Abstract:
A capacitance sensor measures a coin to determine geometrical parameters thereof and to allow assessment of different geometrical parameters of a coin which influence the measured capacitance. In a preferred arrangement, a mechanical arrangement alters the capacitance between two electrodes as a function of the diameter of the coin. This assessment is used in combination with a further measured capacitance which is a function of coin diameter, coin thickness and surface relief of the coin. The initial evaluation of diameter is useful in determining the separate influences of coin thickness and surface relief. The invention is also directed to the method of coin evaluation.

Description:
The present application is a continuation of International Application PCT/CA00/00072 filed Jan. 28, 2000. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to coin evaluators and in particular, to a capacitance sensor for coin evaluation. There are a number of different sensors designed to evaluate metallic coins. Inductive sensors are commonly used and are sensitive to many inherent characteristics of coins simultaneously. Inductive sensors are responsive to magnetic permeability of the coin material, conductivity, quantity of the material and size of the coin. The simplest inductive sensors produce a response that depends on all of these characteristics, and as such, the resulting security is not high. Some inductive sensors attempt to measure the separate contributions of these characteristics, however, this requires a significant increase in the sophistication of the coin acceptor and its electronic evaluation system. 
   An alternate approach to improve the security of coin acceptors, is to have additional non inductive sensors. These sensors can provide a separate evaluation of the same characteristics that are influencing the inductive sensor signal (e.g., the coin diameter) or other characteristics such as coin weight. In the first case, the additional sensors can distinguish some of the characteristics influencing the inductive sensor signal while in the second case additional information is obtained. 
   In general, coin forgery is unprofitable primarily due to the low value of the coins. Unfortunately, coins of two countries which are of drastically different real values may be indistinguishable by coin acceptors because the coins are basically the same, other than the impressed patterns on the coins. Similarly, some industrially produced metallic washers without stamping on their surfaces are also indistinguishable from some coins by many coin acceptors. 
   There remains a need for an effective sensor which is most effective and is effective in distinguishing coins. 
   SUMMARY OF THE INVENTION 
   The capacity sensor of the present invention distinguishes coins by measuring the effect of the coin&#39;s periphery, and the parameters of the pattern impressed on the coin faces. In a preferred embodiment, the characteristics of the edge surface of the coins is measured. 
   According to the present invention, a capacity sensor arrangement evaluates geometrical parameters of coins in combination with a measurement of other parameters which impact capacitance to improve the security level of the coin acceptor. 
   According to the present invention a capacity sensor measures geometrical parameters of coins and utilizes the same electronic evaluation system for measuring all the parameters analyzed. 
   According to a preferred aspect of the invention, the capacity sensor measures the geometry of the periphery of the coin, and in particular, the diameter of the coin, the thickness of the coin, and an assessment of the pattern impressed on the faces of the coin. 
   The preferred capacity sensor includes a measuring capacitor and an auxiliary mechanical system. The measuring unit consists of two flat multilayer electrode systems, mounted parallel to each other on the opposite sides of a coin acceptor channel. The electrode systems are sized to cover the largest coin that the acceptor will accept. Each electrode system includes an active electrode facing the channel of the acceptor that includes a thin insulating covering layer thereover. A screening electrode is situated on the opposite side of the electrode system. It is separated from the active electrode by a thick insulating layer. Active electrodes of the two electrode systems form the measuring capacitor. The walls of the coin acceptor are inclined from vertical so that a coin moves closer to one of the walls. The first electrode system is mounted on the inclined wall and is fixed. The second electrode system selectively is mounted on the opposite wall and selectively moves towards the first electrode system to clutch a coin therebetween. 
   The first electrode system includes an additional electrode that is electrically connected with the active electrode of the same electrode system. An electromechanical system of the coin acceptor halts the coin inside the channel between the electrode systems, shifts the movable electrode system until the coin is clutched between the movable and fastened electrode systems, and subsequently shifts the movable system back releasing the coin for further movement along the channel. The auxiliary mechanical system includes a lever fastened to a passive electrode by a common shaft. The auxiliary mechanical system is mounted so that the lever is situated in the channel of coin acceptor and is displaced when the coin moves inside the channel. The displacement of the lever causes a displacement of the passive electrode relative to an additional electrode and the screening electrode of the first electrode system. With movement of the lever, the ratio of the passive electrode covering the additional and screening electrodes changes. 
   The electronic system is connected with the active electrodes of both electrodes systems; it measures the variation of the measuring capacitor capacity during the movement of a coin along the acceptor channel when the coin is between the electrode system and when the coin is clutched between the electrode systems. 
