PATENT DOCUMENT

Publication Number: US-11803276-B2
Application Number: US-202017116907-A
Country: US
Kind Code: B2

Title: Force sensing architectures

Abstract:
An electronic device with a force sensing device is disclosed. The electronic device comprises a user input surface defining an exterior surface of the electronic device, a first capacitive sensing element, and a second capacitive sensing element capacitively coupled to the first capacitive sensing element. The electronic device also comprises a first spacing layer between the first and second capacitive sensing elements, and a second spacing layer between the first and second capacitive sensing elements. The first and second spacing layers have different compositions. The electronic device also comprises sensing circuitry coupled to the first and second capacitive sensing elements configured to determine an amount of applied force on the user input surface. The first spacing layer is configured to collapse if the applied force is below a force threshold, and the second spacing layer is configured to collapse if the applied force is above the force threshold.

Claims:
What is claimed is: 
     
       1. A sensor assembly for an electronic device, comprising:
 a base; 
 a stack; 
 a deformable element extending from the base or the stack, the deformable element, comprising:
 deformable material; 
 a plurality of protrusions extending from a base portion of the deformable element; and 
 a plurality of sense elements, each sense element of the plurality of sense elements extending from and at a free end of a corresponding protrusion of the plurality of protrusions; and 
 
 a plurality of contact sensing regions positioned on the other of the base or the stack, each contact sensing region of the plurality of contact sensing regions, comprising:
 a plurality of leads positioned to receive a corresponding sense element, wherein: 
 
 each sense element of the plurality of sense elements is partially embedded in the free end of the corresponding protrusion of the plurality of protrusions; 
 a first cross-sectional length of each sense element is less than a second cross-sectional length of the plurality of leads positioned to receive the corresponding sense element; and 
 the first and second cross-sectional lengths are defined with respect to a longitudinal plane corresponding to one of the base or the stack. 
 
     
     
       2. The sensor assembly of  claim 1 , wherein the plurality of sense elements are coated on the plurality of protrusions. 
     
     
       3. The sensor assembly of  claim 1 , wherein the plurality of sense elements comprise a conductive material. 
     
     
       4. The sensor assembly of  claim 1 , wherein the plurality of sense elements comprise a dielectric material. 
     
     
       5. The sensor assembly of  claim 1 , wherein the base portion of the deformable element and the plurality of protrusions are a unitary component. 
     
     
       6. The sensor assembly of  claim 1 , further comprising at least one additional protrusion of the plurality of protrusions that does not include any sense elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/435,649, filed Feb. 17, 2017, and entitled “Force Sensing Architectures,” which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 62/297,676, filed Feb. 19, 2016, and entitled “Force Sensing Architectures, U.S. Provisional Patent Application No. 62/395,888, filed Sep. 16, 2016, and entitled “Force-Sensitive Structure in an Electronic Device,” and U.S. Provisional Patent Application No. 62/382,140, filed Aug. 31, 2016, and entitled “Force Sensing Architectures,” the contents of all of which are incorporated by reference as if fully disclosed herein. 
    
    
     FIELD 
     The disclosure relates generally to sensing a force exerted against a surface, and more particularly to sensing a force through capacitive changes. 
     BACKGROUND 
     Touch devices generally provide for identification of positions where the user touches the device, including movement, gestures, and the like. As one example, touch devices can provide information to a computing system regarding user interaction with a graphical user interface (GUI), such as pointing to elements, reorienting or repositioning those elements, editing or typing, and other GUI features. As another example, touch devices can provide information to a computing system suitable for a user to interact with an application program, such as relating to input or manipulation of animation, photographs, pictures, slide presentations, sound, text, other audiovisual elements, and otherwise. 
     Generally, however, touch inputs are treated as binary inputs. A touch is either present and sensed, or it is not. A force of a touch input may provide another source of input information to a device. For example, a device may respond differently to a touch with a low application force than to a touch with a high application force. Force sensing devices may determine an amount or value of an applied force based on an amount of deformation of a component that is subjected to the force. 
     In devices where force inputs are applied to a touchscreen, such as a multi-touch touchscreen that the user touches to select or interact with an object or application displayed on the display, the noise produced by the display can interfere with the operation of the touchscreen. In some situations, the display noise can electrically couple to the touchscreen and interfere with the operation of the touchscreen. Such display noise can also electrically couple to a force sensing device. The magnitude of the display noise can be much greater than the magnitude of the force signals, making it difficult to discern the force signals from the display noise. 
     SUMMARY 
     An electronic device includes a user input surface defining an exterior surface of the electronic device, a first capacitive sensor comprising a first pair of sensing elements having an air gap therebetween and configured to determine a first amount of applied force on the user input surface that results in a collapse of the air gap, and a second capacitive sensor below the first capacitive sensor comprising a second pair of sensing elements having a deformable element therebetween and configured to determine a second amount of applied force on the user input surface that results in a deformation of the deformable element. 
     The first pair of sensing elements comprises a shared sense element and a first drive element set apart from and capacitively coupled to the shared sense element. The second pair of sensing elements comprises the shared sense element and a second drive element set apart from and capacitively coupled to the shared sense element. The shared sense element may be disposed between the first drive element and the second drive element. The shared sense element may include an array of sensing regions. 
     The electronic device may further include a display element coupled to the first drive element. The electronic device may further include a base structure, wherein the display element is configured to flex relative to the base structure, the deformable element is coupled to the base structure, and the air gap is positioned between the deformable element and the display element. The shared sense element may be coupled to the deformable element. 
     The electronic device may further include a display layer comprising a display element positioned below the user input surface and a back polarizer positioned below the display element. The electronic device may also include a sheet of conductive material formed over a back surface of the back polarizer to produce a conducting surface on the back surface of the back polarizer, and a conductive border formed along at least one edge of the sheet of conductive material. The conductive border may be positioned outside of a user-viewable region of the display layer. The sheet of conductive material may comprise silver nanowire. 
     A capacitive force sensor for an electronic device includes a first drive layer, a second drive layer positioned relative to the first drive layer, a shared sense layer between the first and second drive layers, a first spacing layer between the first drive layer and the shared sense layer, and a second spacing layer between the shared sense layer and the second drive layer. 
     The first spacing layer may comprise an air gap. The capacitive force sensor may further comprise a pair of opposed surfaces defining the air gap, and an anti-adhesion layer configured to prevent adhesion between the opposed surfaces. The air gap may have a thickness of about 1.0 mm or less. The second spacing layer may comprise a deformable material. The second spacing layer may comprise an array of deformable protrusions extending from a base layer. 
     The capacitive force sensor may further include sensing circuitry operatively coupled to the first drive layer, the second drive layer, and the shared sense layer, and configured to determine a first amount of applied force resulting in a change in thickness of the first spacing layer and a second amount of applied force resulting in a change in thickness of the second spacing layer. 
     The first drive layer may include an insulating substrate, a sheet of conductive material formed over a back surface of the insulating substrate to produce a conducting surface on the back surface of the insulating substrate, and a conductive border formed along at least one edge of the sheet of conductive material. The conductive border may include a continuous conductive border that extends along the edges of the sheet of conductive material. The conductive border may include one or more conductive strips formed along a respective edge of the sheet of conductive material 
     An electronic device may include a cover defining a user input surface of the electronic device, a first sensing element coupled to the cover within an interior volume of the electronic device, a frame member coupled to the cover and extending into the interior volume of the electronic device, a second sensing element coupled to the frame member, and a third sensing element coupled to a base structure and set apart from the sense layer. 
     The frame member may define an opening, and the third sensing element may capacitively couple with the second sensing element through the opening. 
     The first sensing element may comprise a continuous layer of transparent conductive material covering substantially an entire surface of a substrate. The second sensing element may comprise a plurality of sensing regions, and the continuous layer of transparent conductive material may overlap multiple sensing regions of the plurality of sensing regions. 
     The third sensing element may comprise a plurality of drive regions, and each drive region may overlap multiple sensing regions of the plurality of sensing regions. The first sensing element may further comprise a connection element electrically coupled to the continuous layer of transparent conductive material, and the electronic device may further comprise sensing circuitry configured to provide an electrical signal to the first sensing element and a connector segment electrically coupling the sensing circuitry to the connection element. 
     An electronic device may include an insulating substrate positioned below a cover layer, a sheet of conductive material formed over a back surface of the insulating substrate to produce a conducting surface on the back surface of the insulating substrate, a conductive border formed along at least one edge of the sheet of conductive material, and an electrode layer positioned below the insulating substrate, wherein the sheet of conductive material and the electrode layer together form a force-sensitive structure that is configured to detect a force input on the cover layer. 
     The electronic device may further include drive circuitry coupled to the sheet of conductive material, and sense circuitry coupled to the electrode layer. The electrode layer may comprise an array of electrodes. The conductive border may comprise a continuous conductive border that extends along the edges of the sheet of conductive material. The conductive border may comprise one or more conductive strips formed along a respective edge of the sheet of conductive material. 
     An electronic device includes a display layer, comprising a display element positioned below a cover layer and a back polarizer positioned below the display element, a sheet of conductive material formed over a back surface of the back polarizer to produce a conducting surface on the back surface of the back polarizer, a conductive border formed along at least one edge of the sheet of conductive material, and a first electrode layer positioned below the display layer. The sheet of conductive material and the first electrode layer together may form a force-sensitive structure that is configured to detect a force input on the cover layer. 
     The electronic device may further comprise a touch-sensitive layer positioned between the cover layer and the front polarizer. The electronic device may further comprise a conductive layer positioned between the touch-sensitive layer and the front polarizer. The conductive border may comprise a continuous conductive border that extends along the edges of the sheet of conductive material. The conductive border may comprise one or more conductive strips formed along a respective edge of the sheet of conductive material. 
     The force-sensitive structure may comprise a first force-sensitive structure, the force input may comprise a first amount of force, and the electronic device may further comprise a second force-sensitive structure comprising a second electrode layer positioned below and spaced apart from the first electrode layer. The second force-sensitive structure may be configured to detect a second amount of force on the cover layer, wherein the second amount of force is greater than the first amount of force. The conductive border may be positioned outside of a user-viewable region of the display layer. 
     The electronic device may further comprise drive circuitry coupled to the sheet of conductive material and sense circuitry coupled to the first electrode layer. The first electrode layer may comprise an array of electrodes. The sheet of conductive material may comprise silver nanowire. 
     A method of forming conductive borders on a surface of a film substrate may include applying a plurality of masks to the surface of the film substrate, each mask defining an area of the surface of the film substrate that will be surrounded by a respective conductive border, forming a conductive material over the surface of the film substrate and the masks, removing each mask from the surface of the film substrate to produce the conductive borders, and singulating the conductive borders to produce individual sections of the film substrate that each includes a respective conductive border. The method may further include forming a protective layer over the surface of the film prior to singulating the conductive borders. 
     Forming the conductive material over the surface of the film substrate and the masks may comprise blanket depositing the conductive material over the surface of the film substrate and the masks. The film substrate may comprise a polarizer film with a sheet of conductive material formed on the surface of the polarizer film. The polarizer film may be attached to a display element in an electronic device. 
     An electronic device may comprise a user input surface defining an exterior surface of the electronic device, a first capacitive sensing element, a second capacitive sensing element capacitively coupled to the first capacitive sensing element, a first spacing layer between the first and second capacitive sensing elements, a second spacing layer between the first and second capacitive sensing elements and having a different composition than the first spacing layer, and sensing circuitry coupled to the first and second capacitive sensing elements configured to determine an amount of applied force on the user input surface. The first spacing layer may be configured to collapse if the applied force is below a force threshold, and the second spacing layer may be configured to collapse if the applied force is above the force threshold. 
     The exterior surface may deflect substantially linearly with respect to force when the applied force is below the force threshold, and the exterior surface may deflect substantially non-linearly with respect to force when the applied force is above the force threshold. The sensing circuitry may determine the amount of applied force using different force-deflection correlations based on whether the first spacing layer is fully collapsed. 
     The first spacing layer may be an air gap, and the second spacing layer may comprise a deformable element. The deformable element may comprise an array of deformable protrusions extending from a base layer. The electronic device may further include a sensor configured to detect whether the first spacing layer is fully collapsed. 
     A force sensing device for an electronic device includes a stack comprising a first capacitive sensing element, a structure below the stack and comprising a second capacitive sensing element capacitively coupled to the first capacitive sensing element, an air gap between the stack and the structure, and a contact sensor. The stack may be configured to move relative to the structure in response to a force applied to a user input surface of the electronic device, thereby causing a change in thickness of the air gap, the first and second capacitive sensing elements may be configured to provide a measure of capacitance corresponding to the change in thickness of the air gap, and the contact sensor may be configured to detect contact between the stack and the structure resulting from the air gap being fully collapsed. The force sensing device may further include a deformable element between the first and second capacitive sensing elements. 
     The contact sensor may comprise sensing regions and conductive elements configured to contact the sensing regions when the stack contacts the structure through the air gap. The force sensing device may further comprise a deformable element on a first side of the air gap, wherein the conductive elements are disposed on the deformable element, and the sensing regions are disposed on a second side of the air gap opposite the first side. The deformable element may comprise protrusions extending from a base layer, and the conductive elements may be coupled to the protrusions. 
     The contact sensor may comprise capacitive sensing regions on a first side of the air gap and dielectric elements on a second side of the air gap opposite the first side and capacitively coupled with the capacitive sensing regions. The capacitive sensing regions may be integrated with the first capacitive sensing element, and the dielectric elements are coupled to the deformable element. 
     A sensor component for an electronic device may include a base, a plurality of protrusions comprising deformable material extending from the base, and a plurality of sense elements disposed at free ends of the protrusions. The sense elements may be at least partially embedded in the protrusions. The sense elements may be coated on the protrusions. The sense elements may comprise a conductive material. The sense elements may comprise a dielectric material. The base and the plurality of protrusions may be a unitary component. The sensor component may further comprise at least one additional protrusion that does not include any sense elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIG.  1    shows an example computing device incorporating a force sensing device. 
         FIG.  2    shows another example computing device incorporating a force sensing device. 
         FIGS.  3 A- 3 E  show partial cross-sectional views of the device of  FIG.  1    viewed along line A-A in  FIG.  1   . 
         FIG.  4    shows a force versus deflection curve of the device of  FIG.  1   . 
         FIG.  5    shows a cross-sectional view of an example force sensing device viewed along line A-A in  FIG.  1   . 
         FIG.  6    shows a force versus deflection curve of the force sensing device of  FIG.  5   . 
         FIG.  7    shows an exploded view of the sensing elements of the force sensing device of  FIG.  5   . 
         FIG.  8    shows a partial cross-sectional view of the sensing elements of  FIG.  7    viewed along line C-C in  FIG.  7   . 
         FIG.  9    shows a sensing element of the force sensing device of  FIG.  5   . 
         FIGS.  10 A- 10 B  show embodiments of another sensing element of the force sensing device of  FIG.  5   . 
         FIG.  11    shows yet another sensing element of the force sensing device of  FIG.  5   . 
         FIG.  12    shows a cross-sectional view of another example force sensing device viewed along line A-A in  FIG.  1   . 
         FIG.  13    shows a force versus deflection curve of the force sensing device of  FIG.  12   . 
         FIG.  14    shows a cross-sectional view of yet another example force sensing device viewed along line A-A in  FIG.  1   . 
         FIG.  15    shows a force versus deflection curve of the force sensing device of  FIG.  14   . 
         FIG.  16    shows a cross-sectional view of yet another example force sensing device viewed along line A-A in  FIG.  1   . 
         FIG.  17    shows a force versus deflection curve of the force sensing device of  FIG.  16   . 
         FIGS.  18 A- 18 B  show expanded cross-sectional views of the force sensing device of  FIG.  17   . 
         FIG.  19    shows a perspective view of a deformable element. 
         FIG.  20    shows a perspective view of a sensing element. 
         FIGS.  21 A- 21 B  show cross-sectional views of an example contact sensor. 
         FIGS.  22 A- 22 B  show cross-sectional views of another example contact sensor. 
         FIGS.  23 A- 23 B  show partial cross-sectional views of the device of  FIG.  1    viewed along line A-A in  FIG.  1   , showing an embodiment with a force sensing system integrated therein. 
         FIG.  24    shows a force versus deflection curve of the force sensing system of  FIGS.  23 A- 23 B . 
         FIG.  25    shows a sensor of the force sensing system of  FIGS.  23 A- 23 B . 
         FIG.  26    shows a cross-sectional view of an example embodiment of the electronic device of  FIG.  1    viewed along line B-B in  FIG.  1   . 
         FIG.  27    depicts a first example arrangement of the conductive border on the polarizer shown in  FIG.  26   . 
         FIG.  28    depicts a second example arrangement of the conductive border on the polarizer shown in  FIG.  26   . 
         FIG.  29    depicts a third example arrangement of the conductive border on the polarizer shown in  FIG.  26   . 
         FIG.  30    shows example components of an electronic device. 
         FIG.  31    shows an example process for determining an amount of force applied to a user input surface. 
         FIG.  32    shows an example process for manufacturing the conductive border on a surface of a polarizer. 
         FIGS.  33 A- 33 B  depict the application of masks to a surface of a film. 
         FIGS.  34 A- 34 B  show the formation of the conductive material over the film and the masks. 
         FIGS.  35 A- 35 B  show the removal of the masks from the film. 
         FIGS.  36 A- 36 B  show the formation of the protective layer over the film and the conductive material. 
         FIGS.  37 A- 37 B  show the production of each individual section of film that is surrounded by a conductive border. 
         FIG.  38    shows a first example technique for determining the geometry of the conductive border. 
         FIG.  39    shows a first example technique for determining the geometry of the conductive border. 
         FIG.  40    shows a first example technique for determining the geometry of the conductive border. 
     
