U.S. patent application number 14/729769 was filed with the patent office on 2016-05-26 for temperature compensating transparent force sensor having a flexible substrate.
The applicant listed for this patent is Apple Inc.. Invention is credited to Christopher J. Butler, Shin John Choi, Sinan Filiz, Martin P. Grunthaner, Brian Q. Huppi, Charley T. Ogata, Dhaval Chandrakant Patel, James E. Pedder, John Stephen Smith.
Application Number | 20160147353 14/729769 |
Document ID | / |
Family ID | 52434986 |
Filed Date | 2016-05-26 |
United States Patent
Application |
20160147353 |
Kind Code |
A1 |
Filiz; Sinan ; et
al. |
May 26, 2016 |
Temperature Compensating Transparent Force Sensor Having a Flexible
Substrate
Abstract
An optically transparent force sensor element is compensated for
effects of environment by comparing a force reading from a first
force-sensitive component with a second force-sensitive components.
The first and second force-sensitive components disposed on
opposite sides of a flexible substrate within a display stack.
Inventors: |
Filiz; Sinan; (Cupertino,
CA) ; Pedder; James E.; (Cupertino, CA) ;
Ogata; Charley T.; (Cupertino, CA) ; Smith; John
Stephen; (Cupertino, CA) ; Patel; Dhaval
Chandrakant; (Cupertino, CA) ; Choi; Shin John;
(Cupertino, CA) ; Huppi; Brian Q.; (Cupertino,
CA) ; Butler; Christopher J.; (Cupertino, CA)
; Grunthaner; Martin P.; (Cupertino, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Apple Inc. |
Cupertino |
CA |
US |
|
|
Family ID: |
52434986 |
Appl. No.: |
14/729769 |
Filed: |
June 3, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14594857 |
Jan 12, 2015 |
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14729769 |
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61926905 |
Jan 13, 2014 |
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61937465 |
Feb 7, 2014 |
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61939257 |
Feb 12, 2014 |
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61942021 |
Feb 19, 2014 |
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62024566 |
Jul 15, 2014 |
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Current U.S.
Class: |
345/174 |
Current CPC
Class: |
G01L 1/16 20130101; G06F
3/044 20130101; G01L 1/18 20130101; G06F 2203/04111 20130101; G01L
1/20 20130101; G06F 3/0418 20130101; G06F 2203/04103 20130101; G01L
1/005 20130101; G06F 3/04144 20190501; G01L 1/205 20130101; G06F
3/0412 20130101; G06F 3/045 20130101; G06F 2203/04105 20130101 |
International
Class: |
G06F 3/041 20060101
G06F003/041 |
Claims
1. An optically transparent force sensor comprising: a
force-receiving layer; a substrate disposed below the
force-receiving layer and formed from an optically transparent
material a first force-sensitive component disposed on a first side
of the substrate and formed from an optically transparent,
strain-sensitive material; and a second force-sensitive component
disposed on a second side of the substrate that is opposite to the
first side, wherein the second force-sensitive component is formed
from the optically transparent, strain-sensitive material.
2. The force sensor of claim 1, further comprising: sensor
circuitry that is operatively coupled to the first force-sensitive
component and the second force-sensitive component, wherein the
sensor circuitry is configured to measure a relative difference
between an electrical response of the first and second
force-sensitive components in response to a force of a touch on the
force-receiving layer, and compute a temperature-compensated force
estimate using the relative difference.
3. The force sensor of claim 1, wherein the substrate is configured
to conduct heat between the first force-sensitive component and the
second force-sensitive component to achieve a substantially uniform
temperature distribution.
4. The force sensor of claim 1, wherein the substrate is disposed
below a display element of an electronic device.
5. The force sensor of claim 1, wherein the substrate is disposed
between a cover and a display element of an electronic device.
6. The force sensor of claim 1, wherein the first force-sensitive
component is placed in compression in response to a force of a
touch on the force-receiving layer, and the second force-sensitive
component is placed in tension in response to the force of the
touch.
7. The force sensor of claim 1, further comprising: a first array
of rectilinear force-sensitive components including the first
force-sensitive component; and a second array of rectilinear
force-sensitive components including the second force-sensitive
component.
8. The force sensor of claim 1, wherein the force-receiving layer
is a cover of a display of a device and is formed from a glass
material.
9. The force sensor of claim 1, wherein the first and second
force-sensitive components have substantially identical temperature
coefficients of resistance.
10. The force sensor of claim 1, wherein the substrate has a
thermal conductivity of greater than 0.5 Watts per square meter per
degree Kelvin.
11. The force sensor of claim 1, wherein the first and second
force-sensitive components are formed from a piezoresistive
material.
12. The force sensor of claim 1, wherein the first and second
force-sensitive components are formed from one or more of: a carbon
nanotube material, graphene, a semiconductor material, a metal
oxide material.
13. The force sensor of claim 1, wherein the first and second
force-sensitive components are formed from a indium oxide material
that is doped with Sn.
14. The force sensor of claim 13, wherein the indium oxide material
doped with Sn to a proportion of less than 5%.
15. An electronic device having an optically transparent force
sensor comprising: a cover; a substrate disposed below the cover
and formed from an optically transparent material; a first array of
force-sensitive components disposed on a first side of the
substrate and formed from an optically transparent,
strain-sensitive material; and a second array of force-sensitive
components disposed on a second side of the substrate that is
opposite to the first side, wherein the second array of
force-sensitive components is formed from the optically
transparent, strain-sensitive material sensor circuitry that is
configured to compare a relative electrical response between
respective components of the first array of force-sensitive
components and the second array of force-sensitive components, and
configured to compute a temperature-compensated force estimate.
16. The electronic device of claim 15, further comprising: a
display element disposed above the first array of force-sensitive
components.
17. The electronic device of claim 15, further comprising: a
display element disposed below the second array of force-sensitive
components.
18. The electronic device of claim 15, wherein the first array of
force-sensitive components includes a subset of edge
force-sensitive components positioned along an edge of the first
array, wherein the edge force-sensitive components are formed from
traces that are oriented along a direction that is substantially
perpendicular to the edge.
19. The electronic device of claim 15, wherein the first array of
force-sensitive components includes a subset of corner
force-sensitive components positioned at corners of the first
array, wherein the corner force-sensitive components are formed
from traces that are oriented along a diagonal direction.
20. The electronic device of claim 15, wherein the first array of
force-sensitive components includes component having a first
portion that includes traces that are substantially oriented along
a first direction and a second portion that includes traces that
are substantially oriented along an second direction, wherein the
first direction is substantially perpendicular to the second
direction.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/594,857, filed Jan. 12, 2015, and titled
"Temperature Compensating Transparent Force Sensor Having a
Flexible Substrate," which claims priority to U.S. Provisional
Patent Application No. 61/926,905, filed Jan. 13, 2014, and titled
"Force Sensor Using a Transparent Force-Sensitive Film," U.S.
Provisional Patent Application No. 61/937,465, filed Feb. 7, 2014,
and titled "Temperature Compensating Transparent Force Sensor,"
U.S. Provisional Patent Application No. 61/939,257, filed Feb. 12,
2014, and titled "Temperature Compensating Transparent Force
Sensor," U.S. Provisional Patent Application No. 61/942,021, filed
Feb. 19, 2014, and titled "Multi-Layer Temperature Compensating
Transparent Force Sensor," and U.S. Provisional Patent Application
No. 62/024,566, filed Jul. 15, 2014, and titled "Strain-Based
Transparent Force Sensor," the disclosure of each of which is
incorporated by reference in its entirety.
TECHNICAL FIELD
[0002] Embodiments described herein generally relate to force
sensing and, more particularly, to a temperature compensating force
sensor having two or more transparent force-sensitive components
disposed on either side of a flexible substrate.
BACKGROUND
[0003] Many electronic devices include some type of user input
device, including, for example, buttons, slides, scroll wheels, and
similar devices or user-input elements. Some devices may include a
touch sensor that is integrated or incorporated with a display
screen. The touch sensor may allow a user to interact directly with
user-interface elements that are presented on the display screen.
However, some traditional touch sensors may only provide a location
of a touch on the device. Other than location of the touch, many
traditional touch sensors produce an output that is binary in
nature. That is, the touch is present or it is not.
[0004] In some cases, it may be advantageous to detect and measure
the force of a touch that is applied to a surface to provide
non-binary touch input. However, there may be several challenges
associated with implementing a force sensor in an electronic
device. For example, temperature fluctuations in the device or
environment may introduce an unacceptable amount of variability in
the force measurements. Additionally, if the force sensor is
incorporated with a display or transparent medium, it may be
challenging to achieve both sensing performance and optical
performance in a compact form factor.
SUMMARY
[0005] Embodiments described herein may relate to, include, or take
the form of an optically transparent force sensor, which may be
used as input to an electronic device. The optically transparent
force sensor may be configured to compensate for variations in
temperature using two or more force-sensitive structures that are
disposed on opposite sides of a flexible substrate.
