Force Input Localisation

A device (15) including a piezoelectric sensor (16). The piezoelectric sensor (16) includes a layer of piezoelectric material (7) disposed between a number of sensing electrodes (4, 12, 13) and at least one counter electrode (3). The device (15) also includes a controller (17) connected to the piezoelectric sensor (16). The sensing electrodes (4, 12, 13) are arranged to form one or more active regions (19). Each active region (19) includes one or more primary sensing electrodes (4,12) and one or more secondary sensing electrodes (4, 13). The secondary sensing electrodes (4, 13) are separated from the primary sensing electrodes (4, 12) by a perimeter (14). The controller (17) is configured, for each active region (19), to monitor primary piezoelectric charges induced on each primary sensing electrode (4, 12) and to monitor secondary piezoelectric charges induced on each secondary sensing electrode (4, 13). The controller (17) is also configured, in response to detecting one or more primary and/or secondary piezoelectric charges, to determine whether a corresponding applied force has a centroid within the perimeter (14) based on comparing the primary piezoelectric charges to the secondary piezoelectric charges.

FIELD OF THE INVENTION

The present invention relates to sensors for piezoelectric force sensors and processing of signals from such sensors. In particular the present invention relates to localising an applied force to one, or a group of, sensing electrodes of a piezoelectric force sensor.

BACKGROUND

Human-machine-interface panels are common interaction method for users to communicate with a wide variety of equipment. Examples include smart-phones, tablet computers, laptops, all-in-one personal computers (PCs), point-of-sale payment devices (automated tills/registers), consumer electronics, white goods (washing machines, tumble dryers), automotive applications (e.g. dashboard), control of industrial machinery, medical devices and so forth.

A full-display touchscreen panel is often an attractive solution for high-end products which may receive a wide variety of input types, for example smart-phones, tablet computers, laptops, all-in-one personal computers (PCs) and so forth. However, for fixed-use panels which do not require the capacity to receive such rich input data, a high resolution touchscreen panel may be too expensive and is usually unnecessary.

Fixed-use panels may find applications in, for example, consumer electronics, white goods (e.g. washing machines), automotive applications (e.g. dashboard controls), control of industrial machinery, medical devices and so forth. For such applications, it may be more straightforward to define fixed buttons, arrays of buttons (e.g. a numeric pad), slider controls, dial controls and so forth.

Such user input controls have previously been implemented using mechanical switches, sliders (e.g. potentiometers) and similar mechanically actuated input controls. Mechanically actuated input controls may be associated with one or more of increased costs, increased complexity of an outer casing/panel of a device, a lack of mechanical robustness and/or increased potential for water/particle ingress into the device. Capacitive-sensing electrodes have been considered as possible replacements for mechanically actuated user controls, sometimes referred to as “buttonless” input panels (in reference to the absence of mechanical buttons). Such “buttonless” panels implemented using capacitive sensing have limitations which may limit the range of suitable applications. Capacitive sensing methods may become inaccurate when liquids are present on or over the sensing electrodes. Further, a capacitive coupling to a user's digit is required, which may not be possible if a user is wearing thicker gloves (e.g. for operation of industrial equipment). User input controls based on capacitive sensing may also be easy to accidentally trigger because no pressure is needed to trigger the response (any grounded conductor may trigger such controls). The requirement for electric fields to be able to extend from capacitive sensing electrodes to interact with a user's digits restricts the possible materials for a casing or cover protecting capacitive input controls to insulating materials.

An example of a projected capacitance touch panel is described in US 2010/0079384 A1. WO 2016/102975 A2 and WO 2017/109455 A1 describe touch panels which are able to combine projected capacitance touch sensing with piezoelectric pressure sensing in a single touch panel. WO 2019/145674 A1 describes a method of processing signals from a touch panel for combined capacitive and force sensing. This method includes determining, based on capacitance signals, a user interaction period during which a user interaction with the touch panel occurs. This information is used as input to a process for conditional integration of piezoelectric signals.

SUMMARY

According to a first aspect of the invention there is provided a device including a piezoelectric sensor. The piezoelectric sensor includes a layer of piezoelectric material disposed between a number of sensing electrodes and at least one counter electrode. The device also includes a controller connected to the piezoelectric sensor. The sensing electrodes are arranged to form one or more active regions. Each active region includes one or more primary sensing electrodes and one or more secondary sensing electrodes. The secondary sensing electrodes are separated from the primary sensing electrodes by a perimeter. The controller is configured, for each active region, to monitor primary JO piezoelectric charges induced on each primary sensing electrode and to monitor secondary piezoelectric charges induced on each secondary sensing electrode. The controller is also configured, in response to detecting one or more primary and/or secondary piezoelectric charges, to determine whether a corresponding applied force has a centroid within the perimeter based on comparing the primary piezoelectric charges to the secondary piezoelectric charges.

Primary piezoelectric charges may correspond to charges induced on (or collected by) the respective primary sensing electrodes. Secondary piezoelectric charges may correspond to charges induced on (or collected by) the respective secondary sensing electrodes.

The relative areas and positions of primary and secondary sensing electrodes within each active region may be configured to enable distinction between an applied force having a centroid within the perimeter and an applied force having a centroid outside the perimeter.

The perimeter may correspond to a locus of positions which are equidistant between the primary sensing electrodes and the secondary sensing electrodes. The perimeter may be defined as a closed curve such that all corresponding primary sensing electrodes are within the closed curve and all corresponding secondary sensing electrodes are outside the closed curve. The perimeter may be defined at one or both ends of a linear array of primary sensing electrodes. The perimeter may be continuous. The perimeter may be discontinuous.

Each secondary sensing electrode may extend at least partway around the perimeter of the respective active region. Each secondary sensing electrode may extend around at least 20%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90% or more than 95% of a length of the perimeter.

Primary sensing electrodes may alternatively be referred to as active electrodes. Secondary sensing electrodes may alternatively be referred to as localisation electrodes. The primary sensing electrodes and/or the secondary sensing electrodes may be co-planar. The primary sensing electrodes and/or the secondary sensing electrodes may all by supported by a single face of a material which may be flat (planar) or curved.

A secondary sensing electrode may belong to two adjacent active regions. For example, a secondary sensing electrode may be useful to distinguish an applied force having a centroid on the boundary between two adjacent active regions.

The one or more secondary sensing electrodes of a first active region of the one or more active regions may include first and second regions of conductive material disposed on opposite sides of the first active region along a first direction, and may include third and fourth regions of conductive material disposed on opposite sides of the first active region along a second direction which is different to the first direction.

The first and second directions may be perpendicular. The first, second, third and fourth regions of conductive material may in total extend around at least 50%, at least 60%, at least 70%, at least 80%, at least 90% or more than 95% of a length of the perimeter.

The first and second regions of conductive material may be electrically connected together to provide a first secondary sensing electrode. The first and second regions of conductive material may be electrically connected together using one or more conductive traces. The first region of conductive material may be electrically connected to a first conductive trace. The second region of conductive material may be electrically connected to a second conductive trace. The first and second conductive traces may be connected together directly or indirectly. The first and second regions of conductive material may be electrically connected together internally to the piezoelectric sensor. In other words, the electrical connection may be provided as part of the structure of the piezoelectric sensor. The first and second regions of conductive material may be electrically connected together externally to the piezoelectric sensor. The first and second regions of conductive material may be electrically connected together at an input to an amplifier, for example a charge amplifier.

Electrically connected refers to ohmic or resistive coupling, rather than capacitive and/or inductive coupling. In other words, electrically connected refers to a physical connection between electrically conductive materials. An electrical connection may be made via any number of different conductive materials, for example, a pair of conductive traces formed of a first conductive material may be soldered to opposite ends of a wire formed from a second conductive material (the solder being a third conductive material).

The third and fourth regions of conductive material may be electrically connected together to provide a second secondary sensing electrode. The third and fourth regions of conductive material may be electrically connected together using one or more conductive traces. The third region of conductive material may be electrically connected to a third conductive trace. The fourth region of conductive material may be electrically connected to a fourth conductive trace. The third and fourth conductive traces may be connected together directly or indirectly. The third and fourth regions of conductive material may be electrically connected together internally to the piezoelectric sensor. In other words, the electrical connection may be provided as part of the structure of the piezoelectric sensor. The third and fourth regions of conductive material may be electrically connected together externally to the piezoelectric sensor. The third and fourth regions of conductive material may be electrically connected together at an input to an amplifier, for example a charge amplifier.

The first, second, third and fourth regions of conductive material may all be electrically connected together to provide a third secondary sensing electrode. The first, second, third and fourth regions of conductive material may be electrically connected together using one or more conductive traces. The first region of conductive material may be electrically connected to a first conductive trace. The second region of conductive material may be electrically connected to a second conductive trace. The third region of conductive material may be electrically connected to a third conductive trace. The fourth region of conductive material may be electrically connected to a fourth conductive trace. The first, second, third and fourth conductive traces may be connected together directly or indirectly. All, or any pair, of the first, second, third and fourth regions of conductive material may be electrically connected together internally to the piezoelectric sensor. In other words, the electrical connection may be provided as part of the structure of the piezoelectric sensor. All, or any pair, of the first, second, third and fourth regions of conductive material may be electrically connected together externally to the piezoelectric sensor. The first, second, third and fourth regions of conductive material may be electrically connected together at an input to an amplifier, for example a charge amplifier.

Each of the first, second, third and fourth regions of conductive material may provide a separate secondary sensing electrode. Each secondary sensing electrode may be electrically connected by a respective conductive trace.

The controller may be configured to sum piezoelectric charges from the first and second regions of conductive material to determine a first secondary piezoelectric charge corresponding to the first and second regions of conductive material.

The controller may be configured to sum piezoelectric charges from the third and fourth regions of conductive material to determine a second secondary piezoelectric charge corresponding to the third and fourth regions of conductive material.

A second active region of the one or more active regions may include a secondary sensing electrode in the form of a fifth region of conductive material extending around all, or a majority of, the perimeter of the second active region.

The fifth region of conductive material may extend completely around the perimeter within the exception of one or more gaps sized to allow passage of one or more conductive traces connecting to the one or more primary sensing electrodes of the second active region. The fifth region of conductive material may extend completely around the perimeter, and one or more conductive traces connecting to the one or more primary sensing electrodes may each be routed over or under the fifth region of conductive material by respective jumpers, internal connections of a multi-layer printed circuit board (PCB) or equivalent structures. A majority may correspond to at least 50%, at least 60%, at least 70%, at least 80%, at least 90% or at least 95% of the length of the perimeter of the second active region.

A third active region of the one or more active regions may include an array of primary sensing electrodes spaced apart along a path, and a pair of secondary sensing electrodes arranged on the path at either end of the array of primary sensing electrodes.

The perimeter of the third active region may take the form of a first line separating the primary sensing electrodes from a first secondary sensing electrode at a first end of the array, and a second line separating the primary sensing electrodes from a second secondary sensing electrode at a second, opposite end of the array. The path may be a straight line. The path may be a curved path. The piezoelectric sensor may be supported on a side of the device. The piezoelectric sensor may form or provide all, or part of, a side of the device. The piezoelectric sensor may underlie a side of the device. The piezoelectric sensor may be integrated with a side of the device. Each sensing electrode of the third active region may substantially span the side of the device in a direction which lies at an angle to the path. The angle may be perpendicular.

The piezoelectric sensor may include a single counter electrode which is common to all of the plurality of sensing electrodes. The single counter electrode may extend to cover an area of the layer of piezoelectric material which partially or completely overlaps with each of the primary sensing electrodes and which partially or completely overlaps with each of the secondary sensing electrodes.

The single counter electrode may be provided by a metal sheet forming at least part of a casing of the device. The layer of piezoelectric material may be supported by the metal sheet. The metal sheet may be substantially flat or planar. The metal sheet may include one or more curved and/or formed portions. The metal sheet may be formed of steel. The metal sheet may be formed of an aluminium alloy.

The piezoelectric sensor may include a separate counter electrode corresponding to each of the plurality of sensing electrodes. Each counter electrode may partially or completely overlap with the respective primary sensing electrode or secondary sensing electrode. Each counter electrode may be co-extensive with the respective primary sensing electrode or secondary sensing electrode.

All of the secondary sensing electrodes of an active region of the one or more active regions may be opposed across the layer of piezoelectric material by a common secondary counter electrode corresponding to that active region. Each common secondary counter electrode may be shaped and dimensioned to partially or completely overlap each of the secondary sensing electrodes of the respective active region. Each common secondary counter electrode may be shaped and dimensioned to fully or partially surround (or enclose) the perimeter of the respective active region. A common secondary counter electrode may take the form or two or more conductive regions which are electrically connected together. Electrical connections between conductive regions forming a common secondary counter electrode may be internal and/or external to the piezoelectric sensor.

All of the secondary sensing electrodes may be opposed across the layer of piezoelectric material by a single common secondary counter electrode. The single common secondary counter electrode may be shaped and dimensioned to partially or completely overlap each of the secondary sensing electrodes. The single common secondary counter electrode may take the form or two or more conductive regions which are electrically connected together. Electrical connections between conductive regions forming the single common secondary counter electrode may be internal and/or external to the piezoelectric sensor.

Each primary sensing electrode of an active region of the one or more active regions may be opposed across the layer of piezoelectric material by a respective primary counter electrode. Every primary sensing electrode may be opposed across the layer of piezoelectric material by a respective primary counter electrode. Each primary counter electrode may partially or completely overlap with the respective primary sensing electrode. Each primary counter electrode may be co-extensive with the respective primary sensing electrode.

The device may be configured for capacitive touch measurements using the primary sensing electrodes corresponding to the active region. The device may be configured for capacitive touch measurements using the primary counter electrodes corresponding to the active region.

All of the primary sensing electrodes of an active region of the one or more active regions may be opposed across the layer of piezoelectric material by a common primary counter electrode corresponding to that active region. Each common primary counter electrode may be shaped and dimensioned to partially or completely overlap all of the primary sensing electrodes of the corresponding active region. Each common primary counter electrode may be shaped and dimensioned to be co-extensive with the perimeter of the corresponding active region. The primary sensing electrodes of every active region may be opposed by a respective common primary counter electrode. A common primary counter electrode may take the form or two or more conductive regions which are electrically connected together. Electrical connections between conductive regions forming a common primary counter electrode may be internal and/or external to the piezoelectric sensor.

The device may be configured for capacitive touch measurements using the primary sensing electrodes corresponding to the active region.

The primary sensing electrodes of an active region of the one or more active regions may be opposed across the layer of piezoelectric material by a number of primary counter electrodes which is different to the number of primary sensing electrodes belonging to that active region. Each primary counter electrode may partially or completely overlap with one or more of the primary sensing electrodes corresponding to that active region.

The device may be configured for capacitive touch measurements using the primary sensing electrodes corresponding to the active region. The device may be configured for capacitive touch measurements using the primary counter electrodes corresponding to the active region.

For each active region, the corresponding primary and secondary sensing electrodes may be configured with relative areas and positions such that it is possible to define a threshold multiplier corresponding to each secondary sensing electrode of the active region. The threshold multipliers for the active region may satisfy, in response to application of a force having a centroid within the perimeter, a secondary piezoelectric charge collected by each secondary sensing electrode is less than a product of the respective threshold multiplier and a total primary piezoelectric charge collected by all of the primary sensing electrodes. The threshold multipliers for the active region may satisfy, in response to application of a force having a centroid outside the perimeter, a secondary piezoelectric charge collected by at least one secondary sensing electrode is greater than the product of the respective threshold multiplier and the total primary piezoelectric charge collected by all of the primary sensing electrodes. The controller may be configured to store pre-calibrated threshold multipliers corresponding to each secondary sensing electrode. The controller may be configured, for each active region, to determine whether an applied force has a centroid within the perimeter by comparing each secondary piezoelectric charge against a product of the respective threshold multiplier with a sum over the primary piezoelectric charges.

The values of threshold multipliers may be pre-calibrated using experimental measurements obtained in response to known applied forces having known centroid locations. Additionally or alternatively, the values of threshold multipliers may be pre-calibrated using theoretical charge values obtained using a model, for example a finite element model. A pair of active regions corresponding to identical layouts of primary and secondary sensing electrodes may have different values of threshold multipliers, depending on the relative locations of each active region belonging to the pair on the casing of a device including or incorporating the piezoelectric sensor.

