Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: SENSOR FOR EVALUATING DIELECTRIC PROPERTIES OF
SPECIALIZED PAPER
FIELD OF THE INVENTION
The present invention relates to validators having
sensors for evaluating dielectric properties of specialized
papers. The invention has particular application for paper
currency evaluation and security appear evaluation.
BACKGROUND OF THE INVENTION
Currency validators are designed to receive a
banknote and move the banknote through an evaluator channel
prior to accepting and storing of the accepted banknote.
From time to time a banknote can become jammed in the
validator which creates problems, particularly for
unsupervised installations.
Jamming of a validator often :Ls due to a wet or
high humidity banknote or due to high density paper
sometimes found in fraudulent banknotes. These conditions
can be recognized by a capacitor sensor.
When the currency bill passes between capacitor
electrodes, the capacitor capacitance increases according
to the effect of the dielectric properties of the currency
note. The deviations from this value will be observed when
the samples with higher or lower density are tested in the
validator.
Water has a dielectric constant almost ten times
higher than the dielectric constant of currency paper.
V~lhen we, currency paper passes between the capacitor
plates, its capacitance is higher than dry paper, the
wetter the paper, the larger the capacitance (as compared
to the authentic currency paper). Therefore, the
capacitive sensor can determine the "humidity" of currency
paper and can be used to evaluate the authenticity of the
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paper, as the currency paper is being evaluated by the
validator.
Many validators are used in a generally non-
supervised application such as a vending machine.
Fraudulent bills often have a high density and if fully
processed by a validator, can become jammed or damage the
validator.
It is important in validators to reject fraudulent
bills, however, it is also important to reject bills which
may become jammed in the validator or which may damage the
validator. A jammed validator causes the operator problems
and also frustrates the user.
Information about the humidit~r and other parameters
of the paper, evaluated by a validator, are important for
the validator's operation.
The design of automatic validators makes
contradictory demands. The size of the sensor should be
small. It should be designed in such a way that it can be
placed anywhere inside the validator channel. Rigid
mechanical and electrical connections between the sensor
elements placed on the opposite sides of the validator
channel lead to complex configurations. The measurement
results should not significantly vary with wobble of the
paper in the validator channel. It is also desirable for
the validator to reject bills which are likely to become
jammed in the validator.
SUMMARY OF THE INVENTION
The present invention is directed to an arrangement
for sensing the dielectric properties of a paper substrate
as the paper substrate moves through an evaluation channel.
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The arrangement includes a generating electrode on
a first side of the channel; a receiving electrode located
on the first side of the channel and spaced from the
generating electrode; a passive conducting electrode
situated on a second side of the channel opposite the first
side and overlapping with the generating electrode and the
receiving electrode; and a electronic processing
arrangement connected to the generating electrode and the
receiving electrode which evaluates the signals thereof for
changes in the detected capacitance sensed by coupling of
the electrodes via the passive conducting electrode.
According to an aspect of the invention, the
arrangement further includes a screening electrode located
on the first side of the channel and connected to the
electronic processing arrangement in a manner to diminish
capacitance due to direct coupling of the generating
electrode and the receiving electrode.
According to a further aspect of the invention, the
generating electrode is provided with an alternating
voltage high frequency signal.
According to yet a further aspect of the invention,
the passive conducting electrode has no electrical
connection with the electronic processing arrangement.
According to yet a further as~~ect of the invention,
the processing arrangement converts any high frequency
signal received by the receiving electrode into a d.c.
voltage which provides a measure of the capacitive coupling
of the generating and receiving electrodes which is greatly
changed in accordance with the banknote currency.
According to yet a further aspect of the invention,
the electronic processing arrangement uses the d.c. voltage
to assess the humidity of a substrate located in the
evaluation channel.
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The electronic processing arrangement in a
preferred aspect of the invention uses a measurement of
capacitance for determining the humidity of the substrate
and rejects the substrate when the determined humidity is
grater than a predetermined level. It also rejects dry
fraudulent bills with deviations of the dielectric
properties relative to known dielectric properties of
authentic bills.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the _~nvention are shown in
the drawings, wherein:
Figure 1 is a perspective view of the sensor
electrode system, located in a validator evaluation
channel;
Figure 2 is the block schemat:~c of the sensing
arrangement; and
Figure 3 is a schematic of thE~ arrangement for
processing the signals of the sensing arrangement.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Currency or banknote validato~_s move a banknote
along a particular path and assuming the banknote is
accepted, typically store the banknote in a stacking
arrangement. The pathway through the validator has a
number of sensors placed there along for evaluating the
banknotes as it passes the sensor. Various drive wheels
advance the banknote from the entrance to the validator to
the banknote stacking arrangement. An example of such a
validator is shown in our United States Patent 5,657,846.
A capacitive sensor 2, is shown in Figure 1 and is
located in the channel 4 through which the banknote 7 is
passed for evaluation in the direction of arrow 8. The
channel 4 includes opposed channel walls 5 and 6 which are
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made of a plastic or similar dielectric isolating material.
The channel walls 5 and 6 include slots therein for
receiving the generator electrode 11 and the receiving
electrode 12 as well as the screening electrode 14 in the
channel wall 5. Directly opposite these electrodes is a
large flat passive electrode 13 located in a slot in the
channel wall 6. This flat passive electrode 13 is situated
directly over and is parallel to the generator electrode 11
and the receiving electrode 12.
