Apparatus and method for determining the validity of a coin

A coin validator is provided with at least two reference positions (U, D) for determining a diameter related characteristic of a coin being validated. In order to reduce the running to the testing station, the timing of a trailing point of the coin passing a first reference position (U) is used to determine the diameter related characteristic. Embodiments using optical inductive and piezo-electric sensors associated with the reference positions are disclosed. An inductive sensor for a coin validator comprises an elongate coil, which, when in use, is arranged such that the magnetic field is substantially constant across the width of the passageway. The use of coils of this type have the advantage of wrap around coils but enable the coin passageway to be shallower and be opened. A coin validator is described wherein the backwall of a coin passageway is movable to and fro so that the depth of the coin passageway can be adjusted. In an embodiment, a cam bears against the backwall of the coin passageway to set the depth thereof.

FIELD OF THE INVENTION 
The present invention relates to a coin validator. 
BACKGROUND TO THE INVENTION 
U.S. Pat. No. 4,474,281 discloses a coin validation apparatus wherein a 
pair of optical beams are directed across the coin path of a validator, 
substantially in the plane of a coin under test. The optical beams are 
spaced along the direction of travel of a coin in the coin path. The 
diameter of a coin is determined by timing the periods during which each 
of the optical beams is interrupted by passing coin, determining a value 
for the speed of the coin as it crosses the beams, deriving two diameter 
values from the timed periods and the speed values, and averaging the 
resultant values. The average produced is proportional to the diameter of 
the coin interrupting the beams. 
If the apparatus of U.S. Pat. No. 4,474,281 is to function correctly, a 
coin to be tested must be in free fall before it encounters the first 
optical beam. A problem arises from this in that it is difficult to 
produce a compact validator with a sufficient run-in for a coin to be in 
free fall, before it interrupts the first optical beam. The problem is 
particularly acute in the case of validators for the large tokens used in 
some casinos. 
DE-A-2 724 868 discloses an apparatus in which the diameter of a coin is 
checked on the basis of the time between the leading edge of the coin 
reaching a lower reference and the trailing edge of the coin leaving an 
upper reference position. However, this apparatus suffers from two 
disadvantages. Firstly, a counter is started when the coin reaches the 
upper reference position. Consequently, the upper reference position must 
be located at the diameter of the largest acceptable coin from the coin 
insertion slot. Secondly, the example, in which the diameter of a coin is 
checked on the basis of the time between the leading edge of the coin 
reaching a lower reference and the trailing edge of the coin leaving an 
upper reference position, cannot be used with coins whose diameters are 
not greater than the separation of the reference positions. 
GB-A-1 405 936 discloses a coin validation apparatus comprising means 
defining first and second reference positions spaced along a coin path, 
sensor means for detecting a trading point on a coin passing the first 
reference position and a leading point on the coin reaching the second 
reference, and processing means for determining the velocity of a coin 
under test on the basis of the output of the sensor means. However, the 
diameter of the coin is checked using additional sensors. 
In the following the term "coin" means coin, token and any similar objects 
representing value. 
SUMMARY OF THE INVENTION 
It is an aim of the present invention to overcome the afore-mentioned 
disadvantages of the prior art. 
According to a first aspect of the present invention, there is provided a 
coin validation apparatus comprising means defining first and second 
reference positions spaced along a coin path, sensor means for detecting a 
trailing point on a coin passing the first reference position and a 
leading point on the coin reaching the second reference position, and 
processing means for checking the diameter of a coin under test on the 
basis of said trailing point passing the first reference position and said 
leading point reaching the second reference position, characterized in 
that the processing means checks the diameter of the coin under test 
without reference to said leading point reaching the first reference 
position. Preferably, the processing means checks the diameter of the coin 
under test on the basis of the time difference between said trailing point 
passing the first reference position and said leading point reaching the 
second reference position. 
In some embodiments of the present invention, the diameter checked is the 
physical diameter of a coin under test. However, in other embodiments the 
diameter is checked on the basis of characterising signal representative 
of a property related to diameter but dependent also on additional factors 
such a the material from which a coin under test is made. The reference 
positions will, in practice, generally have a non-infinitesimal dimension 
in the direction of coin travel. 
Thus, as the diameter-related characteristic determination is based on the 
time of a coin leaving the first reference position, there is no need for 
the run-in required by the prior art. Indeed, the first reference position 
can be located such that a coin extends across it even before a coin is 
full in the validator. 
As a result of friction between a coin under test and the walls of the 
passageway and other factors, the speed of a coin passing through the 
optical beams is indeterminate and some correction for this is normally 
required. However, if the gap between the reference positions is the same 
as the diameter of a coin of interest, no correction is required. This is 
because, for a valid coin, the trailing point leaves the upstream 
reference position at the same time as the leading point enters the 
downstream reference position, regardless of the speed of the coin. 
