U.S. patent application number 11/552546 was filed with the patent office on 2007-03-08 for calibration of force based touch panel systems.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Jerry B. Roberts.
Application Number | 20070052690 11/552546 |
Document ID | / |
Family ID | 29419050 |
Filed Date | 2007-03-08 |
United States Patent
Application |
20070052690 |
Kind Code |
A1 |
Roberts; Jerry B. |
March 8, 2007 |
CALIBRATION OF FORCE BASED TOUCH PANEL SYSTEMS
Abstract
A method and system are provided to correct inaccuracies in
touch location determination associated with mechanical distortion
of the touch screen. Calibration parameters are provided for a
touch screen characterizing an error in an expected touch signal
associated with mechanical distortion of the touch screen. A force
responsive touch signal having the error is detected and the touch
location determined using the calibration parameters to correct the
error in the touch signal. The calibration parameters are
determined by applying mechanical distortion to the touch screen
and characterizing the touch signal error associated with the
mechanical distortion. The calibration parameters are produced
using the characterization of the touch signal error.
Inventors: |
Roberts; Jerry B.;
(Arlington, MA) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
29419050 |
Appl. No.: |
11/552546 |
Filed: |
October 25, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10147604 |
May 17, 2002 |
7158122 |
|
|
11552546 |
Oct 25, 2006 |
|
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Current U.S.
Class: |
345/173 |
Current CPC
Class: |
G06F 3/0418
20130101 |
Class at
Publication: |
345/173 |
International
Class: |
G09G 5/00 20060101
G09G005/00 |
Claims
1. A method for calibrating a touch screen, comprising: applying
mechanical distortion to the touch screen; detecting a force
responsive touch signal arising from the mechanical distortion of
the touch screen; characterizing a touch signal error associated
with the mechanical distortion; and producing calibration
parameters using the characterization of the touch signal
error.
2. The method of claim 1, wherein characterizing the touch signal
error associated with the mechanical distortion comprises: applying
two or more distortion conditions to the touch screen; detecting
sensor signals corresponding to each distortion condition; and
characterizing the effect of distortion using the sensor signals
corresponding to each distortion condition.
3. The method of claim 2, further comprising: determining one or
more basic calibration vectors; and determining one or more
distortion corrected calibration vectors using the one or more
basic calibration vectors and the sensor signals corresponding to
each distortion condition.
4. The method of claim 3, wherein determining the one or more basic
calibration vectors comprises calculating the one or more basic
calibration vectors from nominal sensor locations and known
parameters of the touch screen design.
5. The method of claim 3, wherein determining the one or more basic
calibration vectors comprises measuring sensor locations and touch
screen parameters.
6. The method of claim 3, wherein determining the one or more basic
calibration vectors comprises measuring sensor signals
corresponding to one or more known forces applied at one or more
known locations on the touch screen.
7. The method of claim 3, wherein determining the one or more basic
calibration vectors comprises applying known forces in the vicinity
of each touch sensor location and measuring the sensor signals
responsive to the known forces.
8. The method of claim 2, wherein applying the two or more
distortion conditions comprises: applying a zero distortion as a
first distortion condition; and applying a non-zero distortion as a
second distortion condition.
9. The method of claim 2, wherein applying the two or more
distortion conditions comprises: applying a first non-zero
distortion condition as a first distortion condition; and applying
a second non-zero distortion condition as a second distortion
condition.
10. The method of claim 9, wherein the first distortion condition
has an effect opposite to an effect of the second distortion
condition.
11. The method of claim 2, wherein applying two or more distortion
conditions to the touch screen comprises applying the distortion
conditions to the touch screen at the same time that known forces
are applied to the touch screen.
12. A touch screen calibration system, comprising: a mechanical
distortion system for applying mechanical distortion to the touch
screen; a detection system for detecting force responsive sensor
signals arising from the mechanical distortion; and a processor,
coupled to the detection system, and receiving the sensor signals
detected by the detection system, the processor configured to
detect a force responsive touch signal arising from the mechanical
distortion of the touch screen, characterize a touch signal error
associated with the mechanical distortion of the touch screen, and
produce calibration parameters using the characterization of the
touch signal error.
13. The system of claim 12 wherein the mechanical distortion system
is configured to apply two or more mechanical distortion conditions
to the touch screen.
14. The system of claim 12, wherein the mechanical distortion
applied to the touch screen is torsion.
15. The system of claim 13, wherein the processor is configured to
detect sensor signals arising from each mechanical distortion
condition, and characterize the touch signal error using the sensor
signals arising from each mechanical distortion condition.
16. The system of claim 12, further comprises a force application
system for applying known forces to the touch screen, wherein the
processor is further configured to determine one or more basic
calibration vectors, and determine one or more distortion corrected
calibration vectors using the one or more basic calibration vectors
and the sensor signals corresponding to each distortion
condition.
17. The system of claim 16, wherein the known forces are applied by
the force application system in the vicinity of each touch sensor
location.
18. The system of claim 13, wherein the mechanical distortion
system: applies a zero distortion as a first distortion condition;
and applies a non-zero distortion as a second distortion
condition.
19. The system of claim 13, wherein the mechanical distortion
system: applies a first non-zero distortion condition as a first
distortion condition; and applies a second non-zero distortion
condition as a second distortion condition.
20. The system of claim 19, wherein the first distortion condition
has an effect opposite to an effect of the second distortion
condition.
21. The system of claim 13, wherein the mechanical distortion
system applies two or more distortion conditions at the same time
that known forces are applied to the touch screen.
22. A system for calibrating a touch screen, comprising: means for
applying mechanical distortion to the touch screen; means for
detecting sensor signals associated with the mechanical distortion;
and means for calibrating the touch screen to compensate for the
mechanical distortion.
23. The system of claim 22, wherein means for applying mechanical
distortion to the touch screen comprises means for applying a first
and a second torsion to the touch screen.
24. The system of claim 22, wherein means for calibrating the touch
screen comprises: means for determining a basic calibration; and
means for calibrating the touch signal using the basic calibration
and the detected sensor signals affected by the mechanical
distortion.
25. The system of claim 24, wherein means for determining a basic
calibration comprises: means for applying known forces to the touch
screen; means for detecting sensor signals responsive to the known
forces; and means for determining basic calibration vectors for the
touch screen using the sensor signals responsive to the known
forces.
26. The system of claim 24, wherein means for calibrating the touch
signal using the basic calibration and the detected sensor signals
affected by the mechanical distortion comprises means for
determining torsion corrected calibration vectors for the touch
screen.
27. The system of claim 22, wherein means for applying mechanical
distortion to the touch screen comprises means for applying the
distortion conditions to the touch screen at the same time that
known forces are applied to the touch screen.
28. A computer-readable medium configured with executable
instructions for causing one or more computers to perform a method
of calibrating a touch screen, the method comprising: applying
mechanical distortion to the touch screen; detecting a force
responsive touch signal arising from the mechanical distortion of
the touch screen; characterizing a touch signal error associated
with the mechanical distortion, the touch signal error arising in a
force responsive touch signal; and producing calibration parameters
using the characterization of the touch signal error.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. Ser. No.
10/147,604, filed May 17, 2002, now allowed, the disclosure of
which is incorporated by reference in its entirety herein.
FIELD OF THE INVENTION
[0002] The present invention is directed generally to a touch
sensing system, and more particularly to a method and system for
calibrating a touch screen system for more accurate determination
of the location of a touch on the touch screen.
BACKGROUND
[0003] A touch screen offers a simple, intuitive interface to a
computer or other data processing device. Rather than using a
keyboard to type in data, a user can transfer information through a
touch screen by touching an icon or by writing or drawing on a
screen. Touch screens are used in a variety of information
processing applications. Transparent touch screens are particularly
useful for applications such as cellphones, personal data
assistants (PDAs), and handheld or laptop computers.
[0004] Various methods have been used to determine touch location,
including capacitive, resistive, acoustic and infrared techniques.
Touch location may also be determined by sensing the force of the
touch through force sensors coupled to a touch surface. Touch
screens that operate by sensing touch force have several advantages
over other technologies mentioned above. First, force sensors do
not require the touch surface to be composed of special materials
that may inhibit optical transmission through the touch surface, as
in a resistive touch sensor.
[0005] Further, force sensors do not rely on a lossy electrical
connection to ground, as is required by a capacitive touch screen,
and can be operated by a finger touch, gloved hand, fingernail or
other nonconductive touch instrument. Unlike surface acoustic wave
technology, force sensors are relatively immune to accumulations of
dirt, dust, or liquids on the touch surface. Finally, a force
sensor is less likely to detect a close encounter with the touch
surface as an actual touch, which is a common problem with infrared
touch screens.
