Device for detecting metallic objects in a flow of non-metallic material

A device for detecting metallic objects in a material flow, comprising as sensing member a scanning coil consisting of a primary coil and, for example, two oppositely connected secondary windings arranged symmetrically around the primary winding so that the voltage induced in the secondary winding becomes zero as long as no conducting objects appear in the scanning area. To eliminate irrelevant signals from conducting objects in the vicinity of the scanning coil but not in the material flow or from weakly conducting objects in the material flow, the primary winding is fed with at least two alternating currents having different frequencies. The corresponding secondary voltages are rectified in phase-controlled rectifiers, and in a calculating circuit the difference is derived between the output of one rectifier voltage and the output of another rectifier multiplied by a constant factor which includes the relation between the frequencies of the corresponding alternating voltages.

BACKGROUND OF THE INVENTION 
The present invention relates to a device for detecting metallic objects in 
a flow of non-metallic material which comprises a scanning coil having a 
primary coil and a secondary coil, a current source adapted to feed at 
least two alternating currents with different frequencies to the primary 
coil, and an output circuit for the secondary coil, said output circuit 
delivering a signal when a metal object enters into the sensing range of 
the scanning coil. 
Devices of the above-mentioned kind are used, among other things, for 
detecting metallic objects in a flow of materials such as coal, ore, logs, 
wood chips, wood pulp or the like non-metallic materials, the scanning 
coil being so designed and positioned that the flow of material moves past 
or, even better, through the scanning coil. 
The scanning coil is suitably constructed with a primary coil and a 
secondary coil which is divided into two similar coils located 
symmetrically on opposite sides of the primary coil in the direction of 
material flow through the scanning coil. The secondary coils are connected 
such that the output signal therefrom is balanced, so that it becomes 
approximately zero all the while no metallic object appears within the 
sensing range of the coil. In this way the detector device can be designed 
to operate with very high sensitivity. 
The problem is, however, that a consequence of high sensitivity is a 
greater risk of signals being generated by harmless or uninteresting 
objects, for example objects in the material flow having a low 
conductivity or larger objects of iron or metal in the vicinity of the 
scanning coil, but not in the material flow. As examples of objects with 
low conductivity may be mentioned wet logs of wood or other moist 
material. 
An object of the present invention is to eliminate such erroneous signals 
without significant loss of sensitivity for the detecting of foreign 
matter in the material flow. 
BRIEF STATEMENT OF THE INVENTION 
This desirable object is achieved, according to the invention by supplying 
the primary winding of the scanning coil with an alternating current 
composed of at least two alternating currents having different 
frequencies. The invention is characterised in that the output circuit of 
the secondary coil includes a phase-controlled rectifier for each one of 
the alternating currents, each of the phase-controlled rectifiers being 
controlled with a certain phase angle relative to the respective 
alternating current, and a calculating circuit adapted to derive the 
difference between the output voltage of one rectifier and the product of 
the output voltage of another rectifier and a factor containing the 
relationship between the frequencies of the respective alternating 
currents. 
By phase-controlled rectification, the corresponding secondary voltages are 
filtered away from each other, whereafter the signals corresponding to 
undesired metallic objects in the material flow can be obtained in the 
calculating circuit, whereas uninteresting objects do not generate such 
signals. 
By using currents with different frequencies and matching together the 
corresponding rectified secondary voltages in a proper manner, it will be 
possible according to the invention to distinguish between uninteresting 
objects and the foreign matter in the material flow which it is desired to 
detect. 
Similar principles to those used in this invention are previously known 
from other forms of metal detection, for example for detecting objects of 
value, guns and the like, in the earth, but in those cases the evaluation 
of the signals is different from that proposed by the present invention. 
Desirably, the rectification of the secondary voltages is controlled with a 
phase angle equal to zero or at least close to zero, whereby the imaginary 
parts of the signals are filtered off, which may otherwise give rise to 
disturbances.

DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 shows schematically in the chain line box 1 the scanning coil of a 
device according to the invention. The scanning coil comprises a primary 
coil 2 and two secondary coils 3 interconnected such that the induced 
voltages in each of the secondary coils are oppositely disposed to one 
another and the two secondary coils are connected to an amplifier 4, from 
where the output signal is passed out for further processing. The coils 2 
and 3 are suitably located around a tube, through which the material flow 
is conducted in the direction of the arrow X. The scanning coil is 
illustrated in FIG. 2 and will be described in greater detail later. 
