Abstract:
A device for detecting the presence of a ferromagnetic object comprising at least one coil formed from a multiple winding of a wire and a controller comprising an alarm. The coil is constructed and arranged such that, upon the relative motion of the ferromagnetic object proximate the at least one coil, a voltage induced in the coil is transmitted to the controller, and controller is constructed and arranged to determine whether the induced voltage falls within a predetermine range and, if so, to trigger the alarm.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to ferromagnetic object detectors and more particularly to a passive coil inductor system for detecting ferromagnetic objects which are passed through the system or to which the system is passed in proximity. 
     DISCUSSION OF THE RELATED ART 
     Metal detectors are used for security purposes in a number of locations, such as airports, federal buildings, banks, schools and other high-security installations. Currently, there are two types of metal detectors in use in such installations. The first type includes a transmitting coil located on one side of the detector and a receiving coil located on the opposite side of the detector. Typically, the magnetic field is generated on one side of the detector by the transmitting coil, and the generated field is received on the other side of the detector by the receiving coil. As long as the magnetic field received by the receiving coil is within the predetermined parameters programmed into the detector, an alarm is not actuated. However, the passage or presence of a ferromagnetic object through or in the magnetic field causes a disturbance in the field received by the receiving coil. If this disturbance causes the magnetic field to fall outside of the predetermined parameters, the alarm associated with the detector is actuated. 
     Another type of ferromagnetic metal detector is disclosed in U.S. Pat. No. 3,971,983 to Jaquet. This detector employs a number of gradiometers positioned on both sides of a walk-through portal. While this device does not actively generate a magnetic field within the portal, the gradiometers monitor the magnetic field generated by the earth. Any disturbances in the earth&#39;s magnetic field, such as may be caused by the presence of a ferromagnetic object within the portal, are detected by the gradiometers, resulting in the activation of an alarm. 
     While these types of detector systems can be very accurate, the operation of and hardware associated with the system are very complex and these systems are very expensive to manufacture and operate. Furthermore, these systems can be very sensitive to changes in the magnetic field which occur outside of the detectors and which are not caused by the contraband which the detectors are designed to detect, thereby resulting in undesired false alarms. 
     What is needed therefore is a ferromagnetic metal detecting system which is inexpensive to manufacture and operate and which results in less false detections and consequently, less false alarms. 
     SUMMARY OF THE INVENTION 
     The present invention includes a ferromagnetic metal detector which is simple and inexpensive to manufacture and operate and which is also less prone to interference from ferromagnetic materials which are not passed through the detector. The simple design of the invention also allows the detector to be easily transportable, thereby allowing the detector to be moved between different locations that require monitoring. The detector of the present invention includes a pair of vertically aligned inductive coils located to define a passageway therebetween. The inductive coils are electrically coupled together and to a control device. Movement of a ferromagnetic object in proximity of the inductive coils induces a voltage within the coils, which voltage may then used to activate an alarm. 
     According to one embodiment of the invention, a device for detecting the presence of a ferromagnetic object is disclosed, comprising at least one coil formed from a multiple winding of a wire and a controller comprising an alarm. The coil is constructed and arranged such that, upon the relative motion of the ferromagnetic object proximate the at least one coil, a voltage induced in the coil is transmitted to the controller, and controller is constructed and arranged to determine whether the induced voltage falls within a predetermine range and, if so, to trigger the alarm. 
