U.S. patent application number 15/796985 was filed with the patent office on 2018-02-22 for capacitive touch screen with noise suppression.
The applicant listed for this patent is Atmel Corporation. Invention is credited to Samuel Brunet, Harald Philipp, Peter Sleeman, Matthew Trend.
Application Number | 20180052546 15/796985 |
Document ID | / |
Family ID | 41060854 |
Filed Date | 2018-02-22 |
United States Patent
Application |
20180052546 |
Kind Code |
A1 |
Sleeman; Peter ; et
al. |
February 22, 2018 |
Capacitive Touch Screen with Noise Suppression
Abstract
A capacitive touch sensor wherein the touch sensitive panel has
drive electrodes arranged on the lower side of a substrate and
sense electrodes arranged on the upper side. The drive electrodes
are shaped and dimensioned to substantially entirely cover the
touch sensitive area with individual drive electrodes being
separated from each other by small gaps, the gaps being so small as
to be practically invisible. The near blanket coverage by the drive
electrodes also serves to screen out interference from noise
sources below the drive electrode layer, such as drive signals for
an underlying display, thereby suppressing noise pick-up by the
sense electrodes that are positioned above the drive
electrodes.
Inventors: |
Sleeman; Peter;
(Waterlooville, GB) ; Brunet; Samuel; (North
Boarhunt, GB) ; Trend; Matthew; (Fareham, GB)
; Philipp; Harald; (Zug, CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Atmel Corporation |
San Jose |
CA |
US |
|
|
Family ID: |
41060854 |
Appl. No.: |
15/796985 |
Filed: |
October 30, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12421705 |
Apr 10, 2009 |
9823784 |
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15796985 |
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61044038 |
Apr 10, 2008 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06F 2203/04112
20130101; H05K 1/0296 20130101; G06F 3/0446 20190501; G01B 2210/58
20130101; G01D 5/2405 20130101; G06F 3/0416 20130101; G06F 3/041661
20190501; G06F 3/0445 20190501; G06F 2203/04111 20130101; G06F 1/16
20130101; G06F 3/0412 20130101; G01B 7/003 20130101; H03K 17/9622
20130101; G06F 3/044 20130101 |
International
Class: |
G06F 3/044 20060101
G06F003/044; G06F 3/041 20060101 G06F003/041; G01B 7/00 20060101
G01B007/00; G01D 5/24 20060101 G01D005/24; G06F 1/16 20060101
G06F001/16; H03K 17/96 20060101 H03K017/96; H05K 1/02 20060101
H05K001/02 |
Claims
1-20. (canceled)
21. A sensor comprising: a substrate; and a sensing region
configured to sense a position of an object within the sensing
region, the sensing region comprising: a plurality of drive
electrodes disposed on a first side of the substrate in a first
layer; and a plurality of sense electrodes disposed on a second
side of the substrate in a second layer so that the sense
electrodes intersect the drive electrodes at a plurality of
intersections offset by a thickness of the substrate, wherein the
sensing region further comprises a plurality of isolated conductive
elements disposed on the second side of the substrate between the
sense electrodes so that, together, the plurality of sense
electrodes and the plurality of isolated conductive elements are
substantially area filling within the sensing region.
22. The sensor of claim 21, wherein: a first drive electrode is
coupled to a first voltage source; a second drive electrode is
coupled to a second voltage source; and a third drive electrode,
disposed between the first drive electrode and the second drive
electrode, is not coupled to the first or second voltage
source.
23. The sensor of claim 22, wherein a fourth drive electrode,
disposed between the first drive electrode and the second drive
electrode, is not coupled to the first or second voltage
source.
24. The sensor of claim 22, wherein: the first voltage source
provides a pulse signal; and the second voltage source provides
ground.
25. The sensor of claim 21, wherein each drive electrode is made of
a mesh or filigree pattern of interconnected lines of highly
conductive material which collectively define each electrode.
26. The sensor of claim 21, wherein each drive electrode is coupled
to an adjacent drive electrode through a resistive element.
27. The sensor of claim 21, wherein each sense electrode is made of
a mesh or filigree pattern of interconnected lines of highly
conductive material which collectively define the sense
electrode.
28. The sensor of claim 21, wherein: the sense electrodes have a
line width one quarter or less than a pitch of the drive
electrodes, or a gap between adjacent sense electrodes has a width
that is at least three-fifths of a pitch of the sense
electrodes
29. The sensor of claim 28, wherein the pitch of the sense
electrodes is approximately 5 millimeters or less.
30. The sensor or claim 28, wherein the gap between adjacent sense
electrodes is approximately 3.5 millimeters or more.
31. The sensor of claim 28, wherein the line width of the sense
electrodes is between approximately 100 micrometers and
approximately 250 micrometers.
32. The sensor of claim 21, wherein adjacent drive electrodes are
separated by small gaps, wherein the gaps are less than around 100
micrometers.
33. The sensor of claim 21, wherein the second side of the
substrate is coupled to a display module.
34. The sensor of claim 21, wherein an outermost drive electrode
has a width that is approximately half of a width of an adjacent
drive electrode, the outermost drive electrode being adjacent to
only one drive electrode.
35. The sensor of claim 21, wherein the drive electrodes extend
arcuately and the sense electrodes extend radially so that the
plurality of intersections lie on one or more arcuate paths.
36. A computer-readable non-transitory storage media embodying
logic that is configured when executed to: sense a position of an
object within a sensing region, the sensing region comprising: a
plurality of drive electrodes disposed on a first side of a
substrate in a first layer; and a plurality of sense electrodes
disposed on a second side of the substrate in a second layer so
that the sense electrodes intersect the drive electrodes at a
plurality of intersections offset by a thickness of the substrate;
and communicate a capacitance sensed by the drive and sense
electrodes and indicative of the position of the object within the
sensing region, wherein the sensing region further comprises a
plurality of isolated conductive elements disposed on the second
side of the substrate between the sense electrodes so that,
together, the plurality of sense electrodes and the plurality of
isolated conductive elements are substantially area filling within
the sensing region relative to the plurality of sense
electrodes.
37. The media of claim 36, wherein: each drive electrode is coupled
to an adjacent drive electrode through a resistive element; a first
drive electrode is coupled to a first voltage source; a second
drive electrode is coupled to a second voltage source; and a third
drive electrode, disposed between the first drive electrode and the
second drive electrode, is not coupled to the first or second
voltage source.
38. The media of claim 37, wherein a fourth drive electrode,
disposed between the first drive electrode and the second drive
electrode, is not coupled to the first or second voltage
source.
39. The media of claim 36, wherein: the first voltage source
provides a pulse signal; and the second voltage source provides
ground.
40. The media of claim 36, wherein the sensing region further
comprises a plurality of isolated conductive elements disposed on
the second side of the substrate between the sense electrodes so
that, together, the plurality of sense electrodes and the plurality
of isolated conductive elements are substantially area filling
within the sensing region relative to the plurality of sense
electrodes.
41. The media of claim 36, wherein each drive or sense electrode is
made of a mesh or filigree pattern of interconnected lines of
highly conductive material which collectively define each
electrode.
42. A method comprising: sensing a position of an object within a
sensing region, the sensing region comprising: a plurality of drive
electrodes disposed on a first side of a substrate in a first
layer; and a plurality of sense electrodes disposed on a second
side of the substrate in a second layer so that the sense
electrodes intersect the drive electrodes at a plurality of
intersections offset by a thickness of the substrate; and
communicating a plurality of signals resulting from a capacitance
sensed by the drive and sense electrodes and indicative of the
position of the object within the sensing region, wherein the
sensing region further comprises a plurality of isolated conductive
elements disposed on the second side of the substrate between the
sense electrodes so that, together, the plurality of sense
electrodes and the plurality of isolated conductive elements are
substantially area filling within the sensing region relative to
the plurality of sense electrodes.
43. The method of claim 42, wherein: each drive electrode is
coupled to an adjacent drive electrode through a resistive element;
a first drive electrode is coupled to a first voltage source; a
second drive electrode is coupled to a second voltage source; and a
third drive electrode, disposed between the first drive electrode
and the second drive electrode, is not coupled to the first or
second voltage source.
44. The method of claim 43, wherein a fourth drive electrode,
disposed between the first drive electrode and the second drive
electrode, is not coupled to the first or second voltage
source.
