p95
`United States Patent
`4,293,734
`_
`[11]
`
`[45] Oct. 6, 1981
`Pepper, Jr.
`
`[54] TOUCH PANEL SYSTEM AND METHOD
`
`[75]
`
`Inventor: William Pepper, Jr., Bethesda, Md.
`
`[73] Assignee:
`
`Peptek, Incorporated, Bethesda, Md.
`
`[21] Appl. No.: 14,450
`
`[22] Filed:
`
`Feb. 23, 1979
`
`Int, C13 veces G08C 21/00; GOIN 27/00
`[51]
`[52] US. C0. ceseeccecsccccccccscccccsecccccecesesee 178/18; 178/19
`[58] Field of Search .....cccssssssssssssssssseen 178/18, 19, 20;
`340/365 R, 365 S, 365 C, 365 A, 146.3 SY, 347
`AD; 33/1 M; 250/207, 211 R, 211 K; 324/71
`R, 71 EB
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,691,382
`9/1972 Somer .....cccc 250/211 K
`3,699,439 10/1972 Turner...ccceeccsceseeeeeteeee 178/18
`
`. 178/18
`4,013,835
`3/1977 Eachusetal. ...
`
`4,018,989 4/1977 Snyderetal. ....
`« 178/18
`
`« 178/19
`1/1978 Pepper, Jr. .......
`4,071,691
`
`« 178/18
`4,121,049 10/1978 Roeber.........
`
`4,129,747 12/1978 Pepper, Jr........
`« 178/19
`4,138,592 2/1979 Capehartetal.
`. 178/19
`4,198,539 4/1980 Pepper, Jr. ..sscccscsesseeeees 178/18
`FOREIGN PATENT DOCUMENTS
`
`1133757 11/1968 United Kingdom .
`
`OTHER PUBLICATIONS
`
`chure by Surface Science Laboratories; pages not num-
`bered; before 1/1/79.
`“A Position-sensitive Detector for Electrons”, Moak
`and others; 1975; Journal of Electron Spectroscopy.. .
`, Netherlands; pp. 151-156.
`
`Primary Examiner—Thomas A. Robinson
`Attorney, Agent, or Firm—Jim Zegeer
`
`ABSTRACT
`[57]
`A general methodis disclosed for accurately determin-
`ing the location orposition of a sourceorsink ofelectric
`current on the surface of a resistance element or impe-
`dance layer. Touch panels are described that detect the
`presenceofa user’s finger on such surface, and produce
`output signals corresponding to the position of the fin-
`ger in one or more axes. A preferred embodimentdis-
`closes a touch panel that includes a signal generator and
`determines the position of the user’s finger from current
`flowing through the user’s body to ground. Another
`touch panel embodiment determines the position of the
`user’s finger from currents caused by ambientelectrical
`noise. A pressure-sensitive touch panel, a transparent
`touch panel for use with a video display, a touch-panel-
`controlled audio mixer, and a video gameincorporating
`touch panels are also disclosed.
`
`“What You Can Do With Just 10,000 Electrons”, Bro-
`
`28 Claims, 14 Drawin- “igures
`
`IT,
`
`22.
`
`sil
`
`SUBTRACTOR
`
`
`
`SUBTRACTOR
`
`
`
`
`RECTIFIER
`
`RECTIFIER
`
`
`DIVIDER |
`
`
`
`
`
`OUTPUT
`
`1
`
`APPLE 1015
`
`1
`
`APPLE 1015
`
`

`

`FIG 3
`
`102
`
`plviveR
`
`(22
`
`U.S. Patent
`
`Oct. 6, 1981
`
`Sheet 1 of 8
`
`4,293,734
`
`29~| ADDER
`
`
`OUTPUT
`
`2
`
`

`

`U.S. Patent
`
`Oct
`
`6
`
`, 1981
`
`Sheet 2 of 8
`
`4,293,734
`
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`