[0006] A force based touch screen may be built with a minimum of
three force sensors spaced in a triangular pattern under a touch
surface. Such an arrangement may provide signals sufficient to
determine the net perpendicular force and the two moments necessary
to compute touch location. Touch screen devices also may be built
with a larger number of sensors. Commonly, four corner sensors may
be used, in part to harmonize with the symmetry of the rectangular
touch surface typically required. Upon application of a touch, the
forces sensed by the touch screen sensors may be used to determine
the touch location. However, determination of the touch location
may be affected by a number of factors in addition to the touch
force. Twisting, squeezing or otherwise distorting the touch screen
during a touch may cause inaccuracies in the touch location
determination.
SUMMARY OF THE INVENTION
[0007] In general terms, the present invention relates to a method
and system for detecting the location of a touch on a touch sensor.
Features of the present invention are particularly useful when
combined with a microprocessor-based system operating a display
device enhanced by a transparent touch screen.
[0008] In accordance with one embodiment of the present invention,
a method for determining a touch location on a touch screen is
provided. The touch screen is defined by a plurality of touch
sensors disposed to measure a signal indicative of a touch force
component that is perpendicular to a touch surface. The method
includes providing calibration parameters for the touch screen
acquired using the touch sensors and the touch surface. The
calibration parameters characterize an error in an expected touch
signal associated with mechanical distortion of the touch screen. A
force responsive touch signal having the error is detected and
touch location determined using the calibration parameters to
compensate for the error.
[0009] In another embodiment of the present invention, a method for
calibrating a touch screen includes applying a mechanical
distortion to the touch screen and detecting a force responsive
touch signal arising from the mechanical distortion of the touch
screen. Touch signal error associated with the mechanical
distortion is characterized and calibration parameters are produced
using the characterization of the touch signal error.
[0010] In accordance with a further embodiment of the present
invention, a touch screen system includes a touch surface, a
plurality of force responsive touch sensors mechanically coupled to
the touch surface and producing a sensor signal in response to a
touch applied to the touch surface, and a control system couple to
the touch sensors and receiving the sensor signals. The control
system is configured to provide calibration parameters for the
touch screen acquired using the touch sensors and the touch
surface. The calibration parameters characterize an error in an
expected touch signal associated with mechanical distortion of the
touch screen. The control system detects a force responsive touch
signal having the error and determines a touch location using the
calibration parameters to compensate for the error in the touch
signal.
[0011] In yet another embodiment of the present invention, a touch
screen display system includes a touch surface, a plurality of
touch sensors, a control system and a display for displaying
information through the touch screen system. The control system is
configured to provide calibration parameters for the touch screen
acquired using the touch sensors and the touch surface. The
calibration parameters characterize an error in an expected touch
signal associated with mechanical distortion of the touch screen.
The control system detects a force responsive touch signal having
the error and determines a touch location using the calibration
parameters to compensate for the error in the touch signal.
[0012] In another embodiment of the present invention, a touch
screen calibration system comprises a mechanical distortion system
for applying mechanical distortion to the touch screen, a detection
system for detecting force responsive sensor signals arising from
the mechanical distortion, and a processor coupled to the detection
system. The processor is configured to detect a force responsive
touch signal arising from the mechanical distortion of the touch
screen and characterize a touch signal error associated with the
mechanical distortion of the touch screen. The processor is further
configured to produce calibration parameters using the
characterization of the touch signal error.
[0013] A further embodiment of the present invention includes a
system for determining a touch location on a touch screen. The
touch screen is defined by a plurality of touch sensors
mechanically coupled to a touch surface. The system includes means
for providing touch screen calibration parameters acquired using
the touch surface and the touch sensors, means for detecting a
touch signal having the touch signal error, means for correcting
the touch signal using the touch screen calibration, and means for
determining the touch location using the corrected touch signal.
The touch screen calibration parameters characterize a touch signal
error associated with a mechanical distortion of the touch screen
affecting a touch signal.
[0014] In another embodiment of the present invention, a system for
calibrating a touch screen is provided. The system includes means
for applying mechanical distortion to the touch screen, means for
detecting sensor signals associated with the mechanical distortion,
and means for calibrating the touch screen to compensate for the
mechanical distortion.
[0015] In a further embodiment of the present invention, a
computer-readable medium is configured with executable instructions
for causing one or more computers to perform a method for
determining a touch location on a touch screen. The touch screen
defined by a touch surface and a plurality of touch sensors
disposed to measure a signal indicative of a touch force component
that is perpendicular to the touch screen. The method for
determining touch location includes providing calibration
parameters for the touch screen acquired using the touch sensors
and the touch surface, the calibration parameters characterizing an
error in an expected touch signal associated with mechanical
distortion of the touch screen, detecting a force responsive touch
signal having the error; and determining the touch location using
the calibration parameters to compensate for the error in the touch
signal.
[0016] Yet another embodiment of the present invention includes a
computer-readable medium configured with executable instructions
for causing one or more computers to perform a method of
calibrating a touch screen. The method comprises applying
mechanical distortion to the touch screen, detecting a force
responsive touch signal arising from the mechanical distortion of
the touch screen, characterizing a touch signal error associated
with the mechanical distortion, the touch signal error arising in a
force responsive touch signal, and producing calibration parameters
using the characterization of the touch signal error.
[0017] The above summary of the present invention is not intended
to describe each illustrated embodiment or every implementation of
the present invention. The figures and the detailed description
which follow more particularly exemplify these embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The invention may be more completely understood in
consideration of the following detailed description of various
embodiments of the invention in connection with the accompanying
drawings, in which:
[0019] FIG. 1 schematically illustrates a perspective view of a
touch screen with force sensors located at the corners of the touch
screen in accordance with an embodiment of the invention;
[0020] FIG. 2 schematically illustrates a cross-sectional view of a
capacitive force sensor in accordance with an embodiment of the
invention;
[0021] FIG. 3 is a block diagram of a touch screen and touch screen
control system in accordance with an embodiment of the
invention;
[0022] FIG. 4 schematically illustrates a touch screen under
torsion;
[0023] FIG. 5 is a flowchart conceptually illustrating a method for
determining touch location using a characterization of an error
associated with distortion of the touch screen in accordance with
an embodiment of the invention;
[0024] FIG. 6 is a flowchart conceptually illustrating a method for
characterizing an error caused by distortion of the touch screen in
accordance with an embodiment of the invention;
[0025] FIG. 7 is a more detailed flowchart conceptually
illustrating a method for characterizing an error caused by
distortion of the touch screen in accordance with an embodiment of
the invention;
[0026] FIGS. 8A and 8B schematically illustrate a method of
applying two different support strain configurations to a touch
screen in accordance with an embodiment of the invention;
[0027] FIGS. 9A and 9B schematically illustrate another method of
applying two different support strain configurations to a touch
screen in accordance with an embodiment of the invention;
[0028] FIG. 10 is a flowchart of a method of determining a basic
calibration of the touch screen in accordance with an embodiment of
the invention;
[0029] FIG. 11 is a flowchart conceptually illustrating a method
for characterizing an error caused by distortion of the touch
screen computed in a single step from data responsive to both known
forces and deliberately applied distortions in accordance with an
embodiment of the invention;
[0030] FIG. 12 is a block diagram of a touch screen calibration
system in accordance with an embodiment of the invention;
[0031] FIG. 13 is a block diagram of a data processing system using
a touch sensing interface in accordance with an embodiment of the
invention;
[0032] FIG. 14 illustrates a touch screen controller in accordance
with an embodiment of the invention; and
[0033] FIG. 15 illustrates a touch screen calibration system in
accordance with an embodiment of the invention.
[0034] The invention is amenable to various modifications and
alternative forms. Specific embodiments of the invention have been
shown by way of example in the drawings and will be described in
detail. It should be understood, however, that the intention is not
to limit the invention to the particular embodiments described. On
the contrary, the intention is to cover all modifications,
equivalents, and alternatives falling within the scope of the
invention as defined by the appended claims.
DETAILED DESCRIPTION
[0035] In the following description of the illustrated embodiments,
references are made to the accompanying drawings which form a part
hereof, and various embodiments by which the invention may be
practiced are shown by way of illustration. It is to be understood
that other embodiments may be utilized, and structural and
functional changes may be made without departing from the scope of
the present invention.
[0036] As stated above, and for other reasons stated below which
will become apparent upon reading the present specification, there
is a need for a method and a system for accurately determining the
location of a finger touch or an instrument touch on a touch
surface. There exists a further need for such a method and system
that calculates touch location with correction for mechanical
distortions applied to the touch screen during the time in which
the touch location information is obtained to determine touch
location.
[0037] The present invention is applicable to touch sensing
techniques and is believed to be useful when features of the
present invention are combined with a data processing system
operating a display device enhanced by a transparent touch screen.
For example, a touch screen of the present invention may be used in
a desktop, handheld or laptop computer system, a point-of-sale
terminal, personal data assistant (PDA), or a cell phone. Although
described in combination with a microprocessor-based system, a
touch screen device of the present invention may be combined with
any logic-based system, if desired.