   When the coin moves along the coin acceptor channel past the lever, the maximum rotation angle value of the lever depends on the coin diameter. When the sharp edge of the lever slides over the edge surface of the coin the movement of the lever reflects the form of this surface. The corresponding rotation of the lever causes the shift of the passive electrode and leads to the variation of capacitive coupling between the additional and screening electrodes, and causes a variation of the measuring of the measuring capacitor. Any abrupt changes due to edges of the coin cause a change in this value. The coin continues along the coin acceptor channel until it is located between the electrode systems and the value of the measuring capacitor increases. The measured value is primarily a function of coin diameter and thickness. When the coin is clutched between the electrode systems the main contribution to the capacity of the measuring capacitor is made by the capacities between the measuring electrodes and faces of the coin and depend on the impressed pattern parameters, namely, on the depth of the relief and the ratio of concave and convex surface fragments on both faces of the coin. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are shown in the drawings, wherein: 
       FIG. 1  is a perspective view of the capacity sensor for measuring of coins; 
       FIG. 2  is a perspective view of one half of a coin illustrating the changing cross-section of the coin; 
       FIG. 3  is a schematic figure of the electrode systems positions when the coin is initially received; 
       FIG. 4  is a schematic figure of the electrode systems positions when the coin is clutched between the electrode systems; 
       FIG. 5  is the equivalent scheme of capacities of electrodes systems and the capacities between active electrodes and the coin when located between the electrodes; 
       FIG. 6  is the block diagram of a version of the electronic scheme for registration of a generator frequency shift with frequency output; 
       FIG. 7  is the block diagram of a version of the electronic scheme using a frequency discriminator for registration of the generator frequency shift with analogous output; and 
       FIG. 8  is the block diagram of the electronic scheme using a capacitance bridge. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A perspective view of a capacity sensor for measuring the geometric characteristics of coins is shown in FIG.  1 . Two flat multilayer electrode systems  1  and  2  are positioned on opposite walls  3  and  4  of the coin acceptor channel  33 . The electrode system  1  is fixed on the wall  3  of the channel and the electrode system  2  is movable towards or away from electrode system  1  by an electromechanical system  45  of the coin acceptor in the directions shown by the arrows  5 . The movable electrode system  2  moves between an end position where the electrode system is located in the plane of channel wall  4  and a coin clutching position. Electrode systems  1  and  2  are separated by the width of the coin acceptor channel  33  when electrode system  2  is at the end position and the electromechanical system accurately controls the position of electrode system  2 . The coin clutching position changes according to the thickness of the coin, but allows clutching of any type of coins that can be received in the coin acceptor. 
   There is also an arrangement that halts the coin between the electrode systems  1  and  2  before clutching. An example of the arrangement is the blind  6  that shuts the acceptor channel  33  immediately downstream of the electrode systems  1  and  2 . Movement of the blind  6  is shown by arrows  7  and is controlled by the electromechanical system. 
   An auxiliary mechanical system of the sensor includes the lever  8  fastened to shaft  34  which causes the sympathetic rotation of the passive electrode  9 . The lever is located in the acceptor channel  33  and the passive electrode is located behind the electrode system  1  and parallel to the channel. 
   The sequence of mechanical operations can be appreciated with reference to FIG.  1 . In the initial state the blind  6  shuts the acceptor channel  33 . The channel is inclined from the vertical and any inserted coin  10  moves freely along the channel in the direction shown by the arrow  11  due to a gravity bias. During this movement, one face of the coin slides along the channel wall  3  due to an inclination of the channel. While moving the coin strikes the lever  8  and the sharp edge  12  of the lever  8  slides along the edge surface of the coin and traces the shape thereof. The movement of the lever causes a sympathetic rotation of the passive electrode  9  as the electrode is fastened to the lever  8  by common shaft  34 . As such, any abrupt transitions due to change in the periphery of the coin are translated to abrupt movements of the passive electrode and a change in capacitance. 
   The coin continues along the channel  33  into the space between the electrode systems  1  and  2  and is stopped by the blind  6 . The electrode systems sense the coin (increase in capacitance) and electrode system  2  moves until the coin is tightly clutched. After tight clutching, electrode system  2  moves to the end position of  FIG. 1 , the blind  6  opens t he channel and the coin continues its movement along the channel. 
   For some applications it is possible to use a simplified algorithm where the channel is always open. In this case, the coin is not clutched by the electrode systems and electrode system  2  is fixed or remains in the clear end position. This feature of tightly clutching the coin in some applications can be turned off. For example, bent coins typically would be rejected and it may be desirable to reduce the security level by turning off the clutching feature. 
