CA1114921A - Curved membrane touch panel employing piezoresistant anti-short means - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A location sensitive touch panel for use on a rigid substrate.
The substrate carries a first set of conductive strips. A resilient plastic membrane overlaying it carries a second set of conductive strips orthogonal to and spaced from the first set. Finger pressure can cause electrical contact between any one of the first set of strips and any one of the second set of strips. The substrate may be either flat or curved, with the membrane conforming to its contour.

Description

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The invention is a touch panel device which electrically indicates the X-Y coordinates of contact of an operator's finger on it and is sensitive to pressure only. Frequently, information is displayed on a substrate beneath the touch panel as well. The coordinates of a contact can be related to the displayed information thus providing for inter-active communication between the operator and the device of which the touch panel forms a part.
The prior art includes a variety of techniques for sensing the location of contact on a surface. The most similar device of which the inventors are aware is the stretched drumhead type of membrane. This de-vice employs a membrane spaced from a flat substrate and which can be deflected to cause conductors carried on it to contact those on the sub-strate. Another device is disclosed in an article entitled "CRT Touch Panels Provide Maximum Flexibility in Computer Interaction"~

Control Engineering~ July 1976, pp. 33-34. This article discloses a curved flexible plastic sheet carrying small wires. The sheet can be deflected to cause these wires to come into contact with an orthogonal set of similar wires mounted immediately below. Spacers separate the sets of wires. United States Patent 3,760,360 discloses a quite similar de-20 vice embodied in a flat panel but having no capability of interactivelydisplaying information. United States Patent 3,495,232 discloses a some-what simpler embodiment of a similar device. United States Patent 3,921,167 discloses a panel locati~r-sensitive to the approach of an exter-nal probe sensing change in capacitance.
The touch panel covers a rigid substrate, whose face has a pre-determined radius of curvature ranging from infinite (flat) to 25 inches or less, and comprises in part a resilient membrane of a contour confor-ming to the substrate face and attached about its periphery thereto. A
group of discrete conductive strips adheres to the substrate on the sur-$~Z.~

face facing the membrane. A second group of discrete conductive stripswhich flex with the membrane and which cross the first, is carried by the membrane on its surface facing the substrate. External pressure on a local area of the membrane forces one or more conductive strips on the membrane into electrical contact with one or more conductive strips on the substrate.
By detecting which strips are in contact with each other, the approximate coordinates of the pressure point on the membrane can be determined. To prevent shorting between strips of each group when no external pressure is present, any one of several means can be used. In one embodiment, a thin, transparent insulating grid is interposed between the two groups of conduc-tive strips. A piezo-resistant coating on the surfaces of at least one group of strips also appears to function satisfactorily. When a curved substrate is used, a third anti-short means involves making the radius of ~;
curvature of the membrane somewhat smaller than the substrate~s. It appears that the natural resllience of the membrane is sufficient to support the conductive strips carried by it spaced from the substrate's conductive strips with no interposed element.
In one preferred embodiment, the rigid substrate comprises a curved CRT faceplate or screen, with a resilient membrane curved to conform to the CRT screen. The conductive strips both on the substrate and the membrane are sufficiently thin so as to be transparent and permit viewing of information displayed on the CRT screen. Being transparent, the conduc-tive strips can be relatively wide with respect to the spacing between adjacent ones on the same surface and thus permit a larger area of contact~
The anti-short means comprise an insulating grid preferably formed of one of several photo-resist polym~ now available, thus allowing the grid to be formed in situ on either the substrate or the membrane by masking and exposing to light, followed by the appropriate chemical process. Such photo-resist materials at the small thicknesses contemplated are substan- -

-2-tially transparent.
Accordingly, one purpose of this invention is to provide a passive surface sensitive to low pressure from a finger or stylus.
A second purpose is to provide a touch panel permitting the viewing of a display beneath it.
Another purpose is to provide a touch panel which can be easily integrated with existing display designs.
Thus, in accordance with one broad aspect of the invention, there is provided a transparent switch matrix to be carried on the face of a rigid insulator substrate having a predetermined radius of curvature, and compris-ing:
a) a plurality of spaced apart transparent conductive first strips firmly adhering to the face of the substrate;
b) a transparent resilient membrane having an undistorted contour substantially alike the predetermined contour, and attached about its peri-phery to the face of the substrate in a position matching the membrane con-tour to the substrate contour and spaced apart from the first strips thereon in a predetermined area of the membrane;
c) a plurality of transparent, flexible, spaced apart conductive second strips firmly adhering to the resilient membrane surface facing the substrate, each of said second strips located in the area spaced apart from the first strips and thinner than the spacing therefrom, and each of said second strips crossing at least two first strips; and d) an insulating grid interposed between the first and second strips, and having, within each crossover area between the first and second strips, a plurality of gaps permitting electrical contact between a first and second strip responsive to manual pressure at each said crossover area, said insulating grid being formed of insulating photo-resist material.
In accordance with another broad aspect of the invention there is provided a method of forming a transparent switch matrix to be carried on the face of a rigid insulator substrate having a predetermined radius of curva-ture, said method comprising:

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a) adhering a plurality of spaced apart transparent conductive first strips to the face of the substrate;
b~ coating the conductive strips covered substrate with a photo-resist material;
c) exposing the photo-resist material to a light pattern corres-ponding to a preselected grid pattern;
d) developing the photo-resist material;
e) removing chemically the photo-resist material between the individual grid lines;
f) adhering a plurality of transparent, flexible, spaced apart conductive second strips to a surface of a resilient membrane having an undistorted contour substantially alike the predetermined contour; and g) attaching the membrane about its periphery to the face of the substrate in a position matching the membrane contour to the substrate con-tour and spaced apart from the first strips thereon in a predetermined area of the membrane, each of said second strips being located in the area spaced apart from the first strips and thinner than the spacing therefrom, and each of said second strips crossing at least two first strips.
The invention will now be further described in conjunction with the accompanying drawings, in which:
Figure 1 is a plan view of a corner portion of a typical touch panel assembly, flat or curved, embodying the invention.
Figure 2 is a cross section of a curved embodiment of the touch panel displayed in Figure 1 and incorporating a membrane having a slightly smaller radius of curvature than the substrate.
Figure 3 is a blowup of a portion of Figure 2 detailing the re-lationship of the two sets of conductor strips and the insulating grid - ~when present). ~-Figure 4 is a blowup of a portion of Figure 3 showing in still greater detail the relationship of the two sets of conducting strips and ~ .

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the insulating grid.
Figure 5 is a blowup of a portion of Figure 2 employing a piezo-resistant anti-short means.
In all of these drawings scale between the various parts is not always consistent as this simplifies understanding. Suitable dimensions for the elements of the structure are set out below as needed.
The corner portion of the preferred embodiment shown in Figure 1 comprises a base or substrate 10 which may be flat or, as in Figure 2, curved. Substrate 10 must have an insulating surface. Y conductive strips 20-24 comprise transparent coated areas firmly adhering to the surface of substrate 10 facing the viewer. In a typical application substrate 10 can at least partly comprise a CRT screen. It may not be convenient to directly apply conductive strips to a CRT screen or other substrate, but rather form them on a clear plastic sheet 55, curved if intended to conform to a curved CRT screen, which is then glued or otherwise attached to substrate 10.
Leads 40-44 are attached to ends of strips 20-24 respectively so as to make electrical contact between them and external support electronics. In a typical device, each of conductive strips 20-24 is .5 in. wide and is separated from adjacent strips by .005 in. gaps. Strips 20-24 are in one embodiment preferably formed from indium oxide, tin oxide, or a combination OI both oxides. The strips are easily formed by coating the entire face of substrate 10 with the conductive material using standard techniques.
Standard etching technique using photo-resist material then forms the narrow gaps between adjacent strips.
In one embodiment of this invention, insulating grid 45 forms the next layer of the panel, overlaying at least a portion of conductive strips 20-24, and usually covers strips 20-24 uniformly. When such an insulating grid is used, it can be most easily formed from widely available and well known transparent photo-resist material which is itself inherently non-.. ~ , . , - . : -:: . .

