U.S. patent application number 12/659772 was filed with the patent office on 2010-10-07 for touch sensitive display device.
This patent application is currently assigned to Integritouch Development AB. Invention is credited to Anders Swedin.
Application Number | 20100253641 12/659772 |
Document ID | / |
Family ID | 29398729 |
Filed Date | 2010-10-07 |
United States Patent
Application |
20100253641 |
Kind Code |
A1 |
Swedin; Anders |
October 7, 2010 |
Touch sensitive display device
Abstract
A touch sensor is disclosed comprising a display device having a
substrate on which substrate at least one display electrode is
disposed for the display of a shape on the display device. An
interface is coupled to the at least one display electrode for
receiving display data to the display device. Moreover is a
measuring circuit coupled to the at least one display electrode.
Switching means are provided for connecting the interface to the at
least one display electrode when the switching means is in a first
state of operation and connecting the measuring circuit to the at
least one display electrode when the switching means is in a second
state of operation.
Inventors: |
Swedin; Anders;
(Helsingborg, SE) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Assignee: |
Integritouch Development AB
|
Family ID: |
29398729 |
Appl. No.: |
12/659772 |
Filed: |
March 22, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10575622 |
Nov 6, 2006 |
7705834 |
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PCT/SE2004/001447 |
Oct 12, 2004 |
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12659772 |
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Current U.S.
Class: |
345/173 |
Current CPC
Class: |
G09G 3/04 20130101; G02F
1/1343 20130101; G06F 3/044 20130101; G02F 1/13338 20130101; G06F
3/0412 20130101 |
Class at
Publication: |
345/173 |
International
Class: |
G06F 3/041 20060101
G06F003/041 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 13, 2003 |
SE |
0302711-7 |
Claims
1. A touch sensor (20), comprising: a display device 10 having a
substrate (14, 15) on which substrate at least one display
electrode (11) is disposed for the display of a shape on the
display device (10); an interface (21) coupled to the at least one
display electrode (11) for receiving display data to the display
device (10); a measuring circuit (25, 27) coupled to the at least
one display electrode (11); switching means (22) for connecting the
interface (21) to the at least one display electrode (11) when the
switching means is in a first state of operation and connecting the
measuring circuit (25, 27) to the at least one display electrode
when the switching means is in a second state of operation.
2. A touch sensor (20) according to claim 1, wherein the measuring
circuit (25, 27) is a capacitance measuring circuit.
3. A touch sensor (20) according to claim 1, wherein the measuring
circuit (25, 27) is a resistance measuring circuit.
4. A touch sensor (20) according to any preceding claim, wherein
the measuring circuit (25, 27) comprises a signal generator (25)
coupled to the at least one display electrode (11) for providing a
predetermined test signal to the display electrode, and a signal
evaluating circuit (27) coupled to the at least one display
electrode for receiving the test signal from the signal
generator.
5. A touch sensor (20) according to claim 4, wherein the signal
evaluation circuitry (27) is adapted to detect a deviation in the
test signal when the switching means (22) is in the second state of
operation.
6. A touch sensor (20) according to any of claims 4 or 5, wherein
the signal generator (25) is adapted to apply the test signal to
the segments (11) on a front substrate (14) of the display device
(10).
7. A touch sensor (20) according to any of claims 4 or 4, wherein
the signal generator (25) is adapted to apply the test signal to
the segments (11) on a back substrate (14) of the display device
(10).
8. A touch sensor (20) according to any preceding claim, wherein
the segments (11) on the substrate (14, 15) which are not connected
to the signal generator (25) are left in a high-ohmic state.
9. A method for detecting a touch on a display device (10), said
display device having a substrate (14, 15) on which substrate at
least one display electrode (11) is disposed for the display of a
shape on the display device (10), wherein said display electrode
(11) is coupled to an interface (21) for receiving display data to
the display device, comprising the steps of: disconnecting the at
least one display electrode (11) from the interface (21);
connecting said display electrode (11) to a measuring circuit (25,
27); and detecting a change in an electrical property of the
display electrode (11) due to an electrical coupling towards an
object (17) touching the display device (10) in the vicinity of the
display electrode.
10. A method according to claim 7 comprising the steps of: applying
a predetermined test signal to the display electrode (11) and
detecting a deviation in the test signal due to an electrical
coupling towards an object (17) touching the display device 10 in
the vicinity of the display electrode.
11. A method according to claim 9 or 10, wherein the electrical
coupling is a capacitive coupling.
