CA1157096A - Systems and method for identifying objects having conductive properties - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The type of conductor, its property, and if a metal, its type and cross-sectional area can be obtained from measurements made at different frequencies for the amount of unbalance created in a previously balanced stable coil detection system.
The true resistive component is accurately measured and thus reflects only the voltages loss attributable to eddy currents caused by intro-duction of the test sample to the coil system.
This voltage divided by corresponding applied frequency gives a curve which peaks at a frequency dependent upon type of conductor. For a metal this peak frequency is proportional to the samples resistivity divided by its cross-sectional area.

Description

~9 S7096 SYSTEM AND METHOD FOR IDENTIFYING OBJECTS
HAVING CONDUCTIVE PROPERTIES
BACKGROVND OF THE INVENTION

Metal detection systems have been used for more than thirty years, and have been capable of determining the presence or absence of a metal-lic object. Such systems have found many appli-cations in various fields, and more récently such systems have been finding widespread use as wepons detector devices. However, these systems when used for weapons detection have not been able to readily distinguish between various types of metallic objects.
Thes s tems use an induction coil to which Ose;~a~~ ~
an asoi~ ting signal is applied. Detection readings heretofore have been limited to a general determination as to the presence of a metal object with no precision in the identifi-cation process.
It has now been discovered that information can be developed which will permit this type of detection system to make specific identification - of objects having conductive properties, and to give repeatable data for a specific object.
Previous systems have limited application because o their inability to distinguish between different types of objects, and in the use of these systems for detection at airports, there has been a persistent false alarm problem.
With the development of the system of this invention, it is now possible to accurately ob-tain information with respect to the type of . .

- . . ~ ,; . : :

- 2 - ~57~96 1 conductive object disposed in the coil field, including in-formation as to the various metallic components that are contained in it iE there is more than one metal. This makes it possible to readily screen for different types of metallic objects of interest to preclude false alarms.
In addition, the system of this invention represents a breakthrough in that accurate repetitive readings can be obtained which make it possible to apply such systems to other areas, such as metal classification, sampling, testing of conductive solutions, animal tissues, and for tagging techniques.

SUMMARY OF THE INVENTION
This invention relates to metal detection systems for objects having conductive properties and particularly to a more advanced and sophisticated type of detection system than previously possible.
This system makes it possible to accurately check for a specific object and can be used as a means for sorting different kinds of metal, even making it possible to distinguish between different types of hand guns.
Essentially, this new detection system is based upon the discovery that in a previously balanced coil system, after introduction of a conductive or of a metal sample, the true ~esistive component of the impedance change occurring in the coil system due to eddy current 105s can be determined. When the true resistive component of the impedance change (~R) is divided by the applied frequency ~f), the resultant value varies with frequency and peaks at a single peak frequency. This peak fre~uency value is proportional to the cross-sectional area of the object in a plane transverse ,.~ .

~57~96 to the coil. In addition, the peak frequency, or that occurring at a maximum ~f value, has been found to be proportional to the resistivity of the sample divided by its cross-sectional area.
However, these results will not occur unless very accurate measurements are made and all ex-traneous effects caused by the various system components, such as the frequency generator, coil and detection circuits, are taken into considera- .
tion. That is, in order to obtain a true picture of the effect of the sample, it is necessary to look only at the true resistive component change in the coil system.
The true r~sistive component change can only 15 . be obtained ~ the output signal is referenced to within one degree of the phase of the signal applied to the input coil. Unless this phase relationship, which is hereafter referred to as zero degree phase shift, is kept, the results obtained will not provide the accuracy required for most contemplated uses of the system.
DESCRIPTION OF T~E DRAWINGS
FIGURE 1 shows a mutual inductance detector circuit.
FIGURE 2 is a graph of the secondary coil signal illustrating the shift caused whenametal object is placed between the coils of FIGURE
1.
FIGURE 3 is a vectox diagram of the voltage amplitude vector which shows the resistive com-ponent.
FIGURE 4 is a graph of resistive component divided by frequency versus frequency for a metal object.
FIGURE 5 is a plot of resistive component peak ~5709~

1 values divided by peak frequency versus the reciprocal of the peak ~requency, showing the linear relationship in peak values when cross-section area and cross-section geometry of a metal object vary.
FIGURE 6 is a plot similar to FIGURE 5 showing linear relationship for various types of metal objects.
FIGURE 7 is a plot similar to FIGURE 4, showing peak curve signal where plural pieces are disposed in the coil.

FIGURE 8 shows a second type of detector circuit using a balanced bridge arrangement.

FIGURE 9 shows another type of detection circuit which uses a split coil balanced secondary.
FIGURE 10 is a plot of resistive component divided by frequency versus ~requency which gives the signature for a Smith & Wesson stainless steel revolver.
FIGURE 11 is a plot of resisti~e component divided by frequency versus frequency showing the signature for a Titan .25 revolver.

FIGURE 12 is a block diagram of the detector system where a minicomputer is used for comparison of received signature with those of known objects.
FIGURE 13 is a block diagram of the software elements of the detector system of FIGURE 12, and FIGURE 14 is a block diagram of a detector system showing phase sensitive detection where analog switch circuits and digital logic integra~ed circuits are used and it is located immediately following FIGURE 9.

