United States Patent
`
`[19]
`
`Waldron
`
`[11]
`
`[45]
`
`4,136,291
`
`Jan. 23, 1979
`
`[54] CAPACITIVE TOUCH-PAD DEVICES WITH
`DYNAMIC BIAS
`
`Primary Examiner—Stanley D. Miller, Jr.
`Assistant Examiner-«B. P. Davis
`
`.
`.
`[75] Inventor. Wesley K. Waldron, Scotia, N.Y.
`[73] Assignee: General Electric Company,
`Schenectady, N.Y.
`
`.
`[2]] Appl. No.. 799’345
`‘
`[22] Filed:
`May 23, 1977
`[51]
`Int Cl 2
`H03K 3/26' H03K 5/153,
`'
`' """"""""""""
`’ H01G 5/0;
`[52] US. Cl. .................................... 307/308; 307/360;
`,
`361/280; 361/330
`[58] Field of Search .................... 328/4; 307/308, 360;
`331/65; 340/365 C; 361/278, 271, 280, 330
`
`[56]
`
`3,826,979
`3,986,110
`4,016,490
`
`References Cited
`U.S. PATENT DOCUMENTS
`7/1974
`Steinmann .......................... 324/61 P
`.........
`10/1976 Overall et al.
`324/61 P
`
`........... 324/61 P
`4/1977 Weekenmann et a1.
`
`Attorney, Agent, or Firm—Geoffrey H. Krauss; Joseph
`T. Cohen; Marvin Snyder
`_
`
`ABSTRACT
`[57]
`A dynamic biasing capacitance is formed between a
`transmitting electrode and a receiving electrode of a
`capacitive touch-pad device to couple a portion of the
`scan voltage signal into the sense node of a voltage
`comparator circuit, coupled to the receiving electrode,
`‘0 offset the comparator eireuit threshold voltage The
`dynamic biasing capacitance may be formed by over-
`lapping portions of the electrodes, with a dielectric
`layer positioned therebetween, or by the parasitic ca-
`pacitance between aligned end surfaces of the two elec-
`trodes, with the magnitude of the dynamic biasing ca-
`pacitance being adjusted by variation of interelectrode
`ECOmCtUeS-
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`11 Claims, 5 Drawing Figures
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`CYPRESS 1023
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`US. Patent
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`Jan. 23, 1979
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`4,136,291
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`CAPACITIVE TOUCH-PAD DEVICES WITH
`DYNAMIC BIAS
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates to capacitive touch-pad
`devices and, more particularly, to means for adjusting
`the output of a capacitive touch-pad device to match
`the threshold voltage of a comparator circuit used to
`sense the touch and no-touch conditions.
`Known capacitive touch-pad devices have a pair of
`spatially-separated electrodes, commonly referred to as
`the transmitting electrode and the receiving electrode,
`fabricated upon a first surface of a dielectric layer, with
`a touch electrode fabricated upon the remaining surface
`of the dielectric layer and having an area enclosing the
`boundaries of the transmitting and receiving electrodes
`fabricated upon the opposite substrate surface. A source
`of a scanning voltage, commonly a voltage pulse, is
`coupled to the transmitting electrode and an‘input of a
`voltage comparator means is typically connected to the
`receiving electrode. A pair of series-connected capaci-
`tances (between the transmitting electrode and the
`touch electrode, and between the touch electrode and
`the receiving electrode) allow a certain proportion of
`the scan voltage to reach the voltage comparator input,
`when an object is not contacting or placed adjacent to
`the touch electrode; a different proportion of the scan
`voltage appears at the voltage comparator input when a
`relatively low impedance is placed between the touch
`electrode and a circuit ground. The comparator is con-
`figured to sense the change in signal amplitude at its
`input in the touch and in the no-touch conditions and to
`change the magnitude of its output accordingly.
`The voltage comparator generally has a minimum
`threshold voltage which is typically greater in absolute
`value than the output signal from a capacitance touch-
`pad device which is activated by a relatively low scan-
`ning voltage, up to a magnitude of about 30 volts peak.