45. The method of claim 43, wherein: the first voltage source
provides a pulse signal; and the second voltage source provides
ground.
46. The method of claim 42, wherein the sensing region further
comprises a plurality of isolated conductive elements disposed on
the second side of the substrate between the sense electrodes so
that, together, the plurality of sense electrodes and the plurality
of isolated conductive elements are substantially area filling
within the sensing region relative to the plurality of sense
electrodes.
47. The method of claim 42, wherein the drive electrodes extend
arcuately and the sense electrodes extend radially so that the
plurality of intersections lie on one or more arcuate paths.
Description
BACKGROUND OF THE INVENTION
[0001] The following describes a new invention in the field of
capacitive touch screens or 2-dimensional capacitive transducing
(2DCT) sensors. U.S. Pat. No. 6,452,514, U.S. Pat. No. 7,148,704
and U.S. Pat. No. 5,730,165 disclose a capacitive measurement
technique which makes it possible to create touch responsive
transparent or opaque sensing regions that can detect human touch
through several millimeters of plastic or glass. Described herein
is a new structure for a touch screen that allows significant
enhancement in both operation and appearance of the sensor.
[0002] U.S. Pat. No. 6,452,514 describes a capacitive measurement
technique which is incorporated by reference herein, that uses a
transmit-receive process to induce charge across the gap between an
emitting electrode and a collecting electrode (the transmitter and
the receiver respectively, also referred to as X and Y). The
capacitive sensing described in U.S. Pat. No. 6,452,514 may be
referred to as mutual capacitive or active type 2DCT sensors. As a
finger touch interacts with the resulting electric field between
the transmitter and receiver electrodes, the amount of charge
coupled from transmitter to receiver is changed. A particular
feature of the measurement technique is that most of the electric
charge tends to concentrate near to sharp corners and edges (a well
known effect in electrostatics). The fringing fields between
transmitter and receiver electrodes dominate the charge coupling.
The electrode design therefore tends to focus on the edges and the
gaps between neighboring transmitter and receiver electrodes in
order to maximize coupling and also to maximize the ability of a
touch to interrupt the electric field between the two, hence giving
the biggest relative change in measured charge. Large changes are
desirable as they equate to higher resolution and equally to better
signal to noise ratio.
[0003] A specially designed control chip can detect these changes
in charge. It is convenient to think of these changes in charge as
changes in measured coupling capacitance between transmitter and
receiver electrodes (charge is rather harder to visualize). The
chip processes the relative amounts of capacitive change from
various places around the touch screen and uses this to compute the
absolute location of touch as a set of x and y coordinates. In
order for this to be possible a set of spatially distributed
electrodes must be used. Commonly, these electrodes are required to
be transparent so that the touch screen can operate in front of a
display such as a liquid crystal display (LCD) screen or other
display screen type, for example organic light emitting diode
(OLED) type screens. To achieve this electrodes are often
fabricated from a material known as Indium Tin Oxide (ITO) but
other transparent conductive materials are also suitable. ITO has
desirable properties in optical terms, but can be substantially
ohmic which can have a negative impact on capacitive measurements
if the resistance and capacitance combination leads to time
constants that prevent timely settling of the charge transfer
process.
[0004] Another example 2DCT is disclosed in US20070062739A1.
[0005] In order to create a sensor that can report the absolute
coordinates of the location of the touch (or more than one touch)
on the surface of the sensor or the overlying plastic or glass
panel, the electrode arrangement must be specifically designed to
optimize the following aspects: [0006] accuracy of the reported
touch location i.e. correspondence between real physical location
and reported location. This is broadly known as "linearity" or
"non-linearity" when referring to the measured error. [0007]
immunity of the sensor to external electrical noise sources. [0008]
sensitivity of the sensor to human touch i.e. its ability to detect
a touch through thicker panel materials, or to detect a lighter or
smaller touch. [0009] spatial resolution of the sensor i.e. its
ability to report small changes in touch location. [0010] quality
of the output in terms of the noise or jitter amplitude in the
reported location. [0011] optical quality of the sensor for the
transmission of light, for factors like its transparency, its hue,
its haze, the overall electrode pattern visibility etc. [0012]
optical behavior of the sensor to shallow angle reflected light
i.e. the visibility of the electrode pattern and any color shifts
in the reflected light. [0013] minimizing any errors induced in the
reported location caused by slight mechanical flexing during human
touch. This tends to cause a change in the distance between the
sensor and any underlying display or other mechanical grounded
structure which in turn causes capacitive changes similar to a
touch. [0014] reducing the electrical resistance of the electrodes
to allow efficient capacitive sensing within an acceptable time
(often the overall measurement time of the touch screen needs to be
at or below 10 ms so limiting the amount of settling time that can
be used to make each measurement). [0015] reducing the number of
layers in the physical construction to minimize manufacturing cost
and to improve optical properties. [0016] reducing side-effects in
the quality of reported coordinates or in the ability of the sensor
to detect a touch, near to the edges of the sensor. This region
typically presents difficult challenges in this regard because of
the non-uniformity of the electrode pattern (its ends) and the fact
that interconnecting tracks tend to reside at the edges of the
sensor. [0017] reducing the total number of electrodes used as each
electrode requires some connection to the control chip and so more
electrodes equates to a more complex chip and hence higher
cost.
[0018] In order to optimize linearity, the electrode pattern design
is critical. Linearity is one of the primary measures of quality of
a touch screen because as the linearity degrades, it becomes harder
to report an accurate touch location in some regions of the screen.
A sensor design that offers excellent intrinsic linearity is a key
goal therefore. While it is possible to mathematically correct such
non-linearity via well known techniques such as a look-up table or
piecewise-linear correction, any of these methods actually trades
off spatial resolution for reported linearity, and so is always a
compromise.
[0019] In designing the electrodes a key objective is to arrange
that the electric field that propagates from transmitter to
receiver does so in a way that causes a smooth and progressive
gradation from one electrode to the next. This way, as a touch
moves from region to region, the capacitive change measured by the
control chip also changes in a smooth and progressive way and hence
contributes to good intrinsic linearity. The touch itself actually
influences this process significantly and will tend to "mix" the
fields from neighboring electrodes. This contributes to the overall
smoothness of transition, but does tend to lead to some variation
in linearity depending on the size of the touch applied. Again,
electrode design needs to be carefully considered to optimize the
linearity across a range of touch sizes.
[0020] As described above the quality of the output in terms of the
noise or jitter amplitude in the reported location should be
optimized. However, 2DCT sensors can be sensitive to external
ground loading. Furthermore, electrical noise generated from LCD
screens can interfere with capacitance measurements when a pointing
object approaches the screen. Known methods to minimize the effects
of noise on capacitive coupling is to increase the separation or
air gap between an LCD screen and an overlaying 2DCT sensor.
Alternatively a shielding layer may be incorporated between the LCD
screen and a 2DCT sensor to reduce or block the noise induced by
the LCD screen.
[0021] WO 2009/027629 published on 5 Mar. 2009 describes a
capacitive touch sensor comprising a dielectric panel overlying a
drive electrode with two sense electrodes. One of the sense
electrodes is positioned to be shielded from the drive electrode by
the first sense electrode, so that the first sense electrode
receives the majority of the charge coupled from the drive
electrode and the second sense electrode primarily registers noise.
A sensing circuit including two detector channels is connected to
the first (coupled) and second (noise) sense electrodes to receive
signal samples respectively. The sensing circuit is operable to
output a final signal obtained by subtracting the second signal
sample from the first signal sample to cancel noise.
[0022] However, the methods described above increase the size and
thickness, and may decrease the resolution of a device
incorporating a display screen with a 2DCT sensor when it is more
fashionable and desirable to produce smaller devices. Furthermore,
additional steps are required during manufacture and as a result
there is an increased cost due to further components being
needed.
[0023] European patent EP 1821175 describes an alternative solution
to reduce the noise collected on a 2DCT touch sensor. EP 1821175
discloses a display device with a touch sensor which is arranged so
that the two dimensional touch sensor is overlaid upon a display
device to form a touch sensitive display screen. The display device
uses an LCD arrangement with vertical and horizontal switching of
the LCD pixels. The touch sensing circuit includes a current
detection circuit, a noise elimination circuit as well as a
sampling circuit for each of a plurality of sensors, which are
arranged to form the two-dimensional sensor array. The current
detection circuit receives a strobe signal, which is generated from
the horizontal and vertical switching signals of the LCD screen.
