United States Patent (19)
`Meadows et al.
`
`Patent Number:
`11
`45) Date of Patent:
`
`4,922,061
`May 1, 1990
`
`(75
`
`54 CAPACITIVE TOUCH PANEL SYSTEM
`WITH RANDOMILY MODULATED
`POST ON MEASUREMENT SIGNAL
`Inventors: R. David Meadows, Beaverton; Roger
`J. McCoy, Portland, both of Oreg.
`(73) Assignee: Tektronix, Inc., Beaverton, Oreg.
`21 Appl. No.: 383,113
`22 Filed:
`Jul. 20, 1989
`
`63
`
`Related U.S. Application Data
`Continuation-in-part of Ser. No. 205,896, Jun. 13, 1988,
`Pat. No. 4,853,498.
`51) Int. Cli............................................ G08C 21/OO
`52 U.S. Cl. ..............
`... 178/19; 340/706
`58) Field of Search ........................... 178/19; 340/706
`(56)
`References Cited
`U.S. PATENT DOCUMENTS
`4,476,463 10/1984 Ng et al. ............................. 340/712
`4,680,430 7/1987. Yoshikawa et al.
`... 178/19
`4,698,460 10/1987 Krein et al. ............
`... 178/19
`4,698,46. 10/1987 Meadows et al. .
`... 178/19
`4,707,845 11/1987 Krein et al. ........................... 178/19
`Primary Examiner-Stafford D. Schreyer
`Attorney, Agent, or Firm-John D. Winkelman; Mark
`M. Meininger
`
`ABSTRACT
`57
`A capacitive touch panel system (10) having a faceplate
`(14) with an electrically conductive layer (20) of a con
`sistent resistivity employs a position measurement appa
`ratus (12) to generate an address signal indicative of a
`position (46) on the faceplate in contact with a stylus
`(48). The position measurement apparatus includes a
`position measurement signal source (56) that generates a
`square-wave measurement signal of a frequency that
`varies in a substantially random manner. The position
`measurement signal is applied to a first pair of opposed
`electrodes (36) and (40) and a second pair of opposed
`electrodes (38) and (42) positioned along respective side
`margins (26, 30, 28, and 32) of the faceplate. The resis
`tivity of the conductive layer establishes effective resis
`tances of Rx and Ry between the respective first and
`second pairs of electrodes. Position measurement sub
`circuits (54a-54d) are locked-in with the random mea
`surement signal frequencies to measure currents drawn
`through the electrodes whenever the stylus contacts the
`conductive layer, thereby to form an address signal
`indicative of the location at which the stylus contacts
`the faceplate. The random measurement signal frequen
`cies reduce the susceptibility of the position measure
`ment apparatus to electromagnetic noise and distributes
`over a relatively broad bandwidth the electromagnetic
`noise generated by the position measurement apparatus.
`
`22 Claims, 2 Drawing Sheets
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`U.S. Patent
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`May 1, 1990
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`Sheet 1 of 2
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`4,922,061
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`U.S. Patent May 1, 1990
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`Sheet 2 of 2
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`4,922,061
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`FIG. 2
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`42
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`DIGITAL-TO-
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`MEASURMENT
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`1.
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`CAPACITIVE TOUCH PANEL, SYSTEM WITH
`RANDOMILY MODULATED POSITION
`MEASUREMENT SIGNAL
`This is a continuation-in-part of application Ser. No.
`07/205,896, filed June 13, 1988, U.S. Pat. No. 4,853,498.
`TECHNICAL FIELD
`The present invention relates to touch panel systems
`of the type having electrically conductive faceplates
`and, in particular, to such a touch panel system in which
`a position measurement apparatus generates an address
`signal that is indicative of a location at which a stylus
`contacts the faceplate.
`BACKGROUND OF THE INVENTION
`A touch panel system is a data input device that al
`lows an operator to interact with information rendered
`20
`on a display screen. For example, the operator can
`select one of multiple computer command options ren
`dered at different locations on the display screen by
`touching the screen at one of the locations. A touch
`panel system employs a position measurement apparatus
`that generates an address signal indicative of the
`touched location. The address signal is delivered to a
`computer that determines from the address signal which
`one of the command options is selected. The object
`with which the operator touches the display screen is
`30
`called a stylus and may include, for example, the opera
`tor's finger, a pen, or a pencil.
