Metal detector apparatus utilizing controlled phase response to reject ground effects and to discriminate between different types of metals

Metal detector apparatus includes a controlled phase responsiveness to allow operation with reduced ground effects while discriminating between different types of metals. A phase relationship is predetermined that will reject unwanted signals from mineralized ground and trash metals and will be responsive to the signals from desired metals. The algebraic relationship between two signals is determined so that only input signals falling within the desired algebraic relationship are used to provide an output signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to metal detectors, and, more particularly, to metal 
detectors having the capability of reducing ground effects while 
discriminating between different types of metallic objects. 
2. Description of the Prior Art 
U.S. Pat. No. 4,128,803 uses a metal detector which has a transmit coil 
inductively coupled to a receiving coil and an oscillator connected to the 
transmit coil. The signals detected by the receive coil are processed by 
three synchronous demodulators, the first and second synchronous 
demodulators are used for detecting "R" or eddy current signal components 
and "X" or reactive signal components, respectively, and which two signal 
components are then used as inputs to the third synchronous demodulator. 
The output from the third synchronous demodulator is representative of the 
presence of a metallic object, and the polarity of the output signal 
indicates the particular type of the metallic object detected. 
U.S. Pat. No. 4,024,468 discriminates between different types of metallic 
objects by amplitude discrimination. The amplitude of the received signal 
is representative of the type of metallic object detected. A tuning 
control is used to adjust the amplitude of a signal received from mineral 
soil to substantially eliminate the effects of ferrous mineral soils. 
U.S. Pat. No. 4,030,026 discloses metal detector apparatus which uses a 
sampling technique in which the received signal is sampled and the sample 
voltage is utilized to produce an output signal corresponding to a 
selected component of the received signal. Since the output is based on a 
predetermined component of the received signal, the reactive components in 
the received signal due to mineral soils or other background conditions 
are simply ignored. 
The dominant type of metal detector in contemporary use is a 
transmit-receive detector operating in the very low frequency portion of 
the radio frequency spectrum. This type of detector is usually referred to 
as a VLF/TR detector. The detectors are generally operated by moving a 
search head over the ground to be searched. The search head usually 
contains at least a single conductive coil which is coupled to an 
oscillator A magnetic field is generated by the transmit coil, and the 
generated field provides a sinusoidal wave corresponding to the frequency 
of the oscillator. Any metallic object passing into the magnetic field 
causes a reaction. The reaction, as received by a receive coil in the 
metal detector head, is ultimately presented to the user of the apparatus 
by some type of indicating element, or elements, such as a meter, an 
audible signal, etc. 
In addition to discrete metal elements in the soil, which cause reactions 
from a metal detector, mineralization in the ground, usually ferrous 
oxides, also causes a reaction by metal detector apparatus, and the 
reaction is different from the reaction caused by conductive metals. Early 
metal detectors generally had two operating modes, a first mode for 
detecting conductive metals, and a second mode for detecting mineralized 
ground. 
There is about a ninety degree difference between signals caused by soil 
mineralization and signals caused by conductive metals. By sampling the 
received signals ninety degrees out of phase with the oscillator allows 
minerals to be detected. A peak mineral signal usually occurs where the 
oscillator is crossing zero, which is a phase shift of about ninety 
degrees. This will be discussed in detail below, in conjunction with FIG. 
1. For purposes of the present apparatus, and as is usual in the art, the 
sampling of the mineral signal at the time the oscillator output is zero, 
which is the time of maximum amplitude of the mineral soil signal, will be 
referred to as the mineral sample phase. This is generally referred to as 
the "reactive" or "X" component of the received signal. 
The sampling of the received signal ninety degrees later, when the 
amplitude of the mineral signal is zero, is referred to as the metal 
sample phase, or the "resistive", "eddy current" or "R" component of the 
received signal. As a practical matter, the metal sample phase is usually 
sampled slightly more than ninety degrees after the mineral sample phase. 
However, for purposes of the present application, and as disclosed in the 
drawings to be discussed hereafter, the mineral sample phase and the metal 
sample phase or the "X" and "R" components of the received signal will be 
considered as ninety degrees apart. The practical metal sample phase, 
sampling as shown in FIG. 1, provides a stronger positive response to 
objects, such as coins, which are of prime concern to metal detector 
users. However, this also provides a stronger negative response for 
mineral signals, which is undesirable, than if the metal sample phase is 
sampled ninety degrees after the mineral sample phase, which is the time 
of the zero mineral signal. 
The mass and surface area of a metal element affects the signal or the 
signature of the metal as detected with the metal detector. Once mineral 
soil is detected, the various signatures of the metals are relative to 
each other and generally fall out in the order given herein, as discussed 
below. The characteristics of the oscillator, input circuitry, and coil 
configuration used by a particular metal detector with respect to mineral 
soil is of primary importance and accordingly must first be determined. 