The strobe signal is used to trigger a blanking of the current
detection circuit during a period in which the horizontal switching
voltage signal may affect the measurements performed by the
detection circuit.
[0024] WO 2009/016382 published on 5 Feb. 2009 describes a sensor
used to form a two dimensional touch sensor, which can be overlaid
on a liquid crystal display (LCD) screen. As such, the effects of
switching noise on the detection of an object caused by a common
voltage signal of the LCD screen can be reduced. The sensor
comprises a capacitance measurement circuit operable to measure the
capacitance of the sensing element and a controller circuit to
control charging cycles of the capacitance measurement circuit. The
controller circuit is configured to produce charging cycles at a
predetermined time and in a synchronous manner with a noise signal.
For example, the charge-transfer cycles or `bursts` may be
performed during certain stages of the noise output signal from the
display screen, i.e. at stages where noise does not significantly
affect the capacitance measurements performed. Thus, the sensor can
be arranged to effectively pick up the noise output from a display
screen and automatically synchronize the charge-transfer bursts to
occur during stages of the noise output cycle.
[0025] However, noise reduction techniques such as those described
above require more complex measurement circuitry. This makes the
measurement circuitry more expensive and may increase the time
taken to complete an acquisition cycle.
[0026] It would therefore be desirable to provide an electrode
pattern suitable for mutual capacitive or active type 2DCT sensor
that can be embodied with an electrode pattern with reduced noise
pick-up.
SUMMARY OF THE INVENTION
[0027] According to a first aspect of the invention, a capacitive
touch sensor is provided comprising a touch sensitive panel having
a plurality of drive electrodes arranged on one side of a substrate
in a first layer and a plurality of sense electrodes arranged on
the other side of the substrate in a second layer so that the sense
electrodes cross the drive electrodes at a plurality of
intersections offset from each other by the thickness of the
substrate, wherein the drive electrodes substantially entirely
cover the first layer with individual ones of the drive electrodes
being separated from neighboring drive electrodes by small
gaps.
[0028] This approach has several important advantages. The touch
sensor only requires the two layers of electrodes recited above to
function, so that a third noise-suppressing layer as adopted in
some prior art designs is superfluous. A two layer construction
also leads to improved optical transmission, thinner overall depth
and lower cost compared with designs with a greater numbers of
layers. The area-filling design for the drive electrodes with small
gaps allows for an almost invisible drive electrode pattern, for
example when using ITO, and also isolates the sense lines from
capacitive effects below the first layer, for example noise from an
underlying LCD module or other noise source. The "flooding" of the
first layer with conductive material also allows the second layer
to be implemented with narrow sense electrodes, far narrower than
the dimension of the sensing object. The second layer can also be
made invisible either through in-filling of islands of electrode
material between the sense electrodes to also "flood" the second
layer, or alternatively simply by making the sense electrodes very
thin or very sparse with line widths so small that they are
invisible. This sparse approach using meshes is described further
below. The reduced sense electrode area also reduces susceptibility
to coupling noise from touches.
[0029] The drive electrodes are preferably separated by a pitch of
comparable dimension to the touch size of the touching object for
which the sensor is designed.
[0030] The touching object for which the sensor is designed may be
a finger, e.g. of touch size 8-10 mm diameter, and the pitch is
around 8 mm or less. A stylus could also be used.
[0031] The small gaps between adjacent drive electrodes are
preferably dimensioned to be sufficiently small to be invisible or
almost invisible, for example less than around 100 micrometers,
preferably having dimensions of a few tens of micrometers.
[0032] The sense electrodes are advantageously narrow in comparison
to the size of the touching object. For example, the sense
electrodes may have a line width of one quarter or less of the size
of the touching object. In one embodiment, the touching object for
which the sensor is designed is a finger with a touch size of 8-10
mm diameter, and the sense electrodes have a line width of 2 mm or
less, for example 0.5 mm. The sense electrodes may have a line
width one quarter or less than the pitch of the drive
electrodes.
[0033] In some embodiments, the second layer additionally
accommodates isolated elements arranged between the sensing
electrodes so that the sense electrodes and the isolated elements
together substantially entirely cover the second layer with
individual ones of the sense electrodes and isolated elements being
separated from each other by small gaps. The small gaps have
comparable function and dimensions to the small gaps between the
drive electrodes.
[0034] As mentioned above, the first and second layers of
electrodes may be the only electrode layers provided, a two-layer
electrode construction leading to improved optical transmission for
transparent embodiments such as used for touch-sensitive displays,
thinner overall construction, and lower cost.
[0035] The drive electrodes preferably cover the first layer
sufficiently entirely that the sense electrodes in the second layer
are substantially isolated from capacitive effects below the first
layer.
[0036] An important combination is the above-defined capacitive
touch sensor with a display module. The display module, for example
an LCD or OLED display panel, will typically by arranged below the
first layer and distal the touch surface so that from top to
bottom, or outside to inside the device, the components will
be--dielectric layer the upper surface of which will be the touch
surface-layer 2-substrate-layer 1-display panel, with the display
panel being inside the device housing or outer shell. In a display
application, the electrodes will likely be made of ITO.
[0037] In some embodiments, each drive and/or sense electrode is
made of a continuous sheet of electrically conductive material,
such as ITO or a metal. In other embodiments, each drive and/or
sense electrode is made of a mesh or filigree pattern of
interconnected lines of highly conductive material which
collectively define each electrode. Still further embodiments use
continuous sheets for one of the electrode types and meshes for the
other electrode type. In the mesh approach, the interconnected
lines preferably have a sufficiently small width so as to be
invisible or almost invisible. They can then be made of material
that is not inherently invisible, e.g. a metal such as copper, but
still remain practically invisible.
[0038] The invention can be implemented to form a Cartesian xy grid
of touch sensor locations. In particular, the drive electrodes can
extend in a first linear direction and the sense electrodes in a
second linear direction transverse to the first linear direction so
that the plurality of intersections form a grid pattern, for
example a square, diamond or rectangular grid. The invention can
also be implemented to form a polar `r.theta.` grid, wherein the
drive electrodes extend arcuately and the sense electrodes extend
radially so that the plurality of intersections lie on one or more
arcuate paths.
[0039] A further aspect of the invention relates to a touch
sensitive panel for a capacitive touch sensor, the touch sensitive
panel having a plurality of drive electrodes arranged in a first
layer and a plurality of sense electrodes arranged in a second
layer so that the sense electrodes cross the drive electrodes at a
plurality of intersections offset from each other, wherein the
drive electrodes substantially entirely cover the first layer with
individual ones of the drive electrodes being separated from
neighboring drive electrodes by small gaps. The first and second
layers can be disposed on opposite sides of a common substrate
offset from each other by the thickness of the substrate.
Alternatively, the first and second layers can be disposed on
different substrates which can then be assembled in engagement with
each other to provide an offset between the two layers equal to the
thickness of one of the substrates, or both of them, depending on
which side of the substrates the electrodes are arranged.
[0040] The touch sensitive panel has a plurality of drive
electrodes arranged on one side of a substrate in a first layer and
a plurality of sense electrodes arranged on the other side of the
substrate in a second layer so that the sense electrodes cross the
drive electrodes at a plurality of intersections offset from each
other by the thickness of the substrate, wherein the drive
electrodes substantially entirely cover the first layer with
individual ones of the drive electrodes being separated from
neighboring drive electrodes by small gaps.
[0041] A still further aspect of the invention relates to a method
of manufacturing a touch sensitive panel for a capacitive touch
sensor comprising: [0042] providing a substrate having first and
second sides; [0043] depositing on the first side of the substrate
a first layer of conductive material in a first pattern forming a
plurality of drive electrodes, wherein the drive electrodes
substantially entirely cover the first layer with individual ones
of the drive electrodes being separated from neighboring drive
electrodes by small gaps; and [0044] depositing on the second side
of the substrate a second layer of conductive material in a second
pattern forming a plurality of sense electrodes so that the sense
electrodes cross the drive electrodes at a plurality of
intersections offset from each other by the thickness of the
substrate.
