Abstract:
A suspension system for a differential-pressure touch sensitive panel suspended over force sensors, for use in either fixed or mobile devices such as point of sales terminals, kiosks, laptops, monitors, PDAs, cell phones, UMPCs and more. In one embodiment, each side of the lens is encircled and supported by a looped string, monofilament or flexible wire, which is then looped around the back cover or base plate, forming a figure-8. The  figure 8-loops  bring the lens into a fixed state in the xy-plane without the addition of any friction causing physical contact. Other alternative implementations include continuous suspensions, bender suspensions and 3-dimensional force suspensions. Moreover, the present invention proposes the use of a flexible padding under the force sensors to allow the sensors to be slightly preloaded, which reduces the dependency on extremely tight mechanical tolerances.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/US2008/003374, filed Mar. 14, 2008, which claims the benefit of U.S. Provisional Patent Application No. 60/918,275, filed Mar. 15, 2007, which is incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to mechanical touch input systems such as touch pads and touch screens used in fixed or mobile devices, such as point of sales terminals, kiosks, laptops, monitors, PDAs, cell phones, UMPCs and more, that require the touch component to be fully constrained in two directions (x &amp; y), but requiring unencumbered freedom to translate in the third direction (z-direction). 
     (2) Description of Prior Art 
     The concept of using multiple force sensing sensors to register, measure and triangulate the touched position of a touch screen has been a known concept for more then twenty years, however, to produce a high quality touch screen solution has proven difficult. 
     Over the last few years the performance of force sensors has improved substantially and the component cost has been reduced to commercially viable prices. In addition, there has been software advances, creating an environment ready for high volume product implementations of touch screens based on force sensing. There is however still one major problem that must be overcome, the mechanical implementation. 
     For the typical force based touch screen implementation, the input device consists of a free standing touch screen lens or the actual LCD module. This touch screen lens rests on 3 or more force sensing sensors that are placed on a rear cover or some type of rigid surface, such as a PC Board or a back plane. The overall mechanical housing holds the different components in place as a system with different types of mounting mechanisms, which allow for movement in the lens, yet keeps the lens in place and pressed towards the force sensors. The force sensors are electrically connected, sometimes signal amplified, and converted from analog to digital so that sensor readings may be provided to the touch screen control software running on the device processor or on a separate micro controller. It should also be mentioned that most force sensors are designed to measure vertical forces and have minimal to no tolerances to measure forces accurately that are not applied exactly straight into the sensors measurement plane. 
     The mounting mechanism of the touch screen is an active part of the touch screen and refers to the way the top lens, which is used as an input device, and the sensors that measure the position and the amount of exerted force, are mounted on base plate. Until now, production of force based touch screen devices with high accuracy has been a challenge due to large errors in reading and interpreting the force. The errors occur because the forces introduced by the mechanical devices are not countered sufficiently and very difficult to account for. For example, the lens needs to allow for movement in the z direction, but must be fixed in the xy-plane in order to not introduce side movement and to minimize side forces. The lens also needs to be pre-loaded to ensure that the touch screen lens always remains in contact with the force sensors, even if the unit is moving or being turned upside-down. Adding a pre-loading solution will also introduce non-linear forces that are difficult to correctly account for, especially since the direction of gravity is typically unknown. 
     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 for 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. This mechanical construct is using spring clips to pre-load the sensors and to keep the mechanical assembly in place. These spring clips could theoretically minimize movement in xy-plane, but will provide non-linear additions to the total forces as the lens is touched since the spring loading force will change as the lens is pressed (and moved) in the z-plane. The position of the pre-loading springs will also add to the complexity since they are adding forces that bends the lens over the sensors. 
     U.S. Pat. No. 5,038,142 to Flower et al. (IBM) issued Aug. 6, 1991 shows a touch sensing display screen supported by stiff springs having essentially uniaxial freedom of motion. Strain gauge sensors are applied directly to the springs and a processor calculates the X, Y and Z coordinate information. This configuration is similar to the above-referenced Gawin patent inasmuch as it employs a basic spring construction for allowing movement in z-direction but restricting the xy-plane. Again there are likely a lot of uncontrollable forces. 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 panel LCD including a bottom frame, a backlight panel seated in the frame and that has a plurality of pressure-sensitive transducers mounted thereon, a liquid crystal display 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 bottom and top frame assembly with backlight panel are mounted therein on springs, with an overlying LCD panel. Spring loaded mounting screws will allow for movement in the z-plane and pre-loading, but non-linear forces from pre-loading as well as lens friction will be present. 
