(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2009/0284495 A1
`GEAGHAN et al.
`(43) Pub. Date:
`Nov. 19, 2009
`
`US 20090284.495A1
`
`(54) SYSTEMS AND METHODS FOR ASSESSING
`LOCATIONS OF MULTIPLE TOUCH INPUTS
`
`(75) Inventors:
`
`Bernard O. GEAGHAN, Salem,
`NH (US); Craig A. Cordeiro,
`Westford, MA (US)
`
`Publication Classification
`
`(51) Int. Cl.
`G06F 3/044
`(2006.01)
`(52) U.S. Cl. ...................................... 345/174; 178/18.06
`
`Correspondence Address:
`3M INNOVATIVE PROPERTIES COMPANY
`PO BOX 33427
`ST. PAUL, MN 55133-3427 (US)
`
`(73) Assignee:
`
`3M Innovative Properties
`Company
`
`(21) Appl. No.:
`1-1.
`(22) Filed:
`
`12/465,197.
`
`May 13, 2009
`O
`O
`Related U.S. Application Data
`(60) Provisional application No. 61/127,580, filed on May
`14, 2008.
`
`(57)
`
`ABSTRACT
`
`Matrix-based touch input systems assess touch locations of
`two or more temporally overlapping touch inputs by forming
`valid X-y coordinate pairs from independently determined X
`andy-coordinates. Valid X-y pairs are formed based on com
`paring one or more signal parameters such as signal magni
`tude, signal strength, signal width, and signal rates of change.
`In matrix capacitive systems where capacitance-to-ground
`signals are used to determine the X- and y-coordinates, the
`determined coordinates may be formed into valid X-y pairs
`using mutual capacitance measurements. When resolving
`more than two temporally overlapping touches, information
`gained by resolving a valid X-y coordinate pair of at least one
`of the touches may be used to resolve the remaining touches.
`
`
`
`Corputatior
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`Patent Application Publication
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`Nov. 19, 2009 Sheet 1 of 11
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`Patent Application Publication
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`Nov. 19, 2009 Sheet 2 of 11
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`US 2009/0284.495 A1
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`Patent Application Publication
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`Nov. 19, 2009 Sheet 3 of 11
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`Patent Application Publication
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`Nov. 19, 2009 Sheet 4 of 11
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`US 2009/0284.495 A1
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`Patent Application Publication
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`Nov. 19, 2009 Sheet 6 of 11
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`US 2009/0284.495 A1
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`Patent Application Publication
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`Patent Application Publication
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`Nov. 19, 2009 Sheet 11 of 11
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`12
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`

`

`US 2009/0284.495 A1
`
`Nov. 19, 2009
`
`SYSTEMIS AND METHODS FOR ASSESSING
`LOCATIONS OF MULTIPLE TOUCH INPUTS
`
`CROSS REFERENCE TO RELATED
`APPLICATION
`0001. This patent document claims the benefit, under 35
`U.S.C. S 119(e), of U.S. Provisional Patent Application Ser.
`No. 61/127,580 filed on May 14, 2008, entitled “Systems and
`Methods for Assessing Locations of Multiple Touch Inputs'
`the disclosure of which is incorporated by reference in its
`entirety.
`
`FIELD OF THE INVENTION
`0002 The present invention relates generally to touch
`input systems and methods for assessing and discriminating
`multiple touch inputs, and in particular to discriminating
`multiple touch inputs in touch systems Susceptible to phan
`tom touch errors.
`
`BACKGROUND
`0003 Touch sensitive devices allow a user to conveniently
`interface with electronic systems and displays by reducing or
`eliminating the need for mechanical buttons, keypads, key
`boards, and pointing devices. For example, a user can carry
`out a complicated sequence of instructions by simply touch
`ing an on-display touch screen at a location identified by an
`icon. In many touch sensitive devices, the input is sensed
`when a conductive object in the sensor is capacitively coupled
`to a conductive touch implement such as a user's finger. Such
`devices measure capacitance at multiple locations due to the
`touch disturbance, and use the measured capacitances to
`determine touch position.
`0004. In some applications, multiple touch inputs are
`applied at the same time, for example from multiple users in
`a multiplayer game or from a single person using a virtual
`keyboard or similar interface. Such applications benefit from
`the accurate discrimination of multiple simultaneous touches
`so that the touch position for each input can be determined to
`trigger appropriate actions in the application.
