U.S. patent application number 09/782733 was filed with the patent office on 2001-09-20 for method for capacitive image acquisition.
Invention is credited to Basse, Paul-Werner Von, Marksteiner, Stephan, Scheiter, Thomas, Willer, Josef.
Application Number | 20010022337 09/782733 |
Document ID | / |
Family ID | 7877459 |
Filed Date | 2001-09-20 |
United States Patent
Application |
20010022337 |
Kind Code |
A1 |
Basse, Paul-Werner Von ; et
al. |
September 20, 2001 |
Method for capacitive image acquisition
Abstract
A grid-shaped array of conductor areas is used for capacitive
image acquisition. Shielding conductors are disposed in each case
between the conductors that are provided for measurement. During a
plurality of charging and discharging cycles, the potential is
always carried along on the conductors belonging to a respective
pixel in order to prevent displacement currents between the
shielding capacitors. By way of example, a compensation line with a
feedback operational amplifier can be used for identically altering
the electrical potentials on the conductors.
Inventors: |
Basse, Paul-Werner Von;
(Wolfratshausen, DE) ; Willer, Josef; (Riemerling,
DE) ; Scheiter, Thomas; (Oberhaching, DE) ;
Marksteiner, Stephan; (Munchen, DE) |
Correspondence
Address: |
LERNER AND GREENBERG, P.A.
POST OFFICE BOX 2480
HOLLYWOOD
FL
33022-2480
US
|
Family ID: |
7877459 |
Appl. No.: |
09/782733 |
Filed: |
February 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09782733 |
Feb 13, 2001 |
|
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PCT/DE99/02523 |
Aug 12, 1999 |
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Current U.S.
Class: |
250/208.1 |
Current CPC
Class: |
G06V 40/1306
20220101 |
Class at
Publication: |
250/208.1 |
International
Class: |
H01L 027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 13, 1998 |
DE |
198 36 770.8 |
Claims
We claim:
1. A method for capacitive image acquisition, which comprises: a)
dividing an area to be acquired as an image in a grid-shaped array
into pixels assigned an assembly of electrical conductors
comprising, for each pixel, a measuring conductor and a shielding
conductor; b) placing the area to be acquired as an image opposite
the -measuring conductors, so that a capacitance is present between
the pixels and the measuring conductors in each case dependent on
the relevant pixel; c) at each pixel, connecting the measuring
conductor and the shielding conductor to the same electrical
potential and disconnecting from the potential; d) at each pixel,
discharging a charge present on one of the measuring conductor and
the shielding conductor onto a respective collecting capacitor, and
simultaneously compensating a potential difference between the
measuring conductor and the shielding conductor; and e) repeating
steps c and d until the charges collected on the collecting
capacitors have at least a value predetermined to be sufficient for
a separate measurement of each collecting capacitor.
2. The method according to claim 1, which comprises compensating
the potential difference between the respective measuring conductor
and the respective shielding conductor identically for all pixels
by placing the shielding conductors on the same predetermined
potential.
3. The method according to claim 1, which comprises compensating
the potential difference between the respective measuring conductor
and the respective shielding conductor separately for all the
pixels, by always applying a same potential that is currently
present on the measuring conductor to the respective shielding
conductor.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of copending
International Application PCT/DE99/02523, filed, which designated
the United States.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method for capacitive
image acquisition which is suitable, in particular, for acquiring
fingerprint images by means of sensors effecting capacitive
measurement.
[0004] In the case of capacitive surface sensors, e.g. in the case
of fingerprint sensors, the distance between the object to be
measured (e.g. the surface of the finger) and the sensor is
measured by a grid-shaped array of small conductor areas (pads). In
the case of a fingerprint sensor, these conductor areas are very
small and have a dimension of approximately 50 .mu.m to 100 .mu.m.