`U.S. Patent
`
`Oct. 6, 1981 |
`
`Sheet 3 of 8
`
`4,293,734
`
`FIG 5A
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`Sheet 4 of 8
`
`4,293,734
`
`Oct. 6, 1981
`
`FIG 5E
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`

`

`U.S. Patent
`
`oct. 6, 1981
`
`Sheet 5 of 8
`
`4,293,734
`
`
`
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`

`

`Sheet 6 of 8
`
`4,293,734
`
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`
`Oct. 6, 1981
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`7
`
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`
`
`
`

`

`U.S. Patent
`
`Oct. 6, 1981
`
`Sheet 7 of 8
`
`4,293,734
`
`___Y
`
`BODY SERVES
`
`FIGIO
`
`AS ANTENNA
`
` RESISTIVE
`
`FIGI2
`
`POSITION
`OUTPUT
`
`SURFACE
`
`FIGIS
`
`PLAYER A
`
`PLAYER 8
`
`8
`
`

`

`U.S. Patent
`
`Oct. 6, 1981
`
`Sheet 8 of 8
`
`4,293,734
`
`d3siL)38
`
`
`
`yYAQIAIG
`
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`

`

`1
`
`4,293,734
`
`2
`can be touched at different selected points along its
`length; an oscillator that varies the potential of both
`terminals simultaneously with respect to ground; cur-
`rent-sensing circuitry to measure the current through
`each terminal; and normalizing circuitry to produce an
`output signal proportional to the ratio of one of the
`currents to the sum of both currents. A level detector
`monitors the sum of both currents and provides a Pres-
`ence signal when the sum exceeds a set or predeter-
`mined levei.
`In operation, when the user touches the resistance
`element, his body presents a relatively low impedance
`to ground for the oscillator signal, and a small electric
`current flows through the user’s body. The fraction of
`this current flowing through a boundary is inversely
`proportional to the distance from the boundary of the
`point touched, anddirectly proportional to the distance
`from the other boundary. An output voltage is pro-
`duced proportional to this fraction, and thus, to the
`distance from the other terminal. The Presence output
`providesa binarysignal to theutilization device indicat-
`ing that the resistance elementis being touched.
`In two-axis form, a rectangular surface of uniform
`resistivity is used that has four terminals and a resistive
`coupling network. The characteristics of this network
`are such that the ratio of the sum of the currents
`through two the terminals to the sum of the currents
`throughall four terminals is proportional to the distance
`from one edge. In a mannersimilar to that described for
`the one-axis embodiment, output voltages are simulta-
`neously derived proportional to the X-axis and Y-axis
`coordinates of the point touched. A two-axis touch
`panel can also be constructed using a triangular surface
`with three terminals.
`
`40
`
`45
`
`60
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The above and other objectives, advantages and fea-
`tures of the invention will be fully understood from the
`following detailed description and accompanying draw-
`ings wherein:
`FIG.1 is a schematic diagram illustrating the basic
`position-locating principle incorporated in the inven-
`tion.
`FIG. 2 is a block diagram of a one-axis touch panel
`incorporating the invention.
`FIG. 3 shows a four-terminal pattern of conductive
`segments for use in a two-axis touch panel and corre-
`sponds to FIG. 7 as disclosed and claimed in my appli-
`cation Ser. No. 867,256.
`FIG. 4 is a block diagram of a two-axis touch panel
`incorporating the invention.
`FIG. 5a and 5d taken together constitute a schematic
`diagram of the circuitry shown in block diagram form
`in FIG.4.
`FIG.6 is a cross-sectional diagram showing the con-
`struction of a touch panel incorporating pressure-sens-
`ing means.
`FIG. 7 is a block diagram of circuitry for sensing
