PATENT DOCUMENT

Publication Number: US-10866683-B2
Application Number: US-201916549990-A
Country: US
Kind Code: B2

Title: Force or touch sensing on a mobile device using capacitive or pressure sensing

Abstract:
A device includes a housing defining part of an interior volume and an opening to the interior volume; a cover mounted to the housing to cover the opening and further define the interior volume; a display mounted within the interior volume and viewable through the cover; and a system in package (SiP) mounted within the interior volume. The SiP includes a self-capacitance sense pad adjacent a first surface of the SiP; a set of solder structures attached to a second surface of the SiP, the second surface opposite the first surface; and an IC coupled to the self-capacitance sense pad and configured to output, at one or more solder structures in the set of solder structures, a digital value related to a measured capacitance of the self-capacitance sense pad. The SiP is mounted within the interior volume with the first surface positioned closer to the cover than the second surface.

Claims:
What is claimed is: 
     
       1. A device, comprising:
 a housing defining part of an interior volume and an opening to the interior volume; 
 a cover mounted to the housing to cover the opening and further define the interior volume; 
 a display mounted within the interior volume and viewable through the cover; and 
 a system in package (SiP) mounted within the interior volume, comprising:
 a self-capacitance sense pad adjacent a first surface of the SiP; 
 a set of solder structures attached to a second surface of the SiP, the second surface opposite the first surface; and 
 an integrated circuit (IC) coupled to the self-capacitance sense pad and configured to output, at one or more solder structures in the set of solder structures, a digital value related to a measured capacitance of the self-capacitance sense pad; wherein: 
 
 the SiP is mounted within the interior volume with the first surface positioned closer to the cover than the second surface. 
 
     
     
       2. The device of  claim 1 , further comprising a device stack attached to the cover, wherein:
 the device stack comprises the display and a ground element; 
 the self-capacitance sense pad is separated from the ground element by a compressible gap; 
 the digital value is indicative of an amount of force applied to the cover; and 
 different amounts of force applied to the cover compress the compressible gap to different extents and produce different capacitances at the self-capacitance sense pad. 
 
     
     
       3. The device of  claim 2 , wherein the ground element is separated from the cover by the display. 
     
     
       4. The device of  claim 1 , wherein:
 the digital value is indicative of a proximity of a finger or a stylus to the cover; and 
 different proximities of the finger or the stylus to the cover produce different capacitances at the self-capacitance sense pad. 
 
     
     
       5. The device of  claim 1 , wherein the SiP further comprises an alternating current (AC) shield around the self-capacitance sense pad. 
     
     
       6. The device of  claim 5 , wherein the SiP further comprises an AC shield layer positioned between the self-capacitance sense pad and the IC. 
     
     
       7. The device of  claim 1 , wherein the self-capacitance sense pad is a first self-capacitance sense pad, the device further comprising:
 a second self-capacitance sense pad disposed in a different plane than the first self-capacitance sense pad. 
 
     
     
       8. The device of  claim 7 , wherein the second self-capacitance sense pad is disposed between the first self-capacitance sense pad and the IC. 
     
     
       9. The device of  claim 7 , wherein the SiP further comprises:
 an alternating current (AC) shield around the first and second self-capacitance sense pads; and 
 an AC shield layer positioned between the first and second self-capacitance sense pads and the IC. 
 
     
     
       10. The device of  claim 1 , wherein the SiP is a first SiP, the device further comprising:
 a flex circuit; and 
 a second SiP comprising a same set of components as the first SiP and oriented within the interior volume in a same way as the first SiP; 
 wherein the first SiP and the second SiP are mechanically and electrically coupled to a same side of the flex circuit. 
 
     
     
       11. The device of  claim 1 , wherein the SiP is a first SiP, the device further comprising:
 a printed circuit board; and 
 a second SiP comprising a same set of components as the first SiP and oriented within the interior volume in a same way as the first SiP; wherein; 
 the first SiP and the second SiP are mechanically and electrically coupled to a same side of the printed circuit board. 
 
     
     
       12. A device, comprising:
 a housing defining part of an interior volume and an opening to the interior volume; 
 a cover mounted to the housing to cover the opening and further define the interior volume; 
 a capacitive force sensor disposed within the interior volume and configured to generate a first set of one or more signals related to an amount of force applied to the cover; 
 a pressure sensor disposed within the interior volume and configured to generate a second set of one or more signals related to the amount of force applied to the cover; and 
 a processor configured to determine the amount of force applied to the cover using at least one of the first set of one or more signals or the second set of one or more signals. 
 
     
     
       13. The device of  claim 12 , further comprising:
 a touch sensor disposed to detect one or more touches on the cover; 
 wherein the processor is configured to:
 determine the amount of force applied to the cover using the second set of one or more signals when the amount of force applied to the cover is determined for a first time following a touch detected by the touch sensor; and 
 determine the amount of force applied to the cover using the first set of one or more signals when the amount of force applied to the cover is determined for a second time following the touch detected by the touch sensor. 
 
 
     
     
       14. The device of  claim 13 , wherein the processor is configured to:
 determine the amount of force applied to the cover using the first set of one or more signals and the second set of one or more signals when the amount of force applied to the cover is determined for a third time following the touch detected by the touch sensor; and 
 the third time is between the first time and the second time. 
 
     
     
       15. The device of  claim 12 , wherein the processor is configured to:
 determine a sealing state of the interior volume; 
 determine the amount of force applied to the cover using the first set of one or more signals when the sealing state is determined to be below a threshold; and 
 determine the amount of force applied to the cover using the second set of one or more signals when the sealing state is determined to be above the threshold. 
 
     
     
       16. The device of  claim 12 , wherein the processor is configured to:
 determine a sealing state of the interior volume; 
 determine the amount of force applied to the cover using the first set of one or more signals when the sealing state is determined to be below a first threshold; and 
 determine the amount of force applied to the cover using the first set of one or more signals and the second set of one or more signals when the sealing state is determined to be between the first threshold and a second threshold. 
 
     
     
       17. The device of  claim 12 , wherein the processor is configured to:
 determine a sealing state of the interior volume; 
 determine the amount of force applied to the cover using the first set of one or more signals when the sealing state is determined to be below a first threshold; 
 determine the amount of force applied to the cover using the first set of one or more signals and the second set of one or more signals when the sealing state is determined to be between the first threshold and a second threshold; and 
 determine the amount of force applied to the cover using the second set of one or more signals when the sealing state is determined to be above the second threshold. 
 
     
     
       18. The device of  claim 12 , wherein the capacitive force sensor is a first capacitive force sensor and the device further comprises:
 a set of capacitive force sensors disposed within the interior volume, each disposed within the interior volume and configured to generate a respective first set of one or more signals related to an amount of force applied to the cover; and 
 a fluid immersion detector; wherein: 
 the capacitive force sensors of the set of capacitive force sensors are disposed at different locations beneath the cover; 
 the set of capacitive force sensors includes the first capacitive force sensor; and 
 the processor is further configured to determine a location of a touch on the cover, using the respective first sets of one or more signals, when the fluid immersion detector indicates the device is immersed in a fluid. 
 
     
     
       19. The device of  claim 12 , wherein the pressure sensor comprises a microphone. 
     
     
       20. The device of  claim 12 , further comprising:
 a barometric pressure sensor disposed within the housing and configured to generate a third set of one or more signals related to an ambient pressure of the device; wherein: 
 the processor is configured to determine the amount of force applied to the cover using at least one signal from each of the first set of one or more signals, the second set of one or more signals, and the third set of one or more signals. 
 
     
     
       21. The device of  claim 20 , wherein the processor is configured to:
 determine an internal pressure from the first set of one or more signals; 
 determine a distance between the cover and the capacitive force sensor using the second set of one or more signals; 
 determine the ambient pressure of the device from the third set of one or more signals; and 
 determine a pressure difference between the internal pressure and the ambient pressure; wherein: 
 the processor determines the amount of force applied to the cover from the distance and the pressure difference. 
 
     
     
       22. A method of determining an amount of force applied to a device having a display, comprising:
 determining a sealing state of an interior volume including the display of the device; 
 determining the amount of force applied to the device using a first set of one or more signals when the sealing state is determined to be a first sealing state; 
 determining the amount of force applied to the device using a second set of one or more signals when the sealing state is determined to be a second sealing state; wherein: 
 the first set of one or more signals is provided by a capacitive force sensor disposed within the interior volume; and 
 the second set of one or more signals is provided by a pressure sensor disposed within the interior volume. 
 
     
     
       23. The method of  claim 22 , further comprising:
 measuring capacitances of a first self-capacitance sense pad and a second self-capacitance sense pad of the capacitive force sensor; 
 calibrating a force sensing operation of the capacitive force sensor using the measured capacitances of the first self-capacitance sense pad and the second self-capacitance sense pad; and 
 outputting, from a system in package (SiP) disposed within the interior volume, a digital value related to a measured capacitance of the first self-capacitance sense pad; wherein: 
 the first self-capacitance sense pad and the second self-capacitance sense pad are respectively disposed in different layers of the SiP; 
 the SiP is separated from a ground element within the interior volume by a compressible gap; and 
 the ground element is attached to the display. 
 
     
     
       24. The method of  claim 22 , further comprising:
 measuring the capacitances of the first self-capacitance sense pad and the second self-capacitance sense pad when the compressible gap is in different compression states; wherein: 
 the force sensing operation is calibrated using the measured capacitances for the different compression states. 
 
     
     
       25. A device, comprising:
 a housing defining part of an interior volume and an opening to the interior volume; 
 a cover mounted to the housing to cover the opening and further define the interior volume; 
 an internal pressure sensor disposed within the interior volume and configured to generate a first set of one or more signals related to an amount of force applied to the cover; 
 a barometric pressure sensor disposed within the housing and configured to generate a second set of one or more signals related to an ambient pressure of the device; and 
 a processor configured to determine the amount of force applied to the cover using the first set of one or more signals and the second set of one or more signals. 
 
     
     
       26. The device of  claim 25 , wherein the processor is further configured to:
 determine an internal pressure of the device from the first set of one or more signals; and 
 determine the ambient pressure of the device from the second set of one or more signals; wherein: 
 the processor determines the amount of force applied to the cover using the internal pressure and the ambient pressure. 
 
     
     
       27. The device of  claim 26 , wherein:
 the processor is further configured to adjust the internal pressure using the ambient pressure; and 
 the processor determines the amount of the force applied to the cover using the adjusted internal pressure. 
 
     
     
       28. The device of  claim 27 , wherein the processor is further configured to:
 determine whether the amount of force applied to the cover indicates a user input. 
 
     
     
       29. The device of  claim 26 , wherein the processor is further configured to:
 adjust, using the ambient pressure, a force detection threshold to which the internal pressure is compared; and 
 identify a force input when the internal pressure satisfies the adjusted force detection threshold. 
 
     
     
       30. The device of  claim 26 , wherein the processor is further configured to:
 characterize the ambient pressure; and 
 cease determining the amount of force applied to the cover, for a period of time, in response to the characterization of the ambient pressure. 
 
     
     
       31. The device of  claim 26 , wherein:
 the determined ambient pressure is a time-varying ambient pressure; and 
 the processor is further configured to:
 identify, from the time-varying ambient pressure, an ambient pressure rate of change above a threshold; and 
 adapt, in response to identifying the ambient pressure rate of change above the threshold, how the amount of force applied to the cover is determined. 
 
 
     
     
       32. A device, comprising:
 a housing defining part of an interior volume and an opening to the interior volume; 
 a cover mounted to the housing to cover the opening and further define the interior volume; 
 a pressure sensor disposed within the interior volume and configured to generate a set of pressure samples related to an amount of force applied to the cover; and 
 a processor; wherein the processor is configured to:
 determine, using the set of pressure samples, whether the amount of force applied to the cover exceeds a τ calibration threshold, where the τ calibration threshold is based on an application or release of the force applied to the cover, and τ is a time it takes the interior volume to reach a steady state pressure following the application or release of the force applied to the cover; 
 calibrate τ during field operation of the device, in response to determining the amount of force applied to the cover exceeds the τ calibration threshold; and 
 determine the amount of force applied to the cover, after calibrating τ, using at least one pressure sample obtained from the pressure sensor and the calibrated τ. 
 
 
     
     
       33. The device of  claim 32 , wherein the processor is configured to calibrate τ, during field operation of the device, in response to a combination of both:
 determining the amount of force applied to the cover exceeds the τ calibration threshold; and 
 determining that a number of pressure samples obtained after a release of the force applied to the cover, and before a next touch on the cover, exceeds a threshold number of samples. 
 
     
     
       34. The device of  claim 32 , wherein the processor is further configured to:
 determine whether τ satisfies an interior volume leak threshold; and 
 notifying a user of the device when τ is determined to satisfy the interior volume leak threshold. 
 
     
     
       35. A device, comprising:
 a housing defining part of an interior volume and an opening to the interior volume; 
 a cover mounted to the housing to cover the opening and further define the interior volume; 
 an internal pressure sensor disposed within the interior volume and configured to generate a first set of one or more signals related to an amount of force applied to the cover; 
 a temperature sensor disposed within the housing and configured to generate a second set of one or more signals related to a temperature within the interior volume; and 
 a processor configured to generate an adjusted first set of one or more signals using the second set of one or more signals, and determine the amount of force applied to the cover using at least one adjusted signal from the adjusted first set of one or more signals. 
 
     
     
       36. The device of  claim 35 , wherein the temperature sensor comprises a first temperature sensor and a second temperature sensor. 
     
     
       37. A device, comprising:
 a housing defining part of an interior volume and an opening to the interior volume; 
 a cover mounted to the housing to cover the opening and further define the interior volume; 
 a sealed pocket disposed between the cover and the housing and operating as a spring that opposes an amount of force applied to the cover; 
 a pressure sensor disposed within the interior volume and configured to generate a set of one or more signals related to the amount of force applied to the cover; and 
 a processor configured to determine the amount of force applied to the cover using at least one signal of the first set of one or more signals. 
 
     
     
       38. A device, comprising:
 a housing defining part of an interior volume and an opening to the interior volume; 
 a cover mounted to the housing to cover the opening and further define the interior volume; 
 a pressure chopper configured to modulate a pressure of the interior volume; 
 an internal pressure sensor disposed within the interior volume and configured to generate a first set of one or more modulated signals related to an amount of force applied to the cover; and 
 a processor configured to generate a second set of one or more demodulated signals using a modulation frequency of the pressure chopper, and determine the amount of force applied to the cover using at least one signal from the second set of one or more demodulated signals. 
 
     
     
       39. The device of  claim 38 , wherein the pressure chopper comprises an open back speaker. 
     
     
       40. The device of  claim 38 , wherein the pressure chopper comprises a haptic engine.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a nonprovisional of and claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 62/723,430, filed Aug. 27, 2018, entitled “Force Sensing with Capacitive Sensor in a System in Package,” U.S. Provisional Patent Application No. 62/738,831, filed Sep. 28, 2018, entitled “Force Sensing on a Mobile Device Using Capacitive and/or Pressure Sensing,” and U.S. Provisional Patent Application No. 62/851,632, filed May 22, 2019, entitled “Touch Location Determination for a Device Immersed in a Fluid,” the contents of which are incorporated herein by reference as if fully disclosed herein. 
    
    
     FIELD 
     The described embodiments relate generally to force or touch sensing on a mobile device. More particularly, some of the described embodiments relate to determining an amount of force applied to a device, or determining a location of a touch on a device, using measurements obtained from a capacitive force sensor and/or a pressure sensor, one or both of which may be disposed within an interior volume of the device. In some cases, the amount of force applied to the device or the location of the touch on the device may be determined using additional measurements, such as measurements obtained from a barometric pressure sensor, a temperature sensor, or other sensors. 
     BACKGROUND 
     A device such as a smartphone, tablet computer, or electronic watch may include a touch sensor that indicates where a display of the device is touched, and a force sensor that indicates an amount of force applied to the display by the touch. In some cases, the force sensor may be a capacitive force sensor. A capacitive force sensor may include first and second electrodes disposed in first and second flex circuits. The flex circuits may be separated by a compressible element or gap. As the amount of force applied to the display increases, the compressible element or gap is compressed and the electrodes disposed in the flex circuits move closer to one another, thereby decreasing the capacitance between the electrodes. The flex circuits may be coupled to circuitry (e.g., a microprocessor, an application-specific integrated circuit (ASIC), or a controller) that measures, amplifies, and digitizes the capacitance, and determines an amount of force corresponding to the capacitance. 
     SUMMARY 
     Some embodiments of the systems, devices, methods, and apparatus described in the present disclosure are directed to determining an amount of force applied to a device, or calibrating the sensors used to determine the amount of force, or adjusting the measurements obtained by one sensor using measurements obtained by another sensor. In some embodiments, the amount of force applied to the device may be determined using a capacitive force sensor and/or a pressure sensor disposed within an interior volume of the device. In some embodiments, the measurements obtained from the capacitive force sensor or pressure sensor may be selected or adjusted, for use in determining the amount of force applied to the device, using measurements obtained from another sensor or sensors (e.g., a barometric pressure sensor, a temperature sensor, or other sensors). 
     Some embodiments of the systems, devices, methods, and apparatus described in the present disclosure are directed to enabling a device that is used under water to determine where a user is touching the device. In particular, described systems, devices, methods, and apparatus may use a pressure sensor and one or more displacement sensors to determine where a user is touching a device when the device is used under water. For example, in some embodiments, a device may include a display associated with a touch sensor. When the device is not immersed in water, the touch sensor may indicate a location of a user&#39;s touch with respect to the display, and a pressure sensor and/or displacement sensor(s) positioned within the device (e.g., under the display) may be used to determine an amount of force associated with the touch. However, when the device is immersed in a fluid and the touch sensor becomes inoperable due to saturation of the touch sensor&#39;s pixels (e.g., a condition in which most or all of the touch sensor&#39;s pixels indicate the presence of a touch), measurements generated by the pressure sensor and displacement sensor(s) may be used to determine the location of a user&#39;s touch with respect to the display (e.g., instead of an output of the touch sensor). 
     In a first aspect, the present disclosure describes a device including a housing defining part of an interior volume and an opening to the interior volume; a cover mounted to the housing to cover the opening and further define the interior volume; a display mounted within the interior volume and viewable through the cover; and a SiP mounted within the interior volume. The SiP includes a self-capacitance sense pad adjacent a first surface of the SiP; a set of solder structures attached to a second surface of the SiP, the second surface opposite the first surface; and an integrated circuit (IC) coupled to the self-capacitance sense pad and configured to output, at one or more solder structures in the set of solder structures, a digital value related to a measured capacitance of the self-capacitance sense pad. The SiP is mounted within the interior volume with the first surface positioned closer to the cover than the second surface. 
     In another aspect, the present disclosure describes a device including a housing defining part of an interior volume and an opening to the interior volume; a cover mounted to the housing to cover the opening and further define the interior volume; a capacitive force sensor disposed within the interior volume and configured to generate a first set of one or more signals related to an amount of force applied to the cover; a pressure sensor disposed within the interior volume and configured to generate a second set of one or more signals related to the amount of force applied to the cover; and a processor configured to determine the amount of force applied to the cover using at least one of the first set of one or more signals or the second set of one or more signals. 
     In still another aspect of the disclosure, a method of determining an amount of force applied to a device having a display is described. The method includes determining a sealing state of an interior volume including the display of the device; determining the amount of force applied to the device using a first set of one or more signals when the sealing state is determined to be a first sealing state; and determining the amount of force applied to the device using a second set of one or more signals when the sealing state is determined to be a second sealing state. The first set of one or more signals is provided by a capacitive force sensor disposed within the interior volume, and the second set of one or more signals is provided by a pressure sensor disposed within the interior volume. 
     The disclosure also describes a device including a ground element and a SiP. The SiP includes a first self-capacitance sense pad adjacent a first surface of the SiP and separated from the ground element by a compressible gap; a second self-capacitance sense pad separated from the first self-capacitance sense pad by a non-compressible gap; a set of solder structures attached to a second surface of the SiP, the second surface opposite the first surface and positioned farther from the ground element than the first surface; and an IC coupled to the first self-capacitance sense pad and the second self-capacitance sense pad. The IC is configured to measure capacitances of the first self-capacitance sense pad and the second self-capacitance sense pad; calibrate a force sensing operation using the measured capacitances of the first self-capacitance sense pad and the second self-capacitance sense pad; and output, at one or more solder structures in the set of solder structures, a digital value related to a measured capacitance of the first self-capacitance sense pad. 
     In another aspect of the disclosure, a device includes a housing, a cover, an internal pressure sensor, a barometric pressure sensor, and a processor. The housing defines part of an interior volume and an opening to the interior volume. The cover is mounted to the housing to cover the opening and further define the interior volume. The internal pressure sensor is disposed within the interior volume and configured to generate a first set of one or more signals related to an amount of force applied to the cover. The barometric pressure sensor is disposed within the housing and configured to generate a second set of one or more signals related to an ambient pressure of the device. The processor is configured to determine the amount of force applied to the cover using the first set of one or more signals and the second set of one or more signals. 
     In another aspect, the present disclosure describes a device including a housing, a cover, and a pressure sensor. The housing defines part of an interior volume and an opening to the interior volume, and the cover is mounted to the housing to cover the opening and further define the interior volume. The pressure sensor is disposed within the interior volume and configured to generate a set of one or more pressure samples related to an amount of force applied to the cover. The processor is configured to determine, using at least one signal from the set of one or more signals, whether the amount of force applied to the cover exceeds a τ calibration threshold, where τ is a time it takes the interior volume to reach a steady state pressure following an application or release of a steady-state force to the cover; calibrate τ during field operation of the device, in response to determining the amount of force applied to the cover exceeds the τ calibration threshold; and determine the amount of force applied to the cover, after calibrating τ, using the set of one or more pressure samples and the calibrated τ. 
     A device having a housing, a cover, an internal pressure sensor, a temperature sensor, and a process is described in yet another aspect of the disclosure. The housing defines part of an interior volume and an opening to the interior volume, and the cover is mounted to the housing to cover the opening and further define the interior volume. The internal pressure sensor is disposed within the interior volume and configured to generate a first set of one or more signals related to an amount of force applied to the cover. The temperature sensor is disposed within the housing and configured to generate a second set of one or more signals related to a temperature within the interior volume. The processor is configured to generate an adjusted first set of one or more signals using the second set of one or more signals, and determine the amount of force applied to the cover using at least one adjusted signal from the adjusted first set of one or more signals. 
     In another aspect of the present disclosure, another device is described. The device includes a housing defining part of an interior volume and an opening to the interior volume; a cover mounted to the housing to cover the opening and further define the interior volume; a sealed pocket disposed between the cover and the housing and operating as a spring that opposes an amount of force applied to the cover; a pressure sensor disposed within the interior volume and configured to generate a set of one or more signals related to the amount of force applied to the cover; and a processor configured to determine the amount of force applied to the cover using at least one signal of the first set of one or more signals. 
     In another aspect, the present disclosure describes a device that includes a display, a set of structural components, a pressure sensor, a set of displacement sensors, a fluid immersion detector, and a processor. The set of structural components may be disposed about the display, and may define a cover over the display. The set of structural components may also define a cavity beneath the display. The pressure sensor may be disposed within the cavity. The set of displacement sensors may include a first displacement sensor and a second displacement sensor positioned at different locations beneath the cover. The processor may be configured to determine a location of a touch on the cover, using signals received from the pressure sensor and the set of displacement sensors, when the fluid immersion detector indicates the device is immersed in a fluid. 
     In another aspect, the present disclosure describes a device that includes a housing, a pressure sensor, a set of displacement sensors, and a processor. The housing may define a volume interior to the device. The pressure sensor may be disposed within the volume, and may be configured to generate a time-dependent sequence of pressure measurements in response to a force applied to the device while the device is immersed in a fluid. The set of displacement sensors may include at least a first displacement sensor configured to generate a first time-dependent sequence of displacement measurements in response to the force. The processor may be configured to determine a location where the force is applied to a surface of the device using at least the time-dependent sequence of pressure measurements and the first time-dependent sequence of displacement measurements. 
     In another aspect of the disclosure, the present disclosure describes a method of determining a location of a touch on a surface of a device when the device is immersed in a fluid. The method may include receiving from a pressure sensor within the device, and in response to a force applied to the device while the device is immersed in the fluid, a time-dependent sequence of pressure measurements. The method may also include receiving, from a first displacement sensor within the device, and in response to the force, a first time-dependent sequence of displacement measurements. The first time-dependent sequence of displacement measurements may include displacement measurements for a first location on the surface. The method may further include determining the location of the touch using at least the time-dependent sequence of pressure measurements and the first time-dependent sequence of displacement measurements. 
     In addition to the aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following description. 
    
