United States Patent (19)
`Binstead
`
`54 MULTIPLE INPUT PROXIMITY DETECTOR
`AND TOUCHPAD SYSTEM
`
`76 Inventor: Ronald Peter Binstead, 15 Seely Road,
`Radford, Nottingham, United Kingdom,
`GB NG7 1NU
`
`Notice:
`
`This patent is Subject to a terminal dis
`claimer.
`
`Appl. No.:
`09/179,489
`Oct. 27, 1998
`Filed:
`Related U.S. Application Data
`63 Continuation of application No. 08/718,356, Oct. 3, 1996,
`Pat. No. 5,844,506, which is a continuation of application
`No. PCT/GB95/00767, Apr. 5, 1995.
`Foreign Application Priority Data
`30
`Apr. 5, 1994 GB
`United Kingdom ................... 94O6702
`(51) Int. Cl. ............................................... H03K 17/94
`52 U.S. Cl. .............................. 341/33; 341/34; 34.5/173;
`345/174
`Field of Search .................................. 341/33, 34, 20;
`34.5/173, 174
`
`58
`
`56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`USOO6137427A
`Patent Number:
`11
`(45) Date of Patent:
`
`6,137,427
`*Oct. 24, 2000
`
`5,459,463 10/1995 Gruaz et al. .............................. 341/33
`
`FOREIGN PATENT DOCUMENTS
`0078676A 5/1983 European Pat. Off..
`0609021A 8/1994 European Pat. Off..
`3910977A 10/1990 Germany.
`
`Primary Examiner Michael Horabik
`Assistant Examiner Timothy Edwards, Jr.
`Attorney, Agent, or Firm-Cahill Sutton & Thomas, P.L.C.
`57
`ABSTRACT
`A touchpad is formed of an electrically insulating membrane
`(10) with a first Series of spaced apart conductors (12) on a
`first face of membrane (10) and a Second Series of Spaced
`apart conductors (14) on or proximal thereto, in which there
`is no electrical contact between the first and Second Series of
`conductors (12, 14) Each conductor in the first and Second
`Series of conductors is Sensitive to the proximity of a finger
`to modify the capacitance of the proximate conductor to
`detect the presence of the finger positioned close to that
`conductor. A Scanning System operative to Sample one of the
`conductors in turn from both the first and second series of
`conductors (12, 14) in order to measure and store a capaci
`tance value associated with that respective conductor. The
`Scanning System is operative to maintain all conductors
`(12-n, 14-n) at a common potential equal to the potential of
`the conductor being Sampled when the remaining conductors
`are not actively being Sampled by the Scanning System.
`
`4,686,332 8/1987 Greanias et al. .......................... 178/19
`
`25 Claims, 9 Drawing Sheets
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`U.S. Patent
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`Oct. 24, 2000
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`Sheet 1 of 9
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`1
`MULTIPLE INPUT PROXIMITY DETECTOR
`AND TOUCHPAD SYSTEM
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`6,137,427
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`2
`the use of pressure on two conducting elements at an
`interSection of those conducting elements. Pairs of conduct
`ing elements may be Scanned in Systematic manner to
`determine which, if any, interSection has been pressed.
`Disadvantages of this System are that there are moving parts
`(e.g. the upper Surface presented to the user's finger) which
`can therefore be Subject to damage, and also that the
`positioning of the user's finger must coincide with the
`conducting element interSection. This method employs a Set
`of driver conductors and a set of Sensing conductors.
`The present invention, however, uses only Sensing con
`ductors and has no moving parts. It can thus be well
`protected from damage by users by the glass or other
`dielectric medium covering the touchpad. The electronic
`Scanning of the conducting elements requires connection to
`only one element at a time, and all other elements can be
`placed in condition to reduce interference when not being
`Scanned. The present invention further permits detection of
`the user's finger at any point on the touchpad's active
`Surface, and the electronic Scanning mechanism could be
`arranged to assign predetermined areas of the touchpad to be
`interpreted as discrete keypads, or “boxes'.
`Of fundamental importance to Such a touchpad System is
`the Sensitivity of the apparatus to the proximity of a finger
`when compared with normal variations in capacitance. This
`ensures reliable indication of an intentional “keystroke' as
`previously described, and the determination with a high
`degree of accuracy of the position of that finger. The position
`of the finger may be a digital representation of which “box”
`or predetermined area of the touchpad has been activated
`from a set of possible boxes, or predetermined areas, or
`alternatively an analogue representation of the position by,
`for example, X-y coordinates.
