Coin detection means including a current ramp generator

A coin detection system and method of operation thereof, comprising a sensing circuit portion including a sensor coil, which need be only a single sensor coil, positioned adjacent to the coin path and connected in circuit with a current ramp generator, preferably operable under control of a system control circuit portion, and a detector circuit portion connected to the sensing circuit portion to monitor and detect circuit performance characteristics and changes thereof that are effected by the presence of a coin within the field of the sensor coil at the time a current ramp is applied to the sensor coil by the ramp generator, from which circuit performance characteristics a coin characteristic value, preferably a time constant characteristic, representative of the particular coin present within the field of the sensor coil can be derived and thereafter utilized for coin detection, denomination discrimination, and coin sizing purposes.

BACKGROUND OF THE INVENTION 
The present invention relates to a coin detection means and method, and, 
more particularly, to a coin detection means that employs a derived time 
constant characteristic tau (.tau.) representative of the coin undergoing 
examination, for coin detection and discrimination purposes, and to the 
method of use thereof. 
It will be appreciated that, throughout this application, the term "coin" 
may be employed to mean any coin (whether valid or counterfeit), token, 
slug, washer, or other item which might be used by an individual in an 
attempt to operate a coin-operated device or system. An "acceptable coin" 
is considered to be an authentic coin, token, or the like of the monetary 
system or systems in which or with which the coin-operated device or 
system is intended to operate and of a denomination which the device or 
system is intended selectively to receive and to treat as an item of 
value. 
Over the years a number of coin detection, validation, and verification 
means and techniques have been developed and utilized. Among the more 
effective of such means and techniques have been coin validation systems 
that have utilized a pair of coils positioned in a face-to-face 
arrangement to detect certain performance characteristics of circuits of 
which the coils form a part and the changes that occur in such performance 
characteristics when coins of different values pass between the coils of 
the coil pair. The use in such systems of a pair of coils, as opposed to a 
single coil, was found to be necessary and/or desirable in almost all 
instances in order to minimize system dependency upon coin-to-coil 
distances. By placing a pair of coils in a face-to-face arrangement, and 
by requiring the coin to be examined to pass therebetween, such dependency 
and the overall sensitivity of such validation systems to variations in 
distance between the coin and a given coil could be minimized. However, 
the use of a pair of coils in such systems, as opposed to a single coil, 
has resulted in increased system costs, both in terms of additional 
components and the labor required to install and service such additional 
components. 
Additionally, and for the most part, such systems have required the tuning 
of individual constructions thereof in order to achieve a relative 
uniformity of operational result from individual construction to 
individual construction since the effect upon the circuit performance 
characteristics of a coin passing between the pairs of coils of different 
individual constructions of a given system type has been found to vary 
from construction to construction due to variations in construction 
component values and parameters, many of which variations, though perhaps 
minor, are nonetheless unpredictable, at least to some degree. Such 
variations may typically include deviations from individual construction 
to individual construction in coil inductance, coil resistance, circuit 
capacitance, and power supplies, among other things. By employing some 
type of tuning or programming for such individual constructions during 
system manufacture, which tuning or programming adds further in component 
and labor costs, a relative uniformity in end result from individual 
construction to individual construction can be achieved. However, even 
with such tuning and/or programming during the manufacture of the 
individual constructions, there remains a possibility that component 
values may change over time or that the system may drift out of tune, 
resulting in subsequent performance problems at later times. Still 
further, with many validation systems of the type under discussion, it has 
been found necessary or desirable to employ multiple pairs of coils 
because of an inability to obtain sufficient information for coin 
validation purposes from a single pair of coils, which additional 
components result in still further costs in components and labor. 
The coin detection means of the present invention is designed to eliminate 
or minimize the need for many of such additional components, and the labor 
associated therewith, yet to permit coin detection and validation to be 
accomplished by the use of only a single sensor coil instead of one or 
more pairs of sensor coils. As so designed and constructed, the coin 
detection means of the present invention operates to detect a coin, when 
it is within the field of the sensor coil, essentially independently of 
coin-to-coil distance, and with little or no need for tuning and/or 
individualized construction programming during an individual 
construction's manufacture, to permit a time constant characteristic of a 
detected coin to be derived through the utilization of a single sensor 
coil. 
SUMMARY OF THE INVENTION 
The coin detection means of the present invention comprises a circuit means 
including a sensor coil, which need be only a single sensor coil, 
positioned adjacent to the coin path and connected in circuit with a 
current ramp generator, preferably operable under control of a system 
control means, and detector means connected to said circuit means to 
monitor and detect circuit performance characteristics and changes thereof 
that are effected by the presence of a coin within the field of the sensor 
coil at the time a current ramp is applied to the sensor coil by the ramp 
generator, from which circuit performance characteristics a time constant 
characteristic representative of the particular coin present within the 
field of the sensor coil can be derived and utilized to distinguish 
between different coin denominations. The derived time constant is 
essentially independent of both the coin-to-coil distance and various 
circuit parameters, such as the inductance and resistance of the sensor 
coil and the slope of the current ramp, as a consequence of which (a) such 
coin detection means does not require the use of a pair of coils 
positioned in a face-to-face arrangement, (b) little or no tuning of 
individual constructions is required, and (c) problems associated with 
component value changes or the system drifting out of tune are greatly 
minimized. 
In light of what has been discussed hereinabove, it will be appreciated 
that a principal object of the present invention is to provide an improved 
means and method for detecting and validating coins. 
A further object of the invention is to teach a coin detection means and 
method that requires only a single sensor coil. 
A still further object of the invention is to provide a coin detection 
means wherein changes in circuit performance characteristics are produced 
during coin detection, from which changes a characteristic representative 
of the coin detected, yet essentially independent of circuit component 
parameters, can be derived. 
Another object of the invention is to teach a method of detecting and 
validating coins by deriving, from detected changes effected in the 
performance characteristics of a circuit by the presence of a coin within 
the field thereof, a time constant characteristic representative of such 
detected coin. 
Still another object is to teach the construction and use of a coin 
detection and validation means that includes a single sensor coil wherein 
a characteristic essentially independent of coin-to-coil distance, yet 
representative of a particular coin denomination, is derived when a coin 
is present within the field of the sensor coil. 
A still further object of the invention is to provide a coin detection 
means the separate and individual constructions of which require little or 
no tuning to ensure accurate operation thereof. 
Yet another object of the invention is to provide a coin detection means 
wherein problems associated with component value drift are largely 
eliminated or minimized.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now the drawings, wherein like numbers refer to like items, the 
number 40 in FIG. 1 refers to a coin detection means that includes a 
single sensor coil 42 positioned adjacent to a coin path 44 and connected 
in a circuit 46 with a current ramp generator 48 operable under control of 
a system control means 50, and detector means 52 connected to said circuit 
46 to monitor and detect circuit performance characteristics and changes 
thereof that are effected by the presence of a coin 54 in the field of the 
sensor coil 42, from which changes in circuit performance characteristics 
a time constant characteristic representative of the particular coin 
present in the field of the sensor coil can be derived and utilized to 
distinguish between different coin denominations. 
It will be appreciated by those knowledgeable in the art that a coin may be 
modeled as an inductance L.sub.c and a resistance R.sub.c, the values of 
which are dependent upon the characteristics of the coin, including its 
size and thickness, electrical conductivity, and magnetic permeability. 
Such values may be combined into a single ratiometric parameter L.sub.c 
/R.sub.c. Since the ratio L/R is commonly designated as tau (.tau.), the 
ratiometric parameter L.sub.c /R.sub.c for any given coin will hereafter, 
for ease of reference, be referred to as tau.sub.c (.tau..sub.c). The 
present invention permits the tau.sub.c for the coin 54 in the presence of 
the sensor coil 42 to be derived and utilized to distinguish such coin 
from coins of different denominations. 
It will be understood that the voltage V.sub.s across the sensor coil 42 
can be easily monitored and that, in the absence of a coin, the 
application of a current ramp I=k.sub.1 t (see FIG. 2), where k.sub.1 is a 
constant, by current ramp generator 48 to sensor coil 42 will result in a 
voltage V.sub.s(nc) =k.sub.1 L.sub.s +k.sub.1 R.sub.s t (see FIG. 3) 
across such coil, where V.sub.s(nc) is the voltage V.sub.s across sensor 
coil 42 in the absence of a coin, L.sub.s is the effective inductance of 
the sensor coil 42, and R.sub.s is the effective resistance of such coil. 
However, if a coin 54 is present within the field of the sensor coil 42 at 
the time the current ramp I=k.sub.1 t is applied, the resulting voltage 
V.sub.s across such coil is modified, and has been found to satisfy the 
equation 
EQU V.sub.s(cp) =k.sub.1 L.sub.s +k.sub.1 R.sub.s t-k.sub.2 
e.sup.(-t/tau.sbsp.c.sup.) +k.sub.o (c), 
where V.sub.s(cp) is the voltage across sensor coil 42 when a coin is 
present in the field thereof, k.sub.2 is a constant determined by various 
factors, including the coin-to-coil distance, tau.sub.c is a parameter of 
the coin independent of L.sub.s, R.sub.s, k.sub.1, and k.sub.2, and 
K.sub.o (c) is an offset value attributable to the change in the total 
reluctance of the flux path caused by the presence of the particular coin 
c within the field of the sensor coil. For coins of low magnetic 
permeability K.sub.o (c) has been found to be essentially zero, while for 
coins of high magnetic permeability, i.e., ferromagnetic coins, K.sub.o 
(c) is a coin dependent value which must be taken into consideration, as 
will become better understood from that which follows. 
FIG. 3 depicts typical V.sub.s waveforms across the sensor coil 42 effected 
by the application of current ramps to such sensor coil in situations when 
no coin is present in the field of the coil (waveform V.sub.s(nc)), when a 
coin of low magnetic permeability is present in the field of the coil 
(waveform V.sub.s(L)), and when a coin of high magnetic permeability is 
present in the field of the coil (waveform V.sub.s(H). The detectible 
difference between V.sub.s(nc) and V.sub.s(cp) may be expressed as 
##EQU1## 
where V.sub.diff (t) is the voltage difference at time t. FIG. 4 depicts 
typical V.sub.diff (t) waveforms for a coin of low magnetic permeability 
(waveform V.sub.diff(L)) and for a coin of high magnetic permeability 
(waveform V.sub.diff(H)). 