    
     
       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: 
         FIGS. 1A and 1B  show an example of a device that includes a set of structural components defining an interior volume; 
         FIGS. 2A and 2B  show an example device, which device may be an example of the device described with reference to  FIGS. 1A and 1B ; 
         FIGS. 3A-3D  show various examples of a capacitive force sensor (or capacitive proximity sensor) embodied in a SiP; 
         FIG. 4A  shows an enlarged cross-section of the multi-layer SiP substrate and IC described with reference to any of  FIGS. 3A-3D ; 
         FIG. 4B  shows a schematic view of the conductive layers described with reference to  FIG. 4A ; 
         FIG. 5  shows a plurality of capacitive force sensors within a device, each included in a different SiP; 
         FIGS. 6A-6C  show an example alternative cross-section of the device described with reference to  FIG. 2A ; 
         FIGS. 7A-7C  are graphs of example outputs of the pressure sensor and capacitive force sensors included in the device described with reference to  FIGS. 2A and 6A-6C ; 
         FIG. 8  is a graph showing example displacements of the cover described with reference to  FIGS. 6A-6C ; 
         FIG. 9  is a graph that shows an example relationship between the user facing force noise metrics of a capacitive force sensor and a pressure sensor; 
         FIGS. 10A-10C  provide examples of how capacitance values obtained from a capacitive force sensor, and pressure values obtained from a pressure sensor, may be used to determine an amount of force applied to a cover; 
         FIG. 11A  is a graph showing a relationship between a compressible gap (g) defined between a ground element and a sensing electrode (e.g., the compressible gap described with reference to  FIGS. 2A and 2B ) and capacitances (C) of the sensing and calibration electrodes; 
         FIG. 11B  is another graph showing the relationship between the compressible gap (g) and the capacitances (C) of the sensing and calibration electrodes; 
         FIG. 12  illustrates a gap sensing method for determining the parasitic capacitances, C 0a  and C 0b , effecting sensing and calibration electrodes of a capacitive force sensor; 
         FIG. 13  illustrates a capacitance sensing method for calibrating a differential capacitance between sensing and calibration electrodes of a capacitive force sensor; 
         FIG. 14  shows an example method for calibrating a capacitive force sensor; 
         FIGS. 15A-15D  show various alternative embodiments of a capacitive force sensor; 
         FIGS. 16A-16C  show another example cross-section of the device described with reference to  FIG. 2A ; 
         FIGS. 17A-17C  are graphs of example outputs of the internal pressure sensor and barometric pressure sensor described with reference to  FIGS. 16A-16C , and  FIG. 17D  is a graph of an adjusted internal pressure (adjusted from  FIG. 17C ); 
         FIGS. 18A-18D  show various examples of how a measurement of internal pressure may be adjusted using a measurement of barometric or ambient pressure; 
         FIG. 19  shows another example cross-section of the device described with reference to  FIG. 2A ; 
         FIG. 20  shows how a change in physical τ (the component of τ dependent on the physical characteristics of a device) can translate to accuracy errors in sensed force determinations; 
         FIG. 21  shows an example of how τ may be pre-calibrated and field calibrated for a device; 
         FIG. 22  shows example correlated graphs of touch, force, and pressure on the cover of a device; 
         FIG. 23  shows an example method for performing a field calibration of τ; 
         FIG. 24  shows how τ may be calibrated by continuously monitoring the pressure difference between internal and ambient pressures of a device; 
         FIG. 25  shows another example cross-section of the device  200  described with reference to  FIG. 2A ; 
         FIG. 26  is a graph of the amount of force applied to a device&#39;s cover compared to the amount of force measured by a pressure sensor disposed within an interior volume of the device; 
         FIG. 27  shows a graph of pressure spectral density (PSD) verses frequency, and illustrates how low frequency noise (e.g., red noise) may interfere with a user input; 
         FIG. 28  shows graphs of PSD verses frequency after modulation and demodulation of a user input (e.g., a force input or pressure); 
         FIG. 29  shows another example cross-section of the device described with reference to  FIG. 2A ; 
         FIG. 30  shows example correlated graphs of force, volume, and pressure on the cover of a device; 
         FIG. 31  shows a graph of normalized amplitude response verses frequency for an example microphone; 
         FIGS. 32-36  shows example methods for determining an amount of force applied to a cover of a device; 
         FIG. 37  shows an example top plan view of the device described with reference to  FIGS. 1A and 1B ; 
         FIG. 38  shows an example alternative cross-section of the device shown in  FIG. 2A ; 
         FIGS. 39A-39E  show the device cross-section of  FIG. 38  when, and after, a force is applied to a cover of the device; 
         FIG. 40A-40C  are graphs of example outputs of the pressure sensor and one of the capacitive force sensors described with reference to  FIGS. 38 and 39A-39E , in response to the application and removal (liftoff) of a force on a device; 
         FIG. 41  shows relationships between a force applied to a device, measurements of displacement derived from a capacitive force sensor positioned in or in relation to an interior volume of the device, measurements of pressure derived from a pressure sensor positioned within the interior volume, and estimates of displacement derived from the measurements of pressure; 
         FIG. 42  shows relationships between a force applied to a device, measurements of displacement derived from each of three capacitive force sensors positioned in or in relation to an interior volume of the device, measurements of pressure derived from a pressure sensor positioned within the interior volume, and estimates of displacement derived from the measurements of pressure; 
         FIGS. 43-45  show example plan views of the device described with reference to  FIGS. 1A-1B and 37 , when one or more of its capacitive force sensors are used in combination with its pressure sensor to determine a location of a touch on the device&#39;s cover; 
         FIG. 46  shows an example method of determining a location of a touch on a surface of a device when the device is immersed in a fluid; and 
         FIG. 47  shows a sample electrical block diagram of an electronic device. 
     
    
    
