Page 1 of 22
`
`(cid:58)(cid:76)(cid:81)(cid:87)(cid:72)(cid:78) (cid:40)(cid:91)(cid:75)(cid:76)(cid:69)(cid:76)(cid:87) (cid:20)(cid:19)(cid:20)(cid:20)
`
`

`

`Patent Application Publication
`
`Oct. 4, 2007 Sheet 1 of 12
`
`US 20071'0229469 A]
`
`
`
`FIG. 1
`
`Page 2 of 22
`Page 2 of 22
`
`

`

`Patent Application Publication
`
`Oct. 4, 200’? Sheet 2 of 12
`
`US 2007/0229469 A1
`
`
`
`FIG. 2
`
`Page 3 of 22
`Page 3 of 22
`
`

`

`Patent Application Publication
`
`Oct. 4, 2007 Sheet 3 of 12
`
`US 2007l0229469 A1
`
`300
`
`\
`
`Varying Capacitance Sensor
`Element
`
`
`
`Adiacent Plate
`Capacitor with Shunt
`
`FIG. 3A
`
`Floating
`Ground
`
`Capacitive Sensor
`Element 300
`
`Finger
`303
`
`
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`Insulator
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`304
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`
`Page 4 of 22
`Page 4 of 22
`
`

`

`Patent Application Publication
`
`Oct. 4, 2007 Sheet 4 of 12
`
`US 2007/0229469 A1
`
`
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`namcmficammxm:
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`Page 5 of 22
`Page 5 of 22
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`

`Patent Application Publication
`
`Oct. 4, 200'? Sheet 5 0f 12
`
`US 2007l0229469 A1
`
`Hexagonal Shaped
`Sensor Element
`
`501
`
`
`
`
`
`Gaps 509/
`
`Distance
`
`409
`
`Hexagonal Shaped
`Sensor Element
`
`503
`
`Distance
`
`Octagonal Shaped
`Sensor Element
`
`551
`
`Gaps 559
`
`409
`
`
`
`
`/ /
`
`Octagonal Shaped
`Sensor Element
`
`553
`
`
`
`Distance
`
`409
`
`FIG. 43
`
`Page 6 of 22
`Page 6 of 22
`
`

`

`Patent Application Publication
`
`Oct. 4, 2007 Sheet 6 of ]2
`
`US 2007/0229469 A1
`
`Sensor
`
`Sensor
`Element
`
`Element
`50300
`
`503(1)
`
`Sensor
`Elemenl
`
`501(l)
`
`
`
`
`Row 1
`504( I)
`
`505(1)
`
`onducfive
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`Object
`303
`
`Column M
`
`505(M)
`
`x
`
`Column 1
`
`
`
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`
`
`
`
`Sensor
`)4
`Element Conductive
`
`Processing
`Device
`
`m
`
`Conductive
`Traces
`502
`
`Y
`
`
`
`FIG. 5A
`
`501(L)
`
`Traces
`502
`
`Page 7 of 22
`Page 7 of 22
`
`

`

`Patent Application Publication
`
`Oct. 4, 2007 Sheet 7 of 12
`
`US 2007/0229469 A1
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`FIG. 5D
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`Page 9 of 22
`Page 9 of 22
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`

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`Patent Application Publication
`
`Oct. 4, 200'? Sheet 10 of 12
`
`US 2007l0229469 A1
`
`Dome shaped touch
`sensor pad
`700
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`Hexagon shaped
`sensor elemem
`710
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`element
`"320
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`FIG. 7
`
`Page 11 of 22
`Page 11 of 22
`
`

`

`Patent Application Publication
`
`Oct. 4, 200'? Sheet 11 of 12
`
`US 2007l0229469 A1
`
`
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`Domeshaped[ouch
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`sensorpad 700
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`sensoreleme- 710
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`Page 12 of 22
`Page 12 of 22
`
`

`

`Patent Application Publication
`
`Oct. 4, 2007 Sheet 12 of 12
`
`US 2007/0229469 A1
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`

