Abstract:
A magnetic detection system to be used by security personnel for the purpose of discovering hidden or otherwise concealed objects being brought into or taken out of a defined or screened area employs magnetic induction sensors and, more particularly, a support structure that holds one or more sensors in a defined orientation relative to an object to be screened. The system can also include auxiliary components, such as a cancellation unit for nullifying an interfering environmental field, a camera for taking photographs or video of a subject, and presence sensors for use in verifying or signaling the existence of a subject to be screened.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims the benefit of U.S. Provisional Patent Application Ser. No. 60/611,846 entitled “Passive Magnetic Detection Gateway for Security Screening” filed Sep. 22, 2004, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to a security detection system and, more particularly, to a passive apparatus and method for detecting unauthorized items, specifically items made from ferrous metals, that are passed through a screening system. 
         [0004]    2. Discussion of the Prior Art 
         [0005]    Many types of screening systems for the detection of concealed objects are known and used in a variety of security situations. The majority of these systems are used to detect unauthorized objects that can be used as weapons and are being concealed by a person attempting to gain access to some type of facility, e.g. an airport, a school, or a public forum like a sports stadium. A large percentage of these security devices are based on the use of magnetic sensors to detect metal present in the unauthorized objects. 
         [0006]    Most magnetic metal detectors rely on an applied magnetic field to induce electric currents in metallic objects, and then detect the magnetic field produced by this current. These systems take advantage of their control of the applied field to generate a signal sufficient to discriminate the measured signals from environmental noise, and to detect the metallic objects. 
         [0007]    Passive magnetic-based screening systems do not utilize an applied field and must use detection circuitry with very high sensitivity. In addition, measures must be taken to isolate the magnetic field detectors from environmental interference and the effect of vibration in the Earth&#39;s magnetic field. The standard method to achieve such noise isolation is to produce a magnetic gradiometer by subtracting the output of two calibrated and balanced sensors. The sensors must be rigidly connected so that they move as a common unit and situated such that one couples to the signal of interest more strongly than the other. The latter requirement results in a system structure much larger than it otherwise would need to be. 
         [0008]    The magnetic sensors that have been used to date in prior art passive systems are DC coupled. This means they respond directly to the Earth&#39;s static magnetic field and are accordingly strongly affected by low-frequency motion in that field. The low-frequency motion can be caused by people walking nearby, the operation of vehicles and machinery, and the like, at least some of which is very likely to be present in practical security screening scenarios. In addition, practical, affordable prior art DC-coupled magnetic sensors are limited to a sensitivity of approximately 10 pT/Hz 1/2  at the frequencies of interest for passive security screening. 
         [0009]    Therefore the construction and operation of known magnetic-based screening systems provide for limited accuracy, sensitivity and field of use. To this end, there exists a need in the art for an improved security detection system which overcomes at least the deficiencies set forth above. 
       SUMMARY OF THE INVENTION 
       [0010]    The passive magnetic detection gateway for security screening in accordance with the invention utilizes a set of magnetic sensors mounted on or in a framework or other support structure. In connection with the invention, a preferable type of magnetic sensor is a magnetic induction sensor. This type of sensor is AC coupled and so does not suffer from the problem of coupling to very low frequency signals. Also, it can be easily configured so that it does not respond to signals above a certain defined frequency. The induction sensor has the further advantage of having the highest sensitivity (&lt;1 pT/Hz 1/2 ) of room temperature magnetic field sensors. Until recently, conventional magnetic induction sensors were simply too large and too expensive to be used in most screening applications. However, magnetic induction sensors have now been developed which are small enough to enable multiple units to be built into common structures, such as a gateway of a walk-through screening device, while retaining sensitivity of order 1 pT/Hz 1/2 . Preferably, the sensors are mounted vertically, but can also be mounted along or normal to the direction of transit. The sensors can be placed at specific, predetermined positions in the support structure to advantageously give an indication of the actual location of the detected item on the body of a subject or object being screened. 
