Abstract:
A touchscreen liquid crystal display that includes a liquid crystal display including a viewing surface, a liquid crystal area containing liquid crystal located behind the viewing surface, a plurality of spaced apart elongate first electrodes located on a viewing surface side of the liquid crystal area and a plurality of spaced apart elongate second electrodes located on an opposite side of the liquid crystal area, the first and second electrodes overlapping to form an array of liquid crystal pixel elements, at least some of the first electrodes being displaceable towards the second electrodes in response to external pressure applied to the viewing surface. A control circuit is connected to the first and second electrodes for controlling the operation of the liquid crystal display. The control circuit includes (i) a driver circuit for driving the electrodes for selectively controlling a display state of the pixel elements; and (ii) a measurement circuit for detecting displacement of the at least some of the first electrodes in response to external pressure applied to the viewing surface.

Description:
[0001]    This application claims priority to Provisional U.S. Patent Application No. 60/427,963 filed Nov. 21, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to the field of liquid crystal displays (LCDs). More specifically, this invention relates to touchscreen LCDs.  
           [0004]    2. Description of the State of the Art  
           [0005]    Although there are several types of touchscreens possible, the two most commonly used touchscreens in handheld electronic devices are resistive and capacitive touchscreens.  
           [0006]    Resistive touchscreens use a thin, flexible membrane over a glass substrate. The substrate surface and the facing membrane surface have a transparent metallic coating and are separated by spacers. When a user presses on the outer surface of the membrane, the inner surface of the membrane meets the substrate causing a change in resistance at the point of contact. A touchscreen controller measures this resistance using the membrane and the substrate as a probe. The two resistance measurements provide the x and y coordinates of the point of contact. Resistive touchscreens reduce the reflection and clarity of the LCD because of the added membrane layer and air gap in front of the surface of the LCD. A solution is required that does not require added layers that reduces the LCD visibility.  
           [0007]    Capacitive touchscreens use a metallic coating on a glass sensor. Typically, voltage is applied to the four corners of the sensor. When the screen is not in use, the voltage spreads across the sensor in a uniform field. When the user touches the sensor, the touchscreen controller recognizes a disturbance of the field and sends the x-y coordinate of the point of contact to the CPU of the device. Capacitive touchscreens can only be used with a bare finger or conductive stylus. A touchscreen solution is required that can convert any touch into touchscreen data.  
           [0008]    Resistive and capacitive touchscreens add thickness to the LCD module because of the added layers to provide touchscreen capabilities. With the demand for streamlining and minimizing the size of handheld devices, LCD modules need to be as thin as possible. A touchscreen solution is required to maximize the reflective characteristics of an LCD and to minimize the thickness of an LCD module.  
         SUMMARY  
         [0009]    In one aspect, a touchscreen is integrated into an LCD by using the electrodes that forms the pixels to measure voltage differences to locate a point of contact.  
           [0010]    According to at least one example aspect, a touchscreen liquid crystal display that includes a liquid crystal display including a viewing surface, a liquid crystal area containing liquid crystal located behind the viewing surface, a plurality of spaced apart elongate first electrodes located on a viewing surface side of the liquid crystal area and a plurality of spaced apart elongate second electrodes located on an opposite side of the liquid crystal area, the first and second electrodes overlapping to form an array of liquid crystal pixel elements, at least some of the first electrodes being displaceable towards the second electrodes in response to external pressure applied to the viewing surface. A control circuit is connected to the first and second electrodes for controlling the operation of the liquid crystal display. The control circuit includes (i) a driver circuit for driving the electrodes for selectively controlling a display state of the pixel elements; and (ii) a measurement circuit for detecting displacement of the at least some of the first electrodes in response to external pressure applied to the viewing surface.  
           [0011]    According to at least another example aspect, a method for using a liquid crystal display as a user input, the liquid crystal display having a plurality of first electrodes and a plurality of second electrodes located on opposite sides of a liquid crystal containing area, the first electrodes overlapping with the second electrodes defining an array of liquid crystal display pixel elements, each pixel element being associated with a unique location where an associated one of the first electrodes overlaps with an associated one of the second electrodes, at least some of the first electrodes being displaceable towards the second electrodes when pressure is applied to a viewing surface of the liquid crystal display. The method includes: (a) selectively driving the first and second electrodes to cause the pixel elements to display an image visible from a viewing side of the viewing surface; (b) sampling voltages between the first and second electrodes; and (c) determining based on the sampled voltages if any of the first electrodes have been displaced towards the second electrodes.  
