U.S. patent application number 13/472388 was filed with the patent office on 2013-11-21 for capacitive touch panel device.
This patent application is currently assigned to CHIMEI INNOLUX CORPORATION. The applicant listed for this patent is Serge TOUSSAINT, Frans VERWEG, David YEATES. Invention is credited to Serge TOUSSAINT, Frans VERWEG, David YEATES.
Application Number | 20130307810 13/472388 |
Document ID | / |
Family ID | 49580926 |
Filed Date | 2013-11-21 |
United States Patent
Application |
20130307810 |
Kind Code |
A1 |
VERWEG; Frans ; et
al. |
November 21, 2013 |
CAPACITIVE TOUCH PANEL DEVICE
Abstract
The invention provides a touch panel device comprising, and
arranged above each other in a downward direction, a cover layer
for use as a touch surface, a sensor layer comprising a plurality
of sensing elements, the sensing elements being arranged to
register capacitance, characterized in that between the cover layer
and the sensor layer is an electrically-resistive layer,
electrically resistive layer being a electrically non-insulating
layer and having an electrical resistance is provided, wherein the
electrical resistance of the electrically-resistive layer is lower
than the electrical resistance of the cover layer and higher than
the electrical resistance of the sensing elements. The
electrically-resistive layer can have a resistance suitable for
causing, in response to a touch event above a center of a central
sensing element, a detectable change in the capacitance as measured
by at least two sensing elements adjacent to the central sensing
element.
Inventors: |
VERWEG; Frans; (Chu-Nan,
TW) ; TOUSSAINT; Serge; (Chu-Nan, TW) ;
YEATES; David; (Chu-Nan, TW) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
VERWEG; Frans
TOUSSAINT; Serge
YEATES; David |
Chu-Nan
Chu-Nan
Chu-Nan |
|
TW
TW
TW |
|
|
Assignee: |
CHIMEI INNOLUX CORPORATION
Chu-Nan
TW
INNOCOM TECHNOLOGY (SHENZHEN) CO., LTD.
Shenzhen City
CN
|
Family ID: |
49580926 |
Appl. No.: |
13/472388 |
Filed: |
May 15, 2012 |
Current U.S.
Class: |
345/174 ;
178/18.06 |
Current CPC
Class: |
G06F 3/0446 20190501;
G06F 3/0445 20190501; G06F 3/0412 20130101 |
Class at
Publication: |
345/174 ;
178/18.06 |
International
Class: |
G06F 3/044 20060101
G06F003/044 |
Claims
1. A touch panel device (1), comprising: a cover layer (2) for use
as a touch surface; a sensor layer comprising a plurality of
sensing elements, the sensing elements being arranged to register
capacitance; a electrically-resistive layer is between the cover
layer and the sensor layer, wherein the electrically-resistive
layer is an electrically non-insulating layer and an electrical
resistance of the electrically-resistive is provided; wherein the
electrical resistance of the electrically-resistive layer is lower
than the electrical resistance of the cover layer and higher than
the electrical resistance of the sensing elements.
2. A touch panel device as claimed in claim 1, wherein the
electrically-resistive layer has a electrical resistance suitable
for causing, in response to a touch event above a center of a
central sensing element, a detectable change in the capacitance as
measured by at least two sensing elements adjacent to the central
sensing element.
3. A touch panel device as claimed in claim 2, wherein the
electrically-resistive layer has a sheet resistance between 1 and
10,000 MOhm/sq.
4. A touch panel device as claimed in claim 3, wherein the sheet
resistance of the electrically-resistive layer is preferably
between 20 and 5000 MOhm/sq.
5. A touch panel device as claimed in claim 4, wherein the sheet
resistance of the electrically-resistive layer is even more
preferably between 100 and 1000 MOhm/sq.
6. A touch panel device as claimed in claim 1, wherein the
electrically-resistive layer is formed as a high-resistive ITO
layer.
7. A touch panel device as claimed in claim 1, wherein the
electrically-resistive layer is formed on the cover layer.
8. A touch panel device as claimed in claim 1, wherein a sub-layer
is provided between the cover layer and the sensor layer.