Therefore, in one preferred embodiment, the reference positions are 
separated by the diameter of a coin type to be accepted by the validator. 
Additional reference positions could be added, each spaced from the first 
by the diameter of a coin type to be accepted. However, if more than a few 
denominations of coin are to be accepted, the complexity of this 
arrangement becomes undesirable. 
In order to avoid this undesirable complexity, another preferred embodiment 
includes means to determine a velocity dependent value for a coin passing 
the reference positions, wherein the processing means is further 
responsive to the velocity dependent value for a coin under test to 
produce the characterising signal. 
The means to determine a velocity dependent value may comprise means to 
determine the time elapsing between the trailing point passing the first 
reference position and the trailing point passing the second reference 
position. 
However, the use of the first and second reference positions for velocity 
determination is not ideal if the coin accept gate is only a short 
distance below the second reference position. In such a case there may be 
insufficient time to process coin characterising signals before a decision 
must be made whether to open the accept gate. In order to overcome this 
situation, the means to determine a velocity dependent value may comprise 
a third reference position downstream of the first reference position and 
further sensor means for detecting said leading point reaching the third 
reference position, wherein the processing means is responsive to the 
sensor means to derive said velocity dependent value on the basis of the 
time difference between said leading point reaching the second reference 
position and said leading point reaching the third reference position. 
Thus, all the coin characterising data is obtained before the coin has 
passed fully through the last reference position. 
Preferably, the processing means produces the characterising signal on the 
basis of the result of: 
##EQU1## 
where: t.sub.1 is the time of trailing point passing the upper first 
reference position, and 
t.sub.2 and t.sub.3 are the times of the leading point reaching the second 
and third reference positions. 
The trailing and leading points on a coin under test will be substantially 
on the circumference of the coin with some types of sensor. However, the 
operation of other sensors means the leading and trailing points will be 
located radially inward of the coins circumference with one on either side 
of a diameter of the coin, which runs perpendicular to the coin's 
direction of travel. 
Preferably, the sensor means comprises a beam of optical radiation crossing 
the coin path and a detector therefor for each said reference position. 
More preferably, the coin path has a breadth to accommodate the thickness 
of a coin under test, a width to accommodate the coin's diameter, and a 
length along which coins under test can pass edgewise, wherein the sensor 
means includes emitter means on one side of the passageway for directing 
said beams of optical radiation across the width of the passageway and 
detectors opposite respective emitter means. If the beams are closely 
spaced, it is advantageous that adjacent beams shine in opposite 
directions across the coin passageway. This avoids one beam being detected 
by the photosensor of another beam. 
However, other forms of sensor may be used. For instance, the sensor means 
may comprise inductive sensors. In a preferred embodiment using inductive 
sensors, the coin path has a breadth to accommodate the thickness of a 
coin under test, a width to accommodate the coin's diameter, and a length 
along which coins under test can pass edgewise, wherein the sensor means 
includes an elongate inductor arranged substantially parallel to the width 
direction of the path and having its winding axis substantially parallel 
to the direction of travel of coins along the path. 
In a further embodiment, the sensor means comprises a piezo-electric 
element associated with each reference position, the piezo-electric 
elements being arranged to be stressed by the passage of a coin to produce 
electric signals. Preferably, at least one of the piezo electric elements 
comprises a flap, arranged to stress a piezo-electric film as a passing 
coin displaces it. 
According to the first aspect of the present invention, there is further 
provided a method of validating a coin comprising the steps of: 
(a) moving a coin edgewise past first and second reference positions, the 
reference positions being fixed relative to each other, and 
(b) determining the time difference between a trading point on the coin 
passing the first reference position and a leading point on the coin 
reaching the second reference; characterized by 
(c) checking the diameter of the coin on the basis of said time difference 
without reference to said leading point reaching the first reference 
position. 
Preferably, a method according to the present invention includes the step 
of producing a coin velocity dependent value, wherein said velocity 
dependent value is used to derive the value characteristic of the coin. 
More preferably, such a method comprises the steps of: 
(d) moving a coin edgewise past a third reference position; 
(e) determining the time difference between said leading point reaching the 
second reference position and said leading point reaching the fourth 
reference; 
(f) deriving a value representative of the coin's velocity on the basis of 
said time difference. 
Preferably, optical sensing means are used to detect a trailing point on 
the coin's circumference passing the first reference position and a 
leading point on the coin's circumference reaching the second reference. 
However, inductive sensing means or piezo-electric sensing means could be 
used for determining said time difference or differences. 
In many situations, merely measuring the diameter of a disc will not be 
sufficient to determine whether it is a valid member of a predetermined 
set of coin types. Typically, additional information will be derived using 
inductive sensors. In one type of inductive sensor, a cod is arranged 
beside the coin passageway, with its axis perpendicular to the plane of a 
coin travelling along the passageway. These inductive sensors are 
undesirable for compact coin validators if they are wound in the form of a 
circle or square because this increases the length required for the 
passageway. However, reducing the dimensions of the coil in the direction 
of travel of coins to be tested, produces an unacceptable degradation of 
performance. 
