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Patent 1207024 Summary

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(12) Patent: (11) CA 1207024
(21) Application Number: 1207024
(54) English Title: METHOD AND APPARATUS FOR DETERMINING THE TRACE AND DEPTH OF UNDERGROUND METALLIC CONDUCTORS
(54) French Title: METHODE ET DISPOSITIF DE DEPISTAGE EN SENS ET EN PROFONDEUR DES CONDUCTEURS ENFOUIS
Status: Term Expired - Post Grant
Bibliographic Data
Abstracts

English Abstract


Abstract
The subject matter of the present invention is
a method and apparatus to determine the trace and
depth of metallic conductors (6) located beneath ground
level. The substance of the method is that ferrite
antennas (16,16') are matched to coupled parallel os-
cillating circuits the capacitive element of which
provides resonance frequencies equal to the (cp ? 1)-th,
preferably (cp - l)-th and cp - th, or (2c ? l)-th,
preferably (2c - l)-th and 2c-th upper harmonic of the
basic frequency of the alternating current, where c is
a properly selected integer higher than 1, the value
of p is 1, 2, 3, or 6 as required, while cp is an even
number, thereafter the trace of the conductor (6) is
localized by detecting the vertical component (5) of
the magnetic field at an upper/lower level and the
horizontal component (4) at a lower/upper level, on the
basis of the ratio of the two components the trace is
approximated to a certain extent, then in order to
measure the depth (4) of said conductor (6) the hori-
zontal component (4) of the magnetic field is detected
both at the upper and the lower level and the depth (4)
is determined by the known formula. The apparatus com-
prises an upper and a lower ferrite antenna (16, 16')
at least one of which can be rota-ted in the vertical
plane. By means of the method and apparatus provided
by the present invention the trace and depth of conduc-
tors conducting an alternating current can be determined
with a high selectivity and accuracy.


Claims

Note: Claims are shown in the official language in which they were submitted.


The embodiments of the invention in which an
exclusive property or privilege is claimed are defined
as follows:
1. A method for determining the trace and depth
of an underground metallic conductor in which there
exists an alternating current of a basic frequency which
generates a corresponding electromagnetic field having
horizontal and vertical components, said method comprising:
a) detecting the electromagnetic field by ferrite
antennas at an upper and a lower level, wherein the ferrite
antennas are matched to first and second coupled parallel
oscillating circuits, the capacitance of such circuits
being selected such that each of the circuits has a resonant
frequency equal to the Nth upper harmonic of the basic
frequency when a separate generator is not used to generate
the alternating current in the conductor, and each of the
circuits has a resonant frequency equal to the Mth upper
harmonic of the basic frequency when a separate generator
is used to generate the alternating current in the con-
ductor, wherein (i) N is equal to (CP?1) or (2C?1) (ii) M
is equal to CP or 2C, (iii) C is a properly selected
integer greater than 1, (iv) P is 1, 2, 3, or 6, and (v) the
product CP is an even number;
b) determining the trace of the conductor from
sensing the vertical component of the magnetic field at
either one of the upper and the lower levels, sensing the
horizontal component of the magnetic field at the other of
the levels, and forming the ratio of the two sensed com-
ponents; and
c) determining the depth of the conductor from
sensing the horizontal component of the magnetic field at
both the upper and lower levels.
2. The method of claim 1 wherein the resonant fre-
quency of each of the circuits is the Nth upper harmonic
of the basic frequency when a separate generator is not
16