The primary sensing electrodes of at least one active region may provide one or more buttons. An active region providing one or more buttons may include a single primary electrode. An active region providing one or more buttons may include a number of primary sensing electrodes, each providing a respective button.

The primary sensing electrodes of at least one active region may provide a slider control. An active region providing a slider control may include three or more primary electrodes arranged spaced apart along a straight or curved path.

The primary sensing electrodes of at least one active region may provide a dial control. An active region providing a dial control may include three or more primary electrodes arranged spaced apart along a circular or elliptical path.

The primary sensing electrodes of at least one active region may provide a swipe gesture control. An active region providing a swipe gesture control may include first and second primary sensing electrodes arranged such that along a swipe direction, a width of the first primary sensing electrode perpendicular to the swipe direction decreases and a width of the second primary sensing electrode perpendicular to the swipe direction increases.

The primary sensing electrodes of at least one active region may provide a button pad. An active region providing a button pad may include a number, N, of primary sensing electrodes. The N primary sensing electrodes may be arranged to in an array of rows and columns to form a grid. The N primary sensing electrodes may be arranged and/or dimensioned to correspond to indicia formed or printed onto an exterior casing of the device or an apparatus including the device. The N primary sensing electrodes may be equally sized. The N primary sensing electrodes may be unequally sized.

The primary sensing electrodes of at least one active region may provide a touch pad. An active region providing a touch pad may include a number N of primary sensing electrodes arranged to form a grid. An active region providing a touch pad may include a first number N1 of primary sensing electrodes extending in a first direction and arranged spaced apart in a second, different direction, and a second number N2 of primary sensing electrodes extending in the second direction and spaced apart in the first direction. The first and second directions may be perpendicular.

According to a second aspect of the invention there is provided a piezoelectric sensor includes a layer of piezoelectric material disposed between a number of sensing electrodes and at least one counter electrode. The sensing electrodes are arranged to form one or more active regions. Each active region includes one or more primary sensing electrodes and one or more secondary sensing electrodes. The secondary sensing electrodes are separated from the primary sensing electrodes by a perimeter. The one or more secondary sensing electrodes of at least one active region include first and second regions of conductive material disposed on opposite sides of that active region along a first direction, and third and fourth regions of conductive material disposed on opposite sides of that active region along a second direction which is different to the first direction. At least one pair of the first, second, third and fourth regions are electrically connected together to provide one or the secondary sensing electrodes.

The piezoelectric sensor according to the second aspect may include features corresponding to any features of the device according to the first aspect.

According to the a third aspect of the invention there is provided a piezoelectric sensor including a layer of piezoelectric material disposed between a number of sensing electrodes and at least one counter electrode. The sensing electrodes are arranged to form one or more active regions. Each active region includes one or more primary sensing electrodes and one or more secondary sensing electrodes. The secondary sensing electrodes are separated from the primary sensing electrodes by a perimeter. The one or more secondary sensing electrodes of at least one active region include a secondary sensing electrode in the form of a region of conductive material extending around all, or a majority of, the perimeter of that active region.

The piezoelectric sensor according to the third aspect may include features corresponding to any features of the device of the first aspect and/or the piezoelectric sensor of the second aspect.

According to a fourth aspect of the invention, there is provided a method of monitoring a piezoelectric sensor. The piezoelectric sensor includes a layer of piezoelectric material disposed between a number of sensing electrodes and at least one counter electrode. The sensing electrodes are arranged to form one or more active regions. Each active region includes one or more primary sensing electrodes and one or more secondary sensing electrodes. The secondary sensing electrodes are separated from the primary sensing electrodes by a perimeter. The method includes monitoring primary piezoelectric charges induced on each primary sensing electrode. The method also includes monitoring secondary piezoelectric charges induced on each secondary sensing electrode. The method also includes, in response to detecting one or more first and/or second charges, determining whether a corresponding applied force has a centroid within the perimeter based on comparing the primary and secondary piezoelectric charges.

The method may include features corresponding to any features of the device of the first aspect, the piezoelectric sensor of the second aspect and/or the piezoelectric sensor of the third aspect.

DETAILED DESCRIPTION

In the following description, like parts are denoted by like reference numerals.

In view of the hereinbefore described problems which may be encountered when using capacitive sensing for “buttonless” input panels, the inventors of the present specification have developed approaches to allow replacement of mechanical switches and other mechanically actuated controls (for example dials, sliders and so forth) using piezoelectric sensors. Piezoelectric sensors are described which may provide “buttonless” force-sensing user input panels/controls by introducing a layer of piezoelectric film and using sensing electrodes of the piezoelectric sensors to provide control elements such as buttons, sliders, dials and so forth.

However, the use of piezoelectric sensors to provide “buttonless” force-sensing user input controls is non-trivial, and simply adding a piezoelectric sensor with sensing JO electrodes directly corresponding to mechanically actuated controls and/or capacitive touch sensors will typically not be sufficient. When a force is applied to a panel, casing, or other structure supporting one or more piezoelectric sensors, the strain is typically not localised to a small area surrounding the application point. Instead, the entire panel will usually deform to some extent, leading to non-negligible signals being generated in sensing electrodes of a piezoelectric sensor located an appreciable distance from the centroid of an applied force.

It is not sufficient to simply assume that the sensing electrode of a piezoelectric sensor providing the largest output signal corresponds to a control which a user is interacting with. Simply selecting the largest signal may be practical when considering, for example, a flat touch panel taking up substantially an entire surface. However, when considering input surfaces which need not be planar and/or which may support a number of spaced apart controls, taking the largest signal may lead to false inputs. For example, a user pressing on a portion of a device casing which is away from any intended input controls may still generate detectable signals in sensing electrodes of a piezoelectric sensor which are laterally spaced several centimetres away. There is a need to distinguish a light press on a sensing electrode of a piezoelectric sensor from a stronger press which does not actually corresponding to that sensing electrode. The present specification concerns methods, and apparatuses which may be used to enable correct localisation of inputs received using piezoelectric sensors defining user input controls.

Referring toFIG.1, a simplified cross-section of a first piezoelectric sensor1for defining input controls is shown.

The piezoelectric sensor1may be used for force-only measurements. Other piezoelectric sensors18(FIG.14) described hereinafter may be used for combined force and capacitance measurements.

The piezoelectric sensor1includes a layer structure2, a counter electrode3and a number of sensing electrodes4. The number, shapes, sizes and positions of the sensing electrodes4define the number and types of user input controls which the piezoelectric sensor1may provide.

The layer structure2has a first face5and a second, opposite, face6. The layer structure2includes one or more layers, including at least a layer of piezoelectric material7. Each layer included in the layer structure2is generally planar and extends in first and second directions x, y which are perpendicular to a thickness direction z. The one or more layers of the layer structure2are arranged between the first and second faces5,6such that the thickness direction z of each layer of the layer structure2is perpendicular to the first and second faces5,6. The sensing electrodes4are disposed on, or over, the first face5of the layer structure2, or the first face5may be bonded to the sensing electrodes4(which may be free-standing or supported on a further substrate which is not shown). The counter electrode3is disposed on, or over, the second face6of the layer structure2. Alternatively, the second face6may be bonded to the counter electrode3(which may be free-standing or supported on a further substrate which is not shown).

Preferably, the piezoelectric layer7is formed of a piezoelectric polymer, for example a suitable fluoropolymer such as polyvinylidene fluoride (PVDF). However, the piezoelectric layer may alternatively be formed from a layer of a piezoelectric ceramic such as lead zirconate titanate (PZT). Unlike in touchscreen applications, transparency of the piezoelectric layer7is not required. However, in some applications a transparent piezoelectric layer7may be used to allow visibility of an underlying display, one or more light emitting diodes, or other elements used to provide visual information about the status of a device to a user. Another option is that a piezoelectric layer7may be translucent (partially transparent), for example to act as an optical diffusing layer for an underlying light-emitting diode (LED).

The layer structure2of the first piezoelectric sensor1may include only the layer of piezoelectric material7, such that the first and second opposite faces5,6are faces of the piezoelectric layer7. In some examples, the layer structure may optionally include one or more dielectric layers8between the piezoelectric layer7and the first face5and/or one or more dielectric layers8between the piezoelectric layer7and the second face6. When included, each dielectric layer8is generally planar and extends in first and second directions x, y which are perpendicular to a thickness direction z. Dielectric layer(s)8may include layers of a polymer dielectric material such as polyethylene terephthalate (PET) or layers of pressure sensitive adhesive (PSA) materials. However, dielectric layer(s)8may include layers of a ceramic insulating material such as aluminium oxide. Dielectric layer(s)8may be transparent, opaque or translucent, depending on the intended application.

The counter electrode3and/or the sensing electrodes4may be formed from any conductive materials such as, for example, conductive oxides such as indium tin oxide (ITO), indium zinc oxide (IZO), conductive polymers such as polyaniline, polythiphene, polypyrrole or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS), metals such as aluminium, copper, silver or other metals suitable for deposition and/or patterning, and so forth. The counter electrode3and/or the sensing electrodes4may be formed from a metal mesh, nanowires, optionally silver nanowires, graphene, or carbon nanotubes. When the first piezoelectric sensor1is intended to overlie a display, or is required to be fully or partly transparent for any other reasons, the materials selected should be transparent. In other examples the first piezoelectric sensor1may be opaque or translucent as mentioned hereinbefore.

Although in some examples the counter electrode3may be a thin electrode which is mechanically supported by the layer structure2, this need not be the case. In some first piezoelectric sensors1, the counter electrode3may be a free-standing conductor such as a metal foil, a metal sheet, or a metal casing for a device or appliance. For example, the counter electrode3may be provided by a steel (or other metal) casing of a device which obtains inputs using a first piezoelectric sensor1. For example, a piezoelectric layer7having sensing electrodes4patterned on the first surface5may be bonded or otherwise securely attached to an interior surface of the steel casing providing the counter electrode3. User input controls may be indicated by indicia printed, engraved, embossed, attached or otherwise defined on the exterior surface of the casing overlying the sensing electrodes4. Using a steel casing of a device as part of an input control would be impossible with capacitive sensing (at least where more than a single input is needed), because a metallic, conductive casing would shield electric fields generated using sensing electrodes from interacting with a user.

Referring also toFIG.2, a plan view of a first sensing electrode layout9is shown.

The first sensing electrode layout9includes four sensing electrodes41,42,43,44evenly spaced along a straight line. The sensing electrodes41,42,43,44are supported on the first face5of the layer structure2as described in relation toFIG.1. The sensing electrodes41,42,43,44may provide user input controls corresponding to a line of discrete buttons, or may operate together to provide a slider control by interpolating a position at which the slider control is pressed based on relative signals from the sensing electrodes41,42,43,44.

When a user presses the first piezoelectric sensor1it will deform, and the corresponding straining of the piezoelectric layer7will generate polarisation and cause charges to be induced between each sensing electrode4and the counter electrode3. The piezoelectric charges Q induced on the sensing electrodes41,42,43,44may be detected and amplified using a measurement front end to (FIG.7). Independent piezoelectric charges Q1, Q2, Q3, Q4may be measured for each of the four sensing electrodes41,42,43,44.

Although the first piezoelectric sensor1includes a single counter electrode3, in other examples of piezoelectric sensors16,18(FIGS.14through21) the counter electrode3may be divided into two or more counter electrodes3, each shaped and dimensioned to oppose one, or a group, of sensing electrodes4across the layer of piezoelectric material.

Referring alsoFIG.3, finite element simulations of piezoelectric charges Q1, Q2, Q3, Q4corresponding to the first sensing electrode layout9are shown as a function of the position of a centroid of an applied force.

The approximate extent of each sensing electrode41,42,43,44is indicated onFIG.3using dotted lines for reference. The data were obtained using finite element simulations conducted using the COMSOL® Multiphysics 5.5 software. The modelled piezoelectric sensor (not shown) was an example of the first piezoelectric sensor1in which the sensing electrodes4were defined by a top conductor layer of a conventional 4-layer printed circuit board (PCB) (not shown) and the counter electrode3was provided by a bottom conductor layer of a conventional 2-layer PCB (not shown). The layer structure2modelled included a layer of piezoelectric material7sandwiched between the 4-layer PCB and the 2-layer PCB using respective layers of pressure sensitive adhesive (PSA). The modelled piezoelectric sensor (not shown) was modelled with a rectangular shape and with supports in the form of six elastomeric hemispheres831, . . . ,836(FIG.35), one located at each corner and one located midway along each of the long edges. The applied force was 1 gram-force (gf), or 0.980665 N. The counter electrode3was modelled at zero volts (ground), although similar results would be expected using any fixed potential. The material parameters used for finite element simulation were (Table 1):

(For the purpose of the anisotropic modulus values, the piezoelectric material was modelled as a film in the x-y plane, with the thickness along the z-axis. The elastomeric hemispherical supports were not coupled to the electric field model.)

It may be observed that the strain resulting from the applied force spreads laterally, so that piezoelectric charges Q1, Q2, Q3, Q4may be observed whichever of the sensing electrodes41,42,43,44is directly pressed. For example, if a planar first piezoelectric sensor1is supported at its edges and able to flex in the middle (an arrangement providing strong signals), the signal spread can be several tens of millimetres. It may be observed that a force having a centroid applied over the middle of the second sensing electrode42results in a charge of Q2≈25 pC. The adjacent first and third sensing electrodes41,43have approximately equal charges Q1≈Q3≈20 pC, and the fourth sensing electrode44still has charge Q4≈14 pC which is about half that of the directly pressed second sensing electrode42.

The lateral spreading of the piezoelectric charges means that it is difficult or impossible to distinguish between a soft touch press directly over sensing electrode4providing a discrete user input button and a hard press some distance away from that sensing electrode4. Consequently the localisation of the applied force would be inaccurate. This problem persists beyond sensing electrodes providing individual discrete buttons. In the example of the first sensing electrode layout9, in may be observed fromFIG.3that determining which of the sensing electrodes41,42,43,44could be done simply based on which has the largest charge signal Q1, Q2, Q3, Q4whilst the centroid of an JO applied force is over or between the sensing electrodes41,42,43,44. However, once the applied force is no longer being applied to any of the sensing electrodes41,42,43,44, for example from a position of about 40 mm onwards inFIG.3, the signals Q1, Q2, Q3, Q4decrease for all of the electrodes41,42,43,44whilst maintaining approximately constant ratios between each pairing of the signals Q1, Q2, Q3, Q4. For such an input it is not possible to distinguish between a light press to the fourth sensing electrode44of the array and a hard press off to the side. This could result in detecting an input in error (false positive), for example, because a user is supporting their weight on a part of a device casing which is not defined as an input control (does not correspond to any sensing electrode(s)4). The same issues with input localisation occur when a force moves outside the immediate area of the sensing electrodes41,42,43,44in any direction.

The first sensing electrode layout9is merely one example, and other examples are described and shown in this specification. However, any possible layout of sensing electrodes4on a piezoelectric sensor1will experience similar issues as the centroid of an applied force moves beyond an area containing the sensing electrodes4.

The techniques, methods and apparatuses of the present specification may help to mitigate and/or overcome these localisation issues. This may be accomplished by adding additional sensing electrodes4which do not correspond directly to an intended user input control, and which instead are arranged about (or equivalently around) one or more edges of a perimeter surrounding the sensing electrodes4intended to define one or more user input controls.

Referring also toFIG.4, a second sensing electrode layout11is shown.

Similarly to the first sensing electrode layout9, the second sensing electrode layout11includes four sensing electrodes4in the form of first to fourth primary sensing electrodes121,122,123,124arranged evenly spaced along a straight line. The second sensing electrode layout11also includes first and second secondary sensing electrodes131,132arranged at either end to bracket the linear array of primary sensing electrodes121,122,123,124. A perimeter14separates the primary sensing electrodes121,122,123,124from the secondary sensing electrodes131,132. The second sensing electrode layout11is disposed, supported on, or bonded on or over a layer structure2in the same way as the first sensing electrode layout9.

For the purposes of this example, we shall restrict consideration to varying the location of a centroid of an applied force along a first axis x parallel to the linear array of sensing electrodes12,13, so that the perimeter14takes the form of a pair of lines oriented along a second direction y. As described hereinafter, in general the perimeter14may take the form of any line, set of two or more lines, closed curve, and so forth, which separates the primary sensing electrodes12from the secondary sensing electrodes13. For example, the perimeter14may generally be taken as a locus of points equidistant between a group of primary sensing electrodes12and a corresponding group of secondary sensing electrodes13surrounding or bracketing the primary sensing electrodes12.