As has been previously discussed, for coins of low magnetic permeability 
K.sub.o (c) may be considered to be essentially zero. For such coins, 
then, the voltage differential between V.sub.s(nc) and V.sub.s(L) may be 
expressed as V.sub.diff (t)=k.sub.2 e.sup.(-t/tau.sbsp.c). As so 
expressed, such equation contains four unknowns, viz., V.sub.diff (t), 
k.sub.2, t, and tau.sub.c. However, if the value of V.sub.diff (t) can be 
measured at at least two different specific times relative to time t.sub.o 
=0, where t.sub.o represents the time of application of a current ramp to 
the sensor coil 42, one can obtain two equations in two unknowns, viz., 
V.sub.A =k.sub.2 e.sup.(-T.sbsp.A.sup./tau.sbsp.c.sup.) and V.sub.B 
=k.sub.2 e.sup.(-T.sbsp.B.sup./tau.sbsp.c.sup.), where V.sub.A is the 
measured value of V.sub.diff (t) at time T.sub.A and V.sub.B is the 
measured value of V.sub.diff (t) at time T.sub.B. Since it is well known 
that two equations in two unknowns can be readily solved, it will be 
appreciated that tau.sub.c can thus be derived for coins of low magnetic 
permeability from two or more time-voltage measurement pairs, the values 
of which are determined by the performance characteristics of circuit 46 
when such coin is present within the field of sensor coil 42. By way of 
illustration, if two time-voltage measurement pairs (T.sub.A,V.sub.a) and 
(T.sub.B,V.sub.B) are obtained, in accordance with which V.sub.A =k.sub.2 
e.sup.(-T.sbsp.A.sup./tau.sbsp.c.sup.) and V.sub.B =k.sub.2 
e.sup.(-T.sbsp.B.sup./tau.sbsp.c.sup.), and if k.sub.3 is defined to be 
the ratio of V.sub.A to V.sub.B, i.e., if k.sub.3 =V.sub.A /V.sub.B, then 
V.sub.A =k.sub.3 V.sub.B, as a consequence of which V.sub.A may be 
expressed as V.sub.A =k.sub.2 e.sup.(- T.sbsp.A.sup./tau.sbsp.c.sup.) 
=k.sub.3 k.sub.2 e.sup.(-T.sbsp.B.sup./tau.sbsp.c.sup.). In accordance 
therewith, e.sup.(-T.sbsp.A.sup./tau.sbsp.c.sup.) =k.sub.3 
e.sup.(-T.sbsp.B.sup./tau.sbsp.c.sup.) and k.sub.3 
=e.sup.[(T.sbsp.B.sup.-T.sbsp.A.sup.)/tau.sbsp.c.sup.)]. Therefore, 
1n(k.sub.3)=[(T.sub.B -T.sub.A)/tau.sub.c ], and, solving for tau.sub.c, 
tau.sub.c =[T.sub.B -T.sub.A)/1n(k.sub.3)]=[(T.sub.B -T.sub.A)/1n(V.sub.A 
/V.sub.B)]. 
In light of what has been discussed hereinabove, it will be further 
appreciated that, in many instances, only coins of low magnetic 
permeability may be of interest, as a consequence of which a means for 
detecting or separating coins of high magnetic permeability may sometimes 
be utilized prior to the coin detection means of the present invention to 
eliminate coins of high magnetic permeability from consideration by such 
coin detection means. Since U.S. coins are presently coins of low magnetic 
permeability, any means capable of detecting or separating coins of high 
magnetic permeability before the examination thereof by the present 
invention could be advantageously employed, and, in such cases, since only 
coins of low magnetic permeability would then be undergoing examination by 
the coin detection means of the present invention, the detection means 52 
and the control means 50 could be so designed and/or programmed that 
tau.sub.c for any examined coin could be derived based upon two 
time-voltage measurement pairs of V.sub.diff (t). 
If, however, coins of high magnetic permeability are of interest and/or no 
means for detecting or separating coins of high magnetic permeability is 
employed prior to the coin detection means of the present invention, two 
time-voltage measurement pairs may provide insufficient information to 
permit tau.sub.c to be uniquely derived for any detected coin. It should 
be recalled that the equation representing the voltage difference is 
actually V.sub.diff (t)=k.sub.2 e.sup.(-t/tau.sbsp.c.sup.) -K.sub.o (c) 
and that K.sub.o (c), which is essentially zero for coins of low magnetic 
permeability, is a non-zero coin dependent constant for coins of high 
magnetic permeability. Consequently, K.sub.o (c) must be taken into 
account whenever the coin undergoing examination could possibly be a coin 
of high magnetic permeability. If K.sub.o (c) is unknown for a given 
deposited coin, as would generally be the case if the coin is subjected to 
only a single coin detection operation, the noted equation would then 
contain five unknowns, viz., V.sub.diff (t), k.sub.2, t, tau.sub.c, and 
K.sub.o (c), and two time-voltage measurement pairs would therefore result 
in two equations in three unknowns, from which tau.sub.c could not be 
readily determined. However, if three time-voltage measurement pairs could 
be obtained, tau.sub.c could in such case be uniquely derived, in similar 
fashion to that previously discussed, from three resulting equations in 
three unknowns, viz., V.sub.A =k.sub.2 
e.sup.(-T.sbsp.A.sup./tau.sbsp.c.sup.) -K.sub.o (c), V.sub.B =k.sub.2 
e.sup.(-T.sbsp.B.sup./tau.sbsp.c.sup.) -K.sub.o (c), and V.sub.D =k.sub.2 
e.sup.(-T.sbsp.D.sup./tau.sbsp.c.sup.) -K.sub.o (c), where V.sub.A is the 
measured value of V.sub.diff (t) at time T.sub.A, V.sub.B is the measured 
value of V.sub.diff (t) at time T.sub.B, and V.sub.D is the measured value 
of V.sub.diff (t) at time T.sub.D. 
In view of the foregoing discussions, it should now be readily apparent to 
those skilled in the art that the tau.sub.c of any coin, whether of low or 
high magnetic permeability, can be derived based upon three or more 
time-voltage measurement pairs of V.sub.diff (t). Those skilled in the are 
will further recognize, however, that, while tau.sub.c can be uniquely 
derived for any coin based upon three time-voltage measurement pairs 
obtained during a coin examination operation, if multiple coin examination 
operations are conducted with respect to a given coin, as is often the 
case, it may be possible during the course of such multiple coin 
examination operations of the given coin c to determine K.sub.o (c), or a 
reasonable approximation thereof, as a consequence of which it may then be 
possible during a subsequent coin examination operation with respect to 
such coin c, since K.sub.o (c) is then known for such coin, to uniquely 
derive tau.sub.c for such coin, even if such coin is a coin of high 
magnetic permeability, based upon only two subsequently obtained 
time-voltage measurement pairs. The manner in which this may be 
accomplished will become clearer from that which follows. 
From all of the foregoing, it should now be readily understood that the 
presence of any coin within the field of the sensor coil 42, whether such 
coin is of low or high magnetic permeability, will effect a circuit 
reaction upon application of a current ramp to such coil such that the 
voltage difference V.sub.diff (t) may be expressed in terms of a decaying 
exponential, the time constant of which is the ratio tau.sub.c of the 
coin's inductance L.sub.c to its resistance R.sub.c, which ratio value can 
be utilized to distinguish between different denominations of coins. In 
the embodiment depicted in FIG. 1, detector means 52 and control means 50 
operate in conjunction with one another to derive, from the circuit 
performance characteristics of circuit 46, the tau.sub.c value for any 
given coin within the field of the coil 42 at the time a current ramp is 
applied to such coil, and to then determine whether such derived tau.sub.c 
value is a value representative of a valid coin. The tau.sub.c value may 
be derived from the circuit performance characteristics in a variety of 
ways, including, by way of example only and not by way of limitation, (1) 
by detecting the instantaneous voltage value of V.sub.diff (t) at a 
plurality of known times and by then utilizing such time-voltage pairs to 
calculate the time constant tau.sub.c of the decaying exponential, (2) by 
noting for a plurality of different, selected voltage values the times at 
which V.sub.diff (t) equals such voltage values and by then utilizing such 
time-voltage pairs to calculate t-he time constant tau.sub.c of the 
decaying exponential, (3) by detecting the instantaneous voltage of 
V.sub.diff (t) at a selected time, by thereafter noting the later times at 
which V.sub.diff (t) becomes equal to known lower voltages, and by then 
utilizing such time-voltage pairs to calculate the time constant tau.sub.c 
of the decaying exponential, or (4) by detecting a first instantaneous 
voltage of V.sub.diff (t) at a given time, by thereafter noting the later 
times at which V.sub.diff (t) becomes equal to other voltages that are 
some fractional values of the first voltage, and by then utilizing such 
time-voltage pairs to calculate the time constant tau.sub.c of the 
decaying exponential. With all of such enumerated methods a plurality of 
time-voltage pairs can be readily obtained for use in deriving tau.sub.c. 
With such a background, it is now appropriate to turn our attention to FIG. 
5, which figure depicts in greater detail than FIG. 1 a particular 
embodiment of the coin detection means of the present invention that can 
be advantageously utilized to determine tau.sub.c values for deposited 
coins, especially coins of low magnetic permeability. For discussion 
purposes with regard to FIG. 5, it is most appropriate to presume that 
some means for detecting and/or separating coins of high magnetic 
permeability is employed prior to examination of the remaining low 
magnetic permeability coins by the FIG. 5 embodiment. In such FIG. 5 
embodiment the detector means 52 of the FIG. 1 embodiment is shown 
including a differential amplifier means 60 having a first input 62 
connected via lead 64 to circuit 46 to monitor the voltage present across 
sensor coil 42 and a second input 66 connected via lead 68 to a reference 
signal generator 70 which is controlled by reference adjust data provided 
thereto by control means 50 via data path 72. The output 74 of 
differential amplifier means 60 is connected, first, via lead 76 to an A/D 
input 78 of control means 50, secondly, via leads 80 and 82 to input 84 of 
a comparator 86, and, thirdly, via leads 80 and 88 to a sample and hold 
means 90 which is controlled by a sample control signal supplied thereto 
over lead 92 from control means 50. The output 94 of sample and hold means 
90 is connected to a voltage divider circuit 96 which includes resistors 
98 and 100 and a pick-off point 102 between such resistors, from which 
point 102 a lead 104 is connected to the second input 106 of comparator 
86, the output 108 of which is connected via lead 110 to a timer input 112 
of control means 50. 
