Patent Document

This application is a divisional of U.S. patent application Ser. No. 10/834,361, filed Apr. 29, 2004, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to touch systems and in particular to a tensioned touch panel and method of making the same 
     BACKGROUND OF THE INVENTION 
     Touch panels such as for example digitizers and analog resistive touch screens that make use of one or more tensioned membranes, are known in the art. Tensioned touch panels of this nature typically include a conductive membrane that is stretched tautly over and spaced from a conductive substrate. When a pointer is used to contact the tensioned membrane with sufficient activation force, the tensioned membrane deflects and contacts the conductive substrate thereby to make an electrical contact. Determining voltage changes induced by the electrical contact allows the position of pointer contact on the tensioned touch panel to be determined. 
     In order for such tensioned touch panels to work effectively, the spacing between the tensioned membrane and the conductive substrate must be maintained so that the tensioned membrane only contacts the conductive substrate when a pointer contact is made on the tensioned membrane. 
     As will be appreciated, over time the tensioned membrane may sag creating slack in the tensioned membrane. Changes in environmental conditions such as humidity and/or temperature may also cause the tensioned membrane to expand resulting in slack developing in the tensioned membrane. If the tensioned membrane sags or expands, the slack developed in the tensioned membrane may result in undesirable contact between the tensioned membrane and the conductive substrate. This problem becomes more severe as the size of the touch panel becomes greater. 
     A number of techniques have been considered to avoid undesirable contact between the tensioned membrane and the conductive substrate. For example, electrically insulating spacer dots may be disposed between the tensioned membrane and the conductive substrate at spaced locations over the active contact area of the touch panel to maintain the spacing between the tensioned membrane and the conductive substrate. U.S. Pat. No. 5,220,136 to Kent discloses a contact touchscreen including such insulating spacer dots. 
     Although the use of insulating spacer dots maintains separation between the tensioned membrane and the conductive substrate, the use of insulating spacer dots is problematic. In order to maintain separation between the tensioned membrane and the conductive substrate over the active contact area of the touch panel, the insulating spacer dots must be positioned at locations within the active contact area. Thus, the insulating spacer dots interrupt the active contact area of the touch panel. As a result, contacts with the tensioned membrane over insulating spacer dots will not register as contacts since the tensioned membrane cannot be brought into electrical contact with the conductive substrate at those contact points. Also, the use of insulating spacer dots to separate the tensioned membrane and the conductive substrate is expensive. It is also difficult to maintain an even spacing between the tensioned membrane and the conductive substrate over the active contact area using insulating spacer dots. 
     U.S. Pat. No. 5,838,309 to Robsky et al. discloses a self-tensioning membrane touch screen that avoids the need for insulating spacer dots. The touch screen includes a support structure having a base and a substrate support on which a conductive surface is disposed. A peripheral insulating rail surrounds the conductive surface. A peripheral flexible wall extends upwardly from the base. A conductive membrane is stretched over the conductive surface and is attached to the peripheral flexible wall. The insulating rail acts to space the conductive membrane from the conductive surface. To inhibit sagging and maintain tension on the conductive membrane, during assembly of the touch screen the conductive membrane is attached to the flexible wall when the flexible wall is in a pretensioned state. In the assembled condition, the flexible wall is biased outwardly and downwardly. As a result, tension is continuously applied to the conductive membrane by the flexible wall thereby to inhibit sagging of the conductive membrane. 
