Patent Publication Number: US-6700544-B2

Title: Near-field plasma reader

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of plasma sensors operating in near-field conditions and in particular to a new and useful plasma sensor array used to detect the presence of an interactive element. 
     Near-field readers are generally known for use in scanning systems. Near-field reader systems take advantage of magnetic field interference between a powered transceiver and a powered or passive object to detect the presence of the object by receiving a return signal from the object with the transceiver. 
     Presently, card and label near-field readers are formed by metal loops which read data in the near electromagnetic field. In the near-field situation, for a loop antenna, the electric field is effectively zero and only the magnetic field is present. Thus, near field loop antennas use mutual inductance between active and passive loop antennas to cause the active loop antenna to receive data from the passive loop antenna. That is, the magnetic flux from one loop antenna induces a current in a second loop antenna having properties dependent on the current and voltage in the first loop. The magnetic flux interaction and induced current can be used to transmit information between the loop antennas because of the dependency. The near-field loop antennas can be more correctly considered loop sensors or loop readers, since there is no electric field interaction between the active source and a passive loop. 
     A problem with metal loops used in a sensing array is that even when they are not active, several loops arranged in a multiple orientation array still create unavoidable mutual inductance interferences between loops. That is, even if the metal loop sensors are sequentially activated, they still cause mutual interference with other ones of the loops. The interferences result in detuning of the loops in the array and special considerations must be made when forming arrays. 
     In order to optimize the strength of the mutual inductance field between an active loop sensor and a passive loop antenna, the antennas must be parallel to each other. If the antennas are perpendicular, the magnetic field is zero at the passive loop and there is no mutual induction. The strength of the magnetic field at the passive loop increases as the loops move from a perpendicular to a parallel orientation. For a device to effectively scan a region for a passive loop, a single loop must move through a variety of orientations. The range of effectiveness of an antenna is based on the orientation of the passive and active loops to each other and the diameter of the loop of the active sensor. 
     Patents describing scanning antenna systems using interaction between active and passive antennas include U.S. Pat. No. 3,707,711, which discloses an electronic surveillance system. The patent generally describes a type of electronic interrogation system having a transmitter for sending energy to a passive label, which processes the energy and retransmits the modified energy as a reply signal to a receiver. The system includes a passive antenna label attached to goods that interacts with transmitters, such as at a security gate, when it is in close proximity to the transmitters. The label has a circuit which processes the two distinct transmitted signals from two separate transmitters to produce a third distinct reply signal. A receiver picks up the reply signal and indicates that the label has passed the transmitters, such as by sounding an alarm. 
     U.S. Pat. No. 3,852,755 teaches a transponder which can be used as an identification tag in an interrogation system. An identification tag can be encoded using a diode circuit in which some diodes are disabled to produce a unique code. When the identification tag is interrogated by a transponder, energy from the transponder signal activates the electronic circuit in the tag and the code in the diode circuit is transmitted from the tag using dipole antennas. The transponder uses a range of frequencies to send a sufficiently strong signal to activate a nearby identification tag. 
     A vehicle identification transponder using high and low frequency transmissions is disclosed by U.S. Pat. No. 4,873,531. A transmitting antenna broadcasts both high and low frequency signals that are received through longitudinal slots in a transponder waveguide. Transverse pairs in the waveguide adjacent the longitudinal slots indicate a digital “1”, while the absence of transverse pairs produces a digital “0”. The high and low frequencies are radiated from the transverse pairs to high and low frequency receiving antennas. The transmitting and receiving antennas are fixed relative to each other and move with respect to the transponder. 
     U.S. Pat. No. 5,465,099 teaches a passive loop antenna used in a detection system. The antenna has a dipole for receiving signals, a diode for changing the frequency of the received signal and a loop antenna for transmitting the frequency-altered signal. The original transmission frequency is changed to a harmonic frequency by the diode. 
     As discussed above, near-field loop sensors or readers differ from far field loop antennas by the basic difference that in the near-field, the electric field is effectively zero and the magnetic field of an electromagnetic radiant source is controlling, while in the far field, it is the magnetic field that is effectively zero and the electric field controls. As will be appreciated, the relationships between sources and receivers are different as well due to the different distances and fields which affect communication between them. 
