Search coil assembly for electrically conductive object detection

A search coil assembly for an inductive search device of the transmitter/receiver type is described having advantages relative to manufacture, repeat accuracy and sensitivity in operation. This is made possible by the utilization of printed-circuit board technology in search coil construction in which electrically conductive windings in the form of a conductor band of circuit tracks are arranged side-by-side at the periphery of partial areas on a carrier layer common to the partial areas.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a device for inductively 
detecting electrically conductive objects, and, more particularly, to a 
search coil assembly for use in such devices. 
2. Description of Related Art 
Inductive search devices are used for detecting objects of electrically 
conductive material which are hidden from sight as they are, e.g., buried, 
in the soil or are surrounded by other materials having a lower electrical 
conductivity. It is possible with such devices either to pass the search 
device over a surface to be scanned as well as to pass goods to be tested 
past a stationary search device. In either case, an electrical signal 
voltage is generated as a result of the relative movement between the 
objects sought and the search device, such voltage being transformed into 
a signal capable of human detection, e.g., an acoustic or optical signal. 
Search coil assemblies of the aforementioned species are known in the art 
in various embodiments. For example, there is known an American mine 
search device, wherein the receiver coil of the search coil assembly has 
four partial coils connected pair-wise in opposition to each other and 
arranged in a common plane. Such partial coils are received in depressions 
of a plate-type housing, which are located at the four corners of a 
square. The disadvantage of this search coil assembly is that deformation 
of the housing caused by thermal or other influences will immediately 
affect the position of the individual receiver coils with respect to one 
another and their position with respect to the energizing or excitation 
coil. This will result in lack of stability and in a permanent drift of 
the output voltage of the receiver coils. Another disadvantage is the 
heavy weight of such a search coil assembly. Furthermore, it is 
disadvantageous that the partial coils must be rather accurately identical 
in their coil characteristics requiring high precision of winding. 
SUMMARY OF THE INVENTION 
In contrast to the known prior art, it is an object of the invention to 
provide a search coil assembly of several partial coils having acceptable 
match of the coil characteristics with respect to each other, a high 
repeat accuracy, and relatively low expense of manufacture. 
According to one aspect of the invention, layout of the coil assembly is 
accomplished under magnification providing very high geometrical accuracy. 
The layout once produced can then be reproduced with high reliability and 
in any desired quantity. The cost of producing the layout can be divided 
by the total number of search coil assemblies to be produced and is 
therefore of no great importance. In contrast to a housing exposed to all 
external influences, the carrier layer common to the partial coil areas 
guarantees that the position of the partial areas with respect to each 
other can be maintained with high accuracy. Considerable savings of weight 
are also achieved with the construction according to the invention. 
According to an advantageous embodiment of the invention, coil differential 
connection is not formed only by connection of partial coils in 
opposition, but is formed separately for each winding. This is effected by 
composing the individual coil windings from pairs of conductor loops 
including one partial coil area each, such conductor loops having opposed 
senses of winding. In this way, the induction of high potentials in the 
partial coils corresponding to the number of windings are prevented. 
Instead, now only the potential differences of the individual windings are 
summed up. Low potentials are desired, because the formation of parasitic 
capacitive stray currents to ground and, in particular, to a shield at 
ground potential is the greater, the higher the respective potentials are. 
From the basic relationship, 
EQU Q=C.multidot.U 
Q being the electrical charge, C the stray capacitance and U the existing 
potential, it follows 
EQU I=dQ/dt=C dU/dt. 
Of particular importance are those stray currents I, where higher 
frequencies or different frequencies are involved. 
According to another embodiment of the invention, the crossing at the 
interface between opposed-sense conductor loops takes place on either side 
of a carrier layer, the required electrical connections from one side of 
the carrier layer to the other being effected by through-plated holes. 
From yet another embodiment of the invention, either side of the carrier 
layer can be provided a conductor band with opposed-sense conductor loops. 
A further and advantageous embodiment of the invention describes how higher 
numbers of windings can be achieved in multi-layer coil constructions than 
are possible with only one carrier layer. 