After the mineral soil is determined with respect to its signature and 
with respect to a particular metal detector, the other elements, such as 
discussed below, are generally fixed in order. For purposes of the present 
application, various elements are illustrated and are discussed, and their 
respective signatures are illustrated in wave forms and in vector 
diagrams. The order in which objects appear in phase with respect to each 
other is due to their metallic content and to the makeup of the objects 
and will generally be the same in all transmit/receive metal detectors. 
Since the early detectors used a practical metal sample phase that allowed 
a large negative mineral response, the detectors were difficult to use 
over mineralized ground. When used in the metal mode, they tended to find 
all matter of metallic junk elements, such as nails, tinfoil, hair pins, 
bottle caps, etc. These two drawbacks were first solved on an individual 
basis. 
The manufacturers of metal detectors discovered that by rotating the sample 
axis they could not only select whether the detector would respond to 
mineral or metal, but they could also discriminate between various 
commonly found metallic objects. According to the rotation of the sampling 
axis, various metallic elements may have either a positive component or a 
negative component. 
By configuring the detectors' electronic circuitry to respond with an audio 
output or a meter indication for positive "R" axis signals, and no output 
or indication for negative "R" axis signals, a metal detector can be made 
to be nonresponsive to much of the unwanted and worthless metallic trash 
commonly found where people have congregated. 
The techniques of discrimination discussed above are well known and are 
practiced by virtually all metal detectors in various configurations. 
The type of signal demodulators used in metal detectors may be either 
synchronous or asynchronous, since the received signal in a 
transmit/receive detector is generated by the oscillator and modified in 
phase by the detected object. The receive signal is accordingly 
synchronized to the oscillator. 
The '468 patent discussed above is typical of the type using an 
asynchronous demodulator. The demodulator used is a peak detector which 
responds positively to the decrease in amplitude of the received signals. 
The operating phase of the peak detector is established by biasing the 
receive coil signal with a phase shifted signal from the oscillator. The 
phase of the residual signal is set such that the addition of signals from 
desired objects causes a decrease in amplitude, and signals from unwanted 
objects cause an increase in amplitude. 
Most metal detector apparatus in contemporary use have synchronous 
demodulators to provide the ability to discriminate. A phase variable 
signal is generated from the oscillated signal, and the phase variable 
signal is used as a reference signal for the synchronous demodulator. 
Varying the phase of the reference signal changes the sampling axis to 
provide the ability to discriminate, as discussed above, and as will be 
discussed in detail below. 
The '026 patent discussed above is typical of current detectors using 
synchronous demodulators. The '026 apparatus covers the sampling of the 
received signal where the mineral signal is crossing zero, which is 
discussed above, and which is shown in FIG. 1 as the metal sample phase. 
This sampling point is indicated by the "R" vertical line in FIG. 1 and by 
the "R" axis in FIG. 2. 
Sampling where the mineral components is zero frees the detector from 
adverse detuning effects of mineralized ground. By rotating the sampling 
axis, a metal detector may also be made to discriminate between various 
metal objects, as discussed more in detail herein. The '026 apparatus 
samples the received wave form at the mineral sampling zero crossover 
which frees the apparatus of mineral effects, but the apparatus is 
incapable of discriminating at that time. 
The sampling axis, or "R" axis, is taken ninety degrees out of phase with 
the mineral signal, or "X" axis. Thus, the mineral signals will have no 
effect on the "R" sampling axis, but all of the metallic objects which are 
of common interest will have a positive component along the "R" sampling 
axis, as shown in FIG. 1. 
By rotating the sampling axis more than 90.degree. with respect to the "X" 
axis, discrimination between the various metallic objects can be 
accomplished. However, the mineral component signal will have a negative 
component on the rotated sampling axis. Therefore, rotating the sampling 
axis could either provide mineral-free detection or discrimination between 
various metallic objects, but not simultaneously. 
The general concept of the mineral-free sampling has been known for many 
years. It was first developed for early military mine detectors. Those 
early mine detectors used a pair of synchronous demodulators to detect the 
"X" and "R" components. However, the technique of using the two 
synchronous demodulators and other prior art techniques, were not able to 
provide mineral-free operation while discriminating between various types 
of metals. In the prior art, several methods have been devised to provide 
such discrimination and mineral-free operation, but they have not been 
without undesirable effects. 
Since the received signal is a composite of a response of the magnetic 
field to all objects that have an effect on the receive coil, a readily 
apparent way to differentiate the various components of the composite wave 
form is based on the motion of the search coil over the ground. If the 
search coil were to be swept back and forth at a constant height above the 
ground, the tuning could be adjusted to cancel the mineral component of 
the wave form and to reestablish the operating point of the detector to 
correspond to the origin of vector diagrams such as included in FIGS. 2-6 
herein. However, most operators tend to swing the search coil with a 
pendulum effect, since the pivot point of the detector is the operator's 
hand which is several feet above the ground. The pendulum effect means 
that there is a slow, rhythmic mineral signal that increases in amplitude 
as the coil is raised at each end of the swing. If the contour of the 
ground changes, there will be faster mineral changes with respect to the 
amplitude of the signal. However, this effect is relatively slow. 