[0006] In some example embodiments, an optically transparent force
sensor includes, a force-receiving layer and a substrate comprising
an optically transparent material, the substrate disposed below the
force-receiving layer. The force sensor may also include a first
force-sensitive component disposed on a first side of the substrate
and comprised of an optically transparent, strain-sensitive
material, and a second force-sensitive component disposed on a
second side of the substrate that is opposite to the first side.
The second force-sensitive component is also comprised of the
optically transparent, strain-sensitive material.
[0007] In some embodiments, the sensor also includes sensor
circuitry that is operatively coupled to the first force-sensitive
component and the second force-sensitive component. In some
implementations the sensor circuitry is configured to measure a
relative difference between an electrical response of the first and
second force-sensitive components in response to a force of a touch
on the force-receiving layer. The sensor circuitry may also be
configured to compute a temperature-compensated force estimate
using the relative difference.
[0008] In some implementations, the substrate is configured to
conduct heat between the first force-sensitive component and the
second force-sensitive component to achieve a substantially uniform
temperature distribution. In some cases, the substrate has a
thermal conductivity of greater than 0.5 Watts per square meter per
degree Kelvin. In some cases, the first and second force-sensitive
components have substantially identical temperature coefficients of
resistance.
[0009] The transparent force sensor may be integrated or
incorporated with a display element. In some cases, the substrate
is disposed below a display element of an electronic device. In
some cases, the substrate is disposed between a cover and a display
element of an electronic device. In some cases, the force-receiving
layer is a cover of a display of a device and is formed from a
glass material.
[0010] In some embodiments, the first force-sensitive component is
placed in compression in response to a force of a touch on the
force-receiving layer, and the second force-sensitive component is
placed in tension in response to the force of the touch. In some
embodiments, the sensor includes a first array of rectilinear
force-sensitive components including the first force-sensitive
component and a second array of rectilinear force-sensitive
components including the second force-sensitive component.
[0011] In some embodiments, the first and second force-sensitive
components are formed from a piezoresistive material. In some
embodiments, the first and second force-sensitive components are
formed from one or more of: a carbon nanotube material, graphene,
gallium zinc oxide, indium gallium zinc oxide, a semiconductor
material, a metal oxide material. In some embodiments, the first
and second force-sensitive components are formed from a indium
oxide material that is doped with Sn. The indium oxide material may
be doped with Sn to a proportion of less than 5%.
[0012] Some example embodiments are directed to an electronic
device having an optically transparent force sensor. The electronic
device may include a cover and a substrate comprising an optically
transparent material, the substrate disposed below the cover The
device may also include a first array of force-sensitive components
disposed on a first side of the substrate and comprised of an
optically transparent, strain-sensitive material. The device may
also include a second array of force-sensitive components disposed
on a second side of the substrate that is opposite to the first
side, wherein the second array of force-sensitive components is
comprised of the optically transparent, strain-sensitive material.
In some cases, the device includes sensor circuitry that is
configured to compare a relative electrical response between
respective components of the first array of force-sensitive
components and the second array of force-sensitive components, and
configured to compute a temperature-compensated force estimate. The
device may also include a display element disposed above the first
array of force-sensitive components. In some cases the display
element is disposed below the second array of force-sensitive
components.
[0013] In some embodiments, the first array of force-sensitive
components includes a subset of edge force-sensitive components
that are positioned along an edge of the first array. In some
cases, the edge force-sensitive components are formed from traces
that are oriented along a direction that is substantially
perpendicular to the edge. In some embodiments, the first array of
force-sensitive components includes a subset of corner
force-sensitive components positioned at corners of the first
array. The corner force-sensitive components may be formed from
traces that are oriented along a diagonal direction. In some
embodiments, the first array of force-sensitive components includes
component having a first portion that includes traces that are
substantially oriented along a first direction and a second portion
that includes traces that are substantially oriented along an
second direction. The first direction may be substantially
perpendicular to the second direction.
[0014] Other embodiments described herein may relate to, include,
or take the form of a method of manufacturing a force sensor
including at least the steps of applying a first force-sensitive
film to a first substrate, and applying a second force-sensitive
film to the first substrate or a second substrate resulting in a
force sensor having the first force-sensitive film and the second
force sensitive film disposed on opposite sides of the first
substrate.
BRIEF DESCRIPTION OF THE FIGURES
[0015] Reference will now be made to representative embodiments
illustrated in the accompanying figures. It should be understood
that the following descriptions are not intended to limit the
embodiments to one preferred embodiment. To the contrary, it is
intended to cover alternatives, modifications, and equivalents as
may be included within the spirit and scope of the described
embodiments as defined by the appended claims.
[0016] FIG. 1 depicts an example electronic device.
[0017] FIG. 2A depicts a top view of an example force-sensitive
structure including a grid of optically transparent force-sensitive
films.
[0018] FIG. 2B depicts a top detailed view of an optically
transparent serpentine force-sensitive film which may be used in
the example force-sensitive structure depicted in FIG. 2A.
[0019] FIG. 3 depicts a cross-sectional view of a portion of the
example force-sensitive structure of FIG. 1 taken along section
A-A.
[0020] FIG. 4 depicts a top view of an alternate example of a
force-sensitive structure including two perpendicular layers each
including multiple optically transparent force-sensitive films.
[0021] FIG. 5A depicts a top detailed view of an optically
transparent serpentine force-sensitive component having a first
serpentine pattern and which may be used in the example
force-sensitive structure depicted in FIG. 2A.
[0022] FIG. 5B depicts a top detailed view of an optically
transparent serpentine force-sensitive component having a second
serpentine pattern and which may be used in the example
force-sensitive structure depicted in FIG. 2A.
[0023] FIG. 5C depicts a top detailed view of an optically
transparent serpentine force-sensitive components having a third
serpentine pattern and which may be used in the example
force-sensitive structure depicted in FIG. 2A.
[0024] FIG. 5D depicts a top detailed view of an optically
transparent serpentine force-sensitive component having a fourth
serpentine pattern and which may be used in the example
force-sensitive structure depicted in FIG. 2A.
[0025] FIG. 6 depicts a top view of an example force-sensitive
structure including a grid of optically transparent force-sensitive
components oriented in different directions to detect force.
[0026] FIG. 7A depicts a top detailed view of an optically
transparent serpentine force-sensitive component having a fifth
serpentine pattern and which may be used in the example
force-sensitive structure depicted in FIG. 2A.
[0027] FIG. 7B depicts a top view of an example force-sensitive
structure including a grid of optically transparent force-sensitive
components to detect force.
[0028] FIG. 8 depicts a simplified signal flow diagram of a
temperature-compensating and optically transparent force sensor
circuit.
[0029] FIG. 9 is a process flow diagram illustrating example steps
of a method of manufacturing a temperature-compensating and
optically transparent force sensor.
[0030] FIG. 10 is a process flow diagram illustrating example steps
of a method of operating a temperature-compensating force
sensor.
[0031] FIG. 11 is an additional process flow diagram illustrating
example steps of a method of operating a temperature-compensating
force sensor.
[0032] The use of the same or similar reference numerals in
different figures indicates similar, related, or identical
items.
DETAILED DESCRIPTION
[0033] Embodiments described herein may relate to or take the form
of temperature compensating optically transparent force sensors for
receiving user input to an electronic device. Some embodiments are
directed to a force sensor that can compensate for variations in
temperature and may be optically transparent for integration with a
display or transparent medium of an electronic device. Some
embodiments relate to force-sensitive structures including one or
more force-sensitive components for detecting a magnitude of a
force applied to a device. In one example, a transparent
force-sensitive component is integrated with, or adjacent to, a
display element of an electronic device. The electronic device may
be, for example, a mobile phone, a tablet computing device, a
computer display, a computing input device (such as a touch pad,
keyboard, or mouse), a wearable device, a health monitor device, a
sports accessory device, and so on.
[0034] Generally and broadly, a user touch event may be sensed on a
display, enclosure, or other surface associated with an electronic
device using a force sensor adapted to determine the magnitude of
force of the touch event. The determined magnitude of force may be
used as an input signal, input data, or other input information to
the electronic device. In one example, a high force input event may
be interpreted differently from a low force input event. For
example, a smart phone may unlock a display screen with a high
force input event and may pause audio output for a low force input
event. The device's responses or outputs may thus differ in
response to the two inputs, even though they occur at the same
point and may use the same input device. In further examples, a
change in force may be interpreted as an additional type of input
event. For example, a user may hold a wearable device force sensor
proximate to an artery in order to evaluate blood pressure or heart
rate. One may appreciate that a force sensor may be used for
collecting a variety of user inputs.
[0035] In many examples, a force sensor may be incorporated into a
touch-sensitive electronic device and located proximate to a
display of the device, or incorporated into a display stack.
