GB2534144A

GB2534144A - A capacitive touch panel with matrix electrode pattern

Info

Publication number: GB2534144A
Application number: GB1500508.5A
Authority: GB
Inventors: James Brown Christopher; Kay Andrew
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2015-01-13
Filing date: 2015-01-13
Publication date: 2016-07-20
Also published as: GB201500508D0; WO2016114123A1

Abstract

A capacitive touch sensor has at least a sensing column (410) comprising a plurality of sensing elements (420a-420g). The touch sensor comprises a substrate (10), and a conductive layer (11) disposed over the substrate. The sensing column comprises n first electrodes (D1 - D7), wherein n≥2, and at least a second electrode (Si). The first electrodes and the second electrode are defined in the conductive layer and extend generally parallel to a first direction. The first electrodes comprise a plurality of first electrode unit areas (440) disposed along the first direction and electrically connected with one another and to a voltage supply line by conductive tracks defined in the conductive layer. The sensing elements (420a-420g) extend generally in a second direction crossed with the first direction, each sensing element of the sensing column including a respective first electrode unit area of at least two of the first electrodes of the sensing column and a portion of the second electrode of the sensing column. A method of determining capacitances in a sensing circuit having at least a sensing column comprising a plurality of sensing elements, each sensing element of sensing column including respective portions of two or more first electrodes and a portion of a second electrode is also given.

Description

A Capacitive Touch Panel with Matrix Electrode Pattern

Technical Field and Applications of the Invention

The present invention relates to touch panel devices. In particular, this invention relates to capacitive type touch panels. Such a capacitive type touch panel device may find application in a range of consumer electronic products including, for example, mobile phones, tablet and desktop PCs, electronic book readers and digital signage products.

Background Art

Touch panels have become widely adopted as the input device for a range of electronic products such as smart-phones and tablet devices. Although, a number of different technologies can be used to create these touch panels, capacitive systems have proven to be the most popular due to their accuracy, durability and ability to detect touch input events with little or no activation force.

The most basic method of capacitive sensing for touch panels is the surface capacitive method -also known as self-capacitance -for example as disclosed in US4293734 (Pepper, October 6, 1981). A typical implementation of a surface capacitance type touch panel is illustrated in FIG.1 and comprises a transparent substrate 10, the surface of which is coated with a conductive material that forms a sensing electrode 11. One or more voltage sources 12 are connected to the sensing electrode, for example at each corner, and are used to generate an electrostatic field above the substrate. When an input object 13 that is electrically conductive -such as a human finger -comes into close proximity to the sensing electrode, a capacitor 14 is dynamically formed between the sensing electrode 11 and the input object 13 and this field is disturbed. The capacitor 14 causes a change in the amount of current drawn from the voltage sources 12 wherein the magnitude of current change is related to the distance between the finger location and the point at which the voltage source is connected to the sensing electrode. Current sensors 15 are provided to measure the current drawn from each voltage source 12 and the location of the touch input event is calculated by comparing the magnitude of the current measured at each source. Although simple in construction and operation, surface capacitive type touch panels are unable to detect multiple simultaneous touch input events as occurs when, for example, two or more fingers are in contact with the touch panel.

Another well-known method of capacitive sensing applied to touch panels is the projected capacitive method -also known as mutual capacitance. In this method, as shown in FIG.2, a drive electrode 20 and sense electrode 21 are formed on a transparent substrate (not shown). A changing voltage or excitation signal is applied to the drive electrode 20 from a voltage source 22. A signal is then generated on the adjacent sense electrode 21 by means of capacitive coupling via the mutual coupling capacitor 23 formed between the drive electrode 20 and sense electrode 21. A current measurement means 24 is connected to the sense electrode 21 and provides a measurement of the size of the mutual coupling capacitor 23. When the input object 13 is brought to close proximity to both electrodes, it forms a first dynamic capacitor to the drive electrode 27 and a second dynamic capacitor to the sense electrode 28. If the input object is connected to ground, as is the case for example of a human finger connected to a human body, the effect of these dynamically formed capacitances is manifested as a reduction of the amount of capacitive coupling in between the drive and sense electrodes and hence a reduction in the magnitude of the signal measured by the current measurement means 24 attached to the sense electrode 21.

As is well-known and disclosed, for example, in US 5,841,078 (Bisset et al, October 30, 1996), by arranging a plurality of drive and sense electrodes in a grid pattern to form an electrode array, this projected capacitance sensing method may be used to form a touch panel device. An advantage of the projected capacitance sensing method over the surface capacitance method is that multiple simultaneous touch input events may be detected.

In order to form the electrode grid array in a projected capacitance type touch panel device, a manufacturing process is typically used in which two layers of conductive material are deposited on the touch panel substrate with an electrically insulating layer deposited between them. The sense electrodes and drive electrodes may be formed entirely in separate conductive layers with the insulating layer preventing electrical contact between them at points in the array where they intersect. Alternatively, as disclosed in US 8,274,486 (Atmel, 2008) the sense electrodes and drive electrodes may be substantially formed in the same conductive layer and the other conductive layer used merely to provide bridges at the intersection points to prevent electrical contact. A disadvantage of the projected capacitance sensing method compared to the surface capacitance method is therefore the increased cost of manufacture. Consequently, it is desirable to provide a means of reducing the cost of manufacture of projected capacitance touch screen devices whilst maintaining the performance advantages of the method.

A means of manufacturing a projected capacitance type touch panel device using only a single conductive layer is disclosed, for example, in US 8,319,747 (Apple, 2008). However, although the cost of manufacture is reduced in this device the performance of the sensor is also reduced. Specifically, the accuracy at which the location of objects touching the surface can be calculated is decreased and the minimum size of object that can be detected is increased. A means to reduce the cost of manufacture of touch panel devices without reducing the performance is therefore sought.