`A touch panel system of the capacitive-type typically
`includes a faceplate that has on its outer major surface
`on optically transparent, electrically conductive coat
`ing of a preselected resistivity. The faceplate is posi
`tioned in front of the display screen of a display device
`so that an operator can touch the conductive coating at
`locations aligned with information rendered on the
`display screen.
`The operator touches the conductive coating with a
`stylus having a nonzero, finite capacitance with refer
`ence to electrical ground. The stylus causes a change in
`the characteristics of an amplitude modulated position
`measurement signal applied to the conductive coating.
`The touch panel sysem distinguishes the location the
`stylus contacts from the other locations on the faceplate
`in accordance with the change in the characteristics of
`the position measurement signal.
`The conductive coating on the faceplate causes a
`capacitive touch panel system to suffer from at least two
`disadvantages. First, the conductive coating is receptive
`to electromagnetic noise generated by the display sys
`tem (e.g. cathode-ray tube flyback pulses) or present
`within the environment (e.g. 60 Hz background noise).
`Such noise susceptibility can render difficult the deter
`mination of a touch location. Second, the conductive
`coating causes the touch panel system to generate elec
`tromagnetic interference at the frequency of the posi
`60
`tion measurement signal. As a result, a capacitive touch
`panel system typically generates substantial amounts of
`electromagnetic noise.
`SUMMARY OF THE INVENTION
`An object of the present invention is, therefore, to
`provide a position measurement apparatus for a capaci
`tive touch panel system.
`
`4,922,061
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`Another object of this invention is to provide such an
`apparatus having a reduced susceptibility to electro
`magnetic noise.
`A further object of this invention is to provide such a
`system that generates reduced amounts of electromag
`netic noise at any single specific measurement signal
`frequency.
`The present invention is a touch panel system with a
`position measurement apparatus that generates an ad
`dress signal indicative of a position on the touch panel in
`contact with a capacitive stylus. The stylus has a non
`zero, finite capacitance with reference to electrical
`ground.
`In a preferred embodiment, the touch panel system
`includes a rectangular faceplate with an electrically
`conductive layer. The conductive layer has a consistent
`resistivity, covers the entire outer major surface of the
`faceplate, and carries four bar electrodes. A different
`one of the bar electrodes extends along almost the entire
`length and near each of the side margins of the outer
`major surface of the faceplate. The bar electrodes form
`two pairs of generally parallel opposed electrical
`contacts. Each bar electrode is electrically connected to
`the conductive layer and the position measurement
`apparatus.
`The position measurement apparatus includes a mea
`surement signal source that generates an amplitude
`modulated measurement signal of a frequency that var
`ies in a substantially random manner. The measurement
`signal is applied simiultaneously to each of the bar elec
`trodes. The apparatus identifies a position on the face
`plate in contact with the stylus by measuring the mea
`surement signal current that the stylus draws through
`each electrode. The current drawn through each elec
`trode is inversely proportional to the separation be
`tween the electrode and the position at which the stylus
`touches the faceplate. The current measurements ob
`tained with respect to the four electrodes are analyzed
`by a microprocessor to identify the position.
`The position is identified separately with respect to
`each one of the bar electrodes. The following descrip
`tion relates to the position identification with respect to
`an exemplary one of the bar electrodes. Such descrip
`tion would apply similarly to each of the three remain
`ing electrodes.
`The signal source transmits the measurement signal to
`the bar electrode through the primary coil of a trans
`former. The transformer develops across the output
`0 terminals of a secondary coil a potential difference pro
`portional to the current that the stylus draws through
`the bar electrode. A differential amplifier having first
`and second input terminals is electrically connected to
`the ouput terminals of the secondary coil of the trans
`former. The differential amplifier receives the potential
`difference and generates positive and negative differen
`tial output signals proportional to the current. The
`transformer cooperates, therefore, with the differential
`amplifier to function as a current meter that measures
`the current drawn through the electrode.