used to generate the alternating current in the conductor,
where N is equal to (CP-1) or (2C-1).
3. The method of claim 1 wherein the trace of
the conductor is determined by moving in a direction
determined by the sign of the logarithm of the absolute
value of the quotient of the sensed vertical and horizontal
components of the magnetic field to a corridor including
the trace of the conductor, the width of the corridor being
determined by the magnitude of said absolute value.
4. The method of claim 1 wherein analog circuits
are used to determine the depth of the conductor.
5. The method of claim 1 wherein the resonant fre-
quency of each of the oscillating circuits is equal to the
47th upper harmonic of the basic frequency when a separate
generator is not used to generate the alternating current
in the conductor.
6. The method of claim 1 wherein the resonant
frequency of each of the oscillating circuits is equal to
the 48th upper harmonic of the basic frequency when a
separate generator is used to generate the alternating
current in the conductor.
7. Apparatus for determining the trace and depth
of an underground metallic conductor in which there exists
an alternating current of a basic frequency which generates
a corresponding electromagnetic field having vertical and
horizontal components, comprising:
a) a measuring receiver comprising a transducer,
which transducer comprises an upper and a lower ferrite
antenna, at least one of which is rotatable in a vertical
plane, for converting the magnetic field strength into a
voltage signal;
b) a trace localizing unit connected to the
transducer for sensing the magnetic field and for pointing
out a corridor of a predetermined half-width depending on
the depth of the conductor; and
17

c) a depth measuring unit comprising analog cir-
cuits connected to the transducer;
wherein the transducer comprises:
first and second band-pass filters, each comprising
active and passive filter elements, wherein the upper
ferrite antenna is an inductive element of the first filter
and the lower ferrite antenna is an inductive element of
the second filter and the capacitive elements of the
filters comprise capacitors selected such that the resonant
frequency of both filters is the Nth harmonic of the basic
frequency when a separate generator is not used to generate
the alternating current in the conductor, and the resonant
frequency of both filters is the Mth harmonic of the basic
frequency when a separate generator is used to generate the
alternating current in the conductor, wherein (i) N is
equal to (CP?1) or (2C?1), (ii) M is CP or 2C, (iii) C is
a properly selected integer greater than 1, (iv) P is 1,
2, 3, or 6, and (v) the product CP is an even number.
8. The apparatus of claim 7 further comprising a
first operational amplifier being applied as a rectifier
connected to an active filter stage which is the output
stage of the first band-pass filter, and a second operational
amplifier being applied as a rectifier connected to an
active filter stage which is the output stage of the
second band-pass filter.
9. The apparatus of claim 8 further comprising
a unit for determining the relative position of the
measuring receiver with respect to the conductor, wherein
the unit comprises a multiplier, one input of which is
connected to the first operational amplifier and the other
input of which is connected to the output of a phase
shifter, the input of the phase shifter being connected to
the second operational amplifier.
10. The apparatus of claim 9 wherein the trace
localizing unit comprises:
18

a logarithmic amplifier connected to the outputs of
both the first and second operational amplifiers being
applied as active rectifiers; and
an indicator, connected to the output of the
logarithmic amplifier.
11. The apparatus of claim 10 further comprising
a reference voltage source, wherein an input of the
logarithmic amplifier is switchable between the reference
voltage source and the output of the second operational
amplifier.
12. The apparatus of claim 11 wherein the depth
measuring unit further comprises an exponential amplifier
the input of which is switchably connected to the output
of the logarithmic amplifier, the output of the exponential
amplifier being connected to an analog divider which is in
turn serially connected to the indicator.
13. The apparatus of claim 12 wherein the logar-
ithmic amplifier comprises a transistor and a third
operational amplifier, wherein the output of the third
operational amplifier is connected to the base of the
transistor and the non-inverting input of the third
operational amplifier is connected to the collector of the
transistor.
14. The apparatus of claim 7 further comprising a
current generator for generating an alternating current
in the underground conductor.
15. The apparatus of claim 14 wherein the current
generator comprises:
a) a voltage-controlled oscillator, the output of
which is connected to
b) a transformer current-regenerative circuit,
which is in turn connected to
c) a rectifier, which in turn is connected to
d) controlled switches, the outputs of which are
connected to
19

e) one pole of a load; and wherein another pole
of the load is connected to
f) an operational amplifier applied as an active
rectifier which in turn is connected to the inverting
input of an integrator, which integrator has a reference
voltage connected to its non-inverting input, the output
of the integrator being connected to the input of the
voltage-controlled oscillator.

Description

Note: Descriptions are shown in the official language in which they were submitted.