9. A touch panel device as claimed in claim 8, wherein the
electrically-resistive layer is a deposited layer on the
sub-layer.
10. A touch panel device as claimed in claim 8, wherein the
sub-layer comprises at least one of a polarizer layer, an
anti-splinter film, an airgap layer, and an optical clear adhesive
layer.
11. A touch panel device as claimed in claim 8, wherein the
sub-layer is also arranged as an anti-static layer.
12. A touch panel device as claimed in claim 1, wherein the sensor
layer is attached on a substrate layer.
13. A touch panel device as claimed in claim 12, the substrate
layer comprises: a reference electrode layer attached to the side
opposite the side of the substrate layer to which the sensor layer
is attached.
14. A touch panel device as claimed in claim 1, wherein the touch
panel device comprises a display module and the cover layer is
formed of a transparent material.
15. A touch panel device as claimed in claim 14, wherein a
reference electrode layer is provided on the display module.
16. A touch panel device as claimed in claim 15, wherein the
reference electrode layer is further provided inside the display
module.
17. A touch panel device as claimed in claim 1, wherein the touch
panel preferably comprises a processor which is arranged to the
register capacitance changes of each of the plurality of sensing
elements, wherein the processor is adapted to determine a touch
location by calculating a weighted location average of capacitance
changes registered by a plurality of the touch panel devices.
Description
FIELD OF THE INVENTION
[0001] The invention relates to capacitive touch panel devices. The
invention further relates to a method for determining a touch
location on a capacitive touch panel device, and to an electronic
apparatus comprising a display and a touch panel device.
BACKGROUND OF THE INVENTION
[0002] Capacitive touch panel devices are widely used to allow user
interaction with electronic devices. In particular, a transparent
touch panel can be used on top of a display device to allow a user
to interact with the electronic device via a graphical user
interface presented on the display device. Such touch panels are
used in for example mobile phones, tablet computers, and other
portable devices.
[0003] A known touch panel for use with such devices comprises a
glass plate provided with a first electrode comprising a plurality
of first sensing elements on one face of the glass plate, and a
second electrode on an opposite face of the glass plate. The core
operating principle is that the touch panel is provided with means
for determining (changes in) the capacity between any of the first
sensing elements of the first electrode and the second electrode.
Such change in capacitance is attributed to a touch event,
sometimes also called a gesture or touch gesture. By determining
the position of the sensing element where the change in capacitance
is maximized, the central position of the touch event is
determined.
[0004] In coplanar touch panels the sensors are located in one
single (Indium Tin Oxide, ITO) layer and each sensor has its own
sense circuitry. Coplanar touch technology uses differential
capacitance measurements in combination with a coplanar touch
sensor panel. The sense circuit measures the charge that is
required to load the intrinsic capacitance of each individual
sensor and in addition (if applicable) the finger-touch-capacitance
for those sensors that are covered/activated by the touch event.
The intrinsic capacitance of the sensor depends on the sensor area,
distance to a reference (voltage) layer and the dielectric constant
of the materials between sensor and this reference layer. Assuming
that the intrinsic capacitance is stable and constant over time,
this is accounted for during the tuning/calibration procedure. The
variation of sensor capacitance due to a touch event will then be
the discriminating factor revealing where the touch is located.
[0005] The accuracy performance of a touch panel is the most
important characteristic of the functionality of a touch panel as
it shows the capability of recognizing a touch event on the same
position as the actual spot location of the physical touch. Next to
this, a high accuracy will improve the ability of determining the
shape and size of the touch event. Moreover, a high spatial
accuracy performance of a touch display will enable to correctly
recognize stylus input (i.e. touches with a relative small impact
diameter <4 mm).
[0006] In general, the accuracy of a touch panel with a fixed size
will increase by enlarging the sensor density i.e. the total number
of active touch sensors per display area. With a larger sensor
density per area, not only the position, but also the shape and
size of the touch can be detected with more accuracy. For a typical
touch application of a pixilated display panel, (in which as a
response of the touch event, part of the display will be
activated/selected), the ultimate touch sensor dimension will be
equal to the display pixel sensor or in other words: the maximum
accuracy can be achieved when the touch sensor density is equal to
the Pixels-Per-Inch (PPI) value of the display.
