EP0704825B1 - Device for authenticating coins, tokens or other flat metal objects - Google Patents

Abstract

The coin testing system has the coins (M) introduced into a channel section that is inclined such that the coins roll past a pair of inductive sensors. The channel is of a rectangular cross section and is tilted such that the coins move against one side wall that is ridged to facilitate motion. The inductive sensors have coils (9, 10) set into one side wall and metallic discs (11, 12) set in the other wall such that the coins pass in between. The sensors provide inputs to an electronic circuit (14) that measures the time response of the resistance of each one and determine maximum values. The circuit uses the values to determine the coin thickness.

Description

The invention relates to a device for checking the authenticity of coins, tokens or other flat metallic objects of the type mentioned in the preamble of claim 1.

Such facilities are suitable, for example, as cashier stations in public telephone stations, vending machines, energy meters, etc.

A device for checking the authenticity of coins of the type mentioned in the preamble of claim 1 is known from EP 304 535 B1. The device has three inductive sensors working independently of one another for determining the thickness, the alloy composition and the diameter of the coin to be tested. These inductive sensors are designed as double coils, which are arranged on both sides of the coin channel and are electrically connected in parallel or in series, so that measurement variations due to the coin jumping or jumping in the coin channel can be partially compensated for, with bouncing and jumping lifting off the bottom of the coin channel or a change in the position with respect to the side walls of the coin channel is meant. However, the use of double coils has the disadvantage that the alloy composition and the thickness of the coin cannot be determined independently of one another. The inductive sensors are part of a parallel resonance circuit, in which the shift in the resonance frequency caused by the coin and the changed quality are measured. The measured changes in these parameters serve as decision criteria for the acceptance or rejection of the coin. It is also envisaged to design an inductive sensor for determining the alloy composition as a simple coil, which is attached only on one side of the coin channel.

From GB 1 397 083 a coin detector with inductive sensors is known which are operated at frequencies from 3 kHz to 1 MHz. The inductive sensors are arranged in resonant circuits and in bridge circuits. The resonance frequency in the presence of the coin is used to characterize the coin.

From EP 213 283 it is known to operate several oscillator circuits with inductive sensors with a single amplifier in order to reduce the adjustment effort and drift problems.

From GB 2 266 804 as well as from DE-U1 G 90 13 836.8 the use of energy absorbing elements is known in order to achieve rolling without jumping or jumping of the coin in the area of the sensors. Such energy absorbing elements are preferably ceramic plates which are arranged in the coin channel in such a way that every coin thrown into the coin inlet opening impinges on them.

From DE 30 07 484 it is known to design the lower side wall of a coin channel inclined by a predetermined angle with respect to the vertical along the running direction of the coin with ribs referred to as guide rails.

The invention has for its object to provide a device for checking the authenticity of coins, in which the alloy composition and the thickness of the coin can be determined independently of one another, in which jumping or jumping of the coin is firstly excluded as far as possible and secondly a remaining hopping or Jumping leads to the smallest possible measurement spread.

According to the invention, the stated object is achieved by the features of claims 1 and 2.

Exemplary embodiments of the invention are explained in more detail below with the aid of the drawing, the term coin M also being used below to refer to tokens or other flat metallic objects.

Fig. 1

einen Münzkanal einer Prüfeinrichtung,

Fig. 2

den Münzkanal im Querschnitt,

Fig. 3, 4

Messwertdiagramme,

Fig. 5

ein Sensorsignal und

Fig. 6

eine elektronische Schaltung.

Show it:

Fig. 1

a coin channel of a test facility,

Fig. 2

the coin channel in cross section,

3, 4

Measurement diagrams,

Fig. 5

a sensor signal and

Fig. 6

an electronic circuit.