Such fingerprint sensors effecting capacitive measurement are
specified, for instance, in the overview article by Tartagni and
Guerrieri: "A 390 dpi Live Fingerprint Imager Based on Feedback
Capacitive Sensing Scheme" in ISSCC97, pages 154, 155 and 402. The
capacitances with respect to the measurement object are very small,
so that parasitic capacitances e.g. with respect to the adjacent
conductor or with respect to the support of the relevant sensor
have an interfering effect on the measurement results. Sensitive
amplifiers are necessary in order to be able to isolate the small
measurement signals from the relatively large interference signals.
The interference signals contained in the amplified signals can be
suppressed either directly by measurement technology or after AD
conversion by digital processing of the signal obtained. These
measures are complicated and require a high degree of accuracy.
SUMMARY OF THE INVENTION
[0005] The object of the present invention is to provide a
capacitive image acquisition method which overcomes the above-noted
deficiencies and disadvantages of the prior art devices and methods
of this general kind, and which is suitable, in particular, for
acquiring fingerprints and can be implemented with little technical
complexity.
[0006] With the above and other objects in view there is provided,
in accordance with the invention, a method for capacitive image
acquisition, which comprises:
[0007] a) dividing an area to be acquired as an image in a
grid-shaped array into pixels assigned an assembly of electrical
conductors comprising, for each pixel, a measuring conductor and a
shielding conductor;
[0008] b) placing the area to be acquired as an image opposite the
measuring conductors, so that a capacitance is present between the
pixels and the measuring conductors in each case dependent on the
relevant pixel;
[0009] c) at each pixel, connecting the measuring conductor and the
shielding conductor to the same electrical potential and
disconnecting from the potential;
[0010] d) at each pixel, discharging a charge present on one of the
measuring conductor and the shielding conductor onto a respective
collecting capacitor, and simultaneously compensating a potential
difference between the measuring conductor and the shielding
conductor; and
[0011] e) repeating steps c and d until the charges collected on
the collecting capacitors have at least a value predetermined to be
sufficient for a separate measurement of each collecting
capacitor.
[0012] In accordance with an added feature of the invention, the
potential difference between the respective measuring conductor and
the respective shielding conductor is compensated identically for
all pixels by placing the shielding conductors on the same
predetermined potential.
[0013] In accordance with a concomitant feature of the invention,
the potential difference between the respective measuring conductor
and the respective shielding conductor is compensated separately
for all the pixels, by always applying a same potential that is
currently present on the measuring conductor to the respective
shielding conductor.
[0014] The invention uses an arrangement of individual sensors
effecting capacitive measurement which each comprise conductor
areas, some of which are provided as measuring conductors and some
are used as shielding conductors in order to shield the
capacitances of the individual sensors from adjacent sensors. By
means of transistors used as switches, a predetermined electrical
potential is cyclically applied to all the conductors and the
charge which accumulates thereon on account of the various
capacitances--caused by the image--with respect to the measuring
conductors is dissipated onto a collecting capacitor. During this
operation, a connected compensation line, which has a feedback
comparator in the preferred embodiment, ensures that the potential
on the conductors remains at least approximately compensated, so
that there is no electrical voltage across the capacitors and
charging that is present can have arisen only through a further
external capacitance, but not through undesirable displacement
currents between the conductors.
[0015] A surface of an image which is to be acquired and engenders
a locally variable capacitance relative to the conductors arranged
in the grid, as is the case with the skin surface of a fingerprint,
is arranged parallel to the area of the conductors during the
measurement operation. Thus, the result is different charging of
the individual measuring areas in accordance with the capacitance
of the image present. By means of repeated charging and discharging
of the capacitors of the individual sensors, the charge
respectively accumulating thereon can be added on a further
capacitor to the extent that these charges can be measured with
little technical complexity. In a manner governed by the circuit
used, the conductors, including the conductors provided as guard
ring, are always at the same potential, so that no displacement
currents occur between all the conductor areas present. The effect
achieved in this way is that using a fundamentally known sensor
arrangement for image acquisition, it is also possible to acquire
images such as e.g. fingerprints which engender only very small
capacitive differences.