     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. 
     Additionally, it should be understood that the proportions and dimensions (either relative or absolute) of the various features and elements (and collections and groupings thereof) and the boundaries, separations, and positional relationships presented therebetween, are provided in the accompanying figures merely to facilitate an understanding of the various embodiments described herein and, accordingly, may not necessarily be presented or illustrated to scale, and are not intended to indicate any preference or requirement for an illustrated embodiment to the exclusion of embodiments described with reference thereto. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following description is 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. 
     Described herein are techniques that enable the force of a touch on a device to be determined. Some of the techniques may also be used to determine the proximity of a user&#39;s finger or stylus to a device. The techniques employ capacitive force sensing, pressure sensing, or a combination thereof. Capacitive force sensing may be better suited to determining an amount of force applied to a device when the device has an interior volume that is loosely sealed or not sealed, or after the pressure within the interior volume has reached a steady state following application of the force to the device. Pressure sensing may be better suited to determining an amount of force applied to a device when the device has an interior volume that is well sealed or minimally vented, or when a force is initially applied to the device and the pressure within the interior volume experiences a spike before the pressure exponentially decreases to a steady state. In some of the techniques described herein, an amount of force applied to a device may be determined using a combination of capacitive force sensing and pressure sensing, or by using capacitive force sensing under some conditions and pressure sensing under other conditions. In some cases, the type of force sensing used to determine a force applied to a device may change over the lifetime of the device and/or in response to different conditions (e.g., environments) to which the device is exposed. In some techniques, an amount of force applied to a device may be additionally determined using barometric pressure sensing, temperature sensing, or other types of sensing. 
     Capacitive force sensors that may be easier or cheaper to construct are also described. For example, a capacitive force sensor embodied in a SiP is described. The SiP may contain a self-capacitance sense pad (or mutual-capacitance sense pad), and an IC and passives for sensing, amplifying, digitizing, or otherwise processing a capacitance of the capacitance sense pad, which capacitance is related to an amount of force sensed by the capacitive force sensor. Capacitive force sensors embodied in SiPs are self-contained (e.g., they do not require alignment of different flex circuits containing different electrodes, and connection of the electrodes to an IC). Capacitive force sensors embodied in SiPs may also contain an electrode or other components for calibrating a force sensing operation on-board the SiP. At least in part because capacitive force sensors embodied in SiPs are self-contained, they may be opportunistically positioned within a device. 
     Various techniques for calibrating a force sensor, or for calibrating or adjusting a method for determining a force applied to a device, are also described. 
     Also described herein are techniques that enable a device that is used under water (or otherwise used while immersed in a fluid) to determine where a user is touching the device. The techniques are described primarily with respect to determining the location of a user&#39;s touch with reference to a display (i.e., with reference to a surface of the display). However, the techniques may in some cases be used to determine the location of a user&#39;s touch on a back, side, or non-display surface of the device. 
     These and other techniques are described with reference to  FIGS. 1-47 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. 
     Directional terminology, such as “top”, “bottom”, “upper”, “lower”, “front”, “back”, “over”, “under”, “left”, “right”, etc. is used with reference to the orientation of some of the components in some of the figures described below. Because components in various embodiments can be positioned in a number of different orientations, directional terminology is used for purposes of illustration only and is in no way limiting. The directional terminology is intended to be construed broadly, and therefore should not be interpreted to preclude components being oriented in different ways. The use of alternative terminology, such as “or”, is intended to indicate different combinations of the alternative elements. For example, A or B is intended to include, A, or B, or A and B. 
     As shown in  FIGS. 1A and 1B , a device  100  may include a set of structural components  102  that define an interior volume  104 .  FIG. 1A  shows an isometric view of the device  100 , and  FIG. 1B  shows a cross-section of the device  100 . 
     The set of structural components  102  may include, for example, a housing  102   a  and a cover  102   b . Each structural component (e.g., each of the housing  102   a  and the cover  102   b ) may define a part of the interior volume  104 . The housing  102   a  may further define an opening to the interior volume  104 . In some cases, the housing  102   a  may be a multi-part housing, such as a housing formed by a support plate and one or more edge members extending from the support plate to support the cover  102   b . In some cases, the cover  102   b  may be a transparent cover, such as a glass or plastic cover. The structural components may be semi-permanently or detachably joined to one another by a set of fasteners, adhesives, seals, or other components. 
     In some embodiments, the interior volume  104  may be sealed to prevent gases (e.g., air) or fluids (e.g., water) from entering or leaving the interior volume  104 . In other embodiments, the interior volume  104  may not be sealed, thereby allowing gases and possibly fluids to enter or leave the interior volume  104 . In some embodiments, the interior volume  104  may be vented. For example, an optional port  112  may be defined in one or more of the structural components  102 , and the port may allow gases (e.g., air) but not fluids (e.g., water) to flow between the interior volume  104  and an ambient environment of the device  100 . During the lifetime of the device  100 , the venting state (or sealing state) of the device  100  (e.g., sealed, not sealed, or vented) may change due to clogging of the port  112 , breakage of a seal between the cover  102   b  and the housing  102   a , breakage of the cover  102   b , use of the device  100  under water, and so on. 
     As shown in  FIG. 1B , a touch sensing system  106  (i.e., a system including a touch sensor), a force sensing system  108  (i.e., a system including a force sensor), a processor  110 , or other components may be mounted to the set of structural components  102  (e.g., to the housing  102   a  and/or cover  102   b ) and positioned partly or wholly within the interior volume  104 . Some of the components (e.g., the processor  110 ) may alternatively be positioned entirely outside the interior volume  104  (not shown). The touch sensing system  106  may include, for example, a set of capacitive touch sensing elements, a set of resistive touch sensing elements, or a set of ultrasonic touch sensing elements. When a user of the device  100  touches the cover, the touch sensing system  106  (or touch sensor) may detect one or more touches on the cover  102   b  and determine locations of the touches on the cover  102   b . The touches may include, for example, touches by a user&#39;s finger or stylus. 
     The force sensing system  108  may include, for example, a set of capacitive force sensing elements (i.e., capacitive force sensors) or other types of displacement sensors, a set of resistive force sensing elements (or resistive force sensors), or one or more pressure transducers (i.e., one or more pressure sensors). When a user of the device  100  presses on the cover  102   b  (i.e., applies a force to the cover  102   b ), the force sensing system  108  may determine an amount of force applied to the cover  102   b  (or in some cases, the amount of force applied to a side or sides of the housing  102   a , a surface of the housing  102   a  opposite the cover  102   b , and so on). In some embodiments, the force sensing system  108  may be used alone or in combination with the touch sensing system  106  to determine a location of an applied force, or an amount of force associated with each touch in a set of multiple contemporaneous touches. 
     The processor  110  may be configured to operate the touch sensing system  106  or the force sensing system  108 , and may be configured to receive, evaluate, propagate, or respond to signals obtained from the touch sensing system  106  or the force sensing system  108 . 
     In embodiments in which the device  100  is vented, the port  112  may be covered by an air permeable and fluid impermeable membrane that prevents fluid (e.g., water) from entering the interior volume  104 , thus enabling the device  100  to be used while immersed in a fluid. However, when the device  100  is used while immersed (e.g., under water), the presence of water or water pressure on the cover  102   b  may cause a set of pixels of the touch sensing system  106  (e.g., all or a majority of the pixels of a touch sensor) to saturate. When all or a majority of the pixels saturate, indicating the presence of something touching numerous or all locations of the cover  102   b , the touch sensing system  106  may be unable to distinguish a user&#39;s touch from water pressure or otherwise identify the location of a user&#39;s touch on the cover  102   b . In these cases, the force sensing system  108 , instead of the touch sensing system  106 , may be used to detect the location of a user&#39;s touch on the cover  102   b  (e.g., as described with reference to  FIGS. 37-46 ). 
       FIGS. 2A and 2B  show an example device  200  (e.g., an electronic device, or smartphone), which device  200  may be an example of the device  100  described with reference to  FIGS. 1A-1B .  FIG. 2A  shows an isometric view of the device  200 , and  FIG. 2B  shows a cross-section of the device  200 . 
     The device  200  may include a set of structural components  202  that define a cavity or interior volume  204  of the device  200 . By way of example, the set of structural components  202  may include a housing  202   a  and a cover  202   b . In some embodiments, the housing  202   a  and cover  202   b  may be configured as described with reference to  FIGS. 1A-1B . The cover  202   b  may be mounted to the housing  202   a  to cover an opening defined by the housing  202   a  (i.e., an opening into the interior volume  204 ). The cover  202   b  may be mounted to the housing  202   a  using fasteners, adhesives, seals, or other components. By way of example, the cover  202   b  is shown to be mounted to the housing  202   a  by a gasket  206  that separates the cover  202   b  from the housing  202   a  (see,  FIG. 2B ). A first adhesive may be disposed between the gasket  206  and the housing  202   a , and a second adhesive (which may have the same or different composition as the first adhesive) may be disposed between the cover  202   b  and the gasket  206 . 
     The interior volume  204  may be sealed, or not sealed, or vented, as described with reference to  FIGS. 1A and 1B . In some embodiments, the interior volume  204  may be vented by means of one or more ports  218  in the housing  202   a  or elsewhere, which ports  218  may allow gas or fluid trapped within the interior volume  204  to escape, but not instantaneously. In other embodiments, the interior volume may be vented by one or more ports  218  that allow gas or fluid trapped within the interior volume  204  to escape instantaneously. In some embodiments, the port(s)  218  may allow air (and in some cases water) to flow out of the interior volume  204  and into an ambient environment of the device  200 . The same port(s)  218  may allow air (but not water) to pass from the ambient environment into the interior volume  204 . 
     A display  208  may be mounted within the interior volume  204 . The display may be viewable through the cover  202   b . In some embodiments, a display stack or device stack (hereafter referred to as a “stack”) including the display  208  may be attached to an interior surface of the cover  202   b  and extend into the interior volume  204 . In some cases, the stack may include a touch sensor (e.g., a grid of capacitive touch sensing elements formed at the intersections of different electrodes in orthogonal sets of electrodes), or other layers of optical, mechanical, electrical, or other types of components. 
     A capacitive force sensor  210  may be disposed within the interior volume  204 . The capacitive force sensor  210  may be configured to generate a first set of one or more signals related to an amount of force applied to the cover  202   b  (e.g., by a user&#39;s finger or stylus). In some embodiments, the capacitive force sensor  210  may be provided by a SiP mounted within the interior volume  204 . For example, a SiP may include a self-capacitance sense pad  212  (i.e., an electrode) adjacent (or on) a first surface of the SiP, a set of solder structures (e.g., solder balls or solder posts) attached to a second surface of the SiP (opposite the first surface), and an IC coupled to the self-capacitance sense pad  212 . The IC may be configured to output, at one or more of the solder structures, a digital value related to a measured capacitance of the self-capacitance sense pad  212  (i.e., the capacitive force sensor  210  may be configured in a self-capacitance sensing mode). The digital value may be indicative of a force or amount of force applied to the cover  202   b . The SiP may be mounted upside down with respect to the cover  202   b , such that the first surface of the SiP is positioned closer to the cover  202   b  than the second surface of the SiP. The first surface of the SiP may be oriented parallel (or substantially parallel to) the exterior surface of the cover  202   b.    
     The SiP may be opportunistically positioned within the interior volume  204  (e.g., the SiP may be positioned where space exists or where convenient). For example, the SiP may be attached to a logic board (e.g., a printed circuit board) mounted within the interior volume  204 , or attached to an available area on a flex circuit used for force sensing or other purposes. The SiP need not be positioned under the center of the cover  202   b , but may be. The solder structures may be reflowed to electrically and mechanically attach the SiP to another structure (e.g., a logic board or flex circuit). Some solder structures of the SiP may be used to provide power, signals, or instructions to the SiP (e.g., to the IC). 
     In some embodiments, and as shown in  FIG. 2B , a ground element  214  may be mounted within the interior volume  204 , and the self-capacitance sense pad  212  may be separated from the ground element  214  by a compressible gap  216 . The ground element  214  may be a layer of the stack (or be provided within a layer of the stack), and may be positioned between the display  208  and the self-capacitance sense pad  212 . Alternatively, the ground element  214  may be positioned within the display  208 , or between the display  208  and the cover  202   b . By way of example,  FIG. 2B  shows the ground element  214  positioned between the display  208  and the self-capacitance sense pad  212 . In some examples, the ground element  214  may be provided on a substrate such as a PCB (e.g., a display PCB), on a flex circuit (e.g., a display flex circuit, a touch sensor flex circuit, or a camera flex circuit), or on a surface of a substrate such as the cover  202   b . When a user applies a force to the cover  202   b , the distance between the ground element  214  and the self-capacitance sense pad  212  may decrease, thus changing the capacitance of the self-capacitance sense pad  212  and enabling the force (and in some cases an amount of the force) to be detected by the IC within the SiP. Different amounts of force applied to the cover  202   b  may compress the compressible gap  216  to different extents. In alternative embodiments, and as described with reference to  FIGS. 15A-15C , the capacitive force sensor  210  may be configured in a mutual-capacitance sensing mode, and the ground element  214  may instead be a conductor through which a drive waveform is propagated. 
     In some embodiments, the ground element  214  may not be provided, or may not be provided over the self-capacitance sense pad  212 , and the capacitance of the self-capacitance sense pad  212  may change in relation to the proximity of a user&#39;s finger or conductive stylus to the self-capacitance sense pad  212 . In these embodiments, the SiP may function as a capacitive proximity sensor instead of a capacitive force sensor, and the IC may be configured to output, at one or more of the solder structures, a digital value indicative of a proximity of a user or device (e.g., a finger or a stylus) to the cover  202   b . Different proximities of the user or device to the cover  202   b  may produce different capacitances at the self-capacitance sense pad  212 . 
     In some alternative embodiments, the SiP may be mounted within the interior volume  204  with the first surface oriented toward an edge or the back of the device  200 . With these orientations, the SiP may be used to sense a force, amount of force, or proximity of a user or device to the edge or back of the device  200 . 
     Although  FIGS. 2A and 2B  show a particular form-factor and layout for the device  200 , the structures and techniques described with reference to  FIGS. 2A-2B  may be used to deploy capacitive force or proximity sensing in almost any kind of device, including, for example, a tablet computer or electronic watch. In some embodiments, capacitive force sensing may be deployed in a form factor other than in a SiP. For example, a capacitance sense pad may be disposed on a printed circuit board (PCB) or other substrate, adjacent an ASIC or other controller that senses a capacitance of the capacitance sense pad (see, e.g.,  FIG. 15D ). As another example, a capacitance sense pad may be disposed on a semiconductor wafer (i.e., a semiconductor “chip”), and a circuit for sensing the capacitance of the capacitance sense pad may be formed on the wafer. PCBs or wafers may occupy less z space than a SiP, but may increase the size of the gap  216 . In some embodiments, a PCB or wafer may be elevated using a standoff or disposed on top of another component that positions it closer to the ground element  214  or cover  202   b . A wafer may provide the smallest form factor for a capacitance sense pad and related circuitry, but at a higher cost than a PCB or SiP implementation. 
       FIGS. 3A-3D  show various examples of a capacitive force sensor (or capacitive proximity sensor) embodied in a SiP. In some embodiments, the capacitive force sensor or SiP described with reference to  FIG. 2B  may be configured as described with reference to one of  FIGS. 3A-3C . 
       FIG. 3A  shows a SiP  300  having a substrate  302 , an ASIC  304  that is electrically and mechanically attached to the substrate  302 , a set of solder balls  306  attached to the substrate  302 , and a set of passives  308  (e.g., resistors, capacitors, and so on) attached to the substrate  302 , ASIC  304 , or solder balls  306 . The substrate  302  may include alternating metal and dielectric layers, as shown, for example, in  FIGS. 4A-4B . The metal layers may electrically couple the solder balls  306  to the ASIC  304  and provide one or more self-capacitance sense pads on or near the upper surface of the substrate  302  (i.e., on a surface of the substrate opposite a surface to which the ASIC  304  is attached). In some embodiments, the solder balls  306  may include copper clad solder balls (CCSBs). In some embodiments, the solder balls  306  may include tin-silver-copper (SAC) solder balls. 
       FIG. 3B  shows a SiP  310  having a substrate  312 , an ASIC  314  that is electrically and mechanically attached to the substrate  312 , a set of solder balls  316  attached to the substrate  312 , and a set of passives  320  (e.g., resistors, capacitors, and so on) attached to the substrate  312 , ASIC  314 , or solder balls  316 . The SiP  310  may be configured similarly to the SiP  300  described with reference to  FIG. 3A , but may include an underfill  318  (e.g., a resin or plastic) that surrounds the ASIC  314  and provide alignment bosses for the solder balls  316 . 
       FIG. 3C  shows a SiP  330  having a substrate  332 , an ASIC  334  that is electrically and mechanically attached to the substrate  332 , a set of vias  336  that are electrically connect metal pads or traces in the substrate  332  to a set of solder balls  338 , and a set of passives  342  (e.g., resistors, capacitors, and so on) attached to the substrate  332 , ASIC  334 , or solder balls  338 . The substrate  332  may be configured similarly to the substrate  302  described with reference to  FIG. 3A . An underfill  340  (e.g., a resin or plastic) may surround the ASIC  334 , and the vias  336  may be formed through the underfill  340 . In some embodiments, the vias  336  may include copper, and the solder balls  338  may include CCSBs or SAC solder balls. 
       FIG. 3D  shows a SiP  350  having a substrate  352 , an ASIC  354  that is electrically and mechanically attached to the substrate  352 , and a set of embedded bars (E-Bars)  356  that are electrically connect metal pads or traces in the substrate  352  to a set of solder balls  358 . The substrate  352  may be configured similarly to the substrate  302  described with reference to  FIG. 3A . An underfill  360  (e.g., a resin or plastic) may surround the ASIC  354  and the E-Bars  356 . In some embodiments, the E-Bars  356  may include copper, and the solder balls  358  may include CCSBs or SAC solder balls. 
       FIG. 4A  shows an enlarged cross-section  400  of the multi-layer SiP substrate  322  and IC  324  (or ASIC) described with reference to any of  FIG. 3C . However, any of the substrates described with reference to  FIGS. 3A-3D  may be configured as shown in  FIG. 4A . As shown, the substrate  322  may include a signal routing layer  402 , an alternating current (AC) shield layer  404 , and a self-capacitance sense pad  406 , each layer of which may be separated by a dielectric  408 . The signal routing layer  402  may route signals between the IC  324  and the vias  326  that connect to the SiP&#39;s solder structures. The AC shield layer  404  may be positioned between the self-capacitance sense pad  406  and the IC  324 . 
     The substrate  322  may also include vertical AC shielding, such as an AC shield  410  that surrounds (or substantially surrounds) the self-capacitance sense pad  406  and, in some cases and as shown, all of the conductive layers of the substrate  322 . 
     Optionally, the substrate  322  may also include a second self-capacitance sense pad  412 . The first and second self-capacitance sense pads  406 ,  412  may be disposed in different layers or planes, and the second self-capacitance sense pad  412  may be used to calibrate a force sensing operation of the SiP. In some embodiments, the second self-capacitance sense pad  412  may be disposed between the first self-capacitance sense pad  406  and the IC  324 , and the AC shield  410  may be positioned around both the first and second self-capacitance sense pads  406 ,  412 . The AC shield layer  404  may be positioned between the first and second self-capacitance sense pads  406 ,  412  and the IC  324 . In alternative embodiments, the first and second self-capacitance sense pads  406 ,  412  may partially overlap or not overlap. In some embodiments, the functions of the self-capacitance sense pads  406 ,  412  may be reversed (e.g., the self-capacitance sense pad  406  that is farther from the IC  324  may be used to calibrate the capacitive force sensor). 
     The substrate  322  may be electrically and mechanically connected to the IC  324  and vias  326  by a conductive adhesive or solder  414 . The AC shield layer  404  and vertical AC shield  410  may be electrically connected to a via  326  or solder structure, which in turn may be electrically connected to a device ground. 
     In some embodiments, the self-capacitance sense pad  406  may be covered by an oxide or other dielectric coating  416 . 
     The SiP containing the substrate  322  and IC  324  may be mounted within a device such that the self-capacitance sense pad  406  is separated from a ground element  418  by a gap of height g 1 . 
       FIG. 4B  shows a schematic view  420  of the conductive layers described with reference to  FIG. 4A . As shown, the signal routing layer  402  (or a metal layer 1 (L1)) may route power and ground from the solder structures of the SiP to the IC  324 . The signal routing layer  402  may also route digital signals to or from the IC  324 . 
     Conductive traces  422  in each of second, third, and fourth metal layers (L2, L3, and L4), and vias  424  that electrically connect the conductive traces through the dielectric, may be electrically connected to a system ground external to the chip by means of conductive traces in the IC that electrically connect to the SiP&#39;s solder structures. These grounded traces  422  and vias  424  may provide AC shielding for the self-capacitance sense pad  406  and an optional second self-capacitance sense pad  412 . 
     The self-capacitance sense pad  406  may be coupled to the IC  324  by one or more vias that extend through the dielectric and AC shielding  404 . The optional second self-capacitance sense pad  412  may also be coupled to the IC  324  by one or more vias that extend through the dielectric and AC shielding  404 . 
     As shown in  FIG. 5 , a plurality of capacitive force sensors, each included in a different SiP  502   a ,  502   b ,  502   c , may be opportunistically mounted at various locations within a device  500  (e.g., at various locations under a cover of the device  500 ). The locations may be uniformly or non-uniformly distributed within the device  500 . Similarly, capacitive proximity sensors may be uniformly or non-uniformly distributed within a device. A combination of one or more capacitive force sensors and one or more capacitive proximity sensors may also be uniformly or non-uniformly distributed within a device. 
     In some embodiments, multiple SiPs  502   a ,  502   b ,  502   c  containing capacitive force sensors or proximity sensors (e.g., at least first and second SiPs) may be oriented in the same or different ways within an interior volume, and may be mechanically and electrically coupled to the same side of a flex circuit  504  or printed circuit board. In some embodiments, multiple SiPs  502   a ,  502   b ,  502   c  containing capacitive force sensors or proximity sensors may be mechanically and electrically coupled to different flex circuits or printed circuit boards. 
       FIGS. 6A-6C  show an example alternative cross-section of the device  200  described with reference to  FIG. 2A .  FIG. 6A  shows the cross-section in the absence of a force applied to the cover  202   b , and  FIGS. 6B-6C  show the cross-section after a force is applied to the cover  202   b.    
     The cross-section shown in  FIGS. 6A-6C  differs from the cross-section described with reference to  FIG. 2B  in that the cross-section includes a pressure sensor  602  disposed within the interior volume, in addition to another capacitive force sensor  604 . The pressure sensor  602  may be configured to generate a second set of one or more signals related to an amount of force applied to the cover  202   b . In some embodiments, the device may include a plurality of capacitive force sensors  210 ,  604  (shown) or a plurality of pressure sensors (not shown). 
     One or more (or all) of the capacitive force sensors  210 ,  604  may be constructed and positioned as described with reference to  FIGS. 2A-2B . In some examples, the pressure sensor  602  may be a capacitive pressure sensor or resistive (e.g., strain gauge) pressure sensor. 
     A processor within the device  200  may be configured to determine an amount of force applied to the cover  202   b  using a first set of one or more signals generated by a capacitive force sensor  210  (or by a set of multiple capacitive force sensors  210 ,  604 ) or the second set of one or more signals generated by the pressure sensor  602  (or by a set of multiple pressure sensors). In some cases, the processor may determine the amount of force applied to the cover  202   b  using only the first set of signals or only the second set of signals, and the processor may be configured to use one or the other set in response to evaluating a set of one or more conditions, as described with reference to  FIGS. 7-10 . In some cases, the processor may determine the amount of force applied to the cover  202   b  using both the first set of signals and the second set of signals. The processor may also use a location of a touch (or locations of multiple touches, or a location of touch centroid) to determine the amount of force applied to the cover  202   b . For example, measurements obtained from the capacitive force sensors  210 ,  604  or pressure sensor(s)  602  may be interpreted differently (e.g., associated with different amounts of force) based on where a user is touching the device  200 . 
     When the interior volume  204  is sealed, or when a gas or fluid (e.g., air) contained within the interior volume  204  is unable to escape the interior volume  204  freely, the gas or fluid contained within the interior volume  204  may operate as a spring on the cover  202   b  (e.g., an air spring). The spring effect (e.g., air spring effect) may provide a pressure or resistance that opposes the force (e.g., F finger ) applied to the cover. For example, a force applied to the center of the cover  202   b  may be opposed by the force distribution shown in  FIG. 6B . As shown in  FIG. 6B , the air spring effect may cause portions of the cover  202   b  to bow outward upon application of the force F finger . The portions of the cover  202   b  may bow outward as a result of the pressure within the interior volume  204  increasing, and as a result of gases or fluid within the interior volume  204  being displaced. Also upon application of the force F finger , the gasket  206  that bonds the cover  202   b  to the housing  202   a  (with spring constant k) may apply an inward force to the edges of the cover  202   b . In other cases (not illustrated in  FIG. 6B ), the force distribution inside the interior volume  204  as a result of the air spring effect may differ. For example, the gasket  206  may apply an outward force to the edges of the cover  202   b . More generally, whether or not a gasket such as the gasket  206  applies an inward force, outward force, or any force to the edges of the cover  202   b  may be a function of the interior volume  204 , the aspect ratio (e.g., length, width, and thickness) of the cover  202   b , and other factors. The forces (if any) applied to the edges of the cover  202   b  by the gasket  206  are the result of a force balancing problem involving the force F finger  applied to the cover  202   b , the pressure within the interior volume  204  when the force F finger  is applied (e.g., a back pressure force), and the gasket force. The force balancing problem may be modeled for a particular device using finite element analysis (FEA). In general, a gasket may be expected to apply an inward force to the edges of a cover when a device has a long, thin, glass cover, as may be the case in the device  200 ; and a gasket may be expected to apply an outward force to the edges of a cover when a device has a short, thick, glass cover, as may be the case in an electronic watch. 
     When the interior volume  204  is vented, enabling a gas or fluid contained within the volume  204  to escape after application of the force F finger  (but not escape immediately upon application of the force F finger ), the air spring effect illustrated with reference to  FIG. 6B  may decrease over time, and the pressure within the interior volume  204  may decrease until the steady state shown in  FIG. 6C  is reached. In the steady state, the force F finger  may not be opposed by the pressure or distribution of gases or fluids within the interior volume  204 , and the edges of the cover  202   b  may apply an outward force to the gasket  206 . 
     When the interior volume  204  is not sealed, or when a gas or fluid (e.g., air) contained within the interior volume  204  is able to escape the interior volume  204  freely, the gas or fluid contained within the interior volume  204  may readily escape when the force F finger  is applied to the cover  202   b , and the steady state shown in  FIG. 6D  may be reached immediately after application of the force F finger . 
     Between the time that the force F finger  is initially applied to the cover  202   b  and the time the pressure within the interior volume  204  reaches a steady state following application of the force F finger , the capacitances of the capacitive force sensors  210 ,  604  may change. The changes in the capacitances may be caused by a change in the distance between the ground element  214  attached to the cover  202   b  and the self-capacitance sense pad (e.g.,  212 ) of a capacitive force sensor (e.g.,  210 ). Also between the time the force F finger  is initially applied to the cover  202   b  and the time the pressure within the interior volume  204  reaches a steady state following application of the force F finger , and assuming a well-sealed or slowly vented interior volume  204 , the pressure registered by the pressure sensor  602  may temporarily spike and then returns to its baseline value. 
       FIGS. 7A-7C  are graphs of example outputs of the pressure sensor  602  and capacitive force sensors  210 ,  604  included in the device  200  described with reference to  FIGS. 2A and 6A-6C . More particularly,  FIG. 7A  is a graph  700  of an example output  702  of the pressure sensor  602 , P(t) during the steady state condition shown in  FIG. 6A , in which no force is applied to the cover  202   b .  FIG. 7A  also shows example outputs  704 ,  706  of the capacitive force sensors  210 ,  604 , d(t), during the steady state condition shown in  FIG. 6A . 
       FIG. 7B  is a graph  710  of example outputs  702 ,  704 ,  706  of the pressure sensor  602  and capacitive force sensors  210 ,  604  in response to an initial application of force to the center of the cover  202   b . A touch on the cover  202   b  may be detected at a time t 0 . As shown, P(t) temporarily spikes due to an increase in pressure within the interior volume  204  at time t 0 . Similarly, the output  704  of the capacitive force sensor  210  positioned under the location on the cover  202   b  where the force is applied spikes upward due to a decrease in the compressible gap  216  between the capacitive force sensor  210  and the ground element  214  attached to the cover. A decrease in the compressible gap  216  results in an increased capacitance of the self-capacitance sense pad  212  of the capacitive force sensor  210 . Also upon application of the force, the output  706  of the capacitive force sensor  604  positioned closer to the periphery of the cover  202   b  spikes downward due to an increase in the gap between the capacitive force sensor  604  and the ground element  214  (e.g., an increase in the gap caused by an air spring effect). An increase in the gap results in a decreased capacitance of the self-capacitance sense pad of the capacitive force sensor  604 . 
       FIG. 7C  is a graph  720  of example outputs  702 ,  704 ,  706  of the pressure sensor  602  and capacitive force sensors  210 ,  604  over time, subsequent to the initial application of force to the center of the cover  202   b . As shown, P(t) decreases in a non-linear manner as air or fluid escapes the interior volume  204  and the pressure within the interior volume  204  returns to a steady state. Conversely, the outputs  704 ,  706  of the capacitive force sensors  210 ,  604  rise in a non-linear manner as the pressure opposing the applied force decreases, allowing the force F finger  to move the ground element  214  attached to the cover  202   b  closer to the self-capacitance sense pads of the capacitive force sensors  210 ,  604 . Between times t 0  and t 1 , force measurements obtained using the capacitive force sensors  210 ,  604  may be subject to drift and accuracy errors. At time t 1 , the pressure within the interior volume  204  may once again reach a steady state. 
     At or around time t 0  (e.g., at a first time following a touch detected by a touch sensor), the amount of force applied to the cover  202   b  may be determined using the signal(s) provided by the pressure sensor  602 . At or around time t 1  (e.g., at a second time following the touch detected by the touch sensor), or subsequent to time t 1 , the amount of force applied to the cover  202   b  may be determined using the signal(s) provided by the capacitive force sensors  210 ,  604 . Between times t 0  and t 1  (e.g., at a third time following the touch detected by the touch sensor), the amount of force applied to the cover  202   b  may be determined using a combination of the signal(s) provided by the pressure sensor  602  and the capacitive force sensors  210 ,  604 . 
       FIG. 8  is a graph  800  showing example displacements of the cover  202   b  described with reference to  FIGS. 6A-6C , in view of a steady-state force applied to the cover  202   b  and different sealing/venting characteristics of the interior volume  204 . The vertical axis shows a range of cover displacements in nanometer/gram-force (nm/gf), and the horizontal axis shows time. The graph  800  assumes that the force is applied to the cover  202   b  as a steady-state step input. 
     The different sealing/venting characteristics of the interior volume  204  (e.g., different sealing states) may be defined by a value of tau (τ), which indicates the time it takes the interior volume  204  to reach a steady state pressure following application or release of a steady-state force, F finger , to the cover  202   b . At lower values of τ (e.g., τ=0.6 seconds (s)), indicative of a more loosely sealed or well-vented interior volume  204 , the cover  202   b  may reach its maximum displacement in response to a steady-state force F finger  very quickly (i.e., in 0.6 s). As τ increases to higher values (e.g., to 1.0 s or even 3.0 s), indicating a better sealed or less-vented interior volume  204 , the cover  202   b  may reach its maximum displacement in response to the steady-state force F finger  more slowly. 
     Given the impact of higher values of τ on a capacitive force sensor, the pressure sensor  602  described with reference to  FIG. 6  may be used to provide an initial indication of the amount of force applied to the cover  202   b , and the capacitive force sensor(s)  210 ,  604  may be used to provide a steady state indication of the amount of force applied to the cover  202   b . The pressure sensor  602  may also be used to provide an indication of an amount of force associated with a tap (i.e., a short duration force) on the cover  202   b , while the capacitive force sensor(s)  210 ,  604  may be used to provide an indication of amounts of force associated with a sustained force or force gesture applied to the cover  202   b  (i.e., amounts of force corresponding to a user dragging their finger or stylus across the surface of the cover  202   b  over a longer period of time). 
     Alternatively, a processor of a device (e.g., the device  200  described with reference to  FIGS. 2A and 6A-6C ) may determine a sealing state of the interior volume  204 , and use the determined sealing state to determine the amount of force applied to the cover  202   b  using: the signal(s) provided by the pressure sensor  602 , the signal(s) provided by the capacitive force sensor(s)  210 ,  604 , or a combination thereof. For example, the processor may obtain a pressure of the interior volume  204  from the pressure sensor  602  in the absence of a force applied to the cover  202   b ; evaluate the pressure of the interior volume  204  to determine whether the pressure is below a threshold pressure; and upon determining that the pressure of the interior volume  204  is below the threshold pressure, switch from reliance on the pressure sensor  602  to reliance on (or more reliance on) the capacitive force sensor(s)  210 ,  604  when initially determining an amount of force applied to the cover  202   b . The threshold pressure may be a pressure indicating that the interior volume  204  has transitioned from a sealed state to an unsealed state (e.g., as a result of a drop event). 
       FIG. 9  is a graph  900  that shows an example relationship between the user facing force noise metrics of a capacitive force sensor and a pressure sensor. The vertical axis shows a range of user facing force noise between 4 and 10 (in gram-force milliseconds (gf ms), and the horizontal axis shows a range of values for τ. As shown by the curve  902 , a capacitive force sensor may be less susceptible to force noise at lower values of τ (e.g., τ≤52 ms), and its susceptibility to force noise may increase somewhat quickly for higher values of τ. Conversely, and as shown by the curve  904 , a pressure sensor may be less susceptible to force noise at higher values of τ (e.g., τ&gt;52 ms), and its susceptibility to force noise may increase somewhat quickly with lower values of τ. 
     When a capacitive force sensor (or set of multiple capacitive force sensors) is used to determine the force applied to a cover, the applied force may be determined by either measuring a capacitance change of the capacitive force sensor directly, or by converting the capacitance change to a gap change using the following equation: 
             g   =       ɛ   ⁢           ⁢   A       C   -     C   0               
where C is the capacitance of the sensor, C 0  is a parasitic capacitance, ε is an electrical constant, A is the area of the electrode, and g is the distance from the electrode to the ground element.
 
