Abstract:
A suspension system for mounting a touch surface of a pressure sensitive touch display or panel. The suspension system comprises a frame and a flexible suspension membrane connected to the touch surface and the frame, the membrane allowing frictionless movement of the touch surface along the z-axis and resisting movement of the touch surface within the x-y plane. At least one force sensor is connected beneath the touch surface, whereby the touch surface is pre-loaded against the at least one force sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application No. 61/270,344, filed Jul. 7, 2009, which is a continuation-in-part of International Application No. PCT/US2008/003374, filed Mar. 14, 2008, both of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to input devices for electronics and, more particularly, to a suspension for a touch sensitive input panel&#39;s or display&#39;s touch surface especially suited for use in eBook readers, cellular phones and personal digital assistants (PDAs), PC Tablets, as well as laptops, PCs, office equipment, medical equipment, TVs Monitors, or any other device that uses touch sensitive displays or panels. 
     2. Description of the Background 
     Touch sensitive screens or touch surfaces can detect the application of fingers and other passive objects. Touch surfaces are gaining in popularity and have been deployed in many products in recent years. A number of different technologies have been used to create touch surfaces, such as resistive, capacitive, infrared, surface acoustic wave (SAW), and others. Resistive pads, for example, comprise two conductive plates pressed together. The disadvantage of a resistive pad is that the resistive membrane material will wear out, initially resulting in further reduced clarity followed by dead spots. In addition, the production yield is typically rather poor, and the technology has a few disadvantages such as a fixed (non-user adjustable) actuation force and the light throughput through the resistive membranes is typically only around 70% to 75%. 
     Capacitive touch screens/pads operate by measuring the capacitance of the passive object to ground, or by measuring the alteration of the transcapacitance between different sensors. An example of a capacitive touchpad is described in U.S. Pat. No. 5,495,077 to Miller. Capacitive pads are relatively expensive to manufacture compared to resistive pads, and can only detect objects with sufficient capacitance. Small objects, such as the end of a regular stylus or pen, do not have enough capacitance to ground or transcapacitance to be detected by a capacitive touchpad. The actuation force can also not be set, and the force may be as low as a 0 gram force, that is, the touch screen registers a touch even before the user&#39;s finger touches the screen. This often leads to difficulties in implementing certain end-user features, such as handwriting recognition. 
     SAW devices emit sound along the surface of the pad and measure the interaction of the passive object with the sound. These devices work well, but are generally much too expensive for general applications. Infra red light based displays work in a similar fashion, but this technology typically adds a large size and price. 
     Finally, there are devices that use force sensors to measure the location and magnitude of the force exerted by the passive object on the touchpad. Force sensing technology is very interesting from both feature and cost perspectives. A force sensitive touchpad will sense force applied by any sort of passive object, regardless of the electrical conductivity or composition of the object. Such devices were originally described in U.S. Pat. No. 3,657,475 to Peronneau et al. and U.S. Pat. No. 4,121,049 to Roeber. These devices measure the forces transmitted by the touchpad to a fixed frame at multiple points, for example, at the corners of the pad. Roeber discloses a mathematical formula for deriving the position and magnitude of the force applied by a passive object from the forces measured at the multiple points. 
     For example, U.S. Pat. No. 4,511,760 to Garwin et al., issued Apr. 16, 1985, shows a force sensing data input device responding to the release of pressure force. The input surface is provided with a transparent faceplate mounted on force-sensing piezoelectric transducers. Preferably, four piezoelectric transducers are provided, one at each corner of a rectangular opening formed in the frame. To determine the point of application of force on the input surface, the outputs of the four transducers are first summed. To constitute a valid data entry attempt, the sum must exceed a first threshold while the user is pushing on the input surface. When the user releases his finger, a peak of the sum is detected, which is of opposite polarity from the polarity of the sum in the pushing direction. The individual outputs of the four sensors at the time that the peak of the sum occurs are used to calculate the point of application of the force. 
     United States Patent Application 2003/0085882 by Lu published May 8, 2003, shows a touch pad device having a support layer with a plurality of strain gauges in a matrix configuration. A touch layer is disposed on top of the strain gauge matrix. The touch layer is joined to the top of the strain gauge matrix. Sensor wires connect the strain gauges to a processor that is programmed with an algorithm to measure the location and pressure of simultaneous, multiple touches. 
