U.S. patent application number 09/835040 was filed with the patent office on 2002-10-17 for method and apparatus for force-based touch input.
Invention is credited to Roberts, Jerry B..
Application Number | 20020149571 09/835040 |
Document ID | / |
Family ID | 25268422 |
Filed Date | 2002-10-17 |
United States Patent
Application |
20020149571 |
Kind Code |
A1 |
Roberts, Jerry B. |
October 17, 2002 |
Method and apparatus for force-based touch input
Abstract
A force sensor for sensing a touch force applied to a touch
surface is disclosed. The force sensor includes: a first element
including an elastic element and a first capacitor plate having a
first capacitive surface, the elastic element including at least
part of the first capacitor plate; and a second element including a
second capacitor plate opposed to the first capacitor plate;
wherein transmission of at least part of the touch force through
the elastic element contributes to a change in capacitance between
the first capacitor plate and the second capacitor plate. The
elastic element and the first capacitor plate may be integral.
Other force sensors and methods for manufacturing said force
sensors are also disclosed. Touch location devices suitable for use
with the disclosed force sensors are also disclosed.
Inventors: |
Roberts, Jerry B.;
(Arlington, MA) |
Correspondence
Address: |
William D. Miller, Esq.
3M Office of Intellectual Property Counsel
P.O. Box 33427
St. Paul
MN
55133-3427
US
|
Family ID: |
25268422 |
Appl. No.: |
09/835040 |
Filed: |
April 13, 2001 |
Current U.S.
Class: |
345/174 ;
345/173 |
Current CPC
Class: |
G06F 3/0447 20190501;
G06F 3/04142 20190501 |
Class at
Publication: |
345/174 ;
345/173 |
International
Class: |
G09G 005/00 |
Claims
What is claimed is:
1. A force sensor for sensing a touch force applied to a touch
surface, the force sensor comprising: a first element including an
elastic element and a first capacitor plate having a first
capacitive surface, the elastic element including at least part of
the first capacitor plate; and a second element including a second
capacitor plate opposed to the first capacitor plate; wherein
transmission of at least part of the touch force through the
elastic element contributes to a change in capacitance between the
first capacitor plate and the second capacitor plate.
2. The force sensor of claim 1, wherein the first element is
substantially planar.
3. The force sensor of claim 1, wherein the first capacitor plate
and the elastic element are integral.
4. The force sensor of claim 3, wherein the first capacitor plate
and the elastic element are composed of the same substrate.
5. The force sensor of claim 3, wherein the elastic element
comprises an elevated feature of the first capacitor plate.
6. The force sensor of claim 5, wherein the elevated feature is
located at the elastic center of the first element.
7. The force sensor of claim 1, further comprising force-receiving
means for receiving at least part of the touch force into the first
element.
8. The force sensor of claim 7, wherein the force-receiving means
comprises the elastic element.
9. The force sensor of claim 7, wherein the force-receiving means
comprises a feature formed into the first element.
10. The force sensor of claim 9, wherein the force-receiving means
comprises an elevated feature of the first capacitor plate.
11. The force sensor of claim 7, wherein the touch surface is in
communication with a region of a surface of the force-receiving
means, and wherein the touch surface tends to remain in contact
with the region of the surface of the force-receiving means when
the position of the touch surface changes with respect to the
force-receiving means.
12. The force sensor of claim 1, further comprising force
transmission means for transmitting at least part of the touch
force to at least one structure other than the first element.
13. The force sensor of claim 1: wherein the second element
comprises a planar support surface that includes a plurality of
electrically conductive mechanical bearing contacts; and wherein at
least portions of the first capacitor plate are in contact with the
plurality of mechanical bearing contacts to transmit force
thereto.
14. The force sensor of claim 13, wherein the second capacitor
plate includes a second capacitive surface that is coplanar with
the plurality of mechanical bearing contacts.
15. The force sensor of claim 14, wherein the second capacitive
surface and the plurality of mechanical bearing contacts are
composed of the same substrate.
16. The force sensor of claim 13, wherein the planar support
surface is part of an interconnect system to transmit a signal
developed in response to the change in capacitance between the
first capacitor plate and the second capacitor plate.
17. The force sensor of claim 1, wherein the first and second
capacitor plates are separated by a volume, and wherein the ratio
of the height of the volume to the volume's greatest breadth is
less than 0.05.
18. The force sensor of claim 1, further comprising: force signal
development means for developing a signal in response to the change
in capacitance between the first capacitor plate and the second
capacitor plate.
19. The force sensor of claim 1, wherein the force sensor includes
an axis of sensitivity that passes through the elastic center of
the elastic element.
20. The force sensor of claim 1, further comprising: the touch
surface, wherein the touch surface is a touch surface of a handheld
device.
21. The force sensor of claim 1, wherein the second capacitor plate
is separated by a capacitive gap from the first capacitor plate,
the length of the mechanical path defining the capacitive gap being
no greater than one-fifth of the maximum distance between any two
force sensors that are used in the touch location device to measure
the touch force.
22. A force sensor for sensing a touch force applied to a touch
surface, the force sensor comprising: a first substantially planar
element comprising: a first capacitor plate having a first
capacitive surface; and an elastic element comprising an integral
elevated feature of the first capacitor plate, the elastic element
receiving at least part of the touch force into the first element;
and a second element including a second capacitor plate opposed to
the first capacitor plate; wherein transmission of at least part of
the touch force through the elastic element contributes to a change
in capacitance between the first capacitor plate and the second
capacitor plate.
23. A force sensor for sensing a touch force applied to a touch
surface, the force sensor comprising: a first element including an
elastic element and a first capacitor plate including a first
capacitive surface, the elastic element and the first capacitive
surface being substantially coplanar; a second element including a
second capacitor plate; wherein transmission of at least part of
the touch force through the elastic element contributes to a change
in capacitance between the first capacitor plate and the second
capacitor plate.
24. The force sensor of claim 23, wherein the first capacitor plate
and the elastic element are integral.
25. The force sensor of claim 23, wherein the elastic element is
produced by forming an elevated feature into the first capacitor
plate.
26. The force sensor of claim 23, wherein the first and second
capacitor plates are separated by a volume, the ratio of the height
of the volume to the volume's greatest breadth being less than
0.05.
27. A force sensor for sensing a touch force applied to a touch
surface, the force sensor comprising: a first element including an
elastic element, a first capacitor plate including a first
capacitive surface, force-receiving means for receiving at least
part of the touch force into the first element, force-transmitting
means for transmitting at least part of the touch force to
structures not including the first element; a second element
including a second capacitor plate; and wherein transmission of at
least part of the touch force through the elastic element
contributes to a change in capacitance between the first capacitor
plate and the second capacitor plate; and wherein the smallest
rectangular parallelepiped that encloses the first capacitive
surface, the elastic element, and the second capacitor plate has a
greatest dimension that is at least five times its least
dimension.
28. The force sensor of claim 27, wherein the elastic element
comprises the force-receiving means.
29. The force sensor of claim 27, wherein the elastic element and
the first capacitor plate are integral.
30. The force sensor of claim 27, wherein the second element
comprises a planar support surface that includes a plurality of
electrically conductive mechanical bearing contacts; wherein the
second capacitor plate includes a second capacitive surface that is
coplanar with the plurality of mechanical bearing contacts; and
wherein at least portions of the first capacitor plate are in
contact with the plurality of mechanical bearing contacts to
transmit force thereto.
31. The force sensor of claim 30, wherein the planar support
surface is part of an interconnect system to transmit a signal
developed in response to the change in capacitance between the
first capacitor plate and the second capacitor plate.
32. A force sensor for sensing a touch force applied to a touch
surface, the force sensor comprising: a first element including a
first capacitor plate including a first capacitive surface; a
second element including a second capacitor plate having a second
capacitive surface, at least a portion of the first element being
in contact with at least one support region of the second element
to transmit force thereto, the second capacitive surface being
substantially coplanar with the at least one support region; and
wherein transmission of at least part of the touch force to the
first element contributes to a change in capacitance between the
first capacitor plate and the second capacitor plate.
33. The force sensor of claim 32, wherein the at least one support
region is part of an interconnect system to transmit a signal
developed in response to the change in capacitance between the
first capacitor plate and the second capacitor plate.
34. A force sensor for sensing a touch force applied to a touch
surface, the force sensor comprising: a first element including a
first capacitor plate including a first capacitive surface; a
second element including a second capacitor plate, the second
element being part of an interconnect system to transmit a signal
developed in response to the change in capacitance between the
first capacitor plate and the second capacitor plate, at least a
portion of the first element being in contact with at least one
support region of the second element to transmit force thereto;
wherein transmission of at least part of the touch force to the
first element contributes to a change in capacitance between the
first capacitor plate and the second capacitor plate.
35. The force sensor of claim 34, wherein the second capacitive
surface and the at least one support surface are integral.
36. A force sensor for sensing a touch force applied to a touch
surface, the force sensor comprising: a first element including a
first capacitor plate including a first capacitive surface; a
second element including a second capacitor plate separated by a
capacitive gap from the first capacitor plate, the length of the
mechanical path defining the capacitive gap being no greater than
four times the maximum dimension of the volume of the capacitive
gap; wherein transmission of at least part of the touch force to
the first element contributes to a change in capacitance between
the first capacitor plate and the second capacitor plate.
37. The force sensor of claim 36, wherein the second capacitor
plate is separated from the first capacitor plate in the unloaded
state of the force sensor by not more than 10 mils.
38. A force sensor for sensing a touch force applied to a touch
surface, the force sensor comprising: a first element including a
first capacitor plate including a first capacitive surface; a
second element including a second capacitor plate separated by a
capacitive gap from the first capacitor plate, the aggregate normal
component of the mechanical path defining the capacitive gap being
no greater than twice the size of the capacitive gap; wherein
transmission of at least part of the touch force to the first
element contributes to a change in capacitance between the first
capacitor plate and the second capacitor plate.
39. The force sensor of claim 38, wherein the average width of the
capacitive gap in an unloaded state of the force sensor is not less
than thirty times the average height of the capacitive gap in the
unloaded state of the force sensor.
40. A force sensor for sensing a touch force applied to a touch
surface, the force sensor comprising: a first element including
force-receiving means for receiving at least part of the touch
force into the first element and a first capacitor plate including
a first capacitive surface; a second element including a second
capacitor plate separated by a capacitive gap from the first
capacitor plate, wherein the average width of the capacitive gap in
an unloaded state of the force sensor is not less than thirty times
the average height of the capacitive gap in the unloaded state of
the force sensor; wherein transmission of at least part of the
touch force to the first element contributes to a change in
capacitance between the first capacitor plate and the second
capacitor plate.
41. A force sensor for sensing a touch force applied to a touch
surface, the force sensor comprising: a first element including an
elastic element, and a first capacitor plate including a first
capacitive surface; and a second element including a second
capacitor plate; wherein transmission of at least part of the touch
force through the elastic element contributes to a change in
capacitance between the first capacitor plate and the second
capacitor plate; and wherein the force sensor has a normal
stiffness not less than 0.5 pounds per mil.
42. A force sensing touch location device comprising: a touch
surface; a bezel enclosing a first portion of the touch surface;
and force transmission means including an enclosing portion
enclosing a second portion of the touch surface, said force
transmission means having a stiffness greater than that of the
bezel, wherein the force transmission means includes a path to
transmit force from the bezel to a region not including the touch
surface.
43. The force sensing touch location device of claim 42, wherein
the region comprises a stiff surface.
44. The force sensing touch location device of claim 43, wherein
the touch surface is disposed between the bezel and the stiff
surface.
45. The force sensing touch location device of claim 42, wherein
the portion enclosing the touch surface is narrow.
46. The force sensing touch location device of claim 45, wherein
the force transmission means comprises at least one thin rigid leg
in contact with the bezel and the region not including the touch
surface.
47. The force sensing touch location device of claim 42, wherein a
flange of the force transmission means encloses the second portion
of the touch surface.
48. The force sensing touch location device of claim 42, wherein
the force comprises a force that is perpendicular to the touch
surface.
49. The force sensing touch location device of claim 42, wherein
the path comprises a frame surrounding the touch surface.
50. The force sensing touch location device of claim 49, wherein
the frame comprises the force transmission means.
51. The force sensing touch location device of claim 43, wherein
the stiff surface comprises a surface of a display device.
52. The force sensing touch location device of claim 51, wherein
the display surface comprises an LCD device surface.
53. The device of claim 42, wherein said force transmission means
provides attachment for a vertically compliant seal between said
bezel and said touch surface.
54. The device of claim 53, further comprising the vertically
compliant seal.
55. The device of claim 53, wherein the attachment comprises a
flange of the force transmission means.
56. The force sensing touch location device of claim 53, wherein
the force transmission means comprises a rigid flange coupled to
the bezel.
57. The force sensing touch location device of claim 54, wherein
the force transmission means provides a bearing region to receive
perpendicular forces establishing an additional seal between said
force transmission means and the bezel, said bezel perpendicularly
overlying at least a line of junction of said vertically compliant
seal and said force transmission means.
58. The device of claim 49, wherein said frame provides attachment
for a lateral stiffening means between said frame and said touch
surface.
59. The device of claim 49, wherein said frame provides an
attachment for receiving both a vertically compliant seal and a
lateral stiffening means.
60. The device of claim 59, wherein the seal and the lateral
stiffening means are the same element.
61. The device of claim 59, wherein the attachment comprises a
rigid bearing edge.
62. The device of claim 49, wherein the frame includes an
attachment for receiving both the vertically compliant seal and a
surface of the bezel that acts as a second seal.
63. The device of claim 42, wherein the bezel includes an alignment
feature for aligning the touch surface within the enclosure.
64. The force sensing touch location device of claim 42, wherein
the narrow portion closely invests, but does not touch, the touch
display surface around the periphery of the touch display.
65. The force sensing touch location device of claim 42, further
comprising: a handheld computing device including the touch
surface, the bezel, and the force transmission means.