- l -
A method and apparatus for determining the trace and
depth of underground metallic conductors
Technical field
The subject ma-tter of the pr~sen-t invention is
a method and apparatus for determining the trace and
depth of metallic conduc-tors located beneath ground
level. According -to the method provided by the in-
vention the electromagnetic field generated by the al-
ternating current existing in the metallic conductor is
detected by ferrite antennas at an upper and a lower
level, the trace of the conductor is localized and its
depth is determined by means of the charac-teristics of
the magnetic field detected, while the apparatus pro-
vided by the invention comprises a measuring receiver
consisting of a transducer provided with an upper and
a lower ferrite antenna to convert the strength of the
magnetic field inducted by the alternating current flow-
ing in the metallic conductor into a voltage signaly atrace-localizer unit and a depth finder unit to determine
the depth of said conductor. The method and apparatus
corresponding to the present invention provides for locating
the underground metallic conductors faster and with a
higher accuracy compared to the methods known heretofore
and free of human errors.
Background Art
Surveying and registering a densely located sys-
tem of public u-tilities raises a lot of problems, as by
applying the known me-thods -the determination of the trace.
~.~ ., -

~LZ~7~
and the depth can be accomplished wi-th a relatively
low accuracy only, especially in a very noisy' en-
vironment, namely in the vicinity of power current
lines and equipmen-ts, e.g. tramway and -trolley lines.
The handling of the known equipment is sometimes dif-
ficul-t, portable construc-tions are not available, and
the result of the me~surements may be deteriorated
by significant human errors.
A general method of locating metallic conduc-
-tors is to indicate the variations of the vertical and/
or horizontal components of the magnetic field gener-
ated by the alternating current existing in said con-
ductor by means of a suitable probe, usually a trans-
ducer comprising ferrite antennas. The alternating
current is produced by a generator applied to input an
alternating current of the required amplitude and fre-
quency to the conduc-tor to be located via its available
points. The input can also be accomplished by means
of inductivity. A common method is also to connect one
pole of the generator to the conductor only, the other
pole being grounded.
The locating accomplished by the method des-
cribed is based on the fact that if the conductor to be
located can be considered as an infinite straight lead,
then a concentric magnetic field can be supposed to be
existing around it. In the case of such a field pat-
tern the vertical and horizontal components of the mag-
ne-tic field strength are equal if the lead can be seen
at an angle of 45 from the point of the measurements.
Thereby the direction oE the arrangement of the conduc-
."

o~
tor can be determined, while the position o~ the trace
of the !conductor can be theoretically established based
on the maximum value of -the horizontal component and
the zero level of the vertical component. Both measure-
ments are, however, uncertain; the measurement of the
maximum value may be burdened by serious human errors,
and measuring the zero value is illusory, especially in
noisy environment. An equipment has been developed by
the US company Metrotech utilizing a microcomputer to
realize the locating method described wherein the measur-
ing process is based on a fix program.
Equipments developed to accomplish the method
described above are known from the German Specifications
DE-PS 10 64 655, 25 28 511 or 20 37 865 on circuit level.
The solution of particular partial problems are given in
these Patent Specifications to increase the accuracy or
stability of the detection or to accomplish an energy
saving construction of the generator (powering the gener-
ator from the conductor to be located is recommended by
one of these specifi ations!.
An apparatus is described in the German DE Patent
Specification No. 955 348 comprising two coils arranged
horizontally one above the other and in a fixed position
with respect to each other to find the depth, while a
separate antenna is provided to localize the layout which
is also rigidly fixed in relation to the coils. ~his
apparatus requires a careful calibration when used.
In noisy environments applying the genera-tor
raises particular problems as 100 to 500 W high-pow7er units

~L~O~
-- 4 --
are used in attempting to generate a high level
alternating current in the conductor sufficient
to suppress the effect of the own current of the
conductor considered to be dis-turbing. The power
requirement of this accomplishment is very high,
developing a mobile equipmen-t is difficult and
-the generator must be carefully watched for the
measurements.
A source of additional difficulties is the
measuring frequency, the selection of which can
be commonly considered as incidental~ it is an
odd-order multiple of the mains frequency or
approximates this value (e.g. 1450 Hz in the
case of the instrument of the sEsA company, or
1030 Hz in the RFT equipment), thereby a suffici-
ently selective measuremen-t cannot be always effec-
ted, the relative value of the noise may be signi-
ficant.
Disclosure of the Invention
The object of -the present invention i5 to
eliminate the disadvantages described above.
It has been realised, that localizing the
layout and measuring the depth can be effectively
accomplished by applying ferrite an-tennas placed
at -two different levels and rotatable with respect
to each other for the measurements. Another impor-
tant realization is tha-t a generator is not always
~0 necessary to apply just in a noisy environment if
a sufficiently strong alternating current is
flowing in the conductor, therefore, it is advis-
able to make a dis-tinction between the operation
with and without a
. ~ ,

