U.S. patent number 3,576,244 [Application Number 04/789,872] was granted by the patent office on 1971-04-27 for coin acceptor having resistivity and permeability detector.
This patent grant is currently assigned to The Vendo Company. Invention is credited to John E. Cessna, Jr., James F. Ptacek.
United States Patent |
3,576,244 |
Ptacek , et al. |
April 27, 1971 |
COIN ACCEPTOR HAVING RESISTIVITY AND PERMEABILITY DETECTOR
Abstract
A coin acceptor that detects the bulk resistivity and
permeability of a coin and determines whether or not the coin is
valid. A two-coil inductor forms one arm of a normally unbalanced
Maxwell bridge and is disposed at a test station which is
interposed in a channel through which the coin passes. The two
coils have series aiding magnetic fields and are disposed on
opposite sides of the coin channel with the lines of force crossing
the channel at generally right angles to the opposed faces of a
coin under test. A valid coin moving through the lines of force
causes the inductance to change and effect balancing of the bridge.
If this occurs throughout a predetermined test period, a coin
deflector below the test station is withdrawn to permit the coin to
enter an accept path.
Inventors: |
Ptacek; James F. (Kansas City,
MO), Cessna, Jr.; John E. (Kansas City, MO) |
Assignee: |
The Vendo Company (Kansas City,
MO)
|
Family
ID: |
25148927 |
Appl.
No.: |
04/789,872 |
Filed: |
January 8, 1969 |
Current U.S.
Class: |
194/318 |
Current CPC
Class: |
G07D
5/08 (20130101) |
Current International
Class: |
G07f 003/02 () |
Field of
Search: |
;194/6,100,101,(100.5) |
Foreign Patent Documents
|
|
|
|
|
|
|
152,433 |
|
Jul 1953 |
|
AU |
|
455,362 |
|
Nov 1936 |
|
GB |
|
Primary Examiner: Tollberg; Stanley H.
Claims
We claim:
1. In a coin acceptor:
structure for receiving a coin to be tested for continuous movement
of the coin through a test station;
means communicating with said station and defining an accept path
for the coin and a reject path therefor;
an electrical bridge network provided with a plurality of arms and
having an output,
one of said arms including a variable impedance component at said
station whose impedance is influenced by a characteristic property
of the coin as it travels through the station,
said component being operable to vary the state of the network at
the output thereof between a balanced condition and an unbalanced
condition and, if the coin traveling through said station is valid,
having an impedance in the presence of said coin that causes the
network to assume one of said conditions;
shiftable coin control means downstream from said station for
controlling movement of said coin into said paths after passage
through said station; and
means operably coupling said control means with the output of said
network to cause the coin to enter the accept path when the network
assumes said one condition, and to enter the reject path when the
network fails to assume said one condition as the coin passes
through said station.
2. In a coin acceptor as claimed in claim 1,
said structure presenting a coin-receiving channel,
said component including an inductor for producing a magnetic field
having lines of force crossing said channel at generally right
angles to the opposed faces of a coin under test in said
station.
3. In a coin acceptor as claimed in claim 1,
said coin control means being normally disposed to cause the coin
to enter the reject path after passage through said station,
said coupling means including an electrically responsive actuator
for shifting the coin control means to cause the coin to enter the
accept path after leaving said station, and a control circuit
coupled with said actuator for operating the latter in response to
said network assuming said one condition thereof.
4. In a coin acceptor as claimed in claim 1,
said coin control means being normally disposed to cause the coin
to enter the reject path after passage through said station;
means at said station for sensing the presence of said coin;
and
electrical delay means coupled with said sensing means and having a
predetermined signal output condition assumed after a time period
following sensing of the presence of said coin and during which
period said network is in continuous operation,
said coupling means including an electrically responsive actuator
for shifting the coin control means to cause the coin to enter the
accept path after leaving said station, and a control circuit
coupled with said actuator, said network, and said delay means and
responsive to said output conditions of the network and the delay
means,
said circuit effecting operation of the actuator to accept the coin
if said network is in its one, validity-indicating condition and
remains therein until said delay means assumes said predetermined
signal output condition thereof at the expiration of said time
period.