1 shows a device for checking the authenticity of coins, tokens or other metallic objects with a coin channel 1, which is preferably formed as a recess in a body 2 made of two plastic parts. The coin channel 1 is delimited by the base 3, a lower and an upper side wall 4 or 5 and a ceiling 6. The lower side wall 4 is provided with integrally formed ribs 7 which are formed in the running direction of the coin M. The coin channel 1 is inclined in the running direction of a coin M to be checked and the two side walls 4 and 5 are inclined at an acute angle of typically 10 ° with respect to the vertical V, so that the coin M to be checked rolls down the floor 3 along the coin channel 1 or slides down and ideally lies flat with one side surface on the ribs 7 of the lower side wall 4. The side walls 4 and 5 each have recesses on the side facing away from the coin channel 1 for receiving staggered coils 9 and 10 and, optionally, metallic plates 11, 12. The coil 9 and the plate 12 are located on the lower side wall 4, which is why they are shown in dashed lines. For the sake of clarity, the recesses are only shown in FIG. 2. The plates 11 and 12 are mounted opposite the coils 9 and 10, respectively. They are preferably round or square, but can also have any other geometric shape. Each coil 9, 10 and possibly the metallic plate 11 or 12 arranged in the opposite side wall 5 or 4 form an inductive sensor. The two coils 9 and 10 have two connections, one of which is led to a common electrical ground connection m, the other to a switch 13, so that they can be connected to an electronic circuit 14 for electrically independent operation. The device further contains a control and evaluation unit 15, for example in the form of a microprocessor, for evaluating the output signal supplied by the electronic circuit 14 and for controlling the device. The circuit 14 and the microprocessor 15 are designed to derive discrete values from the signals measured with the coils 9 and 10, which values are a measure of the alloy or the thickness d of the coin M. The coin M is only considered to be genuine and accepted by the test device if these values match predetermined values within predetermined tolerances, otherwise it is rejected.

2 shows the coin channel 1 in cross-section at the height of the coil 10. The ribs 7 are arranged at a mutual distance a, which is preferably a = 7.25 mm. The shape of its surface facing the coin channel 1 is cylindrical, its radius of curvature R being of comparable size to the distance a: R≅a. A somewhat larger value of R = 8 mm is preferred. The ribs 7 are naturally separated by depressions 16, the depth of which is approximately 0.5 mm. The depressions 16 have a flat surface 17 in the area of the greatest depth between the ribs 7, so that the side wall 4 has a minimal wall thickness in the area of the recesses 8, which is only to be expected due to the material properties of the body 2 and the coins M to be expected mechanical loads, but can be determined independently of the radius of curvature R and the distance a. A minimum wall thickness of 0.6 mm is preferably provided, so that the coil 9 mounted in the recess 8 in the lower side wall 4 is at a fixed distance of 1.1 mm from an ideally rolling coin M. The ribs 7 are also there to prevent unwanted sticking or even sticking of a wet coin.

The formation of the ribs 7 as cylindrical surfaces with a comparatively large radius of curvature R results in a larger contact surface between the lower side wall 4 and the coin M than is the case with ribs according to the prior art. This means that an impact of a coin M directed against the lower side wall 4, which is not yet ideally flat, is associated with a relatively high damping, so that the coin M bounces and jumps in the area of the coils 9 and 10 only rarely occurs even if the coin is damaged such as scratches or peaks. The extent to which the jumping and jumping of the coin M can also be achieved with ribs 7, the radius of curvature R of which is smaller than the distance a and, for example, is only a / 2, can easily be determined by means of experiments. The shape does not have to be exactly that of a cylinder.

The high damping of the coin M crashing against the lower side wall 4 also results in significantly lower noise emissions than with the conventional design of the ribs 7.

In another embodiment of the invention, a plate parallel to the side wall 4 is loosely fixed in the lower side wall 4 in the area of the coils 9 and 10 instead of the ribs 7. The plate has a comparatively small mass compared to the masses of the coins to be tested and consists e.g. made of metal or ceramic. It is used to absorb the energy of the jumping coin M if the coin strikes the plate and thereby dampens the jumping of the coin M.

According to a further development of the invention, in addition to the mechanical precautions to prevent the M jumping and / or jumping, metrological improvements are also provided which further reduce the influence of any remaining hopping or jumping on the measurement of the important alloy composition and coin M size.

