A method and apparatus of discrimination detection using multiple frequencies to determine a recognizable profile of an undesirable substance

• US 4,942,360 A
• Filed: 09/08/1987
• Issued: 07/17/1990
• Est. Priority Date: 09/08/1986
• Status: Expired due to Term

• Current Assignee: BHC Consulting Pty Ltd.
• Original Assignee: Bruce H. Candy
• Inventors: Candy, Bruce H.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): Edmonds, Warren S.
• Application Number:
  US07/093,837
• Time in Patent Office: 1,043 Days
• Field of Search: 324/326, 324/330, 324/327, 324/335, 324/328, 324/329, 324/239, 324/233, 324/260, 324/236, 324/238, 324/344, 324/336, 324/337
• US Class Current: 324/329
• CPC Class Codes: G01V 3/104 using several coupled or uncoupled coils G01V3/101 takes precedence
• CPC Class Codes: G06V 10/255 Detecting or recognising potential candidate objects based on visual cues, e.g. shapes

Abstract

A conducting metal discriminating detection apparatus which uses at least two different frequencies and in preference 3 to interrogate a target. The detection apparatus distinguishes reactive and resistant components of each of the received signals and combines these in such a way as to avoid effects of background such as ironstone in one case or salt water in another.

Claims

1. A conducting metal discriminating detection apparatus comprising:

means to transmit at least two alternating magnetic fields of different frequency, each below 100 kHz,

means to receive respective signals arising from each of the transmitted frequencies being retransmitted from a target,

means for demodulating the respective received signals to determine a presence of at least one of reactive and/or resistive components;

means for comparing magnitudes of respective component levels at said different frequencies to produce an output signal indicative thereof an interpretable characteristic of which is substantially independent of a background environment which includes substantial material which has a substantial magnetic effect and has a reactive to resistive response ratio which is substantially independent of an interrogating frequency wherein said means for demodulating includes at least one synchronous demodulator for each transmitted signal component, synchronous demodulator references being derived from said transmitted signal components,

a low pass filter coupled to outputs of the synchronous demodulator, and wherein a low-passed output of each said demodulator is sensitive to one only of the transmitted frequencies, and each said synchronous demodulator having its reference selected whereby each said low-passed synchronously demodulated output includes information proportional to either predominantly reactive components of the magnetically interrogated environment at a frequency Wi, where i is the mathematical label of the i-th transmitted frequency (i=1,2,. . . ), to which the demodulator is sensitive, the output of such a demodulator'"'"'s low-pass filter being called RCI the output of the reactive channel of Wi, or to predominantly resistive components of the magnetically interrogated environment at the frequency, Wi, to which the demodulator is sensitive, the output of such a demodulator'"'"'s low-pass filter being called LCi, the output of the resistive channel of Wi, and further comprising

interpretive means including means adapted to compare by subtraction at least a selected ratio of one of the reactive channel'"'"'s outputs with another to result in a difference signal called a reactive difference signal RDk, where k is the mathematical label of the Kth reactive difference signal (k=1, . . . ), such that RDk yields substantially no change when material that is substantially purely reactive and non-resistive below 100 kHz is moved within the influence of the transmitted fields and this occurs when the reactive channel gains are selected to be substantially equal.

6. An apparatus as in any one of the preceding apparatus claims 1, 3, 4, or 5 characterised in that the apparatus includes means to process the said interpretive signals to give interpretive signals relating to the nature of the interrogated conductiong metal target objects and thereby enabling discrimination.

7. An apparatus as in any one of claims 1, 2, 4, or 5 further characterised in that the apparatus includes means whereby comparisons for interpretive purposes of the received signal components are effected at the synchronous demodulator stages before the respective signal components are low-pass-filtered.

8. An apparatus as in any one of preceding claims 1, 2, 3, or 5 further comprising means for combining said difference signals are adapted to be combined by further interpretive means whereby the said difference signals are each passed through respective high-pass filters, the outputs of these said high-pass filters being each full-wave rectified through respective full-wave rectifiers, the outputs of these said full-wave rectifiers being each added by an adder to produce a final interpretive output.