A solution to this problem is the use of so called "wrap around" coils. 
Wrap around coils are arranged so that a coin to be tested passes along 
the axis of the coil. However, these coils cannot be opened for 
maintenance or rejection of jammed coins. This often necessitates a wider 
than desired gap through which coins under test pass, reducing 
sensitivity. 
It is also an aim of the present invention to overcome the afore-mentioned 
disadvantages of prior art validator coil arrangements. 
According to a second aspect of the present invention, there is provided a 
coin validation apparatus comprising means defining a passageway for coins 
under test, the passageway having a breadth to accommodate the thickness 
of a coin under test, a width to accommodate the coin's diameter, and a 
length along which coins under test can pass edgewise, and an inductive 
coin sensing station including a coil assembly beside the passageway and 
arranged to inductively couple with a major face of a coin therein, 
characterized in that the coil assembly is arranged such that the magnetic 
field produced thereby is substantially constant across the width of the 
passageway. 
Preferably, the inductive coin sensing station comprises first and second 
coils opposite each other across the breadth of the passageway and having 
their axes substantially parallel to the direction of travel of a coin in 
the passageway past the sensing station. With such an arrangement, the 
coils can be switched between in-phase and anti-phase modes of operation. 
This cannot, of course, be achieved using a wraparound coil. 
Preferably, the or each coil is wound in the form of an elongate oval or 
rectangle on a former of magnetic material which is, at least, 
substantially as long as the passageway is wide. Advantageously, the or 
each coil includes an elongate I-section former. However, an E- or 
C-section former may be used. If the former is E-sectioned, the coil may 
be wound around the top, bottom or middle arms. If the former is 
C-sectioned, the coil may be wound around any part. 
Preferably, a validator includes shielding means to magnetically shield 
portions of the or each coil not immediately adjacent the passageway. 
The slim shape of the coils employed in a validator according to this 
second aspect enables a more compact validator to be constructed. 
Alternatively, the space saved can be used for additional sensors of the 
same or different types. Since the windings of these coils include 
portions lying parallel to the coin passageway across its entire width, 
the magnetic field produced in the passageway is substantially constant 
across the width of the passageway. Consequently, the response to the 
passage of a coin, obtained from these coils, is independent of the 
position of a coin across the width of the passageway. This is 
particularly advantageous in the case of validators where coins are in 
free fall past the inductive sensor station because the path followed by a 
coin cannot be rigidly controlled 
Another advantage of the shape of these coils is that they are easier to 
screen than the coils used in prior art validators. 
It has been found that coils of this type are more linear in their response 
to passing coins than prior art designs. 
According to a third aspect of the present invention, there is provided a 
coin validating apparatus comprising a coin path having a breadth 
sufficient to accommodate the thickness of a coin under test, wherein a 
wall, defining in part said breadth, is repositionable to thereby vary 
said breadth. Preferably, a cam is ranged to act on said wall for 
repositioning thereof. More preferably, a sense coil is mounted to said 
wall for sensing a coin moving along the coin path. 
Whilst the different aspects of the present invention provide significant 
advantages when applied individually, a compact validator, particularly 
suited to the validation of large "casino" tokens, can be constructed by 
applying both the first and second aspects. In such a validator, the 
inductive coin sensing station is preferably located between the upstream 
coin sensing station and the or a sequentially first downstream coin 
sensing station.

DETAILED DESCRIPTION OF EMBODIMENTS 
Referring to FIGS. 1 and 2, a coin validator body 1 defines a rectangular 
cross-section coin passageway 2. The passageway 2 comprises a straight, 
vertical upper portion , where various sensor stations 3 are located and a 
wider lower portion 2b. An accept gate 4 is arranged for diverting coins 
along either of two routes A, B. The accept gate 4 normally blocks route A 
but is opened if the signals from the sensor stations 3 indicate that a 
valid coin has been inserted into the validator. The upper portion 2a of 
the passageway 2 has a width w greater than the diameter of the largest 
coin 5 of interest and a depth b greater than the thickness of the 
thickest coin of interest. The entry to the upper portion 2a of the 
passageway is flared so as to simplify alignment of the validator with a 
coin insertion slot (not shown). 
Considering the sensor stations 3 in more detail, an upstream optical 
sensor station comprises a lensed light emitting diode (LED) 6 mounted in 
the validator body 1, so as to shine a beam U of light across the width w 
of the passageway 2 through a slit 7 opening into the passageway 2. The 
slit 7 extends across the full depth b of the upper portion 2a of the 
passageway. A lensed photosensor 8 aligned to receive the beam from the 
LED 6 completes the upstream optical sensor station. A downstream optical 
sensor is similarly constructed from a lensed LED 9, a slit 10 and a 
lensed photosensor 11 to shine a beam D across the passageway 2, and is 
located a short distance below the upstream sensor. Two elongate sense 
coils 12 are located between the upstream and the downstream optical 
sensor stations. The sense coils 12 are press fitted longitudinally into 
respective slots extending transversely across the width w of the upper 
portion 2a of the passageway. The sense coils 12 will be described in more 
detail below. 