Referring also toFIG.5A, a schematic illustration of piezoelectric charges Q induced on the primary and secondary sensing electrodes12,13of the second sensing electrode layout11is shown.

A measurement front end10(FIG.7) monitors and measures primary piezoelectric charges Qp1, Qp2, Qp3, Qp4corresponding to each primary sensing electrode121,122,123,124, and secondary piezoelectric charges Qs1, Qs2corresponding to each secondary sensing electrode131,132.FIG.5Aillustrates an applied force F having a centroid applied centrally over the third primary sensing electrode123. Since the centroid location xFof the force F is applied within the group of primary sensing electrodes121,122,123,124, i.e. within the perimeter14, the primary sensing electrode123being directly pressed has the largest piezoelectric charge Qp3. This makes it straightforward to determine which of the primary sensing electrodes121,122,123,124is being actuated when the second sensing electrode layout11corresponds to a row of button controls and/or the location xFof the centroid of the force F when the second sensing electrode layout11corresponds to a slider control.

Herein we refer to the location of a centroid of an applied force F rather than an application point because in practice any force is applied over a finite contact area.

Forces applied by a user's digit are typically applied over a contact area within which their digit is deformed into contact with an input surface, and this contact area may be irregular and/or may vary with the magnitude of applied force. To a reasonable approximation, pressure over a contact area may be considered constant, so that the effective point of application of the force F coincides with the centroid of the corresponding contact area.

Referring also toFIG.5B, a schematic illustration is shown of piezoelectric charges Q induced on the primary and secondary sensing electrodes12,13of the second sensing electrode layout11for a different centroid location xFto that shown inFIG.5A.

FIG.5Billustrates an applied force F having a centroid location xFwhich is arranged along the first axis x beyond the second secondary sensing electrode132. As explained in relation toFIG.3, relying only on the primary piezoelectric charges Qp1, Qp2, Qp3, Qp4, it would be impossible to distinguish whether these charges correspond to a light press applied to the fourth primary sensing electrode124or a stronger press displaced along the first axis x (the latter being the situation illustrated). However, with the additional information provided by the secondary piezoelectric charges Qs1, Qs2, it may be observed that the second secondary piezoelectric charge Qs2is the largest. This allows inferring that the centroid location xFof the applied force F is offset away from the primary sensing electrodes12, outside of the perimeter14.

A simple condition may be used to generate a flag indicating whether or not a user is interacting with the user input control(s) provided by the second sensing electrode layout11. The maximum value of the piezoelectric charges Qp1, Qp2, Qp3, Qp4, Qs1, Qs2is determined. If the largest is one of the primary piezoelectric charges Qp1, Qp2, Qp3, Qp4, the flag indicates an interaction is occurring with the corresponding input control(s) and the primary piezoelectric charges Qp1, Qp2, Qp3, Qp4are processed to determine which is pressed and/or the centroid location xFof the force F. However, if the largest value is one of the secondary piezoelectric charges Qs1, Qs2, the flag may instead indicate that the primary piezoelectric charges Qp1, Qp2, Qp3, Qp4should be ignored. This is simply one example using the piezoelectric charges Qp1, Qp2, Qp3, Qp4, Qs1, Qs2for localisation, and alternative approaches are described hereinafter.

In this way, by placing secondary sensing electrodes13around the perimeter of a group of primary sensing electrodes12arranged to provide one or more user input controls of a first piezoelectric sensor1, localisation of forces applied to interact with the user input control(s) may be accomplished. The secondary sensing electrodes13may be invisible, or hidden from, a user of the first piezoelectric sensor1. For example, a casing (which may also provide the counter electrode3) of a device may include indicia printed, engraved, embossed or otherwise defined overlying the primary sensing electrodes12, to indicate to a user where input controls have been defined and/or what function they serve. The secondary sensing electrodes13may have no corresponding indicia, or may correspond to a border provided surrounding the indicia corresponding to primary sensing electrodes12.

The piezoelectric charge Qp, Qs collected be a sensing electrode12,13depends on a combination of the polarisation of the layer of piezoelectric material7in the vicinity, and also upon the area of that sensing electrode12,13. Larger sensing electrodes12,13may generally collect larger piezoelectric charges Qp, Qs for the same force F. As the secondary sensing electrodes131,132are only used for localisation, the signal-to-noise requirements from these electrodes may be less than is required for the primary sensing electrodes121,122,123,124defining one or more user input controls. Consequently, the shapes and/or areas of the secondary sensing electrodes13may be different from the shapes and/or areas of the corresponding primary sensing electrodes. One or more weighting factors α (also referred to herein as “scaling factors”) may be used to account for differences in shapes and/or areas between the primary and secondary sensing electrodes12,13.

For example, referring also toFIG.6, a schematic illustration is shown of charges Q induced on the primary and secondary sensing electrodes12,13of a third sensing electrode layout14, for the same centroid location xFshown inFIG.5B.

The third sensing electrode layout14is the same the second sensing electrode layout11, except that each of the secondary sensing electrodes131,132has half the area of one of the primary sensing electrodes121,122,123,124. To account for this, the secondary piezoelectric charges Qs1, Qs2are multiplied by a factor of two, and this adjusted value is shown in the graph ofFIG.6using a dashed outline. With the correction for the relative areas, the charges Qp1, Qp2, Qp3, Qp4and adjusted charges 2Qs1, 2Qs2obtained using the third sensing electrode layout14may be used for localisation in the same way as the charges Qp1, Qp2, Qp3, Qp4, Qs1, Qs2obtained using the second sensing electrode layout11. Reducing the relative areas of one or more secondary sensing electrodes13associated with a group of primary sensing electrodes12may help to reduce an overall area (or footprint) of a piezoelectric panel1for receiving input.

In the general case, the primary sensing electrodes12may have different shapes and/or areas to each other, or to the secondary sensing electrodes13. Sensing electrodes12,13which are identical in shape and area but which are located in different positions on the piezoelectric panel1may still have different responses to the same force F (applied centrally to each electrode) as a result of a bending response of the first piezoelectric sensor1(accounting for mechanical boundary conditions and so forth). Any such variations may be accounted for by multiplying each piezoelectric charge Qp, Qs by an appropriately calibrated weighting factor. For the nthof a number N of primary sensing electrodes121, . . . ,12n, . . . ,12N, a corresponding adjusted charge Apnmay be defined as:

In which βnis a weighting factor corresponding to the nthprimary sensing electrode12n. Similarly, for the mthof a number M of secondary sensing electrodes13marranged around a perimeter14enclosing the N primary sensing electrodes121, . . . ,12N, an adjusted charge Asmmay be defined as:

In which αmis a weighting factor corresponding to the mthsecondary sensing electrode13m. The adjusted charges Ap1, . . . , ApN, As1, . . . , AsMmay then be compared for the purposes of localisation using any method described in this specification.

A simple test to determine which sensing electrode12,13has the maximum piezoelectric charge Qp, Qs or adjusted charge Ap, As has been described in relation to the second and third sensing electrode layouts11,14, these examples are effectively one-dimensional with the centroid coordinate x1confined to movement along the first axis x. Such configurations may be relevant in practice for some devices, for example, to implement one or more button and/or slider controls on the side of a mobile phone, tablet computer, or any other similar device which is relatively thin in one dimension. These configurations may also be useful when sensing electrodes4form a linear array along a first axis x and substantially or completely span a face of a piezoelectric panel along a second axis y.

For examples in which a centroid coordinate (xF, yF) of a force F may be displaced from a group of primary sensing electrodes12in two lateral directions (e.g. x and y), secondary sensing electrodes13may need to be placed around a perimeter14in the form of a closed curve. Such examples are effectively two-dimensional, as the force F may be displaced relative to primary sensing electrodes12on a surface instead of along a line. A simple test to determine which sensing electrode12,13has the maximum piezoelectric charge Qp, Qs or adjusted charge Ap, As may be useable for some effectively two-dimensional situations, but other conditions may be needed. Specific examples are described in relation toFIGS.7to38H.

Device Include Piezoelectric Input Controls

Referring also toFIG.7, a block diagram schematically illustrating a device15including piezoelectric input controls (or simply the “device”) is shown.

The device15includes a piezoelectric sensor16connected to a controller17. The piezoelectric sensor16includes a layer of piezoelectric material7disposed between a number of sensing electrodes4,12,13and at least one counter electrode3. For example, the piezoelectric sensor16may take the form of the first piezoelectric sensor1or the second piezoelectric sensor18(FIG.14). The sensing electrodes4,12,13of the piezoelectric sensor16are arranged to form one or more active regions19. The piezoelectric sensor16includes at least a first active region191, and optionally may include any number K of further active regions192, . . . ,19K.

The primary sensing electrodes12, Pnand secondary sensing electrodes13, Smof each active region19are not shown with actual shapes and positions inFIG.7. Only schematic blocks representing each primary sensing electrode12, Pnand secondary sensing electrode13, Smare shown. Examples of relative shapes and positions of secondary sensing electrodes13, Smfor an active region19may be found inFIGS.8to12, and examples of active region19layouts including relative shapes and positions of primary sensing electrodes12, Pmand/or secondary sensing electrodes13, Smmay be found inFIGS.8to35.

Each secondary sensing electrode13, Snmay extend at least partway around the perimeter14of the respective active region19. For example, each secondary sensing electrode13, Snmay extend around at least 20% of the corresponding perimeter14. In some active regions19, a single secondary sensing electrode13, S1may extend entirely (or almost entirely) around the perimeter14.

In response to detecting one or more primary and/or secondary piezoelectric charges Qp, Qs from a given active region19, the controller17is configured to determine whether a corresponding applied force has a centroid within the perimeter14correspond to that active region19based on comparing the primary piezoelectric charges Qp and the secondary piezoelectric charges Qs. The relative areas and positions of primary and secondary sensing electrodes12, Pn,13, Smwithin each active region19are configured to enable distinction between an applied force F having a centroid within the corresponding perimeter14kand an applied force F having a centroid outside the perimeter14k.

Method Using Threshold Multipliers

If Equation (3) evaluates as true for every one of the M secondary sensing electrodes13, S1, . . . , Sm, . . . , SM, then an applied force F giving rise to the piezoelectric charges Qpn, Qsmhas a centroid coordinate (xF, yF) which is within the corresponding perimeter14of the active regions19under consideration (for example the kthactive region19kof K active regions191, . . . ,19K).

However, if Equation (3) evaluates as false for at least one of the M secondary sensing electrodes13, S1, . . . , Sm. . . , SM, then an applied force F given rise to the piezoelectric charges Qpn, Qsmhas a centroid coordinate (xF, yF) which is outside the corresponding perimeter14of the active region19.

In order for the method explained in relation to Equation (3) to provide accurate localisation, the primary sensing electrodes12, P1, . . . , Pn, . . . , PNand secondary sensing electrodes13, S1, . . . , Sm. . . , SMof each active region19kneed to be configured with relative areas and positions such that it is possible to calibrate threshold multipliers Th1, . . . , Thm, . . . , ThMwhich satisfy the conditions explained hereinbefore in dependence upon a applied force F having centroid coordinates (xF, yF) inside or outside the corresponding perimeter14. Calibration of suitable threshold multipliers Th1, . . . , Thm, . . . , ThMshould be possible provided that the secondary sensing electrodes13, S1, . . . , Sm. . . , SMextend around a sufficient fraction (preferably most or all) of the perimeter14.

The values of threshold multipliers Th1, . . . , Thm, . . . , ThMmay be pre-calibrated using experimental measurements obtained in response to known applied forces F having known centroid coordinates (xF, yF). Additionally or alternatively, the values of threshold multipliers Th1, . . . , Thm, . . . , ThMmay be pre-calibrated using theoretical charge values obtained using a model, for example a finite element analysis (FEA) model. Examples of calibrating threshold multipliers Th1, . . . , Thm, . . . , ThMbased on data from finite element analysis simulations are described hereinafter in relation toFIGS.35to38H, and the same procedures should be applicable to experimentally obtained data.

A pair of active regions19corresponding to identical layouts of primary and secondary sensing electrodes12,13may have different values of threshold multipliers Th1, . . . , Thm, . . . , ThMdepending on the relative locations of those active regions19on the piezoelectric sensor16, for example due to boundary conditions, the shape of the piezoelectric sensor16and so forth.

The method of threshold multipliers Thmdescribed in relation to Equation (3) may be adapted to use the adjusted charges Apn, Asmcalculated according to Equations (1) and (2) instead of piezoelectric charges Qpn, Qsmas measured, by testing the alternative condition:

Alternatively, the weighting factors αmfor the secondary sensing electrodes13, S1, . . . , SMneed not be determined, and the relative weighting of secondary piezoelectric charges Qsmmay be accounted for directly in the calibration of threshold multipliers by testing the condition:

Method Using Maximum Signal

Although primarily useful for linear arrays which are effectively one-dimensional, for example as described in relation to the second and third sensing electrode layouts it,14, a simple determination of the maximum value of piezoelectric charges Qp, Qs may still be useful for some effectively two-dimensional active regions19. For example, the maximum value of the set of all piezoelectric charges {Qp1, . . . , Qpn, . . . , QpN, Qs1, . . . , Qsm, . . . , Qsm} measured from an active region19may be determined. If the maximum value corresponds to one of the primary piezoelectric charges Qp1, . . . , Qpn, . . . , QpNthen the centroid coordinate (xF, yF) of the corresponding applied force F is within the perimeter14. By contrast, if the maximum value corresponds to one of the secondary piezoelectric charges Qs1, . . . , Qsn, . . . , Qsmthen the centroid coordinate (xF, yF) of the corresponding applied force F is outside the perimeter14.

The method of maximum signal may be less sensitive than the threshold multiplier Thm, method to uneven responses of sensing electrodes12, Pn,13, Smdue to factors such as boundary conditions, shape of the piezoelectric sensor16and relative positions of the electrodes12, Pn,13, Smon the piezoelectric sensor16. The applicability of the maximum signal method may be determined for a given active region19of a particular piezoelectric sensor16based on experimental measurements obtained in response to known applied forces F having known centroid coordinates (xF, yF), and/or using theoretical charge values obtained using a model such as a finite element model.

In general, each active region19of a piezoelectric sensor16may be treated independently. Although all of the active regions191, . . . ,19k, . . . ,19Kof a piezoelectric sensor16may be analysed using a single one of the methods described hereinbefore, in some examples different methods could be applied to different active regions19. For example, an active region19kmay achieve sufficient localisation using the maximum signal method, whilst other active regions19h(h*k) may obtain more accurate localisation using the threshold multiplier Thmmethod.

The controller17shown inFIG.7includes a measurement front end10, one or more digital electronic processors21, memory22and non-volatile storage23. The non-volatile storage23stores program code which may be executed by the one or more processors21, utilising the memory22, to carry out any of the methods and functions described hereinbefore. The non-volatile storage23also stores active region information24defining which channels receiving piezoelectric signals20correspond to primary sensing electrodes12, Pn, which channels correspond to secondary sensing electrodes13, Sm, and which active region191, . . . ,19Keach sensing electrode12Pn,13, Smis associated with.

Optionally, the device15may be a combined force and capacitance sensing device and may additionally include a capacitive touch controller25. When the capacitive touch controller25is included, it may be separate from the measurement front end10, or the controller17. Alternatively, the capacitive touch controller25may be integrated with the measurement front end10and/or the controller17as a single package integrated circuit (IC) or chip.

The measurement front end10detects straining of the piezoelectric layer7in response to one or more forces F applied to the piezoelectric sensor16. Depending on the configuration, the measurement front end10may directly detect potentials induced between the counter electrode(s)3and each sensing electrode12, Pn,13, Smby strain-induced polarisation of the layer of piezoelectric material7. Alternatively, the measurement front end10may detect charge or current flow in response to the strain-induced polarisation of the layer of piezoelectric material7. Charge based measurements are preferred, although the methods for localising applied forces F described hereinbefore may be adapted to measurements of currents or voltages instead using the usual conversions between charge Q, current I=dQ/dt and voltage V=Q/C (with C the capacitance between the addressed sensing electrode12, Pn,13, Smand the counter electrode(s)3). The measurement front end10may have a separate input channel corresponding to each sensing electrode12, Pn,13, Sm. Alternatively, the measurement front end may have fewer input channels than a total number of sensing electrodes12, Pn,13, Sm, and the measurement front end10may address the sensing electrodes12, Pn,13, Smaccording to a sequence (for example using time-division multiplexing).