[0007] For various reasons, such as costs, design and process
capability (track/gap capabilities) and display form factor (e.g.
availability for track/routing layout) the number of I/O of the
touch driver/controller will be limited. Consequently, the number
of touch sensors of a touch panel of a display module will, in
general, be much smaller than the actual number of display pixels
which will have its negative impact on the achievable accuracy.
Normally, for stylus input (i.e. with only a small area touching
the surface, <4 mm diameter), a relatively higher accuracy is
requested than for a finger input (with larger area touching the
touch panel, i.e. 9 mm diameter). This is because a stylus input is
related to typical touch display functionalities such as line
drawing and hand-writing which requires a small spatial input (and
recognition).
[0008] Especially, in the situation when the sensor size dimensions
of the touch panel are larger than the physical touch contact area
(e.g. finger touch size, stylus point size) a `death area` can
occur: i.e. the finger or stylus can move around for a certain
small distance at the center of a sensor. In this case, the touch
panel will not register these small movements being located within
one single sensor outline.
SUMMARY OF THE INVENTION
[0009] It is an object of the invention to increase the accuracy
performance of a touch panel with a fixed lay-out (i.e. fixed
number of sensors, with a fixed area per sensor).
[0010] The invention provides a touch panel having a high-resistive
layer at a location above of the sensor layer. The functionality of
this added layer will be to spread out the electrical field between
the physically touch contact area towards the array of sensors that
is positioned underneath the added layer. The number of sensors
that will sense a (part of the) capacitive difference due to the
finger touch capacitance will be increased by this layer. The
typical resistance of the additional layer should be large enough
to avoid that the layer will act as a (conductive) shielding layer
(typically >1-5 MOhm) but small enough in order that it will not
act as fully electrical insulator <<10.sup.8 MOhm). As a
consequence of the added layer, a touch event will not only build
up a capacitance between the physical touch input area and the
sensor array, but via the added resistive layer a larger so called
`fringe field` area will be established, in which a larger area of
the sensor array will be sensing a capacitance variation.
[0011] As a consequence, as more sensors are `affected` by the
touch event, by applying a dedicated algorithm to calculate the
touch position, a higher accuracy can be achieved. Especially in
the case of relative small touch input areas (e.g. at stylus input
<4 mm), the number of sensors involved in the determination of
the position could be increased and consequently the accuracy could
be improved. In effect, the "dead area" of a touch sensor is
reduced through the capacitive spreading effect of the resistive
layer.
[0012] In an embodiment according the invention, such an added
layer with high resistance, but still not totally electrically
insulating, for causing the mentioned `fringe field` around a touch
event is implemented as a (very thin) but high resistive ITO layer
on the cover window.
[0013] Other options are possible, as long as the layer is situated
in between the touched area and the sensor layer. In an embodiment,
an anti-static layer in the polarization stack-up is made resistive
in order to achieve the fringe field, if a polarization layer is
situated between the touch input and sensor array.
[0014] In an embodiment according the invention, a touch panel
device is provided wherein, arranged above each other in a downward
direction, a cover layer for use as a touch surface and a sensor
layer comprising sensing elements are arranged. Between the cover
layer and the sensor layer, an electrically resistive layer is
provided. The resistive layer has an electrical resistance. The
electrical resistance of the resistive layer may be lower than the
electrical resistance of the cover layer and higher than the
electrical resistance of the sensing elements. The electrical
resistance of the resistive layer can be higher than the resistance
of a reference electrode layer below the sensing elements. The
embodiment has the advantages as described above. More advantages
will be described in reference to the exemplary figures.
[0015] In an embodiment according the invention, the
electrically-resistive layer has a resistance suitable for causing,
in response to a touch event above a center of a central sensing
element, a detectable change in the capacitance as measured by
sensing elements adjacent to the central sensing element. For
example, the electrically resistive layer can have a resistance
arranged so that a touch by a finger or stylus will result in a
measurable change in capacitance in at least two, at least three,
four, or five sensors along a line (e.g. a line in the X or Y
direction, wherein X and Y indicate appropriate coordinate axes for
the given sensor grid).