`The present invention is further directed towards the
`achievement of this required Sensitivity, by a number of
`alternative embodiments which may be used Separately or in
`conjunction with one another.
`Applications of Such a touchpad are many and diverse,
`for example:
`as a touchscreen interface for a computer System the
`keyboard being located immediately in front of a
`display unit which may be, for example, a cathode ray
`tube or liquid crystal display;
`a cash till keypad, where there would normally be many
`buttons for Specific or different types of merchandise
`(the present invention is particularly Suited to this
`application where the till operator is likely to have dirty
`or greasy hands, Since the invention can provide a
`Smooth glass for the keypad Surface which is easily
`wiped clean);
`as an equivalent to a “mouse’ input device to a computer
`System where the Screen cursor is moved by moving
`one's finger acroSS the Surface of a touchpad;
`as a Standard layout keyboard for use in a hostile envi
`ronment,
`as many discrete proximity Sensing keys.
`In the environment of a cathode ray tube, or other Static-or
`interference-generating device, it may be necessary to pro
`tect the touchpad from Such Static by known means, for
`example a transparent earthing Shield. Alternatively, an
`actively driven back plane may be used.
`In a general aspect the present invention provides a
`multiple input proximity detector in which the juxtaposition
`of two or more independent Sensor inputs are used to
`determine the proximity of a finger, Such detection only
`being accepted as valid when all the Sensor inputs indicate
`
`The present application is a continuation of an applica
`tion entitled “MULTIPLE INPUT PROXIMITY DETEC
`TOR AND TOUCHPAD SYSTEMS", filed Oct. 3, 1996 and
`assigned Ser. No. 08/718,356, now U.S. Pat. No. 5,844,506
`which is a national application based upon British PCT
`Application entitled “MULTIPLE INPUT PROXIMITY
`DETECTOR AND TOUCHPAD SYSTEM”, assigned Ser.
`No. PCT/GB95/00767, filed Apr. 5, 1995, claiming priority
`to a British application entitled “MULTIPLE INPUT
`PROXIMITY DETECTOR AND TOUCHPAD SYSTEM”,
`filed May 4, 1994 and assigned Ser. No. 9406702.2, all of
`which applications describe and claim inventions made by
`the present inventor.
`The present invention relates to a multiple input proX
`imity detector/touchpad System which may comprise, for
`example, a keypad array, digitising tablet, touchscreen or an
`electronic mouse which may be operated through a variable
`thickness of glass or other dielectric medium, and more
`particularly to the design of a multiple input proximity
`detector/touchpad System in which a large matrix of keys or
`a large touch Sensitive area may be formed using the
`Superposition of, for example, orthogonally arranged con
`ducting elements. The conducting elements, and the elec
`tronic Scanning System to Service the conducting elements
`are particularly arranged to obtain optimized Sensitivity.
`In European Patent No. EP-0185671 there is described a
`touch operated keyboard for attachment to one face of a
`sheet of glass comprising a plurality of keypads disposed
`adjacent each other in a desired pattern, together with
`interrogation means for assessing the condition of the
`keypads, indicating when a keypad, or keypads have been
`operated by a user, and an electronic Scanning and proceSS
`ing System for providing means for threshold value genera
`tion and drift compensation.
`The threshold value generation means is operative to
`determine repeatedly at predetermined intervals the required
`capacitance level associated with any keypad in order to
`indicate that that keypad has been operated by a user.
`The drift compensation means is operative to offset
`variations in capacitance caused by varying background
`conditions.
`The present invention is directed towards the construc
`tion of a multiple input proximity detector/touchpad System,
`which may comprise a keypad array, digitising tablet, touch
`Screen or an electronic mouse, wherein the position of a
`user's finger or other object touching, or in close proximity
`to the "touch sensitive' Surface area, hereinafter referred to
`as a touchpad, is determined by means of the capacitive
`effect of that finger on multiple conductor elements
`(hereinafter referred to as a keystroke), and to the optimi
`sation of Sensitivity of the touchpad, particularly when the
`touch Sensitive area becomes relatively large. It is intended
`that throughout the present Specification, reference to a
`“finger' is intended to include any object that would exert
`Sufficient capacitive influence to be detected by the touch
`pad.
`It should be noted that the activation of a “keypad” or
`area of the touchpad can be achieved without pressure on, or
`even without physical contact with, the Surface of the
`touchpad, although in normal mode of operation, the user's
`finger would contact the touchpad Surface or a Surface
`asSociated therewith.
