Abstract:
A method of differential self-capacitance measurement is used to enhance a signal-to-noise ratio of sense lines in a touch panel display, thereby improving touch sensor accuracy. The differential self-capacitance measurement is implemented for a touch panel using charge sharing between adjacent sense lines of the touch panel matrix. Sequential differential self-capacitance measurements can be compared with one another by computing the difference |C S1 −C S2 |−|C S2 −C S1 | to sense a change caused by an intervening event. By scanning the entire touch panel matrix, events can be tracked across the touch panel.

Description:
BACKGROUND 
     Technical Field 
       [0001]    The present disclosure relates to touch screen technology and, in particular, to improving the sensitivity of touch screens. 
       Description of the Related Art 
       [0002]    Touch screens serve as user interfaces for many types of electronic devices in use today including, for example, smart phones, tablet computers, kiosks, vehicle control panels, and the like. A touch screen includes a touch-sensitive panel overlaid on a display. The display provides visual output to users of the electronic device. The display also presents text and graphics indicating selections, e.g., web links, play buttons, and the like, that can be activated by contact with the overlying touch-sensitive panel. Displays utilize various display technologies such as, for example, liquid crystals (LCD), plasma, organic light emitting diodes (OLED), and the like. 
         [0003]    A touch-sensitive panel, or “touch panel,” typically includes a matrix of drive lines and sense lines arranged transverse to one another and in planes that are separated from one another by a small distance. Sensor electrodes are placed at junctions where the drive lines cross the sense lines. The sensor electrodes may sense touch by a capacitive mechanism. When a finger comes into contact with the touch-sensitive panel at the electrode position, the capacitance of the finger increases the capacitance of the electrode with respect to ground. Detection of such an increase in “self-capacitance” therefore indicates a touch event. However, in some instances, touch events may be missed, or non-events may be misinterpreted as touch events. For example, certain touch panels may not be able to sense multiple touch events at the same time. Furthermore, a self-capacitance detection method may fail to sense a finger hovering above the touch panel. Water droplets falling on the touch panel may be erroneously interpreted as touch events. To improve accuracy in such instances, it is desirable to increase the signal-to-noise ratio of self-capacitance touch panels. 
       BRIEF SUMMARY 
       [0004]    A method of differential capacitance measurement is used to enhance a signal-to-noise ratio of sense lines in a touch panel, thereby increasing touch sensor accuracy over methods that use absolute capacitance measurements. The differential capacitance measurement is a normalized electrical signal value that provides greater sensitivity to noise fluctuations. An increase in sensitivity occurs because differential measurement values are much smaller than absolute measurement values. 
         [0005]    The differential capacitance measurement is implemented for a touch panel by using a charge sharing method to compare capacitance values on adjacent sense lines of the touch panel matrix. In a first step, the charge sharing method entails discharging a first sense line S 1  while charging an adjacent second sense line S 2 . In a second step, the two sense lines are coupled so that they are at a common voltage in sharing the charge stored previously in the sense line S 2 . A differential capacitance measurement, C S2 −C S1 , can then be made comparing the common voltage of the two sense lines to a reference voltage. The sequence can then be reversed as is typically done when using a chopping technique. The first sense line is charged while the second sense line is discharged. After coupling the two sense lines, another differential capacitance measurement can be made comparing the common voltage of the two sense lines to a reference voltage, yielding C S1 −C S2 . Sequential differential capacitance measurements can then be compared with one another by computing the difference |C S1 −C S2 |−|C S2 −C S1 | to sense a change caused by an intervening event. By scanning the entire matrix, events can be tracked across the touch panel. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0006]      FIG. 1A  is an electric circuit schematic of a self-capacitance sensing touch panel, according to an embodiment as described herein. 
           [0007]      FIG. 1B  is an electric circuit schematic of a self-capacitance sensing touch panel during a touch sensing event, according to an embodiment as described herein. 
           [0008]      FIG. 2  is an electric circuit schematic showing junction capacitances of a touch screen, according to an embodiment as described herein. 
           [0009]      FIG. 3  is a flow diagram showing a sequence of steps in a method of operating a self-capacitance sensor according to an embodiment as described herein. 
           [0010]      FIG. 4A  is an electric circuit schematic of a charge sharing circuit with amplification, according to an embodiment as described herein. 
