Sensor for measurement for wet and dry fingers
A sensor device for performing measurements on an at least partially conductive surface includes a number of conductive structures at or directly below the sensor surface. The conductive structures consist of at least one stimulation or current sink electrode and a number of sensor elements coupled to interrogation electrodes in an electronic circuit for measuring impedance between the electrodes and the stimulation electrode. The sensor device also includes at least one additional clamping electrode positioned in the vicinity of the sensor elements and coupled to a chosen voltage.
1. A fingerprint sensor device for performing measurements on an at least partially conductive surface, the sensor comprising a number of conductive structures at or directly below the sensor surface, said conductive structures consisting of at least one stimulation or current sink electrode and a number of sensor elements coupled to interrogation electrodes in an electronic circuit for measuring impedance between the electrodes and said at least one stimulus electrode, the sensor device also comprising at least one additional clamping electrode being positioned in the vicinity of said sensor elements and being coupled to a chosen AC or DC voltage being significantly different from the voltage of the stimulation or current sink electrode.
2. The fingerprint sensor according to claim 1 , wherein the distance between the additional clamping electrode and the sensor elements is less than the distance between the ridges in a fingerprint, preferably in the range of 20-200 μm.
3. The fingerprint sensor according to claim 1 , wherein the additional clamping electrode is coupled to the same voltage as the stimulus electrode.
4. The fingerprint sensor according to claim 1 , wherein the additional clamping electrode is coupled to a voltage supply providing a varying voltage, and said stimulation electrode providing a varying voltage or current to the surface, said varying clamping voltage having an opposite phase relative to the stimulation signal.
5. The fingerprint sensor according to claim 1 , wherein the conductive structures are positioned on the sensor surface, thus to obtain direct contact with the at least partially conductive surface.
6. The fingerprint sensor according to claim 1 , wherein the conductive structures are partly or completely covered by a dielectric material.
7. The fingerprint sensor according to claim 1 , where the sensor elements are constituted by conductive pads on the sensor surface, each of said pads being capacitive coupled to another pad, said pads being connected to interrogation electrodes in an electronic circuit.
8. The fingerprint sensor according to claim 7 , where the sensor element pads are not plane, but have topological features for improving contact with the structures on the partially conductive surface.
9. The fingerprint sensor according to claim 7 , where the sensor element pads are covered with a dielectric material.
10. The fingerprint sensor according to claim 1 , where a part of the conductor tracks between the interrogation electrodes and the sensor pads are capacitively coupled to an electrode, where an AC voltage can be applied to the electrode to partially compensate for the AC signal flowing from finger to sensor element.
11. The fingerprint sensor according to claim 1 , where the local electrode is located between the stimulus electrode and the sensor elements.
12. The fingerprint sensor according to claim 1 , where the stimulus electrode is located between the additional clamping electrode and the sensor elements.
13. The fingerprint sensor according to claim 12 , where the stimulus electrode surrounds a clamping electrode.
14. The fingerprint sensor according to claim 1 , where the sensor is an AC capacitive fingerprint sensor.
15. The fingerprint sensor according to claim 1 , where the sensor is a DC capacitive fingerprint sensor.
16. The fingerprint sensor according to claim 1 , wherein the sensor is provided with one or more ridges or other topological features having heights in the range of 0.5-20 μm placed between the local electrode and the sensor elements.
17. The fingerprint sensor according to claim 1 , wherein the various electrodes are adapted to be turned active or inactive depending on certain measured features of the fingerprint.
18. The fingerprint sensor according to claim 1 , where the stimulus or additional electrodes are coupled to an AC voltage supply delivering a signal at several frequencies.
19. The fingerprint sensor according to claim 1 is used for detecting or compensating for humidity variations in the partially conductive surface.
20. The fingerprint sensor according to claim 5 , where a part of the tracks between the interrogation electrodes and the sensor pads are capacitively coupled to an electrode, where an AC voltage can be applied to the electrode to partially compensate for the AC signal flowing from finger to sensor element.