`A signal demodulator of the lock-in type employs the
`same random frequency measurement signal as a refer
`ence for demodulating the positive and negative differ
`ential output signals. A low-pass filter connected to the
`signal demodulator provides from the demodulated
`signal a substantially steady-state address signal that
`corresponds to an average of the magnitude of the cur
`rent drawn through the bar electrode.
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`tively any resistance. Position measurement apparatus
`The position measurement apparatus of the present
`12 identifies touch location 46 by measuring the separa
`invention has reduced susceptibility to electromagnetic
`tion between location 46 and each one of electrodes 36,
`noise because of the lock-in characteristics of the signal
`demodulator and the low-pass filter. In addition, the
`38, 40, and 42.
`Position measurement apparatus 12 comprises four
`position measurement apparatus of this invention gener
`ates electromagnetic noise that is spread over the band
`similar position measurement subcircuits 54a, 54b, 54c,
`width of the random frequencies of the position mea
`and 54d that are connected to the respective electrodes
`36, 38, 40, and 42 to generate an address signal indica
`surement signal. As a result, the position measurement
`apparatus generates reduced amounts of electromag
`tive of the separation between touch location 46 and
`netic noise at any single specific measurement signal
`each of the electrodes. Subcircuits 54a-54d preferably
`operate simultaneously or "in parallel' to generate the
`frequency.
`address signal. Corresponding components of position
`Additional objects and advantages of the present
`invention will be apparent from the detailed description
`measurement subcircuits 54-a-54d have identical refer
`of a preferred embodiment thereof, which proceeds
`ence numerals with letter suffices "a"-"d", respec
`tively. The following description is directed by way of
`with reference to the accompanying drawings.
`example only to position measurement subcircuit 54a
`BRIEF DESCRIPTION OF THE DRAWINGS
`and is similarly applicable to position measurement
`FIG. 1 is a schematic block diagram of a position
`subcircuits 54b-54d.
`A position measurement signal source 56 generates a
`measurement apparatus of the present invention imple
`bipolar continuous square-wave measurement signal
`mented in a touch panel system.
`20
`FIG. 2 is an enlarged frontal schematic diagram of
`and is delivered to an input 58 of a source output filter
`the faceplate of the touch panel system of FIG. 1.
`60. The measurement signal alternates between a posi
`tive voltage, --V, and a negative voltage level, -V., at
`FIG. 3 is a circuit block diagram of a measurement
`a frequency that varies in a substantially random man
`signal source employed in the position measurement
`apparatus of FIG. 1.
`ner. In the prferred embodiment, a microporcessor 62
`functions as a pseudo-random number generator that
`DETAILED DESCRIPTION OF PREFERRED
`generates a pseudo-random number signal that is deliv
`EMBODIMENT
`ered to an input 64 of measurement signal source 56. In
`FIG. 1 schematically shows a touch panel system 10
`response to the pseudo-random number signal, measure
`and an associated position measurement apparatus 12 of 30
`ment signal source 56 generates the measurement signal
`the present invention. Touch panel system 10 includes
`with a frequency of between 150 and 250 kHz in accor
`an optically transparent faceplate 14 positioned face-to
`dance with the value of the pseudo-random number.
`Microprocessor 62 generates the pseudo-random num
`face with the display screen 16 of a display device 18
`that incorporates, for example, a cathode-ray tube. It
`bers at a frequency of about 50 kHz.
`will be appreciated that the display device 18 could
`Output filer 60 is preferably of the low-pass type and
`alternatively include a liquid crystal display or a sign
`cooperates with an output resistor 66 to establish an
`output impedance of 2 kilohms. This output impedance
`board that displays fixed information such as a numeric
`key pad.
