Abstract:
A membrane switch provides a substantially rigid front fascia without the need for flexure limiting standoffs or the like. The membrane switch may use thin, printed insulating dots whose pattern controls the force required to actuate the switch elements as a function of distance from the switch elements preventing multiple activations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based on and claims the benefit of U.S. Provisional application 60/504,921 filed Sep. 22, 2003, and U.S. Provisional application 60/520,206 filed Nov. 14, 2003, both hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to electrical membrane switches and in particular to a membrane switch having or adhered to a substantially rigid front surface or fascia.  
         [0003]     Membrane switches are well known in the art and normally employ a pair of stacked flexible membranes having opposed contacts printed on their facing surfaces. A spacer layer separates the membranes, except at a region about the contacts, allowing pressure from a finger or the like to deform one of the membranes so that its contact touches the contact of the other membrane closing an electrical switch. The natural resilience of the membranes may separate the contacts once the force of closure is removed. Electrical conductors, also printed on the facing surfaces of the membranes, communicate electrical signals to and from the contacts.  
         [0004]     Normally, a thin plastic decorative trim is adhered to the front surface of the membrane switch to indicate the position of the buttons and their functions to the user.  
         [0005]     A single membrane may support many contacts making membrane switches a cost effective solution for multi-switch control panels and the like. The continuous front membrane of a membrane switch seals the switch contacts from contamination, and for this reason, membrane switches are often used in environments where moisture or contaminants are a problem.  
         [0006]     Membrane switches have some drawbacks. While the membrane itself is resistant to contamination and readily cleaned, it is soft and susceptible to abrasion or damage. The membranes must often be applied over the outer housing of an appliance or other device where they are exposed to damage. The common look and feel of thin plastic membrane can be limiting to designers experimenting with a wider range of design aesthetics.  
         [0007]     The problem of damage to the membranes is addressed in U.S. Pat. No. 5,747,757 to Van Zeeland which describes positioning a membrane switch behind a thin panel of metal to resist vandals. Van Zeeland also suggests alternative use of plastics such as Lucite, Kevlar, or glass. As noted by Van Zeeland, the rigid panel tends to spread the force of actuation by a finger, or the like, over a broader area creating a risk that adjacent switches will be simultaneously actuated by a single touch. Van Zeeland addresses this problem using rigid standoffs or similar structures between the front panel and a back support that resists the deflection of the front panel except at the contact areas, thereby attempting to focus the deflection of the front panel to the contact areas.  
         [0008]     Limiting the natural deflection of the front panel increases the force required to deflect the front panel to an amount which may be unacceptable to the average user.  
         [0009]     The standoff system proposed by Van Zeeland also increases the complexity of manufacture of the membrane switch requiring specialized mechanical components that must be changed for each changed layout of the switch. The problems of supporting these standoffs against the minor deflections they must resist presents additional barriers to the use of the Van Zeeland design.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     The present inventors have created a rigid fascia membrane switch that can work with or without mechanical structure between the fascia and the rest of the membrane switch to restrain the deflection of the fascia, and that may work with a wide variety of fascia including curved fascia, and that provides simplified assembly.  
         [0011]     Generally, the invention employs an ultra-sensitive design where the membranes are separated by thin insulating dots, for example, printed on the membrane, rather than employing a thicker plastic spacer layer. The dots reduce the actuation force (and actuation movement) required to activate the switch and also allow the actuation force and movement to be carefully tailored to accommodate force-spreading by the fascia. This tailoring can be done by changing the density of the dot patterns to decrease the sensitivity of the switch as one moves away from the contact area. The result is a membrane switch that can be used with a variety of fascia materials and with planar or curved fascias without requiring undue finger pressure for actuation.  
         [0012]     Specifically, the present invention provides an electrical switch assembly having a substantially rigid front panel positioned in front of a membrane switch in contact with the front panel, the membrane switch providing a plurality of spatially separated switch elements. A backer plate is positioned behind the membrane switch in contact with the membrane switch and the space between the front panel and the backer plate is substantially free of structure intended to resist deflection of the front panel.  
