Method for acquiring fingerprints

Patent metadata

Field	Value
Publication number	US-11610428-B2
Application number	US-202117147775-A
Country	US
Kind code	B2
Filing date	Jan 13, 2021
Priority date	Jan 21, 2020
Publication date	Mar 21, 2023
Grant date	Mar 21, 2023

Abstract

Method for acquiring a fingerprint, performed by a device comprising an active thermal sensor when it is electrically supplied at a distance by a terminal, said sensor comprising a plurality of pixels, each pixel comprising a pyroelectric capacitor which, when it is subjected to a variation in temperature, generates electrical charges, each pixel being associated with a heating element adapted for heating said pixel and being connected to a reading circuit able to measure the electrical charges generated by said capacitor. The method relies on a taking into account of the heating element received by each pixel of said sensor (33, 34) in order to determine when said pixel is able to provide (36) a signal that can be used for generating information representing a part of a fingerprint.

First claim

The invention claimed is:

1. A method for acquiring information representing a fingerprint performed by a device comprising an active thermal sensor on which a body part carrying a fingerprint is placed at the time of a presentation at a distance of said device in front of a reading terminal producing an electromagnetic field able to generate an electrical supply in said device, said sensor comprising a plurality of pixels, each pixel comprising a thermosensitive element able to convert a variation in temperature into a variation in a parameter representing a measurable electrical property of said thermosensitive element, referred to as an electrical parameter, each pixel being associated with a heating element adapted for heating said pixel and being connected to a reading circuit able to measure the variations in said electrical parameter, the method comprising:
for each pixel:
initiating a heating-activation phase comprising: performing a first reinitialisation of the reading circuit connected to said pixel; and activating a heating of said pixel by the heating element corresponding to said pixel;
ending the first reinitialisation after a predetermined heating period;
ending the heating by the heating element when a heating energy produced in the pixel by said heating element reaches a predetermined heating energy;
measuring a first item of information representing variations in the electrical parameter of said pixel after a predetermined waiting period, the first item of information corresponding to a period, referred to as an integration period, lying between the end of the first initiation and the end of the predetermined waiting period;
performing a second reinitialisation of the reading circuit connected to said pixel;
measuring, without heating by the heating element, a second item of information representing variations in the electrical parameter of said pixel after a period equal to the integration period following the end of the second reinitialisation; and
calculating a third item of information representing variations in the electrical parameter of said pixel in the form of a difference between said first and said second item of information;
the information representing the fingerprint being obtained from the third item of information calculated for each pixel.

2. The method according to claim 1 , wherein the active thermal sensor comprises pixels, referred to as reference pixels, not intended to capture information representing a fingerprint part, and adapted for generating an electrical signal when a finger is present on said active thermal sensor, said signal being insensitive to a presence or not of skin on said reference pixel; and wherein the heating energy produced by the heating element in each pixel intended to generate information representing a part of a fingerprint is estimated using at least one reference pixel.

3. The method according to claim 1 , wherein ending the heating by the heating element when a heating energy produced in the pixel by said heating element reaches a predetermined heating energy comprises:
periodically measuring, by the microcontroller, a heating voltage at the terminals of the heating element;
calculating said heating element produced in the pixel by said heating element from said heating voltage measured periodically at the terminals of the heating element; and
ending the heating by the heating element when the heating energy calculated reaches said predetermined heating energy.

4. The method according to claim 1 , wherein the method comprises, for each pixel:
measuring a voltage provided by the electrical supply of the device;
fixing a supply voltage of the heating element, referred to as the heating voltage, corresponding to said pixel, at a value of no more than the voltage supplied by the electrical supply of the device; and
determining that the heating energy produced in the pixel by said heating element reaches the predetermined heating energy when a period following the end of the first reinitialisation reaches a duration deduced from the heating voltage.

5. The method according to claim 1 , said method comprising,
for each pixel:
gradually increasing a voltage U of supplying the heating element, referred to as the heating voltage, corresponding to said pixel, until a predetermined voltage U H lower than a predetermined minimum value U min is reached or until a voltage U bob provided by the electrical supply of the device becomes lower than said predetermined minimum value;
determining that the heating energy produced in the pixel by said heating element reaches the predetermined heating energy when a period following the end of the first reinitialisation reaches a duration deduced from the heating voltage thus increased.

6. The method according to claim 1 , wherein the method comprises, for each pixel:
measuring a voltage provided by the electrical supply of the device;
fixing a value of a supply voltage of the heating element corresponding to said pixel at a predefined value;
when the predefined value is lower than or equal to the voltage provided by the electrical supply of the device, determining that the heating energy produced in the pixel by said heating element reaches the predetermined heating energy when a period following the end of the first reinitialisation reaches a duration deduced from the predefined value; and
ending said method when the predefined value is higher than the voltage provided by the electrical supply of the device.

7. A non-transitory storage medium storing a computer program comprising instructions for implementing, by a device, the method according to claim 1 , when said program is executed by a processor of said device.

8. A device comprising an active thermal sensor allowing acquisition of information representing a fingerprint when a body part carrying a fingerprint is placed on said thermal sensor, said device being adapted for being electrically supplied at a distance by a reading terminal producing an electromagnetic field, said sensor comprising a plurality of pixels, each pixel comprising a thermosensitive element able to convert a variation in temperature into a variation in a parameter representing a measurable electrical property of said thermosensitive element, referred to as an electrical parameter, each pixel being associated with a heating element adapted for heating said pixel and being connected to a reading circuit able to measure the variations in said electrical parameter, the device being configured to:
for each pixel:
initiate a heating-activation phase comprising: performing a first reinitialisation of the reading circuit connected to said pixel; and activating a heating of said pixel by the heating element corresponding to said pixel;
end the first reinitialisation after a predetermined heating period;
end the heating by the heating element when a heating energy produced in the pixel by said heating element reaches a predetermined heating energy;
measure a first item of information representing variations in the electrical parameter of said pixel after a predetermined waiting period, the first item of information corresponding to a period, referred to as an integration period, lying between the end of the first initiation and the end of the predetermined waiting period;
perform a second reinitialisation of the reading circuit connected to said pixel;
measure, without heating by the heating element, a second item of information representing variations in the electrical parameter of said pixel after a period equal to the integration period following the end of the second reinitialisation; and
calculate a third item of information representing variations in the electrical parameter of said pixel in the form of a d

Assignees

• Idemia Identity & Security France

Inventors

• Fourre Joël-Yann

Classifications

• G06V40/1306 Primary

  non-optical, e.g. ultrasonic or capacitive sensing · CPC title

• G01K7/003

  using pyroelectric elements (radiation pyrometers G01J5/00) · CPC title

• G01K7/34

  using capacitative elements (capacitors per se H01G) · CPC title

• H04B5/79 Primary

  for data transfer in combination with power transfer · CPC title