`functions to improve the uniformity of the signal-to
`Faceplate 14 includes an optically transparent, elec
`noise ratios of the signals on electrodes 36-42, as will be
`trically conductive layer 20 that covers substantially all
`described below in greater detail. Output filter 60 deliv
`of an outer major surface 22 of faceplate 14. Layer 20 is
`ers the square-wave measurement signal to electrode 36
`via the primary coil 68a of a transformer 70a. Primary
`formed from indium tin oxide (ITO) and has a consis
`tent resistivity. Faceplate 14 is typically of a rectangular
`coil 68a is arranged so that its positive terminal 72a and
`shape and includes on major surface 22 a first pair of
`negative terminal 74a are electrically connected to elec
`opposed side margins 26 and 30 and a second pair of 45
`trode 36 and the output resistor 66 of filter 60 respec
`opposed side margins 28 and 32.
`tively.
`8
`Electrodes 36, 38, 40 and 42 in the form of bars or
`With reference to FIGS. 1 and 2, stylus 48 in contact
`strips are positioned on and extend along major portions
`with touch location 46 may be modeled as dividing the
`of the lengths of side margins 26, 28, 30, and 32, respec
`resistance Rx into a first resistance k.R. and a second
`tively. Electrodes 36 and 40 and electrodes 38 and 42 50
`resistance (l-k)Rx, the term ky representing the nor
`form two pairs of opposed electrical contacts that are
`malized distance 76a between touch location 46 and
`electrically connected along their lengths to conductive
`electrode 36. The first resistance represents the resis
`layer 20, Electrodes 36 and 40 define across faceplate 14
`tance of layer 20 between touch location 46 and elec
`an X-axis having its origin 44 located near the bottom
`trode 36, and the second resistance represents the resis
`side of electrode 36, and electrodes 38 and 42 define
`tance of layer 20 between touch location 46 and elec
`55
`across faceplate 14 a Y-axis having its origin 44 located
`trode 40. Stylus 48 in contact with layer 20 draws cur
`rents proportional to the measurement signal to electri
`near the left side of electrode 38.
`The resistivity of layer 20 establishes effective total
`cal ground. Position measurement subcircuits 54a and
`resistances Rx and Ry in the respective X-and Y-axis
`54c measure the currents drawn to electrical ground via
`directions across faceplate 14. Position measurement
`the respective first and second resistances, thereby to
`60
`apparatus 12 identifies a position or location 46 (FIG. 2)
`indicate the position of touch location 46 along the
`on faceplate 14 in contact with a capacitive stylus 48
`X-axis.
`(FIG. 2) such as, for example, a person's finger. (The
`With reference to position measurement subcircuit
`54a, transformer 70a includes two secondary coils 78a
`location 46 is hereinafter referred to as "touch location
`46.") Stylus 48 may be modeled as a capacitor 50 and a
`and 80a that are inductively coupled to primary coil
`65
`resistor 52 electrically connected in parallel to ground.
`68a. Secondary coil 78a has a positive terminal 82a and
`Capacitor 50 can have a capacitance of between 5 and
`a negative terminal 84a that are electrically connected
`to an inverting input 86a of a differential amplifier 88a
`several hundred picofarads. Resistor 52 can have effec
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`system 10 by electromagnetic waves or a system user.
`and ground, respectively. Similarly, secondary coil 80a
`Filter 136a delivers the address signal to an input 138a
`has a negative terminal 92a and a positive terminal 94a
`of a four input-to-one output analog multiplexer 140,
`that are electrically connected to noninverting input
`which transmits in a serial fashion to an analog-to-digi
`96a of differential amplifier 88a and ground, respec
`tal converter 142 the address signals generated by Sub
`tively. Differential amplifier 88a operates in the differ
`circuits 54a-54d. Analog-to-digital converter 142 com
`ential-input, differential-output mode.
`municates with microporcessor 62 that receives the
`The current drawn to ground via the first resistance
`digital address signals and generates in response to them
`flows through primary coil 68a. This current induces in
`secondary coils 78a and 80a a current Ia that flows
`the X- and Y-coordinates of touch location 46.