`pressure on the touch panel shown in FIG.6.
`FIG.8 is a three-terminal pattern of conductive seg-
`ments for use in a two-axis touch panel or in a three-
`channel touch-controlled audio mixer.
`FIG. 9 is a block diagram of a three-channel audio
`mixer using the structure of FIG.8.
`FIG.10 is a block diagram of a one-axis touch panel
`in which the current source is external to the touch
`panel.
`
`TOUCH PANEL SYSTEM AND METHOD
`
`BACKGROUNDOF THE INVENTION
`
`This invention relates to devices for providing infor-
`mation, in the form ofelectrical signals, about the posi-
`tion of a selected touch point serving as a source or sink
`of electric current relative to a current-carrying resis-
`tance or impedance element. In particular, it relates to
`devices for converting selected touch points or posi-
`tions on a surface (reflecting hand motions) into electri-
`cal signals to provide an interface between man and
`machine.
`As used herein, the term ‘“‘a selected touch point”
`means a point on a surface selectively touched by a
`portion of a human body,particularly a finger or toe, or
`a point on a surface touched by an instrument held in
`the hand or other portion of a human body and con-
`trolled by the human,or a point on a surface contacted
`by a mechanical contrivance which is guided by a hu-
`man. In the context of this invention “selected” means
`the direction by the humanintellect of the point on the
`surface that is touched.
`.
`In the context of this invention the term “point” en-
`compasses the area of contact between a humanfinger
`and a surface, or the area of contact of an implement,
`such as the pointed end ofa stylus, with a surface. Thus,
`it is intended that the term “selected touch point” ex-
`clude non-physically contacting transfers of electrical
`energy between the touch panel surface and a sensing
`device or transducer. As used herein the term “current
`collecting” includes electrical current passing to or
`from the impedancesurface.
`This invention is an improvement over the touch
`panels described in my U.S. Pat. Nos. 4,071,691 and
`4,129,747, and my pending patent application Ser. No.
`867,256, filed Jan. 5, 1978, now U.S. Pat. No. 4,198,539
`dated Apr. 15, 1980, discussed in my aforementioned
`patent application Ser. No. 867,256, the phase of the
`field produced in theresistive surface was not a linear
`function of position on the surface, introducing an error
`in the output function unless special techniques were
`used to compensate for the error.
`The present invention substantially overcomes the
`limitations of that earlier system. It does not require a
`pickup surface and is therefore easier to package and
`manufacture than the earlier system. It also has im-
`proved linearity. It will therefore be seen that it is an
`improvementin the art of human-machine interfacing.
`An article, “A Position-Sensitive Detector for Elec-
`trons,” by C. D. Moak, 8. Datz, F. Garcia Santibanez,
`and T. A, Carlson, in the Journal of Electron Spectros-
`copy and Related Phenomena, No.6, 1975, pp. 151-156,
`discloses the principle of locating, on one axis, the point
`on a linear resistive anode at which electrons emerge
`from a chevron multiplier (which was impinged upon
`by an electron beam) in a high-resolution electron spec-
`trometer. In such system, a high D. C. voltage is re-
`quired between the electron multiplier and the resistive
`strip collector.
`BRIEF SUMMARYOF THE INVENTION
`
`The cbject of the present invention is to provide
`improvements in touch panel systems and methods.
`In one-axis form, a touch-panel embodiment of the
`invention includes an extended resistance or impedance
`element, such as a length of nichrome wire, having a
`terminal at each end or boundaryandso disposedthatit
`
`10
`
`10
`
`