12. A method according to claim 9 or 10, wherein the electrical
coupling is a galvanic coupling.
Description
BACKGROUND
[0001] Electrical equipment from various fields of application,
e.g. mobile telephones, personal digital assistants (PDA), and
industrial control equipment often use a display device of some
sort for providing the operator of the device with information. In
simpler applications the display device is a one-way communication
link, i.e. the display is used for providing information to the
operator but not to receive information the other way back. In
order to achieve interaction with the operator, push buttons or
keyboards are normally used. If the electrical equipment is small
sized, for example as with a PDA, normally no room is left on the
device for a keyboard, wherein the manufacturer of the PDA must
provide other means for enabling input of data into the device.
[0002] As is well known in the art the input means may be in form
of a touch sensitive display making it possible to enter data
without the need for a separate keyboard. Many different techniques
for providing touch sensitive devices have been presented and the
most common solution today is to use a separate transparent touch
sensitive layer which is placed on top of the display. The touch
sensitive layer is normally in form of two flexible superimposed
plastic sheets that are separated by a small distance by means of
insulating spacers. On the surfaces of the sheets facing towards
each other, a matrix-like pattern of electrical conductors are
arranged which pattern establishes an electric contact between the
sheets at the location where the touch sensitive layer is
depressed. A control unit scanning the matrix-like pattern on the
plastic sheets may then detect the electric contact between the
sheets and determine the coordinates for the depression on the
display.
[0003] Even though the separate touch sensitive layer makes it
possible to enter data into the device without the need for a
keyboard, it is not an efficient way of realising a touch sensitive
display since the transparency of the touch sensitive layer is not
absolute hence making it difficult to view the information
presented on the display under certain circumstances. The
unsatisfactory transparency of the touch sensitive layer is even
more noticeable when the display device is provided with back
lighter or front lighter technology for making it possible to view
the information on the display under poor lit conditions.
[0004] Another approach for providing a touch sensitive display is
to provide a display with a sensor arranged under the display
rather than on top of the display. The sensor then has to detect a
touch on the display not by means detecting an electric contact
between conductors as with the solution disclosed above, but by
using capacitive or reflective properties of the display. In the
former case, a capacitive coupling through the display to the
finger touching the display makes it possible to detect a touch on
the display as well as determine the position of the touch. In the
latter case light or sound utilizing changes in the reflective
properties of the display at the point of contact may be used for
detecting a touch on the surface of the display.
[0005] Attempts have been made to provide touch sensitivity for
displays without the use of separate sensors arranged on top or
below the display surface. An approach is to use the display
electrodes forming the pixels or the segments of the characters on
the display for sensing the touch.
[0006] U.S. Pat. No. 5,043,710 discloses a touch sensor comprising
a liquid crystal display (LCD), wherein a touch on the display is
sensed by detecting changes in the dielectric properties of the
display. A mechanical force applied to the LCD perpendicular to a
flexible glass substrate over one of the display electrodes gives
rise to a temporary disorganisation of the molecules in the liquid
crystal thereby changing the dielectric constant of the liquid
crystal under the display electrode. Each display electrode of the
LCD is connected to an integrator, wherein a change of the
dielectric constant of the liquid crystal when the segments of the
LCD are in an excited state gives rise to an electric pulse
indicating a touch on the LCD. However, the solution according to
U.S. Pat. No. 5,043,710 becomes complex due to the large amount of
integrators needed for sensing a touch. Moreover, for sensing a
touch the front glass plate needs to be flexible making the display
less durable. In addition to this, the working life of the display
is also decreased due to the repeated compressions of the liquid
crystal in the display.
[0007] U.S. Pat. No. 4,224,615 discloses a LCD with a flexible
front plate, which LCD may be used as a device for receiving data
from a human operator. An operator of a device comprising the touch
sensitive display touches the flexible front plate of the display,
wherein the front plate deflects towards the back substrate thereby
increasing the capacitance between the display electrodes residing
in the area being depressed. The capacitance measured between the
front and back display segment is compared with the capacitance of
a reference cell, wherein it is possible to detect a touch even if
the affected display segments are actuated, i.e. presenting a shape
on the display. As with U.S. Pat. No. 5,043,710 the invention
according to U.S. Pat. No. 4,224,615 uses the change in dielectric
constant of the liquid crystal being compressed for sensing a
touch. The same problems with robustness and life expectancy as
with the invention according to U.S. Pat. No. 5,043,710 exist in
the solution according to U.S. Pat. No. 4,224,615.
