Hlllllllllllllllllllllllllllllllllllllllllllll|||||Illllllllllllllllllllll
`US005572205A
`.
`5,572,205
`[11] Patent Number:
`UnltEd States Patent
`Caldwell et a1.
`[45] Date of Patent:
`Nov. 5, 1996
`
`
`[19]
`
`[54] TOUCH CONTROL SYSTEM
`
`[75]
`
`Inventors: David W. Caldwell; Nicholas W.
`Madendorp’ both 0f Holland, Mmh'
`[73] Assignee: Donnelly Technology, Inc., Holland,
`Mich-
`
`[21] Appl' No": Muss
`~
`.
`[22]
`Filed‘
`Mar’ 29’ 1993
`[51]
`Int. Cl.6 .................................................. H03K 17/955
`[52] US. Cl.
`.............................. 341/33; 341/26; 345/168;
`200/600; 361/181; 219/451
`[58] Field of Search .................................. 341/20, 22, 26,
`341/33; 200/600; 345/168, 173; 331/65;
`361/181; 291/451, 464
`
`[56]
`
`References Cited
`
`U'S' PATENT DOCUMENTS
`11/1974 Foster .................................. 340/365 C
`3,846,791
`
`7/1976 Challonel‘ 6t 31-
`----- 340/337
`3,971,013
`
`3/1976 GOUId 6‘ ‘11-
`341/33
`3,974,472
`
`11’1977 Jordan """
`341/33
`4’056’699
`5/1978 sen-.380
`4’090’092
`" 307,116
`4,119,864 10/1978 PetrlZlo
`.. 307/116
`4,123 631
`10/1978 Lewis ......
`341I33
`4,136:291
`1/1979 Waldron .....................
`.. 307/308
`
`4,145,743
`3/1979 Eichelberger et al.
`_____
`. 364/862
`
`6/1979 Senk ......................... 340/565
`4,159,473
`..... 361/281
`4,161,766
`7/1979 Castleberry et al.
`
`4,175,239
`11/1979 Sandler .................... 307/116
`5/1980 litiglel‘ at al-
`--
`----- 363/187
`4,203,230
`7/1980 M111“ 6‘ 31'
`'
`"""" 371/15
`4,211,915
`
`' 307,116
`4’237’421
`12,1980 “13.1de
`1/1981 Chlang .....
`. 323/349
`4,246,533
`
`4/1981 Hirata et al. .................. 341/33
`4,263,659
`
`9/1981 Eichelberger et al.
`.
`....... 341/33
`4,290,052
`
`............
`4,291,303
`9/1981 Cutler et a1.
`341/33
`....... 341/33
`4,304,976 12/1981 Gottbreht et a1.
`
`
`. 2219/1049 R
`4,308,443
`12/1981 Tucker et a1.
`4/1982 Witney et a1.
`.................. 341/33
`4,323,829
`
`4/1983 Posset .................. 361/280
`4,380,040
`
`....... 341/33
`4,394,643
`7/1983 Williams ..
`8/1983 Frame ........... 361/290
`4,400,758
`
`.......... 340/365 C
`4,405,918 ' 9/1983 Wall et a1.
`4,413,252
`11/1983 Tyler et a1. ................ 341/33
`
`313:: filfiffioeftflalo -
`---21299%57§
`12:23:;
`
`1/1985 Smith ................................ 341/33
`4,495,485
`
`4,527,04
`7/1 85 Thomas et a1.
`219/449
`
`4,529,963
`7/1985 Hilsum et a1.
`340/365 C
`
`4,550,310 10/1985 Yamaguchi et al.
`..... 341/33
`12/1985 Chiu ....................... 341/33
`4,561,002
`
`1/1986 Yoshikawa et a1.
`..... 341/33
`4,567,470
`
`9/1986 Poujois ...................... 341/33
`4,614,937
`
`..... 341/33
`4,651,133
`3/1987 Ganesan et a1.
`..............
`4,665,324
`5/1987 Ogino et a1.