[0016] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0017] Although the invention is illustrated and described herein
as embodied in a method for capacitive image acquisition, it is
nevertheless not intended to be limited to the details shown, since
various modifications and structural changes may be made therein
without departing from the spirit of the invention and within the
scope and range of equivalents of the claims.
[0018] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic diagram of an individual sensor having
a circuit suitable for the method according to the invention;
[0020] FIG. 2 is a collective graph of diagrams of electrical
potentials at different points in the circuit of FIG. 1; and
[0021] FIG. 3 is a plan view onto an arrangement of electrical
conductors suitable for the novel method.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] Referring now to the figures of the drawing in detail and
first, particularly, to the schematic and diagrammatic cross
section of FIG. 1, there is seen an assembly of conductors in two
planes which are coplanar with respect to one another. A detail of
an image surface 1, e.g. a ridge of a fingerprint, is situated
above a measuring conductor 2 of an upper conductor plane. This
measuring conductor 2 in each individual sensor is provided for
measuring the capacitance between the image surface 1 and the
conductor plane (image/conductor capacitance 3). Situated in the
same plane, laterally with respect to the measuring conductor 2,
there are further conductors as upper shielding conductors 8. In a
second plane, a respective further conductor is arranged opposite
the measuring conductor 2. The further conductor forms a lower
shielding conductor 7 and is electrically conductively connected to
the upper shielding conductor 8. The shielding capacitance 5
present between the measuring conductor 2 and the upper shielding
conductor 8 and the shielding capacitance 6 present between the
measuring conductor 2 and the lower shielding conductor 7 are
depicted by broken lines similarly to the image/conductor
capacitance 3. This indicates that there are no capacitor plates
present at these locations, rather that an equivalent circuit
diagram is meant for a capacitor. In this example, the upper
shielding conductor 8 can be imagined such that it surrounds the
measuring conductor 2 all around. The two portions of the upper
shielding conductor 8 illustrated in FIG. 1 then form the cross
section of such a shielding conductor which is present all around
the measuring conductor 2.
[0023] In addition to the structural acquisition system, FIG. 1
illustrates the associated circuit with which the measurement is
carried out. The method according to the invention is carried out
in such a way that firstly, in each pixel of the image array, the
associated conductors (measuring conductor and shielding conductor)
are placed at a specific potential. That is done with the
illustrated circuit in that the upper transistors are turned on via
a clock control .PHI..sub.1, so that the connection potential
V.sub.DD is present at the nodes S and P and thus on the conductors
2, 7, 8 of the individual sensor. Via the second clock control
.PHI..sub.2 and the two lower transistors, the charge on the
conductors is subsequently dissipated in such a way that, to the
extent that is technically possible in terms of circuitry, no
potential difference occurs between the measuring conductor 2 and
the two shielding conductors 7, 8.
[0024] That is preferably achieved by a circuit section 9, which
ensures that the potential present at the points Q and R is always
the same. This circuit section 9 is preferably constructed using a
feedback operational amplifier 10. If the lower transistors are
turned on via the clock control .PHI..sub.2, the effect is that the
same potential is likewise present at the points P and S of the
circuit. In a preferred embodiment of the method, the potential is
tracked separately in the manner described for each image pixel,
i.e. each individual sensor, thereby preventing the generation of a
potential difference on the conductors. A higher sensitivity of the
individual measurement is achieved in this way, because
interference or stray capacitances are shielded and undesirable
displacement currents are prevented. Moreover, the electric field
is homogenized between the two conductor planes at the edge of the
measuring conductor 2. Moreover, the lower shielding conductor 7
shields the measuring arrangement from a parasitic capacitance
caused, for example, by a substrate on which the assembly is
applied (sensor/substrate capacitance 4 in FIG. 1). In principle,
any desired counterelectrodes of a multilayer metallization may be
used as lower shielding conductors 7.