In operation, when no coin is present within the field of the sensor coil 
42, the resulting voltage V.sub.s across the sensor coil following the 
application of a current ramp I=k.sub.1 t to the coil will be k.sub.1 
L.sub.s +k.sub.1 R.sub.s t, and the reference voltage V.sub.ref produced 
by the reference signal generator 70 under control of reference signal 
data provided to such reference signal generator from the control means 50 
will ideally also be maintained at a value essentially equal to k.sub.1 
L.sub.s +k.sub.1 R.sub.s t, as a consequence of which the output of 
differential amplifier means 60 will be maintained at a null reference 
value, which, for the present, can be considered to be essentially zero. 
The control means 50, which may take many forms, including that of a 
programmed microprocessor, is so constructed, designed, and/or programmed 
that, so long as the output of differential amplifier means 60 remains at 
essentially the null reference value, such control means recognizes that 
no detection of a deposited coin by the coin detection means of the 
present invention has occurred and no processing of information therefrom 
is necessary for purposes of coin validation and/or discrimination. 
On the other hand, if a coin is present within the field of the sensor coil 
42 when a current ramp I=k.sub.1 t is produced by the current ramp 
generator under control of control means 50 and applied to the sensor 
coil, the resulting voltage across the coil will be V.sub.s(cp) =k.sub.1 
L.sub.s +k.sub.1 R.sub.s t-k.sub.2 e.sup.-t/tau.sbsp.c while the reference 
voltage will remain V.sub.ref =k.sub.1 L.sub.s +k.sub.1 R.sub.s t, as a 
consequence of which the output of the differential amplifier means 60 
will be V.sub.diff (t)=k.sub.2 e.sup.-t/tau.sbsp.c. The control means 50 
may be so constructed, designed, and/or programmed to respond to such 
change in value of the V.sub.diff (t) signal to effect the production of a 
sample control signal on lead 92 to cause sample and hold means 90 to 
sample the V.sub.diff (t) signal being provided thereto via leads 80 and 
88 at such time, designated T.sub.A, and to thereafter maintain such 
sampled value V.sub.S&H =V.sub.diff (T.sub.A)=V.sub.A on output 94 of 
sample and hold means 90. Such V.sub.A value, which is also available at 
the time of sampling at A/D input 78 of control means 50, may be stored or 
otherwise maintained by the control means 50 for future reference and use. 
The control means may also employ a timer started in some fashion at time 
T.sub.A, or it may store or otherwise maintain a T.sub.A value for future 
reference and use. With the FIG. 5 embodiment, when V.sub.S&H =V.sub.A is 
established and thereafter maintained on output 94 of sample and hold 
means 90, a lower voltage, designated V.sub.B, is established and 
thereafter maintained on input 106 of comparator means 86 due to voltage 
divider circuit 96. Such V.sub.B value may also be computed by control 
means 50, from the V.sub.A value and known component values of resistors 
98 and 100, and stored or otherwise retained by the control means for 
future reference and use. So long as V.sub.diff (t) is greater than 
V.sub.B, i.e., V.sub.diff (t) &gt;V.sub.B, the output of comparator means 86 
will remain HI. When the value of V.sub.diff (t) has decayed such that 
V.sub.diff (t) is less than or equal to V.sub.B, i.e., V.sub.diff 
(t).ltoreq.V.sub.B, the output of comparator means 86 will go LO. The time 
at which such change from a HI to a LO in the output state of comparator 
means 86 occurs is designated T.sub.B. The control means 50 may be so 
constructed, designed, and/or programmed to respond to such change of 
state, detectable at timer input 112, to cause the timer started at time 
T.sub.A to stop, or to otherwise establish and store or otherwise maintain 
a T.sub.B value for future reference and use. 
It will be appreciated by those skilled in the art that the control means 
50 can be so constructed, designed, and/or programmed to readily utilize 
the two time-voltage pairs that are thus obtained for V.sub.diff (t) , 
viz., (T.sub.A,V.sub.A) and (T.sub.B,V.sub.B), to derive a tau.sub.c value 
representative of any coin of low magnetic permeability that undergoes 
examination by such embodiment. The control means 50 can be further 
constructed, designed, and/or programmed in any number of ways to 
thereafter determine whether the particular tau.sub.c value derived 
represents a valid coin and/or to determine the denomination of the 
deposited coin. With a microprocessor based control means it is a 
relatively simple matter to effect comparisons between the derived 
tau.sub.c value and pre-established coin acceptance values, which values 
have typically been entered in read/write memories and stored therein for 
later retrieval and use. 
Many known systems operate, through such use of comparisons, to determine 
whether or not, based upon some measured characteristic of the deposited 
coin, such coin is a valid coin or is a coin of a particular denomination. 
As has been discussed hereinbefore, however, with many such systems, 
because of the tuning requirements associated therewith arising from minor 
variations in component values from individual construction to individual 
construction, and the resultant need to be able to enter comparison data 
that is specific to and correct for a particular construction, the 
provision of some form of read/write or alterable memories for the storage 
of such comparison values has been a necessity. With such types of 
memories, data tailored to a particular construction can be readily 
entered during tuning to ensure that the system will function properly. 
Memory alterability also permits the system to be relatively easily 
adjusted at a later time to correct for circuit drift, which, if not 
compensated for, could result in unacceptable degradation of performance 
by such systems over an extended period of time. 
While it is entirely feasible and possible to employ programmable or 
alterable memories in various embodiments of the present invention for the 
storage of comparison values, such types of memories are not required by 
the present invention since the tuning of individual constructions can 
generally be dispensed with. Consequently, and by way of example, with the 
present invention the comparison values can be provided in a masked ROM or 
can be established by hardwiring, both of which options avoid the 
necessity for programmable or alterable memory as the storage medium for 
the comparison values. It will be understood that, while such advantage is 
not at the heart of the present invention, because of the independence of 
the derived tau.sub.c characteristic from system component values and 
variations therein, from coin-to-coil distances when the coin is within 
the field of the sensor coil, and from other miscellaneous factors which 
may vary from individual construction to individual construction for any 
given system, the realization of such advantage may optionally be readily 
achieved through an appropriate design of the control means. 
FIG. 6 depicts in greater detail an embodiment of the coin detection means 
of the present invention constructed in general accordance with FIG. 5, 
and FIGS. 7-9 are enlargements of certain portions of FIG. 6 provided for 
purposes of clarification and explanation. For explanation purposes, 
certain of the numbered circuit components in FIG. 6 are also identified 
by alternate designations in FIGS. 7-9, which alternate designations will 
generally be found hereinafter set forth in parentheses following the 
numerical designation. 
The current ramp generator 48 is depicted in FIGS. 6 and 7 as including an 
operational amplifier 130 whose inverting input (-) 132 is connected 
through resistor 134 (R.sub.43) to a +12 V source and through circuit 
portion 136, which includes capacitor 138 (C.sub.11) and switch means 140 
connected in parallel with one another, to node 142, which node is shown 
connected to a +8 V source through resistor 144 (R.sub.7), to the emitter 
146 of a transistor 148 (Q.sub.2) which is connected in a Darlington 
configuration with transistor 150 (Q.sub.1), and to the emitter 152 of 
transistor 150 through resistor 154. The base 156 of transistor 150 is 
connected to the output 158 of operational amplifier 130, and the 
collectors 160 and 162 of such transistors 148 and 150 are tied together 
and connected via lead 163 to node 164. The non-inverting input (+) 166 of 
such operational amplifier 130 is connected to a +8 V source through 
attenuator circuit 168, which circuit includes resistors 170 (R.sub.13) 
and 172 (R.sub.12) and a capacitor 174 (C.sub.13) connected in parallel 
with resistor 170. The switch means 140, which might typically take the 
form of an FET, is depicted including a control input 175 connected to 
receive control signals from the system control means 50. 
With particular reference now to FIG. 7, those skilled in the art will 
readily understand that the voltage V.sub.2 at non-inverting input (+) 166 
of operational amplifier 130 satisfies the equation V.sub.2 =[R.sub.12 
/(R.sub.12 +R.sub.13)]8, that I.sub.out =I.sub.R7 due to the high gain of 
the Darlington pair Q.sub.1 and Q.sub.2, and that the operational 
amplifier 130 acts to force the voltage V.sub.1 at the inverting input (-) 
132 thereof to be equal to the voltage V.sub.2 at the non-inverting input 
(+) 166 thereof at all times. So long as the switch means 140 remains 
closed, the shunt path through such switch means ensures that the voltage 
V.sub.3 at node 142 remains essentially equal to the voltage V.sub.1 at 
inverting input (-) 132 of operational amplifier 130, as a consequence of 
which V.sub.3 =V.sub.1 =V.sub.2. The voltage drop across resistor 144 
(R.sub.7) at such time will therefore be 8-V.sub.3 =8-V.sub.2 =8-[R.sub.12 
/(R.sub.12 +R.sub.13)]8=8(1-[R.sub.12 /(R.sub.12 +R.sub.13))]). and the 
current through resistor 144 (R.sub.7) will thus be I.sub.R7 
=8(1-[R.sub.12 /(R.sub.12 +R.sub.13)])/R.sub.7. Since I.sub.out =I.sub.R7 
the output current produced by the current ramp generator 50 of FIGS. 6 
and 7 while the switch means 140 remains closed will be I.sub.out 
=8(1-[R.sub.12 /(R.sub.12 +R.sub.13)])/R.sub.7 =8[R.sub.13 /(R.sub.12 
+R.sub.13)]/R.sub.7. Ideally, I.sub.out would equal zero, which would be 
the case if R.sub.13 were equal to zero or R.sub.12 were equal to 
infinity. As a practical matter, however, R.sub.12 and R.sub.13 are 
selected to have values such that a small bias current, typically on the 
order of approximately 6 ma., is produced, which bias current serves to 
improve the response rate of the current ramp generator. 