[0008] US 2001/0020578 discloses a LCD with touch sensitivity,
wherein the sensor arrangement is placed below a surface of the
display. The sensors are preferably placed below the display in the
regions of the display where no display segments are arranged.
Alternatively, the display segments of the display may be used as
sensors provided that the front and back segment are
short-circuited. When the display electrodes act as touch sensors,
no information may be presented on the screen due to the
short-circuiting of the display electrodes. A microprocessor is
therefore coupled to the display segments for alternating between
presentation of information on the display and touch
sensitivity.
[0009] U.S. Pat. No. 4,910,504 discloses a touch controlled display
device, wherein a touch on the display is sensed by measuring the
capacitance between different display electrodes on the front
substrate. The font substrate may then be rigid protecting the
display from deformation. The detector measuring the capacitance
between the electrodes is coupled to the feeding pins of the
display. A common counter-electrode is arranged on the back
substrate in a manner known per se. As will be disclosed below, the
counter-electrode will act as a short-circuit between the
electrodes on the front substrate thereby deteriorating the
accuracy of the touch sensitive display in regard of where on the
screen the touch is made. Moreover, numerous stray-capacitances in
the needed drive circuitry for the display will interfere with the
capacitance measuring circuitry making it hard to determine where
and if a touch is made.
[0010] DE 19802479 discloses a touch-sensitive display for use in
e.g. elevators. The front surface of display element is provided
with an electrically conducting layer which is so thin that the
display element is visible through the conducting layer. An
evaluation circuit is connected to the conducting layer in order to
detect a touch on the display. However, by arranging a conductive
layer in front of the display element, the visibility of the
display element is deteriorated. Moreover, the conductive layer
will be exposed to wear from users of the display, which implies
that the endurance of the display will be insufficient for many
applications.
[0011] For manufacturers of display driver circuits it is of most
importance that the circuitry used for detecting a touch on the
screen is not affecting the behaviour or the life-expectancy of the
driver circuitry. Hence a touch sensitive display which behaves
like a "normal" display from a drivers point of view is hence
wished for.
SUMMARY OF THE INVENTION
[0012] An object of the present invention is to overcome the above
described problems of the known technologies in regards to
providing a touch sensor which is durable and provides a reliable
detection of touch on the display. The present invention is based
on the understanding that a display is associated with specific
physical characterics which influence the reliability of the
detection of a touch on the display.
[0013] Particular advantages of the present invention are
reliability of the detection of a touch on the screen, improved
robustness of the touch sensor, and the improved matching towards
available display driver circuits.
[0014] A particular feature of the present invention relates to the
provision of a touch sensor with a basic configuration making it
possible to reliably detect a touch on the display without
deforming the display or requiring specially adapted display driver
circuitry. The designer of a system comprising a touch sensor
according to the present inventon may then freely choose driver
circuits thereby lowering the overall cost of the system.
[0015] The above objects, advantages and features together with
numerous other objects, advantages and features, which will become
evident from the detailed description below, are obtained according
to a first aspect of the present invention by a touch sensor
comprising:
[0016] a display device having a substrate on which substrate at
least one display electrode is disposed for the display of a shape
on the display device;
[0017] an interface coupled to the at least one display electrode
for receiving display data to the display device;
[0018] a measuring circuit coupled to the at least one display
electrode;
[0019] switching means for connecting the interface to the at least
one display electrode when the switching means is in a first state
of operation and connecting the measuring circuit to the at least
one display electrode when the switching means is in a second state
of operation.
[0020] A touch sensor according to the present invention will hence
be able to reliably detect a touch on the display by means of a
capacitance measuring circuit even though large stray capacitances
are present in the display.
[0021] A touch sensor according to the present invention may
comprise a measuring circuit which is a capacitance measuring
circuit.
[0022] A touch sensor according to the present invention may
comprise a measuring circuit which is a resistance measuring
circuit.
[0023] According to the present invention the measuring circuit may
comprise a signal generator coupled to the at least one of the
display electrodes for providing a predetermined test signal to the
display electrode, and a signal evaluating circuit coupled to the
at least one display electrode for receiving the test signal from
the signal generator.
[0024] According to the present invention the signal evaluation
circuitry may be adapted to detect a deviation in the test signal
when the switching means is in the second state of operation.
