Abstract:
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.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to systems and methods for electronically sensing biometric features of an object, such as a fingerprint, and, more particularly, to methods and apparatus for electronic fingerprint sensing which utilize differential noise cancellation.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electronic fingerprint sensing has received increased attention as a technique for reliable identification of individuals. Electronic fingerprint sensing may be used in stationary equipment, such as security checkpoints, and in portable devices, such as mobile phones and other wireless devices, and smart cards. Accordingly, electronic fingerprint sensing systems are required to be compact, highly reliable and low in cost.  
         [0003]     Various electronic fingerprint sensing methods have been proposed. Known methods include optical sensing and capacitive sensing with a two-dimensional array of electrodes.  
         [0004]     Capacitive fingerprint sensing using a swiped finger technique is disclosed in International Publication No. WO 02/47018, published Jun. 13, 2002. Conductive elements, or plates, are formed on an insulating substrate to create a one-dimensional capacitive sensing array for detecting topographic variations in an object, such as a finger. The array includes multiple drive plates which are sequentially excited with short duration electronic waveform bursts. An orthogonal pickup plate spaced from each drive plate by a sensor gap and connected to a charge sensing circuit detects the intensity of the electric field created by each drive element. With each complete scan of the drive plates, a one-dimensional slice of the fingerprint is acquired. By swiping a finger across the gap between the drive plates and the pickup plate, and scanning the gap at a much faster rate than the swipe speed, a two-dimensional image based on capacitance is generated. The image represents the fingerprint.  
         [0005]     Fingerprint sensors of this type provide satisfactory performance but are subject to parasitic coupling and noise combined with interference coupled through the body of the finger from finger ridges outside the sensor gap. Accordingly, there is a need for improved electronic fingerprint sensing apparatus and methods wherein the above effects are reduced.  
       SUMMARY OF THE INVENTION  
       [0006]     According to a first aspect of the invention, image sensing apparatus comprises an image pickup plate disposed generally orthogonally with respect to an expected direction of movement of an object, such as a finger, a plurality of image drive plates in spaced relation to the image pickup plate to define a plurality of 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.  
         [0007]     The image sensing apparatus may further comprise a comb plate spaced from the reference plate and coupled to a reference potential, such as ground. The comb plate may comprise substantially parallel, interconnected conductors disposed perpendicular to the reference plate and spaced from the reference plate. In some embodiments, an arrangement of the parallel, interconnected conductors of the comb plate relative to the reference plate may substantially match an arrangement of the image drive plates relative to the image pickup plate.  
         [0008]     The signal on the reference plate is subtracted from the signal on the image pickup plate to provide an image signal in which noise and parasitic signals are substantially cancelled.  
         [0009]     The image pickup plate, the plurality of image drive plates, the reference plate and the comb plate, if present, may comprise conductive traces on a substrate. The conductive traces may be substantially coplanar and may be dimensioned and spaced for sensing a fingerprint.  
         [0010]     According to a second aspect of the invention, a fingerprint sensing system is provided. The fingerprint sensing system comprises an image sensor including an array of sensors for sensing ridge peaks and ridge valleys of a fingerprint on a moving finger, the image sensor configured as described above, 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 the finger sensor with finger drive signals and for detection of finger signals in response to the finger drive signals, and for coordinating the image signals and the finger signals to provide a fingerprint image.  