Of high importance, in particular for multi-frequency devices and when 
applying higher frequencies, is a still further embodiment allowing for 
further reduction of induced potentials. When combining two conductor 
loops to a winding, there is effected a change from one side of the 
conductor band to the other side thereof replacing interior conductor 
loops with exterior ones. The areas surrounded by the conductor loops are, 
therefore, not accurately identical. Accordingly, there is added to the 
positively occurring change of sides another change of sides in the course 
of such winding. 
Yet another version utilizes cut-outs in the carrier layer providing 
additional savings in weight and improvement of mobility.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In FIGS. 1A and 1B there is shown a search coil assembly according to the 
invention in a first embodiment including an excitation coil 10 being 
wound in usual manner onto a coil frame 12. A shoulder 14 in the bore of 
the coil frame 12 bears a printed-circuit board 16 fixed by an adhesive 18 
or in another suitable manner to the coil frame 12. The printed-circuit 
board 16 comprises, in well-known manner, a carrier layer 20 and circuit 
tracks 22 disposed thereon, the latter being arranged closely side-by-side 
in the form of conductor bands 24. For the sake of simplicity, only 6 
circuit tracks are drawn. It is possible without any difficulty, however, 
to have conductor bands of similar width with 20 to 30 circuit tracks. The 
circuit tracks 22 form the windings 26 of two partial coils 28, 30 
combined to form a receiver coil 32. 
The partial coils 28, 30 each include with the conductor tracks 24 partial 
areas 34, 36 which are slightly larger than the areas created by cutouts 
of the carrier layer 20 along lines 38, 40. The partial coil 28 begins at 
a terminal 42 and ends at a central tap 44, while partial coil 30 begins 
at the central tap 44 and ends at a terminal 46. As the two partial coils 
28, 30 are connected in opposition to each other, and as the field of the 
operator coil induces equal voltages in them, there will be no voltages at 
the terminals 42, 46, as long as the symmetry of the magnetic field is not 
disturbed by eddy-current reactions, such as caused by the presence of 
search objects, for example. 
FIGS. 2A and 2B show the printed-circuit board 51 for a receiver coil 50 of 
a search coil assembly in a top plan view and in side elevational view, 
respectively. On both sides of the carrier layer 20 are provided circuit 
tracks 22. The circuit tracks 22 are, in turn, disposed side-by-side in 
the form of a conductor band 24, one-half thereof being located on the top 
side, the other half (shown in dashed lines) on the bottom side of the 
carrier layer 20. Through-plated holes 52', 52", 52"' connect in known 
manner the circuit tracks 22 of top with those on the bottom side, such 
circuit tracks ending at terminals 54 and 56. 
The basic difference from the receiver coil 32 according to FIG. 1 is that 
the windings 26 of FIG. 2 are each composed of two.-conductor loops having 
opposed senses of winding. For example, a conductor loop 60 begins at a 
through-plated hole 52' connected to the terminal 54 and ends at another 
through-plated hole 52". It is continued in another conductor loop 58 
ending in turn at a through-plated hole 52"'. The partial top loops 
collectively enclose a partial area 34 and the bottom loops similarly 
enclose a partial area 36. The resulting crossing of the conductor band 24 
is thus formed on the two sides of the carrier layer 20. Of importance is 
the fact that under certain conditions that the abutting opposed-sense 
partial loops 58 and 60 have identical geometrical shape. Differences in 
length of the individual partial loops 58 and 60 result from the fact that 
for each abutment of two conductor loops, a change from one side of the 
conductor band 24 to the other results. The conductor loop 58 is, e.g., on 
the interior side of the conductor band 24, whereas the conductor loop 60 
is on the exterior side thereof. This will be discussed in detail later. 
FIG. 3 shows in diagrammatical representation a receiver coil 70, wherein 
the through-plated holes 52 are not arranged near to the crossing of the 
conductor band 24, but rather at the center of the top or bottom section 
of the conductor band. In this manner, it is intended to show, on one 
hand, that the arrangement of the through-plated holes 52 can be located 
at any position. On the other hand, the arrangement of FIG. 3 has the 
advantage that the central sections 72 of the conductor band are 
immediately facing each other and, therefore, require little space. For 
the representation of FIG. 3, a wide cross-hatching has been selected for 
the top side of the carrier layer 20, and a narrow one for the bottom 
side. 
FIGS. 4A, 4B and 4C depict a receiver coil 80 in a realistic layout which 
differs, in principle, from the receiver coils according to FIGS. 2 and 3 
in that the through-plated holes 82 are disposed at a different position. 