         [0011]    The induction sensor employed in connection with the invention utilizes a preamplifier that responds directly to the magnetic field at the sensor, rather than responding to the rate of change of magnetic field, as do conventional induction sensors. This new approach is based on reading out the electrical current signal from the induction sensor winding and results in a smaller sensor volume for a given sensitivity than prior art designs, which are optimized to read the coil voltage. 
         [0012]    In addition to the sensors in the support structure, one or more additional sensor(s), positioned relatively remote from the sensing region, can be used to measure existing environmental signals and generate an electrical current signal of opposite sign that is passed to coils wound around sensors in the structure of the same orientation. This current produces a magnetic field in the main sensors with the purpose of canceling the environmental magnetic signal. By using this analog cancellation method, the maximum amplitude of the time-varying magnetic signal that must be collected by subsequent electronics is significantly reduced. In addition, the amplitude of general variations in the background signal is reduced in the recorded signal. This rejection of the background fluctuations greatly reduces the false alarm rate of the overall system when screening for small objects. In particularly high noise environments, such as operation outdoors, it may be necessary to add active cancellation of environmental noise through software. In this case, the output from the remote sensor(s) can be digitized and then subtracted by an appropriate algorithm running on a computer. 
         [0013]    A magnetic field-based security screening system constructed in accordance with the invention has the benefit of not emitting any active probing fields, and has high tolerance to environmental electromagnetic noise and noise due to vibration-induced motion of the support structure. Owing to the use of magnetic induction sensors rather than magnetic gradiometers, the width of the opening afforded by the support structure can be increased considerably over that of prior systems and smaller objects can be detected. 
         [0014]    The small size and high performance of the sensors makes it possible to employ additional sensors to cancel environmental noise. In addition, induction sensors have the benefit that, owing to their simple high-permeability cores, it is relatively simple to null the pickup of external noise by feeding an active signal to a small coil coupled to them. Such active nulling allows cancellation of high amplitude interference such as from power lines that typically limits the dynamic range of magnetic sensors, enabling the full sensitivity of the induction sensors to be exploited. In conjunction, or separately, software-based adaptive nulling methods can be employed with induction sensors to produce effective detection sensitivities well below the environmental magnetic field level. 
         [0015]    Thus, the application of a new magnetic induction sensor system makes possible the construction of an improved passive screening device for ferrous objects. The improved sensitivity allows a reduction in the number of sensors needed and, since the more sensitive sensors do not need to be in such close proximity to an object of interest as with other systems, a wider, more open screening arrangement can be established. The use of noise cancellation methods enables the fall sensitivity of an induction sensor to be used without constructing gradiometer sensing units, while allowing the detection of very small objects in a practical environment. 
         [0016]    Additional features of the invention include adding a presence sensor, such as a light beam, pressure pad or the like, at the support structure to detect the presence of a subject, i.e., person or object to be screened. In addition, a video or still camera can be used to photograph subjects being screened. Furthermore, magnetic or other sensors can be placed adjacent the support structure to sense anyone trying to pass a detectable item around the structure. In general, the sensor system of the invention could be employed in any structure around which people must normally pass such that the detection system operates inconspicuously to detect objects of interest. In any event, additional objects, features and advantages of the present invention will become more fully apparent from the following description of preferred embodiments shown in the figures wherein like reference numerals refer to corresponding parts in the several views. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0017]      FIG. 1  is a perspective view of a security gateway incorporating a passive magnetic detection system constructed in accordance with the present invention; 
           [0018]      FIG. 2  is a perspective view of a compact magnetic induction sensor employed in the passive magnetic detection system of  FIG. 1 ; 
           [0019]      FIG. 3  is a block diagram of a control arrangement employed with the passive magnetic detection system of the invention; 
           [0020]      FIG. 4A  is a schematic representation of a circuit employed with the compact magnetic induction sensor of  FIG. 2 ; 
