Device for suppression and/or separation of signals due to magnetic oxide scales in hot cast billets

Defect detecting equipment includes a device which detects the presence of, for example, disturbing magnetic material such as cold oxide scales in a test object, in order thus to control, for example, that disturbances of a magnetic origin are not confused with surface cracks without being discovered. By utilizing eddy current techniques, phenomena which are harmless to the process can be separated from dangerous surface cracks, thus avoiding the scrapping of crack-free test objects. The invention is based on signal-processing at least two signals, for example H1 and L1, originating from a test object sensing tansducer, by means of a vector transformation method, and distinguished between harmful and harmless phenomena (e.g. cracks and oxide scales) as a function of a comparison of signals, S1 and S2l , from at least two transformation blocks.

TECHNICAL FIELD 
This invention relates to the field of the control and/or supervision of 
defects in bulk material, for example the detection of cracks using eddy 
current techniques in hot continuously cast steel ingots and the like. The 
invention can be considered to be an improvement in the subject-matter 
disclosed in Tornblom's U.S. patent application 085,173 filed on Aug. 14, 
1987. 
DISCUSSION OF PRIOR ART 
In conventional defect (or crack) detection, a vector transformation 
technique is employed using a transformation unit, the aim being to 
combine and sum up different detected analog signals, after the signals 
have been weighted and provided with polarity constants etc., such that 
desired signals (i.e. those representing defects that need to be detected) 
are emphasized whereas undesired signals are suppressed in the resultant 
analog output signal from the transformation unit. However, in those cases 
where it is desired to suppress, for example, signals caused by oxide 
scale deposits on an ingot, the suppression does not function other than 
for a limited number of types of oxide scale deposit. The reason for this 
is that deposits of oxide may vary greatly in size, shape, orientation, 
and so on. In other words, the transformation is only effective for those 
oxide scales which correspond to the current setting of the transformation 
unit. This is an obvious drawback which is difficult to overcome with 
conventional vector transformation techniques. 
The present invention shows how, in a relatively simple manner, simple 
conventional transformation units can be supplemented with a comparison 
unit, which by means of special signal processing is able to distinguish 
between an important defect or crack signal or an unimportant oxide scale 
signal. It should then be noted that the unwanted signals, in this case 
the oxide scale signals, need not be greatly suppressed but that the 
comparison is determining for the classification of the type of signal. 
Since the principle is general by nature, it can also be employed for 
suppression/separation of other signal-generating phenomena in addition to 
oxide scales. However, eliminating spurious defect detection caused by 
oxide scales is valuable, since these scales are indefinite in size and 
shape and thus--at the same time--difficult to suppress using conventional 
vector transformation techniques. The following description is therefore 
to be considered one of many feasible examples of how the present 
invention can be applied and utilized. 
The invention may, for example, be regarded as an important 
reliability-improving complement to crack detection equipment using eddy 
current techniques, which warns of the presence of disturbing magnetic 
material, for example of the oxide scale type, on otherwise substantially 
non-magnetic test objects. 
The fact that the permeability .mu..sub.r of a localised region on a test 
object gives rise to disturbing vectors of varying magnitude, when 
.mu..sub.r &gt;1, has long been a wellknown problem in the eddy current 
testing of non-magnetic material. Heretofore, various attempts have been 
made to remove the magnetic material, for example, magnetic oxide scales 
and the like, for example by flushing the surface of a hot billet with 
water under high pressure, but these removal techniques are expensive and 
time-consuming and sometimes unreliable. 
As far as the applicant knows, the existing specialist literature does not 
describe any method or device corresponding to what is described herein. 
The basic idea behind the invention is to first detect the presence of 
magnetic material and then to alert and control the defect detection 
process. 
By using a simple transformation unit one variable can be suppressed per 
unit. However, if required complex hierarchic networks can then be built 
up with these simple transformation units, so that the resultant output 
signal obtained is not influenced by any of the suppressed signals, an 
arrangement which is felt to be unique to the invention. 
The primary object of the invention is to detect the appearance of magnetic 
material and to warn of its presence and effects in order thus to be able 
to prevent the magnetic disturbances being confused with actual cracks. It 
would, of course, be possible to measure the surface temperature of the 
billet via, for example, one or more radiation pyrometers, thereby 
determining whether colder surface areas exist and whether the material is 
magnetic or not. However, for reasons which are easily understood, this 
method suffers from numerous and considerable drawbacks. 