[0038] The present invention provides for the accurate
determination of a touch location on a force based touch screen in
the presence of mechanical distortions of the touch screen. A touch
may be sensed by a number of touch sensors and represented by one
or more touch signals. Accurate touch location determination
involves measuring the magnitudes of one or more touch signals
during a touch on the touch screen. At the time the touch
information is obtained to determine the touch location, the touch
screen may be influenced by a number of factors, such as those
caused by an operator twisting or squeezing the touch screen
device. Such disturbances of the touch screen during a time the
touch signal is being processed to determine the touch location may
lead to inaccuracies in the calculated touch location.
[0039] A perspective view of a rectangular touch screen is
schematically illustrated in FIG. 1. A touch surface 100 is shown
disposed proximate to force sensors located at respective corners
of the touch surface 100. The touch surface 100 and force sensors
110, 120, 130, 140 are located within a touch screen housing (not
shown).
[0040] As a stylus, finger or other touching device 152 presses the
touch surface 100, a touch force 155 is exerted upon the touch
surface 100 at the touch location 150. The touch force 155 creates
forces F1, F2, F3, F4 on the force sensors 110, 120, 130, 140
perpendicular to the touch surface 100. The force sensors 110, 120,
130, 140 may be driven with an alternating electrical signal. The
perpendicular forces F1, F2, F3, F4 cause a change in the
capacitance of the force sensors 110, 120, 130, 140, thereby
causing the signal coupled through the force sensors 110, 120, 130,
140 to change. The force responsive signals derived from the force
sensors 110, 120, 130, 140 may be used to calculate touch location.
Although the touch screen illustrated in FIG. 1 is rectangular with
sensors located at the corners, various configurations using three
or more touch sensors with differing touch surface shapes may also
be used.
[0041] The sensors 110, 120, 130, 140, may be, for example, small
capacitive force sensors constructed of two capacitor plates
separated by a gap. A capacitive force sensor may be arranged so
that when a touch force of sufficient magnitude and direction is
applied to the touch surface, one capacitor plate deflects towards
the second plate. The deflection alters the distance between the
capacitor plates, changing the capacitance of the sensor. The touch
force may be measured by control system circuitry as a change in an
alternating electrical signal applied to the touch sensor. One
embodiment of a capacitive force sensor appropriate for use in
touch screen applications is described in co-owned U.S. Patent
Application, U.S. Ser. No. 09/835,040, filed Apr. 13, 2001,
entitled "Method and Apparatus for Force-Based Touch Input" (US
publication number 02-0149571-A1, published Oct. 17, 2002), which
is hereby incorporated herein by reference.
[0042] A force sensor is appropriate for use with a liquid crystal
display (LCD), cathode ray tube (CRT) or other electronic display,
and is schematically illustrated in FIG. 2. In this particular
embodiment, the sensor measures the applied force based on the
change of capacitance of a capacitive element. A touch surface 210,
or overlay, is located within a structure or housing 215. The touch
surface 210 is typically transparent to allow viewing of a display
or other object through the touch surface. In other applications,
the touch surface 210 can be opaque.
[0043] The structure or housing 215 may be provided with a large
central aperture through which the display may be viewed. If
desired, the undersurface of the housing 215 may be seated directly
against the surface of such a display, over the border surrounding
its active area. In another embodiment, as mentioned above, the
overlay may be replaced by a structure including a display unit,
such as an LCD.
[0044] A capacitive sensor 220 may be positioned between the touch
surface 210 and the housing 215. An interconnect 225, with
attachment lands 233, may be coupled to the housing 215 by
soldering, cementing, or by other methods. A conductive area forms
a first conductive element 234 on the interconnect 225. A second
conductive element 235 with a central protrusion 240, for example a
dimple, may be attached to the lands 233 of the interconnect 225 by
soldering, for example. A small gap 280 is formed between the first
conductive element 234 and the second conductive element 235,
either by the shape of the second conductive element 235, or by the
process of attaching the second conductive element 235 to the
interconnect 225. The width of the gap 280 may be approximately 1
mil, for example. A capacitor is formed by the conductive elements
234, 235 separated by the gap 280.
[0045] An optional bearing surface 270 may be interposed between
the touch surface 210 and the second conductive element 235. This
may protect the underside of touch surface from indentation or from
damage by the protrusion 240, especially in cases where the overlay
is made of softer material. The bearing surface 270 may also mount
to the touch surface 210 through a thin layer (not shown) of
elastomer or of highly pliable adhesive, thereby providing a
lateral softening function. It will be appreciated that, in normal
operation, the touch surface 210 or bearing surface 270 is in
contact with the protrusion 240: these elements are shown separated
only for clarity in the illustration.
[0046] The second conductive element 235 combines the functions of
a spring and a capacitor plate. As a perpendicular force is applied
to the touch surface 210, the second conductive element 235 flexes,
decreasing the width of the gap 280 and increasing the capacitance
of the sensor 220. This change in capacitance may be measured and
related to the force applied to the touch surface 210. Although a
touch screen using capacitive force sensors is described, other
types of force sensors may be used in a similar manner, including,
for example, piezoelectric sensors and strain gauge sensors.
[0047] One of the advantages of a force-based touch screen is that
the number of optically distinct layers positioned between the
display unit and the user is low. Typically, the overlay positioned
over the display unit is a single layer of glass or relatively
stiff polymer, for example polycarbonate or the like, which may be
chosen for suitable optical qualities. This contrasts with other
types of touch screen, such as resistive or capacitive touch
screens, that require several, potentially optically lossy, layers
over the display unit. The electrically conductive thin films
required in resistive or capacitive touch screens typically have a
high index of refraction, leading to increased reflective losses at
the interface. This is a particular problem in resistive screens
where there are additional solid/air interfaces and where
antireflection coatings are not useful, since the conductive layers
must be able to make physical contact. A screen overlay for a
force-based touch screen, however, has only its upper and lower
surfaces; these may be treated to reduce reflective losses and to
reduce glare. For example, the overlay may be provided with matte
surfaces to reduce specular reflection, and/or may be provided with
anti-reflection coatings to reduce reflective losses.
[0048] Touch signals representing the force of a touch acting on
the touch screen are produced by one or more touch sensors coupled
to a touch surface of the touch screen. A touch signal may be
derived from a single sensor, or by combining sensor signals from
two or more touch sensors. Determination of a touch location
involves analyzing the sensor signals produced by the touch
sensors. A tap touch in a single location characteristically
produces a touch signal that increases in magnitude as the touch is
applied and then decreases in magnitude as the touch is removed. A
touch may be a continuing touch wherein the touch remains on the
touch surface for a period of time. For example, the touch may be
present in a single location for a period of time. Further, the
touch may be a "streaming touch," wherein the touch is applied at
one location, moved across the surface of the touch screen, and
removed at another location, causing the generation of a
continuously changing signal at each sensor.
[0049] Calculation of the touch location at any time, t, may be
performed, for example in a four sensor screen, using combinations
of the force responsive sensor signals f.sub.1(t), f.sub.2(t),
f.sub.3(t), f.sub.4(t). The force responsive signals generated by
the touch sensors may be used to calculate various touch signals,
including the moment about the y-axis, M.sub.Y(t), moment about the
x-axis, M.sub.x(t), and the total z-direction force, F.sub.Z(t).
The coordinates of the touch location may be determined from the
touch sensor signals, as provided in Equation 1. Assuming a
reference point in the center of the touch screen, a perfectly
rigid touch surface, ideal conditions, with no errors, background
fluctuations or disturbances present other than the touch force.
The force and moments employed in Equation 1 may be evaluated as in
Equation 1 b. X .function. ( t ) = M Y .function. ( t ) F Z
.function. ( t ) .times. .times. Y .function. ( t ) = M X
.function. ( t ) F Z .function. ( t ) [ 1 ] ##EQU1##
[0050] where, for this particular case,
M.sub.X(t)=(f.sub.1(t)+f.sub.2(t))-(f.sub.3(t)+f.sub.4(t));
M.sub.Y(t)=(f.sub.2(t)+f.sub.4(t))-(f.sub.1(t)+f.sub.3(t)); and
F.sub.Z(t)=f.sub.1(t)+f.sub.2(t)+f.sub.3(t)+f.sub.4(t). [1b]
[0051] The sensor signals are directed to a control system that
determines a touch location from the force responsive sensor
signals. FIG. 3 schematically illustrates a block diagram of a
touch screen 300 and touch screen control system 350 arranged in
functional blocks in accordance with the principles of the present
invention. It will be appreciated that there exist many possible
configurations in which these functional blocks may be arranged.
The example depicted in FIG. 3 is one possible functional
arrangement.
