Abstract:
There is described a method of forming a touch screen laminate for use in a resistive touch screen display, the method comprising the steps of bonding a layer of ultra-thin glass to a polymer layer by means of an optical adhesive applied uniformly between the glass layer and the polymer layer and applying a non-electrically conductive insulating layer to a lower surface of the polymer layer in a peripherally extending band.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to touch screen technology, and more particularly to resistive touch screen technology. 
       BACKGROUND TO THE INVENTION 
       [0002]    Of various interfaces available for interacting with a computer system one of the easiest to use and understand is the touch screen. This technology allows a user to simply touch an icon or picture to navigate through the system, display the information the user is seeking, and to enter data. For this reason this technology is widely used in many areas, including bank machines, information kiosks, restaurants, cars, etc. 
         [0003]    A number of different methodologies are used to implement touch screen technology, and each has advantages and disadvantages. The three main types of technology used are resistive, capacitive and surface acoustic wave. 
         [0004]    Resistive technology uses a flexible membrane that is affixed over a display. The membrane and display each have a conductive layer, and typically the membrane is energized with an electrical potential. When the membrane is touched, it is brought into contact with the conductive layer on the display, and this creates current flow. Various sensors around the display measure the current and a controller can determine, either through an absolute value or through a ratio with the current measured at other sensors, the location of the touch. One example of this technology is found in U.S. Pat. No. 4,220,815 to Gibson et al. 
         [0005]    One of the advantages of resistive touch screens is that they can be pressed by either a finger or a stylus. The technology responds to pressure and the pressure can be exerted by anything. This is important in some cases where a user may wish to press the screen with the back of a pen or other stylus, with fingernails or with gloved hands. 
         [0006]    A second advantage is that they are sealed and not affected by dirt. Thus they can for example be used in industrial applications where the user&#39;s hands may be greasy or dirty. Further, the touchscreen will work irrespective of whether there is dust or grime on the screen or in the area around the periphery of the screen. 
         [0007]    This technology will also continue to work even when scratches exist on the outer surface of the membrane. 
         [0008]    The main disadvantage of resistive touch screens to date has been the material from which the flexible membrane has been made. The requirement that the membrane be flexible and resistant to breakage has generally meant that polyester films have been used. The problem with these films is that they are easily scratched, torn and melted, and are thus susceptible to vandalism or inadvertent damage. This has generally limited the use of this technology to applications where access to the screens is restricted, and where the general public is not given access to these machines. For example, information kiosks in shopping malls or airports do not typically use resistive touch screens due to the vandalism potential. 
         [0009]    A second technology for touch screens is capacitive. In this technology a layer of glass is used as a dielectric, and typically has a sensor grid on its lower surface. The touch of a user creates a change in capacitance that can be measured by the sensor grid, allowing the controller to determine when and where a touch occurs. 
         [0010]    The advantage of capacitive touch screens is that their outer layer is glass, and thus more resistant to vandalism and damage. 
         [0011]    One disadvantage of capacitive touch screens is that they can be susceptible to electromagnetic interference, and can thus produce false hits. This interference can be caused by a number of things, but most commonly in public locations by cellular telephones and pagers. Due to this potential interference, capacitive touch screen cannot be used in certain applications such as in some military equipment. 
         [0012]    A second disadvantage is that the sensitivity of the screen can be affected by dirt and scratches. These change the capacitance that is sensed, and can create false touch signals. 
         [0013]    Another disadvantage is that skin must be used to make contact with the display. A stylus, fingernail or gloved hand will not produce a sensed touch. Further, in some cases dry hands may not create a sensed touch. 
         [0014]    A third technology that is used is the surface acoustic wave. In this technology ultra-sonic waves are emitted onto the surface of the screen, and microphones situated around the screen detect these waves. The periphery of the screen is generally reflective to the waves. When the screen is touched the waves are affected, and a controller is able to determine the location of the touch based on the information received by the microphones. 
         [0015]    The major problem with this technology is that it is susceptible to dust and dirt. Any particle will affect the waves. Further, when these types of screens are cleaned, the dirt may be pushed to the periphery, where it will affect the reflective surface. The result of the dirt is that a touch may be perceived to be in a different location than the actual touch location. 
         [0016]    What is therefore needed is a touchscreen technology that is robust, so that it can sense the touch of a finger, gloved hand, or any stylus. Further, the technology is required to be unaffected by dirt and scratches. Also, the outer touch surface must be hard and resistant to vandalism. 
       SUMMARY OF THE INVENTION 
       [0017]    The present invention overcomes the shortcomings of the prior art by providing a glass laminate resistive touchscreen. This presents the advantage of having the robustness of resistive touchscreen technologies but overcoming the difficulties of this technology by providing a surface that is resistant to scratching, cutting and burning, and thus is more difficult to vandalize. 