           [0021]      FIG. 4B  is a schematic representation of another circuit employed with the compact magnetic induction sensor of  FIG. 2 ; 
           [0022]      FIG. 4C  is a schematic representation of a further circuit employed with the compact magnetic induction sensor of  FIG. 2 ; 
           [0023]      FIG. 5  is a graphical representation of the response and sensitivity of the magnetic induction sensor detection system; and 
           [0024]      FIG. 6  is a schematic representation of an active noise cancellation assembly employed in connection with the magnetic induction sensor detection system; and 
           [0025]      FIG. 7  is a perspective view of another security gateway arrangement incorporating the passive magnetic detection system of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    A passive magnetic detection system  2  for detecting ferrous objects being carried by a subject, i.e. a person or container, such as a box or crate, into a secured area is shown in  FIG. 1 . Detection system  2  comprises of a rigid structure or gateway  4  and a number of magnetic induction sensors  10 - 19 . As shown, gateway  4  is shown to include laterally spaced pillar units  25  and  26 , as well as an interconnecting overhead unit  28 . In a preferred embodiment of the invention, gateway  4  is intended to be set up on a flat surface  35 , either outside on the ground or inside on a floor. In the embodiment shown, gateway  4  constitutes the type of security screening structure through which people must pass in various places, such as airports, schools, government buildings and the like. To this end, pillar units  25  and  26  are spaced far enough apart so that an average person can easily walk through the center of gateway  4  and below overhead unit  28 . Actually, based on the structure and operation of system  2 , pillar units  25  and  26  can be spaced farther apart than typically found in the prior art. In any case, gateway  4  includes a frame  38  which can be formed in many ways and of various materials. In the most preferred embodiment, frame  38  is composed of hollow tubes, such as that indicated at  39 , that are large enough to contain a respective sensor  10 - 19 . 
         [0027]    Although the embodiment shown in  FIG. 1  includes overhead unit  28  interconnecting pillar units  25  and  26 , as will become more fully evident below, a connection between pillar units  25  and  26  is not required and, in fact, gateway  4  can take various forms, including a single pillar or a lengthened gateway or tunnel with additional sensors along the direction of transit or in a horizontal plane perpendicular to this direction if appropriate structural members are provided. System  2  might be disguised by building sensors  10 - 19  into an existing structure, such as an entry kiosk or into an architectural feature like a column or a planter. Sensors  10 - 19  could also be built into a crowd control structure, like a turnstile or stanchion used with ropes or chains to organize a crowd of people into individual lanes. As will be discussed further below with respect to these embodiments, objects can pass on any side of gateway  4  and still be detected. 
         [0028]    In the most preferred embodiment, sensors  10 ,  12 ,  13  and  15  are attached to gateway  4  in a vertical orientation and constitute primary sensors. Since it is reasonable to assume that there is an equal probability that unauthorized items could be located in any part of gateway  4 , at least sensors  10 ,  12 ,  13  and  15  are preferably, equally provided on each side of gateway  4 . The spacing, both horizontally and vertically, of sensors  10 - 19  not only enables the detection of ferrous items (not shown) which are directed through gateway  4 , either carried by a person or located in a box, bag, crate or the like, but advantageously identifies the location of a detected item relative to the subject (not shown) possessing the item. More specifically, since the magnetic signal produced by a ferrous item decreases with the distance from the item, signals from the various sensors  10 - 19  can be processed to actually indicate the presence and positioning of the item relative to the subject being screened. In addition, the signals from sensors  10 ,  12 ,  13  and  15  can be averaged to increase the spatial resolution of system  2 . It should be obvious to those skilled in the art that the more sensors used, the more accurate the localizing accuracy will be. At this point, it should be understood that the spacing and orientation of the various sensors can vary from that shown, with at least two of the sensors being preferably used in order to establish the presence and positioning of the item relative to the subject being screened as will become more fully evident below. 
         [0029]    Although not shown for the sake of simplicity, additional sensors could be positioned at the rear of gateway  4  such that sensors  10 ,  12 ,  13  and  15 , along with the additional rear sensors, make independent measurements of the subject as it passes through gateway  4 . This arrangement provides improved detection performance by allowing two independent measurements of a given subject. In addition, as the time variation of signals as a person or container carrying an unauthorized object passes through gateway  4  depends in part on the size of the item and the orientation of the sensors relative to the line of travel of the subject, it is possible to estimate the size of the item, in particular to distinguish a small, highly magnetic object from a physically larger object, such as a gun. Analysis of the time variation also provides a method to determine whether a concealed object is at the front or the back of the person carrying the item or the container in which the item is located. 