`

`US 2007:”0229469 A1
`
`Oct. 4, 200'?
`
`NON-PLANAR TOUCH SENSOR PAD
`
`RELATED APPLICATIONS
`
`[0001] The present application claims the benefit of US.
`Provisional Application No. 60t787.983 filed Mar. 31. 2006,
`hereby incorporated by reference.
`
`TECl-INICAL FIELD
`
`[0002] This invention relates to the field of user interface
`devices and. in particular, to a capacitive touch sense device.
`
`BACKGROUND
`
`[0003] Computing devices. such as notebook computers.
`personal data assistants (PDAs). and mobile handsets. have
`user interlace devices. which are also known as human
`interface device (1111)). One user interface device that has
`become more common is a touch—sensor pad. A basic
`notebook touchnsensor pad emulates the function of a perA
`sonal computer (PC) mouse. A touch-sensor pad is typically
`embedded into a PC notebook for built-in portability. A
`touch-sensor pad replicates mouse xt'y movement by using
`two defined axes which contain a collection of sensor
`
`eleinenls that detect the position of a conductive object. such
`as a finger. Mouse rightfleft button clicks can be replicated
`by two mechanical buttons. located in the vicinity of the
`touchpad. or by tapping commands on the touch-sensor pad
`itself. The touch-sensor pad provides a user interface device
`for performing such functions as positioning a pointer. or
`selecting an item on a display. These touch-sensor pads may
`include multi-dimensional sensor arrays for detecting move-
`ment
`in multiple axes. The sensor array may include a
`one—dimensional sensor array. detecting movement in one
`axis. The sensor array may also be two dimensional. detect-
`ing movements in two axes.
`[0004] A touch-sensor pad includes a sensing surface
`having sensing elements (also referred to as electrodes} on
`which a conductive object may be used to position a pointer
`in the to and y~axes. A consideration in the construct of a
`touch—sensor pad is the use of as much of the pad area as
`possible, since unfilled pad is wasted while sensing. One
`conventional shape for a sensor electrode that is suitable for
`increasing surface area of a pad is a circle. However. circular
`shaped electrodes do not eliiciently fill a sensor pad area.
`Seine conventional
`touch sensor pads employ diamond
`shaped electrodes or triangular shaped electrodes, as iIliIs~
`trated in FIG. 1 and FIG. 2 respectively. that have increased
`edge capacitance (represented conceptually by the capaci-
`tors between the triangle shaped electrodes in expanded
`view of FIG. 2) and decreased the sensor pad area. A
`decreased sensor pad area reduces the amount of copper or
`other conductive material with which alt activating elelnent.
`such as a finger. can make contact. Increased edge capaci-
`tance adds parasitic capacitance to the system and decreases
`the proportional change in capacitance when an activating
`element. such as a finger. comes in contact with the sensing
`area.
`
`[0005] Other shaped electrodes have also been described
`in references. such as US. Patent Application Publication
`20080097991. More specifically, U.S. Patent Application
`Publication describes that electrodes may be formed from
`simple shapes (cg, squares. circles. ovals. triangles. rect—
`angles. polygons. and the like) or complex shapes (cg.
`random shapes). U.S. Patent Application Publication states
`
`Page 14 of 22
`Page 14 of 22
`
`that the shapes of the electrodes are generally chosen to
`maximize the sensing area and. in the case of transparent
`electrodes. minimize optical difl'erences between the gaps
`and the transparent electrodes.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`invention is illustrated by way of
`[0006] The present
`example. and not by way of limitation. in the figures of the
`accompanying drawings.
`[0007]
`FIG. 1A illustrates a conventional touch-sensor pad
`having diamond shaped electrodes.
`[0008]
`FIG. 2 illustrates another conventional touch—sen—
`sor pad having triangular shaped electrodes.
`[0009]
`FIG. 3A illustrates how a conductive object may
`aliect the capacitance of a capacitive touch-sensing sensor
`element.
`FIG. 38 is a conceptual cross-section view of the
`[0010]
`capacitive sensor element 300 of FIG. 3A.
`[0011]
`FIG. 4A illustrates hexagonal shaped adjacent sen—
`sor elements within a sensor array according to one embodi-
`ment of the present invention.
`[0012]
`FIG. 48 illustrates two embodiments of the gaps
`between hexagonal and octagonal shaped sensor elements.
`[0013]
`1" 1G. 5A illustrates a top-side view of one embodi-
`ment of a sensor array having a plurality of hexagonal
`shaped sensor elements for detecting a presence of a con-
`ductive object on the sensor array.
`[0014]
`FIG. SB illustrates a block diagram of one embodi-
`ment of a capacitive sensor coupled to the sensor array of
`FIG. 5A.
`
`FIG. 5C illustrates a top-side view ol‘ one embodi-
`[0015]
`ment of a two-layer touch-sensor pad.
`[0016]
`FIG. 5D illustrates a cross section view of one
`embodiment of the twenlayer touch-sensor pad of FIG. SC.
`[0017]
`1" IG. 6 is a cross-sectional view illustrating a non-
`planar touch senor pad according to an alternative embodi-
`ment of the present invention.
`[0018]
`FIG. 7 is a perspective view illustrating a dome-
`shaped touch sensor pad.
`[0019]
`FIG. 8 is a two dimensional view illustrating the
`sensor elements of the dome-shaped touch sensor pad of
`FIG. 7 according to one embodiment of the present inven-
`tion.
`
`F IG. 9 illustrates a block diagram of one embodi-
`[0020]
`ment of an electronic system having a processing device and
`touch-sensor pad for detecting a presence of a conductive
`Object according to one embodiment of the present inven-
`tion.
`
`DETAILED DESCRIPTION
`
`[002]] Described herein is a method and apparatus for
`reducing charge time. and power consumption of sensor
`elements of a sensing device. such as a touch—sensor pad.
`touch—sensor slider. or a touch—sensor button. The following
`description sets forth numerous specific details such as
`examples of specific systems. components, methods, and so
`forth. in order to provide a good understanding of several
`embodiments of the present invention. It will be apparent to
`one skilled in the art, however. that at least some embodi-
`ments of the present invention may be practiced without
`these specific details. In other instances. well—known com—
`ponents or methods are not described in detail or are
`presented in simple block diagram format in order to avoid
`
`

