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Description  |
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BACKGROUND OF THE INVENTION
This invention generally relates to touch actuated capacitance type
switches. More particularly, this invention relates to such switches
wherein the areas which are touched to initiate actions are formed from a
metallic conductive film that has a low degree of abrasion resistance.
Still more specifically, this invention relates to such a switch wherein
the conductive films are given a protective overcoat to prevent abrasion
damage to them.
The use of touch panels as switches has become increasingly common in
household appliances. In particular, such panels have proven popular in
microwave ovens. One problem with such panels is their cost. The front
surface, where the touch sites are located, must be able to withstand
cleaning by harsh, abrasive household cleaners. The only material to date
which has proven usable for such panels is NECCA or Electropane,
tradenames for tin oxide coated plate glass. Such glass is quite expensive
in initial cost and creates further costs in processing since all of the
tin oxide coating must be etched off except for the areas where the touch
sites are to be located. This is a wasteful process. In addition, the
backs of such switches are typically coated, for decorative purposes and
to lay down the other side of a capacitor, with epoxy inks. Such inks are
difficult to handle and have short pot lives, further adding to the
manufacturing costs. I have found that discrete sites or pads of a
conductive material may be placed on the front of a touch switch. This
avoids the necessity of etching away portions of an overall conductive
coating as in the past. However, such discrete sites of conductive
material are quite sensitive to abrasion damage. I have found that they
may be adequately protected without degrading performance by overcoating
them with an open pattern of an abrasion resistant material. Further, the
back face coating and rear conductive sites may be put on using an
organopolysiloxane material which avoids the problems of using epoxy inks.
SUMMARY OF THE INVENTION
My invention resides in an improved touch actuated capacitance type switch.
The switch includes a substrate plate, made of a dielectric material,
which has opposing faces. Formed on and adhered to one face of the plate
are a plurality of conductive touch pads. Each of the touch pads is a
relatively thin metallic area possessing poor resistance to abrasive wear.
A protective overcoat of a relatively abrasion resistance material is
placed over each of the touch pads. The overcoat preferably obscures no
more than about seventy-percent of the total available surface area of the
touch pads. Formed on and adhered to the other face of the plate are a
plurality of conductive sites. The conductive sites are positioned in
general alignment, across the thickness of the plate, with the touch pads
to thus define capacitors. A conductor network interconnecting the
conductive sites in a predetermined logic configuration completes the
switch.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front elevational view of an example of a touch switch of the
present invention;
FIG. 2 is a rear elevational view of the switch of FIG. 1;
FIG. 3 is a cross-sectional view, on an enlarged scale and with some
elements exaggerated in size, taken along the line 3--3 of FIG. 1; and
FIG. 4 is a partial view of the front face of the switch as seen in FIG. 1
with a modified protective overcoat pattern.
DETAILED DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 illustrate the front and back sides of a touch actuated
capacitance type switch 10. With particular reference to FIG. 1, the
switch 10 includes a substrate plate 12 which is of an elongated,
generally rectangular shape in this particular instance. The precise shape
of the substrate plate 12 is not of critical importance, nor is its
precise material nor thickness. By way of example, the substrate 12 may be
plate glass approximately 0.125 inch thick. However, any dielectric
material may be used for the substrate plate 12 provided it allows the
correct functioning of the switch with respect to its capacitive
functions. FIG. 1 illustrates a front face 14 of the plate 12. The front
face 14 is that face which would be viewed by a person desiring to utilize
the switch 10. The front face 14 has on it a plurality of separated,
localized touch pads 16. All of the touch pads 16 are substantially
identical in their construction and configuration, although each of them
control a different function with respect to the operation of the switch
10. However, for the purposes of this invention, it is unnecessary to make
a distinction between any particular one of the pads 16 since it is the
structure of the switch 10 which is the subject of this invention rather
than any particular control configuration. Each of the touch pads 16 is
defined by an area which has been screen printed with gold, palladium,
silver or other conductive type ink and then fired to give a conductive
surface on the front face 14 of the panel 12. The pads could also be
formed by vapor deposition or sputtering techniques. As is known in the
art, such metallic surfaces which are so formed on glass or other
substrates do not possess high abrasion resistance. It is this lack of
abrasion resistance which has in the past limited the utilization of such
techniques in applications such as the conductive pads 16 for the switch
10. To overcome this, as will be seen more clearly with respect to FIG. 3,
each of the touch pads 16 has been over printed, after the gold or other
precious metal portion has been put down, with a protective overcoating.
The protective overcoating which is placed on the pads 16 must be such
that the finger of a person touching the pads 16 can either make contact
with the gold or at least produce enough capacitive effect in the general
area to initiate the control signal. To this end, a half tone dot series
may be printed by a screen technique using a clear, abrasion resistant
material. Additionally, such a half tone pattern could be screen printed
using a relatively low melting glass filled ink which would then be fired
again to extend abrasion resistance to the conductive metal pad. In some
cases, the metallic pad and the protective overcoat material could be
co-fired.