         [0030]    In the preferred embodiment shown, additional sensors  11 ,  14 ,  18  and  19  are placed on gateway  4  in other orientations, e.g., horizontal, skewed, etc., as compared to the primary sensors. These additional sensors  11 ,  14 ,  18  and  19  are designed to provide reference signals used to reduce the detection of spurious signals by primary sensors  10 ,  12 ,  13  and  15 . For example, sensor  18  located on an edge of gateway  4  furthest away from the likely location of the item to be detected is employed to reduce the detection of environmental background magnetic fields. In a similar fashion, one or more sensors  40  could be mounted adjacent gateway  4  near a local interfering source, like computer  45  or a conveyer belt motor (not shown). Similarly, a sensor (not shown) could also be located near the position where a security guard might stand carrying a service revolver. In a situation where multiple systems are in use simultaneously, one or more additional sensors are preferably used to reject signals from people walking through neighboring structures. 
         [0031]    In addition to the above, sensors  16  and  17  are strategically placed on gateway  4  near floor  35  for the purpose of detecting shoe shanks made from ferrous metals. In addition to sensors  18  and  19 , other sensors (not shown) could be mounted on the outside of gateway  4 , preferably in a vertical orientation, to detect unauthorized items that may be passed around gateway  4  in an attempt to escape detection. In any case, it should be readily apparent that the number and positioning of the various sensors can be readily altered in accordance with the invention, with at least a first set of sensors representing primary sensors designed to detect the existence and vertical positioning of ferrous items, a second set of sensors being employed to detect and counter the effects of background and other adjacent magnetic fields, and a third set of sensors being designed to detect magnetic fields at specific adjacent locations. 
         [0032]    Reference will now be made to  FIG. 2  in describing a preferred construction for magnetic induction sensor  10  employed in passive magnetic detection system  2  and it is to be understood that sensors  11 - 19  and  40  are correspondingly constructed. As shown, sensor  10  includes a coil  65 , preferably constituted by copper wire, wrapped around a high permeability core  78 . In addition, an amplifier unit  80  is provided in close proximity to coil  65  and adapted to be linked through wiring  82  to a controller and a power source (not shown) as discussed further below. In general, the operation of sensors  10 - 19  is based on the principle that ferrous materials are sources of static magnetic fields. Movement of the ferrous material with respect to coil  65  will cause a voltage, known as the induced emf, to be generated in coil  65  according to Faraday&#39;s law. The induced emf causes a current to flow in coil  65 . Sensor  10  utilizes amplifier unit  80  that responds directly to the magnetic field at sensor  10 , rather than responding to the rate of change of magnetic field, as do conventional induction sensors. This enhanced approach is based on reading out the electrical current signal from the winding of coil  65  and results in sensor  10  being smaller in volume for a given sensitivity than prior art designs, which are optimized to read coil voltage. The use of this particular induction sensor technology enables a significantly more compact detection arrangement for system  2 . 
         [0033]    With reference to  FIG. 3 , system  2  includes a computerized data acquisition assembly  95  including a CPU  96  for the purpose of measuring the response of each of sensors  10 - 19 , analyzing the sensor data to determine if an unauthorized item is present, storing processed information in memory  97  and displaying the results of the analysis, as well as possibly the raw data, through a display unit  98 . As also depicted in this figure, CPU  96  also preferably receives inputs from other screening instruments. In particular, photo and/or video data is gathered and transmitted from a camera  100  (also see  FIG. 1 ). Further inputs are received from one or more presence sensors, such as a light beam sensor  105 , pressure pad  106  or the like, provided at gateway  4  to detect and verify the actual presence of a subject, i.e., person or object to be screened. 
         [0034]    The essential operation of each of magnetic sensors  10 - 19  including coil  65  and amplifier unit  80  is schematically illustrated with reference to sensor  10  in  FIG. 4A . As shown, coil  65  drives a pre-amplifier  125  with a low input impedance (R i ). The voltage at the input to the pre-amplifier  125  is: 
         [0000]    
       