         [0018]    The laminate of the present invention includes an ultra-thin layer of glass to which a layer of polyester is adhered using an optical laminate material. The three layers are laminated to provide a uniformly transparent yet flexible surface that is resistant to cracking and virtually impossible to shatter. 
         [0019]    One of the problems found with this laminate when used with touch screens is that the different rates of thermal expansion of the various layers can cause rumples at the periphery of the polyester layer, which can cause false touch senses. The present invention also overcomes this difficulty by providing a mounting means that includes an elastic tensioner such as silicon rubber to provide an elastic force ensuring the polyester layer is always taut. 
         [0020]    In a broad aspect, then, the present invention relates to a flexible membrane for a resistive touch screen display, said flexible membrane comprising: a glass laminate, wherein said glass laminate consists of: an ultra-thin glass layer; a polymer layer; and an optical adhesive between said ultra-thin glass layer and said polymer layer, said optical adhesive holding said ultra-thin glass layer to said polymer layer. 
         [0021]    In a further broad aspect, the present invention relates to a touch screen having a flexible outer membrane with a first conducting surface, a backing surface with a second conductive surface, and sensors to detect contact between the first conducting surface and the second conducting surface, the improvement comprising: the flexible outer membrane, wherein the flexible outer layer consists of an ultra-thin glass layer; a polymer layer; and an optical adhesive between said ultra-thin glass layer and said polymer layer, said optical adhesive holding said ultra-thin glass layer to said polymer layer. 
         [0022]    In another broad aspect, the present invention relates to a resistive touch screen display, said display comprising: a flexible membrane, wherein said flexible membrane consists of: an ultra-thin glass layer; a polymer layer, said polymer layer being larger than said glass layer and said polymer layer extending beyond the periphery of said glass layer; and an optical adhesive between said ultra-thin glass layer and said polymer layer, said optical adhesive holding said ultra-thin glass layer to said polymer layer; a backing surface; a pressure sensitive adhesive affixed between the periphery of said polyester layer and said backing surface; an elastic tensioner affixed between the periphery of said polyester layer and said backing surface, said elastic tensioner being adjacent to said pressure sensitive adhesive; a first conductive layer affixed to said polyester layer; a second conductive layer affixed to said backing surface; and sensors used to detect where said first conductive layer contacts said second conductive layer. 
         [0023]    In yet another broad aspect, the present invention relates to a process for the creation of a flexible laminate membrane for a resistive touch screen, the flexible laminate membrane having a glass layer and a polyester layer, the process comprising the steps of: applying an optical adhesive to said glass layer; affixing a polyester layer over said optical adhesive; rolling said optical polyester layer from the center of said polyester layer outwards to remove excess optical adhesive and air bubbles; and pressing said polyester layer, glass layer and optical adhesive combination in a high pressure press to ensure a uniform level of optical adhesive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    For a more complete understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing in which: 
           [0025]      FIG. 1  shows a side elevational cross-sectional view of the glass-polyester laminate of the present invention; 
           [0026]      FIG. 2  shows a side elevational cross-sectional view of a one touch screen assembly using the laminate of  FIG. 1 , in which a false touch is present; 
           [0027]      FIG. 3  shows a side elevational cross-sectional view of one solution to the false touch problem of  FIG. 2 ; and 
           [0028]      FIG. 4  shows a side elevational cross-sectional view of a preferred embodiment of the touch screen assembly of the present invention which overcomes the false touch problem of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]    As discussed above, resistive touch screen technology would be the preferred technology for numerous applications, especially those in which the public needed to use touch screens. The robustness of this technology allows it to function regardless of dirt, dust, or electromagnetic signals. The screen can be touched by a bare hand, gloved hand, or stylus and still function. However, the main problem that needs to be overcome is the vulnerability of the soft upper touchscreen layer. 
         [0030]    It has been found by the inventor that a thin glass layer possesses enough flexibility to allow it to be used for touch screen applications. Glass useful for this purpose includes Schott Borofloat 263™ or Corning 0211™ and is generally about 0.5 mm thick although greater or lesser thicknesses are possible as long as the glass behaves like a film. Further, by having an outer glass layer, the problems of a soft polymer outer layer are overcome. Glass is much harder, and thus not susceptible to being cut or burned. It is also more resistant to scratches and general wear and thus its use increases the life of touch screens. 
         [0031]    The problem with ultra-thin glass however is that it is very brittle, and easily cracks and shatters with very minimal contact. Glass has therefore not been used previously for resistive touch screens. 
