U.S. patent application number 15/464582 was filed with the patent office on 2018-01-11 for fingerprint sensing system with finger detect.
The applicant listed for this patent is Fingerprint Cards AB. Invention is credited to Andreas Larsson, Frank Riedijk, Hans Thornblom.
Application Number | 20180012053 15/464582 |
Document ID | / |
Family ID | 60812839 |
Filed Date | 2018-01-11 |
United States Patent
Application |
20180012053 |
Kind Code |
A1 |
Larsson; Andreas ; et
al. |
January 11, 2018 |
FINGERPRINT SENSING SYSTEM WITH FINGER DETECT
Abstract
The present invention relates to a method of sensing a
fingerprint pattern of a finger using a fingerprint sensing device.
The method comprising: controlling a group of sensing elements to
change a potential of a group of sensing structures comprised in
said group of sensing elements; acquiring, in response to said
variation in potential, a response signal from a finger detecting
circuitry indicative of the capacitive coupling between the group
of sensing structures and the finger detecting structure; comparing
a variation in the response signal with a predefined threshold
value; and providing a signal indicating that a finger is present
when the variation in response signal is greater than the threshold
value. The invention also relates to a corresponding fingerprint
sensing device.
Inventors: |
Larsson; Andreas;
(Herrljunga, SE) ; Riedijk; Frank; (Delft, NL)
; Thornblom; Hans; (Kungsbacka, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fingerprint Cards AB |
Goteborg |
|
SE |
|
|
Family ID: |
60812839 |
Appl. No.: |
15/464582 |
Filed: |
March 21, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06K 9/0002 20130101;
G06K 9/00087 20130101 |
International
Class: |
G06K 9/00 20060101
G06K009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 7, 2016 |
SE |
1651013-3 |
Claims
1. A method of sensing a fingerprint pattern of a finger using a
fingerprint sensing device comprising: providing an array of
sensing elements for sensing said fingerprint pattern, each sensing
element comprising a sensing structure for capacitive coupling with
the finger; controlling a group of sensing elements to change a
potential of the group of sensing structures comprised in said
group of sensing elements; acquiring, in response to said variation
in potential, a response signal from a finger detecting circuitry
connected to an electrically conductive finger detecting structure
arranged adjacent to said array of sensing elements, the response
signal is indicative of the capacitive coupling between the group
of sensing structures and the finger detecting structure; comparing
a variation in the response signal with a predefined threshold
value; and providing a signal indicating that a finger is present
when the variation in response signal is greater than the threshold
value.
2. The method according to claim 1, further comprising: activating,
when the variation in response signal is greater than the threshold
value, at least a subset of said sensing elements to sense at least
a portion of said fingerprint pattern.
3. The method according to claim 2, wherein said step of activating
is only carried out when said variation in the response signal is
greater than the threshold value.
4. The method according to claim 1, further comprising: evaluating
said response signal to determine a background capacitive coupling
between the group of sensing structures and the finger detecting
structure when a finger is not coupled to the finger detecting
structure, wherein said predefined threshold value is indicative of
the background capacitive coupling.
5. The method according to claim 1, wherein said fingerprint
sensing device comprises at least two electrically conductive
finger detecting structures, wherein said response signal is
indicative of the capacitive coupling between the group of sensing
elements and a first finger detecting structure, and wherein a
second group of sensing elements neighboring to a second finger
detecting structure are at least partly inactive, wherein the
method comprises: acquiring, a second response signal from the
finger detecting circuitry of the second finger detecting
structure; and providing a response signal from which the second
response signal has been subtracted.
6. The method according to claim 1, wherein said fingerprint
sensing device comprises at least two electrically conductive
finger detecting structures, wherein said response signal is
indicative of the capacitive coupling between the group of sensing
elements and a first finger detecting structure, wherein a second
group of the sensing elements close to a second finger detecting
structure are at least partly inactive, wherein the method
comprises: acquiring, a second signal from the finger detecting
circuitry of the second finger detecting structure; and discarding
the response signal when the second signal is greater than a
background threshold level, whereby a further response signal is
acquired.
7. The method according to claim 1, wherein the group of sensing
structures and the electrically conductive finger detecting
structure are separated from each other such that the capacitive
coupling between the group of sensing structures and the finger
detecting structure is relatively low when no finger is present and
relatively high when a finger is present, wherein said threshold
value is indicative of the relatively low capacitive coupling.