`Thus, the use of low scanning voltages, e.g. 30 volts
`peak or less, are not practical as the comparator input
`signal, in either the touch or no—touch condition is less
`than the minimum threshold voltage and the Compara-
`tor remains in a single state in either condition.
`It has been suggested to use direct current biasing
`techniques at the voltage comparator input to achieve
`an offset against the minimum threshold voltage and
`thereby reduce the threshold voltage to a value suffi-
`ciently small, to allow low amplitude scan voltages are
`usable. However, in the scanning mode, wherein excita-
`tion signals are applied to the transmitting electrode as
`a pulse occurring only at certain predetermined times,
`the voltage comparator input is typically reset to a fixed
`voltage level (to prevent false activation of the compar-
`ator output by noise and other extraneous signals occur-
`ring between scan pulses) and is released from the fixed
`voltage level immediately prior to the leading edge of
`the scan voltage pulse, whereby any D.C. bias intro-
`duced at the voltage comparator input would be ren-
`dered unusable by the comparator input reset circuit.
`A dynamic biasing scheme which introduces a signal,
`offsetting the minimum threshold voltage, at the start of
`the scan voltage pulse will not be offset by the resetting
`of the comparator input and allows reduction in magni-
`tude of the relatively high amplitude scan voltages pres-
`ently utilized, thus reducing the cost of the electronics
`associated with the capacitive touch-pad device.
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`BRIEF SUMMARY OF THE INVENTION
`
`'In accordance with the invention, a capacitive touch-
`pad device having dynamic biasing means comprises a
`dielectric substrate having a touch electrode fabricated
`on a first surface and having spatially-separated trans-
`mitting and receiving electrodes fabricated upon the
`remaining surface and positioned opposite the touch
`electrode and within the boundaries of the latter; a
`portion of one of the receiving and transmitting elec-
`trodes is geometrically configured to provide a prede-
`termined value of capacitance between the transmitting
`and receiving electrodes to facilitate coupling of a pro-
`portional amount of the scan voltage pulse directly to
`the comparator input to offset the threshold voltage
`thereof.
`In one preferred embodiment, the dynamic biasing
`capacitance is of multilayer configuration and is formed
`by overlapping a portion of one of the receiving and
`transmitting electrodes over the other of the electrodes
`with a layer of dielectric therebetween.
`In another preferred embodiment, at least one portion
`of one of the receiving and transmitting electrodes is
`configured to have an edge thereof extending at lesser
`or greater length adjacent to at least one edge of the
`remaining one of the electrodes to provide greater or
`lesser capacitance, with an air dielectric in the space
`therebetween; dielectric material having a dielectric
`constant differing from the dielectric constant of air,
`may be utilized to achieve a desired value of dynamic
`biasing capacitance.
`Accordingly, it is one object of the present invention
`to provide a capacitive touch-pad device having dy-
`namic biasing means for substantially reducing the
`threshold voltage of a comparator coupled to the de-
`Vice.
`
`It is another object of the present invention to pro-
`vide a capacitive touch-pad device with a dynamic
`biasing means to facilitate scanning of the device with
`relatively low voltage signals.
`These and other objects of the present invention will
`become apparent upon consideration of the following
`detailed description taken in conjunction with the
`drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a schematic diagram illustrating the princi-
`ples of operation of the present invention;
`FIGS. 2a and 2b are a plan view and a sectional view,
`respectively, of one preferred embodiment of the novel
`capacitive touch-pad device with dynamic biasing
`means of the present invention; and
`FIGS. 3a and 3b are a plan view and a sectional view,
`respectively, of another preferred embodiment of the
`present invention.