`
`SUMMARY OF THE INVENTION
`0005. In certain embodiments, the present invention pro
`vides methods for assessing and discriminating touch loca
`tions of two or more temporally overlapping touch inputs for
`use with matrix capacitive touch screen systems. Such sys
`tems include a sensor having a plurality of X-electrodes for
`providing X-signals based on capacitance-to-ground mea
`Surements and indicative of X-coordinates of touch inputs,
`and a plurality of y-electrodes for providing y-signals based
`on capacitance-to-ground measurements and indicative of
`y-coordinates of touch inputs. Such methods include deter
`mining valid X-coordinates of the two or more touch inputs
`from received X-signals, determining valid y-coordinates of
`the two or more touch inputs from received y-signals, and
`forming valid X-y coordinate pairs, the valid X-y pairs being
`indicative of the locations of the touch inputs. In certain
`embodiments, forming valid coordinate pairs may be per
`formed using mutual capacitance measurements, and/or by
`comparing one or more signal parameters such as signal
`magnitude, signal strength, signal width, and signal rates of
`change.
`0006. In certain embodiments, the present invention pro
`vides matrix capacitive touch screen systems that include a
`
`sensor having a plurality of X-electrodes for providing X-sig
`nals based on capacitance-to-ground measurements and
`indicative of X-coordinates of touch input locations, and a
`plurality of y-electrodes for providing y-signals based on
`capacitance-to-ground measurements and indicative of y-co
`ordinates of touch input locations. Such systems further
`include controller circuitry coupled to the X-electrodes to
`receive the X-signals and coupled to the y-electrodes to
`receive the y-signals, the controller configured to determine
`one or more X-coordinates from the X-signals and one or more
`y-coordinates from the y-signals, and responsive to two or
`more touch inputs applied to the touch sensor to form valid
`X-y coordinate pairs from the determined X-coordinates and
`determined y-coordinates.
`0007. In certain embodiments, the present invention pro
`vides methods for assessing touch locations of three or more
`temporally overlapping touch inputs for use in matrix touch
`screen systems that includes a sensor providing X-signals
`indicative of X-coordinates of touch inputs and y-signals
`indicative of y-coordinates of touch inputs. Such methods
`include determining valid x-coordinates of the three or more
`touch inputs from received X-signals, determining validy-co
`ordinates of the three or more touch inputs from received
`y-signals, forming a valid X-y pair from the determined X-co
`ordinates and the determined y-coordinates indicative of a
`valid touch location, thereby resolving one of the touch inputs
`with the other touch inputs remaining unresolved, and
`responsive to the resolved touch input, forming valid X-y pairs
`for one or more of the unresolved touch inputs from the
`remaining X-coordinates and remaining y-coordinates.
`0008. In certain embodiments, this disclosure is also
`directed to, for use in a matrix touch screen having a plurality
`ofnodes at the intersections ofx-andy-sensor bars, each node
`being driven by electronics such that it yields a signal
`strength, a method for assessing touch locations of two or
`more temporally overlapping touch inputs comprising, for
`nodes having signal strength above a defined touch-event
`threshold: (1) associating the node having the highest signal
`strength with a first touch; (2) associating nodes adjacent the
`node with the highest signal strength with the first touch; (3)
`among nodes not associated with the first touch, associating
`the node having the highest signal strength with a second
`touch; and (4) associating nodes adjacent to the node associ
`ated with the second touch with the second touch.
`0009. The above summary of the present invention is not
`intended to describe each embodiment or every implementa
`tion of the present invention. Advantages and attainments,
`together with a more complete understanding of the inven
`tion, will become apparent and appreciated by referring to the
`following detailed description and claims taken in conjunc
`tion with the accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`0010. The present disclosure may be more completely
`understood and appreciated in consideration of the following
`detailed description of various embodiments in connection
`with the accompanying drawings, in which:
`0011
`FIG. 1 schematically illustrates a matrix touchsen
`Sor system for assessing multiple touch inputs in accordance
`with certain embodiments of the present invention;
`0012 FIG. 2 illustrates steps used in assessing multiple
`touch inputs in accordance with certain embodiments of the
`present invention;
`
`13
`
`

`

`US 2009/0284.495 A1
`
`Nov. 19, 2009
`
`0013 FIG. 3A illustrates three temporally overlapping
`touches on a matrix touch sensor that are resolvable in accor
`dance with certain embodiments of the present invention;
`0014 FIG. 3B schematically illustrates time plots of sig
`nals received for the touches indicated in FIG. 3A;
`0015 FIG. 4A illustrates three temporally overlapping
`touches on a matrix touch sensor that are resolvable in accor
`dance with certain embodiments of the present invention;
`0016 FIG. 4B schematically illustrates time plots of sig
`nals received for the touches indicated in FIG. 4A;
`0017 FIG. 5A illustrates three temporally overlapping
`touches on a matrix touch sensor that are resolvable in accor
`dance with certain embodiments of the present invention;
`0018 FIG. 5B schematically illustrates time plots of sig
`nals received for the touches indicated in FIG. 5A;
`0019 FIG. 6 illustrates steps used in assessing multiple
`touch inputs in accordance with certain embodiments of the
`present invention;
`0020 FIG. 7 illustrates signals received at neighboring
`electrodes in a matrix capacitive touch system in the presence
`of two proximately located touches that are resolvable in
`accordance with certain embodiments of the present inven
`tion;
`0021
`FIG. 8A illustrates an example virtual keyboard lay
`out; and
`0022 FIG.8B illustrates an example virtual keyboard lay
`out modified to allow resolution of anticipated key stroke
`combinations in accordance with certain embodiments of the
`present invention.