Summary of the Invention

The present invention provides a capacitive touch sensor having at least a sensing column comprising a plurality of sensing elements; the touch sensor comprising: a substrate; and a conductive layer disposed over the substrate; wherein the sensing column comprises n first electrodes, wherein n 2, and at least a second electrode, the first electrodes and the second electrode defined in the conductive layer and extending generally parallel to a first direction; wherein the first electrodes each comprise a plurality of first electrode unit areas disposed along the first direction and electrically connected with one another and to a voltage supply line by conductive tracks defined in the conductive layer; wherein the sensing elements extend generally in a second direction crossed with the first direction, each sensing element of the sensing column including a respective first electrode unit area of at least two of the first electrodes of the sensing column and a portion of the second electrode of the sensing column.

Each first electrode of the sensing column may have first electrode unit areas included in at least two sensing elements in the sensing column.

A sensing element may include first electrode unit areas from less than all of the first electrodes of the sensing column.

Each sensing element in the sensing column may include first electrode unit areas from m first electrodes, where 2 m < n.

A sensing element in the sensing column may include drive unit areas from m adjacent first electrodes. Preferably this is the case for all sensing elements in the sensing column, however if the term "adjacent" is interpreted as meaning physically adjacent it will be understood that there will be one sensing element that cannot incorporate this feature (although the first and last of the first electrodes may be understood as "adjacent" to one another in that, in a drive scheme in which voltages are applied to the first electrodes in a repeating sequence).

The value of m may be 2 and n may be greater than 2.

The ith sensing element in the sensing column may include first electrode unit areas from an im selection of the first electrodes of the sensing column, the selection being unique in the sensing column.

The sensing column may further comprise a ground electrode. The ground electrode may extend generally along the first direction.

All first electrode unit areas in a sensing element may have the same nominal area as one another, where the term "nominal area" is to be understood as not precluding variations in area that arise from normal manufacturing tolerances.

Alternatively, in a sensing element, a first first electrode unit area nearer the second electrode may a smaller area than a second first electrode unit area further from the second electrode. The area of the first first electrode unit area and the area of the second first electrode unit area may be selected such that the baseline mutual capacitance between the first first electrode unit area and the second electrode is approximately equal to the baseline mutual capacitance between the second first electrode unit area and the second electrode.

The overall area of the first electrode unit areas in a sensing element in the sensing column may increase with distance of the sensor element from the voltage supply line. As the distance of a sensor element from the voltage supply line increases the number of conductive tracks passing through the sensor element generally decreases so that more of the area of the of the sensor element is available for the first electrode unit areas to occupy. Arranging for the first electrode unit areas to occupy as much area of the sensing element as possible maximises the signal-to-noise ratio. In one example, a sensing element in the sensing column may include at least an ith first electrode unit area and a jth first electrode unit area, the ith first electrode unit area having the same area in all sensing elements of the sensing column and the area of the jth first electrode unit area increasing with distance of the sensor element from the voltage supply line.

The touch sensor may comprise another second electrode extending generally parallel to the first direction, and each sensing element of the sensing column may further include a portion of the another second electrode.

A matrix representation G of the sensing elements of the sensing column against the first electrodes of the sensing column, in which a row of the matrix representation G represents the relative areas of first electrode unit areas present in a respective sensing element and a column of the matrix representation G represents the relative areas of first electrode unit areas of a respective first electrode included in the sensing elements of the sensing column, may an invertible matrix.

The matrix representation G may be an orthogonal matrix.

A matrix representation G of the sensing elements of the sensing column against the first electrodes of the sensing column, in which a row of the matrix representation G represents the relative areas of first electrode unit areas present in a respective sensing element and a column of the matrix representation G represents the relative areas of first electrode unit areas of respective first electrode included in the sensing elements of the sensing column, may further represent the relative areas of the interconnecting tracks present in a respective sensing element. Mutual capacitances formed by the interconnecting tracks thus form part of the desired signal, and their width may be increased, thereby reducing their electrical resistance, without introducing unwanted signal artefacts. This in turn increases the possible frequency range of the drive signals.

The touch sensor may comprise a drive circuit for applying a drive voltage to one or more selected first electrodes of the sensor column and a sensing circuit for measuring a current generated by the second electrode of the sensor column.

The drive circuit may be adapted to apply a drive voltage sequentially to successive groups of first electrodes in successive sampling periods.

A matrix representation D of the sequence of drive voltages, in which each column represents the relative magnitude of voltages applied to the first electrodes during a respective sampling period and each row represents the relative magnitude of voltages applied to respective first electrode in successive sampling periods of one frame period, is an invertible matrix.

The product of a matrix representation D of the sequence of drive voltages, in which each column represents the relative magnitude of voltages applied to the first electrodes during a respective sampling period and each row represents the relative magnitude of voltages applied to a respective first electrode in sequential sampling periods of one frame period, and a matrix representation G of the sensing elements of the sensing column against first electrode unit areas of the first electrodes of the sensing column, in which a row of the matrix representation G represents the relative areas of first electrode unit areas present in a respective sensing element and a column of the matrix representation G represents the relative areas of first electrode unit areas of a respective first electrode included in the sensing elements of the sensing column, is an invertible matrix.

