U.S. patent application number 15/661680 was filed with the patent office on 2017-11-09 for method and system for calibration of a fingerprint sensing device.
This patent application is currently assigned to Fingerprint Cards AB. The applicant listed for this patent is Fingerprint Cards AB. Invention is credited to Markus Andersson, Mikael Danielsson.
Application Number | 20170323137 15/661680 |
Document ID | / |
Family ID | 58691126 |
Filed Date | 2017-11-09 |
United States Patent
Application |
20170323137 |
Kind Code |
A1 |
Andersson; Markus ; et
al. |
November 9, 2017 |
METHOD AND SYSTEM FOR CALIBRATION OF A FINGERPRINT SENSING
DEVICE
Abstract
There is provided a method of initiating a calibration operation
of a fingerprint sensing device comprising an array of sensing
elements for sensing a fingerprint pattern, the method comprising
acquiring information indicative of a change of an environmental
parameter influencing the operation of the fingerprint sensing
device; and if the change is larger than a predetermined threshold
value, performing a calibration operation of the fingerprint
sensing device. There is also provided a method of initiating a
calibration operation of a fingerprint sensing device comprising an
array of sensing elements for sensing a fingerprint pattern, the
method comprising: receiving information indicative of an event in
a device in which the fingerprint sensing device is arranged; if
the event is one of a group of predetermined events, performing a
calibration operation of the fingerprint sensing device.
Inventors: |
Andersson; Markus; (LOMMA,
SE) ; Danielsson; Mikael; (HOLLVIKEN, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fingerprint Cards AB |
Goteborg |
|
SE |
|
|
Assignee: |
Fingerprint Cards AB
Goteborg
SE
|
Family ID: |
58691126 |
Appl. No.: |
15/661680 |
Filed: |
July 27, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15193913 |
Jun 27, 2016 |
|
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15661680 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06K 9/209 20130101;
G06K 9/0002 20130101 |
International
Class: |
G06K 9/00 20060101
G06K009/00; G06K 9/20 20060101 G06K009/20 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 13, 2015 |
SE |
15514714 |
Claims
1. A method of initiating a calibration operation of a fingerprint
sensing device comprising an array of sensing elements for sensing
a fingerprint pattern, said method comprising: acquiring
information indicative of a change of an environmental parameter
influencing the operation of said fingerprint sensor; and if said
change is larger than a predetermined threshold value, initiating a
calibration operation of said fingerprint sensor.
2. The method according to claim 1, further comprising: determining
a present value of an environmental parameter; determining a
difference between said present value of said environmental
parameter and a predetermined value of said environmental
parameter; and if said difference is larger than said predetermined
threshold value, performing said calibration operation of said
fingerprint sensor.
3. The method according to claim 2, wherein said predetermined
value of said environmental parameter is a value determined at the
last performed calibration operation.
4. The method according to claim 1, wherein said environmental
parameter is selected from the group comprising humidity,
temperature and electrical field strength.
5. The method according to claim 2, wherein determining a present
value of an environmental parameter comprises acquiring information
from a sensor integrated in said fingerprint sensing device.
6. The method according to claim 2, wherein determining a present
value of an environmental parameter comprises acquiring information
from a sensor external to said fingerprint sensing device.
7. The method according to claim 2, wherein determining a present
value of an environmental parameter comprises acquiring information
via a network connection.
8. The method according to claim 1, further comprising initiating a
second calibration operation of said fingerprint sensing device a
predetermined time after a first operation has been performed
resulting from said change in environmental parameter.
9. A method of initiating a calibration operation of a fingerprint
sensing device comprising an array of sensing elements for sensing
a fingerprint pattern, said method comprising: receiving
information indicative of an event in a user device in which said
fingerprint sensing device is arranged; and if said event is one of
a group of predetermined events, initiating a calibration operation
of said fingerprint sensing device.
10. The method according to claim 9, wherein said group of
predetermined events comprises events requiring a user interaction
with said user device in which said fingerprint sensing device is
arranged.
11. The method according to claim 10, wherein said group of
predetermined events comprises an incoming call, an alarm, an
incoming message and an incoming notification.