A DESCRIPTION OF THE INVENTION

Referring particularly to FIGURE 1, a detector system 10 is shown in which the alternating current signal source 12 supplies a signal to the ~57~96 input ~ primary coil 14. The secondary coil 16 is connectèd to a phase senitive detector 18, which will pick up variations in the secondary coil signal when a metal object 20 is disposed within the field, schematically shown between the input or primary coil 14 and the secondary coil 16. Coil diameter can be any size from a small sample coil of 12" to a 6' walk in coil. The object can be placed either within the coil for maximum response or outside but close to the coil as long as it is within the generated mag-netic field.
It has been found that tests involving metals can be made at frequency ranges from 100 to 10,000 hz. However, if the frequency is increased to the 1-10 megahertz range, test results can be obtained for samples having conductive properties such as metal powder-type explosives, animal tissue, aqueous solutions, ionic solutions and suspensions.
The vector diagram in FIGURE 3 shows the situa-tion when a metal object such as 20 is introduced to the field between the input or primary coil and secondary coils 14 and 16. The vector A is shown at 32. The vector makes an angle 34 with the zero degree phase line and represents the amount of displacement shown in FIGURE 2 on the lower graph at 30. -A reading of interest for purposes of this invention is the resistive component ~R
shown at 36 which runs along the zero degree phase line. This value is the reading that is picked up by the phase sensitive detector 18. It is one of the essential values used in connection with the principles of this invention. It makes it possible to find peak eddy current loss by plotting the resistive component divided by cor-. .
;

~S7~g~ `

respondir.g frequency against frequency. This is shown in FI~URE 4 for a metal sample of three different cross-sectional areas. The larger sample A is represented by the curve 38 which has a peak shown at 40. This plot will giue what is termed the peak frequency, as shown by the dashed line 42. From this plot the value of peak fre-quency and matching resistive component divided by frequency is found.
The sample B, which is of smaller cross-section than that of sample A, but of the same material, gives a peak curve 44 with a peak 46 which is less in amplitude than that of the larger sample A.
The peak frequency line 48 shows that the peak frequency for the smaller sample is hi~her than that of the larger sample.
Similarly, sample C is made of the same metal as samples A and B and is of smaller cross-sectional area than sample B. The peak frequency curve 50 for sample C is somewhat flatter and has a peaX
value 52 of considerably smaller amplitude than either of the other samples. The peak frequency line 54 shows that it also has a considerably higher peak frequency value.
2S It will be noted that the peaks for all three samples shown in FIGURE 4 are in alignment, and a plot using the reciprocal of the frequency, as shown in FIGURE 5, based upon peak frequency value~or these three samples gives a straight line ~. The ordinate for amplitude in this graph is the resistive component divided by the peak frequency, while abscissa is the reciprocal of the peak frequency values.
A plot of peak frequency values for ~amples A, B and C is shown at 58, 60 and 62, respectively.

:

~576~96 The dashed line 64 represents a geometrical factor.
It has been found that the slope of this line will vary slightly with changes in cross-sectional con-figuration. In this graph, line 58 shows readings taken with a test object of square cross-section.
The dashed line 64 indicates the change in slope that will be expected where there is a considerable change in geometry.
It should be noted that these peak frequency values will vary considerably depending upon the type of metal used, inasmuch as metal resistivity is a major factor. This can be clearly seen in a review of the graphs shown in FIGURE 6.
FIGURE 6 is a detailed graph of the same type lS as shown in FIGURE 5 as shows the response charac-teristics for different metal samples. It should be noted that in this graph, coil configuration is taken into consideration for the values given, in that the peak amplitude value includes the resistive component divided by the peak frequency, as well as the reciprocal of the number of turns in the input coil and the reciprocal of the mag-netic induction expressed in Webers per square meter (~f) . ~lB)- The abscissa for this graph is the reciprocal of the peak frequency expressed in hundreths of a second.
The bands shown on the graph for the different types of metal have a wide range of slope values.
The primary factor in determining the slope of the bands is the resistivity of the metal in-volved. Band 66, which represents the linear range of peak values for stainless steel, has a - very high resistivity, as compared to the more conductive metals, such as copper and aluminum.
The band 68 shows the range of peak frequency values . .

.~:
.

, ~, 7~96 for steel. This band as well as all of the other bands shown on the graph fan out from the origin 70. The wide difference in slopes of each of the bands is attributable to the correspondingly wide range of resistivity values for the metals shown.
The following table for metals and their cor-responding resistivity illustrates this:
Metals Resistivity (Micro-ohm - Centimeters) Copper 1.7 Aluminum 4.0 Brass 7.0 Steel 10.0 Stainless Steel 72.0 The slightly diverging lines determining the width of each band, such as lines 72 and 74, re-flect small changes in slope that are due to geometrical cross-section variances of the sample.
Peak frequency for brass, aluminum and copper are shown respectively by bands 76, 78 and 80.
With respect to variation in cross-sectional geometry, the sample under test may be defined as having a geometric ratio, G, which is equal to the width squared divided by the height squared, i. e.
G=a2/b2. This factor is taken into consideration on plots for the bands shown for each metal, where the lower line represents a~ ~ uare block tG=l) ¦ test specimen, while the ~e~ line represents a rectangular block with a width twice that of the height (G=4).
For example, referring to the aluminum band 78 of FIGURE 6, the lower line contains the point 82 at which a one inch square aluminum sample (G=l) could be found. The reciprocal frequency value is approximately .58 hundreths of a second, and 9 ~57~)96 1 the resistive component amplitude, ~f NB has a value of slightly less than 7.5. P
Correspondingly, point 84 lies on the line defining the upper limit (G=4) for the aluminum band 78. This would be the point giving a reading for an aluminum object of one square inch cross-sectional area which has a width twice its heiyht. It will be noted that point 84 has a slightly higher amplitude value and a slightly lower time value for the reciprocal of the frequency. Experimental data for the same cross-sectional dimension blocks for the other metals gave values for all of these metals in which the amplitude for the one inch square test specimen is.about the same as those of points 82 and 84 - specifically, around a value of approximately 7.5. For example, a square copper test sample with.an area of one inch would have an amplitude of 7.5 and a reciprocal peak frequency value of 1.4 hundreths of a second.
Although it is seen that variations in cross-sectional.geometry have a slight effect on slope, changes in cross-sectional area will not affect the slope but will very greatly affect both amplitude and reciprocal frequency values. They will, however, be proportional and fall along the G=l line for each band where the test specimen is square. For example, for a square test specimen of 2S one-half square inch cross-sectional area, the peak requency point will be midway between the origin 70 and point 84. For a square aluminum test object having a cross-sectional area of two square inches, the peak frequency values.will lie along the G=l line at a point twice the distance from the origin 70 and point 82.

