Capacitive fingerprint sensor with adjustable gain

• US 6,538,456 B1
• Filed: 01/11/2000
• Issued: 03/25/2003
• Est. Priority Date: 12/15/1995
• Status: Expired due to Fees

• Current Assignee: Veridicom, Inc. (Veridicom International, Inc.)
• Original Assignee: Veridicom, Inc. (Veridicom International, Inc.)
• Inventors: Dickinson, Alexander G., Ross, Paul C., Mendis, Sunetra, McPherson, Ross
• Primary Examiner(s): Cuneo, Kamand
• Assistant Examiner(s): TANG, MINH NHUT
• Application Number:
  US09/481,129
• Time in Patent Office: 1,169 Days
• Field of Search: 324/627, 324/658, 324/632, 324/719, 324/722, 324/71.1, 324/660, 382/115-116, 382/124-127, 382/108, 382/312, 340/825.34, 257/508, 257/659
• US Class Current: 324/658
• CPC Class Codes: G01B 7/004 for measuring coordinates of points
• CPC Class Codes: G01B 7/34 for measuring roughness or irregularity of surfaces
• CPC Class Codes: G06F 3/041 Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
• CPC Class Codes: G06V 40/1306 non-optical, e.g. ultrasonic or capacitive sensing
• CPC Class Codes: G07C 9/37 using biometric data, e.g. fingerprints, iris scans or voice recognition

Abstract

The present invention relates to a fingerprint sensing device comprising a planar array of closely-spaced capacitive sense elements. When a finger is placed in close proximity to the sensing device, the capacitive sense elements measure a capacitance between the finger and a single capacitor plate in each sense element. This is accomplished by precharging each capacitor plate, and then using a known current source to remove a fixed amount of charge from each capacitor plate. The measured capacitance varies as a function of the distance between the capacitor plate and the finger surface. Thus, a capacitance measurement allows the distance between the capacitor plate and the finger surface to be determined. Distance measurements across the array of sense elements are combined to produce a representation of the pattern of ridges on the finger surface which comprise a fingerprint. After a fingerprint is acquired, the quality of the fingerprint is evaluated, and if necessary, a gain parameter for the sense elements is iteratively adjusted until a satisfactory fingerprint is acquired.

Claims

1. A solid state minutia detector comprising:

an electrode plate;

an insulating surface disposed over said electrode plate;

a switch coupled to said electrode plate;

charge driving circuitry coupled to said electrode plate through said switch;

charge draining circuitry coupled to said electrode plate through said switch;

a clock coupled to said switch, said clock configured to cause said switch to couple, in an alternating manner, said charge driving circuitry and said charge draining circuitry to said electrode plate; and

charge sensing circuitry coupled to said electrode plate, said charge sensing circuitry configured to measure a charge at said electrode plate after a fixed period of time;

wherein said solid state minutia detector comprises a plurality of arrays of said electrode plates.

2. The solid state minutia detector of claim 1, further comprising a gain amplifier configured to adjust a sensitivity of said solid state minutia detector based upon relative values measured through said charge sensing circuitry from a plurality of said electrode plates.

3. The solid state minutia detector of claim 2, wherein said sensitivity is adjusted through said clock by changing said fixed period of time.

4. The solid state minutia detector of claim 2, wherein said sensitivity is adjusted through said clock by changing a duty cycle as measured between said charge driving circuitry and said charge draining circuitry.

5. The solid state minutia detector of claim 2, wherein said sensitivity is adjusted through said charge draining circuitry by changing a rate at which said charge draining circuitry drains charge from said electrode plate.

6. The solid state minutia detector of claim 1, wherein said charge sensing circuitry comprises a precharge sampling circuitry and a discharge sampling circuitry, said precharge sampling circuitry configured to sample charge at said electrode plate at a first time, and said discharge sampling circuitry configured to sample charge at said electrode plate at said fixed time after said first time, said charge sensing circuitry further configured to output a difference between said charge as measured at said precharge sampling circuitry and said charge as measured at said discharge sampling circuitry as a relative value for a minutia.

7. The solid state minutia detector of claim 6, further comprising a gain amplifier configured to adjust a sensitivity of said solid state minutia detector based upon a plurality of said relative values detected through said charge sensing circuitry from a plurality of said electrode plates.

8. A method for detecting minutiae comprising:

coupling charge driving circuitry to a plurality of electrode plates;

charging said plurality of electrode plates with a reference charge through said charge driving circuitry;

coupling charge draining circuitry to said plurality of electrode plates;

draining a charge from said plurality of electrode plates;

sampling said plurality of electrode plates after said charge has been drained from said plurality of electrode plates for a fixed period of time; and

outputting a difference between said reference charge and said sampled charge, for each of said plurality of electrode plates, as an electronic image of a minutia.

9. The method of claim 8, further comprising:

evaluating a quality of said electronic image; and

changing a rate at which said charge draining circuitry drains charge from said plurality of electrode plates, after said step of evaluating.

10. The method of claim 8, further comprising:

evaluating a quality of said electronic image; and

adjusting said step of sampling said plurality of electrode plates, after said step of evaluating.

11. The method of claim 10, wherein said step of adjusting comprises adjusting said fixed period of time.

12. The method of claim 10, wherein said step of adjusting comprises adjusting a duty cycle as measured between said step of charging said plurality of electrode plates and draining said plurality of electrode plates.

13. A solid state fingerprint detector comprising:

a voltage source;

a current source;

a sensor element coupled to said voltage source and said current source through one or more switches, said sensor element comprising a metal plate; and

a readout circuit coupled to said sensor element;

wherein

said voltage source charges said sensor element to a given charge;

said current source drains charge from said sensor element after said voltage source has charged said sensor element to said given charge; and

said readout circuit samples a charge remaining at said sensor element after said current source has drained charge from said sensor element for a period of time.

14. The solid state fingerprint sensor of claim 13, wherein said readout circuit further comprises:

a first sampling circuit configured to measure charge at said sensor element before said current source has drained charge from said sensor element; and

a second sampling circuit configured to measure charge at said sensor element after said period of time.

15. The solid state fingerprint detector of claim 14, further comprising a plurality of said sensor elements,

wherein said readout circuit further comprises a gain adjustment circuit, said gain adjustment circuit configured to adjust said period of time based upon a relative difference between a plurality of values for charge remaining collected from said plurality of sensor elements.

16. The solid state fingerprint detector of claim 14, further comprising a plurality of said sensor elements,

wherein said readout circuit further comprises a gain adjustment circuit, said gain adjustment circuit configured to adjust a rate at which said current source drains said sensor element based upon a relative difference between a plurality of values for charge remaining collected from said plurality of sensor elements.

17. The solid state fingerprint detector of claim 13, further comprising a plurality of said sensor elements,

wherein said readout circuit further comprises a gain adjustment circuit, said gain adjustment circuit configured to adjust said period of time based upon a relative difference between a plurality of values for charge remaining collected from said plurality of sensor elements.

18. The solid state fingerprint detector of claim 13, further comprising a plurality of said sensor elements,

wherein said readout circuit further comprises a gain adjustment circuit, said gain adjustment circuit configured to adjust a rate at which said current source drains said sensor element based upon a relative difference between a plurality of values for charge remaining collected from said plurality of sensor elements.