12. The method according to claim 9, wherein said event comprises a
change in a network connection.
13. The method according to claim 12, wherein said change in
network connection comprises a change in cell ID in a cellular
network.
14. The method according to claim 12, wherein said change in
network connection comprises a change of strength in a wireless
network connection.
15. The method according to claim 9, wherein said event comprises a
change in location of said user device in which said fingerprint
sensing device is arranged.
16. The method according to claim 9, wherein said event comprises
connecting a charger to said user device in which said fingerprint
sensing device is arranged.
17. The method according to claim 9, further comprising waiting for
a minimum period of time between consecutive calibration
operations.
18. The method according to claim 9, further comprising initiating
a second calibration operation of said fingerprint sensing device a
predetermined time after a first operation has been performed
resulting from said indication of said event.
19. A fingerprint sensing device comprising an array of sensing
elements for sensing a fingerprint pattern, said device comprising
a calibration control unit configured to: acquire information
indicative of a change of an environmental parameter influencing
the operating of said fingerprint sensing device; if said change is
larger than a predetermined threshold value, perform a calibration
operation of said fingerprint sensing device; receive information
indicative of an event in a device in which said fingerprint
sensing device is arranged; and if said event is one of a group of
predetermined events, perform a calibration operation of said
fingerprint sensing device.
20. The fingerprint sensing device according to claim 19, further
comprising a temperature sensor, a humidity sensor, and/or an
electromagnetic field sensor.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. application Ser.
No. 15/193,913, filed on Jun. 27, 2016, which claims the benefit of
Swedish Patent Application No. 1551471-4 filed Nov. 13, 2015. The
disclosures of the above applications are incorporated herein by
reference in their entirety.
FIELD
[0002] The present invention relates to calibration of a
fingerprint sensor. In particular, the present invention relates to
a method for determining when to initiate a calibration of a
fingerprint sensor comprised in an electronic device.
BACKGROUND
[0003] Various types of biometric systems are used more and more in
order to provide for increased security and/or enhanced user
convenience. 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] Among the various available fingerprint sensing principles
(such as capacitive, optical, thermal etc.), capacitive sensing is
most commonly used, in particular in applications where size and
power consumption are important issues.
[0005] All capacitive fingerprint sensors provide a measure
indicative of the capacitance between each of several sensing
structures and a finger placed on or moved across the surface of
the fingerprint sensor.
[0006] Electrical properties (determining analogue signal
strengths) of a fingerprint touch sensor depend on external
factors. In particular the presence of moist between the finger and
the sensor can make a significant difference. Moist can come from
the user's finger or be present on the sensor already before the
finger is presented. Moist on the finger is typically due to
sweating or recent contact with water. Moist on the sensor can also
be due to direct contact with water, e.g. after the user has taken
a bath or when the device is used in rainy weather, or due to
condensation of water from humid air at the sensor surface. In the
latter case the amount of condensed water is further dependent on
air pressure, temperature, and temperature difference between the
air and the sensor surface.
[0007] Electromagnetic fields may also affect the electrical
properties of the sensor. An electromagnetic field interacting with
the sensor circuitry may induce power in circuits and connections,
resulting in a shift in the change in electrical potential required
to reach the analogue finger detect threshold. This may lead to
that the fingerprint image is captured too early or too late.
[0008] In order to compensate for analog signal strength variations
depending on external factors, trigger thresholds and parameters
for analogue-to-digital conversion can be calibrated towards the
current conditions. This can be done automatically by various
calibration methods, but typically requires that no finger is
placed on the sensor during the calibration. Furthermore, a
calibration is often performed at system startup and thereafter
repeated with given time intervals.
[0009] However, continuous calibration at given time intervals may
lead to an unnecessarily high power consumption when the
fingerprint sensor is unused for long periods of time. The power
consumption may be decreased by increasing the time between
calibrations. However, this is an undesirable approach since
increasing the time between calibrations may lead to situations
where the sensor is being used just prior to a scheduled
calibration event, increasing the risk that the fingerprint sensor
is not properly calibrated for the current operating
conditions.