The touch sensor may comprise a plurality of sensing columns, and may be adapted to compute C = S.(G.D)-I, where S is a matrix representation of the currents measured by the sensing circuit, where a row of the matrix representation S represents the currents measured from the second electrode of the sensor column in respective sampling periods of one frame period, and a column of the matrix representation S represents the currents measured from second electrodes of respective sensing columns in the touch sensor during one sampling period. (If the sensing circuit has a single sensing column, the matrix representation S of the measured currents has the form of a row vector.) Alternatively, the touch sensor may comprise a drive circuit for applying a drive voltage to the second electrode of a sensor column and a sensing circuit for measuring a current generated by one or more selected first electrodes of the sensor column. In this embodiment the sensing circuit may be adapted to measure a current generated by successive groups of first electrodes in successive sampling periods of one frame period.

The capacitive touch sensor of the present invention provides a means of reducing the cost of manufacture of touch panel devices whilst improving (or at least not worsening) the sensor performance compared to the background art.

The touch sensor includes a sensor substrate and an array of electrodes formed in a single layer of conductive material on the sensor substrate. The array of electrodes is arranged in columns of sensing elements with each sensing element including at least one of a first electrode group comprising at least two drive electrodes and at least one sense electrode, or a second electrode group comprising at least two sense electrodes and at least one drive electrode. Where two or more sense electrodes are provided, the outputs from different sense electrodes, or from different groups of sense electrodes, may be sensed at different time period. Alternatively, the outputs from different sense electrodes may be sensed simultaneously.

The pattern of drive electrode areas in each sensing column may be mathematically represented as a geometry matrix G. The capacitances of all mutual capacitors in the sensing array may be mathematically represented as a capacitance matrix C. The sequence of voltage excitation signals applied to the drive electrodes in order to measure the capacitances of the mutual capacitors may be mathematically represented as a drive matrix D. The signal currents that are measured on the sense electrodes as a result of application of the voltage excitation signals in one frame period of operation may be mathematically represented as a signal matrix S. In order to compute the capacitance matrix C a decoding matrix U is computed as the inverse of the matrix multiple of the drive matrix and geometry matrix, U=(D.0)-1. The capacitance matrix is then computed in each frame period as the matrix multiple of the signal matrix and the decoding matrix, C = S.U. The capacitance matrix is processed to determine the presence and location of one or more objects touching the surface of the touch panel device.

Advantageously, the present invention enables the signal-to-noise ratio of the capacitance measurements to be increased. As a consequence, the accuracy at which the location of an object touching the surface of the touch panel device may be calculated is increased and the minimum size of object that may be detected is decreased.

A second aspect of the invention provides a method of determining capacitances in a sensing circuit having at least a sensing column comprising a plurality of sensing elements, each sensing element including respective portions of two or more first electrodes and a portion of a second electrode, the method comprising: applying a sequence of drive voltages to the first electrodes; measuring a sequence of currents supplied by the second electrode as a consequence of the application of the drive voltages to the first electrodes; and, for each sensing element, determining capacitances between the portions of the first electrodes in the sensing element and the portion of the second electrode in the sensing element according to: C = S.(G.D)-1, where D is a matrix representation of the sequence of drive voltages, in which each column represents relative magnitude of voltages applied to the first electrodes during a respective sampling period and each row represents the relative magnitude of voltages applied to a respective first electrode in sequential sampling periods of one frame period; G is a matrix representation of the sensing elements, in which a row of the matrix representation G represents relative areas of the portions of the first electrodes present in a respective sensing element and a column of the matrix representation G represents relative areas of the portions of a respective first electrode included in the sensing elements; S is a matrix representation of the measured currents, where a row of the matrix representation S represents the currents measured from the second electrode of a sensor column in respective sampling periods, and a column of the matrix representation S represents the currents measured from the second electrodes of respective sensing columns during one sampling period; and C is a matrix representation of mutual capacitances, in which a row represents the relative mutual capacitances for the sensing elements of a sensing column.

If the sensing circuit has a single sensing column, the matrix representation S of the measured currents has the form of a row vector. In this case the matrix representation C of mutual capacitances is also a row vector having one entry for each sensing element of the sensing column, the entry representing the relative mutual capacitance between the portions of the first electrodes in the sensing element and the portion of the second electrode in the sensing element. If the sensing circuit has more than one sensing column, the matrix representation S of the measured currents and the matrix representation C of mutual capacitances both have the same number of rows as there are sensing columns in the sensing circuit form of a row vector. Each row of the matrix representation C of mutual capacitances represents the relative mutual capacitances of the sensing elements of a respective sensing column.

The matrix representation G of the sensing elements may further represent relative areas of interconnecting tracks present in a respective sensing element.

A further aspect of the invention comprises a method of representing the sensing elements of a sensing circuit having at least a sensing column comprising a plurality of sensing elements, each sensing element including respective portions of two or more first electrodes and a portion of a second electrode, the method comprising providing a matrix representation G of the sensing elements in which a row of the matrix representation G represents relative areas of portions of the first electrodes present in a respective sensing element and a column of the matrix representation G represents relative areas of the portions of a respective first electrode included in the sensing elements, and further comprising including in the matrix representation G relative areas of interconnecting tracks present in a respective sensing element.

