Note: Descriptions are shown in the official language in which they were submitted.
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TITLE- EDDY-CURRENT SENSOR FOR COIN EVALUATION
The present invention relates to coin validators
and in particular, relates to coin validators having an
eddy-current sensor for measuring of magnetic features of
coins as they pass the sensor.
A primary magnetic filed produces eddy-currents in
the coin being evaluated and a determination of the
authenticity and denomination of the coin is completed. A
disturbance of the primary magnetic field caused by the
eddy-currents produces a sensor signal. The eddy-currents
(and so the signal) depend on such individual features of
the coin including its shape, specific conductivity and
permeance of the coin material. More information about the
properties of the coin can be derived by using several
frequencies.
With the sensor of the present invention,
information is obtained as the coin moves past the sensor.
The output signal of the sensor depends on the position of
the coin relative to the sensors, the shape of the coin,
and the shape of the primary magnetic field. To make the
dependence more distinct for different positions as the
coin is moving past the sensor, the magnetic field is of
oblate tan shape.
SUMMARY OF THE INVENTION
A coin validator according to the present invention
comprises a pathway for guiding a coin as it moves past any
eddy-current sensor. The eddy-current sensor evaluates the
shape of the coin, the specific conductivity of the coin,
and permeance of the coin. Evaluation do as reason to
decide what denomination the coin has and whether the coin
is authentic or not.
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Based on these evaluations, a prediction of the
coin's denomination and authenticity is made.
The sensor includes a thin rectangle ferrite plate
with a primary coil and a sensing coil. The primary coil
is a short cylindrical coil placed centrally on one side of
the plate with the axis of the coil perpendicular to the
plate's surface. This coil generates a primary magnetic
filed and is a field coil. The sensing coil is wound
around opposite ends of the ferrite plate and has the major
axis of the coil along the plate. The sensing coil is
symmetrical about the middle of the plate. It produces a
signal that is the sensor's response. The sensor has a
plane of symmetry passing through the middle of the plate
and perpendicular to the largest surface of the plate.
According to an aspect of the invention, the field
coil has a cylindrical compact winding, its diameter is
equal to the width of the largest surface of the ferrite
plate, and the sensing coil has two symmetrical halves
placed between the ends of the plate'and the field coil.
According to a further aspect of the invention,
setting of the sensor in the pathway is carried out in such
a manner that the perpendicular to the plane of symmetry
and direction of the coin moving makes an angle of about 45
degrees. '
According to yet a further aspect of the invention,
the guide of the pathway near the sensor is curvilinear.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are shown in
the drawings, wherein:
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Figure 1 is an illustrative view of the design of
the sensor;
Figure 2a is a front view of the sensor and pathway
with Figure 2b being a side view;
Figure 3 is a perspective view of an alternative
version of the sensor; and
Figure 4 is an illustrative drawing illustrating
the shape of the magnetic field near the coin's guideway in
two directions along and across the largest axis of the
ferrite plate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The coin validator includes a pathway 30 having a
guide 10 for supporting and guiding the coin past the
sensor 13. A partition 14 separates the sensor 13 from the
coin. Two different diameter coins 11 and 12 are shown in
Figure 2. ,
The sensor as shown in Figure 1 includes a ferrite
plate 1 supporting a field coil 2 located such that the
coil axis 24 is perpendicular to the surface 22 of the
plate and passes through the center of the surface. A
sensing coil 19 includes two halves 3 and 4 that are wound
around the ferrite plate whereby each axis of the halves 3
and 4 passes along the length of the plate. This sensing
coil is wound symmetrically about the plate 1. The two
half coils 3 and 4 have leads 6 and 8. Field coil 2 has
leads 5 and 7.
,
To operate the sensor 19, the primary coil 2 is
supplied with electrical current that is time-varied during
the measuring time. An AC current of a certain frequency
or a current of a special form is provided to the primary
coil. The current creates a magnetic field around the
sensor that changes according to properties of a coin as it
moves past the sensor.
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The time-dependent magnetic field produced by field
coil 2 runs through half coils 3, and 4 and causes a
voltage to be produced. Due to symmetry of the magnetic
system and the location of the half coils 3 and 4 about the
plane of symmetry 9, the voltage induced in the sensing
coil 19 is zero (the summary voltage acting between leads 6
and 8 is zero) whereas the supply current varies.
When a coin is positioned near the sensor 13, the
response of the half coils 3 and 4 are separately affected
and a voltage appears. This voltage is the signal of the
sensor. Therefore, the half coils 3 and 4 form the sensing
coil 19. The voltage will rise and fall until the center
of the coin passes through the planel9 shown in Figure 2a.
At that time, the symmetry restores itself and a zero
voltage occurs. Further movement of the coin produces a
voltage opposite in polarity due to the position of the
coin.
The geometry of the magnetic system including the
field coil and the ferrite plate placed at the rear of the
coil produces a magnetic field shape having desirable
features. In the region occupied by coins during
evaluation, the value of the magnetic field's tense depends
to a limited extent on the space coordinates, however, the
direction is highly sensitive to the. position. The power
of the eddy-current depends not only on value of the
primary magnetic field but also on the angle between the
direction of the field and the coin's surface. This
arrangement allows the sensor's signal to include more
detailed information about the shape, of the coin including
it diameter. This information simplifies the evaluation of
the coin's denomination and authenticity.
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It is noted that the form of the supplying current
can be chosen to accentuate the coin features being
evaluated.
Determination of each of the'parameters above is
not required. The signal of the sensor is quite distinct
simplifying evaluation of the parameters of the coin.