`0023 FIG.9A illustrates an example of two distinct touch
`down, drag, liftoff events.
`0024 FIG.9B illustrates how the two distinct touchdown
`events of FIG. 9A may be incorrectly interpreted.
`0025 FIG. 10A is a graph illustrating a touch-down event
`with basic ration interpretation.
`0026 FIG. 10B is a graph illustrating a touch-down event.
`0027 FIG. 10C is a graph illustrating a touch-down event.
`0028. While the invention is amenable to various modifi
`cations and alternative forms, specifics thereof have been
`shown by way of example in the drawings and will be
`described in detail. It is to be understood, however, that the
`intention is not to limit the invention to the particular embodi
`ments 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 OF EMBODIMENTS
`0029. In the following description of the illustrated
`embodiments, reference is made to the accompanying draw
`ings which form a part hereof, and in which is shown by way
`of illustration, various embodiments in which the invention
`may be practiced. It is to be understood that the embodiments
`may be utilized and structural changes may be made without
`departing from the scope of the present invention.
`0030 The present invention is generally applicable to
`touch systems and particularly to touch systems where two or
`more touches may be applied by one or more users. The
`present invention is particularly Suited to a touch system
`where some portion of two or more touch inputs may occur
`simultaneously or otherwise temporally overlap. For
`example, the present invention may be Suited for use in an
`electronic game system designed to be played by one or more
`players where, in the course of playing the game, players can
`apply touch input to generate a response in the game, and
`
`where two or more touches may start at the same time and/or
`end at the same time and/or overlap for at least part of the time
`during which each touch is applied. Such touch inputs can be
`referred to as overlapping touches, double touches, or simul
`taneous touches.
`0031. In certain embodiments, the present disclosure is
`directed to touch sensor Systems and methods for assessing
`touch locations of two or more temporally overlapping touch
`inputs. For example, such systems and methods include a
`sensor having a plurality of X-electrodes for providing X-sig
`nals based on capacitance-to-ground measurements and
`indicative of X-coordinates of touch inputs, and a plurality of
`y-electrodes for providingy-signals based on capacitance-to
`ground measurements and indicative of y-coordinates of
`touch inputs. Valid x-coordinates of the two or more touch
`inputs are determined from received X-signals, and valid
`y-coordinates of the two or more touch inputs are determined
`from received y-signals. Valid x-y pairs are formed from the
`determined X-andy-coordinates, the valid pairs being indica
`tive of valid touch locations. The x-y coordinate validation
`may be performed, for example, using mutual capacitance
`measurements, and/or by comparing one or more signal
`parameters such as signal magnitude, signal strength, signal
`width, and signal rates of change. Co-assigned U.S. Pat. No.
`7.254,775, “Touch Panel System and Method for Distin
`guishing Multiple Touch Inputs” (Geaghan, et. al.) includes
`methods and systems for distinguishing multiple temporally
`overlapping touch inputs, and is hereby incorporated by ref
`erence in its entirety.
`0032 Touchpanel sensors, controllers, systems and meth
`ods are disclosed that can distinguish temporally overlapping
`touch inputs from single touch inputs so that valid touch
`position coordinates can be determined. Touch panel systems
`and methods of the present invention can distinguish overlap
`ping touches by comparing signal strengths to specified
`thresholds, by comparing sequential changes of signal mag
`nitudes by comparing the rates of change of signal magni
`tudes or measured positions to determined parameters, and
`the like. As used in this document, signal magnitude is a
`measure that includes signal strength and signal width.
