Capacitive fingerprint sensor with adjustable gain

• US 6,049,620 A
• Filed: 05/13/1997
• Issued: 04/11/2000
• Est. Priority Date: 12/15/1995
• Status: Expired due to Term

• Current Assignee: Apple 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): Boudreau, Leo H.
• Assistant Examiner(s): Werner, Brian P.
• Application Number:
  US08/855,230
• Time in Patent Office: 1,064 Days
• Field of Search: 382/115, 382/124-127, 382/312, 382/108, 340/825.34, 320/166, 257/261, 257/291-293, 257/296, 257/298, 257/300, 257/314, 257/71, 257/72, 358/446, 358/461, 358/406
• US Class Current: 382/124
• 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 method for detecting a fingerprint from a finger surface, the method using a plurality of sense elements, the method comprising the steps of:

placing the finger surface over the plurality of sense elements;

measuring electrical signals from the plurality of sense elements, the electrical signals indicative of an interaction between the plurality of sense elements and the finger surface;

forming the electrical signals into a representation of the fingerprint from the finger surface;

evaluating the quality of the representation of the fingerprint; and

if the representation is of poor quality due to a quality of the finger surface, adjusting a gain of the plurality of sense elements, and measuring the electrical signals again to form another representation of the fingerprint,

wherein adjusting the gain includes adjusting a current feeding into each of the plurality of sense elements or adjusting a time interval before a measurement is made.

2. The method of claim 1, wherein the step of measuring electrical signals from the plurality of sense elements measures a capacitance between the plurality of sense elements and the finger surface.

3. The method of claim 1, wherein adjusting the current feeding into the plurality of sense elements includes adjusting a potentiometer.

4. An apparatus for detecting a fingerprint on a finger surface, comprising:

an array of sense elements, sense elements in the array including, an electrode plate which forms a capacitor with the finger surface, and an amplifier, which amplifies a voltage on the electrode plate;

an insulating and receiving surface disposed over the array of sense elements;

driving circuitry coupled to the sense elements in the array of sense elements for driving a voltage onto the electrode plates in the sense elements;

sensing circuitry coupled to the sense elements for sensing a voltage on the electrode plates in the sense elements, the sensing circuitry producing a first representation of the fingerprint;

means for evaluating the quality of the first representation; and

gain adjusting circuitry coupled to the sense elements for adjusting the gain of the sense elements if the first representation is of poor quality due to a quality of the finger surface, the gain adjusting circuitry including a circuit to adjust a current flowing into each of the sense elements or a circuit to adjust timing of a sampling interval for the sense elements,

wherein if the first representation is of poor quality, the sensing circuitry producing a second representation of the fingerprint once the gain adjusting circuitry has adjusted the gain of the sense elements.

5. The apparatus of claim 4, wherein sense elements in the array of sense elements include a current source, which drains current from the electrode plate at a pre-determined rate.

6. The apparatus of claim 5, wherein sense elements in the array of sense elements include:

a drive input including a drive switch to selectively switch driving current onto the electrode plate, and

a sense output including a sense switch to selectively switch voltage from the electrode plate to the sense output.

7. The apparatus of claim 4, including at least one voltage measurement circuit for measuring a voltage on an electrode plate in a sense element in the array of sense elements.

8. The apparatus of claim 4, wherein the sensing circuitry includes a plurality of lines extending through the array of sense elements, the plurality of lines carrying voltage signals from sense elements in the array of sense elements to voltage measuring circuitry on a periphery of the array, lines in the plurality of lines traversing at most half of the array before arriving at the periphery of the array.

9. The apparatus of claim 4, wherein the driving circuitry includes:

a plurality of lines extending through the array of sense elements, the plurality of lines delivering driving current to the sense elements;

a plurality of drivers on the periphery of the array coupled to the plurality of lines, for supplying the drive current delivered to the sense elements, such that a pair of drivers are attached to opposite ends of lines in the plurality of lines.

10. The apparatus of claim 4, including an addressing mechanism for selectively accessing at least one sense element from the array of sense elements.

11. The apparatus of claim 4, wherein the array of sense elements is located on a semiconductor substrate.

12. The apparatus of claim 4, wherein the array of sense elements is located in a module, the module including:

at least one analog to digital converter coupled to the sensing circuitry;

a digital signal processor coupled to the at least one analog to digital converter for performing computations on a representation of the fingerprint gathered from the array; and

an interface circuit for communicating with devices outside of the module.

13. The apparatus of claim 12, wherein the module is a semiconductor chip.

14. The apparatus of claim 12, wherein the module is a multi-chip module.