`

`US 2007:”0229469 A1
`
`Oct. 4, 200'?
`
`the
`invention. Thus.
`unnecessarily obscuring the present
`specific details set forth are merely exemplary. Particular
`implementations may vary from these exemplary details and
`still be contemplated to be within the spirit and scope of the
`present invention.
`[0022] A touch sensor device having polygonal shaped
`sensor elements having five or more sides is described. The
`five or more sided polygonal shaped sensor elements of the
`touch sensor device may increase sensing element surface
`area while decreasing edge capacitance to yield greater
`packing efficiency and greater proportional capacitance
`change by an activating element. A decreased sensor area
`reduces the amount of copper or other conductive material
`with which an activating conductive object. such as a finger.
`can make contact. Increased edge capacitance adds parasitic
`capacitance to the device and decreases the proportional
`change in capacitance when an activating object. such as a
`linger. comes in contact with the sensing area.
`[0023]
`In one embodiment. the touch sensor deviCe has
`hexagonal shaped sensor elements that operate as capacitive
`sensor elements. The hexagonal shape of the sensor ele-
`ments increases the vertical capacitance of each of the
`sensor elements to the conductive object while not increas-
`ing the fringe. horizontal, capacitance of the sensor elements
`to each other. as is described in further detail below. The
`vertical capacitance is represented in the following equation:
`
`Aswan-covert?)
`(Ivrm'ral =—‘ vertical
`
`l l l
`
`The ratio of perimeter to area is given by the following
`equations for each of the following shapes:
`
`the sensor elements to the conductive object while not
`increasing the fringe, horizontal. capacitance. Similarly. the
`pentagon shape of the sensor element increases the vertical
`capacitance while not increasing the fringe capacitance. For
`example. with a unit area of l. a square or diamond has an
`area given by:
`.4-=.\2. where .\'-'-'-l.
`
`(9}
`
`Therefore. the perimeter of the square or diamond is given
`by:
`
`P—4X. where x'—l.
`
`:10]
`
`Assuming a unit area of l. the perimeter of the square or
`diamond is 4. resulting in an area to perimeter ratio of 0.250.
`
`In comparison. with a unit area of l. a pentagon has
`[0025]
`an area given by:
`
`..
`.‘
`A: ‘I-v15+10v'r5- .whcrex=0.76.
`
`llll
`
`Therefore. the perimeter of the pentagon is given by:
`P=5x. when: xeufffi.
`
`(12}
`
`Assuming a unit area of l, the perimeter of the pentagon is
`3.81. resulting in an area to perimeter ratio 0262. Also, with
`a unit area of l. a hexagon has an area given by:
`
`
`“(as ],r. where .t: = 0.62.
`
`{[3]
`
`’l‘lierefore. the perimeter of the hexagon is given by:
`P=étx. where x=t1.62.
`
`t14}
`
`Assuming a unit area of l, the perimeter of the hexagon is
`3.72. resulting in an area to perimeter ratio 0.270. Accord—
`ingly. the area to perimeter ratio of the hexagon and pent‘ —
`gon are lower than the area to perimeter ratio of the square
`or diamond. This change in ratio increases the vertical
`capacitance measured on a sensor element. For example. the
`capacitance variation measured on the sensor element may
`be as little as 0.1% of the parasitic capacitance of the sensor
`element. so by increasing the vertical capacitance while not
`increasing the fringe capacitance, the capacitance variation
`when a conductive object is present on the device may be a
`easier to detect and measure. As described above.
`the
`capacitance is directly proportional to area. Since the capaci-
`tance is directly proportional to area. an increase in the
`perimeter acts like an increase in area along the cross section
`by adding to one dimension. So increasing the perimeter
`increases the fringe capacitance. and increasing the area of
`the sensor element increases the signal capacitance.
`[0026]
`FIG. 3A illustrates how a conductive object may
`affect the capacitance of a capacitive touch-sensing sensor
`element. The conductive object
`in one embodiment
`is a
`linger. Alternatively. this technique may be applied to any
`conductive object. for example, a stylus. in its basic form, a
`capacitive sensor element 300 is a pair of adjacent plates
`(electrodes) 301 and 302. There is a small edge-to-edge
`capacitance CF. but the intent of sensor element layout is to
`minimize the base capacitance C.» between these plates.
`
`[2]
`
`3]
`
`E
`
`[4]
`
`[51
`
`to]
`
`‘18
`D'
`'
`rt _ .t
`iamon . quare. T’ — E
`
`.'
`A
`\ rr
`Pentagon: — =
`4““igl
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`EJ3—
`4
`
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`if +
`6x
`
`.l'
`
`la
`
`[‘I‘
`LP
`
`A
`Hexagon F =
`A.
`— =
`4Sin[£}"t
`3011: P
`It
`A
`0‘1 "
`I — = —
`Lagon P
`S
`
`where x is a unit of measurement.
`
`The fringe capacitance (without proportions) is given by:
`
`Antietamhnmrdnirr
`Cram.- =—drmr(
`where:
`
`Artur = Pill”
`
`l7]
`
`[81
`
`where h is the thickness of the sensor element.
`
`[0024] As described above. the hexagonal shape of the
`sensor elements increases the vertical capacitance of each of
`
`Page 15 of 22
`Page 15 of 22
`
`