           [0012]    Further features of the invention will be described or will become apparent in the course of the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    In order that the invention may be more clearly understood, one or more example embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:  
         [0014]    [0014]FIG. 1 is a sectional drawing showing the structure of an LCD;  
         [0015]    [0015]FIG. 2 is a graph depicting the voltage change for a segment of pixels due to an applied force on an LCD;  
         [0016]    [0016]FIG. 3 is a front view of the LCD;  
         [0017]    [0017]FIG. 4 is a system diagram showing touchscreen circuitry for an integrated LCD touchscreen; and  
         [0018]    [0018]FIG. 5 is a flow diagram showing the method for sensing a force applied to the LCD. 
     
    
     DETAILED DESCRIPTION  
       [0019]    Turning now to the drawings, FIG. 1 depicts an LCD structure. As it is known in the art, the LCD  100  consists of a sandwich of liquid crystal  112  between a top glass substrate  104  and a bottom glass substrate  118  with polarizers  102 ,  120  on the external surfaces of the glass substrates  104 ,  118 . A user would view information on the LCD  100  through the top glass substrate  104 . The polarizers  102 ,  120  control the light that enters and leaves the LCD  100 . The top polarizer  102  is polarized oppositely or perpendicularly to the bottom polarizer  120 . Polarized light enters the LCD and twists around the liquid crystal molecules  112  so that the light&#39;s polarization becomes oppositely polarized and then exits the LCD  100 . Wherever light passes through all the layers of the LCD  100 , pixels appear white.  
         [0020]    On the internal surface of the top glass substrate  104  is a colour filter  106 . A first layer of strips of transparent electrodes  108  is on the top glass substrate  104 . A second layer of transparent electrodes  116  is attached on the internal surface of the bottom glass substrate  118 , perpendicular to the first layer of electrodes  108 . Therefore if the first layer of electrodes ran in a direction parallel to the width (commons) of the glass substrates  104 ,  118 , then the second layer of electrodes  116  runs in a direction parallel to the length (segments) of the glass substrates  104 ,  118 . These transparent electrodes are usually made using Indium-Tin Oxide (ITO). Wherever a strip of ITO from the first layer of electrodes  108  crosses a second strip of ITO from the second layer of electrodes  116 , a pixel element is formed. Each strip of ITO from the first and second layer of electrodes  108 ,  116  is typically electrically connected to a drive circuit. At each pixel, the drive circuit can control the voltage, which determines the state of the liquid crystal material  112 .  
         [0021]    Over the electrodes are two alignment layers  110 ,  114 , which is usually a thin polymer film that is rubbed to form grooves (grooves not shown). The grooves in the top alignment layer  110  and bottom alignment layer  114  are usually brushed or rubbed so that the liquid crystal  112  will twist in order to align with the grooves. The angles at which the alignment layers  110 ,  114  are brushed to form the twist in the alignment of liquid crystal molecules are typically set depending on the desired contrast, viewing angle, background colour and any other factor that determines such angles. When the electrodes are driven, a voltage is placed across the liquid crystal  112  twisting the molecules out of alignment. The light that enters the LCD  100  does not twist and subsequently cannot exit the LCD  100 . Such pixels appear black.  
         [0022]    The electrical model of a pixel is similar to a capacitor. The intersection of segments and commons of ITO  108 ,  116  form capacitor plates and the liquid crystal  112  acts as the dielectric of a capacitor. The capacitance is determined as follows:  
           C= ( k ε   O    A )/ d   (1)  
         [0023]    where C is the capacitance, k is the dielectric constant, ε O  is the permittivity of free space, A is the area of the plates, and d is the distance between the plates. The dielectric constant of the liquid crystal  112  is determined by the type of liquid crystal  112  used in the LCD  100 . In the pixel model, the area of the plates is equal to the area of the pixel, and the distance between the plates is the distance between the electrodes  108 , 116 . Voltage V across this capacitor is equal to charge Q over capacitance C (V=Q/C), therefore, voltage is proportional to the distance between the plates.  
         [0024]    When a force is applied to the surface of the top glass substrate  104 , such as a press with a finger or stylus, the distance between the top and bottom glass substrates  104 ,  118  changes and thus changes the distance between the strips of ITO electrodes  108 ,  116 . When the distance between the electrodes changes, the capacitance of the pixel changes and the change in capacitance can be detected by the resulting change in voltage at that pixel. Because of the relationship between voltage and distance between the plates, as the electrodes get closer, the pixel voltage will decrease. Using the capacitance change of an LCD pixel to determine the location of an applied force eliminates the need for touchscreen overlays, which add thickness to an LCD module and therefore add thickness to a device which houses such an LCD module. Because overlays also tend to obscure the reflection and clarity of an LCD, eliminating the overlay and using the existing LCD structure improves the visibility for a touchscreen type LCD module. Cost is also reduced since no extra material other than the LCD is required. The solution does not require an extra glass layer, or flex connectors that add to the overall cost of an LCD module.  