   In the preferred embodiment, the coin passes the sensor as it moves along the channel and is then stopped between the electrode systems and evaluated. This evaluation can be appreciated with reference to  FIG. 2  which shows the cross-section  15  of a coin and the relief of the impressed pattern which affects the measured capacitance. Almost any coin has a brim  13  that is higher than the convex fragments of the impressed pattern  14  on both faces of the coin. The maximum thickness of a coin corresponds to the thickness of the brim. 
     FIG. 3  shows the cross-section of the electrode systems of the sensor corresponding to the initial state of the electrode system  2 , aligned with wall  4  of the coin acceptor channel. The planes of electrode systems  1  and  2  are parallel to each other. Each electrode system is a flat multilayer system and contains an active electrode  16 , separated from the acceptor channel by thin insulating covering  17 . The screening electrode  19  is located on the opposite side of the active electrode  16  of electrode system  1 . It is separated from the active electrode  16  by thick insulating layer  18 . The size of the active electrodes is large enough to cover any type of coin that the acceptor can receive. In contrast to the movable electrode system  2 , the electrode system  1  contains an additional electrode  20  that is placed above the screening electrode  19 ; the additional electrode is electrically connected with the active electrode  16  of the same electrode system. The passive electrode  9  of the auxiliary mechanical system is situated over the additional electrode  20  and screening electrode  19  of the electrode system  1 . The separation distance between the passive electrode  9  and the electrode system  1  is constant. The rotation of passive electrode  9  caused by lever  8  is indicated by arrows  21 . While the coin  22  moves between the electrode systems, it slides over the surface of the fixed electrode system  1 . The screening electrodes  19  of each system diminish the influence of both the surrounding electronic elements and movable metallic and dielectrical details of the coin acceptor on the sensor. Another advantage of the screening electrodes for certain types of registering electronic schemes is the diminishing of the radiation of the sensor itself. 
     FIG. 4  illustrates the cross-section of the electrode systems and the cross-section  15  of coin  10 , when the coin is clutched between the electrode systems  1  and  2 . The electrode systems  1  and  2  engage the brim of the coin along its perimeter on both sides of the coin  10  producing a variable spacing of the electrodes from the surface pattern of the coin which is determined by the particular coin. 
     FIG. 5  shows the equivalent scheme of interelectrode capacitances of the electrode systems and the capacitances between active electrodes and the coin when the latter is located between the electrodes. The scheme implies that the screening electrodes and the active electrode  16  of the movable electrode system  2  are electrically interconnected with a common wire of the electronic registering system. The input of the electronic registering system is connected with the active electrode  16  of the stationery electrode system  1 . C 1  and C 2  are the capacitances between the active electrodes  16  of electrode systems  1  and  2  and the nearest coin faces, respectively. C 3  is the capacitance between the active electrodes  16  in free regions were there is no coin. C 4  and C 5  are the capacitances between the passive electrode  9  and the electrodes  20  and  19  of the electrode system  1 , respectively. The capacitance between the active electrodes of both systems and the screening electrodes is not taken into account as it does not change during the coin acceptor operation. The total capacitance of the measuring capacitor is equal to the capacitance between the points  23  and  24  of the complex system of capacitances shown in FIG.  5 . The electronic registering system  25  measures the total capacitance of the measuring capacitor. In the initial state, when there is no coin in the channel, the capacitors C 1  and C 2  should be excluded. 
   When the coin moves along the channel, it strikes and moves the lever of the auxiliary mechanical system causing the movement of the passive electrode. The movement of the passive electrode changes the capacitances of C 4  and C 5 , and, therefore, changes the total capacitance of the measuring capacitor. The maximum change as the coin moves past the lever corresponds to the moment when the sharp edge  12  of the lever achieves the highest point and, thus, characterizes the diameter of the coin. The movement of this sharp edge along the surface of the brim generally traces the shape of the brim and changes the measured capacitance. Analyzing the changes in measured capacitance during the movement of the coin past the lever allows determination of both the diameter of the coin and the shape of the brim. 
   The coin continues to move along the channel until it is stopped between the electrode systems. This causes the emergence of C 1  and C 2  capacitors in the equivalent scheme ( FIG. 5 ) and the decrease of the capacitance of C 3  capacitor. The capacitances C 4  and C 5  return to their initial values as the coin has moved past lever  8  and lever  8  has returned to its initial position. The capacitors C 1  and C 2  represent the capacitances of the active electrodes  19  and the nearest surface of the coin. These capacitances increase as the diameter of the coin increases and is a function of the coin thickness. Therefore total variation of the measured capacitance simultaneously depends on two coin characteristics, the diameter and the thickness. The resulting capacitance variation is registered by the measuring system  25 . As the coin diameter has also been determined independently, the thickness of the coin can be determined from the comparison of the results. When the coin is tightly clutched between the electrode systems, the equivalent scheme of the measuring capacitor is the same as shown in  FIG. 5 , but the interelectrode distance in C 1  and C 2  capacitors, varies in accordance with the impressed relief on the faces of the coin. 
