A capacitive touch screen shield

Abstract: The invention discloses a shield (58) for shielding a capacitive touch screen (54) from external noise arising from stray capacitance. The shield (58) is located between the exterior housing (52) of a touch screen device and the touch screen (54) itself. The shield (58) comprises a thin piece of conductive material (76) such as aluminum foil laminated between two layers of insulating material (78,80) such as a poly-carbonite and is secured to the upper surface of the touch screen (54), as by an adhesive (82). The shield (58) has a central opening (60) which is smaller than an opening (56) in the housing (52) of the touch screen device that is provided to enable the touch screen (54) to be contacted for entry of data, and to enable a user to view a display (62) that is located beneath the touch screen (54), which is transparent. Portions of the shield (58) are thus visible, and the central opening (60) of the shield (58) thus defines the portion of the display (62) which can be viewed. Different sizes of displays (62) can therefore be used in the touch screen device by using shields (58) with different sizes of central openings (60), without the need for redesigning the exterior housing (52) of the touch screen device.

Description: The present invention relates to a capacitive touch screen shield used in a touch screen device to shield the touch screen from external noise due to stray capacitance. Capacitive touch screens have become one of the major touch screen technologies in marketplaces such as the hospitality and quick-service point of sale marketplaces, mainly due to their extremely high durability and their resistance to interference from foreign objects on the screen, such as dirt and grease. Capacitive touch screens calculate touch points by measuring the amount of current that the body of a user of the screen draws from the screen. This current drain is caused by capacitive coupling of the user's finger (and body) from a conductive film layered on the glass of the screen. A capacitor is defined as being two conductive surfaces separated by a thin insulating surface. In the case of a capacitive touch screen, as shown in Fig. 1, the two conductive surfaces are the user's finger 10 and a conductive coating 12 on the glass 14 of the touch screen. The insulating substance is a thin protective covering 16 layered on top of the conductive coating 12. This combination forms a capacitor, represented symbolically in Fig. 1 by the capacitor 18. The touch point is calculated by measuring the current drain at the four corners of the touch screen. The closer a user's touch is to a corner, the more current drain will be measured from that corner. A microprocessor circuit compares the current drain from the four corners and calculates an X, Y coordinate for the touch point. It is the fact that the hand does not actually touch the conductive coating that makes capacitive touch screens so durable, for reasons which include the following. The insulating coating can be made very strong so that it protects the conductive coating from scratching and wearing over time. Any foreign objects on the insulating coating (dirt, grease, etc.) will not degrade the performance of the touch screen. The touch screen assembly has no moving parts; therefore it can be sealed to the bezel of the touch screen device to prevent leakage of liquids into the device. Unfortunately, it is also the fact that the hand does not touch the conductive coating of the touch screen that makes capacitive touch screens susceptible to interference. For example, as shown in Fig. 2, a left hand 20 of a user placed on the bezel 22 of a touch screen device 24 will induce enough stray capacitance to cause error, as shown by point 26, in the X, Y coordinates which are calculated by the system to represent a point 28 touched on the touch screen 30 by a finger 32 of the right hand 34 of the user. The reason that the touch screen 30 so easily picks up the capacitance (C') of the hand 20 is that the active area of the touch screen 30 is only approximately 75% of the entire area of the glass. The 25% unused area of glass (hidden from view under the bezel 22) is required to distribute charge evenly throughout the conductive coating. Although this portion of the touch screen is unused, it is still sensitive to capacitance. Therefore it easily picks up the large capacitance of the hand 20 since only the thin bezel 22 separates the hand 20 from the glass of the touch screen 30. Current methods to shield this sensitive unused portion of the touch screen 30 from stray capacitance induced by the hand 20 of a user of the touch screen device 24, which includes the touch screen 30 and an associated display 34, include covering the unused portion of the screen with a piece of conductive tape 33 beneath the bezel 22, as shown in Fig. 3. The touch screen 30 typically comprises a glass base 36 having a conductive coating 38 and an insulating coating 40 disposed thereover. The conductive tape 33 typically comprises a plastic tape base 42 with a