     U.S. Pat. No. 6,664,950 to Blanchard discloses a resistive touch panel having a removable, tensioned top layer and a base plate. The touch panel may be situated relative to a display screen such that an air gap exists between the base plate and the display screen. The top plate includes a transparent, flexible substrate having a hard transparent coating, one or more anti-reflective coatings and an anti-fingerprint coating thereon. The underside of the substrate is spaced from the upper surface of the base plate by an air gap. To prevent wrinkling of the top plate, a stiff frame is bonded to the anti-fingerprint coating. The stiff frame maintains tension in the top plate despite temperature changes. 
     Although the above references show touch panels having mechanisms to maintain tension in the conductive membrane, manufacturing and labour costs are associated with these tensioning mechanisms. Accordingly, improvements in tensioned touch panels to maintain the spacing between the tensioned membrane and the conductive substrate are desired. 
     It is therefore an object of the present invention to provide a novel tensioned touch panel and method of making the same. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a method of assembling a touch panel including a support structure and a conductive membrane. The support structure has a conductive surface and a peripheral insulating spacer about the conductive surface. The conductive membrane overlies the support structure with the spacer separating the conductive membrane and the conductive surface thereby to define an air gap therebetween. During the method, the conductive membrane is pretensioned and the tensioned conductive membrane is secured to the support structure. 
     The pretensioning in one embodiment is selected to compensate for both the coefficients of thermal expansion and hydroscopic or hygroscopic expansion of the conductive membrane over a variety of temperature and humidity conditions. The stress level is selected to be below the yield point of the conductive membrane and at a level below which the conductive membrane exhibits significant creep i.e. creep where the tension in the conductive membrane drops over time to a level resulting in an unacceptable decrease in activation force and/or unwanted contact between the conductive membrane and the conductive surface. The conductive membrane is bonded to the support structure via an adhesive such as for example an ultraviolet curing or cyanoacrylate (CA) adhesive. 
     The support structure includes a generally planar surface on which the conductive surface is disposed. The spacer is generally continuous and overlies the peripheral region of the planar surface thereby to surround the conductive surface. The conductive membrane may be adhered directly to the spacer or pulled around the spacer and adhered to the support structure. 
     According to another aspect of the present invention there is provided a tensioned touch panel comprising a support structure including a substrate having a generally planar conductive surface disposed thereon and an insulating spacer generally about the periphery of the substrate. A pretensioned conductive membrane overlies the support structure. The spacer separates the conductive membrane and the conductive surface thereby to define an air gap therebetween. The conductive membrane is secured to the support structure under sufficient tension to inhibit slack from developing in the conductive membrane as a result of changes in environmental conditions. 
     According to yet another aspect of the present invention there is provided a tensioned touch panel comprising a support structure having a conductive surface disposed thereon. A conductive membrane overlies the conductive surface in spaced apart relation. The conductive membrane is permanently secured to the substrate while under tension. 
     The present invention provides advantages in that an overall uniform tension can be maintained in the conductive membrane while reducing manufacturing and labour costs of the tensioned touch panel. As a result, slack is inhibited from developing in the conductive membrane regardless of environmental conditions while maintaining activation forces at user acceptable levels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which: 
         FIG. 1  is a side cross-sectional view of a tensioned touch panel; 
         FIG. 2  is an enlarged portion of  FIG. 1 ; 
         FIG. 3  shows steps performed during assembly of the tensioned touch panel of  FIG. 1 ; 
         FIG. 4  is a graph showing the stress versus strain characteristics of a sample length of a conductive membrane and the theoretical stress versus strain characteristics of the conductive membrane film; 
         FIG. 5  is a graph showing the theoretical strain versus activation force characteristics of the conductive membrane; 
         FIG. 6  is a graph showing the creep characteristics of the conductive membrane; 