The primary coil 2 is supplied with a composite a.c. current from a 
summator 5. At least two, and in the case illustrated in FIG. 1 three, AC 
sources 11, 12, 13 are connected to the summator 5. The a.c. currents (i1, 
i2 and i3) have different frequencies f1, f2 and f3. 
The output voltage from the amplifier 4 is connected to rectifiers 21, 22 
and 23 respectively corresponding to each of the supply alternating 
currents. These rectifiers are parallel-connected and are controlled from 
the respective alternating current source with a certain phase angle, 
which is chosen to be the same for all the rectifiers. 
The a.c. currents i1, i2 and i3 are suitably chosen with such an amplitude 
that the voltages e1, e2 and e3 induced thereby in the secondary windings 
3 will have the same root mean square (RMS) value, which means that the 
currents are chosen in relation to the frequencies so that the product of 
their RMS values and frequencies are the same for all, that is, 
i1.times.f1=i2.times.f2=i3.times.f3. 
Theoretically different control angles for the rectifiers might be 
considered. If control is performed with the control angle 0.degree., a 
measure is obtained of the real portion of the output voltage, and if 
control is performed with the control angle 90.degree., the imaginary 
portion of this voltage is obtained. By choosing the control angle 
0.degree., the real portion is obtained, as mentioned which corresponds to 
the resistive losses in conductive materials in the material flow. This is 
normally preferable, since the majority of disturbances, such as 
vibrations in the scanning coil, magnetic background material, etc., for 
the most part influence the imaginary portion of the signal. However, this 
does not exclude that in certain cases it may be convenient to deviate 
from the control angle 0.degree. and possibly in certain cases even use 
the imaginary portion by operating at the control angle 90.degree.. 
By controlling the different rectifiers 21, 22 and 23 with the same phase 
angle or control angle in relation to the corresponding primary current, a 
signal is obtained from each rectifier corresponding to this current 
component, whereas signals of other frequency are suppressed. This effect 
is amplified if the different frequencies are in an integer relationship 
to each other, more particularly, when the relationships between them are 
divisible by two. To avoid disturbing overtones, the primary currents 
should in addition, be as close to sinusoidal as possible. 
The output signals from the rectifiers 21, 22 and 23 are respectively 
smoothed in smoothing filters 31, 32 and 33, from where direct voltage 
signals F(e) are obtained corresponding to the different secondary 
voltages e1, e2, e3. These signals are processed in a calculating circuit 
comprising a summator 6 for the different signals. The signal 
corresponding to the voltage with the lowest frequency, in the illustrated 
case e1, is suitably directly connected to the summator 6. The other 
signals are multiplied by a factor k.times.G((f1/f2),(f1/f3)), where k is 
a constant factor, normally of the order of magnitude 1, and G is a 
function of the relationship between the frequencies, as will be described 
in more detail later. The number of different frequencies and associated 
rectifiers, etc., that are needed and how the corresponding signals are to 
be processed, depends on what disturbances are to be eliminated and this 
will be explained in greater detail hereafter. 
Before that, however, reference will now be made to FIG. 2 which shows how 
the scanning coil can be constructed and how the signal increases when a 
metallic particle 7 (FIG. 1) passes through it. FIG. 2 shows a tube 8 
which supports the primary coil 2 in the center and the secondary coils 3 
disposed symmetrically on either side of the primary coil 2. 
FIG. 2 also shows a system of coordinates in which the abscissa indicates 
the axis of the scanning coil and is marked with the distance from the 
origin in coil diameters. It should thus be imagined that the material 
flow passes along this axis and thus through the tube 8. By the 
symmetrical construction of the scanning coil and the reverse connection 
of its secondary coils 3, the output signal therefrom will be 
approximately zero for as long as no magnetic or electrically conducting 
objects appear in the sensing range of the coil. Now, if a conducting 
particle accompanies the material flow, for example in a direction from 
left to right, this will be influenced by the magnetic field of the 
scanning coil which, initially, will have its strongest influence on the 
lefthand secondary coil so that a negative difference voltage grows up on 
the secondary side. This signal increases to a maximum, the position of 
which depends on the coil dimensions. After that, the signal rapidly 
diminishes and becomes zero when the particle passes the primary coil, 
after which the signal equally rapidly grows up to a positive maximum 
value and then slowly decreases towards zero. Curve I in FIG. 2 shows this 
resultant signal on a linear scale, whereas curve II shows the signal on a 
logarithmic scale. 