`307/126
`
`-------------------- 340/365 E
`4,709,228
`“/1937 Bucking et al-
`-
`-
`0““ confirmed on ”6’“ page)
`Primary Examiner—Jeffery Hofsass
`Assistant Examiner—Daniel J. Wu
`Attomey, Agent, or Firm—Van Dyke, Gardner, Linn &
`BumthLLP
`
`[57]
`ABSTRACT
`A touch control system that is responsive to a user input
`selection includes an electrically non-conducting substrate,
`such as glass ceramic, and at least one capacitive-responsive
`touch pad on the substrate. A source signal having a primary
`frequency that is greater than 150 kHz, and preferably in the
`range of between 150 kHz and 500 kHz, is applied to one
`.
`.
`portion of the touch pad. The touch pad couples the electrical
`Signal to “Other Portion 0m“: t°u°h Pad in order ‘0 dEVCIOP
`a detection signal, which is decoded in order to determine
`the presence of the capacitance of a user. The decoder
`preferably includes a peak detector composed of a low gain
`circuit in order to avoid distortion'of the detection signal.
`Greatly improved performance in the presence of liquids,
`such as water, on the touch pad is provided. This is espe-
`cially useful when the touch pad is applied to a horizontal
`.
`.
`.
`.
`surface, 59°11 as ‘i‘ 9001‘ ‘01” “POP meh hq‘nd smug may
`0001“- A dlsplay 15 Jux‘aPOSed Wlth Fhe glass ceramlc SUb‘
`strate and an Optical correction mammal is Provided between
`the display and the underlying modulated surface that
`imparts mechanical strength to the substrate. The optical
`correction material corrects optical distortion of the visual
`indications of the display caused by the modulated surface_
`
`4,405,917
`
`9/1983 Chai .................................... 340/365 C
`
`60 Claims, 4 Drawing Sheets
`
`//
`
`l
`I
`\
`
`\
`
`//
`
`\\\
`
`‘
`
`\l
`l
`/
`
`//
`
`\\\ w//
`
`\
`l
`/
`
`30
`
`LINE DRIVER DETECTOR
`
`HIGH
`FREQUENCY
`
`44
`
`AMPLITUDE
`
`
`PEAK
`SWITCH
`RESPONSE
`INDICATORS,
`POWER CIRCUITS
`
`APPLE 1004
`
`APPLE 1004
`
`1
`
`

`

`5,572,205
`Page 2
`
`
`US. PATENT DOCUMENTS
`
`4711163 12/1987 0“” """""""""""""""""""""""""""" “2’29
`
`.. 361/280
`4,731,694
`3/1988 Grabneretal
`4,736,190
`4/1988 Fiorella ..............
`.. 341/32
`
`4,740,781
`4/1988 Brown ................
`. 341/33
`..... 341/33
`4,743,895
`5/1988 Alexander
`
`12/1988 Goessler et a1.
`.. 219/464
`4,794,233
`
`.. 280/600
`.....
`4,855,550
`8/1989 Schultz, Jr.
`
`.. 200/600
`4,894,493
`1/1990 Smith et 31‘
`,
`4,901,074
`2/1990 Sinn et 21.
`341/22
`
`4/1990 Hamada et a1.
`.......................... 362/32
`4,914,553
`
`4,920,343
`4,924,222
`4,954,823
`41981812
`530121124
`5,153,572
`5,155,338
`5,157,273
`5,183,996
`5,189,417
`5,270,710
`
`4/1990 Schwartz ................................... 341/33
`5/1990 Antikidis et a1.
`......................... 341/33
`9/1990 Binstead .................................... 341/26
`“1991 Wman ‘3‘ ‘1‘”
`219/451
`
`”41991 H°Haway
`341/22
`10/1992 Caldwell 6t 31-
`-
`-- 341/24
`
`10/1992 Hoffman“ ---------
`219/451
`
`10/1992 Medendom et a1.
`-
`-- 307/147
`219/452
`2/1993 Hazan et a1.
`......
`
`.. 341/26
`2/1993 Caldwell et a1.
`.