[0025] According to the present invention the signal generator may
be adapted to apply the test signal to the segments on a back
substrate or to the segments on a front substrate of the display
device.
[0026] According to the present invention the segments on the
substrate which is not connected to the signal generator may be
left in a high-ohmic state.
[0027] The present invention also relates to a method for detecting
a touch on a display having a substrate, on which substrate at
least one display electrode is disposed for the display of a shape
on the display device, wherein said display electrode is coupled to
an interface for receiving display data to the display device, the
method comprising the steps of:
[0028] disconnecting the at least one display electrode from the
interface;
[0029] connecting said display electrode to a measuring circuit;
and
[0030] detecting a change in an electrical property of the display
electrode due to an electrical coupling towards an object touching
the display device in the vicinity of the display electrode.
[0031] The method according to the present invention may comprise
the steps of applying a predetermined test signal to the display
electrode and detecting a deviation in the test signal due to an
electrical coupling towards an object touching the display device
in the vicinity of the display electrode.
[0032] The method according to the present invention may detect a
capacitive coupling towards an object touching the display device
in the vicinity of the display electrode.
[0033] The method according to the present invention may detect a
galvanic coupling towards an object touching the display device in
the vicinity of the display electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Further objects, features and advantages of the present
invention will become apparent upon consideration of the following
detailed description in conjunction with the appended drawings.
[0035] FIG. 1a illustrates the structure of a display known per
se;
[0036] FIG. 1b illustrates the disposition of some of the stray
capacitances associated with a display known per se;
[0037] FIG. 2 is a schematic diagram of a touch sensor according to
a preferred embodiment of the present invention;
[0038] FIG. 3 is a more detailed illustration of the function of
the touch sensor according to a first embodiment of the present
invention; and
[0039] FIG. 4 is a more detailed illustration of the function of
the touch sensor according to a second embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0040] The most common display used today is the liquid crystal
display (LCD) whose design and operation is well-known to the
skilled person. Variants of the LCD display, e.g. Thin Film
Transistor Displays (TFT) as well as other display techniques, such
as Plasma Display Panels (POP), Vacuum Fluorescent Displays (VFD),
Ferroelectric Liquid Crystal displays (FLC), Surface-stabilized
cholesteric texture-type (SECT) displays, Organic Light-Emitting
Diode (OLED) displays, and Liquid Crystal on Silicon (LCOS)
displays are commonly used depending on the specific field of
application. For the sake of simplicity the following text will
disclose a touch sensitive display in form of a LCD, wherein a
change in capacitance in the display is detected. The present
invention is, however, not limited to such a display, but may be
implemented on a display of any kind comprising at least one
substrate on which at least one display electrode is arranged which
may be capacitively, galvanically or inductively coupled to an
external object.
[0041] FIG. 1a illustrates a top and side view of a portion of a
display 10 known per se. The leftmost figure in FIG. 1a illustrates
the well known seven segment 11 arrangement, wherein different
digits may be presented depending on which segments 11 that are
active. Each segment 11 is reachable by means of thin wires 12
extending from the segment 11 towards electrical terminals 13
normally provided on the edge of the display 10. The segments 11
are formed on the inside of a front substrate 14 and a back
substrate 15 of the display 10. In this context it is emphasized
that the substrates used in the display may be made of glass or
plastic, on which a suitable electrical material, such as Indium
Tin Oxide (ITO), is deposited as to form the segments 11, or one or
more substrates in the display may be made of an electric material,
such as aluminium and shaped as to provide the segments 11. In e.g.
OLED displays a rib structure is pre-formed on patterned ITO anode
lines on a glass substrate. Organic materials and cathode metal are
deposited on the substrate, wherein the rib structure automatically
produces an OLED display with electrical isolation for metallic
cathode lines formed on top of the deposited organic materials.
Depending on the display technique used the display may comprise
further elements besides the front substrate 14 and the back
substrate 15, which elements are not shown for sake of clarity. For
example the display may also comprise a first polarizer arranged on
top of the front substrate 14 and a second polarizer arranged below
the back substrate 15. In addition to the polarizers, the space
between the front substrate 14 and the back substrate 15 may be
filled with liquid crystals 16 in a manner known per se.