When the switch means 140 opens, the shunt path through such switch means 
is eliminated, with the result that the current I.sub.C11 through 
capacitor 138 (C.sub.11) will thereafter be essentially the same as the 
current I.sub.R43 through resistor 134 (R.sub.43), i.e., I.sub.C11 
=I.sub.R43. Since 
EQU V.sub.1 =V.sub.2 =[R.sub.12 /(R.sub.12 +R.sub.13)])8, 
EQU I.sub.R43 =(12-V.sub.1)/R.sub.43 =(12-[R.sub.12 +R.sub.13)]8) /R.sub.43, 
EQU and 
EQU I.sub.C11 =C.sub.11 d(V.sub.1 -V.sub.3)/dt=C.sub.11 d([R.sub.12 /(R.sub.12 
+R.sub.13)]8-V.sub.3)/dt. 
EQU Therefore, 
EQU [R.sub.12 /(R.sub.12 +R.sub.13)]8-V.sub.3 =1/C.sub.11 .sup.t I.sub.C11 dt, 
EQU and 
##EQU2## 
taking the opening of switch means 140 as time t.sub.0 =0. The current 
I.sub.R7 through resistor 144 (R.sub.7) can then be expressed as I.sub.R7 
=(8-V.sub.3)/R.sub.7, or, substituting for V.sub.3, 
EQU I.sub.R7 =[8-([(R.sub.12 /[R.sub.12 +R.sub.13 ]) 8]-1/C.sub.11 
[(12-[R.sub.12 /(R.sub.12 +R.sub.13)]8)/R.sub.43 ]t(]/R.sub.7, 
which can be rewritten in a more simplified form as 
EQU I.sub.R7 =(8/R.sub.7)(1-[R.sub.12 /(R.sub.12 +R.sub.13)])+(8t/R.sub.7 
C.sub.11 R.sub.43)(1.5-[R.sub.12 /(R.sub.12 +R.sub.13)]). 
Since I.sub.out =I.sub.R7 as previously discussed, it is thus the case that 
EQU I.sub.out =(8/R.sub.7)(1-[R.sub.12 /(R.sub.12 +R.sub.13)])+(8t/R.sub.7 
C.sub.11 R.sub.43)(1.5-[R.sub.12 /(R.sub.12 +R.sub.13)]). 
Typically, the values of R.sub.7, R.sub.12, and R.sub.13 might be selected 
to yield a current I.sub.out approximately equal to 0.55 ma. per 
microsecond times the time, i.e., I.sub.out =(0.55 ma./us)t. FIGS. 10 and 
11 depict typical waveforms for V.sub.3 and I.sub.out, consistent with the 
foregoing discussion. 
With reference next to FIGS. 6 and 8, it may be observed that node 164 is 
connected to one side of an RL circuit 176 that includes sensor coil 42 
and a resistor 177 (R.sub.9) connected in parallel therewith, the other 
side of which circuit 176 is connected to ground. It will be appreciated 
by those skilled in the art that resistor 177 (R.sub.9) is not required 
for proper operation of the depicted construction, but is provided as an 
optional element to help reduce self oscillation of the sensor coil 42 and 
to thereby effect an improvement in system performance. 
Node 164 is also shown connected to an input protection circuit 178 that 
includes a resistor 179 (R.sub.16), one side of which is connected to node 
164 and the other side of which is connected both to input 180 of the 
control means 50 and to the cathode 181 of a diode 182 (D.sub.2) The anode 
183 of such diode is connected to node 184 in a voltage divider circuit 
wherein node 184 is connected between a resistor 186 (R.sub.15) tied to a 
+5 V source and a grounded resistor 188 (R.sub.14). Like resistor 177, 
circuit 178 is similarly not required, but is depicted for the purpose of 
showing an optional input protection circuit that may be advantageously 
employed when it is desired to be able to supply V.sub.s (t) to the 
control means 50, which control means might typically include a Motorola 
6805R3 microprocessor, while ensuring that the input voltage supplied to 
the microprocessor will not exceed the input range of the microprocessor. 
Those skilled in the art will readily understand that the circuit 178 acts 
to limit the voltage supplied as an input to system control means 50 when 
the value of V.sub.s (t) exceeds a voltage determined by the values of 
resistors 174 (R.sub.16), 186 (R.sub.15), and 188 (R.sub.14) in order to 
ensure that the voltage level supplied will be within the input range of 
the microprocessor utilized. It should be understood that, for proper 
operation of the disclosed construction, it is not necessary that V.sub.s 
(t) be provided to the control means in any way, and that the provision of 
such signal as depicted in FIGS. 6 and 8 is for the purpose of indicating 
that such signal could be readily provided to the control means, if 
desired for informational purposes. 
In addition to the connections previously noted, node 164 is also connected 
both to a kickback protection circuit 190 and through a resistor 192 
(R.sub.31) to input 62 of differential amplifier means 60. The kickback 
protection circuit 190 includes a diode 194 (D.sub.1), the cathode 196 of 
which is connected to node 164 and the anode 198 of which is connected to 
a grounded RC tank circuit 200 that includes resistor 202 (R.sub.11) and 
capacitor 204 (C.sub.12) It will be readily understood that the circuit 
190 is an optional circuit, the purpose of which is to absorb and 
dissipate the kickback of the sensor coil 42 when the current ramp is 
turned off. As provided, such circuit has no effect upon V.sub.s during 
the time that the ramp current generator 48 is supplying a ramp current to 
sensor coil 42. 
With the foregoing in mind it will be appreciated that FIG. 12 depicts a 
typical V.sub.s(nc) waveform such as might be obtained when resistor 177 
is employed, and also illustrates the kickback that occurs when the 
current ramp is disabled. FIG. 13 illustrates, in combination with FIG. 
12, the effect of kickback protection circuit 190 in providing kickback 
protection. 
With reference now again to FIG. 6, as well as to FIG. 9, the reference 
signal generator 70 is depicted as including an offset reference portion 
210 and a slope reference portion 212. The offset reference portion 210 is 
designed to provide a DC output voltage V.sub.offset, the purpose of which 
is to effectively "cancel out" all DC offsets of the system. Such depicted 
offset reference portion 210 includes an operational amplifier 214 
connected as a voltage follower with its non-inverting input (+) 216 
connected to receive the output of a band pass filter 218 that includes 
capacitor 220 (C.sub.9) and resistor 222 (R.sub.10) and with its inverting 
input (-) 224 connected to amplifier output 226. The input of the band 
pass filter 218 is connected to both the cathode 230 of a first diode 232 
(D.sub.5) and the anode 234 of a second diode 236 (D.sub.6) The anode 238 
of the first diode 232 is connected both to output 240 of system control 
means 50 and through a pull-up resistor 242 (R.sub.23) to a +5 V source, 
and the cathode 246 of the second diode 236 is similarly connected both to 
output 250 of system control means 50 and through pull-up resistor 252 
(R.sub.24) to the +5 V source. Adjustments in the value of V.sub.offset 
are effected by the production of signals at outputs 240 and 250 of the 
system control means 50. If an increase in the value of V.sub.offset is 
desired, HI signals are produced on both of outputs 240 and 250, as a 
consequence of which capacitor 220 will gradually charge thereby resulting 
in a gradual increase in the value of V.sub.offset. If a decrease in the 
value of V.sub.offset is desired, LO signals are produced on both of 
outputs 240 and 250, as a consequence of which capacitor 220 will slowly 
discharge thereby resulting in a gradual decrease in the value of 
V.sub.offset. To maintain the value of V.sub.offset, it is generally 
desirable to provide a LO signal at output 240 and a HI signal at output 
250 in order to ensure that no charging or discharging of the capacitor 
220 will occur. FIG. 14 depicts a typical V.sub.offset waveform. 
The slope reference portion 212 includes an operational amplifier 254 whose 
non-inverting input (+) 256 is connected to receive the output of a band 
pass filter 258 that includes capacitor 260 (C.sub.8) and resistor 262 
(R.sub.6) and whose inverting input (-) 264 is connected through resistors 
266 (R.sub.5) and 268 (R.sub.4) to a +8 V source and through circuit 
portion 270, which includes capacitor 272 (C.sub.10) and switch means 274 
connected in parallel with one another, to amplifier output 276. The input 
of such band pass filter 258 is connected to both the cathode 282 of a 
first diode 284 and the anode 286 of a second diode 288. The anode 290 of 
the first diode 284 is connected both to output 292 of system control 
means 50 and through a pull-up resistor 294 (R.sub.25) to a +5 V source, 
and the cathode 296 of the second diode 288 is similarly connected both to 
output 302 of system control means 50 and through pull-up resistor 304 
(R.sub.26) to the +5 V source. 
Another capacitor 278 is provided with one side connected between resistors 
266 (R.sub.5) and 268 (R.sub.4) and with the other side thereof connected 
between resistor 262 (R.sub.6) and capacitor 260 (C.sub.8), the purpose of 
which capacitor is to minimize noise on the inputs of the differential 
amplifier. Such capacitor 278 is preferably chosen, if employed at all, to 
have such a value in relation to other components that noise minimization 
can be realized through the use of such capacitor and without any other 
appreciable effect upon circuit performance. 
The switch means 274, which might typically take the form of an FET, is 
depicted including a control input 306 connected to receive control 
signals from the system control means 50. At times prior to t.sub.o, i.e., 
at times prior to generation of a current ramp by the current ramp 
generator 48, switch means 274 remains closed, as a consequence of which 
V.sub.slope is maintained at some constant DC voltage, typically 
approximately 3 V, equal to the voltage V.sub.6 present at input 256 of 
amplifier 254 and across capacitor 260, and V.sub.7 =V.sub.6. Those 
skilled in the art will understand that, when the switch means 274 opens 
at time t.sub.o, the shunt across capacitor 272 is removed and the value 
of V.sub.slope will thereafter satisfy the equation V.sub.slope =V.sub.6 
-At, where A is a gain factor approximately equal to (8-V.sub.6)/[(R.sub.4 
+R.sub.5)C.sub.10 ]. Consequently, after the switch means 274 has opened, 
V.sub.slope =V.sub.6 -[(8-V.sub.6)/[(R.sub.4 + R.sub.5)C.sub.10 ]]t. 
The value of V.sub.6 can be readily adjusted under control of system 
control means 50 in much the same way V.sub.offset is adjusted, based upon 
the signals provided at outputs 292 and 302 of system control means 50. 
FIG. 15 depicts a typical V.sub.slope waveform. 
