Abstract:
An improved non-obtrusive weapon detection system and method used in discriminating between a concealed weapon made of a ferromagnetic material. The system provides a high probability of detection of hand guns and other types of weapons with a low false alarm rate. The detection of the weapon is accomplished by measuring a total electromagnetic field. The total field being the sum of an incident electromagnetic field and an electromaetic field scattered from the object. The system uses a magnetic field transmitter, which transmits a low intensity electromagnetic signal. The electromagnetic signal illuminates a volume wherein the weapon, called a target, may or may not be carried by a person. The electromagnetic signal is in a form of a sudden steplike change in a constant magnetic field, called a “time-domain” excitation. The waveform or step pulse of the time-domain excitation is called a Heaviside step pulse. The step pulse creates two signals, which are analyzed and digitally processed using a preprogrammed computer. The analyzed information allows an observer to identify the target as being threatening or non-threatening.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     This invention relates to a system and method for detecting different types of metal objects and more particularly, but not by way of limitation, to a non-obstructive weapon detection system which allows an observer to discriminate between a handgun made of a ferromagnetic material and other objects carried by a person. 
     (b) Discussion of Prior Art 
     In the past, gun detection became an important technological requirement in the 1960s with the first occurrences of hijacking of commercial aircraft by armed terrorists. In short order, an electromagnetic gun detection system was developed and deployed at airports around the World. The technology used was basic, with the gun detector being no more than a coil of wire wrapped around a frame through which a passenger desiring to board an aircraft had to pass. The loop was instrumented to measure a quantity, the impedance of the loop, that was monitored on a control panel. If a person carrying a gun were to pass through the portal, the electromagnetic field from the coil would induce currents in the gun. This extraction of energy would appear on a monitor as a change in impedance of the coil and trigger an alarm. The alarm indicating that the individual was carrying some metallic object and should be searched. 
     The above mentioned metal detection system has been strikingly successful, even though it cannot truthfully be called a gun detection system. The system merely indicates that a person is carrying some small metal article on his person and serves as a cause for that person to be physically searched. The search is the true act of gun detection. The electromagnetic scanning serves only to select the most likely candidates for a physical search. But, this is enough of a deterrent that very few people are found carrying handguns on attempting to board an aircraft. Over the past three decades, the rate of gun detection has been about 1 in 100,000 people and these have almost always been individuals with a legitimate need and who overlook the procedures required to carry a handgun on board the aircraft. 
     More modern versions of airport gun detection systems are improved over the earlier systems but are not significantly different in their capabilities. But, because they have been very effective, there has not been a need for significant changes in airport gun detection systems. However, there are other arenas in which guns pose an even more dangerous threat to the public welfare. These gun threats being in the commission of armed robbery, in acts of political terrorism and in peacekeeping operations in unstable countries. In these situations, the airport gun detection systems have not been effective. 
     The limitation to the general use of electromagnetic gun detection is that the cooperation of the public must be complete. People choosing to board an aircraft are as concerned with gun detection as the authorities and so are willing and even anxious to divest themselves for a few moments of the metallic objects they are carrying, queue up for passage through the gun detection portal and then go on about their business. In everyday life, very few people are willing to do this, and the ones who are willing, do not carry handguns. Thus, electromagnetic gun detectors have met with essentially a zero level of acceptance by the public. Further, those business establishments that have employed them, use them only rarely because they don&#39;t wish to discourage customers from entering their business. 
     Perhaps, wider acceptance of electromagnetic gun detection systems outside an airport setting could be achieved by the development of a system which is discrete. By this, the presence and operation of the system is not obvious to persons being scanned and the system provides a highly reliable discrimination against non-gun metal objects. The subject invention described herein is intended to meet these objectives. 
     In U.S. Pat. No. 5,552,705 to one of the subject inventors, a non-obtrusive weapon detection system and method for discriminating between a concealed weapon and other metal and non-metal objects is disclosed. In this patent, the detection system describes the use of transmitting low intensity electromagnetic step pulses which cause eddy currents to flow in a metal object under observation. The eddy currents create a plurality of scattered signals which are sufficient to plot a “decay curve” due to the relaxation of the eddy currents. A receiver is used to detect the scattered signals from the eddy currents. Computer equipment is used to digitally process the scattered signals for defining the decay curve. The decay curve provides an accurate determination of the conductive properties of the object under surveillance. This processed information allows an observer to identify the object under surveillance as either threatening or non-threatening. 
