Note: Descriptions are shown in the official language in which they were submitted.
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PRESS~ S13NSITIVE CONDUCTI~: EI.ASTIC S~EET
BAC~G~OI~ND OF T~ I~3Vh~lTIUN
Field of the Invention
The present invention relates generally to a
pressure-sensitive, conductive elastic sheet and
particularly to a pressure-sensitive, conductive elastic
sheet sandwiched between a pair of flat electrode plates,
each having a group of straight strip electrodes which is
used as a graphics digitizing tablet through which various
letters or figures can be detected two-dimensionally by a
character reader.
Description of the Prior Art
Pressure-sensitive, conductive elastic sheets
are well-known. Elastic sheets of this kind is usually
used in graphics digitizing tablets, which detect various
letters or figures written thereon with a pen or the like.
Such sheets are sandwiched between a pair of flat electrode
plates provided with a group of straight strip electrodes
~0 in such a way that the strip electrodes on one flat
electrode plate intersect those on the other flat electrode
plate to form a matrix of points. In such a graphics
digitizing tablet when pressure is applied to the pressure-
sensitive conductlve sheet via the upper and lower flat
electrode -plates, the conductive sheet makes electrical
contact at points where pressure is applied by a pen and
the contact points are digitized by the two perpendicular
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strip electrodes on the basis of co-ordinates. The surface
area of the tablet on which letters or figures are written
is relatively large, ranging from 100 mm square to 400 mm
square. Accordingly, when some letters or figures are
written on the tablet with a pen held in the user's hand,
the heel of user's hand or the user's forarm inevitably
depresses the surface of the tablet, with the result that
erroneous operation often occurs in detecting the letters
or figures. In this connection, experiment indicates that
contact pressure (under which some of the upper strip
electrodes are connected electrically to some of the lower
strip electrodes) conventionally decreases with increasing
contact area (throughout which some of the upper strip
electrodes are connected electrically to some o~ the lower
strip electrodes).
In other words, the upper and lower strip
electrodes are easily connected electrically when light
pressure is applied over a large conduction area on the
tablet. To explain in more detail, when a pen is placed
onto the tablet to write some letters, since the contact
area is small, a relatively great contact pressure is
needed to connect the upper and lower strip electrodes; on
the other hand when the heel of the user's hand or the
user's forearm is placed on the tablet, since the contact
area is largei a relatively small contact pressure easily
connects the upper and lower strip electrodes, thus
resulting in erroneous operation.
.
763~2
A more detailed description of the prior-art
pressure-sensitive, conductive elastic sheet for use in
digitizing tablets will be described hereinafter with
reference to the attached drawings in conjunction with the
present invention under DETAILED DESCRIPTION OF THE
PREFERRED EMBOD I MENTS .
SlJMMARY OF TEIE INVh~TION
With these problems in mind therefore, it is the
primary object of the present invention to provide a
10 . pressure-sensitive, conductive elastic sheet provided with
the characteristics that the contact pressure under which
electrical contact is made through the sheet is constant or
increases with increasing contact area.
To achieve the above-mentioned object, the
pressure-sensitive, conductive elastic sheet according to
the present invention comprises a laminar elastomer in
which a great number of coarse ferromagnetic, conductive
metal particles with a diameter of 30 to lS0 micron are
mixed with a great number of fine ferromagnetic, conductive
metal particles with a diameter of 10 micron or less (or
50 micron or less) in a predetermined proportion in such a
way that the coarse particles are aligned perpendicular to
the plane of the sheet and separated slightly from its
surfaces and the fine particles are dispersed near at least
one surface of the sheet.
To form the pressure-sensitive, conductive
elastic sheet according to the present invention, first,
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coarse ferromagnetic, conductive metal particles are
mixed with a liquid-state elastomer; secondly, the
mixed material is formed into an inner sheet within a
uni~orm magnetic field; thirdly, fine ferromagnetic,
conductive metal particles are mixed with another liquid
elastomer; fourthly, the liquid elastomer including fine
particles is laminated onto both of the surfaces of the
inner sheet includins the coarse particles. Additionally,
in another method, firstly, coarse ferromagnetic,
conductive metal particles are coated with a low-molecular
weight elastomer; secondly, the coated coarse particles
are mixed with the fine ferromagnetic, conductive metal
particles in a liquid elastomer; thirdly, the mixed
material is allowed to set or cure within a uniform
magnetic field for a predetermined time period.