As the search coil passes over a metal object, such as a coin, the response 
is very rapid due to the brief time duration that the search coil is 
disposed over the metal object. Most attempts to provide mineral-free 
operation and to simultaneously discriminate have used this frequency 
domain difference to provide the desired results. 
One way to solve the discrimination problem and mineral-free operation at 
the same time is by way of feedback circuitry incorporating time delays in 
the feedback loops. The feedback response is intended to be fast enough to 
cancel the slow mineral signal, and yet at the same time be slow enough to 
react to the higher frequency components of the metallic target response. 
However, this technique has an undesirable effect in that to be of 
substantive value in helping tune out the mineral effects, the feedback 
must be reasonably fast. When a metallic target is passed over, the 
feedback compensates for large portions of the metallic target response, 
which weakens the response to the object itself. At the same time, when 
the search coil passes over an object, the opposing feedback signal tends 
to bias the receive coil in the opposite polarity until the time delay 
allows the demodulator to sense the result and to remove the opposing 
feedback signal. 
To illustrate the statements contained in the preceding paragraph, an 
example may be appropriate. As a detector coil is passed over a nail 
buried in mineral soil, a wave form without feedback would indicate a 
negative response, assuming the "R" sample axis is set at about 
225.degree., which would theoretically eliminate or discriminate between a 
coin and unwanted or undesirable metallic trash objects. With a delayed 
feedback, the effect of the nail is substantially cancelled, but then a 
signal is generated which corresponds to a 180.degree. phase reversal as 
the nail effect is lost but the feedback effect is still present. Thus, 
the opposing wave form generated due to the feedback effect indicates that 
the nail is actually a good or desirable object. 
The feedback concept still results in problems which are undesirable. Other 
attempts have been made to solve the undesirable problems, such as a.c. 
couplings or low pass filtering. These also have resulted in the same 
undesirable effects. 
In the '803 patent, briefly discussed above, a detector is described which 
overcomes the undesirable effects by using three synchronous demodulators 
and matched band pass filters. Two of the synchronous demodulators are 
used to detect the components of the composite signal where the mineral 
signal is zero and at some desired discriminate setting. The mineral-free 
demodulator is referred to as the "R" demodulator, and the discriminating 
demodulator is referred to as the "X" demodulator. The sample axis of the 
"X" demodulator may be rotated to provide the desired degree of 
discrimination, as discussed above. 
In the '803 patent, the outputs of both the "X" and "R" demodulators are 
passed through band pass filters which remove any low frequency signal 
components and provide what is referred to as "ringing" signal outputs at 
a frequency of about twenty Hz. These signals both exhibit the previously 
discussed effects of having a large 180.degree. flyback signal as the 
result of their having lost their d.c. reference component in the filters. 
The purpose of the third synchronous demodulator is to provide d.c. 
restoration to the "X" signal, which substantially eliminates this 
undesirable effect. The "R" signal is used as a reference to demodulate 
the "X" signal. The inputs to the third synchronous demodulator are the 
two filtered "X" and "R" signals. 
Since both filter signals are generated by the same metallic object, the 
"R" signal prior to filtering will be a positive pulse, and the "X" 
signal, prior to filtering, will be positive for a desired object and 
negative for an undesired object. Accordingly, the ringing signal outputs 
of the filters will be in phase if the pulses are both positive, or out of 
phase if the "X" signal is negative. 
The third synchronous demodulator looks for positive "X" signals when "R" 
is positive. It also looks for negative "X" signals when the "R" signal is 
negative. When the "R" signal changes polarity due to the ringing of the 
filtered signals, the third synchronous demodulator has the effect of 
reversing the X demodulator sample axis. Depending on the polarity of the 
R signal, undesirable objects will provide one type of output signal, and 
desirable objects will provide a different type of signal. 
The approach of the '803 patent, while providing a solution of the 
undesirable effects of a.c. coupling or filtering, also has some inherent 
undesirable effects. For example, the filters used must be very well 
matched in response since the elimination of the mineral ground effects is 
accomplished entirely by the filters and since the third synchronous 
demodulator is phase sensitive. Moreover, any phase delay or difference 
between the two ringing signals of ninety degrees or more over the 
duration of the ringing signals may be interpreted by the third 
synchronous demodulator as a 180.degree. phase reversal of one of the 
signals. This would appear as the same effect as a delayed feedback 
discussed above, and an undesirable object would accordingly cause both a 
negative response and a positive response when the phase difference 
exceeds ninety degrees. 
Another problem with the apparatus of the '803 patent is cost. The matching 
of components, including the filters, resistors, and capacitors is 
relatively expensive, and the labor required to match the components is 
also relatively expensive. 