     The subject invention measures both the scattered signals from the eddy currents and a magnetization field signal of the object under observation for an improved weapon detection system. Also the new system and method measures a “crossover” point at which the magnetization signal and the induction current signal are of the same amplitude but opposite in sign. 
     Heretofore, there have been a variety of patents describing the use of electromagnetic detection systems in underground mining applications and exploration. In U.S. Pat. Nos. 5,260,660, 5,185,578, 5,066,917 and 4,994,747 by Stolarczykz, apparatus and methods of detecting underground electrically conductive ore bodies and ore zones are disclosed using transmitted electromagnetic energy. Both downhole receivers and downhole transmitters are used in these detection systems. 
     In U.S. Pat. Nos. 4,821,023 and 4,866,424 to Parks, a current state-of-the-art walk-through metal detector using electromagnetic waves for detecting weapons is described. The metal detector is typical of the detection systems found at airports, court houses and other buildings requiring this type of security system. In U.S. Pat. No. 4,821,023 to Ziolkowski, the use of wave propagation equations is described for producing localized pulses of wave energy. In U.S. Pat. No. 4,978,920 to Mansfield et al., a magnetic screen is developed using a coil surrounded by a set of the electrical conductors. The conductors are placed in a specific region in space. 
     None of the above mentioned patents specifically describe the a system and method of improved detection of hand guns and other types of weapons made of ferromagnetic material with a low false alarm rate. The improved detection system accomplished by measuring a total electromagnetic field. The total electromagnetic field being the sum of both an incident electromagnetic field and an electromagnetic field scattered from the object. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is a primary object of the present invention to provide a non-obtrusive weapon detection system which can accurately discriminate between different types of weapons and other objects carried on a person, carried in packaging, luggage, storage containers and other transportation means where different types of weapons may be hidden or smuggled. 
     Another object of the invention is a person under surveillance need not be aware that he or she is being monitored. The system provides for detection without invading a person&#39;s privacy. 
     Still another object of the system and method of detection is the increased high probability of detecting a weapon made of ferromagnetic material with a low false alarm rate. The weapon detection system is designed to discriminate between a concealed weapon and other metal and non-metal objects passing through an area where they are illuminated by an electromagnetic field. 
     Yet another object of the invention is unlike today&#39;s obtrusive metal detectors, a person is not required to turn over metal objects carried on the person to a security guard or to walk through a portal in order to determine the presence of a potentially dangerous metal object. The subject detection system can be used in various applications in public and private buildings, large and small gathering areas along with other types of military and non-military security requirements. 
     The subject improved weapon detection system includes a magnetic field transmitter which transmits a low intensity electromagnetic signal. The electromagnetic signal illuminates a volume or space wherein the weapon, called a target, may or may not present and carried by a person, in luggage, packaging, etc. The electromagnetic signal is in a form of a sudden steplike change in a constant magnetic field, called a “time-domain” excitation. If a ferromagnetic weapon is detected, a transient waveform is generated by a time-domain excitation. The newly created transient waveform is in the form of two signals. The two signals include a magnetization field signal and a scattered eddy current signal. A receiver is used for detecting the magnetization field signal, the eddy current signal and a crossover point of the step pulse. The two signals and the crossover point are analyzed and digitally processed using a preprogrammed computer. The processed information is then compared with similar values of weapons and non-weapons in a stored data base in the computer to predict the nature of the target. The analyzed information allows an observer to identify the target as being a threatening weapon made of a ferromagnetic material or a non-threatening metal or non-metal object. 