66. A force sensing touch location device comprising: a touch
surface; a bezel enclosing a first portion of the touch surface;
and force transmission means including an enclosing portion
enclosing a second portion of the touch surface and at least one
thin rigid leg in contact with the bezel and a stiff surface not
including the touch surface, said force transmission means having a
stiffness greater than that of the bezel, wherein the force
transmission means includes a path to transmit force from the bezel
to the stiff surface not including the touch surface.
67. A force sensing touch location device comprising: a touch
surface defining a touch plane; a first rigid member; a contoured
first film coupled to the touch surface and the first rigid member
to form a first seal therebetween, the contoured first film being
compliant along an axis normal to the touch plane.
68. The force sensing touch location device of claim 67, wherein
said contoured first film contacts a second rigid member and
wherein said contoured first film is disposed between the second
rigid member and the first rigid member to form a second seal
between the contoured first film and the second rigid member.
69. The force sensing touch location device of claim 68, wherein
the second rigid member contacts said contoured first film over a
portion of said first rigid member.
70. The force sensing touch location device of claim 68, wherein
the first seal comprises a seal between the touch surface and a
surrounding frame.
71. The force sensing touch location device of claim 70, wherein
the first rigid member comprises a portion of the frame.
72. The force sensing touch location device of claim 71, wherein
the second seal comprises a seal between the frame and a bezel
enclosing the touch surface, and wherein the first rigid member
receives perpendicular forces from the bezel to establish the
second seal, a portion of said bezel overlying a line of junction
of said first seal and said frame.
73. The force sensing touch location device of claim 71, wherein
the contoured first film includes a bulge between the touch surface
and the frame, and wherein the bulge is compliant along the axis
normal to the touch plane.
74. The force sensing touch location device of claim 70, wherein
the second seal comprises: a bezel including a slot; an insert
removably engaged in the slot; and a second film covering at least
a portion of the force sensing touch surface.
75. The force sensing touch location device of claim 67, wherein
the contoured first film is transparent.
76. The force sensing touch location device of claim 75, wherein
the contoured first film comprises a transparent film having a
portion overlaying at least part of the touch surface.
77. The force sensing touch location device of claim 76, wherein
the transparent film overlays the entire touch surface.
78. The force sensing touch location device of claim 71, wherein a
portion of the contoured first film extends from the rigid
supporting member to the touch surface, whereby a gap is formed
between the portion of the contoured first film and a portion of
the touch surface.
79. The force sensing touch location device of claim 67, wherein a
portion of the contoured first film extends from the rigid
supporting member to the touch surface in a direction not parallel
to the touch plane.
80. The force sensing touch location device of claim 67, wherein
the contoured first film and the touch surface comprise a
monolithic element.
81. A method for measuring the touch force applied to the touch
surface using the force sensor of claim 1, the method comprising a
step of: (A) developing a signal based on the change in capacitance
between the first capacitor plate and the second capacitor
plate.
82. The method of claim 81, wherein the amplitude of the signal is
a monotonic function of the change in capacitance between the first
capacitor plate and the second capacitor plate.
83. The method of claim 81, further comprising a step of: (B)
measuring a property of the touch force based on the signal.
84. The method of claim 83, wherein the step (B) comprises a step
of measuring the amplitude of a component of the touch force that
is perpendicular to the touch surface.
85. The method of claim 83, wherein the step (B) comprises a step
of measuring a location on the touch surface at which the touch
force is applied.
86. In a force sensor, a method for separating a first capacitor
plate from a second capacitor plate by a desired volume, the method
comprising steps of: (A) disposing a separator between a support
surface and a principal element including the first capacitor plate
to maintain a separation of at least the desired volume between the
first capacitor plate and the second capacitor plate; (B) coupling
at least one region of the principal element to at least one region
of the support surface; and (C) removing the separator, whereby the
first capacitor plate and the second capacitor plate remain
separated by at least the desired volume in an unloaded state of
the force sensor.
87. The method of claim 86, wherein the support surface comprises
the second capacitor plate.
88. The method of claim 86, wherein the support surface is part of
an interconnect system to transmit a signal developed in response
to the change in capacitance between the first capacitor plate and
the second capacitor plate.
89. The method of claim 86, wherein the principal element and the
at least one region of the support surface are substantially
parallel.
90. The method of claim 86, wherein the at least one region of the
principal element and the at least one region of the support
surface are electrically conductive, and wherein the step (B)
comprises a step of coupling the at least one region of the
principal element to at least one region of the support surface
with an electrically conductive substrate.
91. The method of claim 86, wherein the separator comprises a
shim.
92. The method of claim 86, wherein the method further comprises a
step of: (D) prior to the step (B), selecting a substantially
planar sheet of material as the principal element.
93. The method of claim 86, wherein the step (A) comprises
disposing a predetermined substrate between the support surface and
the principal element, and wherein the step (B) comprises a step of
using the predetermined substrate to couple the at least one region
of the principal element to the at least one region of the support
surface.
94. In a force sensor, a method for separating a first capacitor
plate from a second capacitor plate by a desired volume, the method
comprising steps of: (A) disposing a predetermined substrate
containing particles of controlled size between a support surface
and a principal element including the first capacitor plate to
produce a separation of at least the desired volume between the
first capacitor plate and the second capacitor plate; and (B)
coupling at least one region of the principal element to at least
one region of the support surface to maintain the separation of at
least the desired volume between the first capacitor plate and the
second capacitor plate.
95. The method of claim 94, wherein the step (A) comprises a step
of flowing the predetermined substrate in a fluid state between the
principal element and the support surface, and wherein the step (B)
comprises a step of allowing the predetermined substrate to
transition into a solid state.
96. A method for manufacturing a force sensor, the method
comprising steps of: (A) selecting a principle element including a
substantially flat surface and a first capacitive surface; (B)
disposing the first capacitive surface in opposition to a second
capacitive surface; and (C) forming an elevated elastic feature
into the substantially flat surface, whereby transmission of a
force through the elevated elastic feature contributes to a change
in capacitance between the first capacitor plate and the second
capacitor plate.
97. The method of claim 96, wherein the substantially flat surface
and the first capacitive surface are integral.
98. The method of claim 96, wherein the step (A) comprises a step
of selecting a sheet of electrically conductive material as the
principal element.
99. The method of claim 96, further comprising a step of: (D)
placing the elevated elastic feature in communication with a touch
surface to which the force is applied, whereby the elevated elastic
feature provides a region of load transmission from the touch
surface to the principal element.
100. In a force sensor, a method for separating a first capacitor
plate from a second capacitor plate by a desired volume, the method
comprising steps of: (A) disposing a separator between the second
capacitor plate and a substantially planar principal element
including the first capacitor plate to maintain a separation of at
least the desired volume between the first capacitor plate and the
second capacitor plate; (B) coupling at least one region of the
principal element to at least one region of the support surface
that is substantially parallel to the principal element; and (C)
removing the separator, whereby the first capacitor plate and the
second capacitor plate remain separated by at least the desired
volume in an unloaded state of the capacitive force sensor.
101. A force sensing touch location device comprising: a touch
surface structure to which a touch force may be applied, the touch
force including a perpendicular component that is perpendicular to
a touch surface of the touch surface structure and a tangential
component that is tangential to the touch surface of the touch
surface structure; a supporting structure; at least one force
sensor, in communication with the touch surface and the supporting
structure, to measure properties of the touch force; lateral
restraint means, in contact with both the touch surface structure
and the supporting structure, for impeding lateral motion of the
touch surface structure without substantially impeding transmission
of the perpendicular component of the touch force through the at
least one force sensor.
102. The force sensing touch location device of claim 101, wherein
the lateral restraint means comprises a thin member in contact with
both the touch surface structure and the supporting structure.
103. The force sensing touch location device of claim 102, wherein
the thin member joins the touch surface to a surrounding frame.
104. The force sensing touch location device of claim 103, wherein
the thin member comprises at least one strip of tape.
105. The force sensing touch location device of claim 102, wherein
the thin member is constructed of high-modulus material to be
substantially stiff to tangential movement of the touch surface and
substantially compliant to perpendicular motion of the touch
surface.
106. The force sensing touch location device of claim 101, wherein
the touch surface comprises a display surface.
107. The force sensing touch location device of claim 101, wherein
the touch surface comprises a touch overlay overlaying a display
surface.
108. The force sensing touch location device of claim 101, wherein
the lateral restraint means comprises a preload spring.
109. The force sensing touch location device of claim 108, wherein
the preload spring is fastened to an edge of the touch surface.
110. The force sensing touch location device of claim 108, wherein
the preload spring has a non-uniform unloaded curvature.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to concurrently filed and
commonly owned patent application entitled "Tangential Force
Control in a Touch Location Device," hereby incorporated by
reference in its entirety.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to touch sensors and, more
particularly, to force sensing touch location devices.
[0004] 2. Related Art
[0005] The ability to sense and measure the force and/or location
of a touch applied to a surface is useful in a variety of contexts.
As a result, various systems have been developed in which force
sensors are used to measure properties of a force (referred to
herein as a "touch force") applied to a surface (referred to herein
as a "touch surface"). Force sensors typically generate signals in
response to the touch force that may be used, for example, to
locate the position on the touch surface at which the touch force
is applied. A number of particular implementations of this approach
have been proposed, such as that described by Peronneau et al. in
U.S. Pat. No. 3,657,475.
[0006] Such touch location is of particular interest when the touch
surface is that of a computer display, or that of a transparent
overlay in front of a computer display. Furthermore the need for
small, lightweight, and inexpensive devices that are capable of
performing touch location is increasing due to the proliferation of
mobile and handheld devices, such as personal digital assistants
(PDAs). The touch screens which perform this function may be built
with a number of possible technologies. In addition to the force
principle just mentioned, capacitive, resistive, acoustic, and
infrared techniques are among those that have been used.
[0007] The force principle has some strong potential advantages
over these competing techniques. Since force techniques may be
applied to any overlay material, or indeed to the entire display
itself, there is no need to interpose materials or coatings with
low durability or poor optical properties. Also, since force is the
basis of perceived touch, there is no problem with sensitivity
seeming unpredictable to the user. With capacitive measurement, for
instance, touch threshold varies with the condition of the user's
skin, and with interposed materials, such as a glove. Stylus
contact typically gives no response. With resistive measurement,
threshold force depends upon the size of the contact area, and so
is very different between stylus and finger. Acoustic measurement
depends upon the absorptive characteristics of the touching
material; and with infrared, a touch may register when there has
been no contact.
[0008] In spite of these advantages of force-based technologies,
resistive and capacitive technologies have dominated in the touch
screen market. This reflects residual difficulties with known force
techniques, which must be overcome to realize the potential of
force technology.
[0009] Among these difficulties are:
[0010] Excessive force sensor size--especially width and
thickness.
[0011] Excessive sensitivity to transverse forces, leading to
inaccuracy.
[0012] Excessive force sensor cost and complexity.
[0013] Excessive sensitivity to deformations of the touch surface
or its supporting structure, leading to inaccuracy.
[0014] The need to keep the touch surface mechanically independent
of the application bezel that encloses the touch surface, which
makes it difficult to integrate the touch screen into the larger
structure, and makes it difficult to provide a good liquid and dust
seal.
[0015] In modern touch applications, it is extremely important that
provisions for touch force location and/or measurement not increase
the size nor dictate the appearance of the touch-equipped device.
This is especially true in portable and handheld applications.
Conventional force sensors of the type required are typically much
thicker than resistive or capacitive films, thereby potentially
increasing the thickness of devices that incorporate such force
sensors compared to devices that incorporate resistive or
capacitive sensors. Since conventional force sensors of the type
required cannot easily be made transparent, they cannot be placed
in front of an active display area. As a result, devices including
such conventional force sensors must typically be made wider than a
resistive- or capacitive-based device to accommodate the force
sensors. Thus force-based touch is potentially disadvantageous with
respect both to overall device thickness and width, when compared
to other kinds of conventional touch sensors.
[0016] Thus it is seen that the prior art fails to teach how force
sensors may be made sufficiently narrow, thin, and inexpensive.
[0017] A touch force applied to a touch surface has both a
component that is normal to the touch plane (the "perpendicular
component") and a component that is parallel to the touch plane
(the "tangential component"). The presence of a tangential
component can introduce errors in the computed touch location.
Various techniques for reducing the errors introduced by tangential
forces are described in more detail in the co-pending application
entitled "Tangential Force Control in a Touch Location Device."
[0018] In many applications it may be desirable for an application
bezel to press firmly around the edges of a touch-equipped display
or display overlay module. This arrangement provides a dust and/or
liquid seal, and may also serve to stiffen and align the bezel.
With force-sensing touch-location devices, however, the bezel does
not typically rest directly against a force sensitive structure,
since the variable handling forces thereby transmitted would
interfere excessively with touch location accuracy. The prior art
does not teach satisfactory methods for sealing, nor for
sufficiently diverting bezel forces in force-based based touch
systems.
SUMMARY
[0019] In one of its aspects, the invention provides a novel
capacitive force sensor. The sensor comprises a principal element,
and an essentially planar support. The principal element combines
the functions of elastic energy storage and one capacitor plate,
and may be as simple as a plane rectangle of thin spring metal. As
described in more detail below, the sensor may be implemented with
a small number of mechanical parts and a very small capacitive gap,
making the sensor easy and inexpensive to manufacture and making
the sensor particularly applicable for use in mobile and handheld
devices. It should be stressed, however, that sensors made in
accordance with the invention may be of great advantage in a wide
range of devices, sizes, and applications. To date, they have been
successfully used in devices with a working diagonal of from 4"to
15", and supported touch surface assemblies weighing from 0.6
ounces to nearly 4 pounds.
[0020] For example, in one aspect of the invention, a force sensor
for sensing a touch force applied to a touch surface is provided.