         [0025]    [0025]FIG. 2 is a graph depicting the voltage change for a segment of pixels due to an example of a simulated applied force on an LCD. In this example graph  200 , the voltage change  202  across a segment of pixels corresponding to a strip of ITO in a 160×160 pixel LCD is indicated. The segment of pixels was numbered from 0 to 159 across an active area of the LCD. The simulated force on the top glass of the LCD was pressed at pixel  52  on this segment. The voltage change shows a drop  204  in pixel voltage at pixel  52 . If the voltage at each pixel is compared to a reference voltage, then the location of the applied force can be identified by detecting the pixel location of the minimum voltage (maximum voltage difference). This reference voltage may come from a segment of pixels that is not exposed to an external pressure such as a finger or stylus press. As shown in the graph  200 , the voltage drops for several pixels in the vicinity of where the LCD was pressed. These residual voltage drops are due to several factors including but not limited to the size of the finger or stylus, the pixel size, and the deflection of the glass as it is being pressed.  
         [0026]    [0026]FIG. 3 shows a front view of an integrated touchscreen LCD assembly. The LCD assembly  300  comprises a top glass  302 , and a bottom glass  304 . A seal  306  encloses the liquid crystal material between the two glass substrates  302 ,  304 . The seal  306  is preferably a glass epoxy seal. A viewing area  310  is the area of an LCD within the seal  306  that is visible through a bezel or cutout in a device in which the LCD is housed. An active area  312  is defined by a conductive area of ITO segments and commons (not shown) within the viewing area  310 ; that is, the area where the images are displayed. A reference ITO segment  308  is located outside the active area  312 , outside the viewing area  310 , in close proximity to the seal  306 . In this example the reference ITO  308  is a segment, but the reference ITO  308  is not limited to a segment format, and could also be in a common format. When a force is applied to the active area  312  of the LCD  300 , the reference segment  308  is not impacted because of its close proximity to the seal  306  and therefore a voltage drop across the reference segment  308  is negligible. The reference segment  308  is driven using the same data as any segment line that is being measured.  
         [0027]    [0027]FIG. 4 is a system diagram showing touchscreen circuitry for an integrated LCD touchscreen. The system is controlled by an MPU (micro-processing unit)  401 . The circuit  400  comprises measuring circuitry  403  and existing LCD driver circuitry  402 , which is preferably incorporated into an integrated circuit (IC). The components of the measuring circuitry  403  are preferably added to the IC housing the LCD driver circuitry  402 .  
         [0028]    The existing LCD driver circuitry  402  electrically connects to the segments  404  and commons  408  of an LCD  409 , wherein the segment lines  404  have switches  405  to disconnect the pixels of the segment from the driver  402 . These switches  405  are controlled by a logic controller  410 . In this example, one segment  406  is disconnected from the drive circuitry  402  at any given time by opening the segment switch  407 . The LCD  409 , in this example, has 160×160 pixels; therefore there are  160  segment lines (SEG 0 -SEG 159 )  404  and  160  common lines (COM 0 -COM 159 )  408 . A reference segment line (REF SEG)  450  is also controlled by the driver circuitry  402  wherein the REF SEG also has a switch  452  controlled by the logic controller  410 . The system also preferably comprises a multiplexer (MUX)  412 , a correlated double sampler (CDS)  414 , an amplifier  416 , a sample and hold (S/H)  418 , a comparator (C)  420 , an analog-to-digital converter (A/D)  422 , and several registers  426 ,  428 ,  430 ,  432 ,  434 .  
         [0029]    The MPU  401  communicates with the driver circuitry  402 . The driver circuitry preferably comprises an MPU interface  440 , an LCD controller with RAM  442 , SEG drivers  444 , COM drivers  446 , and a display timing circuit  448 . The MPU  401  communicates with the driver circuitry via the MPU interface  440 , which converts the MPU data into LCD driver data. The LCD controller  442  takes the data from the MPU  401  and combines it with data from the display timing circuit  448 . The display timing circuit  448  defines the frame frequency of the LCD and determines when the segments and commons are driven. The LCD controller converts the combination of data from the MPU  401  and the display timing circuit  448  to driver data and sends it to the SEG drivers  444  and the COM drivers  446 , which respectively drive the SEG lines  404  and the COM lines  408 . The SEG lines and COM lines form the pixels on the LCD  409 . The LCD controller uses RAM as a frame buffer for representing data that is to be displayed.  