     Plasma antennas are a type of antenna known for use in far field applications. Plasma antennas generally comprise a chamber in which a gas is ionized to form plasma. The plasma radiates at a frequency dictated by characteristics of the chamber and excitation energy, among other elements. 
     Plasma antennas and their far field applications are disclosed in patents like U.S. Pat. Nos. 5,963,169, 6,118,407 and 6,087,992 among others. Known applications using plasma antennas rely upon the characteristics of electric fields generated by the plasma antenna in far field situations, rather than magnetic fields in near-field conditions. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a near-field scanning loop sensor array which eliminates interference between adjacent loop sensors in the array. 
     It is a further object of the invention to provide a near-field loop reader array which can be arranged to scan in multiple directions without concern for interference between array components. 
     Yet another object of the invention is to provide a near-field scanning array composed of switched plasma loop sensors. 
     A still further object of the invention is to provide an apparatus and method for scanning a volume for an interactive component containing a data using a plasma reader. 
     Accordingly, an array of plasma loop sensors which are sequentially made active to scan a space to identify an interactive object comprising a data source based on mutual inductance interaction of the scanning plasma reader with the data source. The data source can be a passive loop of any type. 
     As used herein, plasma loop sensor and plasma loop reader are intended to both mean a near-field active loop device having at least a section of plasma tube, as will be described further herein. The active loop device is a near-field electromagnetic transducer having a conductive plasma section. That is, the plasma loop reader or sensor can both generate a magnetic field and sense an interfering induction current caused by a nearby passive loop. 
     The array of plasma loop sensors are connected to a power source, which may include a frequency switching circuit, and to a sensor circuit. The power source provides power to each of the plasma loop sensors as determined by a sequential switch circuit to make the loop sensors active in turn. The sensor circuit is used to interpret signals received from the data source by each plasma loop sensor while it is active. 
     One or more plasma loop readers can be arranged in arrays in different orientations to form a sensor and then sequentially activated to simulate a change in orientation of the sensor without any physical movement of the plasma loops in the array. Since the inactive plasma loop sensors are effectively invisible to the active plasma loop reader, there is no interference created between them. The plasma loops can be activated and deactivated in microseconds, so that very rapid switching among several plasma loops is possible. The plasma loop readers in the sensor can be arranged in a variety of configurations, including a sphere, a cylinder or other geometric shape. The terminals of each plasma loop reader in the configuration are connected to the power source via a switching circuit and to the sensor circuit. 
     In a further embodiment of the plasma loop readers, they may have several loops of different diameter joined at a common side. That is, there is a common area at the terminals where a portion of the circumference of each loop is the same. When a frequency switch is used in connection with the power source, the power frequency used to activate the plasma loops can be varied to change the frequency at which the plasma loop reader is active. The particular diameter loop in which the plasma is active in the plasma loop sensor is also changed by changing the active transmission frequency. 
     In yet another alternative of the near-field plasma reader, the plasma loops are replaced by metal loops with sections of plasma loop which can be turned on and off. The plasma loop sections are sufficiently large so that when they are turned off, or made inactive, the metal loop is opened enough that it rendered electromagnetically invisible and no longer interferes with any surrounding active loop readers. The plasma loop sections are connected to the power source in the same manner as the full loops and can be switched in the same way. 