         
           
             
               V 
               a 
             
             = 
             
               
                 
                   R 
                   i 
                 
                 
                   
                     R 
                     i 
                   
                   + 
                   
                     R 
                     s 
                   
                 
               
                
               
                 
                   jω 
                    
                   
                       
                   
                    
                   
                     BA 
                     eff 
                   
                 
                 
                   1 
                   + 
                   
                     jω 
                      
                     
                         
                     
                      
                     
                       L 
                       
                         
                           R 
                           i 
                         
                         + 
                         
                           R 
                           s 
                         
                       
                     
                   
                 
               
             
           
         
       
     
         [0000]    Here ω is the radial frequency, j is the square root of −1, R i  is the amplifier input impedance, R s  is the coil series resistance, A eff  is the effective area of the sensor, L is the inductance of the coil, and B is the component of the field parallel to the axis of the coil. Above a frequency given by 
         [0000]    
       
         
           
             f 
             = 
             
               
                 ( 
                 
                   
                     R 
                     i 
                   
                   + 
                   
                     R 
                     s 
                   
                 
                 ) 
               
               
                 2 
                  
                 π 
                  
                 
                     
                 
                  
                 L 
               
             
           
         
       
     
         [0000]    the response is flat, with the voltage at the input to pre-amplifier  125  being given by: 
         [0000]    
       
         
           
             
               V 
               a 
             
             = 
             
               
                 
                   R 
                   i 
                 
                  
                 
                   BA 
                   eff 
                 
               
               L 
             
           
         
       
     
         [0000]    Thus sensor  10  produces a signal proportional to the ambient B-field and not to its time derivative. Below this characteristic frequency, the response is like the conventional induction sensor, with the response linear in frequency. 
         [0000]    
       
         
           
             
               V 
               a 
             
             = 
             
               jω 
                
               
                   
               
                
               
                 BA 
                 eff 
               
                
               
                 
                   R 
                   i 
                 
                 
                   
                     R 
                     i 
                   
                   + 
                   
                     R 
                     s 
                   
                 
               
             
           
         
       
     
         [0035]    To further emphasize the novel nature of sensor  10 , the analysis above can be repeated in terms of the current produced by sensing coil  65 . As known by those skilled in the art, the current flowing in a coil and the magnetic field produced by a coil are in direct proportion. The coil can thus be viewed as a frequency dependent current source. The optimum way to measure this current is with a low impedance amplifier. The amplitude of the current produced by such a coil and amplifier is: 
         [0000]    
       
         
           
             I 
             = 
             
               
                 ω 
                  
                 
                     
                 
                  
                 
                   BA 
                   eff 
                 
               
               
                 
                   R 
                   i 
                 
                 + 
                 
                   R 
                   s 
                 
                 + 
                 
                   ω 
                    
                   
                       
                   
                    
                   L 
                 
               
             
           
         
       
     
       As for the voltage analysis, this current is frequency independent above a frequency given by 
       [0036]    
       
         
           
             f 
             = 
             
               
                 ( 
                 
                   
                     R 
                     i 
                   
                   + 
                   
                     R 
                     s 
                   
                 
                 ) 
               
               
                 2 
                  
                 π 
                  
                 
                     
                 
                  
                 L 
               
             
           
         
       
     
         [0000]    as was shown for the voltage analysis case. This current is amplified by the circuit shown to produce an output that is frequency independent above the defined circuit dependent frequency value. 
         [0037]    As discussed, the construction of sensor  10 , including the geometry of coil  65  and the configuration of pre-amplifier  125  influence the bandwidth of the sensor response. Specifically, at low frequency, the response of induction sensor  10  is limited by the filter produced by the inductance of sensor  10  in series with its resistance. For sensor  10  constructed in accordance with the invention, a typical lower frequency 3 dB point is 1-2 Hz, which is ideal for guest screening and conventional security screening applications. The highest frequency of interest in such applications is about 10 Hz, and it is relatively easy to arrange for induction sensor  10  to have an upper frequency roll off at this point by changing the capacitance C 2  of a feedback circuit  130  to pre-amplifier  125  such that 
         [0000]    
       
         
           
             
               f 
               2 
             
             = 
             
               
                 1 
                 
                   2 
                    
                   π 
                    
                   
                       
                   
                    
                   
                     R 
                     f 
                   
                    
                   
                     C 
                     2 
                   
                 
               
               = 
               
                 6 
                 - 
                 
                   10 
                    
                   
                       
                   
                    
                   
                     Hz 
                     . 
                   