Accordingly, in some embodiments, the force sensor may be
constructed of optically transparent materials. For example, an
optically transparent force sensor may include at least a
force-receiving layer, a first and second substrate each including
at least an optically transparent material, and each substrate
including, respectively, a first and second force-sensitive
component. In some embodiments, the first and second
force-sensitive components are disposed, or located relative to,
opposite sides of the first substrate. In some cases, the first and
second force-sensitive components are formed on or attached to the
first substrate. In other cases, either or both of the first and
second force-sensitive components may be formed on or attached to a
second substrate. In many examples, the first substrate may be
disposed below the force-receiving layer such that the first
force-sensitive component may experience deflection, compression,
or another mechanical deformation upon application of force to the
force-receiving layer. In this manner, a bottom surface of the
first substrate may experience an expansion and a top surface of
the first substrate may experience a compression. In other words,
the first substrate may bend about its neutral axis, experiencing
compressive and tensile forces. The force-sensitive components may
be used to detect and measure the degree of expansion and
compression caused by the deflection of the first substrate.
[0036] A transparent force-sensitive component is typically a
compliant material that exhibits at least one electrical property
that is variable in response to deformation, deflection, or
shearing of the component. The transparent force-sensitive
component may be formed from a piezoelectric, piezoresistive,
resistive, or other strain-sensitive materials. Potential substrate
materials include, for example, glass, sapphire, diamond,
SiO.sub.2, or transparent polymers like polyethylene terephthalate
(PET) or cyclo-olefin polymer (COP). Example transparent conductive
materials include polyethyleneioxythiophene (PEDOT), indium tin
oxide (ITO), carbon nanotubes, graphene, gallium zinc oxide, indium
gallium zinc oxide, other doped or undated metal oxides,
piezoresistive semiconductor materials, piezoresistive metal
materials, silver nanowire, platinum nanowire, nickel nanowire,
other metallic nanowires, and the like. Transparent materials may
be used in sensors that are integrated or incorporated with a
display or other visual element of a device. If transparency is not
required, then other component materials may be used, including,
for example, Constantan and Karma alloys, doped polycrystalline or
amorphous silicon, or single crystal silicon or other semiconductor
material for the conductive component and a clad metal, ceramic, or
a polyimide may be used as a substrate. Nontransparent applications
include force sensing on track pads or behind display elements. In
general, transparent and non-transparent force-sensitive components
may be referred to herein as "force-sensitive components" or simply
"components."
[0037] Transparent force-sensitive components can be formed by
coating a substrate with a transparent conductive material,
attaching a transparent conductive material, or otherwise
depositing such a material on the substrate. In some embodiments,
the force-sensitive components may be formed relative to the bottom
surface of a first substrate and relative to a top surface of a
second substrate. The force-sensitive components of the first and
second substrates may be oriented to face one another. In some
implementations, the first substrate may deflect in response to a
user touch. The deflection of the first substrate may cause the
bottom surface of the first substrate to expand under tension,
which may cause the transparent force-sensitive component (disposed
relative to the bottom surface) to also expand, stretch, or
otherwise geometrically change as a result of the deflection.
[0038] In some cases, the force-sensitive component may be placed
under tension in response to a downward deflection because the
component is positioned below the neutral axis of the bend of the
substrate. Once under tension, the transparent force-sensitive
component may exhibit a change in at least one electrical property,
for example, resistance. In one example, the resistance of the
transparent force-sensitive component may increase linearly with an
increase in tension experienced by the component. In another
example, the resistance of the transparent force-sensitive
component may decrease linearly with an increase in tension
experienced by the component. One may appreciate that different
transparent materials may experience different changes to different
electrical properties, and as such, the effects of tension may vary
from embodiment to embodiment.
[0039] As previously mentioned, suitable transparent conductive
materials include, for example, polyethyleneioxythiophene (PEDOT),
carbon nanotubes, graphene, silver nanowire, other metallic
nanowires, and the like. In some cases, the transparent conductive
material may include a metal oxide material, including, for
example, SnO.sub.2, In.sub.2O.sub.3, ZnO, Ga.sub.2O.sub.3, and CdO.
The transparent conductive material may also be formed from an
indium oxide material. In some cases, the indium oxide is doped
with tin (Sn) to form an indium-tin oxide. In some implementations,
the indium oxide is doped with Sn to a proportion of less than 5%.
Additionally, in some cases, the transparent conductive material
may be formed from a semiconductor material, including, for
example, a piezoresistive semiconductor material. Potential
substrate materials include, for example, glass or transparent
polymers like polyethylene terephthalate (PET) or cyclo-olefin
polymer (COP). Typically, when a piezoresistive or resistive
component is strained, the resistance of the component changes as a
function of the strain. The resistance can be measured with an
electrical circuit.
[0040] In some embodiments, the force-sensitive components may be
formed from a piezoresistive or resistive material. In some
implementations, when the piezoresistive or resistive material is
strained, the resistance of the component changes as a function of
the strain. The change in resistance can be measured using a
sensing circuit that is configured to measure small changes in
resistance of the force-sensitive components. In some cases, the
sensing circuit may include a bridge circuit configuration that is
configured to measure the differential change in resistance between
two or more force-sensitive components. If the relationship between
electrical resistance, temperature and mechanical strain of the
component material is known, the change in the differential strain
.epsilon..sub.x-.epsilon..sub.y may be derived. In some cases, the
differential strain may account for changes strain or resistance
due to changes in temperature, which may cancel if the two elements
have similar thermal properties and are at similar temperature
while being subjected to differential strain due to a placement
with respect to the neutral axis of a flexible substrate. In this
way, a transparent piezoresistive or resistive component can be
used as a temperature compensating force sensor.
[0041] In certain embodiments, a resistive element may be measured
by using a voltage divider or bridge circuit. For example, a
voltage V.sub.g may be measured across the output of two parallel
voltage dividers connected to a voltage supply V.sub.s. One of the
voltage dividers may include two resistors of known resistance
R.sub.1 and R.sub.2, the other voltage divider may include a first
resistive strain element R.sub.x and a second resistive strain
element R.sub.y. A voltage can be measured between a node between
R.sub.1 and R.sub.2 and a node between R.sub.x and R.sub.y to
detect small changes in the relative resistance between the two
strain elements. In some cases, additional sensor circuitry
(including a processing unit) may be used to calculate the
mechanical strain due to a force on the surface based on the
relative resistance between two strain elements. In some cases, the
sensor circuitry may estimate the mechanical strain while reducing
or eliminating environmental effects, such as variations in
temperature.
[0042] In some embodiments, pairs of voltage dividers may be used
to form a full bridge, so as to compare the output of a plurality
of sensors. In this manner, error present as a result of
temperature differences between sensors may be substantially
reduced or eliminated without requiring dedicated error correction
circuitry or specialized processing software. In some embodiments,
an electrical response due to the force of a touch may be measured
and an algorithm may be used to compare a relative response and
cancel the effects of the temperature changes. In some embodiments,
both differential measurements of the components and measurements
of their individual responses may be made to extract the
corresponding differential strain, and also the temperature. In
some cases an algorithm may use the differential and individual
responses to compute a force estimate that cancels the effects on
strain due to, for example, the differences in the thermal
coefficient of expansion of the two component materials.
[0043] In some embodiments, the force-sensitive component is
patterned into an array of lines, pixels, or other geometric
elements herein referred to as "component elements." The regions of
the force-sensitive component or the component elements may also be
connected to sense circuitry using electrically conductive traces
or electrodes. In some cases, the conductive traces or electrodes
are also formed from transparent conductive materials. In some
embodiments, sense circuitry, may be in electrical communication
with the one or more component elements via the electrically
conductive traces and/or the electrodes. As previously mentioned,
the sense circuitry may be adapted to detect and measure the change
in the electrical property or response (e.g., resistance) of the
component due to the force applied.
[0044] In some cases, the force-sensitive components may be
patterned into pixel elements, each pixel element including an
array of traces generally oriented along one direction. This
configuration may be referred to as a piezoresistive or resistive
strain gauge configuration. In general, in this configuration the
force-sensitive-component may be composed of a material whose
resistance changes in a known fashion in response to strain. For
example, some materials may exhibit a change in resistance linearly
in response to strain. Some materials may exhibit a change in
resistance logarithmically or exponentially in response to strain.
Some materials may exhibit a change in resistance in a different
manner. For example, the change in resistance may be due to a
change in the geometry resulting from the applied strain such as an
increase in length combined with decrease in cross-sectional area
may occur in accordance with Poisson's effect. The change in
resistance may also be due to a change in the inherent resistivity
of the material due to the applied strain.
