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Патент USA US3061785

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Oct. 30, 1962
Filed May 4. 1959
FIG.4 4-2
'United States ipatent
Patented Oct. 30, 1962
FIG. 5 is a circuit diagram of the electrical components
of the apparatus of the present invention.
As has been stated previously, the apparatus of the
invention is particularly useful in detecting the presence
Julian H. Reznowsiti, Chehalis, Wash, assignor to In
dustrial Electronics Incorporated, Chehaiis, Wash, a
corporation of Washington
Filed May 4, 1959, Ser. No. 810,908
1 Claim. (Cl. 324—41)
of metal objects in a mass of wood chips or scrap veneer
and the like as such material is carried along a conveyor.
Accordingly, the apparatus of the invention is shown dia
grammatically in FIG. 1 in connection with a conveyor
The present invention relates to a metal detector and
belt 10 carrying wood chips or wood scraps 12 thereon.
more particularly to apparatus for detecting the presence 10 The apparatus of the invention includes a detector unit
of metal in wood chips or other non-metallic materials
14 which is preferably mounted immediately beneath the
carried on a conveyor.
conveyor and which unit is connected to a suitable ampli
The scrap materials in a lumber operation, that is slabs,
fying and detecting circuit which may be contained in a
chips and the like, are frequently fed to a fuel “hog”
housing 16 positioned at some convenient place remote
which reduces the scrap material to small size so that it
from the detector unit 14 and which circuit may operate
can be fed to wood burners and used to generate power
a visual or audio signal device 18 to indicate the presence
for the operation. Frequently, also, Wood is fed to chip
of metal upon the conveyor and which may also be con
ping apparatus for making chips for chipboard and for
nected to control the motor 20 driving the conveyor 10
other purposes. Obviously, if metal objects are in such
so that the conveyor may be stopped when the presence
wood the cutting knives doing the chipping may be dam 20
aged and injury might be caused to personnel working in
of metal is detected.
the vicinity.
Referring now more particularly to FIGS. 2 to 5, in
clusive, the detector unit comprises a ?at detector coil
It is an object of the present invention to provide a
new and improved detector for detecting the presence of
with the conveyor 10 and a flat energizing coil 26 ar—
24 which is mounted in substantially coplanar relation
metal objects in a mass of Wood or other non-metallic 25 ranged with its plane perpendicular to the plane of the
material on a moving conveyor so that the metal can be
detector coil and with the energizing coil preferably ex
Still another object of the invention is to provide a
metal detecting unit that is relatively insensitive to varia
tions in the moisture content of Wood chips or other ma
terial being assayed.
tending transversely of the conveyor 10. The axes of the
coils 24 and 26 are perpendicular to one another and the
coils are preferably arranged so as to provide minimum
30 coupling therebetween. The coils 24, 26 preferably are
mounted within a suitable housing 28 so as to protect the
A further object of the invention is to provide new
coils against weather and from injury. Mounted adja
and improved means for securing a minimum signal in
cent to one of the coils 24 or 26 is a balancing coil 30.
the detecting unit in the absence of such metal objects.
In the illustrated embodiment, the balancing coil 30 is
Still other objects and advantages of the invention will
mounted slightly out of parallel relation with the ener
become more apparent hereinafter.
gizing coil 26 and with its axis eccentric to such coil.
in accordance with the illustrated embodiment, the
It is apparent that the coil 30 may occupy any other posi
present invention comprises a detector unit including an
tion in the alternating ?eld of the coil 26 which will cause
energizing coil supplied with alternating current so as
adjustable distortion of such ?eld in a manner varying
to provide an alternating magnetic ?eld and a detector 40 the coupling between the coils 24 and 26 when the coil
coil arranged so as to provide substantially minimum
is adjustably loaded. Suitable leads indicated at 32 are
coupling between such coils. The detector coil is con
provided from each of the coils to a connector 34 in a
nected to a suitable detecting and amplifying circuit and
Wall of the housing 28 into which a connecting cable 36
to signaling devices so as to indicate the presence of tramp
‘from the ampli?er 16 may be plugged.