9. An apparatus as in any one of preceding claims 1, 3, 4 or 5 further characterised in that it includes means whereby the said difference signals are adapted to be combined by further interpretive means whereby the said difference signals are each passed through respective high-pass filters, the outputs of these said high-pass filters being each full-wave rectified through respective full-wave rectifiers, the outputs of these said full-wave rectifiers being fed to a selector which passes the largest instantaneous said full-wave rectified output to a final interpretive output.

2. A conducting metal discriminating detection apparatus comprising:

means to transmit at least two alternating magnetic fields of different frequency, each below 100 kHz,

means to receive respective signals arising from each of the transmitted frequencies being retransmitted from a target,

means for demodulating the respective received signals to determine a presence of at least one of reactive and/or resistive components;

means for comparing magnitudes of respective component levels at said different frequencies to produce an output signal indicative thereof, an interpretable characteristic of which is substantially independent of a background environment which includes substantial material which has a substantial magnetic effect and has a reactive to resistive response ratio which is substantially independent of an interrogating frequency,

wherein said means to transmit an alternating magnetic field are adapted to provide three different frequencies, each less than 100 kHz and above 500 Hz, and further including an interpretive means including means adapted to compare by subtraction at least a selected ration of one of the reactive difference signals with another to result in a difference signal RDDm where m is the mathematical label of the mth difference of reactive difference signals (m=1, . . . ), such that RDDm yields no change when magnetic material with frequency independent reactive to resistive component ratios at frequencies below 100 kHz is moved within the influence of the transmitted fields, so that if RDDm α

(the said selected ratio) * RDk-RDn, then d/dt (RDDm)=0 for the interrogation of such said material only and this results when the change gains are selected such that RDDm α

RDk*log(W4/W3)-RDn*log(W2/W1) where the log is a natural log, RDk equals the difference in the resistive channels of W1 and W2, and RDn equals the difference in the resistive channels of W3 and W4, where the channel gains are equal, and W2>

W1 and W4>

W3, and in the case of three simultaneously transmitted signals, W2 may equal W3 or W4, or W3 may equal W1.

3. A conducting metal discriminating detection apparatus comprising:

means to transmit an alternating magnetic field with at least two different frequencies, being each less that 100 kHz and above 500 Hz,

at least one synchronous demodulator for receiving and synchronously demodulating each transmitted signal component as retransmitted by reflections from the ground having synchronous demodulator references which are derived from said transmitted signal components,

low pass filters, one for each synchronous demodulator, wherein a low-passed output of each said demodulator is sensitive to one only of the transmitted frequencies, and each said demodulator having its reference selected whereby each said low-passed synchronously demodulated output will contain information proportional to one of predominantly reactive components of the magnetically interrogated environment at the frequency Wi where i is the mathematical label of the ith transmitted frequency (i=1,2. . . ), to which the demodulator is sensitive, the output of such a demodulator'"'"'s low pass filter being called RCi the output of the reactive channel of Wi, or b) predominantly resistive components of the magnetically interrogated environment at the frequency Wi, to which the demodulator is sensitive, the output of such a demodulator'"'"'s low-pass filter being called LCi the output of the resistive channel of Wi,

an interpretive means including means adapted to compare by subtraction at least a selected ratio of one of the resistive channel'"'"'s outputs with another to result in a difference signal called a resistive difference signal LDk where k is the mathematical label of the kth resistive difference signal (k=1,2. . . ), such that LDk yields substantially no change when magnetic material which has substantial magnetic effect and has a reactive to resistive components response ratio which is substantially independent of any interrogating frequency below 100 kHz is moved within the influence of the transmitted fields and this occurs when the resistive channel gains are selected to be substantially equal.