`The cathode-ray tube of display device 18 forms an
`through positive terminal 82a, negative terminal 92a,
`image by scanning an electron beam (not shown) in a
`and a resistor 98a of 22 ohms that is connected between
`raster pattern across display screen 16. Such an image is
`them. The current I generates across resistor 98a a
`formed of image frames that include between about 200
`potential different porportional to the current drawn
`through electrode 36 by the presence of stylus 48 at
`and 2,000 horizontal scan lines and that are addressed at
`frequencies of between 20 and 90 Hz. The beam current
`touch location 46. The potential difference is applied to
`of the scanning electron beam changes in accordance
`inputs 86a and 96a of differential amplifier 88a, which
`with the magnitude of a video signal that carries infor
`generates at its positive output 106a and negative output
`mation representing the image to be rendered. The
`108a respective positive and negative differential output
`signals representing the current drawn through elec
`video signal further includes for each horizontal scan
`line a flyback or retrace pulse that directs the electron
`trode 36. The differential output signals are modulated
`beam to return to a starting scan position after the com
`at the substantially random measurement signal fre
`quency, and the magnitudes of the differential output
`pletion of each successive horizontal scan line. The
`flyback pulses relate in a one-to-one fashion to the hori
`signals are inversely proportional to the distance 76a
`zontal scan lines and occur, therefore, at frequencies of
`between touch location 46 and electrode 36.
`Outputs 106a and 108a deliver the positive and nega
`about 15-200 kHz.
`25
`Conductive layer 20 of touch panel system 10 has a
`tive differential output signals to different inputs of a
`relatively large surface area (e.g. abou 1100 cm) that
`single pole-double throw demodulation switch 114a
`causes the layer to function as a capacitor. As a conse
`having a switching control input 116a that receives as a
`quence, the flyback pulses on a video signal are capaci
`reference signal the substantially random measurement
`tively induced onto layer 20, thereby introducing noise
`signal generated by measurement signal source 56.
`30
`signals of substantial magnitudes into measurement sys
`Switch 114a generates a demodulated output signal by
`delivering to output 1.18a the positive and negative
`tem 12. For example, the noise signals associated with
`the flyback pulses can have voltage magnitudes up to
`differential output signals during the respective --V
`about 100 times the magnitude of the measurement
`and -V voltage levels of the measurement signal. As a
`signal on conductive layer 20,
`result, differential amplifier 88a and switch 114a coop
`35
`Position measurement apparatus 12 is capable of de
`erate to function as a "lock-in" amplifer that is "locked
`termining the position of touch location 46 by means of
`in' to the measurement signal frequency and coherently
`the cooperation between the random variations in the
`demodulates the differential output signals. The demod
`ulated output signal represents a relatively low-noise,
`measurement signal frequency and the lock-in charac
`teristics of position measurement subcircuits 54a-54d.
`substantially steady-state address signal that corre
`40
`In particular, the noise signals associated with the fly
`sponds to an average of the magnitude of the current
`back pulses as well as other electromagnetic noise
`drawn through electrode 36; is indicative of the distance
`sources are incoherent with the substantially random
`76a between touch location 46 and electrode 36; and is
`measurement signals. Accordingly, low pass filters
`substantially independent of the relative impedances of
`124a-124d and 136a-136d cooperate with the lock-in
`capacitor 50 and resistor 52 of stylus 48.
`45
`Demodulation switch 114a delivers on its oputput the
`characteristics of subcircuits 54a-54d to attenuate the
`noise signals, thereby to allow the detection of touch
`steady-state address signal to a first low pass filter 124a
`having a 3dB cut-off frequency of 3.3 kHz. The cutoff
`location 46.
`Another aspect of the present invention is that the
`frequency of low pass filter 124a is selected so that it
`substantially random variations in the measurement
`rapdily delivers a filtered address signal of substantial
`signal frequency reduces the magnitude of the electro
`magnitude to an automatic gain control circuit 126.
`magnetic noise generated by position measurement ap
`Gain control circuit 126 includes diodes 128a, 128b,
`paratus 2 at any selected frequency. In particular, the
`128c, and 128d that receive at their anodes 130a, 130b,
`measurement signal frequency ranges over a bandwidth
`130c, and 130d the measurement signals generated by
`of 150-250 kHz such that electromagnetic noise gener
`the respective position measurement subcircuits 54a,
`ated by the measurement signal is also distributed over
`54b, 54c, and 54d. The cathodes 132a, 132b, 132c, and
`the same bandwidth. As a result, the electromagnetic
`132d of the respective diodes 128a, 128b, 128c, and 128d
`noise generated by position measurement apparatus 12
`are electrically connected and deliver to a gain normal
`izing stage 134 the one of the address signals generated
`at any selected frequency is of relatively low magni
`tude, thereby reducing the electromagnetic interference
`by subcircuits 54a-54d having the greatest voltage mag
`60
`generated by position measurement apparatus 12.