     Two core problems have been identified in the foregoing references and other existing mechanical solutions for force based touch screens. 
     The first problem identified is the extremely small tolerances required for the mechanical build-up. The issue is that the touch screen needs to be extremely rigid, since if the lens bends part of the force will be captured in the lens material or even lost in heat dissipation. Unfortunately, in making the sensor mounting surface as well as the lens (which rests on top of the sensor) extremely rigid, there is no longer any accommodation for mechanical tolerances. The sensor must fit the components exactly (within 1/100s of a mm), or else the lens will ether not be in contact or must be forced down and be bent) through pre-loading. Due to the very small movement allowed within the force sensor and the use of rigid and parallel surfaces, keeping the top plate at the same distance and parallel to the base consistently, before and after the pressure is applied, remains a challenge both from a production as well as a measurement perspective. 
     The second core problem is the interference from other forces. Typical problems arises from non-linear forces, when there is contact and friction between the lens and other mechanical components, and from pre-loading, where the applied preloading creates non-linear additional forces as the lens is pressed down and some of the pre-loading forces are neutralized through the new and additional forces loading the lens towards the sensors. In addition, there may be bending in the materials, where some of the force is lost into side forces and heat. 
     These problems surface as the following symptoms: 
     1) Low accuracy: Approximately 1-10% of total distance between sensor in positioning error 
     2) Repeating the same operation such as drawing a line, but starting from the opposite direction does not yield the same results (due to interference of lateral forces an/or actual side movement of lens transferred to the force sensors) 
     3) Greater amount of the touch force is required in order to compensate for the mounting mechanisms&#39; pushback forces 
     4) Loss of sensitivity since a high minimum force (approximately &gt;20 gf) is required in order to eliminate inaccuracies from non-linear forces from friction, pre-loading or material bending. 
     5) An elaborate mounting mechanism of the top plate is needed to keep the sensors flat. The mounting adds to the forces that interfere with the measurements. 
     6) Difference in positioning calculation, that is different size and type of accuracy error, at different force levels. 
     Current precision instruments that incorporate precision bearing slide mechanisms try to offset the drag or friction when a shaft or feature is actuated through them. However, these parts can be somewhat bulky and pricey. In much more size constrained assemblies, or high volume manufacturing applications, a smaller or lower cost solution is needed. 
     It would be more advantageous to provide a suspension system for a touch-screen display that does not introduce any additional friction or non-linear forces to the touch screen system. It would also be desirable to reduce the dependency on extremely tight mechanical tolerances, which is a common problem in the above-described prior art force-based touch screens, and it is, therefore, an objective of current invention to address these needs with a more efficient mechanical construction. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to define and illustrate a mechanical suspension for a touch screen display/pad that does not add any additional non-linear forces to a touch screen system. 
     It is also the object of the present invention to provide a method and apparatus for providing frictionless translation ability in one direction, while constraining one or more parts from movement in the two other directions. 
     It is also another object to provide a frictionless motion translation mechanism for a variety of mechanical systems including touch screens that require components to be fully constrained in two directions (x &amp; y), and yet require unencumbered freedom to translate in the third direction (z-direction). 
     It is still another object to provide a frictionless motion translation mechanism as described above that can be contained within the existing parts, and need not require the use of separate bearings or precision constraint parts. 
     Another important object of this invention is to reduce the dependency on extremely tight mechanical tolerances by preloading the sensor. 
