Abstract:
A capacitive touch screen sensing apparatus is provided. The apparatus includes a protecting layer; a sensing layer under the protecting layer for sensing a touch to generate a position signal; and a DC common voltage signal layer electrically connected with a DC voltage for shielding against signal interferences.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
     This patent application is based on Taiwan, R.O.C. patent application No. 098133019 filed on Sep. 29, 2009. 
     FIELD OF THE INVENTION 
     The present invention relates to a touch sensing apparatus and method, and more particularly to a touch sensing apparatus and method applied to a capacitive touch screen. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  is a conventional touch screen. The touch screen comprises a display controller  130 , a touch panel  150 , a sensing circuit  155  and a liquid crystal display (LCD) panel  170 . In general, the touch panel  150  is fabricated on the LCD panel  170 . The display controller  130  receives a video signal and converts the video signal to a panel control signal transmitted to the LCD panel  170  so that the LCD panel  170  displays the image according to the panel control signal. When one touches the touch panel  150 , the touch panel  150  generates a sensing signal to the sensing circuit  155 , and the sensing circuit  155  outputs according to the sensing signal a position signal that represents a touch point where one touches the touch panel  150 . 
       FIG. 2A  is a diagram of the LCD panel. The LCD panel  170  is generally divided into two regions—a display region  112  and a non-display region  114 . The display region  112  comprises a thin film transistor (TFT) array, and the non-display region  114  comprises a gate driver  120  and a source driver  125  for controlling transistors in the TFT array. The panel control signal outputted from the display controller  130  controls the gate driver  120  to generate a gate driving signal and the source driver  125  to generate a source driving signal. The panel control signal further comprises a common voltage signal Vcom for controlling the inversion of a liquid crystal on the LCD panel  170 . The gate driving signal controls respective TFTs within the TFT array to whether turn on or turn off, and the source driving signal provides brightness data to a pixel. In  FIG. 2A , in a portable electronic device application, the display controller  130  can be integrated with a timing controller (TCON), the gate driver  120  and the source driver  125 . 
     The video signal comprises a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a red signal, a green signal and a blue signal. The time to display a scan line on the LCD panel  170  is a period of the horizontal synchronization signal Hsync, while the time to display a frame on the LCD panel  170  is a period of the vertical synchronization signal Vsync. That is, if the LCD panel  170  has M scan lines, the gate driver  120  can generate M gate driving signals, and one period of the vertical synchronization signal Vsync equals M periods of the horizontal synchronization signal Hsync. According to the horizontal synchronization signal Hsync, M gate driving signals can be turned on sequentially. 
       FIG. 2B  is a diagram of the vertical synchronization signal Vsync, the horizontal synchronization signal Hsync, the common voltage signal Vcom and the gate driving signals. The period of the vertical synchronization signal Vsync begins from the start of the low level, and one period of the vertical synchronization signal Vsync equals a plurality of periods of the horizontal synchronization signal Hsync. According to the horizontal synchronization signal Hsync, a plurality of gate driving signals can be turned on sequentially, and a frequency of the common voltage signal Vcom is a half of a frequency of the horizontal synchronization signal Hsync. As mentioned, in the portable electronic device application, the display controller  130  can be integrated with the timing controller, the gate driver  120  and the source driver  125 , with the common voltage signal Vcom being present in the integrated display controller  130 . 
     During a vertical blanking interval (VBI), the red signal, the green signal and the blue signal do not output any data, while the common voltage signal Vcom also remains at the low level. 
       FIG. 3  is a diagram of a conventional capacitive touch panel. The capacitive touch panel comprises a first sensing layer  151 , a second sensing layer  152  and a shielding layer  153 . Generally, the first sensing layer  151  and the second sensing layer  152  respectively comprise a plurality of sensing components, each of which can be viewed as a capacitor. 
     When one touches the capacitive touch panel, an equivalent capacitance of the touch point is changed. By detecting the change in the equivalent capacitance of the touch point, the sensing circuit  155  can detect the actual position of the touch point and output a corresponding position signal. The shielding layer  153  is mainly for isolating the panel control signal from the sensing signal so that the sensing signal is not undesirably affected by noises from the panel control signal. 