     These and other objects of the present invention will become apparent to those familiar with electromagnetic detection systems and weapon detection systems as described in the following detailed description, showing novel construction, combination, and elements as herein described, and more particularly defined by the appended claims, it being understood that changes in the precise embodiments to the herein disclosed invention are meant to be included as coming within the scope of the claims, except insofar as they may be precluded by the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate complete preferred embodiments of the present invention according to the best modes presently devised for the practical application of the principles thereof, and in which: 
     FIG. 1 is a schematic of the electromagnetic weapon detection system. 
     FIG. 2 is a more detailed schematic of the detection system including a transmitter and receiver electronics and transmitter and receiver antennas 
     FIG. 3 is a graph of a curve relationship between magnetization and applied magnetic field strength in a typical ferromagnetic material. 
     FIG. 4 is a graph of a curve of the magnetization of a ferromagnetic material subjected to a magnetic field Heaviside step pulse. The magnetization curve is shown in solid lines and the magnetization curve&#39;s time derivative is shown in dashed lines. 
     FIG. 5 illustrates a target response curve or step pulse of a Jennings 22 caliber handgun made of ferromagnetic material, such as steel. The curve includes a magnetization field signal or measurable magnetization parameter, a scattered eddy current signal or measurable induction parameter and a crossover time or a measurable crossover point of the step pulse. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The basic theory for electromagnetic weapon detection and identification is described in detail in columns 2, 3, 4 and 5 of U.S. Pat. No. 5,552,705 to George V. Keller. Dr. Keller is one of the inventors of the subject application. This material is incorporated herein by reference. Also, the description of using low frequency electromagnetic fields based on Maxwell&#39;s equations are illustrated in the Keller patent. Further, a discussion of computing a magnetic field scattered from a small conductive mass when energized with a low frequency electromagnetic field is analyzed in detail by Kaufman and Keller as published in reference publication to Kaufmann A. A., and Keller, G. V., 1985, title, Inductive Mining Prospecting, Part 1: Theory: Elsevier, Amsterdam 620 pp. 
     In FIGS. 1 and 2, schematics of the electromagnetic weapon detection system are shown with the system having general reference numeral  10 . The system  10  includes a transmitter  12  for generating an electromagnetic signal in the form of a Heaviside step pulse. The transmitter  12  includes solid state electronics  13  with a switch with a 10 usec risetime, a step pulse former with 20 to 50 pulses per second and rated for 20 to 60 ampere. The step pulse is shown as follows:                           
     The 20 to 50 pulses per second of electromagnetic signals are shown as dashed rings  14  in FIG.  1 . The rings  14  emanate from the transmitter antenna  16  shown in FIG. 2. A solid ring  18  indicates an interrogated volume  18 . For example, the interrogated volume  18  might be an entrance area to a store or bank, a walkway between buildings, a departure area at an airport, the boarding area of a bus, etc. 
     A temporal change in the electromagnetic wave field causes current to flow in a metal object  20 . The metal object is also referred to as a target and made of ferromagnetic material. Scattered electromagnetic signals are shown as dashed rings  22  emanating from the metal object  20 . The scattered electromagnetic signals  22  have the form of branches of an exponential curve, with a time constant that is a function of the size, shape and material composition of the metal object  20  observation. It should be mentioned, as described above, not only is the scattered eddy current signal observed but also the magnetization signal is observed along with the crossover point of the electromagnetic signals  22 . 
     The scattered electromagnetic signals  22  are detected by a loop receiver antenna  24  or other fast response magnetic field sensors acting as a magnetometer or by an array of magnetometers. From the antenna  24 , a signal processing unit  26  with receiver electronics  28  conditions the scattered signals  22  in preparation for digital processing of the information. This information is processed using a computer  30  with an A/D converter or equivalent dedicated digital circuitry and wherein the diagnostic characteristics of the time constant of the metal object&#39;s total response curve is analyzed using a software code. The software code is capable of determining time constants and the measuring various signal cross-sections of the objects under observation. 
     The signal processing system using the computer  30  must accomplish three things, they are: 
     1. An accurate determination of the total response curve of the scattered signals  22 . 
     2. Computation of the diagnostic characteristics embedded in the signal  22  and including the magnetization parameter, the crossover point and the time constant for the induction parameter. 