Output 226 of operational amplifier 214 and output 276 of operational 
amplifier 254 are connected through respective resistors 310 (R.sub.33) 
and 316 (R.sub.30) to summing node 314, at which is present the composite 
reference signal V.sub.ref =V.sub.offset +V.sub.slope that is provided via 
lead 68 to input 66 of differential amplifier means 60. The differential 
amplifier means 60 of FIG. 5 is depicted in FIG. 6 as including a 
differential amplifier 320 the inverting input (-) 322 of which is 
connected, in common, at node 324, through summing node 325, to inputs 62 
and 66 of differential amplifier means 60, to cathode 326 of a diode 328 
whose anode 330 is connected to the non-inverting input (+) 332 of such 
differential amplifier 320, and to one side of a circuit portion 336, the 
other side of which is connected to output 338 of such differential 
amplifier 320. Circuit portion 336 includes resistor 340 (R.sub.29) 
connected in parallel circuit with two separate resistor-switch series 
circuits, the first of which includes a resistor 342 (R.sub.28) connected 
in series circuit with an switch means 344 and the second of which 
includes a resistor 346 (R.sub.27) connected in series circuit with a 
switch means 348. The switch means 344 and 348, which might typically take 
the form of FETs, are depicted including respective control inputs 350 and 
352 connected to receive control signals from the system control means 50. 
The non-inverting input (+) 332 of differential amplifier 320, in addition 
to being connected to the anode 330 of diode 328, is connected through 
resistor 360 (R.sub.32) and leads 362 and 364 to node 365 of a voltage 
divider circuit 366 that includes resistors 368 (R.sub.34) and 370 
(R.sub.35) connected between a +5 V source and ground, with a capacitor 
372 connected in parallel circuit with resistor 370. 
Differential amplifier 320 functions as a classical summing amplifier using 
variable gain, where the gain is determined in large part by the status of 
switch means 344 and 348. If both of such switch means are open, the 
effective resistance R.sub.eff of circuit portion 336 is R.sub.29, 
whereas, if switch means 344 is closed and switch means 348 is open, 
R.sub.eff =1/](1/R.sub.29)+(1/R.sub.28)]=(R.sub.29 R.sub.28) /(R.sub.29 
+R.sub.28), and if both switch means are open, R.sub.eff 
=1/[(1/R.sub.29)+(1/R.sub.28)+(1/R.sub.27)]. Those skilled in the art will 
recognize that, with the circuit configuration depicted in FIG. 6, the 
voltage V.sub.o at output 338 of differential amplifier 320 will satisfy 
the equation V.sub.o =V.sub.nulr -[(V.sub.s -V.sub.nulr)/R.sub.31 
+(V.sub.offset -V.sub.nulr)/R.sub.33 +(V.sub.slope -V.sub.nulr)/R.sub.30 
]R.sub.eff, where V.sub.nulr is the null reference voltage value at 
non-inverting input (+) 332 of differential amplifier 320. It will be 
appreciated that component values may be so chosen and V.sub.offset, 
V.sub.slope, and V.sub.nulr so set that, in the absence of a coin, V.sub.o 
=V.sub.nulr, which value, in the embodiment of FIG. 6, may typically be 
approximately 2.5 V. Such values may be further chosen and set, consistent 
with the foregoing teachings, such that V.sub.o (t)=V.sub.nulr +k.sub.2 
e.sub.c.sup.(-t/tau) -K.sub.o (c), which can be considered to be simply 
V.sub.o (t)=V.sub.nulr +k.sub.2 e.sup.(-t/tau.sbsp.c.sup.) for a coin of 
low magnetic permeability since K.sub.o (c) for such coins is essentially 
zero. 
The purpose of the switch means 344 and 348 and the reasons for providing 
for variable gain for the differential amplifier 320 will be addressed in 
greater detail hereinafter. For ease of discussion and understanding at 
this point, it may be presumed that switch means 344 and 348 remain open 
and that the effective resistance R.sub.eff of circuit portion 336 remains 
equal to R.sub.29 at all times of interest. 
The output 338 of differential amplifier 320 is shown connected through 
lead 74, resistor 370, and lead 76 to input 78 of system control means 50, 
the purpose of which input has been previously explained with reference to 
FIG. 5. The voltage divider circuit 372, including resistors 374 and 376 
connected between a +5 V source and ground, and diode 378, is provided as 
shown in order to limit the voltage value supplied to input 78 of system 
control means 50. Output 338 is further connected through leads 74, 80, 82 
and resistor 380 to input 84 of comparator means 86 and through leads 74, 
80, and 88 to one side 381 of a switch means 382 of sample and hold means 
90. 
Depicted within sample and hold means 90 is an operational amplifier 390 
shown connected as a voltage follower with its inverting input (-) 392 
connected to its output 394 and with its non-inverting input (+) 395 
connected both to the second side 396 of switch means 382 and to one side 
of a circuit portion 398 that includes a capacitor 400 (C.sub.20) 
connected in parallel with switch means 402, the other side of which 
circuit portion 398 is connected through leads 404 and 364 to node 365 of 
voltage divider circuit 366. Switch means 382 has a control input 406 
connected to receive control signals from system control means 50, and 
switch means 402 has a control input 408 similarly connected to the system 
control means to receive control signals therefrom. 
The operation of sample and hold means 90 is dependent upon the status of 
switch means 382 and 402. If switch means 382 is maintained open and 
switch means 402 is maintained closed, the sample and hold means 90 is in 
a nulling mode of operation in which the voltage V.sub.12 at non-inverting 
input (+) 395 is equal to V.sub.nulr as established by voltage divider 
circuit 366 due to the shunting action of switch means 402. Since 
operational amplifier 390 is connected in a voltage follower configuration 
the output V.sub.S&H thereof is maintained essentially equal to V.sub.12, 
i.e., V.sub.S&H =V.sub.12 =V.sub.nulr, during such nulling mode of 
operation. If switch means 402 is maintained open, however, sample and 
hold means 90 operates to sample V.sub.o (t) at a given point in time, 
i.e., when switch means 382 briefly closes and re-opens, and sample and 
hold means 90 thereafter maintains or holds a voltage value corresponding 
to such sampled V.sub.o (t) value at output 394 of operational amplifier 
390 until the next sample of V.sub.o (t) is taken. 
Output 394 of operational amplifier 390 is connected to one side of a 
voltage divider circuit 410 that includes resistor 98 (R.sub.60), node 
412, and resistor 100 (R.sub.61), the other side of which circuit is 
connected to node 365, which node is maintained at V.sub.nulr. Node 412 is 
connected to input 106 of comparator 86, which is depicted in FIG. 6 as an 
operational amplifier 420 whose inverting input (-) 422 is connected both 
to input 106 and to one side of a capacitor 424 and whose non-inverting 
input (+) 426 is connected both to the other side of such capacitor 424 
and to input 84 of comparator means 86. Output 428 of such operational 
amplifier 420 is connected both to output 108 of comparator means 86 and 
through a pull-up resistor 430 to a +5 V source. Output 108 is connected 
through lead 110 to timing input 112 of system control means 50 and 
through lead 110 and low pass filter circuit 430, including resistor 432 
and grounded capacitor 434, to input 436 of system control means 50. With 
such a configuration the voltage V.sub.comp (t) at output 428 of 
operational amplifier 420 will remain HI so long as V.sub.o (t) is greater 
than V.sub.k, i.e., V.sub.o (t)&gt;V.sub.k, and will go LO when V.sub.o (t) 
falls below V.sub.k, i.e., V.sub.o (t)&lt;V.sub.k. It will be understood by 
those skilled in the art that V.sub.k =V.sub.nulr when sample and hold 
means 90 is in a nulling mode. By monitoring V.sub.comp (t) during such 
nulling mode system control means 50 can determine whether the values of 
V.sub.offset and V.sub.slope have been appropriately adjusted so that 
V.sub.o (t)=V.sub.nulr. If not, adjustments in such values may be effected 
under control of system control means 50 in the manner previously 
described. 
In light of the preceding discussion, it will be appreciated that, during a 
coin detection operation cycle, if no coin is present within the field of 
coil 42, V.sub.o (t)=V.sub.nulr, as depicted in FIG. 16, but that, if a 
coin of low magnetic permeability is present in such field, V.sub.o 
(t)=V.sub.nulr +k.sub.2 e.sup.(t/tau.sbsp.c.sup.), as depicted in FIG. 17. 
In the latter case, if switch means 382 closes at time T.sub.A, V.sub.S&H 
is set equal to V.sub.o (t) at time T.sub.A, which value is defined as 
V.sub.a, i.e., V.sub.S&H =V.sub.o (T.sub.A)=V.sub.a =V.sub.nulr +V.sub.A, 
as a consequence of which V.sub.k =V.sub.nulr +(V.sub.a 
-V.sub.nulr)[R.sub.61 /(R.sub.60 +R.sub.61)], which becomes V.sub.k 
=V.sub.nulr +(V.sub.nulr +V.sub.A.sup.-V.sub.nulr)[R.sub.61 /(R.sub.60 
+R.sub.61)], or V.sub.k =V.sub.nulr +V.sub.A [R.sub.61 /(R.sub.60 
+R.sub.61)]=V.sub.nulr 30 V.sub.B, where V.sub.B =V.sub.A [R.sub.61 
/(R.sub.60 +R.sub.61)]So long as V.sub.o (t) remains greater than V.sub.k 
=V.sub.nulr +V.sub.B, i.e., V.sub.o (t)&gt;V.sub.nulr +V.sub.B, V.sub.comp 
(t) remains HI, but when V.sub.o (t) falls below V.sub.k =V.sub.nulr 
+V.sub.B, i.e., V.sub.o (t)&lt;V.sub.nulr +V.sub.B, which first occurs at a 
time defined as T.sub.B, V.sub.comp (t) goes LO, as depicted in FIG. 18. 