     It is intended that the sensor circuit connected to the antennas in the array will be capable of interpreting data received from existing types of passive loops commonly used in security devices and the like. The plasma loop sensor interacts with existing passive loops in the same manner as metal loop sensors, but does not suffer from detuning or interference from surrounding loop sensors. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1A is a front elevation view of a plasma loop antenna of the invention; 
     FIG. 1B is a front elevation view of an alternative plasma loop sensor according to the invention; 
     FIG. 2 is a side elevation view diagram of the magnetic field interaction between a plasma loop sensor of FIG. 1 and a passive loop; 
     FIG. 3 is a diagram of an array of plasma loop readers at different orientations; 
     FIG. 4 is a schematic diagram of a transceiver circuit for use with a plasma sensor system; 
     FIG. 5A is a front elevation view of a metal loop sensor with a plasma section; 
     FIG. 5B is a front elevation view of an alternative embodiment of the metal loop sensor and plasma section of FIG. 5A; 
     FIG. 5C is a front elevation view of a second alternative embodiment of the metal loop sensor and plasma section of FIG. 5A; 
     FIG. 6 is a front perspective view of an array of plasma loop readers mounted in a spherical substrate; 
     FIG. 7 is a sectional top plan view of an alternative embodiment of the array of FIG. 6 taken across an equator of the spherical substrate; 
     FIG. 8 is a front perspective view of a cylindrical substrate holding an array of plasma loop sensors; 
     FIG. 9 is a top plan view diagram of a grocery or department store checkout using a plasma loop sensor array of the invention; 
     FIG. 10 is a side elevation view of a diagram of a toll collection system using plasma loop arrays according to the invention; and 
     FIG. 11 is a front perspective view diagram of a security gate system using a plasma loop scanning array according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, in which like reference numerals are used to refer to the same or similar elements, FIG. 1A shows a plasma loop sensor  10  primarily comprising a tube  12  having electrodes  25 ,  27  at each end. The tube  12  is bent into a circular loop. A pair of leads  20 ,  22  are attached to the electrodes  25 ,  27  for connecting the tube  12  to a power source (not shown in FIG.  1 A). 
     The tube  12  of the plasma loop sensor  10  contains a gas  15  inside the plasm loop sensor  10 . The gas  15  may be neon, xenon, argon or other noble gases. The gas  15  can be ionized to form a plasma in the tube  12  by applying energy to the gas  15  using any of several devices including electrodes  25 ,  27 , inductive couplers, capacitive sleeves, lasers or RF heating. 
     When the gas  15  is ionized, a current I begins to flow between the electrodes  25 ,  27 , which in turn generates a magnetic field having a magnetic flux B. The magnetic field is generated in a direction perpendicular to the plane of the loop antenna  10 . The magnetic field is characteristic of the current I and voltage used to power the plasma in the tube  12 . 
     The plasma loop sensor  10  optimal magnetic induction range is equal to the radius r of the loop. The plasma loop sensors  10  may be made any size as is practical and required by a particular application. For purposes of the invention herein, however, the preferred radius for the plasma loop antennas is between 0.5 cm and 100 cm. Further, it should be noted that although the optimal range of the plasma loop sensors  10  is limited by the radius of the loop, the sensors  10  are still effective across a wider range of distances. 
     The plasma loop sensors  10  may be switched on and off in a matter of 1-10 microseconds, with rapid rise and decay times, so that very rapid switching of the plasma loop readers  10  is possible. 
     The frequency of the ionization energy source also affects the plasma magnetic field radiation frequency. It is possible for the sensors  10  to radiate at frequencies in the range of 0.1 MHz to 100 Ghz. 
     The plasma loop reader of FIG. 1B is a multiple loop plasma reader  710  having three different diameter tubes  720 ,  730 ,  740  with a common tangential side  750  and electrodes  722 ,  724 . A gas inside the tubes can be ionized to different excitation levels depending on the energy applied at the electrodes  722 ,  724 . The different ionization levels correspond to different radiant frequencies for the electromagnetic fields generated by the plasma reader  710 . Thus, the multiple loop plasma reader  710  can be used to generate multiple transmission frequencies or to receive on different frequencies from transmission by changing the energy supplied to the plasma loop reader  710 . 
     FIG. 2 illustrates the interaction of a magnetic field  40  of a plasma loop sensor  10  with a passive metal loop  35 . Plasma loop sensor  10  has a plasma current of I A  which generates magnetic field  40  around the loop  10 . The magnetic field  40  is sufficiently strong to at least effectively extend a distance of about twice the radius r of the loop  10  to passive loop  35 . Magnetic field  40  induces a current I i  in the passive loop  35 . 
     Passive loop  35  includes a frequency changing circuit  36 , which operates on induced current I i  to alter the frequency of the received magnetic field and produce a frequency-changed response magnetic field. The frequency changing circuit  36  causes the induced current I i  to have the altered frequency. The circuit  36  may be connected to the terminals of the passive loop  35  in a known manner. Passive loop  35  and frequency changing circuits  36  known in the prior art disclosed herein, for example, may be used for these components. 
