Optel, the company we represent, was established a few years ago on the
basis of the idea that a fingertip structure (reflected in fingerprints) can be better
analyzed with the use of ultrasound than by existing methods. It turns out that our
approach was justified; however its implementation required much more effort than we had
anticipated. Nevertheless, we have managed to create fully-operational prototypes of an
ultrasonic camera which permit the imaging and recognition of finger ridge
patterns. We
are well on our way to developing a plan for mass production. The design of the current
version of the camera and principles behind its operation have been presented in [1]. Its
operation can be briefly summarized as follows:
A very short ultrasonic pulse is sent from a number of different
directions toward a finger surface and from each direction a much longer broadband impulse
response is received. This impulse response is an effect of the contact scattering of the
ultrasonic wave on the surface of the fingertip. Based on a set of such impulse
responses,
an image of the finger surface structure is reconstructed using computer tomography
methods.
In order to create our camera we had to design and make our own ultrasonic
devices, mainly for two reasons: devices available on the market were too expensive and
did not meet a number of requirements important in our applications. Of paramount
importance was that the time skew between different impulse responses must be very small
(less than 5ns). Equally important was that the ultrasonic transducer should be able to
generate a very short pulse (less than 100ns) and should have a sufficiently broad
bandwidth as a receiver (4-16MHz). Finally, all elements used had to be
inexpensive, since
the final product should be suitable for mass production at a reasonable price.
After we have created elements that meet our needs, it has become apparent that they are
suitable for a number of other applications where the use of ultrasonic pulses is
necessary, (e.g., classical defectoscopy, layer thickness measurement, medical
applications). This paper is intended as a presentation of our solutions for the
ultrasonic technology experts and a declaration of our future aims.
We have created following elements based on our own design, partially patented, which
depending on the mechanics and software used can be employed to various
applications.
Ultrasonic PC card, particularly well suited to automatic measurements
using mechanical or electronic scanners. It can directly control such devices
(it only
generates control signals, step motors require an additional driver unit).
Pulser and receiver circuit with an active switch and input amplifier, the size of a
matchbox, powered and controlled by the ultrasonic PC card, capable of generating very
short pulses and having a broad bandwidth as a receiver.
Ultrasonic transducers capable of generating very short pulses while having broad
bandwidths as receivers.
Elements, allowing to produce a very good defined gaussian ultrasonic beam.
The PC card comprises not only an input
amplifier, a high-speed A/D
converter (it has a sampling frequency of 80MHz, a 100 MHz version is under
development)
but also a control circuit. This means that all control signals required for the
measurement process are generated by the card. This kind of design allows us to achieve a
sufficiently small time skew between different channels (or scanner positions) which is
less than 1 ns. Thanks to the independent control system, the CPU is not directly involved
in the measurement process, therefore a high performance PC is not essential. The card has
a simple design achieved through the use of highly integrated circuits and can be offered
at a reasonable price.
Our pulser and receiver circuit also provides an inexpensive
solution. It
is based on a unique design: in order to generate a pulse we first charge the transducer
to a required voltage (max. 600V) which takes a few microseconds, then we discharge it in
a very short period of time (at the moment this time is around 20ns). This approach
results in a simple and inexpensive circuit which generates very clean pulses with
practically any transducer (only transducers with a parallel inductance cannot be
used).
All the electronic circuits we have developed would not be sufficient if
we did not have transducers capable of generating very short pulses while having broad
bandwidths as receivers. We had tested a number of designs proposed by others as well as
products available on the market and we were convinced that there was only one
solution:
to find a new way. It had to lead to a design which would not only satisfy all the
technical requirements but also enable cheap production and repeatable
parameters.
We have succeeded in developing a transducer satisfying the above
requirements for which a patent application has been submitted. Fig. 1 shows a pulse
generated by our transducer (a hydrophone made of PVDF film has been used for the
measurement, 50 ns/div). Fig. 2 presents the signal received in the transceiver mode (100
ns/div). The difference between the two signals results from the fact that our transducer
works differently as a transmitter and as a receiver. This property is also exhibited by
other types of transducers, though the differences are usually much smaller. It is
possible to develop a transducer which would have practically identical signals in both
the transmitter and receiver modes (corresponding roughly to the signal shown in Fig. 1),
but it would have certain drawbacks (particularly when used in the design of our
camera).
Therefore we have decided to use the transducers which receive slightly longer impulse
response (as shown in Fig. 2). How do the signal level and sensitivity compare with other
designs? The signal level generated in the transmitter mode is at least two times higher
and the sensitivity in the receiver mode is slightly lower. Hence, the overall signal
level in the measurement cycle is comparable. Since the development of the new
transceivers has not been completed yet, we expect considerable improvement.
Our technique allows not only the observation of finger ridge patterns but
also other objects. An example of such an application is a device suitable for detecting
material defects in objects as well as for medical measurements. It has been created on
the basis of our electronic circuits, transducers as well as mechanical elements of our
ultrasonic camera. Its basic idea is similar to that behind our ultrasonic
camera. Fig. 3
shows a schematic diagram of the device:
The motor M moves the transducer T around the object S
under measurement, sending toward it an ultrasonic wave. Signals reflected by the object
are received by the same transducer and processed in a similar way as is done in our
camera. Only image reconstruction procedures need to be modified.
The device has been developed for finger
imaging. We would like to see a
cross-section of a finger and subsequently a three dimensional structure (using
tomographic reconstruction procedures). Using the same device, it is possible to observe
other types of objects, both natural and man-made, in order to find material
defects.
The pictures 4 and 5 shows the cross-sections of the
finger. To achieve
this pictures a classical B-scan was used. Echoes, coming from 400 directions are set
together to form a whole picture. In the near future we will try to make tomographic
reconstruction, hoping to became much higher resolution, comparable with the resolution of
our fingerprint devices (at least .1 mm).
Fig.
1
 |
Fig.
2
 |
Fig.
3
 |
Pulse of the
transducer,
received
with a hydrophone (50 ns/div). |
Pulse of the
transducer,
received
with the same transducer. |
Schematic diagram
of the device. |
Fig.
4
 |
Fig.
5
 |
Fig.
6
 |
| View of the device |
Cross-section of the
finger. |
Cross-section of the
finger. |