`Other known types of touchpad, Such as membrane
`Switches having two Sets of conductorS face to face, require
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`3
`a valid detection, where Such inputS may be juxtaposed next
`to a range of other inputs in unique combination Such that
`when any one combination gives a true detection for all its
`individual inputs, a unique valid detection is determined.
`According to the present invention there is provided a
`touchpad comprising an electrically insulating membrane
`with a first Series of Spaced apart conductors on a first face
`of the membrane and a Second Series of Spaced apart
`conductors on or proximal thereto, in which there is no
`electrical contact between the first and Second Series of
`conductors, each conductor in Said Series being Sensitive to
`the proximity of a finger to modify the capacitance of Said
`conductor to detect the presence of Said finger positioned
`close to that conductor. Preferably the first and second
`conductors comprise fine wires preferably of a size between
`10 to 25 microns to be substantially invisible when the
`touchpad is used as a touchscreen.
`Embodiments of the present invention will now be
`described by way of example with reference to the accom
`panying drawings in which:
`FIG. 1 shows in plan view a touchpad according to the
`present invention;
`FIGS. 2a, 2b and 2c show in alternative embodiments
`croSS-Sections through the touchpad of FIG. 1, not to Scale;
`FIGS. 3a and 3b show embodiments of intersection points
`of two conducting elements,
`FIG. 4 shows in plan view an embodiment of the present
`invention Suitable for a large area touchpad with multiple
`conducting elements,
`FIG. 5 shows in cross-section an embodiment of the
`present invention in which connections can be made
`between conducting elements,
`FIG. 6 shows a Small part of a touchpad Surface;
`FIG. 7 shows a part of a touchpad Surface indicating an
`embodiment of the invention in which multiple keypad areas
`are assigned to each interSection;
`FIG. 8 shows schematically an embodiment of scanning
`apparatus Suitable for use with the touchpad;
`FIG. 9 shows a pattern of conductor elements suitable for
`use in the Style of a Standard typewriter keyboard layout; and
`FIG. 10 shows a pattern of conductor elements demon
`Strating one embodiment of a multiplexed touchpad.
`With reference to FIGS. 1 and 2a, and according to one
`embodiment of the invention, there is provided a thin
`dielectric film 10 on which is deposited on one face by an
`appropriate technique Such as Screen printing or Similar
`lithographic process, a pattern of electrically conducting
`material forming a first Series of parallel conductor elements
`12 with appropriate connections at one or both ends. On the
`other face of the thin dielectric film 10, by a similar
`technique, there is provided a pattern of electrically con
`ducting material forming a Second Series of parallel conduc
`tor elements 14 with appropriate connections at one or both
`ends which are orthogonal to, but not in electrical contact
`with the first series. The first and second series of conductor
`elements thus form a plurality of interSections 20. Appro
`priate material for these conductor elements 12, 14 is, for
`example, Silver-based conducting ink. If the conductor ele
`ments are to be of low visibility where the touchpad is being
`used in front of a display System, then indium oxide is an
`appropriate material.
`In other embodiments, the first and Second Series of
`conductor elements need not be parallel, nor is it necessary
`for the first and Second Series of conductor elements to be
`mutually orthogonal. The Second Series of conductor ele
`ments may be deposited onto a Second thin dielectric film,
`the Second film being Superimposed on the first dielectric
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`film in order to achieve similar effect of separation of the
`first and Second Series of conductor elements by a dielectric
`layer.
`It is also possible to effect the Superposition of the
`conducting elements in other ways. For example, in FIG.2b
`the first Series of conductor elements 12 may be deposited
`onto the thin dielectric film 10 and an insulating layer 13
`deposited thereupon. The Second Series of conducting ele
`ments 14 may then be deposited over the insulating layer.
`Thus, the insulating layer 13 forms a membrane Structure
`between the first and Second Series of conducting elements.
`The insulating layer 13 need not, however, be continuous
`over the entire touchpad Surface: it is only necessary to
`insulate the interSections of the first and Second Series of
`conductor elements. In FIG. 2c, this arrangement is shown,
`where Small regions of insulating material 13' are deposited
`over the first Series of conducting elements 12 at the pro
`posed interSection points. The Second Series of conductor
`elements 14 may then be deposited. In this instance, the
`Small regions of insulating material 13" in conjunction with
`the dielectric film 10 form a membrane Structure Separating
`the first and Second Series of conductor elements.