The invention enables, for example, the combination of facilities for the 
detection of cracks and the detection of magnetic material by using, for 
example, a single eddy current transducer. Also, certain parts of the 
electronic measuring equipment associated with the transducer may also be 
common to both facilities. This involves advantages both from the point of 
view of economy and from the point of view of measuring technique. 
In eddy current testing, for example crack detection, on non-magnetic 
material, it is usually assumed that no magnetic permeability is present 
in the material, i.e. that the test object is totally non-magnetic. 
In all the above cases it may occur that foreign magnetic particles and the 
like occur in or on the material, or that, for example, a certain limited 
region on a hot billet surface has become magnetic in spots because of a 
partially low surface temperature. 
In a continuous casting process, the temperature of the cast strand may 
vary as a result of different process parameters, which are changed while 
casting is in progress. In this case, also the billet surface is often 
coated with larger or smaller so-called oxide scales, the Curie 
temperature of which is often somewhat lower than that of the actual 
steel. Still it happens that the oxide scales, which contain FeO, Fe.sub.2 
O.sub.3 and Fe.sub.3 O.sub.4, because of insufficient mechanical contact 
with the surface of the billet, sometimes attain a temperature below their 
Curie temperature, whereby they become magnetic and greatly disturb the 
eddy current testing. 
In this context it is important to realise that irrespective of the reason 
for the occurrence of magnetic material in or on the test object, its 
occurrence is invariably disturbing to an eddy current testing technique 
adapted to non-magnetic test objects, and that this is particularly true 
in the case of so-called multi-frequency testing. 
The present invention aims to provide a solution to the above-mentioned 
problems and other problems associated therewith. 
SUMMARY OF THE INVENTION 
According to the invention there is provided a device for monitoring a test 
object for the presence of a selected phenomenon, which device comprises 
at least one sensor generating an output- signal means to move the sensor 
relative to the test object, and means to process the output signal 
originating from the movement of the sensor relative to the test object to 
detect the presence of the selected phenomenon, which is characterized in 
that the effect on the output signal of a second phenomenon is suppressed 
relative to the effect on the output signal of the selected phenomenon by 
the processing means operating selectively on the said output signal. 
The invention may be regarded as an important complement to the following 
Swedish patent applications and patents: Nos. 7507857-6, 8601785-2, 
8603113-5 and 8603240-6 and U.S. Pat. No. 4,237,419 British patent No. 
2041535, U.S. patent application No. 926850 (filed Nov. 3rd, 1986), U.S. 
Pat. No. 4,646,013, U.S. Pat. No. 4,661,777 and U.S. patent application 
No. 702,314 (filed Feb. 15th, 1985). The terminology and the drawings of 
these patents/applications are applicable, in parts, to the present 
invention as well. Since the majority of the patents/applications 
mentioned above include the detection of cracks on hot non-magnetic 
material, magnetic disturbances ca be expected, which justifies a 
complement according to the present invention. 
Definitions of Terms Used 
In this specification the following definitions apply: 
The term EDDY CURRENT TESTING includes control and/or measurement based on 
the use of frequencies and/or frequency components within a range 
extending from a few Hz to several MHz. 
The term FREQUENCY includes CARRIER FREQUENCY, i.e. the frequency with 
which a transducer/sensor is supplied with electrical power and also 
embraces a frequency component. 
The term TEST OBJECT includes a continuously cast billet, a rod, a tube, a 
sheet or a volume of liquid molten steel and also embraces particles and 
objects on the surface of the test object, for example oxide scales and 
the like. 
The term TRANSDUCER/SENSOR includes a surface transducer coil supplied with 
current, which coil moves, for example, in planes parallel to the surface, 
or part of the surface, of a test object. 
The term LIFT-OFF (LO) means the distance of a transducer/sensor relative 
to the surface of a test object. 
The term MAGNETIC material means that the material is influenced by a 
permanent magnet, i.e. that the relative permeability (.mu..sub.r) is 
greater than unity. 