       FIGS. 10A-10C  provide examples of how capacitance values obtained from a capacitive force sensor (or a set of multiple capacitive force sensor), and pressure values obtained from a pressure sensor (or a set of multiple pressure sensors), may be used to determine an amount of force applied to a device&#39;s cover. As shown in the graph  1000  of  FIG. 10A , capacitance values obtained from the capacitive force sensor may be used to determine an amount of force on a cover at values of τ below a first threshold, τ 1 , and the pressure sensor may not be used to determine an amount of force at values of τ below the first threshold. As values of τ increase from the first threshold to a second threshold, τ 2  (with the second threshold being greater than the first threshold), capacitance values may be given increasingly less weight in determining an amount of force on the cover, and pressure values may be given increasingly more weight. For values of τ above the second threshold, pressure values may be given more weight than capacitive values, but each may be given weight. Above the second threshold, the relative weights given to pressure values and capacitance values may remain fixed. The respective weights given to capacitance values and pressure values between τ 1  and τ 2  may in some cases be determined through modeling or learning. 
     Alternatively, and as shown in the graph  1010  of  FIG. 10B , capacitance values obtained from the capacitive force sensor may be used to determine an amount of force on a cover at values of τ below a first threshold, τ 1 , and the pressure sensor may not be used to determine an amount of force at values of τ below the first threshold. As values of τ increase from the first threshold to a second threshold, τ 2  (with the second threshold being greater than the first threshold), capacitance values may be given increasingly less weight in determining an amount of force on the cover, and pressure values may be given increasingly more weight. For values of τ above the second threshold, pressure values may be used to determine an amount of force on the cover, and the capacitive force sensor may not be used to determine the amount of force. The respective weights given to capacitance values and pressure values between τ 1  and τ 2  may in some cases be determined through modeling or learning. 
     Alternatively, and as shown in the graph of  1020  of  FIG. 10C , capacitance values obtained from the capacitive force sensor may be used to determine an amount of force on a cover at values of τ below a threshold, τ 1 , and the pressure sensor may not be used to determine an amount of force at values of τ below the first threshold. At or above the threshold, pressure values may be used to determine an amount of force on the cover, and the capacitive force sensor may not be used to determine the amount of force. 
     In some embodiments, a value of τ for an interior volume may be preprogrammed into the device that contains the interior volume. In some embodiments, a value of τ for an interior volume may be dynamically determined or estimated using capacitance values obtained from a capacitive force sensor or pressure values obtained from a pressure sensor. 
     As discussed with reference to  FIGS. 4A-4B , a SiP that includes a self-capacitance sense pad (referred to, at times, as a sensing electrode) may also include a second self-capacitance sense pad (referred to, at times, as a calibration electrode). The sensing and calibration electrodes may be operated differentially, such that the calibration electrode can be used to calibrate capacitive measurements obtained using the sensing electrode.  FIGS. 11A-11B  illustrate conditions for which a capacitive force sensor containing sensing and calibration electrodes may be calibrated. 
       FIG. 11A  is a graph  1100  showing a relationship between a compressible gap (g) defined between a ground element and a sensing electrode (e.g., the compressible gap described with reference to  FIGS. 2A-2B ) and capacitances (C) of the sensing and calibration electrodes. The relationship between the gap and the capacitances is dependent on a parasitic capacitance (C 0 ) effecting both the sensing electrode and the calibration electrode. When the parasitic capacitances are properly estimated, a differential capacitance (ΔC) between the sensing and calibration electrodes corresponds to a particular gap (Δg, a sense pad gap) between the sensing and calibration electrodes. When the parasitic capacitances or their relationship has shifted (e.g., increased or decreased) and is not properly estimated, the differential capacitance (ΔC) between the sensing and calibration electrodes corresponds to a particular gap plus or minus an error (e.g., Δg+g error ). To calibrate the parasitic capacitance (C 0 ) of each electrode, the gap sensing method described with reference to  FIG. 12  may be performed. 
       FIG. 11B  is another graph  1110  showing the relationship between the compressible gap (g) and the capacitances (C) of the sensing and calibration electrodes. The sensed capacitances and their difference (ΔC) depends on a baseline value for the compressible gap (i.e., a value of the compressible gap when no force is applied to the device). When the baseline value is properly estimated, a differential capacitance (ΔC) between the sensing and calibration electrodes corresponds to a particular gap (Δg) between the sensing and calibration electrodes. When the baseline value of the gap has shifted (e.g., as a result of damage to or aging of the device), and is not properly estimated, the differential capacitance detected between the sensing and calibration electrodes may not be appropriate given the gap between the sensing and calibration electrodes, and may be measured as ΔC plus or minus an error (e.g., ΔC+C error ). To determine the shifted baseline value of the gap, the capacitance sensing method described with reference to  FIG. 13  may be performed. 
       FIG. 12  illustrates a gap sensing method for determining the parasitic capacitances, C sp  and C cp , effecting sensing and calibration electrodes of a capacitive force sensor. The method is sensitive to shifts in the parasitic capacitances of the sensing and calibration electrodes, but is not sensitive to shifts in the baseline value of a compressible gap between a ground element and the sensing electrode. 
     As shown in  FIG. 12 , a capacitive force sensor  1200  may include a sensing electrode  1202  and a calibration electrode  1204 . Each of the electrodes  1202 ,  1204  may be separated from a ground element  1206  by a compressible gap. The compressible gap may have a baseline value of g 0  with respect to the sensing electrode  1202 , and a value of g 0 +Δg with respect to the calibration electrode  1204 , with Δg being a separation between the electrodes  1202 ,  1204  in a direction perpendicular to the ground element  1206 . Although the electrodes  1202 ,  1204  are shown to be non-overlapping in  FIG. 12 , the electrodes  1202 ,  1204  may alternatively be overlapping, as shown, for example, in  FIGS. 4A-4B . 
       FIG. 12  shows the capacitive force sensor  1200  during different compression states of the compressible gap—a first state in which no force is applied to the ground element  1206  (or to the cover to which the ground element  1206  is attached), and a second state in which the ground element  1206  (or the cover to which the ground element  1206  is attached) is depressed by a distance x. The IC to which the sensing and calibration electrodes  1202 ,  1204  are connected, in-SiP, may measure the capacitance of each electrode (i.e., the sensing electrode  1202  and the calibration electrode  1204 ) in each state, and use the measured capacitances to calibrate a force sensing operation of the capacitive force sensor  1200 . 
     For example, the capacitance (C) of each electrode may be determined using the following equation: 
               C   -     C   0       =           ɛ   ⁢           ⁢   A     g     →   g     =       ɛ   ⁢           ⁢   A       C   -     C   0                 
where C 0  is a parasitic capacitance, ε is an electrical constant, A is the area of the electrode, and g is the distance from the electrode to the ground element. Given the above equation, the capacitance of the sensing electrode  1202 , in the absence of a force applied to the ground element  1206  (i.e., in the first state), may be determined using the following equation:
 
               C   s     =         ɛ   ⁢           ⁢     A   s         g   0       +     C   sp             
where s indicates the sensing electrode  1202 .
 
     Similarly, the capacitance of the calibration electrode  1204 , in the absence of a force applied to the ground element  1206 , may be determined using the following equation: 
               C   c     =         ɛ   ⁢           ⁢     A   c           g   0     +     Δ   ⁢           ⁢   g         +     C   cp             
where c indicates the calibration electrode  1204 .
 
     When the ground element  1206  is depressed by the distance x, the capacitances of the sensing and calibration electrodes  1202 ,  1204  may be determined using the following equations: 
     
       
         
           
             
               
                 C 
                 s 
               
               + 
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   C 
                   s 
                 
               
             
             = 
             
               
                 
                   ɛ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     A 
                     s 
                   
                 
                 
                   
                     g 
                     0 
                   
                   - 
                   x 
                 
               
               + 
               
                 C 
                 sp 
               
             
           
         
       
       
         
           
             
               
                 C 
                 c 
               
               + 
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   C 
                   c 
                 
               
             
             = 
             
               
                 
                   ɛ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     A 
                     c 
                   
                 
                 
                   
                     g 
                     0 
                   
                   + 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     g 
                   
                   - 
                   x 
                 
               
               + 
               
                 C 
                 cp 
               
             
           
         
       
     
     The above equations for C s  may be subtracted, and the above equations for C c  may be subtracted, to yield the following equations: 
     
       
         
           
             
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               
                 C 
                 s 
               
             
             = 
             
               
                 
                   ɛ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     A 
                     s 
                   
                 
                 
                   
                     g 
                     0 
                   
                   - 
                   x 
                 
               
               - 
               
                 
                   ɛ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     A 
                     s 
                   
                 
                 
                   g 
                   0 
                 
               
             
           
         
       
       
         
           
             
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               
                 C 
                 c 
               
             
             = 
             
               
                 
                   ɛ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     A 
                     c 
                   
                 
                 
                   
                     g 
                     0 
                   
                   + 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     g 
                   
                   - 
                   x 
                 
               
               - 
               
                 
                   ɛ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     A 
                     c 
                   
                 
                 
                   
                     g 
                     0 
                   
                   + 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     g 
                   
                 
               
             
           
         
       
     
     These equations may be solved to determine estimates for g 0  and x as follows: 
     
       
         
           
             a 
             = 
             
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   C 
                   s 
                 
                 ⁢ 
                 ɛ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   A 
                   c 
                 
               
               - 
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   C 
                   c 
                 
                 ⁢ 
                 ɛ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   A 
                   s 
                 
               
               - 
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   C 
                   s 
                 
                 ⁢ 
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   C 
                   c 
                 
                 ⁢ 
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 g 
               
             
           
         
       
       
         
           
             b 
             = 
             
               
                 
                   - 
                   2 
                 
                 ⁢ 
                 ɛ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   A 
                   s 
                 
                 ⁢ 
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   C 
                   c 
                 
                 ⁢ 
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 g 
               
               - 
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   C 
                   s 
                 
                 ⁢ 
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   C 
                   c 
                 
                 ⁢ 
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   g 
                   2 
                 
               
             
           
         
       
       
         
           
             c 
             = 
             
               
                 - 
                 ɛ 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 A 
                 s 
               
               ⁢ 
               Δ 
               ⁢ 
               
                   
               
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                 C 
                 c 
               
               ⁢ 
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               
                 g 
                 2 
               
             
           
         
       
       
         
           
             
               g 
               
                 0 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 est 
               
             
             = 
             
               
                 
                   - 
                   b 
                 
                 + 
                 
                   
                     
                       b 
                       2 
                     
                     - 
                     
                       4 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       a 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       c 
                     
                   
                 
               
               
                 2 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 a 
               
             
           
         
       
       
         
           
             
               x 
               est 
             
             = 
             
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   C 
                   s 
                 
                 ⁢ 
                 
                   g 
                   
                     0 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     est 
                   
                   2 
                 
               
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     C 
                     s 
                   
                   ⁢ 
                   
                     g 
                     
                       0 
                       ⁢ 
                       
                           
                       
                       ⁢ 
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                 + 
                 
                   ɛ 
                   ⁢ 
                   
                       
                   
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                     A 
                     s 
                   
                 
               
             
           
         
       
     
     The values of g 0  and x may then be plugged into other equations to determine values for C sp  and C cp . Determinations of C sp  and C cp  can be used to determine a ΔC 0 . These values may be used (e.g., by the IC on-board a SiP) to dynamically calibrate a force sensing operation of the capacitive force sensor  1200 , thereby enabling the capacitive force sensor  1200  to output a digital value related to a capacitance (i.e., an amount of force) sensed by the sensing electrode  1202 . 
       FIG. 13  illustrates a capacitance sensing method for calibrating a differential capacitance (ΔC) between sensing and calibration electrodes of a capacitive force sensor. The method is sensitive to shifts in the baseline value of a compressible gap between a ground element and the sensing electrode, but is not sensitive to shifts in the parasitic capacitances of the sensing and calibration electrodes. 
       FIG. 13  shows the capacitive force sensor  1200  described with reference to  FIG. 12 , but shows the capacitive force sensor  1200  in a single state (e.g., a rest state). The IC to which the sensing and calibration electrodes  1202 ,  1204  are connected, in-SiP, may measure the capacitance of each electrode (i.e., the sensing electrode  1202  and the calibration electrode  1204 ), and use the measured capacitances to calibrate a force sensing operation of the capacitive force sensor  1200 . 
     The capacitance of the sensing electrode  1202  may be determined using the following equation: 
     
       
         
           
             
               C 
               1 
             
             = 
             
               
                 
                   ɛ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   A 
                 
                 
                   g 
                   1 
                 
               
               + 
               
                 C 
                 0 
               
             
           
         
       
     
     Similarly, the capacitance of the calibration electrode  1204  may be determined using the following equation: 
     
       
         
           
             
               C 
               2 
             
             = 
             
               
                 
                   ɛ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   A 
                 
                 
                   
                     g 
                     1 
                   
                   + 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     g 
                   
                 
               
               + 
               
                 C 
                 0 
               
             
           
         
       
     
     The equations for C 1  and C 2  may be subtracted, and the following equation may be solved to determine g 1 : 
     
       
         
           
             
               
                 C 
                 1 
               
               - 
               
                 C 
                 2 
               
             
             = 
             
               
                 