         FIG. 7  is a graph showing cyclical elongation versus time characteristics of the conductive membrane film when subjected to alternating tensions of 8500 psi and zero psi respectively; 
         FIG. 8  is a front plan view of the tensioned touch panel of  FIG. 1  in an interactive display system; 
         FIG. 9  is a side cross-sectional view of another embodiment of a tensioned touch panel; 
         FIG. 10  is a side cross-sectional view of yet another embodiment of a tensioned touch panel; 
         FIG. 11  is a side cross-sectional view of yet another embodiment of a tensioned touch panel; 
         FIG. 12  is a side cross-sectional view of still yet another embodiment of a tensioned touch panel; and 
         FIG. 13  is a side cross-sectional view of still yet another embodiment of a tensioned touch panel. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to  FIGS. 1 and 2 , a tensioned touch panel is shown and is generally identified by reference numeral  10 . Touch panel  10  in this embodiment is generally rectangular and includes a support structure  12 . Support structure  12  comprises a substrate  14  having a major top surface  16 , a major bottom surface  18  and sides  20  bridging the top and bottom surfaces. The substrate  14  is formed of a rigid stable material such as for example aluminum or other suitable rigid material. A conductive carbon resistive layer  22  is bonded to the top surface  16  of the substrate  14  via an adhesive. A peripheral insulating spacer rail  30  is disposed on the top surface  16  of the substrate  14 . The insulating spacer rail  30  is formed of electrically insulating material such as for example rigid polyvinyl chloride (RPVC), acrylonitrile butadiene styrene (ABS), acrylic, fiberglass reinforced plastic (FRP) or coated aluminum and is bonded to the substrate  14  via an adhesive. 
     A flexible, elastic conductive membrane  40  under tension overlies the support structure  12  and is secured to the insulating spacer rail  30  by a fast drying adhesive such as for example, ultraviolet curing or cyanoacrylate (CA) adhesive. The conductive membrane  40  is layered and includes an upper flexible, low creep film  44  such as for example polyethylene terephthalate (PET) and a lower conductive carbon resistive layer  46  bonded to the film  44  by adhesive. The conductive resistive layer  46  overlies the film  44  in the region corresponding to the active area of the tensioned touch panel  10 . Thus, a peripheral region  44   a  of the film is free of the conductive resistive layer  46  allowing the film  44  to be adhered directly to the insulating spacer rail  30 . 
     The tension applied to the conductive membrane  40  maintains separation between the conductive membrane  40  and the conductive resistive layer  22  on the top surface  16  of the substrate to define an air gap  48 . In particular, the tension applied to the conductive membrane  40  before being bonded to the insulating spacer rail  30  is selected to ensure that the air gap  48  is maintained over a significant length of time and over a variety of environmental conditions without significantly increasing the activation force required to bring the conductive resistive layers  22  and  46  into electrical contact in response to a contact made on the tensioned touch panel  10 . In this manner, slack does not develop in the conductive membrane  40  making the tensioned touch panel  10  robust. 
     The tensioned touch panel  10  operates in a manner similar to conventional touch panels. When a pointer is used to contact the tensioned conductive membrane  40  with sufficient activation force, the conductive resistive layer  46  is brought into contact with the conductive resistive layer  22  at the contact location. Voltage changes induced by the electrical contact between the conductive resistive layers  22  and  46  are sensed allowing the position of the pointer contact to be determined. 
