United States Patent
`Makinwaet al.
`
`115
`
`ACAA
`
`[11] Patent Number:
`
`5,510,813
`[45] Date of Patent:
`Apr. 23, 1996
`
`[54] DATA PROCESSING DEVICE COMPRISING
`A TOUCH SCREEN AND A FORCE SENSOR
`
`5,119,079
`5,231,381
`
`6/1992 Hube et al. occeeseeenenee 345/146
`7/1993 Duwaer ...ssesssnessseesssssteetsnssess 345/174
`
`[75]
`
`Inventors: Kofi A. A. Makinwa; Theunis S.
`Baller, both of Eindhoven, Netherlands
`
`[73] Assignee: U.S. Philips Corporation, New York,
`N.Y.
`
`[21] Appl. No.: 289,829
`
`[22] Filed:
`
`Aug. 12, 1994
`
`[30]
`
`Foreign Application Priority Data
`
`Aug. 26, 1993
`
`[BE]
`
`Belgium 0...ssesereecenenes 09300875
`
`[51] Unt, Co iccccecsssssssssecesssssesssscsssssrsessessnssseese G09G 3/02
`[52] OLS. CU. oeccsesssessseceneees 345/173; 345/104; 345/179;
`341/33
`
`[58] Field of Search ou... 345/173, 174,
`345/175, 156, 179, 104; 178/18, 19; 341/33,
`34, 22
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`FOREIGN PATENT DOCUMENTS
`
`0340096
`0419145
`
`11/1989 European Pat. Off..
`3/1991
`European Pat. Off... 345/173
`OTHER PUBLICATIONS
`
`Research Disclosure, No. 302, Jun. 1989, Havant GB p. 456
`RD30289—Touch Screen With Combined Force and Position
`Sensing.
`
`Primary Examiner—Richard Hjerpe
`Assistant Examiner—Doon Chow
`Attorney, Agent, or Firm—Leroy Eason
`
`[57]
`
`ABSTRACT
`
`A data processing device comprises a touch screen with a
`touch position sensor. The position sensor is suitable to
`detect a touch position on the screen from a change in a
`current pattern in a conductive panel. The device also
`comprises a touch force sensor provided with a second
`conductive panel which extends substantially parallel to the
`touch screen. The screen is at least partly movable relative
`to the second panelin a direction transversely of the second
`panel. The force sensoris suitable to determine a force from
`a capacitance value between the touch screen and the second
`panel. The device is suitable for the combined processing of
`the position and force detected in response to touching.
`
`4,293,734 10/1981 Pepper, Jr. on... ceseeeceeeeceeerees 178/18
`4,475,235 10/1984 Graham ......cccceceseesereseseeeensees 178/18
`
`4,723,836
`2/1988 Kone etal.
`....
`w. 345/174
`oe
`4,731,694
`3/1988 Griibmeret al.
`esescceeeee 341/33
`4/1988 Brown .........c000
`wee 345/174
`4,740,781
`
`
`4,853,498 8/1989.Meadowsetal. .. wee 178/19
`
`7/1989 Hilsum et al. 0.2...cseseeeeeeee 345/104
`4,961,968
`13 Claims, 2 Drawing Sheets
`
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`
`Valve Exhibit 1034
`Valve Exhibit 1034
`Valve v. Immersion
`Valve v. Immersion
`
`

`

`U.S. Patent
`
`Apr. 23, 1996
`
`Sheet 1 of 2
`
`5,310,813
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`

`

`U.S. Patent
`
`Apr. 23, 1996
`
`Sheet 2 of 2
`
`5,510,813
`
`
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`CORRECTION CKT. VOLTAGE DRIVE
`
`CKT. 22KHZ
`
`