Brief Description of Drawings

FIG. 1 shows a typical implementation of a surface capacitance type touch panel device; FIG. 2 shows a typical implementation of a projected capacitance type touch panel device; FIG.3 shows the mathematical matrix representation of the signals generated in a touch panel device; FIG.4 shows an electrode arrangement in accordance with a first embodiment of the invention; FIG.5 shows a mathematical matrix representation of an electrode arrangement in accordance with a first embodiment of the invention; FIG. 6 shows a sensing circuit for measurement of the capacitances in a touch panel device; FIG.7 shows a waveform diagram illustrating the operation of a sensing circuit for measurement of the capacitances in a touch panel device; FIG.8 shows a block diagram of a system incorporating a touch panel device; FIG.9 shows an electrode arrangement in accordance with a third embodiment of the invention; FIG.10 shows a mathematical matrix representation of an electrode arrangement in accordance with a third embodiment of the invention; FIG.11 shows an electrode arrangement in accordance with a fourth embodiment of the invention; FIG.12 shows an electrode arrangement in accordance with a fifth embodiment of the invention; FIG.13 shows a mathematical matrix representation of an electrode arrangement in accordance with a fifth embodiment of the Invention; FIG.14 shows a mathematical matrix representation of an electrode arrangement in accordance with a sixth embodiment of the invention FIG.15 shows an electrode arrangement in accordance with a seventh embodiment of the invention;

Description of Reference Numerals

Transparent substrate 11 Sensing electrode 12 Voltage source 13 Conductive object 14 Capacitor

ID

Current sensor Drive electrode 21 Sense electrode 22 Voltage source 23 Mutual coupling capacitor 24 Current measurement means 27 Drive electrode capacitor 28 Sense electrode capacitor 400 sensing array 410 sensing column 420 sensing element 430 sense electrode 435 drive electrode 440 drive electrode area 450 drive electrode interconnecting track 460 ground electrode 600 sensing circuit 605 voltage pulse generator 610 active drive electrode 615 mutual capacitor 620 sense electrode 630 integrator circuit 631 operational amplifier 632 integration capacitor 633 reset switch 640 switching circuit 641 first input switch 642 second input switch 650 analog-to-digital converter 800 touch sensor panel 802 sense electrodes 804 drive electrodes 810 connector 820 controller 822 sense unit 824 drive unit 826 interface unit 830 display device 840 display controller 850 host 910 sensing column 920 sensing element 930 sense electrode 935 drive electrode 940 first drive electrode area 945 second drive electrode area 1110 sensing column 1120 sensing element 1130 sense electrode 1135 drive electrodes 1140 first drive electrode area 1145 second drive electrode area 1200 second drive electrode area 1510 sensing column 1520 sensing element 1530 first sense electrode 1532 second sense electrode 1535 drive electrodes 1540 first drive electrode area 1545 second drive electrode area

Detailed Description of Invention

The present invention provides a capacitive touch sensor that may be used, for example in touch panel display systems or the like. The touch sensor includes a sensor substrate and an array of electrodes formed in a single layer of conductive material on the sensor substrate.

In accordance with a first and most general embodiment of the present invention, the array of electrodes is constructed by a plurality of sensing columns each of which comprises a plurality of drive electrodes and at least one sense electrode. The plurality of drive electrodes and at least one sense electrode are formed into sensing elements wherein each sensing element includes areas of at least two drive electrodes from the plurality of drive electrodes and of the at least one sense electrode. As is described herein, each drive electrode may contain drive electrode areas in at least two sensing elements and the selection of drive electrode areas present in each sensing element may be unique amongst all sensing elements in a sensing column. The physical area of all drive electrode areas in the sensing column may be the same. Further, the pattern of drive electrode areas in each sensing element may be expressed mathematically by an element vector E that represents the physical area of each drive electrode in the respective sensing element. The element vector E has dimension n wherein n denotes the number of drive electrodes in the sensing column. Each element Ei of the element vector E may be an integer or non-integer value. The value may represent an absolute area or a relative area with respect to a known reference area.

The total pattern of drive electrode areas in each sensing column may be represented as a geometry matrix G of dimension m x n wherein m represents the number of sensing elements in a sensing column. A row of the geometry matrix G represents one element vector E. A column of the geometry matrix describes the pattern of electrode areas of a single drive electrode across all sensing elements in one sensing column. The pattern of drive electrodes may be chosen across all sensing elements to form a geometry matrix G that is invertible. Each sensing column in the sensing array may have the same pattern of drive electrodes. Similar drive electrodes from each sensing column may be connected to each by means external to the sensor substrate.

Within each sensing element each drive electrode area forms a mutual capacitor with the sense electrode. The capacitances of all mutual capacitors in one sensing column may therefore be represented by a capacitance vector K of dimension m wherein each element Km is the total capacitance between all drive electrode areas and the sense electrode in the mth sensing element. Further, the capacitances of all mutual capacitors in the sensing array may be represented as a capacitance matrix C of dimension p x m where p is the number of sensing columns in the sensing array. A row of the capacitance matrix represents one capacitance vector K. The capacitance of the mutual capacitors in a sensing element may be changed by the presence of an object in close proximity to or touching the surface of the touch panel device in the region of said sensing element. Measurements of the capacitance of the mutual capacitors in each sensing element of the sensing array may therefore be used to determine the location of any objects touching the surface of the device.

In order to measure the capacitance of each mutual capacitor in the sensing array, voltage excitation signals may be applied to the drive electrodes and the currents that are generated in each sense electrode as a result are measured. A sequence of voltage excitation signals may be time sequentially applied to the drive electrodes in successive sampling periods such that the capacitance of each mutual capacitor in the touch panel may be uniquely measured in one frame period of operation. The sequence of voltage excitation signals may be mathematically represented as a drive matrix D wherein a column of the matrix represents the relative magnitude of the voltage excitation signal applied to each drive electrode during one sample period. Each row of the drive matrix D represents the voltage excitation signals applied in sequential sampling periods to a drive electrode during one frame period. The dimension of the drive matrix D may be n x t wherein t is the number of sample periods in one frame period. In order to uniquely measure the capacitance of all mutual capacitors in the sensing array the minimum value of t is found to equal to m such that the minimum dimension of drive matrix D is n x m. In a simple example the voltage excitation signals may be chosen to form a drive matrix D that is the identity matrix. This simple example corresponds to the case where a voltage excitation signal is time sequentially applied to each drive electrode in turn during one frame period.