     3. Determination of the nature of the metal object  20  scattering the signal  22  with that particular time constant. 
     Accurate determination of the signal  22  can best be accomplished through synchronous detection (averaging) of several signals caused by repeated application of the excitation field, along with application of appropriate linear and non-linear digital filtering schemes for which the design may be specific to the site when the new system is installed. It should be noted, rapid determination of the meaning of the signal  22  is of the essence in an effective security system, with a need to acquire and process data in a very short time being an essential part of the design of the system. 
     Ideally, data acquisition and processing of signals should be accomplished in a time not exceeding one second, though in some applications, times as long as several seconds may be appropriate. For a system operating at a basic frequency of 50 Hertz, 20 step excitations can be achieved in one-tenth of a second. It is expected that synchronous addition and linear filtering will be accomplished with an embedded analog-to-digital (A/D) convertor linked directly to a preprogrammed central processing unit (CPU) to yield a signal curve like the curve shown in FIG. 5 within at most a few microseconds following completion of the excitation cycle of the system, a time which will range from as little as 0.1 second to as great as several seconds. 
     Referring now to FIGS. 3 and 4 and to the addition of the observation of the magnetization curve of the scattered signal  22 , a brief review of the phenomenon of magnetization is as follows. It has long been recognized by physicists that the effect of a magnetic field on a material object is to generate magnetization with an addition of a magnetic field appearing around the object (Maxwell, J. C., 1891, A Treatise on Electricity and Magnetism, 3rd Edition: Constable and Company Ltd. London (republished in 1954 by Dover Publications, New York and Purcell, Edward M. 1963, Electricity and Magnetism, Berkeley Physics Course-Volume 2: McGraw-Hill, New York). This additional field is created by an alignment of elementary particles (atoms) in the material with an external magnetic field. The magnetization may increase the total magnetic field in the vicinity of the object being magnetized, or it may decrease it. Depending on the behavior with a specific material, materials are classified as being paramagnetic (the total magnetic field is increased by magnetization), diamagnetic (the total magnetic field is decreased by magnetization) and ferromagnetic (the total magnetic field is strongly increased). 
     The physical property of a material that characterizes the strength of magnetization is the magnetic susceptibility. If a material existed which exhibited no magnetization when in a magnetic field, its susceptibility would be unity or (1). In reality, this can only happen in a vacuum. In diamagnetic and paramagnetic materials, the susceptibility differs from unity by only tens of part per million. Measurement of magnetization of such small strengths probably is not useful in gun detection systems. On the other hand, susceptibilities in ferromagnetic materials differ from unity by a few percent and up to vary large factors in strongly magnetic alloys such as different types of steel used in weapon manufacturing. Because handguns almost always have components that are ferromagnetic, it seems quite possible that handguns can be distinguished from other non-ferromagnetic metal objects. 
     Referring now to FIG. 3, in ferromagnetic materials magnetization depends in a complex way on the transmitter&#39;s strength of the applied magnetic field and on its history. For example, if an object were initially in a region completely absent of a magnetic field, its magnetization would change with the transmitter&#39;s strength of the applied electromagnetic signals  14  as shown in FIG.  1 . With the first application of the field, the magnetization will increase linearly, as shown as curve  32 . But as the applied field becomes stronger, the rate of increase of magnetization slows. When the magnetization is reversed, the magnetization also is reversed. However, as the magnetization is reversed, a curve  34  does not follow the in initial curve  32  obtained by the first application of the field. With successive reversals of the applied field, the magnetization will follow a sigmoid pair of curves  34  and  36  as shown in FIG.  3 . The phenomenon is known as magnetic hysteresis. 
     In FIG. 4, a graph of a curve  38  of the magnetization of the metal object  20  made of ferromagnetic material is shown. In this example, the object  20  is subjected to a magnetic field Heaviside step pulse. The magnetization curve  38  is shown in solid lines and the magnetization curve&#39;s time derivative  40  is shown in dashed lines. In the embodiment of this behavior in the improved weapon detection system  10 , the magnetization and the eddy current field will normally be measured with a small induction coil or magnetometer which produces a voltage output that is proportional to the time-rate of change of the combined magnetic fields, that from the magnetization of the object and that accompanying the eddy current flow in the object  20 . Electromagnetic theory guarantees that the two transients will be of opposite polarity. 
