Patent Publication Number: US-2009237374-A1

Title: Transparent pressure sensor and method for using

Description:
FIELD 
     The present invention generally relates to electronic devices and more particularly to a transparent pressure sensor. 
     BACKGROUND 
     The market for electronic devices having displays, for example, televisions, computer monitors, cell phones, personal digital assistants (PDA&#39;s), digital cameras, and music playback devices (MP3), is very competitive. Manufactures are constantly improving their product with each model in an attempt to cut costs and production requirements. 
     In many electronic devices, such as portable communication devices, touch panel displays (touch screen) present information to a user and also receive input from the user. A touch screen offers intuitive inputting for a computer or other data processing devices. It is especially useful in portable communication devices where other input devices, such as a keyboard and a mouse, are not easily available. 
     There are many different types of touch sensing technologies, including capacitive, resistive, infrared, and surface acoustic wave. All of these technologies sense the position of touches on a screen. However, they do not respond to the pressure that is applied against the touch screen. 
     It has been previously been disclosed in U.S. Pat. No. 6,492,979 to use a combination of capacitive touch screen and force sensors to prevent false touch. This disclosure however complicates the sensor interface and can not sense different touch forces at the same time. It has also been proposed in U.S. Pat. No. 7,196,694 to use force sensors at the peripherals of the touch screen to determine the position of a touch. This however does not offer a capability of multi-touch. It has also been proposed in US patent publication 2007/0229464 to use a capacitive force sensor array, overlaying a display to form a touch screen. This approach offers multi-touch capability; however, a capacitive pressure sensor has limited spatial resolution. It also is subject to environmental interferences such as EMI and capacitive coupling of fingers and other input devices. 
     Accordingly, it is desirable to provide a transparent pressure sensor to form a force sensing touch screen. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a cross section of a transparent piezoresistive material in accordance with the exemplary embodiment; 
         FIG. 2  is a cross section of the transparent piezoresistive material of  FIG. 1  that is subjected to pressure; 
         FIG. 3  is a partial cross section of an intersection of conductive traces of the exemplary embodiment including a transparent piezoresistive layer; 
         FIG. 4  is a perspective view of the exemplary embodiment of  FIG. 3 ; 
         FIG. 5  is a partial cross section of an intersection of conductive traces of another exemplary embodiment including a patterned transparent piezoresistive layer; and 
         FIG. 6  is as block diagram of a device incorporating the exemplary embodiments; 
         FIG. 7  is a flow chart of a first exemplary method of use of the exemplary embodiments; 
         FIG. 8  is a flow chart of a second exemplary method of use of the exemplary embodiments; and 
         FIG. 9  is a flow chart of a third exemplary method of use of the exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
     A transparent pressure sensing material includes a transparent matrix including, for example, at least one polymer material, and a plurality of transparent conducting particles dispersed in the transparent matrix. The polymer material may comprise, for example, phenoxy resin, polyester, silicone rubber, or polyimide. The transparent conducting particles may be, for example, indium tin oxide, zinc oxide, or tin oxide. The transparent conducting particles dispersed in the transparent matrix preferably have a dimension less than the wavelength of light in the visible range to minimize light scattering. 
     A transparent pressure sensor is formed by applying transparent conducting electrodes to the opposite surfaces of the piezoresistive material. When pressure is applied against the sensor, the resistance across the electrodes decreases and is measured through the electrodes. This change in resistance is then converted into pressure changes. 
     This piezoresistive material may be used in many types of devices, including touch screens, and any other force sensing applications which require underneath features being visible so the transparent nature of the force sensing material is desired. One example would be applications in polishing process where a transparent force sensing device can be applied to a transparent wafer. In this fashion, not only the pressure can be mapped across the wafer, the contact points between the wafer and the polishing pad can be directly observed. 
     There are many different types of touch sensing technologies, including capacitive, resistive, infrared, and surface acoustic wave. All of these technologies sense the position of touches on a screen. However, it is desirable to have a touch sensing device that not only senses the position of the touch, but also the force applied to the touch screen. Force sensing provides an extra dimension of freedom in inputting: it can simplify the input process by enabling different combinations of positions and forces on a touch screen. It also offers the possibility of discriminating against false touches by setting different force thresholds before a touch can register. An additional advantage is that force sensing is not limited to only finger touch as in the case of capacitive sensing, it also accept input from almost all other devices including stylus, glove, and credit cards. It is also more tolerant to environmental noises such as EMI and dirt/oil on surface. 
     The touch screen sensor described herein is formed on a transparent substrate, comprising glass or a polymer, for example. A layer of first patterned conductive traces are deposited over the substrate. A layer of second patterned conductive traces are deposited over the layer of first patterned conductive traces to form an array of addressable intersects (pixels). Scan and read signals are sent and received through tab connectors attached to each of the first and second patterned conductive traces. A piezoresitive material is deposited between the first and second patterned conductive traces at the intersect of each first and second conductive traces. The piezoresistive material may be a continuous layer or may be patterned to be positioned only at the intersects and preferably has a transparent elastomeric matrix, such as polyester, phenoxy resin, or silicone rubber. Transparent conductive or semiconductive particles such as indium tin oxide, zinc oxide, or tin oxide dispersed within the matrix. The dimensions of these particles are smaller than the wavelength of visible light so that scattering of light passing through the matrix is minimum. 
     The resistance at each intersect is controlled by the pressure applied at that intersect. Current flows through the piezoresistive material and through the particles, either directly when the particles are in contact with each other, or by tunneling when the particles are separated by a very small distance. When pressure is applied to the material, it deforms and shortens the tunneling distance between the particles as well as the conductive path of current flow, thus lowering the resistance. 