     Turning to  FIG. 3 , the steps performed during assembly of the tensioned touch panel  10  are illustrated. The conductive membrane  40  is initially placed in an assembly fixture and is stretched in both its lengthwise and widthwise directions to place the conductive membrane  40  under tension. During stretching of the conductive membrane  40 , the conductive membrane  40  is subjected to stress generally in the range of from about 1000 to 1500 pounds per square inch (psi). It has been found that this pretensioning of the conductive membrane  40  is sufficient to ensure effective operation of the tensioned touch panel  10  over a variety of environmental conditions while maintaining the activation force required to bring the conductive resistive layers  22  and  46  into electrical contact at user acceptable levels. With the conductive membrane  40  under the desired amount of tension, adhesive is placed on the peripheral region  44   a  of the film  44  that is free of the conductive resistive layer  46  in a pattern corresponding to the insulating spacer rail  30 . Alternatively the adhesive may be placed on the insulating spacer rail  30  or on both the peripheral region  44   a  of the film  44  and the insulating spacer rail  30 . The support structure  12  is then brought into contact with the tensioned conductive member  40  to enable a secure bond to be formed between the insulating spacer rail  30  and the tensioned conductive membrane  40 . Once the adhesive cures, the conductive membrane  40  is released from the assembly fixture. Excess length of conductive membrane  40  extending beyond the insulating spacer rail  30  is removed. 
     As mentioned above, the tension applied to the conductive membrane  40  is selected to inhibit slack from developing in the conductive membrane by using the conductive membrane  40  itself as the means of maintaining tension. The end result is a highly reliable, robust touch panel  10  that can be easily manufactured in a low cost manner. In particular, the tension applied to the conductive membrane  40  prior to attachment to the insulating spacer rail  30  is selected to compensate for the coefficient of thermal expansion (CTE) and the coefficient of hydroscopic or hygroscopic expansion (CHE) of the conductive membrane  40  without exceeding the yield point of the conductive membrane  40  and while maintaining the activation force at user acceptable levels. By tensioning the conductive membrane  40  in this manner, the conductive membrane  40  remains wrinkle free throughout a wide range of temperature and humidity conditions while ensuring that an adequate, but not excessive, activation force is required to bring the conductive resistive layers  22  and  46  into contact in response to a contact made on the tensioned touch panel  10 . The tension of the conductive membrane  40  simply reduces or increases depending on the temperature and humidity conditions while remaining wrinkle free. 
     A number of tests were performed on the conductive membrane  40  to ensure its suitability. During testing, the effect of the conductive resistive layer  46  on the film  44  was assumed to be negligible to the overall characteristics of the conductive membrane  40  since the conductive resistive layer  46  and bonding adhesive are both very thin and pliable as compared to the film  44 . It was also assumed that the conductive membrane  40  behaves in a linear fashion with respect to CTE and CHE and that the activation force is a linear function of tension applied to the conductive membrane  40 . Creep of the conductive membrane  40  was not considered to be a critical factor at the level of tension applied to the conducive membrane  40  during assembly of the touch panel  10 . The conductive membrane  40  was also assumed to behave the same in both the lengthwise and widthwise directions. 
     Table 1 below shows the amount of elongation of a sample length of the conductive membrane  40  for various stresses applied to the conductive membrane sample. 
                                                           TABLE 1                           Cross sectional area of sample   0.14125   Inches{circumflex over ( )}2           Sample length   41.375   Inches                            Force   Sample elongation   stress   %           (lbs)   (inches)   (psi)   elongation                       0.0   0   0   0.00000           11.8   0.004   84   0.00967           16.0   0.008   113   0.01934           22.0   0.011   156   0.02659           32.4   0.016   229   0.03867           50.4   0.021   357   0.05076           73.0   0.033   517   0.07976           86.0   0.036   609   0.08701           100.8   0.045   714   0.10876           119.4   0.052   845   0.12568           136.0   0.06   963   0.14502           155.8   0.067   1103   0.16193           169.5   0.072   1200   0.17402           178.6   0.076   1264   0.18369           187.0   0.078   1324   0.18852           189.0   0.079   1338   0.19094           199.0   0.086   1409   0.20785                          FIG. 4  is a graph showing the stress versus strain data of Table 1 together with the theoretical stress versus strain characteristics of the film  44 . As will be appreciated, the behaviour of the conductive membrane  40  corresponds very well with the theoretical stress versus strain data.
 