8. A fingerprint sensing device comprising: an array of sensing
elements for sensing a fingerprint pattern, each sensing element
comprising a sensing structure for capacitive coupling with a
finger; an electrically conductive finger detecting structure
arranged adjacent to said array of sensing elements; and finger
detecting circuitry connected to said finger detecting structure
for providing a finger detection signal indicative of a capacitive
coupling between a group of sensing structures and the finger
detecting structure, wherein the fingerprint sensing device is
configured to: control a group of sensing elements to change a
potential of the group of sensing structures comprised in said
group of sensing elements; acquire, in response to said change in
potential, a response signal from the finger detecting circuitry
indicative of the capacitive coupling between the group of sensing
structures and the finger detecting structure; compare a variation
in the response signal with a predefined threshold value; and
provide a signal indicating that a finger is present when the
variation in the response signal is greater than the threshold
value.
9. The fingerprint sensing device according to claim 8, wherein
said fingerprint sensing device comprises at least two finger
detecting structures, wherein said response signal is indicative of
the capacitive coupling between the group of sensing elements and a
first of the finger detecting structures, wherein a second group of
sensing elements neighboring to a second finger detecting structure
are at least partly inactive, wherein the fingerprint sensing
device is configured to: acquire, a second response signal from the
finger detecting circuitry of the second finger detecting
structure; and provide a response signal from which the second
response signal has been subtracted.
10. The fingerprint sensing device according to claim 8, wherein
said fingerprint sensing device comprises at least two finger
detecting structures, wherein said response signal is indicative of
the capacitive coupling between the group of sensing elements and a
first of the finger detecting structures, wherein a second group of
the sensing elements close to a second finger detecting structure
are at least partly inactive, wherein the fingerprint sensing
device is configured to: acquire, a background signal from the
finger detecting circuitry of the second finger detecting
structure; and when the background signal is greater than a
background threshold level, discard the response signal, whereby a
further response signal is acquired.
11. The fingerprint sensing device according to claim 8, wherein
the group of sensing structures and the electrically conductive
finger detecting structure are separated from each other such that
the capacitive coupling between the group of sensing structures and
the finger detecting structure is relatively low when no finger is
present and relatively high when a finger is present.
12. The fingerprint sensing device according to claim 11, wherein
said group of sensing structures is surrounded in a sensing
structure plane by at least partly inactive sensing structures.
13. The fingerprint sensing device according to claim 8, wherein
the electrically conductive finger detecting structure is arranged
to extend around the circumference of the array of sensing
structures.
14-17. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method of sensing a
fingerprint pattern of a finger using a fingerprint sensing device.
The invention further relates to a corresponding fingerprint
sensing device and a computer program product.
BACKGROUND OF THE INVENTION
[0002] Various types of biometric systems are used more and more in
order to provide for increased security and/or enhanced user
convenience.
[0003] In particular, fingerprint sensing systems have been adopted
in, for example, consumer electronic devices, thanks to their small
form factor, high performance and user acceptance.
[0004] To provide for a low energy consumption of the fingerprint
sensing system, the fingerprint sensing system should only operate
when there is a finger on the fingerprint sensing device comprised
in the fingerprint sensing system. On the other hand, the
fingerprint sensing system may be required to independently
determine whether or not a finger is present on the sensing device
and to sense the fingerprint pattern of the finger when the finger
is placed on the sensing device.
[0005] U.S. Pat. No. 8,031,046 discloses a finger sensing device in
which finger sensing electrodes may be bussed together and
connected to a bussed electrode amplifier which is typically
maintained on. A sample and hold and decoder circuit is connected
to the bussed electrode amplifiers and may output an average power
from the bussed group of electrodes that is compared with a
threshold level. If the average power is higher than the threshold
level, it is determined that the finger is present, and a
fingerprint image is acquired.
[0006] Although the solution proposed by U.S. Pat. No. 8,031,046
seems to provide for a rather energy-efficient finger detect
functionality, there still appears to be room for improvement.