The apparatus of the present invention overcomes the problems of the prior 
art, as discussed above, without requiring a third synchronous demodulator 
and the matched components needed in the '803 patent and overcomes the 
problems of the prior art as discussed in general above. 
SUMMARY OF THE INVENTION 
The invention described and claimed herein comprises metal detector 
apparatus with complementary phase response having a pair of demodulators 
into which input signals are received representing two sample axes and the 
output of which is considered in absolute terms. By rotating the sampling 
axes, and by predetermining the limits in absolute terms of the outputs 
from the two demodulators, ground response may be minimized and outputs 
representative of predetermined types of metals may be selectively 
identified. 
Among the objects of the present invention are the following: 
To provide new and useful metal detector apparatus; 
To provide new and useful metal detecting apparatus which may substantially 
eliminate mineralized ground defects; 
To provide new and useful metal detector apparatus which discriminates 
among various types of metals; 
To provide new and useful metal detecting apparatus responsive to the phase 
shift of an output signal in response to the type of metal detected by the 
output signal; 
To provide new and useful metal detection apparatus operating in a very low 
frequency portion of the radio frequency spectrum; 
To provide new and useful metal detection apparatus having selectively 
variable phase angles for an output signal and for selectively determining 
the phase responsiveness to an input signal; and 
To provide new and useful apparatus for detecting preselected types of 
metals.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a waveform drawing showing the relationship of various waveforms 
involved with metal detectors, in general, and illustrating the shifting 
of the phase angle from the output waveform transmitted from the detector 
apparatus by various types of metal and by the soil. An oscillator 
waveform 2 comprises the output signal transmitted by the metal detector 
transmit coil, which is typically in the head of the detector apparatus. 
Waveform 2 is a sinusoidal wave showing two complete cycles of the wave, 
for a total of seven hundred twenty degrees. The oscillator wave 2 begins 
at zero degrees, rises to maximum amplitude at 90.degree., returns through 
zero at 180.degree., reaches a minimum amplitude at 270.degree., and 
passes through zero 360.degree. after the beginning, and which is noted in 
FIG. 1 as zero degrees, again for a complete cycle. The waveform for an 
additional 360.degree., or a second cycle, is shown in FIG. 1 as a 
continuation or a repetition of the 90.degree., 180.degree., 270.degree., 
and zero degrees, again. It is a repeat of the first 360.degree. portion 
of the waveform. Sine waveforms are well known and understood. 
Waveforms 4, 6, 8, 10, 12, and 14 show the relationship of waveforms 
received by bringing a search coil of a detector into proximity with 
various metallic objects. Waveform 16 comprises a composite waveform of a 
nickel coin buried in mineralized soil. 
The waveforms 4-16 are all shown in relation to the oscillator output 
waveform 2. Waveform 4 is an illustration of a reflected wave from mineral 
soil, typically soil having iron oxides therein, and illustrating a 
90.degree. shift from the oscillator or output waveform 2. That is, the 
reflected waveform from mineral soil shifts 90.degree. from the oscillator 
output waveform 2. The mineral waveform 4 leads the oscillator output 
waveform 2 by 90.degree.. The waveform 4 crosses through zero at 
90.degree., reaches its minimum or minus amplitude at 180.degree., passes 
upwardly through zero at 270.degree., and reaches its maximum amplitude 
again at zero degrees. 
Waveform 6 illustrates a waveform reflected by an iron nail, which is 
leading the oscillator waveform 2 by about fifty degrees. That is, the 
maximum, zero, and minimum amplitudes lead by about fifty degrees the 
oscillator waveform 2. 
Waveform 8 represents a typical waveform for a steel bottle cap. It leads 
the oscillator waveform by about thirty degrees. 
Waveform 10 illustrates the waveform responsive to nickel. It lags the 
oscillator output waveform 2 by about thirty degrees. That is, its 
maximum, zero, and minimum amplitude lag the oscillator waveform 2 by 
about thirty degrees. 
Waveform 12 illustrates a waveform for aluminum, such as an aluminum pull 
tab. The waveform 10 lags the oscillator waveform 2 by about sixty 
degrees. 
Waveform 14 represents a typical waveform for a coin, which lags the 
oscillator output waveform 2 by about eighty degrees. The coin for which 
the waveform 12 is responsive may be silver, or a combination of silver 
and some other metal, or copper, such as a dime, a penny, a quarter, etc. 
Waveform 16 represents the waveform for a nickel coin buried in mineralized 
soil. It appears very similar to that of the iron nail, but with less 
amplitude. 
The waveforms illustrated in FIG. 1 are illustrative, and indicate that 
each type of metal, including mineral soil having iron oxide therein, has 
a distinctive waveform when compared to the output or oscillator waveform 
2. It will be noted that the waveforms 4, 6, 8, 10, 12, 14, and 16 are 
sine waves, like the oscillator waveform 2. However, the amplitude of each 
of the waveforms 4-16 is less than the waveform 2 from the oscillator. 