           [0011]      FIG. 4B  is a timing diagram of electric charge on sense lines S 1  and S 2  during operation of the charge sharing circuit shown in  FIG. 4A . 
           [0012]      FIGS. 5A-5B  show operations occurring in a charge sharing differential self-mode used to scan odd numbered sense lines, according to an embodiment as described herein. 
           [0013]      FIGS. 5C-5D  show operations occurring in a charge sharing differential self-mode used to scan even numbered sense lines, according to an embodiment as described herein. 
           [0014]      FIG. 6A  is a differential capacitance profile for a charge-sharing self-mode, according to an embodiment as described herein. 
           [0015]      FIG. 6B  is a plot of a conventional, normalized signal value capacitance profile. 
           [0016]      FIGS. 7-11  are electric circuit schematics for a touch panel implementing a charge sharing differential self-mode, according to various embodiments as described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various aspects of the disclosed subject matter. However, the disclosed subject matter may be practiced without these specific details. In some instances, well-known structures and methods comprising embodiments of the subject matter disclosed herein have not been described in detail to avoid obscuring the descriptions of other aspects of the present disclosure. 
         [0018]    Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising,” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” 
         [0019]    Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “In one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same aspect. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more aspects of the present disclosure. 
         [0020]    In the drawings, identical reference numbers identify similar elements or acts unless the context indicates otherwise. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. 
         [0021]    Specific embodiments are described herein with reference to touch screens that have been produced; however, the present disclosure and the reference to certain materials, dimensions, and the details and ordering of processing steps are exemplary and should not be limited to those shown. 
         [0022]    In the figures, identical reference numbers identify similar features or elements. The sizes and relative positions of the features in the figures are not necessarily drawn to scale. 
         [0023]    Turning now to the drawings,  FIG. 1A  illustrates a self-capacitance sensor  80 , according to an embodiment. The self-capacitance sensor  80 , as part of a touch panel, is configured to detect a touch event, e.g., a finger contacting the touch panel. The self-capacitance sensor  80  includes an AC power supply  82  and a single touch panel electrode  84 . A sense line capacitor C s  represents a capacitance of the touch panel electrode  84 . The self-capacitance sensor  80  detects touch events by monitoring the value of C s  relative to a ground  86 . There is no touch event occurring in  FIG. 1A . 
         [0024]      FIG. 1B  illustrates a touch event  90  in which a human finger  91  is introduced, hovering a distance d above the touch panel electrode  84 . The human finger  91  is aligned with an axis  94  oriented at an angle Φ relative to the plane of the touch panel. The presence of the human finger  91  increases the capacitance of the touch panel electrode  84  relative to the ground  86 . A capacitor C H  represents a change in capacitance of the touch panel electrode  84  introduced by the human finger  91 . Consequently, the self-capacitance sensor  80  is capable of sensing the hovering finger, in addition to sensing a finger in contact with the touch panel. Furthermore, the self-capacitance sensor  80  can distinguish between the human finger  91  and a water droplet that has landed on the touch panel, because a water droplet that is present on the touch panel changes the capacitance of the touch panel electrode  84 , but by an amount that is different from the value of C H . Other types of touch panel sensors, for example, mutual capacitance sensors that measure capacitance between a pair of electrodes, can detect contact with the touch panel. However, mutual capacitance sensors are not capable of detecting a hovering finger or providing a “water rejection” function. 
         [0025]      FIG. 2  represents a portion of a self-capacitance-based touch panel  96 , according to an embodiment of the present disclosure. The self-capacitance-based touch panel  96  is made of glass in which a matrix of drive lines T x  and sense lines S x  are formed as thin conductive wires embedded in the touch panel  96 . Two examples of sense lines, S 1  and S 2 , oriented vertically, and one example of a drive line T x , oriented horizontally, are shown. In the full self-capacitance-based touch panel  96 , there will be several dozens or hundreds of such lines S x  and T x . The sense lines S x  cross over the drive lines T x  at junctions  98 , forming parallel plate capacitors in which the glass touch panel serves as the dielectric material separating the conductors. A touch controller applies drive signals to the drive lines T x  and reads signals from the sense lines S x . Consequently, there exists a panel capacitance C px , between each sense line S x  and the corresponding adjacent drive line T x , wherein x=1, 2, and so on. In addition, when a touch event occurs, a self-capacitance C sx  is created between each sense line S x  and ground. When there is no touch event, the self-capacitance equals zero. The panel capacitances C px  and the self-capacitances C sx  are modeled as parallel-plate capacitors. Values of C px  and C sx  can be monitored by scanning and comparing neighboring sense lines S x . 