     In accordance with the foregoing objects, the present invention is a touch input comprising a touch lens suspended over force sensors by a new and innovative suspension mechanism, for use in either fixed or mobile devices such as point of sales terminals, kiosks, laptops, monitors, PDAs, cell phones, UMPCs and more. The suspension mechanism is scalable and can be varied in size and pre-loading pressure. The solution can also be manufactured at high volumes and at a low cost and is suitable for touch screen based products ranging from cellular phones and personal digital assistance up to computer monitors and plasma TVs/Monitors. The touch sensitive lens may also be either a separate touch lens made out of plastic or computer glass, or it may be the actual display module. 
     More specifically, the foregoing is accomplished by connecting each side of the lens with a looped string, monofilament or flexible wire, which is then looped around the back cover or base plate, forming a figure-8. The figure 8-loops bring the lens into a fixed state in the xy-plane without the addition of any friction causing physical contact. A smaller portion of the force applied by the figure 8-loop also directs the touch screen and the back plate together, providing a pre-loading onto the force sensors. 
     Alternative implementation and manufacturing methods of the above described figure 8-loop suspension system is disclosed one in which the suspension wire is threaded through holes. This manufacturing method not only reduces the manufacturing costs, but also optimizes the performance of the suspension mechanism. 
     Other alternative implementations include continuous suspension, bender suspension and 3-dimensional force suspension. As further described below, these suspension methods also ensures friction-free or close to friction-free movement in the z-plane while locking the lens in the xy-plane and thereby ensuring no movement or introduction of unknown forces to the system. 
     Another important improvement lies in the use of a flexible padding between the rigid back plane and the sensor allowing the sensor to be slightly compressed, e.g., preloaded, which reduces the dependency on extremely tight mechanical tolerances. 
    
    
     
       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  represents part of a touch screen system showing a force sensor between the touch lens and the back cover or LCD module. 
         FIG. 2  illustrates the figure-8 suspension mechanism on one of the 4 sides, holding the touch lens and the back ground aligned 
         FIG. 3  shows the components of  FIG. 2  but from a top-side view, looking down onto the touch screen. 
         FIG. 4  illustrates an alternative implementation from  FIG. 2 , where the touch screen is the actual LCD module. 
         FIG. 5  shows the components of  FIG. 4  but from a top-side view, looking down onto the touch screen. 
         FIG. 6  shows an actual implementation of the system from  FIG. 3 , where both touch lens and back cover are made up from clear glass plates. 
         FIG. 7  shows an actual implementation of the system from  FIG. 2 , where both touch lens and back cover are made up from clear glass plates. 
         FIG. 8  comprises schematic pictures of an HDK™ piezo resistive force sensor. 
         FIG. 9  illustrates an alternative implementation of the figure-8 suspension model, 3-dimensional force suspension. 
         FIG. 10  shows  FIG. 9  from an alternative view. 
         FIG. 11  illustrates an alternative implementation of the 3-dimensional force suspension 
         FIG. 12  illustrates an alternative mechanical implementation of the figure 8-look, using holes in the touch screen and base plate 
         FIG. 13  illustrates the use on one single monofilament for building the figure 8-look suspension 
         FIG. 14  illustrates how the forces are equalized in the suspension system if only a single monofilament is used. 
         FIG. 15  illustrates an alternative mechanical implementation of the figure 8-look, the continuous suspension. 
         FIG. 16  illustrates an alternative mechanical implementation of the figure 8-look, the continuous suspension. 
         FIG. 17  illustrates another alternative mechanical implementation of the figure 8-look, using bender suspension, implemented as an integrated bender. 
         FIG. 18  illustrates another alternative mechanical implementation of the figure 8-look, using a bender suspension, integrated into the lens in a side view. 
         FIG. 19  illustrates a small display module with integrated touch screen suspension mechanism into the display module. 
         FIG. 20  is a side view of a 2 dimensional version of the above-described bender suspension incorporated in a touch screen assembly, which additionally includes a plurality of force sensors  70  mounted atop a circuit board  60 . 
         FIG. 21  is a top view illustrating the touch pad/touch strip of  FIG. 20  as it may appear when integrated into a product such as a laptop PC with a row of touch sensitive keys. 