`

`US 200W0229469 Al
`
`Oct. 4, 200'?
`
`When a conductive object 303 (e.g.. a linger] is placed in
`proximity to the two plates 301 and 302. there is a vertical
`capacitance between one electrode 301 and the conductive
`object 303 and a similar vertical capacitance between the
`conductive object 303 and the other elecu-ode 302.
`'lhe
`vertical capacitance between electrode 301 and the conduc-
`tive object 303 and the vertical capacitance between elec-
`trode 302 and the conductive object 303 add in series to
`yield a capacitance CF. That capacitance adds in parallel to
`the base capacitance Cp between the plates 30] and 302.
`resulting in a change of capacitance CF over the base
`capacitance. Capacitive sensor element 300 may be used in
`a capacitive sensor array where one electrode of each
`capacitor is grounded. Thus. the active capacitor (as con-
`ceptually represented in FIG. SB as CAP 413) has only one
`accessible side. The presence of the conductive object 303
`increases the capacitance (Cp+C F) of the capacitive sensor
`element 300 to ground. Determining sensor element activa-
`tion is then a matter of measuring the change in the capaci-
`tance (CF) or capacitance variation. Capacitive sensor ele-
`ment 300 is also known as a grounded variable capacitor. In
`one exemplary embodiment. Cp may range from approxi-
`mately 10-300 picofarads (pF). and Cf may be approxi-
`mately between 0.5%-3.0% of Cp. Alternatively. Cf may be
`orders of magnitude smaller than Cp. Alternatively. other
`ranges and values may be used.
`[0027]
`FIG. 3B is a conceptual cross-section view of the
`capacitive sensor element 300 of FIG. 3A. The capacitance
`generated by operation of capacitive sensor element 300
`may be measured by a processing device 210. as will be
`discussed in greater detail below. As previously described.
`when a conductive object 303 (e.g._. a finger) is placed in
`proximity to the conductive plates 301 and 302. there is an
`ell‘ective capacitance, C1-‘. between the plates and the con-
`ductive object 303 with respect to ground. Also. there is a
`capacitance. Cp. between the two conductive plates 301 and
`302. Accordingly. the processing device 210 can measure
`the change in capacitance. capacitance variation CF, when
`the conductive object is in proximity to the conductive plates
`301 and 302. Above and below the conductive plate that is
`closest to the conductive object 303 is an insulating dielec-
`tric material 304. The dielectric material 304 above the
`
`conductive plate 301 can be the overlay. as described in
`more detail below. The overlay may be non-conductive
`material (e.g.. plastic. glass. etc.) used to protect the circuitry
`from environmental conditions and to insulate the conduc-
`tive object (cg. the user’s finger) from the circuitry. In one
`embodiment. the conductive plates 301 and 302 may have a
`hexagonal shape and are referred to as sensor elements. as
`discussed below.
`
`FIG. 4A illustrates hexagonal shaped adjacent sen-
`[0028]
`sor elements within a sensor array according to one embodi-
`ment of the present invention. In this embodiment. the shape
`of sensor elements 501 and 503 is substantially hexagonal.
`The use a hexagonal shape for the senor elements 501 and
`503 operates to increase the vertical capacitance of the
`conductive object (e.g.. finger} by increasing the surface area
`407 of the sensor elements as much as possible while
`reducing the amount of perimeter 408 of the sensor ele-
`ments.
`
`[0029] The vertical capacitance is equal to Add (e.g..
`C Acid). The vertical capacitance is, thus. based on three
`primary factors: the area (A) 40'?I of a sensor element. the