The waveforms 4 . . . 16 are not necessarily indicative of actual 
waveforms, since actual waveforms will differ with Virtually each 
instrument. The waveforms 4 . . . 16 are accordingly exemplary only. 
However, once a mineral signal or waveform is found, the other signals are 
relative and they fall into place generally as shown in the waveforms 4 . 
. . 16. 
For metals, either pure or composite metals, the waveforms may vary in 
phase from that illustrated in FIG. 1. Moreover, for metals of various 
types buried in mineral soil, the waveforms may also differ in phase from 
that illustrated. For example, the waveform 16 differs from waveform 10 in 
both phase and amplitude. However, for purposes of illustrating the 
apparatus of the present invention, and the philosophy associated 
therewith, the waveforms illustrated in FIG. 1 will be used herein. 
The waveforms of FIG. 1 are illustrated in vector diagrams in FIGS. 2, 3, 
4, 5, and 6. The vector diagrams of FIGS. 2-6 simplify the illustration 
and explanation of the apparatus of the present invention with respect to 
identifying specific metals while rejecting other metals, and illustrate 
the rejection of the mineralized soil in detecting metal objects. 
As is well known and understood, a metal detector generally uses a search 
head which is moved over the ground to be searched. The search head 
usually contains one or more inductive coils which are part of an 
oscillator tank circuit. The oscillator tank circuit is typically driven 
by an oscillator having a sinusoidal output, such as the waveform 2 in 
FIG. 1. The coils in the search head serve as a transmitter and a receiver 
antenna. A magnetic field is generated by the transmitter coil, and any 
metallic object passing into the magnetic field causes some type of 
measurable reaction. The reaction may be measured as a frequency shift, an 
amplitude change, an inductive change, or a combination of effects. In 
FIG. 1, and in FIGS. 2-6, the effect is illustrated as a change in both 
phase and amplitude. 
By sampling the received signal, or signals, at predetermined time periods 
with respect to the oscillator signal, two components may be detected and 
plotted, as on the vector diagrams of FIGS. 2-6. The sampling of the 
signal is illustrated in FIG. 1 by the vertical lines representing "X" 
sample phase and "R" sample phase. The mineral sample phase is commonly 
referred to as the "reactive" or "X component" of the received signal. In 
the vector diagram of FIG. 2, the two sample phases, which are ninety 
degrees apart,are simply identified as the X axis and the R axis, which 
axes have been discussed above in detail in conjunction with the 
Description of the Prior Art. However, in FIGS. 4, 5, and 6, the sampling 
axes are simply identified as sampling axis A and sampling axis B, since 
it is not necessary to refer to them by R and X designations. The R and X 
designations are not pertinent or relevant to the present invention, as 
will be discussed in detail. 
As has been previously indicated, once mineral soil is detected, the 
signatures of the various metals are relative and they generally fall out 
in the order shown in FIG. 1, Accordingly, the first determination that 
must be made is to determine the relationship of the oscillator of a given 
metal detector with respect to mineral soil. Assuming the relationship, 
for purposes of the present invention, of mineral soil and of the various 
elements as indicated in FIG. 1, the vector diagram of FIG. 2 may then be 
made. 
In FIG. 2, a vector diagram is made with the X sample phase, or X axis, 
taken along the vector for the mineral soil, which corresponds to the 
mineral sample phase of FIG. 1. At the mineral sample phase, the 
oscillator waveform is passing through zero degrees in a positive 
direction. This is at a maximum amplitude for mineral soil. The metal 
sample phase, or R axis, is taken ninety degrees later. At the metal 
sample phase, or along the R axis, the mineral soil waveform passes 
through zero, and the mineral soil accordingly has no R axis component. 
In FIG. 2, the first six object signals from FIG. 1, namely the objects or 
elements for which waveforms 4, 6, 8, 10, 12, and 14 are shown in FIG. 1, 
are plotted as vectors relative to the oscillator phase. The phase angles 
of the vectors are representative of the angle of the peak positive 
response of the object or element, and the vector lengths are indicative 
of the relative amplitude of the received signal. The vectors are 
identified with the reference numerals associated with the waveforms of 
FIG. 1, and designate the element (mineral) or metal obJect associated 
therewith, as identified in FIG. 1. 
The X and R axes provide a relatively simple rectangular coordinate system 
in which the amplitude of an object at various phases may be predicted. 
For example, projecting the terminal point of the iron nail vector 6 from 
FIG. 2 onto the X and R axes, respectively, shows that the response to an 
iron nail will be about eighty percent as strong in the R direction as in 
the X direction. This is indicated by a dotted line in FIG. 2 extending 
from the head of the iron nail vector, which is on the circle, extending 
horizontally to the X axis, and another dotted line extending from the 
vector head on the circle downwardly to the R axis. The conclusion with 
respect to the eighty percent strength in the R direction as in the X 
direction may be confirmed by referring to FIG. 1. The positive iron nail 
response, or amplitude, for waveform 6, at the R sample point, which is 
ninety degrees with respect to the oscillator waveform 2, is less than its 
value (amplitude) at the X sample phase, which is zero degrees for the 
oscillator. The R axis amplitude appears to be about eighty percent of the 
X axis amplitude. 