The signal currents that are measured from the sense electrode in one sensing column during the frame period may be represented by a signal vector R of dimension t wherein each element Rt corresponds to the current measured in the Ph sampling period. The signal currents that are measured from all sensing columns in the sensing array during one frame period may therefore be expressed as a signal matrix S of dimension p x t. A row of the signal matrix represents one signal vector R. A column of the signal matrix represents the signal currents measured from all sense electrodes in the sensing array during one sample period. The signal matrix is therefore given by S = C.G.D as illustrated in FIG 3. Accordingly, it is possible to compute the capacitance matrix C from the measured signal matrix S and the known geometry matrix G and drive matrix D. In particular, the capacitance matrix C = S.(G.D)-'. As a consequence, the matrix product (D.G) must be invertible which requires that the drive matrix D must be invertible and the geometry matrix G must be invertible.

For clarity, the structure and operation of a touch panel device in accordance with the present embodiment is described for an exemplary arrangement of the array of electrodes and an exemplary sequence of voltage excitation signals. As shown in FIG 4, the sensing array 400 in this exemplary arrangement consists of seven sensing columns 410. Each sensing column contains one sense electrode 430 and seven drive electrodes 435 and is arranged into seven sensing elements El to E7, 420a to 420g. Each sensing element further comprises a selection of drive electrode areas 440 from four out of the seven drive electrodes in each column. The physical area occupied by each of the drive electrode areas is the same. Narrow drive interconnection tracks 450 connect electrode areas 440 of the same drive electrode 435 together across different sensing elements 420. Each sensing element may optionally include a ground electrode, N 460. The purpose of the ground electrode 460 is to electrically isolate the drive electrodes of one sensing column from the sense electrode of the neighbouring sensing column. The geometry matrix G associated with the sensing array 400 depicted in FIG 4 is shown in FIG 5. In this exemplary geometry matrix the values in the matrix are expressed as a relative value. Each row of the geometry matrix G shown in figure 5 constitutes an element vector E relating to one sensing element of the sensing array 400 of figure 4, that represents the physical area of each drive electrode in that sensing element.

FIG. 6 shows a schematic diagram of a sensing circuit 600 that may be used to measure the signal currents generated on any one of the sense electrodes in the sensing array during each sampling period. The circuit described herein is provided as an example of a capacitance measurement circuit using a charge transfer technique as is well-known in the field. Alternatively, other known circuits and techniques for capacitance measurement may be used. During one sampling period, a voltage excitation waveform is applied to one drive electrode of the sensing array. A voltage pulse generator 605 supplies the excitation waveform, a series of voltage pulses to the active drive electrode 610, whilst a charge integrator circuit 630 holds the sense electrode 620 at a constant voltage, VREF. Such charge integrator circuits 630 will be well known to one skilled in the art, and typically comprise an operational amplifier 631, an integration capacitor 632 and a reset switch 633. The sensing circuit 600 may additionally have a switching circuit 640 containing a first input switch 641 and a second input switch 642. The first and second input switches are operated so as to accumulate charge onto the integration capacitors 632 over the course of multiple voltage pulses. The amount of charge accumulated on each integration capacitor is indicative of the capacitance of the mutual capacitor 615 between the active drive electrode and the respective sense electrode. An analog-to-digital converter 650 is provided to convert the integrator circuit output voltage to a digital signal at the end of the sampling period.

The operation of the capacitance measurement circuit shown in FIG. 6 during one sample period is now described with reference to the waveform diagram of FIG. 7. The reset switch 633 is firstly closed under the control of a reset switch control signal RST so that the output voltage your begins at a known voltage, VREF, such as the system ground potential. The second input switch 642 is closed under the control of a second input switch control signal S2 at the same time as the reset switch 633 such that the sense electrode 620 is set to a known initial voltage, such as the system ground potential. The second input switch 642 is then opened and the first input switch 641 is subsequently closed under the control of a first input switch control signal S1. The voltage pulse generator 605 now raises the voltage of the drive electrode 610 to a high voltage level causing charge to be injected across mutual capacitor 615 and accumulate on the integration capacitor 632. This causes the output voltage of each integrator circuit \foul-to rise by an amount that corresponds to the capacitance of mutual capacitor 615 between the active drive electrode and the relevant sense electrode. Next, the first input switch 641 is opened and the second input switch 642 is closed. The voltage pulse generator 605 now returns the voltage of the drive electrode 610 to a low voltage level whilst the output voltage of the integrator circuit, which is isolated from the sense electrode 620, is maintained at a constant level. This operation of applying a voltage pulse to the drive electrode 610 and cycling the first and second input switches may be repeated many times (for example 20 times) in order to generate a measurable voltage at the output of each integration circuit.

The capacitive touch panel of the present embodiment may form part of a touch screen system. The system may include a touch sensor panel 800, a connector 810, a touch panel controller 820, a display device 830, a display controller 840 and a host 850. The touch sensor panel may include the sensing array 400 described above and be may be mounted on the display device 830. Electrical connections are made between the touch panel 800 and the touch panel controller 820 via the connector 810. The sense electrodes 802 from each sensing column 410 are routed directly to the sense unit 822 in the touch panel controller 820 via the connector 810. The drive electrodes 804 are routed from the drive unit 824 in the touch panel controller 820 to the sensing array 400 via the connector 810 such that all drive electrodes are connected to each sensing column. Specifically, the drive electrodes 804 are routed so that similar drive electrodes in each sensing column are connected together.