The force sensor comprises: a first element including an elastic
element and a first capacitor plate having a first capacitive
surface, the elastic element including at least part of the first
capacitor plate; and a second element including a second capacitor
plate opposed to the first capacitor plate; wherein transmission of
at least part of the touch force through the elastic element
contributes to a change in capacitance between the first capacitor
plate and the second capacitor plate. Various other force sensors
are also provided, as described in more detail below.
[0021] In yet another aspect of the invention, a force sensing
touch location device is provided. The force sensing touch location
device comprises: a touch surface; a bezel enclosing a first
portion of the touch surface; and force transmission means
including an enclosing portion enclosing a second portion of the
touch surface, said force transmission means having a stiffness
greater than that of the bezel, wherein the force transmission
means includes a path to transmit force from the bezel to a region
not including the touch surface.
[0022] In a further aspect of the invention, a force sensing touch
location device is provided. The force sensing touch location
device comprises: a touch surface defining a touch plane; a first
rigid member; a contoured first film coupled to the touch surface
and the first rigid member to form a first seal therebetween, the
contoured first film being compliant along an axis normal to the
touch plane.
[0023] In another aspect of the invention, a method is provided for
measuring a touch force applied to a touch surface using one of the
force sensors described herein. The method comprises a step of
developing a signal based on the change in capacitance between the
first capacitor plate and the second capacitor plate of the force
sensor. The amplitude of the signal may be a monotonic function of
the change in capacitance between the first capacitor plate and the
second capacitor plate. The method may include a step of measuring
a property of the touch force, such as the amplitude of a component
of the touch force that is perpendicular to the touch surface,
based on the signal. 85. The method may include a step of measuring
a location on the touch surface at which the touch force is
applied.
[0024] In yet another aspect of the invention, a method is provided
for separating a first capacitor plate from a second capacitor
plate in a force sensor by a desired volume. The method comprises
steps of: disposing a separator between a support surface and a
principal element including the first capacitor plate to maintain a
separation of at least the desired volume between the first
capacitor plate and the second capacitor plate; coupling at least
one region of the principal element to at least one region of the
support surface; and removing the separator, whereby the first
capacitor plate and the second capacitor plate remain separated by
at least the desired volume in an unloaded state of the force
sensor. The support surface may, for example, be the second
capacitor plate.
[0025] In a further aspect of the invention, a method is provided
for separating a first capacitor plate from a second capacitor
plate in a force sensor by a desired volume. The method comprises
steps of: disposing a predetermined substrate containing particles
of controlled size between a support surface and a principal
element including the first capacitor plate to produce a separation
of at least the desired volume between the first capacitor plate
and the second capacitor plate; and coupling at least one region of
the principal element to at least one region of the support surface
to maintain the separation of at least the desired volume between
the first capacitor plate and the second capacitor plate.
[0026] In another aspect of the invention, a method for
manufacturing a force sensor is provided. The method comprises
steps of: selecting a principle element including a substantially
flat surface and a first capacitive surface; disposing the first
capacitive surface in opposition to a second capacitive surface;
and forming an elevated elastic feature into the substantially flat
surface, whereby transmission of a force through the elevated
elastic feature contributes to a change in capacitance between the
first capacitor plate and the second capacitor plate.
[0027] In another aspect of the invention, a force sensing touch
location device is provided. The force sensing touch location
device comprises: a touch surface structure to which a touch force
may be applied, the touch force including a perpendicular component
that is perpendicular to a touch surface of the touch surface
structure and a tangential component that is tangential to the
touch surface of the touch surface structure; a supporting
structure; at least one force sensor, in communication with the
touch surface and the supporting structure, to measure properties
of the touch force; lateral restraint means, in contact with both
the touch surface structure and the supporting structure, for
impeding lateral motion of the touch surface structure without
substantially impeding transmission of the perpendicular component
of the touch force through the at least one force sensor.
[0028] Other features and advantages of various embodiments of the
present invention will become apparent from the following
description and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1A is an exploded drawing of a touch screen module of a
first preferred embodiment, as might be used against the face of a
separate LCD module.
[0030] FIG. 1B is a partial cross-section of the module of FIG. 1A,
intersecting the center of a sensor.
[0031] FIG. 2 is a cross sectional view of a first embodiment, in a
typical application installation.
[0032] FIG. 3 is a cross sectional view of a second embodiment, in
a typical application installation.
[0033] FIG. 4 is a partially schematic cross-sectional view of a
general touch-locating system, illustrating reduction of tangential
force errors according to one embodiment of the invention.
[0034] FIGS. 5A through 5C provide partial cross sectional views
illustrating the use of a flat suspension film or beam used as a
lateral stiffening means.
[0035] FIG. 6 is a partial cross sectional view of a lateral
stiffening and/or lateral restraint means with extended range of
vertical motion, and directionally selective lateral
stiffening.
[0036] FIGS. 7A through 7C are partial cross sectional views of
further variations on the lateral stiffening means.
[0037] FIG. 8 is a partial cross sectional view of a touch system
with a field-replaceable touch surface protector and liquid/dust
seal.
[0038] FIG. 9A is a cross sectional view of a larger sensor of a
type built in accordance with the invention.
[0039] FIG. 9B is an exploded perspective view of the sensor
assembly of FIG. 9A.
[0040] FIG. 10A is a cross sectional view of a smaller sensor of a
type built in accordance with the invention.
[0041] FIG. 10B is an exploded perspective view of the sensor
assembly of FIG. 10A.
[0042] FIG. 11 is a vertically exaggerated cross section of a
sensor variation employing a nonuniform gap, built in accordance
with one embodiment of the present invention.
[0043] FIGS. 12A through 12D are plan views depicting possible
variations in the outline and mounting arrangement of principal
elements of sensors according to embodiments of the invention.
[0044] FIGS. 13A and 13B are vertically exaggerated cross sections
depicting possible variations in the thickness distribution of
principal elements of sensors according to embodiments of the
invention.
[0045] FIG. 14A is a plan view of a sensor variation employing a
principal element with simply supported ends, according to
embodiments of the invention.
[0046] FIG. 14B is a side view of the plastic spacer used in the
sensor variation of FIG. 14A.
[0047] FIG. 14C is a partial cross-sectional view of a touch
location device employing variations of aspects of the invention,
including the sensor variation of FIG. 14A.
[0048] FIGS. 15A and 15B are exploded and cross-sectional views,
respectively, of a sensor variation incorporating nonmetallic
elastic portions, according to embodiments of the invention.
[0049] FIG. 15C is a cross-sectional view of a related sensor
variation incorporating nonmetallic elastic portions, according to
one embodiment of the invention.
DETAILED DESCRIPTION
[0050] In one of its aspects, the invention provides a novel
capacitive force sensor. As described in more detail below, the
sensor may be implemented with a small number of mechanical parts
and a very small capacitive gap, making the sensor easy and
inexpensive to manufacture and making the sensor widely applicable,
but particularly so for use in mobile and handheld devices. The
sensor comprises a principal element, and an essentially planar
support. The principal element combines the functions of elastic
energy storage and one capacitor plate, and may be as simple as a
plane rectangle of thin spring metal. The principal element is held
in close parallel alignment with the essentially planar support by
mechanical contacts at one or more bearing points or areas. These
may be under the two ends of the rectangular principal element,
though many other arrangements, such as cantilever, cross, disk,
etc., will readily occur to one of ordinary skill in the art, and
are within the scope of the invention. The support also bears a
thin conductive region opposed to a portion of the principal
element away from the contacts, which functions as a second
capacitor plate, or counter electrode. The mechanical contacts may
provide either simple, or clamped support to the principal element,
viewed as a load bearing beam. The contacts however, should be
designed to minimize dissipative or frictional effects. The
principal element receives forces through an upper loading contact,
at a point or area opposite the counter electrode. Components of
force perpendicular to the support surface deflect the principal
element so as to change the distance separating it from the counter
electrode, thus altering the capacitance therebetween. If the
mechanical contacts provide clamped end constraint, it is desirable
that this be stiff; that is, most of the distance change occasioned
by a force should be due to flexure of the principal element,
rather than twisting of the mechanical contact areas. Although
cleanly elastic clamped end constraints may be tolerable, where
they engender only a systematic change in sensitivity, better
reproducibility and freedom from hysteresis usually may be obtained
if the end constraints are stiff clamped end constraints, or fully
flexible simple supports, such as pivots.
[0051] The essentially planar support surface may be part of an
interconnect system, such as a printed wiring board or flexible
circuit with appropriate stiffeners. The counter electrode may
comprise a land, or foil, within the context of such an
interconnect. The mechanical contacts may also constitute
electrical contacts, and may be accomplished by soldering the ends
of the principal element to other lands in the support plane.
[0052] Force may be measured as the ratio of an exciting AC voltage
to the current it forces through the sensor. As a matter of
practice, a constant current may be applied by a feedback circuit,
and the exciting voltage measured (as in Roberts, U.S. Pat. No.
5,376,948); or a constant exciting voltage may be applied, and the
reciprocal of the resulting current computed. The latter method may
enable use of a somewhat simpler interconnect, and provides a
somewhat more convenient opportunity for subtracting off estimates
of fixed strays which might otherwise degrade linearity of
response. The force-responsive signals derived from the force
sensors may be processed to yield touch location information in
accordance with principles known in the art.
[0053] The curvature resulting from flexure of the principal
element is not ideal, but by confining the counter electrode to an
area near the center of the principal element, potential
nonlinearities of response may be reduced to a level acceptable for
use in a touch locating application. Other provisions for improved
linearity may also be made, as described below.
[0054] A force sensor has a direction of sensitivity, such that a
translational force of given magnitude creates greatest output when
applied in that direction, and no output when applied at right
angles to that direction. A displacement sensor has an analogous
direction of sensitivity with respect to applied pure translational
displacements. A force sensor is said herein to have an axis of
sensitivity that passes through its elastic center in its direction
of sensitivity. A displacement sensor may be taken to have an axis
of sensitivity lying in its direction of sensitivity, and so
located that relative rotation of the two sides about points in the
axis tend to produce no output.
[0055] It is desirable that a force sensor have a precisely
determined axis of sensitivity, and that this axis be easily and
precisely aligned, as desired, with respect to the enclosing
application. The thin, planar nature of the sensor provided in
various embodiments of the invention satisfies this need naturally.
It is also desirable for the force sensor to be unresponsive to any
moment couples passing through it. For a force sensor comprising a
displacement sensor sensing the displacement across an elastic
means, this requires that the displacement sensor's axis of
sensitivity pass through the elastic center of the elastic means.
Sensors provided in various embodiments of the invention accomplish
this goal by making the principal element and its contacts
symmetrical under a 180 degree rotation about the axis of
sensitivity.
[0056] Potential moment sensitivity may be further reduced by
providing a rotational softener at the loading contact. A bump, or
other elevated central feature in the principal element, may serve
as a pivot providing this function. Locating this feature in the
principal element itself has the further advantage of providing the
force sensor with a determined sensitivity. When force is
transmitted from an overlying surface contacting the bump, changes
in relative alignment leave the region of load transmission
unchanged with respect to the force sensor.
[0057] Forces and moments may be transmitted through a sensor that
are not those the sensor is intended to measure. If the sensor is
not perfectly constructed and aligned, it may have some sensitivity
to these, leading to errors of measurement. In addition,
unmonitored forces and moments may be part of a pattern including
monitored forces, such that the equations for locating touch may
not be evaluated accurately without measurements of the full
pattern being available.
[0058] Various aspects of the invention provide for the reduction
or elimination of these unmonitored forces or moments.
[0059] In a first aspect, embodiments of the invention may employ a
rotational softening means to reduce or eliminate moments
transmitted through a force sensor. In one embodiment, such a
rotational softener may comprise a soft elastic body, such as a
small elastomeric slab, or a stiffer element, such as a portion of
a metal stamping, bent or prolonged in the direction of
sensitivity. In another embodiment, it may comprise a pivot,
operating without receptacle against a hard surface, or with
self-forming receptacle, against a softer surface.
[0060] One benefit of rotational softening may obtain where the
touch surface structure is not fully rigid, such that some small
local flexure occurs near a point of touch. Such local flexure may
lead to substantial touch location error, even with perfectly
constructed and aligned sensors, if the sensor on its support from
below is not substantially softer in rotation than the attachment
offered from above by the touch surface structure. In effect, a
sensor connection with excessive rotational stiffness can support a
nearby touching finger in part by using the intervening portion of
the touch surface structure as a cantilever, thereby obtaining more
of the perpendicular force than would be ideally presumed. A
distortion of the position of reported touch locations results,
which distortion is sensitive to details of the stiffness
relationships. Rotational softening may be employed to prevent the
appearance of such a pattern combining unmonitored sensor moment
with balancing spurious perpendicular force components.
[0061] Rotational softening may thus be of particular benefit when
used with a touch surface structure that is thin and flat, and thus
comparatively flexible, such as a flat overlay plate of minimal
thickness.
[0062] Another benefit of rotational softening may obtain where the
sensors are not perfectly constructed. Such sensors may give
spurious responses to transmitted moments. A rotational softener
may offer greatest reduction of the moment actually experienced by
a force sensor, if it is located as close as possible thereto. This
reduces the production of sensor moment in response to any lateral
forces transmitted. Thus rotational softening achieving the
benefits of the invention may be applied away from the plane of
touch, and may be applied close to the force sensors.
[0063] In a second aspect, embodiments of the invention may employ
a lateral softening means to reduce or eliminate forces transmitted
through a force sensor at right angles to its nominal axis of
sensitivity. In one embodiment, such a rotational softener may
comprise an elastic body, such as a small elastomeric slab. In
another embodiment, it may comprise a pin, column, or ball,
offering a pair of pivots, softly elastic ends, or rolling surfaces
offset from each other by at least a small distance.
[0064] One benefit of rotational softening may obtain where
tangential forces applied to the touch surface are prevented from
developing a pattern of forces, such pattern combining spurious
perpendicular sensor forces with lateral sensor force and moment to
maintain overall equilibrium, as described in more detail
below.