[0045] In some embodiments, the orientation of the strain-sensitive
elements may vary from one part of the array to another. For
example, elements in the corners may have traces that are oriented
to be sensitive to strain at 45 degrees with respect to a row (or
column) of the array. Similarly, elements along the edge of the
array may include traces that are most sensitive to strain
perpendicular to the edge or boundary. In some cases, elements may
include one of a variety of serpentine trace configurations that
may be configured to be sensitive to a combination of the strains
along multiple axes. The orientation of the traces in the
strain-sensitive elements may have different angles, depending on
the embodiment.
[0046] The pixel elements may have trace patterns that are
configured to blend the sensitivity to strain along multiple axes
to detect changes in boundary conditions of the sensor or damage to
the device. For example, if an element, component, or substrate
becomes less constrained because of damage to the physical edge of
a device, the sensitivity of the response to strain in the X
direction may become higher, while the sensitivity of the response
to strain in the Y direction may be lower. However, if the pixel
element is configured to be responsive to both X and Y directions,
the combined response of the two or more directions (which may be a
linear combination or otherwise) may facilitate use of the sensor,
even after experiencing damage or changes in the boundary
conditions of the substrate.
[0047] In some embodiments, the force-sensitive component may be
formed from a solid sheet of material and may be placed in
electrical communication with a pattern of electrodes disposed on
one or more surfaces of the force-sensitive component. The
electrodes may be used, for example, to electrically couple a
region of the solid sheet of material to sense circuitry. An
electrode configuration may be used to measure a charge response
when strained. In some cases, the force-sensitive component may
generate different amounts of charge depending on the degree of the
strain. The overall total charge may reflect a superposition of the
charge generated due to strain along various axes.
[0048] In some embodiments, the force-sensitive component may be
integrated with, or placed adjacent to, portions of a display
element, herein generally referred to as a "display stack" or
simply a "stack." A force-sensitive component may be integrated
with a display stack, by, for example, being attached to a
substrate or sheet that is attached to the display stack. In this
manner, as the display stack bends in response to an applied force,
and through all the layers which have good strain transmission
below the neutral axis, a tensile strain is transmitted.
[0049] Alternatively, the force-sensitive component may be placed
within the display stack in certain embodiments. Although certain
examples are herein provided with respect to a force-sensitive
component integrated with a display stack, in other embodiments,
the force-sensitive component may be integrated in a portion of the
device other than the display stack.
[0050] In some embodiments, one or more force-sensitive components
may be integrated with or attached to a display element of a
device, which may include other types of sensors. In some
embodiments, a display element may include a touch sensor included
to detect the location of one or more user touch events. Using a
touch sensor in combination with the transparent force-sensitive
component in accordance with some embodiments described herein, the
location and magnitude of a touch on a display element of a device
can be estimated.
[0051] In some embodiments, the device may include both
touch-sensitive elements and force-sensitive elements relative to a
surface that may cooperate to improve accuracy of the force
sensors. In some cases, the information from the touch-sensitive
elements may be used in combination with stored information about
the responsiveness of the surface to reconstruct the force exerted
on the surface. For example, the location determined by the touch
sensor may be used in conjunction with a set of weighting
coefficients stored in a memory to estimate the force applied at
the corresponding points. A different touch location may be used in
conjunction with a different set of coefficients weighting the
response of the strain sensors to predict a force of touch at that
point. In certain examples, the algorithm used to calculate the
forces at the surface may be based, at least in part, upon the
information provided by the touch sensor, stored information from
calibration of the display, or information collected and stored
during the operational life of the sensors. In some cases, the
sensors may be calibrated to zero force during a time preceding a
touch indication from the touch sensors.
[0052] One challenge associated with using a force-sensitive
component or film within a display stack is that the given
electrical property (for example, resistance) may change in
response to temperature variations as the electronic device is
transported from place to place, or used by a user. For example,
each time a user touches the touch screen, the user may locally
increase the temperature of the screen and force-sensitive
component. In other examples, different environments (e.g., indoors
or outdoors) may subject the electronic device to different ambient
temperatures. In still further examples, an increase in temperature
may occur as a result of heat produced by electronic components or
systems of the device.
[0053] In some cases, the force-sensitive component may also expand
and contract in response to changes in other environmental
conditions, such as changes in humidity or barometric pressure. In
the following examples, the electrical property is a resistance and
the variable environmental condition is temperature. However, the
techniques and methods described herein may also be applied to
different electrical properties, such as capacitance or inductance,
which may be affected by changes in other environmental
conditions.
[0054] In some implementations, a change in temperature or other
environmental conditions, either locally or globally, may result in
expansion or contraction of the force-sensitive component,
electronic device enclosure, and/or other components adjacent to
the component which in turn may change the electrical property
(e.g., resistance) measured by the sense circuitry. In many cases,
the changes in the electrical property due to temperature change
may obfuscate any changes in the electrical property as a result of
an input force. For example, a deflection may produce a reduction
or increase in the resistance or impedance of the force-sensitive
component. A change in temperature may also produce a reduction or
increase in the resistance or impedance of the force-sensitive
component. As a result, the two effects may cancel each other out
or, alternatively, may amplify each other resulting in an
insensitive or hypersensitive force sensor. A similar reduction or
increase in the resistance or impedance of the force-sensitive
component could also be produced by, for example, an increase in
temperature of the force-sensitive component due to heat produced
by other elements of the device.
[0055] In some cases, mechanical changes due to variations in
temperature may also impact the electrical performance of the
sensor. In particular, variations in temperature of the
force-sensing component may result in variations in strain on the
force-sensing elements. For example, a heated force-sensitive
component may expand and a cooled force-sensitive component may
contract producing a strain on the component. This strain may cause
a change in resistance, impedance, current, or voltage that may be
measured by associated sense circuitry and may impact the
performance of the force sensor.
[0056] One solution is to account for environmental effects by
providing more than one force-sensing component that is subjected
to the same or substantially the same environmental conditions. A
first force-sensing component may serve as a reference point or
environmental baseline while measuring the strain of a second
force-sensing component. In some implementations, both of the
force-sensitive components may be constructed of substantially
identical materials such that the reference component reacts to the
environment in the same manner as the component being measured. For
example, in some cases, each of the two components may be adapted
to have identical or nearly identical thermal coefficients of
expansion. In this manner, the mechanical and geometric changes
resulting from temperature changes may be measured as a difference
between the components. In some implementations, because each
sensor has the same or similar thermal coefficient of expansion,
each sensor may expand or contract in a substantially identical
manner. Using appropriate sensor circuitry and/or sensor
processing, effects on the electrical properties of either sensor
as a result of temperature can be substantially compensated,
cancelled, reduced or eliminated.
[0057] In some embodiments, a substrate may be positioned or
disposed below a surface or layer which receives an input force.
The substrate may deflect in response to the input force. A first
force-sensitive component may be disposed with respect to one side
of the substrate and a second force-sensitive component may be
disposed with respect to a second, opposite side of the substrate.
Due to their placement with respect to a neutral axis of the
substrate, the first force-sensitive component may be placed in
compression and the second force-sensitive component may be placed
in tension in response to the deflection. In some cases, the
relative strain response may be compared to estimate or approximate
the input force. For example, in some cases, the second
force-sensitive component may be placed in greater tensile strain
that the first force-sensitive component is placed in compressive
strain. The difference or a comparison between the first and second
force-sensitive component may be used to estimate the input force.
In some embodiments, the first and second force-sensitive films are
subjected to similar temperature conditions and may also be
subjected to similar mechanical influences due to their proximity
to each other. In some cases, the effect of these influences may be
reduced or canceled out and by measuring the difference between or
comparing the output of the first and second force-sensitive
components.
[0058] FIG. 1 depicts an example electronic device 100. The
electronic device 100 may include a display 104 disposed or
positioned within an enclosure 102. The display 104 may include a
stack of multiple elements including, for example, a display
element, a touch sensor layer, a force sensor layer, and other
elements. The display 104 may include a liquid-crystal display
(LCD) element, organic light emitting diode (OLED) element,
electroluminescent display (ELD), and the like. The display 104 may
also include other layers for improving the structural or optical
performance of the display, including, for example, glass sheets,
polymer sheets, polarizer sheets, color masks, and the like. The
display 104 may also be integrated or incorporated with a cover
106, which forms part of the exterior surface of the device 100.
Example display stacks depicting some example layer elements are
described in more detail below with respect to FIGS. 2-5.
[0059] In some embodiments, a touch sensor and or a force sensor
are integrated or incorporated with the display 104. In some
embodiments, the touch and/or force sensor enable a touch-sensitive
surface on the device 100. In the present example, a touch and/or
force sensor are used to form a touch-sensitive surface over at
least a portion of the exterior surface of the cover 106. The touch
sensor may include, for example, a capacitive touch sensor, a
resistive touch sensor, or other device that is configured to
detect the occurrence and/or location of a touch on the cover 106.
The force sensor may include a strain-based force sensor similar to
the force sensors described herein.