         [0030]    The switch  407  on a scanned segment line  406  disconnects the pixels on that segment line from the SEG driver  444 . The voltage of the disconnected segment line may be measured by the measuring circuitry  403 . The logic control  410  determines when the switches  405 ,  407  are opened or closed and only one switch will be opened at a time. A switch  407  is open preferably for approximately one frame, which is when an entire LCD screen is updated or refreshed. A typical frame frequency for a 160×160 LCD is 65 Hz. The SEG driver  444  drives the REF SEG  450  with the same data as the segment that is being sampled.  
         [0031]    The logic controller  410  performs several functions in this system  400 . As previously mentioned, the logic controller  410  opens a segment switch  407  for measurement by the measuring circuitry  403 . The logic controller  410  also addresses the MUX  412  to select a sample segment (in this example, segment  159   406  is sampled and scanned) for scanning such that it is disconnected from the SEG driver  444  by opening the sample segment switch ( 407 ). The logic controller  410  provides the clock signal to the CDS  414  to define when the sampling occurs. The CDS  414  subtracts the reference segment voltage from the voltage of the sample segment line  406 . Using a CDS  414  is a technique commonly used in the field of CCD (charged coupled device) imaging to process the output signal from a CCD image sensor in order to reduce low-frequency noise from components such as the LCD driver circuit, components within the device housing the LCD, and sources outside the device. Using CDS in CCD imaging is well known in the art.  
         [0032]    The CDS  414  sends the voltage difference to the amplifier  416 , which increases the signal since the voltage difference from the CDS  414  will be very small. The amplified signal is sent to the S/H  418 . The S/H  418  stores the maximum voltage difference measured for all the scanned segments. The comparator  420  compares the present voltage difference with the maximum voltage difference stored in the S/H  418 . If the present voltage difference is greater than that stored in the S/H  418 , then the comparator output is asserted and a new maximum voltage difference is stored by the S/H  418 . If the present voltage difference is not greater than the stored voltage difference in the S/H  418 , then no new voltage difference is stored. The logic controller  410  then scans the next segment until all segments are scanned.  
         [0033]    There are two scanning directions being measured in this example. When the sample segment  406  is scanned by the logic controller  410 , the measuring circuit  403  sees  160  different output readings for this sample segment  406  as the common lines  408  are driven one by one. The logic controller  410  then determines the sample segment  406  that has the maximum difference from reference segment  450 . When the logic controller  410  starts scanning segments the location of the force can also be determined along the common lines  408 . The SEG counter register  430  and COM counter register  432  keep track of which segment and common are being measured, respectively. The logic controller  410  saves the value in the SEG counter  430  and COM counter  432  when the comparator  420  triggers the logic controller  410  the counter value for both SEG and COM are saved. These saved values represent the location of the maximum voltage difference.  
         [0034]    If the present voltage difference is higher than the stored voltage difference, the AID  422  converts the voltage difference to a value that represents the force applied to the glass and may save it to a register, Z,  426 . This value may be used for input options. Detecting the amount pressure used in the applied force can indicate what kind of press was used; for example determining the amount of force applied can indicate if the user had made a full press or a double press. As the force applied to the glass increases, the capacitance at the selected pixels increases and subsequently the voltage difference increase. When the voltage at the selected pixels is compared to the REF SEG  450 , the difference will be larger than a segment that has no applied force.  
         [0035]    In an idle mode, where there is no force applied to the LCD glass, the measuring circuit  403  preferably scans only one segment at a slow rate. A slow rate is selected to reduce power consumption that scanning may increase. Another reason for a slow rate of scanning is to reduce the impact on the contrast of the LCD. This segment is preferably located in the middle of the LCD  408 . Therefore, if a 160×160 LCD is used, the middle segment is segment  79   
         [0036]    In an alternative embodiment, the measuring circuitry  403  may scan more than one segment when in idle mode. In this embodiment, the measuring circuitry may alternate the scan for an applied force on the LCD glass by scanning one segment per frame in selected areas of the LCD  408 . For example, if the measuring circuitry scans three segments in the idle mode, the measuring circuitry may scan a segment near one edge of the active area  312  of the LCD in one frame, a segment at the middle of the active area  312  in the next frame, and a segment at the opposite edge of the active area  312  in the next frame.  
         [0037]    When a new maximum voltage difference is measured and saved and the comparator  420  triggers the logic controller  410  from idle mode into scan mode, the logic controller  410  scans the segments of the LCD and compares each segment to the REF SEG  450 . To minimize power consumption and contrast degradation, a percentage of the segments are preferably scanned. For a 160×160 LCD, the minimum percentage of segments scanned to minimize power consumption and contrast degradation is approximately 10%. In an alternative embodiment, for higher accuracy of determining the location of an applied force, a higher percentage of segments or all the segments are scanned when the logic controller is triggered into the scan mode.  