`

`4,293,734
`
`3
`FIG. 11 shows a combination of two one-dimensional
`touch panels similar to those shown in FIG. 2 and FIG.
`10.
`FIG.12 is asketch showing a transparent touch panel
`combined with a display device.
`FIG.13.is a sketch of a video game in whicha repre-
`sentation of the playing area appears on both the video
`display and the two touch panels.
`DETATLED DESCRIPTION
`
`FIG.1 is a schematic diagram illustrating the basic
`position-transducing principle employed by my inven-
`tion. Extended impedanceorresistance element 10 has
`a boundary terminal 11 connected to voltage source 12
`through a current-measuring device, ammeter 13, and a
`boundary terminal 14 connected to voltage source 15
`through a current-measuring device, ammeter 16. The
`other terminals of the voltage sources have a common
`connection C, and impedance 17 is connected between
`common connection C and an arbitrary point 18 on the
`extended resistance element. Let the total resistance of
`resistance element 10 be R andlet the portion between
`point 11 and point 18 be r;, and the portion between
`point 18 and point 14 be ra,as indicated on the diagram.
`Using Kirchoff’s voltage law, equations can be written
`for the two meshesof this circuit:
`an+G+2)Z+V1=0
`
`(1)
`
`hrat+(it+i)Z+v2=0
`
`(2)
`
`30
`
`Subtracting Equation 2 from Equation 1, and substitut-
`ing R—r1}=12, it will be found that:
`
`n=
`
`OR —vy+
`ata
`
`@)
`
`This can be rearranged to express the dimensionless
`ratio r1/R as the sum of two terms:
`
`rt
`Ro
`
`2
`n+
`
`v2 — Vi
`++ DR*
`
`4
`dance, which is schematically represented by a lumped
`impedance 17’, to ground. Operational amplifiers 20 and
`21 maintain the ends 11’ and 14’, respectively, of the
`extended resistance element 10’ at the same instania-
`neous potential as the a-c output of signal generator 24
`by supplying currents through feedbackresistors 22 and
`23, respectively. The current through feedback resistor
`23, which is equal in magnitude and opposite in polarity
`to the current through r2, produces a voltage which is
`added to the output of signal generator 24 to give an
`instantaneous potential at
`the output of operational
`amplifier 21:
`
`¥21=¥24~QRa3
`
`(5)
`
`where v24 is the voltage output of signal generator24, ig
`is the current through r2, and R23 is the resistance of the
`feedback resistor 23. Subtractor 27 instantaneously re-
`moves the output voltage of signal generator 24 from
`v21, and rectifier 28 converts the a-c signal into a d-c
`level proportional to the average magnitude of current
`ia:
`
`= [iz] Roz
`
`©
`
`Summing (adding)circuit 29 adds level v2 to level v1,
`whichis similarly derived from the current through r1
`by meansof similarly connected amplifier 20, subtractor
`25 and rectifier 26. Divider 30 divides vz by the sum of
`(vi+v2) to give an output directly proportional to the
`position of point 18’ on touch surface resistance 10’:
`
`Your =
`
`v2
`n
`V+ 2 ab
`
`(7)
`
`FIG. 3 is a modification of FIG. 7 of my pending
`patent application Ser. No. 867,256. It shows a pattern
`of conductive segments 101 that is inlaid or overlaid (as
`by silk screen printing) on a uniform sheet ofresistive
`material 100 to produce what is termed a linearized
`resistive surface in my application Ser. No. 867,256.
`Whenappropriate voltages are applied to corner termi-
`nations A, B, C, and D, a uniform electric field with
`controllable amplitude and direction is created on the
`surface.
`I have found that the structure of FIG. 3 has the
`following useful properties in addition to the useful
`properties disclosed in patent application Ser. No.
`867,256: If the four terminations A, B, C, and D are held
`at the same instantaneous potential, and a spot on the
`surface is held at a different instantaneous potential,
`currents will flow through the four terminations. If
`there currents are measured with the spot (correspond-
`ing to selected touch point 18’) at various locations, it
`can be shownthat they are related to the X and Y coor-
`dinates of the spot by the equations:
`
`The following three conclusions relevant to my inven-
`tion can be drawn from Equation4. First, the expression
`for ratio r1/R is independent of impedance Z (hence Z
`can vary) except that Z must be finite. Second, if the
`relationship between the position of point 18 and ratio
`11/R is known, then the position of point 18 can be
`determined from measurements of currents i, and iz and
`voltages v; and v2 (and knowledge of resistance R).
`Third, if vj=v2, and the relationship between the posi-
`tion of point 18 and ratio r/R is known,the position of
`point 18 can be determined solely from measurements
`of currents i; and iz. There are other conclusions that
`can be drawn but the foregoing amply demonstrates
`that the position of any point of current flow to or from
`element 10 can be accurately determined without re-
`gard for the impedance Z or the actual resistance of
`element 10.
`FIG.2 is a simplified block diagram of a one-axis
`touch panel using the principle illustrated by FIG. 1.
`Extended resistance element 10’ is in the form of a linear
`touch panel surface, so that resistance r1 is directly
`proportional to the distance between end or boundary
`11’ and any selected touch point 18’. When the user’s
`finger F touches the extended resistance element 10’ at
`point 18’, a small current flows through his body impe-
`
`60
`
`ip+ic
`nak
`x= RS 2h, + igptic+ ip
`ig + ip
`yak +h
`
`(10)
`
`wherein kj is an offset, ko is a scale factor, and i4, ig etc.
`are the currents through the respective terminations. In
`FIG.3, the edges 102 of resistive surface 100 are illus-
`trated as extended solely for purposes of clarity in the
`drawing. In actual practice the resistive layer may be
`
`11
`
`11
`
`

