United States Patent
`
`[19]
`
`[11]
`
`.
`
`4,293,734
`
`
`
`[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] App]. No.: 14,450
`
`[22] Filed:
`
`Feb. 23, 1979
`
`Int. 01.3 ...................... G08C 21/00; GOlN 27/00
`[51]
`[52] US. Cl. ......................................... 178/18; 178/19
`[58] Field of Search ............................. 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 ............................. 250/211 K
`3,699,439 10/ 1972 Turner .................. 178/18
`
`. 178/18
`4,013,835
`3/1977 Eachus et a1.
`.. 178/18
`4,018,989 4/1977 Snyder et a1.
`
`,. 178/19
`4,071,691
`1/1978 Pepper, Jr. .......
`.. 178/18
`4,121,049 10/1978 Roeber .........
`4,129,747 12/1978 Pepper, Jr. .......
`.. 178/19
`
`. 178/19
`4,138,592 2/1979 Capehart et al.
`4,198,539 4/1980 Pepper, Jr. ............................ 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
`
`[57]
`
`ABSTRACT
`
`A general method is disclosed for accurately determin-
`ing the location ‘or position of a source or sink of electric
`current on the surface of a resistance element or impe—
`dance layer. Touch panels are described that detect the
`presence of a 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 embodiment dis-
`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 ambient electrical
`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 game incorporating
`touch panels are also disclosed.
`
`“What You Can Do With Just 10,000 Electrons”, Bro-
`
`28 Claims, 14 Draw-7"" “figures
`
`17'
`
`RECTIHER
`
`
`
`
`
`OUTPUT
`
`1
`
`APPLE 1015
`
`1
`
`APPLE 1015
`
`

`

`U.S. Patent
`
`Oct. 6,1981
`
`Sheet 1 of 8
`
`4,293,734
`
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`

`

`U.S. Patent
`
`Oct
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`U,.S. Patent
`
`Oct. 6, 1981
`
`Sheet 3 of 8
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`4,293,734
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`US. Patent
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`Oct._ 6, 1981
`
`Sheet 4 of 8
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`4,293,734
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`

`U.S. Patent
`
`Oct. 6,1981
`
`Sheet 5 of8
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`4,293,734
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`US. Patent
`
`Oct. 6, 1981
`
`Sheet 7 of8
`
`4,293,734
`
`/____
`
`BODY SERVES
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`

`1
`
`4,293,734
`
`TOUCH PANEL SYSTEM AND METHOD
`
`BACKGROUND OF THE INVENTION
`
`This invention relates to devices for providing infor‘
`mation, in the form of electrical 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 human intellect 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 human finger
`and a surface, or the area of contact of an implement,
`such as the pointed end of a 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 impedance surface.
`This invention is an improvement over the touch
`panels described in my US. Pat. Nos. 4,071,691 and
`4,129,747, and my pending patent application Ser. No.
`867,256, filed Jan. 5, I978, now US. 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 the resistive 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
`improvement in the art of human-machine interfacing.
`An article, “A Position-Sensitive Detector for Elec—
`trons,” by C. D. Moak, S. Datz, F. Garcia Santibanez,
`and T. A. Carlson, in the Journal of Electron Spectros-
`copy and Related Phenomena, No. 6, 1975, pp. lSl—l 56,
`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 SUMMARY OF THE INVENTION
`
`The object 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 boundary and so disposed that it
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`SO
`
`55
`
`60
`
`65
`
`10
`
`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 level.
`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, and directly 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
`provides a binary signal to the utilization device indicat-
`ing that the resistance element is 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
`through all four terminals is proportional to the distance
`from one edge. In a manner similar 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.
`
`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 5b 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.
`
`10
`
`

