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. |