[0016] In an embodiment according the invention, the
electrically-resistive layer has a sheet resistance of between 1
and 10,000 MOhm/sq, preferably between 20 and 5000 MOhm/sq, even
more preferably between 100 and 1000 MOhm/sq.
[0017] The electrical resistance layer can be arranged to have a
resistance between 1 MOhm and 200 MOhm over the area touched by a
finger or stylus.
[0018] In an embodiment according the invention, the
electrically-resistive layer is formed as a high-resistive ITO
layer on the cover layer.
[0019] In an embodiment according the invention, a sub-layer is
provided between the cover layer and the sensor layer. In an
embodiment according the invention, the electrically-resistive
layer is a deposited layer on the sub-layer or the cover layer.
[0020] In an embodiment according the invention, the sub-layer
comprises one or more of a polarizer layer, an anti-splinter film,
an airgap layer, and an optical clear adhesive layer. In an
embodiment according the invention, the sub-layer is also arranged
as an anti-static layer.
[0021] In an embodiment according the invention, the touch panel
device comprises a substrate layer to which the sensor layer is
attached. In an embodiment according the invention, the substrate
layer also comprising a reference electrode layer attached to the
side opposite side the attached sensor layer.
[0022] In an embodiment according the invention, the touch panel
device comprises a display layer and the cover layer is formed of a
transparent material, such as glass. In an embodiment according the
invention, the reference electrode layer is provided on the display
layer. In an embodiment according the invention, the reference
electrode layer is provided inside the display layer.
[0023] The invention further provides a touch panel device as
described above, further comprising a processor arranged to
register capacitance changes of each element of the plurality of
sensing elements wherein the processor is adapted to determine a
touch location by calculating a weighted location average of
capacitance changes registered by a plurality of the touch panel
devices.
[0024] The invention can be applied to various touch panel
configuration, including such variants known as "discrete co-planar
touch variant", "on-cell co-planar touch", and "window integrate
co-planer touch" configurations.
[0025] Where in this application it is stated that a layer is
"above" or "below" another layer, the relative directions above and
below refer to a stack of layers in which the top layer is
typically the cover layer ("touch layer", or outside layer), and
the bottom layer may be a display layer (if any). The exemplary
figures showing a cross section of a touch control panel also
adhere to this convention.
[0026] The invention will now be described in detail in relation to
coplanar touch sensors. However, it will be clear to the skilled
person that the invention may also be applied to other types of
capacitive touch sensor panels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 schematically shows a top view of an electronic
device comprising a touch panel device.
[0028] FIGS. 2a-2c schematically show cross section of touch panel
device variants.
[0029] FIGS. 3a-3c schematically illustrate the functioning of
touch panel device variants.
[0030] FIGS. 4a-4c schematically show cross sections of touch panel
devices according the invention.
[0031] FIG. 5a schematically illustrates the functioning of a
conventional touch panel device and FIG. 5b schematically
illustrates the functioning of a touch panel device according the
invention.
[0032] FIGS. 6a and 6b schematically show charge distributions
corresponding to a touch event on the panels of FIGS. 5a and 5b
respectively.
[0033] FIGS. 7a and 7b schematically show the change in capacitance
as detected by an array of sensing elements along a Y axis.
[0034] FIG. 8 schematically shows neighbouring sensors that are
used in an algorithm for calculating the touch location, according
to an embodiment of the invention.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0035] First, coplanar touch panels will be described in some more
detail.
[0036] FIG. 1 schematically shows a top view of an electronic
device 100 comprising a coplanar capacitive touch panel device.
Examples of applications with such devices are mobile telephones,
tablet computers and other portable devices. In addition,
display-less (input) devices such as mouse pads and graphics
tablets. The touch panel surface 110 of the electronic device 100
can be optimized for finger touches and stylus touches.