The measurement front end10may include a low-frequency cut-off filter configured to reject a pyroelectric response of the layer of piezoelectric material7. The low frequency cut-off may take a value between 1 Hz and 7 Hz. The measurement front end10may include a notch filter configured to reject a mains power distribution frequency, for example, 50 Hz or 60 Hz.

When the optional capacitive touch controller25is included, measurements of the mutual- or self-capacitances of some or all of the primary sensing electrodes12, Pnand/or counter electrode(s)3may be made, either directly from the piezoelectric sensor16, or via the measurement front end10. For example, the measurement front end10may measure piezoelectric force signals and capacitances concurrently as described in WO 2017/109455 A1, or as described in WO 2016/102975 A2, and the entire contents of both documents are hereby incorporated by reference. In particular, suitable combined force and capacitance devices15using piezoelectric sensors16in the form of touch panels are shown in, and described with reference to, FIGS. 4 to 23 of WO 2017/109455 A1. Further, suitable combined force and capacitance devices15using piezoelectric sensors16in the form of touch panels are shown in, and described with reference to, FIGS. 15 to 29 of WO 2016/102975 A2.

When the optional capacitive touch controller25is included, the measurement front end10may also relay and/or modify capacitance measurement signals26between the capacitive touch controller25and one or more primary sensing electrodes12, Pnand/or counter electrodes3. The capacitive signal processing module25may function in the same way as a conventional capacitive touch controller, and may be provided by a conventional capacitive touch controller. In some examples, the capacitive touch controller25may provide driving signals for capacitance measurements to the measurement front end10. The mutual- or self-capacitances of some or all of the primary sensing electrodes12, Pnand/or counter electrodes3may be measured by the capacitive touch controller25according to known methods.

Whilst this specification is concerned with methods for localising user inputs based on piezoelectric measurements alone, examples including the optional capacitive touch controller25do not obviate the need for piezoelectric-based localisation of force F locations (xF, yF). Devices15including a capacitive touch controller25and configured for combined piezoelectric and capacitive measurements may adapted their operation to the prevailing input conditions. For example, when the piezoelectric sensor16and a user's un-gloved digit are both dry, the highly localised signals from capacitance measurements may be used. However, when a user is wearing a glove and/or water is present on the piezoelectric sensor16and/or the user's digit, or when a non-conductive object is used for input, the piezoelectric measurements and the methods of localising input described herein may be used to augment, or entirely replace, capacitance measurements.

Once it has been determined which active region or regions19are being pressed, the controller17outputs user input data27including details of which user input controls (e.g. buttons, sliders, touch pads) defined by the primary sensing electrode12, Pnhave been actuated, and optionally the levels of force F detected for each. Conversion of piezoelectric charges Qp, Qsor adjusted charges Ap, Asto force values F will require additional calibration, because the same force F applied to different locations of a piezoelectric sensor16may result in different amounts of strain, depending on the shape of the piezoelectric sensor16, mechanical boundary conditions and so forth. A look-up table, or other model, calibrated using known forces F applied at known coordinates (xF, yF) may be used for converting piezoelectric charges Q1, Qsor adjusted charges Ap, Asto force values F,

The references to WO 2017/109455 A1 and WO 2016/102975 A2 are provided for the purposes of improving understanding the present specification, however the present specification is not limited to methods or apparatuses described in these documents.

The controller17may be implemented in any way capable of providing the functions described herein, for example, a suitably programmed microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) and so forth. Although shown inFIG.7as separate elements within an integrated controller17, the measurement front end10, processor(s)21, memory (22), non-volatile storage23and optionally the capacitive touch controller25may be implemented as separate components or any group may be integrated as a single component. For example, the measurement front end10may be provided as a separate device coupled to a microcontroller providing the functions of the processor(s)21, memory22and non-volatile storage23.

Primary sensing electrodes12, Pnmay alternatively be referred to as “active” electrodes. Secondary sensing electrodes13, Smmay alternatively be referred to as “localisation”, “perimeter” or “perimetric” electrodes. In general, the primary sensing electrodes12, Pnand/or the secondary sensing electrodes13, Smmay be co-planar, or may all be supported by a single face of a material which may be flat (planar) or curved, for example the casing of a device or appliance as described elsewhere herein.

In some examples, a secondary sensing electrode13, Smmay belong to a pair of active regions19k,19k*, which are closely spaced and adjacent. A shared secondary sensing electrode13, Smmay be useful to distinguish and mask an applied force F having a centroid coordinate (xF, yF) on a boundary between such a pair of adjacent active regions19k,19k1).

First Active Region

Referring also toFIG.8, a first configuration of an active region19,28(also referred to as the “first active region” hereinafter) is shown.

The first active region28has a perimeter14in the form of a generally square perimeter29, which encloses a number N of primary sensing electrodes12, P1, . . . , Pn, . . . , PN. First and second regions301,302of conductive material (first and second “conductive regions” hereinafter) are disposed on opposite sides of the first active region28along a first direction x, bracketing (or sandwiching) the perimeter29and primary sensing electrodes12, Pn. Similarly, third and fourth regions303,304of conductive material (third and fourth “conductive regions” hereinafter) are disposed on opposite sides of the first active region28along a second direction y which is different to the first direction x. Each conductive region301,302,303,304extends substantially along the length of an adjacent edge of the square perimeter29. In other words, the conductive regions301,302,303,304may be considered to at least partly define the perimeter29. Each conductive region301,302,303,304is electrically connected to a respective conductive trace311,312,313,314.

The conductive regions301,302,303,304and the generally square perimeter29are shown in schematic plan view inFIG.8. However, the specific numbers, shapes, relative positions and/or relative areas of the primary sensing electrodes12, P1, . . . , Pn, . . . , PNare not relevant to understanding the first active region28and consequently are shown as representative blocks inFIG.8.

The first active region may be configured with between one and four secondary sensing electrodes13, Sm, depending on the connections of the conductive regions301,302,303,304and/or the processing of signals from the conductive regions301,302,303,304.

Quad Secondary Sensing Electrode Configuration

The first active region28may be configured so that each conductive region301,302,303,304provides a respective secondary sensing electrode13, S1, S2, S3, S4. Measurements from each of the four secondary sensing electrodes13, S1, S2, S3, S4may be readout using the respective conductive traces311,312,313,314.

Dual Secondary Sensing Electrode Configuration

The first active region28may be configured so that the conductive regions301,302,303,304provide a pair of secondary sensing electrodes13, S1, S2.

The first and second conductive regions301,302may be electrically connected together to provide the first secondary sensing electrode13, S1. For example, the first and second conductive traces311,312could be merged between the first active region28and a readout from the piezoelectric sensor16. Alternatively, the first conductive trace31, may be routed to the second conductive region302, while the second conductive trace312is routed to allow connection off the piezoelectric sensor16(for example to a measurement front end10). The electrical connection between the first and second conductive regions301,302may in general be direct or indirect, and either internal or external to the piezoelectric sensor. An internal connection is part of, or supported on, the piezoelectric sensor16itself, for example by merging the first and second conductive traces311,312into a single conductive trace for readout. An external connection may be formed away from the piezoelectric sensor16, for example using wires or by merging the connections leading to the first and second conductive regions301,302at an input to an amplifier forming part of the measurement front end10.

Herein, electrically connected refers to ohmic or resistive coupling, rather than capacitive and/or inductive coupling. In other words, a physical connection between electrically conductive materials. An electrical connection may be made via any number of different conductive materials, for example, a pair of conductive traces formed of a first conductive material may be soldered to opposite ends of a wire formed from a second conductive material, the solder being a third conductive material.

The third and fourth conductive regions303,304may also be electrically connected together to provide the second secondary sensing electrode13, S2. An electrical connection between the third and fourth conductive regions303,304may take any form described in relation to the first and second conductive regions301,302.

In this way, the first active region28may be configured to have a pair of secondary sensing electrodes13, S1, S2. The first secondary sensing electrode13, S1is formed from the first and second conductive regions301,302and may be used to detect when the centroid coordinate (xF, yF) of an applied force F crosses the perimeter29moving parallel to the first direction x. The second secondary sensing electrode13, S2is formed from the third and fourth conductive regions303,304and may be used to detect when the centroid coordinate (xF, yF) of an applied force F crosses the perimeter29moving parallel to the second direction y.

An alternative dual-secondary sensing electrode configuration may be provided by electrically connecting the first conductive region301to the third conductive region303and electrically connecting the second conductive region302to the fourth conductive region304. Another dual-secondary sensing electrode configuration may be provided by electrically connecting the first conductive region301to the fourth conductive region304and electrically connecting the second conductive region302to the third conductive region303.

Single Secondary Sensing Electrode Configuration

The first active region28may be configured so that the conductive regions301,302,303,304provide a single secondary sensing electrode13, S1.

For example, the first, second, third and fourth conductive regions301,302,303,304may all be electrically connected together to provide a single secondary sensing electrode13, S1. The electrical connections amongst the conductive regions301,302,303,304may take any form described hereinbefore, and in particular may be direct or indirect, and internal or external to the piezoelectric sensor.

Number of Secondary Sensing Electrodes to Use

In some implementations of a piezoelectric sensor, the strain resulting from an applied force F, and hence the associated piezoelectric polarisation and signal, decreases reasonably quickly with distance from the centroid coordinate (xF, yF). Consequently piezoelectric signals may be dominated by the charge measured from the sensing electrode12, Pn,13, Smclosest to the centroid coordinate (xF, yF). In such circumstances, it may be sufficient for localisation to employ a single secondary sensing electrode13, S1which completely or substantially encloses the square perimeter29of the first active region28, for example, using the single secondary electrode configuration of the first active region28described hereinbefore (alternatively seeFIGS.11and12). The maximum signal method described hereinbefore may be applied, and a force F may be determined to be outside the perimeter29if the charge Qs1(or adjusted charge As1) on the single secondary sensing electrode13, S1is the largest measured for the first active region28.

Application of the maximum signal method should be contingent on confirming (using calibration experiments and/or simulations) that the charge Qs1(or adjusted charge As1) from the single secondary electrode13, S1will be smaller than at least one of the primary piezoelectric charges Qp1, . . . , QpN(or adjusted charges Ap1, . . . , ApN) when the centroid coordinate (xF, yF) of an applied force F is within the perimeter29. Whether or not this condition can be satisfied may depend on factors including, but not limited to, the relative position of the first active region28on the piezoelectric sensor16, the mechanical support and boundary conditions of the piezoelectric sensor16, the shape and/or curvature of the piezoelectric sensor16, and so forth. The maximum signal method need not be used, and a single secondary sensing electrode13, S1which completely or substantially encloses the perimeter29of the first active region28may alternatively be combined with the threshold multiplier method.

The applicability of a single secondary sensing electrode13, S1for localising inputs to any particular example of the first active region28may be checked through calibration measurements using known forces F applied at known coordinates (xF, yF) and/or by modelling (for example finite element analysis).

Often, the distribution of charges induced by polarisation of a piezoelectric layer7of a piezoelectric sensor16exhibits one or more of lateral spreading, anisotropy and/or a dependence on the centroid coordinate (xF, yF) of an applied force F in addition to the magnitude of the force F itself. Such effects may depend on factors including, but not limited to, the relative position of the first active region28on the piezoelectric sensor16, the mechanical support and boundary conditions of the piezoelectric sensor16, the shape and/or curvature of the piezoelectric sensor16, and so forth. Consequently some implementations of the first active region28may obtain more reliable localisation of a force F generating piezoelectric charges Qpn, Qsmto inside/outside the perimeter29by using a dual- or quad-secondary sensing electrode configuration of the first active region. The most accurate configuration for connecting (or not) the conductive regions301,302,303,304needs to be determined in each case by calibration experiments and or modelling such as finite element analysis.

The quad-secondary sensing electrode configuration may be used for calibration experiments and/or modelling purposes. Each conductive region301,302,303,304then corresponds to a respective secondary sensing electrode13, S1, S2, S3, S4. Piezoelectric charges Qs1, Qs2, Qs3, Qs4, Qp1, . . . , QpNinduced in response to a known force F applied at a range of controlled or known locations spanning the perimeter29in at least first and second directions x, y may be measured and/or modelled. Secondary piezoelectric charges corresponding to the dual secondary electrode configuration may be estimated as Qs1+Qs2and Qs3+Qs4(or appropriate sums for other configurations), and secondary piezoelectric charges corresponding to the single secondary electrode configuration may be estimated as Qs1+Qs2+Qs3+Qs4. Using these measured and/or calculated piezoelectric charges Qpn, Qsm, the possible combinations of secondary electrode configuration (quad, dual or single) and signal processing method (maximum signal, threshold multiplier(s)) may be applied, and those which are capable of localising the centroid coordinate (xF, yF) of an applied force to within the perimeter29can be identified. A transition region may be defined as a measure of quality, representing a locus of centroid coordinates (xF, yF) for which a fixed force F would produce piezoelectric charges Qpn, Qsm(or values derived from them) having a difference smaller than a threshold value such as a measured standard error (or a multiple thereof) of the charge measurements. Preferably, the combination of secondary electrode configuration (quad, dual or single) and signal processing method (maximum signal, threshold multiplier(s)) providing the narrowest transition region should be selected.

For the purpose of conducting such simulations, finite element analysis of mechanical deformation linked to electrostatic coupling of an electric field to electrodes (via the strain computed for a piezoelectric layer7) may be solved using commercial available packages such as Comsol®. Selecting the most appropriate secondary sensing electrode configuration is discussed in relation to three examples hereinafter (seeFIGS.35through38H).

Although quad, dual and single secondary sensing electrode configurations have been described, in general any number M of secondary sensing electrodes13, S1, . . . SMmay be used, for example one, two, three, four, five or more. A triple secondary sensing electrode configuration could be provided using the first active region28by, for example, electrically connecting the first and second conductive regions301,302to provide a first secondary sensing electrode S1, whilst using the third and fourth conductive regions303,304as separate second and third secondary sensing electrodes S2, S3.

Although configurations have been described in which one or more of the conductive regions301,302,303,304of the first active region28are physically, electrically connected together, equivalent functionality may instead by provided by combining piezoelectric signals20and/or piezoelectric charges Qpn, Qsmin the controller17(for example in the measurement front end to or the using the processor(s)21).

Each of the conductive traces311,312,313,314may connect the respective conductive region301,302,303,304to a separate input channel of the measurement front end to. A quad secondary sensing electrode configuration is the same as described hereinbefore. An alternative dual secondary sensing electrode configuration may be obtained by summing piezoelectric charges measured from the first and second conductive regions301,302to determine a first secondary piezoelectric charge Qs1(or adjusted secondary charge As1) corresponding to the first and second regions of conductive material as an effective (or virtual) first secondary sensing electrode S1. Similarly, piezoelectric charges from the third and fourth conductive regions303,304may be summed to determine a second secondary piezoelectric charge Qs2(or adjusted secondary charge As2) corresponding to the third and fourth regions of conductive material as an effective (or virtual) second secondary sensing electrode S2.

Although shown as orthogonal directions x, y inFIG.8, the first direction and second directions need not be perpendicular, and may be oriented at any angle larger than zero degrees. For example, the first and second directions may make an angle of 30 degrees, or 45 degrees.

The first, second, third and fourth regions of conductive material may in total extend around at least 50%, at least 60%, at least 70%, at least 80%, at least 90% or more than 95% of a length of the perimeter29.

Second Active Region

Referring also toFIG.9, a second configuration of an active region19,32(also referred to as the “second active region” hereinafter) is shown.

In the same way as the first active region28, the second active region32has a perimeter14in the form of the a generally square perimeter29, which encloses a number N of primary sensing electrodes12, P1, . . . , Pn, . . . , PN.