[0042] The rightmost figure in FIG. 1a illustrates an alternative
design of the display electrodes 11 on the display 10. Instead of
the seven segment 11 arrangement the display electrodes 11 are
arranged as a matrix of pixels 11'. At the cost of more wires 12
and terminals 13, this arrangement facilitates the presentation of
more complex figures than the seven segment 11 arrangement. The
display functionality of the matrix arrangement of pixels is,
however, the same as with the seven segment 11 arrangement. In this
context it is appreciated that the term segment is used for
describing a display electrode on a substrate or in a metallic
layer in a display. The term shall not be interpreted as only
describing a display electrode in a seven-segment arrangement, but
may be an electrode of any shape, e.g. a pixel in a matrix
arrangement as disclosed above.
[0043] The segments 11 on the back substrate 15 are normally
interconnected so as to minimize the amount of wires 12 and
terminals 13 on the display, i.e. the segments 11 on the back
substrate 15 will always have the same potential, whereas shapes on
the display 10 are presented by means of changing the potential of
the segments 11 on the front substrate 14 in relation to the
potential on the segments 11 on the back substrate 15.
[0044] FIG. 1b is a simplified view of the allocation of some of
the stray-capacitances in an LCD display 10. The spacing of the
substrates 14, 15 in the figure is exaggerated for the sake of
clarity. As can be seen in the figure a first capacitance C1
stretches from the segments 11 on the front substrate 14 towards
the segments 11 on the back substrate 15. The major contribution to
C1 is the capacitance between the segments 11 on front and back
substrates that are on top of each other. It is, however,
appreciated that the capacitance C1 also includes the stray
capacitances between each segment 11 on the front substrate 14 and
all segments 11 on the back substrate 15.
[0045] A second capacitance C2, C2' appears between different
segments 11 on each substrate 14, 15. The major contribution to C2
is the capacitance between adjacent segments, but it is understood
that C2 also includes the capacitance between one specific segment
11 and all other segments 11 on the same substrate 14, 15.
[0046] When a user of the touch sensor touches the display a third
capacitance C3, C3' appears between the segments 11 on the front 14
and back substrate 15 and the finger 17 of the user. The value of
the third capacitance C3, C3' depends inter alia on the thickness
of the substrates and the properties of the object touching the
display 10.
[0047] A fourth capacitance C4 stretches from each and every
segment towards ground potential via the environment and depends on
the distance to the closest ground reference as well as on the
properties of the environment (i.e. the dielectric constant of the
air in the environment, the relative humidity, etc.).
[0048] As to the size of the different stray capacitances the value
of C1 is by far greater than C2 and C3 due to the close spacing
between the front substrate 14 and the back substrate 15. For the
same reason the sizes of C3 and C3' are almost equal whereas the
value of C2 depends on the size display 10 as well as on the
spacing of the segments 11. In case the segments 11 on the back
substrate 15 are interconnected, the stray capacitance C2' becomes
negligible compared to the galvanic contact provided by the thin
interconnecting wires 12 on the substrate 15. An increase in the
capacitance C2 due to a touch on the display covering two adjacent
segments will hence be hard to detect due to the relatively large
capacitance C1 and the short-circuited segments on the back
substrate.
[0049] FIG. 2 illustrates a first embodiment of a touch sensor 20
according to the present invention. An interface 21 is coupled to
the display driver circuitry (not shown). It is emphasized that the
display driver circuitry is not especially adapted for the touch
display according to the present invention, but may on the contrary
be manufactured for driving ordinary displays without touch
sensitivity. The interface may in its simplest form be a contact
providing the display driver circuitry with electric connections to
the display electrodes 11 on the display 10. Alternatively the
interface comprises buffers and impedance matching means for
providing the display driver circuitry with an optimum operating
point thereby increasing the working time of the display
driver.
[0050] The interface 21 is coupled to a set of switches 22 which in
a first state of operation connects the interface 21 to the display
electrodes 11 on the front substrate 14 and the back substrate 15
of the display 10. In FIG. 2 only the switches 22 associated with
one pair of segments 11 are illustrated for the sake of clarity;
however, the dashed lines in the figure indicates that each segment
11 or group of segments 11 in case the segments 11 on the back
substrate 15 are interconnected (partially or completely) on the
front substrate 14 and the back substrate 15 are connected to the
interface 21 by means of a switch 22. In a preferred embodiment of
the present invention, the segments 11 on the back substrate are
not interconnected but are individually reachable within the touch
sensitive device 20. The interface 21 groups the wires 23 from the
segments 11 on the back substrate 15 making it possible to use
standard display driver circuitry adapted for driving displays with
a common electrode on the back substrate 15. As will be disclosed
below the accuracy of the touch sensor is improved by not
interconnecting the wires 23 froth the segments 11 on the back
plane 15 until they reach the interface 21, thereby making it
possible to isolate each segment 11 by means of the switches 22. By
not interconnecting the segments 11 on the back segment 15 it is
also possible to detect two or more touches on the display 10
simultaneously, i.e. it is possible to distinguish a touch by a
finger from an unintentional touch by the whole hand normally
referred to as "palm rejection".
