AU686341B2 – Metal detection system
– Google Patents
AU686341B2 – Metal detection system
– Google Patents
Metal detection system
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Publication number
AU686341B2
AU686341B2
AU18671/95A
AU1867195A
AU686341B2
AU 686341 B2
AU686341 B2
AU 686341B2
AU 18671/95 A
AU18671/95 A
AU 18671/95A
AU 1867195 A
AU1867195 A
AU 1867195A
AU 686341 B2
AU686341 B2
AU 686341B2
Authority
AU
Australia
Prior art keywords
metal
signal
electromagnetic field
coil
mass
Prior art date
1994-01-19
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
AU18671/95A
Other versions
AU1867195A
(en
Inventor
Steven David Frahm
John Edward Turner
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Ranger Security Detectors Inc
Original Assignee
Ranger Security Detectors Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
1994-01-19
Filing date
1995-01-19
Publication date
1998-02-05
Family has litigation
First worldwide family litigation filed
litigation
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https://patents.darts-ip.com/?family=22673939&utm_source=google_patent&utm_medium=platform_link&utm_campaign=public_patent_search&patent=AU686341(B2)
«Global patent litigation dataset” by Darts-ip is licensed under a Creative Commons Attribution 4.0 International License.
1995-01-19
Application filed by Ranger Security Detectors Inc
filed
Critical
Ranger Security Detectors Inc
1995-08-08
Publication of AU1867195A
publication
Critical
patent/AU1867195A/en
1998-02-05
Application granted
granted
Critical
1998-02-05
Publication of AU686341B2
publication
Critical
patent/AU686341B2/en
2015-01-19
Anticipated expiration
legal-status
Critical
Status
Ceased
legal-status
Critical
Current
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Espacenet
Global Dossier
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Classifications
H—ELECTRICITY
H03—ELECTRONIC CIRCUITRY
H03K—PULSE TECHNIQUE
H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
H03K17/94—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the way in which the control signals are generated
H03K17/945—Proximity switches
H03K17/95—Proximity switches using a magnetic detector
H03K17/952—Proximity switches using a magnetic detector using inductive coils
H03K17/9525—Proximity switches using a magnetic detector using inductive coils controlled by an oscillatory signal
G—PHYSICS
G01—MEASURING; TESTING
G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
G01V3/08—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices
G01V3/10—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices using induction coils
G01V3/104—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices using induction coils using several coupled or uncoupled coils
G01V3/105—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices using induction coils using several coupled or uncoupled coils forming directly coupled primary and secondary coils or loops
Abstract
A walk-through metal detection system with split field generation coils excited in phase to generate an electromagnetic field having a substantially uniform vertical field density. A plurality of receive coils are also provided, each receive coil connected to a separate detector circuit for detecting disturbances in the generated field caused by the presence of metal objects. The signals output from the detector circuits are processed in a first embodiment to generate an output signal indicative of the total metal mass detected within the generated electromagnetic field. In a second embodiment, the signals are processed to generate an output signal indicative of the individual lumped metal mass (or masses) detected within the generated electromagnetic field. The output signal is compared to a threshold signal, and if greater than the threshold signal, then an illicit metal object is presumed to be present within the electromagnetic field and an alarm is sounded. Furthermore, the approximate vertical and/or horizontal position of the detected metal object(s) may be determined from further processing of the detector circuit outputs.
Description
WO 95/20205 PCT/US95/00380 -1- METAL DETECTION SYSTEM TECHNICAL FIELD The present invention relates to metal detectors and, in particular, to an improved walk-through metal detection system.
re WO 95/209205 PCT/US95/00380 2 BACKGROUND OF THE INVENTION In recent years, walk-through metal detectors have become a commonly utilized piece of security equipment.
While most people are familiar with, and accustomed to the use of such detection systems in airports, the state of society today has unfortunately necessitated the use of walk-through metal detection systems in such.unconventional locations as schools and courthouses. Regardless of place of use, the primary continuing function of walk-through metal detection systems is to accurately detect the presence of hidden illicit metal objects such as firearms and knives on the body of an individual.