`

`5,510,813
`
`1
`DATA PROCESSING DEVICE COMPRISING
`A TOUCH SCREEN AND A FORCE SENSOR
`
`BACKGROUND OF THE INVENTION
`
`25
`
`30
`
`2
`An embodiment of the data processing device in accor-
`dance with the invention is characterized in that the first and
`the third panel are movable together relative to the second
`panel, the force sensor being suitable to apply to the first and
`the third panel the sameelectric measurementvoltage signal
`relative to the second panel. Because the same measurement
`The invention relates to a data processing device, com-
`voltages are applied to the first and the third panel, it is
`prising a touch screen with a touch position sensor which
`ensured that variations in the current to the first panel are
`comprises a first conductive panel andis suitable to detect a
`touch position on the screen from a change in a current
`caused only by the touch capacitance, whereas variations in
`paltern in the first panel. A data processing device of this
`the current to the third panel are caused exclusively by the
`kind is known from U.S. Pat. No. 4,853,498.
`force exerted. The measurementvoltage signal need concern
`During use the position sensor for example applies the
`only one component oftheelectric voltage present between
`the panels. The measurement voltage signal may be, for
`sameelectric voltage, to a plurality of locations on the first
`panel. Whenthe panel is touched or approachedbyafinger
`example one spectral component of the voltage present
`or any otherat least slightly conductive object, a capacitive
`between the panels and the layer. The other components of
`effect is produced so that current is drained from the panel.
`the voltage need not be the same.
`The position sensor can deduce the position where the touch
`An embodiment of the data processing device in accor-
`dance with the invention is characterized in that the first and
`surface has been touched or approached by measuring the
`amount of current drained from the various locations on the
`the third panel are provided on oppositely situated faces of
`20
`panel whereto the voltage is applied.
`a substrate. The substrate may be, for exampleaglass plate.
`Preferably, the touch force is determined simultaneously
`This can be readily implemented and offers a suitable
`mechanical
`transfer of the touch force from the touch
`with the position. In that case, for example
`surface to the third panel.
`the thickness of lines drawn by means of the position
`sensor can be adjusted in proportion to the force used,
`An embodiment of the data processing device in accor-
`dance with the invention is characterized in that the second
`a distinction can be made between different types of use,
`and the third panelare substantially rigidly arrangedrelative
`such as control of cursor motions or icon selection, on
`to one another, the force sensor being suitable to apply to the
`the basis of different forces,
`second and the third panel the same measurement voltage
`a selection of a menu item can be confirmed by exerting
`signal relative to the first panel. This reduces disturbances of
`additional force.
`the measurement of the touch force which are caused by
`currents to the second panel and which may occur notably
`whenthe second panelis situated substantially near an outer
`side of the device.
`
`SUMMARY OF THE INVENTION
`
`It is an object of the invention to provide a data processing
`device which is suitable to measure the touch position as
`well as the touch force in response to touching.
`To achievethis, the data processing device in accordance
`with the invention is characterized in that the device com-
`prises a touch force sensor which is provided with a second
`conductive panel which is arranged substantially parallel to
`the touch screen which touch screen is at
`least partly
`movable relative to the second panel in a direction trans-
`versely of the second panel, the force sensor being arranged
`to determine a force from a capacitance value between the
`touch screen and the second panel, and the device being
`arranged for the combined processing of the position and
`force detected in response to touching. The touch screen and
`the second panel constitute respective plates of a capacitor.
`When a force is exerted on the touch screen, these plates
`move towards one another, so that the capacitance value of
`the capacitor changes. The capacitance valueis, therefore, a
`fully electronically measurable measure of this force.
`An embodiment of the data processing device in accor-
`dance with the invention is characterized in that the touch
`screen comprises a third conductive panel which is arranged
`between the first and the second panel, the force sensor
`being suitable to determine a variation of the capacitance
`value by means of a current flowing from the first and/or
`second panel
`to the third panel
`in order to charge or
`discharge the capacitance between the third panel on the one
`side and the first and/or second panelon the otherside. The
`measurementof the capacitance value in response to a touch
`force could be disturbed by the capacitance of the finger or
`the-other object touching or approaching the screen. Thus,
`the determination of the force is not unambiguous. These
`touch effects are eliminated by measuring exclusively the
`current to the third panel which constitutes an intermediate
`layer.
`
`35
`
`40
`
`50
`
`55
`
`'
`
`60
`
`65
`
`The edges of the touch screen can in principle be
`mechanically fixed relative to the second panel. The touch
`force then becomes manifest as a difference in bending of
`the touch screen and the conductive layer. A simple and
`rugged force sensor can thus berealised. However, it may be
`that in that case the sensitivity of the force sensoris too low
`when the screen is touched near the edge. The sensitivity
`may also be too low when use is made of a touch screen of
`low flexibility.
`An embodiment of the data processing device in accor-
`dance with the invention is characterized in that the touch
`screen and the second panelareresiliently connected to one
`another. The touch screen can thus move as a wholerelative
`to the second panel and the described sensitivity problems
`will not occur.
`
`An embodiment of the data processing device in accor-
`dance with the invention is characterized in that the touch
`screen is flexible, the force sensor being suitable to deter-
`mine the touch force from the measured capacitance value
`under correction from a detected touch position. The degree
`of bending of a layer is dependent on the force exerted as
`well as on the position where force is exerted. When the
`edges of the layer are fixed, bending will increase as the
`force is exerted further from the edges. Quantitative force
`measurement requires correction for this effect. Preferably,
`the touch position as measured with the touch surface is used
`for this purpose.
`An embodiment of the data processing device in accor-
`dance with the invention which comprises an image display
`face on which there ate provided control electrodes for
`controlling an image pattern is characterized in that the
`control electrodes form part of the second and/orthird panel.
`The force sensor can thus be combined with, for example a
`
`