:LZ1370~
-- 5 --
generator and to select the measuring frequency
accordingly. The main point of this latter realiz-
a-tion is that such multiples of the basic frequency
are selected as measuring frequency when a generator
is applied by which a high effective signal level can
be attained with respect -to -the in-terfering noise
(this is a properly selected even upper harmonic), while
in an operation mode without genera-tor there is suffici-
ent signal level (-this is one of the odd upper harmonics
as a consequence of the Fourier-analysis of the alter-
nating current).
Based on the realizations described above the
object of the present invention is to provide a method
and apparatus applicable to determine the layout and
the depth of underground metallic conductors with a
high accuracy and practically free of human errors.
A method has been developed for the realization
of the object described above, whereby in order to
de-termine the layout and depth of metallic conductors
the magnetic field generated by the alternating cuxrent
flowing said metallic conductor is detected by ferrite
antennas at an upper and a lower level, the trace of
the conductor is localized and its depth is determined
on the basis of the measured characteristics of the
magnetic field, and according -to the present invention
the ferrite antennas are matched to two coupled paral-
lel oscillating circui-ts the capaci-ty of which provides
resonance frequencies equa~ to the (cp + l)--th, pre~
ferably (cp - l)-th and cp-th, or ~2c -~ l)-th, prefer-
ably (2c l)-th and 2c-th upper harmonics of the
basic frequency of the al-terna-ting current, where c is
a properly selected integer higher than 1, the value

Z4
of p is 1, 2, 3 or 6 as required, while cp is an even
number, thereafter the trace of the conductor is local-
ized by sensing the ver-tical component of the magnetic
field at the upper/lower level and the horizontal com-
ponent at the lower/upper level, the trace is approxi-
mated to a cer-tain ex-tent on the basis of the ra-tio of
the two components, -then in order -to de-termine the
depth of the conduc-tor, -the horizontal components of
the magnetic field is detected at bo-th the upper and
the lower level and -the depth is calculated by means
of the known formula. With the proper selection of
the resonance frequencies, preferably selecting the
48-th and 47-th upper harmonics a high selectivity and
thereby the sufficient accuracy of the measurements can
be attained, while the human errors can be prevented by
means of forming ratios, by processing -the ratio as
required, preferably by forming the logarithms of the
absolute values.
The depth is preferably established by means of
analog circuits as thereby a relatively simple construc-
tion can be attained.
The object of the present invention is also
served by the apparatus provided by the invention, which
is applicable to implement the method described above,
having a measuring receiver comprising a transducer pro-
vided with an upper and a lower ferrite antenna to con-
vert the magne-tic field strength generated by the alter-
nating current flowing in a metallic conductor into a
vol-tage signal, a trace-localizing unit and a depth
measuring unit, wherein according to the present inven-
tion said trace-localizing uni-t connected to the trans-
ducer sensing the magnetic field generated by -the alter
nating current induced by a generator
~ .

24
-7
when required is realized as a unit pointing out a
corridor of predetermined half width depending on the
depth of the conductor, the depth-measuring unit also
connected to the transducer is built of analog circuit
elements, the transducer is provided with two band-pass
filters consisting of passive and active filter elements,
one of which comprises the upper ferrite antenna as its
inductive element and the other one comprises the lower
ferrite antenna, wherein at least one of the upper and
lower ferrite antennas can be rotated in the vertical
plane, furthermore their capacitive element contain
capacitors whereby the resonant frequency of each of the
band-pass filters is the Mth upper harmonic of the basic
frequency when a separate generator is used to generate
the alternating current in the conductor, and the
resonant frequency of each of the band-pass filters is
Nth upper harmonic of the basic frequency when no separate
generator is used, where N is equal to (cp+l), preferably
(cp-l), or (2c+1), preferably (2c-1), M is equal to cp or
2c, where c is a properly selected integer and p is 1, 2,
3 or 6 as required, cp being an even number. Preferably
a trace-localizing unit comprising a logarithmic amplifier
is applicable to localize the layout, and an exponential
amplifier is connected to the logarithmic amplifier in
order to determine the depth of the conductor.
If an alternating current has to be generated in
the conductor to be located, then a current generator is
preferably applied for this purpose, as matching of the
generator is not required in this case, the loading im-
pedance is low and therefore a portable instrument canbe constructed. Tile current generator is preferably built
~'