Peculiarities of the signal - position of extremes and
their values, the point where the signal becomes zero and
the timing of the signal are sensitive to the coin's
features. Further advantages are possible if the guide of
the pathway is a special form (not linear). It allows the
signal to be more significant due to guiding the coin
through regions where the shape of the primary magnetic
field is most suitable.
The version of the sensor showed in Figure 1 is
liable to be influenced by extraneous magnetic fields. If
there is a need of protecting the signal from extraneous
magnetic fields, a more complex version of the sensor can
be used as shown in Figure 4. This sensor 113 contains two
equal ferrite plates (main 100 and additional 115) and two
identical sensing coils 120 and 130 where each coil has two
half coils 122, 124 and 132, 134 respectively connected in
such a manner that the voltages induced in them by an
extraneous magnetic field balance each other. The plates
are separated by a short distance. The voltages produced
with coin disturbance are not fully balanced because of the
shielding of the additional parts by'the basic ferrite
plate. This design diminishes the influence of extraneous
magnetic fields if they are significant and if the
frequency ranges of the signal and these fields overlap.
The advantages of the sensor are based on the
following principle. '
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As it is known, the power, being dissipated in the
conducting body due to the eddy-currents caused by
alternating magnetic field in it, depends not only on
magnetic energy located in the body but also on the shape
of both the body and the magnetic field. The dependence
has an integral behavior. With this arrangement, more
precise information about the shape of the body can be
obtained by evaluating how the integral eddy-current
characteristics vary with variation in the shape of the
primary magnetic field.
The spatial distribution of the disturbance of
primary magnetic field caused by edd~r-currents is also
shape-dependent. The reaction of the sensitive coil to the
disturbance is conditioned by several geometric factors.
The sensor signal represents these geometrical relations as
the coin moves past the sensor.
Figure 4 illustrates the spatial shape of the
primary magnetic field. The narrow ferrite plate is locate
with the longest axis perpendicular to the axis of the
field coil to produce the oblete tan shaped primary
magnetic field. Lines of force of the field are prolate
enough along the longest axis of the ferrite plate (see 18)
and are not so prolate along its middle axis (see 19).
Therefore, as a coin moves past the sensor, there are
points where line-of-force's projection going along the
biggest surface of the coin dominates and there are points
where the projection going perpendicular to this surface
dominates. There are also points where the sensing coil
senses the component of magnetic field caused by eddy-
currents, which goes along the coin's surface, better than
the one going along the coin surface and there are points
where a reverse ratio takes place.
Depending on the coin dimension, the time the coin
traverses the symmetry plane 9 varies (see Figure 2).
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Small coins, like 12, traverse it before they come to the
centre of the sensor, large coins 13 traverse it after
that. Therefore, the moment when the signal is zero is
dependent on the coin's size and this moment is an
indication of the coin's diameter. TnThen a coin first
approaches the sensor, the signal rises and subsequently
the signal falls until the coin traverses the symmetry
plane 9. The signal then rises but its polarity is
opposite. The signal caused by the coin up to the point
traversing the symmetry plane and the signal after passing
the symmetry are only similar in the case where the coin is
of a diameter that the centre of the coin passes through
the centre of the sensor. In the other cases, the signals
are not equivalent. Details of deviation of the signal
from a symmetrical signal also provides valuable coin
information.
According to aforesaid, the sensor signal has
several quantitative rates to evaluate a coin's
denomination and authenticity. For instance, value and
position of the first extreme of the signal (maximum)
position of the next zero level and value and position of
the second extreme. Each of them is formed by union of
coin's parameters - shape, specific conductivity and
magnetic properties and of primary magnetic field
parameters - shape and intensity. For various coin types,
combinations of these variables at different points of
coin's way are different from each other. This makes the
task of coin identification and authenticity simpler.
Besides, it is important that the accuracy of the
sensor primarily depends on geometrical factors because the
magnetic features of the ferrite plate do not affect the
primary magnetic field in a wide range of these features.
The magnetic system of the sensor is "wide open" and the
magnetic resistance of the plate is less than the magnetic
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field, there may be some difficulties because a "wide open"
magnetic system has appreciable sensitivity to such fields.
To diminish this effect, a compensation of the field
influence may be performed with using another ferrite plate
and another sensitive coil as it is shown on Figure 3.
Here the second ferrite plate 115 is positioned behind the
first plate 100. The second plate has a sensitive coil 130
being identical to one belong to the first plate (122,124).
Both the coils (122, 124 and 132, 134) are connected in
series in such a manner that the voltages induced in the
coils by the extraneous magnetic field suppress each other.
Because sources of the extraneous magnetic fields are
usually situated far enough from sensor, their influence on
both the sensitive coils is equal. Therefore, suppression
is effective. As to the main signal, the voltages induced,
as a coin, is close to the sensor, are different from each
other. This is a result of shielding of the primary
magnetic field with the first plate due to high permeance
of the plate material and due to non-magnetic gap between
the plates.
The guide of the pathway 10 is designed for the
coin to follow a particular path. This path can be linear
or not linear, horizontal or vertical, and so on, depending
on the applications. The coin position as it moves past
the sensor, is to be predetermined to avoiding change in
the sensor state due to varying of the position of the coin
in the pathway. The best design of the pathway is such
that dependence of the position on time is steady. This
allows processing the sensor signal as a time dependent
signal.
Although various preferred embodiments of the
present invention have been described herein in detail, it
will be appreciated by those skilled in the art, that
variations may be made thereto without departing from the
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Although various preferred embodiments of the
present invention have been described herein in detail, it
will be appreciated by those skilled in the art, that
variations may be made thereto without departing from the
spirit of the invention or the scope of the appended
claims.
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