5. In a coin acceptor having a channel for receiving a coin for
movement to a test station, an accept path for the coin and a
reject path therefor downstream from said station and a
communicating with said channel, and means for controlling movement
of the coin from said station into said paths, the improvement
comprising:
an inductor at said station for producing a magnetic field having
lines of force crossing said channel at generally right angles to
the opposed faces of a coin under test in said station; and
circuitry coupled with said inductor and responsive to a change in
the inductance thereof caused by the presence of said coin under
test,
said circuitry being operably coupled with said coin-controlling
means to cause the coin under test to enter the accept path when
said inductance is of a valve indicative of coin validity, and to
enter the reject path when the inductor fails to have said value of
inductance in the presence of the coin under test.
said inductor including a pair of spaced, aligned coils disposed
with said channel extending therebetween, and means interconnecting
said coils in series-aiding field relationship with said lines of
force bridging said coils,
each of said coils being flat and having multiple turns wound one
inside the other in a common plane,
the planes of respective coils being disposed in substantial
parallelism with said channel extending between said planes.
6. The improvement of claim 5, said coils being oblong and
extending longitudinally in the direction of movement of a coin
traversing said channel, whereby to increase the time a coin in
motion is subjected to the lines of force to facilitate the testing
of coins while in motion.
7. In a coin acceptor:
structure for receiving a coin to be tested for movement of the
coin to a test station;
means communicating with said station for receiving said coin for
movement therefrom and defining an accept path for the coin and a
reject path therefor;
shiftable coin control means downstream from said station and
normally disposed to cause said coin to enter the reject path after
passage from said station;
means at said station for sensing the presence of said coin;
coin-testing means having a component at said station for detecting
a characteristic property of the coin when the latter is at the
station, and having a signal output condition, if the coin is
valid, indicative of validity of the coin;
electrical delay means coupled with said sensing means and having a
predetermined signal output condition assumed after a time period
following sensing of the presence of said coin and during which
period said coin-testing means is in continuous operation;
an electrically responsive actuator coupled with said coin control
means for shifting the latter to cause the coin to enter the accept
path after leaving said station; and
a control circuit coupled with said actuator, said coin-testing
means, and said delay means and responsive to said output
conditions,
said circuit effecting operation of the actuator to accept the coin
if said coin-testing means is in its validity-indicating output
condition and remains therein until said delay means assumes said
predetermined signal output condition thereof at the expiration of
said time period.
8. In a coin acceptor as claimed in claim 2,
said control circuit including a normally circuit-interrupting,
electrically responsive switching device and a silicon-controlled
rectifier in series with said actuator,
said rectifier having a gate;
means coupling said sensing means with said gate for delivering a
momentary pulse to the gate at the beginning of said time
period,
said coin-testing means being connected with said rectifier for
maintaining the latter in conduction after delivery of said pulse
if the coin-testing means is in its validity-indicating output
condition and remains therein during said time period,
said delay means being connected with said switching device for
operating the latter when the delay means assumes said
predetermined signal output condition thereof at the expiration of
said time period.
9. In a coin acceptor as claimed in claim 2,
said inductor including a pair of spaced, aligned coils disposed
with said channel extending therebetween, and means interconnecting
said coils in series-aiding field relationship with said lines of
force bridging said coils,
each of said coils being flat and having multiple turns wound one
inside the other in a common plane,
the planes of respective coils being disposed in substantial
parallelism with said channel extending between said planes.
Description
Coin acceptors in widespread use at the present time are, for the
most part, designed to be compatible with very specific coinage
systems. As commercial usage of vending machines and other
coin-operated apparatus becomes worldwide in scope, the need arises
for coin acceptors that are capable of adaptation to different
coinage systems. Furthermore, any change in a particular system
presently creates a serious situation in that either an existing
acceptor cannot be used at all or, if usable, slugging may become a
problem.