       FIG. 4  is a diagram of a conventional capacitive touch sensing apparatus  40 . The apparatus  40  comprises a protecting layer  420 , a touch panel  450 , a sensing protection layer  457  and a Vcom signal layer  480 . The protecting layer  420  protects the capacitive touch sensing apparatus  40  from scratches that may be caused by touching. The sensing protection layer  457  protects the touch panel  450 . The touch panel  450  comprises a first sensing layer  451 , a second sensing layer  452  and a shielding layer  453 . Generally, the first sensing layer  451  and the second sensing layer  452  comprise a plurality of sensing elements, and each sensing element can be viewed as a capacitor. Since sensing elements of the sensing layers  451  and  452  are usually capacitors, the touch panel  450  is additionally provided with the sensing protection layer  457  for protecting the touch panel  450  from deformation. 
     When one touches the capacitive touch panel, an equivalent capacitance of the touch point is changed. Hence, the capacitive touch sensing apparatus  40  can use such characteristic to detect the actual position of the touch point and output the position signal. Since signals in the conventional Vcom signal layer  480  are alternating current (AC) signals that are constantly transitive, significant noise interference imparted to the touch panel  450 . To render shielding effects against the noise interferences, the shielding layer  453  is provided to isolate the Vcom signal layer  480  and the touch panel  450  from each other. However, helpful to reduce noise interference, the shielding layer  453  increases the manufacturing cost and also reduces transmittance of the touch panel  450 . Further, during the manufacturing process of the touch panel  450 , the shielding layer  453  needs to be adhered with the second sensing layer  452  and the sensing protection layer  457 , and so the manufacturing cost increases from having to discard the entire touch panel  450  in the event of an adherence failure. Further still, due to the additionally provided shielding layer  453 , the touch panel  450  includes not three layers but four layers, which compromises the transmittance of the capacitive touch sensing apparatus  40 . 
       FIG. 5  is a diagram of the relation between a conventional source driving signal and a common voltage signal Vcom. The conventional touch sensing apparatus uses an AC Vcom signal. For example, the AC Vcom signal swings from −1V to 4V, and then a source swing of the source driving signal need an inversion signal of −1V to 4V to provide a voltage difference of 5V between the source driving signal and the AC Vcom signal; that is, the source driving signal provides a working voltage range of 5V. 
     To accommodate the AC Vcom signal adopted in the conventional touch sensing apparatus, the four-layered capacitive touch sensing apparatus is needed. As discussed above, the conventional four-layered capacitive touch sensing apparatus is not only costly but also has unsatisfactory transmittance. Therefore, there is a need for a capacitive touch sensing apparatus that overcomes the above shortcomings. 
     SUMMARY OF THE INVENTION 
     It is one of the objectives of the present invention to provide a capacitive touch sensing apparatus and method applied on a touch screen, so that a capacitive touch panel of the touch screen does not need a shielding layer and can output a position signal correctly. 
     The invention provides a capacitive touch sensing apparatus comprising: a protecting layer; a sensing layer deployed under the protecting layer for sensing a touch to generate a position signal; and a DC common voltage signal layer electrically connected with a DC voltage for shielding against signal interference. 
     The invention further provides a capacitive touch sensing method comprising: applying a DC common voltage signal to a common voltage signal layer on a small-sized panel of a portable device; generating a driving signal symmetrical to the common voltage signal layer for driving a driver; and sensing a touch to output a position signal. 