The term FAULT VECTOR (FV) means that vector which arises in the impedance 
plane of the transducer/sensor when the transducer/sensor moves over a 
defect. 
The term .mu.-VECTOR (.mu.V) means that vector which is caused by the 
influence of magnetic material on the transducer/sensor. 
The term VECTOR LOBE (VL) means that surface in the impedance plane within 
which fault vectors are situated. 
The term NON-MAGNETIC MATERIAL means, for example, steel ingots such as 
slabs, billets and the like, the temperature of which at the time of 
measuring lies above the so-called Curie temperature or Curie point. It 
may also be a magnetic steel pipe which, by way of saturation 
magnetization in conventional manner, has received an apparent 
permeability substantially corresponding to .mu..sub.o. 
It should be pointed out that the following description assumes that the 
reader has a certain basic knowledge of impedance diagrams, etc., and 
therefore the more elementary bases and details have been omitted in order 
to keep the description to acceptable lengths. For the same reason an 
application of the invention will be described in which the device is 
based on eddy current technique. However, it should be appreciated that 
other techniques, for example leakage fluxes, as well as the use of 
sensors of the Hall element type, and so on, are embraced by the invention 
and the following description should be read with that fact in mind.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows a normalized impedance diagram, of conventional character, for 
a transducer/sensor. U.S. Pat. No. 4,646,013 (Tornblom) shows in FIG. 3 a 
corresponding impedance diagram in the case of a test object of 
non-magnetic material, i.e. the impedance curves are based on 
.omega.L/.omega.L.sub.0 =1.0. In the accompanying FIG. 1, however, the 
impedance plane has been supplemented with curves for a magnetic material, 
in other words, .mu..sub.r &lt;1. As will be clear, the permeability, .mu., 
has an amplification effect on the electric impedance, which may greatly 
disturb the eddy current measurement of cracks and the like defects, 
especially when the cracks have an elongate direction in the impedance 
plane which largely coincides with the .mu.-direction. The direction of 
the magnetic permeability in the impedance plane is clear from the 
.mu.-vectors shown by the dot-dashed lines displayed on the graph of FIG. 
1. 
In, for example, crack detection on hot (&gt;780.degree. C.) steel ingots, the 
temperature of which exceeds the Curie temperature, the steel is 
non-magnetic. If a simple surface transducer is used for crack detection 
and the distance of the transducer to the billet surface varies, for 
example between LO.sub.2 and LO.sub.3, the impedance of the transducer 
will also vary. This impedance variation has different magnitudes at 
different carrier frequencies, and for a certain frequency, .omega..sub.L 
in FIG. 1, it is shown as a vector LO between points P2 and P3. Depending 
on the direction of the LO-movement, this vector may reverse its 
direction, that is, it may change polarity. 
The LO.sub.1 -curve in FIG. 1 represents the case where there is strong 
inductive coupling between the transducer and the test object, for example 
coupling such as would occur when LO=0 (i.e. the transducer is in contact 
with the surface). It is also possible, for example, to define the 
LO.sub.1 -curve as the smallest LO-distance which is possible in practice. 
For the non-magnetic material, this means that the impedance curves are 
contained within the sectioned part of FIG. 1. 
Now, let it be assumed that the transducer is at a distance L0.sub.2 from 
the surface of the test object and that the transducer is powered with a 
carrier frequency .omega..sub.L, which means that operation is occurring 
at point P2 in FIG. 1. When the transducer is positioned over a crack, a 
so-called fault vector (FV) is obtained, the direction of which lies near 
the LO-direction, which is described in detail in the above-mentioned U.S. 
Pat. No. 4,646,013. Now, if the test object for some reason should become 
magnetic, i.e. .mu..sub.r &gt;1, a vector would be obtained in a 
corresponding manner, which in FIG. 1 is shown as a vector .mu.V. It 
should be pointed out that the vectors FV and .mu.V after detection have 
different signal frequency contents, i.e. different duration, which is due 
to the fact that the crack has an appearance which is different from 
(shorter than, for example) the magnetic region on the test object, which 
may be an increased thickness of oxide scale. These vectors have, for two 
different carrier frequencies, .omega..sub.L and .omega..sub.H, been 
separated from FIG. 1 and are plotted graphically in FIGS. 2 and 3. These 
vectors can be conventionally transformed into, for example, voltages 
which may be rectified via, for example, phase-controlled rectifiers. In 
this way , it is possible to separate vectors having different directions, 
i.e. different phase positions, in the impedance diagram. 