    
    
     The use of cross-hatching or shading in the accompanying figures is generally provided to clarify the boundaries between adjacent elements and also to facilitate legibility of the figures. Accordingly, neither the presence nor the absence of cross-hatching or shading conveys or indicates any preference or requirement for particular materials, material properties, element proportions, element dimensions, commonalities of similarly illustrated elements, or any other characteristic, attribute, or property for any element illustrated in the accompanying figures. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     The present disclosure is related to force sensing devices that may be incorporated into a variety of electronic or computing devices, such as, but not limited to, computers, smart phones, tablet computers, track pads, wearable devices, small form factor devices, and so on. The force sensing devices may be used to detect one or more user force inputs on an input surface, and then a processor (or processing unit) may correlate the sensed inputs into a force measurement and provide those inputs to the computing device. In some embodiments, the force sensing devices may be used to determine force inputs to a track pad, a touchscreen display, or another input surface. 
     Devices may be configured to respond to or use force inputs in various ways. For example, a device may be configured to display affordances with which a user can interact by touching the surface of a touchscreen. Affordances may include application icons, virtual buttons, selectable regions, text input regions, virtual keys, or the like. The touchscreen may be able to detect the occurrence and the location of a touch event. By incorporating force sensors such as those disclosed herein, the device may be able to not only detect the occurrence and location of a touch, but also an amount of force with which the input is applied. The device can then take different actions based on the amount of applied force. For example, if a user touches an application icon with a force input below a threshold, the device may open the application. If the user touches the application icon with a force above the threshold, the device may open a pop-up menu containing additional affordances related to the application. As another example, force sensors may be used to determine a weight associated with an applied force, such that a device can act as a scale. Other applications for force inputs are also contemplated. 
     The force sensing device may include an input surface, one or more sensing layers (such as capacitive sensing elements, drive layers, sense layers, and the like), one or more spacing layers (e.g., air gaps, deformable elements), and a substrate or support layer. The input surface provides an engagement surface for a user, such as the external surface of a track pad or the cover glass of a display. The force sensing device may be incorporated with other components of an electronic device, such as a touchscreen, a display, or the like. In such cases, the components of the force sensing device, such as the one or more sensing layers, may be interspersed with other layers, such as a cover glass, filters, touch sensing layers, backlighting components, a display element (e.g., a liquid crystal display assembly), or the like. 
     A user input applied to an input surface of the force sensing device may cause one or more layers of the force sensing device to deflect in a direction of the applied force such that a spacing layer (e.g., an air gap) is collapsed. This deflection changes the distance between components of the force sensing device, such as between two complementary sensing layers, which can be detected by the force sensing device and correlated to a particular applied force. When the spacing layer has been fully collapsed (e.g., the components defining opposite sides of the gap have come into contact with each other), additional force applied to the input surface will not result in a significant additional change in distance between the layers of the force sensing device. That is, the force sensing device has reached the maximum value of force that it can detect. 
     Force sensing devices described herein include a first spacing layer, such as an air gap, and a second spacing layer, such as a deformable element, that produce a progressive deformation response to an applied force. For example, an air gap and a deformable element may be disposed between the first and second sensing layers such that an applied force first causes the air gap to collapse, and, once the air gap has fully collapsed, causes the deformable element to compress or otherwise deform. As the applied force increases and the deformable element becomes more compressed, the deformable element imparts a progressively higher reaction force against the applied force. Thus, a force sensing device with a deformable element and an air gap may be able to sense a larger force for a given deflection than would be possible in a similar force sensing device without the deformable element. 
     Force sensing devices described herein may also include contact sensors that indicate when adjacent layers defining an air gap come into contact with each other (e.g., when the air gap has been fully collapsed). Such contact sensors may be used to indicate to a processor or sensing circuitry whether the force sensing device is operating in an air-gap force regime or a deformable-element force regime, which may improve the quality and/or accuracy of the force sensing device. 
     Air gaps, deformable elements, and contact sensors may be used in various different force sensing architectures having various numbers and arrangements of spacing layers, sensing layers, contact sensors, and the like. Examples of such architectures are described herein. 
       FIGS.  1 - 2    show example electronic devices that may incorporate the force sensing devices described herein. For example,  FIG.  1    shows an electronic device  100  (e.g., a mobile computing device) that may incorporate the force sensing devices described herein. The electronic device  100  may include a housing  104  and a display  102 . The display  102  can provide a visual output to a user in a user-viewable region  108 . The display  102  can be implemented with any suitable technology, including, but not limited to, a multi-touch sensing touchscreen that uses a liquid crystal display (LCD) element, a light emitting diode (LED) element, an organic light-emitting display (OLED) element, an organic electroluminescence (OEL) element, and the like. In some embodiments, the display  102  can function as an input device that allows the user to interact with the mobile computing device  100 . For example, the display can be a multi-touch touchscreen LED display. 
     The device  100  may also include an I/O device  106 . The I/O device  106  can take the form of a home button, which may be a mechanical button, a soft button (e.g., a button that does not physically move but still accepts inputs), an icon or image on a display, and so on. Further, in some embodiments, the I/O device  106  can be integrated as part of a cover  110  and/or the housing  104  of the electronic device. The device  100  may also include other types of I/O devices, such as a microphone, a speaker, a camera, a biometric sensor, and one or more ports, such as a network communication port and/or a power cord port. 
     The cover  110  may be positioned over the front surface (or a portion of the front surface) of the device  100 . At least a portion of the cover  110  can function as an input surface that receives touch and/or force inputs. The cover  110  can be formed with any suitable material, such as glass, plastic, sapphire, or combinations thereof. In one embodiment, the cover  110  covers the display  102  and the I/O device  106 . Touch and force inputs can be received by the portion of the cover  110  that covers the display  102  and by the portion of the cover  110  that covers the I/O device  106 . 
     In another embodiment, the cover  110  covers the display  102  but not the I/O device  106 . Touch and force inputs can be received by the portion of the cover  110  that covers the display  102 . In some embodiments, the I/O device  106  may be disposed in an opening or aperture formed in the cover  110 . The aperture may extend through the housing  104  one or more components of the I/O device  106  can be positioned in the housing  104 . 
     A force sensing device may be configured to detect force inputs on the display  102 . A force sensing device may also be configured to detect force inputs on a portion of the housing  104 , such as a back or side of the housing  104 , or a bezel portion surrounding the display  102 . In addition to the force sensing device, the display  102  may also include one or more touch sensors, such as a multi-touch capacitive grid, or the like. In these embodiments, the display  102  may detect both force inputs as well as position or touch inputs. The device  100  in  FIG.  1    is embodied as a tablet computer (e.g., a mobile computing device), but this is merely one example device that may include the force sensing devices described herein. Examples of other devices that may include the force sensing devices described herein include other mobile computing devices, wearable electronic devices (e.g., watches), mobile phones, laptop or desktop computers, computer peripherals (e.g., trackpads that provide input to computers), or the like. 
       FIG.  2    shows a laptop computer  200  that includes a trackpad  206  (or other input surface), a display  202 , and an enclosure  204 . The enclosure  204  may extend around a portion of the trackpad  206  and/or the display  202 . Force sensing devices may be configured to detect force inputs on the trackpad  206 , the display  202 , or both. 
     In another example (not shown), a force sensing device may be incorporated into a trackpad that is connectible to a computer, but housed in a separate enclosure or housing. For example, a standalone trackpad that includes a force sensing device may be configured to be connected to a computer as a peripheral input device, similar to a mouse or trackball. 
       FIG.  3 A  is a cross-sectional view of the device  100  viewed along line A-A in  FIG.  1   , showing an assembly  300  that may provide display, touch sensing, and force sensing functionality to the device  100 , or may be integrated with other components to provide such functionality. For example,  FIGS.  5 ,  12 ,  14 ,  16 ,  23 A, and  26    illustrate examples of force sensing structures and/or devices that may be integrated with the assembly  300  or an assembly similar to the assembly  300 . 
     The device  100  includes a cover  303  coupled to the housing  104  and defining an external surface of the device  100 . The cover  303  may be a single layer or it may include multiple layers, and may be formed from or include any appropriate material(s), such as glass, treated glass, plastic, diamond, sapphire, ceramic, oleophobic coatings, hydrophobic coatings, or the like. The device  100  may also include other internal components, including circuit boards, cameras, sensors, antennas, processors, haptic elements, speakers, or the like, which are omitted from  FIG.  3 A  for clarity. 
     The cover  303  may be coupled to the housing  104  via an interfacing member  305 .  FIG.  3 B  is an expanded view of the area  317  shown in  FIG.  3 A , showing the joint between the cover  303  and the housing  104  in greater detail. 
     The interfacing member  305  may be or may include an adhesive that fixes the cover  303  to a ledge  307  or other feature of the housing  104 . For example, the interfacing member  305  may be a pressure sensitive adhesive (PSA), heat sensitive adhesive (HSA), epoxy, or other bonding agent. The interfacing member  305  may be compliant or rigid. Where the interfacing member  305  is compliant, it may help protect the cover  303  (which may include glass or other breakable materials) from damage due to shocks and impacts. Moreover, as discussed herein with reference to  FIGS.  23 A- 25   , the interfacing member  305  may include or cooperate with sensing elements that, along with appropriate processing circuitry, can detect a degree of deformation of the interfacing member  305 . The detected degree of deformation of the interfacing member  305  can then be used to determine information such as an amount of force applied to the cover  303 . 
     With reference to  FIG.  3 A , the assembly  300  includes an upper stack  304 , which may include one or more layers or components of a display, including a liquid crystal matrix, light emitting diodes (LEDs), light guides, filters (e.g., polarizing filters), diffusers, electrodes, shielding layers (e.g., layers of indium tin oxide), or the like. The upper stack  304  may be coupled to the cover  303 , such as with PSA, HSA, or the like. The upper stack  304  may also include sensing elements for detecting the presence and/or location of a touch input on the cover  303 , including, for example, capacitive sensing elements, resistive sensing elements, and the like. 
     The assembly  300  also includes a lower stack  308 , which may be separated from the upper stack  304  over at least a portion of the lower stack  308  by an air gap  306 . The air gap  306  that separates the upper and lower stacks  304 ,  308  may be approximately 25 microns to approximately 100 microns thick, though other dimensions are also possible. The air gap  306  may help prevent deformation of components in the lower stack  308  in response to an applied force on the cover  303  which may cause undesirable optical artifacts on the display  102 . For example, the lower stack  308  may include light sources, light guides, diffusers, or other optical components that, if rigidly coupled to the upper stack  304 , may deflect when a force is applied to the cover  303 . By separating these elements from the upper stack  304  by the air gap  306 , undesirable deformations may be reduced. 
     The lower stack  308  may include a frame member  309  that supports other components of the lower stack  308  and couples the lower stack  308  to the upper stack  304 . For example, the frame member  309  may support components of the lower stack  308  (including light sources, light guides, diffusers, sensing elements, or the like) in a spaced apart configuration relative to the upper stack  304  and/or the cover  303 . 
     The frame member  309  may be coupled to the upper stack  304  and/or the cover  303  and may extend into an interior volume of an electronic device. The frame member  309  may be coupled to the upper stack  304  and/or the cover  303  by a joining member  311 , which may be or include an adhesive or other bonding agent. The frame member  309  may be formed from or include any appropriate material, such as metal, plastic, or the like. As described herein, the assembly  300  may include sensing elements for sensing an applied force on the cover  303 . Such sensing elements may rely on the ability to electromagnetically interact with other sensing elements in order to determine the applied force. For example, a capacitive sense layer may need to capacitively couple to a capacitive drive layer in order to detect a change in distance between the sense and drive layers. Accordingly, the frame member  309  may define an opening in a central portion of the frame member  309 . The opening may reduce or eliminate interference, shielding, or other negative effects of a solid layer between the sensing elements. As shown, a stiffening member  312  formed from dielectric material (or any other material that does not shield or otherwise interfere with the sense and drive layers) is disposed in the opening. In some embodiments, the stiffening member  312  may be omitted from the frame member  309 , and the opening may remain unfilled. 
     In cases where the frame member  309  defines an opening to facilitate or improve electrical, capacitive, and/or electromagnetic interaction between sensing elements, the opening may be substantially coincident with a display and/or touch-sensitive region of the display  102 . Accordingly, sensing elements may be able to provide force (or other) sensing functionality to substantially the entire display and/or touch-sensitive region of the display  102 . 
     The lower stack  308  may include one or more layers or components of a display. For example, the lower stack  308  may include a light source  313  comprising one or more LEDs, fluorescent lights, or the like. The light source  313  may emit light into an optical stack  315  that includes one or more optical components including but not limited to reflectors, diffusers, polarizers, light guides (e.g., light guide films), and lenses (e.g., Fresnel lenses). The lighting configuration shown in  FIGS.  3 A- 3 B  is merely exemplary, and the lower stack  308  may include lighting configurations other than that shown in  FIGS.  3 A- 3 B . 
     The upper and lower stacks  304 ,  308  are described above as including display elements. In applications where the assembly  300  does not provide display functionality, such as where the assembly  300  is part of or coupled to the trackpad  206 , the upper and lower stacks  304 ,  308  may include different components and/or layers as those described above, or may be omitted or replaced with other components. 
     Below the lower stack  308  are a first spacing layer, such as an air gap  310 , and a second spacing layer, such as a deformable element  314 . The air gap  310  may be approximately 0.5 to approximately 1.0 mm thick, though other dimensions are also possible. 
     The first and second spacing layers are configured to change thickness in response to an applied force. For example, a thickness of the air gap  310  (e.g., the distance between the opposed surfaces that define the air gap) may be decreased as a force is applied to the cover  303 . Similarly, a thickness of the deformable element  314  may be decreased as a force is applied to the cover  303 . 
     The deformable element  314  may include any appropriate material, such as silicone, polyurethane foam, rubber, gels, or the like. Moreover, the deformable element  314  may have any appropriate structure, such as multiple compliant or deformable protrusions (as shown), which may be formed as columns, beams, pyramids, channels with sidewalls, cones, wave-shaped protrusions, bumps, or the like. The deformable element  314  may also or instead comprise open or closed cells, such as a sponge or a foam. The deformable element  314  may also have a substantially homogenous, nonporous composition. As yet another example, the deformable element  314  may include multiple discrete pieces of deformable material, such as dots, pads, or the like. 
     The foregoing materials and configurations for the first and second spacing layers are merely examples, however, and the first and second spacing layers may be formed from any appropriate materials or combinations thereof. For example, the air gap  310  may be replaced with a first foam material, and the deformable element  314  may include a second foam material having a different density, thickness, composition, or spring constant, than the first. As another example, the first and second spacing layers may be substantially identical, and may include or be formed from the same materials. 
     The deformable element  314  may be coupled to or adjacent a base structure or layer  316 . The base structure  316  may be a substrate or support layer dedicated to the assembly  300 , or it may be another component of an electronic device, such as a battery, a portion of a housing or enclosure, a circuit board, or any other component. 
       FIGS.  3 C- 3 E  illustrate a progression of the physical response of the assembly  300  to an input force  302  on the upper stack  304 . As noted above, the input force  302  may correspond to a user contacting a user input surface of an electronic device, such as the cover  303 , with a finger, stylus, or other object. The input force  302  may be transferred through the cover  303  to the surface of the upper stack  304 . 
       FIG.  3 C  illustrates the portion of the assembly  300  represented by area  301  in  FIG.  3 A  prior to the input force  302  being applied to the upper stack  304 .  FIG.  3 D  illustrates the assembly  300  after the force input has caused the upper stack  304  to deflect or flex sufficiently to fully collapse the air gap  306 . In particular, the upper stack  304  has been flexed towards the lower stack  308  such that the upper stack  304  is in contact with the lower stack  308  in at least one location. The stiffness of the upper stack  304  and the size of the air gap  306  may determine the amount of force that causes the upper stack  304  to come into contact with the lower stack  308 . In some cases, even a slight touch from the user will be sufficient (e.g., a touch that a user would not consider to be “pressing” on the cover). 
       FIG.  3 E  illustrates the assembly  300  after the force input has caused the lower stack  308  to deflect sufficiently to fully collapse the air gap  310 , thus bringing the lower stack  308  into contact with and at least partially deforming the deformable element  314 . 
     As used herein, the term “collapse” may refer to a partial collapse of a layer (e.g., corresponding to any reduction in thickness of a material or an air gap at any location), or a full collapse of a layer (e.g., corresponding to opposing surfaces that define an air gap coming into contact with one another at any point, or reaching a maximum deformation of a deformable material). 
       FIG.  4    is an example force versus deflection curve illustrating how a user input surface of the assembly  300  (e.g., the cover  303 ) deflects in response to the force input in  FIGS.  3 C- 3 E . In particular, as the force increases from zero to a force threshold (e.g., corresponding to point  402 ), the deflection increases along a first profile  406 . In some cases, the first profile  406  corresponds to the deflection of the assembly  300  until all of the air gaps in the assembly  300  (e.g., the air gap  306  and the air gap  310 ) have been fully collapsed. As the force increases beyond the force threshold (e.g., point  402 ) and the deformable element  314  compresses, the deflection increases along a second profile  408  extending from point  402  to point  404 . Accordingly, the force threshold corresponds to the amount of force at the transition from collapse of the air gaps only to deformation of the deformable element. 