                 
               
             
           
         
       
     
         [0038]      FIG. 4A  also illustrates the manner in which an output of pre-amplifier  125  is preferably sent to a second stage voltage or differential amplifier  150  of amplifier unit  80  to establish an output signal to be analyzed by CPU  96 .  FIG. 4B  illustrates an alternate circuit which has been found to provide better common mode rejection. In addition,  FIG. 4C  represents a fully differential version of the circuit shown in  FIG. 4A . In this design, the coil contains a center tap  155  connected to ground and each half of the coil is measured by a separate amplifier  125 ,  125 ′ of corresponding design to that of  FIG. 4A . The outputs of amplifiers  125  and  125 ′ are combined and converted to an output referenced to ground in a differential amplifier  150 . In any case, the critical issue is that it is the current flowing in coil  65  that is amplified rather than the voltage produced by the coil. 
         [0039]    The magnetic field sensitivity and bandpass response of induction sensor  10  designed for security screening applications is shown in  FIG. 5 . Tailoring the sensitivity of induction sensor  10  in this manner significantly improves resistance to motion noise and immunity to electromagnetic interference. For the sake of completeness, one preferred embodiment of the invention has R fb =R fb1 =R b2 =50 kOhms; C1=C2=0.47 μF; and f2=6.7 Hz. For a preferred low cutoff frequency using an 18 inch (approximately 45.7 cm) sensor length, R dc =16 Ohm and L coil =1.7 H, then f1=R dc /2πL coil =1.5 Hz. 
         [0040]    The sensitivity data in  FIG. 5  corresponds to the internal noise of sensor  10  and were measured in highly shielded conditions. Even so, significant interference from power line signals at 60 Hz is apparent. To achieve this level of sensitivity, as desired in the practical environments needed for security screening, it is preferable in accordance with the invention to provide active cancellation of external noise that is picked up by sensor  10 . To this end, the present invention preferably employs a noise cancellation unit  200  which is schematically represented in  FIG. 6 . In this embodiment, a remote sensor  205  is located on or adjacent gateway  4 , some distance from the locations of primary sensors  10 ,  12 ,  13  and  15 . Remote sensor  205  is shown to be multi-dimensional, i.e., remote sensor  205  preferably incorporates three orthogonally intersecting sensors  208 - 210 , each of which constitutes a magnetic induction sensor constructed corresponding to any one of sensors  10 - 19 . In any case, remote sensor  205  functions to measure signals from the environment, with the measured signals having minimal contribution from the object or item that system  2  is attempting to detect. This environmental signal is assumed to be substantially common to the total signal measured by each of sensors  10 - 19  in system  2 . The environmental signal from remote sensor  205  is sent to a set of pre-amplifiers  220 , the output of which is inverted at  225  and then used as the input to a current source or driver  230 , which drives secondary coils, such as secondary coils  240 - 243 . 
         [0041]    At this point, it should be noted that, although only one driver  230  is depicted, a separate driver  230  could be employed for each remote sensor  208 - 210  and each secondary coil  240 - 243 . In any case, each secondary coil  240 - 243  is wound around a respective one of coils  65  of primary sensors  10 ,  12 ,  13  and  15 . In series with each secondary coil  240 - 243  is a gain and phase control device  245  that allows the tuning of the cancellation signal. In this manner, the signals from remote sensor  205  null the environmental signals present in the coils  65  of primary sensors  10 ,  12 ,  13  and  15 , thereby enabling improved sensitivity. In general, the amplitude of the nulling signal is adjusted to maximally cancel power line (50/60 Hz) interference. Of course, a similar arrangement could be employed for other sensors utilized in the overall system  2 , including sensors  11 ,  14 ,  16  and  17 - 19 . 
         [0042]    By using this analog cancellation method discussed above, the maximum amplitude of the time-varying magnetic signal that must be collected by subsequent electronics is significantly reduced. In addition, the amplitude of general variations in the background signal is reduced in the recorded signal. This rejection of the background fluctuations greatly reduces the false alarm rate of the overall system when screening for small magnetic objects. As an alternative, the output of remote sensor  205  can be digitized and subtracted in a computer, such as CPU  96 , by an appropriate algorithm. One such active cancellation method that has been shown to work well with induction sensors  10 - 19  employs a Wiener filter which adaptively calculates the coefficients that must be applied to the reference sensor output to cancel signals that are common to the reference and measurement sensors. In particularly high noise environments such as operation outdoors, both analog and software cancellation of environmental noise can be utilized. 
         [0043]    In accordance with another cancellation arrangement, the coefficients for the adaptive cancellation are calculated using data collected prior to the time system  2  is to be used. A defect of this approach is that, in applications such as security screening, the configuration of the conducting objects in the local environment may change throughout the day as staff change their positions and furniture and other objects are moved. As a result, coefficients that gave adequate cancellation of environmental noise at the beginning of the day may not be sufficient at a later time. One means to address this issue, particularly for security screening, is to collect data for cancellation of environmental noise continuously throughout the day, excluding only those times when a person is passing through gateway  4 . Such times can easily be determined by arranging light beam  105 , pressure pad  106 , or an equivalent sensor in gateway  4  to detect the presence of the subject to be screened, with data collected for a brief predetermined time before and after system  2  is triggered being excluded from the overall cancellation scheme. 