         [0013]     Thus, it is one object of one embodiment of the invention to provide a membrane switch for use with a substantially rigid front panel that does not require specialized structure to resist movement of the front panel.  
         [0014]     The front panel may alternatively be a rigid plastic such as a polycarbonate plastic or glass or other rigid material.  
         [0015]     It is thus one object of another embodiment of the invention to provide designers with a variety of different surface materials for membrane switches.  
         [0016]     The front panel may be non-planar, for example, outwardly convex.  
         [0017]     Thus, it is another object of an embodiment of the invention to provide a membrane switch that may be integrated into flowing or curved designs without inset of a flat control panel.  
         [0018]     The separator used in the membrane switch may have a thickness to allow the membrane switch to actuate with a very small deflection of the fascia, for example, 0.001″.  
         [0019]     Thus, it is another object of an embodiment of the invention to provide a highly sensitive membrane switch that may be used with substantially rigid front panel materials.  
         [0020]     The printed insulator elements may have a varying pattern density depending on the distance of the elements from the centers of the switch contacts.  
         [0021]     It is thus another object of an embodiment of the invention to provide a simple method of controlling the actuation force of the membrane switch such as may be used to assist in preventing cross actuation of closely adjacent switch elements.  
         [0022]     A movable switch operator may be positioned in front of the rigid front panel to be pressed by a user and to apply increased pressure to the switch area.  
         [0023]     Thus it is another object of at least one embodiment of the invention to provide a simple mechanism to modify the forces applied to the rigid material required by different applications.  
         [0024]     The switch areas may be separated along a first axis, and the electrically independent conductive switch contacts are proportionally narrower along the first axis than along a perpendicular to the first axis.  
         [0025]     Thus it is another object of the invention to accommodate the force spreading produced by a rigid front panel while preserving desired switch spacings and contact areas.  
         [0026]     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is an exploded, perspective fragmentary view of a washing machine console using the present invention;  
         [0028]      FIG. 2   a  is a cross sectional view through the console of  FIG. 1  showing a first embodiment of the invention not providing indicator lights;  
         [0029]      FIG. 2   b  is a figure similar to that of  FIG. 2   a  of a second embodiment of the invention providing indicator lights;  
         [0030]      FIG. 3  is a front elevational view of the rear membrane of the membrane switch of  FIG. 2   b  and a rear elevational view of the front membrane of the membrane switch of  FIG. 2   b  showing conductive traces, contacts, and opposed shorting pads separated by insulating dots.  
         [0031]      FIG. 4  is a fragmentary view of the membrane switch assembly of  FIG. 1  showing an embodiment with asymmetrical contacts to accommodate force spreading by a rigid front panel;  
         [0032]      FIG. 5  is a figure similar to that of  FIG. 2   b  of an embodiment having a clear front panel and annular switch contacts such as allow central illumination of each switch;  
         [0033]      FIG. 6  is a figure similar to that of  FIG. 2   a  showing an embodiment in which the front panel supports switch contacts;  
         [0034]      FIG. 7  is a top plan view of one membrane of a switch according to one embodiment of the invention showing implementation of multilevel force sensitivity;  
         [0035]      FIG. 8  is a partial cross-sectional view through the switch of the present invention showing additional use of a rocker operator to flex the front panel; and  
         [0036]      FIG. 9  is a figure similar to that of  FIG. 8  showing a button operator used to flex the front panel. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0037]     Referring now to  FIG. 1 , an appliance  10 , for example, a top loading washing machine may provide a rearward upwardly extending console  12  having a fascia  14  facing the user from behind a tub access door  16  or the like.  