     The first profile  406  may be substantially linear, such that an incremental increase in force produces substantially the same incremental increase in deformation of the cover  303  at any point in the first profile  406 . In contrast, the second profile  408  may be non-linear, and may plateau as the force increases. For example, an incremental increase in force at the beginning of the second profile  408  may result in a greater amount of deformation of the cover  303  than the same incremental increase in force at the end of the second profile  408 . However, these profiles are merely exemplary, and the force sensing devices described herein may exhibit any other force versus deflection curves or profiles. 
     The systems and methods described herein, including the force sensing devices  500 ,  700 ,  900 , and  1100  described below, facilitate the detection of whether a force sensing device is operating according to the first profile  406 , such that only air gaps are being collapsed, or the second profile  408 , such that a deformable element is being deformed. By detecting the different profiles, accurate force measurements may be provided. 
     While  FIGS.  3 A- 4    relate to the assembly  300  of the device  100 , the components, structures, and principles of operation of the assembly  300  may apply to other devices as well, such as the display  202  or the trackpad  206  of the device  200  (or any other appropriate device). In cases where a display is not present, such as the trackpad  206 , some components of the assembly  300  may be omitted, replaced, or rearranged. For example, the upper and lower stacks  304 ,  308  may include components other than display elements, or they may be omitted or replaced with spacers or other components. 
       FIG.  5    is a partial cross-sectional view of an example force sensing device  500  that may be incorporated in an electronic device (e.g., the devices  100 ,  200 ), depicting an area similar to the area  301  in  FIG.  3 A . The cover  303  and the housing  104  are omitted for clarity. 
     The force sensing device  500  includes an upper stack  504 , similar to the upper stack  304 , which may include one or more layers or components of a display, including a liquid crystal matrix, light emitting diodes (LEDs), light guides, filters (e.g., polarizing filters), diffusers, electrodes, or the like. The upper stack  504  may be configured to flex or be capable of flexing in response to an applied force on the force sensing device  500 . 
     A first sensing element  505  is coupled to the upper stack  504  (for example, to a cover  303  or to a component that is coupled to the cover  303 , such as a filter) and is within an interior volume of an electronic device. The first sensing element  505  may be a capacitive sensing element that is configured to capacitively couple with another capacitive sensing element. For example, the first sensing element  505  may be a drive layer that is capacitively coupled to a sense layer (e.g., the second sensing element  512 , below) that facilitates detection of a distance between the sense and drive layers using mutual capacitance. As another example, the first sensing element  505  may be a sense layer instead of a drive layer. As yet another example, the first sensing element  505  may be configured to capacitively couple to a ground layer to facilitate detection of a distance between itself and the ground layer using self-capacitance. As yet another example, the first sensing element  505  may be a ground layer that capacitively couples to a separate sense layer. 
     In the presently described examples, the sensing elements are described as elements for capacitive sensing. However, other types of sensors (and sensor components) may be used instead of or in addition to capacitive sensors. Indeed, other types of sensors or sensing technologies that can detect changes in distance, or absolute distance, between components or otherwise detect force may be used. For example, inductive sensors, optical sensors, sonic or ultrasonic sensors, or magnetic sensors may be used. Moreover, the components of the sensors may be integrated in the force sensors as shown herein (e.g., with sensing elements set apart from one another by one or more layers including air gaps, deformable layers, other components, or the like), or they may be integrated in any other manner suitable for that type of sensor (e.g., an optical sensor may include one or more light emitters in place of a sensing layer). 
     The first sensing element  505  may be coupled to the upper stack  504  in any appropriate way, such as with a pressure sensitive adhesive (PSA), heat sensitive adhesive (HSA), or the like. The first sensing element  505  may also be patterned on the upper stack  504 , such as with physical vapor deposition, electron beam evaporation, sputter deposition, or any other appropriate technique. The first sensing element  505  may be formed from or include any appropriate material, such as indium tin oxide (ITO), disposed on a substrate. 
     A lower stack  508  may be disposed below the first sensing element  505  and separated from the first sensing element  505  by an air gap  506 . Like the air gap  306 , the air gap  506  may be any appropriate thickness, such as from about 25 microns to about 100 microns. 
     The lower stack  508  may include any appropriate components or layers, such as those described above with respect to the lower stack  308  (e.g., LEDs, an optical stack, backlights, reflectors, or light guides), and may be coupled to the upper stack  504  and/or the housing  104  as described with respect to the lower stack  308  of  FIG.  3 A  (e.g., via the frame member  309 ). In embodiments where the force sensing device  500  does not include a display or does not provide display functionality, lower stack  508  (as well as the upper stack  504 ) may include different components or be omitted. 
     The lower stack  508  may be coupled to and/or supported by a frame member, which may be similar to the frame member  309  in  FIG.  3 A . The frame member may include a stiffening member  509 , similar to the stiffening member  312  in  FIG.  3 A . The stiffening member  509  may be formed from or include a dielectric material to facilitate or improve electrical, capacitive, and/or electromagnetic interaction between sensing elements (e.g., between the first sensing element  505  and the second sensing element  512 ). 
     The frame member, and in particular the stiffening member  509 , may support the lower stack  508  in a spaced apart configuration relative to the upper stack, a base structure  516 , a deformable element  514 , or other components of the electronic device.  FIG.  5    shows the second sensing element  512  coupled to a deformable element  514 . However, in some cases, the second sensing element  512  may be coupled to the lower stack  508 . In such cases, the second sensing element  512  may be coupled to the frame member, such as to the stiffening member  509  or a component of the lower stack  508 . 
     An air gap  510  separates the lower stack  508  from a second sensing element  512 . The air gap  510  may be any appropriate thickness, such as from about 0.5 mm to 1.0 mm. 
     The second sensing element  512  may be a sense layer for a capacitive sensor, and may be capacitively coupled to the first sensing element  505 . The second sensing element  512  may include an array of discrete capacitive sensing regions that facilitate detection of a location (and/or a magnitude) of a force input on the upper stack  504 . The second sensing element  512  may be formed from or include any appropriate material, such as ITO traces disposed on a substrate. The second sensing element  512  may be coupled to the deformable element  514 , the stiffening member  509  (or other component of the frame member or lower stack  508 ), or any other component or structure in the interior volume of the electronic device such that the second sensing element  512  is between the first sensing element  505  and a third sensing element  515  (discussed below). 
     An optional anti-adhesion layer  511  may be disposed on a surface defining a side of the air gap  510  in order to prevent the opposite sides of the air gap from sticking together, either temporarily or permanently, when they contact each other. Thus, when an applied force is removed from a user input surface, the components of the force sensing device  500  can return to or near their original orientations. The anti-adhesion layer  511  may be formed from or include any appropriate material, and may have any appropriate shape or structure. For example, the anti-adhesion layer  511  may comprise posts, protrusions, channels, or other structures that permit airflow therethrough to reduce or prevent the formation of sealed areas between the surfaces of the air gap  510  when the air gap  510  is fully collapsed. Without the anti-adhesion layer  511 , such sealed areas may result in negative pressure zones that could act similar to “suction cups” that prevent the separation of the sides of the air gap  510 . The anti-adhesion layer  511  may prevent adhesion caused by other mechanisms or forces as well, such as van der Waals forces, electrostatic forces, or the like. 
     The force sensing device  500  includes a deformable element  514  between the second sensing element  512  and a third sensing element  515 . Similar to the deformable element  314 , the deformable element  514  may include any appropriate material (such as silicone, polyurethane foam, rubber, gels, or the like) and may have any suitable structure, such as multiple compliant columns (as shown), beams, pyramids, cones, wave-shaped protrusions, open or closed cells, or the like. The deformable element  514  may deflect non-linearly with respect to an applied force, as described above. 
     The deformable element  514  is shown in  FIG.  5    below the air gap  510  and between the second sensing element  512  and the third sensing element  515 . However, the relative positions of the air gap  510  and the deformable element  514  may be swapped. For example, the deformable element  514  may be coupled to the lower stack  508 . 
     The third sensing element  515 , disposed between the deformable element  514  and a base structure  516 , may be a drive layer for a capacitive sensor, and may be capacitively coupled to the second sensing element  512 . For example, the second sensing element  512  may be a sense layer, and the third sensing element  515  may be a drive layer, thus forming a capacitive sensor spanning the deformable element  514 . 
     The base structure  516  may be a frame, bracket, or support structure of the force sensing device. In some cases, the base structure  516  is a component of an electronic device that is beneath a user input surface, such as a circuit board, a battery, an interior wall of a housing or enclosure, or the like. The base structure  516  may be stiffer or otherwise more resistant to deflection in response to an applied force than the components above it. Thus, once the air gap  510  has been fully collapsed, additional force may primarily deform the deformable element  514  rather than deflecting the base structure  516 . 
     The first, second, and third sensing elements  505 ,  512 , and  515  may form two capacitive sensors. For example, as described above, the first and third sensing elements  505 ,  515  may each act as a distinct drive layer, and the second sensing element  512  may be a sense layer that capacitively couples to (and senses changes in distance to) both the first and third sensing elements  505 ,  515 . 
     Where the second sensing element  512  is a shared sense layer, it may include a first set of sensors for detecting the distance to the first sensing element  505  and a second set of sensors for detecting the distance to the third sensing element  515 . The second sensing element  512  may also or instead use the same sensors to detect the distance to both the first and third sensing elements  505 ,  515 . In the latter cases, the first and third sensing elements  505 ,  515  may be driven with different electrical signals, thus allowing the second sensing element  512  (and/or sensing circuitry coupled to the second sensing element  512 ) to differentiate between capacitance changes that are caused by changes in a size of the air gap  510  and capacitance changes that are caused by changes in a size of the deformable element  514 . In another embodiment (not shown), the second sensing element  512  may be replaced with two discrete sensing elements, each acting as a sense layer for a different one of the first and third sensing elements  505 ,  515 . 
       FIG.  6    is an example force versus deflection curve illustrating how the force sensing device  500  in  FIG.  5    deflects in response to a force input applied (directly or indirectly) to the upper stack  504 . The force response is similar to that shown in  FIG.  4   , with a first profile from point  401  to point  402  (corresponding to collapse of the air gaps  506  and  510 ) and a second profile from point  402  to point  404  (corresponding to deformation of the deformable element  514 ). 
     As noted above, the force sensing device  500  has two capacitive sensors—a first capacitive sensor  518  formed by the first and second sensing elements  505 ,  512 , and a second capacitive sensor  519  formed by the second and third sensing elements  512 ,  515 . The first capacitive sensor  518  spans the air gaps  506  and  510 , and the second capacitive sensor  519  spans the deformable element  514 . Thus, the first capacitive sensor  518  is positioned within the force sensing device  500  to detect deformation of the upper stack  504  along the line  602  in  FIG.  6   , and the second capacitive sensor  519  is positioned within the force sensing device  500  to detect deformation of the upper stack  504  along the line  604  in  FIG.  6   . By detecting the deformation of the air gaps with one sensor and the deformation of the deformable element with a different sensor, sensing circuitry can process the signals according to different force-deflection correlations. For example, deflections from the first capacitive sensor  518  may be correlated to an amount of applied force according to the substantially linear profile between point  401  and point  402 , and the deflections from the second capacitive sensor  519  may be correlated to an amount of applied force according to the non-linear profile between point  402  and point  404 . Of course, the linear and non-linear profiles shown in  FIG.  6    are merely examples, and the deformation of a force sensing device may follow or exhibit different profiles. 
     Sensing circuitry may apply force-deflection correlations in any appropriate manner. For example, force-deflection correlations may be implemented in mathematical functions that output a particular force value for a particular determined amount of deflection (which may in turn have been determined based on a measured or detected capacitance value, or any other electrical measurement or value). As another example, force-deflection correlations may be implemented using lookup tables, where particular deflection values are correlated with particular force values. Other techniques are also possible, and these examples do not limit the mathematical or programmatic techniques that may be used to produce force values from measured or detected electrical properties (e.g., capacitance, resistance, current, signals, etc.). 
       FIG.  7    is an exploded view of the sensing elements  505 ,  512 , and  515  of the force sensing device  500  of  FIG.  5   , illustrating example configurations of the sensing elements in an implementation of the force sensing device  500  that uses capacitive sensing to detect changes in distance between the sensing elements.  FIG.  7    omits components of the force sensing device  500  and the electronic device in which it is configured. For example,  FIG.  7    omits the deformable element  514  that is shown between the second sensing element  512  and the third sensing element  515 . Moreover,  FIG.  7    omits some details of the sensing elements  505 ,  512 ,  515  for clarity, such as conductive traces or leads used to couple the sensing elements (or portions thereof) to other electrical circuitry. 
     As noted above, in the force sensing device  500 , the first and third sensing elements  505 ,  515  may be drive layers for a capacitive sensing scheme, and the second sensing element  512  may be a sense layer. In operation, the first and third sensing elements  505 ,  515  (also referred to as drive layers  505 ,  515 ) may be excited with an electrical signal, such as a substantially sinusoidal signal, a square or edge signal (e.g., a substantially instantaneous transition from a first voltage to a second voltage), or any other appropriate signal. Properties of the signal, such as frequency, voltage, or amplitude, may be selected to avoid or minimize interference with other electronic circuits of a device, such as display circuits, processors, antennas, and the like. Because the second sensing element  512  (also referred to as a sense layer) is capacitively coupled to a drive layer, a corresponding electrical signal may be induced in (or otherwise detected by) the sense layer. For a given electrical signal applied to the drive layers, the induced electrical signal in the sense layer may be different depending on the distance between the drive layer and the sense layer. Thus, the force sensing device  500  (or the associated sensing circuitry) may determine the distance between the sense layer and drive layer by analyzing the signal induced in the sense layer. 
     The first drive layer  505  may include a conductive material coupled or otherwise applied to a substrate. For example, the first drive layer  505  may include a layer of ITO, nanowire (e.g., metallic nanowire, including silver or gold nanowire), or any other appropriate material. As shown in  FIG.  5   , the drive layer  505  is disposed in the light path of the display  102  (e.g., it is above the lower stack  508 , which produces the light used to illuminate the display  102 ). Thus, the conductive material may be substantially transparent. Even when a substantially transparent material is used, if the material is arranged in a regular pattern, such as in a grid or columns, it may be visible on the display  102 . Accordingly, the conductive material of the first drive layer  505  may be substantially uniformly distributed (e.g., as a layer, sheet, coating, or other continuous element) on the first drive layer  505  instead of being arranged in a regular pattern. In some cases, the conductive material may be a continuous layer covering or extending over an entire surface of a substrate of the first drive layer  505  (or substantially an entire surface, such as about 80% or more of the surface area of the substrate). The layer of conductive material may be configured so that there are no borders or edges of the layer positioned within the boundaries of a display in which the force sensing device  500  is incorporated. 
     The first drive layer  505  may also include a connection element  706  that is electrically coupled to the conductive material and facilitates the coupling of the electrical material to other electronic components or circuitry. The connection element  706  may be formed from or include any material, such as silver, copper, nickel vanadium, or any other appropriate material. The connection element  706  may form a continuous frame along an outer portion of the first drive layer  505  (as shown), or it may be formed from discontinuous or distinct segments. In some cases, the connection element  706  does not form a frame, but instead may be a strip along one side of the first drive layer  505 , for example. Other configurations are also possible. Connection elements  706 , such as conductive strips formed on an edge of a drive layer  505  (or any other conductive substrate, layer, coating, etc.) are discussed herein with respect to  FIGS.  26 - 29  and  32 - 40   . 
     The sense layer  512  may include sensing regions  702  formed from (or including) a conductive material and arranged in a substantially regular pattern, such as a grid. The sensing regions  702  may be formed from or include any appropriate material, such as ITO, metallic nanowire, or the like. 
     Each of the sensing regions  702  may act as a discrete area or pixel-like region that may be used to determine a distance between the first drive layer  505  and that particular sensing region. By analyzing all of the sensing regions  702 , the force sensing device  500  can detect an amount of an applied force on the cover  303 . Moreover, pixelating the sense layer  512  as shown may allow the force sensing device  500  to detect force with greater accuracy than if a single, uniform sense layer were used. For example, if a single sense layer were used, it may be difficult or impossible to tell the difference between a large force applied near an edge of the cover  303  and a small force applied near a center of the cover  303 . By using a pixelated sense layer  512 , the force sensing device  500  can account for differences in stiffness among the different regions of the cover  303 . Using a pixelated sense layer  512  may also allow the force sensing device  500  to determine the location of an applied force, detect multi-touch inputs (e.g., corresponding to multiple fingers or styli being applied to the cover  303 ), or the like. 
     The second drive layer  515  may include a plurality of drive regions  704 . Like the first drive layer  505  and the sensing regions  702  of the sense layer  512 , the drive regions  704  may be formed from or include any appropriate conductive material, such as ITO, metallic nanowire, or the like. 