         FIG. 22  is a bottom view of an alternative bender suspension implementation, for a product scenario where the benders can not be integrated into the lens as in  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is a mechanical suspension for a touch screen display that does not add any additional non-linear forces to a touch screen system, and which preloads the touch screen sensor(s) to alleviate the need for extremely tight mechanical tolerances. The invention adapts a unique and different approach for mechanically connecting a touch screen lens to the force sensors, and to also provide a pre-loading force against the sensors with minimal non-linear changes over applied force pressure. 
     The illustrated embodiments of the present invention adhere to the following principles which are required for proper operation.
         The two opposing surfaces defined by the touch lens and sensor mounting plane, need to be flat and parallel.   The touch lens needs to touch the sensors when at rest (non-touch mode), with or without pre-loading applied.   The touch lens needs to be free to move down towards the bottom plate when a force is applied on the top of it.   The touch lens needs to be fixed in the xy-plane, enough to eliminate frictional forces or side movements of the lens.   Pre-loading must not introduce new non-linear forces to the system, especially from bending of the lens and/or the backplane.       

       FIG. 1  represents part of a touch screen system employing the foregoing principles, showing a force sensor  12  between a touch lens  10  and a rigid backplane  14  defined by a back cover or LCD module. The touch lens  10  will typically be made out of computer glass or treated plastic. The force sensor  12  is preferably mounted on a rubberized gasket  13  or padding made of Poron™ for example, to eliminate the need for exact placement or for bending of the lens  10 , as well as to provide additional spring-like preloading force to minimize the impact from shock and vibration. The maximum allowed movement, as allowed by the internal compression of the sensor  12  and the padding  13  is typically between 0.01-0.3 mm, but may be larger depending on sensor, padding material and operational force range. Each force sensor  12  may comprise a conventional piezo-resistive force sensor with a detent steel ball  11  or other force carrier, mounted therein to channel the force to the internal piezo membrane, where the applied force can be detected and measured. 
     For example,  FIG. 8(A , B) comprises top and side illustrations of a suitable HDK™ piezo resistive force sensor. Other force sensors, such as FSR or FTR sensors may also be used. 
     Given a plurality (such as, for example, four) differentially-mounted force sensors  12 , each sensor  12  registers a different force as a function of the two-dimensional (x, y) coordinates along the plane of the lens  10 . By calculating the differential pressure at the corners the exact coordinate of the actual touch can be calculated. 
     In accordance with the present invention (referring back to  FIG. 1 ), a plurality of suspension mechanisms  15  encircle both the lens  10  and backplane  14 , one suspension  15  around each of the four sides. Preferably, four orthogonal suspension mechanisms  15  are used along each of the four sides of the lens/backplane structure and collectively hold the touch lens  10  and the backplane  14  in parallel alignment. 
       FIG. 2(A-C)  illustrates one embodiment of the above-described suspension mechanism  15  formed as a figure-8, and shows how it is applied to one of the four sides of the lens  10 /backplane  14  structure, holding the touch lens  10  and the backplane  14  aligned. The suspension mechanisms  15  may be formed of string, wire, monofilament, leaf-spring metal or plastic, or any other material that provides the desired balance of tightening force versus shape-memory flex. The figure 8-shaped suspension mechanism  15  includes an upper loop ( 15 ′) circled around the lens  10  and integrally joined to a lower loop ( 15 ″) circled around the backplane  14 . All four sides of the lens  10 /backplane  14  structure are bound together with four figure 8-shaped suspension mechanisms  15  as illustrated in  FIG. 2 . Note that by tightening either loop along the direction of arrow A (as seen at FIG.  2 (C)), a majority of the vector tightening force as seen by arrow B pulls the lens  10  towards the center in relation to the backplane  14 . The level of tightening force in the suspension mechanism  15  is adjusted to ensure that a sufficient and correct pre-loading force C is pressing the lens towards the force sensors. 
       FIG. 3  shows the same plurality of suspension mechanisms  15  encircling both the lens  10  and backplane  14  as in  FIG. 2  but from a top-side view, looking down onto the touch screen. As illustrated in  FIG. 3 , the four figure-8 shaped suspension mechanisms  15  loop around and hold the four sides of the lens  10  in place in the xy-plane. As the centering forces A ( FIG. 2 ) pull each of the 4 corners toward the center the need for holding the lens  10  in place through any additional construction (which tends to add unwanted friction forces) is eliminated. A smaller vector component C of the tightening force A (from  FIG. 2 ) pulls the 4 corners of the lens  10  towards the backplane and thereby provides the required pre-loading force. 