silk-screened conductive pattern 44 on its surface. The tape 33 is driven with the same signal that is used to drive the four corners of the touch screen glass. Although the conductive tape certainly is an improvement over the unshielded glass, and insulates the touch screen from a large portion of any outside interference, it still allows some stray capacitance to get through. Two other drawbacks of the tape shield method are that it is visually unattractive and that it must be electrically isolated from grounding sources since it carries a signal. In US-A-3,974,332, there is described a capacitively coupled tablet for use with an electrically conductive pen. The tablet comprises segments on its surface defined by lattice points of a matrix which comprises groups of spaced conductors disposed along coordinate axes. The segments are mutually electrically isolated by an electrically conductive shield layer having apertures which correspond to the surface area of the respective segments, and are marked with corresponding characters to serve as keys. An electrically insulating layer is disposed between the shield layer and the groups of conductors, and a similar layer is disposed over the surface of the shield layer. It is an object of the present invention to produce a capacitive touch screen shield which effectively shields a capacitive touch screen from stray capacitance. According to a first aspect, the present invention consists in a capacitive touch screen shield of generally rectangular configuration and comprising a layer of conductive material laminated between two layers of insulating material, characterized by a central opening defining the size of a display viewable through a touch screen shielded by the shield. According to a second aspect, the invention consists in a touch screen device having a shield of generally rectangular configuration comprising a layer of conductive material laminated between two layers of insulating material and an upper surface and a lower surface, characterized by a transparent touch screen secured to the lower surface of the shield, a housing surrounding the peripheries of the shield and the touch screen, a display positioned beneath the touch screen, and a central opening in the shield through which the touch screen is accessible and defining the size of the display to be viewed through the touch screen. With the present invention, the insulating material can be chosen to be visually attractive and thus the shield need not be hidden from view by a housing. Therefore, one shield size may be utilized for all sizes of touch screen and display. In order that the present invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a diagrammatic view illustrating the operation of a capacitive touch screen; Fig. 2 is a diagrammatic view showing the manner in which errors are introduced in conventional capacitive touch screens when the hand of a user is inadvertently placed on the bezel of a touch screen device; Fig. 3 is a partial sectional view of a prior art capacitive touch screen device, showing a tape shield employed therein; Fig. 4 is a perspective view of a touch screen device. Fig. 5, located on the same sheet as Fig. 3, is a partial sectional view, taken on line 5-5 of Fig. 4, of the capacitive touch screen device of the present invention; Fig. 6 is a plan view of the touch screen device, showing the use of one size of touch screen shield with one type of display; and Fig. 7 is a plan view similar to Fig. 6, showing the use of a different size of touch screen shield with a different type of display. Referring now to Fig. 4, shown there is a touch screen device 50, which may comprise a business terminal, comprising a housing 52 and a touch screen 54, which is normally transparent to permit viewing of a display 62 positioned beneath it, with an insulating element 64 positioned between the touch screen 54 and the display 62. The touch screen 54 may comprise a glass base 66 having a conductive coating 68 and an insulating coating 70 disposed thereon. The housing 52 includes an aperture 56 through which the touch screen 54 is accessible. A touch screen shield 58 is positioned between the housing 52 and the touch screen 54 and is secured to the touch screen 54. The shield 58 extends beyond the housing 52 so that it is visible from the exterior of the touch screen device 50. An opening 60 in the shield 58 provides access to the touch screen 54 and enables the display 62 to be viewed through the touch screen 54 by a user of the touch screen device 50. Located beneath the touch screen 44 and not visible in Fig. 4 is a display 62 (Fig. 5), which typically may be a liquid crystal display (LCD). The internal structure of the device 50 is shown in Fig. 5, which is a partial sectional view taken along line 5-5 of Fig. 4. The touch screen 54, with its associated shield and display 62, is held in position in the interior of the device 50 in any suitable manner, as for example by supports 72 and 74. The touch screen shield 58 comprises a thin layer 76 of conductive material, such as aluminum foil, for example