         [0032]    Reference is now made to  FIG. 1 . The inventor has found that the addition of a polymer substrate layer  30  laminated to the ultra-thin glass layer  20  using an optical adhesive  40  overcomes the brittleness of the glass. The creation of this laminate  10  makes it extremely difficult to crack glass layer  20 , and glass layer  20  can be bent and pressed without risk of breakage. Further, even if cracking does occur, polymer substrate  30  ensures that glass layer  20  does not shatter, and resistive touch screen laminate  10  remains intact and functional. 
         [0033]    In a preferred embodiment, polymer layer  30  of laminate  10  is a polyester, and will be referred to hereinafter as polyester layer  30 . One skilled in the art will however appreciate that other suitable polymers can be used. Polyester layer  30 , in the preferred embodiment, comprises a polyester film, also referred to in the art as PET, with a thickness of approximately 0.007 inches, or 0.175 mm. Suitable films include ICI Melnex™ or Dupont Clear Mylar™. However, the use of other films is contemplated, and in one embodiment it is envisioned that polyester layer  30  may even be opaque to provide a fixed graphic for the touch screen. 
         [0034]    In one embodiment of the invention, a conductive silver buss bar (not shown) may be used to help the transmission of current flow from polyester layer  30 . Such conductive layers are well known in the art and are typically applied using a silk screen process. 
         [0035]    However, it is also contemplated that no buss bar be used in an alternative embodiment, in which polyester layer  30  is used without such a bar. 
         [0036]    Polyester layer  30  and ultra-thin glass layer  20  are laminated together using a liquid or film optical adhesive  40 . One skilled in the art will realize that optical adhesive  40  forms a thin layer between polyester layer  30  and glass layer  20 , and that  FIGS. 1 to 4  show an exaggerated thickness for this layer for illustrative purposes only. 
         [0037]    Optical adhesive  40  is transparent and provides sufficient durability to hold the two layers  20  and  30  together. One suitable optical adhesive has been found to be Norland™ Optical Adhesive  61 . The skilled person will however realize that other suitable adhesives may be used. 
         [0038]    In applying adhesive  40 , it is aesthetically preferable to ensure that the adhesive is applied evenly and with no bubbles or gaps, creating a laminate  10  that is uniformly planar and transparent. This lamination process involves applying a relatively thick layer of optical glue between glass layer  20  and polyester layer  30 . The layer of glue must be thick enough to allow air bubbles to be squeezed out, which is much more difficult to do when thin layers of glue are applied. 
         [0039]    In practice, layers  20  and  30  are laminated together with glue, and a roller is used to squeeze out excess glue and air bubbles. The roller is preferably applied from the centre of laminate  10  and rolls towards the edges of the laminate. A wave of glue and air bubbles is thus propelled to the edges of laminate  10 , leaving a thin layer of glue with fewer or ideally no air bubble behind. 
         [0040]    After rolling, laminate  10  is placed between a pair of ¼″ (0.64 cm) thick steel plates, and the plates are actuated by a press to apply 5-10 tonnes of pressure to the laminate. More or less pressure may be applied as required. The primary purpose of the pressure is to evenly distribute the glue between glass layer  20  and polyester layer  30  to eliminate high and low spots. 
         [0041]    During the application of pressure, an absorbent medium such as tissue is placed between the laminate and the steel plates to protect the laminate and absorb the excess glue that is squeezed out. At the end of the lamination process, the thickness of the glue is preferably limited to 0.001-0.002 inches (0.025-0.05 mm). 
         [0042]    Reference is now made to  FIG. 2 . Laminate  10  is typically made with lower polyester layer  30  being larger than upper glass layer  20 . By creating a larger lower surface the laminate is easier to make. 
         [0043]    Optical adhesive  40  also preferably extends beyond the edges of glass layer  20  and is allowed to build up slightly about the edges of glass layer  20 . This locks glass layer  20  in place and makes it harder to move or separate from polymer layer  30 . The buildup of optical adhesive  40  also prevents microfractures in the glass caused by cutting from propagating into larger fractures. 
         [0044]    Experimenting with the laminate, the inventor has found that a problem can arise due to the different thermal expansion rates of lower polyester layer  30 , adhesive  40  and upper glass layer  20 . Polyester layer  30  and adhesive  40  have similar expansion rates, but glass layer  20  and polyester layer  30  have very different expansion rates, polyester layer  30  having a higher expansion rate than glass layer  20 . 
         [0045]    When applied to a touch screen display  50  these expansion rates can create false touches or shorts  35  between touch screen laminate  10  and the backing display layer  70 . This happens when touch screen display  50  is exposed to different temperature extremes. When it is cold, polyester layer  30  will shrink. 