`

`5,510,813
`
`4
`dependence on which threshold values are exceeded. The
`force can thus be used,for example for selecting icons or for
`implementing a “double click” function as the force is
`greater.
`These and other aspectsof the invention will be described
`in detail hereinafter with reference to Figures herein.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`3
`Liquid Crystal Display (LCD) screen, without a separate
`second panel being required. The control electrodes of an
`LCD consist of a numberofline electrodes. When used for
`the force sensor, all control electrodes carry substantially the
`same electric voltage differencerelative to the other panels,
`at least as far as the measurementvoltage signal is concerned
`(which signal need only be one componentofthe electric
`voltage between the panels as has already been stated).
`Evidently, the further voltage components on the line elec-
`trodes may contain different control voltages for the LCD.
`An embodiment of the data processing device in accor-
`dance with the invention is characterized in that the force
`sensoris suitable to measure the capacitance value by means
`of an alternating voltage of a frequency between harmonics
`of a line frequency for controlling the control electrodes.
`The capacitance value can be simply measured by means of
`an alternating voltage. The use of an alternating voltage
`enables disturbances of the force measurement due to other
`signals, such as the line frequency of the display, to be
`prevented by selective faltering (for example, by means of
`synchronous demodulation). For a line frequency of 15.625
`Hz, for example a measurementfrequency of 22 kHz can be
`used,
`The sources liable to disturb the force measurement, for
`example the line drive of the display interface or the display
`illumination (usually driven at frequencies beyond 25 kHz)
`FIG. 1 is a diagrammatic side elevation of a liquid crystal
`are known in advance. In that case a suitable measurement
`display (LCD) provided with a touch screen. A number of
`frequency can also be chosen in advance.
`components which are not essential to the invention have
`An embodimentof the data processing device in accor-
`been omitted for the sake of clarity. The side elevation shows
`dance with the inventionis characterizedin thatit is suitable
`successively a transparent, conductive panel 10 (for
`to perform interference measurements for several frequen-
`example, of indium tin oxide, or ITO), a glass substrate 11,
`and the LCD 14.Afirst control electrode 15 and a second
`cies of the alternating voltage, in the absence of detection of
`touching of the touch surface, and for selecting a compara-
`control electrode 17 of the LCD 14 are shown, a liquid
`tively low interference frequency for use upon measurement
`crystal layer 16 being shown therebetween. The conductive
`of the capacitance value. In that casc a suitable measurement
`panel 10 and the control electrodes 15, 17 extend across
`frequency can also be chosen in the event of unknown
`substantially the entire surface of the LCD (perpendicularly
`interference sources, for example display illumination of a
`to the plane of drawing). The glass substrate 11 and the LCD
`non-specified frequency. The detection of touching can be
`14 are connectedvia resilient elements 12a,b, for example a
`realised, for example by meansof the touch position sensor.
`tubber ring which extendsalong the entire circumference of
`The measurement frequency can be selected for once and
`the LCD 14 or along parts thereof.
`for all upon assembly or repeatedly during use. Moreover, in
`During operation the panel 10 serves as a touch position
`the absence of detection of touching, the capacitance value
`sensor. Techniques for determining the touch position on a
`can be measured in the absence of an exerted force. This
`resistive, conductive layer are known per se. An example
`value then serves as a reference in calculating the force
`can be found in the cited U.S. Pat. No. 4,853,498 and also
`exerted from the capacitance value.
`in the U.S. Pat. No. 4,293,734, In an embodiment a voltage
`is applied to the panel 10 by means of different electrodes
`An embodiment of the data processing device in accor-
`dance with the invention is characterized in that the second
`(not shown) using these techniques. Under the influence of
`touching,this leads to a current flow through the panel 10,
`or the third panel constitutes a system of conductortracks for
`for example via the capacitance between the panel 10 and a
`a magnetic stylus position sensor. The force sensor can thus
`touching finger and subsequently via the body of the person
`be combined with the touch surface and a magnetic position
`sensor.
`touching to ground. This results in measurable currents
`through the electrodes on the panel 10. The position of
`touching can be calculated from the ratio of these currents.
`Bytouching the panel 10, a force can be exerted on the
`panel 10 (symbolized by an arrow 18 in the Figure). In the
`case ofa freed position of the LCD 14,this force 18 will lead
`to compression of the resilient elements 12a,b and hence to
`displacement (symbolized by an arrow 19) of the panel 10
`and the glass substrate 11 in the direction of the LCD 14.If
`necessary, mechanical guides can be provided so that dis-
`placement is possible exclusively in the direction of the
`panel. The displacement 19 is proportional to the force
`exerted.
`
`An embodiment of the data processing device in accor-
`dance with the invention, comprising an image display face
`and being suitable to gencrate, in response to touching, an
`image pattern in a location on the image display face which
`correspondsto the detected touch position, is characterized
`in that the device is suitable to enlarge the image pattern as
`the detected force is greater. For drawing lines the image
`pattern maybe, for example a brush elementsuch asa circle.
`This pattern remains when the touch position changes; the
`user can thus draw linesof variable thickness by moving the
`touch position across the screen while varying the force.
`An embodimentof the data processing device in accor-
`dance with the invention is characterized in that the device
`is suitable to compare the detected force with several
`threshold values and to execute different operations in
`
`FIG.1 is a diagrammatic side elevation of a liquid crystal
`display (LCD) provided with a touch screen.
`FIG. 2 shows a configuration for voltage application and
`current measurement on a touch screen.
`
`FIG.3 is a diagrammatic cross-sectional view of a further
`assembly of a touch screen and an LCD.
`FIG. 4 showsa further configuration for voltage applica-
`tion and current measurement on a touch screen.
`
`FIG. 5 is a diagrammatic cross-sectional view of a further
`assembly of a touch screen and an LCD.
`FIG. 6 shows an embodimentof a circuit for processing
`the measurement of the exerted force.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`10
`
`20
`
`25
`
`45
`
`50
`
`55
`
`60
`
`65
`
`The panel 10 andthefirst control electrode 15 of the LCD
`constitute the plates of a capacitor. The capacitance value of
`this capacitor is dependent on the distance between the
`
`