It is, therefore, the primary object of this invention to provide
an improved coin acceptor of high flexibility which is capable of
detecting and separating legitimate coins from counterfeit coinage,
without limitation as to the composition of the legitimate
coins.
As a corollary to the foregoing object it is an important aim of
the present invention to provide an acceptor as aforesaid which
detects the bulk resistivity and permeability of a deposited coin
while the same is in motion and then rapidly determines whether or
not the coin is valid.
It is another important object of the invention to provide an
acceptor as aforesaid which may be readily adjusted in accordance
with the composition of legitimate coins of a coinage system, and
has high selectivity so that only coins of predetermined
composition will be accepted.
In the drawings:
FIG. 1 is an electrical schematic diagram of the acceptor
circuitry;
FIG. 2 is a fragmentary, elevational view of a channel of the
acceptor showing the test station therein;
FIG. 3 is a vertical sectional view taken along line 3-3 of FIG.
2;
FIG. 4 is a horizontal, sectional view taken along line 4-4 of FIG.
2;
FIG. 5 is a graph illustrating the operational characteristics and
selectivity of the testing apparatus; and
FIG. 6 is a timing diagram illustrating the operation of various
components of the system.
Referring initially to FIGS. 2--4, a pair of upright, parallel
plates 10 and 12 define a coin channel 14 therebetween through
which a deposited coin (not shown) gravitates. A pair of flat,
oval-shaped coils 16 are received and held in complemental openings
in the respective plates 10 and 12 and are disposed in direct
opposition to each other as is clear in FIG. 3. In FIG. 2 a lead 18
is illustrated connected to the outer end of one of the coils 16,
the latter comprising a single layer of turns wound one within the
other and terminating at the inner end of the coil where a
connection is made to a lead 20. It should be understood that the
turns are illustrated diagrammatically due to the small wire size,
many turns actually being employed in the formation of each of the
coils 16.
A vertical slot 22 in plate 10 is aligned with the longitudinal
axis of the oval coil 16 and is centered with respect thereto,
there being a corresponding slot in plate 12 in alignment with slot
22. A light source 24 is mounted in the slot 22 and may be
adjustably positioned therein depending upon the diameter of the
coin to be tested. The source 24 would commonly comprise a small
electrical lamp encased in a suitable housing.
In similar fashion, the plate 12 serves to mount a coin sensor 26
which, as will be seen hereinafter, may comprise a photo-Darlington
light detector. The sensor 26 is also movable in the slot in plate
12 so that the sensor 26 may be aligned with the source 24 as
required. As the coin gravitates through the channel 14, it passes
between the coils 16 and masks the sensor 26. The two coils 16 are
serially connected and produce magnetic fields in series aiding
relationship to each other, as will be appreciated more fully
hereinafter.
An opening 28 in the plate 12 receives a coin deflector 30 which
normally extends into the channel 14 below the test station. In its
normal position, the deflector 30 diverts gravitating coins into a
reject path 32; when the deflector 30 is withdrawn out of blocking
relationship to the coins, the latter are permitted to gravitate
straight down without diversion and enter an accept path 34. An
electrical actuator in the form of an accept electromagnet 36 is
mounted on plate 12 and, when energized, attracts a magnetic arm 38
connected to deflector 30 that is pivotally carried by a projecting
support member 40.
Referring to FIG. 1, it may be seen that the inductor comprising
the two coils 16 forms one arm of a Maxwell bridge 42 having a
second, resistive arm 44, a third arm 46 containing adjustable
resistors and a parallel capacitor, and a fourth arm 48 containing
a pair of serially connected adjustable resistors. The inputs of
the bridge are presented at 50 (the junction of arms 44 and 46) and
at 52 (the junction of arms 16 and 48). A sinusoidal oscillator
stage 54 has an output connection 56 which extends to the bridge
input 50 through a variable resistor 58. The other output
connection of the oscillator stage 54 is illustrated by the ground
symbol and hence is connected to the grounded bridge input 52. A
power lead for the stage 54 is illustrated at 60. The two terminals
labeled +V represent connections to the positive side of a suitable
DC source, the negative side of which is at ground potential.