[0045] The invention may also be defined by a touch sensitive panel
having an electrode pattern comprising a plurality of drive
electrodes extending in a first direction and spaced apart in a
second direction; wherein the drive electrodes are spaced apart by
a distance of less than 100 .mu.m and have a pitch of less than or
equal to 8 mm.
[0046] The drive electrodes may be spaced apart by a distance 90,
80, 70, 60, 50, 40, 30, 20 or 10 .mu.m. The pitch of the drive
electrodes may be less than or equal to 5 mm.
[0047] The same extent of each drive electrode may be coupled to
adjacent drive electrodes using a resistor. The typical resistor
values used range from a few K.OMEGA. to 10's of K.OMEGA. The
resistors may be discrete resistors, screen printed resistive
elements or meandering patterns formed using the same material as
the drive electrodes.
[0048] The width of the drive electrodes at the outer edges of the
electrode pattern may be half the width of the other drive
electrodes.
[0049] The electrode pattern may further comprise a plurality of
sense electrodes extending in a second direction and spaced apart
in the first direction crossing the drive electrodes.
[0050] The sense electrodes may be spaced apart by a plurality of
isolated electrodes wherein having the same extent in the first and
second direction as the width of the sense electrodes. The space or
gaps between the isolated electrodes is of the order of 10's of
.mu.m.
[0051] The width of the sense electrodes may be substantially less
than the width of the drive electrodes. The width of the sense
electrode is typically in the range of 100 to 1000 .mu.m
[0052] According to another aspect of the present invention there
is provided a two-dimensional position sensor comprising the
electrode pattern of drive electrodes and sense electrodes, wherein
the drive electrodes and the sense electrodes may be disposed on
opposing surfaces of a substrate.
[0053] According to another aspect of the present invention there
is provided a two-dimensional position sensor comprising the
electrode pattern of drive electrodes and sense electrodes, wherein
the drive electrodes and the sense electrodes may be disposed on a
surface of two different substrates.
[0054] The two-dimensional position sensor may further comprise a
controller comprising a drive unit for applying drive signals to
the drive electrodes, and a sense unit for measuring sense signals
received from each of the respective sense electrode representing a
degree of capacitive coupling of the drive signals between the
drive electrodes and each of the sense electrodes.
[0055] The controller may further comprise a processing unit for
calculating a position of an interaction with the sensitive area
from an analysis of the sense signals obtained by applying drive
signals to the drive electrodes.
[0056] The processing unit may be operable to determine position in
the first direction by an interpolation between sense signals
obtained from each of the plurality of sense electrodes.
[0057] The processing unit may be operable to determine position in
the second direction by an interpolation between sense signals
obtained by sequentially driving each of the plurality of drive
electrodes with respective drive signals.
[0058] According to another aspect of the present invention there
is provided a two-dimensional position sensor comprising the
electrode pattern of drive electrodes, further comprising a
plurality of sense electrodes extending in a second direction and
spaced apart in the first direction crossing the drive electrodes;
wherein the drive electrodes and the sense electrodes are disposed
on opposing surfaces of a substrate; the two-dimensional sensor
further comprising a controller comprising: a drive unit for
applying drive signals to the drive electrodes; wherein the drive
electrodes are grouped together into a subset of drive electrodes
such that the drive unit is operable to apply drive signals to the
outer-most drive electrodes of each subset of drive electrodes; and
a sense unit for measuring sense signals received from each of the
respective sense electrode representing a degree of capacitive
coupling of the drive signals between the drive electrodes and each
of the sense electrodes.
[0059] According to another aspect of the present invention there
is provided a method of sensing position of an actuation on a
two-dimensional position sensor comprising: an electrode pattern
comprising a plurality of drive electrodes extending in a first
direction and spaced apart in a second direction; wherein the drive
electrodes are spaced apart by a distance of less than 100 .mu.m
and have a pitch of less than or equal to 8 mm; a plurality of
sense electrodes extending in a second direction and spaced apart
in the first direction crossing the drive electrodes; wherein the
drive electrodes and the sense electrodes are disposed on opposing
surfaces of a substrate; the method comprising: applying drive
signals to the drive electrodes, measuring sense signals received
from each of the respective sense electrodes representing a degree
of capacitive coupling of the drive signals between the drive
electrodes and each of the sense electrodes; determining position
in the first direction by an interpolation between sense signals
obtained from each of the plurality of sense electrodes; and
determining position in the second direction by an interpolation
between sense signals obtained by sequentially driving each of the
plurality of drive electrodes with respective drive signals.
[0060] The invention may alternatively be defined by a
two-dimensional touch screen comprising: a substrate; a plurality
of driven-electrodes extending in a first direction on a first
surface of the substrate; a plurality of Y-electrodes extending in
a second direction being perpendicular to the first direction on a
second surface of the substrate opposing the first surface of the
substrate; wherein the plurality of driven-electrodes substantially
fill an area of the first surface of the substrate, for
example.
[0061] Two-dimensional touch screens are typically used as on
overlay on a display screen. The area filling design of the driven
electrodes leads to an almost invisible electrode pattern. The area
filling design also provides partial attenuation of noise coupled
from an underlying LCD module or other noise source.
[0062] The two-dimensional touch screen may further comprise a
subset of driven-electrodes comprising two outer most
driven-electrodes and two or more intermediate driven-electrodes
connected together using a plurality of resistive elements. This
reduces the interconnecting wiring between the touch screen and the
control chip.
[0063] The width of the two outer most driven electrodes may be
half the width of the other driven-electrodes to improve the
overall linearity of the measured capacitance.
[0064] The width of the Y-electrodes may be substantially less than
the width of the driven-electrodes such that the Y-electrodes are
not easily visible to the human eye and narrower electrodes provide
better noise immunity.
[0065] The spacing between each of the plurality of
driven-electrodes may be less then 100 .mu.m to make the pattern
substantially invisible to the human eye.
[0066] The pitch of the drive-electrodes and the Y-electrodes may
be 8 mm or less to achieve a good intrinsic linearity and to match
the size of a typical finger touch.
[0067] The area between each of said Y-electrodes may be filled
with isolated conductive material such that is it possible to make
narrow Y-electrodes while still have a pattern that is
substantially invisible to the human eye and can reduce the
susceptibility to coupling noise from a touch.
[0068] The Y-electrodes of the two-dimensional touch screen may
further comprise a plurality of equally disposed cross-members
running in the first direction. This can achieve uniform field
patterns that are symmetrical in all regions of the touch screen
leading to good linearity. These cross members effectively act to
spread the electric field further beyond the primary Y-electrode to
overlap the region which can gradate the electric field.
[0069] According to the another aspect of the invention there is
provided a method of determining a touch location adjacent a
two-dimensional touch screen comprising: a substrate; a plurality
of driven-electrodes extending in a first direction on a first
surface of the substrate; a plurality of Y-electrodes extending in
a second direction being perpendicular to the first direction on a
second surface of the substrate opposing the first surface of the
substrate; wherein the plurality of driven-electrodes substantially
fill an area of the first surface of the substrate; the method
comprising the steps of: applying a potential to each of the
plurality of driven-electrodes while the other driven-electrodes
are held at a zero potential; measuring the capacitance at each
intersection formed between the driven electrodes and the Y
electrodes; generating measurements at each intersection formed
between the driven electrodes and the Y electrodes; and computing
the touch location based on the generated measurements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0070] For a better understanding of the invention, and to show how
the same may be carried into effect, reference is now made by way
of example to the accompanying drawings, in which:
[0071] FIG. 1A shows a side view of a two-electrode layer
capacitive touch screen according to an embodiment of the present
invention;
[0072] FIG. 1B shows a perspective view of a two-electrode layer
capacitive touch screen according to an embodiment of the present
invention;
[0073] FIG. 1C shows a side view of a two-electrode layer
capacitive touch screen according to another embodiment of the
present invention;
[0074] FIG. 1D shows a side view of a two-electrode layer
capacitive touch screen according to another embodiment of the
present invention;
[0075] FIG. 1E shows a side view of a two-electrode layer
capacitive touch screen according to an embodiment of the present
invention;
[0076] FIG. 2A shows an electrode pattern of drive electrodes with
resistive elements according to an embodiment of the invention;
[0077] FIG. 2B shows a portion of the electrode pattern shown in
FIG. 2A with a meander pattern of electrode material;
[0078] FIG. 2C shows a portion of the electrode pattern shown in
FIG. 2A with screen printed resistors;
[0079] FIG. 2D shows a portion of the electrode pattern shown in
FIG. 2A with discrete resistors;
[0080] FIG. 3 shows a portion of the electrode pattern shown in
FIG. 2B.