`Connection to the conductor elements 12 and the conduc
`tor elements 14 is made by further conducting elements 32,
`34 respectively deposited and/or defined in Similar manner
`to conductor elements 12, 14. A connection to the touchpad
`Scanning System is made by connector 30 using an appro
`priate connection System.
`In the embodiments of FIGS. 1 and 2a-2c, the width 16
`of the conductor elements 12 and 14 is small compared with
`the inter-element spacing 18. If the conducting material
`being used to form conductor elements 12, 14 is of low
`conductivity, an alternative pattern of conductor element
`may be used as described later.
`In another embodiment, the inter-element spacing 18 need
`not be identical for each adjacent pair of conductor elements.
`The sensitivity of the touchpad and its immunity to
`extraneous interference has been found to be enhanced by
`the encapsulation of the dielectric film 10 and conducting
`elements 12, 14, 32, 34 in a dielectric laminate 50, as shown
`in FIGS. 2a-2c The dielectric laminate may be a plastic film,
`and can be formed using well known techniques Such as heat
`Sealing. This provides a constant dielectric environment in
`the immediate proximity of the conductor elements, elimi
`nates the influence of moisture which might otherwise be
`present on the conductor elements, and further improves the
`robustness of the apparatus.
`High Sensitivity to changes in capacitance of a conductor
`element or group of conductor elements caused by the
`proximity of a finger or other object is achieved by mini
`mising the cross-coupled capacitance between the conductor
`elements 12 and the conductor elements 14. This can be
`achieved in one embodiment by the use of highly conductive
`material (Such as Silver) and the forming of conductor
`elements which have a very narrow width 16 when com
`pared to the conductor Spacing 18 as previously described,
`Such that the capacitance of the interSections 20 is Small. In
`the event that it is desirable that a lower conductivity
`material be used (e.g. indium oxide), or that the dimensions
`of the touchpad become Sufficiently large Such that there is
`Substantial resistance along a conductor element, then alter
`native patterns may be considered Such as those embodied in
`FIGS. 3a and 3b.
`In FIG. 3a, where the conductor elements have a more
`Substantial width 22, at the intersections 20 the width 24 is
`greatly reduced.
`In FIG. 3b, the conductor elements 12 and 14 maintain
`full width 22, but the second conductor element 14 has a
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`“window” area 28 which has no conductive material. This
`“window” allows the necessary capacitive link to the first
`conductive element 12. The window area 28 need not be
`completely open. AS indicated by dotted line 29, an area of
`conductor material electrically isolated from the Second
`conductor element 14 can in fact be left within the window
`28 and still provide the necessary capacitive link to the first
`conductor element 12.
`The relative thicknesses of the conductor elements thus
`can be varied to Suit the conductivity of the material being
`used, the length of the tracks, and other constraining factors.
`It is noted that the Smaller width tracks can result in better
`resolution and higher speed of operation, but use of the
`wider tracks can be acceptable for lower resolution, leSS
`Sensitive or slower requirements.
`In another embodiment, the Sensitivity to changes in
`capacitance caused by the proximity of a finger or other
`object to a large area touchpad is enhanced by connecting
`Several conductor elements 12, 14 together in groups as
`embodied in FIG. 4. This particular embodiment is preferred
`where the required positional resolution of a keystroke can
`be compromised in favour of an increased area of touchpad.
`This particular embodiment conferS upon the apparatus the
`additional benefit that damage to one of the conductor
`elements 12 or 14 causing a break in that element does not
`affect the performance of the System, provided that the
`connection of each group of elements at both ends is made,
`as shown in the embodiment of FIG. 4. If a fine wire is used
`to detect a large area then the wire should be Zig-Zagged over
`that area. The wire could be Zig-Zagged with /4-/5th of an
`inch spacing.
`If required, the conductor elements can be electrically
`connected to elements on the opposite face of the dielectric
`film 10 by the provision of appropriately placed holes 36 in
`the dielectric film as shown in FIGS. 4 and 5, filled with
`conductive material through which, for example, conductor
`element 12 is connected to conductor element 32 in order
`that connector 30 is only required to make contact to one
`face of the dielectric film 10. Such a system may also be
`used to form “underpasses” for the conductor elements if
`required on particularly complex conductor patterns. These
`“underpasses' may be used to effect the interSection points
`of the first series of conductor elements 12 and the second
`Series of conductor elements 14.
`It is further noted that where conductor elements are used
`which have Significant resistance along the length thereof, it
`is possible to minimise the impact this has by providing
`conductor elements 32.34 to contact both ends of conductor
`elements 12.14 respectively. It is further possible to provide
`conductor elements 32.34 in high-conductivity material, and
`conductor elements in the lower conductivity material, the
`elements being coupled together in known manner.