     The induced current I i,  with a different frequency from the plasma current I A , generates a response magnetic field  45  emanating from the passive loop  35 . The response magnetic field  45  is also sufficiently strong so as to interact with the plasma loop sensor  10 . As described further below, the plasma loop sensor  10  can also operate in a receive mode to detect response magnetic field  45 . In the receive mode, the plasma loop sensor  10  has a second induced current that is different from plasma current I A , with characteristics corresponding to the response magnetic field  45 . 
     It should be noted that if the response magnetic field  45  is varied in response to a changing induced current I i  controlled by the frequency changing circuit  36 , that more complex communication is possible, such as transmission of an identifying code in addition to simply indicating the presence of the passive loop  35 . 
     Thus, a single plasma loop sensor  10  can be used to detect the presence of a passive loop  35  and receive communications therefrom. However, the ability of the plasma loop sensor  10  to generate the induced current I i  so that a response magnetic field is subsequently generated and received is dependent in part on the relative orientation of the plasma loop sensor  10  and passive loop  35  to each other. The loops  10 ,  35  must be oriented parallel to each other, as shown in FIG. 2, so that the interaction between the generated magnetic fields  40 ,  45  is maximum. As the relative orientation between the antennas  10 ,  35  changes from parallel to perpendicular, the field interaction with the antennas  10 ,  35  goes from maximum to zero. 
     To solve this problem, there are two primary solutions. One is to physically move the loops  10 ,  35  relative to each other to cover different orientations. The other is to create an array of several differently oriented plasma loop sensors  10  that can be sequentially activated to send and receive magnetic fields  40 ,  45 . 
     In the latter case, plasma loop sensors  10  provide the benefit that they can be easily switched on and off rapidly in sequence. Further, plasma loop sensors  10  can be arranged in any type of sequentially-fired array without affecting adjacent ones of the plasma loop sensors  10  because when the gas  15  is not being ionized to form plasma, the inactive sensor  10  is electromagnetically invisible to another, active plasma loop sensor  10 . 
     An example of an array  100  is shown in FIG. 3, in which seven plasma loop sensors  10  are arranged co-planar directed to different angles at 30° intervals. Although the plasma loop sensors  10  are shown arranged in an arc, this is only for purposes of illustrating the rotation to different angles and is not required. The plasma loop sensors  10  may be arranged co-linear as well, with each loop sensor  10  being rotated 30° from the facing of the previous loop sensor  10 . Further, the angular rotation from one antenna to the next may be more or less than 30°, depending on the number of plasma loop sensors  10  in the array  100  and the desired effective range of each plasma loop sensor  10  based on both the expected distance and angular orientation offset from a passive loop  35 . 
     Each plasma loop sensor  10  has its electrodes connected to a transmitting and receiving circuit (not shown in FIG. 3) with switching between modes and loop sensors  10 , such as will be described in more detail below. 
     FIG. 4 diagrams one possible transceiver circuit  200  for use with an array  100  of plasma loop antennas  10  mounted in substrates  5  for protection during use. A DC power supply  205  is connected to a mixer  210  and an analog to digital converter  230 . The power supply  205  is preferably one which provides standard digital and other voltages needed for operating the circuit components. 
     The transmit segment  215  of the circuit  200  includes RF CW oscillator  210  having its output connected to an RF amplifier  220 . The RF amplifier  220  combines a CW signal from the oscillator  210  with a modulated signal from a connected RF modulator  225  and generates an amplified pulse modulated (PCM) signal having information for transmitting with the plasma loop sensors  10 . The PCM signal is sent to the plasma loop sensor array  100  for energizing an active one of the plasma loop sensors  10  and creating a magnetic field. 
     The PCM signal may be varied using a digital code generator  230  connected to the RF modulator to produce different RF modulated signals. The varying PCM signal in turn provides a time-varying signal to the active plasma loop sensor  10  and results in a time-varying magnetic field being produced by the plasma in the active plasma loop sensor  10 . The digital code generator  230  provides a code word from a look-up table stored in ROM  240 . Changing the code word causes the RF modulator to produce different RF modulated signals. 