SUMMARY
[0007] In view of above-mentioned and other drawbacks of the prior
art, it is an object of the present invention to provide an
improved fingerprint sensing device and method of sensing a
fingerprint pattern, in particular providing for more energy
efficient operation of the fingerprint sensing device.
[0008] According to a first aspect of the present invention, it is
therefore provided a method of sensing a fingerprint pattern of a
finger using a fingerprint sensing device comprising: an array of
sensing elements for sensing the fingerprint pattern, each sensing
element comprising a sensing structure for capacitive coupling with
the finger; an electrically conductive finger detecting structure
arranged adjacent to the array of sensing elements; and finger
detecting circuitry connected to the finger detecting structure for
providing a finger detection signal indicative of a capacitive
coupling between a group of sensing structures and the finger
detecting structure, wherein the method comprises the steps of:
controlling the group of sensing elements to change a potential of
the group of sensing structures comprised in the group of sensing
elements; acquiring, in response to the variation in potential, a
response signal from the finger detecting circuitry indicative of
the capacitive coupling between the group of sensing structures and
the finger detecting structure; comparing a variation in the
response signal with a predefined threshold value and providing a
signal indicating that a finger is present when the variation in
response signal is greater than the threshold value.
[0009] The variation in response signal is caused by the presence
of a finger on or near the finger detecting structure and the
sensing structure. Thus, the presence of the finger causes an
increase or decrease in the capacitive coupling between the finger
detecting structure and the group of sensing structures.
[0010] The sensing elements may, for example, be capacitive sensing
elements, each providing a measure indicative of the capacitive
coupling between that particular sensing element and a finger
surface touching the sensor surface. Sensing elements at locations
corresponding to ridges in the fingerprint will exhibit a stronger
capacitive coupling to the finger than sensing elements at
locations corresponding to valleys in the fingerprint.
[0011] However, the various embodiments of the present invention
are not limited to a fingerprint sensing device comprising sensing
elements utilizing a particular fingerprint sensing technology, but
are equally applicable to, for instance, optical, thermal or
piezo-electric fingerprint sensors etc.
[0012] The fingerprint sensor may comprise additional circuitry for
operating on the sensing signals indicative of the fingerprint
pattern provided by the sensing elements. Such additional
circuitry, which may for instance include sampling circuitry and
analog-to-digital conversion circuitry. Thus, the fingerprint
sensor may thus provide a fingerprint pattern signal as a digital
signal. Alternatively, the fingerprint pattern signal may be
provided as an analog signal.
[0013] For example, the signals may be analog or digital values
indicative of a voltage, which may in turn be proportional to the
capacitance of the capacitor constituted by the finger (or other
conductive object in the vicinity of the finger detecting
structure), the finger detecting structure and the dielectric
material there between.
[0014] The sensed fingerprint pattern may be used for various
purposes, such as biometric enrollment or authentication, or
fingerprint pattern based navigation etc.
[0015] The threshold value may for example be determined by the
coupling strength between the finger and the sensing structures
such that the threshold value is indicative of a fraction of the
coupling strength. Note that this is only an example and there may
be other ways of setting the threshold value appropriate for a
specific implementation.
[0016] The present invention is based upon the realization that a
low-power finger detection on fingerprint sensors is needed and
that this can be achieved in a way that the fingerprint sensing
elements is not in an active mode for acquiring an image. In other
words, the sensing elements can be operated in a low power mode to
save power for the overall fingerprint sensing device. This is
carried out by changing the potential of a group of sensing
structures and monitoring the capacitive coupling with finger
detecting structures.
[0017] An advantage of changing the potential of a group of sensing
structures instead of the finger detecting structures is that the
power consumption of the using the sensing elements for sensing the
finger and thus provide the finger detection signal is relatively
high compared to using the finger detecting structures for doing
the same.
[0018] Furthermore, the finger detecting structures may be used for
detecting that the finger is correctly placed on the fingerprint
sensing structures for fingerprint pattern sensing.
[0019] According to an embodiment, the method may further comprise
activating, when the variation in response signal is greater than
the threshold value, at least a subset of the sensing elements to
sense at least a portion of the fingerprint pattern. Thus, when it
is determine that a finger is present on the fingerprint sensing
device, the fingerprint pattern may be sensed. Advantageously, this
reduces the number of false attempts to sense a fingerprint
pattern.