[0025] The discharging of the measuring arrangement which is
performed after each charging cycle is effected via a collecting
capacitor C.sub.S, on which the charges are collected, until the
charge on this capacitor or the voltage present across this
capacitor is so large that it can be measured with relatively
little technical complexity. On account of the different
image/conductor capacitances 3, different charges result on the
conductors from pixel to pixel. In a corresponding manner, the
charges collected on the collecting capacitors C.sub.S differ for
the individual pixels, so that the image can be reconstructed from
the determination of these charges.
[0026] The individual pixels are preferably selected via read lines
LL after the manner of the cell array of a matrix memory. An
arrangement of this type is complex. It requires, in particular, an
operational amplifier 10 and a compensation line LLN per row of the
grid-shaped arrangement of the individual sensors.
[0027] The method according to the invention can also be carried
out using a simpler conductor structure if complete compensation of
the potential difference between the measuring conductors 2 and the
shielding conductors 7, 8 is dispensed with. A circuit section 9
then suffices for compensation purposes, and it is possible to
manage with one operational amplifier 10 for all of the read lines
LL. This operational amplifier is then driven by this one read line
LL e.g. in the center of the cell array (grid-shaped arrangement of
the individual sensors). Since the potential profile during the
charging and discharging operations corresponds to the average
profile of these operations on the individual sensors, compensation
is effected in each individual sensor with very good accuracy.
[0028] A further possibility for implementing the method using a
relatively simple arrangement is to completely dispense with the
driving by a read line. All of the read lines are simulated by the
sensor capacitance to be measured, whose charge is simply
dissipated onto the collecting capacitor C.sub.S. These charges are
detected by measurement when enough charges have accumulated
thereon after a plurality of charging and discharging cycles. The
simplest form of compensation is fixing at a fixed voltage. To that
end, the point Q of the circuit is put at a fixed potential and it
is then possible to manage without the circuit section 9. This
potential is the same for all of the individual sensors. Although
the compensation line has a voltage swing which is too small at the
beginning and too large at the end, on average the compensation is
balanced. In the two cases described, with identical compensation
for all of the individual sensors, the driving can be performed
from the edge of the sensor array, which greatly simplifies the
outlay on circuitry.
[0029] Referring now to FIG. 2, there are shown the typical
potential profiles at the individual points of the circuit
illustrated in FIG. 1. The discharging clock signals .PHI..sub.2
are in each case temporally staggered relative to the charging
clock signals .PHI..sub.1. On account of the compensation that is
performed, the voltage profiles at the points P and S are identical
or, in the case of the simplified embodiment of the method with a
simplified circuit, at least approximately identical. The voltage
at the points P and/or S falls to a lower and lower value during
the discharges, since the collecting capacitor C.sub.S is charged
to an increasing extent, and the minimal voltage at the points P
and/or S thus increases in the course of time. The potentials at
the point R and at the point Q, which is carried along via the
compensation with the potential at the point R, are likewise
illustrated in FIG. 2.
[0030] With reference to FIG. 3, there is shown the grid-shaped
arrangement of the measuring conductors 2--respectively provided
for the measurement--of the upper conductor plane with the upper
shielding conductors 8 in between. These shielding conductors 8 are
depicted here as a further example as strips between individual
columns 11 of the matrix-type arrangement. Instead of this
shielding between individual columns of the assembly, there may
also be present, all around the measuring conductors 2, a
respective shielding conductor 8 in accordance with the borders
depicted by broken lines.
[0031] The method according to the invention can also be
implemented irrespective of the precise structuring of the
conductors. All that is essential in this case is that a group of
conductors be present for each pixel, of which a specific conductor
faces the image surface and is provided for the measurement, while
the remaining conductors serve for shielding. There has to be a
circuit present which can track the electrical potential on the
shielding conductors during the charging and during the discharging
of the measuring conductor to the potential of the measuring
conductor. The geometrical arrangement of the shielding conductors
can easily be adapted to the respective requirements.
* * * * *