   The smaller the thickness of the insulating coverings  17 , the more sensitive the system is to the influence of surface relief. Care should be exercised as too small a thickness reduces effectiveness. With appropriate coverings  17 , the dominating contribution to the capacitances of C 1  and C 2  is delivered by the brim of the coin. Thus the capacitance of the measuring capacitor during the clutching of the coin between electrode systems depends on the peculiarities of the relief impressed on the faces of the coin, namely on its depth and the ratio of convex and concave fragments. This capacity is also registered by the electronic registering system. Note, that it is integral characteristics of the relief of the coin that are registered. These characteristics can occasionally coincide for different types of coins but the probability of this event is low. 
   There are many types of electronic schemes suitable for application as an electronic registering system of the sensor described. Some examples of electronic arrangements that utilize the capacitance sensor are generally shown in  FIGS. 6 ,  7  and  8 . Other arrangements are possible. 
     FIG. 6  shows a block diagram of an electronic registering system where the measuring capacitor is an element of the oscillatory circuit of the generator, and the variation of its capacity is measured by the variation of the generator frequency. The oscillatory circuit in the generator  26  includes the connected in parallel inductance L 1  and the measuring capacitor C 7  of the sensor. It can also include, if necessary, capacitor C 6  which can be adjusted in value to obtain the necessary initial frequency. The oscillatory circuit is connected via the capacitor C 8  with the diode VD 1  intended for the electronic frequency tuning. As the frequency of the generator  26  depends on the capacitance in the oscillatory circuit, it depends on the capacitance of the measuring capacitor. This frequency can be directly measured by the timer of the microcontroller system of the coin acceptor but in the case of too high frequency it can be divided or shifted. 
   In the arrangement of  FIG. 6  the frequency is transferred by the mixer  27  and the output signal of the crystal generator  28  is used as a second frequency. In this case the timer measures the low frequency corresponding to the differences of the frequencies. It is better to use frequency transfer as compared to frequency division, as the resulting system is more sensitive. The fine tuning of the output frequency of this electronic registering system is carried out by the digital-analog converter mounted on the microcontroller board  29 . 
     FIG. 7  shows a block diagram of an electronic registration system with the same generator  26  as shown in  FIG. 6 , but the measurements of its frequency variations are carried out using the frequency discriminator  30 . The oscillatory circuit L 2 -C 9  is tuned to the initial frequency of the generator  26 . The output analogous signal of the discriminator  30  is registered by the analog-digital converter being a part of the coin acceptor microcontroller board. 
     FIG. 8  shows an electronic registering system containing a measuring capacity bridge with the measuring capacitor C 7  of the sensor in one arm. The ac power is supplied by the generator  31 . To form the capacity bridge the capacitors C 8 , C 9 , C 10 , and C 11  are used. The ac voltage that appears when the bridge is unbalanced is detected by the lock-in detector  32 ; the output voltage of the generator  31  is used as a reference voltage. Similarly to the previous arrangements, the output voltage of the lock-in detector is registered by the analog-digital converter. 
   Application of the described capacitance sensor together with sensors of other coin parameters increases the security of coin acceptors. In many cases, use of the capacity sensor allows simpler versions of other sensors. It is also possible to use the simplified version of the capacitance sensor which does not have the clutching function. It is also possible to allow the switch-on or switch-off of a certain sensor&#39;s functions in accordance with the demands to coins acceptors comprised in a given equipment. For example, to switch-on or switch-off the function of the imprinted relief parameters determination can be implemented simply by changing the algorithm of coin acceptor operation. When this function is switched off, the coin acceptor can accept deformed coins that would have been rejected during the process of impressed relief parameters determination. 
   There are some application where the switch-off of the above function is reasonable in spite of the deterioration of the security of the coin acceptor. 
   It should be noted that the proposed design of electrodes that are associated with the auxiliary mechanical system need not be incorporated into electrode system  1  and other arrangements are possible. For example, the additional and screening electrodes can form a separate electrode system. 
   It should be understood by those skilled in the art that obvious structural modifications can be made without departing from the spirit of the invention. Accordingly, reference should be made primarily to the accompanying claims, rather then the foregoing Specification, to determine the scope of the invention.