Abstract:
A sensor includes a first electrode and a second, compressible electrode. A dielectric layer separates the first electrode from the second electrode. At least one of the first and second electrodes compress responsive to force, increasing capacitance between the first and second electrodes.

Description:
BACKGROUND 
       [0001]    Force sensors in touchscreens have typically utilized a compressible layer between two rigid electrodes. Force generated in touching the screen causes the compressible layer to compress, decreasing the distance between the two rigid electrodes. The decrease in distance results in an increase in capacitance between the two rigid electrodes, which is electrically sensed and representative of the force applied to the screen. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]      FIG. 1  is a block cross sectional diagram of a force sensor having a compressible electrode according to an example embodiment. 
           [0003]      FIG. 2  is a block cross sectional diagram of the force sensor of  FIG. 1  under pressure according to an example embodiment. 
           [0004]      FIG. 3  is a block schematic diagram of a force sensor layout utilizing a spare drive line for a touchscreen according to an example embodiment. 
           [0005]      FIG. 4  is a block top view diagram of a touchscreen device incorporating one or more force sensors according to an example embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0006]    In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description of example embodiments is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
         [0007]    A force detector is shown generally at  100  in  FIG. 1 . In one embodiment, force detector  100  may be located on an upper surface of front panel  110  of a touch panel. A first electrode  115  may be located beneath the front panel  110 , and may be a transmit electrode or a direct current planar structure in various embodiments. The first electrode  115  may be coupled or fixed to a substrate  120  in one embodiment. A second electrode  130  may be located between the substrate  120  and a support member  140 . 
         [0008]    In one embodiment, the second electrode  130  is compressible, such that when force is applied to the front panel  110  above and proximate to the second electrode  130 , the second electrode compresses as seen in  FIG. 2  at  230 , wherein the numbering is consistent with  FIG. 1 . Compression of the second electrode  130  essentially flattens it, increasing the relative surface area between the second electrode  130  and the first electrode  115 . This causes an increase in capacitance between the two electrodes, as capacitance is a function of the area of opposing plates or electrodes and the distance between them. By compressing the second electrode  130  as seen at  230 , both the area of the opposing electrodes is increased, and the average distance is decreased, as more of the second electrode  130  moves toward the substrate  120 . In one embodiment, second electrode  130  may be formed as a compressible element with a cross section that tapers to become narrower close to the first electrode. 
         [0009]    Second electrode  130  in one embodiment is formed of an elastic material that compresses in response to force, and rebounds to its original shape when the force is removed. It thus provides a varying area relative to the first electrode, corresponding to a measurable change in capacitance. In one embodiment, the second electrode is in the shape of a tube or cylinder, that provides sufficient resiliency to return to it original shape when the force from the front panel  110  is removed. In further embodiments, suitable springs may be interspersed about the second electrode to provide the force to return the electrode to its initial shape. The electrode may be retentatively attached to the substrate  120  and support member  140  to facilitates its return to the initial shape. 
         [0010]    In one embodiment, the second electrode  130  is formed as a thin conductive gasket type material in the shape of a tube, perhaps 1-2 mm in diameter. The tube may be formed of carbon impregnated compressible material or metal loaded or coated neoprene or other soft compressible material. Similar materials are typically used for EMC shielding. As the tube is compressed, its surface area and proximity to the first electrode  115  changes, allowing capacitive measurement proportional to the applied force to be made. 
         [0011]    In one embodiment, because the first electrode  115  faces upwards towards the touching/pressing object, and because the first electrode  115  is not itself touch sensitive, then the pressure exerted by the touching/pressing object is all that is detected by the force sensor, not the proximity of the touching/pressing object to the capacitive sensor. The first electrode  115  effectively acts to shield the lower compressible electrode  130  from capacitance changes caused by the proximity of the touching object. This has the advantage that the force sensor does not have to be further shielded from touch effects or placed further away from the touch in the mechanical arrangement. This allows far greater flexibility for the mechanical design and can allow a thinner overall assembly compared to existing solutions. It also allows for the first electrode  115  to be fabricated as part of a touch sensor that is sensitive to touch elsewhere over its upper area. In this way, for example, an XY touch sensor can be fabricated and on another area of the same sensor the first electrode of the force sensor created. This can be done using the same or different material from that used to form the touch sensor itself. The first electrode  115  may serve as a transmit electrode, and the second electrode  130  would then serve as a receive electrode. 
         [0012]    The increase in surface area is due to the compression of the electrode as seen at  230 , causing lateral expansion of the electrode  130 . As the electrode expands laterally, more of the electrode contacts the substrate  120 . The laterally expanded portions of the electrode  130  become more proximate to the first electrode  115 , thus increasing overall capacitance. 
         [0013]    In further embodiments, the positions of the first and second electrodes may be switched. Further layers may be included in further embodiments. 
         [0014]      FIG. 3  is a block schematic diagram of a force sensor layout  300  utilizing a spare drive line for a touchscreen according to an example embodiment. In layout  300 , several receive lines are illustrated at  310 , and labeled Y 0 , Y 1 , Y 2 , Y 3 , Y 4 , Y 5 , and Y 6 . Many of the receive lines may be coupled to an XY sensor indicated generally by broken line  315 . A spare receive line, Y 6 , may be coupled to a compressible electrode  325 . A capacitance sensing circuit  330  may be used for both the XY sensor  315  and compressible electrode  325 , or a separate sensing circuit may be used for the compressible electrode  325 , as the capacitance changes may or may not be consistent with those of the XY sensor. In one embodiment, circuit  330  also serves as a driver circuit for driving the first electrode  115  via drive lines indicated at  335 . 
         [0015]    For self capacitance embodiments, the first electrode  115  may be a conductive plane at a fixed DC potential, and the second compressible electrode  130  may be coupled to a self capacitance sensing circuit, such as circuit  330 . 
         [0016]      FIG. 4  is a block top view diagram of a touchscreen device  400  incorporating one or more force sensors according to an example embodiment. In one embodiment, a compressible electrode  415  may be disposed around the periphery of the touchscreen device  400  and in addition to acting as a force sensor, it may also provide a sealing function to keep moisture out of the touchscreen device  400 . Further compressible electrodes  420 ,  422  and  424  may be disposed in one or more positions about the touchscreen device  400  where desired. An XY touchscreen indicated by broken line  430  may also be included in various embodiments. In one embodiment, an XY touchscreen comprises transmit electrodes and receive electrodes. In further embodiments, compressible electrode force sensors may be integrated with other components on a touchscreen, and may utilize similar or spare capacitive sense channels.