Touch screen device and shielding bracket therefor

A bracket for shielding a capacitive touch screen from external noise arising from stray capacitance comprises upper and lower complementary rectangular conductive metal shield elements which are joined together inside a housing with a transparent touch screen retained therebetween. The upper shield element is provided with a large central aperture so that the touch screen is accessible to a user. The lower shield element is also provided with a large central aperture to enable a display, such as an LCD, to be viewed through the transparent touch screen. The display is secured to the lower shield element with an insulating member interposed between the display and the lower shield element. The shielding bracket provides a full shield for the upper, lower and edge surfaces of the touch screen to prevent the capacitance of a hand resting upon the housing from interfering with the touch screen.

BACKGROUND OF THE INVENTION 
The present invention relates to a touch screen device and more 
particularly to a shielding bracket used in the 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. 
SUMMARY OF THE INVENTION 
The present invention provides an integrated display (such as an LCD) and a 
touch screen mounting bracket that completely shields the touch screen 
from external noise due to stray capacitance. This shielding bracket is 
superior to the conductive tape shielding method because it covers the 
touch screen in a "seamless" manner, thereby shielding the glass much more 
effectively, and it uses highly conductive rigid metal which increases the 
effectiveness of the shield. The unused glass of the touch screen under 
the bezel is shielded with conductive metal, the four signal wires are 
shielded, and the sides of the glass of the touch screen are shielded. 
It is accordingly an object of the present invention to provide a novel and 
effective shielded capacitive touch screen device. 
Another object is to provide a capacitive touch screen shield which 
includes two complementary conductive metal elements which are joined 
together with the touch screen therebetween. 
Another object is to provide an integrated LCD and touch screen glass 
mounting bracket that completely shields the touch screen from external 
noise due to stray capacitance. 
Another object is to provide a shield bracket which is effective to shield 
unused glass of the touch screen, signal wires which are coupled thereto 
and the sides of the touch screen glass. 
Additional benefits and advantages of the present invention will become 
apparent to those skilled in the art to which this invention relates from 
the subsequent description of the preferred embodiment and the appended 
claims, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION 
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. 
Although the invention has been described with particular reference to a 
preferred embodiment thereof, variations and modifications of the present 
invention can be effected within the spirit and scope of the following 
claims.