As far as the considerations apply to both the coil 9 and the coil 10, the following is the For the sake of simplicity, the reference symbol S instead of the reference symbols 9 and 10. A coil S means one of the coils 9 or 10. The coil S is characterized electrically by its inductance L _S and its ohmic internal resistance R _S. It represents an inductive sensor. The above-mentioned combination of the coil S with one of the plates 11 and 12 represents another inductive sensor. When the coin M passes the coil S, the values L _S and R _{S change} briefly due to of physical interactions between the coil S and the coin M. The internal resistance R _S contains a static component R _{S, DC} and a dynamic component R _{S, AC} (ω), which depends on the angular frequency ω of the current flowing through the coil S, of depends on the physical properties of the coin M, on the geometry of the coil S and, in particular, on the distance between the coil S and the coin M. As soon as the coin M comes into the measuring range of the coil S while rolling along the coin channel 1, its internal resistance R _S increases. The typical time course of the internal resistance R _S is shown in FIG. 5. In order to avoid an influence of the diameter of the coin M on the measurements of the thickness d and the alloy composition, the diameter of the coil S is selected to be smaller than the diameter of the smallest coin M to be measured and the coil S is on the side wall 4 or 5 of the coin channel 1 is arranged at an appropriate height, so that the smallest coin M to be tested briefly completely covers the coil S during the passage. The diameter of the coil S is, for example, 14 mm. The resistance of the lead wires is comparatively low. Coils 9 and 10 are particularly suitable for wound coils with a ferrite core. The design of the coils 9 and 10 as single coils arranged on only one side of the coin channel 1 and their complete electrical isolation avoids the loss of sensitivity associated with double coils.

The electronic circuit 14 operates the coil S in a series resonance circuit and delivers at its output an analog signal which is proportional to the internal resistance R _{S of} the coil S. When a coin M passes through the measuring range of the coil S, the course of this output signal over time is recorded and stored by the microprocessor 15 by means of an analog / digital converter as a sequence f1 of digital values. The microprocessor 15 then carries out a detailed analysis, which is explained below, the result of which are two values, for example the values K ₁ and K ₂ described below, which are used to decide whether to accept or reject the coin M.

The coil 9 is located on the lower side wall 4, on which the coin M moves lying along, so that the distance between the coil 9 and the side surface of the coin M is fixed and is, for example, 1.1 mm. The coin M is made of either a single alloy or several alloys. The internal resistance R _{9 of} the coil 9 measured in the presence of the coin M depends almost exclusively on the material of the coin M if the frequency ω of the current flowing through the coil 9 is selected appropriately. 3 shows the internal resistance R ₉ as a function of the thickness d of the coin M for coins made from different alloys L1, L2 and L3, the coin M being in a symmetrical position in front of the coil 9 during the measurement. It can be seen from this that the internal resistance R _{9 is} practically independent of the thickness d. With the coil 9 is therefore an important first characteristic size of the coin M, which is almost exclusively from its Alloy or alloy composition depends, easily determinable.

The distance between the coil 10 and the coin M depends on its thickness d. In the coil 10, the internal resistance R ₁₀ thus depends not only on the material of the coin M, but also on its thickness d. As shown in FIG. 4, the dependence on the thickness d in the region of interest is approximately linear for all alloys L1, L2 and L3 shown. If the alloy of the coin M is known, the thickness d of the coin M can be clearly determined.

In contrast to the use of so-called double coils, the use of the two coils 9 and 10 with or without platelets 11 and 12 as simple coils, which are only arranged on one side wall 4 and 5, respectively, which are arranged on both sides of the coin channel 1 and are electrical are connected in parallel or in series, the mutually independent determination of two parameters of the coin M, which characterize the coin M on the basis of its alloy or alloy composition or thickness.

5 shows the time course of the output signal of the electronic circuit 14 for three coins of the same type. At time t ₁ , the coins come into the measuring range of first coil 9, which they leave again at time t ₂ . At time t ₃ , they enter the measuring range of second coil 10, which they leave approximately at time t ₄ . The output signal of the coil 9 has two maxima M1 and M2 with values U1 and U2, the output signal of the coil 10 has two maxima m1 and m2 with values v1 and v2. The solid line represents the output signal of a coin M, which rolls down the coin channel 1 (FIG. 1) without jumping or jumping and lies flat on the ribs 7. In this case, the measured values U1 and U2 as well as the values v1 and v2 are the same: U1 = U2, v1 = v2. The dash-dotted line shows the output signal of a coin M which jumped or jumped in the measuring range of the first coil 9: the values U1 and U2 are different. The dashed line shows the output signal of a coin M which jumped or jumped in the measuring range of the second coil 10: the values v1 and v2 are different. Experiments have shown that at least one of the values U1 or U2 or v1 or v2 is relatively stable, that is to say has a small scatter, whereas the minimum lying between the corresponding maxima is subject to a larger scatter. In the case of the first coil 9, the value of the larger of the two maxima corresponds to the smallest distance between the coil 9 and the coin M, since the damping of the coil 9 is then greatest. In the example shown in FIG. 5, this is the second maximum M2 with the value U2 for both lines, which is also the more stable of the two maxima. The microprocessor 15 is therefore programmed to determine the greatest value of the output signal in the first coil 9 and to store it as the value K ₁ . The damping of the second coil 10 is smaller, the greater the distance between the coil 10 and the coin M. The microprocessor 15 is therefore programmed to determine the values v1 and v2 of the two maxima m1 and m2 in the second coil 10 and to store the smaller of the two values v1 and v2 as the value K ₂ : K ₂ = min (v1, v2 ). In the example in FIG. 5, the maximum m2 corresponds to this case.