     The drive regions  704  may be arranged in any appropriate pattern or orientation, and may have any appropriate size. For example, the drive regions  704  may be a plurality of substantially rectangular areas of conductive material, and may be substantially aligned with a column of sensing regions  702  in the sense layer  512 , as shown and described with respect to  FIG.  8   . Thus, the drive regions  704  may each overlap multiple ones of the sensing regions  702  of the sense layer  512 . 
     Like the first drive layer  505 , the drive regions  704  may be excited with an electrical signal (e.g., a substantially sinusoidal or edge signal) that induces a corresponding signal in the sensing regions  702  of the sense layer  512  (or that can otherwise be detected by the sense layer  512 ). Because a single sense layer  512  is used to detect the distance between it and two different drive layers  505 ,  515 , the force sensing device  500  needs to differentiate between signals from the first drive layer  505  and the second drive layer  515 . Accordingly, the signals from the first and second drive layers  505 ,  515  may have different frequencies, amplitudes, phases, or other properties such that the signals they induce in the sense layer  512  are differentiable from one another. More particularly, the signal applied to the first drive layer  505  may have a first frequency, and the signal applied to the second drive layer  515  may have a second frequency different from the first frequency. Alternatively or additionally, the first and second drive layers  505 ,  515  may be excited (e.g., with an edge signal) at different times, such that the signal induced in the sense layer  512  can be attributed to one or the other drive layer. For example, sensing circuitry may alternate between exciting the first and second drive layers  505 ,  515 . These (or other) techniques may be used so that the distance between the first drive layer  505  and the sense layer  512  can be detected independently of the distance between the second drive layer  515  and the sense layer  512 . 
     The drive regions  704  may be electrically isolated from one another, or they may be electrically coupled to one another. In embodiments where the drive regions  704  are electrically coupled to one another, all of the drive regions  704  may be simultaneously excited by a single signal. 
     Alternatively, where the drive regions  704  are electrically isolated, they may be driven or excited independently of one another. This may be useful when not all of the sensing regions  702  are analyzed at a time. More particularly, circuitry associated with the force sensing device  500  may cyclically poll subsets of the sensing regions  702 . The drive regions  704  may therefore correspond to the polled groups of sensing regions  702 , and a signal may be provided to drive regions  704  while the corresponding group of sensing regions  702  is being polled. This may help to reduce power consumption by the force sensing device  500  when a cyclic polling technique is used, as not all of the drive regions  704  will be energized when the corresponding sensing regions  702  are not being polled. 
     The drive layers  505 ,  515  and the sense layer  512  may be distinct layers or components, as shown in  FIG.  7   , or they may be incorporated into other layers or components. For example, the first drive layer  505  may be a conductive material coated on, applied to, or otherwise incorporated with a polarizing filter that is part of the upper stack  304  ( FIG.  3 A ). Indeed, the conductive material of any of the sense and drive layers may be incorporated on another component or layer of the electronic device in which it is incorporated. Alternatively, the sense and drive layers may be formed separately, such as by applying a conductive material on substrate such as a flexible circuit material (e.g., polyimide, polyethylene terephthalate, polyether ether ketone, or transparent conductive polyester), and then incorporating the substrate into the electronic device. 
       FIG.  8    is a partial cross-sectional view of the first and second drive layers  505 ,  515  and the sense layer  512 , viewed along line C-C in  FIG.  7   , illustrating relative sizes and positions of the sensing and drive regions  702 ,  704  of the force sensing device  500 . The first drive layer  505  includes a substrate  802 , a conductive layer  804 , and the connection element  706 . The substrate  802  may be any appropriate material or component, such as a flexible circuit material, a polarizing filter, or any other material or component of an electronic device or display stack. The conductive layer  804  may be ITO, a layer of metallic or conductive nanowire, or any other appropriate material, as described above. The conductive layer  804  may be a continuous sheet (e.g., having a single expanse of conductive material, rather than a segmented or pixelated configuration) that overlaps multiple sense regions  702 . The connection element  706  may be a conductive material such as copper, silver, nickel vanadium, or the like. 
     The sense layer  512  may include a substrate  806 , which may be any appropriate material or component, such as flexible circuit material, and the sensing regions  702 . As described above, the sensing regions  702  may be formed from or include any appropriate material, including ITO, conductive nanowire, or the like. 
     The second drive layer  515  may include a substrate  808 , which may be any appropriate material or component, such as flexible circuit material, and the drive regions  704 . The drive regions  704  and the sensing regions  702  of the sense layer  512  may be sized and positioned relative to one another such that the sensing regions  702  shield the drive regions  704  from sources of interference such as the first drive layer  505 . For example, the drive regions  704  may be substantially the same width as, or narrower than, the sensing regions  702 , and may be vertically aligned with the sensing regions  702  (with the positional terms being relative to the orientation of the layers in  FIG.  8   ). In this way, the conductive material of the sensing regions  702  may substantially shield the drive regions  704  from the first drive layer  505  or other potential sources of interference above the sense layer  512 . Some portions of the drive regions  704  may not be directly covered by a sensing region  702 . However, the unshielded area of the substantially rectangular drive regions  704  is significantly less than would be present if the second drive layer  515  were a single continuous sheet of conductive material, such as that on the first drive layer  505 . 
       FIG.  8    shows the sensing regions  702  and the drive regions  704  extending above the surface of their respective substrates. This is merely one example configuration, however. Indeed, the sensing and drive regions  702 ,  704  may be substantially flush with or recessed in their respective substrates. 
       FIG.  9    shows the sense layer  512  with an example distribution of sensing regions  702 .  FIG.  9    also shows conductive paths  902  that may electrically couple the sensing regions  702  to other electronic components or circuits. The conductive paths  902  may be any appropriate material and may be formed in any appropriate way. For example, they may be formed from ITO applied using a photolithography technique. Other materials and techniques are also contemplated. In embodiments where the sensing regions  702  are independently polled to provide unique force values for a particular display location (as shown in  FIG.  9   ), each sensing region  702  may be connected to a unique conductive path  902 . In embodiments where multiple sensing regions  702  are polled or monitored as a single unit, those sensing regions  702  may share or be connected to a common conductive path  902  (not shown). The pattern of sensing regions  702  and conductive paths  902  shown in  FIG.  9    is merely one example of a suitable configuration, and other configurations, including the number and arrangement of the sensing regions  702  and conductive paths  902 , are also contemplated. 
       FIG.  10 A  shows the first drive layer  505 , illustrating an example configuration of an electrical connection to the conductive layer  804  of the first drive layer  505  via the connection element  706  (e.g., a conductive strip or border around the first drive layer  505 ). In particular,  FIG.  10 A  illustrates a pair of connector segments  1002  positioned proximate the connection element  706 . Each connector segment  1002  may be formed from or include an electrical conductor that is electrically connected to a signal generator or other electronic circuitry. For example, the connector segment  1002  may be formed from a flexible circuit material with a metallic or conductive material (e.g., copper, gold, ITO) disposed thereon. In some cases, the connector segment  1002  may be formed substantially entirely of conductive material, such as when the connector segment  1002  is a strip of copper, silver, or any other metal or conductive material. 
     A conductive joining material  1004  may be deposited over connector segments  1002  and a portion of the connection element  706  such that an electrical connection is formed between the connector segments  1002  and the connection element  706 . The conductive material may be any appropriate material, such as silver, gold, copper, conductive adhesives, or the like. 
     As noted above, the connection element  706  is electrically connected to the conductive layer  804 . Accordingly, drive signals can be applied from the connector segments  1002  to the conductive layer  804 . In some cases, more or fewer connector segments  1002  may be used to electrically couple circuitry to the conductive layer  804 , or the connector segments  1002  may be positioned at different locations around the drive layer  505 , such as along opposite edges of the drive layer  505 . 
       FIG.  10 B  shows the first drive layer  505 , illustrating another example configuration of an electrical connection to the conductive layer  804  of the first drive layer  505 . As shown, the first drive layer  505  does not include the connection element  706 . In this example, instead of connecting to the conductive layer  804  via the connection element  706  (as shown in  FIG.  10 A ), the connector segments  1006  connect to the conductive layer  804  via a conductive adhesive  1008 . Like the connector segments  1002  ( FIG.  10 A ), the connector segments  1006  may be formed from or include an electrical conductor that is electrically connected to a signal generator or other electronic circuitry. The connector segments  1006  may be electrically and physically coupled to the conductive layer  804  via the conductive adhesive  1008 , which may be disposed between overlapping portions of the connector segments  1006  and the conductive layer  804 .  FIG.  10 B  illustrates an example embodiment where two connector segments  1006  couple to opposite sides of the first drive layer  505 . Other configurations, including different numbers, sizes, shapes, and coupling locations of the connector segments  1006  are also contemplated. For example, in some cases, only one connector segment  1006  is used. In other cases, four connector segments  1006  are arranged around the first drive layer  505  (e.g., with one connector segment  1006  on each side of the first drive layer  505 ). 
       FIG.  11    shows the second drive layer  515 , with an example distribution of drive regions  704 .  FIG.  11    also shows conductive paths  1102  that may electrically couple the drive regions  704  to other electronic components or circuits. The conductive paths  1102  may be any appropriate material and may be formed in any appropriate way. For example, they may be formed from ITO applied using a photolithography technique. Other materials and techniques are also contemplated. In embodiments where the drive regions  704  are independently driven or excited, as discussed above with respect to  FIG.  8   , each drive region  704  may be connected to a unique conductive path  1102 . In embodiments where multiple drive regions  704  are driven or excited together (e.g., a signal is applied to multiple drive regions  704  simultaneously), those drive regions  704  may share or be connected to a common conductive path (not shown). The pattern of drive regions  704  and conductive paths  1102  shown in  FIG.  11    is merely one example of a suitable configuration, and other configurations, including the number and arrangement of the drive regions  704  and conductive paths  1102 , are also contemplated. 
       FIG.  12    is a partial cross-sectional view of an example force sensing device  1200  that may be incorporated in an electronic device (e.g., the devices  100 ,  200 ), depicting an area similar to the area  301  in  FIG.  3 A . The cover  303  and the housing  104  are omitted for clarity. While the force sensing device  1200  is similar to the force sensing device  500 , the force sensing device  1200  has a different number and arrangement of sensing elements within the electronic device, as described herein. 
     The force sensing device  1200  includes an upper stack  1204 , similar to the upper stack  304 , which may include one or more layers or components of a display, including a liquid crystal matrix, light emitting diodes (LEDs), light guides, filters (e.g., polarizing filters), diffusers, electrodes, or the like. The upper stack  1204  may be configured to flex or be capable of flexing in response to an applied force on the force sensing device  1200 . 
     A lower stack  1208  may be disposed below the upper stack  1204  and separated from the upper stack  1204  by an air gap  1206 . The lower stack  1208  may include a frame member  1207  (similar to the frame member  309 ), an optical stack  1213  (similar to the optical stack  315  described above), and any other appropriate components, such as a light source. As described with respect to the assembly  300 , the air gap  1206  may be any appropriate thickness, such as 25 to 100 microns. In embodiments where the force sensing device  1200  does not include a display or does not provide display functionality, the lower stack  1208  (as well as the upper stack  1204 ) may include different components or be omitted. 
     A first sensing element  1209  is coupled to the lower stack  1208 . The first sensing element  1209  may be a capacitive sensing element that is configured to capacitively couple with another capacitive sensing element. For example, the first sensing element  1209  may be a drive layer that is capacitively coupled to a sense layer (e.g., the second sensing element  1215 , described below) that facilitates detection of a distance between the sense and drive layers using mutual capacitance. As another example, the first sensing element  1209  may be a sense layer instead of a drive layer. As yet another example, the first sensing element  1209  may be configured to capacitively couple to a ground layer and facilitate detection of a distance between itself and the ground layer using self-capacitance. As yet another example, the first sensing element  1209  may be a ground layer that capacitively couples to a sense layer. 
     The first sensing element  1209  may be formed from or include any appropriate material, such as ITO traces disposed on a flexible substrate, and may be coupled to the lower stack  1208  in any appropriate way, such as with a PSA or HSA, or patterned directly onto the lower stack  1208 . Because the first sensing element  1209  is below the lower stack  1208 , the frame member  1207  of the lower stack  1208  may be formed from a conductive material, such as a metal. More particularly, because the frame member  1207  is not between the first sensing element  1209  and a second sensing element  1215  (discussed below), the frame member  1207  may not shield or otherwise negatively interfere with the capacitive coupling between the first and second sensing elements  1209 ,  1215 . Accordingly, more materials may be suitable for use in the frame member  1207 , and the frame member  1207  may define a continuous layer or panel, rather than having an opening therein to avoid undesirable shielding or interference. 
     An air gap  1210  and a deformable element  1214  may be disposed between the first sensing element  1209  and a second sensing element  1215 . The air gap  1210  and the deformable element  1214  correspond to the air gap  510  and deformable element  514 , and may have similar compositions, structures, dimensions, and functions. 
     The second sensing element  1215  may be capacitively coupled to the first sensing element  1209 , and together these components may form a capacitive sensor  1218  that spans the air gap  1210  and the deformable element  1214  to detect deformation of these layers. The second sensing element  1215  may be a sense layer, a drive layer, or a ground layer, depending on the principle of operation of the capacitive sensor  1218  and/or the configuration of the first sensing element  1209 . 
     The second sensing element  1215  may be coupled to a base structure  1216 , which may be a frame, a bracket, a circuit board, a battery, an interior wall of a housing or enclosure, or the like, as described above with respect to the base structure  516  of  FIG.  5   . 
       FIG.  13    is an example force versus deflection curve illustrating how the force sensing device  1200  in  FIG.  12    deflects in response to a force input applied (directly or indirectly) to the upper stack  1204 . The force response is similar to that shown in  FIG.  4   , with a first profile extending from point  401  to point  402  (corresponding to collapse of the air gaps  1206  and  1210 ) and a second profile extending from point  402  to point  404  (corresponding to deformation of the deformable element  1214 ). 
     As noted above, the force sensing device  1200  has one capacitive sensor  1218  formed by the first and second sensing elements  1209 ,  1215 . The first and second sensing elements  1209 ,  1215  span the air gap  1210  and the deformable element  1214 , but do not span the air gap  1206 . Thus, the capacitive sensor  1218  does not detect deflection of the upper stack  1204  that causes the air gap  1206  to collapse (corresponding to line  1302  in  FIG.  13   ), but does detect deflection that causes the air gap  1210  to collapse and the deformable element  1214  to be deformed (corresponding to line  1304  in  FIG.  13   ). Accordingly, the collapse of the air gap  1206  is decoupled from the collapse of the air gap  1210 , and a force detected using the capacitive sensor  1218  of the force sensing device  1200  corresponds to the force required to collapse the air gap  1210 . 
     Because the capacitive sensor  1218  spans both the air gap  1210  and the deformable element  1214 , sensing circuitry coupled to the first and second sensing elements  1209 ,  1215  may be configured to algorithmically determine when the air gap  1210  has fully collapsed. For example, the sensing circuitry may monitor a rate of change of deformation (e.g., a slope of the force versus deflection curve) as a force is applied. If the slope satisfies a first condition (e.g., it is constant or it is below a threshold value), the sensing circuitry may determine that only the air gap  1210  is being or has been collapsed, and may apply a first force-deflection correlation. If the slope satisfies a second condition (e.g., it is increasing or it is above the threshold value), the sensing circuitry may determine that the air gap  1210  has been fully collapsed and the deformable element  1214  is about to be deformed or has been at least partially deformed. In the latter case, the sensing circuitry may apply a second force-deflection correlation to determine a value of the applied force. 
       FIG.  14    is a partial cross-sectional view of an example force sensing device  1400  that may be incorporated in an electronic device (e.g., the devices  100 ,  200 ), depicting an area similar to the area  301  in  FIG.  3 A . In this example, the force sensing device  1400  is the same as the force sensing device  1200  except that the first sensing element  1209  is coupled to the upper stack  1204  such that the capacitive sensor  1402  formed by the first and second sensing elements  1209 ,  1215  spans both the air gap  1206  and the air gap  1210 . Accordingly, as illustrated in the force versus deflection curve in  FIG.  15   , the capacitive sensor  1402  detects deflection of the upper stack  1204  from point  401  to point  404  (corresponding to line  1502 ). Moreover, as described herein, sensing circuitry may be configured to algorithmically determine when the air gap  1210  and optionally the air gap  1206  have fully collapsed in order to apply an appropriate force-deflection correlation. 
     Whereas in  FIG.  12   , the frame member  1207  was not between the first and second sensing elements  1209 ,  1215 , in  FIG.  14    the frame member  1207  is between the first and second sensing elements  1209 ,  1215 . Accordingly, the frame member  1207  may be formed from a dielectric material or may have an opening in which a dielectric material is positioned such that the frame member  1207  does not shield or otherwise interfere with the sensing elements  1209 ,  1215 . 
       FIG.  16    is a partial cross-sectional view of an example force sensing device  1600  that may be incorporated in an electronic device (e.g., the devices  100 ,  200 ), depicting an area similar to the area  301  in  FIG.  3 A . In this example, the force sensing device  1600  includes an upper stack  1604  (corresponding to the upper stack  1204 ), a first sensing element  1605  (corresponding to the first sensing element  1209 ), an air gap  1606  (corresponding to the air gap  1206 ), a lower stack  1608  (corresponding to the lower stack  1208 ), a deformable element  1610 , an air gap  1615 , a second sensing element  1614 , and a base structure  1620  (corresponding to the base structure  1216 ). The lower stack  1608  may include an optical stack  1617  and a frame member  1607  supporting the optical stack  1617  and coupling the lower stack  1608  to the upper stack  1604 . Because the frame member  1607  is between the first and second sensing elements  1605 ,  1614  (similar to the configuration in the force sensing device  500 ,  FIG.  5   ), the frame member  1607  may be formed from or include a dielectric material, such as a dielectric material disposed in an opening in the frame member  1607 . 