As a first example of an interference signal there may be imagined the 
signal coming from weakly conducting materials in the material flow, 
particularly if this weakly conducting material has relatively great 
extension. This may, for example, occur if it is a question of sorting out 
logs of wood in which embedded foreign metal particles that are invisible 
may cause great inconvenience and therefore must be detected and removed, 
but where also a soaking wet log may cause a signal for conducting 
material in spite of the fact that this log is completely harmless in the 
subsequent materials treatment and should not be removed. 
However, it has proved that the signal for different conducting materials 
entering the sensing range of the scanning coil is greatly 
frequency-dependent, and, in addition, this dependence is also a function 
of the conductivity of the material, as indicated in FIG. 3. 
FIG. 3 shows the relationship between the frequency and the real portion of 
the signal from a conducting material, the abscissa indicating the 
frequency in kHz. Curve I represents a material with a low conductivity, 
in the present case a strong salt solution in a container, whereas curve 
II represents a metal particle of an extension of a few millimeters, for 
example a copper cylinder with a diameter of 5 mm. For the salt solution 
it is seen that the curve I increases linearly, whereas the curve II 
increases linearly up to, for example, a frequency of 2 kHz, whereupon it 
reaches saturation because of the current displacement. 
To make use of the above-mentioned conditions, only the first two currents 
i1, i2, and thereby the voltages e1 and e2, are used in the connection 
according to FIG. 1, and the multiplier circuit 42 operates on the basis 
of the factor -k.times.(f1/f2). In this way, the output signal from the 
summator 6 will be: 
EQU F(e1)-k.times.F(e2).times.(f1/f2) (1) 
where G=(f1/f2) 
If the frequencies f1 and f2 are chosen to be, for example, 2 and 8 kHz, 
for a weakly conducting material, such as wet wood material, the two terms 
in the above expression will be approximately equal if the factor k is 
chosen to be about 1, and the output signal will be zero. For a good 
conductor, the second term in the expression becomes small so that a 
substantial resultant output signal, i.e., f(e1), is obtained from the 
summator 6. 
In addition to the fact that output signals from, for example, acidic 
materials are eliminated in this way, signals from very thin, conducting 
objects, such as aluminium foil, are also eliminated, and this is an 
advantage since previously such objects have given rise to unnecessary 
signals. 
Another source of interference, which may cause undesired signals from the 
detecting device, are conducting materials in the vicinity of the scanning 
coil but not in the flow. It is easily understood how difficult it may be 
to avoid such disturbing background material in, for example, an 
industrial plant. It will be just as difficult to screen the scanning coil 
off from such disturbing material. 
From FIG. 2 it will be clear that the sensitivity of the scanning coil is 
greatly reduced with the distance from the coil, and that the conducting 
particles which actually pass through the coil will result in a strong 
signal. It is also seen that the signal level at a distance of twice the 
diameter (D) of the coil is almost negligible in relation to the peak 
value of a genuine signal (FIG. 2). Therefore, if it were possible to 
avoid conducting materials within a zone with a radius of one to two times 
the diameter of the coil, a sufficiently strong signal should be obtained 
from particles passing through the coil compared with signals from similar 
particles outside this zone. However, the problem is that the curves in 
FIG. 2 relate to a particle of a certain size. With increasing size of the 
conducting object, the signal level is greatly increased, so that a large 
object relatively far away from the scanning coil may generate the same 
signal as a smaller particle inside the coil. However, the invention makes 
it possible to compensate for the dependence on the size of objects 
situated within the sensing range of the coil by taking note of the fact 
that the resistive eddy current losses are inversely proportional to the 
square root of the frequency, as shown in FIG. 4. 
The curves in FIG. 4 are, in principle, the same as the curve II in FIG. 3 
with the difference that in FIG. 4 the abscissa and the ordinate have been 
made logarithmic. It will be seen that above a certain frequency, the 
curves decline linearly, corresponding to the inverse proportionality of 
the square root of the frequency. The maximum point of the curves varies 
in such a way that when the extension of the object increases, the maximum 
value will also increase while at the same time the position of the 
maximum occurs at a lower frequency. The curve will thus be displaced 
upwards to the left with increasing size of the object, so that curve a 
corresponds to the smallest object and curve c corresponds to the largest 
object. With increasing resistivity of the object material, the curves are 
displaced to the right so that if curve c represents, for example, copper, 
curve d could represent, for example, stainless steel, or the like, the 
objects having approximately the same size in both cases. For stainless 
steel, other curves corresponding to a and b may be imagined, which are 
only displaced to the right. 