`12/1993 Gaultier et a1. ........................ 741/33
`
`2
`
`

`

`US. Patent
`
`Nov. '5, 1996
`
`Sheet 1 of 4
`
`5,572,205
`
`H/
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`LOW
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`
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`
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`MED
`LOW
`DEEEDEDDD -
`
`-|-
`
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`
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`
`22"
`
`3
`
`

`

`US. Patent
`
`Nov. 5, 1996
`
`Sheet 2 of 4
`
`5,572,205
`
`
`
`4
`
`

`

`US. Patent
`
`Nov. 5, 1996
`
`Sheet 3 of 4
`
`5,572,205
`
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`US. Patent
`
`Nov. 5, 1996
`
`Sheet 4 of 4
`
`5,572,205
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`

`

`1
`TOUCH CONTROL SYSTEM
`
`5,572,205
`
`2
`
`BACKGROUND OF THE INVENTION
`
`This invention relates generally to touch control systems
`and, more particularly, to capacitance-responsive touch con-
`trol input devices for application to horizontal substrates,
`such as glass ceramic panels. The invention is particularly
`adapted for use with smooth-top induction, radiant, and
`halogen burner cooking appliances.
`Touch control input devices that respond to the capaci-
`tance of a user’s contact in order to actuate an appliance are
`typically applied to a vertical surface. While such orientation
`is primarily for the convenience of use, it avoids several
`problems associated with applying touch controls to hori-
`zontal surfaces, such as smooth-top cooking appliances. One
`difficulty with application to horizontal substrates is that
`there is a greater likelihood that liquids will be splashed on
`the touch control applied to a horizontal surface, such as a
`range cook top. Such moisture tends to cause erratic opera»
`tion of the input control, which could be dangerous in the
`case of a cooking appliance. This difficulty is typically
`overcome by separating the touch control from the cooking
`surface in order to provide a physical barrier between the
`two. This solution is not without its drawbacks. The primary
`benefit of a smooth-top cooking appliance is to eliminate the
`difliculty of cleaning up from spills and boi1~over getting
`into burner elements. While separate touch control input
`devices are an improvement over electromechanical con-
`trols, which still allow places where spills can accumulate,
`the requirement for a physical barrier between the cook top
`and the touch control is an impediment to easy cleanup and
`is a compromise in aesthetic appearance.
`An attempt to overcome the problem caused by watery
`spills on the support surface of a smooth-top cooking
`appliance causing erroneous operation of a touch control
`applied directly to the support surface is disclosed in U.S.
`Pat. No. 4,446,350 issued to Takumi Mizukawa et al. for an
`INDUCTION HEATING COOKING APPARATUS.
`In
`Mizukawa et al., touch pads are provided on the upper
`surface of a pan supporting plate and are enclosed by guard
`rings of conductive material composed of a grounded con-
`ductor and an enclosing conductor. A control circuit, which
`is responsive to the touch pads and the guard rings, responds
`to spilled water or the like contacting the guard rings by
`latching a power control circuit at a zero power level. This
`resets the cooking apparatus to a zero heat output condition.
`The solution proposed in Mizukawa et al. has several
`difficulties. At least one of the guard rings must be connected
`with a ground potential in order to be effective. This requires
`conductive leads being applied to the pan support surface,
`which is costly and a potential source of failure. Addition-
`ally, Mizukawa et a1. responds to spilled water by latching
`the cooking apparatus into a zero output condition. This is a
`nuisance to the user by requiting that the spill be wiped up
`and the power level of the cooking apparatus reset in order
`to continue with the cooking operation.
`to a
`Another difficulty with applying a touch control
`horizontal substrate is that code requirements, as well as
`conservative engineering practices, dictate that large hori-
`zontal panels be manufactured using particular materials and
`in a particular manner to avoid breakage due to either
`mechanical impact or thermal shock. In particular, while
`relatively thin soda-lime glass may be utilized for vertical
`touch panels, smooth-top cooking surfaces that are capable
`of supporting multiple pans above multiple burners are made
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`from glass ceramic material having a greater thickness, on
`the order of three (3) to five (5) millimeters and require
`negligible thermal expansion. Additionally, the surface of
`the substrate facing away from the user is modulated, or
`dimpled, in order to add greater mechanical strength to the
`substrate. The thickness of the glass ceramic material and
`the modulated surface have prevented, in the past, applica-
`tion of touch control technology to such large horizontal
`substrates.