This described analysis of the output signals is carried out by the microprocessor 15 in a manner known per se by. In order to compensate for noise effects and to reduce the scatter of the values K ₁ and K ₂ to be determined, it is advantageous to convert the sequence f1 into a sequence f2, each value of the sequence f2 being the moving average averaged over, for example, ten successive values of the sequence f1 . The greatest value of the output signal of the first coil 9 can be determined by numerical comparisons, the maxima m1 and m2 can be determined by forming the first and second derivatives of the sequence f2.

In order to exclude influences of other physical factors such as temperature, humidity, etc. as far as possible on the measurement result, it is advantageous that the microprocessor 15 forms relative values P ₁ = r ₁ / K ₁ and P ₂ = r ₂ / K ₂ , the sizes r ₁ and r _{2 represent} reference resistances which are equal to the internal resistance R _{9 of} the coil 9 and R _{10 of} the coil 10 in the absence of the coin M. The reference resistances r ₁ and r ₂ are advantageously determined each time immediately before or after the passage of the coin M.

As is known, each coin M has two differently minted sides, which are called head and number in German. This asymmetrical embossing of the coin M means that the characteristic sizes K ₁ and K ₂ determined for the coin M depend on the side with which the coin M rests on the side wall 4. The scatter of sizes K ₁ and K ₂ in a given coin type is further increased by this effect. However, the range of size K ₁ remains sufficiently small to be able to clearly determine the alloy of the M coin. On the other hand, the measurement of the thickness d is disturbed by this effect to an extent that makes it difficult to assess the authenticity of the coin M and / or to determine its value, since coins of different values made from the same alloy often differ very little in thickness. With a further measure, which will now be described, the effect of this effect on the determination of the thickness d can be reduced. In the case of a coin M without minting, the measurements of the coils 9 and 10 give, for example, a value K ₁ and a value K ₂ . If the coin M has an asymmetrical embossing and its head side faces the coil 9, the measurements give slightly changed values K ₁ + δr ₁ and K ₂ - δr ₂ . An increase in the size K ₁ leads to a decrease in the size K ₂ , since a reduction in the distance between the coil 9 and the coin M results in an increase in the distance between the coin M and the coil 10. Because of the linearity of the quantities K ₁ and K ₂ as a function of the distance of the coin M from the corresponding coil, when using similar coils 9 and 10 and using the same frequency ω for exciting the coils 9 and 10: δr ₁ = δr ₂ = δr . If, for the same coin M, the number side is facing the coil 10, the measurements, on the other hand, give values K ₁ - δr and K ₂ + δr. The sum H ₂ = K ₁ + K ₂ or the sum I ₂ = P ₁ + P ₂ thus advantageously serves as a measure of the thickness d of the coin M and thus as a decision criterion for the acceptance or rejection of the coin M. The sums H ₂ and I ₂ are independent of which side of the coin M faces the side wall 4, since the values -δr and + δr cancel each other out.