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     The values of g 1  and C 0  may be used (e.g., by the IC on-board a SiP) to statically calibrate a force sensing operation of the capacitive force sensor  1200 , thereby enabling the capacitive force sensor  1200  to output a digital value related to a capacitance (i.e., an amount of force) sensed by the sensing electrode  1202 . 
       FIG. 14  shows an example method  1400  for performing the method described with reference to  FIG. 12 or 13  in response to user input  1402  requesting calibration of a capacitive force sensor (e.g., one of the capacitive force sensors shown in  FIG. 2B, 12 , or  13 ). In response to receiving the user input  1402 , the capacitances C 1  and C 2  may be measured at block  1404 , and a static parasitic calibration may be performed at block  1406 . Performance of the static parasitic calibration may include performance of the method described with reference to  FIG. 13 . As a result of performing the static parasitic calibration method, C 0  may be recalibrated. At block  1408 , the user may depress a cover of a device and determine a change in a compressible gap (e.g., a change by a value x). If x is determined to exceed a threshold at block  1410 , a dynamic parasitic calibration may be performed at block  1412 . Performance of the dynamic parasitic calibration may include performance of the method described with reference to  FIG. 12 . As a result of performing the dynamic parasitic calibration method, the parasitic capacitances C 0a  and C 0b , or ΔC 0 , may be calibrated. 
     Turning now to  FIGS. 15A-15D , there are shown various alternative embodiments of a capacitive force sensor. The capacitive force sensors shown in any of  FIGS. 15A-15D  may be used in place of, or in combination with, any of the capacitive force sensors described herein with reference to other figures. The capacitive force sensors described with reference to  FIGS. 15A-15D  may also be used in combination with pressure sensors, barometric sensors, temperature sensors, touch sensors, and other sensors, as described with reference to other figures. In some embodiments, the capacitive force sensors shown in  FIGS. 15A-15D  may include one or more calibration electrodes and may be calibrated, as described with reference to  FIG. 13 or 14 . 
     In  FIG. 15A , an example cross-section of the device  200  described with reference to  FIG. 2A  is shown. However, in contrast to other example cross-sections of the device  200  shown herein, the capacitive force sensor  1500  is configured for mutual-capacitance sensing (i.e., in a mutual-capacitance sensing mode). For example, a driver  1502  may drive a stimulus (e.g., a signal or waveform) over a conductor  1504  (or set of conductors), and a capacitance sense pad  1506  of the capacitive force sensor  1500  may be configured to sense a mutual capacitance between the conductor  1504  and the capacitance sense pad  1506 . In some examples, the capacitive force sensor  1500  may be provided in a SiP, similarly to the capacitive force sensor  210  or  604  described with reference to  FIGS. 2A, 6A-6C  and other figures. In other examples, the capacitive force sensor  1500  may be provided in another type of package, or on a substrate such as a PCB or semiconductor wafer, as described with reference to  FIG. 2B . In some examples, the conductor  1504  may be included in or on a substrate  1508  such as a flex circuit or PCB, or may be printed on an inside of the cover  202   b . The conductor  1504  may include, for example, an electrode, a pad (including a free pad on a display substrate, touch sensor substrate, or other substrate), a trace, and/or a metal layer or component. 
     The driver  1502  may be a driver that is synchronized with the capacitive force sensor  1500 . In some embodiments, an IC or other circuitry that is packaged with or associated with the capacitive force sensor  1500  may operate (e.g., provide a control signal or drive waveform to) the driver  1502 . In some embodiments, an IC or other circuitry that is packaged with or associated with the driver  1502  may operate, or synchronize with, the capacitive force sensor  1500 . In some embodiments, an IC or other circuitry that is separate from both the capacitive force sensor  1500  and the driver  1502  may operate or synchronize both components. The IC or other circuitry may be programmable (e.g., to change the drive waveform) or non-programmable. 
     In some embodiments, the capacitive force sensor  1500  may be electrically coupled to a substrate  1510  (e.g., a PCB or flex circuit) that is directly or indirectly coupled to the driver  1502 . For example, the capacitive fore sensor  1500  may be electrically coupled to a PCB that is coupled to the driver  1502  by a flex circuit and/or electrical connector, or the capacitive force sensor  1500  may be electrically coupled to a flex circuit that is also electrically coupled to the driver  1502 . 
     As shown in  FIG. 15B , the locations of the capacitive force sensor  1500  (with capacitance sense pad  1506 ) and conductor  1504  may be swapped, with the capacitive force sensor  1500  being mounted closer to the cover  202   b  than the conductor  1504 . Similarly, the positions of the capacitance force sensors  210 ,  604  and ground element  214  described with reference to  FIGS. 2B and 6A-6C  may be swapped. 
     In some embodiments, and as shown in  FIG. 15C , the driver  1502  may have a purpose apart from providing a drive waveform that modulates the capacitance sensed by the capacitance sense pad  1506 . For example, the driver  1502  may be configured to drive a display signal (e.g., a data line signal or gate line signal or clock signal driven over an electrode or conductive layer of a display, such as a thin-film-transistor (TFT) liquid-crystal display (LCD)), a touch sensor signal (e.g., a touch sensor drive signal driven over an indium-tin-oxide (ITO) electrode or layer in a touch sensor), or other signal within the device stack that includes the display  208 . In these embodiments, the capacitive force sensor  1500  may be a mutual-capacitance force sensor, or a self-capacitance force sensor operated in a mutual-capacitance sensing mode. In either case, the capacitive force sensor  1500  may be considered to be operated in a pseudo-mutual-capacitance sensing mode. In the embodiments described in this paragraph, a dedicated driver  1502  may not need to be provided for use with the capacitive force sensor  1500 , and circuitry having other purposes can be used to provide a pseudo-mutual-capacitance sensing mode. 
       FIG. 15D  shows the capacitive force sensor  1500  in an alternative layout, in which the capacitance sense pad  1506  is disposed adjacent an IC or other circuitry  1512  on the substrate  1510  (e.g., a PCB). The capacitive force sensor  1500  shown in  FIG. 15C  may occupy less z space than the capacitive force sensor  1500  described with reference to any of  FIGS. 15A-15C , but may consume more area and/or increase the width of the gap  216 . 
       FIGS. 16A-16C  show another example cross-section of the device  200  described with reference to  FIG. 2A .  FIG. 16A  shows the cross-section in the absence of a force applied to the cover  202   b , and  FIGS. 16B-16C  show the cross-section after a force is applied to the cover  202   b.    
     Similarly to the cross-section shown in  FIGS. 6A-6C , the cross-section shown in  FIGS. 16A-16C  includes a pressure sensor  602  disposed within the interior volume  204  (i.e., an internal pressure sensor). However, in contrast to the cross-section shown in  FIGS. 6A-6C , the cross-section shown in  FIGS. 16A-16C  includes a barometric pressure sensor  1600  instead of a capacitive force sensor  210  or  604 . Alternatively, the device  200  may include a capacitive force sensor  604 , a pressure sensor  602 , and a barometric pressure sensor  1600 , which sensors may be used as described herein or in other ways. 
     The internal pressure sensor  602  may operate similarly to the pressure sensor described with reference to  FIGS. 6A-6C , and in some cases may include a set of multiple pressure sensors distributed within the interior volume  204 . Measurements obtained from multiple pressure sensors may be combined (e.g., averaged) to determine the pressure within the interior volume  204 . The barometric pressure sensor  1600  may be configured to measure the barometric pressure of an environment exterior to the device  200  (e.g., an ambient pressure of the device  200 ). In some cases, the barometric pressure sensor  1600  may be used to measure the barometric pressure while a touch or force is being applied to the cover  202   b . In some embodiments, the barometric pressure sensor  1600  may include a set of multiple pressure sensors, and measurements obtained from the multiple barometric pressure sensors may be combined (e.g., averaged) to determine the ambient pressure of the device  200 . 
     In some embodiments, the barometric pressure sensor  1600  may extend through a sidewall of the housing  202   a , as shown in  FIG. 16A , with a pressure sensing element of the barometric pressure sensor  1600  oriented toward the environment exterior to the housing  202   a , and an electrical interface of the barometric pressure sensor  1600  oriented toward the interior volume  204 . In some cases, an air permeable but liquid water impermeable membrane may be disposed over the pressure sensing element of the barometric pressure sensor  1600 , to protect it from water and debris in the environment exterior to the device  200 . In some embodiments, a gel or other substance may be disposed over the pressure sensing element to protect it from water and debris. 
     A processor within the device  200  may be configured to determine an amount of force applied to the cover  202   b  using a first set of one or more signals generated by the internal pressure sensor  602  (or by a set of multiple internal pressure sensors) and a second set of one or more signals generated by the barometric pressure sensor  1600  (or by a set of multiple barometric pressure sensors). 
     While a pressure sensor alone may be able to detect a force, or amount of force, applied to the cover  202   b , changes in the ambient pressure of the device  200  may be affect the pressure within the interior volume  204 , as a result of the port  218  connecting the interior volume  204  to the environment exterior to the device  200 . Sudden changes in the ambient pressure (e.g., as may be caused by a car door slam, an open window of a moving vehicle, or a moving elevator) may be more likely to cause an abrupt change in the pressure within the interior volume  204 , which pressure change may be indistinguishable from a pressure change caused by a user&#39;s intentional press on the cover  202   b . This can lead to accidental force triggers (e.g., an accidental selection of a function represented by an icon displayed on the device&#39;s display  208 , such as the opening of an email while scrolling through a list of emails) or missed force triggers. 
     When the barometric pressure sensor  1600  is disposed to measure the ambient pressure of the device  200 , and when the pressure sensor  602  disposed within the interior volume  204  may be affected by ambient pressure changes, the ambient pressure measured by the barometric pressure sensor  1600  may be used to adjust the pressure measured by the internal pressure sensor  602 . For example, the ambient pressure may be subtracted from the internal pressure. The resultant pressure may provide a better indication of whether a force has been applied to the cover  202   b , as well as an amount of the applied force. In some cases, the internal pressure may only be adjusted when the ambient pressure undergoes a change, or change above a threshold, within a predetermined period of time. In some cases, the internal pressure may be adjusted for any ambient pressure change. In some embodiments, the ambient pressure may or may not be used to adjust the internal pressure, but may be used as a secondary check to determine whether a force applied to the cover  202   b  is likely a user-applied force. For example, upon detecting a force using a pressure measurement obtained from the internal pressure sensor  602 , an ambient pressure measurement obtained from the barometric pressure sensor  1600  may be used to determine whether a same or similar pressure change occurred external to the device  200 . If so, the pressure change detected by the internal pressure sensor  602  may be ignored as not being associated with a user-applied force to the cover  202   b . Still further, a sudden change in ambient pressure may be used to temporarily disable force sensing functionality of the device  200  or adjust a force sensing threshold. 
       FIG. 16B  shows a user touching the cover  202   b  of the device  200  and applying an amount of force (e.g., F finger ) to the cover  202   b .  FIG. 16C  shows the occurrence of an ambient pressure change while the user is touching the cover  202   b  (indicated by several smaller arrows applied to the external surfaces of the device  200 ). 
       FIGS. 17A-17C  are graphs of example outputs of the internal pressure sensor  602  and barometric pressure sensor  1600  described with reference to  FIGS. 16A-16C . More particularly,  FIG. 17A  shows graphs  1700  of example outputs  1702 ,  1704  of the pressure sensor  602  and barometric pressure sensor  1600 , P 1 (t) and P 2 (t), during the steady state condition shown in  FIG. 16A , in which no force is applied to the cover  202   b .  FIG. 17B  shows graphs  1710  of example outputs  1712 ,  1714  of the pressure sensor  602  and barometric pressure sensor  1600 , P 1 (t) and P 2 (t), in response to a user touching and applying a force to the center of the cover  202   b . The touch event is indicated by a dashed stepped waveform  1716 .  FIG. 17C  shows graphs  1720  of example outputs  1722 ,  1724  of the pressure sensor  602  and barometric pressure sensor  1600 , P 1 (t) and P 2 (t), during a change in ambient pressure that occurs while the user is touching and applying a force to the cover  202   b.    
     As shown in  FIG. 17B , a touch on the cover  202   b  may be detected at a time t 0  (as indicated by a dashed stepped waveform  1716 ). As shown, P 1 (t) temporarily spikes due to an increase in pressure within the interior volume  204  at time t 0 , and then decreases as air in the interior volume escapes through the port  218 . The ambient pressure, P 2 (t), is unaffected by the force applied to the cover  202   b  and does not change. 
     As shown in  FIG. 17C , the ambient pressure of the device, P 2 (t), may experience a sudden change at time t 1 . A similar but lesser change may be experienced within the interior volume  204  of the device  200 , and may cause P 1 (t) to likewise experience a sudden change. In some cases, the change in P 1 (t) may be sufficient to cause P 1 (t) to exceed a force detection threshold. However, if P 1 (t) is adjusted for the pressure change in P 2 (t), the resultant P 1 (t) at t 1  may be as shown in  FIG. 17D , and may not exceed the force detection threshold. 
       FIGS. 18A-18D  show various examples of how a measurement of internal pressure (e.g., a measurement obtained from the internal pressure sensor  602 ) may be adjusted using a measurement of barometric or ambient pressure (e.g., a measurement obtained from the barometric pressure sensor  1600 ). The process flows described with reference to  FIGS. 18A-18D  may be implemented using a processor or other circuit, and may use measurements obtained from an internal pressure sensor  602  and barometric pressure sensor  1600  as inputs. For ease of explanation, the process flows shown in  FIGS. 18A-18D  are described with reference to the device  200  shown in  FIG. 16A . 
     In the flow  1800  shown in  FIG. 18A , a measurement of the pressure within a device&#39;s interior volume  204  may be obtained at block  1802 , and a measurement of the ambient pressure of the device  200  may be obtained at block  1804 . In some embodiments, the measurements may be correlated in time (e.g., the measurements may be obtained at the same or approximately same time). In some cases, each of the measurements may be acquired as a time varying measurement (e.g., P 1 (t) and P 2 (t)). At block  1804 , a change in ambient pressure may be identified. In some embodiments, the change may be a change that exceeds a predetermined threshold (e.g., a change that causes the ambient pressure to exceed a threshold ambient pressure, or a change in ambient pressure that exceeds a predetermined ambient pressure change threshold or a predetermined percentage change in ambient pressure). In response to identifying the change in ambient pressure, a processor may use a transfer function  1806  to determine how the change in ambient pressure may affect the internal pressure, and may estimate (or predict) an internal pressure change  1808  that is likely to occur in response to the ambient pressure change. At block  1810 , the processor may subtract the estimated internal pressure change from the measured internal pressure to yield an adjusted internal pressure  1812 . 
     In the flow  1820  shown in  FIG. 18B , a measurement of the pressure within a device&#39;s interior volume  204  may be obtained at block  1822 , and a measurement of the ambient pressure of the device  200  may be obtained at block  1824 . In some embodiments, the measurements may be correlated in time (e.g., the measurements may be obtained at the same or approximately same time). In some cases, each of the measurements may be acquired as a time varying measurement (e.g., P 1 (t) and P 2 (t)). At block  1824 , a change in ambient pressure may be identified. In some embodiments, the change may be a change that exceeds a predetermined threshold (e.g., a change that causes the ambient pressure to exceed a threshold ambient pressure, or a change in ambient pressure that exceeds a predetermined ambient pressure change threshold or a predetermined percentage change in ambient pressure). 
     Each of the internal pressure and the ambient pressure (or change in ambient pressure) may be adjusted to account for noise and accuracy drift in the sensors (e.g., at blocks  1826  and  1828 ). At blocks  1830  and  1832 , one or both of the internal pressure and ambient pressure (or change in ambient pressure) may be adjusted to account for measurement delay. In other words, the measurement of ambient pressure may be time-synchronized with the measurement of internal pressure, or vice versa; or the measurement of ambient pressure may be adjusted (e.g., interpolated from measurements of ambient pressure made at times other than the time at which the measurement of internal pressure is obtained, or vice versa). In some embodiments, the internal pressure and ambient pressure may be respectively adjusted according to the functions:
 
 H ( s )= e   −sb   ;H ( s )= e   −sa  
 
where H(s) is a sensor measurement, a is an electrical delay of the barometric pressure sensor  1600 , and b is an electrical delay of the internal pressure sensor  602 . In some embodiments, the operations performed through blocks  1830  and  1832  may be performed by the internal pressure sensor  602  and barometric pressure sensor  1600  (or circuitry associated with these sensors).
 
     A processor may use a transfer function  1834  to determine how the change in ambient pressure may affect the internal pressure, and may estimate (or predict) an internal pressure change  1836  that is likely to occur in response to the ambient pressure change. By way of example, the transfer function may be: 
               H   ⁡     (   s   )       =           P   1     ⁡     (   s   )           P   2     ⁡     (   s   )         =       As   +   1           (     A   +   B     )     ⁢   s     +   1               
where A and B are functions of the port resistance, cover area, cover stiffness, and size of the interior volume. At block  1838 , the processor may subtract the estimated internal pressure change from the measured internal pressure to yield an adjusted internal pressure  1840 .
 
     The flows described with reference to  FIGS. 18A-18B  may be considered full differential adjustment flows.  FIGS. 18C-18D  show partial differential adjustment flows. Turning now to  FIG. 18C , a flow  1850  is shown in which measurements of internal pressure and ambient pressure may be obtained, at blocks  1852  and  1854 , as described with reference to  FIG. 18A . The ambient pressure measurement may be separately processed by a low pass filter  1856  and a high pass filter  1858 . The low pass filtered measurement may be used to determine a baseline ambient pressure  1860 , which baseline ambient pressure  1860  may be combined with (e.g., subtracted from) the internal pressure measurement at block  1862  to yield a baselined internal pressure  1864 . The high pass filtered measurement  1858  may be used to identify a sudden ambient pressure change (e.g., at blocks  1866  and  1868 ), which sudden ambient pressure change may be used to determine a gain over the baseline ambient pressure at block  1870 . The determined gain may be used to adjust the measured and baseline-adjusted internal pressure measurement  1864  at block  1872 , to yield an adjusted internal pressure  1874 . 
       FIG. 18D  shows a variation of the flow  1850  described with reference to  FIG. 18C . In the flow  1880 , the high pass filtered measurement of the ambient pressure may be used to identify a sudden ambient pressure change (e.g., at block  1868 ), which sudden ambient pressure change may be flagged at block  1882  and used to increase the force detection threshold to which the measured and baseline-adjusted internal pressure  1864  is compared to determine whether an amount of force applied to the cover  202   b  is sufficient to trigger an action, or to temporarily disable force sensing functionality, or take other actions at block  1884 . 
       FIG. 19  shows another example cross-section of the device  200  described with reference to  FIG. 2A . As shown in  FIG. 19 , the force-sensing mechanism of the device  200  may only include a pressure sensor  602 . In alternative embodiments, the device  200  may also include a capacitive force sensor, a barometric pressure sensor, or other sensors that may be used to determine a force applied to the device. 
     As discussed with reference to  FIGS. 8-10C , the force-sensing capability of a pressure sensor may be depend on a parameter τ. However, the value of τ may depend on the physical characteristics of a device, and may therefore change from a pre-calibrated value as the physical characteristics of the device change (e.g., as a result of device wear over time, or as a result of device damage). As shown in  FIG. 20 , change in physical τ (the component of τ dependent on the physical characteristics of a device) can translate to accuracy errors in sensed force determinations. A positive accuracy error  2002  (in which physical τ is greater than a pre-calibrated value of τ) can make a force-sensing pressure sensor easier to trigger and increase the risk of false force triggers. A negative accuracy error  2004  (in which physical τ is less than the pre-calibrated value of τ) can make a force-sensing pressure sensor harder to trigger and increase the risk of missed force inputs. 
     τ is inversely proportional to the vent flow rate of a device (e.g., the flow rate through the port  218  ( FIG. 19 ) that is used to vent the interior volume, or the flow rate through all vent paths, including cracks in the cover  202   b , damaged seals, and so on). The vent flow rate through the port  218  (and other paths through which air within the interior volume can escape) can change as a result of device wear, device damage, temperature, ambient pressure change, blockage of the port  218 , and so on. Dynamically calibrating τ in the field, and even calibrating τ when a user is using a device, can therefore improve the accuracy of input force detection using a pressure sensor of the device, or can improve an algorithm that determines when a pressure sensor, a capacitive force sensor, another type of sensor, or a combination thereof, is used to determine whether a force has been applied to a device and/or an amount of the force. 
     In some embodiments, the value of τ can be calibrated by monitoring the pressure equalization interior to a device after a user removes a force applied to a cover of the device (as described with reference to  FIGS. 21-23 ), or by continuously monitoring the pressure difference between internal and ambient pressures of the device, or by correlating pressure data with acceleration data. 
       FIG. 21  shows an example of how τ may be pre-calibrated and field calibrated for a device. As shown by a graph  2102  of force over time, a user or calibration device may apply a predetermined amount of force (e.g., a 200 gF force) to a cover of a device at time t 0 . In some embodiments, the device may be the device  200  described with reference to  FIG. 19 . The user may release the force at time t 1 . As shown by a graph  2104  of pressure over time, application of the predetermined force at time t 0  may cause a temporary positive spike in the device&#39;s internal pressure, and release of the predetermined force at time t 1  may cause a temporary negative spike in the device&#39;s internal pressure. A value of τ may be determined from the time it takes the negative spike in internal pressure to increase to a steady state internal pressure. By way of example, the value of τ may be 100 milliseconds (ms), and this may be the pre-calibrated value of τ. Using the pre-calibrated value of τ, the device may later determine that a 200 gF force has been applied to the cover of the device, as shown by graph  2106 . The device may determine when different amounts of force have been applied to the cover of the device by extrapolating from the data shown in graphs  2102 ,  2104 , and  2106 , or by interpolating from data obtained by applying different amounts of force to the cover of the device. Force determinations may also be determined using the ideal gas law:
 
 PV=nRT  
 
     When a user applies a 200 gF force to the cover of the device in the field, the value of τ may have changed, as shown by a graph  2108  of pressure over time. By way of example, the graph  2108  shows τ to have changed to 60 ms. In some cases, the new value of τ may be determined by sampling the pressure at times around the negative spike in internal pressure and subsequent exponential increase in internal pressure, and fitting a curve to sampled internal pressures during the exponential increase in internal pressure, as shown in a graph  2110  of pressure over time. Using the determined new value of τ, the device may determine that a 200 gF has been applied to the cover of the device as shown by graph  2112 . 
       FIG. 22  shows example correlated graphs of touch, force, and pressure on the cover of a device. As shown in graph  2202 , a user may touch a device at time t 0 , stop touching the device at time t 1 , touch the device again at time t 2 , and stop touching the device again at time t 3 . As shown in graph  2204 , an amount of force associated with the touch between times t 0  and t 1  may not exceed a force trigger threshold  2206 . An amount of force associated with the touch between times t 2  and t 3  may exceed the force trigger threshold  2206 , but not until a time t 4 , which time is between times t 2  and t 3 . As shown in graph  2208 , the pressure internal to the device may spike in a positive direction when an amount of force is applied with each touch (e.g., at times t 0  and t 2 ), and spike in a negative direction when an amount of force is released (e.g., at times t 1  and t 3 ). The pressure may increase or decrease as the amount of applied force changes (e.g., the pressure may increase with the increase in applied force between times t 2  and t 4 ). 
     In some embodiments, a field calibration of τ may only be performed when a negative spike in internal pressure exceeds a τ calibration threshold  2210  and/or when a number of internal pressure samples exceeds a threshold number of samples, N. One example method  2300  for performing a field calibration of τ is shown in  FIG. 23 . The method  2300  may include detecting a touch (e.g., touch&gt;no touch) at block  2302 . At block  2304 , data (e.g., internal pressure samples) may be collected for τ calibration. At block  2306 , it may be determined whether enough internal pressure samples has been collected before a next touch (e.g., is the number of internal pressure samples&gt;N). If no, a determination to not calibrate τ may be made at block  2308 . If yes, a peak pressure during the touch, P peak , may be determined at block  2310 . It may be determined whether the peak pressure exceeds a τ calibration threshold at block  2312 . If no, a determination to not calibrate τ may be made at block  2314 . If yes, a curve may be fitted to the sampled internal pressures representing the exponential increase in internal pressure following the negative spike in internal pressure (e.g., at block  2316 ). The curve may be fit using the following equation:
 
 P=−P   peak   −t/τ 
 
     At block  2318 , it may be determined whether the curve fitting satisfies a curve fitting threshold (e.g., a goodness of fit threshold). When the curve fitting threshold is not satisfied, a determination to not calibrate τ may be made at block  2320 . When the curve fitting threshold is satisfied, τ may be calibrated to a new value indicated by the curve fitting at block  2322 . 
     In some cases, τ may alternatively be calibrated by continuously monitoring the pressure difference between internal and ambient pressures of a device, and correcting τ using the equation: 
               H   ⁡     (   s   )       =       θ   ⁡     (       τ   ⁢           ⁢   s     +   1     )           τ   ⁢           ⁢   s     +   1             
where θ represents the difference between the internal and ambient pressures.
 