[0052] In the exemplary embodiment illustrated in FIG. 3, a touch
surface 305 is configured proximate to four force sensors 301, 302,
303, 304 arranged at the respective corners of the touch surface
305. The touch surface 305 and force sensors 301, 302, 303, 304 are
arranged in a touch screen housing (not shown). The sensors 301,
302, 303, 304 may be chosen from a variety of sensing technologies,
including capacitive, piezoelectric and strain gauge sensors. The
sensors 301, 302, 303, 304 measure the force of a touch detected at
the sensor locations and are coupled to drive/sense circuitry 310,
320, 330, 340 located within the control system 350. Alternatively,
some components of the drive/sense circuitry may be located near
the corresponding sensor. An energizing signal developed in the
drive circuitry 312, 322, 332, 342 for each sensor is used to
energize the sensors 301, 302, 303, 304. Each sensor 301, 302, 303,
304 produces a touch force signal corresponding to a touch force
applied to the sensor through the touch surface 305. The touch
force signal developed by each sensor 301, 302, 303, 304 is
detected by sense circuitry 311, 321, 331, 341 located within the
control system 350.
[0053] Analog voltages representing the touch force at each sensor
location are produced by the sense circuitry 311, 321, 331, 341.
The analog voltages are sampled and multiplexed by the sampling
circuitry 360 at a rate sufficient to acquire an adequate
representation of the force responsive sensor signals for
determining touch presence and location. The sampled signals are
digitized by an analog to digital (A/D) converter 370. The
digitized sensor signals are directed to processor circuitry 380.
The processor circuitry 380 performs calculations to determine a
touch location. The processor circuitry 380 may also include
filtering circuitry 382 for signal conditioning and memory
circuitry 386 for storage of touch signal values. The processor
circuitry 380 may also include one or more timers 384 for
determining various interval and delay timing of the touch signal
associated with determination of the preferred time for making the
touch location measurement. The processor circuitry 380 may perform
a number of additional control system functions, including
controlling the touch signal sampling circuitry 360, the
multiplexer circuitry 360, and the A/D converter 370.
[0054] It may be found advantageous to implement the touch screen
control system 350, or its equivalent, on a single mixed-mode
integrated circuit chip. In such an implementation, it may be
advantageous to replace sampling circuitry 360 and A/D converter
370 with a set of delta-sigma converters operating in parallel, one
for each signal channel.
[0055] Consider a force touch screen in a thin portable device,
such as a PDA. Take the case to be one wherein the device is
roughly rectangular in outline, with a rectangular touch surface
supported by four force sensors near the corners as depicted in
FIG. 4. While one hand holds a stylus to apply touches to the
surface, the opposite hand may be holding the device against a
table, or grasping it free of any surface support. Uneven pressure
from this opposite hand may serve to twist the enclosure, leading
to a pattern of forces, such as, for example, F1, F2, F3, and F4
applied to the enclosure at or near its corners. This causes a
torsional distortion of the touch screen device, such that a line
along one edge 410 of the device tends to move very slightly out of
parallel with an opposing edge 420. The touch surface will, to some
degree, resist following this distortion of the support structure
of the touch screen, such that the forces in one diagonally
opposing pair of force sensors, located at diagonally opposing
corners 402 and 403 become less positive (or more negative), while
the forces located at corners 401 and 404 change by an equal amount
in the opposite sense.
[0056] While the grasping or restraining hand applies a force of
perhaps 100 grams downward on opposing device corners 402, 403,
equilibrium may be sustained by equal upward reaction forces at the
other two corners 401, 404. Since a substantial portion of the
entire device stiffness may reside in the touch overlay, the
torsional force applied to it through the sensors may be a
substantial portion of the total applied to the device. If this
portion is one-quarter, for instance, then the sensors in this
example may each experience + or -25 grams of torsionally applied
force. At the same time, a 20-gram touch force applied to the
center of the screen should appear as a 5-gram addition to each
sensor. The force from the restraining hand may fluctuate rapidly.
The signal resulting from the touch force, indicating the touch
location, then, may be several times smaller than a simultaneously
present fluctuating interference.
[0057] In a second example, consider a public-access display
equipped with a robust, vandal-resistant touch screen. Such a
screen may be very thick and rigid. The weight of moving equipment,
the pressure of wind on the building, or even the weight of passing
footsteps may cause small torsional distortions in a kiosk-type or
wall mounted enclosure. The result may again be a significant and
varying torsional force pattern applied to the sensors.
[0058] Difficulties of the sort just discussed may arise whenever a
force-based touch location is to be derived from a surface
supported on more than three sensors. Although touch location may
be determined using a minimum of three force sensors, certain
advantages may mandate the use of a larger number. Additional
points of support for the touch surface, for example, may prevent
it from flexing excessively in response to touch forces.
Conversely, to the extent that the support structure beneath the
touch screen flexes, sensor connections beyond the third sensor may
serve to constrain the touch surface to flex in concert. Both of
these effects reduce relative motion between the edges of the touch
surface and any surrounding frame or bezel, and between the touch
surface structure in general and the structures below. There may be
seals, preload springs, or other connections running between the
touch surface and the larger device that shunt varying forces
around the force sensors in response to such relative motion. As
these unmeasured shunt forces may lead to errors in force location,
their reduction can be advantageous. Reduction is achieved,
however, by passing forces tending to distort one structure, into
another through the force sensors. This may itself become a source
of inaccuracies.
[0059] With three force sensors, each combination of position and
perpendicular touch force may be associated with a specific set of
sensor values. When more than three sensing connections support a
touch surface structure, however, there is no longer a one-to-one
relationship between touch locations and sensor response patterns.
The division of perpendicular touch force among more than three
sensors is a statically indeterminate problem, and the many
different possible response patterns for a given touch are seen to
result from different patterns of device strain.
[0060] In an idealized situation where the support structure and
touch screen are perfectly rigid, strain patterns within the touch
surface structure and support structure remain constant throughout
the course of a touch. In this idealized situation of perfect
rigidity, the change in sensor outputs from a moment before a touch
to one during it would be characterized by a single fixed pattern
as with the case of only three sensors. In this situation, the
relative magnitudes of the different sensor output changes would
depend only on the location touched. Touch location may be computed
from such pre-touch to during-touch changes. Assuming, then, that
forces from internal strain are not excessive for the force
sensors, it is seen that the assumption of perfect rigidity
simplifies the calculation of touch location.
[0061] Slightly flexible structures may, however, be more practical
or more cost effective. Such flexible structures may change strain
pattern during the course of a touch, due either to the stress of
the touch force itself, or due to independently changing stresses
applied to the support structure.
[0062] In an ideally calibrated force-touch screen with somewhat
flexible structure, the total force signal and two moment signals
needed for touch location computation are formed from precise
linear combinations of the sensor outputs, these combinations
having the property of canceling exactly to report zero total force
and moment values for patterns of perpendicular sensor force
arising from indeterminacy and device flexure. The coefficients
employed for such combinations, at whatever level of accuracy
achieved, may be termed a "calibration" for the touch screen in
question. A fully accurate calibration may reflect the exact
locations and sensitivities of the force sensors, along with any
electronic sensitivities or cross-talk. Imprecise calibration
values may lead to location errors. Those resulting from varying
mechanical distortion of the touch screen may be unexpectedly
large, and may benefit from special attention in the calibration
process. For clarity in the discussion below, a calibration
prepared without special attention to potential inaccuracies from
distortion will be termed a "basic calibration".
[0063] For a force-based touch screen with n sensors, Let vector
{right arrow over (F)}(t) represent the set of all sensor values at
time t collected together into a list in a predetermined order:
{right arrow over (F)}(t)=[f.sub.1(t), . . . f.sub.n(t)] [2]
[0064] The variable t may be taken to be the continuous-valued
time, or the discrete-valued sample number, to which the data
correspond, as convenient.
[0065] The coefficients of combination comprising the calibration
may also be collected together into vectors {right arrow over
(C)}.sub.Z, {right arrow over (C)}.sub.Y, and {right arrow over
(C)}.sub.X: {right arrow over (C)}.sub.Z=[c.sub.Z1, . . . c.sub.Zn]
{right arrow over (C)}.sub.Y=[c.sub.Y1, . . . c.sub.Yn] {right
arrow over (C)}.sub.X=[c.sub.X1, . . . c.sub.Xn] [3]
[0066] These associate the proper weights with the sensor channels,
such that: F.sub.Z(t)={right arrow over (C)}.sub.Z{right arrow over
(F)}(t) M.sub.Y(t)={right arrow over (C)}.sub.Y{right arrow over
(F)}(t) M.sub.X(t)={right arrow over (C)}.sub.X{right arrow over
(F)}(t) [4]
[0067] where F.sub.Z(t) represents the perpendicular component of
total touch force, M.sub.Y(t) represents the moment of the touch
force about the desired Y-axis, and M.sub.X(t) represents the
moment of the touch force about the desired X-axis. The desired
axes in question are those of that coordinate grid, lying in the
touch plane, with respect to which touch location is to be
reported. The units in which M.sub.Y(t) and M.sub.X(t) are
represented may be any convenient choice, and may be different for
the two axes. In particular, they may be chosen such that final
touch coordinates may be computed directly using Equation 1,
repeated below for convenience: X .function. ( t ) = M Y .function.