Accordingly, the invention is broadly claimed
herein as a pressure-sensitive, conductive elastic sheet,
which comprises:
(a) coarse ferromagnetic conductive particles
of from 30 to 150 microns in diameter;
(b) fine ferromagnetic conductive particles of
10 microns or less in diameter, said fine particles being
mixed with said coarse particles at a mixture ratio of
from 1:0.1 to 1:1 by weight; and
(c) elastomer mixed with said coarse and fine
particles at a mixture ratio of from 1:0.5 to 1:0.8 by
weight, said coarse ferromagnetic particles being aligned
in the direction perpendicular to the plane of the sheet
through the interior thereof and said fine ferromagnetic
particles being dispersed near at least one surface
ihereof,
whereby the pressure at which an electrically
conductive pathway is formed through the sheet is
almost constant irrespective of contact area to which
pressure is applied or increases with increasing contact
area.
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According to one aspect, the fine ferromagnetic
conductive particles are mixed with the coarse ferro-
magnetic conductive particles at a mixture ratio of from
1:2 to 1:5 by weight.
The invention is also claimed herein as a
method of forming a pressure-sensitive, conductive
elastic sheet in which the pressure at which the sheet
is rendered conductive is almost constant irrespective of
contact area or increases with increasing contact area,
which comprises the following steps of:
(a) mixing coarse ferromagnetic conductive
particles of from 30 to 150 microns in diameter with a
liquid elastomar;
(b) allowing the liquid elastomer mixed with
said coarse particles to solidify into an inner sheet
within a uniform magnetic field;
(c) mixing fine conductive particles of 50
microns or less in diameter with a liquid elastomer; and
(d) laminating the liquid elastomer including
said fine particles onto both of the surfaces of said
inner sheet,-the laminated elastomer being thinner than
said inner sheet.
The invention is also claimed herein as a
method of forming a pressure-sensitive, conductive
elastic sheet in which the pressure at which the sheet is
rendered conductive is almost constant irrespective of
contact area or increases with increasing contact area,
which comprises the following steps of:
(a) coating coarse ferromagnetic conductive
particles of from 30 to 150 microns in diameter with a
low-molecular-weight elastomer;
Ib) mixing the coated coarse particles with a
liquid elastomer in which fine conductive particles of
50 microns or less in diameter have been mixed; and
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1~76342
(c) leaving the elastomer lncluding the
c~oated coarse and fine particles, in the form of a sheet,
within a uniform magnetic field for a predetermined time
sufficiently long to allow the fine ferromagnetic
particles to collect on the bottom surface thereof.
.
BRIEF DESCRIPTION-OF THE DRAWI~GS
.
The features and advantages of the pressure-
sensitive, conductive elastic sheet according to the
present invention will be more clearly appreciated from
the following description taken in conjunction with the
accompanying drawings in which;
Fig. 1 is a pictorial perspective view showing
a typical graphics digitizing tablet for assistance in
explaining ~he present invention;
Fig. 2 is a graphical representation showing
the relationships between conductivity and pressure in a
typical prior-art pressure-sensitive, conductive elastic
6~Z
sheet;
Fig. 3 is an enlarged cross-sectional view of the
pressure-sensitive, conductive elastic sheet according to
the present invention, in which coarse and fine
ferromagnetic, conductive metal particles are shown in an
exaggerated scale; and
Fig. 4 is a graphical representation of the
relationships between contact pressure and contact area in
each of the embodiments of the pressure-sensitive,
conductive elastic sheets according to the present
inventions, including that of a typical prior-art
pressure-sensitive, conductive elastic sheet.
DETAILED DESCRIPTIt~l Ol~ q~ PRE3~:RRED EMBODIME~TS
-
To facilitate understanding of the present
invention, brief reference will be made to a prior-art
pressure-sensitive, conductive elastic sheet with respect
to its application to a graphics digitizing tablet.
As is well-known, pressure-sensitive, conductive
elastic sheets are used for graphics digitizing tablets
which can detect various letters and figures written
thereon with a pen-or the like on the basis of rectangular
co-ordinates and input the detected signaIs indicative of
the co-ordinates to a character reader.
Fig. 1 shows the structure of a t~pical graphic
digitizing tablet. In the figure, reference numeral 1
denotes a tablet frame, reference numeral ~ denotes an
outer flat electrode plate provided with a number of
4;~
horizontally aligned strip electrodes, reference numeral 3
denotes an inner flat electrode plate provided with a
number of vertically aligned strip electrodes, and
reference numeral 4 denotes a pressure-sensitive,
conductive elastic sheet interposed between the outer and
inner 1at electrode plates.
