Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~9 S7096
-- 1 --
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
-- 5 --
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~ `
-- 6 --
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
-- 7 --
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
- 8 -
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
-- 10 --
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
- 12 -
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
- 13 -
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
- 14 -
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
- 15 -
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
- 17 -
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
- 18 -
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
-- 19 --
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
- 20 -
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~ `
- 23 -
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.
;~
.
.