     United States Patent Applications 2004/0108995 and 2004/0021643 both by Hoshino et al. show a display unit with touch panel mounted above a display via four differentially-mounted sensors. The pressure sensors detect force with which a pointing device such as a finger pushes the panel surface, in real time. The force with which the pointing device such as a finger pushes the panel surface is found from the following equation irrespective of the pointing position: P=a+b+c+d−a0+b0+c0+d0, which equation detects dragging of a cursor. 
     United States Patent Application 2005/0156901 by Ma et al., issued Jul. 21, 2005, shows a touch screen display system with a display screen and overlying touch surface. An imaging system determines an angular position on the touch surface of the object coming in contact with the touch surface. 
     United States Patent Application 2006/0016272 by Chang, published Jan. 26, 2006, shows a thin film touch pad with opposed sensor elements that generate an electrical signal that is proportional to both the applied pressure and the surface area at the location of the applied pressure. As a result of the complementary and overlapping orientation of these sensor elements, the first and second sensor elements generate an asymmetric pair of signals that uniquely define the applied pressure by position and magnitude. 
     U.S. Pat. No. 6,879,318 by Chan et al., issued Apr. 12, 2005, shows a touch screen mounting assembly for a liquid crystal display (LCD) panel including a bottom frame having a seated backlight panel and a plurality of mounted pressure-sensitive transducers, an LCD panel, and a top frame for exerting pressure when mounted to the bottom frame such that a plurality of compressible springs biases the LCD panel towards the bottom frame when touched or contacted by a user. The claims require the bottom and top frame assembly with backlight panel mounted therein on springs, and an overlying LCD panel. 
     The market success for force based touch screens and pads has so far been very limited for various reasons. Current implementations employ complex mechanical structures and appropriate force-sensing sensors. A method to overcome the mechanical complexities (promising a low cost and small size penalty) is described in International Application No. PCT US2008/003374 filed Mar. 14, 2008, which employs a figure-8 suspension concept to ensure that the touch screen will not move in the x-y plane. This is therein illustrated in  FIG. 1  which shows the profile of how the wire or line ( 15 ) is wrapped around the touch surface ( 10 ) and the back surface ( 14 ) in a figure-8 loop. The line ( 15 ) is wrapped around all 4 sides, creating 4 separate figure-8 loops or one combined (using 4 or 1 lines). The line suspension can be designed in a number of variants, where the line may be wrapped around the surfaces as in  FIG. 1 , or it may secure the surfaces through hole or channels into the corner or the surfaces, or connected to wire holders which are fastened to the surfaces. Either way, the principles remains the same, the built-in tension in the line pulls the touch surface ( 10 ) towards the base plate ( 14 ) applying a stable, pre-loaded force onto the 4 force sensors ( 41 ). The forces in the string is also centering the touch plane ( 10 ) in relation to the base plate ( 14 ) as the forces in the x-y plane applied to the 4 corners are directed to the center of the touch plane. This centering force will allow the touch plane to remain centered and not move in the x-y plane, but allow for the small friction free movement required in the z-direction, typically less then 0.1 mm distance when using a piezo resistive sensor (other force sensors may require a larger z-direction movement). 
     The foregoing configuration provides a good mechanical suspension with a close to zero loss in the touch force, yet the device is free to move frictionless in the z-direction thereby ensuring that all of the touch force will be distributed to four force sensors. Moreover, the suspension is very low cost in terms of components. However, the assembly process is not optimal. In addition, it is difficult to apply a dust or water seal which typically requires a rubber or silicon material to touch the touch lens, and the seal must typically also be tailor made as well as tested and verified for each new design. 
     It would be greatly advantageous to provide a suspension solution that preserves the mechanical aspects of the force based touch screen system in a way where the mechanical characteristics of the system described above are maintained, but with a design that is optimized for both high volume production as well as for a built in low cost and high performing water seal. The same platform components should also be capable of accommodating different sensor sizes/shapes, should allow for a more cost efficient suspension mechanism, and should be reusable from one product implementation to the next. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present innovation to allow for cost efficient design and manufacturing of force sensing sensor-based touch screen products of different sizes, designs, and applications with one and the same conceptual component. 
     It is another object to provide a suspension solution that can be implemented and assembled at a low cost, and yet support different sensor types and sizes without any conceptual changes and a minimum of component changes. 
     It is another object to provide a suspension solution that also serves as water seal for the touch screen. 
     It is another object to provide a suspension solution that is less sensitive for external interference, such as vibrations. 
     These and other objects are accomplished by a mechanical suspension platform for sensor-based touch surface products that uses a suspension membrane component, for example, a flexible suspension membrane attached between the touch surface and frame for maintaining said touch surface in a suspended configuration, whereby it is afforded frictionless movement along a z-axis but is restrained from movement along the x-y plane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments and certain modifications thereof when taken together with the accompanying drawings in which: 
         FIG. 1  illustrates a touch screen suspended to a rigid frame through a thin membrane, according to the present invention. 