[0060] In some embodiments, each of the layers of the display 104
may be adhered together with an optically transparent adhesive. In
other embodiments, each of the layers of the display 104 may be
attached or deposited onto separate substrates that may be
laminated or bonded to each other. The display 104 may also include
other layers for improving the structural or optical performance of
the display, including, for example, glass sheets, polarizer
sheets, color masks, and the like.
[0061] FIG. 2A depicts a top view of an example force-sensitive
structure 200 including a grid of optically transparent
force-sensitive components. The force-sensitive structure 200 may
be integrated or incorporated with a display of an electronic
device, such as the example described above with respect to FIG. 1.
As shown in FIG. 2A, the force-sensitive structure 200 includes a
substrate 210 having disposed upon it a plurality of independent
force-sensitive components 212. In this example, the substrate 210
may be an optically transparent material, such as polyethylene
terephthalate (PET). The force-sensing components 212 may be made
from transparent conductive materials include, for example,
polyethyleneioxythiophene (PEDOT), carbon nanotubes, graphene,
gallium zinc oxide, indium gallium zinc oxide, other metal oxides,
semiconductor material, silver, nickel, or platinum nanowires,
other metallic nanowires, and the like. In some cases, the
force-sensing components 212 may be formed from a metal oxide,
indium oxide, or indium-tin oxide (ITO) material. As previously
discussed, the ITO may be formed by doping an indium oxide material
with tin (Sn). In some cases, the indium oxide is doped with Sn to
a proportion of less than 5%. In certain embodiments, the
force-sensing components 212 may be selected at least in part on
temperature characteristics. For example, the material selected for
the force-sensing components 212 may have a negative temperature
coefficient of resistance such that, as temperature increases, the
resistance decreases.
[0062] As shown in FIG. 2A, the force-sensing components 212 may be
formed as an array of rectilinear pixel elements, although other
shapes and array patterns could also be used. In many examples,
each individual force-sensing component 212 may have a shape and/or
pattern that depends on the location of the force-sensing component
212 within the array. For example, in some embodiments, the
force-sensing component 212 may be formed as a serpentine pattern
of traces, such as shown in FIG. 2B. The force-sensing component
212 may include at least two electrodes 212a, 212b for connecting
to a sensing circuit. In other cases, the force-sensing component
212 may be electrically connected to sense circuitry without the
use of electrodes. For example, the force-sensing component 212 may
be connected to the sensing circuitry using conductive traces that
are formed as part of the component layer.
[0063] FIG. 3 depicts a cross-sectional view of a portion of a
device taken along section A-A of FIG. 1. In particular, FIG. 3
depicts a cross-sectional view of an example force-sensitive
structure 300 that may be integrated with a display stack and/or a
cover of a device. As depicted in the cross-sectional view, a
substrate 310 has a first and second force-sensing components 311,
312 disposed on opposite sides of the substrate 310. In this
example, the substrate 310 is disposed below a force-receiving
layer 320 as part of the display stack. The force-receiving layer
320 may be formed from of a material such as glass. In some cases,
the force-receiving layer 320 may be formed from a sapphire sheet,
polycarbonate sheet, or other optically-translucent and
structurally rigid material. In some embodiments, the
force-receiving layer 320 also function as a protective cover
element for the device and display stack (e.g., cover 106 of FIG.
1). In some cases, the force-receiving layer 320 may be made from a
material having high strain transmission properties. For example,
the force-receiving layer 320 may be made from a hard or otherwise
rigid material such as glass or metal such that an exerted force
may be effectively transmitted through the force-receiving layer
320 to the layers disposed below. The force-receiving layer 320 may
also be flexible and able to bend or deflect in response to the
force of a touch on the device.
[0064] As shown in FIG. 3, the force-sensitive structure 300
includes a substrate 310 having a first force-sensitive component
311 disposed relative to a first, top side of the substrate 310. As
shown in FIG. 3, a second force-sensitive component 312 is disposed
relative to a second, bottom side of the substrate 310. In this
example, the first and second force-sensitive components 311, 312
are formed on or attached to opposite faces of the substrate 310.
However, in alternative embodiments, the first and second
force-sensitive components may be attached to, or formed on,
multiple substrates or layers that are laminated together in the
stack.
[0065] As shown in FIG. 3, the display stack includes multiple
other layers that are disposed between the force-receiving layer
320 and the substrate 310. In particular, the present configuration
includes a display element 330 and a polarizer layer 324 that are
bonded to a rear face of the force-receiving layer 320 by optically
clear adhesive layer 322. As shown in FIG. 3, a rear polarizer 332
is disposed on a side of the display element 330 that is opposite
to the force-receiving layer 320. The substrate 310 and first
force-sensitive components 311 are attached to the rear polarizer
332 by an optically clear adhesive layer 334. The second
force-sensitive components 312 are disposed relative to a side of
the substrate 310 that is opposite to the first force-sensitive
components 311. Additional structural and/or electrical components
340 may also be disposed below the substrate 310 and the second
force-sensitive components 312. The additional structural and/or
electrical components 340 may include structural supports and/or
shielding to isolate the stack from the other internal electronics
of the device. In some cases, the additional structural and/or
electrical components 340 may include additional sensor or other
electrically active components.
[0066] The display stack of FIG. 3 is provided as one specific
example. However, the number of layers and the composition of the
layers may vary depending on the implementation and the type of
display element that is used. For example, in alternative
embodiments, the substrate 310, the first force-sensitive
components 311 and the second force-sensitive components 312 may be
disposed between the display element 330 and the force-receiving
layer 320. Independent of the particular composition of the display
stack, it may be advantageous that many or all of the layers
disposed between the force-receiving layer 320 and the substrate
310 are substantially rigid or be made from materials having high
strain transmission properties.
[0067] In this example, the force-sensitive components 311, 312 may
be formed as an array of rectilinear pixel elements. For example,
each force-sensitive component 311, 312 may be aligned vertically
(or horizontally) with a respect to a column (or row) of other
force-sensitive components 311, 312. In many examples, each
individual force-sensing component 311, 312 may be formed in a
particular shape or pattern. For example, in certain embodiments,
the force sensing component 311, 312 may be deposited in a
serpentine pattern, similar to the serpentine pattern shown for
force sensing component 212 in FIG. 2B. Other example shapes are
described below with respect to FIGS. 5A-D and 7A.
[0068] The force-sensitive components 311, 312 are typically
connected to sense circuitry 305 that is configured to detect
changes in an electrical property of each of the force-sensitive
components 311, 312. In this example, the sense circuitry 305 may
be configured to detect changes in the resistance of the
force-sensitive component 311, 312 which can be used to estimate a
force that is applied to the device. In some cases, the sense
circuitry 305 may also be configured to provide information about
the location of the touch based on the relative difference in the
change of resistance of the force-sensitive components 311,
312.
[0069] The sensing circuitry 305 may be adapted to determine a
difference between a force experienced by the force-sensitive
component 311 and the force experienced by the force-sensitive
component 312. For example, as described above, a force may be
received at the force-receiving layer 320. As a result of the
rigidity of the force-receiving layer 320, the force received may
be effectively transferred to the substrate 310. Because the
force-sensitive components 311, 312 are affixed to the substrate
310, the force-sensitive components 311, 312 experience the force
as well. Due to the geometry of the display stack, a deflection in
the substrate 310 may result in a lower strain in the first
force-sensitive components 311 as compared to the strain in the
second force-sensitive components 312. Additionally, the first
force-sensitive components 311 may be placed in a compressive
strain mode as compared to the second force-sensitive components
312, which may be placed in a tensile strain mode.
[0070] In some embodiments, the sensing circuitry 305 may be
configured to detect the relative difference in the output of the
first and second force-sensitive components 311, 312 to reduce or
eliminate the effects of temperature and thermal expansion. In
particular, due to their proximity to the same substrate 310, the
first and second force-sensitive components 311, 312 may be
subjected to similar thermal conditions. In some cases, the first
and second force-sensitive components 311, 312 may be at
approximately the same temperature and subjected to substantially
the same thermal expansion conditions. However, when a force is
applied to the force-receiving layer 320, the relative strain
between the first and second force-sensitive components 311, 312
may differ. As discussed above, the amount of strain experienced by
the second force-sensitive components 312 may be greater than the
strain experienced by the first force-sensitive components 311.
Additionally the mode of the strain (compressive vs. tensile) will
be different between the first and second force-sensitive
components 311, 312. In some configurations, the sensing circuitry
305 may be configured to detect the relative difference in the
strain in the first and second force-sensitive components 311, 312
and reduce the impact of thermal and other mechanical influences on
a force measurement.