FIG. 5 shows how the signal representing resistive losses varies with the 
size of the object at two different frequencies f1 and f2, where f2 is 
higher than f1. The abscissa represents the diameter of the object in 
millimeters on a logarithmic scale, whereas the ordinate indicates the 
signal also on a logarithmic scale. As mentioned with reference to FIG. 4, 
the signal increases with the size of the object and, above a certain 
frequency, decreases with the frequency so that curve A at a frequency f1 
lies higher than the curve B at a frequency f2 for one and the same 
object. The curves A and B are approximately parallel over their linear 
parts so that, if a relative difference signal is formed between them, an 
almost constant signal according to curve C is obtained, that is, a signal 
which is approximately independent of the size of the conducting object. 
Such a subtraction of signals is performed in the device shown in FIG. 1 
by the fact that the multiplier circuit 42 contains the square root of the 
frequency relationship so that the resulting signal output from summator 6 
(FIG. 1) is: 
##EQU1## 
As with the function (1) according to FIG. 3, k is of the order of 1; 
however, it may be desirable to let k have a value somewhat greater than 
1, which means that the curve will not be completely horizontal but will 
rise somewhat with increasing size of the object, so that the device 
becomes somewhat sensitive to the size of conducting objects. 
The main thing is, however, that with a signal function according to (2), 
the signal will mainly be dependent on the proximity of the object to the 
scanning coil. It will therefore be easy to restrict the output signal to 
that which occurs when an object passes through the coil, whereas weaker 
signals resulting from external objects are filtered off. 
As will be clear from FIG. 4, the signal according to the function (2) will 
be dependent on the conductivity of the objects which are to be detected 
in the material flow. This means that the curve C in FIG. 5 for materials 
having different conductivity will have different configurations as 
indicated in FIG. 6. In FIG. 6 the curve C' could represent copper and 
curve C" could represent stainless steel. This should be taken into 
consideration when choosing the frequencies f1 and f2. 
However, the situation is often such that the object detected in the 
material flow (nails, pieces of iron and steel) have the same 
characteristic and, thereby, resistivity as the large objects which are 
included in structures in the vicinity of the coil, while at the same time 
objects of low resistivity (aluminium) are of little importance. In such a 
case, the frequencies are suitably chosen in such a way that the device 
becomes less sensitive to objects having low resistivity. On the other 
hand, if it is desired to distinguish good conductors in the material flow 
from large, poor conductors in the vicinity of the coil, it may be 
necessary to match together the signals F(e1) and F(e2) according to 
several different functions. 
One such possibility is to introduce in FIG. 1 (as shown in dotted lines) 
an additional current source 13 with a frequency f3, as well as a 
corresponding phase-controlled rectifier 23, a filter 33 and a multiplier 
circuit 43, and matching these three signals together according to a 
suitable function. It may then be necessary, or at least desirable, to 
also include a multiplier circuit 41 in the output from the filter 31. 
Thus, the signal function will have the following appearance at the input 
to summator 6 in FIG. 1: 
EQU k1.times.F(e1)+k2.times.F(e2)+k3.times.F(e3) (3) 
This signal function will be zero, or almost zero, both in the case of 
signals generated by poor conductors in the material flow and in case of 
signals generated by large objects in the vicinity of the coil but not in 
the flow. 
From the comments given above with reference to FIGS. 3-5 it is clear that 
the voltages F(e) emanating from poorly conducting objects in the material 
flow substantially fulfil the relations 
##EQU2## 
and that the voltages F(e) emanating from larger conducting objects not in 
the flow but in the vicinity of the coil substantially fulfil the 
relations 
##EQU3## 
combining these three expressions, the following signal function results: 
##EQU4## 
in this expression a factor (1-k) has been introduced where k is a 
constant close to zero. The reason for this is that it is not often 
desired to completely compensate for the dependence on the size of the 
objects. It is often desirable to obtain a certain size dependence in the 
signal, and above all it is desirable to ensure that variations in the 
equipment do not cause over-compensation for the size, so that large 
objects in the flow give smaller signals than smaller objects. Experience 
has shown that a suitable value for k is between 0.01 and 0.1. 
Although the above-mentioned expression seems fairly complicated, it is, 
however, often considerably simpler in practice. If, for example, k is 
chosen to be equal to 0 and f3=4f2=16f1, the following simple function is 
obtained after reduction: 
EQU -4F(e1)+9F(e2)-2F(e3)