     The first and second sensing elements  1605 ,  1614  form a capacitive sensor  1619  that spans both the air gap  1606  and the air gap  1615 . Thus, like in the force sensing device  1400 , the capacitive sensor  1619  detects deformation that corresponds to the collapse of both air gaps  1606 ,  1615 , as well as the deformable element  1610 . Accordingly, as illustrated in the force versus deflection curve in  FIG.  17   , the capacitive sensor  1619  detects deflection of the upper stack  1604  from point  401  to point  404 , corresponding to line  1702 . 
     The force sensing device  1600  also includes a contact sensor that is configured to detect contact between the upper and lower stacks. As shown in  FIG.  16   , the contact sensor is integrated with the deformable element  1610  and the second sensing element  1614 . For example, the deformable element  1610  may include protrusions  1611  extending from a base portion of the deformable element  1610 . The protrusions  1611  may include a sense element  1612  that is configured to be sensed or otherwise detected by a contact sensing region (e.g., a contact sensing region  1616 , discussed herein) when the air gap  1615  has been fully collapsed and the deformable element  1610  contacts the second sensing element  1614 . As shown in  FIG.  16   , the sense elements  1612  are disposed at free ends of the protrusions  1611 . 
     The sense elements  1612  may be formed from any appropriate material and may have any appropriate size and shape. These properties, as well as any other property of the sense elements  1612 , may be selected based on the principle of operation of the contact sensor. For example, if a contact sensing region  1616  is a capacitive sensor, the sense elements  1612  may be a conductive material, and/or a dielectric material. A suitable dielectric material may have a dielectric constant (or relative permittivity) greater than about 3.9 (e.g., a high-k dielectric material). Where the contact sensing region  1616  is a continuity sensor, the sense elements  1612  may be a conductive material such as carbon, metal, or the like. 
     The sense elements  1612  may be incorporated in the deformable element  1610  in any appropriate way. For example, the sense elements  1612  may be co-molded with the deformable element  1610 . In another example, the sense elements  1612  may be deposited on the deformable element  1610 . For example, a layer or layers of metal (or any other appropriate material) may be deposited on free ends of the protrusions  1611 . In yet another example, the deformable element  1610  may be formed of a material that is itself configured to be sensed by a corresponding contact sensing region  1616 , and thus discrete sense elements  1612  may not be used. For example, the material may be a silicone or other elastomer with conductive particles, such as carbon, embedded therein. Other materials and techniques for integrating the materials with the deformable element  1610  are also contemplated. 
     The contact sensor of the force sensing device  1600  also includes contact sensing regions  1616  that are configured to detect the sense elements  1612  to determine when the air gap  1615  has been fully collapsed and the deformable element  1610  has begun to be compressed. The contact sensing regions  1616  may be configured to detect the sense elements  1612  in any appropriate way. For example, the contact sensing regions  1616  may include capacitive sensing components that are configured to detect a change in capacitance caused by the proximity of the sense elements  1612  to the contact sensing regions  1616 . As another example, the contact sensing regions  1616  may include electrical switches that are configured to detect a closed circuit when a conductive sense element  1612  contacts the electrical switches. 
     The contact sensing regions  1616  may be integrated with the second sensing element  1614 . For example, the contact sensing regions  1616  for the contact sensor and sensing regions for the capacitive sensor  1619  (e.g., a capacitive force sensor) may be patterned on or otherwise incorporated in the same substrate. As another example, the contact sensing regions  1616  may be disposed on top of the second sensing element  1614 . For example, contact sensing regions  1616  comprising electrical contacts, capacitive sensing components, or the like may be placed on top of and optionally adhered to the second sensing element  1614 . 
     Similar to the force sensing device  1400  in  FIG.  14   , the force sensing device  1600  forms a capacitive sensor  1619  that spans both the air gap  1615  and the deformable element  1610 , and thus the capacitive sensor  1619  exhibits a force response curve (shown in  FIG.  17   ) that extends from point  401  to point  404  (corresponding to line  1702 ). However, the capacitive sensor  1619  may not provide a discrete indication when the force sensing device  1600  is operating in the first force profile (e.g., from point  401  to point  402 ) or the second force profile (e.g., from point  402  to point  404 ). The contact sensor of the force sensing device  1600  provides this indication, allowing sensing circuitry to apply an appropriate force-deflection correlation. For example, before the air gap  1615  is fully collapsed and before the contact sensor indicates a contact event (corresponding to point  1704  in  FIG.  17   ), the sensing circuitry may apply a first force-deflection correlation corresponding to the collapse of the air gap  1615  (from point  401  to point  402 ). After the air gap  1615  has fully collapsed, as detected and indicated by a signal from the contact sensor (at point  1704 ), the sensing circuitry may apply a second force-deflection correlation corresponding to compression of the deformable element  1610  (e.g., from point  402  to point  404 ). 
     While  FIG.  16    illustrates an embodiment where the first sensing element  1605  is disposed on the upper stack  1604 , and thus includes the air gap  1606  in the space between the first and second sensing elements  1605 ,  1614 , other configurations are possible. For example, the first sensing element  1605  may be disposed on the lower stack  1608  on the opposite side of the air gap  1606 , or it may be disposed between the lower stack  1608  and the deformable element  1610 . Regardless of where the first and second sensing elements  1605 ,  1614  are located in the force sensing device  1600 , an air gap, a deformable element, and a contact sensor may be disposed between them. Moreover,  FIG.  16    illustrates the deformable element  1610  positioned on the lower stack  1608 , with the protrusions  1611  extending towards the base structure  1620 , and illustrates the contact sensing regions  1616  positioned on the base structure  1620 . In other embodiments, the relative positioning of these components may be exchanged, such that the deformable element  1610  is positioned on the base structure  1620  with the protrusions  1611  extending towards the lower stack  1608 , and the sensing regions  1616  are positioned on the lower stack  1608 . It will be understood that this modification may produce an equivalent result at least with respect to the operation of the deformable element  1610  and the contact sensing regions  1616 . 
       FIG.  18 A  is an expanded view of the area  1800  in  FIG.  16   , showing an example configuration of the protrusions  1611 , sense elements  1612 , and contact sensing regions  1616  that may form the contact sensor in  FIG.  16   . The second sensing element  1614  may include sensing regions  1810 , such as capacitive plates or leads that capacitively couple to a ground or drive layer, as well as the contact sensing regions  1616 . The contact sensing region  1616  in  FIG.  18 A  includes leads  1802 ,  1804 ,  1806 , and  1808 . The leads may be any appropriate material (such as traces of conductive material (e.g., metal, carbon, ITO), wires, plates, pads, or the like), and may be coupled to appropriate circuitry for detecting contact with or proximity to the sense element  1612 . For example, the leads may be capacitive elements that facilitate detection of a change in capacitance resulting from the sense element  1612  being brought into contact with or proximity to the leads. As another example, the leads may be electrical contacts that facilitate detection of a closed circuit between two or more contacts. 
       FIG.  18 B  illustrates the area  1800  in  FIG.  16    when the air gap  1615  has been fully collapsed and the deformable element  1610  is in contact with the second sensing element  1614 . As shown, the proximity or contact between the sense element  1612  and the leads  1802 ,  1804 ,  1806 , and  1808  results in detection by corresponding pairs of the leads  1802 ,  1804 ,  1806 , and  1808 . While  FIGS.  18 A- 18 B  illustrate four leads, this is merely an example, and more or fewer leads may be used. Moreover, the relative sizes of the contact sensing region  1616 , the sense element  1612 , and the leads  1802 ,  1804 ,  1806 , and  1808  are merely exemplary, and may be selected based on various factors and considerations. For example, the contact sensing regions  1616  may be large enough to accommodate misalignments between the deformable elements  1610  and the contact sensing regions  1616 . Thus, even if the centers of the protrusions  1611  and the contact sensing regions  1616  do not line up exactly, the contact sensor will still effectively detect when the air gap  1615  has fully collapsed. 
       FIG.  19    shows an example of the deformable element  1610 , or a portion thereof. The deformable element  1610  comprises an array of protrusions  1611  extending from a base surface  1900 . The protrusions  1611  may be integrally formed with the base surface  1900 . For example, the deformable element  1610  may be molded (e.g., injection molded) as a unitary, monolithic component of a substantially uniform composition. As noted above, the sense elements  1612  may be co-molded with the deformable element  1610  or they may be applied (e.g., adhered, coated, or deposited) to or on the protrusions  1611  after the deformable element  1610  is formed. In either case, the sense elements  1612  may be at least partially embedded in the protrusions  1611 . Other techniques for securing the sense elements  1612  to the protrusions  1611  are also contemplated. It will be understood that the protrusions  1611  are for illustrative purposes, and are not necessarily to scale relative to the size of the base surface  1900  or any other components depicted in the figures. 
       FIG.  20    shows an example of the second sensing element  1614 , or a portion thereof, that includes both sensing regions  1810  (indicated by plain squares) and contact sensing regions  1616  (indicated by cross-hatched squares), and which may be used in conjunction with the deformable element  1610  shown in  FIG.  19   . Both the sensing regions  1810  and the contact sensing regions  1616  may be formed on the same substrate  2000  (e.g., a flexible circuit material), and may include conductive traces, such as metals, carbon, ITO, or the like. 
     In the examples shown in  FIGS.  19  and  20   , each protrusion  1611  includes a sense element  1612  and corresponds to a contact sensing region  1616  on the second sensing element  1614 . This need not be the case, however, as the considerations that determine the amount, arrangement, and distribution of the protrusions  1611  that provide a suitable resistance to compression may be different than the considerations driving the amount, arrangement, and distribution of contact sensing regions. For example, in some implementations, some of the protrusions  1611  do not correspond to contact sensing regions  1616 . In such cases, the protrusions  1611  that do not correspond to contact sensing regions  1616  may omit the sense element  1612 , but may be formed or shaped to ensure that all of the protrusions  1611  have substantially the same height. Alternatively, all of the protrusions  1611  may include a sense element  1612  regardless of whether they all correspond to a contact sensing region  1616 . This may ensure that all of the protrusions have the same height and contact an opposing surface at substantially the same time. 
       FIG.  21 A  is a cross-sectional view of an example contact sensor  2100 , showing a section similar to those shown in  FIGS.  18 A- 18 B . Whereas the contact sensor formed by the protrusions  1611  and the contact sensing regions  1616  shown in  FIGS.  18 A- 18 B  places the sensing component and the sensed component on opposite sides of the air gap  1615 , the contact sensor  2100  is configured such that both the sensed and sensing components can be disposed on one side of an air gap. 
     The contact sensor  2100  includes a deformable protrusion  2102 , which may be formed of any appropriate deformable material, such as silicone, polyurethane foam, rubber, gel, or the like. A sense element  2104  may be incorporated with the protrusion  2102 . For example, the sense element  2104  may be placed within a cavity  2106  or other internal region of the protrusion  2102 . The sense element  2104  may also be embedded in the material of the protrusion  2102  (e.g., via co-molding or insert molding). Like the sense element  1612 , the sense element  2104  may be formed from or include any appropriate material, such as a dielectric material and/or a conductive material. 
     The contact sensor  2100  also includes leads  2110  in an adjacent layer  2108 . The adjacent layer  2108  may be a sensing element, such as the sensing element  1614 , in or on which the leads  2110  are incorporated. Alternatively, the adjacent layer  2108  may be dedicated to containing the leads  2110 . Like the leads  1802 ,  1804 ,  1806 , and  1808  in  FIGS.  18 A- 18 B , the leads  2110  may be configured to act as capacitive elements (e.g., capacitive plates to capacitively couple to and detect the proximity of the sense element  2104 ), contacts for a continuity sensor, or the like. Moreover, the leads  2110  may be formed from or include any appropriate material, such as traces of conductive material (e.g., metal, carbon, ITO), wires, plates, pads, or the like. The leads  2110  may be coupled to appropriate circuitry for detecting contact with or proximity to the sense element  2104 . 
     Where the contact sensor  2100  is a capacitive sensor, physical contact between the leads  2110  and the sense element  2104  may not be necessary to detect contact between the protrusion  2102  and another component. Rather, when the protrusion  2102  contacts another component (e.g., because an adjacent air gap has been fully collapsed), the leads  2110 , along with associated circuitry, may detect the change in distance between the sense element  2104  and the leads  2110 , thereby triggering the contact sensor  2100 . In such cases, the cavity  2106  may be filled with a deformable material, such as silicone, thereby encapsulating the sense element  2104 . 
       FIG.  21 B  illustrates the contact sensor  2100  after the protrusion  2102  has been deformed by a layer  2112  forming an opposite side of an air gap in which the protrusion  2102  has been disposed. As shown, the sense element  2104  has been brought into contact with the leads  2110 , thus triggering the contact sensor  2100 . It may not be necessary for the sense element  2104  to actually contact the leads  2110 , however, in order for the contact sensor  2100  to be triggered. For example, where the leads  2110  are configured as capacitive sensors (or any other type of sensor capable of detecting a change in distance between it and another object), the contact sensor  2100  may be triggered by any detectable change in distance between the sense element  2104  and the leads  2110  caused by the layer  2112  deforming or otherwise contacting the protrusion  2102 . 
       FIG.  22 A  is a cross-sectional view of an example contact sensor  2200 , showing a section similar to those shown in  FIGS.  18 A- 18 B . Whereas the contact sensor formed by the protrusions  1611  and the contact sensing regions  1616  shown in  FIGS.  18 A- 18 B  places the sensing component and the sensed component on opposite sides of the air gap  1615 , the contact sensor  2200  is configured such that both the sensed and sensing components can be disposed on one side of an air gap. 
     The contact sensor  2200  includes a deformable protrusion  2202 , which may be formed of any appropriate deformable material, such as silicone, polyurethane foam, rubber, gel, or the like. A sense element  2204  may be disposed over the protrusion  2202 . For example, a material may be disposed over at least a portion of the protrusion  2202 , such as by coating, deposition (e.g., physical vapor deposition or chemical vapor deposition), or any other appropriate mechanism. The contact sensor  2200  also includes leads  2208  in a layer  2206  that is proximate the protrusion  2202 . 
     The leads  2208  may be configured to act as capacitive elements that capacitively couple to the sense element  2204 , thereby sensing changes in distance from the leads  2208  to the sense element  2204 . Accordingly, the sense element  2204  may be formed from or include a conductive material, a dielectric material (e.g., a high-k dielectric material), or any other appropriate material that may be capacitively coupled to and sensed by the leads  2208 . 
       FIG.  22 B  illustrates the contact sensor  2200  after the protrusion  2202  has been deformed by a layer  2210  forming an opposite side of an air gap in which the protrusion  2202  is disposed. As shown, the sense element  2204  has been brought into closer proximity to the leads  2208 , thus triggering the contact sensor  2200 . 
     The contact sensors  2100 ,  2200  may be used instead of or in conjunction with the contact sensors described with respect to  FIGS.  18 A- 20   . For example, instead of the protrusions  1611  and the sensing regions  1616 , which together form a contact sensor to detect contact with the deformable element  1610 , the deformable element  1610  may include a plurality of contact sensors  2100  or  2200  that serve the same or a similar function. 
     The contact sensing systems described herein may be applied between any of the layers or components of a force sensing device. For example, while  FIG.  16    depicts a contact sensor to detect when the air gap  1615  has collapsed, a contact sensor may also or instead be configured to detect contact when the air gap  1606  has collapsed. In some cases, multiple air gaps in a stack of a force sensing device may include a contact sensor. By providing additional contact sensors in this manner, an electronic device may determine which layers have been or are being deflected, and may therefore apply force-deflection correlations that are tailored for the particular layer or layers that are being deflected. By providing a distinct force-deflection correlation for each of multiple layers, an amount of force applied to a surface may be determined with a high degree of accuracy. 
     The deformable elements described in each of the foregoing examples above may have different thicknesses and/or different protrusion heights in different areas when the deformable element is in an undeformed state. For example, the base structures and/or the upper or lower stacks of the force sensing devices (or any other layer of a force sensing device) may not have uniformly planar surfaces. Accordingly, in order to provide a relatively constant air gap size across the air gap, the deformable elements may have different thicknesses in different areas. For example, protrusions may be larger in some areas to account for a greater distance between a layer or stack (e.g., the lower stack  308 ) and a base structure (e.g., the base structure or layer  316 ). 
     In some cases, input surfaces may not deflect uniformly across the entire input surface area. For example, a force applied near an edge of the cover  303  (e.g., close to the joint between the housing  104  and the cover  303 ) may cause less deflection of the cover  303  (and hence the upper and lower stacks  304 ,  308 ) than a force of the same magnitude that is applied in the center of the cover  303 . Accordingly, the deformable elements may be thicker in areas where less deformation is expected (e.g., around the edges or perimeter of the cover  303 ) so that the deformable element begins to be compressed at substantially the same magnitude of force regardless of where on the input surface the force is applied. 
       FIG.  23 A  is a cross-sectional view of an embodiment of the device  100 , viewed along line A-A in  FIG.  1   , showing an assembly  2300  that may provide display, touch sensing, and/or force sensing functionality to the device  100 , or may be integrated with other components to provide such functionality. As shown in  FIG.  23 A , the device  100  includes force sensing system in the assembly  2300 , similar to the sensors described above with respect to  FIGS.  5 - 22   , as well as a sensor  2302  ( FIG.  23 B ) positioned between the housing  104  and the cover  303 . The sensor  2302  works in conjunction with the sensing elements in the assembly  2300  to determine an amount of deflection of, and thus an amount of force applied to, the cover  303 . 