`In a particular embodiment of this invention the required
`Sensitivity of the System to changes in capacitance on any
`given element is enhanced by ensuring that all of the
`conductor elements 14-1 . . . 14-n and 12-1 . . . 12-n are
`maintained at the same potential (for example ground
`potential, or V,
`hereinafter referred to as "ground
`potential”) except for the conductor element being Sampled.
`The grounding of all conductor elements not being Sampled
`greatly reduces the effect of Stray capacitance from other
`parts of the touchpad on the element being Sampled, thus
`providing a more reliable measure of any capacitive change
`that may have taken place on the conductor element being
`Scanned.
`An appropriate System for Scanning keyboards, Such as
`that described in European Patent No. 0185671 is readily
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`applicable with Some modification to this touchpad. In one
`particular embodiment as shown in FIG. 8, each of conduc
`tor elements 12-1 to 12-4, and 14-1 to 14-3 is connected at
`one end to a resistor 71 having a high value such as 100
`kohms when compared with the impedance of the detection
`circuit, for example 10 kohms. (The particular values of
`resistance used are exemplary, and may be Substantially
`varied according to the configuration of the System.) Each of
`the resistorS is connected to, for example, ground potential.
`The other end of each of conductor elements 12-1 to 12-4,
`and 14-1 to 14-3 is connected in turn via analogue multi
`plexer 75 to Output line 72.
`Where there is significant resistance along the length of
`the conductor elements 12 and 14, improvements in the
`performance of the detection System can be achieved by the
`placing of the resistorS 71 at the opposite Sides of the
`conductor elements 12 and 14 to that shown in FIG. 8. In
`other words, the resistors are placed at the multiplexer 75 of
`the touchpad and connected to ground or an active ground as
`hereinafter described.
`Output line 72 is connected to the input of a capacitance
`controlled oscillator 85, the output of which is connected to
`a divide-by-n circuit 90, which provides the data output on
`line 92. An indexing counter 80, clocked by a remote clock
`on line 82, is operative to control the analogue multiplexer,
`and to reset capacitance controlled oscillator 85 and divide
`by-n circuit 90. A processing means, not shown, is operative
`to receive the data from divide-by-n counter on line 92, and
`Store it in a plurality of locations, each allocated to a
`particular one of the conductor elements 12 and 14. Divide
`by-n circuit 90 and other components Such as indexing
`counter 80 could be provided by means of a suitable
`Standard microprocessor.
`The Scanning System thus Samples each conductor ele
`ment in turn according to the analogue multiplexer
`Sequence, and Stores each capacitance value in memory.
`These values are compared with reference values from
`earlier Scans, and with other capacitance values in the same
`Scan from the other conductor elements in order to detect a
`keystroke. Keystrokes must be above a threshold value to be
`valid. By having several threshold values it is possible to
`determine the pressure of key preSS or distance that the
`finger is away from the key. This may be useful, for
`example, when moving a cursor acroSS a Screen and then
`making a Selection by pressing harder on a Selected point.
`The remaining features of the Scanning mechanism are
`well described in the cited document and will not be
`discussed further here.
`Detected changes in capacitance on more than one con
`ductor element in any one Scanning Sequence enables inter
`polation of a keystroke between those conductor elements.
`In the two dimensional case, as shown in FIG. 6, conductor
`element 14-3, and conductor element 14-4 cross conductor
`elements 12-1 and 12-2. A finger or other object at position
`40 can be determined in the X-direction by the relative effect
`on the capacitance of element 14-3 compared with element
`14-4, and in the Y-direction by the relative effect on the
`capacitance of element 12-1 compared with element 12-2. In
`a typical application, conductor elements 12-1,12-2. . . 12-n
`and 14-1,14-2 .
`.
`. 14-n will be sampled by the scanning
`System in a Sequential manner. Clearly, the same applies to
`the embodiment of the touchpad where the conductor ele
`ments are arranged in groups where the interpolation is
`made between the centre line 45 of each group of conductor
`elements (FIG. 4).
`It will be clear that the interpolation technique enables not
`only an analogue representation of finger position on the
`
`PANASONIC EX1015, page 013
` IPR2021-01115
`
`

`

`7
`touchpad to be created, but also allows the use of an
`increased number of “boxes” or predetermined key areas
`60.61 over the number of element intersections, as indicated
`in FIG. 7. Such “boxes” or keypad areas could be arranged
`in any number of configurations capable of being resolved
`by the system.