         [0011]     The sensor circuit may comprise an excitation circuit for sequentially energizing the image drive plates with the image drive signals, and a detection circuit for detecting the image drive signals coupled from the image drive plates to the image pickup plate to provide the image signals. The detection circuit may include a differential amplifier having first and second differential inputs coupled to the image pickup plate and the reference plate, respectively.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:  
         [0013]      FIG. 1  is a block diagram of a fingerprint sensing system incorporating features of the present invention;  
         [0014]      FIG. 2  shows a conventional fingerprint image sensor;  
         [0015]      FIG. 3  shows a fingerprint image sensor according to an embodiment of the invention; and  
         [0016]      FIG. 4  is a block diagram of an image sensing circuit according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 1  shows a fingerprint sensing system  100  incorporating features of the present invention. A sensor block  102  receives drive signals from and delivers sense signals to a sensor circuit  108 . Sensor block  102  includes an image sensor  110  and a position sensor  112 . Image sensor  110  and position sensor  112  may be fabricated on a single substrate as described below. Sensor circuit  108  includes an image sensing circuit  124 , a position sensing circuit  122  and a microprocessor and memory  130 . Image sensor  110  receives drive signals  104  from and delivers sense signals  106  to image sensing circuit  124 . Position sensor  112  receives drive signals  105  from and delivers sense signals  107  to position sensing circuit  122 . Microprocessor and memory  130  acquires and processes image data and position data and controls operation of the system. The components of fingerprint sensing system  100  are described below.  
         [0018]     An image sensor as described in International Publication No. WO 02/47018 is shown in  FIG. 2 . An image sensor  150  includes multiple image drive plates  152  and an image pickup plate  154 . Drive plates  152  are substantially parallel to each other and are connected to a drive circuit  156 . Pickup plate  154  is disposed substantially perpendicular to drive plates  152 . Each drive plate  152  is separated from pickup plate  154  by a sensor gap  160 . Thus, image sensor  150  includes a linear array of sensor gaps  160  between respective drive plates  152  and pickup plate  154 .  
         [0019]     A finger is moved, or swiped, in a direction  162  perpendicular to pickup plate  154 . A drive circuit  156  sequentially energizes drive plates  152  with drive signals. As ridge peaks and ridge valleys of the fingerprint pass over sensor gaps  160 , the drive signals applied to drive plates  152  are capacitively coupled to pickup plate  154  according to the capacitances of the individual sensor gaps. The capacitance varies in accordance with the ridge peaks and ridge valleys of the fingerprint passing over the sensor gaps. The capacitively coupled drive signals are coupled to pickup plate  154  and are detected by a sensing circuit  158  to provide a line of the fingerprint image. Multiple slices of the fingerprint image are combined to form a complete fingerprint image.  
         [0020]     Fingerprint image sensors of the type shown in  FIG. 2  provide satisfactory performance but are subject to parasitic coupling and noise picked up by the human body as well as interference coupled through the body of the finger from finger ridges outside the sensor gap. An improved image sensor wherein these effects are at least partially cancelled is shown in  FIG. 3 . An image sensor  200  includes multiple image drive plates  210  and an image pickup plate  212 . Drive plates  212  are substantially parallel to each other and are connected to drive circuit  214 , which may be part of image sensing circuit  124  shown in  FIG. 1 . Pickup plate  212  is disposed substantially perpendicular to drive plates  210 . Each drive plate  210  is spaced from pickup plate by a sensor gap  220 . Thus, image sensor  200  includes a linear array of sensor gaps  220  between respective drive plates  210  and pickup plate  212 . Drive circuit  214  sequentially energizes drive plates  210  with drive signals.  
         [0021]     Image sensor  200  further includes a reference plate  230  that may be substantially parallel to and spaced from pickup plate  212 . Reference plate  230  is located on the opposite side of pickup plate  212  from drive plates  210  and thus is spaced from drive plates  210  by a greater distance than pickup plate  212 . Reference plate  230  should be spaced from drive plates  210  by a distance that is sufficient to provide a noise and parasitic coupling reference for common mode noise cancellation. In some embodiments, reference plate  230  and pickup plate  212  may have equal lengths and widths and may be located in a parallel side-by-side arrangement. Reference plate  230  senses a ridge/valley signal similar to pickup plate  212  but substantially attenuated. Because reference plate  230  and pickup plate  212  are closely spaced and have similar dimensions, the two plates produce approximately equal noise and parasitic signals. Subtracting the signal on pickup plate  212  from the signal on reference plate  230  produces a ridge/valley signal proportional to the difference between the sensed signals, which is significant because of the relative spacings of the two plates from sensor gaps  220 . However, the equally coupled noise and parasitic signals are cancelled by subtracting the signals on the two plates.  