( t ) F Z .function. ( t ) .times. .times. Y .function. ( t ) = M X
.function. ( t ) F Z .function. ( t ) [ 1 ] ##EQU2##
[0068] These coordinates may normally be calculated and reported
only at times when the magnitude of F.sub.Z(t) is such as to
indicate the presence of a deliberate touch that is strong enough
to be accurately located. Only one touch location may be reported,
or many successive locations may be reported for a continuing
touch.
[0069] When there are more than three sensors, the vectors {right
arrow over (C)}.sub.Z, {right arrow over (C)}.sub.Y, and {right
arrow over (C)}.sub.X allow for n-3 additional vectors mutually
orthogonal to these and to each other. These additional vectors may
be added in arbitrary proportion to an existing set of sensor
outputs without changing a computed touch location. Furthermore, if
the calibration vectors are perfectly accurate, these additional
vectors may correspond to distinct patterns of perpendicular sensor
force associated with static indeterminacy, whereby such
indeterminacy forces need not cause error. In particular, when n=4,
the single such vector may correspond to overall torsional flexure
of the device. However, if the calibration vectors are not
perfectly accurate, this one orthogonal vector may not exactly
match the sensor output from torsion. Then fluctuating torsion,
especially arising from potentially large forces applied to the
support structure, may lead to location errors.
[0070] One aspect of the present invention is directed to reducing
the effect of mechanical distortions of the touch screen, such as
torsion, on the determination of the touch location on the touch
screen. Mechanical distortions of the touch screen may arise from
the exemplary situations discussed above, or from other mechanical
distortions affecting the accuracy of the touch location
measurement. FIG. 5 illustrates, in broad and general terms, a
method of reducing the effect of mechanical distortion of the touch
screen to increase touch location accuracy. Calibration parameters
acquired using the touch surface and the touch sensors are provided
510. The calibration parameters characterize an error in an
expected touch signal associated with mechanical distortion of the
touch screen. A touch signal having the error is detected 520. The
touch location is determined using the calibration parameters to
compensate for the error in the touch signal 530.
[0071] Another aspect of present invention is directed to a method
and system for characterizing the effect of mechanical distortions
on the touch signal. FIG. 6 illustrates, in broad and general
terms, a method for determining calibration parameters
characterizing the effect of mechanical distortion on the touch
screen. One or more deliberate mechanical distortions are applied
to the touch screen 610. The force responsive touch signals arising
from the mechanical distortion of the touch screen are detected
620. The touch signal error associated with the mechanical
distortion is characterized 630. Calibration parameters are
produced using the characterization of the touch signal error
640.
[0072] In accordance with one approach, a method for characterizing
the error associated with touch screen torsion is conceptually
illustrated in the flowchart of FIG. 7. A basic calibration may be
obtained in addition to the characterization of the mechanical
distortion. The basic calibration may be calculated from either the
nominal sensor locations and sensitivities of the touch screen
design, or from sensor locations and sensitivities measured on a
unit by unit basis.
[0073] Following basic calibration, two sets of force sensor output
values are accumulated, corresponding to two different states of
torsion. Both are accumulated while no force is externally applied
to the touch surface. The differences formed by subtracting the
second set of sensor output values from the first may then be
normalized to a vector of unit magnitude, and the result taken to
be the normalized response vector to torsion. Thus, for contrasting
sets of values taken at times t.sub.Q1 and t.sub.Q2, the torsion
response vector, {right arrow over (F)}.sub.Q, may be given by: F Q
= .times. [ f Q .times. .times. 1 , f Q .times. .times. 2 , f Q
.times. .times. 3 , f Q .times. .times. 4 ] = .times. [ f 1
.function. ( t Q .times. .times. 2 ) , f 2 .function. ( t Q .times.
.times. 2 ) , f 3 .function. ( t Q .times. .times. 2 ) , f 4
.function. ( t Q .times. .times. 2 ) ] - .times. [ f 1 .function. (
t Q .times. .times. 1 ) , f 2 .function. ( t Q .times. .times. 1 )
, f 3 .function. ( t Q .times. .times. 1 ) , f 4 .function. ( t Q
.times. .times. 1 ) ] [ 5 ] ##EQU3## and a parallel vector of unit
length may be given by: F QN = F Q F Q F Q , .times. whereby
.times. .times. F QN F QN = 1. [ 6 ] ##EQU4##
[0074] More particularly, an embodiment of a method for the
collection of torsion-responsive sensor data may be described as
conceptually illustrated in the flowchart of FIG. 7 and the touch
screen diagrams of FIGS. 8A-B and FIGS. 9A-B. Following a
determination of the basic calibration of the touch screen 710, a
first degree of deliberate mechanical distortion may be applied to
the touch screen 720. A first set of sensor response values may be
measured with the first degree of torsion applied 730 and with no
touch or other force applied to the touch surface. In one example,
the first degree of torsion may simply be a condition of zero
torsion, as illustrated in FIG. 8A. The first set of sensor
response values is taken from an unstressed touch surface 800 where
touch sensors located at corners 801, 802, 803 804 experience no
perpendicular force or mechanical distortion of the touch surface
800.
[0075] A second deliberate torsion may then be imposed on the touch
screen 740, and a second set of force sensor outputs measured 750,
again while no force is externally applied to the touch surface. A
satisfactory deliberate distortion may be achieved, for example, by
an apparatus that applies an upward force under one corner 803 of
the device, as illustrated in FIG. 8B. while the other three
corners 801, 802, 804 are held stationary. Alternatively, the first
set of sensor response values may also be measured with a
deliberate mechanical distortion applied, but with effect opposite
to that of the second set.
[0076] In some configurations, the weight of the device itself is a
sufficient source of distorting force. In this configuration,
illustrated in FIG. 9, the touch sensors may experience the weight
of the touch surface 900 equally distributed to the sensors as
forces f.sub.1(t.sub.1)=F1, f.sub.2(t.sub.1)=F2,
f.sub.3(t.sub.1)=F3 and f.sub.4(t.sub.1)=F4 carried at corners 901,
902, 903, 904 of the touch surface 900. A shim 950 may be inserted
under one corner 904 of the touch screen to apply a first
deliberate torsion to the touch screen corresponding to altered
forces f.sub.1(t.sub.2)=F1', f.sub.2(t.sub.2)=F2',
f.sub.3(t.sub.2)=F3' and f.sub.4(t.sub.2)=F4' at touch sensors
located at corners 901, 902, 903, and 904, respectively. The shim
950 may simply be moved from under one corner to under an adjacent
corner to apply a second deliberate torsion to the touch
screen.
[0077] Turning back to FIG. 7, the difference between the two
different sets of force sensor output values, corresponding to two
different states of torsion are formed by subtracting the second
set from the first 760 and then normalizing the resultant vector
770 to a vector of unit magnitude. The resulting vector represents
the normalized response vector to torsion. Because neither external
force nor moment is present during the application of pure torsion,
calibration vectors {right arrow over (C)}.sub.ZB, {right arrow
over (C)}.sub.YB, and {right arrow over (C)}.sub.XB acquired from
the basic calibration should be orthogonal to the normalized
response vector to torsion: 0?{right arrow over (C)}.sub.ZB{right
arrow over (F)}.sub.Q 0?{right arrow over (C)}.sub.YB{right arrow
over (F)}.sub.Q 0?{right arrow over (C)}.sub.XB{right arrow over
(F)}.sub.Q [7]
[0078] These conditions will hold only within the limits of the
calibration accuracy. However, a torsion-corrected calibration
{right arrow over (C)}.sub.ZT, {right arrow over (C)}.sub.YT,
{right arrow over (C)}.sub.XT may be obtained from a basic
calibration {right arrow over (C)}.sub.ZB, {right arrow over
(C)}.sub.YB, and {right arrow over (C)}.sub.XB by taking each of
its vectors in turn, and removing any part parallel to the pure
torsion response. This may be accomplished by subtracting an
adjustment vector from each basic calibration vector. The
adjustment vector may in each case be formed 780 by multiplying the
normalized response vector to torsion by its own dot product with
the calibration vector in question: k.sub.QZ={right arrow over
(C)}.sub.ZB{right arrow over (F)}.sub.QN k.sub.QY={right arrow over
(C)}.sub.YB{right arrow over (F)}.sub.QN k.sub.QX={right arrow over
(C)}.sub.XB{right arrow over (F)}.sub.QN [8] The distortion
corrected calibration vectors may then be determined 790 by
difference between the basic calibration and the product of the
appropriate adjustment factor by the normalized response of the
touch screen to torsion. {right arrow over (C)}.sub.ZT={right arrow
over (C)}.sub.ZB-k.sub.QZ{right arrow over (F)}.sub.QN {right arrow
over (C)}.sub.YT={right arrow over (C)}.sub.YB-k.sub.QY{right arrow
over (F)}.sub.QN {right arrow over (C)}.sub.XT={right arrow over
(C)}.sub.XB-k.sub.QX{right arrow over (F)}.sub.QN [9]
[0079] Before discussing further embodiments of the method of the
invention, it is appropriate to briefly consider certain methods
for developing a basic calibration for a force touch screen.