`0033. The present invention provides systems and meth
`ods for discriminating valid touch locations among multiple
`possible touch locations resulting when two or more touches
`temporally overlap. Further, the present invention provides
`for storing the signals measured during multiple touch over
`lap so that the information gained by discriminating and
`reporting a least one valid touch location may be used to
`identify, discriminate, and report additional valid touch loca
`tions from the stored signals.
`0034. By the nature of the detection mechanisms, matrix
`touch sensor technologies such as matrix capacitive, infrared
`matrix (IR), and surface acoustic wave (SAW), can readily
`indicate and distinguish valid X-coordinates and valid y-co
`ordinates of multiple touch events. However, it may not be
`apparent which X-coordinate goes with which y-coordinate,
`and as Such the present invention involves distinguishing the
`valid touch locations (that is, valid x-y matches) from the
`phantom touch locations (that is, invalid X-y matches). This
`can be done by comparing and correlating timing of signal
`event sequences, correlating signal magnitude information,
`correlating signal rate of change information, and so forth.
`The basic principle of discrimination is that the X- and y-sig
`nals for a valid touch location point will have similar signal
`timings, signal strengths, signal magnitudes, and signal rates
`
`14
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`

`

`US 2009/0284.495 A1
`
`Nov. 19, 2009
`
`of change, whereas phantom touch points will be associated
`with X- and y-signals that differ in some or all of these signal
`characteristics. As such, any of these signal characteristics
`may be used individually or in any Suitable combination to
`distinguish valid touch locations from phantom touch loca
`tions.
`0035. In certain embodiments, the presence of multiple
`touch points is distinguished from the presence of single
`touches prior to discriminating multiple real touch points
`from phantoms. When two touches are spatially very close to
`one another, their signals may blend together so that discrimi
`nating two from one is not immediately apparent. Later por
`tions of this disclosure describe methods for resolving such
`proximate multiple touches in a matrix capacitive system
`based on touch signal magnitudes as well as describes cali
`bration techniques for interpolation based on scaled measure
`ments to achieve improved touch location accuracy, and may
`be used with the methods described herein for enhanced
`resolution.
`0036. In certain embodiments, multiple touches are suffi
`ciently resolved by determining valid X- and y-coordinates,
`even if the valid coordinates are not matched to form valid
`coordinate pairs. In such embodiments, the bounding poly
`gon defined by the possible touch points is of interest. For
`example, two touches along with their phantoms may define
`the vertices of a bounding box oriented with touch coordinate
`system. The bounding box may be used as a reference to scale
`a displayed rectangle smaller and larger by detecting move
`ment of the two touches closer and further apart. Applications
`include a pinch Zoom operation, which need not rely on
`discriminating real touches from phantom touches. Methods
`that may be used in performing Such operations are described
`below.
`0037. As more temporally overlapping touches are
`applied, for example 3 or more touches, the likelihood
`increases that a touch will land at or near one of the phantom
`touch locations. In accordance with certain embodiments of
`the present invention, Such circumstances may be addressed
`by correlating one or more signal characteristics to find at
`least one valid touch location, and using the valid touch
`location to eliminate one or more of the possible phantom
`touch locations, thereby simplifying analysis of the remain
`ing signals.
`0038. In certain embodiments involving matrix capacitive
`touch sensors, capacitance-to-ground signals are used to
`develop a set of valid X- and y-coordinates, which represent a
`set of possible x-y touch locations. Each of the possible x-y
`touch locations may then be tested for existence of a mutual
`capacitance between the X- and y-electrodes, indicating the
`existence of a valid touch location. For example, given a
`3-touch situation, there exist 3 possible x-coordinates, x1, x2
`and X2, and 3 possible y-coordinates, y1, y2 and y3, giving
`rise to 9 possible touch locations: (x1,y1), (x1, y2), (x1,y3),
`(x2.y1), (x2y2), (x2,y3), (x3.y1), (x3y2), and (x3-y3).
`Assuming the X-electrodes are drive lines and y-electrodes
`are sense lines, each of the X-electrodes associated with a
`valid x-coordinate may be driven in turn, and each of the
`y-electrodes associated with a valid y-coordinate may be
`monitored for a mutual capacitance signal. Such a process
`reveals the valid X-y pairings. Additionally, touch locations
`may be refined by interpolations techniques, for example by
`monitoring for signal strength (capacitance-to-ground or
`mutual capacitance) on the X- and y-electrodes on either side
`
`of the valid touch location, and adjusting the touch location
`accordingly. Appendices A and C describe Such techniques.