         [0026]    A hovering finger causes a small fluctuation in capacitance. In order to increase sensitivity to such small changes, a differential capacitance measurement |C S1 −C S2 |, can be made. In general, small changes are easier to detect using differential measurements than absolute measurements. This is because the fluctuations are large relative to the differential measurement values. Therefore, a differential capacitance measurement will be more sensitive to a touch event than an absolute capacitance measurement would be. A more sensitive measurement also improves noise performance by increasing the signal-to-noise ratio. That is, the ratio of differential signal and noise measurements will be larger than the ratio of absolute signal and noise measurements. 
         [0027]      FIG. 3  shows steps in a method  100  of operating the self-capacitance based touch panel  96 , according to an embodiment that incorporates a charge sharing technique to perform a scan sequence. The method  100  can be carried out by a self-mode charge sharing circuit  130  shown in  FIG. 4A  and described below. The method  100  is further illustrated by a timing diagram  160 , shown in  FIG. 4B , according to which the self-mode charge sharing circuit  130  is operated. The self-mode charge sharing circuit  130  can be used to recycle charge and, by doing so, reduce the demand for power while increasing sensitivity of a touch panel. Charge sharing is a well-known technique and has been used in many areas of electrical design, including low-power digital CMOS circuits, LCD display drivers, and touch sensor panels, for example, as disclosed in U.S. Pat. No. 8,432,364. It is shown herein that charge sharing can be used to facilitate sensing differential self-capacitance of a touch screen associated with a touch event. 
         [0028]    The self-mode charge sharing circuit  130  shown in  FIG. 4A  includes an output buffer  132  coupled to parallel capacitors C S1  and C S2  by switches  154  and  156 . The switch  154 , in particular, acts as charge sharing switch, represented by a bow-tie symbol in  FIG. 4A . The charge sharing switch  154  is used to share charge between the two sense lines S 1  and S 2 . Switch  154  permits charge sharing by coupling C S1  and C S2  to one another. The switch  156  then couples the sense lines S 1  and S 2  to the output buffer  132 . The switches  154 ,  156  can be, for example, standard CMOS transistor gates, or transmission gate switches made from pairs of CMOS transistor gates, as is known in the art. 
         [0029]    In an embodiment, the output buffer  132  is an operational amplifier (op-amp) buffer. The output buffer  132  functions as a differential amplifier that applies a gain factor to the difference between the voltages at the two input terminals, V Cmin  and the voltage at node  92 . A reference capacitor  144  is connected in the feedback loop of the output buffer  132 . The reference capacitor  144  is an internal reference capacitor that can be bypassed by closing a switch  146 . Transistor  122  couples C S1  to the power supply V DD  to charge C S1 . V DD  may be a standard supply voltage within the range of about 3.3 V-12 V. Transistor  152  bypasses C S2  or connects node  92  directly to ground to discharge C S2 . The self-mode charge sharing circuit  130  monitors self-capacitance associated with the sense lines S 1  and S 2 , respectively, according to the timing diagram shown in  FIG. 4B . 
         [0030]    At  102 , at an initial time t 0  in the method  100 , the transistors  122  and  152  are on, so that the capacitor C S1  is set to the voltage V DD  and is charged to q in =C S1 *V DD , while the capacitor C S2  is discharged and is at 0 V, as shown in  FIG. 4B . 
         [0031]    At  104 , charge sharing is engaged. The transistors  122 ,  122 C,  152 , and  152 C are turned off and switch  154  is closed so that sense lines S 1  and S 2  are shorted together at node  92 . During an initial time interval  162 , the capacitor C S1  begins discharging while the capacitor C S2  begins charging. By the end of the time interval  162 , S 1  and S 2  are at approximately the same voltage, e.g., V DD /2. 