         FIG. 2  illustrates a touch screen suspended to a rigid frame and pre-loaded through a thin membrane as in  FIG. 1 , from a side view. 
         FIG. 3  illustrates an alternative embodiment of a touch screen suspended to a rigid frame through a thin membrane, from a side view. 
         FIG. 4  illustrates the alternative embodiment of a touch screen suspended to a rigid frame through a thin membrane as shown in  FIG. 3 , from an enlarged side view. 
         FIG. 5  illustrates the touch screen of  FIG. 3  suspended to a rigid frame and pre-loaded through a thin membrane, from a top view. 
         FIG. 6  illustrates the alternative embodiment of a touch screen suspended to a rigid frame through a thin membrane, from an enlarged side view. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is a suspension platform for a touch sensitive input panel or display having a touch surface, which is especially suited for use in cellular phones, personal digital assistants (PDAs), and PC Tablets, as well as laptops, PCs, office equipment, medical equipment, TV Monitors, or any other device that uses touch sensitive displays or panels. 
       FIG. 1  is an exploded view of a suspension platform according to one embodiment of the present invention incorporated in a touch sensitive input display for an eBook reader, and  FIG. 2  is a side view. 
     In an embodiment, an eBook reader comprises an ePaper display (EPD) that is a touch surface  55  defining an x-y plane having a normal z-axis. The reader housing incorporates a bezel  50  that defines a touch plane on the touch surface  55 . The bezel  50  is held in place by connecting it to a back housing  61  (see  FIG. 2 ) of the eReader housing. The bezel  50  is also connected to a thin suspension membrane  53 . The bezel  50  sits atop the suspension membrane  53 , which in turn sits atop the touch surface  55 . Because the touch surface  55  is an EPD that is a very thin and rather fragile, the touch surface  55  is strengthened and made more rigid by adding a display support  57 . The display support  57  is a flat rigid plate that bears against four under-mounted differential pressure-sensors  58 . The sensors  58  are mounted on a base support  60 , such as a standard printed circuit board (PCB). The base support  60  provides the rigidity and flatness required for the sensors  58 . Depending on the product implementation, the base support  60  may be seated directly onto the back housing  61  (see  FIG. 2 ) of the unit, or if a double sided PCB is used for the base support  60 , space may be available between the bottom of the base  60  and the back housing  61  ( FIG. 2 ) of the unit. As the bezel  50  is mounted with the back housing  61  of the unit, the bezel  50  and the back housing  61  are forced together, which results in the bezel  50  applying a calibrated preload force through the membrane  53  to the touch surface  55  and display support  57  onto the sensors  58 . 
     These differentially-mounted sensors  58  are connected through the base support  60  to an electronic device processor. This way, when a user touches the touch surface  55  at some (x,y) position, the force is transmitted through the touch surface  55  and the display support  57  to the four under-mounted differential pressure sensors  58 , for example, piezo-resistive force sensors. Once the force is applied to the sensors  58 , a change in output voltage is generated and continuously sent to the processor, where the output is registered and processed. The exact (x, y) “touch-coordinate” on the touch surface  55  is calculated. 
     As seen in the inset of  FIG. 1 , the suspension membrane  53  is a substantially rectangular elastomeric frame conforming to the peripheral shape of the touch surface  55 . The bezel  50  is only in contact with the touch surface  55  through the suspension membrane  53 . In the illustrated embodiment, the suspension membrane  53  is formed with three contiguous walls and is inwardly open, leaving a hollow interior air gap. The size of the gap is dependent on the elasticity of the membrane  53 , the size of the touch surface  55 , and the required pre-loading force. The suspension membrane  53  acts like an accordion. The bezel  50  presses down on the membrane  53  that presses on the touch surface  55 , providing the pre-loading. Membrane  53  facilitates frictionless movement of the touch surface  55  towards the sensors  58  (along the z-axis) and yet restricts movement away from sensors (in the x-y plane). Moreover, the membrane  53  creates a dust and water tight seal for the unit. 
     In an alternative embodiment, rather than a bezel  50  of the frame pressing down onto the touch surface  55  via membrane  53 , the bezel pulls the membrane and touch screen down onto the sensors. 
       FIGS. 3 and 4  illustrate an alternative embodiment of a touch sensitive display or panel having a touch screen in which the bezel  150  pulls the membrane  153  down, thereby pulling the touch screen  155  down on the sensors  158 . 