FIG. 2 illustrates a back face 18 of the substrate plate 12. As is known in
the art, the back face 18 contains a plurality of conductive sites or pads
20 which are generally placed in alignment with respective pads 16 on the
front face 14. The conductive pads 20 are laid down in pairs for
convenience in setting up various controlled functions as a result of the
capacitance changes which activate the switch 10. As is well known, the
basic operational principle of such a switch is that the finger of a
person touching one of the pads 16 on the front face 14 will upset the
capacitive balance between the pads 16 and 20, across the thickness of the
plate 12, and result in generation of an electrical signal which can then
be utilized as a logic or control signal. Also note that the pads 20 on
the back face 18 are interconnected by conductors 22 to allow transfer of
a generated signal in various logic patterns on face 18 of the switch 10.
The precise pattern of conductors 22 is immaterial to the present
invention. Each switch 10 will require a different pattern of conductors
22 as well as pads 16 and corresponding pads 20. The examples shown in
FIGS. 1 and 2 are simply by way of presenting an illustrative embodiment
of this invention to enable one to see precisely how such a switch 10 may
be constructed.
FIG. 3 is basically a sectional view taken along the line 3--3 in FIG. 1.
However, the scale of FIG. 3 has been substantially enlarged as respects
the scale of FIG. 1 and the actual thicknesses of the pads 16 and 20 have
been grossly exaggerated with respect to the actual thickness of the plate
12. That is, the proportions of pads 16 and 20 are not accurate in FIG. 3
as regards the thickness of the substrate plate 12. Although such was not
completely evident in FIG. 2, it is evident in FIG. 3 that what was
referred to as the back face 18 of the plate 12 is actually a dielectric
coating which is applied to the substrate plate 12. The coating 18 is
primarily provided for decorative purposes. The coating 18 covers
substantially the entire back face of the plate 12 and is designed to hide
or obscure vision of the pads 20. This is because such pads could present
an objectionable appearance when viewing them through the pads 16 on the
front face 14. To this end then, the coating 18 is preferably an opaque
coating, although, since its primary purpose is decorative rather than
functional, it is possible to omit this particular coating. The coating 18
is preferably an organopolysiloxane resin material of the type which may
be seen in one or more of the following U.S. patents and patent
applications: U.S. Pat. Nos. 3,389,114; 3,389,121; 3,395,117; and
3,457,323; and pending U.S. patent application Ser. No. 713,730, filed
Aug. 12, 1976, having an assignee in common with the present invention.
All of the foregoing U.S. patents and patent application are hereby
incorporated by reference. At the present time, switches of this general
type have their back surfaces covered with epoxy type inks. These epoxy
inks present problems with respect to their pot life and their screening
properties. The organopolysiloxane resins are preferred as opposed to the
epoxy inks because they do not present any pot life problems, they allow a
lower temperature and faster cure, and they also present better screening
properties to allow better quality of the coating 18. The pads 20 and
conductors 22 are also preferably screened on using the same general type
of organopolysiloxane resin material, but filled with a conductive
material such as silver. On the front face 14 of the plate 12 in FIG. 3,
the conductive pad 16 is shown in exaggerated thickness with a plurality
of dots of material 24 overlaying the pad 16. The material 24 is basically
aligned to be screened over the pad 16 in a conventional half tone like
pattern. Such a pattern is approximately 70% open spaces. This is
sufficient to allow proper conductive coupling from the pad 16 to the
finger of a person using the switch 10, yet imparts a sufficient
protective layer over the pad 16 to protect it from abrasion. In general,
no more than 70% of the total surface area of the pad 16 should be covered
by the protective overcoat. The pad 16, as previously explained, is
preferably screened onto the front face 14 as a precious metal filled ink
and fired to give an adherent coating. Then, the half tone dots 24 are
screened over this pad 16. The half tone dots 24 may be screened on from
an organopolysiloxane resin similar to that used on the back of the panel
12 for the coating 18, except that the half tone dots 24 are preferably
transparent; or the half tone dots 24 may be a vitreous type ink which is
screened on and then fired. Whatever the case, the front face 14 is
preferably completed before the back of the panel 12 is given its coating
18 and pads 22 and conductor pattern 22. This is necessary because the
temperature resistance of the organopolysiloxane resins is less than the
temperatures at which the pads 16 must be fired, thus requiring complete
firing of the pads 16 before application of any of the organopolysiloxane
resins.
FIG. 4 is a partial view of the front face 14 of the switch 10 with a
somewhat modified protective pattern for the pads 16. In this case, rather
than using the half tone dots such as those shown at 24 in FIG. 3, a
basket weave or cross hatch like pattern of lines 26 is laid down over
each of the pads 16. The material used for the basket weave pattern 26 may
be substantially identical to any material chosen for the half tone dots
24. The embodiment of FIG. 4 is included to illustrate the basic premise
of protection of the relatively abrasion sensitive pads 16 from contact by
any material which is rubbed over the front face 14. Such touch panels
switches 10 are commonly used in applications in the kitchen such as
microwave ovens. Under these circumstances, it is not unreasonable to
assume that the panel front face 14 could be subjected to cleaning with
harsh, abrasive household cleansers. Under such circumstances, the pads 16
would be quickly abraded and their appearance as well as function would be
seriously affected. The application of some over coating means to these
pads 16 protects them from contact with such abrasive cleaning methods and
allows a longer usable life period.
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