~57~96 To use the graph of FIGURE 6 to determine resistivity and cross-sectional area of an un-known object, values for amp~itude and the peak frequency reciprocal are obtained from a graph line FIGURE 4. Where the reciprocal of a peak fre-quency has a value of .45 hundreths of a second, the vertical reference line 8~ is established.
Where the amplitude (f .NB) is 6.5 x 10 4, the horizontal reference P line 88 is establihsed.
The intersection of both of these lines at 90 in-dicates that the unknown object in the coil system is made of aluminum and has a cross-sectional area of slightly less than one square inch.
As can be seen from the manner in which the respective bands are separated from each other, it is possible to readily distinguish one type of metal from the other with the resistive components and peak frequency reciprocal values even if con-siderable difference in cross-sectional shape exists.
The preceding discussion has assumed a single metal object. In most detection situations, there are several different objects that have several different metallic components which it is desir-able to detect. In these cases, each of the different metals will produce its own peak signal.
For example, in FIGURE 7, three pieces are sensed by the detector system. Piece 1 generates curve 92, piece 2 generates curve 94 and piece 3 generates curve 96. The resultant envelope includes a single trace with three humps representing peak -, frequency and amplitude-value for s ~h pieces.

3~ It is assumed that each of these ~eioe~ could be of a different metal and of different cross-section. The detection system using the change ~57q~9~; `

in resistive component is sufficiently sensitive to distinguish the peaks for each of the different metal pieces as the different frequency values are applied.
FIGURE 8 shows a balanced bridge detection system which can be used. This balanced bridge arrangement is preferred over the detection system of FIGURE 1, in that it is more readily balanced and does not have serious perturbations on measurements. The signal frequency generator 98 is disposed across the bridge as shown at one end of the sensing coil 100, and at the corres-ponding end of matching coil 102. Both coils have similar values. Resistance 104 forms ~K~
other leg of the bridge, and variable resistance 106 which generally matches the value of resis-tance 104 forms the other leg of the bridge. The phase sensitive detector 108 is connected between the common junction of the coils 100 and 102 at one side and the common junction of resistors 104 and 106 on the other side. The reading of the phase sensitive detector will assist in adjustment of the variable resistor 106-to get a balanced con-dition across the bridge prior to introduc~ion of the test object 110. When the test object 110 is introduced to the field surrounding coil 100, an eddy current loss will unbalance the bridge and the phase sensitive detector will read the resis-tive component value which must be referenced to the coil 100 signal.
FIGURE 9 shows another type pf phase sensitive detection circuit arrangement which has proven to be very satisfactory. The signal frequency generator 112 is connected across the input coil 114. The signal frequency generator 112 is connected across .. . . . .

~57~96 the input coil 114. Secondary coils 116 and 118, which are of equal value, are connected at their lower ends. Fixed resistor 120 is connected to the upper end of coil 11~ at one end and has its other end directly connected to a variable balancing resistor 122 which is connected to the upper end of coil 116. A phase sensitive detector 124 is connected across the common connections of coil 116 and 118 at one end and the common connec-tion between resistors 120 and 122 at its other end. The metallic object 126 is disposed between the input coil 114 and the split secondary coil assembly made up of coils 116 and 118. This detector system provides maximum sensitivity and ease of balance with the variable resistance 122.
The actual signature trace that is developed by a complex object, such as a gun, is shown in FIGURES 10 and 11. FIGURE 10 shows the signature S for a Smith & Wesson .38 caliber stainless steel revolver. The ordinate is the resistive component divided by the frequency and the abscissa is the applied frequency values. The trace has a high peak at 128, which indicates the barrel of the revolver, and the low portion 130. In order to obtain such a signature, it is necessary to apply some thirty different frequencies over the 10 Hz range. For automatic analytical purposes, such as used with curve analysis, an average curve en-velope is obtained as shown by the curve 132. This would then be analvzed and compared to a series of stored signatures in the device~
FIGURE 11 shows the signature S for a Titan~
.25 revolver. This signature has a high pronounced spike at 134 and peaks at 136 and 138. Both of these signatures are very dissimilar in appearance.

~' :

:

.