     Here the above-described bezel  50  and back housing  61  are combined in a fixed frame  160  defining an outside structure, such as the housing of the product. There is a thin gap  152  between the frame  160  and the touch surface  155 . A thin membrane  153  spans the gap  152 , and the touch surface  155  is only connected to the fixed frame  160  through the membrane  153 . The size of the gap  152  is dependent on the elasticity of the membrane  153 , the size of the touch surface  155 , and the required pre-loading force. The membrane  153  fixes the touch surface  155  relative to the frame  160  in the x-y plane. The membrane  153  may be a strip of plastic, rubber, silicon, adhesive tape, or similar material depending on the specific characteristics required of the actual product design. The membrane  153  can be kept very thin since it can be applied completely around the touch surface  155 . If a side-force is applied to the touch surface  155 , the 2 parallel sides and the opposite side of the membrane  153  resist the movement, virtually eliminating any side movement. 
       FIG. 5  is a top view of the system of  FIGS. 3 and 4 . The membrane  153  holds the touch surface  155  in place within the outside frame  160 , allowing for small movements in the z-direction while only creating minimal friction forces. As the required movement is typically in the area of 0.01 to 0.05 mm, the forces created from the stretching of the membrane  153  are minimal, if any. Other force sensors, such as traditional strain gauges may require a larger movement in the z-direction, and gap  152  and membrane  153  may be adjusted accordingly. In addition to the x-y-plane control, the suspension solution must also provide a preloading force to ensure that the touch surface  155  is always resting on the force sensors  158  and to reduce jitter from physical vibrations. The force applied in the z-direction towards the sensors  158  from the elasticity/spring effect from the membrane  153  will in most designs be sufficient, but if required, additional pre-loading may be applied by a spring, cantilevered bender, or similar spring-like component  154 . Even when using a thin, elastic membrane  153 , the touch surface  155  is kept fixed in the x-y plane. The membrane  153  provides no support in the z-direction, especially for small movements (&lt;0.1 mm), which affords the touch surface  155  a very close-to-force and friction free movement in the z-direction (again &lt;0.1 mm) as a force is applied through a user&#39;s touch. 
       FIG. 6  illustrates the alternative embodiment of a touch screen suspended to a rigid frame through a thin membrane, from an enlarged side view. In this embodiment, the membrane  153  is connected to the bottom of the touch surface  155 —the side opposite the side of the touch surface  155  that is touched. The membrane  153  pulls the touch screen  155  down on the sensors  158 . 
     Although in the illustrated embodiments, the touch surface is a surface of the display module, the touch surface may be a separate component. The present invention is not limited to any particular display module and may include, for example, EPD, LCD, OLED or other display modules. 
     One skilled in the art should understand that the material options are rather wide, and the complete system; frame (base), membrane, and touch screen can even be manufactured from one material; however, in most applications the three components will consist of different materials, such as a metal incased glass plate assembly for the touch surface, plastic product housing or bezel for the frame, and a plastic membrane part fixed onto the frame and display through strong, permanent adhesive. 
     This invention provides a few important benefits over the existing wire solution. The suspension membrane  53 ,  153  will very effectively seal the touch surface to the outside frame. It is impossible for humidity, dust, or liquid to penetrate in between the touch surface and the frame, even though they are mechanically not fixed connected. 
     The touch sensitive display or panel must have a touch surface and a base plate or base frame that holds the suspension system, which keeps the touch surface fixed in x-y plane with free micro movement in the z-direction. With the membrane approach, there is no need to add mechanical structures in order to fix the wire assembly to the touch surface and the base plate. This can further reduce the overall building height of the final product, which is extremely important from a touch sensitive display or panel competitive position. 
     The suspension membrane  53 ,  153  will also provide required pre-loading force in the z-direction for better sensor performance and minimizing negative effects from vibration and movement impact on the touch screen system. 
     This suspension mechanism, independent of which implementation is used, requires a very large force to further move the display away from the sensors. This will then eliminate a problem sometimes occurring with the wire approach in which the weight of the display overcomes the preloading force when the unit is held up-side down and one or multiple sensors lose contact with the display, resulting in an invalid touch screen input. 
     Finally, the manufacturing and the placement of the membrane is optimal for high volume manufacturing. The material costs are extremely low, and the manufacturing operation is similar to existing operations, such as placing a dust seal into a product. 
     Having now fully set forth the preferred embodiment and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept. It is to be understood, therefore, that the invention may be practiced otherwise than as specifically set forth in the appended claims.