Fifth and sixth regions305,306of conductive material (fifth and sixth “conductive regions” hereinafter) are disposed on opposite sides of the second active region32along a first direction x′, bracketing (or sandwiching) the perimeter29and primary sensing electrodes12, Pn. Similarly, seventh and eighth regions307,308of conductive material (seventh and eighth “conductive regions” hereinafter) are disposed on opposite sides of the second active region32along a second direction y′ which is different to the first direction x′. The edges of the generally square perimeter29are aligned with orthogonal axes labelled x and y inFIG.9, and the first and second directions x′, y′ are orthogonal to one another and rotated 45 degrees anti-clockwise (counter-clockwise) relative to the axes labelled x and y. Each conductive region305,306,307,308is electrically connected to a respective conductive trace315,316,317,318.

In the first active region28, each of the first to fourth conductive regions301,302,303,304extends substantially along the length of an adjacent edge of the square perimeter29. Similar to the first active region28, the fifth to eighth conductive regions305,306,307,308may be considered to at least partly define the perimeter29. However, in contrast to the first active region28, each of the fifth to eighth conductive regions305,306,307,308includes a corner corresponding to a corner of the square perimeter29, and extends along the edges of the square perimeter29which meet at that corner. Each of the fifth to eighth conductive regions305,306,307,308extends substantially to the middle of the edges of the square perimeter29which meet at the corresponding corner, with a gap to provide electrical isolation from the adjacent conductive regions305,306,307,308.

With the exception of the shape and positioning of the conductive regions305,306,307,308relative to the generally square perimeter29, the second active region32is the same as the first active region28.

The conductive regions305,306,307,308and the generally square perimeter29are shown in schematic plan view inFIG.9. However, the specific numbers, shapes, relative positions and/or relative areas of the primary sensing electrodes12, P1, . . . , Pn, . . . , PNare not relevant to understanding the second active region32and consequently are shown as representative blocks inFIG.9.

In the same way as the first active region28, the second active region32may be configured with between one and four secondary sensing electrode13, Sm, depending on the connections of the conductive regions305,306,307,308and/or the processing of signals from the conductive regions305,306,307,308.

The first and second active regions28,32have been described as including perimeters14in the form of generally square perimeter29(and generally square includes square). However, either of the first and second active regions28,32may be simply modified to work with a perimeter14which is rectangular in shape, or which takes the form of an irregular quadrilateral.

The use of four conductive regions spaced around the perimeter14of an active region19is not limited to regular and/or irregular quadrilaterals, and in general may be applied to an active region19having a perimeter14defined by any closed curve, for example, in the shape of any regular or irregular polygon, a circle or ellipse, an irregular curve, and so forth.

Third Active Region

For example, referring also toFIG.10, a third configuration of an active region19,33(also referred to as the “third active region” hereinafter) is shown.

Similar to the first and second active regions28,32, the third active region33includes four conductive regions309,3010,3011,3012. Unlike the first and second active regions28,32, the third active region33includes a perimeter14in the form of elliptical perimeter34which encloses a number N of primary sensing electrodes12, P1, . . . , Pn, . . . , PN.

Ninth and tenth regions309,3010of conductive material (ninth and tenth “conductive regions” hereinafter) are disposed on opposite sides of the third active region33along a first direction35, bracketing (or sandwiching) the perimeter34and primary sensing electrodes12, Pn. Similarly, eleventh and twelfth regions3011,3012of conductive material (eleventh and twelfth “conductive regions” hereinafter) are disposed on opposite sides of the third active region33along a second direction36which is different to the first direction35. Each conductive region309,3010,3011,3012is electrically connected to a respective conductive trace319,3110,3111,3112.

Each of the ninth to twelfth conductive regions309,3010,3011,3012extends around the curve of the elliptical perimeter34along substantially a quadrant of the elliptical perimeter34. Gaps are left between the ninth to twelfth conductive regions309,3010,3011,3012for electrical isolation amongst the ninth to twelfth conductive regions309,3010,3011,3012. The quadrants shown inFIG.10correspond to the semi-major and semi-minor axes of the elliptical perimeter34.

With the exception of the shape and positioning of the perimeter14,34and conductive regions309,3010,3011,3012, the third active region33is the same as the first or second active regions28,32. In particular, the third active region33may be configured with between one and four secondary sensing electrode13, depending on the connections of the conductive regions309,3010,3011,3012and/or the processing of signals from the conductive regions309,3010,3011,3012.

The conductive regions309,3010,3011,3012and the elliptical perimeter34are shown in schematic plan view inFIG.10. However, the specific numbers, shapes, relative positions and/or relative areas of the primary sensing electrodes12, P1, Pn, PNare not relevant to understanding the second active region32and consequently are shown as representative blocks inFIG.10.

Fourth Active Region

For example, referring also toFIG.11, a fourth configuration of an active region19,37(also referred to as the “fourth active region” hereinafter) is shown.

In the same way as the first and second active regions28,32, the fourth active region37has perimeter14in the form of a generally square perimeter29which encloses a number N of primary sensing electrodes12, P1, . . . , Pn, . . . , PN. Unlike the first and second active regions28,32, the fourth active region37includes a single secondary sensing electrode13, S1provided by a thirteenth region of conductive material3013(thirteenth “conductive region” hereinafter) extending entirely around the perimeter29of the fourth active region37. The thirteenth conductive region3013has a hollow square shape enclosing the perimeter29and primary sensing electrodes12, Pn. Functionally, the fourth active region37will operate similarly to the first or second active regions28,32when either is configured for a single secondary sensing electrode13, S1.

The thirteenth conductive region3013and the generally square perimeter29are shown in schematic plan view inFIG.11. However, the specific numbers, shapes, relative positions and/or relative areas of the primary sensing electrodes12, P1, . . . , Pn, . . . , PNare not relevant to understanding the second active region32and consequently are shown as representative blocks inFIG.11.

The fourth active region37has been described as including a perimeter14in the form of generally square perimeter29(and generally square includes square). However, the fourth active region37may be simply modified to work with a perimeter14which is rectangular in shape, or which in general may be defined by any closed curve, for example, in the shape of any regular or irregular polygon, a circle or ellipse, an irregular curve, and so forth. Regardless of the specific shape of the perimeter14, the thirteenth conductive region3013may take a corresponding shape fully enclosing or surrounding that perimeter14.

Fifth Active Region

Instead of extending completely around the perimeter, a conductive region providing a single secondary sensing electrode13, S1may include one or more gaps to allow routing of conductive traces381, . . . ,38n, . . . ,38Nelectrically connecting to the primary sensing electrodes12, PN.

For example, referring also toFIG.12, a fifth configuration of an active region19,39(also referred to as the “fifth active region” hereinafter) is shown.

The fifth active region39is the same as the fourth active region37, except that a fourteenth conductive region3014providing the single secondary sensing electrode13, S1includes a gap40through which the conductive traces381, . . . ,38n, . . . ,38Nelectrically connecting to the primary sensing electrodes12, P1, . . . . Pn, . . . , PNare routed. The gap40should preferably be just large enough to allow passage of all the conductive traces381, . . . ,38n, . . . ,38N. The fourteenth conductive region3014extends around a majority of the perimeter29of the fifth active region39. A majority may correspond to at least 50%, at least 60%, at least 70%, at least 80%, at least 90% or at least 95% of the length of the perimeter29.

The fourteenth conductive region304and the generally square perimeter29are shown in schematic plan view inFIG.12. However, the specific numbers, shapes, relative positions and/or relative areas of the primary sensing electrodes12, P1, . . . , Pn, . . . , PNare not relevant to understanding the second active region32and consequently are shown as representative blocks inFIG.12.

The fourth active region37has been described as including a perimeter14in the form of generally square perimeter29(and generally square includes square). However, the fifth active region39may be simply modified to work with a perimeter14which is rectangular in shape, or which in general may be defined by any closed curve, for example, in the shape of any regular or irregular polygon, a circle or ellipse, an irregular curve, and so forth. Regardless of the shape of the perimeter14, the fourteenth conductive region3014may take a conforming shape which encloses or surrounds that perimeter14with the exception of a gap40.

Casing Integrated Piezoelectric Sensor

As described in relation to the first piezoelectric sensor1, the counter electrode3for a piezoelectric sensor16may take the form of a metal casing of a device which requires input, for example a steel casing.

Referring also toFIG.13, a portion of a metal casing41for a device or appliance (not shown) requiring input controls is shown.

A casing integrated piezoelectric sensor42(hereinafter “integrated piezoelectric sensor”) may be formed by attaching a layer structure2supporting a plurality of sensing electrodes4to the interior surface of the casing41. The layer structure2may include, for example a piezoelectric material layer7having sensing electrodes4deposited on one face, and a pressure sensitive adhesive applied to the opposite face for attachment to the casing41. The sensing electrodes4are divided into primary sensing electrodes12, Pnand secondary sensing electrodes13, Sm, and arranged into active regions19to provide user input controls as described herein.

Whilst the portion(s) of the casing41supporting the integrated piezoelectric sensor42may be substantially flat or planar, the layer structure2may be made thin and flexible, enabling attachment to portions of the casing41which are curved or which include corners and/or edges43.

The metal casing41is made using a metal sheet, typically formed from steel (preferably stainless steel) or aluminium, although any metal suitable for forming a device or appliance casing may be used. Such metal casings41of devices/appliances are typically grounded (or held at a common mode potential), and therefore may be used to provide the counter electrode3of a piezoelectric sensor16such as the integrated piezoelectric sensor42without disturbing normal operation of the device or appliance. Input controls may be indicated to a user by indicia printed, engraved, embossed, attached or otherwise defined on the exterior surface of the casing41overlying the sensing electrodes4intended for use as primary sensing electrodes12, Pn.

Using a metal casing41of a device/appliance as part of a user input panel would be impossible with capacitive sensing (at least where more than a single input is needed), because a metallic, conductive casing would shield electric fields generated using sensing electrodes from interacting with a user. Direct integration of a user input controls into the metal casing41of a device/appliance may be aesthetically appealing, but may also serve a technical purpose since physical interruption of the casing may be avoided, consequently improving the mechanical strength of a device/appliance and the sealing against ingress of liquids, particles and so forth.

First Counter Electrode Configuration

The counter electrode3of the first piezoelectric sensor1may take the form of a single, uniform electrode (which could alternatively be termed a “global” counter electrode). In this configuration, piezoelectric signals20and charges Qpn, Qsmmust be measured using the sensing electrodes4in order to provide any localisation, since charge on a single counter electrode3will indicate only an overall applied force. Overall applied force may be of interest in some applications, and optionally the measurement front end10may include a channel for reading out a total charge induced on the counter electrode3. If optional capacitance measurements are also obtained, these also must be performed using the sensing electrodes4. For capacitance measurements, the sensing electrodes4need to be closest to a user input surface in use, in order to prevent electrostatic screening by the single counter electrode3.

However, piezoelectric sensors16for use in a device15are not limited to using a single counter electrode3.

Second Counter Electrode Configuration

The second piezoelectric sensor18is the same as the first piezoelectric sensor1, except that a single uniform (or blanket) counter electrode3is replaced by a number of separate counter electrodes3. The second piezoelectric sensor18may provide the piezoelectric sensor16of a device15. Each sensing electrode4is opposed across the layer structure2by a respective counter electrode3which is substantially (or completely) coincident and co-extensive with that sensing electrode4. In other words, the counter electrode3corresponding to a given sensing electrode4has the same shape, area, orientation and centroid coordinate as that sensing electrode4, and therefore completely overlaps that sensing electrode4. In some examples, a sensing electrode4and the opposing counter electrode3need not have exactly the same shape, area and/or orientation, but their respective centroids may coincide so that they overlap at least partially.

The second counter electrode configuration, including a separate counter electrode3corresponding to each of the sensing electrodes4, enables differential measurements of piezoelectric charges Qpn, Qsm. This may reduce interference from noise in the form of external electric fields, and may improving the signal-to-noise ratio for measuring piezoelectric charges Qpn, Qsm.

First Counter Electrode Layout

Referring also toFIG.15, a first example of a counter electrode layout43for a user input panel (hereinafter “first counter electrode layout”) is shown.

The slider active region44includes a linear array of six primary sensing electrodes12, P1, P2, P3, P4, P5, P6, separated by a perimeter14in the form of a rectangular perimeter49from four secondary sensing electrodes13, S1, S2, S3, S4provided by first to fourth conductive regions as described in relation to the first active region28. The slider active region44also includes a corresponding linear array of six primary counter electrodes47, Cp1, Cp2, Cp3, Cp4, Cp5, Cp6which are coincident and co-extensive with the primary sensing electrodes12, P1, P2, P3, P4, P5, P6across the piezoelectric layer7, and four secondary counter electrodes48, Cs1, Cs2, Cs3, Cs4which are coincident and co-extensive with the secondary sensing electrodes13, S1, S2, S3, S4across the piezoelectric layer7. Each of the primary counter electrodes47, Cp1, Cp2, Cp3, Cp4, Cp5, Cp6and the secondary counter electrodes48, Cs1, Cs2, Cs3, Cs4is electrically connected a respective conductive trace (not shown).

The button/touch pad active region45includes an array of twelve primary sensing electrodes12, P1, . . . , P12arranged into four rows and three columns, and separated by a perimeter14in the form of a rectangular perimeter50from four secondary sensing electrodes13, S1, S2, S3, S4provided by first to fourth conductive regions as described in relation to the first active region28. The button/touch pad active region45also includes an array of twelve primary counter electrodes47, Cp1, . . . , Cp12which are coincident and co-extensive with the primary sensing electrodes12, P1, . . . , P12across the piezoelectric layer7, and four secondary counter electrodes48, Cs1, Cs2, Cs3, Cs4which are coincident and co-extensive with the secondary sensing electrodes13, S1, S2, S3, S4across the piezoelectric layer7. Each of the primary counter electrodes47, Cp1, . . . , Cp12and the secondary counter electrodes48, Cs1, Cs2, Cs3, Cs4is electrically connected to a respective conductive trace (not shown).

Each of the three discrete button active regions461,462,463includes a single primary sensing electrode12, P1separated by a corresponding perimeter14in the form of a square perimeter511,512,513from a single secondary sensing electrodes13, S1provided by a single conductive region as described in relation to the fifth active region39. As with the slider active region44and the button/touch pad active region45, each discrete button active region461,462,463also includes a single primary counter electrode47, Cp1which is coincident and co-extensive with the single primary sensing electrode12, P1across the piezoelectric layer7, and a single secondary counter electrode48, Cs1which are coincident and co-extensive with the single secondary sensing electrode13, S1across the piezoelectric layer7. The primary counter electrodes47, Cpnand the secondary counter electrode48, Cs1of each discrete button active regions461,462,463is electrically connected to a respective conductive trace (not shown). Each secondary counter electrode48, Cs1of each discrete button active regions461,462,463includes a respective gap401,402,403for routing a conductive trace (not shown) to electrically contact the corresponding primary counter electrode47, Cp1.

Only the primary counter electrodes47, Cpnand the secondary counter electrodes48, Csmare shown inFIG.15, however, the primary sensing electrodes12, Pnand secondary sensing electrodes13, Smhave identical relative sizes, shapes, orientations and positions on, or over, the first face5.

Using sensing electrodes12, Pn,13, Smand corresponding counter electrodes47, Cpn,48, Csm, the first counter electrode layout43allows for differential measurements of the primary piezoelectric charges Qpnbetween the nthpair of primary sensing electrode12, Pnand primary counter electrode47Cpn, and the secondary piezoelectric charges Qsmbetween the mthpair of secondary sensing electrode13, Smand secondary counter electrode38Csm.

The first counter electrode layout43may be used for capacitance measurements using whichever of the primary sensing electrodes12, Pn, and the primary counter electrodes47, Cpnis (or will be) closest to a user providing input during use. Capacitance measurements may be self-capacitance measurements using individual primary sensing electrodes12, Pn(or primary counter electrodes47, Cpn), or capacitive measurements may be mutual-capacitance measurements using pairs of primary sensing electrodes12, Pn(or pairs of primary counter electrodes47, Cpn). Capacitance measurements may be used to provide more precise localisation during optimal input conditions, for example in dry conditions when a user presses using a digit (without any gloves) and/or uses a conductive stylus.

Second Counter Electrode Layout

Referring also toFIG.16, a second example of a counter electrode layout52for a user input panel (hereinafter “second counter electrode layout”) is shown.