3 shows that the measured values K ₁ differ significantly for different alloys. The alloy of the coin M is thus comparatively easy to determine, ie the tolerance values that indicate whether the coin M is accepted or rejected based on its measured alloy, can be set relatively large. The narrower the tolerance limits for the sizes K ₂ or P ₂ or H ₂ or I _{2 are} set, the more coins M can be reliably distinguished due to their thickness d. Preventing the coins from jumping or jumping in the area of the inductive sensors by means of the newly designed ribs 7 in combination with the described, detailed signal analysis now enables the setting of very narrow tolerance values for the sizes K ₂ or P ₂ or H ₂ or I ₂ .

die vom Verhältnis des Widerstandes R_S zur Induktivität L_S der Spule S abhängt (j bezeichnet die imaginäre Einheit). Die Resonanzfrequenz ω₀(L_S) des Serieresonanzkreises RLC ist gegeben durch $${\text{ω}}_{\text{0}}{\text{(L}}_{\text{S}}\text{) =}\frac{\text{1}}{\sqrt{{\text{L}}_{\text{S}}\text{*C}}}\text{.}$$

6 shows an advantageous electronic circuit 14 with a series resonance circuit RLC for the separate detection of the change in the ohmic resistance R _S and the inductance L _{S of} a coil S. The starting point is the knowledge that the coil S and a capacitive element C are formed Series resonance circuit RLC represents a purely ohmic impedance Z _S in the case of resonance, which is equal to the resistance R _{S of} the coil S. In contrast, a parallel resonance circuit in the resonance case, in which the coil S and the capacitive element C are connected in parallel, behaves like an impedance $${\text{Z.}}_{\text{P}}\text{= j}\frac{{\text{C * R}}_{\text{S}}}{{\text{L}}_{\text{S}}}\text{,}$$

which depends on the ratio of the resistance R _S to the inductance L _{S of} the coil S (j denotes the imaginary unit). The resonance frequency ω ₀ (L _S ) of the series resonance circuit RLC is given by $${\text{ω}}_{\text{0}}{\text{(L}}_{\text{S}}\text{) =}\frac{\text{1}}{\sqrt{{\text{L}}_{\text{S}}\text{* C}}}\text{.}$$

The electronic circuit 14 has a differential amplifier 18 with an inverting input 19 and a non-inverting input 20, a resistor 21, a two-stage amplifier circuit 22 and an amplitude detector 23. The series resonance circuit RLC consists of the coil S and a capacitive element C, which are connected in series, and is connected to one terminal with ground m and the other terminal to the inverting input 19 of the differential amplifier 18. The output of differential amplifier 18 is fed back via resistor 21 to inverting input 19 and via amplifier circuit 22 to non-inverting input 20.

The amplifier circuit 22 has the tasks of firstly causing the series resonance circuit RLC to oscillate when the circuit 14 is switched on and secondly of providing an amplitude-stabilized voltage U ₃ (t) for exciting the series resonance circuit RLC. This object is achieved by two inverters 24 and 25 connected in series and a voltage divider 26 connected in series. A capacitor 27 and 28 is connected upstream of the input of the inverters 24 and 25 and the output of the inverters 24 and 25 is connected via a resistor 29 and 30 fed back to the input. The capacitors 27 and 28 are used for DC decoupling. The resistors 29 and 30 determine the DC operating point of the inverters 24 and 25, respectively. When the circuit 14 is switched on, the amplifier circuit 22 behaves like a linear AC voltage amplifier, so that because of the positive feedback of the output voltage U ₁ (t) of the differential amplifier 18 thereon Input 20 of the series resonance circuit RLC begins to oscillate. The amplification of the input signal U ₁ (t) is chosen so high that the second inverter 25 is then always saturated, so that a rectangular voltage U ₂ (t) is present at its output, the two voltage levels of which are positive and negative Voltage level correspond with which the entire electronic circuit 14 is fed in a manner known per se with respect to the mass m bipolar. With the help of the ohmic voltage divider 26, which leads to ground m, the level of the voltage U ₂ (t) is reduced. A rectangular voltage U ₃ (t) is thus present at the output of the amplifier circuit 22 and thus at the input 20 of the differential amplifier 18, which is in phase with the voltage U ₁ (t), the amplitude of which is independent of the amplitude of the voltage U ₁ ( t) is. The voltage divider 26 has two resistors 31 and 32, the resistor 31 being of the order of magnitude of the resistance R _{S of} the coil S. The resistor 32 is dimensioned such that the level of the voltage U ₃ (t) is a few tens to one hundred millivolts. The amplitude detector 23 is used to measure the amplitude of the voltage U ₁ (t) and to pass it on to the microprocessor 15 in a suitable form.