[0071] In one example embodiment, the sensing circuitry 305 may be
configured to measure the relative change in resistance between the
first and second force-sensitive components 311, 312. In
particular, the first and second force-sensitive components 311,
312 may be resistive elements electrically connected as a voltage
divider. In certain examples the first force-sensitive component
311 may be configured as the ground-connected resistor R.sub.ground
of the voltage divider and the second force-sensitive component 312
may be configured as the supply-connected resistor R.sub.supply of
the voltage divider. In accordance with some embodiments, the
voltage V.sub.out at the midpoint of the first force-sensitive
component 312 and the second force-sensitive component 312 may be
calculated by multiplying the supply voltage V.sub.supply by the
ratio of the ground-connected resistor to the total resistance
(i.e., supply-connected resistor R.sub.supply summed with the
ground-connected resistor R.sub.ground). In other words, the
voltage at the midpoint of the voltage divider, V.sub.out may be
found, in a simplified example, by using the equation:
V out = V supply ( R ground R ground + R supply ) . Equation 1
##EQU00001##
[0072] Due to fact that the resistance of resistive elements
R.sub.ground and R.sub.supply (or first force-sensitive component
311 and second force-sensitive component 312, respectively) changes
in response to force and in response to temperature, the resistance
of either element may be calculated as a function of both force
(i.e., strain) and as a function of temperature, using as a
simplified example, the equation:
R.sub.measured.apprxeq.R.sub.baseline(1+.alpha.(T.sub.actual-T.sub.basel-
ine))(1+g.epsilon..sub.applied), Equation 2
where R.sub.baseline a baseline reference resistance, .alpha. is
the temperature coefficient of resistance, g is the strain
coefficient of resistance, and .epsilon..sub.applied is the strain
applied to the structure. The approximation described by Equation 2
illustrates that the base resistance R.sub.baseline of either
R.sub.ground and R.sub.supply may be altered by both the
temperature and the strain applied to the material. In some cases,
the effects of temperature changes may be approximated by the
product of the temperature coefficient of resistance a of the
material selected for the force-sensitive component, and the
difference between the actual temperature T.sub.actual of the
element and a baseline temperature T baseline. Similarly, the
effect of strain may be approximated by the product of the strain
coefficient of resistance g and the strain applied
.epsilon..sub.applied to the element.
[0073] By combining Equation 2 and Equation 1 and entering the
known quantities V.sub.supply, R.sub.baseline, .alpha., and g and
measured quantities V.sub.out, the strain applied to each element
.epsilon..sub.211 and .epsilon..sub.212 and the actual temperature
of each element T.sub.211 and T.sub.212 are the only remaining
unknown variables, which may be further simplified as a difference
in strain .DELTA..epsilon. between the force-sensitive components
311, 312 and a difference in temperature .DELTA.T between the
force-sensitive components 311, 312. As discussed previously, the
substrate 310 may be sufficiently thermally conductive to
substantially normalize or the temperature between the
force-sensitive components 311, 312. Thus, in some cases, the
difference in temperature .DELTA.T may be practically approximated
as zero. Additionally, the strain relationship between the
force-sensitive components 311, 312 may be known due to the
physical constraints of the display stack and the substrate 310.
For example, knowing the thickness of the substrate 310 and one or
more boundary conditions, a strain relationship between the
force-sensitive components 311, 312 may be determined. Thus, strain
.epsilon..sub.211 may be expressed in terms of strain
.epsilon..sub.212 or, alternatively, strain .epsilon..sub.212 may
be expressed in terms of strain .epsilon..sub.211. Accordingly,
.epsilon..sub.211 or (.epsilon..sub.212) may be solved for and
passed to an electronic device or used to calculate an estimate of
the applied force. As discussed previously, the force measurement
or estimate may be used as a user input for the electronic
device.
[0074] In practice, a piezoresistive elements of force-sensitive
components 311, 312 may be effected as the temperature of the
sensor, device, or environment changes. As discussed above, the
electrical properties of the force-sensitive components 311, 312
(e.g., resistance) may change substantially with changes in
temperature. Additionally, in some examples, the electrical
properties of force-sensitive components 311, 312 may also be
impacted by the coefficient of thermal expansion ("CTE") as a
result of temperature of the device. Similarly, one may appreciate
that the strain-sensitive material of the force-sensitive
components 311, 312 may be subject to the changes in resistance due
to the changes in temperature. Such changes may be referred to as
changes resulting from the thermal coefficient of resistance
("TCR") of the material selected for force-sensitive components
311, 312. In this manner, the electrical properties of the
force-sensitive components 311, 312 may be modeled as the sum of
the pyroelectric effect, the CTE effect, the TCR effect, and the
effect of any strain as a result of a force applied by a user.
Thus, the strain measured directly from the force-sensitive
components 311, 312 may be approximated, in one example, as a sum
of three components.
.epsilon..sub.measured.apprxeq..epsilon..sub.user+.epsilon..sub.pyro+.ep-
silon..sub.CTE+.epsilon..sub.TCR, Equation 3
where strain .epsilon..sub.user is due to the force of a touch,
strain .epsilon..sub.pyro is due to pyroelectric effects of the
temperature of the sensor, strain .epsilon..sub.CTE is strain
caused by thermal expansion, and apparent strain .epsilon..sub.TCR
is strain due to the TCR effect. However, as discussed above, the
force-sensitive components 311, 312 may be subjected to similar
thermal and physical conditions, and therefore the strain due to
effects other than the force of a touch (.epsilon..sub.user) may be
accounted for by calculating a force measurement based on the
relative values of the force-sensitive components 311, 312. For
example, the strain .epsilon..sub.user may be estimated or
approximated using a measurement across the force-sensitive
components 311, 312 and a half bridge sensing configuration.
Accordingly, in order to facilitate measuring the force applied by
the user, the pyroelectric effect and the CTE effect may be
reduced, cancelled, eliminated, or otherwise compensated.
[0075] In some cases, the sensor circuitry 305 may be configured to
reduce or compensate for effects due to other components or
electrical influences. For example, in some instances, one or more
of the force-sensitive components 311, 312 may electrically couple
to one or more other components in the device. With respect to the
example depicted in FIG. 3, in some cases, the second
force-sensitive component 312 may capacitively couple to the
additional structural and/or electrical components 340 disposed
below the second force-sensitive components. In one example, the
additional structural and/or electrical components 340 include a
charged conductive element that may capacitively couple to one or
more of the second force-sensitive components 312. As a result, the
second force-sensitive components 312 may carry a residual positive
or electrostatic charge caused by the capacitive coupling to one or
more other components. In some cases, this positive charge may
affect the strain-based measurement. Therefore, in some cases, the
sensor circuitry 305 may be configured to apply a voltage bias to
compensate for the positive charge on one or more of the second
force-sensitive components 312. In one example implementation, the
second force-sensitive components 312 are virtually grounded or
held to a reference voltage to reduce or eliminate the effects of a
positive (or negative) charge caused by other electrical components
in the device.
[0076] In some cases, the sensor circuitry 305 may be configured to
selectively sample the electrical response of the force-sensitive
components 311, 312. For example, the touch of a user's finger may
cause a momentary spike in the electrical output of the
force-sensitive components 311, 312. The spike may be due to a
sudden induced charge or discharge caused by the touch and/or
initial deflection of the substrate 310. Similarly, a spike in the
output may be created when the touch is removed and/or the
substrate 310 is returned to an un-deflected state. Thus, in some
implementations, the sensor circuitry 305 may discard or ignore
electrical output of the force-sensitive components 311, 312 at or
near a time period associated with an initial touch and/or a time
period associated with a removal of a touch. In some cases, the
sensor circuitry 305 may be configured to detect a spike by
comparing the output to an average or normalized output. The
detected spike or spikes may then be discarded or ignored by the
sensor circuitry 305 when estimating the applied force.
[0077] In some cases, the sensor circuitry 305 may be configured to
provide a bias for the electrical response of the force-sensitive
components 311, 312. For example, a user's touch may cause a
momentary spike in the electrical output of the force-sensitive
components 311, 312. The spike may be due to a induced charge or
discharge caused by the initial deflection of the substrate 310.
Similarly, a spike in the output may be created when the substrate
310 is returned to an un-deflected state. Thus, in some
implementations, the sensor circuitry 305 may be biased to a common
mode voltage, which may substantially reduce the induced charge and
voltage change due to deflection of the substrate up or down. For
example, the sensor circuitry 305 may include a voltage divider
bridge and voltage on a bridge-connected pair of strain elements
may be positive with respect to an element below the sensor. In
this case, a positive charge may be induced on the sensor, and if
the sensor is deflected downward, the voltage measured by the
sensor is reduced for a time constant depending on its resistance
and capacitances to other elements. Similarly, if the bridge is
biased sufficiently negative with respect to an element below the
sensor, an negative charge is induced on the sensor, and the
opposite occurs. In some implementations, the sensor may be biased
in between a spikes created by both deflection and un-deflection to
reduce the effect of the induced charge and voltage transient due
to deflections.