-~57~96 They have peaks at different frequencies and the signatures are at different amplitude levels.
Digital and other comparative techniques make it possible to readily distinguish between each of these two signatures. The signatures for other weapons, and other types of objects, are just as distinctive as these two examples.
In airport detection systems, where the in-dividual passing through the coil area may carry 10 numerous types of metal articles, it is also possible to readily pick out the existence of a gun signature. The various articles add to the overall signal envelope, but-the gun signature is still readily distinguishable. In almost 15 every instance, the signal produced by the gun will be the predominant signal.
FIGURE 12 gives a block diagramarrangement of the hardware components of the weapons de-tector system. The multiple frequency source 20 140, which is the equivalent of the frequency ~
signal generators of FIGURES 1, 8 and 9, supplies a signal to the balanced circuit, indicated here at 142. It is also possible to use a balanced system including a bridge with a single coil.
25 The arrangement is similar to that shown in FIGURE
8, except that a resistive element is used in place of coil 102 of FIGURE 8, and the variable resis-tance 106 of FIGURE 8 now contains a variable capacitor in parallel with it~
In FIGURE 12, the multiple frequency source also transmits a frequency to the multiple fre-quency timer and controller 144. The frequency range will be from 100 to 10,000 Hz and can be spanned with approximately thirty different frequencies within this range. This is the fre-:. .

~5763 96 quency range shown in FIGURE 11, and is more than adequate for all of the situations for w~ich the system is designed.
The multiple phase sensitive detector section 146 will receive the signals from the bridge, as well as from the timer and controller circuit //
144. The phase sensitive detector output, as we~
as the output from the multiple frequency timer and controller section 144, are supplied to the minicomputer 148. Typically, this is an 8K memory 16 bit word minicomputer.
With respect to the computer, an analog to digital converter is used to interface the phase sensitive detector circuitry to the computer.
The empty coil response of the induction coil electronics, and its response to a known resistance change are stored in the computer memory. With regard to FIGURE 13, blocks 152 and 154 are used to calibrate the coil electronics and produce true zero degree phase component data.
The computer will have comparison capability with stored data signature values to which the incoming digital signal input is compared. If there is a match for any of the stored signals representing a weapon or other item for which a check is to be made, the computer sends the sig-nal to an alarm circuit 150.
Instead of using the analog phase sensitive detector techniques to separate the zero degree component at each frequency, it is possible to achieve the same results with the use of digital Fourier transform techniques. This would involve replacing the individual analog phase sensitive detector units for each frequency and using a good broad band amplifier at 146 instead of the ., .~
;: ~

~57~96 phase sensitive detector units. This, together with an accurate time base generator for use as a clock_ would make it possible to use the Fourier transform technique. The data would be studied at fixed time intervals and analyzed in the mini-computer by standard Fourier transform techniques.
FIGURE 13 gives the basic logic steps and func-tions of the system of FIGURE 12 using the phase detector. Initially, there is the bridge cali-bration routine illustrated in block 152, followed by the phase sorting routine indicated in block 154.
The zero degree phase values then are sorted as indicated in block 156, producing zero degree phase resistive component values and peak fre-quency values.
Th~ threat data file 158 is fed into the search and compare block 160 for comparison with the input signals which will be supplied from the peak sorting block 156. A comparison is made in block 160 and if there is a match of threat and incoming active data a signal is sent to the alarm block 162.
FIGURE 14 is a more detailed block diagram for a proposed detector system. A crystal oscillator 164 is used to generate a standard frequency which is applied to a variable frequency divider network 166. The output is supplied to a frequency di-vider chain 168 which supplies three outputs.
The first goes to the control divider chain block 170. The second output goes to the square wave adder section 172 and subsequently to the signal condition block 174 and power driver 176. The output from,the power driver is supplied to the sample coil bridge 170, and its output supplied to the phase sensitive datector section 180.

:

~71 57~96 - 16 - ~-With respect to the phase sensitive detector circuitry, it is possible to use standard analog switch devices which can readily be used with the sample and hold circuitry necessary for computer interfacing.
The frequency dividers are J-K flip-flops which provide zero degree phase and 90 reference square waves at eight octave frequencies simultaneously.
Three starting frequencies provided consecutively by the variable divider permit sampling a total of 24 frequencies between about 70 Hz and 12.5 kHz.
The square waves drive the reference channels of the phase sensitive detectors directly, and the in-phase components are analog added to form a composite square wave containing eight frequencies.
This wave form is integrated in a conditioner to form a composite triangle wave. The high frequencies are preamplified in the adder to make the triangle amplitude the same for all frequencies. Power o-perational amplifiers apply this signal to the bridge, and the off-balance signal is amplified and phase detected.
When a fixed number of cycles have occurred, the phase detector output is sampled and held until the computer has accepted it. The starting frequency is changed, and the process repeats.
When all the frequencies have been sampled, the control divider stops the process.
OrI~ TION
As to operational aspects of the system, it has been found that phase relationships are critical in measuring the true resistive compo-nent. Inasmuch as measurements of voltage un-balance are made in the 10 to 100 microvolt range and involves a factor of 1 in 10,000, all equip-. , ,.
' " ' ~lS71D96 ment must be very stable and accurate to pre-clude introduction of phase shifts which would make it impossible to maintain the zero degree phase relation required for measurement of true resistive component impedance change in the sensing coil.
The coil itself must be extremely stable, and it has been found that this stability must be held to at least one part in ten thousand, with a preferred stability of one part in one A hundred thousand. Spacing between adjacent turns o~ th~ o 1, temperature stability of the wire or chioldod rom temperature variation and preclu-sion of displacement of the turns ~ to vibration are factors of importance. The turns in the coil are preferably spaced from one to two centimeters apart to reduce interturn capacitive effects.
The coil should be as free as possible from all extraneous effects.
The oscillator circuit itself must be extremely stable to preclude phase wiggle or shift due to temperature, vibration or instability of its ele-ments. Signal output variation should be held to less than one tenth of a degree when using Fourier transform techniques and one-half degree when using phase sensitive detectors. Oscillator ele-ments must have low thermal change characteristics and be within about one tenth of one percent of their value while operating to preclude unacceptable variation jitter in output signal. Similar rigid requirements are necessary for the bridge and measuring elements.
The phase angle in the input coil is of impor-tance, and all voltage data must be referenced to it. Corrections for phase shift of the various 7~96 circuit elements must be made when measurements are made either upstream or downstream from the input coil.
The several system coils should be as iden-tical as possible and they should be shielded from temperature variation. All of these re-strictions are necessary to give consistent repetitive results where frequently the volume of the sample is on the order of a cubic centi-meter while the coil volume is a cubic meter.
It has been found that this requirement can be met by previously determining what this angle is with respect to other equipment, such as the oscillator, and making a correction for it. The simplest manner of determining the phase angle in the input coil is to place a resistor in series with the input coil and measure the phase angle of the signal passing through it. This will allow determination of the resistive portion of the coil system output signal caused by the unbalance of a metal object only. Using the known resistance in series with the coil makes it possible to obtain corrective data. In this case, data is obtained on both the plain coil response and the coil and resistor resp~nse and is useful in a correction equation which takes into account both the real and imaginary values of the voltage. This infor-mation can readily be programmed into the computer and incoming data normally can be modified to make the correction for zero degree phase shift.
Adjustments for zero degree phase can then be made ei~he~ i~ the instrumentation, such as in the phase ~ ircuitry, or calibration data can be obtained and incoming data modified accordingly, such as with a computer system using a Fourier .
~: .

~5~196 technique to obtain the true resistive component.
The above-described calibration technique which involves introduction of a known pure resi-stance in series with the sensing coil provides knowledge as to the portion of the sensing coil input signal unbalance caused by introduction of a metallic object.
The method used to determine what portion of the unbalanced output signal corresponds to this resistive change will, of course, depenq upon ~,~ the speci~ic balancing circuitry ~p ~ ~ . ;
In the case of the measurement circuit using a bridge arrangement, although it is more easily balan~ed, it has many extra circuit components between the sensing coil and the output signal.
There must be compensation for the extra circuit elements to determine the phase shifts introduced by them, and they must be compensated for, either electrically, or by computation subsequent to measurement.
Once the various phase shifts in the system are known, it is a mattex of applying the appro-priate correction in phase shift so that the re-sistive component values obtained be referenced to zero degree phase existing in the input coil.
As mentioned above, the correction should be made to bring the resistive component vector to within one degree of the resistive component of the coil system i~pedance.
The adjustments for zero degree phase can be made in the instrllmentation, or calibration data applied to the measurement to make the necessary correction or zero degree phase shift.
It has been found that referencing to the oscillator output signal is a most convenient , ~57~96 method of obtaining a good fixed phase base.
Correction for shift between the oscillator and the sensing coil must be made to obtain the zero degree phase line, and once this is obtained, appropriate referencing can be made to the phase of the output signal obtained from the circuit and correction made so that they are within one degree of being completely in phase with each other. It should be kept in mind that the correc-tion equations taken into account both the real and imaginary values of the voltage. This infor-mation can readily be programmed into a computer and the incoming data can normally be modified to make the correction for zero degree phase shift.
It has been found that the value obtained when there is zero degree phase shift will be within plus or minus five percent. Any greater displace-ment than the plus or minus one degree tolerance will result in a substantial loss of accuracy such that the data will not be repetitive for similar samples. The straightline relationships as shown in FIGURE 6, for example, will not be useable.
In a complex object it is unnecessary to use specific peak frequency values, since many peaks, one for each of the various metal components of the object to be checked, will appear. The selec-tion of thirty frequencies in the range of 100 to 10,000 Hz will give a typical range and will produce the results shown for the signatures of interest for guns and also permit easy identifi-cation of other types of objects. To develop the signatures as shown in FIGU~ES 10 and 11, frequency values are chosen for relevancy to both the resis-tivity of the metal being sought, as well as the ,.

- 21 - ~57096 1 estimated cross-sectional area.
With reference to FIGURES 10 and 11 showing the signature traces, there will be a very yreat correlation between the test sample and the actual sample encountered.
As to differen$ types of objects with slight variances in design and makeup, as in di-Eferent types of guns, the signatures will vary significantly because of the differences in cross-sectional area and resistivity of the various components of which the article is made.
For identification, the computer can .store the various signatures for the known objects to be checked for.by the detection system, correct the incoming signal for zero degree phase correlation with the input coil system signal, and then compare the incoming signal data from the unknown object disposed in the sending coil with the stored.signatures to determine whether there is a match.
The object is.physically placed within the sensing coil itself. Frequency reciprocal values change greatly with changes in cross-sectional area. This should not be confused with the changes in cross-sectional.geometry which -have some effect, but not the appreciable effect which results from resistivity and cross-sectional area changes .
: in the sample, i.e. fp = AP . To develop the sig-natures as shown in FIGURES 10 and 11, frequency values are chosen for relèvancy to both the resistivity of the metal being sought, as well as.the estimated cross-sectional area.
Coil configuration and.geometry are important to note since the signature traces will be affected by them. In.this respect, the terms N and Bo will be noted in FIGURE 6. This gives some guidance with respect to coil design which is a factor in ' ' ' "' ;
' ,-- . ,.