`distance (d) 309 (shown in FIG. 33) between the conductive
`
`Page 16 of 22
`Page 16 of 22
`
`(e.g.. plate 301 or sensor
`object and a sensor element
`element 501), and the dielectric properties {E} of the insu-
`lator 304 between the conductive object and the sensor
`element (e.g.. plate 301 or sensor element 501). In one
`embodiment. the distance (d) 309 between the conductive
`objection and the sensor element
`is determined by the
`thickness of insulator overlay 304. as illustrated in FIG. 3B.
`The dielectric properties (5) of the overlay are substantially
`constant. with some minor changes with temperature.
`Accordingly. with larger area. the vertical capacitance to the
`finger increases.
`[0030] The horizontal. or fringe capacitance, comes from
`the very thin edges of the conductive material (cg. copper.
`ITO. etc.) that is used to form the sensor element (e.g.. plate
`302). There is a flat edge 402 and it has its own surface area
`and a distance 409 from an adjacent sensor element. The
`surface area of the flat edge is the height time the width of
`one side times the number of sides (6) ofthe sensor element.
`The use of a hexagonal shape for the sensor elements 501
`and 503 increases the area 407 of each of the sensor
`elements while minimizing the perimeter 408 of each ol'the
`sensor elements (e.g.. as opposed to an array having circular
`shaped sensor elements). Thereby. the vertical capacittmce
`to the conductive object is increased while not increasing the
`horizontal. or fringe, capacitance to the other sensor ele~
`ments in a sensor array as illustrated in FIG. 5A.
`[0031]
`FIG. 413 illustrates two embodiments of the gaps
`between hexagonal and octagonal shaped sensor elements.
`Assuming a unit area and uniform spacing to a ground plane
`(or other sensor elements) for both the hexagonal and
`octagonal shaped sensor elements. each ofthe sides are 0.62
`for the hexagonal shaped sensor elements 501 and 503 and
`0.46 for the octagonal shaped sensor elements 551 and 553.
`The distance 409 between the sensor elements (501. 503.
`551. and 553) is approximately 0.1 linear units. Gaps 509
`and 559 are the area of the unit area of which there is no
`
`sensor elelnent surface area (cg. non-sensor area). The area
`of the gaps 509 that surrotuid and are in between the
`hexagonal sensor elements is 0.19. The area of the gaps 559
`that surround and are in between the octagonal sensor
`elements is 0.43. The area of the gap is given by the
`following equation:
`Agnp "zlmr-‘ln-nm
`
`t 15}
`
`Accordingly. the non-sensor area or gaps 559 of the octago-
`nal sensor elements represents approximately 30% of the
`total unit area. and the non-sensor area or gaps 509 of the
`hexagonal sensor elements represent approximately 16% of
`the total unit area.
`
`FIG. 5A illustrates a top-side view of one embodi-
`[0032]
`ment of a sensor array having a plurality of hexagonal
`shaped sensor elements for detecting a presence of a con—
`ductive object 303 on the sensor array 500. Alternating rows
`and columns in FIG. 5A correspond to. for example, x— and
`y-axis elements. The y-axis sensor elements 503(1 )-503{K)
`are illustrated as black hexagons. The x-axis sensor elements
`501(1)-501(L) are illustrated as white hexagons. Sensor
`array 500 includes a plurality of rows 504(l)-504(N) and a
`plurality of columns 505(1)-505(M). where N is a positive
`integer value representative of the number of rows and M is
`a positive integer value representative of the number of
`columns. liach row includes a plurality of sensor elements
`503(1)-503(K)_. where K is a positive integer value repre-
`
`