When using the term vector, this is often understood to include also a 
signal, for example an alternating voltage, the phase position of which 
represents the direction of the vector and the amplitude of which 
corresponds to the magnitude of the vector. In FIGS. 2 and 3 the socalled 
vector lobes (VL) have been indicated by dashed lines. These lobes 
indicate the limiting surface within which fault vectors of varying depth 
and vertical position are located. In the case of unusually large cracks, 
the length of the lobes may be greater than that shown. 
It is easily realized that in addition to their magnetic vector, oxide 
scales also contain an LO-vector part because of their thickness. 
Therefore, oxide scales appear somewhat indefinitely in the impedance 
diagram. Howver, tests carried out in practice shown that the combined 
vector direction of oxide scales differs from surface cracks at higher 
frequencies, whereas at lower frequencies cracks do exist which are 
somewhat more difficult to phase-discriminate from certain types of oxide 
scale. 
Although much of the impedance diagram according to FIG. 1 is known to the 
person skilled in the art, as far as I know no-one has attempted at or 
succeeded in drawing the conclusions which form the basis of the present 
invention. A probable explanation of this may be that magnetic 
permeability , as opposed to the electrical conductivity, is of no major 
interest to, for example, an end user of steel ingots such as slabs and 
the like, but has only been a problem for users of equipment for eddy 
current testing. 
From FIG. 3 it is clear that the lower right-hand part of the vector lobe 
VL intersects the .mu.-vector, .mu.V. This part of the vector lobe usually 
represents cracks located somewhat deeper in the material, i.e. cracks not 
open to the surface. The consequence of this is that at higher frequencies 
there are cracks whose direction in the impedance diagram coincides with 
the .mu.-direction. In other words, FV and .mu.V cannot be separated in a 
reliable manner merely by using phase discrimination at higher 
frequencies. On the other hand, at a suitably selected low frequency, as 
shown in FIG. 2, separating VL from .mu.V, and inversely, does not present 
any problem, as in this case no intersection occurs. As far as is known, 
this fact has not been made use of by anyone in order to increase the 
reliability in crack detection, as described in the present application. 
As a first step towards a reliable separation of the .mu.-vector from the 
other vectors, a suitable, often low, frequency is chosen which provides a 
complete separation of .mu.V from FV. As a second step, for example, the 
lift-off (LO) vector is suppressed. It is to be noted here that LO may 
change polarity (see the -LO dashed line in FIG. 2). Therefore, if, as 
indicated in FIG. 2, detection is carried out in a direction which is 
horizontal, the respective vector projection on the horizontal line will 
be approximately the same for .mu.V and -LO, which means that it is 
difficult, if not impossible, in this way to separate .mu.V from -LO. On 
the other hand, as can be seen in FIG. 2, at .omega..sub.L horizontal 
projections of FV and .mu.V can be separated from each other without 
difficulty since the projections of FV and .mu.V have different signs and 
can easily be distinguished one from the other by electronic means. At a 
sufficiently low frequency where the angle .alpha. or the sum of the 
angles .alpha. and .beta. is of the order of magnitude of 90.degree., it 
is possible relatively efficiently to suppress the LO-influence by 
detecting the vectors largely perpendicular to the LO-direction or the 
FV-direction, depending on which of these is the more disturbing for the 
.mu. -vector separation. 
In the case of normal surface cracks, the angle .beta. is often 
&lt;18.degree., which means that the fault vector FV is also suppressed 
relatively well when detecting perpendicular to the LO-direction. Because 
of the somewhat incomplete suppression of FV, however, it may be useful to 
improve the suppression via a filtering method. To this end, the fact that 
the frequency contents in the detected and rectified fault vector FV are 
higher than the frequency contents in the corresponding .mu.-vector signal 
and the LO-vector signal, can be employed. The filters for the respective 
vector types are therefore tuned to different signal frequencies, whereby 
they can be more easily separated from each other. The reason for the 
different frequency contents is that cracks and possible magnetic portions 
of the test object have different shape and propagation. The transducer is 
thus likely to be located over a crack and over a magnetic portion for 
different periods of time. 