     The invention further provides a touch screen comprising: an LCD panel; a touch panel for generating a sensing signal in response to a touch; a display controller for processing an image signal to generate a panel control signal comprising a fixed common voltage; and a sensing circuit coupled to the touch panel for receiving the sensing signal to generate a position signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
         FIG. 1  is a conventional touch screen; 
         FIG. 2A  is a diagram of an LCD panel; 
         FIG. 2B  is a diagram of a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a common voltage signal Vcom and gate driving signals; 
         FIG. 3  is a diagram of a conventional capacitive touch panel; 
         FIG. 4  is a diagram of a conventional capacitive touch sensing apparatus; 
         FIG. 5  is a diagram of the relation between a conventional source driving signal and a common voltage signal; 
         FIG. 6  is a capacitive touch sensing apparatus according to one embodiment of the present invention; 
         FIG. 7  is a diagram of the relation between a source driving signal and a common voltage signal according to one embodiment of the present invention; 
         FIG. 8  is a capacitive touch sensing apparatus according to another embodiment of the present invention; 
         FIG. 9  is a flowchart of a capacitive touch sensing method according to one embodiment of the present invention; and 
         FIG. 10  is a touch screen according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 6  is a capacitive touch sensing apparatus  60  according to one embodiment of the present invention. The apparatus comprises a protecting layer  620 , a touch panel  650  and a common voltage (Vcom) signal layer  680 . The touch panel  650  comprises a first sensing layer  651 , a second sensing layer  652  and a sensing protection layer  657 . The protecting layer  620  protects the capacitive touch sensing apparatus  60  from scratches that may be caused by touching. The sensing protection layer  657  protects the touch panel  650 . Since sensing elements of the sensing layers  651  and  652  are both capacitive soft boards (i.e., flexible members), the sensing protection layer  657  is added to protect the touch panel  650  from deformation. Alternatively, the touch panel  650  is directly adhered to the Vcom signal layer  680  without the sensing protection layer  657 . The protecting layer  620  and the sensing protection layer  657  are often made of hardened compounds, such as silicon dioxide or glass. The sensing layers  651  and  652  are made of compounds of good conductivity and high transparency, such as indium-tin oxide (ITO) shielding. 
       FIG. 7  is a diagram of the relation between a source driving signal and a Vcom signal according to one embodiment of the present invention. By realizing the application of a fixed Vcom signal to a small-sized panel, power consumption for driving the Vcom signal can be reduced, which is a significant breakthrough in power saving on portable products. In this embodiment, the fixed voltage (e.g., 0 Volt (0V) to −2V, taking 0V for example in  FIG. 7 ) is provided by a direct current (DC) Vcom signal. An effective voltage difference between the source driving signal and the DC Vcom signal is 5V, and a source displacement can be −5V to 5V. That is, if the source driving signal provides a 10V of working voltage range, then the effective voltage difference between the source driving signal and the DC Vcom signal is also 5V. In this embodiment, the working voltage 10V can be realized by two deep N-wells (DNW) of N-well (NW) and P-well (PW) with a voltage difference of 5V. 
     It is to be noted that, liquid crystal molecules have a characteristic that they cannot be maintained at a constant voltage level for a long time, or else such molecules will be destroyed. In this case, destroyed liquid crystal molecules cannot rotate in response to an electric field to form different gray scales when the voltage is changed. Preferably, the voltage is recovered at a particular interval to prevent the liquid crystal molecules from being impaired. However, if the screen persistently displays the same gray scale while the voltage level cannot remain the same, liquid crystal molecules in a liquid crystal display (LCD) are divided into two polarities, i.e., a positive (P) polarity and a negative (N) polarity. A liquid crystal molecule is positive when the source driving voltage is higher than the Vcom signal, and is negative when the source driving voltage is lower than the Vcom signal. Regardless of the polarity being positive or negative, a gray scale with the same brightness is generated when the displacements from the Vcom signal are the same. That is, a same displacement between the source driving voltage and the Vcom signal renders a same grey scale regardless of which voltage being higher or lower than the Vcom signal. The directions of the liquid crystal molecule are opposite when the polarities are opposite, so it resolves the previous problem of damage to liquid crystal molecules when a constant voltage level is applied for a long period on liquid crystal molecules where the molecules are fixed at a same direction. That is to say, when a currently displayed image stays unchanged, by alternating the voltage level of the liquid crystal molecules, the polarity (i.e., positive and negative) of the liquid crystal molecules are constantly altered with the directions of the liquid crystal molecules continuously changed, so as to preserve the above characteristic of liquid crystal molecules while also keeping the currently displayed image appear as being still. 
     There are five methods applied to changing polarity of an LCD panel: frame inversion, row inversion, column inversion, dot inversion and delta inversion. The polarity can be changed when updating data of a next frame. For example, a 60 Hz refresh frequency means changing the polarity of the image every 16 ms. That is, the polarity of a same point is changed continuously. Further, whether two neighboring points have the same polarity is dependent on the method of changing polarity—with the frame inversion, all points of a whole image have the same polarity; with the column inversion and the row inversion, points on neighboring columns or rows respectively have opposite polarities; with the dot inversion, upper, lower, left and right points adjacent to a particular point have an opposite polarity from the particular point; and with the delta inversion in which a unit of a pixel is formed by three RGB points, and the polarities are similar to those in the dot inversion, i.e., in a unit of a pixel, each pixel has an opposite polarity from pixels at its upper, lower, left and right positions. 