The operation of prior art walk-through detection systems for the purpose of detecting illicit metal objects, however, has been less than satisfactory in at least four ways. First, the detection electromagnetic field generated by prior art transmit-receive coils in walk-through detection systems is plagued by the presence of weak or «dead» spots through which a person may pass an illicit metal object without detection. Alternatively, the detector may be set to respond to objects in the weak or «dead» spots, but unfortunately this causes the detector to be hyper-sensitive in the areas with normal response to trigger undesired metal alarms. Second, the prior art electronic systems provided for processing the signals output from the transmit-receive coils lack the capability WO 95/20205 PCTIUS950380 3 of discriminating between illicit and permissible metal objects. Thus, large metal buttons, pocket change and belt buckles are often identified, quite to the annoyance of the person being scanned, as illicit metal objects. Third, the prior art electronics systems for metal detectors have no provision for discriminating against the cumulative sum total metal mass of small, permissible metal objects versus the metal mass of a single, large, illicit metal object.
This drawback is the largest source of undesired metal alarms, thus necessitating additional security personnel to perform time consuming searches. Fourth, the prior art electronic systems further lack the capability of approximately identifying on the body of an individual the location of the carried metal object triggering the alarm.
Thus, security agents often require a near complete disrobing of the scanned individual to locate the offending metal object and determine whether the object is illicit.
Accordingly, there is a need for an improved walkthrough metal detection system that provides for a more uniform generation of the detection electromagnetic field, is capable of discriminating between illicit and permissible metal objects, is capable of discriminating between cumulative and lumped metal mass, and is capable of identifying the approximate location of the carried metal object triggering the alarm.
WO 95120205 PCTVUS95/00380 4- SUMMARY OF THE INVENTION In accordance with the present invention, a unique configuration of the transmit-receive coil for a walkthrough metal detector is provided. The transmit coil comprises a single coil of wire split in half and excited in phase, with one half of the split coil positioned on either side of a passageway for the walk-through metal detection system. Positioned adjacent each half of the split transmit coil is a separate receive coil. The combined transmit-receive coil is configured with a vertically elongated geometry. Due to mutual coupling between the split transmit coils, an electromagnetic field having a substantially uniform vertical field density virtually no weak or «dead» spots) is generated, the field concentrated in the passageway. With a uniform vertical field, accurate detection of metal objects may be made regardless of relative position within the passageway.
The electronic system for the walk-through metal detection system of the present invention includes a separate detector circuit connected to each receive coil.
The signals output from the detector circuits are processed to generate an output signal indicative of the total metal mass detected within the generated electromagnetic field.
The output signal is compared to a threshold signal, and if greater than the threshold signal, an illicit metal object is presumed to be present within the electromagnetic field WO 95120205 PCT/US95/00380 5 and an alarm is sounded. The use of separate detectors provides maximum sensitivity for discriminating between illicit and permissible metal objects by providing a response to an accurate approximation of true metal mass.
Approximate horizontal position of the detected metal object may also be determined.
In accordance with another embodiment of the present invention, each receive coil on either side of the passageway comprises a plurality of individual receive coils. Each individual receive coil in the plurality of coils is connected to a separate detector circuit outputting a complex phase and amplitude signal indicative of the detection of metal object(s) within the electromagnetic field. Each of the plurality of phase amplitude signals are converted to digital signals and processed using digital signal processing techniques to identify the metal object(s) disrupting the electromagnetic field. The use of dual multiple receive coils with digital signal processing facilitates not only the discrimination between illicit and permissible metal objects by determining individual lumped metal mass, but also the determination of the both the approximate vertical and horizontal position of the metal object detected in the field.
BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the metal detection system of the present invention may be had by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein: FIGURE 1 is a block diagram of a first embodiment of the metal detection system of the present invention; FIGURE 2 is a more detailed block diagram of the first embodiment of the system shown in FIGURE 1; FIGURE 3 illustrates the geometry of one of the transmit-receive coils in the first embodiment of the metal detection system shown in FIGURE 2; FIGURE 4 is a block diagram of a second embodiment of the metal detection system of the pr-sent invention; FIGURE S is a block diagram of the detector circuit for the system of FIGURE 4; FIGURE 6 illustrates the geometry of one of the transmit-receive coils in the second embodiment of the metal detection system shown in FIGURE 4; FIGURE 7 is a circuit diagram for the dual rate, high 20 pass filter for the system of FIGURES 1 and 2; :FIGURE 8 is a circuit diagram for one of the chained logarithmic amplifier circuit with auto-zeroing for the system of FIGURES 1 and 2; and FIGURE 9 is a circuit diagram for dual signal, dual nS rate, high pass filter for the system of FIGURES 4 and WO 95/20205 2PCTIVUS95100380 7 DETAILED DESCRIPTION OF THE DRAWINGS Referring now to FIGURE 1, there is shown a block diagram of a first embodiment of the metal detection system of the present invention. The system 10 utilizes a pair of multiple turn, very low frequency coil sets 11 positioned on either side of a passageway 26 through which persons br articles pass for scanning and detection of illicit metal objects. The system 10 utilizes a field generation coil 12 divided between the coil sets 11 and connected to a single oscillator circuit 14. The single coil of wire for the field generation coil 12 is split at its mid-point 16 into a right coil 12r for one coil set llr and a left coil 121 for the other coil set 111. The split coils are connected in parallel, with the mid-point 16 coupled to one alternating current port 18 of the oscillator 14 and the ends 20 connected together and to another alternating current port 22. The right and left field generation coils 12r and 121 are excited in phase by the oscillator 14 to generate a single alternating current electromagnetic field 24 concentrated with substantially uniform field density in the passageway 26.
The presence of metal objects in the passageway 26.
causes a disturbance in the electromagnetic field 24 generated by the co-phased field generation coils 12. This disturbance is sensed by dual, right and left receive coils 28r and 281, respectively, that, like the split transmit
II
-8coil 12, are also positioned in the coil sets Ilr and 111 on opposite sides of the passageway 26. Each receive coil 28 in a coil set 11 is connected to a corresponding detector circuit 30 that detects the field disturbance. A processor 32 takes the approximate cube roots of the signal outputs from the detector circuits 30, redundantly multiplies and cross-differentiates the cube roots, and selects the lesser result (weaker signal) as an analog approximation of the total mass of the metal object(s) detected in the passageway 26. If the determined metal mass exceeds a predetermined threshold mass level, then an alarm 34 is sounded indicating the presence and detection of an illicit metal object. Although the preferred embodiment of the detector circuit 30 and processor 32 comprises analog circuit devices, it will, of course, be understood that the requisite detection and processing
S..
functions may be implemented using digital signal processing techniques as well.
•Reference in now made to FIGURE 2, wherein there is shown a more detailed block diagram of the first embodiment of the metal detection system 10 of FIGURE 1. A channel selector circuit 36 is connected to the oscillator 14 to i allow for the selection (signal CS) by the system user of different freuencies (preferably approximately 6 kilohertz, with 100 hertz separating the channels) for the T generated alternating current WO 95/20205 PCTIUS95/O0380 9 electromagnetic field 24. Slight phase adjustments in the generated alternating current signal are also possible with the selector circuit 36. With proper frequency and phase selection, multiple systems 10 are operable in close proximity to each other with minimal interference to adversely affect performance. The oscillator 14 further includes two square wave outputs 38 and.40. The first output 38 provides a square wave in phase (signal I) with the generated electromagnetic field 24. The second output 40 provides a square wave in quadrature phase (signal Q) with the signal exciting the generated electromagnetic field 24.
Although illustrated in FIGURE 1 as a single coil, each side of the field generation coil 12 in a coil set 11 is preferably comprised of three separate elements. The first element is a transmit coil 42r and 421, the second element is a null adjust loop 44r and 441, and the third element is a feedback coil 46r and 461. The three elements of the field generation coil 12 in each coil set 11 are connected in series between the mid-point 16 and the coil end Reference is now made to FIGURE 3 wherein there is shown the geometry of one coil set 11 positioned on one side of the passageway 26. It will, of course, be understood that a mirror image of the coil geometry
II
WO 95/20205 PCT/US95/00380 10 illustrated in FIGURE 3 is present in the coil set 11 on the opposite side of the passageway 26.