Since the LO-signal also differs with respect to frequency from other 
signals or vectors, the LO-signal can also be separated or suppressed 
further via a filter method, if required. Another method of separating or 
suppressing both the LO-signals and oxide scale signals is via the type of 
transformation to which the present invention relates. In this case at 
least one LO-signal, or part thereof, of a different carrier frequency 
origin is employed in order to compensate, for example to balance away, 
the LO-vector. The same technique can also be employed for separation and 
suppression of FV-signals and so on. The invention includes both separate 
and combined solutions of the principles mentioned here. 
In order for the vector transformation to operate satisfactorily, the 
relatively definite direction in the impedance diagram of the oxide scale 
vectors is a fundamental condition. It is also important that both the 
absolute direction and the direction relative to other vector types are 
different at different frequencies. The frequencies/carrier frequencies 
used may advantageously be, for example, 10 KHz (L) and 1 MHz (H), 
respectively. To operate with a frequency ratio H/L&gt;5 has in certain cases 
obvious advantages. 
To prevent .mu.-vectors from being confused with fault vectors, it is 
desirable to use the detected presence of magnetic material for 
automatically blocking crack detection so that no false cracks are 
indicated. At the same time, some form of alarm can be given, for example 
automatically, in order to draw attention to the fact that crack detection 
has been temporarily blocked or is unreliable because of the presence of 
magnetic disturbances. 
The presence of magnetic material can also be used as an indication that 
something is wrong in a continuous casting process, for example that 
excessive cooling is occurring in a continuous casting machine. When alarm 
is given indicating the presence of magnetic material, it is also 
possible--for example, automatically and temporarily --to activate devices 
for the removal of oxide scale and the like magnetic material from the 
test object. 
In certain cases, it may be desirable that alarm is given when the 
permeability level exceeds a certain set threshold value. For that reason, 
the permeability signal should be largely constant within the LO operating 
range of the transducer. This can be achieved by signal processing, for 
example by amplifying, the .mu.-signal as a function of the LO-signal. 
In those cases where the same transducer is used both for the detection of 
cracks and for detecting the presence of magnetic material, the following 
advantages, inter alia, may be obtained: The measurement takes place at 
the same time on the same surface part, so the measured values are the 
current ones and are related to each other. The permeability dependence of 
the crack detection is nearly exactly indicated because the same 
transducer is used for both measurements. The transducer arrangement is, 
of course, simpler and less expensive. 
The above-mentioned U.S. Pat. No. 4,661,777 (Tornblom) relating to 
so-called dynamic transformation may, in certain cases, constitute a 
complement to the present invention, or vice versa, since the oxide scales 
because of their thickness often give rise to a disturbing lift-off 
vector. Particularly in the case of thick oxide scales occupying a large 
area, where the lift-off variations are considerable, it may be justified 
to make use of dynamic transformation. 
In crack detection, some form of transducer manipulator is often used to 
move a transducer/sensor over, for example, a hot steel strand. The 
manipulator may be a so-called "whirligig" device, i.e. it moves the 
transducer/sensor along a rotary path superimposed on a slower scanning 
movement. The present invention includes those cases where separate 
transducers for detecting cracks and magnetically disturbing material are 
placed together on or in the same scanning arrangement, and this has 
several advantages. In this way, crack detectors can be blocked to an 
optimum extent, i.e. to precisely the right amount and for precisely the 
right period of time, since the information about the presence of magnetic 
material is both up-to-date and exact. 