Referring to FIG. 3, the LEDs 6,9 are driven by LED driver circuitry 15 in 
order to produce the upstream and downstream beams U,D. The LEDs 6,9 
typically produce optical radiation in the infra-red range although 
visible radiation can also be used. It will thus be appreciated that as 
used herein, the term optical radiation includes both visible and 
non-visible optical radiation. 
The photosensors 8,11 are connected to interface circuitry 16 which 
produces digital signals x.sub.1, x.sub.2 in response to interruptions of 
the upstream and downstream beams U,D, as a coin falls along the 
passageway 2 past the sensor stations 3. The coin signals x.sub.1, x.sub.2 
are fed to a microprocessor 17. As explained in our United Kingdom patent 
application no. 2 169 429, the inductive coupling between the coils 12 and 
a passing coin 5 gives rise to apparent impedance changes for the coil 
which are dependent on the type of coin under test. The apparent impedance 
changes are processed by coil interface circuitry 18 to provide a coin 
parameter signals x.sub.3, x.sub.4, which are a function of the apparent 
impedance changes. 
The microprocessor 17 carries out a validation process on the basis of the 
signals x.sub.1, x.sub.2, x.sub.3, x.sub.4 under the control of a program, 
stored in an EEPROM 19. 
If, as result of the validation processes performed by the microprocessor 
17, the coin is determined to be a true coin, a signal is applied to a 
gate driver circuit 20 in order to operate the accept gate 4 (FIG. 1) so 
as to allow the coin to follow the accept path A. Also, the microprocessor 
17 provides an output on line 21, comprising a credit code indicating the 
denomination of the coin. 
The determination of the validity of coins on the basis of signals from 
sense coils is well known in the art and, accordingly, will not be 
described again here in detail. 
The operation of the coin diameter determining function, according to a 
first embodiment, will now be described with reference to FIGS. 4a to 4e. 
In this embodiment, the upstream and downstream beams U,D are spaced by 
the diameter of the coin or token to be identified by the validator. 
Referring to FIG. 4a, a coin 25, entering the passageway 2 (FIG. 1), first 
intercepts the upstream beam U. Unless the thickness of the coin 
corresponds to the depth b of the passageway 2, the beam U will not be 
fully blocked. However, there will be, in any event, a significant 
reduction in the light intensity detected by the photosensor 8 (FIG. 1). 
Therefore, the output of the photosensor 8 is compared with a reference to 
determine whether the received light intensity has reduce& indicating an 
incursion into the upstream beam U by a coin. If an incursion is detected, 
the state of signal x.sub.1 changes. This change in state is not important 
for coin diameter determination but may conveniently be used as a wake up 
signal for the microprocessor 17 (FIG. 3). 
Referring to FIG. 4b, as the coin 25 continues to fall down the passageway 
2, it continues to block the upstream beam, at least partially, and the 
state of signal x.sub.1 is maintained. 
Referring to FIG. 4c, if the coin 25 is of the desired type, it intercepts 
the downstream beam D just as it is leaving the upstream beam U. This 
results in virtually simultaneous changes in the states of the signals 
x.sub.1 and x.sub.2. In other words, t.sub.1 =t.sub.2. In practice, 
t.sub.1 may not exactly equal t.sub.2 due to component tolerances or 
environmental factors such as temperature. Thus, when the microprocessor 
17 (FIG. 3) detects that either x.sub.1 has returned to its original state 
or that x.sub.2 has changed state to indicate the presence of a coin, it 
waits to see if the other signal makes the appropriate change of state 
within a predetermined window. If the other signal makes the appropriate 
change of state during the window, and inductive test data derived from 
the coils 12 (FIG. 1), is in agreement, the microprocessor 17 (FIG. 3) 
sends a signal to the gate drive circuit 20 (FIG. 3) to open the accept 
gate 4 (FIG. 1). 
FIGS. 4d and 4e show the coin 25 leaving the sensor stations 4. 
It will be appreciated that further downstream beams could be added, spaced 
from the upstream beam by the diameters of other coins or tokens, so that 
a plurality of types of coin or token could be identified. 
A second embodiment of the present invention will now be described with 
reference to FIGS. 3, 5, 6a to 6e and 7a to 7d, wherein like parts have 
the same reference signs as in FIGS. 1 and 2. 
Referring to FIG. 5, the structure of the validator is substantially the 
same as that of FIGS. 1 and 2. However, the accept gate is now located in 
another unit (not shown). As a result there is a larger drop between the 
sensor stations 3 and the accept gate, giving more for the validity of a 
coin to be established. The electronic circuitry for this validator is as 
shown in FIG. 3. However, the EEPROM 19 will store a different program for 
the microprocessor, reflecting the different validation method. 