`nitude. Normalizing stage 134 automatically normalizes
`The following sets forth, by way of example only, a
`further processing of the addressing signals with refer
`description of the manner in which microprocessor 62
`ence to the one measurement signal.
`identifies the position of touch location 46 along only
`Filter 124a also delivers the address signal to a second
`low pass filter 136a having a 3dB frequency of 100 Hz
`the X-axis. The demodulated address signals generated
`65
`for optimizing the signal-to-noise ratio of the address
`by measurement subcircuits 54a and 54c are DC volt
`ages, V and V2, that are proportional to the currents
`signal. Filters 124a and 136a cooperate to reject inco
`herent noise signals that are injected into touch panel
`drawn through electrodes 36 and 40, respectively. With
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`the position of touch location 46 along the X-axis repre
`rable magnitudes and typically differ by less than an
`sented by distance 76a (FIG. 2) and the variable X,
`order of magnitude. Resistor 66 functions, therefore, as
`microprocessor 62 represents the X-coordinate as:
`an automatic gain control that substantially reduces the
`dynamic range requirements of subcircuits 54a-54d and
`allows a system user to "load' signal source 56.
`It will be obvious to those having skill in the art that
`many changes may be made in the above-described
`details of the preferred embodiment of the present in
`vention without departing from the underlying princi
`ples thereof. For example, a measurement signal of
`substantially random frequency could be generated in
`accordance with a pseudo-random number generated
`by a feedback shift register or an analog noise genera
`tor. The scope of the present invention should be deter
`mined, therefore, only by the following claims.
`We claim:
`i. In a display system including a display surface
`positioned adjacent an optically transparent, electri
`cally conductive layer having a first resistivity and first
`and second opposed side margins of preselected lengths,
`a position address encoder for generating an address
`signal indicative of a location on the conductive layer in
`contact with a stylus that is electrically connected to a
`reference potential, comprising:
`signal generating means for generating a time-varying
`position measurement signal of a preselected mag
`nitude and a frequency that varies in a substantially
`random manner;
`first and second electrodes positioned along the
`lengths of the respective first and second opposed
`side margins and receiving the position measure
`ment signal generated by the signal generating
`means, the resistivity of the conductive layer estab
`lishing an effective total resistance between the
`first and second electrodes; and
`current measuring means for measuring currents cor
`responding to the position measurement signal and
`drawn through the first and second electrodes
`whenever a stylus contacts the conductive layer,
`whereby the stylus contacting the conductive layer
`divides the total resistance into first and second
`resistances that cause currents to be drawn through
`the respective first and second electrodes in pro
`portion to the separation between the electrodes
`and the location.
`2. The encoder of claim 1 further comprising a source
`output filter having a high output impedance and being
`electrically connected between the signal generating
`means and the electrodes.
`3. The encoder of claim 2 in which the source output
`filter includes a low-pass filter.
`4. The encoder of claim 1 in which the position mea
`surement signal is a continuous square-wave signal.
`5. The encoder of claim 1 in which the current mea
`Suring means measures in parallel the currents drawn
`through the first and second electrodes.
`6. The encoder of claim 1 in which the current mea
`suring means includes means for generating a first po
`tential difference proportional to a current drawn
`through the first electrode, and a first differential ampli
`fier having first and second inputs across which the first
`potential difference is applied, the first differential am
`plifier forming on a first output a signal representing the
`current drawn through the first electrode.
`7. The encoder of claim 6 in which the current mea
`Suring means further includes means for generating a
`Second potential difference proportional to a current
`
`It will be appreciated that microprocessor 62 would
`employ a similar equation together with the demodu
`lated address signals generated by subcircuits 54b and
`54d to determine only the Y-coordinate of touch loca
`tion 46.