One can see the printed-circuit board 80 in FIGS. 4A and 4B from below and 
above, respectively, and in FIG. 4C from the edge. The through-plated 
holes 82 are arranged on both sides of the conductor band 24 so that cross 
leads to the circuit tracks 22 on the rear side of the carrier layer 20 
can be simple short bridges 84. The signal voltage of the receiver coil 80 
is available at the two terminals 86. 
The construction of the windings 26 from conductor loops 88, 90 so as to be 
connected in opposition is as has been previously described. The 
arrangement of the receiver coil 80 depicted in FIG. 4 offers, 
additionally, the advantage that in a simple way several crossings can be 
disposed side-by-side on the center strip 92 of the carrier layer 81. 
FIG. 5 shows how this is applied for a similar receiver coil 100 with 
terminals 102, 103, such receiver coil being composed of windings 104 and 
106 on the top or on the bottom, respectively, sides of the carrier layer 
20. In FIGS. 5A and 5B, the upper and bottom, respectively, sides of a 
printed-circuit board 101 with receiver coil 100 are shown, and FIG. 5C 
shows a front edge view. The windings 104 on the top side are built up in 
equal manner to those of the receiver coil 80, and are connected via 
through-plated holes 82 and bridges 84. With the windings 104 beginning at 
the terminal 102, the end is obtained after running through all windings 
104, through-plated holes 82 and bridges 84 to a through-plated hole 108. 
The latter coincides with a through-plated hole 108' (FIG. 5B) on the 
bottom side and forms there the beginning of the windings 106. Via 
through-plated holes 110 and bridge 112, the windings 106 arrive at 
through-plated hole 114 and is finally connected to the terminal 103. In 
this way, the two partial areas 34, 36 are surrounded by conductor bands 
24 allowing for a doubled number of windings relative to the earlier 
described receiver coil 80. 
Another increase in the number of windings of the receiver coil can be 
achieved by application of multi-layer technology, as is shown in FIG. 6. 
In the present example, a printed-circuit board 121 includes four layers 
122 of circuit tracks held separated from each other by three carrier 
layers 120. The construction of two layers of circuit tracks can be 
effected substantially according to the embodiment of FIG. 5. The two 
remaining layers of circuit tracks can also be built up in identical 
manner. It is only necessary to provide the remaining layers at locations 
on the center strip 92 free of through-plated holes and bridges. In this 
way, the circuit tracks of, in the present case, four layers 122 can be 
connected in series, resulting in the number of windings being quadrupled 
relative to the previously described receiver coil 80 (FIG. 4). 
FIGS. 7 and 8 show how the capacitive stray currents can effectively be 
further lowered. For example, FIG. 7 shows in diagrammatical manner a 
receiver coil 130, which can be employed in search coil assemblies 
according to the invention and FIG. 8 is an enlarged representation of the 
crossing area of the conductor band 24. In both cases, the bridges 132 are 
disposed on the rear side of the carrier layer and drawn in dashed lines. 
As already explained above, the abutting conductor loops of opposed sense 
for the receiver coils described up to this point are not fully 
coincident. The potential induced in the loops will, therefore, not 
compensate each other completely. Instead, residual potential increases 
linearly from winding to winding up to the center of the conductor band 
24, and is reduced, then, linearly in the same manner to zero at the last 
winding. If conductor loops fitting with respect to each other are 
combined, the generation of residual potentials can be prevented a priori. 
This is possible, e.g., by an arrangement of the receiver coil according 
to FIGS. 7 and 8. If it proceeds from a terminal 134 in the direction of 
arrow 136, the following sequence of conductor loops is achieved: 
top outermost - bottom outermost 
top innermost - bottom innermost 
top outermost but one - bottom outermost but one 
top innermost but one - bottom innermost but one 
and so on. 
This is continued until the receiver coil 130 ends with the terminal 138 in 
the center of the conductor band 24. In this way, only equivalent 
conductor loops will abut each other, and the potentials compensate each 
other for each winding. A simple means allows the change of sides 
additionally required within the conductor band: When changing from one 
side of the carrier layer to the other, an undercut 140 is performed for 
each winding, i.e. part of the windings of the conductor band 24 are 
passed under at this location in order to arrive at the other side of the 
conductor band.