         [0038]     The fascia  14  may be a metal cowling fitting over a recessed portion  11  of the console  12  to cover a recess  13  in a front face of the console  12  that provides a space for a membrane switch assembly  15  that will fit behind the control surface as will be described. The membrane switch assembly  15  provides a tail  44  that may pass through an opening  17  through the front face of the console  12  to connect the membrane switch assembly  15  to control electronics (not shown) positioned within the console  12 .  
         [0039]     The fascia  14  may be outwardly convex, for example, formed of 0.019-inch thick aluminum sheet supported by the console  12 . The fascia  14  is a rigid material, meaning generally that it retains its shape without support and is much stiffer than a conventional plastic membrane of the type used in a membrane switch, for example, to resist folding under light finger pressure. Other metals, plastic, and glass may also be used for the fascia  14 .  
         [0040]     Exposed at the front of the fascia  14  may be a series of actuation positions  18  and indicator lights  20 , the latter providing visual indication that the actuation positions  18  have been activated. The locations of the actuation positions  18  may be indicated by a simple graphics  24  printed on or etched in the fascia of the appliance  10 . The graphic  24  may provide a target location for finger pressure and/or a descriptive legend.  
         [0041]     Small holes may be cut through the fascia  14  for the indicator lights  20 , however, otherwise, the fascia  14  may present a substantially outer surface that is resistant to water and detergent, and that allows drainage of splashed liquids.  
         [0042]     Referring now to  FIG. 2 , the material of the fascia  14  provides a front panel  26  for the actuation positions  18 . Attached to the rear surface of the front panel  26  is a front membrane  32  forming part of a membrane switch assembly  15  and being of conventional material and structure. An adhesive (not shown) may attach the front membrane  32  to the rear of the front panel  26 . Behind front membrane  32  is a rear membrane  36 . The membranes  32  and  36  may be, for example, a polyester film of a type well known in the art.  
         [0043]     The front membrane  32  and rear membranes  36  are held together at their periphery by adhesive  34  and separated within their peripheries by dielectric dots  52  as will be described below. Conductor patterns (not shown in  FIG. 2 ) are printed on the inner, facing surfaces of the front membrane  32  and rear membrane  36 . In use, a person may press the graphic  24  with his or her finger  41  causing a slight deformation of the front panel  26  and corresponding compression of the front membrane  32  against the rear membrane  36  activating the membrane switch.  
         [0044]     A rear support  38 , generally conforming to the curvature of the front panel  26 , stiffens the front membrane  32  and rear membrane  36  and is attached to the front panel  26  by brackets (not shown) or may be a front face of the recess  13  or may be attached to the front panel  26  via the intervening layers of front membrane  32  and rear membranes  36  to provide some resistance to backward motion. The rear membrane  36  and rear support may be combined and replaced as a stiff printed circuit board, particularly when the desired form of the fascia  14  is flat rather than curved in which case a separate rear support  38  is not needed.  
         [0045]     Referring now to  FIG. 2   b , in an alternative embodiment, small holes  28  may be cut in the front panel  26  above the graphic  24  at the locations of the indicator lights  20 , each fitted with a small transparent window  30 . Front membrane  32  and intermediate membrane  36  may be transparent and free of light blocking materials in the region of the indicator lights  20  to allow passage of light therethrough from a light emitting diode (LED)  40 . The LED  40  is attached to and extends from a front surface of a rear membrane, or printed circuit board  39 . A spacer layer  43  attaches the rear membrane or printed circuit board  39  to the rear surface of the intermediate membrane  36  and provides a hole  45  receiving the LED  40  therein to space the front surface of the LED  40  from protruding into the rear surface of the intermediate membrane  36 . Control circuitry (not shown) may be provided that causes the LED  40  to illuminate with alternate pressings of the associated switch to indicate that the switched function is on, as is generally understood in the art.  