         [0044]    A cancellation method to reduce false alarms associated with the subject, i.e., person or object, being screened is to take the difference of the output of any one sensor from the average signal measured by all the signal detection sensors  10 - 17  of the overall array. For example, a system constructed in this manner has been found to be sufficiently sensitive such that, in conditions when a person carries a high static electric charge, system  2  can detect the magnetic field produced by the effect of this charge moving through gateway  4 . This effect produces a signal of almost equal magnitude and phase in all sensors in gateway  4 , and can be removed by subtracting the average signal of sensors from the signal of any one sensor. 
         [0045]    In another variation, a source of active magnetic field can be added to system  2  thereby creating a magnetic field in a vicinity of the support structure so as to induce eddy currents in metal objects, i.e., a magnetic field response in an item of interest. This arrangement is illustrated in  FIG. 1  with the inclusion of a coil  247  in the mat constituting presence sensor  106 . Of course, coil  247  could be placed elsewhere, such as about a tube  42  of gateway  4 . The direct pickup of the field by sensors  10 - 19  can be minimized by adding a corresponding cancellation signal to the active nulling signal applied to each sensor. By these means the sensitivity of system  2  can be increased and the discrimination of metal objects of different kinds can be effected by comparing their responses at different frequencies. 
         [0046]    As discussed above, system  2  may be combined with a video or still camera  100  to photograph the people or other subjects being screened. The video footage or photographs could be stored in memory  97  in association with its corresponding sensor data and can be analyzed for the purpose of identifying the person or persons responsible for transporting an unauthorized object or item. This additional security measure is considered to be particularly advantageous for law enforcement purposes. 
         [0047]    Based on the above, it should be readily apparent that the present invention establishes a magnetic field-based security screening gateway that has the benefit of not emitting any active probing fields, while having high tolerance to environmental electromagnetic noise and noise due to vibration-induced motion of the gateway. Owing to the use of magnetic induction sensors rather than magnetic gradiometers, the width of the opening afforded by the gateway or the spacing between the pillars can be increased considerably over that of prior systems. 
         [0048]    With the above in mind, it should be readily apparent that system  2  can take various forms and be discretely positioned to detect ferrous items unobtrusively. For instance,  FIG. 7  illustrates an embodiment wherein a passive magnetic detection system  2  constructed in accordance with the invention is incorporated into a turnstile  250  typically found at the entrance to an amusement park, subway system or the like. Here, a standard ticket collection or scanning device  255  performs a function corresponding to presence sensors  105  and  106 . In any case, various magnetic induction sensors, such as sensors  260 - 263 , can be unnoticeably carried by turnstile  250  for detection purposes. This arrangement is just intended to be representative of numerous possible implementations of the invention wherein at least one magnetic induction sensor is rendered visually undetectable in a fixture (support structure) commonly found in its environment, with the sensor performing a security screening of people and other objects passing the support structure. For instance, many other common structures can be modified to incorporate system  2  of the invention in order to screen a desired area, including walls of a causeway, fixed refuse containers, light posts, upstanding poles used to rope off or guide individuals, and other standalone structures, with or without ticket or other validation devices. Signals from a succession of support structure mounted sensors could be used to verify the presence of an unauthorized item prior to issuing a warning, such as on a remote display  98  or even an audible alarm. In addition, other types of sensing devices can be used in combination, such as a biometric identification device or a device to read RF or optical tags. 
         [0049]    Further aspects of the invention include the addition of an audio and/or video device mounted at the support structure for communicating messages, such as advertisements, news or instructions, to individuals passing the support structure. This feature of the invention is illustrated with reference to flat screen television  310  shown attached to gateway  4 , although such a message communicating arrangement can be advantageously provided in connection with any support structure. In any event, although the invention has been described with reference to preferred embodiments thereof, it should be understood that various changes and/or modifications can be made to the invention without departing from the spirit thereof. In any case, the invention is only intended to be limited by the scope of the following claims.