metal in the material being conveyed by reason of volt 45
Positioned between the coils and the conveyor 10 is a
ages induced in the detector coil due to distortion of such
grid structure which provides a capacitive shield between
?eld by the metal to thereby increase the coupling be
the detecting unit and‘ the material being conveyed so as
tween such coils. To facilitate balancing of the detector
to minimize the effect of varying moisture content within
and energizing coil circuits to produce apminimum voltage
the wood chips 12 upon the detector unit. In the illus—
input to the ampli?er in the absence of such metal, I
trated embodiment, such a grid comprises a continuous
have provided a small balancing coil which is closely
wire 40 roved back and forth on the under surface of
coupled to one of the other coils and connected in series
the top member 42 of the housing 28 and which grid is
with a variable resistor. Such coil forms a ?eld distort
suitably grounded as indicated at 44 in FIG. 5.
ing element which in conjunction with a suitable circuit
Referring now more particularly to FIG. 5, the energiz
enables such balancing by merely adjusting the variable 55 ing coil 26 is energized from a suitable oscillator circuit
indicated generally at 46 capable of operating at a fre
A capacitive shield in the form of a grounded grid is
quency in the range between 2,000 to 10,000 cycles per
provided between the coils and the conveyor on which the
second. Such oscillating circuits are well known to those
conveyed material is carried, which shield minimizes the
skilled in the art and the coil 26 may, for example, pro
effect of varying moisture content within such material
vide the inductance of the tank circuit of a Colpitts type
upon the sensitivity of the detector unit.
oscillator, which tank circuit also includes the capacitors
For a more detailed description of the invention, refer
31 and 33.
ence is made to the accompanying drawings wherein:
The coils 24 and 26 are connected through a bridge
FIG. 1 is a diagrammatic view in side elevation of a
circuit and transformer 58 to an amplifying circuit indi
conveyor for carrying scrap wood showing a metal de 65 cated generally at 60. The coil 30 has a variable resistor
tector arranged therewith;
62 connected across its terminals. The bridge circuit in
FIG. 2 is a sectional view through a unit taken sub
cludes resistors R1, R2, R3 and R4 and capacitors C1 and
stantially along line 2-—2 of FIG. 1;
C2. It will be apparent that the resistors R1, R2 and the
FIG. 3 is a sectional view taken substantially along
capacitors C1, C2 can be considered to be a ?rst bridge.
line 3-3 of FIG. 2;
70 A portion of the voltage across the energizing coil 26,
FIG. 4 is a sectional view taken substantially along
namely that across the capacitor 33 is reduced by the
line 4-4 of FIG. 1; and
voltage divider made up of the resistors 63 and 64 and
resulting increased current ?ow through the primary wind
ing of the transformer 58 will act through the amplifying
supplied through the series resistor 65 across one pair of
opposed junctions of such ?rst bridge. If such ?rst bridge
is balanced by making the ‘ratio of R1 to R2 equal the
ratio of C2 to C1, no voltage from the coil 24 will appear
circuit 60 to cause the alarm indicated at 18 to be ac
tuated and to also actuate a relay 67 or other suitable
control device to stop the motor 20 and halt the conveyor
10 so that the detected metal may be removed.
across the other two opposed junctions of such ?rst bridge,
i.e., across the detector coil 24.
In the absence of the grid 40, moisture in wood being
conveyed produces spurious response in the system de
scribed above at alternating current frequencies much
Similarly, the resistors R3; R4 and capacitors C1, C2
can be considered to be a second bridge; A portion of
the voltage across the coil 26 is applied to one pair of
above 2000 cycles per second.
opposed terminals of such second bridge through the volt
Apparently this is a
capacitive etfect as the grid 40, constituting a capacitive
shield, eliminates the e?ect of varying moisture content in
the wood chips 12 at considerably higher frequencies so
that the detector does notv produce spurious signals at
made equal to the ratio of C2 to C1, so that none of the
15 such frequencies. It has been found that the frequency
voltage across coil 26 appears across the coil 24.
of the detector oscillator can be increased from 2,000
It will be further apparent that either or both bridges
about 6,000 cycles ,per second and which, of course,
described above can be unbalanced by changing the values
increases the detecting e?iciency of the unit.