`

`4,293,734
`
`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 instanta-
`neous potential as the a-c output of signal generator 24
`by supplying currents through feedback resistors 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:
`
`m = V24 ~12st
`
`(5)
`
`where V24 is the voltage output of signal generator 24, i2
`is the current through I2, and R23 is the resistance of the
`feedback resistor 23. Subtractor 27 instantaneously re-
`moves the ontput voltage of signal generator 24 from
`vz1, and rectifier 28 converts the a-c signal into a d-c
`level proportional to the average magnitude of current
`i2:
`
`V2= “Tl R23
`
`(6)
`
`Summing (adding) circuit 29 adds level vz to level V],
`which is similarly derived from the current through r1
`by means of similarly connected amplifier 20, Subtractor
`25 and rectifier 26. Divider 30 divides V2 by the sum of
`(v1+vz) to give an output directly proportional to the
`position of point 18’ on touch surface resistance 10’:
`
`(7)
`
`
`1'2
`ii + 1‘2
`
`Lv
`
`1+v2
`
`You! :-
`
`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 of resistive
`material 100 to produce what is termed a linearized
`resistive surface in my application Ser. No. 867,256.
`When appropriate 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 shown that they are related to the X and Y coor-
`dinates of the spot by the equations:
`
`iB+iC
`k
`—k
`iA+iB+ic+iD
`x— 1+ 2
`1A+in
`y=k1+k2 iA+i3+ic+iD
`
`(10)
`
`wherein k1 is an offset, k2 is a scale factor, and L4, is 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
`
`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 a sketch showing a transparent touch panel
`combined with a display device.
`FIG. 13-is a sketch of a video game in which a repre-
`sentation of the playing area appears on both the video
`display and the two touch panels.
`DETAILED DESCRIPTION
`
`FIG. 1 is a schematic diagram illustrating the basic
`position-transducing principle employed by my inven—
`tion. Extended impedance or resistance 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 and let the portion between
`point 11 and point 18 be r}, and the portion between
`point 18 and point 14 be r2, as indicated on the diagram.
`Using Kirchoft’s voltage law, equations can be written
`for the two meshes of this circuit;
`i1r[+(i]+i2)Z+ V1=0
`
`(l)
`
`10
`
`15
`
`20
`
`25
`
`i2r2+(i1+i2)Z+v2=0
`
`(2)
`
`3O
`
`Subtracting Equation 2 from Equation 1, and substitut-
`ing R—r1=r2, it will be found that:
`
`n:
`
`izR — v1+ v2
`i1+i2
`
`(3)
`
`35
`
`This can be rearranged to express the dimensionless
`ratio r1/R as the sum of two terms:
`
`"I
`R
`
`
`1'2
`i1 + 1'2
`
`+
`
`v2 —— v1
`(it + i2)R '
`
`The following three conclusions relevant to my inven-
`tion can be drawn from Equation 4. 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
`r1/R is known, then the position of point 18 can be
`determined from measurements of currents i1 and i2 and
`voltages V] and v2 (and knowledge of resistance R).
`Third, if v1=vz, and the relationship between the posi—
`tion of point 18 and ratio r1/R is known, the position of
`point 18 can be determined solely from measurements
`of currents i1 and i2. 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-
`
`45
`
`50
`
`55
`
`60
`
`65
`
`11
`
`