Subject to certain assumptions, it can be shown that a basic
calculated calibration may be obtained from: {right arrow over
(C)}.sub.ZB=[s.sub.1, . . . s.sub.n] {right arrow over
(C)}.sub.YB=[s.sub.1y.sub.1, . . . s.sub.ny.sub.n] {right arrow
over (C)}.sub.XB=[s.sub.1x.sub.1, . . . s.sub.nx.sub.n] [10] where
x.sub.i, y.sub.i is the location at which a touch force passes into
the i.sup.th sensor, as measured in the desired output location
coordinates, and where s.sub.i scales and standardizes the
sensitivity of the i.sup.th sensor and its associated electronics.
That is, if f.sub.test.sub.--.sub.i is the change in sensor i
output in response to a true perpendicular sensor test force
F.sub.test.sub.--.sub.i passing through, then: s i = F test_i f
test_i [ 11 ] ##EQU5##
[0080] The accuracy of such a directly calculated calibration may
be compromised by certain factors. Among these may be inaccuracy in
the measurements of sensitivity or coupling position, the presence
of parallel paths for perpendicular force other than the sensors,
and the presence of significant channel-to-channel cross talk in
the wiring or electronics.
[0081] In addition, the method of obtaining a calculated
calibration, as so far described, makes no provision for especially
low susceptibility to torsional error. This may be improved upon,
however, by applying the torsion response corrections as described
above to the basic calibration vectors, {right arrow over
(C)}.sub.ZB, {right arrow over (C)}.sub.YB, {right arrow over
(C)}.sub.XB, to achieve a torsion corrected calibration vectors,
{right arrow over (C)}.sub.ZT, {right arrow over (C)}.sub.YT,
{right arrow over (C)}.sub.XT.
[0082] A example of a basic calibration and its nominal calculated
value is considered below. With four sensors, the basic form is
given by: {right arrow over (C)}.sub.ZB=[s.sub.1, s.sub.2, s.sub.3,
s.sub.4] {right arrow over (C)}.sub.YB=[s.sub.1y.sub.1,
s.sub.2y.sub.2, s.sub.3y.sub.3, s.sub.4y.sub.4] {right arrow over
(C)}.sub.XB=[s.sub.1x.sub.1, s.sub.2x.sub.2, s.sub.3x.sub.3,
s.sub.4x.sub.4] [12] Returning to FIG. 1, we assume that the four
corner sensors are precisely located, and have the exactly desired
sensitivity, which we will assume to be unity. We further assume
that the desired touch coordinate system should have its origin in
the screen center, that X and Y should each range from -1.00 to
+1.00, and that the edges of this range should extend to the
sensors. The upper left sensor is then located by:
[x.sub.1,y.sub.1]=[-1,+1], the upper right sensor by:
[x.sub.2,y.sub.2]=[+1,+1], the lower left sensor by:
[x.sub.3,y.sub.3]=[-1,-1], and the lower right sensor by:
[x.sub.4,y.sub.4]=[+1,-1]. This yields: {right arrow over
(C)}.sub.ZB.sub.--.sub.FIG1=[1,1,1,1] {right arrow over
(C)}.sub.YB.sub.--.sub.FIG1=[1,1,-1,-1] {right arrow over
(C)}.sub.XB.sub.--.sub.FIG1=[-1,1,-1,1] [13] With this, or with any
other exactly rectangular array of equally sensitive sensors, the
normalized response to torsion is given by: {right arrow over
(F)}.sub.QN.sub.--.sub.FIG1=[-1/2,1/2,1/2,-1/2], [14] which is
orthogonal to the nominal calibration vectors.
[0083] In another approach, basic calibration vectors may be
calculated on a unit-by-unit basis from sets of sensor response
values measured in response to test forces applied to the touch
surface of each completed unit. This approach may be advantageously
simple and accurate. Determination of the basic calibration vectors
by this method is conceptually illustrated in the flowchart of FIG.
10. In accordance with this approach, a known force is applied in
the vicinity of a touch sensor 1010. The force response of each
force sensor is measured 920. The process of applying a known force
at a sensor 910 and measuring the resultant response from each
sensor 1020 is repeated until a known force has been applied in the
vicinity of each of n touch sensors 1030.
[0084] An n.times.n data matrix M.sub.DATA.sub.--.sub.B may be
formed from the responses of the n touch sensors to the known
forces applied at each of n touch sensors 1040. A vector
representing the total force {right arrow over (D)}.sub.ZB the
Y-axis moment {right arrow over (D)}.sub.YB and the X-axis moment
{right arrow over (D)}.sub.XB may be formed from the known force
values 1050. The data matrix M.sub.DATA.sub.--.sub.B may then be
inverted 1060 to form M.sub.DATA.sub.--.sub.B.sup.-1. The
calibration vectors {right arrow over (C)}.sub.ZB, {right arrow
over (C)}.sub.YB, and {right arrow over (C)}.sub.XB may be
calculated 1070 as the dot products of the inverted data matrix
M.sub.DATA.sub.--.sub.B.sup.-1 and the calculated total force
{right arrow over (D)}.sub.ZB, y-axis moment {right arrow over
(D)}.sub.YB, and the x-axis moment {right arrow over (D)}.sub.XB,
respectively.
[0085] In an exemplary embodiment of the above-described method, a
touch surface of a four-sensor unit under test is oriented
horizontally. A known test weight is then placed on the touch
surface such that its center of gravity falls over each of four
known points in succession. These points may be chosen to fall
close to the corner located sensors, but inset somewhat to avoid
edge interferences. For instance, they may be chosen to fall at the
corners of a centered rectangle 15% smaller than the touch surface
itself. The weight, or each of four identical weights, may be
placed with the aid of a fixture or automatic apparatus.
[0086] Four sets of test data are collected, each comprising a
vector of four differences between the sensor readings for a
particular application minus those with no weight applied. Each of
these data vectors is then expected, when dotted with the
calibration vector for total perpendicular force, to yield the
known test weight value (or some convenient scaling thereof).
Equivalently, a 4.times.4 data matrix may be formed from the data
vectors in order as rows. This is expected, when multiplied on the
right by the calibration vector for total perpendicular force, to
yield an expected force vector of four components all equal to the
test force. Thus, the calibration vector for total perpendicular
force may be extracted by multiplying the inverse of the data
matrix by this expected force vector on the right.
[0087] Similarly, there is a vector of expected moments about the
desired Y-axis. These moments are equal to the X-position of each
test point in order times the known weight value. The calibration
vector for Y-axis moment may be extracted by multiplying the
inverse of the data matrix by this expected Y-axis moment vector on
the right.
[0088] Similarly, there is a vector of expected moments about the
desired X-axis. These moments are equal to the Y-position of each
test point in order times the known weight value. The calibration
vector for X-axis moment may be extracted by multiplying the
inverse of the data matrix by this expected X-axis moment vector on
the right.
[0089] For example, calibration forces A, B, C, and D, of 150
grams-force each, may be applied at the following points: [0090]
Test force, upper left: X.sub.A=-0.70 Y.sub.A=0.70 F.sub.A=150 gm.