`
`SUMMARY OF THE INVENTION
`
`The present invention is embodied in a touch control that
`is responsive to a user input selection. The control includes
`an electrically non~conducting substrate and a capacitance-
`responsive touch pad on the substrate. A signal generator is
`provided as a source to generate an electrical signal and to
`apply the signal to one portion of the touch pad. The touch
`pad couples the electrical signal to another portion of the
`touch pad in order to develop a detection signal. The touch
`pad responds to the presence of capacitance of a user in
`order to selectively attenuate the detection signal. A decod»
`ing circuit responds to the detection signal
`in order to
`determine the presence of the capacitance of a user.
`According to one aspect of the invention, a source signal
`generator is provided that generates a high frequency elec-
`trical signal having a primary frequency that is greater than
`150 kHz and preferably in the range of between 150 kHz and
`500 kHz. Such high frequency electrical signal may be a
`square wave, a triangular wave, a sawtooth wave, a sinu-
`soidal wave or some other waveform. This aspect of the
`invention is based upon the discovery that touch controls
`operated at such primary frequencies have improved water
`immunity performance. According to another aspect of the
`invention, the decoding circuit may include a peak detector
`that is coupled directly with the detection signal, in order to
`produce an output, and a switch circuit that is responsive to
`the output of the peak detector in order to determine an
`amplitude of the output indicative of attenuation of the
`source electrical signal. The peak detector is preferably a
`low gain circuit in order to avoid distorting the detection
`signal by exceeding the gain/bandwidth product of the
`circuit and in order to avoid fast signal slew rates. The
`switch circuit is preferably coupled to the output of the peak
`detector by an amplifier circuit. In this manner, amplification
`is performed on the lower frequency signal of the peak
`detector output rather than on the higher frequency of the
`detection signal. The gain/bandwidth product of the system
`is not exceeded at any point in order to provide a more
`accurate detection of the effect of a user contacting a touch
`pad and low slew-rate components may be used to embody
`the invention.
`
`According to another aspect of the invention, a touch
`control is provided for a glass ceramic substrate having a
`user contact surface and an opposite modulated surface. A
`keypad is defined by a plurality of touch pads, each of tile
`touch pads having a pair of electrically conductive elements
`affixed to the modulated surface. A signal generator is
`provided that is adapted to generate an electrical signal and
`to apply the signal to one of the electrically conductive
`elements of at least one of the touch pads. The electrical
`signal is passively coupled to the other one of the electrically
`conductive elements in order to develop a detection signal.
`The touch pads respond to the presence of capacitance of a
`user in order to selectively attenuate the detection signal. A
`decoding circuit is provided that responds to the detection
`7
`
`7
`
`

`

`3
`
`4
`
`5,572,205
`
`circuit in order to determine the presence of capacitance of
`a user.
`
`According to yet another aspect of the invention, a display
`is juxtaposed with the substrate modulated surface in order
`to provide visual indications to a user. An optical correction
`material is provided between the display and the substrate.
`The optical correction material corrects optical distortion of
`the visual indications of the display caused by the modulated
`surface. In a preferred embodiment, the optical correction
`material is a transparent adhesive that adheres a flexible
`carrier carrying the display device and/or the touch pad
`flexible conduCtor to the glass substrate.
`The present invention overcomes tile difficulties of the
`prior art by providing a touch control
`that has greatly
`improved performance in the presence of liquids, such as
`water, on the touch pads positioned on the user interface
`surface of the substrate. Because the system’s immunity to
`water is significantly improved, there is no necessity for a
`specialized guard ring or for latching of the cooking appli—
`ance in an ofl state in the presence of water. Accordingly, the
`user is not bothered by having to reset the power level of the
`burner when a liquid spill occurs. The present invention
`allows, for the first time, a practical application of a touch
`control to a glass ceramic substrate having a modulated rear
`surface. A touch control circuit is provided according to the
`invention which is capable of accommodating the electrical
`characteristics of such substrate. In addition, a unique means
`is provided to correct distortions of the image displayed by
`a display device and viewed through the dimpled, modulated
`surface of the substrate.