[0037] The touch panel surface 110 is divided in a number of touch
sensors 28. In the example of FIG. 1 the sensors 28 form a diamond
pattern, but other patterns are possible as well. Each sensor 28
comprises a touch sensing element 18 (not shown in FIG. 1) which
can be independently read by a processor (not shown).
[0038] The touch panel surface 110 is typically protected by a
glass cover layer. For electronics devices comprising a display 16,
the display is typically provided underneath the touch panel
surface 110, however also variants exist in which display and touch
panel layers are intermixed or shared. More details of the layers
will be disclosed in reference to FIGS. 2a-2c below.
[0039] FIG. 2a schematically shows a cross section of a so-called
"discrete co-planar touch" touch panel, while FIG. 2b shows an
"on-cell co-planar touch" and FIG. 2c shows a "window integrated
co-planar touch" touch panel configuration.
[0040] In FIG. 2a, the top layer is formed by transparent cover
layer 2. This layer, which serves to protect the layers underneath
from damage, is typically made of glass or another hard and
transparent material in case the panel is used on top of a display
layer 16. If no display is present (like in a mouse pad), a
non-transparent protective layer may be used. In some cases, the
glass cover layer is omitted, for example in order to reduce cost.
In this case, the layer immediately below, which may for example be
a polarizer layer, will serve as the cover layer 2 and as the
surface that is to be touched by e.g. a finger or stylus. The term
"cover layer" 2 thus does not necessarily refer to a glass top
surface.
[0041] Beneath the cover window, sub-layer 4 is present. This layer
can for example comprise an anti-splinter film to prevent the cover
layer from falling apart into separate sharp pieces when broken.
Sub-layer 4 can also be a polarizer layer, for example to work with
display layer 16. Sub-layer 4 can also be formed of optical clear
adhesive or simply an airgap (with double sided adhesive at the
edges of the sensor).
[0042] Beneath sub-layer 4, the sensor layer 8 is located. This
layer comprises separate touch sensing elements 18. The sensing
elements 18 are provided on a substrate layer 6. Underneath the
substrate layer 6 reference electrode layer 12 may be provided.
Reference electrode layer 12 can provide a reference voltage. The
touch sensing elements 18 can comprise Indium Tin Oxide (ITO),
which is a suitable material for transparent sensors and
tracks.
[0043] Beneath the substrate 6 to which the sensor layer 8 and
reference electrode layer 12 are attached, another sub-layer 14 may
be provided. This layer could again be an airgap, polarizer,
adhesive layer, etc.
[0044] Below the sub-layer 14, the display layers 16 are provided.
Such a display can for example be a Liquid Crystal Display (LCD) or
organic light-emitting diode (OLED) display.
[0045] Instead of providing reference electrode layer 12 underneath
the substrate 6, the reference voltage layer 12 may also be
provided in other places of the stack, for example as a layer 12'
on top of the display 16 or as a layer 12'' inside the display
stack 16. The function of the reference voltage layer 12, 12', 12''
will be disclosed in reference to FIGS. 3a-3c. The reference
voltage layer 12, 12', 12'' can also be made of ITO.
[0046] As mentioned above, the display layer 16 may be absent, in
which case the substrate 6 with reference electrode layer 12 and
sensor layer 8, together with cover layer 2 forms a touch panel
device, for example for use in mouse pads or graphics tablets.
[0047] FIG. 2b shows an alternative variant to the above described
"discrete co-planar touch variant", the "on-cell co-planar touch".
The main difference is that the sensor layer 8 comprising the touch
sensing elements 18 is not provided on a separate substrate layer
6, but rather on the display layer 16. This saves an additional
layer, and helps to reduce the size and production costs of the
touch-panel display. In this case, the reference voltage layer is a
layer 12'' in the display stack 16.
[0048] FIG. 2c shows a further variant, the "window integrated
co-planar touch" variant. Reference is made to published US patent
application 2010/0 097 344 A1 by the same inventor which details
several embodiments of this variant. Again the separate substrate
layer 6 is absent, and the sensor layer 8 is provided on one of the
sub-layers 4, 14. The sub-layer 4 is not required--the sensing
elements 18 of the sensor layer 8 could also be provided directly
on the cover layer 2 (see for example FIG. 3c). The reference
electrode layer 12', 12'' is provided respectively on or inside the
display stack 16.