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Referring initially to FIGS. 1, 2a and 2b, a first pre-
`ferred embodiment of a capacitive touch-pad device 10
`comprises a substrate 11 of a dielectric material having
`a conductive touch electrode 12 fabricated upon a first
`surface 11a thereof. A pair of electrodes 14 and 15,
`commonly referred to as a transmitting electrode and a
`receiving electrode, reSpectively, are fabricated upon
`the remaining, or inwardly-facing, surface 11b of the
`substrate. Electrodes 14 and 15 are configured and posi-
`tioned to be substantially within the boundaries defined
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`boundaries within which a transmitting electrode 23
`and a receiving electrode 24 are fabricated in planar
`manner, preferably by use of thin film techniques. One
`of electrodes 23 and 24 is of smaller area, e.g. electrode
`24, than the other, whereby the large electrode may be
`fabricated with at least one, and preferably a pair, of
`electrode extensions 23a and 23b extending adjacent to
`the boundaries of the smaller electrode. A channel 25 is
`formed between the adjacent edges of the two elec-
`trodes to provide insulation therebetween. Thus, the
`dynamic biasing capacitance C is formed as a fringing
`capacitance between the edges of the two, relatively
`closely spaced electrodes. Channel 25 may be filled
`with air or may be filled with a solid dielectric material
`having a dielectric constant differing from that of air, to
`increase or reduce the capacitance per unit length of the
`confronting edges.
`As previously explained hereinabove, electrode
`leads, such as a receiving electrode lead means 27 and a
`transmitting electrode lead means 28, may be fabricated
`integral with, and at the same time as, the pair of elec-
`trodes and the electrode extensions. This planar elec-
`trode-extension-lead approach is less costly to fabricate
`than the multilayer embodiment of FIGS. 2a and 2b and
`also provides a certain degree of electrical shielding for
`electrode 24.
`
`3
`by touch electrode 12 upon the opposite surface of the
`dielectric substrate. A layer 16 of dielectric material is
`fabricated over one of the transmitting and receiving
`electrodes, e.g. transmitting electrode 14, to provide an
`insulative supporting surface for an extension 15a of the
`remaining electrode, whereby a capacitance of magni-
`tude C is formed between electrode extension 15a and
`the underlying portion of the remaining electrode 14.
`Capacitance C is generally of lesser magnitude than
`either of the capacitance C,z formed between transmit-
`ting electrode 14 and the underlying touch electrode 12,
`and capacitance C], formed between touch electrode 12
`and the overlying receiving electrode 15. By fabricating
`the multilayered‘combination of electrodes 14 and 15, a
`dielectric layer 16 and electrode extension 15a by thin
`film techniques, the magnitude of overlap between elec-
`trode 14 and extension 150 is relatively easily changed
`to facilitate adjustment of the value of capacitance C.
`Use of thin film fabrication techniques is particularly
`advantageous in that a pair of lead means 16 and 17,
`each coupled to one of electrodes 14 and 15, respec-
`tively, can be integrally fabricated with the planar elec-
`trodes prior to enclosing at least one of the electrodes,
`e.g. electrode 14, in dielectric layer 16.
`In operation, a scan voltage source 19, producing a
`voltage pulse of amplitude V, at predetermined periodic
`intervals, is coupled between transmitting electrode 14
`and a circuit ground 20. A voltage comparator 21 has a
`first input 21a coupled to receiving electrode 15 and has
`a second input 21b coupled to circuit ground. Prior to
`receiving scan voltage pulse, a switch S (coupled be-
`tween comparator input leads 210 and 21b) is closed to
`prevent noise and other extraneous signals from trigger-
`ing the comparator and enabling the output 216 thereof.