     As previously mentioned, a pressure detected by a pressure sensor may depend on temperature, as quantified by the ideal gas law. For example, the pressure internal to a device may depend on the temperature within the device, which may differ from the ambient temperature of the device. The temperature within a well-sealed but vented device may depend, for example, on heat produced by one or more processors or other components within the device, thermal conduction from the exterior surfaces of the device (including the front cover of the device), and so on. In some cases, a change in temperature within a device may lead to a change in pressure that is not caused by a user-applied force, but is instead due to the temperature change. It may therefore be useful to measure the temperature within the interior volume of a device, instead of estimating or factoring out temperature effects. 
       FIG. 24  shows an example distribution of temperature sensors  2402  within a device  2400 . The device  2400  may be an example of the device described with reference to  FIG. 2A, 16A , or  19  (or other devices described herein). Each temperature sensor  2402  may be disposed to measure the temperature of the air within an interior volume  2404  of the device, or the temperature of a component within the interior volume (which component temperature may affect the temperature of the air within the interior volume). In some cases, as few as one temperature sensor may be provided. In other cases, a set of multiple temperature sensors may be provided. Measurements of temperature provided by a set of multiple temperature sensors may be used individually when detecting or determining an amount of force applied to a cover of the device, or measurements of temperature may be combined (e.g., combined to yield an average temperature, or combined to yield a weighted average temperature). A relationship between internal pressure (e.g., one or more internal pressure measurements obtained by one or more pressure sensors  2406 ) and internal temperature (e.g., one or more temperature measurements obtained by one or more temperature sensors  2402 ) may be calibrated using the ideal gas law and/or by empirically measuring and quantifying the relationship between internal pressure and internal temperature. Internal pressure measurements taken in the field may be calibrated based on internal temperature measurement(s). 
     In some embodiments, the device  2400  may include a barometric pressure sensor  2408  and/or other sensors that may be used to detect a force applied to a cover of the device  2400  and determine an amount of the force applied to the cover. 
     In some cases, the linearity and dynamic range of force sensing technologies that use a device cover as the force transduction layer may be limited by the amount of z space into which the cover may deflect. The range of force sensing between a no force condition in which a cover is at rest, and a maximum force condition in which the cover bottoms out against a housing or other component of the device, is defined herein as a device&#39;s dynamic range. Often, larger devices (e.g., a tablet computer) have little or no increase in z space over smaller devices (e.g., a smartphone or electronic watch), and the dynamic range of such larger devices may not provide a good user experience when a device relies on changes in the z space for force sensing. One way to improve a device&#39;s z space is to reduce the thickness of its battery. Another way to improve a device&#39;s z space is to increase the device&#39;s overall thickness. Neither of these alternatives may be attractive. 
     In some embodiments, the z space of a device may be effectively increased by changing the relationship between the amount of force applied to the device&#39;s cover and the amount of deflection (or displacement) of the cover in z space. That is, the cover&#39;s deflection in response to a predetermined amount of force may be reduced. This may be accomplished by increasing the spring force that opposes deflection of the cover in response to an applied force. 
       FIG. 25  shows another example cross-section of the device  200  described with reference to  FIG. 2A .  FIG. 25  also shows an alternative to the cross-section described with reference to  FIGS. 16A-16C . As shown in  FIG. 25 , the spring force that opposes deflection of the cover  202   b  may be increased by defining one or more sealed pockets  2500  within the interior volume  204 . The sealed pocket  2500  may be less deformable than an air-filled space within the interior volume 2-4, and may operate as a spring that opposes forces applied to the cover  202   b . The stiffness of the sealed pocket  2500  may depend on the size (e.g., length, width, and height) of the sealed pocket  2500 , the density of a gas (e.g., air), liquid, or gel within the sealed pocket, and the material(s) used to define the sealed pocket  2500 . In some embodiments, the sealed pocket  2500  may be an air pocket that operates as an air spring. The sealed pocket  2500  may be vented or non-vented, with greater venting tending to decrease the resistance provided by the sealed pocket  2500 . An internal pressure sensor  602  may be disposed outside or inside the sealed pocket  2500 . 
       FIG. 26  is a graph  2600  of the amount of force applied to a device&#39;s cover compared to the amount of force measured by a pressure sensor disposed within an interior volume of the device. As shown, the sealed pocket  2500  described with reference to  FIG. 25  can increase the dynamic range of the device. 
     In some cases, ambient pressure fluctuation and/or ambient temperature fluctuation can add low frequency noise to the pressure sensed by a pressure sensor interior to a device. Ambient pressure fluctuation can produce what is commonly referred to as red noise, and ambient pressure fluctuation an produce what is commonly referred to as pink noise. Red noise and pink noise are often correlated in time with a user&#39;s use of a device, and in some cases can affect the measurement of pressure by a pressure sensor, such that a force input is identified when no force input exists, or such that an intended force input is missed. 
       FIG. 27  shows a graph  2700  of pressure spectral density (PSD) verses frequency, and illustrates how low frequency noise  2702  (e.g., red noise) may interfere with a user input  2704  (i.e., a force input). To enable separation of the low frequency noise  2702  from the user input  2704 , the size of an interior volume of a device may be modulated up to a frequency that is above the cutoff frequency of the low frequency noise  2702 . This sort of modulation may be referred to as chopping (or nested chopping). The modulated volume produces a modulated pressure, in accordance with the ideal gas law. Any change in the interior volume of a device, which change is a result of a user-applied force to the cover of the device, may be detected or determined (e.g., measured) by demodulating the pressure signal using the volume modulation frequency. 
       FIG. 28  shows graphs  2800 ,  2810  of PSD verses frequency after modulation and demodulation of a user input (e.g., a force input or pressure). The user input  2704  may be modulated as modulated user input  2802 , as shown in graph  2800 , by modulating the size of a device&#39;s interior volume. The modulated user input  2802  may be demodulated as demodulated user input  2812 , as shown in graph  2810 , using the volume modulation frequency. As can be appreciated from the graph  2810 , the modulation and demodulation can separate the user input from the low frequency noise. 
       FIG. 29  shows another example cross-section of the device  200  described with reference to  FIG. 2A . As shown in  FIG. 29 , the device  200  may include an internal pressure sensor  602  and a volume modulation device  2900  (e.g., a pressure chopper). In some examples, the volume modulation device  2900  may include a haptic engine, an open back speaker, or an open vent Arc, disposed in or near the interior volume  204 . In some embodiments, the volume modulation frequency may be less than 20 Hertz (Hz), to mitigate ambient pressure drifts. 
       FIG. 30  shows example correlated graphs of force, volume, and pressure on the cover of a device (e.g., on the cover  202   b  of the device  200  described with reference to  FIG. 29 ). A graph  3000  of force over time arbitrarily shows that a force applied to a cover of a device increases over time. A graph  3010  of volume over time shows that the interior volume of the device may be modulated (e.g., in accordance with a volume modulation frequency). A graph  3020  of pressure over time shows that the pressure may modulate over time in relation to an amount of the applied force. By demodulating the pressure using the volume modulation frequency, the amount of the applied force may be determined. 
     In some cases, the pressure (or change in pressure) measured by a pressure sensor disposed within an interior volume of a device may be very small. Conventional pressure sensors are designed to measure absolute pressure, rather than relative pressure change, and may be accurate over the entire atmospheric pressure range on earth (e.g., 30 kPA-110 kPA). When a pressure sensor is designed to measure pressure over a wide dynamic range, the noise floor for measuring relative pressure change may be high. For a pressure sensor disposed within the interior volume of a device and configured to measure small changes in pressure resulting from a user-applied force input to the device, sensitivity to small relative pressure changes may be more critical than absolute pressure accuracy over a wide dynamic range. 
     One type of pressure sensor that may be used as any of the internal pressure sensors or barometric pressure sensors described herein is a microphone. Microphones may be used to measure relative pressure change and may have a much better signal-to-noise ratio (SNR) than a typical pressure sensor (e.g., an SNR on the order of 100× or more greater than that of a typical pressure sensor). In essence, a microphone may be used to ‘listen’ for low frequency acoustic waves generated by pressure changes.  FIG. 31  shows a graph  3100  of normalized amplitude response verses frequency for an example microphone. As shown, the microphone may be sensitive to frequencies less than 20 Hz. 
     The output of a microphone may be low-pass filtered to reject audio noise. In some embodiments, the acoustic-sensing diaphragm of a microphone may include a capacitive, piezoresistive, or piezoelectric film. 
       FIG. 32  shows an example method  3200  for determining an amount of force applied to a cover of a device. 
     At block  3202 , the method  3200  may include measuring a capacitance that is dependent on a distance between a capacitive force sensor and the cover. In some embodiments, the capacitance may be measured using a capacitive force sensor disposed within an interior volume of the device. The capacitive force sensor may be configured to generate a first set of one or more signals related to an amount of force applied to the cover. 
     At block  3204 , the method  3200  may include measuring an internal pressure of the device. In some embodiments, the internal pressure may be measured using a pressure sensor disposed within the interior volume. The pressure sensor may be configured to generate a second set of one or more signals related to the amount of force applied to the cover. 
     At block  3206 , the method  3200  may include determining an amount of force applied to the cover of the device using the measured capacitance and/or the measured internal pressure. The amount of force may be determined by a processor that uses at least one of the first set of one or more signals or the second set of one or more signals. 
     In some embodiments, the method  3200  may include determining the amount of force applied to the cover using the second set of one or more signals when the amount of force applied to the cover is determined for a first time following a touch detected by a touch sensor of the device. The method  3200  may include determining the amount of force applied to the cover using the first set of one or more signals when the amount of force applied to the cover is determined for a second time following the touch detected by the touch sensor. In some embodiments, the method  3200  may include determining the amount of force applied to the cover using the first set of one or more signals and the second set of one or more signals when the amount of force applied to the cover is determined for a third time following the touch detected by the touch sensor. The third time may be between the first time and the second time. 
     In some embodiments, the method  3200  may include determining a sealing state of the interior volume; determining the amount of force applied to the cover using the first set of one or more signals when the sealing state is determined to be below a threshold; and determining the amount of force applied to the cover using the second set of one or more signals when the sealing state is determined to be above the threshold. 
     In some embodiments, the method  3200  may include determining a sealing state of the interior volume; determining the amount of force applied to the cover using the first set of one or more signals when the sealing state is determined to be below a first threshold; and determining the amount of force applied to the cover using the first set of one or more signals and the second set of one or more signals when the sealing state is determined to be between the first threshold and a second threshold. 
     In some embodiments, the method  3200  may include determining a sealing state of the interior volume; determining the amount of force applied to the cover using the first set of one or more signals when the sealing state is determined to be below a first threshold; determining the amount of force applied to the cover using the first set of one or more signals and the second set of one or more signals when the sealing state is determined to be between the first threshold and a second threshold; and determining the amount of force applied to the cover using the second set of one or more signals when the sealing state is determined to be above the second threshold. 
     In some embodiments, the method  3200  may include determining an ambient pressure of the device. The ambient pressure may be determined using a barometric pressure sensor disposed within the housing and configured to generate a third set of one or more signals related to an ambient pressure of the device. In these embodiments, a processor may be configured to determine the amount of force applied to the cover using at least one signal from each of the first set of one or more signals, the second set of one or more signals, and the third set of one or more signals. 
     In some embodiments, the method  3200  may include determining an internal pressure from the first set of one or more signals; determining a distance (d) between the cover and the capacitive force sensor using the second set of one or more signals; determining an ambient pressure of the device from the third set of one or more signals; and determining a pressure difference between the internal pressure and the ambient pressure. Alternatively, the pressure difference may be determined using a differential pressure sensor (e.g., a sensor having a deformable element positioned between the volume interior to the device and an environment exterior to the device). In these embodiments, a processor may determine the amount of force applied to the cover from the distance and the pressure difference. For example, the amount of force may be determined based on a compensated displacement, δ, where:
 
δ= d−s ( P   2   −P   1 )
 
and s is a constant or function.
 