Referring to FIG. 6a, a coin 25, entering the passageway 2 (FIG. 1), first 
intercepts the upstream beam U. When the incursion is detected, the state 
of signal x.sub.1 changes. This change in state is not important for coin 
diameter determination but may conveniently be used as a wake up signal 
for the microprocessor 17. 
Referring to FIG. 6b, as the coin continues to fall down the passageway 2, 
it continues to block the upstream beam U, at least partially, and the 
state of signal x.sub.1 is maintained. 
Referring to FIG. 6c, when the coin 25 leaves the upstream beam U, signal 
x.sub.1 returns to its original value. This change of state is noted by 
the microprocessor 17 which stores a value t.sub.1, representing the 
timing of the event. Shortly thereafter, the coin intercepts the 
downstream beam D, causing a change in state of signal x.sub.2. This 
change of state is also noted by the microprocessor 17 which stores a 
value t.sub.2 representing the timing of the event. 
Referring to FIG. 6d, as the coin continues to fall down the passageway 2, 
it continues to block the downstream beam D, at least partially, and the 
state of signal x.sub.2 is maintained. 
Referring to FIG. 6e, as the coin leaves the downstream beam D, the signal 
x.sub.2 returns to its original state. This change of state is noted by 
the microprocessor 17 which stores a value t.sub.3 representing the timing 
of the event. 
Thus, after a coin has passed both beams U, D, the microprocessor 17 has 
three values t.sub.1, t.sub.2 and t.sub.3 from which to derive a value 
indicative of the diameter of the coin. If it is assumed that the velocity 
u of the coin through the sensing beams U,D, is constant, the distance s 
travelled by a coin in a given time is given by the formula: 
EQU s=ut (1) 
Since the distance s.sub.s between the beams is know and the time taken for 
the coin to travel that distance is known, i.e. the time between the coin 
leaving the upstream beam and the coin leaving the downstream beam, the 
velocity of the coin can be calculated. Thus, from (1): 
EQU u=S/t (2) 
Substituting s.sub.S for s and the measured times for t gives: 
##EQU2## 
Now, the upstream beam U is left when the coin has travelled a dance 
s.sub.0 and the downstream beam is intercepted when the coin has travelled 
s.sub.0 +s.sub.1 d, where d is the diameter of the coin. Thus, from (2) 
and (3) above: 
##EQU3## 
and 
##EQU4## 
Subtracting (4) from (5) gives: 
##EQU5## 
Since s.sub.1 is a constant, only 
##EQU6## 
need be calculated in order to characterise a coin by its diameter. 
Referring to FIGS. 7a to 7d, it can be seen that the coin 25 intercepts the 
downstream beam D before it clears the upstream beam U. This means that 
t.sub.2 is before t.sub.1. Although this produces a negative result when 
(7) is evaluated, no problem arises because, as can be seen from (6), the 
negative sign merely indicates that the diameter of the coin is greater 
than the spacing between the beams. Therefore, the result of the 
evaluation of (7) for a large coin still charcterises the coin by its 
diameter. 
A third embodiment of the present invention will now be described with 
reference to FIGS. 8, 9, 10, 11a to 11e and 12a to 12h, wherein like parts 
have the same reference signs as in FIGS. 1 to 7. 
Referring to FIGS. 8 and 9, a further downstream optical sensor station, 
comprising a LED 30, a slit 31 and a photosensor 32, is provided. 
Referring to FIG. 10, the electronic circuitry is substantially the same as 
that of the first embodiment, described above, the main differences being 
in the program stored in the EEPROM 19. However, the LED driving circuitry 
15 is adapted to drive three LEDs 5,7,30, and the photosensor interface 
circuitry 16 is adapted to process the signals from three photosensors 
6,8,31 and output an additional signal x.sub.3. 
The operation of the validator known in FIGS. 8 and 9 will now be 
described. However, the details of the tests relying on the coils will be 
omitted as suitable techniques are well known in the art. 
Referring to FIG. 11a, a coin 25, entering the passageway 2 (FIG. 8), first 
intercepts the upstream beam U. When the incursion is detected, the state 
of signal x.sub.1 changes. This change in state is not important for coin 
diameter determination but may conveniently be used as a wake up signal 
for the microprocessor 17. 
Referring to FIG. 11b, as the coin 25 continues to fall down the passageway 
2, it continues to block the upstream beam U, at least partially, and the 
state of signal x.sub.1 is maintained until the coin 25 leaves the 
upstream beam U, when of signal x.sub.1 returns to its original value. 