[0091] Test force, upper right: X.sub.B=0.70 Y.sub.B=0.70
F.sub.B=150 gm. [0092] Test force, lower left: X.sub.C=-0.70
Y.sub.C=-0.70 F.sub.C=150 gm. [0093] Test force, lower right:
X.sub.D=0.70 Y.sub.D=-0.70 F.sub.D=150 gm. The changes in sensor
outputs occasioned by the applications of these forces may be
collected together as follows, both as measurement vectors: {right
arrow over (F)}.sub.A=[f.sub.A1, f.sub.A2, f.sub.A3, f.sub.A4]
{right arrow over (F)}.sub.B=[f.sub.B1, f.sub.B2, f.sub.B3,
f.sub.B4] {right arrow over (F)}.sub.C=[f.sub.C1, f.sub.C2,
f.sub.C3, f.sub.C4] {right arrow over (F)}.sub.D=[f.sub.D1,
f.sub.D2, f.sub.D3, f.sub.D4] [15] and as a data matrix: M DATA_B =
[ f A .times. .times. 1 f A .times. .times. 2 f A .times. .times. 3
f A .times. .times. 4 f B .times. .times. 1 f B .times. .times. 2 f
B .times. .times. 3 f B .times. .times. 4 f C .times. .times. 1 f C
.times. .times. 2 f C .times. .times. 3 f C .times. .times. 4 f D
.times. .times. 1 f D .times. .times. 2 f D .times. .times. 3 f D
.times. .times. 4 ] [ 16 ] ##EQU6## The vectors of expected forces
and moments may be similarly collected in the same A, B, C, D
order: D ZB = [ 150 150 150 150 ] .times. .times. D YB = [ - 0.70
.times. 150 0.70 .times. 150 - 0.70 .times. 150 0.70 .times. 150 ]
.times. .times. D XB = [ 0.70 .times. 150 0.70 .times. 150 - 0.70
.times. 150 - 0.70 .times. 150 ] [ 17 ] ##EQU7## The unknown
calibration vectors must render the known data matrix into the
known forces and moments in accordance with:
M.sub.DATA.sub.--.sub.B{right arrow over (C)}.sub.ZB={right arrow
over (D)}.sub.ZB M.sub.DATA.sub.--.sub.B{right arrow over
(C)}.sub.YB={right arrow over (D)}.sub.YB
M.sub.DATA.sub.--.sub.B{right arrow over (C)}.sub.ZB={right arrow
over (D)}.sub.ZB [18]
[0094] Each of these three matrix equations corresponds to a system
of four linear equations with four scalar unknowns. Among other
methods, the unknown calibration vectors may be determined using
the inverse of the known matrix M.sub.DATA.sub.--.sub.B: {right
arrow over (C)}.sub.ZB=M.sub.DATA.sub.--.sub.B.sup.-1{right arrow
over (D)}.sub.ZB {right arrow over
(C)}.sub.YB=M.sub.DATA.sub.--.sub.B.sup.-1{right arrow over
(D)}.sub.ZB [19] {right arrow over
(C)}.sub.XB=.sub.DATA.sub.--.sub.B.sup.-1{right arrow over
(D)}.sub.XB
[0095] This method of calibration, as so far described, provides a
basic calibration with no provision for especially low
susceptibility to torsional error. Indeed, with a rigid touch
surface structure, variable loading of the sensors in a torsional
pattern may be poorly represented in the data matrix
M.sub.DATA.sub.--.sub.B. The susceptibility of the resulting
calibration to torsional error may then be especially high. These
problems may be addressed by applying the torsion response
corrections as described above to the basic calibration vectors
{right arrow over (C)}.sub.ZB,{right arrow over (C)}.sub.YB, {right
arrow over (C)}.sub.XB, or through further embodiments of the
invention, such as those described below.
[0096] In another embodiment of the method of the invention,
illustrated in the flowchart of FIG. 11, torsion corrected
calibration vectors are computed from data responsive both to known
touch surface forces and to deliberately applied or enhanced
torsional distortion. By this method, data matrix
M.sub.DATA.sub.--.sub.B is acquired 1110 by the method discussed
immediately above. A first distortion is applied to the touch
screen and a first set of force response measurements obtained
1120. A second distortion is applied and the resulting second set
of force responsive measurements obtained 1130. The differences,
f.sub.Q1, f.sub.Q2, f.sub.Q3, f.sub.Q4, between the force response
measurements resulting from the second applied distortion,
f.sub.1(t.sub.Q2), f.sub.Q2(t.sub.Q2), f.sub.Q3(t.sub.Q2), f.sub.Q4
(t.sub.Q2), and the first applied distortion, f.sub.1(t.sub.Q1),
f.sub.Q2(t.sub.Q1), f.sub.Q3(t.sub.Q1), f.sub.Q4(t.sub.Q1), are
calculated 1140. These values are used to extend the data matrix
M.sub.DATA.sub.--.sub.B by a fifth row 1150 to form a torsion
extended data matrix M.sub.DATA.sub.--.sub.T. The pseudo-inverse of
the 5.times.4 data matrix M.sub.DATA.sub.--.sub.T is determined
1160. The total expected force vector, Y-axis moment vector, and
the X-axis moment vectors are calculated from known forces and
coordinates 1170. The torsion-corrected calibration vectors {right
arrow over (C)}.sub.ZT, {right arrow over (C)}.sub.YT, {right arrow
over (C)}.sub.XT are then calculated as the as the products of the
pseudo-inverse M.sub.DATA.sub.--.sub.T.sup.PSEUDO-1 of the extended
data matrix M.sub.DATA.sub.--.sub.T times the calculated total
force {right arrow over (D)}.sub.ZT, Y-axis moment {right arrow
over (D)}.sub.YT, and the X-axis moment {right arrow over
(D)}.sub.XT, respectively 1180.
[0097] By this method, the data matrix for basic calibration is
extended by addition of a fifth row. This row may comprise the
response vector to torsion (or some linear scaling of it): M DATA_T
= [ f A .times. .times. 1 f A .times. .times. 2 f A .times. .times.
3 f A .times. .times. 4 f B .times. .times. 1 f B .times. .times. 2
f B .times. .times. 3 f B .times. .times. 4 f C .times. .times. 1 f
C .times. .times. 2 f C .times. .times. 3 f C .times. .times. 4 f D
.times. .times. 1 f D .times. .times. 2 f D .times. .times. 3 f D
.times. .times. 4 f Q .times. .times. 1 f Q .times. .times. 2 f Q
.times. .times. 3 f Q .times. .times. 4 ] [ 20 ] ##EQU8##
[0098] This extended 5.times.4 matrix may be multiplied on the
right by each of the calibration vectors sought:
M.sub.DATA.sub.--.sub.T{right arrow over (C)}.sub.ZT
M.sub.DATA.sub.--.sub.T{right arrow over (C)}.sub.YT={right arrow
over (D)}.sub.YT M.sub.DATA.sub.--.sub.T{right arrow over
(C)}.sub.ZT={right arrow over (D)}.sub.ZT [21]
[0099] In each case, the first four elements of the resulting
5-element vector on the right side should be the same as before,
while the fifth element is expected to be zero: D ZT = [ 150 150
150 150 0 ] .times. .times. D YT = [ - 0.70 .times. 150 0.70
.times. 150 - 0.70 .times. 150 0.70 .times. 150 0 ] .times. .times.
D XT = [ 0.70 .times. 150 0.70 .times. 150 - 0.70 .times. 150 -
0.70 .times. 150 0 ] [ 22 ] ##EQU9##
[0100] While no inverse is defined for a 5.times.4 matrix, a
suitable 4.times.5 pseudo-inverse may be extracted by known methods
employing its singular value decomposition. That is, a matrix M of
m rows by n columns, m.gtoreq.n, may in general be expressed as the
product of three other matrices: M=UWV.sup.T [23] where W is an n
by n diagonal matrix, and U and V are column-orthonormal matrices
of sizes m by n and n by n, respectively. U, W, and V may be found
by standard methods and an n by m pseudo-inverse expressed as:
M.sub.DATA.sub.--.sub.T.sup.PSEUDO-1=VW.sup.-1U.sup.T [24]
[0101] Calibration coefficients may then be extracted by
multiplying this pseudo-inverse on the right by each of the
5-element expected-result vectors, in a manner analogous to that
previously described for the conventional inverse. Note that the
problem solved here is essentially one of achieving a best-fit
solution to an overdetermined set of linear equations. Various
methods to achieve the best-fit solution may be used. For example,
solution by singular value decomposition with back-substitution may
be computationally more efficient than use of an explicitly formed
pseudo-inverse.
[0102] In another such embodiment, varying torsion is applied to
the touch-screen support at the same time that known forces are
applied to the touch surface. Calibration coefficients are then
determined from the data matrix and the expected result vectors as
described previously, although here there need not be any expected
results that are zero, and there need not be more than four data
rows. Additional rows may be added for additional known force
measurements if desired, however. The overdetermination may be
handled as before. It will be evident to those of ordinary skill in
the art that the method of the invention may be adapted to other
procedures for extracting calibration coefficients, including those
that employ a larger number of touch surface forces applied at
known locations, but lacking known force values.
[0103] A first class of methods have been discussed, wherein a
basic calibration is prepared in one step, and refined with respect
to torsion in another. This approach may offer the advantage of
requiring less unit-by-unit data measurement. It may work well for
a certain range of suitable devices, including those with sensors
of roughly similar sensitivity that are close to a rectangular
pattern. It tends to effectively minimize unwanted response to
fluctuating torsion. On the other hand, there is the theoretical
possibility that in the process, it may "spoil" other aspects of
the calibration, in the sense of degrading accuracy in the absence
of torsion. For suitable devices, however, this potential problem
is not significant.
[0104] A second class of methods have also been discussed, wherein
a torsion-refined calibration is prepared in a single step. This
approach may offer the advantage of an optimized calibration over
the full range of force-sensing touch location devices.