The passive electrode 13 is sized and placed within
the channel walls 6 such that the projection of electrode
13 on the wall 5 of the channel covers both the generator
electrode 11 and the receiving electrode 12. The purpose
of the passive electrode is to couple the electrodes in a
manner to be directly influenced by the change in
capacitance caused by the dielectric properties of the
banknote 7 passing between the electrodes.
The screening electrode 14 serves to reduce the
direct coupling between the generating electrode 11 and
receiving electrode 12.
As the banknote 7 is transpor~ed along the channel
4, it is located between the electrodes, and thus
significantly effects the magnitude of the capacitive
coupling of the electrodes. Generally, the banknote is
parallel to electrodes 11, 12 and 13, however, it may be
nonparallel because of some wobble on the banknote. The
exact position of the banknote between the electrodes is
not critical as the net is tolerable because capacitance is
mainly dependent on the presence of the banknote between
the electrodes and the exact location of the banknote
between the electrodes is not as significant.
It can be appreciated the sen;~ing arrangement of
Figure 1 is quite compact and rugged and there is no
requirement to electrically hard wire the passive electrode
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13 to the processing circuitry. This simplifies the
electrical connection of the capacitance sensor as validators
typically open by splitting along the pathway 4 for servicing
of sensors and removing any banknote which may have become
jammed. With a spilt validator, the components on one side of
the pathway remain stationary and components on the opposite
side of the pathway move when the validator is opened. In
this case, the channel wall 5 can be located in the stationary
part of the validator and thus, its electrical connection to
the processing circuitry is simple and straightforward, and
does not have to accommodate movement for service. The
passive electrode 13 is located in the moving part of the
housing.
In Figure 2, a high frequency generator 9 is connected
with the generating electrode 11; the feed of the high
frequency generator is also provided to the locking detector
and is used as a reference signal. The receiving electrode
12 is connected with one of the differential inputs of the
20 lock-in detector 20. Another differential input of the lock-
in detector 20 is supplied with the compensating high
frequency signal formed by the capacitance divider C1-C2.
The screening electrode 14 is connected with the
ground of the system. The signal formed by the lock-in
detector 20 is amplified by amplifier 21 and is subsequently
converted to a digital signal which may be analyzed by the
program of the central processing unit 25. At certain levels
of the signal, the banknote is rejected as having too high a
moisture level, otherwise the signal is compared to the
appropriate standard of authentic currency.
Figure 3 shows a schematic of the capacitance of the
various electrodes of the sensor and the elements of the
electronic processing arrangement that are directly as:~ociated
with the electrodes. C11-12 is the capacitance between the
generating electrode 11 and passive electrode 12; C13-12 is
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the capacitance between the passive electrode 13 and receiving
electrode 12. As evident from Figure 1, these capacitance are
the ones of plane capacitors. C11-12 is negligibly small in
the case of installed screening electrode 14. Figure 3 also
illustrates capacity divider C1, C2 for the signal of the high
frequency generator 9, input capacitance C and input active
resistances R of the inputs of the lock-in detector 20. It
can be seen that the capacitors form a capacitance bridge with
generator 9; the outputs of the bridge are connected to the
inputs of the lock-in detector 20. The bridge may be balanced
by adjusting capacitance divider C1, C2.
When the bridge is unbalanced, a d.c. voltage is
produced at the output of the lock-in-detector 20. The
resulting voltage is a direct function of the unbalanced state
of the bridge.
Since the sensor has small plate sizes, the
interelectrode capacitances are small, generally not exceeding
10 pF. The input capacitances of the lock-in detector are of
the same order of magnitude. To achieve a useful sensitivity,
a high generating frequency is used. It has been determined
that the preferred frequency range is between 50 - 150 MHz.
At these frequencies, the impedances of the bridge
capacitances are smaller than the input active resistances R
of lock-in detector and, therefore, the input resistances only
marginally affect the phase and amplitude characteristics of
the bridge.
It should be noted that the elements C1 and C2 can be
excluded from the circuit if their absence does not saturate
lock-in detector 20. In their absence, the system can be
balanced by varying the input voltage shift of d.c. amplifier
21.
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V~hen the currency paper moves between the
electrodes of the sensor, the capacit;~nces of C11-13 and
C13-12 increase and unbalance the cap~~citance bridge. As
the currency paper is situated in pra~~tically the constant
field of the capacitors C11-13 and C13-12, the magnitude of
the disbalance signal is isolated from effects of wobble
the paper in the validator channel an~~ essentially depends
on the dielectric properties of the currency paper. Thus
by measuring the magnitude of the unbalanced signal, the
system determines the authenticity of the dielectric
properties of the currency paper.
Wet currency paper fed to the validator may jam the
transport mechanism. Therefore, it is important to
evaluate the moisture content of the currency paper as
early as possible. The dielectric constant of water is
approximately 10 times larger than the dielectric constant
of dry currency paper. As such, currency paper having high
humidity provides high capacitance and produces a large
signal in the sensor. Thus, the magnitude of the output
signals gives information about the humidity of the
currency paper. If the measured signal is too high, the
banknote is rejected.
It should be understood by those skilled in the
art, that modifications may be made without departing from
the spirit and scope of the invention as defined in the
claims. Accordingly, reference should be made primarily to
the accompanying claims, rather than the foregoing
specification, to determine the scope of the invention.
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