`At the time that transmitting electrode 14 receives the
`scan voltage pulse, switch S is opened and a minimal
`threshold voltage of magnitude Vt}, appears between
`comparator input terminals 21a and 21b. If a relatively
`high impedance Z exists between touch electrode 12
`1. A capacitive touch pad device comprising:
`and circuit ground 20, a portion of the scan voltage V,
`a substrate of dielectric material having first and sec-
`is coupled to the comparator input via the series capaci-
`ond opposed surfaces;
`tance arrangement of capacitors Ca and C1,, while an
`a single conductive touch electrode responsive to
`additional portion of the scan voltage pulse is coupled
`by dynamic biasing capacitance C between source 19
`human communication fabricated upon said first
`and comparator input 210. As previously stated herein,
`surface over a preselected continuous area;
`a pair of spatially separated electrodes fabricated
`the value of dynamic biasing capacitance C is adjusted
`upon said second surface substantially within an
`to supply a portion of the scan voltage pulse to the
`area overlying and bounded by the area of said
`comparator input to reduce the threshold voltage sub-
`touch electrode;
`stantially to zero, whereby the comparator now senses
`only the absolute value of the signal coupled to its input
`at least one of said pair of electrodes having at least
`via the touch pad capacitors Ca and C1,; a high value of
`one extension therefrom extending parallel to and
`impedance Z allows a relatively large touch~pad volt-
`spaced from at least one surface of the remaining
`one of said pair of electrodes; and
`age to appear at the comparator and maintain compara-
`dielectric material filling the volume defined between
`tor output 21c in the disabled condition. If a relatively
`low magnitude of impedance Z occurs between touch
`said electrode extension and the confronting por-
`plate 12 and ground, the signal coupled to the compara-
`tion of said remaining electrode to provide a first
`tor input via series capacitors Ca and Cb is drastically
`electrical capacitance therebetween of essentially
`constant value;
`reduced, whereby only the dynamic biasing capacitor C
`couples a signal to the comparator input of sufficient
`the dielectric material of said substrate separating said
`magnitude to overcome the threshold voltage. This
`conductive touch electrode and each of said spa-
`relatively low voltage at the comparator input is sensed
`tially separated electrodes providing second and
`to enable comparator output 21c even when relatively
`third electrical capacitances respectively therebe-
`low amplitude scan voltages are utilized.
`tween of essentially constant value.
`Referring now to FIGS. 3a and 3b, another preferred
`2. A device as set forth in claim 1, wherein the dielec—
`embodiment 10' of a capacitive touch-pad device with 65 tric material between said electrode extension and said
`dynamic biasing means is shown. Touch electrode 12,
`remaining electrode is air.
`having been fabricated upon the outwardly-facing sur-
`3. A device as set forth in claim 1, wherein the dielec-
`face lla of the dielectric substrate 11, provides the
`tric material between said electrode extension and said
`
`It should be understood that, in either preferred em-
`bodiment,
`the transmitting and receiving electrodes
`may be interchanged, although electrode 24, being
`shielded by electrode extensions 23a and 23b,
`is best
`connected as the receiving electrode in many applica-
`tions.
`
`While several preferred embodiments of the present
`invention have been dscribed, many variations and
`modifications will now become apparent
`to those
`skilled in the art. It is my intent, therefore, to be limited
`solely by the scope of the appending claims.
`What is claimed is:
`
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`5
`remaining electrode has a dielectric constant differing
`from the dielectric constant of air.
`
`4,136,291
`
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`8. A device as set forth in claim 1, further comprising
`lead means coupled to each of said pair of electrodes.
`9. A device as set forth in claim 8, wherein said lead
`means are thin film members integrally fabricated as
`part of the associated one of said pair of electrodes.
`10. A device as set forth in claim 1 in combination
`with: first means coupled to only one of said pair of
`electrodes for generating a signal; second means cou-
`pled to the remaining one of said electrodes for compar-
`ing, against a reference signal amplitude, the amplitude
`of the signal at said remaining electrode produced re-
`sponsive to the signal from said first means, said second
`means having a threshold signal amplitude; and the
`magnitude of said capacitance between said pair of
`electrodes being adjusted to supply said second means
`with a portion of the signal from said first means se-
`lected to offset said threshold signal amplitude.
`11. A combination as set forth in claim 10, wherein
`the signal from said first means is periodically applied to
`said device; and wherein an input of said second means
`is substantially shortcircuited at all time intervals when
`said first means signal is not present at said device.
`t
`t
`t
`t
`t
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`4. A device as set forth in claim 1, wherein said pair
`of electrodes are planar arranged, said electrode exten-
`sion being positioned above the plane of said electrodes
`and extending over a predetermined area of said remain-
`ing electrode; the volume between confronting surfaces
`of said electrode extension and said remaining electrode
`being filled with a solid dielectric material.
`5. A device as set forth in claim 4, wherein said re-
`maining electrode is completely enclosed by solid di-
`electric material.
`
`6. A device as set forth in claim 5, wherein said pair
`of electrodes and said electrode extension are thin film
`members.
`
`7. A device as set forth in claim 1, wherein said pair
`of electrodes are coplanar, each of said extension elec-
`trodes lying in the same plane thereof and having an
`edge extending adjacent to at least one edge of said
`remaining electrode.
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