     Other features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings which illustrate the best mode presently contemplated for carrying out the present invention: 
     FIG. 1 is a perspective view of a first embodiment of the present invention; 
     FIG. 2 is a cutaway view of the invention, showing a coil inductive sensor including a highly magnetically permeable cylindrical coil, taken along lines  2 — 2  in FIG. 1; 
     FIG. 3 is a cutaway view of the invention, showing a coil inductive sensor without a highly magnetically permeable cylindrical coil, taken along lines  2 — 2  in FIG. 1; 
     FIG. 4 is a partial view of a second embodiment of the present invention, including a large-scale multi-winding coil sized to allow a person to step through the coil; 
     FIG. 5 is a perspective view of a third embodiment of the present invention; 
     FIG. 6 is a perspective view of a fourth embodiment of the present invention; 
     FIG. 7 is a perspective view of a fifth embodiment of the present invention, which includes a hand held device including coil inductive sensor; and 
     FIG. 8 is a block diagram showing the components of a control device of the invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention takes advantage of the relationship between inductive coils and ferromagnetic materials. It is well-known that, when a coil is exposed to a magnet, two things can occur. First, if there is no relative movement between the magnet and the coil, no voltage is generated in the coil, and therefore, no current flows in the coil. However, if there is relative movement between the coil and the magnet, meaning that either the coil is held still and the magnet moved, or the magnet is held still and the coil is moved so that the windings of the coil cross the magnetic lines of force of the magnet, a voltage, called the induced emf, is generated in the coil, resulting in a flow of current in the coil. 
     The present invention takes advantage of this physical phenomena and of the fact that most ferrous weapons, including guns and knives, have built into their domain structure a certain level of magnetic bias which results in a measurable magnetic field which exists in the surrounding space, by using the induced emf to actuate an alarm which indicates the presence of a ferromagnetic material proximate the coils associated with the invention, as will be described in greater detail below. 
     Referring now to the figures, and more particularly to FIGS. 1-3, a first embodiment of the walk-through metal detector, generally indicated at  10 , of the invention will be described. The detector  10  includes a pair of upright columns  12   a  and  12   b,  which are mounted on platforms  14   a  and  14   b,  respectively, for stability. The columns and supports  12   a,    12   b  and  14   a,    14   b,  respectively, are preferably formed from a non-magnetic material such as plastic, so as to not interfere with the operation of the detector  10 . Each column  12   a,    12   b  houses a coil inductive sensor  16 , partially shown in FIG. 2, which is a cutaway view of the column  12   a  as seen along line  2 — 2  in FIG.  1 . An identical coil inductive sensor is housed in column  12   b.    
     As can be seen in FIG. 2, the coil inductive sensor includes a single coil multilayer winding of wire  18  wound onto a highly magnetically permeable core, shown in phantom at  20  in FIG.  2 . The sensor could also be formed without the highly magnetically permeable core, as shown in FIG.  3 . The sensor  16  also includes a shield  22  which completely surrounds the coil to shield the coil sensor from undesired high frequencies which could interfere with the operation of the sensor while allowing the very low frequencies from the intrinsic magnetic fields emitted from personally carrier items to pass through to the sensors relatively unattenuated. Preferably, this non-magnetic highly conductive shield  22  is formed from a material such as aluminum. 
     The coil inductive sensor  16  in each column  12   a  and  12   b  can span the entire length of the column, which preferably is approximately 6 feet in length. Alternatively, the coil inductive sensor  16  can include a series of smaller coil sensors spaced along the column  12   a  and  12   b.  At the upper end  24   a,    24   b  of each column, the wire which forms the coil  18  is fed back down through the coil as indicated at location  26 , for connection to the controller  30  of the system, which will be described in greater detail below. As can be seen in FIG. 1, the sensors located in the columns  12   a  and  12   b  are electrically connected to each other by a shielded cable  32  and then to the controller  30  by a shielded cable  34 . Both ends of the wire  18  which forms the coil sensor  16  located in the column  12   a  are electrically connected to the controller  30  through the shielded cables  32  and  34 . Likewise, both ends of the wire  18  which forms the coil sensor  16  located in the column  12   b  are electrically connected to the controller  30  through the shielded cable  34 . Power is supplied to the controller  30  via a typical wall plug  31 . 