     The RF amplifier  220  outputs the PCM signal to sensor switch  270  connected to plasma loop sensor array  100 . Sensor switch  270  controls switching between the transmit  215  and receive  235  circuit segments. Preferably, the sensor switch  270  cyclically alternates between transmit and receive modes. 
     A switch  105  within array  100  is used to sequentially switch power to the several plasma loop sensors  10  in array  100 . Only one plasma loop sensor  10  is made active at one time; the remaining plasma loop sensors  10  do not receive any power so that they are effectively rendered invisible to the active sensor  10  and do not detune the active sensor  10 . While a plasma loop sensor  10  is active, the sensor switch  270  provides at least one transmit/receive cycle for the active plasma loop sensor  10 . 
     After the sensor switch  270  permits a transmit phase in which the active plasma loop sensor  10  generates a magnetic field, the sensor switch  270  changes to connect the active plasma loop sensor  10  to a receive segment  235  of the transceiver circuit  200 . 
     The receive segment  235  includes a limiter circuit  260  for ensuring the received signal from the array is scaled within the operating range of a receiver  265 . The limiter circuit  260  protects the receiver  265  from over-voltage instances in the received signals. The receiver then demodulates a coded reply RF PCM signal, which can be generated by interaction of the active plasma loop sensor  10  with a nearby passive loop. If necessary, the receiver can also amplify the received RF PCM signal to ensure proper decoding. 
     The transceiver circuit  200  includes components for interpreting the received signal. The demodulated coded reply signal is sent from the receiver  265  to a signal processor  255 . The signal processor  255  conditions the coded reply signal for input into a code comparator  250 . When the conditioned reply signal is input at the code comparator  250 , the coded reply is compared to known or expected replies stored in a look-up table stored in ROM. 
     The result obtained by the code comparator  250  is sent to an output  232 . The result may be information received from the passive loop or it may be a null if no passive loop was detected during the transmit/receive cycle. 
     The output  232  can be connected to any device capable of using the digital signal from the A/D converter. For example, in grocery scanning system, the output  232  may be connected to a cash register to provide price and item information received from a scanned object in a grocery bag. 
     While loop sensors wholly composed of plasma tubes are preferred for use, FIGS. 5A-5C illustrate metal loop sensors  300  having plasma sections  310  which are electromagnetically equivalent to the plasma loop sensors  10  described above. The metal loop sensors  300  with plasma sections  310  are also magnetically invisible to adjacent loops when the plasma sections  310  are deactivated. That is, the plasma sections  310  are sufficiently long that when the ionizing energy is removed from the electrode terminals  315 ,  317 , the loop circuit is broken so that a magnetic field will not generate a current in the metal loop  300 . Since current cannot flow through the loop  300  except when the gas  15  is ionized to form plasma, the metal loop sensor  300  also appears invisible and does not cause detuning of surrounding sensors  10 ,  300  when it is inactive. 
     The plasma sections  310  act like switches for the metal loop sensors  300  to activate and deactivate them in the same manner as the plasma loop sensors  10  are activated and deactivated. When power is supplied to the plasma section  310  through leads  320 ,  322  and electrodes  315 ,  317 , the metal loop sensor  300  is activated and transmits a magnetic field which can interact with other adjacent loop sensors. The metal loop sensors  300  can be connected to a circuit such as that shown in FIG. 4 in the same manner as the plasma loop sensors  10 . Arrays of the metal loop sensors  300  can be connected, oriented and sequentially switched using the plasma sections  310  in the same manner as the plasma loop sensors  10  described herein as well. 
     The plasma section  310  can be as short as a 1° arc segment of the metal loop sensor  300 , up to the entire circumference, less a gap for electrodes, so that it is the same as plasma loop sensor  10 . However, when the metal loop sensor  300  embodiment of the loop sensors  10  is used, it is preferred that the plasma section  310  is an arcuate segment between about 1° and 10° long. 
     FIGS. 6-8 illustrate scanning arrays  100  of plasma loop readers  10  supported in rigid substrates  290 ,  295 . 
     In FIG. 6, a spherical non-magnetic substrate  295  supports an array  100  of plasma loop readers  10  on its surface. The substrate  295  is selected so that it does not interfere with the magnetic fields and electrical properties of the plasma loop sensors  10 . Although non-magnetic substrates are preferred, it should be understood that ferrite materials may be used for the substrate as well. 