`To identify both the X- and Y-coordinates of touch
`location 46, microprocessor 62 communicates with a
`memory circuit 146that stores a look-up table listing the
`X- and Y-axis coordinates corresponding to particular
`measurement signal magnitudes. The look-up table may
`be empirically generated for each touch panel system to
`accommodate any irregularities in the resistivity of the
`conductive layer.
`Touch panel system 10 is of a sturdy or robust config
`20
`uration that also provides low-power operation. The
`configuration of system 10 is robust because of the lim
`ited number of electrodes in contact with conductive
`layer 20 and the inherent sturdiness of transformers.
`The low-power operation of system 10 is obtained be
`25
`cause substantially no power is dissipated from conduc
`tive layer 20 unless it is touched by a stylus.
`With reference to FIG. 3, measurement signal source
`56 includes a digitial-to-analog converter 150 coupled
`to input 64 to receive the pseudo-random number signal
`generated by microprocessor 62. In response to the
`pseudo-random number signal, digital-to-analog con
`verter 150 generates a control voltage of a magnitude
`corresponding to the value of the pseudo-random num
`ber. The control voltage is delivered to a control input
`35
`152 of a voltage controlled oscillator 154, which is
`responsive to the control voltage to generate a measure
`ment signal of a frequency corresponding to the magni
`tude of the control voltage. Accordingly, digital-to
`analog converter 150 and voltage controlled oscillator
`154 cooperate with microprocessor 62 to generate a
`measurement signal of a frequency that varies in a sub
`stantially random manner.
`With reference to FIG. 1, the means by which output
`resistor 66 of filter 60 functions to improve the unifor
`mity of the signal-to-noise ratios of the signals applied to
`measurement subcircuits 54-a-54d is described with ref.
`erence to an exemplary system that does not employ
`output resistor 66. Whenever a user touches the face
`plate of the exemplary system at a location that is very
`close to an electrode that indicates position along a first
`axis, nearly all of the electrical current flowing from the
`faceplate and through the person is drawn from the
`electrode. Under these conditions, very little current is
`drawn from the electrodes that indicate position in the
`other axis. As a consequence, the magnitude of the
`signal-to-noise ratio of signals indicating position along
`the first axis can be up to 1,000 times greater than the
`magnitude of the signal-to-noise ratio of the signals
`indicating position along the orthogonal axis. Such a
`range of signal-to-noise ratios imposes extreme operat
`60
`ing constraints on the dynamic range capabilities of the
`position measurement apparatus.
`Output resistor 66 of filter 60 improves the uniformity
`of the signal-to-noise ratios of the signals applied to
`measurement subcircuits 54a-54d by providing each
`one of electrodes 36-42 with a nonzero resistance
`through which stylus 48 draws current. As a result, the
`currents drawn through electrodes 36-42 are of compa
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`input signal into a position measurement signal
`drawn through the second electrode, and a second dif
`representing the location; and
`ferential amplifier having third and fourth inputs across
`measurement signal demodulating means receiving
`which the second potential difference is applied, the
`the position measurement signal for demodulating
`second differential amplifier forming on a second output
`it in accordance with the input signal, thereby to
`a signal representing the current drawn through the
`generate the address signal indicative of the loca
`second electrode.
`tion on the conductive layer in contact with the
`8. The encoder of claim 1 in which the conductive
`stylus.
`layer is disposed on an otpically transparent faceplate
`13. The encoder of claim 12 further comprising cur
`positioned adjacent the display surface.
`rent measuring means to measure currents correspond
`9. The encoder of claim 1 in which the conductive
`ing to the position measurement signal and drawn
`layer further comprises third and fourth opposed side
`through the first and second electrodes whenever a
`margins of preselected lengths positioned in a trans
`stylus contacts the conductive layer, whereby the stylus
`verse direction to the first and second side margins and
`contacting the conductive layer divides the total resis
`in which the encoder further comprises:
`tance into first and second resistances that cause cur
`third and fourth electrodes positioned along the
`rents to be drawn through t