     The assembly  2300  includes the upper and lower stacks  304 ,  308 , the air gaps  306 ,  310 , and the deformable element  314 , all of which are described above with respect to  FIGS.  3 A- 3 E . The assembly  2300  also includes a first sensing element  2304  positioned on a first side of (e.g., above) the deformable element  314  and a second sensing element  2306  positioned on a second side of (e.g., below) the deformable element  314 . Together, the first and second sensing elements  2304 ,  2306  may be referred to as a force sensor. 
     The first and second sensing elements  2304 ,  2306  may be similar to any of the sensing elements described herein. For example, the first sensing element  2304  may be a capacitive drive layer, and the second sensing element  2306  may be a capacitive sense layer that is capacitively coupled to the drive layer. The first and second sensing elements  2304 ,  2306  and associated circuitry may detect an amount of deformation or deflection of the deformable element  314 , and thus determine an amount of force applied to the cover  303 . While the assembly  2300  shows the first and second sensing elements  2304 ,  2306  positioned on opposite sides of the deformable element  314 , other configurations are also possible. For example, the first sensing element  2304  may be disposed on the bottom of the frame member  309 , on (or in) the upper stack, or the like. In some cases, any of the force sensing devices described herein, such those shown and described with respect to  FIG.  5 ,  12 ,  14   , or  16 , may be used in the assembly  2300 . 
     In addition to the force sensor in the assembly  2300 , the device  100  may include a sensor  2302  disposed between the housing  104  and the cover  303 . The sensor  2302  may include a compliant material that can deflect or deform in response to an applied force on the cover  303 . The sensor  2302 , along with associated sensing circuitry, may be able to detect an amount of deflection of the cover  303  in response to an applied force, and, in conjunction with the sensing elements  2304 ,  2306  in the assembly  2300 , determine an amount of force applied to the cover  303 . 
       FIG.  23 B  shows an exploded view of the area  2308  in  FIG.  23 A , showing details of the sensor  2302 . The sensor  2302  may be positioned between the ledge  307  of the housing  104  and a portion of the cover  303  such that when a force is applied to the cover  303 , the sensor  2302  is pressed between the ledge  307  and the portion of the cover  303 , thus deforming the sensor  2302 . The geometry of the ledge  307  and the cover  303  in  FIG.  23 B  are merely exemplary, and different embodiments of the housing  104  and the cover  303  may have shapes, geometries, and/or features that are different from those shown in  FIG.  23 B . 
     The sensor  2302  includes a deformable portion  2310 . The deformable portion  2310  may be formed from or include any appropriate material, such as silicone, polyurethane foam, rubber, gels, elastomers, or the like. In some cases, the deformable portion  2310  may have adhesive properties, such that the sensor  2302  retains the cover  303  to the housing  104 . 
     The sensor  2302  also includes a first sensing element  2312  and a second sensing element  2314 . The first and second sensing elements  2312 ,  2314  may be positioned on opposite sides of the deformable portion  2310  (e.g., a top and bottom, as shown in  FIG.  23 B ). The first and second sensing elements  2312 ,  2314  may form a capacitive sensor, in which case one of the first or second sensing element  2312 ,  2314  may be a capacitive drive layer, and the other may be a capacitive sense layer. The capacitive sensor may detect an amount of deformation of the deformable portion  2310 , and thus facilitate detection of an amount of applied force, as discussed herein. In some cases, the sensor  2302  may be a resistive sensor (or any other appropriate sensor), in which case the first and second sensing elements  2312 ,  2314  may be omitted or substituted with other components. 
     When a force is applied to the cover  303 , the deformable portion  2310  of the sensor  2302  may deflect or deform such that the first and second sensing elements  2312 ,  2314  are brought closer together. The first and second sensing elements  2312 ,  2314  and associated circuitry may determine the amount of deformation and correlate it with an amount of force applied to the cover  303 . As a certain amount of applied force is reached, however, the deformable portion  2310  may reach a maximum deformation, where greater applied forces may not result in further deformation of the deformable portion  2310 . In some cases, it may be desirable to detect applied forces greater than this amount, however. Accordingly, the sensor  2302  and the sensing elements in the assembly  2300  may sense different ranges of applied forces. 
     For example, the sensor  2302  may be configured to determine forces spanning from no applied force to an amount of force that results in the collapse of the air gaps  306  and  310  in  FIG.  23 A . Up until that point, the sensor in the assembly  2300  (formed by the first and second sensing elements  2304 ,  2306 ) may not detect any force, as the lower stack  308  had not yet been brought into contact with the deformable element  314 . Once the lower stack  308  contacts the deformable element  314 , increasing amounts of force may be determined by the sensing elements  2304 ,  2306  in the assembly  2300 . 
     The first and second sensing elements  2312 ,  2314  may be formed from or include any appropriate material, such as metals, ITO, or the like. Moreover, the first and second sensing elements  2312 ,  2314  may be applied to or otherwise incorporated with the deformable portion  2310  in any appropriate manner. For example, the first and second sensing elements  2312 ,  2314  may be or may include conductive sheets (e.g., copper, silver, or gold) embedded in, positioned on, or otherwise integrated with the deformable portion  2310 . As another example, the first and second sensing elements  2312 ,  2314  may be ITO that is deposited on the deformable portion  2310 . 
     In some cases, either or both of the first and second sensing elements  2312 ,  2314  may not be integrated with the deformable material  2310 , but rather may be separate components. For example, the first and/or second sensing elements  2312 ,  2314  may be layers of material (e.g., flexible circuit material) with conductive materials disposed thereon. The layers may be positioned between the deformable portion  2310  and the cover  303 , and/or between the deformable portion  2310  and the housing  104 , and may be bonded or otherwise adhered to those components. As another example, the first and/or second sensing elements  2312 ,  2314  may be patterned directly on the cover  303  and/or the housing  104 . For example, ITO, conductive nanowires, or any other appropriate material, may be formed directly on the portions of the cover  303  and the housing  104  that are opposite each other when the device  100  is in its assembled configuration. Any combinations of the foregoing examples may be used to integrate the first and/or second sensing elements  2312 ,  2314  with the device  100 . 
       FIG.  24    is an example force versus deflection curve illustrating how the cover  303  of the device illustrated in  FIG.  23 A  deflects in response to a force input applied thereto. The force response is similar to that shown in  FIG.  4   , with a first profile from point  401  to point  402  (corresponding to collapse of the air gaps  306  and  310 ) and a second profile from point  402  to point  404  (corresponding to deformation of the deformable element  314 ). The sensor  2302  may detect deformation of the air gaps  306 ,  310 , as indicated by the line  2402  in  FIG.  24   , while the force sensor in the assembly  2300  detects deformation of the deformable element  314 , as indicated by line  2404 . While the lines  2402 ,  2404  are shown as non-overlapping, this may not be the case. For example, the sensor  2302  may continue to deflect and thus provide meaningful force information even after the air gaps  306 ,  310  have collapsed. In such cases, sensing circuitry associated with the sensors may process the information from both sensors to determine an amount of applied force. 
       FIG.  25    shows a portion of the sensor  2302  as viewed through the cover  303  of the device. The illustrated portion of the sensor  2302  corresponds to a corner portion of the sensor  2302 . The sensor  2302  includes first drive regions  2502  each electrically coupled together (e.g., via conductors  2503 ) and second drive regions  2504  each electrically coupled together (e.g., via conductors  2505 ). The first and second drive regions  2502 ,  2504  may together form the second sensing element  2314  shown in  FIG.  23 B , and may be driven or excited with a signal. As shown, the first and second drive regions  2502 ,  2504  are shown in an alternating, interdigitated pattern, though this is merely one example configuration for the first and second drive regions  2502 ,  2504 . 
     The sensor  2302  also includes sensing regions  2506 . The sensing regions  2506  capacitively couple to the drive regions  2502 ,  2504  and may be connected to circuitry that detects and analyzes signals induced in the sensing regions  2506  by the drive regions  2502 ,  2504 . Each sensing region  2506  may overlap one first drive region  2502  and one second drive region  2504 . As the drive regions may be driven at different times and/or with different signals (e.g., signals having different frequencies), a single sensing region can provide two distinct capacitive measurements, each corresponding to a different location along the sensor  2302 . In this way, the sensor  2302  is pixelated, allowing for more precise force measurements and for detection of a location of an applied force on the cover  303 . 
       FIG.  26    is a cross-sectional view of an embodiment of the device  100 , viewed along line A-A in  FIG.  1   , showing a display stack  2600  positioned below the cover  110 . Force and/or touch sensing systems, or components thereof, may be incorporated with the display stack  2600  to facilitate touch and force input detection on the device  100 . As described herein, device  100  may include conductive sheets (such as the first drive layer  505 ,  FIG.  5   ) that may facilitate sensing force and/or touch inputs on the device  100 . 
     The display stack  2600  may include a touch sensor  2602  positioned between the cover  110  and a display layer  2604 . The touch sensor  2602  can include sensors that are each configured to detect user inputs (e.g., touch and/or force inputs), and the locations of the user inputs, on the cover  110 . Any suitable touch sensor  2602  can be used. For example, in one embodiment, the touch sensor  2602  is formed with a dielectric substrate positioned between two electrode layers. The electrode layers may be made of any suitable optically transparent material. For example, in one embodiment the electrode layers are made of indium tin oxide (ITO). Other suitable materials include, but are not limited to, nanowires or nanowire meshes, a transparent conducting film (e.g., a polymer film), carbon nanotubes, and ultra-thin metal films. 
     Each electrode layer in the touch sensor  2602  can include one or more electrodes. The electrode(s) in one layer are aligned in at least one direction (e.g., vertically) with respective electrodes in the other electrode layer to form one or more capacitive sensors. User inputs, and the locations of the user inputs, are detected through changes in the capacitance of one or more capacitive sensors. As will be described in more detail later, touch and sense circuitry  2632  is coupled to the electrode layers and configured to receive an output signal from each capacitive sensor that represents the capacitance of each capacitive sensor. 
     One or both of the electrode layers in the touch sensor  2602  may be patterned. For example, in one embodiment one electrode layer is patterned into strips positioned along a first axis (e.g., rows) and the other electrode layer is patterned into strips positioned along a second axis that is transverse to the first axis (e.g., columns). Capacitive sensors are formed at the intersections of the strips in the two electrode layers. User inputs, and the locations of the user inputs, can be determined based on the capacitance (or changes in capacitance) of one or more capacitive sensors. 
     The display layer  2604  can include a front polarizer  2606 , a display element  2608  attached to a back surface of the front polarizer  2606 , and a back polarizer  2610  attached to a back surface of the display element  2608 . Any suitable display element  2608  can be used. Example display elements  2608  include, but are not limited to, a LCD element, a LED element, an OLED element, or an OEL element. In the illustrated embodiment, the display element  2608  is a LCD element. 
     In some situations, noise signals that are produced by the display element  2608  can electrically couple with the touch sensor  2602 . This coupling can adversely impact the detection of user inputs by the touch sensor  2602 . To reduce or eliminate the display noise from coupling with the touch sensor  2602 , a conductive layer  2612  can be positioned between the touch sensor  2602  and the front polarizer  2606 . The conductive layer  2612  may be made of any suitable optically transparent material. For example, in one embodiment the conductive layer  2612  is made of ITO. 
     A sheet of conductive material  2614  is formed or coated over the back surface of the back polarizer  2610 . The sheet of conductive material  2614  can be made of any suitable conductive material. For example, in one embodiment, the sheet of conductive material  2614  is made of a silver nanowire film. 
     The back polarizer  2610  may be made of an electrically insulating material. The sheet of conductive material  2614  enables the back surface of the back polarizer  2610  to function as a conducting surface. As will be described in more detail below, the conducting surface of the back polarizer  2610  is used to transmit drive signals for a force sensor that includes the conducting surface. 
     Attached to the back surface of the back polarizer  2610  is a conductive border  2616  (which may be the same or similar in structure, materials, function, etc., to the connection element  706 ,  FIGS.  7 ,  10 A ). The conductive border  2616  is positioned along at least a portion of a perimeter or edge of the back polarizer  2610 . As will be described in more detail in conjunction with  FIGS.  27 - 29   , the conductive border  2616  can be a continuous border that extends around the entire perimeter, or the conductive border  2616  can include one or more discrete conductive strips with each conductive strip positioned along a respective portion of the perimeter of the back polarizer  2610 . 
     In the illustrated embodiment, the display stack  2600  extends across the user-viewable region  108  ( FIG.  1   ) of the display  102  and into non-viewable regions  2618  that do not correspond to a viewable output from the display  102 . Alternatively, in some embodiments, only a subset of the layers in the display stack  2600  extend into the non-viewable regions  2618 . For example, portions of the display layer  2604  can extend into the non-viewable regions  2618  while other layers in the display stack  2600  do not extend into the non-viewable regions  2618 . 
     In some embodiments, the conductive border  2616  can be positioned on the portions of the back polarizer  2610  that reside in the non-viewable regions  2618 , which allows the conductive border  2616  to be formed with any suitable material or materials (e.g., opaque or transparent material(s)). For example, the conductive border  2616  may be formed with a metal or metal alloy, such as copper, aluminum, molybdenum, and nickel vanadium. Other embodiments can form at least a portion of the conductive border  2616  within the user-viewable region  108 . In such embodiments, at least the portion of the conductive border  2616  that is in the user-viewable region  108  may be formed with an optically transparent material, such as ITO. 
     In the illustrated embodiment, a backlight unit  2620  is positioned below the back polarizer  2610  and the conductive border  2616 . The display layer  2604 , along with the backlight unit  2620 , is used to output images on the display  102 . In some implementations, the backlight unit  2620  may be omitted. 
     A first electrode layer  2622  is positioned below and attached to the backlight unit  2620 . In some implementations, the first electrode layer  2622  represents an array of electrodes (e.g., two or more electrodes). In other implementations, the first electrode layer  2622  is a single electrode. The first electrode layer  2622  can be formed with any suitable conductive material (opaque or transparent), such as a metal or metal alloy. Example metals and metal alloys include, but are not limited to, copper, aluminum, titanium, tantalum, nickel, chromium, zirconium, molybdenum niobium, and nickel vanadium. 
     Together, the sheet of conductive material  2614  on the back surface of the back polarizer  2610  and the first electrode layer  2622  form a force sensor. The force sensor can be used to detect a magnitude or an amount of force that is applied to the cover  110 . When the first electrode layer  2622  is implemented as an array of electrodes, the sheet of conductive material  2614  and the first electrode layer  2622  form an array of capacitive sensors. Each capacitive sensor includes an electrode formed by the sheet of conductive material  2614  and a respective electrode in the first electrode layer  2622 . When a user input is applied to the cover  110 , the cover  110  deflects and a distance between the electrodes in at least one capacitive sensor changes, which varies the capacitance of that capacitive sensor. For example, in the illustrated embodiment, the gap  2623  varies based on a user input applied to the cover  110 , which in turn varies the capacitance of at least one capacitive sensor. 
     In some embodiments, the first electrode layer  2622  can be used to detect one or more touches on the cover  110 . In such embodiments, the touch-sensor  2602  (e.g., a touch-sensing layer) may be omitted since the first electrode layer  2622  has a dual function in that it is used to detect both touch and force inputs. 
     The device  100  can also include a support structure  2624  (which may be the same or similar in structure, materials, function, etc., to the frame members  309 ,  1207 , discussed above). In the illustrated embodiment, the support structure  2624  is made from a conductive material (e.g., a metal), although other embodiments can form the support structure  2624  with a different material, such as a plastic, ceramic, or a composite. In the illustrated embodiment, the support structure  2624  extends along a length and a width of the display stack  2600 , although this is not required. The support structure  2624  can have any shape and/or dimensions in other embodiments. For example, the support structure  2624  may have an opening in which a stiffening member may be positioned (as described with respect to the frame member  309  and the stiffening member  312 ,  FIG.  3 A ). 
     In the illustrated embodiment, the support structure  2624  has a U-shaped cross-section and is attached to the cover  110  such that the support structure  2624  is suspended from the cover  110 . In other embodiments, the support structure  2624  may be connected to a component other than the cover  110 . For example, the support structure  2624  can be attached to a housing of the device  100  (e.g., the housing  104  in  FIG.  1   ) or to a frame or other support component in the housing. 
     In some embodiments, the support structure  2624  may be constructed and attached to the cover  110  to define a gap  2626  between the support structure  2624  and the first electrode layer  2622 . The gap  2626  allows the display stack  2600  to flex or move in response to an applied force on the cover  110 . In some embodiments, the first electrode layer  2622  may be attached to the support structure  2624  instead of the backlight unit  2620 . 
     The device  100  may also include a battery  2628 . The battery  2628  provides power to the various components of the device  100 . As shown in  FIG.  26   , a second electrode layer  2630  can be disposed on a top surface of the battery  2628 . In some embodiments, the amount of force applied to the cover  110  may be sufficient to cause the display stack  2600  to deflect such that the back polarizer  2610  contacts the first electrode layer  2622 . When the display stack  2600  is deflected to a point where the back polarizer  2610  contacts the backlight unit  2620  (or first electrode layer  2622  if no backlight unit  2620  is present), the amount of force detected by the force sensor reaches a maximum level (e.g., a first amount of force). The force sensor cannot detect force amounts that exceed the maximum level. The deflection of the display stack  2600  to a point where the back polarizer  2610  contacts the backlight unit  2620  or the first electrode layer  2622  may correspond with the first profile  406  of the force versus deflection curve in  FIG.  4   . For example, the maximum level of force detected by the force sensor that includes the first electrode layer  2622  and the conductive material  2614  may correspond to the point  402  in  FIG.  4   . 
     In such embodiments, the second electrode layer  2630  (in conjunction with the first electrode layer  2622  or other components) can form a second force sensor that detects the force that exceeds the first amount of force by associating an amount of deflection between the first electrode layer  2622  and the second electrode layer  2630  (e.g., a second amount of force). For example, in some embodiments, the second electrode layer  2630  can be used to measure a change in capacitance between the first and the second electrode layers  2622 ,  2630 . Alternatively, the second electrode layer  2630  may be used to detect a change in capacitance between the back surface  2627  of the support structure  2624  and the second electrode layer  2630 . The deflection between the first electrode layer  2622  and the second electrode layer  2630  (or between the back surface  2627  of the support structure and the second electron layer  2630 ) may correspond to the second profile  408  in  FIG.  4   . 