`

`4,293,734
`
`_
`
`6
`5
`FIG. 4), which provides a 20 kHz sine wave with a
`trimmed or limited to the outermost conductive seg-
`ments 101.
`peak-to-peak amplitude of about 1.3 volts. The output
`of operational amplifier Al goes throughafilter 121
`Similar results have been obtained with a rectangular
`comprising capactors C1 and C2 andresistors R3 and
`. linearized resistive surface possessing a non-square as-
`R14. The inverting input of operational amplifier AS is
`pect ratio and fabricated with the construction shown in 5
`used as a summing node (and hence corresponds to
`FIG.3 of mypatent application Ser. No. 867,256. In this
`adder 127 of FIG. 4). A phase shifter, operational ampli-
`case constants k, and k2 differ for the X and Y equations.
`fier AiO (e.g. 126); inverts the oscillator output E; to
`FIG. 4 is a block diagram of the preferred embodi-
`provide an out-of-phase signal E; which is coupled
`ment of the invention: a two-dimensional touch panel
`through capacitor C9 and is summed throughresistor
`using the structure of FIG. 3. This embodiment is an 10
`R13 to cancel E; at the input of A5 (adder 127). The
`extension to two dimensions of the principle embodied
`amplitude and phase of E; are adjustable, so that the
`in FIG. 2, and its mathematical basis is as described in
`the discussion of FIG.3.
`effect of the capacitance between the linearized resis-
`tive surface 110 and ground can also be cancelled.
`Whenthe linearized resistive surface 110 is touched
`The circuit of operational amplifier A5is a precision
`by the user, small currents flow through the four termi-
`rectifier providing a d-c output for constant finger posi-
`nations A, B, C, and D. Voltages proportional to these
`tion, e.g., selected touch point. Identical circuitry is
`currents are developed across the feedback resistors
`provided for the other three terminations B, C and D of
`111, 142, 113, and 114 of the four input amplifiers 116,
`the linearized resistive surface, using operational ampli-
`117, 118 and 119, respectively, as the amplifiers follow
`fiers A2, A3, A4, A6, A7 and A8.
`the output ofoscillator 120. The amplifier outputs are 20
`Operational amplifier A11, constituting the Y axis
`applied to four high-pass filters 121, 122, 123 and 124
`summer 136 of FIG. 4, sumsthe rectified signals corre-
`respectively. These filters are not essential to system
`sponding to terminations A and B; operational amplifier
`operation, but were addedto eliminate the 60 -Hz sig-
`A12, constituting the X-axis summer 137 of FIG. 4,
`nals which may be picked up by the user’s body from
`sums the signals corresponding to terminations B and C;
`power wiring. Filters 121-124 attenuate 60-Hz signals 25
`and operational amplifier A13 constituting the all-chan-
`while passing the oscillator 120 frequency, which is
`nel summerof FIG.4, sums all foursignals A, B, C and
`typically 20 kHz. In a later-disclosed embodiment, the
`D. Resistors R54, R55 and R56 equalizethe loads on the
`60 Hz (or any radiant ambient environmental energy
`four precision rectifiers A5, A6, A7 and A8.
`field ) may be used as a source ofposition signal energy
`Two AD533J analog multipliers 150 and 151 are
`for the touch panel surface.
`connected to divide (e.g., perform the dividing function
`Subtraction of the oscillator signal component from
`of dividers 139 and 140 of FIG. 4) the outputs of opera-
`the filter outputs is accomplished by first shifting the
`tional amplifiers A11 and A12 bv the output of opera-
`phase of the oscillator signal approximately 180° in
`tional amplifier A13 (all-channel summer 138 of FIG.
`phase shifter 126 and then adding the phase-shifted