[0010] Accordingly, there is a need for an improved method of
calibrating a fingerprint sensor.
SUMMARY
[0011] In view of above-mentioned desired properties of a
fingerprint sensor, it is an object of the present invention to
provide a method for determining when to perform a calibration
operation ensuring a more accurate and power efficient operation of
a fingerprint sensor.
[0012] According to a first aspect of the invention, there is
provided a method of initiating a calibration operation of a
fingerprint sensing device comprising an array of sensing elements
for sensing a fingerprint pattern, the method comprising acquiring
information indicative of a change of an environmental parameter
influencing the operation of the fingerprint sensing device; and if
the change is larger than a predetermined threshold value,
performing a calibration operation of the fingerprint sensing
device.
[0013] The fingerprint sensing device, i.e. the fingerprint sensor,
may for example be a capacitive fingerprint sensor where a
fingerprint image is acquired by measuring a capacitive coupling
between a finger placed on the sensor and the sensing element,
thereby determining the distance between the sensing element and
the finger, such that ridges and valleys of the finger are
identified. A fingerprint sensor arranged in an electronic device
is typically calibrated at selected points in time to setup the
fingerprint sensor for image capture under the current
conditions.
[0014] The present invention is based on the realization that the
calibration of the fingerprint sensor can be triggered to occur
only when it is required, e.g. only when the external environment
of the fingerprint sensor has changed to such a degree that it is
unlikely that the latest performed calibration would provide an
optimal, or satisfactory, fingerprint image. Accordingly, it is
suggested to only initiate and perform a calibration of the
fingerprint sensor only when an acquired parameter value exceeds a
predetermined threshold value. The calibration functionality in
itself can be assumed to be a known feature of the fingerprint
sensing device. The present method is thus aimed at controlling
when a calibration operation is initiated, i.e. when the command is
given to perform one or more specific calibration operations. The
calibration operations may comprise various known measures relating
to the functionality of the sensor, and the specific calibration
operations to perform will not be discussed in great detail.
[0015] Through the inventive method, the power efficiency of a
fingerprint sensor can be improved since it is no longer required
to perform calibrations at regular time intervals for the sensor to
be capable of capturing a fingerprint image. Moreover, the
reliability of the sensor is improved since the sensor is more
likely to be calibrated according to current conditions when a
fingerprint image is to be captured.
[0016] According to one embodiment of the invention, the step of
acquiring information may comprise determining a present value of
an environmental parameter, determining a difference between the
present value of the environmental parameter and a predetermined
value of the environmental parameter, and if the difference is
larger than the predetermined threshold value, performing the
calibration operation of the fingerprint sensor. Determining a
present value may comprise controlling a sensor connected to the
fingerprint sensor to measure a parameter value, and to compare the
current value with a predetermined value.
[0017] In one embodiment of the invention, the predetermined value
of the environmental parameter may be a value determined at the
last performed calibration operation. The predetermined value may
also be an average value based on a number of earlier values. The
predetermined value may also represent one or more predetermined
parameter value ranges, such that when the current value is within
a particular predetermined range, a calibration of the fingerprint
sensor is performed.
[0018] According to one embodiment of the invention, the
environmental parameter may be selected from the group comprising
humidity, temperature and electrical field strength, all of which
may influence the operation of a fingerprint sensor. Taking the
example of a capacitive fingerprint sensor, the electromagnetic
field surrounding the sensor can influence the electrical potential
of conductive portions of the sensor, or of the finger, which in
turn may influence the fingerprint capture. Furthermore, moist or
humidity may influence the dielectric environment between the
finger and a sensing element of the sensor, in turn influencing the
capacitive coupling. The temperature may also influence the
fingerprint capture, not in the least indirectly since a higher
temperature leads to a higher risk that the finger is moist due to
sweating. A rapid change in temperature may also affect the
condensation at the surface of the fingerprint sensor.
[0019] According to one embodiment of the invention, determining a
present value of an environmental parameter may comprises acquiring
information from a sensor integrated in the fingerprint sensor. The
fingerprint sensor may comprise dedicated sensors for determining
an environmental parameter. It may also be possible to determine a
value of an environmental parameter, such as electric field, by
using components already present in the fingerprint sensor.