Since a voltage value corresponding to V.sub.o (t) is provided to input 78 
of system control means 50, it is a relatively simple matter for the 
system control means 50 to monitor such signal during a coin detection 
operation cycle, and, if V.sub.o (t) exceeds V.sub.nulr, i.e., if V.sub.o 
(t)&gt;V.sub.nulr, to effect the momentary closure of switch 382 at a time 
T.sub.A. By monitoring the V.sub.comp (t) signal provided to input 112 of 
system control means 50, such system control means can readily determine 
the time T.sub.B at which V.sub.comp (t) goes LO, which change in state of 
V.sub.comp (t) occurs when V.sub.B =V.sub.A [R.sub.61 /(R.sub.60 
+R.sub.61)]. Since times T.sub.A and T.sub.B will then be known, and since 
V.sub.A /V.sub.B =(R.sub.60 +R.sub.61)/R.sub.61, system control means 50 
can then readily derive tau.sub.c for such coin from the equation 
EQU tau.sub.c =[(T.sub.B -T.sub.A)/ln (V.sub.A /V.sub.B)]=((T.sub.B 
-T.sub.A)/ln[(R.sub.60 +R.sub.61)/R.sub.61 ]). 
From the foregoing, it should be readily apparent that the embodiment of 
FIG. 6 can be employed to derive tau.sub.c values for coins of low 
magnetic permeability and to differentiate between valid and invalid coins 
and between denominations of valid coins. Such differentiation can be 
accomplished for any given low magnetic permeability coin from a single 
coin detection operation cycle, although it has been found preferable to 
utilize a plurality of coin detection operation cycles for each coin to 
provide further verification of differentiation results. If such FIG. 6 
embodiment is utilized in a multiple cycle coin detection environment, 
which can be easily effected under control of the system control means 50, 
it then becomes possible with such embodiment to derive tau.sub.c values 
for coins of both low and high magnetic permeability. It will be recalled 
from discussions hereinbefore with regard to FIG. 4 that V.sub.diff(L) 
remains positive while V.sub.diff(H) eventually goes negative with respect 
to the base reference value, there taken to be essentially zero. As a 
consequence, when such V.sub.diff(H) waveform is provided to differential 
comparator means 60 along with V.sub.offset and V.sub.slope , the 
resulting waveform for V.sub.o(H) takes the form depicted in FIG. 19 
instead of the V.sub.o(L) waveform depicted in FIG. 17. As can be readily 
observed from FIG. 19, V.sub.o(L) thus goes negative with respect to 
V.sub.nulr at a time designated t.sub.1. Since a voltage value 
corresponding to V.sub.o (t) is provided to input 78 of system control 
means 50, the system control means 50 can easily be constructed and/or 
programmed to monitor such input value during a coin detection operation 
cycle to determine whether or not a coin is present within the field of 
sensor coil 42 and, if so, whether such coin is a coin of low or high 
magnetic permeability. If no 1 coin is present, V.sub.o (t) should remain 
essentially constant at V.sub.nulr (FIG. 16). On the other hand, if a coin 
is present, V.sub.o (t) should go positive and remain positive with 
respect to V.sub.nulr if the coin is a coin of low magnetic permeability 
(FIG. 17), but should eventually go negative with respect to V.sub.nulr if 
the coin is a coin of high magnetic permeability (FIG. 19). 
It will be appreciated by those skilled in the art that a number of coin 
detection operation cycles may be controllably effected during the time 
that any deposited coin is within the field of sensor coil 42. In such 
circumstances, when the coin first moves within the field of sensor coil 
42 the embodiment of FIG. 6 can operate during a first coin detection 
operation cycle to detect the presence of such coin and to determine 
whether it is a coin of low or high magnetic permeability. System control 
means 50 can be readily constructed and/or programmed to be responsive to 
such determination to thereafter control the operation of switch means 
344, 348, and 382 to permit further coin discrimination with respect to 
both validity and denomination. 
If the coin detected is a coin of low magnetic permeability, the FIG. 6 
embodiment may thereafter operate in the fashion already previously 
described to derive a tau.sub.c value for such detected low magnetic 
permeability coin. On the other hand, if the coin detected is a coin of 
high magnetic permeability, system control means 50 may be so constructed 
and/or programmed to thereafter operate and control the operation of the 
FIG. 6 embodiment during the course of two or more coin detection 
operation cycles whereby a tau.sub.c value may be derived for such 
detected high magnetic permeability coin. 
It may be observed from FIG. 19 that, since V.sub.o (t)=V.sub.nulr +k.sub.2 
e.sup.(-t/tau.sbsp.c.sup.) -K.sub.o (c), V.sub.o(H) decays to an end value 
of essentially V.sub.nulr -K.sub.o (c) at time equal to infinity. In light 
thereof, it has been found that if system control means 50 is so 
constructed and/or programmed to respond to detection of the presence of a 
coin of high, as opposed to low, magnetic permeability by appropriately 
altering the time of operation of switch means 382 of the FIG. 6 
embodiment, an approximate tau.sub.c value for such detected high magnetic 
permeability coin can then be derived during a subsequent coin detection 
operation cycle through utilization of the FIG. 6 embodiment already 
previously described. If, for a high magnetic permeability coin, the 
momentary closure of switch means 382 of the FIG. 6 embodiment is delayed 
during a first coin detection operation cycle from the relatively early 
time it would normally close if a low magnetic permeability coin were 
undergoing examination until a time considerably later in such coin 
detection operation cycle, and if such switch means 382 is then 
momentarily closed at such late time, designated as t.sub.f, to sample 
V.sub.o (t), the sampled value of V.sub.o (t) will approximate such noted 
end value of V.sub.nulr -K.sub.o (c) for V.sub.o (H), i.e., V.sub.S&H 
=V.sub.o (t.sub.f)=V.sub.f =V.sub.nulr -K.sub.o (c). As will be apparent 
from FIG. 6, if V.sub.S&H =V.sub.nulr -K.sub.o (c), V.sub.k then satisfies 
the equation 
##EQU3## 
As can be observed from FIG. 19, at time t.sub.f, when the value of 
V.sub.k is established at the above-noted value, V.sub.o (t) is less than 
V.sub.k, i.e., V.sub.o (t)&lt;V.sub.k, and V.sub.o (t) thereafter remains 
less than V.sub.k for the remainder of such first coin detection operation 
cycle. Since V.sub.o (t) remains less than V.sub.k for the remainder of 
such first coin detection operation cycle, V.sub.comp (t) will remain LO 
for the duration of such coin detection operation cycle. 
From FIGS. 19 and 20 it may be readily observed that, once V.sub.k is 
established at time t.sub.f of a first coin detection operation cycle, 
V.sub.o (t) will not exceed such V.sub.k value until a succeeding coin 
detection operation cycle. At the beginning of a succeeding coin detection 
operation cycle V.sub.o (t) will initially go positive with respect to 
V.sub.nulr, and will then decay exponentially, eventually approaching the 
V.sub.f value of V.sub.nulr -K.sub.o (c) It has been found that, when 
switch means 382, during a first coin detection operation cycle, upon the 
detection of a coin of high magnetic permeability, has operated in the 
fashion described hereinbefore, time t.sub.o in the succeeding coin 
detection operation cycle may be considered, at least for many 
applications where total accuracy in the derivation of tau.sub.c is not 
critical, to be time T.sub.A, and voltage V.sub.A =V.sub.o (T.sub.A) at 
such time t.sub.o =T.sub.A may be considered to be approximately equal to 
V.sub.nulr. Time T.sub.B then becomes that time at which, during such same 
succeeding coin detection operation cycle, V.sub.o (t) falls below the 
value of V.sub.k as established based upon the sampled value of V.sub.o 
(t) at time t.sub.f in the first coin detection operation cycle. 
It will be appreciated that, since V.sub.o (t.sub.f)=V.sub.f and V.sub.f is 
considered to be essentially V.sub.nulr -K.sub.o (c), and since V.sub.A is 
considered to be essentially V.sub.nulr, the difference between V.sub.A 
and V.sub.f is K.sub.o (c), i.e., V.sub.A -V.sub.f =K.sub.o (c). In light 
thereof, and since V.sub.k =V.sub.nulr -K.sub.o (c)(1-[R.sub.60 /(R.sub.60 
+R.sub.61)]), it will further be appreciated that V.sub.k is (1-[R.sub.60 
/(R.sub.60 +R.sub.61)])th of the voltage differential between V.sub.f and 
V.sub.A, using V.sub.f as reference, as a result of which the value 
V.sub.B, at time T.sub.B, may be expressed with reference to V.sub.A as 
V.sub.B =V.sub.A [R.sub.60 /(R.sub.60 +R.sub.61)]. In light of all the 
foregoing, it will be recognized that, since a voltage value corresponding 
to V.sub.o (t) is provided to input 78 of system control means 50, it is a 
simple matter for the system control means 50 to determine a V.sub.f value 
at time t.sub.f of a first coin detection operation cycle and to 
thereafter, during a succeeding coin detection operation cycle, determine 
the time interval T.sub.B -T.sub.A from the V.sub.comp (t) output provided 
to inputs 112 and 436 of system control means 50. As has previously been 
explained, system control means 50 may then readily derive tau.sub.c for 
the detected coin from the equation 
EQU tau.sub.c =[(T.sub.B -T.sub.A)/1n(V.sub.A /V.sub.B)]=((T.sub.B -T.sub.A)/1n 
[(R.sub.60 +R.sub.61)/R.sub.60 ]). 
It may be recalled that it was previously indicated that system control 
means 50 can be so constructed and/or programmed to respond to the input 
signal provided to input 78 to thereafter control the operation of switch 
means 344, 348, and 382. As has previously been explained, the gain of 
differential amplifier means 60 depends, in part, upon the status of 
switch means 344 and 348. In light thereof it has been found desirable to 
be able to change the gain of differential amplifier means 60, depending 
upon whether a low or high magnetic permeability coin is undergoing 
examination, and consistent with the construction and/or programming of 
the system control means 50, to take advantage of the full input range 
available at the inputs for the system control means 50, whereby better 
resolution may be obtainable. It will be readily understood that such 
capability to change the gain of differential amplifier means 60 depending 
upon the magnetic permeability of the coin undergoing examination, though 
desirable, is not necessary for proper operation of the FIG. 6 embodiment. 