     As described above, the detector  10  is a passive detector and therefore only requires power to run the controller  30 . In operation, the columns are set on the floor at an entryway or other security checkpoint. The shielded cable  32  has a length which allows the columns  12   a  and  12   b  to be spaced approximately three feet apart. As people pass between the columns  12   a  and  12   b,  the relative movement of any ferromagnetic materials being carried through the detector will induce a voltage in either or both of the coils  16  of the columns  12   a  and  12   b.  The resulting current in the coils  16  is fed through the shielded cables  32  and  34  to the controller  30  for processing. 
     The controller  30  will now be described with reference to FIG. 8, which is a block diagram of the components of the detector of the present invention. As shown in FIG. 8, the controller  30  includes an amplifier  40 , a tunable filter  42  and an alarm device  44 , which includes a visual output  46  and an audio output  48 . Upon the relative motion of a ferromagnetic device proximate the coil inductive sensors  16 , a voltage is induced in the sensors. The resulting current is then supplied to the amplifier  40  through the shielded cables  32  and/or  34 . The current is amplified and then passed to the filter  42 , which operates to filter out frequencies which do not indicate the presence of a ferromagnetic weapon. In others words, ultra low frequencies of gradually changing magnetic fields and other undesirable direct current and ultra low frequency circuit effects are filtered out and high frequencies resulting from electronic interference are also filtered out by the filter  42 . The filter  42  can be made tunable to allow the operator to adjust the filter band to accommodate for variations in the surrounding area and to compensate for the speed of the people who are passing through the detector. Any signals which are not filtered out by the filter  42  are passed to the alarm  44 , which operates to notify the operator that a potential weapons has passed through the detector. The alarm  44  can notify the operator by either or both of the visual output  46 , which can include a series of lights, and the audio output  48 , which can include a buzzer or other audio indicator. The controller  30  can either be mounted on a nearby wall or placed on a table or other support. Alternatively, the controller  30  can be built into one of the columns  12   a  or  12   b.    
     As described above, the detector  10  only reacts to a ferromagnetic object if there is relative movement between the ferromagnetic object and the coil inductive sensor  16 . Therefore, the detector is less prone than the prior art to false alarms from objects placed in the proximity of the detector. For example, if a ferromagnetic wastebasket was placed next to a prior art detector, because the wastebasket could alter either the magnetic field generated by the detector or the earth&#39;s magnetic field which is monitored by the detector, the detector could signal a false alarm based on the disturbance of the magnetic field caused by the wastebasket. On the other hand, since the detector of the present invention would react only to the relative movement of the wastebasket, as soon as the wastebasket is no longer moving, the detector does not react to its presence. Therefore, the detector is less prone to false alarms. Furthermore, as described above, the design of the detector  10  enables the detector to be portable, as the columns  12   a  and  12   b  and the controller  30  can easily be carried to the desired security location. 
     Alternative embodiments of the invention will now be described with reference to FIGS. 4-7. A second embodiment of the detector of the present invention is generally indicated at  50  in FIG.  4 . The detector  50  includes a large coil  51  formed into a loop by a wire  52  which is located within a housing  54 . Both ends  56   a  and  56   b  of the wire  52  are electrically connected to a controller which is identical to the controller  30  described above. The coil  51  and housing  54  of the detector  50  preferably are approximately 6 feet high and three feet wide, to enable a person to step through the detector. Similar to the detector  10 , any relative motion between a ferromagnetic object and the coil  51  induces a voltage in the coil  51 , which voltage is transmitted to the controller  30  through wire ends  56   a  and  56   b.    
     A third embodiment of the invention is generally indicated at  60  in FIG.  5 . This embodiment includes columns  62   a  and  62   b  connected across the tops thereof by a crosspiece  64 . Columns  62   a  and  62   b  are mounted on platforms  66   a  and  66   b,  respectfully, for stability. A plate  68  interconnects the platforms  66   a  and  66   b.  The detector  60  includes a single coil inductive sensor, similar to that described with respect to FIGS. 1-3, which extends from the bottom of column  62   a  proximate platform  66   a,  upwardly through column  62   a,  across crosspiece  64  and down through to the bottom of column  62   b  proximate platform  66   b.  The beginning and end wires of the coil (not shown) are coupled to the controller  30  through the shielded cable  34 . Similar to the detector  10 , any relative motion between a ferromagnetic object and the coil within detector  60  induces a voltage in the coil, which voltage is transmitted to the controller  30  through the wire ends. 