         [0046]    Touch screen membranes are typically mounted to the display layer  70  using a pressure sensitive adhesive  60  along the periphery of the outer touch screen layer. This adhesive  60  has a bubble-gum like texture and is not elastic. 
         [0047]    When polyester layer  30  shrinks when exposed to cold, pressure sensitive adhesive  60  stretches to allow the polyester layer  30  to contract. The touch screen display  50  will still function at this point. However, when touch screen display  50  is warmed up again, polyester layer  30  will expand, and since pressure sensitive adhesive  60  is not elastic, the polyester will tend to rumple between pressure sensitive adhesive  60  and spacer dots  80  used to maintain a normal spacing between the conductive coating applied to the lower surface of layer  30  and the upper surface of display layer  70 , as illustrated by false short  35 . While not illustrated, one skilled in the art will realize that spacer dots  80  can be affixed to either polyester layer  30  or display layer  70 . 
         [0048]    Glass layer  20  tends to keep the remainder of polyester layer  30  flat, and thus the expansion will be reflected completely or at least primarily along the edge of glass layer  20 . In the prior art, the completely polymer touch screen would distribute this expansion evenly. However, due to adhesive  40  and glass layer  20 , this does not occur in laminate  10 , and the problem of false touches is increased in those cases in which the screens are exposed to temperature extremes. 
         [0049]    Reference is now made to  FIG. 3 . One possible solution to the above problem is to expand glass layer  20  to the edges of polyester layer  30 . This would ensure that polyester layer  30  remains flat against glass layer  20 , to limit or prevent false touches. 
         [0050]    A possible problem with this solution is that adhesive  40  may fail due to repeated expansion or contraction of polyester layer  30  without the outer expansion area shown in  FIG. 2 . In the solution of  FIG. 3 , adhesive layer  40  absorbs all of the stress induced by the differing expansion rates of the glass and polyester. Eventually it is envisioned that optical adhesive  40  could fail and separation of glass layer  20  and polyester layer  30  could occur. 
         [0051]    A preferred solution to the above problem is illustrated in  FIG. 4 . In this embodiment, [ ] polyester layer  30  is larger than glass layer  20 , thus still permitting ease of manufacture. It also allows optical adhesive  40  to be built up about the edges of glass layer  20  to better hold glass layer  20  to polyester layer  30 . 
         [0052]    In order to overcome the false touch problem, an elastic tensioner  110  is added to touch screen display  50  to circumscribe adhesive  60 . Further, an active area insulator  120  is added between polyester layer  30  and elastic tensioner  110 . 
         [0053]    Elastic tensioner  110  preferably comprises silicon rubber. In operation, elastic tensioner  110  creates an elastic force that normally biases or stretches polyester layer  30  outwards. Therefore, if display  50  becomes very cold, polyester layer  50  will shrink, pulling pressure sensitive adhesive  60  inwards, along with elastic tensioner  110 . When the display  50  is later warmed, elastic tensioner  110  pulls polyester layer  30  back to its original configuration, reducing the possibility of rumples, and thus false touches. 
         [0054]    Area insulator  120  further aids in preventing a false short  35  by providing a non-conductive layer in the area most likely to make false contact. Area insulator  120  comprises an ultraviolet ink film printed onto the lower surface of the polyester layer  30  along its outer edges. As one skilled in the art will appreciate, the thickness of area insulator  120  in  FIG. 4  has been exaggerated for illustrative purposes, and in practice area insulator  120  adds no significant spacing between polyester layer  30  and display layer  70 . 
         [0055]    Area insulator  120  reduces the chances of electrical contact between polyester layer  30  and display layer  70 . It has been found that pressure sensitive adhesive  60  is insufficient for this purpose. 
         [0056]    Area insulator  120  bonds aggressively, perhaps covalently, to polyester layer  30 , and thus pressure sensitive adhesive  60  and elastic tensioner  110  are essentially bonded to polyester layer  30  itself. 
         [0057]    One skilled in the art will realize that the embodiments illustrated in  FIGS. 2 and 3  will typically also have an area insulator layer  120  between polyester layer  30  and pressure sensitive adhesive  60 . 
         [0058]    When combined, the above configuration provides a resistive touch screen with an outer glass layer, overcoming the difficulties of the prior art. The above configuration further provides a means to compensate for the different thermal expansion rates of the different materials of the laminate. 
         [0059]    Although the present invention has been described in detail with regard to the preferred embodiment thereof, one skilled in the art will easily realize that other versions are possible, and that the invention is only intended to be limited in scope by the following claims.