[0065] Another benefit of lateral softening may obtain where the
sensors are not perfectly constructed. Such sensors may give
spurious responses to forces at right angles to their nominal axis
of sensitivity. Lateral softening may also reduce extra sensor
moment potentially generated by such lateral forces, where the
associated elastic center is not in the sensor center.
[0066] Combinations of lateral softening, rotational softening, and
lateral stiffening may serve to establish necessary axes of
sensitivity more accurately than can be achieved through the
construction of the sensors themselves. This follows in part from
the large area over which the alignment of the plane of effect of
the lateral stiffening means may be established.
[0067] Many alternative embodiments of lateral and rotational
softening means will be evident to those of ordinary skill in the
art, and are within the scope of the invention.
[0068] The method of forming a force sensor from a principal
element upon an essentially planar supporting surface provides
striking advantages in simplicity and miniaturization.
[0069] In one embodiment of the present invention, where an
essentially planar support surface is comprised of a printed wiring
board or other planar interconnect system, a force sensor is
provided that includes as little as one, single, separately
manufactured and handled part--the principal element described
above. For example, the principal element may be as simple as a
rectangle of plated spring steel, flat but for a small bump pressed
into the center. Mounting may be accomplished by reflowing solder
under the ends of the principal element, while the central region
is spaced from the counter electrode area with a temporary
stainless-steel shim.
[0070] Alternatively, the solder employed may be mixed with a small
quantity of non-fusing particles of controlled size, and the
presence of these may serve to establish the gap width during
soldering. Surface tension of the solder may suffice to draw the
opposing surfaces against the particles. Yet again, the principal
element may be provided with areas of slight offset pressed or
otherwise formed into the ends, and these may rest directly against
the support. Such slightly offset areas may take many forms, one
being a slight displacement of the entire end toward the support
surface. Another entails forming one or more of the smallest
practicable bumps at each end, protruding by the degree necessary
to establish the desired gap. These offer some space under each end
for good solder reflow, and also minimize the likelihood of trapped
solder contaminants enlarging the gap.
[0071] Alternatives to the use of solder will be evident, including
the use of cements, such as conductive epoxy, and methods involving
independent or indirect electrical connection to the mounted
element.
[0072] By allowing construction from starting materials that are
inherently flat, smooth, and true, which are then simply spaced
apart a small distance, various embodiments of the invention
provide an extremely reliable and inexpensive method of achieving a
very small capacitive gap. This small gap provides a high
capacitance per unit area, which allows the sensor area to be very
small. The small gap requires limited mechanical energy storage in
the principal element, allowing the use of thin material. The small
gap implies high sensor stiffness, which in turn implies high
resonant frequencies, and is beneficial for accurate measurement.
The small area of the sensor means that flatness in the materials
need be maintained over only a very short distance, thus making
practical even smaller gaps in a virtuous circle of
miniaturization.
[0073] Sensor designs provided by various embodiments of the
present invention are subject to a simple scaling rule over fairly
wide range of sizes. A new design may be produced, which is N times
shorter, N times narrower, and N squared times smaller in gap. If
the original proportions and material thickness are otherwise
retained, the resulting sensor will retain the same capacitance,
force range, and sensitivity as the original. Since the area is N
squared times smaller and the gap N squared times smaller, the
capacitance is the same. Since the spring rate is N squared times
greater and the gap N squared times smaller, the relative
capacitance change with force, i.e. the sensitivity, is the same.
Since the stressed portion of the principal element is N squared
times smaller in volume, but stores N squared times less energy for
the same applied force, it is exposed to the same stress levels.
Since the deviation of a warped surface from flat, scales as the
square of the distance over which the deviation is taken, the
flatness requirement for the materials used is unchanged. Note that
"flatness" here refers to deviations of low spatial frequency; high
frequency failures of smoothness may ultimately limit
miniaturization scaled this way. It may be noted, however, that
ordinary spring steel and circuit board materials are smooth enough
to support gaps down to {fraction (1/1000)} of an inch, and
probably substantially smaller.
[0074] In another of its aspects, the invention provides a novel
means for performing accurate touch location measurements in the
presence of tangential forces, even when the sensors are located
well behind the plane of touch. This is accomplished with lateral
stiffening means, which direct tangential touch forces away from
the force sensors (e.g., to the surrounding support structure). At
the same time, perpendicular touch force components pass
predominantly through a mechanically separate path to the force
sensors. The lateral stiffening means is typically designed to have
a plane of zero reaction moment to tangential forces, which is
co-incident with, or close to, the touch surface. In cases where
lateral stiffness through other force paths is not insignificant,
the lateral stiffening means may be designed to achieve the same
net effect for all paths collectively.
[0075] To simplify the design, and maximize reproducibility, force
paths other than the lateral stiffening means may be provided with
explicit lateral softening means, such that essentially all
tangential forces pass through the lateral stiffening means. The
perpendicular force path through the sensors may be stiff, while
the perpendicular force path through the lateral stiffening means
may be soft. The latter is particularly desirable in circumstances
where interfering perpendicular displacements may occur across the
lateral stiffening means, as might result from flexing in an
overlay plate or its surrounding frame. Together, both provisions
accomplish full segregation of the tangential and perpendicular
touch forces into separate paths.
[0076] The lateral stiffening means may, for example, be embodied
in a thin member or film, which joins a display or touch overlay to
a surrounding frame. This film may bridge a small gap between the
frame and the edge of the touch surface, where it attaches, or
there may be a lesser gap, and the film may carry above the touch
surface a short ways before attaching to the touch surface. By
being much thinner than it is broad, yet composed of material of
fairly high modulus, this film may be stiff to tangential movement
of the touch surface, yet soft to perpendicular motion. The film
may be made to bulge, or curve, somewhat above and/or below the
touch plane, thus increasing its vertical range of compliance. Such
curvature also has the effect of restricting the lateral stiffening
to the sides parallel to a tangential force, where it is
transmitted through the film as shear, rather than compression or
extension.
[0077] A lateral stiffening means, embodied as a complete
circumferential edge film in or close to the plane of touch, may at
the same time constitute a liquid and/or dust seal.
[0078] Although in the embodiment described above the lateral
stiffening means is a thin member or film, this does not constitute
a limitation of the present invention. Rather, the lateral
stiffening means may take a variety of forms and be constructed
from a variety of materials. The lateral stiffening means need not
be continuous, and is not limited to any particular modulus, aspect
or shape. Rather, the lateral stiffening means may include any
structure that performs the function of lateral stiffening as
described herein.
[0079] In another aspect of the invention, a thin or slender member
or set of members may comprise a lateral restraint means, allowing
assembly of a force-sensing touch-location device to be maintained,
without strong attachments that fix the support surface structure
by paths passing through the force sensors. In such a device, the
exact perpendicular operating position of the touch surface
structure is established by perpendicularly stiff paths, such as
through the force sensors, that provide connection to the support
structure independent of the lateral restraint means. In one
embodiment, the touch surface structure may rest upon force sensors
below, without attachment, and without any special receptacle or
other provision on the touch surface structure for receiving
contact with the force sensors, or needing careful alignment
thereto. The force sensors, whether mounted from the touch surface
structure above, or from the support structure below, may thus be
provided with rotational softeners and/or lateral softeners
offering little or no strength but in compression. Many forms of
curvature or elevated feature from either side at the perpendicular
force contacts may serve as a rotational softener, so long as local
touch surface flexure does not translate the point of contact by
more than the tolerable error in touch location. Perpendicular
contact may be maintained by preload means, which may be separate
from the lateral restraint means.
[0080] A lateral restraint means may be distinguished from a
lateral stiffening means in that tangential touch forces may not
necessarily pass through a lateral restraint means. The small,
incremental forces of a touch may instead follow stiffer paths
through the force sensors or other connections, as dictated by
friction. Larger lateral disturbances, however, overcome friction
and cause minute sliding motions in these paths. These disturbances
may comprise jolts in shipping and handling, for instance, or for
large devices with a heavy touch surface structure, changes in
orientation with respect to gravity. A lateral restraint means may
absorb the brunt of such disturbances tangentially, protecting the
touch device structure, function, and accuracy from significant
alteration. By reaching the upper limit of its perpendicular
motion, a lateral restraint means may also absorb a disturbance
tending to lift the sensors free of contact, although such function
may be performed by separate outward limit stops. A lateral
restraint means may deflect far enough to be assisted by lateral
limit stops during large, temporary forces, but when these cease,
it may restore satisfactory centering, free of continuing
interference from stops.
[0081] A thin member or set of thin members may provide a lateral
restraint means that is simple and compact. It may add little or
nothing to the thickness of a touch-location module or
touch-enabled display module. Such a thin member or set of members
may further offer a favorably high ratio of lateral to
perpendicular stiffness. Absent such a high ratio, members
sufficiently robust to provide good lateral restraint may offer
excessive vertical stiffness. In avoiding such excessive vertical
stiffness, various embodiments of this aspect of the invention
avoid inaccuracy occasioned by parasitic force paths, such as might
pass through a seal. They also avoid the need for an excessively
thick and stiff touch surface structure or support structure in
mitigation. Such thin members may flex softly in response to
perpendicular displacement of the touch surface, but stiffly resist
tangential displacement. Thus, a wire-like member, inclined at most
shallowly to the plane of touch, may serve to resist tangential
forces primarily through end-on compression and extension, while
being softly flexible in transverse beam bending. So too, a
sheet-like member may transmit tangential force stiffly in shear,
and possibly also in compression and extension, while responding to
perpendicular displacement of the touch surface with soft beam
bending transverse to its breadth. Where tangential force
transmission is confined to shear, and where a lateral restraint
means is not also a lateral stiffening means, sheet-like members
may provide an effective lateral restraint means, even though they
incline steeply to the plane of touch. A lateral restraint means
may perform its function, even if not located in the plane of
touch.
[0082] In another aspect of the invention, a thin frame means is
wrapped closely around the periphery of the overlay or supported
display. A major benefit of this construction is the provision of a
module which handles, mounts, and integrates with its surrounding
application in a manner which is familiar from other touch
technologies, is accepted, and is convenient. The frame means
serves to divert perpendicular forces from the application bezel
normally present, so that there is no danger of interference with
the touch surface. The frame lip provides a convenient rigid
bearing edge to receive both a vertically compliant seal passing
outward from the touch surface, and a smooth surface or other seal
provision of the underside of the application bezel. Since
perpendicular forces are of principal concern, a very thin vertical
frame leg, embodies an application bezel support member, and serves
to carry bezel forces back to a stiff surface behind a touch
module, such as the surface of an LCD. With greater section depth,
such a very thin leg also serves to carry back bezel forces when
the frame surrounds a supported LCD.
[0083] FIGS. 1A-1B present a touch sensitive transparent overlay
module 101 including capacitive force sensors according to a first
embodiment of the invention. The module 101 may be used to sense
touches applied by, for example, a finger, stylus, or other object.
As described in more detail below, in various embodiments of the
present invention, the module 101 may be used to sense properties
of a touch force applied to a touch surface, such as the location
at which the touch force is applied to the touch surface and/or the
magnitude of a component of the touch force that is perpendicular
to the touch surface.
[0084] The transparent overlay module 101 is proportioned as might
be appropriate for use on an LCD display with a diagonal of 4
inches, though proportions and variations for other displays of
other sizes will be apparent to those of ordinary skill in the art.
Transparent panel 102, carrying touch surface 103a, rests within
frame 104a. Captured between panel 102 and frame 104a are
interconnect flex print 105, force sensor principal elements 106,
and lateral softening means 107. Preload springs 109 are fastened
to the edges of panel 102 with cement 110. The ends of springs 109
engage holes 112 in frame 104a when assembled, thereby applying a
total compression of approximately two pounds to the structures
captured between panel 102 and frame 104a. The flexed positions of
springs 109, as assembled, place them in straight lines along the
short edges of panel 102. Member 108 is a combination lateral
stiffening means, lateral restraint means, and liquid/dust seal
108. Member 108 may also be referred to elsewhere herein simply as
a lateral stiffening means, lateral restraint means, or seal, for
ease of explication. Member 108 adheres to panel 102 and to the
outer surfaces of the vertical flanges of frame 104a, thereby
securely centering panel 102 within frame 104a. When so centered,
there is a small space between the long sides of panel 102 and
frame 104a, and there is a small space around the nonattached
portions of springs 109. Thus forces applied to touch surface 103a
can produce small perpendicular motions of panel 102 without
occasioning interference or scraping around its edges.
[0085] One embodiment of the capacitive force sensor of the present
invention is now described in more detail. As will become apparent
from the following description, FIG. 1A illustrates four capacitive
force sensors in assembly and FIG. 1B illustrates one of these
capacitive force sensors in cross-section. In assembly,
interconnect 105 is threaded through apertures 111 in frame 104a
and dressed along and above opposing horizontal flanges of frame
104a as shown. Interconnect 105 is fastened securely to frame 104a
under receiving regions of principal elements 106, such that these
regions achieve the effective stiffness of frame 104a. Such
attachment may be achieved by providing interconnect 105 with
backside lands which are soldered to frame 104a, or by cementing
interconnect 105 to frame 104a with epoxy resin, or by other known
means. Elements 106 are secured to interconnect 105 by soldering
their ends to lands 113. By either the shape of principal elements
106, or by the assembly process which attaches them to interconnect
105, there remains a small gap of determined width between
principal elements 106 and counter-electrode lands 114. For the
type of assembly shown, this gap may be 0.0010 inch. A central
dimple, or force bearing 121, is pressed upward into each of
principal elements 106.
[0086] Each of the force sensor principal elements 106 combines the
functions of spring and capacitor plate. As perpendicular force is
applied to one of the bearings 121, flexure of the corresponding
one of the principal elements 106 increases the capacitance between
the central portion of the principal element's underside and the
corresponding one of the counter-electrodes 114 (which are on the
underside of interconnect 105). This change in capacitance may be
measured to measure the force applied to the surface 103a. As shown
in FIGS. 1A and 1B, each bearing 121, corresponding principal
element 106, the corresponding receiving region of interconnect
105, and the stiffening support provided thereto by frame 104a,
thus together constitute a force sensor.