The sensing unit 822 may contain sensor circuits 600 to measure the currents generated on each sense electrode during operation. The drive unit 824 may contain voltage pulse generators 605 to provide voltage excitation signals to each drive electrode as described above. The measured capacitance values are conveyed by the interface unit 826 of the touch panel controller 820 to the host 850 which determines the position and type of input objects touching the surface of the sensor and outputs the result. Alternatively, the calculation of input object position and type may be accomplished within the controller circuit 820, and the calculation result passed to the host electronics 850. The host electronics may generate a video image in response to detected objects, and may pass this video image to the display controller 840 for display on the display device 830.

In general the performance of a touch sensor is related to its signal-to-noise ratio (SNR) which is limited by the frequency at which the excitation signal may be applied. Specifically, as the SNR is decreased, the accuracy at which the location of objects touching the surface of the sensor may be determined is reduced and the minimum size of object that can be detected is increased. The frequency at which the excitation signal may be applied is, in turn, limited by the resistance of the touch panel electrodes. Due to the distribution of wide drive electrode areas along the length of the sensing column, the total resistance of the drive electrode may be reduced compared to conventional means. An advantage of the sensing array of the present embodiment is therefore that the SNR may be increased and the performance improved.

In accordance with a second embodiment of the invention the pattern of drive electrode areas in each sensing column may be arranged so as to form a geometry matrix G that is orthogonal. Examples of orthogonal matrices that may be used are a Hadamard matrix or a Maximum length sequence (M-Sequence) matrix. An advantage of an orthogonal matrix is that the inverse may be simply computed by the matrix transpose. As a result it is computationally cheap to compute the capacitance matrix from the measured signal matrix.

A sensing column of a sensing array in accordance with a third embodiment of the invention is illustrated in FIG 9. Each sensing element 920a-g of the sensing column 910 comprises a sense electrode 930 a first drive electrode area 940 and a second drive electrode area 945. The first drive electrode area 940 may be arranged to be adjacent to the sense electrode 930 and the second drive electrode area 945 may be arranged to be adjacent to the first drive electrode area 940 and separated from the sense electrode 930 by it. The first and second drive electrode areas in one sensing element may be from two adjacent drive electrodes 935. The drive electrodes areas 940, 945 of each successive sensing element 920 may be from each successive drive electrode. The geometry matrix G corresponding to the sensing array of FIG 9 is shown in FIG 10.

In accordance with a fourth embodiment of the invention the physical area of each of the drive electrode areas in a sensing element may be arranged to have the same baseline mutual capacitance. The baseline mutual capacitance is the capacitance of the mutual capacitor between a drive electrode area and a sense electrode in a sensing element when there is no object present in the vicinity of the sensing element. As shown in the exemplary arrangement of FIG 11 which is based on the arrangement shown in FIG 10, each sensing element 1120a-f of the sensing column 1110 comprises a sense electrode 1130 a first drive electrode area 1140 and a second drive electrode area 1145. Since the first drive electrode area 1140 is closer to the sense electrode 1130 than the second drive electrode area 1145, the baseline mutual capacitance per unit area of the first drive electrode area 1140 will be higher than that of the second drive electrode area 1145. The physical area of the first drive electrode 1140 may therefore be chosen to be smaller than the second drive electrode area 1145 such that the baseline mutual capacitance of each is equal.

In accordance with a fifth embodiment of the invention the physical area of the drive electrode areas of each sensing element may be different for each sensing element in a sensing column. In particular, the drive electrode areas may be arranged so as to occupy as much area as possible in the sensing element and hence maximise the signal-to-noise ratio. Due to the presence of the drive electrode interconnecting tracks the available area for the drive electrode areas is different in each sensing element in the sensing column. This is illustrated in the exemplary arrangement of FIG 12 which is based on the arrangement of FIG 11, but in which the overall area of the drive electrode unit areas in a sensing element 1120a-g increases along the sensing column. For example, the physical area occupied by the drive electrode Is areas 1140, 1200a in sensing element 1120a is limited by the presence of the drive electrode interconnecting tracks from drive electrodes D2 to D7. On the other hand, the lack of other drive electrode interconnecting tracks in sensing element 1120g allows the area occupied by the corresponding drive electrode areas 1140, 1200g to be enlarged. FIG 12 shows an arrangement in which the physical area occupied by the first drive electrode areas 1140 is the same in each sensing element while the physical area occupied by the second drive electrode areas 1200a-g is different in each sensing element 1120a-g, but this embodiment is not limited to this and a change in overall area of the drive electrodes from one sensing element to another may be effected by a change in the physical area of all, or any selection of, the drive electrodes. For example the physical area occupied by the first drive electrode areas might vary from one sensing element to another sensing element while the physical area occupied by the second drive electrode areas was the same in each sensing element, and as a further example it would be possible for the physical area occupied by each drive electrode are to vary from one sensing element to another sensing element.

The different sizes of second drive electrode areas may be encoded in the geometry matrix. An example of a geometry matrix, G, corresponding to the arrangement illustrated in FIG 12 is shown in FIG 13. Entries in the matrix may be non-integer values that correspond to the baseline capacitances between the drive electrode areas and the sense electrode. Entries in the matrix that correspond to the second drive electrode areas 1200a-g have different values for each sensing element due to the different physical areas occupied. Values of the entries may be absolute values corresponding to the physical size of the drive electrode area or, as shown in FIG 13, may be values relative to the size of the smallest drive electrode area in the sensing column. Accordingly, it is possible to maximise the signal-to-noise ratio whilst maintaining a consistent response across the sensing column in response to the presence of an object.

In accordance with a sixth embodiment of the invention the mutual capacitance between the drive electrode interconnecting tracks and the sense electrode may be encoded into the geometry matrix. An example of a geometry matrix, G, corresponding to the arrangement illustrated in FIG 12 and including entries in the matrix corresponding to the drive electrode interconnecting tracks is shown in FIG 14. Where a drive electrode interconnecting track of drive electrode Dj is present in a sensing element Ei a non-zero entry is made in the matrix at matrix element Gi,j.

