This project, a coin detector and counter from 1988, shows how to discriminate between different types of coin by measuring the result when they pass through an electromagnetic field.
The system uses an 80kHz oscillating field which allows good discrimination of the different types of coins (US coins).
A coin detector and counter comprising a coreless oblong transmitter coil and a coreless oblong receiver coil spaced apart on opposite sides of a coin path arranged to cause the entire diameter of each coin to pass between the coils. The maximum peak voltage generated in the receiver coil upon passage of each coin is measured as a determination of the conductance of each coin. By comparing the measured conductance of each coin with the known conductance of coins, each coin is thereby identified and counted.
One object of this design is to provide a system for coin identification that does not require close tolerances, and therefore cannot be so readily jammed as prior systems.
Another object is to provide a system that cannot readily be fooled by objects designed to simulate coins.
The system of this design is also much simpler and less expensive than prior systems. One reason is that the path of each coin need not be carefully controlled, as in prior systems.
Several prior systems for coin identification have included the use of a transmitter coil on one side of the coin and a receiver coil on the other side. This design relates to the identification of selected objects within an assorted collection of related and/or unrelated objects, based on the different ability of each successive object to reduce the intensity of an electromagnetic field. More particularly, the design relates to the identification of coins by passing assorted collections of coins and/or other objects through an electromagnetic field, and measuring the different degrees of reduction in field intensity caused by each coin or other object as it traverses the field.
Fare boxes, vending machines, and other coin counters have previously been designed to distinguish coins by sensitive mechanical, electrical and/or electromechanical and optoelectric devices for measuring weight, diameter, and other coin characteristics. The complexity and sensitivity of such prior devices have caused them to be somewhat unreliable because they are too readily jammed by foreign objects such as screws, washers, slugs and other debris maliciously inserted into coin slots.
Various input voltages and frequencies have been connected to the transmitter coil, and various analytical treatments of the output signal from the receiver coil have been tried.
For example, U.S. Pat. No. 4,493,411 covers a "Self Tuning Low Frequency Phase Shift Coin Examination Method And Apparatus" wherein coin identification is achieved by measuring the phase shift between the transmitted signal and the received signal, and then comparing the measured shift with prerecorded phase shift data for coin of known identity.
U.S. Pat. No. 4,086,527 is similar, except that a variable frequency input is connected to the transmitter coil, and the output from the receiver is measured at several different frequencies. Since several frequency-dependent tests are contemplated, each coin would have to be held in the field for whatever time period is necessary for that purpose.
Such systems require phase-shift comparisons because they are designed to measure the capacitance of each coin, as an indication of its denomination. However, the capacitance of one coin does not differ from that of a different coin to as great a degree as conductance differs from one coin to another. Thus, the present design recognizes that it is necessary to test for conductance as the primary or sole indication of the identity of a coin being tested.
The system of the design allows voltage differences in the output signal from the receiving coil to be measured as the primary or sole basis for coin identification. Because of the novel geometry of the coils, such voltage differences are indicative of the conductance of the whole coin; not just the conductivity of the alloy composition used to fabricate the coin.
In a prefered embodiment, each coil of the design is wound on a substantially rectangular base, such that the width of each coil is slightly less than the diameter of a dime, and the length of each coil is slightly greater than the diameter of a half-dollar.
The dime and half-dollar are selected because they are the smallest and the largest coin in the assortment to be tested. When other coin sets are to be counted, the preferred coil width is substantially equal to the diameter of the smallest coin, and the coil length is substantially equal to the diameter of the largest.
The transmitter coil is placed on one side of the coin path, and the receiver coil is placed on the opposite side. Each coil is arranged such that the windings lie substantially in a plane parallel to the coin path, and the length of each coil is perpendicular to the coin path. Thus, each coin passes between the coils in a direction that traverses the field from one side of the coils to the other side.
When the center of a dime reaches the center of the field it momentarily fills substantially the entire width of the field, and therefore the field "sees" only the dime at that instant, and not any significant portion of the coin ahead of it or behind it in the coin path. Each larger coin also interacts with the field across its entire diameter only at the moment its center passes the center of the field, because the length of the coils is substantially equal to or slightly greater than the diameter of the largest coin.
Circular coils are not suitable because a dime-sized coil pair could never effectively test a larger coin, and a half-dollar-sized coil pair would frequently interact with more than one coin at a time.
The system of the design reliably identifies and counts each coin as it passes through the field, regardless of the rate of motion of the coins, which pass through the field in a steady stream, or in an intermittent stream, such as in a fare box, for example. No mechanism is required to control the position or speed of coins as they pass through. This greatly simplifies the coin handling apparatus required for feeding coin through the field. The only requirement is that no two coins be allowed to overlap within the space between the coils.
As shown in FIG. 1, the simplest example of the design consists of an a.c. source 11 connected to transmitter coil 12, and a volt meter 13 and/or an oscilloscope 14 connected to receiver coil 15. The coin path between coils 12 and 15 cuts across the width of the coils, i.e., at an angle to the plane of the paper. The distance between the coils must be slightly greater than the thickness of the thickest coin to be counted. Preferably the distance between the coils is three to five times greater than the minimum required, so that the chance of jamming the slot is minimized.
Each coil is wound on an oblong base, which may be either oval-shaped or rectangular. The dimensions of each coil are selected such that the diameter of the smallest coin to be counted, a dime for example, is slightly greater than the width of each coil, and the diameter of the largest coin, a half dollar for example, is slightly less than the length of each coil. The coils are coaxial and aligned such that all corresponding sides are parallel to each other.
An assortment of coins to be counted is passed one at a time between the coils. No control of coin speed or position is require except that the full diameter of each coin must pass between the coils.
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