         [0046]     Referring now to  FIG. 3 , a front surface of the rear membrane  36  includes a set of conductive traces  42  leading from the tail  44  being an extension of the rear membrane  36 . The conductive traces  42  pass from the tail  44  to a generally rectangular body portion  46  of the rear membrane  36  and there form an interdigitated contact pattern  48  exposed at that front surface of the rear membrane  36  at the location of each pushbutton  18 . The front surface of the rear membrane  36  may also support the LEDs  40  (only one shown for clarity) and associated conductive traces  42  shown by dotted line. The traces  42  may be printed in silver or other suitable material.  
         [0047]     A rear surface of the front membrane  32 , such as is normally adjacent to the front surface of the rear membrane  36 , provides shorting pads  50  spanning the interdigitated contact patterns  48 . When pressure is applied to the front membrane  32  at the points of the shorting pads  50 , the shorting pads  50  contact the interdigitated contact patterns  48  shorting the interdigitated contact patterns  48  and allowing for electrical flow between two associated conductive traces  42 . The shorting pads  50  may be carbon or other suitable material.  
         [0048]     Inadvertent shorting of the interdigitated contact patterns  48  by the shorting pads  50  is prevented not by a spacer layer, but by a series of insulating or dielectric dots  52  printed on the rear surface of the front membrane  32  atop of the shorting pads  50  and the areas around the shorting pads  50 . Alternately the dielectric dots  52  can be printed on the front surface of the rear membrane  36 . As described above, adhesive  34  selectively printed around the perimeter of either the front membrane  32  or the rear membrane  36  may attach the front membrane  32  to the rear membrane  36  as indicated by arrows  54 .  
         [0049]     The spacing between the dielectric dots  52 , describing a “dot density” varies, as will be described below, to control the amount of activation force that will cause the front membrane  32  and rear membrane  36  to contact each other. The number of dielectric dots  52  per square inch may be freely varied to provide accurate control, both of the activation force of the switch and of the change in activation force as a function of location. A solid covering of dielectric can also be placed anywhere it is undesirable to have a switch activation.  
         [0050]     Conventional membrane switches employ a spacer layer that may be as much as 0.005 to 0.01″ thick. In the present invention, the dielectric dots have a thickness of less than 0.002″ and preferably approximately 0.001″ allowing a comparable small deflection to activate the switch formed by the shorting pads  50  and the interdigitated contact patterns  48 .  
         [0051]     It will thus be understood that without necessarily constraining the deflection of front panel  26  against flexure, the activation area around the actuation positions  18  may be controlled simply by the spacing of the dielectric dots  52 . Note that rear support  38  need not be perfectly stiff.  
         [0052]     Other methods to reduce or eliminate false triggering of the switches may also be employed together with or instead of the varying of the spacing of the dielectric dots  52 , for example, including signal processing techniques that assign priorities to particular buttons when multiple buttons are struck or that select the first button to be struck within a predetermined window of time locking out other pressings, or that use anti-bounce techniques or the like to filter false hits.  
         [0053]     Referring now to  FIG. 4 , the rigidity of the front panel  26  will cause some force spreading that requires a margin  60  separating interdigitated contact patterns  48  of the actuation positions  18  to prevent triggering of adjacent actuation positions  18  when a given pushbutton  18  is pressed. For closely spaced actuation positions  18 , this margin  60  can adversely reduce contact area between shorting pads  50  and interdigitated contact patterns  48 . Accordingly, the present invention contemplates that the area of the shorting pads  50  and interdigitated contact patterns  48  can be increased by extending the relative proportion of both along an axis perpendicular to an axis  62  along which actuation positions  18  are separated. As shown in  FIG. 4 , the shorting pads  50  may in one embodiment be oval having their longer axis vertical and perpendicular to a horizontal axis  62  of separation. Other asymmetric shapes may also be used for this purpose.  