This change of state is noted by the microprocessor 17 which stores a 
value t.sub.1 representing the timing of the event. Shortly thereafter, 
the coin intercepts the first downstream beam D1, causing a change in 
state of signal x.sub.2. This change of state is also noted by the 
microprocessor 17 which stores a value t.sub.2 representing the timing of 
the event. 
Referring to FIG. 11c, as the coin continues to fall down the passageway 2, 
it continues to block the first downstream beam D1, at least partially, 
and the state of signal x.sub.2 is maintained. Next, the coin 25 
intercepts the second downstream beam D2, causing a change in state of 
signal x.sub.3. This change of state is noted by the microprocessor 17 
which stores a value t.sub.3 representing the timing of the event. 
Finally, referring to FIG. 11e, as the coin 25 leaves each of the 
downstream beams D1,D2, the corresponding signals x.sub.2, x.sub.3 return 
to their original states. 
In the second embodiment, described above, the speed corrosion is performed 
on the basis of the timings of the coin 25 leaving the two beams U,D. This 
has a disadvantage in that it limits the time available, before the coin 
reaches the accept gate 4, for performing the validation calculations. The 
present embodiment solves this problem by means of the second downstream 
beam D2 which enables the coin's speed to be determined earlier because 
the interception of the downstream beams D1,D2 by the leading edge of the 
coin is detected for this purpose. Thus, the speed of a coin can be 
determined before it has past the second downstream beam D2. 
Now, since the speed correction is based upon the time taken for the 
leading edge of the coin to travel the distance s.sub.s1 between the 
downstream beams D1,D2, equation (6) above becomes: 
##EQU7## 
where .sub.s0 is the distance between the upstream beam U and the first 
downstream beam D1. 
Thus, since s.sub.s0 and s.sub.s1 are constants, a coin can be 
characterised on the basis of its diameter by evaluating: 
##EQU8## 
Referring to FIGS. 12a to 12h, it can be seen that t.sub.2 occurs before 
t.sub.1. If the first form of (9) is used a negative result will be 
obtained. However, as with the case of a large coin in a validator 
according to the second embodiment, the negative sign does not effect the 
validity of the characterisation of the coin by its diameter. 
An advantage of the above-described embodiments is that the beams can be 
positioned such that for coin of interest, the processing means receives 
all the timing information within a window which is short compared with 
the time required for a coin to fall through the sensor stations. 
The coils 12, employed in the validators of FIGS. 1, 2, 5, 8 and 9, will 
now be described in detail. 
Referring to FIG. 13, a coil 12 comprises an elongate, I-section former 42 
about which the winding 43 is wound. The former 42 is formed from a high 
permeability material such as sintered ferrite or iron bonded in a 
polymer, for example 91% oxidised iron bonded in a polymer. Thus, the 
former 42, if it is non-conducting, can serve both as a core and as a 
bobbin onto which the winding 43 is wound directly. 
An electromagnetic shield 44 comprises an elongate member having a flange 
extending perpendicularly at each end. The shield 44 is arranged to be 
attached to the coil 12 such that the winding 43 is wholly covered along 
one long side of the former 42 by the elongate member and at least 
partially covered at the ends of the former 42. The purpose of the shield 
44 is to increase the Q of the coil 12 but also reduces both the 
susceptibility of the coil 40,41 to electromagnetic interference (EMI) and 
the electromagnetic energy emanating from the coil, other than into the 
coin passageway 2 (FIG. 1) of the validator. 
Referring to FIG. 14, when a coil 12 is energized, a magnetic field 45 is 
projected into the coin passageway 2, between primarily the upper and 
lower cross-pieces of the I-section former 42. A coin 25 passing along the 
passageway 2 interacts with the projected magnetic field 45 varying the 
apparent impedance of the coil 12. 
In the foregoing embodiments of the present invention, the diameter of a 
coin is determined by the optical sensor stations as described above. At 
the same time, one or more of the coils 12 are energized as set out in our 
European patent application publication no. 0 599 844. The effects of the 
coin 25 interacting with the magnetic field 45 are detected by the coil 
interface circuitry 18 which outputs signals x.sub.3, x.sub.4 to the 
microprocessor 17. The microprocessor 17 then determines whether the coin 
under test is valid on the basis of the signals x.sub.1, x.sub.2 x, 
generated by the optical sensing process and the signals x.sub.3, x.sub.4 
generated by the inductive sensing process. If the coin is valid the 
microprocessor 17 sends a signal to the gate driver 20 to cause the accept 
gate 4 to open. 
The microprocessor 17 carries out a validation process on the basis of the 
signals x.sub.1, x.sub.2, x.sub.3, x.sub.4 under the control of a program, 
stored in an EEPROM 19. 
If as a result of the validation processes performed by the microprocessor 
17, the coin is determined to be a true coin, a signal is applied to a 
gate driver circuit 20 in order to operate the accept gate 4 (FIG. 1) so 
as to allow the coin to follow the accept path A. Also, the microprocessor 
17 provides an output on line 21, comprising a credit code indicating the 
denomination of the coin. 