[0105] We now reconsider the case wherein the touch surface
structure is relatively rigid, in the sense that most of the small
out-of-plane movements resulting from the application of a
torsional force take place in the force sensors or the supporting
structure. If the touch surface itself always remains plane, its
motions in response to all test forces may explore only three
degrees of freedom: slight vertical motions and rotations, but no
corner-to-corner saddling. Given this, calibration only from a set
of known touch forces may remain an underconstrained problem, no
matter how many forces and locations are used. Adding deliberate
variable torsion in the support resolves this problem. Without
this, however, it is noted that sensitivity to torsional
interference may be particularly high. In other words, a unit with
a rigid touch surface may be calibrated in the factory with a
benign support lacking variable torsion. When that unit is placed
in service in the field, however, it may be vulnerable to large
errors from variable support torsion. A method of the invention is
thus particularly beneficial in this case.
[0106] Such rigid touch surface devices may need only moderate
torsional exposure during calibration to achieve satisfactory
results. In another embodiment of the invention, such moderate
torsional exposure may be achieved by changing the relative
compliance of the support under at least one of the force sensors
during the collection of calibration data. This may be accomplished
in many ways. One approach involves placing materials made of
differing compliance in the regions supporting the different
sensors, and then rotating the overall support surface after half
of the test forces have been applied.
[0107] A system for characterizing error in a touch screen
associated with mechanical distortion is schematically illustrated
in FIG. 12. In this example, a touch screen 1205 includes four
touch sensors 1210, 1220, 1230, 1240 located at four corners of a
rectangular touch surface. The touch screen shown is the device for
which mechanical distortion error is to be characterized. Known
forces may be applied to the touch screen at locations 1215, 1225,
1235, 1245 in the vicinity of the touch sensors 1210, 1220, 1230,
1240 and the force response of the sensors measured in the manner
previously discussed. Additionally, one or more mechanical
distortions of the touch screen may also be applied and the force
response measured. The touch sensors 1210, 1220, 1230, 1240 are
coupled to a touch screen interface 1250 within the touch screen
calibration system 1260. The touch screen interface 1250 provides
drive circuitry for energizing the sensors, as well as sense
circuitry for sensing force responsive touch signals from the
sensors. The touch screen interface drive/sense circuitry is
similar to the sensor drive/sense circuitry 310, 320, 330, 340
schematically illustrated for the touch screen controller FIG. 3.
The touch screen interface 1250 is coupled to a processor 1264
within the touch screen calibration system. The processor 1264
receives force responsive signals from the touch screen interface
1250 and controls the processes of error characterization and
computation of calibration parameters. The processor 1264 may be
coupled to an output interface 1266 for recording or indicating the
calibration parameters 1270 determined by the touch screen
calibration system 1260. The processor 1264 may also be coupled to
memory circuitry 1262 for storing program code and data, including
calibration parameters, for example.
[0108] It is to be appreciated that the calibration parameters 1270
may be grouped and represented in many different ways. Particular
designs may apply additional transformations of touch data. For
instance, known procedures of "registration" may constitute an
additional level of adjustment, allowing a user-applied procedure
to correct for varying alignment of the touch screen with an
underlying display raster. Such a procedure may be combined with
the calibration of the invention without departing from its scope,
either by merging the required adjustment into the calibration
parameters of the invention, or by applying them in a later stage
of calculation. Various computational arrangements may be used to
apply torsion corrected calibration parameters along with
parameters gathered for other purposes, without departing from the
scope of the invention.
[0109] A touch screen calibrated for reduced error from mechanical
distortion as described herein may be advantageously implemented in
various data processing systems. Turning now to FIG. 13, a block
diagram of a data processing system 1300 using an integrated touch
screen and display is shown in accordance with an embodiment of the
present invention. The system 1300 uses a transparent touch screen
1306 arranged above a display 1308 suitable for data processing
applications, such as an LCD display. Other displays may be used,
such as a CRT display, plasma display, LED display or the like. The
display 1308 may require display control system circuitry 1309 for
interfacing the display with the data processor computer 1310. A
touch screen control system 1307 includes the drive/sense circuitry
described above in addition to a touch screen control system
processor according to an embodiment of the present invention.
[0110] The data processor 1310 may include various components
depending upon the computer system application. For example, the
data processor may include a microprocessor 1312, various types of
memory circuitry 1314, a power supply 1318 and one or more
input/output interfaces 1316. The input/output interfaces 1316
allow the data processing system to connect to any number of
peripheral I/O devices 1320 such as keyboards 1321, pointing
devices 1322, and sound devices 1323, including microphone and
speakers. The data processing system may additionally include a
mass data storage device 1330, for example, a hard disk drive or CD
ROM drive, and may be networked to other data processing systems
through a physical or wireless network connection 1340.
[0111] FIG. 14 illustrates a touch screen system 1400 in accordance
with the present invention, wherein the processes of the invention
described herein may be tangibly embodied in a computer-readable
medium or carrier, e.g. one or more of the fixed and/or removable
data storage devices 1410 illustrated in FIG. 14, or other data
storage or data communications devices. One or more computer
programs 1420 expressing the processes embodied on the removable
data storage devices 1410 may be loaded into various memory
elements 1430 located within the touch screen control system 1440
to configure the touch screen system 1400 for operation in
accordance with the invention. The computer programs 1420 comprise
instructions which, when read and executed by the touch screen
system processor 1450 of FIG. 14, cause the touch screen system
1400 to perform the steps necessary to execute the steps or
elements for detecting the location of a touch on a touch screen in
accordance with the principles of the present invention.
[0112] FIG. 15 illustrates a touch screen calibration system 1500
in accordance with the present invention, wherein the processes of
the invention described herein may be tangibly embodied in a
computer-readable medium or carrier, e.g. one or more of the fixed
and/or removable data storage devices 1510 illustrated in FIG. 15,
or other data storage or data communications devices. One or more
computer programs 1520 expressing the processes embodied on the
removable data storage devices 1510 may be loaded into various
memory elements 1530 located within the touch screen calibration
system 1550 to configure the touch screen calibration system 1550
for operation in accordance with the invention. The computer
programs 1520 comprise instructions which, when read and executed
by the touch screen calibration system 1550 of FIG. 15, cause the
touch screen calibration system 1550 to perform the steps necessary
to execute the steps or elements for detecting the location of a
touch on a touch screen in accordance with the principles of the
present invention.
[0113] A touch sensing method and system in accordance with the
principles of the present invention provides for enhanced touch
location accuracy in the presence of mechanical distortions of the
touch screen. Other methods of improving touch location accuracy
may be advantageously combined with the method of the present
invention to further enhance location accuracy.
[0114] One method for timing the touch location measurement for
enhanced touch location accuracy is described in U.S. patent
application entitled "Method for Improving Positioned Accuracy for
a Determined Touch Input," identified under Docket Number
57470US002 (US publication number 03-0206162-A1, published on Nov.
6, 2003), which is hereby incorporated herein by reference in its
entirety. According to this method, touch location may be
calculated from data gathered at a preferred time within the touch
signal time profile. The method of timing the touch location may be
combined with calibration methods of the present invention to
further improve the accuracy of a touch location determination.
[0115] Another method for improving touch location accuracy is
described in co-owned U.S. patent application entitled "Improved
Baselining Techniques in Force-Based Touch Panel Systems,"
identified under Docket Number 57471US002 (US publication number
03-0210235-A1, published on Nov. 13, 2003), which is hereby
incorporated herein by reference in its entirety. One or more
reference levels may be identified for a touch signal. The
reference levels may compensate for various conditions affecting
the touch screen at the time of the touch. Touch location accuracy
may be further enhanced using one or more of the identified touch
signal reference levels for determining the touch location in
combination with the calibration methods provided in the present
invention.
[0116] Yet another method for improving touch location accuracy by
correcting touch signal errors associated with viscoelastic memory
effects is described in co-owned U.S. patent application entitled
"Correction of Memory Effect Errors in Force-Based Touch Panel
Systems," identified under Docket Number 57472US002 (US publication
number 03-0214486-A1, published on Nov. 20, 2003), which is hereby
incorporated herein by reference in its entirety. Correction of
touch signal errors associated with memory effects in combination
with the calibration methods of the present invention may improve
the accuracy of touch location determination.
[0117] The touch sensing approach described herein is well-suited
for use with various data processing systems, including personal
data assistants (PDAs), electronic instruments, cell phones, and
computers, including handheld, laptop and desktop computers.
[0118] The present invention should not be considered limited to
the particular examples described above, but rather should be
understood to cover all aspects of the invention as fairly set out
in the attached claims. Various modifications, equivalent
processes, as well as numerous structures to which the present
invention may be applicable will be readily apparent to those of
skill in the art to which the present invention is directed upon
review of the present specification. The claims are intended to
cover such modifications and processes.
* * * * *