`
`These and other objects, advantages and features of this
`invention will become apparent upon review of the follow-
`ing specification in conjunction with the drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a top plan view of a system incorporating a touch
`control according to the invention;
`FIG. 2 is a sectional view taken along the lines 11—11 in
`FIG. 1;
`
`FIG. 3 is an exploded perspective view from the top of the
`touch control in FIG. 1;
`
`5
`
`10
`
`15
`
`20
`
`25
`
`3O
`
`35
`
`40
`
`FIG. 4 is a block diagram of a touch control according to
`the invention;
`
`45
`
`FIG. 5 is an electrical schematic diagram of a touch
`control according to the invention;
`FIG. 6 is an illustration of an optical indicator without an
`optical correction material; and
`
`FIG. 7 is an illustration of an optical indicator with an
`optical correction material according to the invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`
`Referring now specifically to the drawings, and the illus-
`trated embodiment depicted therein, a touch control 10
`includes a generally planar substrate 12 and a plurality of
`touch pads, generally indicated at 14 applied to substrate 12
`(FIG. 1 ). Each touch pad 14 includes a first portion
`composed of an electrically conducting element 16a and a
`second portion composed of an electrically conducting ele-
`ment 16b aflixed to a surface 18 of substrate 12, which faces
`away from the user (FIGS. 2—4). Each touch pad 14, in the
`illustrative embodiment, also includes a user contact pad 20
`overlying the conductive elements 16a and 16b. User con-
`tact pads 20 are transparent conductive metallic oxide films
`
`50
`
`55
`
`60
`
`65
`
`applied by conventional sputtering or pyrolyric techniques.
`Touch control 10 may additionally include indicators 22 in
`order to provide visual indication to the user of the condition
`of the appliance being controlled (not shown). In the illus-
`trated embodiment, substrate 12 is a glass ceramic member
`having a thickness in the range of three (3) to five (5)
`millimeters in order to provide adequate strength for hori-
`zontal applications in which mechanical stress may be
`applied to the substrate. One such application is a smooth-
`top cooking surface for a 4-burner cooking appliance. In
`order to further enhance the strength of substrate 12, surface
`18 is modulated or dimpled (FIG. 2). Substrate 12 is
`marketed under the mark “Ceran” by Schott Glass Company
`located in Yonkers, NY.
`In order to apply the conductive elements 16a, 16b of
`each touch pad to surface 18 of substrate 12, the conductive
`elements 16a, 16b are mounted to a flexible carrier 24.
`Carrier 24 is adhered to surface 18 by an adhesive layer 26.
`Additionally, indicators 22 are mounted to flexible carrier 24
`in order to locate the indicators in a position where they may
`be viewed through substrate 12. In order to correct optical
`distortion created by the presence of the modulations, or
`dimples, on surface 18, an optical correction material 23 is
`positioned between indicator 22 and modulated surface 18.
`Optical correction material 23 has an index of refraction that
`is compatible with that of substrate 12 and fills in the voids
`between the dimples of surface 18, as well as the space
`between surface 18 and indicator 22. In this manner, light
`emitted by indicator 22 passes through substrate 12 without
`substantial distortion.
`
`Operation of optical correction material 23 may be under-
`stood by comparing an indicator 22' in FIG. 6 with an
`indicator 22" in FIG. 7. Indicator 22 illustrates the optical
`effect of modulated surface 18. The different
`incidence
`angles of light rays caused by the dimples creates a “fish—
`eye” eflect whereby an initially homogeneous indication
`takes on the appearance of numerous circles and the indi-
`cation has serrated edges. In contrast, indicator 22" illus-
`trates the corrective effect of optical correction material 23
`in eliminating distortions to the homogeneous appearance of
`the indicator, including retaining the crisp edges of the initial
`indication.