[0020] According to one embodiment of the invention, determining a
present value of an environmental parameter may comprise acquiring
information from a sensor external to the fingerprint sensor. The
fingerprint sensor device is typically integrated in a consumer
electronics device, which in itself may comprise one or more
sensors for determining parameters such as temperature, humidity
and electromagnetic field. Thereby, the fingerprint sensor may
request information for such external sensors.
[0021] Furthermore, determining a present value of an environmental
parameter may comprises acquiring information via a network
connection. The fingerprint sensor device may be integrated in a
device having a network connection, such as a WiFi, Bluetooth or
cellular data connection. This means that the device may
communication with remote devices or with internet services to
acquire relevant environmental information. To be relevant, the
acquired information is preferably based on the location of the
device, and the location may be determined using a GPS integrated
in the device or by means of location identification via the
network connection.
[0022] According to one embodiment of the invention, the method may
further comprise performing a second calibration operation of the
fingerprint sensor a predetermined time after a first operation has
been performed resulting from the change in environmental
parameter. It may be desirable to perform a second calibration
after a certain time, when the sensor has fingerprint sensor has
reached has stabilized after the detected change. For example, if a
temperature change is detected, triggering a calibration operation,
it may be some delay before all components of the sensor has
reached the new temperature. Therefore, it is advantageous to
perform a second calibration at a time when it is estimated that a
stable state of the sensor is reached.
[0023] According to a second aspect of the invention, there is
provided a method of initiating a calibration operation of a
fingerprint sensor device comprising an array of sensing elements
for sensing a fingerprint pattern, the method comprising: receiving
information indicative of an event in a user device in which the
fingerprint sensor device is arranged; if the event is one of a
group of predetermined events, performing a calibration operation
of the fingerprint sensing device.
[0024] The second aspect of the invention is based on the
realization that it may be sufficient to calibrate the fingerprint
sensor when an event is detected that anticipates the use of the
fingerprint sensor. Taking the example of a fingerprint sensor
arranged in a user device such as a smartphone, there are typically
long periods of inactivity of the smartphone when there is no need
for the fingerprint sensor to be recently calibrated, and where
regular calibrations would only lead to increased power
consumption. Instead, the occurrence of a specific event may be
used to initiate a calibration operation, thereby limiting the
calibrations to when there is a high likelihood that the
fingerprint sensor is about to be used. Information indicative of
an event is thus received by the fingerprint sensing device, or by
a control unit configured to control calibration of the fingerprint
sensing device.
[0025] According to one embodiment of the invention, the group of
predetermined events may comprise events requiring a user
interaction with the device in which the fingerprint sensor is
arranged. Such events may for example comprise an incoming call, an
alarm, an incoming message and an incoming notification, where the
user is prompted to use the device. Thereby, when such an event is
detected, a calibration operation can be initiated and performed
before the user has time to reach the sensor since a calibration
operation can be performed in a very short time compared to the
time it takes for a user to react to an event. Thus, the
fingerprint sensor can be calibrated just prior to being used. To
perform the required calibration operation may take in the range
1-2 seconds. However, the time for performing the calibration
operating may vary depending on the hardware used on the type of
calibration to be performed. Moreover, it may be desirable to
perform different types of calibration operations for different
types of events, or for different changes in an environmental
parameter.
[0026] According to one embodiment of the invention, the event may
comprise a change in a network connection. The change in network
connection may for example be a change in cell ID in a cellular
network, a change in WiFi node connection, a change in a Bluetooth
connection or the like. Furthermore, the event may comprise a
change in location. The change in location may be detected for
example as a change in network connection, where the change from
one WiFi node to another can be used as an indicator of a change in
location. Changing location may in turn indicate that the external
environment changes, such as if a user is moving from indoors to
outdoors, which may require a calibration of the fingerprint
sensor.
[0027] According to one embodiment of the invention, the event may
comprise the connection of a charger to the device in which the
fingerprint sensing device is arranged. When a charger is connected
to the device, the supply voltage may change, and in particular the
drive voltage V.sub.DD of the fingerprint sensor, which in turn
influences the operation of the sensor. Thereby, it may be
desirable to perform a calibration when a charger is connecter.