Referring to FIGS. 1, 5 and 8, reflective strips 100 are provided on the 
walls of the passageway 2 between each of the LEDs 6,9,30 and the 
corresponding photosensors 8,11,32. The reflective strips 100 increase the 
light intensity at the photosensors 8,11,32 in the absence of a coin by 
reducing the amount of light absorbed by the wills of the passageway. As a 
result, the reduction in light intensity at the photosensors 8,11,32, due 
to the passage of a coin, is more profound than would be the case without 
the reflective strips 100. This makes it easier to detect accurately the 
edges of passing coins. 
The reflective strips 100 also solve the problem of the LEDs 6,9,30 not 
directing light directly across the coin passageway making the apparatus 
much less sensitive to the orientation of the LEDs 6,9,30 and the 
direction in which light is actually emitted therefrom. In the absence of 
the reflective strips 100, misaligned LEDs result in regions of the 
passageway 2 which are not illuminated. If a coin passes through one of 
these regions, it will not affect the light intensity at the relent 
photosensor 8,11,32. 
The reflective strips 100 may be, for example, painted onto the walls of 
the passageway 2 with metallic paint or formed from metal foil stuck to 
the walls of the passageway 2. 
A fourth embodiment of the present invention will now be described with 
reference to FIGS. 15 and 16, wherein like parts have the same reference 
signs as in FIGS. 1 and 2. Since, the coils, described above with 
reference to FIGS. 13 and 14, are narrow in the direction of coin travel, 
it is possible to fit a plurality of them along the upper part of the coin 
passageway 2a. Consequently, it is possible to use coils, substantially as 
described, as sensors for determining the diameter of a coin under test. 
Referring to FIG. 15, a validator is substantially as described with 
reference to FIG. 8. However, the coils 12 and the optical sensor stations 
have been replaced by three coil pairs 50,51,52, (one coil of each pair 
not shown) located at positions corresponding to those of the optical 
sensor stations shown in FIG. 8. 
Referring to FIG. 16, a coil interface circuit 18 energizes the coil pairs 
50,51,52 and processes the apparent impedance changes, caused by a passing 
coin, to produce six signals y.sub.1, y.sub.2 y.sub.3, y.sub.4, y.sub.5, 
y.sub.6. The signals y.sub.4, y.sub.5, y.sub.6 are conventional coin 
characteristic data signals and are fed to a microprocessor 17 for 
determination of coin characteristic such as material and thickness. The 
coil interface circuit 18 includes comparators for comparing the outputs 
of, at least, one coil 50,51,52 of each pair with a threshold. 
As a coin passes each of the coil pairs 50,51,52, the amplitude of the 
respective coil signal first falls and then rises. As these signals cross 
the threshold, the outputs of the respective comparators change state, 
producing pulse signals which are similar to those shown in FIGS. 11 and 
12. A diameter value for the coin can then be determined according to 
equation (9) above. However, as the coil signals depend on the material, 
and sometimes the thickness of the coin, the diameter value is for an 
apparent, or "electromagnetic", diameter. 
For instance, a tin coin will appear to have a smaller "electromagnetic" 
diameter than a similarly sized coin made from ferromagnetic material. 
Nevertheless, the apparent diameter determined using equation (9) above 
will differ for differently sized coins of the same material. 
In addition to monitoring the passage of coins into the validator, the 
signals from the coil pairs 50,51,52 are simultaneously used to derive 
additional information about a coin under test, including the nature of 
the material of the coin. For instance, one pair of coils may be driven 
in-phase and another in anti-phase or one coil pair could be switched 
between in-phase and anti-phase configurations. Once the nature of the 
material is known, it is possible to correct the "electromagnetic" 
diameter to derive the coin's physical diameter. However, in practice this 
is not necessary because, for each coin to be accepted, the validator 
could store sets of data defining values indicative of valid coins. The 
stored data would include data representative of coin material thickness, 
and also the "electromagnetic" width. Thus, it is not necessary to 
determine the actual physical diameter of a coin under test but only the 
"electromagnetic" diameter for comparison with a value established 
empirically. 
A fifth embodiment of the present invention will now be described with 
reference to FIGS. 17, 18 and 19, wherein like parts have the same 
reference signs as in FIGS. 1, 2 and 15. 
Referring to FIG. 17, the validator is substantially the same as that shown 
in FIG. 15 but with the lowest coil omitted. The circuit arrangement (FIG. 
18) of this embodiment is simmer to that shown in FIG. 16. However, as 
there are only two coils there are only two conventional coin 
characteristic signal lines y.sub.4, y.sub.5. Three diameter determining 
sign lines y.sub.1, y.sub.2, y.sub.3 are retained but signal y.sub.3 is 
derived differently and the operation of the microprocessor 17 altered in 
consequence. 