To illustrate how a crack detector and a magnetic or .mu.-detector can 
cooperate, two largely equivalent block diagrams are shown in FIGS. 4 and 
5. Let it be assumed that the test object 1 contains a magnetic oxide 
scale flake 2. Two transducers 3 and 4 (which in FIG. 5 are shown as a 
common surface transducer coil) move over the surface of the test object 1 
at a velocity v m/s in the direction of the arrow. The transducer/sensor 
consists of one crack transducer 3 and one magnetic sensor 4. The 
transducer/sensor is respectively connected to a crack detector 5 and a 
.mu.detector 6. The crack detector 5 is connected to a blocking circuit 7, 
from which crack signals can be obtained at an output 11. In FIG. 5 the 
crack signal also passes through a delay circuit 88. The output signal 
from the .mu.-detector 6 controls the blocking circuit 7 via a time delay 
unit 8, which may, for example, extend the control signal from the 
.mu.-detector 6 so as to obtain an optimum blocking. Different types of 
alarm signals 10 are given via an alarm unit 9. A particularly good 
arrangement is to locate the .mu.-transducer 4 immediately in front of the 
crack transducer 3 since in this way the crack detector 5 is blocked just 
before the crack transducer 3 reaches the disturbing region represented by 
the flake 2. The same end is achieved if, as shown in FIG. 5, the crack 
signal is delayed in the delay circuit 88, which may consist of an analog 
shift register or the like. This delay makes it possible for a false crack 
signal, which has arisen as a result of the flake 2 of magnetic material, 
to be blocked in a reliable manner by the signal from the .mu.-detector 6. 
Because of the delay in the circuit 88, the signal from the .mu.-detector 
6 should be extended by the time delay unit 8, for example by a period 
somewhat longer than the delay time set by the delay circuit 88. 
The alarm signals 10, about the presence of magnetic material, can be used 
to activate and/or control, for example, a separate device for the removal 
and/or elimination of the magnetic material and/or a suppression of its 
effects on, for example, the detection of cracks. 
The invention thus embraces the use of a further device, for example 
controlled via an alarm signal 10, for eliminating completely or partially 
the magnetic properties of oxide scales and the like on, for example, hot 
test objects, by heating the oxide scales to a temperature corresponding 
to at least the Curie temperature of the oxide material, which renders the 
oxide scale largely non-magnetic. This heating can, for example, be 
achieved by heating up the oxide scales (e.g. using at least one gas 
burner or gas flame). Another way is to raise the temperature of the oxide 
scales by means of an inductive heating device. The heating device may 
advantageously be mounted on the scanning equipment adjacent to the 
transducer of the measuring and/or control device (e.g. as shown in dashed 
lines at 12 in FIG. 4). Such heating can be initiated. for example, when 
the .mu.-detector indicates that the oxide scales are magnetic or are 
tending to become magnetic. In this way, the heating can take place 
selectively in places here magnetic oxide scales and the like have become 
established. By locating the transducer/sensor for crack detection and 
.mu.-detection and the heating device 12 on the same movable support 
connected to, for example, a billet strand, for example on a cross-travel 
car, a financially attractive overall solution is obtained. 
In summary, the invention comprises providing, for example, a conventional 
eddy-current based crack-detecting equipment, which is adapted to scan 
preferably non-magnetic test objects, with a device which detects--for 
example, advantageously via eddy current technique--the presence of 
disturbing magnetic material, for example relatively cold magnetic oxide 
scales, in and/or on the test object in order thus to monitor, for 
example, that disturbances (i.e. so-called "false" cracks) originating 
from the presence of magnetic material, are not confused with real cracks 
and similar harmful surface defects. 
In order to avoid too large a proportion of the sensed surface becoming 
insensitive for crack detection because of excessive magnetic 
disturbances, the device can be provided with means (for example a heating 
device) to eliminate the magnetic properties of, for example, oxide 
scales. 
It should be noted that the invention can be regarded as a further 
improvement of the invention defined in U.S. patent application 085,173 
previously referred to, the present invention supplementing the earlier 
one insofar as the signal processing is concerned. This means that the 
invention can be realized as a separate device or it can be included as an 
integral part in another device, the task of which need not be to separate 
oxide scales and cracks from each other. 
A detailed example of vector transformation, according to the present 
invention, in diagrammatic form is illustrated in FIG. 6. FIG. 6 can be 
regarded as a functional diagram of block 6 in FIGS. 4 and 5. Measurements 
in practice have shown that the cold oxide scales occur relatively stably 
in phase in the impedance diagram of the transducer; in other words, the 
oxide scale vectors have a relatively definite direction at the 
frequencies used. However, this direction varies between the frequencies 
themselves. In FIG. 6, input signals H1, L1, and H2, L2, respectively, 
consist of signals from, for example, phase-controlled rectifiers, 
so-called synchronous carrier frequency detectors. H1 and H2 originate 
from, for example, a high carrier frequency, for example 1 MHz, whereas L1 
and L2 originate from, for example, a low carrier frequency, for example 
20 KHz. H1, H2, L1 and L2 may be output signals from different detectors. 
For the sake of simplicity, it is assumed in the following that H1=H2 and 
L1=L2, which, however, is to be seen as one of many possible alternatives. 