`
`Optical correction material, in the illustrated embodiment,
`is a transparent acrylic material. While optical correction
`material 23 is clear, it may be also dyed in order to modify
`the color of indicators 22. A clear acrylic material in transfer
`adhesive form is commercially available from the 3M Com-
`pany, Minneapolis, Minn., and marketed under Type 300MP.
`In a most preferred embodiment, a clear acrylic adhesive,
`such as 3M Type 300MP, is applied to the entire interface
`between surface 18 and flexible carrier 24 at a thickness of
`0.013 inches in order to aflix the flexible carrier to the
`substrate and to provide optical correction material for
`indicators 22.
`
`Touch control 10 includes an electronic control 30 having
`a high frequency line driver 32 as a source for producing a
`high frequency pulsed signal at 34, which is applied to
`portion 16a of touch pad 14 (FIGS. 4 and 5). This signal is
`capacitively coupled to portion 16b of touch pad 14 in order
`to produce a detection signal at 36. When there is no user .
`contacting the user contact pad 20 associated with touch pad
`14,
`the very high frequency signal at 34 is coupled to
`detection signal 36 without attenuation by the capacitance of
`the user’s body. This is illustrated as the initial portion (left
`side) of the waveform illustrated at 36. When, however, a
`‘user engages the user contact pad 20 associated with touch
`pad 14, the detection signal becomes attenuated to a lower
`8
`
`8
`
`

`

`5
`
`5,572,205
`
`6
`
`amplitude as illustrated in the second portion (right side) of
`the waveform illustrated at 36 in FIG. 4. In the illustrated
`embodiment, portion 16a and 16b provide a 10—30 picofarad
`(pf) coupling between the high frequency signal at 34 and
`the detection signal at 36. Accordingly, a peak voltage, for
`example, of between five (5) and twelve (12) volts produced
`by the high frequency line driver may develop a detection
`signal at 36 on the order of magnitude of 70 mv.
`The detection signal on line 36 is decoded by a peak
`detector 38, which produces an output 40, and an amplitude-
`responsive switch 42, which responds to the amplitude of
`output 40 in order to actuate appropriate indicators, power
`circuits and the like, as illustrated at 44. High frequency line
`driver 32 has a primary frequency that is at least equal to 150
`kHz. It has been discovered that, for primary frequencies of
`150 kHz and above, touch control 10 has increased immu-
`nity to cross-coupling between adjacent touch pad 14 due to
`liquids, such as water, on substrate 12. While it is believed
`that most
`frequencies above 150 kHz would provide
`improved liquid immunity, considerations of component
`configurations of control system 30 would suggest a prac-
`tical upper limit of approximately 500 kHz on the primary
`frequency for line driver 32. In the illustrated embodiment,
`line driver 32 has a primary frequency of 250 kHz. In the
`illustrated embodiment, the output signal of high frequency
`line driver 32 is a square wave varying between zero volts
`and an upper limit, such as five volts. When coupled through
`portions 16a and 16b of touch pad 14, the detection signal
`at 36 is a square wave which oscillates equally at both
`polarities with respect to signal ground. Alternatively, high
`frequency line driver 32 could produce other waveforms,
`such as a triangle, sawtooth or sinusoidal waveform.
`
`Another advantage of utilizing a high frequency line
`driver as a source for control system 30 is that the waveform
`of the detection signal at 36 retains its original waveform
`notwithstanding the relatively small coupling capacitance
`between conductive elements 16a and 16b. Thus, the ten-
`dency of prior art systems to distort the waveform of the
`detection signal by coupling the higher frequency primary
`component to a noticeably greater extent than the lower
`frequency components of the square wave' is reduced
`because all frequency components are at or greater than the
`primary frequency, which is at a high frequency. This
`provides a more efficient coupling which is desirable to
`oifset the effects of the greater thickness of substrate 12.