The derivation of the signals y.sub.1, y.sub.2, y.sub.3 will now be 
described with reference to FIG. 19. As a coin passes the upper coil 50, 
the amplitude of the respective coil signal rises to a peak and then falls 
again. The coil interface circuit 18 compares the signal for the upper 
coil 50 with a first threshold TH1 and outputs a pulse signal y.sub.1 when 
the coil signal is over the threshold TH1. The microprocessor 17 detects 
the falling edge of the pulse signal y.sub.1 and stores the time t.sub.1. 
As the coin passes the lower coil 51, the amplitude of the respective coil 
signal rises to a peak and then falls again. The coil interface circuit 18 
compares the signal with both the first threshold TH1 and a second higher 
threshold TH2. A pulse signal y.sub.2 is output when the coil signal is 
over the first threshold TH1 and a pulse signal y.sub.3 when the coil 
signal is over the second threshold TH2. 
As described above, the time difference t.sub.2 -t.sub.1 is dependent on 
the diameter of a coin under test but in order to obtain a meaningful 
value, a correction must be made to take account of the velocity of the 
coin. In the present embodiment, the coin's velocity is derived from the 
time difference t.sub.3 -t.sub.2. This time difference depends on the peak 
coil signal which is indicative of the material from which the coin is 
formed. However, the peak coil signal is available as part of the 
conventional inductive testing and can be used to select a predetermined 
correction factor. It should be borne in mind that correction factors are 
required only where the materials and/or thickness indicates that the coin 
may be acceptable. 
Another sensor, suitable for use in place of the optical and inductive 
sensors used in the foregoing embodiments, will now be described with 
reference to FIGS. 20 and 21. 
Referring to FIG. 20, a sensor comprises a flap 55 extending across the 
depth b of the upper part 2a of the coin passageway from the back wall 
thereof. The flap 55 also extends across the full width of the upper part 
2a of the coin passageway. The flap 55 is pivotably mounted to the back 
wall of the coin passageway by a pair of spaced light leaf springs 56,57. 
A piezo-electric film 58 extends from the flap 55 to the back wall of the 
coin passageway between the leaf springs 56,57. The film 58 may be 
polyvinylidene fluoride (PVDF) sold by AMP under the trade mark Kynar*. 
Referring to FIG. 21, as a coin 25 travels down the coin passageway it hits 
the flap 55 causing it to pivot downwardly against the leaf springs. The 
pivoting of the flap 55 stresses the piezo-electric film 58 which 
generates an electrical signal. This electric signal continues to be 
produced as long as the flap 55 is displaced from its rest position. Once 
the coin 25 has passed the flap 55, the leaf springs return it to its rest 
position, relieving the stress in the piezo-electric film 58 and 
terminating the electric signal. 
It will be appreciated that the duration of the electric signal produced by 
the piezo electric film 58 will be dependent on the coin diameter, the 
speed of the coin and the length of the flap 55, perpendicular to the back 
wall of the coin passageway. Consequently, the equations given above will 
need to be modified to take this into account. However, since the length 
of the flap is known, the necessary modifications will be readily apparent 
to the skilled person. 
A modification whereby the depth of the coin passageway can be varied will 
now be described with reference to FIG. 22, wherein like parts have the 
same reference signs as in FIGS. 1 and 2. 
Referring to FIG. 22, the element 60 forming the back wall of the coin 
passageway 2 is provided with a pair of vertical slots 61,62. One slot 
61,62 is provided on each side of the upper portion 2, of the coin 
passageway 2. Since, the element 60 is formed of plastics material, the 
back wall of the upper portion 2a of the passageway 2 is able to bend to 
and fro about a line joining the bottoms of the slots 61,62. 
A cam 63 is mounted behind the element 60 and bears against the back wall 
of the passageway 2. The cam 63 can be rotated which causes the back wall 
of the upper passageway portion 2a to be moved to and fro (as indicated by 
the double headed arrow in FIG. 22), thereby altering the depth b (as 
indicated in FIG. 2) of the upper portion 2a. The bearing surface of the 
cam 63 is formed as a plurality of elongate flats so that the cam 63 will 
not be turned by a force applied to the back wall of the upper passageway 
portion 2a. In use, the cam 63 is rotated into a position which sets the 
depth b of the upper passageway portion 2a to be appropriate for the coins 
for which the validator is designed. Thereafter, the cam 63 is not moved 
unless the validator is to be used with a different coin set. In the 
embodiment shown in FIG. 19, the coil 12 is mounted to the moveable part 
of the element 60 and is dimensioned such that it does not extend beyond 
the slots 61,62. This means that the coil 12 is kept as close as is 
possible to coins travelling through the passageway 2 whatever the 
position of the cam 63. 
In the interests of clarity, only the optical, inductive and piezo-electric 
sensors particular to the present invention have been described. However, 
the skilled person will appreciate that additional sensors and/or 
anti-fraud devices, of which many are known in the art, could be used in 
addition to the sensors described above.