The second counter electrode layout52is for use with an identical layout of primary sensing electrodes12, Pnand secondary sensing electrodes13Smas the first counter electrode layout43. The second counter electrode layout52has identical primary counter electrodes47, Cpnto the first counter electrode layout43. The second counter electrode layout52differs from the first counter electrode layout43in that, for each active region44,45,461,462,463, all of the secondary sensing electrodes13, Smof that active region are opposed across the layer of piezoelectric material7by a single common secondary counter electrode53.

Each common secondary counter electrode53is shaped and dimensioned to partially or completely overlap each of the M≥1 secondary sensing electrodes13, S1, . . . , SMof the respective active region19,44,45,461,462,463. Each of the three discrete button active regions461,462,463only included a single common secondary counter electrode48, Cs1in the first counter electrode layout43, and these are consequently the same for the second counter electrode layout52and provide the corresponding common secondary counter electrodes531,532,543.

In the same way as the first counter electrode layout43, the slider active region44of the second counter electrode layout52includes a linear array of six primary counter electrodes47, Cp1, Cp2, Cp3, Cp4, Cp5, Cp6which are coincident and co-extensive with the primary sensing electrodes12, P1, P2, P3, P4, P5) P6across the piezoelectric layer7. Unlike the first counter electrode layout43, the slider active region44of the second counter electrode layout52includes a common secondary counter electrode534which overlaps with all of the secondary sensing electrodes13, S1, S2, S3, S4across the piezoelectric layer7. Each of the primary counter electrodes47, Cp1, Cp2, Cp3, Cp4, Cp5, Cp6and the common secondary counter electrode534is electrically connected to a respective conductive trace (not shown). The common secondary counter electrode534completely encloses the rectangular perimeter49of the slider active region44, with the exception of a gap404to permit routing of conductive traces (not shown) electrically connecting to the primary counter electrodes47, Cp1, Cp2, Cp3, Cp4, Cp5, Cp6.

In the same way as the first counter electrode layout43, the button/touch pad active region45of the second counter electrode layout52includes an array of twelve primary counter electrodes47, Cp1, Cp12which are coincident and co-extensive with the primary sensing electrodes12, P1, . . . , P12across the piezoelectric layer7. Unlike the first counter electrode layout43, the button/touch pad active region45of the second counter electrode layout52includes a common secondary counter electrode535which overlaps with all of the secondary sensing electrodes13, S1, S2, S3, S4across the piezoelectric layer7. Each of the primary counter electrodes47, Cp1, . . . , Cp12and the common secondary counter electrode535is electrically connected to a respective conductive trace (not shown). The common secondary counter electrode535completely encloses the rectangular perimeter50of the button/touch pad active region45, with the exception of a gap405to permit routing of conductive traces (not shown) electrically connecting to the primary counter electrodes47, Cp1, . . . , Cp12.

In use, a piezoelectric sensor16using the second counter electrode layout52is arranged such that the primary counter electrodes47, Cpnare closest to a user providing input. In this way, the second counter electrode layout52allows for differential measurements of the primary piezoelectric charges Qpnbetween the nthpair of primary sensing electrode12, Pnand primary counter electrode47Cpn. By contrast, differential measurements for the secondary piezoelectric charges Qsmare not possible, and the secondary piezoelectric charges Qsmare obtained using single ended measurements of the secondary sensing electrodes13, Sm. The single ended measurements of secondary piezoelectric charges Qsmwill still have spatial resolution for active regions19which include more than one secondary sensing electrode13, Sm, for example the slider or button/touch pad active regions44,45. The signal-to-noise ratio of secondary piezoelectric charges Qsmmeasured using single ended measurements of the secondary sensing electrodes13, Smmay be improved by holding the common secondary sensing electrodes53at system ground (or another fixed voltage), at least during measurements of the secondary piezoelectric charges Qsm, in order to shield the underlying secondary sensing electrodes13, from external electrical fields.

The second counter electrode layout52may be used for capacitance measurements using the primary counter electrodes47, Cpn. Capacitance measurements may be self-capacitance measurements using individual primary counter electrodes47, Cpn, or capacitive measurements may be mutual-capacitance measurements using pairs of primary counter electrodes47, Cpn. Capacitance measurements may be used to provide more precise localisation during optimal input conditions, for example in dry conditions when a user presses using a digit (without any gloves) and/or uses a conductive stylus.

Third Counter Electrode Layout

Referring also toFIG.17, a third example of a counter electrode layout54for a user input panel (hereinafter “third counter electrode layout”) is shown.

The third counter electrode layout54is the same as the second counter electrode layout52, except that instead of having a common secondary electrode53corresponding to each active region19,44,45,461,462,463and overlapping the secondary sensing electrodes13, Smof that active region19,44,45,461,462,463, the third counter electrode layout54includes a single, global common secondary electrode55which overlaps all of the secondary sensing electrodes13, Sm. For example, the single common secondary electrode55may cover all of the second face6of a layer structure2, with the exception of the perimeters14,49,50511,512,513of each active region and corresponding conduits561,562,563,564,565for routing conductive traces (not shown) to electrically connect to the primary counter electrodes47, Csn.

In use, a piezoelectric sensor16using the third counter electrode layout54is arranged such that the primary counter electrodes47, Cpnare closest to a user providing input. In this way, the third counter electrode layout54allows for differential measurements of the primary piezoelectric charges Qpnand single-ended measurements of the secondary piezoelectric charges Qsmin the same way as the second counter electrode layout52. The more extensive (or blanket) coverage of the single common secondary electrode55may provide more effective shielding of external electrical fields, compared to the multiple common secondary electrodes53of the second counter electrode layout52.

The third counter electrode layout54may be used for capacitance measurements using the primary counter electrodes47, Cpn. Capacitance measurements may be self-capacitance measurements using individual primary counter electrodes47, Cpn, or capacitive measurements may be mutual-capacitance measurements using pairs of primary counter electrodes47, Cpn. Capacitance measurements may be used to provide more precise localisation during optimal input conditions, for example in dry conditions when a user presses using a digit (without any gloves) and/or uses a conductive stylus.

Fourth Counter Electrode Layout

Referring also toFIG.18, a fourth example of a counter electrode layout57for a user input panel (hereinafter “fourth counter electrode layout”) is shown.

The fourth counter electrode layout57is the same as the second counter electrode layout52, except that instead of having a separate primary counter electrode47, Cpncorresponding to each primary sensing electrode12, Pn, each active region19,44,45,461,462,463includes a common primary counter electrode58. Within each active region19,44,45,461,462,463the respective common primary counter electrode58opposes all of the primary sensing electrodes12, Pnbelonging to that active region19,44,45,461,462,463across the layer of piezoelectric material7.

Each of the three discrete button active regions461,462,463only includes a single primary counter electrode47, Cp1in the second counter electrode layout52, and these are consequently the same for the fourth counter electrode layout57and provide the corresponding common primary counter electrodes581,582,583.

The slider active region44of the fourth counter electrode layout57includes an elongated rectangular common primary counter electrode584which partially or completely overlaps each of the primary sensing electrodes12, P1, P2, P3, P4, P5, P6of the linear array across the piezoelectric layer7. The common primary counter electrode584and the common secondary counter electrode53are electrically connected to respective conductive traces (not shown). The common secondary counter electrode534completely encloses the rectangular perimeter49of the slider active region44, with the exception of a gap404to permit routing of a conductive trace (not shown) electrically connecting to the common primary counter electrode584.

The button/touch pad active region45of the fourth counter electrode layout57includes a rectangular common primary counter electrode584which partially or completely overlaps each of the primary sensing electrodes12, P1, . . . , P12across the piezoelectric layer7. The common secondary counter electrode535completely encloses the rectangular perimeter50of the button/touch pad active region44, with the exception of a gap405to permit routing of a conductive trace (not shown) electrically connecting to the common primary counter electrode585.

In use, a piezoelectric sensor16using the second counter electrode layout52is arranged such that the common primary counter electrodes581, . . . ,585are closest to a user providing input. In this way, the fourth counter electrode layout52allows for single ended measurements of the primary piezoelectric charges Qp1, using individual primary sensing electrodes12, Pnand for single ended measurements of the secondary piezoelectric charges Qsmusing individual secondary sensing electrodes13, Sm. The common primary counter electrodes58and the common secondary counter electrodes53may be held at system ground, or another fixed voltage, at least during readout of piezoelectric charges Qpn, Qsm, in order to provide shielding of the primary and secondary sensing electrodes12, Pn,13, Sm, from external electrical fields

The fourth counter electrode layout52may be used for capacitance measurements using the common primary counter electrodes58, though these would be limited to self-capacitance measurements. Although lacking spatial resolution within each perimeter14,49,50,511,512,513, such self-capacitance measurements performed using a common primary counter electrode58may still permit localisation of which active regions19,44,45,461,462,463are being interacted with during optimal input conditions, for example in dry conditions when a user presses using a digit (without any gloves) and/or uses a conductive stylus.

Fifth Counter Electrode Layout

Referring also toFIG.19, a fifth example of a counter electrode layout59for a user input panel (hereinafter “fifth counter electrode layout”) is shown.

The fifth counter electrode layout59combines the single, global common secondary counter electrode55of the third electrode layout54with the common primary counter electrodes581,582,583,584,585of the fourth counter electrode layout57. The primary and secondary sensing electrodes12, Pn,13, Smhave the same layout as any of the first to fourth counter electrode layouts43,52,54,57.

Sixth Counter Electrode Layout

Referring also toFIG.20, a sixth example of a counter electrode layout60for a user input panel (hereinafter “sixth counter electrode layout”) is shown.

The sixth counter electrode layout60is for use with primary and secondary sensing electrodes12, Pn,13, Smhaving the same layout as any of the first to fifth counter electrode layouts43,52,54,57,59. The sixth counter electrode layout60uses the same layout of common secondary counter electrodes531,532,533,534,535as the second counter electrode layout52or the fourth counter electrode layout57.

The sixth counter electrode layout60differs from the first to fifth counter electrode layouts43,52,54,57,59in that the number NC of primary counter electrodes47, Cpnfor each active region19,44,45,461,462,463need not be equal to the number N of primary sensing electrode12, Pn(i.e. NC=N) or one common primary counter electrode58(i.e. NC=1). In the sixth counter electrode layout60, the number NC of primary counter electrodes47, Cpnfor each active region19,44,45,461,462,463may take any value between one and the number N of primary sensing electrodes12, Pnbelonging to that active region19,44,45,461,462,463(between here includes the endpoints so that 1≤NC≤N) In other words, the primary sensing electrodes12, P1, . . . , PNof an active region19,44,45,461,462,463of a piezoelectric sensor16are opposed across the layer of piezoelectric material7by a number NC of primary counter electrodes47, Cp1, . . . , CpNCwhich may be less than the number N of primary sensing electrodes12, P1, . . . , PNbelonging to that active region19,44,45,461,462,463. Additionally and/or alternatively, the primary counter electrodes47, Cpnof one or more active regions19,44,45,461,462,463need not have the same shapes, areas and/or orientations as the corresponding primary sensing electrodes12, Pn.

The three discrete button active regions461,462,463and the button/touch pad active region45have the same configurations of primary counter electrode47, Cpnas the second counter electrode layout52.

The slider active region44includes N=6 primary sensing electrodes12, P1, . . . , P6arranged in a linear array, each of which takes the form of a square electrode. The slider active region44includes NC=5 primary counter electrodes12, Cp1, . . . , Cp5arranged in a linear array spanning the same length as the linear array of primary sensing electrodes12, P1, . . . , P6. A consequence of the smaller number NC<N of primary counter electrodes12, Cp1, . . . , Cp5is that these are spaced more widely than the primary sensing electrodes12, P1, . . . , P6. Additionally the primary counter electrodes12, Cp1, . . . , Cp5are each chevron shaped instead of square.

In use, a piezoelectric sensor16using the sixth counter electrode layout60is arranged such that the primary counter electrodes47, Cpnare closest to a user providing input. The three discrete button active regions461,462,463and the button/touch pad active region45may be used for measurements of piezoelectric charges Qpn, Qsmas described hereinbefore for the second counter electrode layout52. For the slider active region44, measurements of secondary piezoelectric charges Qsmmay be carried out as described in relation to the second counter electrode layout52. Measurements of the primary piezoelectric charges Qpnmay be carried out using single ended measurements from the primary sensing electrodes12, P1, . . . , P6, preferably with all of the primary counter electrodes47, Cp1, . . . , Cp5connected to system ground (or other fixed voltage) to at least partially shield the primary sensing electrodes12, P1, . . . , P6from external electrical fields. During other periods, for example sequentially interspersed with periods for measuring primary piezoelectric charges Qpn, the primary counter electrodes47, Cp1, . . . , Cp5may be used for self-capacitance measurements. The capability to perform capacitive measurements of localisation when input conditions permit this has been described hereinbefore. A potential advantage of the sixth counter electrode layout60is that the electrode pitch (and associated spatial resolution) for measurements of piezoelectric charges Qpnusing primary sensing electrodes12, P1, . . . , P6may be different (finer or coarser) than an electrode pitch (and associated spatial resolution) for measurements of capacitance using the primary counter electrodes47, Cp1, . . . , Cp5.

Although the example of the slider active region44of the sixth counter electrode layout60shows a number NC of primary counter electrodes47, Cpnbeing less than a number N of primary sensing electrodes12, Pn(NC<N), in general the sixth counter electrode layout60may include a mixture of some active regions19in which NC=N, other active regions in which NC<N, and still further active regions19in which NC>N. Regardless of whether NC<N, NC=N or NC>N, each primary counter electrode47, Cpnshould preferably partially or completely overlap with one or more of the primary sensing electrodes Pncorresponding to the same active region19.

Seventh Counter Electrode Layout

Referring also toFIG.21, a seventh example of a counter electrode layout61for a user input panel (hereinafter “seventh counter electrode layout”) is shown.

The seventh counter electrode layout61is for use with primary and secondary sensing electrodes12, Pn,13, Smhaving the same layout as any of the first to sixth counter electrode layouts43,52,54,57,59,60. The seventh counter electrode layout61uses the same, global common secondary counter electrode55as the third counter electrode layout54or the fifth counter electrode layout59. The seventh counter electrode layout61uses the same layout of primary counter electrodes47, Pnas the sixth counter electrode layout60, except for the button/touch pad action region45.

In the sixth counter electrode layout60(and also the first to third counter electrode layouts), the button/touch pad active region45includes an array of twelve primary counter electrodes47, CP1, . . . , Cp12which are coincident and co-extensive with the primary sensing electrodes12, P1, . . . , P12across the piezoelectric layer7. By contrast, in the seventh counter electrode layout61, the button/touch pad active region45includes three (NC=3) primary counter electrodes47, Cp1, Cp2, Cp3. A first primary counter electrode47, Cp1substantially overlaps all of the primary sensing electrodes12, P1, . . . , P9forming the top (relative toFIG.21) three rows of the array. A second primary counter electrode47, Cp2substantially overlaps both the primary sensing electrodes12, P10, P11on the bottom (Relative toFIG.21) row of the array. A third primary counter electrode12, Cp3is coincident and coextensive with the final primary sensing electrode12, P12.

In some examples, each different primary counter electrode47, Cpmmay correspond to a sub-region of an active region19. For example, inFIG.21, the first primary counter electrode47, Cp1corresponding to the primary sensing electrodes12, P1, . . . , P9may correspond to a numeric keypad with the numbers one through9for inputting part of a code, the second primary counter electrode47, Cp2corresponding to the primary sensing electrodes12, P10, P11may correspond to buttons denoting letters “A” and “B” for forming part of a code, and the third primary counter electrode47, Cp3corresponding to the final primary sensing electrode12, P12may correspond to a button for indicating that input of a code to the button/touch pad active region45is completed (“enter” key).

Although the first through seventh counter electrode layouts43,52,54,57,59,60,61have been described and illustrated in relation to specific examples, these examples are only intended to illustrate the underlying principles. Other examples designed according to the principles outlined hereinbefore may include combinations of active regions19providing alternative or further user input controls, and each active region may include more or fewer sensing electrodes12,13and/or counter electrodes47,48,53,55,58than the first through seventh counter electrode layouts43,52,54,57,59,60,61. Sensing electrodes12,13and/or counter electrodes47,48,53,55,58are not limited to the shapes, relative sizes, relative positions, or other specific geometric details of the first through seventh counter electrode layouts43,52,54,57,59,60,61.