`

`5,510,813
`
`5
`plates and hence on the force. Measurement of the capaci-
`tance value enables determination of the force exerted.
`Because this measurementutilizes the panel 10 and thefirst
`control electrode 17, extending substantially across the
`entire surface of the LCD 14, the capacitance value is
`comparatively high so that it can be readily measured.
`The capacitance value can in principle be measured by
`applying a knownelectric voltage between the panel 10 and
`the first control electrode 15 and by measuring the charging
`current starting to flow in response to said voltage (the
`reverse is also possible: applying a known current and
`measuring the resultant voltage).
`The measured capacitance value, however, could be dis-
`turbed by the capacitive effect of, for example a finger
`touching the panel 10. Consequently, in response to the
`voltage an additional current may start to flow to the panel
`10, without a force 18 being exerted yet. Depending on the
`relative magnitude of the effect caused by the capacitor and
`the finger, the measurement of the force could become too
`inaccurate.
`
`This problem is mitigated by choosing a suitable configu-
`ration of voltage application and current measurement. Such
`a configuration is shown diagrammatically in FIG. 2. The
`Figure shows only the components which serve to measure
`the force; voltage sources and current meters for control of
`the LCD 14 andthe position sensor have been omitted for
`the sake of clarity. The panel 10 is represented as a first
`capacitor plate 30, the first control electrode 15 of the LCD
`14 being represented as a second capacitor plate 31 and the
`second control electrode 17 of the LCD 14 as a third
`capacitor plate 32. Between thefirst capacitor plate 30 and
`the second capacitor plate 31 (the panel 10 and the first
`control electrode 15) there is arranged a voltage source 35.
`The second capacitor plate 31 and the third capacitor plate
`32 (the first and second control electrodes 15, 17 of the LCD
`14) are interconnected. (Evidently, this holds only in respect
`of the componentof the voltage between the secondplate 31
`and the third plate 32 whereby the capacitance is measured.
`Asfar as other componentsof the voltage are concerned, for
`example the components required for controlling the LCD,
`a voltage source is present between the second plate 31 and
`the third plate 32). In the supply lead to the second plate 31
`there is inserted a current meter 36; the latter measures the
`current flowing from thefirst plate 30 and the third plate 32
`to the second plate 31 (evidently, in as far as current flows
`from these plates 30, 32). The capacitance value, or at least
`the variation thereof, can be determined from theratio of the
`voltage applied to 35 to the current measured at 36 (evi-
`dently,
`the reverse is also feasible: applying current by
`meansofa current source 36 instead of the current meter and
`measuring the voltage by meansof a voltmeter 35 instead of
`the voltage source).
`The capacitive effect of touching of the panel 10 during
`use is thus eliminated. This is because the current drained
`throughthe pancl 10 ducto the finger touching the panelalso
`has to return to the assembly formed by the much screen and
`the LCD,if no net charge is to be built up in the device. The
`return current flows, for example via the second electrode 17
`of the LCD 14, whenthe latter is held by the user, but not
`via the first control electrode 15 because this electrode is
`situated between the panel 10 and the second control elec-
`trode 17. Thus, the current to the first plate 30 due to
`touching will be compensated for by the current from the
`third plate 32. Touching, therefore, causes only a current.
`flow through the current meter 36 due to the pressing
`together of the panel 10 and thefirst control electrode 15,i.e.
`due to the desired effect.
`
`20
`
`25
`
`40
`
`45
`
`50
`
`35
`