[0049] FIGS. 3a-3c schematically illustrate the functioning of the
respective touch panel device variants of FIGS. 2a-2c. A finger 20
touches the cover layer 2 at a certain point. The cover window
layer 2 is made of a non electrically conducting material,
typically glass. The finger touching the window layer 2 influences
the capacitance of the assembly of the touch panel layers 2 and 4
and finger 20. The capacitance is represented by the Ct ("touch")
capacitance.
[0050] Between touch sensing elements 18 in sensor layer 8 and the
reference voltage layer 12, 12', or 12'', the capacitance is
represented by the Ci ("intrinsic").capacitance. This capacitance
is assumed constant over time.
[0051] In each case of FIGS. 3a-3c, the touch panel configuration
is provided with means (not shown) for determining, for each
sensing element 18, the (change in) the sum of intrinsic
capacitance C.sub.i (which is expected to be constant) and C.sub.t
(the touch capacitance partly caused by finger 20). By detecting a
change in capacitance, a touch event on or near a touch sensor can
be detected. In addition, the magnitude of the change is an
indication of the size or proximity (to the touch sensing element
18) of the touching object 20.
[0052] FIGS. 4a-4c schematically show cross sections of touch panel
devices 1 according the invention. FIG. 4a again shows a "discrete
co-planar touch" variant. The sub-layer 4 is now split into layers
4 and 4'. Between both sub-layers, a resistive layer 10 is
provided. The top sub-layer 4 can comprise an anti-splinter film
and/or a polarizer stack. The resistive layer 10 can be provided on
this layer, e.g. by deposition. In an embodiment of the invention,
the resistive layer has an electrical resistance which will be
specified in more detail in reference to FIG. 5b. The sub-layer 4
may also be absent, and the resistive layer 10 can be provided on
the underside of the window cover 2, i.e. between the window cover
2 and the sensor layer 8, again for example by deposition. The
sub-layer 4' may comprise an airgap (with double-sided-adhesive
only at edges of the sensor or sensor panel), optical clear
adhesive, or an optical polarization stack.
[0053] FIGS. 4b and 4c respectively show an on-cell co-planar touch
and an integrated window co-planar touch variant with the
additional resistive layer 10 according to an embodiment of the
invention. As in FIG. 4a, sub-layer 4 is split into two sub-layers
4 and 4', with resistive layer 10 provided between layers 4 and 4'.
Alternatively, the resistive layer 10 could again be provided as a
layer on the underside of the widow cover layer 2.
[0054] In reference to FIG. 4c, in an embodiment of the invention
the layer 4' between the resistive layer 10 and the touch sensing
elements 18 comprises a polarization stack or optical passivation
coating. The resistive layer 10 can be deposited on the layer
4'.
[0055] FIG. 5a schematically shows a cross section of a
conventional touch panel display, comprising a cover window 2
providing a touch surface for finger 20 (not shown), and sensing
elements 18, 18', and 18''. Here the touch by finger 20 only
significantly influences the capacitance C.sub.i+C.sub.t as
detected by the central element 18. The adjacent elements 18' and
18'' do not register a change in capacitance.
[0056] FIG. 5b schematically shows a cross section of a touch panel
display with sensing elements 18, 18', 18'' according to an
embodiment of the invention. An additional resistive layer 10, here
represented by a series of resistances 11 (R), is provided between
the cover window 2 and sensing elements 18. The resistive layer 10
will cause the touch by a finger to also, through electrical
conduction or, in other words, the fringe effect or capacitance
spreading effect, influence the capacitances as detected by
adjacent sensing elements 18' and 18''. The addition of a high
resistive, but not electrically insulating layer 10 between the
touch input surface (the cover window) and the sensor array 8 thus
increases the accuracy of the touch panel configuration by
effectively increasing the number of sensors that sense a
capacitance variation (induced by the touch event caused by object
20) and consequently contributes to a more accurate position
calculation. The resistive layer 10 thus acts as a capacitance
spreading layer.