[0081] FIG. 4 shows a portion of the electrode pattern of drive
electrodes according to an embodiment of the invention;
[0082] FIG. 5A shows a portion of the electrode pattern shown in
FIG. 2A;
[0083] FIG. 5B shows a typical finger tip;
[0084] FIG. 6 shows an electrode pattern of drive electrodes
according to an embodiment of the invention;
[0085] FIG. 7A shows an electrode pattern of sense electrodes
according to an embodiment of the invention;
[0086] FIG. 7B shows a two-electrode layer capacitive touch screen
according to an embodiment of the present invention with drive and
sense units connected via channels to a controller;
[0087] FIG. 8A shows schematically in plan view a portion of the
electrode pattern shown in FIG. 7A with infilling electrodes;
[0088] FIG. 8B is a cross-section through a part of FIG. 8A
illustrating capacitive paths between infilling electrodes and an X
electrode;
[0089] FIG. 9 shows hand-shadow caused by a proximate location of
the palm, thumb, wrist etc to a touch screen when the user touches
with a finger;
[0090] FIG. 10 shows a portion of the electrode pattern shown in
FIG. 7A with infilling electrodes;
[0091] FIG. 11 shows a portion of an electrode arrangement of sense
electrodes;
[0092] FIG. 12 shows a two-electrode layer capacitive touch screen
according to another embodiment of the present invention; and
[0093] FIG. 13 shows a two-electrode layer capacitive touch screen
according to an embodiment of the present invention with drive and
sense units connected via channels to a controller.
DETAILED DESCRIPTION
[0094] Described herein is a two-electrode layer construction for a
capacitive touch screen or 2DCT sensor.
[0095] FIGS. 1A and 1B are schematic drawings in side view and
perspective view of a two-electrode layer construction for a
capacitive touch screen or 2DCT sensor. The layers 101 can
generally be made of any conductive material and the layers can be
arranged to oppose each other on two sides of any isolating
substrate 102 such as glass, PET, FR4 etc. The thickness of the
substrate 103 is non critical. Thinner substrates lead to higher
capacitive coupling between the layers which must be mitigated in
the control chip. Thicker substrates decrease the layer to layer
coupling and are generally more favorable for this reason (because
the measured change in capacitance is a larger fraction of the
layer-to-layer capacitance so improving signal-to-noise ratio).
Typical substrate thickness' range from 10's to 100's of .mu.m.
Furthermore it will appreciated that a dielectric or isolating
layer may be disposed overlying the two-electrode layer
construction on Layer 2 to prevent an object adjacent the 2DCT
sensor making contact with the surface of the layers. This
isolating layer might be a glass or plastics layer.
[0096] FIG. 1C shows the side view of an alternative arrangement to
the two-electrode layer construction for the capacitive touch
screen or 2DCT sensor shown in FIG. 1A according another embodiment
of the present invention. In FIG. 1C the layers 101 are disposed on
the same surface of the isolating substrate 102, separated by an
isolation layer 108. An additional dielectric or isolating layer
104 is disposed on the electrodes layers to prevent an object
adjacent the 2DCT sensor making contact with the layers
surface.
[0097] FIG. 1D shows the side view of an alternative arrangement to
the two-electrode layer construction for the capacitive touch
screen or 2DCT sensor shown in FIG. 1A according another embodiment
of the present invention. In FIG. 1D the layers 101 are disposed on
the same surface of the isolating substrate 102, separated by an
isolation layer 108. However, the electrode layers 101 are disposed
on the surface of the isolating substrate that is farthest from the
touch surface 106. A display panel 100 is also shown (with
hatching) arranged below the substrate 102 that bears the electrode
layers 101. It will be understood that the display panel in
combination with the touch sensor make a touch screen. A display
panel could also be fitted to an arrangement as shown in FIG. 1C
above.
[0098] FIG. 1E shows the side view of an alternative arrangement to
the two-electrode layer construction for the capacitive touch
screen or 2DCT sensor shown in FIG. 1A according another embodiment
of the present invention. In FIG. 1E each of the layers 101 are
disposed on a surface of two different isolating substrates 102.
The two isolating substrates are brought together such that the two
electrode layers 101 are separated from the touch surface 106 and
are separated by one of the isolating substrates. A display panel
could also be fitted to an arrangement as shown in FIG. 1E.
[0099] FIG. 2A shows an electrode pattern of drive electrodes with
resistive elements according to an embodiment of the invention.
Layer 1 is the layer farthest from the touch surface. On Layer 1 is
an array of transmitting electrodes as shown in FIG. 2A. The
electrodes 201 are arranged as a series of solid bars running along
a first axis 202 or a first direction. A subset of the bars 203 is
connected to the control chip so that they can be driven as the
transmitter in the transmit-receive arrangement described above.
The driven bars 203 include the outer most bars and then an even
gap 204 between the remaining driven bars. The intermediate bars
205 are connected using resistive elements 206 in a chain 210, the
ends of the chain being connected to two adjacent driven 203 bars.
The driven bars 203 will be referred to as driven-X-bars and the
resistively connected bars 205 will be referred to as
resistive-X-bars.
[0100] FIGS. 2B, 2C and 2D show three different ways in which to
form the resistive elements 206. Namely, the resistive elements 206
can be formed using the intrinsic resistance of the electrode
material itself in a "meandered" pattern 207 at the edge of the
touch screen (see FIG. 2B), or can be screen printed resistive
material 208 at the edge (see FIG. 2C), or can be physical discrete
resistors 209 either at the edge of the pattern (see FIG. 2D) or on
a separate circuit. The latter option increases the interconnecting
wiring substantially but can be advantageous in some designs.
[0101] The resistive chain 210 is used to act as a classic
potential divider, such that the amplitude of the transmit signal
is progressively attenuated between one driven-X-bar and the
adjacent driven-X-bar. The set of driven and resistive bars so
described will be referred to as a "segment" 211. Using this chain,
if say driven-X-bar #1 303 is driven with a pulse train 305
relative to 0V 306 with a peak-to-peak voltage V 307, and
driven-X-bar #2 304 is driven to 0V, then resistive-X-bars in
between these two will be ratiometrically attenuated.
[0102] FIG. 3 shows a portion of the electrode pattern shown in
FIG. 2B in which example, if there were 2 resistive-X-bars 205 and
the resistor divider chain 210 is constructed of equal valued
elements R 308, then the resistive-X-bar #1 301 will have a
peak-to-peak voltage of 0.66666V and resistive-X-bar #2 302 will
have a peak-to-peak voltage of 0.33333V. This has the effect of
progressively weakening the electric field emitted from these
resistive electrodes and so forms an interpolating effect for the
capacitive changes within the segment between driven-X-bars. Hence,
the linearity of the capacitive changes when moving within a
segment is improved. Operating without resistive-X-bars is possible
but the linearity is poor because the electric field decays over
distance in a strongly non-linear fashion. By introducing evenly
spaced resistive emitters emitting at an amplitude that is a linear
division from the associated driven-X-bar, the field tends to "fill
in" and form a better approximation to a linear system.
[0103] In the forgoing description Layer 1 is a pattern of
transmit-electrodes, which may also be referred to as drive
electrodes. The electrode pattern of Layer 1 may also be referred
to as X-electrodes. The drive electrodes include the driven-X-bars
203 and the intermediate X bars 205 or resistive-X-bars.
Furthermore, the driven or drive electrodes are defined as being
made up of outer most driven-X-bars 203 and intermediate X bars or
resistive-X-bars 205 connected using resistive elements 206 in a
chain 210. The outer most X bars are referred to as driven-X-bars
203. However, it will be appreciated that all of the X-bars might
be driven X-bars without using resistive elements.
[0104] Typical resistive elements 206 have resistive values ranging
from a few K.OMEGA. up to high 10's of K.OMEGA.. Lower values
require more current (and hence energy) to drive from the control
chip but allow faster capacitive measurements as they have lower
time constants and hence can be charged and discharged faster.
Higher values require less current (and hence energy) to drive but
have higher time constants and hence must be charged and discharged
more slowly. Larger values also help to make any resistance build
up in interconnecting wiring contribute a smaller voltage drop to
the emitted field strength from the X bars, and hence make for a
more efficient system. For this reason, generally higher values are
preferred.