- 22 - ~57~96 response characteristics of the system.
The expression showing the variables associated with the ordinate in FIGURE 6 iæ as follows:
1~ = [(N Bo) 322 P~ (a2/b2)]
N = number of turns of wire on coil Bo = magnetic induction (Webers/square meter) p = resistivity of the metal under test (ohm-meters) ~o = permeability of free space (~ . 10 7- MKS units) K(a2/b2) = dimensionless quantity which depends on geometry through the ratio a2/b2 ~R = in phase component of the detected signal, i.e. see FIGURE 2 (volts) f = peak frequency (Hz) l(N Bo)~322 ~POK(a2/b2)l = slope of the straight lines in FIGURE 4 (note that there is no dependence on the sample cross-section).
With respect to the constant terms in the equa-tion, when a2/b2 equals respectively 1, 2, 3 and 4, K(a2/b2) is 1.248, 1.334, 1.475 and 1.607.
With respect to the coils them~ lv ~ , their J'`~ diameter may be from six inches to/six feet. The input and sensing coils are usually arranged con-centrically in spaced relation with insulation material such as fiberglass disposed between the coils. The coils are shielded from room tempera-ture change by insulation since variation affects output. A further compensating arrangement that has been found to be effective also is the use of special alloy thermal stable metals rather than copper in the conductors to reduce thermal effects.
The six foot coil assembly is used in connection with security at airports in which the individual walks through the coil itself and is scanned for the possession of weapons.

', . ~ -, :
i ~, ~

~ 5709~ `

Throughout this discussion, the relation be-tween resistivity area, peak frequency, and re-sistive component divided by frequency are given as unique values at curve peaks. However, relationships to resistivity and cross-sectional area may be complex. Nevertheless, all that is neeaed is a repetitive signature. ~nd effects~ -shadow effects, and geometric effects and magnetic effects present no problem because signatures for the same objects will be exactly the same.
While this invention has been described, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses and/or adaptations of the invention following in general, the principle of the invention and including such departures from the present disclosure as come within the known or customary practice in the art to which the invention pertains, and as may be applied to the essential features hereinbefore set forth, as fall within the scope of the invention or the limits of the appended claims.

;~

.
.

Claims (45)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A system for identifying objects having conductive properties, comprising:
frequency generating means for providing an output signal having at least one selected periodic driving frequency and a phase;
coil means responsive to said output signal having said at least one selected periodic driving frequency for pro-viding a magnetic field which varies in accordance with said at least one selected periodic driving frequency, said coil means producing an output signal having a phase and being responsive to placement of said conductive object in said magnetic field for undergoing an impedance change having a true resistive component; and means for obtaining the true resistive component of the impedance change at said at least one selected periodic driving frequency by referencing the phase of the output signal of said coil means to which 1° of the phase of the output signal of said frequency generating means for said at least one selected periodic driving frequency, whereby to develop sig-nature data to identify said object.

2. The system of claim 1, wherein said coil means com-prises an input coil connected to said frequency generating means and an output coil connected to said means for obtaining said true resistive component.

3. The system of claim 1, wherein said coil means com-prises a balanced bridge circuit including a sensing coil connected to said frequency generating means, said balanced bridge circuit further comprising a matching coil connected to said means for obtaining said true resistive component.

4. The system of claim 1, wherein said coil means com-prises an input coil connected to said frequency generating means, and at least one secondary coil connected to said means for obtaining said true resistive component.

5. The system of claim 1, wherein said coil means is stable to at least one part in ten thousand.

6. The system of claim 1, wherein said coil means comprises two coils, one of said two coils being an input coil, and wherein said two coils are matched.

7. The system of claim 1, wherein said coil means comprises at least two coils having a rated value, said at least two coils having substantially no variation in said rated value due to change in ambient temperature.

8. The system of claim 1, wherein said output signal generated by said frequency generating means has a phase with a variation of less than one-half of one degree.

9. The system of claim 1, wherein said coil means comprises an input coil connected to said frequency generating means for receiving said output signal thereof, said output signal com-prising a periodic signal, and wherein said means for obtaining said true resistive component includes a phase sensitive detector connected to said coil means and being capable of being calibrated so as to be referenced to be in phase with each said periodic signal applied to said input coil.

10. The system of claim 1, said system further comprising dividing means for dividing said true resistive component by said at least one selected frequency.

11. The system of claim 10, wherein said means for obtaining said true resistive component provides, for a further object positioned in said magnetic field of said coil means, at least one further value of said true resistive component divided by at least one frequency value, said system further comprising storage means for storing values of said true resistive component divided by corresponding said at least one selected frequency, as provided by said dividing means, and comparison means for comparing said provided at least one further values of said true resistive component divided by said at least one frequency value for said second object disposed in said magnetic field of said coil means with said stored values of each said true resistive component divided by corresponding said at least one selected periodic frequency.