The transmit coil 42 for the field generation coil 12 has rounded corners and an elongated rectangular shape having a longitudinal axis 43 of symmetry. The null adjust loop 44 is coaxially aligned with the transmit coil 42 having a similar elongated rectangular shape with rounded corners. The null adjust loop 44 has a narrower width and a slightly shorter length than the transmit coil 42. The feedback coil 46 is co-planar with and also coaxially aligned with the transmit coil 42, and is co-located with and has a substantially identical overall size and shape as the null adjust loop 44. Similarly, the receive coil 28 is also co-planar and coaxially aligned with the transmit coil 42, and is rc-located with and has a substantially identical overall size and shape as the null adjust loop 44 and feedback coil 46.
The transmit coil 42 is shielded with a resistive Faraday split section tubular shield 45 that is terminated to system ground 49. The co-located receive coil 28 and feedback coil 46 are gathered together and also shielded with a resistive Faraday split section tubular shield 47 that is terminated to system ground 49. Termination of the shields 45 and 47 to system ground provides primary electromechanical interference and radio frequency interference suppression. Openings in the tubular shields -11and 47 are selectively located near the longitudinal ends of the coils to minimize loading of the generated electromagnetic field 24.
By co-locating the null adjust loop 44, the receive coil 28 and the feedback loop 46 in the coil set 11, the sensitivity of the system 10 near the longitudinal end regions 51 of the coil structure is enhanced. The null adjust loop 44 further includes a plurality of null adjust wire crossover points for shaping the generated electromagnetic field 24. With proper null adjustment, a substantially uniform electromagnetic field, free from weak or «dead» spots, is generated.
Referring again to FIGURE 2, each detector ciz uit includes a balun 48 for terminating a corresponding one of the receive coils 28r and 281. The balun 48 provides a balanced input 50 connected to the ends 52 and S~ >f the receive coil 28, thus allowing the receive coils 28 to operate with a relatively high resonant frequency providing increased sensitivity. The balun 48 incorporates input low pass filtering circuitry to provide both electromechanical and radio frequency interference suppression. The balun 48 further converts the balanced input 50 to a single ended, unbalanced termination 58. The unbalanced signal at termination 58 is applied to a pre-amp circuit 60 that incorporates further low pass filtering and shifts the phase of the unbalanced signal to output a prec~ R4 ~3 ~Pi iLI it vl* WO 95/20205 PCT/US95/0038(0 12 amp signal on line 62. The phase oa. he pre-amp signal is preferably shifted in comparison to the phase of the two square wave signals 38 and 40 output from the oscillator 14 as needed to provide an all metals response for the detector circuit output.
The pre-amp output signal on line 62 is applied to a quadrature detector 64 having DC coupled phase 66 and amplitude 68 outputs. The in phase and quadrature phase square wave signals output from the oscillator 14 are also applied to the quadrature detector 64. Selection of the static operating point for the detector 64 is made by the channel selector 36 with the channel select signal (CS) to match the frequency selected for the generated alternating current electromagnetic field 24. A slight phase adjustment may also be provided by the channel selector 36 to compensate for a response shift in the receive coils 28 when the system 10 is operated on the various selected operating frequencies.
The DC coupled outputs 66 and 68 are summed at 70, and the dynamic response of the system adjusted at 72 (with respect to the summed amplitude and phase components) to provide a predetermined metal slope response for the receive coil 28. The metal slope response refers to a balancing of the detector circuit 30 operation to detect all metals (both ferrous and non-ferrous) equally well.
The adjusted signal output from summer 70 is applied to a I I, WO 95/20205 WC/IUS95/o3$0 13 low pass filter 74 that filters out high frequency noise components and sets the maximum walk-through rate for metal object detection. This maximum rate allows for typical leg swing velocities.
Following low pass filtering, the filtered signal output on line 76 is AC coupled to a dual rate high pass filter 78 that includes precision comparators for detecting whether an object is approaching or departing from the passageway 26. If the object is detected as approaching, the filter 78 is set to a slow rate for processing the filtered signal on line 76 and generating a detector output signal on line 79. This allows the system to accurately process slow moving objects. Conversely, if the object is detected as departing, the filter 78 is set to a high rate for processing the filtered signal and generating the detector output signal on line 79. This allows the system to quickly reset for a subsequent object to be processed, thus maximizing the object throughput rate. The circuit diagram for the dual rate, high pass filter 78 is shown in FIGURE 7.