`60
`
`63
`
`6
`In this respect it is assumed that the componentvoltages
`for control of the LCD betweenthefirst electrode 15 andthe
`second electrode 17 do not interfere with the measured
`component of the current
`through the current current
`meter 36.
`
`FIG.3 is a diagrammatic cross-sectional view of a further
`assembly of a further touch screen and an LCD. The
`cross-sectional view successively showsa first transparent,
`conductive panel 21, a glass substrate 22, a transparent
`conductive layer 23, resilient clements 24a,b, a transparent
`first control electrode 25, a liquid crystal layer 26, and a
`second control electrode 27. The first contro! electrode 25
`and the transparent conductive layer 23 constitute a second
`panel and a third panel, respectively.
`Whena force is exerted, the touch screen,i.e. the assem-
`bly formed by the first panel 21, the glass substrate 22 and
`the third panel 23, will move in the direction of thefirst
`control electrode 25, so that the capacitance value of the
`capacitor formed by the third panel 23 and thefirst control
`electrode 25 (the second panel) increases.
`An attractive method of measuring this capacitance value
`is illustrated by way of the configuration shownin FIG.4.
`FIG. 4 shows diagrammatically a first, a second and a third
`capacitor plate, 50, 51, 52, respectively, which correspond to
`the first panel 21, the third panel 23 and the second panel
`(the first control electrode 25), respectively. A voltage source
`55 is connected between the second plate 51 and the third
`plate 52. The first plate 50 and the second plate 51 are
`interconnected (which means that no voltage difference
`exists between these two plates in as far as the components
`of the voltage are concerned which are used for the force
`measurement. In the supply lead to the secondplate 51 there
`is inserted a current meter 56; the latter measures the current
`flowing from thefirst plate 50 and the third plate 52 to the
`second plate 51. In addition, a further current meter 57 is
`symbolically shown in the supply lead to thefirst plate 50.
`The operation of this configuration is the same as that
`described with reference to FIG. 2, exceptthat the voltage is
`now applied betweenthe third panel 23 and the second panel
`(the first control electrode 25), i.c. again between the two
`adjoining plates which are movablerelative to one another.
`The voltage sources for the measurement of the touch
`position andthe control of the LCD have again been omitted
`in FIG. 4. During use different voltages are applied to the
`first panel 21 and the control electrode 25 for measuring the
`touch position and for controlling the LCD, respectively.
`The further current meter 57 symbolizes a numberof current
`meters connected to thefirst panel 21 in various locations.
`These current meters serve to measure the currents to the
`first plate wherefrom the touch position is determined.
`Upon measurement of the touch position, the third panel
`23 also serves as a shield between the first panel 21 and the
`control electrode 25, 27. Stray fields from the control
`electrodes 25, 27 are intercepted by the third panel 23 sothat .
`they do not cause a current in the first panel 21. The
`measurement of the touch position, therefore, is not dis-
`turbed by stray fields (conversely, the control of the LCD is
`not disturbed by the measurement voltage on thefirst panel
`21, be it that the latter is a smaller effect).
`FIG.5 is a diagrammatic cross-sectional view of a further
`assembly of a further much screen and an LCD. The
`cross-sectional view successively showsa first conductive
`panel 40, a first control electrode 41, a liquid crystal layer
`42, a second control electrode 43, resilient elements 44a,b,
`a third panel 45, and a second panel (a conductive layer 46).
`When a force is exerted onthefirst panel 40, the assembly
`formed bythefirst panel 40, the first control electrode 41,
`
`