12. The system of claim 1, wherein said coil means com-prises a primary coil and a secondary coil, and wherein said primary coil and said secondary coil are hollow coils having turns spaced apart from, but adjacent to, each other, said primary coil and said secondary coil having a common central axis.

13. The system of claim 1, wherein said coil means com-prises a sensing coil and a matching coil, said matching coil being identical to said sensing coil and connected in series therewith to form a balanced bridge circuit.

14. The system of claim 13, wherein said frequency generating means comprises two output leads, and wherein said sensing coil and said matching coil are connected across said two output leads of said frequency generating means, said coil means further comprising two impedances connected in series and also connected across said two output leads of said frequency generating means, one of said impedances comprising a variable impedance for balancing said coil means, and wherein said means for obtaining

Claim 14 continued....

said true resistive component comprises a phase sensitive detector connected to said sensing coil and said matching coil, on the one hand, and to said two impedances, on the other hand.

15. The system of claim 1, wherein said frequency generating means includes output leads, said coil means including an input coil comprising a hollow coil connected across said output leads of said frequency generating means, said coil means further including a secondary coil arrangement including two matched hollow coils connected in bucking relationship and axially arranged, one with respect to the other, and adjacent to said input coil, said coil means further comprising a balancing variable resistor connected to said secondary coil arrangement, and a phase sensitive detector connected between said two matched hollow coils of said secondary coil arrange-ment for measuring said true resistive component.

16. The system of claim 1, wherein said output signal of said frequency generating means has a phase, and said means for obtaining said true resistive component includes means for providing a correction for zero degree phase shift from the phase of the output signal of said frequency generating means.

17. The system of claim 1, wherein said at least one selected periodic driving frequency comprises a plurality of selected periodic driving frequencies.

18. The system of claim 17, wherein said frequency generat-ing means provides an output signal having frequencies ranging from one hundred to ten thousand hertz, whereby to identify metallic objects.
19. The system of claim 17, wherein said frequency generat-ing means provides output signals having frequencies ranging

Claim 19 continued....

from one to ten megahertz, whereby to identify non-metallic conductive objects.

20. The system of claim 17, wherein said system further comprises a computer for comparing true resistive component values for successive frequencies with known values so as to detect identity therebetween and to issue a corresponding com-parison output, said system further comprising alarm means connected to said computer and responsive to said corresponding comparison output for generating an alarm.

21. A system for identifying objects having conductive properties, comprising:
(a) frequency generating means for generating an output of at least one selected periodic driving frequency, said output having a phase variation of less than one-half of one degree;
(b) coil means responsive to said output of said at least one selected periodic driving frequency from said frequency generating means for producing a magnetic field, said coil means including an input coil and an output coil, said coil means being responsive to introduction of an object between said input coil and said output coil for producing an impedance change having a true resistive component; and (c) means for obtaining, for said applied at least one selected periodic driving frequency, said true resistive com-ponent of said impedance change in said output coil due to said conductive object being placed in said magnetic field of said coil means, whereby to obtain signature data for identifying said object.

22. The system of claim 21, wherein said means for obtaining said true resistive component includes a phase sensitive detector connected to said coil means and being accurately

Claim 22 continued....

referenced to be in-phase with said at least one selected periodic driving frequency of said output of said frequency generating means.

23. The system of claim 21, wherein said input coil and said output coil are connected in series to form a portion of a balanced bridge circuit, and wherein two impedances are connected to said frequency generating means to form another portion of said balanced bridge circuit, one of said impedances being variable for balancing said balanced bridge circuit.

24. The system of claim 21, wherein said frequency generating means has output leads, and wherein said input coil comprises a hollow coil connected across said output leads of said frequency generating means, said coil means further comprising a secondary coil arrangement including two identical matched hollow coils connected in bucking relationship and axially arranged, one with respect to the other, adjacent to said input coil, said coil means further comprising a balancing variable resistor connected to said secondary coil arrangement, and wherein said means for obtaining said true resistive component comprises a computer for calculating the correction to be made for phase shifts introduced in said applied at least one selected periodic driving frequencies.

25. The system of claim 21, wherein said at least one selected periodic driving frequency comprising a plurality of selected periodic driving frequencies.
26. A system for identifying objects having conductive properties, comprising:
frequency generating means for generating at least one selected periodic driving frequency;

Claim 26 continued...

coil means connected to said frequency generating means and responsive to said generated at least one selected periodic driving frequency for producing a magnetic field; and means responsive to said object being introduced to said coil means for obtaining an electrical response of said coil means for said at least one selected periodic driving frequency, said electrical response being a function of a change in a true resistive component of said impedance of said coil means divided by said selected periodic driving frequency applied thereto.

27. The system of claim 26, wherein said at least one selected periodic driving frequency defines an output signal of said frequency generating means having a phase, and said means for obtaining said electrical response of said coil means includes means for providing a correction for zero degree phase shift from the phase of the output signal of said frequency generating means.

28. The system of claim 26, wherein said at least one selected periodic driving frequency comprises a plurality of selected periodic driving frequencies.

29. The system of claim 28, wherein said frequency generating means provides an output signal having frequencies ranging from one hundred to ten thousand hertz, whereby to identify metallic objects.

30. The system of claim 28, wherein said frequency generating means provides output signals having frequencies ranging from one to ten megahertz, whereby to identify non-metallic conductive objects.