[0087] Although four force sensors are shown in FIG. 1A, it should
be appreciated that any number of force sensors may be employed
with a particular device as may be appropriate for a particular
application. Furthermore, although the force sensors are positioned
close to the corners of the overlay panel 102, this is not a
limitation of the present invention.
[0088] Although a particular embodiment of the principal element
106 is shown in FIGS. 1A and 1B, more generally principal element
106 is an electrically conductive elastic element that both stores
elastic energy and acts as a capacitor plate in a force sensor. As
a result of the principal element's elastic properties, the
principal element 106 is deflectable by a touch force applied to
the touch surface 103a. This deflection causes a change in the
capacitance between the principal element and lands 113 (which act
as capacitor plates that oppose the principal element 106). The
principal element 106 thereby combines the functions of elastic
energy storage and a capacitor plate in a small, thin, easily
manufactured part.
[0089] Interconnect 105 provides electrical access to
counter-electrodes 114 and to principal elements 106 via lands 113,
as necessary to provide separate readings from the four force
sensors so constituted. In one embodiment of the present invention,
each of the principal elements 106 is the only component of each
force sensor that must be manufactured and handled
individually.
[0090] In embodiments of the present invention such as that
depicted in FIGS. 1A-1B, lateral softening means 107 may comprise
small punched disks of stainless-steel tape, backed by a typically
soft acrylic adhesive. The adhesive surfaces are applied to the
backside of panel 102, such that the metal surfaces press against
bearings 121 after assembly. The effect of the small area of soft
acrylic adhesive, so confined, is to substantially reduce the
lateral forces generated between principal elements 106 and panel
102, generated in reaction to tiny lateral displacements of the
under surface of panel 102.
[0091] As described above, in one embodiment each of the principal
elements 106 is provided with a bearing 121. The bearing 121 may
provide a region of load transmission from the touch surface 103a
to the corresponding principal element 106. Although the bearings
121 are shown as small bumps located at the centers of the
principal elements 106, it should be appreciated that other
elevated features may be provided in the principal elements 106 to
perform the same function.
[0092] The bearings 121 may serve as pivots. Locating the bearings
121 in the principal elements 106 themselves has the advantage of
providing the force sensor with a determined sensitivity. When
force is transmitted from an overlying surface (such as the touch
surface 103a) contacting one of the bearings 121, changes in
relative alignment of the corresponding one of the principal
elements 106 and the overlying surface (e.g., the undersurface of
panel 102) leave the region of load transmission substantially
unchanged. This effect becomes more pronounced as the size of the
bearings 121 and the corresponding region of contact decreases.
Note that, as shown in the embodiment shown in FIG. 1B, the
bearings 121 need not be disposed within receptacles.
[0093] Further details appropriate for the embodiment depicted in
FIGS. 1A-1B may now be considered. Frame 104a may be of mild steel,
plated or coated for corrosion resistance. It may be made from
0.020 in. sheet, stamped, folded, or drawn by any of a variety of
known techniques. Frame 104a may have flanges around 1/8 in. wide.
Panel 102 may be of either clear plastic or of glass. If of glass,
it may be around 0.050 in. thick. Preload springs 109 may each be a
round steel wire, 0.029 in. in diameter, and 0.080 in. longer than
the matching side of panel 102. In order to adopt the correct
straight form when assembled, each of springs 109 may be given an
unloaded curvature, which from a nil value at the spring's ends,
increases linearly towards the center of the spring where it is
attached to the panel 102.
[0094] Lateral stiffening means 108 may comprise, for example, a
polyester or polyimide film, 0.001 to 0.002 in. thick, with acrylic
adhesive on the under surface in two areas where attachment is
desired. The first such adhesive area 118 lies along the outer
portion of 108 beyond the dashed line, which portion folds down
over the vertical flanges of frame 104a. The second adhesive area
119 lies in a strip about {fraction (1/16)} in. wide around the
inner edge of 108. This area adheres to touch surface 103a slightly
in from the edge of panel 102. The stress in lateral stiffening
means 108, when bent along the dashed line, may be relieved, and
lateral stiffening means 108 may thereby be given a proper final
contour, by a simple thermoforming operation. This may be performed
either before or after assembly. The excess material at the
external corners of lateral stiffening means 108 may be folded
along the diagonal, and laid over to the side against the vertical
flange of the frame 104a. The suitable breadth of the freely
flexing region 120 of lateral stiffening means 108 depends upon its
own stiffness, upon the stiffness of panel 102, and upon the
accuracy required. It may, for example, be in the range of 0.060 to
0.120 in. It should be appreciated that the particular embodiment
of the lateral stiffening means 108 depicted in FIG. 1A is provided
merely for purposes of example and does not constitute a limitation
of the present invention. Rather, lateral stiffening means 108 may
include any structure or structures that limit lateral movement of
the panel 102 in response to touch forces.
[0095] In environments where accurate touch location is required on
a moving or shaking display, accelerometers 115a-b may be employed.
Accelerometers 115a-b may be rectangles of stainless or spring
steel shim stock 1 mil thick, 0.120 in. wide, and 0.250 in long,
plated for solderability. In the embodiment depicted,
accelerometers 115 are soldered to lands 116, so that they carry
over lands 117 as simple cantilevers, with capacitive gaps of about
2 mils. Any number of accelerometers may be used. For example, as
shown in FIG. 1A, the two accelerometers 115 are symmetrically
positioned on opposing sides and are connected in parallel. The
resulting single channel of Z-axis acceleration may then be
measured capacitively, and the results applied to correct the force
sensor channels as taught in Roberts U.S. Pat. No. 5,563,632.
Alternatively, three or four accelerometers driving separate
sensing channels could be used, for example, to encode X and Y
rotational accelerations, as well as the typically larger
acceleration of Z displacement. Since the magnitude of correction
required is generally modest, however, such refinement may not be
necessary in particular embodiments. Where one accelerometer
suffices, it may also be placed externally to module 101, such as
on an accompanying application circuit board. Such mounting may be
parallel to the touch plane, and may be centered approximately
under the centroid of the touch surface. As with the principal
elements of the force sensors, the accelerometer elements may be
constructed in variations with other shape than rectangular. They
may be manufactured and assembled by many of the same techniques as
may be applied to the force sensors.
[0096] Since panel 102 is not secured via the force sensor or the
preload springs 109 in the embodiment depicted in FIG. 1A, lateral
stiffening and restraint means 108 is employed both in its lateral
restraint aspect to maintain basic geometry, and in its lateral
stiffening aspect to define dynamic lateral stiffness. Note,
however, that lateral softening means 107 may be used even though
panel 102 has the potential to slide by tiny amounts with respect
to the sensors beneath. Preload forces, in addition to the touch
force itself, may create sufficient friction to prevent any
plausible tangential force from causing such sliding during a
normal touch. It is, therefore, the ratio of the lateral stiffness
of lateral stiffening means 108 to that of the sensor assemblies
only in the differential sense for small forces that cause no
sliding which determines the path taken by tangential touch
forces.
[0097] Although lateral stiffening means 108 is depicted in FIGS.
1A-1B as a single piece of material, this is simply an example and
does not constitute a limitation of the present invention. For
example, lateral stiffening means 108 may be assembled with 4 tape
segments, butted or overlapped in any of various ways at the
corners. Alternatively, lateral stiffening means 108 may be, for
example, a single sheet of transparent film, attached with an
optically clear adhesive over the full interior area of touch
surface 103a. Lateral softening means 107 may include a thin layer
of a tough but soft elastomer, such as natural rubber. However, the
simpler choice of soft acrylic adhesive has proven sufficiently
tough and compliant, in spite of being somewhat thinned in the
bearing area when the foil is only 0.0015 in. thick. Panel 102 may
be detailed at its edges, especially if made of plastic. For
instance, holes parallel to the surface near the corners of the
panel 102 may retain angled preload spring ends, with hooks bent
inward from frame 104a to hold the preload springs at their
centers.
[0098] FIG. 2 presents touch overlay module 101 as it might be
employed within a typical application device 201. Application
enclosure 202 includes bezel 203, carrying alignment feature 204 on
its inner surface. Alignment feature 204 may, for example, be
continuous, comprise periodic isolated protrusions, or comprise the
ends of periodic stiffening ribs. In addition to touch overlay
module 101, enclosure 202 contains LCD display module 205, and
application electronics 206. LCD 205 and electronics 206 may be
retained and positioned by standoffs as depicted here for
diagrammatic simplicity, or by engagement with molded details in
enclosure 202. Touch module 101 may be retained, centered, and
aligned with respect to the display surface of LCD module 205 by
the pressure of bezel 203, in conjunction with feature 204 and the
rigid support provided by LCD 205. When so retained, upon opening
enclosure 202, touch module 101, and perhaps the other internal
components, may be freely separated. Alternatively, touch module
101 may be permanently or semi-permanently fastened to LCD module
205 by such means as cement, or acrylic transfer adhesive, applied
between frame 104a and the surface of LCD 205. In this instance,
feature 204 may be omitted, or may be employed to better center the
visual opening of bezel 203 by slightly flexing the sides of
enclosure 202.
[0099] The horizontal flanges of frame 104a may receive support
from LCD module 205 by engaging either portions of the bare LCD
glass, portions of the polarizer covering that glass, or portions
of the partial metal enclosure which typically wraps around the
edges of LCD module 205. The highest surface encountered by frame
104a will determine the source of support. The entire horizontal
flange width of frame 104a need not be engaged to provide
satisfactory support, but touch module 101 and frame 104a may be
sized such that engagement occurs in the same plane around all, or
nearly all, of the periphery of touch overlay module 101. Small
gaps in the support of frame 104a are tolerable, but large gaps in
support along the length of 104a are preferably, but not
necessarily, avoided.
[0100] Note that the application of touch module 101 to the surface
of LCD module 205 generates gap 207. Some space (represented by gap
207) may be required for proper operation of touch module 101 so
that vertical displacements of panel 102 created in normal touch
operation do not transfer forces by contact of panel 102 to LCD
205. Gap 207 may also be provided because of the fact that pressure
applied to the surface of an LCD module often results in unpleasant
visual effects, due to displacement of the image-forming fluids
within the LCD. Finally, routine or heavy compression of the LCD
surface may lead to damage, called "bruising". Avoidance of such
bruising may require a larger size of gap 207 than that used to
satisfy the previously stated considerations.
[0101] If, however, the size of gap 207 as otherwise implied by the
construction of touch module 101 is greater than desired, simple
variations of the embodiment depicted in FIG. 2 may be used to
reduce the size of gap 207. In particular, a ledge or step in the
back surface of panel 102 around its edge may be used to engage the
force sensors at their usual height, and provide clearance from
frame 104a, while lowering the back surface of panel 102 over the
display area of module 205, thus narrowing gap 207. Touch surface
103a may be left in the original plane, thus occasioning greater
thickness of panel 102 over the bulk of its area. Alternatively,
touch surface 103a may also be lowered somewhat, and the overall
height of module 101 thereby reduced. This is made possible by the
fact that the strength and stiffness of panel 102 are related
principally to the central portion of its area.
[0102] In a second embodiment of the invention, a force sensing
touch location device is provided in which a surface of an
LCD--rather than an overlay panel (such as the overlay panel 102
shown in FIG. 1A)--serves as the touch surface. For example, the
actual display panel of an LCD assembly may replace the overlay
panel 102 in the touch sensitive transparent overlay module 101
shown in FIGS. 1A-1B. The display panel and possibly other internal
components of the LCD assembly may then be supported by principal
elements 106 in conjunction with lateral stiffening means 108. The
force sensors based upon principal elements 106 may thus be
displaced considerably farther from the touch surface in such an
integrated touch LCD than in the transparent overlay module 101
shown in FIG. 1A; however, the combined use of lateral stiffening
means 108 with lateral softening means 107 may be used to prevent
the introduction of tangential force errors.
[0103] As compared to the transparent overlay module 101, the touch
LCD embodiment described above may benefit from improved optics,
reduced overall thickness, and reduced parallax. Improved optics
result primarily from removing two of the three solid/air
boundaries potentially requiring antiglare treatment. Reduced
thickness may result from eliminating gap 207 and merging of panel
102 with the top-glass of the LCD display to form a single glass
layer of less aggregate thickness. Since these thickness reductions
move the touch surface closer to the image-forming layer of the
LCD, there is also a reduction in touch parallax.
[0104] Although, as previously stated, many LCDs are not
appropriate for direct application of touch, some are; and the
designs of others may be altered for direct application of touch in
combination with the second embodiment of the invention described
above. Such alterations may include, for example, a slight
thickening of the LCD front glass.
[0105] Referring to FIG. 3, a self-contained, touch-enabled LCD
module 305 is shown according to the second embodiment of the
invention. The differences between touch LCD 305 and touch module
101 are best exemplified in the cross-sectional diagram of FIG. 3,
which also exhibits a typical containing application device
301.
[0106] Touch LCD 305 comprises frame 104b, LCD electronics board
304, light diffuser 303, LCD display panel 302, principal elements
106, lateral softening means 107, and lateral stiffening means 108.
Furthermore, preload springs similar in function to springs 109 are
also present, but do not show in the plane of section. Frame 104b
does not require a clear visual opening and so may close across the
back of touch LCD 305, shielding, stiffening, and protecting it,
and in other ways subsuming some of the functions of a conventional
LCD module frame. Frame 104b, though still of thin material, here
has substantially greater section depth than frame 104a, and so
does not include support from behind, continuously or nearly
continuously around its periphery, as frame 104a does in module
101. A separate interconnect 105 is no longer present, as its
function is subsumed by LCD electronics board 304. Board 304 is
firmly supported against frame 104b in the immediate vicinity of
each of the principal elements 106. However, depending upon the
thickness of board 304, the firm support may not require bonding
sufficient to stiffen board 304 under principal elements 106. The
goal must be to achieve sufficient net stiffness under principal
elements 106 that the end constraint, in this sensor embodiment, is
essentially a clamped end constraint. In particular, the residual
elasticity of the end constraint should be sufficiently small
and/or reproducible that the behavior of the force sensor is not
rendered unpredictable.