[0105] Another key reason to include the resistive-X-bars is that
it makes the segment scalable, i.e. by adding more resistive-X-bars
the segment can be made larger. This is at the expense of spatial
resolution; the segment uses the same two driven-X-bars and hence
the resolution of the measurement must be fundamentally the same,
but the segment is now spread across a larger region and so
spatially the resolution degrades. Making the segment scalable
means that fewer driven-X-bars are needed and hence fewer
connections to the control chip. By balancing the trade-off between
spatial resolution and connection cost/complexity an optimal
solution may be found for each design.
[0106] Overall, the bars in Layer 1 can be seen to be substantially
area filling; almost all of the surface area is flooded with
electrode. The gaps between the bars 205 can be made arbitrarily
small and indeed, the smaller the better from a visibility point of
view. Making the gaps larger than around 100 .mu.m is non-ideal as
this leads to increased visibility of the gap to the human eye and
a key goal is often to try and make an invisible touch screen. A
larger gap also tends to increase the possibility of a significant
fringing electric field near the gap to electrodes in Layer 2 which
will lead to worsening non-linearity. Gaps of a few 10's of
micrometers are common as they are almost invisible and can be
easily mass-produced, for example gaps of between 20 and 50
micrometers.
[0107] FIG. 4 shows a portion of the electrode pattern of drive
electrodes according to an embodiment of the invention. Referring
to FIG. 4, it is also desirable to use a gap with a small up/down
wave pattern 401 between driven 402 and resistive-X-bars 403 as
this helps to disguise the gap when viewed through Layer 2 with the
added effect of the parallax caused by the substrate thickness.
Various patterns can be used to help disguise the gap when viewed
in this way, for example a sine wave, triangle wave or square wave
could be used. The frequency and amplitude are chosen to help break
up the otherwise long linear gap when viewed through the complex
but regular pattern in Layer 2. The amplitude must be minimized to
avoid errors in the reported touch coordinate.
[0108] FIG. 5A shows a portion of the electrode pattern shown in
FIG. 2A.
[0109] FIG. 5B shows a typical finger tip.
[0110] The electrode bars (both types) are generally designed so
that they have a fundamental pitch of around 8 mm or less, as shown
in FIG. 5A preferably 5 mm. This is in recognition that, as shown
in FIG. 5B, a typical finger touch 501 creates a generally circular
region 502 (illustrated in FIG. 5B with hashing) of around 8 to 10
mm in diameter and so matching the electrode pitch to the touch
size optimizes the interpolating effect of the touch. Making the
pitch of the electrodes larger than 8 mm can start to lead to
distinct non-linearity in the response as the interpolation is well
below ideal. In essence, by making the electrode bars too wide, as
the touching finger moves perpendicular to the bars its influence
tends to "saturate" over one electrode before it starts to interact
with the next electrode to any significant degree. When the pitch
is optimized, the finger will cause a steadily reducing influence
on one bar while already starting to create a well balanced
increase on the neighboring bar, with the peak influence being
spatially quite distinct i.e. steady increase immediately followed
by steady decrease with no appreciable transition distance from
increase to decrease (or vice-versa).
[0111] FIG. 6 shows an electrode pattern of drive electrodes
according to an embodiment of the invention. Referring to FIG. 6
the driven-X-bars at the outer edges of Layer 1 601 are made to be
half the width of all other bars 602. The overall design is in
essence several identical concatenated segments 603, and the
driven-X-bars on the inside of the layer 604 are also half width
but are butted up to the neighboring segment with its half width
outer bar, so driven-X-bars internal to the pattern appear to be
full width. FIG. 6 shows the virtual division of the internal bars
604 with a dashed line; in practice of course the bars 604 are
one-piece. Having the pattern at its outer two edges with
half-width bars improves the overall linearity; if the pattern were
infinite then the linearity would be perfect in this regard, but of
course the pattern must end and hence there is a natural
non-linearity at the edges.
[0112] FIG. 7A shows an electrode pattern of sense electrodes
according to an embodiment of the invention. Layer 2 is the layer
nearest to the touch surface. Referring to FIG. 7A in its simplest
form, the electrodes on Layer 2 are a uniformly spaced series of
narrow lines running along a second axis at nominally 90 degrees to
the first axis used in Layer 1 herein referred to as a second
direction. That is to say that the Layer 1 or drive electrodes
cross the Layer 2 or sense electrodes. The electrodes on Layer 2
are referred to as sense electrodes, Y-electrodes, Y lines or
receive electrodes. They are arranged to lie directly and
completely over the area 703 occupied by the X bars underneath. The
spacing between the Y lines has a similar influence on the
linearity as does the spacing of the X bars. This means that the Y
lines need to be spaced with a pitch of 8 mm or less 704,
preferably 5 mm for best intrinsic linearity. In a similar way to
the Layer 1 with its half-width outer X bars, the gap from the edge
of the Layer 2 pattern to the first line is half of this pitch 705
to improve the linearity. The width of the Y lines 706 is
important. They need to be narrow enough so that they are not
easily visible to the human eye, but wide enough that they have a
resistance (at their "far-end") that is low enough to be compatible
with capacitive measurements. Narrower is also better as far as
noise immunity is concerned because the surface area of the Y line
has a direct influence on how much electrical noise can be coupled
into the Y lines by a finger touch. Having narrower Y lines also
means that the capacitive coupling between the X and Y layer is
minimized, which, as previously mentioned, helps to maximize
signal-to-noise ratio.
[0113] FIG. 7B shows a touch sensor 10 according to an embodiment
of the invention. The sensor 10 shown in the figure combines the
electrode patterns from FIG. 2A and FIG. 7A. The sensor 10
comprises a substrate 102 bearing an electrode pattern 30 defining
a sensitive area or sensing region of the sensor and a controller
20. The controller 20 is coupled to electrodes within the electrode
pattern by a series of electrical connections which will be
described below. The electrode pattern 30 is made up of Layer 1
electrodes and Layer 2 electrodes on opposing sides of the
substrate 102 as shown in FIG. 1B.
[0114] Referring to FIG. 7B, the controller 20 provides the
functionality of a drive unit 12 for supplying drive signals to
portions of the electrode pattern 30, a sense unit 14 for sensing
signals from other portions of the electrode pattern 30, and a
processing unit 16 for calculating a position based on the
different sense signals seen for drive signals applied to different
portions of the electrode pattern. The controller 20 thus controls
the operation of the drive and sense units, and the processing of
responses from the sense unit 14 in the processing unit 16, in
order to determine the position of an object, e.g. a finger or
stylus, adjacent the sensor 10. The drive unit 12, sense unit 14
and processing unit 16 are shown schematically in FIG. 7B as
separate elements within the controller. However, in general the
functionality of all these elements will be provided by a single
integrated circuit chip, for example a suitably programmed general
purpose microprocessor, or field programmable gate array, or an
application specific integrated circuit, especially in a
microcontroller format.
[0115] In the figure there is provided a number of drive electrodes
60 represented by longitudinal bars extending in the x-direction as
described above and shown in FIG. 2A. On the opposing surface of
the substrate 102, there is provided a number of sense electrodes
62 forming electrode Layer 2 as shown in FIG. 7A and described
above that cross the drive electrodes 60 of Layer 1 in the
y-direction.
[0116] The sense electrodes are then connected to the sense unit 14
via connections or tracks 76 and the drive electrodes are connected
to the drive unit 12 via connections or tracks 72. The connections
to the drive and sense electrodes are shown schematically in FIG.
7B. However, it will be appreciated that other techniques for
routing the connections or tracks might be used. All of the tracks
might be routed to a single connector block at the periphery of the
substrate 102 for connection to the controller 20.
[0117] The operation of the sensor 10 shown in FIG. 7B is described
below. As can be seen there are conflicting requirements for the Y
lines in terms of their width. The strongest requirement tends to
be the minimization of the resistance of the Y line to ensure
successful capacitive measurement within an acceptable overall
measurement time. This leads to wider electrodes, typically in the
region of 100 .mu.m to 1000 .mu.m. Where the visibility of the
electrodes is either not an issue or where the electrodes can be
made practically invisible (such as index matched ITO on PET for
example), then the compromises are all quite easily accommodated
and the width increase is a simple choice. But where the visibility
is an issue and the method used to fabricate the electrodes cannot
be made sufficiently invisible (such as non index matched ITO on
glass) then some alternative arrangement must be found. In this
case, a method called in-filling can be used as now described and
illustrated.