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-- 8 --
of a well known transEormer regenerative circuitO
sr.ief description of the Drawings
The method and apparatus corresponding to the
presen-t invention is examplified by the embodiments
and implementations described in detail referring to
the drawings, where
FIG. 1 is an illustration of the method pro-
vided by the invention;
FIG. 2 is a schematic block diagram of the
appara-tus provided by the invention;
FIG. 3 is the block diagram of the measuring
receive-r provided by the invention;
FIG. 4 is the circuit diagram of the band-pass
filter in the measuring receiver;
FIG. 5 is the circuit diagram the active recti-
fier circui-t and logarithmic amplifier
built of operational amplifiers; and
FIG. 6 is a preferred circuit accomplishment
of the current generator developed for
the generator made of operation of the
apparatus provided by the present invention.
Mode for Carrying out the Invention
When implementing the method provided by -the
present invention an easily available point of the con-
ductor to be located is f.irst connected to a generator
inducing alternating current, if required. Either gal-
vanic or inductive coupling may be applied. When a
current is flowing in the

~207024
9 .
conductor then the strength of the magnetic field induced
may be sufficient to effect the measurements. ~y applying
a reference voltage it can be determined whether the
magnetic field strength is sufficient or not, if required.
Two ferrite antennas are used for the measurements,
two band-pass filters are built of them, one of each. The
resonance frequency of the band-pass filters is selected
according as a generator is or is not applied for the
measurement. In the latter case an odd, preferably 47-th,
and in the former case an even, preferably 48-th upper
harmonic of the basic frequency is adjusted. These values
for the upper harmonics are obtained from theoretical
considerations relating to the determination of the layout
of said conductors.
For the proper measuring the upper ferrite antenna
16 is first set in vertical position and below it the
lower ferrite antenna 16' is set in horizontal position.
Naturally a reverse arrangement can also be applied. The
measurement is theoretically based on the fact the vari-
ations of the horizontal (x-axis) component 4 and the
vertical (y-axis) component 5 of the magnetic field gen-
erated around a conductor 6 positioned in a depth in
the relation to the lower, in this case horizontal ferrite
antenna 16' are different ~see Fig. 1). The horizontal
(x-axis) components 4 form a bell curve, while the verti-
cal ty-axis) components 5 form an S-curve, and consider-
ing the difference of height d between the two positions
where the two different components are measured, the two
curves can be illustrated as seen in Fig. 1. The two
curves are intersecting in point S, the peak of the com-
~.,

~7~
-- 10 --
ponents 5 is at x = h ~ d, while the peak of the com-
ponents 4 is at x = 0, that is at the trace of the con~
ductor to be located. In relation to the point S in
one direction, here in the direction of increasing x
values the componen-t 5 is higher, and in the other
direction the component 4 is the higher one. As for
the absolute value of -the component 5, in -the other branch
of the component 4 point S' is obtained as a ~imilar
limit.
When locating the conductor 6, approximating
the trace 6' of the conductor a measurement is made at
point 7. Here the component 5 is higher than component
4 (if the measurement is made approximating from point
S', then the absolute value of the component 5 is to be
considered). The quotient of the two positive values,
preferably the logarithm formed from the quotient is
followed. Out of the range between S' and S -the value
of this logarithm is either positive or negative depen-
ding on the formation of the quotient. At points S' and
S the value of the quotient equals 1, that of the logarithm
equals 0, and following this the sign of the logarithm
will reverse in the range from S' to S. Thereby the
limits are assigned within which the locating is to be
made with an increased accuracy. The next measurement
is made at point 8 and is continued so that the results
of the measurements should be of higher and higher value
with the required sign. In the vicinity of the trace 6'
of the conduc-tor a corridor of D half-~idth is pointed
out depending on the depth h, the limits of which are
indicated by the value of the quotient or the lo~arithm
exceedi~g a given value. Theoretically the half-width
.~...
,~