`0039. Because touch panel systems and methods of the
`present invention can discriminate multiple touches, they can
`be used in multiple user applications such as multiplayer
`games, in applications that may be subject to rapidly succes
`sive or overlapping touch inputs, and in applications where a
`single user uses multiple fingers (in any combination from
`one or two hands) to input information Such as gestures or
`virtual keyboard touches. The ability to discriminate among
`any arbitrary number of simultaneous touches allows the
`development of applications that take advantage of such func
`tionality. For example, certain handheld devices utilize a
`touch input pinch Zoom and expand operations where a user
`touches the screen with two fingers, and expands or contracts
`the separation between them to Zoom in or out of a document,
`map, image, etc.
`0040 Systems and methods of the present invention may
`also be used to detect and discriminate hover events from
`simultaneous touch or hover events. Hover events occur when
`a touch object is brought close enough to the touch surface to
`capacitively couple with the electrodes while not being suf
`ficiently close to the touch surface to be considered a full
`touch, for example, being in contact with the touchSurface, or
`being in contact with sufficient pressure. When touches and
`touch inputs are referred to in this document, it should be
`considered that both full touches and hover events are
`included, unless the context dictates otherwise.
`0041. In a touch screen system, the location of a touch
`applied by a user is generally determined by measuring sig
`nals generated by the touch input, and using the signals to
`calculate the position of the touch. Application-dependent
`instructions are then carried out based on the determined
`touch position. Assuming a properly calibrated touch system,
`the calculated touch position should be sufficiently close to
`the actual location touched by the user so that the user's
`intended instruction can be carried out. How close the
`reported touch location should be to the actual touch location
`to be sufficiently close is determined, in part, by the resolution
`of the touch system. A reported touch location that suffi
`ciently closely corresponds to an actual location touched by a
`user is referred to as a valid touch. As used in this document,
`reporting a touch location refers to the calculated touch loca
`tion being used by the touch system in an appropriate manner,
`for example by the application software to determine the user
`input instructions. Reporting might include communications
`from a touch screen controller to a central processing unit, or
`in a more integrated system can simply entail touch position
`data being calculated and appropriately used as contemplated
`by the application.
`0042 Methods of the present invention may be suited for
`use with various different touch sensor technologies in which
`X-coordinate data may be determined independent fromy-co
`ordinate data, for example matrix capacitive (capacitance-to
`ground) systems, matrix IR, and SAW. Because each touch
`screen technology differs at least somewhat in the touch input
`signals that are measured, and in the manner that the signals
`are interpreted, the implementation of aspects of the present
`invention can have application-specific elements.
`0043. In certain embodiments, the present invention is
`used to distinguish among multiple temporally overlapping
`touch inputs, in particular when a matrix capacitive touch
`sensor is used as the touch input device. When capacitance
`to-ground signals are used to determine touch coordinates in
`
`15
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`

`

`US 2009/0284.495 A1
`
`Nov. 19, 2009
`
`matrix capacitive touch sensor Systems, the capacitance-to
`ground signal for each individual electrode is measured. As
`Such, valid X-coordinate positions may be directly determined
`from signals gathered from the X-electrodes, and valid y-co
`ordinate positions may be directly determined from signals
`gathered from the y-electrodes. In circumstances where a
`single touch input is applied to the sensor So that a single
`X-coordinate and a singley-coordinate are validated, no coor
`dinate discrepancies exist, and the touch location may be
`directly reported (even though the touch location may be
`refined through interpolation and other methods that may use
`additional information gathered from additional signals. Such
`as rationnetric current measurements, mutual capacitance
`measurements, and so forth, as will be understood to those
`skilled in the art). In circumstances where multiple touch
`inputs are applied to the sensor So that the touch inputs over
`lap in time, there generally exists a valid pairing of x-coordi
`nates and y-coordinates for each touch input (valid touch
`locations), as well as invalid pairings (phantom touch loca
`tions). Aspects of the present invention are drawn to discrimi
`nating valid touch locations from phantom touch locations
`using various techniques to identify valid coordinate pairings
`for any arbitrary number of temporally overlapping touches
`on a sensor that provides X- and y-coordinate information
`independently.