         [0022]     Pickup plate  212  and reference plate  230  are coupled through a differential bandpass filter  240  to a differential amplifier  242 . Bandpass filter  240  and differential amplifier  242  are part of image sensing circuit  124  ( FIG. 1 ). In particular, pickup plate  212  may be coupled through filter  240  to a non-inverting input of differential amplifier  242 , and reference plate  230  may be coupled through filter  240  to an inverting input of differential amplifier  242 . Differential amplifier  242  electronically subtracts the signals on pickup plate  212  and reference plate  230 , so that noise and parasitic signals are cancelled. It will be understood by those skilled in the art that the connections between image sensor  200  and differential amplifier  242  can be reversed within the scope of the invention.  
         [0023]     Image sensor  200  may further include a comb plate  250  spaced from reference plate  230 . As shown in  FIG. 3 , comb plate  250  may include substantially parallel conductors  252  disposed perpendicular to reference plate  230  and spaced from reference plate  230  by gaps  254 . Parallel conductors  252  are electrically interconnected by a conductor  256  and are connected to drive circuit  214  by conductor  258 . In some embodiments, the arrangement of parallel conductors  252  relative to reference plate  230  matches the arrangement of drive plates  210  relative to pickup plate  212 . Thus, the widths of parallel conductors  252 , the spacing between parallel conductors  252  and the dimensions of gaps  254  may be the same as the widths of drive plates  210 , the spacing between drive plates  210  and the dimensions of sensor gaps  220 , respectively.  
         [0024]     Comb plate  250  may be coupled to a reference potential, such as ground, during fingerprint image sensing. Thus, at any instant of time during fingerprint image sensing, one of drive plates  210  may be energized with a drive signal and the remaining drive plates  210  are coupled to a reference potential, such as ground. For the example of an image sensor  200  having  250  drive plates  210 , all but one of the  250  drive plates  210  are coupled to ground at any given time and all of parallel conductors  252  of comb plate  250  are coupled to ground at any given time during image sensing. With this arrangement, noise on the ground conductors is coupled substantially equally to pickup plate  212  and reference plate  230 . The coupled noise is subtracted by differential amplifier  242  and thereby is cancelled. The fingerprint image signal of interest is sensed between pickup plate  212  and reference plate  230 , and is not cancelled by differential amplifier  242 .  
         [0025]     Pickup plate  212 , drive plates  210 , reference plate  230  and comb plate  250  may be substantially coplanar, conductive traces on a substrate  270 . The substrate  270  may be any suitable insulating material. In some embodiments, the substrate may be flexible so that it confirms to the macro contours of the finger. However, a flat substrate may be utilized without impairing the performance of the position sensor. The substrate may be a rigid or flexible printed circuit board, and the drive plates and the pickup plate may be formed using conventional deposition, etching and photolithography techniques.  
         [0026]     In one example of image sensor  200 , drive plates  210  had widths of 25 μm (micrometers) and the spacing between adjacent drive plates  210  was 25 μm. The sensor gaps  220  had dimensions of 32 μm. A spacing between pickup plate  212  and reference plate  230  was 32 μm. Parallel conductors  252  of comb plate  250  had widths of 25 μm and a spacing between adjacent conductors  252  was 25 μm. The gaps  254  had dimensions of 32 μm. It will be understood that these dimensions are given by way of example only and are not limiting as to the scope of the present invention. In general, sensor gaps  220  have dimensions less than the ridge spacing in a typical fingerprint and are typically in a range of 25 to 50 μm.  