When some letters or figures are written on the
tablet with a pen 5, since pressure is applied to the
pressure-sensitive, conductive elastic sheet 4 via the
outer flat electrode plate 2, the conductive elastic
sheet 4 becomes conductive and, thereby, at least one of
horizontal strip electrodes is electrically connected to at
least one of vertical strip electrodes, so that the point
of contact can be identified by its co-ordinates.
lS The graphics digitizing tablet thus constructed
is often as big as 500 mm square. Accordingly, while the
user writes letters or figures on the tablet, the heel of
the user's hand 6 or the user's forearm 7 inevitably
applies pressure to the surface of outer flat electrode
plate 2, so that an unintentional position is detected and
thereby erroneous ooperation often occrus.
In the prior-art pressure-sensitive, conductive
elàstic sheet, however, contact pressure under which the
conductive elastic sheet 4 becomes conductive decreases
with increasing contact area. Since the area of the hand
or the forearm is greater than that of a pen tip, erroneous
input occurs inevitably and readily.
-- 6 --
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Fig. 2 shows the relationships between
conductivity and pressure applied to the tablet of a
prior-art pressure-sensitive, conduc~ive elastic sheet.
As can be seen, when pressure applied to the sheet by a pen
reaches a value PA, the conductivity of the sheet rises
abruptly from insulative to conductive as designated by
curve A. On the other hand, when pressure is applied to
the sheet by something other than the pen, since its area
is usually greater than that of the pen, the conductivity
of the sheet rises abruptly from insulative to conductive
at a value PB less than PA as designated by curve B. That
is to say, the greater the contact area, the higher the
pressure sensitivity of the conductive sheet.
Although the relationships between contact
pressure and contact area of the prior-art
pressure-sensitive, conductive elastic sheet will be
explained hereinafter with reference to Fig. 4 in
conjunction with the conductive sheet according to the
present invention, the contact pressure PA is approximately
50 g/mm2 when the contact area i5 1 mm2; however, the
contact pressure PB is approximately 25 y/mm2 when the
contact area is 50 mm2. That is to say, if the area of the
heel of the user's hand is 50 times larger than that of a
pen, erroneous input occurs when approximately half of the
pen pressure is applied to the pressure~sensitive sheetO
The conductive elastic sheet conventionally
includes conductive metal particles uniformly dispersed in
.
~1~6~42
a rubber material microscopically spaced from each other.
Therefore, when pressure is applied to the sheet, since the
~ubber material is compressed and deformed, particles are
brought into contact with other particles at the point
where pressure is applied. Since the conductive metal
particles dispersed on the surface of the sheet are in
contact with the strip electrodes of the outer and inner
flat electrode plates when pressure is applied to the
surface of the sheet, the two electrode plates are brought
into contact with each other via the pressure-sensitive
elastic sheet.
It is very difficult to explain clearly why
pressure sensitivity increases with increa~ing contact
area; however, it may be due to the following causes:
` 15 tl) variations in the size and the distribution
of conductive metal particles in the conductive sheet;
(2) variations in the modulus of elasticity of
the conductive sheet;
(3) variations in the distance between the
conductive sheet surface and the outermost metal perticles
in the conductive sheet; and
(4) variations in the thickness of the
conductive sheet.
The increase in pressure sensitivity with
respect to increasing contact area may be caused by other
complica~ed factors in combination with the
above-mentioned reasons; however, it is possible to simply
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7~i3~2
consider that the greater the variations in different
conduction factors, the higher the probability of contact
between metal particles. In other words, the greater the
variation in various conduction factors, the greater the
number of points with a relatively low contact pressure.
In order to solve the above-mentioned problem, a
method of normalizing the distribution of metal particles
has aiready been proposed, in which the metal particles
with diameters as small as possible are mixed with a rubber
material in a higher concentration. In such prior-art,
pressure-sensitive, conductive elastic sheet, although it
is possible to manufacture a conductive sheet in which
particles are dispersed uniformly, the working life time is
not sufficiently long. This is because the high
concentration of fine metal particles exerts a harmful
influence upon the characteristics of the rubber with
respect to fatigue and~abrasion resistance. Therefore, the
life time of the prior-art conductive sheet by which a
sufficient pressure sensitivity and proper sheet thickness
can be maintained is as short as one thousand cycles, in
the case where metal particles with a diameter of 10 micron
or less are used. Additionally, there exists another
conductive sheet in which fine insulating particle layers
are formed in order to increase the life time; however, the
pressure sensitivity is too low for practical use~
In view of the above description, reference is
now made to an embodiment of the pressure-sensitive,
~1 7~3~
conductive elastic sheet according to the present
invention.