     Another potential problem with alignment and tolerances is seen in  FIG. 2 . With four (or more) force sensors  12  placed in each corner of the lens  10 , sandwiched between the rigid touch lens  10  and the rigid backplane  14 , there is a risk that the mechanical structure is not perfectly aligned and (without loading) the flat lens  10  will only lie flat against three of the sensors  12 . Unless all sensors  12  are in contact with the lens  10 , the software will not be able to calculate the exact touch coordinates. One alternative is to add enough preload force so that the lens  10  bends down to all force sensors  12 . However, testing shows that the bending adds mechanical strain, which will create additional non-linear forces that will influence the positioning calculation and add accuracy errors that cannot be fully compensated for in software. A better solution to this problem is solved by a layer of padding  13 , which may be a thin layer of flexible material such as Poron™ positioned under each sensor  12 . Typically a layer of Poron™ open celled microcellular polyurethane at 0.2 mm to 0.6 mm provides enough flexibility between the parallel planes, without adding too much movement in the sensor structure. It is important that the padding  13  impart a minimal upward bias to the sensors  12  to keep them in contact with the lens  10 , but the maximum compression of the padding  13  should be kept at a minimum. One skilled in the art should understand that the “padding” material  13  can also be replaced by a more traditional leaf-spring or coil-spring type of component. 
     In actual testing, the above-described Poron™ padding  13  made it possible to distinguish the touch coordinates with substantially higher accuracy than a pre-loaded model without the Poron™ under the force sensors  12 . Actual performance enhancement will depend on the material tolerances and the sensor type. 
     The invention counteracts the interfering forces in a force-based touch screen device, minimizing them to the point that the software formulas start working with much higher accuracy. It also removes the need for an elaborate mounting mechanism for the top plate or lens  10 , and eliminates the need for traditional pre-loading structure over the sensors  12 , such as springs or metal clamps pressing down on the lens  10  over the sensors  12 . 
       FIG. 4  illustrates an alternative embodiment wherein the touch screen  40  is the actual LCD display module (there is no lens  10  or the lens is integral to the LCD). As before, this embodiment is likewise a figure 8-loop suspension mechanism  44 , but here the figure 8-loops are wrapped around the display module  40  and the underlying mechanics, such as the printed circuit board (PCB)  43  of the device. Preferably, four figure-8 suspension mechanisms  44  are used, each encircling one of the four sides of the LCD  40 /PCB  43  structure, holding the two in alignment. 
       FIG. 5  shows the components of  FIG. 4  from a top-side view, looking down onto the touch screen  40 . Unlike the other embodiments where the touch lens  10  is suspended above the display module, here the actual LCD module  40  is the touch sensitive lens. This design approach may be more advantageous for more compact implementations since it can fit in a smaller display housing  45  and is therefore preferred for small footprint implementations such as mobile phones. 
     As illustrated in  FIG. 5 , the LCD module  40  or the frame of the module  45  may be extended above the force sensors  41 , still keeping the figure 8-loop suspension mechanisms  44  in place. The same mechanical concept is also shown in  FIG. 4  where the display module  40  extends over the force sensors  41 . One advantage of this is that the force sensors  41  can now be placed in the same plane as the display module  40  and thereby supporting a more height-compact implementation. Additional height is saved since the display module  40  acts as the actual lens and only a thin computer glass cover (or none at all) needs to be added to the display. This eliminates the need for a thicker freestanding lens as well as the air space between the lens and the display. 
       FIG. 6  shows a top view of the actual implementation of the system from  FIG. 3 , where both touch lens  10  and back cover  14  comprise clear glass plates. The glass provides both a view through the system as well as very flat and rigid planes. 
       FIG. 7  shows a side view of the actual implementation of the system from  FIG. 3 , where both touch lens  10  and back cover  14  are made up from clear glass plates, and figure 8-loop suspension mechanisms  15  each comprise a length of monofilament closed into a loop with a small metal clasp, and then crossed into a figure-8 configuration. 
