A coin sensor performs time-domain eddy-current testing using an inductance in one arm of a Maxwell bridge which is driven by a bipolar square wave having randomly-selected intervals between transitions, the output of the bridge being sampled at a high frequency. A processor stores a model which receives the drive signal and adjusts coefficients of the model so that the output matches the sampled measurements. Acceptance criteria are applied to the adjusted coefficients for coin classification purposes.
1. A method of testing a coin using at least one coin sensor which operates in response to a respective drive signal and a processor storing a transfer function of the sensor(s) in the presence of a coin, the method comprising:
driving the coin sensor(s) using at least one broadband signal as the drive signal(s) and deriving multiple measurement samples from the sensor(s) in the presence of a coin, wherein the measurement samples represent the effects of eddy currents in a coin under test and influenced by a field generated in response to the drive signal;
deriving coefficients of the transfer function using the measurement samples; and
applying acceptance criteria using the coefficients to determine whether the coin falls within a predetermined class of coins.
2. A method as claimed in claim 1 , including the step of applying the drive signal(s) to the transfer function.
3. A method as claimed in claim 1 , wherein at least one drive signal has at least one randomly-determined parameter.
4. A method as claimed in claim 3 , wherein at least one drive signal is a piecewise signal with transitions at randomly-determined instants.
5. A method as claimed in claim 4 , wherein at least one drive signal has substantially instantaneous voltage transitions at randomly-determined instants.
6. A method as claimed in claim 1 , wherein the transfer function is represented by a time difference equation.
7. A method as claimed in claim 6 wherein the time difference equation is:
y
[
k
]
=
1
a
0
(
∑
l
=
1
p
-
a
l
y
[
k
-
l
]
+
∑
l
=
0
q
b
l
x
[
k
-
l
-
m
]
+
n
[
k
]
)
where k is a time-ordered index, x[k] and y[k] are the samples of the sensor input and output, respectively, n[k] represents noise, p and q are predetermined model orders, m is a delay value and a 0 , a 1 and b 1 are coefficients.
8. A method as claimed in claim 1 comprising using the coefficients to derive a frequency spectrum, and applying acceptance criteria using values of the frequency spectrum to determine whether the coin falls within a predetermined class of coins.
9. A method as claimed in claim 1 wherein the sensor comprises a combined transmit/receive coil.
10. A coin sensor comprising:
a combined transmit/receive inductor located in proximity to a coin path;
a drive circuit arranged to drive the inductor with a broadband signal;
a sampler coupled to the inductor for deriving successive measurement samples having values influenced by the times of the samples relative to the drive signal as a result of eddy currents induced in a coin by the field generated by the inductor; and
a processor responsive to the measurement samples to derive coefficients of a transfer function of the inductor in the presence of the coin and to determine whether the coin meets acceptance criteria representing a coin of a particular class;
wherein the inductor is located in an arm of an A.C. bridge circuit having an input for receiving said drive signal and an output from which said measurement samples are derived.
11. A coin sensor as claimed in claim 10 , wherein the drive signal is a piecewise signal.
12. A coin sensor as claimed in claim 11 , wherein the drive signal has substantially instantaneous voltage transitions.
13. A coin sensor as claimed in claim 12 , wherein the drive signal is a square wave.
14. A coin sensor as claimed in claim 10 , wherein the bridge circuit is a Maxwell bridge.
15. A coin sensor as claimed in claim 10 , wherein the bridge circuit is substantially balanced in the absence of a coin.
16. A coin sensor as claimed in claim 10 , wherein the processor is arranged to derive the difference between each measurement sample and an idle sample which was derived in the absence of a coin and at a timing relative to the drive signal which corresponds to the timing of the respective measurement sample.
17. The apparatus of claim 10 wherein the inductor is a single combined transmit/receive coil.
18. A method as claimed in claim 1 , using a coin sensor according to claim 10 .
19. Apparatus comprising a processor arranged to operate according to a method as claimed in claim 1 or claim 18 .