Patent Publication Number: US-6902110-B2

Title: Field creation in a magnetic electronic article surveillance system

Description:
RELATED APPLICATIONS 
   This patent application is a divisional of application Ser. No. 09/880,486, filed Jun. 13, 2001, now U.S. Pat. No. 6,696,951. 

   TECHNICAL FIELD 
   The invention relates generally to security systems and, more particularly, to electronic surveillance systems. 
   BACKGROUND 
   Magnetic electronic article surveillance (EAS) systems are often used to prevent unauthorized removal of articles from a protected area, such as a library or retail store. A conventional EAS system usually includes an interrogation zone located near an exit of the protected area, markers or tags attached to the articles to be protected, and a device to sensitize (activate) or desensitize (deactivate) the markers or tags. Such EAS systems detect the presence of a sensitized marker within the interrogation zone and perform an appropriate security action, such as sounding an audible alarm or locking an exit gate. To allow authorized removal of articles from the protected area, authorized personnel desensitize the marker using the EAS system. 
   An EAS marker typically has a signal producing layer that, when interrogated by a proper magnetic field, emits a signal detectable by the EAS system. Markers of a “dual status” type, i.e., markers capable of being sensitized and desensitized, also have a signal blocking layer that can be selectively activated and deactivated. When the signal blocking layer is activated, it effectively prevents the signal producing layer from providing a signal that is detectable by an EAS detection system. Authorized personnel typically activate and deactivate a magnetic EAS marker by passing the marker near a magnetic field produced by the EAS system. The EAS system may include, for example, an array of magnets or an electric coil that produces a magnetic field of a desired intensity to change the state of the signal blocking layer of the marker. Many conventional EAS systems make use of a high voltage power supply and a tuned resistor-capacitor-inductor (RCL) circuit for controlling the magnetic field when sensitizing and desensitizing markers. 
   SUMMARY 
   In general, the invention is directed to techniques for creating and controlling a magnetic field for use with electronic article surveillance (EAS) markers. Unlike conventional systems that may incorporate an RCL circuit or other circuit for generating the magnetic field, the techniques make use of current switching devices to generate a signal having one or more current pulses for creating the magnetic field. 
   In one embodiment, the invention is directed to an electronic article surveillance (EAS) system having a coil to create a magnetic field for interacting with an electronic marker and a drive unit to output a signal having one or more current pulses for energizing the coil. A programmable processor within the EAS system controls the drive unit to generate the output signal according to a desired profile. To generate the output signal, the processor selectively activates electronic current switching devices within the drive unit. 
   By selectively activating and deactivating the current switching devices, the processor can direct the drive unit to generate the output signal according to a desired profile having a number of current pulses of different amplitudes and polarity. The drive unit may advantageously generate the output signal such that the rate of change of the current (di/dt) is substantially constant and, therefore, the current increases or decreases at substantially constant rates. Furthermore, the frequency of the pulses need not be fixed and can be readily controlled by the processor. These features have many advantages including improved marker detection over conventional systems in which the rate of change of the coil current typically follows a sinusoidal or other non-linear profile. 
   In addition, the programmable processor within the EAS system may dynamically adjust the current pulses of the output signal based on a number of factors including one or more configuration parameters set by a user, a type of article to which the marker is affixed, a sensed drive voltage and intensities of previously generated magnetic fields. In this manner, the EAS system is able to generate magnetic fields suitable for a variety of articles ranging from clothing to books to magnetically-recorded videotapes, and can compensate for effects of the surrounding environment or manufacturing variability. 
   In another embodiment, the invention is directed to a method including generating a signal having one or more current pulses by selectively activating and deactivating current switching devices, and driving the signal through a coil to generate a magnetic field for interacting with an electronic marker. The method may further include determining a profile for the current pulses of the signal, and selectively activating and deactivating the current switching devices according to the profile. 
   In another embodiment, the invention is directed to a computer-readable medium containing instructions. The instructions cause a programmable processor to calculate a target intensity for a magnetic field, and activate and deactivate a set of current switching devices to drive a pulse of current through a coil to create the magnetic field based on the target intensity. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will be apparent from the description, the drawings, and the claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a block diagram illustrating an example embodiment of an electronic article surveillance (EAS) system configured according to the invention. 
       FIG. 2  is a block diagram further illustrating the example EAS system. 
       FIG. 3  is a schematic diagram illustrating an example embodiment of a drive unit of the EAS system. 
       FIGS. 4A and 4B  are graphs illustrating example output signals generated by the EAS system to produce magnetic fields. 