[0078] In some cases, the sensor circuitry 305 may also be
configured to selectively scan the force-sensitive components 311,
312 to improve the sensitivity or responsiveness of the sensor. For
example, the sensor circuitry 305 may be configured to receive
location information associated with the touch location on the
touch on the force-receiving layer 320. In some cases, the location
information may be provided by a separate touch sensor. In some
cases, the location information may be provided by the force sensor
itself. The location information may be used to increase the number
of measurements or sensor reading events for a selected subset of
force-sensitive components 311, 312 that correspond to the touch
location. For example, if the sensor circuitry 305 is configured to
measure the force-sensitive components 311, 312 using a repeating
scan sequence, the number of force-sensitive components 311, 312
may be reduced to a selected subset that may correspond to the
location of a touch. By reducing the number of force-sensitive
components that are scanned, scan rate may be increased thereby
increasing the number of measurements that may be read for the
selected subset. The remaining force-sensitive components 311, 312
may also be periodically scanned, but at a rate that is lower or
reduced with respect to the selected subset. By increasing the
number of measurements for force-sensitive components that
correspond to a touch location, the overall responsiveness and
accuracy of the sensor may be improved.
[0079] In some cases, the force-sensitive structure 300 may be
configured to reduce or eliminate the effects of temperature on an
interconnect formed from dissimilar materials. For example, in some
cases the force-sensitive components 311, 312 may be electrically
connected to the sensor circuitry 305 via one or more electrical
interconnects. In one particular configuration, the first
force-sensitive components 311 are connected to the sensor
circuitry via a first interconnect and the second force-sensitive
components 312 are connected to the sensor circuitry via a second
interconnect. In some cases, the first and second interconnects
include the electrical connection of two or more dissimilar
materials. For example, the interconnects may be formed from an ITO
to aluminum electrical connection. In some cases, the interconnects
may be formed from an ITO to silver paste to copper electrical
connection. Generally, connecting two dissimilar material may
generate a temperature-dependent voltage, which may affect the
force measurement. Therefore, in some instances, it may be
advantageous that the first interconnect and the second
interconnect be formed from the same set of dissimilar materials,
and the junctions placed in proximity, to cancel or reduce the
effects of temperature. In some instances, the sensor circuitry 305
may also be configured to compensate for the effects of a
temperature-dependent voltage on the force measurement. For
example, the sensor circuitry may be configured to measure or
estimate the junction temperatures and perform a compensation
calculation. The compensation calculation may account for
dissimilar materials used in the interconnect and associated
junctions.
[0080] FIG. 4 depicts a top view of an alternate example of a
force-sensitive structure 400 including two angularly-offset
layers, each including multiple optically transparent
force-sensitive components 412, 422. One of the layers may be
arranged as a number of rows while the other is arranged as a
number of columns. Further, one of the sets of rows and/or columns
may be electrically driven while the other set is a sense layer. As
noted with respect to FIG. 2A, other suitable configurations of
transparent force-sensitive components are contemplated. For
example, the angular offset between the layer 412, 422 may be
perpendicular in certain embodiments and a different angle in other
embodiments.
[0081] FIGS. 5A-5C depict a top detailed view of various optically
transparent serpentine geometries for a force-sensitive component
which may be used for either or both of the example force-sensitive
structures depicted in FIG. 3 (311, 312). In the present example,
the force sensing component 512 may include at least two electrodes
512a, 512b for connecting to a sensing circuit. However, in some
cases, the force sensing component 512 may be electrically
connected to sense circuitry without the use of electrodes. For
example, the force sensing component 512 may be connected to the
sense circuitry using conductive traces that are formed as part of
the component layer.
[0082] FIG. 5A depicts a top view of a serpentine geometry which is
sensitive to strain along the Y-axis. In this manner, when the
force-sensing component 512 is strained along the X-axis direction,
the force-sensing component 512 may not experience substantial
tension or strain response. Conversely, when the force-sensing
component 512 is strained along the Y-axis direction, a strain may
be detected and measured. One may appreciate that angular strain
(e.g., strain along a 45 degree path) may strain the force-sensing
component 512 in some amount that may be detected and measured.
Similarly, FIG. 5B depicts a top view of a serpentine geometry
which is sensitive to strain along the X-axis, and may not be
particularly sensitive to strain along the Y-axis. FIG. 5C depicts
a top view of a serpentine geometry which may be sensitive to
strain along both the X and Y axis.
[0083] FIG. 5D depicts a top view of a serpentine geometry which
may be sensitive to strain along a 45 degree angle. One may
appreciate that although shown at 45 degrees, that any angle or
combination of angles may be employed. For example, one embodiment
may include angling a strain sensor 512 along an 80 degree angle.
Another embodiment may include a strain sensor having multiple
distinct portions similar to FIG. 5C, in which one portion is
angled at 25 degrees and another portion is angled at 75 degrees.
In many embodiments, the angle or combination of angles of
orientation for different force-sensitive components may be
selected, at least in part based on the location of the particular
force-sensitive component along the surface of an electronic
device.
[0084] For example, FIG. 6 depicts a top view of an example
force-sensitive structure including an array or grid of optically
transparent force-sensitive components 612a-c oriented to detect
force in different directions. For example, force-sensitive
component 612a may be oriented to detect strain along a 45 degree
angle, whereas force-sensitive component 612b may be oriented to
detect strain along a -45 degree angle. In another examples,
force-sensitive component 612c may be adapted to detect along an
arbitrary angle between 0 and 45 degrees. The force-sensitive
components 612a-c may be used to form a force sensor in accordance
with one or more of the embodiments described herein. In
particular, an array or grid of force-sensitive components similar
to those depicted in FIG. 6 may be used for either or both of the
force-sensitive components 311, 312 discussed above with respect to
FIG. 3.
[0085] In certain embodiments, the material of the force-sensitive
components may be sensitive to strain in the plane of the sensor
along two or more primary axes by producing an output having the
same polarity. For example, a force-sensitive component made of ITO
or other metal oxides, or polysilicon, may respond to strain in
both the primary axes with a charge having the same sign, and thus
may be sensitive to the sum of the net strains. In some cases, the
force-sensitive structures may include traces arranged in
serpentine pattern that be configured to spatially sample and
average strain produced along a particular direction, and/or be
configured to have a resistance within a desired range.
[0086] In certain embodiments, the force-sensitive components may
have a resistance that is configured to maximize the signal to
noise ratio with respect to the power and voltage that is available
on the device. In some cases, it may also be desirable to use
high-resistivity material in order to improve the optical
properties of transparent conductors. The force-sensitive
components may be configured to balance or satisfy these
constraints. For example, a force-sensitive component may be formed
from an ITO or other metal oxides, or polysilicon material having
in a geometry to provide a specific resistance. In some cases, the
serpentine may be configured to spatially sample and average the
desired strains. In some cases, the serpentine and conductive
material may be configured to provide a resistance or electrical
response that falls within a range that satisfies constraints on
the response time of the sensor, the optical properties of the
sensor, and the noise contributed by the sensor and interfaces to
the material of the sensor. In some cases, the desired resistance
of the force-sensitive component may be in the range of 20,000 ohms
to 200,000 ohms, which may be higher than the typical strain gauge
sensor for other applications.
[0087] In certain embodiments, the orientation of force-sensitive
components may be selected based on the position of the
force-sensitive component relative to the housing of an electronic
device. For example, a force-sensitive component positioned
proximate to the edge of a screen within a display stack may be
oriented differently from a force-sensitive component positioned in
the center of the display.
[0088] In some embodiments, as shown in FIG. 6, the grid may be
formed from an array of components that includes a subset of edge
force-sensitive components 612c positioned along an edge of the
first array. In some cases, the edge force-sensitive components
612c are formed from traces that are oriented along a direction
that is substantially perpendicular to the edge. As shown in FIG.
6, the array of force-sensitive components may include a subset of
corner force-sensitive components 612a, 612b positioned at corners
of the array or grid. In some cases, the corner force-sensitive
components 612a, 612b are formed from traces that are oriented
along a diagonal direction.
[0089] FIG. 7A depicts another example force-sensitive component
712 that can be used in a force sensor. As shown in FIG. 7A, the
force-sensitive component 712 includes traces that are
substantially oriented in two primary directions. In particular, a
first portion 712a of the force-sensitive component includes traces
that are substantially oriented along a y-direction and a second
portion 712b includes traces that are substantially oriented along
an x-direction. In this particular example, two first portions 712a
are arranged adjacent to two second portions 712b to form an
alternating pattern. This configuration may be advantageous in
sensing strain in both the x- and y-directions. This configuration
may be further advantageous by increasing the resolution or
sensitivity of each force-sensitive component, as compared, for
example to an arrangement having traces aligned along a single
direction.