     The terminal leads  20 ,  22  of each plasma loop sensor  10  are connected to a switching transceiver (not shown in FIG.  6 ), such as one like that illustrated in FIG. 4, so that each plasma loop sensor  10  may be sequentially activated. 
     The plasma loop sensors  10  are arranged around the surface of the sphere oriented along many different radii of the sphere. The orientation of the plasma loop sensors  10  allows sequential scanning of a broad range of angles for corresponding passive loops  35  within the effective range of the plasma loop sensors  10 . Since the orientations of the plasma loop sensors  10  varies across the surface of the spherical substrate  295 , the substrate itself does not need to rotate. The sequential activation of the plasma loop sensors  10  virtually rotates the scanning angle without moving the substrate  295 . Clearly, when the substrate  295  is spherical, a wide range of angles can be scanned for corresponding receiving loops in objects carrying the receiving loops. 
     FIG. 7 illustrates another embodiment of the spherical substrate  295  having an array  100  of plasma loop readers  10  embedded within the thickness of the substrate  295 . The substrate  295  is shown with the top half of the sphere removed. As can be seen, the plasma loop readers  10  are oriented at different angles along each of several axes of the sphere. The orientations of the plasma loop readers  10  are selected to maximize the scanning coverage of the array  100 . As in FIG. 6, the plasma loop readers  10  are each connected to a switch and transceiver circuit (not shown in FIG. 7) for sequential activation to ensure there is no electromagnetic interference between plasma loop readers  10  in the array  100 . 
     In FIG. 8, a cylindrical substrate  290  has an array of plasma loop sensors  10  arranged around the surface of the substrate  290 . The substrate is selected to have the same properties as the spherical substrate  295 . The cylindrical substrate  290  scans for corresponding receiving passive loops located around the axis of the cylinder within the effective range of the plasma loop sensors  10 . The cylindrical substrate  290  with the plasma loop sensors  10  mounted only on the surface is limited compared to the spherical substrate  295  in that only two axes of receiving passive loop orientations can be fully scanned versus three. 
     However, if the plasma loop sensors  10  are embedded in a cylindrical substrate  290  around the surface and oriented rotated about the cylinder radial axis to different angles, then all three axes can be scanned with a sensor array using the cylindrical substrate  290 . That is, passive loops oriented perpendicular to the longitudinal axis of the cylindrical substrate  290  could be detected as well. 
     Arrays  100  of the plasma loop readers  10  can be used in a variety of scanning applications to detect a receiving passive loop, such as the one shown in FIG.  2 . 
     FIGS. 9-11 depict different scanning applications for arrays of the plasma loop sensors which take advantage of the fact that the array itself does not need to move physically to scan a wide range of angles, as discussed above. 
     In FIG. 9, a checkout lane  340  of a grocery or department store is shown having a cart  330  containing packages or bags  332  containing goods. Depending on the circumstances, either the packages or the goods are each encoded with a unique receiving passive loop (not shown). The lane  340  has two counters  350 ,  355  each having a plasma loop scanner  360 ,  365  located vertically at about the level of the bags  332  in the cart  330 . Each plasma loop scanner includes an array of plasma loop sensors and a switching and transceiver circuit for sequentially activating each sensor in the array to query the goods in the bags  332 . The outputs of the transceiver circuits are connected to a cash register  380  for ringing up each unique goods detected in the cart  330  and completing the sale. 
     The scanners  360 ,  365  use an array such as the spherical or cylindrical arrays of FIGS. 6-8, or a semi-sphere array which scans the 180° in the lane  340 . The semi-sphere array can be created by cutting the spherical substrate  295  in half and using only one half. The arrays are connected to a transceiver circuit like that of FIG. 4, or another circuit having a similar function. 
     When the transceiver of FIG. 4 is used, the ROM  240  provides a look-up table for identifying each uniquely coded object having a receiving passive loop that is detected by the scanners  360 ,  365 . Either of the cash register  380  or the scanners  360 ,  365  includes a logic circuit or computer for determining when the same receiving passive loop is detected by a subsequently activated plasma loop sensor in the array. The logic circuit or computer ignores the duplicate detection, while passing newly detected goods to the cash register  280  for pricing and totaling the purchase. 