     A fourth embodiment of the invention is generally indicated at  70  in FIG.  6 . This embodiment includes panels  72   a  and  72   b  connected across the tops thereof by a crosspiece  74 . Panels  72   a  and  72   b  are mounted on pairs of legs  76   a  and  76   b,  respectfully, for stability. A plate  78  interconnects the legs  76   a  and  76   b.  Large single coils, shown in phantom at  80   a  and  80   b  are disposed within panels  72   a  and  72   b,  respectively. These coils are formed from a single wire into a loop, similar to the coil  51  shown in FIG.  4 . The ends of the wires which form coils  80   a  and  80   b  are electrically coupled to the controller  30  through the shielded cable  34 . Similar to the detector  10 , any relative motion between a ferromagnetic object and the coils  80   a  and  80   b  within detector  70  induces a voltage in the coils, which voltage is transmitted to the controller  30  through the shielded cable  34 . 
     A fifth embodiment of the invention is generally indicated at  100  in FIG.  7 . This embodiment is a hand held version of the detector and includes a handle  102  which is coupled to an elongate housing  104 . Enclosed within housing  104  is a coil inductive sensor  106  which is similar to the coil inductive sensor described with reference to FIGS. 1-3. Again, the ends of the wire which forms the sensor  106  are electrically coupled to a controller  30  through a shielded cable  35 . Alternatively, the controller  30  may be built into the detector  100 , which may be powered with a rechargeable battery, thus eliminating any cords from the detector  100 . 
     The principle of operation of the detector  100  is the same as the detector  10  of FIG.  1 . However, rather than the detector being still and the person passing through the detector, the person remains still and the detector is moved proximate the body of the person. As described above, any relative motion between the ferromagnetic object and the coil inductive sensor will induce a voltage in the sensor. Accordingly, if the ferromagnetic object is still, but the detector is moved past the object, a voltage will be induced in the coil inductive sensor. Once the voltage is induced in the sensor and passed to the controller, the operation of the controller is the same as described with reference to FIGS. 1-3. The filter of this single-sensor hand held detector is configured to suppress the very low frequency signals caused by the passage of the detector through the earth&#39;s magnetic field or any other ambient field and to respond to abrupt changes as the sensor passes close to a ferromagnetic object. Alternatively, the detector could be formed as a two coil sensor in which the coils are wound in a differential manner to cancel the signals caused by movement through any ambient magnetic fields and such that it responds only to ferromagnetic material which is closer to one coil than the other. In this case, the coils are typically separated by several inches and are wired as one sensor. 
     Based on the above, it can be seen that the present invention provides a ferromagnetic metal detector which is inexpensive to manufacture and also to operate, since the detector is passive and the only part of the detector which requires power is the controller. The detector is highly portable and is less prone to false alarms than the prior art. 
     Furthermore, although the invention has been described as a detector for security purposes, it will be understood that the invention may be used to detect the removal of ferromagnetic metal objects such as tools from a work site and may also be used as an anti-pilferage device in locations such as retail outlets and libraries. In this case, small, discreet magnets may be strategically placed on the goods which are to be protected, such that, if a person attempted to carry the protected object through the detector, an alarm would be actuated by the detector. 
     While there is shown and described herein certain specific structure embodying the invention, it will be manifest to those skilled in the art that various modifications and rearrangements of the parts may be made without departing from the spirit and scope of the underlying inventive concept. For instance, the coil inductive sensors can be oriented in any direction, i.e. vertically, horizontally, or diagonally, in order to detect lines of magnetic force which occur at varying orientations. Accordingly, the invention is not limited to the particular forms herein shown and described except insofar as indicated by the scope of the appended claims.