     As described earlier, drive and sense circuitry  2632  is coupled to the touch sensor  2602 . The drive and sense circuitry  2632  may be positioned at any suitable location in the device  100 . The drive and sense circuitry  2632  is configured to provide drive signals to the touch sensor  2602  and to receive output signals from the touch sensor  2602 . For example, when the touch sensor  2602  includes an array of capacitive sensors, the drive and sense circuitry  2632  is coupled to each capacitive sensor and configured to sense or measure the capacitance of each capacitive sensor. A processing device may be coupled to the drive and sense circuitry  2632  and configured to receive signals representing the measured capacitance of each capacitive sensor. The processing device can be configured to correlate the measured capacitances into an amount of force. 
     Similarly, drive circuitry  2634  is coupled to the sheet of conductive material  2614  and is configured to provide drive signals to the back surface of the back polarizer  2610  (e.g., to the sheet of conductive material  2614 ). In some embodiments, the drive circuitry  2634  is coupled to the conductive border  2616 . 
     Sense circuitry  2636  is coupled to the first electrode layer  2622  and is configured to receive one or more output signals from the first electrode layer  2622 . For example, when the first force sensor includes an array of capacitive sensors, the drive circuitry  2634  and the sense circuitry  2636  are coupled to each capacitive sensor and configured to sense or measure the capacitance of each capacitive sensor. A processing device may be coupled to the drive circuitry  2634  and the sense the circuitry  2636  and configured to receive the output signals representing the measured capacitance of each capacitive sensor. The processing device can be configured to correlate the measured capacitances into an amount of force. Like the drive and sense circuitry  2632 , the drive circuitry  2634  and the sense circuitry  2636  may be situated at any suitable location in the device  100 . 
     The drive signals transmitted on the back surface of the back polarizer  2610  (e.g., on the sheet of conductive material  2614 ) can be decoupled from the noise produced by the display element  2608  (e.g., a TFT layer) because the insulating back polarizer  2610  physically separates the sheet of conductive material  2614  from the display element  2608 . Additionally, the conductive border  2616  may reduce the contact resistance between the back polarizer  2610  and the sheet of conductive material  2614 , as well as reduce the sheet resistance of the sheet of conductive material  2614 . Reducing the contact resistance and/or the sheet resistance can increase the suppression of the display noise produced by the display element  2608 . 
     With respect to the second electrode layer  2630 , drive circuitry  2638  is coupled to the second electrode layer  2630  and is configured to provide drive signals to the second electrode layer  2630 . The drive circuitry  2638  can be located at any suitable location in the electronic device  100 . In some embodiments, the sense circuitry  2636  may be configured to receive one or more output signals from the first electrode layer  2622 . A processing device coupled to the sense circuitry  2636  can be configured to receive the output signals and correlate the measured capacitances into an amount of force. 
       FIGS.  27 - 29    depict example arrangements of the conductive border on the polarizer  2610  shown in  FIG.  26   . As shown in  FIG.  27   , the conductive border can include four discrete conductive strips  2702 ,  2704 ,  2706 ,  2708  that are formed on a sheet of conductive material  2710  coated over a polarizer  2700 . Each conductive strip  2702 ,  2704 ,  2706 ,  2708  is formed along a respective edge of the polarizer  2700 . Although  FIG.  27    depicts four conductive strips, other embodiments are not limited to this arrangement. Other embodiments can include one or more conductive strips. The embodiments shown in  FIGS.  27 - 29    may represent embodiments of the conductive material  2614  and the conductive border  2616  on the polarizer  2610  in  FIG.  26   . 
     In some embodiments, the sheet of conductive material  2710  may be formed with an anisotropic material that is more conductive in one direction compared to another direction. In such embodiments, the discrete conductive strip or strips  2702 ,  2704 ,  2706 ,  2708  can be more effective at reducing the sheet and/or contact resistance of the sheet of conductive material  2710 . 
       FIG.  28    depicts a discrete L-shaped conductive strip  2802  that is positioned on a sheet of conductive material  2804  on a polarizer  2800 . In the illustrated embodiment, the conductive strip  2802  is formed along two edges of the polarizer  2800 . Other embodiments can include two “L” shaped conductive strips that are arranged to position a conductive strip along each edge of the polarizer  2800 . 
       FIG.  29    illustrates a continuous conductive border  2902  that is positioned along the entire edge of the polarizer  2900 . In some situations, the continuous conductive border  2902  may reduce the sheet conductivity and/or the contact resistivity of the sheet of conductive material  2904  more effectively than a conductive strip or strips. The conductive strips  2702 ,  2704 ,  2706 ,  2708  and/or the conductive strip  2802  and the continuous conductive border  2902  may form the connection element  706  described above with respect to  FIGS.  7  and  10 A . 
     Although the embodiments shown in  FIGS.  26 - 29    are described in conjunction with a display stack in an electronic device, other embodiments are not limited to displays. A force sensor can be formed below any suitable cover, such as the housing of an electronic device (e.g., the housing  104  in  FIG.  1   , the trackpad  206  in  FIG.  2   ). An insulating substrate may be positioned below the cover. A sheet of conductive material is formed over a back surface of the insulating substrate to produce a conducting surface on the back surface of the insulating substrate. In other words, the sheet of conductive material transforms the back surface of the insulating substrate into a conducting surface. A conductive border is formed along at least one edge of the sheet of conductive material and an electrode layer is positioned below the insulating substrate. The conducting surface of the insulating substrate and the electrode layer together form a force sensor that is configured to detect a force input on the cover. 
     Throughout the foregoing discussion, force sensing devices and contact sensors are described with respect to various examples. However, these examples are not meant to be limiting of the particular elements, layers, or components described. For example, components (e.g., layers of the force sensing devices) that are described herein as being separate and/or distinct may be combined, and components described herein as being combined or integrated may be separated. Moreover, some components may be substituted, added, or removed without departing from the spirit of the disclosure. For example, as noted above, a display structure may be omitted from a force sensing device if the force sensing device is not integrated with or part of a display device. Furthermore, any individual layer or structure described herein may include one or more sub-layers. For example, a cover may include multiple sub-layers, including glasses, coatings, adhesives, filters, and the like. As another example, any of the layers or components of the force sensing devices and contact sensors described herein may be secured to adjacent layers or structures with adhesives, bonding layers, or the like, though such adhesives and bonding layers are not necessarily described herein. 
       FIG.  30    depicts example components of an electronic device in accordance with the embodiments described herein. The schematic representation depicted in  FIG.  30    may correspond to components of the devices depicted in  FIGS.  1 - 2   , and indeed any device in which the force sensing described herein may be incorporated. 
     As shown in  FIG.  30   , a device  3000  includes a processing unit  3002  operatively connected to computer memory  3004  and/or computer-readable media  3006 . The processing unit  3002  may be operatively connected to the memory  3004  and computer-readable media  3006  components via an electronic bus or bridge. The processing unit  3002  may include one or more computer processors or microcontrollers that are configured to perform operations in response to computer-readable instructions. The processing unit  3002  may include the central processing unit (CPU) of the device. Additionally or alternatively, the processing unit  3002  may include other processors within the device including application specific integrated chips (ASIC) and other microcontroller devices. 
     The memory  3004  may include a variety of types of non-transitory computer-readable storage media, including, for example, read access memory (RAM), read-only memory (ROM), erasable programmable memory (e.g., EPROM and EEPROM), or flash memory. The memory  3004  is configured to store computer-readable instructions, sensor values, and other persistent software elements. Computer-readable media  3006  also includes a variety of types of non-transitory computer-readable storage media including, for example, a hard-drive storage device, a solid state storage device, a portable magnetic storage device, or other similar device. The computer-readable media  3006  may also be configured to store computer-readable instructions, sensor values, force-deflection correlations, and other persistent software elements. 
     In this example, the processing unit  3002  is operable to read computer-readable instructions stored on the memory  3004  and/or computer-readable media  3006 . The computer-readable instructions may adapt the processing unit  3002  to perform the operations or functions described above with respect to  FIGS.  1 - 25    or below with respect to the example process  FIG.  31   . In particular, the processing unit  3002 , the memory  3004 , and/or the computer-readable media  3006  may be configured to cooperate with the force sensor  3022 , described below, to determine an amount of force applied to a user input surface by applying different force-deflection correlations based on whether a deflection of the user input surface is collapsing an air gap in a force sensor or compressing a deformable element. The computer-readable instructions may be provided as a computer-program product, software application, or the like. 
     As shown in  FIG.  30   , the device  3000  also includes a display  3008 . The display  3008  may include a liquid-crystal display (LCD), organic light emitting diode (OLED) display, LED display, or the like. If the display  3008  is an LCD, the display  3008  may also include a backlight component that can be controlled to provide variable levels of display brightness. If the display  3008  is an OLED or LED type display, the brightness of the display  3008  may be controlled by modifying the electrical signals that are provided to display elements. The display  3008  may correspond to the upper and/or lower stacks described above. 
     The device  3000  may also include a battery  3009  that is configured to provide electrical power to the components of the device  3000 . The battery  3009  may include one or more power storage cells that are linked together to provide an internal supply of electrical power. The battery  3009  may be operatively coupled to power management circuitry that is configured to provide appropriate voltage and power levels for individual components or groups of components within the device  3000 . The battery  3009 , via power management circuitry, may be configured to receive power from an external source, such as an AC power outlet. The battery  3009  may store received power so that the device  3000  may operate without connection to an external power source for an extended period of time, which may range from several hours to several days. 
     In some embodiments, the device  3000  includes one or more input devices  3010 . The input device  3010  is a device that is configured to receive user input. The input device  3010  may include, for example, a push button, a touch-activated button, a keyboard, a key pad, or the like. In some embodiments, the input device  3010  may provide a dedicated or primary function, including, for example, a power button, volume buttons, home buttons, scroll wheels, and camera buttons. Generally, a touch sensor (e.g., a touchscreen) or a force sensor may also be classified as an input device. However, for purposes of this illustrative example, the touch sensor  3020  and the force sensor  3022  are depicted as distinct components within the device  3000 . 
     The device  3000  may also include a touch sensor  3020  (e.g., the touch sensor  2602 ,  FIG.  26   ) that is configured to determine a location of a touch over a touch-sensitive surface of the device  3000 . The touch sensor  3020  may include a capacitive array of electrodes or nodes that operate in accordance with a mutual-capacitance or self-capacitance scheme. As described herein, the touch sensor  3020  may be integrated with one or more layers of a display stack or a force sensing device to provide the touch-sensing functionality of a touchscreen. The capacitive arrays of the touch sensor  3020  may be integrated with the force sensing devices described above, and may be in addition to the capacitive sensing elements that provide force sensing functionality. 
     The device  3000  may also include a force sensor  3022  that is configured to receive and/or detect force inputs applied to a user input surface of the device  3000 . The force sensor  3022  may correspond to any of the force sensing devices or force sensors described herein, and may include or be coupled to capacitive sensing elements that facilitate the detection of changes in relative positions of the components of the force sensor (e.g., deflections caused by a force input). 
     As described herein, the force sensor  3022  may include contact sensors that are configured to signal when an air gap has been fully collapsed by a force input. The force sensor  3022 , including the contact sensors, may be operatively coupled to the processing unit  3002 , which can process signals from the force sensor  3022  to determine an amount of applied force on the user input surface, as described above. 
     The device  3000  may also include one or more sensors  3024  that may be used to detect an environmental condition, orientation, position, or some other aspect of the device  3000 . Example sensors  3024  that may be included in the device  3000  include, without limitation, one or more accelerometers, gyrometers, inclinometers, goniometers, or magnetometers. The sensors  3024  may also include one or more proximity sensors, such as a magnetic hall-effect sensor, inductive sensor, capacitive sensor, continuity sensor, and the like. 
     The sensors  3024  may also be broadly defined to include wireless positioning devices including, without limitation, global positioning system (GPS) circuitry, Wi-Fi circuitry, cellular communication circuitry, and the like. The device  3000  may also include one or more optical sensors including, without limitation, photodetectors, photosensors, image sensors, infrared sensors, and the like. While the camera  3026  is depicted as a separate element in  FIG.  30   , a broad definition of sensors  3024  may also include the camera  3026  with or without an accompanying light source or flash. The sensors  3024  may also include one or more acoustic elements, such as a microphone used alone or in combination with a speaker element. The sensors may also include a temperature sensor, barometer, pressure sensor, altimeter, moisture sensor, or other similar environmental sensor. 
     The device  3000  may also include a camera  3026  that is configured to capture a digital image or other optical data. The camera  3026  may include a charge-coupled device, complementary metal oxide semiconductor (CMOS) device, or other device configured to convert light into electrical signals. The camera  3026  may also include one or more light sources, such as a strobe, flash, or other light-emitting device. As discussed above, the camera  3026  may be generally categorized as a sensor for detecting optical conditions and/or objects in the proximity of the device  3000 . However, the camera  3026  may also be used to create photorealistic images that may be stored in an electronic format, such as JPG, GIF, TIFF, PNG, raw image file, or other similar file types. 
     The device  3000  may also include a communication port  3028  that is configured to transmit and/or receive signals or electrical communication from an external or separate device. The communication port  3028  may be configured to couple to an external device via a cable, adaptor, or other type of electrical connector. In some embodiments, the communication port  3028  may be used to couple the device  3000  to an accessory, such as a smart case, smart cover, smart stand, keyboard, or other device configured to send and/or receive electrical signals. 
     The device  3000  may determine an amount of force applied to a user input surface using any appropriate techniques or algorithms. For example, the device  3000  may use data, readings, or other information from force sensing devices, and then apply mathematical formulas or consult models or lookup tables to determine an amount of applied force based on the information from the force sensing devices. More particularly, one example technique for determining an amount of force applied to a structure that includes a force sensing device includes consulting a lookup table or other data structure that correlates a sensor value (e.g., a detected capacitance value) to a particular known force. The lookup table may be populated by a calibration process whereby a known force is applied to various locations on the user input surface. For each location, the resulting sensor values, which may be referred to as calibration values, for each pixel or sensing region of the sensor are stored in the lookup table (or other data structure). Accordingly, for each user input location there exists in the lookup table a set of calibration values representing the sensor values of all pixels or sensing regions of the sensor when the sensor is subjected to a known force. In some cases, multiple sets of calibration values exist for each location, such as values associated with forces of different known magnitudes. 
     In order to determine an amount of force applied to the user input surface during normal operation, a location of a touch event on the input surface is determined (e.g., with the touch sensor  3020 ), and calibration values for that location are used in conjunction with the detected sensor values to determine the actual applied force. As one example, if the detected sensor values corresponding to a touch event at a given location are approximately three times the calibration values associated with a touch event at that location, the device  3000  may determine that the applied force is approximately three times larger than the calibration force. 
     Another technique for determining an amount of applied force includes determining an amount of force applied to each pixel or sensing region of a sensor, and then adding the force from each pixel or sensing region to determine the total amount of force applied to that sensor. Where this technique is used, the change in distance between two sensing elements may be used in conjunction with a known stiffness of a material between the two sensing elements to determine the force applied to that pixel or region. As one specific example, a deformable element (e.g., the deformable element  514 ,  FIG.  5   ) may be positioned between capacitive sensing elements. The capacitive sensing elements may correspond to the second and third sensing elements  512 ,  515  in  FIG.  5   , which may be capacitive sense and drive layers, respectively. The capacitive sensing elements may also correspond to the first electrode layer  2622  and the conductive material  2614  in  FIG.  26   . By measuring a capacitance value between the capacitive sensing elements, the device  3000  can determine a distance (or a change in distance) between the sensing elements resulting from a force applied to the deformable element. The change in distance can be multiplied by a stiffness of the deformable element (e.g., a constant correlating an expected deflection or deformation of the material to a given force) to determine the amount of force corresponding to the detected change in distance. As noted above, the second and third sensing elements  512 ,  515  may define a number of different pixels or sensing regions (e.g., regions  702 ,  FIG.  7   ). Accordingly, the foregoing technique can be used to determine the force applied to each individual pixel or sensing region, and those forces can be combined (e.g., added) to determine the total amount of force applied to the user input surface and/or to the sensor. 
     In some cases, the stiffness (e.g., a stiffness constant) of the deformable element may be determined for each sensing region. Thus, the distance measurement for each region may be multiplied by a stiffness constant specific to that region, which may improve the accuracy of the force measurements for each pixel or region, and thus may improve the overall accuracy of the force sensor. The stiffness constant for each pixel or sensing region may be determined manually, for example, by applying a known force to each area of the deformable element corresponding to a pixel or sensing region, and measuring the amount or distance that the deformable element has deflected. In some cases, multiple measurements can be taken at different forces to determine an average stiffness constant or a stiffness profile for the deformable material. This may increase the accuracy of a sensor as compared to using the same stiffness constant for each sensing region, as the stiffness may vary from region to region. 
