Electronic fingerprint sensor with differential noise cancellation
View Patent ↗Image sensing apparatus includes an image pickup plate disposed generally orthogonally with respect to an expected direction of movement of an object, such as a finger, multiple image drive plates in spaced relation to the image pickup plate to define sensor gaps between respective image drive plates and the image pickup plate, and a reference plate disposed substantially parallel to the image pickup plate. The reference plate is spaced from the image pickup plate to permit common mode noise and coupling to be cancelled and is spaced from the image drive plates to permit a differential image signal to develop between the image pickup plate and the reference plate. A differential amplifier coupled to the image pickup plate and the reference plate provides noise cancellation. The apparatus may further include a comb plate spaced from the reference plate and coupled to a reference potential, such as ground.
1. Image sensing apparatus comprising:
an image pickup plate disposed generally orthogonally with respect to an expected direction of movement of an object;
a plurality of image drive plates in spaced relation to said image pickup plate to define a plurality of sensor gaps between respective image drive plates and said image pickup plate; and
a reference plate disposed substantially parallel to the image pickup plate, wherein the reference plate is spaced from the image pickup plate to permit common mode noise and coupling to be cancelled and is spaced from the image drive plates to permit a differential image signal to develop between the image pickup plate and the reference plate.
2. Image sensing apparatus as defined in claim 1 , further comprising a substrate, wherein said image pickup plate, said plurality of image drive plates and said reference plate comprise conductive traces on said substrate.
3. Image sensing apparatus as defined in claim 1 , wherein said image pickup plate, said plurality of image drive plates and said reference plate are substantially coplanar.
4. Image sensing apparatus as defined in claim 1 , wherein said image pickup plate, said plurality of image drive plates and said reference plate are dimensioned and spaced for sensing a fingerprint.
5. Image sensing apparatus as defined in claim 4 , wherein each of said sensor gaps has a dimension that is less than about one half the ridge spacing on a typical fingerprint.
6. Image sensing apparatus as defined in claim 1 , wherein said image drive plates comprise parallel conductors disposed perpendicular to said image pickup plate and said reference plate.
7. Image sensing apparatus as defined in claim 1 , further comprising a comb plate in spaced relation to said reference plate.
8. Image sensing apparatus as defined in claim 7 , wherein said comb plate comprises substantially parallel, interconnected conductors disposed perpendicular to said reference plate and spaced from said reference plate.
9. Image sensing apparatus as defined in claim 8 , wherein an arrangement of the parallel, interconnected conductors relative to said reference plate substantially matches an arrangement of the image drive plates relative to the image pickup plate.
10. Image sensing apparatus as defined in claim 1 , further comprising:
an excitation circuit for sequentially energizing said image drive plates with the drive signals; and
a detection circuit for detecting the drive signals coupled from said image drive plates to said image pickup plate to provide the image signal.
11. Image sensing apparatus as defined in claim 10 , wherein the drive signals comprise signal bursts supplied sequentially to respective ones of said image drive plates and wherein said excitation circuit includes circuitry for coupling non-energized image drive plates to a reference potential.
12. Image sensing apparatus as defined in claim 10 , wherein said detection circuit includes a differential amplifier having first and second differential inputs coupled to the image pickup plate and the reference plate, respectively.
13. Image sensing apparatus as defined in claim 10 , further comprising a comb plate in spaced relation to said reference plate, wherein said excitation circuit includes circuitry for coupling non-energized image drive plates and said comb plate to a reference potential.
14. A fingerprint sensing system comprising:
an image sensor including an array of sensors for sensing ridge peaks and ridge valleys of a fingerprint on a moving finger, said image sensor comprising:
an image pickup plate disposed generally orthogonally with respect to an expected direction of movement of the finger;
a plurality of image drive plates in spaced relation to said image pickup plate to define a plurality of sensor gaps between respective image drive plates and said image pickup plate; and
a reference plate disposed substantially parallel to the image pickup plate, wherein the reference plate is spaced from the image pickup plate to permit common mode noise and coupling to be cancelled and is spaced from the image drive plates to permit a differential image signal to develop between the image pickup plate and the reference plate;
a finger sensor for sensing a speed of the finger as it moves across the image sensor; and
a sensor circuit for excitation of the image sensor with image drive signals and for detection of image signals between the image pickup plate and the reference plate in response to the image drive signals, for excitation of said finger sensor with finger drive signals and for detection of finger signals in response to said finger drive signals, and for coordinating said image signals and said finger signals to provide a fingerprint image.
15. A fingerprint sensing system as defined in claim 14 , further comprising a substrate, wherein said image pickup plate, said plurality of image drive plates and said reference plate comprise conductive traces on said substrate.
16. A fingerprint sensing system as defined in claim 14 , wherein said image pickup plate, said plurality of drive plates and said reference plate are substantially coplanar.
17. A fingerprint sensing system as defined in claim 14 , wherein said image sensor further comprises a comb plate in spaced relation to said reference plate.
18. A fingerprint sensing system as defined in claim 17 , wherein said comb plate comprises substantially parallel, interconnected conductors disposed perpendicular to said reference plate and spaced from said reference plate.
19. A fingerprint sensing system as defined in claim 14 , wherein said sensor circuit comprises:
an excitation circuit for sequentially energizing said image drive plates with said image drive signals; and
a detection circuit for detecting the image drive signals coupled from said image drive plates to said image pickup plate to provide said image signals.
20. A fingerprint sensing system as defined in claim 19 , wherein said detection circuit includes a differential amplifier having first and second differential inputs coupled to said image pickup plate and said reference plate, respectively.
21. A fingerprint sensing system as defined in claim 19 , wherein said image sensor further comprises a comb plate in spaced relation to said reference plate, wherein said excitation circuit includes circuitry for coupling non-energized image drive plates and said comb plate to a reference potential.