     In practical testing, it was found that improved performance can be had by not fixing the figure 8-loop suspension mechanisms  15  in place on the lens  10  or backplane  14 , but instead to allow them to “move” freely around both lens and backplane, as illustrated in  FIG. 7 . Here the structure is simplified so that both lens  10  and backplane  14  are made up by two glass plates with four force sensors  12  placed in each corner. Here the suspension mechanisms  15  each comprise a figure 8-looped monofilament line encircling and holding the plates  10 ,  14  in place on all four sides, also as illustrated in  FIG. 6 . By allowing the figure 8 loops to not be fixed in place, the suspension mechanism  15  material can move small distances without any restrictions. This movement allows for better and adaptive distribution of force, and it the suspension mechanism  15  material is resilient (such as monofilament) this freedom better utilizes the elasticity of the material to more evenly distribute forces. This in turn ensures a more uniform performance and force distribution, minimizes friction and other non-linear forces in the pre-loading and the xy-centering of the lens  10 , and generally improves the accuracy of the total system. In experiments with alternative mechanical solutions, it was observed that side movement introduced into the lens  10 , such as when drawing a line, had a very negative impact on accuracy. The friction between the lens  10  and the user&#39;s finger drags the lens  10  in the direction of the finger movement and the lens is transferring a force both downwards as well as a side fore. Due to the nature of most available force sensors, the measured force must be applied directly (90 degree angle) into the sensor membrane. The side force from the dragging of the finger typically introduced an accuracy error between 0% and 10% of the total distance between the sensors. With the force applied to the line A as illustrated in  FIG. 2(C) , most of this large force is applied to fixating the lens in the xy-plane. The force B in the xy-plane from the user&#39;s finger now became less then 1% of the total pre-loaded force in the xy-plane, basically eliminating or filtering out the impact of the side force, ensuring that only the z-directional force is being measured. 
     It should also be noted that also for a perfect mechanical construct where the lens  10  is resting perfectly on all the force sensors  12 , there is still a need for an additional pre-loading force. By adding a pre-loading force to the lens that is greater than the weight of the lens  10 , the lens  10  will remain in contact with the sensors  12  even if the touch screen system is held upside-down. The added pre-loading will also reduce mechanical interference and jitter. 
     Each of the materials (glass, tape, monofilament line) used in the implementation illustrated by  FIG. 6  and  FIG. 7  were selected due to their physical characteristics and could be substituted by other materials with similar characteristics. 
     One incidental benefit of the above improvements is that the mounting system for the lens  10  or top plate needed previously became unnecessary, since the lens/top plate is now suspended by the monofilament line, reducing the mechanical height of the system and eliminating the number of components. 
     Another important aspect of the invention is that it can be mass-produced at a reasonable cost. Reviewing the description above, it should become clear that the only additional components to the system are the four suspension mechanisms  14  and four semi-flexible padding elements  13  (such as Poron™) for the four force sensors  12 , assuming the system design is using 4 sensors. For high volume production, the suspension mechanisms  15  may be pre-formed and added to the system during production automatically or by hand. Since material and lengths are known, the pre-loading force from the suspension mechanisms  15  can simply be calculated and the lines made accordingly. In addition, the mechanical design will be simpler and traditional pre-loading spring system and lens suspensions systems eliminated. 
     The above described figure 8-loop suspension mechanism provides the needed functionality for locking the lens/display in the xy-plane and yet allowing for small frictionless movement in the z-plan. For high volume production, it is however possible to further enhance the mechanical concept. 