`4). Operational amplifiers A15 and A16 (corresponding
`signal to the filter outputs in adder circuits 127, 128, 129 35
`to output amplifiers 141 and 142 of FIG. 4) are output
`and 130, respectively. The resulting signals are rectified
`scalers allowing variation of output gains and offsets
`in rectifiers 131, 132, 133 and 134, respectively, to pro-
`over a wide range to match the characteristics of vari-
`vide d-c levels proportional to the amplitudes of the a-c
`ous utilization devices. Operational amplifier Al4 (level
`signals. The levels corresponding to the top two termi-
`detector 143 of FIG.4) is a level detector with hystere-
`nations A and B ofthe linearized resistive surface 110 40
`sis and an adjustable trip point. When the linearized
`are summedby the Y-axis summer136, the levels corre-
`resistive surface 110 is touched at any selected touch
`sponding to the right-hand two terminations B and C
`point, a light-emitting diode provides a visual indication
`are summed by the X-axis summer 137, and all four
`thereof and the Presence output switches from 0 to +5
`levels A, B, C and D are summed by theall-channel
`volts. Diodes D12 and D13 and resistors R78 and R82
`summer 138 to provide a denominatorinput for the two 45
`constitute a pull-down circuit to cause the X and Y
`dividers 139 and 140. These dividers 139 and 140 then
`outputs to go off-scale when the linearized resistive
`operate on the Y-axis and X-axis sums to perform the
`surface is not touched; removing a jumper disables this
`divisions of equation 10 above, and output amplifiers
`feature.
`141 and 142 with adjustable offset 143 and 144 and gain
`It will be seen that many alternative techniques can
`provide the desired X-axis and Y-axis outputs described 50
`be used to accomplish the same normalizing function as
`by Equation 10. A level detector 145 monitors the all-
`the dividers in FIGS. 4 and 5. For instance, digital
`channel summer138 output and switches state when the
`outputs can beeasily obtained by applying the X and Y
`user’s finger touches the linearized resistive surface 110.
`axis sums to voltage-to-frequency converters, and
`It is not necessary for the user’s finger to make ohmic
`counting the output pulses for a period of time propor-
`contact with the linearized resistive surface; a thin insu- 55
`tional to the output of the all-channel summer. Analog
`lating layer may be deposited over the resistance mate-
`normalization can be accomplished by controlling the
`rial for protection, and capacitive coupling through the
`gains of the input amplifiers with a feedback loop so as
`insulating layer will still provide adequate current for
`to maintain the output of the all-channel summerat a
`system operation.
`constant value. Another normalization technique is
`FIG.5a and 5é taken together constitute a schematic 60
`illustrated in FIG. 9.
`diagram of the embodimentof the invention shown in
`FIG..6 is a cross-sectional diagram showing a method
`FIG.4. Termination A of the linearizedresistive surface
`of combining pressure-sensing transducers with the
`is connectedto the inverting input of operational ampli-
`two-axis touch panel of FIG. 4. The touch panel’s lin-
`fier Al through isolating resistor R2. The noninverting
`earized resistive surface 40 is deposited on, or bonded
`input is connected to Es, the output of a Wien-bridge 65
`to, a rigid supporting plate 41, each corner of which
`oscillator (operational amplifier A9) through a voltage
`rests on one of four piezoelectric elements. Two of
`divider, consisting of R35 and R37 (all of which corre-
`these, labeled 42 and 43, appear in FIG.6.
`spondsto oscillator 120 shown in the block diagram of
`
`30
`
`12
`
`12
`
`