Furthermore, a charger may introduce noise into the device, and
into the fingerprint sensor, such that a calibration is required to
properly capture a fingerprint despite any induced noise.
[0028] According to on embodiment of the invention, the method may
further comprise waiting for a minimum period of time between
consecutive calibration operations. There may be situations where a
device receives a large number of notifications, or when other
types of events occur, during a short period time, in which case it
is not desirable that a calibration operation of the sensor is
performed after every event. Thereby, a minimum time between
calibrations can be set to both to ensure that the sensor is not
occupied with calibration operations when it is to be used, and to
reduce the power consumption of the sensor.
[0029] Additional effects and features of the second aspect of the
invention are largely analogous to those described above in
connection with the first aspect of the invention.
[0030] According to a third aspect of the invention, there is
provided a fingerprint sensing device comprising an array of
sensing elements for sensing a fingerprint pattern, said device
comprising a calibration control unit configured to acquire
information indicative of a change of an environmental parameter
influencing the operating of said fingerprint sensor, if said
change is larger than a predetermined threshold value, perform a
calibration operation of said fingerprint sensor, receive
information indicative of an event in a device in which said
fingerprint sensor device is arranged; and if the event is one of a
group of predetermined events, perform a calibration operation of
the fingerprint sensor. The fingerprint sensing device may for
example be a capacitive sensing device.
[0031] Furthermore, the fingerprint sensing device may comprise a
temperature sensor, a humidity sensor, and/or an electromagnetic
field sensor. The fingerprint sensing device may also be arranged
in a user device, such as a smartphone, where the above described
sensors are integrated in the smartphone and communicatively
coupled to the calibration control unit of the fingerprint sensing
device.
[0032] Effects and features of the third aspect of the invention
are largely analogous to those described above in connection with
the first and second aspects of the invention.
[0033] Further features of, and advantages with, the present
invention will become apparent when studying the appended claims
and the following description. The skilled person realize that
different features of the present invention may be combined to
create embodiments other than those described in the following,
without departing from the scope of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] 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:
[0035] FIG. 1 schematically illustrates a smartphone comprising a
fingerprint sensing device;
[0036] FIGS. 2A-B are flow charts outlining the general steps of a
method according of an embodiment of the invention;
[0037] FIG. 3 is a schematic illustration of a device comprising a
fingerprint sensing device according to an embodiment of the
invention;
[0038] FIG. 4 is a flow chart outlining the general steps of a
method according of an embodiment of the invention;
[0039] FIG. 5 is a schematic illustration of a device comprising a
fingerprint sensing device according to an embodiment of the
invention;
[0040] FIG. 6 is a schematic illustration of a device comprising a
fingerprint sensing device according to an embodiment of the
invention;
[0041] FIG. 7 is a schematic illustration of a system and a device
comprising a fingerprint sensing device according to an embodiment
of the invention; and
[0042] FIG. 8A-B are timelines schematically illustrating the
method according to embodiments of the invention.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0043] In the present detailed description, various embodiments of
the method and device according to embodiments of the present
invention are mainly described with reference to a capacitive
fingerprint sensing device arranged in a handheld user device, such
as a smartphone. However, the method is applicable also for other
types of fingerprint sensing devices, and for fingerprint sensing
device arranged in other types of consumer electronic devices.
[0044] FIG. 1 schematically illustrates an application for a
fingerprint sensing device 102 according to an example embodiment
of the present invention, in the form of a smartphone 100 with an
integrated fingerprint sensing device 102. The fingerprint sensing
device 102 may, for example, be used for unlocking the smartphone
100 and/or for authorizing transactions carried out using the
smartphone 100, etc. A fingerprint sensing device 102 according to
various embodiments of the invention may also be used in other
devices, such as tablet computers, laptops, smartcards or other
types of consumer electronics.