[0020] To further reduce the number of false attempts to sense a
fingerprint pattern, it may be required that the step of activating
is only carried out when the step of activating is only carried out
when the variation in the response signal is greater than the
threshold value. This advantageously reduces false activation of
the sensing elements and thus reduces power consumption further. In
some embodiments the response signal is required to be greater than
the threshold value for a time duration longer than a predefined
time duration
[0021] In one embodiment, it may be included to evaluate the
response signal to determine a background capacitive coupling
between the group of sensing structures and the finger detecting
structure when a finger is not coupled to the finger detecting
structure, wherein the predefined threshold value is indicative of
the background capacitive coupling. This advantageously reduces the
influence of e.g. dielectrics on the sensor array surface.
Furthermore, it provides a lowest reference level for determining
the presence of a finger.
[0022] According to an embodiment of the invention, the fingerprint
sensing device may comprise at least two electrically conductive
finger detecting structures, wherein the response signal is
indicative of the capacitive coupling between the group of sensing
elements and a first finger detecting structure, and wherein a
second group of sensing elements neighboring to a second finger
detecting structure are at least partly inactive, wherein the
method comprises: acquiring, a second response signal from the
finger detecting circuitry of the second finger detecting
structure; and providing a response signal from which the second
response signal has been subtracted. The at least partly inactive
sensing elements may be disabled such that they are not ready for
fingerprint pattern sensing. In this way, there should in principle
not be any signal measured from the second finger detecting
structure since the coupling between the group of sensing
structures and the second finger detecting structure will be
relatively weak. However, the signal that may still be measureable
on the second finger detecting structure may be external noise
common to the entire fingerprint sensing device, but subtracting
this second signal from the response signal acquired from the first
finger detecting structure, the noise may be reduced and the
detection of the finger may become more accurate.
[0023] In a further embodiment, the fingerprint sensing device may
comprise at least two electrically conductive finger detecting
structures, wherein the response signal is indicative of the
capacitive coupling between the group of sensing elements and a
first finger detecting structure, wherein a second group of the
sensing elements close to a second finger detecting structure are
at least partly inactive, wherein the method may comprise:
acquiring, a second signal from the finger detecting circuitry of
the second finger detecting structure; and discarding the response
signal when the second signal is greater than a background
threshold level, whereby a further response signal is acquired.
This further reduces the number of false triggering events since a
signal may indicate detecting of other objects (e.g. a key in a
pocket of a user's trousers, or jacket) than a finger.
[0024] In yet another embodiment, the group of sensing structures
and the electrically conductive finger detecting structure may be
separated from each other such that the capacitive coupling between
the group of sensing structures and the finger detecting structure
is relatively low when no finger is present and relatively high
when a finger is present, wherein the threshold level is indicative
of the relatively low capacitive coupling. For example, the group
of sensing elements may be an island surrounded by at least partly
inactive sensing elements. Thus, the group of sensing elements is
then separated from the finger detecting structure by the at least
partly inactive sensing elements.
[0025] According to a second aspect of the present invention there
is provided a fingerprint sensing device comprising: an array of
sensing elements for sensing the fingerprint pattern, each sensing
element comprising a sensing structure for capacitive coupling with
the finger; an electrically conductive finger detecting structure
arranged adjacent to the array of sensing elements; and finger
detecting circuitry connected to the finger detecting structure for
providing a finger detection signal indicative of a capacitive
coupling between a group of sensing structures and the finger
detecting structure, wherein the fingerprint sensing device is
configured to: control the group of sensing elements to change a
potential of the group of sensing structures comprised in the group
of sensing elements; acquire, in response to the change in
potential, a response signal from the finger detecting circuitry
indicative of the capacitive coupling between the group of sensing
structures and the finger detecting structure; compare a variation
in the response signal with a predefined threshold value; and
provide a signal indicating that a finger is present when the
variation in the response signal is greater than the threshold
value.
[0026] According to an embodiment, the fingerprint sensing device
may comprise at least two finger detecting structures, wherein the
response signal is indicative of the capacitive coupling between
the group of sensing elements and a first of the finger detecting
structures, wherein a second group of sensing elements neighboring
to a second finger detecting structure are at least partly
inactive, wherein the fingerprint sensing device is configured to:
acquire, a second response signal from the finger detecting
circuitry of the second finger detecting structure; and provide a
response signal from which the second response signal has been
subtracted.