[0051] When the switches 22 are in the first state of operation the
display 10 is not sensitive to touches on the surface thereof, but
acts as an ordinary display. However, a control unit 24 in the
touch sensor 20 operates the switches 21 in the device so as to put
them in a second state of operation, wherein the display 10 is
disconnected from the interface 21. Instead the segments 11 on one
of the substrates are connected to a signal generator 25 which
feeds a test signal to the segments 11. In the figure the segments
11 on the front substrate 14 are connected to the signal generator
25, but in an alternative embodiment the segments 11 on the back
substrate 15 rather than the segments 11 on the front substrate 14
may be connected to the signal generator 25. As disclosed above,
the relatively large capacitance C1 makes it equally possible to
connect either the segments 11 on the front substrate 14 or the
segments 11 on the back substrate 15 to the signal generator 25
without loosing functionality of the touch sensitive device.
[0052] As the segments 11 on the front substrate 14 are connected
to the signal generator 25, the segments 11 on the back substrate
15 are left in a high-impedance state either by simply
disconnecting them from the interface 21 or, as shown in FIG. 2,
connect them to signal ground via a high-ohmic resistor 26. As
disclosed above, the information on the display 10 depends on the
difference in potential between the segments 11 on the front
substrate 14 and the segments 11 on the back substrate 15. The
high-ohmic state of the segments 11 on the back substrate 15 and
the relatively large capacitance C1 will ensure that any difference
in potential between different segments 11 on the substrates 14, 15
is preserved even though a test signal is applied to the segments
11 on one of the substrates 14, 15. A change in potential on a
segment on the front substrate 14 will hence change the potential
on the segment 11 arranged directly below on the back substrate 15.
Consequently, the information presented on the display when the
switches 22 are in the first state of operation will be preserved
when the switches 22 connects the signal generator 25 to the
segments 11 on the front substrate 14 in the second state of
operation.
[0053] A signal evaluation circuit 27 is coupled to the segments 11
on the front substrate 14. Since the capacitances C1 and C2 of the
display are well known and are established when the display is
manufactured, the response to the test signal by the display when
no foreign object touches the display is also well known. When the
operator of the device puts his finger on the display, the
capacitance C3 disclosed above will become part of the load
presented to the signal generator. The response to the test signal
will hence be changed indicating to the signal evaluation circuitry
27 the presence of a touch on one or more of the segments 11 on the
display 10. Since all segments 11 on the front substrate 14 are
connected to the signal evaluation circuitry 27 it may determine
which segment 11 or segments 11 that are affected by the touch. The
signal evaluation circuitry may then respond to the touch by either
providing a general "key-down"-signal or preferably more detailed
information regarding which specific segments 11 that are affected
by the touch to an external control unit (not shown).
[0054] FIG. 3 illustrates the function of the signal generator 25
and the signal evaluation circuitry 27 according to a first
embodiment of the present invention. When the switches are in the
second state of operation, the signal generator 25 feeds a square
wave 31 to the segments 11 on the front substrate 14 via a set of
resistors 32. In the figure only two resistors 32 are shown for the
sake of clarity. The actual number of resistors 32 depends on the
number of segments 11 that are to be used for detecting a touch on
the display. Since the segments 11 on the back substrate 15 are
disconnected and left in a high-ohmic state the load presented by
the segments 11 alone will become the capacitances C1 and C2 in
series with the small capacitance C4 in FIG. 1b. Hence by leaving
the segments 11 on the back plane 15 in a high-ohmic state the
large capacitor C1 will become series-coupled with the small
capacitance C4 making the contribution of C1 less dominant. In case
the electrodes 11 on the back substrate 15 are interconnected the
accuracy of the touch sensor will be somewhat deteriorated due to
the capacity-coupling between different segments 11 on the front
substrate 14 via the short-circuited back segments 11 and the
capacitances C1 between each front and back segment 11. The small
capacitance presented by the coupling of C1, C2 and C4 will
slightly change the appearance of the test signal 33 at a point to
the right of the resistors in FIG. 3. Instead of a square wave, the
test signal exhibits the well know exponential increase in
potential due to the charging of capacitances C1, C2, and C4 via
the fixed resistors 32. Preferably the rise or fall time of the
loaded test signals are measured by the signal evaluation circuitry
27 so as to determine if the capacitive load has changed. Small
variations the rise time may occur due to changes in the
environment in which the touch sensor 20 is operating. These small
changes will not give rise to an output signal from the signal
evaluation circuitry 27 indicating a touch on the display 10, but
are accepted as environment-induced variations in the test
signal.