[0045] FIG. 2A is a flow chart outlining the general steps of a
method according to an embodiment of the invention, with reference
to the block diagram of FIG. 3 illustrating features of a
fingerprint sensing device and of a smartphone in which the sensing
device is arranged. First, the method comprises acquiring 202
information indicative of a change of an environmental parameter
influencing the operation of the fingerprint sensor. The
acquisition of information can be initiated by a request from a
calibration control unit 302 of the fingerprint sensing device 102.
Alternatively, or in combination, information indicative of a
change of an environmental parameter may also be provided to the
control unit 302 of the fingerprint sensing device 102 from the
smartphone 100, without a specific request from the control unit
302. The control unit 302 is illustrated herein in the form of a
microprocessor, connected to the fingerprint sensing device 102,
the control unit 302 being configured to control at least the
initiation of a calibration operation of the fingerprint sensing
device 102. The control unit 302 may also be configured to control
all functionality of the fingerprint sensing device 102. Moreover,
the control unit 302 may be a general purpose control unit of the
smartphone 100, also configured to control the fingerprint sensing
device 102.
[0046] Next, the change in parameter value is compared 204 to a
predetermined threshold value, and if the change is larger than the
threshold value, a calibration operation the fingerprint sensor is
initiated 206 and subsequently performed so that the fingerprint
sensing device 102 is calibrated according to the current
environmental conditions.
[0047] As illustrated in the flow chart of FIG. 2B, determining a
change in an environmental parameter may comprise determining 208 a
present value of an environmental parameter, determining 210 a
difference between the present value of the environmental parameter
and a predetermined value of the environmental parameter; and
comparing 212 the difference with a predetermined threshold value,
and if the difference is larger than the predetermined threshold
value, initiating 214 and subsequently performing the calibration
operation of the fingerprint sensor.
[0048] Accordingly, as illustrated in FIGS. 2A-B, the information
indicative of change may either be provided directly as information
from a source external to the control unit 302 and fingerprint
sensor 102, or the change may be detected by a measurement and
subsequent comparison initiated by the control unit 302 of the
fingerprint sensor 102.
[0049] Parameters that may be calibrated during a calibration
operation of the fingerprint sensor include, but are not limited
to, a threshold for analogue finger detection, analogue gain before
analog-to-digital (AD) conversion of a pixel value, and digital
gain and shift in AD-conversion of a pixel value. In the case where
the AD-conversion parameters are tuned for each image capture, the
default settings or start values for tuning can be subject to
calibration.
[0050] FIG. 3 further illustrates that the control unit is
communicatively connected to a number of sensors for measuring
environmental parameters. The sensors are here illustrated as a
temperature sensor 304, a humidity sensor 306, and an
electromagnetic field sensor 308. The environmental sensors may be
integrated in the fingerprint sensing device circuitry, or they may
be part of the smartphone 100 in which the fingerprint sensor 102
is arranged.
[0051] FIG. 4 is a flow chart outlining the general steps of a
method according to an embodiment of the invention comprising
receiving 402 information indicative of an event in a device 100 in
which the fingerprint sensing device 102 is arranged, and
determining 404 if the event is one of a group of predetermined
events. If the event is among the group of predetermined events a
calibration operation of the fingerprint sensor is initiated 406
and subsequently performed.
[0052] The group of predetermined events comprises events requiring
a user interaction with the smartphone 100. As illustrated in FIG.
5, the group of events may comprise an incoming call 502, an alarm
504, an incoming message 506 and an incoming notification 508. All
of the aforementioned events typically encourage a user to interact
with the smartphone 100, possibly requiring the use of the
fingerprint sensor 102 for unlocking the smartphone 100. Other
types of events may also be comprised in the group of predetermined
events. An example of such an event is the connection of the
smartphone 100 to a charger, which may lead to a change in the
drive voltage, V.sub.DD, of the fingerprint sensing device, which
in turn influences the operating characteristics of the device and
thereby benefiting from a re-calibration of the fingerprint sensor.
Other events triggering a calibration operation may be events
relating to applications of the device, for example if an
application is launched which can be anticipated to require
fingerprint verification.