Comparing the vectors of FIG. 2 with the waveforms of FIG. 1, one readily 
observes that the vectors 6 and 8 are positive in both the mineral sample 
phase (X) and the metal sample phase (R), while vector 4 is positive at 
the X sample phase and zero at the R sample phase. The vectors 10, 12, and 
14 are positive at the metal sample phase (R) and negative at the mineral 
sample phase (X). The positive and negative components are correlated with 
respect to the positive and negative (plus and minus) components of the 
two axes, namely the plus and minus R axis and the plus and minus X axis, 
in the vector diagram of FIG. 2. 
FIG. 3 is a vector diagram illustrating a desirable response for a 
discriminator which eliminates the undesirable effect of "flyback" due to 
A.C. coupling, delayed feedback, or filtering, and which response is used 
by the apparatus of the present invention. The vector diagram includes 
equal and opposite portions of the response spectrum for both desirable 
objects and undesirable objects, and includes the "flyback" signal for 
both desirable and undesirable objects. The "flyback" signal is 
illustrated in FIG. 3 by dotted lines extending in the opposite direction 
from the vectors 6, 8, 10, 12, and 14. The equal and opposite desirable 
and undesirable portions of the vector diagram of FIG. 3 comprise separate 
ninety degree portions of the vector diagram. They are designated in FIG. 
3 by extensions of the respective X and R axes from FIG. 2, but in FIG. 3 
they are not designated as X and R axes. Rather, the extensions of the 
axes, which are at the three o'clock, six o'clock, nine o'clock, and 
twelve o'clock positions in FIG. 3, are simply indicated by the letters 
"D" for desirable and "U" for undesirable. Thus, regardless of the 
polarity of the desirable or the undesirable response, any signal which 
falls in the portion of the spectrum indicated by "D" indicates a 
desirable object, and any signal which occurs in an undesirable or "U" 
portion of the spectrum indicates an undesirable object. The sampling 
method provides that both the primary response and the "flyback" response 
for a desirable object are good signals, by definition. Similarly, by 
definition, both the primary response and the "flyback" response from 
undesirable objects are undesirable signals which may be disregarded. 
A desired response may be achieved by rotating the sampling axes normally 
associated with the mineral sampling, or "X" axis, and the metal sampling, 
or "R" axis, so that their axes are at 45.degree. and 135.degree., 
respectively, with respect to the peak or no response factor. This is 
shown in FIG. 4. The sampling axes are, at these sampling points, no 
longer primarily responsive to one type of signal, but their response is a 
combination of mineral and metal signals. Since they no longer primarily 
define a specific type of response, the definitions of a reactive "X" and 
eddy current "R" components are no longer meaningful. Accordingly, the 
sampling axes may be referred to simply as axis A and axis B, as shown in 
FIGS. 4, 5, and 6. 
FIG. 4 illustrates a sampling method which accomplishes the desired 
response discussed above in conjunction with FIG. 3. The Figure includes a 
pair of sample axes, designated sample axis A and sample axis B, which are 
disposed at 45.degree. and 135.degree., respectively, to the peak mineral 
response vector 4. In FIG. 4, sample axis A is identified by reference 
numeral 20, and sample axis B is designated by reference numeral 22. The 
sample axes are ninety degrees from each other. The desirable portion of 
the spectrum, designated by letter D, is disposed within forty-five 
degrees on either side of sample axis B. The desirable portions of the 
spectrum are defined by the algebraic relationship provided or defined by 
the equation B is greater than A, when only the absolute values of B and A 
are considered. 
By configuring a comparator/amplifier circuit to yield an output of one 
polarity when the absolute value of B is greater than the absolute value 
of A, and to yield an output of the opposite polarity when the absolute 
value of B is less than the absolute value of A, a practical detector may 
be made which would negate the undesirable effects discussed previously. 
Objects which provide a response such that the absolute value of B equals 
the absolute value of A would provide a zero output from the 
comparator/amplifier. 
The desired response is achieved by a comparison of absolute values, and 
the voltage polarities of the sampling A and B axes are accordingly 
unimportant. 
Since a user of a metal detector may want to include different types of 
metallic objects as desirable, the areas of desirable response, such as 
shown in FIGS. 3 and 4, should be variable by the user. Such may be 
accomplished by the apparatus of the present invention by scaling the 
response of one or both of the A and B axes demodulators, or by varying 
the inputs of the comparator/amplifier. This serves to narrow or widen the 
desired response area centered on one of the sample axes, and is 
illustrated in FIGS. 5 and 6. 