[0090] FIG. 7B depicts a top view of an example force-sensitive
structure 700 formed from an array or grid of force-sensitive
structures 712. The force-sensitive structure 700 having an
arrangement of force-sensitive components 712 may be used to form a
force sensor in accordance with one or more of the embodiments
described herein. In particular, an array or grid of
force-sensitive components 712 as depicted in FIG. 7B may be used
for either or both of the force-sensitive components 311, 312
discussed above with respect to FIG. 3.
[0091] FIG. 8 depicts a simplified signal flow diagram of a
temperature-compensating and optically transparent force sensor in
the form of a Wheatstone bridge. In such an embodiment, a voltage
Vg may be measured across the output of two parallel voltage
dividers connected to a voltage supply Vs. One of the voltage
dividers may include two resistors of known resistance R.sub.3,
R.sub.4 and the other voltage divider may include two variable
resistors R.sub.312, R.sub.322 that model the force and temperature
variable resistance of the force-sensitive components 311, 312 as
shown, for example in FIG. 3. By combining, for example, Equation 2
and Equation 1 after entering the known quantities V.sub.supply,
R.sub.baseline, .alpha., g, R.sub.3, and R.sub.4 and measured
quantities V.sub.out, the strain .epsilon..sub.212 applied to the
force-sensitive component 312 may be the only remaining unknown and
accordingly may be determined using, for example, the circuit
depicted in FIG. 8 and potentially other electronics or processors.
The output of the circuit depicted in FIG. 8 may be passed to
another component of an electronic device and/or used to compute a
force measurement.
[0092] FIG. 9 is a process flow diagram illustrating example
operations of a sample process 900 of manufacturing a
temperature-compensating and optically transparent force sensor.
The process 900 may begin at operation 902 in which a substrate may
be selected or obtained. The substrate may include, for example,
glass or transparent polymers like polyethylene terephthalate (PET)
or cyclo-olefin polymer (COP). After the substrate is obtained, a
force-sensitive component may be applied thereto at operation 904.
In one example, an array of force-sensitive components or pixels
are formed on a first surface of the substrate. The force-sensitive
components may be formed from a transparent conductive material,
including, for example, polyethyleneioxythiophene (PEDOT), metal
oxide, indium oxide, indium-tin oxide (ITO), carbon nanotubes,
graphene, semiconductor material, silver nanowire, other metallic
nanowires, and the like. In some cases, the thickness of the
transparent conductive material may be substantially increased if
the substrate is formed from a glass material, which may tolerate
higher temperature manufacturing processes. For example, a
transparent conductive material that is grown at higher
temperatures may exhibit lower sheet resistance and may be used to
form a more sensitive or robust force sensor. During manufacturing,
higher thermal conductivity substrates, such as glass, may allow
for use of thicker substrates while maintaining a substantially
equal temperatures on both sensor planes. For example, in some
embodiments, the substrate may have a thermal conductivity of
greater than 0.5 Watts per square meter per degree Kelvin. In some
cases, the force-sensitive component is formed directly onto the
substrate. In other cases, the force-sensitive component is formed
on another material or substrate, which is bonded or otherwise
attached to the substrate.
[0093] In operation 906 another (second) force-sensitive component
may be applied to a second surface of the substrate. In some cases,
the second surface is opposite to the first surface. The second
force-sensitive component may be formed from the same or a similar
transparent conductive material as used in operation 904. One
advantage to using the same transparent conductive material is that
effects due to temperature and thermal conditions may be
compensated, reduced, or eliminated using, for example, some of the
techniques described herein. Similar to operation 904, the second
force-sensitive component may be formed directly onto the
substrate. In other cases, the second force-sensitive component may
be formed on another material or substrate, which is bonded or
otherwise attached to the substrate. In general, it should be
appreciated that the order of operations may vary between
embodiments.
[0094] FIG. 10 is a process flow diagram depicting example
operations for a process 1000 of operating a
temperature-compensating force sensor. Process 1000 may be used,
for example, to operate one or more of the force sensors described
with respect to FIG. 3, above. In particular, process 1000 may be
used to compute or estimate the force of a touch on a device and
compensate for variations or effects of temperature.
[0095] In operation 1002, an occurrence of a user touch may be
detected. The touch may be detected, for example using a touch
sensor. The touch sensor may include, for example, a
self-capacitive, mutually capacitive, resistive, or other type of
touch sensor. In some embodiments, the occurrence of a touch may be
detected by the force sensor. For example, a change in strain or
resistance or strain of one or more force-sensitive structures of
the sensor may be used to detect the occurrence of a touch. In some
embodiments, operation 1002 is not necessary. For example, the
other operations of process 1000 may be performed on a regularly
repeating or irregular interval without first determining if a
touch is present. For example, process 1000 may be performed and
calculate or estimate a zero applied force, which may be due to the
absence or lack of a touch on the device.
[0096] In operation 1004, a relative measurement between two or
more force-sensitive structure may be obtained. As described
previously with respect to, for example, FIGS. 3 and 8 a relative
measurement may be obtained using a voltage divider, half bridge,
full bridge, or other similar circuit configuration. In some
embodiments, an electrical measurement of each individual
force-sensitive structure is obtained and the measurements are
compared using software, firmware, or combination of
software/firmware and circuit hardware.
[0097] In operation 1006, a force estimate may be computed. In some
embodiments, the force estimate compensates for variations in
thermal effects, including, for example a pyroelectric effect, TCR
effect, and/or CTE effect, as described above with respect to
Equation 3. In particular, the relative measurement obtained in
operation 1004 may be used in combination with Equations 1 and 2 to
compute an estimated strain. The estimated strain may then be used
to estimate an applied force using, for example, a known
correlation between the strain of the corresponding force-sensitive
structure and an applied force. For example, the strain may
correspond to an estimated deflection of the substrate (and other
relevant layers of the display/sensor stack), which may correspond
to a respective force on a surface of the device.
[0098] FIG. 11 is an additional process flow diagram illustrating
example steps of a process 1100 for operating a
temperature-compensating force sensor. In operation 1102, a
location of a user touch may be identified. The location of a user
touch may be determined, using for example a self-capacitive touch
sensor, a mutually capacitive touch sensor, a resistive touch
sensor, and the like.
[0099] In operation 1104 a relative measurement between two or more
force-sensitive structure may be obtained. As described previously
with respect to, for example, FIGS. 3 and 8 a relative measurement
may be obtained using a voltage divider, half bridge, full bridge,
or other similar circuit configuration. In some embodiments, an
electrical measurement of each individual force-sensitive structure
is obtained and the measurements are compared using software,
firmware, or combination of software/firmware and circuit
hardware.
[0100] In operation 1106, a force centroid is calculated. For
example, the relative measurement obtained in operation 1104 may be
used to approximate the centroid of the applied force at 1106. In
some embodiments, the location of the user touch obtained in
operation 1102 may be used to approximate the centroid of the
applied force. In some embodiments, the geometric centroid of all
touches of a multi-touch event may be used to approximate the
centroid of the applied force. Thereafter, the measured force and
the force centroid may be forwarded or otherwise relayed to the
electronic device in operation 1108.
[0101] One may appreciate that although many embodiments are
disclosed above with respect to optically transparent force
sensors, that the systems and methods described herein may apply
equally well to opaque force sensors or force sensors that are not
required to be transparent. For example, the force sensors
described herein may be included below a display stack, or within
the housing of a device. For example, an electronic device may be
adapted to react to a user squeezing or applying pressure to a
housing of an electronic device. Such a force sensor need not, in
all embodiments, be transparent. Still further embodiments may
include a force sensor that is translucent. For example, a force
sensor component may be doped with an ink such that the force
sensor appears as a particular color or set of colors. In still
further embodiments, the force sensor may be optionally
transparent, translucent or opaque.
[0102] Embodiments described herein may be formed in any number of
suitable manufacturing processes. For example, in one embodiment, a
force-sensitive structure may be formed in a roll-to-roll process
which may include depositing a force-sensitive material in a
selected pattern on a substrate, bonding said substrate to one or
more additional layers or components of an electronic device, and
singulating the output of the roll-to-roll process into a plurality
of independent force-sensitive structures.
[0103] One may appreciate that although many embodiments are
disclosed above, that the operations and steps presented with
respect to methods and techniques described herein are meant as
exemplary and accordingly are not exhaustive. One may further
appreciate that alternate step order or, fewer or additional steps
may be required or desired for particular embodiments.
[0104] Although the disclosure above is described in terms of
various exemplary embodiments and implementations, it should be
understood that the various features, aspects and functionality
described in one or more of the individual embodiments are not
limited in their applicability to the particular embodiment with
which they are described, but instead can be applied, alone or in
various combinations, to one or more of the other embodiments of
the invention, whether or not such embodiments are described and
whether or not such features are presented as being a part of a
described embodiment. Thus, the breadth and scope of the present
invention should not be limited by any of the above-described
exemplary embodiments but is instead defined by the claims herein
presented.
* * * * *