aluminum foil No. 7800, marketed by Minnesota Mining and Manufacturing Company, laminated between two layers 78 and 80 of insulating material, such as a plastic material in the form of a poly-carbonite, for example "LEXAN" (Trademark) marketed by General Electric Company. The shield 58 is secured to the touch screen 54 by a layer of adhesive 82, for example adhesive No. 467 marketed by Minnesota Mining and Manufacturing Company. The poly-carbonite material has very good cosmetic qualities. It comes in a wide variety of textures and colors. This allows the shield 58 to extend past the opening 56 in the housing 52 and thus be exposed to view by operators of the touch screen device 50, rather than being hidden beneath the housing 52. It is important to note that since the shield is also electrically insulated, the operator of the touch screen device 50 may physically touch the shield 58 without causing any degradation of its noise shielding characteristics. Non-insulated tape shields will either be rendered useless or will interfere with the operation of the touch screen device if they are touched by the operator. The "viewability" of the shield allows it to be used to cosmetically "adapt" multiple display devices to one size of bezel or housing opening, such as the housing opening 56. This means that one touch screen and one housing can accept several different sizes of displays simply by using touch shields having different sizes of openings, rather than requiring that both the touch screen and the bezel of the housing be retooled to match the size of the display. This is illustrated in Figs. 6 and 7, in which different sizes of shields 58 having different sizes of openings 60 are used with housings 52 having the same size of central opening 56, in order to accommodate different sizes of displays 62, and to provide different sizes of mattes surrounding the viewable portion of the touch screen 54. The current rapid pace of development in displays such as LCDs makes it very advantageous to be able to use the same housing for multiple sizes of LCDs. This allows the manufacturer of business machines such as point of sale terminals to create a single cabinet which can remain unmodified throughout the life of the product. As new and improved LCDs become available, the manufacturer can adapt them to the product by using touch screen shields 58 having different sizes of openings 60. Therefore the tooling costs associated with introducing a new LCD on a touch-based system will be much less than normally expected, which allows manufacturers to take full advantage of the rapid pace of LCD technology, rather than being hindered by it.

Claims: A capacitive touch screen shield (58) of generally rectangular configuration and comprising a layer of conductive material (76) laminated between two layers of insulating material (78,80), characterized by a central opening (60) defining the size of a display (62) viewable through a touch screen (54) shielded by the shield. A touch screen device having a shield (58) of generally rectangular configuration comprising a layer of conductive material (76) laminated between two layers of insulating material (78,80) and an upper surface and a lower surface, characterized by a transparent touch screen (54) secured to the lower surface of the shield (58), a housing (52) surrounding the peripheries of the shield (58) and the touch screen (54), a display (62) positioned beneath the touch screen (54), and a central opening (60) in the shield through which the touch screen (54) is accessible and defining the size of the display to be viewed through the touch screen. A touch screen device according to claim 2, characterized in that the housing (52) has a central opening (56) which is larger than the central opening (60) of the shield (58). A touch screen device according to claim 2 or 3, characterized in that the shield is secured to the touch screen by an adhesive. A touch screen device according to claim 2, 3 or 4, characterized in that the exterior dimensions of the shield are the same as those of the touch screen. A touch screen device according to claim 2, 3, 4 or 5, characterized in that a portion of the shield surrounding its central opening is visible through the opening in the housing from the exterior of the touch screen device. A touch screen shield or device according to any preceding claim, characterized in that the conductive material (76) is formed from aluminium foil. A touch screen shield or device according to any preceding claim, characterized in that the insulating material (78,80) is a plastic material. A touch screen shield or device according to claim 8, characterized in that the insulating material (78,80) is poly-carbonite material. A touch screen shield or device according to any preceding claim, characterized in that the shield (58) has a rectangular shaped central opening (60).

Inventors: FORSYTHE DONALD L, YATES JOSEPH W IV, FORSYTHE, DONALD L., YATES, JOSEPH W. IV
Applicants: NCR INT INC, NCR INTERNATIONAL, INC.
Application year: 1995
Main classification: G06K11
IPC codes: G06F3