[0027] According to another embodiment, the fingerprint sensing
device comprises at least two finger detecting structures, wherein
the response signal is indicative of the capacitive coupling
between the group of sensing elements and a first of the finger
detecting structures, wherein a second group of the sensing
elements close to a second finger detecting structure are at least
partly inactive, wherein the fingerprint sensing device is
configured to: acquire, a background signal from the finger
detecting circuitry of the second finger detecting structure; and
when the background signal is greater than a background threshold
level, discard the response signal, whereby a further response
signal is acquired.
[0028] In yet another embodiment, the group of sensing structures
and the electrically conductive finger detecting structure may be
separated from each other such that the capacitive coupling between
the group of sensing structures and the finger detecting structure
is relatively low when no finger is present and relatively high
when a finger is present.
[0029] For example, in one embodiment, the group of sensing
structures may be surrounded in a sensing structure plane by at
least partly inactive sensing structures.
[0030] In a further embodiment, the electrically conductive finger
detecting structure may be arranged to extend around the
circumference of the array of sensing structures.
[0031] Further embodiments of, and effects obtained through this
second aspect of the present invention are largely analogous to
those described above for the first aspect of the invention.
[0032] There is further provided an electronic device comprising: a
control unit; and a fingerprint sensing device. The fingerprint
sensing device may be a capacitive fingerprint sensor. The
electronic device may be a mobile phone, but may also be e.g. a
desktop computer, tablet etc.
[0033] There is further provided a computer program product
comprising a computer readable medium having stored thereon
computer program means for controlling an electronic device, the
electronic device comprising a control unit, a fingerprint sensor
device comprising: an array of sensing elements for sensing the
fingerprint pattern, each sensing element comprising a sensing
structure for capacitive coupling with the finger; an electrically
conductive finger detecting structure arranged adjacent to the
array of sensing elements; and finger detecting circuitry connected
to the finger detecting structure for providing a finger detection
signal indicative of a capacitive coupling between a group of
sensing structures and the finger detecting structure, wherein the
computer program product comprises: code for controlling the group
of sensing elements to change a potential of the group of sensing
structures comprised in the group of sensing elements; code for
acquiring, in response to the change in potential, a response
signal from the finger detecting circuitry indicative of the
capacitive coupling between the group of sensing structures and the
finger detecting structure; code for comparing a variation in the
response signal with a predefined threshold value; and code for
providing a signal indicating that a finger is present when the
variation in the response signal is greater than the threshold
value.
[0034] In summary, the present invention relates to a method of
sensing a fingerprint pattern of a finger using a fingerprint
sensing device. The method comprising: controlling a group of
sensing elements to change a potential of a group of sensing
structures comprised in the group of sensing elements;
[0035] acquiring, in response to the variation in potential, a
response signal from a finger detecting circuitry indicative of the
capacitive coupling between the group of sensing structures and the
finger detecting structure; comparing a variation in the response
signal with a predefined threshold value; and providing a signal
indicating that a finger is present when the variation in response
signal is greater than the threshold value. The invention also
relates to a corresponding fingerprint sensing device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] These and other aspects of the present invention will now be
described in more detail, with reference to the appended drawings
showing an example embodiment of the invention, wherein:
[0037] FIG. 1 schematically illustrates an application for a
fingerprint sensing device according to an example embodiment of
the present invention;
[0038] FIG. 2 schematically shows the fingerprint sensing device in
FIG. 1;
[0039] FIG. 3 is a block diagram of the fingerprint sensing device
in FIG. 2;
[0040] FIG. 4a is a schematic cross-section view of a portion of
the fingerprint sensing device in FIG. 3;
[0041] FIG. 4b is a schematic cross-section view of a portion of
the fingerprint sensing device in FIG. 3;
[0042] FIG. 5 conceptually illustrates the function of an
embodiment of the invention;
[0043] FIG. 6 conceptually shows a fingerprint sensing device
according to an embodiment;
[0044] FIG. 7 conceptually shows a fingerprint sensing device
according to an embodiment; and
[0045] FIG. 8 is a flow-chart schematically illustrating a method
according to an embodiment of the present invention.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0046] In the present detailed description, various embodiments of
the fingerprint sensing system and method according to the present
invention are mainly described with reference to a mobile phone
having an integrated fingerprint sensing device. However, it should
be noted that many other kinds of electronic devices may have such
a fingerprint sensing device integrated, such as tablets, desktop
computers, laptops etc.