Fig. 3 shows the microscopic structure of the
conductive sheet. In the figure, reference numeral 10
denotes liquid elastomer such as silicon rubber, reference
numeral ll denotes coarse, ferromagnetic, conductive metal
particles, and reference numeral 12 denotes fine,
ferromagnetic, conductive metal particles. The material of
both the coarse and fine particles is ferrite or- carbonic
nickel.
The average diameter o~ the coarse,
ferromagnetic, metal particles 11 ranges from 30 to 150
microns; the average diameter of the fine, ferromagnetic,
metal particles is less than 50 microns.
The mixture ratio by weight of the coarse
particles to the fine particles is from lol to 1:0.1 in the
case where the fine particles with a diameter of about
10 micron or less are used and is about 1:2 to 1:5 in the
case where the fine particles with a diamet~r of about
50 micron or less are used.
The mixture ratio by weight of the elastomer to
the combined coarse and fine particles is from 1:0.5 to
1:0.8.
In order to arrange the coarse, ferromagnetic,
conductive metal particles in the interior of the
conductive sheet and to disperse the fine, ferromagnetic,
conductive metal particles at the two outer surfaces of the
-- 10 --
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.conductive sheet, first, the coarse metal particles are
mixed with a liquid elastomer; secondly, the mixed material
is formed into an inner sheet within a uniform magnetic
field; thirdly, the fine particles are mixed with another
liquid elastomer; fourthly, the liquid elastomer including
fine ferroma~netic conductive metal particles is laminated
onto both of the surfaces of the inner sheet in-such a way
that the thickness of the outer laminated elastomer is less
than that of the inner sheetO
In order to arrange the coarse ferromagnetic,
conductive metal particles through the interior of the
conductivP sheet and to disperse the fine, ferromagnetic
conductive metal particles on one outer surface of the
conductive sheet, first, the coarse particles are coated
with a low-molecular-weight elastomer; secondly, the
coated coarse particles are mixed with the fine particles
in a liquid elastomer; thirdl~, the mixed coarse and fine
particles are allowed to set within a uniform magnetic
field for a predetermined time period. In this method, the
differences in sedimentation velocity and agglutination
rate between the coarse and fine particles ensure formation
of a conductive sheet in which the fine particles are
concentrated on one surface thereof.
The application of the uniform magne~ic field to
the particle-elastomer mixture during setting causes the
coarse metal particles to a li~n verticall~ through the
sheet, as shown in Fig. 3. As a result, the inner sheet is
3~
much more pressure-sensitive than if the coarse particles
were randomly distributed. This allo~s suitable pressure-
sensitivity without the damaging effects of excessively
particle content.
In the conductive sheet according to the present
invention, the choices of the diameters of the coarse and
fine ferromagnetic metal particles and the mixture ratios
of the coarse and fine ferromagnetic metal particles and
elastomer are very impor~ant in order to obtain optimal
relationships between the contact pressure and contact
area. In particular, the mixture ratio RM of the coarse to
fine ferromagnetic metal particles has a great in1uence
upon the characteristics between contact pressure and
contact area. The greater the mixture ratio RM, the
greater the contact pressure with respect to a constant
contact area.
~ Various experiments have been made to determine
the above-mentioned particle diameters and mixture ratios
and the following values have been determined for desirable
characteristics in which the contact pressure is almost
constant irrespective of contact area or increases with
increasing contact area.
(l) The diameter of the coarse ferromagnetic
metal particles is from 30 to 150 micron;
(2) The diameter of the fine ferromagnetic
metal particles is classified into two groups of 10 micron
or less and 50 micron or less;
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42
~3) The mixture ratio of the coarse
ferromagnetic metal particles to the fine ferromagnetic
metal particles is from 1:0.1 to 1:1 by weight in the case
of the fine ferromagnetic metal particles with a diameter
of 10 micron or less and is from 1:2 to 1:5 in the case of
the fine ferromagnetic metal particles with a diameter of
50 micron or less; and
(4) The mixture ratio of the coarse and fine
ferromagnetic metal particles to the elastomer is from
1:0.5 to 1:0.8 by weight.
Fig. 4 shows exemplary relationships between
contact pressure and contact area. In the igure, the
curve A traces the characteristics obtained when the
mixture ratio RM of the coarse to fine ferromagnetic metal
particles is zero, that is, when there are no fine
particles, which are almost the same as those of a
prior-art pressure sensitive conductive sheet illustrated
by curve D.