`

`30
`
`5
`trimmed or limited to the outermost conductive seg-
`ments 101.
`Similar results have been obtained with a rectangular
`- linearized resistive surface possessing a non-square as-
`pect ratio and fabricated with the construction shown in 5
`FIG. 3 of my patent application Ser. No. 867,256. In this
`case constants In and k1 differ for the X and Y equations.
`FIG. 4 is a block diagram of the preferred embodi-
`ment of the invention: a two-dimensional touch panel
`using the structure of FIG. 3. This embodiment is an 10
`extension to two dimensions of the principle embodied
`in FIG. 2, and its mathematical basis is as described in
`the discussion of FIG. 3.
`When the linearized resistive surface 110 is touched
`by the user, small currents flow through the four termi- 15
`nations A, B, C, and D. Voltages proportional to these
`currents are developed across the feedback resistors
`111, 112, 113, and 114 of the four input amplifiers 116,
`117, 118 and 119, respectively, as the amplifiers follow
`the output of oscillator 120. The amplifier outputs are 20
`applied to four high-pass filters 121, 122, 123 and 124
`respectively. These filters are not essential to system
`operation, but were added to eliminate the 60 —Hz sig-
`nals which may be picked up by the user’s body from
`power wiring. Filters 121—124 attenuate 60-Hz signals 25
`while passing the oscillator 120 frequency, which is
`typically 20 kHz. In a later-disclosed embodiment, the
`60 Hz (or any radiant ambient environmental energy
`field ) may be used as a source of position signal energy
`for the touch panel surface.
`Subtraction of the oscillator signal component from
`the filter outputs is accomplished by first shifting the
`phase of the oscillator signal approximately 180“ in
`phase shifter 126 and then adding the phase-shifted
`signal to the filter outputs in adder circuits 127, 128, 129 35
`and 130, respectively. The resulting signals are rectified
`in rectifiers 131, 132, 133 and 134, respectively, to pro-
`vide d-c levels proportional to the amplitudes of the a-c
`signals. The levels corresponding to the top two termi-
`nations A and B of the linearized resistive surface 110 40
`are summed by the Y—axis summer 136, the levels corre-
`sponding to the right-hand two terminations B and C
`are summed by the X-axis summer 137, and all four
`levels A, B, C and D are summed by the all-channel
`summer 138 to provide a denominator input for the two 45
`dividers 139 and 140. These dividers 139 and 140 then
`operate on the Y-axis and X-axis sums to perform the
`divisions of equation 10 above, and output amplifiers
`141 and 142 with adjustable offset 143 and 144 and gain
`provide the desired X-axis and Y-axis outputs described 50
`by Equation 10. A level detector 145 monitors the all-
`channel summer 138 output and switches state when the
`user‘s finger touches the linearized resistive surface 110.
`It is not necessary for the user’s finger to make ohmic
`contact with the linearized resistive surface; a thin insu- 55
`lating layer may be deposited over the resistance mate-
`rial for protection, and capacitive coupling through the
`insulating layer will still provide adequate current for
`system operation.
`FIG. 5a and 5b taken together constitute a schematic 60
`diagram of the embodiment of the invention shown in
`FIG. 4. Termination A of the linearized resistive surface
`is connected to the inverting input of operational ampli-
`fier A1 through isolating resistor R2. The noninverting
`input is connected to Es, the output of a Wien-bridge 65
`oscillator (operational amplifier A9) through a voltage
`divider, consisting of R35 and R37 (all of which corre-
`sponds to oscillator 120 shown in the block diagram of
`
`4,293,734
`
`6
`FIG. 4), which provides a 20 kHz sine wave with a
`peak-to-peak amplitude of about 1.3 volts. The output
`of Operational amplifier A1 goes through a filter 121
`comprising capactors C1 and C2 and resistors R3 and
`R14. The inverting input of operational amplifier A5 is
`used as a summing node (and hence corresponds to
`adder 127 of FIG. 4). A phase shifter, operational ampli-
`fier A10 (cg. 126); inverts the oscillator output E5 to
`provide an out-of-phase signal E: which is coupled
`through capacitor C9 and is summed through resistor
`R13 to cancel E, at the input of AS (adder 127). The
`amplitude and phase of E are adjustable, so that the
`effect of the capacitance between the linearized resis—
`tive surface 110 and ground can also be cancelled.
`The circuit of operational amplifier A5 is a precision
`rectifier providing a d-c output for constant finger posi-
`tion, e.g., selected touch point. Identical circuitry is
`provided for the other three terminations B, C and D of
`the linearized resistive surface, using operational ampli-
`fiers A2, A3, A4, A6, A7 and A8.
`Operational amplifier A11, constituting the Y axis
`summer 136 of FIG. 4, sums the rectified signals corre-
`sponding to terminations A and B; operational amplifier
`A12, constituting the X—axis summer 137 of FIG. 4,
`sums the signals corresponding to terminations B and C;
`and operational amplifier A13 constituting the all-chan-
`nel summer of FIG. 4, sums all four‘signals A, B, C and
`D. Resistors R54, R55 and R56 equalize the loads on the
`four precision rectifiers A5, A6, A7 and A8.
`Two AD5331 analog multipliers 150 and 151 are
`connected to divide (e.g., perform the dividing function
`of dividers 139 and 140 of FIG. 4) the outputs of opera-
`tional amplifiers A11 and A12 bv the output of opera-
`tional amplifier A13 (all-channel summer 138 of FIG.
`4). Operational amplifiers A15 and A16 (corresponding
`to output amplifiers 141 and 142 of FIG. 4) are output
`sealers allowing variation of output gains and offsets
`over a wide range to match the characteristics of vari—
`ous utilization devices. Operational amplifier A14 (level
`detector 143 of FIG. 4) is a level detector with hystere-
`sis and an adjustable trip point. When the linearized
`resistive surface 110 is touched at any selected touch
`point, a light-emitting diode provides a visual indication
`thereof and the Presence output switches from O to + 5
`volts. Diodes D12 and D13 and resistors R78 and R82
`constitute a pull-down circuit to cause the X and Y
`outputs to go off-scale when the linearized resistive
`surface is not touched; removing a jumper disables this
`feature.
`It Will be seen that many alternative techniques can
`be used to accomplish the same normalizing function as
`the dividers in FIGS. 4 and 5. For instance, digital
`outputs can be easily obtained by applying the X and Y
`axis sums to voitage-to—frequency converters, and
`counting the output pulses for a period of time propor-
`tional to the output of the all-channel summer. Analog
`normalization can be accomplished by controlling the
`gains of the input amplifiers with a feedback loop so as
`to maintain the output of the all—channel summer at a
`constant value. Another normalization technique is
`illustrated in FIG. 9.
`FIG.,6 is a cross-sectional diagram showing a method
`of combining pressure-sensing transducers with the
`two-axis touch panel of FIG. 4. The touch panel’s lin-
`earized resistive surface 40 is deposited on, or bonded
`to, a rigid supporting plate 41. each corner of which
`rests on one of four piezoelectric elements. Two of
`these, labeled 42 and 43, appear in FIG. 6.
`
`12
`
`12
`
`