         [0054]     Referring now to  FIG. 5  in one embodiment, the front panel  26  may be a transparent material such as glass or plastic. In this case, the shorting pad  50  and interdigitated contact patterns  48  may be constructed to have an annular form when printed on the rear surface of membrane  32  and front surface of membrane  36 . The annular form of the shorting pad  50  and interdigitated contact patterns  48  allows light from LED  40  (described above) to pass through transparent membrane  32  and  36  and through the center of the shorting pad  50  and interdigitated contact patterns  48  to provide a visible illumination centered in the area of the actuation positions  18 . In this example, the rear support  38  is formed by rigid material of the printed circuit board  39 . The printed circuit board  39  may also hold other electrical components  47  such a resistors, diodes or transistors or the like and may stand in lieu of the second membrane  36  to support electrical contacts.  
         [0055]     Referring now to  FIG. 6 , in another embodiment, the front membrane  32  may be eliminated by using the front panel  26  to support the shorting pad  50  or in the case of a metallic front panel  26  to serve as the shorting pad  50  itself. In the case that the front panel  26  is an insulating material such as plastic, the shorting pad  50  may be printed on the rear surface of the front panel  26  using techniques similar to those used to print the membrane  32 .  
         [0056]     Referring now to  FIG. 7 , the interdigitated contact patterns  48  associated with one pushbutton  18  may be constructed to provide three electrically isolated sets of interdigitated contact patterns  48   a - 48   c , all operating in the region of one pushbutton  18  with a common shorting pad  50 . Each electrically isolated set of interdigitated contact patterns  48   a - 48   c  may have a different activation pressure threshold defined as the pressure at which they contact electrically upon compression on the membranes  32  and  36 .  
         [0057]     In one embodiment, these different pressure thresholds may be produced by using dielectric dots  52  of different heights above the conductors of the interdigitated contact patterns  48 . For interdigitated contact pattern  48   a , taller dielectric dots  52  require greater activation pressure thresholds than the shorter dielectric dots  52  associated with interdigitated contact pattern  48   c.    
         [0058]     Alternatively or in addition, as also shown in  FIG. 7 , the separation distance between the dielectric dots  52  may be changed to provide differences in activation pressure thresholds among the interdigitated contact patterns  48   a - 48   c  with a greater separation distance between the dielectric dots  52  corresponding to lower activation pressure thresholds.  
         [0059]     In these ways, a single pushbutton  18  may distinguish among no pressure and at least two compressive different activation pressures applied to membranes  32  and  36 .  
         [0060]     In an alternative embodiment, the different interdigitated contact patterns  48   a - 48   c  may be arranged on different layers of the switch to be separated along the axis of the pressing of the pushbutton  18 .  
         [0061]     Referring now to  FIG. 8 , the front panel  26  may have a switch operator  64  attached to it, in this case, a rocker operator  66  pivoting about a pivot  68  attached to the front panel  26 . The rocker operator  66  has a rearwardly extending cam  70  positioned so that tipping of the rocker operator  66  presses the cam  70  against the front panel  26  concentrating force of a finger pressure at the region of the pushbutton  18  as well as increasing that force by mechanical advantage.  
         [0062]     Alternatively as shown in  FIG. 9 , a pushbutton operator  71  may be employed having a rearward extending point  72  held by a cowling  74  against the outward urging of a biasing compression spring  76 . Pressing the pushbutton operator  71  pushes the point  72  against front panel  26  concentrating force at the location of the pushbutton  18 .  
         [0063]     Referring now to  FIG. 10 , the dielectric dots  52  are of arbitrary shape providing discrete islands of insulation that may be varied both in height and in spatial density. In one embodiment the dielectric dot  52   c  may be printed using an insulating ink or adhesive. Alternatively the dielectric dots  52   b  may be an element of insulating film, for example, polyester, die- or otherwise cut or perforated to provide for the necessary regions of insulation. In this case, the discrete dielectric dots  52   b  may be joined by a network of material to position them with respect to each other and to simplify assembly. Alternatively dielectric dots  52   a  may be embossments or deformations in either of membranes  32  or  36 . The dielectric dots  52  need not be of a particular shape or arranged at regular locations.  
         [0064]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.