`0044) The present Applicant has recognized efficiencies
`that result from distinguishing multiple touches on a matrix
`capacitive touch sensor by using capacitance-to-ground mea
`Surements. In making capacitance-to-ground measurements
`using a matrix capacitive touch sensor, signals produced by
`each X-electrode and each y-electrode are measured to inde
`pendently determine the X-position and y-position of touch
`inputs. As discussed above, Such independent determination
`can result in phantom touches when multiple touches are
`applied during the same time frame. Matrix capacitive touch
`sensors can also be measured by monitoring mutual capaci
`tance between each X-electrode and each y-electrode. In this
`way, each X-y electrode intersection is monitored, automati
`cally resolving valid touches (that is, phantom touches do not
`arise). However, for a sensor that includes NX-electrodes and
`My-electrodes, mutual capacitance involves making NXM
`measurements for full characterization of the sensor, whereas
`capacitance-to-ground involves making N--M measurements.
`Moreover, capacitance-to-ground measurements may be
`made on all X or Y electrodes simultaneously, so all elec
`trodes may be measured in two steps, whereas mutual capaci
`tance involves driving each electrode of one dimension
`sequentially, and measuring electrodes or the other dimen
`sion either simultaneously or sequentially.
`0045 FIG. 1 schematically illustrates a touch sensor sys
`tem 100 in which x-coordinate data and y-coordinate data are
`independently measured and then matched, or correlated, to
`form valid X-y pairs that represent touch positions on the
`sensor 110. For ease of illustration, touch sensor 110 will be
`described as a matrix capacitive touch sensor, although it will
`be appreciated that sensor 110 and the concepts described
`herein are equally applicable to IR, SAW, and other matrix
`based touch sensing technologies. Touch sensor system 100
`may be incorporated into any suitable device, including
`mobile devices such as tablet computers, PDAs, cell phones,
`and so forth, as well as gaming and entertainment machines,
`public kiosks, or any other device that utilizes touch input and
`may include applications that utilize multiple touch locations
`that may be applied at the same time from a single user or
`
`from multiple users. Many of these concepts described herein
`are Suited for analogous implementation in a variety of Suit
`able ways for matrix systems that measure X and Y indepen
`dently, including matrix capacitive touch systems, IR touch
`systems, and SAW touch systems.
`0046) Matrix capacitive touch screens include a grid of
`multiple electrodes so that when a touch object is capacitively
`coupled to electrodes in close proximity to the touch location,
`the resulting signals can be measured to determine the touch
`location. Multiple electrode sensor arrangements include
`mutually orthogonal sets of linear X-electrodes and linear
`y-electrodes, although other arrangements are possible. Sig
`nals may be based on capacitance-to-ground measurements
`where the effect of a touch on each individual electrode is
`measured, or based on mutual capacitance measurements
`where the effect of a touch on each electrode pair is deter
`mined by driving one electrode of the pair and sensing the
`other. In exemplary matrix capacitive touch screens, indi
`vidual electrodes are activated, for example sequentially, with
`an AC signal. A finger or other conductive touch object that is
`in sufficient proximity with one or more of the electrodes
`capacitively couples to them and alters the signal on the
`electrode in proportion to the strength of the capacitive cou
`pling. This signal change is measured on each electrode, and
`the relative changes are used to calculate touch position.
`0047 Touch position is a function of signal strength and
`signal width. Signal strength is the maximum signal mea
`Sured on one capacitive electrode, for example, item 62
`shown in FIG. 7. Signal width is the width of the bell shaped
`signal envelope measured across several capacitive elec
`trodes, for example, the width of curve 70 as shown in FIG. 7.
`Capacitive signal magnitude may include signal strength and/
`or width.
`0048. The operating principles of matrix IR touchscreens
`are disclosed, for example, in U.S. Pat. No. 4,868,912. Matrix
`IR touch systems typically have arrays of light emitters (for
`example LED's) on two adjacent edges (horizontal and Ver
`tical) of a rectangular active touchSurface, and arrays of light
`receivers on the two opposite edges of the active Surface. In its
`simplest form, each emitter sends light to a receiver directly
`opposite from it, across the active Surface. A touch on the
`active surface breaks at least one horizontal lightbeanandone
`vertical beam. Location of a touch is determined by which
`light receivers have a reduction in received light. Signal mag
`nitude in an IR system refers to the width of a shadow caused
`by a touch interrupting IR light beams.
`0049. The operating principles of SAW touch screens are
`disclosed, for example, in U.S. Pat. No. 6,225,985. In SAW
`systems, acoustic waves ar