`

`5,510,813
`
`7
`the liquid crystal layer 42, and the second control electrode
`43 will move towards the third panel 45, so that the
`capacitance value of the capacitor formed bythe third panel
`45 and the second control electrode 43 increases. This
`capacitance value is preferably measured by means of a
`configuration as shown in FIG. 2, in which the first, the
`second and the third plate 30, 31, 32, respectively, corre-
`spond to the second control electrode 43, the third panel 45
`and the conductive layer 46 (the second panel), respectively.
`As described above,
`the conductive layer 46 serves to
`prevent undesirable capacitive effects, for example when the
`LCDis placed on a metal table.
`If necessary, a conductor pattern of a magnetic position
`sensor (also known as a digitizer tablet) can be provided
`between the second electrode 43 and the third panel 45. This
`sensor serves to measure the location where a stylus gener-
`ating magnetic fields touches the display. In that case the
`conductor pattern of the sensor can serve as a capacitor plate
`instead of the second control electrode 45.
`
`FIG. 6 shows an embodimentofa circuit for processing
`the measurement of the force exerted. This Figure com-
`prises, by way of example, the configuration of FIG. 4 with
`the first plate 50, the secondplate 51, the third plate 52, the
`voltage source 55 and the current meter 56.
`The circuit comprises a drive circuit 60 for driving the
`voltage source. The current meter is connected to a cascade
`of successively a narrow-band detector 63, a subtraction
`means 64, and a position-dependent correction means 65. A
`memory 67 for the zero force capacitance is coupled to the
`subtraction means 64. A position sensor 68 is coupled to the
`position-dependent correction means 65.
`During operation the drive circuit 60 generates an alter-
`nating voltage of a given frequency, for example 22 kHz,
`which is applied between the second plate 51 and the third
`plate 52 by means of the voltage source 55. In response
`thereto a current flows through the current meter 56. The
`current measured is applied to the narrow-band detector 62.
`The amplitude ofthe current is a measure of the capacitance
`value. The narrow-band detector 62 detects substantially
`exclusively signals of the generated frequency (22 kHz), and
`outputs a signal which is proportional to the amplitude ofthe
`current. The subtraction meanssubtracts the quiescent value
`therefrom, i.e. the current amplitude in the absence of an
`exerted force.
`
`The signal at the output of the subtraction means 64is in
`principle a measure of the force exerted. However, it has
`been found that the capacitance variation can depend not
`only on the force but also on the location wherethe forceis
`exerted. This is notably the case when the resilient elements
`24a,b are replaced by non-flexible or hardly flexible con-
`nections. In that case the capacitance variation is due mainly
`to bending ofthe glass substrate 22. The degree of bending
`of the glass substrate 22 for a given force is dependent on the
`location wherethe force is exerted. Typical bending amounts
`to 1 mm in response to a force of 10N exerted at the center
`of the substrate 22, and to substantially 0 mm in response to
`a force exerted on the connection between the second panel
`23 and the first electrode 25. For the sake of compactness of
`the device and in order to minimize parallax between the
`first panel 21 and the LCD layer 26, the distance between the
`second panel 23 and the first electrode 25 preferably is
`hardly greater than this bending; in that case the bending
`determines a substantial part of the touch force effect.
`The position-dependent correction means 65 and the
`position sensor 68 are provided to correct for the location-
`dependencyofthe relation between the touch force and the
`
`20
`
`25
`
`40
`
`45
`
`50
`
`55
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`60
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`65
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`8