When the coin M passes the coil S, the resonance frequency ω ₀ (L _S ) changes with the change in the inductance L _S. The circuit 14 described operates in such a way that the series resonance circuit RLC oscillates at a frequency ω which is always equal to the resonance frequency ω ₀ (L _S ). When the coin M passes the coil S, its resistance R _{S also changes} . Since the series resonance circuit RLC has the ohmic impedance Z _S = R _S at resonance and since the voltage U ₃ (t) which serves to excite the series resonance circuit RLC is a periodic voltage with constant amplitude, the current flowing through the series resonance circuit RLC is i (t) and thus the amplitude of the voltage U ₁ (t) at the output of the differential amplifier 18 directly measures the resistance R _{S of} the coil S. The evaluation of the signal U ₁ (t) is now carried out by the microprocessor 15 as previously described.

The frequency ω of the square-wave voltage U ₂ (t) present at the output of the second inverter 25 can be determined in a simple manner, not shown, for example with a counter module which is counted by the microprocessor 15 in accordance with the time course of the amplitude of the voltage U ₁ (t) can be released while the coin M covers the coil S. The frequencies ω ₁ and ω ₂ determined in this way in the coil 9 or in the coil 10 correspond to the resonance frequencies when the coin M passes and represent a third and fourth characteristic variable K ₃ and K ₄ , which are further decision criteria for the acceptance or reject the coin M can serve.

With the device described, the sizes K ₁ and K ₂ and thus the alloy composition and the thickness d of the coin M can be determined with an accuracy which is sufficient to be able to distinguish a large number of coins M. In order to exclude the possibility of fraud that a coin M2 of a certain alloy and greater thickness d can be simulated with a coin M1 of small thickness d or with a thin metallic plate by deliberately increasing the distance of the coin M1 or the metallic plate from the coil 9 is, for example by inserting a non-metallic layer between the coin M1 and the coil 9, it is sufficient to determine whether the resonance frequency ω ₀ (L ₂ ) of the coil 9 is greater or less during the passage of the coin M than in the absence of a coin. The sign of the change in resonance frequency ω ₀ (L ₂ ) of the coil 9 thus advantageously serves as a further decision criterion for the acceptance or rejection of the coin M. A precise determination of the resonance frequency ω ₀ (L ₂ ) in the presence of the coin M is not necessary.

The arrangement of the coil 9 or 10 in the series resonance circuit RLC offers the advantage that a quantity characterizing the alloy composition or the thickness d characterizing quantity can be determined with a simply constructed circuit which measures the damping of the series resonance circuit RLC in the presence of the coin M. The series resonance circuit RLC therefore represents a particularly suitable means for measuring the change in resistance induced in the coil S. This also makes it possible to identify coins which, when using a parallel resonance circuit, result in no or an inadequate signal change if the changes in the inductance L _S and the resistance R _S compensate one another.

The inductance L _{S of} the coil S and the value of the capacitive element C are selected such that the resonance frequency ω ₀ (L _S ) of the resonant circuit RLC is in the range from 50 to 200 kHz, a typical value being 90 kHz. At these frequencies, the depth of penetration of the magnetic field generated by the coil S into the coin M is sufficiently large that the material composition of the coin M can be determined sufficiently selectively.

Fluctuations in the level of the voltage U ₃ (t) which excites the resonance circuit RLC, which result, for example, from fluctuations in the operating voltage which serves to supply the circuit 14 with power, have no influence on the quantities P ₁ and P ₂ , since these have a ratio of two represent successive resistance measurements.

Inverters 24 and 25 can be, for example, inverters of the known type 4007. In a special embodiment of the circuit 14, at least one of the inverters 24 or 25 is replaced by a NAND or a NOR module with an additional input, the additional input being connected to an output of the microprocessor 15. The circuit 14 can be switched on and off in a simple manner via the logic potential at this output of the microprocessor 15. The circuit 14 can therefore only be switched on for a short time just for checking a coin M. The replacement of both inverters 24 and 25 by a NAND or a NOR module offers the advantage that the circuit 14 requires very little energy when switched off.

FIG. 6 shows only one example of an electronic circuit 14 which is suitable for detecting the change in the resistance R _{S of} the coil S by means of a series resonance circuit RLC. In the technical literature there are countless other examples of electrical circuits of the series resonance circuit RLC which excite the series resonance circuit RLC with a voltage or a current.