[0118] FIG. 8A shows a portion of the electrode pattern shown in
FIG. 7A with infilling electrodes. This method fills all "unused"
801 space with isolated squares of conductor 802 (ITO for example),
separated with gaps 803 to its neighbors that are small enough to
be practically invisible and small enough to cause significant
square-to-square capacitance. Another key factor in designing the
isolated elements or islands is to make them the same size 804 in
each axis as the width of the Y lines 805. In this way, the
uniformity of the overall pattern is optimal, and the only
irregularity is in the length of the Y lines. This pattern is
substantially invisible to the human eye. The gaps between
neighboring squares, and the gaps between squares and neighboring Y
lines can be made arbitrarily small, typically in the region of
10's of .mu.m as they are almost invisible and can be easily
mass-produced. The in-filling is generated during manufacture at
the same time, and using the same process steps, as the sense
electrodes, so they are made of the same material and have the same
thickness and electrical properties as the sense electrodes. This
is convenient, but not essential. The in-filling could be carried
out separately in principle.
[0119] The isolated squares 802 serve to obscure the overall
pattern but they also act as a capacitive interpolator (somewhat
analogous to the resistive interpolator used in Layer 1). The
capacitive interpolator so formed has the effect of only minimally
impacting the fringing fields between the Y line and the underlying
X bars. This is important because the field must spread out down to
the X bars sufficiently from the edges of the Y lines to allow a
substantial touch influence over at least half the pitch of the Y
lines. This holds true so long as the capacitance from square to
square is substantially higher (at least .times.2) the capacitance
of a square down to the X bars. The reason for this is that under
these conditions the electric field tends to propagate from square
to square more easily than it is shunted down to the X layer. As a
result, the field distributions of a design with no in-fill
compared to one with in-fill are similar enough that the linearity
is preserved. If the square-to-square gaps are increased, the
linearity degrades because the field tends to pass via the first
couple of squares away from a Y line down to the X bars and so does
not propagate far from the Y line.
[0120] FIG. 8B illustrates these capacitive paths between example
infilling electrodes and between an example infilling electrode and
an example X electrode. Capacitance from square 808 to square 808
is shown with nominal capacitors 806 and capacitance from one of
the squares 808 down to an adjacent X bar 809 is shown with nominal
capacitor 807.
[0121] It should be noted that the in-fill is not actually needed
in this design, but it can be used to minimize pattern visibility
without compromising the linearity of the output.
[0122] In operation the transmitting or drive electrodes are
sequenced such that only one driven-X-bar 203 is ever active at one
time, all others being driven to a zero potential. The field
emitted therefore only radiates from one segment at a time. This
radiated field couples locally into all of the Y lines 701 above
the segment in question. The control chip then takes a capacitive
measurement for each of the "intersections" or "crossings" formed
between the X and the Y electrodes in this segment. Each XY
intersection is also known as a node. In sequence, each
driven-X-bar is activated, holding all others at zero potential. In
this way, each segment is sequentially scanned. Once all segments
have been completed, a total of N x M nodes will have been measured
where N is the number of driven-X-bars and M is the number of Y
lines. It should be stressed that the node measurements are all
independent of each other making it possible to detect several
touch locations simultaneously. Another important point in the way
the XY array is scanned is that because only one segment is active
at any one time, the others being driven to zero potential, only
touches in the active segment can influence the measured node
capacitances in that segment (at least to a first approximation).
This means that an effect known as "hand-shadow" is strongly
minimized. Hand-shadow is an effect caused by the proximate
location of the palm, thumb, wrist etc to the touch screen when the
user touches with a finger.
[0123] FIG. 9 shows hand-shadow caused by a proximate location of
the palm, thumb, wrist etc to a touch screen when the user touches
with a finger. The nature of capacitive measurement means that the
electric fields tend to radiate or project from the surface of the
device and so can be influenced even by objects that are not in
direct contact with the surface. This influence would normally
serve to distort the reported touch location, as the combined
capacitive readings of the finger together with readings caused by
the "hand shadow" would slightly corrupt the computed coordinates
reported by the control chip. By activating only one segment at a
time this normally problematic effect is drastically reduced.
[0124] Having scanned the entire touch screen, generating N.times.M
node measurements, it is a simple task to compute the touch
location, in both of the axes, of one or more objects, as described
in U.S. patent application 60/949,376 published as WO 2009/007704
on 15 Jan. 2009, using a combination of logical processing to
discover the node at the approximate centre of each touch, and
standard mathematical centroid computations of the relative signal
strengths around each touch detected. The touch location along the
first axis is resolved using the touch's centre node signal and the
immediately adjacent node signal to each side that lie along the
first axis. Likewise, the location in the second axis is resolved
using the centre node and the immediately adjacent node signals
that lie along the second axis.
[0125] A key design advantage in having the entire Layer 1 almost
entirely covered or flooded with emitting X electrodes is that
because these electrodes are virtually immune to changes in
parasitic capacitive loading (they are relatively low impendence
drivers, even the resistively coupled X bars still only have DC
resistances of a few 10's of K.OMEGA. and so can charge and
discharge any moderate parasitics very quickly) any change in the
distance between the rear (non-touch side) of Layer 1 and a nearby
ground load will make no difference to the measured capacitances of
the nodes. The touch screen is thus touch-sensitive only on one
side, Layer 2. This has major benefits when slightly flexible front
panels are used that can bend relative to an LCD placed below the
touch screen. The separation between Layer 1 and Layer 2 is fixed
by the substrate material and hence the capacitance between these
two is fixed even if the substrate is bent during touch causing the
rear of Layer 1 to experience a change in its ambient
conditions.
[0126] A further advantage to using the flooded X design is that it
provides an inherent amount of noise attenuation for radiated
emissions that are present behind Layer 1. This is common with LCD
modules that tend to have large amplitude drive signals present on
their outer layers. These drive waveforms will normally couple to
the Y lines and disturb the momentary reported capacitance of the
associated nodes. However, because the Y lines are effectively
shielded by the flooded X layer, the only remaining mechanism for
the noise to couple to the Y lines is capacitively via the X layer
itself. The X bars, as already described, are reasonably low
resistance and so can only be disturbed by the interfering noise
waveform in proportion to the ratio of the impedance of the noise
coupling vs. the impedance of the X bar. Hence, the amount of noise
coupled onward to the Y lines is attenuated by this ratio. The
coupling of the noise waveform to X bars is purely capacitive and
so decreasing this coupling capacitance helps to attenuate the
interference even more. This can be achieved by arranging an air
gap between the LCD and the back of Layer 1, or by using a
transparent dielectric spacer layer instead of the air gap that
will yield a higher capacitance of coupling but has the advantage
of being mechanically robust. In a traditional capacitive touch
screen an entire extra "shielding" layer below Layer 1 must often
be used to mitigate this LCD noise. This layer is often driven to
zero potential or is actively driven with a facsimile or copy of
the capacitive acquisition waveform, which serves to isolate the
noise from the capacitive nodes. This has the disadvantage of
adding cost and complexity, worsens optical properties and also
tends to attenuate the size of the change in capacitance during
touch (leading to lower resolution and worse signal-to-noise
ratio). The flooded X design described herein will often produce
sufficient inherent attenuation of the coupled noise that no extra
layer is required, offering a substantial commercial advantage.
[0127] Another advantage found with this design is that the Y lines
can be made narrow in comparison to the size of the touching
object. In particular, the Y lines can have a width of one quarter
or less than the size of the touching object, or equivalently the
pitch of the X electrodes. For example, a Y line width of 0.5 mm is
16 times narrower than the width of a typical finger touch. The
implication of this is related to the surface area available for
interaction with the touching finger. A narrow Y line has a very
small surface area to couple capacitively to the touch object; in
the example cited, the coupled area is around 4 mm.sup.2 compared
with the total "circular" touch area of around 50 mm.sup.2. With
such a small area coupled to the touch, the amount of noise
injected into the Y line from the finger is minimized because the
coupling capacitance is small. This has an attenuating effect on
any differential noise between the touch object and device using
the touch screen. Furthermore, by making narrow Y lines the
resistance is reduced. Reducing the resistance of the Y lines
reduces the acquisition times and decreases the power
dissipation.