`variation of the capacitance value.For the position sensor 68
`use is preferably made ofthe first panel 10. In the correction
`means the measured capacitance variation is multiplied by a
`correction factor which is dependent on the location where
`the touch force is exerted.
`
`The correction factor can in principle be calculated by
`utilizing the equations for elastic deformation of thin plates.
`This calculation can be performed either for each touch or
`only once and for all, after which the correction factor is
`stored in a look-up table in the device as a function of the
`location. However, it has been found in practice that a simple
`approximated relation between the location and the correc-
`tion factor suffices.
`
`In a simple but very suitable embodiment the correction
`factor is taken to be proportional to the distance between the
`location where the force is exerted and the nearest location
`where the third panel 23 is mechanically connected to the
`first electrode 25. In another suitable version the correction
`factor is taken to be proportional to the sum of the absolute
`values of the x and y coordinates of the touch location,
`measured relative to the centre of the screen.
`
`For the measurement frequency, for example 22 kHz is
`used; at this frequency no disturbance is experienced from
`the customary drive frequency of the control signals for the
`display (for example, 15,625 Hz) or harmonics thereof and,
`moreover, this frequency is below the frequencies used for
`background illumination (backlighting) of the LCD (typi-
`cally 25 kHz and higher). This prevents other signals from
`disturbing the force measurement. Conversely,
`this fre-
`quency is also chosen to prevent disturbances in the image
`producedonthe display by the control electrodes 25, 27, but
`this effect is smaller.
`
`The control electrode 25 at oneside ofthe liquid crystal
`layer 26 is composed of a numberof line electrodes. In as
`far as the measurement frequency (22 kHz)is concerned,all
`these electrodes receive the samepotential. Evidently, as far
`as image frequencies (necessary for generating the image)
`are concerned, different potentials are usually applied to
`differentline electrodes. This can be realised, for example by
`using respective voltage sources for controlling the image
`between a common junction and the variousline electrodes,
`said commonjunction being connectedto the third panel 23
`via the voltage source 55.
`The bandwidth of the narrow band detector 62 corre-
`spondsto the speed of force variations to be measured and
`typically amounts to a few tens of Herz, for example 50 Hz.
`The narrow-band detector 62 preferably utilizes synchro-
`nous detection under the control of the drive circuit 60.
`
`The memory 67 for the zero force is preferably loaded by
`storing the output signal of the narrow-band detector at an
`instant at which no force is exerted on the screen. The
`absence of an exerted force can be detected by meansof the
`position sensor when the latter does not measure a variation
`of the current pattern through the first panel 21, The mea-
`surement ofthe currentflow to the outer plate 50 thus serves
`for touchdetection and the measurementofthe current to the
`intermediate plate 51 serves for force measurement.
`The measurement frequency of the force sensor, gener-
`ated by the drive circuit 60, is chosen so that the other
`signals in the LCD andthe position sensor causeaslittle
`interference as possible. If necessary, in the absence of a
`touch force the interference amplitude can be measured at
`different measurement frequencies, after which one of the
`least disturbed frequencies can be chosen for force measure-
`ments. This choice of frequency can either be performed
`once and for all upon design or assembly, or be repeated at
`
`

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`5,510,813
`
`9
`regular intervals when no force is exerted. The latter can be
`performed, for example under the control of a processor in
`the device.
`
`The measurement has been explained in conjunction with
`frequency select