4. A conducting metal discriminating detection apparatus comprising:

means to transmit an alternating magnetic field with at least three different frequencies, being each less than 100 kHz, and

an interpretive means, including means adapted to compare by subtraction at least a selected ratio of one of the resistive difference signals with another to result in a difference signal LDDm where m is a mathematical label of the mth difference of resistive difference signals (m=1. . .), such that LDDm yields substantially no change when either magnetic material which has a substantial magnetic effect and has a reactive to resistive component response ratio which is substantially independent of any interrogating frequency at frequencies below 100 kHz or non-magnetic material that is mildly electrically conductive or both is moved within the influence of the transmitted magnetic fields, so that if LDDm α

(the said selected ratio)*LDk-LDn, then d/dt(RDDm)=0 for the interrogation of such said material types only;

which results when the resistive channel gains are selected such that the sum of the channel gains resulting in LDDm equals zero and the sum of the channel gains, each multiplied by the value of its associated transmitted frequency, equals zero.

5. A conducting metal discriminating detection apparatus comprising means to transmit an alternating magnetic field with at least two different frequencies, being each less than 100 kHz, and being such to cause a retransmitted signal from the ground and/or electrically conducting target metal object,

means for synchronously demodulateing said retransmitted signal, including at least one synchronous demodulator for each transmitted signal component, having synchronous demodulator references which are derived from said transmitted signals components,

low pass filters, connected to outputs of said synchronous demodulating means, where a low-passed output of each said demodulator is sensitive to one only of the transmitted frequencies, and each said demodulator having its reference selected whereby each said low-passed synchronously demodulated output includes information proportional to either (a) predominantly reactive components of the magnetically interrogated environment at the frequency Wi where i is the mathematical label of the ith transmitted frequency (i=1,2. . . ), to which the demodulator is sensitive, the output of such a demodulator'"'"'s low-pass filter being called RCi the output of the reactive channel of Wi, or b) predominantly resistive components of the magnetically interrogated environment at the frequency Wi, to which the demodulator is sensitive, the output of such a demodulator'"'"'s low-pass filter being called LCi the output of the resistive channel of Wi, or, c) a linear combination of resistance and reactive components of the magnetically interrogated environment at the frequency Wi, to which the demodulator is sensitive, the output of such a demodulator'"'"'s low-pass filter being called RLCi the output of the reactive/resistive channel of Wi, and

an interpretive means including means adapted to produce a signal that is linear combination of, either two different reactive channel outputs and a resistive channel output or a reactive/resistive channel output RLCi and reactive channel output RCj where i is different from j, to result in a reactive difference resistive difference signal LRDk, where k is the mathematical label of the kth such signal (k=1,2. . . ), such the LRDk yields substantially no change when magnetic material which has a substantial magnetic effect and has a reactive to resistive response ratio which is substantially independent of any interrogating frequency below 100 kHz is moved within the influence of the transmitted fields;

adapted whereby this results when the channel gains are selected so that LRDk α

LCj-π

/(2log(Wh/Wi)*RDk equals the difference in the reactive components of Wh and Wi, Wh>

Wi, where the channel gains are equal, which for two transmitted signals is equivalent to substracting a reacttive channel output RCi from RLCj, such that when RLCj is mathematically split into a predominantly reactive component, RCj and a predominantly resistive component LCj, where j=h and i=i, or, j=l and i=h, then for effective gain of RCi being equal to RCj, the effective gain of LCi is selected whereby LRDk α

LCj-π

/(2log(Wh/Wi)*RDk where RDk equals the difference in the reactive components of RCi and RCj, Wh>

Wi. then for the selective gain of RCi being equal to RCj, the effective gain of LCi is selected whereby LRDkα

LCJ-π

/(2log(Wh/Wi)*RDk where RDk equals the difference in the reactive components of RCi and RCj, Wh>

Wi.