`In an alternative embodiment, an active backplane may be
`incorporated into the touchscreen. For example, a plastic
`sheet upon which is coated a conductive film is laminated to
`the touchscreen. The output 72 is connected to a unity gain
`non-inverting amplifier 73. The output of this amplifier 73 is
`connected to the backplane conductor (not shown) which
`may cover all or part of the keypad. The backplane will be
`active Since the Voltage thereon will vary with the output on
`line 72.
`The backplane could also extend to areas in front of the
`keypad to "shield' keys which are non-operative.
`The backplane potential thus created could also be Suit
`ably connected to maintain all conductors 12-n, 14-n which
`are not actively being Sampled at a common potential equal
`to the active backplane potential rather than the common
`ground potential as previously described herein.
`This can in certain uses of the touchscreen eliminate the
`requirement for a completely conductive backplane film.
`In FIG. 9 there is shown an example of an appropriate
`pattern of elements for Simulating a keypad configuration
`Such as that normally associated with a typewriter keyboard.
`This particular embodiment comprises the horizontal con
`ducting elements 12, Vertical conducting elements 14, con
`ducting elements 32, 34 for connection to connector 30 in
`similar manner to the embodiments described with reference
`to the FIGS. 1 and 2. The sensitivity of the system can be
`further enhanced by the addition of further conducting
`elements 42, 44; elements 42 being in electrical connection
`with conducting elements 12, and elements 44 being in
`electrical connection with conducting elements 14, elements
`42 being positioned such that centre of a box defined by the
`elements 42 is Superimposed on the centre of a box defined
`by elements 44, the elements 12, 42 being on one face of the
`thin dielectric film 10, and the elements 14, 44 being on the
`other face of the thin dielectric film 10. The separate
`elements 42, 44 are indicated schematically to the side of the
`drawing of FIG. 9.
`AS indicated earlier, the first and Second Series of con
`ductor elements 12 and 14 need not be deposited on opposite
`faces of the same dielectric membrane, but might be depos
`ited on Separate dielectric membranes, with Said membranes
`being Superimposed one on the other. This principle may be
`extended to include a plurality of membranes, each having
`a separate pattern of conductor elements. These could, for
`example be PCB's (printed circuit boards) of known type.
`The conductor elements 12,14.32 and 34 could be formed
`from fine conducting wires which would preferably be
`insulated by, for example, an enamel coating. The wires
`12, 14 could be allowed to touch at intersections 20, electri
`cal contact being prevented by the insulating coating.
`Alternatively, the wires could be arranged on either side of
`a Suitable membrane for mounting purposes. The wires may
`be from 10 to 25 microns in diameter thereby being invisible
`to the naked eye when the invention is used as a touchscreen.
`In a further embodiment of the present invention, particu
`larly an embodiment including, for example, a plurality of
`membranes having conductor elements thereon, multiplex
`ing techniques can be used. Duplicate Sets of N Small
`touchpads are arranged to form a large touchpad array. This
`array is Superimposed upon a larger touchpad with M keys
`(M could be equal to N). The position of a finger or other
`
`15
`
`25
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`6,137,427
`
`8
`object proximate to the first touchpad is interpretable by the
`System as a key Stroke in any one of N possible positions.
`The Second, larger grid pattern is used to determine which of
`the M possible duplicate key pads has been touched,
`enabling unambiguous determination of the position of the
`finger.
`With reference to FIG. 10, there is a first grid pattern
`comprising repeating pattern of elements Ato D and W to Z,
`that is to Say that all A elements are electrically connected,
`all B elements are electrically connected, and So on. It is thus
`apparent that there will be four first grid horizontal connec
`tions A,B,C,D, and four first grid vertical connections W.X.,
`YZ. A Second grid is placed directly over the first grid, the
`Second grid having four horizontal elements with four
`connections a,b,c,d, and four vertical elements with four
`connections w.x.y.z. A finger placed at the position marked
`with a square on the first grid will be indicated by the first
`grid as interference with elements A and Z. Such interfer
`ence would be the same for Sixteen positions on this grid, but
`the Second grid will indicate interference with elements c
`and b, with c Stronger than b, and interference with elements
`X and y, with X Stronger than y. This enables unique
`determination of the position of the interfering object. It is
`readily apparent that 256 positions can thus be resolved by
`just 16 electrical connections. If interpolation techniques are
`used, more than 256 positions can be resol