Abstract:
A capacitor sensor for assessing dielectric properties of currency paper uses a transmitting dielectrode on a first side of an evaluation channel and a receiving electrode on the same first side of the evaluation channel. A passive electrode is located on the opposite side of the evaluation channel and overlays with the transmitting and receiving electrodes. An electronic processing arrangement is connected to the transmitting and receiving electrodes and evaluates the signals for changes in the capacitance coupling of the electrode. This coupling is directly related to the properties of the paper passing between the passive electrode on one side and then transmitting and receiving electrodes on the other side.

Description:
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 is 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. When 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 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 humidity and other parameters of the paper, evaluated by a validator, are important for the validator&#39;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. 
     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 aspect 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. 
     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 invention are shown in the drawings, wherein: 
     FIG. 1 is a perspective view of the sensor electrode system, located in a validator evaluation channel; 
     FIG. 2 is the block schematic of the sensing arrangement; and 
     FIG. 3 is a schematic of the arrangement for processing the signals of the sensing arrangement. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Currency or banknote validators 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 U.S. Pat. No. 5,657,846. 
     A capacitive sensor  2 , is shown in FIG.  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 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 transported 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 sensing arrangement of FIG. 1 is quite compact and rugged and there is no requirement to electrically hard wire the passive electrode  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 FIG. 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  10  and is used as a reference signal. The receiving electrode  12  is connected with one of the differential inputs of the locking detector  10 . Another differential input of the locking detector  10  is supplied with the compensating high frequency signal formed by the capacitance divider C 1 -C 2 . 
     The screening electrode  14  is connected with the ground of the system. The signal formed by the locking detector  10  is amplified by amplifier  11  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. 
     FIG. 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 associated with the electrodes. C 11 - 12  is the capacitance between the generating electrode  11  and passive electrode  12 ; C 13 - 12  is the capacitance between the passive electrode  13  and receiving electrode  12 . As evident from FIG. 1, these capacitance are the ones of plane capacitors. C 11 - 12  is negligibly small in the case of installed screening electrode  14 . FIG. 3 also illustrates capacity divider C 1 , C 2  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  10 . 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  10 . The bridge may be balanced by adjusting capacitance divider C 1 , C 2 . 
     When the bridge is unbalanced, a d.c. voltage is produced at the output of the lock-in-detector  10 . 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 effect the phase and amplitude characteristics of the bridge. 
     It should be noted that the elements C 1  and C 2  can be excluded from the circuit if their absence does not saturate lock-in detector  10 . In their absence, the system can be balanced by varying the input voltage shift of d.c. amplifier  11 . when the currency paper moves between the electrodes of the sensor, the capacitances of C 11 - 13  and C 13 - 12  increase and unbalance the capacitance bridge. As the currency paper is situated in practically the constant field of the capacitors C 11 - 13  and C 13 - 12 , the magnitude of the disbalance signal is isolated from effects of wobble the paper in the validator channel and 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.