FIGS. 21 and 22 depict waveforms corresponding to V.sub.o (t) for various 
coins detected within the field of sensor coil 42 by a particular FIG. 6 
embodiment in which [R.sub.61 /(R.sub.60 +R.sub.61)]=3/7, and under 
certain conditions. FIG. 21 depicts the different waveforms obtained when 
a low magnetic permeability coin was detected, within the field of the 
sensor coil 42, during separate coin detection operation cycles, at three 
different distances, viz., 20 mills (20/1000th of an inch), 30 mills 
(30/1000th of an inch), and 40 mills (40/1000th of an inch), from the 
sensor coil 42. Since the same coin was utilized at the three different 
distances, the T.sub.A sampling time for the coin detection operation 
cycle at each of such distances, as effected by the momentary closure of 
switch means 382 under control of the system control means 50, occurred at 
the same time during each such coin detection operation cycle. The voltage 
V.sub.A sampled at such time T.sub.A, i.e., V.sub.A =V.sub.o (T.sub.A), 
was used to establish a V.sub.B voltage value for each such coin detection 
operation cycle in accordance with the equation V.sub.B =V.sub.A [R.sub.61 
/(R.sub.60 +R.sub.61)]=3/7)V.sub.A. It may be readily observed from FIG. 
21 that, for each waveform, the V.sub.B value associated with such 
waveform occurred at essentially the same time during the coin detection 
operation cycle as the V.sub.B values associated with the other two 
waveforms occurred during such other coin detection operation cycles, 
indicating that the tau.sub.c values for all of such waveforms are 
essentially equal. Such essential equality of tau.sub.c values for all of 
such waveforms verifies the relative independence of the derived tau.sub.c 
value with regard to coin-to-coil distance for a coin present within the 
field of the sensor coil 42, which independence, as previously discussed, 
permits coin detection means according to the present invention to be 
designed and constructed, if one so desires, utilizing only a single 
sensor coil instead of the plurality of sensor coils required with many 
prior art constructions. 
FIG. 22 depicts portions of different waveforms corresponding to V.sub.o 
(t) obtained when coins of different denominations, viz., nickel, dime, 
and quarter, were detected during respective coin detection operation 
cycles by a particular FIG. 6 embodiment in which [R.sub.61 /(R.sub.60 
+R.sub.61)]=3/7. Since all of such coins are coins of low magnetic 
permeability, the T.sub.A sampling times for the respective coin detection 
operation cycles were all effected at the same time point in each 
respective coin detection operation cycle. The voltage V.sub.A sampled at 
time T.sub.A in each respective coin detection operation cycle, i.e., 
V.sub.A =V.sub.o (T.sub.A), was used to establish a V.sub.B voltage value 
for such respective coin detection operation cycle in accordance with the 
equation V.sub.B =V.sub.A [R.sub.61 /(R.sub.60 +R.sub.61)]=(3/7)V.sub.A. 
For each of the waveforms a T.sub.B value was thus determined in 
accordance with previous discussions, and tau.sub.c was then derived from 
the equation 
##EQU4## 
where (T.sub.B -T.sub.A)=19 us. for a nickel, (T.sub.B -T.sub.A)=70 us. 
for a dime, and (T.sub.B -T.sub.A)=180 us. for a quarter. Based upon such 
determinations the tau.sub.c values for such coins, when such particular 
FIG. 6 embodiment was utilized, were calculated as tau.sub.nickel =22.4 
us., tau.sub.dime =82.6 us., and tau.sub.quarter =212 us. 
Those knowledgeable in the art will recognize that the same or similar 
results could be obtained from other embodiments, including embodiments 
that make greater use of digital techniques and microprocessor programming 
than does the embodiment of FIG. 6. FIG. 23 depicts one such embodiment 
that includes many circuit portions which are the same as or similar to 
those depicted in FIG. 6. The FIG. 23 embodiment includes a current ramp 
generator 48, many components of which are essentially identical to 
components employed in the current ramp generator 48 depicted in FIG. 6. 
In the FIG. 23 embodiment, however, switch means 140 is depicted as being 
an FET 450 with a resistor 452 connected between its gate (G) input 454 
and its source (S) connection 456. The gate (G) input 454 is also 
connected to control input 175 of switch means 140, which control input is 
connected to receive the output of an inverter 458 whose input 469 is 
connected to system control means 50 to receive ramp enable signals 
produced thereby. Resistor 452 is selected to have a value that is large 
compared to the value of resistor 144 (R.sub.7) so that it will have a 
negligible effect with respect to the operation of the current ramp 
generator 48, as previously described with regard to FIGS. 6 and 7, yet 
will ensure that the FET 450 properly and effectively turns off. 
With the FIG. 23 embodiment, reference signal generation is effected both 
through the employment of a resistor 462 connected between node 142 and 
summing node 325 and through the employment of other resistors 464-470 all 
connected in parallel with one another between summing node 325 and 
respective D/A outputs 474-480 of system control means 50. Resistor 462 is 
selected to have such a value that the effect thereof with respect to the 
operation of the current ramp generator 48, as previously described with 
regard to FIGS. 6 and 7 will be negligible, but to permit the negative 
going ramp produced at node 142 during a coin detection operation cycle, 
as shown in FIG. 24, to be provided to summing node 325 along with the 
signals present on outputs 474-480 of system control means 50 and the 
value of V.sub.S (t) present across sensor coil 42, the V.sub.S (nc) 
waveform of which is shown in FIG. 25. 
It may be readily observed that the particular components employed in the 
differential amplifier means 60 of the FIG. 23 embodiment are quite 
similar to those components employed in the differential amplifier means 
60 of the FIG. 6 embodiment, except for the deletion of the switch means 
344 and 348 and the resistors 342 and 348 associated therewith and the 
addition of a resistor 490 connected to the output 338 of differential 
amplifier 320, which resistor is connected within the feedback loop of 
such differential amplifier 320. As will be apparent to those skilled in 
the art, the purpose of such resistor 490 is to limit the output current 
and thereby prevent damage to the A/D converter circuitry within system 
control means 50. In light of previous discussions, it will readily 
understood that the V.sub.o (t) signal present at node 492 at any point in 
time during a coin detection operation cycle depends, in large part, upon 
the value of V.sub.S (t) at such time. For purposes of reference and 
comparison, FIG. 26 depicts a typical V.sub.o (t) waveform that is 
realized when no coin is present within the field of sensor coil 42. If a 
coin were present in such field during the application of a current ramp 
to the coil 42, the resulting waveform would include a decaying 
exponential factor, as has been previously discussed, the time constant of 
which is characteristic of such coin. 
From the foregoing discussions, and especially the discussions regarding 
the operation of the FIG. 6 embodiment, those skilled in the art will 
recognize that a microprocessor may be included in system control means 50 
and may be so programmed to obtain from the V.sub.o (t) signal provided to 
the analog-to-digital (A/D) input 78 of system control means 50 a 
plurality of time-voltage pairs corresponding to the values of V.sub.o (t) 
at specified points in time, to then utilize such time-voltage pairs to 
derive a tau.sub.c value for such particular coin, and to thereafter 
compare such derived tau.sub.c value against selected stored tau values 
indicative of acceptable coins and/or denomination values of acceptable 
coins to determine the acceptability and/or denomination value of such 
particular coin. The particular programming steps or techniques that would 
be employed in any particular instance will depend upon the particular 
system control means 50 utilized. 
At this stage in the discussion of the present invention it should now be 
apparent that there have been described and discussed hereinabove several 
embodiments of coin detection means that can be employed to detect coins 
of both low and high magnetic permeability and which fulfill the various 
objects sought therefor and achieve the advantages sought, especially the 
advantage that only a single sensor coil need be employed, which advantage 
is made possible due to the relative independence from the coin-to-coil 
distance of the derived coin characteristic tau.sub.c for a coin detected 
within the field of the sensor coil. Those skilled in the art will 
recognize that the present invention is not restricted to utilization of 
only a single sensor coil, however, and will understand that in various 
instances and constructions the use of multiple sensor coils may be 
desirable or advantageous. They will also recognize that, although the 
embodiments discussed hereinbefore have employed current ramp generators 
that operate under control of the system control means, it is possible to 
employ current ramp generators that operate asynchronously with respect to 
the system control means, so long as the system is otherwise so designed, 
constructed, and/or programmed to permit the development by such system of 
data corresponding to a plurality of time-voltage pairs for use in 
deriving tau.sub.c values for coins within the field of the sensor coil. 
All such variations on and modifications of the disclosed embodiments are 
considered to fall within the spirit and scope of the present invention. 
From all the foregoing, it should be abundantly clear that the tau.sub.c 
value that is derived by the present invention for any coin within the 
field of the sensor coil is essentially independent of the distance 
between such coin and such coil, so long as such coin remains within the 
field, i.e., so long as there is total overlap between such coin and the 
field of the coil, and that such independence is important with regard to 
the present invention and an understanding thereof. It should be clearly 
understood, however, that such independence does not apply if the coin is 
not totally within the field of the coil, i.e., if there is not total 
overlap between the coin and the field of the coil. In such event, the 
derived tau.sub.c value is clearly dependent upon coin position relative 
to the sensor coil. 
When a coin is deposited it moves along a coin path that carries it into 
and through the field of the coil, but it is totally within the field of 
the coil for only a relatively short period of time. As the coin follows 
such coin path tau.sub.c values can be continuously iteratively derived by 
coin detection means constructed according to the present invention, but 
such derived tau.sub.c values will vary depending upon the extent to which 
the coin has entered into the field of the sensor coil, as is graphically 
illustrated by FIG. 27, which figure depicts typical tau.sub.c values 
derived as a given coin moves into and through the field of a typical pot 
core sensor coil such as might be employed in the present invention. Such 
figure vividly illustrates that, although tau.sub.c is essentially 
independent of coin-to-coil distance for any given coin while such coin is 
positioned within the field of the sensor coil, tau.sub.c is not 
independent of coin position when the coin is not totally within the field 
of such sensor coil but is passing into and through such field. As will be 
discussed in more detail hereinafter, such dependence of tau.sub.c upon 
coin position as the coin passes by the sensor coil, as opposed to the 
independence of tau.sub.c from coin-to-coil distance while the coin is 
totally within the field of the sensor coil, permits tau.sub.c values 
derived by the present invention to also be utilized with certain 
embodiments of the present invention for coin sizing purposes. In light of 
FIG. 27, it will be readily understood by those skilled in the art that 
the maximum derived tau.sub.c value for a given coin, which tau.sub.c 
value is obtained from an application of a current ramp to the sensor coil 
at a time when such coin is totally within the field of the sensor coil, 
is the value of tau.sub.c that is generally utilized, as described 
hereinbefore, to determine the acceptability and/or denomination of such 
coin. 