H1=H2 and L1=L2, despite its simplicity, functions very well in practice. 
FIG. 6 is divided into three functional units, namely a crack detection 
channel (SPR), an oxide scale detection channel (GSK) and a comparison 
channel (JMF). The task of the SPR unit is to generate a signal S1 and to 
suppress the effect of Lift-Off (LO) and signals arising from minor 
surface irregularities (often referred to as oscillation marks (OSCM)). 
The task of the GSK unit is to generate a signal S2. The task of the JMF 
unit is to compare the signals S1 and S2 with one another. An output 
signal S3 from the JMF unit is employed, for example, to block the crack 
detector unit in the presence of so-called false crack signals caused by 
oxide scales. The term "block" is to be construed in a broad manner and 
includes, for example, the meanings of "bar", "damp", "separate" or 
"short-circuit". It would, however, be possible to combine several signals 
of the same type as S3 into superordinate condition complexes, whereby it 
is of course, possible to carry out, for example, advanced signal 
separations or the like by using several S3-signals as digital input 
signals to one or more digital condition networks. In other words, FIG. 6. 
can be regarded as a building block, from which, by multiplying and 
combining more complex structures or networks can be built. By detecting 
the carrier frequency signals, as indicated in FIG. 6, so that the H- and 
L-signals assume different polarity, these can be weighted relative to 
each other by means of NORM-potentiometers 20 (NORM.sub.SPR) and 21 
(NOR.sub.GSK), respectively. By grounding the center slide of the 
potentiometers 20, 21, as shown in FIG. 6, the respective transformation 
block can be normalized which significantly simplifies signal processing. 
The same is true of the balancing potentiometer 22 in unit JMF. For the 
SPR unit a position of potentiometer 20 must be chosen which gives a 
minimum output signal S1 for LO and OSCM. For the GSK unit, in a similar 
manner, a position of potentiometer 21 must be chosen which gives a 
minimum output signal S2 for oxide scale (GSK). The SPR and GSK units are 
to be regarded as separate, simple transformation blocks, usually 
utilizing different potentiometer settings to suppress the one or more 
variables requiring suppression. By thereafter combining several units, 
the number of suppressed variables can, of course, be increased in spite 
of the fact that the method for setting for each does not become more 
difficult. The optimum setting for the respective variable, of course, 
normally takes place by means of a setting of the separate potentiometer. 
It is also worth noting that the respective units may otherwise 
advantageously be identically constructed. The units may also suitably be 
designated subtransformation complexes or the like. In practice, 
sometimes, signals from one unit are utilized in another unit. The 
important thing, however, is that the signals S1 and S2 originate from at 
least partially different normalizations in the two units generating them. 
By the simple circuit solution, in which a resistance network R1, R2, R3, 
R4, a summer 30 and the potentiometer 20 constitute the weighting portion 
together with an amplifier 31 and a filter 32, the signals which are to be 
minimized can thus be suppressed individually by their separate 
normalization settings. This, of course, facilitates the signal 
processing. 
By using the same input signals to the transformation units (H1=H2 and 
L1=L2) and normalizing the units for suppression of OSCM and of cold GSK, 
it is often adequate to use only two input signals. This, in combination 
with the above, forms the basis of a very simple setting. FIG. 7 shows how 
the different signals S1, S2 and S.sub.D, because of the different NORM- 
and BAL-settings, vary in amplitude and shape depending on their origin, 
the phenomenon causing each signal being marked at the top of each column 
of S1, S2 and S.sub.D signals. The S1-signal exhibits a low amplitude for 
LO and OSCM whereas the S2-signal in a corresponding manner exhibits a low 
amplitude for GSK. The arrow heads between the S1- and S2-signals point to 
the larger signal for each respective phenomenon. As will be clear, the 
direction of the arrow head for GSK differs from the other four, which is 
important to note. 