[0107] In the variation depicted in FIG. 3, diffuser 303 and
display panel 302 interlock, or are otherwise so attached, as to
travel together for purposes of positioning and force transmission.
They are supported--in a manner similar to panel 102 of module
101--by lateral softening means 107, coupling to bearings 121 of
principal elements 106; and by lateral stiffening means 108.
[0108] Note that diffuser 303 is depicted with shallow bosses
extending downward to establish contact with the force sensors.
This is because the force sensing assemblies will generally be
thinner than the thickest components mounted to board 304. In other
variations, the diffuser 303 may be carried with board 304 and move
independently of display panel 302. Perpendicular forces applied to
panel 302 are then transmitted back to the force sensors through
columns, bosses, or tabs extending between. These roughly columnar
structures may have sufficient flexibility in both transverse
directions to perform the same function as lateral softening means
107, obviating the need to entrain a thin layer of soft material as
previously depicted. Such columnar structures may be molded as part
of the same component comprising diffuser 303, being softly
connected thereto by thin molded connections.
[0109] Display panel 302 may connect to electronics 304 with either
flex cable, or a sufficiently compliant elastomeric connector. If
the connection is hard-docked, as with screws, a cantilever tab may
be routed into the edge or interior of the PC board of 304 to carry
the connection with sufficient perpendicular compliance.
[0110] Various embodiments of the invention employ no permanent
connection between the force sensing assemblies and the floated
structures they support, whether overlay plate or display
components. This simplifies assembly, relaxes requirements for
precision and dimensional stability, and provides a simple means
whereby the force sensors may be protected from unwanted rotational
sensitivities. In such embodiments of the invention, provision may
be made to establish a static perpendicular preload force to keep
the floated components firmly seated against the force sensors
during all ordinary conditions of operation.
[0111] In one embodiment of the invention, the preload means apply
sufficient total preload force, are possessed of a sufficiently low
spring constant, do not provide unwanted lateral stiffening
significantly removed from the plane of touch, and are coupled to
the floated components with sufficient symmetry to preload the
sensors more-or-less equally.
[0112] In various embodiments, a minimum sufficient preload force
may be established by factors including but not limited to the
following. If it is desired that the touch apparatus operate in any
orientation, a total preload force may be provided that exceeds the
weight of the floated overlay plate or display components. In the
case of large, statically mounted displays, this may be the main
consideration. In other cases, other total preload forces may be
provided if particular resistance to vibration and/or enclosure
torsion is desired. In automotive applications, for instance, the
need to avoid buzzing under at least several g's of vibration may
lead to use of a total preload force of at least several times the
floated weight. In all applications, there is the potential for
unsymmetrical loading, as may occur when the application enclosure
is seated against an uneven surface. This can lead to torsion
extending to frame 104a, such that the corners of frame 104a no
longer lie in a common plane.
[0113] Modest preload forces prevent torsional problems with touch
module 101. This is due to the relative flexibility of plate
102--which is generally made as thin as possible--and to the
stiffness of the underlying LCD. Greater preload forces, and/or a
surrounding structure more resistant to torsion may be used with
touch LCD 305.
[0114] In one embodiment, the preload force changes very little as
a function of ordinary touch displacements, in order that
essentially all of the perpendicular force change occasioned by a
touch may pass through the sensor assemblies. Thus, the preload
force may be applied by elastic means which, in use, are deflected
a long distance from their unloaded state. The "long distance" in
question is considered in comparison to the distance through which
the preload force deflects the common path shared by both the
preload force and perpendicular touch forces. In one embodiment,
each of the preload springs 109 of module 101 applies a total force
of about 1 lb. when its ends have been flexed through the
approximately one inch displacement required to place them in
assembled position. For example, a touch close to the location at
which the spring 109 is attached by cement 110 shares the maximum
common path with the preload force--conversely, it tends to
generate the greatest flex in preload spring 109. For a one pound
touch force, the deflection generated at the location of cement 110
is not more than a few thousandths of an inch, the great bulk of
which occurs in panel 102 itself, rather than in principal elements
106. Since the preload force is a roughly linear function of
preload spring deflection, it can be seen that well under 1 percent
of the perpendicular touch force is diverted through springs 109,
and therefore "not seen" at the force sensors.
[0115] Since the ends of springs 109 press upward against the inner
surfaces of holes 112 at points very close to the plane of touch,
it is immaterial that springs 109 may provide significant
additional lateral stiffness against displacements parallel to
their length. Other embodiments of the preload springs 109,
however, with an end or ends retained significantly out of the
plane of touch, might include some ancillary lateral softening
means.
[0116] Alternatively, in other embodiments, preload spring may be
applied along all four edges of an overlay or other touch surface
structure, and by appropriate attachment of their ends, serve also
as either lateral stiffening means or lateral restraint means.
Furthermore, such springs may be located wholly or in part below
the touch surface. With an appropriate shallow sigmoid shape, and
supporting ends attached somewhat below centers affixed to the
touch surface structure, such springs may further comprise a
lateral stiffening means in accordance with an angled stiffness
structure, as described in the co-pending application entitled
"Tangential Force Control in a Touch Location Device."
[0117] Touch LCD 305 may be preloaded with a design identical to
that of touch module 101. Free access to areas behind the display
panel 302, however, creates other opportunities for locating the
preload means. A single spring, for instance, might be attached to
the back of the LCD panel 302 near its center. A spring wire,
having the assembled shape of a "Z", might attach at its ends to
the frame sides, and at its center to the back of the LCD panel
302. A nearly closed "C" shape might connect at its ends between
the opposed centers of the frame back and of the floated assembly.
Many other variations will be apparent to those of ordinary skill
in the art. Note that the preload spring attachments may be well
removed from the plane of touch; therefore the shape of the spring
may allow relatively soft flexure in all directions, in which case
additional lateral softening may not be used.
[0118] Note that preload may be accomplished with a larger number
of smaller elastic devices. For example, such elastic devices may
attach to the floated components at points adjacent to each sensor.
In one embodiment, the perpendicular deflection of the sensor
assembly is smaller than the deflection that will occur in the
overlay or LCD panel 302 at points far from support. Thus, springs
located very close to the sensors may have smaller unloaded
displacements, less assembled energy storage, and substantially
smaller size, and yet still divert insignificant touch force. In
the situation just described, where the sensors are stiff compared
to the touch surface, a preload spring near one sensor will do
little to load others. Therefore, it may be advantageous to use one
preload spring per sensor.
[0119] An application bezel, such as application bezel 203,
typically applies forces to a touch module such as touch module 101
or a display module such as display module 305. These will include
static forces associated with assembly and seal maintenance, and
variable forces from handling. A force-based touch system should be
designed such that operation is not disturbed by these forces. In
various embodiments of the present invention, components such as
frames 104a and 104b receive and transmit these forces. For
example, frame 104a may be provided with an elevated lip; that is,
a vertical flange which rises above the level of touch surface 103
by a small amount. This facilitates use of an application bezel of
simple design, that may have a flat and parallel undersurface,
without danger of touching the overlay panel 102 (or, similarly,
the floated LCD panel 302 in FIG. 3). In application, bezel forces
applied to touch module 101 are transmitted directly to the surface
below, which may be a very stiff LCD module. Thus by "borrowing"
the stiffness of the surface below, frame 104a resists significant
deflections. The bezel forces of greatest concern are predominantly
perpendicular. Thus the greater section depth of frame 104b also
allows the bezel forces to be successfully resisted in touch LCD
305 (FIG. 3), in spite of the use of thin material.
[0120] The thinness of the vertical legs of frames 104a and 104b
maximizes the active touch area in relationship to the overall
module dimensions. In touch LCD 305, for example, rearward
placement of the force sensors, combined with the thinness of the
vertical leg of frame 104b, allows the lateral dimensions of touch
LCD 305 to be scarcely greater than those of an LCD with the same
image size not equipped for touch. Since frame 104b replaces a
partial metal enclosure normally present, increase of width is
limited to the introduction of a small clearance gap, plus any
differential in material thickness. Since touch LCD 305 also avoids
much or all of the increase in thickness usually associated with
touch input, touch LCD 305 is of particular benefit in portable, or
other space-constrained, applications.
[0121] Thus, in addition to other functions, vertical legs of frame
104a and 104b are seen to comprise application bezel support
members.
[0122] In other variations of the invention, an additional
application bezel support member may be provided, that both closely
invests the force-sensitive structure, and that transmits
application bezel forces to support behind. For instance, in one
variation, a continuous rib or flange member may be molded into the
application bezel. This flange member may extend perpendicularly
downward from the undersurface of the application bezel, emerging
from the lateral body of the application bezel along a line spaced
slightly in from the visible edge of the bezel opening, and resting
along its lower edge along the LCD display or other stiff support
surface beneath. The height of the flange member is such as to
provide necessary clearance between any inwardly protruding
extension of the application bezel and force sensitive structures.
The flange member may be fully continuous; however, it may also be
interrupted into a sequence of segments or a row of bosses, closely
enough spaced to "borrow" the necessary stiffness from below.
[0123] In another variation, an additional application bezel
support member may comprise a vertical leg of a metal stamping
coupled to, or part of, an LCD or other display assembly, but
distinct from the vertical leg of frame 104a or its equivalent,
while wrapping around and closely investing it. In yet another
variation frame 104 may take the form of a "U" channel, with the
interconnect and force sensors attached directly to the display
surface just inside this channel. The inner vertical leg may then
provide support for a lateral stiffening and restraint means, seal,
and preload means, while the outer vertical leg comprises an
application bezel support member.
[0124] In yet another variation of the invention, an additional
application bezel support member may, while remaining laterally
thin, be extended perpendicularly so as to closely invest an entire
force-sensitive display structure or more, thereby achieving from
depth of section greater stiffness against flexure from
perpendicular bezel forces. Such member may then receive support
for such forces from localized attachments to structures behind, or
from other structures not constituting a continuous stiff surface
of support.
[0125] In various embodiments of the present invention, the
application bezel support member of the invention comprises a path
for bezel support that closely invests force-sensitive structures;
cantilevered support from the outer edges of an application
enclosure is thus avoided, and disturbance to force-sensitive
structures is minimized. A novel opportunity for forming an overall
liquid and/or dust seal may also be thus obtained.
[0126] The lip of the vertical leg of frame 104a provides a line
against which the application bezel 203 may achieve a dust and/or
liquid seal, and also provides a convenient point of attachment for
continuing a flexible liquid and dust seal from the frame 104a to
the touch surface 103a. By providing a separation of the sealing
function into an internal flexible seal and an external application
seal, various embodiments of the invention simplify application
assembly. The vertical frame leg also provides a point of
attachment for a lateral stiffening means (such as lateral
stiffening means 108) which is close to the plane of touch. While
the lateral stiffening means, lateral support means, and the
sealing means are embodied within the same physical element in the
particular embodiment depicted in FIGS. 1A-1B, FIG. 2, and FIG. 3,
this need not be the case. In some applications, for example, there
may be advantage in confining the lateral stiffening means and/or
lateral restraint means to the vicinity of the sensors, where less
vertical flexure is encountered, while distributing a thinner seal
film around the entire periphery.
[0127] Various embodiments of the present invention advantageously
reduce the introduction of touch location errors by tangential
forces. For example, referring to FIG. 4, touch surface 103 (which
may, for example, be the touch surface 103a shown in FIG. 2 or the
touch surface 103b shown in FIG. 3) resides upon floated structure
401, which may represent, for example an overlay (such as overlay
panel 102 shown in FIG. 1A) or display unit (such as LCD panel 302
in FIG. 3). A finger 402, applies a touch force comprising
tangential component 403 and perpendicular component 404. Structure
401 is supported by a lateral stiffening means 405, and by force
sensors 407 through lateral softening means 406. Receiving all
forces is surrounding structure 408. Tangential component 403 of
the touch force applied by the finger 402 generates reactions 409,
and perpendicular component 404 of the touch force applied by the
finger 402 generates reactions 410a and 410b. Due to the
construction and positioning of lateral stiffening means 405, the
combination of component 403 and reactions 409 generate no net
moment. In the absence of such extraneous moments, then, the
partitioning of the reaction to perpendicular component 404 between
410a and 410b accurately locates the touch position in accordance
with force and moment equations that are well-known to those of
ordinary skill in the art.
[0128] Although lateral stiffening means 405, force sensors 407,
lateral softening means 406, and surrounding structure 408 are
illustrated in FIG. 4 in generalized form, it should be appreciated
that these elements may be implemented, for example, as shown in
FIG. 1A, FIG. 2, and FIG. 3. For example, lateral stiffening means
405 may be lateral stiffening means 108, force sensors 407 may be
the force sensors shown in FIG. 1A, lateral softening means 406 may
be lateral softening means 107, and surrounding structure 408 may
be enclosure 202 and/or frame 104a or 104b. Note also that lateral
softening means 107 may be provided below sensors 407, rather than
above as shown, and also provide the desired function. Further, if
the lateral stiffness of the force path through structure 401,
sensors 407, and supporting structure 408 is low enough compared to
that of the force path passing through lateral stiffening means
405, then lateral softening means 407 may be omitted.
[0129] Lateral stiffening means 405 is in part so named because it
rests where a void might well exist in a conventional force-based
touch device, while lateral softening means 406 is in part so named
because it is inserted where a rigid coupling typically exists in
conventional force-based touch devices. Note that in both cases,
though, a coupling may be desired which is much stiffer to forces
applied in one direction than to another at right angles. Columns,
beams, plates, and membranes of high aspect ratio, for example,
have this property, as do high aspect layers of elastomer trapped
between rigid flat surfaces. Classical bearings do also, of course,
but here it is better, as well as simpler, to avoid rubbing
surfaces that may exhibit stiction at small force levels.