Examples of User Input Controls

A large variety of different shapes and configurations of user input controls may be defined using a combination of one or more primary sensing electrodes12, Pndisposed within a perimeter14and one or more secondary sensing electrodes13, Smarranged about the perimeter14.

Button Controls

For example, referring also toFIG.22, an active region19providing a first button control62is shown.

The first button control62includes one primary sensing electrode12, P1in the form of a square with rounded corners. The first button control62uses the fourth configuration of active region37, and includes one secondary sensing electrode13, S1extending entirely around the primary sensing electrode12, P. The secondary sensing electrode13, S1has a shape conforming to the primary sensing electrode12, P1, namely a square with rounded corners, with an internal space enclosing the primary sensing electrode12, P1also having the shape of a square with rounded corners. A perimeter14separating the primary sensing electrode12, P1from the secondary sensing electrode13, S1takes the form of the locus of points equidistant between the primary and secondary sensing electrodes12, P1,13, S1.

The first button control62may be used to provide a discrete, pressure sensing button. When no capacitance measurements are obtained (or when capacitance measurements are rendered inoperable by environmental conditions such as a wet input surface), an input to the first button62may be localised by comparing the primary piezoelectric charge Qp1(or adjusted charge Ap1) measured from the primary sensing electrode12, P1with the secondary piezoelectric charge Qs1(or adjusted charge As1) measured from the secondary sensing electrode13, S1, for example using one or more of the methods described hereinbefore.

Referring also toFIG.23, an active region19providing a second button control63is shown.

The second button control63is similar to the first button control62, except that the primary sensing electrode12, P1takes the form of a square, and that the second button63uses a secondary sensing electrode13, S1configured according to the fifth active region39. The perimeter14separating the primary sensing electrode12, P1and the secondary sensing electrode13, S1is also square.

Referring also toFIG.24, an active region19providing a third button control64is shown.

The third button control64is the same as the second button control63, except that each of the primary sensing electrode12, P1, the secondary sensing electrode13, S1and the perimeter14are substantially circular (and substantially concentric).

Referring also toFIG.25, an active region19providing a fourth button control65is shown.

The fourth button control65is the same as the third button control64, except that instead of a single secondary sensing electrode13, S1extending around the entire perimeter14(excepting a gap40), the fourth button65includes four secondary sensing electrodes13, S1, S2, S3, S4, each extending around substantially a quarter of the circular perimeter14. The secondary sensing electrodes13, S1, S2, S3, S4of the fourth button65are configured as described in relation to the first to third active regions28,32,33.

User input controls provided by an active region19are not limited to single buttons, and in other examples, two or more primary sensing electrodes12, Pnmay be disposed within the perimeter14of an active region19, each primary sensing electrodes12, Pnproviding a corresponding discrete button.

Slider Controls

Referring also toFIG.26, an active region19providing a first slider control66is shown.

The first slider control66includes four primary sensing electrodes12, P1, P2, P3, P4each in the form of a square with rounded corners. The primary sensing electrodes12, P1, P2, P3, P4are arranged evenly spaced along a straight line. A perimeter14having the shape of a rectangle with rounded corners encloses the primary sensing electrodes12, P1, P2, P3, P4, and is in turn enclosed by a single secondary sensing electrode13, S1configured as described in relation to the fourth active region37. The perimeter14is substantially coincident with the locus of points equidistant between the four primary sensing electrodes12, P1, P2, P3, P4and the secondary sensing electrode13, S1.

The four primary sensing electrodes12, P1, P2, P3, P4may be used together to provide a slider control, for example by interpolating a pressed position on the first slider control66based on comparing and/or interpolating the primary piezoelectric charges Qp1, Qp2, Qp3, Qp4(or adjusted charges Ap1, Ap2, Ap3, Ap4) measured from the respective primary sensing electrodes12, P1, P2, P3, P4. When no capacitance measurements are obtained (or when capacitance measurements are rendered inoperable by environmental conditions such as a wet input surface), an input to the first slider control66may be localised by comparing the primary piezoelectric charges Qp1, Qp2, Qp3, Qp4(or adjusted charges Ap1, Ap2, Ap3, Ap4) measured from the respective primary sensing electrodes12, P1, P2, P3, P4with the secondary piezoelectric charge Qs1(or adjusted charge As1) measured from the secondary sensing electrode13, S1, for example using one or more of the methods described hereinbefore.

Additionally or alternatively, each of the four primary sensing electrodes12, P1, P2, P3, P4may be used as a discrete button. For example, the first slider control66may be used as an array of four discrete buttons, instead of as a slider.

Although shown inFIG.26with four primary sensing electrodes12, P1, P2, P3, P4evenly spaced along a first direction x, in general the first slider control66(or first button array66when used as discrete buttons) may include any number N>2 of primary sensing electrodes12, P1, . . . , PNevenly or irregularly spaced along a line oriented in any direction. For use as a slider control, at least three primary sensing electrodes12, P1, P2, P3are preferable.

Referring also toFIG.27, an active region19providing a second slider control67is shown.

The second slider control67is the same as the first slider control66, except that the second slider control67includes six primary sensing electrodes12, P1, . . . , P6evenly spaced along a first direction x instead of four, and in that the second slider control67includes four secondary sensing electrodes13, S1, S2, S3, S4configured as described in relation to the first to third active regions28,32,33.

In the same way as the first slider control66, each of the primary sensing electrodes12, P1, . . . , P6of the second slider control67may be configured to provide a discrete force sensing button, instead of functioning as an element of a slider control.

As discussed hereinafter in relation toFIGS.35and38A to38H, depending on the configuration and relative position within a piezoelectric sensor16, the second slider control67including four secondary sensing electrodes13, S1, S2, S3, S4arranged top, bottom, left and right (relative to the directions shown inFIG.27) may provide improved localisation compared to a single secondary sensing electrode13, S1used for the first slider control66.

Although shown inFIG.27with six primary sensing electrodes12, P1, . . . , P6evenly spaced along a first direction x, in general the second slider control67(or second button array67when used as discrete buttons) may include any number N>2 of primary sensing electrodes12, P1, . . . , PNevenly or irregularly spaced along a line oriented in any direction. For use as a slider control, at least three primary sensing electrodes12, P2, P2, P3are preferable. The four secondary sensing electrodes13, S1, S2, S3, S4may be arranged analogously relative to the orientation of the line along which the primary sensing electrodes12, P1, . . . , PNare arranged.

Referring also toFIG.28, an active region19providing a third slider control69is shown.

The third slider control68is the same as the first slider control66, except that the second slider control67includes five primary sensing electrodes12, P1, . . . , P5evenly spaced along a arcuate path69, instead of four spaced along a direction x. In the same way as the first slider control66, the third slider control68includes a single secondary sensing electrode13, S1enclosing the primary sensing electrodes12, P1, . . . , P5and configured as described in relation to the fourth active region37.

In the same way as the first or second slider controls66,67, each of the primary sensing electrodes12, P1, . . . , P5of the third slider control68may be configured to provide a discrete force sensing button, instead of functioning as an element of a slider control.

Although shown inFIG.28with five primary sensing electrodes12, P1, . . . , P5evenly spaced along an arcuate path, in general the third slider control68(or third button array68when used as discrete buttons) may include any number N>2 of primary sensing electrodes12, P1, . . . , PNevenly or irregularly spaced along any curved and/or straight path (or a path including straight segments and curved segments). For use as a slider control, at least three primary sensing electrodes12, P1, P2, P3are preferable. A single secondary sensing electrode13, S1need not be used, and instead a number M of secondary sensing electrodes13, S1, . . . , SMmay be spread about the perimeter14. For example, four secondary sensing electrodes13, S1, S2, S3, S4configured as described in relation to the first to third active regions28,32,33may be used.

Referring also toFIG.29, an active region19providing a first button pad control70is shown.

The first button pad control70includes a number, N, of primary sensing electrodes12, P1, . . . , PN. The primary sensing electrodes12, P1, PNare arranged in an array of rows and columns to form a grid. In the example shown inFIG.29, the first button pad70includes an array of twelve primary sensing electrodes12, P1, . . . , P12arranged into four rows and three columns. The first button pad control70has a substantially rectangular perimeter14which encloses the primary sensing electrodes12, P1, . . . , P12and separates them from four secondary sensing electrodes13, S1, S2, S3, S4provided by first to fourth conductive regions as described in relation to the first active region28.

Each of the primary sensing electrodes12, P1, . . . , PNprovides a corresponding pressure sensing button, and each may correspond to a different user input. For example, the twelve primary sensing electrodes12, P1, . . . , P12shown inFIG.29may correspond to respective inputs “1”, “2”, “3”, “4”, “5”, “6”, “7”, “8”, “9”, “o”, “#”, “*” to provide a numeric keypad. In some examples, the first button pad70may form part of a piezoelectric sensor16bonded or otherwise supported on the interior of a casing of a device15, or an apparatus including the device15. Each of the primary sensing electrodes12, P1, . . . , PNmay be positioned to correspond to indicia printed, engraved, embossed or otherwise formed onto an exterior surface of the casing.

When no capacitance measurements are obtained (or when capacitance measurements are rendered inoperable by environmental conditions such as a wet input surface), an input to the first button pad70may be localised by comparing the primary piezoelectric charges Qp1, . . . , Qp12(or adjusted charges Ap1, . . . , Ap12) measured from the respective primary sensing electrodes12, P1, . . . , P12with the secondary piezoelectric charges Qs1. . . , Qs4(or adjusted charges As1, . . . , As4) measured from the secondary sensing electrodes13, S1, S2, S3, S4, for example using one or more of the methods described hereinbefore.

Additionally or alternatively, the primary sensing electrodes12, P1, . . . , P12may be used together to provide a first touch panel control, for example by interpolating a coordinate (xF, yF) for a force F having a centroid applied within the perimeter14based on comparing and/or interpolating the primary piezoelectric charges Qp1, . . . , Qp12(or adjusted charges Ap1, . . . , Ap12) measured from the respective primary sensing electrodes12, P1, . . . , P12. For example, when an input is localised to within the perimeter14(either using capacitive sensing or by comparison of primary Qpnand secondary Qsmcharges), the primary sensing electrode12, Pncorresponding to the peak primary piezoelectric charge Qpnmay be identified. The neighbouring primary sensing electrodes12, Pnalong the row and the column containing the peak primary piezoelectric charge Qpnare identified, and a coordinate (xF, yF) interpolated based on the corresponding primary piezoelectric charges Qpn.

The N primary sensing electrodes12, Pnof the first button pad control70may be equally or unequally sized. When configured to provide discrete buttons, the primary sensing electrodes12, Pnneed not be arranged in a regular array, and may instead be positioned arbitrarily within the perimeter14depending on a desired layout of discrete force sensing buttons. When configured to provide a first touch panel, the N primary sensing electrodes12, Pnof the first button pad control70are preferably (if not essentially) of equal sizes and arranged in a regular lattice (which need not be square or rectangular).

Although illustrated inFIG.29with four secondary sensing electrodes13, S1, S2, S3, S4provided by first to fourth conductive regions as described in relation to the first active region28, the first button pad control70may alternatively use any other configuration of one or more secondary sensing electrodes13, Smdescribed herein, for example in relation any of the second to fifth active regions32,33,37,39. In generally, the first button pad control70may include more or fewer than four secondary sensing electrodes13, Sm, though preferably the secondary sensing electrodes13, Smtaken together will completely or substantially enclose the perimeter14.

Although the first button pad70may be used as a touch pad/panel, other arrangements of primary sensing electrodes12, Pnmay provide touch pads/panels.

Referring also toFIG.30, an active region19providing a first touch panel control71is shown.

The first touch panel control71includes a number N of primary sensing electrodes12, Pn, including a first number N1 of primary sensing electrodes12, P1, . . . , PN, extending in a first direction x and arranged spaced apart in a second, different direction y, and a second number N2 of primary sensing electrodes12, PN−N2+1, . . . , PNextending in the second direction y and spaced apart in the first direction x (where N1+N2=N). In the example shown inFIG.30, the first touch panel71includes a first number N1=6 of primary sensing electrodes12, P1, . . . , P6extending in the first direction x and a second number N2=5 of primary sensing electrodes12, P7, . . . , P11extending in the second direction y. The first touch panel71also includes a single secondary sensing electrode13, S1configured as described in relation to the fourth active region37.

Each of the five primary sensing electrodes12, P7, . . . , P11extending in the second direction y is formed of a continuous (or unitary) region of conductive material including diamond shaped regions evenly spaced along the second direction y and connected by narrow bridging segments. Each of the six primary sensing electrodes12, P1, . . . , P6extending in the first direction x is formed from a number of diamond shaped regions (or portions thereof) evenly spaced along the first direction x and connected together by jumpers (or equivalent structures) which are insulated from the primary sensing electrodes12, P7, . . . , P11at intersections. In this way, the primary sensing electrodes12, P1, . . . , P11may adopt a diamond-patterned configuration extensively used in conventional projected capacitance touch panels.

When no capacitance measurements are obtained (or when capacitance measurements are rendered inoperable by environmental conditions such as a wet input surface), an input to the first touch pad/panel71may be localised by comparing the primary piezoelectric charges Qp1, . . . , Qp1, (or adjusted charges Ap1, Ap11) measured from the respective primary sensing electrodes12, P1, . . . , P11with the secondary piezoelectric charge Qs1(or adjusted charge As1) measured from the secondary sensing electrode13, S1, for example using one or more of the methods described hereinbefore.

When the coordinate (xF, yF) of the centroid of an applied force F is localised to within the perimeter14, a more precise estimate of the coordinate (xF, yF) may be obtained using the primary sensing electrodes12, P1, . . . , P11. Piezoelectric charges Qp1, . . . , Qp6(or adjusted charges Ap1, . . . , Ap6) from the six primary sensing electrodes12, P1, . . . , P6spaced apart along the second direction y may be interpolated to estimate the coordinate yFalong the second direction y, whilst piezoelectric charges Qp7, . . . , Qp11(or adjusted charges Ap1, . . . , Ap6) from the five primary sensing electrodes12, P7, . . . , P11spaced apart along the first direction x may be interpolated to estimate the coordinate xFalong the first direction x.

When the capacitive touch controller25is present (and enabled), the capacitive touch controller25may be used to determine a touched location from measurements of mutual capacitances at the intersections of the primary sensing electrodes12, P1, . . . , P6and the primary sensing electrodes12, P7, . . . , P11.

Although the first and second directions x, y are shown as perpendicular inFIG.30, this is not necessary. The piezoelectric sensor16, or at least the portion of it corresponding to a first touch panel control71, may be transparent if it is overlying a display. However, the piezoelectric sensor16and a portion of it corresponding to a first touch panel control71may also be opaque.

Dial Controls

Referring also toFIG.31, an active region19providing a first dial control72is shown.

The first dial control72includes four primary sensing electrodes12, P1, P2, P3, P4each in the form of one quadrant of a circle. In other words, the four primary sensing electrodes12, P1, P2, P3, P4are arranged evenly spaced along a circular path. A circular perimeter14encloses the primary sensing electrodes12, P1, P2, P3, P4, and is in turn enclosed by a single secondary sensing electrode13, S1configured as described in relation to the fourth active region37. The perimeter14is substantially coincident with the locus of points equidistant between the four primary sensing electrodes12, P1, P2, P3, P4and the secondary sensing electrode13, S1.

The four primary sensing electrodes12, P1, P2, P3, P4may be used together to provide a dial control, for example by interpolating a pressed position on the first dial control72based on comparing and/or interpolating the primary piezoelectric charges Qp1, Qp2, Qp3, Qp4(or adjusted charges Ap1, Ap2, Ap3, Ap4) measured from the respective primary sensing electrodes12, P1, P2, P3, P4. In this way, an angle at which the first dial control72is being pressed relative to a centre of the circular path may be measured. A dial control such as the first dial control72is functionally similar to a slider control such as the first to third slider controls66,67,68, except that a location is measured along a closed path (circular inFIG.31) as opposed to a straight or curving open path.