       FIG. 5  is a graph illustrating an output signal generated by the EAS system to produce a magnetic field for desensitizing a marker. 
       FIG. 6  is a flow chart illustrating an example mode of operation of the EAS system. 
       FIG. 7  is a schematic diagram illustrating another example embodiment of a drive unit. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram illustrating a system  2  in which a user  4  interacts with an electronic article surveillance (EAS) system  3  to detect or change a state of, or otherwise interact with, an EAS marker  10 . User  4  may, for example, sensitize or desensitize marker  10  when checking in or checking out, respectively, a protected article (not shown) to which marker  10  is affixed. Marker  10  may be affixed to a variety of different articles such as books, videos, compact discs, clothing and the like. 
   EAS system  3  includes a control unit  6  that energizes coil  8  to create a magnetic field  7 . Coil  8  may be any inductor capable of generating a magnetic field  7 . Coil  8  may be, for example, a generally round, solenoid-type coil that provides a substantially uniform magnetic field  7  suitable to activate and deactivate marker  10 . Other types of coils may also be used including non-solenoid-type coils or other devices that provide magnetic fields. 
   To create magnetic field  7 , control unit  6  outputs a signal having one or more current pulses and drives the signal through coil  8  to energize coil  8  and produce magnetic field  7 . Magnetic field  7 , therefore, increases and decreases in intensity based on a “profile” of the pulsed output signal. Control unit  6  controls the intensity and orientation of magnetic field  7  by controlling an amplitude, duty cycle and polarity for each current pulse of the output signal. More specifically, control unit  6  determines a target intensity and orientation for magnetic field  7  and, based on the determined target intensity and orientation, controls a number of current pulses within the output signal, as well as an amplitude, duty cycle and polarity for each pulse. Control unit  6  may calculate the target intensity based on a number of factors. User  4  may, for example, set one or more configuration parameters within EAS system  3  to adjust the intensity. Control unit  6  may also adjust the target intensity based on a type of article to which the electronic marker  4  is affixed. Control unit  6  may, for example, calculate a lower target intensity for magnetically-recorded videotapes than for books or clothing. Control unit  6  may also incorporate an analog-to-digital converter (ADC) to sense a drive voltage and adjust the current pulses based on the sensed voltage. 
   In addition, EAS system  3  may incorporate feedback that enables control unit  6  to dynamically adjust the target intensity for magnetic field  7  based on a sensed intensity of magnetic field  7  or previously generated magnetic fields. More specifically, detector  11  senses an intensity of magnetic field  7  and provides control unit  6  a corresponding signal indicative of the sensed intensity. Based on the signal received from detector  11 , control unit  6  may adjust the output signal to increase or decrease the intensity of magnetic field  7 . In this manner, control unit  6  is able to compensate for effects on magnetic field  7  due to the surrounding environment or manufacturing variability. 
     FIG. 2  is a block diagram illustrating the example EAS system  3  in further detail. In the illustrated embodiment, EAS system  3  includes user interface  13 , processor  12 , drive interface  14  and drive unit  16 . User interface  13  includes hardware and software for interacting with user  4 . User interface  13  may include, for example, a display or other output for presenting information to user  4 , and a keyboard, keypad, mouse, trackball, custom panel or other suitable input device for receiving input. User interface  13  may also include one or more software modules executing in an operating environment provided by processor  12 . The software modules may present a command line interface or a graphical user interface having a variety of menus or windows by which user  4  controls and configures EAS system  3 . 
   EAS system  3  is not limited to a particular processor type. Processor  12  may be, for example, an embedded processor from a variety of manufacturers such as Intel Corporation, Cypress Corporation and Motorola Incorporated. Furthermore, Processor  12  may be a reduced instruction set computing (RISC) processor, a complex instruction set computing (CISC) processor, or variations of conventional RISC processors or CISC processors. In addition, the functionality carried out by Processor  12  may be implemented by dedicated hardware, such as one or more application specific integrated circuits (ASIC&#39;s) or other circuitry. 
   Control unit  6  may include a computer-readable memory (not shown) such as, for example, volatile and nonvolatile memory, or removable and non-removable media for storage of information such as instructions, data structures, program modules, or other data. The memory may comprise random access memory (RAM), read-only memory (ROM), EEPROM, flash memory, or any other medium that can be accessed by the Processor  12 . 