The processing circuit 32 is connected to receive the detector output signals generated by both detector circuits for the right and left receive coils 28r and 281, respectively. The processing circuit 32 includes a pair of chained logarithmic amplifier circuits 80 and 82, each one connected to process one of the received detector output WO 9 /20205 PCT/US9$/00380 14 signals on lines 79r and 791. The chained logarithmic amplifier circuits 80 and 82 are implemented with operational transconductance amplifiers, stabilized with a windowed threshold type comparator. The transconductance amplifiers further provide a multiple-pole, variable low pass filter function that enhances system noise immunity.
Each logarithmic amplifier circuit 80 and 82 further includes an auto-zeroing feedback circuit 84 to compensate for thermal drift and background noise. The circuit diagram for one of the logarithmic amplifier circuit 80 and 82 along with auto-zeroing feedback circuit 84 is shown in FIGURE 8.
When an object moves through the passageway 26 closer to one receive coil 28 than another, the signal derived from the close coil receives a relatively intense and quick processing response from the logarithmic amplifier circuits and 82, while the signal derived from the distant coil receives a relatively weaker and slower response. A weak detector output signal indicative of the detection of a small or distant metal object in the passageway 26 is significantly amplified by the variable low pass filters of the circuits 80 and 82 in accordance with a long integration time constant. Conversely, a strong detector output signal indicative of the detection of a large or close metal object is subjected to a short integration time constant and is only slightly amplified. This selective SWO 95/20205 I CIUS95/(0(380 15 amplification of the detector output signals is in effect taking the cube root of the output detector signal, and functions to compensate for the known near field inverse cube law attenuation characteristics of the receive coils 28. Thus, the system 10 will possess a wide dynamic range.
It will, of course, be understood that in a digital implementation of the processor circuit 32, a precise cube root of the detector output signal will be taken.
Up to this point, the signals derived by the system from the sensing of the electromagnetic field 24 by the right and left receive coils 28r and 281, respectively, have been maintained separate from each other. The processing circuit further includes a pair of multiplier circuits 86 and 88. The first multiplier circuit 86 multiplies a cube root signal A output from the logarithmic amplifier 80 by a cube root signal B output from the logarithmic amplifier 82. This signal multiplication operation generates an output signal C in accordance with the following equation: C Similarly, the second multiplier circuit 88 multiplies the signal A by the signal B. However, the multiplication generates an output signal D in accordance with the following equation: D WO 95120205 PCTUS95/003 0 16 The multiplication coefficients x and y in the two above equations are selected empirically as calibration factors.
With correct selection, a predetermined voltage level output at C and D will be achieved regardless of placement of the detected metal object in the field 24.
The cube root signals A and B output from the logarithmic amplifiers 80 and 82 may be further processed to identify the approximate horizontal location of any detected object. The strength of the signals A and B is related to both the proximity of an object to the receive coils 28r and 281, respectively, and the metal mass of the object. A dual channel LED bar graph device 85 is connected to outputs from the chained logarithmic amplifiers 80 and 82 and includes a signal strength measuring circuit driven by the output cubed root signals A and B. The higher the strength of the output cubed root signal, the more LEDs in the device 85 that will be lit and the more likely that the detected object is proximate to corresponding receive coil 28′. When the signals are approximately equal, the object is most likely centrally located in the field 24. The bar graph device 85 further includes a display hold circuit for holding LED display of the peak signal strength, thus allowing security personnel to review the location indication provided by the LED display after the person or article has passed through the field 24.
WO 95/20205 PCUS95/00380 17 The multiplication output signals C and D are cross coupled by a pair of difference amplifiers 90 and 92. The first difference amplifier 90 subtracts the output signal D from the output signal C to generate an output signal E in accordance with the following equation: E 2C D.
Similarly, the second difference amplifier 92 subtracts the output signal c from the output signal D to generate an output signal F in accordance with the following equation: F 2D C.
The processing circuit 32 further includes a select circuit 94 that receives the difference output signals E and F from the difference aplifiers 90 and 92, respectively, and outputs a signal G comprising the one of the signals E or F that has the weaker signal level. Thus, if select circuit 94 determines that E