`Furthermore, the output of peak detector 38, at 40, exhibits
`a relatively flat amplitude for both the attenuated and
`non-attenuated conditions of detection signal at 36. This
`improves the reliability of the system by increasing the
`distinction between touched and non-touched conditions of
`touch pad 14. Furthermore, in contrast to prior systems in
`which the detection signal at 36 is amplified prior to decod-
`ing, peak detector 38 is coupled directly with the detection
`signal at 36. Because, in the illustrated embodiment, peak
`detector 38 has a low gain, the gain/bandwidth product of
`the system accorrunodates the very high frequency of line
`driver 32 without distortion of the signal. The gain/band-
`width product, as is well understood to those skilled in the
`art,
`is a constant for each system and dictates that the
`frequency band multiplied by the gain of each amplifier
`cannot exceed a predefined constant without distorting the
`signal. The output 40 of peak detector 38, in contrast, is a
`relatively low frequency signal having a significant DC
`component. Accordingly, desirable amplification can be
`applied to output 40 by arnplitude—responsive switch 42
`without compromising the gain/bandwidth product of the
`system. In addition, the output 40 of peak detector 38 has a
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`limited amplitude range. Therefore, components utilized in
`peak detector 38 are not required to have a high slew-rate
`capability, which reduces the expense of such components.
`Although control system 30 has been described as it
`applies to an individual touch pad 14, the same principles are
`applicable to a keypad 15 composed of multiple touch pads '
`14 wherein known multiplexing techniques are utilized to
`apply high frequency line driver 32 and peak detector 38
`sequentially to touch pads 14a, 14b .
`.
`. 14n in a strobed
`fashion. Such a system is illustrated in FIG. 5 in which a 250
`kHz pulsed signal generator 46 has an output 47 that is
`applied to a demultiplexing circuit 48. Under the control of
`an output line 49 of a microcomputer 50, demultiplexer 48
`sequentially applies the output of signal generator 46 to
`driver lines 52a, 52b and 52c. Because of the sequential
`nature of the application of the drive signal to the drive lines,
`the drive signal appears as a burst of high frequency pulses
`on each of the drive lines 52a—52c. The pulse bursts, in turn,
`produce detection signals on sense lines 54a—54d, which are
`connected to different groups of touch pads 14a—l4n than
`the pads connected to driver lines 52a—52c. The sense lines
`54a—54d are provided as inputs to a multiplex circuit 56,
`which is under the control of microcomputer 50, via line 55,
`in order to synchronize multiplex circuit 56 with the opera-
`tion of the demultiplex circuit 48. Because multiplex circuit
`56 is a switching device, which sequentially applies each
`sense line 54a—54d to detection signal line 57 connected
`with peak detector 38, the peak detector is directly coupled
`sequentially with each of the sense lines 54a—54d. Touch
`pads 14a .
`.
`. 14):, which are sequentially coupled to signal
`generator 46 and to decoding circuit 38, are thereby multi—
`plexed with the signal generator and the decoding circuit.
`Peak detector 38, in the illustrated embodiment, is com-
`posed of an operational amplifier 58 whose output 59 is
`connected by a diode 60 with its inverting input 61. Diode
`60 is also connected, through a resistor 63, with output 40.
`Detection signal line 57 is connected with the non-inverting
`input of amplifier 58. Detection signal line 57 is additionally
`connected to signal ground through a pulldown resistor 62.
`This configuration is a low gain peak detector because the
`direct connection feedback provides amplifier 58 with a
`unity gain. Alternatively, diode 60 could be replaced with a
`short circuit and a diode connected between output 59 and
`resistor 63. Output 40 is filtered by a filter 64 in order to
`complete the peak detect function of peak detector 38. Filter
`64 is composed of a parallel combination of resistor 66 and
`capacitor 68 connected between output 40 and signal
`ground.
`In the illustrated'embodiment, pull-down resistor 62 is a
`10 kohm resistor. This provides very low input impedance to
`peak detector 38 which, advantageously,
`imparts excep-
`tional static electricity resistance to touch control 10. Static
`charges applied to substrate 12 are rapidly dissipated
`through resistor 62. This low input impedance is possible
`because of the exceptional signal strength of the detection
`signal as a result of the very high frequency primary
`component of the drive signal, as well as the equivalent
`resistance/capacitance value (RC) of this portion of the
`circuit.