`

`US 200W0229469 Al
`
`Oct. 4, 200'?
`
`sentative ol' the ttttmber of sensor elements in the row. Each
`colttmn includes a plurality of sensor elements 501(1)-501
`(L). where L is a positive integer value representative of the
`number of sensor elements in the column. Accordingly,
`sensor array is an NXM sensor matrix. The NXM sensor
`matrix. itt conjunction with the processing device 210.
`is
`configured to detect a position of a presence of the conduc-
`tive object 303 in the x-. and y-directions. In one embodi-
`ment. the sensor array is a lxM or le sensor matrix that
`can be configured to operate as a touchwsensor slider.
`[0033]
`In one embodiment, the process device 210 may
`include a capacitive switch relaxation oscillator ((TSR). It
`should be noted that there are various known methods for
`
`embodiments
`the
`capacitance. Although
`measuring
`described herein are described using a relaxation oscillator,
`the present embodiments are not limited to using relaxation
`oscillators. but may include other methods. such as current
`versus voltage phase shift measurement. resistor—capacitor
`charge liming. capacitive bridge divider. charge transfer. or
`the like. For example. the current versus voltage phase shift
`measurement may include driving the capacitance through a
`fixed-value resistor to yield voltage and current waveforms
`that are out of phase by a predictable amount. The drive
`frequency can be adjusted to keep the phase measurement in
`a readily measured range. The resistor—capacitor charge
`timing may include charging the capacitor through a lixed
`resistor and measuring timing on the voltage ramp. Small
`capacitor values may require very large resistors for reason-
`able tinting. The capacitive bridge divider may include
`driving the capacitor under test througlt a fixed reference
`capacitor. The reference capacitor and the capacitor under
`test form a voltage divider. The voltage signal is recovered
`with a synchronous demodulator. which may be done in the
`processing device 210. The charge transfer may be concep-
`tually similar to an R-C‘ charging circuit. In this metltod. C P
`is the capacitance being sensed. C SUM is the summing
`capacitor.
`into which charge is transferred on successive
`cycles. At the start of the measurement cycle. the voltage on
`CSUM is reset. The voltage on CSUM increases exponentially
`[and only slightly] with each clock cycle. The time for this
`voltage to reach a specific threshold is measured with a
`counter. Additional details
`regarding these alternative
`embodiments have [lot been included so as to not obscure the
`present embodiments. and because these alternative embodi-
`ments for measuring capacitance are known by those of
`ordinary skill in the art.
`[0034]
`FIG. 5B illustrates a block diagram of one embodi—
`ment of a capacitive sensor coupled to sensor array 500. It
`should be noted that only two sensor elements from sensor
`array 500 are shown in FIG. 53 liar ease of illustration.
`Capacitive sensor 410 includes a relaxation oscillator 450.
`and a digital counter 440. The sensor array 500 is coupled to
`relaxation oscillator 450 via an analog bus 401 having a
`plurality of pins 401(1)—401(N). The multi—dimension sensor
`array 500 provides output data to the analog bus 401 of the
`processing device 210.
`[0035] The selection circuit 430 is coupled to the plurality
`of sensor elements 355(l}-355(N). the reset switch 454. the
`current source 452. and the comparator 453. Selection
`circuit 430 may be used to allow the relaxation oscillator 450
`to measure capacitance on multiple sensor elements (e.g..
`rows or columns). The selection circuit 430 may be config-
`tlred to sequentially select a sensor element of the plurality
`of sensor elements to provide the charge current and to
