US20170140233A1

US20170140233A1 - Method and system for calibration of a fingerprint sensing device

Info

Publication number: US20170140233A1
Application number: US15/193,913
Authority: US
Inventors: Markus Andersson; Mikael Danielsson
Original assignee: Fingerprint Cards AB
Current assignee: Fingerprint Cards AB
Priority date: 2015-11-13
Filing date: 2016-06-27
Publication date: 2017-05-18
Also published as: US20170323137A1; US10108840B2; US20170323138A1; EP3374929A1; CN107111760A; WO2017082800A1

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.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Swedish Patent Application No. 1551471-4 filed Nov. 13, 2015. The disclosure of the above application is incorporated herein by reference in its entirety.

FIELD

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

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.
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.
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.
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.
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.
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.
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.
Accordingly, there is a need for an improved method of calibrating a fingerprint sensor.

SUMMARY

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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_DDof 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.
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.
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.
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.
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.
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.
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

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:

FIG. 1 schematically illustrates a smartphone comprising a fingerprint sensing device;

FIGS. 2A-B are flow charts outlining the general steps of a method according of an embodiment of the invention;

FIG. 3 is a schematic illustration of a device comprising a fingerprint sensing device according to an embodiment of the invention;

FIG. 4 is a flow chart outlining the general steps of a method according of an embodiment of the invention;

FIG. 5 is a schematic illustration of a device comprising a fingerprint sensing device according to an embodiment of the invention;

FIG. 6 is a schematic illustration of a device comprising a fingerprint sensing device according to an embodiment of the invention;

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

FIGS. 8A-B are timelines schematically illustrating the method according to embodiments of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

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.
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.
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.
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.
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.
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.
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.
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.
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.
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_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.
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.
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.
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 t₁after 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° C. to −10° 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.
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 t₂, 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 t₂is specific excluded events occur, such as a change in the temperature. Once t₂has passed, the next event triggering a calibration can be handled normally and a calibration performed.
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.
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.
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.

Claims

1-18. (canceled)

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.

21. The device according to claim 19, wherein said control unit is further configured to:

determine a present value of an environmental parameter;

determine 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, perform said calibration operation of said fingerprint sensor.

22. The device according to claim 21, wherein said predetermined value of said environmental parameter is a value determined at the last performed calibration operation.

23. The device according to claim 21, wherein said environmental parameter is selected from the group comprising humidity, temperature and electrical field strength.

24. The device according to claim 21, wherein determining a present value of an environmental parameter comprises acquiring information from a sensor integrated in said fingerprint sensing device.

25. The device according to claim 21, wherein determining a present value of an environmental parameter comprises acquiring information from a sensor external to said fingerprint sensing device.

26. The device according to claim 21, wherein determining a present value of an environmental parameter comprises acquiring information via a network connection.

27. The device according to claim 19, wherein said control unit is further configured to initiate 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.

28. The device according to claim 19, wherein said group of predetermined events comprises events requiring a user interaction with said user device in which said fingerprint sensing device is arranged.

29. The device according to claim 19, wherein said group of predetermined events comprises an incoming call, an alarm, an incoming message and an incoming notification.

30. The device according to claim 19, wherein said event comprises a change in a network connection.

31. The device according to claim 30, wherein said change in network connection comprises a change in cell ID in a cellular network.

32. The device according to claim 30, wherein said change in network connection comprises a change of strength in a wireless network connection.

33. The device according to claim 19, wherein said event comprises a change in location of said user device in which said fingerprint sensing device is arranged.

34. The device according to claim 19, wherein said event comprises connecting a charger to said user device in which said fingerprint sensing device is arranged.

35. The device according to claim 19, wherein said control unit is further configured to wait for a minimum period of time between consecutive calibration operations.

36. The method according to claim 19, wherein said control unit is further configured initiate 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.