The bridge 42 has a pair of output terminals 62 and 64, formed by
the junction of arms 16 and 44 and the junction of arms 46 and 48
respectively. Coupling capacitor 66 connects the output terminal 62
to one input 68 of an integrated circuit differential amplifier 70,
such as a GE PA230. A coupling capacitor 72 connects the other
bridge output terminal 64 to the second input 74 of amplifier 70.
The various other resistive and capacitive elements shown connected
to the amplifier 70 comprise conventional nomenclature for
amplifiers of this type and are employed for purposes of frequency
compensation.
The amplifier 70 has an output 76 which is connected by a coupling
capacitor 78 to the base of a PNP transistor 80. The emitter of
transistor 80 is connected to the positive DC power lead 82, the
collector thereof being connected in series with the anode of a
silicon-controlled rectifier 84 by a resistor 86. The cathode of
the SCR 84 is connected to a common ground lead 88. A voltage
divider comprising a pair of series resistors 81 and 83 is
connected across leads 82 and 88, the common junction of the two
resistors 81 and 83 being connected to the base of transistor 80 to
set the base bias voltage. Similarly, with respect to the input 68
of the differential amplifier 70, a voltage divider comprising a
pair of series resistors 67 and 69 is connected to the input 68 at
the common junction of resistors 67 and 69.
The photo-Darlington coin sensor 26 mentioned previously is
connected across the positive power lead 82 and the ground lead 88
through an emitter resistor 90. The output of the photo-Darlington
26 is taken at the emitter connection and delivered to the base of
an NPN transistor 92 by a capacitor 94. Bias resistors for the
collector and base of transistor 92 are illustrated at 96 and
98.
A Darlington amplifier 100 has its input base connected to the
collector of transistor 92 by a capacitor 102, and a resistor 104
is connected between the input and the positive lead 82. The output
of amplifier 100 is taken at the collector, a resistor 106
connecting the collector to the positive lead 82. The emitter
output of the amplifier 100 is directly connected to the ground
lead 88.
The collector of transistor 92 is also connected to the gate of SCR
84 by a capacitor 108, a resistor 110 being connected from the SCR
gate to the ground lead 88. Thus, SCR 84 and the Darlington
amplifier 100 are both responsive to the state of the transistor
92.
A diode 112 connects the collector output of amplifier 100 to the
base of an NPN accept transistor 114, the emitter thereof being
connected to the anode of the SCR 84. An input resistor 116 is
connected across the emitter and base of the transistor 114, the
collector thereof being connected to the positive lead 82 through
the accept electromagnet 36. A diode 118 is connected in parallel
with the electromagnet 36 and is poled in the reverse direction
with respect to the DC potential to serve as an inductive transient
suppressor.
To illustrate the adaptation of the present invention to the
testing of coins of a particular denomination that may be of two
different but legitimate compositions, the bridge 42 is provided
with two additional, adjustable arms 120 and 122 which are
identical to arms 46 and 48 except for the values of the resistive
elements thereof. The two arms 120 and 122 are connected across the
two bridge inputs 50 and 52 and thus function independently with
the common arms 16 and 44. An output terminal 124 is formed at the
junction of the two arms 120 and 122 and is connected by a lead 126
to test circuitry 128. A lead 130 connects the common bridge output
terminal 62 to the other input of the test circuitry 128. It should
be understood that the circuitry 128 includes a differential
amplifier and other circuit components as described above and,
therefore, constitutes an identical arrangement which is responsive
to the second bridge formed by the arms 16, 44, 120 and 122. The
accept electromagnet 36 is common to the two circuitries and is
connected to test circuitry 128 by a lead 132. The photo-Darlington
coin sensor 26 may also be employed as a common component in the
two circuitries, in which case a capacitor (not shown) would also
connect the emitter output of sensor 26 to the appropriate point in
the circuitry 128.