~L2(370Z4
D may be selected as small as required, but because
of the disturbing fields always existing and resulting
in a high distortion of the vertlcal (y-axis) component
5 here, measuring of the component 5 and thereby form-
ing the quotient and its logarithm provides perfectlyreliable results within the specified range, as it
has been already mentioned in connection with the known
methods, too. With applying the method provided by the
present invention this range has been experienced to be
smaller than 2 to 3 percent of the depth, which means
that in the case of a depth of 1 m the trace 6' can be
localized with an accuracy of 2 to 3 cm with respect
to the cen-terline of the conductor and this is ~uite
sufficient.
At the limi-t of the corridor of half-width D the
value of the quotient and the logarithm, respectively,
attains a given limit value specified for example by
the overflow of a visual display and then the measuring
is continued at point 9 by setting also the upper ferrite
antenna 16 in horizontal position. Then, on the basis
of the two horizontal (x-axis) components and the well
known relations of the physics the depth of the conduc-
tor is determined and indicated preferably by analog
computing methods based on the electronic circuits app-
lied for locating the trace 6'.
Several me-thods can be applied for indicating:
visual~ e.g. with arrows and/or digits, possibly display-
ing additional information, too, or acoustic, e.g. vary-
ing pitch of the -tone and/or tone modulation, or both
visual and acoustic, etc.
An appara-tus suitable to implement the method

~20~V~L
12 -
provided by the presént invention is comprised of a
measuring receiver and a generator applie~ when required.
The schematic block diagram of the measuring receiver is
shown in Fig. 2 and more detailed in Fig. 3. In the
measuring receiver the transducer 1 comprises band-pass
filters 23 and 23' built of active and passive filter
elements. Their output is c~nnected to the tracelocal-
izing unit A and the depth measuring unit B being con-
nected to a display unit 14 (Fig.2~. The ba~d-pass
filters 23 and 23' provide voltage signals corresponding
to the field strength detected and this voltage signal
is the basis of the measurements.
The upper 16 and lower 16' ferrite antennas for-
- ming the inductive element sensing the magnetic field
strength in the band-pass filters 23 and 23' accomplished
with coupled parallel oscillating circuits are connected
to the unit C determining the position of the measuring
receiver in relation to the conductor 6 to be located
via the filter stages 22 and 22' and operational ampli-
fiers 12 and 12' formed as rectifiers (see Fig.3) being
thereafter connected to the logic unit 27. The output
of the logic unit 27 is connected to the indica-tor 14.
The indicator 14 may be for example a unit providing acous~
tic and/or visual signals. One output of the operational
amplifiers 12 and 12' each is connected to a logarithmic
amplifier 13 being a fundamental part of the trace-local-
ising unit A. An alternately switchable modified expon-
ential amplier 15 and serially an analog divider 24 is
connec~ed to the logarithmic amplifier 13, the output of
said divider being also connected to the indicator 14.
The function of the modified exponential amplifier 15 is

lZ~7~
- 13 -
to restore the value logarithmized by the logarithmic
amplifier 13 and to reduce the value obtained by 1.
Breaking the interconnection between the opera-
tional amplifier 12 and the logarithmic amplifier 13 a
reference voltage source 37 can be interposed applying
a voltage of Ur to the logarithmic amplifiex 13. The
reference voltage source 37 provides a basiS for evalu-
ating the measured magnetic field strength when neces-
sary and makes possible to decide about the possible appli-
cation of a generator (i-t is required for example when the
conductors are densely laid side by side, the current flow-
ing in the conductor to be located is small, etc.1
The unit C comprises in series connection a phase
shifter 25 and a multiplier unit 26, whëre the phase shif-
ter 26 is connected to -the operational amplifier 12 and
one output of the multiplier unit 26 is connected to the
operational amplifier 12'.
The band-pass filter 23 (see Fig. 4) comprises
following the ferrite antenna 16 a tuning stage 17 inclu-
ding parallel capacitors switchable as required, a coup-
ling stage 18 provided with an inductance 19 and a capa-
citor unit 20. The active filter stage 22 comprises
active filter elements 21. The switchable capacitors of
the coupling stage 18 provide fox adjusting the tuning
frequencies rela-ted to the operating modes with or with
out generator. The band-pass filter 23l has a similar
construction.
The circuit realization of the operational ampli-
fier 12 applied as a rectifier (Fig. 5) is well known in
ttself and requires no comments. The band~pass filter
23' is connec-ted to the operational amplifier while the
band-pass