[0057] The requirements for resistance of the capacitance spreading
layer depend on the capacitance of the insulating layer 4' between
sensors and the conductive layer formed by reference electrode 12,
12' or 12''. The required resistance increases as the capacitance
of the insulating layer decreases. For example, for a high
capacitance (thin) insulating layer, where the capacitance is
typically in the order of 1 mF/m.sup.2, a typical sheet resistance
of 30M Ohm/sq. over the touched area can be used. For a medium
capacitance insulating layer, where the capacitance is typically in
the order of 220 nF/m.sup.2, a resistive layer sheet resistance of
150 MOhm/sq over the touched area can be used. Finally, for Low
Capacitance (thick) Insulating Layers, with Capacitance in the
order of 30 nF/m.sup.2, a resistive layer resistance of 1.2 GOhm/sq
over the touched area can be used. The resistance will thus
typically be in the range 30 MOhm/sq .about.1.2 GOhm/sq. This
translates for typical cases of panel and finger sizes (determining
the touched area) to a layer 10 sheet resistance in the range 20
MOhm/sq .about.5000 MOhm/sq. Further example values are provided in
table 1:
TABLE-US-00001 TABLE 1 Example electrical resistance values of the
resistive layer 10 as a function of capacitance of the material
between the cover surface 2 and the reference electrode 12
Capacitance Resistance (.mu.F/m.sup.2) (MOhm/sq) 900 20-100 0.22
100-500 0.03 1000-5000
[0058] FIGS. 6a and 6b schematically show curves indicative of
charge distributions corresponding to a touch event on the panels
of FIGS. 5a and 5b respectively. Spot 30 indicates the touched area
on the touch surface 110. In FIG. 6a, the spot 30 effectively
covers only one sensor 28. In FIG. 6b, fringe area 32 schematically
indicates the fringe area, overlapping a number of touch sensors
28, where the capacitance is significantly influenced due to the
capacitance spreading effect of the resistive layer 10. Thus, the
total area covered by spot 30 and fringe area 32 overlaps a
plurality of sensors 28. In FIG. 6b, a total of 9 sensors 28 is
influenced by the touch event (three sensors in a row in the X
direction and three sensors in a row in the Y direction).
[0059] If in FIG. 6a, the spot 30 is located at a corner point of a
sensor 28, then up to four sensors 28 may be influenced by the
capacitance change. But if the finger moves just a little bit away
from the corner, up to three of the four sensors may suddenly stop
registering the capacitance change. This "on-off" behavior of
conventional touch sensors is disadvantageous. In the case of FIG.
6b, always approximately 9 sensors are active, regardless of
whether the touch is on the center of a sensor or on a corner.
[0060] FIG. 7a shows the change in capacitance as detected by a
range of sensing elements along a Y axis (the North-South direction
in FIG. 8) in a touch panel without the resistive layer 10. The
arrow indicates the "true" position of the touch event, and the
dashed line 70 corresponds to the "true" distribution of the
capacitance change (of which FIG. 6a shows a top view). In this
case, the change in capacitance is so limited that only the central
sensor 28 registers a change, and the sensors to the north and the
two sensors to the south register no change at all. This results in
a "dead area", as the arrow can move over a distance within the
sensor 28 without influencing the capacitance detection.
[0061] FIG. 7b shows the change in capacitance in a touch panel
with resistive layer 10. Due to the resistive layer 10, the change
in capacitance is more widely spread (the dashed curve 70' again
represents the "true" capacitance distribution), so that now five
sensors in a row detect a change. In principle it is possible to
fit a suitable curve to the five measurements (e.g. a Bell curve)
and determine the X,Y coordinates of the central position of the
touching element.
[0062] Other, less computationally expensive methods are also
available for finding the maximum of the measured capacitance
change distribution, such as weighted averages and parabolic or
linear line fitting. In this case, there is no "dead area", since a
small movement of the arrow in the central sensor 28 will
immediately influence the balance between the capacitance
detections in neighboring sensors 28' and 28'', and thus be
detectible.