       FIG. 12  represents a display  110  where the touch glass  112 , standard part of a touch LCD, extends beyond the boarders of the display module. The glass is also manufactured with one hole  111  in each corner. The underlying base plate (which may be a second glass plate, B-cover, PCB or other material serving as base plate to the system) is also equipped with corresponding holes. During the manufacturing process, the holes  111  make it a much simpler task to monofilament the system with one piece of wire/line  112 . The result is:
         Only one (1) monofilament to apply tension to and to clamp (into one continuous loop. Speeding up the production process   Stringing the system through these holes is also simple and time quick   By only using one monofilament, it is now possible to ensure that the tension in the system is the same everywhere, as illustrated in  FIG. 14 . This will ensure that the system equilibrium is a state where the preloading forces are the same over each of the sensor and the forces in the xy-plane are pulling the lens towards the middle of the lens, not accidentally pushing it closer towards one side and thereby increasing the risk of material interference with other components. Since the monofilament is connected and allowed to move freely, the tension in the monofilament F 1  is equal to the tension anywhere else in the monofilament F 2 . Note that the lens must have low friction by the holes in order to allow the monofilament to move freely and thereby equalize the monofilament tension.       

       FIG. 13  illustrates one pattern for stringing the unit with only one continuous monofilament wire  112 . The monofilament/wire  112  is strung according to the pattern in  FIG. 13  through both the touch lens and the base plate. As is shown, the monofilament will pass through each hole twice to the opposite hole. For example the monofilament reaches from the bottom plate top left (B-TL) hole to the top plate top right (T-TR) and bottom left (T-BL) holes 
     For both basic suspension concepts described above, it shall be noted that the force sensor  12  shall be placed as close as possible to hole or the wrapping of the monofilament. If there is a significant distance, the pre-loading (the downwards force) from the monofilament can case the lens or base plane to bend. This material bend would then add or subtract unpredictable forces when the lens is pressed and thereby distort the measured touch coordinate. 
     It is possible to implement alternative systems based on the principles of the figure 8-loop, at least in part. It is not possible to fully replicate the force distribution between the xy-plane and the z-plane that is supported by the figure 8-loop, but it is possible to implement part of the principles. 
       FIG. 9  illustrates an alternative implementation of the figure 8 suspension model, and  FIG. 10  shows  FIG. 9  from an alternative view. As seen in  FIG. 9  the display is suspended by 8 strong spring elements  92 , 2 in each corner. These 8 springs will apply large forces in the xy-plane F xy . A small pre-loading force F z  is applied directly above the sensor. The spring elements  92  will restrict movement in the xy-plane, but have no impact on any movement in the z-plane. This construct provides necessary pre-loading force, restriction of movement in the xy-plane as well as elimination of non-linear force from friction when pressure is applied. This solution can be made good enough for many implementation, although it can not reproduce the preciseness of the figure-8 loop construction. It shall also be noted that the forces in the xy-plane F xy  are typically 20 to 40 times the size of the pre-loading forces F z . The major difference with this suspension concept compared to the figure 8-loop is that each of the spring elements act as individual forces, while the wire in the figure 8-loop suspension ensures that the relationship between the forces in the xy-plane and the z-direction is maintained, as illustrated in  FIG. 2 . 
     It is also possible to implement the same type of mechanical construction described in  FIG. 9 , but using a reversed force in the xy-plane. Again, the important factor is that the xy forces are high enough to eliminate the xy-movement of the lens. 
     For example,  FIG. 11  illustrates an alternative implementation of the figure-8 suspension model, where the lens  90  is pulled towards the corners with a force F xy , substantially larger then the applied pre-loading force F z . 
     One additional suspension mechanism implementation is described in  FIG. 15 . It also captures the figure 8-concept to a high degree, although the concept is building on no additional side forces applied to the lens  150  in its equilibrium state, but allowing for a fast increase in opposite forces in case the lens is moved in a side direction. In this embodiment, the lens is attached to an outside frame  151  with a small airgap  152  between the bodies. The lens  150  is fixed in position relative to the frame  151  through a thin film  153 , such plastic or adhesive tape. This tape can be kept very thin since it can be applied around the complete lens  150 . If a side-force is applied to the lens  150 , the tape is resisting the movement in the 2 parallel sides and the opposite side, virtually elimination any side movement. 
     As can be observed in a side view of the system in  FIG. 16 , it is obvious that the tape  153  holding the lens  150  in place within the outside frame  151  will allow for small movements in the z-direction with only creating minimal friction forces. As the required movement is in the area of 0.01 to 0.2 mm, the forces created from the stretching of the tape will be minimal if any. In addition to the xy-plane control, the suspension solution must also provide a preloading force to ensure that the lens is always resting on the force sensor  41  and to reduce jitter from physical vibrations. A small preloading force is then applied directly over the sensor  41  through a spring or spring like material  154 . 