..i :~\
~2~7~
-19-
filter 23 is connected to the operational amplifier 12
being applied as a rectifier and having a similar construc-
tion, and ~oth band-pass filters are so connected to the
logarithmic amplifier 13. The logarithmic amplifier 13 is
built of operational amplifiers 2 and 3, where the output
of -the operational amplifier 3 is fed back to its in-
verting input and the output of the operational amplifier
is fed back to its noninverting output by a transistor
each, the emitters of the two transistors being connected.
Thereby the processing of the voltage signal is facilitated
from the measuring point of view, and measured signal re-
tains its sign relating to its natural value given in dBs.
The units described herein can also be accomplished
in different ways by utilizing known circuits and elements.
When developing the apparatus and method provided
by the present invention one of the fundamental realiza-
tions was the fact well known from the theory of the rec-
tifying circuits that the n = c.p + 1 order upper harmonics
exist in the current I of a p cycle rectifying circuit.
20 Here the value of c is 1, 2, 3,....... , while in the case
of the practicable circuits p is 1, 2, 3, and 6. Con-
sidering these for example at the frequencies of the 24-th
and 48-th upper harmonics no upper harmonic exists at any
value of p, therefore these are preferable to be selected when 25 a generator is applied. Because of the application of
ferrite antennas and band-pass filters the 48-th upper
harmonic is more favourable. However, if a generator is not
applied the 47-th upper harmonic is always present and
therefore the band-pass filters ~3 and 23' can be easily
retuned to the 47-th upper harmonics with small auxiliary
capacitors.
~.

~z(37~6~24
The current obtained ls only one 47-th part of that
obtained at the basic frequency, but it is measure-
able quite well.
For the apparatus provided by the invention
5 . the generator is preferred to be realized AS a cur-
rent generator exe.mplified by the circuit diagram
shown in Fig. 6. In this case a transformer current
regenerative circuit 29 is connected to the output of
a voltage-controlled oscillator 28 to drive the rec-
tifier unit 30 and controlled switches 31. On pole
of the loading 34 is connected to one pole of said
switches, while the other pole is connected to the in-
put of the voltage-controlled oscillator 28 via an
operational amplifier 38 as a rectifier and an inte-
grator 36. The controlled switch 31 is connected to
a frequency divider 33 and a quartz-controlled oscil-
lator 32. The inverting input of the integrator 36
is connected to a source providing a reference volt-
age UR.
The method and apparatus corresponding to the
present invention provides for determining the trace
and depth of conductors conducting an alternating cur-
rent with a high selectivity and accuracy.
- ~f
~; ~
I D-

Representative Drawing

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Administrative Status

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Event History

Description Date
Inactive: Expired (old Act Patent) latest possible expiry date 2003-07-29
Grant by Issuance 1986-07-02

Abandonment History

There is no abandonment history.

Owners on Record

Note: Records showing the ownership history in alphabetical order.

Current Owners on Record
None
Past Owners on Record
LASZLO SZEDLMAJER
Past Owners that do not appear in the "Owners on Record" listing will appear in other documentation within the application.
Documents

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Document
Description 
Date
(yyyy-mm-dd) 
Number of pages   Size of Image (KB) 
Cover Page 1993-06-27 1 12
Claims 1993-06-27 5 162
Abstract 1993-06-27 1 32
Drawings 1993-06-27 5 75
Descriptions 1993-06-27 15 511