For purposes of illustration, it is assumed that the coin channel
14 receives quarters (25 cents) of U.S. coinage; thus, the
apparatus described above is utilized to determine the validity of
deposited quarters. In applications where channels for additional
denominations are required, the adaptability of the present
invention to multidenominational use is illustrated in FIG. 1 by
the blocks 134 and 136, each of such blocks comprising apparatus
identical to that as described above for the quarter channel. The
same oscillator 54 is utilized and delivers its output to the ten
cent testing apparatus 134 through a variable resistor 138. The
accept electromagnet for the 10 cent channel is illustrated at 140.
Similarly, the oscillator 54 is coupled to the 5 cent testing
apparatus 136 through a variable resistor 142, and an accept
electromagnet is illustrated at 144 for the 5 cent coin channel. At
the present time in U.S. coinage, only a single bridge and test
circuitry arrangement would be needed for the 10 cent and 5 cent
denominations.
OPERATION
In the example to follow, it is assumed that the coin channel 14 is
adapted to receive U.S. quarters which may be of either silver
alloy or clad composition. Referring to FIG. 5, the relative
inductance of the coils 16 is plotted against the frequency in Hz
of the output signal from the oscillator stage 54. A frequency of
2000 Hz is assumed, although it should be understood that
frequencies as low as approximately 60 Hz or as high as
approximately 4000 Hz may be utilized, depending upon the
composition of the legitimate coin under consideration. It will be
appreciated, however, from viewing the graph of FIG. 5, that a
frequency on the order of 2000 Hz enables separation of various
materials, both magnetic and nonmagnetic, due to the different
value of inductance assumed by the coils 16 in the presence of the
respective materials depicted.
To adjust the testing circuitry for operation, the light source 24
and coin photo sensor 26 are moved to a position relative to the
coils 16 which will cause the validity test to be accomplished
during the brief interval (on the order of 4 msec. for example)
that the coin is approximately centered with respect to the coils
16. Thus, for coins of larger diameter, the source 24 and sensor 26
will be disposed closer to the bottom of the coils 16 as
illustrated, while for coins of lesser diameter the source 24 and
sensor 26 will be shifted into closer spaced relationship to the
center of the coils 16.
Alignment of the electrical circuitry may be effected by
positioning a coin (a quarter in the instant example) between the
two coils 16 and adjusting the bridge arms 46 and 48 until a
balanced condition is realized at the bridge output terminals 62
and 64. This is effected utilizing a quarter of one legitimate
composition such as silver alloy. Then the clad quarter is inserted
and the other two adjustable bridge arms 120 and 122 are set to
obtain a balanced condition at the second bridge output terminals
62 and 124. Thus, it may be appreciated that either a silver alloy
or a clad quarter in the test station causes the appropriate bridge
to change from an unbalanced to a balanced condition. In the
absence of a quarter, it is evident that both bridges will be
unbalanced since the coils 16 will present the "air" inductance
characteristic seen in the graph of FIG. 5.
Since both bridges and their associated testing circuitry operate
in the same manner, it is assumed that a silver alloy quarter is
deposited and gravitates through the quarter channel 14. In
standby, the light from source 24 maintains the photosensor 26 in
conduction. When the leading edge of the quarter masks the sensor
26, the latter changes to its nonconductive state and a negative
spike is delivered to the base of transistor 92 via the capacitor
94. The right-hand plate of capacitor 94, as it appears in the
schematic, was at essentially the positive potential of the
positive power lead 82 while the sensor 26 was in conduction, but
once the sensor 26 turns off, this plate assumes ground potential
thereby effecting the delivery of the negative spike to the base of
the transistor 92.
The emitter-collector circuit of the transistor 92 conducts in
standby and is rendered nonconductive in response to the mentioned
negative spike at its base. Due to the time constant of capacitor
94 and resistor 90, the nonconductive state of transistor 92 is
maintained for a predetermined test period, assumed to be
approximately 4 msec. in the instant example.