   Processor  12  controls drive unit  16  to output a signal having one or more current pulses and drives the signal through coil  8  to energize coil  8  and produce magnetic field  7 . In particular, drive unit  16  comprises a plurality of current switching devices for driving current pulses through coil  8 . Drive unit  16  may comprise a number of N-Type MOSFET transistors for switching the current through coil  8 . 
   In one embodiment, Processor  12  activates a first set of electronic current switching devices of drive unit  16  to drive the signal through coil  8  in a first direction, thereby creating magnetic field  7  in a first orientation. To create magnetic field  7  in an opposite orientation, processor  12  deactivates the first set of current switching devices and activates a second set of electronic current switching devices to drive the signal through the coil in the opposite direction. In this manner, control unit  6  can control the intensity and orientation of magnetic field  7  by selectively activating and deactivating the first and second set of current switching devices of drive unit  16  to generate the output signal having current pulses of calculated amplitudes and duty cycles. 
   Drive interface  14  includes circuitry for interfacing processor  12  with drive unit  16 . Drive interface  14  may include, for example, programmable logic devices and one or more voltage comparators for providing control signals to drive unit  16  in response to signals received from processor  12 . 
     FIG. 3  is a schematic diagram illustrating an example embodiment of drive unit  16  of EAS system  3 . In this embodiment, drive unit  16  includes two sets of current switching devices  20  and  22  that processor  12  and drive interface  14  can selectively activate and deactivate using control lines C 1  and C 2 , respectively. Based on control lines C 1  and C 2 , voltage level shifters  23 A and  23 B apply suitable voltages to the corresponding gates of current switching devices  20  and  22 . More specifically, processor  12  can direct drive interface  14  to enable control line C 1  and thereby activate a first set of current switching devices  20 A and  20 B. In this mode, current flows from VDC through device  20 A, through coil  8  in a first direction, and through device  20 B to GND, thereby creating magnetic field  7 . Upon deactivating devices  20 A and  20 B, energy is captured from magnetic field  7  and the current flow through coil  7  drops. Similarly, processor  12  can activate a second set of current switching devices  22 A and  22 B by enabling control line C 2 . In this mode, current flows from VDC through device  22 B, through coil  8  in a second direction, and through device  22 A to GND, thereby creating magnetic field  7  in an opposite orientation. 
   Thus, in this exemplary embodiment, processor  12  and drive interface  14  can alternatively enable control lines C 1  or C 2  for activation durations. In this manner, processor  12  can selectively activate and deactivate the first and second set of current switching devices  20  and  22  to direct drive unit  16  to output a signal having one or more current pulses. In response, coil  8  creates a magnetic field  7  having an intensity based on the amplitude of the current pulses and an orientation based on the direction in which the current flows through coil  8 . 
     FIG. 4A  is a graph illustrating an example output signal  30  generated by drive unit  16  ( FIG. 2 ) to sensitize (demagnetize) marker  10 , thereby activating marker  10  for detection by EAS system  3 . In particular,  FIG. 4  plots the current of output signal  30  versus time. For exemplary purposes, reference is made to  FIGS. 1-3 . 
   To demagnetize marker  10 , processor  12  selectively activates and deactivates the first and second set of current switching devices  20 ,  22  ( FIG. 3 ) to generate the output signal  30  having a plurality of pulses  32 A through  32 I, collectively referred to as pulses  32 . Furthermore, by selectively activating and deactivating the current switching devices  20 ,  22  at calculated times, processor  12  can generate the output signal  30  to follow a desired profile. Signal  30  illustrates, for example, a decaying profile in which the amplitudes of the current pulses  32  decay over time. More specifically, processor  12  reduces the amplitudes of pulses  32  over time by shortening the corresponding duty cycle of each pulse, i.e., by activating and deactivating the corresponding current switching devices  20 ,  22  for shorter periods. In this manner, the time period from T 3  to T 5 , for example, is shorter than the time period from T 0  to T 2 . In one embodiment, processor  10  calculates a duty cycle of each subsequent pulse  32  that is 92% of the previous pulse. 
   To generate output signal  30 , processor  12  activates the first set of current switching devices  20  at a time T 0 , forming a first current pulse  32  within the output signal and causing current to flow through coil  8  (FIG.  3 ). At a time T 1 , processor  12  deactivates the first set of current switching devices  20 , causing current to drop from peak  33  until a time T 2  at which time current is no longer flowing through coil  8 . 