         [0032]    At  106 , the switch  146  is turned off and the switch  156  is closed. During an amplification period coinciding with time interval  164 , a portion of the charge shared by C S1  and C S2  is transferred to the capacitor  144 , as is typical in a switched capacitor circuit. 
         [0033]    At  108 , at the end of the time interval  164 , a differential capacitance |C S1 −C S2 | is determined by extracting the transferred charge, q out +. Using, for instance, V CMIN =V DD /2 and q out +=V DD /2(C S1 −C S2 ), then the equation V OUT −V DD /2=V DD /2(C S2 −C S1 ) can be used to compute the differential capacitance |C S1 −C S2 |. Computations can be carried out by a microprocessor and an associated memory that are part of the touch screen controller. 
         [0034]    At  110 , the process can be reversed, as is usual in a chopped circuit to improve flicker noise rejection. During the time interval  166 , the switch  146  is opened to reset the previous value of the output signal, and the transistors  122 C,  152 C are turned on so that S 1  is discharged to 0 V and S 2  is pre-charged to V DD . Then the transistors  122 C,  152 C are turned off and the two sense lines S 1  and S 2  are shorted together so that C S2  discharges while C S1  charges to a common voltage during the time interval  168 . During an amplification and charge sharing time interval  170 , charge is equalized on the two capacitors C S1  and C S2 . Switches  146  and  154  are closed during time interval  170 , permitting the charge q out − to be transferred by the output buffer  132 . A differential capacitance |C S2 −C S1 | can then be computed at the end of the time interval  170  by extracting the amplified charge, q out − and using the equation q out +=V DD /2(C S2 −C S1 ) when V CMIN =V DD /2. During a final time interval  172 , the charge sharing switch  154  is opened and the capacitor C S2  is discharged while C S1  is restored to the supply voltage V DD . 
         [0035]    The differential capacitance measurements that are made at the end of the charge transfer intervals  164 ,  170  can be made by sampling using an A-to-D converter. By monitoring differential capacitance values over time and across the touch panel, a touch event can be detected by the disturbance that the touch event creates in the value of the differential capacitance. 
         [0036]    At  112 , the method  100  can be repeated to monitor an adjacent pair of sense lines, and so move across the touch panel in a serial fashion. Alternatively, the method  100  can be carried out in parallel by replicating the self-mode charge sharing circuit  130  to monitor a plurality of pairs of sense lines simultaneously. 
         [0037]      FIGS. 5A-5D  illustrate various charge sharing scan sequences involving up to five sense lines S 1 -S 5 , or channels, as examples. There may be 32 channels, for example, or any arbitrary number of channels. Each instance of charge sharing indicated by a solid line bow-tie switch  154  represents execution of the method  100  described above. A dotted line bow-tie switch indicates an open switch. The number of sense lines being scanned depends on the size of the touch screen. 
         [0038]    In a first scan shown in  FIG. 5A , charge sharing is applied to sense lines S 1  and S 2  to determine the differential capacitance |C S2 −C S1 | and to sense lines S 3  and S 4  to determine a differential capacitance |C S3 −C S4 |. In a second scan shown in  FIG. 5B , charge sharing is applied to sense lines S 2  and S 3  to determine a differential capacitance |C S2 −C S3 | and to sense lines S 4  and S 5  to determine a differential capacitance |C S4 −C S5 |. In a third scan shown in  FIG. 5C , charge sharing is applied to sense lines S 1  and S 2  to determine the differential capacitance |C S1 −C S2 | and to sense lines S 3  and S 4  to determine the differential capacitance |C S3 −C S4 |. In a fourth scan shown in  FIG. 5D , charge sharing is applied to sense lines S 2  and S 3  to determine the differential capacitance |C S2 −C S3 |. 
         [0039]      FIGS. 6A and 6B  compare differential capacitances with absolute capacitances determined during a touch event. The touch event occurs at the origin ( 0 ) and is measurable out to about two radial units away from the touch site. Values of absolute capacitance  174  obtained for the example shown in  FIG. 6B  are about 1000 times greater than corresponding normalized differential capacitance measurements  173  shown in  FIG. 6A . 