[0047] FIG. 1 schematically illustrates an application for a
fingerprint sensing device according to an example embodiment of
the present invention, in the form of a mobile phone 1 with an
integrated fingerprint sensing device 2. The fingerprint sensing
device 2 may, for example, be used for unlocking the mobile phone 1
and/or for authorizing transactions carried out using the mobile
phone, etc.
[0048] FIG. 2 schematically shows the fingerprint sensing device 2
comprised in the mobile phone 1 in FIG. 1. As can be seen in FIG.
2, the fingerprint sensing device 2 comprises a sensor array 5, and
finger detecting structures 4a-d. Although not shown in FIG. 2, the
fingerprint sensing device 2 further comprises a power supply
interface 6 and a communication interface 7. The sensor array 5
comprises a large number of sensing elements 8 (only one of the
sensing elements has been indicated with a reference numeral to
avoid cluttering the drawing), each being controllable to sense a
distance between a sensing structure comprised in the sensing
element 8 and the surface of a finger contacting the top surface of
the sensor array 5.
[0049] In a battery powered electronic device, such as the mobile
phone 1 in FIG. 1, keeping the energy consumption of various
sub-systems, including the fingerprint sensing device 2, as low as
possible is of utmost importance. In particular, the energy
consumption of sub-systems that are not used during a given time
period should be zero or close to zero during that time period.
[0050] Therefore, although not shown in FIG. 2, the fingerprint
sensing device 2 in FIG. 2 is also provided with circuitry for
determining if there is a finger present on the fingerprint sensing
device and to activate the fingerprint sensing device when a finger
is determined to be present so that the fingerprint sensing device
can be in a very low power state between finger detection
events.
[0051] Referring to the block diagram in FIG. 3, the fingerprint
sensing device 2 comprises, in addition to the sensor array 5 and
finger detecting structures 4a-d shown in FIG. 2, a finger detector
circuit 9, and operation control circuitry 10 including a finger
detection evaluation circuit 14 and an image acquisition control
circuit 12. The finger detector circuit 9 is connected to the
finger detecting structure 4a for providing a finger detection
signal S.sub.d indicative of a capacitive coupling between the
finger detecting structure 4a and a group of sensing structures. In
FIG. 3, only one of the finger detecting structures 4a is shown.
The additional finger detecting structures 4b-d may all be
connected to the same finger detector circuit 9, or each finger
detecting structure may be connected to its own finger detector
circuit.
[0052] As is schematically shown in FIG. 3, the finger detection
evaluation circuit 14 is connected to the finger detector circuit 9
and to the image acquisition control circuit 12. The image
acquisition control circuit 12 is connected to the sensor array
5.
[0053] Next, an example configuration of the finger detector
circuit 9 and the sensing elements 8 will be described with
reference to FIG. 4.
[0054] FIG. 4a is a schematic cross section of a portion of the
fingerprint sensing device 2 in FIG. 2 taken along the line A-A' as
indicated in FIG. 2 with a finger 11 placed on top of a protective
dielectric top layer 13 covering the sensor array 5 and the finger
detecting structure 4a. Referring to FIG. 4a, the fingerprint
sensing device 2 comprises an excitation signal providing circuit
19 electrically connected to the finger via a conductive finger
drive structure (not shown in FIG. 4), a plurality of sensing
elements 8, and a finger detection arrangement comprising the
finger detecting structure 4a and a finger detection circuit 9
connected to the finger detecting structure 4a.
[0055] As is schematically indicated in FIG. 4a, each sensing
element 8 comprises a conductive sensing structure, here in the
form of a metal plate 17 underneath the protective dielectric top
layer 13, a charge amplifier 18, and selection circuitry, here
functionally illustrated as a simple selection switch 21 for
allowing selection/activation of the sensing element 8.