of resistors therein so that a portion of the voltage across
age divider including resistors 63 and 64 and through the
series ‘capacitor 66. Again it will be apparent that the
second bridge will be balanced if the ratio of R3 to R4 is
While I have described, my invention in particular
relation to detection of wood chips moving on a conveyor,
it will be obvious that the detector unit can be utilized
the coil 26 does appear across the terminals of the
coil 24. The dual bridges supplied with voltage from the
coil 26 through the resistor 65 and capacitor 66 constitute
in other environments. Thus, having illustrated and de
scribed a preferred embodiment of the invention, it
also a phase shifting circuit so that such voltage may be
made to have substantially any amplitude and phase
within wide limits by selecting suitable values for the
resistors R1, R2, R3 and R4.
should be apparent to those skilled in the art that the
25 invention permits of modi?cation in arrangement and
detail. I claim as my invention all such modi?cations
The detector coil 24 will not, in general, have zero
as come within the true spirit and scope of the appended
coupling with the coil 26 so that a voltage is induced in
such detector coil. If the coil 30 has the terminals open
I claim:
it 'will have little effect on the coupling between the coils
Apparatus for detecting metal objects in a mass of
24 and 26 but loading such coil with the variable re 30
wood chips on a moving conveyor comprising a detector
sistor 62 will increase the coupling between the coils
unit mounted beneath said conveyor and including a
24 and 26 by distorting the alternating ?eld produced
detector coil disposed in coplanar relation with said con
by the energization of coil 26. In general, the lower the
resistance of resistor 62, the greater the coupling between
the coils 2'4 and 26 and the greater the voltage induced
veyor, an oscillator coil mounted with its axis perpen
35 dicular to said detector coil and arranged to provide
minimum coupling therebetween, a balancing coil
in coil 24. By selecting values of resistance for the re
mounted adjacent to one of the aforesaid coils, means
sistors R1, R2, R3 and R4 producing a voltage of the same
for providing an alternating current of a frequency in
amplitude and 180° out of phase with the voltage induced
the range between 2,000 and 10,000 cycles per second
in the coil 24 by mutual coupling with the coil 26, the 40 to
said oscillator coil, means for detecting the induced
current through the primary of the transformer 58 can be
voltage in said detector coil, a variable resistance con
brought to a minimum value when no metal is present in
nected in series to said balancing coil for producing a
balance between said detector and oscillator coils in the
the material being conveyed. Any metal in such material
will further distort the ?eld produced by the coil 26 to
absence of a metal object on said conveyor, a grid struc
induce ‘different voltage in the coil 24 so that the current 45 ture disposed between said coils and said conveyor, and
through the primary of the transformer 58 increases to
means connecting said grid to ground whereby said grid
operate the detector or control devices through the am
pli?er 60.
structure provides a capacitive shield for said detector
unit and minimizes the effect of varying moisture content
within said wood chips upon the sensitivity of said de
In actual practice the values of the resistors R1,- R2‘, R3
and R4 are selected at the point of manufacture of the 50 tector unit.
metal detector to produce a minimum current in the pri
mary winding of the transformer 58 when the variable
resistor has its movable contact in an intermediate posi- ,
References Cited in the ?le of this patent
tion. Any changes in surrounding conditions caused by
installation of the metal detector at some other location 55 2,237,254
which affect the coupling between the two coils 24 and
26 may then be compensated for by merely adjusting the
variable resistor to produce minimum current in the pri
mary winding of the transformer 58 as determined by
any suitableindicating instrument.
Details of the amplifying and detecting circuit 60 have
not been shown since any conventional circuit may be
utilized so that‘when a, piece of metal on the conveyor
10 comes within the ?eld of the detector coil 24, the
Broekhuysen __________ __ Apr. 1, 1941
Doll __' ________ __;__;____ Mar. 1,
Nelson _______________ __ Apr. 3,
Kurtz et al. __________ __ May 29,
Gossick ______________ __ May 27,
Zimmerman __________ __ Dec. 30,
Great Britain _________ __ May 11, 1955
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