         [0027]     An embodiment of image sensing circuit  124  and microprocessor and memory  130  of  FIG. 1  are shown in  FIG. 4 . A master clock  302  provides a clock signal to mux scanning logic  304  and a detector  306 . Master clock  302  can operate over a range of frequencies, for example, 20-80 MHz, but is not limited to this range. Microprocessor and memory  130  generates control signals for mux scanning logic  304 . Outputs of mux scanning logic  304  serve as control inputs to switches  310 .  
         [0028]     The clock signal from master clock  302  is gated by switches  310  to provide signal bursts. A low impedance buffer  314  activates each image drive plate  210  with a signal burst  312 . Signal bursts  312  are generated by standard circuit elements known to those skilled in the art and are derived from a common reference frequency or master clock  302 .  
         [0029]     Mux scanning logic  304  may sequentially activate switches  310  to scan the image drive plates. In one embodiment, master clock  302  operates at 48 MHz, and is divided down to 16 MHz before being supplied to buffers  314 . Each switch  310  is gated on for about 0.5-5 microseconds. The sequential signal bursts  312  applied to the drive plates provide a scan of the image sensor  200 . Because the scan speed is fast in comparison with the finger swipe speed, multiple lines of a fingerprint image can be acquired.  
         [0030]     The drive plates  210  of image sensor  200  are energized sequentially, but need not be energized in any particular order. Further, the drive plates need not be energized with bursts of master clock  302 , but may be energized by any periodic signal, such as a sinewave.  
         [0031]     When its control input is activated, each switch  310  supplies a signal burst from master clock  302  to buffer  314 . Signal burst  312  output by buffer  314  is coupled from one of the drive plates  210  to pickup plate  212 . The coupled signal is a function of the fingerprint features of a finger in contact with the image sensor  200 . When the control input to switch  310  is not activated, buffer  314  drives its connected drive plate to ground. Any parasitic fields between the energized drive plate and the non-energized drive plates are therefore shorted to ground. Pickup plate  212  detects the signal bursts and provides the coupled signals to bandpass filter  240 . In addition, a buffer  320  drives comb plate  250  to ground during image sensing.  
         [0032]     Bandpass filter  240  may be centered at the frequency of master clock  302  and may have a Q of 10. The output from bandpass filter  240  is supplied to differential amplifier  242 , which may have variable gain. The gain of differential amplifier  242  may be controlled by microprocessor and memory  130 . The gain may be adjusted to provide a desired output level despite variable sensing conditions.  
         [0033]     The output of differential amplifier  242  is demodulated in detector  306 . Detector  306  performs synchronous envelope detection of signal bursts  312 . The output of detector  306  is a baseband pulse that represents the envelope of the coupled signal burst. In an alternative embodiment, synchronous rectification may be used for envelope extraction. The amplitude of the pulse output by detector  306  is a function of the magnitude of the signal coupled from the drive plate  210  to the pickup plate  212 . The pulse amplitude modulated signal is supplied to a low pass filter  322 . Low pass filter  322  removes unwanted high frequency harmonics produced by the demodulation process. Low pass filter  322  may have group delay characteristics that compensate for phase distortions that occurred in the previous signal processing stages. Low pass filter  322  may be optimized for processing the information coming out of the detector  306  at the rate at which the drive plates are scanned.  
         [0034]     An analog-to-digital converter  324  converts the output of low pass filter  322  to a digital value. Analog-to-digital converter  324 , for example, may have a resolution of 8-12 bits and is therefore capable of resolving the output of low pass filter  322  into 256 to 4096 values in this example. Analog-to-digital converter  324  operates at a sufficient speed (e.g. 1 megasamples/sec) to accommodate the scanning of image sensor  200 . Microprocessor and memory  130  receives the output of analog-to-digital converter  324  and stores it in a buffer. Each stored digital value represents the coupled signal between a drive plate  210  and the pickup plate  212  when the drive plate was energized by signal burst  312 .  
         [0035]     Each scan of image sensor  200  acquires a slice of a fingerprint image. The stored digital values represent multiple slices of the fingerprint image and are used to generate a fingerprint image.  
         [0036]     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.