[0130] Some additional aspects should be noted which are not shown
directly in FIG. 4. Lateral stiffening means 405 may also be
present along the edges above and below the plane of the FIG. 4. In
various embodiments of the invention, reaction forces 409 are
developed primarily through shear in these other portions of
lateral stiffening means 405.
[0131] FIGS. 5A, 5B, and SC illustrate one embodiment of the
lateral stiffening means 405. Generalized floating structure 401a,
which may represent an overlay (such as overlay panel 102 shown in
FIG. 1A) or display unit (such as LCD panel 302 in FIG. 3),
receives perpendicular support from generalized force sensor 407
through lateral softening means 501, portrayed in this variation as
an elastomeric sheet. Lateral stiffening means 502 is a sheet of
material, with its freely flexing region intended to rest as close
as possible to the plane of touch. Lateral stiffening means 502 may
be carried around the full periphery of 401a, or may be confined to
certain regions, such as those near the sensor mountings. There are
two independent degrees of tangential force; one directed along the
left/right axes of FIGS. 5A-5C, and tending to place the portion of
lateral stiffening means 502 visible in these sections into tension
or compression, and another perpendicular to the plane of FIGS.
5A-5C, and tending to place the portions of lateral stiffening
means 502 visible in these sections into shear. If lateral
stiffening means 502 is kept essentially flat, both degrees are
effectively resisted by all portions of lateral stiffening means
502. For most of the materials of which lateral stiffening means
502 might be composed, the ratio of Young's modulus to the modulus
of rigidity is such that about 3 to 4 times as much stiffening will
come from portions of lateral stiffening means 502 in tension or
compression as from equal lengths in shear.
[0132] Referring to FIG. 5B, perpendicular force 503 may cause a
perpendicular deflection of touch surface 103 through distance 506,
such that the flexing portion of lateral stiffening means 502
becomes tilted and stretched. This distance 506 may be particularly
large at points midway between the support offered by the sensors,
as is depicted in this cross-section. Tension in lateral stiffening
means 502 rises as the square of distance 506. Due to the tilting
of lateral stiffening means 502, this tension has a vertical
component 504, which becomes part of the balancing reaction to
applied force 503. This diminishes the reaction component 505,
passing through the out-of-section sensors, to below the expected
value, causing some error.
[0133] FIG. 5C depicts a situation in which the flexing portion of
lateral stiffening means 502 is tilted in the absence of
perpendicular load. Distance 510 may represent, for example, either
an intentionally raised lip of frame 104, or the effect of
component and assembly tolerances. Tangential force 507 causes
compression in lateral stiffening means 502. Since this compression
is tilted, it contains a perpendicular component balancing reaction
509, in addition to a tangential component that balances the
tangential force 507. A similar situation in tension occurs along
the opposing edge. Error force 509 and its equal but opposite
counterpart acting upon sensors along the opposing edge, together
represent a substantial moment generated in reaction to tangential
force 507. This "jamming" effect represents another characteristic
of the configurations depicted in FIGS. 5A-5C.
[0134] FIG. 6 depicts another lateral stiffening means 601, which
is provided everywhere with a modest contour. Because lateral
stiffening means 601 is compliant vertically (i.e., in a direction
substantially normal to the touch surface 103), this contour allows
surface 103 to be deflected substantially without placing lateral
stiffening means 601 into tension. This improves the range of touch
forces which may be located accurately, especially for touches near
the edge between sensors. The contour of lateral stiffening means
601 also greatly decreases the lateral stiffening effect in tension
and compression. Since the lateral stiffness provided by the sides
of lateral stiffening means 601 in shear may still be made
sufficient, however, this is advantageous in greatly decreasing
error from imperfections which have effect selectively through the
tension and/or compression of the lateral stiffening means
(referred to herein as the "jamming effect").
[0135] The structure of lateral stiffening means 601, and of others
discussed here, may also be employed as lateral restraint means. In
such use, contouring conveys similar benefits with regard to
increasing the perpendicular range over which perpendicular
stiffness is slight, while retaining a high ratio of lateral to
perpendicular stiffness throughout.
[0136] Floating structure 401b is depicted with beveled edge 602.
This allows the force sensors and the lateral stiffening means 601
to share the same narrow border width, while preserving clearance
for the flexing portion of the latter. Application bezel 203 is
depicted with additional feature 604 intended to guarantee
clearance between the bezel 203 and both lateral stiffening means
601 and surface 103. Bezel 203 is depicted as carrying fully over
the border structures, both to conceal them cosmetically, and to
protect lateral stiffening means 601 from damage.
[0137] An additional point may be noted with regard to the contour
of lateral stiffening means 601. The elastic axis of rotation for
lateral stiffening means 601 in shear lies at the level of dashed
line 603. For roughly circular contour, the offset of dashed line
603 from the plane of touch is approximately twice the maximum
offset of lateral stiffening means 601 itself. If the contour of
lateral stiffening means 601 were that of a shallow "V," dashed
line 603 would lie at the level of its point. Since the plane of
accuracy lies at the level of dashed line 603, tangential force
rejection is not perfect; it is, however, still substantial.
[0138] FIGS. 7A-7C depict additional variations 108a-c of the
lateral stiffening means 108, as may be applied, for example, to
the first and second embodiments depicted in FIGS. 1A-1B, FIG. 2,
and FIG. 3. In these variations, frame 104 is depicted with an
intentional elevation, or lip, which may rise 0.020 in. above touch
surface 103. Lateral stiffening means 108a also acts as a seal and
is provided with a fairly abrupt "dog leg" contour 701a. Most of
the flexing region of 108a is backed up by overlay 102. This
portion achieves the advantage of becoming quite resistant to
damage, and need not necessarily be covered by the application
bezel 203a. It should be appreciated that in other embodiments,
lateral stiffening means 108a may not provide a seal between frame
104a and touch surface 103.
[0139] In FIG. 7A, contour 701a is placed close to the point 702 at
which lateral stiffening means 108a attaches to surface 103. Bezel
203a is of minimal width. Lateral stiffening means 108a may be
opaque, and of a color suitable for a visible detail of the border.
Note that there is little or no exposed cavity under the bezel 203a
where contamination may collect, so that this arrangement may be
particularly suitable for dirty environments. In FIG. 7B, contour
701b is placed close to the lip of frame 104. Bezel 203b is
depicted concealing the border structures. Lateral stiffening means
108a and 108b in FIGS. 7A and 7B, respectively, may be applied as,
for example either four separate tapes, or as a single die cut
piece.
[0140] For the dog-leg lateral stiffening means 108a-b of FIGS.
7A-7B, the elastic axes 603 for rotation in reaction to shear lie
at approximately the average height of the flexing portion of the
lateral stiffening means above touch surface 103. The resulting
plane of accuracy may be sufficiently close to the touch plane for
many purposes. Note, however, that any residual jamming effect
tends to put the plane of accuracy below the touch surface 103,
whereas the axes 603 here lie above it. Thus by adjusting the
position of contour 701 and/or the lip height, the two opposing
effects may be adjusted to cancel out. This constitutes one example
of a lateral stiffening means that creates tangential reaction
forces much more closely confined to the plane of touch than is the
lateral stiffening means itself.
[0141] In FIG. 7C, lateral stiffening means 108c comprises a
transparent film which passes over the entire touch surface 103.
The area of lateral stiffening means 108c interior to the point of
attachment 702 is fastened with optical adhesive. If bezel 203a is
minimal as shown, and if floating structure 401 is otherwise
transparent, it may be cosmetically advantageous to coat the upper
or lower surface of floating structure 401 along the edges with
opaque material (so as to conceal sensors and other edge structures
from user view). If floating structure 401 is a glass overlay or
fragmentable display, lateral stiffening means 108c provides an
advantageous safety effect in case of breakage. Since surface 103
is of uniform optical quality right up to the point of attachment
702, this point may now be placed farther inward without increasing
the border width. Since the full border width is now available for
the flexing portion of 108c, the advantage is gained that the
lateral stiffening means 108c may now be made thicker, and
therefore tougher, without giving it excessive perpendicular
stiffness.
[0142] Turning to FIG. 8, molded plastic bezel insert 801 carries
transparent protective film 802. Flange 803 on insert 801 engages
slot 804 in application bezel 203b. Together, insert 801, film 802,
and bezel 203b provide a liquid/dust seal. Film 802 also protects
the upper surface of structure 401 from scratches, especially if it
is a plastic overlay or an LCD polarizer film. If 401 is a bare
glass overlay, film 802 provides some protection from shards in
case of breakage. The combination of insert 801 with protective
film 802 constitute a part easily replaced in the field. Tiny holes
805 are present at the center of each side, and provide purchase
for a needle or pointed tool to draw the insert inward out of slot
804. Flange 803 and slot 804 are engaged maximally at the mid
sides, but taper to negligible engagement at the corners to
facilitate replacement.
[0143] Touch pressure brings film 802 into firm contact with the
surface of 401 under the point of contact, allowing the touch
module below to locate the point accurately. Film 802 may be made
quite thick, since perpendicular force transmitted to its
attachment at 801 does not generate a reaction force passed to 401.
That is, there is no analog to the problem depicted in FIG. 5B.
Lateral stiffening/restraint means 806 is also provided, but this
no longer need perform a combined seal function, and may be
implemented in a wide range variations.
[0144] Turning to FIGS. 9A-9B, a larger sensor according to another
embodiment of the invention is depicted. Principal element 106b is
made from spring steel strap 1/4 inch wide and 10 mils thick. This
is cut to a length of 3/4 in. and pressed in a die to the shape
shown. The capacitive gap is 5 mils, but has been drawn with some
exaggeration for clarity. The free span of principal element 106b
is 550 mils, the central 300 mils of which are opposed by land 114.
A substantially planar support surface is found on the epoxy glass
PC board 901, which is only slightly larger than principal element
106b. Discrete wiring 105b provides interconnection. PC board 901
is mounted against underlying support 408 with segments of acrylic
tape 902, which also constitute a lateral softening means. PC board
901 is of sufficient stiffness that lateral softening means may be
placed thereunder. This configuration has the advantage that should
support 408 flex, its curvature is very poorly transmitted to board
901, thus preventing enclosure forces from disturbing force
readings. Pivoted force bearing 121b is in the form of a ridge, and
suffices to fix sensor sensitivity, while providing good strength
against extreme overloads. Unloaded capacitance is about three
picofarads, and the bottoming-out force is between four and five
pounds.
[0145] Although the use of different materials may require other
choices of dimensions, the principal element 106b may be made from
other materials, such as plastic with a electrically conductive
coating.
[0146] Turning to FIGS. 10A-10B, a smaller sensor according to one
embodiment of the invention is depicted. Principal element 106 is
cut from spring steel 6 mils thick. It is 120 mils wide and 230
mils long. Alternatively, principal element 106 may be of
phosphor-bronze 8 mils thick, with the same length and breadth. The
capacitive gap is 1 mil, formed by spacing the gap with a temporary
shim while lands 113 are reflowed with solder. Alternatively, the
solder may contain particles of controlled size that act to space
the principal element 106 from lands 113.
[0147] Bearing dimple 121 may be created with a spring-loaded
center punch while principal element 106 is pressed against an
aluminum backing. The free span of principal element 106 is 150
mils, the central 86 mils of which are opposed by land 114.
Unloaded capacitance is about three picofarads, and the
bottoming-out force is between three and four pounds.
[0148] Other details of assembly are as described for the sensors
shown in FIG. 1A.
[0149] Capacitive force sensors exhibit a change in capacitive
reactance as a function of a change in applied force. For the
sensors of FIGS. 9A-9B and 10A-10B, this change is substantially
linear for smaller forces, where the relative gap change is small.
With larger forces, however, the center of the capacitive region
closes up while the edges remain more widely spaced; this leads to
a drop in reactance that becomes more rapid than linear. To
increase the range of force sensing that may be accomplished with
high accuracy, compensation for the response characteristic just
described may be accomplished in the processing of the sensor
signal; alternatively, varied embodiments of the sensor of the
invention may be provided which have an inherently greater range of
linear reactance change.
[0150] Thus in another novel aspect of the invention, a capacitive
force sensor of nonuniform gap may provide improved linearity of
measurement with simple processing of the signal, even where one or
more capacitor plates are flexing in response to applied force.
[0151] For example, FIG. 11 depicts a sensor 1100 with overall
dimensions similar to those of the sensor of FIGS. 10A-10B.
Principal element 106c, however, has been provided with a slight
bend of controlled shape. Because this bend would otherwise be too
subtle to depict with clarity, the vertical dimensions of the
sensor 1100 are exaggerated tenfold in FIG. 11 with respect to the
sensor's horizontal dimensions. The bend is such that the ends of
element 106c may attach to lands 113 with a minimal solder film,
while the center provides a maximum capacitive gap (between point
1102 and the upper surface of land 114) of about 1.5 mils.
[0152] There is a level of force that may be applied to coupling
121c which is just sufficient to first bring element 106c into
contact with the land 114. The tapering of the capacitive gap away
from the exact center point 1102 of element 106c may be so shaped
that this contact tends to happen simultaneously at all points
where element 106c opposes land 114.
[0153] Such a nonuniform gap design may help to provide a force
sensor with optimal linearity. Call a general applied force "F",
and call the minimum force to bottom out the sensor "F_max".
Subject to the assumptions that the gap is thin compared to its
lateral dimensions, and that Hooke's law applies, the stated
condition upon the gap shape requires that the gap spacing be
everywhere proportional to F_max-F. Each small region then adds to
the total capacitance a contribution proportional to 1/(F max-F).
This expression of the functional dependence upon applied force is
not itself a function of position, and so factors out of the area
integral defining the total capacitance. The overall sensor
capacitance thus varies in proportion to 1/(F max-F), and its
capacitive reactance at a given frequency is proportional to
F_max-F. This is, of course, the expected behavior for an ideal
parallel plate capacitor spaced by an ideal spring. Thus a linear
measure of the perpendicular force transmitted may be obtained by
differencing the reactance before and during a touch, for the full
range of gap closure.