The sample axes may be rotated to cover the desired response portion of the 
phase spectrum, as shown in FIGS. 5 and 6. In FIG. 5, the sampling axes, 
or A and B axes, are rotated to thirty degrees and 120.degree., 
respectively, with respect to the maximum mineral soil signal, and the 
scale is changed such that the comparator/amplifier provides a desired 
output response when the absolute value of B is greater than the absolute 
value of 0.57A. Thus, the "D" portion of the spectrum in the vector 
diaphragm of FIG. 5 is widened to an amount which is substantially greater 
then 90.degree.. This results in the metal detector providing a desirable 
response for all of the objects shown except the iron nail. That is, the 
iron nail alone is excluded as providing an undesirable response. The 
other elements, the bottle cap 8, the nickel 10, the pull tab 12, and the 
coin 14, all result in a signal response which is defined as being a 
desirable output. 
In FIG. 6, the A and B axes are rotated to 77.5.degree. and 167.5.degree., 
respectively, from the maximum mineral signal, and the scale is changed to 
obtain a desired output when the absolute value of B is greater than 4.5 
times the absolute value of A. This provides means for obtaining a 
desirable response for only the coin among the objects shown. The 
desirable portion of the output spectrum is thus substantially narrowed to 
eliminate all responses except the response from the coin. Or, in other 
words, the response from the coin alone falls in the desirable range, and 
the response from all other items falls within the undesirable response. 
It will be noted that the flyback signals, illustrated by dotted lines 
extending 180.degree. from the primary vector in FIG. 3, have not been 
included in FIGS. 4-6. However, since only absolute values are considered 
in the apparatus of the present invention, such flyback signals are 
considered in the present apparatus as good signals. They are desirable 
signals if they fall within a stated or defined equation, and they are 
undesirable signals if they fall without such equation, as illustrated in 
FIGS. 4, 5, and 6 by the respective "D" and "U" portions of the spectra. 
FIG. 7 is a block diagram of apparatus 30 of the present invention. The 
apparatus 30 includes an oscillator 32 which provides a sine wave signal 
on conductor 34 to a transmit coil 36. The oscillator 32 is preferably 
similar to that which has been discussed above, which includes a transmit 
coil as part of its resonant circuit. The oscillator also preferably 
operates in the very low frequency portion of the radio frequency 
spectrum, between about three KHZ and about thirty KHZ. 
A conductor 38 is connected to conductor 34 and transmits part of the 
oscillator output to a variable phase shift 40. The phase shift circuitry 
40, well known and understood in the art, provides reference signals to a 
pair of demodulators, namely an A-axis demodulator 60 and a B-axis 
demodulator 70 through a pair of conductors 42 and 44, respectively. The 
A-axis and B-axis demodulators 60 and 70 are ninety degrees out of phase 
with each other, and they maintain this phase difference as the sample 
axes are varied or rotated. 
Input to the A-axis demodulator 60 and to the B-axis demodulator 70 is from 
a receive coil 50. The receive or search coil 50 may be of any 
contemporary design, generally well known and understood in the art. As 
has been discussed above, various types of coils may provide different 
basic phase responses to various metallic objects, as referenced to a 
particular oscillator. Such effects are well known and understood, and 
only require an adjustment to the phase shift circuitry 40 in the 
implementation of the present invention. 
A conductor 52 extends from the search or receive coil 50 to a pair of 
conductors 54 and 56. The conductors 54 and 56 extend from conductor 52 to 
the A-axis demodulator 60 and the B-axis demodulator 70, respectively. 
Thus part of the output from the receive coil 50 is transmitted to both 
demodulators. 
The demodulators 60 and 70 are preferably synchronous demodulators, well 
known and understood in the art, although asynchronous demodulators may 
also be used. 
From the A-axis demodulator 60, a conductor 62 extends to a variable scale 
factor 64. The purpose of the variable scale factor is to vary the 
relationship between A and B in absolute values, as discussed above in 
conjunction with FIGS. 4, 5, and 6. That is, in the apparatus 30, the 
absolute value of A is varied by the variable scale factor 64, while the 
absolute value of B is not varied. The variable scale factor 64 may 
accomplish scaling in several different ways, such as changing the gain of 
an amplifier, resistive attenuation, etc. The particular type of scaling 
is relatively unimportant, so long as the proper scale factors are 
achieved. 
In practice, the variable phase shift circuitry 40 and the variable scale 
factor circuitry 64 are preferably ganged together. Adjustment of the 
variable scale factor also adjusts the variable phase shift to keep them 
lined up. 
From the variable scale factor 64, a conductor 66 extends to an absolute 
value reference block 68. It is preferable that the scale factoring be 
accomplished prior to the absolute value function. The absolute value 
circuitry 68 preferably includes active gain elements to minimize 
different types of errors which may occur, such as offset and zero 
crossover errors. As is well known and understood, various types of 
existing circuits may be usable for providing the absolute value function. 