`

`7
`FIG. 7 is a block diagram of the circuitry for sum-
`ming the voltages developed by the four piezoelectric
`elements 42, 43, 44 and 45 of FIG. 6 to produce an
`output signal. One terminal of each of the four piezo-
`electric elements 42’, 43’, 44 and 45 is connected in
`common,with the same polarity being observed for all
`four elements. The other terminals are connected to
`four inputs of voltage-summing circuit 46. The voltage
`outputs of the four piezoelectric elements are summed
`by voltage-summingcircuit 46 to provide a vertical axis
`output proportional to the net downward pressure ex-
`erted on the linearized resistive surface by the user’s
`finger.
`FIG. 8 showsa triangular linearized resistive surface
`200 using the edge termination system disclosed in my
`application Ser. No. 867,256. The geometry of the con-
`ductive segments 201 between cach two cornersis the
`same as the geometry of the conductive segments 101
`between adjacent corners in FIG. 3. A uniform electric
`field can also be established in this triangular surface in
`a manner analogousto that described for the rectangu-
`lar surfaces as described in patent application Ser. No.
`867,256. In fact, it is possible to provide a uniform field,
`as disclosed in my above-identified patent application,
`in conjunction with the location-or position-detecting
`apparatus and method of the present invention.
`I have foundthatif all three terminals A’, B’, C’ are
`held at the same potential and a spot or selected touch
`point on the linearized resistive surface 200 is held at a
`different potential,
`the currents flowing through the
`terminations at corners A’, B’ and C’ follow the rela-
`tionship:
`
`25
`
`i4
`as+ Baie
`
`(H)
`
`35
`
`wherein dy is the perpendicular distance from the side
`opposite corner A‘ to the current source; i4, igand icare
`the currents through the corresponding terminations;
`and ky and kg are offset and scale constants.
`It will be seen that, given any twoofthe three dis-
`tances d4, dg and dc, the two-dimensional location of
`the selected touch point is determined. FIG. 9 is a block
`diagram of an audio mixer using the triangular linear-
`ized resistive surface 200 of FIG. 8. Whenthe linearized
`resistive surface 200 is touched by the user at any se-
`lected touch point 202, currents flow through the termi-
`nations at corners A’, B’ and C’. Voltages proportional
`to these currents are developed across the feedback
`resistors 203, 204 and 206 of the three input amplifiers
`207, 208 and 209, respectively as the amplifiers follow
`the outputofoscillator 210. The amplifier outputs go to
`high-pass filters 211, 212 and 213, and the outputs of
`these filters are supplied to adders 214, 216 and 217
`whichreceive the E; signal from inverter 218 to remove
`the oscillator signal component as described above for
`the embodimentillustrated in FIG. 4. The resulting
`signals are rectified by rectifiers 219, 220 and 221 to
`produce d-c levels proportional to the currents through
`the three terminations A’, B’ and C’ ofthe triangular
`linearized resistive surface 200. An adder 222 sums the
`three d-c levels.
`In this implementation of my invention, the ratios
`required by Equation 11 are obtained by maintaining
`the sum of the three d-c levels at a constant amplitude
`with a feedback loop. As shown in FIG.9, a differential
`amplifier 223 compares the sum ofthe three levels with
`a reference voltage. The outputofdifferential amplifier
`
`40
`
`45
`
`65
`
`4,293,734
`
`8
`223 is used as an automatic gain control voltage to con-
`trol the amplitude of the output of oscillator 210.
`Each of the three d-c levels also goes to a digital
`track-and-hold circuit 224, 226 and 227. These circuits,
`the details of which are described in the literature (See
`the article by Eugene L. Zuch, entitled “Designing
`With A Sample—Hold Won’t Be A Problem If You
`Use The Right Circuit’, Electric Design Vol. 26 Num-
`ber and 23 Nov.8, 1978 page 84) of analog-digitalinter-
`faces, use a combination ofdigital and analog circuitry
`to follow an analog inputin the track mode and provide
`- a corresponding outputin either analog or digital form.
`Whenthesecircuits switch from track to hold mode, a
`stored digital number maintains the output at its last .
`value. The digital outputs of the track-and-hold circuits
`are connected to logarithmic attentuators 228, 229 and
`230. These are nonlinear digital-to-analog converters
`whose details are described in the literature (See the
`article by Walter Jung and Will Retmamch entitled
`“Get Wide Range Digitally Controlled Audio Attenua-
`tion With A Companding D/A,” Electronic Design,
`Vol. 26 Number 23 Nov.8, 1978, page 92). Adder 232
`combines the’ outputs of the attenuators to produce the
`mixer output.
`Switching of the operating mode of the track-and-
`hold circuits 224, 226 and 227is controlled by a level
`detector 231 analogous in function to the level detector
`of FIG. 4. Whenthe user touches the linearized resistive
`surface 200, the presence ofhis finger is sensed by the
`level detector and the track-and-hold circuits 224, 226
`and 227 are switched to the track mode of operation.
`When he removeshis finger, the track-and-holdcircuits
`224, 226 and 227 switch to the hold mode and maintain
`the last attenuator settings until the user touches the
`touch panel again. It will be appreciated that besides use
`as an audio mixer this technique can be applied to vari-
`ous other arts where it is desired that a plurality of
`signal levels be provided at a single touch or input by
`the user.
`FIG.10 is a block diagram of another embodiment of
`the invention (shownin one-dimensional form) in which
`the terminations of the linearized resistive surface are
`maintained at ground potential and the bodyof the user
`serves as an antenna and picks up ambient electrical
`noise which causes current to flow through selected
`touch point 18” to the terminations 11” and 14”.
`This technique has the advantage that it imposes no
`voltage on the bodyofthe user. Although the voltages
`and currents imposed on the user by the previously
`discussed embodiments are far below levels that can be
`felt, and even farther below levels that can do bodily
`harm, some corporations prefer that products they use
`impose no voltages or currents whatsoeveron the user.
`Touch panels made using this technique should also be
`less expensive, as fewer circuit elements are required. In
`the touch panel of FIG.10, stray voltageson the user’s
`body cause currents to flow through extended resis-
`tance element 10” when it is touched at selected point
`18". These currents flow to virtual ground at ends 11”
`and 14”of the resistance element. Ground potential is
`maintained at ends 11” and 14” by currents supplied
`through feedback resistors 22' and 23’ by operational
`amplifiers 20’ and 21’. Rectifiers 26’ and 27’ convert the
`output voltages of operational amplifiers 20’ and 21’ to
`d-c levels, which are summed by adder 29’. Divider 30’
`divides the amplitude of one level by the sum of both to
`
`13
`
`13
`
`