At the outset of the test period, another action takes place as a
result of transistor 92 being rendered nonconductive. The collector
of transistor 92, which was at essentially ground potential during
standby, now rises to the potential of the positive power lead 82
and a positive spike is thus delivered to the gate of SCR 84 via
the capacitor 108. However, the SCR 84 will not be gated into
conduction for any substantial period of time unless the deposited
coin is legitimate, as will be explained.
In standby before the deposited coin reaches the test station, the
differential amplifier 70 delivers a train of 2 kHz. pulses
illustrated at 146 in FIG. 6. This occurs because the bridge is
unbalanced, the output terminals 62 and 64 thereof being coupled to
respective differential inputs 68 and 74 of the amplifier 70. The
train of pulses 146, being positive, pulses the transistor 80 off
at the 2 kHz. oscillator frequency. Therefore, the
emitter-collector circuit of the transistor 80 is in conduction
only during the intervals between the successive pulses 146. For
this reason, if the deposited coin is not legitimate and is hence
incapable of balancing the bridge, the SCR 84 will not remain on
after its gate receives the positive spike via capacitor 108. This
is also depicted in FIG. 6. Arrival of the leading edge of the
deposited coin (turnoff of the sensor 26) is illustrated at 148.
The second timing graph illustrates turn on of the SCR 84 at 150,
coincident in time with the sensing of the leading edge of the
deposited coin. If the coin is legitimate, the pulse train 146
ceases as illustrated at 152; thus the transistor 80 is permitted
to remain in conduction and, accordingly, the SCR 84 remains
conductive. In the event that the coin is counterfeit, the SCR 84
will not remain on for any significant period of time, as
illustrated by the broken line 154 in the graph of the SCR
operation.
It will be recalled that the transitor 92 is rendered nonconductive
for approximately 4 msec. (the test period). In standby, the
Darlington amplifier 100 is in its conductive state, and this is
not changed by the turning off of the transistor 92. However, when
the transistor 92 resumes conduction at the end of the test period,
the capacitor 102 transmits a negative pulse to the input base of
the Darlington amplifier 100, causing the latter to assume its
nonconductive state. When the amplifier 100 turns off, its
collector output goes positive and this potential is impressed upon
the base of the accept transistor 114 which, in standby, is
nonconductive. Thus, the accept transistor 114 is placed in
conduction and, if the SCR 84 is also in conduction to indicated
that the coin is valid, electrical continuity now exists from the
positive supply lead 82 to the ground lead 88 through the accept
electromagnet 36. It may thus be appreciated that the serially
arranged accept transistor 114 at the SCR 84 constitute a control
circuit for the accept electromagnet 36, which circuit requires
that two conditions coexist in order to energize the electromagnet
36. Energization of the electromagnet 36 attracts the arm 38 (FIG.
4) to shift the coin deflector 30 from between the plates 10 and
12, thereby permitting the deposited coin to gravitate straight
down from the test station and enter the accept path 34. The coin
is conducted along the path 34 to a coin box (not shown) or the
coin tubes of a coin changer in appropriate applications. On the
other hand, if the coin is counterfeit, the electromagnet 36
remains deenergized and the coin deflector 30 diverts the coin to
the reject path 32 for return to the customer.
Referring to FIG. 6, the third graph depicts the operation of the
accept transistor 114. The transistor 114 is rendered conductive at
a time 156 approximately 4 msec. after the leading edge of the coin
is sensed as indicated at 148. The accept transistor 114 remains on
for a period of approximately 150 msec. due to the action of a
pulse-stretching arrangement constituting resistor 104 and
capacitor 102. The resistor 104, in standby, supplies base current
to the amplifier 100 to maintain the same in conduction, but
establishes the necessary time constant with capacitor 102 to hold
the amplifier 100 off for approximately 150 msec. in response to
delivery of the negative turnoff pulse to amplifier 100 by the
capacitor 102.