         [0038]    In a further alternative embodiment, if more than one segment is scanned in idle mode, then when a force is applied to the LCD glass, the segment that is continuously scanned closest to the force has the maximum voltage difference measurement. The measuring circuit may only scan the segments in close proximity to the scanned segment with the lowest voltage difference measurement.  
         [0039]    When a force is applied to the LCD glass, the logic controller sends an interrupt signal to the MPU interface  440 , which in turn sends the signal to the MPU  401 . The MPU reads the location value of the applied force and interprets the corresponding input made by the user. The location registers are cleared.  
         [0040]    In an alternative embodiment, the center of deflection of an applied force may be calculated by the device operating system by taking a weighted averaged of all deflections and calculating the centroid of the force. Such a calculation is made to determine the location of an applied force with greater accuracy. In the mode previously described, a location of an applied force can be defined by which segment and common have the lowest voltage. Using a centroid calculation allows the location to be determined to a fraction of a pixel. This method is preferred for applications that require high resolution such as hand writing recognition. The centroid calculations are determined using the following formulae:  
               Seg   centroid     =       X   o     +         ∑     com   =   0     com                       ∑     seg   =   0     seg                     SEG_counter   ×     Z        (     seg   ,   com     )                 ∑     com   =   0     com                       ∑     seg   =   0     seg          Z        (     seg   ,   com     )                       (   2   )                 Com   centroid     =       Y   o     +         ∑     seg   =   0     seg                       ∑     com   =   0     com                     COM_counter   ×     Z        (     seg   ,   com     )                 ∑     com   =   0     com                       ∑     seg   =   0     seg          Z        (     seg   ,   com     )                       (   3   )                               
 
         [0041]    The location of the applied force is found as previously described. The pixels around the location of the lowest recorded voltage are scanned again, for example in a 10×10 matrix around the location. A 10×10 matrix is an example of the size of a typical finger press; however, the matrix is not limited to such a matrix size. Smaller matrix sizes may be used to represent a typical force applied from a stylus press.  
         [0042]    In equation (2), X O  is the segment number for the starting location of the matrix. Typically, this segment is the leftmost segment of the matrix, but may also be the rightmost segment. SEG counter is the value in the SEG counter register  430 . Z(seg, com) is the value representing the amount pressure of the applied force, which is stored in the Z register  426 .  
         [0043]    In equation (3), Y O  is the common number for the starting location of the matrix. Typically, this common is the topmost segment of the matrix, but may also be the bottom most segment. COM counter is the value in the COM counter register  432 . Z(seg, com) is the value representing the amount pressure of the applied force, which is stored in the Z register  426 .  
         [0044]    The centroid calculation is analogous to a center-of-mass calculation for an object if the local mass density is represented, in this case, as the pressure of an applied force.  
         [0045]    [0045]FIG. 5 is a flow diagram illustrating the method  500  used to determine the location of an applied force to an LCD. In step  502 , the measuring circuit  403  is in idle mode and scans at least one segment in the active area of an LCD. The measuring circuit  403  compares the voltage of the scanned segment  406  to the reference segment  450 . In step  504 , a user applies a force to the LCD glass to make an input to a device housing the LCD. In step  506 , a voltage drop is detected across the scanned segment  406 . The measuring circuit  403  is triggered into scan mode in step  508 . The measuring circuit  403  scans a percentage of the segments to find the segment with the maximum drop in voltage. The percentage is preferably between 10% and 100% of segments. The maximum drop in voltage represents the location of the applied force. In step  510 , the maximum voltage difference between a scanned segment and the reference segment  450  is stored. The segment and common locations of the maximum voltage difference is also stored. In step  512 , the measuring circuit  403  sends the MPU  401  the location value. In step  514 , the MPU  401  translates the location into input data. In step  516 , the MPU  401  clears the location value and returns the measuring circuit  403  to the idle mode. The method  500  returns to step  502  where the measuring circuit continuously scans at least one segment.  
         [0046]    The presently described invention can be applied to display panels of both passive matrix and active matrix type displays. In active matrix type displays the control and measurement circuitry can be conveniently incorporated as part of the display panel  
         [0047]    It will be appreciated that the above description relates to the preferred embodiment by way of example only. Many variations on the invention will be obvious to those knowledgeable in the field, and such obvious variations are within the scope of the invention as described and claimed, whether or not expressly described.