     In some embodiments of the method  3200 , a measured internal pressure may be used to adjust or compensate a measured capacitance. In other embodiments, a measured capacitance may be used to adjust or compensate a measured internal pressure. In some embodiments, the type of adjustment or compensation applied, or whether force sensing is enabled or temporarily disabled, may depend on a processor&#39;s assessment of the condition(s) of the environment exterior to the device (e.g., is the ambient pressure changing quickly, has there been a sudden change in ambient pressure, and so on). 
       FIG. 33  shows another example method  3300  for determining an amount of force applied to a cover of a device. 
     At block  3302 , the method  3300  may include measuring an internal pressure of the device. The internal pressure may be measured using an internal pressure sensor disposed within the interior volume and configured to generate a first set of one or more signals related to an amount of force applied to the cover. 
     At block  3304 , the method  3300  may include measuring an ambient pressure of the device. The ambient pressure may be a barometric pressure sensor disposed within the housing and configured to generate a second set of one or more signals related to an ambient pressure of the device. 
     At block  3306 , the method  3300  may include determining an amount of force applied to the cover of the device using the measured internal pressure and/or the measured ambient pressure. For example, the amount of force applied to the cover may be determined by a processor that uses the first set of one or more signals and the second set of one or more signals. 
     In some embodiments, the method  3300  may further include determining an internal pressure of the device from the first set of one or more signals, determine an ambient pressure of the device from the second set of one or more signals, and using the processor to determine an amount of force applied to the cover using the internal pressure and the ambient pressure. In some embodiments, the processor may be further configured to determine whether the amount of force applied to the cover indicates a user input. 
     In some embodiments of the method  3300 , the processor may be further configured to adjust a force detection threshold to which an internal pressure is compared. The internal pressure may be adjusting using (or based on) the ambient pressure. In these embodiments, the processor may identify a force input (i.e., an intended force input or force input trigger) when the internal pressure satisfies the adjusted force detection threshold. 
     In some embodiments, the method  3300  may further include characterizing the ambient pressure, and ceasing to determine the amount of force applied to the cover, for a period of time, in response to the characterization of the ambient pressure. For example, if the ambient pressure characterized as indicative of a door slam (e.g., a car door slam), force sensing operations may be temporarily disabled due to the effect that a door slam may have on the internal pressure sensor. 
     In some embodiments, the ambient pressure determined at block  3304  may be a time-varying ambient pressure, and the method  3300  may further include identifying, from the time-varying ambient pressure, an ambient pressure rate of change above a threshold; and adapting, in response to identifying the ambient pressure rate of change above the threshold, how the amount of force applied to the cover is determined. 
     In some embodiments, the type of adjustment or compensation applied to an internal pressure, or whether force sensing is enabled or temporarily disabled, may depend on a processor&#39;s assessment of the condition(s) of the environment exterior to the device (e.g., is the ambient pressure changing quickly, has there been a sudden change in ambient pressure, and so on). 
       FIG. 34  shows an example method  3400  for determining an amount of force applied to a cover of a device. 
     At block  3402 , the method  3400  may include determining, using a pressure sensor within an interior volume of the device, whether an amount of force applied to the cover exceeds a τ calibration threshold (i.e., a predetermined amount of force applied to the cover). In some embodiments, the determination may be made from a set of pressure samples obtained from the pressure sensor. The τ calibration threshold may be based on application or release of an applied force. As previously discussed, τ is the time it takes the interior volume to reach a steady state pressure following the application or release of the force applied to the cover; 
     At block  3404 , the method  3400  may include calibrating τ during field operation of the device, in response to determining that the amount of force applied to the cover exceeds the τ calibration threshold. 
     At block  3406 , the method  3400  may include determining an amount of force applied to the cover of the device, after calibrating τ, using at least one pressure sample obtained from the pressure sensor and the calibrated τ. 
     In some embodiments, the method  3400  may further include determining that a number of pressure samples obtained after a release of the force applied to the cover, and before a next touch on the cover, exceeds a threshold number of samples. In these embodiments, the operations at block  3404  may be performed in response to a combination of both 1) determining the amount of force applied to the cover exceeds the τ calibration threshold, and 2) determining that a number of pressure samples obtained after a release of the force applied to the cover, and before a next touch on the cover, exceeds a threshold number of samples. 
     In some embodiments, the method  3400  may further include determining whether τ satisfies an interior volume leak threshold, and notifying a user of the device when τ is determined to satisfy the interior volume leak threshold. For example, a notification may be displayed on a display of the device, or an audible or haptic notification may be provided to the user. The notification may enable the user to take extra precautions in situations where their device might be exposed to water or other contaminants. 
       FIG. 35  shows another example method  3500  for determining an amount of force applied to a cover of a device. 
     At block  3502 , the method  3500  may include measuring a pressure within an interior volume of the device. In some cases, an internal pressure sensor (e.g., a pressure sensor within the volume that is interior to the device) may generate a first set of one or more modulated signals related to an amount of force applied to the cover. 
     At block  3504 , the method  3500  may include measuring a temperature of the interior volume. In some cases, a temperature sensor disposed within the interior volume may generate a second set of one or more signals related to a temperature within the interior volume. 
     At block  3506 , the method  3500  may include adjusting the internal pressure based on the temperature (e.g., adjusting at least one signal in the first set of signals using at least one signal in the second set of signals). 
     At block  3508 , the method  3500  may include determining an amount of force applied to the cover of the device using the adjusted internal pressure (e.g., at least one adjusted signal from the first set of signals). 
       FIG. 36  shows another example method  3600  for determining an amount of force applied to a cover of a device. 
     At block  3602 , the method  3600  may include modulating a pressure of a volume interior to the device. The pressure may be modulated using a pressure modulation device (or pressure chopper), as described with reference to  FIG. 29 . 
     At block  3604 , the method  3600  may include measuring an internal pressure of the interior volume. In some cases, an internal pressure sensor (e.g., internal to the volume that is interior to the device) may generate a first set of one or more modulated signals related to an amount of force applied to the cover. 
     At block  3606 , the method  3600  may include demodulating the measured internal pressure. In some cases, a processor may generate a second set of one or more demodulated signals using a modulation frequency of the pressure modulation device. 
     At block  3608 , the method  3600  may include determining an amount of force applied to the cover of the device using the demodulated internal pressure (e.g., the demodulated signal(s) from block  3606 ). 
     Various configuration of capacitive force sensors and/or pressure sensors within a device may provide various advantages, depending on their configuration. For example, a pressure sensor may be used to provide rate-based click detection (e.g., of a tap force), whereas a capacitive force sensor may be used to provide absolute force detection. A device having both a capacitive force sensor and a pressure sensor may provide both rate-based click detection and absolute force detection. Rate-based click detection can enable click detection on a low signal-to-noise ratio (SNR) or poorly sealed device. A capacitive force sensor and/or pressure sensor may in some cases provide for low-latency click detection or tap discrimination using temporal features of a measured signal. 
     In some examples, an applied force or rate of applied force may be determined using a pressure sensor and inverting a system using τ, whereas a capacitive force sensor may determine an applied force or rate of applied force using displacement sensing. In some examples, and when a device includes both a capacitive force sensor and a pressure sensor), the displacement of a capacitive force sensor may be compensated using an overpressure (e.g., from an interior/exterior pressure differential) when τ is not known, or using sensor fusion when τ is known and capacitance and pressure measurements are available. 
     In some embodiments, a device including a capacitive force sensor and/or pressure sensor may determine τ when an applied pressure is removed from a device, and/or τ may be classified as high or low after one or more taps on a device. In a device including both a capacitive force sensor and a pressure sensor, τ may be determined by correlating the outputs of both sensors when the device is touched. 
     In some embodiments, a device having a capacitive force sensor and/or pressure sensor may also have a barometric pressure sensor, which sensor may enable low-noise tracking of the device&#39;s ambient pressure. The ambient pressure may be used to detect high-intensity ambient pressures that may affect the capacitive force sensor and/or pressure sensor and, for example, 1) compensate for the ambient pressure, 2) adjust a force sensing threshold or force trigger threshold, 3) or temporarily disable force detection. In this manner, false force triggers may be avoided in scenarios such as, when a device is used in an elevator, in a vehicle or other room in which a door is slammed, on a high-speed train, in a vehicle with an open window, and so on. 
     In some cases, one of the devices described herein may be immersed in a fluid. When the device is immersed in a fluid (e.g., water), the pixels of the device&#39;s touch sensor may saturate, rendering the touch sensor unable to determine where a user is touching the device&#39;s cover or display. In this regard,  FIG. 37  shows an example top plan view of the device  100  described with reference to  FIGS. 1A-1B , wherein the device  100  has been provided with a collection of sensors that enable the device  100  to determine a location of a user&#39;s touch on the cover  102   b  when the device is immersed in a fluid. 
     As shown in  FIG. 37 , and in some embodiments, the force sensing system  108  of the device  100  may include a set of pressure sensors (e.g., at least one pressure sensor  602 ) and a set of displacement sensors  210  (e.g., one or more displacement sensors, including, for example, a first displacement sensor  210   a , a second displacement sensor  210   b , and a third displacement sensor  210   c ). The displacement sensors  210   a ,  210   b ,  210   c  may be positioned at different locations beneath the cover  102   b . In some cases, the displacement sensors  210   a ,  210   b ,  210   c  may be capacitive force sensors. When a fluid immersion detector indicates that the device  100  is immersed in a fluid, the processor  110  ( FIG. 1B ) may determine a location of a touch on the cover  102   b  using signals received form the pressure sensor  602  and set of displacement sensors  210   a ,  210   b ,  210   c , as described, for example, with reference to any of  FIGS. 42-46 . When the fluid immersion detector indicates that the device  100  is not immersed in a fluid, the processor  110  may determine an amount of force applied to the cover  102   b , as described, for example, with reference to any of  FIGS. 1-36 . 
     The fluid immersion detector may in some cases take the form of a particular output or state of the touch sensing system  106 . For example, the fluid immersion detector may be an output of a touch sensor, which output indicates that a set of pixels of the touch sensor (e.g., a majority or all of the pixels of the touch sensor) is saturated. In other cases, the fluid immersion detector may take the form of a fluid detector, such as a fluid detector mounted in a port of the device  100 . The fluid immersion detector may also or alternatively include a barometric pressure sensor that indicates when the device  100  is likely to be under water; or a processor that analyzes the outputs of one or more types of fluid immersion detectors. Analysis of the outputs or multiple fluid immersion detectors can reduce the likelihood that the device  100  is falsely determined to be immersed. 
     Building on the principles introduced with respect to  FIG. 37 ,  FIG. 38  shows an example alternative cross-section of the device  200  described with reference to  FIG. 2A . In the alternative cross-section, different numbers and/or types of sensors are included in the interior volume  204 . For example, a pressure sensor  602  may be disposed within the interior volume  204 . The pressure sensor  602  may be configured to generate (and output) a first set of one or more signals, or time-dependent sequence of pressure measurements, related to (i.e., in response to) an amount of force applied to the cover  202   b  (e.g., a force applied to the cover  202   b  by a user&#39;s finger or stylus). In some cases, the first set of one or more signals may also or alternatively be related to an amount of force applied to a side or back of the device  200 . In some examples, the pressure sensor  602  may be a capacitive pressure sensor or resistive (e.g., strain gauge) pressure sensor. The pressure sensor  602  may be opportunistically positioned within the interior volume  204  (e.g., the pressure sensor  210  may be positioned where space exists or where convenient). For example, the pressure sensor  602  may be attached to a logic board (e.g., a printed circuit board) mounted within the interior volume  204 , or attached to an available area on a flex circuit used for force sensing or other purposes. The pressure sensor  602  need not be positioned toward a side of the device  200 , but may be. In some embodiments, the pressure sensor  602  may be positioned under the center of the cover  202   b  (e.g., the positions of the sensor  210   a  and pressure sensor  602  may be swapped, or the sensors  210   a ,  602  may be positioned near each other under the center of the cover  202   b ). 
     A capacitive force sensor  210   a  or other type of displacement sensor may also be disposed within the interior volume  204 , or otherwise disposed to detect displacement (e.g., distortion) of the interior volume  204 . The capacitive force sensor  210   a  may be configured to generate (and output) a second set of one or more signals, or first time-dependent sequence of displacement measurements, related to an amount of force applied to the cover  202   b . In some embodiments, the capacitive force sensor  210   a  may be provided by a SiP mounted within the interior volume  204 . For example, a SiP may include a self-capacitance sense pad  212  (i.e., an electrode) adjacent (or on) a first surface of the SiP, a set of solder structures (e.g., solder balls or solder posts) attached to a second surface of the SiP (opposite the first surface), and an integrated circuit (IC) coupled to the self-capacitance sense pad  212 . The IC may be configured to output, at one or more of the solder structures, an analog or digital value related to a measured capacitance of the self-capacitance sense pad  212  (i.e., the capacitive force sensor  210   a  may be configured in a self-capacitance sensing mode). The digital value may be indicative of a force or amount of force applied to the cover  202   b , or a displacement of the cover  202   b  at the location of the capacitive force sensor  210   a . The SiP may be mounted upside down with respect to the cover  202   b , such that the first surface of the SiP is positioned closer to the cover  202   b  than the second surface of the SiP. The first surface of the SiP may be oriented parallel (or substantially parallel) to the exterior surface of the cover  202   b.    
     The SiP may be opportunistically positioned within the interior volume  204  (e.g., the SiP may be positioned where space exists or where convenient). For example, the SiP may be attached to a logic board (e.g., a printed circuit board) mounted within the interior volume  204 , or attached to an available area on a flex circuit used for force sensing or other purposes. The SiP need not be positioned under the center of the cover  202   b , but may be. The solder structures may be reflowed to electrically and mechanically attach the SiP to another structure (e.g., a logic board or flex circuit). Some solder structures of the SiP may be used to provide power, signals, or instructions to the SiP (e.g., to the IC). 
     In some embodiments, and as shown in  FIG. 38 , a ground element  214  may be mounted within the interior volume  204 , and the self-capacitance sense pad  212  may be separated from the ground element  214  by a compressible gap  216 . The ground element  214  may be a layer of the stack (or may be provided within a layer of the stack), and may be positioned between the display  208  and the self-capacitance sense pad  212 . Alternatively, the ground element  214  may be positioned within the display  208 , or between the display  208  and the cover  202   b , or in various positions as described with reference to  FIGS. 15A-15C  for example). By way of example,  FIG. 38  shows the ground element  214  positioned between the display  208  and the self-capacitance sense pad  212 . In some examples, the ground element  214  may be provided on a substrate such as a PCB (e.g., a display PCB), on a flex circuit (e.g., a display flex circuit, a touch sensor flex circuit, or a camera flex circuit), or on a surface of a substrate such as the cover  202   b . When a user applies a force to the cover  202   b , the distance (i.e., displacement) between the ground element  214  and the self-capacitance sense pad  212  may decrease, thus changing the capacitance of the self-capacitance sense pad  212  and enabling an amount of force to be detected by the IC within the SiP. Different amounts of force applied to the cover  202   b  may compress the compressible gap  216  to different extents. In alternative embodiments, the capacitive force sensor  210   a  may be configured in a mutual-capacitance sensing mode, and the ground element  214  may instead be a conductor through which a drive waveform is propagated. 
     In some alternative embodiments, the SiP may be mounted within the interior volume  204  with the first surface oriented toward an edge or the back of the device  200 . With these orientations, the SiP may be used to sense a force or amount of force applied to the edge or back of the device  200  (though the SIP may also sense a force or amount of force applied to the edge or back of the device  200  when positioned as shown in  FIG. 38 ). 
     In some embodiments, the device  200  may include a plurality of pressure sensors and/or a plurality of displacement sensors. When the device  200  includes a plurality of pressure sensors, the measurements of the pressure sensors may be averaged, combined, or selected in various ways to determine the pressure within the internal volume. A plurality of pressure sensors may enable the device  200  to adjust a time-dependent sequence of pressure measurements for the effects of high heat producing components within the device  200 , ambient effects, and so on. 
     By way of example, the device  200  is shown to have three displacement sensors (e.g., a first capacitive force sensor  210   a , a second capacitive force sensor  210   b , and a third capacitive force sensor  210   c ). The capacitive force sensors  210   a ,  210   b ,  210   c  may be positioned at different locations beneath the cover  202   b , and may generate different signals (or different time-dependent sequences of displacement measurements) in response to displacement of the cover  202   b  at different locations with respect to a surface of the cover  202   b . In some embodiments, the capacitive force sensors  210   a ,  210   b ,  210   c  may be generally distributed as shown with reference to  FIG. 37 . The capacitive force sensors  210   a ,  210   b ,  210   c  may also be distributed beneath the cover  202   b  in other ways. 
     In some embodiments, the device  200  may include a barometric pressure sensor  1600  that extends through the housing  202   a  (e.g., with a pressure sensing element of the barometric pressure sensor  1600  oriented toward an environment exterior to the housing  202   a , and an electrical interface of the barometric pressure sensor  1600  oriented toward the interior volume  204 ). The barometric pressure sensor  1600  may be configured to measure the barometric pressure of an environment exterior to the device  200  (e.g., an ambient pressure of the device  200 ). In some cases, the barometric pressure sensor  1600  may be used to measure the barometric pressure while a touch or force is being applied to the cover  202   b . In some embodiments, the barometric pressure sensor  1600  may include a set of multiple pressure sensors, and measurements obtained from the multiple barometric pressure sensors may be combined (e.g., averaged) to determine the ambient pressure of the device  200 . 
     In some cases, an air permeable but liquid water impermeable membrane may be disposed over the pressure sensing element of the barometric pressure sensor  1600 , to protect it from water and debris in the environment exterior to the device  200 . In some embodiments, a gel or other substance may be disposed over the pressure sensing element to protect it from water and debris. 
     A processor within the device  200  may be configured to determine an amount of force applied to the cover  202   b  (or in some cases, an amount of force applied to an edge or back of the device  200 ) using the time-dependent sequence of pressure measurements received from the pressure sensor  602  (or from a set of multiple pressure sensors) and/or the time-dependent sequence of displacement measurements received from the capacitive force sensor  210   a  (or time-dependent sequences of displacement measurements received from multiple capacitive force sensors  210   a ,  210   b ,  210   c ). In some cases, the processor may determine the amount of force applied to the cover  202   b  using only the time-dependent sequence of pressure measurements, or a subset thereof, or only the time-dependent sequence(s) of displacement measurements, or a subset thereof. The processor may be configured to use one or the other time-dependent sequences of measurements in response to evaluating a set of one or more conditions (including, for example, a current or past value of a venting state of the interior volume  204 ). In some cases, the processor may determine the amount of force applied to the cover  202   b  using time-dependent sequences of pressure and displacement measurements. The processor may also use a location of a touch (or locations of multiple touches, or a location of a touch centroid) to determine the amount of force applied to the cover  202   b . For example, measurements obtained from the pressure sensor  602  or capacitive force sensor(s)  210   a ,  210   b ,  210   c  may be mapped to different amounts of force based on where a user touches the device  200 . 
     The capacitive force sensors  210   a ,  210   b ,  210   c  or other displacement sensors may in some cases be used for different purposes, depending on whether the device  200  is immersed in a fluid (e.g., water). For example, when the device  200  is not immersed in fluid, the capacitive force sensors  210   a ,  210   b ,  210   c  may be used to determine an amount of force applied to the cover  202   b . However, when the device  200  is immersed in fluid, the capacitive force sensors  210   a ,  210   b ,  210   c  may be used to determine the location of a user&#39;s touch on the cover  202   b . When the device  200  is immersed in a fluid, the presence and/or pressure of the fluid on the cover  202   b  may saturate a majority or all of the pixels of the device&#39;s touch sensor (e.g., a touch sensor included in the stack that includes the display  208 ), rendering the touch sensor inoperable for the purpose of detecting a location of a user&#39;s touch. However, in such a case, the device&#39;s processor may determine a location of a user&#39;s touch using signals received form the pressure sensor  602  and set of capacitive force sensors (or other displacement sensors)  210   a ,  210   b ,  210   c , as described, for example, with reference to  FIGS. 42-46 . 
     The signals produced by the pressure sensor  602  and capacitive force sensor(s)  210   a ,  210   b ,  210   c  may be selectively used for amount of force determination and/or touch location detection based on the output of a fluid immersion detector. The fluid immersion detector may in some cases take the form of a particular output or state of the device&#39;s touch sensor. For example, an output of the touch sensor indicating that a set of pixels of the touch sensor (e.g., a majority or all of the pixels of the touch sensor) is saturated may cause the processor to use the signals produced by the pressure sensor  602  and capacitive force sensor(s)  210   a ,  210   b ,  210   c  to determine a location of a touch on the cover  202   b . In other cases, the fluid immersion detector may take the form of a fluid detector, such as a fluid detector mounted in a port  218  of the device  200 . The fluid immersion detector may also or alternatively include the barometric pressure sensor  1600 , which may generate an output indicating that the device  200  is likely immersed in a fluid; or a processor that analyzes the outputs of one or more types of fluid immersion detectors. 
     Although  FIGS. 2A and 38  show a particular form-factor and layout for the device  200 , the structures and techniques described with reference to  FIGS. 2A and 38  may be used to deploy pressure-based force sensing alone, capacitive (or displacement-based) force sensing alone, a combination of pressure-based and capacitive force sensing, or combinations of other types of force sensing. A single force sensing type, or combination of force sensing types, may be deployed in almost any kind of device having an interior volume  204  that is capable of compressing or otherwise changing shape, including, for example, a tablet computer or electronic watch. In some embodiments, capacitive force sensing may be deployed in a form factor other than in a SiP. For example, a capacitance sense pad may be disposed on a PCB or other substrate, adjacent an ASIC or other controller that senses a capacitance of the capacitance sense pad. As another example, a capacitance sense pad may be disposed on a semiconductor wafer (i.e., a semiconductor “chip”), and a circuit for sensing the capacitance of the capacitance sense pad may be formed on the wafer. PCBs or wafers may occupy less z space than a SiP, but may increase the size of the gap  216 . In some embodiments, a PCB or wafer may be elevated using a standoff, or disposed on top of another component that positions it closer to the ground element  214  or cover  202   b . A wafer may provide the smallest form factor for a capacitance sense pad and related circuitry, but at a higher cost than a PCB or SiP implementation. 
       FIGS. 39A-39E  show the device cross-section of  FIG. 38  when, and after, a force is applied to the cover  202   b . The force may be applied to the cover  202   b  while the device  200  is immersed in a fluid or surrounded by air. When the device  200  is immersed in a fluid, the cover  202   b  may begin at a position that is pushed inward, or somewhat flexed, with respect to the cover position shown in  FIG. 39A . 
       FIG. 39A  shows an initial application of force (e.g., F finger ) to the cover  202   b . That is,  FIG. 39A  shows an initial application of a force related to a user&#39;s touch, whether the touch be with a user&#39;s finger, stylus, or other body part or implement. When the port  218  is small in size compared to the interior volume  204 , and a gas (e.g., air) contained within the interior volume  204  is unable to escape the interior volume  204  freely, the air contained within the interior volume  204  may operate as a spring on the cover  202   b  (e.g., an air spring). The spring effect (e.g., air spring effect) may provide a pressure or resistance that opposes the force (e.g., F finger ) applied to the cover  202   b . For example, and referring to  FIG. 39A , a force applied to the center of the cover  202   b  may be opposed by the forces represented by the small arrows within the interior volume  204 . 
     As the force continues to be applied to the cover  202   b , and as shown in  FIG. 39B , the air spring effect may cause portions of the cover  202   b  to bow outward upon continued application of the force F finger . The portions of the cover  202   b  may bow outward as a result of the pressure within the interior volume  204  increasing, and as a result of air within the interior volume  204  being compressed. Also upon application of the force F finger , the gasket  206  that bonds the cover  202   b  to the housing  202   a  (with spring constant k) may apply an inward force to the edges of the cover  202   b . In other cases (not illustrated in  FIG. 39B ), the force distribution inside the interior volume  204  as a result of the air spring effect may differ. For example, the gasket  206  may apply an outward force to the edges of the cover  202   b . More generally, whether or not a gasket such as the gasket  206  applies an inward force, outward force, or any force to the edges of the cover  202   b  may be a function of the interior volume  204 , the aspect ratio (e.g., length, width, and thickness) of the cover  202   b , and other factors. The forces (if any) applied to the edges of the cover  202   b  by the gasket  206  are the result of a force balancing problem involving the force F finger  applied to the cover  202   b , the pressure within the interior volume  204  when the force F finger  is applied (e.g., a back pressure force), and the gasket force. The force balancing problem may be modeled for a particular device using finite element analysis (FEA). In general, a gasket may be expected to apply an inward force to the edges of a cover when a device has a long, thin, glass cover, as may be the case in the device  200 ; and a gasket may be expected to apply an outward force to the edges of a cover when a device has a short, thick, glass cover, as may be the case in an electronic watch. 
     When the interior volume  204  is vented, thereby enabling air contained within the volume  204  to escape after application of the force F finger  (but not escape immediately upon application of the force F finger ), the air spring effect illustrated with reference to  FIG. 39B  may decrease over time (assuming that the device  200  is not immersed in a fluid). Air flow out of the interior volume  204  is indicated by the arrow directed away from the port  218  in  FIG. 39B . As air escapes the interior volume  204 , the pressure within the interior volume  204  may exponentially decrease until the steady state shown in  FIG. 39C  is reached. In the steady state, the force F finger  may not be opposed by the pressure or distribution of air within the interior volume  204 , and the edges of the cover  202   b  may apply an outward force (or alternatively, an inward force) to the gasket  206 . As shown in  FIG. 39C , different capacitive force sensors  210   a ,  210   b ,  210   c  may sense different displacements of the cover  202   b , depending on the locations of the capacitive force sensors  210   a ,  210   b ,  210   c  beneath the cover  202   b.    
     When the device  200  is immersed in a fluid, air may not be able to escape the interior volume  204  through the port  218  when the force F finger  is applied to the cover  202   b , and a steady state similar to what is shown in  FIG. 39C  may be reached very more quickly, without air escaping the interior volume  204  as shown in  FIG. 39B . Also, the cover  202   b  may not deflect as much as shown in  FIG. 39C  when air is unable to escape the interior volume  204 . 
       FIG. 39D  shows the device  200  immediately after removal (liftoff) of the force F finger . As shown, the ambient environment of the device  200  (air or water) may initially oppose return of the cover  202   b  to a steady state position. As air flows back into the interior volume  204  (or in the case of a device immersed in fluid, as the force F finger  is lifted off), the device  200  may return to a steady state, as shown in  FIG. 39E . 
     Between the time that the force F finger  is initially applied to the cover  202   b  (as shown in  FIG. 39A ) and the time the pressure within the interior volume  204  reaches a steady state following application of the force F finger  (as shown in  FIG. 39C ), the capacitance measurements generated by the capacitive force sensors  210   a ,  210   b ,  210   c  may change. The changes in the capacitance measurements may be caused by a change in the distance between the ground element  214  attached to the cover  202   b  and the self-capacitance sense pads of the capacitive force sensors  210   a ,  210   b ,  210   c . Also between the time the force F finger  is initially applied to the cover  202   b  and the time the pressure within the interior volume  204  reaches a steady state following application of the force F finger , and assuming a fluid sealed or slowly vented interior volume  204 , the pressure measurements generated by the pressure sensor  602  may change. The capacitance measurements and pressure measurements may also change between the time the force is lifted off of the cover  202   b  and the time the pressure within the interior volume  204  reaches a steady state following lift off of the force. 
       FIGS. 40A-40C  are graphs of example outputs of the pressure sensor  602  and a capacitive force sensor (e.g., the capacitive force sensor  210   a ) described with reference to  FIGS. 38 and 39A-39E , in response to application and removal (liftoff) of a force on the cover  202   b , when the device  200  is not immersed in a fluid.  FIG. 40A  shows a graph  4000  of the force over time, F(t).  FIG. 40B  shows a graph  4010  of the pressure sensor&#39;s response to the force, P(t).  40 C shows a graph  4020  of the capacitive force sensor&#39;s response to the force, in terms of displacement, d 1 ( t ), of a ground element (e.g., the ground element  214  described with reference to  FIGS. 38 and 39A-39E ). 
     As shown in  FIG. 40A , a steady state force may be applied to the cover  202   b  at a time t 0 , and lifted off at a time t 1 . As shown in  FIG. 40B , the pressure measured by the pressure sensor  602  may spike upward when the force is initially applied to the cover  202   b  at time t 0 , as a result of an air spring effect, and then exponentially decay as air (or another gas) flows out of the interior volume  204 . Similarly, the pressure measured by the pressure sensor  602  may spike downward when the force is initially lifted off at time t 1 , and exponentially increase after liftoff. As shown in  FIG. 40C , the capacitance (or displacement) measured by the capacitive force sensor  210   a  may decrease exponentially, beginning when the force is applied to the cover  202   b  at time t 0 . Similarly, the capacitance (or displacement) measured by the capacitive force sensor  210   a  may increase exponentially after liftoff of the force, beginning at time t 1 . When the device to which the force is applied is immersed in a fluid (e.g., water), the pressure spike shown in  FIG. 40B  at time t 0  may be smaller; there may be less or no decay of the pressure between times t 0  and t 1 ; and the pressure before the force is lifted off at time t 1  may be a positive pressure. Also when the device is immersed in a fluid, the cover  202   b  may experience a smaller change in displacement and the capacitance (or displacement) change measured by the capacitive force sensor  210   a  in  FIG. 40C  may be less. 
       FIG. 41  shows relationships between a force  4100  applied to a device (e.g., the device  100  described with reference to  FIGS. 1A-1B and 37 , or the device  200  described with reference to  FIGS. 2A, 38, and 39A-39E ), measurements of displacement  4102  derived from a capacitive force sensor positioned in or in relation to an interior volume of the device, measurements of pressure  4104  derived from a pressure sensor positioned within the interior volume, and estimates of displacement  4106  derived from the measurements of pressure  4104 . The relationships shown in  FIG. 41  may be used, when the device is surrounded by air or otherwise not immersed in a fluid, to determine the amount of force applied to the device. The relationships shown in  FIG. 41  may be used, when the device is immersed in a fluid, to determine a location of a touch on the device (e.g., the location of a touch on a cover of a display), and in some cases may also or alternatively be used to determine an amount of force associated with the touch. 
     As shown, the measured displacement  4102  may be related to the applied force  4100  by a force-to-displacement transfer function H d (z)  4108 , the measured pressure  4104  may be related to the applied force  4100  by a force-to-pressure transfer function H p (z)  4110 , and the estimated displacement  4106  may be related to the measured pressure  4104  by a displacement-to-pressure transfer function G d/p (z)  4112 . Each of the transfer functions may depend on various system parameters, such as a venting state (τ) of the interior volume; a uniform load (uniform pressure) sensitivity S up ; a steady-state sensitivity (S ss   (x,y) ) of a displacement sensor (e.g., a capacitive force sensor) in response to a force applied to the device; a steady-state sensitivity (S p2   (x,y) ) of a pressure sensor in response to a force applied to the device; and/or a relative sensitivity 
             (       S   r     (     x   ,   y     )       =       S   ss     (     x   ,   y     )         S     p   ⁢           ⁢   2       (     x   ,   y     )           )         
between the steady-state sensitivity of the displacement sensor and the steady-state sensitivity of the pressure sensor. In each of these quantities, (x,y) is a location on a surface for which touch detection is provided.
 