`

`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
`trackend-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-digital inter—
`faces, use a combination of digital and analog circuitry
`to follow an analog input in the track mode and provide
`- a corresponding output in either analog or digital form.
`When these circuits 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 227 is controlled by a level
`detector 231 analogous in function to the level detector
`of FIG. 4. When the user touches the linearized resistive
`surface 200, the presence of his 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 removes his finger, the track-and-hold circuits
`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 (shown in one-dimensional form) in which
`the terminations of the linearized resistive surface are
`maintained at ground potential and the body of 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 body of the 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 whatsoever on 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 voltages on 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
`
`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-summing circuit 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 shows a 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 each two corners is 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 analogous to 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 found that if 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:
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`[A
`«1., = M “W
`
`(11)
`
`35
`
`wherein dA is the perpendicular distance from the side
`opposite corner A’ to the current source; iA, i3 and ic are
`the currents through the corresponding terminations;
`and k1 and k2 are offset and scale constants.
`It will be seen that, given any two of the three dis‘
`tances dA, dB 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. When the linearized
`resistive surface 200 is touched by the user at any se-
`lected touch point 202, currents flow through the termi-
`nations at comers 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 output of oscillator 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
`which receive theE signal from inverter 218 to remove
`the oscillator signal component as described above for
`the embodiment illustrated 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' of the 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 of the three levels with
`a reference voltage. The output of differential amplifier
`
`4O
`
`45
`
`50
`
`55
`
`60
`
`65
`
`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. 11 is 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
`body and also insenstive to ambient electrical 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
`my invention in which the extended resistance element
`is a transparent linearized resistive surface 300 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 poly