     In FIG. 5, a target response curve or step pulse of a Jennings 22 caliber handgun made of ferromagnetic material, such as steel, is illustrated. A step pulse curve  42  includes a magnetization field signal or measurable magnetization parameter curve  44 , a scattered eddy current signal or measurable induction parameter curve  46  and a crossover time or a measurable crossover point  48  of the step pulse curve  42 . 
     The levels of the two signals  44  and  46  depend on the geometric parameters such as the distance of the object  20  from the Heaviside step pulse transmitter  12 , the transmitters strength, and the orientation of the object  20  as well as the magnetic properties of the object. Thus, the amplitudes of the two signals  44  and  46  give only very qualitative information about the object  20 . However, these parameters  44 ,  46  and  48  can be derived and processed to separate non relevant factors from those which specifically separate non-guns from, in this example, the step pulse of the Jennings 22 caliber handgun made of ferromagnetic material. 
     Three informative and measurable factors can be identified which are independent of signal strength. 
     1. The “negativity” of the signal. The negativity of the signal is defined as the ratio of the lowest initial level of the step pulse curve  42  to the peak level or the crossover point  48  of the step curve  42 . This quantity is unity for an object  20  which is not ferromagnetic, and hence, not a gun. 
     2. The “crossover time” or crossover point  48  of the step pulse curve  42  is defined as the delay time at which the signal  44  from the magnetization equals the signal  46  from the induction currents. With the use of a sensor that measures a time-rate of change of the magnetic field, the crossover time is the time at which the signal passes through an external. This quantity can be used to estimate the product of the electrical conductivity and the magnetic susceptibility, and hence, the metallic composition of the object. 
     3. The “slope” of the late part of the decay curve or the scattered eddy current signal  46 , which is the ratio of signal strength at two points along the descending part of the signal. This quantity defines the scattering cross section of the object  20 , which is in a general way a parameter related to the object&#39;s shape. 
     Experimental testing has shown that the above three mentioned measurable factors can be determined with sufficient accuracy to separate ferromagnetic objects that are weapons from other metal objects and non-metal objects under surveillance. 
     From the above mentioned three measured factors, reliable determination that a detected metal object is in fact a gun can be established. 
     If there in no negativity detected, it is very unlikely that the metal object  20  being observed is a gun. Therefore, the computer analysis, using the computer  30 , can immediately indicate a “no risk” target and no further analysis is required. 
     If there is negativity to the detected signal, the likelihood that the signal is from a gun, and in particular some class of gun, the class being calibre, size, weight, etc., can be done using the slope of the magnetization parameter signal  44  and the crossover point  48 . Both are dimensionless parameters, requiring no additional information such as signal strength or location of the metal object  20  in the volume  18  being searched. 
     Theory shows that the slope of the magnetization parameter signal  44  is related by a simple algebraic formula to a quantity known in electromagnetic theory as “cross section” and is a measure of the size and shape of the target. The crossover point  48  is the point on the step pulse curve  42  at which the time rate of change of the magnetization of the metal object  20  equals the time rate of change of the eddy current density. Also, theory shows that the ratio of these two quantities depends algebraically only on the ratio of the magnetic susceptibility of the metal object  20  to its electrical conductivity. This ratio is a sensitive indicator of the metallurgical composition of the object. 
     A decision as to the probable nature of the detected target can be made based on the size/shape factor and the metallurgical composition with considerable confidence. For rapid decision making, perhaps the best approach is to search a data base in the computer  30  wherein these numbers have been compiled for all sorts of guns and various types of weapons. Many standard approaches to a rapid search of a two-parameter data-base are available and well known in the art and need not be discussed herein. 
     While the invention has been particularly shown, described and illustrated in detail with reference to the preferred embodiments and modifications thereof, it should be understood by those skilled in the art that equivalent changes in form and detail may be made therein without departing from the true spirit and scope of the invention as claimed, except as precluded by the prior art.