From absolute value circuitry 68, a conductor 78 extends to a comparator 
80. The comparator 80 includes comparator and amplifier functions which 
may be of a differential type, summing type, or various other types of 
configurations. The comparator 80 preferably is configured to yield a 
smooth, continuous response as its output changes from one polarity to the 
other. 
From the B-axis demodulator 70, a conductor 72 extends to a second absolute 
value circuitry 74. The absolute value 74 is preferably substantially 
identical to the absolute value circuitry 68. A conductor 76 then extends 
from the absolute value circuitry 74 to the comparator 80. 
The variable scale factor 64 is in the A-axis demodulator circuitry, but it 
could well be included in the B-axis demodulator circuitry, if desired. In 
the alternative, scaling could be accomplished in both axes if desired. 
For example, in addition to the variable scale factor 64, extending 
between conductors 62 and 66, a second variable scale factor, not shown, 
could be inserted in conductor 72 between the B-axis demodulator 70 and 
the absolute value circuitry 74. Moreover, if desired, the variable 
scaling could be accomplished between the absolute value circuitry and the 
comparator. 
From the comparator 80, a conductor 82 extends to indicator means 84. The 
indicator means may be of any contemporary type, such as a meter, an audio 
output, etc. 
The comparator 80 considers only magnitude, and not polarity, from its 
input signals, and the magnitude only is thus used as a basis for its 
output on conductor 82 to the indicator means 84. The absolute value 
circuitry elements 68 and 74 for the A-axis and B-axis, respectively, 
provide the inputs to the comparator on conductors 78 and 76, 
respectively. Since magnitude alone is considered as a basis for the 
indicator means 84, differences in filters, coupling circuits, or feedback 
delays cause only apparent scale factor errors. These errors may be 
adjusted by the operator of the apparatus by altering the sample axes 
slightly to compensate for the scale factor error. An adjustment of the 
sample axis is part of the initial procedure by the operator in setting up 
the apparatus, and is known and understood. 
In FIGS. 4, 5, and 6, in accordance with the discussion pertaining thereto, 
the variable scale factor 64 is implemented along the A-axis, as is shown 
in FIG. 7. As has been discussed above, the variable scale factor could be 
in the B-axis, if desired, or in both the A and B axes to provide greater 
flexibility with respect to discriminating between various types of 
metallic targets, and at the same time eliminating the effects of 
mineralized soil. 
The algebraic relationship between the A-axis signal and the B-axis signal 
is predetermined by the variable scale circuitry 64, and may be varied, as 
discussed above in conjunction with FIGS. 4, 5, and 6. For discriminating 
between different types of metals, the algebraic relationship, or the 
equation defining the algebraic relationship between the A and B axes, may 
be varied, to selectively provide a response to the desired metal. The 
algebraic relationship may be greater than unity or less than unity in 
order to provide the desired response. As indicated, the algebraic 
relationship is in terms of magnitude or absolute value, with polarity 
being immaterial. 
There are two variables in the apparatus of the present invention, the 
first variable being the rotation of the sampling axis accomplished by the 
variable phase shift element 40, and the second variable being the 
variable scale factor circuitry 64. By predetermining the variable phase 
shift, or the location of the A and B sampling axes, and the algebraic 
relationship between the absolute values of the signal sampled at the two 
axes, the apparatus of the present invention provides for the elimination 
of mineral ground effects and for the discrimination of various types of 
metals. As has been discussed, the discrimination may be broad enough to 
include several types of metals, or it may be narrowed to provide an 
output response to only a single type of metal. 
The individual blocks included in FIG. 7 represent various circuit elements 
or components, each of which is relatively well known and understood. 
Moreover, the apparatus 30 as embodied in FIG. 7 represents only the 
minimum required for the apparatus to function. Other elements may be 
added without detracting from the basic theory of operation as set out 
herein. For example, variable feedback could be added by feeding the 
output of the demodulators back to the input of their input. The inputs of 
the absolute value elements could be capacitively coupled, or bandpass 
filters could be inserted in place of capacitors. Filters have the 
advantage of cut-off speed over capacitors. Persons skilled in the art 
understand the use of feedback, filters, capacitors, etc. 
The alternatives discussed in the preceding paragraph are all well known 
ways of eliminating the mineral signal and of discriminating at the same 
time. All of them have a flyback signal. The apparatus of the present 
invention utilizes the flyback signal and simplifies the components or 
elements involved, and at the same time reduces the cost of the apparatus. 
While the principles of the invention have been made clear in illustrative 
embodiments, there will be immediately obvious to those skilled in the art 
many modifications of structure, arrangement, proportions, the elements, 
materials, and components used in the practice of the invention, and 
otherwise, which are particularly adapted for specific environments and 
operative requirements without departing from those principles. The 
appended claims are intended to cover and embrace any and all such 
modifications, within the limits only of the true spirit and scope of the 
invention. This specification and the appended claims have been prepared 
in accordance with the applicable patent laws and the rules promulgated 
under the authority thereof.