10. A conducting metal discriminating detection apparatus comprising:

means to transmit at least two alternating magnetic fields of different frequency, each below 100 kHz,

means to receive respective retransmitted signals arising from each of said transmitted frequencies from a target,

means to synchronously demodulate, for each frequency, the respective received signals to provide an output of a selected linear combination of both reactive and resistive components, and

comparison means to compare information, from the synchronously demodulate means for respective frequencies, to produce an output,

wherein said synchronously demodulate means includes a first synchronous demodulator responding to a first frequency and adjusted to a first setting, and

a second synchronous demodulator responding to a second frequency and adjusted to a second setting,

wherein said settings are adjusted to effect a nonchanging output when interrogation is effected across a solely ferrite background.

11. A conducting metal discriminating detection apparatus as in claim 10 further characterized in that said output comprises an average of synchronous demodulated contributions formed from both reactive and resistive components from both transmitted frequencies,

where the gains and phase angles of the said synchronous demodulator component channels forming said comparison output are selected such that,

(a) the sum of gains of the respective resistive components of each respective channel, added to, the product of pi divided by 2 (2 times arctan (1) in radians) and

(b) the sum each of the gains of the respective reactive component of each respective channel each multiplied by the natural logarithm of the transmit frequency from which the said reactive component is derived,

substantially equals zero, and also such that

the sum of all the gains of the respective reactive components of each respective channel substantially equal zero.

12. A conducting metal discriminating detection apparatus as in claim 11 wherein gains of the respective reactive and resistive components channels are selected such that the condition ##EQU3## is satisfied where LC_i is the ith contributing resistive channel gain (n in total), RC_j is the jth contributing reactive channel gain (m in total), π

is pi, log is the natural logarithm, and the summation sums over all contributing channels and ##EQU4## where each said synchronous demodulators may contribute both resistive (gain LC_j and reactive components (gain Rc_i), where for the ith channel, the net gain of the contribution from the averaged ith synchronous demodulator is ##EQU5##

13. A method of effecting discriminating detection of a conducting metal target which comprises the steps of:

interrogating the target with at least two discrete frequencies,

detecting any resultant generated magnetic field in respect of each of said discrete frequencies,

distinguishing reactive and resistive components of such detected received signals in respect of each of the said discrete frequencies and then

combining the results in a predetermined manner whereby the said output is substantially independent of selected background materials in the target environment,

further characterized in that at least three discrete frequencies used to interrogate the target and each of the at least three frequencies are detected and combined in predetermined manner whereby the said output is substantially independent of selected background materials in the target environment.

14. A method of effecting discriminating detection of a conducting metal target as in claim 13 further characterized in that the interrogating frequencies are within the range of 500 Hz to 100 kHz.

15. A method of effecting discriminating detection of a conducting metal target which comprises the steps of:

interrogating the target with at least two discrete frequencies,

detecting any resultant generated magnetic field in respect of each of said discrete frequencies,

distinguishing reactive and resistive components of such detected received signals in respect of each of the said discrete frequencies and then

combining the results in a predetermined manner whereby the said output is substantially independent of selected background materials in the target environment,

further characterized in that the frequencies are transmitted simultaneously.

16. A method of effecting discriminating detection of a conducting metal target as in claim 15 further characterized in that the interrogating frequencies are within the range of 500 Hz to 100 kHz.

17. A method of effecting discriminating detection of a conducting metal target which comprises the steps of:

interrogating the target with at least two discrete frequencies,

detecting any resultant generated magnetic field in respect of each of said discrete frequencies,

distinguishing reactive and resistive components of such detected received signals in respect of each of the said discreter frequencies and then

combining the results in a predetermined manner whereby the said output is substantially independent of selected background materials in the target environment,

further characterized in that the manner of combining the signals comprises, in the case of two discrete frequencies, subtracting the magnitude of the one signal from the magnitude of the other.

18. A method of effecting discriminating detection of a conducting metal target as in claim 17 further characterized in that the interrogating frequencies are within the range of 500 Hz to 100 kHz.