[0128] In summary, the advantages of the described touch screen
are: [0129] 1. Only two layers are required for construction
leading to; (i) improved optical transmission (ii) thinner overall
construction (iii) lower cost. [0130] 2. Area filling design for
electrodes on Layer 1 leading to; (i) almost invisible electrode
pattern when using ITO (ii) isolation of the Y lines on Layer 2
from capacitive effects below Layer 1 (iii) partial attenuation of
noise coupled from an underlying LCD module or other noise source.
[0131] 3. Narrow Y lines on Layer 2 with optional area filling
isolated squares leading to; (i) almost invisible electrode pattern
when using ITO (ii) reduced electrode area reduces susceptibility
to coupling noise from touch.
[0132] In some designs it may be desirable to minimize the number
of Y lines used across Axis 1--labeled the first axis in FIG. 7A.
This generally leads to a lower cost control chip and simplifies
interconnection of the electrodes. With the described Y line
design, the fundamental pitch between lines needs to be 8 mm or
below to achieve good linearity. Spacing the lines further apart
rapidly compromises linearity in Axis 1. To enable the Y lines to
have a greater "reach" there are several adaptations that can be
made to the Layer 2 design.
[0133] FIG. 10 shows a portion of the electrode pattern shown in
FIG. 7A with infilling electrodes according to a first option. The
first option shown in FIG. 10 is to use the capacitive interpolator
technique previously described with the square-to-square gap 1001
reduced to allow the electric field to propagate further away from
the Y line and so allow a larger pitch 1002 between Y lines 1003.
This technique may require that the ratio of capacitance between
squares vs. square to X bars must be carefully tuned to achieve the
best linearity.
[0134] FIG. 11 shows a portion of an electrode arrangement of sense
electrodes according to a second option and more flexible option
which modifies the Y line 1101 design to add a series of
cross-members 1102 running along the first axis 1103 and equally
disposed 1104 so as to be centered about the Y line. The cross
members span approximately 1/2 to 3/4 of the gap to the next Y line
1105 in both directions. The cross members on each successive Y
line are arranged so that they overlap the cross members of those
on the neighboring Y lines 1106 with the gap 1107 between the
overlapping sections chosen to be a few 10's of .mu.m to minimize
visibility and prevent any substantial fringing fields from forming
along the inside of the overlapped region. The cross members are
spaced by a distance 1108 along the Y line on a pitch of 8 mm or
less, and ideally they are spaced to lie with a uniform
relationship to the gaps in the underlying X bars. This ensures
that the field patterns are uniform and symmetrical in all regions
of the touch screen, leading to good linearity. The cross members
effectively act to spread the electric field further beyond the
primary Y line and the overlapped region helps to gradate the field
from one Y region to the next in a linear fashion.
[0135] Embodiments of the invention shown in FIGS. 2A, 7A, 7B and
11 may further comprise connections to both extents of the drive
and sense electrodes or transmitting electrodes and Y lines
respectively. That is to say that a connection is made at both ends
of each of the drive and sense electrodes. This may increase the
linearity of the electric filed along the drive electrodes and
improve the shielding of the flooded electrode design.
[0136] Embodiments of the invention may also be applied to
non-display applications, for example touch pads on a laptop or
control panels on domestic appliances.
[0137] FIG. 12 shows a sensor 80 comprising an electrode pattern
according to an embodiment of the invention. For simplicity the
electrode pattern shown in the figure does not include any
circuitry. However, it will be appreciated that drive and sense
circuitry may also be used as described above for the FIG. 7B
embodiment. The figure shows an electrode pattern on opposing sides
of a substrate 82, viewed from above to show the relative position
of the electrode patterns.
[0138] The electrode pattern comprises two annular electrodes of
the type described above referred to as Layer 1 or transmit
electrodes. The transmit electrodes may also be referred to as
drive electrodes. The drive electrodes 84 shown in the figure are
effectively the transmit electrodes shown in FIG. 2A and have been
wrapped around arcuately to form a complete, or near complete, ring
or annulus, as might be used by a scroll wheel sensor for example.
Connected to each of the drive electrodes 84 is a connection or
track 90 to provide a drive signal from an appropriate drive unit
(not shown). The drive unit described above may be used. The
electrode pattern further comprises a number of sense electrodes
referred to above as Layer 2 electrodes 86 which extend radially
from a central point. The Layer 2 electrodes may also be referred
to as sense electrodes or receive electrodes. The sense electrodes
86 are in the form shown in FIG. 11 and described above. The sense
electrodes are connected to a sense unit (not shown) via
connections or tracks (not shown). The operation of the sensor 80
is similar to that described above. However, the readout from a
processing unit (not shown) connected to the drive and sense units
will be different. The output of the processing unit will provide a
polar co-ordinate of an object adjacent the sensor 80. The sensor
80 shown in FIG. 12 may be used in an application where two
circular controls are typically used in combination, for example
the bass and treble controls or the left/right and front/rear fade
controls on a hi-fi amplifier. It will be appreciated that further
annular shaped drive electrodes may be implemented in the sensor 80
shown in the figure. This embodiment may therefore be summarized as
following a polar coordinate grid, with the two electrode types
extending radially and arcuately, in contrast to the other
embodiments which follow a Cartesian coordinate grid, with the two
electrode types extending along the x- and y-axes.
[0139] In a modification of the FIG. 12 design, the arcuate path
may extend over a smaller angle for example a quarter or half
circle instead of a full circle, or another angular range.
[0140] FIG. 13 is a view of a front side of a position sensor 10
according to an embodiment of the invention. The same reference
numerals used in FIG. 7B are used in reference to the sensor 10
shown in FIG. 13 where appropriate. The position sensor shown in
FIG. 13 is similar to the sensor shown in FIG. 7B in layout and
operation. However, the position sensor shown in the figure has an
alternative arrangement of electrodes. The drive and sense
electrodes shown in the figure are made up of thin wires or a mesh
of wire instead of the continuous layer of electrode material shown
in FIG. 7B. The drive electrodes 60 are made up of a rectangular
perimeter to define the shape of the drive electrode with a series
of diagonal lines going across the rectangular perimeter. The
diagonal lines are typically arranged at an angle, preferably
approximately 45.+-.15 degrees, to an axis in extending in the
x-direction. The diagonal lines and the rectangular perimeter of
each drive electrode are electrically connected and connected to
the drive unit 12 via the drive channels 72. The wires or mesh are
manufactured from high electrical conductivity material such as
metal wires, where the metal is preferably copper, but could also
be gold, silver or another high electrical conductivity metal. The
sense electrodes are manufactured in a similar way using thin metal
traces that follows the perimeter of the sense electrode pattern
shown in FIG. 7B. The sense electrodes 62 are relatively narrow
compared to the drive electrodes 60, so there is no need to use
in-filling with diagonal lines. However, some extra wires are added
within the sense electrode mesh structure as shown in FIG. 13 by
lines 64 which bridge between peripheral wires in each electrode.
These bridge wires add redundancy in the pattern in the sense that
if there is a defect in a peripheral wire at one location, the
current has an alternative path along the electrode. By defect we
mean a break, local narrowing or other feature that causes a severe
reduction in the local conductivity along a wire. Such defects can
occur, for example, as a result of errors in the electrode
patterning process. For example, if there is a defect in the
optical mask used to pattern the wires or if there is debris on the
surface of the wires during processing then defects can arise.
[0141] It will be understood that the "mesh" or "filligrane"
approach to forming each electrode out of a plurality of
interconnected fine lines of highly conducting wire or traces may
be used for either Layer 1 (flooded X drive), Layer 2 (sense) or
both. The FIG. 13 embodiment uses meshes for both layers. However,
a particularly preferred combination for display applications or
other applications where invisibility is important is that Layer 1
is made with non-mesh, i.e. "solid" electrodes with the small,
invisible gaps, for example from ITO, and Layer 2 is made with mesh
electrodes, for example out of copper, having line widths
sufficiently small to be invisible also.
[0142] It will also be understood that the mesh approach of the
embodiment of FIG. 13 can be used in a design of the kind
illustrated in FIG. 11 and FIG. 12 in which the sense electrodes
have overlapping branches.
* * * * *