Fingerprint sensing system with liveness detection

1. A method of controlling a fingerprint sensing system for liveness authentication, said fingerprint sensing system comprising:

a device connection interface for connection of said fingerprint sensing system to an electronic device, said device connection interface including a device reference potential input for receiving a device reference potential from said electronic device;

a sensing arrangement including:

a plurality of sensing structures, each being covered by a dielectric structure and being arranged to capacitively couple to said finger when said finger is placed on said dielectric structure; and

read-out circuitry 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 sensing structure potential of said sensing structure and a finger potential of said finger, said read-out circuitry being connected to each of said sensing structures in such a way that said sensing structure potential follows a sensing reference potential of said sensing arrangement,

a finger drive structure arranged adjacent to said sensing structures for making electrical contact with said finger when sensing said fingerprint pattern; and

supply circuitry connected to said sensing arrangement for providing, to said sensing arrangement, said sensing reference potential,

wherein said method comprises:

a) providing said sensing reference potential to the sensing arrangement in the form of a sensing reference signal alternating, in relation to said device reference potential, between a first sensing reference potential and a second sensing reference potential, a change in potential difference, in relation to said device reference potential, between said first sensing reference potential and said second sensing reference potential resulting in said change in potential difference between said finger potential and said sensing structure potential;

b) providing a first set of sensing signals to a control circuitry;

c) providing said sensing reference potential to the finger drive electrode in the form of a sensing reference signal alternating, in relation to said device reference potential, between a third sensing reference potential and a fourth sensing reference potential, a change in potential difference, in relation to said device reference potential, between said third sensing reference potential and said fourth sensing reference potential resulting in said change in potential difference between said finger potential and said sensing structure potential,

d) providing a second set of sensing signals to said control circuitry;

e) providing a liveness authentication signal based on the first set of sensing signals and the second set of sensing signals.

2. The method according to claim 1 , wherein said first set of sensing signals and said second set of sensing signals are acquired from capacitive coupling between the finger and the same set of sensing structures.

3. The method according to claim 1 , wherein said liveness authentication signal is based on a liveness parameter determined based on a comparison between a first signal of said first set of sensing signals and a second signal of said second set of sensing signals, wherein the method comprises:

when said liveness parameter is in a threshold interval, providing said liveness authentication signal indicative of a live finger.

4. The method according to claim 1 , wherein said liveness authentication signal is based on a liveness parameter determined based on a comparison between a first accumulated value of said first set of sensing signals and a second accumulated value of said second set of sensing signals, wherein the method comprises:

when said liveness parameter is in a threshold interval, providing said liveness authentication signal indicative of a live finger.

5. The method according to claim 1 , comprising:

determining a liveness parameter based on the first set of sensing signals and the second set of sensing signals,

including said liveness parameter in a set of liveness features,

applying a liveness test on the set of liveness features, and based on the test, provide said liveness authentication signal indicative of a live finger or a spoof finger.

6. The method according to claim 1 , comprising:

determining a liveness parameter based on the first set of sensing signals and the second set of sensing signals,

determining a verification representation based on said first set of sensing signals and said liveness parameter,

performing a fingerprint authentication procedure based on said verification representation and an enrolment representation indicative of a fingerprint pattern, said enrolment representation including an enrolment liveness parameter based on a first set of enrolment sensing signals provided in said first operational mode and a second set of enrolment sensing signals provided in said second operational mode.

7. A fingerprint sensing system for sensing a fingerprint pattern of a finger, said fingerprint sensing system comprising:

a device connection interface for connection of said fingerprint sensing system to an electronic device, said device connection interface including a device reference potential input for receiving a device reference potential from said electronic device;

a sensing arrangement including:

a plurality of sensing structures, each being covered by a dielectric structure and being arranged to capacitively couple to said finger when said finger is placed on said dielectric structure;

read-out circuitry 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 sensing structure potential of said sensing structure and a finger potential of said finger,

a finger drive structure arranged adjacent to said sensing structures for making electrical contact with said finger when sensing said fingerprint pattern,

supply circuitry connected to said sensing arrangement for providing, to said sensing arrangement, said sensing reference potential,

wherein said sensing arrangement further comprises a sensing arrangement controller for controlling said sensing arrangement between:

a first sensing arrangement operational mode in which said read-out circuitry is connected to each of said sensing structures in such a way that said sensing structure potential follows a sensing reference potential of said sensing arrangement, wherein said sensing reference potential is provided in the form of a sensing reference signal alternating, in relation to said device reference potential, between a first sensing reference potential and a second sensing reference potential, a change in potential difference, in relation to said device reference potential, between said first sensing reference potential and said second sensing reference potential resulting in said change in potential difference between said finger potential and said sensing structure potential, wherein a first set of sensing signals is provided by said read-out circuitry, and