   After generating current pulse  33 , processor  12  activates the second set of current switching devices  22  at a time T 3 , forming a second current pulse  35  and causing current to flow through coil  8  in an opposite direction from the current flow of pulse  33 . At a point T 4 , processor  12  deactivates the second set of current switching devices  20 , causing current to drop from peak  35  until a time T 5  when current is no longer flowing through coil  8 . 
   Notably, the increase and subsequent decrease of current flow of pulse  32  has a substantially constant rate of change. In other words, current flow increases and decreases in substantially linear fashion from T 0  to T 1  and from T 1  to T 2 , respectively. Unlike conventional RCL circuits that follow a sinusoidal profile, drive unit  16  outputs a signal in which the rate of change of the current (di/dt) is substantially constant, according to the following equation: 
         V   =       L   ⁢       ⅆ   i       ⅆ   t         +   iR       ,       
 
in which iR is small compared to Ldi/dt. As a result, magnetic field  7  increases and decreases at constant rates in like manner. This has many advantages including improved marker detection.
 
   In order to detect a sensitized marker  10 , control unit  6  senses a signal emitted by marker  10  when marker  10  is exposed to magnetic field  7 . The strength of the signal produced by marker  10  is a function of the location of marker  10  within magnetic field  7  and the rate of change of the current flowing through coil  8 . Because the rate of change of the output signal produced by drive unit  16  is substantially constant, the strength of the signal does not vary as magnetic field  7  increases and decreases. Because control unit  6  need not compensate for signal variability due to changes in the slope of magnetic field  7  versus time, detecting the presence of marker  10  is simplified. 
   In addition, control unit  6  may determine whether marker  10  is sensitized or desensitized based on the harmonic content of the signal produced by marker  10 . The harmonic content of a signal emitted by a marker, however, can be greatly affected by the rate of change of a surrounding magnetic field. Because the rate of change of the output signal produced by drive unit  16  is substantially constant, the harmonic content does not vary due to increases and decreases in magnetic field  7 . As a result, control unit  6  can more readily detect markers and distinguish between sensitized and desensitized markers than conventional systems in which the rate of change follows a sinusoidal or other non-linear profile. 
     FIG. 4B  is a graph illustrating another example output signal  36  generated by drive unit  16  (FIG.  2 ). Processor  12  selectively activates and deactivates the first and second set of current switching devices  20 ,  22  ( FIG. 3 ) to generate the output signal  36  having a plurality of pulses  38 A through  38 E, collectively referred to as pulses  38 . In particular, processor  12  generated pulses  38  to have substantially equal magnitudes  37 ,  40  and substantially equal durations T D . Notably, processor  12  can control current switching devices  20 ,  22  to vary the time periods ΔT 1 , ΔT 2 , ΔT 3 , ΔT 4 , between subsequent pulses  38  to affect a total time for the output signal  36 , and hence change the effective frequency of the output signal  36 . 
   This embodiment can be particularly advantageous for avoiding ambient noise localized at particular frequencies. EAS system  3  may incorporate circuitry similar to drive unit  16  to produce, for example, an interrogation field having a high frequency, beneficial for interrogating EAS marker  10 . In particular, the high frequency interrogation field may give rise to greater signal strength received from EAS marker  10  than magnetic field  7 , which may be primarily used for sensitizing and desensitizing marker  10 . In addition, control unit  6  can also change the effective frequency of the interrogation field by varying a DC supply voltage VDC (FIG.  3 ). 
     FIG. 5  is a graph illustrating an example output signal  49  generated by drive unit  16  ( FIG. 2 ) to desensitize (magnetize) marker  10 , and thereby deactivate marker  10 . To magnetize marker  10 , processor  12  selectively activates and deactivates the first set of current switching devices  20  ( FIG. 3 ) to generate the output signal  49  to have a single pulse  48 . To generate output signal  49 , processor  12  activates the first set of current switching devices  20  at a time T 0 , forming a first current pulse  48  within the output signal  49  and causing current to flow through coil  8 . At a point T 1 , processor  12  deactivates the first set of current switching devices  20 , causing current to drop from peak  47  until a point T 2  at which time current is no longer flowing through coil  8 . 
     FIG. 6  is a flow chart illustrating an example mode of operation of the EAS system  3  when creating magnetic field  7 . For exemplary purposes, reference is made to output signal  30  of FIG.  4 . 