[0056] The charge amplifier 18 comprises at least one amplifier
stage, here schematically illustrated as an operational amplifier
(op amp) 24 having a first input (negative input) 25 connected to
the sensing structure 17, a second input (positive input) 26
connected to sensor ground or another reference potential, and an
output 27. In addition, the charge amplifier 18 comprises a
feedback capacitor 29 connected between the first input 25 and the
output 27, and reset circuitry, here functionally illustrated as a
switch 30, for allowing controllable discharge of the feedback
capacitor 29. The charge amplifier 18 may be reset by operating the
reset circuitry 30 to discharge the feedback capacitor 29.
[0057] As is often the case for an op amp 24 in a negative feedback
configuration, the voltage at the first input 25 follows the
voltage at the second input 26. Depending on the particular
amplifier configuration, the potential at the first input 25 may be
substantially the same as the potential at the second input 26, or
there may be a substantially fixed offset between the potential at
the first input 25 and the potential at the second input 26. In the
configuration of FIG. 4, the first input 25 of the charge amplifier
is virtually grounded.
[0058] When a time-varying potential is provided to the finger 11
by the excitation signal providing circuitry 19, a corresponding
time-varying potential difference occurs between the sensing
structure 17 and the finger 11.
[0059] The above-described change in potential difference between
the finger 11 and the sensing structure 17 results in a sensing
voltage signal V.sub.s on the output 27 of the charge amplifier
18.
[0060] When the indicated sensing element 8 is selected for
sensing, the selection switch 21 is closed to provide the sensing
signal to the readout line 33. The readout line 33, which may be a
common readout line for a row or a column of the sensor array 5 in
FIG. 2, is shown in FIG. 4 to be connected to a multiplexer 36. As
is schematically indicated in FIG. 4a, additional readout lines
from other rows/columns of the sensor array 5 may also be connected
to the multiplexer 36.
[0061] The output of the multiplexer 36 is connected to a
sample-and-hold circuit 37 and an analog-to-digital converter 38 in
series for sampling and converting the analog signals originating
from the sensing elements 8 to a digital representation of the
fingerprint pattern of the finger 11 on the sensor 2.
[0062] As is schematically indicated in FIG. 4a, the finger
detecting circuit 9 is here provided in the form of a charge
amplifier similar in principle to the charge amplifier 18 comprised
in the sensing element 8 described above.
[0063] Accordingly, the finger detecting circuit 9 comprises at
least one amplifier stage, here schematically illustrated as an
operational amplifier (op amp) 44 having a first input (negative
input) 45 connected to the finger detecting structure 4a, a second
input (positive input) 46 connected to sensor ground or another
reference potential, and an output 47. In addition, the charge
amplifier comprises a feedback capacitor 49 connected between the
first input 45 and the output 47, and reset circuitry, here
functionally illustrated as a switch 50, for allowing controllable
discharge of the feedback capacitor 49. The charge amplifier may be
reset by operating the reset circuitry 50 to discharge the feedback
capacitor 49. As is also indicated in FIG. 4, the output of the
finger detecting circuitry is a finger detection signal S.sub.d (in
the form of a voltage) indicative of the capacitive coupling
between a group of sensing structures 17 and the finger detecting
structure 4a.
[0064] In FIG. 4a, the finger 11 is shown as being connected to an
excitation circuit 19 for providing the desired potential
difference between the finger 11, and the sensing plates 17 of the
sensor array 5. It should be noted that this desired potential
difference may alternatively be provided by changing the ground
level of the fingerprint sensing device in relation to the ground
level of the electronic device (such as mobile phone 1) in which
the fingerprint sensing device 2 is included. Furthermore, the
potential difference may also be provided by changing the potential
of part of the sensing structures 17, i.e. also the potential of
the sensing structures (or at least a portion of the sensing
structures) changes.
[0065] For detecting the presence of a finger on the finger
detecting structure 4a, a group of sensing elements is controlled
to change a potential of the group of sensing structures comprises
in the group of sensing elements. This is done by changing the
potential of the sensing structures in the group of sensing
elements. As an example shown in FIG. 4a this is performed by
switching from a first power supply 51 connected to the sensing
structures to a second power supply 53 through a switch 55. This
switching may be performed by each sensing element separately or by
a set of power supplies and switches for changing the potential of
the entire group of sensing structures.
