilled in the art will have no difficulty devising countless obvious variations and improvements upon the invention as described here in connection with a particular embodiment, which variations and improvements are intended to be encompassed within the claims which follow.