     Since the polarity change method of an LCD panel relates to the LCD performance, two important phenomena occurring in the LCD are introduced. The first is that crosstalk exists in the LCD, i.e., data of neighboring points on the LCD interfere with one another to result incorrectness in a displayed image. Although crosstalk is caused by many reasons, the phenomenon can be reduced if polarities of neighboring points are different. Hence, the dot inversion has its advantage of providing such characteristic. The other phenomenon is flicker, which means the image appears to flicker when one observes the image on the LCD. The flicker here is not an intended visual effect, but instead is an inevitable phenomenon that occurs when the pixels on the image are changed whenever the image is refreshed. Again, the dot inversion can also reduce this phenomenon. 
     However, it is noted that not all the polarity change methods can match the above two Vcom signals, i.e., the AC Vcom signal and the DC Vcom signal. When the DC Vcom signal is used, all the polarity change methods can be used. However, if the AC Vcom signal is used, the polarity change methods can only be the frame inversion or the row inversion. That is to say, if the column inversion or the dot inversion is needed, the DC Vcom signal should be used. 
       FIG. 8  is a capacitive touch sensing apparatus  80  according to another embodiment of the present invention. The apparatus  80  comprises a protecting layer  820 , a touch panel  850  and a Vcom signal layer  880 . The touch panel  850  comprises a sensing layer  851 .  FIG. 8  is similar to  FIG. 6 , and the differences are that the touch panel  650  in  FIG. 6  has two sensing layers  651  and  652  but the touch panel  850  on  FIG. 8  only has one sensing layer  851 , and the touch panel  650  in  FIG. 6  has the sensing protection layer  657  while the touch panel  850  on  FIG. 8  is directly attached to the Vcom signal layer  880 . Other operation details are similar to those in the foregoing description, and shall not be again described for brevity. 
       FIG. 9  is a flowchart of a capacitive touch sensing method according to one embodiment of the present invention. In Step  920 , a DC Vcom signal is applied to a Vcom signal layer of a small-sized screen on a portable device. Since the DC Vcom signal is at a constant DC voltage level, signal performance of the sensing layer is unaffected by the voltage displacement to provide good shielding effects for reducing signal interferences. In Step  940 , a driving signal symmetric to the Vcom signal layer is generated to drive the driver. Preferably, the driving signal is generated by a plurality of deep N-wells formed by the voltage difference with 5V of N-wells and P-wells. In Step  960 , a touch to output a position signal is sensed. 
       FIG. 10  is a touch screen  100  according to one embodiment of the present invention. The touch screen  100  comprises a display controller  1030 , a touch panel  1050 , a sensing circuit  1055  and an LCD panel  1070 . The touch panel  1050  comprises a first sensing layer  1051  and a second sensing layer  1052 . The touch panel  1050  is disposed on the LCD panel  1070 . The display controller  1030  receives an image signal and converts the image signal to a panel control signal to the LCD panel  1070 , so that the LCD panel  1070  displays images according to the panel control signal. When one touches the touch panel  1050 , the touch panel  1050  generates a sensing signal to the sensing circuit  1055 . The sensing circuit  1055  can output according to the sensing signal a position signal that represents the touch point where one touches the touch panel  1050 . The first sensing layer  1051  and the second sensing layer  1052  comprises a plurality of sensing components, and each component can be viewed as a capacitor. 
     The touch panel  1050  according to the present invention can eliminate the shielding layer needed in the prior art. According to the present invention, the panel control signal of the DC Vcom signal is applied by the LCD controller  1030  to the LCD panel  1070 , so as to provide good shielding effects that prevent the panel control signal from coupling to the sensing signal. Hence, the present invention offers advantages of providing a capacitive touch panel on the touch screen without adopting the shielding layer in a way that the position signal is still sensed correctly as well as reducing the manufacturing cost and increasing the transmittance of the screen. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not to be limited to the above embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.