31. The system of claim 28, wherein said system further comprises a computer for comparing true resistive component values for successive frequencies with known values so as to detect identity therebetween and to issue a corresponding comparison output, said system further comprising alarm means connected to said computer and responsive to said corresponding comparison output for generating an alarm.

32. A method for identifying objects having conductive properties, comprising the steps of:
(a) providing a coil system having an input coil;
(b) generating at least one signal frequency;
(c) applying said at least one signal frequency to said input coil of said coil system to produce a magnetic field;
(d) introducing an object into said magnetic field;
of said coil system so as to cause an eddy current loss;
(e) obtaining, for said at least one signal frequency applied to said input coil, a true resistive component of said eddy current loss so as to develop at least one true resistive component value;
(f) dividing said at least one true resistive component value by said at least one signal frequency applied to said input coil to develop at least one quotient value; and (g) using said at least one quotient value as a function of signal frequency applied to said input coil to obtain signature data for identifying said object.

33. The method of claim 32, wherein said coil system provided in step (a) comprises a balanced coil system, said method further comprising the step of balancing said coil system to provide maximum sensitivity for detection of said at least one true resistive component value caused by said eddy current loss.

34. The method of claim 32, further comprising the steps of storing a plurality of signature data for comparison purposes, and comparing each of the signature data with the signature data for the object, as obtained in step (g), whereby to identify the object

35. The method of claim 32, further comprising the step of introducing a specific uniform type of metal sample of given cross-sectional area to the magnetic field of said coil system to obtain reference data values, whereby to calibrate said coil system.

36. The method of claim 32, wherein said at least signal frequency comprises a plurality of signal frequencies.

37. A method for determining response characteristics for objects having conductive properties, comprising the steps of:
(a) providing a coil assembly;
(b) applying at least one periodic signal frequency to said coil assembly so as to produce a magnetic field;
(c) balancing said coil assembly as said at least one periodic signal frequency is applied;
(d) introducing a metal sample of known cross-section and uniform composition into said magnetic field of said coil assembly;
(e) obtaining, for said at least one periodic signal frequency applied to said coil assembly, a corresponding true resistive component of impedance unbalance caused by said in-troduction of said metal object;

(f) compensating for phase shift with respect to the phase of said at least one periodic signal frequency, such that said corresponding true resistive component is obtained at zero degree phase shift;

Claim 37 continued....

(g) dividing said obtained true resistive component by said at least one periodic signal frequency applied to said coil assembly to obtain at least one corresponding quotient value; and (h) using said at least one quotient value as a function of said at least one periodic signal frequency applied to said coil assembly to identify said object.

38. The method of claim 37, wherein said at least one periodic signal frequency comprises a plurality of periodic signal frequencies, and said at least one quotient value com-prises a plurality of quotient values, further comprising the steps of:
(i) identifying a peak quotient value and corresponding said frequency at which resistivity of said metal sample divided by its cross-sectional area is proportional to said true resistive component of said impedance unbalance divided by said corresponding frequency.

39. The method of claim 37, wherein said coil assembly produces an output signal having a phase, and wherein said at least one periodic signal frequency has a phase, said step (e) comprising referencing the phase of the output signal of said coil assembly to within 1° of the phase of said at least one periodic signal frequency 40. A method for determining response characteristics for objects having conductive properties, comprising the steps of:
(a) providing a coil assembly characterized by a phase and an amplitude;
(b) applying at least one periodic signal frequency to said coil assembly so as to provide a magnetic field;

Claim 40 continued...

(c) balancing said coil assembly with respect to both said phase and said amplitude thereof;
(d) introducing a plurality of the same kind of metal samples of different cross-sectional areas to said magnetic field of said coil assembly;
(e) measuring, for said at least one periodic signal frequency applied to said coil assembly, the true resistive component of voltage unbalance caused by introduction of each of the metal objects; and (f) compensating for phase-shift in the coil assembly with respect to the phase of said at least one periodic signal frequency applied to said coil assembly, whereby said true resistive component value is obtained at zero degree phase-shift.

41. The method of claim 40, wherein said at least one periodic signal frequency has a phase, and wherein said coil assembly produces an object signal having a phase, said step (e) comprising referencing the phase of the output signal of said coil assembly to within 1° of the phase of said at least one periodic signal frequency.

42. The method of identifying objects having conductive properties, comprising the steps of:
(a) generating a plurality of signal frequencies, the variation of which are less than one-half of one degree, (b) applying the signal frequencies to the input coil of a balanced highly stable coil system, (c) introducing an object having conductive properties into the field of the coil system, (d) obtaining the true resistive component of the eddy current loss which is within one degree of being in complete phase with the frequency signal introduced to the input coil,

Claim 42 continued.,.

(e) dividing the resistive component by the frequency applied to the input coil, and (f) using these values as a function of frequency to determine a set of signature data for the object.

43. The method of identifying objects having conductive properties as set forth in claim 42, including the step of:
(a) balancing the coil system to provide maximum sensitivity for detection of the true resistive component caused by eddy current loss.

44. The method of identifying objects having conductive properties as set forth in claim 42, including the steps of:
(a) storing a plurality of sets of signature data for comparison purposes, and (b) comparing the sets of signature data with data obtained from an unknown object.

45. The method of identifying objects having conductive properties as set forth in claim 42, including the step of:
(a) introducing a specific uniform type of metal sample of given cross-sectional area to the field of the coil system to obtain reference data values.