     The scanner system of FIG. 9 provides a checkout line in which it is unnecessary for a customer to unload the cart  330  for a clerk to individually scan items in the bags  332 . The contents of the bags  332  can be determined solely by using the scanners  360 ,  365 . Further, depending on the effective range of the arrays in the scanners  360 ,  365 , only one of the scanners may be needed. Where the distance across the lane  340  is too great for a scanner  360  from one side to effectively detect receiving sensors on the far side of the lane, the second scanner  365  can be used as well. 
     Used in combination with a known debit and credit card terminal  385  connected to the cash register  380 , a single clerk can effectively manage several checkout lanes  340  at once, since the checkout is fully automated except when cash or a check is used as payment. Consumers can bag their goods as they shop since it is not necessary to remove the items for checkout, further eliminating wasted checkout time. 
     FIG. 10 illustrates a toll collection system in which a toll gate  450  is equipped with a scanner  400  connected to a transaction manager  410 . The scanner  400  includes an array of plasma loop readers  10 ,  300  as in the checkout lane scanners  360 ,  365 . The array is used to rapidly sequentially scan for receiving passive loops oriented in a range of angles on cars  420 ,  425 ,  430  passing underneath the toll gate  450 . 
     Each car  420 ,  425 ,  430  that will use the system is assigned a unique receiving sensor for identifying the car. The transaction manager  410  contains logic programming for determining whether a particular car  420 ,  425 ,  430  has been scanned already or if it is unique from prior scanned cars. The toll gate  450  may contain anti-fraud devices as well, such as weight-triggered checks against whether a receiving passive loop was detected or human toll collectors who can monitor the system. 
     As will be appreciated, the horizontally and vertically oriented scanners described above can be used in wide range of applications where an object coded with a unique receiving passive loop passes below or adjacent a scanning array of plasma loop sensors. Further, the particular vertical or horizontal orientation shown in the examples is not intended to be limiting, as the scanners could be oriented to any fixed position which is more practical, subject to ensuring the plasma loop readers in the scanner are oriented to scan the appropriate area. 
     And, when a unique identification is not required, but merely detection, the receiving passive loop in the object to be detected does not need to include a unique code. The scanning array is used to simply detect the presence of the receiving passive loop and generate an alert, such as in a store security system or another gated area for holding animals or objects carrying receiving passive loops having a scanner at the gate. 
     As an example, in another embodiment of a scanning system, FIG. 11 shows a gate  515  having two walls containing scanners  520 ,  525  connected to an alarm system  530 . A person  500  has a card  510  or other substrate carrying a receiving passive loop. If the person  500  passes through the gate  515  with the card  510 , the plasma loop sensors in the scanners  520 ,  525  will detect the presence of the card  510  by interaction with the passive loop and the alarm system  530  will generate a response, such as shutting the gate  515 , sounding a siren or making a light flash. Such a scanning system can be used for ensuring certain persons do not exit a gated area, provided compliance with carrying the card  510  can be guaranteed. 
     Alternatively, the card  510  may contain a uniquely coded identifier for the person  500 . The card  510  can be coded to permit access through some gates  515  without sounding an alarm, while passing others will activate the alarm. In such cases the scanners  520 ,  525  and alarm system  530  include a code table for interpreting which card  510  is passing the gate  515  and determining the permissions associated with the card  510  before sounding an alarm or preventing passage. 
     It should be understood that any one or a combination of the plasma loop sensor  10 , metal loop sensor  300  with plasma section  310  or multiple loop plasma sensor  710  can be used in the arrays and scanning systems described herein. 
     Further, although the sensed loops  35  are referred to herein as passive loops, it is envisioned that the sensed loops can be active also, so as to produce their own magnetic field. For example, a lithium battery source could be connected with the sensed loop and frequency changing circuit like that shown in FIG. 2 to power the sensed loop and circuit. The principles of near-field induction are not changed and the plasma loop sensors  10 ,  300  can still detect the presence or absence of such active sensed loops, as well as receive information from the sensed loops. 
     While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.