     As for most mechanical concepts, the solution can typically be designed in multiple ways, each with advantages and disadvantages. The implementation described in  FIG. 15  and  FIG. 16  can also be implemented through integrated benders manufactured into the lens or added to the lens as described in  FIG. 17 . 
       FIG. 17  illustrates one corner of the touch sensitive lens  170 . In this embodiment, the lens  170  is notched with two parallel V-shaped notches to define a V-shaped beam or “bender.” A screw  172  is inserted in the bender  171 . As is better illustrated in  FIG. 18 , as the screw  172  is tightened, the bender  171  will bend towards the base plane  173 , pulling the complete lens structure towards the base plane  173  and the force sensor  41 . Since there are often conflicts between the materials the lens  170  should be rigid, while the bender  171  should act as a spring in the z-plane and rigid in terms of any movement in the xy-plane, it is often a preferred solution to use a flat metal spring for the bender  171 . 
     It shall be noted that also this concept fulfills most of the benefits from the original 8-loop suspension mechanism, however, there will be minor forces within this beam suspension that will hinder the solution from performing as well as the 8-loop suspension, however, for mechanical implementations where a wired solution can not be used, this solution will provide a close performance, although the accuracy of the touch system is expected to be between 0% and 20% lower due to energy absorption in the benders and the change in pre-loading force as pressure is applied to the touch lens (the minor movement of the lens  170  will slightly reduce the built-in preloading forces from the bender  171 . Obviously, using a known spring-like material for the bender  171  with a known spring constant (k) it is possible to adjust for much of the pre-loading changes due to the lens movement during a touch. 
       FIG. 19  illustrates a modular implementation of the figure 8-loop concept. Especially for small devices, such as mobile phones, it is vitally important that the added size is kept at a minimum and smaller design modifications may be required. In this integration example, the display “can”  196 , that is the module packaging of the display, is extended around 1 mm to 2 mm in the x and y dimension, enough to create a small cavity  193  for the 8-loop string  199 . The string/wire  199  may be looped in the recommended 8-loop or in a simplified (and slightly less accurate) 0-loop in order to reduce the required space. The string/wire  199  is still looped around a touch lens  190  and a supporting back plane  197 , preloading the force sensors  195 . For this particular application, these sensors are likely thin force resistive material based sensors, which can be made less then 0.5 mm thick. The sensor connection  192  to the outside electronics can be a separate connector or shared with the existing display and backlight connection system. 
       FIG. 20  illustrates an alternative implementation of the bender  213  for a touch pad. In this implementation, the bender can be placed underneath the touch pad  211  since there is no need for a transparent lens. The benders  213  are mounted underneath the touch pad  211  and held in place with molded posts  212 . The benders  213  are also fixed in the outside frame  214 . The four force sensors  215  are located in each corner. 
       FIGS. 22 and 21  illustrate a 2 dimensional implementation, such as a key strip shown in  FIG. 22 . In this embodiment, there is only a need for two force sensors  237  and two benders  233  and the slightly higher position error from using a bender concept would be acceptable in such application. 
     It should now be apparent that the above-described embodiments all provide a mechanical suspension for a touch screen display/pad that does not add any additional non-linear forces to a touch screen system, provides frictionless translation ability along one axis, and yet requires components to be fully constrained along the other two axes. 
     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. 
     INDUSTRIAL APPLICABILITY 
     Touch screens are being deployed in an increasing number of products using an array of several types of technology. As the products continually decrease in size, the demand for inexpensive, low-profile and precise touch screens is increasing. One type of touch screen senses differential-pressure of the lens or LCD using four corner-mounted force sensors. However, this type of touch screen also requires a suspension system for the lens/LCD, and existing suspension systems introduce friction or non-linear forces into the system that disrupts the measurements of the force sensors. Therefore, there is significant industrial applicability in the present invention which provides a differential-pressure force sensing suspension system which overcomes some of the deficiencies of the prior art.