     Either of the foregoing techniques (e.g., consulting a lookup table or calculating the force based on a stiffness constant) may be used to determine the force applied to a given sensor or sensing device described herein. In embodiments where a device includes multiple sensors, a different technique may be used for each sensor. For example, for the force sensing device  500 , which includes first and second capacitive sensors  518 ,  519  ( FIG.  5   ), a lookup table may be used to determine the force applied to the first capacitive sensor  518 , and a stiffness-based force calculation may be used to determine the force applied to the second capacitive sensor  519 . As another example, the device of  FIGS.  23 A- 23 B  includes a sensor  2302  positioned between a housing and a cover, as well as a sensor within the housing (e.g., including the first and second sensing elements  2304 ,  2306  with a deformable element  314  therebetween). In this case, a lookup table may be used to determine the force applied to the sensor  2303 , and a stiffness-based calculation may be used to determine the force applied to the sensor within the housing (e.g., the first and second sensing elements  2304 ,  2306 ). Alternatively, a lookup table technique may be used for both sensors. 
     Where two or more sensors are used, the force values that are determined for each sensor may be combined to produce a single value that represents the force applied to the user input surface. For example, with reference to the force sensing device  500  ( FIG.  5   ), the first and second capacitive sensors  518 ,  519  may deflect in response to different applied forces. More particularly, the air gaps  506  and  510  (between the first and second sensing elements  505 ,  512 ) may collapse in response to an applied force having a particular value. Because the air gaps  506 ,  510  are between the first and second sensing elements  505 ,  512 , the first capacitive sensor  518  defined by these sensing elements can be used to determine the force up to the particular value. Because the distance between the first and second sensing elements  505 ,  512  cannot be further reduced, however, the first capacitive sensor  518  will not detect values of applied forces in excess of the particular value. The second capacitive sensor  519 , however, may detect force after the collapse of the air gaps  506 ,  510 . Accordingly, where both the first and second capacitive sensors  518 ,  519  produce force values, the values may be added together to determine the overall force applied to the force sensing device  500 . The same or a similar process may be used in conjunction with the force sensors described with respect to  FIG.  26   , in which the conductive material  2614  and the first electrode layer  2622  form a first force sensor, and the first electrode layer  2622  and the second electrode layer  2630  form a second force sensor. 
       FIG.  31    depicts an example process  3100  for determining an amount of force applied on a user input surface of an electronic device. The process  3100  may be implemented on any of the example devices discussed herein. The process  3100  may be used, for example, to determine what actions (if any) the electronic device should perform in response to the force input, and may be implemented using, for example, the processing unit and other hardware elements described with respect to  FIG.  30   . The process  3100  may be implemented as processor-executable instructions that are stored within the memory of the electronic device. 
     In operation  3102 , it is determined whether a sensor signal corresponds to a deformation of a first spacing layer (e.g., an air gap, as described above) or of a second spacing layer (e.g., a deformable element, as described above), or a combination of both. For example, the device may monitor a rate of change of a sensor signal. If the rate of change of the sensor signal satisfies a first condition (e.g., it is constant over a particular deformation range or it is below a threshold value), the device may determine that an air gap is being or has been collapsed. If the rate of change of the sensor signal satisfies a second condition (e.g., it is increasing over a particular deformation range or it is above the threshold value), the device may determine that an air gap has been fully collapsed and a deformable element has been or is about to be at least partially compressed. As another example, the device may determine whether a sensor signal corresponds to a collapse of a first spacing layer or a second spacing layer based on whether or not a contact sensor (e.g., the contact sensors described with respect to  FIGS.  16  and  18 A- 22 B ) indicates that the first spacing layer has fully collapsed. 
     In operation  3104 , a force-deflection correlation is selected. As described herein, a different force-deflection correlation may be used to determine an amount of applied force, depending on whether the deflection of the force sensor corresponds to a collapse of a first spacing layer (e.g., an air gap) or deformation of a second spacing layer (e.g., a deformable element). Thus, if the device determines at operation  3102  that the sensor signal corresponds to a deformation of the first spacing layer, such as the collapse of an air gap, the device may at operation  3104  select a first force-deflection correlation. If the device determines at operation  3102  that the sensor signal corresponds to a deformation of the second spacing layer, such as compression of a deformable element, the device may at operation  3104  select a second force-deflection correlation that is different than the first. 
     In embodiments where the device includes multiple sensors spanning different spacing layers (such as the first and second capacitive sensors  518 ,  519 ,  FIG.  5   ), the device may select and use multiple force-deflection correlations. For example, if the device determines at operation  3102  that the deflection corresponds to an at least partial collapse of both a first and a second spacing layer, the device may select an appropriate force-deflection correlation for each sensor. 
     In operation  3106 , an amount of applied force is determined based on the selected force-deflection correlation(s). For example, the device correlates the amount of deflection indicated by the sensor signal to a particular applied force by using a lookup table, a stiffness-based force calculation, or another technique that implements the selected force-deflection correlation. In embodiments where the device includes multiple sensors, the device may correlate the amount of deflection indicated by each sensor with a force value, and then add the force values from each sensor to determine the total amount of applied force. 
     Based on the determined amount of applied force, the device may perform (or not perform) certain actions. For example, if the applied force is lower than a threshold value, the device may perform one action, and if the applied force is higher than the threshold value, the device may perform another action. As one example, if the force is lower than the threshold value, the device may move a cursor to a position corresponding to the location of the touch event, whereas if the force is higher than the threshold value, the device may register a selection (e.g., a mouse click) at the location of the cursor. This is merely one example, however, and the range of possible actions that the device can perform based on the determined amount of applied force are limited only by the capabilities of the device. 
     As noted above, force sensors may use sheets or layers with conductive borders. For example, as described with respect to  FIGS.  7 ,  10 A, and  26 - 29   , conductive sheets may be used as drive layers for capacitive force sensing systems. Conductive borders may be applied to or otherwise included in the conductive sheets.  FIG.  32    shows a flowchart of a method of manufacturing the conductive borders on a surface of a sheet, such as a polarizer as described with respect to  FIGS.  26 - 29    or a force sensing element  505  described with respect to  FIGS.  5 ,  7   , and  10 A.  FIG.  32    will be described in conjunction with  FIGS.  33 - 37   . The method is described in conjunction with a roll-to-roll production process. Although described in conjunction with a polarizer, the process can be used to produce a conductive border on any suitable film or substrate. Additionally, the method is described in conjunction with forming continuous conductive borders (e.g., see  FIGS.  7 ,  29   ), although embodiments are not limited to this type of conductive border. 
     In other embodiments, a conductive border can be fabricated on a polarizer or substrate using other manufacturing processes. Example manufacturing processes include, but are not limited to, physical or chemical vapor deposition, screen printing or inkjet coating technology using a shadow mask, and film mask and photolithography. 
     Initially, as shown in block  3200 , masks are applied to a surface of a film. In one embodiment, the film is a polarizer film that includes a sheet of conductive material formed or coated over a surface of the polarizer film. As describe earlier, the polarizer film will be attached (e.g., laminated) to the back surface of a display element and function as a polarizer for the display (e.g., display element  2608  and back polarizer  2610  in  FIG.  26   ). 
     Each mask defines the area that will be surrounded by, or inside of, the conductive border. For example, the masks can define the user-viewable region (e.g., the user-viewable region  108 ) of a display. Although depicted as having a rectangular shape, a mask can have any given shape and/or dimensions. 
     In some embodiments, each mask can be one of multiple masks. For example, when forming multiple conductive strips (e.g., see  FIG.  27   ) on a film substrate, a mask defines the area that will not include the conductive strips. 
       FIGS.  33 A- 33 B  depict the application of masks to a surface of a film. As shown in  FIG.  33 A , the application process  3300  includes moving the film  3302  from a first roller  3304  towards a second roller  3306  in a roll-to-roll production system. This movement is represented in  FIGS.  33 A and  33 B  by arrow  3308 . In one embodiment, the second roller  3306  includes the finished product of the method shown in  FIG.  32    (e.g., a collection of conductive borders formed on the surface of the polarizer film). In another embodiment, the second roller  3306  includes a collection of masks formed on the surface of the film (e.g., the finished product of block  3200 ). 
     A third roller  3310  is positioned between the first and the second rollers  3304 ,  3306 . The third roller  3310  includes a collection of masks  3312  that are applied to the film  3302  as the film  3302  moves below the third roller  3310 .  FIG.  33 B  illustrates a top view of the film  3302  after the masks  3312  have been applied to the film  3302  by the third roller  3310 . 
     Referring now to block  3202  in  FIG.  32   , a conductive material is formed over the masks and the surface of the film. The conductive material is the material used to form the conductive borders.  FIGS.  34 A- 34 B  show the formation of the conductive material over the film and the masks. The formation process  3400  includes moving the film  3302  from a fourth roller  3402  towards a fifth roller  3404  (movement represented by arrow  3406 ). In one embodiment, the fourth roller  3402  corresponds to the first roller  3304  and the fifth roller  3404  corresponds to the second roller  3306 . In such embodiments, the fifth roller  3404  includes a collection of conductive borders formed on the surface of the polarizer film (e.g., the finished product of the method shown in  FIG.  32   ). In other embodiments, the fourth roller  3402  includes the finished product of block  3200 . 
     In the illustrated embodiment, the film  3302  with the masks  3312  enters a deposition chamber  3408  where a nozzle  3410  deposits the conductive material  3412  onto the film  3302  and the masks  3312 . The deposition can be a blanket deposition such that the entire film  3302  and masks  3312  have conductive material deposited thereon.  FIG.  34 B  illustrates a top view of the film  3302  after the conductive material  3412  has been deposited onto the film  3302  and the masks  3312  by the deposition chamber  3408 . 
     Referring now to block  3204  in  FIG.  32   , the masks are removed from the surface of the film after the conductive material has been formed over the masks and the film.  FIGS.  35 A- 35 B  show the removal of the masks  3312  from the film  3302 . The removal process  3500  includes moving the film  3302  from a sixth roller  3502  towards a seventh roller  3504  (movement represented by arrow  3506 ). In one embodiment, the sixth roller  3502  corresponds to the first roller  3304  and the seventh roller  3504  corresponds to the second roller  3306 . In such embodiments, the seventh roller  3504  includes the finished product of the method shown in  FIG.  32   . In other embodiments, the sixth roller  3502  includes the finished product of block  3202 . 
     An eighth roller  3508  is positioned between the sixth and seventh rollers  3502 ,  3504 . The eighth roller  3508  removes the masks  3312 , which leaves regions  3514  that include only the film  3302 . The conductive material is disposed on the areas around the regions  3514 .  FIG.  35 B  illustrates a top view of the film  3302  after the masks  3312  have been removed by the eighth roller  3508 . 
     Any suitable process can be used to remove the masks  3312 . For example, in one embodiment, the eighth roller  3508  employs an electrostatic technique to remove the masks  3312 . 
     In some embodiments, an imaging system (e.g., a camera) can be positioned over the film  3302  between the eighth roller  3508  and the seventh roller  3504 . The imaging or automated optical inspection system may be used to inspect the film for defects after the masks have been removed by the eighth roller  3508 . 
     Referring now to block  3206  in  FIG.  32   , a protective layer is formed over the surface of the film and the conductive material.  FIGS.  36 A- 36 B  show the formation of the protective layer over the film and the conductive material. The formation process  3600  includes moving the film  3302  from a ninth roller  3602  towards a tenth roller  3604  (movement represented by arrow  3606 ). In one embodiment, the ninth roller  3602  corresponds to the first roller  3304  and the tenth roller  3604  corresponds to the second roller  3304 . In such embodiments, the tenth roller  3604  includes the finished product of the method shown in  FIG.  32   . In other embodiments, the ninth roller  3602  includes the finished product of block  3204 . 
     An eleventh roller  3608  is positioned between the ninth and tenth rollers  3602 ,  3604 . The eleventh roller  3608  applies the protective layer  3610  over the film  3302  and the conductive material  3412 .  FIG.  36 B  illustrates a top view of the film  3302  after the protective layer  3610  has been applied by the eleventh roller  3608 . 
     Referring now to block  3208  in  FIG.  32   , the conductive borders are cut (e.g., singulated) to produce individual sections of film that are each surrounded by a conductive border.  FIGS.  37 A- 37 B  show the production of each individual section of film that is surrounded by a conductive border. The cutting process  3700  includes moving the film  3302  from a twelfth roller  3702  towards a thirteenth roller  3704  (movement represented by arrow  3706 ). In one embodiment, the twelfth roller  3702  corresponds to the first roller  3304  and the thirteenth roller  3704  corresponds to the second roller  3306 . In such embodiments, the thirteenth roller  3704  includes the finished product of the method shown in  FIG.  32   . In other embodiments, the twelfth roller  3702  includes the finished product of block  3206 . 
     In the illustrated embodiment, a singulation system  3708  is positioned over the film  3302  between the twelfth roller  3702  and the thirteenth roller  3704 . The singulation system  3708  includes a precision die cut tool  3710  that is aligned by one or more alignment cameras  3712 . 
     In one embodiment, the precision die cut tool  3710  uses one or more corners of the regions  3514  ( FIG.  35   ) as a cut reference  3714  to position the die cut pattern  3716 .  FIG.  37 B  illustrates top view of the film  3302  with the cut references  3714  and die cut pattern  3716  before the die cut tool  3710  cuts the individual sections. Two singulated sections  3718  are also depicted in  FIG.  37 B . Each singulated section  3718  includes a section of film  3720  surrounded by a conductive border  3722 . As described earlier, the section of film  3720  includes a sheet of conductive material formed over a polarizer film (e.g., the sheet of conductive material  2614  coated over the back polarizer  2610  in  FIG.  26   ). 
     Referring to block  3210  in  FIG.  32   , each singulated section may then be attached to a display layer. In particular, each singulated section can be laminated to a back surface of a back polarizer in the display layer. 
     The geometry of the mask (e.g., mask  3312  in  FIG.  33 B ) and/or the geometry of the die cut pattern (e.g., die cut pattern  3716  in  FIG.  37 B ) can be varied to adjust the geometry of the conductive border.  FIGS.  38 - 40    show example techniques for determining the geometry of the conductive border. In  FIG.  38   , the die cut pattern  3800  is a rectangular shape that is situated to center the mask  3802  in the center of the die cut pattern  3800 . After the singulation process is performed, the film  3806  includes a continuous rectangular conductive border  3804  that extends along the edges of the film  3806 . 
     As shown in  FIG.  39   , the die cut pattern  3900  is offset from the mask  3902  such that one edge of the mask  3902  is outside the die cut pattern  3900 . After the singulation process is performed, the film  3906  includes a U-shaped conductive border  3904 . In the illustrated embodiment, the top edge of the mask  3902  is located outside the die cut pattern  3900  to produce a U-shaped conductive border  3904  that extends along the two side edges and the bottom edge of the film  3906 . However, other embodiments are not limited to this presentation. The shape and orientation of the conductive border  3804  determines which edge (or edges) of the mask  3902  are located outside of the die cut pattern  3900 . 
       FIG.  40    illustrates a die cut pattern  4000  that situates three of the four edges of the mask  4002  outside of the die cut pattern  4000 . After the singulation process is performed, the film  4006  includes a linear conductive border  4004  that extends along one edge of the film  4006 . In the illustrated embodiment, only a portion of the bottom edge of the mask  4002  is positioned within the die cut pattern  4000  to produce a linear conductive border  4004  that extends along the bottom edge of the film  4006 . However, other embodiments are not limited to this configuration. The shape and orientation of the conductive border determines which edge (or edges) of the mask  4002  are located outside of the die cut pattern  4000 . 
     The foregoing description, for purposes of explanation, uses specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. Also, when used herein to refer to positions of components, the terms above and below, or their synonyms, do not necessarily refer to an absolute position relative to an external reference, but instead refer to the relative position of components with reference to the figures.

Metadata:
Filing Date: 20201209
Publication Date: 20231031
Grant Date: 20231031
Priority Date: 20160219
Inventors: PATEL, DHAVAL C.
CHEUNG, Eugene C.
KO, Pey-Jiun
CHEN, PO-JUI
RUMFORD, ROBERT W.
TERRY, STEVE L.
LIN, WEI
NIU, XIAOFAN
ZHOU, XIAOQI
GU, YI
CHUO, YINDAR
DAS, RASMI R.
SCARDATO, STEVEN M.
AHN, SE HYUN
YIN, VICTOR H.
BAE, WOOKYUNG
BOITNOTT, CHRISTOPHER L.
TUNG, CHUN-HAO
SON, Mookyung
KANG, SUNGGU
GUPTA, NATHAN K.
ZHONG, JOHN Z.
Assignee: APPLE INC
CPC Classifications: [{"code": "G06F3/0443", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01L1/146", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13394", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13398", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/041", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0412", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0446", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0447", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L9/0052", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/04146", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04102", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0443", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0443", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0446", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0447", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/041", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0446", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0447", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/04146", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/13398", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/146", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0412", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13394", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L9/0052", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04102", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 58191689