a second sensing arrangement operational mode in which said sensing reference potential is provided to the finger drive electrode in the form of a sensing reference signal alternating, in relation to said device reference potential, between a third sensing reference potential and a fourth sensing reference potential, a change in potential difference, in relation to said device reference potential, between said first sensing reference potential and said second sensing reference potential resulting in said change in potential difference between said finger potential and said sensing structure potential, wherein a second set of sensing signals is provided by said read-out circuitry, and

control circuitry configured to determine a liveness parameter based on the first set of sensing signals and the second set of sensing signals, and based on said liveness parameter, provide a liveness authentication signal.

8. The fingerprint sensing system according to claim 7 , wherein the same set of sensing structures are used in said first mode and in said second mode for sensing said first sensing signals and said second sensing signals.

9. The fingerprint sensing system according to claim 7 , wherein said liveness authentication signal is based on a liveness parameter determined based on a comparison between a first signal of said first set of sensing signals and a second signal of said second set of sensing signals, wherein, when said liveness parameter is determined to be in a threshold interval, said control circuitry is configured to provide said liveness authentication signal indicative of a live finger.

10. The fingerprint sensing system according to claim 7 , wherein said liveness parameter is a comparison between a first accumulated value of said first set of sensing signals and a second accumulated value of said second set of sensing signals, wherein when said liveness parameter is in a threshold interval, said control circuitry is configured to provide said liveness authentication signal indicative of a live finger.

11. The fingerprint sensing system according to 7 , wherein the control circuitry is configured to:

include said liveness parameter in a set of liveness parameters,

apply a liveness test on the set of liveness parameters, and based on the test, provide a liveness authentication signal indicative of a live finger or a spoof finger.

12. The fingerprint sensing system according to claim 7 , wherein said control circuitry is configured to:

determine a verification representation based on said first set of signals and said liveness parameter,

perform a fingerprint authentication procedure based on said verification representation and an enrolment representation indicative of a user's fingerprint pattern, said enrolment representation including an enrolment liveness parameter based on a first set of enrolment sensing signals provided in said first operational mode and a second set of enrolment sensing signals provided in said second operational mode.

13. The fingerprint sensing system according to claim 7 , wherein said read-out circuitry is configured to sense a change in charge carried by each of said sensing structures resulting from said change in potential difference, said read-out circuitry comprises:

a plurality of charge amplifiers, each being connected to at least one of said sensing structures for providing a sensing signal indicative of a change in charge carried by said at least one sensing structure, wherein each of said charge amplifiers comprises:

a first input connected to said at least one sensing structure;

a second input connected to said supply circuitry to receive said sensing reference potential;

an output providing said sensing signal;

a feedback capacitor connected between said first input and said output; and

at least one amplifier stage between said first and second inputs, and said output,

wherein said charge amplifier is configured in such a way that a potential at said first input substantially follows said sensing reference potential at said second input.

14. The fingerprint sensing system according to claim 13 , wherein said read-out circuitry further comprises:

sampling circuitry connected to the output of each of said charge amplifiers for sampling said sensing signals at sampling times being related to changes of said sensing reference potential between said first sensing reference potential and said second sensing reference potential or between said third sensing reference potential and said fourth sensing reference potential.

15. The fingerprint sensing system according to claim 14 , wherein the sampling circuitry may be controlled to sample the output a first time when the sensing reference potential is at either the first sensing reference potential or the second sensing reference potential, and a second time when the sensing reference potential is at the other one of the first sensing reference potential and the second sensing reference potential, and wherein the sampling circuitry may be controlled to sample the output a third time when the sensing reference potential is at either the third sensing reference potential or the fourth sensing reference potential, and a third time when the sensing reference potential is at the other one of the third sensing reference potential and the fourth sensing reference potential.

16. The fingerprint sensing system according to claim 7 , wherein:

said sensing arrangement is included in a first integrated circuit; and

said device connection interface and said supply circuitry are included in a second integrated circuit coupled to said first integrated circuit.

17. The fingerprint sensing system according to claim 7 , wherein:

said sensing arrangement, said device connection interface, and said supply circuitry are included in a single integrated circuit.

18. An electronic device comprising:

the fingerprint sensing system according to claim 7 ; and

processing circuitry configured to:

acquire said sensing signals and said liveness parameter from the fingerprint sensing system via said device connection interface;

authenticate a user based on said fingerprint pattern signal; and

perform at least one action only if said user is authenticated based on said sensing signals and said liveness parameter.