A capacitive touch screen shield

Abstract: This invention discloses a capacitive touch screen shield for shielding a capacitive touch screen (44) from external noise arising from stray capacitance, the shield comprising upper (50) and lower (52) complementary rectangular conductive metal shield elements which are joined together inside a housing with a transparent touch screen (44) retained therebetween. The upper shield element (50) is provided with a large central aperture (62) so that the touch screen (44) is accessible to a user. The lower shield element (52) is also provided with a large central aperture (64) to enable a display (48), such as an LCD, to be viewed through the transparent touch screen (44). The display (48) is secured to the lower shield element (52) with an insulating member (72) interposed between the display (48) and the lower shield element (52). The shield provides a full shield for the upper, lower and edge surfaces of the touch screen (44).

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 characteristic that the hand does not touch the conductive coating 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, and then driving the tape 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, it still allows some stray capacitance to get through. Another problem is that the four wires that connect to the corners of the touch screen glass are also sensitive to stray capacitance, and the tape does not shield these wires at all. It is an object of the present invention to provide a capacitive touch screen shield which effectively shields a capacitive touch screen from stray capacitance. According to the first aspect of the present invention there is provided a capacitive touch screen shield, characterized by a first shield element of rigid conductive material having an upper surface and a lower surface, also having a central opening through which a touch screen positioned beneath said shield element is accessible, and having a peripheral portion which extends past the outer edges of said touch screen; and a second shield element of rigid conductive material which is located beneath said touch screen and supports the touch screen, said second shield element including an upturned portion which shields the edges of the touch screen, the second shield element being secured to the lower surface of the first shield element. According to a second aspect of the present invention there is provided a touch screen device, characterized by a top shield of conductive material of generally rectangular configuration, having an upper surface and a lower surface, and also having a central opening through which a touch screen positioned beneath said top shield is accessible; a touch screen holder shield of conductive material having an upper surface and a lower surface, also having an indented portion for receiving and retaining said touch screen, and a central opening in said indented portion through which a display positioned below the indented portion is visible, the holder shield being secured to the top shield; said transparent touch screen being located in the indented portion of said holder shield; a bezel surrounding the peripheries of said top shield and said holder shield; and a display positioned beneath the holder shield and secured to said holder shield. An embodiment of the present invention will now be described, by way of example, with reference 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 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 top shield of the touch screen device of the present invention and Fig. 7 is a plan view of the touch glass holder shield of the touch screen device of the present invention;. Referring now to Fig. 4, shown there is a touch screen device 40, which may comprise or be included as a part of a business terminal, comprising a bezel 42 and a touch screen 44, which is normally transparent to permit viewing of a display positioned beneath it. The bezel 42 includes an aperture 46 through which the touch screen 44 is accessible. Located beneath the touch screen 44 and not visible in Fig. 4 is a display 48 (Fig. 5), which typically may be a liquid crystal display (LCD). The internal structure of the device 40 is shown in Fig. 5, which is a partial sectional view taken along line 5-5 of Fig. 4. As shown in Fig. 5, the touch screen 44 is held in position in the interior of the device 40 by being placed between two shield elements, namely a touch screen top shield 50 (Fig. 6) and a touch screen holder shield 52 (Fig. 7). The shields may be made of any suitable conductive relatively rigid metal, such as aluminum or steel, and will most commonly be fabricated in a stamping operation. The touch screen holder shield 52 has an indented portion 54 to receive and retain in position the touch screen 44. After the touch screen 44 has been inserted between them, the two shields 50 and 52 are held together by a plurality of studs, screws or other suitable fasteners 56, which also secure the assembled shields and touch screen to the bezel 42. The fasteners 56 extend through matching apertures 58 and 60 in the shields 50 and 52, respectively. As shown most clearly in Figs. 6 and 7, the two shields 50 and 52 are also provided with large central apertures 62 and 64, respectively. The aperture 62 provides access by a user to the touch screen 44, to enable it to be touched or otherwise written upon for the purpose of data entry. The aperture 64 in the shield 52 enables the display 48 to be seen from the exterior of the touch screen device 40 through the transparent touch screen 44. The display 48 is retained in operative relation to the touch screen 44 by a plurality of screws or other fasteners 66 made of suitable non-conducting material which extend through apertures in the display 48 and through apertures 68 in the touch screen holder shield 52. A rectangular insulating sheet 70 with a suitable rectangular aperture 72 is positioned between the shield 52 and the display 48 to provide the necessary electrical insulation between the shield and the display, and is provided with apertures through which the fasteners 66 extend. The entire assembly of the shields 50 and 52, together with the touch screen 44, is driven with an electrical signal applied to the assembly. Insulated bushings and foam tape may also be used to keep the bracket or shield assembly from grounding the display 48. It will be seen that the shielding bracket comprising the shields 50 and 52 provides a full 180 degree shielding surface to prevent the capacitance of a user's hand resting on the side of the bezel from interfering with the touch screen 44, on the top, bottom or edge surfaces.

Claims: A capacitive touch screen shield, characterized by a first shield element (50) of rigid conductive material having an upper surface and a lower surface, also having a central opening (62) through which a touch screen (44) positioned beneath said shield element (50) is accessible, and having a peripheral portion which extends past the outer edges of said touch screen (44); and a second shield element (52) of rigid conductive material which is located beneath said touch screen (44) and supports the touch screen, said second shield element (52) including an upturned portion (54) which shields the edges of the touch screen (44), the second shield element (52) being secured to the lower surface of the first shield element (50). A capacitive touch screen shield according to claim 1, characterized in that said first and second shield elements (50,52) are generally rectangular in configuration. A capacitive touch screen shield according to claim 1 or claim 2, characterized in that second shield element (52) has a central opening (64). A capacitive touch screen according to claim 3, in which said second shield element (52) includes at least one mounting device (66) for mounting a display (48). A capacitive touch screen shield according to any one of the preceding claims, characterized in that said first and second shield elements (50,52) are formed from aluminum. A capacitive touch screen shield according to any one of claims 1 to 4, characterized in that said first and second shield elements (50,52) are formed from steel. A capacitive touch screen shield according to any one of the preceding claims, characterized in that said first and second shield elements (50,52) have the same external dimensions. A capacitive touch screen shield according to claim 3, characterized in that said central openings (62,64) of said first and second shield elements (50,52) have the same dimensions. A capacitive touch screen shield according to any one of the preceding claims, characterized in that said second shield element (52) is secured to said first shield element (50) by a plurality of fasteners (56). A touch screen device, characterized by a top shield (50) of conductive material of generally rectangular configuration, having an upper surface and a lower surface, and also having a central opening (62) through which a touch screen (44) positioned beneath said top shield (50) is accessible; a touch screen holder shield (52) of conductive material having an upper surface and a lower surface, also having an indented portion (54) for receiving and retaining said touch screen (44), and a central opening (64) in said indented portion (54) through which a display (48) positioned below said indented portion (54) is visible, the holder shield (52) being secured to the top shield (50); said transparent touch screen (44) being located in the indented portion (54) of said holder shield (52); a bezel (42) surrounding the peripheries of said top shield (50) and said holder shield (52); and a display (48) positioned beneath the holder shield (52) and secured to the holder shield (52).

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