Classifications

• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
  • G06V40/00—Recognition of biometric, human-related or animal-related patterns in image or video data
  • G06V40/10—Human or animal bodies, e.g. vehicle occupants or pedestrians; Body parts, e.g. hands
  • G06V40/12—Fingerprints or palmprints
  • G06V40/13—Sensors therefor
  • G06V40/1306—Sensors therefor non-optical, e.g. ultrasonic or capacitive sensing

Definitions

• the present invention relates to improvements in capacitance sensor systems as are suitable to be fabricated with integrated circuit technology for use in sensors and sensing arrays that measure the position of fingerprint artifacts or other structures near the sensor surface.
• the capacitance between a target electrode and a sense electrode varies inversely proportional to their relative distance, inversely proportional to their relative distance squared, or some functional dependence between inverse and inverse quadratic depending on the nature of the physical structure.
• the maximum distance at which a capacitance sensor system can detect target conductors in the vicinity of its sensor area is dependent on the minimum capacitance the system can resolve. If the capacitance of the sensor electrode relative to its ambient environment, its reference capacitance, is large compared to the capacitance between the target electrode and the sensor electrode, the capacitance sensor system sensitivity is significantly degraded.
• the size of the sensor electrode is dictated by the size of fingerprint artifacts, which is typically about the size of a 100 micrometer square. Being part of an integrated circuit whose vertical dimensions are small compared to 100 micrometers, the sensor electrode itself has significant capacitance to the substrate on which it mechanically rests.
• a sensor array composed of an array of sensor electrodes was disclosed by Knapp in U.S. Patent No. 5,325,442.
• Each sense electrode is connected through a passive switch to array wiring that is the length of the array.
• the array wiring is connected to a charge sensing circuit to determine the capacitance.
• the capacitance sensitivity is degraded by the array wiring as the effective reference capacitance on each sensor electrode increased.
• semiconductor switches are introduced into the sensor area where they may be damaged by mechanical contact with the target electrode, or may leak due to photocurrent when the sensor is operated in a high-light-level environment.
• Additional coatings may be applied to the sensor surface to reduce the sensor's susceptibility to damage, but at an increase in the sensor to target electrode distance.
• Dickinson et al. disclose a technique to measure the capacitance at each sensor electrode using a low value current source and additional active circuitry. A signal proportional to the total sensor capacitance is switched onto the array wiring after being passed through a source follower thereby isolating the wiring capacitance from the sensor electrode. With this technique the reference capacitance value is dominated by the sensor electrode capacitance and the capacitance of the circuitry connected to the sensor electrode itself.
• Ackland et al. disclose a method to reduce the sensor electrode capacitance by introducing a shield electrode between the sensor electrode and the grounded physical support structure. This reference capacitance cancellation technique is applied individually to each sensor electrode, resulting in a significant reduction in the reference capacitance and a proportional increase in the sensor capacitance sensitivity.
• a unity gain amplifier is connected between the sensor electrode and the shield electrode with one amplifier used per sensor electrode. The increase in sensor complexity increases the sensor cost and the risk of damage from the target structures.
• the present invention provides a capacitance sensor system that overcomes the shortcomings of known technology as described above.
• the present invention eliminates the need for reference capacitance cancellation circuitry while reducing the sensor's power consumption and increasing its mechanical robustness.
• each sensor cell has one electrode at or near the surface of the integrated circuit. Physically below the surface electrode is a buried stimulus electrode.
• the drive capacitance is the capacitance between the stimulus electrode and the sensor electrode.
• the capacitance from the sensor electrode to ground is the sensed capacitance.
• a measurement of the time-varying output voltage at the sense electrode is used to determine the sensed capacitance relative to the drive capacitance.
• This series arrangement of capacitors forms a capacitive voltage divider. If there are no structures proximate to the sensor electrode, the sense capacitance value is the reference value.
• Target structures such as fingerprint artifacts, that are proximate to the sense electrode increase the sense capacitance by adding a target capacitance to the reference capacitance.
• the increased capacitance reduces the time- varying output voltage at the sense electrode, which in turn identifies the target structure's proximity.
• An array of sensors cells is able to identify the location of target structures above each element of the array as required for the location of fingerprint artifacts.
• the number of sources and voltmeters is reduced from one pair per cell using an appropriate switching matrix.
• An arrangement of n rows with n sources and m columns with m voltmeters provides a suitable tradeoff in circuit power, circuit complexity, and overall capacitance measurement rate.
• the array of electrodes, switches, time-varying sources, time- varying voltmeters and ancillary circuits are combined in the same integrated circuit to form the capacitance sensor system.
• the time-varying voltmeter is shared by all sensor elements in the same column and the time-varying voltage source is shared by all sensor elements in the same row.
• the tim.e-varying input voltage source and time-varying voltmeter are implemented in CMOS technology. Cost reductions result from reductions in sensor circuit size as well as the use of CMOS technology that has not been modified for sensor applications. Mechanical robustness and resistance to optically induced currents are increased in the invention by removing all MOS transistor components from the sensor electrode area.