`

`9
`produce an output proportional to the position of the
`point touched.
`two one-
`the combination of
`FIG.
`11
`shows
`. dimensional touch panels similar to the embodiments of
`FIG. 2 and FIG. 10. Applications are envisioned for a
`multiplicity of one-axis touch panels, each operated by
`a different finger of the user, and for combinations of
`one-, two- and three-dimensional touch panels. Two
`one-axis touch panels can be maintained orthogonally
`related to provide X and Y coordinate readouts for
`various manually controlled devices. The circuit of
`FIG. 11is preferably floating relative to. ground, unlike
`the embodiments of FIG. 2 and FIG.10, so that it is
`insensitive to the impedance to ground of the user’s
`bodyand also insenstive to ambientelectrical noise. The
`circuit relies instead on impedance 29,the impedance of
`the user’s body between the points 18'” and 18’””, to
`simultaneously complete the circuits of FIG. 2 and
`FIG. 10.
`FIG.12 is a sketch of a contemplated embodiment of
`myinvention in which the extended resistance element
`is a transparent linearized resistive surface 360 with a
`configuration similar to that of FIG. 3, and is combined
`with a display device. A suitable resistive surface for
`this purpose can be made by depositing indium tin oxide
`on transparent polyester film. The pattern of conduc-
`tive segments 301 is then silk-screened on t