conductive. The areas to be covered by insulating grid 45 are covered with the photo-resist material, an appropriate mask is placed on these areas, the ; resist is exposed according to usual procedures, and the unexposed photo-resist is removed with standard chemical procedures. A suitable grid 45 comprises a crosshatch of lines or strips formed of the photo-resist mate-rial, where each line is .005 in. wide (w in Figure 4) and all lines, in both the vertical and horizontal directions are on .025 in. centers (s in Figure 3). Thickness of insulating grid 45 can vary depending on the pres-sure desired to form contact, but a nominal value of 0.0001 in. appears to be suitable for the 0.003 MYLAR (reg. trademark of Dupont Corp.) polyester membrane described below. In general, a ratio of from 1:5 to 1:100 for the width w of the insulating grid lines to the centerline spacing s of adjacent pairs is suitable for this grid thickness. The width w of individual lines should never exceed a few thousandths of an inch. Insulating grid 45 can ~ -also be formed on membrane 11 after strips 12-16 are formed as described below.
Resilient insulating membrane 11 forms the tactile surface which the operator presses at a desired point to create an electrical contact indicating the coordinates of the pressure point. Membrane 11 carries con-ductive strips 12-16 on its surface facing substrate 10~ which strips are formed before attaching membrane 11 to substrate 10. Strips 12-16 must be flexible enough to easily bend with membrane 11. Transparent polyester film of 0.003 in. thickness with a transparent conductive gold film on one sur-face available from Sierracin Corp., 12780 San Fernando Road, Sylmar, CA, 91342 is suitable, as well as other thicknesses to at least 0.007 in.
Conductive strips 12-16 are conveniently formed by removing (through etching) narrow strips of gold in parallel lines from such a film. Typical ; dimensions of the gold-free lines defining gold strips 12-16 are 0.002 in.
on .5 in. centers. Vent 50 allows membrane 11 to assume its natural shape :

more quickly after pressure on it by allowing air to rapidly flow into the space between membrane 11 and substrate 10. It may be desirable to place a filter in vent 50 to prevent the entrance of dirt. This vent prevents the slow return of membrane 11 to its natural shape when deflected over a large area at one instant. It also prevents shorts caused by changes in ambient atmospheric pressure.
If substrate 10 is curved, it is necessary to mold membrane 11 and strips 12-16 already formed on it to a smooth contour which conforms to substrate 10. ~embrane 11, after etching of the gold layer to form conductive strips 12-16, is formed into a shape substantially conforming to the topology of substrate 10. If substrate 10 is curved it is preferable that the curva-ture of membrane 11 when unstressed be slightly greater than that of sub-strate 10. When substrate 10 comprises a typical curved CRT implosion shield, curvature is approximately spherical with a radius of approximately 20-30 in.
In such a case membrane 11 preferably is molded to a radius of curvature of from 1-4 in. less than that of substrate 10. The slightly greater curvature prevents strips 12-16 on membrane 11 from being drawn down tightly onto strips 20-24 and possibly shorting to them. Further, such dimensioning is essential if anti-short means other than grid 45 are employed on a curved substrate, as described infra. Membrane 11 is securely fastened around its periphery to substrate 10 by tape strips 54 in such a position that con-ductive strips 12-16 pass across each of conductive strips 20-24 and are spaced therefrom by grid 45 and the natural tendency of membrane 11 to assume its molded-in spherical shape when unstressed. Conductive strips 12-16 are connected to leads 32-36 by a conductive adhesive. Leads 32-36 .~ -7-.' ; ' '' ~ .

may be formed in situ on substrate 10 at the same time conductive leads 20-24 are formed. The support electronics can thus be easily connected to strips 12-16. Spacer 53 (Figure 2), though often not essential, can be em-ployed advantageously in certain cases to prevent shorting around the peri-phery of membrane 11, particularly if anti-short means other than grid 45 are used. Spacer 53 need not be placed on strips 20-24 and may extend to the edge of membrane 11.
In operation, a contact between any one of conductive strips 20-24 and any one of conductive strips 12-16 can be made by gentle finger or stylus pressure on membrane 11 above the desired point of intersection.
Because of the relatively wide contact surfaces the pressure point need not precisely in the center of the desired intersection. With either insulating grid 45 or the other anti-short means described nfra, gentle finger pressure forms an essentially zero resistance contact between the two selected strips. The wide contact surfaoes also add reliability in forming each contact between the strips.
Figure 5 discloses one alternative to ins~lating grid 45 as the anti-short means. The aforementioned gold covered polyester film from Sierracin Corp. is available optionally with a "proprietary ceramic coating which serves to increase visible light transmission and to provide a measure of mechanical protection to the conductive metal deposit".
(Sierracin Corp. brochure entitled Sierracin_Intrex (TM) Electricall Conductive Film Components.~ This coating has been determined to have a piezoresistant characteristic of high resistance under very light pressure, and a very low resistance under pressure no heavier than that generated by gentle finger pressure. In Figure 5~ coatings 51 and 52 indicate use of this alternative. As now available, both coating 51 and 52 must be present to yield sufficiently high resistance at very low pressures to allow functioning as an anti-short means. It is probable that a coating 51 .: . - ~ . .