• FIG. 1 illustrates an embodiment of the capacitive divider elements
• FIG. 2 is an embodiment of a single sensor cell showing the stimulus, sense, reference and target electrodes
• FIG. 3 is a schematic representation of an embodiment of the capacitive sensor illustrating a representative portion of the capacitive sensor system
• FIG. 4 is an embodiment of a portion of a capacitive sensor system using individual row-stimulus, time-varying sources; and shared-column, time-varying voltmeters corresponding to the schematic representation of FIG. 3; and
• FIG. 5 is another embodiment of a portion of a capacitive sensor system using individual row-stimulus, time-varying sources; and shared-column, time-varying voltmeters corresponding to the schematic representation of FIG. 3 in which the column switch matrix is external to the sensor array.
• FIG. 1 illustrates an embodiment of an elementary method for measuring the value of an unknown capacitance (1).
• the sense capacitance (1) is shown with one connection to the reference potential (5).
• the connection (5) represents the reference potential for the time-varying voltage source (3) and for the time-varying voltmeter (4).
• the capacitance value, cl, of sense capacitor (1) may be determined relative to the drive capacitance value, c2 of drive capacitor (2) by connecting the capacitors as shown.
• Node (6) is a common node between the voltmeter (4) and the two capacitors, (1) and (2).
• the unknown capacitance (1) is determined by the simple formula cl equals the product of c2 and the calculated quantity (v3 - v4) / v4, in which v4 is the voltage measured by the time- varying voltmeter (4).
• FIG. 2 A physical implementation of the capacitive divider of FIG. 1 is illustrated in FIG. 2.
• the reference capacitance of value c3 is the capacitance between the sense electrode (10) and the reference electrode (8).
• the target capacitance of value c4 is the electrical capacitance between the sense electrode (10) and the target electrode (11).
• Sense capacitance value c2 is the sum of c3 and c4.
• the drive capacitor (2) of value c2 is the capacitance between the stimulus electrode (9) and the sense electrode (10).
• the time-varying voltage source (3) is connected between the drive electrode (9) and the reference electrode (8), which is shown connected to ground potential (5) without loss of generality.
• the target electrode (11) is also at the ground potential.
• the target electrode (11) is the unknown shaped electrode above the sense electrode (10).
• the sense electrode (10), node (6) of FIG. 1, serves as part of both drive and sense capacitors, (1) and (2). Both the time-varying voltage source (3) and the time-varying voltmeter (4) may be shared between multiple capacitance measuring sensor cells.
• FIG. 3 shows a portion of the electrical schematic of embodiment of the invention.
• Nine identical sense electrodes in a 3 x 3 array are shown, (21), (22), (23), (31), (32), (33), (41), (42), and (43).
• These sense electrodes are part of both the drive and the sense capacitors.
• the other terminal of the drive capacitor is the stimulus electrode. While the stimulus electrodes beneath the sensor electrodes are physically unique, all stimulus electrodes in the same row are electrically connected.
• Common nodes (14) and (17) are driven by sources, (13) and (12) respectively. To save power, only one source is active during capacitance measurement of a particular row of sensor cells. In FIG.
• FIG. 3 shows nine switches, (15) and (18), added to the portion of the sensor array. Each switch, (15) and (18), is connected to a wire oriented perpendicular to the direction of the commonly driven stimulus electrodes, (14) and (17). When node (17) is driven by a source (13), the switches (18) connected to sense electrodes (22), (32), and (42) are closed. All other switches (15) are open. The voltmeters (16) are connected to only one sensor electrode each. This perpendicular arrangement of drive nodes and voltmeter connections allows for a simultaneous measurement of the sensor capacitance along a row of sensor electrodes.
• Sense electrodes (22), (32), and (42) are connected to their respective voltmeters (16).
• the voltage on the voltmeter (16) connected to electrode (32) is lower than the voltage on the other voltmeters (16) connected to either sense electrodes (22) or (42). In this way both the presence of a target electrode at sense electrode (32) is detectable as well as a measurement of the capacitance between electrodes (11) and (32), and hence information about the distance between electrodes (11) and (32).
• FIG. 4 shows the embodiment of the invention corresponding to the schematic of FIG. 3.
• the switches (15) and (18) are shown physically within the sensor electrode array area. Each switch is shown physically adjacent to a corresponding sensor electrode. All switches in the same column are connected to one voltmeter (16). Multiple target electrodes (11) or the complex shape of a single target electrode (11) are deduced by sequentially measuring all the sense capacitance values.
• FIG. 5 shows another embodiment of the invention. All the switches, and hence all transistor devices, are located external to the sensor electrode array area.

Landscapes

• Engineering & Computer Science (AREA)
• Human Computer Interaction (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Multimedia (AREA)
• Theoretical Computer Science (AREA)
• Measurement Of Length, Angles, Or The Like Using Electric Or Magnetic Means (AREA)

Applications Claiming Priority (3)

Publications (2)

Family

ID=23126513

Family Applications (1)

Country Status (5)

Families Citing this family (1)

Family Cites Families (12)

• 2002
  • 2002-05-22 EP EP02737174A patent/EP1412765A4/de not_active Withdrawn
  • 2002-05-22 WO PCT/US2002/016533 patent/WO2002095439A2/en not_active Ceased
  • 2002-05-22 JP JP2002591858A patent/JP4102672B2/ja not_active Expired - Lifetime
  • 2002-05-22 AU AU2002310124A patent/AU2002310124A1/en not_active Abandoned
  • 2002-05-22 TW TW091110792A patent/TWI225932B/zh not_active IP Right Cessation

Also Published As

Similar Documents

Legal Events