[0055] When the operator 17 of the device 20 touches the display
10, the capacitive load presented to the signal generator 25 will
increase due to the capacitance C3 hence increasing the rise and
fall time of the test signal 34 to the right of the resistors 32.
The capacitance C3 is large compared to the series connection C1,
C2, and C4 thereby making a great contribution to the overall
capacitance presented to the signal generator. The exact magnitude
of the increase in the rise time is not critical as long as it is
large enough for making it possible to distinguish a touch on the
display from the small environment-induced variations disclosed
above. The signal evaluation circuitry maybe in form of a simple
comparator providing an output signal in case the rise time exceeds
a predetermined value, or may be intelligent in that it analyses
the long time behaviour of the rise time and compensates for
changes in the environment.
[0056] The control unit 24 is adapted to alternate the switches 22
between the first and second state of operation. The rate at which
the control unit 24 change the state of the switches 22 depends on
the capacitance C1, the resistance between each segment and signal
ground, and the inertia in the liquid crystal, i.e. how long it
takes for the crystal in the display to turn in the absence of an
external electric field.
[0057] The resistances 32 in FIG. 3 may be implemented in form of
traditional resistors 32 or as shown in an alternative embodiment
in FIG. 4 as switched capacitors 42. The switched capacitor 42
known per se is well suited for integration on a chip making it
possible to combine the electronics of the touch sensor with the
display 10 as an integral unit.
[0058] In the above embodiment of the present invention a
capacitance measuring circuit in form of a signal generator 25 and
a signal evaluation circuit 27 is disclosed, wherein the signal
evaluation circuit 27 measures the rise or fall time of the test
signal. It is however appreciated that the capacitance measuring
circuit as well may measure the current fed to the display at a
fixed or varied voltage and frequency, measure the phase difference
between current and voltage applied to the display, or measure the
capacitance between at least one segment 11 of the display 10 and
the environment in any other suitable way.
[0059] In an alternative embodiment of the present invention the
display electrode 11 may be arranged on a substrate 14, 15 so as to
make it possible to detect a galvanic contact between the display
electrode 11 and an object touching the display device 10. For
example, a display may be formed by a matrix of light emitting
diodes (LED), wherein each diode of the display is soldered to a
pair of pads on a printed circuit board (PCB). Each pad then
constitutes a display electrode 11 which may be disconnected from
the display driver circuitry and used for detecting a touch on the
display device 10. Hence, if a person touches the display electrode
11, the person will act as a capacitor receiving charge from the
display electrodes 11. A small, detectable current will flow from
the display device 10 through the finger of the person indicating a
touch on the display device 10.
[0060] In yet an alternative embodiment of the present invention
the display electrode 11 may be provided with a high-voltage when
the switches are in the second state of operation. The display
electrodes 11 on the substrates 14, 15 may then be arranged between
the substrates 14, 15, as disclosed with regards to the LCD display
above, wherein a very large resistance of the substrate still may
allow the flow of a current large enough to be detected when a
person touches the front of the display device 10.
[0061] In yet an alternative embodiment of the invention, touch
sensitive areas are formed on one side of a third substrate in
accordance with the description in relation to FIG. 1 (i.e. a
substrate e.g. made of glass or plastic, on which a suitable
electrical material, such as Indium Tin Oxide (ITO), is deposited
as to form the desired touch-sensitive areas). The third substrate
may then be arranged in front of any kind of display in order to
provide touch-sensitivity for the display. The touch-sensitive
areas are preferably arranged on the inside of the substrate, i.e.
the side which is facing the display and is not in direct contact
with the users of the touch-sensitive substrate, in order to
provide a long operational life of the touch-sensitive substrate
even under exposure to hard wear. The third substrate will, in
addition to provide touch sensitivity for the display, also act as
a protective cover since it is arranged in front of the
display.
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