Method for determining a finger motion on a fingerprint sensor

1. A method for determining a finger motion on a fingerprint sensor comprising a sensing area, the method comprises:

receiving a finger on the sensing area of the fingerprint sensor;

when an amount of sensor area coverage caused by the finger touching the sensing area is below a coverage threshold, acquiring a sequence of images of a first type using a first image acquisition setting with the fingerprint sensor,

when the amount of sensor area coverage is equal to or exceeds the coverage threshold, acquiring a sequence of images of a second type using a second image acquisition setting with the fingerprint sensor, wherein the image quality is higher with the second image acquisition setting than with the first image acquisition setting, and

determining a present motion of the finger on the fingerprint sensor based on the images of the first type or images of the second type depending on a present amount of sensor area coverage.

2. The method according to claim 1 , comprising dynamically switching between determining the motion of the finger based on the images of the first type or images of the second type depending on a present amount of sensor area coverage.

3. The method according to claim 1 , comprising determining a motion of the finger based on a combination of an images of the first type and images of the second type.

4. The method according to claim 1 , comprising determining a motion of the finger based on a sequence of images of the first type, a sequence of images of the second type, or any combination of sequences of images of the first type and the second type.

5. The method according to claim 1 , wherein the frame rate is higher with the first image acquisition setting than with the second image acquisition setting.

6. The method according to claim 1 , comprising:

determining the motion of the finger based on image registration of the images of the second type.

7. The method according to claim 1 , comprising:

determining the motion of the finger based on changes in sensor area coverage between the images of the first type.

8. The method according to claim 1 , comprising:

acquiring the first type of images using a selected sub-group of sensing elements in an array of sensing elements comprised in the fingerprint sensor.

9. The method according to claim 1 , comprising:

acquiring the second type of images using only a selected sub-group of sensing elements in an array of sensing elements comprised in the fingerprint sensor, the selected sub-group of sensing elements is larger than the selected sub-group of sensing elements used for acquiring the first type of images.

10. The method according to claim 8 , wherein the selected sub-group of sensing elements comprises selected rows or columns of sensing elements separated by rows or columns not comprised in the selected sub-group of sensing elements.

11. A fingerprint sensing system comprising:

a finger sensing area for receiving a finger having a fingerprint topography;

an array of sensing elements;

fingerprint image acquisition circuitry connected to said array of sensing elements for acquiring fingerprint images indicating local distances between said sensing elements and said fingerprint topography; and

image processing circuitry connected to said fingerprint image acquisition circuitry, the image processing circuitry is configured to:

determine a present amount of sensor area coverage caused by the finger touching the sensing area;

when the present amount of sensor area coverage caused by the finger touching the sensing area is below a coverage threshold, control the fingerprint image acquisition circuitry to acquire a sequence of images of a first type using a first image acquisition setting,

when the present amount of sensor area coverage is equal to or exceeds the coverage threshold, control the fingerprint image acquisition circuitry to acquire a sequence of images of a second type using a second image acquisition setting, wherein the image quality is higher with the second image acquisition setting than with the first image acquisition setting, and

provide a finger motion signal indicative of a motion of the finger on the fingerprint sensor sensing area based on the first type of images or the second type of images depending on a present amount of sensor area coverage.

12. The fingerprint sensing system according to claim 11 , wherein the image acquisition frame rate is higher with the first image acquisition setting than with the second image acquisition setting.

13. The fingerprint sensing system according to 12 claim 11 , wherein:

each sensing element in said array of sensing elements comprises an electrically conducting sensing structure; and

the fingerprint acquisition circuitry is connected to each of said sensing structures for providing sensing signals indicative of a capacitive coupling between each sensing structure and said finger in response to a change in potential difference between a finger potential of said finger and a sensing structure potential of said sensing structure.

14. An electronic device comprising:

the fingerprint sensing system according to claim 11 ; and

a device control unit configured to:

execute a finger gesture event on the electronic device based on the determined finger motion.

15. The electronic device according to claim 14 , wherein the device controller is configured to:

interact with the fingerprint sensing system to authenticate a user based on a fingerprint representation; and

perform at least one action only if the user is authenticated based on said fingerprint representation.