     When a device is immersed in fluid, the venting state of the device&#39;s interior volume may be considered to approach infinity. The steady-state sensitivity of the displacement sensor may nonetheless be characterized in terms of the steady-state displacement seen at the sensor&#39;s location from each gram Force (gF) applied at a touch location on a device (e.g., nanometer/gF=nm/gF). The steady-state sensitivity of the pressure sensor may be characterized in terms of the steady-state pressure change in the interior volume from each gF applied at the touch location (e.g., Pascal/gF=pa/gF). 
       FIG. 42  shows relationships between a force  4200  applied to a device (e.g., the device  100  described with reference to  FIGS. 1A-1B and 37 , or the device  200  described with reference to  FIGS. 2A, 38 and 39A-39E ), measurements of displacement  4202 ,  4204 ,  4206  derived from each of three capacitive force sensors positioned in or in relation to an interior volume of the device, measurements of pressure  4208  derived from a pressure sensor positioned within the interior volume, and estimates of displacement  4210 ,  4212 ,  4214  derived from the measurements of pressure  4208 . The relationships shown in  FIG. 42  may be used, when the device is surrounded by air or otherwise not immersed in a fluid, to determine the amount of force applied to the device. The relationships shown in  FIG. 42  may be used, when the device is immersed in a fluid, to determine a location of a touch on the device (e.g., the location of a touch on a cover over a display). 
     Similarly to what is described with reference to  FIG. 41 , the displacement (d 1 )  4202  measured by a first capacitive force sensor (or other type of displacement sensor) may be related to the applied force  4200  by a first force-to-displacement transfer function H d1 (z)  4216 , the displacement (d 2 )  4204  measured by a second capacitive force sensor (or other type of displacement sensor) may be related to the applied force  4200  by a second force-to-displacement transfer function H d2 (z)  4218 , and the displacement (d 3 )  4206  measured by a third capacitive force sensor (or other type of displacement sensor) may be related to the applied force  4200  by a third force-to-displacement transfer function H d3 (z)  4220 . Also, the measured pressure  4208  may be related to the applied force  4200  by a force-to-pressure transfer function H p (z)  4222 . The estimated displacement d 1   4210  may be related to the measured pressure  4208  by a first displacement-to-pressure transfer function F d1/p (z)  4224 , the estimated displacement d 2   4212  may be related to the measured pressure  4208  by a second displacement-to-pressure transfer function G d2/p (z)  4226 , and the estimated displacement d 3   4214  may be related to the measured pressure  4208  by a third displacement-to-pressure transfer function G d3/p (z)  4228 . Each of the transfer functions may depend on various system parameters, such as a venting state (τ) of the interior volume; the steady-state sensitivity (S ss     i     (x,y) ) of each displacement sensor (e.g., each capacitive force sensor) in response to a force applied to the device, for i∈{1,2,3}; the uniform-load (uniform pressure) sensitivity (S up     i   =α i ) of each displacement sensor; a steady-state sensitivity (S p2   (x,y) ) of the pressure sensor in response to a force applied to the device; and/or a relative sensitivity 
             (       S     r   i       (     x   ,   y     )       =       S     ss   i       (     x   ,   y     )         S     p   ⁢           ⁢   2       (     x   ,   y     )           )         
between the steady-state sensitivity of a displacement sensor and the steady-state sensitivity of the pressure sensor.
 
     When a device is immersed in fluid, the venting state of the device&#39;s interior volume may approach infinity. The steady-state sensitivity of each displacement sensor may nonetheless be characterized in terms of a steady-state displacement seen at the sensor&#39;s location from each gram Force (gF) applied at a touch location on a device (e.g., nanometer/gF=nm/gF). The steady-state sensitivity of the pressure sensor may be characterized in terms of the steady-state pressure change in the interior volume from each gF applied at the touch location (e.g., Pascal/gF=pa/gF). The uniform-load sensitivity is independent of touch location. 
     The transfer functions H di (z), H p (z), and G di/p (z) can be characterized as follows: 
                       H     d   i       ⁡     (   z   )       =       ⁢       lim     τ   →   ∞       ⁢           S     ss   i       (     x   ,   y     )       ⁡     (         T   s       2   ⁢           ⁢   τ       +   1     )       +       S     up   i       ⁢     S     p   ⁢           ⁢   2       (     x   ,   y     )         +       (         S     ss   i       (     x   ,   y     )       ⁡     (         T   s       2   ⁢           ⁢   τ       -   1     )       -       S     up   i       ⁢     S     p   ⁢           ⁢   2       (     x   ,   y     )           )     ⁢     z     -   1               (         T   s       2   ⁢           ⁢   τ       +   1     )     +       (         T   s       2   ⁢           ⁢   τ       -   1     )     ⁢     z     -   1                           =       ⁢       S     ss   i       (     x   ,   y     )       +       S     up   i       ⁢     S     p   ⁢           ⁢   2       (     x   ,   y     )                                     ⁢         H   p     ⁡     (   z   )       =         lim     τ   →   ∞       ⁢       S     p   ⁢           ⁢   2       (     x   ,   y     )       ⁢       1   -     z     -   1             (         T   s       2   ⁢           ⁢   τ       +   1     )     +       (         T   s       2   ⁢           ⁢   τ       -   1     )     ⁢     z     -   1                 =     S     p   ⁢           ⁢   2       (     x   ,   y     )                           G       d   i     /   p       ⁡     (   z   )       =           D   i     ⁡     (   z   )         P   ⁡     (   z   )         =           H   p     -   1       ⁡     (   z   )       ⁢       xH     d   i       ⁡     (   z   )         =           S     ss   i       (     x   ,   y     )       +       S     up   i       ⁢     S     p   ⁢           ⁢   2       (     x   ,   y     )             S     p   ⁢           ⁢   2       (     x   ,   y     )         =         S     r   i       (     x   ,   y     )       +     S     up   i         =       S     r   i       (     x   ,   y     )       +     α   i                     
where T s  is the sampling period.
 
     A displacement measurement for a capacitive force sensor may be estimated using pressure measurements and past displacement measurements for the capacitive force sensor, or a pressure measurement may be estimated using past displacement measurements and past pressure measurements, at any point in time. The estimation may be simpler when each of the transfer functions H d1 (z), H d2 (z), H d3 (z), H p (z), G d1/p (z), G d2/p (z), and G d3/p (z) have the same poles between application and removal of a force to a device (e.g., between times t 0  and t 1  in  FIGS. 40A-40C ). 
     Under ideal conditions (e.g., known values of system parameters and no coexistence issues caused by ambient noise (e.g., ambient pressure change) or internal noise (e.g., heat, or electrical or magnetic noise), estimated measurements such as the estimated displacement measurements should equal the actual displacement measurements. Under non-ideal conditions, the difference (or error) between estimated measurements and actual measurements can be quantified (and minimized) as follows: 
               min   ⁢              [             d   i     ⁡     (   n   )               ⋮               d   i     ⁡     (   0   )             ]     -     [               d   ^     i     ⁡     (   n   )               ⋮                 d   ^     i     ⁡     (   0   )             ]            2       =       min   ⁢              d   i     -       d   ^     i            2       =     min   ⁢              d   i     -       (       S     r   i       +     α   i       )     ⁢   p            2               
where d(n) corresponds to the actual displacement and d{circumflex over ( )}(n) corresponds to the displacement estimated from pressure, and where in the time domain, {circumflex over (d)} i (n)=(S r     i     (x,y) +α i )p(n).
 
     A relative sensitivity of the pressure sensor and a particular displacement sensor may be determined using the quantified error in the below convex optimization problem, as follows: 
     
       
         
           
             
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     Using the relative sensitivity of the pressure sensor and a particular displacement sensor, or the relative sensitivities of the pressure sensor and each displacement sensor in a set of multiple displacement sensors, a touch location may be determined as described with reference to any of  FIGS. 43-45 . 
       FIG. 43  shows a plan view of the device described with reference to  FIGS. 1A-1B and 37 , but only shows one of the displacement sensors  210  (i.e., the displacement sensor  210   a ). The relative sensitivities of the displacement sensor  210   a  and the pressure sensor  602  may be determined before the device  100  is immersed in liquid. A relative sensitivity may be determined for each or several touch location on the cover  102   b . In some cases, relatively sensitivities within different ranges may be grouped into relative sensitivity zones, such as the relative sensitivity zones  4302 ,  4304 ,  4306 , and  4308 . Regardless of whether relative sensitivities are grouped into zones, a map that associates relative sensitivities of the displacement sensor  210   a  and the pressure sensor  602  with locations (e.g., discrete locations or zones) may be generated and electronically stored. 
     When the device  100  is immersed in a fluid, a location of a user&#39;s touch on the cover  102   b  may be determined by retrieving, from the map, a location associated with a relative sensitivity determined using the techniques described with reference to  FIG. 42 . Given measurements generated by only one displacement sensor  210   a , the “location” may be limited to a “zone” or region of the cover  102   b.    
       FIG. 44  builds on what is shown in  FIG. 43  and shows how determining a relative sensitivity for each of first and displacement sensors  210   a ,  210   b  in relation to the pressure sensor  602  enables a touch location on the cover  102   b  to be determined as one of two discrete locations  4404   a  or  4404   b  where boundaries of the relative sensitivity zones  4306 ,  4402  for the first and second displacement sensors  210   a ,  210   b  intersect. 
       FIG. 45  builds on what is shown in  FIG. 44  and shows how determining a relative sensitivity for each of first, second, and third displacement sensors  210   a ,  210   b ,  210   c  in relation to the pressure sensor  602  enables a discrete touch location on the cover  102   b  to be determined as a location  4504  where boundaries of the relative sensitivity zones  4306 ,  4402 ,  4502  for the first, second, and third displacement sensors  210   a ,  210   b ,  210   c  intersect (i.e., the location  4504  is based on a relationship of three candidate locations defined by the relative sensitivity zones  4306 ,  4402 , and  4502 ). 
       FIG. 46  shows an example method  4600  of determining a location of a touch on a surface of a device, when the device is immersed in a fluid (e.g., water). The operations included in the method  4600  may in some cases be performed by a processor of the device. 
     At block  4602 , the method  4600  may include receiving, from a pressure sensor within the device and in response to a force applied to the device while the device is immersed in the fluid, a time-dependent sequence of pressure measurements. 
     At block  4604 , the method  4600  may include receiving, from a first displacement sensor within the device and in response to the force, a first time-dependent sequence of displacement measurements. The first time-dependent sequence of displacement measurements may include displacement measurements for a first location on the surface. 
     At block  4606 , the method  4600  may include determining the location of the touch using at least the time-dependent sequence of pressure measurements and the first time-dependent sequence of displacement measurements. 
     In some embodiments of the method  4600 , determining the location of the touch may include estimating a set of measurements in the first time-dependent sequence of displacement measurements using at least the time-dependent sequence of pressure measurements, as described with reference to  FIG. 42 . An error between generated and estimated measurements in the first time-dependent sequence of displacement measurements may then be quantifies; a relative sensitivity of the first displacement sensor and the pressure sensor may be quantified using the quantified error; and the determined relative sensitivity may be used to retrieve, from a map that associates relative sensitivities of the first displacement sensor and the pressure sensor with locations on the surface of the device, the location of the touch. 
     In some embodiments of the method  4600 , the method  4600  may further include receiving, from a second displacement sensor within the device and in response to the force, a second time-dependent sequence of displacement measurements. The second time-dependent sequence of displacement measurements may include displacement measurements for a second location of the surface to which the force is applied. The method  4600  may also include receiving, from a third displacement sensor within the device and in response to the force, a third time-dependent sequence of displacement measurements. The third time-dependent sequence of displacement measurements may include displacement measurements for a third location of the surface to which the force is applied. A set of measurements in each of the first time-dependent sequence of displacement measurements, the second time-dependent sequence of displacement measurements, and the third time-dependent sequence of displacement measurements may be estimated using at least the time-dependent sequence of pressure measurements. Errors between generated and estimated measurements in each of the first time-dependent sequence of displacement measurements, the second time-dependent sequence of displacement measurements, and the third time-dependent sequence of displacement measurements may be quantified. A first relative sensitivity of the first displacement sensor and the pressure sensor using a first quantified error, a second relative sensitivity of the second displacement sensor and the pressure sensor using a second quantified error, and a third relative sensitivity of the third displacement sensor and the pressure sensor using a third quantified error may be determined. The determined first, second, and third relative sensitivities may be used to retrieve, from a set of maps that associate relative sensitivities with locations on the surface of the device, three candidate locations of the touch. The location of the touch may then be identified based on a relationship of the three candidate locations. 
       FIG. 47  shows a sample electrical block diagram of an electronic device  4700 , which may be the electronic device described with reference to  FIGS. 1A-1B, 2A-2B, 5, 6A-6C, 15A-15C, 16A-16C, 19, 24, 25, 29, 37, 38, 39A-39E , or  43 - 45 . The electronic device  4700  may include a display  4702  (e.g., a light-emitting display), a processor  4704 , a power source  4706 , a memory  4708  or storage device, a sensor system  4710 , and an input/output (I/O) mechanism  4712  (e.g., an input/output device and/or input/output port). The processor  4704  may control some or all of the operations of the electronic device  4700 . The processor  4704  may communicate, either directly or indirectly, with substantially all of the components of the electronic device  4700 . For example, a system bus or other communication mechanism  4714  may provide communication between the processor  4704 , the power source  4706 , the memory  4708 , the sensor system  4710 , and/or the input/output mechanism  4712 . 
     The processor  4704  may be implemented as any electronic device capable of processing, receiving, or transmitting data or instructions. For example, the processor  4704  may be a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), or combinations of such devices. As described herein, the term “processor” is meant to encompass a single processor or processing unit, multiple processors, multiple processing units, or other suitably configured computing element or elements. The processor  4704  may be an example of the processor  110  described with reference to  FIG. 1B . 
     In some embodiments, the components of the electronic device  4700  may be controlled by multiple processors. For example, select components of the electronic device  4700  may be controlled by a first processor and other components of the electronic device  4700  may be controlled by a second processor, where the first and second processors may or may not be in communication with each other. 
     The power source  4706  may be implemented with any device capable of providing energy to the electronic device  4700 . For example, the power source  4706  may be one or more batteries or rechargeable batteries. Additionally or alternatively, the power source  4706  may be a power connector or power cord that connects the electronic device  4700  to another power source, such as a wall outlet. 
     The memory  4708  may store electronic data that may be used by the electronic device  4700 . For example, the memory  4708  may store electrical data or content such as, for example, audio and video files, documents and applications, device settings and user preferences, timing signals, control signals, data structures or databases, image data, or focus settings. The memory  4708  may be configured as any type of memory. By way of example only, the memory  4708  may be implemented as random access memory, read-only memory, Flash memory, removable memory, other types of storage elements, or combinations of such devices. 
     The electronic device  4700  may also include one or more sensors defining the sensor system  4710 . The sensors may be positioned substantially anywhere on the electronic device  4700 . The sensor(s) may be configured to sense substantially any type of characteristic, such as but not limited to, touch, force, pressure, light, heat, movement, relative motion, biometric data, and so on. For example, the sensor system  4710  may include a touch sensor, a force sensor, a heat sensor, a position sensor, a light or optical sensor, an accelerometer, a pressure sensor (e.g., a pressure transducer), a gyroscope, a magnetometer, a health monitoring sensor, and so on. Additionally, the one or more sensors may utilize any suitable sensing technology, including, but not limited to, capacitive, ultrasonic, resistive, optical, ultrasound, piezoelectric, and thermal sensing technology. The sensor system  4710  may include any of the sensors described herein. 
     The I/O mechanism  4712  may transmit and/or receive data from a user or another electronic device. The I/O mechanism  4712  may include a display, a touch sensing input surface such as a track pad, one or more buttons (e.g., a graphical user interface “home” button), one or more cameras, one or more microphones or speakers, one or more ports such as a microphone port, and/or a keyboard. Additionally or alternatively, the I/O mechanism  4712  (e.g., an interface or port) may transmit electronic signals via a communications network, such as a wireless and/or wired network connection. Examples of wireless and wired network connections include, but are not limited to, cellular, Wi-Fi, Bluetooth, IR, and Ethernet connections. 
     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, after reading this description, 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, after reading this description, that many modifications and variations are possible in view of the above teachings.

Metadata:
Filing Date: 20190823
Publication Date: 20201215
Grant Date: 20201215
Priority Date: 20180827
Inventors: GUPTA, PAVAN O.
JOYCE, ANDREW W.
DREVNIOK, BENEDICT
LI, MO
GRAFF, DAVID S.
LIN, ALBERT
SHUTZBERG, JULIAN K.
MOUSAVI, HOJJAT SEYED
Assignee: APPLE INC
CPC Classifications: [{"code": "H04M1/0266", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/0277", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M2250/22", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04M2250/22", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L1/142", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04106", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L19/0092", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/026", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01L9/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F2203/04107", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04M2250/12", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F2203/04106", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2203/04107", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04M2250/22", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0414", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L19/0092", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/0266", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L9/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F3/044", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F2203/04106", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L1/142", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/0277", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 69586866