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`Output 40 is provided to the non-inverting input of an
`amplifier 70. A biasing network composed of resistors 72a
`and 72b connected in a conventional fashion between an
`inverting input of amplifier 70 and its output 88 establishes
`the gain of amplifier 70. As previously described, because
`the signal on output 40 of peak detector 38 has a relatively
`low frequency content, the gain of amplifier 70 may be set
`at a relatively high level without exceeding the gain/band-
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`width product of the system and without creating high slew
`rates. In the illustrated embodiment,
`the voltage gain of
`amplifier 70 is between 80 and 100.
`In order to detect the capacitance of a user contacting one
`of the touch pads 14a—14n, output 88 of amplifier 70 is
`applied to an amplitude-responsive switching circuit, which
`in the illustrated embodiment is a successive approximation
`register 74 controlled with microcomputer 50. Successive
`approximation register (SAR) 74 provides a highly accurate
`means to allow microcomputer 50 to determine relative
`amplitude of output 88 of amplifier 70. SAR 74 includes a
`resistance network 76, which is composed of a ladder of
`resistances which vary from each other in multiples of two.
`Therefore, such network is referred to as an RZR network.
`The RZR network 76 is utilized by microcomputer 50 in
`order to produce an analog signal at 78 as a function of the
`combination of output lines 75 actuated by the microcom-
`puter. The analog signal at 78 is scaled by an amplifier 80
`whose scaling factor is established by bias resistors 82a and
`8212 connected in conventional fashion with respect to its
`output 84 and inverting input. Output 84 is connected with
`the inverting input of a comparator 86 whose non-inverting
`input is connected with output 88. The output of 90 of
`comparator 86 is provided as an input to microcomputer 50.
`SAR 74 operates as follows. In order to determine the
`relative analog voltage at output 88, microcomputer 50
`actuates a combination of output lines 75 representative of
`a known relative analog voltage at output 84 of scaling
`amplifier 80. Comparator 86 will compare this analog volt—
`age at 84 with the analog voltage at output 88 of amplifier
`70. Output 90 of comparator 86 will assume one of two
`alternate states depending upon whether the approximate
`voltage produced at output 84 is greater than, or less than,
`the output voltage 88 of amplifier 70. Microcomputer 50
`interprets the state of output 90 in order to adjust the states
`of output line 75 and vary the analog voltage at 84 until it
`is substantially equal to the analog voltage at 88. Micro-
`computer 50 then makes a determination whether such
`analog voltage represents a touched or non-touched condi—
`tion for the associated touch pads 14a—14n. Such determi-
`nation is made in software resident in microcomputer 50 in
`the manner disclosed in U.S. Pat. No. 5,189,417 issued to
`David Caldwell and Nicholas Medendorp for a DETEC-
`TION CIRCUIT FOR MATRIX TOUCH PAD, the disclo-
`sure of which is hereby incorporated herein by reference,
`except
`that relative amplitude is used in circuit 30 to
`determine a touch/no touch condition rather than relative
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`pulse-width used as in the ’417 patent. Microcomputer 50
`may then actuate the appropriate elements of indicator 22,
`power relays or the like at output 44 depending upon the
`responses programmed within the microcomputer for vari-
`ous combinations of input selections by the user contacting
`touch pads 14a—14n.
`Successive approximation registers (SARs) are well
`known in the art. The advantage of such an SAR is that it
`provides a higher resolution detection of the relative analog
`voltage at output 88 of amplifier 70 than would be possible
`by coupling output 88 directly to an analog input port of
`microcomputer 50. However, in certain applications, it may
`be possible to read the analog level of output 88 of amplifier
`70 at an analog input port to microcomputer 50. In the
`illustrated embodiment, SAR 74 is capable of reading the
`relative analog voltage level at output 88 to an accuracy of
`nine bits. In contrast, present commercially available micro-
`processors that are capable of eight-bit accuracy or greater,
`in a determination of analog input voltages, are relatively
`expensive.
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`The unique principles of the present invention allow the
`application of touch control
`technology directly to the
`unique environment of a large horizontal substrate of the
`type found in smooth cook tops and the like. The hurdle of
`the use of a substrate that not only has an exceptionally large
`thickness but also a modulated rear surface is overcome