`
`Page 17 of 22
`Page 17 of 22
`
`measure the capacitance of each sensor element. In one
`exemplary embodiment. the selection circuit 430 is a null-
`tiplexer array of the relaxation oscillator 450. Alternatively.
`Selection cirCuit may be other circuitry outside the relaxation
`oscillator 450. or even outside the capacitive sensor 410 to
`select the sensor element to be measured. Capacitive sensor
`410 may include one relaxation oscillator and digital counter
`for the plurality of sensor elements of the sensor array.
`Alternatively. capacitive sensor 410 may include multiple
`relaxation oscillators and digital counters to measure capaci—
`lance on the plurality of sensor elements of the sensor array.
`The multiplexer array may also be used to grotutd the sensor
`elements that are not being measured.
`[0036]
`In another embodiment. the capacitive sensor 410
`may be configured to simultaneously scan the sensor ele-
`ments. as opposed to being configured to sequentially scan
`the sensor elements as described above. For example, the
`sensing device may include a sensor array having a plurality
`of roves and columns. The rows may be scanned simulta-
`neously. and the columns may be scanned simultaneously.
`[0037]
`I11 one exemplary embodiment. the voltages on all
`oi” the rows of the sensor array are simultaneously moved.
`while the voltages at the columns are held at a constant
`voltage. with the complete set of sampled points simult‘ ~
`neously giving a profile of the conductive object in a first
`dimension. Next. the voltages on all of the rows are held at
`a constant voltage, while the voltages on all the roWs are
`simultaneously moved. to obtain a complete set of sampled
`points simultaneously giving a profile of the conductive
`object in the other dimension.
`[0038]
`In another exemplary embodiment. the voltages on
`all of the rows of the sensor array are simultaneously moved
`in a positive direction. while the voltages of the colurruis are
`moved in a negative direction. Next. the voltages on all of
`the rows of the sensor array are simultaneously moved in a
`negative direction. while the voltages of the columns are
`moved in a positive direction. This technique doubles the
`efl'ect of any transcapacitance between the two dimensions.
`or conversely. halves the effect of any parasitic capacitance
`to the ground. In both methods. the capacitive information
`from the sensing process provides a profile of the presence
`of the conductive object
`to the sensing device in each
`dimension. Alternatively. other methods tor scanning known
`by those of ordinary skill in the art may be used to scan the
`sensing device.
`[0039] Digital counter 440 is coupled to the output of the
`relaxation oscillator 450. Digital counter 440 receives the
`relaxation oscillator output signal 456 (lion-'1‘). Digital
`counter 440 is configured to count at least one ofa frequency
`or a period of the relaxation oscillator output received from
`the relaxation oscillator.
`
`[0040] As previously described with respect to the relax-
`ation oscillator 450. when a finger or conductive object is
`placed on the sensor element. the capacitance increases from
`Cp to Cp+Cf so the relaxation oscillator output signal 456
`(l--‘otJ't‘) decreases. The relaxation oscillator output signal 356
`(Four) is fed to the digital counter 440 for measurement.
`There are two methods for counting the relaxation oscillator
`output signal 456. frequency measurement and period mea-
`surement. In one embodiment. the digital counter 440 may
`include two multiplexers 423 and 424. Multiplexers 423 and
`424 are configured to select the inputs for the PWM 421 and
`the timer 422 for the two measurement methods. frequency
`and period measurement methods. Alternatively. other selec-
`
`