         [0040]      FIGS. 7-12  show variations of the self-mode charge sharing circuit  130  in which panel capacitances C P1  and C P2  are taken into account.  FIGS. 7-12  therefore include the addition of panel capacitor C P1  in parallel with C S1 , panel capacitor C P2  in parallel with C S2 , and an error cancellation stage CX. The error cancellation stage CX is used to cancel out charge on the capacitors C P1 , C P2  so it will not affect the charge measurements q out + and q out − associated with self-capacitance during a touch event. The error cancellation stage CX is a switching capacitor circuit that includes a capacitor and a driver that can remove charge from the S 2  line, thereby compensating for error introduced by the differential capacitance |C P1 −C P2 |. 
         [0041]      FIG. 7  shows a low-power self-mode circuit  175 , according to an embodiment of the present disclosure. The low-power self-mode circuit  175  can be substituted for the self-mode charge sharing circuit  130  when the touch panel is in “wake-up” mode following an idle period. In use, the power supply V DD  for the low-power self-mode circuit  175  is set to 3.3 V and the V CMIN  input to the output buffer  132  is then 3.3 V/2=1.65 V. Due to charge sharing, the voltages used in the self-mode circuits described herein are low, and therefore error cancellation occurs quickly and efficiently. Use of the low-power self-mode circuit  175  also incurs less noise than a conventional touch panel circuit that detects capacitance instead of differential capacitance. 
         [0042]      FIG. 8  shows a low-power self-mode circuit  180 , according to an embodiment. The low-power self-mode circuit  180  is also intended for use, for example, when the touch panel is in “wake-up” mode following an idle period. The low-power self-mode circuit  180  is a complimentary, reverse circuit corresponding to the low-power self-mode circuit  175  in which the power supply V DD  and the transistor  122  are coupled to the sense line S 2 , and the transistor  152  is coupled to the sense line S 1 . As explained above, the low-power self-mode circuit  180  can be used in conjunction with a chopping technique. In use, the power supply V DD  for the low-power self-mode circuit  180  is set to 3.3 V and the V CMIN  input to the output buffer  132  is then V DD /2=1.65 V. 
         [0043]      FIG. 9  shows a self-mode circuit  200 , according to an embodiment of the present disclosure. The self-mode circuit  200  is intended for use during normal operation of the touch panel and provides for cancellation of the differential panel capacitance for a faster scan. The self-mode circuit  200  resembles the self-mode circuit  175  except that a switch  202  has been added to the output of the output buffer  132 , the switch  146  has been removed from the feedback loop of the output buffer  132 , and a switch  192  has been added between the inputs of the output buffer  132 . As is usual in a switched capacitor circuit, such components permit discrimination between the output and the input common mode of the operational amplifier  132 . In use, the power supply V DD  for the self-mode circuit  200  is set to 5.0 V and the V CMIN  input to the output buffer  132  is then V DD /2=2.5 V. In addition, the switch  202  couples to a 1.65 V power supply. 
         [0044]      FIG. 10  shows a self-mode circuit  210 , according to an embodiment of the present disclosure. The self-mode circuit  210  is intended for use during normal operation of the touch panel, and provides an alternative to differential C p  cancellation. The self-mode circuit  210  resembles the self-mode circuit  200  except that the error cancellation stage CX has been removed and a tunable voltage divider  212  is used to generate a reference voltage, V ref . In use, the power supply V DD  for the self-mode circuit  210  is set to 5.0 V and the V ref  input to the output buffer  132  is then V DD /2=2.5 V. In place of the error cancellation stage CX, V ref  is coupled to the voltage divider  212  that includes a resistance R coupled to an intermediate power supply V dd  and a variable resistance R 1 . V DD  may be set to 3.3 V, for example. 
         [0045]      FIG. 11  shows a self-mode circuit  220 , according to an embodiment of the present disclosure. The self-mode circuit  220  is intended for use during normal operation of the touch panel. The self-mode circuit  210  resembles the self-mode circuit  200  shown in  FIG. 11  except that the transistor  152  is connected to a negative voltage instead of being grounded. In use, the power supply V DD  for the self-mode circuit  220  is set to 5.0 V and the V CMIN  node is effectively the ground. In addition, the switch  202  couples to a 1.65 V power supply. 
         [0046]    All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entireties. 
         [0047]    It will be appreciated that, although specific embodiments of the present disclosure are described herein for purposes of illustration, various modifications may be made without departing from the spirit and scope of the present disclosure. The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
         [0048]    These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.