thicker than now available would allow omission of coating 52. As previous-ly mentioned, when no insulating grid 45 is used, spacer 53 may be necessary to prevent shorting adjacent the edges.
Another means for preventing shorting between the X and Y conductor strips 12-16 and 20-24 is available for use with a substrate 10 having a finite radius of curvature. ~y selecting membrane ll's radius of curvature smaller than substrate 10's (for membr-Qnes mounted on substrate lO's convex side, of course), as shown in Figure 2, the natural resiliency of membrane 11 and its arched shape supports X strips 12-16 in spaced ~elationship with Y strips 20-24 and prevents their shorting absent external pressure.
Although a wide variety of radius of curvature will undoubtedly work, it is known that a substrate of 25 in. radius of curvature and a .003 in thick polyester membrane molded with a form having a 22 in. radius of curvature are satisfactory. As shown in Figures 1 and 2, it is desirable with this anti-short means, to bond the periphery of membrane 11 to substrate 10 out-side Y strips 20-24 to increase the clearance between the peripheral X and Y strip~ areas. Spacer 53 may also be used for this purpose. It is likely~
although not confirmed, that use of membrane 11's natural resiliency and curvature to provide the necessary anti-short spacing between X and Y strips requires a greater difference in radihc of curvature for substrate 10 and membrane 11 than do the previously mentioned anti-short means. Thus, while a 3 in. smaller radius works with a 25 in. substrate radius in all 3 cases, a 1 in. difference or less may well be satisfactory when grid 45 or piezo- -resistant coating 51 is used.
During the manufacture of this apparatus, it is important that the surfaces of strips 20-24 and 12-16 be relatively free of dust and other foreign matter during attachment of membrane 11 to substrate 10. However, the relatively wide contact areas between crossing strips does tolerate a small amount of such foreign matter, particularly as long as the foreign ~J màtter is non-conductive. 9 ~ -.. . .
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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A transparent switch matrix to be carried on the face of a rigid insulator substrate having a predetermined radius of curvature, and com-prising:
a) a plurality of spaced apart transparent conductive first strips firmly adhering to the face of the substrate;
b) a transparent resilient membrane having an undistorted contour substantially alike the predetermined contour, and attached about its periphery to the face of the substrate in a position matching the membrane contour to the substrate contour and spaced apart from the first strips thereon in a predetermined area of the membrane;
c) a plurality of transparent, flexible, spaced apart conductive second strips firmly adhering to the resilient membrane surface facing the substrate, each of said second strips located in the area spaced apart from the first strips and thinner than the spacing therefrom, and each of said second strips crossing at least two first strips; and d) an insulating grid interposed between the first and second strips, and having, within each crossover area between the first and second strips, a plurality of gaps permitting electrical contact between a first and second strip responsive to manual pressure at each said crossover area, said insulating grid being formed of insulating photo-resist material.

2. The switch matrix of claim 1, wherein the substrate contour is con-vex, and wherein the contour of the membrane is slightly more convex than the contour of the substrate.

3. The switch matrix of claim 1, wherein the insulating grid comprises orthogonal lines for which the ratio of width of individual lines forming the grid to the spacing between adjacent lines is in the range of 1:5 to 1:100.

4. The switch matrix of claim 3, wherein the thickness of the lines comprising the grid is approximately 0.0001 in.

5. A method of forming a transparent switch matrix to be carried on the face of a rigid insulator substrate having a predetermined radius of curvature, said method comprising:
a) adhering a plurality of spaced apart transparent conductive first strips to the face of the substrate;
b) coating the conductive strips covered substrate with a photo-resist material;
c) exposing the photo-resist material to a light pattern corres-ponding to a preselected grid pattern;
d) developing the photo-resist material;
e) removing chemically the photo-resist material between the individual grid lines;
f) adhering a plurality of transparent, flexible, spaced apart conductive second strips to a surface of a resilient membrane having an undistorted contour substantially alike the predetermined contour, and g) attaching the membrane about its periphery to the face of the substrate in a position matching the membrane contour to the substrate con-tour and spaced apart from the first strips thereon in a predetermined area of the membrane, each of said second strips being located in the area spaced apart from the first strips and thinner than the spacing therefrom, and each of said second strips crossing at least two first strips.