When no capacitance measurements are obtained (or when capacitance measurements are rendered inoperable by environmental conditions such as a wet input surface), an input to the first dial control72may be localised by comparing the primary piezoelectric charges Qp1, Qp2, Qp3, Qp4(or adjusted charges Ap1, Ap2, Ap3, Ap4) measured from the respective primary sensing electrodes12, P1, P2, P3, P4with the secondary piezoelectric charge Qs1(or adjusted charge As1) measured from the secondary sensing electrode13, S1, for example using one or more of the methods described hereinbefore.

Additionally or alternatively, each of the four primary sensing electrodes12, P1, P2, P3, P4may be used as a discrete button. For example, the first dial control72may be used as an array of four discrete buttons, instead of as a dial.

Although shown inFIG.31with four primary sensing electrodes12, P1, P2, P3, P4evenly spaced about a circular path, in general the first dial control72may include any number N>2 of primary sensing electrodes12, P1, . . . , PNevenly or irregularly spaced along a closed path. For use as a dial control, at least three primary sensing electrodes12, P1, P2, P3are preferable, spaced along a circular or elliptical path.

Referring also toFIG.32, an active region19providing a second dial control73is shown.

The second dial control73is the same as the first dial control72, except that it includes eight primary sensing electrodes12, P1, . . . , P8evenly spaced about a circular path, and in that the single secondary sensing electrode13, S1is configured with a gap40according to the fifth active region39.

Swine Gesture Controls

Referring also toFIG.33, an active region19providing a first swipe gesture control74is shown.

The first swipe gesture control74includes first and second primary sensing electrodes12, P1, P2arranged such that along a swipe direction75, a width of the first primary sensing electrode12, P1perpendicular to the swipe direction75decreases and a width of the second primary sensing electrode12, P2perpendicular to the swipe direction75increases. In the example shown inFIG.33, the swipe direction75corresponds to the y direction, and the first primary sensing electrode12, P1takes the form of first triangular protrusions76tapering along the positive y direction. Similarly, the second primary sensing electrode12, P2takes the form of second triangular protrusions77tapering along the negative y direction and interdigitated with the first triangular protrusions76. A rectangular perimeter14encloses the first and second primary sensing electrodes12, P1, P2, and is in turn surrounded by four secondary sensing electrodes13, S1, S2, S3, S4configured as described in relation to the first to third active regions28,32,33.

An applied force F moving along the swipe direction75in the positive y direction from the bottom (relative toFIG.33) of the first swipe gesture control74will initially induce a larger first piezoelectric charge Qp1(or adjusted charge Ap1) than a second piezoelectric charge Qp2(or adjusted charge AA) because the first primary sensing electrode P1has a relatively larger area to collect charges at that end of the first swipe gesture control74. As the force F is moved upwards (relative toFIG.44) along the swipe direction75, the relative area of the second primary piezoelectric electrode P2and the relative size of the second piezoelectric charge Qp2(or adjusted charge Ap1) increase, and eventually exceed the first piezoelectric charge Qp1. By comparing the first and second primary piezoelectric charges Qp1, Qp2(or adjusted charges Ap1, Ap2), a user swiping along the swipe direction can be detected. In some implementations, it may be possible to estimate the relative position along the swipe direction75, for example based on a ratio Qp1/Qp2of the first and second primary piezoelectric charges Qp1, Qp2(or adjusted charges Ap1, Ap2).

When no capacitance measurements are obtained (or when capacitance measurements are rendered inoperable by environmental conditions such as a wet input surface), an input to the first swipe control74may be localised by comparing the primary piezoelectric charges Qp1, Qp2(or adjusted charges Ap1, Ap2) measured from the first and second primary sensing electrodes12, P1, P2with the secondary piezoelectric charges Qs1, Qs2, Qs3, Qs4(or adjusted charges As1, As2, As3, As4) measured from the secondary sensing electrodes13, S1, S2, S3, S4, for example using one or more of the methods described hereinbefore.

Referring also toFIG.34, an active region19providing a second swipe gesture control78is shown.

The second swipe gesture control78is the same as the first swipe gesture control74, except that the different numbers and shapes of particular protrusions76,77are used, and that the swipe direction75is aligned with the x axis as shown instead of the y axis as shown.

Simulations

Referring also toFIG.35, a model piezoelectric sensor79is illustrated.

The model piezoelectric sensor79was used for finite element analysis modelling to obtain the simulation results discussed hereinafter. The model piezoelectric sensor79includes three active regions19including a swipe control region80, a slider control region81and a discrete button region82. The swipe control region80has the layout shown inFIG.35and is configured substantially as described in relation to the first and second swipe gesture controls74,78. The slider control region81is configured in the same way as the second slider control67, and includes a linear array of ten primary piezoelectric electrodes12, P1, . . . , P10. The discrete button region82includes a single, circular primary piezoelectric electrode12, P1providing a button, and is configured as described in relation to the fourth button control65. The perimeters14of the swipe control region80, the slider control region81and the discrete button region82are omitted inFIG.35for visual purposes, but in each case can be considered as the locus of points equidistant between the primary sensing electrodes12, Pnand the corresponding secondary sensing electrodes13, Sm.

The model piezoelectric sensor79was modelled as being physically constrained by mechanical boundary conditions in the form of six hemispherical supports831, . . . ,836dispersed around the edges of the model piezoelectric sensor79. Four of the hemispherical supports831,832,834,835are disposed in corners of the model piezoelectric sensor79, which is generally rectangular with rounded corners. The remaining hemispherical supports833,836are disposed at the centres of the long edges (parallel to the first direction x). Each of the hemispherical supports831, . . . ,836was movelled as being formed from an elastomeric material (eg. Rubber).

Localisation to an active region19may be implemented using one or more secondary sensing electrodes13, Smand associated secondary piezoelectric charges Qsm(or adjusted charges Asm) as described hereinbefore. Determining the number and configuration of secondary sensing electrodes13, Smto use for a particular active region19may depend on a number of factors including, but not limited to, the size and shape of that active region19, the relative location of that active region19on a piezoelectric sensor16,79, the mechanical boundary conditions experienced by the piezoelectric sensor16,79, and so forth.

Simulations shall be described which were conducted using finite element analysis applied to the model piezoelectric sensor79. Simulations were conducted using the COMSOL® Multiphysics 5.5 software package, and linked mechanical deformation of the model piezoelectric sensor79to piezoelectric charges Qpn, Qsmvia strain induced polarisation of a modelled layer of piezoelectric material7. Aside from the layout of sensing electrodes4,12,13, the simulations were conducted as described hereinbefore in relation toFIG.3.

Discrete Button Region

Referring also toFIG.36A, normalised secondary piezoelectric charges Qs1, Qs2, Qs3, Qs4are plotted for a force F modelled with a centroid coordinate (xF, yF) traversing the discrete button region82along a first axis x.

Referring also toFIG.36B, normalised secondary piezoelectric charges Qs1, Qs2, Qs3, Qs4are plotted for a force F modelled with a centroid coordinate (xF, yF) traversing the discrete button region82along a second axis y perpendicular to the first axis x.

The data forFIGS.36A and36Bwere obtained assuming that the centroid coordinate (xF, yF) of the applied force passes above the centre of the circular primary sensing electrode12, P1. The normalisation referred to for the series ofFIGS.36A and36Bis with respect to the primary piezoelectric charge Qp1calculated for the primary sensing electrode12, P1. For example, a normalised secondary piezoelectric charge Qsm=1 corresponds to Qsm=Qp1.

It may be observed that when the centroid coordinate (xF, yF) of the force F moves along the first direction x, as shown inFIG.36A, the response from the secondary sensing electrodes13, S1, S2, S3, S4is relatively symmetric about a mid-point of the primary sensing electrode12, P1. Similarly symmetric responses may be observed for moving the centroid coordinate (xF, yF) of the force F along the second direction y, as shown inFIG.36B.

Considering both motions, it may be observed that it is possible to use the “Net” signal obtained as the sum Qs1+Qs2+Qs3+Qs4of all the secondary piezoelectric charges. For example, the conductive regions providing the secondary sensing electrodes13, S1, S2, S3, S4as modelled could instead all be electrically connected to provide a single secondary sensing electrode13, S1. Alternatively, the sum Qs1+Qs2+Qs3+Qs4may be evaluated in the controller17or front end to of a device15. In a further example, four conductive regions spaced around the perimeter14could be replaced with more or fewer conductive regions to form a single overall secondary sensing electrode13, S1, for example by electrically connecting all of the conductive regions together, or by having the corresponding piezoelectric charges summed by the controller17or front end to of a device15. At minimum a single secondary sensing electrode13, Smmay substantially or completely surround the perimeter14(see for example the fourth and fifth active regions37,39).

FromFIGS.36A and36B, a suitable threshold multiplier for application of the method using threshold multipliers described hereinbefore to an overall secondary sensing electrode would be in the region of Th1=1.4. This value of the threshold multiplier Th1=1.4 is determined from the approximate intersection of the “Net” series with the projection of the perimeter14.

Swipe Control Region

Referring also toFIG.37A, normalised secondary piezoelectric charges Qs1, Qs2, Qs3, Qs4are plotted for a force F modelled with a centroid coordinate (xF, yF) traversing the swipe control region80along the first axis x.

Referring also toFIG.37B, normalised secondary piezoelectric charges Qs1, Qs2, Qs3, Qs4are plotted for a force F modelled with a centroid coordinate (xF, yF) traversing the swipe control region80along the second axis y.

Referring also toFIG.37C to37E, contour plots of charge density resulting from straining of the piezoelectric layer7are shown corresponding to the movement along the first axis x shown inFIG.37A.

Referring also toFIG.37F to37H, contour plots of charge density resulting from straining of the piezoelectric layer7are shown corresponding to the movement along the second axis y shown inFIG.37B.

The series labelled “Top”, “Bottom”, “Left” and “Right” inFIGS.37A and37Bcorrespond respectively to the secondary sensing electrodes13, S1, S2, S3, S4of the swipe control region80. The series labelled “Top+Bottom”, “Left+Right” and “All” correspond respectively to the sums Qs1+Qs2, Qs3+Qs4and Qs1+Qs2+Qs3+Qs4of the modelled secondary piezoelectric charges. The projection of the perimeter14of the swipe control region80is indicated inFIGS.37A and37Bwith dashed lines, and the projection of an exterior boundary of the secondary sensing electrodes13, S1, S2, S3, S4is indicated by chained lines (not labelled).

The data forFIGS.37A and37Bwere obtained assuming that the centroid coordinate (xF, yF) of the applied force F passes above the centre of the swipe control region80. The normalisation referred to for the series ofFIGS.37A and37Bis with respect to a sum Qp1+Qp2over the primary sensing electrodes12, P1, P2.

It may be observed fromFIGS.37A and37C to37Ethat when the centroid coordinate (xF, yF) of the force F moves along the first direction x, the response from the secondary sensing electrodes13, S1, S2, S3, S4is not symmetric. This is a result of the proximity to an edge of the model piezoelectric sensor79, in particular because the motion of the modelled force F is not symmetric with respect to the hemispherical supports831, . . . ,836.

On the other hand, it may be observed fromFIGS.37B and37F to37Hthat when the centroid coordinate (xF, yF) of the force F moves along the second direction y, the response from the secondary sensing electrodes13, S1, S2, S3, S4is more symmetric. This results from the motion of the modelled force F being relatively more symmetric with respect to the hemispherical supports831, . . . ,836.

Based on the modelling results inFIGS.37A to37H, to localise a force F to within the swipe control region80along the first axis x, the signals from the “Left” S3and “Right” S4electrodes could be summed, either by electrically connecting the corresponding conductive regions or by summation in the measurement front end10or controller17. The resulting first overall (or effective) secondary sensing electrode may be associated with a first threshold multiplier of approximately Th1≈0.65 for application of the method using threshold multipliers described hereinbefore.

Based on the modelling results inFIGS.37A to37H, to localise a force F within the swipe control region80along the second axis y, a second overall secondary sensing electrode may be formed by summing the signals from the “Top” S1and “Bottom” S2electrodes, in combination with setting a second threshold multiplier of Th2≈0.25 for the method using threshold multipliers described hereinbefore. Alternatively, the second overall secondary sensing electrode may be formed by summing the signals from all four of the secondary sensing electrodes13, S1, S2, S3, S4in combination with the a second threshold multiplier of Th2≈0.94 for application of the method using threshold multipliers. For practical reasons, the latter combination for the second overall secondary sensing electrode can only be used in combination with the first overall secondary sensing electrode (sum S3+S4) when signals are combined in the front end10or controller17.

Slider Control Region

Referring also toFIG.38A, normalised secondary piezoelectric charges Qs1, Qs2, Qs3, Qs4are plotted for a force F modelled with a centroid coordinate (xF, yF) traversing the slider control region81along the first axis x.

Referring also toFIG.38B, normalised secondary piezoelectric charges Qs1, Qs2, Qs3, Qs4are plotted for a force F modelled with a centroid coordinate (xF, yF) traversing the slider control region81along the second axis y.

Referring also toFIG.38C to38E, contour plots of charge density resulting from straining of the piezoelectric layer7are shown corresponding to the movement along the first axis x shown inFIG.38A.

Referring also toFIG.38F to38H, contour plots of charge density resulting from straining of the piezoelectric layer7are shown corresponding to the movement along the second axis y shown inFIG.38B.

The series labelled “Top”, “Bottom”, “Left” and “Right” inFIGS.38A and38Bcorrespond respectively to the secondary sensing electrodes13, S1, S2, S3, S4of the slider control region81. The series labelled “Top+Bottom”, “Left+Right” and “All” correspond respectively to the sums Qs1+Qs2, Qs3+Qs4and Qs1+Qs2+Qs3+Qs4of modelled secondary piezoelectric charges. The projection of the perimeter14of the slider control region80is indicated inFIGS.38A and38Bwith dashed lines, and the projection of an exterior boundary of the secondary sensing electrodes13, S1, S2, S3, S4is indicated by chained lines (not labelled).

The data forFIGS.38A and38Bwere obtained assuming that the centroid coordinate (xF, yF) of the applied force passes above the centre of the slider control region81. The normalisation referred to for the series ofFIGS.38A and38Bis with respect to a sum Qp1+Qp2over the primary sensing electrodes12, P1, . . . , P10.

It may be observed fromFIGS.38A and38C to38Ethat when the centroid coordinate (xF, yF) of the force F moves along the first direction x, the response from the secondary sensing electrodes13, S1, S2, S3, S4is substantially symmetric. Similarly, it may be observed fromFIGS.38B and38F to38Hthat when the centroid coordinate (xF, yF) of the force F moves along the second direction y, the response from the secondary sensing electrodes13, S1, S2, S3, S4is also substantially symmetric.

Based on the modelling results inFIGS.38A to38H, to localise a force F within the slider control region81along the first axis x, the signals from the “Left” S3and “Right” S4electrodes could be summed, either by electrically connecting the corresponding conductive regions or by summation using the measurement front end10or controller17. The resulting first overall (or effective) secondary sensing electrode may be associated with a first threshold multiplier of Th1≈0.016 for application of the method using threshold multipliers described hereinbefore. Similarly, a second overall secondary sensing electrode may be formed by summing the signals from the “Top” S1and “Bottom” S2electrodes in combination with setting a second threshold multiplier of Th2≈0.028 for application of the method using threshold multipliers described hereinbefore.

Alternatively, a single overall secondary sensing electrode may be formed by summing the signals from all four of the secondary sensing electrodes13, S1, S2, S3, S4in combination with a threshold multiplier of Th≈0.034 for application of the method using threshold multipliers. This latter option would lose some resolution at the extremal ends of the slider control region81along the first axis x. A resolution loss estimated fromFIG.38Awould be in the region of 2.5 mm either side. This may be acceptable in some applications, for example, if input is expected from a user's digit having a contact area with a diameter in the region of 10 mm.

Modifications

It will be appreciated that many modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of piezoelectric force sensors, buttons and/or touch panels, and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

The counter electrodes3,47,48,53,55,58and/or sensing electrodes4,12,13may be defined using one or more conductor layers of a multi-layer printed circuit board (PCB). For example, one or more sensing electrodes4may be defined by a conductor layer of a two-layer PCB or a four layer PCB. Similarly, one or more counter electrodes3may be defined by a conductor layer of a two-layer PCB or a four layer PCB. The sensing electrodes4and the counter electrode(s)3may be defined using separate multi-layer PCBs. The sensing electrodes4and the counter electrode(s)3may be defined using separate conductor layers of the same multi-layer PCB.