   Initially, processor  12  calculates a peak amplitude  33  for the first current pulse  32 A based on a target intensity for magnetic field  7  ( 52 ). In determining the target peak amplitude, processor  12  may consider a number of factors including a measured drive voltage VDC, one or more configuration parameters set by user  4 , a type article to which market  10  is affixed, and sensed intensities of previously generated magnetic fields, as described above. Typical configuration parameters that a user might set, for example, includes the type of media being processed, such as audio tapes, videotapes, books, compact discs, and the like, setting EAS system  3  in a check-in or check-out mode, setting EAS system  3  to verify the status of marker  10 , and setting EAS system  3  in a non-processing mode to read radio frequency (RF) information from marker  10 . In determining the target peak amplitude, processor  12  may, for example, read a radio frequency identification (RFID) tag fixed to an article or media in order to determine proper parameters for sensitizing or desensitizing the particular tag. 
   Based on the calculated peak, processor  12  determines an activation time TIME ON  and a deactivation time TIME OFF  for the current switching devices of drive unit  16  in order to generate a current pulse having the calculated peak ( 54 ). Next, processor  12  determines a direction for which current should flow through coil  8  according to the desired signal profile ( 56 ). Output signal  30  of  FIG. 4 , for example, has a profile in which a number of current pulses  32  alternate in polarity, yielding current flow in alternating directions. 
   Based on the directions, processor  12  selectively activates the first or second set of current switching devices  20 ,  22 . More specifically, to drive current through coil  8  in a first direction, processor  12  activates the first set of current switching devices  20  by driving control line C 1  high ( 58 ) until the activation TIME ON  has elapsed ( 62 ). In current pulse  32 A, for example, the activation time TIME ON  equals T 1 . Upon expiration of TIME ON , processor  12  deactivates the first set of current switching devices  20  by driving control line C 1  low ( 66 ) until the deactivation TIME OFF  has elapsed ( 70 ). In current pulse  32 A, for example, the deactivation time TIME OFF  equals T 3 −T 1 . 
   After generating the pulse in the first polarity, processor  12  determines whether the target peak amplitude has dropped to a minimum level ( 74 ) and, if so, terminates the process. Current pulse  33 I, for example, has an amplitude below a defined minimum level, causing Processor  12  to stop generating the series of pulses  32 . 
   If, however, the target amplitude has not yet reached the minimum level, processor  14  repeats the process by calculating a new target amplitude ( 52 ) and a corresponding activation time TIME ON  and a deactivation time TIME OFF  ( 54 ). In this iteration, Processor  12  may elect to drive current through coil  8  in a second direction ( 56 ) by driving control line C 2  high to activate the second set of current switching devices  22  ( 60 ) until the activation TIME ON  has elapsed ( 64 ). In current pulse  32 B, for example, the activation time TIME ON  equals T 4 −T 3 . Upon expiration of TIME ON , processor  12  deactivates the second set of current switching devices  22  by driving control line C 1  low ( 68 ) until the deactivation TIME OFF  has elapsed ( 72 ). In this manner, processor  12  may repeat the process to generate an output signal having one or more current pulses according to a desired profile. 
   The above-describe process is for exemplary purposes, and may be readily modified by EAS system  3 . For example, processor  14  may repetitively interrogate the marker and generate magnetic fields of higher intensities until a signal received from the marker indicates that the measured residual value of the marker meets an acceptable level. When sensitizing the marker, processor  12  may control drive circuit  16  to subject the marker to a series of magnetic fields of higher and higher intensities until the residual value for the marker drops and reaches a specified minimum level. Similarly, when desensitizing a marker, processor  12  may control drive circuit  16  to subject the marker to a series of magnetic fields having higher and higher magnetic intensities until the residual value for the marker reaches to a specified maximum level. 
   In this manner, with the ability to interrogate the marker and the ability to control the magnetic field, EAS system  3  can ensure that the marker is subjected to the minimum field necessary to obtain the desired result. Processor  12  may terminate the process when the targeted level has been reached or when a maximum limit on field intensity has been achieved. 
   The ability to finely control the magnetic field offers many advantages, including enhanced detection capabilities if all markers are brought to approximately the same level of residual value. Furthermore, such features may be advantageous in markets with heavy regulations regarding magnetic fields. 
     FIG. 7  is a schematic diagram illustrating another example embodiment of a drive unit  76  that includes capacitor  78  in parallel with coil  8 . In this embodiment, drive unit  76  may provide an output signal having one or more current pulses to charge capacitor  78 , causing magnetic field  7  to resonate at very high frequencies. In this manner, drive unit  76  may be useful in generating magnetic fields for verifying a change of state of an EAS marker and, therefore, detecting whether an EAS marker is present. 
   Various embodiments of the invention have been described. These and other embodiments are within the scope of the following claims.