`

`US 20020229469 A1
`
`Oct. 4, 200'?
`
`lion circuits may be used to select the inputs for the PWM
`421 and the time 422. ln another embodiment. multiplexers
`423 and 424 are not included in the digital counter. for
`example. the digital counter 440 may be configured in one,
`or the other. measurement configuration.
`[0041]
`in the frequency measurement method. the relax-
`ation oscillator output signal 356 is counted for a fixed
`period of time. The counter 422 is read to obtain the number
`of counts during the gate time. This method works well at
`low frequencies where the oscillator reset time is small
`compared to the oscillator period. A pulse width modulator
`[PWM) 441 is clocked for a fixed period by a derivative of
`the system clock. \r'(_‘3 426 (which is a divider from system
`clock 425. e.g.. 24 MHz). Pulse width modulation is a
`modulation tecturique that generates variable-length pulses
`to represent the amplitude of an analog input signal: in this
`case V'C3 426. The output of PWM 421 enables timer 422
`[e.g.. 16—bit). The relaxation oscillator output signal 456
`clocks the timer 422. The timer 422 is reset at the start of the
`sequence. and the count value is read ottt at the end of the
`gate period.
`[0042]
`In the period measurement method. the relaxation
`oscillator output signal 356 gates a counter 422. which is
`clocked by the system clock 425 {e.g.. 24 MHZ). In order to
`improve sensitivity and resolution. multiple periods of the
`oscillator are counted with the PWM 421. The output of
`PWM 421 is used to gate the timer 422. In this method. the
`relaxation oscillator output signal 356 drives the clock input
`of PWM 421. As previously described pulse width modu-
`lation is a modulation technique that generates variable-
`length pulses to represent the amplitude of an analog input
`signal; in this case the relaxation oscillator output signal
`456. The output of the PWM 421 enables timer 422 {c.g..
`16-bit). which is clocked at the system clock frequency 425
`[e.g.. 24 MHZ). When the output of PWM 421 is asserted
`[e.g.. goes high). the count starts by releasing the capture
`control. When the terminal count of the PWM 421 is
`reached.
`the capture signal
`is asserted (e.g.. goes high].
`stopping the count and setting the PWM‘s interrupt. The
`timer value is read in this interrupt. The relaxation oscillator
`450 is indexed to the next sensor eletnent to be measured and
`
`the count sequence is started again.
`[0043] The two counting methods may have equivalent
`performance in sensitivity and signal-to-noise ratio (SNR).
`The period measurement method may ltave a slightly faster
`data acquisition rate. but this rate is dependent on software
`loads and the values of the capacitances on the sensor
`elements. The frequency measurement method has a Exod-
`sensor element data acquisition rate.
`[0044] The length of the counter 422 and the detection
`time required for the sensor element are determined by
`sensitivity requirements. Small changes in the capacitance
`on capacitor 351 result in small changes in frequency. In
`order to find these small changes,
`it may be necessary to
`count for a considerable titne.
`
`[0045] Using the selection circuit 430. multiple sensor
`elements may be sequentially scanned to provide current to
`and measure the capacitance from the capacitors (cg.
`sensor elements). as previously described.
`[11 other words.
`while one sensor element is being measured. the remaining
`sensor elements are grounded. The capacitor charging cur—
`rent (e.g.. current source 452) and reset switch 453 are
`connected to the analog mux bus 411. This may limit the
`pin-count requirement
`to simply the number of sensor
`
`Page 18 of 22
`Page 18 of 22
`
`elements to be addressed. In one exemplary embodiment. no
`external resistors or capacitors are required inside or outside
`the processing device 210 to enable operation.
`[0046]
`FIGS. 5C and 5D i