[0066] FIG. 4b is a schematic cross section of a portion of an
embodiment of a fingerprint sensing device. The difference between
the embodiment shown in FIG. 4b and the one in FIG. 4a is that
there is a finger excitation structure 40a connected to power
supplies 51' and alternatingly to power supply 53' via a switch
55'. Note that the power supplies 51 and 53 may be the same as the
power supplies 51' and 53'. For detecting the presence of a finger
on the finger detecting structure 4a, a group of sensing elements
and the finger excitation structure are controlled to change a
potential of the group of sensing structures comprises in the group
of sensing elements and a potential of the finger excitation
structure. This way, the finger detecting signal may be enhanced.
Each of the finger detecting structures 4a-d may have a
corresponding adjacent finger excitation structure.
[0067] FIG. 5 schematically shows a close up of a group of sensing
structures (some denoted 502) comprised in a group of sensing
elements. Fields lines 504 indicate the capacitive coupling between
the finger detecting structure 4a and the sensing structures 502.
As is further shown, the finger 11 interrupts some of the fields
lines, thereby changing the capacitive coupling between the group
of sensing structures 502 and the finger detecting structure
4a.
[0068] FIG. 6 schematically shows another fingerprint sensing
device 600. As can be seen in FIG. 6, the fingerprint sensing
device 2 comprises a sensor array 5, and finger detecting
structures 4a-d. Furthermore, a second group of sensing elements
602 (individual sensing elements not shown) neighboring to a second
finger detecting structure 4c is at least partly inactive. Thus,
the capacitive coupling between the group of structures comprised
in the sensing elements which are not disabled will only very
weakly, if at all couple to the second finger detecting structure
4c. With this embodiment shown in FIG. 6, it is advantageous to
acquiring a second signal from the finger detecting circuitry of
the second finger detecting structure 4c and to provide a response
signal from which the second response signal has been subtracted.
In other words, the second response signal is used for removing
background noise. Alternatively or additionally, when the second
signal is greater than a background threshold level, the response
signal may be discarded, whereby a further response signal may be
acquired.
[0069] FIG. 7 schematically shows another fingerprint sensing
device 700. As can be seen in FIG. 7, the fingerprint sensing
device 700 comprises a sensor array 5, and finger detecting
structures 4a-d. Furthermore, the group of sensing elements 702 is
separated from the finger detecting structures 4a-d by disabled (or
at least partly inactive) sensing elements 704. Thus, the
capacitive coupling between the group of sensing elements 702 and
each of the finger detecting structures 4a-d is a coupling through
the finger 11 as is indicated by the capacitance C.sub.ref between
the finger and the finger detecting structures 4a-d and the
capacitance C.sub.pm between the finger 11 and the group of sensing
elements 702. When the finger 11 is not present, there is no or
close to no signal (e.g. only noise) since the coupling between the
group of sensing elements 702 and each of the finger detecting
structures 4a-d is relatively weak.
[0070] In a further embodiment, the finger detecting structures
4a-d are connected to each other for increasing the sensing
area.
[0071] FIG. 8 shows a flow-chart of method steps according to an
embodiment of the invention. In a first step S802, controlling a
group of sensing elements to change a potential of the group of
sensing structures comprised in the group of sensing elements.
Subsequently in step S804, acquire, in response to the variation in
potential, a response signal from the finger detecting circuitry
indicative of the capacitive coupling between the group of sensing
structures and the finger detecting structure. Then in step S806,
comparing a variation in the response signal with a predefined
threshold value. When the variation in response signal is greater
than the threshold value S808, providing S810 a signal indicating
that a finger is present and activating S812 at least a subset of
the sensing elements to sense at least a portion of the fingerprint
pattern.
[0072] In the claims, the word "comprising" does not exclude other
elements or steps, and the indefinite article "a" or "an" does not
exclude a plurality. The mere fact that certain measures are
recited in mutually different dependent claims does not indicate
that a combination of these measures cannot be used to advantage. A
computer program may be stored/distributed on a suitable medium,
such as an optical storage medium or a solid-state medium supplied
together with or as part of other hardware, but may also be
distributed in other forms, such as via the Internet or other wired
or wireless telecommunication systems. Any reference signs in the
claims should not be construed as limiting the scope.
* * * * *