Entries in the matrix may be values relative to the smallest drive electrode area in the sensing column. In a conventional touch panel device the mutual capacitance associated with the drive electrode tracks is unwanted and acts to lower the signal-to-noise ratio. However, by encoding the mutual capacitances of the drive electrode tracks into the geometry matrix, such capacitances form part of the desired signal and the signal-to-noise ratio may be increased.

Further, since the mutual capacitances associated with the drive electrode interconnecting tracks now form part of the desired measurement signal it is possible to increase their width without introducing unwanted signal artefacts. This may bring an additional advantage as is now described. As stated above, the frequency of the excitation signal applied to the touch panel drives electrodes, and hence the signal-to-noise ratio of the capacitance measurement, may be limited by the resistance of the touch panel electrodes. Also, the drive electrode interconnecting tracks may form the major component of the drive electrode resistance. Accordingly, by increasing the width of the drive electrode interconnecting tracks the signal-to-noise ratio of the capacitance measurement may be further increased.

In accordance with a seventh embodiment of the invention, each sensing element includes two sense electrodes. Fig 15 illustrates an exemplary arrangement of a sensing column 1510 in accordance with the present embodiment. The sensing column comprises a plurality of sensing elements 1520. Each sensing element comprises portions of a first sense electrode, SA 1530, portions of a second sense electrode, SB 1532 and first and second drive electrode areas 1540, 1545 chosen from amongst a plurality of drive electrodes 1535. The first sense electrode, SA 1530, is arranged to be adjacent to the first drive electrode areas 1540 and the second sense electrode, SB 1532, may be arranged to be adjacent to the first sense electrode, SA 1530 and separated from the first drive electrode areas 1530 by it. Within each sensing element, a first set of capacitors is formed between the first sense electrode, 1530, and the drive electrode areas and a second set of capacitors is formed between the second sense electrode, 1532, and the drive electrode areas. The capacitances of said capacitors may be independently measured using the aforementioned methods. For example, separate sensing circuits 600 may be connected to each of the first sense electrode, 1530, and the second sense electrode 1532, such that both sets of capacitances may be measured simultaneously. From these capacitance measurements, it is possible to compute a first and second capacitance matrix C for the first and second set of capacitances respectively in one 2() sensing column. Since the first and second capacitors have different separations between the drive and sense electrodes it is possible to use the respective capacitance measurements to measure additional properties of the input object. For example, by comparing the first and second capacitance matrices it is possible to measure the height of objects above the surface of the touch panel, as disclosed in US 20140009428 (to Coulson et al, July 3, 2014) which is hereby incorporated in full by reference. Alternatively or additionally, it is possible to use the first and second capacitance matrices to detect input events from non-conductive objects, as disclosed in US Patent Application No. 14/135,639 (to Brown et al, December 20, 2013) which is hereby incorporated in full by reference.

The invention has been described above with respect to embodiments in which a sensing column includes one sense electrode and a plurality of drive electrodes. The invention may however be implemented using a plurality of sense electrodes and a single drive electrode. In essence this would require no change in the electrode structure, but what has been described as a "drive electrode" in the above embodiments would be used as a sense electrode and what has been described as a "sense electrode" in the above embodiments would be used as a drive electrode. Thus, in the embodiment of figures 4 and 6, for example, the voltage pulse generator 605 would be arranged to apply a voltage pulse to the electrode line Si in each sampling period of a time frame (so that the electrode line S, acted as the drive electrode). The sensing circuit 600 would be arranged to, in each sampling period, sample the current generated by a different group of the electrodes DI to D7 (so that the electrodes DI to D7 acted as sense electrodes), such that the capacitance of each mutual capacitor in the sensing column may be measured over one frame period. In a simple example, the sensing circuit 600 could be arranged to time-sequentially sample the current generated by each of the electrodes D1 to D7 in turn during a frame period. Alternatively, each electrode D1 to D7 could be provided with a separate sensing circuit, so that the currents generated by each of the electrodes D1 to D7 could be sensed simultaneously.