The test period terminates prior to the time that the coin, which
is constantly moving, can no longer affect the inductance of the
coils 16. This is illustrated in FIG. 6 where it may be seen that
the pulse train 146 recommences a short time after the termination
of the test period. This test period may also be considered to be a
delay during which time the coin must continue to maintain the
bridge in a balanced condition. If the bridge became momentarily
unbalanced before the end of the test period, the SCR 84 would
return to its nonconductive state since, it will be recalled, only
a single positive spike is delivered to the gate of the SCR 84 at
the commencement of the test period. It may be appreciated,
therefore, that this delay assures that the acceptor will have a
high percentage of rejection of counterfeit coins, since a spurious
indication of coin validity caused by a momentary balancing of the
bridge will not be interpreted as a true indication that the coin
is valid and should be accepted.
The above description of operation assumed that a U.S. quarter of
silver alloy composition was deposited or, alternatively, a
counterfeit coin or slug. However, if a quarter of clad composition
is deposited, the accept electromagnet 36 will likewise be
energized since a balanced bridge condition will occur in the
bridge formed by the four arms 16, 44, 120 and 122. It may thus be
appreciated that the present invention is quite selective since
each bridge may be rendered operable to indicate validity only
within a relatively narrow range inductance. However, the invention
is nonetheless equally applicable to coinage systems where two
different valid coin compositions of the same denomination cause a
wide variance in the inductive arm 16. An example is the proposed
change in the composition of Canadian dimes and quarters from
nonmagnetic, silver alloy composition to pure nickel, the latter
being magnetic. Note that the characteristic curves for nickel and
silver are widely spaced in the graph of FIG. 5, yet a dual bridge
arrangement adjusted to these two materials would also be equally
capable of rejecting counterfeit coins of iron, brass, aluminum or
copper.
It should be understood that the graph of FIG. 5 is somewhat
idealized for the purpose of portraying the operational
characteristics of the invention, in that the resistive component
of the impedance of the two coils 16 also changes in the presence
of the coin under test. However, the inductive reactance of the
coils 16 is the major component of the impedance at the selected
frequency and thus is the parameter which primarily determines the
condition of the bridge. Different compositions of a given metal
alloy will also have respectively different characteristic curves,
but the variation from alloy to alloy is normally sufficiently
insignificant to present no major difficulty in selecting a test
frequency where the legitimate inductance or inductances are
readily separable from the inductances imparted to the coils 16 by
the various counterfeit materials from which the legitimate coinage
is to be separated.
The configuration and arrangement of the coils 16 as seen in FIGS.
2 and 3 are especially important to the operation of the instant
invention. The turns of each of the coils 16 are disposed in a
common vertical plane, the two coils 16 being disposed in aligned
relationship on opposite sides of the coin channel 14 with the
planes thereof in parallelism and the channel 14 extending
therebetween. Being serially interconnected to produce
series-aiding magnetic fields, the lines of force of the composite
magnetic field bridge the opposed inner faces of the coils and thus
cross the coin channel 14 at generally right angles to the opposed
faces of a coin under test (not shown) as the latter traverses the
test station. This relationship between the lines of force and the
faces of the coin serves to maximize the eddy current effect
resulting from an interaction of the magnetic field with the
material of the coin. With the lines of force being directed
broadside into the coin, the longest paths for eddy current flow
are established and hence the test apparatus is highly sensitive to
the eddy current effect. It may be noted in FIG. 5 that the
permeability of magnetic materials such as nickel and iron is also
a factor in the total effect of coins on the inductance of the two
coils 16, the characteristic curves serving to illustrate that the
magnetic properties of iron and nickel offset eddy current losses
to a degree depending upon the frequency of the alternating
magnetic field.
It should also be appreciated that the oblong configuration of the
coils 16 is of importance in the present invention. The
determination of validity or invalidity of each deposited coin is
accomplished in a dynamic test as the coin remains in motion in the
channel 14 as it passes through the test station. The oblong
configuration enables the coin to affect the magnetic field of the
coils 16 for a greater period of time than if the coils were
circular, thereby contributing to the reliability of sensing the
change of inductance required for bridge balancing.
* * * * *