The curve B shows the characteristics o~tained
when the mixture ratio RM of coarse to fine ferromagnetic
metal particles is 1:0.1 in the case where the fine
ferromagnetic metal particles with a diameter of 10 micron
or less are used or when the RM is 1:2 in the case where the
fine particles of 50 micron or less are used.
The curve C shows the characteristics obtained
when the mixture ratio RM of coarse to fine ferromagnetic
metal particles is 1:0.4 in the case where the fine
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ferromagnetic metal particles with a diameter of 50 micron
or less are used or when the RM is 1:4 in the case where the
fine particles of 50 micron or less are used.
As explained with reference to Fig. 4, it is
possible to obtain desirable characteristics between
contact pressure and contact area by controlling the
mixture ratio of coarse to fine ferromagnetic metal
particles at a predetermined value and by aligning the
coarse, conductive, ferromagnetic metal particles through
the interior of the conductive sheet and dispersing the
fine, conductive, ferromagnetic metal particles in at least
one outer surface of the conductive sheet.
It is not easy to explain the above-mentioned
effect; however, it may be due to the following reasons:
(1) since a great number o~ fine particles are
dispersed near the surface of the conductive sheet, the
hardness of the sheet surface is high and additionally
more uniform over a wider contact area than that of the
interior of the sheet. Therefore, more uniform pressure is
required to bring the fine particles into contact with each
other completely throughout the conduction area, thus
preventing local conduction. As a result, the sheet can be
easily rendered conductive by a pen tip with a small
contact area but not by a hand with a large contact area.
This phenomenon can more readily be understood by imagining
a model in which a relatively hard plastic sheet is placed
on plastic foam. In this case, although it is easy to
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contact the hard sheet through the foamed plastic in a
small area, it is difficult to compress the foamed plastic
over a large area.
(2) since the diameter of the fine particles is
small, the electrical contact resistance between particles
is large. Therefore, the phenomenon described under (1)
above is additionally emphasized.
Therefore, the higher the mixture ratio RM of the
coarse to fine ferromagnetic metal particles, the higher
the contact pressure required over the contact area.
Additionally, the larger the contact area, the higher the
contact pressure required with the mixture ratio RM at a
constant value.
As opposed to the prior-art conductive sheet in
which there is a high probability of contact between metal
particles at local positions in the contact area, in the
conductive sheet according to the present invention, since
the variations in various conduction factors are small, a
more uniform pressure is required to make the sheet
conductive. Furthermore, since the hardness of the sheet
surface is high, a relatively high pressure is required to
make a large conduction area conductive.
In the case where the pressure-sensitive,
conductive elastic sheet according to the present invention
is applied to a 200 mm square tablet, the tablet can detect
contact when a letter is written with a ballpoint pen
(small contact area of 10 mm2 or less), but cannot detect
~7~
pressure due to contact with the heel of userls hand or the
user's forearm (larage contact area).
Furthermore, in this embodiment described above,
the elasticity of the sheet is almost the same as in the
prior-art rubber conductive sheet, because the ~ine
ferromagnetic metal particles are concentrated near the
surface of the sheet and the coarse ferromagnetic metal
particles are arranged in the interior of the sheet.
As described above, in the pressure-sensitive,
conductive elastic sheet according to the present
invention, since the coarse ferromagnetic metal particles
and the fine ferromagnetic metal particles are first mixed
at a predetermined ratio and next mixed with liquid
elastomer and since the elastomer including the particles
is formed into a sheet wlthin a uniform magnetic field, the
coarse ferromagnetic metal particles can be aligned in the
direction perpendicular to the plane of the sheet and
additionally the fine ferromagnetic metal particles can be
dispersed near one surface or near both surfaces of the
conductive sheet. Therefore, it is possible to provide a
pressure-sensitive, conductive elastic sheet in which the
contact pressure is constant irrespective of the contact
area or the contact pressure increases with increasing
contact area.
Furthermore, when the pressure-sensitive,
conductive elastic sheet is applied to a graphics
digitizing tablet through which various letters or figures
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can be detected by a character reader, it is possible to
provide a tablet which is sensitive only to, for instance,
a pencil or a ballpoint pen with a small contact area and
not sensitive to, for instance, the heel of the user's hand
or the user's forearm, which have a large contact area.
It will be understood by those skilled in the art
that the foregoing description is in terms of preferred
embodiments of the present invention wherein various
changes and modifications may be made without departing
from the spirit and scope of the invention, as set forth in
the appended claims.