Claims

CLAIMS: 1. A capacitive touch sensor having at least a sensing column comprising a plurality of sensing elements; the touch sensor comprising: a substrate; and a conductive layer disposed over the substrate; wherein the sensing column comprises n first electrodes, wherein n a 2, and at least a second electrode, the first electrodes and the second electrode defined in the conductive layer and extending generally parallel to a first direction; wherein the first electrodes comprise a plurality of first electrode unit areas disposed along the first direction and electrically connected with one another and to a voltage supply line by conductive tracks defined in the conductive layer; wherein the sensing elements extend generally in a second direction crossed with the first direction, each sensing element of the sensing column including a respective first electrode unit area of at least two of the first electrodes of the sensing column and a portion of the second electrode of the sensing column.
2. A touch sensor as claimed in claim 1 wherein the first electrodes of the sensing column have first electrode unit areas included in at least two sensing elements in the sensing column.
3. A touch sensor as claimed in claim 1 or 2 wherein a sensing element in the sensing column includes first electrode unit areas from less than all of the first electrodes of the sensing column.
4. A touch sensor as claimed in claim 1, 2 or 3 wherein each sensing element in the sensing column includes first electrode unit areas from m first electrodes, where 2 m < n.
5. A touch sensor as claimed in claim 4 wherein a sensing element in the sensing column includes drive unit areas from m adjacent first electrodes.
6. A touch sensor as claimed in claim 4 or 5 wherein m = 2 and n > 2.
7. A touch sensor as claimed in any preceding claim wherein the ith sensing element in the sensing column includes first electrode unit areas from an selection of the first electrodes of the sensing column, the ith selection being unique in the sensing column.
8. A touch sensor as claimed in any preceding claim wherein the sensing column further comprises a ground electrode.
9. A touch sensor as claimed in any preceding claim wherein all first electrode unit areas in a sensing element have the same area as one another.
10. A touch sensor as claimed in any one of claims 1 to 8 wherein, in a sensing element, a first first electrode unit area nearer the second electrode has a smaller area than a second first electrode unit area further from the second electrode.
11. A touch sensor as claimed in claim 10 wherein the area of the first first electrode unit area and the area of the second first electrode unit area are selected such that the baseline mutual capacitance between the first first electrode unit area and the second electrode is approximately equal to the baseline mutual capacitance between the second first electrode unit area and the second electrode.
12 A touch sensor as claimed in any preceding claim wherein the overall area of the first electrode unit areas in a sensing element in the sensing column increases with distance of the sensor element from the voltage supply line.
13 A touch sensor as claimed in any preceding claim and comprising another second electrode extending generally parallel to the first direction, each sensing element of the sensing column further including a portion of the another second electrode.
14. A touch sensor as claimed in any preceding claim wherein a matrix representation G of the sensing elements of the sensing column against the first electrodes of the sensing column, in which a row of the matrix representation G represents the relative areas of first electrode unit areas present in a respective sensing element and a column of the matrix representation G represents the relative areas of first electrode unit areas of a respective first electrode included in the sensing elements of the sensing column, is an invertible matrix.
15. A touch sensor as claimed in claim 14 wherein the matrix representation G is an orthogonal matrix.
16. A touch sensor as claimed in any one of claims 1 to 13 wherein a matrix representation G of the sensing elements of the sensing column against the first electrodes of the sensing column, in which a row of the matrix representation G represents the relative areas of first electrode unit areas present in a respective sensing element and a column of the matrix representation G represents the relative areas of first electrode unit areas of respective first electrode included in the sensing elements of the sensing column, further represents the relative areas of the interconnecting tracks present in a respective sensing element.
17. A touch sensor as claimed in any preceding claim and further comprising a drive circuit for applying a drive voltage to one or more selected first electrodes of the sensor column and a sensing circuit measuring a current generated by the second electrode of the sensor column.
18. A touch sensor as claimed in claim 17 wherein the drive circuit is adapted to apply a drive voltage sequentially to successive groups of first electrodes in successive sampling periods.
19. A touch sensor as claimed in claim 18 in which a matrix representation D of the sequence of drive voltages, in which each column represents the relative magnitude of voltages applied to the first electrodes during a respective sampling period and each row represents the relative magnitude of voltages applied to respective first electrode in successive sampling periods of one frame period, is an invertible matrix.
20. A touch sensor as claimed in any preceding claim in which the product of a matrix representation D of the sequence of drive voltages, in which each column represents the relative magnitude of voltages applied to the first electrodes during a respective sampling period and each row represents the relative magnitude of voltages applied to a respective first electrode in sequential sampling periods of one frame period, and a matrix representation G of the sensing elements of the sensing column against first electrode unit areas of the first electrodes of the sensing column, in which a row of the matrix representation G represents the relative areas of first electrode unit areas present in a respective sensing element and a column of the matrix representation G represents the relative areas of first electrode unit areas of a respective first electrode included in the sensing elements of the sensing column, is an invertible matrix.
21. A touch sensor as claimed in claim 19 and comprising a plurality of sensing columns, and adapted to compute C = S.(G.Dy1, where S is a matrix representation of the currents measured by the sensing circuit, where a row of the matrix representation S represents the currents measured from the second electrode of the sensor column in respective sampling periods of one frame period, and a column of the matrix representation S represents the currents measured from second electrodes of respective sensing columns in the touch sensor during one sampling period.
22. A touch sensor as claimed in any one of claims 1 to 16 and further comprising a drive circuit for applying a drive voltage to the second electrode of a sensor column and a sensing circuit for measuring a current generated by one or more selected first electrodes of the sensor column.
23. A touch sensor as claimed in claim 22 wherein the sensing circuit is adapted to measuring a current generated by successive groups of first electrodes in successive sampling periods of one frame period.
24. A method of determining capacitances in a sensing circuit having at least a sensing column comprising a plurality of sensing elements, each sensing element of sensing column including respective portions of two or more first electrodes and a portion of a second electrode, the method comprising: applying a sequence of drive voltages to the first electrodes; measuring a sequence of currents supplied by the second electrode as a consequence of the application of the drive voltages to the first electrodes; and for each sensing element, determining capacitances between the portions of the first electrodes in the sensing element and the portion of the second electrode in the sensing element according to C = S.(G.D)1, where D is a matrix representation of the sequence of drive voltages, in which each column represents relative magnitude of voltages applied to the first electrodes during a respective sampling period and each row represents the relative magnitude of voltages applied to a respective first electrode in sequential sampling periods of one frame period; G is a matrix representation of the sensing elements, in which a row of the matrix representation G represents relative areas of the portions of the first electrodes present in a respective sensing element and a column of the matrix representation G represents the relative areas of the portions of a respective first electrode included in the sensing elements; S is a matrix representation of the measured currents, where a row of the matrix representation S represents the currents measured from the second electrode of a sensor column in respective sampling periods, and a column of the matrix representation S represents the currents measured from the second electrodes of respective sensing columns during one sampling period; and C is a matrix representation of mutual capacitances, in which a row represents the relative mutual capacitances in a sensing element between the portions of the first electrodes in the sensing element and the portion of the second electrode in the sensing element.
25. A method as claimed in claim 24 wherein the matrix representation G of the sensing elements further represents relative areas of interconnecting tracks present in a respective sensing element.