As has previously been noted, although the present invention eliminates the 
need for multiple sensor coils due to the independence of tau.sub.c from 
coin-to-coil distance when a coin is within the field of the sensor coil, 
it does not restrict embodiments thereof to the use of a single sensor, 
nor does it require the sensor coil to be of any particular configuration. 
In practice, it has been found desirable to employ a "U" shaped core with 
windings upon the legs thereof connected in series with one another to 
form a single inductor, as depicted in FIG. 28, since such a sensor coil 
configuration permits derived tau values to be used not only in the manner 
set forth in detail hereinbefore to determine coin acceptability and/or 
denomination but also for coin sizing checks, as will become evident from 
that which follows. 
With the sensor coil configuration depicted in FIG. 28 a deposited coin is 
caused to effectively pass by two sensor stations as it follows its coin 
path, as pictorially illustrated in FIG. 29. If tau.sub.c values are 
continuously iteratively derived during the time that such coin is passing 
the two sensor stations, the system control means 50 can utilize such 
derived tau.sub.c values to derive a coin sizing value S.sub.c 
characteristic of such particular coin, which value can be compared 
against pre-established stored coin size characteristic values to 
determine coin acceptability and/or denomination based upon coin sizing. 
The use of various electrical or electronic means for determining or 
measuring coin dimensions, and the effect of various factors, such as coin 
speed and the height of sensors relative to true coin diameters, with 
regard to such determinations and measurements, have been previously 
described and discussed in various U.S. Patents, including U.S. Pat. Nos. 
3,653,481; 3,739,895; 3,797,307; 3,797,628; 4,509,633; and 4,646,904. In 
view of the teachings therein, and for ease of discussion in providing a 
basic understanding of the manner in which derived tau.sub.c values can be 
utilized in coin sizing determination checks, it will here be assumed that 
deposited coins move past the sensor stations at constant speed, that the 
sensor stations are so positioned that the true diameters of all coins as 
they pass thereby are centered with respect to the sensor stations, and 
that the coins and sensors may be considered to have square faces when 
discussing overlap thereof. It will be readily recognized by those skilled 
in the art that, although such conditions do not apply in practice, they 
permit simplified discussion of the pertinent basic concepts. 
In light of what has already been said, it will be recognized that the 
tau.sub.c values derived for a given coin 502 as it moves past the two 
sensor stations S.sub.1 and S.sub.2, denoted by the numbers 504 and 506 in 
FIG. 29, can be plotted as a function of coin position. By way of example, 
if the sensor stations S.sub.1 and S.sub.2 are positioned a distance d 
apart (as measured center to center) and are considered to have effective 
respective radii of r.sub.1 and r.sub.2, the expected waveform for 
tau.sub.c (x) for a coin whose diameter D.sub.c =2R.sub.c, where R.sub.c 
is the radius of such coin, and where the coin diameter is such that no 
simultaneous overlap of both sensors S.sub.1 and S.sub.2 occurs, i.e., 
D.sub.c =2R.sub.c &lt;d-r.sub.1 -r.sub.2, would be expected to be similar to 
that waveform depicted in FIG. 30, where j.sub.1 is some fraction of 
P.sub.1 (peak 1) and j.sub.2 is some fraction of P.sub.2 (peak 2), and 
where, for the sake of simplicity, j.sub.1 =j.sub.2. Those skilled in the 
art will understand that a coin sizing characteristic S.sub.c can be 
selected based upon such characteristic being some function of X.sub.1, 
X.sub.2, X.sub.3, and X.sub.4, i.e., S.sub.c =f(X.sub.1, X.sub.2, X.sub.3, 
X.sub.4), and a characteristic value can be derived for each deposited 
coin as it passes by the sensor stations S.sub.1 and S.sub.2. 
For example, the characteristic could be selected to be S.sub.c1 =(X.sub.4 
-X.sub.1)/(X.sub.3 -X.sub.2). If X.sub.1 and X.sub.2 are positions at 
which the sensor station S.sub.1 is half-covered by the coin as it moves 
past such sensor station and X.sub.3 and X.sub.4 are positions at which 
the sensor station S.sub.2 is half-covered by the coin as it moves past 
such sensor station, the values of X.sub.1, X.sub.2, X.sub.3, and X.sub.4 
relative to some arbitrary X.sub.o position would be X.sub.1 =X.sub.k 
+r.sub.1 X.sub.2 =X.sub.k +r.sub.1 +2R.sub.c, X.sub.3 =X.sub.k +r.sub.1 
+d, and X.sub.4 =X.sub.k +r.sub.1 +d+2R.sub.c, as a consequence of which 
S.sub.c1 would be S.sub.c1 =(d+2R.sub.c)/(d-2R.sub.c). It will be readily 
apparent that, if S.sub.c1 is selected as the characteristic, the larger 
the radius of the deposited coin, the larger will be the derived value 
S.sub.c1. Alternatively, the characteristic could be selected to be 
S.sub.c2 =[(X.sub.2 -X.sub.1)+(X.sub.4 -X.sub.3)]/[(X.sub.3 
-X.sub.1)+(X.sub.4 -X.sub.2)] or some other function of X.sub.1, X.sub.2, 
X.sub.3, and X.sub.4. With the position values for X.sub.1, X.sub.2, 
X.sub.3, and X.sub.4 as set forth previously, S.sub.c2 =2R.sub.c /d. Those 
skilled in the art will recognize that, although the characteristic 
S.sub.c2 is proportional to R.sub.c and S.sub.c1 is not, either or both 
could be advantageously utilized as coin sizing characteristics since they 
both exhibit a one-to-one correspondence between each derived 
characteristic value thereof and the radius of the particular coin with 
which such derived characteristic value is associated. 
It should be noted that, if the relationship between the coin radius 
R.sub.c, the distance d, and the radii r.sub.1 and r.sub.2 are different 
than specified hereinabove, the waveform for tau.sub.c (x) may have a 
different appearance from that depicted in FIG. 29. For example, if the 
coin diameter is greater than the distance between the sensor stations, 
i.e., D.sub.c =2R.sub.c &gt;d, the expected waveform would be as depicted in 
FIG. 31. It will be recognized, however, that coin size characteristics 
can be derived in much the same fashion for such a waveform as was true 
with regard to the waveform depicted in FIG. 29. 
In practice, it has been found that, if the distance d between the sensor 
stations is so selected that acceptable coins produce waveforms similar to 
those depicted in FIG. 30, which, for example, is the case if d-r.sub.1 
-r.sub.2 &lt;2R.sub.c&lt;d+r.sub.1 +r.sub.2, a coin sizing parameter S.sub.c3 
based upon peak values P.sub.1 and P.sub.2 and intermediate nadir value N 
can be advantageously employed. Since the derived value of tau.sub.c at 
P.sub.1 can be considered to be the value that occurs when the overlap 
between sensor station S.sub.1 and the deposited coin is greatest, since 
the derived value of tau.sub.c at P.sub.2 can be considered to be the 
value that occurs when the overlap between sensor station S.sub.2 and the 
deposited coin is greatest, and since the derived value of tau.sub.c at N 
can be considered to be the value that occurs when the deposited coin is 
positioned centrally intermediately with respect to sensor stations 
S.sub.1 and S.sub.2, sizing parameter S.sub.c3 can be chosen to be a 
function of P.sub.1, P.sub.2, and N, i.e., S.sub.c3 =f(P.sub.1, P.sub.2, 
N), where the values of P.sub.1, P.sub.2, and N are determined by the 
overlap of the deposited coin with sensor stations S.sub.1 and S.sub.2. 
Those skilled in the art will appreciate that the area overlap for sensor 
S.sub.1 at P.sub.1 is a function of 2r.sub.1, that the area overlap of 
sensor S.sub.2 at P.sub.2 is a function of 2r.sub.2, and that the area 
overlap of sensors S.sub.1 and S.sub.2 at N is a function of 2R.sub.c 
-(d-r.sub.1 -r.sub.2), as a consequence of which S.sub.c3 =f(P.sub.1, 
P.sub.2, N)=(P.sub.1 +P.sub.2) /N can be considered to be approximately 
S.sub.c3 =(2r.sub.1 +2r.sub.2)/[2R.sub.c -(d-r.sub.1 -r.sub.2)], which, 
for r.sub.1 =r.sub.2, becomes S.sub.cs =4r.sub.1 /(2R.sub.c +2r.sub.1 -d). 
In view of the foregoing it will be understood that the ratio (P.sub.1 
+P.sub.2)/N can therefore be utilized to derive a coin sizing factor 
S.sub.c3 for a deposited coin, which coin sizing factor can be compared 
against stored coin sizing factors for acceptably sized coins to determine 
the coin size acceptability of the deposited coin. 
Although the foregoing discussion with respect to the derivation of coin 
sizing characteristics has focused upon the use of derived tau.sub.c 
values as a deposited coin passes by two sensor stations, it will be 
readily understood that other derived or measured values, such as the 
amplitude of a particular signal as the coin passes by the sensor 
stations, could similarly be employed in the derivation of coin sizing 
characteristics. Those skilled in the art will recognize the value in and 
the advantages of utilizing a variety of parameters and coin validation 
and verification techniques to combat the increasingly sophisticated 
attempts to "cheat" coin-operated devices and systems and will readily 
understand and appreciate that many combinations of validation and 
verification techniques may be used in conjunction with one another to 
good effect. 
In light of all the foregoing it should be evident that certain embodiments 
constructed according to the present invention can be utilized not only 
for coin detection and denomination discrimination based upon the 
independence of a particular tau.sub.c value derived for a coin within the 
field of the sensor coil from the distance between such coin and such 
coil, but also for coin size checking based upon the dependence upon coin 
position relative to such sensor coil of a series of tau.sub.c values 
derived for such coin as it passes into and through the field of such 
sensor coil. Such embodiments offer advantages even beyond those 
advantages listed hereinbefore and sought for the present invention, as a 
consequence of which such embodiments are considered to be of significant 
practical and economic benefit. 
From all that has been said, it should now be clear that there has been 
shown and described a coin detection means and method, including various 
embodiments of such coin detection means, which fulfills the various 
objects and advantages sought therefor. It will be apparent to those 
skilled in the art, however, that many changes, modifications, variations, 
and other uses and applications of the subject coin detection means and 
method are possible and contemplated. All such changes, modifications, 
variations, and other uses and applications which do not depart from the 
spirit and scope of the invention are deemed to be covered by the 
invention, which is limited only by the claims which follow.