Claims (9)

1. Device for testing the authenticity of coins (M), tokens or other flat metallic objects, having a coin channel (1) with a lower (4) and an upper (5) side wall, the coin channel (1) being inclined by a predetermined angle with respect to the vertical (V) and the coin (M), in the ideal case, moving along the lower side wall (4) in contact, having two inductive sensors arranged along the coin channel (1), an electronic circuit (14) and having a control and evaluation unit (15), characterised in that the first inductive sensor is a coil (9) fitted to the lower side wall (4), in that the second inductive sensor is a coil (10) fitted to the upper side wall (5), in that means (13, 14) are provided to operate the two coils (9, 10) electrically independently, in that the electronic circuit (14) is equipped to measure the variation with time of the ohmic resistance R₉(t) and R₁₀(t) of the two coils (9, 10) during the passage of a coin (M), in that the control and evaluation unit (15) determines the greatest value of the resistance R₉(t) of the first coil (9) as the value K₁, in that the control and evaluation unit (15) determines the local maxima (m1, m2) of the resistance R₁₀(t) assumed by the second coil (10) and determines the greater of the two values (v1, v2) of the two maxima (m1, m2) as the value K₂, and in that the values K₁ and K₂ or the values K₁ and H₂ = K₁ + K₂ serve to decide about the acceptance or rejection of the coin (M).
2. Device for testing the authenticity of coins (M), tokens or other flat metallic objects, having a coin channel (1) with a lower (4) and an upper (5) side wall, the coin channel (1) being inclined by a predetermined angle with respect to the vertical (V) and the coin (M), in the ideal case, moving along the lower side wall (4) in contact, having two inductive sensors arranged along the coin channel (1), an electronic circuit (14) and having a control and evaluation unit (15), characterised in that the first inductive sensor is a coil (9) fitted to the lower side wall (4), in that the second inductive sensor is a coil (10) fitted to the upper side wall (5) in that means (13, 14) are provided to operate the two coils (9, 10) electrically independently, in that the electronic circuit (14) is equipped to measure the variation with time of the ohmic resistance R₉(t) and R₁₀(t) of the two coils (9, 10) during the passage of a coin (M), in that the control and evaluation unit (15) determines the greatest value of the resistance R₉(t) of the first coil (9) as the value K₁, in that the control and evaluation unit (15) determines the local maxima (m1, m2) of the resistance R₁₀(t) assumed by the second coil (10) and determines the greater of the two values (v1, v2) of the two maxima (m1, m2) as the value K₂, in that the control and evaluation unit (15) determines the internal resistance r₁ of the first coil (9) and the internal resistance r₂ of the second coil (10) directly before or after the passage of the coin (M), and in that the values P₁ = r₁/K₁ and P₂ = r₂/K₂ or the values P₁ and I₂ = P₁ +P₂ serve to decide about the acceptance or rejection of the coin (M).
3. Device according to Claim 1 to 2, characterised in that the coils (9, 10) are arranged to measure the resistance in a series resonant circuit (RLC).
4. Device according to Claim 3, characterised in that the electronic circuit (14) comprises a differential amplifier (18) and an amplifier circuit (22), the output of the differential amplifier (18) being fed back via a resistor (21) to the inverting input (19) and via the amplifier circuit (22) to the non-inverting input (20), and the amplifier circuit (22), firstly, on switching on the electronic circuit (14), brings the series resonant circuit (RLC) into oscillation and, secondly, makes available an amplitude-stabilised voltage (U₃(t)) for exciting the series resonant circuit (RLC).
5. Device according to Claim 4, characterised in that the amplifier circuit (22) has two inverters (24, 25) connected in series or NAND or NOR components.
6. Device according to one of Claims 3 to 5, characterised in that means are provided in order, during the passage of the coin (M), to determine the sign of the change in the resonant frequency (ω₀(L_S)) in the first coil (9), and in that this sign serves as a further decision criterion for the acceptance or rejection of the coin (M).
7. Device according to one of Claims 1 to 6, characterised in that a metallic platelet (11, 12) is fitted in each case on that side wall (5, 4) lying opposite the coils (9, 10).
8. Device according to one of Claims 1 to 7, characterised in that the lower side wall (4) is provided with ribs (7) in the running direction of the coin and that the radius of curvature (R) of the ribs (7) is at least half as large as the spacing (a) of adjacent ribs (7).
9. Device according to Claim 8, characterised in that the radius of curvature (R) of the ribs (7) is approximately comparable to the spacing (a) of adjacent ribs (7).

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