[0053] FIG. 6 schematically illustrates another group of events
which may be comprised in the group of events initiating a
calibration operation. FIG. 6 illustrates that the smartphone 100
in which the fingerprint sensor 102 and control unit 302 is
arranged may comprise various additional functionality, such as
WiFi 602, Bluetooth 604, Navigation 606 and cellular network 608
connection functionality. The described functionalities can all be
used to indicate a change of location of the smartphone 100, which
in turn may require a calibration of the smartphone 100. For
example, a change in signal strength, or the loss of a WiFi
connection may indicate that the user moves from indoors to
outdoors, thereby suggesting a change in environmental conditions
requiring a calibration operation. The same applies for a change in
a Bluetooth connection or the change in a cellular network
connection. Furthermore, a device is often wirelessly connected via
two or more technologies, in which case the combination of changes
in connection properties can be used to determine for example if
the user moves from indoors to outdoors. Navigation functionality
using systems such as GPS, Glonass or Galileo can also be used to
determine a change of location of the smartphone 100 requiring a
calibration of the fingerprint sensor 102.
[0054] FIG. 7 illustrates a user device 700 comprising at least one
of a WiFi 602, Bluetooth 604 and a cellular network 608 connection
and which is configured to connect to a remote service provider 702
to acquire information regarding an environmental parameter, a
change in environmental parameter, and/or of an event which can be
used to determine if a calibration operation should be initiated.
The remote service may for example be an internet based service
providing up-to-date local humidity and temperature information.
The remote service may also be connected to specific sensors 704
located in the vicinity of the user device 700, which may be of any
of the types discussed above. Moreover, the user device 700 may
also use a peer-to-per connection to exchange relevant
environmental information with nearby devices, thereby receiving
relevant information indicative of a change in an environmental
parameter. Thereby, a change in an environmental parameter may be
detected even if neither of the fingerprint sensing device 102 or
the user device 700 in which the sensor is arranged comprises any
sensors.
[0055] FIGS. 8A-B schematically illustrate timelines elaborating
further steps of a method according to embodiments of the
invention. FIG. 8A illustrates that a second calibration operation
is performed a predetermined time t.sub.1 after a first calibration
operation has been performed, where the first calibration operation
was initiated by a change in environmental parameter or by a
specific event. If the change in environmental parameter triggering
the first calibration is a rapid change in temperature, it may be
advantageous to perform a second calibration once the properties of
the fingerprint sensing device have stabilized to the new
conditions. Taking the example that a change in temperature is
detected where the temperatures changes from +20.degree. C. to
-10.degree. C., indicating a movement from indoors to outdoors, a
first calibration is advantageously initiated and performed to
compensate for any change in humidity. After a certain time has
elapsed, such as one or a few minutes, it can be assumed that the
temperature of the surface and of the components in the sensing
device has stabilized after which a second calibration is
performed.
[0056] FIG. 8B illustrates the situation where a first calibration
is initiated, for example based on an event such as a message
notification, and where a number of subsequent event occur in rapid
succession, and where the events normally would trigger new
calibrations. To avoid performing many calibrations repeatedly in a
short time interval, a time period t.sub.2, starting after the
first calibration is finalized, is set during which no calibration
may be performed. It may also be possible to override the no
calibration condition during t.sub.2 is specific excluded events
occur, such as a change in the temperature. Once t.sub.2 has
passed, the next event triggering a calibration can be handled
normally and a calibration performed.
[0057] It is also easy to realize that a combination of the above
described methods for initiating a calibration operation of a
fingerprint sensing device may be used to an advantage. For
example, if a calibration has been recently performed resulting
from a change in an environmental parameter, it may not be required
to perform a new calibration if an event from the group of events
is detected.
[0058] Even though the invention has been described with reference
to specific exemplifying embodiments thereof, many different
alterations, modifications and the like will become apparent for
those skilled in the art. Also, it should be noted that parts of
the method may be omitted, interchanged or arranged in various
ways, the method yet being able to perform the functionality of the
present invention.
[0059] Additionally, variations to the disclosed embodiments can be
understood and effected by the skilled person in practicing the
claimed invention, from a study of the drawings, the disclosure,
and the appended claims. 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.
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