The task of the JMF-block in FIG. 6 is to compare the signals S1 and S2 
with each other. By rectifying S1 and S2 via rectifiers 33 and 34, two 
signals of different polarity are obtained independently of whether S1 and 
S2 have an inverted appearance, as is shown in FIG. 6, or otherwise differ 
in shape. In other words, the full-wave rectification has an equalizing 
effect on the curve shapes of S1 and S2. If the rectifiers 33 and 34 are 
provided with capacitors C on their outputs, the equalizing effect will be 
further improved. A resistance network and summer (similar to those used 
in units SPR and GSK) feeds an amplifier 35, the output S.sub.D of which 
is fed to a filter 36. In FIG. 7 the S.sub.D -signals are shown with a 
discharge curve shape, which is due to the capacitors C. The signals S1 
and S2 can be weighted relative to each other by way of the balancing 
potentiometer 22, which can be seen as a superordinate normalization. By 
selecting a suitable setting for the potentiometer 22, the appearances of 
the S.sub.D -signals obtained will be as shown in FIG. 7. As will be clear 
from FIG. 7, the signal from an area of oxide scale (GSK) has a positive 
polarity whereas the other input parameters/quantities each give rise to a 
negative polarity S.sub.D signal. 
The consequence of the above is that, after character generation, weighting 
and summing of the signals S1 and S2 via, inter alia, the amplifier 35, 
the filter 36, and a suitable threshold voltage (TR) on an amplifier 37, 
it is possible to separate the oxide scale signals (GSK) from the other 
signals. In the block 38, the signal from the amplifier 37, for example, 
is digitized in order to impart a suitable shape and level to the output 
S3-signal. Thereafter, the S3-signal may, for example, be used for 
controlling/blocking the crack detection unit, by blocking the signal path 
or by, for example, resetting an error code register, and the like. The 
superordinate S4-signal outputting from a weighting unit (LV) can be used 
in a corresponding manner. 
In those cases where it is desired to suppress several 
variables/quantities, it may be justified to extend the number of 
transformation complexes and a further one is partially shown at 100 in 
FIG. 6. Several S3-signals, S3', are then obtained, as is indicated in 
FIG. 6. These S3'-signals constitute input signals to the weighting unit, 
LV, which, for example, comprises logic conditions, i.e. a type of 
superordinate condition function. The output signal S4 from LV can be 
looked upon as a more sophisticated control signal than the S3-signal. The 
weighting unit LV may incorporate everything from simple threshold 
conditions to advanced and complex logic conditions, comprising, for 
example, AND- and OR-function gates or comparators. In this way, the 
transformation function thus used and supplemented becomes special in 
nature and may, for example, be used in spite of the fact that the 
quantity being monitored (in this case oxide scales) varies in shape and 
dimension and is thus difficult to suppress in a conventional manner. 
Successively--i.e. unit by unit--suppressing a variable with one 
potentiometer per variable and/or unit provides for a simplicity of signal 
processing which is superior to other known methods. 
In a conventional suppression of unwanted phenomena by means of vector 
transformation, as described in the above-mentioned patents/applications, 
the starting-points are continuous linear functions which are then 
normally represented by analog voltages, and via weighting methods, the 
unwanted phenomenon is suppressed. In contrast to this, a method involving 
comparison and conditions is utilized to determine the type of phenomenon, 
for example whether the surface crack on the billet surface is a genuine 
or a false crack. 
The condition-based separation described above can also be used for 
detecting and/or suppressing phenomenon other than oxide scales and 
magnetic material so that these other applications are also embraced by 
this invention. 
Since the transformation includes some kind of conditions, the requirement 
for accuracy of setting up is often reduced, since in many cases it is 
sufficient to determine the polarity of the S.sub.D -signal to determine 
the type of signal occurring, for example whether or not it is a question 
of an oxide scale (see FIG. 7). This can also be expressed as follows. 
Instead of aiming at a complete suppression of the unwanted analog signal, 
as in the case of conventional transformation, according to the present 
invention a comparison is carried out, for example of the amplitude, 
between two separate signals derived from different transformation 
settings. 
The invention can be varied in many ways within the scope of the following 
claims.