[0154] Principal element 106c is substantially rectangular and of
uniform thickness, and is mounted rigidly at its ends through lands
113 to interconnect 105 or other support. Also, all deflections to
be considered are small compared to the thickness of element 106c.
Therefore, perpendicular force applied to coupling 121c will
deflect element 106c in a pattern closely approximating that of a
centrally loaded uniform beam with clamped end constraint. This
deflection pattern may be expressed as
d.multidot.(3.multidot.x.sup.2-2.multidot.x.sup.3), where d is the
maximum deflection, and x is the fractional position along element
106c, measured from the last clamped point 1101, where x=0, to the
center of element 106c at point 1102, where x=1. The curve from
point 1102 to point 1103 then continues as the mirror image of
this.
[0155] The desired shape for element 106c in its unloaded condition
is, therefore, the negative of this deflection pattern, extended
with flat ends for mounting. In cases where the end constraint has
significant rotational flexibility, the correct shape for element
106c may be derived from the stated deflection pattern by
associating with point 1101 a value of x that is somewhat larger
than zero. In the limiting case of simply supported ends, x=0.5 may
be assigned to point 1101, while x=1 is still assigned to point
1102.
[0156] For convenience of exposition, the curve for element 106c
has been defined here over the entire span between attachments at
point 1101 and point 1103. Only the area of element 106c opposing
the second capacitor plate (i.e., land 114) needs to follow this
curve, however, so long as other regions do not bottom out before
the capacitive areas do.
[0157] Although providing substantial improvement, this
one-dimensional analysis is not fully precise, given that coupling
121c approximates a point feature, rather than a linear one as does
bearing 121b of FIGS. 9A-9B. Further degrees of refinement,
however, may be obtained as desired through methods of analysis
well known in the art, as well as by empirical means. Such methods
may also be similarly employed to linearize the reactance response
of a wide range of other capacitive force sensor variations that
fall within the scope of the invention. Such variations include,
for example, complex outlines, nonuniform thickness, flexure in one
capacitor plate or both, multiple areas of support or single
cantilevered support, etc. In all cases, the desired effect is
achieved by shaping the surfaces of one or both capacitor plates to
produce a gap that "bottoms out" simultaneously at all points.
[0158] Turning to FIGS. 12A-12D, additional outline and mounting
arrangements of force sensor principal elements are shown according
to various embodiments of the present invention. All of the
elements shown in FIGS. 12A-12D may, for example, be made with
uniform thickness. Principal elements 106d, 106e, and 106f provide
regions variously narrowed so as to concentrate flexure in areas
1203a-c not serving as capacitor plates. This reduces flexure in
capacitive areas 1202a-c, improving linearity of reactance change.
Couplings 121d-f receive perpendicular force, which is passed to
structures beyond via support areas 1201a-c. With the thicknesses
of the principal elements 106d-f being greater, for a given
stiffness, than elements of similar size without narrowed regions,
clamped support of areas 1201a-c may receive less concentrated
twisting stress. Conversely, the concentration of flexure into
areas 1203a-c means that simple support of areas 1201a-c will see
greater rotation. Couplings 121d-f may be elevated features as
described previously, elastic features as described below, or any
other coupling feature providing a defined path for entrance of the
force to be measured.
[0159] Referring to FIG. 12C, principal element 106f is provided
with three areas of support 1201c, whereas principal element 106g
(shown in FIG. 12D) is a simple cantilever with a single area of
support 1201d. Cantilevered element 106g must, of course, receive
clamped support in area 1201d; whereas the other elements 106d-f
may be adapted for either simple or clamped support in areas
1201a-c, respectively.
[0160] Turning to FIGS. 13A-13B, additional variations for the
cross-sectional shape and thickness of a principal element of a
force sensor are shown according to embodiments of the present
invention. For example, referring to FIG. 13A, a sensor 1300 is
shown according to one embodiment of the present invention. The
vertical dimensions of the sensor 1300 (and the sensor 1310, shown
in FIG. 13B) are exaggerated approximately tenfold in FIG. 13A with
respect to the sensor's horizontal dimensions. Principal element
106h has relatively thin regions 1303 between mounting regions 1301
and capacitive region 1302. These may be produced from planar
feedstock by a process such as, for instance, coining. They may
again serve to reduce the relative amount of flexure in capacitive
area 1302, thereby improving linearity. Referring to FIG. 13B,
principal element 106i of sensor 1310 achieves a similar relative
stiffening of capacitive region 1302 by laminating this portion. As
depicted for principal element 106h, a principal element relatively
thicker in support regions 1301 may advantageously reduce stress in
the support attachments caused by the moments passing through
them.
[0161] Referring to FIGS. 14A-14C, an embodiment of a sensor
according to another embodiment of the present invention is
depicted in which the principal element is simply supported, and in
which the second element is a discrete element of identical
manufacture to the principal element.
[0162] More specifically, turning to FIG. 14A, principal element
106j (shown in solid outline) may be 300 mils wide and may be
stamped or photoetched from beryllium-copper 15 mils thick. Tabs
1401a-b engage plastic spacers 1402, allowing principal element
106j to be assembled opposite another identically manufactured
element 1403, which, flipped end-for-end with respect to 106j, is
inserted into the same pair of spacers 1402.
[0163] FIG. 14B presents a side view of plastic spacer 1402.
Rectangular holes 1404a receive tabs 1401a of one element (such as
element 106j), while rectangular hole 1404b receives tab 1401b of
the opposing element (such as element 1403). Elevations 1405, on
the sides of the spacers 1402 away from the principal element 106j,
locate the force sensor by engaging holes (not shown) in the
support surface. Thus at one end of the force sensor, the support
surface corresponds to the plane of 1406a, and at the other, of
1406b.
[0164] FIG. 14C presents a partial cross-section in which principal
element 106j and element 1403 are employed as a force sensor in a
touch location device. Spacers 1402 are employed above and below
the plane of section, and seat against the immediate support
surface provided by outer frame 104c. Transparent touch overlay
1408 is secured within the inner frame 1407 by cement 1411. The
combination is then supported perpendicularly by plastic force
transmission couplings 121h, one of which is associated with each
sensor. Couplings 121h may be press fit into holes in inner frame
1407, which align over the centers of the square capacitive areas
afforded by each of the principal elements 106j employed. Inner
frame 1407 is supported laterally by combination seal and lateral
restraint means 1409. Oversize clearance holes may be provided in
inner frame 1407, if necessary, to guarantee that there is no
contact with the unused elevations 1405 that are on the surfaces of
spacers 1402 directed upward. Discrete wiring 1410 may connect to
the upper surfaces of tabs 1401 by soldering or wire welding.
Application bezel 1412 seats against lateral restraint means 1409
and frame 104c.
[0165] When unloaded, principal element 106j rests about 10 mils
above the surface of non-flexing element 1403. Holes 1404a-b are
somewhat larger at the surface of spacer 1402, and taper to minimal
cross-section at its center, which cross-section just matches tabs
1401a-b. Thus as force is applied to coupling 121h, principal
element 106j flexes as a member having simply-supported end
constraint with minimal friction.
[0166] The arrangement of FIG. 14C offers a touch location device
of minimal thickness, but the inclusion of inner frame 1407
increases border width. The sensor is scalable to other, including
smaller, sizes.
[0167] Since principal element 106j may be located quite close to
the plane of touch, special provisions for handling tangential
forces may be omitted without significant adverse consequences. For
instance, the aggregate lateral stiffness of lateral restraint
means 1409 need not substantially exceed the aggregate lateral
stiffness of the force sensors and their couplings 121h.
Nevertheless, it should be noted that lateral restraint means 1409
provides a novel means of lateral assembly alignment having high
perpendicular compliance.
[0168] We now consider sensors of a type made in accordance with
embodiments of the invention where the principal element is made of
an insulating material with a conductively coated area or
areas.
[0169] Turning to FIG. 15A, epoxy glass PC board 1501 includes a
region comprising principal element 106k. Principal element 106k
comprises lands 113 and 114, and such portions of the epoxy glass
substrate as store significant elastic energy associated with
changes in the capacitive gap.
[0170] As may be seen more clearly from cross sectional FIG. 15B, a
predefined path carries applied force from touchable structure 401,
through force-coupling elastomeric pad 121i, upper capacitor plate
1503, and spacing/connecting solder film 1505, to central region
1506 of principal element 106k. Central region 1506 is flanked by
slots 1502, which serve both to increase and to relatively localize
the flexure in the PC substrate. From central region 1506, force
passes both out and around the ends of slots 1502, eventually
reaching PC board supports 1504. As force passes away from the
immediate vicinity of the capacitive area and the slots 1502, any
additional flexure it produces ceases to relate to force-induced
changes in the capacitive gap, and so is no longer passing through
the force sensor. If present, supports 1504 placed close to the
sensor may have some effect upon sensitivity and symmetry of
response. Such close supports may be given a symmetrical
disposition, such as that shown, not excessively close to central
region 1506. More remote supports may be placed in any pattern
desired.
[0171] Elastomeric pad 121i provides both lateral softening and a
degree of rotational softening. As such, pad 121i may serve as an
alternative to the combination of raised feature 121 and lateral
softener 107 shown in FIG. 10B. Pad 121i may be fastened adhesively
to the capacitor plate 1503 below, but not attached above.
Structures above may then be aligned and preloaded shown as
elsewhere herein. Alternatively, pad 121i offers the possibility of
maintaining alignment and assembly through adhesive attachments
both above and below.
[0172] The variation presented in FIG. 15C alters the force path,
as it now passes through the length of the upper capacitor plate
1503. This upper plate 1503 may now make a significant contribution
to the elastic energy storage associated with the capacitive gap;
in which case, it is appropriate to view the upper plate 1503 as an
additional principal element 106q,working in concert with lower
principal element 106m. Force from element 106q through solder
1505b into element 106m, continues around slots 1502, into central
region 1506, and thence to support 1504b.
[0173] Thus, many variations on the capacitive force sensor of the
invention will be evident to one of ordinary skill in the art.
These variations may share certain features, such as:
[0174] Major components of the sensor may be substantially planar,
and may be manufactured from planar materials. This provides
inexpensive access to high-precision flat surfaces, and to surfaces
that are designed to deviate from flat by slight but precisely
controlled amounts. Sensors according to various embodiments of the
invention may involve one or more substantially planar principal
elements. These receive and pass on forces through a predefined
path, and respond to the normal component of such forces by a
normal displacement of a capacitive surface that they expose. The
capacitive surface so exposed may itself be subject to some degree
of flexure. Note that the point at which force enters a principal
element may be considered to be that point beyond which force
transmitted may produce flexure directly affecting the measured
capacitive gap.
[0175] Sensors according to various embodiments of the invention
may have a very small gap; for this reason, in part, they may be
made small in comparison with the containing touch location device.
The gap-defining mechanical path of such sensors is small compared
to the dimensions of the touch location device; as a direct result,
the gap suffers only tiny error deflections due to device flexure.
Furthermore, the small size of the gap-defining path may
effectively provide additional error reduction through local
stiffening and/or structural isolation.
[0176] To more precisely understand the meaning of the term
"gap-defining path" as used herein, draw a curve through space that
originates at the center of one capacitive area and terminates at
the center of the opposing capacitive area. Pass this curve
entirely within solid material fully contributing to the mechanical
coupling between the two opposing capacitive areas. The term
"gap-defining path" refers to the length of the shortest such
curve.
[0177] In sensors according to various embodiments of the
invention, the extent of the gap-defining path projected along a
line normal to the sensor (referred to herein as the aggregate
normal component of the gap-defining path) may be scarcely greater
than the thickness of the gap itself. Since the sensor spring lies
in the same plane as its corresponding capacitive area (e.g., both
are embodied in the principal element 106), and is a continuation
of the same planar material defining the plane of that area, some
means of directly spacing the width of the gap is all that may be
required to construct the capacitor. In prior art designs of
capacitive force sensors, wherein the normal component of the
gap-defining path is substantially larger than the gap itself, the
gap is effectively determined by the small difference of two larger
numbers. This has previously limited the precision, stability, and
economy with which a very small gap may be employed.
[0178] The precision with which the directly-spaced gaps of sensors
of various embodiments of the invention may be made allows for a
capacitive gap of high aspect ratio. Width and length that are
large compared to the gap spacing itself allow an adequate absolute
capacitance to be maintained as the sensor is miniaturized.
[0179] In some embodiments, some original material may be removed
from regions of originally substantially planar materials. Thus, 1
or 2 mils of copper may be etched from between the support lands
113 and counter-electrode land 114, to isolate them electrically.
The surfaces of lands 113 and 114 remain highly coplanar, however.
Thus, in spite of this, and similar operations that may be
performed between the capacitive and support areas of substantially
planar principal elements 106, which operations may superficially
increase the normal component of the gap defining path, the end
surfaces continue to afford the same opportunity for establishing
highly precise, directly-spaced gaps using offsets or spacing means
of roughly the same perpendicular extent as the gap spacing
itself.
[0180] Capacitive force sensor stiffness in the direction of
measurement may be inversely related to the gap width. Thus,
sensors according to various embodiments of the invention provide
very high stiffness, raising the resonant frequencies of the
supported structure and improving the performance of the unit
housing the force sensor. Keeping sensor motions very small also
reduces the problem of force transmission on parasitic paths (those
not passing through a sensor).
[0181] Some variations of the sensor of the invention further
exploit an interconnect, such as a PC board, to provide both a
substantially planar support surface and coplanar second capacitor
plate for a principal element.
[0182] It is to be understood that although the invention has been
described above in terms of particular embodiments, the foregoing
embodiments are provided as illustrative only, and do not limit or
define the scope of the invention. Other embodiments are also
within the scope of the present invention, which is defined by the
scope of the claims below.
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