     The relative change in dimension between the conductive membrane  40  and the support structure  12  at a variety of environmental conditions were calculated for a tensioned touch panel  10  having an active contact area 60 inches in length and 48 inches in width. For the purpose of these calculation, the following assumptions were made: 
     Conductive membrane CTE: 0.000017 in/in/° C. 
     Conductive membrane CHE: 0.00006 in/in/% RH 
     Support structure CTE: 0.0000237 in/in/° C. 
     Temperature during assembly: 21° C. 
     Humidity during assembly: 44% 
     Based on the above assumptions and looking at the longest dimension of the conductive membrane  40  where changes are greater than in the shorter dimension, the change in the size of the conductive membrane  40  for each 1° C. increase in temperature above the assembly temperature can be calculated as follows:
 
0.000017″/″/°*60″*1°=0.00102″
 
     The change in size of the support structure  12  for each 1° C. increase in temperature above the assembly temperature can be calculated as follows:
 
0.0000237″/″/°*60″*1°=0.00142″
 
     The change in size of the conductive membrane  40  for each 1% increase in relative humidity (RH) above the assembly humidity can be calculated as follows:
 
0.000006″/″/% *60″*1%=0.00036″
 
     The effects of the CTE and CHE are cumulative for the conductive membrane  40 , so for a 1° C. temperature increase and a 1% increase in relative humidity, the net change in size of the conductive membrane  40  is:
 
0.00102″+0.00036″=0.00138″
 
     The relative change in size between the conductive membrane  40  and the support structure  12  for a 1° C. temperature increase and a 1% increase in relative humidity above the assembly conditions is therefore:
 
0.00138″−0.00142″=−0.00004″
 
     The negative number indicates that the conductive membrane  40  grew less than the support structure  12 . Since the conductive membrane  40  is rigidly and permanently bonded to the support structure  12 , the conductive membrane  40  was stretched by the support structure  12  an amount equal to 0.00004″. 
     An interactive analysis of the effects of temperature and humidity was performed using the above calculations to allow the changes in size of the conductive membrane to be calculated over a variety of environmental conditions differing from assembly conditions. For example, consider the following assembly and in service conditions where the in service conditions represent a typical office environment:
     Assembly conditions: 20° C.@40% RH   In service conditions: 23° C.@60% RH   In these in service conditions, the size of the conductive membrane  40  would increase by 0.006″.   

     Consider more severe in service conditions that may represent a shipping environment:
     Assembly conditions: 20° C.@35% RH   In service conditions: 50° C.@95% RH   In these in service conditions, the size of the conductive membrane  40  would increase by 0.009″.   

     Consider opposite end extreme in service conditions that may also represent a shipping environment:
     Assembly conditions: 20° C.@35% RH   In service conditions: −40° C.@15% RH   In these in service conditions, the size of the conductive membrane would increase by 0.016″.   