[0063] While FIGS. 7a and 7b illustrate the principles of the
capacitance distribution in 1 dimension (along axis Y), the same
principles apply in 2 dimensions (X-Y directions).
[0064] FIG. 8 schematically shows a 2D overview of neighbouring
sensors 28 (each comprising at least one sensing element 18) of
which capacitance change values are used in an algorithm for
calculating the touch location according to an embodiment of the
invention. The algorithm for calculating the touch location may use
not only the sensor with the highest `capacitive touch response
signal` but also adjacent sensors that may have been partly
activated, as is illustrated in FIG. 7b.
[0065] FIG. 8 shows a central sensor C, which is indicated by the
sensor which shows the maximum registered change in capacity.
Neighboring or adjacent sensors are E (East) in the negative X
direction, and W (West) in the positive X direction, N (North) in
the negative Y direction and S (South) in the positive Y direction.
Between S and E sensors, sensor SE (South-East) is located.
Likewise NE (North-East) between N and E, SW (South-West) between S
and W and NW (North-West) between N and W.
[0066] With S as the signal per sensor (in counts), the total of
counts of sensors contributing to the X or Y location determination
become respectively (the subscript refers to the sensor as
described above):
S.sub.X-total=[S.sub.E+S.sub.NE+S.sub.SE+S.sub.C+S.sub.NW+S.sub.SW+S.sub-
.W]
S.sub.Y-total=[S.sub.N+S.sub.NE+S.sub.NW+S.sub.C+S.sub.SE+S.sub.SW+S.sub-
.S]
[0067] The values for the S-coefficients will typically depend on
the proximity of the respective sensor to the central sensor along
the axis (X or Y) of interest.
[0068] The weighted centroid locations in X and Y directions are
now defined as:
X centroid = [ ( X E S E ) + ( X NE S NE ) + ( X SE S SE ) + ( X C
S C ) + ( X NW S NW ) + ( X SW S SW ) + ( X W S W ) ] / S X - total
##EQU00001## Y centroid = [ ( Y N S N ) + ( Y NE S NE ) + ( Y NW S
NW ) + ( Y C S C ) + ( Y SE S SE ) + ( Y SW S SW ) + ( Y S S S ) ]
/ S Y - total ##EQU00001.2##
[0069] The coordinate pair (Xcentroid, Ycentroid) is thus a
floating point value rather than a discrete value. Therefore, the
use of a weighted algorithm in combination with a resistive layer
10 according the invention improves the accuracy of the touch event
location detection.
[0070] While the above example is given in reference to the diamond
pattern of FIGS. 1 and 9, the skilled person will be able to apply
the example to other patterns, e.g. square grids, rectangular
grids, etc. The skilled person will also be aware that other
algorithms exist for calculating a floating point coordinate pair
(X,Y) from a series of measurements at different discrete locations
(C, E, SE, S, etc).
[0071] It is observed that, in the above specification, at several
locations reference is made to "controllers" or "processors". It is
to be understood that such controllers/processors may be designed
in any desired technology, i.e. analogue or digital or a
combination of both. A suitable implementation would be a software
controlled processor where such software is stored in a suitable
memory present in the touch panel device and connected to the
processor/controller. The memory may be arranged as any known
suitable form of RAM (random access memory) or ROM (read only
memory), where such ROM may be any form of erasable ROM such as
EEPROM (electrically erasable ROM). Parts of the software may be
embedded. Parts of the software may be stored such as to be
updatable e.g. wirelessly as controlled by a server transmitting
updates regularly over the air.
[0072] It is to be understood that the invention is limited by the
annexed claims and its technical equivalents only. In this document
and in its claims, the verb "to comprise" and its conjugations are
used in their non-limiting sense to mean that items following the
word are included, without excluding items not specifically
mentioned. In addition, reference to an element by the indefinite
article "a" or "an" does not exclude the possibility that more than
one of the element is present, unless the context clearly requires
that there be one and only one of the elements. The indefinite
article "a" or "an" thus usually means "at least one".
* * * * *