     By scanning the rows and columns of the conductive traces and mapping the resistance of the piezoresistive materials at each intersection, a corresponding pressure map of the touch screen may be obtained. This map provides both the position and the force of the corresponding touch. The touch screen sensor is also multitouch capable. When multiple fingers or objects are placed on the screen, each individual position and force can be distinguished, thus enabling greater freedom of inputting. 
     Referring to  FIG. 1 , a transparent matrix  100  includes a material  102  including at least one polymer. For example, the material  102  may comprise a transparent elastomeric matrix such as polyester, phenoxy resin, polyimide, or silicone rubber. Transparent conductive or semiconductive particles  104  such as indium tin oxide, zinc oxide, or tin oxide dispersed within the material  102 . 
     This transparent matrix  100  may be used as a pressure sensor in many electronic applications. When pressure is applied to the transparent matrix  100  in a direction  106  ( FIG. 2 ), the matrix  100  is compressed, reducing the distance between adjacent particles  104  as well as the conductive path between electrodes (not shown), thereby lowering the resistance. Current flows through the material  102  and through the particles  104 , either directly through the particles  104  when the particles  104  are in contact with each other, or by tunneling through the material  102  when the particles  104  are separated by a very small distance. 
     Referring to  FIGS. 3 and 4 , a transparent pressure sensor  300  includes a transparent substrate  302  preferably is a rigid material of, for example, glass or a polymer, but may be a flexible material. A patterned layer  304  of transparent conductive traces  305  is deposited on the substrate  302 . The traces  305  are preferably aligned in a first direction and have a pitch of 0.05-10 mm, (preferably 1.0 mm), a width less than the pitch but larger than 0.001 mm, a thickness of 1.0-1000 nm, (preferably 80 nm). The transparent traces  305  may be a transparent conductive oxide, for example, indium tin oxide, zinc oxide, and tin oxide. A tab  306  is electrically coupled to each trace for providing connection to other circuitry as is known in the industry. 
     Transparent matrix  100  is disposed on the traces  305  as a layer or in a predetermined pattern. The transparent material  102  preferably is a transparent elastomeric matrix such as polyester, phenoxy resin, or silicone rubber. Transparent conductive or semiconductive particles  104  such as indium tin oxide, zinc oxide, or tin oxide dispersed within the matrix  110  as discussed above. 
     A patterned layer  312  of transparent conductive traces  313  is deposited over the layer  308  of the transparent matrix  100 . The placement of the transparent conductive traces  313  creates a plurality of intersections, each including one of the transparent conductive traces  313 , the transparent matrix  100  and the transparent conductive traces  305  ( FIG. 4 ). The layer  308  may be patterned to form a plurality of islands  502 , with each island formed between an intersect of the transparent conductive traces  305  and  313  ( FIG. 5 ). An optional layer  314  of a transparent protective material, such as glass or a polymer, is disposed over the patterned layer  312 . 
     When pressure is applied to the transparent matrix  100  by applying pressure to the layer  314 , the matrix  100  is compressed, reducing the distance between adjacent particles  104  as well as the conductive path, thereby lowering the resistance between conductive traces  305  and  313 . Current flows through the matrix  100  and through the particles  104 , either directly when the particles  104  are in contact with each other, or by tunneling through the matrix  100  when the particles  104  are separated by a very small distance. 
     By being able to sense this change in resistance due to pressure being applied to the transparent pressure sensor  300 , the selection of modes, or functions, may be accomplished. This selection of modes by applying pressure may be accomplished alone or in combination with a conventional imaging device  301 , for example a liquid crystal display. Those skilled in the art will appreciate that other types of imaging devices  301  may be utilized as exemplary embodiments, including, for example, transmissive, reflective or transflective liquid crystal displays, cathode ray tubes, micromirror arrays, and printed panels. 
     While the transparent pressure device described herein may be used in electronic devices in general, a block diagram of a force imaging system  600  as an example using the transparent pressure sensor is depicted in  FIG. 6 . A touch screen controller  606  provides drive signals  610  to a force sensing touch screen  602 , and a sense signal  604  is provided from the force sensing touch screen  602  to the touch screen controller  606 , which periodically provides a signal  608  of the distribution of pressure to a processor  612 . The processor interprets the controller signal  608 , determines a function in response thereto, and provides a display signal  614  to a display  616  (display  103  in  FIG. 3 ). 
     A first exemplary embodiment, shown in  FIG. 7 , includes determining  702  if pressure applied to a specific location on the pressure sensor of an electronic device exceeds a threshold and enabling  704  a function of the electronic device if the threshold is exceeded. This prevents inadvertent light pressure, such as imparted by touching clothing, from enabling the function. For example, in phone dialing mode, when the finger lightly touches on a key shown in the display, the system will sense the touch, but only when the touch force exceeds a preset value, and the system will trigger the dialing action. 
     In a second exemplary embodiment ( FIG. 8 ), a determination  802  is made if pressure applied to the pressure sensor exceeds a first or a second threshold, and enabling  804  a first function if the pressure exceeds only the first threshold and a second function if the pressure exceeds the first and second thresholds. For example, when the force exceeds first threshold, one can select and move an object across the screen and drop the object when the force exceeds second threshold (drag and drop). Another example is, in the game mode (car race game), one can drive the car at different speed by controlling the press force at different threshold values. 
     In a third exemplary embodiment ( FIG. 9 ), a determination  902  is made of whether pressure is applied beyond a threshold at two different locations, thereby reducing the resistance at two regions of intersections, and enabling  904  a function if the pressure at both of the two regions exceed the threshold. For example, two different objects, such as pictures and windows, can be selected simultaneously for alignments or operations. This example may be expanded by applying different levels of pressure at the two locations to select additional functions. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.