     During assembly of the touch panel  10 , the conductive membrane  40  is stretched by more than the above calculated amounts prior to being attached to the insulating spacer rail  30  of the support structure  12 . As a result, changes in environmental conditions causing the conductive membrane  40  to expand do not create slack in the conductive membrane  40 . Rather these environmental changes affect the tension, or stress in the conductive membrane  40  and therefore, simply alter the activation force. Since the activation force generated by a certain strain is known, the activation force can be plotted as a line as shown in  FIG. 5 . 
     Line  60  in the graph of  FIG. 5  shows the theoretical relationship between strain of the conductive membrane  40  and the resulting activation force. The intersection point of line  62  and line  60  represents the activation force required to bring the conductive resistive layers  22  and  46  into electrical contact at assembly conditions of 21° C.@44% RH. The intersection point of line  64  and line  60  represents the activation force required to bring the conductive resistive layers  22  an  46  into electrical contact at environmental conditions of 40° C.@85% RH. The difference along the x-axis between the two intersection points represents the resulting change in activation force. In the above example, there is a decrease in activation force equal to about 6 or 7 grams. 
     Creep of the conductive membrane  40  after assembly of the touch panel  10  is also of concern. If the conductive membrane  40  were to creep significantly after assembly of the touch panel  10 , the activation force would drop gradually as the internal stress of the conductive membrane  40  relaxed. Creep data for the film  44  is shown in  FIG. 6 . The graph depicts creep as the change in elongation over time at a fixed stress. The flatter the line, the less creep exhibited by the film  44 . As can be seen, creep is very low at the tension used to pretension conductive membrane  40  during assembly. The line is very flat at stresses in the 1000 to 1500 psi range. 
     The effect of cyclical, or alternating stresses is also of concern in that the touch panel  10  may encounter many changes in environmental conditions during shipping.  FIG. 7  shows data for the film  44  when the film is subjected to alternating tensions of 8500 psi and zero psi respectively. As can be seen, the film  44  exhibits a slight creep under these conditions as the bottom of each cycle is slightly higher than the previous cycle. Since the strain applied to the conductive membrane  40  during assembly of the touch panel  10  and over a variety of environmental conditions is significantly less than 8500 psi, it is believed that the effect of cyclical, or alternating stresses will be negligile. 
       FIG. 8  shows the tensioned touch panel  10  in an interactive touch system  100  of the type disclosed in U.S. Pat. No. 5,448,263 to Martin, the content of which is incorporated herein by reference. As can be seen, the tensioned touch panel  10  is coupled to a computer  102 . Computer  102  provides image data to a projector  104  which in turn projects an image  106  on the touch panel  10 . Sensed pointer contacts on the touch panel  10  that are sufficient to bring the conductive resistive layers  22  and  46  into electrical contact are conveyed to the computer  102 , which in turn updates the image data conveyed to the projector  104  so that the image  106  projected on the touch panel  10  reflects pointer contacts. The touch panel  10 , computer  102  and projector  104  thus form a closed loop. Alternatively, the tensioned touch panel  10  may be used in conjunction with a rear projection system. 
       FIG. 9  shows another embodiment of a tensioned touch panel  110  similar to that of  FIG. 1 . In this embodiment, the conductive resistive layer  146  adhered to the film  144  overlies the entire surface of the film  144  that faces the support structure  12  thereby eliminating the peripheral margin  44   a.    
       FIG. 10  shows yet another embodiment of a tensioned touch panel  210 . In this embodiment, the insulating spacer rail  230  is generally L-shaped in section. One arm  230   a  of the insulating spacer rail  230  overlies the periphery of the top surface  216  of the substrate  214 . The other arm  230   b  of the insulating spacer rail  230  abuts the sides  220  of the substrate  214 . 
       FIG. 11  shows yet another embodiment of a tensioned touch panel  310 . In this embodiment the insulating spacer rail  330  is C-shaped in section and completely surrounds the sides  320  of the substrate  314 . The upper arm  330   a  of the insulating spacer rail  330  overlies the periphery of the top surface  316  of the substrate  314 . The lower arm  330   b  of the insulating spacer rail  330  overlies the periphery of the bottom surface  318  of the substrate  314 . The bight  330   c  of the insulating spacer rail  330  abuts the sides  320  of the substrate  314 . The conductive membrane  340  is similar to that shown in  FIG. 1  and is bonded to the top surface of the upper arm  330   a.    
       FIGS. 12 and 13  show still yet further embodiments of tensioned touch panels  410  and  510  respectively similar to that of  FIG. 11 . In  FIG. 12 , the conductive membrane  440  overlies the entire outer surface of the insulating spacer rail  430  and is bonded to the upper and lower arms  430   a  and  430   b  as well as the bight  430   c  of the insulating spacer rail  430 . In the embodiment of  FIG. 13 , the conductive membrane  540  also overlies the entire outer surface of the insulating spacer rail  530  but extends beyond the lower arm  530   b  of the insulating spacer rail  530  and is bonded to the bottom surface  518  of the substrate  514 . 
     Although the conductive membranes illustrated in  FIGS. 10 to 13  show the conductive resistive layer covering the entire surface of the film that faces the support structure, conductive membranes of the form shown in  FIG. 1  can of course be used. 
     Although a number of embodiments of the tensioned touch panel have been described and illustrated, those of skill in the art will appreciate that other variations and modifications may be made without departing from the spirit and scope thereof as defined by the appended claims. For example, the support structure need not be rectangular. The present method allows tensioned touch panels of virtually any shape to be constructed. Ultraviolet and CA adhesives were selected to secure the conductive membrane to the support structure due to their fast cure times. Other suitable adhesives can of course also be used. The peripheral insulating spacer rails need not to be adhered to the support structure. Other suitable fastening means may of course be used to secure the insulating spacer rails to the support structure.

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